Direct effect of parathyroid hormone on the proliferation of osteoblast-like cells; a possible involvement of cyclic AMP

Effects of tenascin-W on osteoblasts in vitro

Tenascin-W is a glycoprotein secreted into the extracellular matrix of developing bones. Here, we have examined possible roles for tenascin-W in osteogenesis. Purified recombinant tenascin-W, like tenascin-C, increases the number of mineralized foci in primary cultures of avian osteoblasts and increases alkaline phosphatase activity in vitro. In addition, tenascin-W in solution promotes the migration of primary osteoblasts across fibronectin-coated filters. The sixth fibronectin type III domain of chicken tenascin-W contains a phylogenetically conserved KGD motif that is predicted to be available to integrin binding. To determine whether this motif is potentially functional, we have cultured osteoblasts on KGD-containing peptides and control peptides. Osteoblasts cultured on peptides with the KGD motif acquire a multipolar phenotype with pseudopods tipped with actin-rich ruffles, which is similar to the morphology of osteoblasts cultured on recombinant tenascin-W. Moreover, the KGD peptides, but not the control peptides, promote proliferation in cultured osteoblasts but not alkaline phosphatase activity or migration. Finally, explanted embryonic frontal bones are significantly thicker when cultured in the presence of tenascin-W, suggesting that tenascin-W can accelerate the formation of new bone in a complex multicellular environment.

Avian tenascin-W: expression in smooth muscle and bone, and effects on calvarial cell spreading and adhesion in vitro

Tenascins are glycoproteins found primarily in the embryonic extracellular matrix. Here we have characterized the fourth and final member of the tenascin family in birds: tenascin-W. Avian tenascin-W has 3.5 epidermal growth factor-like repeats, 6 fibronectin type III domains, and a C-terminal fibrinogen-related domain. Immunohistochemistry reveals that avian tenascin-W is expressed transiently in developing smooth muscle, tendons, and ligaments, but the primary site of tenascin-W expression during development is in the extracellular matrix of bone and the cellular periosteum. In bony matrix, tenascin-W-coated fibrils partly overlap with fibrils that contain tenascin-C. The anti-tenascin-W also labels fibrils in cultures of osteogenic embryonic chicken calvarial cells. Primary calvarial cells cultured on purified tenascin-W become rounded, and fewer of these cells spread on fibronectin when tenascin-W is added to the medium when compared with calvarial cells cultured on fibronectin alone. Moreover, tenascin-W reduces the adhesion of calvarial cells to collagen type I in a shear force assay. We conclude that tenascin-W is likely to play a phylogenetically conserved role in developing bone and that it shares some of the basic anti-adhesive and matrix modulatory properties as tenascin-C.

PTH2 receptor-mediated inhibitory effect of parathyroid hormone and TIP39 on cell proliferation

The parathyroid hormone (PTH) has dual mitogenic and inhibitory effects on cell proliferation, depending on the cell type and experimental conditions. PTH can signal via two different receptors, both positively coupled to the adenylyl cyclase/cyclic AMP pathway which can mimic some of the proliferative effects of PTH. We evaluated the role of the type-2 PTH (PTH2) receptor on cell proliferation in clonal human embryonic kidney HEK293 cells stably expressing the human PTH2 receptor. Using a cyclic AMP-responsive gene-reporter, we confirmed that the tuberoinfundibular peptide (TIP39) and various human (h) PTH fragments including hPTH-(1-34) were potent agonists (EC(50) in the range of 0.01-0.3 nM) whereas the bovine (b) PTH peptides b(Tyr(34))PTH-(7-34) and its tryptophan derivative b[D-Trp(12),Tyr(34)]PTH-(7-34) behaved as antagonists (IC(50)=117 and 249 nM, respectively). hPTH-(1-34) produced a dose-dependent inhibition of cell proliferation (EC(50)=8.5+/-0.4 nM) after 3 days and this effect was fully reversed by the tryptophan derivative antagonist. The same effect was observed with TIP39 which caused a 30% maximal inhibition. These findings reveal that PTH2 receptor activation can inhibit cell proliferation and might explain the dual functionality of PTH on cell proliferation.

Expression of serotonin receptors in bone

The 5-hydroxytryptamine (5-HT) receptors 5-HT(2A), 5-HT(2B), and 5-HT(2C) belong to a subfamily of serotonin receptors. Amino acid and mRNA sequences of these receptors have been published for several species including man. The 5-HT(2) receptors have been reported to act on nervous, muscle, and endothelial tissues. Here we report the presence of 5-HT(2B) receptor in fetal chicken bone cells. 5-HT(2B) receptor mRNA expression was demonstrated in osteocytes, osteoblasts, and periosteal fibroblasts, a population containing osteoblast precursor cells. Pharmacological studies using several agonists and antagonists showed that occupancy of the 5-HT(2B) receptor stimulates the proliferation of periosteal fibroblasts. Activity of the 5-HT(2A) receptor could however not be excluded. mRNA for both receptors was shown to be equally present in adult mouse osteoblasts. Osteocytes, which showed the highest expression of 5-HT(2B) receptor mRNA in chicken, and to a lesser extent osteoblasts, are considered to be mechanosensor cells involved in the adaptation of bone to its mechanical usage. Nitric oxide is one of the signaling molecules that is released upon mechanical stimulation of osteocytes and osteoblasts. The serotonin analog alpha-methyl-5-HT, which preferentially binds to 5-HT(2) receptors, decreased nitric oxide release by mechanically stimulated mouse osteoblasts. These results demonstrate that serotonin is involved in bone metabolism and its mechanoregulation.

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.

Regulation of connexin43 expression and function by prostaglandin E2 (PGE2) and parathyroid hormone (PTH) in osteoblastic cells

Connexin43 (Cx43) forms gap junctions that mediate intercellular communication between osteoblasts. We have examined the effects of prostaglandin E2 (PGE2) and parathyroid hormone (PTH) on gap junctional communication in the rat osteogenic sarcoma cells UMR 106-01. Incubation with either PGE2 or PTH rapidly (within 30 min) increased transfer of negatively charged dyes between UMR 106-01 cells. This stimulatory effect lasted for at least 4 h. Both PGE2 and PTH increased steady-state levels of Cx43 mRNA, but only after 2-4 h of incubation. Transfection with a Cx43 gene construct linked to luciferase showed that this effect of PTH was the result of transcriptional upregulation of Cx43 promoter. Stimulation of dye coupling and Cx43 gene transcription were reproduced by forskolin and 8Br-cAMP. Exposure to PGE2 for 30 min increased Cx43 abundance at appositional membranes in UMR 106-01, whereas total Cx43 protein levels increased only after 4-6 h of incubation with either PGE2 or PTH. Inhibition of protein synthesis by cycloheximide did not affect this early stimulation of dye coupling, but it significantly inhibited the sustained effect of PTH and forskolin on cell coupling. In summary, both PTH and PGE2, presumably through cAMP production, enhance gap junctional communication in osteoblastic cell cultures via two mechanisms: initial rapid redistribution of Cx43 to the cell membrane, and later stimulation of Cx43 gene expression. Modulation of intercellular communication represents a novel mechanism by which osteotropic factors regulate the activity of bone forming cells.

