Effects of prostaglandins and other drugs on the cyclic AMP content of cultured bone cells

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.

Regulation of 25-hydroxyvitamin D3 metabolism in cultures of osteoblastic cells

This study was designed to investigate the mechanisms involved in the regulation of the conversion of 25-hydroxyvitamin D3 (25-OHD3) to 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] and 24,25-dihydroxyvitamin D3 [24,25-(OH)2D3] in primary cultures of osteoblastlike cells from neonatal mouse calvariae. These cells, when incubated with tritiated 25-OHD3 ([3H]25-OHD3), spontaneously synthesized [3H]24,25-(OH)2D3 20-50 times more efficiently than [3H]1,25-(OH)2D3 at a rate of conversion that was substrate dependent and linear from 1 to 36 h. Gas chromatography-mass spectrometry verified the identity of the dihydroxylated metabolites. The calcium ionophore A23187 (5 microM) consistently stimulated the synthesis of 1,25-(OH)2D3 while suppressing the production of 24,25-(OH)2D3. This effect was sustained for 36 h and was dose dependent for concentrations from 0.05 to 10 microM. Furthermore, A23187 stimulated cAMP production and indomethacin (50 ng/ml) blocked the A23187-induced production of cAMP and 1,25-(OH)2D3 but had no effect on the suppression of 24,25-(OH)2D3 by A23187. This led to other experiments to find out whether the stimulative effect of A23187 on 1,25-(OH)2D3 synthesis is mediated by prostaglandins or cAMP, or both. PGE2 (10(-8)-10(-6) M) increased the production of 1,25-(OH)2D3 and of 24,25-(OH)2D3. Forskolin (0.01-10 microM) and dibutyryl cAMP (0.1-10 mM) increased the production of both metabolites but to a lesser degree than PGE2. These data suggest that osteoblastlike cells are stimulated by A23187 to increase the synthesis of 1,25-(OH)2D3 through mechanisms involving prostaglandins and cAMP. The synthesis of 24,25-(OH)2D3 is suppressed by A23187 through different mechanisms.

Prostaglandin E specifically upregulates the expression of the mannose-receptor on mouse bone marrow-derived macrophages

The macrophage mannose receptor (MMR) facilitates the binding and internalization of microorganisms and glycoproteins with terminal mannose residues. The receptor is progressively upregulated as bone marrow precursor cells mature into macrophages and thus may serve as a marker of differentiation. Prostaglandins of the E series (PGE) are known inhibitors of monocyte and macrophage precursor proliferation, an effect often associated with cellular maturation. MMR expression was therefore assessed after exposure of bone marrow macrophage precursor (BMMP) cells to these prostanoids. Receptor expression was determined by ligand binding and via immunoprecipitation of newly synthesized receptor molecules. PGE1 and PGE2 at 10(-9)-10(-6) M upregulated MMR surface expression and biosynthesis four- to sixfold in a dose-dependent manner. BMMPs responsive to prostaglandins were characterized by plastic adherence, F4/80 antigen expression, and nonspecific esterase activity. Prostaglandins accelerated the expression of the MMR in cells by 48-72h, with maximal levels of receptor expression being identical in control or treated cells. Thus, prostaglandins enhanced mannose receptor expression in adherent but not fully differentiated macrophage precursors. This effect is specific for PGE and is mimicked by dibutyrl cyclic AMP. These results indicate that prostaglandins accelerate MMR expression and hence the differentiation of macrophage precursor cells. Cells resident in the bone marrow secrete abundant prostaglandins, suggesting that a paracrine mechanism may exist to regulate MMR expression and function.

Involvement of prostaglandin E2 in the inhibition of osteocalcin synthesis by human osteoblast-like cells in response to cytokines and systemic hormones

The stimulation of the production of osteocalcin by human osteoblast-like cells in response to 1,25(OH)2D3 is antagonized by several agents that induce the synthesis of prostaglandin E2 (PGE2) including interleukin 1 (IL-1), tumour necrosis factor (TNF) and parathyroid hormone (PTH). The mechanism whereby these agents inhibit the synthesis of osteocalcin is not known. In this report we show that exogenous PGE2 inhibits this stimulatory action of 1,25(OH)2D3 on human osteoblast-like cells in a dose-dependent manner, suggesting that PGE2 may contribute to the inhibition of osteocalcin synthesis in response to these agents. Assessment of the inhibitory role of endogenous PGE2 synthesis in the action of rhIL-1 alpha, rhIL-1 beta and rhTNF alpha on the production of osteocalcin demonstrated that the inhibition by these agents could be partially overcome by the addition of indomethacin, an inhibitor of PGE2 synthesis. In contrast, the inhibitory action observed with bPTH (1-84) was unaffected by indomethacin. These observations indicate that endogenous PGE2 synthesis mediates, in part, some of the inhibitory actions of the cytokines on the induction of osteocalcin synthesis in response to 1,25(OH)2D3, but not of PTH. Since the antagonism of the synthesis of osteocalcin by rhIL-1 alpha, rhIL-1 beta and rhTNF alpha was not completely abolished following the inhibition of PGE2 synthesis this would indicate that additional PGE2-independent mechanisms also account for the action of these cytokines on osteocalcin production. The nature of these mechanisms is currently not known.

