A proposed mechanism of the mitogenic action of fluoride on bone cells: inhibition of the activity of an osteoblastic acid phosphatase

Fluoride (F) is a potent inhibitor of osteoblastic acid phosphatase activity with an apparent Ki value (10 to 100 mumol/L) that corresponds to F concentrations that increase bone cell proliferation and bone formation in vivo and in vitro. This high sensitivity of acid phosphatase to F inhibition appeared to be specific for skeletal tissues. Mitogenic concentrations of F did not increase cellular cAMP levels but significantly stimulated net protein phosphorylation in intact calvarial cells and in isolated calvarial membranes. These concentrations of F also stimulated net membrane-mediated phosphorylation of angiotensin II (which contains tyrosyl but no seryl or threonyl residues), suggesting that some of the F-stimulated protein phosphorylations could occur on tyrosyl residues. F had no apparent effect on thiophosphorylation of membrane proteins, suggesting that the F-stimulated net protein phosphorylation in bone cells was probably not mediated via activation of protein kinases. Orthovanadate or molybdate at concentrations that inhibit bone acid phosphatase activity also stimulated bone cell proliferation, supporting the idea that inhibition of bone acid phosphatase would lead to stimulation of bone cell proliferation. Mitogenic concentrations of F potentiated the mitogenic activities of insulin, EGF, and IGF-1 (ie, growth factors the receptors of which are tyrosyl kinases) to a greater extent than they potentiated the action of basic FGF (a growth factor that does not appear to stimulate tyrosyl protein phosphorylation). Based on these findings, a model is proposed for the biochemical mechanism of the osteogenic action of F in which F stimulates bone cell proliferation by a direct inhibition of an osteoblastic acid phosphatase/phosphotyrosyl protein phosphatase activity, which in turn increases overall cellular tyrosyl phosphorylation, resulting in a subsequent stimulation of bone cell proliferation.     

Growth stimulation of rat calvaria osteoblastic cells by acidic fibroblast growth factor

Purified acidic fibroblast growth factor (aFGF) from bovine brain stimulates the proliferation of calvaria-derived osteoblastic cells. Maximum stimulation, relative to corresponding controls, was seen at 0.2% serum (2- to 3-fold), and no stimulation was seen in the absence of serum or under serum replete conditions. The effect was dose-dependent with an ED50 of around 750 pg/ml (47 pM). aFGF (5 ng/ml) sustained the growth of calvaria cells in culture during multiple passages (72 days) at 0.2% serum. In DNA synthesis assays aFGF produced 2- to 4-fold stimulation; insulin-like growth factor I had a slight effect on DNA synthesis on its own, but enhanced the effect of aFGF 2-fold. In cells fully stimulated by epidermal growth factor (5-fold), aFGF had no further effect. Stimulation of DNA synthesis peaked at 5 ng/ml, while higher concentrations were inhibitory. Recombinant aFGF (bovine sequence) also stimulated cell proliferation (1.5-fold), and its potency was augmented by heparin (50 micrograms/ml), about 2-fold. Using simultaneous histochemical staining for alkaline phosphatase activity and [3H]thymidine nuclear uptake we found that aFGF stimulates DNA synthesis to the same extent in alkaline phosphatase-rich (osteoblastic) and alkaline phosphatase-poor (nonosteoblastic) cells. However, after cell division there is a significant decrease in PTH-responsive adenylate cyclase (2- to 3-fold) and in alkaline phosphatase levels (4- to 8-fold). These findings indicate that aFGF is mitogenic to rat calvaria osteoblastic cells, its action requires additional factors, and its growth stimulation is associated with a reduction in phenotypic expression.     

