Autoradiographic localization of target cells for 1 alpha, 25-dihydroxyvitamin D3 in bones from fetal rats

Vitamin D and bone

All cells comprising the skeleton-chondrocytes, osteoblasts, and osteoclasts-contain both the vitamin D receptor and the enzyme CYP27B1 required for producing the active metabolite of vitamin D, 1,25 dihydroxyvitamin D. Direct effects of 25 hydroxyvitamin D and 1,25 dihydroxyvitamin D on these bone cells have been demonstrated. However, the major skeletal manifestations of vitamin D deficiency or mutations in the vitamin D receptor and CYP27B1, namely rickets and osteomalacia, can be corrected by increasing the intestinal absorption of calcium and phosphate, indicating the importance of indirect effects. On the other hand, these dietary manipulations do not reverse defects in osteoblast or osteoclast function that lead to osteopenic bone. This review discusses the relative importance of the direct versus indirect actions of vitamin D on bone, and provides guidelines for the clinical use of vitamin D to prevent/treat bone loss and fractures.

Vitamin D metabolism, cartilage and bone fracture repair

The 1,25-(OH)(2)D metabolite mediates the endocrine actions of vitamin D by regulating in the small intestine the expression of target genes that play a critical role in intestinal calcium absorption. The major role of the vitamin D hormone on bone is indirect and mediated through its endocrine function on mineral homeostasis. However, genetic manipulation of the expression of Cyp27b1 or the VDR in chondrocytes strongly support a direct role for locally synthesized 1,25(OH)(2)D, acting through the VDR, in vascular invasion and osteoclastogenesis during endochondral bone development. Cells from the growth plate respond to the 24,25-(OH)(2)D and 1,25-(OH)(2)D metabolites in a cell maturation-dependent manner and the effects of 1,25-(OH)(2)D are thought to be mediated through binding to the membrane-associated receptor PDIA3 (protein disulfide isomerase associated 3). The physiological relevance of membrane-mediated 1,25-(OH)(2)D signaling is emerging and is discussed. Finally, preliminary results suggest that mice deficient for Cyp24a1 exhibit a delay in bone fracture healing and support a role for 24,25-(OH)(2)D in mammalian fracture repair.

Chondrocyte-specific modulation of Cyp27b1 expression supports a role for local synthesis of 1,25-dihydroxyvitamin D3 in growth plate development

The Cyp27b1 enzyme (25-hydroxyvitamin D-1alpha-hydroxylase) that converts 25-hydroxyvitamin D into the active metabolite, 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], is expressed in kidney but also in other cell types such as chondrocytes. This suggests that local production of 1,25(OH)(2)D(3) could play an important role in the differentiation of these cells. To test this hypothesis, we engineered mutant mice that do not express the Cyp27b1 gene in chondrocytes. Inactivation of both alleles of the Cyp27b1 gene led to decreased RANKL expression and reduced osteoclastogenesis, increased width of the hypertrophic zone of the growth plate at embryonic d 15.5, increased bone volume in neonatal long bones, and increased expression of the chondrocytic differentiation markers Indian Hedgehog and PTH/PTHrP receptor. The expression of the angiogenic marker VEGF was decreased, accompanied by decreased platelet/endothelial cell adhesion molecule-1 staining in the neonatal growth plate, suggesting a delay in vascularization. In parallel, we engineered strains of mice overexpressing a Cyp27b1 transgene in chondrocytes by coupling the Cyp27b1 cDNA to the collagen alpha(1)(II) promoter. The transgenic mice showed a mirror image phenotype when compared with the tissue-specific inactivation, i.e. a reduction in the width of the hypertrophic zone of the embryonic growth plate, decreased bone volume in neonatal long bones, and inverse expression patterns of chondrocytic differentiation markers. These results support an intracrine role of 1,25(OH)(2)D(3) in endochondral ossification and chondrocyte development in vivo.

Bone parameters are improved with intermittent dosing of vitamin D3 and calcitonin

Intermittent combination of an anabolic agent to promote bone formation and an antiresorptive agent that would prevent further bone loss is a theoretically attractive approach for restoring bone mass. We tested the potential of intermittently dosed calcitriol and calcitonin (CT) to restore bone properties in ovariectomized (Ovx) rats. Rats had Ovx or sham surgery at 8 weeks old and 4 weeks later were assigned to experimental groups: (1) sham vehicle, (2) Ovx vehicle, (3) Ovx + parathyroid hormone (PTH, 40 microg/kg), and (4) Ovx + calcitriol (2 microg/kg) + CT (2 microg/kg). Group 3 received PTH every week throughout the study, and group 4 received calcitriol at weeks 1, 3, 5, and 7 and CT at weeks 2, 4, 6, and 8. Dosing was carried out for 8 weeks with serum, and micro-computed tomographic analysis was done at 0, 4, and 8 weeks. Femurs and tibias were used for radiological analyses and for mechanical testing. Dosing with PTH improved bone mass and structure of cancellous bone at metaphyses of tibias and femurs as well as properties of cortical bone including geometry and strength. Intermittent dosing with calcitriol and CT was less potent in correcting loss of cancellous bone relative to treatment with PTH and had no effect on cortical bone parameters. However, intermittent dosing with calcitriol and CT was robust enough to improve cancellous bone mass and structure through bone formation without causing deleterious side effects. Our data provide additional evidence that therapies can be devised to ameliorate the skeletal defects associated with established osteoporosis.

Incorporation of Cestrum diurnum leaf improves intestinal Ca transport in broilers

The economy of Ca utilization is under the control of vitamin D(3), particularly its active metabolite 1,25-dihydroxy cholecalciferol [1,25(OH)(2)D(3)]. In sufficient Ca absorption leads to tibial dyschondroplasia resulting in not attaining optimum body weight. Our earlier studies [T.P. Prema, N. Raghuramulu, Phytochemistry 37 (1994) 167] have shown that the Cestrum diurnum (CD) leaves contain vitamin D(3) metabolites. It was felt whether incorporation of CD as a source of 1,25(OH)(2)D(3) could improve the Ca absorption in broilers. Four groups of 60 birds each were fed with either normal diet or normal diet+0.25% CD or normal diet without vitamin D(3) or normal diet without vitamin D(3)+0.25% CD leaf powder for 45 days. In subsample of six birds it was observed that incorporation of CD leaves in the feed had the maximal effect on all the parameters studied. The results indicate that the intestinal Ca transport as represented by Serosa/Mucosa (S/M) ratio was found to be significantly (p<0.01) higher in broilers fed diet with CD leaf powder and the 1alpha hydroxylase activity in kidney is significantly (p<0.001) higher in negative controls. On the other hand the supplementation of CD leaves enhanced the serum Ca, body weight, tibia weight, density and strength resulting in the disappearance of tibial dyschondroplasia. No lesions of toxicity were observed in any of the soft tissue examined. The results suggest that the incorporation of CD leaf powder in poultry feed could be beneficial to the poultry.

