1. health and fitness
2. disease
3. diabetes

REVIEWS
Mechanisms by which
Thiazolidinediones Enhance Insulin
Action
Mauricio J. Reginato and Mitchell A. Lazar
Thiazolidinediones (TZDs) are an exciting new class of insulinsensitizing drugs being used currently for the treatment of non-insulindependent diabetes mellitus. The molecular target of these compounds
is thought to be the nuclear hormone receptor, peroxisome proliferatoractivated receptor γ (PPARγ). PPARγ is expressed predominantly in
adipose tissue, yet a major site of TZD-responsive glucose disposal is
skeletal muscle. Potential explanations for this paradox are discussed
in this review.
Type 2 diabetes mellitus (DM) is a
common, chronic disease that is a
major cause of morbidity and mortality in industrialized societies. Type 2
DM has a strong genetic component
and is linked tightly to obesity. Although impaired insulin secretion contributes to type 2 DM, early in the
course of the disease insulin levels are
often increased. A major difference between type 2 DM and insulin-dependent diabetes is that type 2 DM is
characterized by peripheral insulin resistance1. The resistance occurs despite
qualitatively and quantitatively normal
insulin receptors, thus implicating one
or more defective steps in the insulin
signaling pathway downstream from
insulin binding to its receptor. Nevertheless, until recently the only available
M.J. Reginato is Research Fellow at the
Departments of Pharmacology and Medicine,
University of Pennsylvania Medical Center,
Philadelphia, PA 19104, USA. M.A. Lazar is
Chief of the Division of Endocrinology,
Diabetes, and Metabolism, Department of
Medicine, and Director of Penn Diabetes
Center, University of Pennsylvania Medical
Center, Philadelphia, PA 19104, USA.
TEM Vol. 10, No. 1, 1999
pharmacological treatments for type 2
DM were insulin or agents that increase insulin secretion. New pharmacological approaches to treating type 2
DM have been developed recently that
target other metabolic abnormalities2.
The thiazolidinediones (TZDs) are a
new class of orally active drugs that
are particularly exciting because they
decrease insulin resistance by enhancing the actions of insulin at a level distal to the insulin receptor3.
•
Effects of TZDs
TZDs, which include troglitazone, pioglitazone and rosaglitazone (Fig. 1),
are thought to sensitize target tissues
to the action of insulin. Indeed, they
are ineffective at lowering serum glucose levels in the absence of insulin3.
In animal models of type 2 DM, TZDs
reduce plasma glucose levels, and decrease insulin and triglycerides to near
normal levels4,5.
In human studies, about 75% of patients with type 2 DM responded to
troglitazone treatment6. In addition to
reduced plasma glucose levels, insulin
levels and/or dose requirements also
drop7. This reduction in insulin levels
is also associated with improved metabolic status of patients with the syndrome of insulin resistance and polycystic ovaries8,9. TZDs also significantly
reduce serum concentrations of
triglycerides and free fatty acids, with
a small rise in high-density lipoprotein
(HDL) cholesterol10,11. Recent hyperinsulinemic-euglycemic clamp studies
have suggested that troglitazone works
primarily by increasing the rate of peripheral glucose disposal in skeletal
muscle12.
In general, TZDs are well tolerated
by patients, although troglitazone
treatment has been associated with
hepatic dysfunction that has been fatal
in a few cases13–15. Thus, it is recommended that liver function tests be
monitored frequently, although the
causal relationship and mechanism
have not been established. Chronic
TZD treatment also leads to modest
weight gain in rodents and humans16.
•
Peroxisome Proliferator-activated
Receptor g: Molecular Target of
TZDs
TZDs were developed originally by
screening analogs of clofibric acid for
antilipidemic and antihyperglycemic
effects17. The antidiabetic effects of
these compounds were not understood,
but the discovery that TZDs enhanced
adipocyte differentiation18,19 was an
important clue to identifying their molecular target. Activators of a member
of the nuclear hormone receptor superfamily, peroxisome proliferator-activated receptor (PPAR), were also found
to induce adipogenesis20, and PPARγ
was shown to be expressed predominantly in adipose tissue21, and to function as a key transcription factor in
adipocyte differentiation22. Shortly
thereafter, TZDs were demonstrated to
be direct ligands for PPARγ (Ref. 23).