Androgens directly stimulate proliferation of bone cells in vitro

This report describes the first observation of a direct mitogenic effect of androgens on isolated osteoblastic cells in serum-free culture. [3H]thymidine incorporation into DNA and cell counts were used as measures of cell proliferation. The percentage of cells that stained for alkaline phosphatase was used as a measure of differentiation. Dihydrotestosterone (DHT) enhanced mouse osteoblastic cell proliferation in a dose dependent manner over a wide range of doses (10(-8) to 10(-11) molar), and was maximally active at 10(-9) M. DHT also stimulated proliferation in human osteoblast cell cultures and in cultures of the human osteosarcoma cell line, TE89. Testosterone, fluoxymesterone (a synthetic androgenic steroid) and methenolone (an anabolic steroid) were also mitogenic in the mouse bone cell system. The mitogenic effect of DHT on bone cells was inhibited by antiandrogens (hydroxyflutamide and cyproterone acetate) which compete for binding to the androgen receptor. In addition to effects on cell proliferation, DHT increased the percentage of alkaline phosphatase (ALP) positive cells in all three bone cell systems tested, and this effect was inhibited by antiandrogens. We conclude that androgens can stimulate human and murine osteoblastic cell proliferation in vitro, and induce expression of the osteoblast-line differentiation marker ALP, presumably by an androgen receptor mediated mechanism.     

Ghrelin stimulates proliferation of human osteoblastic TE85 cells via NO/cGMP signaling pathway

Ghrelin regulates bone formation and osteoblast proliferation, but the detailed signaling pathway for its action on osteoblasts remains unclear. In human osteoblastic TE85 cells, we observed the effects and intracellular signaling pathway of ghrelin on cell proliferation using BrdU incorporation method. Ghrelin, at 10⁻¹⁰–10⁻⁸ M concentration, significantly increased BrdU incorporation into TE85 cells. The action of ghrelin was inhibited by d-Lys3-GHRP-6, a selective antagonist of GHS-R. Nitric oxide (NO) scavenger hemoglobin and the NO synthase inhibitor NAME eliminated the stimulatory action of ghrelin on proliferation, while NO donor SNAP and NO synthase substrate L-AME stimulated proliferation of osteoblastic TE85 cells. The cGMP analogue, 8-Br-cGMP, stimulated TE85 cell proliferation, and ghrelin did not enhance proliferation in the presence of 8-Br-cGMP. Inhibition of cGMP production by the guanylate cyclase inhibitor prevented ghrelin-induced osteoblastic TE85 cell proliferation. In conclusion, ghrelin stimulates proliferation of human osteoblastic TE85 cells via intracellular NO/cGMP signaling pathway. The authors Deng-Hu Wang and Yun-Sheng Hu contributed equally to this work.     

Growth factor-induced expression of macrophage migration inhibitory factor in osteoblasts: relevance to the plasminogen activator system

Macrophage migration inhibitory factor (MIF) is a cytokine that mediates inflammatory processes in a variety of tissues. In this study, we examined the expression of MIF mRNA in the mouse osteoblastic cell line MC3T3-E1, whose proliferation is promoted by various growth factors. In the subconfluent state, transforming growth factor-beta, basic fibroblast growth factor, insulin-like growth factor-II, and fetal calf serum markedly upregulated MIF mRNA expression. The upregulation of MIF mRNA was less extensive when the cells were stimulated by the same growth factors in the overconfluent state. We also investigated the expression of MIF mRNA through a whole cell cycle from G0 phase when the osteoblastic cells were synchronized by serum starvation. The MIF mRNA expression, which gradually increased from the G0 and reached its maximum at the S phase, was nonperiodic. Moreover, human recombinant MIF upregulated the expression of urokinase plasminogen activator inhibitor-1 (PAI-1) precursor mRNA in human osteoblastic Saos-2 cells. Plasminogen activator (PA) is known to play an important role in bone metabolism, for example, in activation of procollagenase or growth factors indirectly via the formation of plasmin, and in mitogenic activity for osteoblastic cells. Our results suggest that MIF modulates the proliferation of osteoblasts and, moreover, bone tissue remodeling through the PA and plasmin system.     

