A two-receptor model for the action of parathyroid hormone on osteoblasts: a role for intracellular free calcium and cAMP

β-Arrestin-biased signaling of PTH analogs of the type 1 parathyroid hormone receptor

Parathyroid hormone (PTH) is an anabolic agent that mediates bone formation through activation of the Gα(s)-, Gα(q)- and β-arrestin-coupled parathyroid hormone receptor type 1 (PTH1R). Pharmacological evidence based on the effect of PTH(7-34), a PTH derivative that is said to preferentially activate β-arrestin signaling through PTH1R, suggests that PTH1R-activated β-arrestin signaling mediates anabolic effects on bone. Here, we performed a thorough evaluation of PTH(7-34) signaling behaviour using quantitative assays for β-arrestin recruitment, Gα(s)- and Gα(q)-signaling. We found that PTH(7-34) inhibited PTH-induced cAMP accumulation, but was unable to induce β-arrestin recruitment, PTH1R internalization and ERK1/2 phosphorylation in HEK293, CHO and U2OS cells. Thus, the β-arrestin bias of PTH(7-34) is not apparent in every cell type examined, suggesting that correlating in vivo effects of PTH(7-34) to in vitro pharmacology should be done with caution.

Hormonal regulation of bone growth and remodelling

Many systemic and local hormones influence bone growth and remodelling. These include calcium regulating hormones, systemic growth regulators and local growth factors. Parathyroid hormone (PHT) is a potent stimulator of osteoclastic bone resorption and a direct inhibitor of osteoblastic collagen synthesis. However, intermittent low-dose PTH administration can increase bone formation in vivo. PTH may act indirectly via local factors. It has been shown to increase prostaglandin E2 (PGE2) and transforming growth factor beta (TGF-beta) release from bone. Both PGE2 and TGF-beta have complex effects on bone metabolism and are likely to be physiological regulators of bone remodelling. Oestradiol has been shown to inhibit bone resorption in vivo but not in vitro. While there is evidence for oestrogen receptors in cultured bone cells, the effect could still be indirect. Oestradiol can inhibit bone PGE2 release in an in vivo-in vitro model in the rat. Glucocorticoids are potent inhibitors of bone formation and inhibit PGE2 and interleukin 1 production both in vivo and in vitro. While many regulatory factors affect prostaglandin production in bone, the complex effects of PGE2 on bone metabolism make it difficult to predict the ultimate response. The major effects of PGE2 are stimulation of bone formation and resorption and an increase in bone turnover. However, opposite effects can occur at certain times and concentrations. Interactions among these factors could explain some physiological, pathological, and therapeutic responses in skeletal tissue.

Modeling the interactions between osteoblast and osteoclast activities in bone remodeling

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

Mechanisms of regulation of G(11)alpha protein by dexamethasone in osteoblastic UMR 106-01 cells

We have previously demonstrated that glucocorticoids increased G(q/11)alpha protein expression and phospholipase C activity in the rat osteosarcoma cell line UMR 106-01. In this study, we demonstrated that G(11)alpha is the primary G(q)-subtype family member expressed in UMR cells. Dexamethasone treatment increased the expression of G(11)alpha protein in both a time- and a dose-dependent manner. Glucocorticoid treatment significantly increased the half-life of G(11)alpha protein from 20.3 to 63 h. Steady-state G(11)alpha mRNA level was also increased by glucocorticoid treatment by approximately 70%. This change was not the result of changes in RNA stability but rather the result of increased transcription, because the glucocorticoid-mediated upregulation of G(11)alpha mRNA was blocked by the transcription inhibitor actinomycin D. The dexamethasone induction of G(11)alpha mRNA occurred after a time lag of 12-24 h and was blocked by the protein synthesis inhibitor cycloheximide. These results suggest that the dexamethasone-induced rise in G(11)alpha protein results primarily from changes in the degradation rate of the protein, whereas changes in G(11)alpha mRNA play a smaller role and require de novo synthesis of regulatory protein(s).

Functional parathyroid hormone receptors are present in an umbilical vein endothelial cell line

Acute parathyroid hormone exposure induces vascular smooth muscle relaxation. In contrast, continuous infusion of parathyroid hormone leads to vasoconstriction and an elevation of blood pressure. Despite the known effects of parathyroid hormone on vascular smooth muscle, possible direct effects on the vascular endothelium have not previously been investigated. Using a human umbilical vein endothelial cell line, we found that parathyroid hormone increased both intracellular calcium and cellular cAMP content in these endothelial cells. Furthermore, exposure of these cells to increasing concentrations of parathyroid hormone stimulated both [(3)H]thymidine incorporation and endothelin-1 secretion. Parathyroid hormone/parathyroid hormone-related peptide receptor mRNA could be detected at low levels in these cells. In summary, these data demonstrate that endothelium-derived cells contain functional parathyroid hormone receptors. The potential physiological role of these receptors remains to be determined.

Induction of rat interstitial collagenase (MMP-13) mRNA in a development-dependent manner by parathyroid hormone in osteoblastic cells

The purpose of this study was to determine whether the production of interstitial collagenase mRNA in response to parathyroid hormone (PTH) changes with osteoblast phenotypic development. To accomplish this, cells derived from fetal rat calvaria were examined. The calvarial osteoblasts, which proliferate when placed in culture, can be made to differentiate after confluence. Studies were performed on cells while they were proliferating, at confluence, and during the differentiation process. The cells were treated with PTH for various times, and interstitial collagenase mRNA was quantified by RNase protection assay. We concluded that the ability of PTH to induce interstitial collagenase mRNA in these cells increased with osteoblast phenotypic development. We also determined that the response could be mimicked by combining the effect of 8-bromo-cAMP and 12-O-tetradecanoyl-phorbol-13-acetate, stimulators of the protein kinase A and protein kinase C pathways, respectively, both known to be activated by PTH. The binding of nuclear factors to two regions previously reported to be important for PTH induction of the gene in UMR 106-01 cells was also examined. These data indicated that the binding of nuclear factors to oligonucleotides encompassing the TRE (-51) or the PEA3 (-80) elements changed with development of the osteoblast phenotype. The latter was also shown to be PTH responsive.

Parathyroid hormone inhibits collagen synthesis and the activity of rat col1a1 transgenes mainly by a cAMP-mediated pathway in mouse calvariae

We examined the effect of parathyroid hormone and various signaling molecules on collagen synthesis and chloramphenicol acetyltransferase activity in cultured transgenic mouse calvariae carrying fusion genes of the rat Col1a1 promoter and the chloramphenicol acetyltransferase reporter. After 48 h of culture, parathyroid hormone, forskolin, dibutyryl cAMP, 8-bromo cAMP, and phorbol myristate acetate inhibited transgene activity, while the calcium ionophore ionomycin had no effect. Pretreatment of calvariae with the phosphodiesterase inhibitor isobutylmethylxanthine potentiated the inhibitory effect of 1 nM parathyroid hormone on transgene activity and collagen synthesis. Parathyroid hormone further inhibited transgene activity and collagen synthesis in the presence of phorbol myristate acetate. Parathyroid hormone inhibition of transgene activity and collagen synthesis was not affected by indomethacin or interleukin-6. After 48 h of culture, parathyroid hormone inhibited chloramphenicol acetyltransferase activity by 50-85% in cultured calvariae carrying transgenes having progressive 5' upstream deletions of promoter DNA down to -1683 bp. These data show that the inhibitory effect of parathyroid hormone on Col1a1 expression in mouse calvariae is mediated mainly by the cAMP signaling pathway. Prostaglandins and IL-6 are not local mediators of the parathyroid hormone response in this model. Finally, regions of the Col1a1 promoter downstream of -1683 bp are sufficient for parathyroid hormone inhibition of the Col1a1 promoter.

