Autophagy impairment aggravates the inhibitory effects of high glucose on osteoblast viability and function

Programmed cell death of periodontal ligament cells

The periodontal ligament is a crucial tissue that provides support to the periodontium. Situated between the alveolar bone and the tooth root, it consists primarily of fibroblasts, cementoblasts, osteoblasts, osteoclasts, periodontal ligament stem cells (PDLSCs), and epithelial cell rests of Malassez. Fibroblasts, cementoblasts, osteoblasts, and osteoclasts are functionally differentiated cells, whereas PDLSCs are undifferentiated mesenchymal stem cells. The dynamic development of these cells is intricately linked to periodontal changes and homeostasis. Notably, the regulation of programmed cell death facilitates the clearance of necrotic tissue and plays a pivotal role in immune response. However, it also potentially contributes to the loss of periodontal supporting tissues and root resorption. These findings have significant implications for understanding the occurrence and progression of periodontitis, as well as the mechanisms underlying orthodontic root resorption. Further, the regulation of periodontal ligament cell (PDLC) death is influenced by both systemic and local factors. This comprehensive review focuses on recent studies reporting the mechanisms of PDLC death and related factors.

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

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

Calcitriol-enhanced autophagy in gingival epithelium attenuates periodontal inflammation in rats with type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM)-associated periodontitis is a common disease with high prevalence, associated with persistent infection and complicated manifestations. Calcitriol (1 alpha, 25-dihydroxyvitamin D3, 1,25D) is the active form of vitamin D that plays a protective role in immune regulation, bone metabolism, and inflammatory response. In this study, we constructed a T2DM model in rats by combining a high-fat diet with low-dose streptozotocin. The periodontitis model in rats was developed by ligation and Porphyromonas gingivalis (ATCC 33277) inoculation. Rats were randomly divided into five groups: non-diabetic blank, diabetic blank, diabetes with calcitriol treatment, diabetes with 3-methyladenine (3-MA) treatment, or diabetes with calcitriol and 3-MA treatment. The diabetic rats exhibited an intense inflammatory response and decreased autophagy compared with the non-diabetic rats. Intraperitoneal injection of calcitriol and autophagy inhibitor (3-MA) allowed us to explore the effect of calcitriol on inflammation in the gingival epithelium and the role of autophagy in this process. Treatment with calcitriol resulted in the decreased expression of NFκB-p65, p62/SQSTM1 and inflammatory response and increased expression of LC3-II/LC3-I. Application of 3-MA significantly suppressed autophagy, which was apparently retrieved by calcitriol. Antibacterial peptide (LL-37) is the only antimicrobial peptide in the cathelicidin family that is found in the human body, and it exhibits a broad spectrum of antibacterial activity and regulates the immune system. In the present study, our findings indicated that calcitriol-enhanced autophagy may attenuated periodontitis and the decrease of LL-37 was rescued by calcitriol treatment in the gingival epithelial cells of T2DM rats. Our study provides evidence for the application of calcitriol as an adjunctive treatment for T2DM-associated periodontitis.

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.

Suppressing phosphoinositide-specific phospholipases Cγ1 promotes mineralization of osteoarthritic subchondral bone osteoblasts via increasing autophagy, thereby ameliorating articular cartilage degeneration

INTRODUCTION

Phosphoinositide-specific phospholipases C-γ1 (PLC-γ1) signaling has been shown to modulate osteoarthritis (OA) chondrocyte metabolism. However, the role of PLC-γ1 in OA osteoblasts remains unclear. Herein, whether and how PLC-γ1 was involved in mineralization in OA subchondral bone osteoblasts were investigated.

METHODS

Primary non-OA and OA osteoblasts of human and rat isolated from the subchondral bone or the calvaria were cultured in vitro, as well as mouse pre-osteoblastic cell line MC3T3-E1 cells. Rat knee OA model was induced by anterior cruciate ligament transection (ACLT), in which bone canal was carried out from the surface of lateral epicondyle of femur using micro-electric drill. Morphological characteristics of subchondral bone structure and articular cartilage were assessed using CT, micro-CT, and Safranin O/Fast green staining, respectively. Mineralization was measured by alizarin red staining. The expression and production of genes involved in osteoblastic phenotype and mineralization were evaluated by qPCR, western blotting, and immunohistochemistry assays, respectively. The inhibitions were performed using inhibitors and ShRNAs.

RESULTS

The decreased relative bone density and thickness in the early stage of OA and the increased one in the late stage of OA were observed in subchondral bone of ACLT-rat model. Decreased ALP and OCN levels and absorbance values of ARS content were observed in in vitro osteoblasts isolated from 2 w post-ACLT rat model, as well as IL-1β-treated (for maintaining and mimicking inflammatory status) human OA and rat osteoblasts. Decreased Atg7 level and LC3BII/I ratio in combination with an increase in the P62 level, was concomitant with decreased ALP and OCN mRNA levels and absorbance values of ARS content in OA or IL-1β-treated osteoblasts. Specific inhibition of PLC-γ1 by ShRNAs or inhibitor (U73122) elevated ALP and OCN mRNA levels and absorbance values of ARS content accompanied with increased Atg7 level and LC3BII/I ratio in combination with a decrease in the P62 level in OA osteoblasts. Furthermore, the promoting effect of PLC-γ1 inhibition on ALP and OCN mRNA levels and absorbance values of ARS content was reversed by endoplasmic reticulum (ER) stress activator HA15, as well as autophagic inhibitors CQ and 3MA. Injection with PLC-γ1 inhibitor U73122 from the surface of lateral epicondyle of femur reduced aberrant subchondral bone formation and attenuated articular cartilage degeneration in ACLT-rat.

