1,25-Dihydroxyvitamin D receptors in an established bone cell line. Correlation with biochemical responses

Is Osteogenic Differentiation of Human Nucleus Pulposus Cells a Possibility for Biological Spinal Fusion?

OBJECTIVE

The purpose of this study was to investigate whether a simple, biologically robust method for inducing calcification of degenerate intervertebral discs (IVD) could be developed to provide an alternative treatment for patients requiring spinal fusion.

DESIGN

Nucleus pulposus (NP) cells isolated from 14 human IVDs were cultured in monolayer and exposed to osteogenic medium, 1,25-dihydroxyvitamin D₃ (VitD₃), parathyroid hormone (PTH), and bone morphogenic proteins (BMPs) 2/7 to determine if they could become osteogenic. Similarly explant cultures of IVDs from 11 patients were cultured in osteogenic media with and without prior exposure to VitD₃ and BMP-2. Osteogenic differentiation was assessed by alkaline phosphatase activity and areas of calcification identified by alizarin red or von Kossa staining. Expression of osteogenic genes during monolayer culture was determined using polymerase chain reaction and explant tissues assessed for BMP inhibitors. Human bone marrow-derived mesenchymal stromal cells (MSCs) were used for comparison.

RESULTS

Standard osteogenic media was optimum for promoting mineralization by human NP cells in monolayer. Some osteogenic differentiation was observed with 10 nM VitD₃, but none following application of PTH or BMPs. Regions of calcification were detected in 2 of the eleven IVD tissue explants, one cultured in osteogenic media and one with the addition of VitD₃ and BMP-2.

CONCLUSIONS

Human NP cells can become osteogenic in monolayer and calcification of the extracellular matrix can also occur, although not consistently. Inhibitory factors within either the cells or the extracellular matrix may hinder osteogenesis, indicating that a robust biological fusion at this time requires further optimization.

Calcium intake and serum calcium status in Mongolian children

Dietary calcium intake in relation to calcium status in Mongolian children was investigated. This survey was a cross-sectional survey. A total of 835 children were randomly selected from 4 economic regions and Ulaanbaatar city. Information on dietary intake was collected from 835 children in the 1-3, 4-7, and 8-14 year old groups by a 24-h recall method. The average daily intake of calcium from diet was calculated for individuals. Blood samples were collected from 104 children. The mean of daily calcium intakes as 273±30.0mg in 1-3-year old children, 309.0±30.0mg in 4-7 year old children, and 317.0±31.0mg in 8-14 year old children, respectively. There were statistically significant differences in calcium intakes between the age groups 1-3 years, 4-7 years, and 1-3 years, 8-14 years of children (p<0.001). Calcium intakes in all studied children of all age groups were lower (39%, 30.9%, and 24.4%) than the recommended level of calcium intakes. In 22.1% of studied children, the serum total calcium concentration levels were below the normal range. Based on the total serum calcium, the prevalence of hypocalcemia was higher among children in the age group 8-14 years (27.6%) than the prevalence among children in the age group <1 year (p<0.05). Based on the corrected serum calcium values, the prevalence of hypocalcemia was higher (52.4%, 63.6%, and 51.1%) among children in the age groups 1-3, 4-7, and 8-14 years. The mean level of corrected serum calcium were low (2.02±0.04, 2.05±0.73, and 1.99±0.64mg) in children in the age groups 1-3, 4-7, and 8-14 years. These findings suggest that low dietary calcium intakes may be reflected by hypocalcemia in Mongolian children. In conclusion, there is a need to improve a consumption of milk, dairy products in Mongolian children. In addition, there is need to use a vitamin D supplementation with a calcium supplementation in children with severe vitamin D deficiency rickets. This article is part of a Special issue entitled "16th Vitamin D Workshop".

Vitamin D up-regulates the vitamin D receptor by protecting it from proteasomal degradation in human CD4+ T cells

The active form of vitamin D3, 1,25(OH)2D3, has significant immunomodulatory properties and is an important determinant in the differentiation of CD4+ effector T cells. The biological actions of 1,25(OH)2D3 are mediated by the vitamin D receptor (VDR) and are believed to correlate with the VDR protein expression level in a given cell. The aim of this study was to determine if and how 1,25(OH)2D3 by itself regulates VDR expression in human CD4+ T cells. We found that activated CD4+ T cells have the capacity to convert the inactive 25(OH)D3 to the active 1,25(OH)2D3 that subsequently up-regulates VDR protein expression approximately 2-fold. 1,25(OH)2D3 does not increase VDR mRNA expression but increases the half-life of the VDR protein in activated CD4+ T cells. Furthermore, 1,25(OH)2D3 induces a significant intracellular redistribution of the VDR. We show that 1,25(OH)2D3 stabilizes the VDR by protecting it from proteasomal degradation. Finally, we demonstrate that proteasome inhibition leads to up-regulation of VDR protein expression and increases 1,25(OH)2D3-induced gene activation. In conclusion, our study shows that activated CD4+ T cells can produce 1,25(OH)2D3, and that 1,25(OH)2D3 induces a 2-fold up-regulation of the VDR protein expression in activated CD4+ T cells by protecting the VDR against proteasomal degradation.

New insights on membrane mediated effects of 1α,25-dihydroxy vitamin D3 signaling in the musculoskeletal system

1α,25-Dihydroxy vitamin D3 [1α,25(OH)2D3] acts on cells via classical steroid hormone receptor-mediated gene transcription and by initiating rapid membrane-mediated signaling pathways. Two receptors have been implicated to play roles in 1α,25(OH)2D3 mediated rapid signaling, the classical nuclear vitamin D receptor (VDR) and protein disulfide isomerase, family A, member 3 (Pdia3). Long term efforts to investigate the roles of these two receptors demonstrated thatPdia3 is located in caveolae, where it interacts with phospholipase A2 (PLA2) activating protein (PLAA) and caveolin-1 (Cav-1) to initiate rapid signaling via Ca(++)/calmodulin-dependent protein kinase II (CaMKII), PLA2, phospholipase C (PLC), protein kinase C (PKC), and ultimately the ERK1/2 family of mitogen activated protein kinases (MAPK). VDR is present on the plasma membrane, and it is required for 1α,25(OH)2D3 induced rapid activation of Src. PDIA3+/- mice demonstrate an impaired musculoskeletal phenotype. Moreover, our studies examining mineralization of pre-osteoblasts in 3D culture have shown the physiological importance of Pdia3 and VDR interaction: knockdown of Pdia3 or VDR is characterized by impaired mineralization of the constructs.

