Wnt/β-catenin Signaling Controls Maxillofacial Hyperostosis

The roles of Wnt/β-catenin signaling in regulating the morphology and microstructure of craniomaxillofacial (CMF) bones was explored using mice carrying a constitutively active form of β-catenin in activating Dmp1-expressing cells (e.g., daβcatOt mice). By postnatal day 24, daβcatOt mice exhibited midfacial truncations coupled with maxillary and mandibular hyperostosis that progressively worsened with age. Mechanistic insights into the basis for the hyperostotic facial phenotype were gained through molecular and cellular analyses, which revealed that constitutively activated β-catenin in Dmp1-expressing cells resulted in an increase in osteoblast number and an increased rate of mineral apposition. An increase in osteoblasts was accompanied by an increase in osteocytes, but they failed to mature. The resulting CMF bone matrix also had an abundance of osteoid, and in locations where compact lamellar bone typically forms, it was replaced by porous, woven bone. The hyperostotic facial phenotype was progressive. These findings identify for the first time a ligand-independent positive feedback loop whereby unrestrained Wnt/β-catenin signaling results in a CMF phenotype of progressive hyperostosis combined with architecturally abnormal, poorly mineralized matrix that is reminiscent of craniotubular disorders in humans.

Molecular Basis for Craniofacial Phenotypes Caused by Sclerostin Deletion

Some genetic disorders are associated with distinctive facial features, which can aid in diagnosis. While considerable advances have been made in identifying causal genes, relatively little progress has been made toward understanding how a particular genotype results in a characteristic craniofacial phenotype. An example is sclerosteosis/van Buchem disease, which is caused by mutations in the Wnt inhibitor sclerostin (SOST). Affected patients have a high bone mass coupled with a distinctive appearance where the mandible is enlarged and the maxilla is foreshortened. Here, mice carrying a null mutation in Sost were analyzed using quantitative micro-computed tomographic (µCT) imaging and histomorphometric analyses to determine the extent to which the size and shape of craniofacial skeleton were altered. Sost^-/- mice exhibited a significant increase in appositional bone growth, which increased the height and width of the mandible and reduced the diameters of foramina. In vivo fluorochrome labeling, histology, and immunohistochemical analyses indicated that excessive bone deposition in the premaxillary suture mesenchyme curtailed overall growth, leading to midfacial hypoplasia. The amount of bone extracellular matrix produced by Sost^-/- cells was significantly increased; as a consequence, osteoid seams were evident throughout the facial skeleton. Collectively, these analyses revealed a remarkable fidelity between human characteristics of sclerosteosis/van Buchem disease and the Sost^-/- phenotype and provide clues into the conserved role for sclerostin signaling in modulating craniofacial morphology.

Genetic deletion of Sost or pharmacological inhibition of sclerostin prevent multiple myeloma-induced bone disease without affecting tumor growth

Multiple myeloma (MM) causes lytic bone lesions due to increased bone resorption and concomitant marked suppression of bone formation. Sclerostin (Scl) levels, an osteocyte-derived inhibitor of Wnt/β-catenin signaling, are elevated in MM patient sera and are increased in osteocytes in MM-bearing mice. We show here that genetic deletion of Sost, the gene encoding Scl, prevented MM-induced bone disease in an immune-deficient mouse model of early MM, and that administration of anti-Scl antibody (Scl-Ab) increased bone mass and decreases osteolysis in immune-competent mice with established MM. Sost/Scl inhibition increased osteoblast numbers, stimulated new bone formation and decreased osteoclast number in MM-colonized bone. Further, Sost/Scl inhibition did not affect tumor growth in vivo or anti-myeloma drug efficacy in vitro. These results identify the osteocyte as a major contributor to the deleterious effects of MM in bone and osteocyte-derived Scl as a promising target for the treatment of established MM-induced bone disease. Further, Scl did not interfere with efficacy of chemotherapy for MM suggesting that combined treatment with anti-myeloma drugs and Scl-Ab should effectively control MM growth and bone disease, providing new avenues to effectively control MM and bone disease in patients with active MM.

Hyponatremia and fractures: should hyponatremia be further studied as a potential biochemical risk factor to be included in FRAX algorithms?

The Fracture Risk Assessment Tool (FRAX®) was developed by the WHO Collaborating Centre for metabolic bone diseases to evaluate fracture risk of patients. It is based on patient models that integrate the risk associated with clinical variables and bone mineral density (BMD) at the femoral neck. The clinical risk factors included in FRAX were chosen to include only well-established and independent variables related to skeletal fracture risk. The FRAX tool has acquired worldwide acceptance despite having several limitations. FRAX models have not included biochemical derangements in estimation of fracture risk due to the lack of validation in large prospective studies. Recently, there has been an increasing number of studies showing a relationship between hyponatremia and the occurrence of fractures. Hyponatremia is the most frequent electrolyte abnormality measured in the clinic, and serum sodium concentration is a very reproducible, affordable, and readily obtainable measurement. Thus, we think that hyponatremia should be further studied as a biochemical risk factor for skeletal fractures prediction, particularly those at the hip which carries the greatest morbidity and mortality. To achieve this will require the collection of large patient cohorts from diverse geographical locations that include a measure of serum sodium in addition to the other FRAX variables in large numbers, in both sexes, over a wide age range and with wide geographical representation. It would also require the inclusion of data on duration and severity of hyponatremia. Information will be required both on the risk of fracture associated with the occurrence and length of exposure to hyponatremia and to the relationship with the other risk variables included in FRAX and also the independent effect on the occurrence of death which is increased by hyponatremia.

Role of connexin 43 in the mechanism of action of alendronate: dissociation of anti-apoptotic and proliferative signaling pathways

Bisphosphonates (BPs) inhibit osteocyte and osteoblast apoptosis via opening of connexin (Cx) 43 hemichannels and activating the extracellular signal regulated kinases ERKs. Previously, we hypothesized that intracellular survival signaling is initiated by interaction of BPs with Cx43. However, using whole cell binding assays with [(3)H]-alendronate, herein we demonstrated the presence of saturable, specific and high affinity binding sites in the Cx43-expressing ROS 17/2.8 osteoblastic cells, authentic osteoblasts and MLO-Y4 cells expressing Cx43 or not, as well as in HeLa cells lacking Cx43 expression and ROS 17/2.8 cells pretreated with agents that disassemble Cx channels. In addition, both BPs and the PTP inhibitor Na(3)VO(4) increased proliferation of cells expressing Cx43 or not. Furthermore, although BPs are internalized and inhibit intracellular enzymes in osteoclasts, whether the drugs penetrate non-resorptive bone cells is not known. To clarify this, we evaluated the osteoblastic uptake of AF-ALN, a fluorescently labeled analog of alendronate. AF-ALN was rapidly internalized in cells expressing Cx43 or not indicating that this process is not mediated via Cx43 hemichannels. Altogether, these findings suggest that although required for triggering intracellular survival signaling by BPs, Cx43 is dispensable for cellular BP binding, its uptake, as well as the proliferative effects of these agents.