The relationship of prostaglandins to cAMP, IgG, IgM and alpha-2-macroglobulin in gingival crevicular fluid in chronic adult periodontitis

Gingival crevicular fluid, collected from 8 patients with chronic adult periodontitis before and 21 days after root planing and scaling, was analysed for prostaglandin E2, 6KPGF1 alpha, cAMP, IgG, IgM and alpha-2-macroglobulin, and their inter-relationship evaluated. There was a significant decrease in the levels of prostaglandin E2, IgG, IgM and alpha-2-macroglobulin after treatment, whereas the levels of 6KPGF1 alpha and cAMP remained essentially unchanged. The level of prostaglandin E2 decreased by 35%, IgG by 32%, IgM by 90%, and alpha-2-macroglobulin by 79%. There was a significant degree of correlation between prostaglandin E2 and 6KPGF1 alpha and cAMP before treatment but not after, but no correlation between prostaglandin E2 and IgG, IgM and alpha-2-macroglobulin either before or after. This correlation pattern indicates the involvement of E2, prostaglandin 6KPGF1 alpha and cAMP in inflammation in the periodontium. The changes in IgG, IgM and alpha-2-macroglobulin reflect yet another mechanism of host response which appears to be independent of prostaglandins.

Agents affecting adenylate cyclase activity modulate the stimulatory action of 1,25-dihydroxyvitamin D3 on the production of osteocalcin by human bone cells

The stimulation of osteocalcin synthesis by human osteoblast-like cells in response to 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is antagonised by several bone regulatory agents. We have shown that agents which activate adenylate cyclase inhibit this action of 1,25(OH)2D3 on human osteoblast-like cells. Activation of adenylate cyclase, either via the stimulatory GTP-binding protein using cholera toxin, or directly at the catalytic via the stimulatory GTP-binding protein using cholera toxin, or directly at the catalytic subunit using forskolin, results in a suppression of osteocalcin synthesis. Whilst the activation of adenylate cyclase induces this inhibitory response, neither exogenous dibutyryl cyclic AMP nor the phosphodiesterase inhibitor, IBMX, exerted any apparent effect on the production of osteocalcin. The tumour promoting phorbol ester, 4 beta-phorbol 12,13-dibutyrate, also inhibited 1,25(OH)2D3-stimulated osteocalcin production. This was not apparent in response to the non-tumour promoting phorbol ester 4 beta-phorbol suggesting the involvement of protein kinase C.

Leukotrienes in orthodontic tooth movement

Prostaglandins (PGs) and leukotrienes (LTs) are products of arachidonic acid conversion. PGs have an established role in mediating orthodontic tooth movement. The role of LTs in modulating or mediating orthodontic tooth movement was investigated in this study. One hundred thirty-two Sprague-Dawley rats were used; the animals weighed 300 to 400 gm with equal numbers of male and female rats. They were divided into five main groups of 24 animals each and a sham group of 12 animals. An orthodontic appliance was placed and activated on all the animals except the sham group; in this group the appliances were not active. Each main group was given one of the following treatments daily: distilled water, 5% gum arabic solution, PG synthesis inhibitor indomethacin, LT synthesis inhibitor AA861, and a combination of both drugs. Each group was divided into six subgroups of four animals; the animals were killed at either 1, 3, 5, 7, 10, or 14 days, and tooth movement measured. The three sham subgroups received distilled water and were killed at 1, 7, or 10 days. The first maxillary molar (the moved tooth) and surrounding tissues were removed from all animals in the sham group and the subgroups killed at 1, 7, and 10 days in the gum arabic solution group and the LT synthesis inhibitor group. Prostaglandin E2 (PGE2) and leukotriene B4 (LTB4) were extracted, measured with radioimmunoassay (RIA), and standardized per milligram of protein in the sample. A significant inhibition of tooth movement occurred beginning on day 7 in the indomethacin, AA861, and combination groups; there was no significant difference among these groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Diphenylhydantoin inhibits parathyroid hormone and prostaglandin E2-stimulated bone resorption in mouse calvaria without affecting cyclic AMP formation

The effect of diphenylhydantoin (DPH) on mouse calvarial bone metabolism was studied in vitro. DPH caused a dose-dependent, reversible inhibition of PTH and PGE2-stimulated bone resorption at concentrations above 20-30 micrograms/ml without affecting cyclic AMP formation. The inhibition was observed already after 60 min and was accompanied by a reduced release of the lysosomal enzymes beta-glucuronidase and beta-N-acetylglucosaminidase. The calcium antagonist Verapamil had similar effects on bone resorption and lysosomal enzyme release and it is suggested that DPH influences bone resorption by interfering with calcium fluxes across osteoclastic cell membranes resulting in low intracellular calcium levels and reduced exocytotic processes.

