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Biomarkers of Metabolic Syndrome: Biochemical Background and Clinical Significance

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Biomarkers of the metabolic syndrome are divided into four subgroups. Although dividing them in groups has some limitations, it can be used to draw some conclusions. In a first part, the dyslipidemias and markers of oxidative stress are discussed, while inflammatory markers and cardiometabolic biomarkers are reviewed in a second part. For most of them, the biochemical background and clinical significance are discussed, although here also a well-cut separation cannot always be made. Altered levels cannot always be claimed as the cause, risk, or consequence of the syndrome. Several factors are interrelated to each other and act in a concerted, antagonistic, synergistic, or modulating way. Most important conclusions are summarized at the end of every reviewed subgroup. Genetic biomarkers or influences of various food components on concentration levels are not included in this review article.
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REVIEW
Biomarkers of Metabolic Syndrome:
Biochemical Background and Clinical Significance
Harry Robberecht, DrSc, and Nina Hermans, PharmD
Abstract
Biomarkers of the metabolic syndrome are divided into four subgroups. Although dividing them in groups has
some limitations, it can be used to draw some conclusions. In a first part, the dyslipidemias and markers of
oxidative stress are discussed, while inflammatory markers and cardiometabolic biomarkers are reviewed in a
second part. For most of them, the biochemical background and clinical significance are discussed, although
here also a well-cut separation cannot always be made. Altered levels cannot always be claimed as the cause,
risk, or consequence of the syndrome. Several factors are interrelated to each other and act in a concerted,
antagonistic, synergistic, or modulating way. Most important conclusions are summarized at the end of every
reviewed subgroup. Genetic biomarkers or influences of various food components on concentration levels are
not included in this review article.
Introduction
Metabolic syndrome (MetS) is characterized by ab-
dominal obesity, hypertriglyceridemia, low high-
density lipoprotein cholesterol (HDL-C) level, increased
blood pressure, and elevated fasting glucose level.
1,2
It has become a significant public health problem and its
prevalence is likely to increase. Approximately 20%–30%
of the adult population in most countries is affected by
metabolic syndrome.
3
The prevalence is dependent on age,
gender, race, and diagnostic criteria.
4
It has been associated with an increased mortality from
cardiovascular disease (CVD), diabetes mellitus (DM) type
2, and various other causes.
5–7
Traditionally, assessment of MetS and risk for CVD has in-
volved the analysis of serum or plasma biomarkers, including
total cholesterol (TC), triglycerides (TGs), HDL-C, low-density
lipoprotein cholesterol (LDL-C), insulin, and C-peptide levels.
8
Biomarkers such as apolipoprotein A1 (apo-A1) and apo-
lipoprotein B (apo-B) have been proposed as more precise
predictors of atherogenicity and CVD risk.
9
Similarly, leptin,
adiponectin, free fatty acids (FFAs), and ghrelin are emerging
biomarkers of insulin resistance.
10–13
In the latter group,
adiponectin, ghrelin, and FFA have also been implicated as
biomarkers of coronary artery disease (CAD).
14–16
Although the exact mechanism underlying (MetS) has not
yet been elucidated completely, many cross-sectional or
longitudinal studies have shown that MetS is strongly asso-
ciated with inflammation,
17–19
insulin sensitivity,
20
endo-
thelial dysfunction,
21
renal dysfunction,
22
oxidative
stress,
23,24
and hepatic dysfunction.
25
Therefore, some re-
search groups use a mixture of several biological markers
as risk factors for metabolic syndrome. Additionally to the
traditional markers, white blood cell (WBC) count,
17
high-
sensitivity C-reactive protein
18
(hsCRP; as a marker of
inflammation), homeostasis model assessment of insulin
resistance (HOMA-IR) index
19
(as a marker of insulin re-
sistance), homocysteine
21
(as a marker of endothelial dys-
function), cystatin C
22
(as a marker of renal dysfunction), uric
acid
23
(as a marker of oxidative stress), gamma-glutamyl
transferase
24
(GGT or g-GT; as a marker of oxidative stress),
and alanine aminotransferase
25
(ALT; as a marker of hepatic
dysfunction) are assessed.
26
Therefore, we will not only review the traditional markers
(lipoprofiles or dyslipidemias) but also some new markers
related to the inflammatory response and consequent cardio-
vascular risk. Various cytokines are included in this group.
Due to the complexity of the syndrome with various in-
fluences and consequences for other diseases, it is hard to
make a well-defined distinction between various groups of
biomarkers.
MetS can be considered as a proinflammatory state,
27
therefore pro- and anti-inflammatory markers can be in-
cluded. Since oxidative stress, resulting in inflammation, has
consequences for cardiovascular risk of various types also,
here the different markers cannot be well separated.
Department of Pharmaceutical Sciences, NatuRA (Natural Products and Food Research and Analysis), University of Antwerp, Wilrijk,
Antwerp, Belgium.
METABOLIC SYNDROME AND RELATED DISORDERS
Volume 14, Number 2, 2016
Mary Ann Liebert, Inc.
Pp. 1–47
DOI: 10.1089/met.2015.0113
111
On the other side, metabolism-related peptides can also
be indicative for the risk of cardiovascular problems.
Due to the great amount of markers, cited in literature as
being related to the metabolic syndrome, we intend to
classify or subdivide them into various groups, according to
their biological role played in human metabolism and
pathological processes. Nevertheless, classifying the mark-
ers into groups has limitations for obvious reasons: the
complexity of the syndrome, the interactions of the various
biochemical pathways, and the overlap of these markers.
Therefore, some of these can be placed in various groups.
We have chosen to divide them into four major groups:
dyslipidemias, markers of oxidative stress, inflammatory
markers, and the cardiometabolic markers. The latter group
is quite extended and includes the classical parameters and
some new and promising ones.
In part 1, we discuss the dyslipidemias and markers of
oxidative stress, while in part 2, the inflammatory and car-
diometabolic markers are reviewed.
For several biomarkers, we make a distinction between
the biochemical background and the clinical significance,
although it is sometimes not that clear-cut.
Analytical aspects and reference intervals for the pa-
rameter are not discussed for obvious reasons.
In addition, DNA is becoming a molecular biomarker, as is
summarized in a recent brief review on biomarkers in medi-
cine.
28
Therefore, and also due to the length already of this
article, we will not discuss these various genetic markers (gene
polymorphisms, epigenetic aspects, including the miRNAs).
In a third review article, we will deal with the various
food components and dietary factors, influencing the bio-
markers, related to MetS.
29
Metabolic Syndrome
MetS comprises a cluster of cardiovascular risk factors.
The term was introduced in the early 1990s by Reaven,
30
who also called it syndrome X or insulin resistance syn-
drome. Insulin resistance, detected by various methods, is a
key factor associated with the deadly quartet: increased
blood glucose, excess of body fat, increase in blood pres-
sure, and cholesterol abnormalities.
Resistance to the metabolic action of insulin results in
various biological effects as summarized in Table 1.
All these features are promoting factors of atherosclerosis
and thus MetS constitutes a significant risk for coronary
heart disease.
6,7,19,31–33
Therefore, the syndrome can also be
described as a cardiometabolic syndrome, which stimulates
us to distinguish a group of cardiometabolic markers or risk
factors.
Other synonyms used in the past are hypertension-
hyperglycemia-hyperuricemia syndrome, dysmetabolic syn-
drome X, metabolic dyslipidemia, the deadly quartet and civ-
ilization syndrome.
The vast majority of the definitions
1,2
share the same
components: a surrogate of obesity [body mass index (BMI)
or waist circumference], hypertension, altered glucose me-
tabolism (elevated fasting glucose level or impaired glucose
tolerance), elevated triglyceride, and low HDL-C.
34
The World Health Organization and Programs, including
the National Cholesterol Education Program (NCEP) and
Adult Treatment Program III (ATP III), have agreed
35,36
to
consider MetS as a disease with the constellation of metabolic
abnormalities requiring at least three of the following symp-
toms: increased waist circumference (>102 cm for men,
>88 cm for women), elevated serum triglyceride levels
(150 mg/dL), reduced HDL-C (<40 mg/dL for men, <50 mg/
dL for women), increased blood pressure (130/85 mmHg),
and increased fasting blood glucose levels (100 mg/dL).
37
It is always important to control quite well the definitions
used since adiposity is used frequently instead of obesity.
There is an incomplete overlap between the metabolic syn-
drome and obesity,
38
although various interactions between
obesity, diabetes, and inflammation may result in a better
understanding of the pathogenesis of various cardiovascular
problems.
39
Several pathophysiological explanations for the metabolic
syndrome have been proposed involving insulin resistance,
chronic inflammation, and ectopic fat accumulation.
38
Significant heterogeneity exists between men and women
developing the metabolic syndrome, in large part related to
hormonal regulation of body fat distribution and attendant
metabolic abnormalities.
40
More details can be traced from a review article.
41
Biomarkers
A biomarker is a measured characteristic that may be
used as an indicator of some biological state or condition. In
health and disease, biomarkers have been used not only for
clinical diagnosis purposes but also as tools to assess ef-
fectiveness of a nutrition or drug intervention. When con-
sidering nutritional or pharmacological studies, evaluating
the appropriate biomarker is a useful tool to assess com-
pliance of a particular dietary component or drug in the
biochemistry of an organism.
42
According to the National Institutes of Health (NIH)
Biomarker Working Group (USA), a diagnostic biomarker
is defined as a characteristic that is objectively measured
and evaluated to identify disease progress in patients. Al-
ternatively, prognostic biomarkers serve as tools to classify
and evaluate the disease progression. Finally, efficacy
Table 1. Some Biological Effects
of Insulin Resistance
Metabolism of Effects
Carbohydrates Hyperglycemia
Hyperinsulinemia
Lipids Lipolysis [
FFA and glycerol [
Synthesis of triglycerides
and apo-B-protein [
Triglycerides [
HDL levels Y
Small dense LDL [
Proteins Gluconeogenesis [
Protein catabolism [
Protein synthesis Y
Purines Uric acid formation [
Uric acid clearance Y
Hyperuricemia
FFA, free fatty acid; HDL, high-density lipoprotein; LDL, low-
density lipoprotein.
arrow up, enhanced; arrow down, decreased.
2 ROBBERECHT AND HERMANS
biomarkers are used to monitor clinical responses and ben-
eficial effects of new therapeutic intervention strategies.
43
Biomarkers can reflect the entire spectrum of disease from
the earliest manifestations to the terminal stages and are one
of the driving forces of pharmaceutical research and drug
development.
The biological phenotype (insulin resistance, dyslipide-
mia) results in a clinical phenotype ( MetS), which contributes
to the development of a proinflammatory state characterized
by increased oxidative stress (e.g., oxidized lipoproteins) and
subclinical vascular inflammation (e.g., increased CRP).
The subclinical inflammatory state modulates an ath-
erosclerotic process at different stages, resulting in (1)
endothelial dysfunction and increased expression of en-
dothelial adhesion molecules; (2) an enhanced recruitment
of monocytes within the arterial wall in the early stages of
atherogenesis; leading to (3) the formation of an unstable
atherosclerotic plaque, rich in inflammatory cells. Since the
metabolic syndrome can be considered as a chronic cardio-
vascular inflammatory condition,
44
we feel that inflammatory
and cardiometabolic markers are two important groups of
biomarkers for this syndrome.
However, due to the fact that the syndrome is chronic,
multifactorial, and consists of various stages, it remains hard
to indicate at what stage or in which group a parameter must
be classified at that moment. Furthermore, the metabolic
syndrome is not only nonstatic but it may also progress and
cohabit with other related pathologies with high mortality
risk. An early diagnosis of the syndrome is crucial to avoid
the progression to severe pathologies, such as diabetes,
atherosclerosis, and liver fibrosis.
Several efforts have been made in the discovery of non-
invasive biomarkers capable of scanning and identifying
specific signs of the progress of disease, both in treated and
untreated patients.
45
Circulating factors have been associated with the devel-
opment and progression of MetS and its related co-
morbidities.
46
They can unravel the underlying pathophys-
iology of MetS and serve to identify novel targets for early
preventions.
34
Part 1: Dyslipidemias and Markers
of Oxidative Stress
Dyslipidemias
Dyslipidemia is characterized by a spectrum of qualitative
and quantitative lipid abnormalities reflecting perturbations in
the structure, metabolism, and biological activities of both
atherogenic lipoproteins and antiatherogenic HDL-C. This
includes an elevation of chylomicrons, VLDL, LDL, lipo-
proteins containing apo-B, elevated TGs, and low levels of
HDL-cholesterol.
47
Classical metabolic dyslipidemias
Dyslipidemia is since long related to metabolic syn-
drome.
48,49
Traditional serum or plasma biomarkers include
TC, TG, and HDL-C (or the TC/HDL-C ratio).
50–57
Therefore,
these biomarkers can be referred to as classical dyslipidemias.
A more extended list is presented in Table 2.
A study in Europe used the classical lipid parameters in a
Metabolic Syndrome and Cancer Project ( Me-Can) and found
a decreased risk of incident breast cancer in women below age
50 and an increased risk of breast cancer mortality in women
above 60.
58
The biomarkers, apo-A1 and apo-B, have been proposed
as more precise predictors of atherogenicity and CVD risk.
9
Therefore, the metabolic dyslipidemias are quite frequently
described as atherogenic dyslipidemia.
59,60
The essential part of managing dyslipidemia consists of
lifestyle intervention such as maintaining an adequate diet
and a physical activity program. When pharmacologic therapy
becomes necessary, HMG-CoA (3-hydroxy-3-methylglutamyl-
coenzyme A) reductase inhibitors (statins) and fibrates are
the most effective drugs in controlling the metabolic syn-
drome hyperlipidemia. Fibrates
60
are effective in lowering
triglycerides and increasing HDL-C levels, the two most
frequent abnormalities associated with metabolic syndrome.
Statins are effective in lowering the LDL-C level. A combi-
nation of fibrates and statins is highly effective in controlling
abnormalities of lipid profile in patients with metabolic syn-
drome.
61
Apolipoprotein A1
Biochemical background. Apo-A1 is the main structural
protein found in HDL. This lipoprotein plays a critical role
in the transport of cholesterol by moving excess cholesterol
from tissues to the liver. There are two major forms of apo-
A in humans, apo-A1 and apo-A2. The former is the most
abundant form and is normally present in high concentra-
tions in plasma and extravascular tissue. Both forms are
synthetized by the gut and the liver. Besides its structural
role in HDL, apo-A1 also acts as a cofactor for the enzyme
lecithin-cholesterol acyltransferase, which converts choles-
terol to cholesterylesters.
