The OPG/RANKL/RANK system in metabolic bone diseases

The OPG/RANKL/RANK cytokine system is essential for osteoclast biology. Various studies suggest that human metabolic bone diseases are related to alterations of this system. Here we summarize OPG/RANKL/RANK abnormalities in different forms of osteoporoses and hyperparathyroidism. Skeletal estrogen agonists (including 17beta-estradiol, raloxifene, and genistein) induce osteoblastic OPG production through estrogen receptor-alpha activation in vitro, while immune cells appear to over-express RANKL in estrogen deficiency in vivo. Of note, OPG administration can prevent bone loss associated with estrogen deficiency as observed in both animal models and a small clinical study. Glucocorticoids and immunosuppressants concurrently up-regulate RANKL and suppress OPG in osteoblastic cells in vitro, and glucocorticoids are among the most powerful drugs to suppress OPG serum levels in vivo. As for mechanisms of immobilization-induced bone loss, it appears that mechanical strain inhibits RANKL production through the ERK 1/2 MAP kinase pathway and up-regulates OPG production in vitro. Hence, lack of mechanical strainduring immobilization may favor an enhanced RANKL-to-OPG ratio leading to increased bone loss. As for hyperparathyroidism, chronic PTH exposure concurrently enhances RANKL production and suppresses OPG secretion through activation of osteoblastic protein kinase A in vitro which would favour increased osteoclastic activity. In sum, the capacity for OPG to antagonize the increases in bone loss seen in many rodent models of metabolic bone disease implicates RANKL/OPG imbalances as the likely etiology and supports the potential role for a RANKL antagonist as a therapeutic intervention in these settings.     

仙灵骨葆对骨质疏松大鼠OPG/RANK/RANKL 表达的影响

目的:观察仙灵骨葆治疗骨质疏松模型大鼠后,对大鼠体内OPG/RANKL/RANK表达的影响。方法:卵巢摘除法建立SD大鼠骨质疏松模型,设立假手术组、对照组(单纯去卵巢组)、雌激素组(给予17β-雌二醇)和治疗组(给予仙灵骨葆)。术后1周开始给药,给药12周后检测各组大鼠股骨骨密度,ELISA法检测血清中OPG/RANKL含量,RT-PCR检测骨组织中OPG/RANK/RAN-KL mRNA表达,免疫组化检测骨组织中RANK的表达。结果:对照组大鼠骨密度显著低于假手术组;治疗组和雌激素组大鼠O-PG表达显著高于对照组,RANK及RANKL的表达显著低于对照组。结论:采用卵巢摘除法成功建立大鼠骨质疏松模型;仙灵骨葆可促进骨质疏松大鼠OPG的表达,并抑制RANK及RANKL的表达,对骨质疏松模型大鼠有治疗作用。     

Disturbance of the OPG/RANK/RANKL pathway and systemic inflammation in COPD patients with emphysema and osteoporosis

Background

Osteoporosis is one of the systemic features of COPD. A correlation between the emphysema phenotype of COPD and reduced bone mineral density (BMD) is suggested by some studies, however, the mechanisms underlying this relationship are unclear. Experimental studies indicate that IL-1β, IL-6 and TNF-α may play important roles in the etiology of both osteoporosis and emphysema. The OPG/RANK/RANKL system is an important regulator of bone metabolism, and participates in the development of post-menopausal osteoporosis. Whether the OPG/RANK/RANKL pathway is involved in the pathogenesis of osteoporosis in COPD has not been studied.

Methods

Eighty male patients (current or former smokers) completed a chest CT scan, pulmonary function test, dual x-ray absorptiometry measurements and questionnaires. Among these subjects, thirty patients with normal BMD and thirty patients with low BMD were selected randomly for measurement of IL-1β, IL-6, TNF-α (flow cytometry) and OPG/RANK/RANKL (ELISA). Twenty age-matched healthy volunteers were recruited as controls.

Results

Among these eighty patients, thirty-six had normal BMD and forty-four had low BMD. Age, BMI and CAT score showed significant differences between these two COPD groups (p < 0.05). The low-attenuation area (LAA%) in the lungs of COPD patients was negatively correlated with lumbar vertebral BMD (r = 0.741; p < 0.0001). Forward logistic regression analysis showed that only LAA% (p = 0.005) and BMI (p = 0.009) were selected as explanatory variables. The level of IL-1β was significantly higher in the COPD patients as compared to the normal controls (p < 0.05), but the difference between the two COPD groups did not reach significance. The levels of IL-6 and TNF-α among the three groups were significantly different (p < 0.05). The level of RANKL and the RANKL/OPG ratio were significantly higher in COPD patients with low BMD compared to those with normal BMD and the normal controls (p < 0.05), and correlated negatively with lumbar vertebral BMD, but positively with LAA%.