The effects of calcium regulating hormones on bone resorption by isolated human osteoclastoma cells

Cells were disaggregated from osteoclastomas, and the response of the giant cells to calcium-regulating hormones, prostaglandin (PG)E1 and dibutyryl cyclic AMP (dbcAMP) was observed by phase-contrast time-lapse video microscopy. The pattern and nature of their response was very similar to that previously found to be characteristic of osteoclasts: calcitonin (CT), PGE1 and dbcAMP induced cytoplasmic quiescence, while parathyroid hormone (PTH) showed no influence on cytoplasmic motility or behaviour. The cells were also cultured on slices of devitalized cortical bone for 5 or 18 h. After this time the giant cells were associated with the appearance in the scanning electron microscope of characteristic resorption pits, the volume of which was calculated by computer-assisted morphometric and stereophotogrammetric techniques after removal of cells. Calcitonin caused a dramatic reduction in the volume of bone resorbed by these isolated cells compared with control cultures, while PTH was without significant effect. This result supports the view that PTH does not increase bone resorption in intact bone through a direct effect on osteoclasts. PGE1, which stimulates bone resorption when added to intact bone, paradoxically reduced resorption in our cultures. It thus appears possible that PGE1 acts as a direct inhibitor of osteoclastic bone resorption but has an additional effect on other cells in bone, which are induced by PGE1 to cause osteoclastic stimulation.

The pathobiology of the osteoclast

This article reviews recent information concerning the origin of osteoclasts and the local and systemic regulation of their activity. It appears that much of the environmental responsiveness of osteoclasts is mediated by cells of the osteoblastic lineage, which exert a major influence on the localisation, induction, stimulation, and inhibition of osteoclastic bone resorption. Some of the mechanisms by which osteoclast function may be disturbed by inflammatory and neoplastic diseases are discussed, and it is suggested that many pathological disturbances of osteoclastic bone resorption may be explicable as mimicry of physiological regulatory mechanisms by local hormones introduced into bone as the local regulators of the diseased tissue.

Effect of levamisole on bone resorption in cultured mouse calvaria

The effects of levamisole (LT), dexamisole (DT), levo-p-bromotetramisole (LBT) and dextro-p-bromotetramisole (DBT) on bone were examined in an organ culture system using calvarial bones from newborn mice. LBT and DBT at concentrations 30 microM and greater and LT and DT at concentrations 100 microM and greater caused a dose-dependent, reversible inhibitory effect on mineral mobilization and matrix degradation. LBT, DBT (100 and 300 microM) as well as LT and DT (greater than or equal to 100 microM) reduced the spontaneous release of beta-glucuronidase without having any marked effect on the release of lactate dehydrogenase (LDH). LT and DT did not influence protein synthesis but LBT and LBT were inhibitory in concentrations at and above 100 microM. Mitotic activity, as assessed by incorporation of [3H]thymidine, was inhibited by LBT and DBT (0.1, 1 mM). LT and LBT caused a stereospecific inhibition of GPase, PPiase and ATPase. It is concluded that tetramisoles are potent, non-stereospecific inhibitors of bone resorption in vitro.

Clinical application of prostaglandin E1 (PGE1) upon orthodontic tooth movement

Chemically produced prostaglandin E1 (PGE1) was administered in clinical cases of orthodontic tooth movement. In the first phase, lingual arch springs were applied on both sides of the maxilla to upper first premolars which were scheduled for extraction. One side received submucosal injections of PGE1 and the other received vehicle injections. The rate of tooth movement in the buccal direction approximately doubled on the side of several PGE1 injections as compared to the control side. In the second phase, the PGE1 injections were applied in canine-retraction cases for up to 3 weeks in first-premolar-extraction cases. The rate of distal canine movement was almost double on the side receiving PGE1 injections as compared to the vehicle-injected side. In the third phase, the PGE1 injections were applied on routine canine retraction in first-premolar-extraction cases. The rate of distal canine movement was almost 1.6-fold on the side of PGE1 injections as compared to the vehicle-injected side. Throughout this study, no side effects were observed macroscopically in the gingiva and roentgenographically in the alveolar bone, except for a slight pain reaction consistent with orthodontic tooth movement.

The morphology and hormonal responsiveness of developing skeletal elements in chick limb buds

While parathyroid hormone (PTH), calcitonin (CT), and certain prostaglandins (PGs) are known to regulate the metabolism of both osteogenic and osteolytic cells of the adult skeleton through an adenosine 3', 5'-monophosphate-dependent mechanism, little is known about the development of this hormonally mediated response in embryonic skeletal tissues. In the present study, the responsiveness of embryonic skeletal elements to PTH and PGE2 was examined during various stages of development utilizing cAMP concentrations as an indicator of hormone-receptor interaction. The cytology of the limb skeletal system was examined also at each stage tested in order to compare the differentiated cellular phenotypes with their hormonal responsiveness. Prior to differentiation of cartilaginous elements in developing limb buds (stage 20-21), cells were responsive to PGE2 and epinephrine (EPI) but not to PTH. The first consistent response to PTH occurred coincident with the initial differentiation of the cartilage phenotype in limb buds (stage 24-25). A responsiveness to both PTH and PGE2 was progressively increased as maturation of cartilaginous and osteogenic elements occurred (stage 26-35). The initial response to CT was detected within cartilage rods in which osteogenic cells had differentiated (stage 33-35). The results of this study indicate that PGE2-sensitive cells exist within the developing limb prior to cytodifferentiation. The development of PTH responsiveness within embryonic chick limb buds is correlated with the onset of both chondrogenesis and osteogenesis in vivo.

