Chemistry of 1α,25-(OH)₂ D₃. 1α,25-(OH)₂ D₃ is derived from the 4 cyclopentanoperhydro-phenanthrene ring structure (A, B, C, and D rings) for steroids. In 1α,25-(OH)₂ D₃, the 9,10 carbon–carbon bond of ring B is broken between ring A and rings C and D (arrow, a) and the molecule is technically classified as a seco-steroid. The molecule may then rotate along the bond between ring A and rings C and D (arrow), to provide the structure of 1α,25-(OH)₂ D₃ (a). Stepwise modification of the molecule, involving location of a oxygen atom at position 23 on the C and D ring side chain or removal of the terminal –OH group can have important biological effects (b).

Transcriptional regulation by vitamin D. Classical action of 1α,25-(OH)₂ D₃ is mediated by binding of the VDR/RXR complex at VDREs. (a) Transcriptional activation involves the co-activator molecules SRCs, NCoA62, HATs, CBP p300 and other molecules to derepress chromatin. (b) Binding of DRIP to the AF2 domain attracts a complex containing transcription factor 2B (TF2B) and RNA polymerase II (RNA Pol II) for transcription initiation. The presence of the multiprotein complex facilitates increased gene transcription. (c) 1α,25(OH)₂ D₃-mediated transcriptional repression involves VDR–RXR heterodimer association with VDR-interacting repressor (VDIR) bound to E-box-type negative VDREs (nVDREs), dissociation of the HAT co-activator and recruitment of histone deacetylase (HDAC) co-repressor. Williams syndrome transcription factor (WSTF) potentiates transrepression by interacting with a multifunctional, ATP-dependent chromatin-remodelling complex (WINAC) and chromatin. This leads to the repression of genes, such as CYP27B1 (which encodes 1α-OHase) and PTH (which encodes parathyroid hormone). (d) Non-genomic, rapid actions of 1α,25-(OH)₂ D₃ are though activation of mitogen-activated protein kinase (MAPK)–extracellular signal-regulated kinase (ERK) 1 and 2 cascade through the phosphorylation (P) and activation of Raf by protein kinase C (PKC), partly through induced changes of intra-cellular Ca²⁺ concentration (reproduced from Deeb et al. [126] with kind permission of Dr CS Johnston Roswell Park CI, New York and the publisher).

Schematic representation of the vitamin D receptor (VDR) domain structure. (a) VDR protein backbone and 1α,25-(OH)₂ D₃ ligand-binding pocket. The VDR protein backbone is represented by a ribbon. A space-filling representation of 1α,25-(OH)₂ D₃ is shown within the VDR ligand-binding pocket by an atom-based structure. Conformational modification of the vitamin D side chain may influence ligand binding and transcriptional activity [125]. (b) The human VDR gene domain structure. The human VDR gene is composed of 9 exons that encode domains (A–F). Upon 1α,25(OH)₂ D₃ binding to the hormone ligand-binding domain, VDR is stabilized by the phosphorylation of serine 51 in the DNA-binding domain and serine 208 in the hinge region. VDR associates with the retinoic acid receptor (RXR) through the dimerization domains in E/F. The 1α,25-(OH)₂ D₃–VDR–RXR complex binds to the vitamin D response elements (VDREs) through the DNA-binding domain in the promoters of target genes. Conformational change in VDR results in co-repressor dissociation and enables interaction of the AF2 transactivation domain with stimulatory coactivators, such as steroid receptor coactivators (SRCs), vitamin D receptor-interacting proteins complex and nuclear coactivators.