The Phosphorylation Site Located in the A Region of Retinoic X Receptor α Is Required for the Antiproliferative Effect of Retinoic Acid (RA) and the Activation of RA Target Genes in F9 Cells

Mouse F9 embryocarcinoma cells constitute a well established cell autonomous model system for investigating retinoic acid (RA) signaling in vitro. RA induces the differentiation of F9 cells grown as monolayers into endodermal-like cells and decreases their rate of proliferation. Knock-out of the retinoic X receptor alpha (RXRalpha) gene abolishes endodermal differentiation and the induction of several endogenous RA-responsive genes. RXRalpha null cells are also drastically impaired in their antiproliferative response to RA. The role of the RXRalpha phosphorylation site located in the N-terminal A region (Ser(22)) has been investigated here by establishing cell lines re-expressing RXRalpha either wild type or mutated at the phosphorylation site (RXRalphaS22A) in a RXRalpha-null background. We show that Ser(22) is dispensable for RA-induced endodermal differentiation but is crucial for the expression of several RA-responsive genes. Ser(22) is also indispensable for the antiproliferative effect of RA and necessary for the RA-induced down-regulation of p21(CIP) and p27(KIP) CKIs proteins that are known to be involved in the control of cell cycle progression.