CONCLUSION

Aberrant changes of OA subchondral bone structure were concomitant with altered osteoblastic phenotype and mineralization. Impaired autophagy contributed to decreased osteoblastic mineralization in the early stage of OA. PLC-γ1 inhibition promoted osteoblastic mineralization through increasing autophagy in OA osteoblasts, which was partially attributed to suppression of ER stress. Targeting PLC-γ1 in subchondral bone osteoblasts could be more efficacious for OA therapy through treating the bone and cartilage at the same time. In summary, we hypothesize that suppressing PLCγ1 promotes mineralization of osteoarthritic subchondral bone osteoblasts via increasing autophagy, thereby ameliorating articular cartilage degeneration.

Novel regulatory roles of UCP1 in osteoblasts

AIMS

The bone-adipose axis requires complex homeostasis in energy and global metabolism. The bioenergetics of bone establishes the necessary energy balance to coordinate endocrine functions that are affected by various factors and is not limited to matrix proteins only. UCP1 is an uncoupling protein of adipocytes, commonly known for its unique feature of promoting thermogenesis, mainly in brown fat; however, the effects of UCP1 in other cell types remain unreported.

MAIN METHODS

In the current study, we determined the roles of UCP1 in osteoblasts by silencing the Ucp1 gene in MC-3T3-E1 cells, as well as C3H10T1/2 mesenchymal stem cells, and explored its functional activities.

KEY FINDINGS

Our results demonstrate for the first time the presence of UCP1 in osteoblast cells. We identified that UCP1 regulates ATP and oxidative phosphorylation in MC-3T3-E1 cells. In addition, our data reveal that the lack of Ucp1 results in reduced expressions of regulatory proteins involved in scavenging of ROS by enhancing an autophagic event to balance osteogenic differentiation.

SIGNIFICANCE

In conclusion, this study highlights a novel perspective on the importance of UCP1 in bone cells.

Arbutin ameliorates glucocorticoid-induced osteoporosis through activating autophagy in osteoblasts

Chronic long-term glucocorticoid use causes osteoporosis partly by interrupting osteoblast homeostasis and exacerbating bone loss. Arbutin, a natural hydroquinone glycoside, has been reported to have biological activities related to the differentiation of osteoblasts and osteoclasts. However, the role and underlying mechanism of arbutin in glucocorticoid-induced osteoporosis are elusive. In this study, we demonstrated that arbutin administration ameliorated osteoporotic disorders in glucocorticoid dexamethasone (Dex)-induced mouse model, including attenuating the loss of bone mass and trabecular microstructure, promoting bone formation, suppressing bone resorption, and activating autophagy in bone tissues. Furthermore, Dex-stimulated mouse osteoblastic MC3T3-E1 cells were treated with arbutin. Arbutin treatment rescued Dex-induced repression of osteoblast differentiation and mineralization, the downregulation of osteogenic gene expression, reduced autophagic marker expression, and decreased autophagic puncta formation. The application of autophagy inhibitor 3-MA decreased autophagy, differentiation, and mineralization of MC3T3-E1 cells triggered by arbutin. Taken together, our findings suggest that arbutin treatment fends off glucocorticoid-induced osteoporosis, partly through promoting differentiation and mineralization of osteoblasts by autophagy activation.

Effects of osteoblast autophagy on glucocorticoid-induced femoral head necrosis

Objectives

This study aims to explore the mechanism by which osteoblast autophagy participated in glucocorticoid-induced femoral head necrosis (FHN).

Materials and methods

Thirty male specific-pathogen-free C57 mice (age, one month; weighing 20-25 g) were randomly divided into blank control, dexamethasone and rapamycin-dexamethasone groups (n=10). After six weeks of intervention, right femoral head was obtained to observe morphology and to calculate percentage of empty lacunae. MC3T3-E1 cells were randomly divided into normal, dexamethasone, rapamycin and dexamethasone-rapamycin groups, and cultured for 24 h. Microtubule-associated protein 1 light chain 3 (LC3)-I, LC3-II, mammalian target of rapamycin (mTOR) and Beclin-1 protein expressions were detected by Western blot.

Results

In rapamycin-dexamethasone group, some bone trabeculae in medullary cavity ruptured and atrophied, and subchondral bone underwent local necrosis. The total apoptosis rates of dexamethasone and rapamycin-dexamethasone groups surpassed that of blank control group, and the former two groups had significantly different rates (p<0.001). LC3-II/LC3-I of dexamethasone group was lower than those of rapamycin and dexamethasone-rapamycin groups (p<0.001), and the ratio of rapamycin group surpassed that of dexamethasone-rapamycin group (p<0.001). Dexamethasone group had higher mTOR protein expression than those of rapamycin and dexamethasone- rapamycin groups (p<0.001), and the expression of rapamycin group was lower than that of dexamethasone-rapamycin group (p<0.001). The Beclin-1 protein expression of dexamethasone group was lower than those of rapamycin and dexamethasone- rapamycin groups (p<0.001), and the expression of rapamycin group exceeded that of dexamethasone-rapamycin group (p<0.05).

Conclusion

Osteoblast autophagy may play a crucial protective role in dexamethasone-induced FHN. The attenuation of autophagy may be related to the affected expressions of key autophagy regulators mTOR and Beclin-1.

