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Adiponectin, a Therapeutic Target for Obesity, Diabetes, and Endothelial Dysfunction

Authors:

Abstract

Adiponectin is the most abundant peptide secreted by adipocytes, whose reduction plays a central role in obesity-related diseases, including insulin resistance/type 2 diabetes and cardiovascular disease. In addition to adipocytes, other cell types, such as skeletal and cardiac myocytes and endothelial cells, can also produce this adipocytokine. Adiponectin effects are mediated by adiponectin receptors, which occur as two isoforms (AdipoR1 and AdipoR2). Adiponectin has direct actions in liver, skeletal muscle, and the vasculature.Adiponectin exists in the circulation as varying molecular weight forms, produced by multimerization. Several endoplasmic reticulum ER-associated proteins, including ER oxidoreductase 1-α (Ero1-α), ER resident protein 44 (ERp44), disulfide-bond A oxidoreductase-like protein (DsbA-L), and glucose-regulated protein 94 (GPR94), have recently been found to be involved in the assembly and secretion of higher-order adiponectin complexes. Recent data indicate that the high-molecular weight (HMW) complexes have the predominant action in metabolic tissues. Studies have shown that adiponectin administration in humans and rodents has insulin-sensitizing, anti-atherogenic, and anti-inflammatory effects, and, in certain settings, also decreases body weight. Therefore, adiponectin replacement therapy in humans may suggest potential versatile therapeutic targets in the treatment of obesity, insulin resistance/type 2 diabetes, and atherosclerosis. The current knowledge on regulation and function of adiponectin in obesity, insulin resistance, and cardiovascular disease is summarized in this review.
International Journal of
Molecular Sciences
Review
Adiponectin, a Therapeutic Target for Obesity,
Diabetes, and Endothelial Dysfunction
Arunkumar E. Achari and Sushil K. Jain *
Department of Pediatrics, Louisiana State University Health Sciences Center, 1501 Kings Highway,
Shreveport, LA 71103, USA; aeluma@lsuhsc.edu
*Correspondence: sjain@lsuhsc.edu; Tel.: +1-318-675-6086; Fax: +1-318-675-6059
Received: 7 April 2017; Accepted: 13 June 2017; Published: 21 June 2017
Abstract:
Adiponectin is the most abundant peptide secreted by adipocytes, whose reduction
plays a central role in obesity-related diseases, including insulin resistance/type 2 diabetes and
cardiovascular disease. In addition to adipocytes, other cell types, such as skeletal and cardiac
myocytes and endothelial cells, can also produce this adipocytokine. Adiponectin effects are mediated
by adiponectin receptors, which occur as two isoforms (AdipoR1 and AdipoR2). Adiponectin has
direct actions in liver, skeletal muscle, and the vasculature.Adiponectin exists in the circulation as
varying molecular weight forms, produced by multimerization. Several endoplasmic reticulum
ER-associated proteins, including ER oxidoreductase 1-
α
(Ero1-
α
), ER resident protein 44 (ERp44),
disulfide-bond A oxidoreductase-like protein (DsbA-L), and glucose-regulated protein 94 (GPR94),
have recently been found to be involved in the assembly and secretion of higher-order adiponectin
complexes. Recent data indicate that the high-molecular weight (HMW) complexes have
the predominant action in metabolic tissues. Studies have shown that adiponectin administration
in humans and rodents has insulin-sensitizing, anti-atherogenic, and anti-inflammatory effects, and,
in certain settings, also decreases body weight. Therefore, adiponectin replacement therapy in humans
may suggest potential versatile therapeutic targets in the treatment of obesity, insulin resistance/type
2 diabetes, and atherosclerosis. The current knowledge on regulation and function of adiponectin
in obesity, insulin resistance, and cardiovascular disease is summarized in this review.
Keywords: adiponectin; obesity; type 2 diabetes; endothelial dysfunction
1. Introduction
Life style modification and rapid urbanization has triggered the obesity epidemic,
which is associated with a number of health problems. Obesity is often summarized together as
the metabolic syndrome and increases the risk of insulin resistance, type 2 diabetes, fatty liver disease and
cardiovascular disease [
1
]. Adiponectin, also known as adipocyte complement-related protein of 30 kDa
(Acrp30), a type of adipokine, was identified by different groups [
2
4
]. Adiponectin is an endocrine
factor synthesized and released from adipose tissue [
5
]. Basic scientific studies have demonstrated
that adiponectin has insulin-sensitizing [
5
], anti-atherogenic, and anti-inflammatory properties [
6
,
7
].
Therefore, it is important for investigators to have a thorough understanding of adiponectin.
Such knowledge may lead to new therapeutic approaches for diseases, such as type 2 diabetes, metabolic
syndrome, cardiovascular disease, and obesity.
2. Adipose Tissue Biology: A Brief Overviews
Adipose tissue, commonly called “fat”, is a type of loose connective tissue comprised of lipid-filled
cells (adipocytes) surrounded by a matrix of collagen fibers, blood vessels, fibroblasts, and immune
cells [
8
]. Two adipose tissues with different functions coexist in humans: white adipose tissue (WAT)
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Int. J. Mol. Sci. 2017,18, 1321 2 of 17
and brown adipose tissue (BAT). WAT represents the vast majority of adipose tissue in the organism.
Adipose tissue stored energy in the form of triglycerides and cholesterol as a single large lipid droplet
(unilocular appearance), whereas brown adipose tissue is involved in nonshivering thermogenesis
as a result of fat burning. Brown adipocytes are particularly present in small mammals and human
neonates, and contain several smaller lipid droplets (multilocular appearance) [
9
]. WAT is composed
of many cell types, adipocyte being the most abundant. The other cells, collectively referred to as
the stromalvascular fraction (SVF), are a heterogeneous population of endothelial cells, macrophages,
fibroblasts, stem cells, and lymphocytes [10].
3. Adiponectin: Biosynthesis, Structure and Downstream Signaling
Adiponectin (also known as Acrp30 [
2
], AdipoQ [
11
], GBP-28 [
12
], and apM1 [
3
]) is a 244-amino
acid protein secreted mainly by the adipose tissue. It was identified almost simultaneously by
fourdifferent groups using different approaches [
2
,
3
,
11
,
12
]. Initially, it was thought that adiponectin
was exclusively produced only by adipose tissue. However, later it has proven, from different
research groups, that adiponectin is expressed in other tissues including human and murine
osteoblasts [
13
], liver parenchyma cells [
14
], myocytes [
15
], epithelial cells [
16
], and placental tissue [
17
].
Human adiponectin is encoded by the Adipo Q gene, which spans 17 kb on chromosome locus 3q27.
