Beclin-1 Is Required for RANKL-Induced Osteoclast Differentiation

ERK inhibits osteoclast differentiation in RAW 264.7 cells through the osteoprotegerin-mediated autophagy

Osteoclasts (OCs) are bone-resorbing cells derived from the monocyte/macrophage lineage. The extracellular signal-regulated kinase (ERK) pathway controls cellular responses such as proliferation, differentiation, and survival, including those of OCs. In the present study, ERK inhibitors reduced the proliferation of bone marrow-derived macrophages (BMMs) and RAW 264.7 cells. However, ERK inhibitors decreased OC differentiation in BMMs but increased it in RAW 264.7 cells. ERK downregulation using small interfering RNA transfection also increased the OC differentiation and the expression of receptor activator of nuclear factor-κB, OC-specific markers, and OC-associated transcription factors in RAW 264.7 cells. These findings suggest ERK regulates OC differentiation in RAW 264.7 cells differently than in BMMs. Thus, we further investigated the mechanism by which ERK negatively regulates OC differentiation in RAW 264.7 cells. ERK inhibition decreased the expression of osteoprotegerin (OPG), a negative regulator of OC differentiation. OPG knockdown increased OC formation. ERK inhibitors activated the Akt/mammalian target of the rapamycin (mTOR) signaling pathway while inhibiting unc-51-like autophagy activating kinase 1 (ULK1). This resulted in decreased levels of microtubule-associated protein 1A/1B-light chain 3-II (LC3-II) and increased levels of p62, thereby reducing autophagy. In addition, OPG knockdown reduced autophagy by activating Akt/mTOR and inhibiting ULK1, resulting in decreased LC3-II and accumulated p62. Therefore, ERK inhibition promoted OC differentiation by downregulating OPG-mediated inhibition of osteoclastogenesis and autophagy in RAW 264.7 cells. These findings highlight ERK's complex role in OC differentiation and suggest that understanding ERK's dual impact on OC differentiation can provide insights into novel treatment strategies for bone-related disorders.

Beclin 1 of megakaryocytic lineage cells is locally dispensable for platelet hemostasis but functions distally in bone homeostasis

The crosstalk between megakaryocytic lineage cells and the skeletal system has just begun to be explored but remains largely elusive. Using conditional gene knockout mouse models, we demonstrated that loss of Beclin 1 (Becn1), a major regulator of mammalian autophagy, exclusively in the megakaryocytic lineage disrupted autophagy in platelets but did not compromise megakaryopoiesis or the formation and function of platelets. Unexpectedly, conditional Becn1 deletion in male mice led to a remarkable increase in bone mass with improved bone quality, in association with a decrease in sex hormone binding globulin (SHBG) and an increase in free testosterone (FT). In vivo Becn1 overexpression in megakaryocytic lineage-specific cells reduced bone mass and quality, along with an increase in SHBG and a decrease in FT. Transplantation of wild-type bone marrow cells into megakaryocytic lineage Becn1-deficient male mice restored bone mass and normalized SHBG and FT. Furthermore, bilateral orchiectomy of Becn1f/f;Pf4-iCre mice, which are crippled with the production of testosterone, resulted in a reduction in bone mass and quality, whereas in vivo overexpression of SHBG, specifically in the liver of Becn1f/f;Pf4-iCre mice, decreased FT and reduced bone mass and quality. In addition, metformin treatment, which induces SHBG expression, reduced FT and normalized bone mass in Becn1f/f;Pf4-iCre mice. We thus concluded that Becn1 of the megakaryocytic lineage is dispensable locally for platelet hemostasis but limits bone mass by increasing SHBG, which in turn reduces the FT of male mice. Our findings highlight a mechanism by which Becn1 from megakaryocytic lineage cells distally balances bone growth.

Targeting the mTOR-Autophagy Axis: Unveiling Therapeutic Potentials in Osteoporosis

Osteoporosis (OP) is a widespread age-related disorder marked by decreased bone density and increased fracture risk, presenting a significant public health challenge. Central to the development and progression of OP is the dysregulation of the mechanistic target of the rapamycin (mTOR)-signaling pathway, which plays a critical role in cellular processes including autophagy, growth, and proliferation. The mTOR-autophagy axis is emerging as a promising therapeutic target due to its regulatory capacity in bone metabolism and homeostasis. This review aims to (1) elucidate the role of mTOR signaling in bone metabolism and its dysregulation in OP, (2) explore the interplay between mTOR and autophagy in the context of bone cell activity, and (3) assess the therapeutic potential of targeting the mTOR pathway with modulators as innovative strategies for OP treatment. By examining the interactions among autophagy, mTOR, and OP, including insights from various types of OP and the impact on different bone cells, this review underscores the complexity of mTOR's role in bone health. Despite advances, significant gaps remain in understanding the detailed mechanisms of mTOR's effects on autophagy and bone cell function, highlighting the need for comprehensive clinical trials to establish the efficacy and safety of mTOR inhibitors in OP management. Future research directions include clarifying mTOR's molecular interactions with bone metabolism and investigating the combined benefits of mTOR modulation with other therapeutic approaches. Addressing these challenges is crucial for developing more effective treatments and improving outcomes for individuals with OP, thereby unveiling the therapeutic potentials of targeting the mTOR-autophagy axis in this prevalent disease.

The role of autophagy in odontogenesis, dental implant surgery, periapical and periodontal diseases

Autophagy is a cellular process that is evolutionarily conserved, involving the sequestration of damaged organelles and proteins into autophagic vesicles, which subsequently fuse with lysosomes for degradation. Autophagy controls the development of many diseases by influencing apoptosis, inflammation, the immune response and different cellular processes. Autophagy plays a significant role in the aetiology of disorders associated with dentistry. Autophagy controls odontogenesis. Furthermore, it is implicated in the pathophysiology of pulpitis and periapical disorders. It enhances the survival, penetration and colonization of periodontal pathogenic bacteria into the host periodontal tissues and facilitates their escape from host defences. Autophagy plays a crucial role in mitigating exaggerated inflammatory reactions within the host's system during instances of infection and inflammation. Autophagy also plays a role in the relationship between periodontal disease and systemic diseases. Autophagy promotes wound healing and may enhance implant osseointegration. This study reviews autophagy's dento-alveolar effects, focusing on its role in odontogenesis, periapical diseases, periodontal diseases and dental implant surgery, providing valuable insights for dentists on tooth development and dental applications. A thorough examination of autophagy has the potential to discover novel and efficacious treatment targets within the field of dentistry.

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.

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.

