Disruption of Osteoprotegerin has complex effects on medial destruction and adventitial fibrosis during mouse abdominal aortic aneurysm formation

Aortic aneurysm refers to dilatation of the aorta due to loss of elasticity and degenerative weakening of its wall. A preventive role for osteoprotegerin (Opg) in the development of abdominal aortic aneurysm has been reported in the CaCl₂-induced aneurysm model, whereas Opg was found to promote suprarenal aortic aneurysm in the AngII-induced ApoE knockout mouse aneurysm model. To determine whether there is a common underlying mechanism to explain the impact of Opg deficiency on the vascular structure of the two aneurysm models, we analyzed suprarenal aortic tissue of 6-month-old ApoE^-/-Opg^-/- mice after AngII infusion for 28 days. Less aortic dissection and aortic lumen dilatation, more adventitial thickening, and higher expression of collagen I and Trail were observed in ApoE^-/-Opg^-/- mice relative to ApoE^-/-Opg^+/+ mice. An accumulation of α-smooth muscle actin and vimentin double-positive myofibroblasts was noted in the thickened adventitia of ApoE^-/-Opg^-/- mice. Our results suggest that fibrotic remodeling of the aorta induced by myofibroblast accumulation might be an important pathological event which tends to limit AngII-induced aortic dilatation in ApoE^-/-Opg^-/- mice.

Regulation of Vascular Calcification by Growth Hormone-Releasing Hormone and Its Agonists

RATIONALE

Vascular calcification (VC) is a marker of the severity of atherosclerotic disease. Hormones play important roles in regulating calcification; estrogen and parathyroid hormones exert opposing effects, the former alleviating VC and the latter exacerbating it. To date no treatment strategies have been developed to regulate clinical VC.

OBJECTIVE

The objective of this study was to investigate the effect of growth hormone-releasing hormone (GHRH) and its agonist (GHRH-A) on the blocking of VC in a mouse model.

METHODS AND RESULTS

Young adult osteoprotegerin-deficient mice were given daily subcutaneous injections of GHRH-A (MR409) for 4 weeks. Significant reductions in calcification of the aortas of MR409-treated mice were paralleled by markedly lower alkaline phosphatase activity and a dramatic reduction in the expression of transcription factors, including the osteogenic marker gene Runx2 and its downstream factors, osteonectin and osteocalcin. The mechanism of action of GHRH-A was dissected in smooth muscle cells isolated from human and mouse aortas. Calcification of smooth muscle cells induced by osteogenic medium was inhibited in the presence of GHRH or MR409, as evidenced by reduced alkaline phosphatase activity and Runx2 expression. Inhibition of calcification by MR409 was partially reversed by MIA602, a GHRH antagonist, or a GHRH receptor-selective small interfering RNA. Treatment with MR409 induced elevated cytosolic cAMP and its target, protein kinase A which in turn blocked nicotinamide adenine dinucleotide phosphate oxidase activity and reduced production of reactive oxygen species, thus blocking the phosphorylation of nuclear factor κB (p65), a key intermediate in the ligand of receptor activator for nuclear factor-κ B-Runx2/alkaline phosphatase osteogenesis program. A protein kinase A-selective small interfering RNA or the chemical inhibitor H89 abolished these beneficial effects of MR409.

CONCLUSIONS

GHRH-A controls osteogenesis in smooth muscle cells by targeting cross talk between protein kinase A and nuclear factor κB (p65) and through the suppression of reactive oxygen species production that induces the Runx2 gene and alkaline phosphatase. Inflammation-mediated osteogenesis is thereby blocked. GHRH-A may represent a new pharmacological strategy to regulate VC.

