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.

Glucocorticoids act directly on osteoclasts to increase their life span and reduce bone density

Glucocorticoid administration to mice results in a rapid loss of bone mineral density due to an imbalance in osteoblast and osteoclast numbers. Whereas excess glucocorticoids reduce both osteoblast and osteoclast precursors, cancellous osteoclast number surprisingly does not decrease as does osteoblast number, presumably due to the ability of glucocorticoids to promote osteoclast life span. Whether glucocorticoids act directly on osteoclasts in vivo to promote their life span and whether this contributes to the rapid loss of bone with glucocorticoid excess remains unknown. To determine the direct effects of glucocorticoids on osteoclasts in vivo, we expressed 11beta-hydroxysteroid dehydrogenase type 2, an enzyme that inactivates glucocorticoids, specifically in the osteoclasts of transgenic mice using the tartrate-resistant acid phosphatase promoter. Bone mass, geometry, and histomorphometry were similar in untreated wild-type and transgenic animals. Glucocorticoid administration for 7 d caused equivalent increases in cancellous osteoblast apoptosis, and equivalent decreases in osteoblasts, osteoid, and bone formation, in wild-type and transgenic mice. In contrast, glucocorticoids stimulated expression of the mRNA for calcitonin receptor, an osteoclast product, in wild-type but not transgenic mice. Consistent with the previous finding that glucocorticoids decrease osteoclast precursors and prolong osteoclast life span, glucocorticoids decreased cancellous osteoclast number in the transgenic mice but not wild-type mice. In accord with this decrease in osteoclast number, the loss of bone density observed in wild-type mice was strikingly prevented in transgenic mice. These results demonstrate for the first time that the early, rapid loss of bone caused by glucocorticoid excess results from direct actions on osteoclasts.

Classical genotropic versus kinase-initiated regulation of gene transcription by the estrogen receptor alpha

Elucidation of kinase-initiated routes by which the estrogen receptors alpha and beta (ERalpha and ERbeta) control gene transcription, along with evidence of distinct biologic outcomes in response to ligands that can selectively activate nongenotropic signaling of the ERs or the androgen receptor, suggest that the ERs control a range of genes wider than that regulated by their direct association with DNA. To ascertain the extent and significance of nongenotropic ER-mediated transcription, we employed transduced HeLa cells expressing wild-type ERalpha or the ligand binding domain of ERalpha localized to the cell membrane (E-Mem), the OB-6 osteoblastic cell line, MCF-7 breast carcinoma cells and uteri from mice treated with 17beta-estradiol (E(2)), or the nongenotropic signaling activator 4-estren-3alpha,17beta-diol (estren). E(2) and estren induced ERK1/2 and Akt phosphorylation in ERalpha or E-Mem stably transfected HeLa cells; however, the phosphorylation kinetics differed between the two cell lines. In all four models, nongenotropic ER actions regulated a population of genes distinct from those regulated by genotropic ER actions. Specifically, the expression of Wnt2, Frizzled10, Egr-1, and c-Fos was strongly up-regulated in E-Mem-containing HeLa cells treated with E(2) or estren, or in ERalpha-containing HeLa cells treated with estren. Up-regulation of Frizzled10 by estren was reproduced in MCF-7 cells. Egr-1 was up-regulated by both estren and E(2); but complement 3, only by E(2) in the uteri. Estren had no effect on complement 3, cathepsin D, progesterone receptor, bcl-2, and cyclin D1 in MCF-7 cells, whereas E(2) up-regulated all these estrogen response element or activating protein-1-containing genes. These results support an extensive divergence in gene expression depending on the mode of ER activation.

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.

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].

Sex steroids and bone

The adult skeleton is periodically remodeled by temporary anatomic structures that comprise juxtaposed osteoclast and osteoblast teams and replace old bone with new. Estrogens and androgens slow the rate of bone remodeling and protect against bone loss. Conversely, loss of estrogen leads to increased rate of remodeling and tilts the balance between bone resorption and formation in favor of the former. Studies from our group during the last 10 years have elucidated that estrogens and androgens decrease the number of remodeling cycles by attenuating the birth rate of osteoclasts and osteoblasts from their respective progenitors. These effects result, in part, from the transcriptional regulation of genes responsible for osteoclastogenesis and mesenchymal cell replication and/or differentiation and are exerted through interactions of the ligand-activated receptors with other transcription factors. However, increased remodeling alone cannot explain why loss of sex steroids tilts the balance of resorption and formation in favor of the former. Estrogens and androgens also exert effects on the lifespan of mature bone cells: pro-apoptotic effects on osteoclasts but anti-apoptotic effects on osteoblasts and osteocytes. These latter effects stem from a heretofore unexpected function of the classical "nuclear" sex steroid receptors outside the nucleus and result from activation of a Src/Shc/extracellular signal-regulated kinase signal transduction pathway probably within preassembled scaffolds called caveolae. Strikingly, estrogen receptor (ER) alpha or beta or the androgen receptor can transmit anti-apoptotic signals with similar efficiency, irrespective of whether the ligand is an estrogen or an androgen. More importantly, these nongenotropic, sex-nonspecific actions are mediated by the ligand-binding domain of the receptor and can be functionally dissociated from transcriptional activity with synthetic ligands. Taken together, these lines of evidence strongly suggest that, in sex steroid deficiency, loss of transcriptional effects may be responsible for the increased osteoclastogenesis and osteoblastogenesis and thereby the increased rate of bone remodeling. Loss of nongenotropic anti-apoptotic effects on mature osteoblasts and osteocytes, in combination with an opposite effect on the lifespan of mature osteoclasts, may be responsible for the imbalance between formation and resorption and the progressive loss of bone mass and strength. Elucidation of the dual function of sex steroid receptors has important pathophysiologic and pharmacologic implications. Specifically, synthetic ligands of the ER that can evoke the nongenotropic but not the genotropic signal may be bone anabolic agents, as opposed to natural estrogens or selective estrogen receptor modulators that are antiresorptive agents. The same ligands may also circumvent the side effects associated with conventional hormone replacement therapy.