Conclusions

Radiographic emphysema is correlated with low BMD in current and former smokers with COPD. IL-1β, IL-6, TNF-α, and the osteoporosis-related protein system OPG/RANK/RANKL may have some synergetic effects on emphysema and bone loss in COPD.     

Serum osteoprotegerin and RANKL are not specifically altered in women with postmenopausal osteoporosis treated with teriparatide or risedronate: a randomized, controlled trial.

Risedronate and teriparatide have opposite actions on the osteoblast-osteoclast dipole and are expected to influence the RANK/RANKL/osteoprotegerin (OPG) system. We aimed to evaluate changes in serum OPG and RANKL after risedronate or teriparatide administration in postmenopausal osteoporotic women. Seventy-four postmenopausal Caucasian women (age 64.1+/-1.0 years) were studied. Women with osteopenia served as controls (group 1, n=30). Women with osteoporosis were randomly assigned to either risedronate 35 mg once weekly (group 2, n=21) or teriparatide 20 microg once daily (group 3, n=23) for six months. Blood samples for serum RANKL, OPG, N-terminal propeptide of type 1 collagen (P1NP), and C-terminal telopeptide of type 1 collagen (CTx) were obtained before treatment and three and six months after treatment. P1NP and CTx levels remained unchanged in group 1, decreased in group 2 (p<0.001), and increased in group 3 women (p<0.001) throughout the treatment. OPG levels remained unchanged while RANKL decreased gradually in all groups (p<0.001). There was no correlation between OPG or RANKL and P1NP or CTx. Our data suggest that neither antiresorptive nor osteoanabolic therapy causes specific alterations of serum OPG/RANKL levels; therefore, these cytokines cannot substitute for the established markers of bone turnover in the monitoring of response to osteoporosis treatment.     

Gene–gene interactions in RANK/RANKL/OPG system influence bone mineral density in postmenopausal women

Receptor activator of nuclear factor κB (RANK) is one of the proteins in regulation of osteoclastogenesis via RANK/RANKL/OPG. Gene that codes for RANK protein (TNFRSF11A) was associated with osteoporotic fractures in a recent genome-wide association study. As variations in the RANK gene could alter its expression and activity, the aim of our study was to evaluate the influence of four RANK gene polymorphisms on bone mineral density (BMD) and biochemical markers.We evaluated 467 postmenopausal women and 117 elderly men. All subjects were genotyped for the presence of RANK polymorphisms ?670G>C, +34694C>T, +34901G>A and +35966insdelC. BMD and biochemical markers were measured.Significant associations of +35966insdelC with BMD at lumbar spine (BMD-ls), total hip (BMD-th) and femoral neck (BMD-fn) were found in postmenopausal women (p = 0.020, 0.024 and 0.034), but not in men. Significant gene–gene interaction was proved for two RANK polymorphisms in combination with OPG and RANKL polymorphisms studied previously in postmenopausal women. Firstly, RANK/RANKL (+34901G>A/?290C>T) combination was associated with BMD-fn, BMD-th and BMD-ls (p = 0.034, 0.016 and 0.050), and secondly, RANK/OPG combination (+35966insdelC/K3N) showed influence on BMD-fn and BMD-ls (p = 0.043 and 0.039).Our results suggest that gene–gene interactions between RANK and OPG, and RANK and RANKL influence BMD in postmenopausal women.     

Assessment and clinical implications of RANK/RANKL/OPG pathway as markers of bone tumor progression in patients with NET harboring bone metastases

Abstract

Introduction: The impact on the survival of bone metastases (BM) in patients with neuroendocrine tumor (NET) is a matter of debate. BM have a key role in causing symptoms and in decreasing patients’ quality of life. Although the mechanisms of the development of BM are not completely clear, it is now well understood that the Receptor Activator of Nuclear factor Kappa-B-/Ligand (RANK/RANKL)/osteoprotegerin (OPG) pathway plays a relevant role.

Aim: To characterize the RANK/RANKL/OPG pathway in patients affected with NET.

Patients and methods: Two cohorts of 15 patients each were enrolled in the study; one cohort was affected with NET without BM and the second cohort was affected with NET with BM. The serum RANK/RANKL/OPG pathway was assessed in both the groups.