Treatment of OPG-deficient mice with WP9QY, a RANKL-binding peptide, recovers alveolar bone loss by suppressing osteoclastogenesis and enhancing osteoblastogenesis

Osteoblasts express two key molecules for osteoclast differentiation, receptor activator of NF-κB ligand (RANKL) and osteoprotegerin (OPG), a soluble decoy receptor for RANKL. RANKL induces osteoclastogenesis, while OPG inhibits it by blocking the binding of RANKL to RANK, a cellular receptor of RANKL. OPG-deficient (OPG^–/–) mice exhibit severe alveolar bone loss with enhanced bone resorption. WP9QY (W9) peptide binds to RANKL and blocks RANKL-induced osteoclastogenesis. W9 is also reported to stimulate bone formation in vivo. Here, we show that treatment with W9 restores alveolar bone loss in OPG^–/–mice by suppressing osteoclastogenesis and enhancing osteoblastogenesis. Administration of W9 or risedronate, a bisphosphonate, to OPG^–/–mice significantly decreased the osteoclast number in the alveolar bone. Interestingly, treatment with W9, but not risedronate, enhanced Wnt/β-catenin signaling and induced alveolar bone formation in OPG^–/–mice. Expression of sclerostin, an inhibitor of Wnt/β-catenin signaling, was significantly lower in tibiae of OPG^–/–mice than in wild-type mice. Treatment with risedronate recovered sclerostin expression in OPG^–/–mice, while W9 treatment further suppressed sclerostin expression. Histomorphometric analysis confirmed that bone formation-related parameters in OPG^–/–mice, such as osteoblast number, osteoblast surface and osteoid surface, were increased by W9 administration but not by risedronate administration. These results suggest that treatment of OPG^–/–mice with W9 suppressed osteoclastogenesis by inhibiting RANKL signaling and enhanced osteoblastogenesis by attenuating sclerostin expression in the alveolar bone. Taken together, W9 may be a useful drug to prevent alveolar bone loss in periodontitis.

EPA Prevents the Development of Abdominal Aortic Aneurysms through Gpr-120/Ffar-4

Abdominal aortic aneurysms (AAAs), which commonly occur among elderly individuals, are accompanied by a risk of rupture with a high mortality rate. Although eicosapentaenoic acid (EPA) has been reported to prevent AAA formation, the mechanism by which EPA works on vascular smooth muscle cells is unknown. This study aimed to investigate the mechanism by which orally-administered EPA prevents the formation of severe AAAs that develop in Osteoprotegerin (Opg) knockout (KO) mice. In the CaCl₂-induced AAA model, EPA attenuated the enhanced progression of AAAs in Opg-KO mice, including the increase in aortic diameter with destruction of elastic fibers in the media. Immunohistochemical analyses showed that EPA reduced the phosphorylation of transforming growth factor beta-activated kinase-1/Map3k7 (Tak-1) and c-Jun NH2-terminal kinase (JNK), as well as the expression of Matrix metalloproteinase-9 (Mmp-9) in the media of the aorta. In smooth muscle cell cultures, rh-TRAIL-induced activation of the Tak-1-JNK pathway and increase in Mmp-9 expression were inhibited by EPA. Moreover, GW9508, a specific ligand for G-protein coupled receptor (Gpr)-120/Free fatty acid receptor (Ffar)-4, mimicked the effects of EPA. The effects of EPA were abrogated by knockdown of the Gpr-120/Ffar-4 receptor gene. Our data demonstrate that the Trail-Tak-1-JNK-Mmp-9 pathway is responsible for the enhancement of AAAs in Opg-KO mice, and that EPA inhibits the Tak-1-JNK pathway by activating Gpr-120/Ffar-4, which results in the attenuation of AAA development.

Denosumab treatment for juvenile Paget's disease: results from two adult patients with osteoprotegerin deficiency ("Balkan" mutation in the TNFRSF11B gene)

CONTEXT

Most patients with juvenile Paget's disease (JPD) have homozygous loss-of-function mutations in the TNFRSF11B gene resulting in osteoprotegerin deficiency. Because recombinant osteoprotegerin is not available for clinical use, an alternative therapeutic approach could be denosumab, which acts on the same pathway.

MAIN OBJECTIVE

The aim was to study the effect of denosumab on bone turnover markers in two adult patients with JPD ("Balkan" mutation) previously treated with calcitonin and bisphosphonates.

