Osteoclast precursor differentiation by MCPIP via oxidative stress, endoplasmic reticulum stress, and autophagy

RANKL promotes MT2 degradation and ROS production in osteoclast precursors through Beclin1-dependent autophagy

ROS produced under oxidative stress are crucial for osteoclast differentiation. Metallothionein (MT) is a ROS-scavenging molecule. As a member of MT family, MT2 can clear ROS in osteoclast precursors (OCPs) and contributes to osteoclast differentiation. RANKL can promote OCP autophagy. Given the molecular-degrading effect of autophagy, the relationship between RANKL-dependent autophagy, MT2 and ROS during osteoclast differentiation is worth exploring. We depended in vitro RANKL administration and RANKL-overexpressing (Tg-RANKL) mice to observe the effects of RANKL on ROS production, MT2 protein expression, Beclin1 expression and autophagic activity in OCPs. Spautin1 was used to investigate the relationship between Beclin1-dependent autophagy and RANKL-regulated MT2 expression. Osteoclast-targeting MT2-cDNA-AAVs were applied to assess the therapeutic effect of MT2 on Tg-RANKL-related bone loss. The results showed that RANKL promoted ROS production but reduced MT2 protein expression in OCPs. RANKL also enhanced Beclin1 expression and LC3-puncta abundance. Decreased Beclin1 expression with spautin1 blocked RANKL-increased ROS production and osteoclast differentiation and recovered RANKL-decreased MT2 expression. MT2 selective overexpression with CD11b-promoter-MT2-cDNA-AAVs attenuated ROS production and osteoclastogenesis in Tg-RANKL mice and improved bone loss. Overall, RANKL can reduce MT2 protein expression through Beclin1-dependent autophagy, thereby promoting ROS production and osteoclast differentiation; this suggests that MT2-overexpressing small molecule drugs have the potential to treat RANKL-related bone loss.

Role of oxidative stress in impaired type II diabetic bone repair: scope for antioxidant therapy intervention?

Impaired bone healing is a significant complication observed in individuals with type 2 diabetes mellitus (T2DM), leading to prolonged recovery, increased risk of complications, impaired quality of life, and increased risk of patient morbidity. Oxidative stress, resulting from an imbalance between the generation of reactive oxygen species (ROS) and cellular/tissue antioxidant defence mechanisms, has been identified as a critical contributor to the pathogenesis of impaired bone healing in T2DM. Antioxidants have shown promise in mitigating oxidative stress and promoting bone repair, particularly non-enzymic antioxidant entities. This comprehensive narrative review aims to explore the underlying mechanisms and intricate relationship between oxidative stress, impaired bone healing and T2DM, with a specific focus on the current preclinical and clinical evidence advocating the potential of antioxidant therapeutic interventions in improving bone healing outcomes in individuals with T2DM. From the ever-emerging evidence available, it is apparent that exogenously supplemented antioxidants, especially non-enzymic antioxidants, can ameliorate the detrimental effects of oxidative stress, inflammation, and impaired cellular function on bone healing processes during uncontrolled hyperglycaemia; and therefore, hold considerable promise as novel efficacious therapeutic entities. However, despite such conclusions, several important gaps in our knowledge remain to be addressed, including studies involving more sophisticated enzymic antioxidant-based delivery systems, further mechanistic studies into how these antioxidants exert their desirable reparative effects; and more extensive clinical trial studies into the optimisation of antioxidant therapy dosing, frequency, duration and their subsequent biodistribution and bioavailability. By enhancing our understanding of such crucial issues, we can fully exploit the oxidative stress-neutralising properties of these antioxidants to develop effective antioxidant interventions to mitigate impaired bone healing and reduce the associated complications in such T2DM patient populations.

Formation of multinucleated osteoclasts depends on an oxidized species of cell surface-associated La protein

The bone-resorbing activity of osteoclasts plays a critical role in the life-long remodeling of our bones that is perturbed in many bone loss diseases. Multinucleated osteoclasts are formed by the fusion of precursor cells, and larger cells - generated by an increased number of cell fusion events - have higher resorptive activity. We find that osteoclast fusion and bone resorption are promoted by reactive oxygen species (ROS) signaling and by an unconventional low molecular weight species of La protein, located at the osteoclast surface. Here, we develop the hypothesis that La's unique regulatory role in osteoclast multinucleation and function is controlled by an ROS switch in La trafficking. Using antibodies that recognize reduced or oxidized species of La, we find that differentiating osteoclasts enrich an oxidized species of La at the cell surface, which is distinct from the reduced La species conventionally localized within cell nuclei. ROS signaling triggers the shift from reduced to oxidized La species, its dephosphorylation and delivery to the surface of osteoclasts, where La promotes multinucleation and resorptive activity. Moreover, intracellular ROS signaling in differentiating osteoclasts oxidizes critical cysteine residues in the C-terminal half of La, producing this unconventional La species that promotes osteoclast fusion. Our findings suggest that redox signaling induces changes in the location and function of La and may represent a promising target for novel skeletal therapies.

IRE1α inhibits osteogenic differentiation of mouse embryonic fibroblasts by limiting Shh signaling

OBJECTIVES

This study aimed to investigate the effect of endoplasmic reticulum (ER) stress sensor inositol-requiring enzyme 1α (IRE1α) on the sonic hedgehog N-terminus (N-Shh)-enhanced-osteogenic differentiation process in mouse embryonic fibroblasts (MEFs).

MATERIALS AND METHODS

Osteogenesis of MEFs was observed by alkaline phosphatase (ALP) staining, alizarin red staining, and Von Kossa staining assays. Activation of unfolded protein response and Shh signaling were examined using real-time quantitative PCR and western blot assays. IRE1α-deficient MEFs were used to explore the effect of IRE1α on N-Shh-driven osteogenesis.

RESULTS

N-Shh increased ALP activity, matrix mineralization, and the expression of Alp and Col-I in MEFs under osteogenic conditions; notably, this was reversed when combined with the ER stress activator Tm treatment. Interestingly, the administration of N-Shh decreased the expression of IRE1α. Abrogation of IRE1α increased the expression of Shh pathway factors in osteogenesis-induced MEFs, contributing to the osteogenic effect of N-Shh. Moreover, IRE1α-deficient MEFs exhibited elevated levels of osteogenic markers.

CONCLUSIONS

Our findings suggest that the IRE1α-mediated unfolded protein response may alleviate the ossification of MEFs by attenuating Shh signaling. Our research has identified a strategy to inhibit excessive ossification, which may have clinical significance in preventing temporomandibular joint bony ankylosis.

IRE1α pathway: A potential bone metabolism mediator

Osteoblasts and osteoclasts collaborate in bone metabolism, facilitating bone development, maintaining normal bone density and strength, and aiding in the repair of pathological damage. Endoplasmic reticulum stress (ERS) can disrupt the intracellular equilibrium between osteoclast and osteoblast, resulting in dysfunctional bone metabolism. The inositol-requiring enzyme-1α (IRE1α) pathway-the most conservative unfolded protein response pathway activated by ERS-is crucial in regulating cell metabolism. This involvement encompasses functions such as inflammation, autophagy, and apoptosis. Many studies have highlighted the potential roles of the IRE1α pathway in osteoblasts, chondrocytes, and osteoclasts and its implication in certain bone-related diseases. These findings suggest that it may serve as a mediator for bone metabolism. However, relevant reviews on the role of the IRE1α pathway in bone metabolism remain unavailable. Therefore, this review aims to explore recent research that elucidated the intricate roles of the IRE1α pathway in bone metabolism, specifically in osteogenesis, chondrogenesis, osteoclastogenesis, and osteo-immunology. The findings may provide novel insights into regulating bone metabolism and treating bone-related diseases.