Ca(2+) and extracellular acidification rate responses to parathyroid hormone fragments in rat ROS 17/2 and human SaOS-2 cells

To examine the importance of the N- or C-termini of PTH(1-34) the effects of truncated fragments of PTH on human receptors in osteoblast-like SaOS-2 cells and rat receptors in rats ROS 17/2 cells were examined. Fura-2-loaded cells were used to monitor cytosolic free Ca(2+) concentration ([Ca2+]i), and the Cytosensor microphysiometer was used to monitor extracellular acidification rate (ECAR). C-terminally truncated fragments (1-31) and (1-28) of hPTH(1-34)NH(2) stimulated an increase in [Ca(2+)](i) and ECAR in both cell lines. hPTH(3-34)NH(2) and other N-terminally truncated fragments did not stimulate [Ca(2+)](i) or ECAR in either cell type. The signal transduction pathway of PTH-induced ECAR in ROS 17/2 cells was investigated to compare with previous results in SaOS-2 cells. Potentiation by IBMX, attenuation by 8Br-cAMP and lack of effect of the PKC inhibitor chelerythrine chloride support a cAMP/PKA-mediated signal transduction pathway in ROS 17/2, while the protein kinase C pathway was predominant in SaOS-2 cells. We conclude that the intact N-terminus of PTH is essential in PTH signaling mediated via either the cAMP/PKA or inositol lipid/Ca(2+)/PKC pathways in osteoblast-like cells.

Bone resorption induced by A23187 is abolished by indomethacin: implications for second messenger utilised by parathyroid hormone

Parathyroid hormone acts on the osteoblast to induce osteoclastic bone resorption. Parathyroid hormone utilises cyclic AMP as a second messenger in osteoblasts, but may also cause an increase in cytoplasmatic free calcium ions ([Ca2+]i) in the same cell. To investigate the role of osteoblastic [Ca2+]i in the induction of bone resorption, we have compared the effects of parathyroid hormone and the Ca2+-ionophore, A23187, as well as the adenylate cyclase stimulating agent, forskolin, and the phorbol ester, phorbole 12,13 dibutyrate (PDB), on bone resorption in neonatal mouse calvarial bones. Parathyroid hormone (0.1 and 1 nM) dose dependently stimulated the release of prelabelled 45Ca2+ in 72 h culture. Parathyroid hormone-induced bone resorption was not affected by the addition of 1 microM indomethacin to the incubation media, and was therefore, not mediated by local prostaglandin formation. A23187 stimulated the release of 45Ca2+ at 1-10 nM. Above 100 nM, A23187 inhibited bone resorption. The A23187 (3 and 10 nM)-induced bone resorption was abolished by the cyclooxygenase inhibitor, indomethacin (1 microM), indicating that the stimulatory effect was mediated via prostaglandin formation. The adenylate cyclase stimulating agent, forskolin, dose dependently stimulated bone resorption at and above 1 microM. There was no additive or synergistic effect of forskolin and A23187 on 45Ca2+ release. Forskolin-induced bone resorption was, as with parathyroid hormone but in contrast to ionophore-induced bone resorption, not abolished by indomethacin (1 microM). The protein kinase C activator, PDB, at 10 and 1000 nM stimulated the release of prelabelled 45Ca2+. The stimulatory effect of the protein kinase C stimulating phorbol ester, PDB, on bone resorption was abolished by the addition of indomethacin. In summary, bone resorption induced by a Ca2+-ionophore is abolished by indomethacin. This indicates that bone resorbing agents known to increase [Ca2+]i subsequently enhance local prostaglandin formation. Bone resorption induced by the protein kinase C activator, PDB, was also abolished by indomethacin, whereas, forskolin and parathyroid hormone-induced bone resorption was unaffected. These data indicate that cyclic AMP, but not [Ca2+]i, is involved as a second messenger in parathyroid-induced bone resorption.

Pharmacological and biochemical evidence for the regulation of osteocalcin secretion by potassium channels in human osteoblast-like MG-63 cells

Previous reports have suggested the involvement of voltage-activated calcium (Ca2+) channels in bone metabolism and in particular on the secretion of osteocalcin by osteoblast-like cells. We now report that potassium (K+) channels can also modulate the secretion of osteocalcin by MG-63 cells, a human osteosarcoma cell line. When 1,25-dihydroxyvitamin D3(1,25(OH)2D3)-treated MG-63 cells were depolarized by step increases of the extracellular K+ concentration ([K+]out) from 5-30 mM, osteocalcin (OC) secretion increased from a control value of 218 +/- 13 to 369 +/- 18 ng/mg of protein/48 h (p < 0.005 by analysis of variance). In contrast, in the absence of 1,25(OH)2D3, there is no osteocalcin secretion nor any effect of cell depolarization on this activity. The depolarization-induced increase in 1,25(OH)2D3-dependent osteocalcin secretion was totally inhibited in the presence of 10 microM Nitrendipine (a Ca2+ channel blocker, p < 0.005) without affecting cellular alkaline phosphatase nor cell growth. Charybdotoxin, a selective blocker of Ca2+-dependent K+ channels (maxi-K) present in MG-63 cells, stimulated 1,25(OH)2D3-induced osteocalcin synthesis about 2-fold (p < 0.005) after either 30, 60, or 120 minutes of treatment. However, Charybdotoxin was without effect on basal release of osteocalcin in the absence of 1,25(OH)2D3 pretreatment. Using patch clamp technique, we occasionally observed the presence of a small conductance K+ channel, compatible with an ATP-dependent K+ channel (GK[ATP]) in nonstimulated cells, whereas multiple channel openings were observed when cells were treated with Diazoxide, a sulfonamide derivative which opens GK(ATP). Western blot analysis revealed the presence of the N-terminal peptide of GK(ATP) in MG-63 cells, and its expression was regulated with the proliferation rate of these cells, maximal detection by Western blots being observed during the logarithmic phase of the cycle. Glipizide and Glybenclamide, selective sulfonylureas which can block GK(ATP), dose-dependently enhanced 1,25(OH)2D3-induced OC secretion (p < 0.005). Reducing the extracellular calcium concentration with EGTA (microM range) totally inhibited the effect of Glipizide and Glybenclamide on osteocalcin secretion (p < 0.005), which remained at the same levels as controls. Diazoxide totally prevented the effect of these sulfonylureas. These results suggest that voltage-activated Ca2+ channels triggered via cell depolarization can enhance 1,25(OH)2D3-induced OC release by MG-63 cells. In addition, OC secretion is increased by blocking two types of K+ channels: maxi-K channels, which normally hyperpolarize cells and close Ca2+ channels, and GK(ATP) channels. The role of these channels is closely linked to the extracellular Ca2+ concentration.

Humoral factors of ascites sarcoma 180 stimulate osteoblastic UMR 106-01 cell proliferation and bone resorption via signal transduction pathways, which are clearly different from those of parathyroid hormone

Ascites sarcoma 180 (S180A) is a transplantable tumor that induces hypercalcemia in tumor-bearing mice and stimulates bone resorption in cultured neonatal mouse calvaria without parathyroid hormone (PTH)-like activity. The serum-free conditioned media of S180A cell cultures (S180A-CM) stimulated [3H]thymidine incorporation (178.3% of the control) and inhibited alkaline phosphatase activity (39.0% of the control) in the osteoblastic osteosarcoma cell line UMR 106-01, contrary to PTH. To investigate signal transduction by S180A-CM, we determined the levels of intracellular free calcium ([Ca2+]i), inositol 1,4,5-triphosphate (IP3), 1,2-diacylglycerol (DAG), phosphatidylcholine (PC) and protein kinase (PK) C activity in UMR 106-01 cells. PTH and PTH-related protein (PTHrP), both potent bone-resorbing factors (BRFs), caused an increase in [Ca2+]i and stimulated IP3 production, whereas S180A-CM had little or no effect on these parameters. On the other hand, S180A-CM stimulated DAG production, accompanied by PC breakdown, and the translocation of PKC activity from the cytosol to the membrane fraction. Sphingosine, a specific PKC inhibitor, inhibited bone-resorbing activity (BRA) in S180A-CM more effectively than PTH or PTHrP-stimulated resorption. H-7, an inhibitor of both cAMP-dependent PKA and PKC, completely inhibited BRA in S180A-CM. These results suggest that BRFs of S180A-CM stimulate osteoblastic cell proliferation and bone resorption via two signal transduction pathways, which are different from those of PTH: 1) activation of PKC by DAG resulting from PC hydrolysis and 2) activation of PKA subsequent to prostaglandin E2 production by bone.

