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doi:10.1111/j.1745-7270.2006.00128.x |
Role of Peroxisome
Proliferator-Activated Receptor Gamma in Glucose-induced Insulin Secretion
Ze-Kuan XU1,2, Neng-Guin CHEN1, Chang-Yan MA1, Zhuo-Xian MENG1, Yu-Jie SUN1, and Xiao HAN1*
1 Key Laboratory of Human
Functional Genomics of Jiangsu Province, Department of Biochemistry and
Molecular Biology, Nanjing Medical University, Nanjing 210029, China;
2 Department of Normal
Surgery, the First Affiliated Hospital of Nanjing Medical University, Nanjing
210029, China
Accepted: August
15, 2005
Received: November
2, 2005
This work was
supported by a grant from the Key Program of Natural Science Foundation of
Jiangsu Province (No. BK2003003)
*Corresponding author: Tel, 86-25-86862731; Fax, 86-25-86862731;
E-mail, [email protected]
Abstract Peroxisome proliferator-activated receptor (PPAR) isoforms
(a and g)
are known to be expressed in pancreatic islets as well as in insulin-producing
cell lines. Ligands of PPAR have been shown to enhance glucose-induced insulin
secretion in rat pancreatic islets. However, their effect on insulin secretion
is still unclear. To understand the molecular mechanism by which PPARg exerts its effect on glucose-induced
insulin secretion, we examined the endogenous activity of PPAR isoforms, and
studied the PPARg function and its target gene
expression in INS-1 cells. We found that: (1) endogenous PPARg was activated in a ligand-dependent
manner in INS-1 cells; (2) overexpression of PPARg
in the absence of PPARg ligands enhanced
glucose-induced insulin secretion, which indicates that the increased
glucose-induced insulin secretion is a PPARg-mediated
event; (3) the addition of both PPARg and
retinoid X receptor (RXR) ligands showed a synergistic effect on the
augmentation of reporter activity, suggesting that the hetero-dimerization of
PPARg and RXR is required for the
regulation of the target genes; (4) PPARs upregulated both the glucose
transporter 2 (GLUT2) and Cb1-associated protein (CAP) genes in INS-1 cells.
Our findings suggest an important mechanistic pathway in which PPARg enhances glucose-induced insulin
secretion by activating the expression of GLUT2 and CAP genes in a
ligand-dependent manner.
Key words PPARg; ligand; glucose-induced insulin
secretion; glucose transporter 2; Cb1-associated protein
The peroxisome
proliferator-activated receptors (PPARs) are ligand-dependent transcription
factors that regulate gene networks involved in cellular development,
differentiation and metabolism [1]. PPARs exist in three forms in rat: PPARa, g
and d, which are members of the
nuclear hormone receptor superfamily. PPARs heterodimerize with retinoid X
receptor (RXR) in order to bind to DNA recognition sequences, which contain a
direct repeat core-site separated by one nucleotide (NNN-AGGTCA-N-AGGTCA).
These complexes destabilize chromatin and activate transcription [2,3]. Through
this mechanism, PPARs directly regulate transcription in response to their
specific ligands. In addition to ligand-dependent transcriptional activation,
PPARg activity is also regulated by
mitogen-activated protein (MAP) kinase [4] or c-Jun N-terminal kinase signaling
pathways [5]. Phosphorylation of PPARg at a
consensus MAP kinase site inhibits the ligand-independent and ligand-dependent
transactivation functions [6]. These findings provide an important mechanism
for cross-talk between PPARg and other cellular signaling
pathways in a physiological context [7].
Recently, PPARs have been
shown to be involved in diabetes, cancer and inflammatory diseases. The
thiazolidinedione (TZD) class of antidiabetic drugs alleviates insulin
resistance and hyperglycemia in human diabetes [8,9]. Several antidiabetic
agents in the TZD class such as rosiglitazone, troglitazone and pioglitazone,
have been identified as ligands of PPARg
[10–13]. There is evidence that the effect
of TZD on increased insulin sensitivity is mediated through PPARg [14,15]. PPARg
ligands are shown to augment glucose disposal in peripheral tissues by
increasing expression of the glucose transporter genes glucose transporter 1
(GLUT1) and GLUT4 [16]. Several clinical studies linked the mutation in
different regions of PPARg with insulin resistance,
diabetes and hypertension [17–21]. It is generally accepted
that PPARg increases glucose transport
activity and transporter expression in adipose tissues and muscles.
In pancreatic islet, GLUT2 was
reported to act as a glucose sensor [22]. Pancreatic islets treated with
troglitazone increased the expression level of GLUT2 in Zucker Diabetic Fatty
rats [23]. A functional PPAR response element (PPRE) has been identified in
the rat GLUT2 gene promoter and it is suggested that PPARs may be involved in
the regulation of glucose-induced insulin secretion [24]. However, the direct
correlation between PPARs and glucose-induced insulin secretion needs to be
established.