Clinical significance. Measurements of apo-AI levels
have been very useful in the diagnosis and monitoring of
genetic diseases associated with low HDL-C levels, in-
cluding familial low HDL concentration and familial apo-AI
deficiency.
There is conflicting evidence regarding the advantage of
apo-AI over HDL-C as a risk factor of CVD, as is discussed
Table 2. Dyslipidemia Components Associated
with Metabolic Syndrome
57
Increased TG and TG-rich lipoproteins
Increased postprandial TG
Low HDL-C
Increased TC/HDL-C ratio
Low apo-A1
Small HDL, prebeta-1 HDL, and alpha-3 HDL
Increased apo-B
Increased LDL particle number
Small dense LDL
Increased apo C-III
Increased non-HDL-C
Increased oxidized and glycated lipids
Apo-A1, apolipoprotein A1; apo-B, apolipoprotein B; HDL-C,
high-density lipoprotein cholesterol; TC, total cholesterol; TG,
triglyceride.
BIOMARKERS OF METS 3
in some prospective studies. One found that apo-A1 is able
to discriminate between patients with and without CVD.
Two studies did not find that apo-AI provided additional
value.
62–64
However, many reports in the literature discuss the
complementary predictive value of apo-B-to-apo-A1 ratio.
This seems to be higher in metabolic syndrome and a good
predictive marker of metabolic and premetabolic syn-
drome.
65–68
The ratio was associated with carotid artery
intima-media thickness,
69,70
an early marker of atherogen-
esis
71,72
and a risk factor of myocardial infarction in older
aldults.
73
Apolipoprotein B
Biochemical background. Apo-B plays a critical role in
cholesterol metabolism. One molecule of apo-B is found
in each chylomicron, VLDL, and LDL particle.
74
Apo-B
constitutes a large proportion of the total protein in LDL.
The two main forms are apo-B100 and apo-B48. Apo-B100
is mainly synthesized by the liver and facilitates the trans-
port of cholesterol from LDL to various tissues throughout
the body by acting as a ligand for LDL receptors. Apo-B48
is only produced by the intestine
75
and is involved in the
catabolism of chylomicron remnants by the liver.
Clinical significance. As with apo-AI, the role of apo-B
in CVD risk assessment is controversial and needs to be
further investigated. A large number of studies support the
hypothesis that there is a correlation between lipoproteins con-
taining apo-B and the development of atherosclerosis.
64,76–79
Furthermore, studies have shown that apo-B concentrations
may be more discriminative for CVD than traditional bio-
markers.
80–82
Moreover, a few prospective studies have shown
the ability of apo-B concentrations to predict CVD risk, but
whether their ability is more precise than serum lipid mea-
surements is inconclusive.
76–78,83,84
With regard to metabolic syndrome, an association between
a number of features of the syndrome and higher apo-B100
levels has been found.
85
A Mediterranean diet reduces LDL-
apo-B100 concentrations primarily by increasing the catabo-
lism of LDL in male metabolic syndrome participants.
86
In
addition, soy nut consumption improved lipid profiles and
lowered apo-B100 concentration.
87
Lipoprotein (a)
Biochemical background. Lipoprotein (a) [Lp(a)] is an
LDL-like particle with a specific apolipoprotein (a) [apo(a)],
bound to apo-B100 by a disulfide bridge.
88
Sequencing the
apo(a) cDNA shows high homology with plasminogen.
89
Therefore, there is considerable interest in the apolipoprotein
that links altered fibrinolysis, a procoagulant phenotype and
atherosclerosis.
90
Lp(a) is a glycoprotein with a large genetic
polymorphism. The abbreviation may not be confused with
Lp-PLA
2
(lipoprotein-associated phospholipase A
2
).
Clinical significance. Lp(a) is a known risk factor for
CVD,
91
but its influence on metabolic syndrome is still
controversial.
92
In recent literature, Lp(a) levels have been
investigated in MetS and were found to be higher in the MetS
group
93,94
and associated with increased proinflammatory
status in newly diagnosed MetS patients.
95,96
Only one pub-
lication could be traced in which Lp(a) levels were inversely
related to MetS and its components.
97
An explanation for these contradictory observations may be
that the plasma concentration is affected by several pathologies
(e.g., diseases of liver and kidney), hormonal factors (e.g.,
sexual steroids, glucosteroids, thyroid hormones), and indi-
vidual and environmental factors (e.g.,age,
98
cigarette smok-
ing), as well as pharmaceuticals (e.g., derivatives of nicotinic
acid) and therapeutic procedures (lipid apheresis).
99
Free fatty acids
Biochemical background. FFAs, also known as nones-
terified fatty acids, are an important energy source for the
body, especially in the fasting state.
100
When triglycerides
within the adipose tissue undergo lipolysis, FFAs are re-
leased into circulation. This process is inhibited by insulin
and stimulated by catecholamines, glucagon, and adrenal
corticoids.
101
FFAs within the blood are bound to albumin
and transported to various tissues in the body, including the
liver, muscle, and heart. Muscle and heart utilize FFAs as a
major source of energy. The liver takes up a portion of the
circulating FFAs and either oxidizes them or forms tri-
glycerides. Furthermore, FFAs form esters with cholesterol
and ultimately become fundamental structural components
of LDL-cholesterol and VLDL-cholesterol cores, as well as
membrane phospholipids.
96
Clinical significance. Many studies have associated in-
creased plasma FFA concentrations with type 2 DM, obesity,
and other insulin-resistant states.
102,103
FFAs have been im-
plicated in the inflammatory process as activators of the IkB/
NF-kBpathway.
104
Those studies suggest that elevated FFAs
may not only contribute to hepatic and peripheral insulin
resistance but also to the development of CADs.
Fatty acid-binding protein 4 (FABP4) chaperones FFAs in
the adipocytes during lipolysis. Serum FABP4 is a biomarker,
which will be discussed as an adipokine, and is more related
to metabolic risk than FFA.
105
In the latter study, the FFA
level measured after glucose loading provided a better risk
assessment than the fasting values of this marker.
Serum fatty acid pattern reflecting low linoleic acid and
high saturated fatty acids was strongly associated with MetS
in individuals with chronic kidney disease.
106
The higher the concentration of saturated fat, monoun-
saturated fat, and trans fat, the higher the risk of MetS.
107
It
was also observed that a diet rich in saturated fatty acids
induces the same fatty acid pattern in serum lipid esters as
seen in persons with MetS.
108,109
Consequently, the fatty acid composition in serum samples
can be used not only as a biomarker of fat quality but also as
an indicator of disease risk
110
because an altered fatty acid
composition has been related to metabolic disease and car-
diovascular disorders in observational and intervention stud-
ies.
108,111
Erythrocyte fatty acid profile not only shows an
association with metabolic syndrome but also correlates with
most of its individual components.
112
Interestingly trans fatty
acids displayed an antagonistic behavior. However, elaidic
acid isomer (C18:1n9t) had a strong association with MetS,
while the vaccenic isomer (C18:1n7t) confers a protective
effect. Therefore, a separate analysis of the various trans fatty
acids is a more accurate approach to determine their role in the
pathogenesis of MetS or other related metabolic disorders.
112
All these findings support current dietary recommendations
to increase polyunsaturated fatty acids (PUFA) intakes and
restrict saturated fatty acid intakes.
113
4 ROBBERECHT AND HERMANS
Table 3 summarizes most remarkable results on lipid markers.
Markers of Oxidative Stress (Antioxidative
Profile)
Introduction
Metabolic overload evokes oxidative stress, a condition in
which an imbalance exists between the production and inac-
tivation of reactive oxygen species (ROS). These substances
can be best described as double-edged swords. They play an
essential role in multiple physiological systems, but under
conditions of increased oxidative stress, they contribute to
cellular dysfunction.
114–116
The potential role of oxidative stress in MetS is rapidly
evolving
117
and results support the concept that increased
stress may play an important role in metabolic syndrome-
related manifestations (atherosclerosis, hypertension, and
type 2 diabetes).
118–120
This suggests that oxidative stress
could be an early event in the pathology of these chronic
diseases rather than merely a consequence or an innocent
bystander.
121
MetS is accompanied by a chronic low-grade inflamma-
tory condition that does not completely fit into the classical
definition of acute or chronic inflammation as it is not ac-
companied by infection. There is no massive tissue injury
and the dimension of the inflammatory activation is not that
large. So, it is often called low-grade chronic inflammation
or metainflammation, meaning metabolically triggered in-
flammation
122
or even parainflammation, an intermediate
state between basal and inflammatory state.
123
Whatever the
term used, the inflammatory process that characterizes
metabolic syndrome has its own unique features, but its
causes are far from being fully understood.
124
The low-grade inflammatory condition is a major factor
both in the installation and the associated pathophysiologi-
cal consequences.
124–126
Some findings
127,128
prove that
inflammation explains some of the risks for CVDs, but other
mechanisms may be more important.
Since both processes (oxidative stress and low-grade in-
flammation) are quite well related to each other, we feel that
there is a sequence in processes and activity and therefore
we first discuss the oxidative markers and afterward the
markers of inflammation.
Some publications deal with pro-oxidative markers, markers
of oxidative stress, while others mentioned antioxidative
markers, an antioxidative profile, or oxidative status.
129
A similar remark can be made at the level of inflammation,
where proinflammatory, inflammatory, and anti-inflammatory
markers are mentioned.
19,114,130
Sometimes the words, in-
flammation burden, are used.
131
Antioxidant systems
The natural antioxidant system consists of a series of anti-
oxidant enzymes and numerous endogenous and dietary anti-
oxidant compounds that react with and inactivate ROS. The
primary antioxidant enzymes include, but are not limited to,
superoxide dismutase (SOD), catalase (CAT), and glutathione
peroxidase (GPx). Meanwhile, the nonenzymatic antioxidants
include, among others, vitamin C, vitamin E, b-carotene, re-
duced glutathione (GSH), and numerous phytochemicals. Cells
must maintain their levels of antioxidants and coenzyme Q10,
often defined as their antioxidant potential, through dietary
intake and/or de novo synthesis.
SOD catalyzes the dismutation of peroxides to O
2
and the
less active H
2
O
2
.
132
In addition to being produced by the
SOD-catalyzed dismutation, H
2
O
2
is synthetized by many
other enzymatic reactions.
Since this compound is a powerful oxidizing agent, cells
express abundant CAT, GPx, and thioredoxin that convert
H
2
O
2
to water and molecular oxygen. Peroxides are gener-
ally inactivated by either CAT or GPx. CAT is a peroxidase:
2H
2O242H2OþO2
GPx, on the other hand, is a selenium-containing tetra-
meric enzyme that reduces H
2
O
2
, lipid peroxides, and other
organic hydroperoxides to their corresponding hydroxylated
compounds using glutathione as a hydrogen donor:
ROOH þ2 GSH4ROH þGSSG þH2O133
SOD enzymes deal with the toxic superoxide radical by
alternately adding or removing an electron from the super-
oxide molecules it encounters, thus changing the O
2
-
into
one of two less damaging species, either molecular oxygen
(O
2
) or hydrogen peroxide (H
2
O
2
). This SOD-catalyzed
dismutation of superoxide may be written, for Cu/Zn SOD,
with the following half-reactions:
Cu2þSOD þO2
/Cu þSOD þO2
CuþSOD þO2
þ2H þ/Cu2þSOD þH2O2
Markers of oxidative stress
Table 4 summarizes the most cited markers for oxidative
stress in blood.
Most of the time, plasma thiobarbituric acid-reactive sub-
stances (TBARS), erythrocyte glutathione, and plasma GPx
were chosen because they all are implicated in the potential
pathways linking oxidation to pathological processes. In
particular, plasma TBARS were chosen because they have
been hypothesized to represent a composite number of oxida-
tive damage products, including malondialdehyde ( MDA).
134
Although these substances have been criticized as being non-
specific markers of lipid peroxidation, some studies have
shown good correlation between increases in TBARS and
increases in levels of isoprostane (a potentially more spe-
cific marker of lipid peroxidation) in response to induced
oxidative stress.
135
However, some publications deal with
Table 3. Summary of Literature Results Found
for Lipid Biomarkers
Marker Observation
Dyslipidemia Table 2
Apo-A1 Unclear
Better: apo-B/apo-A1 ratio
Apo-B100 Higher concentration
Lp(a) Controversial
FFA More interesting: FABP4 (see adipokines)
Better: fatty acid composition
FABP4, fatty acid-binding protein 4; Lp(a), lipoprotein (a).
BIOMARKERS OF METS 5
MDA as a more reliable marker compared with TBARS since
it is measured more accurately by high-performance liquid
chromatography–fluorescence detection.
136,137
MDA measurement in metabolic syndrome has been
discussed elsewhere.
138,139
Erythrocyte glutathione has direct radical scavenging
ability, and levels of erythrocyte glutathione may reflect the
glutathione activity in other tissues.
140
It also represents a
substrate for GPx.
141
GPx appears to be sensitive to body
selenium stores.
142
A thorough review of the function of
both glutathione and GPx can be found elsewhere.
141
In this article, we intend to make some distinction between
enzymes and other markers of oxidative stress. The more
interested reader can be referred to an extensive review ar-
ticle on origin, link, measurement, mechanisms of action, and
biomarkers of oxidative stress in various human diseases.
143
In this study, the authors analyzed articles in which 71 dif-
ferent biomarkers of oxidative stress are mentioned.
Enzymes and enzyme inhibitors
GPx and selenium
Biochemical background. Selenium (Se) is an essential
trace element in a number of selenoproteins and enzymes.
144,145
There are four seleno-GPx isoenzymes encoded by different
genes, which vary in cellular location and substrate speci-
ficity. Among them, GPx3 is circulating in human plasma and
a defect in GPx3 expression in adipocytes is associated with
reduced systemic GPx activity in obesity.
146
Clinical significance. Although many questions still
remain to be solved, there is an increasing interest in the
clinical significance of this trace element,
147
and blood se-
lenium levels are influenced by different factors.
148
Serum selenium has been found to be associated with an
MetS component HDL-C since nearly 30 years.
149
Serum Se
concentration increased significantly with TC, triglycerides,
LDL-C, and serum glucose in Taiwanese elderly patients
150
as well as in Serbian MetS patients with schizophrenia.
151
This is contrary to a study in Caucasian patients in England
where serum levels decreased significantly with accumu-
lating features of MetS.
