A receptor-like binding macromolecule for 1 alpha, 25-dihydroxycholecalciferol in cultured mouse bone cells

Coffee consumption and caffeine intake in relation to risk of fractures: a systematic review and dose-response meta-analysis of observational studies

BACKGROUND

Conflicting reports are available about the association of coffee or caffeine intake and risk of fracture. We performed the current updated systematic review and dose-response meta-analysis of coffee consumption and caffeine intake and risk of fracture to quantify this association.

MATERIALS AND METHODS

PubMed/Medline, ISI Web of Science, and Scopus, Cochrane database were searched up to July 2021. Random-effects model or fixed-effects model was used to pool the study-specific effect sizes (ESs) and 95% confidence intervals (CIs). Dose-response relationship was examined using linear and non-linear dose-response analyses. The certainty of evidence was assessed using NutriGrade tool.

RESULTS

Out of 22 eligible studies included in the meta-analysis, 15 had cohort and 7 had case-control design. We found no significant association between coffee consumption and risk of fracture, either based on pooling cohort (RR: 0.99; 95% CI: 0.88, 1.12; I2 = 71.4%, Pheterogeneity < 0.01) or case-control studies (OR: 1.13; 95% CI: 0.87, 1.46; I2 = 49.0%, Pheterogeneity=0.08). In the subgroup analysis of cohort studies, we observed that higher coffee intake was inversely associated with risk of fracture in men (RR: 0.85; 95% CI: 0.76 to 0.94). In addition, a positive association was seen between coffee consumption and risk of fracture in studies with less than 12 years of follow-up (RR: 1.14; 95% CI: 1.02 to 1.27). With regard to caffeine intake, a statistically significant positive association was seen with risk of fracture (RR: 1.15; 95% CI, 1.08 to 1.23; I2=26.6%, n = 8). In the dose-response analysis, we found that each additional 100 mg caffeine intake was marginally associated with 2% greater risk of fracture (RR: 1.02; 95% CI: 1 to 1.05; I2= 70.3%, n = 6).

CONCLUSION

High coffee consumption was protectively associated with risk of fracture in men, while caffeine intake was positive associated with risk.

The small molecule phenamil induces osteoblast differentiation and mineralization

Stimulation of osteoblast differentiation from mesenchymal stem cells is a potential strategy for bone repair. Bone morphogenetic proteins (BMPs) that induce osteoblastic differentiation have been successfully used in humans to treat fractures. Here we outline a new approach to the stimulation of osteoblast differentiation using small molecules that stimulate BMP activity. We have identified the amiloride derivative phenamil as a stimulator of osteoblast differentiation and mineralization. Remarkably, phenamil acts cooperatively with BMPs to induce the expression of BMP target genes, osteogenic markers, and matrix mineralization in both mesenchymal stem cell lines and calvarial organ cultures. Transcriptional profiling of cells treated with phenamil led to the identification of tribbles homolog 3 (Trb3) as a mediator of its effects. Trb3 is induced by phenamil selectively in cells with osteoblastic potential. Both Trb3 and phenamil stabilize the expression of SMAD, the critical transcription factor in BMP signaling, by promoting the degradation of SMAD ubiquitin regulatory factor 1. Small interfering RNA-mediated knockdown of Trb3 blunts the effects of phenamil on BMP signaling and osteogenesis. Thus, phenamil induces osteogenic differentiation, at least in part, through Trb3-dependent promotion of BMP action. The synergistic use of small molecules such as phenamil along with BMPs may provide new strategies for the promotion of bone healing.

Caffeine decreases vitamin D receptor protein expression and 1,25(OH)2D3 stimulated alkaline phosphatase activity in human osteoblast cells

Of the various risk factors contributing to osteoporosis, dietary/lifestyle factors are important. In a clinical study we reported that women with caffeine intakes >300 mg/day had higher bone loss and women with vitamin D receptor (VDR) variant, tt were at a greater risk for this deleterious effect of caffeine. However, the mechanism of how caffeine effects bone metabolism is not clear. 1,25-Dihydroxy vitamin D(3) (1,25(OH)(2)D(3)) plays a critical role in regulating bone metabolism. The receptor for 1,25(OH)(2)D(3), VDR has been demonstrated in osteoblast cells and it belongs to the superfamily of nuclear hormone receptors. To understand the molecular mechanism of the role of caffeine in relation to bone, we tested the effect of caffeine on VDR expression and 1,25(OH)(2)D(3) mediated actions in bone. We therefore examined the effect of different doses of caffeine (0.2, 0.5, 1.0 and 10mM) on 1,25(OH)(2)D(3) induced VDR protein expression in human osteoblast cells. We also tested the effect of different doses of caffeine on 1,25(OH)(2)D(3) induced alkaline phosphatase (ALP) activity, a widely used marker of osteoblastic activity. Caffeine dose dependently decreased the 1,25(OH)(2)D(3) induced VDR expression and at concentrations of 1 and 10mM, VDR expression was decreased by about 50-70%, respectively. In addition, the 1,25(OH)(2)D(3) induced alkaline phosphatase activity was also reduced at similar doses thus affecting the osteoblastic function. The basal ALP activity was not affected with increasing doses of caffeine. Overall, our results suggest that caffeine affects 1,25(OH)(2)D(3) stimulated VDR protein expression and 1,25(OH)(2)D(3) mediated actions in human osteoblast cells.