1α,25-Dihydroxyvitamin D3 ameliorates diabetes-induced bone loss by attenuating FoxO1-mediated autophagy

Autophagy is vital for maintaining cellular homeostasis through removing impaired organelles. It has recently been found to play pivotal roles in diabetes mellitus (DM), which is associated with increased bone fracture risk and loss of bone density. However, the mechanism whereby autophagy modulates DM-induced bone loss is not fully elucidated. Previous work has shown that 1α,25-Dihydroxyvitamin D3 (1,25D) exerts positive effects on autophagy, thus affecting bone metabolism. Here, we investigated whether autophagy was involved in the regulation of diabetic bone metabolism. Using Micro-CT, Elisa, histology, and histomorphometry analysis, we demonstrated that 1,25D rescues glucose metabolism dysfunction and ameliorates bone loss in diabetic mice. In vitro, 1,25D alleviated primary osteoblast dysfunction and intracellular oxidative stress through reducing prolonged high-glucose-mediated excessive autophagy in primary osteoblasts, reflected by decreased protein level of Beclin1 and LC3. Of note, the autophagy activator rapamycin (RAP) ablated the positive effects of 1,25D in diabetic environment, leading to a marked increase in autolysosomes and autophagosomes, examined by mRFP-GFP-LC3 fluorescence double labeling. The excessive autophagy induced by high glucose was deleterious to proliferation and differentiation of primary osteoblasts. Additionally, biochemical studies identified that PI3K/Akt signaling could be activated by 1,25D, resulting in the inhibition of FoxO1. We confirmed that FoxO1 deficiency alleviated high-glucose-induced autophagy and improved biological functions of primary osteoblasts. Together, our results suggest that the PI3K/Akt/FoxO1 signaling pathway is involved in the osteoprotective effect of 1,25D by attenuating autophagy in diabetes, providing a novel insight for the prevention and treatment of diabetes-caused bone loss.

Bone Autophagy: A Potential Way of Exercise-Mediated Meg3/P62/Runx2 Pathway to Regulate Bone Formation in T2DM Mice

BACKGROUND

Meg3 has been shown to attenuate T2DM bone autophagy by activating p62 to inhibit bone formation. However, whether exercise can reverse this process to promote T2DM bone formation and its mechanism remains unknown.

METHODS

A T2DM mouse model was established by a high-fat diet and STZ injection, and the mice were trained with 8-week HIIT and downhill running exercise. Micro-CT was used to scan the bone microstructure. Bone morphology was observed by HE staining, and the osteoblast (OB) activity in bones was observed by AKP staining. Calcium ion and phosphorus concentration in serum was detected by ELISA; RT-PCR was used to detect the mRNA level, and Western blot was used to detect the protein level of related indexes in Meg3/p62/Runx2 pathway.

RESULTS

The inhibition of bone autophagy, in the bones of T2DM mice, resulted in the degradation of the bone tissue morphology and structure, with the increase of the expressions of Meg3, PI3K, Akt, mTOR, p62 and NF-κB. However, 8-week HIIT and downhill running could reverse this process, especially downhill running, manifested with the up-regulation of miR-16 mRNA level, along with Beclin-1, LC3 II and Runx2 mRNA and protein level.

CONCLUSION

T2DM leads to pathology in model mice. Eight-week HIIT and downhill running exercise can inhibit Meg3, activate autophagy of osteoblasts and promote bone formation in T2DM mice.

Role of Metabolism in Bone Development and Homeostasis

Carbohydrates, fats, and proteins are the underlying energy sources for animals and are catabolized through specific biochemical cascades involving numerous enzymes. The catabolites and metabolites in these metabolic pathways are crucial for many cellular functions; therefore, an imbalance and/or dysregulation of these pathways causes cellular dysfunction, resulting in various metabolic diseases. Bone, a highly mineralized organ that serves as a skeleton of the body, undergoes continuous active turnover, which is required for the maintenance of healthy bony components through the deposition and resorption of bone matrix and minerals. This highly coordinated event is regulated throughout life by bone cells such as osteoblasts, osteoclasts, and osteocytes, and requires synchronized activities from different metabolic pathways. Here, we aim to provide a comprehensive review of the cellular metabolism involved in bone development and homeostasis, as revealed by mouse genetic studies.

Proteomic study of in vitro osteogenic differentiation of mesenchymal stem cells in high glucose condition

Patients with diabetes have been widely reported to be at an increased risk of secondary osteoporosis. Osteoporosis is caused by an imbalance in bone remodeling due to increased bone resorption and/or decreased osteoblast-dependent bone formation. In this study, mesenchymal stem cells (MSCs) were used as a disease model to determine the effects of high glucose levels on MSC-osteoblast development. The results indicated that under high glucose conditions, MSCs had reduced cell viability and increased number of β-galactosidase-positive cells. Furthermore, in vitro osteogenesis was shown to be reduced in MSCs cultured in osteogenic differentiation medium at 10, 25, and 40 mM glucose as demonstrated by Alizarin red S staining and alkaline phosphatase activity assay. Moreover, a proteomic study was performed in MSCs cultured with 25 and 40 mM glucose. The proteomic results demonstrated that 12 proteins were up- and downregulated in bone marrow-derived mesenchymal stem cells cultured with high glucose in a dose-dependent manner. The findings presented here contribute to our understanding of the mechanism of diabetes mellitus responsible for bone loss. However, the exact mechanism of action of hyperglycemia on bone deformability requires additional studies.