A new vitamin D analog, 2MD, restores trabecular and cortical bone mass and strength in ovariectomized rats with established osteopenia

An orally active, highly potent analog of 1alpha,25-dihydroxyvitamin D3, 2MD, restores trabecular and cortical bone mass and strength by stimulating periosteal bone formation and decreasing trabecular bone resorption in OVX rats with established osteopenia.

INTRODUCTION

The purposes of this study were to determine the effects of long-term treatment with 2-methylene-19-nor-(20S)-1alpha,25(OH)2D3 (2MD) on restoring bone mass and bone strength in ovariectomized (OVX) rats with established osteopenia and 2MD effects on bone formation and bone resorption on trabecular and cortical bone surfaces.

MATERIALS AND METHODS

Sprague-Dawley female rats were sham-operated (sham) or OVX at 4 months of age. Beginning at 8 weeks after OVX, OVX rats were orally dosed with 2MD at 0.5, 1, 2.5, 5, or 10 ng/kg/day for 16 weeks. Serum calcium was measured at 6, 13, and 16 weeks after treatment, and bone mass and structure, bone formation, bone resorption, and bone strength were determined at the end of the study.

RESULTS

Serum calcium did not change significantly with 2MD at 0.5 or 1 ng/kg/day, whereas it significantly increased at 2.5, 5, or 10 ng/kg/day. 2MD significantly and dose-dependently increased total body BMD, total BMC, and stiffness of femoral shaft (FS), maximal load and stiffness of femoral neck, and toughness of the fifth lumbar vertebral body (L5) at all doses compared with OVX controls. In 2MD-treated OVX rats, there was a dose-dependent increase in total BMD and total BMC of the distal femoral metaphysis (DFM), trabecular bone volume of L3, ultimate strength and stiffness of L5, and maximal load of FS compared with OVX controls at dosages>or=1 ng/kg/day. At dosages>2.5 ng/kg/day, most of the bone mass and bone strength related parameters were significantly higher in 2MD-treated OVX rats compared with sham controls. Bone histomorphometric analysis of L3 showed dose-dependent decreases in osteoclast number and osteoclast surface on trabecular bone surface and a dose-dependent increase in periosteal bone formation associated with 2MD treatment.

CONCLUSIONS

2MD not only restored both trabecular and cortical bone mass but also added bone to the osteopenic OVX rats beyond that of sham controls by stimulating bone formation on the periosteal surface and decreasing bone resorption on the trabecular surface. 2MD increased bone mass and strength at doses that did not induced hypercalcemia.

Cell lines and primary cell cultures in the study of bone cell biology

Bone is a metabolically active and highly organized tissue consisting of a mineral phase of hydroxyapatite and amorphous calcium phosphate crystals deposited in an organic matrix. Bone has two main functions. It forms a rigid skeleton and has a central role in calcium and phosphate homeostasis. The major cell types of bone are osteoblasts, osteoclasts and chondrocytes. In the laboratory, primary cultures or cell lines established from each of these different cell types provide valuable information about the processes of skeletal development, bone formation and bone resorption, leading ultimately, to the formulation of new forms of treatment for common bone diseases such as osteoporosis.

Comparison of the effects of 1,25 dihydroxycholecalciferol and prostaglandin E2 on orthodontic tooth movement

This study compared the effects of local administrations of prostaglandin E2 (PGE2) and 1,25-dihydroxycholecalciferol (1,25-DHCC) on orthodontic tooth movement in rats. Thirty-seven 6-week-old male Sprague-Dawley rats, weighing 160 +/- 10 g were used. Five rats served as the baseline control group. A fixed appliance system exerting 20 g of distally directed force was applied on the maxillary incisors of 32 animals for 9 days. Eight rats served as the appliance control group; 8 received a 20-microL injection of dimethyl sulfoxide (solvent for 1,25-DHCC) on days 0, 3, and 6; 8 received 20 microL of 10(-10) mol/L 1,25-DHCC on days 0, 3, and 6; 8 received a single injection of 0.1 mL of 0.1 microg PGE2 only on day 0. There was no significant difference in tooth movement between the PGE2 and the 1,25-DHCC groups. Both PGE2 and 1,25-DHCC enhanced the amount of tooth movement significantly when compared with the control group. The numbers of Howship's lacunae and capillaries on the pressure side were significantly greater in the PGE2 group than in the 1,25-DHCC group. On the other hand, the number of osteoblasts on the external surface of the alveolar bone on the pressure side was significantly greater in the 1,25-DHCC group than in the PGE2 group. Thus, 1,25-DHCC was found to be more effective in modulating bone turnover during orthodontic tooth movement, because its effects on bone formation and bone resorption were well balanced.

Modulation of growth factor/cytokine synthesis and signaling by 1alpha,25-dihydroxyvitamin D(3): implications in cell growth and differentiation

Distinct from its classic functions in the regulation of calcium and phosphorus metabolism as a systemic hormone, 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)] is involved in the local control and regulation of cellular growth and differentiation in various tissues, including epidermis (keratinocytes) and bone (osteoblasts and osteoclasts). In this review, the impact of 1alpha,25(OH)(2)D(3) on growth factor/cytokine synthesis and signaling is discussed, particularly as it pertains to bone cells and keratinocytes. 1alpha,25(OH)(2)D(3) not only regulates growth factor/cytokine synthesis but may also alter growth factor signaling. Recently discovered examples for such interactions are the interactions between the vitamin D receptor and the mothers against decapentaplegic-related proteins that function downstream of TGFbeta receptors. Inhibitory effects of 1alpha,25(OH)(2)D(3) on keratinocytes through TGFbeta activation and IL-1alpha, IL-6, and IL-8 suppression may provide a rationale for its beneficial effects in the treatment of hyperproliferative skin disorders, whereas stimulatory effects through the epidermal growth factor-related family members and platelet-derived growth factor may be operative in its beneficial effects in skin atrophy and wound healing. Modulation of cytokines and growth factors by 1alpha,25(OH)(2)D(3) during bone remodeling plays an important role in the coupling of osteoblastic bone formation with osteoclastic resorption to maintain bone mass.

