Stimulation of bone formation by prostaglandin E2

Loading-related increases in prostaglandin production in cores of adult canine cancellous bone in vitro: a role for prostacyclin in adaptive bone remodeling?

Cyclic mechanical loading sufficient to engender strains of physiologic magnitude applied to recently excised canine cancellous bone cores in vitro increased the release of prostaglandin E (PGE) and prostacyclin (PGI2, measured as its breakdown product 6-keto-PGF1 alpha), during a 15 minute loading period in which PG levels were measured in perfusing medium at 5 minute intervals. Peak production occurred in the 0-5 minute sample. Mean levels preload compared to during load were PGE, 2.66 and 3.67 ng/ml (p less than 0.002); and 6-keto-PGF1 alpha, 543 and 868 pg/ml (p less than 0.007). The elevated levels then declined to preload levels during the loading period. However, the 5-10 minute but not the 10-15 minute samples still contained levels greater than preload values. A second 15 minute period of load, 1 h following the end of the first, produced smaller increases in the levels of release that were statistically significant only for the first 0-5 minute sample during load (preload compared to load mean values, PGE, 1.09-1.66 ng/ml, p less than 0.02; 6-keto-PGF1 alpha, 401-558 pg/ml, p less than 0.04). Immunolocalization revealed PGE and 6-keto-PGF1 alpha in lining cells and 6-keto-PGF1 alpha but not PGE in osteocytes. Addition to the medium of 1 microM PGE2, approximating the concentration produced by loading, had no significant effect on the specific activity of the extractable RNA fraction labeled with [3H]uridine, whereas 1 microM PGI2 produced an increase similar to that seen previously with loading.(ABSTRACT TRUNCATED AT 250 WORDS)

Toward a cure for osteoporosis: reversal of excessive bone fragility

While estrogen replacement therapy and calcium supplementation appear to be effective at preventing postmenopausal osteoporosis, therapy for established osteoporosis is far less effective. The reduction of bone fragility should be a goal of a treatment for established osteoporosis. To this end, increases in cortical bone mass by subperiosteal new bone formation may produce the greatest mechanical advantage. Anti-resorptive drugs, such as etidronate, have shown potential for reducing the incidence of osteoporotic fracture in the short term, but their ability to produce a long-term benefit may be limited. An alternative approach might be to develop drug therapies that substantially increase cortical bone strength, namely by stimulating periosteal bone formation. Although sodium fluoride has proved to be problematic, there are several other potential osteoporosis therapies. They include treatment with anabolic hormones (e.g. growth hormone and anabolic steroids) and targeted delivery of growth factors. Also, anti-resorptive and formation-stimulating drugs might be combined in a new form of ADFR (coherence) therapy where the new acronym means: Activate-Depress-Formation stimulation-Repeat.

Interleukin-1 alpha stimulates the release of prostaglandin E2 and phospholipase A2 from fetal rat calvarial cells in vitro: relationship to bone nodule formation

We have shown previously that interleukin-1 (IL-1) has biphasic effects on the formation of bone nodules in long-term cultures of fetal rat calvarial (RC) cells (Ellies and Aubin, Cytokine 2:430-437, 1990). To determine the role of arachidonic acid metabolism in this process, we have examined the release of prostaglandin E2 (PGE2) and phospholipase A2 (PLA2) from RC cells under conditions that allowed concomitant analysis of the formation of bone nodules. Recombinant human IL-1 alpha (rhIL-1 alpha) stimulated PGE2 and PLA2 release in a time- and dose-dependent manner. PGE2 release was highest in preconfluent cultures (days 1-6) and was stimulated up to 8.5-fold in response to 50 U/ml of rhIL-1 alpha. In contrast, extracellular PLA2 activity was maximal in postconfluent cultures, with 50 U/ml of rhIL-1 alpha causing a 20-fold increase by day 15. PLA2 release by RC cells was not significantly affected by PGE2, the glucocorticoid dexamethasone, or the cyclooxygenase inhibitor indomethacin. Indomethacin partially blocked the inhibition of bone nodule formation caused by rhIL-1 alpha, and exogenous PGE2 reversed this effect. Addition of group I PLA2 from Naja naja venom to RC cells had no effect on bone nodule development; however, group II PLA2 from Crotalus adamanteus venom inhibited the formation of bone nodules in a dose range similar to that induced by rhIL-1 alpha. These results indicate that PGE2 release does not have a direct temporal correlation with increases in PLA2 activity. In addition, the data show that only part of the inhibition of bone formation seen with rhIL-1 alpha is mediated by PGE2 and suggest that extracellular PLA2 also accounts for part of the inhibition.

Immobilization increases bone prostaglandin E. Effect of acetylsalicylic acid on disuse osteoporosis studied in dogs

The effect of acetylsalicylic acid (aspirin) on bone mass and bone prostaglandin E (PGE) in immobilization osteoporosis was studied in 12 growing dogs using a unilateral hind limb cast-fixation model. Osteoporosis was induced by fiberglass-cast immobilization of the right hind limb for 4 weeks, with the left hind limb as a control. Six dogs received buffered aspirin at 25 mg/kg body weight per os every 8 hours; 6 dogs received no treatment. All the dogs were killed after 4 weeks, and bone samples were collected. Bone mineral content of the distal tibial metaphysis was measured by single-photon absorptiometry. In vitro release of PGE from the calcaneus, tibial cortical bone, tibial cancellous bone, and ilium were measured using a specific radioimmunoassay for PGE. Compared with the controls, the casted limb of untreated dogs had half the bone mass and a twofold increase in bone PGE. Aspirin treatment was associated with a 65 percent reduction in bone PGE and a 13 percent bone mass sparing effect. These results provide indirect evidence that PGE plays a role in immobilization osteoporosis.

Prostaglandin release by normal and osteomyelitic human bones

The release of prostaglandin E (PGE) and prostacyclin (as 6-keto PGF1 alpha) by human osteomyelitic bone, compared with normal (control) bone, incubated in vitro was evaluated. Prostacyclin was the main arachidonic acid metabolite released by normal human bone, and similar quantities were released by osteomyelitic bone. However, PGE production was 5-30-fold higher in osteomyelitic bone, compared with control, thus becoming the major prostanoid in this disease. It is concluded that PGE production is probably involved in the inflammatory and/or bone resorption processes that occur in osteomyelitis.