The mitogenic effect of parathyroid hormone is associated with E2F-dependent activation of cyclin-dependent kinase 1 (cdc2) in osteoblast precursors

Injections of parathyroid hormone (PTH) have been reported to stimulate skeletal accretion through increased bone formation in several species, and osteoblast proliferation is a critical component of bone formation. However, the biological mechanisms of PTH-stimulated bone cell proliferation are largely unknown. In this study, we demonstrated that PTH stimulates proliferation of the osteoblast precursor cell line, TE-85, in association with increasing cdc2 protein levels and its kinase activity. cdc2 antisense oligonucleotides blocked PTH-induced DNA synthesis and cell cycle progression. Analysis of the time course of PTH-stimulated cdc2 message levels demonstrated that cdc2 mRNA levels were increased 1.5- to 4-fold between 3-18 h following release from cell synchronization. Transfections of TE-85 cells with a series of cdc2 promoter-luciferase deletion constructs revealed PTH stimulation of the cdc2 promoter. Promoter constructs containing a mutation in the E2F binding site were not stimulated by PTH. Gel mobility shift assays demonstrated increased free E2F levels in TE-85 nuclear extracts in response to PTH. Furthermore, the ratios of hyperphosphorylated to hypophosphorylated forms of Rb protein were increased by PTH treatment. These results demonstrate that PTH-stimulated cdc2 expression was associated with TE-85 cell proliferation and that the mechanism of stimulating cdc2 gene expression involved increasing the levels of free E2F.

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.

Phenotypic heterogeneity of osteoblast-like MC3T3-E1 cells: changes of bradykinin-induced prostaglandin E2 production during osteoblast maturation

We have examined clonal murine calvarial MC3T3-E1 cells obtained from different sources to compare their osteoblastic features (alkaline phosphatase [ALP], cyclic adenosine monophosphate [cAMP] response to parathyroid hormone, prostaglandin E2 (PGE2) and PGE1, bradykinin-induced production of PGE2). It was found that the sublines investigated showed large variation of the above-mentioned parameters, which may be attributed to distinct differentiated stages of osteoblast development. Increase of ALP activity was paralleled by an increase in cAMP accumulation in response to the above-mentioned agents. The most striking difference was observed with bradykinin-induced production of PGE2. Early stage cells (low ALP) produced high levels of PGE2, whereas cells with high ALP activity showed no bradykinin stimulation at all. This was consistent with the results of specific binding of 3H-bradykinin to its receptor and also correlated well with the bradykinin-induced signal transduction sequence (inositol triphosphate liberation and elevation of intracellular calcium levels). This was confirmed by Northern blot analysis of bradykinin receptor mRNA expression. These results indicate that the widely used osteoblast-like cell line MC3T3-E1 is synonymous for multiple sublines, representing different stages of osteoblast development. These sublines were most likely emerging from the early stage cell line due to the applied culture conditions. Moreover, distinct biochemical features are displayed in correlation to the differentiation stage, thus providing a useful model to study the molecular mechanism of osteoblast maturation.

An intact N terminus is required for the anabolic action of parathyroid hormone on adult female rats

Intermittent administration of parathyroid hormone (PTH) peptides increases bone density in animal and human models of osteoporosis. In vitro studies have demonstrated that PTH analogs lacking the first two amino acids can stimulate cell proliferation in certain cell systems, whereas fragments with an intact N terminus can be antimitogenic. We have tested whether the truncated PTH(3-38) fragment may be a better "anabolic analog" than PTH(1-38) by monitoring bone density and biomechanical properties of the femur in 6-month-old ovariectomized (OVX) rats. Either PTH fragment was administered subcutaneously (8 micrograms/100 g of body weight) 5 days/week, for 4 weeks, starting 1 week after surgery. During the entire study, untreated OVX rats lost 12.1 +/- 4.4% of their initial bone density. PTH(1-38) reversed the initial bone loss, leading to complete restoration of presurgery values after 4 weeks of treatment. Conversely, administration of PTH(3-38) resulted in 13.2 +/- 5.8% bone loss, while continuous estrogen infusion (10 micrograms/kg/day) prevented bone loss but did not reverse it. Sham-operated animals also experienced significant bone loss in the vehicle and PTH(3-38)-treated groups (-4.5 +/- 6.7%, and -7.6 +/- 2.8%, respectively), whereas a significant gain in bone density (+4.4 +/- 5.6%) was observed in the rats treated with PTH(1-38). A bone quality factor (index of strain energy loss) and the impact strength (resistance to fracture) were 25% and 44% lower in femurs explanted from OVX animals treated with either vehicle or PTH(3-38), compared with sham-operated animals. On the contrary, no difference was observed between OVX and control animals after treatment with PTH(1-38), indicating a preservation of the capacity to withstand mechanical stress. Thus, PTH(1-38) counteracts estrogen-dependent loss of mineral density and bone biomechanical properties and increases bone density in estrogen-replete animals. An intact N terminus sequence is necessary for this anabolic action of PTH.

Effect of activating and inactivating mutations of Gs- and Gi2-alpha protein subunits on growth and differentiation of 3T3-L1 preadipocytes

Previous investigations have demonstrated that both Gs- and the Gi-family of GTP-binding proteins are implicated in differentiation of the 3T3-L1 preadipocyte. In order to further analyze the role of Gs alpha vs. Gi2 alpha, which are both involved in adenylate cyclase modulation, we transfected undifferentiated 3T3-L1 cells with two sets of G-protein cDNA: the pZEM vector with either wild type, the activating (i.e., GTP-ase inhibiting) R201C-Gs alpha or the inactivating G226A(H21a)-Gs alpha point mutations, or the pZIPNeoSV(X) retroviral vector constructs containing the Gi2 alpha wild type or the missense mutations R179E-Gi2 alpha, Q205L-Gi2 alpha, and G204A(H21a)-Gi2 alpha. The activating [R201C]Gs alpha-mutant did not significantly affect the differentiation process, i.e., increase in the steady-state levels of G-protein subunits, gross appearance, or insulin-elicited deoxy-glucose uptake into 3T3-L1 adipocytes, despite a marked initial increase in hormone-elicited adenylate cyclase activity. The [H21a]Gs alpha-mutant, on the other hand, enhanced the degree of differentiation slightly, as evidenced by an augmented production of lipid vesicles and insulin-stimulated deoxy-glucose uptake. However, an expected increase in mRNA for hormone-sensitive lipase was not seen. Secondly, it appeared that both activating [R179E]Gi2 alpha or [Q205L]Gi2 alpha mutants reduced cell doubling time in non-confluent 3T3-L1 cell cultures, while [H21a]Gi2 alpha slowed proliferation rate. Furthermore, it seemed that cell proliferation, as evidenced by thymidine incorporation, ceased at a much earlier stage prior to cell confluency when cultures were transfected with the [R179E]Gi2 alpha or [Q205L]Gi2 alpha mutants. Upon differentiation with insulin, dexamethasone, and iBuMeXan, the following cell characteristics emerged: the [R179E]Gi2 alpha and [Q205L]Gi2 alpha mutants consistently enhanced adenylate cyclase activation and cAMP accumulation stimulated by isoproterenol and corticotropin over controls. Deoxy-glucose uptake was also super-activated by the [R179E]Gi2 alpha and [Q205L]Gi2 alpha mutants. Finally, steady-state levels of hormone sensitive lipase mRNA were dramatically increased by [R179E]Gi2 alpha and [Q205L]Gi2 alpha over differentiated controls. The inactivating [H21a]Gi2 alpha-mutant obliterated all signs of preadipocyte differentiation. It is concluded that Gi2 plays a positive and much more important role than Gs in 3T3-L1 preadipocyte differentiation. Cyclic AMP appears to play no role in this process.

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.

Cytokine regulation of bone cell differentiation

Systemic hormones and cytokines play important roles in regulating both osteoblast and osteoclast activity. These cytokines can have either positive or negative effects on the growth and differentiation of bone cells. These effects appear to be dependent on the model systems use to assess them, as well as the species tested. In the near future, other autocrine-paracrine factors will be identified that enhance osteoblast and osteoclast activity, and model systems should be available to further delineate their effects on cells in the osteoblast lineage. Use of transgenic mice with genes targeted to the osteoblast and osteoclast may further reveal the mechanisms responsible for the growth and differentiation of these cells, as well as produce immortalized cell lines that more accurately reflect the cell biology of the osteoclast and osteoblast in vivo.

