Increased bone formation by intermittent parathyroid hormone administration is due to the stimulation of proliferation and differentiation of osteoprogenitor cells in bone marrow

Regional responsiveness of the tibia to intermittent administration of parathyroid hormone as affected by skeletal unloading

To determine whether the acute inhibition of bone formation and deficit in bone mineral induced by skeletal unloading can be prevented, we studied the effects of intermittent parathyroid hormone (PTH) administration (8 micrograms/100 g/day) on growing rats submitted to 8 days of skeletal unloading. Loss of weight bearing decreased periosteal bone formation by 34 and 51% at the tibiofibular junction and tibial midshaft, respectively, and reduced the normal gain in tibial mass by 35%. Treatment with PTH of normally loaded and unloaded animals increased mRNA for osteocalcin (+58 and +148%, respectively), cancellous bone volume in the proximal tibia (+41 and +42%, respectively), and bone formation at the tibiofibular junction (+27 and +27%, respectively). Formation was also stimulated at the midshaft in unloaded (+47%, p < 0.05), but not loaded animals (-3%, NS). Although cancellous bone volume was preserved in PTH-treated, unloaded animals, PTH did not restore periosteal bone formation to normal nor prevent the deficit in overall tibial mass induced by unloading. We conclude that the effects of PTH on bone formation are region specific and load dependent. PTH can prevent the decrease in cancellous bone volume and reduce the decrement in cortical bone formation induced by loss of weight bearing.

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.

Systemic administration of an anabolic dose of PGE2 in young rats increases the osteogenic capacity of bone marrow

Prostaglandin E2 (PGE2) possesses significant anabolic properties when administered systemically (i.e., it increases bone formation and, consequently, bone mass). We recently characterized the effects of a 3 week administration of 6 mg/kg PGE2 into young rats and showed it increases cortical and cancellous bone mass and mechanical strength in long bones and bone density in the calvaria. We also found that a single dose of PGE2 induces the expression of early-response genes (c-fos, c-jun, and egr-1) in bone marrow cells within these two types of bone. These observations, together with findings by others of new cancellous bone formation in PGE2-treated animals, suggested that recruitment of osteoblasts from their precursors is a major mechanism of the anabolic effect of PGE2. To test this hypothesis directly, we injected PGE2 (6 mg/kg) or vehicle into 4-week-old rats for 2 weeks and then assessed the osteogenic potential of bone marrow in an ex vivo culture system. Primary and first-passage bone marrow cultures were established in the presence of beta-glycerophosphate, ascorbate, and dexamethasone, and osteogenic differentiation was measured by bone nodule formation and alkaline phosphatase activity. This regimen increased bone mass expressed as femoral ash weight by 4.7% and tibial cancellous bone area by 38.3%. Nodule formation at 21 days was increased in both primary and first-passage cultures from PGE2-treated rats despite seeding of the same number of marrow cells. Alkaline phosphatase activity was elevated in both primary and first-passage cultures from PGE2-treated rats beginning 6-10 days after culture initiation. Cell proliferation was only slightly elevated in cultures from PGE2-treated rats. These data strongly suggest that in vivo administration of PGE2 induces the proliferation or differentiation of osteoprogenitor cells in bone marrow, and this effect takes a major part in its anabolic effect in vivo.

Anabolic effects of estrogen and parathyroid hormone on skeletal tissues: the use of creatine kinase B activity as a response marker

