Severe osteoporosis in mice lacking osteoclastogenesis inhibitory factor/osteoprotegerin

The relationship between four single nucleotide polymorphisms in the promoter region of the osteoprotegerin gene and aortic calcification or coronary artery disease in Koreans

OBJECTIVE

Osteoprotegerin (OPG) is a recently identified cytokine that acts as a decoy receptor for the receptor activator of the NF-kappaB ligand (RANKL). OPG has been shown to be an important inhibitor of osteoclastogenesis and arterial calcification in animal models. OPG has been proposed as a link molecule between osteoporosis and arterial calcification, but the relationship between the OPG gene and the cardiovascular system in human populations is unclear. Thus, the aim of this study was to investigate the relationship between OPG gene polymorphisms and aortic calcification or coronary artery disease in Koreans.

DESIGN AND PATIENTS

Genotyping of four polymorphisms, A163G, G209A, T245G and T950C, in the promoter region of the OPG gene was performed in 251 healthy Korean women (mean age 51.3 +/- 6.9 years) and in a second study population consisting of 100 patients who underwent coronary angiography (mean age 57.0 +/- 11.9 years), by allelic discrimination using the 5' nuclease polymerase chain reaction assay. Cardiovascular risk factors and serum OPG levels were measured and aortic calcification in thoracic and abdominal aorta was examined by simple radiological methods.

RESULTS

In the first study population, the prevalence of aortic calcification increased significantly as the subjects grew older. The frequencies of mutant alleles were significantly higher in the subjects with aortic calcification compared with those without aortic calcification in G209A and T950C polymorphisms, although these significances were lost after adjustment for age. No significant relationship was found between OPG gene polymorphisms and serum OPG levels or cardiovascular risk factors. In the second study group, there were no associations between OPG promoter genotypes and aortic calcification, serum OPG levels, or coronary artery disease.

CONCLUSIONS

We observed that the four polymorphisms in the promoter region of the OPG gene were not associated with aortic calcification or coronary artery disease in Koreans. Further studies are needed to clarify this relationship.

NFATc1 expression in the developing heart valves is responsive to the RANKL pathway and is required for endocardial expression of cathepsin K

NFATc1 is necessary for remodeling endocardial cushions into mature heart valve leaflets and is also an essential effector of receptor activator of NFkappaB ligand (RANKL) signaling required for transcriptional activation of bone matrix remodeling enzymes during osteoclast differentiation. Therefore, developing heart valves were examined to determine if NFATc1 functions in the RANKL pathway during leaflet remodeling. Key components of RANKL signal transduction including RANKL, its receptor RANK, and the downstream remodeling enzyme cathepsin K (Ctsk) are expressed in the heart during valve remodeling and colocalize with NFATc1 in developing valve endocardium. However, the absence of tartrate-resistant acid phosphatase (TRAP) activity and the lack of F4/80-positive macrophage lineage contribution to the remodeling valves demonstrate that certain aspects of osteoclast RANKL function are not shared during valve formation. Analysis of NFATc1-/- mouse embryos shows that NFATc1 is specifically required for endocardial expression of RANKL and Ctsk during valve formation. In addition, RANKL treatment augments expression of NFATc1 and Ctsk in embryonic heart cultures, and the RANKL-mediated increase in Ctsk expression is dependent on NFATc1. Together, these results support a role for RANKL signaling during heart valve development and suggest that valve leaflet morphogenesis involves NFATc1-dependent expression of remodeling enzymes including Ctsk.

Hearts and bones: shared regulatory mechanisms in heart valve, cartilage, tendon, and bone development

The mature heart valves are dynamic structures composed of highly organized cell lineages and extracellular matrices. The discrete architecture of connective tissue within valve leaflets and supporting structures allows the valve to withstand life-long functional demands and changes in hemodynamic forces and load. The dysregulation of ECM organization is a common feature of heart valve disease and can often be linked to genetic defects in matrix protein structure or developmental regulation. Recent studies have identified specific regulatory pathways that are active in the developing valve structures and also control cartilage, tendon, and bone development. This review will focus on the regulatory hierarchies that control normal and abnormal heart valve development in parallel with other connective tissue cell types.

Osteoprotegrin knockout mice demonstrate abnormal remodeling of the otic capsule and progressive hearing loss

OBJECTIVES

The otic capsule, when compared with other bones in the body, is unique in that it undergoes no significant remodeling of bone after development. We previously demonstrated that osteoprotegerin (OPG), which inhibits formation and function of osteoclasts, is produced at high levels in the inner ear of normal mice and secreted into the perilymph from where it diffuses into the surrounding otic capsule bone through a lacunocanalicular system. To test our hypothesis that the high level of OPG may be important in the inhibition of otic capsule remodeling, we studied the light microscopic histology of the otic capsule in OPG knockout mice for evidence of abnormal remodeling of bone. We also tested the hearing in OPG knockout mice to determine whether OPG and its influence on surrounding bone is important for auditory function.

METHODS

Temporal bone histopathology and pathophysiology were compared in homozygous OPG knockout mice and C57BL/6 (B6) mice, the background strain for the knockouts. Auditory function in age-matched animals from each group was evaluated at approximately 4-week intervals from 8 to 21 weeks using frequency-specific auditory brainstem responses (ABR) and distortion product otoacoustic emissions (DPOAE). After each of the last three evaluations, the cochleae from one mouse of each group were harvested, processed, and examined by light microscopy.

RESULTS

Osteoprotegerin knockout mice demonstrated abnormal remodeling of bone within the otic capsule with multiple foci showing osteoclastic bone resorption and formation of new bone. Such changes were not seen in the age-matched B6 controls. The active bone remodeling process in the knockout animals showed many similarities to otosclerosis seen in human temporal bones. Over the time period that we monitored, auditory function was significantly and progressively compromised in the knockout animals relative to B6 controls. At the earliest age of test (8 wk), the loss was apparent as a mild, high-frequency reduction in sensitivity by ABR. In contrast, DPOAE losses in the knockouts were substantial even at 8 weeks, and by 21 weeks, these losses exceeded our equipment limits. Results of ABR testing showed hearing sensitivity changes in the animals of the background strain were confined largely to the high frequencies, whereas OPG knockouts demonstrated substantial low-frequency shifts in addition to those at high frequencies.

CONCLUSIONS

The histopathological and pathophysiological findings in OPG knockout mice support the hypothesis that OPG is important in the inhibition of bone remodeling within the otic capsule and the maintenance of normal auditory function. This mouse may provide a valuable animal model of human otosclerosis.

New prospects for the management of renal bone disease

The last decade has been a remarkably productive one in the field of bone biology. New insights into the maintenance of a normal bone microenvironment have led to significant advances in our understanding of many important skeletal disorders, including renal osteodystrophy. Novel targets for therapeutic manipulation have been exposed and encouraging progress made towards new treatments. In addition, just as clinical studies have alerted us to the potential hazards of vascular calcification, basic science has unearthed the intimate nature of the relationship between the previously separate disciplines of bone and vascular biology. The clinical nephrologist, however, may be forgiven a little cynicism at this point. If such progress has been made, why do the same proverbial difficulties confront us in day-to-day practice? Control of phosphate remains inadequate, despite a literature which constantly reaffirms its crucial importance, and parathyroid hyperplasia seems inevitable in many patients. Furthermore, even the satisfaction of successful control of serum parathyroid hormone concentration must now be tempered by disquiet regarding the skeletal and cardiovascular consequences of oversuppression. This review aims to provide an update of the latest developments in relevant skeletal research and to assess how recently acquired knowledge may improve clinical nephrological practice over the next five years.

Osteoprotegerin and bone mineral density in hemodialysis patients

INTRODUCTION

Osteoprotegerin is a soluble glycoprotein that belongs to the tumor-necrosis-factor receptor superfamily. In vitro, osteoprotegerin blocks osteoclastogenesis in a dose-dependent manner. The serum osteoprotegerin level shows a positive correlation with bone metabolism markers and a negative correlation with bone mineral density in healthy persons, but these relationships are unclear in hemodialysis patients. We investigated the role of osteoprotegerin in bone loss in hemodialysis patients.

METHODS

We measured baseline serum osteoprotegerin, bone metabolism markers, and bone mineral density in hemodialysis patients. A total of 201 patients (114 men and 87 women) were followed for 12 months, and bone mineral density was measured again to calculate the annual percent change in bone mineral density. Serum osteoprotegerin was also measured in 20 healthy persons.

