The TNF receptor superfamily: role in immune inflammation and bone formation

RANK, RANKL, and OPG in Dentigerous Cyst, Odontogenic Keratocyst, and Ameloblastoma: A Meta-Analysis

The aim of this study was to assess and compare RANK, RANKL, and OPG immunoexpression in dentigerous cyst, odontogenic keratocyst, and ameloblastoma. The protocol was registered in PROSPERO (CRD42018105543). Seven databases (Embase, Lilacs, LIVIVO, PubMed, Scopus, SciELO, and Web of Science) were the primary search sources and two databases (Open Grey and Open Thesis) partially captured the "grey literature". Only cross sectional studies were included. The JBI Checklist assessed the risk of bias. A meta-analysis with random effects model estimated the values from the OPG and RANKL ratio reported by the individual studies and respective 95% confidence intervals. The heterogeneity among studies was assessed with I2 statistics. Only nine studies met the inclusion criteria and were considered in the analyses. The studies were published from 2008 to 2018. Two studies presented low risk of bias, while seven studies presented moderate risk. The meta-analysis showed the highest OPG>RANKL ratio for dentigerous cyst (ES=43.3%; 95% CI=14.3-74.8) and odontogenic keratocyst (ES=36.8%; 95% CI=18.8-56.7). In contrast, the highest OPG<RANKL ratio was found for ameloblastoma (ES=73.4%; 95% CI=55.4-88.4) and it was higher in the stromal region compared to the odontogenic epithelial region. The results may explain the aggressive potential of ameloblastoma from the higher OPG<RANKL ratio in this tumor, while it was lower for dentigerous cyst and odontogenic keratocyst.

Extracellular Vesicle-Mediated Bone Remodeling and Bone Metastasis: Implications in Prostate Cancer

Bone metastasis is the tendency of certain primary tumors to spawn and dictate secondary neoplasia in the bone. The process of bone metastasis is regulated by the dynamic crosstalk between metastatic cancer cells, cellular components of the bone marrow microenvironment (osteoblasts, osteoclasts, and osteocytes), and the bone matrix. The feed-forward loop mechanisms governs the co-option of homeostatic bone remodeling by cancer cells in bone. Recent developments have highlighted the discovery of extracellular vesicles (EVs) and their diverse roles in distant outgrowths. Several studies have implicated EV-mediated interactions between cancer cells and the bone microenvironment in synergistically promoting pathological skeletal metabolism in the metastatic site. Nevertheless, the potential role that EVs serve in arbitrating intricate sequences of coordinated events within the bone microenvironment remains an emerging field. In this chapter, we review the role of cellular participants and molecular mechanisms in regulating normal bone physiology and explore the progress of current research into bone-derived EVs in directly triggering and coordinating the processes of physiological bone remodeling. In view of the emerging role of EVs in interorgan crosstalk, this review also highlights the multiple systemic pathophysiological processes orchestrated by the EVs to direct organotropism in bone in prostate cancer. Given the deleterious consequences of bone metastasis and its clinical importance, in-depth knowledge of the multifarious role of EVs in distant organ metastasis is expected to open new possibilities for prognostic evaluation and therapeutic intervention for advanced bone metastatic prostate cancer.

Lipopolysaccharide- TLR-4 Axis regulates Osteoclastogenesis independent of RANKL/RANK signaling

BACKGROUND

Lipopolysaccharide (LPS) is an endotoxin and a vital component of gram-negative bacteria's outer membrane. During gram-negative bacterial sepsis, LPS regulates osteoclast differentiation and activity, in addition to increasing inflammation. This study aimed to investigate how LPS regulates osteoclast differentiation of RAW 264.7 cells in vitro.