Results: Serum OPG levels and RANKL/OPG ratio were lower and higher, respectively, in NET patients harboring BM than in those without BM. During the ROC analysis, a cut-off value of 1071?pg/ml for OPG and 0.62 for RANKL/OPG ratio were able to significantly distinguish between the two groups.

Conclusions: This study indicates that RANK/RANKL/OPG pathway is imbalanced in patients with NET harboring BM. Specific alterations of this pathway could predict an early development of BM.     

RANKL/RANK/OPG: new therapeutic targets in bone tumours and associated osteolysis

The emergence of the molecular triad osteoprotegerin (OPG)/Receptor Activator of NF-kB (RANK)/RANK Ligand (RANKL) has helped elucidate a key signalling pathway between stromal cells and osteoclasts. The interaction between RANK and RANKL plays a critical role in promoting osteoclast differentiation and activation leading to bone resorption. OPG is a soluble decoy receptor for RANKL that blocks osteoclast formation by inhibiting RANKL binding to RANK. The OPG/RANK/RANKL system has been shown to be abnormally regulated in several malignant osteolytic pathologies such as multiple myeloma [MM, where enhanced RANKL expression (directly by tumour cells or indirectly by stromal bone cells or T-lymphocytes)] plays an important role in associated bone destruction. By contrast, production of its endogenous counteracting decoy receptor OPG is either inhibited or too low to compensate for the increase in RANKL production. Therefore, targeting the OPG/RANK/RANKL axis may offer a novel therapeutic approach to malignant osteolytic pathologies. In animal models, OPG or soluble RANK was shown both to control hypercalcaemia of malignancy and the establishment and progression of osteolytic metastases caused by various malignant tumours. To this day, only one phase I study has been performed using a recombinant OPG construct that suppressed bone resorption in patients with multiple myeloma or breast carcinoma with radiologically confirmed bone lesions. RANK-Fc also exhibits promising therapeutic effects, as revealed in animal models of prostate cancer and multiple myeloma. If the animal results translate to similar clinical benefits in humans, using RANK-Fc or OPG may yield novel and potent strategies for treating patients with established or imminent malignant bone diseases and where standard therapeutic regimens have failed.     

OPG, RANK and RANK ligand expression in thyroid lesions

Receptor activator of NF-kappaB (RANK), RANK ligand (RANKL) and osteoprotegerin (OPG) play essential roles in bone metabolism. RANKL binds to RANK, which is expressed by osteoclasts whereas OPG acts as its decoy receptor blocking the RANK-RANKL interaction. OPG/RANK/RANKL are produced by variety of tissues including epithelial and mesenchymal cells. However, the role of RANKL/OPG in thyroid pathophysiology remains unclear. The aim of this study was to determine the expression pattern of RANK/RANKL/OPG in primary neoplastic thyroid lesions and in lymph node metastases. 27 specimens from total thyroidectomy were studied by immunohistochemistry: 9 papillary carcinomas (PC), 9 medullary carcinomas (MC), 9 macrovesicular adenomas (MA). Immunohistochemical evidence of RANKL was found in 30 % of MC, 22% of PC while RANKL has never been detected in PC. The expression of RANK is closely related to RANKL. OPG was restricted to the cytoplasm of epithelial in 1 MA and 1 MC. In contrast to pathological tissues, any expression of OPG/RANK/RANKL was detected in healthy thyroid tissue. This work reveals for the first time that OPG/RANK/RANKL are expressed in the pathological thyroid gland by follicular cells, by malignant parafollicular cells as well as in metastatic lymph node microenvironment. Thus OPG/RANK/RANKL molecular triad might play a role during pathogenesis of follicular and parafollicular tumors.     

Functions of RANKL/RANK/OPG in bone modeling and remodeling

The discovery of the RANKL/RANK/OPG system in the mid 1990s for the regulation of bone resorption has led to major advances in our understanding of how bone modeling and remodeling are regulated. It had been known for many years before this discovery that osteoblastic stromal cells regulated osteoclast formation, but it had not been anticipated that they would do this through expression of members of the TNF superfamily: receptor activator of NF-κB ligand (RANKL) and osteoprotegerin (OPG), or that these cytokines and signaling through receptor activator of NF-κB (RANK) would have extensive functions beyond regulation of bone remodeling. RANKL/RANK signaling regulates osteoclast formation, activation and survival in normal bone modeling and remodeling and in a variety of pathologic conditions characterized by increased bone turnover. OPG protects bone from excessive resorption by binding to RANKL and preventing it from binding to RANK. Thus, the relative concentration of RANKL and OPG in bone is a major determinant of bone mass and strength. Here, we review our current understanding of the role of the RANKL/RANK/OPG system in bone modeling and remodeling.     