152
No relationship was found between Se concentration and
MetS in the United States,
153
elderly patients in Croatia,
154
a
limited study in Greece,
155
and the Su.Vi.Max trial for a
much larger population in France.
156
A recent study in Eu-
ropean MetS patients showed that selenium was positively
associated in women, but not in men.
157
Although for the Croatian population no association with
the serum Se level was found, significantly higher GPx ac-
tivity was observed in subjects with MetS.
154
In a Japanese
population, a metabolic syndrome group revealed selenium
levels negatively correlating with monocyte chemoattractant
protein-1 (MCP-1).
158
This protein (MCP-1) is critical for the
initiation and development of atherosclerotic lesions.
159
An-
other study
160
reported high MCP-1 concentration associated
with endothelial dysfunction.
All these studies reported simple correlations in various
ethnic and age groups and do not allow to infer any causal
relationship between selenium levels and MetS.
Elevated levels of another selenoprotein P (SeP) are
independently associated with a reduced risk of MetS in
children.
161
Further study is needed to examine the molecular
mechanism of selenium (in which enzyme or what pro-
tein) in adipocytokine dysregulation and other inflamma-
tory markers.
162–164
Superoxide dismutase and xanthine oxidase. Serum
SOD activity negatively correlates with the components of
metabolic syndrome,
165
while another research group
154
claimed that no clear influence of MetS on SOD activity
could be found. Nevertheless, it must be stated that nearly
all publications traced deal with a significantly lower SOD
activity in subjects with metabolic syndrome.
139,165–168
Most of the time, other enzymes (GPx
164
and the oxida-
tive enzyme, xanthine oxidase
139,168
) are measured as well.
The activity of the latter enzyme was found to be signifi-
cantly higher in subjects with metabolic syndrome com-
pared with a control group.
139,168
Gamma-glutamyl transferase
Biochemical background. GGT is a cell surface protein
contributing to the extracellular catabolism of GSH.
169
This
enzyme is produced in many tissues, but most GGT in serum is
derived from the liver.
170
In serum, GGT is carried primarily by
lipoproteins or albumin.
171
Serum levels of this enzyme are
influenced by several factors: alcohol intake, body fat content,
plasma lipoprotein and glucose levels, and various medica-
tions.
169,172,173
Clinical significance. In the Framingham study, an in-
creased serum GGT level predicted the onset of MetS, the
occurrence of CVD, and finally death.
174
The elevation of GGT is closely related to hepatic
steatosis,
175–177
the latter in turn strongly associated with
MetS.
178–182
High levels of GGT are not the only hepatic
biomarkers of this syndrome. High levels of serum trans-
aminases have been associated with metabolic syndrome as
well.
183–185
Other lines of evidence support the relationship
between elevated serum GGT and MetS,
24,186–193
hyper-
tension,
194–198
and enhanced risk of diabetes.
199,200
This
was reviewed in an editorial
201
and later extended in other
studies.
202,203
Table 4. Markers of Oxidative Stress in Blood
DNA/RNA damage
8-Hydroxyguanosine (8-OHG)
8-Hydroxydeoxyguanosine (8-OHdG)
Lipid peroxidation
8-Iso-prostaglandin F2 alpha (8-isoprostane)
Malondialdehyde (MDA)
Thiobarbituric acid-reactive substances (TBARS)
Protein oxidation/nitration
3-Nitrotyrosine
Advanced glycation end products
Reactive oxygen species
ROS/RNS (O
2
,HO
, HOO
)
H
2
O
2
NO
Antioxidants
Catalase (CAT)
Superoxide dismutase (SOD)
Glutatione peroxidase
Glutathione
Oxygen radical antioxidant capacity
Total antioxidant capacity
ROS, reactive oxygen species; RNS, reactive nitrogen species.
6 ROBBERECHT AND HERMANS
The elevated level of serum GGT in the list of biomarkers
linked to MetS largely reflects ectopic liver fat or secondary
inflammation.
169
Measuring this marker is simple and in-
expensive and it is used as a sensitive marker of increased
oxidative stress and MetS.
204
Pigment epithelium-derived factor
Biochemical background. The pigment epithelium-
derived factor (PEDF) belongs to the superfamily of serine
protease inhibitors (serpins), and the major sources of cir-
culating PEDF are liver and adipose tissues.
205
Therefore,
this factor can be classified as an adipokine,
206
but the an-
tioxidant and anti-inflammatory properties of this marker
convinced us to discuss this factor here.
207
Clinical significance. PEDF is a multifunctional pro-
tein possessing antiangiogenic, antitumorogenic, antioxi-
dant, anti-inflammatory, antithrombotic, and neuroprotective
properties. Demonstrated as a highly effective antiangio-
genic factor, PEDF is not only capable of inhibiting vascu-
lar endothelial growth and migration but can also suppress
secretion of angiogenic factors
207
as well as activate the
FAS-FAS ligand death pathway to stimulate endothelial cell
apoptosis.
208
Serum levels of PEDF are elevated in morbidly obese pa-
tients
209
and are associated with the visceral fat depot in
Caucasian
210
as well as Japanese people.
211
Circulating PEDF
concentrations were shown to be higher in subjects with met-
abolic syndrome and to be correlated with the extent of MetS
components.
212–214
Most of these studies are cross-sectional
and thus could not assess the question whether elevation of
PEDF was a cause or a consequence of the metabolic syn-
drome. The elevation may be a protective response against
vascular damage in the metabolic syndrome due to the anti-
oxidant and anti-inflammatory properties of the protein.
212
Serum PEDF levels are independently positively associated
with CAD in a Chinese population and may act as a protective
response against vascular damage and subsequent CAD.
215
Therefore, the factor is considered as a biomarker for athero-
sclerosis
216
and became a therapeutic target in CVD.
217
Due
to these characteristics, this factor could be classified and
discussed under cardiometabolic markers as well.
Other markers
Oxidized LDL. Oxidized or in other ways modified
LDL particles have antigenic properties leading to an antibody
response and an inflammatory reaction.
218,219
This is a driving
force in the formation of atherosclerotic lesions.
220
The small
dense LDL particles have a tendency to be easily oxidized.
221
Therefore, these oxidized lipoproteins are considered to be
associated with the cardiovascular risk factors in MetS.
222,223
This was extensively studied by a group in Sweden
222,224
and in
the Health, Aging, and Body Composition study, a cohort of
well-functioning older adults in the United States.
225,226
In the
Unites States, the observed elevated level of circulating oxi-
dized LDL was in contrast to the results in Sweden on healthy
men with MetS.
227
This discrepancy has been extensively
discussed in the literature.
228,229
Since then, however, elevated circulating oxidized LDL
levels are found for different groups with MetS in various
countries.
131,230–236
The levels have been found to be reduced by specific
nutritional therapy
237
or by drinking citrus-based juice
238
or
taking certain food supplements. A recent study describes
the oxLDL lowering effect of a red yeast rice–olive extract
on MetS patients.
239
However, more clinical trials have to
be conducted in this area.
Quite recently, a soluble lectin-like oxidized LDL receptor
1 is added to the list of mediators of endothelial dysfunction
in patients with MetS.
240
For this receptor, protein gene
polymorphism seems to play a role.
241
Uric acid
Biochemical background. Uric acid is a product of the
metabolic breakdown of purine nucleotides from the genetic
material. The enzyme, xanthine oxidase, forms uric acid
from xanthine and hypoxanthine, which in turn are produced
from other purines.
High intake of dietary purine, high-fructose corn syrups,
and table sugar can cause increased levels of uric acid
242,243
while reduced excretion by the kidneys can also be impli-
cated.
High blood concentrations of this acid can lead to gout or
calculi formation in kidneys and the bile bladder.
Clinical significance. Uric acid is a strong reducing agent
and hence a potent antioxidant. In humans, over half the an-
tioxidant capacity of blood plasma is due to uric acid.
244
Elevation of this marker has been long recognized in
obesity and its close association with metabolic syndrome
has been established.
245–248
Elevated uric acid is a strong
predictor of the onset of metabolic syndrome in longitudinal
studies.
242,249–251
It seems to be the most reliable biomarker
to identify obese young females with metabolic syn-
drome.
252
Even salivary uric acid can be used as a nonin-
vasive biomarker of MetS and its elevation correlates with
several cardiovascular risk factors.
253
As seen in other studies, the relationship between uric
acid in serum and metabolic syndrome was stronger in fe-
males than in males.
252,254–256
Uric acid further tends to worsen insulin resistance and
related abnormalities, including hyperlipidemia and fatty
liver. It could mediate unfavorable metabolic effects of
excess fructose intake.
257–260
It is quite contradictory that uric acid induces oxidative
stress and unfavorable adipokine secretion patterns in cul-
tured mouse 3T3-L1 adipocytes, a murine model of meta-
bolic syndrome.
259
Therefore, there is still no consensus if
uric acid is a protective or a risk factor. It seems that acute
elevation is a protective factor, whereas chronic elevation is
a risk for disease.
261
Uric acid is not an independent factor to predict the
metabolic syndrome
262
and whether the association is causal
remains to be investigated.
263,264
Some authors claim that
uric acid is an independent risk factor for incidence of ca-
rotid atherosclerosis in elderly patients without MetS
265
or
obese children.
266
After all, most of the observations provide some indica-
tion for a more aggressive lowering of circulating uric acid
in obese and metabolic syndrome patients.
267
Bilirubin. Bilirubin has been regarded as a potentially
toxic metabolite of the heme catabolism, but recent data
have suggested numerous beneficial effects.
268
Bilirubin has well-known antioxidative and anti-
inflammatory properties, as evidenced by its ability to
scavenge peroxyl radicals, to inhibit LDL oxidation, and to
downregulate expression of the cellular adhesion molecules,
BIOMARKERS OF METS 7
vascular cell adhesion molecule 1 (VCAM-1) and intercel-
lular adhesion molecule 1 (ICAM-1) in vitro.
269
In line with these beneficial effects, mildly elevated bil-
irubin has been negatively associated with oxidative stress-
mediated diseases such as MetS.
270–272
One publication
specifies that in Korean adults, the fasting direct bilirubin is
more related to MetS than the other types of bilirubin.
273
Most of these studies have been carried out with populations in
the Asian part of the world (China, Korea, Japan).
270,272–276
Although the inverse associations in these various in-
vestigations do not mean that low bilirubin is a risk factor
for MetS,
275
they suggest that serum total bilirubin concen-
tration in the upper normal range may provide some protec-
tion against MetS
277
and diminish a future risk of CAD.
278
The fact that hsCRP is inversely related to bilirubin fur-
ther suggests that low bilirubin is implicated in enhanced
low-grade systemic inflammation.
279
The role of bilirubin in MetS is more extensively dis-
cussed somewhere else.
280
Ferritin. Ferritin is the major intracellular iron storage
protein in all organisms. This protein, indicating the cellular
iron status, is mainly synthetized in the liver.
281
Iron over-
load (measured by serum ferritin levels) is a risk factor for
oxidative damage and can therefore be included as a marker
of oxidative stress.
Serum ferritin levels are often elevated in MetS (dys-
metabolic hyperferritinemia) and sometimes associated with
a true-to-moderate hepatic iron overload (dysmetabolic iron
overload syndrome). Therefore, ferritin
282–285
or ferritin and
transferrin,
286
both markers of iron metabolism, or even
serum iron
287
are increased in MetS.
The levels of hepcidin, a key iron regulatory hormone,
increased linearly with increasing number of MetS features,
paralleling the trend of serum ferritin.
288
Studies in Korea revealed that elevated iron stores were
positively associated with serum ALT levels and metabolic
syndrome in postmenopausal women.
289
Elevated serum
ferritin concentrations were associated with an increased
risk of MetS in a representative sample of the adult South
Korean population
290
or middle-aged Korean men.
291
For the same country, some other researchers
292
found
that serum concentration of this marker was only associated
with insulin resistance and glucose metabolism among men,
but not among women. A similar sex-dependent association
was not found for Chinese adults.
293
Quite remarkably, a study in Spain revealed that serum
ferritin is commonly increased in familial combined hy-
perlipidemia and familial hypertriglyceridemia.
294
For India, a high prevalence of metabolic syndrome in
young patients with acute myocardial infarction was regis-
tered and serum ferritin was found to be elevated.
94
Since serum ferritin is an independent predictor of ad-
vanced fibrosis in patients with nonalcoholic fatty liver
disease (NAFLD),
295
the link between ferritin, metabolic
syndrome, and NAFLD offers not only elective attractions
but also dangerous liaisons. Trombini and Piperno
296
sum-
marized these links among ferritin (and iron), metabolic
syndrome, and NAFLD in a nice figure. They argued that
studies on iron-related gene expression and genetic poly-
morphisms may help to clarify the alteration of iron ho-
meostasis associated with metabolic syndrome.
Vitamin E. Alpha-tocopherol, the major form of vitamin
E, acts as an antioxidant vitamin in the human body. Supple-
mentation studies have revealed a significant decrease of bio-
markers of oxidative stress and inflammation.
297,298
However,
supplementation of the alpha form in combination with
gamma-tocopherol appears to be superior in this action.
299
Several studies reported that serum vitamin E concen-
tration is lower in patients with metabolic syndrome than in
controls, showing unbalanced serum redox status with de-
creased lipid antioxidant capacity.
138,153,300
In a study in
Taiwan, the higher concentrations of vitamin E in subjects
with MetS were not significant after normalizing for tri-
glyceride level.
301
In Chinese women with metabolic syndrome, receiving
vitamin E supplements, reduced oxidative stress and im-
proved lipid status were observed.
302
Table 5 summarizes most remarkable findings about
concentration levels of markers of oxidative stress.
Part 2: Markers of Inflammation
and Cardiometabolic Biomarkers
Inflammatory Markers
Acute-phase reaction markers
WBC count and erythrocyte sedimentation rate are
markers of an acute-phase reaction in a later stage as also is
the case for quite high levels of CRP, as is mentioned below.
Therefore, WBC and sedimentation rate are quite nonspe-
cific markers for the metabolic syndrome and hence not
included. Since the high-sensitive CRP determination is
frequently mentioned as a marker, this protein is discussed
here.
C-reactive protein and hsCRP. CRP was so named
because it was first identified as a substance in the serum of
patients with acute inflammation that reacted with the C-
polysaccharide of Pneumococcus. CRP is an acute-phase
protein and the levels rise in response to inflammation. The
concentration is elevated in a variety of illnesses, including
cancer, and also in metabolic syndrome. In this study,
however, it is important to check carefully in every article
whether it consists of a CRP test or an hsCRP determination.