Histone deacetylation mediates the rejuvenation of osteoblastogenesis by the combination of 25(OH)D3 and parathyroid hormone in MSCs from elders

Vitamin D metabolites are important effectors of bone and mineral homeostasis. Human bone marrow stromal cells (hMSCs) are targets of 1α,25-dihydroxyvitamin D [1α,25(OH)2D] action to promote their differentiation to osteoblasts. Osteoblastogenesis is also stimulated by 25-hydroxyvitamin D [25(OH)D], an effect that requires conversion to 1α,25(OH)2D3 by 25-hydroxyvitamin D3 1α-hydroxylase (CYP27B1). These findings support an autocrine/paracrine role of vitamin D metabolism in osteoblastogenesis of hMSCs. In this study, we assessed whether and by what mechanisms osteoblastogenesis could be rejuvenated with hMSCs from elders. First, knockdown studies with VDR-siRNA showed that both the pro-differentiation and anti-proliferative effects of 1α,25(OH)2D3 required VDR. Second, 100nM 25(OH)D3 (p<0.01 vs. control, ANOVA) and 100nM PTH1-34 (p<0.05) significantly stimulated alkaline phosphatase (ALP) activity (a measure of osteoblastogenesis), with a synergistic effect when combined (p<0.001). Scriptaid, an inhibitor of histone deacetylase, blocked the effect of 25(OH)D3 and PTH on osteoblastogenesis. Scriptaid alone downregulated VDR in hMSCs. These data demonstrate that histone deacetylation is required for the synergistic effect of 25(OH)D3 and PTH on osteoblastogenesis in hMSCs. Both VDR siRNA and Scriptaid dowregulated VDR mRNA and inhibited osteoblastogenesis. Thus, epigenetic regulation of the VDR may be central to rejuvenating osteoblastogenesis in hMSCs from elders. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

Vitamin D nutritional policy needs a vision for the future

Historically vitamin D is known to be essential for normal bone growth and quality, and thus appropriate dietary vitamin D supplementation can eliminate vitamin D deficiency childhood rickets and adult osteomalacia. In spite of many government and medical associations' worldwide guidelines for the reference daily intake (RDI) of vitamin D, scientists and nutritionists from many countries agree that at present about half of elderly North Americans and Western Europeans and probably also of the rest of the world are not receiving enough vitamin D to maintain healthy bone. In addition, over the past decade there has been a dramatic increase in our understanding of the many biological actions that result from vitamin D acting through its daughter steroid hormone, 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)] in collaboration with its cognate vitamin D receptor (VDR). Consequently, evidence has accumulated that beside intestine and bone, there are five additional physiological systems where the VDR with 1alpha,25(OH)(2)D generates biological responses. These include the immune system (both the innate and adaptive), pancreas and metabolic homeostasis, heart-cardiovascular, muscle and brain systems as well as the control of the cell cycle, and thus of the disease process of cancer. Acting through the VDR, 1alpha,25(OH)(2)D(3) can produce a wide array of favorable biological effects that collectively are projected to contribute to the improvement of human health. Responsible medicine demands that worldwide vitamin D nutritional guidelines reflect current scientific knowledge about vitamin D's spectrum of activities. Thus, worldwide vitamin D nutritional policy is now at a crossroads. This paper presents several proposed policy changes with regard to the amount of vitamin D daily intake that if implemented will maximize vitamin D's contribution to reducing the frequency of many diseases, which would then increase the quality and longevity of life and significantly reduce the cost of medical care worldwide.

Effect of 1,25-dihydroxy vitamin D3 on fracture healing and bone remodeling in ovariectomized rat femora

Osteoporosis is a major health problem characterized by compromised bone strength that predisposes patients to an increased risk of fracture, more and more investigations are focusing on the treatment of osteoporotic fracture healing. However, there are few studies elucidating the efficacy of vitamin D, 1,25-dihydroxy vitamin D(3) (1,25(OH)(2)D(3)), on osteoporotic fracture healing. In the present study we have established an osteoporotic fracture rat model to evaluate the effects of 1,25(OH)(2)D(3) on fracture healing. Female SD rats of six-month-old (n=40) allocated randomly into two groups were given ovariectomy. Bilateral midshaft femoral osteotomy was performed 12 weeks post-ovariectomy. Then treatment was begun at the second day after osteotomy and continued until sacrifice at 6 and 16 weeks post-fracture with middle chain triglyceride (MCT) vehicle and 1,25(OH)(2)D(3) at 0.1 microg/kg/day by oral gavage. Fracture callus was evaluated by soft X-ray radiography, micro-computed tomography (micro-CT), biomechanical testing and histology. Soft X-ray radiography, at 6 weeks post-fracture, showed a less distinct fracture line in the 1,25(OH)(2)D(3) group compared with the MCT-vehicle group, however, the fracture line was invisible in both groups at 16 weeks post-fracture. Micro-CT based histomorphometric data, at 6 weeks post-fracture, showed that the total volume of callus (TV) was approximately 23% higher in the 1,25(OH)(2)D(3) group than that in the MCT-vehicle group (P<0.001), and the new bone volume (BV), BV/TV, the trabecular number (Tb.N), and density of TV also showed the same trend. At 16 weeks post-fracture, the increment still existed as shown by Tb.Th and density of TV (P<0.001, vs control). Biomechanical testing data, at 6 weeks post-fracture, showed that the ultimate load at failure and energy absorption of the 1,25(OH)(2)D(3) group were nearly one fold higher than that of the MCT-vehicle group (P<0.001). At 16 weeks post-fracture, the ultimate load and energy absorption were also higher with the treatment of 1,25(OH)(2)D(3) (P<0.01 vs control). Histology showed that the fracture callus in the 1,25(OH)(2)D(3) group was remodeled better compared to the control group. In conclusion, 1,25(OH)(2)D(3) could promote fracture healing by improving the histomorphometric parameters, mechanical strength and tendency to increase transformation of woven bone into lamellar bone in an ovariectomized rat model.

