FOXO1 and peroxisome proliferator-activated receptor-␥ (PPAR␥) are crucial transcription factors that regulate glucose metabolism and insulin responsiveness in insulin target tissues. We have shown that, in primary rat adipocytes, both factors regulate transcription of the insulin-responsive GLUT4 gene and that PPAR␥2 detachment from the GLUT4 promoter upon thiazolidinedione binding up-regulates GLUT4 gene expression, thus increasing insulin sensitivity (Armoni, M., Kritz, N., Harel, C., Bar-Yoseph, F., Chen, H., Quon, M. J., and Karnieli, E. (2003) J. Biol. Chem. 278, 30614 -30623). However, the mechanisms regulating PPAR␥ gene transcription are largely unknown. We studied the effects of FOXO1 on human PPAR␥ gene expression in primary rat adipocytes and found that both genes are endogenously expressed. FOXO1 coexpression dosedependently repressed transcription from either the PPAR␥1 or PPAR␥2 promoter reporter by 65%, whereas insulin (100 nM, 20 -24 h) either partially or completely reversed this effect. Phosphorylation-defective FOXO1 mutants T24A, S256A, S319A, and T24A/S256A/S319A still repressed the PPAR␥1 promoter and partially lost their effects on the PPAR␥2 promoter in either basal or insulin-stimulated cells. Use of DNA binding-defective FOXO1 (H215R) indicated that this domain is crucial for FOXO1 repression of the PPAR␥2 (but not PPAR␥1) promoter. Progressive 5-deletion and gel retardation analyses revealed that this repression involves direct and specific binding of FOXO1 to the PPAR␥2 promoter; chromatin immunoprecipitation analysis confirmed that this binding occurs in cellulo. We suggest a novel paradigm to increase insulin sensitivity in adipocytes in which FOXO1 repression of PPAR␥, the latter being a repressor of the GLUT4 promoter, consequently leads to GLUT4 derepression/up-regulation, thus enhancing cellular insulin sensitivity. The newly identified FOXO1-binding site on the PPAR␥2 promoter may serve as a therapeutic target for type 2 diabetes.The peroxisome proliferator-activated receptor (PPAR) 2 family of nuclear receptors and the FOXO (forkhead box class O) family of winged helix/forkhead box factors are two key families of transcription factors that dominate the regulation of glucose metabolism and insulin responsiveness in insulin target tissues. Members of both families are crucial for a multitude of biological processes, including the cell cycle, cell death, differentiation, and metabolism, and have prominent roles in insulin signaling pathways. A convergence of nuclear receptors and forkhead pathways in general and of FOXO1 and PPAR␥ in particular has been implicated in the pathophysiological states of insulin resistance and diabetes, supporting the importance of these transcription factors (1, 2). However, despite their importance to glucose homeostasis and adipocyte differentiation, the molecular mechanism(s) regulating transcription of the PPAR␥ gene and the roles of both PPAR␥ and FOXO1 transcription factors in these processes are not fully known.The PPAR family of ligand-activ...

The synthetic thiazolidinedione ligands of peroxisome proliferator-activated receptor-␥ (PPAR␥) improve insulin sensitivity in type II diabetes and induce GLUT4 mRNA expression in fat and muscle. However, the molecular mechanisms involved are still unclear. We studied the regulatory effects of PPAR␥ and its ligands on GLUT4 gene expression in primary rat adipocytes and CHO-K1 cells cotransfected with PPAR␥ and the GLUT4 promoter reporter. PPAR␥1 and PPAR␥2 repressed the activity of the GLUT4 promoter in a dose-dependent manner. Whereas this repression was augmented by the natural ligand 15⌬-prostaglandin J 2 , it was completely alleviated by rosiglitazone (Rg). Ligand binding-defective mutants PPAR␥1-L468A/E471A and PPAR␥2-L496A/ E499A retained the repression effect, which was unaffected by Rg, whereas the PPAR␥2-S112A mutant exhibited a 50% reduced capacity to repress GLUT4 promoter activity. The ؊66/؉163 bp GLUT4 promoter region was sufficient to mediate PPAR␥ inhibitory effects. The PPAR␥/retinoid X receptor-␣ heterodimer directly bound to this region, whereas binding was abolished in the presence of Rg. Thus, we show that PPAR␥ represses transcriptional activity of the GLUT4 promoter via direct and specific binding of PPAR␥/retinoid X receptor-␣ to the GLUT4 promoter. This effect requires an intact Ser 112 phosphorylation site on PPAR␥ and is completely alleviated by Rg, acting via its ligand-binding domain. These data suggest a novel mechanism by which Rg exerts its antidiabetic effects via detaching PPAR␥ from the GLUT4 gene promoter, thus leading to increased GLUT4 expression and enhanced insulin sensitivity.