重组人结缔组织生长因子对人成骨细胞增殖、分化及凋亡的影响

目的 研究重组人结缔组织生长因子(rCTGF)对人成骨细胞增殖、分化及凋亡的影响.方法 用rCTGF干预体外培养的人成骨细胞,采用[3H]-TdR掺入法检测人成骨细胞增殖率,α-磷酸萘酚法测定细胞内碱性磷酸酶活性变化,放射免疫法测定骨钙素含量的变化,Western印迹法检测Ⅰ型胶原表达的变化,应用流式细胞仪检测rCTGF埘成骨细胞凋亡的影响.结果 rCTGF呈剂量依赖性促进人成骨细胞增殖,200 ng/ml rCTGF达最大效应;rCrrGF干预显著促进人成骨细胞分化,rCTGF旱剂量依赖性增加Ⅰ型胶原、骨钙素表达及碱件磷酸酶活性;rCTGF干预可显著减少成骨细胞凋亡.结论 rCTGF可显著促进人成骨细胞增殖、分化,并抑制人成骨细胞凋亡.     

Studies of the mechanism by which androgens enhance mitogenesis and differentiation in bone cells

Recently, we reported a direct effect of androgens on murine and human bone cells to stimulate bone cell proliferation and differentiation. To test whether this effect of androgenic steroids might be mediated by growth factors, we measured relative concentrations of insulin-like growth factor-I and -II (IGF-I and IGF-II) and transforming growth factor-beta (TGF beta) in the conditioned medium from androgen-treated murine calvarial cell cultures. Only the concentration of TGF beta was increased. Consistent with the increased secretion of TGF beta in the mouse calvarial cell system, we observed an increased expression of TGF beta mRNA in a normal human osteoblastic cell system. We also determined whether androgens alter the response to growth factors. We found that dihydrotestosterone (DHT) treatment enhanced the mitogenic effects of fibroblast growth factor (FGF) and IGF-II but not those of IGF-I. The enhanced effect of FGF and IGF-II after DHT pretreatment was not affected by addition of TGF beta-blocking antibodies or by changing the culture medium. This indicated that in addition to increased release of TGF beta, another mechanism might be involved in the action of DHT on human and murine bone cells. Thus, we investigated the binding of human IGF-II to human osteoblastic cells and observed an increase in IGF-II binding after DHT treatment. Our results are consistent with a mechanism of action of androgens on bone cells that involves the induction of TGF beta and, in addition, may sensitize the cells to show an enhanced response to FGF and IGF-II, possibly by changing the receptor binding of mitogenic growth factors.     

Human platelet-derived transforming growth factor-beta stimulates parameters of bone growth in fetal rat calvariae

Human platelet-derived transforming growth factor type beta (TGF beta) is mitogenic for fetal rat calvariae in serum-free organ culture. It enhances DNA synthesis in short (24-h) and long (48- to 96-h) term cultures, but produces no significant stimulatory effects on bone collagen synthesis or alkaline phosphatase activity (two parameters of differentiated osteoblastic cell-type function) when present continuously in culture. Transitory treatment with TGF beta, however, induces a subsequent stimulation of collagen and noncollagen protein synthesis that depends on prior cell replication, suggesting an increase in the number of newly differentiated bone cells. In addition, TGF beta increases prostaglandin release, but this effect is probably unrelated to its mitogenic function. TGF beta activity is also found in culture medium conditioned by fetal rat calvariae, and the bone-derived factor produces effects similar to those of the human platelet factor. This polypeptide, therefore, may have an important function in early stages of bone development as well as bone repair after trauma-induced platelet degranulation.     

