Bipedal stance exercise and prostaglandin E2 (PGE2) and its synergistic effect in increasing bone mass and in lowering the PGE2 dose required to prevent ovariectomized-induced cancellous bone loss in aged rats

The Effects of 8-Week Resistance and Endurance Trainings on Bone Strength Compared to Irisin Injection Protocol in Mice

Background

Osteoporosis is a prevalent elderly complication that is characterized by decreased bone mineral density and increased fracture risk because of dysregulation in bone mineralization and resorption. Physical activity can enhance bone strength by exerting mechanical forces and myokines. Irisin is a myokine that is increased following physical exercise and can affect bones. In this study, 8 weeks of resistance and endurance exercises are applied in mice compared to irisin injection to assess the contribution of the protocols and this myokine to bone strength.

Materials and Methods

Thirty-five male NMRI mice were separated into five groups; control, placebo, irisin injection, resistance exercise, and endurance exercise. 8-week of exercise protocols and irisin injection protocol (100 μg/kg/week) was applied. Plasma irisin concentration and bone strength were measured using enzyme-linked immunoassay and 3-point bending assay, respectively. Statistical analyses were done through one-way ANOVA and Tukey test, and P < 0.05 was considered the significant difference.

Results

Serum irisin concentration and bone strength in resistance exercise and irisin-injected groups were significantly higher than control and placebo groups (P < 0.0001). Serum irisin concentration, but not bone strength, of the endurance exercise group was also significantly higher than control and placebo groups (P < 0.0001) but lower than resistance and irisin-injected groups.

Conclusion

Resistance exercise and irisin injection, but not endurance exercise, are likely to be effective in increasing bone strength. There may be a threshold for plasma irisin level to affect bones which the applied protocols of irisin injection and resistance exercise but not endurance exercise can reach.

Irisin Has a Protective Role against Osteoporosis in Ovariectomized Rats

The reduction in estrogen levels results in a decrease in bone density at menopause. Irisin is a myokine that modulates the benefits of exercise, which may include bone health. This study was planned to examine irisin's impact in preventing osteoporosis after ovariectomy. 4 groups of female albino rats (10 rats/group): control, sham-operated, ovariectomized (OVX-control), and OVX-irisin-treated. Serum levels of bone markers [osteocalcin (OC), bone alkaline phosphatase (BALP), tartrate-resistant acid phosphatase (TRAP), calcium (Ca⁺⁺), phosphorus (P)], glucose, and insulin were being measured. Body mass index, Homeostatic Model Assessment of Insulin Resistance (HOMA-IR), dry and ash femur weight, and bone contents of Ca⁺⁺ and P were investigated. The femur was examined histopathologically. The OVX-control group showed an increase in serum levels of OC, BALP, TRAP, calcium, phosphorus, BMI, glucose, insulin, and HOMA-IR (P < 0.05) and a reduction in dry and ash weight of the femur, the concentration of calcium and phosphorus content in bone ash (P < 0.05). The OVX-irisin-treated group exhibited a decrease in serum levels of OC, BALP and TRAP, calcium, phosphorus, BMI, glucose, insulin, HOMA-IR (P < 0.05), and a rise in dry and ash weight of the femur, the concentration of calcium and phosphorus in bone ash (P < 0.05). Histological examination of the distal femur diaphysis of the OVX-irisin-treated group exhibited proper bone architecture and density compared with that of the OVX-control group. It is concluded that irisin treatment in the OVX rats safeguarded the regular bone architecture and normal levels of serum bone biomarkers. Irisin may be a possible novel target in the prohibition of postmenopausal osteoporosis.

The bone anabolic effects of irisin are through preferential stimulation of aerobic glycolysis

