Ovariectomy enhances and estrogen replacement inhibits the activity of bone marrow factors that stimulate prostaglandin production in cultured mouse calvariae

Aging and Bone Metabolism

Changes in bone architecture and metabolism with aging increase the likelihood of osteoporosis and fracture. Age-onset osteoporosis is multifactorial, with contributory extrinsic and intrinsic factors including certain medical problems, specific prescription drugs, estrogen loss, secondary hyperparathyroidism, microenvironmental and cellular alterations in bone tissue, and mechanical unloading or immobilization. At the histological level, there are changes in trabecular and cortical bone as well as marrow cellularity, lineage switching of mesenchymal stem cells to an adipogenic fate, inadequate transduction of signals during skeletal loading, and predisposition toward senescent cell accumulation with production of a senescence-associated secretory phenotype. Cumulatively, these changes result in bone remodeling abnormalities that over time cause net bone loss typically seen in older adults. Age-related osteoporosis is a geriatric syndrome due to the multiple etiologies that converge upon the skeleton to produce the ultimate phenotypic changes that manifest as bone fragility. Bone tissue is dynamic but with tendencies toward poor osteoblastic bone formation and relative osteoclastic bone resorption with aging. Interactions with other aging physiologic systems, such as muscle, may also confer detrimental effects on the aging skeleton. Conversely, individuals who maintain their BMD experience a lower risk of fractures, disability, and mortality, suggesting that this phenotype may be a marker of successful aging. © 2023 American Physiological Society. Compr Physiol 13:4355-4386, 2023.

Lowering of lysophosphatidylcholines in ovariectomized rats by Curcuma comosa

Decline of ovarian function in menopausal women increases metabolic disease risk. Curcuma comosa extract and its major compound, (3R)-1,7-diphenyl-(4E,6E)-4,6-heptadien-3-ol (DPHD), improved estrogen-deficient ovariectomized (OVX) rat metabolic disturbances. However, information on their effects on metabolites is limited. Here, we investigated the impacts of C. comosa ethanol extract and DPHD on 12-week-old OVX rat metabolic disturbances, emphasizing the less hydrophobic metabolites. Metabolomics analysis of OVX rat serum showed a marked increase compared to sham-operated rat (SHAM) in levels of lysophosphatidylcholines (lysoPCs), particularly lysoPC (18:0) and lysoPC (16:0), and of arachidonic acid (AA), metabolites associated with inflammation. OVX rat elevated lysoPCs and AA levels reverted to SHAM levels following treatments with C. comosa ethanol extract and DPHD. Overall, our studies demonstrate the effect of C. comosa extract in ameliorating the metabolic disturbances caused by ovariectomy, and the elevated levels of bioactive lipid metabolites, lysoPCs and AA, may serve as potential biomarkers of menopausal metabolic disturbances.

The Role of Mast Cells in Bone Metabolism and Bone Disorders

Mast cells (MCs) are important sensor and effector cells of the immune system that are involved in many physiological and pathological conditions. Increasing evidence suggests that they also play an important role in bone metabolism and bone disorders. MCs are located in the bone marrow and secrete a wide spectrum of mediators, which can be rapidly released upon activation of mature MCs following their differentiation in mucosal or connective tissues. Many of these mediators can exert osteocatabolic effects by promoting osteoclast formation [e.g., histamine, tumor necrosis factor (TNF), interleukin-6 (IL-6)] and/or by inhibiting osteoblast activity (e.g., IL-1, TNF). By contrast, MCs could potentially act in an osteoprotective manner by stimulating osteoblasts (e.g., transforming growth factor-β) or reducing osteoclastogenesis (e.g., IL-12, interferon-γ). Experimental studies investigating MC functions in physiological bone turnover using MC-deficient mouse lines give contradictory results, reporting delayed or increased bone turnover or no influence depending on the mouse model used. By contrast, the involvement of MCs in various pathological conditions affecting bone is evident. MCs may contribute to the pathogenesis of primary and secondary osteoporosis as well as inflammatory disorders, including rheumatoid arthritis and osteoarthritis, because increased numbers of MCs were found in patients suffering from these diseases. The clinical observations could be largely confirmed in experimental studies using MC-deficient mouse models, which also provide mechanistic insights. MCs also regulate bone healing after fracture by influencing the inflammatory response toward the fracture, vascularization, bone formation, and callus remodeling by osteoclasts. This review summarizes the current view and understanding of the role of MCs on bone in both physiological and pathological conditions.

Cytokines and Bone: Osteoimmunology

Cytokines and hematopoietic growth factors have traditionally been thought of as regulators of the development and function of immune and blood cells. However, an ever-expanding number of these factors have been discovered to have major effects on bone cells and the development of the skeleton in health and disease (Table 1). In addition, several cytokines have been directly linked to the development of osteoporosis in both animal models and in patients. In order to understand the mechanisms regulating bone cells and how this may be dysregulated in disease states, it is necessary to appreciate the diverse effects that cytokines and inflammation have on osteoblasts, osteoclasts, and bone mass. This chapter provides a broad overview of this topic with extensive references so that, if desired, readers can access specific references to delve into individual topics in greater detail.

An estrogen receptor β-selective agonist inhibits non-alcoholic steatohepatitis in preclinical models by regulating bile acid and xenobiotic receptors

Non-alcoholic steatohepatitis (NASH) affects 8-10 million people in the US and up to 75% of obese individuals. Despite this, there are no approved oral therapeutics to treat NASH and therefore the need for novel approaches exists. The estrogen receptor β (ER-β)-selective agonist, β-LGND2, inhibits body weight and white adipose tissue, and increases metabolism, resulting in higher energy expenditure and thermogenesis. Due to favorable effects of β-LGND2 on obesity, we hypothesized that β-LGND2 will prevent NASH directly by reducing lipid accumulation in the liver or indirectly by favorably changing body composition. Male C57BL/6 mice fed with high fat diet (HFD) for 10 weeks or methionine choline-deficient diet for four weeks and treated with vehicle exhibited altered liver weights by twofold and increased serum transaminases by 2-6-folds. These changes were not observed in β-LGND2-treated animals. Infiltration of inflammatory cells and collagen deposits, an indication of fibrosis, were observed in the liver of mice fed with HFD for 10 weeks, which were effectively blocked by β-LGND2. Gene expression studies in the liver indicate that pregnane X receptor target genes were significantly increased by HFD, and the increase was inhibited by β-LGND2. On the other hand, metabolomics indicate that bile acid metabolites were significantly increased by β-LGND2. These studies demonstrate that an ER-β agonist might provide therapeutic benefits in NASH by directly modulating the function of xenobiotic and bile acid receptors in the liver, which have important functions in the liver, and indirectly, as demonstrated before, by inhibiting adiposity. Impact statement Over 75-90% of those classified as clinically obese suffer from co-morbidities, the most common of which is non-alcoholic steatohepatitis (NASH). While there are currently no effective treatment approaches for NASH, data presented here provide preliminary evidence that an estrogen receptor β-selective ligand could have the potential to reduce lipid accumulation and inflammation, and protect liver from NASH.

