F9 embryocarcinoma cells: a cell autonomous model to study the functional selectivity of RARs and RXRs in retinoid signaling

Mouse F9 embryocarcinoma (EC) cells constitute a well established cell-auto nomous model system for investigating retinoid signaling in vitro as, depen ding on culture conditions, retinoic acid (RA) can induce their differentia tion into either primitive, parietal or visceral extraembryonic endoderm-li ke cells. These RA-induced differentiations are accompanied by decreases in proliferation rates, modifications of expression of subsets of RA-target g enes, and induction of apoptosis. To elucidate the roles played by the mult iple retinoid receptors (RARs and RXRs) in response to RA treatments, F9 EC cells lacking one or several RARs or RXRs were engineered through homologo us recombination. Mutated RARs and/or RXRs were then reexpressed in given R AR or RXR null backgrounds. WT and mutant cells were also treated with diff erent combinations of ligands selective for RXRs and/or for each of the thr ee RAR isotypes. These studies lead to the conclusion that most RA-induced events (e.g. primitive and visceral differentiation, growth arrest, apoptos is and activation of expression of a number of genes) are transduced by RAR gamma /RXR alpha heterodimers, whereas some other events (e.g. parietal di fferentiation) are mediated by RAR alpha /RXR alpha heterodimers. They also demonstrate that both AF-1 and AF-2 activation functions of RARs and RXRs, as well as their phosphorylation, are differentially required in these RA- induced events. In RAR gamma /RXR alpha heterodimers, the phosphorylation o f RAR gamma is necessary for triggering primitive differentiation, while th at of RXR alpha is required for growth arrest. On the other hand, phosphory lation of RAR alpha is necessary for parietal differentiation. Thus, retino id receptors are sophisticated signal integrators that transduce not only t he effects of their cognate ligands, but also those of ligands that bind to membrane receptors.