A chimeric form of osteoprotegerin inhibits hypercalcemia and bone resorption induced by IL-1beta, TNF-alpha, PTH, PTHrP, and 1, 25(OH)2D3

Osteoprotegerin (OPG) is a secreted protein that inhibits osteoclast formation and activity and appears to be a critical regulator of bone mass and metabolism. In the current study, mice were challenged with various cytokines and hormones (interleukin-1beta, tumor necrosis factor-alpha, parathyroid hormone, parathyroid hormone-related protein, and 1alpha,25-dihydroxyvitamin D3) that are known to increase bone resorption and cause hypercalcemia and treated concurrently with either a recombinant chimeric Fc fusion form of human OPG, with enhanced biological activity (cOPG) (2.5 mg/kg/day) or vehicle. Mice receiving these bone-resorbing factors became hypercalcemic by day 3 after commencing treatment and had increased bone resorption as evidenced by elevated osteoclast numbers on day 5. Concurrent cOPG treatment prevented hypercalcemia (p < 0.05) and maintained osteoclast numbers in the normal range (p < 0.001). The demonstration that cOPG can inhibit bone resorption suggests that this molecule may be useful in the treatment of diseases including hyperparathyroidism, humoral hypercalcemia of malignancy, osteoporosis, and inflammatory bone disease, which are characterized, in part, by increases in osteoclastic bone resorption.

Culture of stromal cells derived from medullary cavity of human long bone in the presence of 1,25-dihydroxyvitamin D3, recombinant human bone morphogenetic protein-2, or ipriflavone

We previously showed that stromal cells derived from bone marrow specimens formed at the fracture site of human long bone differentiated during culture to polygonal cells and spindle cells, and polygonal cells, but not spindle cells, produced calcified matrix. To clarify the origin of polygonal and/or spindle cells, and factors necessary for differentiation of marrow stromal cells to osteogenic cells, we cultured stromal cells derived from the normal (unfractured) medullary cavity (SCN) as well as stromal cells from the medullary cavity distant from the fracture site (SCF). After 3 weeks of primary culture and 2 days of secondary culture, the cells were cultured in medium containing 1,25-dihydroxyvitamin D3 (VD), recombinant human bone morphogenetic protein-2 (BMP), or ipriflavone (IF) for 3 weeks. For biochemical analysis, cells reaching confluence after 3 weeks of secondary culture were cultured with one of the factors for 3 days. Some of SCF cultured with VD or IF were transformed to polygonal cells, and showed high alkaline phosphatase (ALPase) activity and high osteocalcin and insoluble calcium production. Cloned polygonal cells from the SCF formed nodules and aggregates consisting of calcium. Other SCF cultured with VD or IF and SCF cultured with BMP were spindle shaped. Some spindle-shaped cells from SCF cultured with BMP or IF revealed high ALPase activity and high osteocalcin production, comparable with the spindle cells from the fracture site. However, spindle-shaped cells from SCF cultured with VD and other spindle-shaped cells from SCF cultured with BMP or IF showed low ALPase activity and low osteocalcin production. The results show that SCF probably contain at least three subpopulations: (a) cells that differentiate to polygonal cells by the influence of VD or IF; (b) cells that differentiate to the spindle cells by the influence of BMP or IF; and (c) cells that are not transformed by the influence of VD, BMP, or IF.

Receptor localization of steroid hormones and drugs: discoveries through the use of thaw-mount and dry-mount autoradiography

The history of receptor autoradiography, its development and applications, testify to the utility of this histochemical technique for localizing radiolabeled hormones and drugs at cellular and subcellular sites of action in intact tissues. Localization of diffusible compounds has been a challenge that was met through the introduction of the "thaw-mount" and "dry-mount" autoradiographic techniques thirty years ago. With this cellular receptor autoradiography, used alone or combined with other histochemical techniques, sites of specific binding and deposition in vivo and in vitro have been characterized. Numerous discoveries, some reviewed in this article, provided information that led to new concepts and opened new areas of research. As an example, in recent years more than fifty target tissues for vitamin D have been specified, challenging the conventional view about the main biological role of vitamin D. The functions of most of these vitamin D target tissues are unrelated to the regulation of systemic calcium homeostasis, but pertain to the (seasonal) regulation of endo- and exocrine secretion, cell proliferation, reproduction, neural, immune and cardiovascular responses, and adaptation to stress. Receptor autoradiography with cellular resolution has become an indispensable tool in drug research and development, since information can be obtained that is difficult or impossible to gain otherwise.

Regulation of connexin43 expression and function by prostaglandin E2 (PGE2) and parathyroid hormone (PTH) in osteoblastic cells

Connexin43 (Cx43) forms gap junctions that mediate intercellular communication between osteoblasts. We have examined the effects of prostaglandin E2 (PGE2) and parathyroid hormone (PTH) on gap junctional communication in the rat osteogenic sarcoma cells UMR 106-01. Incubation with either PGE2 or PTH rapidly (within 30 min) increased transfer of negatively charged dyes between UMR 106-01 cells. This stimulatory effect lasted for at least 4 h. Both PGE2 and PTH increased steady-state levels of Cx43 mRNA, but only after 2-4 h of incubation. Transfection with a Cx43 gene construct linked to luciferase showed that this effect of PTH was the result of transcriptional upregulation of Cx43 promoter. Stimulation of dye coupling and Cx43 gene transcription were reproduced by forskolin and 8Br-cAMP. Exposure to PGE2 for 30 min increased Cx43 abundance at appositional membranes in UMR 106-01, whereas total Cx43 protein levels increased only after 4-6 h of incubation with either PGE2 or PTH. Inhibition of protein synthesis by cycloheximide did not affect this early stimulation of dye coupling, but it significantly inhibited the sustained effect of PTH and forskolin on cell coupling. In summary, both PTH and PGE2, presumably through cAMP production, enhance gap junctional communication in osteoblastic cell cultures via two mechanisms: initial rapid redistribution of Cx43 to the cell membrane, and later stimulation of Cx43 gene expression. Modulation of intercellular communication represents a novel mechanism by which osteotropic factors regulate the activity of bone forming cells.

Short-term treatment of rats with high dose 1,25-dihydroxyvitamin D3 stimulates bone formation and increases the number of osteoblast precursor cells in bone marrow

Using an experimental rat model, this study was undertaken to assess the effects of a short-term application of high dose 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] on calcium homeostasis, cancellous bone formation, and numbers of osteoblast precursors in ex vivo bone marrow cultures. For Exp 1 and 2, 6-month-old female rats were sc injected with either 0.2 microg 1,25-(OH)2D3/kg x day or vehicle on days 1, 2, and 3 of the studies. Serum calcium and urinary excretion of calcium were monitored for 12 days in Exp 1. In Exp 2, the rats were ip labeled with five different fluorochromes on days 0, 5, 10, 15, and 20, respectively. Half of the rats in each group were killed on day 7, the rest of the rats were killed on day 24, and the first lumbar vertebrae were processed for histomorphometry. In Exp 3, 0.2 microg 1,25-(OH)2D3/kg BW or vehicle was sc administered to 6-month-old male rats on days 1, 2, and 3, and the number of colony-forming units with the ability to express alkaline phosphatase, to calcify, and/or to synthesize collagen were enumerated sequentially on days 4, 6, 8, 10, 12, and 14 in bone marrow cultures. Short-term 1,25-(OH)2D3 treatment resulted in increased values for serum and urinary calcium during the treatment phase in Exp 1, depressed osteoclast numbers and strongly elevated osteoblast perimeter by day 7, and stimulated mineral apposition rate and bone formation rate in the interval between days 5-15 of Exp 2. Moreover, 1,25-(OH)2D3 administration to rats significantly enhanced the number of mesenchymal precursor cells in bone marrow with the ability to differentiate into an osteoblastic phenotype in ex vivo bone marrow cultures on day 4 of Exp 3. These studies provide evidence that short-term 1,25-(OH)2D3 treatment creates new bone remodeling units and augments osteoblast recruitment and osteoblast team performance in rat cancellous bone.