Targeting autophagy in osteoporosis: From pathophysiology to potential therapy

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

The Role of Autophagy and Mitophagy in Bone Metabolic Disorders

Bone metabolic disorders include osteolysis, osteoporosis, osteoarthritis and rheumatoid arthritis. Osteoblasts and osteoclasts are two major types of cells in bone constituting homeostasis. The imbalance between bone formation by osteoblasts and bone resorption by osteoclasts has been shown to have a direct contribution to the onset of these diseases. Recent evidence indicates that autophagy and mitophagy, the selective autophagy of mitochondria, may play a vital role in regulating the proliferation, differentiation and function of osteoblasts and osteoclasts. Several signaling pathways, including PINK1/Parkin, SIRT1, MAPK8/FOXO3, Beclin-1/BECN1, p62/SQSTM1, and mTOR pathways, have been implied in the regulation of autophagy and mitophagy in these cells. Here we review the current progress about the regulation of autophagy and mitophagy in osteoblasts and osteoclasts in these bone metabolic disorders, as well as the molecular signaling activated or deactivated during this process. Together, we hope to draw attention to the role of autophagy and mitophagy in bone metabolic disorders, and their potential as a new target for the treatment of bone metabolic diseases and the requirements of further mechanism studies.

Hyperglycemia impairs osteoblast cell migration and chemotaxis due to a decrease in mitochondrial biogenesis

Diabetes is associated with an increase in skeletal fragility and risk of fracture. However, the underlying mechanism for the same is not well understood. Specifically, the results from osteoblast cell culture studies are ambiguous due to contradicting reports. The use of supraphysiological concentrations in these studies, unachievable in vivo, might be the reason for the same. Therefore, here, we studied the effect of physiologically relevant levels of high glucose during diabetes (11.1 mM) on MC3T3-E1 osteoblast cell functions. The results showed that high glucose exposure to osteoblast cells increases their differentiation and mineralization without any effect on the proliferation. However, high glucose decreases their migratory potential and chemotaxis with a decrease in the associated cell signaling. Notably, this decrease in cell migration in high glucose conditions was accompanied by aberrant localization of Dynamin 2 in osteoblast cells. Besides, high glucose also caused a shift in mitochondrial dynamics towards the appearance of more fused and lesser fragmented mitochondria, with a concomitant decrease in the expression of DRP1, suggesting decreased mitochondrial biogenesis. In conclusion, here we are reporting for the first time that hyperglycemia causes a reduction in osteoblast cell migration and chemotaxis. This decrease might lead to an inefficient movement of osteoblasts to the erosion site resulting in uneven mineralization and skeletal fragility found in type 2 diabetes patients, in spite of having normal bone mineral density (BMD).

MnTBAP inhibits bone loss in ovariectomized rats by reducing mitochondrial oxidative stress in osteoblasts

The development of postmenopausal osteoporosis is thought to be closely related to oxidative stress. Mn(III)tetrakis (4-benzoic acid) porphyrin (MnTBAP), a novel superoxide dismutase (SOD) mimetic, could protect osteoblasts from cytotoxicity and dysfunction caused by oxidative stress. However, it is still unclear whether MnTBAP has effect on the development of postmenopausal osteoporosis. Here, we demonstrated that MnTBAP can inhibit bone mass loss and bone microarchitecture alteration, and increase the number of osteoblasts while reducing osteoclasts number, as well as improve the BMP-2 expression level in ovariectomized rat model. Additionally, MnTBAP can also prevent oxidative stress status up-regulation induced by ovariotomy and hydrogen peroxide (H₂O₂). Furthermore, MnTBAP reduced the effect of oxidative stress on osteoblasts differentiation and increased BMP-2 expression levels with a dose-dependent manner, via reducing the levels of mitochondrial oxidative stress in osteoblasts. Taken together, our findings provide new insights that MnTBAP inhibits bone loss in ovariectomized rats by reducing mitochondrial oxidative stress in osteoblasts, and maybe a potential drug in postmenopausal osteoporosis therapy.

Hedgehog signaling regulates osteoblast differentiation in zebrafish larvae through modulation of autophagy

Impaired osteoblast differentiation may result in bone metabolic diseases such as osteoporosis. It was reported recently that hedgehog (Hh) signaling and autophagy are two important regulators of bone differentiation. In order to further dissect their relationship in bone development, we used a zebrafish larvae model to investigate how disruption of one of these signals affects the function of the other and impacts osteoblast differentiation. Our results showed that activation of Hh signaling negatively regulated autophagy. However, suppression of autophagy by knocking down atg5 expression did not alter Hh signaling, but dramatically upregulated the expression of osteoblast-related genes and increased bone mineralization, especially in the den region. On the contrary, inhibition of the Hh signaling pathway by cyclopamine treatment suppressed the expression of osteoblast-related genes and decreased bone mineralization. In agreement with these findings, blocking Hh signaling through knockdown SHH and Gli2 genes led to defective osteoblast differentiation, while promoting Hh signaling by knockdown Ptch1 was beneficial to osteoblast differentiation. Our results thus support that activation of the Hh signaling pathway negatively regulates autophagy and consequentially promotes osteoblast differentiation. On the contrary, induction of autophagy inhibits osteoblast differentiation. Our work reveals the mechanism underlying Hh signaling pathway regulation of bone development.

Summary: Our report of an essential regulation role of hedgehog signaling and autophagy on osteoblast differentiation may contribute to research on bone development biology, hedgehog signaling and the autophagy pathway.

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 Protects MC3T3-E1 Cells upon Aluminum-Induced Apoptosis

Aluminum (Al) exposure has adverse effects on osteoblasts, and the effect might be through autophagy-associated apoptosis. In this study, we showed that aluminum trichloride (AlCl₃) could induce autophagy in MC3T3-E1 cells, as demonstrated by monodansylcadaverine (MDC) staining and the expressions of the ATG3, ATG5, and ATG9 genes. We found AlCl₃ inhibited MC3T3-E1 cell survival rate and caused apoptosis, as evidenced by CCK-8 assay, Annexin V/PI double staining, and increased expressions of Bcl-2, Bax, and Caspase-3 genes. In addition, increased autophagy induced by rapamycin further attenuated the MC3T3-E1 cell apoptosis rate after AlCl₃ exposure. These results support the hypothesis that autophagy plays a protective role in impeding apoptosis caused by AlCl₃. Activating autophagy may be a strategy for treatment of Al-induced bone disease.