Inflammatory Markers and the Risk of Hip and Vertebral Fractures in Men: the Osteoporotic Fractures in Men (MrOS)

Cytokines play major roles in regulating bone remodeling, but their relationship to incident fractures in older men is uncertain. We tested the hypothesis that men with higher concentrations of pro-inflammatory markers have a higher risk of fracture. We used a case-cohort design and measured inflammatory markers in a random sample of 961 men and in men with incident fractures including 120 clinical vertebral, 117 hip, and 577 non-spine fractures; average follow-up 6.13 years (7.88 years for vertebral fractures). We measured interleukin (IL)-6, C-reactive protein (CRP), tumor necrosis factor alpha (TNFα), soluble receptors (SR) of IL-6 (IL-6SR) and TNF (TNFαSR1 and TNFαSR2), and IL-10. The risk of non-spine, hip, and clinical vertebral fracture was compared across quartiles (Q) of inflammatory markers using Cox proportional hazard models with tests for linear trend. In multivariable-adjusted models, men with the highest (Q4) TNFa cytokine concentrations and their receptors had a 2.0-4.2-fold higher risk of hip and clinical vertebral fracture than men with the lowest (Q1). Results were similar for all non-spine fractures, but associations were smaller. There was no association between CRP and IL-6SR and fracture. Men in the highest Q of IL-10 had a 49% lower risk of vertebral fracture compared with men in Q1. Among men with ≥3 inflammatory markers in the highest Q, the hazard ratio (HR) for hip fractures was 2.03 (95% confidence interval [CI] 1.11-3.71) and for vertebral fracture 3.06 (1.66-5.63). The HRs for hip fracture were attenuated by 27%, 27%, and 15%, respectively, after adjusting for appendicular lean mass (ALM), disability, and bone density, suggesting mediating roles. ALM also attenuated the HR for vertebral fractures by 10%. There was no association between inflammation and rate of hip BMD loss. We conclude that inflammation may play an important role in the etiology of fractures in older men. © 2016 American Society for Bone and Mineral Research.

BMP signaling is required for adult skeletal homeostasis and mediates bone anabolic action of parathyroid hormone

Bmp2 and Bmp4 genes were ablated in adult mice (KO) using a conditional gene knockout technology. Bones were evaluated by microcomputed tomography (μCT), bone strength tester, histomorphometry and serum biochemical markers of bone turnover. Drill-hole was made at femur metaphysis and bone regeneration in the hole site was measured by calcein binding and μCT. Mice were either sham operated (ovary intact) or ovariectomized (OVX), and treated with human parathyroid hormone (PTH), 17β-estradiol (E2) or vehicle. KO mice displayed trabecular bone loss, diminished osteoid formation and reduced biomechanical strength compared with control (expressing Bmp2 and Bmp4). Both osteoblast and osteoclast functions were impaired in KO mice. Bone histomorphomtery and serum parameters established a low turnover bone loss in KO mice. Bone regeneration at the drill-hole site in KO mice was lower than control. However, deletion of Bmp2 gene alone had no effect on skeleton, an outcome similar to that reported previously for deletion of Bmp4 gene. Both PTH and E2 resulted in skeletal preservation in control-OVX, whereas in KO-OVX, E2 but not PTH was effective which suggested that the skeletal action of PTH required Bmp ligands but E2 did not. To determine cellular effects of Bmp2 and Bmp4, we used bone marrow stromal cells in which PTH but not E2 stimulated both Bmp2 and Bmp4 synthesis leading to increased Smad1/5 phosphorylation. Taken together, we conclude that Bmp2 and Bmp4 are essential for maintaining adult skeletal homeostasis and mediating the anabolic action of PTH.

The novel bisphosphonate disodium dihydrogen-4-[(methylthio) phenylthio] methanebisphosphonate increases bone mass in post-ovariectomy rats

The novel bisphosphonate (BP) disodium dihydrogen-4-[(methylthio) phenylthio] methanebisphosphonate (MPMBP) is a non-nitrogen-containing BP with an antioxidant side chain that possesses anti-inflammatory properties. We investigated the systemic effects of this compound on bone loss induced by ovariectomy (OVX) in adult rats. Micro-computed tomography revealed that MPMBP increased bone mass and density in both the metaphysis and diaphysis, and improved the structural properties important for mechanical strength of osteoporotic bone. Sequential bone labeling with tetracycline and calcein indicated that MPMBP decreased longitudinal growth of the primary spongiosa (PS), but stimulated cortical bone formation in the diaphysis. MPMBP increased type I collagen accumulation in the PS, and decreased the number and size of adipocytes in the bone marrow, suggesting inhibition of increased bone marrow adipogenesis induced by OVX. Furthermore, MPMBP reduced the number of bone resorbing cathepsin K-positive osteoclasts induced by OVX. These results suggest that MPMBP could improve bone loss induced by estrogen deficiency. Both stimulation of bone formation and inhibition of bone resorption might play a role in the increase in bone mass and bone density after MPMBP treatment.

Non-steroidal anti-inflammatory drugs and the risk of a second hip fracture: a propensity-score matching study

Background

Non-steroidal anti-inflammatory drugs (NSAIDs) are frequently prescribed for elderly patients, particularly after a hip fracture. However, we are not clear about the effect of NSAIDs on the risk of a second hip fracture because of confounding factors.

Methods

This was a Taiwan National Health Insurance Research Database-based study using propensity-score matching (PSM) to control for confounding. Enrollees were selected from patients with a hip fracture during 1996–2004 and followed longitudinally until December 2009. After PSM for comorbidities and bisphosphonate therapy, 94 patients with a second hip fracture were assigned to the Cases group and 461 without it to the Controls group. The target drugs are NSAIDs; paracetamol and dexamethasone are used for comparison.

Results

The correlation between the mean daily-dose (MDD) ratios of NSAIDs and the probability values of the current statistical tests were highly negative (Pearson’s r = −0.920, P = 0.003), which indicated that the higher the MDD ratios, the greater the risks of a second hip fracture. A Kaplan-Meier survival analysis showed a time-dependent trend of increasing risk of a second hip fracture in patients taking NSAIDs (P < 0.001). Moreover, patients ≥60 years old had a higher risk of a second hip fracture than did those <60 and taking the NSAIDs diclofenac (P = 0.016) and celecoxib (P = 0.003) and the corticosteroid dexamethasone (P = 0.018), but not those taking analgesic paracetamol (P = 0.074).

Conclusions

We conclude that taking NSAIDs after a fragility hip fracture dose- and time-dependently significantly increases the risk of a second hip fracture, especially in elderly patients. To lower the risk of a second hip fracture, any underlying causes for excessively using NSAIDs should be treated and thus fewer NSAIDs prescribed after a first hip fracture.

Myricetin Prevents Alveolar Bone Loss in an Experimental Ovariectomized Mouse Model of Periodontitis

Periodontitis is a common chronic inflammatory disease, which leads to alveolar bone resorption. Healthy and functional alveolar bone, which can support the teeth and enable their movement, is very important for orthodontic treatment. Myricetin inhibited osteoclastogenesis by suppressing the expression of some genes, signaling pathways, and cytokines. This study aimed to investigate the effects of myricetin on alveolar bone loss in an ovariectomized (OVX) mouse model of periodontitis as well as in vitro osteoclast formation and bone resorption. Twenty-four healthy eight-week-old C57BL/J6 female mice were assigned randomly to four groups: phosphate-buffered saline (PBS) control (sham) OVX + ligature + PBS (vehicle), and OVX + ligature + low or high (2 or 5 mg∙kg⁻¹∙day⁻¹, respectively) doses of myricetin. Myricetin or PBS was injected intraperitoneally (i.p.) every other day for 30 days. The maxillae were collected and subjected to further examination, including micro-computed tomography (micro-CT), hematoxylin and eosin (H&E) staining, and tartrate-resistant acid phosphatase (TRAP) staining; a resorption pit assay was also performed in vitro to evaluate the effects of myricetin on receptor activator of nuclear factor κ-B ligand (RANKL)-induced osteoclastogenesis. Myricetin, at both high and low doses, prevented alveolar bone resorption and increased alveolar crest height in the mouse model and inhibited osteoclast formation and bone resorption in vitro. However, myricetin was more effective at high dose than at low dose. Our study demonstrated that myricetin had a positive effect on alveolar bone resorption in an OVX mouse model of periodontitis and, therefore, may be a potential agent for the treatment of periodontitis and osteoporosis.