1043-2760/99/$ – see front matter © 1999 Elsevier Science. All rights reserved. PII: S1043-2760(98)00110-6
9
O
S
O
O
NH
O
O
HO
Pioglitazone
O
S
NH
O
Troglitazone
Figure 1. Thiazolidinedione structures. The structures of pioglitazone and troglitazone are
shown.
in target genes25. The DNA sequences
recognized by the PPAR–RXR heterodimer are referred to as PPAR-response elements (PPREs). PPAR–RXR
heterodimers bind to PPREs in the absence of ligand, but ligand leads to a
conformational change that results in
activation of transcription of the target
gene. The active conformation recruits
a multiprotein coactivator complex
(reviewed in Ref. 26) that acetylates histones (leading to an open, more active
conformation of the nucleosome), as
well as interacting directly with the
basal transcription machinery (Fig. 2).
PPREs have been found in the regulatory regions of a number of genes involved in lipid metabolism and energy
balance27,28.
PPARγ is a member of the nuclear
hormone receptor superfamily of transcription factors that are activated by
small, lipophilic, non-genomically encoded ligands24. There are two PPARγ
isoforms, γ1 and γ2, derived from alternative promoter usage. PPARγ2 contains an additional 31 amino acids at its
N-terminus, but the functional significance is unclear. Interestingly, PPARγ2
is found exclusively in adipocytes,
whereas PPARγ1 is expressed predominantly in adipocytes, but is also expressed in other tissues (see below).
PPARγ belongs to a subset of nuclear receptors that form heterodimers
with the retinoid X receptor (RXR),
greatly enhancing the ability of the receptor to bind specific DNA sequences
SRC1
CBP
P/CAF
Others
Coactivator complex:
TZD
PPARγ
LBD
RXR
LBD
9-cis
RA
HAT
PPARγ
DBD
RXR
DBD
AANTAGGTCANAGGTCA
PPRE
Basal transcription machinery
Others
H
F
TAF
Nucleosome
AA A
A
c c cA
c
A
c
cAA A A
cc
cc
TBP
RNA polymerase II
B
E
TATA
Ectopic expression of PPARγ in
preadipocytes, fibroblasts and myoblasts induces adipocyte differentiation in the presence of the TZD ligand22,29. The ability of TZDs acting via
PPARγ to induce adipocyte differentiation might explain the modest weight
increases observed in vivo. However, it
is not clear how to reconcile the fact
that excess fat cell mass is a major risk
factor for insulin resistance and type 2
DM with the antihyperglycemic effects
of TZDs.
•
Evidence that PPARg Mediates the
Antidiabetic Effects of TZDs
Nevertheless, there is strong evidence
that TZDs function via PPARγ. PPARγ
has been shown to bind to a number of
different ligands, including a number of
fatty acids, as well as prostaglandin J
derivatives, such as 15-deoxy-∆12, 14prostaglandin J2 (Refs 30–32). However, none of these compounds binds to
PPARγ with affinities in the nanomolar
range. By contrast, TZDs have been
shown to bind to PPARγ with an affinity in the range of 40–200 nM (Refs
30,31). Not only are TZDs activating ligands for PPARγ at nanomolar concentrations, but there is a remarkable correlation between TZD potencies for in
vivo plasma glucose lowering and their
order of potency for both PPARγ activation and direct binding to PPARγ
(Refs 33,34). RXR ligands can also activate the PPARγ–RXR heterodimer35,36,
and synthetic RXR agonists increase insulin sensitivity in obese mice and work
in combination with TZDs to enhance
antidiabetic activity37. This further suggests that the PPARγ–RXR heterodimer
complex is the molecular target for
treatment of insulin resistance in vivo.