The rapid stimulation of the specific activity of the brain type isozyme of creatine kinase (CK BB) is an almost universal marker of cell stimulation. We have studied its stimulation in skeletal-derived cells and shown that the increase in its activity is closely correlated with the biochemical parameter of cell proliferation - [(3)thymidine incorporation into DNA - and with the morphological parameters of bone growth, increase in thickness of cortical bone and of the number of cells in the proliferating zone of the epiphyseal growth plate. We have used the increase in CK activity to demonstrate sex specific stimulation of diaphyseal bone, exclusively by estrogens in females and by androgens in males, and the dependence of sex steroid stimulation on an adequate level of vitamin D. After finding that parathyroid hormone can act as a mitogen via a phospholipase-C-phosphoinositide turnover pathway, we collaborated with colleagues at the GBF in Braunschweig to find that mid-region fragments of PTH could act exclusively as mitogens, without stimulating cAMP production leading to bone resorption. hPTH (28-48) variants designed to be resistant to proteolysis were efficient in stimulating CK specific activity in vitro and in vivo and increased cortical bone thickness and the number of proliferating epiphyseal cartilage cells in rat long bones. These results are put into an historical context and compared with recent studies, in this short, selective review.

Establishment and characterization of conditionally immortalized stromal cell lines from a temperature-sensitive T-Ag transgenic mouse

We established bone marrow stromal cell lines from a transgenic mouse that harbors a temperature-sensitive mutant of the simian virus 40-derived large T-antigen under the control of a major histocompatibility complex (MHC) I promotor. These cell lines were screened for their ability to induce the formation of osteoclasts in a spleen cell/stromal cell coculture system. By means of this screen, five clones, referred to as marine bone marrow stromal clone 1 (mBMS-B1) mBMS-B2, mBMS-B14, mBMS-B18, and mBMS-B21, were selected for detailed characterization. Cell growth depends on culture conditions, i.e., cells grow at 33 degrees C in the presence of murine interferon-gamma, whereas cell proliferation ceases at 39 degrees C. The phenotype of the cells is also correlated with the culture conditions because the osteoclast inductive capacity is only seen at 39 degrees C, indicating that the cells undergo differentiation when the transforming agent is inactivated. These conditionally immortalized stromal cells can be induced to express a variety of markers that are typical for mature osteoblasts, e.g., alkaline phosphatase activity and expression of functional parathyroid hormone receptor after stimulation with soluble osteogenic protein 1 (sOP-1). mRNA analysis revealed the expression and regulation of osteopontin, osteonectin, and collagen alpha 1(I) as well as the inducibility of osteocalcin upon treatment with sOP-1. The cells have the potential to form mineralized nodules in supplemented medium. We observed expression of vascular cell adhesion molecule-1, which is stimulated upon treatment of the cells with 1 alpha,25-dihydrocholecalciferol after 4 days, indicating the presence of the receptor for this steroid. These cell lines represent a model to study mechanisms and factors involved in osteoblast differentiation.

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

The effects of parathyroid hormone (1-34) (PTH (1-34) on osteoblast differentiation were investigated using primary osteoblast-like cells isolated from newborn mouse calvaria. The osteoblast-like cells cultured at low cell densities, in which the cells remained in a subconfluent state at the end of culture, were exposed for 7 days to PTH. This stimulated alkaline phosphatase (ALP) activity in a dose-dependent manner. In contrast, PTH dose-dependently inhibited both ALP activity and osteocalcin production in cells inoculated at high cell densities, in which they had reached a confluent state before the end of culture. The changes of ALP activity by PTH were accompanied with the expression of ALP messenger RNA. PTH induced no changes of the hydroxyproline content in the cell layer when the cells were exposed to the hormone at a subconfluent state, but reduced the content at a postconfluent state. The stimulation of ALP activity by PTH at a preconfluent state was retained even after the removal of PTH from the culture media. The opposite effect of PTH, observed between the preconfluent and the postconfluent state, was reproduced by adding dibutyryl cyclic adenosine monophosphate (cAMP) or forskolin, but not by adding phorbol myristate acetate. In a colony-forming unit fibroblastic (CFU-F) assay, using bone marrow cells isolated from tibiae of 10-week-old mice, PTH induced no changes in the total number of CFU-Fs, but increased the proportion of ALP-positive colonies. These results indicate that PTH exerts opposite effects on the phenotypic expression of osteoblasts, depending on their differentiation stages of osteoblasts. PTH may preferentially stimulate osteoblast differentiation in immature osteoblasts but inhibit it in more mature cells.

