Receptor activator of NF-kappa B and osteoprotegerin expression by human microvascular endothelial cells, regulation by inflammatory cytokines, and role in human osteoclastogenesis

Expression and regulation of osteoprotegerin in adipose tissue

PURPOSE

Osteoprotegerin (OPG), a potent inhibitor of osteoclastic bone resorption, has a variety of biological functions that include anti-inflammatory effects. Adipocytes and osteoblasts share a common origin, and the formation of new blood vessels often precedes adipogenesis in developing adipose tissue microvasculature. We examined whether OPG is secreted from adipocytes, therefore contributing to the prevention of neovascularization and protecting the vessels from intimal inflammation and medial calcification.

MATERIALS AND METHODS

The mRNA expression of OPG and receptor activator of NF-kappaB ligand (RANKL) was measured in differentiated 3T3L1 adipocytes and adipose tissues.

RESULTS

OPG mRNA expression increased with the differentiation of 3T3L1 adipocytes, while RANKL expression was not significantly altered. OPG mRNA was expressed at higher levels in white adipose tissue than in brown adipose tissue and was most abundant in the epididymal portion. In differentiated 3T3L1 adipocytes, Rosiglitazone and insulin reduced the OPG/RANKL expression ratio in a dose- and time- dependent manner. In contrast, tumor necrosis factor-alpha (TNF-alpha) increased the expression of both OPG and RANKL in a time-dependent manner. The OPG/RANKL ratio was at a maximum two hours after TNF-alpha treatment and then returned to control levels. Furthermore, OPG was abundantly secreted into the media after transfection of OPG cDNA with Phi C31 integrase into 3T3L1 cells.

CONCLUSION

Our results indicate that OPG mRNA is expressed and regulated in the adipose tissue. Considering the role of OPG in obesity-associated inflammatory changes in adipose tissue and vessels, we speculate that OPG may have both a protective function against inflammation and anti-angiogenic effects on adipose tissue.

Expression of receptor activator of nuclear factor kappa B ligand relates to inflammatory bone resorption, with or without occlusal trauma, in rats

BACKGROUND AND OBJECTIVE

Receptor activator of nuclear factor kappa B ligand (RANKL) is an important factor in osteoclast differentiation, activation and survival; however, its involvement in inflammatory bone resorption, with or without occlusal trauma, is unclear. The purpose of the present study was to investigate the distribution of RANKL-expressing cells in rat periodontium during lipopolysaccharide-induced inflammation with or without occlusal trauma.

MATERIAL AND METHODS

Lipopolysaccharide was injected into rat gingiva of the lower left first molar to induce inflammation. In addition, the occlusal surface of the upper left first molar of rat was raised by placing a gold inlay to induce occlusal trauma in the lower left first molars. The distribution of RANKL-expressing cells was immunohistochemically observed.

RESULTS

In the inflammatory model, many osteoclasts were observed at the apical inter-radicular septum on day 5 and they were reduced by day 10. On the other hand, in the inflammatory model with occlusal trauma, many osteoclasts were still observed on day 10. RANKL expression was similar to the changes in osteoclast number. The expression of RANKL increased in endothelial cells, inflammatory cells and periodontal ligament cells.

CONCLUSION

These findings clearly demonstrated that RANKL expression on endothelial cells, inflammatory cells and periodontal ligament cells is involved in inflammatory bone resorption and the expression is enhanced by traumatic occlusion. These results suggest that RANKL expression on these cells is closely involved in the increase of osteoclasts induced by occlusal trauma.

Pepstatin A, an aspartic proteinase inhibitor, suppresses RANKL-induced osteoclast differentiation

Pepstatin A is well known to be an inhibitor of aspartic proteinases such as pepsin, cathepsins D and E. Except for its role as a proteinase inhibitor, however, the pharmacological action of pepstatin A upon cells remain unclear. In this study, we found that pepstatin A suppressed receptor activator of NF-kappaB ligand (RANKL)-induced osteoclast differentiation. Pepstatin A suppressed the formation of multinuclear osteoclasts dose-dependently. This inhibition of the formation only affected osteoclast cells, i.e., not osteoblast-like cells. Furthermore, pepstatin A also suppressed differentiation from pre-osteoclast cells to mononuclear osteoclast cells dose-dependently. This inhibition seems to be independent of the activities of proteinases such as cathepsin D, because the formation of osteoclasts was not suppressed with the concentration that inhibited the activity of cathepsin D. Cell signaling analysis indicated that the phosphorylation of ERK was inhibited in pepstatin A-treated cells, while the phosphorylation of IkappaB and Akt showed almost no change. Furthermore, pepstatin A decreased the expression of nuclear factor of activated T cells c1 (NFATc1). These results suggest that pepstatin A suppresses the differentiation of osteoclasts through the blockade of ERK signaling and the inhibition of NFATc1 expression.

Role of RANKL inhibition in osteoporosis

When the rate of bone resorption exceeds that of bone formation, destruction of bone tissue occurs, resulting in a fragile skeleton. The clinical consequences, namely osteoporosis and fragility fractures, are common and costly problems. Treatments that normalize the balance of bone turnover by inhibiting bone resorption preserve bone mass and reduce fracture risk. The discovery of receptor activator of nuclear factor-κB ligand (RANKL) as a pivotal regulator of osteoclast activity provides a new therapeutic target. Early studies have demonstrated that denosumab, an investigational, highly specific anti-RANKL antibody, rapidly and substantially reduces bone resorption. Pharmacokinetics of the antibody allow dosing by subcutaneous injection at an interval of 6 months. Inhibiting RANKL appears to be a promising new treatment for osteoporosis and related disorders. More information about the effectiveness of denosumab in reducing fracture risk, its tolerability and safety, and the response to discontinuing therapy will be provided by ongoing clinical studies.

Circulating leptin and osteoprotegerin levels affect insulin resistance in healthy premenopausal obese women

We investigated the relationship between circulating leptin and osteoprotegerin (OPG) levels and insulin resistance assessed by HOMA-IR in premenopausal obese and normal weight women. Thirty four obese women (age 31 +/- 8 years) (BMI 35 +/- 4 kg/m(2)) with 19 healthy controls (age 31 +/- 7 years) (BMI <25 kg/m(2)) (BMI 21 +/- 2 kg/m(2)) were included in the study. Women were healthy and had no osteoporosis. Circulating leptin levels were significantly higher in obese women (17.11 +/- 2.05 ng/mL vs. 8.38 +/- 4.71 ng/mL, p <0.0001) and decreased OPG levels were found (14.7 +/- 7.15 pg/mL vs. 19.17 +/- 6.37 pg/mL, p = 0.03). Leptin showed a positive correlation with BMI (r = 0.851, p <0.0001), waist-to-hip ratio (r = 0.692, p <0.0001), fasting insulin (r = 0.441, p <0.001), HOMA-IR (r = 0.412, p = 0.002), fibrinogen (r = 0.387, p = 0.004), uric acid (r = 0.293, p = 0.033), hematocrit (r = 0.394, p = 0.003), systolic (r = 0.504, p <0.0001), and diastolic blood pressure (r = 0.363, p = 0.008). OPG showed a negative correlation with insulin (r = -0.341, p = 0.013) and HOMA-IR (r = -0.324, p = 0.018). In obese women group, the regression equation of HOMA-IR was (HOMA-IR = [0.095 x leptin]-[0.051 x OPG] + 1.71). However, there was no relation between leptin and OPG levels. In conclusion, circulating leptin and OPG levels were related to insulin resistance in premenopausal obese women. However, leptin had no interference in OPG in premenopausal women.

Mitogen-activated protein kinases mediate interleukin-1?-induced receptor activator of nuclear factor-?B ligand expression in human periodontal ligament cells

BACKGROUND AND OBJECTIVE

Interleukin-1beta-stimulated receptor activator of nuclear factor-kappaB ligand (RANKL) expression in human periodontal ligament cells is partially mediated by endogenous prostaglandin E2, whereas mitogen-activated protein kinases (MAPKs) are implicated in regulating various interleukin-1-responsive genes. We investigated herein the involvement of MAPKs in interleukin-1beta-stimulated RANKL expression in human periodontal ligament cells.

MATERIAL AND METHODS

Human periodontal ligament cells were pretreated separately with specific inhibitors of MAPKs, including extracellular signal-regulated kinase, p38 MAPK and c-Jun N-terminal kinase, and subsequently treated with interleukin-1beta. Following each treatment, the phosphorylation of each MAPK, the expression of RANKL, and the production of prostaglandin E2 were determined. RANKL activity was evaluated using an assay to determine the survival of prefusion osteoclasts.

RESULTS

Interleukin-1beta induced RANKL expression at the mRNA and protein levels, as well as RANKL activity in human periodontal ligament cells. Interleukin-1beta also activated extracellular signal-regulated kinase, p38 MAPK, and c-Jun N-terminal kinase. Pretreatment with each MAPK inhibitor partially, but significantly, suppressed interleukin-1beta-induced RANKL expression and its activity, as well as prostaglandin E2 production.

CONCLUSION

In human periodontal ligament cells, three types of MAPK inhibitor may abrogate RANKL expression and activity induced by interleukin-1beta, directly or indirectly through partial suppression of prostaglandin E2 synthesis. In addition, extracellular signal-regulated kinase, p38 MAPK, and c-Jun N-terminal kinase signals may co-operatively mediate interleukin-1beta-stimulated RANKL expression and its activity in those cells.