A bisphosphonate that does not affect osteoclasts prevents osteoblast and osteocyte apoptosis and the loss of bone strength induced by glucocorticoids in mice

Although a major effect of bisphosphonates on bone is inhibition of resorption resulting from their ability to interfere with osteoclast function, these agents also prevent osteoblast and osteocyte apoptosis in vitro and in vivo. However, the contribution of the latter property to the overall beneficial effects of the drugs on bone remains unknown. We compared herein the action on glucocorticoid-induced bone disease of the classical bisphosphonate alendronate with that of IG9402, a bisphosphonate analog that preserves osteoblast and osteocyte viability but does not induce osteoclast apoptosis in vitro. The bisphosphonates were injected daily (2.3 μmol/kg) to 5-month-old Swiss Webster mice (6-11 per group), starting 3 days before implantation of pellets releasing the glucocorticoid prednisolone (2.1 mg/kg/day). IG9402 did not affect levels of circulating C-telopeptide or osteocalcin, markers of resorption and formation, respectively, nor did it decrease mRNA levels of osteocalcin or collagen 1a1 in bone. On the other hand, alendronate decreased all these parameters. Moreover, IG9402 did not reduce cancellous mineralizing surface, mineral apposition rate, or bone formation rate, whereas alendronate induced a decrease in each of these bone formation measures. These findings demonstrate that, in contrast to alendronate, IG9402 does not inhibit bone turnover. Both alendronate and IG9402, on the other hand, activated survival kinase signaling in vivo, as evidenced by induction of ERK phosphorylation in bone. Furthermore, both bisphosphonates prevented the increase in osteoblast and osteocyte apoptosis as well as the decrease in vertebral bone mass and strength induced by glucocorticoids. We conclude that a bisphosphonate that does not affect osteoclasts prevents osteoblast and osteocyte apoptosis and the loss of bone strength induced by glucocorticoids in mice.

Sex steroids, cytokines and the bone marrow: new concepts on the pathogenesis of osteoporosis

Osteoclasts and osteoblasts, originating in the bone marrow from haemopoietic progenitors and mesenchymal stromal cells, respectively, are responsible for the remodelling of the skeleton throughout adult life. Upon loss of sex steroids, the production of osteoclasts in the bone marrow is increased. This is mediated by an increase in the production of interleukin 6 (IL-6), as well as an increase in the sensitivity of the osteoclastic precursors to the action of cytokines such as IL-6, owing to an up-regulation of the gp130 signal transduction pathway. Consistent with this, oestrogens as well as androgens inhibit IL-6 production through an indirect effect of their specific receptors on the transcriptional activity of the IL-6 gene promoter, and inhibit the expression of the gp130 gene. With advancing age, the ability of the marrow to maintain the high rate of osteoclastogenesis caused by the acute loss of sex steroids is diminished. This is probably the result of the negative effect of senescence on the ability of the marrow to produce stromal/osteoblastic cells, which provide the essential support for osteoclastogenesis. These observations suggest that inappropriate production of osteoclasts or inadequate production of osteoblasts in the bone marrow are fundamental cellular changes in the pathogenesis of postmenopausal and senescence-associated osteoporosis, respectively.

Chronic elevation of parathyroid hormone in mice reduces expression of sclerostin by osteocytes: a novel mechanism for hormonal control of osteoblastogenesis

Both chronic excess of PTH, as in hyperparathyroidism, and intermittent elevation of PTH (by daily injections) increase the number of osteoblasts; albeit, the former is associated with bone catabolism and the later with bone anabolism. Intermittent PTH increases osteoblast number by attenuating osteoblast apoptosis, an effect that requires the transcription factor Runx2. However, chronic elevation of PTH does not affect osteoblast apoptosis because it stimulates the proteasomal degradation of Runx2. Here, we studied the effects of PTH on Sost, a Runx2 target gene expressed in osteocytes (former osteoblasts embedded in the bone matrix), which antagonizes the pro-osteoblastogenic actions of bone morphogenetic proteins and Wnts. We report that continuous infusion of PTH to mice for 4 d decreased Sost mRNA expression in vertebral bone by 80-90%. This effect was accompanied by a comparable reduction of sclerostin, the product of Sost, in osteocytes, as determined by quantitative immunoblot analysis of bone extracts and by immunostaining. In contrast, a single injection of PTH caused a transient 50% reduction in Sost mRNA at 2 h, but four daily injections had no effect on Sost mRNA or sclerostin. PTH strongly decreased Sost expression in osteocytes formed in primary cultures of neonatal murine calvaria cells as well as in osteocytic MLO-A5 cells, demonstrating a direct effect of PTH on this cell type. These results, together with evidence that sclerostin antagonizes bone morphogenetic proteins and Wnts, strongly suggest that suppression of Sost by PTH represents a novel mechanism for hormonal control of osteoblastogenesis mediated by osteocytes.

Mechanical stimulation prevents osteocyte apoptosis: requirement of integrins, Src kinases, and ERKs

Osteocytes, former osteoblasts entombed in the bone matrix, form an extensive cell communication network that is thought to detect microdamage and mechanical strains and to transmit signals leading to repair and compensatory bone augmentation or reduction. Bone active hormones and drugs control the integrity of this network by regulating osteocyte apoptosis, which might be a determinant of bone strength. Herein we demonstrate that mechanical stimulation by stretching activates the ERKs, which in turn are responsible for the attenuation of osteocyte apoptosis. The effect of osteocyte stretching is transmitted by integrins and cytoskeletal and catalytic molecules, such as Src kinases. Stretch-induced antiapoptosis also requires nuclear translocation of ERKs and new gene transcription. The evidence linking mechanical stimulation, activation of an integrin/cytoskeleton/Src/ERK signaling pathway, and osteocyte survival provides a mechanistic basis for the profound role of mechanical forces, or lack thereof, on skeletal health and disease.