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.

1,25-Dihydroxyvitamin D3-induced calcium efflux from calvaria is mediated by protein kinase C

1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is an important regulator of bone metabolism involved in both formation and resorption. Traditionally it was assumed that vitamin D receptors are intracellular. Recent data indicate that vitamin D may also act through a membrane receptor, specifically raising intracellular calcium and inositol 1,4,5 triphosphate. The present study was undertaken to explore further the mechanism(s) of vitamin D-induced bone resorption in cultured bone. 1,25(OH)2D3 induced a dose-dependent increase of calcium efflux from cultured bone. This increase was completely obliterated by inhibition of protein kinase C (PKC) with either staurosporine or calphostin C. In cultured rat calvariae, 1,25(OH)2D3 also induced a dose-dependent translocation of PKC from cytosol to membrane. The activation of PKC by 1, 25(OH)2D3 occurred following a 30-s incubation, peaked at 1 minute, and disappeared by 5 minutes. 1,25(OH)2D3 did not increase cAMP production in similarly cultured calvaria. These results suggest that the action of 1,25(OH)2D3 on calcium flux from cultured bone is mediated, in part, via activation of PKC.

1,25 (OH)2D3 enhances PTH-induced Ca2+ transients in preosteoblasts by activating L-type Ca2+ channels

We previously demonstrated electrophysiologically that 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] shifts the activation threshold of L-type Ca2+ channels in osteoblasts toward the resting potential and prolongs mean open time. Presently, we used single-cell Ca2+ imaging to study the combined effects of 1,25(OH)2D3 and parathyroid hormone (PTH) during generation of Ca2+ transients in fura 2-loaded MC3T3-E1 cells. Pretreatment with 1,25(OH)2D3 concentrations, which alone did not produce Ca2+ transients, consistently enhanced Ca2+ responses to PTH. Enhancement was dose dependent over the range of 1 to 10 nM and was blocked by pretreatment with 5 microM nitrendipine during pretreatment. A 1,25(OH)2D3 analog that activates L-type channels and shifts their activation threshold also enhanced PTH responses. In contrast, an analog devoid of membrane Ca2+ effects did not enhance PTH-induced Ca2+ transients. The PTH-induced Ca2+ transient involved activation of a dihydropyridine-insensitive cation channel that was inhibited by Gd3+. Together, these data suggest that 1,25(OH)2D3 increases osteoblast responsiveness to PTH through rapid modification of L-type Ca2+ channel gating properties, whose activation enhances Ca2+ entry through other channels such as the PTH-responsive, Gd(3+)-sensitive cation channel.

Effects of osteoclastic resorption on bone surface ion composition

Osteoclasts are responsible for resorption of bone mineral. To determine how osteoclasts alter bone surface ion composition, neonatal mouse bone cells were isolated and cultured in the presence of parathyroid hormone (PTH) on bovine cortical bone. Surface ion composition of the resulting osteoclastic resorption pits was compared with that of unresorbed bone, utilizing a high-resolution scanning ion microprobe. Cortical bone cultured with cells in the presence of PTH had numerous resorption pits. The unresorbed area adjacent to the pits had a ratio of surface 23Na/40Ca of 18.7 + 1.6 (mean counts per second of detected secondary ions +95% confidence interval) and 39K/40Ca of 2.3 + 2.2. At the base of the pits, the ratio of 23Na/40Ca was 1.0 + 2.0 and 39K/40Ca was 0.1 + 1.0 (each different from area adjacent to the pit, P < 0.001). The ratio of 23Na/39K in the unresorbed area was not different from that at the base of the pit. Thus osteoclasts induce a decrease in the ratio of surface ion composition of both 23Na/40Ca and 39K/40Ca but not 23Na/39K in bovine cortical bone. The elevated ratios of 23Na/40Ca and 39K/40Ca on the surface, but not at the base of the pits, indicate adsorption of medium ions onto the mineral. Because osteoclasts foster the release of bone Ca, these results indicate that osteoclastic resorption causes a greater, and approximately equal, release of both 23Na and 39K compared with 40Ca from bone mineral. Osteoclasts appear to remove nonselectively the surface mineral that had been exposed to the medium, uncovering underlying mineral.

