Parathyroid hormone regulates osteoblast differentiation positively or negatively depending on the differentiation stages

Parathyroid hormone modifies human periodontal ligament cell proliferation and survival in vitro

BACKGROUND AND OBJECTIVE

Periodontal ligament (PDL) cells show traits that are typical of osteoblasts, such as osteoblastic marker gene expression and the ability to respond to parathyroid hormone (PTH) stimulation in an osteoblast-like manner with respect to differentiation and local factor production. In the present study, we hypothesized that human PDL cells might respond to PTH stimulation with changes in proliferation and cell survival and thereby provide another mechanism by which PTH might affect the reparative potential of PDL cells. We speculated that the maturation state of the cells and the mode of PTH(1-34) administration would have an impact on the cellular response.

MATERIAL AND METHODS

PDL cells were challenged with PTH(1-34) intermittently or continuously at different maturation states. Cell number, 5-bromo-2-deoxyuridine (BrdU) incorporation, DNA fragmentation, nitric oxide production and the duration of the PTH(1-34) effect were determined.

RESULTS

Intermittent PTH(1-34) treatment of preconfluent cells caused a significant increase in proliferation and DNA fragmentation, whereas in more mature cells, proliferation was less enhanced while apoptosis was more pronounced than in immature cells. Continuous PTH(1-34) exposure did not alter proliferation in any maturation state but increased DNA fragmentation in preconfluent cells. PTH(1-34) prevented etoposide-induced apoptosis after 6 h but no longer after 24 h. Nitric oxide production was unaffected.

CONCLUSION

These results indicate that human PDL cells respond to PTH(1-34) with changes in proliferative and apoptotic signaling in a maturation-state-dependent manner. Besides changes in local factor production, these findings provide a further possible mechanism to support the idea that PDL cells possess the potential to be involved in the regulation of dental hard tissue repair.

Influence of hormones on osteogenic differentiation processes of mesenchymal stem cells

Bone development, regeneration and maintenance are governed by osteogenic differentiation processes from mesenchymal stem cells through to mature bone cells, which are directed by local growth and differentiation factors and modulated strongly by hormones. Mesenchymal stem cells develop from both mesoderm and neural crest and can give rise to development, regeneration and maintenance of mesenchymal tissues, such as bone, cartilage, muscle, tendons and discs. There are only limited data regarding the effects of hormones on early events, such as regulation of stemness and maintenance of the mesenchymal stem cell pool. Hormones, such as estrogens, vitamin D-hormone and parathyroid hormone, besides others, are important modulators of osteogenic differentiation processes and bone formation, starting off with fate decision and the development of osteogenic offspring from mesenchymal stem cells, which end up in osteoblasts and osteocytes. Hormones are involved in fetal bone development and regeneration and, in childhood, adolescence and adulthood, they control adaptive needs for growth and reproduction, nutrition, physical power and crisis adaptation. As in other tissues, aging in mesenchymal stem cells and their osteogenic offspring is accompanied by the accumulation of genomic and proteomic damage caused by oxidative burden and insufficient repair. Failsafe programs, such as apoptosis and cellular senescence avoid tumorigenesis. Hormones can influence the pace of such events, thus supporting the quality of tissue regeneration in aging organisms in vivo; for example, by delaying osteoporosis development. The potential for hormones in systemic therapeutic strategies is well appreciated and some concepts are approved for clinical use already. Their potential for cell-based therapeutic strategies for tissue regeneration is probably underestimated and could enhance the quality of tissue-engineering constructs for transplantation and the concept of in situ-guided tissue regeneration.

Identification of a PTH regulated gene selectively induced in vivo during PTH-mediated bone formation

The biological activities of parathyroid hormone (PTH) on bone are quite complex as demonstrated by its catabolic and anabolic activities on the skeleton. Although there have been many reports describing genes that are regulated by PTH in osteoblast cells, the goal of this study was to utilize a well-established in vivo PTH anabolic treatment regimen to identify genes that mediate bone anabolic effects of PTH. We identified a gene we named PTH anabolic induced gene in bone (PAIGB) that has been reported as brain and acute leukemia cytoplasmic (BAALC). Therefore, using the latter nomenclature, we have discovered that BAALC is a PTH-regulated gene whose mRNA expression was selectively induced in rat tibiae nearly 100-fold (maximal) by a PTH 1-34 anabolic treatment regimen in a time-dependent manner. Although BAALC is broadly expressed, PTH did not regulate BAALC expression in other PTH receptor expressing tissues and we find that the regulation of BAALC protein by PTH in vivo is confined to mature osteoblasts. Further in vitro studies using rat UMR-106 osteoblastic cells show that PTH 1-34 rapidly induces BAALC mRNA expression maximally by 4 h while the protein was induced by 8 h. In addition to being regulated by PTH 1-34, BAALC expression can also be induced by other bone forming factors including PGE(2) and 1,25 dihydroxy vitamin D(3). We determined that BAALC is regulated by PTH predominantly through the cAMP/PKA pathway. Finally, we demonstrate in MC3T3-E1 osteoblastic cells that BAALC overexpression regulates markers of osteoblast differentiation, including downregulating alkaline phosphatase and osteocalcin expression while inducing osteopontin expression. We also demonstrate that these transcriptional responses mediated by BAALC are similar to the responses elicited by PTH 1-34. These data, showing BAALC overexpression can mimic the effect of PTH on markers of osteoblast differentiation, along with the observations that BAALC is induced selectively with a bone anabolic treatment regimen of PTH (not a catabolic treatment regimen), suggest that BAALC may be an important mediator of the PTH anabolic action on bone cell function.

Influence of the parathyroid glands on bone metabolism

Bone is a classic target tissue for parathyroid hormone (PTH), whose calciotropic effect is mediated largely via catabolic actions on this tissue. Paradoxically, PTH also exerts anabolic actions, with intermittent injections of PTH or its amino-terminal fragments causing an increase in bone formation and bone mass, actions that form the basis for the use of PTH in the treatment of osteoporosis. Besides vitamin D, PTH is the only other known bone anabolic agent. High-affinity PTH receptors (PTH-1R) have been detected on osteoblasts and osteoclasts (albeit in lower numbers). Bone turnover, which includes activation of osteoclasts and osteoblasts, appears to be best reflected not by absolute concentrations of PTH (which can vary based on the assay and antibody used) but by a balance of circulating full-length PTH-(1-84) and amino-terminally truncated C-PTH fragments. When PTH-(1-84) is predominant, bone turnover is promoted. Among PTH fragments, PTH-(7-84) appears to be the most potent antagonist of PTH-(1-84). The mechanisms involved in these effects are unclear although mediation via unique C-terminal receptors has been suggested. We propose that, within the range of total PTH (100-1000 pg mL(-1)), the ratio of PTH-(1-84)/C-PTH fragment is a valuable tool for diagnosis of bone turnover. Data indicate that at PTH levels < 100-150 pg mL(-1) and > 1000 pg mL(-1), the ratio looses its predictive power. Assay type, patient characteristics (race, underlying renal disease) and treatment attributes (vitamin D, corticosteroids, phosphate binders) have an impact on the PTH ratio, and care should be used in interpreting assay results and making subsequent treatment decisions.

HMGB1 expression and release by bone cells

Immune and bone cells are functionally coupled by pro-inflammatory cytokine intercellular signaling networks common to both tissues and their crosstalk may contribute to the etiologies of some immune-associated bone pathologies. For example, the receptor activator of NF-kappaB ligand (RANKL)/osteoprotegerin (OPG)/receptor activator of NF-kappaB (RANK) signaling axis plays a critical role in dendritic cell (DC) function as well as bone remodeling. The expression of RANKL by immune cells may contribute to bone loss in periodontitis, arthritis, and multiple myeloma. A recent discovery reveals that DCs release the chromatin protein high mobility group box 1 (HMGB1) as a potent immunomodulatory cytokine mediating the interaction between DCs and T-cells, via HMGB1 binding to the membrane receptor for advanced glycation end products (RAGE). To determine whether osteoblasts or osteoclasts express and/or release HMGB1 into the bone microenvironment, we analyzed tissue, cells, and culture media for the presence of this molecule. Our immunohistochemical and immunocytochemical analyses demonstrate HMGB1 expression in primary osteoblasts and osteoclasts and that both cells express RAGE. HMGB1 is recoverable in the media of primary osteoblast cultures and cultures of isolated osteoclast precursors and osteoclasts. Parathyroid hormone (PTH), a regulator of bone remodeling, attenuates HMGB1 release in cultures of primary osteoblasts and MC3T3-E1 osteoblast-like cells but augments this release in the rat osteosarcoma cell line UMR 106-01, both responses primarily via activation of adenylyl cyclase. PTH-induced HMGB1 discharge by UMR cells exhibits similar release kinetics as reported for activated macrophages. These data confirm the presence of the HMGB1/RAGE signaling axis in bone.

Recent advances in physiological calcium homeostasis

A constant extracellular Ca2+ concentration is required for numerous physiological functions at tissue and cellular levels. This suggests that minor changes in Ca2+ will be corrected by appropriate homeostatic systems. The system regulating Ca2+ homeostasis involves several organs and hormones. The former are mainly the kidneys, skeleton, intestine and the parathyroid glands. The latter comprise, amongst others, the parathyroid hormone, vitamin D and calcitonin. Progress has recently been made in the identification and characterisation of Ca2+ transport proteins CaT1 and ECaC and this has provided new insights into the molecular mechanisms of Ca2+ transport in cells. The G-protein coupled calcium-sensing receptor, responsible for the exquisite ability of the parathyroid gland to respond to small changes in serum Ca2+ concentration was discovered about a decade ago. Research has focussed on the molecular mechanisms determining the serum levels of 1,25(OH)2D3, and on the transcriptional activity of the vitamin D receptor. The aim of recent work has been to elucidate the mechanisms and the intracellular signalling pathways by which parathyroid hormone, vitamin D and calcitonin affect Ca2+ homeostasis. This article summarises recent advances in the understanding and the molecular basis of physiological Ca2+ homeostasis.

