Down-regulation of the receptor for parathyroid hormone (PTH) and PTH-related peptide by PTH in primary fetal rat osteoblasts

We studied the effects of parathyroid hormone (PTH) on PTH parathyroid hormone related peptide (PTHrP) receptor mRNA level, PTHrP binding and PTH-stimulated cyclic adenosine monophosphate (cAMP) accumulation in osteoblasts, derived from fetal rat calvariae (ROB). Cells isolated during 10-70 minutes of collagenase treatment were seeded at a density of 25,000 cells/cm2 and cultured for 4 days. These cells show a fast increase in cAMP production after stimulation for 5 minutes with 20 nM bovine parathyroid hormone(1-34) (bPTH(1-34)). When ROB are incubated with bPTH(1-34) (0.04-40nM) for 24 h, a dose-dependent decrease of the PTH/PTHrP receptor mRNA level, PTHrP binding, and PTH-stimulated cAMP accumulation can be observed. Pretreatment of ROB with a high concentration of bPTH(1-34) (40 nM) leads within 15 minutes to a decrease in PTH-stimulated cAMP accumulation. However, it takes > or = 3 h before a significant decrease in PTH/PTHrP receptor mRNA level can be observed. Also a significant decrease in PTHrP binding is observed after only 4 h of incubation with bPTH(1-34). Compared with bPTH(1-34), pretreatment of ROB with bPTH(3-34) (40 and 100 nM) for 24 h causes smaller decreases in PTH-stimulated cAMP accumulation, PTHrP binding, and in the PTH/PTHrP receptor mRNA level. We investigated the possible involvement of the protein kinase A signaling pathway in the regulation of the PTH/PTHrP receptor mRNA expression. Both forskolin and (Bu)2cAMP decreased PTHrP binding and PTH/PTHrP mRNA levels. These observations suggest that chronic activation of the PKA signaling pathway may down-regulate PTH/PTHrP receptor expression and thus hormone responsiveness in "normal" osteoblasts. In short, we found that the decrease of the PTH-stimulated cAMP accumulation after long-term pretreatment with bPTH(1-34) is correlated with both PTH/PTHrP receptor mRNA level and PTHrP binding. These data also suggest that the initial desensitization (< 30 minutes) of PTH-stimulated cAMP responsiveness by pretreatment with a high concentration of bPTH(1-34) (40 nM) is not dependent on the number of available PTH/PTHrP receptors. The protein kinase A signaling pathway is involved in the regulation of the PTH/PTHrP receptor, but, regarding the effect of bPTH(3-34), other signaling systems are also involved.

Expression of the parathyroid hormone receptor and correlation with other osteoblastic parameters in fetal rat osteoblasts

Primary fetal rat calvarial cell cultures were examined for the expression of different osteoblastic parameters at the single cell level and in the whole population. The presence of the parathyroid hormone (PTH) receptor was studied by employing receptor autoradiography. After 3 days of culture, 10% of the cells expressed the PTH receptor. Immunolocalization of osteocalcin in 3-day-old cell cultures was found to be strongly correlated with the presence of the PTH receptor. Alkaline phosphatase (APase) localization in 3-day-old cultures correlated with only 69% of the PTH receptor expressing cells. Our results show that in 3-day-old rat calvarial cell cultures, only about 10% of the cells show markers of osteoblastic differentiation. The presence of the PTH receptor is strongly correlated with the presence of osteocalcin, but less with the presence of APase, indicating that it is the mature osteoblast that expresses the PTH receptor. After 7 days of culture, most receptor labeling, APase, and osteocalcin expression was found in multilayered areas of cells (nodules).

Down-regulation of the receptor for parathyroid hormone (PTH) and PTH-related peptide by transforming growth factor-beta in primary fetal rat osteoblasts