Differentiating the new rheumatoid arthritis biologic therapies

Current understanding of the mechanisms behind the pathogenesis of rheumatoid arthritis (RA) has led to the development of therapies involving biologic agents that target specific mediators of the disease process. Although the biologic agents used to treat RA share the ability to alter the cytokine cascade, they differ in ways that are clinically important. For example, they vary with regard to how they block cytokine activity (ie, as receptors, as receptor blockers, or as anticytokine antibodies) and the particular cytokine they target (eg, tumor necrosis factor [TNF] versus interleukin-1). Biologic therapies for RA also differ in mode of administration. Several are administered subcutaneously, whereas others are given intravenously. They also have molecular dissimilarities that result in differences in pharmacokinetics (ie, long versus medium half-life) and that may influence their safety profiles.Some biologic agents, such as the TNF inhibitors etanercept and infliximab, have been rigorously examined for long-term safety and efficacy, whereas other agents, like the TNF inhibitor adalimumab, have not. Differences between the various biologic agents may relate to the usefulness of each individual drug as a long-term treatment in RA. For example, the need for physician visits may impact the practicality of drugs that are administered intravenously. Also, physicians should be aware that use of cytokine inhibitors increases the risk of infection. The prevalence of tuberculosis during therapy with infliximab exceeds the background rate in patients with RA. Accordingly, physicians should be familiar with the updated warnings in the package inserts that accompany these drugs.

Mechanisms of bone loss in rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune inflammatory disease in which destruction of bone in the joints causes major morbidity. Recent research has shed light on the cell and molecular mechanisms that lead to this osteolysis, all due directly or indirectly to the chronic inflammation. The aspects of this research covered in this review include the alteration of cell proliferation and survival that results in growth of the RA synovium. This process depends upon an increase in angiogenesis and local blood flow, which is also a feature of increased bone turnover. In addition, the inflammatory environment increases expression of chemokines, which are involved in the recruitment of monocytic osteoclast precursors. Chronic inflammation also promotes an overall catabolic state, with increased osteoclast differentiation and resorptive activity, driven by disregulation of receptor activator of NF-kappaB ligand (RANKL) and the synergistic activity of inflammatory cytokines such as tumor necrosis factor-alpha and interleukin-1. Osteoclast survival is increased in this environment, but osteoblast differentiation and survival are decreased, with a consequent reduction in bone formation and a net loss of bone. Recognition of these processes and the factors involved will enable more effective and targeted treatments for RA.

Osteoprotegerin increases leukocyte adhesion to endothelial cells both in vitro and in vivo

Recombinant osteoprotegerin (OPG) promoted the adhesion of both primary polymorphonuclear neutrophils (PMNs) and leukemic HL60 cells to endothelial cells. Leukocyte/endothelial cell adhesion was promoted by short (peak at 1 hour) preincubation of either endothelial cells or PMNs with OPG, and the peak of proadhesive activity was observed in the same range of OPG concentrations detected in the sera of patients affected by cardiovascular diseases. Although the cognate high-affinity ligands for OPG, membrane receptor activator of nuclear factor-kappaB ligand (RANKL) and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), were detected at significant levels on both PMNs and HL60 cells, they were not expressed on the surface of endothelial cells. However, preincubation of OPG with heparin abrogated its proadhesive activity, whereas pretreatment of endothelial cells with chondroitinase plus heparinases significantly decreased the proadhesive activity of OPG. Taken together, these findings suggest the involvement of both the ligand binding and the N-terminal heparin-binding domains of OPG in mediating its pro-adhesive activity. The relevance of these in vitro findings was underscored by in vivo experiments, in which the topical administration of recombinant OPG increased leukocyte rolling and adhesion to rat mesenteric postcapillary venules. Our data suggest that a pathological increase of OPG serum levels might play an important role in promoting leukocyte/endothelial cell adhesion.

Role of osteoprotegerin and its ligands and competing receptors in atherosclerotic calcification

Vascular calcification significantly impairs cardiovascular physiology, and its mechanism is under investigation. Many of the same factors that modulate bone osteogenesis, including cytokines, hormones, and lipids, also modulate vascular calcification, acting through many of the same transcription factors. In some cases, such as for lipids and cytokines, the net effect on calcification is positive in the artery wall and negative in bone. The mechanism for this reciprocal relation is not established. A recent series of reports points to the possibility that two bone regulatory factors, receptor activator of NF-kappaB ligand (RANKL) and its soluble decoy receptor, osteoprotegerin (OPG), govern vascular calcification and may explain the phenomenon. Both RANKL and OPG are widely accepted as the final common pathway for most factors and processes affecting bone resorption. Binding of RANKL to its cognate receptor RANK induces NF-kappaB signaling, which stimulates osteoclastic differentiation in preosteoclasts and induces bone morphogenetic protein (BMP-2) expression in chondrocytes. A role for RANKL and its receptors in vascular calcification is spported by several findings: a vascular calcification phenotype in mice genetically deficient in OPG; an increase in expression of RANKL, and a decrease in expression of OPG, in calcified arteries; clinical associations between coronary disease and serum OPG and RANKL levels; and RANKL induction of calcification and osteoblastic differentiation in valvular myofibroblasts.

An increased osteoprotegerin serum release characterizes the early onset of diabetes mellitus and may contribute to endothelial cell dysfunction

Serum osteoprotegerin (OPG) is significantly increased in diabetic patients, prompting expanded investigation of the correlation between OPG production/release and glycemic levels. Serum levels of OPG, but not of its cognate ligand receptor activator of nuclear factor-kappaB ligand (RANKL), were significantly increased in type 2 diabetes mellitus patients compared with healthy blood donors. Serum OPG was also significantly elevated in a subgroup of recently diagnosed diabetic patients (within 2 years). The relationship between serum OPG and diabetes mellitus onset was next investigated in apoE-null and littermate mice. Serum OPG increased early after diabetes induction in both mouse strains and showed a positive correlation with blood glucose levels and an inverse correlation with the levels of free (OPG-unbound) RANKL. The in vitro addition of tumor necrosis factor-alpha to human vascular endothelial cells, but not human peripheral blood mononuclear cells, markedly enhanced OPG release in culture. In contrast, high glucose concentrations did not modulate OPG release when used alone or in association with tumor necrosis factor-alpha. Moreover, the ability of soluble RANKL to activate the extracellular signal-regulated kinase/mitogen-activated protein kinase and endothelial nitric-oxide synthase pathways in endothelial cells was neutralized by preincubation with recombinant OPG. Altogether, these findings suggest that increased OPG production represents an early event in the natural history of diabetes mellitus, possibly contributing to disease-associated endothelial cell dysfunction.

Osteoimmunology

Osteoimmunology is an interdisciplinary research field combining the exciting fields of osteology and immunology. An observation that contributed enormously to the emergence of osteoimmunology was the accelerated bone loss caused by inflammatory diseases such as rheumatoid arthritis. Receptor activator of nuclear factor kappaB ligand (RANKL), which is the main regulator of osteoclastogenesis, was found to be the primary culprit responsible for the enhanced activation of osteoclasts: activated T cells directly and indirectly increased the expression of RANKL, and thereby promoted osteoclastic activity. Excessive bone loss is not only present in inflammatory diseases but also in autoimmune diseases and cancer. Furthermore, there is accumulating evidence that the very prevalent skeletal disorder osteoporosis is associated with alterations in the immune system. Meanwhile, numerous connections have been discovered in osteoimmunology beyond merely the actions of RANKL. These include the importance of osteoblasts in the maintenance of the hematopoietic stem cell niche and in lymphocyte development as well as the functions of immune cells participating in osteoblast and osteoclast development. Furthermore, research is being done investigating cytokines, chemokines, transcription factors and co-stimulatory molecules which are shared by both systems. Research in osteoimmunology promises the discovery of new strategies and the development of innovative therapeutics to cure or alleviate bone loss in inflammatory and autoimmune diseases as well as in osteoporosis. This review gives an introduction to bone remodeling and the cells governing that process and summarizes the most recent discoveries in the interdisciplinary field of osteoimmunology. Furthermore, an alternative large animal model will be discussed and the pathophysiological alterations of the immune system in osteoporosis will be highlighted.

Receptor activator of nuclear factor (NF)-kappaB ligand (RANKL) increases vascular permeability: impaired permeability and angiogenesis in eNOS-deficient mice

Receptor activator of nuclear factor (NF)-kappaB ligand (RANKL) is emerging as an important regulator of vascular pathophysiology. Here, we demonstrate a novel role of RANKL as a vascular permeability factor and a critical role of endothelial nitric oxide synthase (eNOS) in RANKL-induced endothelial function. RANKL increased the vascular permeability and leukocyte infiltration in vivo and caused the breakdown of the blood-retinal barrier in wild-type mice but not in eNOS-deficient mice. In vitro, it increased endothelial permeability and reduced VE-cadherin-facilitated endothelial cell-cell junctions in a NO-dependent manner. RANKL also led to the activation of Akt and eNOS and to NO production in endothelial cells (ECs). These effects were suppressed by the inhibition of TRAF6, phosphoinositide 3'-kinase (PI3K), Akt, or NOS by genetic or pharmacologic means. Inhibition of the TRAF6-mediated NO pathway reduced EC migration and capillary-like tube formation in response to RANKL. Moreover, the effects of RANKL on ECs sprouting from the aorta, and neovessel formation in both the mouse Matrigel plug assay and corneal micropocket assay, were impaired in eNOS-deficient mice. These results demonstrate that RANKL promotes vascular permeability and angiogenesis by stimulating eNOS by a TRAF6-PI3K-Akt-dependent mechanism. These properties may be relevant to the pathogenesis of angiogenesis-dependent and inflammatory vascular diseases.