Effects of estrogens on striatal damage after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxicity in male and female mice

Nurr1, NGFI-B, and Nor1 form the NR4A subfamily of orphan nuclear receptors. The NR4A receptors are immediate early genes that can be rapidly induced in response to a variety of stimuli in many cell types, for example, in osteoblasts. Nurr1 regulates the differentiation of osteoblasts and the expression of several osteoblastic genes. Fibroblast growth factor 8b (FGF-8b) regulates osteoblastic differentiation. We show here that treatment of preosteoblastic MC3T3-E1 cells or mouse bone marrow mesenchymal cells with FGF-8b induces the expression of NR4A receptors rapidly and in a dose-dependent manner. This induction involves mitogen-activated protein kinase (MAPK), phosphatidylinositol-3-kinase (PI-3K), and protein kinase C (PKC) pathways. FGF-8b stimulates the proliferation of MC3T3-E1 cells. This effect is enhanced by overexpression of Nurr1 and NGFI-B whereas it is abolished by a dominant negative Nurr1 variant. In conclusion, FGF-8b induces the expression of NR4A orphan nuclear receptors that are involved in mediating the growth promoting effect of FGF-8b in osteoblasts.     

Evidence that fluoride-stimulated 3[H]-thymidine incorporation in embryonic chick calvarial cell cultures is dependent on the presence of a bone cell mitogen, sensitive to changes in the phosphate concentration, and modulated by systemic skeletal effectors

In previous studies we have shown that clinically effective concentrations of fluoride (5 to 30 mumol/L) could also have direct effects in vitro on skeletal tissues to increase embryonic chick bone formation and bone cell proliferation (3[H]-thymidine incorporation into DNA). From these observations, we hypothesized that fluoride-stimulated bone formation might be mediated by a direct effect of fluoride to increase bone cell proliferation. The current studies were intended to investigate the mechanism of fluoride-stimulated 3[H]-thymidine incorporation, in chick calvarial cell cultures, by assessing mitogenic interactions between fluoride and inorganic phosphate, bone-derived growth factors, and systemic skeletal effectors. With respect to fluoride-phosphate interactions, the results of our studies indicate that the effect of fluoride was dependent on the phosphate concentration in the medium. Fluoride did not increase 3[H]-thymidine incorporation in BGJb medium containing 1 mmol/L (total) phosphate; but, in 1.6 mmol/L phosphate medium, fluoride caused a dose-dependent increase in 3[H]-thymidine incorporation, between 1 and 20 mumol/L (P less than .001). The action of fluoride was also dependent on the presence of a bone cell mitogen. Fluoride increased 3[H]-thymidine incorporation when added to calvarial cell cultures in the cell-conditioned medium, but had no effect in unconditioned (ie, fresh) medium. The action of fluoride could be restored by adding an exogenous growth factor (ie, concentrated cell-conditioned medium, bone-derived growth factors, or a systemic bone cell mitogen) to the unconditioned culture medium, P less than .05 for each effector.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential activation of mitogen-activated protein kinase in response to basic fibroblast growth factor in skeletal muscle cells.

In the MM14 mouse myoblast cell line, fibroblast growth factor (FGF) stimulates proliferation and represses differentiation. However, the intracellular signaling pathways used by FGF to affect these cellular processes are unknown. The predominant FGF receptor present on MM14 cells, FGFR1, is a receptor tyrosine kinase capable of activating the mitogen-activated protein kinase (MAPK) cascade in fibroblast and neuronal cell lines. To determine whether the FGF signal is mediated via the MAPK cascade in myoblasts, MM14 cells were stimulated with basic FGF (bFGF) and activities of the various kinases were measured. After withdrawal from serum and bFGF for 3 hr, bFGF stimulated MAPK kinase (MAPKK) activity, but MAPK and S6 peptide kinase activities were not detected. In contrast, when serum and bFGF were withdrawn for 10 hr, the activities of MAPKK, MAPK, and S6 peptide kinase were all stimulated by bFGF treatment. The inability of bFGF to stimulate MAPK after 3 hr of withdrawal may be due, in part, to the presence of a MAPK phosphatase activity that was detected in MM14 cell extracts. This dephosphorylating activity diminishes during commitment to terminal differentiation and is inhibited by sodium orthovanadate. Thus, the ability of bFGF to stimulate MAPK in MM14 cells is correlated with the loss of a MAPK phosphatase activity. These results show that although bFGF activates MAPKK in proliferating myoblasts, the mitogenic signal does not progress to the downstream kinases, providing a physiological example of an uncoupling of the MAPK cascade.     