SETTING

The study was conducted at two tertiary hospitals in Greece.

PATIENTS

Patient 1 (a 36-year-old woman) developed a severe and long-term hypocalcemia after a single dose (3.5 mg) of zoledronic acid. Her bone disease remained active despite treatment. Patient 2 (a 67-year-old man) had satisfactorily controlled bone disease with only intermittent risedronate treatment during the last 10 years, but suffered from progressive loss of hearing and vision. Low doses (20-40 mg) of denosumab every 3-6 months were administered in both patients.

RESULTS

Bone markers (including total and bone-specific alkaline phosphatase, procollagen I N-terminal peptide, and osteocalcin) were reduced to normal levels in both patients, with nadir observed 2-4 months after each denosumab injection. Retinal and hearing involvement remained unchanged, but patient 2 developed a rapid progression of cataract in the right eye.

CONCLUSIONS

Low-dose denosumab every 3-6 months for about 2 years in two patients with JPD successfully controlled their bone disease. The long-term effect of denosumab on the nonskeletal complications remains to be elucidated.

The beneficial effect of Icariin on bone is diminished in osteoprotegerin-deficient mice

BACKGROUND

Osteoprogeterin (OPG) plays an important role in regulating bone homeostasis by inhibiting osteoclastogenesis and bone resorption. Icariin is the major ingredient of Herba Epimedii, which exerts anabolic and anti-resorptive effects on bone, but the mechanism remains unknown. In this study, we evaluated the role of OPG in Icariin-mediated beneficial effects on bone.

MATERIALS AND METHODS

Twelve-week-old Opg knockout (KO) male mice and their wild type (WT) littermates were orally administered with Icariin (0.3 mg/g) everyday for 8 weeks. Bone mass and microstructure in the right proximal tibiae were analyzed with micro-computed tomography (μCT). Bone remodeling was evaluated with serum biochemical analyses and bone histomorphometry. The colonies of fibroblast and osteoblast from bone marrow derived cells were quantified. The mRNA expressions of osteoblast and osteoclast related genes in trabecular bone from the femora were analyzed by real-time PCR.

RESULTS

Icariin treatment led to greater trabecular bone volume and trabecular number compared with vehicle treatment in WT mice. Icariin treatment increased bone formation parameters while it decreased bone resorption parameters in WT mice; however, the anabolic response of trabecular bone to Icariin treatment was diminished in KO mice. At cellular and molecular levels, Icariin significantly increased the formation of osteoblast colonies from bone marrow derived cells and the Opg gene expression in trabecular bone of WT mice.

CONCLUSIONS

These data suggest that Icariin treatment exerted anabolic and anti-resorptive effects on trabecular bone of WT mice, in which the effects were diminished in KO mice. The effects of Icariin treatment on bone are dependent on up-regulation of Opg, therefore, OPG plays an essential role in Icariin-mediated beneficial effects on trabecular bone.

[Animal models for bone and joint disease. Bone disease of osteoprotegerin deficient mouse]

Osteoprotegerin (OPG) acts as a decoy receptor for the receptor activator of the nuclear factor κ B (RANK) ligand (RANKL) , preventing its association with RANK and inhibiting osteoclastogenesis. Therefore, mice homozygous for targeted disruption of the OPG gene reveal stimulated bone resorption and bone formation, resulting in enhanced bone remodeling. The OPG deficient (OPG( - / - )) mouse showed the diturbed distribution of collagen fibers and complex meshwork of cement lines, which implies weakened strength of OPG( - / - ) bone against mechanical stress. In addition, the abnormally promoted remodeling of the OPG( - / - ) bone caused the disorganized distribution of osteocyte lacunar canalicular system (OLCS) . Histochemical assessment revealed the markedly reduced synthesis of sclerostin in the OPG( - / - ) OLCS while the synthesis of dentin matrix protein-1 was not extremely affected by the OPG deficiency. Taken together, OPG deficient mouse appears to be a valid model for extremely-stimulated bone remodeling, and would provided important clues for better understanding for activities of bone cells in a pathological state in bone.

