Osteocyte apoptosis: the roles and key molecular mechanisms in resorption-related bone diseases

The Roles of Fibrinolytic Factors in Bone Destruction Caused by Inflammation

Chronic inflammatory diseases, such as rheumatoid arthritis, spondyloarthritis, systemic lupus erythematosus, Crohn's disease, periodontitis, and carcinoma metastasis frequently result in bone destruction. Pro-inflammatory cytokines such as tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), IL-6, and IL-17 are known to influence bone loss by promoting the differentiation and activation of osteoclasts. Fibrinolytic factors, such as plasminogen (Plg), plasmin, urokinase-type plasminogen activator (uPA), its receptor (uPAR), tissue-type plasminogen activator (tPA), α2-antiplasmin (α2AP), and plasminogen activator inhibitor-1 (PAI-1) are expressed in osteoclasts and osteoblasts and are considered essential in maintaining bone homeostasis by regulating the functions of both osteoclasts and osteoblasts. Additionally, fibrinolytic factors are associated with the regulation of inflammation and the immune system. This review explores the roles of fibrinolytic factors in bone destruction caused by inflammation.

Ferroptosis in Osteocytes as a Target for Protection Against Postmenopausal Osteoporosis

Ferroptosis is a necrotic form of iron-dependent regulatory cell death. Estrogen withdrawal can interfere with iron metabolism, which is responsible for the pathogenesis of postmenopausal osteoporosis (PMOP). Here, it is demonstrated that estrogen withdrawal induces iron accumulation in the skeleton and the ferroptosis of osteocytes, leading to reduced bone mineral density. Furthermore, the facilitatory effect of ferroptosis of osteocytes is verified in the occurrence and development of postmenopausal osteoporosis is associated with over activated osteoclastogenesis using a direct osteocyte/osteoclast coculture system and glutathione peroxidase 4 (GPX4) knockout ovariectomized mice. In addition, the nuclear factor erythroid derived 2-related factor-2 (Nrf2) signaling pathway is confirmed to be a crucial factor in the ferroptosis of osteocytic cells. Nrf2 regulates the expression of nuclear factor kappa-B ligand (RANKL) by regulating the DNA methylation level of the RANKL promoter mediated by DNA methyltransferase 3a (Dnmt3a), which is as an important mechanism in osteocytic ferroptosis-mediated osteoclastogenesis. Taken together, this data suggests that osteocytic ferroptosis is involved in PMOP and can be targeted to tune bone homeostasis.

Pip5k1c expression in osteocytes regulates bone remodeling in mice

Research background

The role of osteocytes in maintaining bone mass has been progressively emphasized. Pip5k1c is the most critical isoform among PIP5KIs, which can regulate cytoskeleton, biomembrane, and Ca2+ release of cells and participate in many processes, such as cell adhesion, differentiation, and apoptosis. However, its expression and function in osteocytes are still unclear.

Materials and methods

To determine the function of Pip5k1c in osteocytes, the expression of Pip5k1c in osteocytes was deleted by breeding the 10-kb mouse Dmp1-Cre transgenic mice with the Pip5k1cfl/fl mice. Bone histomorphometry, micro-computerized tomography analysis, immunofluorescence staining and western blotting were used to determine the effects of Pip5k1c loss on bone mass. In vitro, we explored the mechanism by siRNA knockdown of Pip5k1c in MLO-Y4 cells.

Results

Pip5k1c expression was decreased in osteocytes in senescent and osteoporotic tissues both in humans and mice. Loss of Pip5k1c in osteocytes led to a low bone mass in long bones and spines and impaired biomechanical properties in femur, without changes in calvariae. The loss of Pip5k1c resulted in the reduction of the protein level of type 1 collagen in tibiae and MLO-Y4 cells. Osteocyte Pip5k1c loss reduced the osteoblast and bone formation rate with high expression of sclerostin, impacting the osteoclast activities at the same time. Moreover, Pip5k1c loss in osteocytes reduced expression of focal adhesion proteins and promoted apoptosis.

Conclusion

Our studies demonstrate the critical role and mechanism of Pip5k1c in osteocytes in regulating bone remodeling.