Effects of BMP-2 on osteoblastic cells and on skeletal growth and bone formation in unloaded rats.

A previous study showed that skeletal unloading induced by hindlimb suspension for 14 days in rats reduces osteoblastic cell proliferation, inhibits skeletal growth and bone formation and induces metaphyseal bone loss. This study investigated the effect of recombinant human bone morphogenetic protein-2 (rhBMP-2) in this model. In vitro analysis showed that rhBMP-2 (25-100 ng/ml, 48-96 h) increased alkaline phosphatase activity, an early marker of osteoblast differentiation, in rat neonatal calvaria cells and adult marrow stromal cells, showing that rhBMP-2 induced the differentiation of osteoblast precursor cells in vitro. In contrast, rhBMP-2 did not increase rat calvaria or marrow stromal cell proliferation. Biochemical and histomorphometric analysis showed that systemic infusion with rhBMP-2 (2 microg/kg/day) in unloaded rats had no significant effect on serum osteocalcin levels and on histomorphometric indices of bone formation. Accordingly, rhBMP-2 infusion did not prevent the decreased skeletal growth, trabecular bone bone volume and bone mineral content induced by unloading. The present data indicate that, although rhBMP-2 stimulates osteoblastic cell differentiation, rhBMP-2 infusion is not effective in increasing bone formation and in preventing trabecular bone loss induced by unloading in rats.     

A potential role for adrenomedullin as a local regulator of bone growth

Bone remodeling is a complex process of coordinated resorption and formation of bone, which is regulated by systemic hormones and by local factors. We have previously shown that the peptide hormone adrenomedullin is mitogenic to osteoblastic cells in vitro and that it promotes bone growth in vivo. The aim of the present study was to characterize the expression of molecules that may mediate adrenomedullin signaling in osteoblasts and to investigate the expression of adrenomedullin itself in these cells. The first adrenomedullin receptor that was cloned is the seven-transmembrane G protein-coupled receptor, L1. Two additional receptors for adrenomedullin, which arise from interactions between calcitonin receptor-like receptor and receptor activity modifying proteins 2 or 3, have now been described. In the current study, we used RT-PCR and Northern blot analysis to demonstrate that messenger RNA for the three adrenomedullin receptors, as well as for adrenomedullin itself, is expressed in primary rat osteoblasts. Treating primary osteoblasts with transforming growth factor-beta and insulin-like growth factor-I moderately reduced adrenomedullin RNA levels, whereas PTH had no effect. We have shown by immunocytochemistry that adrenomedullin peptide is present in osteoblasts, and by competitive binding assays that (125)I-adrenomedullin binds with high affinity to intact osteoblasts and to osteoblast cell membranes. Coexpression of adrenomedullin and adrenomedullin receptors in osteoblasts, taken together with our previous finding that adrenomedullin is mitogenic to these cells, raises the possibility that this peptide is a local regulator of bone growth.     

Effects of fibroblast growth factors on deoxyribonucleic acid and collagen synthesis in rat parietal bone cells

Acidic fibroblast growth factor (aFGF) and basic FGF (bFGF) are related molecules that are extractable from bone matrix and may be important in the maintenance of normal bone physiology. The influence of each agent on DNA and protein synthesis was studied using bone-derived primary cell cultures. Both forms of FGF were relatively more mitogenic for bone cell populations with fewer osteoblastic (Ob) characteristics than for Ob-enriched cultures. However, in the Ob cultures, bFGF was intrinsically 10-fold more stimulatory than aFGF, whereas heparin enhanced the mitotic response only to aFGF. An optimal dose of either aFGF or bFGF (100 ng/ml) decreased alkaline phosphatase activity and increased the rate of noncollagen and collagen protein synthesis in Ob cultures. The stimulatory effect was relatively greater on noncollagen than on collagen synthesis, which resulted in a decrease in percent collagen synthesis. Neither factor altered the rate of collagen degradation. Furthermore, hydroxyurea diminished, but did not prevent, the stimulatory effect of each factor on rates of protein synthesis. In contrast, polyacrylamide gel analysis of newly synthesized protein and Northern blot analysis of steady state alpha 1 type I procollagen mRNA indicated differential effects by each agent on procollagen synthesis and processing. These studies suggest that the FGFs may produce their effects on Ob cells through both shared and disparate mechanisms, with the net result being a decrease in the expression of the osteoblastic phenotype.     