Osteogenic growth peptide enhances the proliferation of bone marrow mesenchymal stem cells from osteoprotegerin-deficient mice by CDK2/cyclin A

To promote bone formation is one of the fundamental strategies in osteoporosis treatment and fractures repair. As one of the stimulators on bone formation, osteogenic growth peptide (OGP) increases both proliferation and differentiation of the osteoblasts in vitro and in vivo, in which osteoprotegerin (OPG) has been suggested being involved. In this study, we evaluated the effects of OGP on bone marrow mesenchymal stem cells (MSCs) from OPG-deficient mice in vitro by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, alkaline phosphatase (ALP) activity assay, real-time polymerase chain reaction, and western blot analysis. Results showed that OGP stimulated MSC proliferation and increased the expression of CDK2 and cyclin A in MSCs both at mRNA and protein levels. However, no differentiative effect of OGP was shown as ALP activity and the expression levels of Runx2 and Osterix were not increased significantly by OGP. Our study suggested that OGP may increase the bone formation in OPG-deficient mice by stimulating MSC proliferation rather than differentiation, and probably by triggering CDK2/cyclin A pathway.

[Animal models for vascular calcification]

Analysis of animal models is indispensable to elucidate the molecular mechanism in vascular calcification (VC) as well as to develop new therapies for VC. Various gene-modified mice that show VC have been reported, and considerable progress has been made through the analyses of these animals. Mice of which bone-calcification regulatory factors were modified are the representative animal models for VC, indicating that these factors certainly regulate VC as well as bone-calcification. Inducible VC in wild-type animals is also an important research tool for developing preventive and therapeutic approach for VC.

Accelerated cartilage resorption by chondroclasts during bone fracture healing in osteoprotegerin-deficient mice

Receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG), a decoy receptor of RANKL, maintain bone mass by regulating the differentiation of osteoclasts, which are bone-resorbing cells. Endochondral bone ossification and bone fracture healing involve cartilage resorption, a less well-understood process that is needed for replacement of cartilage by bone. Here we describe the role of OPG produced by chondrocytes in chondroclastogenesis. Fracture healing in OPG(-/-) mice showed faster union of the fractured bone, faster resorption of the cartilaginous callus, and an increased number of chondroclasts at the chondroosseous junctions compared with that in wild-type littermates. When a cultured pellet of OPG(-/-) chondrocytes was transplanted beneath the kidney capsule, the pellet recruited many chondroclasts. The pellet showed the ability to induce tartrate-resistant acid phosphatase-positive multinucleated cells from RAW 264.7 cells in vitro. Finally, OPG(-/-) chondrocytes (but not wild-type chondrocytes) cultured with spleen cells induced many tartrate-resistant acid phosphatase-positive multinucleated cells. The expression of RANKL and OPG in chondrocytes was regulated by several osteotropic factors including 1,25-dihydroxyvitamin D(3), PTHrP, IL-1alpha, and TNF-alpha. Thus, local OPG produced by chondrocytes probably controls cartilage resorption as a negative regulator for chondrocyte-dependent chondroclastogenesis.

[Osteoclastogenesis and bone resorption]

Osteoclasts, the multinucleated cells that resorb bone, originate from monocyte-macrophage lineage cells. Various hormones, cytokines and growth factors are involved in osteoclastogenesis, via interaction with osteoblasts. Deficiency of osteoprotegerin (OPG), a soluble decoy receptor for receptor activator of NF-kappa B ligand (RANKL), in mice induces osteoporosis caused by enhanced bone resorption but also accelerates bone formation. OPG-deficient mice exhibite high serum alkaline phosphatase activity and osteocalcin concentration, both of which are decreased to the levels of wild-type mice by the bisphosphonate injection. This suggests that bone formation is coupled with bone resorption in vivo. RANKL expressed by osteoblasts is a requirement for osteoclastogenesis, osteoblasts also play important roles in osteoclastogenesis through offering the critical microenvironment for the action of RANKL.