Osteoprotegerin and receptor activator of nuclear factor-kappaB ligand system in the early post-operative period of liver transplantation

BACKGROUND

The precise mechanism that leads to accelerated bone resorption in the early post-transplant period remains unclear. Recent data suggest that osteoprotegerin (OPG) and its ligand receptor activator of nuclear factor-kappaB ligand (RANKL) constitute a novel cytokine system that can influence the function of both bone and immune cells. The aim of our study was to assess OPG and RANKL concentrations in the early post-operative period of liver transplantation.

METHODS

Serum OPG and RANKL levels were measured in 30 patients who underwent liver transplantation at 1, 7 and 14 d post-operatively. These values were compared with 22 age- and sex-matched healthy controls. Plasma sodium, creatinine, aspartate-aminotransferase, alanine-amino transferase, gamma-glutamyl transferase, alkaline phosphatase, bilirubin, albumin, prothrombin time, tacrolimus and cyclosporine levels were measured in each patient.

RESULTS

We found a significant increase in OPG levels in the early post-operative period compared with the control group: day 1 (10.42 pmol/L, range 3.80-17.50 vs. 3.91 pmol/L, range 1.20-6.60; p = 0.0001), day 7 (6.90 pmol/L, range 3.00-15.30 vs. 3.91 pmol/L, range 1.20-6.60; p = 0.0001) and day 14 (5.76 pmol/L, range 2.60-10.70 vs. 3.91 pmol/L, range 1.20-6.60; p = 0.001). Similarly, serum RANKL levels were significantly higher than in the control group in this period, day 1 (0.123 pmol/L, range 0.010-0.420 vs. 0.054 pmol/L, range 0.010-0.300; p = 0.02), day 7 (0.236 pmol/L, range 0.010-0.720 vs. 0.054 pmol/L, range 0.010-0.300; p = 0.0004) and day 14 (0.137 pmol/L, range 0.010-0.520 vs. 0.054 pmol/L, range 0.010-0.300; p = 0.007). No correlation was found between OPG levels and RANKL, ischemic times, liver function tests, albumin, sodium or creatinine concentrations and tacrolimus or cyclosporine levels.

CONCLUSIONS

A significant amount of OPG and RANKL is released in the early post-transplant period of liver transplantation. This might be explained by an activation of the immune system caused by the allograft. Therefore, the RANKL/OPG system may be involved in the pathophysiological evolution of transplantation osteoporosis.

Statins decrease TNF-alpha-induced osteoprotegerin production by endothelial cells and smooth muscle cells in vitro

Recent reports have implicated osteoprotegerin (OPG) in cardiovascular disease processes. Endothelial and smooth muscle cells produce OPG and its expression in these cells is upregulated by inflammatory mediators. Statins, which besides their lipid lowering properties have various vasculoprotective effects, have been shown to regulate OPG expression in osteoblasts. We investigated whether statins affect the expression of OPG in human endothelial and smooth muscle cells. Using an ELISA we could demonstrate that statins reduce tumor necrosis factor-alpha (TNF-alpha)-induced OPG production in cultured human endothelial cells and smooth muscle cells. Atorvastatin also downregulated interleukin-1alpha (IL-1alpha)-induced OPG production in endothelial cells. A significant reduction of TNF-alpha-induced OPG was seen when statins were used in the nanomolar range. These results were confirmed at the level of specific mRNA expression by real-time-PCR. Using LDH leakage as a marker of cell damage we show that cell viability was not affected by statins at concentrations used in our study. The effect of statins on TNF-alpha-induced OPG production was reversed by mevalonate and geranyl-geranyl pyrophosphate at the level of protein production and at the level of mRNA expression, suggesting that it was brought about by inhibition of the mevalonic acid pathway and protein prenylation. Through our results we have added OPG to the list of molecules whose TNF-alpha-induced upregulation is counteracted by statins. If such an effect is also operative in the in vivo setting, one could postulate a role for statins in the modulation of cardiovascular disease processes possibly regulated by OPG.

Inflammatory bowel diseases as secondary causes of osteoporosis

Inflammatory bowel disease (IBD), comprising Crohn's disease and ulcerative colitis, is associated with an increased risk of osteoporosis and bone fractures. Initial studies suggested very high rates of osteoporosis in IBD, but more recent studies have suggested that bone mineral density (BMD) is often normal in patients with IBD and typically changes little over time. Nonetheless, IBD is associated with an increased risk of fractures. Doctors managing patients with IBD must consider a variety of risk factors, not just BMD measurements, in assessing fracture risk. Advances have been made in exploring the pathogenesis of osteoporosis in IBD. The evolution of knowledge regarding receptor for activated factor of nuclear factor kappaB (RANK), its ligand RANKL, and osteoprotegerin (OPG), which serves as a decoy receptor, has enhanced the understanding of both osteoporosis and T-cell immunobiology. Recent clinical studies in patients with IBD have revealed that serum OPG levels may be elevated and inflamed intestinal tissue secretes increased amounts of OPG. It is suspected that OPG levels are elevated as a counterregulatory response to low BMD, as serum OPG levels in IBD have been found to be inversely associated with BMD. Finally, in animal models of IBD, exogenous OPG has reversed both the osteopenia and the enterocolitis, suggesting that it may have a therapeutic role in human IBD.

Morphometric evaluation of the frontal sinus in relation to age

To investigate the development of the frontal sinus size during life, we studied the planar morphometry in 60 frontal radiographs of patients of different age and gender. A professional software (Bersoft Image 4.02) was used to measure the frontal area of the right and left frontal sinuses on radiographic images. A frontal sinus was already evident in 4-year-old children. Unilateral or bilateral absence of the frontal sinus was seen in 5% of cases. The size of the sinusal area increases up to 19-year-old patients, synchronous with general craniofacial growth. In adults, individual differences in size and shape occurred in relation to environmental factors. In some elderly patients, osseous resorption led to an enlargement of the frontal sinus that might complicate surgical procedures performed in this area.

Inhibition of p38 mitogen-activated protein kinase prevents inflammatory bone destruction

Mitogen-activated protein kinase (MAPK) pathways are implicated in joint destruction in rheumatoid arthritis (RA) by modulating the production and functions of inflammatory cytokines. Although p38 MAPK (p38) participates in signaling cascades leading to osteolysis in arthritis, the mechanisms of its action in this process remain incompletely understood. Here, we found that the osteoclast (Ocl) precursors expressed p38alpha, but not p38beta, p38delta, and p38gamma isoforms. Treatment of these cells with receptor activator of nuclear factor (NF)-kappaB ligand (RANKL) resulted in p38 activation. Importantly, Ocl development induced by RANKL or RANKL and tumor necrosis factor (TNF)-alpha was blocked with the novel p38 inhibitor 4-(3-(4-chlorophenyl)-5-(1-methylpiperidin-4-yl)-1H-pyrazol-4-yl)pyrimidine (SC-409). To validate in vitro data, p38 role was further investigated in streptococcal cell wall (SCW)-induced arthritis in rats. We found that SCW-induced joint swelling and bone destruction were attenuated by SC-409. Mechanistically, the data show that SCW-stimulated DNA binding activity of the transcription factor myocyte-enhancing factor 2 C, which is downstream of p38, was inhibited by SC-409. In addition, SC-409 inhibited SCW-stimulated expression of numerous factors, including TNF-alpha, interleukin-1beta, and RANKL. Although c-Jun NH2-terminal kinase and NF-kappaB pathways were activated in vitro by RANKL and in vivo by SCW, SC-409 had no significant effect on these pathways. In conclusion, our data show that p38 modulates the production and signaling of cytokines, thus providing a mechanism of the bone-sparing effect of SC-409 in rat arthritis. These data present SC-409 as a novel potent p38 inhibitor and suggest that p38-based therapies may be beneficial in preventing bone loss associated with RA.

Circulating osteoprotegerin is correlated with lipid profile, insulin sensitivity, adiponectin and sex steroids in an ageing male population

OBJECTIVE

The relationship between osteoprotegerin (OPG) and lipid profile, insulin sensitivity, adipocytokines and sex steroids has been poorly studied and subject to controversy. The purpose of this study was to look at the correlates of OPG in an elderly male population.

DESIGN

One hundred and fifty-one nondiabetic, elderly Lebanese men (age range 50-83) were recruited in this cross-sectional study based on voluntary enrolment.

MEASUREMENTS

In all the subjects, serum OPG levels were measured and related to clinical parameters (age, waist, body mass index (BMI), systolic and diastolic blood pressure), as well as to metabolic and hormonal parameters. The following fasting laboratory measurements were performed: plasma glucose and insulin levels, total cholesterol, triglycerides and HDL cholesterol, adiponectin, leptin, as well as sex steroids (testosterone, SHBG, free androgen index, ooestradiol, DHEAS), GH and IGF-1. QUICKI index was calculated as a measure of insulin sensitivity.