Bisphosphonate-induced, hemichannel-mediated, anti-apoptosis through the Src/ERK pathway: a gap junction-independent action of connexin43

Preservation of the mechanosensory function of osteocytes by inhibiting their apoptosis might contribute to the beneficial effects of bisphosphonates in bone. We report herein a mechanism by which connexin43 hemichannel opening by bisphosphonates triggers the activation of the kinases Src and ERKs and promotes cell survival. Bisphosphonate-induced anti-apoptosis requires connexin channel integrity, but not gap junctions. Osteocytic cells express functional hemichannels that are opened by bisphosphonates, as demonstrated by dye uptake, regulation by established agonists and antagonists, and cell surface biotinylation. The anti-apoptotic effect of bisphosphonates depends on connexin43 expression in mouse embryonic fibroblasts and osteoblastic cells. Transfection of connexin43, but not other connexins, into connexin43 naïve cells confers de novo responsiveness to the drugs. The signal transducing property of connexin43 requires the pore-forming, as well as the C-terminal domains of the protein, the interaction of connexin43 with Src. and the activation of both Src and ERK kinases. These studies establish a role for connexin43 hemichannels in bisphosphonate action, and a novel function of connexin43--beyond gap junction communication--in the regulation of survival signaling pathways.

Reversal of bone loss in mice by nongenotropic signaling of sex steroids

We show that sex steroids protect the adult murine skeleton through a mechanism that is distinct from that used to preserve the mass and function of reproductive organs. The classical genotropic actions of sex steroid receptors are dispensable for their bone protective effects, but essential for their effects on reproductive tissues. A synthetic ligand (4-estren-3alpha,17beta-diol) that reproduces the nongenotropic effects of sex steroids, without affecting classical transcription, increases bone mass and strength in ovariectomized females above the level of the estrogen-replete state and is at least as effective as dihydrotestosterone in orchidectomized males, without affecting reproductive organs. Such ligands merit investigation as potential therapeutic alternatives to hormone replacement for osteoporosis in both women and men [corrected].

Extracellular signal-regulated kinases and calcium channels are involved in the proliferative effect of bisphosphonates on osteoblastic cells in vitro

Bisphosphonates (BPs) are analogues of pyrophosphate, which are widely used for the treatment of different pathologies associated with imbalances in bone turnover. Recent evidence suggested that cells of the osteoblastic lineage might be targets of the action of BPs. The objective of this work was to determine whether BPs induce proliferation of osteoblasts and whether this action involves activation of the extracellular signal-regulated kinases (ERKs). We have shown that three different BPs (olpadronate, pamidronate, and etidronate) induce proliferation in calvaria-derived osteoblasts and ROS 17/2.8 as measured by cell count and by [3H]thymidine uptake. Osteoblast proliferation induced by all BPs diminished to control levels in the presence of U0126, a specific inhibitor of the upstream kinase MEK 1 responsible for ERK phosphorylation. Consistent with this, BPs induced ERK activation as assessed by in-gel kinase assays. Phosphorylation of ERK1/2 was induced by the BPs olpadronate and pamidronate within 30 s, followed by rapid dephosphorylation, whereas etidronate induced phosphorylation of ERKs only after 90 s of incubation and returned to basal levels within 15-30 minutes. In addition, both BP-induced cell proliferation and ERK phosphorylation were reduced to basal levels in the presence of nifedipine, an L-type voltage-sensitive calcium channel (VSCC) inhibitor. These results show that BP-induced proliferation of osteoblastic cells is mediated by activation of ERKs and suggest that this effect requires influx of Ca2+ from the extracellular space through calcium channels.

Nongenotropic, sex-nonspecific signaling through the estrogen or androgen receptors: dissociation from transcriptional activity

The relationship of the classical receptors and their transcriptional activity to nongenotropic effects of steroid hormones is unknown. We demonstrate herein a novel paradigm of sex steroid action on osteoblasts, osteocytes, embryonic fibroblasts, and HeLa cells involving activation of a Src/Shc/ERK signaling pathway and attenuating apoptosis. This action is mediated by the ligand binding domain and eliminated by nuclear targeting of the receptor protein; ERalpha, ERbeta, or AR can transmit it with similar efficiency irrespective of whether the ligand is an estrogen or an androgen. This antiapoptotic action can be dissociated from the transcriptional activity of the receptor with synthetic ligands, providing proof of principle for the development of function-specific-as opposed to tissue-selective-and gender-neutral pharmacotherapeutics.

Expression levels of gp130 in bone marrow stromal cells determine the magnitude of osteoclastogenic signals generated by IL-6-type cytokines

Interleukin-6 (IL-6)-type cytokines stimulate osteoclast formation by activating the glycoprotein 130 (gp130) receptor subunit on stromal/osteoblastic cells, which in turn leads to signal transducer and activator of transcription 3 (STAT3)-mediated expression of receptor activator of NF-kappaB ligand (RANKL). Based on evidence that gp130 expression is regulated by a variety of cytokines and hormones, we have determined here whether changes in gp130 levels directly contribute to the magnitude of the osteoclastogenic stimulus delivered by IL-6-type cytokines. To accomplish this, gp130 protein levels were modulated using a tetracycline-regulated expression system in a stromal/osteoblastic cell line, UAMS-32, which supports osteoclast formation. Removal of doxycycline from the culture medium elevated gp130 expression and increased the responsiveness of a STAT-responsive promoter-luciferase construct to IL-6 complexed with its soluble receptor (IL-6+sIL-6R), but diminished the responsiveness to oncostatin M (OSM). IL-6+sIL-6R-stimulated osteoclast formation was greater when osteoclast precursors were cocultured with the cells expressing elevated gp130 levels than when cells expressing low gp130 levels were used. However, increased gp130 levels reduced OSM-stimulated osteoclast formation. These results establish that the level of gp130 in stromal/osteoblastic cells directly modulates the magnitude of the osteoclastogenic response to IL-6-type cytokines such that an increase in gp130 increases the cellular responsiveness to IL-6+sIL-6R but reduces responsiveness to OSM.

