Chloroquine reduces osteoclastogenesis in murine osteoporosis by preventing TRAF3 degradation

Coenzyme Q10 prevents RANKL-induced osteoclastogenesis by promoting autophagy via inactivation of the PI3K/AKT/mTOR and MAPK pathways

Coenzyme Q10 (CoQ10) is a potent antioxidant that is implicated in the inhibition of osteoclastogenesis, but the underlying mechanism has not been determined. We explored the underlying molecular mechanisms involved in this process. RAW264.7 cells received receptor activator of NF-κB ligand (RANKL) and CoQ10, after which the differentiation and viability of osteoclasts were assessed. After the cells were treated with CoQ10 and/or H2O2 and RANKL, the levels of reactive oxygen species (ROS) and proteins involved in the PI3K/AKT/mTOR and MAPK pathways and autophagy were tested. Moreover, after the cells were pretreated with or without inhibitors of the two pathways or with the mitophagy agonist, the levels of autophagy-related proteins and osteoclast markers were measured. CoQ10 significantly decreased the number of TRAP-positive cells and the level of ROS but had no significant impact on cell viability. The relative phosphorylation levels of PI3K, AKT, mTOR, ERK, and p38 were significantly reduced, but the levels of FOXO3/LC3/Beclin1 were significantly augmented. Moreover, the levels of FOXO3/LC3/Beclin1 were significantly increased by the inhibitors and mitophagy agonist, while the levels of osteoclast markers showed the opposite results. Our data showed that CoQ10 prevented RANKL-induced osteoclastogenesis by promoting autophagy via inactivation of the PI3K/AKT/mTOR and MAPK pathways in RAW264.7 cells.

Hydroxychloroquine and a low activity bisphosphonate conjugate prevent and reverse ovariectomy-induced bone loss in mice through dual antiresorptive and anabolic effects

Osteoporosis is incurable because there are no dual antiresorptive and anabolic therapeutic agents that can be administered long-term. The most widely used antiresorptive agents, bisphosphonates (BPs), also inhibit bone formation and thus have limited effect in preventing osteoporotic fracture. Hydroxychloroquine (HCQ), which is used to treat rheumatoid arthritis, prevents the lysosomal degradation of TNF receptor-associated factor 3 (TRAF3), an NF-κB adaptor protein that limits bone resorption and maintains bone formation. We attempted to covalently link HCQ to a hydroxyalklyl BP (HABP) with anticipated low antiresorptive activity, to target delivery of HCQ to bone to test if this targeting increases its efficacy to prevent TRAF3 degradation in the bone microenvironment and thus reduce bone resorption and increase bone formation, while reducing its systemic side effects. Unexpectedly, HABP-HCQ was found to exist as a salt in aqueous solution, composed of a protonated HCQ cation and a deprotonated HABP anion. Nevertheless, it inhibited osteoclastogenesis, stimulated osteoblast differentiation, and increased TRAF3 protein levels in vitro. HABP-HCQ significantly inhibited both osteoclast formation and bone marrow fibrosis in mice given multiple daily PTH injections. In contrast, HCQ inhibited fibrosis, but not osteoclast formation, while the HABP alone inhibited osteoclast formation, but not fibrosis, in the mice. HABP-HCQ, but not HCQ, prevented trabecular bone loss following ovariectomy in mice and, importantly, increased bone volume in ovariectomized mice with established bone loss because HABP-HCQ increased bone formation and decreased bone resorption parameters simultaneously. In contrast, HCQ increased bone formation, but did not decrease bone resorption parameters, while HABP also restored the bone lost in ovariectomized mice, but it inhibited parameters of both bone resorption and formation. Our findings suggest that the combination of HABP and HCQ could have dual antiresorptive and anabolic effects to prevent and treat osteoporosis.

Human cytomegalovirus degrades DMXL1 to inhibit autophagy, lysosomal acidification, and viral assembly

Human cytomegalovirus (HCMV) is an important human pathogen that regulates host immunity and hijacks host compartments, including lysosomes, to assemble virions. We combined a quantitative proteomic analysis of HCMV infection with a database of proteins involved in vacuolar acidification, revealing Dmx-like protein-1 (DMXL1) as the only protein that acidifies vacuoles yet is degraded by HCMV. Systematic comparison of viral deletion mutants reveals the uncharacterized 7 kDa US33A protein as necessary and sufficient for DMXL1 degradation, which occurs via recruitment of the E3 ubiquitin ligase Kip1 ubiquitination-promoting complex (KPC). US33A-mediated DMXL1 degradation inhibits lysosome acidification and autophagic cargo degradation. Formation of the virion assembly compartment, which requires lysosomes, occurs significantly later with US33A-expressing virus infection, with reduced viral replication. These data thus identify a viral strategy for cellular remodeling, with the potential to employ US33A in therapies for viral infection or rheumatic conditions, in which inhibition of lysosome acidification can attenuate disease.

Curcumin suppresses RANKL-induced osteoclast precursor autophagy in osteoclastogenesis by inhibiting RANK signaling and downstream JNK-BCL2-Beclin1 pathway

BACKGROUND

Curcumin ameliorates bone loss by inhibiting osteoclastogenesis. Curcumin inhibits RANKL-promoted autophagy in osteoclast precursors (OCPs), which mediates its anti-osteoclastogenic effect. But the role of RANKL signaling in curcumin-regulated OCP autophagy is unknown. This study aimed to explore the relationship between curcumin, RANKL signaling, and OCP autophagy during osteoclastogenesis.

METHODS

We investigated the role of curcumin in RANKL-related molecular signaling in OCPs, and identified the significance of RANK-TRAF6 signaling in curcumin-treated osteoclastogenesis and OCP autophagy using flow sorting and lentiviral transduction. Tg-hRANKL mice were used to observe the in vivo effects of curcumin on RANKL-regulated bone loss, osteoclastogenesis, and OCP autophagy. The significance of JNK-BCL2-Beclin1 pathway in curcumin-regulated OCP autophagy with RANKL was explored via rescue assays and BCL2 phosphorylation detection.

RESULTS

Curcumin inhibited RANKL-related molecular signaling in OCPs, and repressed osteoclast differentiation and autophagy in sorted RANK+ OCPs but did not affect those of RANK- OCPs. Curcumin-inhibited osteoclast differentiation and OCP autophagy were recovered by TRAF6 overexpression. But curcumin lost these effects under TRAF6 knockdown. Furthermore, curcumin prevented the decrease in bone mass and the increase in trabecular osteoclast formation and autophagy in RANK+ OCPs in Tg-hRANKL mice. Additionally, curcumin-inhibited OCP autophagy with RANKL was reversed by JNK activator anisomycin and TAT-Beclin1 overexpressing Beclin1. Curcumin inhibited BCL2 phosphorylation at Ser70 and enhanced protein interaction between BCL2 and Beclin1 in OCPs.

CONCLUSIONS

Curcumin suppresses RANKL-promoted OCP autophagy by inhibiting signaling pathway downstream of RANKL, contributing to its anti-osteoclastogenic effect. Moreover, JNK-BCL2-Beclin1 pathway plays an important role in curcumin-regulated OCP autophagy.

The effects of hydroxychloroquine-induced oxidative stress on fracture healing in an experimental rat model

OBJECTIVES

The purpose of this study was to investigate whether hydroxychloroquine (HCQ) sulfate causes oxidative stress (OS) and its effect on fracture healing in an experimental rat model.

MATERIALS AND METHODS

In this experimental study, open diaphyseal femur fractures were induced in 24 eight-week-old male rats (mean weight: 225±25 g; range, 200 to 250 g) and then fixed with K-wire. The rats were divided into four groups: HCQ-2, control-2 (C-2), HCQ-4, and control-4 (C-4). During the study period, rats in the HCQ groups received an HCQ solution (160 mg/kg/day), whereas rats in the control groups received saline. The HCQ-2 and C-2 groups were sacrificed on the 14th day, and the HCQ-4 and C-4 groups were sacrificed on the 28th day. After sacrifice, malondialdehyde levels induced by OS were calculated for each rat, and fracture healing was evaluated radiographically, histomorphometrically, histopathologically, and immunohistochemically.

RESULTS

Malondialdehyde levels were higher in the HCQ groups than in the control groups (p<0.05). Hydroxychloroquine caused OS in rats. The ratio of total callus diameter to femur bone diameter was lower in HCQ groups compared to control groups (p<0.05). No differences were observed when comparing radiological and histological healing results between the control and HCQ groups. Alkaline phosphatase levels were lower in the HCQ-4 group than the C-4 group at week four (p<0.05), although osteocalcin and osteopontin levels did not differ between groups (p>0.05). Oxidative stress had no adverse effects on histologic healing outcomes and osteoblast functions. Cathepsin K and tartrate-resistant acid phosphatase-5b levels were higher in the HCQ-4 group than in the C-4 group (p<0.05). While the number and function of osteoclasts increased due to OS in callus tissue, a decrease in the number of chondrocytes was observed.

CONCLUSION

Hydroxychloroquine-induced OS increases the number and function of osteoclasts and decreases the number of hypertrophic chondrocytes and endochondral ossification but has no significant effect on mid-late osteoblast products and histological fracture healing scores.

