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

Effects of arachidonic acid and docosahexaenoic acid on differentiation and mineralization of MC3T3-E1 osteoblast-like cells

Osteoblasts in culture can differentiate into mature mineralizing osteoblasts when stimulated with osteogenic agents. Clinical trials and in vivo animal studies suggest that specific polyunsaturated fatty acids (PUFAs) may benefit bone health. The aim of this study was to investigate whether arachidonic acid (AA) and docosahexaenoic acid (DHA) affect osteogenesis in osteoblasts and the transdifferentiation into adipocytes. Results from this study show that long-term exposure to AA inhibited alkaline phosphatase (ALP) activity in these cells, which might be prostaglandin E(2) (PGE(2))-mediated. DHA exposure also inhibited ALP activity which was evident after both short- and long-term exposures. The mechanism whereby DHA inhibits ALP activity is not clear and needs to be investigated. Although long-term exposure to PUFAs inhibited ALP activity, the mineralizing properties of these cells were not compromised. Furthermore, PUFA exposure did not induce adipocyte-like features in these cells as evidenced by the lack of cytoplasmic triacylglycerol accummulation. More research is required to elucidate the cellular mechanisms of action of PUFAs on bone.

Hypoxia regulates PGE(2) release and EP1 receptor expression in osteoblastic cells

Changes in regional O(2) tension that occur during fracture and skeletal unloading may stimulate local bone cell activity and ultimately regulate bone maintenance and repair. The mechanisms by which bone cells sense and respond to changes in O(2) tension are unclear. In this study we investigated the effects of low O(2) on activation of the hypoxia response element (HRE), prostaglandin E(2) (PGE(2)) production, PGE(2) receptor (EP) expression and proliferation in MC3T3-E1 osteoblastic cells. Cells were cultured for up to 72 h in 2% O(2) (considered hypoxic), 5% O(2) (in the range of normal O(2) tension in vivo) or 21% O(2) (commonly used for cell culture). Cells cultured in 2% O(2) showed activation of the HRE, increased PGE(2) release, increased EP1 expression, and reduced cell proliferation compared to cells grown at 21% O(2). Similarly, cells cultured in 5% O(2) showed increased expression of EP1 and a trend toward a decrease in proliferation, but no activation of the HRE or increase in PGE(2) levels. Expression of EP2, EP3 and EP4 were not affected by O(2) tension. The differences in EP receptor profile observed in cells grown at 5% compared to 21% O(2) suggest that bone cell phenotype may be altered under routine cell culture conditions. Furthermore, our data suggest that hypoxia-dependent PGE(2) production and EP1 expression in bone cells may play a role in bone remodeling and repair in regions of compromised or damaged bone, where O(2) tension is low.

Cyclooxygenase-2 gene disruption promotes proliferation of murine calvarial osteoblasts in vitro

Cyclooxygenase-2 (COX-2) is highly expressed in osteoblasts, and COX-2 produced prostaglandins (PGs) can increase osteoblastic differentiation in vitro. The goal of this study was to examine effects of COX-2 expression on calvarial osteoblastic proliferation and apoptosis. Primary osteoblasts (POBs) were cultured from calvariae of COX-2 wild-type (WT) and knockout (KO) mice. POB proliferation was evaluated by (3)H-thymidine incorporation and analysis of cell replication and cell cycle distribution by flow cytometry. POB apoptosis was evaluated by annexin and PI staining on flow cytometry. As expected, PGE(2) production and alkaline phosphatase (ALP) activity were increased in WT cultures compared to KO cultures. In contrast, cell numbers were decreased in WT compared to KO cells by day 4 of culture. Proliferation, measured on days 3-7 of culture, was 2-fold greater in KO than in WT POBs and associated with decreased Go/G1 and increased S cell cycle distribution. There was no significant effect of COX-2 genotype on apoptosis under basal culture conditions on day 5 of culture. Cell growth was decreased in KO POBs by the addition of PGE(2) or a protein kinase A agonist and increased in WT POBs by the addition of NS398, a selective COX-2 inhibitor. In contrast, differentiation and cell growth in marrow stromal cell (MSC) cultures, evaluated by ALP and crystal violet staining respectively, were increased in MSCs from WT mice compared to MSCs from KO mice, and exogenous PGE(2) increased cell growth in KO MSC cultures. We conclude that PGs secondary to COX-2 expression decrease osteoblastic proliferation in cultured calvarial cells but increase growth of osteoblastic precursors in MSC cultures.