RESULTS

Herein, we revealed that RAW cells failed to differentiate into mature osteoclasts in vitro in the presence of LPS. However, differentiation occurred in cells primed with receptor activator of nuclear factor-kappa-Β ligand (RANKL) for 24 h and then treated with LPS for 48 h (henceforth, denoted as LPS-treated cells). In cells treated with either RANKL or LPS, an increase in membrane levels of toll-like receptor 4 (TLR4) receptor was observed. Mechanistically, an inhibitor of TLR4 (TAK-242) reduced the number of osteoclasts as well as the secretion of tumor necrosis factor (TNF)-α in LPS-treated cells. RANKL-induced RAW cells secreted a very basal level TNF-α. TAK-242 did not affect RANKL-induced osteoclastogenesis. Increased osteoclast differentiation in LPS-treated osteoclasts was not associated with the RANKL/RANK/OPG axis but connected with the LPS/TLR4/TNF-α tumor necrosis factor receptor (TNFR)-2 axis. We postulate that this is because TAK-242 and a TNF-α antibody suppress osteoclast differentiation. Furthermore, an antibody against TNF-α reduced membrane levels of TNFR-2. Secreted TNF-α appears to function as an autocrine/ paracrine factor in the induction of osteoclastogenesis independent of RANKL.

CONCLUSION

TNF-α secreted via LPS/TLR4 signaling regulates osteoclastogenesis in macrophages primed with RANKL and then treated with LPS. Our findings suggest that TLR4/TNF-α might be a potential target to suppress bone loss associated with inflammatory bone diseases, including periodontitis, rheumatoid arthritis, and osteoporosis.

Intranuclear Delivery of Nuclear Factor-Kappa B p65 in a Rat Model of Tooth Replantation

After avulsion and replantation, teeth are at risk of bone and root resorption. The present study aimed to demonstrate that the intra-nuclear transducible form of transcription modulation domain of p65 (nt-p65-TMD) can suppress osteoclast differentiation in vitro, and reduce bone resorption in a rat model of tooth replantation. Cell viability and nitric oxide release were evaluated in RAW264.7 cells using CCK-8 assay and Griess reaction kit. Osteoclast differentiation was evaluated using quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and tartrate-resistant acid phosphatase (TRAP) staining. Thirty-two maxillary rat molars were extracted and stored in saline (n = 10) or 10 µM nt-p65-TMD solution (n = 22) before replantation. After 4 weeks, specimens were scored according to the inflammatory pattern using micro-computed tomography (CT) imaging and histological analyses. nt-p65-TMD treatment resulted in significant reduction of nitric oxide release and osteoclast differentiation as studied using PCR and TRAP staining. Further, micro-CT analysis revealed a significant decrease in bone resorption in the nt-p65-TMD treatment group (p < 0.05). Histological analysis of nt-p65-TMD treatment group showed that not only bone and root resorption, but also inflammation of the periodontal ligament and epithelial insertion was significantly reduced. These findings suggest that nt-p65-TMD has the unique capabilities of regulating bone remodeling after tooth replantation.

TRAIL inhibits RANK signaling and suppresses osteoclast activation via inhibiting lipid raft assembly and TRAF6 recruitment

Human osteoclast formation from mononuclear phagocyte precursors involves interactions between members of the tumor necrosis factor (TNF) ligand superfamily and their receptors. Recent evidence indicated that TNF-α-related apoptosis-inducing ligand (TRAIL) induces osteoclast differentiation via a TRAF6-dependent signaling pathway; but paradoxically, it inhibits RANK ligand (RANKL)-induced osteoclast differentiation. Although a number of signaling pathways were linked to the RANK and osteoclastogenesis, it is not known how TRAIL regulates RANK signaling. In this study, we demonstrate that TRAIL regulates RANK-induced osteoclastogenesis in terms of the assembly of lipid raft-associated signaling complexes. RANKL stimulation induced recruitment of TRAF6, c-Src, and DAP-12 into lipid rafts. However, the RANKL-induced assembly of lipid raft-associated signaling complexes and TRAF6 recruitment was abolished in the presence of TRAIL. TRAIL-induced dissociation of RANKL-induced lipid raft signaling complexes was reversed by treatment with TRAIL receptor (TRAIL-R) siRNA or an anti-TRAIL-R blocking antibody, indicating that TRAIL mediates suppression of RANKL-induced lipid raft signaling via interactions with TRAIL-R. Finally, we demonstrated that TRAIL suppressed inflammation-induced bone resorption and osteoclastogenesis in a collagen-induced arthritis (CIA) rat animal model. Our results provide a novel apoptosis-independent role of TRAIL in regulating RANK signaling and suppresses osteoclast activation via inhibiting lipid raft assembly and TRAF6 recruitment.