The evidence supporting PPARγ as
the target of TZD is summarized in
Table 1.
PPARγ target gene
•
Figure 2. Mechanism of thiazolidinedione (TZD) activation of transcription by peroxisome
proliferator-activated receptor γ (PPARγ). PPARγ binds to specific DNA sequences in target
genes as a heterodimer with retinoid X receptor (RXR). TZDs [and/or an RXR ligand, indicated
as 9-cis retinoic acid (RA)] recruit coactivator complexes to the target gene, resulting in increased transcription through inherent histone acetylase (HAT) activity or via interactions with
the basal transcription machinery. CBP, CREB-binding protein; CREB, cyclic AMP response
element-binding protein; DBD, DNA-binding domain; LBD, ligand-binding domain; PPRE,
PPAR-response element; P/CAF, p300/CBP-associated factor; SRC1, steroid receptor coactivator
1; TAF, TBP-associated factor; TBP, TATA-binding protein.
10
The Paradox
There is general agreement that TZDs
are effective antidiabetic agents because they enhance insulin-responsive
glucose disposal in vivo. It is also clear
that TZDs are high-affinity, activating
ligands for PPARγ. However, the
mechanism by which PPARγ mediates
the antidiabetic actions of TZDs is
TEM Vol. 10, No. 1, 1999
controversial. The problem is that the
main site of TZD-enhanced glucose disposal occurs primarily in skeletal muscle, whereas the main site of PPARγ
expression is in adipose tissue. Although PPARγ is expressed predominantly in adipocytes, PPARγ expression
has been demonstrated in a variety of
extra-adipose tissues, including liver38,
colon39,40, breast41, type II pneumocytes
of the lung42 and macrophages43–45.
PPARγ expression in skeletal muscle
has also been reported46. By northern
analysis, the level of PPARγ mRNA is
>50-fold higher in adipose tissue than
in skeletal muscle21,47, although protein
levels have not been compared quantitatively. Therefore, one missing piece
of the puzzle is the exact tissue site at
which TZDs function to promote insulin action in muscle.
•
Mechanisms
It is possible that mechanisms other
than PPARγ activation explain the effects of TZDs on glucose disposal in
muscle. However, given the nanomolar
binding to PPARγ and the remarkable
correlation between PPARγ activation
and enhancement of insulin action, it
seems likely that PPARγ binding and
activation are related to the in vivo actions of TZDs. A number of potential
mechanisms could link the activation
of PPARγ to insulin action. These are
summarized in Table 2.
The abundance of PPARγ in
adipocytes suggests that this is the site
of action of TZDs. Consistent with
this, TZD treatment increases the
number of small adipocytes in diabetic
rats; these small adipocytes might
have altered properties that promote
insulin action either directly or indirectly48. One possibility is that TZD activation of PPARγ directly induces
genes involved in glucose metabolism
in adipocytes. Indeed, TZDs increase
GLUT4 mRNA levels and glucose uptake in cultured adipocytes49,50. However, the increases in GLUT4 levels following TZD treatment are modest (twoto threefold). In addition, glucose disposal into adipocytes is unlikely to be
of sufficient quantitative impact to
explain the dramatic effects of TZDs
(Ref. 51).
TEM Vol. 10, No. 1, 1999
Table 1. Evidence that the mechanism of glucose lowering by thiazolidinediones (TZDs) in vivo involves peroxisome proliferator-activated
receptor g (PPARg)
• TZDs bind to PPARγ with affinities in the nanomolar range
• The rank order potency of TZDs for blood glucose lowering in vivo correlates
strongly with TZD binding and activation of PPARγ in vitro
• Activating ligands for retinoid X receptor (RXR), which forms PPARγ–RXR
heterodimers also have antidiabetic activity in vivo
Another possibility is that TZDdependent activation of PPARγ induces
adipocytes to send an endocrine signal
to muscle that enhances insulin action. This signal could be the decrease
in a factor that promotes insulin resistance, or the increase in a factor that
enhances insulin action. One potential
factor is adipocyte-derived tumor
necrosis factor α (TNF-α), which has
been shown to be associated with insulin resistance52–54. TZDs can block the
inhibitory effects of TNF-α on insulin
action55,56 and reduce TNF-α levels57.
