Mark R. Haussler

FIG. 1. Calcemic and phosphatemic biological actions of vitamin D in mammals.(A) Effects of vitamin D and its metabolites to ensure skeletal integrity, especially when calcium is limiting.(Central open box) Vitamin D 3 , obtained from diet or derived from sunlight-initiated photobiogenesis in skin, is converted via two hydroxylation reactions to the 1,25(OH) 2 D 3 hormonal form that circulates in blood.The final step in bioactivation of vitamin to hormone is catalyzed by the renal 1-OHase when stimulated by PTH under conditions of low calcium.(Lower portion) Integrated actions of the 1,25(OH) 2 D 3 metabolite, via binding to the intracellular VDR, to control calcium homeostasis in bone, intestine, kidney, and parathyroid as explained in the text.(Top left) Action of 1,25(OH) 2 D 3 -VDR in skin cell differentiation.(Top center) Conversion of 1,25(OH) 2 D 3 or the preceding 25(OH)D 3 metabolite to 24-hydroxylated forms in response to 1,25(OH) 2 D 3 -VDR induction of the 24-OHase gene.This conversion serves to initiate catabolism of the vitamin D molecule, but may also produce 24-hydroxylated metabolites with novel hormonal activity with respect to chondrocyte differentiation and bone mineralization (see text).(B) The vitamin D bioactivation-phosphate homeostatic loop: proposed novel roles for phosphatonin, the PEX gene product, and NPT2.(Left and lower portion) Under normal physiologic conditions, low PO 4 enhances the synthesis of 1,25(OH) 2 D 3 , which then acts through VDR to effect phosphate reclamation by suppressing PTH as well as inducing NPT2 and PEX gene expression.NPT2 acts directly to reabsorb PO 4 , while the PEX enzyme eliminates phosphatonin.(Top right) Tumor-induced osteomalacia and XLH each elicit increased phosphatonin, an uncharacterized phosphaturic hormone that is postulated to inhibit both NPT2 and the 1-OHase, to cause severe phosphate wasting.

The vitamin D receptor (VDR) mediates the effects of the calcemic hormone 1alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3]. We show that VDR also functions as a receptor for the secondary bile acid lithocholic acid (LCA), which is hepatotoxic and a potential enteric carcinogen. VDR is an order of magnitude more sensitive to LCA and its metabolites than are other nuclear receptors. Activation of VDR by LCA or vitamin D induced expression in vivo of CYP3A, a cytochrome P450 enzyme that detoxifies LCA in the liver and intestine. These studies offer a mechanism that may explain the proposed protective effects of vitamin D and its receptor against colon cancer.

Complementary DNA clones encoding the human vitamin D receptor have been isolated from human intestine and T47D cell cDNA libraries. The nucleotide sequence of the 4605-base pair (bp) cDNA includes a noncoding leader sequence of 115 bp, a 1281-bp open reading frame, and 3209 bp of 3' noncoding sequence. Two polyadenylylation signals, AATAAA, are present 25 and 70 bp upstream of the poly(A) tail, respectively. RNA blot hybridization indicates a single mRNA species of approximately equal to 4600 bp. Transfection of the cloned sequences into COS-1 cells results in the production of a single receptor species indistinguishable from the native receptor. Sequence comparisons demonstrate that the vitamin D receptor belongs to the steroid-receptor gene family and is closest in size and sequence to another member of this family, the thyroid hormone receptor.

High specific activity estradiol labeled with iodine-125 was used to detect approximately 200 saturable, high-affinity (dissociation constant ≅ 1.0 n M ) nuclear binding sites in rat (ROS 17/2.8) and human (HOS TE85) clonal osteoblast-like osteosarcoma cells. Of the steroids tested, only testosterone exhibited significant cross-reactivity with estrogen binding. RNA blot analysis with a complementary DNA probe to the human estrogen receptor revealed putative receptor transcripts of 6 to 6.2 kilobases in both rat and human osteosarcoma cells. Type I procollagen and transforming growth factor-β messenger RNA levels were enhanced in cultured human osteoblast-like cells treated with 1 n M estradiol. Thus, estrogen can act directly on osteoblasts by a receptor-mediated mechanism and thereby modulate the extracellular matrix and other proteins involved in the maintenance of skeletal mineralization and remodeling.