Characterization of calcium translocation across the plasma membrane of primary osteoblasts using a lipophilic calcium-sensitive fluorescent dye, calcium green C18

The synthesis of Calcium Green C18, a lipophilic fluorescent calcium-sensitive dye, and its use as a monitor of Ca2+ efflux from cells is described. This indicator consists of a Calcium Green-1 molecule conjugated to a lipophilic 18-carbon alkyl chain which will intercalate into cell membranes. The Kd of the indicator for Ca2+ in aqueous solution (pH 7.2, 22 degrees C, ionic strength 0.1 M) is 0.23 +/- 0.04 microM and in the presence of liposomes is 0.062 +/- 0.007 microM. Due to its high negativity, the calcium chelating fluorophore faces the cell exterior, when loaded under a defined set of conditions. The dye was found largely on the surface of the cells when loaded at a concentration of 5 microM for 10 min at 37 degrees C. Five minutes after introduction of EGTA, 83-95% fluorescence disappeared, indicating that most of the fluorophore was on the cell surface. Photobleaching was minimal (3-13%). A confocal laser scanning microscope was used to detect and quantify fluorescence. Internalized dye was apparent in cells loaded for longer times (30-60 min) and in membrane-impaired cells, as shown by uptake of propidium iodide. Under defined confocal laser scanning microscope settings, a transient fluorescence at the periphery of approximately 30% of the cells was observed following 10(-8) M parathyroid hormone treatment, indicating the presence of outwardly directed calcium transport across the plasma membrane. Calcium efflux usually lasted 7-10 min, peaking at around 2-3 min. Changes in cell shape were also observed. Calcium efflux was shown to be sensitive to (a) 10 microM quercetin and 10 microM vanadate, partially specific inhibitors of plasma membrane Ca(2+)-ATPase, to (b) 0.1 mM trifluoperazine, an agent which renders calmodulin ineffective, and to (c) 10 mM neomycin sulfate, which blocks release of Ca2+ from intracellular stores. Thapsigargin (5 microM), an inhibitor of Ca(2+)-ATPase of the endoplasmic reticulum, prolonged fluorescence. These observations indicate that cell surface fluorescence was due to the capture of Ca2+ by Calcium Green C18 after Ca2+ had been translocated across osteoblast plasma membranes. Involvement of the plasma membrane Ca(2+)-ATPase, known to be present in osteoblasts in substantial amounts, is implicated.

The 5-lipoxygenase inhibitor BWA4C impairs osteoclastic resorption in a synchronized model of bone remodeling

The role of leukotrienes on bone resorption was tested in a well-standardized model of bone remodeling by inhibiting their biosynthesis with BWA4C, a specific inhibitor of 5-lipoxygenase. After extraction of their upper molars unilaterally, 30 Wistar rats were divided into three groups; the first remained untreated (control group), the second received 80 mg/kg/day of BWA4C dissolved in polyethylene glycol 300 (experimental group), and the third received only the vehicle (sham-treated group). After four days of experiment, the animals were killed and the resorption profile was assessed along the antagonist mandibular buccal cortex. The main result was a dramatic decrease in the number of TRAP-positive mononucleated preosteoclasts in the experimental group (-69%, p < 0.0005 and p < 0.003 vs. the control and sham-treated groups, respectively). This drop was related to a significant decrease in the number of osteoclasts. Neither the activation of the differentiated osteoclasts nor their mean interface with the bone surface were affected by BWA4C. Concomitantly, the mast cell population residing near the vascular network limiting the periosteum was markedly and significantly increased by the treatment. These mast cells were mostly degranulating, i.e., were in a state of activation that we previously found to related to resorption. These data suggest (1) that the leukotrienes are involved in the recruitment of osteoclast progenitors, and/or their differentiation into preosteoclasts, and (2) that mast cells responded to leukotriene inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Effects of the Pasteurella multocida toxin on osteoblastic cells in vitro

Pasteurella multocida toxin induces localized osteolysis in the turbinate bones of swine. Osteolysis appears to be due to an increased level of osteoclastic bone resorption, although osteoblast activity may also be impaired. We studied the effects of purified toxin on the osteoblastic phenotype of the ROS 17/2.8 rat osteoblastic osteosarcoma cell line. Treatment of both embryonic bovine lung cells and a nonosteoblastic rat osteosarcoma cell line (ROS 25/1) with nanomolar doses of toxin produced marked cytotoxic actions. In the osteoblastic ROS 17/2.8 cells, this level of toxin reduced expression of an osteoblastic marker (alkaline phosphatase), was associated with matrix mineralization, but had no cytopathologic action. The osteoblastic cell population may be resistant to a direct cytotoxic effect but is nevertheless a target for toxin action.

Heparin-insensitive calcium release from intracellular stores triggered by the recombinant human parathyroid hormone receptor

1. In the present study we have characterized the parathyroid hormone (PTH)-induced calcium signalling in 293 cells stably transfected with the human PTH receptor cDNA. In these cells, human PTH-1(1-38) strongly stimulates adenosine 3':5'-cyclic monophosphate (cyclic AMP) formation (EC50 = 0.39 nM) but fails to activate phosphoinositide (PI) turnover. The latter pathway is strongly activated, however, by carbachol (CCh) acting through endogenous M3-muscarinic receptors. 2. Despite the lack of detectable inositol phosphate (IP) formation, hPTH-(1-38) elicited calcium transients (EC50 = 11.2 nM) which were comparable to the signals evoked by CCh. These signals are independent of cyclic AMP generation as cyclic AMP elevating agents did not mimic or modify the PTH response. 3. The PTH-stimulated calcium signal still occurred in calcium-free medium but was absent in cells pretreated with thapsigargin, an inhibitor of the calcium pump of the endoplasmic reticulum (ER). hPTH-(1-38) did not accelerate Mn(2+)-influx through the plasma membrane. These data indicate that PTH releases calcium from intracellular stores. 4. Using heparin, an inhibitor of the IP3-activated calcium release channel of the ER, we tested whether the formation of a low amount of IP3, escaping detection by our biochemical assay, might be the origin of the PTH-induced calcium response. However, intracellular infusion of heparin through patch pipettes in voltage clamp experiments failed to block hPTH-(1-38)-induced calcium signals, whereas it abolished the CCh response. 5. The PTH response, like the CCh response, was insensitive to micromolar concentrations of ryanodine and ruthenium red, eliminating the possibility that hPTH-(1-38) stimulates calcium-induced calcium release through ryanodine receptors.6. We conclude that the recombinant human PTH receptor stimulates calcium release from intracellular stores through a novel pathway not involving IP3- or ryanodine receptors.

S-phase independence of parathyroid hormone-induced calcium signalling in primary osteoblast-like cells

Cell cycle-dependence of parathyroid hormone (PTH)-induced signal transduction was studied at the single cell level in fetal rat osteoblast-like cells (ROB) in primary culture. Responsiveness to 10(-7) M rPTH1-34 was measured as changes in the intracellular calcium concentration ([Ca2+]i) of cells loaded with Fura-2 using a video imaging setup. Cells in S-phase were identified by staining for incorporated 5-bromo-2'-deoxyuridine (BrdU). ROB were cultured on glass coverslips which had alphanumerically marked gratings so that cells could be located. Directly after measurement of the PTH-induced [Ca2+]i response of the individual cells, cells were fixed and stained for incorporated BrdU. Phase-contrast images taken with the video imaging setup were compared with phase-contrast micrographs and fluoro-micrographs taken after staining for BrdU incorporation. We found that 43% of the cells responding to PTH with a rise in [Ca2+]i had also incorporated BrdU. This percentage was not different from the percentage BrdU positive cells in the whole culture, showing that calcium responsiveness was randomly distributed between ROB in S-phase and those not in S-phase. We therefore conclude that PTH does not induce calcium responses preferentially during the S-phase of the cell cycle in ROB in primary culture.

