ENDOGENOUS RETINOIC-ACID RECEPTOR (RAR) RETINOID-X RECEPTOR (RXR) HETERODIMERS ARE THE MAJOR FUNCTIONAL FORMS REGULATING RETINOID-RESPONSIVE ELEMENTS IN ADULT HUMAN KERATINOCYTES - BINDING OF LIGANDS TO RAR ONLY IS SUFFICIENT FOR RAR-RXR HETERODIMERS TO CONFER LIGAND-DEPENDENT ACTIVATION OF HRAR-BETA-2 RARE (DR5)/

We have examined how retinoic acid receptors (RARs) and retinoid X rec eptors (RXRs) at physiological concentrations regulate distinct retino id-responsive elements, hRAR beta 2/beta RARE (DR5) and rCRBPII/RXRE ( DR1), in keratinocytes from human skin, a major retinoid target. In vi tro, endogenous RAR gamma and RXRs bound to these elements as heterodi mers (RAR-RXR) but not homodimers (RAR.RAR or RXR.RXR). In cultured ke ratinocytes, all-trans retinoic acid, 9-cis retinoic acid, and CD367 a ctivated beta RARE but not RXRE via endogenous RAR.RXR (ED(50) = 2.3, 3.8, and 0.3 nM respectively) whereas SR11237 showed no significant ef fect. All-trans retinoic acid, 9-cis retinoic acid, and SR11237 activa ted RXRE via overexpressed RXR.RXR (ED(50) = 110, 120, and 11 nM, resp ectively), indicating interconversion between retinoic acid isomers, w hereas co-overexpression of RAR alpha or RAR gamma suppressed this act ivation. Unlike 9cRA, CD367 neither induced formation of nor activated RXR.RXR. Overexpression of RAR or RXR mutated in transactivation doma in AF-2 suppressed endogenous receptor activity over beta RARE. Our da ta suggest that 1) in keratinocytes, RAR.RXR-mediated pathway dominate s over that mediated by RXR.RXR; 2) RAR-selective CD367 and RXR-select ive SR11237 can be used to identify these two distinct pathways, respe ctively; 3) beta RARE is mainly regulated by RAR.RXR, in which RAR alo ne confers ligand inducibility whereas AF-2 of unliganded RXR is requi red for transactivation by liganded RAR AF-2; 4) lack of RXRE activity in keratinocytes is due to low endogenous levels of RXR.RXR and inhib ition by RAR.RXR; and 5) interaction among RXRs is much lower than tha t between RAR and RXR.