Cbfa1 does not regulate RANKL gene activity in stromal/osteoblastic cells

The rates of osteoblast and osteoclast formation are tightly balanced, possibly due to the requirement of mesenchymal osteoblast progenitors for osteoclastogenesis. Osteoblast differentiation requires the transcription factor Cbfa1, whereas osteoclastogenesis results from the interaction between receptor activator of NF kappa B ligand (RANKL), expressed on stromal/osteoblastic cells, and RANK, a surface receptor on hematopoietic precursors. A striking decrease in the number of osteoclasts in Cbfa1-deficient mice suggested that Cbfa1 might be involved in RANKL expression. To investigate this possibility and to elucidate the mechanisms regulating RANKL expression, we isolated the 5'-flanking region of the murine RANKL gene and found that it contains two potential binding sites for Cbfa1 (OSE2-like sites). Cbfa1 bound to either of these sites in gel shift assays and stimulated the activity of a chimeric promoter consisting of multimerized RANKL OSE2-like sites inserted upstream from a minimal thymidine kinase (tk) promoter in transient transfections. However, Cbfa1 cotransfection did not stimulate murine RANKL promoter-luciferase constructs. Further analysis revealed that removal of these sites from the RANKL promoter by either site-directed mutagenesis or 5'-deletion did not alter the basal activity of promoter-reporter constructs. Conditional expression of Cbfa1 in a stromal/osteoblastic cell line stimulated osteocalcin mRNA by fivefold, but had no significant effect on RANKL mRNA levels. Conversely, conditional expression of a dominant-negative form of Cbfa1 in the same cell line inhibited osteocalcin mRNA by threefold, but had no effect on RANKL mRNA. Although these results cannot rule out a novel function for Cbfa1 in RANKL expression, they demonstrate that Cbfa1 does not regulate RANKL gene activity in the same manner as known targets of this transcription factor, such as osteocalcin.

Inhibin suppresses and activin stimulates osteoblastogenesis and osteoclastogenesis in murine bone marrow cultures

Using primary murine bone marrow cell cultures, we demonstrate that inhibin suppresses osteoblastogenesis and osteoclastogenesis. In contrast, activin supports osteoblast formation (by alkaline phosphatase-positive and mineralized colony formation); and activin also stimulates osteoclast formation (as measured by staining tartrate-resistant acid phosphatase-positive multinucleated cells). Inhibin, the activin antagonist follistatin, and the bone morphogenetic protein antagonist noggin can all suppress endogenous activin accumulation in bone marrow cultures. Associated with this decrease in activin is the loss of mineralized osteoblastic colony formation (colony forming unit-osteoblast; CFU-OB). However, exogenous activin administration, even in the presence of noggin, permits both alkaline phosphatase-positive and CFU-OB colony formation in vitro. In contrast, the stimulatory effects of locally produced activin on osteoblast and osteoclast development are not likely to be dominant over the suppressive effects of gonadally derived inhibin. The suppressive effect of inhibin is maintained in the presence of either activin or bone morphogenetic protein, suggesting the presence of a distinct inhibin-specific receptor. Taken together, the direct regulation of osteoblastogenesis and osteoclastogenesis by inhibin and activin in vitro suggest that changes in the inhibin/activin ratio detected by bone marrow cells, during the perimenopausal transition, contribute to altered cell differentiation and may be associated with the increased bone resorption observed at this time.

Breast cancer increases osteoclastogenesis by secreting M-CSF and upregulating RANKL in stromal cells

BACKGROUND

Breast cancer metastasis to bone causes resorption of the mineralized matrix by osteoclasts. Macrophage colony stimulating factor (M-CSF)and receptor activator of the NF-kappaB ligand (RANKL) are produced by stromal cells and are essential for osteoclast formation. The human breast cancer cell line, MDA-MB-231, reliably forms bone metastases in a murine model and stimulates osteoclast formation in culture. We hypothesized that MDA-MB-231 stimulates osteoclast formation through secretion of M-CSF and/or RANKL.

MATERIALS AND METHODS

We cocultured MDA-MB-231 and a bone marrow derived cell line, UAMS-33, and evaluated the expression of M-CSF and RANKL mRNA. Osteoclast formation was assessed using these cells added to hematopoietic cell cultures.

RESULTS

MDA-MB-231 exhibited constitutive expression of M-CSF mRNA. As expected, addition of recombinant M-CSF (30 ng/ml) and RANKL (30 ng/ml) to hematopoietic osteoclast precursors supported osteoclast formation, while the addition of soluble RANKL alone or MDA-231 without added RANKL did not. Notably, coculture of MDA-231 with hematopoietic cells and added soluble RANKL stimulated significant osteoclast formation, indicating that MDA-231 served as an effective source for M-CSF. MDA-231 did not express RANKL. However, when cocultured with the murine bone marrow stromal cell line UAMS-33, RANKL expression was significantly increased in the latter cells. MDA-231 also stimulated osteoclast formation in coculture with UAMS-33 and hematopoietic cells.

CONCLUSIONS

We conclude that MDA-MB-231 increases osteoclast formation by secreting adequate amounts of M-CSF protein and enhancing the expression of RANKL by stromal support cells. The ability to stimulate osteoclasts may explain the ability to metastasize to bone.

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.

