Specific and redundant functions of retinoid X receptor retinoic acid receptor heterodimers in differentiation, proliferation, and apoptosis of F9 embryonal carcinoma cells

We have generated F9 murine embryonal carcinoma cells in which either the retinoid X receptor (RXR)alpha and retinoic acid receptor (RAR)alpha genes or the RXR alpha and RAR gamma genes are knocked out, and compared their phenotypes with those of wild-type (WT), RXR alpha(-/-), RAR alpha(-/-), and RAR gamma(-/-) cells. RXR alpha(-/-)/RAR alpha(-/-) cells were resistant to retinoic acid treatment for the induction of primitive and parietal endodermal differentiation, as well as for antiproliferative and apoptotic responses, whereas they could differentiate into visceral endoderm-like cells, as previously observed for RXR alpha(-/-) cells. In contrast, RXR alpha(-/-)/RAR gamma(-/-) cells were defective for all three types of differentiation, as well as antiproliferative and apoptotic responses, indicating that RXR alpha and RAR gamma represent an essential receptor pair for these responses. Taken together with results obtained by treatment of WT and mutant F9 cells with RAR isotype- and panRXR-selective retinoids, our observations support the conclusion that RXR/RAR heterodimers are the functional units mediating the retinoid signal in vivo. Our results also indicate that the various heterodimers can exert both specific and redundant functions in differentiation, proliferation, and apoptosis. We also show that the functional redundancy exhibited between RXR isotypes and between RAR isotypes in cellular processes can be artifactually generated by gene knockouts. The present approach for multiple gene targeting should allow inactivation of any set of genes in a given cell.