RESULTS

The osteoprotegerin levels of the hemodialysis patients were about three times higher than those of the healthy controls. The osteoprotegerin level showed a negative correlation with various bone metabolism markers. In multiple regression analysis, the annual percent change in bone mineral density showed a positive correlation with osteoprotegerin level, while there was a negative correlation with duration of hemodialysis and intact parathyroid hormone level. The osteoprotegerin levels of the hemodialysis patients were about three times higher than those of the healthy controls. The osteoprotegerin level showed a negative correlation with various bone metabolism markers. In multiple regression analysis, the annual percent change in bone mineral density showed a positive correlation with osteoprotegerin level, while there was a negative correlation with duration of hemodialysis and intact parathyroid hormone level.

CONCLUSIONS

These correlations of osteoprotegerin are opposite to those found in healthy persons. However, osteoprotegerin might act to prevent bone loss even in hemodialysis patients.

Deletion of aspartate 182 in OPG causes juvenile Paget's disease by impairing both protein secretion and binding to RANKL

Mutations in the OPG gene cause idiopathic hyperphosphatasia. We characterized the effects of one such mutation and found that the mutant OPG is poorly secreted and has reduced biological activity compared with the wildtype protein. Therefore, correct structure and cellular processing of OPG is essential for normal bone remodeling.

INTRODUCTION

Inactivating mutations in osteoprotegerin (OPG) cause juvenile Paget's disease (JPD). We recently reported a family with JPD in which affected members were homozygous for an in-frame mutation resulting in the deletion of aspartate 182 in OPG. Here we report the structural and functional characterization of the OPGdeltaD182 mutant protein.

MATERIALS AND METHODS

Inhibition of osteoclastogenesis by the recombinant OPG proteins was studied in a murine bone marrow culture. Binding of wildtype and mutant OPG to RANKL was measured in two experimental systems: glutathione-S-transferase (GST) pull-down assay and surface plasmon resonance. Site-directed mutagenesis was used to study the glycosylation of OPGdeltaD182 in two potential glycosylation sites adjacent to the deleted aspartate residue at position 182. ELISA and Western blots were used to determine OPG concentrations in cell lysates and conditioned media from transiently transfected cells.

RESULTS

OPGdeltaD182 inhibited the generation of osteoclasts less effectively than the wildtype protein and had a reduced ability to bind to RANKL. The apparent higher molecular weight of OPGdeltaD182 compared with the wildtype is a result of hyperglycosylation of asparagine residues at positions 178 and 183. Glycosylation at N183 has the potential to disrupt OPG structure by interfering with disulphide bond formation and correct protein folding. Transient transfection experiments in SaOS2 cells suggest that OPGdeltaD182 is retained within the cell, a typical response to unstable or incorrect protein folding.

CONCLUSIONS

Taken together, these data suggest that the deletion of aspartate 182 impairs both the secretion and activity of OPG, which in turn provides an explanation for the increased osteoclastogenesis and high bone turnover observed in JPD patients with this mutation.

Expression of osteoprotegerin (OPG), TNF related apoptosis inducing ligand (TRAIL), and receptor activator of nuclear factor kappaB ligand (RANKL) in human breast tumours

BACKGROUND

Osteoprotegerin (OPG) is involved in the regulation of bone turnover through binding to the receptor activator of nuclear factor kappaB ligand (RANKL), and has also been reported to be a potential survival factor for several different cell types. The survival effects are mediated through inhibition of the activity of tumour necrosis factor related apoptosis inducing ligand (TRAIL). Both breast and prostate cancer cells produce sufficient amounts of OPG to be protected against the effects of TRAIL in vitro.

AIMS

To investigate the spatial expression of OPG, RANKL, and TRAIL in non-neoplastic breast tissue and breast cancer, and its relation with oestrogen receptor (ER) expression.

METHODS

Forty breast cancers (20 ER+, 20 ER-) and five non-neoplastic breast tissue samples were stained with antibodies against OPG, RANKL, and TRAIL.

RESULTS

OPG was not expressed in non-neoplastic breast tissue except when colocalised with altered columnar epithelium. RANKL was expressed at the apical surface of luminal epithelial cells and TRAIL was expressed in myoepithelial cells. All three proteins were expressed in some breast cancers but showed no significant association with tumour type. OPG expression showed a significant positive correlation with ER expression (p = 0.011).

CONCLUSIONS

This is the first published study of the spatial expression of OPG, RANKL, and TRAIL in breast tissue and breast cancer. The localisation of each protein was specific and they were not colocalised. This specificity may provide a useful marker of functional differentiation in breast cancer; for example, TRAIL expression as a marker of myoepithelial differentiation.

Immunolocalization of receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG) in Meckel's cartilage compared with developing endochondral bones in mice

We examined the immunolocalization of receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG) in areas of resorption caused by osteoclasts/chondroclasts on embryonic days 14-16 (E14-16) in Meckel's cartilage, and compared the results with those in endochondral bones in mice. Intense RANKL and OPG immunoreactivity was detected in the chondrocytes in Meckel's cartilage. On E15, when the incisor teeth were closest to the middle portion of Meckel's cartilage, tartrate-resistant acid phosphatase (TRAP)-positive cells appeared on the lateral side of the cartilage. Furthermore, the dental follicle showed moderate immunoreactivity for RANKL and OPG, whereas osteoblasts derived from perichondral cells were immunonegative for RANKL and OPG in that area. On E16, cartilage resorption by TRAP-positive cells had progressed at the differential position, and intensely immunoreactive products of RANKL were overlapped on and found to exist next to TRAP-positive cells in the resorption area. In developing metatarsal tissue, OPG immunoreactivity was intense in periosteal osteoblasts, whereas RANKL was only faintly seen in some of the periosteal cells. In epiphyseal chondrocytes of the developing femur, RANKL immunoreactivity was moderate, and OPG scarcely detected. These results indicate a peculiarity of RANKL and OPG immunolocalization in resorption of Meckel's cartilage. Growth of the incisor teeth may be involved in the time- and position-specific resorption of Meckel's cartilage through local regulation of the RANKL/OPG system in dental follicular cells and periosteal osteoblasts, whereas RANKL and OPG in chondrocytes seem to contribute to resorption through regulation of the chondroclast function.

Role of RANKL in physiological and pathological bone resorption and therapeutics targeting the RANKL-RANK signaling system

Osteoclasts are primary cells for physiological and pathological bone resorption, and receptor activator of nuclear factor-kappaB ligand (RANKL) is critically involved in the differentiation, activation, and survival of these cells. Recently, therapeutics for pathological bone destruction targeting RANKL pathways has attracted a great deal of attention. Herein, we review the recent advances in the research on osteoclast biology and discuss the advantages and disadvantages of anti-RANKL therapies.

B cells and osteoblast and osteoclast development

The molecules that regulate bone cell development, particularly at the early stages of development, are only partially known. Data are accumulating that indicate a complex relationship exists between B cells and bone cell differentiation. Although the exact nature of this relationship is still evolving, it takes at least two forms. First, factors that regulate B-cell growth and development have striking effects on osteoclast and osteoblast lineage cells. Similarly, factors that regulate bone cell development influence B-cell maturation. Second, a series of transcription factors required for B-cell differentiation have been identified, and these factors function in a developmentally ordered circuit. These transcription factors have unpredicted, pronounced, and non-overlapping effects on osteoblast and/or osteoclast development. These data indicate that at least a regulatory relationship exists between B lymphopoiesis, osteoclastogenesis, and osteoblastogenesis.

Comparison of Fc-osteoprotegerin and zoledronic acid activities suggests that zoledronic acid inhibits prostate cancer in bone by indirect mechanisms

Zoledronic acid (ZA) has been shown to inhibit prostate tumor growth in vitro and have beneficial effects in patients with advanced prostate cancer (CaP). The aim of this study was to determine whether ZA exhibits direct anti-tumor effects on CaP cells in vivo. To distinguish the effects of inhibition of osteolysis and direct anti-tumor activity of ZA in vivo, we compared the results of treatment with ZA and osteoprotegerin (Fc-OPG), which inhibits osteolysis, but without significant direct anti-tumor effects. In vitro Fc-OPG had no significant effects on C4-2 proliferation, whereas ZA decreased proliferation. However, both agents decreased tumor growth in bone. Moreover, both increased bone volume and prevented the overall decreases in BMD associated with growth of C4-2 cells in bone. Our study provides novel and significant observations that the in vivo effects of ZA are consistent with indirect effects mediated by osteoclasts.

