TRANSACTIVATION PROPERTIES OF RETINOIC ACID AND RETINOID-X RECEPTORS IN MAMMALIAN-CELLS AND YEAST - CORRELATION WITH HORMONE-BINDING AND EFFECTS OF METABOLISM

The binding affinities of 9-cis-retinoic acid (9-cis-RA) and all-trans -retinoic acid (t-RA) for retinoic acid receptors (RAR) alpha, beta, a nd gamma and for retinoid X receptors (RXR) alpha, beta, and gamma wer e determined using the recombinant receptor proteins and were compared with each hormone's ability to activate transcription through the rec eptors in mammalian and yeast cell systems. 9-cis-RA bound to both the RXRs (K(d) = 1.4-2.4 nM) and the RARs (K(d) = 0.2-0.8 nM). The abilit y of 9-cis-RA to bind to the RARs and RXRs correlated with its ability to produce similar transactivation profiles with these receptors in m ammalian and yeast cell assays. t-RA bound to the RARs (K(d) = 0.2-0.4 nM) and activated transcription through the RARs in mammalian and yea st cells. In contrast, while t-RA did not bind to the RXRs, it did act ivate the RXRs, albeit less potently than 9-cis-RA, in mammalian cells . In yeast, however, the RXRs activated transcription only in the pres ence of 9-cis-RA, not with t-RA. While RARgamma is activated in yeast by either t-RA or 9-cis-RA, the overall level of transcription was inc reased upon the addition of hormone-occupied RXR. Metabolism studies s uggest that while there was no cell-dependent interconversion between t-RA and 9-cis-RA in yeast, there was cell-dependent conversion of 9-c is-RA to t-RA in mammalian cells.