Prostaglandin-induced changes in calcium uptake and cAMP production in osteoblast-like cells: role of protein kinase C

Phorbol esters were used to evaluate the putative effect of protein kinase C (PKC) activation on prostaglandin E2 (PGE2)-induced increases in calcium uptake and cAMP production in the human osteoblastic osteosarcoma cell line, Saos-2. The cells were pretreated for 15 min with phorbol myristate acetate (PMA) followed by a 5 min incubation with PGE2. Calcium uptake was measured with 45Ca and cAMP by radioimmunoassay. A significant increase in calcium uptake was noted in the PGE-treated cells compared with controls and preincubation with the PMA caused a significant decrease in this response. Preincubation with PMA also inhibited the PGE2-induced increase in cAMP under identical conditions. The effect of PMA on the cAMP response was not influenced by the addition of a phosphodiesterase inhibitor. PMA had no effect on the basal levels of either calcium uptake or cAMP production. Likewise, the inactive phorbol esters, phorbol 12,13-didecanoate (PDD) and 4 alpha-phorbol 12-myristate, 13-acetate (4 alpha), had no effect on either basal levels of these parameters or on the PGE2-induced increases. These results suggest that PKC is involved in the down-regulation of PGE2-induced increases in calcium uptake and cAMP production in the Saos-2 osteoblastic cell line.

Prostaglandin D2 metabolite stimulates collagen synthesis by human osteoblasts during calcification

We investigated the effect of the prostaglandin D2 metabolite delta 12-PGJ2 (9-Deoxy-delta 9, delta 12-13,14-dihydroprostaglandin 2D) on collagen synthesis in human osteoblasts. delta 12-PGJ2 at 10(-5) M enhanced collagen synthesis in the presence of 2 mM alpha-glycerophosphate-2Na. The stimulative effect appeared as early as 3 days after addition and continued until 22 days. The enhancement of type I collagen synthesis was confirmed by polyacrylamide gel electrophoresis. The potency was the same as 10(-8) M 1 alpha, 25 dihydroxy vitamin D3 (1,25(OH)2D3). Northern blot analysis showed that 10(-5) M delta 12-PGJ2 and 10(-8) M 1,25(OH)2D3 enhanced the transcription of type I procollagen (alpha 1) mRNA levels in osteoblasts.

Homeostatic control of bone structure: an application of feedback theory

The regulation of bone mass and structure in the weight-bearing skeleton is governed to a great extent by the mechanical demands placed upon the bone tissue. The apparent biological goal is the maintenance of a minimum adequate structure, in which the margin of safety between normal mechanical demands and fracture is balanced by the cost of excessive bone mass on mobility. Frost has developed two powerful postulates concerning bone adaptation: (a) there exist threshold levels of mechanical strain, above or below which bone adaptation is turned on, and (b) the set point for normal bone structure can be modulated by hormones. A model was developed, using Frost's postulates and simple feedback theory, that describes the interaction between biochemical influences and mechanical influences on bone structure. The model predicts that biochemical agents that influence bone structure independently of the mechanical feedback system (e.g., calcitonin) are capable of only limited anabolic effects on bone mass because their influences conflict with mechanical influences. However, biochemical agents that influence bone structure by changing the set point of the mechanical feedback system (e.g., estrogen) will provide lasting changes in bone structure. Age-related changes occur within the effector and transduction components of the mechanical feedback system that tend to make it sluggish. These changes may lead to increased bone fragility because the system is no longer capable of maintaining adequate bone structure.

Platelet-activating factor stimulates production of prostaglandin E2 in murine osteoblast-like cell line MC3T3-E1

We found that platelet-activating factor (PAF) stimulated the production of prostaglandin (PG) E2 in MC3T3-E1 cells in a time- and dose-dependent manner. 1.0 microM PAF gave a maximal stimulation of PGE2 production by MC3T3-E1 cells after a 4 hr PAF-treatment. Furthermore, the PAF-induced PGE2 production was abolished by the pre-treatment of the cells with a PAF receptor antagonist, 1-O-hexadecyl-2-acetyl-sn-glycero-3-phospho(N,N,N-trimethyl)hexanolamine, which occupied the same receptor site as PAF. These results suggest that PAF stimulates the PGE2 synthesis through a PAF receptor mediated pathway. Possibly PAF modulates bone metabolism by stimulating PGE2 synthesis.

Case report 701: Prostaglandin E1 (PGE1) periostitis

A case of periosteal new bone in a newborn is presented. The periostitis resulted from long-term therapy with PGE1, which was administered to maintain patency of the ductus in a neonate with ductal-dependent cyanotic congenital heart disease. The features of PGE1 periostitis and the differential diagnosis are reviewed.

The role of prostaglandins in bone in vivo

Prostaglandins of the E series, primarily E2 and E1, have the greatest activity in bone. Following discovery of their potent ability to stimulate bone resorption in vitro, clinical investigations have placed prostaglandins at sites of localized bone resorption associated with inflammatory or space occupying lesions in vivo. These studies have shown that prostaglandin production at such sites may be increased by cytokines such as interleukin-1 but the mechanisms by which prostaglandins stimulate bone resorption are not yet known. Observation of periosteal bone formation in patients given, pharmacological doses of prostaglandin has led to investigation of its bone forming activity. Young, growing rats have increased metaphyseal bone formation and this is accompanied by increased periosteal and endocortical bone formation in older animals. In the mature animals there is a generalized activation of remodelling with increased formation in the remodeling cycle. This is also seen in oophorectomized rats and results in repletion of the lost bone in this model of osteoporosis. In animal models of localized disuse osteopenia, prostaglandins are found to be elevated at the site of bone loss and prostaglandin inhibitors at least partially protect against the exaggerated resorption that occurs. This is also seen in models of orthodontic tooth movement, periodontitis and osteomyelitis. Prostaglandin synthesis inhibitors have been shown to delay healing of bone and this has led to limitations on their use clinically in some situations. Exogenously administered prostaglandins have been found to enhance periosteal callus formation, but healing is not uniformly enhanced. Prostaglandins have also been associated with hypercalcemia in certain animal tumors that model human hypercalcemia of malignancy but are probably most important in this condition as mediators in the localized resorption of bone at tumor sites. These in vivo studies have shown that prostaglandins are involved with increases in both bone formation and bone resorption. In vitro studies have shown that prostaglandins stimulate osteoblasts as well as osteoclastic bone resorption but understanding these effects under in vivo conditions will require further investigation.

In vivo effects of human recombinant transforming growth factor beta on bone turnover in normal mice

Reports of the effects of TGF-beta on bone cells are conflicting and controversial. Different cell culture and organ culture models for both osteoblasts and osteoclasts have given different responses. In some the effects are dependent on prostaglandin synthesis, and in others they are prostaglandin independent. To determine the effects of TGF-beta on osteoblasts and osteoclasts in vivo and the role of prostaglandins in mediating these effects, we injected 2.5-5 micrograms TGF-beta into the subcutaneous tissue overlying the calvariae of normal mice for 2-5 days anc compared the morphologic responses in underlying calvarial bone with those in mice injected caused a marked increase in periosteal thickness (fivefold) and cellularity, morphologic changes in osteoblasts, and new mineralized bone formation. These effects were localized to the site of injection and were partially inhibited by concomitant indomethacin treatment. There was a parallel increase in osteoclast numbers in adjacent marrow spaces, and the osteoclasts formed were unusually large. In contrast, no increase in the numbers of osteoclasts was seen in indomethacin-treat animals. These data show that TGF-beta has powerful effects on local bone cell function in vivo and that these effects may be mediated, in part, by prostaglandin generation.