RANKL-RANK signaling in osteoclastogenesis and bone disease

Hundreds of millions of people worldwide are affected by bone-related diseases, such as osteoporosis and rheumatoid arthritis. Understanding the molecular mechanisms of bone metabolism is crucial for developing novel drugs for treating such diseases. In particular, genetic experiments showing that the receptor activator of NF-kappaB (RANK), its ligand RANKL, and the decoy receptor OPG are essential, central regulators of osteoclast development and osteoclast function were significant turning points in our understanding of bone diseases. RANKL-RANK signaling activates a variety of downstream signaling pathways required for osteoclast development. Moreover, molecular cross-talk between RANKL-RANK and other ligand-receptor systems fine-tunes bone homeostasis in normal physiology and disease. Designing novel drugs that target RANKL-RANK and their signaling pathways in osteoclasts could potentially revolutionize the treatment of many diseases associated with bone loss such as arthritis, tooth loss, cancer metastases or osteoporosis.

Effects of osteoprotegerin administration on osteoclast differentiation and trabecular bone structure in osteoprotegerin-deficient mice

Osteoprotegerin (OPG)-deficient mice exhibit severe bone loss including the destruction of growth plate cartilage. Using OPG-deficient mice, we attempted to clarify the differentiation and ultrastructure of osteoclasts located on the destroyed growth plate cartilage and trabecular bone matrix in long bones. In (-/-) homozygous OPG knockout mice, adjacent to the growth plate cartilage, the formation of bone trabeculae without a calcified cartilaginous core resulted in an irregular chondrocyte distribution in the growth plate cartilage. At the metaphyseal ossification center, TRAP-positive osteoclasts showed unusual localization on both type-II collagen-positive cartilage and type-I collagen-positive bone matrix. Osteoclasts located on cartilage matrix lacked a typical ruffled border structure, but formed resorption lacunae. During growth plate cartilage destruction, osteoclasts formed ruffled border structures on bone matrix deposited on the remaining cartilage surfaces. These findings suggest that, in OPG (-/-) mice, osteoclast structure differs, depending on the matrix of either cartilage or bone. Then, we examined the effects of OPG administration on the internal trabecular bone structure and osteoclast differentiation in OPG (-/-) mice. OPG administration to OPG (-/-) mice significantly inhibited trabecular bone loss and maintained the internal trabecular bone structure, but did not reduce the osteoclast number on bone trabeculae. For most osteoclasts, OPG administration caused disappearance or reduction of the ruffled border, but induced neither necrotic nor apoptotic damages. These results suggest that OPG administration is an effective means of maintaining the internal structure and volume of trabecular bone in metabolic bone diseases by inhibition of osteoclastic bone resorption.

The influence of Lys3Asn polymorphism in the osteoprotegerin gene on bone mineral density in Chinese postmenopausal women

The objective was to identify single nucleotide polymorphisms (SNPs) in exons of the osteoprotegerin gene and to analyze the relationship between the SNPs and bone mineral density in postmenopausal women. We used polymerase chain reaction (PCR) and direct sequencing methods to identify SNPs and genotypes in 205 postmenopausal women. BMD at the lumbar spine (L2-4) and femoral neck (FN) were measured by dual-energy X-ray absorptiometry (DEXA). Serum osteocalcin (OC), osteoprotegerin (OPG), receptor activator of nuclear factor kappaB ligand (RANKL) and urinary N-telopeptide of type I collagen (NTx) were also measured. In exon 1 of the OPG gene, we found the Lys3Asn SNP. In 205 postmenopausal women, the Asn-allele frequency was 26.0%, and the distribution of Lys3Asn genotypes was Lys-Lys 56.6%, Lys-Asn 34.6% and Asn-Asn 8.8%, respectively. BMD at the lumbar spine (L2-4) of the Asn-Asn genotype was significantly higher (9.5-12.6%) than Lys-Asn and Lys-Lys genotypes (P=0.012), with evidence for an allele dose effect (P=0.008). Results remained similar after adjustment for age and body mass index. The Lys3Asn polymorphism of the OPG gene alone accounted for 7.7% of the variance of the L2-4 BMD in a multiple regression model. Logistic regression analysis revealed that the OPG genotype Lys-Lys had a 2.7 times (95% CI: 0.83-9.11) greater risk for osteopenia/osteoporosis than the Asn-Asn genotype. The Lys3Asn polymorphism in the OPG gene is associated with L2-4 BMD in postmenopausal women. The Lys-allele is associated with lower BMD and an increased risk for osteoporosis.

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.

Enhancement of crude bone morphogenetic protein-induced new bone formation and normalization of endochondral ossification by bisphosphonate treatment in osteoprotegerin-deficient mice

Osteoprotegerin (OPG) is a novel secreted member of the tumor necrosis factor receptor family which plays a crucial role in negative regulation of osteoclastic bone resorption. We investigated both the quantity and quality of heterotopic new bone induced by crude bone morphogenetic protein (BMP) as a means of examining bone metabolism by bisphosphonate administration in OPG-/- mice. Four weeks after implantation of crude BMP, the volume of heterotopic new bone in OPG-/- mice without alendronate was significantly less than in wild-type (WT) mice. Alendronate treatment of OPG-/- mice resulted in enhancement of the volume of heterotopic new bone. Histological findings revealed that WT mice showed normal bone formation with persistent cartilage that was interspersed with islands of bone. In contrast, the cartilage was replaced by trabecular bone and bone marrow adipocytes in OPG-/- mice without alendronate. However, some cartilage was still present in OPG-/- mice with alendronate compared to those without alendronate. All bone formation-related parameters and bone resorption-related parameters were significantly lower in OPG-/- mice with alendronate than in those without alendronate. These findings suggest that in stimulated osteoclastogenesis without OPG, osteoinductive activity induced by crude BMP is inhibited and endochondral ossification induced by crude BMP is accelerated. On the other hand, alendronate treatment of OPG-/- mice caused osteoinductive activity induced by crude BMP to increase and endochondral ossification induced by crude BMP to be decelerated. In conclusion, inhibition of stimulated osteoclastogenesis results in the enhancement of new bone formation and normalization of endochondral ossification.

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.

Aging increases stromal/osteoblastic cell-induced osteoclastogenesis and alters the osteoclast precursor pool in the mouse

Stromal/osteoblastic cell expression of RANKL and M-CSF regulates osteoclastogenesis. We show that aging is accompanied by increased RANKL and M-CSF expression, increased stromal/osteoblastic cell-induced osteoclastogenesis, and expansion of the osteoclast precursor pool. These changes correlate with age-related alterations in the relationship between osteoblasts and osteoclasts in cancellous bone.

INTRODUCTION

Bone mass is maintained through a balance between osteoblast and osteoclast activity. Osteoblasts regulate the number and activity of osteoclasts through expression of RANKL, osteoprotegerin (OPG), and macrophage-colony stimulation factor (M-CSF). To determine whether age-related changes in stromal/osteoblastic cell expression of RANKL, OPG, and M-CSF are associated with stimulation of osteoclastogenesis and whether the osteoclast precursor pool changes with age, we studied cultures of stromal/osteoblastic cells and osteoclast precursor cells from animals of different ages and examined how aging influences bone cell populations in vivo.

MATERIALS AND METHODS

Osteoclast precursors from male C57BL/6 mice of 6 weeks (young), 6 months (adult), and 24 months (old) of age were either co-cultured with stromal/osteoblastic cells from young, adult, or old mice or treated with M-CSF, RANKL, and/or OPG. Osteoclast precursor pool size was determined by fluorescence-activated cell sorting (FACS), and osteoclast formation was assessed by measuring the number of multinucleated TRACP(+) cells and pit formation. The levels of mRNA for RANKL, M-CSF, and OPG were determined by quantitative RT-PCR, and transcription was measured by PCR-based run-on assays. Osteoblast and osteoclast numbers in bone were measured by histomorphometry.