Increased sensitivity to 1,25(OH)2D3 in bone from genetic hypercalciuric rats

As a model of human hypercalciuria, we have selectively inbred genetic hypercalciuric stone-forming (GHS) Sprague-Dawley rats whose mean urine calcium excretion is eight to nine times greater than that of controls. A large component of this excess urine calcium excretion is secondary to increased intestinal calcium absorption, which is not due to an elevation in serum 1,25(OH)2D3, but appears to result from an increased number of intestinal 1,25(OH)2D3 receptors (VDR). When GHS rats are fed a low-calcium diet, the hypercalciuria is only partially decreased and urine calcium excretion exceeds intake, suggesting that an additional mechanism contributing to the hypercalciuria is enhanced bone demineralization. To determine if GHS rat bones are more sensitive to exogenous 1,25(OH)2D3, we cultured calvariae from neonatal (2- to 3-day-old) GHS and control rats with or without 1,25(OH)2D3 or parathyroid hormone (PTH) for 48 h at 37 degrees C. There was significant stimulation of calcium efflux from GHS calvariae at 1 and 10 nM 1,25(OH)2D3, whereas control calvariae showed no significant response to 1,25(OH)2D3 at any concentration tested. In contrast, PTH induced similar bone resorption in control and GHS calvariae. Immunoblot analysis demonstrated a fourfold increase in the level of VDR in GHS calvariae compared with control calvariae, similar to the increased intestinal receptors described previously. There was no comparable change in VDR RNA levels as measured by slot blot analysis, suggesting the altered regulation of the VDR occurs posttranscriptionally. That both bone and intestine display an increased amount of VDR suggests that this may be a systemic disorder in the GHS rat and that enhanced bone resorption may be responsible, in part, for the hypercalciuria in the GHS rat.

Cell-matrix interaction in bone: type I collagen modulates signal transduction in osteoblast-like cells

Cell interaction with extracellular matrix (ECM) modulates cell growth and differentiation. By using in vitro culture systems, we tested the effect of type I collagen (Coll-I) on signal transduction mechanisms in the osteosarcoma cell line UMR-106 and in primary cultures from neonatal rat calvariae. Cells were cultured for 72 h on Coll-I gel matrix and compared with control cells plated on plastic surfaces. Agonist-dependent and voltage-dependent rises in cytosolic Ca2+ concentration ([Ca2+]i; measured by fura 2 fluorometry) were significantly blunted in cells cultured on Coll-I compared with cells grown on plastic. In UMR-106 cells, the collagen matrix effect was mimicked by 24-h incubation with soluble Coll-I or short peptides containing the arginine-glycine-aspartate motif. Accumulation of cellular adenosine 3',5'-cyclic monophosphate (cAMP) stimulated by parathyroid hormone, cholera toxin, and forskolin was augmented (50-150%) in cells plated on Coll-I vs. control. The collagen effect on both [Ca2+]i- and adenylate cyclase-signaling pathways in UMR-106 cells was abrogated in the presence of protein kinase C (PKC) depletion or inhibition. Also, Coll-I induced a twofold increase in membrane-bound PKC without changing cytosolic PKC activity. Thus, by altering PKC activity, Coll-I modulates the [Ca2+]i- and cAMP-signaling pathways in osteoblasts. This, in turn, may influence bone remodeling processes.

Transforming growth factor beta-induced dissociation between vitamin D receptor level and 1,25-dihydroxyvitamin D3 action in osteoblast-like cells

In the present study the interaction between a locally produced factor in bone, transforming growth factor beta (TGF beta) and a systemic regulator of bone metabolism, 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) was investigated. In rat (UMR 106, ROS 17/2.8) and human (MG-63) osteoblastic cell lines and in isolated fetal rat osteoblasts TGF beta caused a comparable increase in vitamin D receptor (VDR) level. A maximum was observed after 6 h at 1 ng/ml TGF beta. Scatchard analysis revealed that up-regulation of VDR is due to an increase in receptor number and not to a change in affinity. This was supported by Northern blot analysis which showed a dose- and time-dependent increase in VDR mRNA by TGF beta. To assess the significance of the TGF beta-induced increase in VDR level for 1,25-(OH)2D3 effects cells were preincubated with TGF for 4 h (causing a 2-3-fold increase of the VDR level) and subsequently incubated with 1,25-(OH)2D3 for 4 h and 24 h. TGF beta preincubation potently inhibited subsequent 1,25-(OH)2D3 stimulation of osteocalcin production in both ROS 17/2.8 and MG-63 cells on protein as well as mRNA level. A similar inhibition by TGF beta was observed on the 1,25-(OH)2D3-induced increase in osteopontin mRNA. The current study demonstrates dissociation between regulation of VDR level and modulation of two 1,25-(OH)2D3 biological responses by TGF beta in osteoblast-like cell lines of different origin. This dissociation shows that, besides interaction at VDR level also at other levels in the cell interaction(s) exist between TGF beta and 1,25-(OH)2D3. Besides, these data emphasize the potential importance of the interplay of locally produced factors and systemic calciotrophic hormones in the regulation of bone metabolism.