Activation of protein kinase C inhibits ATP-induced [Ca2+]i elevation in rat osteoblastic cells: selective effects on P2Y and P2U signaling pathways

Extracellular ATP elicits transient elevation of cytosolic free Ca2+ concentration ([Ca2+]i) in osteoblasts through interaction with more than one subtype of cell surface P2-purinoceptor. Elevation of [Ca2+]i arises, at least in part, by release of Ca2+ from intracellular stores. In the present study, we investigated the possible roles of protein kinase C (PKC) in regulating these signaling pathways. [Ca2+]i of indo-1-loaded UMR-106 osteoblastic cells was monitored by spectrofluorimetry. In the absence of extracellular Ca2+, ATP (100 microM) induced transient elevation of [Ca2+]i to a peak 57 +/- 7 nM above basal levels (31 +/- 2 nM, means +/- S.E.M., n = 25). Exposure of cells to the PKC activator 12-O-tetradecanoyl-beta-phorbol 13-acetate (TPA, 100 nM) for 2 min significantly reduced the amplitude of the ATP response to 13 +/- 4 nM (n = 11), without altering basal [Ca2+]i. Inhibition was half-maximal at approximately 1 nM TPA. The Ca2+ response to ATP was also inhibited by the PKC activators 1,2-dioctanoyl-sn-glycerol or 4 beta-phorbol 12,13-dibutyrate, but not by the control compounds 4 alpha-phorbol or 4 alpha-phorbol 12,13-didecanoate. Furthermore, exposure of cells to the protein kinase inhibitors H-7 or staurosporine for 10 min significantly attenuated the inhibitory effect of TPA. However, these protein kinase inhibitors did not prolong the [Ca2+]i response to ATP alone, indicating that activation of PKC does not account for the transient nature of this response. When the effects of other nucleotides were examined, TPA was found to cause significantly greater inhibition of the response to the P2Y-receptor agonists, ADP and 2-methylthioATP, than the response to the P2U-receptor agonist, UTP. These data indicate that activation of PKC selectively inhibits the P2Y signaling pathway in osteoblastic cells. In vivo, endocrine or paracrine factors, acting through PKC, may regulate the responsiveness of osteoblasts to extracellular nucleotides.

Parathyroid hormone-induced retraction of MC3T3-E1 osteoblastic cells is attenuated by the calpain inhibitor N-Ac-Leu-Leu-norleucinal

Parathyroid hormone (PTH) binding to its osteoblastic receptors stimulates cytoplasmic retraction within minutes. We hypothesized that the calpains (calcium-activated papain-like enzymes) contribute to PTH-induced osteoblastic retraction by catalyzing regulatory hydrolysis of cytoskeletal structural proteins or enzymes important in cytokinesis. N-Ac-Leu-Leu-norleucinal (ALLN), a reversible calpain inhibitor, was tested for its ability to inhibit PTH-induced retraction in murine MC3T3-E1 osteoblastic cells. ALLN inhibited PTH-induced retraction for 30 minutes in cells cultured on polystyrene cultureware or gelatin-coated glass cover slips, supporting the hypothesis that PTH-induced activation of the calpains contributes to short-term changes in MC3T3-E1 cell shape. Inhibition of PTH-induced retraction occurred on two substrata, suggesting that interactions between the extracellular matrix and cell surface proteins are not the sole determinants of morphology. Intracellular events, such as hydrolysis of focal adherens junction proteins on the cytoplasmic face of the plasma membrane, may contribute to PTH-induced retraction.

Single-cell analysis of cyclic AMP response to parathyroid hormone in osteoblastic cells

We previously demonstrated that the [Ca2+]i response to PTH is heterogeneous in single UMR-106-01 osteogenic sarcoma cells. To verify whether response heterogeneity is a universal feature of PTH signal transduction, cAMP production was monitored in monolayer cultures of UMR-106-01 cells and human trabecular bone osteoblasts (HOB) using the cAMP-sensitive fluorescent indicator FlCRhR. FlCRhR was microinjected into single cells, and the 500-530/> 560 nm fluorescence ratio was monitored by confocal laserscanning video imaging as a measure of cAMP concentration ([cAMP]). Virtually all UMR-106-01 cells exposed to bovine PTH(1-34) (10(-7) M) exhibited an increase in intracellular [cAMP], with an average fluorescence ratio change of 145 +/- 17% of baseline (n = 15), corresponding to nearly maximal dissociation of protein kinase A. In the continued presence of the hormone (10(-7) M), [cAMP] remained elevated for at least 30 minutes. This effect was accompanied by a slow translocation of the fluorescein-labeled catalytic subunit of protein kinase A from the cytoplasm to the nucleus. In contrast, PTH(1-34) caused no detectable increase in [cAMP] in HOB cells, although PGE2 (3 x 10(-6) M) stimulation was able to increase the FlCRhR ratio (154 +/- 27%, n = 10). The truncated fragment PTH(2-34) was only 67% as potent at PTH(1-34), but deletion of the first two amino acids at the N terminus abolished the hormone's ability to stimulate cAMP production in UMR-106-01 cells. Brief exposure to 10(-7) M of either PTH(3-34) or PTH(7-34) did not affect the amplitude of the fluorescence ratio change induced by equimolar doses of PTH(1-34). Thus, in osteoblast-like cells stimulated with PTH, the [cAMP] response is much more homogeneous from cell to cell than the [Ca2+]i response.

Dual modulation of the L-type calcium current of rat osteoblastic cells by parathyroid hormone: opposite effects of protein kinase C and cyclic nucleotides

Using whole-cell voltage-clamp recording of rat osteoblastic cells, we show that PTH-(1-34), known to stimulate protein kinase C (PKC) and adenylate cyclase, has a dual effect on the L-type calcium current. It induces a long-lasting increase and a superimposed reversible decrease, which can be separated by repeating hormone applications. The stimulatory effect is the only effect induced by the (3-34) fragment, able to stimulate PKC but unable to stimulate adenylate cyclase. The L current is stimulated by an active phorbol ester and is reduced by permeable analogues of cyclic AMP. Thus, the effect of PTH-(1-34) can be explained by the opposite effects of PKC and cyclic AMP. Dibutyryl cyclic GMP reduces the L current even more potently than dibutyryl cyclic AMP. The above modulations are all voltage-insensitive. These results led us to reinvestigate the effects of some vitamin D3 metabolites known to stimulate PKC and/or guanylate cyclase, and previously reported to affect the voltage-sensitivity of the L current. We only detected voltage-insensitive effects.

Inhibition of prostanoid synthesis depresses alveolar bone resorption but enhances root resorption in the rat

Tooth drift requires the deformation of the root socket and the adjustment of the other components of the attachment apparatus, namely, the periodontal ligament (PDL) and the cementum. Indomethacin (7.5 mg/kg/d), an inhibitor of prostanoid synthesis, provoked in rats a depression in the bone resorption effecting the deformation of the socket (Lasfargues and Saffar, Anat. Rec., 234:310-316, 1992). In the present paper we examined the consequence of this treatment both on the PDL and the root surface. After 3 days of treatment, when osteoclastic resorption was not yet disturbed, the root had been markedly resorbed (P < 0.05) opposite the resorbing bone surface; at that time the PDL width remained in the normal range. After 7 days, i.e., when the bone resorption was depressed, the PDL was widened as the result of the ongoing root resorption. Despite the extensive root resorption, the anchorage of the PDL fibers appeared to remain effective, suggesting that it was rapidly restored. On day 14 at the time of the bone resorption recovery, cementum was deposited in the root resorption lacunae and the PDL width had returned to its control value. As early as day 3 the daily rate of dentine formation increased in the pulp area subjacent to the root resorption lacunae (P < 0.01). These data demonstrate that i) the responses of the different components of the periodontal apparatus are coordinated to allow for the maintainance of the PDL width so that when bone resorption is disturbed, root resorption compensates for it, and ii) the odontoclasts can differentiate and resorb under prostanoid inhibition whilst osteoclastic resorption of the bone socket is inhibited.

