The effects of 1,25-dihydroxyvitamin D3 and dexamethasone on rat osteoblast-like primary cell cultures: receptor occupancy and functional expression patterns for three different bioresponses

The effects of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] and dexamethasone to regulate collagen and osteocalcin synthesis and induction of 25-hydroxyvitamin D3-24-hydroxylase (24-hydroxylase) activity were studied in rat osteoblast-like cell primary cultures. In this culture system, the basal levels of collagen and osteocalcin synthesis increased with rising cell density in culture. At maximal doses, both 1,25-(OH)2D3 (8.1 nM) and dexamethasone (130 nM) reduced collagen synthesis to about 50% of the control levels, 1,25-(OH)2D3 affected osteocalcin synthesis in a biphasic manner: stimulatory at low doses, which peaked near 0.33 nM to reach 3- to 5-fold the basal level, followed by a gradual return to the basal level at higher concentrations. Dexamethasone had only a slight stimulatory effect on osteocalcin. 1,25-(OH)2D3 also induced 24-hydroxylase activity in rat osteoblast-like cells, while dexamethasone had no effect on the enzyme. Induction of enzyme activity achieved a 4- to 6-fold rise, but required higher concentrations of 1,25-(OH)2D3 to achieve maximal levels (16 nM). The half-maximal doses (ED50) of 1,25-(OH)2D3 required for each bioresponse were different. The approximate ED50 for the inhibition of collagen synthesis was near the Kin (0.4 nM; apparent dissociation constant of receptor nuclear internalization), while the ED50 for osteocalcin synthesis (0.08 nM) was below the Kin, and the ED50 for 24-hydroxylase induction (20 nM) was greater than the Kin. The ED50 for dexamethasone on collagen synthesis (20 nM) was about 5-fold higher than the Kin (4 nM) of dexamethasone receptor binding. The potencies of various vitamin D3 metabolites in all three functional responses followed their abilities to compete for the 1,25-(OH)2D3 receptor, indicating that these actions were 1,25-(OH)2D3 receptor mediated. In summary, these studies explored bone cell bioresponses to 1,25-(OH)2D3 and dexamethasone and examined the relationship between receptor occupancy and functional expression. Each action exhibited a different dose-response pattern, implying that different levels of control are required for each individual response.     

Regulation of 1,25-dihydroxyvitamin D3 production by cultured alveolar macrophages from normal human donors and from patients with pulmonary sarcoidosis

Regulation of the production of the biologically active vitamin D3 sterol 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] by cultured pulmonary alveolar macrophages (PAM) obtained from 6 patients with pulmonary sarcoidosis and from 9 normal subjects was studied. The sarcoid cells, all collected from patients with normal calcium metabolism, synthesized 1,25-(OH)2-[3H]D3 from the substrate 25-hydroxyvitamin [3H]D3 (25OH-[3H]D3), whereas in vitro incubation with recombinant human interferon-gamma (IFN gamma) or lipopolysaccharide (LPS) was required for induction of synthesis of the hormone by normal PAM. Exogenous 1,25-(OH)2D3 (10-100 nmol/L) decreased endogenous hormone production by normal PAM by approximately 45%. The relative inhibitory effect of 1,25-(OH)2D3 was less pronounced in sarcoid PAM, in which 10-100 nmol/L 1,25-(OH)2D3 inhibited 250HD3-1-hydroxylase by approximately 25%. An accompanying induction of the 250HD3-24-hydroxylase, which is typical for renal cells, was found at low levels in only 3 of 10 experiments; in this regard, no differences between sarcoid and normal PAM were apparent. PTH or forskolin did not influence 250HD3 metabolism by PAM. 1,25-(OH)2D3 production by sarcoid PAM was enhanced by lipopolysaccharide and IFN gamma. Likewise, recombinant human interleukin-2 stimulated 1,25-(OH)2D3 production by sarcoid PAM, suggesting a possible role for both IFN gamma and interleukin-2 in the induction of 1,25-(OH)2D3 synthesis by sarcoid PAM in vivo. Recombinant human IFN alpha, IFN beta, and granulocyte-macrophage colony-stimulating factor had little effect. Dexamethasone and chloroquine, which have in vivo antihypercalcemic activity in sarcoidosis, both inhibited 1,25-(OH)2D3 synthesis by sarcoid PAM; chloroquine simultaneously stimulated the 24-hydroxylase. Our studies suggest that the 250HD3-metabolizing system in PAM is in some respects different from renal metabolism of 250HD3.     