Calbindin-D28k is expressed in osteoblastic cells and suppresses their apoptosis by inhibiting caspase-3 activity

The rate of osteoblast apoptosis is a critical determinant of the rate of bone formation. Because the calcium-binding protein calbindin-D(28k) has anti-apoptotic properties in neuronal cells and lymphocytes, we searched for the presence of this protein in osteoblastic cells and investigated whether it can modify their response to proapoptotic signals. Calbindin-D(28K) was expressed at low levels in several osteoblastic cell lines and at high levels in primary cultures of murine osteoblastic cells. Transient transfection of rat calbindin-D(28k) cDNA blocked tumor necrosis factor alpha (TNFalpha)-induced apoptosis in osteoblastic MC3T3-E1 cells, as determined by cell viability and nuclear morphology of cells cotransfected with the green fluorescent protein targeted to the nucleus, whereas transfection of the empty vector had no effect. Calbindin-D(28k) levels in several stably transfected MC3T3-E1 lines were directly related to protection from TNFalpha-induced apoptosis. Purified rat calbindin-D(28k) markedly reduced the activity of caspase-3, a critical molecule for the degradation phase of apoptosis, in a cell-free assay. In addition, cell extracts from MC3T3-E1 cells expressing high levels of calbindin-D(28k) decreased caspase-3 activity, compared with extracts from vector-transfected cells. This effect was apparently unrelated to the calcium binding properties of calbindin, as chelation of calcium by EGTA or addition of other calcium-binding proteins such as calbindin-D(9k), S100, calmodulin, and osteocalcin, did not affect caspase-3 activity. Last, calbindin-D(28k) interacts with the active form of caspase-3 as demonstrated by a GST pull-down assay. These results demonstrate that calbindin-D(28k) is a biosynthetic product of osteoblasts with a role in the regulation of apoptosis. They also reveal that the antiapoptotic properties of calbindin-D(28k) may result not only from calcium buffering but also from the ability of the protein to interact with and to inhibit caspase-3 activity, a property that is independent of its calcium binding capability.

Prevention of osteocyte and osteoblast apoptosis by bisphosphonates and calcitonin

Glucocorticoid-induced osteoporosis may be due, in part, to increased apoptosis of osteocytes and osteoblasts, and bisphosphonates (BPs) are effective in the management of this condition. We have tested the hypothesis that BPs suppress apoptosis in these cell types. Etidronate, alendronate, pamidronate, olpadronate, or amino-olpadronate (IG9402, a bisphosphonate that lacks antiresorptive activity) at 10(-9) to 10(-6) M prevented apoptosis of murine osteocytic MLO-Y4 cells, whether it was induced by etoposide, TNF-alpha, or the synthetic glucocorticoid dexamethasone. BPs also inhibited apoptosis of primary murine osteoblastic cells isolated from calvaria. Similar antiapoptotic effects on MLO-Y4 and osteoblastic cells were seen with nanomolar concentrations of the peptide hormone calcitonin. The antiapoptotic effect of BPs and calcitonin was associated with a rapid increase in the phosphorylated fraction of extracellular signal regulated kinases (ERKs) and was blocked by specific inhibitors of ERK activation. Consistent with these in vitro results, alendronate abolished the increased prevalence of apoptosis in vertebral cancellous bone osteocytes and osteoblasts that follows prednisolone administration to mice. These results suggest that the therapeutic efficacy of BPs or calcitonin in diseases such as glucocorticoid-induced osteoporosis may be due, in part, to their ability to prevent osteocyte and osteoblast apoptosis.

Increased bone formation by prevention of osteoblast apoptosis with parathyroid hormone

The mass of regenerating tissues, such as bone, is critically dependent on the number of executive cells, which in turn is determined by the rate of replication of progenitors and the life-span of mature cells, reflecting the timing of death by apoptosis. Bone mass can be increased by intermittent parathyroid hormone (PTH) administration, but the mechanism of this phenomenon has remained unknown. We report that daily PTH injections in mice with either normal bone mass or osteopenia due to defective osteoblastogenesis increased bone formation without affecting the generation of new osteoblasts. Instead, PTH increased the life-span of mature osteoblasts by preventing their apoptosis - the fate of the majority of these cells under normal conditions. The antiapoptotic effect of PTH was sufficient to account for the increase in bone mass, and was confirmed in vitro using rodent and human osteoblasts and osteocytes. This evidence provides proof of the basic principle that the work performed by a cell population can be increased by suppression of apoptosis. Moreover, it suggests novel pharmacotherapeutic strategies for osteoporosis and, perhaps, other pathologic conditions in which tissue mass diminution has compromised functional integrity.

Decrease in protein tyrosine phosphorylation is associated with F-actin reorganization by retinoic acid in human endometrial adenocarcinoma (RL95-2) cells

Transformed cells often express elevated levels of tyrosine-phosphorylated proteins. Inhibition of protein tyrosine kinases causes reversion of malignant cells to the normal phenotype. In the present study, we evaluated the possibility that the reversion of human endometrial adenocarcinoma RL95-2 cells to a stationary phenotype induced by retinoic acid was associated with inhibition of tyrosine phosphorylation of cellular proteins. We found that retinoic acid decreased the levels of tyrosine-phosphorylated proteins, as assessed by immunostaining and immunoprecipitations using specific anti-phosphotyrosine antibodies. In addition, the inhibitors of tyrosine kinases herbimycin A and tyrphostin mimicked retinoic acid, inducing F-actin reorganization and increasing the size of RL95-2 cells, as determined by measurement of cell perimeters. Because focal adhesions that connect actin filaments with the plasma membrane are major sites of tyrosine phosphorylation, we further investigated whether selected focal adhesion proteins were affected by retinoic acid. We found that retinoic acid altered the localization of focal adhesion kinase. All-trans retinoic acid was effective in reducing the levels of focal adhesion kinase and paxillin protein. Thirteen-cis retinoic acid increased the levels of vinculin protein in the cytosolic fraction of cells. These changes are consistent with actin reorganization and reversion toward a stationary phenotype induced by retinoic acid in endometrial adenocarcinoma RL95-2 cells. Our results indicate that the differentiating effects of retinoids on endometrial cells are associated with decreases in tyrosine phosphorylation and changes in the levels and distribution of focal adhesion proteins. These findings suggest that signaling pathways that involve tyrosine kinases are potential targets for drug design against endometrial cancer.