IL-17A regulates autophagy and promotes osteoclast differentiation through the ERK/mTOR/Beclin1 pathway

Bone is a frequent target of tumor metastasis, with high incidence rate and poor prognosis. Osteoclasts play a key role in the process of tumor bone metastasis. Interleukin-17A (IL-17A) is an inflammatory cytokine, highly expressed in a variety of tumor cells, that can alter the autophagic activity of other cells, thereby causing corresponding lesions. Previous studies have shown that low concentration IL-17A can promote osteoclastogenesis. The aim of this study was to clarify the mechanism of low concentration IL-17A promoting osteoclastogenesis by regulating autophagic activity. The results of our study showed that IL-17A could promote the differentiation of osteoclast precursors (OCPs) into osteoclasts in the presence of RANKL, and increase the mRNA levels of osteoclast-specific genes. Moreover, IL-17A increased the expression of Beclin1 by inhibiting the phosphorylation of ERK and mTOR, leading to enhanced autophagy of OCPs, accompanied by decreased OCP apoptosis. Furthermore, knockdown of Beclin1 and suppression of autophagy by 3-methyladenine (3-MA) significantly attenuated the enhanced osteoclastogenesis induced by IL-17A. In summary, these results indicate that low concentration IL-17A enhances the autophagic activity of OCPs through the ERK/mTOR/Beclin1 pathway during osteoclastogenesis, and further promotes osteoclast differentiation, suggesting that IL-17A may serve as a potential therapeutic target for cancer-related bone resorption in cancer patients.

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.

Autophagy Plays Multiple Roles in the Soft-Tissue Healing and Osseointegration in Dental Implant Surgery-A Narrative Review

Dental endo-osseous implants have become a widely used treatment for replacing missing teeth. Dental implants are placed into a surgically created osteotomy in alveolar bone, the healing of the soft tissue lesion and the osseointegration of the implant being key elements to long-term success. Autophagy is considered the major intracellular degradation system, playing important roles in various cellular processes involved in dental implant integration. The aim of this review is an exploration of autophagy roles in the main cell types involved in the healing and remodeling of soft tissue lesions and implant osseointegration, post-implant surgery. We have focused on the autophagy pathway in macrophages, endothelial cells; osteoclasts, osteoblasts; fibroblasts, myofibroblasts and keratinocytes. In macrophages, autophagy modulates innate and adaptive immune responses playing a key role in osteo-immunity. Autophagy induction in endothelial cells promotes apoptosis resistance, cell survival, and protection against oxidative stress damage. The autophagic machinery is also involved in transporting stromal vesicles containing mineralization-related factors to the extracellular matrix and regulating osteoblasts' functions. Alveolar bone remodeling is achieved by immune cells differentiation into osteoclasts; autophagy plays an important and active role in this process. Autophagy downregulation in fibroblasts induces apoptosis, leading to better wound healing by improving excessive deposition of extracellular matrix and inhibiting fibrosis progression. Autophagy seems to be a dual actor on the scene of dental implant surgery, imposing further research in order to completely reveal its positive features which may be essential for clinical efficacy.

A Review of Signaling Transduction Mechanisms in Osteoclastogenesis Regulation by Autophagy, Inflammation, and Immunity

Osteoclastogenesis is an ongoing rigorous course that includes osteoclast precursors fusion and bone resorption executed by degradative enzymes. Osteoclastogenesis is controlled by endogenous signaling and/or regulators or affected by exogenous conditions and can also be controlled both internally and externally. More evidence indicates that autophagy, inflammation, and immunity are closely related to osteoclastogenesis and involve multiple intracellular organelles (e.g., lysosomes and autophagosomes) and certain inflammatory or immunological factors. Based on the literature on osteoclastogenesis induced by different regulatory aspects, emerging basic cross-studies have reported the emerging disquisitive orientation for osteoclast differentiation and function. In this review, we summarize the partial potential therapeutic targets for osteoclast differentiation and function, including the signaling pathways and various cellular processes.

Loureirin B downregulates osteoclast differentiation of bone marrow macrophages by targeting the MAPK signaling pathway

Excessive absorption of osteoclasts will break the balance between osteoclasts and osteoblasts, leading to bone loss, decreased bone density, and increased bone fragility. We have shown that Loureirin B (LrB) can inhibit osteoclasts. In this study, we demonstrated the targeting-inhibitory mechanism of LrB acting on osteoclast precursor. Using SPR, HPLC and MALDI-TOF-MS to capture and analyze the target protein of Loureirin B in bone marrow macrophages (BMMs), we used this method to detect all target proteins that LrB acts on BMMs, and analyzed the distribution and enrichment rate of the target protein by DAVID enrichment analysis. Ledock molecular docking was used to detect the binding of LrB. We used Western Blot for verification. The target proteins of LrB acting on BMMs were Serpine1, Atp6ap1, Dvl1, Rhd, Fzd2, MAPK1, MAP2K2, MAPK3 and so on. MAPK1, MAP2K2 and MAPK3 were the most relevant. LrB treatment attenuated the expression of phosphorylated JNK and p38 kinases of the MAPK signaling pathway. Our research further confirmed that LrB affects the MAPK signaling pathway in BMMs, thereby inhibiting the differentiation of BMMs into osteoclasts. This discovery can confirm the mechanism by which LrB acts on BMMs.

SPHK Inhibitors and Zoledronic Acid Suppress Osteoclastogenesis and Wear Particle-Induced Osteolysis

Background: Inflammatory osteolysis induced by wear particles is the major cause of prosthetic loosening after artificial joint replacement, and its prevention and treatment are difficult worldwide. Our previous study confirmed that sphingosine kinases (SPHKs) are important mediators regulating the wear particle-induced macrophage inflammatory response. However, it is unclear whether SPHKs can modulate chronic inflammation and alleviate osteolysis. Zoledronic acid (ZA), an imidazole-containing bisphosphonate, directly affects osteoclasts and prevents bone mineral-related diseases. However, the effects of SPHK inhibitors and ZA used to treat periprosthetic osteolysis are unknown. Methods: We applied tartrate-resistant acid phosphatase (TRAP) staining to evaluate bone destruction in the interface membranes of patients with aseptic loosening and a control group. A murine calvarial osteolysis model was used to examine the preventative effect of SPHK inhibitors and ZA on osteolysis. Micro-CT scanning, immunohistochemistry (IHC), and histomorphometric analysis were conducted to determine the variations in inflammatory osteolysis. The effects of different drug concentrations on cell viability were evaluated using the Cell Counting Kit-8 (CCK-8) assay. Real-time quantitative polymerase chain reaction (RT-qPCR) analysis was performed to confirm the reduced expression of osteoclast-specific genes after drug and titanium treatment. The osteoclast formation and functions of the drugs were analyzed using TRAP staining in vivo and in vitro. The effect of SPHKs/S1P-TRAF2-BECN1 signaling pathways was verified via RT-qPCR and tissue IHC. Results: In this study, we found that SPHK inhibitors (ABC294640 and FTY720) combined with ZA decreased the degree of inflammatory osteolysis in vivo. However, ABC294640 and ZA suppressed osteoclast differentiation and osteoclast-specific genes in vitro. SPHKs regulate the inflammatory osteolysis induced by wear particles by increasing the expression of SPHKs/S1P-TRAF2-BECN1. Conclusion: Our study revealed that wear particles could induce inflammatory osteolysis by upregulating SPHKs/S1P-TRAF2-BECN1 and SPHK inhibitors/ZA inhibit osteoclastogenesis in vitro and prevent inflammatory osteolysis in vivo, suggesting that SPHK inhibitors and ZA can be a new perspective and scientific basis for the prevention and treatment of prosthesis loosening.

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.