Downregulation of cAMP-dependent protein kinase inhibitor gamma is required for BMP-2-induced osteoblastic differentiation

Osteoblasts, normally derived from undifferentiated mesenchymal precursor cells, acquire their characteristic phenotypes when induced by various regulatory factors, one of which is bone morphogenetic protein-2 (BMP-2). Our recent studies suggest that expression of cAMP-dependent protein kinase (PKA) inhibitor G (PKIG) is down-regulated as human mesenchymal stromal cells (MSCs) undergo BMP-2-induced osteoblastic differentiation. This raises our hypothesis that the PKA pathway is involved in osteogenesis. In this report, we demonstrated that PKIG in human MSCs and its murine homologue PKA inhibitor gamma (PKIgamma) in murine pre-myoblast C2C12 cells were down-regulated when these cells were treated with BMP-2. On the contrary, the PKA activity of C2C12 cells was increased upon BMP-2 treatment. Overexpression of PKIgamma in C2C12 cells was shown to repress mRNA expression of early osteoblastic markers osterix and type I collagen while inhibiting the PKA activity. This correlated with decreased alkaline phosphatase (ALP) activities. Furthermore, inhibition of the PKA activity using its specific inhibitor KT5720 was found to have the similar effect, whereas 8-Br-cAMP, a specific PKA activator, accelerated BMP-2-induced ALP activities. Finally, this study showed that BMP-2 treatment promoted activities of transcription regulatory elements including cAMP response element (CRE) and activating protein-1 (AP1). This effect of BMP-2 was diminished in PKIgamma-overexpressed C2C12 cells. Taken together, our results indicate that the activation of the PKA pathway may be one of key BMP-2-activated signaling events that lead to osteogenesis and that downregulation of PKIgamma may be prerequisite for the PKA activation during the osteoblastic differentiation of precursor cells.

Runx2 regulates the expression of GNAS on SaOs-2 cells

Runx2 is a key regulator of osteoblast-specific gene expression and controls the expression of multiple target genes during osteoblast differentiation. Although some transcriptional targets for Runx2 are known, it is believed that the osteogenic action of Runx2 is mediated by additional target genes, and increasing studies are performed in order to identify such Runx2-responsive genes. To identify genes following the inhibition of Runx2 in osteoblastic cell line, SaOs-2 was stably transfected with a dominant negative mutant of Runx2 (Deltacbfa1) under the control of a strong promoter. Comparison of gene expression patterns by differential display on selected SaOs-2 clones allowed us to observe that GNAS mRNA which encodes for the Gsalpha protein is overexpressed (5 to 8 fold) in cells presenting high levels of Deltacbfa1. This overexpression was also observed at the protein level and seemed to be reflected by an increased basal cAMP level. Gel shift experiments performed in this study indicate that Runx2 is able to bind to the promoter of GNAS, suggesting a direct regulation at the transcriptional level. Well-described GNAS mutations like fibrous dysplasia or Albright hereditary osteodystrophy are linked to abnormality in osteoblast function, and numerous evidences showed that Gsalpha coupled adrenergic receptors increase the expression of osteotrophic factors and regulate bone mass. Regulation of Gsalpha protein by Runx2 seems to be of particular interest considering the increasing evidences on bone metabolism regulation by G proteins.

Osteoblastic Activation in the Hematopoietic Stem Cell Niche

Hematopoietic stem cells (HSC) are rare primitive cells capable of reconstituting all blood cell lineages throughout the life of an individual. The microenvironment in which stem cells reside is essential for their survival, self-renewal, and differentiation. This microenvironment, or HSC niche, has been difficult to define in bone and bone marrow, but recent studies from our laboratory and others have shown that osteoblasts, the bone-forming cells, are an essential regulatory component of this complex cellular network. We established that parathyroid hormone (PTH), through activation of the PTH/PTHrP receptor (PTH1R) in osteoblastic cells, could alter the HSC niche resulting in HSC expansion in vivo and in vitro and improving dramatically the survival of mice receiving bone marrow transplants. These findings are of great clinical appeal, because they suggest that a strategy aimed at modifying supportive cells in a stem cell niche can expand HSC. While a number of molecules have been found to be important for hematopoietic/osteoblastic interactions, we have focused on the Jagged1/Notch signaling pathway, which was necessary for the PTH-dependent HSC expansion. Since the Jagged1/Notch signaling pathway has been implicated in the microenvironmental control of stem cell self-renewal in several organ systems, definition of Jagged1 modulation, which is currently poorly understood, should provide additional molecular targets for stem cell regulation and advance the understanding of stem cell-microenvironmental interactions.

Maturation-state dependent response of human periodontal ligament cells to an intermittent parathyroid hormone exposure in vitro

BACKGROUND AND OBJECTIVES

Parathyroid hormone (parathyroid hormone) has been shown to be capable of exerting anabolic effects on bone when administered intermittently. We hypothesized that parathyroid hormone will modulate the response of periodontal ligament cells in terms of anabolic effects with respect to proliferation, differentiation and the production of key regulatory factors of bone remodeling such as osteoprotegerin and receptor activator of nuclear factor kappaB ligand (RANKL) in a maturation-state dependent manner.

METHODS

Periodontal ligament cells were cultured from human bicuspids obtained from six patients. Following characterization, confluent and preconfluent periodontal ligament cells were challenged with parathyroid hormone (1-34) for 0, 1, 3, 6 or 24 h within three incubation cycles of 48 h each. At harvest, the cell number, alkaline phosphatase specific activity and osteocalcin, osteoprotegerin and RANKL production were determined by means of semiquantitative polymerase chain reaction (PCR) and immunoassays. Dermal fibroblasts and MG63 osteoblast-like cells served as a reference.

RESULTS

Intermittent parathyroid hormone treatment of confluent periodontal ligament cells caused a significant increase in proliferation (+32% maximum) whereas alkaline phosphatase activity, osteocalcin and osteoprotegerin decreased at the transcriptional and translational level (-59.7% maximum). In preconfluent periodontal ligament cells, parathyroid hormone induced a decrease in proliferation (-66.3% maximum) but an increase in differentiation and osteoprotegerin production (+49.2% maximum). RANKL was hardly detectable and unaffected by parathyroid hormone treatment. Similar results were obtained in MG63 cells, whereas parathyroid hormone stimulation did not alter any of the parameters examined in dermal fibroblasts.

CONCLUSION

These results indicate that human periodontal ligament cells respond to an intermittent parathyroid hormone exposure with changes in proliferation, differentiation and osteoprotegerin production in a maturation-state dependent manner and therefore might be regulatorily involved in periodontal regeneration.

Connections between vascular calcification and progression of chronic kidney disease: therapeutic alternatives

We have shown that renal injury and chronic kidney disease (CKD) directly inhibit skeletal anabolism, and that stimulation of bone formation decreases the serum phosphate. Most recently, these observations were rediscovered in low-density lipoprotein receptor null mice fed high-fat/cholesterol diets, a model of the metabolic syndrome (hypertension, obesity, dyslipidemia, and insulin resistance). We had demonstrated that these mice have vascular calcification (VC) of both the intimal atherosclerotic type and medial type. We have shown that VC is worsened by CKD and ameliorated by bone morphogenetic protein -7 (BMP-7). The finding that high-fat-fed low-density lipoprotein receptor null animals without CKD have hyperphosphatemia led us to examine the skeletons of these mice. We found significant reductions in bone formation rates, associated with increased VC and superimposing CKD results in the adynamic bone disorder (ABD), while VC was worsened and hyperphosphatemia persisted. A pathological link between abnormal bone mineralization and VC through the serum phosphorus was demonstrated by the partial effectiveness of directly reducing the serum phosphate by a phosphate binder that had no skeletal action. BMP-7 treatment corrected the ABD and corrected hyperphosphatemia, compatible with BMP-7-driven stimulation of skeletal phosphate deposition reducing plasma phosphate and thereby removing a major stimulus to VC. Thus, in the metabolic syndrome with CKD, a reduction in bone-forming potential of osteogenic cells leads to ABD producing hyperphosphatemia and VC, processes ameliorated by the skeletal anabolic agent BMP-7, in part through increased bone formation and skeletal deposition of phosphate, and in part through direct actions on vascular smooth muscle cells. We have demonstrated that the processes leading to vascular calcification begin with even mild levels of renal injury before demonstrable hyperphosphatemia, and they are preventable and treatable. Therefore, early intervention in CKD is warranted and may affect mortality of the disease.

Intermittent parathyroid hormone therapy to increase bone formation

Clinical data suggested that parathyroid hormone (PTH) might be effective in improving bone mass in patients with osteoporosis, providing its resorptive effects, which are particularly marked at cortical sites, were kept under control. We reviewed the evidence that intermittent PTH therapy is a valid treatment option whose predominant effect is bone anabolism. In cell culture studies, PTH affected both bone formation and bone resorption, suggesting that the net result of PTH therapy may be either bone gain or bone loss depending on the dosage, mode of administration, bone site, and animal species. Histological studies established that intermittent PTH therapy was associated with an increase in trabecular bone and, importantly, with improvements in trabecular and cortical microarchitectural parameters that have not been reported with antiresorptive drugs. This anabolic effect of intermittent PTH therapy translates into increased biomechanical strength, despite the increase in endocortical porosity seen in humans and nonhuman primates. The biochemical response profile to intermittent PTH therapy in clinical trials indicated a phase of isolated anabolism followed by an overall increase in bone remodeling that predominantly affected bone formation, the result being a large increase in spinal bone mineral density as early as the first treatment year. Thus, intermittent PTH therapy exerts predominantly anabolic effects on bone.