Transcriptional activation of the p21(WAF1,CIP1,SDI1) gene by interleukin-6 type cytokines. A prerequisite for their pro-differentiating and anti-apoptotic effects on human osteoblastic cells

The cyclin-dependent kinase inhibitor p21(WAF1,CIP1,SDI1) plays a critical role in cell differentiation, and it has been shown to confer resistance to apoptosis. Based on this, and on evidence that activation of the gp130/signal transducer and activator of transcription (STAT) signal transduction pathway by interleukin (IL)-6 type cytokines promotes differentiation and prevents apoptosis in osteoblastic cells, we have investigated the possibility that p21 is a downstream effector of this signaling pathway in osteoblasts. We report that either oncostatin M (OSM) or IL-6 plus soluble IL-6 receptor increased the levels of p21 mRNA and protein in the osteoblast-like human osteosarcoma cell line MG63 and stimulated the activity of a 2.4-kilobase pair segment of the human p21 gene promoter. Further, nuclear extracts from cytokine-stimulated MG63 cells formed protein-DNA complexes with a 19-base pair nucleotide fragment of the p21 promoter containing a single STAT response element. The identity of the binding proteins as Stat3 and Stat1 was demonstrated with specific antibodies. In addition, and in support of a mediating role of STATs in the activation of the p21 promoter, overexpression of Stat3 potentiated the cytokine effect on the p21 promoter; whereas a dominant negative Stat3, or a mutation of the STAT response element on the promoter, significantly reduced the cytokine effect. Finally, antisense oligonucleotides complementary to p21 mRNA inhibited OSM-induced stimulation of alkaline phosphatase expression and antagonized the protective effect of OSM on anti-Fas-induced apoptosis. These results demonstrate that p21 is a downstream effector of gp130/Stat3 activation and a critical mediator of the pro-differentiating and anti-apoptotic effects of IL-6 type cytokines on human osteoblastic cells.

Osteoblast programmed cell death (apoptosis): modulation by growth factors and cytokines

Once osteoblasts have completed their bone-forming function, they are either entrapped in bone matrix and become osteocytes or remain on the surface as lining cells. Nonetheless, 50-70% of the osteoblasts initially present at the remodeling site cannot be accounted for after enumeration of lining cells and osteocytes. We hypothesized that the missing osteoblasts die by apoptosis and that growth factors and cytokines produced in the bone microenvironment influence this process. We report that murine osteoblastic MC3T3-E1 cells underwent apoptosis following removal of serum, or addition of tumor necrosis factor (TNF), as indicated by terminal deoxynucleotidyl transferase-mediated dUTP-nick end labeling and DNA fragmentation studies. Transforming growth factor-beta and interleukin-6 (IL-6)-type cytokines had antiapoptotic effects because they were able to counteract the effect of serum starvation or TNF. In addition, anti-Fas antibody stimulated apoptosis of human osteoblastic MG-63 cells and IL-6-type cytokines prevented these changes. The induction of apoptosis in MG-63 cells was associated with an increase in the ratio of the proapoptotic protein bax to the antiapoptotic protein bcl-2, and oncostatin M prevented this change. Examination of undecalcified sections of murine cancellous bone revealed the presence of apoptotic cells, identified as osteoblasts by their proximity to osteoid seams and their juxtaposition to cuboidal osteoblasts. Assuming an osteoblast life span of 300 h and a prevalence of apoptosis of 0.6%, we calculated that the fraction that undergo this process in vivo can indeed account for the missing osteoblasts. These findings establish that osteoblasts undergo apoptosis and strongly suggest that the process can be modulated by growth factors and cytokines produced in the bone microenvironment.

Regulation of the gp80 and gp130 subunits of the IL-6 receptor by sex steroids in the murine bone marrow

Both estrogen and androgen exert their antiosteoporotic effects, at least in part, by inhibiting IL-6 production, thereby suppressing osteoclastogenesis. Several observations, however, suggest that besides increased IL-6 production, sensitivity of the osteoclastogenic process to this cytokine is altered after ovariectomy. Based on this and evidence that the ligand-binding subunit of the IL-6 receptor (gp80) is a limiting factor for the actions of IL-6 on bone, we hypothesized that sex steroids regulate expression of the IL-6 receptor as well. We report that 17beta-estradiol or dihydrotestosterone in vitro decreased the abundance of the gp80 mRNA as well as the mRNA of the signal-transducing subunit of the IL-6 receptor (gp130) in cells of the bone marrow stromal/osteoblastic lineage, and also decreased gp130 protein levels. These effects did not require new protein synthesis. In contrast to sex steroids, parathyroid hormone stimulated gp130 expression; this effect was opposed by sex steroids. Consistent with these findings, ovariectomy in mice caused an increase in expression of gp80, gp130, and IL-6 mRNAs in ex vivo bone marrow cell cultures as determined by quantitative reverse transcription (RT)-PCR, and confirmed on an individual cell basis using in situ RT-PCR. The demonstration of increased expression of the IL-6 receptor after loss of sex steroids provides an explanation for why IL-6 is important for skeletal homeostasis in the sex steroid-deficient, but not replete, state.

Activation of the Janus kinase/STAT (signal transducer and activator of transcription) signal transduction pathway by interleukin-6-type cytokines promotes osteoblast differentiation

We have previously established that stromal/osteoblastic cells collectively express receptors for all members of the cytokine subfamily that share the gp130 signal transducer and that different receptor repertoires may be expressed at different stages of differentiation of this lineage. We have now used human (MG-63) and murine (MC3T3-E1) osteoblastic cell lines as well as primary murine calvaria cells to test the hypothesis that these receptors mediate effects of the cytokines on the biology of osteoblasts. We report that as in other cell types, all of the osteoblastic cell models responded to interleukin-6 (IL-6)-type cytokines with activation of both the JAK/STAT (Janus kinase/signal transducer and activator of transcription) and the mitogen-activated protein kinase (MAPK) pathways. In addition, IL-6-type cytokines stimulated alkaline phosphatase activity and osteocalcin expression and inhibited (MG-63), stimulated (MC3T3-E1), or had no effect (calvaria cells) on the rate of cell proliferation. The ability of a given cell type to respond to a particular member of this family of cytokines was strictly dependent on the presence of the corresponding ligand-binding subunit (alpha) of the cytokine receptor, and the magnitude of all the effects was closely correlated with the concentration of this subunit. The relative contribution of the JAK/STAT and MAPK pathways to the biological effects of the cytokines was evaluated using kinase inhibitors. Cytokine-mediated modulation of cell proliferation as well as stimulation of alkaline phosphatase activity were abrogated by tyrosine kinase inhibitors as well as a threonine/serine kinase inhibitor, but were only minimally affected by a specific inhibitor of MAPK phosphorylation. These results demonstrate that IL-6-type cytokines, besides their osteoclastogenic properties, promote differentiation of committed osteoblastic cells toward a more mature phenotype and that this action is mediated primarily via the activation of the JAK/STAT pathway.