RESULTS

Osteoclast formation increased dramatically when stromal/osteoblastic cells from old compared with young donors were used to induce osteoclastogenesis. Regardless of the origin of the stromal/osteoblastic cells, the number of osteoclasts formed from the nonadherent population of cells increased with increasing age. Stromal/osteoblastic cell expression of RANKL and M-CSF increased, whereas OPG decreased with aging. Exogenously administered RANKL and M-CSF increased, dose-dependently, osteoclast formation from all donors, but the response was greater in cells from old donors. Osteoclast formation in vitro positively, and the ratio of osteoblasts to osteoclasts in vivo negatively, correlated with the ratio of RANKL to OPG expression in stromal/osteoblastic cells for all ages. The effects of RANKL-induced osteoclastogenesis in vitro were blocked by OPG, suggesting a causal relationship between RANKL expression and osteoclast-inducing potential. The osteoclast precursor pool and expression of RANK and c-fms increased with age.

CONCLUSIONS

Our results show that aging significantly increases stromal/osteoblastic cell-induced osteoclastogenesis, promotes expansion of the osteoclast precursor pool and alters the relationship between osteoblasts and osteoclasts in cancellous bone.

Hepatic osteodystrophy: does the osteoprotegerin/receptor activator of nuclear factor-kB ligand system play a role?

Multiple factors can contribute to the development of osteodystrophy in patients with chronic liver disease (CLD). Recently, two new cytokines, osteoprotegerin (OPG) and the receptor activator of nuclear factor-kB ligand (RANKL), have been implicated in the pathogenesis of postmenopausal osteoporosis and other metabolic bone diseases. Therefore, the aim of our study was to evaluate bone metabolism, bone mineral density (BMD) and OPG/RANKL system in 65 male patients with CLD and in 65 healthy controls. Our patients showed lower BMD values than controls both at lumbar and femoral levels. Moreover, they had an unbalanced bone turnover with an increased resorption phase, as shown by high levels of urinary deoxypyridinoline and a decreased formation phase, as shown by the slightly, but significant, low levels of bone-alkaline phosphatase. Patients showed lower plasma levels of free-testosterone than controls and higher - although not significantly so - plasma levels of 17 beta-estradiol. Furthermore, patients with CLD had higher levels of sex hormone-binding globulin and OPG, and lower levels of 25-hydroxyvitamin D (25-HOD) and IGF-I than the control group, while RANKL levels were similar in the two groups. In conclusion, our data do not confirm the hypothesis that the OPG/RANKL system could exert a key role in the pathogenesis of hepatic osteodystrophy, but rather that the observed increase in OPG levels may represent either the result of the inflammatory process per se or a compensation for the observed enhanced bone resorption.

Recombinant osteoprotegerin for juvenile Paget's disease

Juvenile Paget's disease, a genetic bone disease characterized by accelerated bone turnover, results from inactivating mutations in the gene encoding osteoprotegerin--a key regulator of osteoclastogenesis. The effects of recombinant osteoprotegerin were investigated in two adult siblings with juvenile Paget's disease. Bone resorption (assessed by N-telopeptide excretion) was suppressed by once-weekly subcutaneous doses of 0.3 to 0.4 mg per kilogram of body weight. After 15 months of treatment, radial bone mass increased in one patient by 9 percent and in the other by 30 percent, skeletal bisphosphonate retention decreased by 37 percent and 55 percent, respectively, and there was radiographic improvement. Apart from mild hypocalcemia and hypophosphatemia, no apparent adverse events occurred.

Association of the osteoprotegerin gene polymorphisms with bone mineral density in postmenopausal women

OBJECTIVES

Osteoprotegerin (OPG) is a recently discovered member of the tumour necrosis factor receptor superfamily. It plays a crucial role in the control of bone resorption and its gene could therefore be a good candidate gene for osteoporosis. The aim of our work was to find polymorphisms in the OPG gene and to investigate their possible contribution to the genetic susceptibility to osteoporosis by testing for their association with bone mineral density (BMD).

METHODS

The whole OPG gene coding region was screened for the presence of polymorphisms in a group of 60 osteoporotic women by single-strand conformation polymorphism analysis (SSCP) approach. Association of the discovered polymorphisms with bone mineral density was investigated in 136 Slovenian postmenopausal women.

RESULTS

We detected eight OPG gene polymorphisms that were confirmed by direct DNA sequencing, deletion 4752_4753delCT and nucleotide substitutions 1181G>C, 1217C>T, 1284G>A, 4501C>T, 6893A>G, 6950A>C and 8738T>A. Nucleotide substitutions 1284G>A and 8738T>A have not been previously described. Polymorphisms 4752_4753delCT, 6893A>G and 6950A>C were in complete linkage and the same was true for 1217C>T and 4501C>T. The association with BMD was found only for polymorphism 1181G>C. Subjects with genotype 1181GG had significantly lower lumbar spine BMD than subjects displaying 1181GC.

CONCLUSIONS

By our approach we detected eight polymorphisms in the OPG gene. According to our analysis polymorphism 1181G>C is associated with BMD and could therefore be considered as an element of genetic susceptibility to osteoporosis.

Gamma interferon positively modulates Actinobacillus actinomycetemcomitans-specific RANKL+ CD4+ Th-cell-mediated alveolar bone destruction in vivo

Recent studies have shown the biological and clinical significance of signaling pathways of osteogenic cytokines RANKL-RANK/OPG in controlling osteoclastogenesis associated with bone pathologies, including rheumatoid arthritis, osteoporosis, and other osteolytic disorders. In contrast to the inhibitory effect of gamma interferon (IFN-gamma) on RANKL-mediated osteoclastogenesis reported recently, alternative new evidence is demonstrated via studies of experimental periodontitis using humanized NOD/SCID and diabetic NOD mice and clinical human T-cell isolates from diseased periodontal tissues, where the presence of increasing IFN-gamma is clearly associated with (i) enhanced Actinobacillus actinomycetemcomitans-specific RANKL-expressing CD4(+) Th cell-mediated alveolar bone loss during the progression of periodontal disease and (ii) a concomitant and significantly increased coexpression of IFN-gamma in RANKL(+) CD4(+) Th cells. Therefore, there are more complex networks in regulating RANKL-RANK/OPG signaling pathways for osteoclastogenesis in vivo than have been suggested to date.

Osteoprotegerin (OPG) and receptor activator of NF-kB ligand (RANK-L) serum levels in patients on chronic hemodialysis

The mechanisms underlying the skeletal resistance to PTH in patients on chronic hemodialysis (CHD) are not yet fully clarified. Osteoprotegerin (OPG) and receptor activator of NF-kB ligand (RANK-L) modulate the genesis and activity of osteoclasts, however their role in renal osteodystrophy pathogenesis has not been clarified so far. The present study aimed to evaluate OPG and RANK-L serum levels in hemodialysis patients and whether OPG/RANK-L system could have a role in the skeletal resistance to PTH. In fasting blood samples obtained from 60 patients (36 males and 24 females) on CHD for at least 2 yr and from 40 healthy subjects of similar age and gender distribution as controls (CTRs), we measured serum OPG, RANK-L, bone alkaline phosphatase (B-ALP), N-terminal telopeptide of type I collagen (NTx), PTH(1-84), calcium and phosphate. In 30 of 60 hemodialysis patients, a blood sample was also drawn soon after the dialytic session. Serum levels of RANK-L, but not OPG, showed a slight but significant (p<0.05) decrease after the dialytic session. OPG resulted being about six times higher in CHD patients than in CTRs (38.7 +/- 16.2 vs 6.3 +/- 0.17 pg/ml), whereas RAN K-L serum levels were only slightly increased with respect to controls (0.88 +/- 0.47 vs 0.64 +/- 0.38 pmol/l). CHD patients showed serum PTH(1-84) and bone turnover higher than in CTRs. No correlation was found between OPG/RANK-L system and PTH or bone turnover markers. Instead, in the patients with high osteoclast activity (no.=21) OPG/RANK-L ratio was correlated (r=-0.41, p<0.01) with NTx serum levels, whereas in patients with decreased osteoclast activity (no.=39) no relationship was found. In conclusion, our findings showed that, although both OPG and RANK-L are accumulated in hemodialysis patients, only RANK-L and the balance between OPG and RANK-L seem to be related to osteoclast activity.

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.