Multiple mechanisms are involved in inhibition of osteoblast differentiation by PTHrP and PTH in KS483 Cells

We examined the mechanism by which PTHrP and PTH inhibit KS483 osteoblastic differentiation. We show that PTHrP and PTH inhibit differentiation downstream of early BMP signaling and downregulated components of the hedgehog (Hh) signaling cascade. In addition, PTHrP and PTH repressed RunX2 and osx expression. Overexpression of either gene, however, could not relieve PTHrP and PTH's inhibitory actions. Our data suggest that multiple parallel mechanisms are involved in the inhibition of osteoblast differentiation and matrix mineralization by PTHrP and PTH.

INTRODUCTION

PTH-related peptide (PTHrP) and PTH are potent inhibitors of osteoblast differentiation in vitro by as yet unexplained mechanisms.

MATERIALS AND METHODS

We treated murine bone marrow stromal cells and the mesenchymal progenitor cell line KS483 with PTHrP and PTH in combination with either BMPs or hedgehog (Hh) and measured early and late markers of osteoblast differentiation and studied the expression of RunX2 and Osterix (osx). In addition, we examined the PTHrP and PTH response in stable KS483 cells overexpressing either RunX2 or osx.

RESULTS

PTHrP and PTH inhibited BMP- and Hh-induced osteogenesis downstream of early BMP signaling and by downregulation of components of the Hh signaling cascade. PTHrP and PTH prevented the upregulation of RunX2 expression associated with osteoblast differentiation in an indirect response. However, PTHrP and PTH could still inhibit differentiation, and particularly matrix mineralization, of cells expressing RunX2. In addition, PTHrP and PTH potently downregulated osx expression only in mature osteoblasts in an intermediate early response, but osx overexpression could not relieve the inhibitory effects of PTHrP and PTH on matrix mineralization.

CONCLUSIONS

Our data suggest that, besides transcriptional repression of RunX2 and osx, other mechanisms in parallel with or downstream of RunX2 and osx are involved in the inhibition of osteoblast differentiation and matrix mineralization by PTHrP and PTH in vitro.

Downregulation of osteoblast Phex expression by PTH

Human/murine X-linked hypophosphatemia is a dominant disorder associated with renal phosphate wasting and defective bone mineralization. This disorder results from mutations in the PHEX/Phex (Phosphate-regulating gene with homologies to endopeptidases on the X chromosome) gene, which is expressed in fully differentiated osteoblasts. The purpose of the present study was to assess whether PTH, a major regulator of bone development and turnover, modulates osteoblastic Phex expression. The effects of different concentrations of PTH (rat fragment 1-34) were determined on Phex mRNA and protein expression in vitro using MC3T3-E1 osteoblastic cells and mouse primary osteoblasts; and in vivo using 45-day-old mice infused for 3 days with PTH. Phex mRNA levels were quantitated on Northern blots by densitometric analysis relative to GAPDH mRNA levels. Phex protein levels were analyzed by immunoprecipitation of 35S-methionine-labeled osteoblast lysates or by immunoblotting of calvaria membrane extracts using a polyclonal rabbit antiserum raised against a mouse Phex carboxy-terminal peptide. Fully differentiated MC3T3-E1 cells were incubated for 4 to 48 h with increasing concentrations of PTH (10(-11) to 10(-7) M). PTH inhibited Phex mRNA expression in both mineralizing and nonmineralizing osteoblast cultures in a dose- and time-dependent manner with a maximal inhibition at 10(-7) M PTH after 24 h (15+/-7% of control levels, n=5, P<0.001). The PTH-mediated downregulation of Phex mRNA levels was associated with corresponding decreases in Phex protein synthesis and suppression at 10(-7) M PTH. Similar results were obtained with primary osteoblasts isolated from newborn mouse calvaria. Consistent with the in vitro findings, continuous PTH infusion to mice elicited decreases in Phex expression in calvaria. The effect of PTH was also assessed on matrix mineralization by mature MC3T3-E1 cells by measuring 45Ca accumulation in cell layers. PTH (10(-7) M) inhibited the initiation (57+/-2% of control levels, n=5, P<0.001) and the progression of matrix mineralization (75+/-1% of control levels, n=5, P<0.001). In summary, PTH inhibits osteoblastic Phex expression in vitro and in vivo. The downregulation of Phex expression by PTH in vitro is associated with inhibition of matrix mineralization, consistent with a role for Phex in bone mineralization.

SOST is a target gene for PTH in bone

Intermittent parathyroid hormone (PTH) application is an established pharmacological principle to stimulate bone formation. Yet, the molecular mechanisms underlying this bone anabolic action are not fully understood. Recently, SOST (sclerostin) was identified as a potent osteocyte expressed negative regulator of bone formation in vitro, in murine models and in patients with the bone overgrowth disorders Sclerosteosis and Van Buchem disease. Therefore, we have studied the impact of PTH on SOST regulation. First, we analyzed SOST expression during PTH-induced bone formation in a classical model of local bone formation. 8-month-old mice received intermittently 100 nM hPTH(1-34) or vehicle onto the calvaria for 5 days. PTH stimulated bone formation as assessed by fluorochrome-marker-based histomorphometry. SOST expression was reduced in PTH-treated calvariae 4 h after the last administration as evaluated by real-time quantitative PCR. Next, we observed a decrease of SOST expression in femoral cortical bone of 6-month-old rats following single subcutaneous systemic administration of 80 microg/kg PTH(1-34). Finally, we studied SOST mRNA expression in bone of 11-month-old osteopenic estrogen-deprived (OVX) rats following 8-week systemic intermittent administration of 5 microg/kg PTH(1-34). PTH-treated animals displayed increases in bone mineral density as detected by pQCT, while SOST mRNA levels were decreased compared to vehicle-treated OVX and SHAM controls. PTH decreased SOST expression also in vitro. 100 nM PTH(1-34) inhibited expression in mouse calvaria organ cultures and in osteoblastic UMR-106 cells within 6 h by 95%. An IC50 of 1 nM was determined for PTH(1-34) in UMR-106 cells, whereas the PTH antagonist (d-Trp12,Tyr34)-bPTH(7-34) did not efficiently reduce SOST expression. Furthermore, SOST inhibition by PTH was not blocked by the protein synthesis inhibitor cycloheximide, indicating direct regulation, and PTH did not influence SOST mRNA degradation, implying transcriptional regulation. Finally, we observed full suppression of SOST by the cAMP inducer forskolin, partial inhibition by ionomycin, and no effect with PMA, indicating that PTH action is mainly mediated via the cAMP/PKA pathway. In summary, we have shown that PTH directly inhibits SOST transcription in vivo and in vitro, suggesting that SOST regulation may play a role in mediating PTH action in bone.

Parathyroid hormone induces the NR4A family of nuclear orphan receptors in vivo

Parathyroid hormone (PTH) has both anabolic and catabolic effects on bone metabolism, although the molecular mechanisms mediating these effects are largely unknown. Among the transcription factors induced by PTH in osteoblasts are the nerve growth factor-inducible factor B (NR4A; NGFI-B) family of orphan nuclear receptors: Nurr1, Nur77, and NOR-1. PTH induces NR4A members through the cAMP-protein kinase A (PKA) pathway in vitro. We report here that PTH rapidly and transiently induced expression of all three NR4A genes in PTH-target tissues in vivo. In calvaria, long bones, and kidneys, NR4A induction was maximal 0.5-1 h after a single intraperitoneal (i.p.) injection of 80 microg/kg PTH. Nur77 demonstrated the highest expression, followed, in order, by Nurr1 and NOR-1. In calvaria and long bone, PTH-induced expression of each NR4A gene was detectable at 10 microg/kg i.p. with maximum induction at 40-80 microg/kg. PTH (3-34) did not induce NR4A mRNA levels in calvaria, long bone, and kidney in vivo, confirming our in vitro results that NR4A genes are induced primarily through the cAMP-PKA pathway. The magnitude of PTH-induced NR4A expression was comparable in vivo and in vitro. However, NR4A mRNA levels peaked and returned to baseline faster in vivo. Both in vivo and in vitro, PTH induced NR4A pre-mRNA levels suggesting that induction of these genes is, at least in part, through activation of mRNA synthesis. The in vivo induction of the NR4A family members by PTH suggests their involvement in, at least some, PTH-induced changes in bone metabolism.

Identification of parathyroid hormone-regulated proteins in mouse bone marrow cells by proteomics

The ability of parathyroid hormone (PTH) to enhance bone formation has recently been exploited in the treatment of osteoporosis. Several studies have suggested that the activation of bone marrow stromal cells could be preceded to show the anabolic effect of PTH on bone formation, but little is known of PTH-regulated proteins in bone marrow cells. Therefore, protein profiling in the intermittent PTH-treated bone marrow cells was evaluated using proteomics. Daily treatment for 5 days consisting of subcutaneous injection of either 150 microg/kg per day of mouse PTH (1-84) or vehicle (0.9% normal saline) was performed on the ICR mouse. At the end of the treatment period, bone marrow cells were separated and used in proteomics. The expression levels of seven proteins including vimentin were decreased, but those of four proteins including calreticulin and thioredoxin domain containing 7 protein (Txnde7) were increased. Among these, the decrease of vimentin and the increase of both calreticulin Txnde7 in mRNA levels were confirmed by semi-quantitative RT-PCR. In PTH-treated mouse MC3T3-E1 osteoblast cells, mRNA expression levels were not totally consistent with the results observed in proteomics. In conclusion, the differentially expressed proteins in bone marrow cells depending on PTH could be highly linked to the differentiation of osteoprogenitor cells in the bone marrow into preosteoblast cells.