Regulation of arachidonic acid turnover by 1,25-(OH)2D3 and 24,25-(OH)2D3 in growth zone and resting zone chondrocyte cultures

Previous studies have shown that phospholipase A2 activity in rat costochondral chondrocyte cultures is differentially regulated by 1,25-(OH)2D3 and 24,25-(OH)2D3. 1,25-(OH)2D3 stimulates enzyme activity in growth zone chondrocytes but has no effect on the resting zone chondrocyte enzyme activity. 24,25-(OH)2D3 inhibits the resting zone enzyme but has no effect on the growth zone chondrocyte phospholipase A2. This study examined whether the metabolites affect arachidonic acid turnover in their target cell populations. Incorporation and release of [14C]arachidonate was measured at various times following addition of hormone to the cultures. Acylation and reacylation were measured independently by incubating half of the [14C]arachidonate-labeled cultures with p-chloromercuribenzoate. The results demonstrated that the distribution of [14C]arachidonate in membrane phospholipids differed between growth zone and resting zone chondrocytes and between the plasma membranes and matrix vesicles isolated from the growth zone chondrocyte cultures. Plasma membrane phospholipids were more susceptible to the release of [14C]arachidonic acid by exogenous phospholipases than were matrix vesicle phospholipids. The effect of 1,25-(OH)2D3 on growth zone chondrocytes was observed within 5 min. Incorporation was greatest after 60 min; release was greatest after 30 min. 24,25-(OH)2D3 stimulated consistently elevated incorporation throughout the incubation period, peaking at 15 min. Peak release was at 60 min. The results confirm that resting zone chondrocytes and growth zone chondrocytes retain a differential phenotype in culture and demonstrate that matrix vesicles are distinct from the plasma membrane in terms of lipid composition and arachidonic acid incorporation. 1,25-(OH)2D3 and 24,25-(OH)2D3 appear to stimulate arachidonic acid turnover in their target cells by different mechanisms. Changes in fatty acid acylation and reacylation may be one mode of vitamin D-3 action in cartilage.

Cyclic AMP-dependent inhibition of collagen synthesis in avian epiphyseal cartilage cells: effect of chicken and human parathyroid hormone and parathyroid hormone-related peptide

Avian cartilage cells derived from epiphyseal growth-plate and avian skin fibroblasts were cultured in vitro. Production of cAMP by cartilage cells was stimulated by the synthetic fragments (1-34) of chicken (cPTH), human (hPTH) parathyroid hormone and by parathyroid hormone-related peptide (PTHrP). The enhancement of cAMP production by any of the peptides could be blocked by the parathyroid hormone analogue (3-34)PTH, suggesting interaction with PTH specific receptors. When incubated with [3H]proline, both cell types released radiolabelled collagenase-digestible and non-digestible proteins into the medium. cPTH, hPTH, PTHrP, forskolin, prostaglandin E2 (PGE2) and 8-bromo cAMP inhibited collagen production in cartilage cells with only minor effects on non-collagenase digestible proteins. No effect of cAMP on collagen production by fibroblasts was observed. The present results provide additional evidence that avian growth-plate cartilage cells are targets for PTH, and are first to demonstrate the response of a non-mammalian system to mammalian PTHrP. The data suggest that collagen production by epiphyseal growth-plate cartilage cells is inhibited by PTH and that this inhibition is mediated by cAMP.

Effect of heparin on bone formation in cultured fetal rat calvaria

To assess the effects of heparin on bone formation we measured [3H]proline incorporation into collagenase-digestible (CDP) and noncollagen protein (NCP), [3H]thymidine (TdR) incorporation into DNA, and DNA content in 21-day-old fetal rat calvaria cultured in BGJ medium with bovine serum albumin for 24-96 hours. Heparin at 5-125 micrograms/ml decreased TdR incorporation by 26-51% at 24 and 96 hours. At 96 hours, heparin 5, 25, and 125 micrograms/ml decreased [3H]proline incorporation into CDP by 41, 48, and 32%, respectively, with no significant change in NCP. To evaluate the possible role of PGE2 in these inhibitory responses, media PGE2 concentration was measured and the effects of heparin on CDP labeling and DNA synthesis were tested in the presence of indomethacin, piroxicam, and flurbiprofen to inhibit endogenous prostaglandin E2 (PGE2) production and in the presence of a high concentration (10(-7) M) of exogenous PGE2. Heparin did not alter PGE2 production at 24 hours but at 48 hours there was a significant reduction. At 96 hours, indomethacin (10(-6) M) inhibited [3H]-proline incorporation into CDP by 38% but had no effect on the labeling of NCP. Heparin had no further significant inhibitory effect in the presence of indomethacin. Piroxicam and flurbiprofen did not alter DNA content and had a smaller inhibitory effect than indomethacin on the labeling of CDP. Moreover, addition of heparin produced a further inhibition of CDP and DNA content and finally, heparin decreased CDP labeling by 71% in the presence of PGE2.(ABSTRACT TRUNCATED AT 250 WORDS)

Transient effects of subcutaneously administered prostaglandin E2 on cancellous and cortical bone in young adult dogs

The transient effects of prostaglandin E2 (PGE2) on cancellous and cortical bone in iliac crests and mid-tibial shafts of nine intact young adult dogs were evaluated following 31 days of treatment. Histomorphometric bone changes were characterized from in vivo fluorescent double-labeled undecalcified bone specimens. PGE2 caused an increase in cancellous bone remodeling evidence by increased in activation frequency; increased percent eroded and formation surfaces; increased mineral apposition and bone formation rates; and shortened resorption, formation, and total bone remodeling periods. Activated cancellous bone remodeling did not lead to decreased cancellous bone mass, indicating an imbalance between bone resorption and formation in favor of formation (activation----resorption----stimulated formation; A----R----F increases) at remodeling sites. The PGE2 treatment activated bone modeling in the formation mode (activation----formation; A----F) at the periosteal and endocortical surfaces and increased activation frequency of intracortical bone remodeling in the tibial shaft. Increased modeling activation converted quiescent bone surfaces to formation surfaces with stimulated osteoblastic activity (i.e., increased percent labeled periosteal and endocortical surfaces, mineral apposition rates, and woven and lamellar trabecular bone formation) leading to 9- to 26-fold increases in newly formed bone mass in subperiosteal, subendosteal, and marrow regions, compared to controls. However, increased intracortical bone remodelling elevated remodeling space (i.e., increased cortical porosity), producing a bone loss that partially offsets the bone gain. The combined events lead to a positive bone balance in PGE2-treated cortical bone, compared to a negative bone balance in control bones. Collectively our data suggest that in vivo PGE2 is a powerful activator of cancellous and cortical bone formation, which may be able to build a peak bone mass to prevent and/or correct the skeletal defects to cure osteoporosis.