Stimulation of creatine kinase activity in rat skeletal tissue in vivo and in vitro by protease-resistant variants of parathyroid hormone fragments

We have reported that mid-region fragments of human parathyroid hormone (hPTH), exemplified by hPTH-(28-48), stimulated [3H]thymidine incorporation into DNA and increased the specific activity of the brain-type isoenzyme of creatine kinase (CK) in both skeletal-derived cell cultures (ROS 17/2.8 cells) and immature rat epiphyseal cartilage and diaphyseal bone, without stimulating cyclic AMP synthesis which is a prerequisite for bone resorption. In the present study, substitution of amino acids in hPTH-(28-48), which resulted in increased resistance to proteolysis, produced variants that stimulated skeletal systems at two orders of magnitude lower concentration than the wild-type fragment. We modified hPTH-(28-48) at Leu-37 by replacement with Met, Thr or Val. Under conditions in which 20% of the native hPTH-(28-48) resisted proteolysis by cathepsin D for 6 h, approx. 40% of the L37V mutant and 70% of the L37T mutant remained intact. Substitution of Met for Phe-34 in addition to Thr for Leu-37, or the substitution of Met for Phe-34 alone, produced 100%-resistant fragments. These variants at residue 34 caused maximal stimulation of CK in ROS 17/2.8 cells at 0.24 nM compared with 24 nM for hPTH-(28-48). The double mutant stimulated CK activity significantly in immature rats, at a minimum dose of 12.5 ng/rat, and caused maximal stimulation at 125 ng/rat, a 10-fold lower dose than for hPTH-(28-48). The effect of the double mutant lasted up to 24 h which differs from the stimulation by hPTH-(28-48) in which CK specific activity returns to the control level at 24 h. This same dose also significantly stimulated CK activity in gonadectomized rats. These results show the advantage of using protease-resistant mid-region variants of hPTH-(28-48) to stimulate bone cells, in terms of lower doses and longer duration of effectiveness, both in vitro and in vivo.

Bone marrow cells are targets for the anabolic actions of prostaglandin E2 on bone: induction of a transition from nonadherent to adherent osteoblast precursors

Although prostaglandin E2 (PGE2) is known to stimulate bone formation in vivo, its mechanism of action is not well understood. Circumstantial evidence suggests that bone marrow cells (BMC) may well be involved in this, and in order to investigate this further we have studied the effect of PGE2 on proliferation and matrix synthesis in high-density BMC cultures and on colony-forming unit (CFU-f) formation efficiency by BMC in vitro. High-density cultures of BMC formed a collagenous, calcified matrix, synthesized osteocalcin and expressed alkaline phosphatase activity. The addition of PGE2 caused a concentration-dependent increase in total (but not specific) APase activity, cell number, and collagen accumulation. It was found that PGE2 need only be present during the first 48 hours of the culture period and that longer exposure had no additional effect. PGE2 also caused a concentration-dependent increase in CFU-f formation, and it was found that this was due to the recruitment of new mesenchymal precursor cells from the nonadherent fraction of the BMC. Once again, the presence of PGE2 for only the first 48 hours of the culture period was enough to precipitate a maximal response. We conclude that one mechanism for the anabolic actions of PGE2 may be the recruitment of OB precursors from a population of nonadherent mesenchymal precursor cells present in the bone marrow.

Second messenger signaling of c-fos gene induction by parathyroid hormone (PTH) and PTH-related peptide in osteoblastic osteosarcoma cells: its role in osteoblast proliferation and osteoclast-like cell formation

The present study was performed to clarify second messenger signaling in parathyroid hormone (PTH)-induced c-fos gene expression, to characterize the participation of the c-fos gene in the regulation of osteoblast proliferation and function as well as osteoclast-like cell formation by PTH and to compare these effects of PTH with those of PTH-related peptide (PTHrP). Both human (h) PTH-(1-34) and hPTHrP-(1-34) at 10(-8) M induced a transient c-fos gene expression to a similar degree in osteoblastic osteosarcoma cells, UMR-106. N6,O2'-dibutyryl adenosine 3',5'-cyclic monophosphate (dbcAMP) as well as Sp-diastereoisomer of adenosine cyclic 3',5'-phosphorothioate (Sp-cAMPS), an activator of cAMP-dependent protein kinase (PKA), induced a weak c-fos gene expression. Although Rp-diastereoisomer of adenosine cyclic 3',5'-phosphorothioate (Rp-cAMPS), an inhibitor of PKA, almost completely antagonized dbCAMP- and Sp-cAMPS-induced expression of c-fos gene, it did not cause an obvious inhibition of PTH- or PTHrP-induced expression. Phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), induced an intense expression of the c-fos gene, while 4 alpha-phorbol 12,13-didecanoate (4 alpha PDD), incapable of activating PKC, and calcium ionophores (A23187 and ionomycin) did not. Protein kinase C inhibitor (H-7, 50 microM) completely blocked the expression of the c-fos gene by PTH as well as by PTHrP). Antisense oligodeoxynucleotides (as-ODN) complementary to c-fos mRNA, which have been shown to inhibit its mRNA translation, at 1 microM significantly antagonized PTH- and PTHrP-induced inhibition of [3H] thymidine incorporation and stimulation of osteoclast-like cell formation in the presence of osteoblasts, but not an increase in alkaline phosphatase activity, compared to control oligodeoxynucleotides with same nucleotides as as-ODN but with a random sequence. The present study indicates the involvement of PKC system in c-fos gene expression by PTH as well as PTHrP and also indicates the involvement of the c-fos gene in the regulation of bone cell physiology by PTH and PTHrP.

Time-dependent effects of parathyroid hormone and prostaglandin E2 on DNA synthesis by periosteal cells from embryonic chick calvaria

Both PGE2 and PTH (1-34) caused a time- and concentration-dependent stimulation of proliferation by embryonic chick periosteal cells. Cells were exposed to the agents for different periods of time, the medium was replaced with fresh medium, and 3H-TdR incorporation was measured after 16 hours. Challenge with 10(-6) M prostaglandin E2 (PGE2) or 10(-7) M parathyroid hormone (1-34) (PTH) for 5 minutes produced 4- and 5.5-fold increases in 3H-TdR incorporation, respectively. Longer exposures, however, produced diminishing responses and after 45 minutes, only minimal effects or slight inhibitions were seen. These time-dependent effects were also seen with forskolin and dibutyryl-cAMP; TPA on the other hand stimulated DNA synthesis after both short- and long-term exposure. Both PGE2 and PTH (1-34) stimulated cAMP synthesis in periosteal cells but neither could be shown to stimulate protein kinase-C (PKC) at concentrations required for stimulation of proliferation, and dibutyryl-cyclic AMP (cAMP) effectively inhibited endogenous PKC activity. It is possible that the stimulation of proliferation by short-term exposure to PGE2 and PTH (1-34) is mediated by cAMP and that the time dependency possibly stems from the inhibition of endogenous PKC activity by increased intracellular cAMP levels.