Another peptide hormone secreted by
adipocytes is leptin58. The concentration of leptin is proportional to fat
cell mass, which is itself correlated
directly with type 2 DM (Ref. 59). This
raises the possibility of a link between
leptin and type 2 DM (Ref. 60), and
TZDs do reduce leptin gene expression
in vitro and in vivo61–63. Increased free
fatty acid (FFA) levels have also been
implicated in the pathogenesis of insulin resistance64,65. TZDs lower plasma
FFA levels, both by increasing β-oxidation in the liver and by increasing
adipocyte FFA uptake66. Other, as yet
undiscovered adipocyte factors, whose
gene expression and/or secretion is altered by TZDs, could also lead to insulin action in muscle.
It is also possible that the effects
of TZDs are independent of adipose
tissue. Lipodystrophic mice with little
or no adipose tissue develop insulin resistance and diabetes that responds
well to TZD treatment67. In this
model, TZDs might act upon hepatic
cells; these mice develop massive, fatty
livers that express PPARγ. It is also
possible that, despite its low abundance, PPARγ in skeletal muscle is the
target of TZDs. PPARγ mRNA was undetectable in skeletal muscle of the fatablated mice67. Nevertheless, activation of PPARγ in muscle, the main site
of glucose disposal, would provide a
direct mechanism to explain TZD action and, indeed, TZDs reportedly
stimulate glucose uptake and enhance
Table 2. Potential mechanisms by which thiazolidinediones (TZDs)
enhance insulin action
Mechanisms involving peroxisome proliferator-activated receptor g (PPARg)
Via PPARγ in adipocytes
• Direct stimulation of increased glucose disposal in adipocytes
• Stimulation of increased glucose disposal in skeletal muscle
• Reduced tumor necrosis factor α
• Reduced leptin
• Reduced free fatty acids
• Alteration of other adipocyte factors
Via extra-adipocytic PPARγ
• Direct stimulation of increased glucose disposal in skeletal muscle
• Action on other target tissue (such as liver) leading to increased glucose disposal
in skeletal muscle
Other mechanisms not involving PPARγ
11
GLUT4 mRNA levels in cultured
muscle cells68.
•
Future Directions
TZDs represent a breakthrough in the
treatment of type 2 DM. New insights
into the mechanism of TZD action in
type 2 DM are likely to result from
basic research in a variety of directions.
The discovery of a physiological ligand
for PPARγ might provide clues to the
site and substrates of the normal hormonal or metabolic pathways regulating insulin action. Tissue-specific
knockouts of PPARγ will not only test
the hypothesis that PPARγ is the molecular target of TZDs, but will support or
eliminate various cell types as candidate sites of TZD action. The discovery
of new TZD-dependent PPARγ target
genes will also contribute to a conceptual bridge between TZD activation of
PPARγ and insulin action. Finally, better understanding of the mechanistic
relationship between TZD binding to
PPARγ and enhanced insulin action in
vivo might lead to the development of
additional treatments directed at this
TZD receptor. For example, phosphorylation of PPARγ negatively regulates its
function69–72, suggesting that therapies
aimed at increasing the dephosphorylated state might synergize with TZDs in
potentiating insulin action.
•
Acknowledgements
Our work was supported by NIH grants
DK49780 and DK49210. We thank Dalei
Shao for valuable discussions.
5 Fujiwara, T., Yoshioka, S., Yoshioka, T.,
Ushiyama, I. and Horikoshi, H. (1988)
Characterization of new oral antidiabetic agent CS-045. Studies in KK
and ob/ob mice and Zucker fatty rats.