Zoledronate reverses mandibular bone loss in osteoprotegerin-deficient mice

To characterize the changes in osteoprotegerin-deficient (OPG-/-) mice mandibles and the possible mandibular bone loss prevention by zoledronate. This preventive effect in the mandible differed from that in the proximal tibia and was independent of the OPG pathway.

INTRODUCTION

The study aimed to characterize both the changes in the mandible in osteoprotegerin-deficient (OPG-/-) mice and possible mandibular bone loss prevention by zoledronate.

METHODS

Twenty-eight 6-week-old female mice (C57BL/6J), including OPG-/- (n = 21) and wild-type (WT) (n = 7) mice, were assigned to four groups after 2 weeks of acclimatization to local vivarium conditions: wild mice with vehicle (WT group); OPG-/- mice with vehicle (OPG-/- group); and OPG-/- mice that were subcutaneously injected with either 50 or 150 microg/kg zoledronate (Zol-50 and Zol-150 groups, respectively). Mice were sacrificed at 4 weeks after these treatments and after fasting for 12 h. Sera were harvested for biochemical analyses. The right mandible and tibia of each mouse were selected for microCT analysis. Student's t-test was performed for comparisons of bone parameters at different sites in the WT group. Analysis of variance (ANOVA) was used to compare the biomarkers and bone parameters in the different treatment groups.

RESULTS

Serum bone-specific alkaline phosphatase (B-ALP) and tartrate-resistant acid phosphatase 5b (TRACP-5b) were significantly decreased in WT mice as compared to the levels in the OPG-/- mice (P < 0.05). Zoledronate treatment decreased the high serum B-ALP activity observed in OPG-/- mice to the levels seen in WT mice, while serum TRACP-5b concentrations were decreased to levels even lower than those in WT mice. There were substantial variations in BMD and microstructure of the mandibular and proximal tibial trabeculae. Mandibular bone loss was less affected by OPG gene deprivation than the proximal tibia was. Both zoledronate groups showed greater BMD, trabecular BV/TV, Tb.Th, Tb.N, and Conn.D and a significant decrease in Tb.Sp and SMI as compared to the findings in OPG-/- mice (P < 0.05). However, higher apparent BMD and more compact plate-like trabeculae were observed in the mandible after treatment with zoledronate as compared to the findings in the proximal tibia. No significant differences were found in any parameter in both zoledronate groups.

CONCLUSIONS

The present study showed that zoledronate could reverse the significant bone loss in mice mandibles that was induced by OPG gene deficiency. This preventive effect, which was accompanied with considerable inhibition of bone turnover, differed in the mandible and in the proximal tibia and was independent of the OPG pathway.

Dexamethasone promotes osteoclastogenesis by inhibiting osteoprotegerin through multiple levels

Increased bone fragility attributed to osteopenia is a serious side effect of glucocorticoid treatment. Glucocorticoid-induced bone loss is caused primarily by hypofunction and apoptosis of osteoblasts, and secondarily by accelerated bone resorption. To explore the mechanism whereby dexamethasone (Dex) stimulates osteoclastogenesis in the coculture system, we analyzed the effect of Dex on the expression of both mouse osteoprotegerin (OPG) and receptor activator of NF-kappaB ligand (RANKL). Dex reduced OPG transcripts and OPG protein secretion by the ST2 osteoblastic cells. Since mainly the c-Jun homodimer maintains the steady-state transcription of the OPG gene, we examined the effect of Dex on c-Jun signaling in ST2 cells. Western blotting disclosed that Dex decreased the amount of phospho-c-Jun protein (p-c-Jun) and, correspondingly, the amount of the phosphorylated p46 isoform of Jun N-terminal kinase (JNK). The amount of phospho-SEK1 also decreased after Dex treatment, while the amounts of phospho-ERK and p38 remained constant. Among mitogen-activated protein (MAP) kinase inhibitors, the JNK inhibitor mimicked the inhibitory effect of Dex on OPG promoter activity. On the other hand, Dex treatment per se showed a nominal increase of RANKL gene expression. A part of Dex-mediated OPG gene suppression was achieved by the suppression of beta-catenin signaling. We speculate therefore that the bone resorptive action of Dex is mediated mainly by the inhibition of OPG by transrepressing the OPG gene through the AP-1 site, with a reduction (mediated mainly by the decrease in the p46 isoform of JNK) in the proportion of p-c-Jun in a JNK-dependent manner.

