Bile Acid Sequestrants for Lipid and Glucose Control

Abstract

Bile acids are generated in the liver and are traditionally recognized for their regulatory role in multiple metabolic processes including bile acid homeostasis, nutrient absorption, and cholesterol homeostasis. Recently, bile acids emerged as signaling molecules that, as ligands for the bile acid receptors farnesoid X receptor (FXR) and TGR5, activate and integrate multiple complex signaling pathways involved in lipid and glucose metabolism. Bile acid sequestrants are pharmacologic molecules that bind to bile acids in the intestine resulting in the interruption of bile acid homeostasis and, consequently, reduction in low-density lipoprotein cholesterol levels in hypercholesterolemia. Bile acid sequestrants also reduce glucose levels and improve glycemic control in persons with type 2 diabetes mellitus (T2DM). This article examines the mechanisms by which bile acid-mediated activation of FXR and TGR5 signaling pathways regulate lipid and glucose metabolism and the potential implications for bile acid sequestrant-mediated regulation of lipid and glucose levels in T2DM.

Full text

Bile Acid Sequestrants for Lipid and Glucose Control
Bart Staels &Yehuda Handelsman &Vivian Fonseca
Published online: 22 January 2010
#The Author(s) 2010. This article is published with open access at Springerlink.com
Abstract Bile acids are generated in the liver and are
traditionally recognized for their regulatory role in multiple
metabolic processes including bile acid homeostasis,
nutrient absorption, and cholesterol homeostasis. Recently,
bile acids emerged as signaling molecules that, as ligands
for the bile acid receptors farnesoid X receptor (FXR) and
TGR5, activate and integrate multiple complex signaling
pathways involved in lipid and glucose metabolism. Bile
acid sequestrants are pharmacologic molecules that bind to
bile acids in the intestine resulting in the interruption of bile
acid homeostasis and, consequently, reduction in low-
density lipoprotein cholesterol levels in hypercholesterol-
emia. Bile acid sequestrants also reduce glucose levels and
improve glycemic control in persons with type 2 diabetes
mellitus (T2DM). This article examines the mechanisms by
which bile acid–mediated activation of FXR and TGR5
signaling pathways regulate lipid and glucose metabolism
and the potential implications for bile acid sequestrant–
mediated regulation of lipid and glucose levels in T2DM.
Keywords Bile acid receptor .Bile acid sequestrant .
Cholesterol .Glucose control .Lipid control .
Type 2 diabetes mellitus
Introduction
Bile acid synthesis is the major pathway of cholesterol
catabolism in the liver; approximately 500 mg of choles-
terol is converted into bile acids daily in the adult human
liver [1].
The first and rate-limiting step in the primary (classic
or neutral) pathway of conversion of cholesterol to bile
acids is catalyzed by cytochrome P450 enzyme choles-
terol 7 α-hydroxylase (CYP7A1) [1]. Traditionally
recognized for their role in regulating lipid absorption,
bile acids are now known to be involved in the regulation
of multiple metabolic processes including lipid and
glucose metabolism and energy homeostasis through the
activation of multiple signaling pathways. This article
examines the effects of interrupting bile acid enterohepatic
recirculation using bile acid sequestrants not only on lipid
but also on glycemic control and the potential mechanism
(s) of action involved in the bile acid–mediated regulation
of these processes.
Clinical Effects of Bile Acid Sequestrants
Bile acid sequestrants (cholestyramine, colestipol, colesti-
mide, and colesevelam) are positively charged nondiges-
tible resins that bind to bile acids in the intestine to form an
insoluble complex that is excreted in the feces. This process
reduces bile acid levels in the liver, promoting the increased
synthesis of bile acids from cholesterol and reducing
hepatic cholesterol levels [2].
The clinical lipid-lowering utility of bile acid seques-
trants was demonstrated more than two decades ago during
the Lipid Research Clinics Coronary Primary Prevention
Trials, in which long-term administration of cholestyramine
in men with hypercholesterolemia resulted in overall
reductions in total cholesterol and low-density lipoprotein
B. Staels (*)
Institut Pasteur de Lille,
1 rue Calmette BP245,
59019 Lille cedex, France
e-mail: Bart.Staels@pasteur-lille.fr
Y. Handelsman
Metabolic Institute of America,
18372 Clark Street, #212,
Tarzana, CA 91356, USA
e-mail: yhandelsman@pacbell.net
V. Fonseca
Tulane University Health Sciences Center,
1430 Tulane Avenue, SL53,
New Orleans, LA 70118, USA
e-mail: vfonseca@tulane.edu
Curr Diab Rep (2010) 10:70–77
DOI 10.1007/s11892-009-0087-5

cholesterol (LDLC) of 13.4% and 20.3%, respectively,
compared with placebo (4.9% and 7.7%, respectively).
These reductions were accompanied by a 19% reduction in
the incidence of coronary heart disease [3,4]. Similarly,
treatment with colestipol (2 or 8 g twice daily) reduced
LDLC levels in patients with mild hypercholesterolemia by
12% or 24%, respectively (P≤0.05) [5]. Monotherapy with
colesevelam resulted in mean reductions in LDLC of 15%
to 19% in adults with hypercholesterolemia [6,7]. Further-
more, the addition of colesevelam to therapy with atorva-
statin (10 mg) or simvastatin (10 mg) reduced LDLC levels
by 48% or 42%, respectively, an increase over colesevelam
(12%) or atorvastatin (38%) alone or colesevelam (16%) or
simvastatin (26%) alone, respectively [8,9].