The Pathophysiology and Treatment of Osteoporosis

PURPOSE

The objectives of this article are to review the pathophysiology of bone loss associated with aging and to review current pharmacologic approaches for the treatment of osteoporosis.

METHODS

A literature search with PubMed was performed with the terms osteoporosis and pathophysiology and osteoporosis and treatment and limited to studies written in English that were published within the preceding 10 years. Given the large number of studies identified, we selectively reviewed those studies that contained primary data related to osteoporosis pathophysiology or osteoporosis pharmacologic treatments and references included within selected studies identified from abstract review.

FINDINGS

Published studies have consistently reported that osteoporosis in older adults is caused by an imbalance of bone resorption in excess of bone formation. The dominant factor leading to bone loss in older adults appears to be gonadal sex steroid deficiency, with multiple genetic and biochemical factors, such as vitamin D deficiency or hyperparathyroidism, that may accelerate bone loss. Conditions that adversely affect growth and development may limit development of peak bone mass and accelerate subsequent bone loss. Studies of bone microarchitecture have shown that trabecular bone loss begins in the third decade of life, before gonadal sex steroid deficiency develops, whereas cortical loss typically begins in the sixth decade, about the time of menopause in women and about the same age in men. Antiresorptive agents for the treatment of osteoporosis act primarily by limiting osteoclast activity, whereas osteoanabolic agents, such as teriparatide, act primarily by stimulating osteoblastic bone formation. Clinical investigation of new compounds for the treatment of osteoporosis is mainly directed to those that stimulate bone formation or differentially decrease bone resorption more than bone formation. Therapies for osteoporosis are associated with adverse effects, but in patients at high risk of fracture, the benefits generally far outweigh the risks.

IMPLICATIONS

Current osteoporosis therapies mitigate or reverse the loss of bone associated with age-related decreases of gonadal sex steroids, increase bone strength, and reduce fracture risk. With improved knowledge of the pathophysiology of osteoporosis, new targets for therapeutic intervention have been identified. Clinical investigations of potential new treatments for osteoporosis are primarily directed to stimulating osteoblastic bone formation or to modulating the balance of bone resorption and formation in ways that improve bone strength.

Celecoxib, a selective cyclooxygenase-2 inhibitor, reduces level of a bone resorption marker in postmenopausal women with rheumatoid arthritis

AIM

Celecoxib (CEL), a selective cyclooxygenase-2 (COX-2) inhibitor, has been reported to suppress osteoclastogenesis in vitro, reduce levels of bone resorption markers in ovariectomized (OVX) mice, and prevent bone destruction in rheumatoid arthritis (RA) model mice; however, no clinical data has been reported. Here, we prospectively evaluated the changes in bone turnover markers in RA patients who switched from nonsteroidal anti-inflammatory drugs (NSAIDs) to CEL, to examine the effects of selective COX-2 inhibitor on bone metabolism.

METHODS

RA patients who had been treated with NSAIDs for more than 12 weeks were switched to CEL (400 mg/day) without any other changes in previously prescribed medications. Urinary type I collagen cross-linked N-telopeptide (uNTX), serum bone alkaline phosphatase (BAP), C-reactive protein (CRP), erythrocyte sedimentation rate (ESR) and matrix metalloproteinase-3 (MMP-3) were evaluated before switching to CEL and 16 weeks later.

RESULTS

Significant reductions in uNTX, a bone resorption marker, were observed in 60 female patients (P = 0.042), especially in 52 postmenopausal women (P = 0.033). However, uNTX level did not significantly change in premenopausal women or in men. There were no significant changes in BAP, a bone formation marker. CRP significantly decreased (P = 0.007), while ESR and MMP-3 were unchanged.

CONCLUSION

CEL reduced the levels of a bone resorption marker in postmenopausal RA patients, suggesting that this drug may attenuate the accelerated osteoclastic bone resorption associated with menopause.

Disruption of claudin-18 diminishes ovariectomy-induced bone loss in mice

Claudin-18 (Cldn-18), a member of the tight junction family of proteins, is a negative regulator of RANKL-induced osteoclast differentiation and bone resorption (BR) in vivo. Since estrogen deficiency decreases bone mass in part by a RANKL-mediated increase in BR, we evaluated whether estrogen regulates Cldn-18 expression in bone. We found that Cldn-18 expression was reduced in the bones of estrogen deficient mice, whereas it was increased by estrogen treatment in osteoblasts and osteoclasts in vitro. We next evaluated the role of Cldn-18 in mediating estrogen-induced bone loss. Cldn-18 knockout (KO) and littermate wild-type (WT) mice were ovariectomized (OVX) or sham operated at 6 wk of age, and the skeletal phenotype was evaluated at 14 wk of age. PIXImus revealed that total body, femur, and lumbar BMD were reduced 8-13% (P < 0.05) after 8 wk of OVX compared with sham in WT mice. As expected, total body, femur, and lumbar BMD were reduced 14-21% (P < 0.05) in Cldn-18 KO sham mice compared with sham WT mice. However, ovariectomy failed to induce significant changes in BMD of total body, femur, or vertebra in the Cldn-18 KO mice. μCT analysis of the distal femur revealed that trabecular (Tb) bone volume was decreased 50% in the OVX WT mice compared with sham that was caused by a 26% decrease in Tb number and a 30% increase in Tb separation (all P < 0.05). By contrast, none of the Tb parameters were significantly different in OVX Cldn-18 KO mice compared with sham KO mice. Histomorphometric analyses at the Tb site revealed that neither osteoclast surface nor osteoclast perimeter was increased significantly as a consequence of OVX in either genotype at the time point examined. Based on our findings, we conclude that the estrogen effects on osteoclasts may in part be mediated via regulation of Cldn-18 signaling.

Examination of ERα signaling pathways in bone of mutant mouse models reveals the importance of ERE-dependent signaling

The mechanisms of estrogen receptor (ER)-α activity can be categorized into those involving direct (classical) or indirect (nonclassical) DNA binding. Although various mouse models have demonstrated the importance of ERα in bone, the specific gene expression patterns affected by these modes of ERα action are unknown. In this report, the gene expression patterns of ERα-deficient (ERKO) mice and nonclassical ER knock-in (NERKI) mice, which can function only by nonclassical means, were analyzed. Three-month-old mice were ovariectomized and implanted with estrogen pellets for 1 month to normalize estrogen levels. Microarray analysis of flushed cortical bone revealed 28% (210 of 763) of the genes differentially expressed in ERKO mice were altered in NERKI mice, suggesting estrogen response element-dependent regulation of these genes in bone. Pathway analysis revealed alterations in genes involved in focal adhesion and extracellular matrix interactions. However, the majority of genes regulated in ERKO mice (72%) were unique (i.e. not altered in NERKI mice), suggesting these are regulated by nonclassical mechanisms. To further explore the pathways affected in ERKO mice, we performed focused quantitative PCR arrays for genes involved in various aspects of bone physiology. Genes involved in bone formation, senescence, apoptosis, and autophagy were significantly regulated. Overall, the majority of the genes regulated by ERα in bone are via nonclassical pathways. However, because NERKI mice display an osteoporotic phenotype, it can be deduced that the minority of the estrogen response element-dependent genes/pathways play critical roles in the regulation of bone physiology. These data demonstrate the importance of classical ERα signaling in regulating bone metabolism.