Attenuation of the self-renewal of transit-amplifying osteoblast progenitors in the murine bone marrow by 17 beta-estradiol

In agreement with evidence that estrogens slow the rate of bone remodeling by suppressing the production of both osteoclasts and osteoblasts, loss of estrogens leads to an increase in the number of osteoclast as well as early osteoblast progenitors (CFU-osteoblasts; CFU-OBs) in the murine bone marrow. Here we show that CFU-OBs are early transit-amplifying progenitors, i.e., dividing cells capable of limited self-renewal, and that 17 beta-estradiol acts in vivo and in vitro to attenuate their self-renewal by approximately 50%. Consistent with a direct receptor-mediated action of estrogens on early mesenchymal cell progenitors, anti-estrogen receptor-alpha (anti-ER alpha) Ab's stain a small number of marrow cells that exhibit characteristics of primitive undifferentiated cells, including a high nucleus/cytoplasm ratio and lack of lineage-specific biochemical markers; the effect of 17 beta-estradiol on CFU-OB self-renewal is absent in mice lacking ER alpha. Because both osteoblasts and the stromal/osteoblastic cells that are required for osteoclast development are derived from CFU-OBs, suppression of the self-renewal of this common progenitor may represent a key mechanism of the anti-remodeling effects of estrogens.

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.

Apoptosis of osteocytes in glucocorticoid-induced osteonecrosis of the hip

An increase in osteoblast and osteocyte apoptosis has been demonstrated in mice and humans receiving glucocorticoids and may be involved in the pathogenesis of the associated osteonecrosis. To examine the spatial relationship between osteocyte apoptosis and glucocorticoid-induced osteonecrosis, we determined the prevalence of osteocyte apoptosis in whole femoral heads obtained from patients who underwent prosthetic hip replacement because of osteonecrosis due to chronic glucocorticoid treatment (n = 5), alcoholism (n = 3), and trauma (n = 1) as well as in femoral neck cores from patients with sickle cell disease (n = 5). Abundant apoptotic osteocytes and cells lining cancellous bone were found juxtaposed to the subchondral fracture crescent in femurs from the patients with glucocorticoid excess. In contrast, apoptotic bone cells were absent from the specimens taken from patients with trauma or sickle cell disease and were rare with alcohol abuse. These results indicate that glucocorticoid-induced osteonecrosis is a misnomer. The bone is not necrotic; instead, it shows prominent apoptosis of cancellous lining cells and osteocytes. Glucocorticoid-induced osteocyte apoptosis, a cumulative and irreparable defect, could uniquely disrupt the mechanosensory function of the osteocyte network and thus start the inexorable sequence of events leading to collapse of the femoral head.

Birth and death of bone cells: basic regulatory mechanisms and implications for the pathogenesis and treatment of osteoporosis

The adult skeleton regenerates by temporary cellular structures that comprise teams of juxtaposed osteoclasts and osteoblasts and replace periodically old bone with new. A considerable body of evidence accumulated during the last decade has shown that the rate of genesis of these two highly specialized cell types, as well as the prevalence of their apoptosis, is essential for the maintenance of bone homeostasis; and that common metabolic bone disorders such as osteoporosis result largely from a derangement in the birth or death of these cells. The purpose of this article is 3-fold: 1) to review the role and the molecular mechanism of action of regulatory molecules, such as cytokines and hormones, in osteoclast and osteoblast birth and apoptosis; 2) to review the evidence for the contribution of changes in bone cell birth or death to the pathogenesis of the most common forms of osteoporosis; and 3) to highlight the implications of bone cell birth and death for a better understanding of the mechanism of action and efficacy of present and future pharmacotherapeutic agents for osteoporosis.

Birth and death of bone cells: basic regulatory mechanisms and implications for the pathogenesis and treatment of osteoporosis

The adult skeleton regenerates by temporary cellular structures that comprise teams of juxtaposed osteoclasts and osteoblasts and replace periodically old bone with new. A considerable body of evidence accumulated during the last decade has shown that the rate of genesis of these two highly specialized cell types, as well as the prevalence of their apoptosis, is essential for the maintenance of bone homeostasis; and that common metabolic bone disorders such as osteoporosis result largely from a derangement in the birth or death of these cells. The purpose of this article is 3-fold: 1) to review the role and the molecular mechanism of action of regulatory molecules, such as cytokines and hormones, in osteoclast and osteoblast birth and apoptosis; 2) to review the evidence for the contribution of changes in bone cell birth or death to the pathogenesis of the most common forms of osteoporosis; and 3) to highlight the implications of bone cell birth and death for a better understanding of the mechanism of action and efficacy of present and future pharmacotherapeutic agents for osteoporosis.

Chromosomal mapping of osteopenia-associated quantitative trait loci using closely related mouse strains

Peak bone mineral density (BMD) is a highly heritable trait in humans and is currently the best predictor of skeletal fragility underlying osteoporosis. The SAMP6 mouse strain displays unusually low BMD at maturity, and age-dependent osteopenia associated with defective osteoblastogenesis. To identify quantitative trait loci (QTLs) influencing bone density, we constructed crosses between SAMP6 and either AKR/J or SAMP6, two related mouse strains of higher peak BMD. Due to common ancestry of these strains, intercross parents differed at only 39-40% of 227 highly-polymorphic genotyping markers, thus restricting our search to this informative portion of the genome and reducing the number of mice required for QTL significance. Using dual energy X-ray absorptiometry (DEXA), we measured spinal BMD in F2 cross progeny at 4 months of age, and selectively genotyped those in the highest and lowest quartiles for BMD. Based on linear regression of bone density on genotype, including Composite Interval Mapping to enhance mapping precision while adjusting for effects of distal markers, we identified multiple QTLs significantly affecting spinal BMD; these were mapped to regions of chromosomes 2 (two sites, one confirmed in both crosses), 7, 11, 13 and 16. One of these loci had been previously identified as a significant bone-density QTL, while 3 substantiate QTLs suggested by a low-power study of 24 recombinant-inbred mouse lines. Such recurrent appearance of QTLs, especially in crosses involving distantly-related strains, implies that polymorphism at these loci may be favored by evolution and might underlie variation in peak bone density among humans.

