The identification and characterization of a STAT 1 binding site in the PPARgamma2 promoter

Interferon-gamma (IFNgamma) has been shown to decrease the expression of peroxisome proliferator activated receptor-gamma (PPARgamma) in fat cells by blocking the synthesis and increasing the degradation of this transcription factor. Since IFNgamma is a potent activator of STAT 1, we searched for IFNgamma-sensitive binding sites in the PPARgamma promotors. A region of the murine PPARgamma2 promoter was identified that bound nuclear protein from adipocyte nuclei that had been acutely treated with IFNgamma. Supershift analysis revealed that STAT 1, and no other STATs present in the adipocyte nucleus, was capable of binding to this site within the PPARgamma2 promoter. NIH 3T3 and 3T3-L1 cells were transiently transfected with a PPARgamma2 promoter reporter construct, which contained the STAT 1 binding site. Treatment of these cells with IFNgamma resulted in a decrease in reporter activity, demonstrating the modulation of the PPARgamma2 promoter by IFNgamma. We also examined the ability of leukemia inhibitory factor (LIF) to regulate binding at this site. LIF, a potent activator of STAT3 and a weak activator of STAT 1 in these cells, resulted in some binding to the IFNgamma responsive element in the PPARgamma2 promoter that was mediated by STAT 1. Therefore, we examined the ability of LIF to regulate PPARgamma mRNA and observed that LIF, unlike IFNgamma, had little effect on PPARgamma expression. These results and our previous work suggest that cytokine induced STAT 1 homodimers modulate the transcriptional repression of PPARgamma2 in adipocytes.