Regulation of ER molecular chaperone prevents bone loss in a murine model for osteoporosis

Endoplasmic reticulum (ER) stress response is important for protein maturation in the ER. Some murine models for bone diseases have provided significant insight into the possibility that pathogenesis of osteoporosis is related to ER stress response of osteoblasts. We examined a possible correlation between osteoporosis and ER stress response. Bone specimens from 8 osteoporosis patients and 8 disease-controls were used for immunohistochemical analysis. We found that ER molecular chaperones, such as BiP (immunoglobulin heavy-chain binding protein) and PDI (protein-disulfide isomerase) are down-regulated in osteoblasts from osteoporosis patients. Based on this result, we hypothesized that up-regulation of ER molecular chaperones in osteoblasts could restore decreased bone formation in osteoporosis. Therefore, we investigated whether treatment of murine model for osteoporosis with BIX (BiP inducer X), selective inducer BiP, could prevent bone loss. We found that oral administration of BIX effectively improves decline in bone formation through the activation of folding and secretion of bone matrix proteins. Considering these results together, BIX may be a potential therapeutic agent for the prevention of bone loss in osteoporosis patients.

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.

A novel carborane analog, BE360, with a carbon-containing polyhedral boron-cluster is a new selective estrogen receptor modulator for bone

Carboranes are a class of carbon-containing polyhedral boron-cluster compounds with globular geometry and hydrophobic surface that interact with hormone receptors. Estrogen deficiency results in marked bone loss due to increased osteoclastic bone resorption in females, but estrogen replacement therapy is not generally used for postmenopausal osteoporosis due to the risk of uterine cancer. We synthesized a novel carborane compound BE360 to clarify its anti-osteoporosis activity. BE360 showed a high binding affinity to estrogen receptors (ER), ERalpha and ERbeta. In ovariectomized (OVX) mice, femoral bone volume was markedly reduced and BE360 dose-dependently restored bone loss in OVX mice. However, BE360 did not exhibit any estrogenic activity in the uterus. BE360 also restored bone loss in orchidectomized mice without androgenic action in the sex organs. Therefore, BE360 is a novel selective estrogen receptor modulator (SERM) that may offer a new therapy option for osteoporosis.

Identification of inflammatory and proresolving lipid mediators in bone marrow and their lipidomic profiles with ovariectomy and omega-3 intake

Newly described lipoxygenase (LOX)-generated lipid mediators, that is, resolvins and protectins as well as lipoxins, are both anti-inflammatory and proresolving. We aimed to determine whether these lipid mediators are present in bone marrow and whether their lipidomic profiles are altered following ovariectomy or dietary supplementation with eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) ethyl esters. Female rats were ovariectomised or sham-operated. Shams and one ovariectomised group received a diet devoid of omega-3 long-chain polyunsaturated fatty acids. The remaining ovariectomised rats received either 0.5 g EPA or DHA ethyl ester/kg body weight/day for 4 months. Bone marrow was analyzed using both GC to determine fatty acid composition and mediator lipidomics by LC/MS/MS profiling for the presence of LOX-pathway lipid mediators derived from arachidonic acid (AA), EPA, and DHA. LOX-derived products including lipoxins, resolvin D1, resolvin E1, and protectin D1 were identified in bone marrow by the presence of diagnostic ions in their corresponding MS-MS spectra. The proportion of AA relative to DHA and of AA-derived relative to DHA-derived mediators in bone marrow was higher in ovariectomised compared to sham-operated rats. DHA or EPA ethyl ester supplementation increased the percentage of DHA and EPA in bone marrow and increased the proportion of LOX mediators biosynthesized from DHA or EPA, respectively. Given the potent bioactivities of the lipoxins, resolvins, and protectins, the presence and changes in profile postovariectomy and with EPA and DHA ethyl ester supplementation may be of interest in bone marrow function and as a potential source of these mediators in vivo.

Osteoimmunology: interactions of the bone and immune system

Bone and the immune system are both complex tissues that respectively regulate the skeleton and the body's response to invading pathogens. It has now become clear that these organ systems often interact in their function. This is particularly true for the development of immune cells in the bone marrow and for the function of bone cells in health and disease. Because these two disciplines developed independently, investigators in each don't always fully appreciate the significance that the other system has on the function of the tissue they are studying. This review is meant to provide a broad overview of the many ways that bone and immune cells interact so that a better understanding of the role that each plays in the development and function of the other can develop. It is hoped that an appreciation of the interactions of these two organ systems will lead to better therapeutics for diseases that affect either or both.

Inflammatory markers and incident fracture risk in older men and women: the Health Aging and Body Composition Study

The inflammation of aging hypothesis purports that aging is the accumulation of damage, which results, in part, from chronic activation of inflammation process. We tested this hypothesis in relationship to fractures in 2985 men and women enrolled in the Health ABC study. Results showed that subjects with the greatest number of inflammatory markers have the highest risk of fracture.

INTRODUCTION

Cytokines play major roles in regulating bone remodeling in the bone microenvironment, but their relationship to fractures is uncertain.

MATERIALS AND METHODS

The study population includes 2985 well-functioning white and black women and men (42%, black; 51%, women) 70-79 yr of age enrolled in the Health Aging and Body Composition Study. Inflammatory markers were measured in frozen serum using standardized assays. We measured interleukin (IL-6), TNFalpha, C-reactive protein (CRP), and soluble receptors (IL-2 sR, IL-6 sR, TNF sR1and TNF sR2).Cytokine-soluble receptors were measured in a subset (n = 1430). Total hip BMD was measured by DXA. During 5.8 +/- 1.6 yr of 95% complete follow-up, incident fractures were confirmed in 268 subjects. The risk of fracture was compared among subjects with the highest inflammatory markers (quartile 4) versus lower levels (quartiles 1, 2, and 3) using proportional hazard models.

RESULTS AND CONCLUSIONS

Subjects who fractured were more likely to be white and female. Baseline markers of inflammation were higher among subjects who subsequently experienced an incident fracture. In multivariate models, the relative risk of fracture (95% CIs) for subjects with the highest inflammatory markers (quartile 4) compared with those with lower inflammatory markers (quartiles 1, 2, and 3) was 1.34 (0.99, 1.82) for CRP; 1.28 (0.95-1.74) for IL-6; 1.28 (0.97-1.70) for TNFalpha; 1.52 (1.04-2.21) for IL-2 sR; 1.33 (0.90-1.96) for IL-6 sR; 1.73 (1.18-2.55) for TNF sR1 and 1.48 (1.01-2.20) for TNF sR2. In subjects with three or more (out of seven) high inflammatory markers, the relative risk of fracture was 2.65 (1.44-4.89) in comparison with subjects with no elevated markers. (p trend = 0.001). We conclude that elevated inflammatory markers are prognostic for fractures, extending the inflammation hypothesis of aging to osteoporotic fractures.