Essential requirement of BMPs-2/4 for both osteoblast and osteoclast formation in murine bone marrow cultures from adult mice: antagonism by noggin

Bone morphogenetic proteins (BMPs) have been heretofore implicated in the induction of osteoblast differentiation from uncommitted progenitors during embryonic skeletogenesis and fracture healing. We have tested the hypothesis that BMPs are also involved in the osteoblastogenesis that takes place in the bone marrow in postnatal life. To do this, we took advantage of the properties of noggin, a recently discovered protein that binds BMP-2 and -4 and blocks their action. Addition of human recombinant noggin to bone marrow cell cultures from normal adult mice inhibited both osteoblast and osteoclast formation; these effects were reversed by exogenous BMP-2. Consistent with these findings, BMP-2 and -4 and BMP-2/4 receptor transcripts and proteins were detected in these primary cultures, in a bone marrow-derived stromal/osteoblastic cell line, as well as in murine adult whole bone; noggin expression was also documented in all these preparations. Moreover, addition of antinoggin antibody caused an increase in osteoblast progenitor formation. These findings suggest that BMP-2 and -4 are expressed in the bone marrow in postnatal life and serve to maintain the continuous supply of osteoblasts and osteoclasts; and that, in fact, BMP-2/4-induced commitment to the osteoblastic lineage is a prerequisite for osteoclast development. Hence, BMPs, perhaps in balance with noggin and possibly other antagonists, may provide the tonic baseline control of the rate of bone remodeling on which other inputs (e.g., hormonal, biomechanical, etc.) operate.

Apoptosis and osteoporosis

During normal bone remodeling, the rate of supply of new osteoblasts and osteoclasts and the timing of the death of osteoclasts, osteoblasts, and osteocytes by apoptosis are critical determinants of the initiation of new BMUs and the extension or reduction of the lifetime of existing ones. Disruption of the fine balance among these processes may be an important mechanism behind the deranged bone turnover found in most metabolic disorders of the adult skeleton. Like most armies, the amount 5 of work done by bone cells is far more dependent on numbers than vigor. Therapeutic agents that alter the prevalence of apoptosis of osteoblasts and osteoclasts can correct the imbalance in cell numbers that is the basis of the diminished bone mass and increased risk of fractures in osteoporosis.

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.

A role for interleukin-6 in parathyroid hormone-induced bone resorption in vivo

Parathyroid hormone (PTH) exerts its regulatory effects on calcium homeostasis in part by stimulating the release of calcium from the skeleton. PTH stimulates bone resorption indirectly, by inducing the production by stromal/osteoblastic cells of paracrine agents which recruit and activate the bone-resorbing cell, the osteoclast. The identity of the stromal cell/osteoblast-derived paracrine factor(s) responsible for mediating the effects of PTH on osteoclasts is uncertain. Recently, it has been demonstrated that the cytokine interleukin-6 (IL-6), which potently induces osteoclastogenesis, is produced by osteoblastic cells in response to PTH. Further, we have reported that circulating levels of IL-6 are elevated in patients with primary hyperparathyroidism, and correlate with biochemical markers of bone resorption. Thus, IL-6 may play a permissive role in PTH-induced bone resorption. In the current studies, we demonstrate that low-dose PTH infusion in rodents increased serum levels of IL-6, coincident with a rise in biochemical markers of bone resorption. In mice, both acute neutralization and chronic deficiency of IL-6 were associated with markedly lower levels of biochemical markers of bone resorption in response to PTH infusion than were observed in animals with normal IL-6 production. Acute neutralization of IL-6 did not affect PTH-induced changes in markers of bone formation. These findings demonstrate that PTH regulates systemic levels of IL-6 in experimental animals, that IL-6 is an important mediator of the bone-resorbing actions of PTH in vivo and suggest that IL-6 plays a role in coupling PTH-induced bone resorption and formation.

Inhibition of Osf2/Cbfa1 expression and terminal osteoblast differentiation by PPARgamma2

Cells of the bone marrow stroma can reversibly convert among different phenotypes. Based on this and on evidence for a reciprocal relationship between osteoblastogenesis and adipogenesis, we have isolated several murine bone marrow-derived clonal cell lines with phenotypic characteristics of osteoblasts or adipocytes, or both. Consistent with a state of plasticity, cell lines with a mixed phenotype synthesized osteoblast markers like type I collagen, alkaline phosphatase, osteocalcin, as well as the adipocyte marker lipoprotein lipase, under basal conditions. In the presence of ascorbic acid and beta-glycerophosphate-agents that promote osteoblast differentiation-they formed a mineralized matrix. In the presence of isobutylmethylxanthine, hydrocortisone, and indomethacin-agents that promote adipocyte differentiation-they accumulated fat droplets, but failed to express adipsin and aP2, markers of terminally differentiated adipocytes. Furthermore, they were converted back to matrix mineralizing cells when the adipogenic stimuli were replaced with the osteoblastogenic ones. A prototypic cell line with mixed phenotype (UAMS-33) expressed Osf2/Cbfa1-a transcription factor required for osteoblast differentiation, but not PPARgamma2-a transcription factor required for terminal adipocyte differentiation. Stable transfection with a PPARgamma2 expression construct and activation with the thiazolidinedione BRL49653 stimulated aP2 and adipsin synthesis and fat accumulation, and simultaneously suppressed Osf2/Cbfa1, alpha1(I) procollagen, and osteocalcin synthesis. Moreover, it rendered the cells incapable of forming a mineralized matrix. These results strongly suggest that PPARgamma2 negatively regulates stromal cell plasticity by suppressing Osf2/Cbfa1 and osteoblast-like biosynthetic activity, while promoting terminal differentiation to adipocytes.

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.