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 κB 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, Théoleyre S, Heymann D, Fortun Y. IL-6, RANKL, TNF-/IL-1: interrelations in bone resorption pathophysiology. Cytokine Growth Factor Rev 2004;15:49–60].     

Regulation of RANKL by biomechanical loading in fibrochondrocytes of meniscus

OBJECTIVE: We sought to determine whether fibrochondrocytes from menisci express receptor activator of NF-kappaB (RANK), its ligand (RANKL), or osteoprotegerin (OPG) and, if so, whether their expression is modulated by dynamic mechanical loading under inflammatory and normal conditions. METHODS: Fibrochondrocytes from rat menisci were subjected to cyclic tensile strain (CTS) at various magnitudes and frequencies in the presence or absence of interleukin (IL)-1beta for up to 24 h. In order to determine whether a possible regulatory effect of mechanical loading on RANKL and its receptors under inflamed conditions is sustained, cells were stimulated with IL-1beta for 24 h while being subjected to CTS only for the initial 4 and 8h, respectively. Regulation of RANKL, RANK, and OPG expression and synthesis were determined by semiquantitative and real-time PCR, Western blotting, and immunofluorescence. RESULT: Fibrochondrocytes constitutively expressed low levels of RANKL and RANK but marked levels of OPG. IL-1beta upregulated expression and synthesis of RANKL and RANK significantly (p<0.05), whereas expression of OPG was unaffected following 4 and 24 h. When fibrochondrocytes were simultaneously subjected to CTS and IL-1beta, expression of RANKL and RANK was significantly (p<0.05) downregulated as compared to that of IL-1beta-stimulated unstretched cells. The inhibitory effect of CTS on the IL-1beta-induced upregulation of RANKL and RANK was sustained as well as magnitude and frequency dependent. CONCLUSIONS: Our study provides evidence that RANKL and its receptors are expressed in fibrochondrocytes from meniscus. These data also demonstrate that dynamic mechanical loading can modify the expression of RANKL and RANK in inflammatory conditions.     

The relationship between osteoclastogenic and anti-osteoclastogenic pro-inflammatory cytokines differs in human osteoporotic and osteoarthritic bone tissues

Background

Pro-inflammatory cytokines possess osteoclastogenic or anti-osteoclastogenic activities. They influence osteoclasts directly or via the receptor activator of nuclear factor κB (RANK), RANK ligand (RANKL) and osteoprotegerin (OPG) system. Recent evidence suggests that inflammation may play a role in osteoporosis (OP) and osteoarthritis (OA). We aimed therefore to determine whether there is a difference between both groups: first, in the expression of the osteoclastogenic and anti-osteoclastogenic cytokines, second, in correlation of these cytokines with bone mineral density (BMD) and levels of bone turnover markers (BTM) and third, in correlation between the expression of these cytokines and osteoclast specific genes and RANK/RANKL/OPG genes.

Methods

Human bone samples from 54 age and sex matched patients with OP or OA were collected during hip arthroplasty surgery. The expression of 25 genes encoding pro-inflammatory cytokines, their receptors, osteoclast specific genes and RANK/RANKL/OPG genes was measured using quantitative real-time PCR. Total hip, femoral neck and lumbar spine BMD and BTM in blood samples were measured. The comparison between OP and OA was assessed using Student''s t-test or Mann-Whitney U test and correlations between gene expression, BMD and BTM were determined using nonparametric correlation.

Results

The results demonstrated a higher expression of interleukin (IL)-6 and IL-1α in OP, and interferon (IFN)-γ in OA (p < 0.0005). Negative correlations of total hip BMD with tumor necrosis factor-α (TNF-α) in OA and with RANKL/RANK in OP were found (p < 0.05). Significant correlations with BTM were shown for IL-1α and IFN-γ in OP (rho = 0.608 and -0.634) and for TNF-α, IL-6 and transforming growth factor-β1 (TGF-β1) in OA (rho = 0.591, -0.521 and 0.636). Results showed OP specific negative correlations (IFN-γ with ITGB3, IFN-β1 with CTSK, tartrate resistant acid phosphatase (TRAP), CALCR, RANK, RANKL, IL-1α with CTSK, OPG, IL-17A with CALCR) and positive (TGF-β1 with CTSK, TRAP, RANK), and OA specific negative (IL-1α with osteoclast associated immunoglobulin-like receptor (OSCAR), TNF-α with RANK, RANKL, OPG) and positive (IL-6 with RANK, RANKL, OPG) correlations.

