The Retinoid X receptor-alpha (RXR alpha) is a nuclear hormone receptor and can function as a ligand inducible transcription factor capable of acting as a co-repressor and/or coactivator for gene expression. Nuclear receptors contain a series of conserved domains or regions. These domains/regions include a variable NH2-domain (A/B region), a conserved DNA-binding domain (DBD or region C), a linker region (region D), a ligand binding domain (LBD or region E), and in some receptors a variable COOH-terminal (region F). RXR alpha belongs to the family of retinoid X receptors, one of two retinoic receptor families (retinoic acid receptors and retinoid X receptors). RXR alpha is one of three members of the RXR family which consists of RXR alpha, RXR beta, and RXR gamma. The A/B and D regions of RXR alpha are involved in dictating the cell dependent transcriptional response. RXR receptors are able to form both homo- and heterodimers. RXR receptors have been reported to form heterodimers with TRs (thyroid hormone receptors), RARs retinoic acid receptors), VDR (vitamin D receptor), PPARs (peroxisome proliferators activated receptor), LXR (liver X receptor), and FXR (farnesoid X receptor). These heterodimers can be classified as permissive and nonpermissive heterodimers. Addition of a RXR agonist, such as 9-cis-retinoic acid, can result in transcriptional activity of permissive heterodimers while activation of nonpermissive heterodimers occurs independent of the RXR agonist. RXR alpha agonist LG100268 activates the transcriptional response RXR:PPAR gamma and
RXR:LXR heterodimers alone and synergistically with PPAR gamma and LXR agonists, but does not activate RXR:RAR and RXR:TR heterodimers whose activity is dependent upon RAR and TR agonists. RXR alpha is expressed in the liver, spleen, placenta, epidermis, central nervous system, and is implicated in embryo development and differentiation. With its ability to form heterodimers with other nuclear receptors, RXR alpha has potential roles in lipid metabolism, skin alopecia, dermal cysts, cardiac development, insulin sensitization, and gene regulation. RXR activation has been shown in the spinal cord, brain, and epithelia of transgenic Xenopus laevis embryos. The development of a loss of function mutation of RXR alpha in a mouse germ line resulted in embryonic lethality due to defects in the ventricular walls of the heart.The endogenous ligands for RXR alpha include 9-cis-retinoic acid, phytanic acid, and docosahexaenoic acid. Synthetic agonists for RXR alpha have been termed rexinoids and include LG100268.