We studied the effects of transforming growth factor-beta 2 (TGF beta 2) on the level of PTH/PTH-related peptide-(PTHrP) receptor messenger RNA (mRNA), PTHrP binding, and PTH-stimulated cAMP accumulation in cultured osteoblasts derived from fetal rat calvariae (ROB). When ROB were pretreated with TGF beta 2 at concentrations ranging from 1-100 pM for 24 h, dose-dependent decreases in the level of PTH/PTHrP receptor mRNA, PTHrP binding, and PTH-stimulated cAMP accumulation were observed. For the PTH/PTHrP receptor mRNA level and PTH-stimulated cAMP accumulation, the half-maximal effective concentration was approximately 4 pM. For the inhibition of PTHrP binding, the half-maximal effective concentration was much higher. A 50% decrease in both PTH/PTHrP receptor mRNA level and PTH-stimulated cAMP accumulation was obtained when ROB were treated with 100 pM TGF beta 2 for 4 h. A comparable decrease in PTHrP binding was only observed after 24 h of incubation with 100 pM TGF beta 2. Actinomycin D induced a rapid decrease in the PTH/PTHrP receptor mRNA level (70% after 4 h), indicating a half-life for the receptor mRNA of 2-3 h. Under the same conditions, PTHrP binding and PTH-stimulated cAMP accumulation did not change. When ROB were treated with cycloheximide for the same period, only a small decrease in PTHrP binding (20%) was observed, suggesting that PTH/PTHrP receptors do not have a rapid turnover. Cycloheximide also reduced PTH-stimulated cAMP production; after coincubation of cycloheximide with TGF beta 2, this inhibition was smaller than that in ROB cultures treated with TGF beta 2 exclusively. From these observations we conclude that TGF beta 2 induces a decrease in steady state levels of PTH/PTHrP receptor mRNA that results in decreased PTHrP receptor binding. The PTH-stimulated cAMP accumulation is at least to some extent independent of the PTH/PTHrP receptor availability. Furthermore, there is a high turnover of PTH/PTHrP receptor mRNA, whereas turnover of the receptor protein is much slower. Finally, protein synthesis is required for TGF beta 2-induced desensitization of cAMP responsiveness to PTH.

Down-regulation and differential restoration of cAMP responses upon transient phorbol ester treatment of primary osteoblastic cells

We studied cAMP responses induced by parathyroid hormone (PTH), prostaglandin E2 (PGE2) and forskolin in foetal rat calvariae-derived osteoblastic cells after 24 h treatment with a protein kinase C (PKC) activating phorbol ester. After this treatment, meant to down-regulate PKC activity, all tested cAMP responses were attenuated and were indeed accompanied by a decline in PKC activity. PTH receptor affinity was not altered and PTH receptor number was only slightly lowered after 24 h phorbol ester treatment. These results indicate that modulation of the cAMP responses by 24 h PMA treatment was mainly caused by a general impairment of adenylyl cyclase activity. Removal of the phorbol ester and subsequent culture for 2 days rendered the cells hyper-responsive to PTH: the PTH-induced cAMP response was 2 to 3 times higher than in control cells. Again no change in binding affinity of the PTH receptor was observed and receptor number was just 10% lower than in control cells. The PGE2- and forskolin-induced cAMP responses were not higher than normal. So, transient phorbol ester treatment leads to a differential, agonist-dependent restoration of the cAMP signalling system.

Regulation of protein kinase C activity by phorbol ester, thrombin, parathyroid hormone and transforming growth factor-beta 2 in different types of osteoblastic cells

We investigated the role of protein kinase C (PKC) in osteoblast function using a set of putative PKC modulating factors and an in situ peptide substrate-based kinase assay in different types of osteoblastic cells. Primary calvarial rat osteoblastic cells (ROB) and ROS 17/2.8 osteosarcoma cells showed an equally high PKC activity when a maximal dose of PKC-activating phorbol ester was applied. The osteosarcoma cell line UMR 106-01 showed only 5-10% of this maximal PKC activity. All 3 cell types responded to 10 U/ml thrombin with a 2-fold stimulation of PKC activity. However, no distinct direct effects of parathyroid hormone (bPTH (1-34)) or transforming growth factor-beta 2 (TGF-beta 2) were found in either of the cell types. The thrombin-induced stimulation of PKC was associated with an increase in the PTH-mediated cAMP response of ROB. Down-regulation of PKC-activity was found when ROB were treated for 24 h with phorbol ester and, interestingly, also after a 24 h treatment with bPTH (1-34) and TGF-beta 2. We conclude that differences in PKC activity exist among osteoblastic cell types, which may be related to their different proliferative activity. Direct PKC activation may lead to modulation of the cAMP signaling pathway. Down-regulation of PKC activity by bPTH (1-34) and TGF-beta 2 provides an interesting possible mechanism for the long-term regulation of signal transduction.

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.

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).

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.