Biomaterials Designed to Modulate Reactive Oxygen Species for Enhanced Bone Regeneration in Diabetic Conditions

Diabetes mellitus, characterized by enduring hyperglycemia, precipitates oxidative stress, engendering a spectrum of complications, notably increased bone vulnerability. The genesis of reactive oxygen species (ROS), a byproduct of oxygen metabolism, instigates oxidative detriment and impairs bone metabolism in diabetic conditions. This review delves into the mechanisms of ROS generation and its impact on bone homeostasis within the context of diabetes. Furthermore, the review summarizes the cutting-edge progress in the development of ROS-neutralizing biomaterials tailored for the amelioration of diabetic osteopathy. These biomaterials are engineered to modulate ROS dynamics, thereby mitigating inflammatory responses and facilitating bone repair. Additionally, the challenges and therapeutic prospects of ROS-targeted biomaterials in clinical application of diabetic bone disease treatment is addressed.

FORMATION OF MULTINUCLEATED OSTEOCLASTS DEPENDS ON AN OXIDIZED SPECIES OF CELL SURFACE ASSOCIATED LA PROTEIN

The bone-resorbing activity of osteoclasts plays a critical role in the life-long remodeling of our bones that is perturbed in many bone loss diseases. Multinucleated osteoclasts are formed by the fusion of precursor cells, and larger cells - generated by an increased number of cell fusion events - have higher resorptive activity. We find that osteoclast fusion and bone-resorption are promoted by reactive oxygen species (ROS) signaling and by an unconventional low molecular weight species of La protein, located at the osteoclast surface. Here, we develop the hypothesis that La's unique regulatory role in osteoclast multinucleation and function is controlled by a ROS switch in La trafficking. Using antibodies that recognize reduced or oxidized species of La, we find that differentiating osteoclasts enrich an oxidized species of La at the cell surface, which is distinct from the reduced La species conventionally localized within cell nuclei. ROS signaling triggers the shift from reduced to oxidized La species, its dephosphorylation and delivery to the surface of osteoclasts, where La promotes multinucleation and resorptive activity. Moreover, intracellular ROS signaling in differentiating osteoclasts oxidizes critical cysteine residues in the C-terminal half of La, producing this unconventional La species that promotes osteoclast fusion. Our findings suggest that redox signaling induces changes in the location and function of La and may represent a promising target for novel skeletal therapies.

Osteogenic mechanism of chlorogenic acid and its application in clinical practice

Natural polyphenols may have a role in counteracting oxidative stress, which is associated with aging and several bone-related diseases. Chlorogenic acid (CGA) is a naturally occurring polyphenolic compound formed by the esterification of caffeic and quininic acids with osteogenic, antioxidant, and anti-inflammatory properties. This review discusses the potential of CGA to enhance osteogenesis by increasing the osteogenic capacity of mesenchymal stem cells (MSCs), osteoblast survival, proliferation, differentiation, and mineralization, as well as its ability to attenuate osteoclastogenesis by enhancing osteoclast apoptosis and impeding osteoclast regeneration. CGA can be involved in bone remodeling by acting directly on pro-osteoclasts/osteoblasts or indirectly on osteoclasts by activating the nuclear factor kB (RANK)/RANK ligand (RANKL)/acting osteoprotegerin (OPG) system. Finally, we provide perspectives for using CGA to treat bone diseases.

Bacillus amyloliquefaciens attenuates the intestinal permeability, oxidative stress and endoplasmic reticulum stress: transcriptome and microbiome analyses in weaned piglets

Endoplasmic reticulum (ER) stress is related to oxidative stress (OS) and leads to intestinal injury. Bacillus amyloliquefaciens SC06 (SC06) can regulate OS, but its roles in intestinal ER stress remains unclear. Using a 2 × 2 factorial design, 32 weaned piglets were treated by two SC06 levels (0 or 1 × 108 CFU/g), either with or without diquat (DQ) injection. We found that SC06 increased growth performance, decreased ileal permeability, OS and ER stress in DQ-treated piglets. Transcriptome showed that differentially expressed genes (DEGs) induced by DQ were enriched in NF-κB signaling pathway. DEGs between DQ- and SC06 + DQ-treated piglets were enriched in glutathione metabolism pathway. Ileal microbiome revealed that the SC06 + DQ treatment decreased Clostridium and increased Actinobacillus. Correlations were found between microbiota and ER stress genes. In conclusion, dietary SC06 supplementation increased the performance, decreased the permeability, OS and ER stress in weaned piglets by regulating ileal genes and microbiota.

Inhibition of Protein Disulfide Isomerase Attenuates Osteoclast Differentiation and Function via the Readjustment of Cellular Redox State in Postmenopausal Osteoporosis

Due to the accumulation of reactive oxygen species (ROS) and heightened activity of osteoclasts, postmenopausal osteoporosis could cause severe pathological bone destruction. Protein disulfide isomerase (PDI), an endoplasmic prototypic thiol isomerase, plays a central role in affecting cellular redox state. To test whether suppression of PDI could inhibit osteoclastogenesis through cellular redox regulation, bioinformatics network analysis was performed on the causative genes, followed by biological validation on the osteoclastogenesis in vitro and ovariectomy (OVX) mice model in vivo. The analysis identified PDI as one of gene targets for postmenopausal osteoporosis, which was positively expressed during osteoclastogenesis. Therefore, PDI expression inhibitor and chaperone activity inhibitor were used to verify the effects of PDI inhibitors on osteoclastogenesis. Results demonstrated that PDI inhibitors could reduce osteoclast number and inhibit resorption function via suppression on osteoclast marker genes. The mechanisms behind the scenes were the PDI inhibitors-caused intracellular ROS reduction via enhancement of the antioxidant system. Micro-CT and histological results indicated PDI inhibitors could effectively alleviate or even prevent bone loss in OVX mice. In conclusion, our findings unveiled the suppressive effects of PDI inhibitors on osteoclastogenesis by reducing intracellular ROS, providing new therapeutic options for postmenopausal osteoporosis.

Restrained Mitf-associated autophagy by Mulberroside A ameliorates osteoclastogenesis and counteracts OVX-Induced osteoporosis in mice

Bone and mineral metabolism homeostasis accounts for the maintenance of normal skeletal remodeling. However, with aging and changes in hormone levels, over-activated osteoclasts disrupt homeostasis, induce osteoporosis, and even cause osteoporotic fractures, leading to an enormous economic burden. Despite the rapid development of pharmacological therapy for osteoporosis, safer and more effective treatments remain to be explored. Here, we demonstrate that Mulberroside A (Mul-A), a natural component extracted from mulberry bark and branches, effectively suppresses osteoclastogenesis in vitro and counteracts bone loss caused by ovariectomy (OVX). The mechanism underlying this effect involves the repression of autophagic flux during osteoclastogenesis by Mul-A, which can be attributed to the restrained expression of microphthalmia-related transcription factor (Mitf) and its nuclear translocation. Importantly, Mitf overexpression partially reverses the inhibitory effects of Mul-A on autophagy and osteoclastogenesis. Moreover, applying two autophagy agonizts, rapamycin and Torin 1, attenuates the osteoclastogenic regulatory role of Mul-A. Collectively, our study demonstrates that Mul-A damages osteoclast differentiation and ameliorates osteoporosis caused by estrogen deficiency by modulation of Mitf-associated autophagy, indicating its therapeutic potential against osteoporosis.