Diabetes 37, 1549–1558
6 Suter, S.L., Nolan, J.J., Wallace, P.,
Gumbiner, B. and Olefsky, J.M. (1992)
Metabolic effects of new oral hypoglycemic agent CS-045 in NIDDM subjects. Diabetes Care 15, 193–203
7 Kumar, S. et al. (1996) Troglitazone, an
insulin action enhancer, improves
metabolic control in NIDDM patients.
Troglitazone study group [published
erratum appears in Diabetologia (1996)
39, 12451]. Diabetologia 39, 701–709
8 Ehrmann, D.A. et al. (1997) Troglitazone
improves defects in insulin action, insulin secretion, ovarian steroidogenesis, and ribrinolysis in women with
polycystic ovary syndrome. J. Clin.
Endocrinol. Metab. 82, 2108–2116
9 Dunaif, A., Scott, D., Finegood, D., Quintana, B. and Whitcomb, R. (1996) The
insulin-sensitizing agent troglitazone
improves metabolic and reproductive
abnormalities in the polycystic ovary
syndrome. J. Clin. Endocrinol. Metab. 81,
3299–3306
20 Chawla, A. and Lazar, M.A. (1994) Peroxisome proliferator and retinoid signalling pathways coregulate preadipocyte phenotype and survival. Proc.
Natl. Acad. Sci. U. S. A. 91, 1786–1790
21 Chawla, A., Schwarz, E.J., Dimaculangan,
D.D. and Lazar, M.A. (1994) Peroxisome proliferator-activated receptor g
(PPARg ): adipose predominant expression and induction early in adipocyte differentiation. Endocrinology 135,
798–800
22 Tontonoz, P., Hu, E. and Spiegelman,
B.M. (1994) Stimulation of adipogenesis
in fibroblasts by PPARg2, a lipidactivated transcription factor. Cell 79,
1147–1156
23 Lehmann, J.M. et al. (1995) An antidiabetic thiazolidinedione is a high
affinity ligand for the nuclear
peroxisome proliferator-activated receptor g (PPARg). J. Biol. Chem. 270,
12953–12956
24 Mangelsdorf, D.J. et al. (1995) The nuclear receptor superfamily: the second
decade. Cell 83, 835–839
11 Spencer, C.M. and Markham, A. (1997)
Troglitazone. Drugs 54, 89–101
26 Glass, C.K., Rose, D.W. and Rosenfeld,
M.G. (1997) Nuclear receptor coactivators. Curr. Opin. Cell. Biol. 9, 222–232
12 Inzucchi, S.E. et al. (1998) Efficacy and
metabolic effects of metformin and
troglitazone in type II diabetes mellitus. New Engl. J. Med. 338, 867–872
13 Watkins, P.B. and Whitcomb, R.W. (1998)
Hepatic dysfunction associated with
troglitazone. New Engl. J. Med 338,
916–917
References
1 Olefsky, J.M., Ciaraldi, T.P. and
Kolterman, O.G. (1985) Mechanisms
of insulin resistance in non-insulindependent (type III) diabetes. Am. J.
Med. 79, 12–22
15 Gitlin, N., Julie, N.L., Spurr, C.L.,
Lim, K.N. and Juarbe, H.M. (1998) Two
cases of severe clinical and histological
hepatotoxicity associated with troglitazone. Ann. Intern. Med. 129, 36–38
2 Larkins, R.G. (1997) New concepts for
treatment of non-insulin-dependent
diabetes mellitus. Trends Endocrinol.
Metab. 8, 187–191
16 Imura, H. (1997) A novel antidiabetic
drug, troglitazone – reason for hope
and concern. New Engl. J. Med. 338,
908–909
3 Henry, R.R. (1997) Thiazolidinediones.
Endocrinol. Metab. Clin. North Am. 26,
553–573
17 Kawamatsu, Y., Saraie, T., Imamiya, E.,
Nishikawa, K. and Hamuro, Y. (1980)
Studies on antihyperlipidemic agents.