Bone morphogenetic proteins in development and homeostasis of kidney

Bone morphogenetic proteins play a key role in kidney development and postnatal function. The kidney has been identified as a major site of bone morphogenetic protein (BMP)-7 synthesis during embryonic and postnatal development, which mediates differentiation and maintenance of metanephric mesenchyme. Targeted disruption of BMP-7 gene expression in mice resulted in dysgenic kidneys with hydroureters, causing uremia within 24h after birth. Several experimental animal models of acute and chronic renal injury have all unequivocally shown beneficial effect of BMP-7 in ameliorating the severity of damage by preventing inflammation and fibrosis. Apart from the beneficial effect on kidney disease itself, BMP-7 improves important complications of chronic renal impairment such as renal osteodystrophy and vascular calcification.

Insulin receptor substrate-1 is required for bone anabolic function of parathyroid hormone in mice

Bone anabolic action of PTH has been suggested to be mediated by induction of IGF-I in osteoblasts; however, little is known about the molecular mechanism by which IGF-I leads to bone formation under the PTH stimulation. This study initially confirmed in mouse osteoblast cultures that PTH treatment increased IGF-I mRNA and protein levels and alkaline phosphatase activity, which were accompanied by phosphorylations of IGF-I receptor, insulin receptor substrate (IRS)-1 and IRS-2, essential adaptor molecules for the IGF-I signaling. To learn the involvement of IRS-1 and IRS-2 in the bone anabolic action of PTH in vivo, IRS-1-/- and IRS-2-/- mice and their respective wild-type littermates were given daily injections of PTH (80 mug/kg) or vehicle for 4 wk. In the wild-type mice, the PTH injection increased bone mineral densities of the femur, tibia, and vertebrae by 10-20% without altering the serum IGF-I level. These stimulations were similarly seen in IRS-2-/- mice; however, they were markedly suppressed in IRS-1-/- mice. Although the PTH anabolic effects were stronger on trabecular bones than on cortical bones, the stimulations on both bones were blocked in IRS-1-/- mice but not in IRS-2-/- mice. Histomorphometric and biochemical analyses showed an increased bone turnover by PTH, which was also blunted by the IRS-1 deficiency, though not by the IRS-2 deficiency. These results indicate that the PTH bone anabolic action is mediated by the activation of IRS-1, but not IRS-2, as a downstream signaling of IGF-I that acts locally as an autocrine/paracrine factor.

Parathyroid hormone secretion and action: evidence for discrete receptors for the carboxyl-terminal region and related biological actions of carboxyl- terminal ligands

PTH is a major systemic regulator of the concentrations of calcium, phosphate, and active vitamin D metabolites in blood and of cellular activity in bone. Intermittently administered PTH and amino-terminal PTH peptide fragments or analogs also augment bone mass and currently are being introduced into clinical practice as therapies for osteoporosis. The amino-terminal region of PTH is known to be both necessary and sufficient for full activity at PTH/PTHrP receptors (PTH1Rs), which mediate the classical biological actions of the hormone. It is well known that multiple carboxyl-terminal fragments of PTH are present in blood, where they comprise the major form(s) of circulating hormone, but these fragments have long been regarded as inert by-products of PTH metabolism because they neither bind to nor activate PTH1Rs. New in vitro and in vivo evidence, together with older observations extending over the past 20 yr, now points strongly to the existence of novel large carboxyl-terminal PTH fragments in blood and to receptors for these fragments that appear to mediate unique biological actions in bone. This review traces the development of this field in the context of the evolution of our understanding of the "classical" receptor for amino-terminal PTH and the now convincing evidence for these receptors for carboxyl-terminal PTH. The review summarizes current knowledge of the structure, secretion, and metabolism of PTH and its circulating fragments, details available information concerning the pharmacology and actions of carboxyl-terminal PTH receptors, and frames their likely biological and clinical significance. It seems likely that physiological parathyroid regulation of calcium and bone metabolism may involve receptors for circulating carboxy-terminal PTH ligands as well as the action of amino-terminal determinants within the PTH molecule on the classical PTH1R.

Comparison of the action of transient and continuous PTH on primary osteoblast cultures expressing differentiation stage-specific GFP

Primary calvarial osteoblast cultures derived from type I collagen promoter-GFP reporter transgenic mice were used to examine progression of the osteoblast lineage. This system was validated by assessing the effect of PTH on osteoblast growth in real time. The anabolic effect of PTH seemed to be the result of enhanced osteoblast differentiation rather than expansion of a progenitor population.

INTRODUCTION

Activation of green fluorescent protein (GFP) marker genes driven by Col1a1 promoter fragments has been associated with the level of osteoblast differentiation. GFP-marked cultures provide an approach to continuously monitor the level of osteoblast differentiation in real time without the termination of cultures.

MATERIALS AND METHODS

Neonatal calvarial cells transgenic for pOBCol2.3GFP and pOBCol3.6GFP were used to establish calvarial osteoblast cultures. Parathyroid hormone (PTH) was added either continuous (days 1-21) or transient (days 1-7) to examine its diverse effect on osteoblast differentiation in cultures for 21 days. Three fluorescent markers were used: (1) pOBCol3.6GFP, which is activated in preosteoblastic cells; (2) pOBCol2.3GFP, which is restricted to differentiated osteoblasts; and (3) xylenol orange (XO), which stains the mineralized nodules. Progression of osteoblast differentiation indicated by fluorescent markers was documented throughout the entire period of culture. Recorded fluorescent images were analyzed in the patterns of expression and quantitated in the area of expression.

RESULTS

Continuous PTH blocked osteoblast differentiation, which was evident by the attenuation of pOBCol3.6GFP and an absence of pOBCol2.3GFP. In contrast, transient PTH inhibited the initial osteoblast differentiation but ultimately resulted in a culture with more mineralized nodules and enhanced osteoblast differentiation expressing strong levels of pOBCol3.6GFP and pOBCol2.3GFP. Quantitative analysis showed that transient PTH first decreased then later increased areas of GFP expression and XO staining, which correlated with results of Northern blot and alkaline phosphatase activity. Transient PTH caused a decrease in DNA content during the treatment and after the removal of PTH.

CONCLUSION

GFP-marked cultures combined with fluorescent image analysis have the advantage to assess the effect of PTH on osteoblast differentiation in real time. Results suggest that the anabolic effect of transient PTH is caused by an enhancement in osteoblast differentiation rather than an increase in the population of progenitor cells.

Modeling the interactions between osteoblast and osteoclast activities in bone remodeling

We propose a mathematical model explaining the interactions between osteoblasts and osteoclasts, two cell types specialized in the maintenance of the bone integrity. Bone is a dynamic, living tissue whose structure and shape continuously evolves during life. It has the ability to change architecture by removal of old bone and replacement with newly formed bone in a localized process called remodeling. The model described here is based on the idea that the relative proportions of immature and mature osteoblasts control the degree of osteoclastic activity. In addition, osteoclasts control osteoblasts differentially depending on their stage of differentiation. Despite the tremendous complexity of the bone regulatory system and its fragmentary understanding, we obtain surprisingly good correlations between the model simulations and the experimental observations extracted from the literature. The model results corroborate all behaviors of the bone remodeling system that we have simulated, including the tight coupling between osteoblasts and osteoclasts, the catabolic effect induced by continuous administration of PTH, the catabolic action of RANKL, as well as its reversal by soluble antagonist OPG. The model is also able to simulate metabolic bone diseases such as estrogen deficiency, vitamin D deficiency, senescence and glucocorticoid excess. Conversely, possible routes for therapeutic interventions are tested and evaluated. Our model confirms that anti-resorptive therapies are unable to partially restore bone loss, whereas bone formation therapies yield better results. The model enables us to determine and evaluate potential therapies based on their efficacy. In particular, the model predicts that combinations of anti-resorptive and anabolic therapies provide significant benefits compared with monotherapy, especially for certain type of skeletal disease. Finally, the model clearly indicates that increasing the size of the pool of preosteoblasts is an essential ingredient for the therapeutic manipulation of bone formation. This model was conceived as the first step in a bone turnover modeling platform. These initial modeling results are extremely encouraging and lead us to proceed with additional explorations into bone turnover and skeletal remodeling.

Kidney-bone, bone-kidney, and cell-cell communications in renal osteodystrophy

The relationship between bone and the kidney in renal osteodystrophy is a complex interplay of kidney to bone connections, bone to kidney connections, and cell to cell connections. In addition, such interactions have a profound effect on the vasculature. In this review, we discuss the role of the bone morphogenetic proteins (BMPs) in the skeleton, kidney, and vasculature. In addition, we propose that deficiencies of these BMPs seen in chronic kidney disease (CKD) result in decreased bone remodeling and a compensatory secondary hyperparathyroidism (high turnover state). Treatment of the hyperparathyroidism blocks this compensatory arm and thus decreased bone remodeling occurs (low turnover). We review animal models of CKD in which treatment with BMP-7 resulted in normalization of both high and low turnover states. Finally, we discuss vascular calcification as it relates to bone metabolism. We discuss the roles of BMP-7 and 2 other bone regulatory proteins, osteoprotegerin (OPG) and alpha2-HS glycoprotein (AHSG, human fetuin), in the human vasculature and their implications for vascular calcification.

Effect of raloxifene and its interaction with human PTH on bone formation

We studied the of effects raloxifene alone or in combination with human PTH (hPTH) 1-34 in mineralizing cultures of SaOS-2 cells. Raloxifene (10(-8)-10(-6) M) increased bone nodule formation in cultures of SaOS-2 cells when added intermittently from day 8 to day 17. A single 24-h treatment with 10(-8) M hPTH (1-34) at day 8 reduced the nodule area by 75.6% at day 17, and raloxifene added concomitantly with hPTH (1-34) reduced this inhibitory effect in a dose-dependent manner. Raloxifene also reduced the hPTH (1-34)-induced inhibition of alkaline phosphatase (ALP) activity. The 10-fold stimulation of c-fos mRNA expression by hPTH (1-34) was not influenced by raloxifene co-treatment. The protein kinase A (PKA) inhibitor 6-22 amide (1.7 nM) and the protein kinase C (PKC) inhibitor-bisindolylmaleimide 1 (10 nM) did not influence the separate effects of PTH and raloxifene on mineralized bone nodule formation. This is the first report on the interaction of PTH and raloxifene in an osteoblast culture system.