MCP-5 suppresses osteoclast differentiation through Ccr5 upregulation

Human monocyte chemoattractant protein-1 (MCP-1) in mice has two orthologs, MCP-1 and MCP-5. MCP-1, which is highly expressed in osteoclasts rather than in osteoclast precursor cells, is an important factor in osteoclast differentiation. However, the roles of MCP-5 in osteoclasts are completely unknown. In this study, contrary to MCP-1, MCP-5 was downregulated during receptor activator of nuclear factor kappa B ligand (RANKL)-induced osteoclast differentiation and was considered an inhibitory factor in osteoclast differentiation. The inhibitory role of MCP-5 in osteoclast differentiation was closely related to the increase in Ccr5 expression and the inhibition of IκB degradation by RANKL. Transgenic mice expressing MCP-5 controlled by Mx-1 promoter exhibited an increased bone mass because of a decrease in osteoclasts. This result strongly supported that MCP-5 negatively regulated osteoclast differentiation. MCP-5 also prevented severe bone loss caused by RANKL.

Epigenetic Regulation of Autophagy in Bone Metabolism

The skeletal system is crucial for supporting bodily functions, protecting vital organs, facilitating hematopoiesis, and storing essential minerals. Skeletal homeostasis, which includes aspects such as bone density, structural integrity, and regenerative processes, is essential for normal skeletal function. Autophagy, an intricate intracellular mechanism for degrading and recycling cellular components, plays a multifaceted role in bone metabolism. It involves sequestering cellular waste, damaged proteins, and organelles within autophagosomes, which are then degraded and recycled. Autophagy's impact on bone health varies depending on factors such as regulation, cell type, environmental cues, and physiological context. Despite being traditionally considered a cytoplasmic process, autophagy is subject to transcriptional and epigenetic regulation within the nucleus. However, the precise influence of epigenetic regulation, including DNA methylation, histone modifications, and non-coding RNA expression, on cellular fate remains incompletely understood. The interplay between autophagy and epigenetic modifications adds complexity to bone cell regulation. This article provides an in-depth exploration of the intricate interplay between these two regulatory paradigms, with a focus on the epigenetic control of autophagy in bone metabolism. Such an understanding enhances our knowledge of bone metabolism-related disorders and offers insights for the development of targeted therapeutic strategies.

MT2 INHIBITS OSTEOCLASTOGENESIS BY SCAVENGING ROS

Context and objective

Reactive oxygen species (ROS) produced under oxidative stress is important for osteoclastogenesis. As a major member of the metallothionein (MT) family, metallothionein2 (MT2) can scavenge ROS in osteoblasts. However, the role of MT2 in osteoclastogenesis and ROS production in osteoclast precursors (OCPs) is unknown.

Material and methods

In this study, we first investigated MT2 expression level in osteoporotic model mice. Next, we explored the roles of MT2 in osteoclastic differentiation and ROS production in OCPs. Ultimately, via rescue assays based on hydrogen peroxide (H2O2), the significance of ROS in MT-2-regulated osteoclastic differentiation was further elucidated.

Results

Compared with sham operated (Sham) mice, ovariectomized (OVX) mice displayed bone marrow primary OCPs (Ly6C+CD11b-) having higher ROS levels and lower MT2 expression. MT2 overexpression inhibited the formation of mature osteoclasts, while MT2 knockdown was contrary. Moreover, MT2 overexpression inhibited ROS production in OCPs, while MT2 knockdown exhibited the opposite effects. Notably, the inhibitory effect of MT2 overexpression on osteoclastogenesis and ROS production was blocked by the addition of H2O2.

Conclusion

MT2 inhibits osteoclastogenesis through repressing ROS production in OCPs, which indicates that the strategy of upregulating MT2 in OCPs may be applied to the clinical treatment of osteoclastic bone loss.

Advances in the interaction between endoplasmic reticulum stress and osteoporosis

The endoplasmic reticulum (ER) is the main site for protein synthesis, folding, and secretion, and accumulation of the unfolded/misfolded proteins in the ER may induce ER stress. ER stress is an important participant in various intracellular signaling pathways. Prolonged- or high-intensity ER stress may induce cell apoptosis. Osteoporosis, characterized by imbalanced bone remodeling, is a global disease caused by many factors, such as ER stress. ER stress stimulates osteoblast apoptosis, increases bone loss, and promotes osteoporosis development. Many factors, such as the drug's adverse effects, metabolic disorders, calcium ion imbalance, bad habits, and aging, have been reported to activate ER stress, resulting in the pathological development of osteoporosis. Increasing evidence shows that ER stress regulates osteogenic differentiation, osteoblast activity, and osteoclast formation and function. Various therapeutic agents have been developed to counteract ER stress and thereby suppress osteoporosis development. Thus, inhibition of ER stress has become a potential target for the therapeutic management of osteoporosis. However, the in-depth understanding of ER stress in the pathogenesis of osteoporosis still needs more effort.

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.

ER Stress, the Unfolded Protein Response and Osteoclastogenesis: A Review

Endoplasmic reticulum (ER) stress and its adaptive mechanism, the unfolded protein response (UPR), are triggered by the accumulation of unfolded and misfolded proteins. During osteoclastogenesis, a large number of active proteins are synthesized. When an imbalance in the protein folding process occurs, it causes osteoclasts to trigger the UPR. This close association has led to the role of the UPR in osteoclastogenesis being increasingly explored. In recent years, several studies have reported the role of ER stress and UPR in osteoclastogenesis and bone resorption. Here, we reviewed the relevant literature and discussed the UPR signaling cascade response, osteoclastogenesis-related signaling pathways, and the role of UPR in osteoclastogenesis and bone resorption in detail. It was found that the UPR signal (PERK, CHOP, and IRE1-XBP1) promoted the expression of the receptor activator of the nuclear factor-kappa B ligand (RANKL) in osteoblasts and indirectly enhanced osteoclastogenesis. IRE1 promoted osteoclastogenesis via promoting NF-κB, MAPK signaling, or the release of pro-inflammatory factors (IL-6, IL-1β, and TNFα). CREBH promoted osteoclast differentiation by promoting NFATc1 expression. The PERK signaling pathway also promoted osteoclastogenesis through NF-κB and MAPK signaling pathways, autophagy, and RANKL secretion from osteoblasts. However, salubrinal (an inhibitor of eIF2α dephosphorylation that upregulated p-eIF2α expression) directly inhibited osteoclastogenesis by suppressing NFATc1 expression and indirectly promoted osteoclastogenesis by promoting RANKL secretion from osteoblasts. Therefore, the specific effects and mechanisms of p-PERK and its downstream signaling on osteoclastogenesis still need further experiments to confirm. In addition, the exact role of ATF6 and BiP in osteoclastogenesis also required further exploration. In conclusion, our detailed and systematic review provides some references for the next step to fully elucidate the relationship between UPR and osteoclastogenesis, intending to provide new insights for the treatment of diseases caused by osteoclast over-differentiation, such as osteoporosis.