This work is designed to
investigate the molecular role of PPARa and g in glucose-induced insulin secretion and
to focus on the function and target genes of PPARg.
Materials and Methods
Reagents
Rosiglitazone (BRL 49653) was
obtained from Biomol (Plymouth Meeting, USA). Wy 14643 was purchased from
Cayman Chemical (Ann Arbor, USA). 9-cis-retinoic acid was obtained from Sigma
(St. Louis, USA). Cell culture reagents were from Invitrogen (Carlsbad, USA).
Fetal bovine serum (FBS) was from Hyclone (Logan, USA). Expression plasmids
pCMX-mPPARa, pCMX-mPPARg and pCMX-VP-mPPARg were modified by the cDNA constructs
obtained from Invitrogen. PPRE3-TK-Luc reporter construct was made as
previously described [25]. pCMX-mPPARg-S84A
was constructed according to published procedures [6].
Cell culture
INS-1 cells, a widely used rat
insulinoma b cell line for insulin secretion
studies [26], were from Dr. Han (Diabetes
and Genetics Research Center, City of Hope National Medical Center, Duarte,
USA) and were cultured to near 100% confluence in RPMI 1640 medium supplemented
with 11 mM D-glucose, 10% FBS, 100 U/ml penicillin, 100 mg/ml streptomycin, 10 mM HEPES, 2 mM L-glutamine,
1 mM sodium pyruvate and 50 mM-mercaptoethanol. All tissue
cultures were performed in a Forma Scientific tissue culture incubator that
provided an environment of 5% CO2.
Transient transfection and
luciferase assay
One day before transfection,
INS-1 cells were dispersed with trypsin-EDTA solution and counted. The cells
were seeded into 12-well plates at a density of 2´105 per well to attain 90%
confluence the next day. PPRE3-TK-Luc, a reporter construct containing PPRE
(0.8 mg/well) and b-gal plasmid (0.4 mg/well, Clontech, Palo Alto, USA) were
incubated with Lipofectamine 2000 reagent (Gibco, Grand Island, USA) for 30 min
at room temperature. The cell culture medium was removed and replaced with 1 ml
of RPMI 1640 containing 10% FBS and the lipid/DNA complex, and the cells were
cultured for 18 h. Then the medium was changed to phenol red-free RPMI 1640
with 5% stripped-FBS (FBS deprived of the growth factors by charcoal) and the
cells were incubated for an additional 18 h. Then ligands were added, and the
cells were harvested after a further 18 h of incubation. In addition, we
followed a similar transfection procedure with the pEGFP-N1 (Clontech) and
counted the enhanced green fluorescent protein-positive cells versus the total
cell number in order to estimate the transfection efficiency, which was
approximately 75% here. The luciferase activity was measured with a TD-20/20
luminometer (Turner Designs, Sunnyvale, USA), using 100 ml of whole cell lysate and the same
volume of luciferase assay reagent (Promega, Madison, USA). An aliquot of the
same cell lysate for each sample was used to measure b-galactosidase
activity to normalize luciferase activity. Luciferase assays were performed in
triplicate and repeated four times.
RNA extraction, real-time
quantitative polymerase chain reaction (RT-QPCR) and insulin secretion
measurement
Cells were transfected with
various expression vectors overnight. Culture plates were washed with 1´PBS and then treated with
ligands in RPMI 1640 (11 mM glucose and 10% stripped-FBS) for 24 h. To
stabilize the insulin secretion, the transfected cells were incubated in RPMI
1640 (3 mM glucose and 0.1% bovine serum albumin) for 1 h, then the supernatant
was removed and the cells were incubated with the same medium for 2 h, at the
time point, the supernatant was collected for later insulin measurement. The
glucose concentration in the medium was then increased to 20 mM and the
transfected cells were incubated for an additional 2 h. Supernatant was frozen
at –70 ºC and insulin determination assay was
performed later. The amount of insulin in the supernatant was detected by rat
insulin ELISA kit (Crystal Chem, Chicago, USA). Total RNA was isolated from
cells by RNeasy (Qiagen, Carlsbad, USA). The RT-QPCR was performed with the
following forward and reverse primers: GLUT2, 5‘-CTCGGGCCTTACGTGTTCTT-3‘
and 5‘-TAGGCAGCTCATCCTCACACA-3‘; CAP, 5‘-CGCTGCTCCGACAGGTG-3‘
and 5‘-CTCGAAGTGCCAAACCAT-3‘. The reaction was carried out using
an ABI PRISM 7700 sequence detection system (Applied Biosystems, Foster City,
USA) with CYBER Green I according to the manufacturer’s instructions.