The protein is the same in both determinations, but hsCRP is
more sensitive.
303
The high sensitivity refers to the lower
Table 5. Summary of Literature Results Found
for Biomarkers of Oxidative Stress
Parameter Observation
Se and GSH-Px Controversial
SOD Lower values
GGT Higher values
PEDF Higher values
Oxidized LDL Higher values
Uric acid Chronic elevation
Bilirubin Upper normal range protects
Ferritin Higher values
Vitamin E Lower levels
GGT, gamma-glutamyl transferase; PEDF, pigment epithelium-
derived factor; Se, selenium.
8 ROBBERECHT AND HERMANS
detection limit of the assay procedures than previous com-
mercial assays routinely used for clinical measurements.
304
The regular CRP determination is used in detecting sig-
nificant inflammatory conditions, while the interpretation
related to hsCRP is used in coronary risk assessment in
MetS. In literature, the description of the test used is not
always that clear.
305,306
Biochemical background. CRP is a member of the
pentraxin family of calcium-dependent ligand-binding plas-
ma proteins, which appears to be a phylogenetically highly
conserved protein. It is a 115-kDa protein that consists of
five identical, noncovalently associated 23-kDa protomers,
arranged symmetrically around a central pore.
307,308
Clinical significance. In humans, CRP induction in the
liver is part of the nonspecific acute-phase response to most
forms of inflammation, infection, and tissue damage. This
protein is primarily synthetized by hepatocytes and regu-
lated by inflammatory cytokines, mostly tumor necrosis
factor-alpha (TNF-a) and interleukin (IL)-6.
307–309
Plasma levels of CRP in the human body may rise rapidly
and markedly, as much as 100- to 1000-fold or more, during
the acute phase of infection or injury response.
309
A review article intensively discussed the clinical importance
of hsCRP,
310
where it is considered as a marker associated with
ahigherriskofdevelopingMetS.
311,312
hsCRP has limited
capacity to predict the presence of MetS in a population
with central obesity.
313
Quite a lot of publications reported higher hsCRP levels
in MetS than in patients without MetS.
314–318
A significant
dose-related association was confirmed between a number
of metabolic syndrome conditions and increasing odds of
elevated hsCRP concentrations.
319
The biomarker concen-
tration is also higher in children with excess weight com-
pared with normal-weight individuals.
320
Obese patients
with obstructive sleep apnea syndrome with higher hsCRP
have an increased rate of metabolic syndrome than obese
subjects without sleep apnea.
321
hsCRP is used as a good marker of low-grade inflam-
mation in Arabs with metabolic syndrome
322
and is posi-
tively associated with the inflammatory marker, lipocalin-2
(LCN-2).
323
Knowledge of hsCRP status improves cardiovascular risk
prediction. In the Women’s Health Study,
324
hsCRP levels of
below 1, 1–3, and above 3 mg/L successfully differentiated
women with metabolic syndrome into low-, moderate- and
high-risk groups. A concentration level above 10 mg/L is an
indicator of an acute infection. A similar pattern of risk was
observed in the West Scotland Coronary Prevention Study,
which followed 6447 middle-aged men for 5 years.
305
hsCRP,
code as >3mg/L versus <3 mg/L, was strongly predictive of
incident CHD after stratification bymetabolic syndrome status.
Because MetS has been linked with greater chance of
future CVD events,
324–329
CRP levels can be an important
predictor of infavorable outcomes in MetS.
330–332
Although a high CRP level predisposes to increased
cardiovascular risk in MetS, future investigation is war-
ranted on the in vivo role of CRP in mediating vascular
effects and resulting in increased cardiovascular events in
MetS patients.
333
Since statin therapy decreases the vascular
events among persons with elevated hsCRP by half, even
when cholesterol levels are low,
334
the inclusion of infor-
mation on hsCRP with other cardiovascular risk factors may
assist physicians in medical decision-making.
335
There is a strong negative correlation between adipo-
nectin mRNA and CRP mRNA expression in human adipose
tissue, suggesting that a decrease in adiponectin leads to a
rise in CRP.
336,337
Serum amyloid A
Biochemical background. Serum amyloid A (SAA) is
an acute-phase reactant, such as CRP, which has been as-
sociated with systemic inflammation, linked to atheroscle-
rosis, and used as a predictor for CAD and cardiovascular
outcome.
338
SAA is a multigene family consisting of four
genes (SAA1–4) that are conserved in major vertebrates.
339
In humans, three of the four genes (SAA1, SAA2, and
SAA4), but not SAA3 (a pseudogene), are expressed.
340
In
response to acute inflammatory stimuli, SAA1 and SAA2
levels in plasma can increase as much as 1000-fold and
therefore both proteins are collectively known as acute-
phase SAA (A-SAA).
341
Clinical significance. A-SAA is a proinflammatory and
lipolytic protein. The increased expression of A-SAA by
adipocytes in obesity (the protein can also be classified as an
adipokine) suggests that it may play a critical role in local
and systemic inflammation and FFA production.
342
Locally,
it alters cytokine production and fat metabolism, while
systematically, it acts on liver, muscle, cells of the immune
system, and vasculature with impact on insulin resistance
and atherosclerosis.
SAA levels are associated with (central) obesity and are
increased in subjects with metabolic syndrome.
343–346
The
level correlates positively with the number of components of
MetS.
347
SAA could participate in lipoprotein metabolic alterations,
promoting the linking of HDL-C to macrophages and thus
reducing their cardioprotective effect.
348
It displaces apo-A1
from HDL and increases binding of these particles to mac-
rophages. In this way, the cardioprotective effect of HDL-C
is decreased.
349,350
The HDLs contain the antioxidative enzyme,
paraoxonase-1 (PON-1), which is important for atheroprotec-
tion. Decreased serum PON-1 activity in MetS may, in part, be
attributed to higher SAA levels.
345
Sialic acid. The serum sialic acid concentration is a
marker of the acute-phase response since many of the acute-
phase proteins (e.g., haptoglobin, alpha-glycoprotein, fi-
brinogen, transferrin, and complement) are glycoproteins
with sialic acid as the terminal sugar of the oligosaccha-
ride chain.
351
Serum sialic acid is a possible risk factor for
CVD.
352–354
Total sialic acid levels are also increased in
type 2 diabetes
355
and have been found to be significantly
associated with the development of diabetes.
356
Elevated
serum and urinary sialic acid concentration were strongly
related to the presence of microvascular complications in
type 2 diabetic subjects.
357
Therefore, elevated serum levels
were strongly correlated with the presence of MetS.
358–364
In a normal population, a positive correlation was found
between sialic acid and CRP.
365
While CRP and sialic acid
are both univariately associated with individual features of
the metabolic syndrome, sialic acid, but not CRP, was sig-
nificantly associated with the metabolic syndrome.
359
The
authors concluded that this marker identifies a subgroup of
overweight individuals with an inflammatory phenotype,
who are at the greatest risk of metabolic syndrome.
A negative association between serum sialic acid con-
centration and selenium intake, a recognized antioxidant
trace element, in healthy young subjects reinforces the view
BIOMARKERS OF METS 9
of selenium as a potential anti-inflammatory nutrient, and
the role of the interaction between antioxidant intake and
inflammatory process is envisaged.
Cytokines
Cytokines, secreted by adipose tissue, modulate the immune
system in favor of chronic systemic inflammation. Especially
classical proinflammatory cytokines, IL-6, IL-18, and TNF-a,
are mentioned as well as the anti-inflammatory cytokine, IL-10.
Interleukines. Although cytokines predominantly func-
tion as paracrine or autocrine factors, IL-6 is unusual, in that
it is a true endocrine cytokine. Most cellular targets are dis-
tant from the site of release, and effects of IL-6 are correlated
with the serum concentration.
366
Fat accounts for roughly
30% of circulating IL-6 concentrations in humans.
366
IL-6 is
a highly pleiotropic cytokine, with hormonal effects on many
tissues, but the effects on the liver, bone marrow, and endo-
thelium are thought to be most significant in contributing to
the metabolic effects of obesity.
367
IL-6 and the soluble IL-6 receptor (sIL-6r) were signifi-
cantly elevated in subjects with MetS.
368–370
This cytokine
exerts its biological function through a complex orchestration
of soluble and membrane-bound receptors.
371
The key com-
ponents of the IL-6 system are the soluble components, IL-6,
sIL-6r, soluble glycoprotein 130 (sGP130), the membrane-
bound IL-6 receptor (IL-6r), and the membrane-bound ubi-
quitary glycoprotein 130 (GP130).
372,373
An in vivo clinical treatment with plant food supplement
observed a decline in IL-6 concentration in MetS patients.
374
This effect, however, could not be approved in a 6-month
nutritional and physical activity intervention in France.
375
In
addition, garlic has no effect on this inflammatory biomarker
in women with metabolic syndrome.
376
A growing body of evidence suggests that IL-18 levels may
be closely related to MetS and its consequences.
377,378
In-
creasing levels of circulating IL-18 have been reported to be
closely associated with the components of MetS and to predict
type 2 diabetes, cardiovascular events, and mortality,
378,379
although Zirlik et al.
377
claimed that IL-18 does not add
independently to detection of atherosclerotic burden in
asymptomatic individuals. IL-18 may be, however, a useful
biomarker of the clinical manifestations of MetS and for the
management of risk factors of CVD.
380,381
Polymorphisms
in the IL-18 gene have been shown to be associated with
circulating IL-18 levels.
382,383
Troseid et al.
384
outlined the role of IL-18 in diabetes
and metabolic syndrome, with particular emphasis on car-
diovascular risk and the potential effect of lifestyle inter-
ventions such as diet and exercise.
Circulating levels of the anti-inflammatory IL-10 are el-
evated in obese women, but low in patients with MetS.
385
Tumor necrosis factor-alpha. TNF-ais, with IL-6, an
important cytokine, which alters adipose tissue function, in-
fluences adipogenesis, and is involved in the complications of
obesity.
386,387
Since TNF-ais a (pro)-inflammatory cytokine,
the concentration of this marker is higher in MetS.
388–391
High levels of TNF-aare not only observed in MetS but
also in a lot of pathologies where inflammation is implicated
(f.i. psoriasis
392–394
). For psoriasis, another chronic inflam-
matory process, a lot of publications significantly associate
this pathology with MetS.
395–402
However, no relation-
ship between disease severity and presence of MetS was
found.
403
The fact that untreated patients with psoriasis have
no, significantly higher, prevalence of MetS than healthy
controls suggests that systematic use of antipsoriatic drugs
may play an important role in the pathogenesis of MetS.
404
Another research group claimed that especially the hy-
perleptinemia in psoriasis may contribute to MetS.
405
Various other biochemical markers have been quite re-
cently reviewed to link psoriasis to a metabolic disease.
406
Circulating levels of TNF-awere related with MetS in
overweight hemodialysis patients.
389
Higher TNF-aconcentrations were found in nonobese
nondiabetic Mexican Americans, a population group at in-
creased risk for MetS,
407
compared with non-Hispanic white
adults.
408
Osteoprotegerin
Biochemical background. Osteoprotegerin (OPG) is a
soluble glycoprotein member of the TNF receptor super-
family, originally discovered as an inhibitor of osteoclasto-
genesis. Biochemically, OPG is a basic secretory glycoprotein,
comprising 380 amino acids and seven structural domains, and
exists as a more active monomeric form or as a homodimeric
one.
409
OPG acts as a decoy receptor for the receptor acti-
vator of nuclear factor-kB ligand (RANKL), inhibiting
binding of RANKL to its receptor (RANK).
410–412
Clinical significance. In addition to the role in bone
metabolism,
410,411,413
several studies suggest a potential role
of OPG in mediating cardiovascular damage.
411–414
In a cohort of patients with peripheral artery disease,
serum concentrations of OPG were elevated in those indi-
viduals with obesity and MetS.
415
In contrast, in an aging
male population, there was no statistical difference in OPG
values between men with or without MetS.
416
Since then,
the debate on the role of OPG in the pathogenesis of met-
abolic syndrome, type 2 diabetes, and CVD is still going
on.
95
Some authors find increased levels,
417–419
while others
claimed that no significant difference was found between the
mean serum OPG levels of those with metabolic syndrome
and those without.
420,421
Up to now, it is concluded that OPG is not a useful
marker of all components of MetS.
422
CD40 ligand. The proinflammatory mediator, CD40 li-
gand (CD40L), is expressed on CD4
+
T cells and activated
platelets. Both cell-bound and soluble CD40 (sCD40L) in-
teract with CD40L expressed on vascular cells and both of
these fractions increase in metabolic syndrome.
370,423–428
Enhanced CD40L–CD40 interaction plays an important
role in a cascade of inflammatory and proatherothrombotic
functions.
429,430
The anti-inflammatory effect of adiponectin is described
through the modulation of the CD40/CD40L system.
426
sCD40L is predominantly derived from platelets.
427
Pre-
diabetic subjects with MetS have significantly higher levels
of sCD40L compared with those without MetS.
429
The levels were also higher in metabolic syndrome pa-
tients with overt ischemic heart disease.
370
Monocyte chemoattractant protein-1. MCP-1 has been
regarded as a key cytokine in the recruitment of monocytes
from the blood into early atherosclerotic lesions and plays a
crucial role in atherosclerosis.
431–433
Studies showed that adi-
pocytes secrete MCP-1.
430,431
The concentration of this protein
was higher in subjects with MetS and associated with a low-
grade systemic inflammatory reaction.
433–436
Type 2 diabetic
patients with MetS exhibit higher serum MCP-1 levels.
437–440
10 ROBBERECHT AND HERMANS
Associations were found between polymorphisms in the
MCP-1 gene and components of MetS.
441–443
Eotaxin-1. Eotaxin-1 is produced by a variety of cells,
including macrophages and adipocytes.
444
It shares 66% amino
acid sequence homology with MCP-1
445
and has been found
within the smooth muscle cells of atherosclerotic plaques.
446
Vasudevan et al.
447
reported that its plasma levels are increased
in obesity. More recently, eotaxin-1 significantly increased in
metabolic syndrome in comparison with lean controls.
439
Exercise training with weight loss induced a significant
reduction of eotaxin levels in obese nondiabetic Korean
women.
448
Cystatin-C. Cystatin-C (cys-C) is a 15-kDa protein that
acts as a negative regulator of proatherogenic cysteine prote-
ases.
449
It has recently been shown that cys-C is also a novel
sensitive marker of kidney and cardiovascular function.
