Tm. Underhill et al., CONSTITUTIVELY ACTIVE RETINOID RECEPTORS EXHIBIT INTERFAMILY AND INTRAFAMILY PROMOTER SPECIFICITY, Molecular endocrinology, 8(3), 1994, pp. 274-285
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.