The gene for human adiponectin contains three exons, with the start codon in exon 2 and stop codon
in exon 3 [
18
,
19
]. This human chromosome 3q27 has been identified as a region carrying a susceptibility
gene for type 2 diabetes and metabolic syndrome [
20
,
21
]. Serum levels of adiponectin decrease with
obesity and are positively associated with insulin sensitivity [
22
,
23
]. Because of these positive actions,
adiponectin has attracted tremendous scientific interest in recent years, and has been extensively
studied both in human and animal models.
3.1. Structural Features, Synthesis and Post Translational Modification of Adiponectin
Adiponectin is a 30 kDa multimeric protein and is secreted mainly by white adipose tissue,
although other tissues express low levels of adiponectin too. Full-length human adiponectin comprises
244 amino acid residues, including a NH
2
-terminal hyper-variable region (amino acids from 1–18),
followed by a collagenous domain consisting of 22 Gly-XY repeats, and a COOH-terminal C1q-like
globular domain (amino acids from 108–244). In contrast to humans, mouse adiponectin is a 247 amino
acid long protein [
2
]. Adiponectin is secreted from adipocytes into the bloodstream as three oligomeric
complexes, including trimer (67 kDa), hexamer (140 kDa), and a high molecular weight (300 kDa)
multimer comprising of at least 18 monomers (Figure 1). The monomeric form of adiponectin
is undetectable in native conditions. Homotrimer, also known as low molecular weight (LMW),
is a basic building block of oligomeric adiponectin. The interaction between the collagenous domains
results in formation of highly ordered trimer, which is further stabilized by an intratrimer disulfide
bond mediated by Cys
39
(or Cys
22
, if the N-terminal 17-amino acid secretory peptide is excluded).
The formation of a disulfide bond between two trimers mediated by the free Cys
39
in each leads to
the formation of the hexameric form of adiponectin. This hexameric form serving as the building
block for the HMM form, which consists of 12–18 hexamers existing in a bouquet-like structure [
24
].
Post-translational modifications, especially hydroxylation and subsequent glycosylation of several
highly conserved lysine residues within its collagenous domain, are crucial for the formation of HMW
oligomeric adiponectin, which is the major bioactive isoform contributing to its insulin-sensitizing and
cardiovascular protective effects [
25
]. Globular adiponectin, the globular C1q domain of adiponectin
generated from full-length protein by proteolysis, is also biologically active [26].
The biosynthesis and secretion of adiponectin in adipocytes are tightly controlled
by several molecular chaperones in the endoplasmic reticulum, including: ERp44 (Endoplasmic
Reticulum resident protein 44), Ero1-La (ER oxidoreductase 1-La), and DsbA-L (disulfide-bond
A oxidoreductase-like protein) [
27
29
]. Scherer and his colleagues proved that ERp44 retains
adiponectin oligomers in the endoplasmic reticulum via a thiol-mediated mechanism [
2
]. ERp44 forms
Int. J. Mol. Sci. 2017,18, 1321 3 of 17
a mixed disulfide bond with adiponectin through the cysteine residue within its variable region
(Cys
36
in humans, and Cys
39
in mice) [
27
]. In contrast with the inhibitory effects of ERp44, Ero1-L
α
selectively enhances the secretion of HMW adiponectin. Ero1-L
α
can displace the ERp44-retained
HMW adiponectin, and, therefore, release this oligomeric complex trapped by ERp44 [
27
].
DsbA-L functions as a protein disulfide isomerase to regulate adiponectin disulfide bond formation,
which is essential for multimerization. Sialic acids also modified adiponectin through O-linked
glycosylation situated on threonine residues within the hypervariable region [
22
], which determines
the half-life of adiponectin in the circulation by modulating its clearance from the bloodstream.
In addition, succination of the highly conserved cysteine residues (Cys
36
) within the hypervariable
region of adiponectin blocks adiponectin multimerization, and it may contribute to the decrease
in plasma adiponectin in diabetes [
30
]. Therefore, extensive post-translational modifications of
adiponectin are essential for efficient maturation, oligomerization, and secretion of adiponectin,
and are also important for maintaining its stability in the circulation.
Int. J. Mol. Sci. 2017, 18, 1321 3 of 17
Figure 1. Domains and structure of adiponectin: Full-length adiponectin is composed of 244 amino
acids, including a collagen-like fibrous domain at the N-terminus and a C1q-like globular domain at
the C-terminus. In circulation, adiponectin forms low-molecular weight (LMW) homotrimers and
hexamers, and high-molecular weight (HMW) multimers of 12–18 monomers. A smaller form of
adiponectin that consists of globular domain also exists in plasma in negligible amounts. Each
adiponectin subunit in the basic trimeric building block represented in a different color.
The biosynthesis and secretion of adiponectin in adipocytes are tightly controlled by several
molecular chaperones in the endoplasmic reticulum, including: ERp44 (Endoplasmic Reticulum
resident protein 44), Ero1-La (ER oxidoreductase 1-La), and DsbA-L (disulfide-bond A
oxidoreductase-like protein) [27–29]. Scherer and his colleagues proved that ERp44 retains
adiponectin oligomers in the endoplasmic reticulum via a thiol-mediated mechanism [2]. ERp44
forms a mixed disulfide bond with adiponectin through the cysteine residue within its variable region
(Cys36 in humans, and Cys39 in mice) [27]. In contrast with the inhibitory effects of ERp44, Ero1-Lα
selectively enhances the secretion of HMW adiponectin. Ero1-Lα can displace the ERp44-retained
HMW adiponectin, and, therefore, release this oligomeric complex trapped by ERp44 [27]. DsbA-L
functions as a protein disulfide isomerase to regulate adiponectin disulfide bond formation, which is
essential for multimerization. Sialic acids also modified adiponectin through O-linked glycosylation
situated on threonine residues within the hypervariable region [22], which determines the half-life of
adiponectin in the circulation by modulating its clearance from the bloodstream. In addition,
succination of the highly conserved cysteine residues (Cys36) within the hypervariable region of
adiponectin blocks adiponectin multimerization, and it may contribute to the decrease in plasma
adiponectin in diabetes [30]. Therefore, extensive post-translational modifications of adiponectin are
essential for efficient maturation, oligomerization, and secretion of adiponectin, and are also
important for maintaining its stability in the circulation.
3.2. Adiponectin Receptors
AdipoR1 and AdipoR2, two structurally related seven transmembrane receptors, have been
identified to function as adiponectin receptors. They are structurally and functionally distinct from
classical G-protein coupled receptors (GPCR). Unlike all other GPCRs reported, AdipoR1 and
AdipoR2 have an inverted membrane topology with a cytoplasmic NH2 terminus and a short,
Figure 1.
Domains and structure of adiponectin: Full-length adiponectin is composed of 244 amino
acids, including a collagen-like fibrous domain at the N-terminus and a C1q-like globular domain
at the C-terminus. In circulation, adiponectin forms low-molecular weight (LMW) homotrimers
and hexamers, and high-molecular weight (HMW) multimers of 12–18 monomers. A smaller form
of adiponectin that consists of globular domain also exists in plasma in negligible amounts.