Comparison of parathyroid hormone receptors in rat osteosarcoma cells and kidney

Parathyroid hormone/parathyroid-hormone-related peptide (PTH/PTHrP) receptors have been characterized with chicken parathyroid hormone related protein [Tyr36]chPTHrP(1-36)amide (chPTHrP(1-36)) as radioligand in rat UMR-106 osteosarcoma (UMR) cells and in rat renal cortical membranes (RCM). Binding of 125 pM [125I][Tyr36]chPTHrP(1-36) was displaced by chPTHrP(1-36) with ID50 values of 2.6 +/- 0.22 nM (mean +/- S.E.) and 0.9 +/- 0.03 nM in UMR cells and RCM, respectively. ID50 values in membranes from UMR cells and RCM were the same in the presence and absence of 10 microM guanosine-5'-O-(3-thiotriphosphate). Rat [Nle8,18] PTH(1-34) was 5-fold more potent than chPTHrP(1-36) in RCM, but not in UMR cells. Hill coefficients derived from binding inhibition were 0.93 and 0.35 in UMR and RCM, respectively. For affinity labeling, N-hydroxysuccinimidyl-4-azidobenzoate-modified [125I]chPTHrP(1-36) was used. Specifically-labeled PTH/PTHrP-binding proteins had a molecular mass of 83 kDa in UMR cells and RCM. Treatment with N-endoglycosidases lowered the molecular mass of chPTHrP binding proteins to 54 kDa in UMR and RCM. In conclusion, skeletal UMR-106 cells and renal cortical membranes of the rat reveal PTH/PTHrP receptors with no apparent tissue specific differences in molecular mass of the polypeptide backbone and polysaccharide chains. Higher affinity of rat PTH(1-34) binding and lower Hill coefficients in kidney compared to bone are consistent with tissue specific receptor-ligand interactions.

Heterogeneity of intracellular calcium responses to parathyroid hormone and thrombin in primary osteoblast-like cells and UMR106-01 cells: correlations with culture conditions, intracellular calcium concentration and differentiation state

The present study evaluates the effect of parathyroid hormone (PTH) on intracellular calcium. Intracellular calcium ion concentrations ([Ca2+]i) in fetal rat osteoblasts in primary culture (ROB) and in UMR106-01 osteogenic sarcoma cells were monitored as changes in the ratio (R) of Fura-2 fluorescence intensities in single cells as well as populations of cells. In both single ROB and UMR106-01 cells, addition of 10(-7) M rat PTH1-34 and 3 NIH units/ml human thrombin resulted in heterogeneous responses in R values and therefore [Ca2+]i. PTH-induced calcium responsiveness of ROB was dependent on culture conditions, such that response frequencies were positively correlated with the percentage of fetal calf serum in the culture medium. PTH responsive ROB and UMR106-01 cells had significantly higher resting [Ca2+]i than unresponsive cells. PTH- or thrombin-mediated calcium signalling appeared not to be correlated to alkaline phosphatase activity in single ROB. Low percentages of cells responded to PTH in comparison to thrombin suggesting that an increase in [Ca2+]i is not a common PTH signalling pathway in osteoblasts in primary culture. Our data suggest that activation of this signalling pathway by PTH is culture condition dependent, possibly via a cell-cycle related increase in sensitivity of the pathway.

Human parathyroid hormone (1-34) and (1-84) increase the mechanical strength and thickness of cortical bone in rats

An anabolic effect on bone of intermittent parathyroid hormone (PTH) treatment has been found in patients with osteoporosis and also in experimental animals. Controversies exist, however, about whether the positive effect on the trabecular bone balance occurs at the expense of the cortical bone. We examined the biomechanical quality of cortical bone after intermittent treatment with different doses of PTH and, furthermore, compared the effects of PTH-(1-34) and PTH-(1-84). Groups of rats were treated with biosynthetic human PTH-(1-34) or PTH-(1-84), 1.1, 3.3, 10, or 30 nmol/kg/day for 30 days. No changes in the body weights and no changes in the lengths of the femora were observed after the PTH treatments. The biomechanical properties were analyzed by means of a materials-testing machine. A dose-related increase in the bending strength and stiffness of the femora was found, and this increase in mechanical strength corresponds with a 9-12% increase in the cross-sectional area of the femoral diaphyses. The deflection capability and energy absorption were not influenced by any of the PTH treatments. No differences were found between the effects of PTH-(1-34) or PTH-(1-84) on the biomechanical properties of the femora. Consequently, intermittent treatment with biosynthetic PTH-(1-34) or PTH-(1-84) increased the formation of cortical bone, and the biomechanical competence of the femora was found to be preserved.

Pertussis toxin-sensitive activation of phospholipase A2 can be resolved from phosphoinositidase C in primary cultures of mouse osteoblasts using indomethacin

Recent work has established that various bone-resorbing hormones are able to activate phosphoinositide metabolism as well as eicosanoid production in osteoblast-like cells, although the relationship between these pathways is unclear. We used pertussis toxin and indomethacin to inhibit the stimulation of [3H]arachidonic acid release and [3H]phosphoinositide turnover caused by treating primary cultures of mouse osteoblasts with fetal calf serum. We found (1) that pertussis toxin and indomethacin each inhibited both pathways and (2) that although pertussis toxin inhibited [3H]arachidonic acid release to a greater extent than indomethacin, [3H]inositol phosphate accumulation was inhibited rather more effectively by indomethacin. These data suggest that whereas ligands in fetal calf serum activate [3H]arachidonic acid release largely directly via the action of a pertussis-sensitive G protein, activation of phosphoinositidase C is indirect, being substantially dependent upon eicosanoid production. These experiments suggest that serial activation of phospholipase A2 and phosphoinositidase C may occur in osteoblasts and that only the former enzyme is regulated by a pertussis toxin-sensitive G protein.

Parathyroid hormone-induced changes in alkaline phosphatase expression in fetal calvarial osteoblasts: differences between rat and mouse

We studied the effects of parathyroid hormone (PTH) on two markers of the osteoblast phenotype: alkaline phosphatase (AP) (activity and mRNA) and cyclic adenosine monophosphate (cAMP) accumulation. Osteoblast-like cells derived from fetal rat (ROB) and mouse (MOB) calvariae were isolated by collagenase treatment. Cells were cultured in alpha-Minimal Essential Medium (MEM) with 2% fetal calf serum (FCS) for 4 days. In ROB and MOB bPTH(1-34) induced a fast increase (up to 5 minutes) in cAMP accumulation. When equal amounts of cells were seeded, the cAMP accumulation was higher in MOB than in ROB. No difference in basal AP activity was observed between ROB and MOB. When bpTH (1-34) was added to ROB for the last 24 or 48 hr, AP activity decreased dose dependently. However, MOB treated with bPTH(1-34) for the last 24 or 48 hours showed an increase of AP activity. Basal AP activity was positively correlated with the seeding density of ROB and MOB cultures. Basal AP activity influenced the degree of inhibition (ROB) or stimulation (MOB) after incubation with bPTH(1-34).

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)

Hyperparathyroidism, platelet intracellular free calcium and hypertension in chronic renal failure

To investigate possible relationships between hyperparathyroidism, alterations in intracellular free calcium concentration ([Ca2+]i) and hypertension in chronic renal failure, serum concentrations of intact parathyroid hormone (PTH) were measured by two-site immunometric assay, and platelet ([Ca2+]i) was assessed using the fluorescent indicator fura-2. Thirty-six patients with chronic renal failure were studied, 10 with normal serum PTH concentrations (mean 8.0 +/- 0.6 pmol/liter), 17 with elevated serum PTH (35.0 +/- 7.2 pmol/liter) and 9 patients with elevated PTH (36.2 +/- 5.9 pmol/liter) who were receiving nifedipine. Platelet [Ca2+]i was increased in patients with elevated PTH, compared with those in whom PTH was normal (138 +/- 16 vs. 83 +/- 7 nmol/liter, P < 0.01). A linear relation was observed between serum PTH and platelet [Ca2+]i in these patients (r = 0.818, P < 0.001). In contrast, platelet [Ca2+]i was not elevated (84 +/- 9 nmol/liter) in the patients with elevated PTH who were receiving nifedipine. A linear relation was also present between both serum PTH (r = 0.616, P < 0.001) and platelet [Ca2+]i (r = 0.576, P < 0.005) and mean blood pressure. Nine patients with hyperparathyroidism were restudied after treatment with the vitamin D analogue alfacalcidol. This resulted in significant decreases in serum PTH (P < 0.01), platelet [Ca2+]i (P < 0.02), and mean blood pressure (P < 0.05). These studies indicate that [Ca2+]i may be increased early in renal failure, and that this increase occurs in association with both hyperparathyroidism and hypertension. Furthermore, treatment of hyperparathyroidism with alfacalcidol may result in reductions in both [Ca2+]i and blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Recent advances in understanding mechanically induced bone remodeling and their relevance to orthodontic theory and practice

This review highlights recent developments in bone cell biology, evaluates previous research, and offers future direction toward improving our understanding of events that mediate orthodontic tooth movement. The in vivo and in vitro models that have been developed to examine the responses of connective tissues and how they have contributed to our understanding of the mechanisms involved in mechanically induced bone remodeling are discussed in detail. Osteoblasts are now recognized as the cells that control both the resorptive and the formative phases of the remodeling cycle, and receptor studies have shown them to be the target cells for resorptive agents in bone. The osteoblast is perceived as a pivotal cell, controlling many of the responses of bone to stimulation with hormones and mechanical forces. It is apparent that not all the cellular responses induced by mechanically deformed tissues can be explained by the current paradigm emphasizing the importance of prostaglandin production and cAMP elevation; the mobilization of membrane phospholipids giving rise to inositol phosphates offers an alternative second messenger pathway. It is also argued from circumstantial evidence that changes in cell shape produce a range of effects mediated by membrane integral proteins (integrins) and the cytoskeleton, which may be important in transducing mechanical deformation into a meaningful biologic response.