Identification of a six-gene signature with prognostic value for patients with endometrial carcinoma

Uterine corpus endometrial carcinoma (UCEC) is frequently diagnosed among women worldwide. However, there are different prognostic outcomes because of heterogeneity. Thus, the aim of the current study was to identify a gene signature that can predict the prognosis of patients with UCEC. UCEC gene expression profiles were first downloaded from the The Cancer Genome Atlas (TCGA) database. After data processing and forward screening, 11 390 key genes were selected. The UCEC samples were randomly divided into training and testing sets. In total, 996 genes with prognostic value were then examined by univariate Cox survival analysis with a P-value <0.01 in the training set. Next, using robust likelihood-based survival modeling, we developed a six-gene signature (CTSW, PCSK4, LRRC8D, TNFRSF18, IHH, and CDKN2A) with a prognostic function in UCEC. A prognostic risk score system was developed by multivariate Cox proportional hazard regression based on this six-gene signature. According to the Kaplan-Meier curve, patients in the high-risk group had significantly poorer overall survival (OS) outcomes than those in the low-risk group (log-rank test P-value <0.0001). This signature was further validated in the testing dataset and the entire TCGA dataset. In conclusion, we conducted an integrated study to develop a six-gene signature for the prognostic prediction of patients with UCEC. Our findings may provide novel biomarkers for prognosis and have significant implications in the understanding of therapeutic targets for UCEC.

Current Understanding of RANK Signaling in Osteoclast Differentiation and Maturation

Osteoclasts are bone-resorbing cells that are derived from hematopoietic precursor cells and require macrophage-colony stimulating factor and receptor activator of nuclear factor-κB ligand (RANKL) for their survival, proliferation, differentiation, and activation. The binding of RANKL to its receptor RANK triggers osteoclast precursors to differentiate into osteoclasts. This process depends on RANKL-RANK signaling, which is temporally regulated by various adaptor proteins and kinases. Here we summarize the current understanding of the mechanisms that regulate RANK signaling during osteoclastogenesis. In the early stage, RANK signaling is mediated by recruiting adaptor molecules such as tumor necrosis factor receptor-associated factor 6 (TRAF6), which leads to the activation of mitogen-activated protein kinases (MAPKs), and the transcription factors nuclear factor-κB (NF-κB) and activator protein-1 (AP-1). Activated NF-κB induces the nuclear factor of activated T-cells cytoplasmic 1 (NFATc1), which is the key osteoclastogenesis regulator. In the intermediate stage of signaling, the co-stimulatory signal induces Ca2+ oscillation via activated phospholipase Cγ2 (PLCγ2) together with c-Fos/AP-1, wherein Ca2+ signaling facilitates the robust production of NFATc1. In the late stage of osteoclastogenesis, NFATc1 translocates into the nucleus where it induces numerous osteoclast-specific target genes that are responsible for cell fusion and function.

Structural studies of death receptors

This chapter describes reports of the structural characterization of death ligands and death receptors (DRs) from the tumor necrosis factor (TNF) and TNF receptor families. The review discusses the interactions of these proteins with agonist ligands, inhibitors, and downstream signaling molecules. Though historically labeled as being implicated in programmed cell death, the function of these proteins extends to nonapoptotic pathways. The review highlights, from a structural biology perspective, the complexity of DR signaling and the ongoing challenge to discern the precise mechanisms that occur at the point of DR activation, including how the degree to which the receptors are induced to cluster may be related to the nature of the impact upon the cell. The potential for posttranslational modification and receptor internalization to play roles in DR signaling is briefly discussed.

