Specific activation of retinoic acid receptors (RARs) and retinoid X receptors reveals a unique role for RAR gamma in induction of differentiation and apoptosis of S91 melanoma cells

Retinoic acid (RA) and 9-cis-RA induce growth arrest and differentiation of S91 melanoma cells. RA activates retinoic acid receptors (RARs), whereas 9-cis-RA activates both RARs and retinoid X receptors (RXRs). Both classes of receptors function as ligand-dependent transcription factors. S91 melanoma cells contain mRNA for RXR alpha, RXR beta, RAR alpha, RAR gamma, and RAR beta in low levels. Among these, only RAR beta gene transcription is induced by retinoids. However, at present the individual role(s) for each RXR and RAR isoform in these processes is unclear. We assessed the function of all isoforms in the S91 melanoma model by using RXR and RAR isoform-specific retinoids to study their effects on cell growth, RAR beta expression, and differentiation. Activation of each of the endogenous RXR or RAR isoforms induces RAR beta gene expression, and blocks cellular proliferation. However, only the RAR gamma-ligands cause additional differentiation toward a melanocytic phenotype, which coincides with substantial apoptosis well before morphological changes are apparent. Apoptosis is completely dependent on de novo protein synthesis but cannot be induced by changes in activities of AP-1, protein kinase C, and protein kinase A, nor can it be blocked by the presence of the antioxidant glutathione. These results argue against a specific role for RAR beta, but suggest that RAR gamma has a critical role in a genetic switch between melanocytes and melanoma, and induction of ligand-dependent apoptosis.