Effect of the X-linked Hyp mutation and vitamin D status on induction of renal 25-hydroxyvitamin D3-24-hydroxylase

The present study was undertaken to evaluate the response of Hyp mice to regulators known to inhibit renal 25-hydroxyvitamin D3-1-hydroxylase (1-hydroxylase) and stimulate renal 25-hydroxyvitamin D-24-hydroxylase (24-hydroxylase). Renal mitochondrial metabolism of 25-hydroxyvitamin D3 (25OHD3) was initially examined in vitamin D- and calcium-deprived normal and mutant mice (with no detectable 24-hydroxylase) treated with either calcium, 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], or both calcium + 1,25-(OH)2D3. In normal mice, 1,25-(OH)2D3 treatment was more effective than calcium in turning off 1-hydroxylase and turning on 24-hydroxylase activity; serum calcium, however, was similarly increased by both treatments. Although calcium + 1,25-(OH)2D3 did not result in a further change in 25OHD3 metabolism in normal mice, a further elevation in serum calcium was apparent. In Hyp mice, treatment with calcium + 1,25-(OH)2D3 resulted in a greater decrease in 1-hydroxylase and a greater increase in 24-hydroxylase and in serum calcium than treatment with either agent alone. In spite of similar serum calcium levels in both genotypes, 24-hydroxylase was 20-fold, 3-fold, and 8-fold greater in Hyp mice relative to normals treated with calcium, 1,25-(OH)2D3, and calcium + 1,25-(OH)2D3, respectively. Kinetic studies revealed that the maximum velocity (Vmax) for induced 24-hydroxylase was 6-fold greater than normal in Hyp mice whereas the apparent Michaelis-Menten constant (Km) was not different in the two groups of calcium + 1,25-(OH)2D3-treated mice. The effect of 1,25-(OH)2D3 treatment on the above serum and renal parameters was also examined in vitamin D replete normal and Hyp mice. A sharp rise in serum phosphate was observed in 1,25-(OH)2D3-treated Hyp mice whereas normal littermates experienced marked hypercalcemia in response to treatment. Renal 24-hydroxylase was significantly stimulated by 1,25-(OH)2D3 treatment in both normal and Hyp mice and genotype differences were not apparent. The present study demonstrates that vitamin D- and calcium-deprived Hyp mice are more responsive to signals which induce 24-hydroxylase than normal littermates; Vmax for induced 24-hydroxylase is 6-fold greater in Hyp mice than in normal littermates whereas apparent Km is unchanged; the inhibitory control of 1-hydroxylase appears to be intact in the mutant strain; induced 24-hydroxylase is similar in vitamin D replete normal and Hyp mice; and vitamin D status can thus modify the response of both genotypes to treatment with 1,25-(OH)2D3.(ABSTRACT TRUNCATED AT 400 WORDS)     

Vitamin D metabolites regulate osteocalcin synthesis and proliferation of human bone cells in vitro

The effects of six natural vitamin D metabolites of potential biological and therapeutic interest, 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3), 25-hydroxyvitamin D3 (25-OH-D3), 24R,25-dihydroxyvitamin D3 (24R,25-(OH)2D3), 1,24R,25-trihydroxyvitamin D3 (1,24R,25-(OH)3D3), 25S,26-dihydroxyvitamin D3 (25S,26-(OH)2D3) and 1,25S,26-trihydroxyvitamin D3 (1,25S,26-(OH)3D3) on cell replication and expression of the osteoblastic phenotype in terms of osteocalcin production were examined in cultured human bone cells. At a dose of 5 X 10(-12) mol/1, 1,25-(OH)2D3 stimulated cell proliferation, whereas at higher doses (5 X 10(-9)-5 X 10(-6) mol/1) cell growth was inhibited in a dose-dependent manner. The same pattern of effects was seen for the other metabolites in a rank order of potency: 1,25-(OH)2D3 greater than 1,25S,26-(OH)3D3 = 1,24R,25-(OH)3D3 greater than 25S,26-(OH)2D3 = 24R,25-(OH)2D3 = 25-OH-D3. Synthesis of osteocalcin was induced by 1,25-(OH)2D3 in doses similar to those required to inhibit cell proliferation. Biphasic responses were observed for some of the metabolites in terms of osteocalcin synthesis, inhibitory effects becoming apparent at 5 X 10(-6) mol/1. The cells did not secrete osteocalcin spontaneously. These results indicate that vitamin D metabolites may regulate growth and expression of differentiated functions of normal human osteoblasts.     

IGF-I and GH stimulate Phex mRNA expression in lungs and bones and 1,25-dihydroxyvitamin D(3) production in hypophysectomized rats.