Early Atherosclerosis in HIV Infected Subjects on Suppressive Antiretroviral Treatment: Role of Osteoprotegerin

Cardiovascular disease is increased in HIV-infected patients. Cytokines such as osteoprotegerin are implicated in atherosclerosis. The aim of our study was to evaluate the role of osteoprotegerin in the development and progression of atherosclerosis in HIV infected subjects on suppressive antiretroviral treatment. We enrolled 76 patients; 35 HIV infected men on suppressive Highly Active Antiretroviral Therapy with Framingham score <10%; 21 HIV negative individuals matched for age, gender, and Framingham score, and 20 subjects with Framingham score >10% as control groups. HIV positive subjects underwent echocardiography, electrocardiography, and heart multidetector computed tomography, whereas in HIV negative subjects, tomography was only performed in case of any abnormalities either in echocardiography or electrocardiography. In HIV positive patients, computed tomography showed stenosis in 51.4% of the subjects. Osteoprotegerin plasma levels were higher in HIV-infected patients than those in healthy controls but lower than in HIV negative subjects with Framingham score >10%. Higher osteoprotegerin plasma levels were found in HIV positive patients with grade I stenosis than in patients with grade II/III stenosis. In conclusion, in HIV infected subjects with Framingham score <10%, osteoprotegerin plasma concentrations are associated with atherosclerosis, in particular at the early stage of the process.

Novel tumor suppressor function of glucocorticoid-induced TNF receptor GITR in multiple myeloma

Glucocorticoid-induced TNF receptor (GITR) plays a crucial role in modulating immune response and inflammation, however the role of GITR in human cancers is poorly understood. In this study, we demonstrated that GITR is inactivated during tumor progression in Multiple Myeloma (MM) through promoter CpG island methylation, mediating gene silencing in primary MM plasma cells and MM cell lines. Restoration of GITR expression in GITR deficient MM cells led to inhibition of MM proliferation in vitro and in vivo and induction of apoptosis. These findings were supported by the presence of induction of p21 and PUMA, two direct downstream targets of p53, together with modulation of NF-κB in GITR-overexpressing MM cells. Moreover, the unbalanced expression of GITR in clonal plasma cells correlated with MM disease progression, poor prognosis and survival. These findings provide novel insights into the pivotal role of GITR in MM pathogenesis and disease progression.

TRAF-6 dependent signaling pathway is essential for TNF-related apoptosis-inducing ligand (TRAIL) induces osteoclast differentiation

Human osteoclast formation from mononuclear phagocyte precursors involves interactions between tumor necrosis factor (TNF) ligand superfamily members and their receptors. Recent evidence indicates that in addition to triggering apoptosis, the TNF-related apoptosis-inducing ligand (TRAIL) induces osteoclast differentiation. To understand TRAIL-mediated signal transduction mechanism in osteoclastogenesis, we demonstrated that TRAIL induces osteoclast differentiation via a Tumor necrosis factor receptor-associated factor 6 (TRAF-6)-dependent signaling pathway. TRAIL-induced osteoclast differentiation was significantly inhibited by treatment with TRAF-6 siRNA and TRAF6 decoy peptides in both human monocytes and murine RAW264.7 macrophage cell lines, as evaluated in terms of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells and bone resorption activity. Moreover, TRAIL-induced osteoclast differentiation was also abolished in TRAF6 knockout bone marrow macrophages. In addition to induction of NFATc1, treatment of TRAIL also induced ubiquitination of TRAF6 in osteoclast differentiation. Thus, our data demonstrate that TRAIL induces osteoclastic differentiation via a TRAF-6 dependent signaling pathway. This study suggests TRAF6-dependent signaling may be a central pathway in osteoclast differentiation, and that TNF superfamily molecules other than RANKL may modify RANK signaling by interaction with TRAF6-associated signaling.