An immunohistochemical investigation of the expression of parathyroid hormone receptors in rat cementoblasts

Cementoblasts share many of the features of the osteoblast phenotype. To investigate their expression of cell surface receptors for parathyroid hormone (PTH) (i) unlabelled PTH was bound to tissue sections and subsequently detected with anti-PTH monoclonal antibodies; and (ii) digoxigenin (DIG)-labelled PTH was applied to the sections and the bound hormone detected with anti-DIG antibodies. The use of non-radioactive DIG-labelled PTH represents a novel approach for the immunodetection of PTH receptors in situ. The expression of PTH binding sites by cementoblasts of cellular, but not acellular, cementum was demonstrated. The immunoreactivity was weaker than that seen in osteoblasts, and mainly confined to cementoblasts of fully formed roots. These results suggest that cementoblasts of functional erupted teeth may be responsive to PTH stimulation and further support the idea that cementoblasts and osteoblasts share a similar phenotype.

Perspectives: a proposed general model of the "mechanostat" (suggestions from a new skeletal-biologic paradigm)

This provisional general model for the skeleton's mechanostat spans the biologic "distance" between the organ and macromolecule. It could apply to bone, cartilage and fibrous tissue, and to bones, joints, ligaments and other organs made wholly or in part from the basic tissues. It suggests where small things such as a cytokine effect on some cell should fit in the overall scheme of skeletal physiology. It proposes that interlocking negative feedback loops provide mechanical-usage-dedicated message traffic routes on which nonmechanical agents could act to optimize or impair postnatal skeletal adaptations to varied mechanical and nonmechanical challenges, and treatments of disease too. It suggests that future research must try to understand the mechanostat's cell- and molecular-biologic roots.

Maintaining bone mass by bisphosphonate incadronate disodium (YM175) sequential treatment after discontinuation of intermittent human parathyroid hormone (1-34) administration in ovariectomized rats

Intermittent treatment with human parathyroid hormone (1-34) [hPTH(1-34)] stimulates bone formation and increases cancellous bone mass in ovariectomized (OVX) rats. But PTH-induced cancellous bone rapidly disappears upon cessation of treatment. The fate of cortical bone treated by PTH has not been well characterized. Incadronate disodium (disodium cycloheptylaminomethylenedisphosphonate monohydrate, YM175) was expected to be antiresorptive without inhibiting bone formation. The purposes of this study were to determine (1) whether PTH treatment increases new cancellous and cortical bone mass and bone formation, (2) whether the new bone could be maintained by YM175 sequential treatment, and (3) whether the maintenance effect is persistent after YM175 withdrawal. Eighty-eight 11-week-old Sprague-Dawley rats were divided into sham operation and OVX groups. The OVX rats were treated for 8 weeks with the subcutaneous intermittent injection of 30 micrograms/kg of hPTH(1-34) three times a week beginning 4 weeks after surgery, then PTH treatment was withdrawn and YM175 (10 micrograms/kg) was injected subcutaneously three times a week for 4 weeks. YM175 treatment was withdrawn for the last 8 weeks of the protocol. The results of microstructural assessment in proximal tibial metaphysis and bone mineral density in distal and proximal femur demonstrated that PTH treatment for 8 weeks restored bone mass to the sham control level. However, after cessation of PTH treatment, the PTH-induced tibial cancellous bone mass showed a decrease at 4 weeks and almost totally disappeared after 12 weeks. Conversely, YM175 treatment maintained the PTH-induced tibial cancellous bone mass, and the bone continued to be maintained after 8 weeks of withdrawal of the YM175. Cortical bone was not lost during PTH treatment. YM175 maintained the PTH-induced new tibial cancellous bone in OVX rats by suppressing remodeling.