The translational potential of this article

Osteocyte has been considered to a key role in regulating bone homeostasis. The present study has demonstrated that the significance of Pip5k1c in bone homeostasis by regulating the expression of collagen, sclerostin and focal adhesion expression, which provided a possible therapeutic target against human metabolic bone disease.

Regulator of G protein signalling 18 promotes osteocyte proliferation by activating the extracellular signal‑regulated kinase signalling pathway

Osteocyte function is critical for metabolism, remodelling and regeneration of bone tissue. In the present study, the roles of regulator of G protein signalling 18 (RGS18) were assessed in the regulation of osteocyte proliferation and bone formation. Target genes and signalling pathways were screened using the Gene Expression Omnibus (GEO) database and Gene Set Enrichment Analysis (GSEA). The function of RGS18 and the associated mechanisms were analysed by Cell Counting Kit 8 assay, 5‑ethynyl‑2'‑deoxyuridine assay, flow cytometry, reverse transcription‑quantitative PCR, western blotting and immunostaining. Overlap analysis of acutely injured subjects (AIS) and healthy volunteers (HVs) from the GSE93138 and GSE93215 datasets of the GEO database identified four genes: KIAA0825, ANXA3, RGS18 and LIPN. Notably, RGS18 was more highly expressed in peripheral blood samples from AIS than in those from HVs. Furthermore, RGS18 overexpression promoted MLO‑Y4 and MC3T3‑E1 cell viability, proliferation and S‑phase arrest, but inhibited apoptosis by suppressing caspase‑3/9 cleavage. Silencing RGS18 exerted the opposite effects. GSEA of GSE93138 revealed that RGS18 has the ability to regulate MAPK signalling. Treatment with the MEK1/2 inhibitor PD98059 reversed the RGS18 overexpression‑induced osteocyte proliferation, and treatment with the ERK1/2 activator 12‑O‑tetradecanoylphorbol‑13‑acetate reversed the effects of RGS18 silencing on osteocyte proliferation. In conclusion, RGS18 may contribute to osteocyte proliferation and bone fracture healing via activation of ERK signalling.

Effects of Weight Bearing on Marrow Adipose Tissue and Trabecular Bone after Anterior Cruciate Ligament Reconstruction in the Rat Proximal Tibial Epiphysis

The effects of mechanical unloading after anterior cruciate ligament (ACL) reconstruction on bone and marrow adipose tissue (MAT) are unclear. We investigated weight bearing effects on bone and MAT after ACL reconstruction. Rats underwent unilateral knee ACL transection and reconstruction, followed by hindlimb unloading (non-weight bearing), no intervention (low-weight bearing, the hindlimb standing time ratio (STR; operated/contralateral) during treadmill locomotion ranging from 0.55 to 0.91), or sustained morphine administration (moderate-weight bearing, STR ranging from 0.80 to 0.95). Untreated rats were used as controls. At 7 or 14 days after surgery, changes in trabecular bone and MAT in the proximal tibial were assessed histologically. Histological assessments at 7 or 14 days after surgery showed that ACL reconstruction without post-operative intervention did not significantly change trabecular bone and MAT areas. Hindlimb unloading after ACL reconstruction induced MAT accumulation with adipocyte hyperplasia and hypertrophy within 14 days, but did not significantly affect trabecular bone area. Increased weight bearing through morphine administration did not affect trabecular bone and MAT parameters. Our results suggest that early weight bearing after ACL reconstruction is important in reducing MAT accumulation, and that reduction in weight bearing alone is not sufficient to induce bone loss early after ACL reconstruction.

Metformin alleviates genetic and traumatic heterotopic ossification by inhibiting infiltration and mitochondrial metabolism of myeloid cells

OBJECTIVES

Heterotopic ossification (HO), whether hereditary or traumatic, refers to the abnormal formation of bone in extraskeletal sites, often triggered by inflammation or flare-ups. Unfortunately, there are currently no effective treatments for HO. Metformin is well-known for its anti-diabetic, anti-inflammatory, anti-aging, and anti-cancer effects. However, its potential role in treating HO remains uncertain.

