Pathogenesis of vascular calcification in dialysis patients

Soft-tissue and vascular calcification are highly prevalent in end-stage renal disease (ESRD). Vascular calcifications manifest as both medial and intimal calcification of arteries and are a hallmark of the accelerated atherosclerosis observed in uremia. The nature of vascular calcification is progressive, and is associated with arterial stiffness and increased cardiovascular mortality. Age, duration of dialysis, and diabetes mellitus are clear determinants of the severity of vascular calcification; however, more recently novel insights into the pathomechanisms of unwanted calcification processes have been gained. Disturbances of mineral metabolism such as hyperphosphatemia and hypercalcemia appear to contribute to progressive calcification, not only by passive precipitation but by actively inducing changes in vascular smooth muscle cell behavior toward an osteoblast-like phenotype. Specific calcium-regulatory proteins may act locally or systemically as calcification inhibitors. Dysregulations of calcification inhibitors, including fetuin-A, matrix Gla protein, osteoprotegerin, and pyrophosphates may also be pathophysiologically relevant factors in the context of uremic extraosseous calcification. In this context, low serum fetuin-A levels were recently found to be associated with increased mortality in cohorts of dialysis patients. This overview intends to summarize current knowledge of the scientific concepts involved in the pathogenesis of extraosseous calcification in ESRD.     

Gene therapy with human osteoprotegerin decreases callus remodeling with limited effects on biomechanical properties

Osteoprotegerin (OPG) is a naturally occurring protein, which prevents bone resorption by inhibition of osteoclastogenesis, function, and survival. Therefore, recombinant OPG may be an attractive drug in the treatment of chronic bone resorptive diseases such as osteoporosis. Gene therapy has the potential to achieve long-term treatment by delivering genes of anti-resorptive proteins to the recipient. The effects of OPG gene therapy on fracture healing have not been described previously.

The influence of OPG gene therapy on callus formation, callus tissue structural strength, apparent material properties, and histology of tibia fractures in rats was investigated after 3 weeks and 8 weeks of healing. Intramuscular administration of adeno-associated virus (AAV) vector-mediated OPG resulted in increased levels of OPG in serum of approximately 100 ng/ml throughout the study period. Control animals with fractures received transduction with an AAV reporter gene construct (AAV-enhanced green fluorescent protein (eGFP)), and in this group serum OPG levels remained at baseline (<10 ng/ml). After 3 weeks of healing, AAV-OPG treatment reduced the number of osteoclasts in the callus tissue (33%, P < 0.001). However, AAV-OPG treatment did not influence callus dimensions, callus bone mineral content (BMC), fracture structural strength, or apparent callus tissue material properties. After 8 weeks of healing, AAV-OPG treatment reduced the number of osteoclasts in the callus tissue (31%, P < 0.001) compared with AAV-eGFP fractures. Furthermore, deposition of new woven bone at the fracture line of the original cortical bone was hampered (new woven bone present: in all AAV-eGFP animals, in 41% of AAV-OPG-treated animals, P < 0.001). AAV-OPG treatment also increased callus BMC (18%, P = 0.023) compared with AAV-eGFP fractures. AAV-OPG did not influence callus dimensions, structural strength of the fractures, or ultimate stress, whereas elastic modulus was reduced in the AAV-OPG groups (37%, P = 0.039). The experiment demonstrates that AAV-OPG gene therapy decreases the fracture remodeling, but this does not influence the structural strength of healing fractures.     

Decreased osteoprotegerin and increased bone turnover in young female patients with major depressive disorder and a lifetime history of anorexia nervosa

Low bone mineral density (BMD) is a frequent, often persistent complication in patients with major depressive disorder (MDD) and anorexia nervosa (AN) that increases the risk of pathologic fractures. The pathogenetic process underlying osteopenia in MDD and AN is still unclear, although several factors, including a dysbalance of cytokines, are associated with loss of bone mass. Alterations in the serum levels of cytokines have been observed in patients with MDD, AN, and other psychiatric disorders. Therefore, we examined serum levels of cytokines, markers of bone turnover, and BMD in 13 patients with MDD and a lifetime history of AN. Bone turnover markers (osteocalcin and C-terminal degradation products of type I collagen) and tumor necrosis factor (TNF-) in patients were significantly increased compared with those of the control group. Osteoprotegerin (OPG) in patients was significantly decreased. Eight of 13 patients (62%) displayed osteopenia at the lumbar spine. TNF- correlated significantly with C-terminal degradation products of type I collagen, an osteoclastic marker, but significantly negatively with OPG. Our data suggest that TNF- and OPG may play a role in the pathogenetic process underlying osteopenia in these patients.     