Dual effect of flurbiprofen on cell proliferation and agonist-induced Ca(2+) movement in human osteosarcoma cells

In human MG63 osteosarcoma cells, the effect of flurbiprofen on intracellular Ca(2+) concentrations ([Ca(2+)](i)) and proliferation was explored. The proliferation was enhanced by 20-120 microM flurbiprofen, and was decreased by 140-200 microM flurbiprofen. The effect of flurbiprofen on the increases in cytosolic free Ca(2+) levels ([Ca(2+)](i)) induced by ATP, bradykinin, histamine and thapsigargin (an inhibitor of the endoplasmic reticulum Ca(2+) ATPase), was examined. In cell preincubated with 20 or 80 microM flurbiprofen, the [Ca(2+)](i) increases induced by all agonists were attenuated. In the presence of 20 microM flurbiprofen, the decreased [Ca(2+)](i) responses with the agonists were attributed to a defective Ca(2+) influx because this decrease was unobserved in agonists-induced [Ca(2+)](i) increases in the absence of extracellular Ca(2+). In the presence of 80 microM flurbiprofen, both the Ca(2+) influx component and the Ca(2+) releasing (from organelles) component were defective. These results suggest that flurbiprofen could alter proliferation and inhibit [Ca(2+)](i) increases.

The relationship between serum resistin, leptin, adiponectin, ghrelin levels and bone mineral density in middle-aged men

OBJECTIVE

Body weight is a significant predictor of bone mass. Hormonal factors such as sex hormones, insulin, leptin and adiponectin are thought to play a role in the mechanisms controlling the association of body weight and fat mass with bone mass. However, contradictory results have been reported for the association between serum adipocytokines and bone mineral density (BMD). We therefore examined whether the serum adipocytokine and ghrelin levels, markers of fat metabolism, are associated with BMD in male adults.

PATIENTS AND MEASUREMENTS

For 80 male adults (average age 54.5 +/- 6.4 years; average body mass index (BMI) 24.4 +/- 2.5 kg/m2), the correlations between serum resistin, leptin, adiponectin and ghrelin levels with BMD were investigated.

RESULTS

Among the adipocytokines, serum resistin levels were negatively correlated with lumbar spine BMD (r = -0.237, P = 0.05). After adjustment was made for age and BMI, log-transformed serum leptin showed a significant negative correlation with lumbar spine BMD, which was not seen on bivariate analysis (r = -0.237, P = 0.039). Femoral neck BMD was marginally associated only with serum adiponectin levels (r = -0.226, P = 0.062). In multiple regression analyses, among the adipokines, only resistin was a significant determinant of lumbar spine BMD, although the variance was small (R2 = 0.256). Serum ghrelin levels were not correlated with the BMD of either body site.

CONCLUSIONS

Serum resistin level showed a significant negative correlation with lumbar spine BMD, although the variance was small. Further studies are needed to elucidate the role of adipocytokines in bone metabolism.

Extracellular calcium is a potent inducer of cyclo-oxygenase-2 in murine osteoblasts through an ERK signaling pathway

[Ca2+]e may be important in bone turnover. We found [Ca2+]e induces COX-2 transcription and PGE2 production in primary calvarial osteoblasts through an ERK signaling pathway. Inhibition of PGE2 production inhibited the [Ca2+]e stimulation of osteoblastic differentiation but not the increase in cell number. Hence, some effects of [Ca2+]e on bone may be mediated by COX-2.

INTRODUCTION

Local changes in extracellular calcium ([Ca2+]e) may play an important role in bone turnover. We examined the possibility that prostaglandins produced by cyclo-oxygenase-2 (COX-2) could mediate some of the effects of [Ca2+]e on osteoblasts.