Vascular Pericyte-Derived Exosomes Inhibit Bone Resorption via Traf3

Purpose

Blood vessels distribute cells, oxygen, and nutrients throughout the body to support tissue growth and balance. Pericytes and endothelial cells form the inner wall of blood vessels, crucial for organ development and tissue homeostasis by producing paracrine signaling molecules. In the skeletal system, pericyte-derived vascular factors along with angiogenic factors released by bone cells regulate angiogenesis and bone formation. Although the involvement of angiogenic factors and skeletal blood vessels in bone homeostasis is relatively clear, the role of pericytes and the underlying mechanisms remain unknown. Here, our objective was to elucidate the significance of pericytes in regulating osteoclast differentiation.

Methods

We used tissue staining to detect the coverage of pericytes and osteoclasts in femoral tissues of osteoporotic mice and mice of different ages, analyzing their correlation. We developed mice with conditionally deleted pericytes, observing changes in bone mass and osteoclast activity using micro-computer tomography and tissue staining to detect the regulatory effect of pericytes on osteoclasts. Pericytes-derived exosomes (PC-EVs) were collected and co-cultured with monocytes that induce osteoclast differentiation to detect the effect of the former on the exosomes. Finally, the specific mechanism of PC-EVs regulating osteoclast differentiation was verified using RNA sequencing and Western blotting.

Results

Our study indicates a significant correlation between pericytes and age-related bone resorption. Conditional deletion of pericytes activated bone resorption and led to osteopenia in vivo. We discovered that PC-EVs inhibited the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathway, which is mediated by tumor necrosis factor receptor-associated factor 3 (Traf3), negatively regulating osteoclast development and bone resorption. Silencing Traf3 in PC-EVs canceled their inhibitory effect on osteoclast differentiation.

Conclusion

Our study provides a novel perspective into the regulatory role of pericytes on bone resorption and may provide potential strategies for developing novel anti-bone resorption therapies.

Bioactive silica nanoparticles target autophagy, NF-κB, and MAPK pathways to inhibit osteoclastogenesis

Spherical 50 nm silica-based nanoparticles (SiNPs) promote healthy bone homeostasis and maintenance by supporting bone forming osteoblast lineage cells while simultaneously inhibiting the differentiation of bone resorbing osteoclasts. Previous work demonstrated that an intraperitoneal injection of SiNPs in healthy mice - both young and old - increased bone density and quality, suggesting the possibility that SiNPs represent a dual action therapeutic. However, the underlying mechanisms governing the osteoclast response to SiNPs have yet to be fully explored and defined. Therefore, the goals of this study were to investigate the cellular and molecular mechanisms by which SiNPs inhibit osteoclastogenesis. SiNPs strongly inhibited RANKL-induced osteoclast differentiation within the first hours and concomitantly inhibited early transcriptional regulators such as Nfatc1. SiNPs simultaneously stimulated expression of autophagy related genes p62 and LC3β dependent on ERK1/2 signaling pathway. Intriguingly, SiNPs were found to stimulate autophagosome formation while inhibiting the autophagic flux necessary for RANKL-stimulated osteoclast differentiation, resulting in the inhibition of both the canonical and non-canonical NF-κB signaling pathways and stabilizing TRAF3. These results suggest a model in which SiNPs inhibit osteoclastogenesis by inhibiting the autophagic machinery and RANKL-dependent functionality. This mechanism of action defines a novel therapeutic strategy for inhibiting osteoclastogenesis.

Nuclear Factor-Kappa B Regulation of Osteoclastogenesis and Osteoblastogenesis

Maintenance of skeletal integrity requires the coordinated activity of multinucleated bone-resorbing osteoclasts and bone-forming osteoblasts. Osteoclasts form resorption lacunae on bone surfaces in response to cytokines by fusion of precursor cells. Osteoblasts are derived from mesenchymal precursors and lay down new bone in resorption lacunae during bone remodeling. Nuclear factorkappa B (NF-κB) signaling regulates osteoclast and osteoblast formation and is activated in osteoclast precursors in response to the essential osteoclastogenic cytokine, receptor activator of NF-κB ligand (RANKL), which can also control osteoblast formation through RANK-RANKL reverse signaling in osteoblast precursors. RANKL and some pro-inflammatory cytokines, including tumor necrosis factor (TNF), activate NF-κB signaling to positively regulate osteoclast formation and functions. However, these cytokines also limit osteoclast and osteoblast formation through NF-κB signaling molecules, including TNF receptor-associated factors (TRAFs). TRAF6 mediates RANKL-induced osteoclast formation through canonical NF-κB signaling. In contrast, TRAF3 limits RANKL- and TNF-induced osteoclast formation, and it restricts transforming growth factor β (TGFβ)-induced inhibition of osteoblast formation in young and adult mice. During aging, neutrophils expressing TGFβ and C-C chemokine receptor type 5 (CCR5) increase in bone marrow of mice in response to increased NF-κB-induced CC motif chemokine ligand 5 (CCL5) expression by mesenchymal progenitor cells and injection of these neutrophils into young mice decreased bone mass. TGFβ causes degradation of TRAF3, resulting in decreased glycogen synthase kinase-3β/β-catenin-mediated osteoblast formation and age-related osteoporosis in mice. The CCR5 inhibitor, maraviroc, prevented accumulation of TGFβ+/CCR5+ neutrophils in bone marrow and increased bone mass by inhibiting bone resorption and increasing bone formation in aged mice. This paper updates current understanding of how NF-κB signaling is involved in the positive and negative regulation of cytokine-mediated osteoclast and osteoblast formation and activation with a focus on the role of TRAF3 signaling, which can be targeted therapeutically to enhance bone mass.

Rubiadin-1-methyl ether inhibits BECN1 transcription and Beclin1-dependent autophagy during osteoclastogenesis by inhibiting NF-κB p65 activation

As an active substance isolated from the root of Morinda officinalis How., rubiadin-1-methyl ether (RBM), can improve osteoporosis due to its inhibition on osteoclastogenesis. Autophagy plays a key role in osteoclastogenesis. Our research aims to explore the relationship between RBM, autophagy, and osteoclastogenesis. Our results showed that RBM not only inhibited the differentiation level of osteoclasts and the proliferation ability of osteoclast precursors (OCPs), but also repressed the autophagic activity in OCPs (LC3 transformation and the number of autophagosomes observed by transmission electron microscopy). However, RBM-inhibited osteoclast differentiation and OCP autophagy (LC3 transformation and LC3-puncta formation) could be reversed by the application of TAT-Beclin1. Moreover, RBM administration reduced RANKL-induced p65 phosphorylation and p65 nuclear translocation in OCPs. In addition, the addition of RBM inhibited Beclin1 protein level and BECN1 (the gene form of Beclin1) mRNA level in OCPs increased by RANKL. Importantly, the reduction in the expression of BECN1 and Beclin1, LC3 transformation, and osteoclastic differentiation in OCPs caused by RBM were reversed by p65 overexpression. In conclusion, RBM may reduce the transcription of BECN1 by inhibiting the activation of nuclear factor kappa B (NF-κB) p65, thereby inhibiting Beclin1-dependent autophagy and RANKL-induced osteoclastogenesis.

Targeting proteostasis network in osteoporosis: Pathological mechanisms and therapeutic implications

As the most common bone disease, osteoporosis (OP) increases bone fragility and makes patients more vulnerable to the threat of osteoporotic fractures. With the ageing population in today's society, OP has become a huge and growing public health problem. Unfortunately, the clear pathogenesis of OP is still under exploration, and effective interventions are still scarce. Therefore, exploring new targets for pharmacological interventions to develop promising therapeutic drugs for OP is of great clinical value. Previous studies have shown that normal bone remodeling depends on proteostasis, whereas loss of proteostasis during ageing leads to the dysfunctional proteostasis network (PN) that fails to maintain bone homeostasis. Nevertheless, only a few studies have revealed the pathophysiological relationship between bone metabolism and a single component of PN, yet the role of PN as a whole in the pathogenesis of OP is still under investigation. This review comprehensively summarized the role of PN in the pathogenesis of OP and further discussed the potential of PN as innovative drug targets for the therapy of OP.

TGFβ1+CCR5+ neutrophil subset increases in bone marrow and causes age-related osteoporosis in male mice

TGFβ1 induces age-related bone loss by promoting degradation of TNF receptor-associated factor 3 (TRAF3), levels of which decrease in murine and human bone during aging. We report that a subset of neutrophils (TGFβ1+CCR5+) is the major source of TGFβ1 in murine bone. Their numbers are increased in bone marrow (BM) of aged wild-type mice and adult mice with TRAF3 conditionally deleted in mesenchymal progenitor cells (MPCs), associated with increased expression in BM of the chemokine, CCL5, suggesting that TRAF3 in MPCs limits TGFβ1+CCR5+ neutrophil numbers in BM of young mice. During aging, TGFβ1-induced TRAF3 degradation in MPCs promotes NF-κB-mediated expression of CCL5 by MPCs, associated with higher TGFβ1+CCR5+ neutrophil numbers in BM where they induce bone loss. TGFβ1+CCR5+ neutrophils decreased bone mass in male mice. The FDA-approved CCR5 antagonist, maraviroc, reduced TGFβ1+CCR5+ neutrophil numbers in BM and increased bone mass in aged mice. 15-mon-old mice with TGFβRII specifically deleted in MPCs had lower numbers of TGFβ1+CCR5+ neutrophils in BM and higher bone volume than wild-type littermates. We propose that pharmacologic reduction of TGFβ1+CCR5+ neutrophil numbers in BM could treat or prevent age-related osteoporosis.

