Background |
The human retinoid X receptors (RXRs) are encoded by three distinct genes (RXRalpha, RXRbeta, and RXRgamma) and bind selectively and with high affinity to the vitamin A derivative, 9-cis-retinoic acid. RXRs are type-II nuclear hormone receptors that are largely localized to the nuclear compartment independent of ligand binding. Nuclear RXRs form heterodimers with nuclear hormone receptor subfamily 1 proteins, including thyroid hormone receptor, retinoic acid receptors, vitamin D receptor, peroxisome proliferator-activated receptors, liver X receptors, and farnesoid X receptor. Since RXRs heterodimerize with multiple nuclear hormone receptors, they play a central role in transcriptional control of numerous hormonal signaling pathways by binding to cis-acting response elements in the promoter/enhancer region of target genes. Retinoid X receptor alpha (RXRalpha) is the founding RXR family member and is predominantly expressed in liver, kidney, epidermis, intestine and a variety of tissues. Knockout of the murine rxralpha gene results in embryonic lethality tentatively due to myocardial hypoplasia, which demonstrates the importance of RXRalpha to retinoid signaling in vivo. Biochemical evidence suggests that RXRalpha transcriptional activity is post-translationally regulated through the Ras-Raf-MAPK signaling cascade. MAPK-dependent phosphorylation of RXRalpha directly abrogates the ability of RXRalpha to associate with nuclear receptor coactivators. |