Sustained In Vitro Expansion of Bone Progenitors Is Cell Density Dependent

Osteogenic cells are an integral part of the dynamic tissue-remodeling process in bone and are potential tools for tissue engineering and cell-based therapies. We examined the role of glucocorticoids and cell density in the expansion of primary rat calvaria cell populations and osteoprogenitor subpopulations in adherent cell culture. Osteoprogenitor response to dexamethasone (dex, a synthetic glucocorticoid known to stimulate bone formation in vitro) supplementation and long-term osteoprogenitor cell proliferation and differentiation were quantified using functional (colony forming unit-osteoblast [CFU-O]) and phenotypic analyses. Although osteoprogenitor self-renewal occurred at both standard and high initiating cell densities, progenitor cell expansion (measured by changes in CFU-O number relative to input) was sustained and dramatically increased at high initiating cell densities (30-fold CFU-O expansion for standard-density cultures compared with a greater than 10,000-fold CFU-O expansion in high-density cultures). Cell density was also found to impact upon the potential of dex to recruit additional progenitors towards bone development. These multifaceted effects appeared to be independent of cell proliferation rates or population phenotypic expression. Together, our results emphasize a roll for cell-cell interactions and/or community effects in the control and maintenance of progenitor cells during in vitro culture.

Modeling of bone formation and resorption mediated by parathyroid hormone: response to estrogen/PTH therapy

Bone, a major reservoir of body calcium, is under the hormonal control of the parathyroid hormone (PTH). Several aspects of its growth, turnover, and mechanism, occur in the absence of gonadal hormones. Sex steroids such as estrogen, nonetheless, play an important role in bone physiology, and are extremely essential to maintain bone balance in adults. In order to provide a basis for understanding the underlying mechanisms of bone remodeling as it is mediated by PTH, we propose here a mathematical model of the process. The nonlinear system model is then utilized to study the temporal effect of PTH as well as the action of estrogen replacement therapy on bone turnover. Analysis of the model is done on the assumption, supported by reported clinical evidence, that the process is characterized by highly diversified dynamics, which warrants the use of singular perturbation arguments. The model is shown to exhibit limit cycle behavior, which can develop into chaotic dynamics for certain ranges of the system's parametric values. Effects of estrogen and PTH administrations are then investigated by extending on the core model. Analysis of the model seems to indicate that the paradoxical observation that intermittent PTH administration causes net bone deposition while continuous administration causes net bone loss, and certain other reported phenomena may be attributed to the highly diversified dynamics which characterizes this nonlinear remodeling process.

PTH-dependent adenylyl cyclase activation in SaOS-2 cells: passage dependent effects on G protein interactions

Parathyroid hormone (PTH) sensitive adenylyl cyclase activity (ACA) in SaOS-2 cells varies as a function of cell passage. In early passage (EP) cells (< 6), ACA in response to PTH and forskolin (FOR) was relatively low and equivalent, whereas in late passage (LP) cells (> 22), PTH exceeded FOR dependent ACA. Potential biochemical mechanisms for this passage dependent change in ACA were considered. In EP, prolonged exposure to pertussis toxin (PT) markedly enhanced ACA activity in response to PTH, Isoproterenol and Gpp(NH)p, whereas ACA in response to FOR was decreased. In contrast, the identical treatment of LP with PT diminished all ACA in response to PTH, Gpp(NH)p, and FOR. The dose dependent effects of PT on subsequent [(32)P]ADP-ribosylation of its substrates, GTPase activity, as well as FOR-dependent ACA, were equivalent in EP and LP. The relative amounts of G(alpha)i and G(alpha)s proteins, as determined both by Western blot, PT and cholera toxin (CT) dependent [(32)P]ADP-ribosylation, were quantitatively similar in EP and LP. Western blot levels of G(alpha)s and G(alpha)i proteins were not influenced by prior exposure to PT. Both PT and CT dependent [(32)P]ADP-ribosylation were dose-dependently decreased following exposure to PT. However, the PT-dependent decline in CT-dependent [(32)P]ADP-ribosylation occurred with enhanced sensitivity in LP. The protein synthesis inhibitor cycloheximide partially reversed the PT associated decrease in FOR dependent ACA in EP. In contrast, cycloheximide completely reversed the PT associated decrease in FOR and as well as PTH dependent ACA in LP. G(alpha)s activity, revealed by cyc(-) reconstitution, was not altered either by cell passage or exposure to PT. The results suggest that the coupling between the components of the complex may be pivotally important in the differential responsiveness of early and late passage SaOS-2 cells to PTH.

A phosphodiesterase inhibitor, pentoxifylline, enhances the bone morphogenetic protein-4 (BMP-4)-dependent differentiation of osteoprogenitor cells

Bone morphogenetic protein-4 (BMP-4), a member of the transforming growth factor-beta superfamily, is capable of initiating differentiation of uncommitted mesenchymal cells into a chondro/osteogenic pathway. This study reports the effects of pentoxifylline (PTX), a nonspecific inhibitor of phosphodiesterases (PDEs), that causes elevation of the intracellular cyclic adenosine monophosphate (cAMP) level on the BMP-4-induced chondro/osteogenic differentiation of a mesenchymal cell line, C3H10T1/2; a bone marrow stromal cell line, ST2; and an osteoblastic cell line, MC3T3-E1. It was found that PTX enhanced BMP-4-induced chondro/osteogenic differentiation in C3H10T1/2 and ST2 cells. Similar effects were observed when adding dibutyryl-cAMP and forskolin. These results indicate that cAMP may potentiate the action of BMP-4 on osteoprogenitor cells, highlighting the possibility that PDE inhibitors could be used as therapeutic agents to enhance bone formation through this effect.

Transactivation of core binding factor alpha1 as a basic mechanism to trigger parathyroid hormone-induced osteogenesis

During 28-day culture of bone marrow- and calvaria-derived osteoblasts, the constant presence of parathyroid hormone (PTH)(1-34) retarded differentiation and nodule formation (NF) in a dose-dependent fashion (C-phase). In contrast, addition of PTH(1-34) in late stage cultures (from day 10 to 21) accelerated NF (A-phase). The stable production of such an A-phase allowed us to study the mechanism of bone anabolic action of PTH(1-34). Subcellular localization studies of core binding factor alpha1 (Cbfa1) and reporter assays provided the results indicating that in the A-phase, PTH(1-34) triggers its bone anabolic action via enhancement of Cbfa1 transactivation. RT-PCR and Northern blot analyses revealed that alkaline phosphatase, osteocalcin and bone sialoprotein expression decreased in the C-phase and increased in the A-phase; however, expression of other bone proteins (Cbfa1, PTH/PTH-related peptide-receptor, osteopontin, collagen I alpha1, collagen I alpha2, vitamin K-dependent gamma-glutamyl carboxylase) did not change in a phase transition-related manner. Ovariectomized osteopenic mice, treated with PTH(1-34) (4 and 40 microg/kg, s.c., every other day, 4 or 6 weeks), recovered lost bone, displayed elevated nuclear localization of Cbfal in tibiae without alteration of its cytosolic level and exhibited upregulation of expressions of the same set of proteins (alkaline phosphatase, osteocalcin and bone sialoprotein) in femora. These results obtained by a concerted study in vitro and in vivo suggest that PTH triggers its osteogenic action via promotion of the transactivation of Cbfa1.

Parathyroid hormone/parathyroid hormone-related peptide type 1 receptor in human bone

The parathyroid hormone/parathyroid hormone-related peptide (PTH/PTHrP) receptor (denoted as PTH-1R) is a key signaling factor through which calcium-regulating hormones PTH and PTHrP exert their effects on bone. There are contradictory reports regarding the capability of osteoclasts to express PTH-1R. To address this issue in humans, bone biopsy specimen samples from 9 normal controls and 16 patients with moderate to severe secondary renal hyperparathyroid bone disease (2 degrees HPT) with elevated PTH levels were studied to determine whether osteoclasts in the bone microenvironment express PTH-1R messenger RNA (mRNA) and protein. We report that osteoclasts express the PTH-1R mRNA but the protein is detected only in patients with 2 degrees HPT. The PTH-1R mRNA and protein also were found in osteoblasts, osteocytes, and bone marrow cells. Receptor expression was higher in osteoclasts and osteoblasts of patients with 2 degrees HPT than normal controls (98.0 +/- 1.1% vs. 65.7 +/- 14.3% and 65.8 +/- 3.4% vs. 39.1 +/- 6.2%; p < 0.01, respectively). Approximately half of osteoclasts found in bone of patients with 2 degrees HPT have the PTH-1R protein. In patients with 2 degrees HPT, a positive relationship exists between erosion depth, a parameter of osteoclastic activity, and the percentage of osteoclasts with PTH-1R protein (r = 0.58; p < 0.05). In normal controls, an inverse relationship exists between the percentage of osteoblasts with receptor mRNA, mRNA signals/cell, and serum PTH levels (r = -0.82 and p < 0.05 and r = -0.78 and p < 0.01, respectively). The results provide the novel evidence of PTH-1R in human osteoclasts and suggest a functional role for the receptors in 2 degrees HPT.