A histological assessment on the distribution of the osteocytic lacunar canalicular system using silver staining

Giving the complexity that characterizes the mechanisms of bone remodeling and the number of events that have to be in absolute harmony for it to occur flawlessly, the postulation that temporospatial distribution of osteocytes and their lacunar canalicular system might influence and be influenced by bone remodeling can be regarded, at least, as feasible. In this study, using Schoen's silver staining, we have examined the distribution of the osteocytic lacunar canalicular system (OLCS) in bones of developing mice. Trabecular bones of 3-day-old, 2-week-old, and 3-week-old mice displayed osteocytic cytoplasmic processes without any perceptible alignment. Also, many plump osteocytes were embedded in the mineralized bone matrix in a disorderly manner. At 4 weeks of age, however, mice bones showed some osteocytic processes that reached the bone surface on a right angle, while other osteocytes displayed the same features seen on 3-week specimens. Samples at 8 weeks of age featured osteocytes with their usual spindle shape, organized so as to parallel the longitudinal axis of trabecular bone. They also extended their cytoplasmic processes perpendicularly to the bone surface. However, several osteocytes immersed in older bone, i.e., a residual mix of cartilage and bone matrices, still showed a random pattern of distribution of their cytoplasmic processes. Up to 12 weeks of age, the majority of the osteocytes became flattened and were shown to be aligned with their long axis paralleling the bone surface. This tendency for such a gradual arrangement was also observed in cortical bones. We have further demonstrated that 8-week-old osteoprotegerin-deficient mice, which demonstrated histological evidence of higher than average bone turnover, revealed a disorganized OLCS. Given the data gathered in this work, the OLCS appears to assume an organized, probably function-related spatial distribution as normal bone remodeling goes on.

Urinary gamma-glutamyltransferase (GGT) as a potential marker of bone resorption

We recently identified gamma-glutamyltransferase (GGT) as a novel bone-resorbing factor. The present study was undertaken to determine whether GGT is a marker of bone resorption in two genetic models of hyper- and hypo-function of osteoclasts, as well as in postmenopausal women with accelerated bone resorption, using type I collagen N-telopeptide (NTX) and deoxypyridinoline (DPD) as established biochemical markers. Urinary excretion of GGT, corrected for creatinine, was found to be increased in osteoprotegerin (OPG)-deficient osteoporotic mice as well as in patients with postmenopausal osteoporosis (67-83 years of age); in both cases the urinary level decreased after treatment of patients or mice with alendronate, a selective inhibitor of bone resorption, concomitantly with a reduction in DPD and NTX. Conversely, in osteopetrotic op/op mice, urinary GGT increased in parallel with DPD after induction of osteoclasts with M-CSF injection. Constant infusion of parathyroid hormone (PTH) also increased urinary GGT along with DPD. In a survey of 551 postmenopausal women (50-89 years of age) at their regular health checkup, urinary GGT excretion exhibited a high correlation with DPD (rho = 0.49, p < 0.0001). The calculated sensitivity and specificity for diagnosing elevated bone resorption, as determined by a DPD value higher than 7.6 nM/mM Cr, were 61% and 92%, respectively, when a cut-off value of 40 IU/g Cr was assigned for urinary GGT. Since GGT activity can be measured inexpensively in large numbers in a very short time, the measurement of urinary level may provide a convenient and useful method for mass screening to identify those with increased bone turnover and hence at increased risk for bone fracture.