Demonstration of vitamin D receptor transcripts in actively resorbing osteoclasts in bone sections

The effects of the active metabolite of vitamin D, 1,25 dihydroxyvitamin D3 (1,25D), are mediated via the vitamin D receptor (VDR). 1,25D is known to have profound effects on bone resorption, but proof that the human osteoclast expresses VDR in vivo is absent. Receptors have been demonstrated in osteoblasts, and it has been generally accepted that the effects of 1,25D on formed osteoclasts are mediated via osteoblasts. Using conventional riboprobe in situ hybridization, VDR transcripts were readily detectable in osteoblasts within sections taken from normal bone and several actively remodelling bone tissues, namely, Paget's disease, renal hyperparathyroidism, and healing fracture callus. However, VDR transcripts also appeared to be present at low levels within osteoclasts from two pagetic samples and two hyperparathyroid samples. To examine this latter finding further, we have used the novel technique of in situ-reverse transcriptase-polymerase chain reaction (IS-RT-PCR) for specific amplification and detection of VDR mRNA within sections taken from the same conditions described above, and also from osteoclastoma samples. As expected, VDR transcripts were amplified and detected in osteoblasts and marrow cells, but were also prominently found in osteoclasts at approximately 50% of the level detected in osteoblasts in normal bone and at 60% in the active bone tissues. This suggests that in addition to effects on osteoclast precursors and those mediated via osteoblasts, 1,25D could exert direct effects on the active bone resorbing cells in vivo.

Estrogen pretreatment increases arachidonic acid release by bradykinin stimulated normal human osteoblast-like cells

Eicosanoids are multifunctional autocrine/paracrine regulators of bone that are enzymatically derived from arachidonic acid (AA). The rate-limiting step in the eicosanoid biosynthetic pathways may be the release of AA from membrane glycerophospholipids by activated phospholipases. Free AA can serve as the substrate for cyclooxygenase(s) or lipoxygenases that catalyze the commitive steps in eicosanoid synthesis; alternatively, free AA may be used in reacylation processes, resulting in its reincorporation into cellular lipids. The hormones 17 beta-estradiol (17 beta-E2), dexamethasone (a synthetic glucocorticoid), and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) have been identified as regulators of AA metabolism, at various levels, in several tissues including bone. The possibility that these osteotropic steroids modulate the availability of free AA in bone cells was studied in the human osteoblast-like (hOB) cell model system. Following a 48-h steroid pretreatment, bradykinin or the calcium ionophore A23187 were used as agonists to stimulate hOB cell release of AA. The principal findings from these investigations were that (1) 17 beta-E2 pretreatment potentiated the appearance of free AA following bradykinin stimulation of the cells but, did not alter their response to A23187 stimulation; (2) dexamethasone pretreatment limited bradykinin-induced increases in free AA levels but did not alter cell response to A23187 stimulation; (3) hOB cells derived from different trabecular bone compartments (manubrium of the sternum, femoral head) differed quantitatively in their responses to bradykinin stimulation of AA release; and (4) 1,25(OH)2D3 did not effect AA release stimulated by either agonist. The ability of the steroids to modulate AA release by hOB cells suggests that these hormones may indirectly mediate bone cell responses to other osteotropic hormones that act through eicosanoid-dependent processes.

Cytokine regulation of bone cell differentiation

Systemic hormones and cytokines play important roles in regulating both osteoblast and osteoclast activity. These cytokines can have either positive or negative effects on the growth and differentiation of bone cells. These effects appear to be dependent on the model systems use to assess them, as well as the species tested. In the near future, other autocrine-paracrine factors will be identified that enhance osteoblast and osteoclast activity, and model systems should be available to further delineate their effects on cells in the osteoblast lineage. Use of transgenic mice with genes targeted to the osteoblast and osteoclast may further reveal the mechanisms responsible for the growth and differentiation of these cells, as well as produce immortalized cell lines that more accurately reflect the cell biology of the osteoclast and osteoblast in vivo.

Ontogeny of the 1,25-dihydroxyvitamin D3 receptor in fetal rat bone

To gain insights into 1,25-dihydroxyvitamin D3 receptor (VDR) function during fetal bone development, we examined fetal rat tissues from gestational days 13-21 for the presence and distribution of VDR using immunohistochemistry. Prior to ossification, VDR epitopes were observed in the mesenchyme condensing to form skeletal tissues, on day 13 in the developing vertebral column and limbs, and on day 17 of gestation in developing calvaria. Immunostaining for VDR was seen in proliferating and hypertrophic chondrocytes and in osteoblasts of limb buds and the vertebral column by day 17 of gestation. In calvaria, VDR epitopes were observed in osteoblasts by gestational day 19. VDR immunostaining was also evident in the skin of fetal limbs at all gestational ages examined. We show for the first time that the VDR appears very early in the developing fetal rat skeleton, suggesting that the VDR, in concert with its ligand, 1,25-dihydroxyvitamin D3, may play a role in the differentiation of mesenchymal precursors into bone tissue.

1,25-Dihydroxyvitamin D3 and 22-oxa-1,25-dihydroxyvitamin D3 in vivo nuclear receptor binding in developing bone during endochondral and intramembranous ossification

Target cells for 3H-labeled 1 alpha, 25(OH)2 vitamin D3 [1,25(OH)2D3, vitamin D] and its analog 3H-labeled 22-oxa-1 alpha, 25(OH)2 vitamin D3 (OCT) have been identified during endochondral and intramembranous ossification in developing, undecalcified, unembedded bone, using thaw-mount autoradiography. Two-day-old neonatal rats were injected with [3H]1,25(OH)2D3 or [3H]OCT; after 2 h leg, spine, and head were frozen and sectioned. In the epiphyseal-metaphyseal region specific nuclear concentrations of [3H]1,25(OH)2D3 and [3H]OCT were observed in identical cell populations, being low in cells of the articular and resting zone, intermediate in the proliferating zone, and highest in hypertrophic chondrocytes and in osteoblasts and precursor cells. In the primary spongiosa intertrabecular spaces there were a large number of cells with nuclear labeling--probably osteoblasts and precursor cells. In contrast, in the secondary spongiosa intertrabecular spaces, apparent blood-forming cells were mostly unlabeled. Osteoblasts along bone spicules and compact bone in long bones, vertebrae, and head also showed strong nuclear labeling, as did cells of the periosteum. These data suggest that 1,25(OH)2D3 and OCT regulate development, differentiation, and activities of chondrocytes and osteoblasts, including differentiation of resting chondrocytes into proliferating and hypertrophic chondrocytes that involve "chondroclastic" enlargement of lacunae and "trans-differentiation" of surviving hypertrophic chondrocytes; differentiation of stroma cells into osteoblasts; and in periosteum and other regions of intramembranous ossification differentiation of precursor cells and osteoblasts. Nuclear receptor binding and their selective and hierarchical distribution during cell differentiation appear to correspond to multiple genomic effects toward growth, regeneration and repair. The findings indicate a physiological significance and therapeutic potential of 1,25(OH)2D3 and in particular of its less hypercalcemic analog OCT.