STAT3 activation in stromal/osteoblastic cells is required for induction of the receptor activator of NF-kappaB ligand and stimulation of osteoclastogenesis by gp130-utilizing cytokines or interleukin-1 but not 1,25-dihydroxyvitamin D3 or parathyroid hormone

Interleukin (IL)-6-type cytokines stimulate osteoclastogenesis by activating gp130 in stromal/osteoblastic cells and may mediate some of the osteoclastogenic effects of other cytokines and hormones. To determine whether STAT3 is a downstream effector of gp130 in the osteoclast support function of stromal/osteoblastic cells and whether the gp130/STAT3 pathway is utilized by other osteoclastogenic agents, we conditionally expressed dominant negative (dn)-STAT3 or dn-gp130 in a stromal/osteoblastic cell line (UAMS-32) that supports osteoclast formation. Expression of either dominant negative protein abolished osteoclast formation stimulated by IL-6 + soluble IL-6 receptor, oncostatin M, or IL-1 but not by parathyroid hormone or 1,25-dihydroxyvitamin D3. Because previous studies suggested that IL-6-type cytokines may stimulate osteoclastogenesis by inducing expression of the tumor necrosis factor-related protein, receptor activator of NF-kappaB ligand (RANKL), we conditionally expressed RANKL in UAMS-32 cells and found that this was sufficient to stimulate osteoclastogenesis. Moreover, dn-STAT3 blocked the ability of either IL-6 + soluble IL-6 receptor or oncostatin M to induce RANKL. These results establish that STAT3 is essential for gp130-mediated osteoclast formation and that the target of STAT3 during this process is induction of RANKL. In addition, this study demonstrates that activation of the gp130-STAT3 pathway in stromal/osteoblastic cells mediates the osteoclastogenic effects of IL-1, but not parathyroid hormone or 1, 25-dihydroxyvitamin D3.

Meltrin-alpha, a fusion protein involved in multinucleated giant cell and osteoclast formation

The formation of multinucleated cells such as myotubes, macrophage-derived giant cells (MGC), and osteoclasts is the result of cell-cell fusion of mononuclear precursors. Meltrin-alpha, -beta, and -gamma are members of a recently discovered family of proteins that contain disintegrin and metalloprotease domains and are related to fertilin, a protein involved in egg-sperm fusion. Based on this and evidence implicating meltrin-alpha in myoblast formation, we have investigated the possibility that meltrins may also play a role in the formation of MGC and osteoclasts. Using in situ RT-PCR, we have determined that murine mononuclear alveolar macrophages cultured under basal conditions express the transcript for meltrin-beta, but not for meltrin-alpha. However, meltrin-alpha mRNA appeared in mononuclear cells before cell fusion after treatment with 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], a potent inducer of giant cell and osteoclast formation. Moreover, addition of meltrin-alpha antisense oligonucleotides to the cultures caused a 50% inhibition of giant cell formation. Similarly, meltrin-alpha antisense oligonucleotides inhibited by 70% the formation of multinucleated osteoclast-like cells expressing tartrate-resistant acid phosphatase (TRAP) in co-cultures of bone marrow cells and osteoblastic cells (2107) in the presence of 1,25(OH)2D3. Mononucleated TRAP-positive cells, induced by 1,25(OH)2D3 in the co-cultures, also expressed meltrin-alpha mRNA, but their number was not changed in the presence of meltrin-alpha antisense oligonucleotide. In contrast to mononuclear macrophages and osteoclast-like cells, murine bone marrow stroma and calvaria derived-cell lines (+/+ LDA.11 and 2107), primary cultures of calvaria cells, and primary cultures of bone marrow cells expressed both meltrin-alpha and -beta mRNA under basal conditions; whereas embryonic fibroblasts (NIH3T3) expressed only the meltrin-beta transcript. Upregulation of meltrin-alpha protein expression during cell fusion in alveolar macrophages and expression in osteoblastic cell lines were confirmed by Western blot analysis. These observations demonstrate that meltrins play a role in MGC and osteoclast formation from mononuclear precursors, as in the case with myotubes.

Biologic and therapeutic determinants of bone mineral density in multiple myeloma

The net impact of malignancy and anti-tumor therapy on bone resorption in myeloma is poorly understood because conventional skeletal radiographs are relatively insensitive for the diagnosis and monitoring of bone disease. We performed determinations of bone mineral density (BMD) at the lumbar spine, femoral neck and radial diaphysis by dual energy X ray absorptiometry (DEXA) in 168 consecutive patients with myeloma seen at our institution. Follow up studies were performed in 41 of these patients. A detailed analysis of patient and disease characteristics was performed to identify the determinants of BMD. Compared to normal age and sex matched controls, mean (+/- SE) BMD was significantly decreased at the lumbar spine (Z score -0.4 +/- 0.10) and femoral neck (Z score -1.0 +/- 0.10), but was surprisingly above normal at the radial diaphysis (Z score +0.35 +/- 0.10), a cortical bone site devoid of hematopoietic marrow, suggesting a differential bone preserving effect at this site. Lack of correlation between the BMD findings and the presence or extent of radiographically evident osteolytic lesions suggested the presence of a systemic bone disease. On multivariate analysis, duration of disease >12 months (p = 0.003) and female sex (p = 0.01) were independently associated with a lower BMD at the femoral neck/lumbar spine. On follow up DEXA (n = 41), BMD increased at > or = 1 site in 9 of 20 patients receiving bisphosphonates and in only 2 of 21 patients not receiving such therapy (p = 0.02). Similarly a decline in BMD at > or = 1 site was seen in 9 of 21 patients not receiving bisphosphonates, irrespective of the disease response status. Interval pamidronate therapy (p = 0.0007) and a low serum beta-2-microglobulin (< 2.5 mg/l) (p = 0.04) were the two most significant variables associated with an increase in BMD on multivariate analysis. These data suggest that myeloma is associated with a systemic bone disease with progressive generalized cancellous bone loss and a bone preserving effect on the radial cortical bone. The early use of bisphosphonates may improve myeloma related bone disease.