RESULTS

OPG levels were significantly correlated with age (r = 0.28, P < 0.0001) but not with BMI, waist, systolic or diastolic blood pressure. There was a trend towards higher OPG levels in subjects without, compared to subjects with the metabolic syndrome (3.58 +/- 1.28 vs. 3.26 +/- 1.04 pmol/l, P = 0.09). OPG was negatively correlated with fasting glucose and triglyceride levels (r = -0.18, P = 0.031 and r = -0.19, P = 0.02, respectively) and positively correlated with the QUICKI index (r = 0.17, P = 0.033), HDL cholesterol (r = 0.21, P = 0.009) and adiponectin levels (r = 0.27, P = 0.001). No significant correlations were reported with total or LDL cholesterol levels and with leptin levels. After adjustment for age, OPG is still correlated with triglycerides (r = -0.19, P = 0.02), glucose (r = -0.21, P = 0.011) and adiponectin (r = 0.19, P = 0.02). Finally, OPG was positively associated with SHBG (r = 0.31, P < 0.001) and negatively associated with free androgen index (r =-0.346, P < 0.001); both correlations persisted after adjustment for age (r = 0.21, P = 0.009 and r = -0.23, P = 0.005, respectively). No significant correlation was found between OPG and oestradiol levels while a weak negative correlation was demonstrated with DHEAS (r = -0.18, P = 0.025). Also, no significant correlation was found between OPG and GH or IGF-1 values. In a multiple regression analysis with a stepwise model, the main determinants of OPG were free androgen index and adiponectin (P < 0.0001 and P = 0.015, respectively).

CONCLUSION

Our results show that circulating OPG levels are favourably associated with some components of the metabolic syndrome. Also, for the first time, an association between OPG and adiponectin is described. Finally, the negative correlation we found between OPG and free androgen index may suggest a potential role of OPG in the increase in cardiovascular disease related to ageing and sex steroid deficiency.

RANKL stimulates inducible nitric-oxide synthase expression and nitric oxide production in developing osteoclasts. An autocrine negative feedback mechanism triggered by RANKL-induced interferon-beta via NF-kappaB that restrains osteoclastogenesis and bone resorption

Nitric oxide (NO) is a multifunctional signaling molecule and a key vasculoprotective and potential osteoprotective factor. NO regulates normal bone remodeling and pathological bone loss in part through affecting the recruitment, formation, and activity of bone-resorbing osteoclasts. Using murine RAW 264.7 and primary bone marrow cells or osteoclasts formed from them by receptor activator of NF-kappaB ligand (RANKL) differentiation, we found that inducible nitric-oxide synthase (iNOS) expression and NO generation were stimulated by interferon (IFN)-gamma or lipopolysaccharide, but not by interleukin-1 or tumor necrosis factor-alpha. Surprisingly, iNOS expression and NO release were also triggered by RANKL. This response was time- and dose-dependent, required NF-kappaB activation and new protein synthesis, and was specifically blocked by the RANKL decoy receptor osteoprotegerin. Preventing RANKL-induced NO (via iNOS-selective inhibition or use of marrow cells from iNOS-/- mice) increased osteoclast formation and bone pit resorption, indicating that such NO normally restrains RANKL-mediated osteoclastogenesis. Additional studies suggested that RANKL-induced NO inhibition of osteoclast formation does not occur via NO activation of a cGMP pathway. Because IFN-beta is also a RANKL-induced autocrine negative feedback inhibitor that limits osteoclastogenesis, we investigated whether IFN-beta is involved in this novel RANKL/iNOS/NO autoregulatory pathway. IFN-beta was induced by RANKL and stimulated iNOS expression and NO release, and a neutralizing antibody to IFN-beta inhibited iNOS/NO elevation in response to RANKL, thereby enhancing osteoclast formation. Thus, RANKL-induced IFN-beta triggers iNOS/NO as an important negative feedback signal during osteoclastogenesis. Specifically targeting this novel autoregulatory pathway may provide new therapeutic approaches to combat various osteolytic bone diseases.

Human microvascular endothelial cell activation by IL-1 and TNF-alpha stimulates the adhesion and transendothelial migration of circulating human CD14+ monocytes that develop with RANKL into functional osteoclasts

Circulating pre-OCs may be recruited to locally inflamed sites through specific interactions with activated microvasculature. We found that HMVECs stimulated the adhesion and TEM of circulating pre-OCs, in an ICAM-1- and CD44-dependent manner, leading to greater RANKL-induced OC formation and bone pit resorption.

INTRODUCTION

Inflammation is critical for healing processes but causes severe tissue destruction when chronic. Local osteoclast (OC) formation and bone resorption may increase at inflammatory sites through multiple mechanisms, including direct stimulation by inflamed microvasculature of circulating OC precursor (pre-OC) migration through a blood vessel barrier into bone or joint tissue. How this might occur is not yet well understood.

MATERIALS AND METHODS

Cytokine-activated human microvascular endothelial cell (HMVEC) monolayers, with or without IL-1 and TNF-alpha preactivation (24 h), were incubated in adhesion (1-3 h) or porous transwell transendothelial migration (TEM; 3 h) assays with human peripheral blood mononuclear cells (hPBMCs) or CD14+ monocyte or CD14- lymphocyte subsets. The number of cells that adhered or transmigrated, and their ability to thereafter develop with macrophage-colony stimulating factor (M-CSF) + RANKL into bone pit-resorbing OCs, were analyzed. Immunostaining and neutralizing antibodies to key cell adhesion molecules were used to determine their potential involvement in stimulated CD14+ monocyte TEM.

RESULTS

M-CSF + RANKL caused OC and bone pit formation only from hPBMCs and CD14+ cells but not CD14- cells. Adhesion of hPBMCs or CD14+ cells but not CD14- cells was stimulated by cytokine preactivation of HMVECs and led to the full capture of all circulating pre-OCs capable of developing into OCs. Cytokine-preactivated HMVECs also promoted the postadhesion TEM of hPBMCs and CD14+ populations, resulting in markedly greater OC formation and bone pit resorption by transmigrated cells. Immunodetectable vascular cell adhesion molecule (VCAM-1), intercellular adhesion molecule (ICAM-1), and CD44 levels increased on cytokine-treated HMVEC surfaces, and neutralizing antibodies to ICAM-1 or CD44, but not VCAM-1 or platelet endothelial cell adhesion molecule (PECAM-1), inhibited stimulated CD14+ cell TEM through activated HMVECs.

CONCLUSIONS

This is the first demonstration that cytokine-activated HMVECs efficiently capture and promote the TEM of circulating pre-OCs capable of differentiating into bone-resorbing OCs. Thus, direct pre-OC recruitment by activated microvasculature at inflammatory sites may significantly contribute to normal OC bone remodeling during fracture healing or exacerbate pathological bone loss in various chronic inflammatory disorders.

Osteostat/tumor necrosis factor superfamily 18 inhibits osteoclastogenesis and is selectively expressed by vascular endothelial cells

Vascular endothelial cells (EC) participate in the process of bone formation through the production of factors regulating osteoclast differentiation and function. In this study, we report the selective expression in primary human microvascular EC of Osteostat/TNF superfamily 18, a ligand of the TNF superfamily. Osteostat protein is detectable in human microvascular EC and is highly up-regulated by IFN-alpha and IFN-beta. Moreover, an anti-Osteostat antibody strongly binds to the vascular endothelium in human tissues, demonstrating that the protein is present in the EC layers surrounding blood vessels. Functional in vitro assays were used to define Osteostat involvement in osteoclastogenesis. Both recombinant and membrane-bound Osteostat inhibit differentiation of osteoclasts from monocytic precursor cells. Osteostat suppresses the early stage of osteoclastogenesis via inhibition of macrophage colony-stimulating factor-induced receptor activator of NF-kappaB (RANK) expression in the osteoclast precursor cells. This effect appears to be specific for the differentiation pathway of the osteoclast lineage, because Osteostat does not inhibit lipopolysaccharide-induced RANK expression in monocytes and dendritic cells, or activation-induced RANK expression in T cells. These findings demonstrate that Osteostat is a novel regulator of osteoclast generation and substantiate the major role played by the endothelium in bone physiology.

Serum levels of osteoprotegerin and RANKL in patients with ST elevation acute myocardial infarction

OPG (osteoprotegerin) has been suggested to have an important role in atherogenesis and vascular calcification. In the present study, we have investigated serum OPG and RANKL (receptor activator of nuclear factor kappaB ligand) concentrations in patients with ST elevation AMI (acute myocardial infarction) and established CAD (coronary artery disease). OPG and RANKL were measured in 58 male patients hospitalized in the coronary care unit with ST elevation AMI, in 52 asymptomatic male patients with an established diagnosis of CAD and in 52 healthy male controls. These last two groups were matched with the AMI patients for age and body mass index. OPG was significantly (P<0.05) higher in patients with AMI at 1 h after AMI (8.04+/-4.86 pmol/l) than in both patients with established CAD (4.92+/-1.65 pmol/l) and healthy subjects (3.15+/-1.01 pmol/l). Subjects with established CAD had significantly (P<0.05) increased OPG levels compared with controls. RANKL levels in patients with established CAD (0.02+/-0.05 pmol/l) and with AMI (0.11+/-0.4 pmol/l) were significantly (P<0.05) lower compared with controls (0.32+/-0.35 pmol/l). In the AMI group, OPG decreased significantly (P<0.05) at 1 and 4 weeks after infarction (8.04+/-4.86 compared with 6.38+/-3.87 and 6.55+/-2.6 pmol/l respectively), but OPG levels, either at 1 h or 1-4 weeks after AMI, remained significantly (P<0.05) higher compared with established CAD (4.92+/-1.65 pmol/l) and controls (3.15+/-1.01 pmol/l). Our data show for the first time that OPG levels are increased in ST elevation AMI within 1 h of infarction. Whether the increase in OPG is a consequence or a causal factor of plaque destabilization deserves further investigation.