Conclusions

Our results demonstrate that the relationship between osteoclastogenic and anti-osteoclastogenic pro-inflammatory cytokines differs in human OP and OA bone and could present an important factor for characteristics of OP and OA bone phenotypes.     

OPG/RANKL/RANK系统与骨破坏性疾病

近年来发现的OPG/RANKL/RANK系统在破骨细胞生成中起着至关重要的作用,是骨骼生理研究领域的重大进展。成骨细胞、骨髓基质细胞、激活的T淋巴细胞表达RANKL,与破骨细胞前体细胞或成熟破骨细胞表面上的RANK结合后,促进破骨细胞的分化及骨吸收活性。成骨细胞及骨髓基质细胞分泌表达OPG可与RANKL竞争性结合,从而阻断RANKL与RANK之间的相互作用。体内多种激素或因子通过影响骨髓微环境内的OPG/RANKL比率来调节骨代谢。此外,乳腺上皮细胞表达有RANK,孕期在性激素的诱导下可表达RANKL,OPG/RANKL/RANK系统在孕期乳腺发育以及母体向胎儿的钙转运过程中发挥重要作用。阻断RANKL/RANK通路有望给骨质疏松、类风湿关节炎及癌症骨转移等骨破坏性疾病的治疗开辟新的途径。进一步研究应了解OPG/RANKL/RANK系统与其它信号传导途径的关系,重视骨骼、免疫及内分泌系统之间的相互作用。目前,开发与OPG功能相似或促进其表达的合成药物有可能成为具有良好经济效益和社会效益的产业。     

The effects of fatty acids consumption on OPG/RANKL/RANK system in cardiovascular diseases: Current status and future perspectives for the impact of diet-gene interaction

Cardiovascular disease (CVD) is an overall term that comprises a number of related pathologies, these include peripheral arterial disease, cerebrovascular disease, coronary heart disease (CHD), venous thromboembolism, and rheumatic and congenital heart diseases. Fatty acids in the diet have been reported to affect CVD. The OPG/RANKL/RANK system appears to have a role in CVD outcomes. However, there have been few studies on the impact of diet-gene interaction for effects of fatty acids consumption on the OPG/RANKL/RANK system in CVD. This review focuses on the effects of fatty acids on OPG/RANKL/RANK in CVD.     

Variation in genes involved in the RANKL/RANK/OPG bone remodeling pathway are associated with bone mineral density at different skeletal sites in men

In order to assess the contribution of polymorphisms in the RANKL (TNFSF11), RANK (TNFRSF11A) and OPG (TNFRSF11B) genes to variations in bone mineral density (BMD), a population-based cohort with 1,120 extreme low hip BMD cases or extreme high hip BMD controls was genotyped on five SNPs. We further explored the associations between these genetic variations and forearm BMDs by genotyping 266 offspring and 309 available parents from 160 nuclear families. A family-based association test was used. Significantly positive associations were found for A163G polymorphisms in the promoter regions of the OPG gene, a missense substitution in exon 7 (Ala192Val) of the RANK gene and rs9594782 SNP in the 5′ UTR of the RANKL gene with BMD in men only. Men with TC/CC genotypes of the rs9594782 SNP had a 2.1 times higher risk of extremely low hip BMD (P=0.004), and lower whole body BMD (P<0.001). Subjects with the TC genotype of the Ala192Val polymorphism had a 40% reduced risk of having extremely low hip BMD (P<0.01), and higher whole body BMD (P<0.01). Subjects with the GG genotype of the A163G polymorphism had a 70% reduced risk of having extremely low hip BMD (P<0.05), and higher whole body BMD (P<0.01). Significant gene–gene interactions were also observed among the OPG, RANK and RANKL genes. Our findings suggest that genetic variation in genes involved in the RANKL/RANK/OPG bone remodeling pathway are strongly associated with BMD at different skeletal sites in adult men, but not in women. Electronic Supplementary Material Supplementary material is available for this article at     

RANKL/OPG in primary cultures of osteoblasts from post-menopausal women. Differences between osteoporotic hip fractures and osteoarthritis