Each adiponectin subunit in the basic trimeric building block represented in a different color.
3.2. Adiponectin Receptors
AdipoR1 and AdipoR2, two structurally related seven transmembrane receptors, have been
identified to function as adiponectin receptors. They are structurally and functionally distinct from
classical G-protein coupled receptors (GPCR). Unlike all other GPCRs reported, AdipoR1 and AdipoR2
have an inverted membrane topology with a cytoplasmic NH
2
terminus and a short, extracellular
COOH terminal domain of approximately 25 amino acids [
31
]. AdipoR1 and AdipoR2 encoded by
genes situated on the 1p36.13-q41 and 12p13.31 chromosomal regions, respectively [
31
]. AdipoR1
is a high affinity receptor for globular adiponectin and a low affinity receptor for full length adiponectin.
It is expressed ubiquitously, but most abundantly, in skeletal muscle. On the other hand, AdipoR2
mainly recognizes full length adiponectin and is predominantly expressed in the liver [31].
Int. J. Mol. Sci. 2017,18, 1321 4 of 17
Apart from AdipoR1 and AdipoR2, another receptor has also been identified for adiponectin,
called T-cadherin. It acts as a receptor for hexameric and HMW forms of adiponectin but not for other
forms [
32
]. T-cadherin is a unique cadherin molecule that is anchored to the surface membrane,
not through a transmembrane domain, but, instead, via a glycosyl phosphatidyl inositol (GPI)
moiety [3335]
. Recent studies have identified that cadherin-deficient mice showed elevated plasma
adiponectin levels, especially HMW form [34].
3.3. Role of APPL1 and APPL2 in Adiponectin Signaling
APPL1, an adaptor protein, binds to the adiponectin receptors and positively mediates
adiponectin signaling in mammals. APPL1 has three functional domains, which play an important
role in the intracellular signal transduction of adiponectin receptors pathway. This includes
NH
2
-terminal Bin1/amphiphysin/rvs167 (BAR) domain (initially identified as the leucine zipper
motif, 18–226 amino acids), followed by a pleckstrin homology (PH) domain (278–377 amino acids)
and a phosphotyrosine binding (PTB) domain (597–636 amino acids) near the COOH terminus [
36
].
APPL1 acts as an interacting partner of both AdipoR1 and AdipoR2. Broadly, the BAR domain
is associated with multiple biological processes like sensing and inducing membrane curvature,
small GTPase binding [
37
39
] transcriptional repression, apoptosis, and secretory vesicle fusion [
40
,
41
],
and it is located near to the NH
2
terminus. In general, the PH domain targets proteins to specific
membrane compartments by increasing the lipid specificity of the BAR domain [
42
]. The general
function of PTB domain is to act as an adaptor or scaffold for the binding of proteins. The PTB domain of
APPL1 is located near the COOH terminus, away from BAR-PH domain, making it an easily accessible
structure for its binding partners. APPL2 is an isoform of APPL1, and these two proteins display
54% identity in protein sequences [
43
]. Similar to APPL1, APPL2 has an N-terminal BAR domain,
central PH domain, and C-terminal PTB domain. APPL2 involves follicle-stimulating hormone signal
transduction pathway by binding to APPL1 via their respective BAR domains [44].
3.4. Downstream Signaling Events of Adiponectin
Adiponectin elicits a number of downstream signaling events. APPL1 acts as a signaling pathway
mediator in cross-talk with adiponectin and insulin, and it interacts directly with insulin receptor
substrates. Activation of insulin receptor substrate proteins serve as docking platforms for the p85
regulatory subunit of the phosphatidylinositol 3-kinase (PI3K), which results in the generation
of phosphatidylinositol 3,4,5-triphosphate at the plasma membrane. This activation of the PI3K
pathway activates Akt and its downstream targets, which in turn exhibits a biological response [
45
].
Reports from various research groups demonstrate that APPL1 is involved in the activation of AMP
activated protein kinase (AMPK) [
46
,
47
]. Upon binding of adiponectin to its receptor, APPL1 binds
and activates protein phosphatase 2A, resulting in dephosphorylation and inactivation of protein
kinase Cz (PKCz). This in turn dephosphorylates liver kinase B1 (LKB1) at its Ser
307
, allowing LKB1
to translocate from nucleus to cytoplasm, and activate AMPK [
46
]. Activation of AMPK is a key
step in mediating the most of the effects of adiponectin at cellular level. AMPK, is a fuel-sensing
enzyme that responds to decreases in cellular energy state by activating pathways that generate
energy (e.g., oxidation of fats), and inhibiting energy consuming pathways; however, AMPK is not
acutely necessary for survival (e.g., fatty acid, triglyceride, and protein synthesis). Adiponectin
drastically increases the expression and activity of PPAR-
α
, a key transcription factor in metabolic
regulation, which in turn upregulates acetyl CoA oxidase (ACO) and uncoupling proteins (UCPs);
thereby, promoting fatty acid oxidation and energy expenditure [
31
]. Interestingly, the action of APPL1
by adiponectin on p38 MAPK [
48
] and Rab5 a GTPase downstream of APPL1, improves glucose
metabolism in various metabolic tissues [
36
]. Activated AMPK, in response to adiponectin, is also
involved in nitric oxide production through the activation of eNOS, resulted in vasodilation [
49
].
In addition, activated AMPK by adiponectin inhibits IKK/NF
κ
B/PTEN triggered apoptosis [
50
]
(Figure 2).
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Int. J. Mol. Sci. 2017, 18, 1321 5 of 17
is also involved in nitric oxide production through the activation of eNOS, resulted in vasodilation
[49]. In addition, activated AMPK by adiponectin inhibits IKK/NFκB/PTEN triggered apoptosis [50]
(Figure 2).
Figure 2. Schematic representation of adiponectin signal transduction implicating a cross talk with
the insulin signaling pathway: Insulin and adiponectin interact with their respective receptors, which
trigger a cascade of signaling events. Metabolic actions of the insulin are mainly carried out by
PI3K/AKT pathway, resulting in increased protein synthesis, lipogenesis, glucose uptake and
utilization, glycogen synthesis, and reduced lipolysis and gluconeogenesis. Interaction of adiponectin
with its receptors (Adipo R1 and R2) results in the activation of multiple signaling pathways including
IRS1/2, AMPK, and p38 MAPK. Activation of IRS1/2 by adiponectin signaling is a major mechanism
by which adiponectin sensitizes insulin action in insulin responsive tissues.
