THE VITAMIN-D HORMONE AND ITS NUCLEAR RECEPTOR - MOLECULAR ACTIONS AND DISEASE STATES

Vitamin D plays a major role in bone mineral homeostasis by promoting the transport of calcium and phosphate to ensure that the blood levels of these ions are sufficient for the normal mineralization of type I collagen matrix in the skeleton. In contrast to classic vitamin D-defi ciency rickets, a number of vitamin D-resistant rachitic syndromes are caused by acquired and hereditary defects in the metabolic activation of the vitamin to its hormonal form, 1,25-dihydroxyvitamin D-3 (1,25( OH)(2)D-3), or in the subsequent functions of the hormone in target ce lls. The actions of 1,25(OH)(2)D-3 are mediated by the nuclear vitamin D receptor (VDR), a phosphoprotein which binds the hormone with high affinity and regulates the expression of genes via zinc finger-mediate d DNA binding and protein-protein interactions. In hereditary hypocalc emic vitamin D-resistant rickets (HVDRR), natural mutations in human V DR that confer patients with tissue insensitivity to 1,25(OH)(2)D-3 ar e particularly instructive in revealing VDR structure/function relatio nships. These mutations fall into three categories: (i) DNA binding/nu clear localization, (ii) hormone binding and (iii) heterodimerization with retinoid X receptors (RXRs). That all three classes of VDR mutati ons generate the HVDRR phenotype is consistent with a basic model of t he active receptor as a DNA-bound, 1,25(OH)(2)D-3-liganded heterodimer of VDR and RXR. Vitamin D responsive elements (VDREs) consisting of d irect hexanucleotide repeats with a spacer of three nucleotides have b een identified in the promoter regions of positively controlled genes expressed in bone, such as osteocalcin, osteopontin, beta(3)-integrin and vitamin D 24-OHase. The 1,25(OH)(2)D-3 Ligand promotes VDR-RXR het erodimerization and specific, high affinity VDRE binding, whereas the ligand for RXR, 9-cis retinoic acid (9-cis RA), is capable of suppress ing 1,25(OH)(2)D-3-stimulated transcription by diverting RXR to form h omodimers. However, initial 1,25(OH)(2)D-3 Liganding of a VDR monomer renders it competent not only to recruit RXR into a heterodimer but al so to conformationally silence the ability of its RXR partner to bind 9-cis RA and dissociate the heterodimer. Additional probing of protein -protein interactions has revealed that VDR also binds to basal transc ription factor IIB (TFIIB) and, in the presence of 1,25(OH)(2)D-3, an RXR-VDR-TFIIB ternary complex can be created in solution. Moreover, fo r transcriptional activation by 1,25(OH)(2)D-3, both VDR and RXR requi re an intact shea amphipathic alpha-helix, known as AF-2, positioned a t their extreme C-termini. Because the AF-2 domains participate neithe r in VDR-RXR heterodimerization nor in TFIIB association, it is hypoth esized that they contact, in a ligand-dependent fashion, transcription al coactivators such as those of the steroid receptor coacti vator fam ily, constituting yet a third protein-protein interaction for VDR. The refore, in VDR-mediated transcriptional activation, 1,25(OH)(2)D-3 bin ding to VDR alters the conformation of the Ligand binding domain such that it: (i) engages in strong heterodimerization with RXR to facilita te VDRE binding, (ii) influences the RXR ligand binding domain such th at it is resistant to the binding of 9-cis RA but active in recruiting coactivator to its AF-2 and (iii) presents the AF-2 region in VDR for coactivator association. The above events, including bridging by coac tivators to the TATA binding protein and associated factors, may posit ion VDR such that it is able to attract TFIIB and the balance oi the R NA polymerase II transcription machinery, culminating in repeated tran scriptional initiation of VDRE-containing, vitamin D target genes. Suc h a model would explain the action of 1,25(OH)(2)D-3 to elicit bone re modeling by stimulating osteoblast and osteoclast precursor gene expre ssion, while concomitantly triggering the termination of its hormonal signal by inducing the 24-OHase catabolizing enzyme.