PTH circadian rhythm and PTH target-organ sensitivity is altered in patients with adult growth hormone deficiency with low BMD

AGHD is associated with osteoporosis. We examined PTH circadian rhythmicity and PTH target-organ sensitivity in 23 patients with AGHD with low BMD and 20 patients with AGHD with normal BMD. Patients with low BMD had a blunted nocturnal rise in PTH concentration and reduced PTH target-organ sensitivity compared with patients with normal BMD; these factors may be important in the pathogenesis of AGHD-related osteoporosis.

INTRODUCTION

Adult growth hormone deficiency (AGHD) is associated with decreased BMD. Reduced parathyroid gland sensitivity to changes in calcium and reduced PTH target-organ sensitivity may underlie the pathogenesis of AGHD-related osteoporosis. A blunted nocturnal PTH rise has been reported in AGHD and may contribute to the reduction in BMD. We examined the difference in PTH concentration and markers of bone metabolism in patients with AGHD with normal and low BMD.

MATERIALS AND METHODS

Forty-three patients with AGHD consented to the study. Twenty-five patients were growth hormone (GH) naïve (GH-N, 13 had BMD femoral neck or lumbar spine T-score < -1.0), and 18 patients had received GH for >2 yr (GH-R, 10 had BMD T-score < -1.0). Patients were hospitalized for 24 h, where blood samples were collected every 0.5 h and urine samples were collected every 3 h for PTH, calcium, phosphate, NcAMP, 1,25-dihydroxyvitamin D [1,25(OH)(2)D], type-I collagen beta C-telopeptide (betaCTX), and procollagen type-I amino-terminal propeptide (PINP). Serum calcium was adjusted for albumin (ACa).

RESULTS

Low BMD GH-N and GH-R patients exhibited a reduced nocturnal rise in PTH concentration compared with patients with normal BMD (p < 0.001). GH-N low BMD patients had significantly higher 24-h mean PTH (p < 0.001) than GH-N normal BMD patients, with significantly lower 24-h mean NcAMP, ACa, and 1,25(OH)(2)D (p < 0.01), suggesting a reduction in renal PTH sensitivity. GH-R low BMD patients had significantly lower 24-h mean PTH, NcAMP, ACa, and 1,25(OH)(2)D (p < 0.01) than GH-R normal BMD patients, suggesting reduced renal PTH action. Lower PTH concentration in the presence of lower ACa may reflect reduced sensitivity of the parathyroid calcium-sensing receptor to changes in ACa concentration in the GH-R low BMD patients.

CONCLUSIONS

Low BMD in GH-N and GH-R AGHD patients may be a consequence of abnormalities in PTH circadian rhythmicity together with reduced parathyroid gland and target-organ sensitivity. Further studies are needed to determine the potential benefit of therapeutic manipulation of PTH rhythmicity and sensitivity on BMD.

Ligand structure-function relationships in the vitamin D endocrine system from the perspective of drug development (including cancer treatment)

It has become readily apparent to many scientists and pharmaceutical companies that the vitamin D endocrine system offers a wide array of drug development opportunities. There are already successes, as noted by 1alpha,25(OH)2D3 (Roche, and Abbott) for renal osteodystrophy and osteoporosis and 1alpha(OH)D3 (Leo, Chugai, Teijin) for renal osteodystrophy and (in Japan) osteoporosis, 1alpha,24(OH)2-24-cyclopropyl-D3 (Dovonex) and 1alpha,24(OH)2D3 (Teijin) for psoriasis, and 19-nor-1alpha,25(OH)2D2 (Abbott) for renal osteodystrophy, as well as drugs under active development. Yet there are still many important and challenging drug development frontiers, particularly in the area of cancer treatment and immune system disorders where exploration is only in the initial early stages. In addition, the application of vitamin D-related drugs in neurology and brain pathology should not be overlooked. It is to be hoped that the cellular and molecular basis for the vexing problem of analog-induced hypercalcemia will be elucidated. Given that there are believed to be over 2000 analogs of 1alpha,25(OH)2D3 already available for consideration, it is to be expected that over the next decade a significant number of new vitamin D structure-function drug development projects will be brought to conclusion.

Evidence for a 1 alpha,25-dihydroxyvitamin D3 receptor/binding protein in a membrane fraction isolated from a chick tibial fracture-healing callus

Previous biological studies have implicated two vitamin D metabolites, 1 alpha,25(OH)2-vitamin D3[1 alpha,25(OH)2-D3] and 24R,25(OH)2-vitamin D3 [24R,25(OH)2D3] in the process of skeletal fracture-healing. While a nuclear receptor for 1 alpha,25(OH)2D3 is known to be present in osteoblast and absent in osteoclast cell lines, no systematic study has been carried out on the callus tissue which is formed during fracture-healing. The present report shows that a binding protein/receptor for 1 alpha,25(OH)2D3 resides both in a postnuclear membrane fraction and in a high speed cytosol fraction of the callus tissue obtained 10 days after imposition of a tibial fracture. The dissociation constant, KD, for 1 alpha,25(OH)2D3 was 0.83 +/- 0.34 M and 0.66 +/- 0.38 nM respectively, for the membrane and cytosol fractions. Results from a panel of steroid competition assays indicate that both receptor/binding proteins greatly prefer 1 alpha-hydroxylated ligands as compared to 1 alpha-deoxy or 24-hydroxylated ligands. The presence of 1 alpha,25(OH)2D3 receptors in the fracture-healing callus is consistent with the known biological effects of the metabolite on the fracture-healing process.