Nox4 Promotes RANKL-Induced Autophagy and Osteoclastogenesis via Activating ROS/PERK/eIF-2α/ATF4 Pathway

Receptor activator of nuclear factor-κB ligand (RANKL) has been found to induce osteoclastogenesis and bone resorption. However, the underlying molecular mechanisms remain unclear. Via conducting a series of biochemical experiments with in vitro cell lines, this study investigated the role and mechanism of NADPH oxidase 4 (Nox4) in RANKL-induced autophagy and osteoclastogenesis. In the current study, we found that RANKL dramatically induced autophagy and osteoclastogenesis, inhibition of autophagy with chloroquine (CQ) markedly attenuates RANKL-induced osteoclastogenesis. Interestingly, we found that the protein level of Nox4 was remarkably upregulated by RANKL treatment. Inhibition of Nox4 by 5-O-methyl quercetin or knockdown of Nox4 with specific shRNA markedly attenuated RANKL-induced autophagy and osteoclastogenesis. Furthermore, we found that Nox4 stimulated the production of nonmitochondrial reactive oxygen species (ROS), activating the critical unfolded protein response (UPR)-related signaling pathway PERK/eIF-2α/ATF4, leading to RANKL-induced autophagy and osteoclastogenesis. Blocking the activation of PERK/eIF-2α/ATF4 signaling pathway either by Nox4 shRNA, ROS scavenger (NAC) or PERK inhibitor (GSK2606414) significantly inhibited autophagy during RANKL-induced osteoclastogenesis. Collectively, this study reveals that Nox4 promotes RANKL-induced autophagy and osteoclastogenesis via activating ROS/PERK/eIF-2α/ATF4 pathway, suggesting that the pathway may be a novel potential therapeutic target for osteoclastogenesis-related disease.

Beclin 1 positively regulates osteoprotegerin-induced inhibition of osteoclastogenesis by increasing autophagy in vitro

Osteoclastogenesis is induced by receptor activator of nuclear factor-κB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF), and can be suppressed by osteoprotegerin (OPG). Beclin1 has a dual role in osteoclastogenesis. However, the role of Beclin1-mediated autophagy during OPG-induced inhibition of osteoclastogenesis remains unclear. Here, we found that Beclin1 and matrix metalloproteinase 9 (MMP-9) expression were increased during osteoclastogenesis. OPG (20, 40, and 80 ng/mL) decreased Src and MMP-9 expression, but augmented Beclin1 expression and fluorescence intensity. Similarly, treatment with the autophagy activator rapamycin increased Beclin1 expression during OPG-induced inhibition of osteoclastogenesis. Further, Beclin1 knockdown restored osteoclast numbers by reducing autophagy during OPG-induced inhibition of osteoclastogenesis. These results indicate that Beclin1 has a positive role during OPG-induced inhibition of osteoclastogenesis by regulating autophagy, which might provide a potential basis for osteoclastogenesis.

Amino acid metabolism and autophagy in skeletal development and homeostasis

Bone is an active organ that is continuously remodeled throughout life via formation and resorption; therefore, a fine-tuned bone (re)modeling is crucial for bone homeostasis and is closely connected with energy metabolism. Amino acids are essential for various cellular functions as well as an energy source, and their synthesis and catabolism (e.g., metabolism of carbohydrates and fatty acids) are regulated through numerous enzymatic cascades. In addition, the intracellular levels of amino acids are maintained by autophagy, a cellular recycling system for proteins and organelles; under nutrient deprivation conditions, autophagy is strongly induced to compensate for cellular demands and to restore the amino acid pool. Metabolites derived from amino acids are known to be precursors of bioactive molecules such as second messengers and neurotransmitters, which control various cellular processes, including cell proliferation, differentiation, and homeostasis. Thus, amino acid metabolism and autophagy are tightly and reciprocally regulated in our bodies. This review discusses the current knowledge and potential links between bone diseases and deficiencies in amino acid metabolism and autophagy.

Simvastatin intervention mitigates hypercholesterolemia-induced alveolar bone resorption in rats

Simvastatin promotes bone formation and increases bone mineral density in patients with hyperlipidemia and ameliorates hypercholesterolemia-induced microstructure changes in the jaw bone of animals. However, whether and how treatment with simvastatin can modulate the hypercholesterolemia-induced alveolar bone resorption is unclear. The present study aimed to examine the therapeutic efficacy and potential mechanisms of simvastatin application in hypercholesterolemia-induced alveolar bone resorption. The association between hyperlipidemia and alveolar bone resorption in 100 patients with periodontitis was examined. Additionally, male Sprague-Dawley rats were fed a standard rodent chow (NC) for 32 weeks or a high cholesterol diet (HCD) for 24 weeks. The HCD-fed rats were randomized, continually fed with HCD and treated with vehicle saline (HC) or simvastatin by gavage (5 mg/kg; SIM, n=10/group) for 8 weeks. The morphological changes to alveolar bone resorption in rats were analyzed by linear measurements. The relative levels of osteoprotegerin (OPG), receptor activator of nuclear factor-κB ligand RANKL, nuclear factor-κB (NF-κB), microtubule-associated protein 1 light chain 3 (LC3) and p62 in the alveolar bone tissues were determined by reverse transcription-quantitative PCR and/or immunohistochemistry. Sulcus bleeding index (SBI), clinical attachment loss (CAL), probing depth (PD) and the distance of cemantoenamel junction-alveolar bone crest (CEJ-ABC) in patients with hyperlipidemia were significantly greater than that in the controls (P<0.001). The levels of hyperlipidemia were positively correlated with the values of SBI, CAL, PD and CEJ-ABC in this population. Compared with the NC rats, higher levels of alveolar bone resorption, NF-κB expression, higher ratios of RANKL/OPG mRNA transcripts and LC3 to p62 expression were detected in the alveolar bone tissues of HC group. Simvastatin intervention significantly mitigated hypercholesterolemia-induced alveolar bone loss and RANKL mRNA transcription, but increased the ratios of LC3/p62 protein expression in the alveolar bone tissues of rats. Hyperlipidemia is associated with alveolar bone resorption and simvastatin treatment alleviated the hypercholesterolemia-related alveolar bone loss by down-regulating the NF-κB expression.

Syndecan 4 contributes to osteoclast differentiation induced by RANKL through enhancing autophagy

Periodontitis is a common chronic disease. Osteoclast differentiation contributes to alveolar bone resorption which is a distinct phenomenon during periodontitis. Syndecan 4 (SDC4), a member of the syndecan family, was found to be highly expressed during periodontitis. However, little is known about its role in periodontitis. Herein, we explored the role of SDC4 in osteoclast differentiation. An experimental periodontitis rat model was established by ligating the right first molar. The SDC4 expression in periodontium was detected by western blot and immunofluorescence. Our study demonstrated that SDC4 was highly expressed in the periodontium of periodontitis rats. It was positively transcriptionally regulated by NF-κB. SDC4 silencing abrogated osteoclast differentiation induced by RANKL, while SDC4 overexpression enhanced osteoclast differentiation. Moreover, SDC4 enhanced autophagy induced by RANKL. 3-MA, an autophagy inhibitor, was employed to explore whether SDC4 impacts osteoclast differentiation through activating autophagy. Treatment with 3-MA abolished osteoclast differentiation which was enhanced by SDC4, indicating that SDC4 promotes osteoclast differentiation through activating autophagy. This study reveals that SDC4 may contribute to osteoclast differentiation during periodontitis through activating autophagy. It sheds light on the important role of SDC4 in periodontitis.