Effects of prostaglandins on rat calvarial bone-cell calcium

Prostaglandin E2 (PGE2) over the concentration range 10(-5)-10(-7) M stimulated calcium uptake in osteoclastic-enriched populations isolated by sequential collagenase digestions of newborn rat calvaria. This effect was on initial calcium uptake occurring at 5 min at 37 degrees C but was not present when isotopic equilibrium was approached (60 min). Prostacyclin (PGI2, PGE1 and PGF2 alpha) stimulated osteoclastic calcium uptake in a similar manner, but with slightly smaller effects than PGE2. Under identical conditions, significant effects of PG were not observed in osteoblastic cells isolated from the same bones by extended collagenase digestions. Combined treatment with PGE2 and parathyroid hormone (PTH) at concentrations which produced no individual effects resulted in a significant increase in calcium uptake in osteoclastic cells. During a 48-h culture period, osteoblastic populations released significantly greater amounts of PGE2 than osteoclastic populations. Pre-incubation for 1 h at 37 degrees C with the prostaglandin cyclo-oxygenase antagonists, indomethacin and flufenamic acid, had no effect on calcium uptake in osteoclastic cells, but resulted in significant decreases in osteoblastic cells. The PGE2-induced increase in calcium uptake on osteoclastic cells was not altered by indomethacin or flufenamic-acid pretreatment. However, after treatment with these inhibitors, a significant response to PGE2 was observed in osteoblastic cells.

[Hypocalcemia in malignant diseases]

Hypercalcemia accompanies often malignant diseases. The majority of cases of malignancy complicated by hypercalcemia is induced by metastases involving bone, hypercalcemia may also accompany localised tumors. Various hormones have been implicated in the genesis of malignant hypercalcemia: ectopic secretion of parathyroid hormone by tumor or orthotopic secretion by concomitant primary hyperparathyroidism, prostaglandin activating osteoclasts, production of hypercalcemic factor other than these hormones. This review summarizes current knowledge about endocrine-mediated mechanisms which produce hypercalcemia and about its frequency and mechanism in different types of tumors.

The role of cyclic AMP, calcium, and prostaglandins in the induction of osteoclastic bone resorption associated with experimental tooth movement

Administration of imidazole and verapamil inhibited the appearance of osteoclasts and bone resorption induced by experimental tooth movement in rats. On the other hand, administration of theophylline and ouabain enhanced the appearance of osteoclasts. A mixed solution of DB cyclic AMP and A-23187 injected in gingiva near the orthodontically-treated teeth caused enhancement of the appearance of osteoclasts and bone resorption.

Differential sensitivity of osteoclasts and osteoblasts suggests that prostaglandin E1 effects on bone may be mediated primarily through the osteoclasts

Prostaglandin E (PGE) stimulates resorption in bone. Since osteoblast-like osteosarcoma cells secrete PGE2, the possibility that osteoclasts were the major target for PGE was considered. To study this question, it was first established that in isolated bone cells enriched for either osteoclastic (OC) or osteoblastic (OB) characteristics, PGE1 can induce biochemical effects similar to those seen with bovine parathyroid hormone 1-84 (PTH), another potent stimulator of bone resorption. These changes include increased cAMP and hyaluronate synthesis in OC cells, and increased cAMP but decreased citrate decarboxylation in OB cells. By following these markers, it is demonstrated that PGE1 can activate OC cells at doses as low as 1 nM, whereas OB cells require 250 nM. Bone cell responses to various doses of PTH and PGE1 were also compared. In OC cells the lowest effective dose of PGE1 and PTH was similar (1 nM), but increasing response to PGE1 was seen up to 1000 nM in contrast to PTH response which peaked at 20 nM. In addition, the magnitude of PGE1-induced OC cell hyaluronate was two to four times greater than that of PTH at all doses tested. In OB cells, PTH induced significant decreases in citrate decarboxylation at 0.1 nM, compared to 250 nM for PGE1. Half-maximal inhibition of citrate decarboxylation (19% of control) by PTH occurred at 0.5 nM, whereas 500 nM of PGE1 was required for an equivalent effect. Thus, (i) OC cells responded to PGE1 doses that were approximately 200 times lower than the minimum required by OB cells, and (ii) OB cells responded to 100 times lower doses of PTH than PGE1.

