ACTIVATION OF THE PROLACTIN GENE BY PEROXISOME PROLIFERATOR-ACTIVATEDRECEPTOR-ALPHA APPEARS TO BE DNA BINDING-INDEPENDENT

Although the effects of the peroxisome proliferator-activated receptor s (PPARs) have been studied primarily in adipocytes and liver, the wid e distribution of these receptors suggests that they might also play a role in other cell types, We present evidence that PPAR activators st imulate the expression of the prolactin gene in pituitary GH4C1 cells. Transfection assays in non-pituitary HeLa cells showed that stimulati on of the prolactin promoter by PPAR alpha requires the presence of th e transcription factor GHF-1 (or Pit-1), Proximal promoter sequences c onfer responsiveness to PPAR alpha, and activation by this receptor is lost concomitantly with the response to GHF-1. Surprisingly, expressi on of the retinoid X receptor (RXR) abolishes stimulation by PPAR alph a, Furthermore, the promoter region that confers PPAR alpha responsive ness does not contain a PPAR response element. This suggests that the transcriptional effect of PPAR alpha might be mediated by protein-prot ein interactions rather than by binding of PPAR/RXR to the promoter. A direct interaction between PPAR alpha and GHF-1 was confirmed by in v itro binding studies, Expression of the coactivators SRC-1 and CREB-bi nding protein, which bind to PPAR, also enhanced the responsiveness of the prolactin promoter to PPAR alpha. Furthermore, CREB-binding prote in also significantly increased activation by GHF-1, and both proteins associated in vitro, Thus, PPAR alpha, a receptor that normally acts as a ligand-dependent transcription factor by binding to specific DNA sequences in one context, can also stimulate the prolactin promoter by association with GHF-1 and coactivator proteins.