The autophagic response to oxidative stress in osteoarthritic chondrocytes is deregulated

It has been reported that oxidative stress (OS) is involved in the pathogenesis of osteoarthritis (OA) and that defective autophagy is accompanying this age-related disease. Moreover, it has been proposed that induction of autophagy could serve as therapeutic mean, as it was shown to alleviate several symptoms in OA animal models. On the contrary, it is also known that autophagic death, which results from over-activation of autophagy, is also a contributor in the development of this disease. Given this discrepancy, in this study we aimed at analysing the autophagic response against acute exogenous oxidative insult of chondrocytes from healthy individuals (control) and OA patients (OA). Cells were treated with sublethal concentrations of hydrogen peroxide (H₂O₂) and then allowed to recover for different periods of time. Firstly, mRNA levels of autophagy-related genes (ATG5, Beclin-1 and LC3) were found significantly reduced in OA chondrocytes compared to control chondrocytes under physiological conditions. After the exposure to OS, in control cells mRNA and protein levels of these genes initially increased and decreased back to their basal levels 6-24 h after treatment. On the contrary, in OA chondrocytes the levels of autophagy-related genes remained high even 24 h post-treatment, indicating their inability to attenuate autophagy. Under the same conditions, the staining pattern of LC3, known marker of autophagosome formation, was analysed, and possible morphological differences between mitochondria of control and OA cells were microscopically assessed. These analyses revealed higher number of impaired mitochondria as well as increased autophagosome formation in OA cells as compared to control cells at all time points. Taken together, our results demonstrate a deregulation of the autophagic response against the oxidative insult in OA chondrocytes and offers insights on autophagy's role in the progression of OA.

Skeletal Fragility in Type 2 Diabetes Mellitus

Type 2 diabetes (T2D) is associated with an increased risk of fracture, which has been reported in several epidemiological studies. However, bone mineral density in T2D is increased and underestimates the fracture risk. Common risk factors for fracture do not fully explain the increased fracture risk observed in patients with T2D. We propose that the pathogenesis of increased fracture risk in T2D is due to low bone turnover caused by osteocyte dysfunction resulting in bone microcracks and fractures. Increased levels of sclerostin may mediate the low bone turnover and may be a novel marker of increased fracture risk, although further research is needed. An impaired incretin response in T2D may also affect bone turnover. Accumulation of advanced glycosylation endproducts may also impair bone strength. Concerning antidiabetic medication, the glitazones are detrimental to bone health and associated with increased fracture risk, and the sulphonylureas may increase fracture risk by causing hypoglycemia. So far, the results on the effect of other antidiabetics are ambiguous. No specific guideline for the management of bone disease in T2D is available and current evidence on the effects of antiosteoporotic medication in T2D is sparse. The aim of this review is to collate current evidence of the pathogenesis, detection and treatment of diabetic bone disease.

Glucosamine promotes osteoblast proliferation by modulating autophagy via the mammalian target of rapamycin pathway

Glucosamine is effective in the treatment of osteoarthritis; however, its effect on osteoporosis remains unclear. Decreased activity of osteoblasts is the main cause of osteoporosis. Here, we examined the effects of glucosamine on osteoblasts. The potential underlying mechanisms were explored. The results showed that glucosamine had a biphasic effect on the viability of hFOB1.19 osteoblasts. At low concentrations (<0.6 mM), glucosamine induced hFOB1.19 cell proliferation, whereas at high concentrations (>0.8 mM) it induced apoptosis. The autophagy inhibitor 3-methyladenine (3-MA) was used to verify that glucosamine modulated hFOB1.19 cell viability via autophagy. The induction of apoptosis by high concentrations of glucosamine was significantly exacerbated by 3-MA, whereas the promotion of cell proliferation by low concentrations of glucosamine was significantly suppressed by 3-MA. Autophagy was examined by western blot detection of autophagy-related proteins including LC3, Beclin-1, and SQSTM1/p62 and by immunofluorescence analysis of autophagosomes. Glucosamine activated autophagy in a time- and concentration-dependent manner. Investigation of the underlying mechanism showed that glucosamine inhibited the phosphorylation of m-TOR in a concentration-dependent manner within 48 h, and rapamycin significantly inhibited the phosphorylation of m-TOR. These results demonstrated that glucosamine promoted hFOB1.19 cell proliferation and increased autophagy by inhibiting the m-TOR pathway, suggesting its potential as a therapeutic agent for osteoporosis.

Defective autophagy in osteoblasts induces endoplasmic reticulum stress and causes remarkable bone loss