Effects of a tyrosine phosphatase inhibitor on chloride secretion in human intestinal epithelia.

AIMS: Transcellular chloride transport is the key event underlying epithelial hydration in the intestine. Little is known about the role of protein tyrosine phosphatases in the regulation of basal and stimulated secretion in human intestinal epithelia. The aim of our study was to investigate the effects of the protein tyrosine phosphatase inhibitor sodium orthovanadate on vectorial chloride transport in native human colon. METHODS: An electrophysiological technique was used to measure changes in short-circuit current via a dual voltage/current clamp in native human colon mucosa and in T84 (ATCC) human intestinal cells mounted in modified Ussing chambers. RESULTS: Orthovanadate (1 mM) added to the serosal side of native human colon caused a net rise in short circuit current, reflecting the stimulation of serosal-to-mucosal chloride movement. Epithelial cells responded similarly to the same concentration of the compound. The stimulatory effect of orthovanadate was enhanced by pretreatment with the tyrosine kinase inhibitor genistein, but only when orthovanadate was added to the basolateral chamber. In contrast, the synergistic interaction did not occur when epithelial cells were previously exposed to the cAMP agonist forskolin. CONCLUSIONS: We show that tyrosine phosphatases may be involved in the regulation of epithelial chloride transport, and that orthovanadate stimulates secretion in the human colon.     

Fluoride stimulates [3H]thymidine incorporation and alkaline phosphatase production by human osteoblasts

The effect of sodium fluoride on alkaline phosphatase (ALP) release and [3H]thymidine uptake by human osteoblasts in culture was investigated. Sodium fluoride stimulated both ALP release and [3H]thymidine uptake at concentrations of sodium fluoride greater than 250 mumol/L. This stimulation was similar in magnitude to that induced by 1,25-dihydroxycholecalciferol. The fluoride-induced increase in ALP was inhibited by verapamil, a calcium channel blocker. We conclude that sodium fluoride stimulates osteoblasts to proliferate and to release ALP. This stimulation by fluoride is dependent on calcium influx. Fluoride-induced stimulation of human osteoblasts may be relevant to its effect in enhancing bone formation in patients with osteoporosis.     

Regulation of osteoblast differentiation: a novel function for fibroblast growth factor 8

Several members of the fibroblast growth factor (FGF) family have an important role in the development of skeletal tissues. FGF-8 is widely expressed in the developing skeleton, but its function there has remained unknown. We asked in this study whether FGF-8 could have a role in the differentiation of mesenchymal stem cells to an osteoblastic lineage. Addition of FGF-8 to mouse bone marrow cultures effectively increased initial cell proliferation as well as subsequent osteoblast-specific alkaline phosphatase production, bone nodule formation, and calcium accumulation if it was added to the cultures at an early stage of osteoblastic differentiation. Exogenous FGF-8 also stimulated the proliferation of MG63 osteosarcoma cells, which was blocked by a neutralizing antibody to FGF-8b. In addition, the heparin-binding growth factor fraction of Shionogi 115 (S115) mouse breast cancer cells, which express and secrete FGF-8 at a very high level, had an effect in bone marrow cultures similar to that of exogenous FGF-8. Interestingly, experimental nude mouse tumors of S115 cells present ectopic bone and cartilage formation as demonstrated by typical histology and expression of markers specific for cartilage (type II and IX collagen) and bone (osteocalcin). These results demonstrate that FGF-8 effectively predetermines bone marrow cells to differentiate to osteoblasts and increases bone formation in vitro. It is possible that FGF-8 also stimulates bone formation in vivo. The results suggest that FGF-8, which is expressed by a great proportion of malignant breast and prostate tumors, may, among other factors, also be involved in the formation of osteosclerotic bone metastases.     