3.5. Interactions between the AMPK and Insulin Signaling Pathways
The insulin signaling pathway is activated when nutrients are available, whereas the AMPK
pathway is activated when cells are starved for a carbon source. One would therefore expect these 2
pathways to oppose each other, and this is often the case. In mammals, insulin promotes lipid,
protein, and glycogen synthesis, whereas AMPK inhibits these biosynthetic pathways. Proteiogenic
effect of insulin is mediated in part by activation of the target of rapamycin TOR pathway via
phosphorylation of TSC2, whereas AMPK activation causes phosphorylation of different sites on
TSC2 and inhibits TOR [51,52]. In some tissues, such as heart, insulin inhibits AMPK and this action
was mediated by the activation of the protein kinase B. In other cases, the insulin and AMPK signaling
pathways work in the same direction, particularly in processes that regulate plasma glucose levels.
In skeletal muscle, both insulin and AMPK activation promotes glucose uptake by increasing GLUT4
translocation to the plasma membrane. However, the fate of the glucose is different in either case. In
the case of insulin, glucose can be stored as glycogen (anabolic), whereas, in the case of AMPK,
glucose can be entered in oxidative (catabolic) pathway. These two pathways appear to converge on
the phosphorylation of AS160, which further involved in GLUT-4 translocation [53,54]. A second case
in which insulin and AMPK act in the same direction occurs in the liver, in which both repress the
expression of enzymes of gluconeogenesis, such as phosphoenolpyruvate carboxykinase and
glucose-6-phosphatase [55]. It makes sense that insulin, a hormone released in response to high blood
glucose, should repress hepatic glucose production, whereas in the case of AMPK it may perhaps
have evolved as among its anti-anabolic actions.
Figure 2.
Schematic representation of adiponectin signal transduction implicating a cross talk with
the insulin signaling pathway: Insulin and adiponectin interact with their respective receptors,
which trigger a cascade of signaling events. Metabolic actions of the insulin are mainly carried
out by PI3K/AKT pathway, resulting in increased protein synthesis, lipogenesis, glucose uptake and
utilization, glycogen synthesis, and reduced lipolysis and gluconeogenesis. Interaction of adiponectin
with its receptors (Adipo R1 and R2) results in the activation of multiple signaling pathways including
IRS1/2, AMPK, and p38 MAPK. Activation of IRS1/2 by adiponectin signaling is a major mechanism
by which adiponectin sensitizes insulin action in insulin responsive tissues.
3.5. Interactions between the AMPK and Insulin Signaling Pathways
The insulin signaling pathway is activated when nutrients are available, whereas the AMPK
pathway is activated when cells are starved for a carbon source. One would therefore expect these 2
pathways to oppose each other, and this is often the case. In mammals, insulin promotes lipid, protein,
and glycogen synthesis, whereas AMPK inhibits these biosynthetic pathways. Proteiogenic effect of
insulin is mediated in part by activation of the target of rapamycin TOR pathway via phosphorylation
of TSC2, whereas AMPK activation causes phosphorylation of different sites on TSC2 and inhibits
TOR [
51
,
52
]. In some tissues, such as heart, insulin inhibits AMPK and this action was mediated by
the activation of the protein kinase B. In other cases, the insulin and AMPK signaling pathways work
in the same direction, particularly in processes that regulate plasma glucose levels. In skeletal muscle,
both insulin and AMPK activation promotes glucose uptake by increasing GLUT4 translocation to
the plasma membrane. However, the fate of the glucose is different in either case. In the case of insulin,
glucose can be stored as glycogen (anabolic), whereas, in the case of AMPK, glucose can be entered
in oxidative (catabolic) pathway. These two pathways appear to converge on the phosphorylation of
AS160, which further involved in GLUT-4 translocation [
53
,
54
]. A second case in which insulin and
AMPK act in the same direction occurs in the liver, in which both repress the expression of enzymes
of gluconeogenesis, such as phosphoenolpyruvate carboxykinase and glucose-6-phosphatase [
55
].
It makes sense that insulin, a hormone released in response to high blood glucose, should repress
hepatic glucose production, whereas in the case of AMPK it may perhaps have evolved as among its
anti-anabolic actions.
4. Adiponectin Signaling in Key Metabolic Tissues
Adiponectin exhibits anti-diabetic, anti-inflammatory, and anti-atherogenic effects, and it also
functions as an insulin sensitizer. Hence, it is a novel therapeutic target for diabetes and metabolic
Int. J. Mol. Sci. 2017,18, 1321 6 of 17
syndrome [
6
]. Adiponectin also plays a central role in energy homeostasis through its action
in hypothalamus, and a new role for adiponectin as a “starvation gene” has been proposed [
56
,
57
].
The following section discusses the signal transduction of adiponectin in different tissues and the role
of APPL1 in mediating the effects of adiponectin.
4.1. Skeletal Muscle
Many studies have clearly demonstrated that the skeletal muscle is an important peripheral
target tissue for adiponectin to exert its beneficial metabolic effects. Previous work reported that
adiponectin improved glucose utilization and fatty acid oxidation in C2C12 myocytes [
58
]. In addition,
in mice fed with high fat/sucrose diet, adiponectin showed to increase energy expenditure by
increasing fatty acid oxidation, and to increases glucose uptake in skeletal muscle [
26
]. Studies with rat
skeletal muscle cells have shown that globular adiponectin increases glucose transporter-4 (GLUT-4)
translocation and glucose uptake [
59
]. AdipoR1 is the most abundant adiponectin receptor in skeletal
muscle, and globular adiponectin showed high affinity towards AdipoR1. By this reason, most of
the adiponectin effects in skeletal muscle are carried out by globular form [31].
The binding of adiponectin to its membrane receptors, such as AdipoR1 and AdipoR2, leads to
the activation of two major signal pathways in muscle cells, the AMPK and the p38 mitogen-activated
protein kinase (MAPK) pathways [
31
,
43
]. Activation of these pathways has been shown to be essential
for adiponectin-induced glucose uptake and fatty acid oxidation. Globular and full length adiponectin
was found to increase phosphorylation and activation of AMPK; thereby, it increases fat oxidation
and glucose uptake in C2C12 myoctyes [
58
]. Similar to previous studies, mice injected with globular
adiponectin showed increased expression of molecules involved in fatty-acid transport, combustion,
and energy dissipation, such as such as CD36, ACO and UCP2. This turn leads to decrease in tissue
triglyceride content in mice skeletal muscle [5].
Recently it has been shown that cellular ceramide levels were lowered by adiponectin through
the activation of ceramidase, which inturn converts ceramide to sphingosine. This effect appears to
be dependent on activation of AdipoR1 or AdipoR2 [
60
]. In addition, overexpression of adiponectin,
AdipoR1, or AdipoR2 in liver reduces hepatic ceramide levels and improves insulin sensitivity,
while deficiency of adiponectin increases hepatic ceramide levels and exacerbates insulin resistance.