Macroautophagy/autophagy is a highly regulated process involved in the turnover of cytosolic components, however its pivotal role in maintenance of bone homeostasis remains elusive. In the present study, we investigated the direct role of ATG7 (autophagy related 7) during developmental and remodeling stages in vivo using osteoblast-specific Atg7 conditional knockout (cKO) mice. Atg7 cKO mice exhibited a reduced bone mass at both developmental and adult age. The trabecular bone volume of Atg7 cKO mice was significantly lower than that of controls at 5 months of age. This phenotype was attributed to decreased osteoblast formation and matrix mineralization, accompanied with an increased osteoclast number and the extent of the bone surface covered by osteoclasts as well as an elevated secretion of TNFSF11/RANKL (tumor necrosis factor [ligand] superfamily, member 11), and a decrease in TNFRSF11B/OPG (tumor necrosis factor receptor superfamily, member 11b [osteoprotegerin]). Remarkably, Atg7 deficiency in osteoblasts triggered endoplasmic reticulum (ER) stress, whereas attenuation of ER stress by administration of phenylbutyric acid in vivo abrogated Atg7 ablation-mediated effects on osteoblast differentiation, mineralization capacity and bone formation. Consistently, Atg7 deficiency impeded osteoblast mineralization and promoted apoptosis partially in DDIT3/CHOP (DNA-damage-inducible transcript 3)- and MAPK8/JNK1 (mitogen-activated protein kinase 8)-SMAD1/5/8-dependent manner in vitro, while reconstitution of Atg7 could improve ER stress and restore skeletal balance. In conclusion, our findings provide direct evidences that autophagy plays crucial roles in regulation of bone homeostasis and suggest an innovative therapeutic strategy against skeletal diseases.

Dioscin enhances osteoblastic cell differentiation and proliferation by inhibiting cell autophagy via the ASPP2/NF-κβ pathway

Dioscin, a typical steroid saponin, has been reported to promote osteoblastic cell differentiation. However, the underling mechanisms remain to be elucidated. In the present study, it was identified that dioscin (0.5, 1, 5, 10 and 25 µg/ml) promoted MC3T3‑E1 cell proliferation and differentiation in a dose‑dependent manner. Western blot analysis showed that dioscin regulated autophagy‑associated protein expression in MC3T3‑E1 cells; it promoted the expression of apoptosis stimulated protein of p53‑2 (ASPP2), and inhibited the expression of nuclear factor (NF)‑κβ and microtubule‑associated protein 1 light chain 3β, in a concentration‑dependent manner. Caffeic acid phenethyl ester (CAPE) was used to inhibit the activation of NF‑κB and examine the effect of the ASPP2/NF‑κβ pathway on osteoblastic cell differentiation, proliferation and autophagy. It was identified that CAPE reversed the regulation of dioscin on osteoblastic cell differentiation, proliferation and autophagy. In conclusion, the present study revealed that dioscin promoted osteoblast proliferation and differentiation by inhibiting cell autophagy via the ASPP2/NF‑κβ pathway. These results are the first, to the best of our knowledge, to reveal the involvement of autophagy in the effects of dioscin on the prevention and therapy of osteoporosis.

Involvement of Endoplasmic Reticulum Stress, Autophagy, and Apoptosis in Advanced Glycation End Products-Induced Glomerular Mesangial Cell Injury

Advanced glycation end-products (AGEs)-induced mesangial cell death is one of major causes of glomerulus dysfunction in diabetic nephropathy. Both endoplasmic reticulum (ER) stress and autophagy are adaptive responses in cells under environmental stress and participate in the renal diseases. The role of ER stress and autophagy in AGEs-induced mesangial cell death is still unclear. Here, we investigated the effect and mechanism of AGEs on glomerular mesangial cells. AGEs dose-dependently decreased mesangial cell viability and induced cell apoptosis. AGEs also induced ER stress signals in a time- and dose-dependent manner. Inhibition of ER stress with 4-phenylbutyric acid effectively inhibited the activation of eIF2α and CHOP signals and reversed AGEs-induced cell apoptosis. AGEs also activated LC-3 cleavage, increased Atg5 expression, and decreased p62 expression, which indicated the autophagy induction in mesangial cells. Inhibition of autophagy by Atg5 siRNAs transfection aggravated AGEs-induced mesangial cell apoptosis. Moreover, ER stress inhibition by 4-phenylbutyric acid significantly reversed AGEs-induced autophagy, but autophagy inhibition did not influence the AGEs-induced ER stress-related signals activation. These results suggest that AGEs induce mesangial cell apoptosis via an ER stress-triggered signaling pathway. Atg5-dependent autophagy plays a protective role. These findings may offer a new strategy against AGEs toxicity in the kidney.

Bone morphogenetic protein-induced cell differentiation involves Atg7 and Wnt16 sequentially in human stem cell-derived osteoblastic cells

We established a differentiation method for homogeneous α7 integrin-positive human skeletal muscle stem cell (α7(+)hSMSC)-derived osteoblast-like cells with bone morphogenetic protein (BMP)-2. To explore the early signaling cascade for osteoblastic differentiation, we examined the upregulation of autophagy-related gene (Atg) and wingless/int1 (Wnt) signaling during BMP-2-mediated human osteoblastic differentiation. In a screening experiment, BMP-2 increased the mRNA and protein levels of Atg7, Wnt16, and Lrp5/Fzd2 (a Wnt receptor), but not microtubule-associated protein 1 light chain (LC3; a mammalian homolog of yeast Atg8), TFE3, Beclin1, Atg5, Atg12, Wnt3a, or Wnt5, together with the amounts of autophagosomes and autophagy fluxes. Treatment with siRNAs against Atg7 and Wnt16 individually suppressed the BMP-2-induced increase in osteoblastic differentiation. The osteoblastic phenotype, involving osteocalcin (BGLAP), osteopontin (SPP1), and osterix (SP7) expression, decreased when autophagy was inhibited by chloroquine (an autophagy inhibitor), but increased after treatment with rapamycin (an autophagy enhancer). Taken together with our previous findings, we have revealed a unique sequential cascade of BMP-2→Atg7→Wnt16→Lrp5/Fzd2→matrix metalloproteinase-13→osteoblastic differentiation. This cascade results in a potent increase in osteoblastic cell differentiation, indicating the unique involvement of Atg7, autophagy, and Wnt16 signaling in BMP-2-induced differentiation of α7(+)hSMSCs into osteoblast-like cells at a relatively early stage.