Relationship between prostacyclin biosynthesis and cyclic AMP in cultured rabbit mesothelial cells

Calcium ionophore A23187 (10 microM) as well as thrombin (10 U/ml) stimulated the biosynthesis of prostacyclin in cultured rabbit mesothelial cells; in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine (Mix, 1 mM) the cyclic AMP (cAMP) content was also elevated. Both effects were inhibited by indomethacin (28 microM). Exogenous prostacyclin elicited by itself a clear enhancement of intracellular cAMP. An increased cAMP content was also obtained with isoproterenol (10 microM), whose activity was antagonized by propranolol (10 microM). These two products however, had no effect on the prostacyclin release. In all these experiments, inhibition of phosphodiesterase with Mix, was necessary to obtain detectable cAMP levels. In the presence of Mix, the stimulation of prostacyclin production by A23187 and thrombin was significantly lower as compared to the stimulation in the absence of Mix. Our results suggest that increased prostacyclin biosynthesis results in adenylate cyclase stimulation. This rise in intracellular cAMP in the presence of Mix, is accompanied by a downward regulation of further prostacyclin production.

Inhibition of osteoclastic motility by prostaglandins I2, E1, E2 and 6-oxo-E1

We have recently found that calcitonin (CT), a hormone which inhibits osteoclastic bone resorption, completely abolishes the normally intense cytoplasmic motility of isolated osteoclasts. Here we report that prostaglandin (PG)I2, PGE1, PGE2 and 6-oxo-PGE1 cause an identical change in behaviour to that induced by CT. The order of potency was PGI2 greater than PGE1 greater than 6-oxo-PGE1 greater than PGE2. We found that, unlike CT which causes prolonged immotility in osteoclasts, the effect of these PGs was transient. The transient nature of the inhibition was not caused by their decay or inactivation, nor was it due to production in the cultures of a stimulator of osteoclast motility. Osteoclasts refractory to one PG were also less sensitive to the others, but showed no loss of sensitivity to CT, suggesting that the PGs share a common receptor system, distinct from that for CT. The PGs, like CT, appear to operate by increasing the cyclic AMP level in osteoclasts. The identical nature of the response of osteoclasts to PGs and CT, and the shared use of cyclic AMP as second messenger, suggest that the PGs, like CT, act directly on osteoclasts to inhibit bone resorption by these cells. Osteoblasts are known to make PGs, and we suggest that osteoblasts make them as agents of the local control of osteoclastic bone resorption. Paradoxically, when PGs are added to bone in organ culture they stimulate bone resorption. Like PTH they increase osteoblastic cyclic AMP levels, and the effect of adding PGs to bone may be a transient direct inhibition of osteoclasts followed by a sustained PTH-like stimulation of osteoclasts through osteoblasts. This mechanism may account for the bone resorption seen in inflammatory and malignant disease.

A high affinity, calmodulin-responsive (Ca2+ + Mg2+)-ATPase in isolated bone cells

Although acute alterations in Ca2+ fluxes may mediate the skeletal responses to certain humoral agents, the processes subserving those fluxes are not well understood. We have sought evidence for Ca2+-dependent ATPase activity in isolated osteoblast-like cells maintained in primary culture. Two Ca2+-dependent ATPase components were found in a plasma membrane fraction: a high affinity component (half-saturation constant for Ca2+ of 280 nM, Vmax of 13.5 nmol/mg per min) and a low affinity component, which was in reality a divalent cation ATPase, since Mg2+ could replace Ca2+ without loss of activity. The high affinity component exhibited a pH optimum of 7.2 and required Mg2+ for full activity. It was unaffected by potassium or sodium chloride, ouabain or sodium azide, but was inhibited by lanthanum and by the calmodulin antagonist trifluoperazine. This component was prevalent in a subcellular fraction which was also enriched in 5'-nucleotidase and adenylate cyclase activities, suggesting the plasma membrane as its principal location. Osteosarcoma cells, known to resemble osteoblasts in their biological characteristics and responses to bone-seeking hormones, contained similar ATPase activities. Inclusion of purified calmodulin in the assay system caused small non-reproducible increases in the Ca2+-dependent ATPase activity of EGTA-washed membranes. Marked, consistent calmodulin stimulation was demonstrated in membranes exposed previously to trifluoperazine and then washed in trifluoperazine-free buffer. These results indicate the presence of a high affinity, calmodulin-sensitive Ca2+-dependent ATPase in osteoblast-like bone cells. As one determinant of Ca2+ fluxes in bone cells, this enzyme may participate in the hormonal regulation of bone cell function.

