CONSTITUTIVELY ACTIVE RETINOID RECEPTORS EXHIBIT INTERFAMILY AND INTRAFAMILY PROMOTER SPECIFICITY

Retinoid receptors are ligand activated transcription factors that reg ulate gene transcription through a complex network of interactions wit h members of the nuclear hormone receptor superfamily. Although ligand is required for trans-activation, addition of ligand to mammalian cel ls in vitro complicates the study of individual activated retinoid rec eptors. In order to circumvent this problem we have constructed a seri es of retinoid receptors which do not require ligand for trans-activat ion. This was accomplished by fusing the acidic activation domain of t he herpes simplex viral protein VP16 to the carboxyl terminus of indiv idual retinoid receptors. All of the chimeric receptors were found to exhibit constitutive trans-activation activity in CV-1 and P19 cells w hen cotransfected with a reporter that contained a trimerized retinoic acid receptor-beta 2 (RAR beta 2) retinoic acid response element. Fur ther analysis conducted on reporters containing either the RAR beta 2 promoter or the rat cellular retinol binding protein II (rCRBPII) prom oter showed that promoter specificity was well conserved between the c himeric receptors in the absence of exogenous retinoid and their ligan d-induced native counterparts. Moreover, on the RAR beta 2 promoter re porter construct, the chimeric retinoid receptors displayed both cell type and inter- and intrafamily differences in trans-activation, where as, trans-activation of the rCRBPII in the absence of exogenous ligand in CV-1 and P19 cells was found to be stimulated only by chimeric ret inoid X receptor-alpha (RXR alpha). In P19 cells trans-activation of t he rCRBPII promoter by RXR alpha v in the absence of exogenous ligand was inhibited by RAR alpha and the constitutive forms of RAR alpha, RA R beta, RAR gamma, RXR beta, and to a lesser extent RXR gamma.