TSC2 N-terminal lysine acetylation status affects to its stability modulating mTORC1 signaling and autophagy

There is a growing evidence of the role of protein acetylation in different processes controlling metabolism. Sirtuins (histone deacetylases nicotinamide adenine dinucleotide-dependent) activate autophagy playing a protective role in cell homeostasis. This study analyzes tuberous sclerosis complex (TSC2) lysine acetylation, in the regulation of mTORC1 signaling activation, autophagy and cell proliferation. Nicotinamide 5mM (a concentration commonly used to inhibit SIRT1), increased TSC2 acetylation in its N-terminal domain, and concomitantly with an augment in its ubiquitination protein status, leading to mTORC1 activation and cell proliferation. In contrast, resveratrol (RESV), an activator of sirtuins deacetylation activity, avoided TSC2 acetylation, inhibiting mTORC1 signaling and promoting autophagy. Moreover, TSC2 in its deacetylated state was prevented from ubiquitination. Using MEF Sirt1 +/+ and Sirt1 -/- cells or a SIRT1 inhibitor (EX527) in MIN6 cells, TSC2 was hyperacetylated and neither NAM nor RESV were capable to modulate mTORC1 signaling. Then, silencing Tsc2 in MIN6 or in MEF Tsc2-/- cells, the effects of SIRT1 modulation by NAM or RESV on mTORC1 signaling were abolished. We also observed that two TSC2 lysine mutants in its N-terminal domain, derived from TSC patients, differentially modulate mTORC1 signaling. TSC2 K599M variant presented a lower mTORC1 activity. However, with K106Q mutant, there was an activation of mTORC1 signaling at the basal state as well as in response to NAM. This study provides, for the first time, a relationship between TSC2 lysine acetylation status and its stability, representing a novel mechanism for regulating mTORC1 pathway.

High glucose induces autophagy of MC3T3-E1 cells via ROS-AKT-mTOR axis

In the present study, we investigate the function of ROS-AKT-mTOR axis on the apoptosis, proliferation and autophagy of MC3T3-E1 cells, and the proliferation of MC3T3-E1 cells after autophagy inhibition under high glucose conditions. MC3T3-E1 cells cultured in vitro were divided into the following groups: normal control group, N-acetylcysteine (NAC) group, 11.0 mM high glucose group, 11.0 mM high glucose + NAC group, 22.0 mM high glucose group, 22.0 mM high glucose + NAC group, CQ group, 22.0 mM high glucose + CQ group, 3-MA group and 3-MA + 22.0 mM high glucose group. ROS production was measured by DCFH-DA fluorescent probe. Cell proliferation was measured by MTT assay. Cells in different groups were stained with Annexin V-FITC/PI, and then apoptosis rate was detected by flow cytometry. Nucleus morphology was observed under fluorescence microscope after being incubated with Honchest33258. Protein expression was measured using Western blotting and immunofluorescence. Cell apoptosis and proliferation in high glucose group were increased and decreased, respectively, in a dose-dependent manner. Autophagy was significantly induced in high glucose group, even though different concentration of glucose induced autophagy in different stages of autophagy. ROS production in MC3T3-E1 cells was remarkably increased in high glucose group, but not in a dose-dependent manner. NAC, as an antioxidant, reduced ROS production and ameliorated cell apoptosis, proliferation abnormity and autophagy caused by high glucose. Expression of p-AKT and p-mTOR proteins were dramatically decreased in high glucose group, and NAC reversed their expression. In addition, 3-MA, an inhibitor of autophagy, significantly decreased the proliferation of MC3T3-E1 cells. When cocultured with 22.0 mM glucose that induced autophagy, proliferation of MC3T3-E1 cells was not affected compared to 22.0 mM high glucose group. Our present findings reveal that high glucose affects apoptosis, proliferation and autophagy of MC3T3-E1 cells through ROS-AKT-mTOR axis. In addition, autophagy inhibition does not affect the proliferation of MC3T3-E1 cells under high glucose conditions.

Autophagy ameliorates cognitive impairment through activation of PVT1 and apoptosis in diabetes mice

The underlying mechanisms of cognitive impairment in diabetes remain incompletely characterized. Here we show that the autophagic inhibition by 3-methyladenine (3-MA) aggravates cognitive impairment in streptozotocin-induced diabetic mice, including exacerbation of anxiety-like behaviors and aggravation in spatial learning and memory, especially the spatial reversal memory. Further neuronal function identification confirmed that both long term potentiation (LTP) and depotentiation (DPT) were exacerbated by autophagic inhibition in diabetic mice, which indicating impairment of synaptic plasticity. However, no significant change of pair-pulse facilitation (PPF) was recorded in diabetic mice with autophagic suppression compared with the diabetic mice, which indicated that presynaptic function was not affected by autophagic inhibition in diabetes. Subsequent hippocampal neuronal cell death analysis showed that the apoptotic cell death, but not the regulated necrosis, significantly increased in autophagic suppression of diabetic mice. Finally, molecular mechanism that may lead to cell death was identified. The long non-coding RNA PVT1 (plasmacytoma variant translocation 1) expression was analyzed, and data revealed that PVT1 was decreased significantly by 3-MA in diabetes. These findings show that PVT1-mediated autophagy may protect hippocampal neurons from impairment of synaptic plasticity and apoptosis, and then ameliorates cognitive impairment in diabetes. These intriguing findings will help pave the way for exciting functional studies of autophagy in cognitive impairment and diabetes that may alter the existing paradigms.