Modulating glycosphingolipid metabolism and autophagy improves outcomes in pre-clinical models of myeloma bone disease

Patients with multiple myeloma, an incurable malignancy of plasma cells, frequently develop osteolytic bone lesions that severely impact quality of life and clinical outcomes. Eliglustat, a U.S. Food and Drug Administration-approved glucosylceramide synthase inhibitor, reduced osteoclast-driven bone loss in preclinical in vivo models of myeloma. In combination with zoledronic acid, a bisphosphonate that treats myeloma bone disease, eliglustat provided further protection from bone loss. Autophagic degradation of TRAF3, a key step for osteoclast differentiation, was inhibited by eliglustat as evidenced by TRAF3 lysosomal and cytoplasmic accumulation. Eliglustat blocked autophagy by altering glycosphingolipid composition whilst restoration of missing glycosphingolipids rescued autophagy markers and TRAF3 degradation thus restoring osteoclastogenesis in bone marrow cells from myeloma patients. This work delineates both the mechanism by which glucosylceramide synthase inhibition prevents autophagic degradation of TRAF3 to reduce osteoclastogenesis as well as highlighting the clinical translational potential of eliglustat for the treatment of myeloma bone disease.

TET2 regulates osteoclastogenesis by modulating autophagy in OVX-induced bone loss

Increased bone resorption by osteoclasts after estrogen deficiency is the main cause of postmenopausal osteoporosis. TET2 (tet methylcytosine dioxygenase 2) is a DNA demethylase that regulates cellular function and differentiation potential. Macroautophagy/autophagy maintains cellular homeostasis by recycling unnecessary and damaged organelles. This study revealed that TET2 promoted bone loss in oophorectomized (OVX) mice and that TET2 promoted osteoclast differentiation by regulating autophagy. Tet2 knockdown inhibited autophagy and osteoclast differentiation in vitro. Mechanistically, Tet2 knockdown increased BCL2 (B cell leukemia/lymphoma 2) expression and BCL2 exhibited increased binding to BECN1 and negatively regulated autophagy. Small interfering RNA specific to Bcl2 interfered with BCL2 expression in Tet2-knockdown bone marrow cells/precursors, partially reversing autophagy dysregulation and promoting osteoclast differentiation. Moreover, the LV-shTet2 lentivirus prevented bone loss in OVX mice. In summary, our findings provide evidence that TET2 promotes osteoclast differentiation by inhibiting BCL2 expression and positively regulating BECN1-dependent autophagy.Abbreviations: ACP5/TRAP: acid phosphatase 5, tartrate resistant; ATP6V0D2: ATPase, H+ transporting, lysosomal V0 subunit D2; BCL2: B cell leukemia/lymphoma 2; BECN1: beclin 1, autophagy related; BMs: bone marrow cells; CTSK: cathepsin K; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; MMP9: matrix metallopeptidase 9; OVX: oophorectomy; RUNX1: runt related transcription factor 1; SOCS3: suppressor of cytokine signaling 3; SPI1/PU.1: Spi-1 proto-oncogene; TNFSF11/RANKL: tumor necrosis factor (ligand) superfamily, member 11; TET2: tet methylcytosine dioxygenase 2.

Metformin alleviates bone loss in ovariectomized mice through inhibition of autophagy of osteoclast precursors mediated by E2F1

Background

Postmenopausal bone loss, mainly caused by excessive bone resorption mediated by osteoclasts, has become a global public health burden. Metformin, a hypoglycemic drug, has been reported to have beneficial effects on maintaining bone health. However, the role and underlying mechanism of metformin in ovariectomized (OVX)-induced bone loss is still vague.

Results

In this study, we demonstrated for the first time that metformin administration alleviated bone loss in postmenopausal women and ovariectomized mice, based on reduced bone resorption markers, increased bone mineral density (BMD) and improvement of bone microstructure. Then, osteoclast precursors administered metformin in vitro and in vivo were collected to examine the differentiation potential and autophagical level. The mechanism was investigated by infection with lentivirus-mediated BNIP3 or E2F1 overexpression. We observed a dramatical inhibition of autophagosome synthesis and osteoclast formation and activity. Treatment with RAPA, an autophagy activator, abrogated the metformin-mediated autophagy downregulation and inhibition of osteoclastogenesis. Additionally, overexpression of E2F1 demonstrated that reduction of OVX-upregulated autophagy mediated by metformin was E2F1 dependent. Mechanistically, metformin-mediated downregulation of E2F1 in ovariectomized mice could downregulate BECN1 and BNIP3 levels, which subsequently perturbed the binding of BECN1 to BCL2. Furthermore, the disconnect between BECN1 and BCL2 was shown by BNIP3 overexpression.

Conclusion

In summary, we demonstrated the effect and underlying mechanism of metformin on OVX-induced bone loss, which could be, at least in part, ascribed to its role in downregulating autophagy during osteoclastogenesis via E2F1-dependent BECN1 and BCL2 downregulation, suggesting that metformin or E2F1 inhibitor is a potential agent against postmenopausal bone loss.

The Evaluation of Xiaozeng Qianggu Tablets for Treating Postmenopausal Osteoporosis via up-Regulated Autophagy

Objective. Postmenopausal osteoporosis (PMOP) is a common age-associated disease in the life course. Clinically, Xiaozeng Qianggu Tablets (XQT) have a potent therapeutic effect on the PMOP. However, the bioactive components and the mechanism of XQT underlying the PMOP treatment were unclear and it should be explored to discover the scientific connotation in traditional medical practice. Methods. The components in XQT were identified by UPLC-Q-TOF/MS. The animal model of PMOP was established by surgical ovariectomy in the female Sprague-Dawley rats. After treatment of XQT, the therapeutic effect was assessed by the determination of bone metabolism biomarkers in serum and histopathological examination. The effect of XQT on the autophagy and bone micro-situation were tested using western blot, RT-qPCR, and transmission electron microscope. Results. There were 27 compounds identified in XQT, including catalpol, monotropein, verbascoside, cryptochlorogenic acid, 5,7-dihydroxychromone 7-rutinoside, biorobin, and so on. The bone metabolism markers (alkaline phosphatase, bone alkaline phosphatase, procollagen type I intact N-terminal propeptide, cross-linked carboxy-terminal telopeptide of type I collagen, and tartrate-resistant acid phosphatase) were significantly increased in the PMOP rats and reversed by XQT administration. Moreover, the width of bone trabeculae and the ratio of the area of calcium deposition to bone trabeculae were also improved after treating the middle dose of XQT. Meanwhile, the bone micro-structure was improved by XQT. The mRNA and protein expression of unc-51 like kinase 1, beclin-1, and microtubule-associated protein 1B-light chain 3 in PMOP rats were down-regulated and up-regulated by XQT administration. Conclusions. The compounds in XQT, including catalpol, monotropein, verbascoside cryptochlorogenic acid, and so on, were valuable for further pharmacy evaluation. The pathological changes and bone micro-structure were improved by XQT, and the down-regulated autophagy level was also restored, which suggested a potent effect of XQT on treating PMOP, corresponding to its clinic use.

Exercise for osteoporosis: A literature review of pathology and mechanism

Osteoporosis (OP) is a disease that weakens bones and has a high morbidity rate worldwide, which is prevalent among the elderly, particularly, women of postmenopausal age. The dynamic balance between bone formation and resorption is necessary for normal bone metabolism. Many factors, including aging, estrogen deficiency, and prolonged immobilization, disrupt normal apoptosis, autophagy, and inflammation, leading to abnormal activation of osteoclasts, which gradually overwhelm bone formation by bone resorption. Moderate exercise as an effective non-drug treatment helps increase bone formation and helps relieve OP. The possible mechanisms are that exercise affects apoptosis and autophagy through the release of exercise-stimulated myohormone and the secretion of anti-inflammatory cytokines via mechanical force. In addition, exercise may also have an impact on the epigenetic processes involved in bone metabolism. Mechanical stimulation promotes bone marrow mesenchymal stem cells (BMSCs) to osteogenic differentiation by altering the expression of non-coding RNAs. Besides, by reducing DNA methylation, the mechanical stimulus can also alter the epigenetic status of osteogenic genes and show associated increased expression. In this review, we reviewed the possible pathological mechanisms of OP and summarized the effects of exercise on bone metabolism, and the mechanisms by which exercise alleviates the progression of OP, to provide a reference for the prevention and treatment of OP.