Emerging and potential therapies for osteoporosis

Osteoporotic fractures are an important public health problem, contributing substantially to morbidity and mortality in an ageing world population and consuming considerable health resources. Currently available pharmacological therapies for prevention of fragility fractures are limited in scope, efficacy and acceptability to patients. Considerable efforts are being made to develop new, more effective treatments for osteoporosis and to refine/optimise existing therapies. These novel treatments include an expanding array of drugs that primarily inhibit osteoclastic bone resorption; oestrogenic compounds, bisphosphonates, inhibitors of receptor activator of nuclear factor-kappaB ligand signalling, cathepsin K inhibitors, c-src kinase inhibitors, integrin inhibitors and chloride channel inhibitors. The advent of intermittent para-thyroid hormone (PTH) therapy has provided proof-of-principle that osteo-blast-targeted (anabolic) agents can effectively prevent osteoporotic fractures, and is likely to be followed by the introduction of other therapies based upon PTH, such as orally active PTH analogues, antagonists of the calcium sensing receptor, PTH-related peptide analogues, and/or agents that induce osteoblast anabolism via pathways involving key, recently identified, molecular targets (wnt low-density lipoprotein receptor-related protein-5 signalling, sclerostin and matrix extracellular phosphoglycoprotein).

Osteoprotegerin in prostate cancer bone metastasis

Osteoprotegerin (OPG), a critical regulator of osteoclastogenesis, is expressed by prostate cancer cells, and OPG levels are increased in patients with prostate cancer bone metastases. The objective of this study was to investigate the effects of OPG overexpression on prostate cancer cells and prostate cancer/bone cell interactions in vitro and in vivo. OPG-transfected C4-2 cells expressed 8.0 ng OPG per mL per 10(6) cells, whereas no OPG was detected in the media of C4-2 cells transfected with a control plasmid. OPG overexpressed by C4-2 cells protected these cells from tumor necrosis factor-related apoptosis-inducing ligand-induced apoptosis and decreased osteoclast formation. Subcutaneous OPG-C4-2 and pcDNA-C4-2 tumors exhibited similar growth and take-rate characteristics. However, when grown in bone, tumor volume was decreased in OPG-C4-2 versus pcDNA-C4-2 (P=0.0017). OPG expressed by C4-2 cells caused increases in bone mineral density (P=0.0074) and percentage of trabecular bone volume (P=0.007), and decreases in numbers of osteoblasts and osteoclasts when compared with intratibial pcDNA-C4-2 tumors (P=0.003 and P=0.019, respectively). In summary, our data show that increased expression of OPG in C4-2 cells does not directly affect proliferation of prostate cancer cells but indirectly decreases growth of C4-2 tumors in the bone environment. Our data also show that OPG expressed by C4-2 cells inhibits bone lysis associated with C4-2 bone metastasis, which results in net increases in bone volume. We therefore hypothesize that OPG expressed in prostate cancer patient bone metastases may be at least partially responsible for the osteoblastic character of most prostate cancer bone lesions.

Circulating amounts of osteoprotegerin and RANK ligand: genetic influence and relationship with BMD assessed in female twins

Osteoprotegerin (OPG) is a circulating receptor that inhibits osteoclastogenesis by binding to RANK ligand (RANKL). OPG knock-out animals develop severe osteoporosis. Treatment with OPG lowers bone resorption and increases BMD. OPG production is influenced by a wide range of hormones and cytokines. The influence of genetic factors on circulating amounts of OPG and RANKL is not known. BMD has been demonstrated to have a high heritability and there is evidence also that bone turnover and bone loss rates are controlled at least in part by genetic factors.

OBJECTIVE

Assessing the genetic impact on serum OPG and RANKL in women and estimation of the relative contribution of this inheritance to the total heritability of BMD.

METHODS

188 female twins (52 DZ and 42 MZ pairs) from the Danish Twin Registry were included in the study. Mean age was 35 years (range 19-64 years), average spine BMD was 1.04 +/- 0.11 g/cm2. Serum levels of OPG and RANKL were measured by ELISA (Biomedica, Vienna, Austria). This register covers twins born in Denmark since 1870. Heritability and environmental influence was assessed using a maximum-likelihood model for genetic pleiotropy.

RESULTS

RANKL levels showed a negative correlation with age and lower values in smokers. OPG levels were higher in postmenopausal women. Heritability (h(2)) was 85% for spine BMD and 52% for serum RANKL after adjustment for age, smoking and BMI. By contrast, there was no significant genetic influence on OPG levels (h(2) = 0, 95% CI: 0 to 0.31). Serum OPG was determined almost exclusively by individual environment (e(2) = 0.79), with a small, non-significant contribution from shared environment (c(2) = 0.21). Restricting analyses to the 158 premenopausal twins did not alter the findings.

CONCLUSIONS

Serum OPG and RANKL levels have only a weak relation to BMD in healthy women. Phenotype correlations indicate that the genes that contribute to twin similarity for BMD are not genes regulating serum levels of RANKL or OPG. The weak correlation with BMD appears to consist in shared environmental factors.

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 alpha (TNF-alpha) 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-alpha correlated significantly with C-terminal degradation products of type I collagen, an osteoclastic marker, but significantly negatively with OPG. Our data suggest that TNF-alpha and OPG may play a role in the pathogenetic process underlying osteopenia in these patients.

The effect of osteoprotegerin administration on the intra-tibial growth of the osteoblastic LuCaP 23.1 prostate cancer xenograft

Osteoprotegerin (OPG) plays a central role in controlling bone resorption. Exogenous administration of OPG has been shown to be effective in preventing osteolysis and limiting the growth of osteolytic metastasis. The objective of this study was to investigate the effects of OPG on osteoblastic prostate cancer (CaP) metastases in an animal model. LuCaP 23.1 cells were injected intra-tibially and Fc-OPG (6.0 mg/kg) was administered subcutaneously three times a week starting either 24 hours prior to cell injection (prevention regimen) or at 4 weeks post-injection (treatment regimen). Changes in bone mineral density at the tumor site were determined by dual x-ray absorptiometry. Tumor growth was monitored by evaluating serum prostate specific antigen (PSA). Fc-OPG did not inhibit establishment of osteoblastic bone lesions of LuCaP 23.1, but it decreased growth of the tumor cells, as determined by decreases in serum PSA levels of 73.0 +/- 44.3% (P < 0.001) and 78.3 +/- 25.3% (P < 0.001) under the treatment and prevention regimens, respectively, compared to the untreated tumor-bearing animals. Administration of Fc-OPG decreased the proliferative index by 35.0% (P = 0.1838) in the treatment group, and 75.2% (P = 0.0358) in the prevention group. The results of this study suggest a potential role for OPG in the treatment of established osteoblastic CaP bone metastases.

Inducible nitric oxide synthase mediates bone development and P. gingivalis-induced alveolar bone loss

The role of inducible nitric oxide synthase (iNOS) in bone development and bacterially induced periodontal bone loss was examined using mice with targeted mutation of the iNOS gene. Femurs of iNOS KO mice showed 30% and 9% higher bone mineral density compared to wild type (WT) at 4 and 9 weeks of age, respectively. Micro-computed tomography revealed that cortical thickness and cortical bone density is increased in the absence of iNOS, while trabecular bone thickness and bone density remains unchanged. Histochemical analysis using TRAP staining showed that osteoclast numbers are lower by 25% in iNOS KO femurs compared to WT femurs. When bone marrow cells were stimulated with M-CSF and RANKL in vitro, iNOS KO cultures developed 51% fewer TRAP-positive multinuclear cells compared to WT cultures. When similar cultures were grown on dentine discs, resorption pit area was decreased by 54% in iNOS KO cultures. Gene expression studies showed that iNOS expression is induced by M-CSF and RANKL in WT bone marrow cultures, while no iNOS transcript was detected in iNOS KO. No compensatory change was detected in the expression of neuronal or endothelial NOS isoforms. There was no difference in RANK and osteoprotegerin expression between iNOS KO and WT bone marrow cultures after M-CSF and RANKL-treatment, while Traf6 expression was significantly lower in the absence of iNOS. In the alveolar bone of the maxilla, the distance between the cementoenamel junction and the alveolar bone crest was larger in iNOS KO compared to WT mice from 6 to 14 weeks of age, indicating a developmental effect of iNOS in oral tissues. Oral administration of the periodontal pathogen Porphyromonas gingivalis caused alveolar bone loss in the maxilla of WT mice, but failed to do so in iNOS KO mice. Expression of the osteoclast marker cathepsin K was 25% lower in iNOS KO alveolar bone. These data indicate that iNOS promotes bone resorption during bone development as well as after bacterial infection, and that iNOS is an important signal for normal osteoclast differentiation.