I. Synthesis and hypolipidemic activities of phenoxyphenyl alkanoic acid
derivatives. Arzneimittelforschung 30,
454–459
12
19 Kletzien, R.F., Clarke, S.D. and Ulrich,
R.G. (1992) Enhancement of adipocyte
differentiation by an insulin-sensitizing
agent. Mol. Pharmacol. 41, 393–398
10 Schwartz, S., Raskin, P., Fonseca, V. and
Graveline, J.F. (1998) Effect of troglitazone in insulin-treated patients with
type II diabetes mellitus. Troglitazone
and exogenous insulin study group.
New Engl. J. Med. 338, 861–866
14 Neuschwander-Tetri, B.A. et al. (1998)
Troglitazone-induced hepatic failure
leading to liver transplantation. A case
report. Ann. Intern. Med. 129, 38–41
4 Fujita, T. et al. (1983) Reduction of insulin resistance in obese and/or diabetic animals by 5-[4-(1-methylcyclohexylmethoxy)benzyl]-thiazolidine-2,
4-dione (ADD-3878, U-63,287, ciglitazone), a new antidiabetic agent. Diabetes 32, 804–810
adipogenic agent. J. Cell. Physiol. 134,
124–130
18 Hiragun, A., Sato, M. and Mitsui, H.
(1988) Preadipocyte differentiation in
vitro: identification of a highly active
25 Mangelsdorf, D.J. and Evans, R.M. (1995)
The RXR heterodimers and orphan receptors. Cell 83, 841–850
27 Lemberger, T., Desvergne, B. and
Wahli, W. (1996) Peroxisome proliferator-activated receptors: a nuclear receptor signaling pathway in lipid physiology. Annu. Rev. Cell Dev. Biol. 12,
335–363
28 Martin, G., Schoonjans, K., Lefebvre,
A.M., Staels, B. and Auwerx, J. (1997)
Coordinate regulation of the expression of the fatty acid transport protein
and acyl-CoA synthetase genes by
PPARa and PPARg activators. J. Biol.
Chem. 272, 28210–28217
29 Hu, E., Tontonoz, P. and Spiegelman, B.M.
(1995) Transdifferentiation of myoblasts
by the adipogenic transcription factors
PPARg and C/EBPa. Proc. Natl. Acad. Sci.
U. S. A. 92, 9856–9860
30 Forman, B.M. et al. (1995) 15-deoxy,
delta 12, 14-prostaglandin J2 is a ligand for the adipocyte determination
factor PPARg. Cell 83, 803–812
31 Kliewer, S.A. et al. (1995) A prostaglandin
J2 metabolite binds peroxisome proliferator-activated receptor g and promotes
adipocyte differentiation. Cell 83, 813–819
32 Yu, K. et al. (1995) Differential activation of peroxisome proliferatoractivated receptors by eicosanoids.
J. Biol. Chem. 270, 23975–23983
TEM Vol. 10, No. 1, 1999
33 Willson, T.M. et al. (1996) The structure–activity relationship between peroxisome proliferator-activated receptor
g and the antihyperglycemic activity of
thiazolidinediones. J. Med. Chem. 39,
665–668
34 Berger, J. et al. (1996) Thiazolidinediones produce a conformational
change in peroxisome proliferatoractivated receptor-g: binding and activation correlate with antidiabetic
actions in db/db mice. Endocrinology
137, 4189–4195
35 Kliewer, S.A., Umesono, K., Noonan, D.J.,
Heyman, R.A. and Evans, R.M. (1992)
Convergence of 9-cis retinoic acid and
peroxisome proliferator signalling
pathways through heterodimer formation of their receptors. Nature 358,
771–774
36 Schulman, I.G., Shao, G. and Heyman,
R.A. (1998) Transactivation by retinoid
X receptor-peroxisome proliferatoractivated receptor γ (PPARg) heterodimers: intermolecular synergy requires
only the PPARg hormone-dependent
activation function. Mol. Cell. Biol. 18,
3483–3494
37 Mukherjee, R. et al. (1997) Sensitization
of diabetes and obese mice to insulin
by retinoid X receptor agonists. Nature
386, 407–410
38 Vidal-Puig, A.J. et al. (1997) Peroxisome
proliferator-activated receptor gene expression in human tissues: effects of
obesity, weight loss, and regulation by
insulin and glucocorticoids. J. Clin.