CST6 suppresses osteolytic bone disease in multiple myeloma by blocking osteoclast differentiation

Osteolytic bone disease is a hallmark of multiple myeloma (MM). A significant fraction (~20%) of MM patients do not develop osteolytic lesions (OL). The molecular basis for the absence of bone disease in MM is not understood. We combined PET-CT and gene expression profiling (GEP) of purified bone marrow (BM) CD138+ MM cells from 512 newly diagnosed MM patients to reveal that elevated expression of cystatin M/E (CST6) was significantly associated with the absence of OL in MM. An enzyme-linked immunosorbent assay revealed a strong correlation between CST6 levels in BM serum/plasma and CST6 mRNA expression. Both recombinant CST6 protein and BM serum from patients with high CST6 significantly inhibited the activity of the osteoclast-specific protease cathepsin K, and blocked osteoclast differentiation and function. Recombinant CST6 inhibited bone destruction in ex vivo and in vivo myeloma models. Single cell RNA-sequencing identified that CST6 attenuates polarization of monocytes to osteoclast precursors. Furthermore, CST6 protein blocks osteoclast differentiation by suppressing cathepsin-mediated cleavage of NF-κB/p100 and TRAF3 following RANKL stimulation. Secretion by MM cells of CST6, an inhibitor of osteoclast differentiation and function, suppresses osteolytic bone disease in MM and probably other diseases associated with osteoclast-mediated bone loss.

Bone remodeling: an operational process ensuring survival and bone mechanical competence

Bone remodeling replaces old and damaged bone with new bone through a sequence of cellular events occurring on the same surface without any change in bone shape. It was initially thought that the basic multicellular unit (BMU) responsible for bone remodeling consists of osteoclasts and osteoblasts functioning through a hierarchical sequence of events organized into distinct stages. However, recent discoveries have indicated that all bone cells participate in BMU formation by interacting both simultaneously and at different differentiation stages with their progenitors, other cells, and bone matrix constituents. Therefore, bone remodeling is currently considered a physiological outcome of continuous cellular operational processes optimized to confer a survival advantage. Bone remodeling defines the primary activities that BMUs need to perform to renew successfully bone structural units. Hence, this review summarizes the current understanding of bone remodeling and future research directions with the aim of providing a clinically relevant biological background with which to identify targets for therapeutic strategies in osteoporosis.

Low-dose cadmium exposure promotes osteoclastogenesis by enhancing autophagy via inhibiting the mTOR/p70S6K1 signaling pathway

Cadmium (Cd)-induced bone damage may be mediated through activating osteoclastogenesis. However, the underlying mechanism is unknown. The purpose of this study was to explore the effect and possible mechanism of CdCl₂-induced osteoclastogenesis in RAW264.7 cells. We found that a low concentration of CdCl₂ (0.025 and 0.050 µM) did not affect the viability of RAW264.7 cells, but promoted osteoclastogenesis. A low concentration of CdCl₂ increased the mRNA and protein expression of osteoclastogenesis-related genes. TRAP staining and transmission electron microscopy (TEM) also demonstrated that CdCl₂ promoted osteoclastogenesis. A low concentration of CdCl₂ upregulated the levels of LC3-II and Beclin-1, and decreased p62 expression. TEM showed relatively abundant autophagic vacuoles (autophagosomes) after CdCl₂ exposure. A low concentration of CdCl₂ downregulated the expression levels of Mtor and p70S6K1, and the relative protein expression ratios of p-mTOR/mTOR and p-p70S6K1/p70S6K1. When cells were treated with the autophagy inhibitor chloroquine (CQ) or mTOR activator MHY1485 combined with CdCl₂, the expressions of osteoclastogenesis related-genes were decreased and autophagy was attenuated compared with cells treated with CdCl₂ alone. Deficiencies in autophagosomes and osteoclasts were also observed. Taken together, the results indicate that a low concentration of CdCl₂ promotes osteoclastogenesis by enhancing autophagy via inhibiting the mTOR/p70S6K1 signaling pathway.

Mechanisms of autophagy and mitophagy in skeletal development, diseases and therapeutics

Autophagy is a highly evolutionarily conserved process in the eukaryotic cellular system by which dysfunctional organelles are selectively degraded through a series of processes of lysosomal activity and then returned to the cytoplasm for reuse. All cells require this process to maintain cellular homeostasis and promote cell survival during stress responses such as deprivation and hypoxia. Osteoblasts and osteoclasts are two cellular phenotypes in the bone that mediate bone homeostasis. However, an imbalance between osteoblastic bone formation and osteoclastic bone resorption contributes to the onset of bone diseases. Recent studies suggest that autophagy, mitophagy, and selective mitochondrial autophagy may play an essential role in regulating osteoblast differentiation and osteoclast maturation. Autophagic activity dysregulation alters the equilibrium between osteoblastic bone creation and osteoclastic bone resorption, allowing bone disorders like osteoporosis to develop more easily. The current review emphasizes the role of autophagy and mitophagy and their related molecular mechanisms in bone metabolic disorders. In the current review, we emphasize the role of autophagy and mitophagy as well as their related molecular mechanism in bone metabolic disorders. Furthermore, we will discuss autophagy as a target for the treatment of metabolic bone disease and future application in therapeutic translational research.

Plumbagin is a NF-κB-inducing kinase inhibitor with dual anabolic and antiresorptive effects that prevents menopausal-related osteoporosis in mice

Osteoporosis is caused by enhanced bone resorption and relatively reduced bone formation. There is an unmet need to develop new agents with both antiresorptive and anabolic effects to treat osteoporosis, although drugs with either effect alone are available. A small molecular compound, plumbagin, was reported to inhibit receptor activator of nuclear factor kappa-B ligand-induced osteoclast (OC) differentiation by inhibiting IκBα phosphorylation-mediated canonical NF-κB activation. However, the key transcriptional factor RelA/p65 in canonical NF-κB pathway functions to promote OC precursor survival but not terminal OC differentiation. Here, we found that plumbagin inhibited the activity of NF-κB inducing kinase, the key molecule that controls noncanonical NF-κB signaling, in an ATP/ADP-based kinase assay. Consistent with this, plumbagin inhibited processing of NF-κB2 p100 to p52 in the progenitor cells of both OCs and osteoblasts (OBs). Interestingly, plumbagin not only inhibited OC but also stimulated OB differentiation in vitro. Importantly, plumbagin prevented trabecular bone loss in ovariectomized mice. This was associated with decreased OC surfaces on trabecular surface and increased parameters of OBs, including OB surface on trabecular surface, bone formation rate, and level of serum osteocalcin, compared to vehicle-treated mice. In summary, we conclude that plumbagin is a NF-κB-inducing kinase inhibitor with dual anabolic and antiresorptive effects on bone and could represent a new class of agent for the prevention and treatment of osteoporosis.

Advances in pathogenesis and therapeutic strategies for osteoporosis

Osteoporosis, is the most common bone disorder worldwide characterized by low bone mineral density, leaving affected bones vulnerable to fracture. Bone homeostasis depends on the precise balance between bone resorption by osteoclasts and bone matrix formation by mesenchymal lineage osteoblasts, and involves a series of complex and highly regulated steps. Bone homeostasis will be disrupted when the speed of bone resorption is faster than bone formation. Based on various regulatory mechanisms of bone homeostasis, a series of drugs targeting osteoporosis have emerged in clinical practice, including bisphosphonates, selective estrogen receptor modulators, calcitonin, molecular-targeted drugs and so on. However, many drugs have major adverse effects or are unsuitable for long-term use. Therefore, it is very urgent to find more effective therapeutic drugs based on the new pathogenesis of osteoporosis. In this review, we summarize novel mechanisms involved in the pathological process of osteoporosis, including the roles of gut microbiome, autophagy, iron balance and cellular senescence. Based on the above pathological mechanism, we found promising drugs for osteoporosis treatment, such as: probiotics, alpha-ketoglutarate, senolytics and hydrogen sulfide. This new finding may provide an important basis for elucidating the complex pathological mechanisms of osteoporosis and provide promising drugs for clinical osteoporosis treatment.

Phosphorylation of BCL2 at the Ser70 site mediates RANKL-induced osteoclast precursor autophagy and osteoclastogenesis

Background

Phosphorylation modification of BCL2 is involved in receptor activator of nuclear factor-κB ligand (RANKL)-induced autophagy of osteoclast precursors (OCPs) and osteoclastogenesis. As an antiapoptotic molecule, the role of BCL2 phosphorylation in osteoclastogenesis is unknown. This study aimed to explore how BCL2 phosphorylation at specific sites regulates osteoclastogenesis.

Methods

We first examined the effects of RANKL on BCL2 phosphorylation at different sites (Ser70 and Ser87) in OCPs. In vivo, transgenic mice overexpressing RANKL (Tg-hRANKL mice) were used to observe the effects of RANKL on phosphorylated BCL2 at different sites in OCPs of trabecular bone. Subsequently, using site-directed mutagenesis, we observed the respective effect of BCL2 mutations at different phosphorylation sites in OCPs on osteoclastogenesis, apoptosis, autophagy and the affinity between BCL2 and Beclin1/BAX under RANKL intervention.