450
Epidemiological analyses have found that patients with
metabolic syndrome are at high risk for developing renal
abnormalities.
451–453
Cystatin C has been suggested as a sen-
sitive endogenous serum marker of changes in the glomeral
filtration rate.
454–456
It turned out that the cystatin value is higher in metabolic
syndrome patients than in nonmetabolic syndrome persons,
independently of creatinine clearance or even where creat-
inine does not exceed the normal.
457
The progressive in-
crease in cystatin, as a function of the number of MetS
components,
458
could indicate an increased cardiovascular
risk. This is registrated for children with MetS,
459
a hyper-
tensive population,
460,461
as well as elderly patients,
462
even
without recognized chronic kidney disease.
463
This suggests that cystatin is probably more than a marker
of glomerular filtration rate.
464
A prospective study in Sweden proved that high levels of
cystatin C can predict the metabolic syndrome.
465
Very recently, it was claimed that cystatin C might not
only play a role in the diagnosis of CAD but also serve as a
marker of the severity of CAD.
466
In addition, urinary cystatin C can function as a potential
risk marker in obesity and MetS.
467
Hemostatic markers
Plasminogen activator inhibitor-1. Plasminogen activa-
tor inhibitor-1 (PAI-1) is a member of the serine protease
inhibitor (serpin) superfamily.
468
It is always synthesized in
an inactive form, but spontaneously converts into an active
state with a half-life of *1–2 hr in physiological environ-
ments.
469
PAI-1 is the main and fast-acting inhibitor of the fibri-
nolytic system
470
and is sometimes called a prothrombotic
adipokine
471
and is considered as a component of MetS.
The mechanisms linking PAI-1 to this syndrome are
complex and reviewed in literature.
472,473
An elevated plasma level of PAI-1 is a common feature of
patients with metabolic syndrome
470–476
and directly related
to the severity of the disease.
477
CRP induces PAI-1 expression in human aortic endo-
thelial cells.
478
Some,
478,479
but not all,
479
gene polymorphisms are as-
sociated with MetS. Other researchers claimed that meta-
bolic determinants are much more important than genetic
polymorphisms in lowering PAI-1 activity.
480
Moderate intensity exercise
481
and weight reduction by a
hypocaloric diet
482
induce favorable changes in lowering
lipid profile and PAI-1 levels and hence may reduce risk of
CVDs. A similar reduction in the PAI-1 level was observed
in a study on treatment of metabolic syndrome by irbesartan
and/or lipoic acid.
483
Fibrinogen. Together with PAI-1, a high plasmatic level
of fibrinogen contributes to the increased cardiovascular risk
that characterizes individuals with MetS.
475
Hyperfibrino-
gemia is since long considered as a component of this
syndrome
484
and higher levels are found in people with
MetS than in those without.
485
In contrast to a control group, the offspring of hyperten-
sive men revealed a correlation between higher levels of
fibrinogen and metabolic syndrome variables.
486
Obese patients with obstructive sleep apnea syndrome
had an increased rate of metabolic syndrome and higher
fibrinogen serum levels than obese subjects without sleep
apnea.
321
However, this study was questioned for some
details.
487
Endothelial dysfunction
Homocysteine
Biochemical background. Homocysteine is a nonprotein
amino acid that is produced in humans by the demethylation
of nutritional methionine, catabolyzed by methyltransferases.
488
There are different genetic–nutrient interactions that can
predispose an individual to hyperhomocysteinemia.
489,490
Most cases of mild hyperhomocysteinemia are due to nu-
tritional folate and vitamin B12 deficiency and/or reduced
glomerular filtration rate. In severe cases, mutations in the
key enzyme of homocysteine metabolism, methylenete-
trahydrofolate reductase (MTHFR), can be found.
491
It is
well known that several widely used drugs, such as lipid-
lowering substances (f.i. fibrates and niacin) or oral hypo-
glycemic drugs (such as metformin), insulin, drugs used in
rheumatoid arthritis (corticosteroids and nonsteroidal anti-
inflammatory drugs), and anticonvulsants (carbamazepine,
phenytoin, and phenobarbital), can cause elevated homo-
cysteine concentrations.
492
Clinical significance. Hyperhomocysteinemia has emerged
as an independent and graded risk factor for the develop-
ment of CVD,
493
which is, at the same time, the primary
outcome of MetS.
Although in most studies higher values of homocysteine
are found in patients with the syndrome,
494–502
the associ-
ation between homocysteine levels and MetS is not well
characterized.
503
Most of the publications
494–498
claim to
find no association. For one publication,
499
the sampled
population (patients with bipolar disorder and schizophrenia
treated with second-generation antipsychotics) is too com-
plicated to draw definite conclusions. Small sample size,
lack of healthy controls, the cross-sectional design, and no
data on physical activity, smoking, coffee consumption, and
dietary habits (daily intake of folate and vitamin B12) can
lead to preliminary indications only.
One publication
500
mentioned an increased risk of MetS
with elevatedhomocysteine in age and gender-adjusted logistic
regression models for 6- to 8-year-old children in rural Nepal.
Most of the time, the authors claimed that hyperhomo-
cysteinemia and MetS could work together in increasing
CVD risk,
495,496,501,502
but this risk seems to share no
common mechanisms.
498,503
Only one publication mentioned that high levels of
homocysteine are not associated with an increased risk for
BIOMARKERS OF METS 11
new cardiovascular events in metabolic syndrome patients.
Quite remarkably, elevated homocysteine levels confer in-
creased risk in patients without metabolic syndrome.
Lipoprotein-associated phospholipase A
2
Biochemical background. Lp-PLA
2
is characterized as a
novel inflammatory biomarker correlated with several com-
ponents constituting the MetS and atherosclerosis and inci-
dent CVD.
504,505
Lp-PLA
2
is preferentially secreted by
monocytes and macrophages and hydrolyzes oxidatively
modified LDL by cleaving the oxidized phosphatidylcholines,
generating lysophosphatidylcholines and oxidized FFAs.
506
A
lot of prospective and nested case cohort studies suggest that
this enzyme is proatherogenic.
507
Clinical significance. Lp-PLA
2
is associated with
MetS.
508
Elevated Lp-PLA
2
activity may identify especially
high-risk individuals.
509
The enzyme activity is strongly influenced by ferritin
levels, LDL-C, and apo-B100. The increased susceptibil-
ity to oxidation is manifested by increased ferritin levels,
low HDL-C, and diminished antioxidant vitamin levels in
an environment of pro-oxidant stress manifested by in-
creased substrate (LDL-C) and insulin resistance.
510
Ferritin
is the main intracellular storage protein of iron. This cat-
ion is a strong catalyst for oxidation of lipoproteins, re-
sulting in a number of biological properties that could
make oxidized LDL proatherogenic.
511
All these observa-
tions are arguments for an integrated role of Lp-PLA
2
in
lipid peroxidation.
Lp-PLA
2
was significantly increased in patients with ca-
rotid intima-mediated thickness in arteries compared with
those without plaques. Maybe this enzyme is a modulating
factor through age and LDL-C.
512
Endothelin-1. The measurement of circulating endothelin-
1 is a well-recognized marker of endothelial atherosclerotic
and CVD.
513
The level of this biomarker correlated inde-
pendently with triglyceride content and glycosylated hemo-
globin in patients with the insulin resistance syndrome.
514
Overweight and obesity are associated with endothelin-1-
mediated vasoconstriction that may play a role in the
increased prevalence of hypertension with increased ad-
iposity.
515
In patients with metabolic syndrome, the adipokine
ghrelin tends to normalize the balance between vasocon-
strictor (endothelin-1) and vasodilating (NO) mediators and
hence preserves vascular homeostasis in humans.
516
Cell adhesion molecules
Biochemical background. Proinflammatory states en-
hancing innate immune signaling result in activation of NF-
kB. This transcription factor regulates the production of
adhesion molecules, such as VCAM-1 and ICAM-1. It also
activates transcription of genes encoding chemoattracting
factors, including monocyte chemoattracting protein (MCP-
1) and macrophage stimulatory factor that attract monocytes
into vessel cells.
517
MCP-1 is secreted by monocytes/macrophages, lympho-
cytes, endothelial cells, and epithelial cells, among others.
Their targets are T lymphocytes, basophils, eosinophils,
NK-cells, etc.
Endothelial cells, macrophages, lung, and intestinal cells
secrete eotaxin. The primary targets are eosinophils.
518
Both
(MCP-1 and eotaxin) function in leukocyte activation, re-
cruitment, and adhesion (induction of cell adhesion mole-
cule expression: ICAM-1, VCAM-1, VLA, etc).
519
MCP-1 and eotaxin are already discussed. In this part, the
adhesion molecules, L-selectin and especially E- and P-
selectin, are discussed in relation to MetS, together with
ICAM-1 and VCAM-1.
Selectins. Selectins are cell surface proteins that enable
adhesion between endothelial cells and leukocytes. E- and
P-selectin are expressed on the surface of endothelial cells;
L-selectin on the leukocyte surface. P-selectin is also found
on the surface of activated platelets.
520
Platelet activation, which initiates thrombus formation, is
considered to be one of the major risks for prothrombotic
state in metabolic syndrome patients.
521
Among 17 inflammatory biomarkers, soluble E-selectin
(together with VCAM-1) demonstrated the strongest asso-
ciation with MetS in a community-based sample of elderly
patients,
522
as was also demonstrated in a multiethnic pop-
ulation in the United Kingdom.
523
In this study, the stron-
gest association was with the waist–hip ratio. Higher plasma
E-selectin values were found in women with polycystic
ovary syndrome with MetS
524
and in adults from Inner
Mongolia, China.
525
In obese women, the higher E-selectin values correlated
negatively with metabolic flexibility.
526
Plasma E-selectin levels were decreased in young women
with metabolic syndrome after exercise training.
527
Increased levels of P-selectin were found in Asian Indian
subjects with MetS.
425
This gives more strength to another
observation that most of the associations between MetS and
inflammatory markers were eliminated after adjusting for
each of its components, except for P-selectin.
528
The observation that the adipokine resistin induces
platelet activation by increasing P-selectin expression
through the p38 MAPK-dependent pathway provides one
mechanism for the prothrombotic state in individuals with
metabolic syndrome.
529
Cellular adhesion molecules. The concentration of sol-
uble cellular adhesion molecules, ICAM-1 and VCAM-1, in
healthy men is strongly associated with traits of the meta-
bolic syndrome,
530,531
becoming evident in the postprandial
response to a lipid-rich meal.
531
Aizawa et al.,
532
however,
claimed that both markers are similar in MetS
+
and MetS
-
persons and that these parameters remained unchanged
following a lifestyle modification.
Higher values were found for VCAM-1 and ICAM-
1
524,525,533,534
in patients with MetS. sICAM-1 was signifi-
cantly associated with a greater prevalence of detectable
coronary artery calcium, measured using computed tomog-
raphy.
534
Elevated sICAM-1 concentration was associated
with increased odds of MetS
+
prediabetes or diabetes, but
not with MetS alone.
525
Table 6 summarizes most remarkable results for bio-
markers of inflammation.
Cardiometabolic Markers
With the increasing recognition that MetS also leads to
cardiovascular complications (myocardial infarction, stroke),
the syndrome is sometimes changed to the notion car-
diometabolic syndrome.
535
Microvascular abnormalities
were completely ignored for a long time, but more recent
investigations have revealed that modifications such as
12 ROBBERECHT AND HERMANS
microalbuminurea,
536
retinal blood flow change, and signs
of nonenzymatic glycation can be found, although far less
pronounced than in type 2 diabetes.
The cardiometabolic syndrome is an extremely compli-
cated issue due to the fact that it encompasses a mesh of
metabolic pathways (mainly glycemia and lipids) and in-
volves several tissues (liver, fat, muscle, and others).
537
The liver is closely involved not only in regulation of
glycemia and lipids but also in inflammation and hemosta-
sis, which are main players in the metabolic syndrome, and
many processes operate in a bidirectional manner.
535
The risk of CVD in metabolic syndrome patients was
reviewed after a meta-analysis.
538
The authors claimed that
MetS is not only a risk factor for CVD incidence and
mortality but also all-cause mortality.
In this review, we divide the cardiometabolic markers
into two main categories: the classical metabolic biomark-
ers [glucose, glycated hemoglobin (HbA1c), insulin, and
parameters related to insulin resistance] and metabolism-
related peptides. In the latter group, a lot of attention is paid
to the ever increasing importance of the adipocytokines or
adipokines. In this family of proteins, adiponectin is well
studied and plasma levels revealed a negative relationship
with the MetS score.
539
More recent vascular biomarkers in the metabolic syn-
drome (asymmetric dimethylarginine, advanced glycated
end products or sRAGE,
540
and endothelial progenitor
cells) are reviewed,
541
while quite recently, biomarkers of
endothelial cell activation (asymmetric dimethylarginine,
angiopoietin-2, osteopontin, gelsolin, and OPG) have been
compiled elsewhere.
542,543
Classical metabolic markers
Glucose and HbA1c. Hyperglycosemia, most of the
time, is a marker later in the process of MetS and related to
diabetes type 2. Nevertheless, in multiparameter analysis,
the glucose concentration in a fasting blood sample is ele-
vated in MetS patients.
544–546
A good agreement exists between fasting plasma glucose
(FPG) and HbA1c for the enforcing of diagnosis of MetS,
and appreciably different groups of subjects were classified
using each method.
547
Compared with the use of FGP, the HbA1c significantly
increased the identification rate of MetS. In a Korean pop-
ulation, an appropriate cutoff value of HbA1c is put at the
5.65% level.
548
Insulin
Biochemical background. Insulin is the most important
hormone involved in glucose homeostasis and increased
peripheral glucose uptake. The glucose load stimulates in-
sulin secretion from the b-cells of the pancreatic islets. The
hormone acts through its main receptors, IRS-1 and IRS-2,
to (1) stimulate the activity of the GLUT-4 glucose trans-
porter, resulting in transmembrane glucose uptake by skel-
etal and cardiac muscle, as well as by adipose tissue; (2)
increase glycogen and triglyceride synthesis; and (3) inhibit
lipolysis and proteolysis.
549
By clearing glucose from the blood and limiting the
amount of gluconeogenic substrates available to the liver,
insulin effectively decreases blood glucose levels. In addition
to its peripheral effects, insulin also acts to stimulate hepatic
glycogen synthesis and suppress hepatic gluconeogenesis and
glycogenolysis. Additionally, FFA, ketone bodies, and some
amino acids can also stimulate insulin secretion.