Parathyroid hormone fragments may stimulate bone growth in ovariectomized rats by activating adenylyl cyclase

PTH is regarded conventionally as a catabolic hormone that stimulates osteoclastic resorption of bone. However, it has been known since 1932 that intermittent pulses of PTH stimulate bone formation in animals and humans. PTH independently activates two signal mechanisms: one that stimulates adenylyl cyclase and one that stimulates protein kinase C (PKC). The goal of this study was to use the 3- to 5-month-old ovariectomized (OVX) rat model to determine which of the two signal mechanisms is responsible for the anabolic action of PTH on bone. OVX triggered a large loss of trabecular bone without significantly affecting the normal slow growth of cortical bone in the distal halves of the femora. Daily injections of human hPTH(1-34) fragment (1 nmol/100 g body weight), which stimulated both adenylyl cyclase and membrane-associated PKC activity in osteoblast-like ROS 17/2 rat osteosarcoma cells, stimulated the growth of both cortical and trabecular bone in the OVX rats. Daily injections of the same dose of hPTH(1-31), which stimulated adenylyl cyclase but not PKC in ROS 17/2 cells, stimulated trabecular bone growth in the OVX rats less effectively than hPTH(1-34), but it stimulated cortical bone growth as rapidly and as dramatically as hPTH(1-34). Injections of equimolar amounts of desamino-hPTH(1-34) [N-propionyl(2-3)hPTH-amide], which stimulated PKC as strongly as hPTH(1-34) in ROS 17/2 cells but had a drastically reduced ability to stimulate adenylyl cyclase, or injections of recombinant hPTH(8-84) which stimulated PKC only in the ROS 17/2 cells, did not stimulate cortical or trabecular bone growth in the OVX animals. Thus, cyclic AMP and cyclic AMP-dependent protein kinases may be the primary mediators of the anabolic action of intermittent pulses of PTH on bone in OVX rats.

Stimulation of signal transduction pathways in osteoblasts by mechanical strain potentiated by parathyroid hormone

Second-messenger systems have been implicated to transmit mechanical stimulation into cellular signals; however, there is no information on how mechanical stimulation is affected by such systemic factors as parathyroid hormone (PTH). Regulation of adenylyl cyclase and phosphatidylinositol pathways in rat dentoalveolar bone cells by mechanical strain and PTH was investigated. Two different cell populations were isolated after sequential enzyme digestions from dentoalveolar bone (group I and group II) to study potential differences in response. Mechanical strain was applied with 20 kPa of vacuum intermittently at 0.05 Hz for periods of 0.5, 1, 5, 10, and 30 minutes and 1, 3, and 7 days using the Flexercell system. Levels of cAMP, measured by RIA, and levels of inositol 1,4,5-triphosphate (IP3) and protein kinase C activity (PKC), measured by assay systems, increased with mechanical strain. When PTH was added to the cells, there was a significant increase in levels of all the intracellular signals, which appeared to potentiate the response to mechanical strain. IP3 levels (0.5 minute) peaked before those of PKC activity (5 minutes), which in turn peaked before those of cAMP (10 minutes). Group II cells showed higher levels of cAMP and IP3 than the group I cells. This suggests that the former may ultimately play the predominant roles in skeletal remodeling in response to strain. Immunolocalization of the cytoskeleton proteins vimentin and alpha-actinin, focal contact protein vinculin, and PKC showed a marked difference between strained and nonstrained cells. However, the addition of PTH did not cause any significant effect in cytoskeleton reorganization. Staining of PKC and vimentin, alpha-actinin, and vinculin suggests that PKC participates actively in the transduction of mechanical signals to the cell through focal adhesions and the cytoskeleton, although only PKC seemed to change with short time periods of strain. In conclusion, dentoalveolar osteoblasts responded to mechanical strain initially through increases in levels of IP3, PKC activity, and later cAMP, and this response was potentiated when PTH was applied together with mechanical strain.

In vitro effects of growth hormone and other hormones on chondrocytes and osteoblast-like cells

The influence of growth hormone (GH), insulin-like growth factor I (IGF-I), parathyroid hormone(1-34) (PTH(1-34)), 1,25-dihydroxycholecalciferol (1,25(OH)2D3) and 17 beta-oestradiol on proliferation and on production of cytokines, such as interleukin-1 beta (IL-1 beta), IL-6, IL-8 and transforming growth factor-beta (TGF-beta), was studied in chondrocytes obtained from the growing cartilage of the iliac crest and in the osteoblast-like cell clone SaOS-2. GH and IGF-I were mitogenic for chondrocytes and SaOS-2 cells, as indicated by the dose-related increase in uptake of [3H]thymidine. PTH(1-34) was also mitogenic, while 1,25(OH)2D3 inhibited the proliferation of both chondrocytes and SaOS-2 cells in a dose-dependent manner. 17 beta-oestradiol was stimulatory in SaOS-2 cells, but gave a biphasic pattern in chondrocytes; it was stimulatory at low concentrations (0.1 nmol/l) and inhibitory at supraphysiological doses (10 nmol/l). Using the cDNA polymerase chain reaction, specific mRNAs for IL-1 beta, IL-6, IL-8 and TGF-beta were found in chondrocytes, while SaOS-2 cells had a positive signal only for TGF-beta. Specific enzyme immunoassays revealed detectable levels of IL-1 beta, IL-6 and IL-8 only in chondrocytes. IL-6 was increased by GH and IGF-I, and lowered by 1,25(OH)2D3 and supraphysiological doses of 17 beta-oestradiol, while PTH(1-34) had no effects. IL-8 was not influenced by GH or IGF-I, was slightly but not significantly increased by PTH(1-34) and was reduced by 1,25(OH)2D3 and 17 beta-oestradiol at supraphysiological doses.(ABSTRACT TRUNCATED AT 250 WORDS)

Cross talk of dual-signal transduction systems in the regulation of DNA synthesis by parathyroid hormone in osteoblastic osteosarcoma cells

There has been recent evidence that calcium/protein kinase C (Ca/PKC) messenger system as well as adenylate cyclase are involved in the signal transduction stimulated by PTH. We therefore examined the role of these dual-signal transduction systems and the interaction of these systems in the regulation of DNA synthesis by PTH in the osteoblastic osteosarcoma cells, UMR-106. As recently reported, 10(-4) M Sp-cAMPS, a direct activator of cAMP-dependent protein kinase (PKA), and 10(-4) M dibutyryl-cAMP, as well as hPTH-(1-34), caused the significant inhibition of [3H]thymidine incorporation (TdR). Both A23187 and ionomycin (10(-8)-10(-6) M) inhibited TdR in a dose-dependent manner, with a minimal effective dose at 10(-7) M. Although 10(-6) M phorbol 12-myristate 13-acetate (PMA) caused slight but significant stimulation of TdR by itself, it augmented not only dibutyryl-cAMP- but also Sp-cAMPS-induced inhibition of TdR. On the other hand, 4 alpha-phorbol 12,13-didecanoate, incapable of activating PKC, failed to augment these cAMP analogs-induced effects. Pretreatment with 50 microM H-7, an inhibitor of PKC, not only abolished the PMA-induced augmentation of effect by cAMP analogs but also significantly blocked the PTH-induced inhibitory effect on TdR. Pretreatment with 10(-6) M PMA, which downregulates PKC, significantly inhibited the PTH-induced suppression of TdR. Combined treatment with cAMP analog (dibutyryl-cAMP or Sp-cAMPS) and calcium ionophore (A23187 or ionomycin) caused additive effects on TdR, and PMA used in combination with both cAMP analog and calcium ionophore induced the further inhibition of TdR.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of local prostaglandin E2 on fracture callus in rabbits

We investigated the effect of local infusion with prostaglandin E2 (PGE2) in doses of 0.0003 to 4.0 mg/hour per kg body weight for 6 weeks on a plated unilateral osteotomy in rabbits. PGE2 caused a dose-dependent stimulation of callus formation. Total bone mineral content increased, although the mineral content per volume of the callus was reduced. In another experiment, PGE2 was infused either in the first half or in the second half of the healing period. No effect of PGE2 infusion could be observed in the first half of the 6-week healing period, whereas PGE2 infusion during the second half caused callus stimulation.