Parathyroid hormone sensitizes long bones to the stimulation of bone resorption by 1,25-dihydroxyvitamin D3

In response to hypocalcemia the serum PTH level increases rapidly followed by a PTH-induced rise in 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] production. Therefore, bone is first exposed to increased PTH levels before increased 1,25-(OH)2D3 levels. In the present study the effect of pretreatment with PTH on 1,25-(OH)2D3-induced bone resorption was examined. Bone resorption was measured as release of prelabeled 45Ca during culture from 17-day-old fetal mice radii/ulnae and metatarsals. Radii/ulnae and metatarsals are characterized by differences in development. In radii/ulnae mature osteoclasts are present, whereas in metatarsals only different stages of preosteoclasts can be found. Preincubation for 24 h but not 4 h with PTH increases the stimulation of bone resorption by 1,25-(OH)2D3 in fetal radii/ulnae but not in metatarsals. Coincubation of PTH and 1,25-(OH)2D3 did not result in a significant change in bone resorption compared to 1,25-(OH)2D3 alone. The observed difference in the effect of pretreatment with PTH between radii/ulnae and metatarsals indicates that PTH does not stimulate the development of early osteoclast precursors but that a certain level of differentiation of the osteoclast precursor is required. Pretreatment with prostaglandin E2 resulted in an effect similar to that of PTH. Inhibition of prostaglandin synthesis by indomethacin prevented the potentiation of 1,25-(OH)2D3-induced bone resorption by pretreatment with PTH. Thus, the present study demonstrates that PTH sensitizes responses to 1,25-(OH)2D3. PTH must be present before 1,25-(OH)2D3 to observe a potentiation of 1,25-(OH)2D3-induced bone resorption.(ABSTRACT TRUNCATED AT 250 WORDS)

Salmon stanniocalcin and bovine parathyroid hormone have dissimilar actions on mammalian bone

Stanniocalcin (STC), a calcium-regulating glycoprotein hormone isolated from the corpuscles of Stannius of salmon, was tested for effects on bone and calcium metabolism in mammalian species (rats and mice). STC generally failed to alter serum calcium of parathyroidectomized rats at concentrations equimolar with effective concentrations of parathyroid hormone (PTH). STC did not increase cAMP in ROS 17/2.8 or UMR-108 osteosarcoma cells, OK kidney cells, fetal rat limb bones, or neonatal mouse calvariae, and similarly failed to increase urinary cAMP in rats. STC did not consistently stimulate resorption in any of the rodent bone culture systems, although variable resorptive responses were elicited in fetal mouse calvariae. The results indicate that this fish hormone has limited, if any, PTH-like activity on calcium metabolism in mammalian systems.

Modulation of responsiveness to cAMP stimulating agonists by phorbol ester in fetal rat osteoblasts

We studied the effect of activation of protein kinase C (PKC) by a phorbol ester on cAMP accumulation in fetal rat osteoblasts. Activation of PKC by phorbol 12-myristate 13-acetate (PMA) caused a potentiation of cAMP accumulation induced by parathyroid hormone (PTH), forskolin, and cholera toxin. The results suggest that the potentiating effect of PMA on PTH-induced cAMP accumulation was not due to an effect on the PTH-receptor nor to an effect on cAMP degradation, as the effect of PMA persisted in the presence of a phosphodiesterase inhibitor. Pretreatment of the cells with pertussis toxin did not prevent the action of PMA, indicating that PMA does not act via the inhibitory G-protein. PMA had a biphasic effect on prostaglandin E2 (PGE2)-induced cAMP accumulation; i.e., at concentrations greater than or equal to 10(-6) M, PMA potentiated the PGE2-induced cAMP response but PMA attenuated cAMP accumulation induced by concentrations of PGE2 less than or equal to 5.10(77) M. From our data we conclude that PKC can interact with a stimulated cAMP pathway in a stimulatory and inhibitory manner. Potentiation of cAMP accumulation is probably due to modification of the adenylate cyclase complex, whereas attenuation of stimulated cAMP accumulation appears to be due to an effect on a different site of the cAMP generating pathway, which may be specific to PGE2-induced cAMP accumulation.