Hepatic MCPIP1 protein levels are reduced in NAFLD patients and are predominantly expressed in cholangiocytes and liver endothelium

BACKGROUND AND AIMS

NAFLD is characterized by the excessive accumulation of fat in hepatocytes. NAFLD can range from simple steatosis to the aggressive form called NASH, which is characterized by both fatty liver and liver inflammation. Without proper treatment, NAFLD may further progress to life-threatening complications, such as fibrosis, cirrhosis, or liver failure. Monocyte chemoattractant protein-induced protein 1 (MCPIP1, alias Regnase 1) is a negative regulator of inflammation, acting through the cleavage of transcripts coding for proinflammatory cytokines and the inhibition of NF-κB activity.

METHODS

In this study, we investigated MCPIP1 expression in the liver and peripheral blood mononuclear cells (PBMCs) collected from a cohort of 36 control and NAFLD patients hospitalized due to bariatric surgery or primary inguinal hernia laparoscopic repair. Based on liver histology data (hematoxylin and eosin and Oil Red-O staining), 12 patients were classified into the NAFL group, 19 into the NASH group, and 5 into the control (non-NAFLD) group. Biochemical characterization of patient plasma was followed by expression analysis of genes regulating inflammation and lipid metabolism. The MCPIP1 protein level was reduced in the livers of NAFL and NASH patients in comparison to non-NAFLD control individuals. In addition, in all groups of patients, immunohistochemical staining showed that the expression of MCPIP1 was higher in the portal fields and bile ducts in comparison to the liver parenchyma and central vein. The liver MCPIP1 protein level negatively correlated with hepatic steatosis but not with patient body mass index or any other analyte. The MCPIP1 level in PBMCs did not differ between NAFLD patients and control patients. Similarly, in patients' PBMCs there were no differences in the expression of genes regulating β-oxidation (ACOX1, CPT1A, and ACC1) and inflammation (TNF, IL1B, IL6, IL8, IL10, and CCL2), or transcription factors controlling metabolism (FAS, LCN2, CEBPB, SREBP1, PPARA, and PPARG).

CONCLUSION

We have demonstrated that MCPIP1 protein levels are reduced in NAFLD patients, but further research is needed to investigate the specific role of MCPIP1 in NAFL initiation and the transition to NASH.

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 Role of Endoplasmic Reticulum Stress in Differentiation of Cells of Mesenchymal Origin

Endoplasmic reticulum (ER) is a multifunctional membrane-enclosed organelle. One of the major ER functions is cotranslational transport and processing of secretory, lysosomal, and transmembrane proteins. Impaired protein processing caused by disturbances in the ER homeostasis results in the ER stress. Restoration of normal ER functioning requires activation of an adaptive mechanism involving cell response to misfolded proteins, the so-called unfolded protein response (UPR). Besides controlling protein folding, UPR plays a key role in other physiological processes, in particular, differentiation of cells of connective, muscle, epithelial, and neural tissues. Cell differentiation is induced by the physiological levels of ER stress, while excessive ER stress suppresses differentiation and can result in cell death. So far, it remains unknown whether UPR activation induces cell differentiation or if UPR is initiated by the upregulated synthesis of secretory proteins during cell differentiation. Cell differentiation is an important stage in the development of multicellular organisms and is tightly controlled. Suppression or excessive activation of this process can lead to the development of various pathologies in an organism. In particular, impairments in the differentiation of connective tissue cells can result in the development of fibrosis, obesity, and osteoporosis. Recently, special attention has been paid to fibrosis as one of the major complications of COVID-19. Therefore, studying the role of UPR in the activation of cell differentiation is of both theoretical and practical interest, as it might result in the identification of molecular targets for selective regulation of cell differentiation stages and as well as the potential to modulate the mechanisms involved in the development of various pathological states.

Cholesterol inhibits autophagy in RANKL-induced osteoclast differentiation through activating the PI3K/AKT/mTOR signaling pathway

BACKGROUND

A dysregulated balance between bone formation and bone resorption controlled by osteoblast and osteoclast will lead to osteoporosis. Cholesterol (CHO) is a crucial factor leading to osteoporosis, and autophagy appears to involve it. Therefore, we aimed to study the molecular mechanism of autophagy in CHO-induced osteoclasts differentiation.

METHODS

Nuclear factor-κ B ligand as a receptor activator was used to induce osteoclasts differentiation of murine macrophage RAW264.7 treated with CHO, PI3-kinase inhibitor (LY294002), and Rapamycin (RAPA), respectively. Western blot assay was used to detect the expression of TRAP/ACP5 and the proteins involved in autophagy and the PI3K/AKT/mTOR signaling pathway. In addition, TRAP staining, bone resorption assay, and F-actin immunofluorescence were performed to evaluate the ability of osteoclast formation. Transmission electron microscopy and immunofluorescence were also executed to observed the expression of LC3B, and autophagosome.

RESULTS

When RAW264.7 was treated with 20 μg/mL CHO for 5 consecutive days, It exhibited the optimal osteoclast activity. In addition, CHO could inhibit autophagy and activate the PI3K/AKT/mTOR signaling pathway. Moreover, the effects of CHO on osteoclast differentiation and autophagy could partially be reversed by LY294002 and RAPA.

CONCLUSION

Therefore, our results demonstrated that CHO could inhibit autophagy during osteoclast differentiation by activating the PI3K/AKT/mTOR signaling pathway. These findings provided important theoretical basis for CHO in bone resorption and formation.

Cinacalcet Improves Bone Parameters Through Regulation of Osteoclast Endoplasmic Reticulum Stress, Autophagy, and Apoptotic Pathways in Chronic Kidney Disease-Mineral and Bone Disorder

The possible mechanisms underlying the quantitative and qualitative effects of cinacalcet on bone were explored in a chronic kidney disease-mineral and bone disorder (CKD-MBD) mouse model in relation to the influence of the interactions among the osteoclast (OC) endoplasmic reticulum (ER) stress, autophagy and apoptosis pathways on OC differentiation. Body weight and biochemical parameters improved significantly in the CKD + cinacalcet groups compared to the CKD group. Micro-computed tomography (μCT) revealed both cortical and trabecular parameters deteriorated significantly in the CKD group and were reversed by cinacalcet in a dose-dependent manner. Nanoindentation analysis of bone quality proved that both cortical hardness and elastic modulus improved significantly with high dose cinacalcet treatment. In vitro studies revealed that cinacalcet inhibited receptor activator of NF-κB ligand (RANKL)/receptor activator of NF-κB (RANK)-induced OC differentiation in a concentration-dependent manner through a close interaction between activation of caspase-related apoptosis, reversal of OC autophagy through the protein kinase B (Akt)/mammalian target of rapamycin (mTOR) and adenosine monophosphate-activated protein kinase (AMPK) pathways, and attenuation of the OC ER stress/CREBH/NFATc1 signaling pathway. Cinacalcet improves both bone quantity and bone quality in CKD mouse model and inhibits OC differentiation through regulation of the interactions among the apoptosis, ER stress, and autophagy pathways within OCs. © 2021 American Society for Bone and Mineral Research (ASBMR).