METHODS

We examined the [Ca2+]e induction of COX-2 expression and prostaglandin E2 (PGE2) production in primary osteoblasts (POBs) obtained by sequential enzymatic digestion of mouse calvariae. We measured mRNA and protein levels by Northern and Western analyses and PGE2 production in culture medium by radioimmunoassay (RIA). COX-2 promoter activity was measured as luciferase activity in calvarial osteoblasts derived from mice transgenic for 371 bp of the COX-2 promoter fused to a luciferase reporter gene.

RESULTS AND CONCLUSIONS

COX-2 mRNA and protein expression were induced by 3-40 mM of [Ca2+]e. [Ca2+]e (5 mM) induced COX-2 mRNA within 30 minutes; levels peaked at 6-9 h and remained elevated at 24 h. Cumulative medium PGE2 was increased at 3 h, with levels rising to 30 nM at 24 h. PGE2 production in POBs from mice with only COX-1 gene expression was 1/40th of that in POBs from mice with both COX-1 and COX-2 gene expression. [Ca2+]e increased alkaline phosphatase activity and osteocalcin mRNA, and this increase was blocked by inhibiting PGE2 production. [Ca2+]e stimulation of COX-2 promoter activity correlated with the induction of COX-2 mRNA expression. [Ca2+]e induced rapid and transient phosphorylation of extracellular signal-regulated kinase (ERK) in POBs, which peaked at 5-10 minutes. Inhibition of ERK phosphorylation with the specific inhibitors, PD-98059 and U-0126, decreased the [Ca2+]e induction of both COX-2 mRNA and luciferase activity by 70-80%. Although less effective than [Ca2+]e, strontium [Sr2+]e also induced COX-2 mRNA and promoter activity in POBs through an ERK signaling pathway. We conclude that [Ca2+]e is a potent transcriptional inducer of COX-2 expression and PGE2 production in osteoblasts through an ERK signaling pathway.

Relation of fatty acid composition in lead-exposed mallards to fat mobilization, lipid peroxidation and alkaline phosphatase activity

The increase of n-6 polyunsaturated fatty acids (PUFA) in animal tissues has been proposed as a mechanism of lead (Pb) poisoning through lipid peroxidation or altered eicosanoids metabolism. We have studied fatty acid (FA) composition in liver and brain of mallards (Anas platyrhynchos) feeding for 3 weeks on diets containing combinations of low or high levels of vitamin E (20 or 200 UI/kg) and Pb (0 or 2 g/kg). Saturated FA, n-6 PUFA and total concentrations of FA were higher in livers of Pb-exposed mallards, but not in their brains. The percentage of n-6 PUFA in liver and brain was slightly higher in Pb-exposed mallards. The increase of n-6 PUFA in liver was associated with decreased triglycerides and increased cholesterol in plasma, thus could be in part attributed to feed refusal and fat mobilization. The hepatic ratios between adrenic acid (22:4 n-6) and arachidonic acid (20:4 n-6) or between adrenic acid and linoleic acid (18:2 n-6) were higher in Pb exposed birds, supporting the existing hypothesis of increased fatty acid elongation by Pb. Among the possible consequences of increased n-6 PUFA concentration in tissues, we found increased lipid peroxidation in liver without important histopathological changes, and decreased plasma alkaline phosphatase activity that may reflect altered bone metabolism in birds.