Stimulation of the growth of femoral trabecular bone in ovariectomized rats by the novel parathyroid hormone fragment, hPTH-(1-31)NH2 (Ostabolin)

The human parathyroid hormone, hPTH-(1-84), and its hPTH-(1-34) fragment are promising anabolic agents for treating osteoporosis because they can strongly stimulate the production of biomechanically effective cortical and trabecular bone in osteopenic ovariectomized (OVX) rats and trabecular bone in osteoporotic postmenopausal humans. The ideal PTH fragment for treating osteoporosis would be the smallest and functionally simplest fragment that activates only one signal mechanism and still strongly stimulates trabecular bone growth. A new PTH fragment, hPTH-(1-31)NH2, which only stimulates adenylyl cyclase instead of stimulating both adenylyl cyclase and phospholipase-C as do hPTH-(1-84) and hPTH-(1-34), is this minimum, high-potency anabolic fragment. hPTH-(1-31)NH2 (which we have named Ostabolin) can greatly thicken trabeculae and increase the dry weight and calcium content of trabecular bone in the distal femurs of osteopenic, young, sexually mature OVX Sprague-Dawley rats when injected subcutaneously each day for 6 weeks at doses between 0.4 and 1.6 nmole/100 g of body weight.

Effects of parathyroid hormone and agonists of the adenylyl cyclase and protein kinase C pathways on bone cell proliferation

The anabolic effect of parathyroid hormone (PTH) on bone is partly due to a stimulation of osteoblast proliferation. The PTH signal is transduced by the pathways of adenylyl cyclase (AC)/protein kinase (PK) A and phospholipase C/PKC/Ca++. There is still uncertainty about the relative contribution of the two pathways to the proliferative effects of the hormone. In our study, PTH(1-34), AC/PKA agonists, and phorbol 12-myristate-13-acetate (PMA, a PKC activator) stimulated cell proliferation in cultured mouse calvariae. In isolated osteoblasts, only PMA stimulated proliferation, whereas AC/PKA agonists and PTH(1-34) inhibited it. As already known, PTH in the presence of supramaximal concentrations of transforming growth factor-beta (TGF-beta) stimulated osteoblast growth; under these same conditions, AC/PKA agonists reproduced the stimulatory effect of PTH(1-34), whereas PMA became inhibitory. PTH(1-31), which stimulates AC without affecting PKC, acted similarly to the fully active PTH(1-34) in both calvaria and isolated osteoblasts. On the contrary, midregion fragments that activate only PKC stimulated calvaria cell proliferation faintly in comparison with PTH(1-34); no effect was seen in osteoblasts, either with or without TGF-beta. Our study shows that the effects of PTH on proliferation can be mimicked by agonists of the AC/cAMP pathway. Although PMA is indeed able to stimulate cell growth in tissue explants, its effects on isolated osteoblasts markedly diverge from those of PTH. We conclude that activation of the AC/PKA pathway is the main component of the proliferative effects of PTH.