Irisin, a recently identified hormone secreted by skeletal muscle in response to exercise, exhibits anabolic effects on the skeleton primarily through the stimulation of bone formation. However, the mechanism underlying the irisin-stimulated anabolic response remains largely unknown. To uncover the underlying mechanism, we biosynthesized recombinant irisin (r-irisin) using an Escherichia coli expression system and used it to treat several osteoblast cell types. Our synthesized r-irisin could promote proliferation and differentiation of osteoblasts as evidenced by enhanced expression of osteoblast-specific transcriptional factors, including Runt-related transcription factor-2 (Runx2), Oster (Osx), as well as early osteoblastic differentiation markers such as alkaline phosphatase (Alp) and collagen type I alpha 1 (Col1a1). Furthermore, we showed that the promotion of r-irisin on the proliferation and differentiation of osteoblast lineage cells are preferentially through aerobic glycolysis, as indicated by the enhanced abundance of representative enzymes such as lactate dehydrogenase A (LDHA) and pyruvate dehydrogenase kinase 1 (PDK1), together with increased lactate levels. Suppression of r-irisin-mediated aerobic glycolysis with Dichloroacetate blunted its anabolic effects. The favorite of the aerobic glycolysis after r-irisin treatment was then confirmed in primary calvarial cells by metabolic analysis using gas chromatography-mass spectrometry. Thus, our results suggest that the anabolic actions of r-irisin on the regulation of osteoblast lineage cells are preferentially through aerobic glycolysis, which may help to develop new irisin-based bone anabolic agents.

Bone mechanotransduction may require augmentation in order to strengthen the senescent skeleton

Physical exercise is thought to hold promise as a non-invasive countermeasure against skeletal fragility arising from post-menopausal and age-related osteoporosis. Importantly, mechanical loading and exercise are capable of increasing bone size via periosteal expansion, which by far, is the most effective means of strengthening the structure of a given bone. The focus of this review was to therefore explore whether exercise has the potential to increase periosteal modeling and bone size in the senescent skeleton. A survey of exercise trials in humans suggests that exercise interventions that enhance periosteal modeling in the young skeleton fail to do the same in the elderly skeleton. Underlying this ineffectiveness, in vitro studies indicate that aging lowers basal levels of cell function and degrades bone mechanotransduction at a variety of levels from altered second messenger signaling to gene expression driving proliferation and/or differentiation. Given these age-related alterations, the ultimate efficacy of an exercise intervention may depend upon concurrent supplementation that directly address deficits in signaling and/or cell function. In this context, in vivo animal models of mechanical loading that simulate the muted periosteal adaptation in the elderly hold potential to examine the efficacy of countermeasures. Preliminary in vivo experiments suggest that pharmacologically counteracting age-related deficits in cellular function can restore exercise induced periosteal modeling in the senescent skeleton to levels observed in young animals. If the safety and efficacy of this strategy were to be confirmed for human use, it would enable the utilization of exercise as a viable countermeasure against skeletal fragility at senescence.

Physical activity improves age-related decline in the osteogenic potential of rats' bone marrow-derived mesenchymal stem cells

AIM

To examine whether physical activity increases osteogenic differentiation of bone marrow mesenchymal stem cells (BMMSCs) from adult rats compared with young rats.

METHODS

Eighteen female Wistar rats were divided into three groups and the following cells isolated: (1) differentiated BMMSCs from young donors, (2) differentiated BMMSCs from sedentary adult donors and (3) differentiated BMMSCs from active adult donors. We analysed MTT conversion, percentage of cells per field, mineralized nodule number and gene expression for telomerase reverse transcriptase (TERT), alkaline phosphatase, caspase 3, osteocalcin, bone sialoprotein and collagen I.

RESULTS

Telomerase reverse transcriptase expression and the percentage of cells per field in BMMSCs cultures from adult rats were smaller than those observed in young donors. However, levels of caspase 3 expression were higher in BMMSCs from adult donors (P < 0.05). Despite the fact that physical activity was associated with an increase in expression of caspase 3 (P < 0.05), there was no difference in the percentage of cells per field between groups of adult BMMSCs (active or sedentary). However, physical activity increased the number of mineralized nodules and osteocalcin expression after 21 days, and alkaline phosphatase expression at 7, 14 and 21 days in the BMMSCs of adult donors (P < 0.05). However, those values were smaller when compared with young donors BMMSCs (P < 0.05). Only the expression levels of alkaline phosphatase were similar to young donors BMMSCs (P ≥ 0.05).

CONCLUSION

Physical activity increases osteogenic differentiation of BMMSCs from adult donors but does not increase the differentiation to the levels observed in BMMSCs from young donor rats.

Effects of strontium ranelate, raloxifene and misoprostol on bone mineral density in ovariectomized rats

OBJECTIVES

To investigate the effects of strontium ranelate, raloxifene and misoprostol on bone mineral density (BMD) in ovariectomized rats to contribute to the individualization of the treatment of postmenopausal osteoporosis.