Invest. 99, 2416–2422
39 DuBois, R.N. et al. (1998) The nuclear
eicosanoid receptor PPARg is aberrantly expressed in colonic cancers.
Carcinogenesis 19, 49–53
40 Fajas, L. et al. (1997) The organization,
promoter analysis, and expression of
the human PPARg gene. J. Biol. Chem.
272, 18779–18789
41 Mueller, E. et al. (1998) Terminal differentiation of human breast cancer
through PPARg. Mol. Cell. 1, 465–470
42 Michael, L.F., Lazar, M.A. and Mendelson,
C.R. (1997) PPARg1 expression is induced during cyclic AMP-stimulated
differentiation of alveolar type II pneumocytes. Endocrinology 138, 3695–3703
43 Greene, M.E. et al. (1995) Isolation of the
human peroxisome proliferator activated receptor gamma cDNA: expression in hematopoietic cells and chromosomal mapping. Gene Expr. 4,
281–299
44 Ricote, M., Li, A.C., Willson, T.M., Kelly,
C.J. and Glass, C.K. (1998) The peroxisome proliferator-activated receptor-g
is a negative regulator of macrophage
activation. Nature 391, 79–82
45 Tontonoz, P., Nagy, L., Alvarez, J.G.,
Thomazy, V.A. and Evans, R.M.
(1998) PPARg promotes monocyte/
macrophage differentiation and uptake
of oxidized LDL. Cell 93, 241–252
TEM Vol. 10, No. 1, 1999
46 Parks, K.S. et al. (1997) PPAR-g gene expression is elevated in skeletal muscle
of obese and type II diabetic subjects.
Diabetes 46, 1230–1234
59 Considine, R.V. et al. (1996) Serum immunoreactive-leptin concentrations in
normal-weight and obese humans. New
Engl. J. Med. 334, 292–295
47 Tontonoz, P., Hu, E., Graves, R.A.,
Budavari, A.I. and Spiegelman, B.M.
(1994) mPPARg2: tissue-specific regulator of an adipocyte enhancer. Genes
Dev. 8, 1224–1234
60 Zimmet, P.Z. et al. (1998) Is there a relationship between leptin and insulin
sensitivity independent of obesity? A
population-based study in the Indian
Ocean nation of Mauritius. Int. J. Obes.
Relat. Metab. Disord. 22, 171–177
48 Okuno, A. et al. (1998) Troglitazone increases the number of small adipocytes
without the change of white adipose
tissue mass in obese Zucker rats.
J. Clin. Invest. 101, 1354–1361
49 Wu, Z., Xie, Y., Morrison, R.F., Bucher,
N.L. and Farmer, S.R. (1998) PPARg induces the insulin-dependent glucose
transporter GLUT4 in the absence of
C/EBPa during the conversion of 3T3
fibroblasts into adipocytes. J. Clin.
Invest. 101, 22–32
50 Young, P.W. et al. (1995) Repeat treatment of obese mice with BRL49653, a
new potent insulin sensitizer, enhances
insulin action in white adipocytes.
Association with increased insulin
binding and cell-surface GLUT4 as
measured by photoaffinity labeling.
Diabetes 44, 1087–1092
51 Marin, P. et al. (1992) Uptake of glucose
carbon in muscle glycogen and adipose
tissue triglycerides in vivo in humans.