Effect of local prostaglandin E2 on periosteum and muscle in rabbits

We assessed the target tissue for the stimulatory effect of prostaglandin E2 (PGE2) on bone formation previously observed during fracture healing. PGE2 was infused into tibial periosteal tissue in the right leg of 7 rabbits and into the anterior tibial muscle in the right leg of 7 other rabbits for 6 weeks. Solvent solution was infused into the left leg. PGE2 infusion at the periosteum caused the formation of primitive woven bone with large amounts of connective tissue; solvent infusion caused small amounts of normal periosteal bone formation. In the neighboring cortical bone, remodeling was increased after PGE2 infusion compared to solvent infusion. In the muscle, PGE2 infusion caused the formation of connective tissue with small amounts of woven bone. Thus, the major effects of PGE2 infusion at the site of the periosteum was the formation of primitive woven bone and in muscles the formation of connective tissue.

Regulation of ornithine decarboxylase by parathyroid hormone in osteoblastic cell systems

Parathyroid hormone (PTH) has been shown to induce osteoblastic activity via a complex signal transduction process which is mediated either by cAMP or cytosolic calcium ([Ca2+]i), or a combination thereof. One of the PTH functions in osteoblasts is the induction of ornithine decarboxylase (ODC) activity. We have analyzed the second messengers involved in this process. 8-Bromo cAMP, a cAMP derivative, enhanced ODC activity in UMR106-01 osteoblastic cell system. The calcium ionophore A23187 and the protein kinase stimulator phorbol-12-myristate 13-acetate did not alter ODC activity. ODC activity was increased by bPTH-(1-34), PGE1, and PGE2 which stimulated both cAMP and [Ca2+]i. In contrast, PTH-(2-34), propionyl bPTH-(2-34), bPTH-(3-34), bPTH-(7-34), and PGF2 alpha, which only enhanced [Ca2+]i but not cAMP, had no effect on ODC activity. Thus, the stimulation of ODC in UMR106 cells by PTH appeared to be mediated primarily via the cAMP signal transduction pathway, and the mere increase in intracellular calcium could not account for the stimulation of ODC activity. ODC mRNA level was found to be increased by PTH treatment. Therefore, translation of ODC may be stimulated by PTH. Moreover, PTH also stimulated ODC antizyme activity, suggesting that the ODC degradation rate was increased.

Effects of prostaglandin inhibition on the bone activities associated with the spontaneous drift of molar teeth in the rat

Little is known about local bone regulation that enables spontaneous molar tooth drift. In this study the role of prostaglandins (PGs) were investigated in the rat by inhibiting PG-synthesis with indomethacin (7 mg/kg/d). Untreated animals were killed at the start of the experiment, while treated ones were killed after 3, 7, or 14 days of treatment. Mandibles were processed for histomorphometry without demineralization. Changes in osteoclasts and extent of the different steps of the bone remodeling sequence (resorption, reversal, and formation) were assessed along the remodeling side of the socket of the buccal root of the first molar. The total number of osteoclasts decreased after 7 days of PG inhibition (P < 0.01 vs controls) and then partially recovered. This change was due to a sharp decrease in active cells on day 7 (P < 0.01), while inactive cells remained unchanged throughout the experimental period. The extent of resorption fell on day 7 (P < 0.01) and then recovered almost to the control level on day 14. Reversal at first increased insignificantly and thereafter decreased (P < 0.02) for the remaining 7 days. Formation was modified only on day 14; at that time it had doubled compared with controls. These results show that PGs are involved in the local regulation of bone remodeling accompanying tooth drift. Resorption inhibition was partial, indicating that other factors participate with PGs in this regulation; in addition, the trend to recovery observed at the end of the experimental period suggests that these factors can take over from PGs to achieve the necessary remodeling of the socket.

Tenascin in bone morphogenesis: expression by osteoblasts and cell type-specific expression of splice variants

The extracellular matrix glycoprotein, tenascin, is associated in vivo with mesenchyme undergoing osteogenesis and chondrogenesis, but is absent from mature bone and cartilage matrix. The expression of tenascin by osteoblastic cells in vitro has been investigated by immunoblotting and immunocytochemistry. Tenascin was secreted into the medium and deposited in the matrix by human and rat osteoblast-like cell lines, as well as by primary osteoblast-enriched cultures from chick embryo calvarial bones. In primary osteoblast-enriched cultures, extracellular tenascin was found only in cell aggregates expressing the osteoblast marker alkaline phosphatase. Chicken osteoblast cultures synthesized almost exclusively the largest tenascin subunit, whereas fibroblast cultures from periostea of chicken calvariae synthesized approximately equal amounts of all three subunits. In situ hybridization studies of developing chicken bones, using a cDNA probe that hybridizes to all chicken tenascin splice variants, showed specific labelling of both osteogenic and chondrogenic regions of developing endochondral bones. In contrast, a cDNA probe specific for the large tenascin splice variant showed specific hybridization in osteogenic but not chondrogenic regions. Within osteogenic regions, tenascin mRNA was expressed by osteoblasts. A comparison of in situ hybridization and immunohistochemical studies demonstrated that tenascin mRNA and protein were codistributed in osteogenic regions of endochondral and membrane bones, whereas protein was retained in regions of differentiating cartilage where mRNA was no longer detectable. The results presented here demonstrate that tenascin is synthesized by osteoblasts. Moreover, within developing bones, there are at least three different cell type-specific patterns of expression of tenascin splice variants.

Prostaglandin E2 stimulates synthesis of insulin-like growth factor binding protein-3 in rat bone cells in vitro

Prostaglandin E2 is produced by bone cells and increases cyclic AMP in these cells. Like PTH and dibutyryl cyclic AMP, PGE2 is a potent stimulator of IGF-I synthesis in cultured rat osteoblasts and inhibits DNA synthesis and type I procollagen gene expression. In addition, PGE2 inhibits the response of the cells toward IGF-I after 1 day but not after 4 days of incubation. Rat calvaria osteoblasts constitutively release IGFBPs into the culture medium, in particular IGFBP-2 and IGFBP-3. Like growth hormone, PGE2 stimulates the accumulation of IGFBP-3. PGE2 rapidly increases IGF-I and IGFBP-3 mRNA expression in calvaria cells, with a time course clearly different from that observed in response to growth hormone. Thus, PGE2 modifies not only the synthesis of IGF-I but also that of IGFBP-3 in skeletal tissue.

Regulation of gene transcription and proliferation by parathyroid hormone is blocked in mutant osteoblastic cells resistant to cyclic AMP

We employed a cyclic AMP-resistant subclone of UMR 106-01 osteoblastic osteosarcoma cells (UMR 4-7) with a regulated, dominant-negative mutation of cyclic AMP-dependent protein kinase (PK-A), to examine the mechanism(s) whereby parathyroid hormone (PTH) regulates growth of these cells. Expression of a transiently transfected CAT reporter gene controlled by the cAMP response element of the rat somatostatin gene ('SST-CAT') was used to monitor PK-A activation in intact cells. Agonist-stimulated SST-CAT expression was specific for agents known to activate adenylate cyclase, required an intact cAMP response element and was specifically blocked following induction of the mutant cAMP-resistant phenotype in UMR 4-7 cells. Inhibition of the proliferation of UMR 106-01 cells by PTH, which is mimicked by forskolin and 8-bromo-cAMP, was blocked completely in mutant cyclic AMP-resistant UMR 4-7 cells. We conclude that control of proliferation in UMR 106-01 cells by PTH involves the cAMP messenger system and requires activation of PK-A.