Osteoprotegerin (OPG)-a potential new role in the regulation of endothelialcell phenotype and tumour angiogenesis?

The progression of cancer depends on the establishment of a tumour blood supply, and therefore tumour angiogenesis has been identified as a major target for new anticancer agents. Recent reports have suggested that osteoprotegerin (OPG) is involved in the control of endothelial cell survival through the inhibition of the activity of tumour necrosis factor- (TNF) related apoptosis-inducing ligand (TRAIL). The role of OPG in human tumour development and angiogenesis is currently unknown. In the present study we demonstrate the ability of OPG to support endothelial cell survival, as well as the formation of cord-like structures in vitro using a matrigel tubule formation assay. Investigation of various human cancers demonstrated endothelial OPG expression in 59% of malignant tumours (n=512), but in contrast, OPG was absent in endothelial cells associated with benign tumours and normal tissues (n=178). In a series of 400 breast tumours, endothelial OPG expression was associated with high tumour grade and certain histological types. Our data show a clear separation in endothelial OPG expression between malignant tumours and nonmalignant tissues, supporting a potential biological role for this molecule in the development and/or maintenance of the tumour vasculature. This is the first study to report the proangiogenic effects of OPG in vitro, as well as correlating expression of OPG by tumour endothelial cells with clinicopathological data in human tumours.

TNF-alpha and pathologic bone resorption

Chronic inflammatory bone diseases, such as rheumatoid arthritis, periodontal disease and aseptic periprosthetic osteolysis, are characterized by bone loss around affected joints and teeth caused by increased osteoclastic bone resorption. This resorption is mediated largely by the increased local production of pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNFa). These cytokines may induce resorption indirectly by affecting the production of the essential osteoclast differentiation factor, receptor activator of NF-kB ligand, and/or its soluble decoy receptor, osteoprotegerin, by osteoblast/stromal cells or directly by enhancing proliferation and/or activity of cells in the osteoclast lineage. The importance of TNFa in the pathogenesis of various forms of bone loss is supported by both experimental and clinical evidence. However, TNFa is not absolutely required for osteoclastogenesis, erosive arthritis, or osteolysis, as all these events could occur in the absence of TNFa and whether TNFa promotes osteoclast formation independently of RANK signaling is still a topic of debate. Here we review our current understanding of the mechanisms whereby TNFa increases osteoclastogenesis in vitro and in vivo.

Osteosclerosis in advanced chronic idiopathic myelofibrosis is associated with endothelial overexpression of osteoprotegerin

Advanced chronic idiopathic myelofibrosis (IMF) with osteosclerosis and increase and thickening of bone trabeculae is typically contrasted by the absence or sparse presence of osteoclasts. Because osteoclast formation can be inhibited by osteoprotegerin (OPG) we investigated OPG expression in IMF with severe fibrosis and osteosclerosis, which expressed significantly higher (up to 71-fold) OPG mRNA levels when compared with prefibrotic cellular IMF and control cases. The receptor activator of nuclear factor kappaB ligand (RANKL), a positive regulator of osteoclast differentiation and putative antagonist of OPG was overexpressed by up to 34-fold exclusively in advanced IMF. Case-specific calculation of the RANKL/OPG ratio in advanced IMF showed a wide range without significant differences when compared with the prefibrotic IMF and non-neoplastic haematopoiesis. Immunohistochemical detection of OPG protein revealed strong labelling of endothelial cells within proliferating vessels in fibrotic IMF and heterogeneously labelled megakaryocytes, and fibroblasts. Osteosclerosis and impaired osteoclast function in IMF appears to be associated with upregulated endothelial OPG expression but concomitant reduction of the antagonist RANKL could not be demonstrated. We conclude that osteosclerosis in IMF is associated with increased endothelial OPG expression without concomitant RANKL downregulation.

The insulin-like growth factor-I gene and osteoporosis: a critical appraisal

Osteoporosis, a disorder of skeletal fragility, is common in the elderly, and its prevalence is increasing as more individuals with low bone mineral density (BMD), the strongest predictor of fracture risk, are detected. Previous basic and clinical studies imply there is a significant role for insulin-like growth factor-I (IGF-I) in determining BMD. Recently, polymorphisms upstream of the P1 promoter region of the human IGF-I gene have been found to be associated with serum levels of IGF-I, BMD and fracture risk in various ethnic groups. Multiple quantitative trait loci (QTLs) have been identified that underlie serum IGF-I in a mouse intercross between two inbred strains. The most promising QTL on mouse chromosome 6 has provided clues for unraveling the molecular mechanisms that regulate osteoblast differentiation. Genomic engineering resulting in IGF-I deficient mice, and mice with targeted over-expression of IGF-I reinforce the essential role of IGF-I in bone development at both the embryonic and postnatal stages. Thus, it is apparent that significant new insights into the role of the IGF-I gene in bone remodeling occur through several distinct mechanisms: (1) the skeletal IGF regulatory system; (2) the systemic growth hormone/IGF-I axis; (3) parathyroid hormone signaling; (4) sex steroids; and (5) the OPG/RANKL/RANK cytokine system. Molecular dissection of the IGF regulatory system and its signaling pathway in bone may reveal novel therapeutic targets for the treatment of osteoporosis.

Osteoprotegerin is associated with silent coronary artery disease in high-risk but asymptomatic type 2 diabetic patients

OBJECTIVE

Osteoprotegerin (OPG) is an inhibitor of osteoclastogenesis, which has been recently involved in atherosclerosis. The relationship between coronary atherosclerosis and OPG has never been studied in asymptomatic type 2 diabetic patients.

RESEARCH DESIGN AND METHODS

This is a nested case-control study; 162 asymptomatic type 2 diabetic patients were evaluated for silent myocardial ischemia using stress myocardial perfusion imaging; of 50 patients with positive results, 37 underwent coronary angiography, 20 of whom showed significant coronary artery disease (CAD group). Of 112 patients without silent myocardial ischemia, 20 subjects (NO-CAD group) were selected and matched by age and sex to patients with CAD. OPG, C-reactive protein, adiponectin, lipoprotein(a), albuminuria, and classical risk factors were measured.

RESULTS

The percentages of subjects with OPG levels above median and with nephropathy were higher in the CAD group than in the NO-CAD group (70 vs. 25%, P = 0.004 and 50 vs. 5%, P = 0.001, respectively). LDL cholesterol levels were higher and HDL cholesterol levels lower in the CAD compared with the NO-CAD group (P = 0.033 and P = 0.005, respectively). No other variables were associated with CAD. Logistic regression analysis showed that OPG values above median (odds ratio 8.31 [95% CI 1.18-58.68], P = 0.034) and nephropathy (21.98 [1.24-388.36], P = 0.035) were significant independent predictors of asymptomatic CAD in type 2 diabetic patients.

CONCLUSIONS

Our investigation reports the first evidence of an independent association of OPG with asymptomatic CAD in type 2 diabetic patients. The results of this nested case- control study with 20 cases need to be confirmed in a larger population.

TNF-related activation-induced cytokine enhances leukocyte adhesiveness: induction of ICAM-1 and VCAM-1 via TNF receptor-associated factor and protein kinase C-dependent NF-kappaB activation in endothelial cells

Inflammation is a basic pathological mechanism leading to a variety of vascular diseases. The inflammatory reaction involves complex interactions between both circulating and resident leukocytes and the vascular endothelium. In this study, we report evidence for a novel action of TNF-related activation-induced cytokine (TRANCE) as an inflammatory mediator and its underlying signaling mechanism in the vascular wall. TRANCE significantly increased endothelial-leukocyte cell interactions, and this effect was associated with increased expression of the cell adhesion molecules, ICAM-1 and VCAM-1, on the endothelial cells. RT-PCR analysis and promoter assays revealed that expression of these cell adhesion molecules was transcriptionally regulated mainly by activation of the inflammatory transcription factor, NF-kappaB. TRANCE induced IkappaB-alpha phosphorylation and NF-kappaB activation via a cascade of reactions involving the TNFR-associated factors, phospholipase C, PI3K, and protein kinase C (PKC-alpha and PKC-zeta). It also led to the production of reactive oxygen species via PKC- and PI3K-dependent activation of NADPH oxidase in the endothelial cells, and antioxidants suppressed the responses to TRANCE. These results demonstrate that TRANCE has an inflammatory action and may play a role in the pathogenesis of inflammation-related diseases.

Regulation of vascular calcification by osteoclast regulatory factors RANKL and osteoprotegerin

Vascular calcification often occurs with advancing age, atherosclerosis, various metabolic disorders such as diabetes mellitus and end-stage renal disease, or in rare genetic diseases, leading to serious clinical consequences. Such mineralization can occur at various sites (cardiac valves, arterial intima or media, capillaries), involve localized or diffuse widespread calcification, and result from numerous causes that provoke active inflammatory and osteogenic processes or disordered mineral homeostasis. Although valuable research has defined many key factors and cell types involved, surprising new insights continue to arise that deepen our understanding and suggest novel research directions or strategies for clinical intervention in calcific vasculopathies. One emerging area in vascular biology involves the RANKL/RANK/OPG system, molecules of the tumor necrosis factor-related family recently discovered to be critical regulators of immune and skeletal biology. Evidence is accumulating that such signals may be expressed, regulated, and function in vascular physiology and pathology in unique ways to promote endothelial cell survival, angiogenesis, monocyte or endothelial cell recruitment, and smooth muscle cell osteogenesis and calcification. Concerted research efforts are greatly needed to understand these potential roles, clarify whether RANKL (receptor activator of nuclear factor kappaB ligand) promotes and osteoprotegerin (OPG) protects against vascular calcification, define how OPG genetic polymorphisms relate to cardiovascular disease, and learn whether elevated serum OPG levels reflect endothelial dysfunction in patients. Overall, the RANKL/RANK/OPG system may mediate important and complex links between the vascular, skeletal, and immune systems. Thus, these molecules may play a central role in regulating the development of vascular calcification coincident with declines in skeletal mineralization with age, osteoporosis, or disease.

