Mouse F9 embryocarcinoma (EC) cells
constitute a well established cell-autonomous model
system for investigating retinoid signaling in vitro as,
depending on culture conditions, retinoic acid (RA) can
induce their differentiation into either primitive, parietal
or visceral extraembryonic endoderm-like cells. These
RA-induced differentiations are accompanied by
decreases in proliferation rates, modifications of
expression of subsets of RA-target genes, and induction
of apoptosis. To elucidate the roles played by the
multiple retinoid receptors (RARs and RXRs) in
response to RA treatments, F9 EC cells lacking one or
severa1 RARs or RXRs were engineered through
homologous recombination. Mutated RARs and/or
RXRs were then reexpressed in given RAR or RXR null
backgrounds. WT and mutant cells were also treated
with different combinations of ligands selective for
RXRs and/or for each of the three RAR isotypes. These
studies lead to the conclusion that most RA-induced
events (e.g. primitive and visceral differentiation, growth
arrest, apoptosis and activation of expression of a
number of genes) are transduced by RARy/RXRa
heterodimers, whereas some other events (e.g. parietal
differentiation) are mediated by RARa/RXRa
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 RARy/RXRa heterodimers, the
phosphorylation of RARy is necessary for triggering
primitive differentiation, while that of RXRa is required
for growth arrest. On the other hand, phosphorylation of
RARa is necessary for parietal differentiation. Thus,
retinoid receptors are sophisticated signal integrators that
transduce not only the effects of their cognate ligands,
but also those of ligands that bind to membrane
receptors