Bile acid sequestrants have also been shown to improve
glycemic control in patients with type 2 diabetes mellitus
(T2DM). In initial studies in patients with T2DM and
dyslipidemia, cholestyramine therapy decreased LDLC levels
by 28% and resulted in improved glycemic control, as defined
by a 13% reduction in mean plasma glucose levels, a median
reduction of 0.22 g/d in urinary glucose excretion, and a
tendency toward a lower concentration of glycosylated
hemoglobin A
1c
(HbA
1c
)[10]. In three 26-week, double-
blind, placebo-controlled studies, the addition of colesevelam
to the antidiabetes regimen of patients with T2DM who were
inadequately controlled on their current metformin, sulfo-
nylurea, or insulin therapies alone or in combination with
other oral antidiabetes agents resulted in additional mean
reductions in HbA
1c
of 0.54% (metformin and sulfonylurea)
and 0.50% (insulin). These patients also experienced mean
reductions in their LDLC levels of 15.9%, 16.7%, and
12.8% (P<0.001 for all) for those receiving colesevelam
added to ongoing metformin, sulfonylurea, and insulin
therapy, respectively [11–13]. Colesevelam is the only bile
acid sequestrant approved for glycemic control in patients
with T2DM who are uncontrolled on their current treatment
regimen.
Bile Acid Receptors
In addition to facilitating the dietary absorption of lipids, bile
acids act as ligands for the nuclear receptor farnesoid X
receptor (FXR) and the G-protein-coupled receptor (GPCR)
TGR5 in the liver and intestine. However, in addition to their
role as receptor-mediated signaling molecules, certain bile
acids also exert non–receptor-mediated effects on cellular
responses such as improvement of the endoplasmic reticulum
(ER) stress response. Obesity-induced ER stress can lead
to the development of insulin resistance and T2DM.
However, administration of the bile acid taurine-
conjugated ursodeoxycholic acid (TUDCA) alleviated ER
stress in cells and whole animals [14]. In addition,
administration of TUDCA to obese and diabetic mice
resulted in lowered glucose levels, improved insulin
sensitivity, and resolution of fatty liver disease [14].
Evaluation of the mechanisms regulating these non–
receptor-mediated effects of bile acids and their contribution
to the lipid- and glucose-lowering effects of bile acid
sequestrants is beyond the scope of this article.
FXR and Bile Acid Synthesis
FXR is a member of the nuclear receptor superfamily of
ligand-activated transcription factors. FXR is expressed at
high levels in the liver and intestine and is most potently
activated by the primary bile acid chenodeoxycholic acid
[15]. In the liver, as the bile acid pool increases in size, bile
acid activation of FXR upregulates expression of the gene
encoding the inhibitory nuclear receptor small heterodimer
partner (SHP) [15]. SHP represses activation of several
transcription factors including liver X receptor, liver
receptor homologue-1 (LRH-1), and hepatocyte nuclear
factor-4α(HNF-4α), subsequently suppressing, in humans,
the LRH-1–mediated activation of CYP7A1, thereby
inhibiting the first step in cholesterol catabolism (Fig. 1).
Bile acid–mediated repression of HNF-4αalso inhibits
transcription of CYP7A1 [16].
A second pathway of bile acid–mediated repression of
its own synthesis involves FXR-mediated induction of
fibroblast growth factor-19 (FGF-19; [FGF-15 in mice]) in
the intestine. In the intestine, transintestinal transport of bile
acids after a meal induces activation of intestinal FXR,
resulting in expression of FGF-19 [17]. Binding of FGF-19
to surface hepatocyte fibroblast growth factor receptor 4
results in a c-Jun N-terminal kinase–mediated repression of
CYP7A1 transcription, a potent SHP-independent alterna-
tive pathway of CYP7A1 repression (Fig. 1)[17].
TGR5 and Bile Acid Synthesis
Bile acids can also bind to and activate TGR5 (also known as
GPBAR1, M-BAR, and BG37), a member of the rhodopsin-
like superfamily of transmembrane GPCRs [18,19].
Expression of TGR5 is detected in multiple tissues, with
the highest level of expression detected in the gallbladder,
ileum, and colon, and with lower levels of expression in
brown adipose tissue, liver, and intestine; TGR5 expression
has not been detected in murine or human hepatocytes [19].
TGR5 is activated by multiple bile acids, with lithocholic
acid being the most potent natural agonist (EC
50
of 530 nM)
[18]. Bile acid–mediated activation of TGR5 results in
internalization of the receptor, activation of extracellular-
regulated kinase, the mitogen-activated protein-kinase
pathway, and ultimately stimulation of cyclic adenosine
monophosphate (cAMP) synthesis. This, in turn, results in
Curr Diab Rep (2010) 10:70–77 71

the activation of protein kinase A, phosphorylation of the
cAMP-response element-binding protein, and transactiva-
tion of target gene expression [20]. Activation of TGR5
may play a role in energy and glucose homeostasis.
