THE OPPOSING EFFECTS OF RETINOIC ACID AND PHORBOL ESTERS CONVERGE TO A COMMON RESPONSE ELEMENT IN THE PROMOTER OF THE RAT CHOLESTEROL 7-ALPHA-HYDROXYLASE GENE (CYP7A)

The activity of the rat CYP7A/luciferase reporter gene was increased f ive-fold by all-trans retinoic acid (atRA) or 9-cis retinoic acid (9cR A) in transient transfection assay in HepG2 cells. Cotransfection with retinoid X receptor (RXR) stimulated the promoter activity in the abs ence of ligand. however, addition of atRA inhibited the transcriptiona l activity. Cotransfection with retinoic acid receptor (RAR) did not h ave much effect on CYP7A promoter activity. The CYP7A promoter, when l inked upstream to the SV40/luciferase reporter gene, strongly represse d the phorbol 12-myristate 13-acetate (PMA)-stimulated SV40/luciferase reporter gene activity. The regions conferring the effects of RA and PMA were mapped to nt - 176/-117 and nt - 148/-129, respectively. Seve ral direct repeats of hormone response element (AGTTCA) in this region are required for RA response. AP-1 like sequences are located within the region responding to both RA and PMA. Site-directed mutagenesis of the AP-1 site abolished the effects of both RA and phorbol esters. Re tinoic acid effect was antagonized by PMA. Moreover, cotransfection of Fos and Jun expression vectors blunted the stimulatory effect of reti noic acid on the CYP7A/luciferase gene activity. Therefore, effects of two different signal transduction pathways converge to a common respo nse element. This regulatory cross-talk may be involved in bile acid r epression and regulates CYP7 gene transcription in the liver. (C) 1996 Academic Press, Inc.