Effects of cyclic AMP and butyrate on cell cycle, DNA, RNA, and purine synthesis of cultured astrocytes

Dibutyryl cyclic monophosphate (dBcAMP) has been shown to inhibit growth, and alter the morphology of astrocytes. However, the potential contribution of its hydrolytic product, butyrate, in inducing some of the changes that have been attributed to dBcAMP, is not clear. DNA, RNA, and purine synthesis were therefore studied in primary astrocyte cultures after 24 hours of exposure to varying concentrations of butyrate, dBcAMP, and agents that increase intracellular cAMP levels. Progression of cells through cell cycle was also studied by flow cytometry. Dibutyryl cAMP partially arrested cells in Go/G1 phase of cell cycle while sodium butyrate increased the percentage population of cells in G2/M phase. DNA synthesis and de novo purine synthesis were inhibited after treatment with dBcAMP, sodium butyrate, and various drugs that increase intracellular cAMP levels. RNA synthesis was increased with cAMP but was not affected by sodium butyrate. Our study shows that at millimolar concentrations, butyrate is capable of altering the cell cycle and inhibiting DNA synthesis in primary astrocyte cultures, in a manner that is similar although not identical to the effects of dBcAMP.

PTH stimulates the proliferation of TE-85 human osteosarcoma cells by a mechanism not involving either increased cAMP or increased secretion of IGF-I, IGF-II or TGF beta

Injections of parathyroid hormone (PTH) result in increased bone formation in several species. Work in our laboratory and others has shown a stimulation of bone cell proliferation and growth factor production by PTH. Our purpose was to study the effects of PTH on a human bone cell line using TE-85 human osteosarcoma cells as a model. After 24 h treatment, PTH caused an increase in cell proliferation as measured by cell counts and [3H]-thymidine incorporation. Proliferation was not inhibited by an anti-transforming growth factor beta (TGF beta) antibody which could abolish stimulation by exogenous TGF beta. PTH did not stimulate cAMP production, alkaline phosphatase activity or production of insulin-like growth factors I or II (IGF-I or IGF-II) in TE-85 cells. Although basal TE-85 proliferation was slowed by incubation with the calcium channel blocking agent verapamil, PTH still caused an increase in growth rate. We conclude that PTH directly stimulates TE-85 proliferation via a mechanism not involving increased adenylate cyclase activity or increased secretion of IGF-I, IGF-II or TGF beta and may stimulate bone formation in vivo by activating some other mitogenic signal to increase bone cell proliferation.

Two biochemical indices of mouse bone formation are increased, in vivo, in response to calcitonin

In a series of four studies, adult female Swiss-Webster mice were used to measure the effects of salmon calcitonin on two biochemical indices of local and systematic bone formation: (1) skeletal alkaline phosphatase activity--in serum and in extracts of calvaria and tibiae, and (2) calvarial collagenase-digestible protein synthesis--measured, acutely, in vitro. Subcutaneous calcitonin doses ranged from 50 to 400 mU/mouse/day (0.95-18.1 U/kg/day), and treatment schedules were continuous (daily) for 2-14 days, acute, or intermittent (2 days/week for 6 weeks). The effects of calcitonin on these bone formation indices (skeletal alkaline phosphatase and collagenase-digestible protein synthesis) were biphasic with respect to dose and treatment time, being increased in response to short-term, low-dose treatment, but not long-term, continuous treatment. The effects of long-term intermittent calcitonin treatment were dose-dependent increases in skeletal alkaline phosphatase in calvaria and serum (r = 0.948, P less than 0.02, and r = 0.960, P less than 0.01, respectively).

Stimulation of DNA synthesis in Jurkat cells by synergistic action between adenine and guanine nucleotides

P2-purinoceptor agonists stimulated the DNA synthesis of Jurkat cells via a pathway independent of cAMP and intracellular free calcium. The response was greatly enhanced by the synergistic action between adenine and guanine nucleotides, suggesting that binding sites of these nucleotides are different from each other, and the proliferation is stimulated by a novel interaction between adenine and guanine nucleotide receptors. The stimulatory effects of P2-agonists on proliferation was completely abolished by cholera toxin and attenuated by pertussis toxin, which suggests that substrates for cholera toxin and pertussis toxin are involved in the proliferative pathways associated with P2-purinoceptors.

The activation of cAMP-dependent protein kinase is directly linked to the regulation of osteoblast proliferation (UMR-106) by parathyroid hormone

In order to characterize the direct involvement of cAMP in the change of osteoblast proliferation by parathyroid hormone (PTH), we employed the diastereoisomers of adenosine 3',5'-cyclic phosphorothioate, Sp-cAMPS and Rp-cAMPS, which have been recently shown to act directly as agonist and antagonist, respectively in the activation of cAMP-dependent protein kinase (PKA). Dibutyryl cAMP (dbcAMP) and cholera toxin as well as human(h) PTH-(1-34) significantly inhibited [3H]thymidine incorporation (TdR) in osteoblastic osteosarcoma cells, UMR-106. Sp-cAMPS (10(-6)-10(-4) M) inhibited TdR in a dose-dependent manner. Although Rp-cAMPS (10(-6)-10(-4) M) itself did not affect TdR, it significantly blocked dbcAMP-, cholera toxin- and Sp-cAMPS-induced suppression of TdR. Moreover, Rp-cAMPS (10(-6)-10(-4) M) dose-dependently antagonized hPTH-induced suppression of TdR. Present studies first indicated that the activation of PKA is directly linked to the change of osteoblast proliferation by PTH.

Vitamin D: a modulator of cell proliferation and differentiation

1,25-Dihydroxyvitamin D3, [1,25(OH)2D3], the biologically most active metabolite of vitamin D3, is involved in the regulation of calcium homeostasis and bone metabolism. Recently, receptors for 1,25(OH)2D3 have also been shown in cells and tissues not directly related to calcium homeostasis. Experimental data obtained with leukaemic and cancer cell lines, both in vitro and in vivo, showed the effects of 1,25(OH)2D3 on cell differentiation and proliferation. However, high doses of the sterol have to be used to observe these effects. Additional studies are needed to establish whether 1,25(OH)2D3 or suitable analogues have a therapeutic potential in malignant diseases without unacceptable toxicity like the development of hypercalcemia.

Stimulation by defined parathyroid hormone fragments of cell proliferation in skeletal-derived cell cultures

We have reported previously that parathyroid hormone (PTH) acts on cultured bone cells to stimulate creatine kinase (CK) activity and [3H]thymidine incorporation into DNA via phosphoinositide turnover, in addition to its other actions via increased cyclic AMP production. We also found that mid-region fragments of PTH stimulate [3H]thymidine incorporation into avian chondrocytes. In the present study of mammalian systems, we demonstrate differential effects of defined synthetic PTH fragments on CK activity and DNA synthesis, as compared with cyclic AMP production, in osteoblast-enriched embryonic rat calvaria cell cultures, in an osteoblast-like clone of rat osteosarcoma cells (ROS 17/2.8) and in chondroblasts from rat epiphysial cartilage cell cultures. Unlike full-length bovine (b)PTH-(1-84) or the fully effective shorter fragment human (h)PTH-(1-34), fragments lacking the N-terminal region of the hormone did not increase cyclic AMP formation, whereas they did stimulate increases in both DNA synthesis and CK activity. Moreover, the PTH fragment hPTH-(28-48) at 10 microM inhibited the increase in cyclic AMP caused by 10 nM-bPTH-(1-84). The increase of CK activity in ROS 17/2.8 cells caused by bPTH-(1-84) or hPTH-(28-48) was completely inhibited by either cycloheximide or actinomycin D, as was shown previously for rat calvaria cell cultures. These results indicated the presence of a functional domain of PTH in the central part of the molecule which exerts its mitogenic-related effects on osteoblast- and chondroblast-like cells in a cyclic AMP-independent manner. Since cyclic AMP formation by PTH leads to bone resorption, specific mid-region fragments of PTH might prove suitable for use in vivo to induce bone formation without concomitant resorption.