Since accumulation of ceramide in skeletal muscle has been reported to be associated with impaired
insulin sensitivity [
61
], decreased ceramide concentration by adiponectin may be a mechanism for
increasing insulin sensitivity in skeletal muscles. In addition, it has been suggested that adiponectin
decreases insulin resistance by decreasing muscular lipid content in obese mice [5].
p38 MAPK, which belongs to the group of stress-activated kinases, has been connected with a large
number of cellular processes, including cell growth and differentiation, apoptosis, and inflammation;
in addition, it gets activated in response to a numerous extracellular stimuli [
62
]. Previous studies have
suggested that the activation of p38 MAPK and PPAR-
α
along with AMPK by globular adiponectin
was found to induce fatty acid oxidation in C2C12 skeletal muscle cells [
63
]. Similarly, p38 MAPK
acts downstream of AMPK in cardiomyocytes, and the inhibition of the AMPK/p38 MAPK signaling
pathway partially abolishes the stimulation of glucose uptake in response to hypoxia [
64
]. It has
recently been reported that adiponectin enhances fatty acid oxidation in muscles cells by stimulating
PPAR
α
transcriptional activity via sequential activation of AMPK and p38 MAPK [
63
].These results
suggest the presence of multiple pathways that could mediate the effect of adiponectin on glucose and
the fatty acid metabolism in muscles.
From the various studies, it was evident that APPL1 plays an important role in adiponectin signaling.
In cultured skeletal muscle cells, overexpression of APPL1 enhances the phosphorylation and activation
of AMPK and p38 MAPK. On the other hand, suppression of APPL1 inhibited adiponectin meditated
AMPK as well as p38 MAPK and ACC phosphorylation, and, thereby, limited fat oxidation in C2C12
myotubes, implies APPL1 plays a crucial role in adiponectin signaling in the skeletal muscle [43].
Int. J. Mol. Sci. 2017,18, 1321 7 of 17
Adiponectin reduces plasma glucose levels in mice subjected to high fat meal [
26
], and directly
regulates glucose metabolism and insulin sensitivity in C57BL6J mice [
58
]. This beneficial effect of
adiponectin on glucose metabolism was mainly via the activation of AMPK [
58
], which leads to
the translocation of GLUT4 to the cell membrane. Both forms of adiponectin upregulates GLUT-4
membrane translocation in rat skeletal muscle cells, and the adaptor protein, APPL1, acts as the first
signaling molecule that binds to the adiponectin receptors and positively mediates adiponectin
signaling in muscle cells [
43
]. Further, overexpression of APPL1 increases, and suppression of
APPL1 level reduces its action. Thus, adiponectin signaling and adiponectin-mediated downstream
events in mouse skeletal muscle cells include: lipid oxidation, glucose uptake, and the GLUT-4
membrane translocation [
43
]. Therefore, binding of adiponectin with APPL1 mediates phosphorylation
of AMPK and p38 MAPK, thereby adiponectin stimulates GLUT-4 translocation in muscle cells.
In addition, recently small GTPase, Rab5, has been shown to interact with APPL1 and to regulate
GLUT4 internalization in an insulin-dependent mechanism [43].
4.2. Vascular Endothelium
Atherosclerosis is the process of vascular wall thickening and hardening, and it is the primary
cause of coronary heart disease, ischemic stroke, and peripheral arterial disease [
65
]. Numerous
epidemiological studies suggest that adiponectin deficiency (hypoadiponectinemia) is associated with
coronary artery disease and hypertension [
66
], left ventricular hypertrophy [
67
], and a greater risk of
myocardial infarction [
68
]. Experimental studies with cell cultures and animal models have shown
cardioprotective action of adiponectin in cell types, including: vascular endothelial cells, smooth muscle
cells, and cardiac myocytes and adiponectin-deficient mice [
7
]. The vasculoprotective and angiogenic
properties of adiponectin have demonstrated in adiponectin-deficient mice in which adiponectin
improves revascularization of ischemic limbs [
69
] and rescues from cerebral ischemia-reperfusion [
70
].
Additionally, adiponectin supplementation attenuates neointimal thickening in mechanically injured
arteries through the suppressive action of adiponectin on the proliferation and migration of vascular
smooth muscle cells [
7
]. On the high salt diet, adiponectin knock-out mice developed severe blood
pressure due in part to a reduction of endothelial nitric oxide synthase activity [
71
]. In addition,
studies had shown that overexpression of adiponectin protects arties from atherosclerotic plaques
formation [
72
], whereas deficiency of adiponectin results in the higher incidence of atherosclerosis [
73
].
Mechanistically, many of the adiponectin benefits connected to its vasculoprotective are carried out
via its ability to increases nitric oxide production through the activation of eNOS by AMPK-dependent
manner [
74
,
75
]. It has also been proven that adiponectin prevents endothelial apoptosis through
the AMPK mediated pathway [
75
]. Adiponectin supplementation reduces TNF-
α
mediated vascular
cell adhesion molecule-1 and interleukin-8 by suppressing the nuclear factor kappa-b activation
in endothelial cells [
76
,
77
]. Furthermore, cyclooxygenase-2 expression was increased by adiponectin
treatment in cultured endothelial cells, and deletion of cyclooxygenase-2 inhibits adiponectin meditated
growth in endothelial cell migration, differentiation, and survival [
78
]. Studies supports the idea that
induction of cyclooxygenase-2 expression is mediated by sphingosine kinase-1 in cardiomyocytes
by adiponectin [
79
]. Based on previous studies, it is evident that pressure overload or angiotensin
II-induced cardiac hypertrophy, was inhibited through AMPK activation by adiponectin treatment
in myocytes [
56
,
69
]; and, in animal models, adiponectin has been shown to be protective for systolic
and diastolic dysfunction of myocardial infarction [
80
,
81
]. Thus, adiponectin might utilize AMPK
pathway and cyclooxygenase-2 to improve endothelial function.
Although AdipoR1 and AdipoR2 are mainly involved in the metabolic action of adiponectin,
some studies have investigated other receptors for adiponectin in heart [
31
,
82
]. Studies have shown
that T-cadherin is a GPI-anchored adiponectin-binding protein involved in the cardioprotective action
of adiponectin. T-cadherin is highly expressed in the vasculature, including endothelial cells [
83
],
smooth muscle cells [
84
], and pericytes [
85
]. Studies demonstrate that ablation of T-cadherin abolishes
adiponectin mediated cardioprotective effects in both short and long term cardiac hypertrophy as
Int. J. Mol. Sci. 2017,18, 1321 8 of 17
well as myocardial ischemia-reperfusion injury. This suggests that T-cadherin is a physiological
adiponectin binding receptor that enables the association of adiponectin within the heart [
34
].
T-cadherin is also essential for the provascularization effects of adiponectin in mice [
35
]. Furthermore,
hypoadiponectinemia was observed in T-cadherin-deficient mice, thus supporting the impairment of
adiponectin recruiting in cardiovascular tissues of these mice [
34
]. Conversely, low T-cadherin tissue
expression was observed in adiponectin-deficient mice, suggesting regulatory axis between T-cadherin
and adiponectin [35].