MAPK usage in periodontal disease progression

In periodontal disease, host recognition of bacterial constituents, including lipopolysaccharide (LPS), induces p38 MAPK activation and subsequent inflammatory cytokine expression, favoring osteoclastogenesis and increased net bone resorption in the local periodontal environment. In this paper, we discuss evidence that the p38/MAPK-activated protein kinase-2 (MK2) signaling axis is needed for periodontal disease progression: an orally administered p38α inhibitor reduced the progression of experimental periodontal bone loss by reducing inflammation and cytokine expression. Subsequently, the significance of p38 signaling was confirmed with RNA interference to attenuate MK2-reduced cytokine expression and LPS-induced alveolar bone loss. MAPK phosphatase-1 (MKP-1), a negative regulator of MAPK activation, was also critical for periodontal disease progression. In MPK-1-deficient mice, p38-sustained activation increased osteoclast formation and bone loss, whereas MKP-1 overexpression dampened p38 signaling and subsequent cytokine expression. Finally, overexpression of the p38/MK2 target RNA-binding tristetraprolin (TTP) decreased mRNA stability of key inflammatory cytokines at the posttranscriptional level, thereby protecting against periodontal inflammation. Collectively, these studies highlight the importance of p38 MAPK signaling in immune cytokine production and periodontal disease progression.

Phospholipase Cγ2 is required for basal but not oestrogen deficiency-induced bone resorption

BACKGROUND

Osteoclasts play a critical role in bone resorption under basal conditions, but they also contribute to pathological bone loss during diseases including postmenopausal osteoporosis. Phospholipase Cγ2 (PLCγ2) is an important signalling molecule in diverse haematopoietic lineages. Here, we tested the role of PLCγ2 in basal and ovariectomy-induced bone resorption, as well as in in vitro osteoclast cultures using PLCγ2-deficient (PLCγ2(-/-) ) mice.

MATERIALS AND METHODS

The trabecular architecture of long bone metaphyses was tested by micro-CT and histomorphometric analyses. Postmenopausal osteoporosis was modelled by surgical ovariectomy. Osteoclast development and function, gene expression and PLCγ2 phosphorylation were tested on in vitro osteoclast and macrophage cultures.

RESULTS

  PLCγ2(-/-) mice had significantly higher trabecular bone mass under basal conditions than wild-type mice. PLCγ2 was required for in vitro development and resorptive function of osteoclasts, but not for upregulation of osteoclast-specific gene expression. PLCγ2 was phosphorylated in a Src-family-dependent manner upon macrophage adhesion but not upon stimulation by M-CSF or RANKL. Surprisingly, ovariectomy-induced bone resorption in PLCγ2(-/-) mice was similar to, or even more robust than, that in wild-type animals.

CONCLUSIONS

Our results indicate that PLCγ2 participates in bone resorption under basal conditions, likely because of its role in adhesion receptor signalling during osteoclast development. In contrast, PLCγ2 does not appear to play a major role in ovariectomy-induced bone loss. These results suggest that basal and oestrogen deficiency-induced bone resorption utilizes different signalling pathways and that PLCγ2 may not be a suitable therapeutic target in postmenopausal osteoporosis.

Increased osteopontin plasma levels in multiple sclerosis patients correlate with bone-specific markers

The pro-inflammatory cytokine osteopontin has been found to be highly expressed in multiple sclerosis lesions and plasma levels are increased during relapses in relapse-onset multiple sclerosis patients.

The objective was to determine the relationship between osteopontin plasma and cerebrospinal fluid levels in relation to the immunoglobulin G index. In addition, osteopontin plasma levels were compared with osteopontin mRNA levels in peripheral blood mononuclear cells and bone-specific markers to analyse whether osteopontin may be peripherally produced.

Osteopontin and bone-specific markers were determined in paired plasma—cerebrospinal fluid samples and serum samples of relapse-onset multiple sclerosis patients ( n = 36), respectively. Osteopontin mRNA levels were determined by quantitative polymerase chain reaction analysis.