Results

RANKL promoted BCL2 phosphorylation at the Ser70 (S70) site, but not the Ser87 (S87) site, in OCPs. Moreover, Tg-hRANKL mice had stronger BCL2 phosphorylation capacity at S70, not S87, in the OCPs of trabecular bone than wild-type mice in the same nest. Furthermore, BCL2 mutation at S70, not S87, inhibited RANKL-induced osteoclast differentiation and bone resorption activity. In addition, BCL2 mutation at S70 promoted OCP apoptosis, while BCL2 mutation at S87 showed the opposite effect. Remarkably, the BCL2 mutation at S70, not S87, inhibited OCP autophagic activity. Furthermore, BCL2 mutation at S70 enhanced the coimmunoprecipitation of BCL2 and Beclin1, whereas BCL2 mutation at S87 enhanced the coimmunoprecipitation of BCL2 and BAX in OCPs. More importantly, OCP autophagy, osteoclast differentiation and resorption pits inhibited by BCL2 mutation at S70 could be reversed by Beclin1 upregulation with TAT-Beclin1.

Conclusion

RANKL activates OCP autophagy through BCL2 phosphorylation at S70, thereby promoting osteoclastogenesis, which indicates that the inactivation of BCL2 at S70 in OCPs may be a therapeutic strategy for pathological bone loss.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10020-022-00449-w.

Regulation of TNF-Induced Osteoclast Differentiation

Increased osteoclast (OC) differentiation and activity is the critical event that results in bone loss and joint destruction in common pathological bone conditions, such as osteoporosis and rheumatoid arthritis (RA). RANKL and its decoy receptor, osteoprotegerin (OPG), control OC differentiation and activity. However, there is a specific concern of a rebound effect of denosumab discontinuation in treating osteoporosis. TNFα can induce OC differentiation that is independent of the RANKL/RANK system. In this review, we discuss the factors that negatively and positively regulate TNFα induction of OC formation, and the mechanisms involved to inform the design of new anti-resorptive agents for the treatment of bone conditions with enhanced OC formation. Similar to, and being independent of, RANKL, TNFα recruits TNF receptor-associated factors (TRAFs) to sequentially activate transcriptional factors NF-κB p50 and p52, followed by c-Fos, and then NFATc1 to induce OC differentiation. However, induction of OC formation by TNFα alone is very limited, since it also induces many inhibitory proteins, such as TRAF3, p100, IRF8, and RBP-j. TNFα induction of OC differentiation is, however, versatile, and Interleukin-1 or TGFβ1 can enhance TNFα-induced OC formation through a mechanism which is independent of RANKL, TRAF6, and/or NF-κB. However, TNFα polarized macrophages also produce anabolic factors, including insulin such as 6 peptide and Jagged1, to slow down bone loss in the pathological conditions. Thus, the development of novel approaches targeting TNFα signaling should focus on its downstream molecules that do not affect its anabolic effect.

TIFAB accelerates MLL-AF9-Induced acute myeloid leukemia through upregulation of HOXA9

We previously showed stabilization of NIK-induced activation of NF-κB non-canonical signaling suppresses MLL-AF9-induced AML. In the current study, we demonstrate that deletion of NF-κB non-canonical RelB prevents the inhibitory effect of NIK stabilization in MLL-AF9 AML. Mechanistically, RelB suppresses its direct target, TIFAB, which is upregulated in human AML and correlates negatively with the survival of AML patients. Forced expression of TIFAB reverses NIK-induced impaired AML development through downregulation of RelB and upregulation of HOXA9. Consistent with upregulation of HOXA9, gene set enrichment analysis shows that forced expression of TIFAB blocks myeloid cell development, upregulates leukemia stem cell signature and induces similar gene expression patterns to those of HOXA9-MEIS1 and HOXA9-NUP98, and upregulates oxidative phosphorylation. Accordingly, forced expression of HOXA9 also largely releases the inhibitory impact of NIK stabilization via downregulation of RelB and upregulation of RelA. Our data suggest that NIK/RelB suppresses MLL-AF9-induced AML mainly through downregulation of TIFAB/HOXA9.

Chloroquine increases osteoclast activity in vitro but does not improve the osteopetrotic bone phenotype of ADO2 mice

Autosomal Dominant Osteopetrosis type II (ADO2) is a bone disease of impaired osteoclastic bone resorption that usually results from heterozygous missense mutations in the chloride channel 7 (CLCN7) gene. We created mouse models of ADO2 by introducing a knock-in (p.G213R) mutation in the Clcn7 gene, which is analogous to one of the common mutations (G215R) found in humans. The mutation leads to severe osteopetrosis and lethality in homozygous mice but produces substantial phenotypic variability in heterozygous mice on different genetic backgrounds that phenocopy the human disease of ADO2. ADO2 is an osteoclast-intrinsic disease, and lysosomal enzymes and proteins are critical for osteoclast activity. Chloroquine (CQ) is known to affect lysosomal trafficking, intracellular signaling and the lysosomal and vesicular pH, suggesting it might improve ADO2 osteoclast function. We tested this hypothesis in cell culture studies using osteoclasts derived from wild-type (WT or ADO2+/+) and ADO2 heterozygous (ADO2+/-) mice and found that CQ and its metabolite desethylchloroquine (DCQ), significantly increased ADO2+/- osteoclasts bone resorption activity in vitro, whereas bone resorption of ADO2+/+ osteoclasts was increased only by DCQ. In addition, we exploited our unique animal model of ADO2 on 129 background to identify the effect of CQ for the treatment of ADO2. Female ADO2 mice at 8 weeks of age were treated with 5 doses of CQ (1, 2.5, 5, 7.5 and 10 mg/kg BW/day) via drinking water for 6 months. Bone mineral density and bone micro-architecture were analyzed by longitudinal in vivo DXA and micro-CT at baseline, 3 and 6 months. Serum bone biomarkers (CTX, TRAP and P1NP) were also analyzed at these time points. CQ treatment at the doses tested failed to produce any significant changes of aBMD, BMC (whole body, femur and spine) and trabecular BV/TV (distal femur) in ADO2 mice compared to the control group (water only). Further, levels of bone biomarkers were not significantly changed due to CQ treatment in these mice. Our findings indicate that while CQ increased osteoclast activity in vitro, it did not improve the osteopetrotic bone phenotypes in ADO2 heterozygous mice.

Recent Advances in Understanding the Pathogenesis of Rheumatoid Arthritis: New Treatment Strategies

Rheumatoid arthritis (RA) is considered a chronic systemic, multi-factorial, inflammatory, and progressive autoimmune disease affecting many people worldwide. While patients show very individual courses of disease, with RA focusing on the musculoskeletal system, joints are often severely affected, leading to local inflammation, cartilage destruction, and bone erosion. To prevent joint damage and physical disability as one of many symptoms of RA, early diagnosis is critical. Auto-antibodies play a pivotal clinical role in patients with systemic RA. As biomarkers, they could help to make a more efficient diagnosis, prognosis, and treatment decision. Besides auto-antibodies, several other factors are involved in the progression of RA, such as epigenetic alterations, post-translational modifications, glycosylation, autophagy, and T-cells. Understanding the interplay between these factors would contribute to a deeper insight into the causes, mechanisms, progression, and treatment of the disease. In this review, the latest RA research findings are discussed to better understand the pathogenesis, and finally, treatment strategies for RA therapy are presented, including both conventional approaches and new methods that have been developed in recent years or are currently under investigation.

A multiethnic whole genome sequencing study to identify novel loci for bone mineral density

At present, there have only been a few DNA sequencing-based studies to explore the genetic determinants of bone mineral density (BMD). We carried out the largest whole genome sequencing analysis to date for femoral neck and spine BMD (n = 4981), with one of the highest average sequencing depths implemented thus far at 22×, in a multiethnic sample (58% Caucasian and 42% African American) from the Louisiana Osteoporosis Study (LOS). The LOS samples were combined with summary statistics from the GEFOS consortium and several independent samples of various ethnicities to perform GWAS meta-analysis (n = 44 506). We identified 31 and 30 genomic risk loci for femoral neck and spine BMD, respectively. The findings substantiate many previously reported susceptibility loci (e.g. WNT16 and ESR1) and reveal several others that are either novel or have not been widely replicated in GWAS for BMD, including two for femoral neck (IGF2 and ZNF423) and one for spine (SIPA1). Although we were not able to uncover ethnicity specific differences in the genetic determinants of BMD, we did identify several loci which demonstrated sex-specific associations, including two for women (PDE4D and PIGN) and three for men (TRAF3IP2, NFIB and LYSMD4). Gene-based rare variant association testing detected MAML2, a regulator of the Notch signaling pathway, which has not previously been suggested, for association with spine BMD. The findings provide novel insights into the pathophysiological mechanisms of osteoporosis.