Group III metabotropic glutamate receptors in rat cultured calvarial osteoblasts

Reverse transcription polymerase chain reaction revealed expression of mRNAs for particular receptors for the central neurotransmitter l-glutamic acid (Glu) in primary cultures of rat calvarial osteoblastic cells under premature to mature states according to the duration of days in vitro. These included metabotropic Glu receptors (mGluR) such as mGluR4 and mGluR8, in addition to several ionotropic Glu receptor subunits including NR1 and NR2D. Expression of mRNAs was not detected with other mGluR and NR2A-C subunits irrespective of the maturity of cultured cells. The agonist for group III mGluR L-(+)-2-amino-4-phosphonobutyric acid significantly inhibited the forskolin-induced accumulation of cAMP in premature osteoblasts, which occurred in a manner sensitive to prevention by the group III mGluR antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine. These results suggest that Glu may at least in part play a role in mechanisms associated with cellular proliferation and/or differentiation through group III mGluR functionally expressed in rat calvarial osteoblastic cells.

Parathyroid hormone-related protein regulates extracellular matrix gene expression in cementoblasts and inhibits cementoblast-mediated mineralization in vitro

Parathyroid hormone-related protein (PTHrP) has been implicated in regulating tooth eruption and/or development. Formation of cementum, a mineralized tissue covering the tooth root surface, is a critical biological event for tooth root development. To test the hypothesis that PTHrP targets cementoblasts (CMs) and acts to regulate cementogenesis, CM cell lines were established and their responsiveness to PTHrP stimulation was determined, in vitro. First, subclones were derived from two immortalized murine cell populations that contained CMs; SV-CM/periodontal ligament (PDL) cells were obtained from the root surface of first mandibular molars of CD-1 mice and immortalized with SV40 T-antigen (TAg), and OC-CM cell population was established from OC-TAg transgenic mice in which their cells harbor an osteocalcin (OC and/or OCN) promoter-driving immortal gene SV40 TAg. Based on our previous in situ studies, CM subclones were identified as cells expressing bone sialoprotein (BSP) and OCN transcripts, while PDL cell lines were designated as cells lacking BSP and OCN messenger RNA (mRNA). CMs exhibited a cuboidal appearance and promoted biomineralization, both in vitro and in vivo. In contrast, PDL cells (PDL subclones) displayed a spindle-shaped morphology and lacked the ability to promote mineralized nodule formation, both in vitro and in vivo. Next, using these subclones, the effect of PTHrP on cementogenesis was studied. CMs, not PDL cells, expressed PTH/PTHrP receptor mRNA and exhibited PTHrP-mediated elevation in cyclic adenosine monophosphate (cAMP) levels and c-fos gene induction. PTHrP stimulation repressed mRNA expression of BSP and OCN in CMs and blocked CM-mediated mineralization, in vitro. Collectively, these data suggest that CMs possess PTH/PTHrP receptors and, thus, are direct targets for PTHrP action during cementogenesis and that PTHrP may serve as an important regulator of cementogenesis.

Long-term effects of parathyroid hormone, 1,25-dihydroxyvitamin d(3), and dexamethasone on the cell growth and functional activity of human osteogenic alveolar bone cell cultures

The proliferation-differentiation behaviour of human alveolar bone cell cultures grown for 32 days in conditions that allowed the complete expression of the osteoblastic phenotype was significantly affected by the continuous presence of parathyroid hormone, 1, 25-dihydroxyvitamin D(3), or dexamethasone. Parathyroid hormone and, in particular, dexamethasone significantly induced the differentiation of osteoblastic cells. Moreover, cultures exposed to these hormones presented an earlier appearance and higher levels of alkaline phosphatase, and an increased ability to form calcium phosphate deposits in the extracellular matrix.

Species-divergent regulation of human and mouse osteocalcin genes by calciotropic hormones

Although osteocalcin is the most abundant noncollagenous protein in bone, its role remains undefined. Recent studies have reported diametrically opposing responses in the vitamin D regulation of the mouse vs the human and rat osteocalcin genes. The aim of this study was to increase the understanding of these differences and further elucidate the physiological function and regulation of osteocalcin. Direct comparison of the regulation of both the endogenous mouse osteocalcin gene (mOC) and a human osteocalcin promoter-chloramphenicol acetyl transferase (hOC-CAT) reporter as integrated templates was undertaken in primary osteoblastic cultures from OSCAT transgenic mice. Expression of both genes was up-regulated with the onset of mineralization. Long-term chronic 1, 25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) treatment and acute (2 day) PTH treatment inhibited both mOC and hOC-CAT expression. At all stages of osteoblastic development studied, hOC-CAT was up-regulated by acute 1,25-(OH)(2)D(3), whereas mOC was unaffected or inhibited. Mouse osteopontin was strongly up-regulated by acute 1, 25-(OH)(2)D(3) treatment. Thus, the divergence of the osteocalcin responses to 1,25-(OH)(2)D(3) is specific for the osteocalcin gene and for an acute 1,25-(OH)(2)D(3) treatment regime. Elucidation of this unique aspect of bone physiology will provide valuable insights into the still incompletely understood roles of osteocalcin and 1, 25-(OH)(2)D(3) in bone.

Inhibition of osteoblast-specific transcription factor Cbfa1 by the cAMP pathway in osteoblastic cells. Ubiquitin/proteasome-dependent regulation

The cAMP pathway, a major intracellular pathway mediating parathyroid hormone signal, regulates osteoblastic function. Parathyroid hormone (through activation of protein kinase A) has also been shown to stimulate ubiquitin/proteasome activity in osteoblasts. Since the osteoblast-specific transcription factor Osf2/Cbfa1 is important for differentiation of osteoblastic cells, we examined the roles of the cAMP and ubiquitin/proteasome pathways in regulation of Cbfa1. In the osteoblastic cell line, MC3T3-E1, continuous treatment with cAMP elevating agents inhibited both osteoblastic differentiation based on alkaline phosphatase assay and DNA binding ability of Cbfa1 based on a gel retardation assay. Cbfa1 inhibition was paralleled by an inhibitory effect of forskolin on Cbfa1-regulated genes. Northern and Western blot analyses suggested that the inhibition of Cbfa1 by forskolin was mainly at the protein level. Pretreatment with proteasome inhibitors prior to forskolin treatment reversed the effect of forskolin. Furthermore, addition of proteasome inhibitors to forskolin-pretreated samples resulted in recovery of Cbfa1 protein levels and accumulation of polyubiquitinated forms of Cbfa1, indicating a role for the proteasome pathway in the degradation of Cbfa1. These results suggest that suppression of osteoblastic function by the cAMP pathway is through proteolytic degradation of Cbfa1 involving a ubiquitin/proteasome-dependent mechanism.

Anabolic or catabolic responses of MC3T3-E1 osteoblastic cells to parathyroid hormone depend on time and duration of treatment

We have investigated signaling (cAMP) and anabolic responses (mineralization of extracellular matrix [ECM]) to parathyroid hormone (PTH) in long-term (30 days) cultures of MC3T3-E1 cells, a murine model of osteoblast differentiation. Expression of PTH/PTH-related peptide receptor (PTH1R) mRNA is detected early and remains relatively constant for 2 weeks with somewhat higher levels observed during the second half of the culture period. In contrast to the relatively stable PTH1R mRNA expression, the cAMP response to PTH varies markedly with no response at day 5 and a marked response (80-fold versus control) by day 10. Responsiveness to PTH remains elevated with fluctuations of 30- to 80-fold stimulation throughout the remainder of the culture period. The timing and duration of PTH treatment to achieve in vitro mineralization of ECM was evaluated. When continuous PTH treatment was initiated before day 20, mineralization decreased. If continuous PTH treatment began on or after day 20, mineralization was unaffected. However, if treatment began on day 20 and then stopped on day 25, mineralization on day 30 was increased 5-fold. This mineralization response to intermittent PTH was confirmed in primary cultures of murine and human osteoblastic cells. These data provide a potential basis for understanding the differential responses to PTH (anabolic versus catabolic) and indicate the developmental temporal variance of anabolic and catabolic responses. Since cAMP signaling was relatively unchanged during this interval (day 10-30) and stimulation of adenylate cyclase only partially mimicked the PTH effect on increased mineralization, other signaling pathways are likely to be involved in order to determine the specific anabolic response to short-term PTH treatment during the differentiation process.

Parathyroid hormone [PTH(1-34)] and parathyroid hormone-related protein [PTHrP(1-34)] promote reversion of hypertrophic chondrocytes to a prehypertrophic proliferating phenotype and prevent terminal differentiation of osteoblast-like cells

The effects of parathyroid hormone/parathyroid hormone-related protein (PTH/PTHrP) on late events in chondrocyte differentiation were investigated by a dual in vitro model where conditions of suspension versus adhesion culturing are permissive either for apoptosis or for the further differentiation of hypertrophic chondrocytes to osteoblast- like cells. Chick embryo hypertrophic chondrocytes maintained in suspension synthesized type II and type X collagen and organized their extracellular matrix, forming a tissue highly reminiscent of true cartilage, which eventually mineralized. The formation of mineralized cartilage was associated with the expression of alkaline phosphatase (ALP), arrest of cell growth, and apoptosis, as observed in growth plates in vivo. In this system, PTH/PTHrP was found to repress type X collagen synthesis, ALP expression, and cartilage matrix mineralization. Cell proliferation was resumed, whereas apoptosis was blocked. Hypertrophic chondrocytes cultured in adherent conditions in the presence of retinoic acid underwent further differentiation to osteoblast-like cells (i.e., they resumed cell proliferation, switched to type I collagen synthesis, and produced a mineralizing bone-like matrix). In this system, PTH addition to culture completely inhibited the expression of ALP and matrix mineralization, whereas cell proliferation and expression of type I collagen were not affected. These data indicate that PTH/PTHrP inhibit both the mineralization of a cartilage-like matrix and apoptosis (mimicked in the suspension culture) and the production of a mineralizing bone-like matrix, characterizing further differentiation of hypertrophic chondrocytes to osteoblasts like cells (mimicked in adhesion culture). Treatment of chondrocyte cultures with PTH/PTHrP reverts cultured cells in states of differentiation earlier than hypertrophic chondrocytes (suspension), or earlier than mineralizing osteoblast-like cells (adhesion). However, withdrawal of hormonal stimulation redirects cells toward their distinct, microenvironment-dependent, terminal differentiation and fate.