Compared to healthy controls ( n = 20), plasma osteopontin levels were significantly increased in relapsing-remitting multiple sclerosis patients and correlated ( r = 0.43, p = 0.013) with the immunoglobulin G index. In contrast, cerebrospinal fluid osteopontin levels correlated neither with plasma osteopontin in paired samples nor with the immunoglobulin G index. Since osteopontin mRNA levels in peripheral blood mononuclear cells of relapsing-remitting multiple sclerosis patients did not correlate with osteopontin plasma levels, peripheral blood mononuclear cells might not be the major source for the increased osteopontin plasma levels. Osteopontin plasma levels correlated ( r = 0.42, p = 0.035) with the bone-specific degradation product C-telopeptide of type-1 collagen. In addition, another immunomodulatory molecule involved in bone metabolism, 25-OH vitamin D, correlated negatively ( r = —0.359, p = 0.048) with the immunoglobulin G index.

This study suggests that bone-related molecules like osteopontin and vitamin D with important immunomodulatory functions are related to the immunoglobulin G index in relapsing-remitting multiple sclerosis patients.

Genomic and functional uniqueness of the TNF receptor-associated factor gene family in amphioxus, the basal chordate

The TNF-associated factor (TRAF) family, the crucial adaptor group in innate immune signaling, increased to 24 in amphioxus, the oldest lineage of the Chordata. To address how these expanded molecules evolved to adapt to the changing TRAF mediated signaling pathways, here we conducted genomic and functional comparisons of four distinct amphioxus TRAF groups with their human counterparts. We showed that lineage-specific duplication and rearrangement were responsible for the expansion of amphioxus TRAF1/2 and 3 lineages, whereas TRAF4 and 6 maintained a relatively stable genome and protein structure. Amphioxus TRAF1/2 and 3 molecules displayed various expression patterns in response to microbial infection, and some of them can attenuate the NF-kappaB activation mediated by human TRAF2 and 6. Amphioxus TRAF4 presented two unique functions: activation of the NF-kappaB pathway and involvement in somite formation. Although amphioxus TRAF6 was conserved in activating NF-kappaB pathway for antibacterial defense, the mechanism was not the same as that observed in humans. In summary, our findings reveal the evolutionary uniqueness of the TRAF family in this basal chordate, and suggest that genomic duplication and functional divergence of the TRAF family are important for the current form of the TRAF-mediated signaling pathways in humans.

Disconnect between inflammation and joint destruction after treatment with etanercept plus methotrexate: results from the trial of etanercept and methotrexate with radiographic and patient outcomes

OBJECTIVE

To determine the relationship between disease activity and radiographic progression of joint destruction in patients with rheumatoid arthritis (RA) treated with methotrexate (MTX), those treated with etanercept, and those treated with the combination of MTX plus etanercept.

METHODS

Baseline, 12-month, and 24-month data from the Trial of Etanercept and Methotrexate with Radiographic and Patient Outcomes database were analyzed. The dependent variable was the 1-year change in the modified Sharp/van der Heijde score (Sharp score); therefore, 2 interval changes per patient were available. Interval change in the Sharp score was modeled by time (years), treatment, disease activity, and the interaction (disease activity x treatment). Disease activity was reflected by the time-averaged Disease Activity Score (taDAS) and the time-averaged C-reactive protein (taCRP) level, which were calculated per 1-year interval. Generalized mixed linear modeling (GMLM) was used to adjust for within-patient correlation.

RESULTS

GMLM confirmed a significant interaction between treatment and the taCRP level and taDAS with respect to the change in Sharp score (P = 0.012 and P = 0.03, respectively). In patients treated with MTX alone, radiographic progression increased with an increasing taCRP level or taDAS, although progression rates were low in patients whose disease was in remission and in those with low-to-moderate disease activity. This relationship was less clear in patients treated with etanercept and was absent in those who received combination therapy.