Autophagy in Bone Remodeling: A Regulator of Oxidative Stress

Bone homeostasis involves bone formation and bone resorption, which are processes that maintain skeletal health. Oxidative stress is an independent risk factor, causing the dysfunction of bone homeostasis including osteoblast-induced osteogenesis and osteoclast-induced osteoclastogenesis, thereby leading to bone-related diseases, especially osteoporosis. Autophagy is the main cellular stress response system for the limination of damaged organelles and proteins, and it plays a critical role in the differentiation, apoptosis, and survival of bone cells, including bone marrow stem cells (BMSCs), osteoblasts, osteoclasts, and osteocytes. High evels of reactive oxygen species (ROS) induced by oxidative stress induce autophagy to protect against cell damage or even apoptosis. Additionally, pathways such as ROS/FOXO3, ROS/AMPK, ROS/Akt/mTOR, and ROS/JNK/c-Jun are involved in the regulation of oxidative stress-induced autophagy in bone cells, including osteoblasts, osteocytes and osteoclasts. This review discusses how autophagy regulates bone formation and bone resorption following oxidative stress and summarizes the potential protective mechanisms exerted by autophagy, thereby providing new insights regarding bone remodeling and potential therapeutic targets for osteoporosis.

SIS3 suppresses osteoclastogenesis and ameliorates bone loss in ovariectomized mice by modulating Nox4-dependent reactive oxygen species

Osteoporosis is a metabolic disorder of reduced bone mass, accompanied by the deterioration of the bone microstructure, resulting in increased brittleness and easy fracture. Its pathogenesis can be explained by mainly excessive osteoclast formation or bone resorption hyperfunction. Oxidative stress is intricately linked with bone metabolism, and the maturation and bone resorption of osteoclasts respond to intracellular ROS levels. SIS3 is a small-molecule compound that selectively suppresses Smad3 phosphorylation in the TGF-β/Smad signaling pathway and attenuates the ability to bind to target DNA. Several studies have reported that Smad3 plays a significant role in bone metabolism. However, whether SIS3 can modulate bone metabolism by affecting osteoclastogenesis and the specific molecular mechanisms involved remain unknown. Here, we demonstrated that SIS3 could suppress osteoclastogenesis and ameliorate bone loss in ovariectomized mice. Mechanistically, SIS3 inhibited Smad3 phosphorylation in BMMs, and the deficiency of phosphorylated Smad3 downregulated ROS production and Nox4-dependent expression during osteoclast formation, thereby blocking MAPK phosphorylation and the synthesis of downstream osteoclast marker proteins. Similarly, Nox4 plasmid transfection significantly alleviated osteoclast formation inhibited by SIS3. In addition, we identified the interaction region between Smad3 and Nox4 by ChIP and dual luciferase reporter assays. Collectively, we found that SIS3 could inhibit Smad3 phosphorylation, reduce Nox4-dependent ROS generation induced by RANKL, and prevent osteoclast differentiation and maturation, making it a promising alternative therapy for osteoporosis.

DNA damage-inducible transcript 3 restrains osteoclast differentiation and function

DNA damage-inducible transcript 3 (DDIT3), a member of the CCAAT/enhancer-binding protein (C/EBP) family, is involved in cellular apoptosis and differentiation. DDIT3 participates in the regulation of adipogenesis and osteogenesis in vitro and in vivo. However, the role of DDIT3 in osteoclastogenesis is not yet known. In this study, the involvement of DDIT3 in osteoclast differentiation and function was reported for the first time. CRISPR/Cas9-mediated DDIT3 knockout (KO) mice were generated for functional assessment. Tartrate-resistant acid phosphatase (TRAP) staining of distal femurs showed increased positive cells in DDIT3 KO mice. DDIT3 expression was downregulated during the receptor activator of nuclear factor κB ligand (RANKL)-induced osteoclast differentiation of bone marrow-derived macrophages (BMMs). The loss of DDIT3 increased the expression of osteoclast-specific markers, including nuclear factor of activated T-cells cytoplasmic 1 (NFATc1), TRAP, cathepsin K (CTSK), and dendritic cell-specific transmembrane protein (DC-STAMP) and promoted the formation of TRAP-positive multinucleated osteoclasts. The actin ring number and resorption area of bone slices were also increased in DDIT3 KO BMMs. Lentivirus-mediated DDIT3 overexpression significantly inhibited the osteoclast differentiation of RAW264.7 cells. In the tumor necrosis factor-α-induced osteolysis model, DDIT3 deficiency enhanced osteoclast formation and aggravated bone resorption. DDIT3 inhibited osteoclast differentiation by regulating the C/EBPα-CTSK axis. Furthermore, DDIT3 KO intensified the RANKL-triggered activation of the MAPKs and Akt signaling pathways. Taken together, the results revealed the essential role of DDIT3 in osteoclastogenesis in vitro and in vivo and its close relationship with osteoclast-associated transcription factors and pathways.

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.

A Novel Autophagy-Related Marker for Improved Differential Diagnosis of Rheumatoid Arthritis and Osteoarthritis

Rheumatoid arthritis (RA) and osteoarthritis (OA) are two most common rheumatic diseases in the world. Although there are standard methods for the diagnosis of both RA and OA, the differentials in some cases are poor. With deepening research, the role of autophagy in maintaining cell homeostasis and thus enabling cells adapt to external environments has become increasingly prominent. Both RA and OA, two diseases with inherent differences in pathogenesis, gradually show differences in autophagy levels. Our study therefore aims to further understand differences in pathogenesis of RA and OA through in-depth studies of autophagy in RA and OA. We also define appropriate autophagy-related markers as recognition indicators. Differences in autophagy levels between RA and OA were found based on analysis of the Kyoto Encyclopedia of Genes and Genomes (KEGG) and single-sample gene set enrichment (ssGSEA). These differences were mainly caused by 134 differentially expressed genes (DEGs). In two autophagy-related genes, CXCR4 and SERPINA1, there existed significant statistical difference between RA and OA. An autophagy related index (ARI) was thus successfully constructed based on CXCR4 and SERPINA by binary logistic regression of the generalized linear regression (GLR) algorithm. Pearson analysis indicated that the expression of CXCR4, SERPINA1, and ARI were closely correlated with autophagy scores and immune infiltration. Moreover, ARI showed high disease identification through receiver operating characteristic (ROC) analysis (AUCtesting cohort = 0.956, AUCtraining cohort = 0.867). These results were then verified in GSE12021 independent cohort. In conclusion, ARI associated with autophagy and immune infiltration was successfully constructed for accurately identifying OA and RA. The index, thus, has great potential in clinical applications.

Site-1 protease controls osteoclastogenesis by mediating LC3 transcription

Site-1 protease (S1P) is a Golgi-located protein that activates unique membrane-bound latent transcription factors, and it plays an indispensable role in endoplasmic reticulum stress, lipid metabolism, inflammatory response and lysosome function. A patient with S1P mutation exhibits severe skeletal dysplasia with kyphoscoliosis, dysmorphic facial features and pectus carinatum. However, whether S1P regulates bone remodeling by affecting osteoclastogenesis remains elusive. Here, we show that S1P is indeed a positive regulator of osteoclastogenesis. S1P ablation in mice led to significant osteosclerosis compared with wild-type littermates. Mechanistically, S1P showed upregulated during osteoclastogenesis and was identified as a direct target of miR-9-5p. S1P deletion in bone marrow monocytes (BMMs) inhibited ATF6 and SREBP2 maturation, which subsequently impeded CHOP/SREBP2-complex-induced LC3 expression and autophagy flux. Consistently, transfection of LC3 adenovirus evidently rescued osteoclastogenesis in S1P-deficient BMMs. We then identified the interaction regions between CHOP and SREBP2 by Co-immunoprecipitation (Co-IP) and molecular docking. Furthermore, S1P deletion or inhibitor efficaciously rescued ovariectomized (OVX)- and LPS-induced bone loss in vivo. Collectively, we showed that S1P regulates osteoclast differentiation in a LC3 dependent manner and so is a potential therapy target for osteoporosis.