A Retrospective Analysis of Longitudinal Changes in Bone Mineral Density in Women with Systemic Lupus Erythematosus

Most prospective studies of bone mineral density (BMD) in systemic lupus erythematosus (SLE) patients have been of relatively short duration, with a maximum of 6 years. To describe long-term changes in BMD in women with SLE and identify risk factors associated with BMD loss. We retrospectively evaluated 132 adult Mexican-Mestizo women with SLE who underwent dual X-ray absorptiometry (DXA). Demographic and clinical data were collected and BMD at the lumbar spine (L1-L4) and total hip were collected at baseline and during the follow up. At baseline, the mean age of participants was 43.4 ± 12.5 years, 50.8% had osteopenia and 11% osteoporosis. The median follow-up was 13 (IQR 10.2-14.0) years. During follow up, 79% of patients used glucocorticoid (GCT). The mean percentage of changes in BMD during follow up were: - 14.03 ± 11.25% (- 1.49%/year) at the lumbar spine, and - 15.77 ± 11.57% (- 1.78%/year) at the total hip, with significant changes (p < 0.001 for both comparisons). Multivariate analysis showed older age, GCT use at baseline, and transition to the menopause during the follow-up were significantly associated with greater reductions in BMD. This retrospective longitudinal study found significant BMD loss at the lumbar spine and hip. Older age, menopausal transition and GCT use were independently associated with BMD decline in women with SLE.

Rictor Is a Novel Regulator of TRAF6/TRAF3 in Osteoclasts

Tumor necrosis factor receptor-associated factors (TRAFs) are crucial for receptor activator of nuclear factor-κB (RANK) activation in osteoclasts. However, the upstream mechanisms of TRAF members in the osteoclastic lineage remain largely unknown. Here, we demonstrated that Rictor, a key component of mechanistic target of rapamycin complex 2 (mTORC2), was crucial for TRAF6/TRAF3 expression in osteoclasts. Our ex vivo and in vivo studies showed that Rictor ablation from the osteoclastic lineage reduced osteoclast numbers and increased bone mass in mice. Mechanistically, we found that Rictor ablation restricted osteoclast formation, which disrupted TRAF6 stability and caused autophagy block in a manner distinct from mTORC1, resulting in reduced TRAF3 degradation. Boosting TRAF6 expression or knockdown of TRAF3 levels in Rictor-deficient cells could both overcome the defect. Moreover, Rictor could interact with TRAF6 upon RANK ligand (RANKL) stimulation and loss of Rictor impaired TRAF6 stability and promoted its ubiquitinated degradation. These findings established an innovative link between Rictor, TRAF protein levels, and autophagic block. More importantly, mTOR complexes in the osteoclastic lineage are likely switches for coordinating TRAF6 and TRAF3 protein levels, and Rictor may function as an essential upstream regulator of TRAF6/TRAF3 that is partially independent of mTORC1 activity. Inhibitors targeting Rictor may therefore be valuable for preventing or treating osteoclast-related diseases. © 2021 American Society for Bone and Mineral Research (ASBMR).

1α,25-(OH)2-D3 promotes the autophagy during osteoclastogenesis by enhancing RANKL-RANK-TRAF6 signaling

As the active form of vitamin D3, 1α,25-(OH)2-D3 promotes receptor activator for nuclear factor-κB ligand (RANKL)-induced autophagy in osteoclast precursors (OCPs). However, the relationship between 1α,25-(OH)2-D3 and RANKL signaling is still unknown. This study aimed to explore whether 1α,25-(OH)2-D3 regulates OCP autophagy and osteoclastogenesis through RANKL signaling. Our results showed that 1α,25-(OH)2-D3 directly decreased OCP autophagy while significantly enhancing the ability of RANKL to promote OCP autophagy. Moreover, 1α,25-(OH)2-D3 not only promoted the expression of key signaling proteins in OCPs induced by RANKL but also enhanced the coimmunoprecipitation levels of RANK and TRAF6. Notably, 1α,25-(OH)2-D3 significantly enhanced the autophagic activity and osteoclast differentiation of RANK-positive OCPs but did not affect the autophagic activity or osteoclast differentiation of RANK-negative OCPs. More importantly, 1α,25-(OH)2-D3 had no effect on autophagy or osteoclastogenesis in TRAF6-silenced OCPs. Overall, 1α,25-(OH)2-D3 could upregulate RANKL-RANK-TRAF6 signaling in OCPs, thereby promoting OCP autophagy and osteoclastogenesis.

The Chx10-Traf3 Knockout Mouse as a Viable Model to Study Neuronal Immune Regulation

Uncontrolled inflammation is associated with neurodegenerative conditions in central nervous system tissues, including the retina and brain. We previously found that the neural retina (NR) plays an important role in retinal immunity. Tumor necrosis factor Receptor-Associated Factor 3 (TRAF3) is a known immune regulator expressed in the retina; however, whether TRAF3 regulates retinal immunity is unknown. We have generated the first conditional NR-Traf3 knockout mouse model (Chx10-Cre/Traf3^f/f) to enable studies of neuronal TRAF3 function. Here, we evaluated NR-Traf3 depletion effects on whole retinal TRAF3 protein expression, visual acuity, and retinal structure and function. Additionally, to determine if NR-Traf3 plays a role in retinal immune regulation, we used flow cytometry to assess immune cell infiltration following acute local lipopolysaccharide (LPS) administration. Our results show that TRAF3 protein is highly expressed in the NR and establish that NR-Traf3 depletion does not affect basal retinal structure or function. Importantly, NR-Traf3 promoted LPS-stimulated retinal immune infiltration. Thus, our findings propose NR-Traf3 as a positive regulator of retinal immunity. Further, the NR-Traf3 mouse provides a tool for investigations of neuronal TRAF3 as a novel potential target for therapeutic interventions aimed at suppressing retinal inflammatory disease and may also inform treatment approaches for inflammatory neurodegenerative brain conditions.

The E3 ubiquitin ligase Itch limits the progression of post-traumatic osteoarthritis in mice by inhibiting macrophage polarization

OBJECTIVE

Osteoarthritis (OA) is characterized by articular cartilage loss, associated with synovial inflammation. We recently reported increased pro-inflammatory macrophages in murine post-traumatic OA (PTOA) joints, and blockade of the ubiquitin-proteasome system alleviates PTOA progression. However, the mechanisms whereby protein ubiquitination influences PTOA pathology are not well studied. We hypothesized that loss of the negative regulator of inflammation, E3 ligase Itch, in macrophages contributes to joint OA tissue damage by promoting pro-inflammatory polarization of macrophages.

METHODS

Mice deficient Itch in macrophages (MΔItch) were generated by crossing Itchfl/fl mice with LysM-Cre mice. PTOA surgery was performed on global Itch knockout, Itch-/-, mice and MΔItch mice. Joint tissue damage and synovial macrophages were examined. Itch-/- cells were treated with IL-1 and pro-inflammatory polarization was determined. Expression of Itch protein and mRNA in PTOA synovium were assessed at different time points post PTOA.

RESULTS

Similar to Itch-/- mice, MΔItch mice developed more severe joint damage than control mice following PTOA surgery (mean difference of OARSI score: 1.17 (95% CI 0.31-2.03) between MΔItch and Itchfl/fl mice), accompanied by increased the inflammatory macrophage infiltration in the synovium (mean difference of % F4/80 + CD86 + CD36-inflammatory macrophages: 14.81 (95% CI 8.90-20.73) between MΔItch and Itchfl/fl mice). Itch-/- macrophages exerted pro-inflammatory phenotype in response to IL-1β treatment. Itch protein, but not mRNA levels decreased during PTOA progression.

CONCLUSION

The negative regulator of inflammation, Itch, limits PTOA progression by inhibiting macrophage pro-inflammatory polarization. Itch protein degradation may contribute to PTOA pathology.

Bisphosphonates for delivering drugs to bone

Advances in the design of potential bone-selective drugs for the treatment of various bone-related diseases are creating exciting new directions for multiple unmet medical needs. For bone-related cancers, off-target/non-bone toxicities with current drugs represent a significant barrier to the quality of life of affected patients. For bone infections and osteomyelitis, bacterial biofilms on infected bones limit the efficacy of antibiotics because it is hard to access the bacteria with current approaches. Promising new experimental approaches to therapy, based on bone-targeting of drugs, have been used in animal models of these conditions and demonstrate improved efficacy and safety. The success of these drug-design strategies bodes well for the development of therapies with improved efficacy for the treatment of diseases affecting the skeleton. LINKED ARTICLES: This article is part of a themed issue on The molecular pharmacology of bone and cancer-related bone diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.9/issuetoc.

Chloroquine ameliorates bone loss induced by d-galactose in male rats via inhibition of ERK associated osteoclastogenesis and antioxidant effect

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Cloroquine (CQ) has reduced the adverse bone changes caused by d-galactose.

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It improved bone health, switched off nuclear factor kappa-B ligand (RANKL) receptor activator activation and decreased ERK bone expression.

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CQ treatment inhibited osteoclastogenesis and consequently restored the RANKL/OPG ratio.

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CQ demonstrated an antioxidant effect in bone where it increased both catalase (CAT) and superoxide dismutase (SOD).

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CQ is a possible anti-osteoporotic agent through the suppression of osteoclastogenesis associated with ERK.