PTH-induced osteoblast contraction is mediated by cysteine proteases

E-64d, a membrane-permeable cysteine protease inhibitor, was tested for its ability to inhibit PTH-induced contraction in intact mouse MC-3T3-E1 osteoblastic cells. Incubation of MC-3T3-E1 cells with vehicle (DMSO) or E-64c, a nonpermeant cysteine protease inhibitor, in the presence or in the absence of PTH had no effects on cAMP production or on morphology from 0 to 90 minutes after addition. In contrast, treatment with E-64d markedly attenuated PTH-induced contraction in these cells. These findings suggest that cysteine proteases, such as the calcium-activated neutral proteases (calpains), are involved in PTH-induced osteoblastic contraction. The observation that cysteine protease activity mediates PTH-induced osteoblastic contraction also suggests that endogenous inhibitors, such as calpastatin, may also be present in the osteoblast and play a role in the regulation of stimulus-response coupling in bone. This mechanism may provide another regulatory point at which bone cells may be pharmacologically manipulated in clinical situations characterized by excessive bone resorption.

Verapamil decreases cyclic load-induced calcium incorporation in ROS 17/2.8 osteosarcoma cell cultures

Bone is a tissue that responds to mechanical load by changing its internal architecture. However, the mode of transmission of mechanical stimuli into biological signals and the effect of load at the cellular level are still not clear. An in vitro system, a Flexercell Strain Unit, was used to apply cyclic load to osteoblast-like cells in culture. In the first series of experiments, ROS 17/2.8 rat osteosarcoma cells, cultured on Flex I, flexible bottomed culture plates, were subjected to a 0.05 Hz, 0.24 STRAIN cyclic load regime for 3 and 7 days, in vitro. One group subjected to load received verapamil, a calcium channel blocker, throughout the experimental period. A second group was exposed to load but received no verapamil. A third group had no drug or load and a fourth group had no load but received verapamil. Cultures were incubated for 24 hours prior to collection with 10 microCi of 45CaCl in the medium, then well bottoms were divided to yield outer (maximum) and inner (minimum) load zones for assay of radioactivity. The effect of verapamil during a 7-day loading period was studied by adding the drug to individual cultures at daily intervals. Results indicated that mechanical loading stimulates calcium incorporation in ROS 17/2.8 cell cultures by day 7 but not by day 3. Only early verapamil addition decreased load-induced calcium incorporation when drug was added prior to day 4. If verapamil was added after 4 days, the channel blocker did not diminish load-induced calcium incorporation.

Protein kinase C modulates parathyroid hormone- but not prostaglandin E2-mediated stimulation of cyclic AMP production via the inhibitory guanine nucleotide binding protein in UMR-106 osteosarcoma cells

In UMR-106 osteosarcoma cells we found that PTH activated both the cAMP/protein kinase A and the Ca(2+)-dependent phosphoinositide/protein kinase C (PKC) pathways, but prostaglandin E2 (PGE2) activated only the cAMP pathway. Activation of PKC by the phorbol ester PMA had no effect on cAMP production but enhanced PTH-stimulated cAMP production by 50% or more; the effect on PGE2-induced cAMP was negligible. Inhibition of the alpha-subunit of the inhibitory guanine nucleotide binding protein (Gi) by pertussis toxin pretreatment also enhanced PTH-mediated cAMP production but had no effect on PGE2-induced cAMP production. These results suggest that although PTH-mediated adenylate cyclase activity is regulated via both the stimulatory (Gs) and inhibitory (Gi) guanine nucleotide binding proteins, only Gs regulates PGE2-mediated adenylate cyclase activity in UMR-106 cells. Costimulation with pertussis toxin and PMA did not increase PTH-stimulated cAMP production above that obtained with PMA alone. This implies a similar target of action for pertussis toxin and PMA, that is, the alpha-subunit of Gi. The alpha-subunit of Gi was found to be a substrate for in vitro PKC phosphorylation of membrane fractions from UMR-106 cells, seen as a +/- 40 kD band on SDS-PAGE. Stimulation of in situ 32P-labeled cells with either PMA or PTH also enhanced incorporation of 32P into the 40 kD band. Using the peptide antisera AS/7 and EC/2, we showed that pertussis toxin-labeled subunits of both Gi1 alpha/Gi2 alpha and Gi3 alpha could be immunoprecipitated, respectively, but immunoprecipitation of membrane proteins after in situ phosphorylation and stimulation with PMA precipitated only Gi2 alpha.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormone responses of in vitro bone nodule cells: studies on changes of intracellular calcium and membrane potential in response to parathyroid hormone and calcitonin

We used two techniques to study the responses of individual in vitro bone nodule cells to parathyroid hormone (PTH) and calcitonin (CT). These techniques are laser scanning confocal imaging with a fluorescent indicator to measure intracellular free [Ca2+], and microelectrode impalement to measure the electrical potential difference across the cell membrane. We applied these measurement techniques to cells in the top cellular layer of nodules that form in vitro in cultures of cells obtained from fetal rat calvaria. Our measurements showed a transient increase in intracellular free [Ca2+] following application of PTH or CT. The duration of the increase in fluorescent intensity following PTH application varied from about 100 to more than 300 s, and the duration following CT application was from 30 to 80 s. In some measurements we applied both hormones in sequence, and observed that some cells showed an intracellular [Ca2+] response to both hormones, while other cells apparently responded to only one or the other of the hormones, or to neither. We also observed membrane potential changes in response to PTH and to CT. The membrane potential response to CT was quite small. The time courses of these membrane potential changes consisted of a depolarizing phase lasting about 100 s (with both hormones) followed by a hyperpolarizing phase (with PTH). Control measurements using only the vehicle solutions were carried out with both techniques, producing negligible responses.

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.

Dissociation between parathyroid hormone-stimulated cAMP and calcium increase in UMR-106-01 cells

We used the osteogenic sarcoma cell line, UMR-106-01, to determine whether the rise in free cytosolic Ca2+ concentration ([Ca2+]i) and cellular cAMP following PTH stimulation are able to be regulated independently. For this purpose, we compared the effect of a PTH antagonist, stimulation of protein kinase C, augmentation by prostaglandins, and the time course of desensitization of the two cellular responses. Two x 10(-7) M of the PTH antagonist 8,18Nle 34Tyr-bPTH(3-34) amide ([Nle,Tyr]bPTH(3-34)A) was required to inhibit 10(-9) M bPTH(1-34)-stimulated cAMP generation by 50%. 10(-7) M bPTH(1-34) completely overcame the inhibition induced by 10(-6) M [Nle,Tyr]bPTH(3-34)A. Only 7 x 10(-8) M and 2.7 x 10(-7) M [Nle,Tyr]bPTH(3-34)A were required to half maximally inhibit the [Ca2+]i increase evoked by 3 x 10(-8) and 10(-7) M bPTH(1-34), respectively. In addition, dissociation between [Ca2+]i and cAMP signals was observed when modulation by protein kinase C and prostaglandins was tested. Preincubation of the cells with 10 nM TPA for 5 minutes markedly inhibited the PTH-evoked [Ca2+]i increase. Short incubation with PGF2 alpha augmented the PTH-evoked [Ca2+]i increase. Similar pretreatments had no effect on the PTH-stimulated cAMP increase. Finally, preincubation with 1.5 x 10(-9) M bPTH(1-34) for 20 minutes almost completely blocked the effect of 10(-7) M bPTH(1-34) on [Ca2+]i, while preincubation with 5 x 10(-9) M bPTH(1-34) for 4 hours was required to inhibit the effect of 10(-8) M bPTH(1-34) on cAMP production by 50%. The differences in the regulation of the two PTH-stimulated cellular signaling systems, in particular, the response to antagonists and the time course of desensitization, could be at the level of the PTH receptor(s) or at a postreceptor domain.