Archetypal autophagic players through new lenses for bone marrow stem/mature cells regulation

The bone marrow landscape consists of specialized and stem/progenitor cells, which coordinate important tissue-related and systemic physiological features. Within the marrow cavity, stem/progenitor and differentiated hematopoietic and skeletal cells congregate into dynamic functional assemblies throughout specific anatomical regions, termed niches. There is a need for better understanding of the bone marrow microareas, through exploration of the intramural physical and molecular interactions of the distinctive cell populations. The elective liaisons established among the mesenchymal/stromal stem cell and hematopoietic stem cell lineage trees play a key role in orchestrating the stem/mature cell behavior and customized hierarchies within bone marrow cell populations. Recently, the autophagic apparatus has been discovered to be an important feature of bone marrow homeostasis. Autophagy-related factors involved in the labyrinthic and highly dynamic bone marrow workshop redesign the niche framework by coordinating the operational schedule of pluripotent stem and mature cells. The following report summarizes the most recent breakthroughs in our understanding of the intramural relationships between bone marrow cells and key autophagic mediators. Doubtless, the consideration of the autophagy-related and unrelated functions of main players, such as p62, Atg7, Atg5, and Beclin-1 remains a compelling task to thoroughly understand the complex relations between the heterogenic cell types that populate bone marrow.

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.

Immunoexpression pattern of autophagy mediators in alveolar bone osteoclasts following estrogen withdrawal in female rats

It is known that estrogen deficiency increases osteoclast formation and activity. Autophagy, a cell survival pathway, has been shown to be crucial for osteoclast function. However, little is known about the effects of estrogen depletion on osteoclast autophagy. Here, we evaluated the effects of estrogen deficiency in the immunoexpression of autophagy mediators in alveolar bone osteoclasts of ovariectomized rats. Twelve adult female rats were ovariectomized (OVX-group) or SHAM-operated (SHAM-group). After three weeks, the rats were euthanized and maxillary fragments containing alveolar bone of the first molars were processed for light microscopy or transmission electron microscopy (TEM). Paraffin-sections were subjected to the TRAP method (osteoclast marker) or to the immunohistochemical detections of beclin-1, LC3α, and p62 (autophagy mediators); araldite-sections were processed for TEM. The number of TRAP-positive osteoclasts and the number of immunolabeled-multinucleated cells (MNCs) along the alveolar bone surface of the first molar were computed. The number of TRAP-positive osteoclasts and the number of beclin-1-, LC3α- and p62-immunolabelled osteoclasts were significantly higher in OVX-group than the SHAM-group. MNCs were frequently located juxtaposed to Howship lacunae along the alveolar bone surface, indicating that these cells are osteoclasts. TEM revealed osteoclasts exhibiting autophagosomes. Our data indicate that autophagy plays an important role during estrogen deficiency-induced osteoclastogenesis. Thus, our results contribute to a better understanding on the role of autophagy on osteoclasts under estrogenic deficiency, and reinforce the idea that modulation of autophagy may be a useful tool to inhibit excessive oral bone resorption in post-menopausal women.

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.

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.

Long Non-Coding RNA H19 Participates in Periodontal Inflammation via Activation of Autophagy

Purpose

Periodontitis is the leading cause of tooth loss. The role of long non-coding RNA (lncRNA) in periodontal inflammation remains unclear. The aim of this study was to investigate the role of lncRNA H19 in periodontitis and its possible regulation of autophagy in periodontitis.

Material and Methods

Inflammation level was determined by quantitative reverse-transcription polymerase chain reaction (qRT-PCR) and enzyme-linked immunosorbent assay (ELISA) in periodontal ligament cells (PDLCs). Western blotting, flow cytometric analysis, and immunofluorescence staining were used to detect the autophagy flux. Overexpression or knockdown of H19 was used to confirm its function. Ligature-induced periodontitis model in mice and periodontitis-affected human gingival tissue were used in vivo. RNA sequencing was performed to determine the differentially expressed genes.

Results

Autophagy was significantly increased in PDLCs after inflammatory stimulation as well as in a ligature-induced periodontitis model in mice and periodontitis-affected human gingival tissue. During the inflammatory process, H19 expression was also significantly upregulated. Further, the levels of autophagic markers were significantly upregulated after overexpressing H19 in PDLCs, and the increased autophagic activity induced by inflammatory stimulation was reversed by H19 knockdown. RNA sequencing showed that the expression profiles of mRNAs were significantly altered after H19 overexpression, and the differentially expressed genes were enriched in the PI3K/AKT signaling pathway, which was confirmed by the decreased p-AKT protein expression in the H19 overexpression group.

Conclusion

Periodontal inflammation activates autophagy flux, and H19 mediates the activation of autophagy via AKT pathway in periodontitis. This study expands our understanding of molecular regulation in periodontitis.

Knockout of NRAGE promotes autophagy-related gene expression and the periodontitis process in mice

BACKGROUND AND OBJECTIVE

Neurotrophin receptor-interacting MAGE homologue (NRAGE) plays a crucial role in the regulation of bone metabolism. The present study investigated the regulation role of NRAGE on autophagy activation and periodontitis process during experimental periodontitis.

MATERIALS AND METHODS

Six-week-old wild-type (WT) and NRAGE-/- mice were randomly divided into three time points in the periodontitis groups (0, 2, and 4 weeks). Histopathological changes were determined using the tooth mobility, hematoxylin and eosin (H&E) staining, and micro-computed tomography (micro-CT). Osteoclasts activation and number were investigated using tartrate-resistant acid phosphatase (TRAP) staining, immunohistochemistry, and real-time quantitative PCR (RT-PCR). The level of autophagy-related gene expression was measured using immunohistochemistry, immunofluorescence, and RT-PCR.

RESULTS

H&E staining and Micro-CT showed that the destruction of the alveolar bone was considerably more severe in the NRAGE-/- group than the WT group after ligation. Tooth mobility in the NRAGE-/- group was obviously higher than that in the WT group. The activation and number of osteoclasts and the level of autophagy-related gene expression in NRAGE-/- group were significantly higher than that in WT group.

CONCLUSIONS

The present study showed that knockout of NRAGE induced autophagy-related gene expression and accelerated the process of periodontitis disease via increasing the activity and differentiation of osteoclast.