Chloroquine (CQ); a lysosomotropic agent used for decade ago as anti-malarial, was tested against aging induced osteoporosis. Osteoporosis in male rats was induced using d-galactose (D-gal) as a reducing sugar at a dose of 200 mg/kg/day; i.p. Osteoporotic rats were orally treated with CQ (10 mg/kg/day) for four successive weeks. Bone densitometry of tibia and femur were evaluated. Bone formation biomarkers; osteoprotegrin (OPG), bone specific alkaline phosphatse (BALP), and osteocalcin (OCN), and bone resorption biomarker; receptor activator of nuclear factor kappa-B ligand (RANKL), cathepsin-k (CTSK), tartrate-resistant acid phosphatase (TRAP) were estimated. Moreover, the expression of extracellular regulated kinase (ERK) in bone was determined. CQ ameliorated the bone detrimental changes induced by d-galactose. It enhanced bone health as revealed by measurement of bone densitometry, halted the activation of receptor activator of nuclear factor kappa-B ligand (RANKL) and reduced bone manifestation of ERK. Furthermore, CQ treatment abated serum cathepsin-k (CTSK) and serum tartrate-resistant acid phosphatase (TRAP) thus inhibited osteoclastogenesis and consequently restored the RANKL/OPG ratio. CQ demonstrated an antioxidant effect in bone where it increased both Catalase (CAT) and Superoxide dismutase (SOD). These CQ preserving effect in rats treated with d-galactose were confirmed by the histopathological examination. The present study points to the potential therapeutic effect of CQ as anti-osteoporotic agent possibly through its antioxidant effects and suppression of ERK associated osteoclastogenesis.

The role of autophagy in bone homeostasis

Autophagy is an evolutionarily conserved intracellular process and is considered one of the main catabolism pathways. In the process of autophagy, cells are digested nonselectively or selectively to recover nutrients and energy, so it is regarded as an antiaging process. In addition to the essential role of autophagy in cellular homeostasis, autophagy is a stress response mechanism for cell survival. Here, we review recent literature describing the pathway of autophagy and its role in different bone cell types, including osteoblasts, osteoclasts, and osteocytes. Also discussed is the mechanism of autophagy in bone diseases associated with bone homeostasis, including osteoporosis and Paget's disease. Finally, we discuss the application of autophagy regulators in bone diseases. This review aims to introduce autophagy, summarize the understanding of its relevance in bone physiology, and discuss its role and therapeutic potential in the pathogenesis of bone diseases such as osteoporosis.

Glucocorticoid-induced autophagy and apoptosis in bone

Glucocorticoids are widely prescribed to treat various allergic and autoimmune diseases; however, long-term use results in glucocorticoid-induced osteoporosis, characterized by consistent changes in bone remodeling with decreased bone formation as well as increased bone resorption. Not only bone mass but also bone quality decrease, resulting in an increased incidence of fractures. The primary role of autophagy is to clear up damaged cellular components such as long-lived proteins and organelles, thus participating in the conservation of different cells. Apoptosis is the physiological death of cells, and plays a crucial role in the stability of the environment inside a tissue. Available basic and clinical studies indicate that autophagy and apoptosis induced by glucocorticoids can regulate bone metabolism through complex mechanisms. In this review, we summarize the relationship between apoptosis, autophagy and bone metabolism related to glucocorticoids, providing a theoretical basis for therapeutic targets to rescue bone mass and bone quality in glucocorticoid-induced osteoporosis.

Overexpression of c-Fos reverses osteoprotegerin-mediated suppression of osteoclastogenesis by increasing the Beclin1-induced autophagy

Osteoclastogenesis requires the involvement of transcription factors and degrading enzymes, and is regulated by upstream and downstream signalling. However, c-Fos how regulates osteoclastogenesis through autophagy remain unclear. This study aimed to explore the role of c-Fos during osteoprotegerin (OPG)-mediated suppression of osteoclastogenesis. We found that the number of osteoclasts and the expression of c-Fos, MMP-9, CAⅡ, Src and p62 were decreased after treated with OPG, including attenuation the PI3K/Akt and the TAK1/S6 signalling pathways, but the expression of Beclin1 and LC3Ⅱ were increased. Knockdown of Beclin1 could reverse the expression of c-Fos and MMP-9 by activating the PI3K/Akt signalling pathway, but inhibiting the autophagy and the TAK1/S6 signalling pathway. In addition, inhibition of autophagy using the PI3K inhibitor LY294002 did not rescues OPG-mediated suppression of osteoclastogenesis, but caused reduction of the expression of c-Fos and CAⅡ by attenuating the autophagy, as well as the PI3K/Akt and the TAK1/S6 signalling pathways. Furthermore, continuous activation of c-Fos could reverse OPG-mediated suppression of osteoclastogenesis by activating the autophagy and the PI3K/Akt and the TAK1/S6 signalling pathways. Thus, overexpression of c-Fos could reverse OPG-mediated suppression of osteoclastogenesis via activation of Beclin1-induced autophagy, indicating c-Fos might serve as a new candidate for bone-related basic studies.

Carrier-Free Platinum Nanomedicine for Targeted Cancer Therapy

Numerous nanomedicines have been developed to improve the efficiency and safety of conventional anticancer drugs; however, the complexities in carrier materials and functional integration make it challenging to promote these candidates for clinical translation. In this study, a facile method to prepare carrier-free anticancer nanodrug with inherent bone targeting and osteoclastogenesis inhibition capabilities is reported. Phytic acid, a naturally occurring and nontoxic product, is reacted with cisplatin to form uniform nanoparticles of different sizes. The prepared nanoparticles possess high drug loading and pH-responsive drug release behaviors. Phytic acid in the nanomedicine ensures high bone targeting and osteoclastogenesis inhibition, and the released platinum drugs triggered by tumor extracellular acidity eradicate tumor cells. The nanomedicine around 100 nm shows high anticancer activity and much reduced side effects in a subcutaneous breast cancer model when compared with cisplatin. In addition, it shows high accumulation at osteolytic lesions, and efficiently inhibits tumor growth and tumor-associated osteolysis in a bone metastatic breast cancer model. Here, a facile and efficient strategy to prepare carrier-free nanomedicines with high anticancer drug loading, inherent bone targeting, and osteoclast inhibitory activities for cancer therapy is provided.

Deletion of the proton receptor OGR1 in mouse osteoclasts impairs metabolic acidosis-induced bone resorption

Metabolic acidosis induces osteoclastic bone resorption and inhibits osteoblastic bone formation. Previously we found that mice with a global deletion of the proton receptor OGR1 had increased bone density although both osteoblast and osteoclast activity were increased. To test whether direct effects on osteoclast OGR1 are critical for metabolic acidosis stimulated bone resorption, we generated knockout mice with an osteoclast-specific deletion of OGR1 (knockout mice). We studied bones from three-month old female mice and the differentiated osteoclasts derived from bone marrow of femurs from these knockout and wild type mice. MicroCT demonstrated increased density in tibiae and femurs but not in vertebrae of the knockout mice. Tartrate resistant acid phosphatase staining of tibia indicated a decrease in osteoclast number and surface area/bone surface from knockout compared to wild type mice. Osteoclasts derived from the marrow of knockout mice demonstrated decreased pit formation, osteoclast staining and osteoclast-specific gene expression compared to those from wild type mice. In response to metabolic acidosis, osteoclasts from knockout mice had decreased nuclear translocation of NFATc1, a transcriptional regulator of differentiation, and no increase in size or number compared to osteoclasts from wild type mice. Thus, loss of osteoclast OGR1 decreased both basal and metabolic acidosis-induced osteoclast activity indicating osteoclast OGR1 is important in mediating metabolic acidosis-induced bone resorption. Understanding the role of OGR1 in metabolic acidosis-induced bone resorption will provide insight into bone loss in acidotic patients with chronic kidney disease.

Nepetin inhibits osteoclastogenesis by inhibiting RANKL-induced activation of NF-κB and MAPK signalling pathway, and autophagy

Aseptic prosthetic loosening due to wear particle-induced inflammatory osteolysis is the main cause of failure for artificial joint replacement. The inflammatory response and the production of pro-osteoclastic factors lead to elevation of osteoclast formation and excessive activity results in extensive bone destruction around the bone-implant interface. Here we showed that Nepetin, a natural bioactive flavonoid with proven anti-inflammatory and anti-proliferative properties, potently inhibited RANKL-induced osteoclast differentiation, formation and bone resorption in vitro, and protected mice against the deleterious effects of titanium particle-induced calvarial osteolysis in vivo. Mechanistically, Nepetin attenuated RANKL-induced activation of NF-κB and MAPK signalling pathways and TRAF6-dependent ubiquitination of Beclin 1 which is necessary for the induction of autophagy. In brief, our study demonstrates the potential therapeutic application of Nepetin against osteoclast-mediated osteolytic diseases.