The OPG/RANKL/RANK system is important in the balance between bone formation and resorption.We used primary human osteoblasts (hOBs) cells to examine the impact of 17-β-estradiol (E2) or/and 1,25-dihydroxyvitamin D (1,25D) in OPG/RANKL system in 28 post-menopausal (PM) women; (a) with hip fracture (OP) or (b) with osteoarthritis (OA). The hOB from OP patients proliferated slower during the first stage, than the OA women (31.5 ± 2.6 and 21.4 ± 1.3 days, respectively, p < 0.05). The OP group secreted significantly higher OPG protein levels than the OA women (10.1 ± 2.6 and 4.4 ± 0.8 pmol/L, respectively, p < 0.05). The 1,25D and 1,25D+E2 induce an increase in RANKL and RANKL/OPG mRNA expression in OP patients above 200% (p < 0.05).HOBs from the osteoporotic hip initially proliferate slower but after reaching the first cellular confluence, the proliferation rate is equal in both groups. Furthermore, hOBs from hips with OP present a higher protein secretion of OPG, and higher RANKL and RANKL/OPG expression ratio in response to 1,25D and 1,25D+E2, than hOBs from OA women. All this could suggest that the greater bone loss that characterizes OP patients can be mediated due to differences in the secretion and expression of the RANKL/OPG system in response to different stimuli.     

Altered osteoprotegerin/RANKL ratio and low bone mineral density in celiac patients on long-term treatment with gluten-free diet.

Skeletal demineralization and mineral metabolism derangement are well-recognized features of untreated celiac disease (CD). Although treatment with a gluten-free diet appears to prevent bone loss while correcting skeletal demineralization in childhood, there is evidence that bone mineral density does not return to normal in celiacs diagnosed in adulthood. Osteoprotegerin (OPG), a member of the tumor necrosis factor receptor family, and ligand of receptor activator of NFkB (RANKL) are involved in the process of bone turnover and have been implicated in the pathogenesis of osteoporosis and other metabolic bone diseases. We measured OPG, RANKL, bone mineral density (BMD), and biochemical markers of bone turnover in 32 adult female premenopausal celiac patients on a gluten-free diet, and thirty age-matched healthy women. We correlated the OPG/RANKL ratio with the severity of bone loss. Celiac patients had a mean BMD lower than controls in lumbar spine and in the femoral neck. Serum levels of bone alkaline phosphatase (BAP, marker of bone formation), and urinary excretion of telopeptides of type I collagen (a marker of bone resorption) were significantly higher than in controls. Serum OPG and RANKL levels were significantly higher in CD patients than in controls, while the OPG/RANKL ratio was significantly lower in CD patients than in controls and was positively correlated with BMD at the spine. The role of elevated OPG in CD patients is unclear, but it might represent a compensatory mechanism against other factors that promote bone damage. Further studies are required to assess a possible therapeutic potential of osteoprotegerin in optimally treated celiacs with persistent osteopenia.     

Imbalance of RANK,RANKL and OPG expression during tibial fracture repair in diabetic rats

To clarify the mechanisms of altered bone repair in the diabetic state, we investigated RANK, RANKL and OPG expression by immunohistochemistry and RT-PCR in the fracture sites of rats that were either healthy or made diabetic by alloxan. Histomorphometric analysis of the fracture site at 7 days after fracture revealed that diabetic rats (db) have significantly less hard tissue formation at the fracture site, compared to controls. The number of RANK, RANKL and OPG positive cells was decreased in the db group; however, the RANKL/OPG ratio was similar in controls and db at this time. At day 14, numbers of RANKL and OPG positive cells and the mRNA expression for these markers were higher in the control group than in db (P = 0.008). The RANKL/OPG ratio in the db group was greater than in controls. Our results demonstrate an imbalance of RANKL/OPG expression associated with diabetes that may contribute to the delay of fracture repair during the course of diabetes.     

Role of OPG/RANKL/RANK axis on the vasculature

Vascular calcification, a degenerative process considered in the past to be a passive procedure, has now been suggested to be related to ossification. Many proteins responsible for bone formation have been identified on the arterial wall. The OPG/RANKL/RANK axis, responsible for ossification and bone mineralization, seems to play a major role in vasculature and atherosclerosis. Mice lacking OPG gene present osteoporosis and arterial calcification, while overexpression of OPG gene leads to osteopetrosis. In the present review the latest knowledge related to the effects of the OPG/RANKL/RANK axis on vasculature, including atherosclerosis, will be analyzed. The clinical significance of circulating OPG and RANKL levels in vascular diseases will also be referred.