Detection of receptors for interleukin-6, interleukin-11, leukemia inhibitory factor, oncostatin M, and ciliary neurotrophic factor in bone marrow stromal/osteoblastic cells

The functional receptor complexes assembled in response to interleukin-6 and -11 (IL-6 and IL-11), leukemia inhibitory factor (LIF), oncostatin M (OSM), and ciliary neurotrophic factor (CNTF), all involve the signal transducer gp130: IL-6 and IL-11 induce homodimerization of gp130, while the rest heterodimerize gp130 with other gp130-related beta subunits. Some of these cytokines (IL-6, IL-11, and CNTF) also require a specificity-determining alpha subunit not directly involved in signaling. We have searched for functional receptor complexes for these cytokines in cells of the bone marrow stromal/osteoblastic lineage, using tyrosine phosphorylation of the beta subunits as a detection assay. Collectively, murine calvaria cells, bone marrow-derived murine cell lines (+/+LDA11 and MBA13.2), as well as murine (MC3T3-E1) and human (MG-63) osteoblast-like cell lines displayed all the previously recognized alpha and beta subunits of this family of receptors. However, individual cell types had different constellations of alpha and beta subunits. In addition and in difference to the other cell types examined, MC3T3-E1 cells expressed a heretofore unrecognized form of gp130; and MG-63 displayed an alternative form (type II) of the OSM receptor. These findings establish that stromal/osteoblastic cells are targets for the actions of all the members of the cytokine subfamily that shares the gp130 signal transducer; and suggest that different receptor repertoires may be expressed at different stages of differentiation of this lineage.

Down-regulation of NF-kappa B protein levels in activated human lymphocytes by 1,25-dihydroxyvitamin D3

The effect of 1,25-dihydroxyvitamin D3 [1,25(OH)2)D3], a steroid hormone with immunomodulating properties, on nuclear factor kappa B (NF-kappa B) proteins was examined in in vitro activated normal human lymphocytes by Western blot analysis. Over a 72-hr period of activation, the expression of the 50-kDa NF-kappa B, p50, and its precursor, p105, was increased progressively. When cells were activated in the presence of 1,25(OH)2D3, the levels of the mature protein as well as its precursor were decreased. The effect of the hormone on the levels of p50 was demonstrable in the cytosolic and nuclear compartments; it required between 4 and 8 hr and was specific, as 25-hydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 were ineffective. Besides p50, 1,25(OH)2D3 decreased the levels of another NF-kappa B protein, namely c-rel. In addition, 1,25(OH)2D3 decreased the abundance of a specific DNA-protein complex formed upon incubation of nuclear extracts from activated lymphocytes with a labeled NF-kappa B DNA binding motif. Further, 1,25(OH)2D3 inhibited the transcriptional activity of NF-kappa B in Jurkat cells transiently transfected with a construct containing four tandem repeats of the NF-kappa B binding sequence of the immunoglobulin kappa light chain gene linked to the chloramphenicol acetyltransferase reporter gene. These observations demonstrate directly that there is de novo synthesis of NF-kappa B during human lymphocyte activation and suggest that this process is hormonally regulated.

Regulation of interleukin-6, osteoclastogenesis, and bone mass by androgens. The role of the androgen receptor

Interleukin-6 is an essential mediator of the bone loss caused by loss of estrogens. Because loss of androgens also causes bone loss, we have examined whether the IL-6 gene is regulated by androgens, and whether IL-6 plays a role in the bone loss caused by androgen deficiency. Both testosterone and dihydrotestosterone inhibited IL-6 production by murine bone marrow-derived stromal cells. In addition, testosterone, dihydrotestosterone, and adrenal androgens inhibited the expression of a chloramphenicol acetyl transferase reporter plasmid driven by the human IL-6 promoter in HeLa cells cotransfected with an androgen receptor expression plasmid; however, these steroids were ineffective when the cells were cotransfected with an estrogen receptor expression plasmid. In accordance with the in vitro findings, orchidectomy in mice caused an increase in the replication of osteoclast progenitors in the bone marrow which could be prevented by androgen replacement or administration of an IL-6 neutralizing antibody. Moreover, bone histomorphometric analysis of trabecular bone revealed that, in contrast to IL-6 sufficient mice which exhibited increased osteoclast numbers and bone loss following orchidectomy, IL-6 deficient mice (generated by targeted gene disruption) did not. This evidence demonstrates that male sex steroids, acting through the androgen-specific receptor, inhibit the expression of the IL-6 gene; and that IL-6 mediates the upregulation of osteoclastogenesis and therefore the bone loss caused by androgen deficiency, as it does in estrogen deficiency.

New insights into the cellular, biochemical, and molecular basis of postmenopausal and senile osteoporosis: roles of IL-6 and gp130

It is well established that osteoclasts, the cells responsible for bone resorption, are derived from hematopoietic progenitors (CFU-GM), whereas the bone-forming osteoblasts are of the same lineage as the mesenchymal stromal cells of the bone marrow. Moreover, it is widely accepted that osteoclast formation depends on cells of the stromal/osteoblastic lineage. The appreciation of the ontogeny of osteoclasts and osteoblasts, the interaction between them, and the role of local factors that regulate their development has led to the emergence of new insights into the pathophysiology of the osteopenias associated with estrogen deficiency and senescence. Consistent with histomorphometric data from humans, there is now evidence from studies in animal models suggesting that a critical cellular change caused by the loss of ovarian, as well as testicular, function is an increase in osteoclastogenesis. This change is apparently mediated by an increase in the production of the osteoclastogenic cytokine interleukin-6 by cells of the bone marrow, which follows the removal of an inhibiting control of estrogens or androgens on IL-6. The inhibiting effect of sex steroids on IL-6 production is mediated by their respective receptors and is exerted indirectly on the transcriptional activity of the proximal 225 bp sequence of the IL-6 gene promoter. Besides its effects on IL-6 production, loss of gonadal function may also cause an increase in the sensitivity of the osteoclastic precursors to the action of cytokines such as IL-6, due to an upregulation of the gp130 signal transduction pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of calcium uptake in cultured cardiac muscle cells by 1,25-dihydroxyvitamin D3

Recent studies have provided evidence indicating that 1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] rapidly stimulates calcium influx through Ca2+ channels in isolated chick heart tissue and cells. Studies were performed both to evaluate the characteristics of the hormone action in cultured myocytes and to obtain information on the metabolic pathway which mediates its effects. Treatment of 70-80% confluent chick embryo myocyte monolayers with 1,25(OH)2D3 induced a fast (within 3-5 min) stimulation of 45Ca uptake which was dose-dependent, maximum responses (130% above controls) being elicited at a concentration of 10(-10) M. Physiological levels of 25(OH)D3 and 24,25(OH)2D3, and the synthetic analog 1 alpha (OH)D3, had lower activity. Coincident with the rapid changes in Ca uptake, 1,25(OH)2D3 significantly increased cAMP levels. The hormone-induced increase in cAMP was not blocked by nifedipine. Compound SQ 22536, a specific inhibitor of adenylate cyclase, completely suppressed the effects of the sterol on cAMP and Ca uptake. Furthermore, GDP-beta-S inhibited the increase in Ca uptake by 1,25(OH)2D3. These results involve the adenylate cyclase pathway and the participation of G proteins in 1,25(OH)2D3 stimulation of Ca influx in chick heart cells.