Bone cell responsiveness to transforming growth factor beta, parathyroid hormone, and prostaglandin E2 in normal and postmenopausal osteoporotic women

We have shown previously that the decreased trabecular bone formation in osteoporotic postmenopausal women results from a reduced ability of osteoblastic cells to proliferate. In this study we have tested the possibility that bone cells from osteoporotic women with low bone formation have an abnormal responsiveness to hormonal or local mitogenic factors. Primary cultures of bone cells with osteoblastic characteristics were obtained by migration from the trabecular bone surface in osteoporotic postmenopausal women with high (n = 7) or low (n = 7) bone formation as evaluated histomorphometrically by the extent of double tetracycline-labeled surface (DLS). Control bone cells were obtained under identical conditions from eight normal age-matched postmenopausal women. Parameters of osteoblastic differentiation (alkaline phosphatase activity and osteocalcin production) were found to be normal and similar in bone cells from osteoporotic women with low or high DLS. In contrast, cell replication as evaluated by [3H]thymidine into DNA was 3.4-fold lower in the low DLS group compared to the high DLS group, confirming our previous findings. Treatment of quiescent bone cells with TGF-beta (0.5-1 ng/ml) for 24 h significantly stimulated DNA synthesis in osteoblastic cells from normal women and in bone cells from osteoporotic patients with low or high DLS, indicating a normal responsiveness to TGF-beta in these patients. We have compared the effect of parathyroidhormone (PTH) on bone cells from normal and osteoporotic women. Basal cAMP levels and the cAMP accumulation in response to (1-34)-hPTH were similar in bone cells from patients with low or high DLS and were not different from normal values.(ABSTRACT TRUNCATED AT 250 WORDS)

Parathyroid hormone fragment [3-34] stimulates protein kinase C (PKC) activity in rat osteosarcoma and murine T-lymphoma cells

The parathyroid hormone (PTH) fragment [1-34] strongly stimulated both adenylate cyclase and membrane-associated PKC activities in rat 17/2 osteosarcoma cells. By contrast, the PTH [3-34] fragment, which was unable to stimulate adenylate cyclase, remained a potent stimulator of membrane-associated PKC activity in these cells. Both PTH fragments also strongly stimulated membrane-PKC activity in cyc-S49T-lymphoma cells possessing a defective adenylate cyclase system. This ability of PTH [3-34] to stimulate membrane-associated PKC activity could explain the residual bioactivity of this fragment.

Distinct effects of calcium- and cyclic AMP-enhancing factors on cytoskeletal synthesis and assembly in mouse osteoblastic cells

Previous studies have indicated that the effects of parathyroid hormone (PTH) on osteoblastic function involve alteration of cytoskeletal assembly. We have reported that after a transitory cell retraction, PTH induces respreading with stimulation of actin, vimentin and tubulins synthesis in mouse bone cells and that this effect is not mediated by cAMP. In order to further elucidate the role of intracellular cAMP and calcium on PTH action on bone cell shape and cytoskeleton we have compared the effects of calcium- and cAMP-enhancing factors on actin, tubulin and vimentin synthesis in relation with mouse bone cell morphology, DNA synthesis and alkaline phosphatase activity as a marker of differentiation. Confluent mouse osteoblastic cells were treated with 0.1 mM isobutylmethylxanthine (IBMX) for 24 h. This treatment caused an increase in the levels of cytoskeletal subunits associated with an elevation of cAMP. Under these conditions, PTH (20 nM) and forskolin (0.1 microM) produced persistent cytoplasmic retraction. PTH and forskolin treatment in presence of IBMX (24 h) induced inhibitory effects on actin and tubulin synthesis evaluated by [35S]methionine incorporation into cytoskeletal proteins identified on two-dimensional gel electrophoresis. Under these culture conditions PTH and forskolin also caused disassembly of microfilament and microtubules as shown by the marked reduction in Triton X soluble-actin and alpha- and beta-tubulins. In contrast, incubation of mouse bone cells with 1 microM calcium ionophore A23187 (24 h) resulted in increased monomeric and polymeric forms of actin and tubulin while not affecting intracellular cAMP. Alkaline phosphatase activity was increased in all conditions while DNA synthesis evaluated by [3H]thymidine incorporation into DNA was stimulated by PTH combined with forskolin and inhibited by the calcium ionophore. These data indicate that persistent elevation of cAMP levels induced by PTH and forskolin with IBMX cause cell retraction with actin and tubulin disassembly whereas rising cell calcium induces cytoskeletal protein assembly and synthesis in mouse osteoblasts. The results point to a distinct involvement of calcium and cAMP in both cytoskeletal assembly and DNA synthesis in mouse bone cells.

Adenylate cyclase blockers dissociate PTH-stimulated bone resorption from cAMP production

It is uncertain whether adenosine 3',5'-cyclic monophosphate (cAMP) or the inositol-calcium pathway mediates the stimulation of bone resorption by parathyroid hormone (PTH). Incubation of bone organ cultures with cAMP analogues and forskolin has not resolved this question because of the cellular inhomogeneity of bone and the consequent presence of adenylate cyclase-linked receptors for both PTH and calcitonin, hormones with opposite effects on bone resorption. We have used two new inhibitors of adenylate cyclase, 9-(tetrahydro-2-furyl)adenine (SQ 22536) and 2',5'-dideoxyadenosine (DDA), to directly reassess the role of cAMP in PTH-stimulated osteolysis. SQ 22536 (0.01-1.0 mM) and DDA (0.01-1.0 mM) completely blocked PTH stimulation of cAMP production measured in the absence of a phosphodiesterase blocker. In the presence of 1 mM 3-isobutyl-1-methylxanthine, half-maximal inhibition of PTH-induced cAMP production occurred with 0.2 mM SQ and 0.1 mM DDA, respectively. These concentrations of SQ and DDA had no effect on PTH-stimulated 45Ca release from calvaria, although both agents inhibited bone resorption when present at concentrations of 1-2 mM. At these levels, SQ and DDA caused equivalent inhibition of 45Ca release stimulated by 1,25-dihydroxyvitamin D3 but did not affect basal 45Ca release or [3H]-phenylalanine incorporation. It is concluded that substantial blockade of PTH-induced cAMP production does not affect this hormone's stimulation of bone resorption, which is therefore likely to be mediated by another intracellular messenger system, possibly calcium. In millimolar concentrations, SQ and DDA appear to be nonspecific blockers of osteoclastic bone resorption.

Fluid shear stress as a mediator of osteoblast cyclic adenosine monophosphate production

Effects of interstitial fluid flow on osteoblasts were investigated. Intracellular cyclic adenosine monophosphate (cAMP) levels were monitored in cultured osteoblasts subjected to shear rates ranging from 10 to 3,500 sec-1. Cyclic AMP levels were significantly increased at all shear rates from 1 pmole/mg protein to 10-16 pmole/mg protein. Osteoblasts subjected to a shear rate of 430 sec-1 for 0.5-15 minutes exhibited elevated levels (12-fold) of intracellular cAMP, which were sustained throughout the perfusion period. Osteoblasts were three times more sensitive to flow stimulation than human umbilical vein endothelial cells and baby hamster kidney fibroblasts, which also displayed higher cAMP levels (4-fold) after exposure to flow. To distinguish streaming potential effects from shear stress effects, viscosity was increased 5-fold by addition of neutral dextran to the perfusing medium. Shear stress is a function of viscosity, and streaming potentials are not for a given shear rate. The mechanism of this cellular response to flow was shown to be shear stress dependent. Inhibition of cyclooxygenase by 20 microM ibuprofen completely inhibited the flow-dependent cAMP response, indicating the cAMP response is mediated by prostaglandins. Our results suggest that fluid flow induced by mechanical stress may be an important mediator of bone remodeling.