Oscillations in inositol 1,4,5-trisphosphate and diacyglycerol induced by vitamin D3 metabolites in confluent mouse osteoblasts

For the last 5 years, attention has focused on the nongenomic effects of 1,25-(OH)2D3, but considerably less is known about the mechanisms of the nonnuclear actions of 24,25-(OH)2D3. The present study examines and compares the rapid (5-90 s) effects of 100 pM to 10 nM 24,25-(OH)2D3, 10 pM to 1 nM 1,25-(OH)2D3, and 1-100 nM 25-OHD3 on the formation of inositol phosphates and lipids in confluent mouse osteoblasts. 24,25-(OH)2D3 and 25-OHD3 effects were dose dependent; those of 1,25-(OH)2D3 were dose dependent in a bell-shaped manner. The two dihydroxylated metabolites induced a multiphasic response in inositol 1,4,5-trisphosphate (IP3) formation with three stimulation peaks; the IP3 response to 25-OHD3 was monophasic. The amplitude of the IP3 response to 24,25-(OH)2D3 was greater and its oscillation period was slower than that induced by 1,25-(OH)2D3. The diacylglycerol (DAG) responses to secosteroids showed two stimulation peaks that appeared at different times depending on the secosteroid used. Pretreatment with neomycin totally inhibited the first DAG response; neomycin had no effect on the second peak of DAG induced by 25-OHD3, whereas it partially blocked the second response of DAG to 24,25-(OH)2D3 and 1,25-(OH)2D3. These data show for the first time that 24,25-(OH)2D3 can modulate phospholipid metabolism in confluent mouse osteoblasts as early as 5-10 s. The first pathway used by all three secosteroids is that of the hydrolysis of phosphatidylinositol 4,5-bisphosphate via phospholipase C activation, leading to the formation of the two second messengers, IP3 and DAG, since neomycin totally blocked the response. Thus, the action of these secosteroids on the osteoblast membrane may also implicate several steps of the phosphatidylcholine cycle, according to the metabolite tested. Finally, these data point to a direct interaction of vitamin D metabolites with specific membrane recognition moieties.

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.

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.

Differentiation-related regulation of 1,25-dihydroxyvitamin D3 receptor mRNA in human leukaemia cells HL-60

Vitamin D receptor (VDR) is a nuclear protein which mediates the physiological actions of its hormone ligand, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). While it appears that the receptor-hormone complex regulates the expression of hormone-dependent genes involved in mineral homeostasis, its role in induction of differentiation of leukaemic cells is less clear. We have studied the expression of the VDR gene in several sublines of HL-60 leukaemic cells with varying responsiveness to 1,25(OH)2D3. Sublines which rapidly differentiated to monocytic forms were shown to contain elevated steady-state levels of VDR mRNA within 1 h of exposure to high concentration of 1,25(OH)2D3. This up-regulation of the expression of VDR was not apparent in sublines in which monocytic differentiation occurred after a delay of several days. Beginning at approximately 3 h after exposure to 1,25(OH)2D3 in most cases, there was a gradual decline in VDR mRNA levels. Measurement of steady-state levels of mRNA for c-myc and c-fos showed that in sublines of HL-60 cells which respond rapidly to 1,25(OH)2D3, elevation of VDR mRNA is evident prior to the changes in proto-oncogene expression. These data are consistent with the hypothesis that a change in VDR gene expression is one of the steps that promote monocytic 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.

Steroid hormone receptors

In the three decades since the original discovery of receptors for steroid hormones, much has been learned about the biochemical processes by which these regulatory agents exert their effects in target tissues. The intracellular receptor proteins are potential transcription factors, needed for optimal gene expression in hormone-dependent cells. They are present in an inactive form until association with the hormone converts them to a functional state that can react with target genes. Transformation of the receptor protein to the nuclear binding form appears to involve the removal of both macromolecular and micromolecular factors that act to keep the receptor form reacting with DNA. Much of the native receptor is present in the nucleus, loosely bound and readily extractable, but for some and possibly all steroid hormones, some receptor is in the cytoplasm, perhaps in equilibrium with a nuclear pool. Methods have been developed for the stabilization, purification, and characterization of receptor proteins, and through cloning and sequencing of their cDNAs, primary structures for these receptors are now known. This has led to the recognition of structural similarities among the family of receptors for the different steroid hormones and to the identification of regions in the protein molecule responsible for the various aspects of their function. Monoclonal antibodies recognizing specific molecular domains are available for most receptors. Despite the knowledge that has been acquired, many important questions remain unsolved. How does association with the steroid remove factors keeping the receptor protein in its native state, and how does binding of the transformed receptor to the response element in the promoter region enhance gene transcription? Once it has converted the receptor to the nuclear binding state, is there a further role for the steroid in modulating transcription? Still not entirely clear is the involvement of phosphorylation and/or dephosphorylation in hormone binding, receptor transformation, and transcriptional activation. Less vital to basic understanding but important in the overall picture is whether the native receptors for gonadal hormones are entirely confined to the nucleus or whether there is an intracellular distribution equilibrium. With the effort now being devoted to this field, and with the application of new experimental techniques, especially those of molecular biology, our understanding of receptor function is progressing rapidly. The precise mechanism of steroid hormone action should soon be completely established.

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 alpha,25-dihydroxyvitamin D3 stimulates colony formation of chick embryo chondrocytes in soft agar

The effect of vitamin D metabolites on the growth of chick embryo chondrocytes in soft agar was examined. 1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] at 10(-8)-10(-7) M induced colony formation by chick embryo chondrocytes in soft agar in the presence of 10% fetal bovine serum. Furthermore, 1,25(OH)2D3 increased the number of colonies in the presence of a maximal dose of basic fibroblast growth factor, a potent mitogen for chondrocytes in soft agar. However, 24R,25 (OH)2D3 and other metabolites had little effect on the soft agar growth of chondrocytes in the presence or absence of basic fibroblast growth factor. These results suggest that 1,25(OH)2D3 is an active metabolite which may be involved in supporting cartilage growth.