Dietary ratio of (n-6)/(n-3) polyunsaturated fatty acids alters the fatty acid composition of bone compartments and biomarkers of bone formation in rats

The effects of dietary polyunsaturated fatty acids (PUFA) on ex vivo bone prostaglandin E(2) (PGE(2)) production and bone formation rate were evaluated in rats. Weanling male Sprague-Dawley rats were fed AIN-93G diet containing 70 g/kg of added fat for 42 d. The dietary lipid treatments were formulated with safflower oil and menhaden oil to provide the following ratios of (n-6)/(n-3) fatty acids: 23.8 (SMI), 9.8 (SMII), 2.6 (SMIII), and 1.2 (SMIV). Ex vivo PGE(2) production in liver homogenates and bone organ cultures (right femur and tibia) were significantly lower in rats fed diets with a lower dietary ratio of (n-6)/(n-3) fatty acids than in those fed diets with a higher dietary ratio. Regression analysis revealed a significant positive correlation between bone PGE(2) and the ratio of arachidonic acid (AA)/eicosapentaenoic acid (EPA), but significant negative correlations between bone formation rate and either the ratio of AA/EPA or PGE(2) in bone. Activities of serum alkaline phosphatase isoenzymes, including the bone-specific isoenzyme (BALP), were greater in rats fed a diet high in (n-3) or a low ratio of (n-6)/(n-3), further supporting the positive action of (n-3) fatty acids on bone formation. These results demonstrated that the dietary ratio of (n-6)/(n-3) modulates bone PGE(2) production and the activity of serum BALP in growing rats.

Significant loss of bone mass in patients with early, active ankylosing spondylitis: a followup study

OBJECTIVE

To analyze whether inflammatory disease activity plays a substantial role in the loss of bone mass observed in ankylosing spondylitis (AS) patients who have not yet developed ankylosis.

METHODS

A longitudinal cohort study of 34 patients with early AS (duration <10 years) without ankylosis was conducted. The mean followup was 19 months. Loss of bone mass in defined regions of the lumbar spine and femoral neck was analyzed by dual x-ray absorptiometry. Patients were grouped according to biologic parameters of disease activity (erythrocyte sedimentation rate or C-reactive protein level). Group 1 consisted of 14 patients with active disease; group 2 comprised 20 patients with inactive disease. Serum levels of interleukin-6 (IL-6) and of hormones (sex, thyroid, and calciotropic), vertebral mobility (Schober test), daily physical activity, and treatment administered were recorded every 6 months for all patients.

RESULTS

At the end of the followup period, patients with active AS showed a significant reduction in bone mass in the lumbar spine (mean 1.01 gm/cm2 at study entry versus 0.961 gm/cm2 at followup [P = 0.005]) and femoral neck (0.849 gm/cm2 versus 0.821 gm/cm2 [P = 0.015]), which represented losses of 5% and 3%, respectively. In contrast, no significant reduction in bone mass was observed in patients with inactive AS. As expected, serum IL-6 levels were significantly higher in patients with active AS than in those with inactive disease (mean +/- SD 8.3 +/- 9 pg/ml versus 2.8 +/- 5 pg/ml [P = 0.008]). No significant differences were observed between the 2 groups in any of the other variables analyzed.

CONCLUSION

The observation that loss of bone mass in AS occurred only in patients with persistent active disease strongly suggests that inflammatory activity of the disease itself plays a major role in the pathophysiology of the early bone mineral disorders observed in these patients.

Interaction between nitric oxide synthase and cyclooxygenase pathways in osteoblastic MC3T3-E1 cells