Glucocorticoids Enhanced Osteoclast Autophagy Through the PI3K/Akt/mTOR Signaling Pathway

Autophagy is an evolutionarily conserved dynamic process and present in variety of cells at basal levels to maintain homeostasis and to promote cell survival in response to stresses. The early bone loss with excessive glucocorticoids (GCs) was reported to be related with the extension of the life span of osteoclasts. However, the connection between GCs induced bone loss and osteoclast autophagy remains to be elucidated. Autophagy was detected in a Dexamethasone (Dex) induced osteoporotic mice model and primary osteoclast cultures by autophagosome detection kit, and autophagy-related proteins were assayed by Western blotting and Immunostaining. The bone morphology was examined by micro-CT and TRAP staining. The trabecular bone micro-architecture was deteriorated, and the osteoclast number and spread area were increased in the Dex-treated mice compared with the control group (P < 0.01). Meanwhile, autophagy in pre-osteoclasts was increased in mice under Dex administration evidenced by the increased number of autophagosome and up-regulation of autophagy-related protein levels. Further, the enhanced autophagy under Dex treatment was verified in primary cultured osteoclasts, as shown by the increased levels of Beclin 1 and LC3-II/LC3-I and the autophagy complex formation members including Atg1, Atg13, and Atg7. However, the expressions of PI3K, p-Akt and p-mTOR in primary cultured osteoclasts were inhibited under Dex induced autophagy. Using the selective PTEN inhibitor SF1670 to activate the PI3K/Akt/mTOR pathway reversed this osteoclast autophagy under Dex treatment. Our study suggests that osteoclast autophagy was enhanced in glucocorticoids induced bone loss, and the PI3K/Akt/mTOR signaling pathway mediated the increased autophagy in primary cultured osteoclasts under glucocorticoids treatment.

Autophagy: A Promising Target for Age-related Osteoporosis

Autophagy is a process the primary role of which is to clear up damaged cellular components such as long-lived proteins and organelles, thus participating in the conservation of different cells. Osteoporosis associated with aging is characterized by consistent changes in bone metabolism with suppression of bone formation as well as increased bone resorption. In advanced age, not only bone mass but also bone strength decrease in both sexes, resulting in an increased incidence of fractures. Clinical and animal experiments reveal that age-related bone loss is associated with many factors such as accumulation of autophagy, increased levels of reactive oxygen species, sex hormone deficiency, and high levels of endogenous glucocorticoids. Available basic and clinical studies indicate that age-associated factors can regulate autophagy. Those factors play important roles in bone remodeling and contribute to decreased bone mass and bone strength with aging. In this review, we summarize the mechanisms involved in bone metabolism related to aging and autophagy, supplying a theory for therapeutic targets to rescue bone mass and bone strength in older people.

Kynurenine inhibits autophagy and promotes senescence in aged bone marrow mesenchymal stem cells through the aryl hydrocarbon receptor pathway

Osteoporosis is an age-related deterioration in bone health that is, at least in part, a stem cell disease. The different mechanisms and signaling pathways that change with age and contribute to the development of osteoporosis are being identified. One key upstream mechanism that appears to target a number of osteogenic pathways with age is kynurenine, a tryptophan metabolite and an endogenous Aryl hydrocarbon receptor (AhR) agonist. The AhR signaling pathway has been reported to promote aging phenotypes across species and in different tissues. We previously found that kynurenine accumulates with age in the plasma and various tissues including bone and induces bone loss and osteoporosis in mice. Bone marrow mesenchymal stem cells (BMSCs) are responsible for osteogenesis, adipogenesis, and overall bone regeneration. In the present study, we investigated the effect of kynurenine on BMSCs, with a focus on autophagy and senescence as two cellular processes that control BMSCs proliferation and differentiation capacity. We found that physiological levels of kynurenine (10 and 100 μM) disrupted autophagic flux as evidenced by the reduction of LC3B-II, and autophagolysosomal production, as well as a significant increase of p62 protein level. Additionally, kynurenine also induced a senescent phenotype in BMSCs as shown by the increased expression of several senescence markers including senescence associated β-galactosidase in BMSCs. Additionally, western blotting reveals that levels of p21, another marker of senescence, also increased in kynurenine-treated BMSCs, while senescent-associated aggregation of nuclear H3K9me3 also showed a significant increase in response to kynurenine treatment. To validate that these effects are in fact due to AhR signaling pathway, we utilized two known AhR antagonists: CH-223191, and 3',4'-dimethoxyflavone to try to block AhR signaling and rescue kynurenine /AhR mediated effects. Indeed, AhR inhibition restored kynurenine-suppressed autophagy levels as shown by levels of LC3B-II, p62 and autophagolysosomal formation demonstrating a rescuing of autophagic flux. Furthermore, inhibition of AhR signaling prevented the kynurenine-induced increase in senescence associated β-galactosidase and p21 levels, as well as blocking aggregation of nuclear H3K9me3. Taken together, our results suggest that kynurenine inhibits autophagy and induces senescence in BMSCs via AhR signaling, and that this may be a novel target to prevent or reduce age-associated bone loss and osteoporosis.

Curcumin-activated autophagy plays a negative role in its anti-osteoclastogenic effect

BACKGROUND/PURPOSE

It remains unclear what role curcumin plays in the autophagy of osteoclast precursors (OCPs) during osteoclastogenesis, since some researchers found that curcumin has the ability to inhibit osteoclastogenesis. While others have considered it as an autophagy activator. This study aimed to determine the effect of curcumin-regulated autophagy on osteoclastogenesis.

RESULTS

The results revealed that direct administration of curcumin enhanced the OCP autophagy response in bone marrow-derived macrophages (BMMs). Curcumin could also abate RANKL's stimulatory effect on OCP autophagy and osteoclastogenesis. Autophagic suppression related to pharmacological inhibitors or gene silencing could further enhance the inhibitory effect of curcumin on osteoclastogenesis. As expected, curcumin ameliorated ovariectomy (OVX)-induced bone loss and its effect could be promoted by an autophagy inhibitor (chloroquine).

CONCLUSIONS

In conclusion, curcumin can directly enhance the autophagic activity of OCPs, which inhibits its anti-osteoclastogeneic effects. Autophagy inhibition-based drugs are expected to enhance curcumin's efficacy in treating osteoporosis.

Puerarin inhibits the osteoclastogenesis by inhibiting RANKL-dependent and -independent autophagic responses

Background

Puerarin exerts therapeutic effect on osteoporosis due to its inhibitory effect on the formation of osteoclasts. Puerarin is also widely established as an autophagy inhibitor. The study aimed to investigate the significance of autophagy in Puerarin-treated osteoclast formation.

Methods

Osteoclast precursors (OCPs) derived from bone marrow-derived macrophages (BMMs) were treated with Puerarin along with RANKL or without RANKL, and then the autophagic parameters of OCPs (including autophagic proteins, LC3 transformation, autophagosome or LC3-puncta) were observed through Western Blotting, Transmission Electron Microscopy and Immunofluorescence assays. Next, after using overexpression vectors of autophagic genes (Atg7, Atg5 and BECN1) to alter autophagy activity, OCP proliferation was measured by Ethynyl deoxyuridine (EdU) assays and Cell Counting Kit-8 (CCK-8) kit, and osteoclast differentiation was assessed by Tartrate-resistant acid phosphatase (TRAP) staining.

Results

The results showed that Puerarin could directly inhibit the autophagy and proliferation of OCPs. Importantly, overexpression of autophagic genes Atg5, Atg7 and BECN1 reversed Puerarin-inhibited OCP autophagy and proliferation. What’s more, RANKL could promote the autography of OCPs, which was recovered by Puerarin treatment. Interestingly, different from single-Puerarin treatment, we found that in the presence of RANKL, only BECN1 overexpression significantly reversed Puerarin-inhibited osteoclast differentiation and OCP autophagy.