The relationship between insulin resistance assessed by HOMA-IR and serum osteoprotegerin levels in obesity

AIM

We investigated the relationship between insulin resistance and serum osteoprotegerin (OPG) levels in healthy obese subjects and healthy lean controls.

METHODS

Fifty obese subjects (age: 31+/-8 years) and 24 lean controls (age: 30+/-7 years) were included in the study. We used the homeostasis model assessment for insulin resistance (HOMA-IR) index as the index of insulin resistance. OPG levels were measured with the commercial ELISA kit. Obese subjects were studied in three groups: Group I (n = 25) HOMA-IR index < 2.24, Group II (n = 13) index 2.24-3.59, Group III (n = 12) index > 3.59. Group IV (n = 24) was the lean controls with HOMA-IR index < 2.24.

RESULTS

Obese subjects with increased insulin resistance (Group III) had lower OPG values than other groups (11.88+/-7.43 pg/ml, 16.39+/-6.39 pg/ml, 17.37+/-9.61 pg/ml, and 18.1+/-6.65 pg/ml, respectively; Group I versus Group III p = 0.036; Group III versus Group IV p = 0.012). We also found significant inverse correlations between OPGc (corrected for BMI) and fasting glucose (r = -0.325, p = 0.005), fasting insulin (r = -0.404, p = 0.0001) as well as HOMA-IR (r = -0.428, p = 0.0001). Increased fibrinogen level was found in Group III than Group IV (9.32+/-1.97 micromol/l versus 7.47+/-1.65 micromol/l, respectively; p = 0.005). In conclusion, insulin resistance in obesity is associated with decreased serum OPG levels and increased fibrinogen levels. The relationship between serum OPG levels and HOMA-IR may provide an insight into vascular endothelial dysfunction in obesity.

Histopathological and ultrastructural features of dermal telangiectasias in systemic sclerosis

AIMS

To investigate the histological, ultrastructural and immunohistochemical features of the vascular lining of dermal telangiectasia, a characteristic clinical finding in scleroderma.

METHODS

Standard histological, electron microscopic and immunohistological techniques were used to examine dermal telangiectasias in five patients with limited scleroderma, the most common scleroderma variant in Caucasian populations.

RESULTS

The telangiectasias were dilated postcapillary venules located in the papillary and superficial reticular dermis. The vessel walls consisted of non-fenestrated endothelial cells surrounded by a variable number of pericytes and smooth muscle cells. There were no unique ultrastructural features. Thickened collagen fibres in the reticular or deep dermis were seen in all but one patient, although in variable and generally minimal quantities. Surrounding infiltrating inflammatory cells were scarce. No enhanced endothelial staining was obtained with antibodies directed against endoglin, endothelin, E-selectin and ICAM-1 suggesting a resting or inactivated state.

CONCLUSION

The immunohistological and ultrastructural features of the lining endothelium of established telangiectasias in long-standing, limited scleroderma appear benign. It would be of interest to examine telangiectasias in the early phase of their formation. Alternatively, other explanations need to be explored in understanding the aetiopathogenesis of telangiectasia in scleroderma.

Osteoprotegerin and the receptor activator of NF-kappa B ligand in the serum and synovial fluid. A comparison of patients with longstanding rheumatoid arthritis and osteoarthritis

We examined OPG and soluble RANKL in the serum (sOPG, sRANKL) and synovial fluid (synOPG, synRANKL) in patients with rheumatoid arthritis (RA) and osteoarthritis (OA). OPG and RANKL were measured in 85 patients (44 with RA, 41 patients with OA) in serum and synovial fluid as well. For measuring of OPG and RANKL ELISA tests were used. The results of OPG and RANKL were compared with clinical and radiological scores. We found a negative correlation for OPG and RANKL in synovial fluids: not only for the whole group of patients (P < 0.003, r = -0.32), but also for the subgroups (RA: P < 0.04, r = -0.28, OA: P < 0.002, r = -0.54). SRANKL and synRANKL were positively correlated in the whole group (P < 0.01, r = 0.25) and in the OA group (P < 0.02, r = 0.35); the RA group was showing a trend (P < 0.063, r = 0.24), however. Serum OPG was lower in RA, synOPG higher in OA. The difference between the two patient groups was only significant for synOPG (P < 0.03, r = 0.056), but not for sOPG (P < 0.09, r = 0.19), sRANKL (P < 0.43, r = 0.85) or synRANKL (P < 0.11, r = 0.22). The synOPG:synRANKL ratio was significantly correlated with the Larsen score (P < 0.004, r = 0.38). Synovial OPG is significantly decreased in rheumatoid joints, whereby synovial RANKL is increased. Lower synOPG could reflect a lower protective effect on bone, thus leading to an earlier and more pronounced bone destruction in RA. However, the effect of different mediators for joint destruction in RA and OA seems not to be important to the pathophysiological changes in the joints. The upregulation of serum OPG might be the result of the inflammation; in contrast, an upregulation of RANKL could not be found in the serum of patients with RA and OA.

Serum osteoprotegerin is increased in Crohn's disease: a population-based case control study

BACKGROUND

There is a potential interface between osteoporosis and the chronic inflammation of inflammatory bowel disease (IBD), and the osteoprotegerin (OPG)/receptor for activated nuclear factor-kappaB (RANK)/RANK ligand (RANKL) signaling pathway may be an important mediator, although data are limited.

METHODS

We conducted a population-based case-control seroassay study to look for alterations in serum OPG and soluble RANKL (sRANKL). The study population included IBD patients who were 18 to 50 years old with Crohn's disease (CD; n = 287) or ulcerative colitis (UC; n = 166), age-matched healthy controls (n = 368), and nonaffected siblings of IBD patients (n = 146). Serum OPG and sRANKL were measured by enzyme-linked immunoassay. Sex-specific reference ranges were derived from the healthy controls.

RESULTS

Analysis of variance (ANOVA) confirmed significant group differences in women for mean serum OPG (P = 0.018). CD women had higher values of OPG than UC women (P = 0.028) or healthy controls (P = 0.045), whereas the other groups were similar. OPG levels were above the reference range in 13/173 (8%) of CD women, exceeding the expected proportion (P = 0.032). In contrast, no differences in OPG were seen in men between controls, CD, or UC. Estrogen use in women (P = 0.000002) and corticosteroid use in men (P = 0.026) were associated with higher OPG levels. In multivariate analysis, CD diagnosis (P = 0.031) and estrogen use (P = 0.000002) were independently associated with higher OPG levels. No group differences were seen in mean serum sRANKL measurements.

CONCLUSIONS

An OPG:sRANKL imbalance with OPG exceeding sRANKL should inhibit osteoclastogenesis and promote bone formation. CD is associated with increased fracture risk, and possibly, the paradoxically higher OPG is a counterregulatory response to factors such as inflammatory cytokines, promoting high bone turnover. Alternatively, elevated OPG in CD may reflect T-cell activation.

Osteoprotegerin (OPG) is localized to the Weibel-Palade bodies of human vascular endothelial cells and is physically associated with von Willebrand factor

Recent studies demonstrate roles for osteoprotegerin (OPG) in both skeletal and extra-skeletal tissues. Although its role in preventing osteoclast (OC) formation and activity is well documented, emerging evidence suggests a role of OPG in endothelial cell survival and the prevention of arterial calcification. In this communication, we show that vascular endothelial cells in situ, and human umbilical vein endothelial cells (HUVEC) in vitro, express abundant OPG. In HUVEC, OPG co-localizes with P-selectin and von Willebrand factor (vWF), within the Weibel-Palade bodies (WPB). Treatment of HUVEC with the pro-inflammatory cytokines, tumor necrosis factor (TNF)-alpha and IL-1beta, resulted in mobilization from the WPBs and subsequent secretion of OPG protein into the culture supernatant. Furthermore, TNF-alpha treatment of HUVEC resulted in a sustained increase in OPG mRNA levels and protein secretion over the 24-h treatment period. Reciprocal immunoprecipitation experiments revealed that while not associated with P-Selectin, OPG is physically complexed with vWF both within the WPB and following secretion from endothelial cells. Interestingly, this association was also identified in human peripheral blood plasma. In addition to its interaction with vWF, we show that OPG also binds with high avidity to the vWF reductase, thrombospondin (TSP-1), raising the intriguing possibility that OPG may provide a link between TSP-1 and vWF. In summary, the intracellular localization of OPG in HUVEC, in association with vWF, together with its rapid and sustained secretory response to inflammatory stimuli, strongly support a modulatory role in vascular injury, inflammation and hemostasis.