Bone remodeling: new aspects of a key process that controls skeletal maintenance and repair

Bone remodeling is the concerted interplay of two cellular activities: osteoclastic bone resorption and osteoblastic bone formation. Bone remodeling is the physiologic process that maintains bone mass, skeletal integrity and skeletal function. A molecular understanding of this process is therefore of paramount importance for almost all aspects of skeletal physiology and many facets of bone diseases. Based on the morphological observation of the BMU-"bone multicellular unit" or "bone metabolic unit"-and a wide body of in vitro data, bone remodeling was thought to be controlled locally through functional coupling of resorption and formation and vice versa. However, recent genetic studies have shown that there is no obligatory tight cross-control of bone formation and bone resorption in vivo and that there is also a central axis controlling bone formation, one aspect of bone remodeling. The molecule that inhibits bone formation through a hypothalamic relay is leptin. Following binding to its receptor located on the ventromedial nuclei of the hypothalamus, leptin's action on bone formation is mediated via a neuronal signaling cascade that involves the ss-adrenergic system. The overall goal of this review is to show how the dialogue between clinical medicine and mouse genetics helped to uncover a new concept in skeletal physiology.

Recombinant human TSH modulates in vivo C-telopeptides of type-1 collagen and bone alkaline phosphatase, but not osteoprotegerin production in postmenopausal women monitored for differentiated thyroid carcinoma

In women monitored for thyroid carcinoma, short-term stimulation with rhTSH induced an acute decrease in serum C-telopeptides of type-1 collagen and an increase in serum BALP levels without any effect on OPG production. The inhibitory effect of TSH on bone resorption occurred only in postmenopausal women who showed low BMD and a high bone turnover rate as an effect of L-thyroxine suppressive therapy.

INTRODUCTION

It has been recently shown that thyrotropin (TSH) has an inhibitory activity on skeletal remodeling in in vitro conditions. Here, we have aimed at evaluating whether TSH has similar effects in vivo. For this purpose, we have evaluated the sequential profile of serum bone metabolism markers during acute stimulation with recombinant human TSH (rhTSH) in thyroidectomized women monitored for thyroid carcinoma.

MATERIALS AND METHODS

The study group included 66 thyroidectomized patients, of whom 38 were premenopausal and 28 postmenopausal, who underwent routine rhTSH-assisted whole body radioactive iodine scanning for differentiated thyroid carcinoma. The patients were sequentially evaluated for TSH, free triiodothyronine (FT3), free thyroxine (FT4), bone alkaline phosphatase (BALP), C-telopeptides of type-1 collagen (CrossLaps), and osteoprotegerin (OPG) levels during rhTSH stimulation. The samples were drawn just before and 2 and 7 days after the first administration of rhTSH. BMD was evaluated by ultrasonography at baseline. Seventy-one healthy women (41 premenopausal and 30 postmenopausal) acted as a control group.

RESULTS AND CONCLUSIONS

At study entry, all patients had subclinical thyrotoxicosis as effect of L-thyroxine (L-T4) treatment. The patients had higher serum CrossLaps and OPG levels and lower BMD than healthy subjects. Postmenopausal patients showed comparable serum FT4 and FT3 concentrations with those found in premenopausal patients. However, postmenopausal patients showed higher serum CrossLaps (p < 0.001), OPG (p = 0.03), and BALP (p < 0.001) levels and lower BMD (p < 0.001) than those measured in premenopausal patients. Two days after the first administration of rhTSH, all patients had serum TSH values >100 mUI/liter. At this time, serum CrossLaps levels decreased significantly (p < 0.001) and BALP values increased (p = 0.001) with respect to the baseline values in postmenopausal but not in premenopausal patients. rhTSH did not induce any significant change in serum OPG values either in premenopausal or in postmenopausal patients. One week after the first rhTSH administration, serum CrossLaps values decreased again to values comparable with those measured at baseline, whereas serum BALP values remained high. This study shows that subclinical thyrotoxicosis is accompanied by high bone turnover rate with an increase in serum OPG levels compared with euthyroid healthy subjects. Acute increase in serum TSH levels is accompanied by a reversible inhibition of bone resorption. This effect is characterized by a decrease in serum CrossLaps and an increase in BALP levels without any evident effect on OPG production. The activity of TSH occurs specifically in postmenopausal women in whom the negative effects of L-T4 suppressive therapy on bone mass and metabolism are more marked compared with premenopausal women.

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.

Relationships between the changes of serum levels of OPG and RANKL with age, menopause, bone biochemical markers and bone mineral density in Chinese women aged 20-75

The correlations between the serum levels of OPG, RANKL with age, menopause, bone markers, and bone mineral densities (BMDs) at the lumbar spine and proximal femur were studied in 504 pre- and postmenopausal Chinese women aged 20-75 years. We found that age was positively and negatively correlated with serum concentrations of OPG (r = 0.442, P < 0.001) and RANKL (r = -0.263, P < 0.001), respectively. Compared with premenopausal women, postmenopausal women showed higher serum OPG levels (107.6 +/- 3.0 vs 72.0 +/- 1.8 pg/ml, P < 0.001), lower serum RANKL concentrations (4.7 +/- 0.4 vs. 5.8 +/- 0.3 pg/ml, P < 0.001) and RANKL/OPG ratios (0.045 +/- 0. 004 vs. 0.099 +/- 0.008, P < 0.001). Neither serum levels of OPG nor RANKL or RANKL/OPG ratio correlated with BMDs after adjustment of age and menopause. They also showed no differences among normal, osteopenic and osteoporotic postmenopausal women. Serum levels of OPG were positively correlated with urinary excretion of NTx (r = 0.1453, P = 0.006). Serum levels of RANKL (r = -0.1928, P < 0.001) and RANKL/OPG ratio (r = -0.1303, P = 0.013) were inversely correlated with serum concentrations of OC. In multiple regression analysis, up to 20% variance (R(2) = 0.106-0.224) of the OPG-RANKL system in peripheral circulation can be explained by age, menopause and bone markers. These results suggest that although serum OPG and RANKL concentrations were unrelated with BMDs, the age- and menopause- dependent changes of serum OPG and RANKL might be a protective mechanism against the accelerated bone loss in postmenopausal women.

Runx-2 is not essential for the vitamin D-regulated expression of RANKL and osteoprotegerin in osteoblastic cells

The differentiation and activity of osteoclasts are positively and negatively controlled by receptor activator of nuclear factor-kappaB ligand (RANKL), which is expressed on the surface of osteoblasts and stromal cells, and its decoy receptor osteoprotegerin (OPG), which is secreted by osteoblasts and stromal cells, respectively. The expression of the genes for RANKL and OPG is regulated by 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)]. Runt-related gene-2 (Runx-2) is essential for osteoblast differentiation and there are several reports that Runx-2 is involved in osteoclast formation. Therefore, to clarify the role of Runx-2 in osteoclastogenesis, we designed a series of experiments using C2 cells and C6 cells, which are derived from calvariae of runx2-deficient mice. Treatment of C2 cells and C6 cells with 1alpha,25(OH)(2)D(3) for 2-4 days increased and decreased the levels of expression of the mRNAs for RANKL and OPG, respectively, and the effects were dose-dependent. However, by day 8, the level of RANKL mRNA had fallen and that of OPG mRNA had risen. Furthermore, C6 cells induced the differentiation of mouse spleen cells into tartrate-resistant acid phosphatase-positive (TRAP-positive) multinucleated cells (osteoclast-like cells) in the presence of 10(-7)M 1alpha,25(OH)(2)D(3). Such formation of osteoclast-like cells was inhibited by exogenous OPG in a dose-dependent manner. Thus, our findings indicate that Runx-2 is not essential for the expression of RANKL and OPG, and the formation of osteoclast-like cells.

Osteosclerosis in idiopathic myelofibrosis is related to the overproduction of osteoprotegerin (OPG)

OBJECTIVE

The aim of this study is to investigate the mechanism of osteosclerosis in IMF in relation to OPG derangement.

METHODS

Plasma OPG level was assayed by OPG ELISA in 19 patients with IMF, 15 patients with other myeloproliferative disorders (MPDs), and 12 normal volunteers as controls and correlated with the degree of osteosclerosis. Furthermore, the level of OPG mRNA, in the cultured bone marrow stromal (BMS) cells of patients with IMF and anemia patients used as controls, in the presence or absence of TGF-beta1, was studied by real-time RT-PCR.