Thrombin's effects on osteoblastic cells. I. Cytosolic calcium and phosphoinositides

Thrombin, a blood coagulation factor, has been shown to be a very effective in vitro bone resorbing agent whose mechanism of action on osteoblastic cells remains to be elucidated. In the present study, the effects of highly purified human thrombin on Saos-2 and G292 cells, two human osteoblast-like osteosarcoma cell lines, were investigated. Thrombin (0.6-16 U/ml) caused a significant, dose-dependent increase in osteoblastic cell proliferation. Thrombin also elicited a dose-dependent increase in cytosolic calcium concentration in both Saos-2 and G292 cells (maximal increases were 38% and 200% over baseline, respectively). Addition of thrombin to the osteoblast-like cells resulted in significant time- and dose-dependent changes in phosphoinositide levels: the percentage of inositol monophosphate levels were decreased, whereas the percentage of inositol bisphosphate, inositol trisphosphate and inositol tetrakisphosphate levels were increased. The relative magnitude of the changes in phosphoinositide levels was similar to the changes in cytosolic calcium concentration. These results suggest that thrombin's mechanism of action on bone cells may involve increases in cytosolic calcium levels and in phosphoinositide metabolism.

Cyclooxygenase inhibitors enhance cell growth in an osteoblastic cell line, MC3T3-E1

To elucidate the significance of endogenous prostaglandin E2 (PGE2) in osteoblastic cell function, we studied the effects of cyclooxygenase inhibitors on cell growth and alkaline phosphatase (ALP) activity in MC3T3-E1 cells. UMR-106 cells were also used as references in our experiments. MC3T3-E1 cells, cultured in alpha-minimal essential medium containing 10% fetal bovine serum, were shown to produce PGE2, which was markedly suppressed in the presence of indomethacin. Addition of indomethacin resulted in an increase in DNA content and [3H]thymidine incorporation. A similar growth stimulatory effect was observed when structurally different cyclooxygenase inhibitors, that is, acetyl salicylic acid (ASA), flurbiprofen, and piroxicam, were added. These cyclooxygenase inhibitors, however, differed in their effects on ALP activity. Indomethacin and ASA enhanced ALP activity, whereas flurbiprofen and piroxicam suppressed it. We then examined the effects of exogenous addition of PGE2. Although exogenous PGE2 at 6 x 10(-6) M slightly stimulated cell growth, it inhibited cell growth at 6 x 10(-8) M and 6 x 10(-7) M. ALP activity was reduced in a dose-dependent fashion by exogenous PGE2. These results suggest that PGE2 produced by MC3T3-E1 may be suppressing cell proliferation and that cyclooxygenase inhibitors, per se, may stimulate cell growth by inhibiting endogenous PGE2 production in MC3T3-E1 cells. UMR-106 cells also produced PGE2, although less than MC3T3-E1 cells. In UMR-106 cells, the cyclooxygenase inhibitors did not influence DNA content or ALP activity as distinctly as in MC3T3-E1 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of transforming growth factor alpha and interleukin-1 on DNA synthesis, collagen synthesis, procollagen mRNA levels, and prostaglandin E2 production in cultured fetal rat calvaria

Transforming growth factor alpha (TGF-alpha) and interleukin-1 (IL-1) have been shown to affect bone metabolism in vitro by prostaglandin-dependent and PG-independent mechanisms. We assessed the effects of the combination of these two agents on [3H]thymidine (TdR) incorporation into DNA, DNA content, [3H]proline incorporation into collagenase-digestible (CDP), noncollagen protein (NCP), and PGE2 production in 21 day fetal rat calvaria cultured for 24-96 h. We also determined whether TGF-alpha plus IL-1 altered procollagen mRNA levels at 96 h. TGF-alpha, 1-30 ng/ml, produced a 41-59% increase in TdR incorporation into DNA, but the effect was partially blocked by human recombinant IL-1. At 96 h TGF-alpha alone or in combination with IL-1 significantly increased the DNA content of calvaria. At 96 h, TGF-alpha inhibited CDP labeling and the addition of IL-1 further enhanced this inhibitory effect. The enhanced inhibitory effect of TGF-alpha plus IL-1 on collagen synthesis was associated with a synergistic increase in prostaglandin accumulation in the medium. Addition of indomethacin blocked PGE2 accumulation and partially reversed the inhibitory effect of TGF-alpha alone or in combination with IL-1 on collagen synthesis. TGF-alpha decreased procollagen mRNA levels by 55%, but the combination of TGF-alpha plus IL-1 decreased procollagen mRNA levels by 82%. Our results show that TGF-alpha and IL-1, which are both produced by certain tumors as well as activated macrophages, appear to act synergistically to increase prostaglandin synthesis and inhibit collagen synthesis in vitro. Thus these agents may have a regulatory role on bone formation in vivo.

The metabolism and immunology of bone

Many cells and their cytokines produce a significant effect on bone metabolism. Bone matrix synthesis is a function of the osteoblast (Fig 1), influenced directly by numerous local and systemic factors (Tables 1 and 2). Locally synthesized factors such as SGF, BMP, and BDGF may be particularly important in stimulating new bone formation at sites of bone resorption or following bony injury. Of the systemic factors, GH; somatomedin C (IGF-1); high concentrations of insulin, testosterone, PDGF and TGF beta; and low concentrations of PGE2 and IL-1 appear to stimulate bone formation in vitro. These latter factors may be more important in maintaining skeletal growth and bone mass. Bone resorption by osteoclasts (Figs 2 and 3) is also controlled by the osteoblast, as this cell produces a leukotriene-dependent polypeptide that stimulates osteoclastic bone resorption. Osteoblasts cover the periosteal and endosteal bone-surfaces and limit exposure of the underlying bone to osteoclasts. PTH, vitamin D, PGE2, and other systemic factors interact directly with the osteoblast, not the osteoclast. Surface receptor binding of PTH increases intracellular cAMP and calcium and results in release of the factor that stimulates osteoclastic bone resorption. PGE2 induces osteoblasts to activate osteoclasts and is a major controlling factor in bone metabolism; the osteoblast produces PGE2, which can then modify osteoblastic function by positive feedback. Although low concentrations of PGE2 stimulate bone formation, higher concentrations promote osteoblast-mediated bone resorption. Furthermore, many of the systemic factors stimulate bone resorption via a PGE2-associated mechanism. Immune cytokines also appear to exert a profound influence on bone metabolism. INF-gamma inhibits osteoclastic resorption, whereas IL-1, TNF, and LT strongly stimulate bone resorption. However, low concentrations of IL-1 paradoxically result in stimulation of bone formation. These cytokines, particularly in various combinations, may prove extremely important in understanding and treating the bone loss associated with malignancies, osteoporosis, and rheumatoid arthritis.