Statistical analysis
The data were presented in the
mean+/–SEM format and compared by
one-way ANOVA and Tukey’s
post-hoc comparison. P values of less than 0.01 were considered
statistically significant. The analysis was conducted using Prism software
(Version 4.0; GraphPad Software, San Diego, USA).
Results
Ligand activation of
endogenous PPARg
To test whether endogenous
PPARa and g
are activated by their specific ligands, INS-1 cells were transiently
transfected with a PPRE-reporter plasmid and treated with various ligands. As
shown in Fig. 1, BRL 49653, a specific ligand for PPARg, increased the luciferase activity up
to 2.5-fold in a dose-dependent manner, whereas Wy 14643, a specific ligand for
PPARa, had no effect on reporter
gene activity. Notably, BRL 49653-stimulated luciferase activity was increased
4-fold in the presence of 9-cis-retinoic acid, an endogenous ligand for RXR (P<0.01 vs. control). These results not only
indicated the presence of PPARg in INS-1 cells, but also
demonstrated that the activation of PPARg
resulted from ligand-dependent heterodimerization of RXR and PPARg.
Elevated glucose-stimulated
insulin secretion (GSIS) by ligand-dependent activation of PPARg
To investigate the function of
PPARg in the regulation of GSIS,
the interaction between GSIS, PPARg ligands
and PPARg activity was studied. We
observed that BRL 49653 enhanced GSIS up to 2-fold compared with the cells
incubated with 20 mM glucose only, and overexpression of PPARg further enhanced GSIS to 2.5-fold (Fig.
2). These observations strongly suggested that PPARg
ligand BRL 49653 can elevate GSIS by activating both endogenous and exogenous
PPARg.
The effects of Wy 14643 (a
PPARa ligand) and PPARa on GSIS were also studied. Compared with
BRL 49653, Wy 14643 had less effect on GSIS. Overexpression of PPARa have no effect on GSIS. Combining these
results with the observations shown in Fig. 1, we concluded that Wy
14643 could not activate endogenous or exogenous PPARa
in INS-1 cells.
Function of PPARg on GSIS
The functional role of PPARg was further investigated by the
overexpression of a constitutively active chimeric form of PPARg, VP-PPARg,
in INS-1 cells. We found that the overexpression of VP-PPARg resulted in a 2-fold increase of GSIS
in the absence of PPARg ligand (Fig. 3). This
increment is comparable to the GSIS elevation when cells were treated with PPARg ligand.
To confirm that PPARg is directly involved in the regulation
of GSIS, INS-1 cells were transiently transfected with PPARg-S84A, a PPARg
mutant, which contains a point mutation at the serine phosphorylation site to
avoid the activation of PPARg
by MAP kinase. As
expected, BRL 49653 did not increase GSIS in cells expression PPARg-S84A to the same extent as that in cells
overexpression wild-type PPARg (Fig. 3). These
findings provided direct evidence that PPARg
played a functional role in GSIS regulation.
PPARg
target genes on GSIS
A previous report has
identified the promoter region of the GLUT2 gene containing PPRE, which
indicates that PPAR may regulate the expression of the GLUT2 gene [24]. To
further explore the molecular mechanism of GSIS regulated by PPARg, we investigated the role of PPARg in GLUT2 gene activity regulation, the
INS-1 cells were transiently transfected and overexpressed with wild-type PPARg or PPARa
plasmid and treated with BRL 49653 or Wy 14643, and then exposed to 3 mM
glucose for 2 h, followed by challenged with 20 mM glucose for 2 h as described
in materials and methods. We observed that the expression of the GLUT2 gene
induced by BRL 49653 was increased up to 5-fold by both endogenous and
exogenous PPARg when the cells were exposed
to high glucose concentration (20 mM) for 2 h (Fig. 4).
We further examined the
expression level of CAP that is known to facilitate GLUT4 translocation in
insulin-sensitive tissues [27] with the same research system as above. The CAP
expression was found to be induced by BRL 49653 and enhanced by overexpression
of PPARg (Fig. 4). These
observations suggested that GLUT2 and CAP were involved in the GSIS pathway
regulated by PPARg.
Discussion
There have been many reports
on the biological role of PPARa and g
in pancreatic b-cells. PPARs have been
reported to express in the human pancreatic islet cells, rodent pancreatic
islet cells, INS-1 cells and insulin-producing cell lines including HIT-T15
[28–30]. Moderate amounts of PPARg are expressed in pancreatic b cells, and its expression is increased
in the diabetic state [28,31]. But the fundamental role of PPARg in b-cells
is not fully understood. Reports on the effects of PPARg
on insulin secretion are contradictory. PPARg
agonists can protect the pancreatic b cells
from apoptosis and restore the function of b
cells, including GSIS [23]. However, it is reported that PPARg agonists can also decrease insulin
secretion in diabetic animal models [32]. The activation of PPARg did not improve insulin secretion in
isolated human islets [33,34]. Our results suggested that the activation of
PPARg elevated GSIS and increased
the expression of the GLUT2 and CAP genes in rat pancreatic b-cell line, INS-1 cells.