Growth hormone induces expression of c-jun and jun B oncogenes and employs a protein kinase C signal transduction pathway for the induction of c-fos oncogene expression

Although the structure of several members of the GH receptor family has been defined, signal transduction following GH binding to its receptor has not been elucidated. Mouse osteoblasts were used to study the effect of GH on immediate early gene expression and, subsequently, the cellular signal(s) mediating this expression were analysed. GH rapidly and transiently induced the expression of c-jun and jun B in concert with the already reported expression of c-fos. The GH-induced expression of c-fos was completely blocked by the protein kinase inhibitors staurosporine and H7, indicating that the action of GH is mediated by one or several protein kinases. We next analysed the identity of the putative protein kinases in more detail by using a more specific protein kinase inhibitor, namely the ether-lipid 1-O-alkyl-2-O-methylglycerol, understood to be an inhibitor of protein kinase C (PKC). Data obtained from these studies revealed that GH-induced expression of c-fos is mediated by PKC. In addition, we observed a profound increase in formation of the PKC activator diacyglycerol upon addition of GH, a natural activator of PKC. In conclusion, upon binding of GH to mouse osteoblasts, the receptor-mediated cellular signal involves diacyglycerol formation and activation of PKC, leading to the induction of oncogene expression. Finally, the expression of c-fos, c-jun and jun B results in an increased binding of protein complexes to AP-1 binding sites.

Characterization of growth factor activity produced by fetal mouse osteoblasts

Bone matrix contains a variety of growth factors, but little is known of osteoblastic production of such materials. The present study assesses growth factor activity, chromatographed on acidic Bio-Gel P-100, secreted into conditioned media of primary cultures of fetal mouse calvaria. The cultures produced insulin-like growth factor-I (IGF-I), determined by radioimmuno-assay of molecular weights 20 and 10 kDa. IGF-II, determined by radioreceptor assay, was present at 20-29 and 7 kDa. The IGF peaks at 20, 10 and 7 kDa were all mitogenic in MCF-7 cells. Proteins of several different molecular weights were also present that specifically bound IGF-I and IGF-II. Transforming growth factor-beta (TGF-beta), assayed in a system for inhibition of growth, was also produced. Both activated and latent forms were present, and part of the TGF-beta was TGF-beta 2. The absence of mitogenic activity in the bmolecular range of platelet-derived growth factor, assayed in 3T3 fibroblasts, makes it unlikely that mouse osteoblasts produce this growth factor.

Parathyroid hormone-induced ornithine decarboxylase activity in fetal rat osteoblasts

Induction of ornithine decarboxylase (ODC, E.C. 4.1.1.17) activity by parathyroid hormone (PTH) in cultured fetal rat osteoblasts was studied. PTH induced ODC activity and stimulated cAMP production in a dose-dependent manner, the ED50 for cAMP being five times as high as that for ODC. Induction of ODC activity by PTH was partly inhibited by actinomycin D and cycloheximide, with 40 and 55% inhibition, respectively. PTH increased the intracellular ionized calcium concentration ([Ca2+]i), which was absent in a Ca2+-free medium. Blocking calcium influx, lowering the extracellular calcium concentration, and adding trifluoperazine inhibited both induction of ODC activity and stimulation of cAMP production by PTH. A23187 (100 nM and 1 microM), combined with a low dose of PTH (4 nM), resulted in a synergistic induction of ODC activity and an inhibition of cAMP production. A23187 inhibited induction of ODC activity as well as stimulation of cAMP production by the dose of PTH (20 nM) maximally effective in inducing ODC activity. Forskolin together with this maximal dose of PTH resulted in an additive effect on ODC activity and a synergistic stimulation of cAMP production. The current results show similarities and differences with respect to results obtained with osteoblasts from other species and osteoblast cell lines. The present data indicate that (1) PTH stimulates ODC activity and this is partly due to new enzyme synthesis; (2) calcium is involved in induction of ODC activity and stimulation of cAMP production by PTH; furthermore, it is suggestive that calmodulin and/or protein kinase C are involved; and (3) stimulation of cAMP production by PTH depends on an optimal intracellular calcium concentration range.

Rainbow trout hypocalcin stimulates bone resorption in embryonic mouse calvaria in vitro in a PTH-like fashion

Hypocalcin, the major hormone with hypocalcaemic action in fish, was isolated from trout corpuscles of Stannius (SCs). The bioactivity of hypocalcin was assessed in a parathyroid hormone (PTH) bioassay involving bone resorption in embryonic mouse calvaria. Calcium and phosphate release and lactate production were stimulated in a dose-dependent manner by hypocalcin. On a molar basis about equal amounts of hypocalcin and PTH were required to obtain similar effects in this assay. Hypocalcin did not stimulate cyclic AMP production either in mouse calvaria or in cultured osteoblasts. In this respect hypocalcin resembles shortened or N-terminus-modified PTH molecules that induce bone resorption without increasing cyclic AMP levels. Since hypocalcin and PTH have comparable bioactivity in this mammalian bioassay (as well as in fish bioassays), we tentatively suggest that both hormones are structurally similar and that both hormones may act via the same receptors. The two hormones show no resemblance to one another in primary structure, so we suggest that they have similarities in tertiary structure.