Vitamin D and the hematolymphopoietic tissue: a 1994 update

Monocytes/macrophages and activated (but not resting) lymphocytes as well as certain subsets of thymocytes express the VDR. This protein is indistinguishable from the classical 50-kDa VDR and is encoded by an mRNA with identical nucleotide sequence to that of the human intestinal VDR. Acting via the VDR, 1,25(OH)2D3 modulates the production of a plethora of monocyte, lymphocyte, and bone marrow stromal cell products, including several interleukins and other cytokines, as well as various oncogenes and transcription factors. However, these hormonal effects vary depending on the signals used to activate blood mononuclear cells; moreover, each of the effects of the hormone can be either attenuated, abolished, or even reversed from negative to positive in the presence of phorbol esters. Lymphocytes also express a previously unrecognized 80-kDa cytosolic protein that shares immunologic cross-reactivity with the VDR. This protein is induced on activation and is downregulated by 1,25(OH)2D3, whereas the VDR is upregulated by 1,25(OH)2D3. In contrast to the signal-dependent effects of the hormone on cytokine production and lymphocyte proliferation, the effects of 1,25(OH)2D3 on the 80-kDa protein and VDR are independent of the activation signals. This apparent mechanistic distinction raises the possibility that the signal-independent effects of 1,25(OH)2D3 on the 80-kDa protein and the VDR might be due to direct interactions of the 1,25(OH)2D3-VDR complex with specific response elements (negative and positive VDREs, respectively) on these two genes; as opposed to the signal-dependent effects that might be due to influences of the 1,25(OH)2D3-VDR complex on other transcription factors that are generated in response to the different activation stimuli. Consistent with the second part of this contention, we have recently found that 1,25(OH)2D3 regulates the 50-kDa DNA binding subunit of the pleiotropic transcription factor NF-kappa B and the 105-kDa precursor of this subunit; as well as other members of the rel-related family of proteins, including v-rel and its normal cellular homolog c-rel, in activated normal human lymphocytes. Besides its influence on immune cell products, 1,25(OH)2D3 is a potent agent for the differentiation of cells of the myeloid lineage. In addition, 1,25(OH)2D3 stimulates the fusion and differentiation of hematopoietic progenitors into osteoclasts, an effect which accounts for the potent role of the hormone in bone resorption.(ABSTRACT TRUNCATED AT 400 WORDS)

17 beta-Estradiol inhibits expression of human interleukin-6 promoter-reporter constructs by a receptor-dependent mechanism

We previously reported that 17 beta-estradiol inhibits cytokine-stimulated bioassayable IL-6 and the steady-state level of IL-6 mRNA. To determine the molecular basis of this effect, the transient expression of chloramphenicol acetyltransferase (CAT) reporter plasmid driven by the human IL-6 promoter was studied here in HeLa or murine bone marrow stromal cells (MBA 13.2). 17 beta-estradiol (10(-8) M) completely suppressed stimulated CAT expression in HeLa cells cotransfected with IL-6/CAT constructs and a human estrogen receptor (hER) expression plasmid; but had no effect on reporter expression in HeLa cells not transfected with hER. 17 beta-estradiol also inhibited stimulated expression in MBA 13.2 cells (which express the estrogen receptor constitutively) without the requirement of cotransfection of the hER plasmid. The hormonal effects were indistinguishable between constructs containing a 1.2-kb fragment of the 5' flanking region of the IL-6 gene or only the proximal 225-bp fragment. However, yeast-derived recombinant hER did not bind to the 225-bp segment in DNA band shift assays, nor did the 225-bp fragment compete for binding of an estrogen response element oligonucleotide to yeast-derived estrogen receptor. These data suggest that 17 beta-estradiol inhibits the stimulated expression of the human IL-6 gene through an estrogen receptor mediated indirect effect on the transcriptional activity of the proximal 225-bp sequence of the promoter.

Demonstration of estrogen and vitamin D receptors in bone marrow-derived stromal cells: up-regulation of the estrogen receptor by 1,25-dihydroxyvitamin-D3

We have shown earlier that 17 beta-estradiol inhibits cytokine-induced interleukin-6 (IL-6) production by bone marrow-derived stromal cells as well as osteoblasts, two types of cells with a critical influence on osteoclast development, and that ovariectomy causes an IL-6-mediated up-regulation of osteoclastogenesis in mice. Prompted by this, we have searched here for the presence of estrogen receptors (ERs) in two murine bone marrow-derived stromal cell lines, +/+ LDA11 and MBA 13.2, and the osteoblast-like cell line MC3T3-E1. All three cell lines exhibited high affinity saturable binding for [125I]17 beta-estradiol with a dissociation constant of approximately 10(-10) M and concentration of binding sites of 260 +/- 30, 170 +/- 10, and 90 +/- 10 sites per cell, respectively. In addition, we amplified complementary DNA from the stromal cell lines by polymerase chain reaction using oligonucleotide primers flanking the DNA binding domain of the murine uterine ER. The amplified product showed an identical nucleotide sequence to the DNA binding domain of the murine uterine receptor. Consistent with the functionality of the ER in stromal cells, and specifically its role in the regulation of IL-6 by 17 beta-estradiol, we found that the pure estrogen antagonist ICI 164,384 completely prevented the effect of 17 beta-estradiol on IL-6. All three cell lines also expressed receptors for 1,25-dihydroxyvitamin-D3 [1,25(OH)2D3] (dissociation constant, approximately 10(-10) M), with a concentration of binding sites of 490 +/- 20, 920 +/- 20, and 1110 +/- 70 sites per cell, respectively. 1,25(OH)2D3 treatment of the stromal cells caused a 2-fold increase in the concentration of ERs and a decrease in cell proliferation. These data establish that bone marrow-derived stromal cells express functional estrogen as well as vitamin D receptors, which serve to mediate actions of their respective ligands on the biosynthetic activity of these cells and presumably the effects of these two steroid hormones on osteoclastogenesis.