Demonstration of sodium/calcium exchange in rodent osteoblasts

Based on the inhibition of stimulated Ca release from cultured bone by several different agents that alter Na transport, we proposed that hormonally stimulated bone resorption requires Na/Ca exchange. Calcemic hormones appear to interact primarily directly with the osteoblast, which then mediates the activation of osteoclast activity. In organ culture it is not possible to determine whether Na/Ca exchange is involved in this initiating step in the osteoblast or directly in osteoclast-mediated Ca release, and there have been no prior direct measurements of Na/Ca exchange in bone or bone cells. The purpose of this study was to demonstrate the presence of Na/Ca exchange transport in the osteoblast. Thus, we characterized Na-dependent Ca transport in osteoblast-like rat osteosarcoma cells (UMR-106) and primary bone cells isolated from neonatal mouse calvaria. Cells were loaded with the Ca-sensitive dye fura-2 in the presence of physiologic NaCl and the absence of Ca with or without 0.3 mM ouabain. Changes in free cytosolic Ca after the extracellular addition of 1.5 mM Ca were measured spectrofluorimetrically. An outward Na gradient was generated by decreasing extracellular Na while maintaining isotonicity. UMR-106 cells that were Na loaded by ouabain inhibition of Na,K-ATPase activity exhibited 30% greater Ca uptake than control cells. Similar results were obtained with primary bone cells. This uptake required extracellular Ca, was not inhibited by 200 microM verapamil, and was reversible upon reversal of the Na gradient. These data demonstrate the presence of a Na/Ca exchange transport system in osteoblasts.

Lack of effect of ovariectomy on divalent cation regulation of skeletal adenylate cyclase

Both estrogen and androgen have been reported to attenuate cyclic AMP responses to parathyroid hormone stimulation in cultured bone cells. The present study examines the effect of in vivo estrogen deficiency on skeletal adenylate cyclase (AC) activity. The AC activity was compared in bone membranes prepared from normal female guinea pigs and from age-matched guinea pigs 3 weeks after ovariectomy. Histomorphometric analysis of femoral specimens from the ovariectomized guinea pigs demonstrated significant decreases in percentage bone volume, the percentage eroded surfaces and osteoclast numbers, and increased osteoid thickness, compared with the normal controls. No differences were found in basal AC activity, the ability of bone AC to be stimulated by parathyroid hormone (bPTH(1-34)) or isoproterenol, or in the regulation of AC activity by calcium and magnesium. We conclude that bone AC is not a direct target for estrogen effects on bone cells and that the reported effects of sex steroids on cAMP levels in bone cells probably act via an indirect mechanism.

Ipriflavone inhibits phosphoinositide hydrolysis and Ca2+ uptake in the osteoblast-like UMR-106 cells

The mechanism of action of ipriflavone, an isoflavone derivative, was studied in the osteoblastic-like UMR-106 cell line. Ipriflavone affected both phosphoinositide hydrolysis and 45Ca2+ uptake. A repeated treatment of UMR-106 cells (once a day, for 3 days) with ipriflavone decreased, in a concentration-dependent manner, [3H]inositol monophosphate accumulation. This effect was also achieved after single addition of high concentrations of ipriflavone or 100 nM [Asu1,7]eel-calcitonin, a semi-synthetic analog of eel calcitonin. When repeatedly added to UMR-106 cells, 17 beta-estradiol produced a marked inhibition of [3H]inositol monophosphate accumulation, an effect which appeared significant only at a concentration of 1 microM and which was accompanied by a reduced incorporation of [3H]inositol into membrane phospholipids. A repeated treatment with ipriflavone reduced 45Ca2+ uptake as well. This effect was observed also after a single addition of [Asu1,7]eel-calcitonin but not following single or repeated treatment with 17 beta-estradiol. The present data indicate the osteoblast as a direct and specific target for ipriflavone and suggest that this compound may share intracellular transducing mechanisms with other antiosteoporotic hormones such as estrogen and calcitonin.

Effects of parathyroid hormone on cyclic AMP-formation and cytoplasmic free Ca2+ in the osteosarcoma cell line UMR 106-01

The effects of parathyroid hormone (PTH) on cytoplasmic free Ca2+ (Cai2+) and cAMP-formation were investigated in the rat osteosarcoma cell line UMR 106-01. In fura-2 loaded adherent single cells bPTH 1-34 (10 nM - 1 microM) induced a rapid transient increase in Cai2+ in 11% of the studied cells. In fura-2 tracings from UMR 106-01 cells in suspension, bPTH 1-34 (0.1 microM) induced a transient increase in Cai2+ in 20% of the experiments. The transient increase in Cai2+ seen in suspensions of cells was not abolished by addition of EGTA (2.5 mM) prior to challenge with PTH, suggesting that the increase in Cai2+ was derived from intracellular stores. A marked rapid increase in cAMP-formation was observed in all experiments with cells in suspension, also in the experiments where PTH did not affect Cai2+. These data show that PTH causes a release of Ca2+ from intracellular stores in a small percentage of osteosarcoma UMR 106-01 cells, and that PTH is capable of inducing an increase in cAMP-formation without affecting Cai2+ in osteoblasts.

Role of protein kinase C (PKC) in bone resorption: effect of the specific PKC inhibitor 1-alkyl-2-methylglycerol

The specific inhibitor of protein kinase C, 1-O-alkyl-2-O-methylglycerol (AMG), was studied for its effect on bone resorption, measured as 45Ca-release, in fetal mouse calvariae. AMG (1 to 50 microM) had no effect on basal bone resorption. AMG inhibited parathyroid hormone (40 nM) induced bone resorption in a dose-dependent manner. Resorption induced by 1,25 (OH)2-vitamin D3 (10 nM) or prostaglandin E2 (5 microM) was also inhibited by AMG. The release of beta-glucuronidase activity paralleled the course of the 45Ca-release. The production of interleukin 6, induced by parathyroid hormone, in fetal rat calvarial osteoblasts was not affected by AMG. AMG (1 to 50 microM) had no cytotoxic effects on cells or calvariae. From these results it is concluded that protein kinase C may have an important role in the regulation of bone resorption.

Protein kinase A-dependent inhibition of alkaline phosphatase release by SaOS-2 human osteoblastic cells: studies in new mutant cell lines that express a cyclic AMP-resistant phenotype

We have established mutant SaOS-2 cell lines that express a cyclic AMP (cAMP)-resistant phenotype to investigate the regulation and functional importance of orthophosphoric-monoester phosphohydrolase alkaline optimum (ALPase) in the action of parathyroid hormone (PTH). Cells were stably transfected with a plasmid that directs the synthesis of a mutant form of the type I regulatory subunit of protein kinase A (PKA) under the control of the metallothionein promotor. There was no significant difference between parental SaOS-2 cells and the mutant lines in the affinity or number of receptors for 125I-Nle8,18Tyr34bPTH1-34NH2, either in the absence or presence of Zn2+. When cAMP-dependent gene transcription was examined using transient transfection with a somatostatin promoter-chloramphenicol acetyl transferase (CAT) reporter plasmid, CAT activity stimulated by human PTH and dibutyryl cAMP (DBcAMP) was inhibited by greater than 90% in the presence of Zn2+ in the mutant cell lines. In contrast, activation by a phorbol ester of a pentameric collagenase promoter/CAT construct containing five tandem copies of the AP-1 response element (5x-TRE-CAT) was unaffected in Zn(2+)-treated mutant cells. The inhibitory actions of PTH and DBcAMP on ALPase release were blunted by up to 80-90% in the mutant cell lines in the presence of Zn2+; there were no significant differences in the magnitude of inhibitory effects between these agonists. We conclude that the inhibitory action of PTH on ALPase release in SaOS-2 cells is mediated via activation of PKA. These cAMP-resistant cell lines will be especially useful in elucidating signal transduction mechanism(s) for PTH in human osteoblastic cells.