Recombinant human lymphotoxin effects on osteoblastic cells

Lymphotoxin, or tumor necrosis factor beta, has been shown to be a potent bone resorbing cytokine. In the present study, the effect of recombinant human lymphotoxin on osteoblastic cell proliferation and prostaglandin synthesis was investigated. Lymphotoxin (10(-10)-10(-7) M) caused a significant, dose-dependent decrease of rat osteoblastic cell proliferation. This appeared to be an indirect, prostaglandin-dependent action, since in the presence of indomethacin (1 microM) the lymphotoxin effect was reversed. Subsequently, prostaglandin E2 and prostacyclin (assayed as 6-keto-prostaglandin F1 alpha) levels produced by the osteoblastic cells in response to lymphotoxin were measured. The cytokine caused a dose-dependent increase of these arachidonic acid metabolites, with the maximum effect at 10(-8) M. These results suggest that lymphotoxin's mechanism of action on bone may involve increases in arachidonic acid metabolite synthesis and an indirect, prostanoid-mediated decrease in the proliferation rate of osteoblastic cells.

Regulation of bone cell metabolism

Bone formation and resorption are normal physiologic processes. In pathologic states such as in periodontal disease or osteoporosis a shift in the balance of these two processes occurs, resulting in a net loss of mineralized tissue. Osteoclasts have historically been considered to be the primary bone resorbing cells, but current research has lead to the hypothesis that osteoblastic cells play an integral role in bone resorption as well. It appears that osteoblasts respond to bone resorbing agents via a series of intracellular responses after interactions with specific surface receptors. Two basic pathways involving different "second messengers" have been identified. The first pathway involves cyclic 3',5' adenosine monophosphate (cAMP) production and the second involves membrane phospholipids, diacylglycerol and calcium. A cytosolic enzyme, protein kinase C (PKC), has been shown to affect both cAMP as well as calcium fluxes and may act to regulate both these pathways. It is the purpose of this paper to discuss current studies and hypotheses concerning the nature of mechanisms involved in regulation of bone metabolism with emphasis on second messenger systems. Information of this nature is critical to the development of rationale regarding diagnosis, treatment and management of systemic and local pathoses of bone.

Indomethacin modulation of load-related stimulation of new bone formation in vivo

The capacity of bone to organize and reorganize its structure in response to changing mechanical demands is well recognized. However, the mechanism by which the changing mechanical environment is detected, and the means by which this information is translated into a stimulus for structural modification, are not understood. A group of substances suggested to be involved in the initial transduction of strain information are the prostaglandins. In this experiment we used a single period of dynamic loading to stimulate an adaptive osteogenic response in vivo. Loading was performed in the presence and absence of indomethacin. Measurements of the periosteum 5 days after loading showed that the presence of indomethacin at the time of loading reduced the osteogenic response. Though consistent with the hypothesis that prostaglandins are involved in the initial transduction of tissue strain into a biochemical response, this result is not sufficient to demonstrate this conclusively because reduced prostaglandin levels during the 24 hours immediately after the period of loading may affect many other points in the cascade of events between strain transduction and adaptive new bone formation. Furthermore, indomethacin at the relatively high levels we used (40 mg/kg) may have effects other than those on prostaglandin synthesis.

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

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

Evidence for an osteoblast-activating factor in a patient with peripheral T-cell lymphoma and osteosclerosis

A patient with peripheral T-cell Lymphoma and acquired, systemic osteosclerosis is described. Bone histology showed a spectacular activation of osteoblasts accompanyed by massive new bone formation. Alkaline phosphatase in serum was elevated and increased to greater than 2000 U/l when the lymphoma became refractory to chemotherapy. In the patient's serum an osteoblast-activating factor could be demonstrated using a rat osteogenic osteosarcoma cell line (ROS 17/2.8). The factor was absent during remission of the tumor. We conclude that osteosclerosis was a paraneoplastic syndrome in this patient due to the secretion of an osteoblast-stimulating factor by the T-cell lymphoma. This situation is similar to the secretion of osteoclast-activating factors described in B-cell lymphomas, particularly multiple myeloma. The characterization of such a factor could be of therapeutic relevance.

Prostaglandins in inflammatory bone pathology: mechanism and therapeutic benefit of etodolac

To investigate the role of PGE2 in the development of bone and joint pathology in rat adjuvant arthritis, hindlimb paws were evaluated by calcified tissue histologic techniques focusing on histochemical visualization of cartilage and bone lesions. Case studies of hindlimbs from normal, adjuvant arthritic, and etodolac-treated arthritic rats demonstrated the association of disease severity with inflammation, chondromalacia, replacement of adipose bone marrow with a fibroid marrow, osteoclastic bone resorption, synovial cysts, and pannus formation within the joints. Extensive periosteal intramembranous bone formation was temporally associated with joint destruction and medullary tissue pathology. In vivo data were correlated with in vitro effects of inflammatory mediators (IL-1, PGE2) on bone resorption. Etodolac blocked bone explant PGE2 accumulation at concentrations of 10(-7) M and higher, and inhibited bone resorption at concentrations of 10(-5) M and higher. The data indicate that in vitro and in vivo models of bone metabolism are well correlated regarding prostaglandin synthesis; that the inflammatory mediator PGE2 is largely responsible for the involvement of skeletal tissue in the adjuvant arthritis model; and that the effects of etodolac are specifically mediated by its ability to inhibit PGE2 accumulation in vivo.

Hormonal regulation of collagen gene expression in osteoblastic cell-overview and new findings

Type I collagen synthesis in osteoblasts is regulated by a variety of systemic hormones, growth factors and locally produced factors. With the use of cDNA probes and hybridization analysis, it appears that many of these agents alter collagen synthesis by a pretranslational mechanism.

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

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

Systemic effects of prostaglandin E2 on vertebral trabecular remodeling in beagles used in a healing study

Prostaglandin E2 (PGE2) at a dose of 10 mg/kg was administered orally to beagles used in a study of rib fracture and drill hole defect healing. Double fluochrome labels were given prior to surgical manipulation and before necropsy at 30 days. Bone remodeling was evaluated in trabecular bone of the fourth lumbar vertebra. There was a decrease in the number and extent of posttreatment labels (P less than 0.05) in the controls, with decreased mineral apposition rate (P less than 0.05) and decreased active bone formation rate (P less than 0.01). In dogs given PGE2 for 30 days following surgery, the extent of posttreatment labels (P less than 0.05) and bone formation rate (P less than 0.01) were increased. There was no difference found, however, in static morphometric parameters, including osteoid and osteoblast-covered surface, indicating that the stimulation of bone formation may have been transitory and matrix synthesis had declined. In another group of dogs given PGE2 for 5 days prior to surgical manipulation and between the first and second pretreatment labels, the extent of the double-labeled surface was increased (P less than 0.05) indicating an acute PG effect to sustain formation at remodeling sites. These studies show that PGE2 given orally has a systemic effect on bone remodeling in vertebral trabeculae that involves the stimulation of formation activity.