Conclusion

In conclusion, Puerarin could inhibit the OCP autophagy in the presence or absence of RANKL, which blocked the OCP proliferation and osteoclast differentiation respectively. Moreover, BECN1 plays an essential role in Puerarin-inhibited osteoclastogenesis. Our study provides potential clue to further complete the intrinsic mechanism of Puerarin in treating osteoporosis.

Beclin1 Modulates Bone Homeostasis by Regulating Osteoclast and Chondrocyte Differentiation

Autophagy (ATG), an important cellular recycling process whereby macromolecules or organelles are encapsulated by autophagosome and degraded upon merging with lysosome, has recently been shown to play an essential role in bone biology. However, the involvement of ATG in bone and bone-related cells remains unclear. Here, we show that Beclin1, an ATG-related protein involved in ATG initiation, plays a pivotal role in osteoclasts. ATG was activated during osteoclast differentiation in vitro. Beclin1 was enhanced and required for osteoclast differentiation. Mechanistically, we found that TRAF6-mediated ubiquitination of Beclin1 at K117, but not ULK1-mediated phosphorylation, is required for RANKL-stimulated osteoclast differentiation. In vivo, mice lacking Beclin1 in CstK-expressing cells exhibited an increased cortical bone thickness caused by impaired osteoclasts' function. Interestingly, these mice also exhibited diminished trabecular bone mass, which was associated with a defect in cartilage formation and chondrocyte differentiation. Collectively, our study highlights the functional importance of ATG in osteoclasts and chondrocytes, and identifies ATG as a potential therapeutic target for managing bone-related diseases. © 2019 American Society for Bone and Mineral Research.

Tanshinone IIA Can Inhibit Angiotensin II-Induced Proliferation and Autophagy of Vascular Smooth Muscle Cells via Regulating the MAPK Signaling Pathway

To examine the effect of tanshinone IIA on Angiotensin II (Ang II)-induced proliferation and autophagy in vascular smooth muscle cells (VSMCs) and the related mechanism. VSMCs were treated with Ang II with or without tanshinone IIA (1, 5 and 10 µg/mL), and the proliferation, apoptosis in cells with different treatment were examined by methylthiazolyl tetrazolium (MTT) and flow cytometry methods. Moreover, the expression of autophagy related proteins and mitogen-activated protein kinase (MAPK) signaling molecules were examined by RT-quantitative (q)PCR and Western blot methods. Ang II induced significantly increase in the proliferation and autophagy of VSMCs, and the MAPK signaling was activated. Tanshinone IIA can attenuate Ang II-induced effects via down-regulating the MAPK signaling pathway. Tanshinone IIA can inhibit Ang II-induced proliferation and autophagy of VSMCs via regulating the MAPK signaling pathway.

JNK1 regulates RANKL-induced osteoclastogenesis via activation of a novel Bcl-2-Beclin1-autophagy pathway

JNK1 plays an important role in osteoclastogenesis in response to the osteoclastogenic cytokine receptor activator for nuclear factor-κB ligand (RANKL). JNK1 is widely accepted as an autophagy regulator under stress conditions. However, the role of JNK1-mediated autophagy in osteoclastogenesis remains largely unknown. In the current study, our data showed that JNK1 inhibition by a pharmacological inhibitor or RNA interference significantly reduced the autophagic response induced by RANKL in osteoclast precursors (OCPs) derived from bone marrow-derived macrophages. Overexpression of the key autophagy protein Beclin1 rescued autophagy deficiency and osteoclastogenesis in the presence of a JNK inhibitor (SP600125). In contrast, JNK activator (anisomycin)-induced autophagy was blocked by Beclin1 knockdown in OCPs. In addition, JNK1 inhibition increased apoptosis and blocked autophagy, whereas overexpression of Beclin1 reversed the enhanced apoptosis induced by JNK1 inhibition in OCPs. Furthermore, RANKL could induce the phosphorylation of Bcl-2, subsequently dissociating Beclin1 from the Bcl-2-Beclin1 complex, which could be blocked by JNK1 inhibition. Collectively, this study revealed that JNK1 regulated osteoclastogenesis by activating Bcl-2-Beclin1-autophagy signaling in addition to the classic c-Jun/activator protein 1 pathway, which provided the first evidence for the contribution of JNK1 signaling to OCP autophagy and the autophagic mechanism underlying JNK1-regulated osteoclastogenesis. An important osteoclastogenesis-regulating signaling pathway (JNK1-Bcl-2-Beclin1-autophagy activation) was identified, which provides novel potential targets for the clinical therapy of metabolic bone diseases.-Ke, D., Ji, L., Wang, Y., Fu, X., Chen, J., Wang, F., Zhao, D., Xue, Y., Lan, X., Hou, J. JNK1 regulates RANKL-induced osteoclastogenesis via activation of a novel Bcl-2-Beclin1-autophagy pathway.

KLF2 (kruppel-like factor 2 [lung]) regulates osteoclastogenesis by modulating autophagy

Macroautophagy/autophagy is involved in myeloid cellular repair, destruction, and osteoclast differentiation; conversely, KLF2 (kruppel-like factor 2 [lung]) regulates myeloid cell activation and differentiation. To investigate the specific role of KLF2 in autophagy, osteoclastic differentiation was induced in monocytes in presence or absence of the autophagy inhibitor 3-methyladenine (3-MA), KLF2 inducer geranylgeranyl transferase inhibitor (GGTI298), and adenoviral overexpression of KLF2. We found that the number of autophagic cells and multinucleated osteoclasts were significantly decreased in presence of 3-MA, GGTI298, and KLF2 overexpressed cells indicating involvement of KLF2 in these processes. In addition, autophagy-related protein molecules were significantly decreased after induction of KLF2 during the course of osteoclastic differentiation. Furthermore, induction of arthritis in mice reduced the level of Klf2 in monocytes, and enhanced autophagy during osteoclastic differentiation. Mechanistically, knocking down of KLF2 increased the level of Beclin1 (BECN1) expression, and conversely, KLF2 over-expression reduced the level of BECN1 in monocytes. Moreover, 3-MA and GGTI298 both reduced myeloid cell proliferation concomitantly upregulating senescence-related molecules (CDKN1A/p21 and CDKN1B/p27^kip1). We further confirmed epigenetic regulation of Becn1 by modulating Klf2; knocking down of Klf2 increased the levels of histone activation marks H3K9 and H4K8 acetylation in the promoter region of Becn1; and overexpression of Klf2 decreased the levels of H4K8 and H3K9 acetylation. In addition, osteoclastic differentiation also increased levels of H3K9 and H4K8 acetylation in the promoter region of Becn1. Together these findings for the first time revealed that Klf2 critically regulates Becn1-mediated autophagy process during osteoclastogenesis.Abbreviations: ACP5/TRAP: acid phosphatase 5, tartrate resistant; Ad-KLF2: adenoviral construct of KLF2; ATG3: autophagy related 3; ATG5: autophagy related 5; ATG7: autophagy related 7; ATG12: autophagy related 12; BECN1: beclin 1, autophagy related; C57BL/6: inbred mouse strain C57 black 6; ChIP: chromatin immunoprecipitation; CSF1/MCSF: colony stimulating factor 1 (macrophage); CTSK: cathepsin K; EV: empty vector; GGTI298: geranylgeranyl transferase inhibitor; H3K9Ac: histone H3 lysine 9 acetylation; H4K8Ac: histone H4 lysine 8 acetylation; K/BxN mice: T cell receptor (TCR) transgene KRN and the MHC class II molecule A(g7) generates K/BxN mice; KLF2: kruppel-like factor 2 (lung); 3MA: 3-methyladenine; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; MDC: monodansylcadaverine; NFATc1: nuclear factor of activated T cells 1; NFKB: nuclear factor of kappa light polypeptide gene enhancer in B cells; p21/CDKN1A: cyclin dependent kinase inhibitor 1A; p27^kip1/CDKN1B: cyclin-dependent kinase inhibitor 1B; PCR: polymerase chain reaction; PtdIns3K: phosphoinositide 3-kinase; RA: rheumatoid arthritis; siKlf2: small interfering KLF2 ribonucleic acid; NS: non-specific; RAW 264.7: abelson murine leukemia virus transformed macrophage cell line; TNFSF11/RANKL: tumor necrosis factor (ligand) superfamily, member 11; TSS: transcriptional start site; UCSC: University of California, Santa Cruz.