Role of calcium and cAMP in heterologous up-regulation of the 1,25-dihydroxyvitamin D3 receptor in an osteoblast cell line

To understand further the mechanism of action of parathyroid hormone (PTH) in the stimulation of the number of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) binding sites in UMR 106-01 cells we studied the role of cAMP and calcium. In addition to PTH other agents known to act via the cAMP signal pathway, prostaglandin E2, forskolin and dibutyryl cAMP, caused an increase in 1,25(OH)2D3 binding. Addition of the adenylate cyclase inhibitor 9-(tetrahydro-2-furyl)adenine resulted in a marked decrease of PTH-stimulated cAMP production but this was not followed by a reduction of 1,25(OH)2D3 receptor up-regulation by PTH. Increasing the intracellular calcium concentration by Bay K 8644 and A23817 independent of an activation of the cAMP signal pathway did not result in an increased 1,25(OH)2D3 binding. The calcium channel blockers nitrendipine and verapamil and chelating extracellular calcium with EGTA all reduced cAMP-mediated stimulation of 1,25(OH)2D3 binding. This reduction was not due to a reduce cAMP production as verapamil even potentiated PTH- and forskolin-stimulated cAMP production in a dose-dependent manner. The present study provides evidence for an interrelated action of calcium and cAMP in the heterologous up-regulation of the 1,25(OH)2D3 receptor. The current data show an interaction between the cAMP and calcium signal pathway at (1) the level of cAMP generation/degradation, and (2) a level located distal in the cascade leading to 1,25(OH)2D3 receptor up-regulation.

Effect of dihydrotachysterol on bone induction in ovariectomized rats

We have demonstrated via marrow stromal cell cultures and the osteoinductive response to demineralized bone grafts (DBM) that the cortical bone deficit in the ovariectomized (OVX) rat (6 weeks postop) is primarily due to impaired osteoprogenitor cell proliferation, and that dihydrotachysterol (DHT) treatment can be protective. In cultured marrow stromal cells from OVX rats, short-term DHT-Rx exaggerated the already subnormal pattern of marrow stromal cell proliferation. However, in DBM grafts, DHT treatment benefited the time-course of mesenchymal cell DNA synthesis as measured by tritiated thymidine incorporation and osteogenic cell maturation as measured by alkaline phosphatase concentration, and established a suggestive trend toward normalization of bone formation/mineralization (24 h 45Ca incorporation). The data from this animal model infer that DHT could moderate the bone loss normally seen in ovariectomized rats via an activation of the osteoprogenitor cell population.

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)

Osteoinduction by implants of demineralized allogeneic bone matrix is diminished in vitamin D-deficient rats

Experimental heterotopic bone formation was produced by subcutaneous implants of demineralized allogeneic bone matrix (DABM) in vitamin D-deficient (-D) animals that were either not treated or given vitamin D3 (+D) or 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) to determine the role of vitamin D and its most active metabolite in osteoinduction and implant remodeling. Histologically, implants in both +D and -D groups caused a similar acute inflammatory response, formation of a fibrous capsule, and chondrogenesis by 1 to 2 weeks after implantation. However, by 3 weeks after implantation implants in the -D animals had formed less bone matrix, had developed a defect in matrix mineralization, had reduced bone forming and bone resorbing surfaces, and had altered bone architecture resulting from defective bone remodeling. The altered histology in -D animals was not corrected by 10 weeks after implantation. Treatment of vitamin D-deficient rats with 1,25(OH)2D3, 65 pmol/day for 3 weeks, corrected both the defect in mineralization and the abnormal histology. The results indicate that (1) vitamin D deficiency does not alter either the timing or the sequence of histologic events associated with osteoinduction but dramatically reduces the magnitude of the response, (2) vitamin D deficiency not only impairs mineralization but also reduces bone formation and resorption, and (3) 1,25(OH)2D3 mimics all of the actions of vitamin D with regard to correcting the abnormal osteoinductive response and bone histomorphometry.