Recombinant interleukin-1 stimulates prostaglandin E2 production by osteoblastic cells: synergy with parathyroid hormone

Recombinant mouse IL-1 (Interleukin-1) has been shown to be capable of stimulating prostaglandin E2 (PGE2) production by isolated rat osteoblastic cells in a dose-dependent manner. The rapidity of the effect (1 hour) and the potency of IL-1 (5 x 10(-12) M) in producing this effect suggest that IL-1 may exert some of its effects on bone via PGE2. Parathyroid hormone (PTH) appears to have a strong synergistic effect with IL-1. These data further substantiate the role of IL-1 in bone physiology.

Differentiation of muscle, fat, cartilage, and bone from progenitor cells present in a bone-derived clonal cell population: effect of dexamethasone

RCJ 3.1, a clonally derived cell population isolated from 21-d fetal rat calvaria, expresses the osteoblast-associated characteristics of polygonal morphology, a cAMP response to parathyroid hormone, synthesis of predominantly type I collagen, and the presence of 1,25-dihydroxyvitamin D3-regulated alkaline phosphatase activity. When cultured in the presence of ascorbic acid, sodium beta-glycerophosphate, and the synthetic glucocorticoid dexamethasone, this clone differentiated in a time-dependent manner into four morphologically distinct phenotypes of known mesenchymal origin. Multinucleated muscle cells were observed as early as 9-10 d in culture, lipid-containing adipocytes formed after 12 d, chondrocyte nodules were observed after 16 d, and mineralized bone nodules formed after 21 d in culture. The differentiated cell types were characterized morphologically, histochemically, and immunohistochemically. The formation of adipocytes and chondrocytes was dependent upon the addition of dexamethasone; the muscle and bone phenotypes were also expressed at low frequency in the absence of dexamethasone. The sex steroid hormones progesterone and 17 beta-estradiol had no effect on differentiation in this system, suggesting that the effects of dexamethasone represent effects specific for glucocorticosteroids. Increasing concentrations of dexamethasone (10(-9)-10(-6) M) increased the numbers of myotubes, adipocytes, and chondrocytes; however, when present continuously for 35 d, the lower concentrations appeared to better maintain the muscle and adipocyte phenotypes. Bone nodules were not quantitated because the frequency of bone nodule formation was too low. Single cells obtained by plating RCJ 3.1 cells at limiting dilutions in the presence of dexamethasone, were shown to give rise to subclones that could differentiate into either single or multiple phenotypes. Thus, the data suggest that this clonal cell line contains subpopulations of mesenchymal progenitor cells which can, under the influence of glucocorticoid hormones, differentiate in vitro into four distinct cell types. It is, therefore, a unique cell line which will be of great use in the study of the regulation of mesenchymal stem cell differentiation.

Inhibition of bone collagen synthesis by the tumor promoter phorbol 12-myristate 13-acetate

We characterized the effect of the tumor promoter phorbol 12-myristate 13-acetate (PMA) on osteoblast function and DNA synthesis in 21-day-old fetal rat calvaria maintained in organ culture. Protein synthesis was determined by measuring the incorporation of [3H]proline into collagenase-digestible (CDP) and noncollagen protein (NCP), respectively. Alkaline phosphatase activity was assessed as the release of p-nitrophenol from p-nitrophenol phosphate. DNA synthesis was determined by the incorporation of [3H]thymidine into acid-insoluble bone and total DNA content. PMA at 3-100 ng/ml (4-133 nM) caused a dose-related inhibition of collagen synthesis that was observed 6 hours after adding PMA to calvaria. PMA inhibited collagen synthesis in the osteoblast-rich central bone of calvaria but did not alter collagen synthesis in the periosteum. There was little effect of PMA on noncollagen protein synthesis in the central bone or periosteum. Phorbol esters that do not promote tumor formation in vivo did not alter collagen synthesis in calvaria. PMA stimulated prostaglandin E2 (PGE2) production in calvaria, but indomethacin did not alter the inhibitory effect of PMA on bone collagen synthesis. PMA decreased alkaline phosphatase activity measured after 48 hr of culture and increased the incorporation of [3H]thymidine into bone and DNA content after 96 hr of culture. These data indicate that PMA inhibits collagen synthesis and alkaline phosphatase activity, while stimulating DNA synthesis, suggesting that activation of protein kinase C might regulate osteoblast function and bone cell replication.

Effects of orally administered prostaglandin E-2 on cortical bone turnover in adult dogs: a histomorphometric study

The effects of Prostaglandin E-2 (PGE-2) on cortical bone turnover in ribs and femurs of 32 intact adult dogs were evaluated following 3 months treatment. Static and dynamic histomorphometric skeletal changes were characterized using terminal in vivo tetracycline double labeling. PGE-2 caused a dose dependent increase in the formation of subperiosteal fibrous-lamellar new bone in femurs, and an increase in bone remodeling within the (original) cortical compacta of both femurs and ribs. Increased cortical remodeling resulted in a new steady state, but only in ribs. Increased Haversian remodeling in ribs and femurs was characterized by increases in the activation frequency, the number of bone resorbing and forming foci, the percent of osteons with single labels, and the radial closure and bone formation rates, with no effect on appositional rate. While the mean ratios of the number of resorption to formation foci (R/F) were unremarkable in femurs of treated versus control males, the R/F ratios in treated females were approximately 50% lower than matched controls. In treated males, both femoral osteon resorption and formation times were 50% shorter than matched controls. In treated females, femoral osteon resorption time was 2-4-fold shorter than the decrease in osteon formation time. Calcium and phosphorus levels were normal in all treated dogs. Serum alkaline phosphatase levels were increased approximately two-fold in high dose (10.0 mg/kg) dogs and correlated well with the histologic findings of increased skeletal turnover and bone formation.

The role of bone cells in increasing metaphyseal hard tissue in rapidly growing rats treated with prostaglandin E2

The skeletal effects of graded doses of prostaglandin E2 (PGE2) given to weanling Sprague-Dawley rats for 3 weeks were investigated to elucidate the role of bone cells in increasing hard tissue mass. Decalcified (3 micron) sections were quantified in the light microscope by point hit and intersect counting using a Merz grid. Hard tissue mass (bone and calcified cartilage) and osteoblast, osteoclast and osteoprogenitor cell numbers were counted in metaphyseal tissue bands 0.24, 0.48, 0.72, 1.20, 1.68, 2.16, 2.64, 3.12, 3.60 and 4.08 mm from the growth plate metaphyseal junction. Changes were different and more marked in the secondary spongiosa than the primary spongiosa of the proximal tibial metaphysis of treated rats. In the primary spongiosa of rats treated with 3 or 6 mg PGE2/kg/d (1) an increase in bone and hard tissue masses and (2) a decrease in osteoclasts, osteoprogenitor cell numbers and surface to volume ratio was observed. In the secondary spongiosa (lower metaphysis) of rats treated with 2 same dose levels (1) an increase in bone mass, calcified cartilage cores, and hard tissue mass and perimeter, an elevation of osteoprogenitor cell and osteoblast numbers, a depression of osteoclast, osteoclast nuclei numbers and surface to volume ratio and new sites of intramembranous ossification (woven bone formation) originating from the cortico-endosteal envelope was observed. In this growing rat skeletal model, we showed that PGE2 increases metaphyseal calcified tissue mass by depressing hard tissue resorption and stimulating the replication and differentiation of osteoblast precursors to form new foci of woven bone.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of prostaglandins on regional remodeling changes during tibial healing in beagles: a histomorphometric study