Development of an in vitro culture system for the study of osteoclast activity and function

Resorption is a difficult process to study in vivo because of the complex organization of mineralized tissue. The objective of this study was to develop an in vitro culture system for studying osteoclast attachment and activity. Transparent, microporous substrata were prepared using radio frequency-sputtering techniques to apply a thin layer of hydroxyapatite on polycarbonate filters previously coated with Type I collagen. Dentinoclasts isolated from the root surface of rat molar teeth undergoing inflammatory resorption and osteoclasts isolated from the endosteal surfaces of rat long bones were incubated on the experimental substrata. The culture system proved successful, supporting osteoclast viability and activity as demonstrated by cellular attachment and spreading seen by scanning electron microscopy. Activity was further demonstrated by monitoring acid production using a fluorescent probe, acridine orange. Isolated osteoclasts or dentinoclasts, when combined with synthetically prepared substrata, were observed to respond by mechanisms comparable to those that occur in vivo. Therefore, the system developed permits basic observations of osteoclasts in an in vitro system.

Effects of 1,25-dihydroxyvitamin D3 on bone resorption and natural immunity in osteopetrotic (ia) rats

Osteopetrois is an inherited bone disease characterized by an excessive accumulation of bone throughout the skeleton. The disease in the ia (incisors absent) rat is the result of reduced bone resorption caused by defective, although numerous osteoclasts. In addition to the bone defects, ia rats have suppressed natural immunity, even though these animals have excessive numbers of natural killer (NK) cells. The osteopetrotic condition also appears to have an associated abnormality in vitamin D metabolism. Because 1,25-dihydroxyvitamin D3[1,25-(OH)2D3] stimulates bone resorption and has a role in the immunoregulation of NK cells, mutant and normal rats were infused with 1,25-(OH)2D3 for 14 days in an attempt to correct the defects in this mutant. Serum levels of osteocalcin, 25-OHD3, and 1,25-(OH)2D3, as well as NK function and parameters of bone resorption, were evaluated after the infusion period. Serum levels of osteocalcin and 1,25-(OH)2D3 were elevated in both ia and normal rats treated with 1,25-(OH)2D3. Serum 25-OHD3 levels were significantly reduced in the treated animals. The elevated percentage of NK cells normally found in ia rats was reduced to normal in the treated mutants, and NK cell function was elevated to normal levels of lytic activity. The percentage of NK cells and NK function remained unchanged in the treated normal rats. The bone marrow cavity size was significantly increased in the 1,25-(OH)2D3-treated mutants, as was the percentage of osteoclasts exhibiting normal morphology. Radiographically, the mutant bones were less dense.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of 1,25-dihydroxyvitamin D3 in the generation of the acute-phase response in rats with talc-induced granulomatosis

Subcutaneous injection of nonspecific irritants such as magnesium silicate (talc) provokes granulomatous inflammation in the rat. Part of the acute phase response (APR) in these animals is the loss of trabecular bone at sites distant from the site of inflammation. To assess the possible involvement of vitamin D in the bone loss, we studied the development of the acute phase response in vitamin D-deprived rats. The serum APR provoked by subcutaneous inflammation in rachitic rats consisted of hypozincemia, hypercupremia, increased alkaline phosphatase activity and adrenocorticotropic hormone (ACTH) concentration, and was similar to that in control animals except for the absence of hypoferremia. Control rats with talc-induced subcutaneous inflammation also had splenomegaly and decreased total and mononuclear peripheral blood cell counts, while subcutaneous inflammation did not induce spleen changes in rachitic rats. Subcutaneous inflammation induced the loss of trabecular bone and decreased the osteoblastic cell count in tibial metaphyses in control animals. Rachitic rats had abundant osteoid on trabecular surfaces, and the number of osteoblasts and osteoclasts was comparable to that of the controls. Subcutaneous inflammation did not affect any of the bone parameters in rachitic rats. These results indicate that vitamin D plays an important role in the generation of the acute phase response during inflammation, particularly in the induction of spleen and bone cell changes. The discrepancy of the blood on one hand and bone and spleen indices of the APR on the other, indicate that they may be divergent pathways in the generation of the inflammatory response, some of which may be dependent on vitamin D.

Cellular biology of bone resorption

Past knowledge and the recent developments on the formation, activation and mode of action of osteoclasts, with particular reference to the regulation of each individual step, have been reviewed. The following conclusions of consensus have emerged. 1. The resorption of bone is the result of successive steps that can be regulated individually. 2. Osteoclast progenitors are formed in bone marrow. This is followed by their vascular dissemination and the generation of resting preosteoclasts and osteoclasts in bone. 3. The exact pathways of differentiation of the osteoclast progenators to mature osteoclasts are debatable, but there is clear evidence that stromal cells support osteoclast generation. 4. Osteoclasts are activated following contact with mineralized bone. This appears to be controlled by osteoblasts that expose mineral to osteoclasts and/or release a factor that activates these cells. 5. Activated osteoclasts dissolve the bone mineral and digest the organic matter of bone by the action of agents secreted in the segregated microcompartments underlying their ruffled borders. The mineral is solubilized by protons generated from CO2 by carbonic anhydrase and secreted by an ATP-driven vacuolar H(+)-K(+)-ATPase located at the ruffled border. The organic matrix of the bone is removed by acid proteinases, particularly cysteine-proteinases that are secreted together with other lysosomal enzymes in the acid environment of the resorption zone. 6. Osteoclastic bone resorption is directly regulated by a polypeptide hormone, calcitonin (CT), and locally, by ionized calcium (Ca2+) generated as a result of osteoclastic bone resorption. 7. There is new evidence that osteoclast activity may also be influenced by the endothelial cells via generation of products including PG, NO and endothelin.