Changes in Osteoprotegerin/RANKL System, Bone Mineral Density, and Bone Biochemicals Markers in Patients with Recent Spinal Cord Injury

This study analyzed the temporal and regional variations in bone loss and explored bone cell activities via biochemical markers during an extended follow-up in patients with spinal cord injury (SCI). In parallel, the possible role of the osteoprotegerin (OPG)/RANKL system in disuse osteoporosis was investigated. Seven male patients with acute and complete SCI (31.3 ± 9.5 years) and 12 able-bodied (AB) men (26.9 ± 4.2 years) participated in the study. Measurements were performed 16, 24, 36, 48, and 71 weeks after injury. At week 16, marked calcium homeostasis disturbance and a concomitant increase in bone resorption markers were observed, reflecting an intense bone degradation process. Resorption activity decreased continuously with time. Contrasting with the great rise in the resorption markers, the bone formation markers showed little variation. During the period of investigation, a loss in bone mineral density (BMD) was demonstrated for the total body (−4.3%), pelvis (−15.7%) and lower limbs (−15.2%), whereas BMD did not change at the lumbar spine, upper limbs, or skull. At all stages, SCI patients had lower serum RANKL levels and higher serum OPG levels than did AB controls, but no significant variation with time was observed for either cytokine. These findings suggest that bone resorption persisted long after SCI and specifically affected BMD at sublesional sites. The marked modification of serum OPG/RANKL levels in SCI patients suggests that this system is affected, in disuse osteoporosis. However, the precise biologic role of the OPG/RANKL system in the bone tissue of SCI patients has yet to be determined.     

OPG/RANKL system imbalance in a case of hepatitis C-associated osteosclerosis: the pathogenetic key?

Hepatitis C-associated osteosclerosis (HCAO) is an impressive example of acquired diffuse osteosclerosis in adults, recently described in ten patients infected with hepatitis C virus (HCV). Its hallmark is a painful and generalized increase of bone mass. Bone biopsies show enhanced accretion rate, usually without histological abnormalities. The HCAO pathogenesis is hitherto unknown. HCV might induce a slow bone cell infection and the production of bone growth factors, such as insulin-like growth factors. Recently, receptor activator of nuclear factor-B (RANK), its ligand (RANKL), and soluble decoy receptor osteoprotegerin (OPG) have been identified as a pivotal cytokine system in the bone remodeling control. We describe the 11th case of HCAO. Notably, the patients bone biopsy showed the presence of a high number of OPG-positive osteoblasts, a slight increase of RANKL-positive stromal cells, and a dramatic reduction of the osteoclasts. Moreover, OPG serum levels were increased. These findings reported here for the first time are consistent with a pathogenetic role of the OPG/RANKL system imbalance in HCAO.     

Low bone density and abnormal bone turnover in patients with atherosclerosis of peripheral vessels.

Patients with vascular calcifications often have low bone mineral density (BMD), but it is still uncertain if osteoporosis and peripheral vascular disease (VD) are interrelated and linked by a common pathomechanism. Moreover, data on bone turnover in patients with advanced atherosclerosis are lacking. We measured BMD by dual-energy X-ray absorptiometry (DXA) and quantitative bone ultrasound (QUS), as well as the serum levels of osteocalcin (OC), bone-specific alkaline phosphatase (BAP), osteoprotegerin (OPG) and its ligand RANKL, and the urinary concentration of the C-terminal telopeptides of type I collagen (CrossLaps), in 36 patient (20 male and 16 female) with serious atherosclerotic involvement of the carotid and/or femoral artery to investigate the underlying mechanism of vascular and osseous disorders. Thirty age-matched and gender matched healthy individuals served as controls. After adjustment for age, BMD was significantly reduced at the lumbar spine in 23/36 (63%) patients (mean T score –1.71±1.42) and at the proximal femur in 34/36 (93%) patients (neck mean T score –2.5±0.88). Ten patients (27%) had abnormal QUS parameters. Gender and diabetes had no effect on the relationship between vascular calcification and bone density at any site measured. VD subjects had OC and BAP serum levels lower than controls (13.3±3.1 vs 27.7±3.3 ng/ml, P<0.01, and 8.4±2.3 vs 12.5±1.4 g/l, P<0.01, respectively). Urinary CrossLaps excretion was not significantly different in patients with VD and in controls (257.9±138.9 vs 272.2±79.4 µg/mmol Cr, respectively). Serum OPG and RANKL levels were similar in patients and in controls (3.5±1.07 vs 3.4±1.05 pmol/l, and 0.37±0.07 vs 0.36±0.06 pmol/l, respectively). We proved high occurrence of osteoporosis in VD, with evidence of age and gender independence. Negative bone remodelling balance would be a consequence of reduced bone formation, with no apparent increased activation of the OPG–RANKL system.     