Advanced Glycation End Products Affect Osteoblast Proliferation and Function by Modulating Autophagy Via the Receptor of Advanced Glycation End Products/Raf Protein/Mitogen-activated Protein Kinase/Extracellular Signal-regulated Kinase Kinase/Extracellular Signal-regulated Kinase (RAGE/Raf/MEK/ERK) Pathway

The interaction between advanced glycation end products (AGEs) and receptor of AGEs (RAGE) is associated with the development and progression of diabetes-associated osteoporosis, but the mechanisms involved are still poorly understood. In this study, we found that AGE-modified bovine serum albumin (AGE-BSA) induced a biphasic effect on the viability of hFOB1.19 cells; cell proliferation was stimulated after exposure to low dose AGE-BSA, but cell apoptosis was stimulated after exposure to high dose AGE-BSA. The low dose AGE-BSA facilitates proliferation of hFOB1.19 cells by concomitantly promoting autophagy, RAGE production, and the Raf/MEK/ERK signaling pathway activation. Furthermore, we investigated the effects of AGE-BSA on the function of hFOB1.19 cells. Interestingly, the results suggest that the short term effects of low dose AGE-BSA increase osteogenic function and decrease osteoclastogenic function, which are likely mediated by autophagy and the RAGE/Raf/MEK/ERK signal pathway. In contrast, with increased treatment time, the opposite effects were observed. Collectively, AGE-BSA had a biphasic effect on the viability of hFOB1.19 cells in vitro, which was determined by the concentration of AGE-BSA and treatment time. A low concentration of AGE-BSA activated the Raf/MEK/ERK signal pathway through the interaction with RAGE, induced autophagy, and regulated the proliferation and function of hFOB1.19 cells.

Sirt3-MnSOD axis represses nicotine-induced mitochondrial oxidative stress and mtDNA damage in osteoblasts

Increasing evidence has suggested an important role played by reactive oxygen species in the pathogenesis of osteoporosis. Tobacco smoking is an important risk factor for the development of osteoporosis, and nicotine is one of the major components in tobacco. However, the mechanism by which nicotine promotes osteoporosis is not fully understood. Here, in this study, we found that nicotine-induced mitochondrial oxidative stress and mitochondrial DNA (mtDNA) damage in osteoblasts differentiated from mouse mesenchymal stem cell. The activity of MnSOD, one of the mitochondrial anti-oxidative enzymes, was significantly reduced by nicotine due to the reduced level of Sirt3. Moreover, it was also found that Sirt3 could promote MnSOD activity by deacetylating MnSOD. Finally, Mn(III)tetrakis (4-benzoic acid) porphyrin (MnTBAP, a MnSOD mimetic) was found to markedly reduce the effect of nicotine on osteoblasts. In summary, Sirt3-MnSOD axis was identified as a negative component in nicotine-induced mitochondrial oxidative stress and mtDNA damage, and MnTBAP may serve as a potential therapeutic drug for osteoporosis.

Transforming growth factor β and insulin signal changes in stromal fibroblasts of individual keratoconus patients

Keratoconus (KC) is a complex thinning disease of the cornea that often requires transplantation. The underlying pathogenic molecular changes in this disease are poorly understood. Earlier studies reported oxidative stress, metabolic dysfunctions and accelerated death of stromal keratocytes in keratoconus (KC) patients. Utilizing mass spectrometry we found reduced stromal extracellular matrix (ECM) proteins in KC, suggesting ECM-regulatory changes that may be due to altered TGFβ signals. Here we investigated properties of stromal cells from donor (DN) and KC corneas grown as fibroblasts in serum containing DMEM: F12 or in serum-free medium containing insulin, transferrin, selenium (ITS). Phosphorylation of SMAD2/3 of the canonical TGFβ pathway, was high in serum-starved DN and KC fibroblast protein extracts, but pSMAD1/5/8 low at base line, was induced within 30 minutes of TGFβ1 stimulation, more so in KC than DN, suggesting a novel TGFβ1-SMAD1/5/8 axis in the cornea, that may be altered in KC. The serine/threonine kinases AKT, known to regulate proliferation, survival and biosynthetic activities of cells, were poorly activated in KC fibroblasts in high glucose media. Concordantly, alcohol dehydrogenase 1 (ADH1), an indicator of increased glucose uptake and metabolism, was reduced in KC compared to DN fibroblasts. By contrast, in low glucose (5.5 mM, normoglycemic) serum-free DMEM and ITS, cell survival and pAKT levels were comparable in KC and DN cells. Therefore, high glucose combined with serum-deprivation presents some cellular stress difficult to overcome by the KC stromal cells. Our study provides molecular insights into AKT and TGFβ signal changes in KC, and a mechanism for functional studies of stromal cells from KC corneas.

Autophagy: a two-edged sword in diabetes mellitus

A fragility fracture is a serious complication in patients with diabetes mellitus as a result of hyperglycaemia, insulin resistance and the production of AGEs (advanced glycation end-products). In their paper published in the Biochemical Journal, Bartolomé et al. identified a role for autophagy in the differentiation, function and survival of osteoblastic cells in a high-glucose environment, and they also demonstrated that osteoblastic cell survival was limited by chemical and genetic inhibition of autophagy. These novel findings show the possibility of investigating a therapeutic strategy of maintaining autophagy in osteoblasts to lead to the prevention of diabetes-related osteopaenia. Autophagy is one of the common functions for maintaining cellular health, and the regulation of autophagy that is perturbed by diabetes mellitus may induce improvement of cellular functions not only for diabetes-related osteopaenia, but also for other systemic complications. However, systemic activation of autophagy may not always induce beneficial effects for non-targeted healthy cells, and autophagy should be controlled at a proper level at each disease stage in each target organ.