Effects of inositol trisphosphate on calcium mobilization in bone cells

The effect of inositol 1,4,5 trisphosphate (IP3) on calcium mobilization was studied in human osteosarcoma lines, Saos-2 and G292, as well as isolated rat osteoblastic and osteoclastic cells. Cells were permeabilized with saponin and calcium mobilization was studied with the fluorescent dye, fura-2 in a recording spectrofluorometer. IP3 (10 microM) increased calcium release in all cell types studied. The effect was dependent on ATP and occurred in the presence of mitochondrial inhibitors. The effect was not seen with inositol 1-phosphate (IP) or inositol 1,4-diphosphate (IP2). Inositol 1,3,4,5 tetrakisphosphate (IP4) appeared to elicit a decrease in the calcium released. Depletion of the intracellular pool with the calcium ionophore, ionomycin, as well as incubation with the inhibitor of intracellular calcium mobilization, TMB-8, obliterated the IP3 effect. The results are consistent with the hypothesis that increases in IP3 can cause a rapid elevation of bone cell cytosolic calcium.

Pathophysiology of primary hyperparathyroidism

Primary hyperparathyroidism (PHPT) is characterized by hypersecretion of parathyroid hormone (PTH) leading to hypercalcemia and relative hypophosphatemia. PTH acts by binding to cell surface receptors coupled to G proteins. Cyclic AMP is the classic second messenger of PTH action, but substantial evidence indicates that PTH also acts to stimulate formation of the dual second messengers, inositol trisphosphate and diacylglycerol, thereby mobilizing intracellular calcium. The physiologic actions of PTH include (1) an increase in extracellular fluid ionized calcium through direct actions on kidney and bone, the classic target organs for PTH, and (2) a decrease in extracellular fluid phosphate primarily through renal action. The pathophysiologic effects of PTH arise from (1) direct actions of PTH on bone and kidney, and possibly on nonclassic target organs, and (2) indirect effects of altered mineral homeostasis. PTH hypersecretion in PHPT can lead to bony demineralization, nephrolithiasis, and hypercalcemic crisis. PHPT may also be associated with mental disturbances, neuromuscular disease, hypertension, and glucose intolerance.

Prostaglandins enhance parathyroid hormone-evoked increase in free cytosolic calcium concentration in osteoblast-like cells

Prostaglandins (PGs) are autocrine or paracrine hormones that may interact with circulating hormones such as parathyroid hormone (PTH) in bone. We examined the interaction of the PGs, PGF2 alpha, PGE2, and 6-keto-PGF1 alpha with PTH to enhance the rapid, initial transient rise in free cytosolic calcium ([Ca2+]i) and cAMP levels stimulated by PTH. Pretreatment of UMR-106, MC3T3-E1, and neonatal rat calvarial osteoblast-like cells by PGs resulted in an enhancement of the early transient rise in [Ca2+]i stimulated by PTH. PGF2 alpha was approximately 100 times more potent than PGE2. PGE2 itself was more potent than 6-keto-PGF1 alpha in enhancing PTH-stimulated rise in [Ca2+]i. Near-maximal augmentation was achieved at PGF2 alpha doses of 10 nM and PGE2 of 1 microM. The degree of augmentation in [Ca2+]i by PGF2 alpha was independent of preincubation time. PGF2 alpha pretreatment did not alter the EC50 for the PTH-induced [Ca2+]i increase but only the extent of rise in [Ca2+]i at each dose of PTH. The augmented increase in [Ca2+]i was mostly due to enhanced PTH-mediated release of Ca2+ from intracellular stores. PGF2 alpha did not stimulate an increase in PTH receptor number as assessed by [125I]-PTH-related peptide binding. PG pretreatment partially reversed PTH inhibition of cell proliferation, suggesting that an increase in [Ca2+]i may play a role in tempering the anti-proliferative effect of PTH mediated by cAMP. These studies suggest a new mode by which PGs can affect cellular activity.

Parathyroid hormone selectively inhibits L-type calcium channels in single vascular smooth muscle cells of the rat

1. The active synthetic N-terminal fragment of bovine parathyroid hormone, bPTH-(1-34) at a concentration of 1 microM, decreased the peak amplitude of the long-lasting (L-type) calcium channel current by 37% (n = 14, P less than 0.01) in rat tail artery smooth muscle cells. By contrast, this fragment of parathyroid hormone (PTH) (1 microM) had no effect on the transient (T-type) calcium channel current in the same cell preparation. 2. The inhibitory effect of bPTH-(1-34) on L-channel currents was reversible and could be antagonized by the L-channel agonist, Bay K 8644. In contrast, bPTH-(1-34) inhibited Bay K 8644-induced amplification of L-channel currents. 3. The inhibitory effect of bPTH-(1-34) on L-Channel currents was dose dependent with a threshold concentration of less than 10(-7), and voltage dependent with increased inhibition at more positive holding potentials. However, this effect of bPTH-(1-34) was not dependent on different pulse lengths or interpulse intervals. 4. The kinetics of deactivation of L-channel currents were not changed although the instantaneous amplitude of the L-channel tail current was reduced by bPTH-(1-34). 5. Application of bPTH-(1-34) antagonists (10(-6) M-bPTH-(3-34) and 10(-5) M-bPTH-(7-34] did not result in any significant change in the magnitude of L-channel currents (n = 15 and n = 7, respectively). 6. Pre-incubation of cells with bPTH-(3-34) for more than 15 min abolished the inhibitory effect of bPTH-(1-34) on L-channel currents. 7. The present study provides direct evidence to demonstrate the PTH, an endogenous circulating hormone, is a selective inhibitor of L-channel currents in vascular smooth muscle cells.

Regulation of osteocalcin secretion by human primary bone cells and by the human osteosarcoma cell line MG-63

We present evidence that the regulation of osteocalcin secretion by PTH and PGE2 in normal human bone cells can be produced in the human osteoblast-like cell line MG-63. Both cell cultures showed time- and dose-dependent stimulation of osteocalcin secretion in response to 1,25(OH)2D3. Bovine parathyroid hormone (PTH) amino acid fragment 1-34 (40 nM) and prostaglandin E2 (PGE2, 5 nM) significantly inhibited 1,25(OH)2D3-induced osteocalcin secretion by these cells. The inhibition reached 20 and 36%, respectively. In contrast, PTH 3-34 had no effect on osteocalcin secretion. Both cell cultures produced cAMP in response to PTH. Dexamethasone (Dex) (100 nM) potentiated PTH-induced (40 nM) cAMP synthesis in subconfluent MG-63 cells (1.5-fold increase, P less than 0.05). This treatment with Dex resulted in a greater inhibition of 1,25(OH)2D3-induced osteocalcin secretion (-30%, P less than 0.005) by PTH in MG-63 cells as compared to cells exposed to PTH and 1,25(OH)2D3 alone. Pretreatment of subconfluent MG-63 cells with Dex (100 nM) for 48 h also increased 1,25(OH)2D3-induced osteocalcin secretion by 40% (P less than 0.025). In contrast, treatments of confluent MG-63 cells with Dex inhibited osteocalcin secretion regardless of the 1,25(OH)2D3 doses used. Forskolin (10(-7)-10(-5) M) and dibutyryl cAMP (10(-6)-(10(-3) M) both reproduced the effects observed with PTH and PGE2 in the two cell cultures. Forskolin's action was time-dependent: addition of forskolin (10(-6) M) 12 h after 1,25(OH)2D3 (50 nM) resulted in a progressively weaker inhibition of osteocalcin secretion. Increasing the extracellular calcium concentration of the incubation media resulted in a dose-dependent increase in osteocalcin secretion (P less than 0.01). These results indicate that PTH and PGE2 inhibit osteocalcin secretion by a mechanism involving cAMP production. In contrast, an increase in extracellular calcium stimulated osteocalcin release. Thus the human osteosarcoma cell line MG-63 is a useful osteoblast-like cell model to study the regulation of osteocalcin secretion. Furthermore, a factor (or factors) between hormone-receptor coupling and gene induction can regulate the expression of the osteocalcin gene or affect pre- or posttranslational mechanisms implicated in osteocalcin synthesis and secretion.