Peroxisome proliferator-activated receptor γ down-regulates follistatin in intestinal epithelial cells through SP1

Activation of peroxisome proliferator-activated receptor γ (PPARγ) down-regulates the expression of follistatin mRNA in intestinal epithelial cells in vivo. The mechanism of PPARγ-mediated down-regulation of follistatin was investigated using non-transformed, rat intestinal epithelial cells (RIE-1). RIE cells expressed activin A, the activin receptors ActRI and ActRII, and the follistatin-315 mRNA. RIE-1 cells responded to endogenous activin A, and this response was antagonized by follistatin, as evidenced by changes in cell growth and regulation of an activin-responsive reporter. Using RIE-1 cells, we show that activation of PPARγ by rosiglitazone reduced follistatin mRNA levels in a dose- and concentration-dependent manner. Down-regulation of follistatin by rosiglitazone required the DNA binding domain of PPARγ and was dependent upon dimerization with the retinoid X receptor. Inhibition of follistatin expression by rosiglitazone was not associated with decreased follistatin mRNA stability, suggesting that regulation may be at the promoter level. Analysis of the follistatin promoter revealed consensus binding sites for AP-1, AP-2, and Sp1. Targeting the AP-1 pathway with SP600125, an inhibitor of JNK, and TAM67, a dominant negative c-Jun, had no effect on PPARγ-mediated down-regulation of follistatin. However, the follistatin promoter was dramatically regulated by Sp1, and this regulation was inhibited by PPARγ expression. Knockdown of Sp1 expression relieved repression of follistatin levels by rosiglitazone. Moreover, PPARγ was found to interact with Sp1 and repress its transcriptional activation function. Collectively, our data indicate that repression of Sp1 transcriptional activity by PPARγ is the underlying mechanism responsible for PPARγ-mediated regulation of follistatin expression.