Differentiation of human osteoblastic cells in culture: modulation of proteases by extracellular matrix and tumor necrosis factor-alpha

We have followed the synthesis and secretion of urokinase-type plasminogen activator (u-PA) and its inhibitor, PAI-1, and matrix metalloproteinases (MMPs) and tissue inhibitor of metalloproteinases (TIMP-1) during differentiation of a human osteoblastic cell line, HOS TE85, and the effect of TNF-alpha on this process. Our results show that the ratio of u-PA/PAI-1 associated with the cell-matrix components increases during differentiation of these cells over a 14-day period. Although TNF-alpha suppresses the induced increase in steady-state mRNA levels of u-PA and PAI-1 during maturation of extracellular matrix (ECM), the u-PA/PAI-1 ratio is altered in such a way that PA activity associated with the ECM is higher than control cells. The expression of MMP-1 is low and remains essentially invariant over a culture period of 14 days. TNF-alpha enhances MMP-1 transcription nearly 12-fold initially, after which mRNA levels drop off but remain significantly higher than the controls. Activities and steady-state mRNA levels of MMP-2 and MMP-9 increase nearly 15-fold during maturation of the ECM, but the level of TIMP-1 mRNA is not appreciably altered. The presence of TNF-alpha suppresses maturation-induced transcription of MMP-2, enhances TIMP-1 transcription, but has little effect on MMP-9 mRNA levels. The data show that chronic exposure to TNF-alpha alters the balance between u-PA/PAI-1 and MMPs/TIMP-1, which favors higher activity of proteinases. Accordingly, the presence of TNF-alpha in chronic inflammatory episodes would be expected to alter bone remodeling by inhibiting maturation of ECM and formation of bone.

Interactions between retinoic acid and 1,25(OH)2D in mouse immortalized osteoblastic C1 cells

The interactions between 1,25-dihydroxyvitamin D3 [1,25(OH)2D] and retinoic acid (RA) on alkaline phosphatase (ALP) activity and vitamin D receptors (VDR) were determined in the SV40 immortalized osteoblastic cell line C1. Biochemical and cytochemical analysis showed that ALP activity increased with C1 cell density and that 1,25(OH)2D inhibited ALP activity at high density, whereas RA increased ALP activity at low density. The interactions of the two hormones were also cell density dependent, with a predominant stimulatory effect of RA at low density and a predominant inhibitory effect of 1,25(OH)2D at high density. In contrast, the two hormones inhibited C1 cell proliferation independently of cell density. Scatchard analysis and immunocytochemical staining showed that nuclear VDR levels increased with cell density and that RA, 1,25(OH)2D, and their combination upregulated VDR levels mainly at high cell density. Although RA increased VDR levels, RA did not potentiate the effect of 1,25(OH)2D on ALP activity. The results indicate that the effects and interactions of 1,25(OH)2D and RA on ALP activity in osteoblastic C1 cells depend on the state of phenotypic maturation.

Differentiation of heterogeneous phenotypes in human osteoblast cultures in response to 1,25-dihydroxyvitamin D3

1,25(OH)2D3 (1,25-dihydroxyvitamin D3) inhibits the cell proliferation of human osteoblast-like cell cultures, but stimulates the synthesis of two of the phenotypic markers of the osteoblast, alkaline phosphatase and osteocalcin. It is not known whether all cells which synthesize alkaline phosphatase also synthesize osteocalcin in response to 1,25(OH)2D3. In this study we addressed this question by examining the response of human osteoblast-like cell cultures to 1,25(OH)2D3, using concurrent histochemical and immunochemical staining for alkaline phosphatase and osteocalcin, respectively. The cells were grown in the presence or absence of 1,25(OH)2D3 (10(-9) M) for 48 h. Co-localisation of osteocalcin and alkaline phosphatase in osteoblast-like cell cultures showed that not all cells which synthesize osteocalcin (about 9%) in response to 1,25(OH)2D3 synthesize alkaline phosphatase (about 24%) and vice versa. There was also a proportion of osteoblast-like cells which produce both osteocalcin and alkaline phosphatase simultaneously (about 12%). These findings suggest that during differentiation of bone-derived cells in cultures, in response to 1,25(OH)2D3, heterogeneous phenotypes with respect to expression of alkaline phosphatase and osteocalcin appear.

Regulation of expression of the type I collagen genes

The identification and functional analysis of DNA-protein interactions in the intronic and 5' flanking regions of the type I collagen genes has begun to define a series of cis-elements and trans-acting factors which regulate transcription of these genes. Studies such as these will eventually be expected to elucidate the mechanisms responsible for coordinate transcription of the alpha 1 and alpha 2 genes, a question which remains central to the field of collagen research. Although it is relatively straightforward to define sites of DNA-protein binding, interpretation of the functional importance of such interactions can be extremely complex. Furthermore, while mutation or deletion of a particular binding site may alter the functional activity of a construct transfected into cultured cells, there is no guarantee that a similar change will have the same effect in vivo, where the entire gene locus is present in its native chromosomal context. Nevertheless, these kinds of in vitro studies offer the best current approach to defining and isolating transcription factors that control expression of the alpha 1 and alpha 2 genes. Ultimately, it will be necessary to test the activity of such factors (and their respective cis-elements) in defined systems in vivo.

Physiological effects of 1,25-dihydroxyvitamin D3 in TM4 Sertoli cell line

1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] receptors have been previously described in Sertoli cells. This study was performed to assess biological activity of the receptor in the mouse Sertoli cell line TM4. A 2-h preincubation with 0.01-25 nM 1,25(OH)2D3 resulted in a dose-dependent rapid uptake of 45Ca2+ within 5 min of addition of the isotope to the cells (27 +/- 8%, n = 4 experiments; P less than 0.05). This response was specific for 1,25(OH)2D3, in that it was not induced by 25-hydroxyvitamin D3, estradiol, cortisol, R 5020 (promegestone), or testosterone. However, a combination of testosterone and 1,25(OH)2D3 inhibited uptake by 23 +/- 8% (n = 3 experiments, P less than 0.01). That the mechanism responsible for 1,25(OH)2D3-stimulated uptake may involve 1,25(OH)2D3 receptor interaction is supported by the observation that cycloheximide inhibited the response. Conversely, there was no detectable change in uptake by 1,25(OH)2D3-treated cells after 24-h incubation with 0.1-5 nM 1,25(OH)2D3. Increased levels of DNA and protein content also resulted from a 2-h incubation with the steroid and were sustained up to 24 h without a concomitant increase in cell number or a detectable change in cell morphology. The presence of specific 1,25(OH)2D3 receptor-like binding sites was demonstrated by sucrose gradient analysis and hydroxylapatite assay. These data demonstrate that 1,25(OH)2D3 may play an important role in testicular function through regulation of receptor-mediated events.

