Glucocorticoid decreases circulating osteoprotegerin (OPG): possible mechanism for glucocorticoid induced osteoporosis

Changes in osteoprotegerin and markers of bone metabolism during glucocorticoid treatment in patients with chronic glomerulonephritis

It is well known that long-term glucocorticoid treatment causes osteoporosis, but the precise mechanism remains unclear. Recently, osteoprotegerin (OPG) has been identified as a cytokine that inhibits osteoclast differentiation. We have previously demonstrated that serum OPG is suppressed by glucocorticoids. Therefore, the present study was carried out to clarify the interrelationships between OPG and other markers of bone metabolism during glucocorticoid treatment. Thirteen patients (7 men, 6 women; 44.1 +/- 5.9 years old) with chronic glomerulonephritis who were to be treated with glucocorticoids for the first time were chosen for this study. Markers of bone metabolism, including serum OPG, osteocalcin (OC), bone-specific alkaline phosphatase activity (bAP), parathyroid hormone (PTH), tartrate-resistant acid phosphatase (TRAP), and bone mineral density (BMD), were measured before and during the treatment period. Glucocorticoids significantly reduced BMD of the lumbar spine in the 6 month treatment period (p < 0.01). Serum OPG was decreased significantly by glucocorticoids within 2 weeks (p < 0.001), and serum TRAP, a marker of bone resorption, was markedly increased (p < 0.001). On the other hand, there were no remarkable changes in serum PTH. Serum OC and bAP, markers of bone formation, were transiently reduced during the treatment period (p < 0.01). Furthermore, only serum OPG was positively and independently correlated with percentage BMD of age-matched reference (%AMR). These findings imply that glucocorticoid-induced bone loss develops rapidly via enhanced bone resorption and suppressed bone formation. Moreover, the increased bone resorption caused by glucocorticoids may be, at least in part, mediated by inhibition of OPG, not increment of PTH.

Increased circulating levels of osteoclastogenesis inhibitory factor (osteoprotegerin) in patients with chronic renal failure

Skeletal resistance to parathyroid hormone (PTH) is one of the major abnormalities underlying bone diseases in uremia, the mechanism of which has not yet been fully elucidated. Osteoclastogenesis inhibitory factor (OCIF), or osteoprotegerin, is a natural decoy receptor for osteoclast differentiation factor (ODF), produced by osteoblasts in response to PTH. To elucidate the kinetics and roles of OCIF in chronic renal failure, serum OCIF levels were measured in 46 predialysis patients and 21 dialysis patients by means of enzyme-linked immunosorbent assay (ELISA). Serum OCIF levels in predialysis patients increased as renal function declined (OCIF = 1.178 + 0.233 x creatinine; r2 = 0.413; P < 0.0001). Twenty-four-hour creatinine clearance and 1/OCIF in predialysis patients showed a clear positive correlation and a straight line regression (1/OCIF = 0.443 + 0.004 x creatinine clearance; r2 = 0.425; P < 0.0001). In dialysis patients, serum OCIF levels were significantly elevated (5.18 +/- 1.48 ng/mL) to a level that would inhibit 50% osteoclast formation in vitro. These findings suggest that OCIF accumulates in serum of patients with renal dysfunction. Because serum levels of OCIF with the ability to bind ODF in vitro (active OCIF) correlated well with those of OCIF detected by standard ELISA (active OCIF = 0.251 + 0.877 x OCIF; r2 = 0.829; P < 0.0001), OCIF accumulated in serum may be a candidate uremic toxin responsible for the skeletal resistance to PTH seen in chronic renal failure. Further studies with serum parameters and bone histological evaluation are needed to assess this possibility.

Osteoprotegerin and parathyroid hormone as markers of high-turnover osteodystrophy and decreased bone mineralization in hemodialysis patients