OBJECTIVE: X-linked hypophosphatemia, a renal phosphate (Pi)-wasting disorder with defective bone mineralization, is caused by mutations in the PHEX gene (a Pi-regulating gene with homology to endopeptidases on the X chromosome). We wondered whether changes in Phex and neprilysin (NEP) (another member of the family of zinc endopeptidases) mRNA expression could be observed in relation to vitamin D and Pi metabolism during GH- and IGF-I-stimulated growth of hypophysectomized rats. DESIGN: Animals were infused s.c. for 2 days with vehicle, 200 mU (67 microg) GH or 300 microg IGF-I/rat per 24 h. We determined serum osteocalcin and osteocalcin mRNA in bone, Phex mRNA in bone and lungs, serum 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) and serum Pi levels, and renal expression of 25-hydroxyvitamin D(3)-1alpha-hydroxylase (1alpha-hydroxylase), of 25-hydroxyvitamin D(3)-24-hydroxylase (24-hydroxylase) and of the Na-dependent Pi-cotransporter type I and II (Na(d)Pi-I and -II). RESULTS: As compared with vehicle-treated controls, body weight and tibial epiphyseal width significantly increased in GH- and IGF-I-treated animals. Serum osteocalcin and osteocalcin mRNA levels in bone, Phex mRNA in bone and lungs, serum 1,25-(OH)(2)D(3) and renal 1alpha-hydroxylase mRNA rose concomitantly, whereas expression of NEP in lungs was barely affected and renal 24-hydroxylase mRNA decreased. Na(d)Pi-I and -II gene expression in the kidney and serum Pi levels remained unchanged. CONCLUSIONS: Our findings suggest a coordinate regulation of Phex mRNA expression in lungs and bone and vitamin D metabolism during GH- and IGF-I-stimulated growth.     

Expression of 25-hydroxyvitamin D3-24-hydroxylase activity in Caco-2 cells. An in vitro model of intestinal vitamin D catabolism

The C-24 oxidation pathway plays a major role in the degradation of vitamin D metabolites in kidney and other target tissues. The aim of the present study was to establish an intestinal cell culture system to study the mechanisms regulating the vitamin D catabolic pathway. 25-Hydroxyvitamin D3-24-hydroxylase (24-hydroxylase), the first enzyme in the catabolic sequence, was examined in Caco-2 cells, a human colon adenocarcinoma cell line which exhibits differentiated functions of absorbing intestinal epithelial cells. While untreated Caco-2 cells did not exhibit 24-hydroxylase activity, significant catabolic activity was induced by prior treatment of cell monolayers with 1,25-dihydroxyvitamin D3(1,25-(OH)2D3). Induced 24-hydroxylase D3 (25OHD3) and 1,25-(OH)2D3 was detected 6 h after treatment of cells with 10(-8)M 1,25-(OH)2D3, peaked at 16 h, and decreased thereafter. Treatment of cells with 10(-7) M 1,25-(OH)2D3 elicited a maximal 24-hydroxylase response. Comparable time courses of induction by 1,25-(OH)2D3 and 1,25-(OH)2D3-dose response curves were observed in cultured human skin fibroblasts and Caco-2 cells. 25OHD3 was not as good an inducer of the vitamin D catabolic pathway in Caco-2 cells as 1,25-(OH)2D3. Induction of 24-hydroxylase activity by 1,25-(OH)2D3 was inhibited by pretreatment of Caco-2 cells with either actinomycin D, alpha-amanitin, or cycloheximide suggesting that mRNA and protein synthesis are required for induction. The present study demonstrates that 1,25-(OH)2D3-treated Caco-2 cells express the vitamin D catabolic pathway and, therefore, constitute a useful in vitro model to study the mechanism of induction by 1,25-(OH)2D3.     

Studies on the hydroxylation of vitamin D in man

The authors have studied some of the factors influencing vitamin D hydroxylases in man, using two indirect experimental approaches. In the first study they have considered the effect of a long-term treatment with 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) on the serum levels of 25-hydroxyvitamin D (25-OHD) in postmenopausal osteoporosis, a condition in which high serum levels of 25-OHD and low mean levels of 1,25(OH)2D have been observed. In the second study the effects of the infusion of physiological doses of human parathyroid hormone (PTH) on the serum levels of 1,25(OH)2D and 24,25(OH)2D have been investigated. In the first study a decrease in the circulating levels of 25-OHD was observed during 1,25(OH)2D3 treatment. This could be considered as an indirect evidence of an inhibitory action of 1,25(OH)2D3 on 25-hydroxylase: in this view 1,25(OH)2D3 treatment decreases 25-hydroxylase activity, which is higher than normal in postmenopausal osteoporosis due to the low levels of 1,25(OH)2D. In the second study PTH infusion was followed by a remarkable increase in 1,25(OH)2D serum levels as a result of 1 alpha-hydroxylase stimulation, which was much higher in patients with hypoparathyroidism. The determination of 24,25(OH)2D levels during PTH infusion indicated an inhibitory effect on 24-hydroxylase.     