A histomorphometric study was carried out on bone samples in the region of healing defects in the tibias of beagles of various ages. A 5 mm diameter drill hole defect was made in the mid-shaft of the tibia. Eleven of the beagles received either vehicle (n = 6) or prostaglandin E2 orally (n = 5) for the 30-day period from surgery to time of necropsy. Ten dogs received local injections of vehicle (n = 4) or prostaglandin E1 directly into the defects for the first 10 days after surgery. Double labels were given with each of two fluochrome markers, calcein prior to surgical treatment, and oxytetracycline hydrochloride prior to 30-day sacrifice. The regional remodeling changes were evaluated in 40-50 micron thick cross-sections taken 2 cm proximal to the defect and matching samples from the contralateral side. In the controls, the changes were variable and reflected primarily increased formation on the surfaces of the cortex. Cortical endosteal bone formation, as indicated by oxytetracycline labeling, was increased in both control groups and there was an increase in labeling in the periosteal and haversian envelopes as well, in the local injection control group. In the dogs given prostaglandin E2 orally, there was increased periosteal bone formation in addition to increased cortical-endosteal formation. When healing sides were compared to controls, prostaglandin E2-treated dogs also had increased osteoid formation in all three envelopes and increased resorption surface in the cortical endosteal envelope, indicating accelerated remodeling.(ABSTRACT TRUNCATED AT 250 WORDS)

Prostaglandin E2 stimulates DNA synthesis by a cyclic AMP-independent pathway in osteoblastic clone MC3T3-E1 cells

The effect of prostaglandin E2 (PGE2) on osteoblastic cell proliferation was investigated using osteoblastic clone MC3T3-E1 cells cultured in serum-free medium. PGE2 at 2 micrograms/ml increased the number of the cells by 2 days after its addition. PGE2 raised the level of DNA synthesis in a dose-related fashion after a constant lag time, the maximal effect being at 2-10 micrograms/ml and the level about fourfold over that of the control at 36 hr after its addition. However, at low doses (below 0.2 microgram/ml), PGE2 rather depressed DNA synthesis. Isobutyl methylxanthine counteracted the stimulation of DNA synthesis by PGE2, and forskolin depressed the synthesis, which was inversely correlated with increasing intracellular cAMP content. These results indicate that an increase in cAMP content inhibits DNA synthesis. In addition, 2',5'-dideoxyadenosine did not negate the stimulatory effect of PGE2 on DNA synthesis, suggesting that PGE2 increases DNA synthesis, probably via a pathway different from the adenylate cyclase/cAMP system. Moreover, at a high dose, PGE2 stimulated both the production and degradation of cAMP; the elevation of cAMP content was rapidly depressed by the stimulated degradation system. Consequently, the stimulatory effect of PGE2 on DNA synthesis would be released from the inhibition by cAMP, resulting in an increase in DNA synthesis. Taken together with data from our previous reports, these results indicate that PGE2 enhances both the proliferation and differentiation of osteoblastic cells in vitro, which are probably mediated by two different second messengers dependent on the concentration of PGE2.

Effect of 16,16-dimethyl prostaglandin E2 methyl ester on weanling rat skeleton: daily and systemic administration

The effects of 0, 0.3, 1.0, and 3.0 mg of 16,16-dimethyl prostaglandin E2 methyl ester (Di-M-PGE2) per kilogram per day administered subcutaneously for 21 days to fluorescent-labeled weanling rats were studied, by single-photon absorptiometric and static and dynamic histomorphometric techniques, to determine possible alterations in growth and mineralized tissue mass and their mechanisms of response. Specimens of femurs, proximal tibia, and tibial shaft were analyzed. Di-M-PGE2 caused a reduction in bone elongation and a dramatic accumulation in metaphyseal trabecular hard tissue mass. At high doses, the growth cartilage exhibited reduced thickness and degenerative cell size and cell production rate. The increased metaphyseal trabecular hard tissue mass was restricted to the secondary spongiosa region and was observed at all dose levels. The metaphysis was further characterized by an increase in bone and calcified cartilage cores, a marked elevation in osteoblast and osteoclast numbers, in osteoblast-to-osteoclast ratios, and in ratios of differentiated cells to osteoprogenitor cells. These findings were consistent with the interpretations that Di-M-PGE2 depressed bone elongation by delaying the division and maturation of growth plate chondrocytes; stimulated the differentiation of osteoblasts and osteoclasts, thus generating more differentiated bone cells but suppressing their activities; and increased metaphyseal trabecular hard tissue by creating an imbalance in osteoblasts over osteoclasts and suppressing hard tissue resorption.

Skeletal alkaline phosphatase activity as a bone formation index in vitro

These studies were intended to examine the relationship between skeletal collagen formation and skeletal alkaline phosphatase (ALP) activity in vitro. Embryonic chick calvaria were exposed to skeletal effectors (including high and low pH, a range of [pi] and [Ca], PTH, NaF, etc), and collagen formation was assessed by the incorporation of 3[H]-proline as 3[H]-hydroxyproline (3[H]-hyp). ALP activity was measured in the serum-free conditioned medium and in 20% butanol extracts of the bones. We found that ALP activity and 3[H]-hyp incorporation were coordinately increased from pH 5.5 to pH 7.2 (r = .99, P less than 0.001). Calvarial ALP was not increased in response to low [Pi], but low [Ca] increased ALP and coordinately decreased collagen formation (r = -.81, P less than 0.05). Although calvarial ALP and 3[H]-hyp incorporation were coordinately increased by NaF, vanadate, PGE2, calcitonin, and insulin, the slopes of the correlations were not the same for all effectors (eg, NaF: r = .97, P less than 0.01, slope = 0.90; vanadate, r = .95, P less than 0.005, slope = 0.20), indicating differential actions on ALP and 3[H]-hyp incorporation. When a variety of effectors, including low [Ca], were used to treat different groups of calvaria, ALP activity was not correlated with 3[H]-hyp incorporation (r = .35), but when the exposure to effectors was limited to a preincubation, or when the low [Ca] data were excluded, a correlation was observed (r = .87, P less than 0.001, and r = .64, P less than 0.02, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Clonal differences in prostaglandin synthesis among osteosarcoma cell lines