Bile acid sequestrants cause alterations in the bile acid
pool, the size of which is tightly regulated in the liver and the
intestine by a negative feedback mechanism that prevents
cytotoxic accumulation of bile acids [15]. Early studies into
the effects of bile acid sequestrants showed that they
interrupted the enterohepatic circulation of bile acids and
increased fecal excretion of bile acids, resulting in decreased
hepatic bile acid levels. As bile acid levels decreased, the
FXR-mediated repression of CYP7A1 was reduced, upregu-
lating expression of CYP7A1 and resulting in an increase in
bile acid synthesis and reduced hepatic cholesterol levels.
Reduced hepatic cholesterol levels promote activation of the
sterol regulatory element-binding protein (SREBP)-2, ulti-
mately resulting in increased expression of LDL receptors
and increased clearance of LDLC from the blood. In parallel,
increased secretion of very low-density lipoprotein (VLDL)
particles and a transient rise in triglyceride levels are often
observed [2,21].
Bile acid sequestrants also increase high-density lipo-
protein cholesterol (HDLC) levels. In murine studies, as
bile acid levels decrease, the FXR-mediated activation of
scavenger receptor BI (SR-BI) expression is also decreased.
This receptor is responsible for the hepatic uptake of HDLC. It
can be hypothesized that the diminished expression of SR-BI
and resultant decrease in uptake of HDLC may contribute to
the increase in HDLC levels seen after administration of bile
acid sequestrants in humans [15,22]. In addition, bile acid–
induced downregulation of apolipoprotein (apo) A-I expres-
sion may be counteracted by bile acid sequestrants [23],
resulting in increased apo A-I plasma levels. These effects
may be mediated in part via the bile acid receptors FXR and
TGR5, which regulate genes involved in lipid and glucose
metabolism.
FXR and Lipid Metabolism
Studies have shown that activation of FXR alters the
transcription of several genes involved in triglyceride
synthesis and lipid metabolism. Mice deficient in FXR
(Fxr
-/-
) have increased plasma non-HDLC and triglyceride
levels, increased synthesis of apo B–containing lipopro-
teins, mainly VLDL, and increased absorption of intestinal
cholesterol. Fxr
-/-
mice also have increased levels of plasma
HDLC coincident with a reduced rate of plasma HDLC
ester clearance [22]. Accordingly, mice deficient in the
FXR target gene SHP (Shp
-/-
) have increased levels of the
transcription factor SREBP-1c, which controls the expres-
sion of genes involved in lipogenesis (Table 1)[24].
Fig. 1 FXR- and TGR5-mediated regulation of bile acid synthesis
and lipid and glucose metabolism in the liver and intestine. Bile acid
synthesis and lipid and glucose metabolism are regulated in the liver
and intestine via pathways involving the bile acid receptors FXR and
TGR5. In the liver, bile acids activate FXR resulting in upregulation
of SHP, an inhibitor of bile acid synthesis, gluconeogenesis, and fatty
acid synthesis. In the intestine, bile acid activation of FXR upregulates
FGF-15/19 and ultimately inhibits bile acid synthesis. CYP7A1—
cytochrome P450 enzyme cholesterol 7 α-hydroxylase; FGF-15/19—
fibroblast growth factor 15/19; FGFR4—fibroblast growth factor
receptor 4; FXR—farnesoid X receptor; GLP-1—glucagon-like
peptide-1; GR—glucocorticoid receptor; HNF-4—hepatocyte nuclear
factor-4; JNK—c-jun N-terminal kinase; LRH-1—liver receptor
homologue-1; PEPCK—phosphoenolpyruvate carboxykinase; SHP—
small heterodimer partner; SREBP-1c—sterol regulatory element-
binding protein-1c; TGs—triglycerides; VLDL—very low-density
lipoprotein. (Adapted from Reasner [50] and Thomas et al. [51].)
72 Curr Diab Rep (2010) 10:70–77

In addition, Fxr
-/-
mice have approximately twofold
higher levels of circulating free fatty acids (FFAs), com-
pared with wild-type mice, and accumulate fat in the liver
[25]. Elevated levels of FFAs and excessive accumulation of
fat in the liver can lead to the development of nonalcoholic
fatty liver disease and hepatic steatosis. Interestingly, studies
have shown that activation of FGF-19 can increase fatty
acid oxidation and inhibit fatty acid synthesis via activation
of SHP, inhibition of SREBP-1c, and changes in signal
transducer and activator of transcription 3 and peroxisome
proliferator-activated receptor (PPAR) coactivator-1βsig-
naling pathways in murine models of diet-induced diabetes
and obesity [26,27]. These studies suggest a potential role
for FGF-19–mediated signaling effects in fatty liver disease
and in the resolution of some effects associated with the
metabolic syndrome. The mechanism(s), if any, that
integrates the effects of FGF-19 on obesity and diabetes
and the lipid- and glucose-lowering effects of bile acid
sequestrants remain to be investigated.
In murine models predisposed to develop hypertri-
glyceridemia (KK/A
y
and ob/ob), administration of the
primary bile acid cholic acid (CA) or the synthetic FXR
agonist (GW4064) resulted in decreased plasma FFAs,
decreased triglycerides (VLDL triglycerides), decreased
cholesterol (HDLC), and increased LDLC levels [24]. The
decreased cholesterol levels observed following FXR acti-
vation are due largely to a decrease in circulating HDLC
levels. Results from several studies in murine models of
hypercholesterolemia (ApoE
-/-
,ob/ob, and db/db) confirm
that FXR activation resulted in a reduction in HDLC
plasma levels [24,28•].