STUDY DESIGN

Sixty sexually mature female Sprague-Dawley rats weighing 250 g were used. The 60 rats were divided into six groups of 10 rats each: SR, MISO, RAL, SHAM, DW and OVX. All except the SHAM rats were subjected to bilateral ovariectomy. Three days after surgery, rats were administered strontium ranelate (Protelos, 2 g, Servier, Istanbul), 1800 mg/kg/day; misoprostol (Cytotec, 200 mcg, Ali Raif, Istanbul), 200 mcg/kg/day; raloxifene (Evista, 60 mg, Lily and Company, Istanbul), 3 mg/kg/day and 1 cc of distilled water by gavage for 8 weeks. Bone mineral density measurements were then performed.

RESULTS

The strontium ranelate (SR) group had significantly higher vertebral BMD than all other groups. Femoral density in the SR group was also significantly higher than in other groups and there was no difference between femoral density in the strontium ranelate and sham groups.

CONCLUSIONS

Strontium ranelate, raloxifene and misoprostol can prevent bone loss in the vertebrae, whereas strontium ranelate can also prevent bone loss in the femur of ovariectomized rats. Strontium ranelate increases greater than raloxifene and misoprostol BMD in the vertebrae.

CONDENSATION

Strontium ranelate may increase both vertebral and femur BMD in ovariectomized rats while raloxifene and misoprostol may only increase lumbar spine BMD.

P2X7 nucleotide receptor plays an important role in callus remodeling during fracture repair

The P2X7 nucleotide receptor (P2X7R) is an ATP-gated ion channel expressed in bone cells. Homozygous null P2X7R (P2X7R(-/-)) mice have reduced bone formation, so we hypothesized that P2X7R(-/-) mice have impaired fracture healing compared to P2X7R(+/+) control mice. To test the hypothesis, adult P2X7R(-/-) mice and P2X7R(+/+) mice were studied. Osteotomy of the right femur was performed and a stainless-steel pin was inserted into the medullary cavity to stabilize the fracture site. No differences in callus development were seen in the radiograph, micro computed tomography, or dual-energy x-ray absorptiometry measurements. Mechanical testing showed that the recovery of ultimate force, stiffness, and energy to failure were slightly decreased in P2X7R(-/-) mice compared with the control. Histomorphometric measurements of the callus revealed that mineralizing surface and bone formation were significantly decreased, by 22% (p < 0.001) and 29% (p < 0.05), respectively, in P2X7R(-/-) mice in comparison with the wild-type control. These data show that a null mutation of the P2X7R does not affect the amount of callus formed in our osteotomy fracture model. However, callus remodeling was significantly delayed. Our data suggest the different role of the P2X7R in woven bone and lamellar bone formation.

Feasibility of using a bone-targeted, macromolecular delivery system coupled with prostaglandin E(1) to promote bone formation in aged, estrogen-deficient rats

PURPOSE

Macromolecular delivery systems have therapeutic uses because of their ability to deliver and release drugs to specific tissues. The uptake and localization of HPMA copolymers using Asp(8) as the bone-targeting moiety was determined in aged, ovariectomized (ovx) rats. PGE(1) was attached via a cathepsin K-sensitive linkage to HPMA copolymer-Asp(8) conjugate and was tested to determine if it could promote bone formation.

MATERIALS AND METHODS

The uptake of FITC-labeled HPMA copolymer-Asp(8) conjugate (P-Asp(8)-FITC) on bone surfaces was compared with the mineralization marker, tetracycline. Then a targeted PGE(1)-HPMA copolymer conjugate (P-Asp(8)-FITC-PGE(1)) was given as a single injection and its effects on bone formation were measured 4 weeks later.

RESULTS

P-Asp(8)-FITC preferentially deposited on resorption surfaces, unlike tetracycline. A single injection of P-Asp(8)-FITC-PGE(1) resulted in greater indices of bone formation in aged, ovx rats.

CONCLUSIONS

HPMA copolymers can be targeted to bone surfaces using Asp(8), with preferential uptake on resorption surfaces. Additionally, PGE(1) attached to the Asp(8)-targeted HPMA copolymers and given by a single injection resulted in greater bone formation measured 4 weeks later. This initial in vivo study suggests that macromolecular delivery systems targeted to bone may offer some therapeutic opportunities and advantages for the treatment of skeletal diseases.