Interleukin 1 (IL-1) and tumor necrosis factor alpha (TNF-alpha) have been implicated in the pathogenesis of osteoporosis. These proinflammatory cytokines induce both cyclooxygenase (COX) and nitric oxide synthase (NOS) with the release of prostaglandin (PG) and NO, respectively. The present study was undertaken to examine the interaction between COX and NOS pathways and their role in the regulation of osteoblastic function in MC3T3-E1 cells. Addition of IL-1 alpha and TNF-alpha induced a marked increase in the production of both NO and PGE2. Reverse transcription-polymerase chain reaction analysis showed that the increase in NO production was preceded by the expression of inducible NOS mRNA. The temporal profile of PGE2 production revealed a biphasic pattern: the first small peak at 3 h was caused by de novo synthesis of PGE2 through inducible COX (COX-2) mRNA, while the subsequent progressive accumulation of PGE2 was mediated through the activation of COX pathway by NO since (1) aminoguanidine (AG), a selective inhibitor of inducible NOS, significantly suppressed the PGE2 production by IL-1 alpha and TNF-alpha, (2) NOC-18, an NO donor, reversed this suppression, and (3) NOC-18 increased PGE2 production by itself. The increase in NO production in response to IL-1 alpha and TNF-alpha was further stimulated by aspirin and inhibited by exogenous addition of PGE2, suggesting that PGE2 produced by the cytokines, in turn, negatively modulates NO production. IL-1 alpha and TNF-alpha inhibited alkaline phosphatase (ALP) activity, which was significantly reversed by AG. NOC-18 not only suppressed ALP activity by itself but also blocked the effect of AG, suggesting the role of NO in the inhibition of ALP activity. PGE2 decreased ALP activity, and the inhibitory effect of NOC-18 was attenuated in the presence of aspirin, suggesting the involvement of PGE2 in the negative modulation of ALP activity by NO. These results suggest that NO produced in response to proinflammatory cytokines participates in the modulation of ALP activity via the activation of COX pathway. The interaction between NO and the COX pathways may play an important role in the regulation of osteoblastic functions under physiologic as well as pathologic conditions.

Dietary (n-3) and (n-6) polyunsaturates and acetylsalicylic acid alter ex vivo PGE2 biosynthesis, tissue IGF-I levels, and bone morphometry in chicks

This study examined the effects of dietary (n-6) and (n-3) polyunsaturated fatty acids (PUFA) and acetylsalicylic acid (ASA) on bone ash content, morphometry, fatty acid composition, ex vivo PGE2 biosynthesis, tissue IGF-I concentration, and serum alkaline phosphatase (ALPase) activity in chicks. Newly hatched chicks were fed a semipurified diet containing soybean oil (S) or menhaden oil / safflower oil (M) at 90 g/kg. At 4 days of age, chicks were divided into four equal treatment groups receiving 0 mg [symbol: see text] or 500 mg [symbol: see text] of ASA/kg of diet: S[symbol: see text]ASA, M[symbol: see text]ASA, S[symbol: see text]ASA, and M[symbol: see text]ASA. Lipid and ASA treatments did not affect bone length, bone ash, or bone mineral content in chicks. Chicks fed M had increased fractional labeled trabecular surface and tissue level bone formation rates, independent of ASA treatment, compared with those given S. A significant fat x ASA interaction effect was found for trabecular bone volume, thickness, separation, and number. Chicks fed S had higher 20:4(n-6) but lower 20:5(n-3) concentrations in liver and bone compared with those given M. Ex vivo PGE2 biosynthesis was higher in liver homogenates and bone organ cultures of chicks fed S compared with the values for those given M at 17 days. ASA treatment decreased ex vivo PGE2 production in liver homogenates and bone organ cultures of chicks, independent of the dietary lipids. Chicks fed ASA had a lower concentration of IGF-I in tibiotarsal bone compared with those not given ASA at 19 days. Serum ALPase activity was higher in chicks given M compared with those fed S, but the values were reversed with ASA feeding. This study demonstrated that both dietary fat and ASA modulated bone PGE2 biosynthesis, and that (n-3) PUFA and fat x ASA interactions altered bone morphometry.

Fibroblastic regulation of osteoblast function by prostaglandins

The effects of osteogenic inhibitory factors secreted by human periodontal ligament fibroblasts were studied in rat bone marrow stromal cell cultures. Serum-free conditioned medium from cultures of fibroblasts strongly depressed formation of mineralized tissue by bone marrow cell cultures. The inhibitory activity was reduced by treatment of fibroblast cultures with indomethacin or by pretreatment of conditioned medium with specific antibodies to prostaglandins (PGs) E2 and F2 alpha. Passage of conditioned medium over octadecyl columns enriched PGs four-fold and significantly increased inhibitory activity. Inhibition of mineralization was replicated by treatment of bone-cell cultures with PGs B2, D2, E2, F2 alpha and I2 at concentrations of 350 ng/ml to 350 pg/ml. All combinations of these agents were inhibitory but PGE2 and PGF2 alpha exhibited the greatest inhibition at low concentrations (350 pg/ml). These experiments indicate that fibroblasts secrete PGs which can inhibit bone formation, and this may be one mechanism whereby fibroblasts can modulate osteogenesis at the interfaces of soft and mineralizing connective tissues.

