Modulation of adipocyte genes by signal transducers and activators of transcription

Abstract

Members of the STAT transcription factor family are expressed in adipocytes, including STATs 1, 3, 5A, 5B, and 6. Although STATs 1 and 5 proteins are known to be induced during adipogenesis, the functions of these STATs in mature adipocytes are not known. Hence we have sought to identify adipocyte genes which are transcriptionally regulated by STATs to elucidate a role of these proteins in fat cells. We have characterized STAT binding sites in the promoters of four adipocyte genes, PPARγ2, LPL, FAS and C/EBPδ. PPARγ2 expression decreases in adipocytes following exposure to IFNγ, an activator of STAT1. IFNγ induces the binding of STAT1 to a site in the PPARγ2 promoter. Furthermore, the STAT1 binding site is required for IFNγ regulation of the PPARγ2 promoter in vitro. Although CT-1 and LIF induced STAT1 binding to the PPARγ2 promoter, only CT-1 substantially modulated expression of PPARγ2. We have also identified a STAT1 binding site in the promoter of LPL, which is bound by STAT1 following treatment of 3T3-L1 adipocytes with IFNγ. In addition, LPL protein levels are downregulated by IFNγ treatment. Treatment with PRL, an activator of STAT5A, decreased levels of FAS protein and mRNA in adipocytes. Regulation of the rat FAS promoter by PRL required a region of the promoter which contained a STAT5 binding site. Binding to this site by STAT5A was activated by PRL treatment and was highly specific. Finally, in our analysis on the effects of LIF on adipocytes, we determined that the expression SOCS3 and C/EBPδ mRNA transiently increases following treatment of adipocytes with LIF. We identified three STAT1 binding sites within the C/EBPδ promoter, which we hypothesize mediate the induction of C/EBPδ by LIF. Although LIF did not profoundly affect adipogenesis or basal and insulin-stimulated glucose uptake, chronic treatment with LIF abrogated the level of SREBP1 and FAS proteins. In summary, our studies suggest that STAT1 and STAT5 serve to limit synthesis of lipids in mature fat cells, limiting expansion of adipocytes and accretion of adipose tissue. The identification of PPARγ, FAS, and LPL as STAT-regulated genes provides insight into the molecular mechanisms of energy homeostasis, adipocyte physiology and the action of cytokines in fat.