Connexin43 mediates direct intercellular communication in human osteoblastic cell networks

We have examined cell coupling and expression of gap junction proteins in monolayer cultures of cells derived from human bone marrow stromal cells (BMC) and trabecular bone osteoblasts (HOB), and in the human osteogenic sarcoma cell line, SaOS-2. Both HOB and BMC cells were functionally coupled, since microinjection of Lucifer yellow resulted in dye transfer to neighboring cells, with averages of 3.4 +/- 2.8 (n = 131) and 8.1 +/- 9.3 (n = 51) coupled cells per injection, respectively. In contrast, little diffusion of Lucifer yellow was observed in SaOS-2 monolayers (1.4 +/- 1.8 coupled cells per injection, n = 100). Dye diffusion was inhibited by octanol (3.8 mM), an inhibitor of gap junctional communication. All of the osteoblastic cells expressed mRNA for connexin43 and connexin45, but not for connexins 26, 32, 37, 40, or 46. Whereas all of the osteoblastic cells expressed similar quantities of mRNA for connexin43, the poorly coupled SaOS-2 cells produced significantly less Cx43 protein than either HOB or BMC, as assessed by immunofluorescence and immunoprecipitation. Conversely, more Cx45 mRNA was expressed by SaOS-2 cells than by HOB or BMC. Thus, intercellular coupling in normal and transformed human osteoblastic cells correlates with the level of expression of Cx43, which appears to mediate intercellular communication in these cells. Gap junctional communication may serve as a means by which osteoblasts can work in synchrony and propagate locally generated signals throughout the skeletal tissue.

The necessity for calcium for increased renal vitamin D receptor in response to 1,25-dihydroxyvitamin D

To further investigate the regulation of the vitamin D receptor in the kidney of the rat, we analyzed the response of the receptor to 1,25-dihydroxyvitamin D-3 under conditions of calcium supplementation and calcium restriction. Vitamin D-deficient, male weanling rats, fed a calcium-restricted or calcium-supplemented, vitamin D-deficient diet, were treated for 4 weeks with vitamin D (orally) or 1,25-dihydroxyvitamin D-3 (60 pmol/d by mini-osmotic pump). We also extended the treatment to 8 weeks for one group of animals fed the calcium-supplemented diet. Vitamin D compounds decreased the level of renal receptor in rats fed the low calcium diet. However, in animals given a 1.2% calcium diet, both vitamin D and 1,25-dihydroxyvitamin D-3 increased receptor levels by 130%. After 8 weeks, 1,25-dihydroxyvitamin D-3 had increased the receptor level by 260% while vitamin D had no further effect. Thus, dietary calcium is required for vitamin D to up-regulate the renal vitamin D receptor level. Further, 1,25-dihydroxyvitamin D-3 itself appears to have a marked ability to increase vitamin D receptor levels. These results suggest a complex mechanism of homologous regulation of the vitamin D receptor in the kidney.

Differentiating and antitumor activities of 1 alpha,25-dihydroxyvitamin D3 in vitro and 1 alpha-hydroxyvitamin D3 in vivo on human osteosarcoma

The differentiating and antitumor activities of 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25(OH)2D3) in vitro and 1 alpha-hydroxyvitamin D3 (1 alpha(OH)D3) in vivo were studied with a human osteosarcoma cell line (OST strain). Anti-tumor activity was estimated with the use of 3-(4,5-dimethylthiazol)-2, 5-diphenyltetrazolium bromide (MTT) assay, colony-forming assay, and athymic mouse assay. The intracellular alkaline phosphatase (ALP) activity of tumor cells and production of bone Gla protein (BGP) in culture media were measured to mark osteoblastic differentiation. In addition, the combination of 1 alpha,25(OH)2D3 and cis-dichlorodiammineplatinum(II) (CDDP) was tested by the colony-forming assay and the measurement of ALP activity and BGP production for differentiating and antitumor effects. The assays revealed that 1 alpha,25(OH)2D3 exerted a dose-related, growth-inhibitory influence. In the colony-forming assay, the 1 alpha,25(OH)2D3-treated colonies were smaller than the untreated colonies. The ALP activity and the BGP production also increased in relation to dose. In the assay in athymic mice, the relative weight of tumors treated with 1 alpha(OH)D3 at 2.5 nmol/kg was significantly smaller than that of the controls, and no side effects were observed in the 1 alpha(OH)D3-treated mice. Marked tumor chondrogenesis was observed in human osteosarcoma treated with 1 alpha(OH)D3 in athymic mice. The combination of 1 alpha,25(OH)2D3 at 10(-8) M and CDDP at 2 micrograms/ml significantly enhanced both the differentiation and the growth inhibition in vitro. Our study apparently is the first demonstration that vitamin D3 metabolites have an antitumor and differentiating effect on human osteosarcoma cells in vitro and in athymic mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of age on the rate of tooth movement in combination with local use of 1,25(OH)2D3 and mechanical force in the rat

The purpose of this study was to compare the amount and rate of tooth movement in young and mature rats administered 1,25(OH)2D3 simultaneous with application of mechanical force. In 30 seven-week-old and 30 28-week-old male Wistar rats, the right maxillary first molar was moved buccally with a fixed appliance. The appliances delivered forces ranging from 5 to 20 g. Twenty microL of 1,25(OH)2D3 (10(-10) and 10(-8) mol/L) was injected locally into the submucosal palatal area of the root bifurcation of the right first molar. The left side was injected with phosphate-buffered saline (PBS). In young rats receiving 10(-10) mol/L 1,25(OH)2D3 every three days, tooth movement significantly increased to 126% of that in PBS-injected control rats on day 20. In 1,25(OH)2D3-injected mature rats, tooth movement was stimulated markedly and increased with 10(-10) mol/L to 245% and with 10(-8) mol/L to 154% of the amount of tooth movement seen in the PBS-injected controls by the end of the experiment. PBS-injected rats had a plateau stage where tooth movement did not occur at all, while there was no such lag-time in the 1,25(OH)2D3-injected group which showed continuous tooth movement. The local injection of 1,25(OH)2D3 did not change serum calcium, phosphate, and alkaline phosphatase activity, and there were no apparent clinical or microscopic side-effects.