Cytokines regulating osteoclast formation and function

PURPOSE OF REVIEW

The osteoclast is the principal bone-resorbing cell. Because of its unique ability to efficiently remove both the mineral and the organic matrix of bone, the osteoclast is an important element of the homeostatic mechanisms that maintain skeletal integrity and serum calcium levels. Over the past 30 years, a number of immune cell modulators have been shown to have effects on osteoclast formation and function. This review will briefly summarize the roles that cytokines have in osteoclast regulation.

RECENT FINDINGS

A large number of cytokines have been shown to regulate osteoclast formation and function. In addition, a number of additional cytokines are now known to have a major influence on the ability of osteoclasts to resorb bone. Interactions of the immune system with bone, which has been recently labeled 'osteoimmunology', appear to be mediated mainly by cytokine signals. Cytokines are known to regulate many of the responses of bone to inflammatory conditions; however, they also may regulate physiologic responses of bone.

SUMMARY

In the future it is hoped that therapies that target cytokine actions may be used to reduce the effects of inflammatory diseases on bone, as well as to regulate normal bone physiology.

Alteration of femoral bone morphology and density in COX-2-/- mice

A role of COX-2 in pathological bone destruction and fracture repair has been established; however, few studies have been conducted to examine the involvement of COX-2 in maintaining bone mineral density and bone micro-architecture. In this study, we examined bone morphology in multiple trabecular and cortical regions within the distal and diaphyseal femur of 4-month-old wild-type and COX-2-/- mice using micro-computed tomography. Our results demonstrated that while COX-2-/- female mice had normal bone geometry and trabecular microarchitecture at 4 months of age, the male knockout mice displayed reduced bone volume fraction within the distal femoral metaphysis. Furthermore, male COX-2-/- mice had a significant reduction in cortical bone mineral density within the central cortical diaphysis and distal epiphysis and metaphysis. Consistent with the observed reduction in cortical mineral density, biomechanical testing via 4-point-bending showed that male COX-2-/- mice had a significant increase in postyield deformation, indicating a ductile bone phenotype in male COX-2-/- mice. In conclusion, our study suggests that genetic ablation of COX-2 may have a sex-related effect on cortical bone homeostasis and COX-2 plays a role in maintaining normal bone micro-architecture and density in mice.

Inhibitors of cyclo-oxygenase-2 and secretory phospholipase A2 preserve bone architecture following ovariectomy in adult rats

Epidemiological evidence and in vitro data suggest that COX-2 is a key regulator of accelerated remodeling. Accelerated states of osteoblast and osteoclast activity are regulated by prostaglandins in vitro, but experimental evidence for specific roles of cyclooxygenase-2 (COX-2) and secretory phospholipase A2 (sPLA2) in activated states of remodeling in vivo is lacking. The aim of this study was to determine the effect of specific inhibitors of sPLA2-IIa and COX-2 on bone remodeling activated by estrogen deficiency in adult female rats. One hundred and twenty-four adult female Wistar rats were ovariectomized (OVX) or sham-operated. Rats commenced treatment 14 days after surgery with either vehicle, a COX-2 inhibitor (DFU at 0.02 mg/kg/day and 2.0 mg/kg/day) or a sPLA2-group-IIa inhibitor (KH064 at 0.4 mg/kg/day and 4.0 mg/kg/day). Treatment continued daily until rats were sacrificed at 70 days or 98 days post-OVX. The right tibiae were harvested, fixed and embedded in methylmethacrylate for structural histomorphometric bone analysis at the proximal tibial metaphysis. The specific COX-2 or sPLA2 inhibitors prevented ovariectomy-induced (OVX-induced) decreases in trabecular connectivity (P<0.05); suppressed the acceleration of bone resorption; and maintained bone turnover at SHAM levels following OVX in the rat. The sPLA2 inhibitor significantly suppressed increases in osteoclast surface induced by OVX (P<0.05), while the effect of COX-2 inhibition was less marked. These findings demonstrate that inhibitors of COX-2 and sPLA2-IIa can effectively suppress OVX-induced bone loss in the adult rat by conserving trabecular bone mass and architecture through reduced bone remodeling and decreased resorptive activity. Moreover, we report an important role of sPLA2-IIa in osteoclastogenesis that may be independent of the COX-2 metabolic pathway in the OVX rat in vivo.

Effects of hormone replacement therapy on plasma pro-inflammatory and anti-inflammatory cytokines and some bone turnover markers in postmenopausal women

OBJECTIVE

The present study was undertaken to evaluate plasma TNFalpha, IL-1beta, IL-10; and urinary hydroxyproline (Hyp) and calcium (Ca) as bone resorption markers in postmenopausal women compared with premenopausal ones; and to assess the effects of HRT upon these cytokines and bone turnover markers.

PATIENTS AND METHODS

The study involved 50 healthy postmenopausal women, and 25 healthy premenopausal women (control group). Postmenopausal women were randomly divided into two subgroups: women receiving cycle HRT schedule (0.625 mg conjugated estrogen from days 1 to 28+5 mg medroxyprogesterone acetate from days 18 to 28) for 2 months (n=25); and second subgroup consisted of women receiving continue HRT schedule (0.625 mg conjugated estrogen+2.5 mg medroxyprogesterone acetate from days 1 to 28) for 2 months (n=25). Plasma TNFalpha, IL-1beta and IL-10 concentrations were measured with ELISA kits. Fasting urinary Hyp was measured by Ehrlich's spectrophotometric reaction. Ca was determined by oxalate precipitation and the redox titration procedure. Statistical significance was analysed by Kruskal-Wallis plus post hoc Mann-Whitney U-tests for multiple comparisons, Wilcoxon signed ranks test for paired data, and Pearson correlation test.

RESULTS

Compared with premenopausal individuals, postmenopausal women have increased plasma TNFalpha, IL-1beta, IL-10 (p<0.0001, p<0.0001, and p<0.001, respectively); and increased urinary Hyp and Ca concentrations (p<0.05). HRT (both cycle and continue schedules) lead to a significant decrease in TNFalpha, IL-1beta and urinary Hyp concentrations, and has no effect uppon IL-10 levels. HRT reverses increased urinary Hyp and Ca excretion to the premenopausal level. There is a significant positive correlation between pre- and post-HRT IL-1beta levels in both cycle and continue subgroups (r=0.437, p<0.05; and r=0.656, p<0.01, respectively), and between pre-HRT IL-1beta and urinary Ca (r=0.509, p<0.01; and r=0.415, p<0.05). There is a significant negative correlation between post-HRT IL-10 and TNFalpha levels in continue HRT receiving group (r=-0.446, p<0.05). Urinary Hyp in cycle and continue HRT received subgroups are correlated with post-treatment values (r=0.455, p<0.05; and r=0.776, p<0.01).

CONCLUSIONS

Plasma TNFalpha, IL-1beta, IL-10; and urinary Hyp and Ca increase with menopause. We suggest that the increase of IL-10 is secondary to the elevation of TNFalpha and IL-1beta and that the increase of IL-10 is a compensatory mechanism, by which this anti-inflammatory cytokine counteracts to pro-inflammatory TNFalpha and IL-1beta, and thus balances their osteoclast activating and oxidative stress-related effects. Two months duration HRT (cycle and continue schedule) lead to the significant decrease in plasma TNFalpha, IL-1beta and urinary Hyp concentrations. HRT reverses increased Hyp and Ca excretion to the premenopausal level. So, HRT, decreasing Th1 cytokines (TNFalpha, IL-1beta) probably improve the aberation of Th1/Th2 balance that is implicated in various pathological conditions. However, because of the relatively small number of participants and short duration of the therapy, further studies are necessary to establish a risk/benefit ratio for HRT to view effects on cytokine pattern and bone metabolism.