1,25-dihydroxyvitamin D resistance, rickets, and alopecia: analysis of receptors and bioresponse in cultured fibroblasts from patients and parents

To investigate further the cellular defects of vitamin D-dependent rickets type II with alopecia, we studied 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] receptors and the response to 1,25-(OH)2D3 in cultured skin fibroblasts from rachitic patients. Our studies included cells from four affected patients from three kindreds and their parents and cells from five normal subjects. We measured total 1,25-(OH)2D3 receptor binding in cell extracts and the capacity of 1,25-(OH)2D3 to induce the enzyme 25-hydroxyvitamin D3-24-hydroxylase (24-hydroxylase) as a marker of functional response. In normal fibroblasts, the 1,25-(OH)2D3 maximal binding capacity was 52 +/- 5 fmol/100 micrograms DNA (mean +/- SE), and the apparent dissociation constant (Kd) was 0.05 +/- 0.01 nM. The maximal induced 24-hydroxylase activity after 1,25-(OH)2D3 treatment was 11.5 +/- 1 fmol/10(6) cells X 30 min, and the dose of 1,25-(OH)2D3 that achieved half-maximal induction was 2.3 +/- 0.3 nM. Fibroblasts from all four rachitic patients had the same defect: no measurable 1,25-(OH)2D3 receptor binding and no detectable response above basal activity even after high doses of 1,25-(OH)2D3. Cells from all parents except one had normal 1,25-(OH)2D3 binding characteristics and normal 24-hydroxylase bioresponse to 1,25-(OH)2D3. One parent despite a normal phenotype had only half the normal level of binding sites and only half the normal bioresponse. In summary, the cultured fibroblasts from four affected children representing three different kindreds with 1,25-(OH)2D3 resistance failed to exhibit detectable 1,25-(OH)2D3 receptors. We postulate that this biochemical defect produced both the inability to respond to 1,25-(OH)2D3 in vitro and the 1,25-(OH)2D3 resistance in vivo. The obligate heterozygotic parents were normal, except for one who had both half the normal number of receptors and half the normal response to 1,25-(OH)2D3. The data confirm the critical role of the receptor in 1,25-(OH)2D3 action and the close coupling of receptor content and functional responsiveness.     

Liarozole acts synergistically with 1alpha,25-dihydroxyvitamin D3 to inhibit growth of DU 145 human prostate cancer cells by blocking 24-hydroxylase activity

1Alpha,25-dihydroxyvitamin D3 [1,25-(OH)2D3] inhibits the proliferation of many cancer cells in culture, but not the aggressive human prostate cancer cell line DU 145. We postulated that the 1,25-(OH)2D3-resistant phenotype in DU 145 cells might result from the high levels of expression of 25-hydroxyvitamin D-24-hydroxylase (24-hydroxylase) induced by treatment with 1,25-(OH)2D3. As this P450 enzyme initiates 1,25-(OH)2D3 catabolism, we presumed that a high level of enzyme induction could limit the effectiveness of the 1,25-(OH)2D3 antiproliferative action. To examine this hypothesis we explored combination therapy with liarozole fumarate (R85,246), an imidazole derivative currently in trials for prostate cancer therapy. As imidizole derivatives are known to inhibit P450 enzymes, we postulated that this drug would inhibit 24-hydroxylase activity, increasing the 1,25-(OH)2D3 half-life, thereby enhancing 1,25-(OH)2D3 antiproliferative effects on DU 145 cells. Cell growth was assessed by measurement of viable cells using the MTS assay. When used alone, neither 1,25-(OH)2D3 (1-10 nM) nor liarozole (1-10 microM) inhibited DU 145 cell growth. However, when added together, 1,25-(OH)2D3 (10 nM)/liarozole (1 microM) inhibited growth 65% after 4 days of culture. We used a TLC method to assess 24-hydroxylase activity and demonstrated that liarozole (1-100 microM) inhibited this P450 enzyme in a dose-dependent manner. Moreover, liarozole treatment caused a significant increase in 1,25-(OH)2D3 half-life from 11 to 31 h. In addition, 1,25-(OH)2D3 can cause homologous up-regulation of the vitamin D receptor (VDR), and in the presence of liarozole, this effect was amplified, thus enhancing 1,25-(OH)2D3 activity. Western blot analyses demonstrated that DU 145 cells treated with 1,25-(OH)2D3/liarozole showed greater VDR up-regulation than cells treated with either drug alone. In summary, our data demonstrate that liarozole augments the ability of 1,25-(OH)2D3 to inhibit DU 145 cell growth. The mechanism appears to be due to inhibition of 24-hydroxylase activity, leading to increased 1,25-(OH)2D3 half-life and augmentation of homologous up-regulation of VDR. We raise the possibility that combination therapy using 1,25-(OH)2D3 and liarozole or other inhibitors of 24-hydroxylase, both in nontoxic doses, might serve as an effective treatment for prostate cancer.     