1,25-(OH)2D3 and 24,25-(OH)2D3 regulation of arachidonic acid turnover in chondrocyte cultures is cell maturation-specific and may involve direct effects on phospholipase A2

Previous studies have shown that 1,25-(OH)2D3 stimulates phospholipase A2 (PA2) activity in growth zone chondrocytes (GC), but has no effect on the resting zone chondrocyte (RC) enzyme activity. 24,25-(OH)2D3 inhibits the RC enzyme but has no effect on the GC. This study examined whether the vitamin D metabolites affect arachidonic acid turnover in their contra-target cell populations. Incorporation and release of [14C]arachidonate was measured at various times following addition of hormone. Acylation and reacylation were measured independently by incubating with p-chloromercuribenzoate. The results demonstrated that 1,25-(OH)2D3 has no effect on arachidonic acid turnover in RC, but stimulates turnover in GC. In contrast, 24,25-(OH)2D3 stimulates arachidonic acid turnover in RC, but inhibits both incorporation and release in GC. To determine whether direct interaction with PA2 is one mechanism by which 1,25-(OH)2D3 and 24,25-(OH)2D3 regulate arachidonic acid release, snake venom (Niger niger) PA2 was incubated with the vitamin D metabolites. Enzyme specific activity was inhibited by 24,25-(OH)2D3 and stimulated by 1,25-(OH)2D3 in a time- and dose-dependent manner. These results suggest that at least part of the direct effect of vitamin D3 metabolites on cell membranes may be related to changes in PA2 activity. The regulation is related to the stage of differentiation of the target cell population. Changes in fatty acid acylation and reacylation may be one mode of vitamin D3 action in cartilage.

Differential metabolism of 25-hydroxyvitamin D3 by cultured synovial fluid macrophages and fibroblast-like cells from patients with arthritis

Differential metabolism of 25-hydroxyvitamin D3 (25(OH)D3) has been shown for macrophages and fibroblast-like cells (possibly synoviocytes) cultured for two to 50 days after isolation from the synovial fluid of 12 patients with various forms of inflammatory arthritis. Macrophages synthesised the active metabolite of vitamin D3, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), the synthesis of which was increased by bacterial lipopolysaccharide, a known macrophage activating factor. In contrast, fibroblast-like cells formed 24, 25-dihydroxyvitamin D3 (24,25(OH)2D3), synthesis of which was stimulated by 1,25(OH)2D3 and inhibited by lipopolysaccharide. The synthesis of 1,25(OH)2D3 and 24,25(OH)2D3 by macrophages and fibroblast-like cells respectively was inhibited by ketoconazole, indicating that both hydroxylases are dependent on cytochrome P-450. Mean (SEM) synovial fluid and serum 25(OH)D3 concentrations were 16.7 (1.7) and 22.2 (2.6) ng/ml and those of 1,25(OH)2D3 were 29.4 (4.8) and 43.3 (4.0) pg/ml respectively. In most cases concentrations were lower in synovial fluid than in paired serum samples, but in two patients 1,25(OH)2D3 concentrations were greater in synovial fluid than in serum, suggesting local synthesis within the affected joints.

The effect of local application of 1,25-dihydroxycholecalciferol on osteoclast numbers in orthodontically treated rats

Orthodontic tooth movement requires remodeling of periodontal tissues, especially alveolar bone. 1,25-(OH)2D3, the active form of vitamin D3, is known to be a potent stimulator of osteoclastic bone resorption. The purpose of this study was to investigate the effect of local application of 1,25-(OH)2D3 on osteoclast numbers induced by experimental tooth movement. A piece of orthodontic elastic band was inserted between the first and second upper molars of male Wistar rats weighing about 200 g each. Twenty microL of 1,25-(OH)2D3 (10(-12)-10(-7) mol/L) was injected locally into the submucosal palatal area of the root bifurcation of the right first molar. The left side was injected with phosphate-buffered saline (PBS). The number of osteoclasts was counted in a 700 x 1050 microns 2 area of the interradicular septum. The local injection of 1,25-(OH)2D3 caused a dose-dependent increase in osteoclast number. The effect of 1,25-(OH)2D3 reached a response plateau at 10(-10) mol/L when greater than a three-fold rise in osteoclast number was attained compared with the PBS-treated controls. While the insertion of a piece of elastic band for three days induced a significant increase in osteoclasts in the alveolar bone, daily injections of 20 microL of 10(-10) mol/M 1,25-(OH)2D3 for three days markedly stimulated the numbers of osteoclasts induced by the insertion of an elastic band. 1,25-(OH)2D3 was apparently synergistic with mechanical stimuli, resulting in enhancement of the numbers of osteoclasts induced by mechanical stimuli alone.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Inhibition of 1,25-(OH)2D3- and 24,25-(OH)2D3-dependent stimulation of alkaline phosphatase activity by A23187 suggests a role for calcium in the mechanism of vitamin D regulation of chondrocyte cultures

This study used the ionophore, A23187, to examine the hypothesis that the regulation of alkaline phosphatase and phospholipase A2 activity by vitamin D3 metabolites in cartilage cells is mediated by changes in calcium influx. Confluent, fourth-passage cultures of growth zone and resting zone chondrocytes from the costochondral cartilage of 125 g rats were incubated with 0.01-10 microM A23187. Specific activities of alkaline phosphatase and phospholipase A2 were measured in the cell layer and in isolated plasma membranes and matrix vesicles. There was an inhibition of alkaline phosphatase specific activity at 0.1 microM A23187 in resting zone cells and at 0.1 and 1 microM in growth zone chondrocytes. At these concentrations of ionophore, the 45Ca content of the chondrocytes was shown to increase. Both the plasma membrane and matrix vesicle enzyme activities were inhibited. There was no effect of ionophore on matrix vesicle or plasma membrane phospholipase A2 in either cell type. In contrast, alkaline phosphatase activity is stimulated when growth zone chondrocytes are incubated with 1,25-(OH)2D3 and in resting zone cells incubated with 24,25-(OH)2D3. Phospholipase A2 activity is differentially affected depending on the metabolite used and the cell examined. Addition of ionophore to cultures preincubated with 1,25-(OH)2D3 or 24,25-(OH)2D3 blocked the stimulation of alkaline phosphatase by the vitamin D3 metabolites in a dose-dependent manner. The effects of ionophore were not due to a direct effect on the membrane enzymes since enzyme activity is isolated membranes incubated with A23187 in vitro was unaffected. These results suggest a role for calcium in the action of vitamin D metabolites on chondrocyte membrane enzyme activity but indicate that mechanisms other than merely Ca2+ influx per se are involved.

Murine recombinant leukemia inhibitory factor modulates inhibitory effect of 1,25 dihydroxyvitamin D3 on alkaline phosphatase activity in MC3T3-E1 cells

We demonstrated murine leukemia inhibitory factor (mLIF) mRNA in osteoblastic MC3T3-E1 cells, but not mLIF in their conditioned medium. Recombinant mLIF had an inhibitory effect on alkaline phosphatase (ALP) activity, but not on DNA synthesis, in these mLIF-free cells. This inhibitory effect was not prostaglandin E2 dependent. mLIF also modulated the inhibitory effect of 1,25 dihydroxyvitamin D3 [1,25(OH)2D3] on ALP activity, partly via down regulation of 1,25(OH)2D3 binding sites. These results suggest that LIF may play a role in regulating osteoblast differentiation.

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.

