Binding of 9-cis-retinoic acid and all-trans-retinoic acid to retinoic acid receptors alpha, beta, and gamma. Retinoic acid receptor gamma binds all-trans-retinoic acid preferentially over 9-cis-retinoic acid

Both 9-cis-retinoic acid (RA) and all-trans-RA (t-RA) compete for [3H]9-cis-RA binding to RA receptors (RAR alpha, beta, and gamma) in nucleosol fractions from transiently transfected COS-1 cells with IC50 values of approximately 12 and 5 nM, respectively. Curiously, 9-cis-RA competes for [3H]t-RA binding to mouse RAR alpha, beta, and gamma with IC50 values of 31, 8, and 60 nM, respectively, while t-RA itself does not exhibit such differential competition (IC50 values for RARs, 5 nM). A similar pattern is observed with human retinoic acid receptors (RARs). Differential binding of 9-cis-RA to the RAR beta and gamma receptors is also found following in vitro transcription and translation of these receptors. Displacement assays demonstrate that t-RA exhibits similar off-rates for RAR alpha, beta, and gamma. However, 9-cis-RA is 6-fold more rapidly displaced from RAR gamma than from RAR beta. When RAR-transfected COS-1 cells are incubated with [3H]t-RA, [3H]-9-cis-RA or various mixtures of these two radioligands, high performance liquid chromatography analysis demonstrates that the ligands bound in nucleosol fractions from RAR beta-transfected cells reflect the isomer content of the media. However, in identical whole cell assays, nucleosol fractions from RAR gamma-transfected cells preferentially bind t-RA over 9-cis-RA, consistent with the in vitro data. These binding kinetics in vitro and in whole cells suggest that there could be differences in the interactions of the receptor subtypes with the endogenous retinoic acids under physiologic conditions.