Modulatory function of protein kinase C in the activation of ornithine decarboxylase and in cAMP production in rat osteoblasts

The effect of activation of protein kinase C on stimulation of ornithine decarboxylase (ODC) activity and cAMP production was studied in fetal rat osteoblasts. Both phorbol 12-myristate, 13-acetate (PMA), an activator of protein kinase C, and 4 alpha-phorbol, ineffective in activating protein kinase C, failed to stimulate ODC activity and cAMP production. We tested the effect of protein kinase C on stimulation of ODC activity by parathyroid hormone (PTH) and forskolin. In contrast to PTH-stimulated ODC activity, which was not affected by PMA, forskolin-stimulated (1 and 10 microM) ODC activity was dose dependently reduced. PMA (400 nM) reduced both 1 and 10 microM forskolin-stimulated ODC activity to the same level, approximately 3 nmol CO2/mg protein, which suggests a controlling role of protein kinase C in forskolin-stimulated ODC activity. The study of the effect of protein kinase C on PTH- and forskolin-stimulated cAMP production also revealed differences between PTH and forskolin. When PMA was added simultaneously with PTH (4 and 20 nM) or forskolin (1 and 10 microM) the PTH-stimulated cAMP production was dose-dependently potentiated by PMA, whereas forskolin-stimulated cAMP production was not affected. However, both PTH- and forskolin-stimulated cAMP production was dose-dependently augmented when PMA was added 3 min prior to PTH or forskolin. With increasing preincubation periods (up to 24 h) with PMA instead of a potentiation an inhibition was observed. This inhibition is not due to PTH receptor desensitization, although, on basis of the present results desensitization can not completely be excluded. In all cases 4 alpha-phorbol was without effect. The present results show that protein kinase C modulates stimulation of ODC activity and cAMP production in fetal rat osteoblasts. The modulation of both ODC activity and cAMP production appears to be dependent on the nature of the stimulator. The present data suggest a role for protein kinase C in limiting the cAMP-mediated stimulation of ODC activity in these cells. Furthermore, it is suggested that protein kinase C can interfere at more than one site in the cAMP-generating system.

Independent and interrelated regulation of ornithine decarboxylase by calcium and cAMP in fetal rat osteoblasts

The present study was undertaken to determine in fetal rat osteoblasts whether and how the intracellular messengers calcium and cAMP are involved in stimulation of ornithine decarboxylase (ODC) activity. For that purpose we used different drugs affecting [Ca2+]i and cAMP concentration. A23187 stimulates ODC activity in a biphasic way, with maximal stimulation at 100 nM A23187. At that concentration no stimulation of cAMP production was observed. Basal and A23187-stimulated (100 nM) ODC activity were inhibited by EGTA and trifluoperazine. Forskolin stimulated dose-dependently both ODC activity and cAMP production. Besides these effects forskolin (1 and 10 microM) increased the [Ca2+]i via an increased calcium influx. Addition of La3+, verapamil or EGTA, but not of trifluoperazine, significantly inhibited the forskolin-stimulated (10 microM) ODC activity. When forskolin (100 nM and 1 microM) was added together with 1 microM A23187, a synergistic stimulation of ODC activity was observed. These results implicate that calcium is involved in basal ODC activity, and that ODC activity can be stimulated via (1) a cAMP-independent calcium pathway, and (2) a calcium-dependent, cAMP pathway. It is proposed that ODC activity can be stimulated via interaction between calcium and cAMP.