Role of Mcpip1 in obesity-induced hepatic steatosis as determined by myeloid and liver-specific conditional knockouts

Monocyte chemoattractant protein-induced protein 1 (MCPIP1, alias Regnase 1) is a negative regulator of inflammation, acting through cleavage of transcripts coding for proinflammatory cytokines and by inhibition of NFκB activity. Moreover, it was demonstrated that MCPIP1 regulates lipid metabolism both in adipose tissue and in hepatocytes. In this study, we investigated the effects of tissue-specific Mcpip1 deletion on the regulation of hepatic metabolism and development of nonalcoholic fatty liver disease (NAFLD). We used control Mcpip1^fl/fl mice and animals with deletion of Mcpip1 in myeloid leukocytes (Mcpip1^fl/fl LysM^Cre ) and in hepatocytes (Mcpip1^fl/fl Alb^Cre ), which were fed chow or a high-fat diet (HFD) for 12 weeks. Mcpip1^fl/fl LysM^Cre mice fed a chow diet were characterized by a significantly reduced hepatic expression of genes regulating lipid and glucose metabolism, which subsequently resulted in low plasma glucose level and dyslipidemia. These animals also displayed systemic inflammation, demonstrated by increased concentrations of cytokines in the plasma and high Tnfa, Il6, IL1b mRNA levels in the liver and brown adipose tissue (BAT). Proinflammatory leukocyte infiltration into BAT, together with low expression of Ucp1 and Ppargc1a, resulted in hypothermia of 22-week-old Mcpip1^fl/fl LysM^Cre mice. On the other hand, there were no significant changes in phenotype in Mcpip1^fl/fl Alb^Cre mice. Although we detected a reduced hepatic expression of genes regulating glucose metabolism and β-oxidation in these mice, they remained asymptomatic. Upon feeding with a HFD, Mcpip1^fl/fl LysM^Cre mice did not develop obesity, glucose intolerance, nor hepatic steatosis, but were characterized by low plasma glucose level and dyslipidemia, along with proinflammatory phenotype. Mcpip1^fl/fl Alb^Cre animals, following a HFD, became hypercholesterolemic, but accumulated lipids in the liver at the same level as Mcpip1^fl/fl mice, and no changes in the level of soluble factors tested in the plasma were detected. We have demonstrated that Mcpip1 protein plays an important role in the liver homeostasis. Depletion of Mcpip1 in myeloid leukocytes, followed by systemic inflammation, has a more pronounced effect on controlling liver metabolism and homeostasis than the depletion of Mcpip1 in hepatocytes.

MCPIP-1 Restricts Inflammation via Promoting Apoptosis of Neutrophils

Monocyte chemoattractant protein-induced protein-1 (MCPIP-1) is a potent inhibitor of inflammatory response to pathogens. Acting as endonuclease against transcripts of inflammatory cytokines or transcription factors MCPIP-1 can significantly reduce the cytokine storm, thus limiting the tissue damage. As the adequate resolution of inflammation depends also on the efficient clearance of accumulated neutrophils, we focused on the role of MCPIP-1 in apoptosis and retention of neutrophils. We used peritoneal neutrophils from cell-specific MCPIP-1 knockout mice and showed prolonged survival of these cells. Moreover, we confirmed that MCPIP-1-dependent degradation of transcripts of antiapoptotic genes, including BCL3, BCL2A1, BCL2L1, and for the first time MCL-1, serves as an early event in spontaneous apoptosis of primary neutrophils. Additionally, we identified previously unknown miRNAs as potential binding partners to the MCPIP-1 transcript and their regulation suggest a role in MCPIP-1 half-life and translation. These phenomena may play a role as a molecular switch that balances the MCPIP-1-dependent apoptosis. Besides that, we determined these particular miRNAs as integral components of the GM-CSF-MCPIP-1 axis. Taken together, we identified the novel anti-inflammatory role of MCPIP-1 as a regulator of accumulation and survival of neutrophils that simultaneously promotes an adequate resolution of inflammation.

PERK signaling pathway in bone metabolism: Friend or foe?

Osteoblasts and osteoclasts participate in the process of bone remodelling to meet the needs of normal growth and development or repair pathological damage. Endoplasmic reticulum stress (ER stress) can break the intracellular homeostasis of osteoclasts and osteoblasts, which is closely related to abnormal bone remodelling. The double‐stranded RNA‐dependent protein kinase (PKR)‐like ER kinase (PERK) is a key transmembrane protein that regulates ER stress, and growing evidence suggests that the PERK pathway plays a crucial role in regulating bone metabolism under both physiological and pathological conditions. Based on the current findings, we summarized the main mechanisms involved in bone metabolism downstream of the PERK pathway, among which elF2α, FOXO1, CaN, Nrf2 and DAG play a role in regulating the differentiation of osteoblasts and osteoclasts. Importantly, strategies by the regulation of PERK pathway exert beneficial effects in preclinical trials of several bone‐related diseases. Given the importance and novelty of PERK pathway, we provide an overview and discuss the roles of PERK pathway in regulating bone metabolism and its impact on bone‐related diseases. We hope that the development of research in this field will bring a bright future for the treatment of bone‐related diseases.

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.

circRNA expression pattern and ceRNA network in the pathogenesis of aseptic loosening after total hip arthroplasty

Increasing evidence has demonstrated that circular RNA (circRNA) exerts important function in the pathogenesis of some diseases. While, the contributions of circRNAs to aseptic loosening after total hip arthroplasty (THA) remain largely unknown. Our research is to explore the differentially expressed circRNAs (DEcircRNAs) and elucidate complex regulated mechanism of circRNAs in aseptic loosening. The DEcircRNAs were identified by RNA sequencing (RNA-seq) analysis. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) was adopted to corroborate these DEcircRNAs. The potential function of circRNAs in aseptic loosening tissue was identified by competing endogenous RNA (ceRNA) analysis. Enrichment analysis was performed for target mRNAs and host genes of the DEcircRNAs by Gene Oncology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG). 257 DEcircRNAs were obtained from RNA-seq results. Following the RT-qPCR corroboration, 6 circRNAs (hsa_circ_0007482, hsa_circ_0005232, hsa_circ_0000994, hsa_circ_0000690, hsa_circ_0058092 and hsa_circ_0004496) were selected for further analysis. By circRNA-miRNA and miRNA-mRNA prediction, 6 circRNAs, 138 miRNAs and 1667 mRNAs were identified. Then, circRNA-miRNA-mRNA network was established. The result of GO and KEGG enrichment analysis suggested that the circRNAs were related with some biological functions and pathways of aseptic loosening. A novel pathogenesis and treatment strategy about aseptic loosening after THA was revealed from our study of circRNA-miRNA-mRNA network.

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.