Osteodystrophy in liver cirrhosis: detection and treatment evaluation using 99Tcm methylene diphosphonate bone scintigraphy

To investigate osteodystrophy in liver cirrhosis, 99Tcm MDP (methylene diphosphonate) bone scintigraphy was performed on 36 patients with liver cirrhosis. Abnormal lesions were detected in 17 out of 36 scans (47.2%). On the other hand, areas of increased uptake were uncommon in patients with chronic active hepatitis (1/11 cases). Plasma vitamin D3 fractions [25(OH)D3, 24.25(OH)2D3 and 1.25(OH)2D3] were decreased. Statistically significant depletion of 1.25(OH)2D3 was observed in cases with positive bone scintigraphy. 1 alpha(OH)D3 (1-2 micrograms/day) was administered for 6 months to nine patients having abnormal bone scans. Six of them showed improvement without any apparent side-effects. We conclude that hepatic cirrhotic osteodystrophy can be diagnosed positively by 99Tcm MDP bone scintigraphy and can be treated accordingly.

Osteoclasts: structure and function

Osteoclasts are multinucleated giant cells showing specialized membrane structures, clear zones and ruffled borders, which are responsible for the process of bone resorption. These cells arrive at the resorption site via the bloodstream as mononuclear cells, derived from haemopoietic precursors in the spleen or bone marrow, which fuse prior to resorption. The osteoclast may share an early progenitor cell, the granulocyte macrophage colony-forming unit (GM-CFU) with monocytes, macrophages and granulocytes, implying that osteoclasts share the pluripotent haemopoietic stem cell with all other haemopoietic cells. In the past, elucidation of the structure of these cells relied upon traditional ultrastructural techniques. Transmission electron microscopic studies revealed details of the unique ultrastructure of these cells and, in combination with stereological techniques, showed the response of cells to various hormonal stimuli. Scanning electron microscopy not only demonstrated the surface appearance of osteoclasts, and their predilection for spreading on various substratum components, but has also been used as an adjunct in resorption assays in which areas of resorption lacunae are measured as indicators of cell activity. Recent advances in fields such as immunocytochemistry and freeze fracture techniques have contributed towards a more detailed delineation of antigenic profile, cytoskeletal structure and localization of enzymatic pathways. The osteoclast is subject to extensive regulatory mechanisms and it has been established that the osteoblast plays a major rôle in mediating the effects of osteotropic hormones and local mediators on these cells. Hence, research aimed at elucidating the coupling mechanisms between these two cells may result in new therapies for bone disease.

Physiological and pharmacological regulation of biological calcification

Biological calcification is a highly regulated process which occurs in diverse species of microorganisms, plants, and animals. Calcification provides tissues with structural rigidity to function in support and protection, supplies the organism with a reservoir for physiologically important ions, and also serves in a variety of specialized functions. In the vertebrate skeleton, hydroxyapatite crystals are laid down on a backbone of type I collagen, with the process being controlled by a wide range of noncollagenous proteins present in the local surroundings. In bone, cells of the osteoblast lineage are responsible for the synthesis of the bone matrix and many of these regulatory proteins. Osteoclasts, on the other hand, are continually resorbing bone to both produce changes in bone shape and maintain skeletal integrity, and to establish the ionic environment needed by the organism. The proliferation, differentiation, and activity of these cells is regulated by a number of growth factors and hormones. While much has already been discovered over the past few years about the involvement of various regulators in the process of mineralization, the identification and functional characterization of these factors remains an area of intense investigation. As with any complex, biological system that is in a finely tuned equilibrium under normal conditions, problems can occur. An imbalance in the processes of formation and resorption can lead to calcification disorders, and the resultant diseases of the skeletal system have a major impact on human health. A number of pharmacological agents have been, and are being, investigated for their therapeutic potential to correct these defects.(ABSTRACT TRUNCATED AT 250 WORDS)

1,25(OH)2D3 and calcipotriol (MC903) have similar effects on the induction of osteoclast-like cell formation in human bone marrow cultures

MC903 is a novel analogue of 1,25(OH)2D3 which exhibits similar inhibitory effects on cell proliferation and like, 1,25(OH)2D3, stimulates synthesis of osteoblast specific proteins by osteoblast-like cells in vitro. It is less active than 1,25(OH)2D3 in causing hypercalcemia in vivo. Since 1,25(OH)2D3 is known to stimulate bone resorption and increase the number of osteoclasts in several systems (in vivo and in vitro) we examined the effects of MC903 on the formation of osteoclast-like cells in vitro. As reported previously 1,25(OH)2D3 promoted the formation of multinucleated cells with phenotypic and functional characteristics of osteoclasts from adult human bone-marrow cultures at concentrations between 10(-8)M to 10(-12)M. Higher doses consistently suppressed multinucleated cell formation to values seen in the absence of 1,25(OH)2D3. Cells cultured in the presence of MC903 or for three weeks consistently induced the formation of multinucleated cells at concentrations 10(-8)M to 10(-12)M. As seen with 1,25(OH)2D3, MC903 also inhibited multinucleated cell formation at very high concentrations (10(-6)M). In two separate experiments MC903 appeared to be more potent than 1,25(OH)2D3 at lower concentrations (10(-10)M - 10(-12)M). From this study we conclude that MC903 is at least as potent as 1,25(OH)2D3 in inducing the formation human osteoclast-like cells in vitro. The decreased ability of MC903 to induce hypercalcemia in vivo is not therefore a result of a less marked effect than 1,25(OH)2D3 on the regulation of osteoclast formation.

1,25-Dihydroxyvitamin D3 metabolism in a human osteosarcoma cell line and human bone cells

The metabolism of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] by a human osteoblastic sarcoma cell line, U-2 OS, and by primary cultures of human bone-derived cells was examined at physiologic (5 x 10(-11) M) and pharmacologic (3.5 x 10(-7) M) substrate concentrations. For metabolite identification purposes, cells nearing confluency were incubated for 18 h with 3.5 x 10(-7) M 1,25-(OH)2D3 in serum-free medium. The putative vitamin D metabolites produced during this incubation were isolated from a total lipid extract of cells and medium. Identification of the metabolites was achieved by comigration with authentic standards on three high-performance liquid chromatography systems, UV spectral analysis, mass spectrometry, and chemical modification by sodium borohydride and sodium metaperiodate. The identified metabolites produced from 1,25-(OH)2D3 by the human osteosarcoma cells include 1,24,25-trihydroxyvitamin D3; 24-oxo-1,25-dihydroxyvitamin D3; 24-oxo-1,23,25-trihydroxyvitamin D3; and 24,25,26,27-tetranor-1,23-dihydroxyvitamin D3. Evidence is presented that (1) 1,25-(OH)2D3 metabolism occurs constitutively in U-2 OS osteosarcoma cells at a physiologic substrate concentration (5 x 11(-11) M), (2) the pathway can be further induced by pharmacologic 1,25-(OH)2D3 concentrations (10(-7) M), and (3) this pathway is present in primary cultures of normal human bone-derived cells.