FOXO1 Represses Peroxisome Proliferator-activated Receptor-γ1 and -γ2 Gene Promoters in Primary Adipocytes

Abstract

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...

FOXO-binding partners: it takes two to tango

“…Reducing transcription of the glucose reporter GLUT4 by PPARg can lead to a decrease in insulin sensitivity in adipocytes (Armoni et al, 2003). Thus in addition to inhibiting differentiation, FOXO1 activation leads to upregulated GLUT4 levels and a further increase in cellular insulin sensitivity (Armoni et al, 2006(Armoni et al, , 2007. Evidence also suggests that FOXO1 might function as a coactivator of PPARa in myocytes.…”

The FoxO transcription factors and metabolic regulation

“…Therefore, reducing FoxO1 mRNA expression may lead to improvements in Pparc activity. Furthermore, it has been reported that FoxO1 can inhibit PPARc1 and PPARc2 promoter activity through direct interaction between FoxO1 and PPARc promoter [94]. Therefore, FoxO1 may suppress PPARc expression at the transcriptional level.…”

Peroxisome Proliferator-activated Receptor γ (PPARγ) and Its Target Genes Are Downstream Effectors of FoxO1 Protein in Islet β-Cells

“…In adipocytes, PPAR␥ gene expression is under inhibitory control by FoxO1 (11,12). FoxO1 is a member of the family of winged-helix/forkhead transcription factors that serve important roles in cellular differentiation, proliferation, apoptosis, and the response to cellular stress in many tissues.…”