Age-related changes in bone morphology are accelerated in group VIA phospholipase A2 (iPLA2beta)-null mice

Phospholipases A(2) (PLA(2)) hydrolyze the sn-2 fatty acid substituent, such as arachidonic acid, from phospholipids, and arachidonate metabolites are recognized mediators of bone modeling. We have previously generated knockout (KO) mice lacking the group VIA PLA(2) (iPLA(2)beta), which participates in a variety of signaling events; iPLA(2)beta mRNA is expressed in bones of wild-type (WT) but not KO mice. Cortical bone size, trabecular bone volume, bone mineralizing surfaces, and bone strength are similar in WT and KO mice at 3 months and decline with age in both groups, but the decreases are more pronounced in KO mice. The lower bone mass phenotype observed in KO mice is not associated with an increase in osteoclast abundance/activity or a decrease in osteoblast density, but is accompanied by an increase in bone marrow fat. Relative to WT mice, undifferentiated bone marrow stromal cells (BMSCs) from KO mice express higher levels of PPAR-gamma and lower levels of Runx2 mRNA, and this correlates with increased adipogenesis and decreased osteogenesis in BMSCs from these mice. In summary, our studies indicate that age-related losses in bone mass and strength are accelerated in iPLA(2)beta-null mice. Because adipocytes and osteoblasts share a common mesenchymal stem cell origin, our findings suggest that absence of iPLA(2)beta causes abnormalities in osteoblast function and BMSC differentiation and identify a previously unrecognized role of iPLA(2)beta in bone formation.

Alfacalcidol-stimulated focal bone formation on the cancellous surface and increased bone formation on the periosteal surface of the lumbar vertebrae of adult female rats

PURPOSE

To investigate the skeletal effects of alfacalcidol alone or in combination with exercise in intact adult female rats.

METHODS

Seventy-four 8.5-month-old rats were orally administered 0, 0.005, 0.025, 0.05 or 0.1 microg/kg of alfacalcidol for 12 weeks, alone or in combination with exercise. Cancellous bone histomorphometric measurements were performed on the second lumbar vertebra.

RESULTS

At 0.05 and 0.1 microg/kg, alfacalcidol caused a significant increase in cancellous bone volume, accompanied by an increase in trabecular architecture. Percent eroded surface, bone resorption and formation were suppressed by alfacalcidol treatment. However, mineral apposition rate was significantly increased, indicating osteoblast activity was increased. A positive balance between bone formation and resorption was observed in the rats treated with the highest dose of alfacalcidol. Alfacalcidol induced a unique bone formation site ("bouton") on the cancellous surface. These boutons connected adjacent trabeculae and increased trabecular thickness. They exhibited both smooth and scalloped cement lines, suggesting that they were formed by minimodeling- and remodeling-based bone formation. Furthermore, alfacalcidol at 0.1 microg/kg increased periosteal bone formation of the lumbar transverse processes. Bipedal stance exercise alone did not have an effect on bone balance and bone turnover. There were no interactions between alfacalcidol and bipedal stance exercise except for a decrease in bone resorption.

CONCLUSION

Alfacalcidol exhibited both anti-catabolic and anabolic effects on bone in intact female rats. The effect of combined treatment with alfacalcidol and bipedal stance exercise was no better than that of alfacalcidol alone.

Alfacalcidol treatment increases bone mass from anticatabolic and anabolic effects on cancellous and cortical bone in intact female rats

It has been reported that alfacalcidol had an anticatabolic and anabolic effect on bone in ovariectomized and aged male rat models, but this has not been tested on intact female rats. The current study was to determine the effects of alfacalcidol on cancellous and cortical bone in intact female rats with or without exercise. Seventy-four, 8.5-month-old, intact female rats were orally treated with 0, 0.005, 0.025, 0.05, or 0.1 microg/kg alfacalcidol alone or in combination with raised cage (RC) exercise for 3 months. In vivo peripheral quantitative computerized tomography (pQCT) of the proximal tibial metaphyses (PTM) and ex vivo histomorphometric analyses of the PTM and tibial shaft (TX) were performed. Only the 0.1 microg alfacalcidol/kg dose proved to be anabolic. pQCT analysis showed that this dose increased total and cortical bone mineral content and density and trabecular bone mineral density. Histomorphometrically, it induced an anabolic response by increased trabecular mass and microarchitecture from stimulated cancellous bone and bone bouton formations, and suppressed bone resorption more than bone formation on the trabecular and endocortical surfaces, to produce a positive bone balance. A positive correlation between trabecular connectivity and bone bouton numbers occurred. These findings suggest alfacalcidol treatment augments bone mass by increased cancellous bone mass and improved trabecular architecture through its anticatabolic and anabolic properties in the intact adult female rat. Last, raised cage exercise alone or the combination of raised cage and alfacalcidol was no more effective than alfacalcidol alone.