Suppression of adjuvant-induced arthritic bone destruction by cyclooxygenase-2 selective agents with and without inhibitory potency against carbonic anhydrase II

In vitro assays revealed that COX-2 inhibitors with CA II inhibitory potency suppressed both differentiation and activity of osteoclasts, whereas that without the potency reduced only osteoclast differentiation. However, all COX-2 inhibitors similarly suppressed bone destruction in adjuvant-induced arthritic rats, indicating that suppression of osteoclast differentiation is more effective than that of osteoclast activity for the treatment.

INTRODUCTION

Cyclooxygenase (COX)-2 and carbonic anhydrase II (CA II) are known to play important roles in the differentiation of osteoclasts and the activity of mature osteoclasts, respectively. Because several COX-2 selective agents were recently found to possess an inhibitory potency against CA II, this study compared the bone sparing effects of COX-2 selective agents with and without the CA II inhibitory potency.

MATERIALS AND METHODS

Osteoclast differentiation was determined by the mouse co-culture system of osteoblasts and bone marrow cells, and mature osteoclast activity was measured by the pit area on a dentine slice resorbed by osteoclasts generated and isolated from bone marrow cells. In vivo effects on arthritic bone destruction were determined by radiological and histological analyses of hind-paws of adjuvant-induced arthritic (AIA) rats.

RESULTS

CA II was expressed predominantly in mature osteoclasts, but not in the precursors. CA II activity was inhibited by sulfonamide-type COX-2 selective agents celecoxib and JTE-522 similarly to a CA II inhibitor acetazolamide, but not by a methylsulfone-type COX-2 inhibitor rofecoxib. In vitro assays clearly revealed that celecoxib and JTE-522 suppressed both differentiation and activity of osteoclasts, whereas rofecoxib and acetazolamide suppressed only osteoclast differentiation and activation, respectively. However, bone destruction in AIA rats was potently and similarly suppressed by all COX-2 selective agents whether with or without CA II inhibitory potency, although only moderately by acetazolamide.

CONCLUSIONS

Suppression of osteoclast differentiation by COX-2 inhibition is more effective than suppression of mature osteoclast activity by CA II inhibition for the treatment of arthritic bone destruction.

The effect of cyclooxygenase-2 inhibitors on bone mineral density: results from the Canadian Multicentre Osteoporosis Study

INTRODUCTION

The use of cyclooxygenase-2 (COX-2) inhibitors has been demonstrated to not only impair load-induced bone formation but also prevent menopause-associated bone loss. We hypothesized that COX-2 inhibitor use would be associated with increased bone mineral density (BMD) in postmenopausal women not using estrogen therapy and, conversely, with decreased BMD in men.

METHODS

The Canadian Multicentre Osteoporosis Study is a longitudinal, randomly selected, population-based community cohort. We present data from men (n=2,004) and postmenopausal women age 65 and older (n=2,776) who underwent a BMD measurement and structured interview in the 5th year of the study. The outcome measure was percent difference in BMD (g/cm(2)).

RESULTS

Daily COX-2 inhibitor use was reported by 394 subjects. In men, daily use of COX-2 inhibitors was associated with a lower BMD at all hip sites, with a percent difference of -3.1% [95% confidence interval (CI), -6.0, -0.3] between users and nonusers at total hip. In postmenopausal women not using estrogen replacement therapy, daily COX-2 inhibitor use was associated with higher BMD at most sites [percent difference at total hip: +3.0% (95% CI, 0.3, 5.8)]. These effects appeared to be dose-dependent.

CONCLUSION

COX-2 inhibitor use was associated with a lower BMD in men and, on the other hand, with a higher BMD in postmenopausal women not using estrogen replacement therapy. Men who have used COX-2 inhibitors may wish to seek BMD measurement to assess their fracture risk. However, COX-2 inhibitors may have utility in postmenopausal women if bone-selective analogs can be developed.

Pathogenesis of osteoporosis: concepts, conflicts, and prospects

Osteoporosis is a disorder in which loss of bone strength leads to fragility fractures. This review examines the fundamental pathogenetic mechanisms underlying this disorder, which include: (a) failure to achieve a skeleton of optimal strength during growth and development; (b) excessive bone resorption resulting in loss of bone mass and disruption of architecture; and (c) failure to replace lost bone due to defects in bone formation. Estrogen deficiency is known to play a critical role in the development of osteoporosis, while calcium and vitamin D deficiencies and secondary hyperparathyroidism also contribute. There are multiple mechanisms underlying the regulation of bone remodeling, and these involve not only the osteoblastic and osteoclastic cell lineages but also other marrow cells, in addition to the interaction of systemic hormones, local cytokines, growth factors, and transcription factors. Polymorphisms of a large number of genes have been associated with differences in bone mass and fragility. It is now possible to diagnose osteoporosis, assess fracture risk, and reduce that risk with antiresorptive or other available therapies. However, new and more effective approaches are likely to emerge from a better understanding of the regulators of bone cell function.

Immunolocalization of the prostaglandin E2 receptor subtypes in human bone tissue: differences in foetal, adult normal, osteoporotic and pagetic bone

PGE(2) is an important mediator of bone metabolism, but the precise localization of its receptors in human bone remains unknown. The present study used specific antibodies against EP(1), EP(2), EP(3) and EP(4) receptors for immunolocalization in normal, osteoporotic and pagetic human adult bone and in human foetal bone. No labelling was obtained for the EP(1) and EP(2) receptors. The EP(3) receptor was detected in foetal osteoclasts, osteoblasts and osteocytes, but only in osteoclasts and some osteoblasts from adult bone. The EP(4) receptor was detected in foetal osteoclasts, osteoblasts and osteocytes and in adult osteoclasts and osteoblasts, but not in adult osteocytes. Our results show differences in PGE(2) receptor expression in foetal and adult human bone but no difference in adult normal compared to pathologic bone. Finally, these results show that the distribution of EP receptors in human osteoblasts in bone corresponds in part to what we recently described in human osteoblasts in culture.

Contribution of membrane-associated prostaglandin E2 synthase to bone resorption

This study initially confirmed that, among prostaglandins (PGs) produced in bone, only PGE(2) has the potency to stimulate osteoclastogenesis and bone resorption in the mouse coculture system of osteoblasts and bone marrow cells. For the PGE(2) biosynthesis two isoforms of the terminal and specific enzymes, membrane-associated PGE(2) synthase (mPGES) and cytosolic PGES (cPGES) have recently been identified. In cultured mouse primary osteoblasts, both mPGES and cyclooxygenase-2 were induced by the bone resorptive cytokines interleukin-1, tumor necrosis factor-alpha, and fibroblast growth factor-2. Induction of mPGES was also seen in the mouse long bone and bone marrow in vivo by intraperitoneal injection of lipopolysaccharide. In contrast, cPGES was expressed constitutively both in vitro and in vivo without being affected by these stimuli. An antisense oligonucleotide blocking mPGES expression inhibited not only PGE(2) production, but also osteoclastogenesis and bone resorption stimulated by the cytokines, which was reversed by addition of exogenous PGE(2). We therefore conclude that mPGES, which is induced by and mediates the effects of bone resorptive stimuli, may make a target molecule for the treatment of bone resorptive disorders.