Integrative genomics reveal a role for MCPIP1 in adipogenesis and adipocyte metabolism

Obesity is considered a serious chronic disease, associated with an increased risk of developing cardiovascular diseases, non-alcoholic fatty liver disease and type 2 diabetes. Monocyte chemoattractant protein-1-induced protein-1 (MCPIP1) is an RNase decreasing stability of transcripts coding for inflammation-related proteins. In addition, MCPIP1 plays an important role in the regulation of adipogenesis in vitro by reducing the expression of key transcription factors, including C/EBPβ. To elucidate the role of MCPIP1 in adipocyte biology, we performed RNA-Seq and proteome analysis in 3T3-L1 adipocytes overexpressing wild-type (WTMCPIP1) and the mutant form of MCPIP1 protein (D141NMCPIP1). Our RNA-Seq analysis followed by confirmatory Q-RT-PCR revealed that elevated MCPIP1 levels in 3T3-L1 adipocytes upregulated transcripts encoding proteins involved in signal transmission and cellular remodeling and downregulated transcripts of factors involved in metabolism. These data are consistent with our proteomic analysis, which showed that MCPIP1 expressing adipocytes exhibit upregulation of proteins involved in cellular organization and movement and decreased levels of proteins involved in lipid and carbohydrate metabolism. Moreover, MCPIP1 adipocytes are characterized by decreased level of insulin receptor, reduced insulin-induced Akt phosphorylation, as well as depleted Glut4 level and impaired glucose uptake. Overexpression of Glut4 in 3T3-L1 cells expressed WTMCPIP1 rescued adipogenesis. Interestingly, we found decreased level of MCPIP1 along with an increase in body mass index in subcutaneous adipose tissue. The presented data show a novel role of MCPIP1 in modulating insulin sensitivity in adipocytes. Overall, our findings demonstrate that MCPIP1 is an important regulator of adipogenesis and adipocyte metabolism.

Rab GTPases in Osteoclastic Bone Resorption and Autophagy

Small guanosine triphosphate hydrolases (GTPases) of the Rab family are involved in plasma membrane delivery, fusion events, and lysosomal and autophagic degradation pathways, thereby regulating signaling pathways and cell differentiation and function. Osteoclasts are bone-resorbing cells that maintain bone homeostasis. Polarized vesicular trafficking pathways result in the formation of the ruffled border, the osteoclast’s resorptive organelle, which also assists in transcytosis. Here, we reviewed the different roles of Rab GTPases in the endomembrane machinery of osteoclasts and in bone diseases caused by the dysfunction of these proteins, with a particular focus on autophagy and bone resorption. Understanding the molecular mechanisms underlying osteoclast-related bone disease development is critical for developing and improving therapies.

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.

MCPIP1 RNase and Its Multifaceted Role

Inflammation is an organism’s physiological response to harmful septic and aseptic stimuli. This process begins locally through the influx of immune system cells to the damaged tissue and the subsequent activation and secretion of inflammatory mediators to restore homeostasis in the organism. Inflammation is regulated at many levels, and one of these levels is post-transcriptional regulation, which controls the half-life of transcripts that encode inflammatory mediators. One of the proteins responsible for controlling the amount of mRNA in a cell is the RNase monocyte chemoattractant protein-induced protein 1 (MCPIP1). The studies conducted so far have shown that MCPIP1 is involved not only in the regulation of inflammation but also in many other physiological and pathological processes. This paper provides a summary of the information on the role of MCPIP1 in adipogenesis, angiogenesis, cell differentiation, cancer, and skin inflammation obtained to date.

Zebrafish as a model to study autophagy and its role in skeletal development and disease

In the last twenty years, research using zebrafish as a model organism has increased immensely. With the many advantages that zebrafish offer such as high fecundity, optical transparency, ex vivo development, and genetic tractability, they are well suited to studying developmental processes and the effect of genetic mutations. More recently, zebrafish models have been used to study autophagy. This important protein degradation pathway is needed for cell and tissue homeostasis in a variety of contexts. Correspondingly, its dysregulation has been implicated in multiple diseases including skeletal disorders. In this review, we explore how zebrafish are being used to study autophagy in the context of skeletal development and disease, and the ways these areas are intersecting to help identify potential therapeutic targets for skeletal disorders.

Role of autophagy in the pathogenesis of rheumatoid arthritis: Latest evidence and therapeutic approaches

Autophagy is considered as an important intracellular mechanism that degrades cytoplasmic components to furnish additional energy. It has cytoprotective effects through the degradation of intracellular pathogens, damaged organelles, and protein aggregates. On the other hand, there are reports of an association between autophagy and autoimmune diseases. Indeed, it has been evident that autophagy is dysregulated in various autoimmune diseases including rheumatoid arthritis (RA). Autophagy is implicated in the maturation survival and proliferation of various immune and non-immune cells, which play pivotal roles in RA pathogenesis. Additionally, autophagy seems to be involved in citrullination and presentation of citrullinated peptides to T lymphocyte cells. Presentation of citrullinated peptides through MHC compartments to the T cells leads to immune response and chronic inflammation. Evidence suggests that autophagy could be implicated in apoptosis resistance of RA fibroblast-like synoviocyte (RA FLS), osteoclastogenesis, and finally severe bone and cartilage destruction. Since autophagy could be an important phenomenon in RA pathogenesis, we summarized the roles of autophagy in citrullination, osteoclastogenesis, RA FLS cells survival, apoptosis resistance of cells, lymphocyte homeostasis and its clinical outcomes in RA disease.

TGF-β-activated kinase 1 (TAK1) mediates cadmium-induced autophagy in osteoblasts via the AMPK / mTORC1 / ULK1 pathway

Cadmium (Cd) is one of worldwide environmental pollutants that causes bone homeostasis, which depends on the resorption of bones by osteoclasts and formation of bones by the osteoblasts (OB). However, the Cd toxicity on OB and its mechanism are unclear. Autophagy is an evolutionarily conserved degradation process in which domestic intracellular components are selectively digested for the recycling of nutrients and energy. This process is indispensable for cell homeostasis maintenance and stress responses. Dysregulation at the level of autophagic activity consequently disturbs the balance between bone formation and bone resorption and mediates the onset and progression of multiple bone diseases, including osteoporosis. TAK1 has been recently emerged as an activator of AMPK and hence an autophagy inducer. AMPK is a key molecule that induces autophagy and regulates cellular metabolism to maintain energy homeostasis. Conversely, autophagy is inhibited by mTORC1. In this study, we found that Cd treatment caused the formation of autophagosomes, LC3-II lipidation and p62 downregulation, and the increased autophagic flux, indicating that Cd treatment induced autophagy in OBs. Cd treatment induced TAK1 activation mediated AMPK phosphorylation, which promoted autophagy via phosphorylation of ULK1 at S317. Meanwhile, Cd treatment dramatically decreased mTORC1, S6K1, 4E-BP1, S6, ULK1^S555 and ULK1^S757 phosphorylation, suggesting that mTORC1 activity was inhibited and inactive mTORC1 prevents ULK1 activation by phosphorylating ULK1 at SerS555 and Ser757. Our data strongly suggest that TAK1 mediates AMPK activation, which activates ULK1 by phosphorylating ULK1^S317 and suppressing mTORC1-mediated ULK1^S555 and ULK1^S757 phosphorylation. Our study has revealed a signaling mechanism for TAK1 in Cd-induced autophagy in OBs.

Inhibition of CCL2 by bindarit alleviates diabetes-associated periodontitis by suppressing inflammatory monocyte infiltration and altering macrophage properties

Diabetes-associated periodontitis (DP) aggravates diabetic complications and increases mortality from diabetes. DP is caused by diabetes-enhanced host immune-inflammatory responses to bacterial insult. In this study, we found that persistently elevated CCL2 levels in combination with proinflammatory monocyte infiltration of periodontal tissues were closely related to DP. Moreover, inhibition of CCL2 by oral administration of bindarit reduced alveolar bone loss and increased periodontal epithelial thickness by suppressing periodontal inflammation. Furthermore, bindarit suppressed the infiltration of proinflammatory monocytes and altered the inflammatory properties of macrophages in the diabetic periodontium. This finding provides a basis for the development of an effective therapeutic approach for treating DP.

