Phosphorylation of the Human Retinoid X Receptor α at Serine 260 Impairs Coactivator(s) Recruitment and Induces Hormone Resistance to Multiple Ligands.

The retinoid X receptor α (RXRα) is a member of the nuclear receptor superfamily that regulates transcription of target genes through heterodimerization with several partners, including peroxisome proliferator-activated receptor, retinoic acid receptor, thyroid receptor, and vitamin D receptor (VDR). We have shown previously that signaling through VDR·RXRα heterodimers was attenuated in ras-transformed keratinocytes due to phosphorylation of serine 260 of the RXRα via the activated Ras-Raf-MAPK cascade in these cells. In this study we demonstrate that phosphorylation at serine 260, a site located in the omega loop-AF-2 interacting domain of RXRα, inhibits signaling through several heterodimeric partners of the RXRα. The inhibition of signaling results in reduced transactivational response to ligand presentation and the reduced physiological response of growth inhibition not only of 1,25-dihydroxyvitamin D3 but also of retinoic acid receptor a ligands and LG1069 (an RXRα ligand). This partial resistance to ligands could be reversed by inhibition of MAPK activity or by overexpression of a non-phosphorylable RXRα mutant at serine 260 (RXRα Ser-260 → Ala). Importantly, phosphorylation of RXRα at serine 260 impaired the recruitment of DRIP205 and other coactivators to the VDR·RXRα complex. Chromatin immunoprecipitation and pulldown assays further demonstrated that coactivator recruitment to the VDR·RXR complex could be restored by treatment with a MAPK inhibitor. Our data suggest that phosphorylation at serine 260 plays a critical role in inducing hormone resistance of RXRα-mediated signaling likely through structural changes in the H1-H3 omega loop-AF2 coactivator(s) interacting domain. [ABSTRACT FROM AUTHOR]