Similar to retinoic acid, the vitamin D receptor (VDR) is a member of the steroid/thyroid hormone superfamily of receptors. These act as ligand-dependent transcription factors that regulate gene expression. In intestine, bone, kidney, and other tissues, calcitriol binds to VDR localized in the cytoplasm, and the complex forms a heterodimer with a retinoid RXR. RXRs are also involved in vitamin A action and the reader is referred to that section for more information. This complex translocates to the nucleus and binds to vitamin D response elements (VDREs) associated with the promoter region of target genes and, together with coactivators, regulates target gene transcription. Over 50 genes have been identified that are controlled by calcitriol.
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