Mechanotransduction and the functional response of bone to mechanical strain

Mechanotransduction plays a crucial role in the physiology of many tissues including bone. Mechanical loading can inhibit bone resorption and increase bone formation in vivo. In bone, the process of mechanotransduction can be divided into four distinct steps: (1) mechanocoupling, (2) biochemical coupling, (3) transmission of signal, and (4) effector cell response. In mechanocoupling, mechanical loads in vivo cause deformations in bone that stretch bone cells within and lining the bone matrix and create fluid movement within the canaliculae of bone. Dynamic loading, which is associated with extracellular fluid flow and the creation of streaming potentials within bone, is most effective for stimulating new bone formation in vivo. Bone cells in vitro are stimulated to produce second messengers when exposed to fluid flow or mechanical stretch. In biochemical coupling, the possible mechanisms for the coupling of cell-level mechanical signals into intracellular biochemical signals include force transduction through the integrin-cytoskeleton-nuclear matrix structure, stretch-activated cation channels within the cell membrane, G protein-dependent pathways, and linkage between the cytoskeleton and the phospholipase C or phospholipase A pathways. The tight interaction of each of these pathways would suggest that the entire cell is a mechanosensor and there are many different pathways available for the transduction of a mechanical signal. In the transmission of signal, osteoblasts, osteocytes, and bone lining cells may act as sensors of mechanical signals and may communicate the signal through cell processes connected by gap junctions. These cells also produce paracrine factors that may signal osteoprogenitors to differentiate into osteoblasts and attach to the bone surface. Insulin-like growth factors and prostaglandins are possible candidates for intermediaries in signal transduction. In the effector cell response, the effects of mechanical loading are dependent upon the magnitude, duration, and rate of the applied load. Longer duration, lower amplitude loading has the same effect on bone formation as loads with short duration and high amplitude. Loading must be cyclic to stimulate new bone formation. Aging greatly reduces the osteogenic effects of mechanical loading in vivo. Also, some hormones may interact with local mechanical signals to change the sensitivity of the sensor or effector cells to mechanical load.

A histomorphometric study on effects of single and concurrent intermittent administration of human PTH (1-34) and bisphosphonate cimadronate on tibial metaphysis in ovariectomized rats

This study compared the single administration of hPTH(1-34), bisphosphonate cimadronate (YM-175), and concurrent therapy of these two for restoration of lost bone mass in ovariectomized (OVX) rats. Animals were untreated for 4 weeks after surgery, and then injected s.c. with vehicle (OVX+V), hPTH(1-34) (30 micrograms/kg) (OVX+P), YM-175 (5 micrograms/kg) (OVX+Y), or a combination of these two (OVX+P+Y), 3 days a week, for 8 weeks, and sacrificed. Their proximal tibia were processed undecalcified for quantitative bone histomorphometry. Although OVX+Y showed a reduction of bone turnover compared to OVX+V, it failed to restore lost bone mass in OVX rats. In contrast, OVX+P exhibited a stimulation of bone formation and restored cancellous osteopenia due to OVX. OVX+P+Y also resulted a recovery of osteopenia, however, stimulation of bone formation by PTH was suppressed by YM-175.

The anabolic effect of human PTH (1-34) on bone formation is blunted when bone resorption is inhibited by the bisphosphonate tiludronate--is activated resorption a prerequisite for the in vivo effect of PTH on formation in a remodeling system?

Parathyroid hormone (PTH) and its (1-34) fragment are stimulators of bone turnover that have an anabolic effect increasing trabecular bone mass when administered intermittently by daily subcutaneous injections. Its clinical use in osteoporosis, however, has been limited by the concomitant increased bone resorption and deleterious effect on cortical bone. To evaluate if a treatment combining PTH and a potent inhibitor of bone resorption would retain the anabolic effect of PTH without increasing bone resorption, we analyzed the effects of PTH (1-34) (500 IU/d) with or without the bisphosphonate tiludronate (1 mg/kg per day) for 3 months on biochemical and histological indices of bone turnover in old female sheep, an animal model which has a slow bone remodeling activity that resembles the one of elderly women. As expected, PTH (1-34) induced a significant increase of urinary pyridinoline and hydroxyproline (reflecting bone resorption), and of serum osteocalcin and alkaline phosphatase (reflecting bone formation), that were consistent with an increase of resorption and tetracycline-based formation of bone measured on iliac crest biopsy. In contrast, all biochemical and histological indices of bone turnover were decreased in sheep receiving tiludronate, a potent inhibitor of bone resorption. Surprisingly, in the combined therapy group, biochemical and histological indices of both resorption and formation did not differ from the control groups. Thus, the model of old sheep, which closely resembles the situation in old human, shows that the anabolic effect of PTH on bone is not maintained when PTH is coadministered with a bisphosphonate, in marked contrast to results noted in the growing rat.(ABSTRACT TRUNCATED AT 250 WORDS)