Interleukin-6-type cytokines stimulate mesenchymal progenitor differentiation toward the osteoblastic lineage

Cytokines that transduce their signals either through glycoprotein 130 (gp130) homodimers or gp 130/leukemia inhibitory factor (LIF) receptor beta heterodimers are potent inducers of osteoclast development in vitro as well as in vivo; and interleukin (IL)-6 has been recognized as an important pathogenic factor in diseases characterized by increased bone remodeling, such as the osteoporosis of sex steroid deficiency. Based on evidence that the same cytokines can also promote committed osteoblast differentiation and stimulate bone formation in vitro and in vivo and that mesenchymal cell differentiation toward the osteoblast lineage may be a prerequisite for osteoclastogenesis, we have investigated whether gp130 activation can affect the differentiation of uncommitted mesenchymal progenitors. Using as our model murine embryonic fibroblasts (EF), we found that IL-6 or IL-11 in combination with their soluble receptors (sIL-6R or sIL-11R) increased dose-dependently the number of alkaline phosphatase (AP)-positive cells in 3-6-day-long cultures. Moreover, EF cells maintained with IL-6/sIL-6R in the presence of ascorbic acid and beta-glycerophosphate expressed osteocalcin messenger RNA (mRNA) by 2 weeks and formed a matrix containing mineralized collagen fibers by 3 weeks. This prodifferentiation effect was specific for the osteoblastic lineage, as we found no evidence for increased differentiation of chondrocytes, adipocytes, or muscle cells. Unlike IL-6/sIL-6R, LIF, oncostatin M (OSM), and ciliary neurotrophic factor (CNTF) did not promote osteoblastic differentiation of EF cells. This pattern of specificity was accounted for by the finding that EF cells express gp130, but not the ligand-binding subunit of the IL-6 receptor (gp80) nor the LIF receptor beta. These observations add credence to the contention that increased production of gp130-utilizing cytokines and their receptors in pathological conditions like sex steroid deficiency is indeed responsible for not only the increased osteoclastogenesis, but also the increased osteoblastogenesis, and thereby for the increased rate of bone remodeling.

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.

Inhibition of osteoblastogenesis and promotion of apoptosis of osteoblasts and osteocytes by glucocorticoids. Potential mechanisms of their deleterious effects on bone

Glucocorticoid-induced bone disease is characterized by decreased bone formation and in situ death of isolated segments of bone (osteonecrosis) suggesting that glucocorticoid excess, the third most common cause of osteoporosis, may affect the birth or death rate of bone cells, thus reducing their numbers. To test this hypothesis, we administered prednisolone to 7-mo-old mice for 27 d and found decreased bone density, serum osteocalcin, and cancellous bone area along with trabecular narrowing. These changes were accompanied by diminished bone formation and turnover, as determined by histomorphometric analysis of tetracycline-labeled vertebrae, and impaired osteoblastogenesis and osteoclastogenesis, as determined by ex vivo bone marrow cell cultures. In addition, the mice exhibited a threefold increase in osteoblast apoptosis in vertebrae and showed apoptosis in 28% of the osteocytes in metaphyseal cortical bone. As in mice, an increase in osteoblast and osteocyte apoptosis was documented in patients with glucocorticoid-induced osteoporosis. Decreased production of osteoclasts explains the reduction in bone turnover, whereas decreased production and apoptosis of osteoblasts would account for the decline in bone formation and trabecular width. Furthermore, accumulation of apoptotic osteocytes may contribute to osteonecrosis. These findings provide evidence that glucocorticoid-induced bone disease arises from changes in the numbers of bone cells.

Cellular and molecular mechanisms of osteoporosis

Osteoblasts and osteoclasts are derived from progenitors originating in the bone marrow, and the process of bone remodeling is controlled by growth factors and cytokines which regulate the birth and death of these cells. An overproduction of osteoclasts relative to the need for remodeling, and an undersupply of osteoblasts relative to the need for cavity repair, represent the fundamental pathophysiologic changes in postmenopausal and age-related osteopenia, respectively. As in these two forms of the disease, the osteoporosis induced by glucocorticoid excess is also caused by changes in the birth and death of bone cells, and in particular a decrease in osteoblastogenesis in the bone marrow, and an increased rate of osteoblast and osteocyte apoptosis.

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.

The role of IL-6 type cytokines and their receptors in bone

Substantial evidence indicates that the IL-6 type of cytokines have profound effects on bone metabolism by regulating osteoclast and osteoblast development and function. In addition, there is evidence that the gp130 signal transduction pathway may be a critical site for the regulation of the rate of bone remodeling, and probably the coupling of bone resorption to bone formation. Sex steroids inhibit the expression of the genes encoding IL-6, gp80, and gp130, most likely by repressing the activity of transcription factors such as NF kappa B and NF-IL-6. Considering this and the evidence that IL-6 autoregulates its own production and can upregulate the components of its receptor, removal of the direct inhibitory effects of sex steroids on IL-6, gp80, and gp130 could unleash a self-amplifying cascade of events responsible for increasing not only the production of IL-6, but also the responsiveness of osteoclast progenitors, osteoblast progenitors, and stromal/osteoblastic cells that support osteoclastogenesis, or combinations of these cells, to IL-6 type cytokines. Such a scenario could explain both the increased osteoclastogenesis and osteoblastogenesis that follows loss of gonadal function and thereby the effect of such loss on the rate of bone remodeling and skeletal homeostasis. Manipulation of the effects of IL-6 type cytokines, by selectively targeting to specific bone cell precursors, may allow means of altering the balance between bone resorption and formation in favor of the latter.

