Molecular mechanism of vitamin D receptor action.

The vitamin D system is unique in that distinct calcium homeostatic functions and cell growth regulatory activities are mediated through a single ligand, calcitriol, acting through a specific receptor exhibiting ubiquitous tissue expression, the vitamin D receptor (VDR). The VDR is a member of a superfamily of nuclear steroid hormone receptors which regulate gene transcription by interacting with response elements in gene promoters. Structure-function analysis of the VDR protein has defined distinct domains involved in DNA binding, ligand binding, receptor dimerisation and gene transactivation, including a C-terminal activation function domain (AF-2) that is important for cofactor interaction. A model for regulation of gene transcription by the VDR is evolving and proposes VDR interaction with various components of the basal transcriptional machinery, including newly defined coactivators and corepressors, which may act to regulate gene transcription by altering histone acetylation and chromatin structure. This review describes the vitamin D endocrine system and the role of the VDR in regulating this system, including the molecular basis for the diverse actions of synthetic calcitriol analogues in the treatment of autoimmune disease and cancer.