Osteoprotegerin (OPG) has a profound inhibitory effect on osteoclast differentiation and bone resorption. Because high-turnover renal osteodystrophy (ROD) is characterized by increased osteoclast activity, serum OPG concentrations might be used to distinguish between forms of ROD. Twenty-six patients on maintenance hemodialysis therapy underwent a transiliac crest biopsy for evaluation of histopathologic characteristics and histomorphometric studies. ROD was diagnosed as type II (normal or low turnover) or type III (high turnover plus osteoidosis) disease. Bone mineralization density distribution (BMDD) was characterized by measuring the mean trabecular calcium concentration in the biopsy specimen with quantitative backscattered electron imaging. Patients underwent additional dual-energy x-ray absorptiometry (DEXA) of the spine and hip and measurement of such biochemical markers of bone turnover as OPG, intact parathyroid hormone (iPTH), osteocalcin, calcitonin, bone alkaline phosphatase, and cross-laps. OPG levels were significantly reduced in patients with ROD III compared with ROD II (118 +/- 38 versus 204 +/- 130 pg/mL; P < 0.05) and correlated with BMDD (r = 0.43; P < 0.05). Patients with ROD III showed significantly lower BMDD compared with healthy controls (21.42% +/- 0.12% versus 22.17% +/- 0.81% weight; P < 0.01). Besides iPTH, which showed significantly greater levels in patients with ROD III than ROD II (382 +/- 322 versus 136 +/- 156 pg/mL; P < 0.05), none of the serological markers or DEXA was useful in separation of the groups. Discriminant function analysis showed that a combination of OPG and iPTH correctly classifies ROD II in 72% and ROD III in 88% of patients. We conclude that OPG in combination with iPTH can be used as a marker for noninvasive diagnosis of ROD in hemodialysis patients. Furthermore, OPG serum levels might be used to estimate trabecular bone mineralization in these subjects.

Involvement of receptor activator of NFkappaB ligand and tumor necrosis factor-alpha in bone destruction in rheumatoid arthritis

Bone loss represents a major unsolved problem in rheumatoid arthritis (RA). The skeletal complications of RA consist of focal bone erosions and periarticular osteoporosis at sites of active inflammation, and generalized bone loss with reduced bone mass. New evidence indicates that osteoclasts are key mediators of all forms of bone loss in RA. TNF-alpha is one of the most potent osteoclastogenic cytokines produced in inflammation and is pivotal in the pathogenesis of RA. Production of tumor necrosis factor-alpha (TNF-alpha) and other proinflammatory cytokines in RA is largely CD4(+) T-cell dependent and mostly a result of interferon-gamma (IFN-gamma) secretion. Synovial T cells contribute to synovitis by secreting IFN-gamma and interleukin (IL)-17 as well as directly interacting with macrophages and fibroblasts through cell-to-cell contact mechanisms. Activated synovial T cells express both membrane-bound and soluble forms of receptor activator of NF-kappaB ligand (RANKL). In rheumatoid synovium, fibroblasts also provide an abundant source of RANKL. Furthermore, TNF-alpha and IL-1 target stromal-osteoblastic cells to increase IL-6, IL-11, and parathyroid hormone-related protein (PTHrP) production as well as expression of RANKL. In the presence of permissive levels of RANKL, TNF-alpha acts directly to stimulate osteoclast differentiation of macrophages and myeloid progenitor cells. In addition, TNF-alpha induces IL-1 release by synovial fibroblasts and macrophages, and IL-1, together with RANKL, is a major survival and activation signal for nascent osteoclasts. Consequently, TNF-alpha and IL-1, acting in concert with RANKL, can powerfully promote osteoclast recruitment, activation, and osteolysis in RA. The most convincing support for this hypothesis has come from in vivo studies of animal models. Protection of bone in the presence of continued inflammation in arthritic rats treated with osteoprotegerin (OPG) supports the concept that osteoclasts mediate bone loss, providing further evidence that OPG protects bone integrity by downregulating osteoclastogenesis and promoting osteoclast apoptosis. Modulation of the RANKL/OPG equilibrium in arthritis may provide additional skeletal benefits, such as chondroprotection. The nexus between T-cell activation, TNF-alpha overproduction, and the RANKL/OPG/RANK ligand-receptor system points to a unifying paradigm for the entire spectrum of skeletal pathology in RA. Strategies that address osteoclastic bone resorption will represent an important new facet of therapy for RA.

Immunological study on circulating murine osteoprotegerin/osteoclastogenesis inhibitory factor (OPG/OCIF): possible role of OPG/OCIF in the prevention of osteoporosis in pregnancy

Osteoprotegerin (OPG)/osteoclastogenesis inhibitory factor (OCIF) is a soluble member of the tumor necrosis factor receptor family and plays a crucial role in the negative regulation of osteoclastic bone resorption. We have immunized OPG/OCIF knockout mice with murine rOPG/rOCIF and established a panel of hybridomas producing monoclonal antibodies (mAbs) to murine rOPG/rOCIF. Utilizing the mAbs, we developed enzyme-linked immunosorbent assay (ELISA) systems: one detecting both homodimeric and monomeric forms of murine OPG/OCIF and the other detecting only dimeric form of murine OPG/OCIF. With the aid of these ELISA systems we showed that OPG/OCIF is present mainly as a monomer in murine blood. The concentration of OPG/OCIF in normal mouse sera was approximately 500 pg/ml and there was no statistical difference in the serum concentration of OPG/OCIF among genders, age, and strains. Interestingly, the concentration of circulating OPG/OCIF in mouse markedly increased during pregnancy. The result indicated that circulating OPG/OCIF plays an important role in the protection of bone from excess resorption during pregnancy in mammals.