3',5'-Cyclic adenosine monophosphate activation in osteoblastic cells: effects on parathyroid hormone-1 receptors and osteoblastic differentiation in vitro

PTH has anabolic and catabolic effects in bone through activation of the PTH-1 (PTH/PTHrP) receptor and the cAMP/protein kinase A pathway. The effects of agents that regulate cAMP in nontransformed osteoblasts in relation to cell differentiation have not been described. The purpose of this study was to determine the effects of PTH fragments with differing cAMP-stimulating activity, and nonPTH cAMP regulators on PTH-1 receptor expression and activity, and osteoblast differentiation in vitro using MC3T3-E1 and primary rat calvarial cells. PTH (1-34), but not PTH (53-84), (7-34), or PTHrP (107-139) treatment (24 h) resulted in down-regulation of steady-state messenger RNA for the PTH-1 receptor. Forskolin (a stimulator of cAMP accumulation) also down regulated the PTH-1 receptor, whereas 9-(tetrahydro-2-furyl) adenine (THFA) (an inhibitor of adenylyl cyclase) had no effect. Similarly, PTH (1-34) treatment for 48 h abolished PTHrP binding to cell surface receptors; however, neither the PTH analogs nor the cAMP regulating agents altered PTH binding or numbers of binding sites on osteoblastic cells. Basal levels of cAMP were reduced in cultured cells treated for 6 days with PTH (7-34) or THFA compared with controls. In contrast, PTH-stimulated cAMP levels were significantly increased in cultures treated with PTH (7-34) and THFA for 6 days during osteoblast differentiation and were decreased in cultures treated with PTH (1-34) and forskolin compared with controls. To evaluate effects of the cAMP pathway on osteoblast differentiation, cultures were treated continuously with PTH analogs and cAMP regulators during an 18-day differentiation regime, total RNA was isolated at multiple time points, and Northern blot analysis for osteocalcin (OCN) was performed. THFA and PTH (7-34)-treated cultures had increased OCN expression; whereas, PTH (1-34) and forskolin reduced OCN expression. Interestingly, PTH (7-34) and THFA-treated cultures had increased mineralized nodule formation, in contrast to PTH (1-34) and forskolin treatment, which reduced nodule formation. Similarly, calcium accumulation in cultures was significantly increased in the PTH (7-34) and THFA-treated cultures and reduced in the PTH (1-34) and forskolin-treated cultures. These data demonstrate that agents that increase cAMP down regulate PTH-1 receptor messenger RNA and inhibit osteoblast differentiation in vitro. Agents that reduce or block adenylyl cyclase or cAMP activity do not alter PTH-1 receptor expression or binding, but have striking effects on promoting osteoblast differentiation. We conclude that many effects of PTH on osteoblasts may be mimicked or antagonized by agents that alter cAMP activity and bypass the PTH-1 receptor.

Bone morphogenetic protein-2 enhances osteoclast formation mediated by interleukin-1alpha through upregulation of osteoclast differentiation factor and cyclooxygenase-2

Bone morphogenic protein-2 (BMP-2) is a member of the transforming growth factor beta (TGF-beta) superfamily. While BMP-2 is capable of inducing bone formation ectopically, little is known about its role on osteoclastogenesis. In this study, we examined the effect of BMP-2 on osteoclast-like multinucleated cell (OCL) formation in cocultures of osteoblast-like cells and hematopoietic cells of bone marrow origin. BMP-2 alone did not stimulate OCL formation in this culture system; however, it strongly enhanced OCL formation in a dose-dependent fashion in the presence of interleukin-1alpha (IL-1alpha). Western blot analysis showed that a simultaneous addition of BMP-2 and IL-1alpha synergistically enhanced cyclooxygenase-2 (COX-2) expression in osteoblast-like cells. Moreover, Northern blot analysis revealed that the level of osteoclast differentiation factor (ODF) mRNA increased by treatment with BMP-2 and IL-1alpha in osteoblast-like cells. It is noted that BMP-2 alone did cause an increase in the expression of both COX-2 and ODF genes. The stimulatory effect of BMP-2 was abolished by adding nonsteroidal anti-inflammatory drugs, such as indomethacin and a selective COX-2 inhibitor NS-398. Addition of NS-398 inhibited the expression of the ODF gene in osteoblast-like cells treated with BMP-2 and IL-1alpha. These results indicated that the combination of BMP-2 and IL-1alpha stimulated osteoblast-like cells to elevate the expression of both COX-2 and ODF genes, resulting in an enhanced OCL formation. Since BMP-2 alone induced the expression of COX-2 and ODF genes in osteoblast-like cells, it appears to be one of the regulating factors of osteoclastogenesis.

Effects of new analogues of vitamin D on bone cells: implications for treatment of uremic bone disease

BACKGROUND

The use of calcitriol in the treatment of uremic hyperparathyroidism and renal osteodystrophy is limited in many patients by hypercalcemic side-effects. New less calcemic analogues of calcitriol are being developed, and some are under clinical evaluation. To investigate whether these compounds possess important differences in their action on bone cells, we have studied their effects [with and without parathyroid hormone (PTH)] on the release and synthesis of the resorptive osteotropic cytokine, interleukin-6 (IL-6).

METHODS

MG 63 and SaOS-2 human osteoblastic cell lines were cultured for 6 or 24 hours in media containing calcitriol, the sterols of interest, or 1-34 synthetic PTH. IL-6 release was assayed by commercially available enzyme-linked immunosorbent assay. IL-6 mRNA levels were assessed by reverse transcriptase-polymerase chain reaction.

RESULTS

We found that calcitriol and paricalcitol behaved in a similar fashion, resulting in increased IL-6 release only at higher concentrations (10(-7) to 10(-9) M). In contrast, 22-oxacalcitriol and 1,25-dihydroxydihydrotachysterol2 stimulated release to a similar extent but at concentrations three to four orders of magnitude lower (10(-11) to 10(-13) M), despite being less potent as suppressers of parathyroid function than calcitriol. Studies of IL-6 mRNA showed a similar pattern of concentration and cell line-dependent transcription.

CONCLUSIONS

Compounds stimulating IL-6 release at concentrations achievable during the treatment of uremic hyperparathyroidism might favor continuing linked bone formation and resorption and thereby avoid adynamic bone disease while still allowing profound suppression of PTH.

Exposure of macrophage-like cells to titanium particles does not affect bone resorption, but inhibits bone formation

We examined the capacity of culture supernatants of macrophage-like cells exposed to titanium particles to influence bone formation and bone resorption, our aim being to elucidate the mechanism of implant loosening. A mouse macrophage-like cell line, J774, was exposed to titanium particles and the concentrations of prostaglandin E2, tumor necrosis factor-alpha, interleukin-1alpha, and interleukin-6 in the supernatants were measured. Titanium particles stimulated the J774 cells to release tumor necrosis factor-alpha, whereas prostaglandin E2, interleukin-1alpha and interleukin-6 concentrations remained low. The bone resorptive activity of the supernatants was measured by determining 45Ca release from cultured pre-labeled newborn mouse calvariae. The culture supernatants of J774 cells exposed to titanium particles showed no significant difference in bone resorptive activity in mouse calvariae from that of culture supernatants of J774 cells not exposed to titanium particles. The bone-forming activity of the supernatant was evaluated by determining bone nodule formation and alkaline phosphatase activity in cultured mouse calvaria cells. The bone-forming activity of the supernatants exposed to titanium particles was significantly decreased compared with the supernatants of unexposed J774 cells. This inhibition was reversed by the addition of anti-tumor necrosis factor-alpha neutralizing antibody. We conclude that tumor necrosis factor-alpha released from J774 cells exposed to titanium particles played an important role in the inhibition of bone formation rather than in the stimulation of bone resorption.

Conditionally immortalized murine osteoblasts lacking the type 1 PTH/PTHrP receptor

Osteoblasts synthesize and mineralize bone matrix and are principal target cells for parathyroid hormone (PTH). The type 1 PTH/PTH-related protein (PTHrP) receptor (PTH1R), cloned from rat osteoblastic cells, activates multiple intracellular signaling mechanisms. The specific roles of these PTH1R signals, or of responses to other types of PTH receptors that may be expressed, in regulating osteoblast function are incompletely understood. Use of established mammalian osteoblastic cell lines has led to much understanding of PTH action in bone, although such cells are of neoplastic origin or have other characteristics that compromise their validity as models of normal osteoblasts. To examine the role of the PTH1R in osteoblast biology, we have isolated a series of clonal murine calvarial osteoblastic cell lines that are only conditionally immortalized, via expression of a transgene encoding the tsA58 temperature-sensitive SV40 large T antigen, and that lack both functional alleles of the PTH1R gene. When cultured under nontransforming conditions, these cells stopped proliferating, expressed a series of characteristic osteoblastic genes (including the nonfunctional remnant of the PTH1R gene), and, after 3-4 weeks, produced mineralized bone nodules in a manner that was regulated by 1,25-dihydroxyvitamin D3 but not by PTH(1-84). Cyclic AMP measurements revealed no evidence of expression of alternate species of Gs-linked PTH receptors. Stable transfection with PTH1R cDNA reconstituted both PTH binding and adenylyl cyclase activation, increased basal osteocalcin expression, and supported PTH stimulation of c-Fos expression and matrix mineralization. These conditionally transformed, PTH1R(-/-) clonal osteoblastic cell lines should prove useful for studies of the regulation of osteoblast differentiation and function by both endogenous nonclassical species of PTH (or PTHrP) receptors and mutant signal-selective PTH1Rs.