Loss of estrogen upregulates osteoblastogenesis in the murine bone marrow. Evidence for autonomy from factors released during bone resorption

Loss of sex steroids causes an increase in both the resorption and formation of bone, with the former exceeding the latter. Based on evidence that the increased bone resorption after estrogen loss is due to an increase in osteoclastogenesis, we hypothesized that estrogen loss also stimulates osteoblastogenesis. We report that the number of mesenchymal osteoblast progenitors in the murine bone marrow was increased two- to threefold between 2 and 8 wk after ovariectomy and returned to control levels by 16 wk. Circulating osteocalcin, as well as osteoclastogenesis and the rate of bone loss, followed a very similar temporal pattern. Inhibition of bone resorption by administration of the bisphosphonate alendronate led to a decrease of the absolute number of osteoblast progenitors; however, it did not influence the stimulating effect of ovariectomy on osteoblastogenesis or osteoclastogenesis. These observations indicate that the increased bone formation that follows loss of estrogen can be explained, at least in part, by an increase in osteoblastogenesis. Moreover, they strongly suggest that unlike normal bone remodeling, whereby osteoblast development is stimulated by factors released from the bone matrix during osteoclastic resorption, estrogen deficiency unleashes signals that can stimulate the differentiation of osteoblast progenitors in a fashion that is autonomous from the need created by bone resorption, and therefore, inappropriate.

The pathophysiologic roles of interleukin-6 in human disease

Interleukin-6, an inflammatory cytokine, is characterized by pleiotropy and redundancy of action. Apart from its hematologic, immune, and hepatic effects, it has many endocrine and metabolic actions. Specifically, it is a potent stimulator of the hypothalamic-pituitary-adrenal axis and is under the tonic negative control of glucocorticoids. It acutely stimulates the secretion of growth hormone, inhibits thyroid-stimulating hormone secretion, and decreases serum lipid concentrations. Furthermore, it is secreted during stress and is positively controlled by catecholamines. Administration of interleukin-6 results in fever, anorexia, and fatigue. Elevated levels of circulating interleukin-6 have been seen in the steroid withdrawal syndrome and in the severe inflammatory, infectious, and traumatic states potentially associated with the inappropriate secretion of vasopressin. Levels of circulating interleukin-6 are also elevated in several inflammatory diseases, such as rheumatoid arthritis. Interleukin-6 is negatively controlled by estrogens and androgens, and it plays a central role in the pathogenesis of the osteoporosis seen in conditions characterized by increased bone resorption, such as sex-steroid deficiency and hyperparathyroidism. Overproduction of interleukin-6 may contribute to illness during aging and chronic stress. Finally, administration of recombinant human interleukin-6 may serve as a stimulation test for the integrity of the hypothalamic-pituitary-adrenal axis.

Increased adipogenesis and myelopoiesis in the bone marrow of SAMP6, a murine model of defective osteoblastogenesis and low turnover osteopenia

Bone formation and hematopoiesis are anatomically juxtaposed and share common regulatory mechanisms. However, little is known about the interrelationship between these two processes. We have previously shown that the senescence accelerated mouse-P6 (SAMP6) exhibits decreased osteoblastogenesis in the bone marrow that is temporally linked with a low rate of bone formation and decreased bone mineral density. Here we report that in contrast to decreased osteoblastogenesis, ex vivo bone marrow cultures from SAMP6 mice exhibited an increase in the number of colony-forming unit adipocytes, as well as an increase in the number of fully differentiated marrow adipocytes, compared with SAMR1 (nonosteopenic) controls. Further, long-term bone marrow cultures from SAMP6 produced an adherent stromal layer more rapidly, generated significantly more myeloid progenitors and produced more IL-6 and colony-stimulating activity. Consistent with this, the number of myeloid cells in freshly isolated marrow from SAMP6 mice was increased, as was the number of granulocytes in peripheral blood. The evidence that SAMP6 mice exhibit decreased osteoblastogenesis, and increased adipogenesis and myelopoiesis, strongly suggests that a switch in the differentiation program of multipotential mesenchymal progenitors may underlie the abnormal phenotype manifested in the skeleton and other tissues of these animals. Moreover, these observations support the contention for the existence of a reciprocal relationship between osteoblastogenesis and adipogenesis that may explain the association of decreased bone formation and the resulting osteopenia with the increased adiposity of the marrow seen with advancing age in animals and humans.

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.

The effects of androgen deficiency on murine bone remodeling and bone mineral density are mediated via cells of the osteoblastic lineage

Both estrogens and androgens act on bone marrow stromal/osteoblastic cells to inhibit the production of local factors that promote osteoclast development. Based on this and the evidence that loss of sex steroids up-regulates not only osteoclastogenesis but also osteoblastogenesis, we have hypothesized that cells of the osteoblastic lineage are the mediators of the adverse effects of sex steroid deficiency on bone. To test this hypothesis, we used the senescence-accelerated mouse (SAMP6), a model of defective osteoblast development, and examined the effects of orchidectomy on static and dynamic histological features of bone remodeling and on bone mineral density. After orchidectomy in SAMP6 mice, the expected increases in osteoblast precursors, cancellous osteoclasts and osteoblasts, frequency of remodeling events, trabecular spacing, and rate of bone formation were absent or greatly attenuated. Moreover, whereas bone mineral density decreased in orchidectomized controls, it did not change in SAMP6. Our data indicate that when osteoblast development is defective, orchidectomy fails to result in bone loss. This evidence suggests that cells of the osteoblastic lineage are essential mediators of the changes in the rate of bone remodeling and loss of bone mass that ensue following loss of androgens.