CONCLUSION

Combination therapy with MTX plus etanercept uncouples the classic relationship between disease activity and radiographic progression in patients with RA.

Caution in Treating Transsphenoidal Encephalocele with Concomitant Moyamoya Disease

A basal encephalocele is a rare congenital malformation involving a cranial bone defect and cystic-like herniation through the defect. Moyamoya is a rare cerebrovascular disease of unclear etiology involving occlusion of the distal internal carotids and formation of collateral vascular networks. Both diseases have been correlated with optic disc anomalies, hypopituitarism, and midfacial defects. We present a case of a 2-year-old boy with a midline facial cleft who underwent surgical correction of a basal encephalocele. His moyamoya disease may have contributed to a vascular complication. There is growing evidence indicating an overlap in disease profiles for these two rare diseases. In addition, molecular evidence indicates elevated levels of fibroblast growth factor and transforming growth factor in both diseases, suggesting common molecular pathways.

Evolution of the mammary gland from the innate immune system?

The mammary gland is a skin gland unique to the class Mammalia. Despite a growing molecular and histological understanding of the development and physiology of the mammary gland, its functional and morphological origins have remained speculative. Numerous theories on the origin of the mammary gland and lactation exist. The purpose of the mammary gland is to provide the newborn with copious amounts of milk, a unique body fluid that has a dual role of nutrition and immunological protection. Interestingly, antimicrobial enzymes, such as xanthine oxidoreductase or lysozyme, are directly involved in the evolution of the nutritional aspect of milk. We outline that xanthine oxidoreductase evolved a dual role in the mammary gland and hence provide new evidence supporting the hypothesis that the nutritional function of the milk evolved subsequent to its protective function. Therefore, we postulate that the mammary gland evolved from the innate immune system. In addition, we suggest that lactation partly evolved as an inflammatory response to tissue damage and infection, and discuss the observation that the two signaling pathways, NF-kB and Jak/Stat, play central roles in inflammation as well as in lactation.

The receptor activator of nuclear factor-kappaB ligand inhibitor osteoprotegerin is a bone-protective agent in a rat model of chronic renal insufficiency and hyperparathyroidism

Osteoprotegerin (OPG) acts by neutralizing the receptor activator of nuclear factor-kappaB ligand (RANKL), the primary mediator of osteoclast differentiation, function, and survival. We examined whether OPG could affect the bone loss associated with chronic kidney disease (CKD) in a rodent model of CKD and secondary hyperparathyroidism (SHPT). SHPT was induced in rats by 5/6 nephrectomy (5/6 Nx) and a 1.2% P/0.6% Ca(2+) diet. Starting 1 week after 5/6 Nx, rats were treated with vehicle (veh) or OPG-Fc (3 mg/kg, intravenously) every 2 weeks for 9 weeks. At baseline, 3, 6, and 9 weeks, blood was taken and bone mineral density (BMD) and bone mineral content (BMC) were assessed by dual-energy X-ray absorptiometry. Serum parathyroid hormone (sPTH) levels reached 912 pg/ml in 5/6 Nx rats vs. 97 pg/ml in shams at 9 weeks. OPG-Fc had no effect on sPTH or Ca(2+) levels throughout the 9-week study, indicating that SHPT was a renal effect independent of bone changes. At 3 weeks, 5/6 Nx-veh rats had osteopenia compared with sham-veh rats and 5/6 Nx-OPG-Fc rats had significantly higher percent changes in whole-body BMC, leg BMD, and lumbar BMD versus 5/6 Nx-veh rats. By 6-9 weeks, elevated sPTH was associated with reversal of bone loss and osteitis fibrosa in the proximal tibial metaphysis. OPG-Fc decreased this sPTH-driven high bone turnover, resulting in augmented thickness of proximal tibial trabeculae in 5/6 Nx rats. Thus, RANKL inhibition with OPG-Fc can block the deleterious effects of continuously elevated sPTH on bone, suggesting that RANKL may be an important therapeutic target for protecting bone in patients with CKD and SHPT.