Inhibition of ligand induced promoter occupancy in vivo by a dominant negative RXR

Background: Retinoid X receptors (RXRs) heterodimerize with other nuclear hormone receptors and control ligand mediated transcription. To address how RXRs function as heterodimers, we investigated activities of truncated RXRα and RXRβ that lack ≈ 20 conserved C-terminal amino acids. Results: The truncated RXRs formed heterodimers and bound to respective DNA elements in vitro. By transient reporter assays we found that these RXRs act as dominant negative receptors and inhibit ligand dependent transcription by the retinoic acid receptor (RAR) and vitamin D receptor. P19 embryonal carcinoma cells stably expressing the truncated RXRβ (termed ΔC2) were deficient in activating the endogenous RARβ gene and an RA responsive reporter. To study the dominant negative activity of ΔC2 further, genoniic footprinting analysis was performed for the RARβ₂ promoter. In control P19 clones, the RA responsive element (RARE) and other elements in the promoter were protected after RA treatment. However, in ΔC2 clones RA-induced protection was markedly inhibited at all elements. Conclusions: These results indicate that the C-terminal region of RXR is required for full RARE occupancy in vivo, a RA dependent process that leads to the recruitment of other factors to the promoter and the subsequent transcriptional activation. Thus, RXRs play an integral role in ligand dependent transcription.