Osteoprotegerin levels before and after renal transplantation

Osteoprotegerin (OPG) is a newly identified glycoprotein that belongs to the tumor necrosis factor receptor superfamily and regulates bone mass by inhibiting osteoclastic bone resorption. The regulatory mechanism of OPG is still unclear after successful renal transplantation (RTX), however, resulting in resolution of uremia. The present study was designed to clarify the potential role of OPG in uremia and after RTX under immunosuppressive therapy. We evaluated circulating OPG levels by measuring them before and after RTX (postoperative days 2, 14, and 28). Our protocol of immunosuppressive drugs was dual therapy using cyclosporine and steroids. Serum OPG was quantitated using enzyme-linked immunosorbent assay. After successful RTX, serum OPG levels decreased significantly on day 14 and day 28 compared with the baseline level (P < 0.05). Creatinine clearance dramatically increased until day 14 and decreased thereafter. Serum OPG declines for the first 2 weeks after RTX owing to functioning allograft and decreases again for the next 2 weeks because of steroids and possible immunosuppressive agents.

Skeletal resistance to pth as a basic abnormality underlying uremic bone diseases

Skeletal resistance to parathyroid hormone (PTH) was suggested initially as a mechanism of PTH hypersecretion in uremia. Because of the effective suppression of PTH by recently developed therapeutic modalities, this background abnormality has been uncovered and currently recognized as relative hypoparathyroidism in terms of its relation to bone turnover. Thus, PTH levels two to three times greater than normal are usually required to keep bone turnover normal in uremia. Recent studies suggested that PTH activity may be overestimated using the conventional intact PTH assay. In addition, several steps to osteoclastogenesis are suspected to be disturbed in uremia. Additional studies at cellular and molecular levels are needed to establish preventive and therapeutic modalities for this abnormality.

Bisphosphonates in renal osteodystrophy

The present review considers the role that bisphosphonates might have in patients with renal failure. Although bisphosphonates are widely used to reduce fracture risk in patients with osteoporosis, few studies have documented their effect in patients with renal osteodystrophy. The pathogenesis of bone loss after renal transplantation and the role of the recently identified osteoprotegerin/receptor activating nuclear factor-kappaB system is described. Inhibition of bone resorption may prove beneficial when high bone turnover is present, but there are potential drawbacks to widespread use of bisphosphonates. These issues are discussed, with emphasis placed on reports published within the past 18 months.

Receptor activator of nuclear factor-kappaB ligand and osteoprotegerin: potential implications for the pathogenesis and treatment of malignant bone diseases

BACKGROUND

The current review summarizes the roles of the ligand, receptor activator of nuclear factor-kappaB ligand (RANKL), its receptor, receptor activator of nuclear factor-kappaB (RANK), and its decoy receptor, osteoprotegerin (OPG), on osteoclast biology and bone resorption. Furthermore, it highlights the impact of these compounds on the pathogenesis of malignant bone diseases, including tumor metastasis, humoral hypercalcemia of malignancy, and multiple myeloma. Finally, the authors discuss the therapeutic potential of OPG in the management of malignancies involving the skeleton.

METHODS

After its discovery and cloning, the biologic effects of RANKL, RANK, and OPG have been characterized by in vitro experiments and in vivo studies. The generation of knock-out mice and transgenic mice has produced animal models with absent or excessive production of these cytokine components that display opposite abnormal skeletal phenotypes (osteoporosis or osteopetrosis). The potential effect of RANKL and OPG has been assessed by evaluating these compounds in various animal models of metabolic and malignant bone disease and by administering OPG to humans.

RESULTS

Abnormal bone resorption due to local or systemic stimulation of osteoclast differentiation and activation is a hallmark of various benign and malignant bone diseases. RANKL, RANK, and OPG form an essential cytokine system that is capable of regulating all aspects of osteoclast functions, including proliferation, differentiation, fusion, activation, and apoptosis. The balance of bone resorption depends on the local RANKL-to-OPG ratio, which is enhanced in bone metastases and humoral hypercalcemia of malignancy. The exogenous administration of OPG to tumor-bearing animals corrects the increased RANKL-to-OPG ratio, and reverses the skeletal complications of malignancies.

CONCLUSIONS

Abnormalities of the RANKL/OPG system have been implicated in the pathogenesis of various primary and secondary bone malignancies. The systemic administration of OPG appears to be a potent novel therapeutic agent for treatment of these disorders.