Soybean isoflavones inhibit tumor necrosis factor-alpha-induced apoptosis and the production of interleukin-6 and prostaglandin E2 in osteoblastic cells

The effects of individual soybean isoflavones, genistein (4',5,7-trihydroxyisoflavone) and daidzein (4',7-dihydroxyisoflavone), on tumor necrosis factor-alpha (TNF-alpha)-induced apoptosis and the production of local factors in osteoblastic cells has been investigated. Soybean isoflavones increased DNA synthesis and the number of viable cells. When cells were treated with TNF-alpha, the number of viable cells dose-dependently decreased. The decrease in cell number caused by TNF-alpha treatment was due to apoptosis, which was confirmed by TUNEL and cell death ELISA analyses. Soybean isoflavones inhibited apoptosis of osteoblastic cells subjected to TNF-alpha treatment. MC3T3-E1 osteoblastic cells secrete interleukin-6 (IL-6), interleukin-1beta (IL-1beta), nitric oxide (NO) and prostaglandin E(2) (PGE(2)) constitutively, but at low levels. Soybean isoflavones had no effect on the constitutive production of these local factors. When cells were treated with TNF-alpha (10(-10)M), the production of IL-6 and PGE(2), but not that of IL-1beta and NO, significantly increased. Treatment with soybean isoflavones (10(-5)M), in the presence of TNF-alpha (10(-10)M), for 48 h inhibited production of IL-6 and PGE(2), suggesting the antiresorptive action of soy phytoestrogen may be mediated by decreases in these local factors. The findings of this study thus suggest that soybean isoflavones may promote the function of osteoblastic cells and play an important role in bone remodeling.

Pathogenesis of osteoporosis

There are many pathways that might lead to decreased bone mass, skeletal fragility, and increased fracture risk in osteoporosis. Some of these have been clearly identified, such as estrogen deficiency. Others that were conceived on the basis of experimental findings and recent scientific discoveries such as abnormalities of cytokines, bone growth factors, and osteoblast transcription factors remain interesting but speculative. The recent revolution in genomics and proteomics opens new avenues for pursuing in great depth the pathways leading to osteoporosis. Animal models developed largely in rodents can suggest specific factors that can be further studied in primate models and in osteoporotic patients. Identification of specific pathogenetic mechanisms should lead to new approaches to the diagnosis and management of this disorder.

Sex steroids and the construction and conservation of the adult skeleton

Here we review and extend a new unitary model for the pathophysiology of involutional osteoporosis that identifies estrogen (E) as the key hormone for maintaining bone mass and E deficiency as the major cause of age-related bone loss in both sexes. Also, both E and testosterone (T) are key regulators of skeletal growth and maturation, and E, together with GH and IGF-I, initiate a 3- to 4-yr pubertal growth spurt that doubles skeletal mass. Although E is required for the attainment of maximal peak bone mass in both sexes, the additional action of T on stimulating periosteal apposition accounts for the larger size and thicker cortices of the adult male skeleton. Aging women undergo two phases of bone loss, whereas aging men undergo only one. In women, the menopause initiates an accelerated phase of predominantly cancellous bone loss that declines rapidly over 4-8 yr to become asymptotic with a subsequent slow phase that continues indefinitely. The accelerated phase results from the loss of the direct restraining effects of E on bone turnover, an action mediated by E receptors in both osteoblasts and osteoclasts. In the ensuing slow phase, the rate of cancellous bone loss is reduced, but the rate of cortical bone loss is unchanged or increased. This phase is mediated largely by secondary hyperparathyroidism that results from the loss of E actions on extraskeletal calcium metabolism. The resultant external calcium losses increase the level of dietary calcium intake that is required to maintain bone balance. Impaired osteoblast function due to E deficiency, aging, or both also contributes to the slow phase of bone loss. Although both serum bioavailable (Bio) E and Bio T decline in aging men, Bio E is the major predictor of their bone loss. Thus, both sex steroids are important for developing peak bone mass, but E deficiency is the major determinant of age-related bone loss in both sexes.

Effects of prostaglandin E2 on gene expression in primary osteoblastic cells from prostaglandin receptor knockout mice

Recent studies have shown that stimulation of osteoclastogenesis in cocultures of osteoblasts and spleen cells in response to prostaglandin E2 (PGE2) is markedly decreased when the osteoblasts are derived from cells lacking either the EP2 or the EP4 receptor. Induction of osteoclast formation requires upregulation of receptor activator of nuclear factor-kappaB ligand (RANKL) on cells of the osteoblastic lineage, which then binds to the RANK receptor on cells of the osteoclast lineage. Osteoprotegerin (OPG) is a decoy receptor for RANKL that can block its interaction with RANK. In addition, macrophage-colony stimulating factor (M-CSF) is essential for osteoclast formation. Finally, PGE2 can increase interleukin-6 (IL-6), which may further enhance osteoclastogenesis. To study the relative influence of the EP2 and EP4 receptors on response of these factors to PGE2, we examined mRNA levels for RANKL, OPG, M-CSF, and IL-6 in primary osteoblastic cell cultures derived from two lines of EP2 knockout mice (EP2-/-) and one line of EP4 knockout mice (EP4-/-) and the relevant wild-type controls (EP2+/+ and EP4+/+). The responses of cells from wild-type animals of all three lines were similar. After PGE2 treatment, RANKL mRNA levels were increased at 2 h, and this was sustained over 72 h. Basal RANKL expression was moderately reduced in EP2-/- cells and markedly reduced in EP4-/- cells. PGE2 increased RANKL mRNA in EP2-/- cells and EP4-/- cells, but the levels were significantly reduced compared with wild-type cells. There were no consistent changes in expression of M-CSF or OPG in the different genotypes or with PGE2 treatment. IL-6 mRNA was variably increased by PGE2 in both wild-type and knockout cells, although the absolute levels were somewhat lower in both EP2-/- and EP4 -/- cultures. Parathyroid hormone (PTH) increased RANKL and IL-6 and decreased OPG mRNA levels similarly in both wild-type and EP2-/- or EP4-/- cells. The major defect in the response to PGE2 in animals lacking either EP2 or EP4 receptors is a reduction in basal and stimulated RANKL levels. Loss of EP4 receptor appears to have a greater effect on basal RANKL expression than EP2.

Changes in proinflammatory cytokine activity after menopause

There is now a large body of evidence suggesting that the decline in ovarian function with menopause is associated with spontaneous increases in proinflammatory cytokines. The cytokines that have obtained the most attention are IL-1, IL-6, and TNF-alpha. The exact mechanisms by which estrogen interferes with cytokine activity are still incompletely known but may potentially include interactions of the ER with other transcription factors, modulation of nitric oxide activity, antioxidative effects, plasma membrane actions, and changes in immune cell function. Experimental and clinical studies strongly support a link between the increased state of proinflammatory cytokine activity and postmenopausal bone loss. Preliminary evidence suggests that these changes also might be relevant to vascular homeostasis and the development of atherosclerosis. Better knowledge of the mechanisms and the time course of these interactions may open new avenues for the prevention and treatment of some of the most prevalent and important disorders in postmenopausal women.