Am. J. Physiol. 263, E473–E480
52 Hotamisligil, G.S., Shargill, N.S. and
Spiegelman, B.M. (1993) Adipose expression of tumor necrosis factor-a: direct
role in obesity-linked insulin resistance. Science 259, 87–91
53 Uysal, K.T., Wiesbrock, S.M., Marion,
M.W. and Hotamisligil, G.S. (1997) Protection from obesity-induced insulin
resistance in mice lacking TNF-a
function. Nature 389, 610–614
54 Hofmann, C. et al. (1994) Altered gene expression for tumor necrosis factor-alpha
and its receptors during drug and
dietary modulation of insulin resistance. Endocrinology 134, 264–270
55 Szalkowski, D., White-Carrington, S.,
Berger, J. and Zhang, B. (1995) Antidiabetic thiazolidinediones block the
inhibitory effect of tumor necrosis
factor-a on differentiation, insulinstimulated glucose uptake, and gene
expression in 3T3-L1 cells. Endocrinology 136, 1474–1481
56 Peraldi, P., Xu, M. and Spiegelman, B.M.
(1997) Thiazolidinediones block tumor
necrosis factor-a-induced inhibition of
insulin signaling. J. Clin. Invest. 100,
1863–1869
57 Murase, K., Odaka, H., Suzuki, M., Tayuki,
N. and Ikeda, H. (1998) Pioglitazone timedependently reduces tumour necrosis
factor-a level in muscle and improves
metabolic abnormalities in Wistar fatty
rats. Diabetologia 41, 257–264
58 Zhang, Y. et al. (1994) Positional cloning
of the mouse obese gene and its human
homologue. Nature 372, 425–432
61 Kallen, C.B. and Lazar, M.A. (1996)
Antidiabetic thiazolidinediones inhibit
leptin (ob) gene expression in 3T3-Ll
adipocytes. Proc. Natl. Acad. Sci. U. S. A.
93, 5793–5796
62 DeVos, P. et al. (1996) Thiazolidinediones repress ob gene expression in
rodents via activation of peroxisome
proliferator-activated receptor gamma.
J. Clin. Invest. 98, 1004–1009
63 Zhang, B. et al. (1996) Down-regulation
of the expression of the Obese gene by
an antidiabetic thiazolidinedione in
Zucker diabetic fatty rats and db/db
mice. J. Biol. Chem. 271, 9455–9459
64 Boden, G. (1997) Role of fatty acids in
the pathogenesis of insulin resistance
and NIDDM. Diabetes 46, 1–10
65 Groop, L.C. et al. (1991) The role of
free fatty acid metabolism in the pathogenesis of insulin resistance in obesity
and noninsulin-dependent diabetes
mellitus. J Clin. Endocrinol. Metab. 72,
96–107
66 Schoojans, K., Martin, G., Staels, B. and
Auwerx, J. (1997) Peroxisome proliferator-activated receptors, orphans with
ligands and functions. Curr. Opin.
Lipidol. 8, 159–166
67 Burant, C.F. et al. (1997) Troglitazone
action is independent of adipose tissue.
J. Clin. Invest. 100, 2900–2908
68 Park, K.S. et al. (1998) Troglitazone
regulation of glucose metabolism in
human skeletal muscle cultures from
obese type II diabetic subjects. J. Clin.
Endocrinol. Metab. 83, 1636–1643
69 Adams, M., Reginato, M.J., Shao, D.,
Lazar, M.A. and Chatterjee, V.K.
(1997) Transcriptional activation by
PPARg is inhibited by phosphorylation
at a consensus mitogen-activated
protein kinase site. J. Biol. Chem. 272,
5128–5132
70 Reginato, M.J., Krakow, S.L., Bailey, S.T.
and Lazar, M.A. (1998) Prostaglandins
promote and block adipogenesis
through opposing effects on peroxisome proliferator-activated receptor g.
J. Biol. Chem. 273, 1855–1858
71 Hu, E., Kim, J.B., Sarraf, P. and
Spiegelman, B.M. (1996) Inhibition of
adipogenesis through MAP kinasemediated phosphorylation of PPARg.
Science 274, 2100–2103
72 Camp, H.S. and Tafuri, S.R. (1997)
Regulation of peroxisome proliferatoractivated receptor g activity by
mitogen-activated protein kinase.
J. Biol. Chem. 272, 13452–13457
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