Prostaglandin E2 binding and cyclic AMP production in isolated bone cells

The binding of prostaglandin E2 (PGE2) to bone cells was studied to provide direct evidence for the existence of specific receptors in bone. Bone cells were isolated by collagenase digestion of fetal and newborn rat calvaria. Isolated cells were incubated with 3H-PGE2 and collected on Millipore filters. Specific binding was determined by subtracting the binding that occurred with 10(-6) M non-radioactive PGE2 and 3H-PGE2 from that with 3H-PGE2 alone. With heterogeneous cell preparations and at PGE2 concentrations from 10(-9) - 1.7 X 10(-8) M at 37 degrees C, specific binding reached steady state within 10 min. Bound 3H-PGE2 was displaced by the addition of increasing amounts of unlabeled PGE2. Inhibition of PGE2 binding was observed with PGE1 and the endoperoxide analog, U44069, but not with PGF2 alpha, a lipopolysaccharide, or 13,14-dihydro 15-keto PGE2. Studies with bone cell populations, obtained by sequential digestions, indicated that an osteoclastic population binds 30-fold more PGE2 than osteoblastic cells. Scatchard analyses revealed that the osteoclastic cells have an affinity constant for PGE2 binding similar to that obtained with heterogeneous populations. However, the PGE2 binding capacity in this osteoclastic population was fivefold greater than in the heterogeneous population. The osteoclastic population responded with an increase in cyclic AMP to lower concentrations of PGE2 than the osteoblastic populations. These studies suggest that differences in the binding capacity of PGE2 receptors exist among bone cell-types and that these differences are reflected in the cellular cyclic AMP response.

Some properties of prostaglandin E2 receptors in rabbit alveolar bone cell membranes

[3H]prostaglandin E2 (PGE2) binding receptors exist in rabbit alveolar bone cell membranes. The presence of high (Kd = 3.9 X 10(-9) M) and low (Kd = 8.8 X 10(-8) M) affinity binding sites of [3H]PGE2 was demonstrated. The saturation values of [3H]PGE2 for high and low affinity binding sites were 0.13 pmol/mg protein and 1.22 pmol/mg protein, respectively. The digestion of the membranes with pronase, phospholipase C, D and neuraminidase led to a decrease of [3H]PGE2 binding but phospholipase A2 did not.

Inhibition of 1 alpha-hydroxy-vitamin D3 stimulated bone resorption in tissue culture by the calcium antagonist verapamil

The effect of the calcium antagonist verapamil on 1 alpha-hydroxy-vitamin D3 (1 alpha (OH)D3) stimulated bone resorption in tissue culture has been investigated. It was found that verapamil in concentrations above 20 mumol/l reduced 1 alpha (OH)D3-stimulated mineral mobilization, as measured by release of in vivo incorporated 45Ca from mouse calvarial bones. The inhibition of verapamil could be seen already 3 h after exposure to the drug. The increased degradation by 1 alpha (OH)D3 of the organic matrix in the calvaria, as assessed by the release of in vivo 3H-proline labelled collagen, was also reduced by verapamil. The inhibitory effect of the drug on 45Ca release was reversible after withdrawal. 1 alpha (OH)D3 increased the release of stable calcium and beta-glucuronidase, and these effects could be blocked by verapamil. Increasing medium calcium concentration from 1.8 to 5 mmol/l slightly reduced the inhibitory capacity of 50 mumol/l verapamil on 1 alpha (OH)D3-stimulated 45Ca release. These data indicate that stimulation of osteoclasts by hydroxylated metabolites of vitamin D to resorb bone and secrete lysosomal enzyme is dependent on an increased availability of free intracellular calcium.

Role of natural prostaglandins in the control of murine mammary tumor virus expression

The regulation of murine mammary tumor virus (MuMTV) production by mammotropic hormones, hormonomimetic substances, and cyclic nucleotides was investigated. The virus produced in control and treated mammary tumor cell cultures was quantitated by measuring the supernatant reverse transcriptase activity in exogenous reaction using poly(rC).oligo(dG) as template-primer. Two days after exposure, the synthetic glucocorticoid, dexamethasone (DXMT), increased spontaneous MuMTV production at optimal concentration (0.1 mumol) up to ten times. Dibutyryl derivative of cyclic AMP had no effect on spontaneous MuMTV production, whereas the drug potentiated suboptimal concentrations of the glucocorticoid. Natural prostaglandins, potent agonists of adenylate cyclase catalyzing intracellular synthesis of cyclic AMP, enhanced both basal (up to five times) and DXMT-stimulated (up to 1.6 times) MuMTV replication. The MuMTV-stimulating activity of prostaglandins decreased in the order of PGA1 greater than PGE1 greater than PGB1 greater than PGF2 alpha. Prostaglandins can be replaced partially by norepinephrine and isoproterenol by enhancing the DXMT-mediated MuMTV stimulation, whereas these drugs remained without effect on spontaneous MuMTV production. Theophylline, an antagonist of cAMP-phosphodiesterase converting cAMP to AMP, enhanced the virus-stimulating activity of DXMT as well as of prostaglandins. The enhancement of MuMTV production by adenylate cyclase agonists do not correlate absolutely with the estimates of intracellular cAMP levels, since the highest amounts of cAMP has been repeatedly observed in cells treated with PGE1 and norepinephrine. The results indicate that besides hormones, other hormone-like substances and cyclic nucleotides may be involved in the complex mechanism of hormone-regulated MuMTV genome expression.