25-Hydroxyvitamin D-24-hydroxylase in phytohemagglutinin-stimulated lymphocytes: intermediate bioresponse to 1,25-dihydroxyvitamin D3 of cells from parents of patients with vitamin D-dependent rickets type II

A method for assay of 25-hydroxyvitamin D-24-hydroxylase (24-hydroxylase) activity in phytohemagglutinin (PHA)-stimulated lymphocytes was applied to determine whether vitamin D-dependent rickets type II (VDDR II) is hereditary. In normal lymphocytes incubated with PHA for 3 days, maximal and half-maximal responses of 24-hydroxylase were observed after exposure to 10(-8) mol/L and (1.3 +/- 0.4) x 10(-9) mol/L 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], respectively. These responses were similar to those of cultured skin fibroblasts. In contrast, after exposure to 10(-8), 10(-7), and 10(-6) mol/L 1,25-(OH)2D3, no 24-hydroxylase activity was detected in cells from patients with VDDR II, and intermediate activity was observed in cells from their parents. These findings indicated the presence of an intracellular receptor-effector system for 1,25-(OH)2D3 in peripheral lymphocytes. Heterozygotes of VDDR II could be identified, and autosomal recessive inheritance of the disease was demonstrated. Detection of heterozygotes of this disease was not possible by assay of inhibition of thymidine incorporation, another marker of the function of 1,25-(OH)2D3 in PHA-stimulated lymphocytes. Therefore, assay of 24-hydroxylase induction reflected the receptor status more closely than assay of inhibition of DNA biosynthesis. The assay of 24-hydroxylase activity in PHA-stimulated lymphocytes described here will be useful for diagnosis of VDDR II and study of families of patients with this disease.     

1,25-dihydroxyvitamin D3 receptors and hormonal responses in cloned human skeletal muscle cells

Although skeletal muscle is a major calcium-regulated organ, there remains uncertainty about whether muscle is a target organ for the action of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. In this study we examine pure populations of clonally derived human muscle cells for the presence of 1,25-(OH)2D3 receptors and direct responses to the hormone. All of the clones tested exhibited specific [3H]1,25-(OH)2D3 binding, with values ranging from 5-70 fmol/mg protein. Scatchard analysis of binding data revealed a dissociation constant (approximately 100 pM) comparable to that of classical receptors in other target organs. The 1,25-(OH)2D3 receptors sedimented at 3.3S on hypertonic sucrose gradients. Specificity for [3H]1,25-(OH)2D3 was demonstrated on gradients by substantially better competition by 1,25-(OH)2D3 than 25-hydroxyvitamin D3 for the 3.3S receptor binding peak. The 1,25-(OH)2D3 receptor complex bound to DNA-cellulose and eluted as a single peak at 0.2 M KCl. Myoblasts and myotubes did not show significant differences in either the amount or characteristics of the 1,25-(OH)2D3 receptor. In addition to the presence of receptors, cells were tested for functional responsiveness to 1,25-(OH)2D3. Both cell types exhibited a dose-dependent induction of 25-hydroxyvitamin D3-24-hydroxylase enzyme activity after treatment of monolayers with 1,25-(OH)2D3. Incorporation of both leucine and thymidine into growing myoblasts and fused myotubes was inhibited in a dose-dependent fashion after treatment with 1,25-(OH)2D3. In summary, cloned human skeletal muscle cells contain a binding protein compatible with classical 1,25-(OH)2D3 receptors as well as functional responsiveness to 1,25-(OH)2D3 at physiological concentrations of hormone.     