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.

Therapeutic Potential of Tauroursodeoxycholic Acid for the Treatment of Osteoporosis

Tauroursodeoxycholic acid (TUDCA) is a US FDA-approved hydrophilic bile acid for the treatment of chronic cholestatic liver disease. In the present study, we investigate the effects of TUDCA on the proliferation and differentiation of osteoblasts and its therapeutic effect on a mice model of osteoporosis. Following treatment with different concentrations of TUDCA, cell viability, differentiation, and mineralization were measured. Three-month-old female C57BL/6 mice were randomly divided into three groups (n = 8 mice per group): (i) normal mice as the control group, (ii) ovariectomy (OVX) group (receiving phosphate-buffered saline (PBS) treatment every other day for 4 weeks), and (iii) OVX group with TUDCA (receiving TUDCA treatment every other day for 4 weeks starting 6 weeks after OVX). At 11 weeks post-surgery, serum levels of procollagen type I N-terminal propeptides (PINP) and type I collagen crosslinked C-telopeptides (CTX) were measured, and all mice were sacrificed to examine the distal femur by micro-computed tomography (CT) scans and histology. TUDCA (100 nM, 1 µM) significantly increased the proliferation and viability of osteoblasts and osteoblast differentiation and mineralization when used in vitro. Furthermore, TUDCA neutralized the detrimental effects of methylprednisolone (methylprednisolone-induced osteoblast apoptosis). In the TUDCA treatment group the PINP level was higher and the CTX level was lower, but these levels were not significantly different compared to the PBS treatment group. Micro-CT and histology showed that the TUDCA treatment group preserved more trabecular structures in the distal femur compared to the PBS treatment group. In addition, the TUDCA treatment group increased the percentage bone volume with respect to the total bone volume, bone mineral density, and mice distal femur trabeculae compared with the PBS treatment group. Taken together, our findings suggest that TUDCA may provide a favorable effect on bones and could be used for the prevention and treatment of osteoporosis.

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.

Biological Factors, Metals, and Biomaterials Regulating Osteogenesis through Autophagy

Bone loss raises great concern in numerous situations, such as ageing and many diseases and in both orthopedic and dentistry fields of application, with an extensive impact on health care. Therefore, it is crucial to understand the mechanisms and the determinants that can regulate osteogenesis and ensure bone balance. Autophagy is a well conserved lysosomal degradation pathway, which is known to be highly active during differentiation and development. This review provides a revision of the literature on all the exogen factors that can modulate osteogenesis through autophagy regulation. Metal ion exposition, mechanical stimuli, and biological factors, including hormones, nutrients, and metabolic conditions, were taken into consideration for their ability to tune osteogenic differentiation through autophagy. In addition, an exhaustive overview of biomaterials, both for orthopedic and dentistry applications, enhancing osteogenesis by modulation of the autophagic process is provided as well. Already investigated conditions regulating bone regeneration via autophagy need to be better understood for finely tailoring innovative therapeutic treatments and designing novel biomaterials.

Suppression of AMP-activated protein kinase reverses osteoprotegerin-induced inhibition of osteoclast differentiation by reducing autophagy

Objectives

Osteoclasts (OC) are unique terminally differentiated cells whose primary function is bone resorption. We previously showed that osteoprotegerin (OPG) inhibits OC differentiation in vitro by enhancing autophagy via the adenosine monophosphate‐activated protein kinase (AMPK)/mTOR/p70S6K signalling pathway in vitro. Here, we aimed to elucidate the mechanism of AMPK mediated autophagy to regulate OPG‐mediated inhibition of OC differentiation and identify potential therapeutic targets associated with bone loss.

Materials and Methods

We used the AMPK activator AICAR to determine the relationship between AMPK activation and OC differentiation, and studied the role of AMPK‐mediated autophagy in OPG‐mediated inhibition of OC differentiation by using autophagy inhibitors or AMPK knockdown.

Results

AMP‐activated protein kinase activation caused LC3II accumulation and weakened OC differentiation activity. In contrast, inactivation of autophagy by 3‐methyladenine or Bafilomycin A1 could attenuate OPG‐mediated inhibition of OC differentiation via the AMPK/mTOR/p70S6K signalling pathway. Furthermore, the AMPK inhibitor compound C and knockdown of AMPK impaired OPG‐mediated inhibition of OC differentiation by inducing autophagy.

Conclusions

These results demonstrated that the AMPK signalling pathway functions as a critical regulator in the OPG‐mediated inhibition of OC differentiation, by inducing autophagy. Our results provide a basis for future bone‐related studies on the AMPK signalling pathway.

Autophagy in bone homeostasis and the onset of osteoporosis

Autophagy is an evolutionarily conserved intracellular process, in which domestic cellular components are selectively digested for the recycling of nutrients and energy. This process is indispensable for cell homeostasis maintenance and stress responses. Both genetic and functional studies have demonstrated that multiple proteins involved in autophagic activities are critical to the survival, differentiation, and functioning of bone cells, including osteoblasts, osteocytes, and osteoclasts. Dysregulation at the level of autophagic activity consequently disturbs the balance between bone formation and bone resorption and mediates the onset and progression of multiple bone diseases, including osteoporosis. This review aims to introduce the topic of autophagy, summarize the understanding of its relevance in bone physiology, and discuss its role in the onset of osteoporosis and therapeutic potential.

RNase MCPIP1 regulates hepatic peroxisome proliferator-activated receptor gamma via TXNIP/PGC-1alpha pathway

Monocyte chemoattractant protein-1-induced protein-1 (MCPIP1) acts as an endonuclease that degrades selected mRNAs, viral RNAs and pre-miRNAs. MCPIP1 inhibits adipogenesis by degradation of C/EBPβ mRNA and adipogenesis-related miRNA, however its role in the regulation of hepatic lipid homeostasis is unknown. In this study, we investigated the role of MCPIP1 in the regulation of lipid metabolism in hepatocytes. C57BL/6 mice were fed a high-fat diet (HFD) for 2-20 weeks and next primary hepatocytes and adipose tissue were isolated. For in vitro experiments we used murine primary hepatocytes, control HepG2 cells and HepG2 with overexpressed or silenced MCPIP1. We found that Mcpip1 levels were lower in primary hepatocytes isolated from HFD-fed mice than in control cells starting at 4 weeks of a HFD. Level of Mcpip1 was also depleted in visceral fat isolated from obese and glucose-intolerant mice characterized by fatty liver disease. We showed that MCPIP1 overexpression in HepG2 cells treated with oleate induces the level and activity of peroxisome proliferator-activated receptor γ (PPARγ). This phenotype was reverted upon silencing of MCPIP1 in HepG2 cells and in primary hepatocytes lacking Mcpip1 protein. MCPIP1 activated the PPARγ transcription factor via the thioredoxin-interacting protein (TXNIP)/peroxisome proliferator-activated receptor γ coactivator 1- α (PGC-1α) pathway. MCPIP1 contributes to lipid metabolism in hepatocytes by regulating the TXNIP/PGC-1α/PPARγ pathway.