Administration of bile acids also repressed expression of
the transcription factor SREBP-1c and its lipogenic target
genes in murine hepatocytes and liver in an SHP-dependent
manner (Table 1)[24,29]. Other mechanisms implicated in
FXR-mediated hypotriglyceridemia include FXR-activated
expression of PPAR-αand its target gene pyruvate
dehydrogenase kinase 4, which promote fatty acid oxida-
tion, increased expression of apo C-II (an activator of
lipoprotein lipase activity), and decreased expression of apo
C-III and angiopoietin-like protein 3, which are both
lipoprotein lipase inhibitors (Table 1)[30–32]. The relative
contribution of triglyceride production versus triglyceride
clearance is unknown.
Hypothetical Glucose-lowering Mechanisms
FXR and Hepatic Glucose Metabolism
The liver plays a central role in the control of blood glucose
homeostasis by maintaining a balance between glucose
production and utilization (Fig. 1). Evidence for a role of
bile acids in glucose metabolism was revealed by studies
showing that bile acid composition and pool size are altered
in animal and human models of diabetes [33,34]. A link
between FXR and glucose metabolism was provided by the
observation that hepatic expression of the gene encoding
FXR was decreased following quantitative analysis of hepatic
FXR mRNA expression in a rat model of type 1 diabetes.
Accordingly, this was accompanied by increased expression
of CYP7A1, which likely contributes to the enlarged bile acid
pool that is also characteristic of such diabetic animals [35].
As seen in the rat model of type 1 diabetes, hepatic expres-
sion of FXR also decreased with age in a rat model of T2DM
[35]. Hepatocytes from nondiabetic rats were incubated with
Table 1 Positive and negative FXR- and TGR5-mediated modulation of genes involved in lipid, glucose, and energy metabolism
Study Function Gene Regulation
FXR
Goodwin et al. [16] Bile acid metabolism CYP7A1 Repressed (through SHP induction)
Lambert et al. [22],
Watanabe et al. [24]
Lipogenesis SREBP-1c Repressed (through SHP induction)
Lambert et al. [22] TG metabolism Apo B Repressed
Kast et al. [31] TG metabolism Apo C-II Induced
Kast et al. [31] TG metabolism Apo C-III Repressed
Cariou and Staels [52] TG metabolism VLDLR Induced
Claudel et al. [23] HDL metabolism Apo A-1 Repressed
Ma et al. [25] Glucose metabolism PEPCK Induced and repressed
Ma et al. [25] Glucose metabolism G6Pase Repressed
TGR5
Watanabe et al. [44] Energy metabolism Iodothyronine deiodinase type 2 (D2) Induced
Apo—apolipoprotein; CYP7A1—cytochrome P450 enzyme cholesterol 7 α-hydroxylase; FXR—farnesoid X receptor; G6Pase—glucose-6
phosphatase; HDL—high-density lipoprotein; PEPCK—phosphoenolpyruvate carboxykinase; SHP—small heterodimer partner; SREBP-1c—sterol
regulatory element-binding protein-1c; TG—triglyceride; VLDLR—very low-density lipoprotein receptor. (Adapted from Cariou and Staels [52].)
Curr Diab Rep (2010) 10:70–77 73

glucose and insulin to assess the effects of glucose and
insulin on hepatic FXR expression. Incubation of rat hepato-
cytes with insulin repressed expression of FXR, an effect that
is reduced by glucose. These results suggested that diabetes
impairs the normal regulation of FXR expression [35].
A potential role for FXR in the regulation of glucose
metabolism was provided by the observation that pharmaco-
logic treatment with CA for 5 days resulted in decreased
expression of phosphoenolpyruvate carboxykinase (PEPCK),
glucose-6-phosphatase (G6Pase), and fructose 1,6-bisphos-
phatase, enzymes critically involved in the regulation of
hepatic gluconeogenesis (Table 1), in nondiabetic wild-type
but not Fxr
-/-
mice [25]. In contrast to the results obtained
with CA, in vivo treatment of nondiabetic wild-type mice
with the synthetic FXR agonist GW4064 induced expres-
sion of PEPCK, whereas activation of FXR with GW4064
in the diabetic db/db mouse model reduced expression of
PEPCK and G6Pase. Together, these results suggest that bile
acids may regulate hepatic glucose utilization and produc-
tion [15,36].
Fxr
-/-
mice also have decreased levels of glycogen in the
liver and develop transient hypoglycemia upon fasting [37].
In keeping with a potential role for FXR in regulation of the
kinetics of glucose metabolism, analysis of Fxr
-/-
mice
subject to an overnight fast followed by re-feeding with a
high-carbohydrate diet resulted in increased L-pyruvate
kinase and lipogenic gene expression, potentially revealing
an additional role for FXR in regulating hepatic carbohy-
drate metabolism [38]. Together, these data provide
evidence of a critical role for bile acids and FXR in the
dynamic regulation of glucose metabolism [15]. However,
given the contradictory evidence (different animal models
and tested ligands), a precise mechanism of FXR regulation
of glucose metabolism remains unclear.