Interleukin‑17A facilitates osteoclast differentiation and bone resorption via activation of autophagy in mouse bone marrow macrophages

Interleukin 17A (IL-17A) exerts pleiotropic effects on periodontitis, partially through enhancement of alveolar bone loss. Osteoclasts are the main culprits that absorb alveolar bone. However, studies describing the correlation between IL-17A and osteoclasts are not conclusive. Previously, autophagy was revealed to be involved in osteoclast differentiation and bone resorption. However, the role of autophagy in IL-17A-mediated osteoclast formation is yet to be clarified. In the present study, bone marrow macrophages (BMMs) were treated with or without IL-17A. 3-Methyladenine (3-MA) was applied to inhibit autophagy. Osteoclast formation was detected by tartrate-resistant acid phosphatase (TRAP) staining, immunofluorescence, and scanning electron microscope. The effects of IL-17A on osteoclast-specific genes and autophagy-related genes during osteoclast differentiation were examined by real-time quantitative polymerase chain reaction and western blot analysis. Autophagosomes were observed by transmission electron microscope. Hematoxylin and eosin (H&E), and TRAP staining was adopted to assess alveolar bone destruction and the number of osteoclasts, respectively in a rat periodontitis model. Consequently, IL-17A stimulated osteoclast differentiation and bone resorption of BMMs accompanied by an increase in the mRNA expression of osteoclast-specific genes. Furthermore, IL-17A increased the levels of autophagy-related genes and proteins, and inhibition of autophagy with 3-MA attenuated the IL-17A-mediated osteoclastogenesis. In addition, there was an increase in the number of osteoclasts and alveolar bone resorption with IL-17A treatment in the periodontitis rat model. Collectively, these findings indicated that IL-17A facilitated osteoclast differentiation and bone resorption in vitro and in vivo, which may contribute to the understanding of the molecular basis of IL-17A in alveolar bone destruction and provide insight on the clinical therapeutic targets for periodontitis.

Role of autophagy in osteosarcoma

Osteosarcoma (OS) is the most common primary bone tumour in children and adolescents. It is a highly aggressive tumor with a tendency to spread to the lungs, which are the most common site of metastasis. Advanced osteosarcoma patients with metastasis share a poor prognosis. Despite the use of chemotherapy to treat OS, the 5-year overall survival rate for patients has remained unchanged at 65–70% for the past 20 years. In addition, the 5-year survival of patients with a metastatic disease is around 20%, highlighting the need for novel therapeutic targets.

Autophagy is an intracellular degradation process which eliminates and recycles damaged proteins and organelles to improve cell lifespan. In the context of cancer, numerous studies have demonstrated that autophagy is used by tumor cells to repress initial steps of carcinogenesis and/or support the survival and growth of established tumors. In osteosarcoma, autophagy appears to be deregulated and could also act both as a pro or anti-tumoral process. In this manuscript, we aim to review these major findings regarding the role of autophagy in osteosarcoma.

Autophagy plays an essential role in bone homeostasis

Autophagy is very critical for multiple cellular processes. Autophagy plays a critical role in bone cell differentiation and function.

IL-17A modulates osteoclast precursors' apoptosis through autophagy-TRAF3 signaling during osteoclastogenesis

Osteoclasts play an important role in bone remodeling. The inflammatory cytokine IL-17A could modulate the RANKL-induced osteoclastogenesis by regulating the autophagic activity. It is well accepted that protective autophagy has an anti-apoptotic effect. It is necessary to elucidate whether IL-17A can influence the apoptosis of osteoclast precursors (OCPs) through autophagy responses during osteoclastogenesis. The results showed that apoptosis of RAW264.7-derived OCPs was promoted by high levels of IL-17A, but the opposite anti-apoptotic function was shown by low levels of IL-17A. Furthermore, the enhanced apoptosis by high levels of IL-17A was reversed by overexpression of autophagy protein Beclin1; conversely, the inhibited apoptosis by low levels of IL-17A was restored by knockdown of Beclin1. It was also found that Beclin1 suppression with Beclin1 inhibitor (spautin1) could block the reduced apoptosis by low levels of IL-17A, which was recovered by TRAF3 knockdown. Moreover, the enhanced apoptosis by high levels of IL-17A decreased following the downregulation of TRAF3. Importantly, overexpression of caspase3 further attenuated osteoclastogenesis treated by high levels of IL-17A, without significantly affecting osteoclastogenesis stimulated by low levels of IL-17A. In conclusion, IL-17A modulates apoptosis of OCPs through Beclin1-autophagy-TRAF3 signaling pathway, thereby influencing osteoclastogenesis. Therefore, our study sheds lights on the improvement of clinical strategies of dental implantation or orthodontic treatment by revealing the novel targets in the bone remodeling.

Roles of Mitogen-Activated Protein Kinases in Osteoclast Biology

Bone undergoes continuous remodeling, which is homeostatically regulated by concerted communication between bone-forming osteoblasts and bone-degrading osteoclasts. Multinucleated giant osteoclasts are the only specialized cells that degrade or resorb the organic and inorganic bone components. They secrete proteases (e.g., cathepsin K) that degrade the organic collagenous matrix and establish localized acidosis at the bone-resorbing site through proton-pumping to facilitate the dissolution of inorganic mineral. Osteoporosis, the most common bone disease, is caused by excessive bone resorption, highlighting the crucial role of osteoclasts in intact bone remodeling. Signaling mediated by mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38, has been recognized to be critical for normal osteoclast differentiation and activation. Various exogenous (e.g., toll-like receptor agonists) and endogenous (e.g., growth factors and inflammatory cytokines) stimuli contribute to determining whether MAPKs positively or negatively regulate osteoclast adhesion, migration, fusion and survival, and osteoclastic bone resorption. In this review, we delineate the unique roles of MAPKs in osteoclast metabolism and provide an overview of the upstream regulators that activate or inhibit MAPKs and their downstream targets. Furthermore, we discuss the current knowledge about the differential kinetics of ERK, JNK, and p38, and the crosstalk between MAPKs in osteoclast metabolism.