Human articular chondrocytes acquire 1,25-(OH)2 vitamin D-3 receptors in culture

The vitamin D endocrine system is crucial in calcium homeostasis in mammalian species. Central to this role 1,25-dihydroxyvitamin D-3 (1,25-(OH)2D3) receptors have been detected in freshly isolated osteoblast-like bone cells and it has been shown that the active metabolite of vitamin D-3 1,25-(OH)2D3, increases bone resorption in vitro and in vivo. The requirement of 1,25-(OH)2D3 for the normal development of growth plate cartilage can be seen in vitamin D deficient rickets. However, there is still considerable controversy regarding the presence of 1,25-(OH)2D3 receptors in chondrocytes. In this paper, we report the presence of a 3.5-S 1,25-(OH)2D3-binding macromolecule in freshly isolated human costal but not articular chondrocytes. After subculture, both articular and costal chondrocytes have receptors. Saturation binding analysis revealed a single class of binding sites with an apparent Kd of 0.09 nM and approx. 2700 receptor molecules per cell for articular chondrocytes and a Kd of 0.1 nM and approx. 2000 receptor molecules per cell for costal chondrocytes. The presence of 1,25-(OH)2D3 receptors did not correlate with the switch from synthesis of cartilage-specific type II collagen to types I and III collagens. The acquisition of 1,25-(OH)2D3 receptors by articular chondrocytes may, therefore, be another phenotypic characteristic of cultured cells or may appear in vivo when chondrocytes are exposed to vascular or inflammatory cell products.

Update on secondary forms of hyperparathyroidism

Recent information has shed a new light on the control of parathyroid hormone (PTH) secretion by calcium and 1,25-(OH)2D. These new data have permitted a better understanding of the pathogenesis and management of secondary hyperparathyroidism in end-stage renal disease. Emerging evidence has suggested a role for secondary hyperparathyroidism in the development of certain forms of hypertension and osteoporosis. Recent insights have been obtained regarding the occurrence of secondary hyperparathyroidism in obese and black subjects, in patients with multiple endocrine neoplasia type I, and in manic-depressive patients receiving lithium therapy. This review examines some of these recent gains in knowledge concerning secondary hyperparathyroidism, as well as their clinical implications.

The effect of 1,25-dihydroxyvitamin D3 on the cytoskeleton of rat calvaria and rat osteosarcoma (ROS 17/2.8) osteoblastic cells

1,25-dihydroxyvitamin D3 produces pronounced shape changes in fetal rat calvaria and osteosarcoma-derived (ROS 17/2.8) osteoblastic cells, characterized by retracting processes and cell rounding followed by aggregation of cells. The 1,25(OH)2D3 effect on ROS 17/2.8 morphology was determined morphometrically on scanning electron micrographs. The hormone effect was found to be dose dependent between 10(-12) and 10(-9) M. The shape changes appeared 12 h after hormone (10(-10) M) addition and were present in 80% of the ROS 17/2.8 cells and in 50% of the calvaria cells at 72 h. Cycloheximide at 1 microM, inhibited the hormone-dependent change in morphology. The 1,25(OH)2D3 effects were partially mimicked by 10(-8) M 25(OH)D3 but not by 10(-10) M 25(OH)D3 or 10(-11)-10(-8) M 24,25(OH)2D3. 1,25-dihydroxyvitamin D3 also increased cell proliferation twofold at 14 days in serum-free medium. 1,25(OH)2D3 treatment produced changes in microfilament organization, visualized with rhodamine-conjugated phalloidin. Microfilaments were localized at the terminal attachment points and in the perinuclear region, and few if any, were seen in the retracting processes themselves. Estimation of cytoskeletal actin and myosin by gel electrophoresis of Triton X-100 nonextractable proteins showed a 30% reduction in these proteins in the hormone-treated cells. Microtubules visualized by indirect immunofluorescence showed no major changes in organization. Both colchicine and cytochalasin D altered the hormone-induced shape change, suggesting that both microfilaments and microtubules were required for this process. Thus, 1,25(OH)2D3 had pronounced effects on cell shape in osteoblastic cells, probably via de novo protein synthesis. These changes lead to rearrangement of the cytoskeleton, primarily the microfilaments.

Avian medullary bone in organ culture: effects of vitamin D metabolites on collagen synthesis

A new organ culture system for the study of bone metabolism has been developed using chicken medullary bone. The presence of viable bone cells in culture was demonstrated by histological and histochemical techniques. Incorporation of 3H-proline into collagenase-digestible protein (CDP) and noncollagen protein (NCP) was determined using purified bacterial collagenase. Collagen accounted for approximately 10-15% of the total protein labeled. The addition of 1,25-dihydroxycholecalciferol (1,25(OH)2D3) resulted in a dose-dependent inhibition of 3H-proline incorporation into CDP at doses from 10(-10)M to 10(-7)M, with maximal suppression reaching 30% of control. The effect was specific for collagen, since 3H-proline incorporation into NCP was unaffected. Hydroxyproline analysis of bone explants and culture medium revealed a 1,25(OH)2D3-induced decrease in the 3H-hydroxyproline content of the system (bone + medium), suggesting that the effect of 1,25(OH)2D3 is due to inhibition of collagen synthesis rather than enhanced collagen degradation, impaired incorporation of collagen into bone matrix, or bone resorption. Medullary bone collagen synthesis was not affected by 24,25(OH)2D3, either alone or in combination with 1,25(OH)2D3. Structure-activity studies of vitamin D metabolites showed that 1,25(OH)2D3 and 1,24,25(OH)3D3 were the most potent metabolites tested, followed by 1-alpha(OH)D3. 25(OH)D3 and 24,25(OH)2D3 had no effect at concentrations as high as 10(-7)M. These results indicate a possible role for vitamin D in the regulation of medullary bone formation during the reproductive cycle of the egg-laying hen, and suggest the potential utility of medullary bone as an in vitro model for the study of bone formation.