Prenatal diagnosis of vitamin D-dependent rickets, type II: response to 1,25-dihydroxyvitamin D in amniotic fluid cells and fetal tissues

Vitamin D-dependent rickets type II (VDDR-II; hereditary resistance to 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]), an autosomal recessive genetic disease that results from a failure to respond to 1,25-(OH)2D3, is characterized by severe rickets, hypocalcemia, growth retardation, and high prevalence of alopecia. We used amniotic fluid cells in the 17th week of gestation to detect VDDR-II in fetuses at risk for the defect. First, we demonstrated in cells obtained from 15 control pregnancies the presence of a specific high affinity 1,25-(OH)2D3 receptor (Kd = 0.3 x 10(-11) mol/L; maximal number of binding sites, 6.1 fmol/mg protein) and 1,25-(OH)2D3-induced 25-hydroxyvitamin D3-24-hydroxylase activity (up to 30-fold increase). Amniotic fluid cells from a woman who had already given birth to a child with VDDR-II contained receptors that bound [3H]1,25-(OH)2D3 normally and responded to 1,25-(OH)2D3 stimulation with a 10-fold increase in 24-hydroxylase activity. The fetus was, therefore, judged unaffected, and a normal baby girl was born. At the age of 16 months she did not demonstrate clinical or biochemical features of VDDR-II. Amniotic fluid cells from another mother of a child with VDDR-II were unable to bind [3H]1,25-(OH)2D3, and the hormone failed to stimulate 24-hydroxylase activity. VDDR-II in this fetus was confirmed after termination of pregnancy by the total inability of 1,25-(OH)2D3 to stimulate 24-hydroxylase activity in tissue explants and cell cultures prepared from the fetus's kidney and skin. In contrast, tissues from dead control fetuses responded to stimulation by 1,25-(OH)2D3 with a 3- to 10-fold increase in 24-hydroxylase activity. Fetal kidney and skin explants and cell cultures also synthesized a [3H]1,25-(OH)2D3-like metabolite from [3H]25-OHD3 as early as the 17th week of gestation. 1,25-(OH)2D3 (10 nM) decreased the in vitro synthesis of the [3H]1,25-(OH)2D3-like metabolite in tissues from dead control fetuses, but not from the affected fetus. Thus, human fetuses at midgestation already have the regulatory mechanisms responsive to 1,25-(OH)2D3 present postnatally. The prenatal diagnosis of VDDR-II is now possible and is indicated in a high risk family.     

Effects of dexamethasone and vitamin D3 on cartilage differentiation in a clonal chondrogenic cell population

We have investigated the regulation of chondroblast/chondrocyte differentiation using a unique clonal cell population, designated RCJ 3.1C5 (C5), which differentiates into discrete three-dimensional cartilage nodules when grown in the presence of 15% fetal calf serum. Histologically, the nodules resembled hyaline cartilage; they contained large rounded chondrocytes surrounded by a refractile matrix which stained intensely with Alcian blue, exhibited metachromasia after Toluidine blue staining, and stained with an antibody against type II collagen. The cartilage nodules that formed did not mineralize, despite the presence of organic phosphate in the culture medium. The synthetic glucocorticoid dexamethasone (DEX) increased the number of cartilage nodules formed in a dose-dependent manner (ED50, approximately 10(-9) M), with a maximal stimulatory dose of 10(-8) M. DEX had no effect on the population doubling time and saturation density. The effects of DEX on the number of cartilage nodules were similar whether it was added from the beginning of the culture period (starting during exponential growth) or at confluence. In contrast, 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] inhibited cartilage nodule formation in a dose-dependent manner (IC50, approximately 5 x 10(-10) M), with maximum inhibition at 10(-7) M. In addition, 1,25-(OH)2D3 decreased cell proliferation and saturation density. Equimolar doses of the vitamin D3 metabolites 24,25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3 had no effect. C5 cells treated with 1,25-(OH)2D3 in the absence of DEX during the exponential growth phase exhibited a reduced capacity to form cartilage nodules upon subsequent exposure to DEX. At confluence, before cartilage nodules had formed, C5 cells responded to PTH and prostaglandin-E2 with increases in intracellular cAMP of about 10- and 95-fold respectively. After cartilage nodules were present, prostaglandin-E2 responsiveness decreased to about 25-fold, whereas there was no significant change in PTH responsiveness. DEX decreased the population alkaline phosphatase levels at all times measured, whereas 1,25-(OH)2D3 had a biphasic effect: an increase at 5 days in culture, followed by a decrease at later times in culture. These data indicate that the clonal cell line RCJ 3.1C5 is a useful model system in which to investigate cartilage differentiation.     