549
Endogenous insulin has a half-life within the circulation
of 3–5 min.
550
Clinical significance. DM encompasses a heterogeneous
group of disorders of carbohydrate metabolism that are due to
a defect in either insulin secretion or insulin action.
551
The
two main types of DM are termed types 1 and 2 and have
been categorized based on the underlying cause of hyper-
glycemia.
551
Type 1 DM, previously known as juvenile-onset
DM or insulin-dependent DM, usually involves autoimmune
destruction of the pancreatic b-cells, resulting in insulin de-
ficiency.
551
Type 2 DM is characterized by insulin resistance,
defective insulin secretion, and increased glucose production.
Insulin resistance is defined as the inability of normal insulin
concentrations to adequately decrease blood glucose lev-
els.
552
Type 2 diabetic patients are initially hyperinsulinemic
as their pancreas tries to compensate for the decreased insulin
sensitivity. When the response to insulin fails, hyperglycemia
ensues. Although previously considered a disease exclusive
to adults, type 2 DM is rapidly becoming a common diag-
nosis in the pediatric population.
553,554
Fasting insulin levels have been shown to be positively
related to obesity, systolic and diastolic blood pressures, and
triglyceride levels, as well as LDL-C and VLDL-C.
555,556
In
contrast, insulin levels have been negatively associated with
HDL-C.
555
In a study on young Arab females, insulin significantly
correlated with leptin and diastolic blood pressure; however,
more specifically, a possible role for the C-peptide in the
prediction of CVD was argued.
557
In an obese population with an elevated waist circum-
ference, insulin was claimed to be the only reliable bio-
marker to differentiate MetS status.
558
Concentrations in humans of fetuin-A, a multifunctional
hepatic secretory protein that inhibits the action of insulin in
Table 6. Summary of Literature Results Found
for Biomarkers of Inflammation
Parameter Observation
hsCRP Higher values
SAA Higher values
Sialic acid Higher values
Interleukins Depends on species
TNF-aHigher values
Osteoprotegerin Not that useful marker
CD40 ligand Higher values
Monocyte chemoattractant
protein-1
Higher values
Eotaxin Increased levels
Cystatin Higher values
PAI-1 Higher values
Fibrinogen Higher values
Homocysteine Higher values
Lipoprotein-associated
phospholipase A
2
Higher values
Endothelin-1 Unclear
Selectins Higher values
Soluble cell adhesion
molecules
Higher values
hsCRP, high-sensitivity C-reactive protein; PAI-1, plasminogen
activator inhibitor-1; SAA, serum amyloid A; TNF-a, tumor
necrosis factor-alpha.
BIOMARKERS OF METS 13
experimental animals, were strongly associated with MetS
and an atherogenic profile.
559
C-peptide
Biochemical background. C-peptide is a by-product of
insulin synthesis. Insulin is initially synthesized as a single
polypeptide chain known as preproinsulin. Proteolytic
cleavage of the amino-terminal of preproinsulin forms pro-
insulin. Subsequently, cleavage of an internal fragment of
proinsulin produces connecting peptide as well as an A and B
chain of insulin. Therefore, C-peptide is produced in equal
molar amounts to insulin. C-peptide has no known biological
function. It is degraded or excreted mainly by the kidney and
has a half-life three to four times that of insulin.
560
Clinical significance. Despite the lack of biological
function, C-peptide is useful clinically for the purposes of
diagnosis and monitoring of individuals with insulin-related
diseases, particularly DM.
Since for every molecule of insulin, a molecule of C-
peptide is produced, it is a good marker of the amount of
endogenous insulin. The measurement of C-peptide levels is
very useful in documenting insulin synthesis capacity in
insulin-treated individuals, in whom measurement of insulin
would registrate both endogenous and exogenous insulin.
560
The same applies when C-peptide measurements are used
to assess the response of type 2 diabetic patients to metfor-
min, a drug that stimulates the pancreas to produce insulin.
561
C-peptide levels were higher in patients with MetS.
562,563
In young Arab females, the possible role of C-peptide in the
prediction of CVD is mentioned.
557
The assessment of C-peptide levels is a useful tool to
monitor the progress of MetS among patients also suffering
from type 2 DM and Alzheimer’s disease.
564
Homeostasis model assessment of insulin resistance in-
dex. Measurement of insulin resistance is a constitutive
element of MetS.
565,566
The euglycemic insulin clamp technique is generally
considered a reference method for assessing insulin sensi-
tivity or resistance,
567
but is expensive, laborious, and not
routinely available. Since the oral glucose tolerance test
(OGTT), the most commonly used method to evaluate
whole-body glucose tolerance in vivo, is instead simple and
cheap, it is used to calculate insulin resistance.
However, other different methods are published. The
quantitative insulin sensitivity check index (QUICKI) used
fasting glucose and insulin concentrations,
568
while the in-
sulin sensitivity index (ISI-gly) is calculated from basal and
OGTT-induced insulin and glucose levels.
569
The HOMA-IR index, described by Matthews et al.,
570
is
used in large epidemiological studies most of the time.
571–574
HOMA-IR is a convenient way for quantifying insulin
resistance. It is calculated by multiplying fasting plasma
insulin (FPI) by FPG, then dividing by the constant factor
22.5; that is, HOMA-IR =(FPI ·FPG)/22.5.
575
For interpopulation comparisons, it is necessary to know
normal values for each population. Although HOMA-IR has
been widely used, there is hardly any consensus on the cutoff
points for classification of insulin resistance. Some authors
have tried to find HOMA-IR cutoffs in subjects who had
increased tendencies toward insulin resistance or MetS, but
their findings were not consistent.
576–588
Age and gender
587
and puberty
588
can be of some influence.
In general, a higher value of the index was found in
MetS.
589–591
The HOMA-IR was proven to be an independent pre-
dictor of incident CVD
592
and correlated significantly with
triglycerides and fasting glucose and the adiponectin/leptin
ratio.
593
Only one publication could be traced, where ISI-gly,
which includes postload glucose and insulin concentrations,
tends to provide a more accurate estimate of whole-body
insulin sensitivity than HOMA-IR.
594
Metabolism-related markers
Thyroid function. MetS is a status where most features of
hypothyroidism can be seen.
595
A thyroid-stimulating hor-
mone (TSH) increase was shown to be associated with in-
creased cholesterol and triglyceride levels and with decreased
HDL-C.
596
A PubMed search for any combination of hyper-
thyroidism, thyrotoxicosis or hypothyroidism and metabolic
syndrome, blood pressure, hypertension, hyperlipidemia,
cholesterol, HDL-C, glucose, DM, body weight, or visceral fat
was performed.
597
There was convincing evidence for a major
impact of thyroid function on all components of MetS.
Thyroid-stimulating hormone. In the range of TSH,
which is considered clinically normal, an increasing level of
TSH was associated with less favorable lipid concentration
and this was linear across the entire reference range of
TSH.
598
In addition, slightly elevated serum TSH concen-
trations are associated with an increase in the occurrence of
obesity.
599
A lot of publications mention that higher concentrations
of TSH are associated with MetS.
600–605
The relationship
was especially found for TSH levels in the upper quartile
600
or upper normal range.
602
Other authors mentioned that the
TSH level has to be used in combination with (lower) FT4
level.
606
Saleem et al. claimed that there may be increased
risk of CVDs with high-normal TSH levels.
607
In another
publication, higher TSH levels and subclinical hypothy-
roidism are associated with increased odds of prevalent, but
not incident, MetS.
608
Parathyroid hormone. Decreased 25-hydroxyvitamin D
[25(OH)D]
609,610
and elevated parathyroid hormone (PTH)
plasma concentration
611
have been associated with MetS
and with each of its individual components. It has been
proposed that low 25(OH)D levels in MetS are accounted
for decreased hormonal bioavailability because of seques-
tration of 25(OH)D in body fat.
612
In this context, PTH
elevation has been viewed as a compensatory mechanism
for the low 25(OH)D. Since then, a lot of publications have
dealt with the association of PTH, 25(OH)D, and MetS, with
conflicting conclusions. Most of the time, PTH, but not vi-
tamin D, is associated with the metabolic syndrome.
613–617
In community-dwelling older adults, the increased risk for
MetS was only found in men.
611
A recent study supports the role of PTH since an asso-
ciation was found between the PTH levels and inflammatory
markers among US adults,
618
while also higher PTH levels
were found to be associated with a higher prevalence of the
cardiorenal metabolic syndrome.
619
Some authors claim that serum 25(OH)D, but not PTH,
was significantly associated with MetS and a number of its
components after multivariate adjustment.
620,621
Only one study could be traced, where the authors con-
cluded that an independent contribution of 25(OH)D or PTH
in the pathogenesis of MetS in severely obese subjects could
14 ROBBERECHT AND HERMANS
not be found.
621
Probably the role of PTH as a mediator can
be explained not only by 25(OH)D but also by more vari-
ables, including magnesium and phosphate.
616
Neurohormonal activity markers
Aldosterone. Aldosterone, a mineralocorticoid hormone
classically involved in regulation of the sodium balance, is
increased in patients with MetS.
522,622–624
It could contrib-
ute to the hypertension associated with this disease.
625
Aldosterone excess might predispose or aggravate the met-
abolic and cardiovascular complications of patients with
obstructive sleep apnea syndrome.
626
The role of aldosterone in MetS is discussed in a brief
627
or a more extended review article.
628
Testosterone. Biochemical evidence indicates that tes-
tosterone is involved in promoting glucose utilization by
stimulating glucose uptake, glycolysis, and mitochondrial
oxidative phosphorylation. Testosterone is also involved in
lipid homeostasis in the major insulin-responsive target
tissues, such as liver, adipose tissue, and skeletal muscle.
There is a complex interplay between adipocytokines,
proinflammatory cytokines, and hypothalamic hormones
that control the pituitary-testicular axis.
629
Testosterone levels are reduced in the metabolic syndrome.
630
Interventional studies have shown beneficial effects of
testosterone and components of the metabolic syndrome
631
and improvement of body composition.
632
The observational and interventional clinical data in re-
lation to testosterone and metabolic syndrome are reviewed
elsewhere.
633
Cortisol. Circulating cortisol
634
and psychosocial
stress
635,636
may contribute to the pathogenesis of obesity
and metabolic syndrome. In addition, hypercortisolemia
within depressed persons may increase the risk of MetS.
637
The overlap between some clinical features of MetS and
Cushing’s disease has prompted the hypothesis that this
adrenal steroid may be associated with the development of
MetS.
634
Studies investigating the role of cortisol and
DHEAS in MetS presented conflicting results
634,638–641
and
a role of ethnic differences in the prevalence of MetS was
claimed.
642
The cortisol/DHEAS ratio was positively associated with
only one component of MetS, diastolic blood pressure, in a
sample of bipolar and schizophrenia patients.
643
Few studies demonstrated a relationship between corti-
sol and insulin resistance or MetS in children and adoles-
cents.
640,644–646
Normal physiological differences in long-term cortisol
secretion, as assessed in hair, showed relevant relationship
with MetS and individual cardiometric parameters.
647
Reviewing the literature, Abraham et al.
648
claimed that
there was no evidence for a strong relationship between
systemic cortisol or stress and obesity or metabolic syn-
drome, as was also recently published for obese children and
adolescents.
646
Arginine vasopressin activity. Most of the time, plasma
levels of the C-terminal proarginine vasopressin (AVP) frag-
ment, also known as copeptin, are used as a surrogate for
circulating AVP levels. Copeptin is secreted in equimolar
amounts to AVP and therefore directly reflects the release of
AVP into the circulation. Unlike AVP, it remains stable ex vivo
for several days at room temperature in serum or plasma.
649
The association between plasma copeptin and measures
of MetS is published
650,651
and the AVP system can be
modified in the treatment of MetS and the prevention of
CVD.
651
Other authors claim that copeptin independently
predicts DM and abdominal obesity, but not the cluster of
MetS.
652
Natriuretic peptides. The metabolic action of natriuretic
peptides and therapeutic potentials in MetS has been re-
cently reviewed.
653
It is quite important to keep in mind to check carefully the
terms mentioned. Due to partial proteolysis, several pre-
forms, peptides, and fragments are described: plasma B-
type, long-acting, and the atrial and brain or N-terminal
probrain natriuretic peptide (NT-proBNP).
Most of the publications deal with a reduced natriuretic
peptide activity as a manifestation of the metabolic syn-
drome.
654
This is published for the N-terminal pro-B-type
natriuretic peptide
655–657
as well as for the long-acting na-
triuretic peptide.
658–660
NT-proBNP and high-sensitivity troponin T (hs-TnT) act
as direct indicators of functional and structural damage in
the cardiovascular system
661
and are both directly related to
the number of criteria of MetS.
662
The strong relationship between atrium and brain natri-
uretic peptide and insulin and the insulin resistance index
allowed considering these parameters as markers in the early
diagnosis of MetS.
663
The nonlinear U-shaped relationship of plasma B-type
natriuretic peptide with MetS is unexpected and warrants
replication.
664
Table 7 summarizes most remarkable results for cardio-
metabolic markers, except the adipokines (Table 8).
Adipokines. Adipose tissue has been recognized as an
active endocrine organ in addition to its role as the main
storage depot for triglycerides.
665
An increasing number of
adipocyte-derived secretory factors (adipocytokines or adi-
pokines) are described in the literature.
666–670
This high-
lights the central role of adipose tissue (adipocytes and
nonfat cells) in regulating whole body energy homeostasis.
This is obtained not only by partitioning lipids into various
depots but also through adipokine-mediated modulation of a
number of signaling cascades in target tissues.
671
More than 50 adipokines have been identified, including
adiponectin, leptin, resistin, chemerin, IL-6, PAI-1, retinol-
binding protein 4 (RBP-4), TNF, and visfatin, among others. A
Table 7. Summary of Literature Results Found
for Cardiometabolic Markers
Parameter Observation
C-peptide Higher values
HOMA-IR Higher values
TSH Higher values
T3 and T4 Conflicting results
PTH Higher values
25-Hydroxyvitamin D Lower values
Aldosterone Higher values
Testosterone Lower values
Cortisol Unclear observations
AVP Unclear observations
Natriuretic peptide Lower values
AVP, arginine vasopressin; HOMA-IR, homeostasis model
assessment of insulin resistance; PTH, parathyroid hormone; TSH,
thyroid-stimulating hormone.
BIOMARKERS OF METS 15
more extensive listing is published elsewhere.