The influence of ibuprofen on fracture repair: biomechanical, biochemical, histologic, and histomorphometric parameters in rats

Ibuprofen is a widely used cyclo-oxygenase inhibitor in clinical practice. It has been demonstrated by others to have an inhibitory effect on fracture repair in animals. In the present study, we were unable to demonstrate any significant alterations in fracture biomechanics as measured by torsion testing and fracture stage in mature Sprague-Dawley rats treated with 30 mg/kg/day oral dose of ibuprofen, starting 3 days following fracture, over a 12-week time interval. Fracture histology and serum osteocalcin levels were no different in treated animals than control animals. Furthermore, histomorphometric parameters of bone remodeling, including bone volume and bone formation rate in the intact tail vertebrae of these animals with unilateral femur fractures, were no different between treated and control animals.

Physiological and pharmacological regulation of biological calcification

Biological calcification is a highly regulated process which occurs in diverse species of microorganisms, plants, and animals. Calcification provides tissues with structural rigidity to function in support and protection, supplies the organism with a reservoir for physiologically important ions, and also serves in a variety of specialized functions. In the vertebrate skeleton, hydroxyapatite crystals are laid down on a backbone of type I collagen, with the process being controlled by a wide range of noncollagenous proteins present in the local surroundings. In bone, cells of the osteoblast lineage are responsible for the synthesis of the bone matrix and many of these regulatory proteins. Osteoclasts, on the other hand, are continually resorbing bone to both produce changes in bone shape and maintain skeletal integrity, and to establish the ionic environment needed by the organism. The proliferation, differentiation, and activity of these cells is regulated by a number of growth factors and hormones. While much has already been discovered over the past few years about the involvement of various regulators in the process of mineralization, the identification and functional characterization of these factors remains an area of intense investigation. As with any complex, biological system that is in a finely tuned equilibrium under normal conditions, problems can occur. An imbalance in the processes of formation and resorption can lead to calcification disorders, and the resultant diseases of the skeletal system have a major impact on human health. A number of pharmacological agents have been, and are being, investigated for their therapeutic potential to correct these defects.(ABSTRACT TRUNCATED AT 250 WORDS)

A clonal prostaglandin-responsive cell line (RDP 4-1) derived from rat dental pulp

The RDP 4-1 cell line was established from isolated rat dental pulp and cloned with limiting dilution in 96-multiwell plates. Despite their fibroblast-like appearance, RDP 4-1 cells required more than 5% (v/v) heat-inactivated fetal calf serum to maintain the cell growth and function. Type I collagen, but not type II and type III collagen, was immunocytochemically detected in the cells in subconfluent cultures. Prostaglandin E2 (PGE2, 5 ng/ml-5 micrograms/ml) dose-dependently increased cAMP accumulation, while prostaglandin F2 alpha (PGF2 alpha, 5 micrograms/ml) slightly increased it, in the presence of 0.1 mM isobutylmethyl xanthine. PGE2 (5 ng/ml-5 micrograms/ml) also stimulated phosphoinositide hydrolysis to produce inositol phosphates (IPs) in a time- and dose-dependent manner in the presence of 10 mM LiCl. PGF2 alpha was a more potent stimulator of phosphoinositide hydrolysis. Both prostaglandins increased cytoplasmic free calcium, independent of the level of extracellular calcium. Prolonged exposure (48 h) of the cells to PGE2 at higher concentrations (greater than or equal to 0.5 micrograms/ml) increased DNA synthesis. In these RDP 4-1 cells, thus, PGE2 receptors appeared to be associated with both cAMP and IPs-Ca2+ signaling systems, and that PGF2 alpha receptors are poor stimulators of the former but more potent ones of the latter. Our results suggest that both prostaglandins have some roles in regulating dental pulp metabolism.