Differential effects of 1,25-dihydroxyvitamin D3 on human lymphocytes and monocyte/macrophages: inhibition of interleukin-2 and augmentation of interleukin-1 production

Human peripheral blood monocytes and activated, but not resting, lymphocytes possess specific intracellular receptors for the active metabolite of vitamin D3, 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3). The effects of 1,25-(OH)2D3 on the function of these cells was therefore examined. The addition of physiologic concentrations of the hormone (0.001-0.1 nM) to lectin- or antigen-activated lymphocytes resulted in inhibition of lymphocyte proliferation. Supernatants from lectin-activated lymphocytes incubated with 1,25-(OH)2D3 had reduced interleukin-2 (IL-2) activity. The immediate biological precursor of 1,25-(OH)2D3, 25-hydroxyvitamin D3, did not affect function of lymphocytes or monocytes. The ability of exogenous recombinant IL-2 to reverse the inhibitory effects of the hormone on lymphocyte proliferation suggest that 1,25-(OH)2D3 does not alter the generation of IL-2 receptors. In contrast to its effects on IL-2 production, 1,25-(OH)2D3 caused a dose-dependent increase in the production of interleukin-1 (IL-1) by monocyte/macrophages. These results suggest that immune cells and their products can be regulated in a specific but diverse fashion by the vitamin D3-endocrine system.

Hormone-dependent phosphorylation of the 1,25-dihydroxyvitamin D3 receptor in mouse fibroblasts

Experimental results, employing several immunologic techniques, suggest that the mouse receptor for 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) undergoes hormone-dependent phosphorylation in intact cells. Treatment of monolayer cultures of mouse 3T6 fibroblasts with 1,25(OH)2D3 reveals that the occupied 1,25(OH)2D3 receptor displays a minor reduction in electrophoretic mobility as compared to its unoccupied 54,500 dalton counterpart, a change consistent with covalent modification. Similar results were obtained by immunoprecipitation of metabolically-labeled receptors after incubation of 3T6 cells with [35S]methionine. This technique also provided greater insight into the precursor-product relationship between the two receptor forms. [32P]Orthophosphate-labeling of 3T6 cells, followed by immunoprecipitation indicated that only the form exhibiting covalent modification was phosphorylated. The temporal correspondence between the binding of 1,25(OH)2D3 to its cellular receptor and its phosphorylation suggests that the biochemical role of 1,25(OH)2D3 may be to induce a conformational change susceptible to phosphorylation and possibly functional activation.

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.

10-Keto or 25-hydroxy substitution confer equivalent in vitro bone-resorbing activity to vitamin D3

The biological activities of 10-keto derivatives of vitamin D3 and 25-hydroxyvitamin D3 were determined in bone organ culture. Fetal rat limb bones prelabeled with 45Ca were incubated for 60 h with 10-keto-25-hydroxyvitamin D3, 10-keto-vitamin D3, 1,25-dihydroxyvitamin D3, 25-hydroxyvitamin D3, or vitamin D3. Resorption was quantified by release of 45Ca. Substitution of a keto group in the 10 position of the vitamin D3 molecule resulted in a compound equal in potency to 25-hydroxyvitamin D3. When a 10-keto group was substituted in the 25-hydroxy vitamin D3 molecule, the potency was increased 20- to 40-fold. In contrast, 1,25-dihydroxyvitamin D3 was 7500-fold more potent than 25-hydroxyvitamin D3. Since 10-keto-25-hydroxyvitamin D3 has a retention time close to that of 1,25-dihydroxyvitamin D3 on normal-phase HPLC eluted with isopropanol:hexane, it is a possible artifact in the assay of 1,25-dihydroxyvitamin D3. Based upon the observed relative activities of the two compounds, the concentration of 10-keto-25-hydroxyvitamin D3 would have to be greater than 0.8 ng/ml for it to interfere in the bioassay of 1,25-dihydroxyvitamin D3.

1 alpha,25-dihydroxyvitamin D3 receptors and their action in embryonic chick chondrocytes

The role of vitamin D in the maturation of epiphyseal chondrocytes was investigated in the developing chick embryo. Cartilage tissues were divided into two parts: resting cartilage and growth cartilage. A cytosol component to which 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25(OH)2D3) is specifically bound first appeared in the growth cartilage on day 15, rapidly increased, and attained a maximum on day 19. The calcium content of the growth cartilage also began to increase on day 15 and continued to increase in parallel with the 1 alpha,25(OH)2D3 receptor levels. Glycosaminoglycan (GAG) synthesis by the growth cartilage cells increased from day 11-17 and rapidly declined thereafter reciprocally with the increase in calcium and receptor levels. In the resting cartilage, no cytosol receptor for 1 alpha,25(OH)2D3 was detected up to hatching time. The calcium content and GAG synthesis in the resting cartilage were very low and did not change appreciably throughout development. No receptor-like macromolecule for 24R,25-dihydroxyvitamin D3 (24R,25(OH)2D3) was recognized in either the resting or growth cartilage. 1 alpha,25(OH)2D3 added to the culture of chondrocytes from the epiphyseal growth cartilage inhibited GAG synthesis and stimulated its release from the cell layer into the medium in a dose-dependent manner. These in vitro effects of 1 alpha,25(OH)2D3 were not observed in chondrocytes obtained from 13-day-old growth cartilage and 19-day-old resting cartilage. 25-Hydroxyvitamin D3 and 24R,25(OH)2D3 had no effect on chondrocytes in any of the preparations. These results suggest that 1 alpha, 25 (OH)2D3 is directly involved in the maturation of chondrocytes and possibly in the calcification of growth cartilage.