FXR and Insulin Sensitivity
Hyperinsulinemic-euglycemic clamp studies in Fxr
-/-
mice
have led to the observation that FXR may also play a partial
role in regulating insulin sensitivity [25,39]. Fxr
-/-
mice
exhibit peripheral insulin resistance and impaired insulin
signaling in insulin-sensitive tissues such as skeletal muscle
and white adipose tissue. Moreover, activation of FXR in
db/db mice and treatment of db/db and ob/ob mice with
GW4064 resulted in significant improvement in insulin
sensitivity [36,39]. However, thus far, no studies have been
performed in genetically predisposed or diet-induced models
of diabetes.
TGR5 and Glucose Metabolism
Mice deficient in TGR5 (Tgr5
-/-
) have normal glucose
levels and do not develop overt diabetes [40]. However, the
observation that murine intestinal cells secrete the gut-
derived incretin hormone glucagon-like peptide-1 (GLP-1)
in a Tgr5-dependent manner following in vitro stimulation
with the bile acids lithocholic acid and deoxycholic acid
revealed a role for TGR5 in glucose homeostasis [41,42•].
Additional evidence implicating TGR5 in glucose homeo-
stasis was provided by the observation that treatment of mice
fed a high-fat diet with oleanolic acid (an extract of Olea
Europaea [olive leaves] and a TGR5 agonist) protected
against weight gain and resulted in reduced plasma glucose
and insulin levels compared with controls [43].
Moreover, in vivo overexpression of TGR5 in a transgenic
(TGR5-Tg) mouse model markedly improved glucose
tolerance in mice fed a high-fat diet compared with controls.
This improved glucose tolerance in TGR5-Tg mice was
associated with robust secretion of GLP-1 and increased
insulin release in response to an oral glucose load. Interest-
ingly, there was an even greater postprandial effect on GLP-1
release and insulin secretion after a test meal than in response
to the glucose challenge. The researchers hypothesized that
this was because of an increased bile acid flux triggered by
the test meal compared with the glucose challenge [42•].
These findings establish a role for TGR5 in bile acid–
mediated regulation of glucose homeostasis.
TGR5 and Energy Homeostasis
Supplementation of high-fat-fed mice with CA decreased
obesity and insulin resistance (through an increase in
energy expenditure in brown adipose tissue) and increased
the bile acid pool size [44]. These observations implicate
bile acids in the regulation of the metabolic process
through increases in energy expenditure via modulation of
thermogenesis.
Evidence of a role for TGR5 in the regulation of energy
expenditure was provided by observations that TGR5 is
expressed in brown adipose tissue and that the bile acid–
mediated increase in expression of the D2 gene that
controls energy expenditure in brown adipose tissue is
regulated primarily through the cAMP–protein kinase A
signaling pathway [44]. TGR5 and D2 are also expressed in
human skeletal muscle. Expression of D2 in skeletal muscle
increased in a dose-dependent manner in response to
incubation of skeletal muscle tissue with a TGR5 agonist
but not with the FXR agonist GW4064, providing further
confirmation for the TGR5-dependent regulation of energy
expenditure [44]. However, despite the data supporting a
role for a TGR5-cAMP–dependent pathway in regulation of
energy homeostasis, the observation that Tgr5
-/-
mice have
normal triglyceride levels and do not gain weight when fed
a regular diet suggests that more analyses are necessary to
fully uncover the role of bile acids in the regulation of
energy homeostasis [40].
74 Curr Diab Rep (2010) 10:70–77

Bile Acid Sequestrants in Lipid and Glycemic Control
Bile acid sequestrants are well known for their effects on
lipid levels, particularly for reducing LDLC. Results from
studies examining the effects of FXR on bile acid synthesis
and lipid metabolism implicate FXR-dependent signaling
pathways in the control of lipid metabolism by bile acid
sequestrants [22,24,26,27].
Less is known about the mechanisms involved in the
glucose-lowering effects of bile acid sequestrants [10–13,
45•]. It can be hypothesized, given the results of studies
examining the mechanisms of action involved in bile acid–
mediated regulation of hepatic gluconeogenesis, that bile acid
sequestrants may modulate FXR-dependent signaling path-
ways that regulate expression of PEPCK and other enzymes
involved in hepatic gluconeogenesis [25,33–35,37].
Preliminary studies suggested that administration of the
bile acid sequestrant colesevelam may act on glucose
homeostasis by reducing insulin resistance and/or improving
clearance of plasma glucose [46,47]. In a rat model that
develops insulin resistance and diet-induced obesity (DIO)
when fed a high-energy diet, treatment with 2% colesevelam
lowered plasma glucose levels following a 2-hour oral
glucose tolerance test, whereas the insulin response was
normalized (insulin peak in 15–30 min with return to baseline
in 30–60 min) compared with DIO rats or DIO rats treated
with the apical sodium codependent bile acid transport
inhibitor SC-435 [47]. These results suggest that in this rat
model of insulin resistance, colesevelam may reduce glucose
levels by reducing insulin resistance and normalizing the
first-phase insulin release [47].
In clinical studies, improved glycemic control in
patients treated with colesevelam (3.75 g/d) compared
with placebo was directly correlated with an increase in
whole-body insulin sensitivity as measured by the
Matsuda index, and not with an increase in peripheral
insulin sensitivity as measured by the hyperinsulinemic-
euglycemic clamp method [48]. In a separate study, stable
isotope infusion analyses showed that significant reduc-
tions in fasting plasma glucose and HbA
1c
levels in patients
receiving colesevelam compared with placebo were accom-
panied by an increase in plasma glucose clearance without the
increase in endogenous glucose production seen in patients
receiving placebo [46].