The effects of prostaglandin E2 on DNA and collagen synthesis in osteoblasts in vitro

The effects of prostaglandin E2(PGE2) on DNA and collagen synthesis in two separate cell populations were investigated. In view of their morphology, ALPase activity, DNA and collagen synthesis, and response to PGE2, one population was in an undifferentiated state consisting of preosteoblast-like (PL) cells and the other was in a differentiated state consisting predominantly of osteoblast-like (OB) cells. As parameters of bone-forming activity, the incorporation of 3H-thymidine into DNA and the incorporation of 3H-proline into collagenase digestible protein were measured to assess DNA and collagen synthesis. The cells were treated with PGE2 in the presence of indomethacin (IM) to avoid the influence of endogenous prostaglandins. At 24 hours, IM stimulated the DNA synthesis in both cell populations. Furthermore, a greater stimulation was found in the PL cells than in the OB cells. On the other hand, exogenously supplemented PGE2 reversed the IM-induced stimulation of DNA synthesis. In contrast, high concentrations of PGE2 alone increased the DNA synthesis. With respect to collagen synthesis, IM showed an inhibitory effect, especially in the PL cells. This inhibitory effect was also reversed by the addition of PGE2. In addition to the stimulation of collagen synthesis, PGE2 enhanced the proportion of protein synthesized as collagen. In the PL cells, the percentage of collagen synthesis was markedly decreased when cultured with IM for 48 hours. These results suggested that the effects of IM were mediated in part via its ability to reduce biosynthesis of prostaglandins, and that PGE2 is a multifunctional autocrine regulator of bone formation.

Effects of insulin and parathyroid hormone on DNA synthesis and ornithine decarboxylase activity in cultured bovine dental pulp

The effects of insulin and parathyroid hormone (PTH) on the proliferation of developing bovine dental pulp in an explant culture system were studied. Dental pulp explants were cultured on siliconized lens paper floating on the serum-free medium for up to 72 h. Ornithine decarboxylase (ODC) activity increased and reached a peak after 24 h. DNA synthesis increased continuously after a lag period of 24 h. Insulin (10 milliunits per ml) stimulated ODC activity 1.3-fold and DNA synthesis 1.5-fold. PTH alone (1 unit per ml) stimulated ODC activity in 1.7-fold, but did not affect DNA synthesis. PTH plus insulin caused greater increases in ODC activity and DNA synthesis in dental pulp explants than insulin alone (ODC, 2.6-fold; DNA, 3.7-fold). These results suggest that insulin and PTH are involved in the regulation of growth of dentinogenically active bovine dental pulp.

Bone cell biology: the regulation of development, structure, and function in the skeleton

Bone cells compose a population of cells of heterogeneous origin but restricted function with respect to matrix formation, mineralization, and resorption. The local, mesenchymal origin of the cells which form the skeleton contrasts with their extraskeletal, hemopoietic relatives under which bone resorption takes place. However, the functions of these two diverse populations are remarkably related and interdependent. Bone cell regulation, presently in its infancy, is a complicated cascade involving a plethora of local and systemic factors, including some components of the skeletal matrices and other organ systems. Thus, any understanding of bone cell regulation is a key ingredient in understanding not only the development, maintenance, and repair of the skeleton but also the prevention and treatment of skeletal disorders.

Effect of parathyroid hormone and forskolin on cytoskeletal protein synthesis in cultured mouse osteoblastic cells

Parathyroid hormone (PTH) has been shown to cause transient cell shape changes in bone cells. We have examined the effects of parathyroid hormone and forskolin on the organization and expression of cytoskeletal proteins in cultured mouse endosteal osteoblastic cells. Analysis of [35S]methionine-labeled cytoskeletal proteins isolated on two-dimensional gel electrophoresis showed that PTH treatment (24 h) stimulated the de novo biosynthesis of actin, vimentin and tubulins in confluent cells, whereas forskolin had a minor effect despite a huge stimulation of cAMP production. This PTH-induced stimulation was associated with cell respreading following a mild and transitory cell retraction. PTH increased the synthesis of monomeric subunits of actin and beta-tubulins in subconfluent bone cells, whereas both monomeric and polymeric levels of beta-tubulins were increased in confluent osteoblasts. Under conditions reducing cell spreading, osteoblastic cells had initially high levels of unpolymerized subunits. In these poorly spread cells, parathyroid hormone or forskolin had no effect on the de novo synthesis of cytoskeletal proteins despite a marked elevation in intracellular cAMP levels. It is concluded that PTH affects the biosynthesis of cytoskeletal proteins in osteoblastic cells and that cAMP production does not seem to be directly involved. In addition, the effect of PTH is modulated by cell spreading and by the initial pool of cytoskeletal subunits.

Induction of ornithine decarboxylase activity in isolated chicken osteoblasts by parathyroid hormone: the role of cAMP and calcium

We investigated the role of cAMP and Ca2+ as mediators in parathyroid hormone (PTH)-induced ornithine decarboxylase (ODC) activity in primary cultures of chicken osteoblasts. We present evidence that the induction of ODC activity by PTH is most likely a receptor-mediated process and that cAMP is a mediator. However, using three different approaches we have strong indications that cAMP is not the exclusive mediator of PTH-induced ODC activity. First, when the dose-response curve of PTH-induced ODC activity is compared with that of PTH-stimulated cAMP production, the ED50 for cAMP production is about five times as high as that for the induction of ODC activity. Second, 1 mM 9-(tetrahydro-2-furanyl) adenine (SQ 22.536) almost completely inhibited PTH-stimulated cAMP production whereas there was only a small inhibitory effect on PTH-induced ODC activity. Third, some PTH fragments unable to stimulate cAMP production were still able to induce ODC activity. We therefore propose that apart from cAMP, an additional messenger, most likely Ca2+, must be present. Evidence for this concept are the observations that substances affecting extracellular and intracellular Ca2+ levels (EGTA, A23187, CoCl2, verapamil) or antagonizing calmodulin (Trifluoroperazin, Compound 48/80) also strongly affect PTH-induced ODC activity. These effects could not be explained by a positive interaction of Ca2+ with the hormone-stimulated cAMP system as 2 mM EGTA strongly enhanced PTH-stimulated cAMP production but at the same time completely inhibited PTH-induced ODC activity. A similar dissociation between hormone-induced cAMP production and induction of ODC activity was found with the Ca2+ -ionophore A23187 (10(-7) M) which significantly inhibited PTH-stimulated cAMP production but strongly enhanced PTH-induced ODC activity. Our results suggest that intracellular Ca2+, and possibly calmodulin, in addition to cAMP, are involved in PTH-induced ODC activity in chicken osteoblasts. Most probably Ca2+ is the initial messenger and cAMP acts in a coordinate pattern as a synarchic messenger making the induction of ODC activity by PTH more sensitive to Ca2+. Furthermore, the present findings are in agreement with our concept of the existence of two receptors or two receptor-sites for PTH on osteoblasts. One receptor is coupled to the production of cAMP and is presumably activated when the first two aminoacids of the NH2-terminus of the hormone are present and the other, suggested to be responsible for the increase in intracellular Ca2+, is thought to be activated by a region of the hormone sequence between amino acid 3 and 34.(ABSTRACT TRUNCATED AT 250 WORDS)