The molecular basis for resistance to 1,25-dihydroxyvitamin D: studies in cells cultured from patients with hereditary hypocalcemic 1,25(OH)2D3-resistant rickets

1,25-Dihydroxyvitamin D₃ contributes to regulating mammary calcium transport and modulates neonatal skeletal growth and turnover cooperatively with calcium

To assess the interaction of 1,25(OH)(2)D(3) and dietary calcium on mammary calcium transport in lactating dams and skeletal growth and turnover in the neonate, female lactating 1α(OH)ase(+/-) or 1α(OH)ase(-/-) mice were fed either a high-calcium diet containing 1.5% calcium in the drinking water or a "rescue diet." Dietary effects on the expression of molecules mediating mammary calcium transport were determined in the dams, and the effects of milk calcium content were assessed on skeletal growth and turnover in 2-wk-old 1,25(OH)(2)D(3)-deficient pups. Results showed that the reduction of milk calcium levels in the 1α(OH)ase(-/-) dams and the elevation of milk calcium levels in dams fed the rescue diet were associated with the down- or upregulation of calbindin D(9k) and plasma membrane Ca(2+) ATPase isoform 2b expression, respectively, in mammary epithelial cells. The action of ambient calcium in stimulating skeletal growth in the neonates appeared to supercede the direct action of 1,25(OH)(2)D(3), and the response of chondrocytes in the neonates to elevated calcium was more sensitive in hypocalcemic animals. Osteopenia was more apparent in pups nursed by dams with lower milk calcium than in 1,25(OH)(2)D(3)-deficient pups nursed by dams with higher milk calcium. Bone formation parameters were increased significantly in all pups fed by dams on the rescue diet but were still lower in 1α(OH)ase(-/-) pups than in 1α(OH)ase(+/-) pups. Consequently, there is an important contributory role of calcium in conjunction with 1,25(OH)(2)D(3) to mammary calcium transport in lactating dams and skeletal growth and turnover in the neonate.

The skeleton as an intracrine organ for vitamin D metabolism

The endocrine hormone, 1alpha,25-dihydroxyvitamin D(3) (1,25D) is an important regulator of calcium and phosphorus homeostasis. In this context, 1,25D is generally recognized as necessary for the maintenance of a healthy skeleton through its actions on the small intestine. In this review, we highlight the direct effects of 1,25D on the constituent cells of the bone, actions that are independent of effects on the intestine and kidney. We also consider the evidence that 25D levels, not 1,25D levels, correlate best with parameters of bone health, and that the bone itself is a site of metabolic conversion of 25D into 1,25D, by virtue of its expression of the 25-hydroxyvitamin D 1alpha-hydroxylase, CYP27B1. We review the evidence that at least osteoblasts and chondrocytes, and possibly also bone resorbing osteoclasts, are capable of such metabolic conversion, and therefore that these cells likely participate in autocrine and paracrine loops of vitamin D metabolism. We conclude that the skeleton is an intracrine organ for vitamin D metabolism, challenging the long-held notion that 1,25D is solely an endocrine hormone.

Genetic models show that parathyroid hormone and 1,25-dihydroxyvitamin D3 play distinct and synergistic roles in postnatal mineral ion homeostasis and skeletal development

In humans, loss-of-function mutations in parathyroid hormone (PTH) and 25-hydroxyvitamin D3-1alpha-hydroxylase [1alpha(OH)ase] genes lead to isolated hypoparathyroidism and vitamin D-dependent rickets type I, respectively. To better understand the relative contributions of PTH and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] to skeletal and calcium homeostasis, we compared mice with targeted disruption of the PTH or 1alpha(OH)ase genes to the double null mutants. Although PTH-/- and 1alpha(OH)ase-/- mice displayed only moderate hypocalcemia, PTH-/-1alpha(OH)ase-/- mice died of tetany with severe hypocalcemia by 3 weeks of age. At 2 weeks, PTH-/- mice exhibited only minimal dysmorphic changes, whereas 1alpha(OH)ase-/- mice displayed epiphyseal dysgenesis which was most severe in the double mutants. Although reduced osteoblastic bone formation was seen in both mutants, PTH deficiency caused only a slight reduction in long bone length but a marked reduction in trabecular bone volume, whereas 1alpha(OH)ase ablation caused a smaller reduction in trabecular bone volume but a significant decrease in bone length. The results therefore show that PTH plays a predominant role in appositional bone growth, whereas 1,25(OH)2D3 acts predominantly on endochondral bone formation. Although PTH and 1,25(OH)2D3 independently, but not additively, regulate osteoclastic bone resorption, they do affect the renal calcium transport pathway cooperatively. Consequently, PTH and 1,25(OH)2D3 exhibit discrete and collaborative roles in modulating skeletal and calcium homeostasis and loss of the renal component of calcium conservation might be the major factor contributing to the lethal hypocalcemia in double mutants.