Glucose is the most important
physiological stimulus for insulin secretion, and the process requires glucose
sensing [35]. The glucokinase in pancreatic b-cells
(b-GK) is the rate-limiting step in
glycolitic flux for insulin secretion, and a small change in b-GK activity sharply affects the
threshold for GSIS [36]. GLUT2 is known to play an important role in allowing
rapid equilibration of glucose across the plasma membrane. However, it is also
essential in GSIS because normal glucose uptake and subsequent metabolic
signaling for GSIS can not be achieved without GLUT2. The expression of GLUT2
and b-GK is decreased in diabetes
subjects before the loss of GSIS. The b-cell-specific
knockout of the GLUT2 or GK gene results in infant death because of severe
hyperglycemia [37].
The direct involvement of PPARg in GSIS was tested by a PPARg construct which has constitutive
activity. Transient transfection of this chimeric receptor has been shown to
increase GSIS up to 2-fold in the absence of specific ligands, which suggests
that this elevation was directly mediated by PPARg.
In addition, insulin is a stimulator in the MAP kinase signaling pathway. It
has been shown that PPARg activity is downregulated by
MAP kinase in preadipocyte 3T3-L1 cells [6]. One possible explanation for such
downregulation is that the secreted insulin released into culture media may
cause feedback inhibition of PPARg
activity by the phosphorylation of the serine residue at amino acid position 84
in INS-1 cells. However, our mutant PPARg-S84A
construct showed no significant effect on PPARg
activity by the insulin-activated MAP kinase pathway in INS-1 cells. It may
provide the information regarding the differential biological effects of PPARg activity between 3T3-L1 preadipocyte and
insulin-producing cells.
GLUT2 is a major form of
glucose transporter in pancreatic b-cells
and plays a key role in GSIS. Suppression of GLUT2 in pancreatic b-cells is correlated with the loss of
high-Km glucose transport and GSIS [38]. Several approaches, including
antisense blocking of GLUT2 activity and GLUT2-null islets, have suggested an
exclusive role of GLUT2 on glucose uptake, utilization and signaling in
pancreatic islets [37,39,40]. Our approach of either stimulating endogenous
PPARg receptor by specific ligands,
or overexpressing the constitutively active receptor, suggested that these
manipulations induced GLUT2 gene expression, thus demonstrating a direct link
with elevated GSIS. This finding is also consistent with an earlier report that
the promoter of GLUT2 contains a functional PPRE [24].
Significant progress has been
made over the past several years to address the role of insulin receptor on
pancreatic b-cells [41]. Interestingly, in
mice, the tissue-specific knockout of the insulin receptor in muscle failed to
produce diabetes, but the disruption of the gene in b-cells
produced a diabetic phenotype [42]. The positive coupling between insulin
secretion and insulin receptor action has been suggested by way of PI3
kinase-dependent action [27]. CAP, which associates with Cb-l proto-oncoprotein
in a PI3-kinase-independent pathway through insulin receptor signaling, appears
to be induced during the adipocyte differentiation by the PPARg ligands treatment. In the present study,
we first reported that CAP was expressed in INS-1 cells. The role of CAP in
INS-1 cells is still unknown, and GLUT2 does not undergo insulin-stimulated
translocation as compared to GLUT4 in non-insulin producing cells [43,44]. The
role that CAP might play in the post-insulin receptor signaling pathway and in
promoting GLUT2 translocation in INS-1 cells needs to be investigated.
Our study also revealed
distinct mechanisms of glucose-induced insulin secretion, which were mediated
by PPARa and PPARg. Our results suggested that the
overexpression of PPARa had no incremental effects on
GSIS or the expression of GLUT2 and CAP genes. It is consistent with our
findings that the elevated glucose markedly downregulated the expression of the
PPARa gene in pancreatic b-cell [45]. The action of glucose on PPARa mRNA expression occurs in less than 2 h
and does not require de novo protein synthesis. The rapidity of this
effect and the absence of a requirement for protein synthesis indicate that the
PPARa behaves as an early response
gene in INS-1 cells [45].
Rosiglitazone, as well as
other TZD antidiabetic drugs, is known to improve insulin resistance by
reducing hyperglycemia, hyperinsulinemia and hypertriglyceridemia in human and
rodent. Our findings provide a new evidence that PPARg
directly acts on the pancreatic b-cells
to enhance GSIS.
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