Isoflavone-supplemented soy yoghurt associated with resistive physical exercise increase bone mineral density of ovariectomized rats

OBJECTIVE

To determine the effects produced by the ingestion of a fermented soy product (soy yoghurt), supplemented with isoflavones and associated with a resistive exercise program, on the bone metabolism of mature ovariectomized (Ovx) and sham-ovariectomized (sham-Ovx) rats.

METHODS

A total of 56 rats were used. They were divided into 2 sedentary control groups, the Ovx control group (C-Ovx) and the sham-Ovx control group (C-Sovx), each with 7 sedentary animals, and 2 treated groups, Ovx and sham-Ovx, with 21 animals each. These two treated groups were subdivided into three subgroups of seven animals each, which received the following treatments: consuming the soy yoghurt+sedentary, only subjected to resistive exercise, and consuming the soy yoghurt+resistive exercise. Both the program of resistive exercise and the consumption of soy yoghurt (at 3 mL/(kg body weight day)) continued for 12 weeks. The soy yoghurt was supplemented with isoflavones at 50mg/100g of product. The animals were sacrificed and their right-side femurs and tibias removed and assessed for bone mineral density (BMD). The alkaline phosphatase activity (AP) was determined in the blood serum.

RESULTS

There was a significant increase in both femur and tibia BMD values and in serum alkaline phosphatase activity in all the treated subgroups, compared with the control groups (p<0.05).

CONCLUSION

The ingestion of the soy yoghurt supplemented with isoflavones was capable of preventing a loss of bone mass in Ovx rats and of increasing bone mass in sham rats, whilst the resistive exercise program was effective in augmenting the bone mass in sham and Ovx rats.

Efeito da atividade física no osso normal e na prevenção e tratamento da osteoporose

A osteoporose é uma doença cada vez mais diagnosticada em mulheres e homens de todo o mundo. Embora os esteróides sexuais sejam importantes na gênese da osteoporose, a inatividade física constitui um fator de risco. O exercício físico atua no osso por efeito direto, via força mecânica, ou indireto, mediado por fatores hormonais. Mas os mecanismos pelos quais a atividade física melhora a massa óssea ainda não são totalmente conhecidos. Baseando-se nos resultados que demonstram os efeitos benéficos da atividade física no tecido ósseo, a prática de esportes vem sendo cada vez mais indicada na prevenção e até mesmo no tratamento da osteoporose. O objetivo desta revisão é descrever os efeitos da atividade física no tecido ósseo normal e na prevenção e tratamento da osteoporose.

Effect of misoprostol on bone mineral density in women with postmenopausal osteoporosis

OBJECTIVE

To evaluate the effect of misoprostol on bone mineral density in postmenopausal women.

MATERIALS AND METHODS

The study was performed in a randomized controlled prospective manner in 90 women with menopause at Süleymaniye Maternity and Women's Diseases Teaching and Research Hospital between January and December 2003. Cases were divided into three groups each consisting of 30 women who were in menopause for at least 1 year and had t-scores less than -1 by dual energy X-ray densitometry (DEXA). Group I was treated with misoprostol and calcium, Group II received tibolone and calcium and Group III was given calcium only and considered as control group. In all patients, bone mineral density in L1-L4 vertebrae, femur neck and Ward triangle were measured by DEXA and t and z scores were calculated.

RESULTS

All groups were similar demographically. Bone mineral density in L1-L4 vertebrae, femur neck and Ward triangle in the group treated with misoprostol, increased by 5, 8.1 and 3.6%, respectively. In the tibolone group, bone mineral density in L1-L4 vertebrae, femur neck and Ward triangle increased by 8.3, 5.3 and 7.8%, respectively. There was not a significant difference in t and z-scores and bone mineral density measurements between misoprostol and tibolon groups.