Endothelial cell reaction on a biological material

The successful clinical application of materials should involve detailed investigations on interaction between them and tissue with which they will contact. We examined herein the behavior of endothelial cells (ECs) on a collagen material, using histological and immunohistochemical methods. We used isolated human umbilical cord vein cells (HUVECs) identified by means of endothelial-specific antibodies. Cells were seeded in a standard density on a collagen membrane (Lycoll, Resorba, Nuernberg, Germany) and on gelatin-coated, control plastic surfaces, after two passages. These were then maintained for periods of 1, 7, or 14 days. The cells adhered, spread, and proliferated, and within 24 h started forming a subconfluent monolayer. We observed that the cultured cells expressed integrins (alpha5beta1 and alpha(v)beta3) and synthesized fibronectin. After 14 days, we could observe a confluent layer of ECs. We could conclude that the collagen material supported growth and attachment of endothelial cells. In addition, the attachment seemed to be most related to the fibronectin synthesized by the cells and to its highly expressed receptor (the alpha5beta1 integrin); even though this is not the only protein related to this adhesion, we observed that our cultured HUVECs did not synthesize vitronectin.

Circulating osteoprotegerin levels are associated with age, waist-to-hip ratio, serum total cholesterol, and low-density lipoprotein cholesterol levels in healthy Korean women

Osteoprotegerin (OPG) is a recently identified cytokine that acts as a decoy receptor for the receptor activator of nuclear factor I masculine B ligand. Osteoprotegerin has been shown to be an important inhibitor of osteoclastogenesis and arterial calcification in animal models. Recently, OPG has been proposed as a link molecule between osteoporosis and arterial calcification, but the relationship between circulating OPG levels and cardiovascular disease in human populations is unclear. Thus, the aim of this study was to investigate the relationship between circulating OPG levels and cardiovascular risk factors in healthy Korean women. The subjects were 286 women aged 37 to 73 (mean +/- SD, 51.5 +/- 6.9 years). We examined blood pressure, body mass index, and waist-to-hip ratio. Serum concentrations of OPG were determined by enzyme-linked immunosorbent assay. Fasting plasma glucose levels, serum lipid profiles, insulin levels, and serum follicle-stimulating hormone (FSH) levels were determined by standard methods and homeostasis model assessment of insulin resistance was calculated. We observed a significant association between serum OPG levels and age, waist-to-hip ratio, total cholesterol, low-density lipoprotein cholesterol, and FSH levels (P < .05). Among the metabolic components, the older, obese, and hypercholsterolemic subjects had higher serum OPG levels (P < .05). However, no significant relationship was found between serum OPG levels and blood pressure and fasting plasma glucose levels. We found that mean serum OPG levels were about 11% greater in postmenopausal women (mean +/- SD, 1358.5 +/- 380.0 pg/mL) than in premenopausal women (mean +/- SD, 1228.8 +/- 407.7 pg/mL, P < .001). In multiple regression analysis with OPG as the dependent variable, serum FSH and low-density lipoprotein cholesterol levels were the significant predictor for serum OPG level (R(2) = 0.051, P < .05). In conclusion, our results show that circulating OPG levels are partly associated with cardiovascular risk factors and menopausal status in healthy Korean women. Out findings suggest that OPG may be an important paracrine factor of cardiovascular disease in the female population.

The molecular triad OPG/RANK/RANKL: involvement in the orchestration of pathophysiological bone remodeling

The past decade has seen an explosion in the field of bone biology. The area of bone biology over this period of time has been marked by a number of key discoveries that have opened up entirely new areas for investigation. The recent identification of the receptor activator of nuclear factor kappaB ligand (RANKL), its cognate receptor RANK, and its decoy receptor osteoprotegerin (OPG) has led to a new molecular perspective on osteoclast biology and bone homeostasis. Specifically, the interaction between RANKL and RANK has been shown to be required for osteoclast differentiation. The third protagonist, OPG, acts as a soluble receptor antagonist for RANKL that prevents it from binding to and activating RANK. Any dysregulation of their respective expression leads to pathological conditions such as bone tumor-associated osteolysis, immune disease, or cardiovascular pathology. In this context, the OPG/RANK/RANKL triad opens novel therapeutic areas in diseases characterized by excessive bone resorption. The present article is an update and extension of an earlier review published by Kwan Tat et al. [Kwan Tat S, Padrines M, Theoleyre S, Heymann D, Fortun Y. IL-6, RANKL, TNF-alpha/IL-1: interrelations in bone resorption pathophysiology. Cytokine Growth Factor Rev 2004;15:49-60].

CCR1 chemokines promote the chemotactic recruitment, RANKL development, and motility of osteoclasts and are induced by inflammatory cytokines in osteoblasts

Chemoattractants that recruit OC precursors to locally inflamed sites of resorption are not well known. A chemokine receptor, CCR1, was expressed in OC precursors and elevated in mature OCs, and its ligands promoted OC precursor recruitment, RANKL development, and OC motility. Cytokines induced OB release of such chemokines, which may therefore significantly contribute to inflammatory bone loss.

INTRODUCTION

Chemokines, primarily of two major (CXC, CC) families, are essential signals for the trafficking and localization of circulating hematopoietic cells into tissues. However, little is known about their potential roles in osteoclast (OC) recruitment, development, or function. Previously, we analyzed CXC receptors in murine OC precursors and found high expression of CXCR4 that mediated their stromal-derived factor-1(SDF-1)-induced chemotaxis and collagen invasion. Here, we investigated if CC receptors and ligands, which are elevated in inflammatory and other osteolytic diseases, also play important roles in the recruitment, formation, or activity of murine bone-resorptive OCs.

MATERIALS AND METHODS

CC chemokine receptor (CCR) mRNA expression was analyzed during OC formation induced by RANKL in murine RAW 264.7 cells and primary marrow cells. Corresponding CC chemokines were tested for their ability to elicit precursor chemotaxis or OC development, or to influence motility, bone resorption, adhesion, or survival in RANKL-differentiated OCs. Constitutive and inflammatory cytokine-induced release of the chemokines macrophage inflammatory protein-1alpha (MIP-1alpha) and regulated on activation, normal T-cell expressed and secreted (RANTES) was measured by ELISA for OCs, osteoblasts (OBs), and their precursor cells.

RESULTS

CCR1 was expressed in murine marrow cells, the most prominent CCR in RAW cells, and upregulated by RANKL in marrow or RAW cells. Chemokines that bind CCR1 (MIP-1alpha, RANTES, and monocyte chemoattractant protein-3 [MCP-3]) were produced to varying degrees by murine OCs, OBs, and their precursors, and markedly increased by interleukin (IL)-1alpha and TNFalpha in differentiating OBs. RANTES, and especially MIP-1alpha, increased mature OC motility, but did not alter OC resorption activity, adhesion, or survival. All three chemokines stimulated chemotaxis of marrow or RAW cell precursors, leading to the greater formation of OCs (in number and size) after RANKL development of such chemoattracted marrow cells. All three chemokines also directly and dramatically enhanced OC formation in marrow cultures, through a pathway dependent on the presence of RANKL but without altering RANK expression.

CONCLUSIONS

Pathological increases in secretion of these chemokines from activated OBs or other cells may potently stimulate the chemotactic recruitment and RANKL formation of bone-resorptive OCs, thereby exacerbating local osteolysis in multiple skeletal diseases.

Decreased serum osteoprotegerin levels in patients with cardiac syndrome X

Receptor activator of nuclear factor KB (RANK) and osteoprotegerin (OPG) represent the ligand and decoy receptor, respectively, of a pleiotropic cytokine system that regulates bone metabolism and vascular biology. Several studies supported systemic microvascular abnormalities in patients with cardiac syndrome X (CSX). This study investigates serum OPG levels in healthy obese subjects and healthy lean controls affected by cardiac syndrome X.

METHODS

We compared the OPG levels in 8 patients with cardiac syndrome X [2 males, 6 females; age: 46+/-6 yr; body mass index (BMI): 30+/-5 kg/m2] with 24 obese subjects (8 males, 16 females; age: 38+/-5 yr; BMI: 35+/-5 kg/m2) and 15 healthy lean controls (6 males, 9 females; age: 36+/-5 yr; BMI: 23+/-2 kg/m2; BMI<25kg/m2).

RESULTS

Serum OPG levels in patients with cardiac syndrome X were lower than those in obese subjects and lean controls (11.45+/-8.36 pg/ml, 14.78+/-8.22 pg/ml, 19.24+/-6.96 pg/ml, respectively, cardiac syndrome X vs lean controls, p=0.039).

CONCLUSIONS

Serum OPG levels are lower in patients with CSX. Further studies on the mechanisms of OPG in microangiopathy may help to evaluate the OPG system role as a marker for disease activity, prognosis and response to therapy in cardiovascular diseases.

Osteoprotegerin (OPG) acts as an endogenous decoy receptor in tumour necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis of fibroblast-like synovial cells

We examined the role of osteoprotegerin (OPG) on tumour necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in rheumatoid fibroblast-like synovial cells (FLS). OPG protein concentrations in synovial fluid from patients with rheumatoid arthritis (RA) correlated with those of interleukin (IL)-1beta or IL-6. A similar correlation was present between IL-1beta and IL-6 concentrations. Rheumatoid FLS in vitro expressed both death domain-containing receptors [death receptor 4 (DR4) and DR5] and decoy receptors [decoy receptor 1 (DcR1) and DcR2]. DR4 expression on FLS was weak compared with the expression of DR5, DcR1 and DcR2. Recombinant TRAIL (rTRAIL) rapidly induced apoptosis of FLS. DR5 as well as DR4 were functional with regard to TRAIL-mediated apoptosis induction in FLS; however, DR5 appeared be more efficient than DR4. In addition to soluble DR5 (sDR5) and sDR4, OPG administration significantly inhibited TRAIL-induced apoptogenic activity. OPG was identified in the culture supernatants of FLS, and its concentration increased significantly by the addition of IL-1beta in a time-dependent manner. Neither IL-6 nor tumour necrosis factor (TNF)-alpha increased the production of OPG from FLS. TRAIL-induced apoptogenic activity towards FLS was reduced when rTRAIL was added without exchanging the culture media, and this was particularly noticeable in the IL-1beta-stimulated FLS culture; however, the sensitivity of FLS to TRAIL-induced apoptosis itself was not changed by IL-1beta. Interestingly, neutralization of endogenous OPG by adding anti-OPG monoclonal antibody (MoAb) to FLS culture restored TRAIL-mediated apoptosis. Our data demonstrate that OPG is an endogenous decoy receptor for TRAIL-induced apoptosis of FLS. In addition, IL-1beta seems to promote the growth of rheumatoid synovial tissues through stimulation of OPG production, which interferes with TRAIL death signals in a competitive manner.