671
In general,
they regulate intermediate and energy metabolism (adipo-
nectin, leptin), inflammation (pro- and anti-inflammatory
cytokines, ILs), endothelial function, and hemostasis (mono-
cyte chemoattracting protein-1, PAI-1, intracellular adhesion
molecules).
672,673
Some authors regard obesity as a systemic, low-grade
inflammatory state
674
and inflammation as a link between
obesity, metabolic syndrome, and CVD.
667,675,676
Adipo-
nectin, leptin, resistin, and visfatin are known to contribute
to inflammatory responses.
677
Among these adipokines, adiponectin is one of the most
potent molecules with respect to its insulin-sensitizing
activity. However, unlike the vast majority of adipocyte-
derived factors, the levels of adiponectin in circulation
display an inverse relationship with adiposity. Biomarkers
of systemic inflammation other than adipokines include
proinflammatory cytokines and chemokines, which have
been suggested to be closely related to excess adipose tissue
accumulation.
678
The elucidation of the molecular mechanisms by which
adipokines are produced and secreted from adipocytes will
be a challenge toward a more comprehensive understanding
of the role of adipose tissue physiology in whole body en-
ergy homeostasis.
Adiponectin
Biochemical background. Adiponectin, also referred to
as adipocyte complement-related protein of 30 kDa
(Acrp30),
679
adipo Q,
680
adipose most abundant gene tran-
script 1,
681
and gelatin-binding protein of 28 kDa,
682
is ex-
clusively synthetized by adipose tissue. It plays a central
role in glucose and lipid metabolism.
683
Its secretion is
stimulated by insulin and is induced during fat cell differ-
entiation. It shows structural similarity to collagen VII,
collagen X, and complement C1q
679–681
and assembles into
homotrimers and other higher order structures by interaction
of its collagen-like regions.
684
It is present in blood, constituting *0.01% of total
plasma protein.
685
Two receptors for adiponectin have been
cloned, AdipoR1 ad AdipoR2.
686
The first one is primarily
expressed in skeletal muscle, whereas AdipoR2 is mainly
expressed in the liver.
Clinical significance. Circulating levels of adiponectin are
negatively associated with insulin resistance, type 2 diabetes,
and dyslipidemia.
687–692
An inverse relationship between the
serum adiponectin level and MetS and its components has been
observed for various groups in different countries.
669,693–700
Quite recently, the adiponectin/leptin ratio was claimed to be
a better biomarker of acute metabolic stress.
701
Adiponectin appears to have anti-inflammatory and anti-
atherogenic effects, therefore plasma adiponectin levels
have been inversely related to a number of CAD risk factors
in obese and overweight individuals.
687,691,701–712
The levels of this markerare inversely associatedwith serum
hsCRP, IL-6, and IL-10 levels. This suggests that adiponectin
might be associated with the metabolic syndrome through
regulation of pro-/anti-inflammatory cytokines.
713
This protein
also has been shown to suppress TNF-aproduction by the
macrophages
714
and to reduce monocyte attachment.
The anti-inflammatory effects of adiponectin suggest a
therapeutic role in protecting against the development of
atherosclerosis and CAD. This may be particularly im-
portant in patients that present low circulating adiponectin
levels. Serum adiponectin is an independent protective
factor for incidence of MetS.
692
A prospective study supports the central role of adipo-
nectin in the development of cardiometabolic abnormality in
obesity.
715
Only one study could be traced where the traditional
markers of adiposity such as BMI or waist circumference
remain superior for predicting metabolic syndrome com-
pared with CRP, adiponectin, or the combination of both.
716
Quite recently, Japanese researchers examined the accuracy
of MetS screening by adiponectin and claimed that this was
possible.
717
High adiponectin levels have been correlated with in-
creased insulin sensitivity and glucose tolerance.
718
There-
fore, recombinant adiponectin may represent a potential
treatment for insulin resistance in patients with diseases
such as MetS and type 2 DM.
719
The use of adiponectin as a therapeutic agent as well as a
powerful biomarker of insulin sensitivity, components of
MetS, and risk of CAD, seems likely. This is reviewed
720
and updated.
721
Adiponectin
716
and adiponectin receptors represent po-
tential versatile therapeutic targets to combat obesity-linked
diseases characterized by insulin resistance.
710
Fenofibrate raised adiponectin levels in patients with
hypertriglyceridemia and metabolic syndrome. Changes in
the concentration were significantly and inversely associ-
ated with changes in multiple inflammatory markers. This
suggests that adiponectin may contribute to the anti-
inflammatory effects of fenofibrate.
722
TNF-aantagonism with etanercept decreases glucose and
increases the proportion of high-molecular-weight adiponectin
in obese subjects with features of metabolic syndrome.
723
Table 8. Some of the Established Actions
of Adipokines in Metabolic Syndrome
Adipokine Effect
Adiponectin Negatively correlated with insulin resistance
Negatively correlated plasma levels
with MetS score
Leptin Positively correlated with insulin resistance
Related to leptin resistance
Resistin Controversial
Proinflammatory role in MetS
Ghrelin Lower values
A-FABP Higher values
RBP-4 Higher values
Lipocalin-2 No definite conclusion
FGF-21 Somewhat higher values
Chemerin Higher values, early role in pathogenesis
Omentin Lower values
Visfatin Controversial
Proposed as a proinflammatory factor
for development of insulin resistance
Sfrp5 No definite conclusions
Vaspin No definite conclusions
Apelin No definite conclusions
A-FABP, adipocyte fatty acid-binding protein; FGF-21, fibroblast
growth factor 21; RBP-4, retinol-binding protein 4; Sfrp5, secreted
frizzled-related protein 5.
16 ROBBERECHT AND HERMANS
Leptin
Biochemical background. Leptin is derived from the
Greek word, leptos, which means thin. Leptin, the product
of the ob gene
724
is a 16 kDa proteohormone that is be-
lieved to be involved in the regulation of food intake, energy
balance, and body weight.
725
Various isoforms of leptin
have been identified with diverse biological actions that
range from affecting blood pressure to immune func-
tions.
726–728
Loss of leptin results in obesity.
729
Specifically, leptin acts at the level of the hypothalamus to
suppress food intake and increase energy expenditure.
730–732
In addition, it is also involved in various hypothalamus-
pituitary-endocrine axes.
733–735
Leptin was the first identified
adipocytokine, therefore leptin is called the classical adipo-
cytokine. The previous biomarker (adiponectin) is considered
as the promising one, while the next one (resistin) is called
the most controversial one.
736
Its primary structure comprises 167 amino acids and it is
not only predominantly synthesized by differential adipo-
cytes
735,736
but has also been documented in other tissues,
including skeletal muscle, liver, and the placenta.
737
It exerts
pleiotropic effects by binding and activating specific leptin
receptors (obRA and obRB) in the hypothalamus
738,739
and
other organs.
740
Leptin receptors belong to the cytokine class
I receptor family.
741
They are found ubiquitously in the
body,
742
including a circulating form, which acts as a leptin-
binding protein.
743
It has direct and indirect effects in meta-
bolically active tissues and regulates several neuroendocrine
axes.
744–746
Leptin has been implicated as a growth factor
747
for its
ability to promote proliferation, inhibit apoptosis,
748
and
stimulate angiogenesis in metastatic breast cancer hypoxic
conditions.
749
Clinical significance. During periods of energy balance
(weight maintenance), leptin concentrations in the blood
reflect total body fat in humans.
750,751
Leptin concentrations
act not only as an adipostat by signaling information about
the amount of body fat to the brain but also as a sensor of
energy imbalance.
Leptin is implicated in peripheral insulin resistance by
attenuating insulin action on insulin-sensitive cells
752
or
may cause this resistance by influencing insulin secretion.
For example, leptin receptors have been found on pancreatic
b-cells and have also been shown to inhibit b-cell secretion
of insulin by interfering with ion channel function.
753
More
peripheral actions and interactions, illustrating the com-
plexity of the leptin axis, are reviewed elsewhere.
746,754
In human liver, leptin has been shown to attenuate a
number of insulin-induced activities ultimately resulting in
insulin resistance.
752
Furthermore, a proinflammatory and
profibrinogenic activity of leptin has been reported.
755
There are inconsistencies in published literature con-
cerning the role of leptin in pathological conditions such as
NAFLD
756–759
or nonalcoholic steatohepatitis.
760,761
Subjects with MetS had higher leptin levels compared
with individuals without MetS.
762–765
In Korean populations,
the serum leptin levels were associated with MetS indepen-
dent of BMI.
766
Various authors claim that the plasma leptin
level was a significant predictor of the risk of MetS.
767,768
Despite the thorough understanding of leptin’s actions
and numerous attempts to target leptin for obesity and
metabolic disorders,
769
leptin’s clinical applications have
been very limited.
Leptin is used to treat genetically obese subjects carrying
leptin mutations, but such mutations are extremely rare.
770
Leptin is largely ineffective for treating regular obese pa-
tients due to leptin resistance caused by hyperleptinemia,
and leptin administration into these individuals does not
generate anorexic effects.
771
Leptin is successfully used to treat insulin resistance and
hepatic steatosis in patients with congenital severe lipody-
strophy who have very low levels of circulating leptin.
772,773
Resistin
Biochemical background. Resistin is a 10 kDa cysteine-
rich protein of 94 amino acids, highly expressed in the ad-
ipose tissue. It is also known as ADSF (adipocyte-secreted
factor) or FIZZ3 (found in inflammation zone 3). It is a
major determinant of hepatic insulin resistance induced by
high-fat diet in animal models.
774
In humans, it is predomi-
nantly expressed in macrophages
775
and is named because of
its resistance to the action of insulin. The role of resistin in
human susceptibility to disease is discussed in a review ar-
ticle and nicely summarized.
776
Clinical significance. Human data regarding the role of
this adipokine in insulin sensitivity, obesity, and the meta-
bolic syndrome are controversial.
777,778
Part of it could be explained by the fact that this protein
circulates in two distinct assembly states, which exhibit
differential activities in metabolic regulation.
779
Resistin is considered a proinflammatory molecule, which
also plays an important role in the pathogenesis of diabetes
and its complications.
780,781
The release of resistin is often
stimulated by the inflammatory process, IL-6, hyperglyce-
mia, and hormones such as growth hormone (GH) and go-
nadal hormones.
782
Some authors indicated that increased serum resistin levels
are associated with increased obesity,
783
visceral fat,
784
in-
sulin resistance, and type 2 DM,
785–787
while other groups
failed to observe such correlations.
778,788
Most of the time, higher values for resistin are found in
MetS
789–792
and this can contribute to early atherosclerosis
in obese Chinese children
793
since resistin plays an active
role in CD40
+
lymphocyte chemotaxis, an important process
in early atherosclerosis.
794
Only a few publications could
be traced where no differences with a control group were
found,
764,795
therefore researchers claim that although re-
sistin or resistin gene variation is associated with systemic
inflammation, the protein has little influence on adiposity,
metabolic syndrome, or atherosclerosis.
796
As MetS is associated with inflammation, there might be
the possibility that resistin and the correlation with other
metabolic parameters
797
are just a secondary effect.
777,790,791
Resistin, together with leptin, adiponectin, and RBP-4, is
claimed as a marker for predicting metabolic syndrome and
NAFLD.
798,799
In addition, an adiponectin–resistin index,
taking into account both adiponectin and resistin levels,
seems to provide a useful integrated diagnostic biomarker
for screening subjects with increased risk of future devel-
opment of type 2 DM and metabolic syndrome.
800
Since human resistin stimulates hepatic overproduction of
atherogenic apo-B-containing lipoprotein particles by enhanc-
ing apo-B stability and impairing intracellular insulin signaling,
BIOMARKERS OF METS 17
this adipokine might become a novel therapeutic target for
mitigating common hepatic pathophysiological processes.
801
However, since (1) resistin exerts a modulatory effect in
the expression and activity of various adipocytokines, which
is not fully understood, (2) the protein acts differently in
human and experimental animals, and (3) the cascade of
activation of receptors for resistin is poorly understood,
802
this strategy is quite premature.
A more promising observation was that resistin may af-
fect the improvement of insulin sensitivity in the metabolic
syndrome patient treated with metformin, a first-line medi-
cation used for MetS to reduce insulin resistance.
803
Finally, recent investigations on gene polymorphisms in the
promoter site as well as in the gene for resistin
796,804–807
may
shed some light on the debate and conclusions on associations
of resistin levels and MetS. This will be of great interest to
determine the therapeutic potential of resistin inhibition.
Ghrelin
Biochemical background. Ghrelin is a 28-amino acid
peptide, which plays a major role in appetite regulation and
energy balance.
808–810
Postsynthetic modification of the
peptide, in which an n-octanoic acid residue is bound to one
of its amino acids, confers biological activity.
Ghrelin is predominantly produced by the enteroendocrine
cells of the fundus of the stomach.
810
It is named for its strong
GH-releasing ability through the type 1a GH secretagogue
receptor (GHS-R)
809
and named after ghre, meaning grow.
Ghreling has widespread effects on various tissues, both
endocrine and nonendocrine in nature, including (1) stimu-
lation of lactotroph and corticotroph secretion; (2) influence
on gastroenteropancreatic function; (3) regulation of insulin
secretion and glucose and lipid metabolism; (4) influence on
behavior, sleep, and cardiovascular effect; and (5) anti-
proliferative effects.
809–814
Clinical significance. Ghrelin characteristically in-
creases food intake and body weight through its orexigenic,
adipogenic, and somatotrophic properties.
809
In humans,
ghrelin concentration is decreased in states of obesity
815
and
increased in anorexia nervosa.
816
In diabetic children, the
low ghrelin levels may represent a defense mechanism
against hyperglycemia.
817
Progressive decrease of basal ghrelin levels with increase
of BMI, waist-to-hip ratio, and waist circumference was
revealed.
818
Low ghrelin concentrations are also associated with
higher prevalence of the metabolic syndrome with pro-
gressively lower ghrelin levels in relation to the number of
components of the MetS.
819,820
This is mostly explained by
a higher BMI in subjects with lower ghrelin levels because
adiposity influences all other features of MetS.
819–823
Total plasma ghrelin is lower in obese patients with the
metabolic syndrome compared with nonobese counter-
parts.
822
Among obese subjects, the ghrelin levels are lower
in insulin-resistant persons compared with insulin-sensitive
persons.
824
A similar inverse association between fasting ghrelin
and metabolic syndrome was observed in peritoneal dialy-
sis patients, where the HDL level was related to the serum
ghrelin concentration.