In addition, bile acid sequestrants may decrease glucose
levels by effecting regulation of the TGR5-dependent
secretion of the gut-derived incretin hormone GLP-1 [45•].
A study by Suzuki et al. [45•] showed that administration of
colestimide (1500 mg, twice daily) resulted in decreased
postprandial plasma glucose levels and increased secretion
of GLP-1 in patients with T2DM and dyslipidemia.
Although no comparisons were made with patients not
receiving colestimide and the time to measurement of GLP-
1 may have been too long to accurately reflect changes in
GLP-1 levels, this study does provide some evidence to
suggest that bile acid sequestrants may act in a TGR5-
dependent manner to increase secretion of GLP-1 in patients
with T2DM. Further studies are necessary to determine
whether bile acid sequestrants act in an FXR- and/or TGR5-
dependent manner to improve insulin sensitivity and insulin
secretion in patients with T2DM.
Unlike cholestyramine and colestipol, colesevelam has
a high affinity for trihydroxy and dihydroxy bile acids in
the intestine, leading to increased fecal excretion with
colesevelam [49]. This differential bile acid–binding ability
may allow colesevelam to have an alternative effect on the
bile acid pool and on bile acid–mediated regulation of
glucose metabolism. The metabolism of bile acids by the
intestinal flora is altered in T2DM, and modification of the
bile acid pool seen by bacteria by bile acids may influence
secretion of GLP-1 by L cells. As such, differential bile acid–
binding properties of distinct bile acid sequestrants may exist
and may result in pathophysiologically relevant pharmaco-
logic differences in action. However, definitive studies are
necessary to examine the effect of alteration of the bile acid
pool induced by bile acid sequestrants on activation of the
bile acid receptors FXR and TGR5 and the potential effects
on glycemic control in patients with T2DM.
Conclusions
Bile acid sequestrants cause alterations in the bile acid
pool with resultant effects on lipid and glucose metabo-
lism. Studies suggest that alterations of the enterohepatic
circulation may regulate glucose homeostasis by modu-
lating FXR- and TGR5-mediated pathways. Further
studies examining the role of FXR- and TGR5-mediated
signaling pathways on lipid and glucose metabolism in
animal and human models of T2DM and obesity are
needed to elucidate how these signals are integrated to
mediate the lipid- and glucose-lowering effects of bile
acid sequestrants.
Acknowledgments Editorial assistance was provided by Luana
Atherly, PhD, and funded by Daiichi Sankyo, Inc.
Disclosure Dr. Bart Staels has received grant/research support from
Daiichi Sankyo, Inc. Dr. Yehuda Handelsman has received grant/
research support from Daiichi Sankyo, Inc., GlaxoSmithKline, Novo
Nordisk, and Takeda Pharmaceuticals; has been a consultant for
Bristol-Myers Squibb, Daiichi Sankyo, Inc., GlaxoSmithKline, Med-
tronic, Merck, Xoma, and Tethys; and has been on the speakers’bureau
for Daiichi Sankyo, Inc., GlaxoSmithKline, Merck, Bristol-Myers
Squibb, and AstraZeneca. Dr. Vivian Fonseca has received research
support (to Tulane)/grants from GlaxoSmithKline, Novartis, Novo
Nordisk, Takeda Pharmaceuticals, AstraZeneca, Pfizer, sanofi-aventis,
Eli Lilly, Daiichi Sankyo, Inc., Novartis, the US National Institutes of
Curr Diab Rep (2010) 10:70–77 75

Health, and the American Diabetes Association. He has also received
honoraria for consulting and lectures from GlaxoSmithKline, Novartis,
Takeda Pharmaceuticals, Novo Nordisk, sanofi-aventis, Eli Lilly, and
Daiichi Sankyo, Inc.
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References
Papers of particular interest, published recently, have been
highlighted as:
•Of importance
1. Russell DW. The enzymes, regulation, and genetics of bile acid
synthesis. Annu Rev Biochem. 2003;72:137–174.
2. Insull Jr W. Clinical utility of bile acid sequestrants in the
treatment of dyslipidemia: a scientific review. South Med J.
2006;99:257–273.
3. The Lipid Research Clinics Coronary Primary Prevention Trial
results. I. Reduction in incidence of coronary heart disease [no
authors listed]. JAMA. 1984; 251:351–364.
4. The Lipid Research Clinics Coronary Primary Prevention Trial
results. II. The relationship of reduction in incidence of coronary
heart disease to cholesterol lowering [no authors listed]. JAMA.
1984; 251:365–374.
5. Insull Jr W, Davidson MH, Demke DM, et al. The effects of
colestipol tablets compared with colestipol granules on plasma
cholesterol and other lipids in moderately hypercholesterolemic
patients. Atherosclerosis. 1995;112:223–235.
6. Insull Jr W, Toth P, Mullican W, et al. Effectiveness of colesevelam
hydrochloride in decreasing LDL cholesterol in patients with
primary hypercholesterolemia: a 24-week randomized controlled
trial. Mayo Clin Proc. 2001;76:971–982.