Abnormal response of osteoblasts from Hyp mice to 1,25-dihydroxyvitamin D3

To further explore the hypothesis of an osteoblast inappropriate response to 1,25-(OH)2D3 in hypophosphatemic vitamin D-resistant rickets (HYP), osteoblasts were isolated from Hyp mice, the animal model for human HYP, and their response to a physiologic dose of 1,25-(OH)2D3 (10(-10) M) was investigated with respect to alkaline phosphatase (ALP) activity and cell proliferation, and compared to that of normal osteoblasts. Cells in secondary culture were incubated for 72 h while in log phase, with or without 1,25-(OH)2D3, at various medium phosphate (P) concentrations ranging from 0.5 to 4.5 mM. Stimulation of ALP activity and inhibition of cell proliferation was induced by 10(-10)M 1,25-(OH)2D3 in normal cells exposed to medium P concentration corresponding to serum levels observed in normal mice (2.1-2.7 mM P). By contrast, Hyp cells failed to respond to 1,25-(OH)2D3 in that range of P concentrations. Stimulation of ALP activity and inhibition of proliferation of mutant cells were evident at higher medium P concentrations (over 3 mM). 1,25-(OH)2D3 at the supraphysiologic level of 10(-9)M had no consistent effect on ALP activity in normal and Hyp mouse osteoblasts, but inhibited cell proliferation in cultures of both genotypes at all P concentrations tested. These results indicate that extracellular P modulates the action of 1,25-(OH)2D3 on osteoblasts, and that this modulation was altered in osteoblasts from Hyp mice. The failure of Hyp cells to respond to a physiologic dose of 1,25-(OH)2D3 upon normal P concentration may reflect the abnormal response of bone to 1,25-(OH)2D3 observed in Hyp mice and HYP patients.

Dose-related effects of 1,25-dihydroxyvitamin D3 on growth, modeling, and morphology of fetal mouse metatarsals cultured in serum-free medium

A serum-free, fetal bone organ culture model that permits the simultaneous determination of modeling and growth parameters was used to examine the effects of a near physiologic and a pharmacologic dose of 1,25-dihydroxycholecalciferol [1,25-(OH)2D3]. The fetuses of pregnant mice were removed on day 17 of gestation, and three medial metatarsal rudiments were cleaned and after preculturing were cultured as pair-matched groups for 4 days in MEM supplemented with 0.2% BSA. 1,25-(OH)2D3 was added to the cultures at concentrations of 10(-12) or 10(-6) M. Cultures treated with the carrier and devitalized bones served as controls. For resorption studies, pregnant mice were given 45Ca on day 17 of pregnancy and fetal metatarsals harvested 24 h later. Resorption was determined by the amount of 45Ca released into the media. DNA synthesis was estimated by determining the incorporation of [3H]thymidine, collagen synthesis by measuring the incorporation of [3H]proline, mineralization by the incorporation of 45Ca, and proteoglycan synthesis by the incorporation of 35S. The amount of radiolabel was detected in media, as well as in noncultured, dead, and cultured rudiments. The total length of the rudiments and length of the calcified diaphyses were measured daily. In addition, rudiments from all experimental groups were prepared for light and electron microscopy. The high dose (10(-6) M) of 1,25-(OH)2D3 suppressed total rudiment growth but not the growth of the calcified diaphysis, 1,25-(OH)2D3 also decreased DNA, collagen, and proteoglycan synthesis, reduced calcification, and increased bone resorption in a dose-related manner. There were morphologic and ultrastructural changes in the osseous tissues and cells, particularly with the high dose of vitamin D, that supported the biochemical findings of suppressed activity of the osteogenic and chondrogenic cells. However, the suppression of collagen production and bone cell proliferation observed with the pharmacologic dose of vitamin D may be partially attributable to the decrease in bone mass (from increased resorption), thus resulting in less osseous tissue surface for these events to occur as endochondral osteogenesis progressed. The lower dose of vitamin D, however, had effects on 35S and 45Ca incorporation that could not be attributed to a decreased osseous tissue mass. This study emphasizes the importance of measuring specialized activities of the various cell populations in bone rudiment culture models to more fully understand the changes in tissue metabolism that result in changes in rudiment growth and modeling.

Actions of calcipotriol (MC 903), a novel vitamin D3 analog, on human bone-derived cells: comparison with 1,25-dihydroxyvitamin D3

The actions of a novel vitamin D3 analog calcipotriol (MC 903), on human bone-derived cells were compared to those of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. Both calcipotriol and 1,25-(OH)2D3 inhibited the proliferation of human osteoblast-like cells in a dose-dependent manner (10(-10)-10(-6) M), an effect observed at different cell densities. Lower concentrations of either agent exerted no marked effect on the growth of the cells compared to untreated cultures. Calcipotriol and 1,25-(OH)2D3 were equipotent in stimulating the activity of alkaline phosphatase and the synthesis of osteocalcin in human osteoblast-like cells. The stimulation of alkaline phosphatase activity and osteocalcin synthesis by both compounds was evident by 24 h and was increased progressively up to 96 h in a dose-dependent manner over the concentration range of 10(-10)-10(-6) M. The increment in both proteins was dependent on cell density and was attenuated at higher cell densities. In contrast to these actions, neither calcipotriol nor 1,25-(OH)2D3 (10(-14)-10(-6) M) affected the synthesis of prostaglandin E2. These studies indicate that calcipotriol and 1,25-(OH)2D3 exhibit a similar spectrum of activity on human osteoblast-like cells in vitro.