CONCLUSION

Misoprostol may be an alternative treatment for patients with osteopenia and osteoporosis who are not suitable for hormone replacement therapy.

Intermittently administered parathyroid hormone 1-34 reverses bone loss and structural impairment in orchiectomized adult rats

Male osteoporosis is emerging as a central theme in bone research. As in females, hypogonadism appears as a principal risk factor in men that leads to bone loss and increased fracture incidence. Intermittently administered parathyroid hormone (PTH) reverses bone loss in sex hormone-deprived women and female animals and increases bone mass in elderly men and normal male animals. This study was carried out to assess whether the PTH anabolic activity is also effective in adult castrated males and to gain insight into the underlying tissue processes. Bilateral orchiectomy (ORX) or sham-ORX was performed in 13-week old rats. Five weeks later, the ORX rats were treated intermittently with human PTH(1-34), 80 microg/kg/day or vehicle for 6 weeks. Femora were evaluated by quantitative micro-computed tomography followed by dynamic histomorphometry. The trabecular bone volume density showed 40% and 56% ORX-induced loss in the distal metaphysis at 6 weeks and 12 weeks post-ORX, respectively. PTH(1-34) induced supraphysiologic recovery of this bone loss (155% recovery) consequent to a vast increase in trabecular thickness (174% over sham-ORX controls) and a partial reversal (62%) of the decrease in trabecular number. As compared with the results in 12-week, orchiectomized vehicle-administered rats, the PTH(1-34) treatment induced a significant decrease in osteoclast number (20%) and twofold increase in bone formation rate. While ORX did not affect the femoral diaphysis, PTH(1-34) induced marked cortical thickening via the stimulation of endosteal mineral appositional rate (154% over ORX rats). These data portray PTH(1-34) as a highly potent bone anabolic agent in adult ORX rats, mainly by increasing both the trabecular and cortical thicknesses through its effect on osteoblasts and osteoclasts. The adult ORX rat is useful for investigating the processes involved in bone anabolic activity in castrated osteoporotic males and for the development of bone anabolic agents for treating this condition.

Prostaglandin E2 stimulates fibronectin expression through EP1 receptor, phospholipase C, protein kinase Calpha, and c-Src pathway in primary cultured rat osteoblasts

Fibronectin (Fn) is involved in the early stages of bone formation, and prostaglandin E (PGE) is an important factor regulating osteogenesis. Here we found that PGE(2) enhanced extracellular Fn assembly in rat primary osteoblasts, as shown by immunofluorescence staining and enzyme-linked immunosorbent assay. PGE(2) also increased the protein levels of Fn by using Western blotting analysis. By using pharmacological inhibitors or activators or genetic inhibition by the EP receptor, antisense oligonucleotides revealed that the EP(1) receptor but not other PGE receptors is involved in PGE(2)-mediated up-regulation of Fn. At the mechanistic level, Ca(2+) chelator (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester)), phosphatidylinositol-phospholipase C inhibitor (U73122), or Src inhibitor (PP2) attenuated the PGE(2)-induced Fn expression. Protein kinase C (PKC) inhibitor (GF109203X) also inhibited the potentiating action of PGE(2). Furthermore, treatment with antisense oligonucleotides of various PKC isoforms, including alpha, beta, epsilon, and delta, demonstrated that alpha isozyme plays an important role in the enhancement action of PGE(2) on Fn assembly. Flow cytometry and reverse transcription-PCR showed that PGE(2) and 17-phenyl trinor PGE(2) (EP(1)/EP(3) agonist) increased the surface expression and mRNA level of alpha5 or beta1 integrins. Fn promoter activity was enhanced by PGE(2) and 17-phenyl trinor PGE(2) in cells transfected with pGL2F1900-Luc. Cotransfection with dominant negative mutants of PKCalpha or c-Src inhibited the potentiating action of PGE(2) on Fn promoter activity. Local administration of PGE(2) or 17-phenyl trinor PGE(2) into the metaphysis of the tibia via the implantation of a needle cannula significantly increased the Fn and alpha5beta1 integrin immunostaining and bone volume of secondary spongiosa in tibia. Taken together, our results provided evidence that PGE(2) increased Fn and promoted bone formation in rat osteoblasts via the EP(1)/phospholipase C/PKCalpha/c-Src signaling pathway.