Systemic prostaglandin E2 increases cancellous bone formation and mass in aging rats and stimulates their bone marrow osteogenic capacity in vivo and in vitro

Prostaglandin E(2) (PGE(2)) has been shown to exert a bone anabolic effect in young and adult rats. In this study we tested whether it possesses a similar effect on bone formation and bone mass in aging rats. Fifteen-month-old rats were injected daily with either PGE(2) at 5 mg/kg or vehicle for 14 days. PGE(2) treatment stimulated the rate of cancellous bone formation (a approximately 5.5-fold increase in bone formation rate), measured by the incorporation of calcein into bone-forming surfaces at the tibial proximal metaphysis. This effect resulted in increased cancellous bone area (+54%) at the same site. Since PGE(2) treatment resulted in a much higher proportion of bone surface undergoing bone formation and thus lined with osteoblasts, we tested the hypothesis that PGE(2) stimulates osteoblast differentiation from bone marrow precursor cells both in vivo and in vitro. We found that ex vivo cultures of bone marrow stromal cells from rats injected for 2 weeks with PGE(2) at 5 mg/kg per day yielded more ( approximately 4-fold) mineralized nodules and exhibited a greater (by 30-40%) alkaline phosphatase activity compared with cultures from vehicle-injected rats, attesting to a stimulation of osteoblastic differentiation by PGE(2). We also compared the osteogenic capacity of bone marrow from aging (15-month-old) versus young (5-week-old) rats and its regulation by PGE(2) in vitro. Bone marrow stromal cell cultures from aging rats exhibited a greatly diminished osteogenic capacity, reflected in reduced nodule formation ( approximately 6% of young animals) and lower alkaline phosphatase activity ( approximately 60% of young animals). However, these parameters could be stimulated in both groups of animals by incubation with 10-100 nM PGE(2). The magnitude of this stimulation was greater in cultures from aging rats (+550% vs +70% in nodule formation of aging compared with young rats). In conclusion, we demonstrate here that PGE(2) exerts a bone anabolic effect in aging rats, similar to the effect we and others have reported in young, growing rats. The PGE(2)-stimulated bone formation, which augments bone mass, most likely results from recruitment of osteoblasts from their bone marrow stromal precursors.     

Lysophosphatidic acid is an osteoblast mitogen whose proliferative actions involve G(i) proteins and protein kinase C, but not P42/44 mitogen-activated protein kinases

The simple glycerophospholipid lysophosphatidic acid (LPA) acts both as an intermediary in phospholipid metabolism and as an intercellular signaling molecule in its own right. In various cell types, LPA signals through its membrane-bound, G protein-coupled receptors to influence cellular processes such as proliferation, survival, and cytoskeletal function. Its actions in bone cells have not been studied. Here we show that the LPA receptor, LP(A1)/edg-2/vzg-1, is expressed in primary rat osteoblasts and the UMR 106-01 osteoblastic cell line. LPA potently induces DNA synthesis and an increase in cell number in cultures of osteoblastic cells. LPA rapidly (within 10 min) stimulates phosphorylation of p42/44 mitogen-activated protein (MAP) kinases in osteoblastic cells, an effect that is sensitive to inhibition of G(i) proteins, inhibition of influx of extracellular calcium, and inhibition of protein kinase C. LPA-induced DNA synthesis is partially inhibited by either pertussis toxin or calphostin C, but is insensitive to specific inhibitors of MEK, the kinase upstream of p42/44 MAP kinases, or of phosphatidylinositol-3 kinases. These data demonstrate that LPA is an osteoblast mitogen whose signaling effects in osteoblastic cells include activation of p42/44 MAP kinases. However, the LPA mitogenic signal in osteoblastic cells, while requiring G(i) proteins and protein kinase C, is independent of the activity of p42/44 MAP kinases.