Evidence for the participation of protein kinase C and 3',5'-cyclic AMP-dependent protein kinase in the stimulation of muscle cell proliferation by 1,25-dihydroxy-vitamin D3

Treatment with 1,25-dihydroxy-vitamin D3 (1,25(OH)2D3) (1-12 h, 10(-10) M) stimulates DNA synthesis in proliferating myoblasts, with an early response at 2-4 h of treatment followed by a maximal effect at 10 h. To investigate the mechanism involved in the mitogenic action of the hormone we studied the possible activation of intracellular messengers by 1,25(OH)2D3. The initial phase of stimulation of [3H]thymidine incorporation into DNA by the sterol was mimicked by the protein kinase C activator tetradecanoylphorbol acetate (TPA) in a manner which was dose dependent and specific as the inactive analog 4 alpha-phorbol was without effect. Maximal responses to TPA (100 nM) were obtained at 4 h. Staurosporine, a protein kinase C inhibitor, blocked the effect of 1,25(OH)2D3 on myoblast proliferation at 4 h. In addition, a fast (1-5 min) elevation of diacylglycerol levels and membrane-associated protein kinase C activity was observed in response to 1,25(OH)2D3. The adenylate cyclase activator forskolin (20 microM) and dibutyryl-cAMP (50 microM) increased DNA synthesis reproducing the second 1,25(OH)2D3-dependent stimulatory phase at 10 h. Inhibitors of protein kinase A blocked the increase in muscle cell DNA synthesis induced by 1,25(OH)2D3 at 10 h. Significant increases in cyclic AMP levels were detected in myoblasts treated with the sterol for 1-10 h. The calcium channel antagonist nifedipine (5-10 microM) abolished both the effects of 4-h treatment with 1,25(OH)2D3 or TPA and 10-h treatment with 1,25(OH)2D3 or dibutyryl-cAMP. Similar to the calcium channel agonist Bay K8644, 1,25(OH)2D3 stimulated myoblast 45Ca uptake and its effects were blocked by nifedipine.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of 1,25-dihydroxy-vitamin D3 on phosphate accumulation by myoblasts

The effects of 1,25-dihydroxy-Vitamin D3 on phosphate uptake by cultured chick embryonic muscle cells were investigated. Preincubation of primary myoblast cultures during 4-24 hours with physiological levels of 1,25(OH)2D3 resulted in a significant stimulation of velocity and total capacity of phosphate accumulation by the cells. Maximal responses were obtained at 8 hours of treatment with the sterol. In agreement with previous studies, 25-hydroxy-Vitamin D3 also stimulated myoblast phosphate uptake. 24,25-dihydroxy-Vitamin D3 and vitamin D3 were ineffective. Evidence was obtained indicating that 1,25(OH)2D3 affects the Na(+)-linked component of muscle cell phosphate uptake through a mechanism dependent on "de movo" protein and RNA synthesis.

Stimulation of myoblast membrane protein synthesis by 25-hydroxy-vitamin D3

The effects of 25-hydroxy-vitamin D3 (25 OHD3) on myoblast protein synthesis were studied in connection with its role on muscle cell phosphate metabolism. The sterol markedly increased leucine incorporation into total cell proteins in cultured chick embryo myoblasts. This enhancement was greater than that produced by 1,25-dihydroxy-vitamin D3 (1,25(OH)2D3) and occurred prior to a significant stimulation of cell phosphate accumulation. Maximum effects of 25 OHD3 (8 h) on myoblast phosphate uptake were suppressed by cycloheximide indicating that they are mediated by de novo protein synthesis. At a similar treatment period, labelling of myoblasts with [3H]leucine (control) and [14C]leucine (+25 OHD3) followed by co-electrophoresis of total protein extracts on SDS-PAGE and isoelectrofocusing gels revealed that the sterol selectively affects the synthesis of proteins of 20 kDa and 50 kDa. These macromolecules were recovered in the microsomal fraction after differential centrifugation of homogenates. Further fractionation of myoblast microsomes on sucrose density gradients showed co-localization of the 50 kDa and 20 kDa proteins with microsomal subfractions which preferentially bind [3H-alpha]bungarotoxin, suggesting that the proteins induced by 25 OHD3 are associated to plasma membranes and may play a role in the effects of the sterol on cell phosphate uptake.

The phospholipid and fatty acid composition of skeletal muscle cells during culture in the presence of vitamin D-3 metabolites

The phospholipid and fatty acid composition of primary cultures (24 h) of chick embryo skeletal muscle myoblasts treated for 4-24 h with physiological concentrations of 1,25-dihydroxyvitamin D-3 and 25-hydroxyvitamin D-3 were analyzed. 25-Hydroxyvitamin D-3 did not alter the relative amounts of individual muscle cell phospholipids whereas 1,25-dihydroxyvitamin D-3 significantly increased phosphatidylcholine content, mainly at the expense of a decrease in phosphatidylethanolamine concentration. The increase in phosphatidylcholine occurred at a faster rate during the first 8 h than in the subsequent 8-24 h treatment period. A similar time course in 1,25-dihydroxyvitamin D3-dependent changes in myoblast calcium uptake has been observe. In addition, this metabolite markedly increased (100%) the arachidonate content of myoblast phosphatidylcholine near the fusion stage of the cells (24 h of treatment). The levels of docosahexaenoate, a minor polyunsaturated fatty acid, in phosphatidylcholine and phosphatidylethanolamine were also substantially elevated by 1,25-dihydroxyvitamin D-3. No significant changes in fatty acid composition in response to 25-hydroxyvitamin D-3 were observed. Modifications in phospholipids and polyunsaturated fatty acids may play a role in the effects of 1,25-dihydroxyvitamin D-3 on muscle cell calcium transport and differentiation.

Phosphate accumulation by muscle in vitro and the influence of vitamin D3 metabolites

Phosphate accumulation by muscle in vitro and the effects of vitamin D3 metabolites thereupon were studied in cultures of chick embryo skeletal muscle myoblasts and intact chick soleus muscles. A significant proportion of phosphate accumulation by the cells was Na+-dependent, saturable with respect to phosphate, energy-dependent and inhibited by ouabain and arsenate, in agreement with the operation of a Na+-phosphate cotransport system in the muscle cell plasma membrane as has been described for intestine and kidney. This was further supported by the demonstration of substrate-saturable phosphate uptake in sarcolemma vesicles isolated from chick skeletal muscle. Preincubation of myoblast and soleus muscle cultures with physiological levels of 25-hydroxy-vitamin D3 resulted in a significant stimulation of phosphate accumulation by cultures. 1,25-dihydroxy-vitamin D3 had no effects on the differentiated tissue whereas it markedly increased phosphate accumulation by embryonic muscle cells. In addition, it could be shown that 25-hydroxy-vitamin D3 affects the Na+-linked component of cell phosphate uptake through a mechanism dependent on de novo protein synthesis.