Prostaglandins enhance parathyroid hormone-evoked increase in free cytosolic calcium concentration in osteoblast-like cells

Prostaglandins (PGs) are autocrine or paracrine hormones that may interact with circulating hormones such as parathyroid hormone (PTH) in bone. We examined the interaction of the PGs, PGF2 alpha, PGE2, and 6-keto-PGF1 alpha with PTH to enhance the rapid, initial transient rise in free cytosolic calcium ([Ca2+]i) and cAMP levels stimulated by PTH. Pretreatment of UMR-106, MC3T3-E1, and neonatal rat calvarial osteoblast-like cells by PGs resulted in an enhancement of the early transient rise in [Ca2+]i stimulated by PTH. PGF2 alpha was approximately 100 times more potent than PGE2. PGE2 itself was more potent than 6-keto-PGF1 alpha in enhancing PTH-stimulated rise in [Ca2+]i. Near-maximal augmentation was achieved at PGF2 alpha doses of 10 nM and PGE2 of 1 microM. The degree of augmentation in [Ca2+]i by PGF2 alpha was independent of preincubation time. PGF2 alpha pretreatment did not alter the EC50 for the PTH-induced [Ca2+]i increase but only the extent of rise in [Ca2+]i at each dose of PTH. The augmented increase in [Ca2+]i was mostly due to enhanced PTH-mediated release of Ca2+ from intracellular stores. PGF2 alpha did not stimulate an increase in PTH receptor number as assessed by [125I]-PTH-related peptide binding. PG pretreatment partially reversed PTH inhibition of cell proliferation, suggesting that an increase in [Ca2+]i may play a role in tempering the anti-proliferative effect of PTH mediated by cAMP. These studies suggest a new mode by which PGs can affect cellular activity.

Immunoreactive calbindin-D9K in bone matrix vesicle

This electron microscope study describes the subcellular occurrence and distribution of immunoreactive calbindin-D9K in the trabecular metaphyseal and compact cortical bone of normal rats, rachitic vitamin-D-deficient rats, and rachitic rats given 1,25-(OH)2D3. Undecalcified bones were embedded in Lowicryl K4M and calbindin-D9K antigenicity was detected by the protein A-gold method. Immunoreactive calbindin-D9K was localized in the cytoplasm and cell processes of osteoblasts and osteocytes. Immunoreactive calbindin-D9K was also found within matrix vesicles and calcifying matrix vesicles, where it lay over the needle-shaped crystallites, at the apparent site of initial crystal formation, but not along the whole crystallites. In fully mineralized bone it occurred at the same site, over the crystallites. Calibindin-D9K was vitamin-D-dependent in the osteoblasts and matrix vesicles, where its presence was correlated with the reappearance of crystallites in 1,25-(OH)2D3-treated vitamin-D-deficient rats. This suggests that immunoreactive calbindin-D9K is involved in mineral deposition in bone matrix vesicles. Abnormal intracellular calcification associated with calbindin-D9K antigenicity in the osteoblasts of 1,25-(OH)2D3-treated vitamin-D-deficient rats indicates that immunoreactive calbindin-D9K may also play a part in abnormal intracellular mineral deposition.

Stimulation by 1,25-dihydroxyvitamin D3 of in vitro mineralization induced by osteoblast-like MC3T3-E1 cells

Although vitamin D is essential for mineralization of bone, it is as yet unclear whether vitamin D has a direct stimulatory effect on the bone mineralization process. In the present study, the effect of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] on in vitro mineralization mediated by osteoblast-like MC3T3-E1 cells was examined. MC3T3-E1 cells continued to grow after they reached confluency, and DNA content and alkaline phosphatase activity increased linearly until about 16 days of culture, whereas 45Ca accumulation into cell and matrix layer remained low. After this period, DNA content plateaued, and 45Ca accumulation increased sharply. Histological examination by von Kossa staining revealed that calcium was accumulated into extracellular matrix. In addition, needle-shaped mineral crystals similar to hydroxyapatite crystals could be demonstrated in between collagen fibrils by electron microscopy. Thus, MC3T3-E1 cells differentiate in vitro into cells with osteoblastic phenotype and exhibit mineralization. When MC3T3-E1 cells were treated with 1,25(OH)2D3 at this stage of culture, there was a dose-dependent stimulation of 45Ca accumulation by 1,25(OH)2D3, and a significant stimulation of 45Ca accumulation was observed with 3 x 10(-10) M 1,25(OH)2D3. Although 1,25(OH)2D3 enhanced alkaline phosphatase activity and collagen synthesis at the early phase of culture, it did not affect any of these parameters at the late phase when 1,25(OH)2D3 stimulated mineralization. Neither 24,25-dihydroxyvitamin D3 nor human PTH(1-34) affected mineralization in the presence or absence of 1,25(OH)2D3. These results demonstrate that 1,25(OH)2D3 stimulates matrix mineralization induced by osteoblastic MC3T3-E1 cells, and are consistent with the possibility that 1,25(OH)2D3 has a direct stimulatory effect on bone mineralization process.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of 1,25-(OH)2-vitamin D3 on growth, homologous receptor and c-myc regulation in C3H/10T1/2 cells

1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3) receptor concentration, cell proliferation, and the steady-state level of c-myc mRNA were examined in the C3H/10T1/2 mouse embryo fibroblasts, before and after exposing the cells to 1,25-(OH)2D3. The non-transformed, logarithmically growing C3H/10T1/2 Cl 8 cells contained a high concentration of 1,25-(OH)2D3 receptor (164 fmol/mg of protein). An up-regulation of the 1,25-(OH)2D3 receptor and a potent inhibition of cell growth were observed by exposing the cells to 10 nM 1,25-(OH)2D3. The concentration of 1,25-(OH)2D3 receptor in the two chemically transformed, tumorigenic cell lines. C3H/10T1/2 Cl 16 and C3H/10T1/2 TPA 482, was 218 and 63 fmol/mg of protein, respectively. In the two transformed cell lines, 10 nM 1,25-(OH)2D3 had only negligible effect on cell growth. In the Cl 16 cells, an up-regulation of the 1,25-(OH)2D3 receptor was demonstrated, but only a weak up-regulation was found in the TPA 482 cells by the 1,25-(OH)2D3 treatment. No major changes were found in c-myc mRNA levels by the 1,25-(OH)2D3 treatment. Despite inhibition of cell growth, the steady-state level of c-myc mRNA was slightly induced (35%, mean) in the Cl 8 cells compared to control cells. In the transformed cells, no consistent change of the c-myc level was found. In contrast to earlier reports, we did not find any correlation between the 1,25-(OH)2D3 receptor and c-myc level, nor did we find any decrease of c-myc mRNA by 1,25-(OH)2D3 treatment in the C3H/10T1/2 fibroblasts.