RESULTS

The present study showed that blood OPG level was significantly elevated in patients with IMF as compared to patients with other MPDs (p < 0.01) or normal volunteer controls (p < 0.05), and there was no significant difference in the level between patients with MPDs and controls. In addition, there was a positive correlation (r=0.67, p=0.04) between plasma OPG levels and the degree of osteosclerosis. There was no difference in the OPG mRNA in patients with IMF as compared with controls even on TGF-beta1 stimulation.

CONCLUSION

These results suggest that osteosclerosis in IMF may be related to overproduction of OPG and enhanced level of OPG is not due to the effect of TGF-beta1 on the BMS cells. It could be due to the effect of TGF-beta1 or other growth factors on cells other than BMS cells such as the osteoblasts.

MyD88 but not TRIF is essential for osteoclastogenesis induced by lipopolysaccharide, diacyl lipopeptide, and IL-1alpha

Myeloid differentiation factor 88 (MyD88) plays essential roles in the signaling of the Toll/interleukin (IL)-1 receptor family. Toll-IL-1 receptor domain-containing adaptor inducing interferon-beta (TRIF)-mediated signals are involved in lipopolysaccharide (LPS)-induced MyD88-independent pathways. Using MyD88-deficient (MyD88-/-) mice and TRIF-deficient (TRIF-/-) mice, we examined roles of MyD88 and TRIF in osteoclast differentiation and function. LPS, diacyl lipopeptide, and IL-1alpha stimulated osteoclastogenesis in cocultures of osteoblasts and hemopoietic cells obtained from TRIF-/- mice, but not MyD88-/- mice. These factors stimulated receptor activator of nuclear factor-kappaB ligand mRNA expression in TRIF-/- osteoblasts, but not MyD88-/- osteoblasts. LPS stimulated IL-6 production in TRIF-/- osteoblasts, but not TRIF-/- macrophages. LPS and IL-1alpha enhanced the survival of TRIF-/- osteoclasts, but not MyD88-/- osteoclasts. Diacyl lipopeptide did not support the survival of osteoclasts because of the lack of Toll-like receptor (TLR)6 in osteoclasts. Macrophages expressed both TRIF and TRIF-related adaptor molecule (TRAM) mRNA, whereas osteoblasts and osteoclasts expressed only TRIF mRNA. Bone histomorphometry showed that MyD88-/- mice exhibited osteopenia with reduced bone resorption and formation. These results suggest that the MyD88-mediated signal is essential for the osteoclastogenesis and function induced by IL-1 and TLR ligands, and that MyD88 is physiologically involved in bone turnover.

Osteoprotegerin production by human intestinal epithelial cells: a potential regulator of mucosal immune responses

Receptor activator of NF-κB (RANK) and its ligand (RANKL) are important members of the TNF receptor (TNFR) and TNF superfamilies, respectively. RANK is expressed on osteoclasts, T-lymphocytes, and dendritic cells, and its ligation with RANKL leads to cellular activation. However, another member of the TNFR family, osteoprotegerin (OPG), acts as a decoy receptor, binding to RANKL and preventing its interaction with RANK. Furthermore, OPG also binds TNF-related apoptosis-inducing ligand (TRAIL), an important regulator of cell survival. OPG is therefore an important regulator of bone metabolism and immune responses. Although intestinal epithelial cells (IEC) express some members of the TNF/TNFR superfamilies, the roles of OPG and RANKL in the intestinal mucosa has not been investigated. Here, we report that various human IEC lines constitutively express OPG mRNA and protein as well as mRNA for RANKL. Furthermore, human colonic epithelium constitutively expressed OPG, and this expression was increased in inflamed tissue. All of the IEC lines tested released OPG into the culture supernatant under standard culture conditions. Whereas TNF-α increased OPG protein secretion by HT29 cells, the cytokines IL-1β and IFN-γ had little, if any, effect. Furthermore, the culture supernatant from untreated HT29 cells abrogated TRAIL-induced inhibition of Jurkat T-cell proliferation and inhibited osteoclast activity in an in vitro model of bone resorption. Taken together, our data indicate that OPG is constitutively produced by IEC, could be upregulated by TNF-α, and is biologically active. Thus IEC-derived OPG may represent an important mucosal immunoregulatory factor and may be involved in bone physiology.

Vitamin A differentially regulates RANKL and OPG expression in human osteoblasts

All-trans-retinoic acid (ATRA) induces bone resorption, but the molecular mechanisms are unknown. We have studied the effect of ATRA on osteoprotegerin (OPG) and receptor activator of NF-kappaB ligand (RANKL) expression in human MG-63 osteosarcoma cells and primary osteoblast-like cultures. ATRA dose-dependently down-regulated protein levels of OPG in MG-63 cells, with a maximum (-56%) observed at a dose of 10(-6)M. This effect was confirmed with quantitative real-time PCR, where OPG mRNA was decreased after 4h (-68%) in primary cultures and after 8h (-87%) in MG-63 cells. The reduction in OPG expression was inhibited by a retinoic acid receptor (RAR)-antagonist and was mimicked by a RARbeta,gamma-agonist, indicating that the ATRA effect is mediated by these receptors. In primary cultures we found a threefold induction of RANKL mRNA expression. Thus, the RANKL/OPG ratio was markedly increased, suggesting a potential mechanism of ATRA-induced bone resorption.

1 alpha,25 dihydroxyvitamin D3 rapidly regulates the mouse osteoprotegerin gene through dual pathways

1 alpha,25(OH)(2)D(3) rapidly and transiently suppressed OPG gene expression both by accelerating the degradation of mRNA and by suppressing promoter activity. The latter process was mediated through the AP-1 binding site by a reduction in the proportion of phospho-c-Jun in a JNK-independent manner.

INTRODUCTION

Osteoclastogenesis is regulated by an integrated network of numerous bone metabolic factors, among which 1 alpha,25-dihydroxyvitamin D(3) [1 alpha,25(OH)(2)D(3)] promotes osteoclastogenesis by reciprocally upregulating the expression of RANKL and downregulating that of osteoprotegerin (OPG).

MATERIALS AND METHODS

To analyze the mechanism by which 1 alpha,25(OH)(2)D(3) suppresses OPG, we characterized cis-acting elements of the mouse OPG gene and assessed the post-transcriptional modifications by actinomycin D assays.

RESULTS

1 alpha,25(OH)(2)D(3) rapidly and transiently suppressed OPG expression and shortened the half-life of OPG mRNA; additionally, the c-Jun homodimer bound to the AP-1 binding site (TGACTGA, -293/-287) and maintained steady-state transcription of the OPG gene. Furthermore, mutation of the AP-1 site negated 1 alpha,25(OH)(2)D(3)-driven OPG suppression. Moreover, 1 alpha,25(OH)(2)D(3) treatment of ST2 cells decreased the amount of phosphorylated c-Jun protein (phospho-c-Jun), while the total amount of c-Jun remained constant; however, the amount of phosphorylated Jun N-terminal kinase (JNK) was nearly unchanged by 1 alpha,25(OH)(2)D(3) treatment.

CONCLUSION

Taken together with the observation that the OPG promoter has no consensus negative vitamin D-responsive elements, these data suggest that 1 alpha,25(OH)(2)D(3) transrepresses mouse OPG by reducing the proportion of phospho-c-Jun in a JNK-independent manner. Our data indicated that short-term treatment with 1 alpha,25(OH)(2)D(3) effectively downregulated OPG expression both by accelerating the degradation of OPG mRNA and by transrepressing the OPG gene through its AP-1 binding site in the catabolic phase. The OPG gene became insensitive to 1 alpha,25(OH)(2)D(3) treatment, however, and reverted to its steady-state expression level over time, leading to the anabolic phase of the effect of 1 alpha,25(OH)(2)D(3) on bone.