Osteoprotegerin and Osteoprotegerin-Ligand Balance: a New Paradigm in Bone Metabolism Providing New Therapeutic Targets

Osteoprotegerin-ligand, also called Osteoclast Differentiation Factor or RANK-ligand, its receptor RANK and its decoy receptor Osteoprotegerin are key molecules regulating osteoclast differentiation and activation. In this view we discuss structure and expression of these molecules, the genetic models addressing their function and their role in in vivo models of osteoclast differentiation and activation. The new paradigm that has evolved from these studies, is not only important in normal bone homeostasis but also appears to play a role in different diseases that affect the skeleton, such as osteoporosis, inflammatory joint disease and cancer. It has opened a new era in bone research by increasing our molecular knowledge and providing new therapeutic targets in bone disease. 5 November 2000 / Accepted: 15 December 2000     

绝经后妇女血清骨保护蛋白和基质金属蛋白酶-2水平与骨密度和骨代谢指标的关系

目的:研究显示血清骨保护蛋白(OPG)和基质金属蛋白酶-2(MMP-2)在骨代谢中降解骨基质方面起到作用。本研究主要比较武汉地区的绝经后妇女的以上2项指标与骨密度和3个代谢指标的关系。方法:用酶联免疫吸附试验(ELISA)测定78名48~65岁绝经后妇女的血清OPG、MMP-2、血清骨碱性磷酸酶(BAP)和血清骨钙素(OC),用双能X线吸收法(DEXA)测定腰椎,股骨颈,Ward's三角,大粗隆的骨密度。结果:(1)绝经后骨质疏松患者血清OPG和MMP-2水平均高于年龄匹配的正常对照组。(2)绝经后妇女血清OPG和MMP-2均与骨密度呈负相关。在校正年龄和体质指数后,血清MMP-2与股骨颈相关性消失。(3)血清OPG和MMP-2与骨钙素含量呈负相关。结论:绝经后骨质疏松妇女血清OPG、MMP-2与高骨转换代谢相关联,血清OPG与MMP-2水平升高可能为高骨代谢转换过程(如绝经后骨质疏松症)中的一种伴随表现。因此血清骨保护蛋白和基质金属蛋白酶-2可以作为绝经后骨质疏松妇女临床骨代谢的检测指标。     

Increased serum OPG in atrophic nonunion shaft fractures

Background  Bone repair alteration is hypothesized for nonunion fracture pathogenesis. Since it is involved in osteoclast regulation, the RANK/RANKL/OPG system (receptor activator of nuclear factor kB/its ligand/osteoprotegerin) may play a role. Materials and methods  Serum OPG, free RANKL, bone alkaline phosphatase (BAP), osteocalcin (OC), and urinary deoxypyridinoline (DPD) were determined in 16 male patients (20–39 years) with long bone atrophic nonunion fractures. Serum markers were also measured in 18 age-matched male controls who healed from the same type of fractures within six months, and in 14 age-matched male controls who were healing from the same fractures one month after injury. One-way ANOVA and Bonferroni’s test were used for statistical analysis. Results  Only OPG was significantly higher (0.56 sd 0.11 ng/ml) in the patients compared to healed (0.26 sd 0.04 ng/ml; P < 0.001) and healing (0.29 sd 0.09 ng/ml; P < 0.001) controls. The patients’ DPD levels were normal. No correlations were found between bone markers and the characteristics of the subjects in all groups. Conclusions  A normal steady state of bone metabolism seems to be present in patients with atrophic nonunion fractures, despite the high serum OPG. The reason for the inability of the patients’ OPG to inhibit osteoclastic activity is unknown. Osteoblast activity also appears normal, so another cellular source of OPG can be hypothesized.