Prostaglandin E2 and cyclic AMP in tumor and plasma of breast cancer patients

Prostaglandin E2 and cyclic AMP (cAMP) levels were measured in tumors and plasma of 78 patients with benign and malignant breast tumors. Two groups of malignant tissues were found, one with a high level of PGE2 (M = 55.4 pg/mg) and one with a low level (M = 10.7 pg/mg). The low level did not differ significantly from the benign tissue level (M = 8.7 pg/mg). Two malignant groups could not be detected in the plasma levels. Plasma PGE2 concentration (in form of the 13,14-dihydro-15-Keto metabolite) did not reflect the tissue levels, and no difference was found between the benign (M = 59.9 pg/ml) and the malignant (M = 62.3 pg/ml) patients, but both concentrations were higher than those of healthy controls (M = 34.4 pg/ml). The stage of the cancer, the histological classification and, most important, the period of survival, could not be related to the differences in the PGE2 tissue levels. Neither could plasma cAMP be nominated as a breast cancer market because no difference was found between the cAMP levels of benign tumor patients (M = 16.48 pmol/ml), of malignant tumor patients (M = 21.14 pmol/ml) and of healthy controls (M = 19.07 pmol/ml). The conclusion is that although high amounts of PGE2 appear in some malignant breast tumors, they do not affect the clinical situation. These results may explain the failure to treat human breast cancer patients with prostaglandin synthetase inhibitors.

Characterization of osteoblast-like cells from fetal rat calvaria

An improved method is described for the isolation of osteoblast-like cells from fetal rat calvaria. The cells are mononuclear, 8.3 mu in diameter, alkaline phosphate positive, and possess rough endoplasmic reticulum, mitochondria, Golgi, and cytoplasmic processes. Isolated bone cells synthesize collagen and cholesterol, grow to confluence in primary culture, and respond to hormonal stimulation.

Prostaglandin as a mediator of bone resorption induced by experimental tooth movement in rats

Administration of indomethacin inhibited the appearance of osteoclasts and bone resorption induced by experimental tooth movement in rats. Effects were limited to groups injected within 12 h after treatment. Prostaglandin E1 or E2 solutions injected in gingiva lying near the upper first molar caused the appearance of osteoclasts and bone resorption.

Influence of diphenylhydantoin on lysosomal enzyme release during bone resorption in vitro

The effect of diphenylhydantoin (DPH) on the release of lysosomal enzymes during resorption of cultured mouse calvarial bone was studied. The enzyme activities of beta-glucuronidase and beta-galactosidase in the culture medium was taken as indicators for lysosomal enzyme release. In concentrations 50 micrograms/ml or higher, DPH inhibited the release of beta-glucuronidase and beta-galactosidase in parallel with bone resorption as indicated by reduced release of 45Ca, Ca2+, Pi and hydroxyproline. The release of the cytosolic enzyme lactate dehydrogenase was not influenced by concentrations of DPH up to 50 micrograms/ml but higher concentrations caused an increased release indicating cell injury. When bone resorption was stimulated by prostaglandin E2, DPH (50 micrograms/ml) also reduced the mobilization of bone mineral and the release of beta-glucuronidase without influencing the release of lactate dehydrogenase. It is suggested that DPH by interfering with cellular release processes reduces the resorption of bone.

Effects of a bone resorptive factor from human cancer ascites fluid on rat bone cell calcium and cyclic AMP

The effects of a bone resorptive protein isolated from human cancer ascites fluid on bone cell calcium and cyclic AMP were studied with fetal rat cells. The osteoclast-activating factor increased bone cell calcium uptake at 37 degrees C and 4 degrees C with no direct effects on calcium efflux. Concentrations of the resorptive factor that increased in vitro bone resorption and cell calcium uptake had no effect on cyclic AMP. The effects of the protein on calcium uptake were not specific for bone cells, and large increases were also observed in isolated fetal rat skin cells. These studies suggest that increases in the permeability of the cell membrane to calcium are involved in the mechanism of action of the ascites fluid resorptive protein.

Prostaglandin E2 and cyclic AMP levels in human breast tumors

Prostaglandin E2 (PGE2) and cyclic adenosine-3', 5'-monophosphate (cAMP) concentrations were measured in human benign and malignant breast tumors by radioimmunoassay. Two groups were found among the malignant tissues, one with high PGE2 (M = 65.89 pg/mg) and high cAMP (M = 0.704 pmole/mg) concentrations and one with low concentrations (M = 9.24 pg/mg and M = 0.299 pmole/mg, respectively). The low PGE2 levels in the malignant tumors did not differ significantly from the levels found in benign tumors (M = 8.06 pg/mg). cAMP levels were positively and highly correlated (r = +0.81) with PGE2 levels. No bone osteolysis could be discovered in any of the patients a few weeks after mastectomy operation, but PGE2 analysis of breast tumors may have prognostic value for the future.