Prostaglandin E2 receptors EP2 and EP4 are down-regulated during differentiation of mouse osteoclasts from their precursors

Prostaglandin E2 (PGE2) has been proposed to be a potent stimulator of bone resorption. However, PGE2 itself has been shown to directly inhibit bone-resorbing activity of osteoclasts. We examined the role of PGE2 in the function of mouse osteoclasts formed in vitro. Bone marrow macrophage osteoclast precursors expressed PGE2 receptors EP1, EP2, EP3beta, and EP4, and the expression of EP2 and EP4 was down-regulated during osteoclastic differentiation induced by receptor activator of NF-kappaB ligand and macrophage colony-stimulating factor. In contrast, functional EP1 was continuously expressed in mature osteoclasts. PGE2 as well as calcitonin caused intracellular Ca2+ influx in osteoclasts. However, PGE2 and 17-phenyltrinol-PGE2 (an EP1 agonist) failed to inhibit actin-ring formation and pit formation by osteoclasts cultured on dentine slices. When EP4 was expressed in osteoclasts using an adenovirus carrying EP4 cDNA, both actin-ring and pit-forming activities of osteoclasts were inhibited in an infectious unit-dependent manner. Treatment of EP4-expressing osteoclasts with PGE2 further inhibited their actin-ring and pit-forming activities. Such inhibitory effects of EP4-mediated signals on osteoclast function are similar to those that are calcitonin receptor-mediated. Thus, osteoclast precursors down-regulate their own EP2 and EP4 levels during their differentiation into osteoclasts to escape inhibitory effects of PGE2 on bone resorption.

Regulation of bone morphogenetic protein-2 expression by endogenous prostaglandin E2 in human mesenchymal stem cells

Cyclooxygenase (COX)-2 is generally known as an inducible enzyme, and it produces arachidonic acid to prostaglandin E2 (PGE2), which modulates bone metabolism. Here, we investigated the expression and role of COX isomers in human mesenchymal stem cells. Human mesenchymal stem cells constitutively expressed COX-2 as well as COX-1, and secretion of PGE2 was completely inhibited by NS-398, a specific inhibitor of COX-2. Levels of secreted PGE2 were strikingly higher in human mesenchymal stem cells than in osteoblastic cells differentiated from the mesenchymal cells. This higher production of PGE2 in mesenchymal stem cells was due to higher expression of membrane-associated PGE synthase (mPGES) regulated by early growth response factor-1 (Egr-1). Treatment of human mesenchymal stem cells with NS-398 suppressed expression of bone morphogenetic protein-2 (BMP-2). The suppression of BMP-2 by NS-398 was abrogated by an EP4 receptor agonist as well as by PGE2. Moreover, BMP-2 expression was suppressed by an EP4 receptor antagonist. These data indicate that PGE2 produced by COX-2 increases BMP-2 expression via binding the EP4 receptor.

Administration of Parathyroid Hormone, Prostaglandin E2, or 1-alpha,25-Dihydroxyvitamin D3 Restores the Bone Inductive Activity of rhBMP-2 in Aged Rats

Bone morphogenetic protein (BMP) induces bone formation in young rodents, but aging causes a reduction in the bone-forming ability of BMP. Most patients who require bone reconstruction are relatively old. Accordingly, we examined whether anabolic hormones could restore the bone inductive activity of rhBMP-2 in aged rats. rhBMP-2 in a carrier pellet was implanted subcutaneously in both 4- and 50-week-old female Wistar rats. PTH, PGE2, or 1,25(OH)2D3 was injected every day during the period of BMP implantation. The pellets were harvested, and were examined both histologically and biochemically 2 weeks after implantation. Bone-forming ability was measured by alkaline phosphatase (ALP) activity and calcium (Ca) content. Pellets in 50-week-old rats showed a significant reduction in bone formation compared to pellets in 4-week-old rats. However, daily injections of PTH into 50-week-old rats restored both ALP activity (103 +/- 4.6%) and Ca content (105 +/- 2.6%). 1,25(OH)2D3 and PGE2 also restored Ca content (103 +/- 4.5% and 98 +/- 3.8%, respectively) and stimulated ALP activity (142 +/- 2.3% and 133 +/- 3.6%). These results show that the administration of these hormones restores bone-forming ability in aged rats. A combination treatment of these hormones with rhBMP-2 might be applicable to the reconstruction of bone defects in elderly patients.