Osteocalcin secretion by the human osteosarcoma cell line MG-63

The human osteosarcoma cell line MG-63 has been used to study the production of the bone-specific protein, osteocalcin. In the absence of any stimuli, MG-63 cells secreted very low levels of osteocalcin. The secretion of osteocalcin started after a lag time of 10-12 h upon 1,25-(OH)2D3 treatment. Osteocalcin secretion was measured at doses as low as 0.03 nM (fourfold increase, p less than 0.05), and this activity increased further with higher doses of 1,25-(OH)2D3 to reach a plateau at 50 nM. The secretion increased transiently from very low levels in sparse cell cultures to peak values in subconfluent cultures (+/- 40%), two- to threefold above values obtained for confluent cells. Values for confluent cells average 55.9 +/- 2.0 ng/ml protein per 48 h. A similar behavior is observed for 1,25-(OH)2D3 receptor concentration under similar experimental conditions. Bmax increased transiently from sparse to subconfluent cell cultures (40-60% confluent) and reached values 50% lower in confluent cells. However, the receptor affinity was not affected by cell density. MG-63 cells also possessed an alkaline phosphatase isoenzyme of the bone-liver-kidney type that was stimulated by 1,25-(OH)2D3 treatment (two- to threefold) and inhibited by parathyroid hormone (40 nM, -25%, p less than 0.025). PTH and PGE2 increased cAMP production in a dose-dependent manner, but the cells were irresponsive to salmon calcitonin. Basal and PTH-responsive cyclic AMP production were also modulated by cell density. Dexamethasone pretreatment (100 nM, 48 h) stimulated the PTH-dependent cAMP production but failed to influence the response to PGE2.(ABSTRACT TRUNCATED AT 250 WORDS)

Calbindin-D9K immunolocalization and vitamin D-dependence in the bone of growing and adult rats

This report presents evidence for the presence of the vitamin D-dependent calcium-binding protein, calbindin-D9K, in bone cells and matrix. In undecalcified frozen sections of growing and adult rat bone, calbindin-D9K was immunohistochemically localized in trabecular bone of the epiphysis and metaphysis and in cortical bone of the diaphysis. It was found within the cytoplasm of osteocytes, of osteoblasts lining the osteoid, and osteoblasts inside the osteoid seams. It was also found in the osteoblast processes and the anastomosed reticulum of the processes connecting the osteocytes with each other. Extracellularly, calbindin-D9K immunoreactivity was present in compact cortical bone in the areas of the mineralized matrix surrounding the osteocyte lacunae, and in the pericanalicular walls containing the cell processes. Calbindin-D9K immunoreactivity was low or absent from the cytoplasm of osteocytes in trabecular bone from severely vitamin D-deficient rats and restored in vitamin D-deficient rats given a single dose of 1,25(OH)2-VitD3. Thus, the synthesis of immunoreactive calbindin-D9K by osteoblasts and osteocytes in trabecular bone is vitamin D-dependent. The presence of immunoreactive calbindin-D9K in the osteocytes and their cell processes suggests that this calcium-binding protein is involved in the calcium fluxes regulating bone calcium homeostasis. Its localization in osteoblasts involved in bone formation and in their cell processes suggests that it has a role in the calcium transport from these cells towards the sites of active bone mineralization.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormonal responses to 1,25-dihydroxyvitamin D3 in cultured mouse osteoblast-like cells--modulation by changes in receptor level

The level of 1,25(OH)2D3 receptors in cultured mouse osteoblast-like (OB) cells is modulated by the rate of cell proliferation. We have studied two 1,25(OH)2D3-induced bioresponses to ascertain whether the changes in receptor levels during growth in culture alter cell responsiveness. Nuclear receptor levels were high (127 fmol/100 micrograms DNA) in rapidly dividing (log) cells and low (25 fmol/100 micrograms DNA) in quiescent (confluent) cells. The bioresponses we studied were induction of 25(OH)D3-24-hydroxylase activity (24-hydroxylase) and inhibition of collagen synthesis. The basal levels of 24-hydroxylase were low and similar in cells at log growth phase and confluence. At a maximal induction dose of 13 nM, 1,25(OH)2D3 induced a three-fold rise in enzyme activity at long growth phase, but only caused less than two-fold rise at confluence. The half-maximal dose (ED50) was slightly shifted from 0.6 nM to 0.8 nM. Daily measurement of 1,25(OH)2D3 receptor levels and maximal induction of 24-hydroxylase activity throughout the culture cycle showed a strong correlation between receptor abundance and enzyme induction. The basal level of collagen synthesized by cells in log growth phase was approximately 5% and increased to approximately 8% at confluence. Maximal inhibition of collagen synthesis by 1,25(OH)2D3 reached 80% of control levels in log cells, but was only 40% of control in confluent cells. The ED50 was approximately 0.1 nM in the log cells and increased to approximately 1 nM at confluence. Daily assay of 1,25(OH)2D3 receptor levels and 1,25(OH)2D3 responses during the culture cycle indicated a correlation between changes in receptor level and the extent of inhibition of collagen synthesis. These changes in bioresponse at various growth phases did not occur in rat OB cells where the 1,25(OH)2D3 receptor levels were independent of cell proliferation. The results indicate that cell proliferation rate, via change in receptor levels, determines the magnitude and sensitivity of the cellular responses to 1,25(OH)2D3.