7. Davidson MH, Dillon MA, Gordon B, et al. Colesevelam
hydrochloride (cholestagel): a new, potent bile acid sequestrant
associated with a low incidence of gastrointestinal side effects.
Arch Intern Med. 1999;159:1893–1900.
8. Hunninghake D, Insull Jr W, Toth P, et al. Coadministration of
colesevelam hydrochloride with atorvastatin lowers LDL choles-
terol additively. Atherosclerosis. 2001;158:407–416.
9. Knapp HH, Schrott H, Ma P, et al. Efficacy and safety of
combination simvastatin and colesevelam in patients with primary
hypercholesterolemia. Am J Med. 2001;110:352–360.
10. Garg A, Grundy SM. Cholestyramine therapy for dyslipidemia in
non-insulin-dependent diabetes mellitus. A short-term, double-blind,
crossover trial. Ann Intern Med. 1994;121:416–422.
11. Bays HE, Goldberg RB, Truitt KE, Jones MR. Colesevelam
hydrochloride therapy in patients with type 2 diabetes mellitus
treated with metformin: glucose and lipid effects. Arch Intern
Med. 2008;168:1975–1983.
12. Fonseca VA, Rosenstock J, Wang AC, et al. Colesevelam HCl
improves glycemic control and reduces LDL cholesterol in
patients with inadequately controlled type 2 diabetes on
sulfonylurea-based therapy. Diabetes Care. 2008;31:1479–1484.
13. Goldberg RB, Fonseca VA, Truitt KE, Jones MR. Efficacy and
safety of colesevelam in patients with type 2 diabetes mellitus and
inadequate glycemic control receiving insulin-based therapy. Arch
Intern Med. 2008;168:1531–1540.
14. Ozcan U, Yilmaz E, Ozcan L, et al. Chemical chaperones reduce
ER stress and restore glucose homeostasis in a mouse model of
type 2 diabetes. Science. 2006;313:1137–1140.
15. Lefebvre P, Cariou B, Lien F, et al. Role of bile acids and bile
acid receptors in metabolic regulation. Physiol Rev. 2009;89:
147–191.
16. Goodwin B, Jones SA, Price RR, et al. A regulatory cascade of
the nuclear receptors FXR, SHP-1, and LRH-1 represses bile acid
biosynthesis. Mol Cell. 2000;6:517–526.
17. Inagaki T, Choi M, Moschetta A, et al. Fibroblast growth factor 15
functions as an enterohepatic signal to regulate bile acid
homeostasis. Cell Metab. 2005;2:217–225.
18. Kawamata Y, Fujii R, Hosoya M, et al. A G protein-coupled receptor
responsive to bile acids. J Biol Chem. 2003;278:9435–9940.
19. Maruyama T, Miyamoto Y, Nakamura T, et al. Identification of
membrane-type receptor for bile acids (M-BAR). Biochem
Biophys Res Commun. 2002;298:714–719.
20. Nguyen A, Bouscarel B. Bile acids and signal transduction: role in
glucose homeostasis. Cell Signal. 2008;20:2180–2197.
21. Einarsson K, Ericsson S, Ewerth S, et al. Bile acid sequestrants:
mechanisms of action on bile acid and cholesterol metabolism.
Eur J Clin Pharmacol. 1991;40 Suppl 1:S53–S58.
22. Lambert G, Amar MJ, Guo G, et al. The farnesoid X-receptor is
an essential regulator of cholesterol homeostasis. J Biol Chem.
2003;278:2563–2570.
23. Claudel T, Sturm E, Duez H, et al. Bile acid-activated nuclear
receptor FXR suppresses apolipoprotein A-I transcription via a
negative FXR response element. J Clin Invest. 2002;109:961–971.
24. Watanabe M, Houten SM, Wang L, et al. Bile acids lower triglyceride
levels via a pathway involving FXR, SHP, and SREBP-1c. J Clin
Invest. 2004;113:1408–1418.
25. Ma K, Saha PK, Chan L, Moore DD. Farnesoid X receptor is essential
for normal glucose homeostasis. J Clin Invest. 2006;116:1102–1109.
26. Bhatnagar S, Damron HA, Hillgartner FB. Fibroblast growth
factor-19, a novel factor that inhibits hepatic fatty acid synthesis. J
Biol Chem. 2009;284:10023–10033.
27. Tomlinson E, Fu L, John L, et al. Transgenic mice expressing
human fibroblast growth factor-19 display increased metabolic rate
and decreased adiposity. Endocrinology. 2002;143:1741–1747.
28. •Hartman HB, Gardell SJ, Petucci CJ, et al. Activation of
farnesoid X receptor prevents atherosclerotic lesion formation in
LDLR-/- and apoE-/- mice. J Lipid Res. 2009; 50:1090–1100.
This paper defines a role for FXR in atherosclerosis and the
potential mechanisms involved by evaluating the in vivo effect of
FXR activation on the formation of atherosclerotic lesions.
29. Zhang Y, Castellani LW, Sinal CJ, et al. Peroxisome proliferator-
activated receptor-gamma coactivator 1alpha (PGC-1alpha) regu-
lates triglyceride metabolism by activation of the nuclear receptor
FXR. Genes Dev. 2004;18:157–169.
30. Claudel T, Inoue Y, Barbier O, et al. Farnesoid X receptor agonists
suppress hepatic apolipoprotein CIII expression. Gastroenterolo-
gy. 2003;125:544–555.