PTH(1-34) suppresses appositional bone formation by cultured rat cranial osteoblasts

Parathyroid hormone (PTH) provokes cell division of osteoblasts in vitro and is anabolic in vivo when administered intermittently. We studied the effect on bone formation in vitro by rat primary calvarial osteoblasts of the short-term addition of rat PTH(1-34) (10(-12), 10(-9), and 10(-7) mol/L) to the medium. Our aim was to determine whether intermittent PTH(1-34) would promote osteogenesis in an experimental system designed to promote appositional bone formation. Unexpectedly, whether the exposure to PTH(1-34) was for 24 h only on days 3 or 12, or 24 h on both days 3 and 12, or a pulse for 6 h every 48 h, we observed suppression of bone formation even at the lowest concentration of added PTH(1-34). Our finding that cells which are of the osteoblastic phenotype can be entirely prevented from making appositional bone by high concentrations of PTH(1-34) for 1 day in a week, and have their osteogenic capacity suppressed by lower concentrations even when the PTH(1-34) is administered in brief pulses every other day, suggests that pulses of PTH administered in vivo may not increase bone formation by already differentiated osteoblasts.

Parathyroid hormone enhances early and suppresses late stages of osteogenic and chondrogenic development in a BMP-dependent mesenchymal differentiation system (C3H10T1/2)

The role of parathyroid hormone (PTH) upon osteo-/chondrogenic development was investigated in a bone morphogenetic protein (BMP)-dependent differentiation system involving the recombinant expression of BMPs in mesenchymal progenitor cells (C3H10T1/2). The constitutive expression of the PTH/PTH related protein receptor in this system led to a marked stimulation of chondrogenic and osteogenic development, while the permanent application of the ligand PTH(1-34) resulted in opposite responses by stimulating the early and suppressing the late stages of osteo-/chondrogenic development. These contrasting effects of PTH(1-34) on osteogenic and chondrocytic development seem, therefore, to depend on the cellular state of differentiation. The osteogenic and chondrocytic differentiation potential was substantiated histologically and by genetic analyses of marker genes like c-fos, alkaline phosphatase, osteocalcin, collagen alpha1(I), and collagen alpha1(II). The capacity to regulate osteogenic and chondrogenic development is located in the amino-terminal (1-34) region of the PTH molecule and seems to be mediated by the cyclic adenosine monophosphate signaling cascade. The application of other PTH domains like PTH(28-48) and PTH(53-84) did not exhibit significant responses. PTH acts as an essential factor in mesenchymal development controlling rates of differentiation into the osteogenic or chondrogenic lineage. The analysis of PTH effects in this system demonstrates the value of recombinant mesenchymal progenitor cells in the in vitro analysis of osteo-/chondrogenic development.

Parathyroid hormone up-regulation of connexin 43 gene expression in osteoblasts depends on cell phenotype

Accumulating evidence indicates that gap junctions, primarily composed of connexin 43 (Cx43), are distributed extensively throughout bone. We have previously reported that in osteoblastic cells parathyroid hormone (PTH) increases both the steady-state levels of transcripts for Cx43 and gap-junctional intercellular communication in a process involving cyclic adenosine monophosphate (cAMP). We now present data showing that the mechanism of stimulation of Cx43 gene expression by PTH involves an increased rate of Cx43 gene transcription without affecting Cx43 transcript stability in UMR 106 osteoblastic cells. Activation of the protein kinase C pathway is not involved in this process. Inhibiting translation consistently decreases the PTH-mediated stimulation of Cx43 gene expression at all the times we tested (1-3 h). However, this effect is only partial, demonstrating that de novo protein synthesis is required for full stimulation. PTH increases the steady-state levels of Cx43 mRNA in several osteoblastic cell lines, albeit to different levels. We were unable to detect PTH stimulation in ROS 17/2.8 osteoblastic cells, suggesting that the effect of PTH on Cx43 gene expression may depend on the developmental state of the cell along the osteoblastic differentiation pathway. In the MC3T3-E1 preosteoblastic cell line, we find that PTH increases Cx43 gene expression in proliferating and maturing osteoblastic cells, but not in nondividing, differentiated osteoblasts, where the basal level of Cx43 gene expression is elevated. Unlike PTH, the osteotropic hormones 1,25-dihydroxyvitamin D3 and 17beta-estradiol do not appear to affect Cx43 gene expression in UMR 106 osteoblastic cells.

Purinergic transmitters inhibit bone formation by cultured osteoblasts

Adenosine triphosphate (ATP) and other purinoceptor agonists cause a transient rise in [Ca2+]i in cultured osteoblast-like cells and have a mitogenic effect, as does parathyroid hormone (PTH), and there is evidence that ATP and PTH can act synergistically on osteoblasts. The likelihood that nucleotides, acting through purinoceptors, are important local factors in bone remodeling is therefore considerable. However, their effect on bone formation is unknown. We recently developed a culture system in which appositional bone formation occurs only in narrow grooves cut in a substratum. We have used this as an assay to measure the effects of ATP (50 and 500 mumol/L), ATP gamma S (20 mumol/L), 2-MeSATP (2 and 20 mumol/L), uridine triphosphate (UTP) (0.2, 2, and 20 mumol/L), adenosine (20 mumol/L), bovine PTH (0.25 and 0.5 IU/mL), rat PTH1-34 (10(-8) and 10(-7) mol/L), and rat PTHrP1-40 (10(-9) and 10(-8) mol/L) on bone formation by rat calvarial osteoblasts. The culture medium was renewed 3 times/week (every 2 or 3 days), and the number of bone loci and length and area of Alizarin red-stained mineralized bone formed in the grooves of each specimen in 16-29 days were measured. Compared with controls, ATP gamma S, 2-MeSATP, and ATP reduced the amount of bone formed in a 2-3 week culture period. Adenosine had no effect, and UTP either had no effect or at 2 mumol/L stimulated bone formation. PTH and PTHrP completely abolished bone formation in 4 week cultures. Our findings are consistent with evidence for more than one P2 purinoceptor subtype in bone, and show for the first time that the effect of ATP on appositional bone formation by osteoblasts in vitro is, like PTH and PTHrP, inhibitory.

Parathyroid hormone exerts disparate effects on osteoblast differentiation depending on exposure time in rat osteoblastic cells

It has been reported that PTH exerts bone-forming effects in vivo when administered intermittently. In the present study, the anabolic effects of PTH(1-34) on osteoblast differentiation were examined in vitro. Osteoblastic cells isolated from newborn rat calvaria were cyclically treated with PTH(1-34) for the first few hours of each 48-h incubation cycle. When osteoblastic cells were intermittently exposed to PTH only for the first hour of each 48-h incubation cycle and cultured for the remainder of the cycle without the hormone, osteoblast differentiation was inhibited by suppressing alkaline phosphatase activity, bone nodule formation, and mRNA expression of alkaline phosphatase, osteocalcin, and PTH/PTHrP receptor. Experiments using inhibitors and stimulators of cAMP/protein kinase A (PKA) and Ca2+/PKC demonstrated that cAMP/PKA was the major signal transduction system in the inhibitory action of PTH. In contrast, the intermittent exposure to PTH for the first 6 h of each 48-h cycle stimulated osteoblast differentiation. Both cAMP/ PKA and Ca2+/PKC systems appeared to be involved cooperatively in this anabolic effect. Continuous exposure to PTH during the 48-h incubation cycle strongly inhibited osteoblast differentiation. Although both cAMP/PKA and Ca2+/PKC were involved in the effect of continuous exposure to PTH, they appeared to act independently. A neutralizing antibody against IGF-I blocked the stimulatory effect on alkaline phosphatase activity and the expression of osteocalcin mRNA induced by the 6-h intermittent exposure. The inhibitory effect induced by the 1-h intermittent exposure was not affected by anti-IGF-I antibody. These results suggest that PTH has diverse effects on osteoblast differentiation depending on the exposure time in vitro mediated through different signal transduction systems. These in vitro findings explain at least in part the in vivo action of PTH that varies with the mode of administration.

Expression of p27Kip1 in osteoblast-like cells during differentiation with parathyroid hormone

PTH is a major systemic regulator of bone metabolism and plays an important role in both bone formation and resorption. PTH either inhibits or stimulates osteoblastic cell proliferation depending on the model that is studied. We analyzed the cell cycle of the UMR-106 cell line, a relatively differentiated osteoblastic osteogenic sarcoma line in which PTH is known to inhibit proliferation but the mechanism of action is unknown. PTH decreased the proportion of cells in S phase and increased the number of G1 phase cells. We examined the effect of PTH on the regulators of the G1 phase cyclin-dependent kinases and found that PTH increased p27Kip1, but not p21Cip1, levels. This effect was mimicked by 8-bromo-cAMP, but not by phorbol 12-myristate 13-acetate. The protein kinase A inhibitor KT5720 abolished the effect of PTH on the increase in p27Kip1 expression. PTH increased CDK2-associated p27Kip1 without affecting the levels of CDK2. CDK2 activity was down-regulated by both PTH and 8-bromo-cAMP treatment. These data suggest that PTH blocks entry of cells into S phase and inhibits cell proliferation as the consequence of an increase in p27Kip1, which is mediated through the protein kinase A pathway. The inhibition of G1 cyclin-dependent kinases by p27Kip1 could cause a reduction of phosphorylation of key substrates and inactivation of transcription factors essential for entry into S phase. The inhibition of cell cycle progression through PKA-mediated p27Kip1 induction might play an important role in PTH-induced differentiation of osteoblasts.