PERK controls bone homeostasis through the regulation of osteoclast differentiation and function

Osteoclasts are multinucleated giant cells with the ability to degrade bone tissue, and are closely related to abnormal bone metabolic diseases. Endoplasmic reticulum (ER) is an organelle responsible for protein modification, quality control, and transportation. The accumulation of unfolded or misfolded proteins in ER cavity induces ER stress. Double-stranded RNA-dependent protein kinase-like ER kinase (PERK) is an ER stress-sensing protein, which is ubiquitous in eukaryotic cells. Systemic PERK knockout mice show severe bone loss, suggesting that PERK is of great significance for maintaining the normal growth and development of bone tissue, but the role of PERK in osteoclastogenesis is still unclear. In this study, we found that PERK was significantly activated during RANKL-induced osteoclast differentiation; knockdown of PERK by siRNA and inhibition of PERK by GSK2606414, respectively, had significant negative regulatory effects on the formation and bone resorption of osteoclasts. PERK inhibitor GSK2606414 down-regulated the mRNA levels and protein expression of osteoclast differentiation marker genes, and inhibited RANKL-induced activation of Mitogen-activated protein kinase (MAPK) and nuclear factor κB (NF-κB) pathways. Treatment with PERK inhibitor GSK2606414 in ovariectomized mouse model significantly suppressed bone loss and osteoclast formation. Thapsigargin activated ER stress to enhance autophagy, while GSK2606414 had a significant inhibitory effect on autophagy flux and autophagosome formation. Antioxidant N-acetylcysteine (NAC) could inhibit the expression of PERK phosphorylation, osteoclast-related proteins and autophagy-related proteins, but the use of PERK activator CCT020312 can reverse inhibition effect of NAC. Our findings demonstrate a key role for PERK in osteoclast differentiation and suggest its therapeutic potential.

The Role of Autophagy in Osteoclast Differentiation and Bone Resorption Function

Autophagy is an evolutionary conserved and highly regulated recycling process of cellular wastes. Having a housekeeping role, autophagy through the digestion of domestic cytosolic organelles, proteins, macromolecules, and pathogens, eliminates unnecessary materials and provides nutrients and energy for cell survival and maintenance. The critical role of autophagy and autophagy-related proteins in osteoclast differentiation, bone resorption, and maintenance of bone homeostasis has previously been reported. Increasing evidence reveals that autophagy dysregulation leads to alteration of osteoclast function and enhanced bone loss, which is associated with the onset and progression of osteoporosis. In this review, we briefly consolidate the current state-of-the-art technology regarding the role of autophagy in osteoclast function in both physiologic and pathologic conditions to have a more general view on this issue.

Targeting nanoparticles for diagnosis and therapy of bone tumors: Opportunities and challenges

A variety of targeted nanoparticles were developed for the diagnosis and therapy of orthotopic and metastatic bone tumors during the past decade. This critical review will focus on principles and methods in the design of these bone-targeted nanoparticles. Ligands including bisphosphonates, aspartic acid-rich peptides and synthetic polymers were grafted on nanoparticles such as PLGA nanoparticles, liposomes, dendrimers and inorganic nanoparticles for bone targeting. Besides, other ligands such as monoclonal antibodies, peptides and aptamers targeting biomarkers on tumor/bone cells were identified for targeted diagnosis and therapy. Examples of targeted nanoparticles for the early detection of bone metastatic tumors and the ablation of cancer via chemotherapy, photothermal therapy, gene therapy and combination therapy will be intensively reviewed. The development of multifunctional nanoparticles to break down the "vicious" cycle between tumor cell proliferation and bone resorption, and the challenges and perspectives in this area will be discussed.

4-Acetylantroquinonol B Inhibits Osteoclastogenesis by Inhibiting the Autophagy Pathway in a Simulated Microgravity Model

Astronauts suffer from 1–2% bone loss per month during space missions. Targeting osteoclast differentiation has been regarded as a promising strategy to prevent osteoporosis in microgravity (μXg). 4-acetylantroquinonol B (4-AAQB), a ubiquinone from Antrodia cinnamomea, has shown anti-inflammatory and anti-hepatoma activities. However, the effect of 4-AAQB on μXg-induced osteoclastogenesis remains unclear. In this study, we aimed to explore the mechanistic impact of 4-AAQB on osteoclast formation under μXg conditions. The monocyte/macrophage-like cell line RAW264.7 was exposed to simulated μXg (Rotary Cell Culture System; Synthecon, Houston, TX, USA) for 24 h and then treated with 4-AAQB or alendronate (ALN) and osteoclast differentiation factor receptor activator of nuclear factor kappa-B ligand (RANKL). Osteoclastogenesis, bone resorption activity, and osteoclast differentiation-related signaling pathways were analyzed using tartrate-resistant acid phosphatase (TRAP) staining, actin ring fluorescent staining, bone resorption, and western blotting assays. Based on the results of TRAP staining, actin ring staining, and bone resorption assays, we found that 4-AAQB significantly inhibited μXg-induced osteoclast differentiation. The critical regulators of osteoclast differentiation, including nuclear factor of activated T-cells cytoplasmic 1 (NFATc1), c-Fos, and dendritic cell-specific transmembrane protein (DC-STAMP), were consistently decreased. Meanwhile, osteoclast apoptosis and cell cycle arrest were also observed along with autophagy suppression. Interestingly, the autophagy inhibitors 3-methyladenine (3-MA) and chloroquine (CQ) showed similar effects to 4-AAQB. In conclusion, we suggest that 4-AAQB may serve as a potential agent against μXg-induced osteoclast formation.

Targeting autophagy in osteoporosis: From pathophysiology to potential therapy

Osteoporosis is a highly prevalent disorder characterized by the loss of bone mass and microarchitecture deterioration of bone tissue, attributed to various factors, including menopause (primary), aging (primary) and adverse effects of relevant medications (secondary). In recent decades, knowledge regarding the etiological mechanisms underpinning osteoporosis emphasizes that bone cellular homeostasis, including the maintenance of cell functions, differentiation, and the response to stress, is tightly regulated by autophagy, which is a cell survival mechanism for eliminating and recycling damaged proteins and organelles. With the important roles in the maintenance of cellular homeostasis and organ function, autophagy has emerged as a potential target for the prevention and treatment of osteoporosis. In this review, we update and discuss the pathophysiology of autophagy in normal bone cell life cycle and metabolism. Then, the alternations of autophagy in primary and secondary osteoporosis, and the accompanied pathological process are discussed. Finally, we discuss current strategies, limitations, and challenges involved in targeting relevant pathways and propose strategies by which such hurdles may be circumvented in the future for their translation into clinical validations and applications for the prevention and treatment of osteoporosis.

Targeting anti-cancer agents to bone using bisphosphonates

The skeleton is affected by numerous primary and metastatic solid and hematopoietic malignant tumors, which can cause localized sites of osteolysis or osteosclerosis that can weaken bones and increase the risk of fractures in affected patients. Chemotherapeutic drugs can eliminate some tumors in bones or reduce their volume and skeletal-related events, but adverse effects on non-target organs can significantly limit the amount of drug that can be administered to patients. In these circumstances, it may be impossible to deliver therapeutic drug concentrations to tumor sites in bones. One attractive mechanism to approach this challenge is to conjugate drugs to bisphosphonates, which can target them to bone where they can be released at diseased sites. Multiple attempts have been made to do this since the 1990s with limited degrees of success. Here, we review the results of pre-clinical and clinical studies made to target FDA-approved drugs and other antineoplastic small molecules to bone to treat diseases affecting the skeleton, including osteoporosis, metastatic bone disease, multiple myeloma and osteosarcoma. Results to date are encouraging and indicate that drug efficacy can be increased and side effects reduced using these approaches. Despite these successes, challenges remain: no drugs have gone beyond small phase 2 clinical trials, and major pharmaceutical companies have shown little interest in the approach to repurpose any of their drugs or to embrace the technology. Nevertheless, interest shown by smaller biotechnology companies in the technology suggests that bone-targeting of drugs with bisphosphonates has a viable future.

A Loop-Based and AGO-Incorporated Virtual Screening Model Targeting AGO-Mediated miRNA-mRNA Interactions for Drug Discovery to Rescue Bone Phenotype in Genetically Modified Mice

Several virtual screening models are proposed to screen small molecules only targeting primary miRNAs without selectivity. Few attempts have been made to develop virtual screening strategies for discovering small molecules targeting mature miRNAs. Mature miRNAs and their specific target mRNA can form unique functional loops during argonaute (AGO)-mediated miRNA-mRNA interactions, which may serve as potential targets for small-molecule drug discovery. Thus, a loop-based and AGO-incorporated virtual screening model is constructed for targeting the loops. The previously published studies have found that miR-214 can target ATF4 to inhibit osteoblastic bone formation, whereas miR-214 can target TRAF3 to promote osteoclast activity. By using the virtual model, the top ten candidate small molecules targeting miR-214-ATF4 mRNA interactions and top ten candidate small molecules targeting miR-214-TRAF3 mRNA interactions are selected, respectively. Based on both in vitro and in vivo data, one small molecule can target miR-214-ATF4 mRNA to promote ATF4 protein expression and enhance osteogenic potential, whereas one small molecule can target miR-214-TRAF3 mRNA to promote TRAF3 protein expression and inhibit osteoclast activity. These data indicate that the loop-based and AGO-incorporated virtual screening model can help to obtain small molecules specifically targeting miRNA-mRNA interactions to rescue bone phenotype in genetically modified mice.