Nano-sized Al2O3 particle-induced autophagy reduces osteolysis in aseptic loosening of total hip arthroplasty by negative feedback regulation of RANKL expression in fibroblasts

Aseptic loosening is mainly caused by wear debris generated by friction that can increase the expression of receptor activation of nuclear factor (NF)-κB (RANKL). RANKL has been shown to support the differentiation and maturation of osteoclasts. Although autophagy is a key metabolic pathway for maintaining the metabolic homeostasis of cells, no study has determined whether autophagy induced by Al₂O₃ particles is involved in the pathogenesis of aseptic loosening. The aim of this study was to evaluate RANKL levels in patients experiencing aseptic loosening after total hip arthroplasty (THA) and hip osteoarthritis (hOA) and to consequently clarify the relationship between RANKL and LC3II expression. We determined the levels of RANKL and autophagy in fibroblasts treated with Al₂O₃ particles in vitro while using shBECN-1 interference lentivirus vectors to block the autophagy pathway and BECN-1 overexpression lentivirus vectors to promote autophagy. We established a novel rat model of femoral head replacement and analyzed the effects of Al₂O₃ particles on autophagy levels and RANKL expression in synovial tissues in vivo. The RANKL levels in the revision total hip arthroplasty (rTHA) group were higher than those in the hOA group. In patients with rTHA with a ceramic interface, LC3II expression was high, whereas RANKL expression was low. The in vitro results showed that Al₂O₃ particles promoted fibroblast autophagy in a time- and dose-dependent manner and that RANKL expression was negatively correlated with autophagy. The in vivo results further confirmed these findings. Al₂O₃ particles induced fibroblast autophagy, which reduced RANKL expression. Decreasing the autophagy level promoted osteolysis and aseptic prosthetic loosening, whereas increasing the autophagy level reversed this trend.

A paeonol derivative, YPH-PA3 promotes the differentiation of monocyte/macrophage lineage precursor cells into osteoblasts and enhances their autophagy

Previous studies have indicated that paeonol inhibits RANKL-induced osteoclastogenesis by inhibiting the ERK, p38, and NF-κB pathway. We modified paeonol to form a new compound, YPH-PA3, and found that it promoted osteoclastogenesis rather than inhibited it the way paeonol does. The aim of this study is to investigate the mechanisms involved in YPH-PA3-promoted osteoclastogenesis. YPH-PA3-promoted differentiation of RAW264.7 cells (human monocytes) into osteoclasts is activated through ERK/p38/JNK phosphorylation, affecting c-FOS, NF-κB, and NFATc2. Real-time quantitative PCR and western blot revealed an increased expression of autophagy-related markers during YPH-PA3-induced osteoclastogenesis. We also demonstrated the relationship between p62/LC3 localization and F-actin ring formation by double-labeling immunofluorescence. Knockdown of p62 small-interfering RNA (siRNA) attenuated YPH-PA3-induced expression of autophagy-related genes. Our study results indicated that p62 may play a role in YPH-PA3-induced autophagy and osteoclastogenesis, which may help to develop a novel therapeutic strategy against osteoclastogenesis-related diseases.

Overactive autophagy is a pathological mechanism underlying premature suture ossification in nonsyndromic craniosynostosis

Nonsyndromic craniosynostosis (NSC) is the most common craniosynostosis with the primary defect being one or more fused sutures. In contrast to syndromic craniosynostosis, the etiopathogenesis of NSC is largely unknown. Here we show that autophagy, a major catabolic process required for the maintenance of bone homeostasis and bone growth, is a pathological change associated with NSC. Using calvarial suture mesenchymal cells (SMCs) isolated from the fused and unfused sutures of NSC patients, we demonstrate that during SMC differentiation, the level of the autophagosomal marker LC3-II increases as osteogenic differentiation progresses, particularly at differentiation day 7, a stage concurrent with mineralization. In fused SMCs, autophagic induction was more robust than that in unfused SMCs, which consequently led to enhanced mineralized nodule formation. Perturbation of autophagy with rapamycin or wortmannin promoted or inhibited the ossification of SMCs, respectively. Our findings suggest that autophagy is essential for the osteogenic differentiation of SMCs and that overactive autophagy is a molecular abnormality underlying premature calvarial ossification in NSC.

IL-17A regulates the autophagic activity of osteoclast precursors through RANKL-JNK1 signaling during osteoclastogenesis in vitro

Interleukin-17A(IL-17A), a proinflammatory cytokine, may have effects on osteoclastic resorption in inflammation-mediated bone loss, including postmenopausal osteoporosis. IL-17A could alter autophagic activity among other tissues and cells, thereby causing corresponding lesions. The aim of this study was to clarify how IL-17A influenced osteoclastogenesis by regulating autophagy. The present study showed that IL-17A could facilitate osteoclast precursors (OCPs) autophagy and osteoclastogenesis at a low concentration. Furthermore, suppression of autophagy with chloroquine (CQ) or 3-MA could significantly attenuate the enhanced osteoclastogenesis by a low level of IL-17A. It was also found that a low level of IL-17A couldn't up-regulate OCPs autophagy after removal of RANKL(Receptor Activator for Nuclear Factor-κB Ligand), and JNK(c-Jun N-terminal kinase) inhibitor only inhibited autophagy at a low level of IL-17A. These results suggest that a low concentration of IL-17A is likely to promote autophagic activity via activating RANKL-JNK pathway during osteoclastogenesis.

Osteoporosis and autophagy: What is the relationship?

Autophagy is a survival pathway wherein non-functional proteins and organelles are degraded in lysosomes for recycling and energy production. Therefore, autophagy is fundamental for the maintenance of cell viability, acting as a quality control process that prevents the accumulation of unnecessary structures and oxidative stress. Increasing evidence has shown that autophagy dysfunction is related to several pathologies including neurodegenerative diseases and cancer. Moreover, recent studies have shown that autophagy plays an important role for the maintenance of bone homeostasis. For instance, in vitro and animal and human studies indicate that autophagy dysfunction in bone cells is associated with the onset of bone diseases such as osteoporosis. This review had the purpose of discussing the issue to confirm whether a relationship between autophagy dysfunction and osteoporosis exits.

Mechanism of deoxynivalenol effects on the reproductive system and fetus malformation: Current status and future challenges

Deoxynivalenol (DON) is a toxic fungal secondary metabolite produced by molds of the Fusarium genus, and it is known to cause a spectrum of diseases both in humans and animals, such as emesis, diarrhea, anorexia, immunotoxicity, hematological disorders, impairment of maternal reproduction, and fetal development. The recently revealed teratogenic potential of DON has received much attention. In various animal models, it has been shown that DON led to skeletal deformities of the fetus. However, the underlying mechanisms are not yet fully understood, and toxicological data are also scarce. Several animal research studies highlight the potential link between morphological abnormalities and changes of autophagy in the reproductive system. Because autophagy is involved in fetal development, maintenance of placental function, and bone remodeling, this mechanism has become a high priority for future research. The general aim of the present review is to deliver a comprehensive overview of the current state of knowledge of DON-induced reproductive toxicity in different animal models and to provide some prospective ideas for further research. The focus of the current review is to summarize toxic and negative effects of DON exposure on the reproductive system and the potential underlying molecular mechanisms in various animal models.