Ketoconazole decreases the serum 1,25-dihydroxyvitamin D and calcium concentration in sarcoidosis-associated hypercalcemia

Ketoconazole is an antifungal agent capable of inhibiting human steroid hormone synthesis, including renal 1,25-dihydroxyvitamin D [1,25-(OH)2D] synthesis. The ability of this drug to inhibit the extrarenal production of 1,25-(OH)2D, as occurs in human granuloma-forming disease states, including sarcoidosis, has not been evaluated. We examined the effect of ketoconazole on the 1,25-(OH)2D-calcium homeostatic mechanism in a hypercalcemic patient with sarcoidosis and on the synthesis of 1,25-(OH)2D3 in vitro by cultured pulmonary alveolar macrophages (PAM) from this and another host. Oral ketoconazole therapy (800 mg/day) decreased the serum 1,25-(OH)2D concentration 73% within 4 days; this was associated with a 15% decrease in the serum calcium concentration and a 57% decrease in the fractional urinary calcium excretion rate. In vitro, ketoconazole had a rapid onset, concentration-dependent inhibitory effect on sarcoid PAM 1,25-(OH)2D3 synthesis (ED50 = 0.1 mumol/L) that was not reversible by exposure to leukotriene C4, a potent stimulator of PAM 1,25-(OH)2D3 synthesis. Kinetic analysis of ketoconazole's action on the macrophage 1 alpha-hydroxylation reaction was examined at concentrations achieved in vivo when the drug is given orally. The velocity of the 1 alpha-hydroxylation reaction at ketoconazole concentrations of 0.01-1.0 mumol/L increased as the concentration of substrate 25-hydroxyvitamin D3 increased from 12-2000 nmol/L.     

25-hydroxyvitamin D(3)-1alpha-hydroxylase expression in normal and pathological parathyroid glands

Active vitamin D, 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], plays a pivotal role in calcium homeostasis and bone metabolism. Circulating levels of 1,25(OH)(2)D(3) are thought to be dependent mainly on the activity of the renal cytochrome P450 enzyme 25-hydroxyvitamin D(3)-1alpha-hydroxylase (1alpha-hydroxylase), which is potently induced by PTH. However, 1alpha-hydroxylase activity or expression has also been reported at several extrarenal sites, at which local synthesis of 1,25(OH)(2)D(3) appears to fulfill autocrine or paracrine functions. This includes tissues such as placenta and brain that also express LRP-2/megalin, an endocytic receptor for multiple ligands, which is involved in the renal uptake of the substrate for 1alpha -hydroxylase, 25-hydroxyvitamin D(3). We have previously demonstrated LRP-2/megalin in parathyroid cells, and here we present results from RT-PCR and immunohistochemical analyses showing coincident expression of 1alpha-hydroxylase in normal and pathological parathyroid tissue. With real-time quantitative RT-PCR analysis, the expression of 1alpha-hydroxylase mRNA was higher in the majority of parathyroid adenomas and secondary hyperplastic glands but lower in parathyroid carcinomas, compared with normal parathyroid tissue. The findings imply that in addition to feedback control by circulating 1,25(OH)(2)D(3) levels, parathyroid cells may also be influenced by local 1alpha -hydroxylase activity with possible growth regulatory and differentiating effects.     

HIV-protease inhibitors impair vitamin D bioactivation to 1,25-dihydroxyvitamin D

BACKGROUND: A high prevalence of bone demineralization occurs in people living with HIV/AIDS. The contribution of HIV itself and its treatment is still unclear. Protease inhibitors (PIs) are potent inhibitors of the cytochrome p450 enzyme system. Three cytochrome p450 mixed function oxygenases control serum levels of 1,25-dihydroxyvitamin D (1,25(OH) D ), which is responsible for vitamin D actions in target tissues including bone. The 25- and 1alpha-hydroxylases regulate 1,25(OH) D synthesis and 24-hydroxylase 1,25(OH) D catabolism. OBJECTIVE: To assess whether HIV-protease inhibitors (ritonavir, indinavir, nelfinavir) impair the activity of the main enzymes involved in 1,25(OH) D homeostasis. DESIGN AND METHODS: Studies were conducted in the human hepatocyte (H3B)- and monocyte (THP-1) cell lines, expressing 25-hydroxylase and 1alpha-hydroxylase, respectively. The 24-hydroxylase expression was induced in macrophages by exposure to 1,25(OH) D. Conversion rates of vitamin D to 25-hydroxyvitamin D [25(OH)D ]; 25(OH)D to 1,25(OH) D or 24,25(OH) D, and 1,25(OH) D degradation were quantified in untreated and HIV-PI-treated cells after C -cartridge extraction and high-performance liquid chromatography purification of 25(OH)D - 24,25(OH) D - and 1,25(OH) D fractions. RESULTS: The PIs impair hepatocyte 25(OH)D - and macrophage 1,25(OH) D synthesis in a reversible, dose-dependent manner. Furthermore, PIs inhibit 1,25(OH) D -degradation in macrophages with lower potency than that elicited on 1alpha-hydroxylase. Thus, reduced macrophage 1,25(OH) D production is the net effect of PIs action. CONCLUSIONS: In intact cells, HIV-PIs markedly suppress the activities of 25- and 1alpha-hydroxylase, which are critical in 1,25(OH) D synthesis, while exerting mild inhibition of 24-hydroxylase, responsible for 1,25(OH) D catabolism. If PIs elicit a similar potency in inhibiting these critical steps for 1,25(OH) D homeostasis, defective 1,25(OH) D production could contribute to the bone demineralization in HIV patients.     