Generation of bone-resorbing osteoclasts from B220+ cells: its role in accelerated osteoclastogenesis due to estrogen deficiency

Estrogen deficiency stimulates both osteoclastic bone resorption and pre-B lymphopoiesis, the interrelationships between which remain unknown. To investigate the involvement of an increase in the number of B220+ cells in accelerated osteoclastogenesis after estrogen deficiency, we first examined whether ovariectomy (OVX) increased the frequency of clonogenic osteoclast precursors in bone marrow. The results were that after OVX, the frequency of clonogenic osteoclast precursors is increased in bone marrow, suggesting that accumulated osteoclast precursors contribute to accelerated osteoclastogenesis. Further, we found that cocultures of B220+ cells purified from bone marrow cells and stromal ST2 cells in the presence of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] gave rise to osteoclasts that can resorb bone and express calcitonin receptors. When the frequencies of clonogenic osteoclast precursors in the purified B220+ and B220- cell fractions were compared, it was found that the fractions gave rise to osteoclasts at similar frequencies, which rules out the possibility of cross-contamination and suggests that the two fractions contain comparable numbers of osteoclast precursors. Furthermore, we identified cells that are positive for both tartrate-resistant acid phosphatase (TRAP) and B220, not only in cocultures of B220+ and ST2 cells, but also in freshly isolated unfractionated bone cells. Therefore, it is concluded that at least a subfraction of B220+ cells are capable of generating osteoclasts and that the increase in the number of B220+ cells caused by estrogen deficiency may contribute to accelerated bone resorption by this novel osteoclastogenesis pathway.

Human prostate cancer cells adhere specifically to hemoglobin: a possible role in bone-specific metastasis

From the supernatant of rabbit bone marrow, we isolated an organ-specific factor, which was related with the metastasis of prostate cancer to the bone and examined its adhesion to prostate cancer cells (PC-3). Molecular weight and amino acid sequence analyses of the active component obtained by high performance liquid chromatography revealed that a component identical to the alpha chain of hemoglobin accounted for 80% of the biological activity. Hemoglobin showed over 50% adhesion to PC-3 cells but only 10% adhesion to human colon cancer cell lines, representative of organ non-specific metastasis, and leukemia cells line, representative of a non-solid tumor. Some substance in the bone marrow may promote the first step of adhesion of cancer cells to bone marrow in the metastasis of prostate cancer to the bone, possibly an amino acid sequence or some tertiary structure similar to hemoglobin.

Estrogen prevents the lipopolysaccharide-induced inflammatory response in microglia

After neuronal injury and in several neurodegenerative diseases, activated microglia secrete proinflammatory molecules that can contribute to the progressive neural damage. The recent demonstration of a protective role of estrogen in neurodegenerative disorders in humans and experimental animal models led us to investigate whether this hormone regulates the inflammatory response in the CNS. We here show that estrogen exerts an anti-inflammatory activity on primary cultures of rat microglia, as suggested by the blockage of the phenotypic conversion associated with activation and by the prevention of lipopolysaccharide-induced production of inflammatory mediators: inducible form of NO synthase (iNOS), prostaglandin-E(2) (PGE(2)), and metalloproteinase-9 (MMP-9). These effects are dose-dependent, maximal at 1 nm 17beta-estradiol, and can be blocked by the estrogen receptor (ER) antagonist ICI 182,780. The demonstration of ERalpha and ERbeta expression in microglia and macrophages and the observation of estrogen blockade of MMP-9 mRNA accumulation and MMP-9 promoter induction further support the hypothesis of a genomic activity of estrogen via intracellular receptors. This is the first report showing an anti-inflammatory activity of estrogen in microglia. Our study proposes a novel explanation for the protective effects of estrogen in neurodegenerative and inflammatory diseases and provides new molecular and cellular targets for the screening of ER ligands acting in the CNS.

Omega-3 polyunsaturated fatty acids and skeletal health

This minireview on skeletal biology describes the actions of prostaglandins and cytokines involved in the local regulation of bone metabolism, it documents the role of lipids in bone biology, and it presents relationships between fatty acids and other factors that impact skeletal metabolism. The data presented herein show consistent and reproducible beneficial effects of omega-3 (n-3) fatty acids on bone metabolism and bone/joint diseases. Polyunsaturated fatty acids modulate eicosanoid biosynthesis in numerous tissues and cell types, alter signal transduction, and influence gene expression. These effects have not been explored in the skeletal system. Future research on n-3 fatty acids in bone biology should focus on the following two aspects. First, the further elucidation of how n-3 fatty acids alter biochemical and molecular processes involved in bone modeling and bone cell differentiation, and second, the evaluation of the potential pharmaceutical applications of these nutraceutical fatty acids in maintaining bone mineral status and controlling inflammatory bone/joint diseases.

Estrogen decreases osteoclast formation by down-regulating receptor activator of NF-kappa B ligand (RANKL)-induced JNK activation

The differentiation of cells of the monocytic lineage into mature osteoclasts (OC) is specifically induced by the tumor necrosis factor-related factor, RANKL (receptor activator of NF-kappaB ligand; also known as OPGL, ODF, or TRANCE). Because inhibition of osteoclastogenesis is one of the main mechanisms by which estrogen (E2) prevents bone loss, it is likely that E2 may regulate either the production of, or the target cell responsiveness to RANKL. We found that E2 decreases the differentiation into OC of both murine bone marrow monocytes and RAW 264.7 cells, a monocytic line, by down-regulating the activation of Jun N-terminal kinase 1 (JNK1). Diminished JNK1 activity results in decreased nuclear levels of the key osteoclastogenic transcription factors, c-Fos and c-Jun, and lower binding of these transcriptional inducers to DNA. Thus, one novel mechanism by which E2 down-regulates osteoclastogenesis is by decreasing the responsiveness of OC precursors to RANKL.

Estrogen prevents the lipopolysaccharide-induced inflammatory response in microglia

After neuronal injury and in several neurodegenerative diseases, activated microglia secrete proinflammatory molecules that can contribute to the progressive neural damage. The recent demonstration of a protective role of estrogen in neurodegenerative disorders in humans and experimental animal models led us to investigate whether this hormone regulates the inflammatory response in the CNS. We here show that estrogen exerts an anti-inflammatory activity on primary cultures of rat microglia, as suggested by the blockage of the phenotypic conversion associated with activation and by the prevention of lipopolysaccharide-induced production of inflammatory mediators: inducible form of NO synthase (iNOS), prostaglandin-E(2) (PGE(2)), and metalloproteinase-9 (MMP-9). These effects are dose-dependent, maximal at 1 nm 17beta-estradiol, and can be blocked by the estrogen receptor (ER) antagonist ICI 182,780. The demonstration of ERalpha and ERbeta expression in microglia and macrophages and the observation of estrogen blockade of MMP-9 mRNA accumulation and MMP-9 promoter induction further support the hypothesis of a genomic activity of estrogen via intracellular receptors. This is the first report showing an anti-inflammatory activity of estrogen in microglia. Our study proposes a novel explanation for the protective effects of estrogen in neurodegenerative and inflammatory diseases and provides new molecular and cellular targets for the screening of ER ligands acting in the CNS.

Potential impact of selective cyclooxygenase-2 inhibitors on bone metabolism in health and disease

Prostaglandins are potent local regulators of bone cell function and may play a critical role in both physiologic and pathologic changes in the human skeleton. The availability of cyclooxygenase-2 (COX-2)-specific inhibitors, which are likely to be used frequently and for long periods because of their improved safety profile, mandates an investigation of their possible effects on bone. This is particularly important in older individuals, who are at risk of developing bone loss as a result of estrogen deficiency and aging. This review describes the major effects of prostaglandins on bone resorption and formation and outlines an approach to further studies on the potential effect of COX-2-specific inhibitors on bone remodeling, thereby possibly answering some of the key questions about their benefits and potential risks.