Inactivation of the 25-Hydroxyvitamin D 1α-Hydroxylase and Vitamin D Receptor Demonstrates Independent and Interdependent Effects of Calcium and Vitamin D on Skeletal and Mineral Homeostasis

We employed a genetic approach to determine whether deficiency of 1,25-dihydroxyvitamin D (1,25(OH)2D) and deficiency of the vitamin D receptor (VDR) produce the same alterations in skeletal and calcium homeostasis and whether calcium can subserve the skeletal functions of 1,25(OH)2D and the VDR. Mice with targeted deletion of the 25-hydroxyvitamin D 1alpha-hydroxylase (1alpha(OH)ase-/-) gene, the VDR gene, and both genes were exposed to 1) a high calcium intake, which maintained fertility but left mice hypocalcemic; 2) this intake plus three times weekly injections of 1,25(OH)2D3, which normalized calcium in the 1alpha(OH)ase-/- mice only; or 3) a "rescue" diet, which normalized calcium in all mutants. These regimens induced different phenotypic changes, thereby disclosing selective modulation by calcium and the vitamin D system. Parathyroid gland size and the development of the cartilaginous growth plate were each regulated by calcium and by 1,25(OH)2D3 but independent of the VDR. Parathyroid hormone secretion and mineralization of bone reflected ambient calcium levels rather than the 1,25(OH)2D/VDR system. In contrast, increased calcium absorption and optimal osteoblastogenesis and osteoclastogenesis were modulated by the 1,25(OH)2D/VDR system. These studies indicate that the calcium ion and the 1,25(OH)2D/VDR system exert discrete effects on skeletal and calcium homeostasis, which may occur coordinately or independently.

Vitamin D deficiency and disorders of vitamin D metabolism

The disorders of vitamin D metabolism are inherited metabolic abnormalities involving mutations of the vitamin D receptor or enzymes involved in the metabolism of vitamin D to its biologically active form 1,25-dihydroxyvitamin D. Although these mutations are rare, studies in affected patients and animal models have helped to identify critical actions of vitamin D and the mechanism by which it exerts its effects. Vitamin D deficiency, however, is an increasingly recognized problem among the elderly and in the general population. Screening for vitamin D deficiency only in those patients with known risk factors will result in a large proportion of unrecognized affected patients.

Transcriptional synergism between vitamin D-responsive elements in the rat 25-hydroxyvitamin D3 24-hydroxylase (CYP24) promoter

Transcription of the CYP24 gene is induced by 1,25-(OH)2D3 through a vitamin D receptor-dependent process. The functional activities of three possible vitamin D response elements (VDREs), located on the antisense strand of the rat CYP24 promoter, were investigated by transient expression of native and mutant promoter constructs in COS-1, JTC-12, and ROS 17/2.8 cells. A putative VDRE with a half-site spacing of 6 base pairs at -249/-232 (VDRE-3) did not contribute to 1,25-(OH)2D3 induced expression in the native promoter, although activity has been reported when the element was fused to the heterologous thymidine kinase promoter. Two VDREs with half-site spacings of 3 base pairs at -150/-136 and -258/-244 (VDRE-1 and VDRE-2, respectively), showed transcriptional synergism in COS-1 cells when treated with 1,25-(OH)2D3 (10(-7) to 10(-11) M). The contribution of both VDREs was hormone-concentration dependent from 10(-10) to 10(-12) M, with VDRE-1 demonstrating greatest sensitivity to 1,25-(OH)2D3. Transactivation by VDRE-1 was always greater than VDRE-2, but the converse was observed for the binding of vitamin D receptor-retinoid X receptor complex by each VDRE in gel mobility shift assays. The synergy observed between VDRE-1 and VDRE-2 may have important implications in cellular responses to different circulating levels of 1,25-(OH)2D3.

Physiology of endocrine skin interrelations

Hormones influence the skin and play a role in normal biologic processes. Keratinocytes can convert and synthesize endocrine hormones. Endocrine dysregulation of the skin and abnormalities of endocrine functions of keratinocytes may produce abnormal changes in the skin. Knowledge of the cutaneous metabolism of thyroid hormones, steroids, peptide hormones, and vitamin A derivatives is being rapidly updated. Skin manifestations of endocrine disorders result from imbalance in feedback loops maintaining endocrine homeostasis. Define molecular mechanisms of hormonal action on target cells underlie functional agonism and antagonism of hormonal signals aimed at governing epidermal turnover. The molecular synergism between vitamin A and other hormones may explain the therapeutic efficiency of combining retinoids with other therapies.

Overproduction of rat 1,25-dihydroxyvitamin D3 receptor in insect cells using the baculovirus expression system

The rat 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] receptor has been expressed at elevated levels in Spodoptera frugiperda cells using the baculovirus expression vector system. The recombinant 1,25-(OH)2D3 receptor is full-length, binds 1,25-(OH)2D3, and is recognized by a monoclonal antibody specific for 1,25-(OH)2D3 receptor. Densitometric scanning of Coomassie brilliant blue-stained SDS/polyacrylamide gels indicated a recombinant receptor protein level comprising 5% of the total soluble protein from the insect cells. The hydroxylapatite binding assay revealed average levels of 2 nmol of unoccupied 1,25-(OH)2D3 receptor per mg of protein in insect cells at 72 hr after infection with recombinant baculovirus. A measure of total 1,25-(OH)2D3 receptor using a ligand-independent, immunoradiometric assay disclosed average levels of 2.3 nmol of receptor per mg of protein produced by these same cells. A monoclonal antibody directed against the 1,25-(OH)2D3 receptor, and reported to cross-react with this receptor derived from several species, recognized the recombinant rat 1,25-(OH)2D3 receptor upon Western analysis. A monoclonal antibody directed specifically against the porcine receptor failed to recognize the recombinant rat 1,25-(OH)2D3 receptor protein. The cytosolic preparation of insect cells infected with recombinant baculovirus exhibited an equilibrium dissociation constant of 1 x 10(-11) M as determined by a 1,25-(OH)2D3 saturation analysis plotted by the method of Scatchard. This expression system provides an adequate source from which abundant quantities of 1,25-(OH)2D3 receptor can be purified for subsequent x-ray crystallographic analyses.