Factors regulating osteoclast formation in human tissues adjacent to peri-implant bone loss: expression of receptor activator NFkappaB, RANK ligand and osteoprotegerin

Aseptic bone loss adjacent to orthopedic joint implants is a common cause of joint implant failure in humans. This study investigates the expression of key regulators of osteoclast formation, receptor activator NFkappaB (RANK), Receptor activator of NFkappaB ligand (RANKL) and osteoprotegerin (OPG), in the peri-implant tissues of patients with osteolysis compared with levels in synovial tissues from osteoarthritic and healthy subjects. Immunohistochemical studies demonstrated that significantly higher levels of RANKL protein (p<0.05) were found in the peri-implant tissues of patients with implant failure than in similar tissues from osteoarthritic and healthy subjects. In contrast, OPG protein levels were similar in all tissues. RANKL, expressed as mRNA and protein, was predominantly associated with cells containing wear particles. Dual labeling studies showed that the cells expressing RANKL protein were macrophages. In situ hybridization studies confirmed that mRNA encoding for these proteins is also expressed by cells in the peri-implant tissues. In addition, RANK mRNA was expressed in cells that contained wear particles. These findings show that abnormally high levels of RANKL are expressed in peri-implant tissues of patients with prosthetic loosening and that these abnormal levels of RANKL may significantly contribute to aseptic implant loosening.

Modeling the interactions between osteoblast and osteoclast activities in bone remodeling

We propose a mathematical model explaining the interactions between osteoblasts and osteoclasts, two cell types specialized in the maintenance of the bone integrity. Bone is a dynamic, living tissue whose structure and shape continuously evolves during life. It has the ability to change architecture by removal of old bone and replacement with newly formed bone in a localized process called remodeling. The model described here is based on the idea that the relative proportions of immature and mature osteoblasts control the degree of osteoclastic activity. In addition, osteoclasts control osteoblasts differentially depending on their stage of differentiation. Despite the tremendous complexity of the bone regulatory system and its fragmentary understanding, we obtain surprisingly good correlations between the model simulations and the experimental observations extracted from the literature. The model results corroborate all behaviors of the bone remodeling system that we have simulated, including the tight coupling between osteoblasts and osteoclasts, the catabolic effect induced by continuous administration of PTH, the catabolic action of RANKL, as well as its reversal by soluble antagonist OPG. The model is also able to simulate metabolic bone diseases such as estrogen deficiency, vitamin D deficiency, senescence and glucocorticoid excess. Conversely, possible routes for therapeutic interventions are tested and evaluated. Our model confirms that anti-resorptive therapies are unable to partially restore bone loss, whereas bone formation therapies yield better results. The model enables us to determine and evaluate potential therapies based on their efficacy. In particular, the model predicts that combinations of anti-resorptive and anabolic therapies provide significant benefits compared with monotherapy, especially for certain type of skeletal disease. Finally, the model clearly indicates that increasing the size of the pool of preosteoblasts is an essential ingredient for the therapeutic manipulation of bone formation. This model was conceived as the first step in a bone turnover modeling platform. These initial modeling results are extremely encouraging and lead us to proceed with additional explorations into bone turnover and skeletal remodeling.

Localization of osteoprotegerin, tumor necrosis factor-related apoptosis-inducing ligand, and receptor activator of nuclear factor-kappaB ligand in Mönckeberg's sclerosis and atherosclerosis

Vascular calcification may occur at different areas of the vessel wall, including the intima in atherosclerosis and the media in Mönckeberg's sclerosis. Medial calcification of arteries is common in patients with diabetes mellitus or chronic renal failure. Osteoprotegerin (OPG) and receptor activator of nuclear factor-kappaB ligand are essential modulators of bone homeostasis and may be involved in the process of vascular calcification. In this study we investigated arteries from patients with Mönckeberg's sclerosis and atherosclerosis. Apoptosis, which precedes vascular calcification in vitro, was assessed by an in situ ligation assay and was localized to the medial layer of arteries (Mönckeberg's sclerosis) and the neointima (atherosclerosis). Immunohistochemistry and in situ hybridization revealed OPG immunoreactivity and mRNA expression surrounding calcified areas in the medial layer (Mönckeberg's sclerosis), whereas OPG was mainly expressed adjacent to calcified neointimal lesions (atherosclerosis). Receptor activator of nuclear factor-kappaB ligand protein and mRNA were barely or not detectable. Of note, TNF-related apoptosis-inducing ligand, an inducer of apoptosis that is also blocked by OPG, displayed a similar spatial distribution as OPG. In summary, we demonstrate enhanced apoptosis adjacent to vascular calcification, and the concurrent expression of regulators of apoptosis and osteoclastic differentiation, TNF-related apoptosis-inducing ligand and OPG, suggesting their involvement in the pathogenesis of vascular calcification.

In vivo real-time imaging of TGF-beta-induced transcriptional activation of the RANK ligand gene promoter in intraosseous prostate cancer

BACKGROUND

Current animal models of prostate cancer (CaP) bone metastasis do not allow measurement of either tumor growth in bone over time or activation of gene promoters in intraosseous tumors. To develop these methods, we used bioluminescent imaging (BLI) to determine if expression of receptor activator of NF-kappaB ligand (RANKL), a pro-osteoclastogenic factor that promotes CaP bone metastases, is modulated by the bone matrix protein transforming growth factor-beta (TGF-beta) in vivo.

METHODS

C4-2B human CaP cells were treated with TGF-beta in vitro and RANKL mRNA and protein production were measured by polymerase chain reaction (PCR) and ELISA, respectively. Then C4-2B cells stably transfected with the RANKL promoter driving luciferase (lux) were injected intra-tibially into severe combined immundeficient (SCID) mice. Tumors were subjected to BLI every 2 weeks for 6 weeks and serum prostate specific antigen (PSA) was measured using ELISA. Vehicle (V), 1,25 dihydroxyvitamin D (VitD), or TGF-beta was administered to mice with established tumors and BLI to measure RANKL promoter activity was performed. Tumors were then subjected to immunohistochemistry for lux and assayed for RANKL mRNA levels.

RESULTS

TGF-beta induced RANKL protein and mRNA expression and activated the RANKL promoter activity in a dose-dependent manner in vitro. BLI demonstrated an increase in intraosseous tumor size over time, which correlated with serum PSA levels. Administration of TGF-beta and VitD to mice with established intraosseous tumors increased lux activity compared to V. Intratibial tumor RANKL mRNA expression paralleled the increased promoter activity. Immunohistochemistry confirmed the presence of lux in the intraosseous tumors.

CONCLUSIONS

These results demonstrate the ability to measure intraosseous tumor growth over time and gene promoter activation in an established intraosseous tumor in vivo and also demonstrate that TGF-beta induces activates the RANKL promoter. These results provide a novel method to explore the biology of CaP bone metastases.

Nitric oxide donor increases osteoprotegerin production and osteoclastogenesis inhibitory activity in bone marrow stromal cells from ovariectomized rats

Nitric oxide (NO) has emerged as a potent regulator useful in alleviating estrogen deficiency bone loss. Osteoprotegerin (OPG) and receptor activator of nuclear factor-kappaB ligand (RANKL) play important roles in regulating osteoclastogenesis. Although recent studies have reported NO donor attenuation of bone loss, the effect of NO donor on OPG and RANKL expression of osteogenic stromal cells and bone microenvironment in ovariectomized rats is not fully understood. Here, we showed that optimal NO donor treatment [2,2'-(hydroxynitrosohydrazino)bis-ethanamine; 15 microm] increased OPG, but not RANKL, levels in bone marrow stromal cells from ovariectomized rats. NO donor augmentation of OPG synthesis was transcriptionally mediated. The stimulatory action of NO donor on OPG expression appeared to be regulated by tyrosine kinase-dependent activation of Cbfa1/Runx2 binding to the OPG promoter, because cell cultures pretreated with tyrosine kinase inhibitor (herbimycin A), but not with protein kinase A inhibitor (calphostain C) or protein kinase C inhibitor [(Rp)-cAMP] significantly reduced NO-augmented Runx2 activation and OPG levels. Conditioned medium from NO donor-treated cells inhibited macrophage-colony-stimulating factor and RANKL-induced osteoclast formation of macrophage-colony-stimulating factor-dependent bone marrow macrophages. Neutralization with anti-OPG antibodies abolished the inhibitory effect of conditioned medium on osteoclastogenesis. Immunohistochemical observation also showed that 2,2'-(hydroxynitrosohydrazino)bis-ethanamine increased OPG expression of osteochondral cells located at metaphyseal endosteum and calcified cartilage of proximal femurs in ovariectomized rats. These findings suggest that NO donor can be an alternative pharmacological strategy for regulating bone resorption.