Osteopontin expression by osteoclast and osteoblast progenitors in the murine bone marrow: demonstration of its requirement for osteoclastogenesis and its increase after ovariectomy

Osteoclast development requires cell-to-cell contact between hematopoietic osteoclast progenitors and bone marrow stromal/osteoblastic support cells. Based on this, we hypothesized that osteopontin, an adhesion protein produced by osteoclasts and osteoblasts, plays a role in osteoclastogenesis. Using in situ hybridization, we demonstrate that cells expressing the osteopontin messenger RNA (mRNA) appear after 3 days of culturing murine bone marrow cells. The number of these cells increases thereafter, reaching a peak on day 5. In the same cultures, cells expressing alkaline phosphatase (AP) or tartrate resistant acid phosphatase (TRAP), phenotypic markers for osteoblastic and osteoclast-like cells, respectively, appeared subsequent to the appearance of the osteopontin-positive cells. By means of a combination of in situ hybridization and histostaining, it was shown that the osteopontin mRNA was localized in 30-50% of the AP-positive or the TRAP-positive, as well as in nonspecific esterase (NSE)-positive, cells. The number of cells expressing both the osteopontin mRNA and either one of the three phenotypic markers was significantly increased in bone marrow cultures from estrogen-deficient mice, as compared with controls. Conversely, the number of all three populations of double positive cells was decreased in cultures treated with a specific antimouse rabbit osteopontin antibody or an RGD peptide. These findings indicate that osteopontin is expressed during the early stages of the differentiation of osteoclast and osteoblast progenitors in the bone marrow and that its cell adhesion properties are required for osteoclastogenesis.

Isolation and characterization of the human gp130 promoter. Regulation by STATS

Glycoprotein 130 (gp130), a shared component of all the receptors for the interleukin-6 cytokine family, transduces cytokine signals in part by activating latent cytoplasmic signal transducers and activators of transcription (STATs). STATs subsequently translocate into the nucleus and stimulate gene expression. In the studies reported here, the 5'-flanking region of the human gp130 gene was isolated and the transcription initiation sites were mapped. To demonstrate that the isolated DNA fragment contained a functional promoter, a plasmid construct containing 2433 base pairs of the gp130 5'-flanking region, inserted upstream from the firefly luciferase gene, was transiently transfected into HepG2 hepatoma cells. The construct exhibited constitutive promoter activity. In addition, a 5-h treatment with interleukin-6 or oncostatin M stimulated the activity of this promoter severalfold. Localization of the cytokine response element by 5'-deletion analysis and site-directed mutagenesis revealed a cis-acting binding site for activated STAT complexes. Furthermore, DNA binding analysis demonstrated that this element binds activated STAT1 and STAT3 homo- and heterodimers. This STAT-binding element was sufficient to confer cytokine stimulation to a minimal herpesvirus thymidine kinase promoter. These results establish that the DNA fragment we have isolated contains the human gp130 promoter and that interleukin-6 type cytokines may influence the activity of this promoter via activated STATs.

Quantification of vitamin D receptor mRNA by competitive polymerase chain reaction in PBMC: lack of correspondence with common allelic variants

It has been recently claimed that polymorphism for the vitamin D receptor (VDR) influences several aspects of calcium and bone metabolism. To evaluate the physiologic plausibility of these claims, we compared the abundance of the VDR mRNA in peripheral blood mononuclear cells (PBMCs) between different VDR genotypes using a quantitative reverse transcribed polymerase chain reaction-based method. The method is based on the coamplification of VDR cDNA and an internal standard consisting of known concentrations of a human VDR CDNA mutated at a BglII restriction site; the interassay coefficient of variation is 11%. To validate the method, we made use of earlier receptor binding studies indicating that normal human monocytes and activated, but not resting, lymphocytes expressed the VDR. The concentration of the VDR mRNA was 10(-8) to 10(-7) g/g of total RNA in cell-sorted monocytes and in in vitro activated lymphocytes, but only 10(-12) g/g of total mRNA in resting lymphocytes, establishing that the VDR mRNA determined by our method in PBMCs is due to constitutive expression in monocytes. Following an initial genotype screening of 85 normal volunteers by polymerase chain reaction or restriction fragment length polymorphism analysis, 14 individuals with the Bb genotype, 12 with the bb genotype, and 12 with the BB genotype were selected. The concentration of the VDR mRNA, corrected for the number of monocytes, was similar among the three genotype groups, as were the other variables examined: serum calcitriol, serum osteocalcin, and vertebral and hip bone density. We conclude that VDR polymorphism does not affect the abundance of the VDR mRNA.

Pathogenesis and management of bone lesions in multiple myeloma

Osteolytic lesions are among the hallmarks of multiple myeloma (MM). On the other hand, they are rare in B cell malignancies other than MM. Data have shown a close relationship between myeloma cell growth factors and factors involved in the occurrence of bone lesions in MM. Biophosphonates, which are potent inhibitors of bone resorption to control MM-induced hypercalcemia, are of interest due to their ability to slow the progression of MM bone disease.

Linkage of decreased bone mass with impaired osteoblastogenesis in a murine model of accelerated senescence

Bone marrow is the principal site for osteoclastogenesis and osteoblastogenesis; and an increase in the former has been linked with bone loss caused by acute loss of gonadal steroids. We have now used an established murine model of accelerated senescence and osteopenia (SAMP6) to test the hypothesis that reduced osteoblastogenesis is linked with decreased bone mass. At 1 mo of age, the number of osteoblast progenitors in SAMP6 marrow was indistinguishable from controls; however a threefold decrease was found at 3-4 mo of age. Impaired osteoblast formation was temporally associated with decreased bone formation and decreased bone mineral density, as determined by histomorphometric analysis of tetracycline-labeled cancellous bone and dual-energy x-ray absorptiometry, respectively. Osteoclastogenesis determined in ex vivo bone marrow cultures was also decreased in these mice, as was the number of osteoclasts in histologic sections. Moreover, unlike controls, senescence-accelerated mice failed to increase osteoclast development after gonadectomy. The osteoclastogenesis defeat was secondary to impaired osteoblast formation as evidenced by the fact that osteoclastogenesis could be restored by addition of osteoblastic cells from normal mice. These findings provide the first demonstration of a link between low bone mineral density and decreased osteoblastogenesis in the bone marrow and validate the senescence-accelerated mouse as a model of involutional osteopenia.

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.