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)

Effect of parathyroid hormone and parathyroid hormone fragments on the intracellular ionized calcium concentration in an osteoblast cell line

In the present study the effect of parathyroid hormone and parathyroid hormone fragments on the intracellular ionized calcium concentration in an osteoblastic cell line (UMR 106-01) is examined. In these cells the intact hormone, bPTH(1-84) and three fragments (bPTH(1-34), hPTH(1-34) and bPTH(3-34) induce an initial increase of the ionized calcium concentration [( Ca2+]i). The increase of the [Ca2+]i due to bPTH(1-84) is more pronounced and of longer duration compared to the increase due to PTH fragments. For two parathyroid hormone fragments with a modified amino acid sequence [( Nle-8,Nle-18,Tyr-34]bPTH(1-34), [Nle-8,Nle-18,Tyr-34]bPTH(3-34], no effect was found. Our observations suggest: (1) a second messenger role for intracellular ionized calcium in the action of bPTH(1-84) in osteoblasts; (2) a functional role for the COOH-terminal part in the action of bPTH(1-84); and (3) the existence of a specific part of the hormone being responsible for the increase of the intracellular ionized calcium concentration.

Direct stimulatory effect of growth hormone on DNA synthesis of fetal chicken osteoblasts in culture

GH action on cartilage is thought to be mainly mediated through the somatomedins produced by the liver. However, recent studies using cultured chondrocytes also point to a direct mitogenic action of GH. Besides, a direct action of GH on fetal rat tibiae has been demonstrated. This study is the first to show stimulation of thymidine uptake as well as DNA synthesis under the influence of physiological concentrations of human and chicken GH in fetal chicken osteoblasts in culture.

Involvement of cAMP and calcium in the induction of ornithine decarboxylase activity in an osteoblast cell line

The role of cAMP and calcium in the induction of ornithine decarboxylase (ODC, E.C.4.1.1.17) activity in the osteogenic sarcoma cell line, UMR 106-01, was studied, with particular interest for parathyroid hormone (PTH). PTH and forskolin dose-dependently induced the ODC activity and the cAMP production. Protein synthesis is involved in the effect of PTH and forskolin on ODC activity but not on cAMP production. Using quin2 we showed that 20 nM PTH and 10 microM forskolin increased the intracellular ionized calcium concentration ([Ca2+]i), thereby offering the possibility for calcium to play a role as cellular mediator in the action of PTH and forskolin in bone. Data obtained with A23187 showed that solely an increase of the [Ca2+]i is not sufficient to stimulate basal or potentiate PTH- and forskolin-induced ODC activity. However, the effects of calcium channel blockers and EGTA on basal and PTH- and forskolin-induced ODC activity point to a specific role for calcium. Moreover, the effects of calcium channel blockers and EGTA on basal and PTH- and forskolin-induced cAMP production indicate that the involvement of calcium in the induction of ODC activity is primarily located at another site than the adenylate cyclase. These data indicate that calcium is involved in the control of basal ODC activity. Furthermore, these data suggest that both cAMP and calcium are involved in the induction of ODC activity by PTH and forskolin. More precisely, ODC activity in UMR 106-01 cells can be induced by PTH and forskolin via a calcium-dependent cAMP messenger system.

Vitamin K is no antagonist for the action of warfarin in rat osteosarcoma UMR 106

The recycling of vitamin K in the liver occurs via one or two dithiol-dependent reductases, which are strongly inhibited by coumarin derivatives such as warfarin. This inhibition may be partly overcome by the action of a NADH-dependent reductase, which is relatively insensitive for warfarin. In this paper we demonstrate that the osteoblast-like osteosarcoma UMR 106 does not contain the NADH-dependent reductase. Assuming that a similar enzyme distribution occurs in normal osteoblasts this explains the observation of Price and Kaneda, that the administration of vitamin K to rats efficiently counteracted the effect of warfarin on blood coagulation, but that the vitamin had no effect on the Gla-content of serum osteocalcin.

Growth hormone is mitogenic for fetal mouse osteoblasts but not for undifferentiated bone cells

More evidence has recently been obtained indicating that growth hormone (GH) has a direct effect on bone. However, it is not clear which cell type reacts to the hormone. The present study used osteoblast-like cells derived from sequentially digested fetal mouse calvaria. Separately cultured tractions resulted in populations enriched in cells with a more or a less differentiated phenotype. The results showed that GH acts on the cells released last, i.e. those with more characteristics of the osteoblast. In these cells, GH induced strong mitogenic activity. Prolactin was not active.