4.3. Adipocyte/Adipose Tissue
Adipose tissue plays a central role in regulating whole-body energy and glucose homeostasis
through its subtle functions at both organ and systemic levels [
86
]. Adipose tissue, which is primarily
composed of adipocytes as well as pre-adipocytes, macrophages, endothelial cells, fibroblasts, and
leucocytes, has been increasingly recognized as a major player of systemically metabolic regulation [
87
].
Adipose tissue acts as an endocrine organ and produces numerous bioactive factors such as adipokines
that communicate with other organs and modulate a range of metabolic pathways. On the other hand,
adipose tissue stores energy in the form of lipid and controls the lipid mobilization and distribution
in the body [88].
Studies reported that 3T3L1 adipocytes showed high expression of adiponectin. Adiponectin,
through its autocrine activity, helps in adipocytes cell differentiation. In adipocytes, C/EBP
α
, PPAR
γ
,
and sterol regulatory element-binding protein (SREBP)-1c are involved in promoting adipogenesis,
and increasing lipid content and insulin directed glucose transport [
89
]. Transgene-mediated
overexpression of adiponectin in ob/ob mice lead to morbid obesity due to decreased energy
expenditure; however, there is marked improvement in glucose metabolism, accompanied by
a reduction in macrophage numbers in adipose tissue and decreased expression of TNF
α
in fat
pads. In addition, over expression of adiponectin in ob/ob mice showed improved vascularization and
expansion of the subcutaneous fat pad in experimental animals. Collectively, chronic over expresson
of adiponectin leads to massive increase in subcutaneous fat, and it protects against diet induced
insulin resistance [90].
Overexpression of adiponectin protects against both the acute and the chronic effects of high
fat diet HFD-induced lipotoxic effects of lipid accumulation, and, in mice, it increases the metabolic
flexibility of the adipose tissue [
91
]. Adiponectin, through its receptor signaling, is also involved
in the metabolic action of adipocyte/adipose tissue. Adiponectin levels in bloodstream play a key
role in reflecting its metabolic action on adipocytes and adipose tissue. There is evidence showing
that low adiponectin receptors are expressed in visceral adipocytes in adipose tissues of humans and
rats, and decreased adiponectin receptor expression is detected in adipose tissues of insulin-resistant
animals. These results indicate an impairment of adiponectin action in the insulin-resistant animals by
low adiponectin receptor activity [
92
]. Additionally, it was reported that activation of PPAR
α
with its
agoinists in obese diabetic KKAy mice can stimulates the potency of adiponectin through upregulating
the adiponectin and its receptor expressions in adipocyte/adipose tissue, which ultimately rescued
these animals from obesity induced insulin resistance [57].
4.4. Liver
The liver plays a major role in blood glucose homeostasis by maintaining a balance between
the uptake and storage of glucose via glycogenesis, and the release of glucose via glycogenolysis and
gluconeogenesis [
93
]. Injection of recombinant adiponectin in both wild type and diabetic mouse
models lowers serum glucose to near normal levels [
45
]. Intraperitoneal injection of HMW and
LMW adiponectin lowers plasma glucose in healthy mice as well as mice with Type 1 Diabetes (T1D)
and Type 2 Diabetes (T2D) [
9
]. In addition, high doses of adiponectin did not show hypoglycemic
episodes in mice, which implies that the glucose lowering effect of adiponectin is primarily
mediated by suppressing gluconeogensis or glycogenolysis. Short term infusion of adiponectin led
Int. J. Mol. Sci. 2017,18, 1321 9 of 17
to marked suppression of endogenous glucose production in conscious mice by suppressing glucose
6 phosphatase mRNA and phospho enol pyruvate carboxy kinase mRNA in the liver [
94
]. The insulin
sensitizing action of adiponectin can also be mediated by up regulating PPAR
α
, and its target genes
include CD36, ACO, and UCP-2 in liver [
5
]. In addition, adiponectin supplementation has suppressed
glucose output in primary rat hepatocytes [
45
]. Thiazolidinedione drugs restore the glycemic status
by increasing the circulating levels of adiponectin in type 2 diabetic patients, adiponectin transgenic
models, and knockout mouse models [91,95,96].
Adiponectin is not “insulin mimetic”. Adiponectin is effective in alleviating both alcohol and
obesity associated liver abnormality, including hepatomegaly, steatosis, and the elevated levels
of serum alanine aminotransferase. These therapeutic effects resulted partly from the ability of
adiponectin to increase carnitine palmitoyltransferase I activity and enhance hepatic fatty acid
oxidation, while decreased the activities of two key enzymes involved in fatty acid synthesis, including
acetyl-CoA carboxylase and fatty acid synthase [
97
]. HMW and LMW adiponectin supplementation
induces AMPK activation in in rat McArdle 7777 hepatoma cells [
98
]. Activated AMPK downregulates
lipogenic genes and activates fat oxidative pathways [
99
]. Using noninvasive methods, it has been
demonstrated that adiponectin levels in circulation were inversely correlated with liver fat content [
100
].
As patients with nonalcoholic steatohepatitis also have dysregulation of postprandial glucose and
lipid homeostasis, we hypothesized that serum adiponectin levels in patients with nonalcoholic
steatohepatitis would respond suboptimally to a mixed meal compared with the response in obese
controls [
101
]. However, other studies have reported a lower staining of AdipoR2 in biopsies of patients
with non-alcoholic steatohepatitis when compared to simple steatosis, which might be explained by
post translational deregulation [102].
Mitochondrial dysfunction represents a central mechanism linking obesity with associated
metabolic complications. In nonalcoholic steatoheaptitis, the liver mitochondria showed ultrastructural
lesions and low activity of the respiratory chain complexes. This attenuation in the activity of
the respiratory chain would results from an accumulation of reactive oxygen species (ROS) that
oxidize stored fats to form lipid peroxidation products, which ultimately leads to steatohepatitis,
necrosis, inflammation, and fibrosis [
103
]. Studies reported that mice lacking adiponectin resulted
in high fat accumulation even under normal chow consumption [
104
]. In fact, adiponectin itself has
been described as a PPAR-
γ
target gene [
105
]. This hepatic steatotic condition may be responsible
for the malfunction of mitochondria. Supplementation of adiponectin rescues the mitochondrial
functions, thereby lowering the mitochondrial lipid peroxidation products [
82
], which might represent
a common mechanism underlying the multiple beneficial activities of this hormone in various
obesity-related pathologies.
5. Can We Increase Circulatory Adiponectin Status?
Research demonstrated that pharmacological elevation of circulating adiponectin will become
the promising therapeutic strategy to ameliorate obesity related diseases. The PPAR
γ
agonists
thiazolidinediones (TZDs), such as rosiglitazone and pioglitazone, have been shown to elevate
the circulating levels of adiponectin in both animals and humans [
5
,
106
]. Troglitazone, is an oral
antihyperglycemic agent, which increases adiponectin production in isolated human adipocytes [
107
].