This study compares the metabolism of [14C]-arachidonic acid between PGE2 synthesizing (ROS 17/2.8) and nonsynthesizing (ROS 25/1) osteosarcoma cell lines. In both cell lines: (a) 90% of [14C]-arachidonic acid was taken up at 24 h. (b) More than 90% of the label was associated with phospholipids. (c) [14C]-arachidonic acid was rapidly taken up by phosphatidylcholine which reached the highest specific activity around 5 h while the labeling of other phospholipids was still increasing at 24 h. (d) Twenty-four hours after addition of [14C]-arachidonic acid only 4% of the label was associated with triacylglycerols in ROS 25/1 and 0.3% in ROS 17/2.8 cells. The calcium ionophore A23187 enhanced the release of [14C]-arachidonic acid from phospholipids in the PGE synthesizing osteoblastic cells (ROS 17/2.8 and 2/3) but had no effect in nonosteoblastic cells (ROS 24/1 and 25/1). ROS 17/2.8 and 2/3 cells converted the released arachidonic acid as well as exogeneously added arachidonic acid into PGE2. PGE2 synthesis depended on arachidonic acid concentration. Among bone resorbing agents, parathyroid hormone and 1,25(OH)2D3 had no effect on PGE synthesis, whereas thrombin and rabbit serum stimulated PGE2 production. The effect of rabbit serum was abolished by heat inactivation. The findings of this study indicate that the difference in PGE production between the osteoblastic and nonosteoblastic osteosarcoma cells are due mainly to differences in arachidonic acid conversion to PGE2.

Simultaneous measurement of bone resorption and collagen synthesis in neonatal mouse calvaria

A tissue culture technique which permits the simultaneous measurement of collagen synthesis and bone resorption has been developed. Cultured neonatal mouse calvaria undergo resorption when stimulated by a number of agents including parathyroid hormone, vitamin D, and prostaglandin E2. Mouse calvaria are of sufficient size to measure the extent of proline incorporation into collagenase-digestible protein. Four chemically diverse stimulators of bone resorption were tested for their effect on collagen synthesis. For each stimulator tested, the dose-response relationships for the stimulation of resorption and the inhibition of collagen synthesis were found to coincide.

Effects of acetylsalicylic acid and naproxen on the synthesis and mineralization of collagen in the rat femur

The influence of acetylsalicylic acid (ASA) and naproxen on the biochemical properties of intact growing femora in young male rats was studied. The medication periods were 9 and 18 days. At an ASA dose of 150 mg/kg/12 h the rate of collagen synthesis and the rate of mineral incorporation decreased and were impaired by about 10% compared with controls after 18 days. The dry weights and contents of collagen and calcium were not influenced after 9 days, but were reduced by 4%-7% after 18 days. A higher solubility of collagen (7%) was also found at the end of the study. In rats that received ASA at 100 mg/kg/12 h no significant differences were observed. A naproxen dose of 20 mg/kg/12 h reduced the rate of mineral deposition after 18 days, but had no other detectable effects on bone. The results indicate that ASA inhibits bone formation.

Effects of exogenous prostanoids on the proliferation of osteoblast-like cells in vitro

The effects of several prostaglandins on the proliferation of secondary cultures of osteoblast-like cells, as measured by the incorporation of [3H]-thymidine into DNA and total DNA content of the cultures, were studied. PGE2 in the concentration range of 10(-8) to 10(-5) M caused a direct, dose-related stimulation of proliferation, while PGF2 alpha and PGD2 were less effective. PGA2 and 6-keto-PGF1 alpha were inactive in the osteoblasts in concentrations of 10(-7) to 10(-6) M. A similar stimulation profile was observed for the induction of ornithine decarboxylase (ODC, L-ornithine decarboxy-lyase, EC 4.1.1.17): the order of potency of the different prostaglandins in the induction of the ODC activity was PGE2 greater than PGF2 alpha = PGD2; again, PGA2 and 6-keto-PGF1 alpha were without effect in concentrations up to 10(-6) M. These results show that the primary prostaglandins, in order of potency PGE2 greater than PGF2 alpha = PGD2, can have a direct, stimulatory effect on the proliferation of osteoblasts, which is closely related to the induction of ODC activity.

Prostaglandin-related bone resorption in cultured neonatal mouse calvaria: evaluation of biopotency of nonsteroidal anti-inflammatory drugs

The objective of this study was the development of an assay based on suppression of endogenous prostaglandin synthesis in cultured neonatal mouse calvaria for evaluation of the biopotency of nonsteroidal anti-inflammatory drugs in bone. In preliminary trials, osteolytic activity due to spontaneous prostaglandin production over a 72 h culture period was found highly variable, and could not be stabilized by addition of the common precursor arachidonic acid to the culture medium. Eventually, continuous exposure of mouse calvaria to moderate concentrations of thrombin (greater than or equal to 14 U/ml medium) proved to be satisfactory to achieve stable rates of bone resorption through continuous stimulation of prostaglandin synthesis from endogenous sources. Notably, the extent of net calcium release into the medium was highly reproducible in different experiments. As an example for possible applications of the bioassay, the ability of acemetacin to interfere with prostaglandin synthesis in bone, which had not been assessed before, was evaluated in a comparative assay with indomethacin and acetylsalicylic acid. While 1 X 10(-8) M acemetacin appeared to augment thrombin-induced bone resorption, as did 5 X 10(-6) M acetylsalicylic acid, a dose-dependent inhibition of calcium release was observed between 10(-7)-10(-5) M acemetacin. In this respect, the biopotency of indomethacin was 50 times higher than that of acemetacin and exceeded that of acetylsalicylic acid by a factor of more than 2000. These data could be useful for the appraisal of multiple effects of the investigated drugs on prostaglandin-related bone turnover.

Bone changes from prostaglandin therapy

Prostaglandin E therapy in infants causes periosteal elevation. Although the changes usually take 30-40 days to become visible, we have seen them as early as nine days. In 15 infants who had prostaglandin E therapy for over six days, three developed periosteal elevation. Three other cases are described in greater detail, with long-term follow-up in two in which the bone remodeled to normal. Gallium scan in one showed increased uptake in areas involved. The periosteal cloaking may mimic Caffey disease but the pattern of involvement is different, since the mandible, which is commonly affected in Caffey disease, is rarely involved in prostaglandin E therapy.

Prostaglandin E2 stimulates collagen and non-collagen protein synthesis and prolyl hydroxylase activity in osteoblastic clone MC3T3-E1 cells

We investigated the stimulative effect of prostaglandin E2 (PGE2) on an osteoblastic cell line, clone MC3T3-E1, in serum-free medium. PGE2 elevated collagen and non-collagen protein syntheses in a dose-related fashion up to 2 micrograms/ml, the maximal increases being 2- and 3-fold, respectively, over that in the control. Its stimulative effect was evident as early as 12 h. PGE2 slightly increased DNA content, but its effect was less than that on collagen and non-collagen protein syntheses. Moreover, PGE stimulated an increase in prolyl hydroxylase activity with a maximal effect at 1-2 micrograms/ml, the activity being 15-fold over that of the control. These results strongly indicate that PGE2 directly enhances total protein synthesis including that of collagen in osteoblasts in vitro, suggesting its direct effect on bone formation in vivo as well.