Different roles for calcium and cyclic AMP in the action of PTH: studies in bone explants and isolated bone cells

In fetal mouse calvaria forskolin (0.1-100 microM), like PTH, stimulated cyclic AMP production in a dose-dependent way. The dose-response curve for forskolin-induced bone mineral release (24 hrs), however, demonstrated a biphasic character, showing stimulation at 0.1 microM and inhibition at 5 and 10 microM. In addition, forskolin-stimulated bone resorption reached a plateau after 48 hrs of incubation, a phenomenon which did not occur with PTH. Forskolin (0.1 microM) strongly stimulated PTH-induced cyclic AMP production in fetal mouse calvaria. However, PTH-stimulated bone resorption and PTH-induced increase in cytosolic free Ca2+ in bone fetal rat cells were not stimulated by forskolin (0.1 microM). 9-(Tetrahydro-2-furyl) adenine (100 microM) completely blunted PTH-stimulated cyclic AMP response in fetal mouse calvaria. PTH-stimulated bone resorption was also completely inhibited, but only after 6 hrs and not after 24 hrs of incubation. With nifedipine and varabamil PTH-stimulated bone resorption was significantly inhibited after 24 hrs of incubation and not significantly after 6 hrs of incubation. A23187 (1 microM) significantly stimulated PTH-stimulated cyclic AMP level and increased basal cytosolic free Ca2+ concentration in cultured rat bone cells. In calvaria, however, it had no effect on either basal and PTH-stimulated cyclic AMP production or on basal and PTH-stimulated bone resorption (6 and 24 hrs). From these observations it follows that in calvaria manipulation of intracellular cyclic AMP only (partially) affects bone resorption. This observation points to a role for an additional second messenger in establishing full blown bone resorption. Some of the results are published in short elsewhere.

Identification of hypocalcin (teleocalcin) isolated from trout Stannius corpuscles

We have isolated and purified a glycoprotein from the corpuscles of Stannius (CS) of trout, which we consider hypocalcin (also called teleocalcin), the major hypocalcemic hormone of fish. This product is present in relatively large amounts in the CS of several species (i.e., European eel, tilapia, goldfish, and carp). Hypocalcin is typically released from the CS in response to an experimentally induced increase of the blood calcium concentration. Ultrastructural observations show that after this treatment the type 1 cells, reportedly the hypocalcin-producing cell type of the CS, are almost completely degranulated. The isolated glycoprotein has an apparent molecular weight of 54 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This molecule appears susceptible to breakdown and is recovered upon concanavalin-A affinity chromatography as a 41 kDa product. Reducing agents such as mercaptoethanol or dithiothreitol employed, e.g., during standard electrophoretic techniques or during amino acid sequence analysis, allow only the recovery of 28 or 18 kDa products. Evidence is given that the 54 and 41 kDa products are dimer molecules, with the 28 and 18 kDa products as their respective monomeric constituents. The sequence of the first 33 N-terminal amino acids of these products and the composition of the sugar component are presented.

Parathyroid hormone-like effects of rainbow trout Stannius products on bone resorption of embryonic mouse calvaria in vitro

Products of the Stannius corpuscles (SC) of rainbow trout were tested in an established PTH bioassay involving bone resorption in embryonic mouse calvaria. Aqueous extracts from Stannius corpuscles (SC-homogenate) showed a bone-resorbing activity comparable to PTH in 24-h cultures of calvaria, indicated by a dose-dependent stimulation of lactate production and of calcium, phosphate as well as beta-glucuronidase release. Moreover, SC-homogenates induced an increase in osteoclastic activity. The PTH-like SC-principle is released during in vitro incubations of the glands. These results and the lack of an additive effect of SC-products and PTH on bone resorption suggest that both products activate the same receptor. We hypothesize that the hypocalcemic hormone of the SC of fish shares structural resemblance with PTH, the major hypercalcemic hormone of terrestrial vertebrates.

An in situ assay system to measure ornithine decarboxylase activity in primary cultures of chicken osteoblasts: effects of bone-seeking hormones

We present a rapid and uncomplicated in situ assay for measuring ornithine decarboxylase activity in small cell quantities. This method is more economic than the in situ methods described by others. In addition, our system is faster and less complicated since it avoids manipulation of the CO2-trapping paper. Applying this method we demonstrate that parathyroid hormone, PGE1, and other inducers of intracellular cAMP levels, like IBMX and forskolin can induce ODC activity in primary cultures of chicken osteoblasts. Salmon calcitonin does not induce ODC activity, and 1.25 (OH)2D3 at higher concentrations can even give an inhibition of ODC activity. We confirm the recent findings that ODC activity is also dependent on calcium.