Co-chaperone potentiation of vitamin D receptor-mediated transactivation: a role for Bcl2-associated athanogene-1 as an intracellular-binding protein for 1,25-dihydroxyvitamin D3

The constitutively expressed member of the heat shock protein-70 family (hsc70) is a chaperone with multiple functions in cellular homeostasis. Previously, we demonstrated the ability of hsc70 to bind 25-hydroxyvitamin D3 (25-OHD3) and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). Hsc70 also recruits and interacts with the co-chaperone Bcl2-associated athanogene (BAG)-1 via the ATP-binding domain that resides on hsc70. Competitive ligand-binding assays showed that, like hsc70, recombinant BAG-1 is able to bind 25-OHD3 (Kd=0.71+/-0.25 nM, Bmax 69.9+/-16.1 fmoles/microg protein) and 1,25(OH)2D3 (Kd=0.16+/-0.07 nM, Bmax = 38.1+/-3.5 fmoles/microg protein; both n=3 separate binding assays, P<0.001 for Kd and Bmax). To investigate the functional significance of this, we transiently overexpressed the S, M, and L variants of BAG-1 into human kidney HKC-8 cells stably transfected with a 1,25(OH)2D3-responsive 24-hydroxylase (CYP24) promoter-reporter construct. As HKC-8 cells also express the enzyme 1alpha-hydroxylase, both 25-OHD3 (200 nM) and 1,25(OH)2D3 (5 nM) were able to induce CYP24 promoter activity. This was further enhanced following overexpression of all the three BAG-1 isoforms. By contrast, BAG-1 isoforms had no effect on metabolism of 25-OHD3 by HKC-8 cells (either via 1alpha- or 24-hydroxylase activities). Further studies showed that a mutant form of BAG-1S exhibited decreased binding of 1,25(OH)2D3 and this resulted in a concomitant loss of potentiation of CYP24 promoter transactivation. Similar effects were not observed for 25-OHD3. These data highlight a novel role for BAG-1 as an intracellular-binding protein for 1,25(OH)2D3 and further suggest that BAG-1 is able to potentiate vitamin D receptor-mediated transactivation by acting as a nuclear chaperone for 1,25(OH)2D3.     

Vitamin D resistance and alopecia: a kindred with normal 1,25-dihydroxyvitamin D binding, but decreased receptor affinity for deoxyribonucleic acid

A new kindred exhibiting vitamin D resistance and alopecia is described. Clinically, three of seven sisters demonstrated rickets, hypocalcemia, elevated serum 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] levels, and alopecia. Biochemical analysis of cultured fibroblasts from skin biopsy explants in two affected and one normal sister revealed normal [3H]1,25-(OH)2D3 binding to receptors (Kd = 0.05 nM; Nmax = 30-50 fmol/mg protein). Despite normal steroid binding, cells from the two affected sisters failed to respond to 1,25-(OH)2D3 in vitro, as measured by induction of the enzyme 25-hydroxyvitamin D-24-hydroxylase. The cells from the normal sister showed a response within the range of five normal cell lines. Sucrose gradient analysis yielded a typical 3.2S protein under high salt conditions in extracts from the three siblings, but with reduced capacity to aggregate to a 6S moiety in low salt gradients in the two affected cells. Whole cell [3H]1,25-(OH)2D3 binding studies revealed nearly normal localization of bound receptor to the nuclear compartment. Elution of bound receptors by KCl gradients from both DNA-cellulose or fibroblast nuclei demonstrated that the receptors from the affected sisters exhibited decreased affinity for DNA compared to those from normal subjects. We conclude that 1,25-(OH)2D3 receptors from these resistant fibroblasts have a normal steroid-binding domain, but a defective nuclear binding domain. We believe that this abnormality may be responsible for the vitamin D resistance observed both in vivo and in vitro.