No abstract

Here, we examined the chronic effects of two cannabinoid receptor-1 (CB1) inverse agonists, rimonabant and ibipinabant, in hyperinsulinemic Zucker rats to determine their chronic effects on insulinemia. Rimonabant and ibipinabant (10 mg·kg⁻¹·day⁻¹) elicited body weight-independent improvements in insulinemia and glycemia during 10 wk of chronic treatment. To elucidate the mechanism of insulin lowering, acute in vivo and in vitro studies were then performed. Surprisingly, chronic treatment was not required for insulin lowering. In acute in vivo and in vitro studies, the CB1 inverse agonists exhibited acute K channel opener (KCO; e.g., diazoxide and NN414)-like effects on glucose tolerance and glucose-stimulated insulin secretion (GSIS) with approximately fivefold better potency than diazoxide. Followup studies implied that these effects were inconsistent with a CB1-mediated mechanism. Thus effects of several CB1 agonists, inverse agonists, and distomers during GTTs or GSIS studies using perifused rat islets were unpredictable from their known CB1 activities. In vivo rimonabant and ibipinabant caused glucose intolerance in CB1 but not SUR1-KO mice. Electrophysiological studies indicated that, compared with diazoxide, 3 μM rimonabant and ibipinabant are partial agonists for K channel opening. Partial agonism was consistent with data from radioligand binding assays designed to detect SUR1 K(ATP) KCOs where rimonabant and ibipinabant allosterically regulated ³H-glibenclamide-specific binding in the presence of MgATP, as did diazoxide and NN414. Our findings indicate that some CB1 ligands may directly bind and allosterically regulate Kir6.2/SUR1 K(ATP) channels like other KCOs. This mechanism appears to be compatible with and may contribute to their acute and chronic effects on GSIS and insulinemia.

Hyperlipidemia (HL) impairs cardiac glucose homeostasis, but the molecular mechanisms involved are yet unclear. We examined HL-regulated GLUT4 and peroxisome proliferator-activated receptor (PPAR) ␥ gene expression in human cardiac muscle. Compared with control patients, GLUT4 protein levels were 30% lower in human cardiac muscle biopsies from patients with HL and/or type 2 diabetes mellitus, whereas GLUT4 mRNA levels were unchanged. PPAR␥ mRNA levels were 30 -50% lower in patients with HL and/or diabetes mellitus type 2 than in controls. Reporter studies in H9C2 cardiomyotubes showed that HL in vitro, induced by high levels of arachidonic (AA) stearic, linoleic, and oleic acids (24 h, 200 M) repressed transcription from the GLUT4 promoter; AA also repressed transcription from the PPAR␥1 and PPAR␥2 promoters. Co-expression of PPAR␥2 repressed GLUT4 promoter activity, and the addition of AA further enhanced this effect. 5-Deletion analysis revealed three GLUT4 promoter regions that accounted for AAmediated effects: two repression-mediating sequences at ؊443/؊423 bp and ؊222/؊197 bp, the deletion of either or both of which led to a partial derepression of promoter activity, and a third derepression-mediating sequence at ؊612/؊587 bp that was required for sustaining this derepression effect. Electromobility shift assay further shows that AA enhanced binding to two of the three regions of cardiac nuclear protein(s), the nature of which is still unknown. We propose that HL, exhibited as a high free fatty acid level, modulates GLUT4 gene expression in cardiac muscle via a complex mechanism that includes: (a) binding of AA mediator proteins to three newly identified response elements on the GLUT4 promoter gene and (b) repression of GLUT4 and the PPAR␥ genes by AA.Glucose transport is the rate-limiting step for glucose metabolism in the heart (1). Under resting conditions, the heart derives about 70% of its energy from the oxidation of lipids and only 30% from glycolysis and glucose oxidation (1). Pathological states such as ischemia, hypertrophy, and congestive heart failure render the heart increasingly dependent on glucose to meet its metabolic demands (2-4). Although patients with DM2 4 and/or HL are at higher risk of developing coronary atherosclerosis, little is known about how these risk factors affect glucose homeostasis in hCM. In critically hospitalized patients, intensive normalization of plasma glucose levels was shown to be most beneficial way to reduce the size of the ischemic zone during coronary ischemia (5).Insulin stimulation of glucose uptake in muscle and adipose tissue includes translocation of insulin-sensitive glucose transporters (GLUT4) from intracellular pools to the cell surface (6 -8). Reduced cellular content of GLUT4 is characteristic of DM2 and insulin resistance (9, 10). Stresses such as ischemia, hypoxia, and high frequency contraction can also modulate GLUT4 expression (11). Although GLUT4 knock-out mice are not diabetic, they do exhibit abnormalities in glucose and lipid metabolism, an...