Studies have shown that the insulin-sensitizing effects of TZDs are mediated at least in part by
the acceleration of adiponectin production in adiponectin-null mice [
108
]. The phytochemicals
astragaloside II and isoastragaloside I, isolated from the medicinal herb radix, were shown to alleviate
hyperglycemia, and improve glucose tolerance and insulin sensitivity, presumably by augmenting
adiponectin secretion [
109
]. Zataria multiflora improved insulin sensitivity and reduced glucose levels
through adiponectin secretion, in fructose fed insulin resistant rats. This increase in adiponectin levels
might be due to increase in PPAR
γ
protein levels [
110
]. Recently, it was demonstrated that garlic extract,
after 12 weeks, improves adiponectin levels in patients with metabolic syndrome [
111
]. The treatment
of diabetic mice with cobalt, a heme oxygenase inducer, reduces visceral and subcutaneous obesity,
Int. J. Mol. Sci. 2017,18, 1321 10 of 17
and increases insulin sensitivity through the upregulation of adiponectin [
112
]. Studies have suggested
that supplementation of L-cysteine increases the adiponectin secretion in adipoctyes [
113
] and
manganese supplementation increases adiponectin secretion in adipocytes and in Zucker type 2
diabetic rats too [
114
]. In addition, studies indicate that adiponectin levels increase in healthy,
nondiabetic volunteers treated with PPAR
α
/
γ
agonists [
115
]. In addition, it has been reported that
temocapril decreases plasma glucose level and this action may be partly due to increase in adiponectin
levels in patients with essential hypertension [
116
]. Thus, drugs targeting adiponectin synthesis would
be helpful in treating obesity, diabetes, and cardiovascular disease.
In addition to pharmacotherapy, an acute bout of aerobic exercise results in a significant increase
in plasma adiponectin levels in abdominally obese individuals. Adiponectin levels are inversely
proportional to both total and abdominal fat mass [
117
,
118
]. Evidence suggests that an acute bout of
vigorous aerobic exercise may result in a significant increase in plasma adiponectin levels in trained
athletes [
119
,
120
]. In contrast, immediately following the cessation of exercise, adiponectin levels
are reported to be unchanged [
119
] or even reduced [
120
] in trained individuals. Kriketos et al. [
121
]
report that one week of aerobic training results in increased adiponectin levels in abdominally obese
men. In addition, Numao and colleagues report that although circulating levels of both medium- and
low-molecular weight adiponectin decreased immediately following a bout of vigorous aerobic exercise,
the proportion of high molecular weight adiponectin was significantly increased [
122
]. Thus, vigorous
whole-body exercise leads to an acute increase in plasma adiponectin levels in subjects.
Although the multiple benefits of thiazolidinediones are well established, there are a number
of safety concerns and limitations that must be taken into consideration when selecting TZDs for
the management of metabolic diseases. Troglitazone, the first agent of this class to be approved,
was effective in controlling glycemia but was removed from the market because of serious liver toxicity.
Heart failure is one of the most common side effects of TZDs [
123
] and as a result the FDA added
a related “black box” warning. Pioglitazone treatment has been associated with increased risk of
the development of edema in type 2 diabetic patients [
124
]. Another important safety issue of TZDs
is their effect on bone metabolism [
125
]. Indeed, TZDs have been shown to decrease bone density
and thus increase fracture risk [
126
]. Studies have shown that pioglitazone usage increased the risk of
bladder cancer [
127
]. Moreover, treatment with TZDs is associated with an increase in body weight
even though insulin resistance is reduced [
128
]. Glitazones have been associated with macular edema,
a serious form of diabetic retinopathy that leads to vision loss [129].
6. Conclusions
Adiponectin is a fat-derived hormone that appears to play a crucial role in protecting against
insulin resistance/diabetes and atherosclerosis. Decreased adiponectin levels are thought to play
a central role in the development of type 2 diabetes, obesity and cardiovascular disease in humans.
Research in humans and rodent models has consistently demonstrated the role of adiponectin as
an important physiological regulator of insulin sensitivity, glucose, and lipid metabolism as well as
cardiovascular homeostasis. Current studies conducted in human and animal models for obesity,
diabetes, and atherosclerosis have reported on the potential role of adiponectin and adiponectin
receptors for these metabolic diseases. As the production of endogenous adiponectin is impaired as
an effect of obesity and related pathologies, a practical therapeutic approach is to use pharmacological
or dietary interventions to restore the capacity of adipose tissue in secreting adiponectin. In the future,
this unique strategy can probably serve as a potential novel and innovative therapeutic approach for
treatment of the metabolic diseases.
Acknowledgments:
The authors are supported by grants from the National Institutes of Health RO1 AT007442.
This work was also supported by the Malcolm Feist Endowed Chair in Diabetes and by a fellowship from
the Malcolm Feist Cardiovascular Research Endowment, LSU Health Sciences Center, Shreveport.
Conflicts of Interest: The authors declare no conflict of interest.
Int. J. Mol. Sci. 2017,18, 1321 11 of 17
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... The lack of a significant interaction effect between s-ADP and PTSD among rape exposed participants could be explained by cohort characteristics (e.g metabolic) or other unexamined factors. Adiponectin has been shown to be inversely associated with obesity phenotypes as well as several obesity-related diseases (Achari and Jain, 2017;Nigro et al., 2014). In this study, at baseline, adiposity indices (BMI, WC and WHR) were found to be significantly inversely related to adiponectin levels among the RE, but not the RUE, group. ...
... On the other hand; adiponectin is an anti-inflammatory, anti-diabetic and anti-atherogenic protein secreted from adipocytes. It was reported to be quantitatively higher in the state of periodontal health, whereas their levels were significantly reduced in the case of inflammation (Achari & Jain, 2017). ...
... Adiponectin is a hormone secreted by adipose tissues and it appears to have a variety of functions (Achari and Jain, 2017;Di Zazzo et al., 2019;Lee and Shao, 2014). It mediates its effects through interactions with two receptors: AdipoR1 and AdipoR2. ...
... APN increases glucose and fatty acid metabolism by activating adenosine monophosphateactivated protein kinase (AMPK), leading to an increase in insulin sensitivity (Atawia et al., 2019). The reduction of APN plays a key role in obesity (Achari and Jain, 2017). According to a cross-sectional study in Taiwan (Hwang et al., 2011), there is a negative association between APN levels and hearing thresholds, especially at high frequencies. ...