Identification of a monoclonal antibody which interacts with the parathyroid hormone receptor-adenylate cyclase system in murine bone

We have produced monoclonal antibodies which bind specifically to mouse bone cells and then selected these monoclonal antibodies for their ability to inhibit parathyroid hormone (PTH) responses in mouse cranial bone treated with the (1-34) amino terminal peptide of bovine PTH [bPTH(1-34)]. One clone, designated 3-6, characterized as an IgM(kappa), significantly inhibited the accumulation of cAMP in response to bPTH(1-34) at concentrations of hormone between 10(-9) and 10(-7) M. This antibody was subsequently isolated by gel filtration and shown to bind to intact mouse calvariae, with saturation binding occurring at 3 micrograms/ml IgM. A maximal inhibition of approximately 70% of the cAMP accumulation produced in response to 2.5 X 10(-9) M (100 ng/ml) bPTH(1-34) was obtained with 7 micrograms/ml of the purified 3-6 IgM. At this concentration of 3-6 IgM, the half-maximal dose of PTH for activation of cAMP accumulation was increased from 5 X 10(-9) M to 2 X 10(-8) M with no reduction in maximal levels of cAMP production. The utility of this antibody as an inhibitor was further tested by its ability to block the binding of an iodinated PTH analogue, 125I-[Nle8, Nle18, Tyr34]-bPTH(1-34) to mouse cranial bone. The 3-6 IgM at a concentration of 5 X 10(-8) M inhibited 70% of the specific binding of the 125I-labeled analogue. In the absence of parathyroid hormone, 2 X 10(-8) M 3-6 IgM produced a 4-fold increase in cAMP above basal levels, as compared to 40-fold maximal increases observed with PTH, indicating a partial PTH agonist activity of this antibody. When tested for effects on other hormones, 3-6 IgM did not inhibit cAMP accumulation produced in response to salmon calcitonin, epinephrine, prostaglandin E2 or cholera toxin. We propose that the 3-6 monoclonal IgM is specific for the PTH receptor or a component of the PTH receptor-adenylate cyclase system and that this or similar antibodies will serve as useful reagents for future molecular characterization of this receptor.

Vitamin D and skeletal tissues

It is now accepted that vitamin D is an integral part of a complex endocrine system, one with far-reaching implications in mineral metabolism. Reviews of the sources, functions and metabolism of vitamin D, as currently understood, are presented as a prelude to discussions of the role of vitamin D in calcium and phosphorous homeostatis and possible specific roles for vitamin D in mineralized tissues. Data describing a possible regulatory function for vitamin D in bone and bone protein metabolism are presented. Some of the controversy which presently exists regarding the biochemical mechanism of the action of this vitamin is discussed. Finally, the possible relationship of vitamin D and disorders of skeletal tissues is described.

Biological activity assessment of the 26,23-lactones of 1,25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3 and their binding properties to chick intestinal receptor and plasma vitamin D binding protein

The binding of the natural and unnatural diastereoisomers 25-hydroxyvitamin D3-26,23-lactone and 1,25 dihydroxyvitamin D3-26,23-lactone to the vitamin D-binding protein (DBP) and 1,25 dihydroxyvitamin D3 [1,25(OH)2D3] chick intestinal receptor have been investigated. Also, the biological activities, under in vivo conditions, of these compounds, in terms of intestinal calcium absorption (ICA) and bone calcium mobilization (BCM), in the chick are reported. The presence of the lactone ring in the C23-C26 position of the seco-steroid side chain increased two to three times the ability of both 25(OH)D3 and 1,25(OH)2D3 to displace 25(OH)[3H]D3 from the D-binding protein; however, the DBP could not distinguish between the various diastereoisomers. In contrast, the unnatural form (23R,25S) of the 25-hydroxy-lactone was found to be 10-fold more potent than the natural form, and the unnatural (23R,25S)1,25(OH)2D3-26,23-lactone three times more potent than the natural 1,25-dihydroxy-lactone in displacing 1,25(OH)2[3H]D3 from its intestinal receptor. While studying the biological activity of these lactone compounds, it was found that the natural form of the 25-hydroxy-lactone increased the intestinal calcium absorption 48 h after injection (16.25 nmol), while bone calcium mobilization was decreased by the same dose of the 25-hydroxy-lactone. The 1,25-dihydroxyvitamin D3-26,23-lactone in both its natural and unnatural forms was found to be active in stimulating ICA and BCM. These results suggest that the 25-hydroxy-lactone has some biological activity in the chick and that 1,25(OH)2D3-26,23-lactone can mediate ICA and BCM biological responses, probably through an interaction with 1,25-(OH)2D3 specific receptors in these target tissues.

Ontogenesis of the rabbit intestinal receptor for 1,25-dihydroxyvitamin D3--evidence for increased receptor content during late suckling and lactating periods

The 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) intestinal receptor was not detected in term fetal rabbits. This receptor was present at 2 weeks postpartum and its concentration reached a maximum at 4 weeks of age, and declined to adult levels by 10 weeks postpartum. The 1,25(OH)2D3 intestinal receptor concentration was elevated at 2 weeks postpartum in lactating rabbits, but returned to normal adult concentrations by 4 weeks postpartum. In rabbits of various ages, only minor changes in the equilibrium dissociation constant of this receptor were observed. These data indicate that increasing the small intestine 1,25(OH)2D3 receptor concentration is one mechanism by which the rabbit adapts to periods of increased calcium demand.

Possible significance of new target tissues for 1,25-dihydroxyvitamin D3

A sensitive sucrose gradient procedure provided evidence for specific 3.6S 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] receptor-like binding components with low affinity for 25(OH)D3 in several reproductive target tissues in the rat, including testis, uterus and probably epididymis. Thus, the presence of 1,25(OH)2D3 receptors in non-vitamin D endocrine tissues is not restricted to the hormone secreting glands. Estrogen-stimulated uterine growth paralleled increased levels of the putative 1,25(OH)2D3 receptors, providing in vivo evidence for the concept of a relationship between these phenomena. However, a similar correlation was not observed in the growing testis (prepubertal vs mature rats). Whether this result stems from receptor localization in a cell type prevalent in the adult or a true dissociation between testis growth and receptor levels is unclear. Finally, significant levels (10.3 +/- 1.0% vs intestinal mucosa) of the putative 1,25(OH)2D3 receptors were found in the heart, an organ in which calcium mediates many specialized functions. Taken collectively, these observations on new target tissues of quite different overall function suggest that 1,25(OH)2D3 and its receptors may play a role in intracellular calcium homeostasis and possibly in regulating specialized intracellular functions of calcium.