Retinoid receptors are
ligand activated
transcription factors that regulate gene transcription through a complex network of interactions with members of the
nuclear hormone receptor superfamily. Although
ligand is required for trans-activation, addition of
ligand to mammalian cells in vitro complicates the study of individual activated
retinoid receptors. In order to circumvent this problem we have constructed a series of
retinoid receptors which do not require
ligand for trans-activation. This was accomplished by fusing the acidic activation domain of the
herpes simplex viral protein VP16 to the carboxyl terminus of individual
retinoid receptors. All of the chimeric receptors were found to exhibit constitutive trans-activation activity in
CV-1 and P19 cells when cotransfected with a reporter that contained a trimerized
retinoic acid receptor-beta 2 (
RAR beta 2)
retinoic acid response element. Further analysis conducted on reporters containing either the
RAR beta 2 promoter or the rat
cellular retinol binding protein II (rCRBPII) promoter showed that promoter specificity was well conserved between the chimeric receptors in the absence of exogenous
retinoid and their
ligand-induced native counterparts. Moreover, on the
RAR beta 2 promoter reporter construct, the chimeric
retinoid receptors displayed both cell type and inter- and intrafamily differences in trans-activation, whereas, 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
retinoid X receptor-alpha (RXR alpha). In P19 cells trans-activation of the rCRBPII promoter by RXR alpha v in the absence of exogenous
ligand was inhibited by RAR alpha and the constitutive forms of RAR alpha,
RAR beta,
RAR gamma, RXR beta, and to a lesser extent RXR gamma.