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With the increase in life expectancy in the global population, aging societies have emerged in many countries, including China. As a common sensory defect in the elderly population, the prevalence of age-related hearing loss and its influence on society are increasing yearly. Metabolic syndrome is currently one of the main health problems in the world. Many studies have demonstrated that metabolic syndrome and its components are correlated with a variety of age-related diseases of the peripheral sensory system, including age-related hearing loss. Both age-related hearing loss and metabolic syndrome are high-prevalence chronic diseases, and many people suffer from both at the same time. In recent years, more and more studies have found that mitochondrial dysfunction occurs in both metabolic syndrome and age-related hearing loss. Therefore, to better understand the impact of metabolic syndrome on age-related hearing loss from the perspective of mitochondrial dysfunction, we reviewed the literature related to the relationship between age-related hearing loss and metabolic syndrome and their components to discern the possible role of mitochondria in both conditions.
... Furthermore, our result suggests that the proportions of obesity, smoking, and older people with kidney stones are growing gradually, and these types of participants are likely to have a lower content of adiponectin (Kadowaki et al. 2006;Achari and Jain 2017;Higham et al. 2018;Chełchowska et al. 2020;Komiyama et al. 2018). The decline of adiponectin, an adipocytokine with the ability of anti-inflammation and anti-lipid peroxidation, contributes to a relatively high risk of stone diseases. ...
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Background Polycystic ovary syndrome (PCOS) is a common endocrine disorder characterized by hyperandrogenism and follicular arrest. Electroacupuncture (EA) has been shown to be effective at improving hyperandrogenism and follicular arrest in PCOS; however, its mechanism of action remains to be deciphered. Objective In this study, we investigated whether EA improved follicular development in an obese rat model of PCOS and regulated the expression of adiponectin, AMP-activated protein kinase (AMPK) and acetyl-CoA carboxylase (ACC). Methods EA was administered at CV3, CV4 and ST40. Changes in body weight, paraovarian fat, estrus cycle, ovarian morphology, levels of related hormones, and glucose and lipid metabolism were evaluated. In addition, protein and mRNA expression of adiponectin, AMPK and ACC was measured. Results The body weight and paraovarian fat of rats in the EA group were reduced, while estrus cyclicity and ovarian morphology improved. Levels of free fatty acids, triglycerides, total cholesterol and low-density lipoprotein cholesterol were significantly reduced in the EA group, as well as blood glucose levels. Furthermore, levels of testosterone and luteinizing hormone were reduced in the EA group, while estradiol levels were increased. Protein and mRNA expression of adiponectin, AMPKα1 and liver kinase B1 (LKB1) was found to be increased in the EA group, while protein and mRNA expression of ACC were significantly reduced. Conclusion: Our findings suggest that EA improved follicular development and metabolism and regulated expression levels of adiponectin, AMPKα1, LKB1 and ACC in our obese rat model of PCOS.
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Chapter
The cellular composition of adipose tissue is highly variable and is shown to be dependent on anatomical site, species, and a host of environmental factors. Recent studies also demonstrate that adipocytes perform essential physiological functions in addition to triglyceride storage. An eventual description of the full physiological role of adipose tissue in man and animals is equally dependent on both biochemical and anatomical research. This chapter highlights some new perspectives in adipose tissue anatomy. The relationship between adipose tissue cellularity and human obesity has been extensively reviewed, which shows that early-onset obesity is associated with increased cell numbers, whereas only cell hypertrophy is associated with adult-onset obesity. Typically the stromal-vascular cells consist of macrophages, fibroblasts, pericytes, and various types of blood cells. Electron microscopic studies on cells that develop into adipocytes describe a wide variety of cellular ultrastructures. In some species, such as dog, man, mouse, and rat, mast cells are a normal component of adipose tissue that are deployed along the blood vessels. The full range of cells common to mature red bone marrow have been located in immature and mature white adipose tissue. The capillaries and primitive blood cells are formed in a process of blood island formation.
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Hypoadiponectinemia is closely associated with endothelial dysfunction and insulin resistance and microvasculature plays a critical role in the regulation of insulin action in muscle. Here we tested whether adiponectin replenishment could improve metabolic insulin sensitivity in male rats fed a high-fat diet (HFD) via modulation of microvascular insulin responses. Male Sprague-Dawley rats were fed either a HFD or low-fat diet (LFD) for 4 weeks. Small resistance artery myograph changes in tension, muscle microvascular recruitment and metabolic response to insulin were determined. Compared with rats fed a LFD, HFD feeding abolished globular adiponectin (gAd)'s and insulin's vasodilatory actions on pre-constricted distal saphenous arteries. Pretreatment with gAd improved insulin responses in arterioles isolated from HFD rats, which was blocked by AMP-activated protein kinase (AMPK) inhibition. Similarly, HFD abolished microvascular responses to either gAd or insulin and decreased insulin-stimulated glucose disposal by ∼60%. However, supplementing gAd fully rescued insulin's microvascular action and significantly improved the metabolic responses to insulin in HFD male rats and these actions were abolished by inhibition of either AMPK or nitric oxide production. We conclude that HFD induces vascular adiponectin and insulin resistance but gAd administration can restore vascular insulin responses and improve insulin's metabolic action via an AMPK and nitric oxide-dependent mechanism in male rats. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.
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Aims/hypothesis Thiazolidinediones (TZDs) are associated with an increased risk of fracture but the mechanism is unclear. We sought to determine the effect of TZDs on bone mineral density (BMD) and bone turnover markers. Methods PubMed, EMBASE and Cochrane CENTRAL databases were searched from inception until January 2015 for randomised controlled trials comparing TZDs with metformin, sulfonylureas or placebo, and those reporting changes in BMD and/or bone turnover markers. The primary outcome was percentage change in BMD from baseline and results were pooled with random effects meta-analyses. Results In all, 18 trials were included in the primary analyses and another two were included in the sensitivity analyses (n = 3,743, 50% women, mean age 56 years, median trial duration 48 weeks). TZDs decreased BMD at the lumbar spine (difference −1.1% [95% CI −1.6, −0.7]; p
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Adipose differentiation is accompanied by changes in cellular morphology, a dramatic accumulation of intracellular lipid and activation of a specific program of gene expression. Using an mRNA differential display technique, we have isolated a novel adipose cDNA, termed adipoQ. The adipoQ cDNA encodes a polypeptide of 247 amino acids with a secretory signal sequence at the amino terminus, a collagenous region (Gly-X-Y repeats), and a globular domain. The globular domain of adipoQ shares significant homology with subunits of complement factor C1q, collagen α1(X), and the brain-specific factor cerebellin. The expression of adipoQ is highly specific to adipose tissue in both mouse and rat. Expression of adipoQ is observed exclusively in mature fat cells as the stromal-vascular fraction of fat tissue does not contain adipoQ mRNA. In cultured 3T3-F442A and 3T3-L1 preadipocytes, hormone-induced differentiation dramatically increases the level of expression for adipoQ. Furthermore, the expression of adipoQ mRNA is significantly reduced in the adipose tissues from obese mice and humans. Whereas the biological function of this polypeptide is presently unknown, the tissue-specific expression of a putative secreted protein suggests that this factor may function as a novel signaling molecule for adipose tissue.