High serum osteoprotegerin levels in patients with hyperthyroidism: effect of medical treatment

This study was aimed at evaluating serum osteoprotegerin (OPG) concentrations in a cohort of patients with hyperthyroidism before and after methimazole (MMI) treatment. One hundred fourteen hyperthyroid patients [93 with Graves disease (GD) and 21 with toxic nodular goitre (TNG)] and 68 matched for sex and age healthy subjects were evaluated for serum free-thyroxine (FT4), free-triiodiothyronine (FT3), thyrotropin (TSH), TSH receptor antibodies (TRAb), bone alkaline phosphatase (BALP), C-telopeptides of type-1 collagen (CrossLaps), OPG levels, and bone mineral density (BMD). In hyperthyroid patients, the biochemical evaluations were performed before and after 6 and 12 months of MMI treatment, whereas BMD was measured at baseline and after 12 months of treatment. Hyperthyroidism was more severe in GD than TNG patients. Serum OPG levels were found to be significantly higher in hyperthyroid patients than in the healthy subjects (4.3 pmol/l, range: 1.6-12.0, vs. 2.2 pmol/l, range: 1.4-6.0; P < 0.001), the values being higher in GD patients than TNG. A significant correlation between serum OPG levels and age was found in the healthy subjects (r: 0.48; P < 0.001) but not in hyperthyroid patients (r: -0.03; P = 0.8). In the healthy subjects, serum OPG levels were also positively correlated with both serum FT4 (r: 0.23; P = 0.03) and FT3 (r: 0.24; P = 0.04) levels. In hyperthyroid patients, however, serum OPG was still correlated with FT3 levels (r: 0.38; P < 0.001), whereas the correlation with serum FT4 was lost (r: 0.19; P = 0.06). In hyperthyroid patients, but not in the healthy subjects, serum OPG levels were correlated positively with CrossLaps (r: 0.20; P = 0.03) and negatively with BALP (r: -0.24; P = 0.01) and BMD (r: -0.33; P = 0.01). After 6 months of MMI treatment, serum OPG concentrations decreased significantly in TNG patients (from 3.5 pmol/l, range: 1.6-8.0, to 2.3 pmol/l, range: 1.0-4.3; P < 0.001), whereas a not significant change in OPG levels occurred in GD patients (from 4.8 pmol/l, range: 1.8-12.0, to 4.2 pmol/l, range: 1.0-14.0; P = 0.7). At Month 12 of treatment, serum OPG concentrations were significantly lower than those measured at baseline in both TNG (2.5 pmol/l, range: 1.0-3.1, vs. 3.5 pmol/l, range: 1.6-8.0; P < 0.001) and GD (2.1 pmol/l, range: 1.0-8.6, vs. 4.8 pmol/l, range: 1.8-12.0; P < 0.001). At this time, no significant differences in serum OPG, CrossLaps, and BALP values were found between patients and control subjects. At the end of follow-up, BMD was higher than those measured at baseline but still significantly lower than those measured in the control subjects. This study shows that hyperthyroid patients have serum OPG concentrations significantly higher in comparison with euthyroid subjects, in relation to thyroid hormone excess and high bone turnover. Medical treatment of hyperthyroidism normalizes serum OPG levels in temporal relationship with the normalization of bone metabolism markers, even in presence of persistent abnormal bone structure as determined by ultrasonography.

Increased expression of IL-6 and RANK mRNA in human trabecular bone from fragility fracture of the femoral neck

Previous studies have implicated pro-inflammatory cytokines in the bone loss of estrogen deficiency. The aim of this study was to investigate the expression of key regulatory molecules of bone remodeling in the trabecular bone microenvironment in osteoporosis. Bone samples were taken from the intertrochanteric region of the proximal femur of patients undergoing total hip arthroplasty for a subcapital fragility fracture of the femoral neck (#NOF). For comparison, samples were taken from age-matched control individuals at routine autopsy. Expression of RANKL, RANK, osteoprotegerin (OPG), IL-6, IL-11, osteocalcin (OCN), and calcitonin receptor (CTR) messenger RNA (mRNA) species were analyzed and the data were nonparametrically distributed. The median expression of the proresorptive genes, RANK and IL-6, were significantly elevated in the fracture group compared to an age-matched control group (2.2 [1.9-2.9; 25th-75th percentiles] > 1.0 [0.4-2.1], P < 0.03; 3.9 [1.8-6.2] > 0.8 [0.7-1.5], P < 0.002, respectively). In contrast, there were no significant differences in expression of RANKL, OPG, CTR, or OCN mRNA between the #NOF and control groups. The median RANKL/OPG mRNA ratio was significantly greater in hip fracture bone than in bone from controls (4.8 [3.8-7.6] > 3.2 [2.1-4.0], P < 0.05). IL-6 mRNA levels associated strongly with RANKL mRNA levels in the #NOF group (r = 0.77, P < 0.001), but not in the control group. A strong positive association was found between IL-11 mRNA levels and RANKL mRNA levels in the #NOF group (r = 0.81, P < 0.001), consistent with the apparent coordinated regulation of IL-6 and IL-11 in bone samples from the #NOF group (r = 0.93, P < 0.0001). These data suggest a relative increase in the expression of the molecular promoters of osteoclast formation and activity in #NOF bone, which may lead to the imbalance between bone formation and resorption associated with fragility fracture.

Osteoprotegerin in human milk: a potential role in the regulation of bone metabolism and immune development

Osteoprotegerin (OPG) is a member of the tumor necrosis factor superfamily. It is a soluble "decoy" receptor for tumor necrosis factor-related apoptosis-inducing ligand and ligand of the receptor activator of NF-kappaB. As such, OPG inhibits osteoclast activity and regulates the immune system. Human milk is a complex biologic fluid that supplies nutritional and protective factors to the breast-fed infant. In the present study, human milk samples at various times postpartum were assessed for the presence of OPG. Using biochemical as well as immunologic and biologic techniques we showed that human milk contains OPG at a level that is 1000-fold higher than that found in normal human serum. We observed that human breast milk cells and the human mammary epithelial cell line MCF-7 express OPG, indicating that both cell types are possible sources of milk OPG in vivo. In vitro studies demonstrated that milk OPG is biologically active and suggested that it may contribute to the antiresorptive activity of milk on bone, as well as tumor necrosis factor-related apoptosis-inducing ligand-induced inhibition of T cell proliferation. OPG-like activity was also observed in bovine colostrum and milk. Furthermore, we were able to detect human OPG in the sera of rats gavaged with human milk. We discuss the relevance of our findings for the breast-fed infant and for the prevention of immune and bone disorders.

Osteoprotegerin-Receptor Activator of Nuclear Factor-κB Ligand Ratio: A New Approach to Osteoporosis Treatment?

Osteoporosis, the most commonly occurring bone disease, is characterized by enhanced bone fragility and increased risk of fracture. Bone remodeling is the process in which bone is broken down by osteoclasts and then built back again by osteoblasts. In healthy adult bone, these two processes are balanced and a constant level of bone mass is maintained. Some of the proteins involved in the interaction between osteoblasts and osteoclasts have recently been identified. Receptor activator of nuclear factor-kappaB (RANK) ligand is produced by osteoblasts and exerts its effects through binding to its receptor (RANK) on osteoclast precursor cells. Binding results in activation of osteoclasts. Osteoblasts also produce osteoprotegerin (OPG), a potent inhibitor of osteoclast formation and a decoy receptor for RANK. The relative ratio of OPG and RANK ligand in the bone marrow microenvironment may determine the number of active osteoclasts, bone resorption rate, and bone mass. OPG is currently under investigation for osteoporosis treatment.

The role of osteoprotegerin and tumor necrosis factor-related apoptosis-inducing ligand in human microvascular endothelial cell survival

Endothelial cell survival and antiapoptotic pathways, including those stimulated by extracellular matrix, are critical regulators of vasculogenesis, angiogenesis, endothelial repair, and shear-stress-induced endothelial activation. One of these pathways is mediated by alpha(v)beta(3) integrin ligation, downstream activation of nuclear factor-kappaB, and subsequent up-regulation of osteoprotegerin (OPG). In this study, the mechanism by which OPG protects endothelial cells from death was examined. Serum-starved human microvascular endothelial cells (HMECs) plated on the alpha(v)beta(3) ligand osteopontin were protected from cell death. Immunoprecipitation experiments indicated that OPG formed a complex with tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) in HMECs under these conditions. Furthermore, inhibitors of TRAIL, including recombinant soluble TRAIL receptors and a neutralizing antibody against TRAIL, blocked apoptosis of serum-starved HMECs plated on the nonintegrin attachment factor poly-d-lysine. Whereas TRAIL was unable to induce apoptosis in HMECs plated on osteopontin, the addition of recombinant TRAIL did increase the percentage of apoptotic HMECs plated on poly-d-lysine. This evidence indicates that OPG blocks endothelial cell apoptosis through binding TRAIL and preventing its interaction with death-inducing TRAIL-receptors