9-CIS-RETINOIC ACID IS A NATURAL ANTAGONIST FOR THE RETINOIC ACID RECEPTOR RESPONSE PATHWAY

The pleiotropic activities of retinoids are mediated by two types of n uclear receptors, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs). All-trans-retinoic acid (RA) transcriptionally act ivates RARs, but not RXRs, whereas its natural stereoisomer, 9-cis-RA, is the ligand for RXRs. Here, we demonstrate that 9-cis-RA did not tr anscriptionally activate RARs, whereas in the presence of all-trans-RA the transactivation of RARs was inhibited in a dose-dependent manner by 9-cis-RA. RAR homodimer complexes were destabilized in vitro in the presence of 9-cis-RA. This suggests that 9-cis-RA may be a natural an tagonist of all-trans-RA for binding to RAR complexes. The levels of 9 -cis-RA may determine by which pathway the transcription of retinoid-r esponsive genes is modulated.