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

It has been suggested that intracellular Ca2+, in addition to cAMP, plays an important role in PTH-stimulated bone resorption. There is now strong evidence indicating that the osteoblast is the main target cell for PTH action, regulating indirectly, via cell-cell communication, osteoclastic bone resorption. In order to investigate the possible role of free cytosolic calcium in stimulated bone resorption, we studied the effects of the intact hormone (bPTH 1-84) and some of its fragments (bPTH (1-34), bPTH(3-34,) (Nle-8, Nle-18,Tyr-34) bPTH (3-34) amide) on their capacity to modify the cytosolic Ca2+ concentration in rat osteoblast-like cells. The experiments were performed using Quin-2, a fluorescent indicator of free calcium. We found an excellent correlation between the ability of PTH and PTH fragments to transiently increase cytosolic Ca2+ concentration in rat osteoblast-like cells and their ability to stimulate bone resorption in embryonic rat calvaria in vitro. On the other hand, no direct correlation was found for the cAMP and bone-resorbing responses. On the ground of these data we propose a two-receptor model for PTH action in osteoblasts, in which one receptor is coupled to the production of cAMP, whereas the other is involved in the increase of cytosolic Ca2+. Activation of both receptors by PTH (1-84) or PTH (1-34) leads to the full physiological response in osteoblasts, most probably the release of one or more factors which stimulate the activity of existing osteoclasts and others which stimulate the recruitment of additional osteoclasts.

The effects of 1,25-dihydroxycholecalciferol on embryonic bone in vitro: a biochemical and histological study

Explants from embryonic rat and mouse calvaria were cultivated in the presence of different concentrations of 1, 25-dihydroxycholecalciferol (1, 25-(OH)2D3). The bone resorbing effects of the vitamin D3 metabolite were evaluated by measuring the release of calcium, phosphate, lactate and citrate into the culture medium after 24 h of cultivation. The influence on bone morphology was studied using embryonic mouse radii, in which histological phenomena in the bony, cartilagenous and connective tissue compartments were observed. Both kinds of experiments show that 1, 25-(OH)2D3 has effects on embryonic bone which are typical for high concentrations of parathyroid hormone (PTH). However, the maximal effects on calcium release and on histology are 2.5 times less than those of PTH. In addition, some of the histological features such as the effects on epiphyseal cartilage and on osteoclasts were not observed. 1, 25-(OH)2D3 in concentrations up to 2 X 10(-9) M does not affect basal or PTH-stimulated cAMP levels in embryonic rat calvaria. In a concentration of 1 X 10(-8) M, however, a significant decrease in PTH-stimulated cAMP production was found. It is concluded from these in vitro experiments that (1) 1, 25-(OH)2D3 stimulates bone resorption, and (2) in 1, 25-(OH)2D3-induced bone resorption cAMP is apparently not involved as a second messenger.

The effect of verapamil on the action of parathyroid hormone on embryonic bone in vitro

Short term in vitro experiments showed that, added alone, verapamil inhibited both glycolysis and Ca uptake in embryonic chicken and rat bone cells. Added together with PTH, verapamil (0.02 MM) enhanced cAMP production, had no effect on lactate production, but significantly inhibited citrate, calcium and phosphate release from embryonic rat and mouse calvaria incubated under hypocalcemic conditions. The depression by verapamil of PTH-stimulated demineralization was confirmed histologically. It is concluded that in addition to cAMP, Ca plays a key role in the action of PTH on bone.

Effects of bone in vitro of bovine parathyroid hormone and synthetic fragments representing residues 1-34, 2-34 and 3-34

The biological activities of bovine parathyroid hormone (BPTH) and fragments comprising portions of its amino-terminal sequence have been compared in three different assay systems using embryonic rat bone in vitro. Whereas the 3-34 fragment was without significant activity the 1-34 fragment caused all the actions characteristic of BPTH 1-84, extending to bone previous evidence that the amino-terminal residues are sufficient for expression of the biological effects of intact parathyroid hormone. However, the relative potencies of the fragment and the intact hormone were different in the various systems. BPTH 1-34 showed relatively low osteolytic activity and induced anabolic effects in both osteoblasts and cartilage cells of cultivated embryonic mouse radii which were not evoked by the intact hormone. Further work is required to determine the mechanisms responsible for these interesting alterations in relative potency of fragment and native hormone.