Time-dependent stimulatory and inhibitory effects of prostaglandin E1 on exudative and tissue components of granulomatous inflammation in rats

1 The effects of prostaglandin (PGE(1)), following local administration during different phases of developing sponge-induced granulomata, were studied in normal and essential fatty acid deficient (EFAD) rats.2 In normal rats, a single dose of 1 mug PGE(1) on implantation (day 1) increased exudate production without altering total leucocyte counts after 6 h and stimulated granulomatous tissue formation after 8 days.3 Repeated daily administration of the same dose of PGE(1) on days 1 to 3 had no effect, while administration on days 4 to 7 (i.e. when tissue growth is already in progress) inhibited granuloma formation.4 In EFAD rats, which are known to produce only very small amounts of endogenous prostaglandins, acute (6 h) exudate formation was unaffected by 0.05 mug PGE(1). However, early stimulatory and later inhibitory effects of 0.05 mug PGE(1) per day were obtained on the granulomatous tissue, similar to those obtained with the 20 fold higher dose in normal rats.5 The early stimulatory action of PGE(1) on granulomatous tissue formation was enhanced, in normal rats, by concomitant administration of 10 mug theophylline. This latter compound did not influence the later inhibitory effect of PGE(1).6 These results indicate that PGE(1) exerts either pro- or anti-inflammatory actions on the proliferative (tissue) component of the inflammatory process, depending on the time of administration. While the stimulatory effect following early administration may have been secondary to an initial cyclic adenosine 3',5'-monophosphate-mediated, vascular response, such a mechanism is unlikely to have been responsible for the later anti-inflammatory action of PGE(1).7 The implications of these results are discussed in relation to the postulated negative-feedback role of endogenous PGE in chronic inflammation.

Cyclic AMP formation and release by cultured bone cells stimulated with prostaglandin E2

PGE2 produced a marked and dose-related increase in cAMP content of cultured bone cells and in the release of cAMP into the incubation medium. The amount of cAMP released from the cells by PGE2 was proportional to the cellular concentration, and was dependent upon the time of incubation with PGE2. The cAMP levels released into the media increased slowly at a linear rate during a 60 min treatment with PGE2. This release was blocked by theophylline, probenecid, ouabain and dinitrophenol, suggesting that the release of cAMP was not a simple diffusive process and required energy. SC-19220 reduced the formation of cAMP more than the release, suggesting that the formation and the release may arise from separate events. Inability of D600 to inhibit PGE2-induced release of cAMP indicates that the release does not require calcium.

Prostaglandin-mediated hypercalcemia: a paraneoplastic syndrome

Evidence has been presented for prostaglandin-mediated hypercalcemia and bone resorption in malignancies of both, experimental animals and man. Occurence of hypercalcemia in cancer patients is known for a long time, but its pathogenesis has been poorly understood so far. Besides ectopic parathyroid hormone secretion by tumors, an osteoclast-activating factor released from leukocytes and direct bone destruction by tumor cells, prostaglandins of the E series have to be considered as one of the candicates involved in the pathomechanism of hypercalcemia and osteoclastic osteolysis in cancer patients. This new concept on the pathophysiology of cancer-associated hypercalcemia has implications for the diagnosis and management of this common complication of neoplastic disease.

Changes in red cell space of bone in hypocalcemic states: studies in rat femur

With the use of technetium-99m-labeled red cells, the red cell space of the femur was found to be increased in rats fed a 0.01 percent Ca diet for 4 wk, at which time they were also hypocalcemic. Rats fed the same diet for 2 wk did not demonstrate hypocalcemia nor hypervascularity of the femur. Thyroparathyroidectomized (TPTX) rats fed the calcium-deficient diet produced hypocalcemia nor hypervascularity as did TPTX rats fed a normal diet for 4 wk. However, bone vascularity was normal at 2 wk post-TPTX (normal diet) when the degree of hypocalcemia was less marked. Inhibition of prostaglandin synthetase with oral indomethacin 0.2 mg/100 g per 12 h for 60 h did not alter the red cell space estimates but did increase bone blood flow in both normal and calcium-deficient animals. Acute restoration of normocalcemia with CaCl2 orally in the calcium-deficient rats did not revert the bone hypervascularity. The evidence suggests that in these models of hypocalcemic states, the rise in the vascular space of bone does not result from the influence of PTH, calcitonin, diet, or prostaglandins.

Rat osteogenic sarcoma cells:effects of some prostaglandins, their metabolites and analogues on cyclic AMP production

Cyclic AMP production by freshly isolated cells, from a 32P-induced transplantable rat osteogenic sarcoma, was stimulated by PGE1, PGE2 and to a less extent by PGF2alpha and PGA2. In the case of PGE2, the cyclic AMP content of cells was maximal within 5 min. The 13,14-dihydroderivatives of PGE1, PGE2 and PGF2alpha had approximately 40% of the activity of the parent prostaglandin whilst, in every case, the metabolites (15-keto and 13,14-dihydro-15-keto) had very little activity. Two prostaglandin endoperoxide analogues (U44069 and U46619) had only 10% of the activity of an equimolar dose of PGE2. The data presented in this paper demonstrates similarities between the responses of these cells and cells derived from bony tissue in terms of the ability of prostaglandins to stimulate bone resorption in tissue culture.