Effects of 1 alpha,25-dihydroxyvitamin D3 and glucocorticoids on the growth of rat and mouse osteoblast-like bone cells

The effects of 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) and its interaction with glucocorticoids to regulate bone cell growth were studied in osteoblast-like (OH) cell cultures. Owing to our earlier findings that species difference and cell density at the time of treatment modified hormonal responses, comparisons were made between rat and mouse cells and sparse and dense cultures. 1,25(OH)2D3 inhibited cell proliferation in both species regardless of cell density. The magnitude of inhibition was larger in mouse cells, but the sensitivity to 1,25(OH)2D3 was the same for both species. Other metabolites, 25(OH)D3 and 24R,25(OH)2D3, were greater than 100-fold less potent than 1,25(OH)2D3 even in serum-free medium, which is similar to their ratio of affinity for the 1,25(OH)2D3 receptor. Dexamethasone, as previously shown, inhibited sparse and dense mouse cell cultures and sparse rat cell cultures while stimulating dense rat cell cultures to grow. The inhibitory actions of 1,25(OH)2D3 were not additive to the inhibitory dexamethasone effects. However, 1,25(OH)2D3 addition resulted in attenuation of the stimulatory effect of dexamethasone. These responses to 1,25(OH)2D3 and dexamethasone were dependent on cell density and not selective attachment of certain cell types at either plating density. In conclusion, the findings demonstrated that 1,25(OH)2D3 exerts an inhibiting action on both mouse and rat bone cell proliferation. This effect must be reconciled with the in vivo beneficial actions of 1,25(OH)2D3 on bone metabolism. Also, the likelihood of decreased cell number must be considered when biochemical activities are assessed after vitamin D treatment in vitro.

Effects of phosphate and 1,25(OH)2D3 on in vitro bone collagen synthesis in the hypophosphatemic mouse

Calvarial bones from hypophosphatemic (Hyp) mice and normal littermates were cultured in a chemically defined medium to determine: (a) the effect of medium phosphate (Pi) concentration (1, 2, and 3 mM) on collagen synthesis; (b) the effect of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] (10(-12)M-10(-7)M) on collagen synthesis; and (c) whether bone responsiveness to 1,25(OH)2D3 was affected by changes in medium Pi concentration. Bone collagen synthesis was evaluated by measuring [ 3H ]hydroxyproline formation. The distribution of labeled hydroxyproline between bone explant and culture medium (total and dialyzable fraction) was studied. These experiments confirm that 1,25(OH)2D3 inhibits specifically bone collagen synthesis in vitro. We did not detect any effect of medium Pi concentration on basal collagen synthesis but were able to demonstrate that lowering medium Pi concentration increased the 1,25(OH)2D3-induced inhibition of collagen synthesis. Bones from both genotypes responded to 1,25(OH)2D3, but modulation of this response by changes in Pi concentration was altered in Hyp bone as, in contrast to normal bone, its response to 1,25(OH)2D3 was unaffected when medium Pi concentration was decreased from 3 to 2 mM. These findings support the hypothesis of an altered response of bone to 1,25(OH)2D3 in the Hyp mouse.

Vitamin D receptors in isolated rat parotid gland acinar cells

Rat parotid gland was examined for the presence of 1 alpha, 25-dihydroxycholecalciferol receptors using sucrose density gradient ultracentrifugation techniques. [3H] DHCC bound specifically and with high affinity to a 3.2 S protein present in nuclear and cytosolic fractions of isolated parotid acinar cells. Values for the equilibrium dissociation constant and for the receptor concentration were determined to be approx. 0.1 nM, and 12 fmol/mg protein, respectively. In competitive inhibition experiments, the 3.2 S protein displayed 100-fold lower affinity for 25-hydroxycholecalciferol than for DHCC, and did not bind estradiol or methylprednisolone. These results suggest that rat parotid gland acinar cells contain classical DHCC receptors. A similar approach failed to provide evidence of DHCC receptors in isolated pancreas acinar cells, lacrimal gland or submandibular gland. It has been previously reported that vitamin D is essential for normal exocrine secretion from the rat parotid gland (Tenenhouse, A. and Afari, G. (1978) Biochim. Biophys. Acta 538, 631-634). The present findings suggest that this effect is the result of a direct action of DHCC on the parotid gland acinar cell. The absence of DHCC receptors in other exocrine cells suggests that tissue sensitivity to DHCC is not a general property of exocrine systems.