825
Ghrelin increases in response to overweight reduction and
negative energy balance, resulting from either exercise in-
tervention or reduction in food intake in prepubescent obese
children.
826
Quite recently, no significant difference was found in
ghrelin concentration between postmenopausal women with
and without MetS.
827
In a review article, the role of ghrelin in diabetes and
metabolic syndrome was further discussed, including the
putative role of genetic variation in the ghrelin and ghrelin
receptor genes.
828
Adipocyte fatty acid-binding protein
Biochemical background. Adipocyte fatty acid-binding
protein (A-FABP) is a 15 kDa small lipid-binding protein,
also known as FABP4 and aP2. In addition to leptin
829
and
adiponectin,
830
it is another adipokine that is preferentially
produced in and released from adipocytes. A-FABP ac-
counts for up to 6% of the total cystolic protein in cultured
differentiated fat cells.
831
In addition to production in adi-
pocytes, A-FABP is also produced in significant amounts
in macrophages
832
and endothelial cells.
833
Some excel-
lent reviews on functionality of A-FABP have been pub-
lished.
834,835
Clinical significance. The first description of A-FABP
as a circulating protein was published in 2006.
836,837
The
relationship between metabolic syndrome diagnosis and
serum A-FABP was defined with 40% sensitivity and 99%
specificity at a protein level of 16 mg/L.
837
FABP4 knockout
mice are protected against almost every aspect of metabolic
syndrome.
838,839
Several cross-sectional studies demonstrated that A-FABP
is independently and positively associated with markers of
metabolic syndrome, most notably obesity-related vari-
ables.
836,837,840–848
Furthermore, circulating A-FABP has
been reported to be elevated in patients with familial com-
bined hyperlipidemia
849
and in those with NAFLD, a dis-
order regarded as the hepatic manifestation of metabolic
syndrome.
850,851
The prevalence of metabolic syndrome is significantly
higher in liver transplant recipients than in a general pop-
ulation. A-FABP4 proved to serve as a circulating bio-
marker in the clinical diagnosis of metabolic syndrome in
postliver transplantation patients.
852
Circulating levels of the protein are also increased in
patients with atherosclerosis
853,854
and CVD,
855,856
but this
is out of the scope of this review.
The fact that FABP4 levels were significantly associated
with left ventricular diastolic dysfunction in morbid obese
subjects, when MetS is present, may indicate that FABP4 is
one of the links between obesity and cardiometabolic dis-
orders.
857
Prospective studies suggest that high baseline circulating
A-FABP concentrations predict the risk of developing
metabolic and vascular diseases independent of established
risk markers.
858
In agreement with these findings, baseline
serum A-FABP concentrations are significantly higher in
children who develop MetS compared with controls.
859
Interventional studies suggest that circulating A-FABP
decreases after weight loss and statin therapy and may con-
tribute to the beneficial effects of these interventions.
860–864
Taken together, high baseline circulating A-FABP con-
centrations predict the risk of developing metabolic (and
cardiovascular) disease states independently of established
18 ROBBERECHT AND HERMANS
risk markers. Whether the A-FABP directly contributes to
the disease needs to be determined. Other important un-
answered questions concerning the role of this adipokine are
as follows: (1) Is circulating A-FABP only a marker of the
disease or does it directly influence the disease? (2) In the
latter case, does A-FABP mediate its effects through ligand-
dependent and/or independent mechanisms? (3) What are
the target tissues of the circulating A-FABP?
These questions have to be resolved before pharmaceu-
tical intervention can be started by using A-FABP inhibitors.
Only after that this could be a promising strategy to tone
down the effects of metabolic diseases such as obesity and
type 2 DM on atherosclerotic disease progression.
865
Retinol-binding protein 4
Biochemical background. RBP-4 is a protein that is a
specific carrier for retinol in the blood. It is one of a large
number of proteins that solubilize and stabilize the hydro-
phobic and labile metabolites of retinoids in aqueous phases
in both extra- and intracellular spaces. It belongs to the li-
pocalin family.
866
Its physiological function appears to be to bind retinol and
prevent its loss through the kidneys.
RBP-4, although largely produced in the liver, is also
made by adipocytes in increased levels in obesity, contrib-
uting to impaired insulin action.
867,868
It acts as a signal to
other cells when there is a decrease in plasma glucose
concentration.
868
RBP-4-knockout mice showed enhanced
insulin sensitivity.
867
Clinical background. A cross-sectional study of a large
middle-aged population in China showed that plasma RBP-4
levels increased gradually with increasing number of met-
abolic syndrome components.
869
Moreover, plasma RBP-4
levels were associated with an adverse profile of oxidative
stress and inflammatory markers.
870
Similar to other adipokines, circulating levels of RBP-4
were associated with body fat distribution rather than body
weight per se. This adipokine was reported to be highly
correlated with the waist-to-hip ratio or visceral fat areas
than with BMI.
871–873
RBP-4, adiponectin, and in particular resistin may be
used as suitable predictable biomarkers of metabolic syn-
drome.
798
Clinical studies on children and adolescents are
quite consistent in supporting a role for RBP-4 in obesity
and insulin resistance, suggesting that the protein is more
involved in the early stages of MetS.
874
RBP-4 and LCN-2
were suggested to be relevant mediators of the trend toward
MetS in psoriatric patients.
875
A marked decrease of RBP-4 levels after bariatic surgery,
which correlates with reduction in visceral fat mass, is
demonstrated and the extent of changes correlates with the
severity of the MetS.
876
Weight loss with a low-fat diet
could partly involve changes in RBP-4 and adiponectin
levels.
877
Other adipokines
Lipocalin-2
Biochemical background. LCN-2, or neutrophil gelatinase-
associated lipocalin, is a 25 kDa secretory glycoprotein
originally identified in human neutrophils.
878
LCN-2 be-
longs to the superfamily of proteins called lipocalins, which
also includes RBP-4. These small proteins are characterized
by a hydrophobic pocket that allows them to function as
transport proteins for hydrophobic molecules such as iron,
retinoids, and steroids.
866
Clinical significance. LCN-2 has been identified as an
adipokine present in the circulation and related to insulin
resistance, obesity, atherosclerotic diseases, and type 2 di-
abetes.
323
The initial hope for LCN-2 as an indicator of
inflammation and a link to metabolic syndrome and CVD, as
pointed out in a review article,
879
could not be established in
later studies.
880,881
LCN-2 and psoriasis are assumed to be closely linked
together
882
and some publications deal with the relationship
between psoriasis and the metabolic syndrome.
399,883
In this
study also, conflicting data are published since recently no
correlation was found.
884
Higher LCN-2 levels are only found in men with
MetS,
885
which illustrates that no definite role for this
adipokine can been established in the MetS.
875,884–886
LCN-2 should be related to inflammation rather than adi-
posity.
884,885,887,888
Fibroblast growth factor 21
Biochemical background. The fibroblast growth factor
(FGF) family comprises 22 members with a wide range of
biological activities, including cell growth, development,
angiogenesis, and wound healing.
889
FGF is an adipocyte-
derived protein with multifunctional nature. Three members
of the family have emerged as endocrine factors involved in
metabolic regulation.
890
FGF-21, a polypeptide with 210
amino acid residues, produced preferentially in liver tis-
sue,
891
has been shown to possess potent beneficial effects
on glucose and lipid metabolism and insulin sensitivity in
animal models.
892
Clinical significance. Serum FGF-21 levels in over-
weight/obese subjects were significantly higher than in lean
objects. The level correlated positively with adiposity, fasting
insulin, and triglycerides and negatively with HDL-C. An
association between FGF-21 and the metabolic syndrome was
found in adults,
848,893–896
but not in children.
897
Elevated
levels are associated with carotid atherosclerosis in humans,
independent of established risk factors, including adverse li-
pid profiles and CRP.
898
More prospective studies are necessary to prove whether
the elevated levels are causally associated with obesity and
its related cardiometabolic complications or simply are a
compensatory upregulation in response to the disease.
899
However, several reviews already discussed the potential
utility of FGF-21-based pharmacotherapy.
900,901
Chemerin. Chemerin is an adipokine with chemoat-
tractant activity. It is secreted as an 18 kDa inactive pro-
protein and it is activated by post-translational C-terminal
cleavage.
902
Chemerin and its receptor are mainly, but not
exclusively, expressed in adipose tissue.
Bozaoglu et al.
903
identified, for the first time, chemerin
as a novel adipokine with a role in the pathophysiology of
MetS. They showed that plasma chemerin concentrations
were strongly associated with BMI, plasma triglycerides,
and blood pressure. This raises the possibility that chemerin
BIOMARKERS OF METS 19
may be of value as a biomarker for the disorder, even before
it is clinically symptomatic.
904,905
Recent studies confirmed this by revealing that plasma
levels of chemerin were high in subjects with nascent MetS,
suggesting an early role of this adipokine in the pathogen-
esis of MetS.
904–908
Chemerin is sometimes,
909
but not always,
910
considered
as a predictor of atherosclerosis in metabolic syndrome. The
elevated level is claimed to be an independent predictive
marker of the presence of CAD in patients with MetS.
911
A recent meta-analysis of studies on serum chemerin con-
centrations and clinical indices of MetS suggests that this adi-
pokine plays an important role in the pathophysiology of MetS.
912
Chemerin and adiponectin may reciprocally participate in
the development of MetS.
913
A 12-week intensive lifestyle intervention significantly de-
creased the serum chemerin level compared with usual care.
914
Omentin. Human omentin is a peptide of 313 amino
acids, and contains a secretory sequence and a fibrinogen-
related domain,
915
originally identified in the omental fat
cDNA library. While omentin is highly expressed in human
visceral fat tissue, circulating omentin is reduced in obese
subjects.
916
Omentin is also downregulated in association
with obesity-linked metabolic disorders.
917,918
Circulating omentin levels negatively correlated with the
multiplicity of metabolic risk factors, suggesting that it
acts as a biomarker for the assessment of metabolic disor-
ders.
919,920
It correlates positively with adiponectin and
HDL in healthy Amisch
916
and Chinese subjects.
917
It seems to play an anti-inflammatory role by prevent-
ing the TNF-a-induced COX-2 expression in endothelial
cells
921
and therefore it is claimed as a biomarker of en-
dothelial dysfunction.
922
Sex appears to influence the relationship between plasma
omentin-1 concentrations and components of the metabolic
syndrome.
923
For Turkey, plasma levels were similar in
nondiabetic MetS patients and healthy subjects.
924
Omentin-1 levels are reduced by pharmacologic doses of
leptin, but remain unaffected by energy deprivation, and
display no day–night variation.
925
Visfatin. Visfatin is a protein of *471 amino acids and
52 kDa. It is reported to be increased in obesity.
926
The relationship of visfatin with MetS is still confusing.
Some authors showed that serum visfatin was increased in
subjects with MetS,
927,928
others claimed that the serum
concentration was not associated with the accumulation of
MetS factors or the diagnosis of MetS.
929
In obese women
also, no relationship was found between visfatin levels and
the presence of MetS.
930
A multifactorial analysis on the value of visfatin in the
prediction of MetS revealed that there was only a correlation
with circulating adiponectin levels.
928
This unclear topic needs further investigation since visfatin
could be a proinflammatory factor favoring the development of
insulin resistance
931
and remains promising, as claimed by
Chang et al. after a meta-analysis and a systemic review.
927
Some newer adipokines
Secreted frizzled-related protein 5. Secreted frizzled-
related protein 5 (Sfrp5) was some years ago identified as an
anti-inflammatory adipocytokine.
932
This protein is highly
expressed in adipose tissue of lean mice, but downregulated
in obese mice.
Human studies regarding Sfrp5 in metabolic disease gave
rise to conflicting data,
933–935
although there is a trend in
lower serum levels in obese patients, especially in those
with MetS.
936
Vaspin. Vaspin is an adipokine identified as a member
of the serine protease inhibitor family.
937,938
It is strongly
expressed in visceral adipose tissue and is stimulated in
mouse and human obesity.
939,940
However, serum levels in morbidly obese were not sig-
nificantly different from those in controls.
919
Metabolic
syndrome-positive patients had higher vaspin levels com-
pared with MetS-negative patients,
941,942
while a research
group in Korea found this relationship only in men.
943
Apelin. Apelin is a new adipokine produced by diverse
cell types, including adipocytes and endothelial cells.
543,944
It is the endogenous ligand of orphan G protein-coupled
receptor.
945
In addition, here, literature data on serum apelin levels
and MetS are conflicting. No association between the se-
rum concentration and metabolic syndrome features is ob-
served,
946
while others find significantly higher apelin
levels.
942,947
Some other factors, such as f.i. testosterone
levels in men,
948
seem to be of importance.
As can be seen from the adipokines discussed, the action
in MetS is most of the time controversial and not fully
understood.
Table 8 summarizes the effects of several of them.
Methylation related
Homocysteine and endothelin are already previously
discussed. Both markers are mentioned as methylation-
related components through the MTHFR gene.
949
In addi-
tion, the vitamins, B12 and folate, are mentioned in relation
to the metabolic syndrome.
950
In literature, data on vitamin B12 in obese patients, with
or without MetS, are normal
951,952
or lower.
953
The homo-
cysteine level was higher in most studies.
953
Folate and vitamin B12 treatment improved insulin re-
sistance along with decreasing homocysteine levels in
patients with metabolic syndrome.
954
Conclusion
Due to the complexity of MetS with the various influ-
ences and consequences for other diseases, it is hard to make
a well-defined distinction between the various groups of
biomarkers. Subdividing has limitations: the complexity of
the syndrome, the interaction of the various biochemical
pathways, and the overlap of the markers. Nevertheless, we
have chosen to divide them into four major groups: dysli-
pidemias, markers of oxidative stress, inflammatory mark-
ers, and the cardiometabolic markers. The latter group is
quite extended and includes the classical parameters and
some new and promising ones, f.i. the adipokines.
Whether changes in concentration levels are causes or
consequences of the metabolic syndrome cannot be defined.
An inverse association of a marker with the metabolic
20 ROBBERECHT AND HERMANS
syndrome does not mean that low values are a risk factor for
MetS and high values are protective. Sometimes it turned out
that an acute elevation is a protective factor, whereas chronic
elevation is a risk for disease, such as f.i. for uric acid.
Most important observations are compiled at the end of
every subgroup.
In the group of the adipokines, adiponectin can be indi-
cated as an independent protective factor for incidence of
MetS.
Author Disclosure Statement
No competing financial interests exist.
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