31. Kast HR, Nguyen CM, Sinal CJ, et al. Farnesoid X-activated
receptor induces apolipoprotein C-II transcription: a molecular
mechanism linking plasma triglyceride levels to bile acids. Mol
Endocrinol. 2001;15:1720–1728.
32. Pineda Torra I, Claudel T, Duval C, et al. Bile acids induce the
expression of the human peroxisome proliferator-activated recep-
tor alpha gene via activation of the farnesoid X receptor. Mol
Endocrinol. 2003;17:259–272.
33. Andersen E, Karlaganis G, Sjovall J. Altered bile acid profiles in
duodenal bile and urine in diabetic subjects. Eur J Clin Invest.
1988;18:166–172.
34. Uchida K, Makino S, Akiyoshi T. Altered bile acid metabolism in
nonobese, spontaneously diabetic (NOD) mice. Diabetes. 1985;34:
79–83.
76 Curr Diab Rep (2010) 10:70–77

35. Duran-Sandoval D, Mautino G, Martin G, et al. Glucose regulates
the expression of the farnesoid X receptor in liver. Diabetes.
2004;53:890–898.
36. Zhang Y, Lee FY, Barrera G, et al. Activation of the nuclear
receptor FXR improves hyperglycemia and hyperlipidemia in
diabetic mice. Proc Natl Acad Sci U S A. 2006;103:1006–1011.
37. Cariou B, van Harmelen K, Duran-Sandoval D, et al. Transient
impairment of the adaptive response to fasting in FXR-deficient
mice. FEBS Lett. 2005;579:4076–4080.
38. Duran-Sandoval D, Cariou B, Percevault F, et al. The farnesoid X
receptor modulates hepatic carbohydrate metabolism during the
fasting-refeeding transition. J Biol Chem. 2005;280:29971–29979.
39. Cariou B, van Harmelen K, Duran-Sandoval D, et al. The
farnesoid X receptor modulates adiposity and peripheral insulin
sensitivity in mice. J Biol Chem. 2006;281:11039–11049.
40. Maruyama T, Tanaka K, Suzuki J, et al. Targeted disruption of G
protein-coupled bile acid receptor 1 (Gpbar1/M-Bar) in mice. J
Endocrinol. 2006;191:197–205.
41. Katsuma S, Hirasawa A, Tsujimoto G. Bile acids promote glucagon-
like peptide-1 secretion through TGR5 in a murine enteroendocrine
cell line STC-1. Biochem Biophys Res Commun. 2005;329:386–390.
42. •Thomas C, Gioiello A, Noriega L, et al. TGR5-mediated bile
acid sensing controls glucose homeostasis. Cell Metab. 2009;
10:167–177. This paper evaluates the role of the TGR5 signaling
pathway and a TGR5-specific agonist in the in vivo regulation of
secretion of intestinal GLP-1.
43. Sato H, Genet C, Strehle A, et al. Anti-hyperglycemic activity of a
TGR5 agonist isolated from Olea europaea. Biochem Biophys Res
Commun. 2007;362:793–798.
44. Watanabe M, Houten SM, Mataki C, et al. Bile acids induce
energy expenditure by promoting intracellular thyroid hormone
activation. Nature. 2006;439:484–489.
45. •Suzuki T, Oba K, Igari Y, et al. Colestimide lowers plasma
glucose levels and increases plasma glucagon-like PEPTIDE-1
(7–36) levels in patients with type 2 diabetes mellitus complicated
by hypercholesterolemia. J Nippon Med Sch. 2007; 74:338–343.
This paper implicates TGR5-mediated secretion of GLP-1 in the
mechanism governing the glucose-lowering effect of bile acid
sequestrants.
46. Beysen C, Murphy E, Deines K, et al. Colesevelam HCl Reduces
Fasting Plasma Glucose Concentrations by Improving Plasma
Glucose Clearance in Subjects With Type 2 Diabetes.Presented
at the 69th Annual Meeting and Scientific Sessions of the
American Diabetes Association. New Orleans, LA; June 5–9,
2009.
47. Shang Q, Saumoy M, Holst JJ, et al. Colesevelam improves
insulin resistance in a diet-induced obesity (F-DIO) rat model by
increasing the release of GLP-1. Am J Physiol Gastrointest Liver
Physiol. 2009 Dec 31. [Epub ahead of print].
48. Schwartz SL, Lai YL, Xu J, et al. The effect of colesevelam
hydrochloride on insulin sensitivity and secretion in patients with
type 2 diabetes: a pilot study. Metab Syndr Relat Disord. 2010 Jan 8.
[Epub ahead of print].
49. Staels B. A review of bile acid sequestrants: potential mechanism
(s) for glucose-lowering effects in type 2 diabetes mellitus.
Postgrad Med. 2009;121:25–30.
50. Reasner CA. Reducing cardiovascular complications of type 2
diabetes by targeting multiple risk factors. J Cardiovasc Pharma-
col. 2008;52:136–44.
51. Thomas C, Pellicciari R, Pruzanski M, et al. Targeting bile-acid
signalling for metabolic diseases. Nat Rev Drug Discov. 2008;7:
678–693.
52. Cariou B, Staels B. FXR: a promising target for the metabolic
syndrome? Trends Pharmacol Sci. 2007;28:236–243.
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