METHODS

Metformin was dissolved into water and given to mice. All the mice in this study were examined by microCT and myeloid cell quantification using flow cytometry. Complex activity kit was used to examine the activity of mitochondrial complexes of myeloid cells.

RESULTS

In this study, we discovered that metformin effectively inhibits genetic and traumatic HO formation and progression. Additionally, we observed a significant increase in myeloid cells in the genetic and traumatic HO mouse model compared to uninjured mice. Notably, metformin specifically reduced the infiltration of myeloid cells into the injured sites of the genetic and traumatic HO model mice. Further investigations revealed that metformin targets mitochondrial complex I and suppresses mitochondrial metabolism in myeloid cells.

CONCLUSION

These findings suggest that metformin suppresses HO development by potentially downregulating the mitochondrial metabolism of myeloid cells, offering a promising therapeutic option for HO treatment.

Effects of 625 nm light-emitting diode irradiation on preventing ER stress-induced apoptosis via GSK-3β phosphorylation in MC3T3-E1

Prolonged endoplasmic reticulum (ER) stress contributes to cell apoptosis and interferes with bone homeostasis. Although photobiomodulation (PBM) might be used for ER stress-induced diseases, the role of PBM in relieving cell apoptosis remains unknown. During ER stress, glycogen synthase kinase-3β (GSK-3β) is critical; however, its functions in PBM remain uncertain. Thus, this study aimed to investigate the role of GSK-3β in 625 nm light-emitting diode irradiation (LEDI) relieving tunicamycin (TM)-induced apoptosis. Based on the results, pre-625 nm LEDI (Pre-IR) phosphorylated GSK-3β via ROS production. Compared with the TM group, Pre-IR + TM group reduced the phosphorylation of the α-subunit of eukaryotic translation initiation factor 2 (eIF-2α) and B-cell lymphoma protein 2 (Bcl-2)-associated X (Bax)/Bcl-2 ratio through regulating GSK-3β. Furthermore, a similar tendency was observed between Pre-IR + TM and Pre-LiCl+TM groups in preventing TM-induced early and late apoptosis. In summary, this study suggests that the Pre-IR treatment in TM-induced ER stress is beneficial for preventing cell apoptosis via GSK-3β phosphorylation.

Crosstalk Between the Neuroendocrine System and Bone Homeostasis

The homeostasis of bone microenvironment is the foundation of bone health and comprises 2 concerted events: bone formation by osteoblasts and bone resorption by osteoclasts. In the early 21st century, leptin, an adipocytes-derived hormone, was found to affect bone homeostasis through hypothalamic relay and the sympathetic nervous system, involving neurotransmitters like serotonin and norepinephrine. This discovery has provided a new perspective regarding the synergistic effects of endocrine and nervous systems on skeletal homeostasis. Since then, more studies have been conducted, gradually uncovering the complex neuroendocrine regulation underlying bone homeostasis. Intriguingly, bone is also considered as an endocrine organ that can produce regulatory factors that in turn exert effects on neuroendocrine activities. After decades of exploration into bone regulation mechanisms, separate bioactive factors have been extensively investigated, whereas few studies have systematically shown a global view of bone homeostasis regulation. Therefore, we summarized the previously studied regulatory patterns from the nervous system and endocrine system to bone. This review will provide readers with a panoramic view of the intimate relationship between the neuroendocrine system and bone, compensating for the current understanding of the regulation patterns of bone homeostasis, and probably developing new therapeutic strategies for its related disorders.

Mechanical loading intensities affect the release of extracellular vesicles from mouse bone marrow-derived hematopoietic progenitor cells and change their osteoclast-modulating effect

Low-intensity loading maintains or increases bone mass, whereas lack of mechanical loading and high-intensity loading decreases bone mass, possibly via the release of extracellular vesicles by mechanosensitive bone cells. How different loading intensities alter the biological effect of these vesicles is not fully understood. Dynamic fluid shear stress at low intensity (0.7 ± 0.3 Pa, 5 Hz) or high intensity (2.9 ± 0.2 Pa, 1 Hz) was used on mouse hematopoietic progenitor cells for 2 min in the presence or absence of chemical compounds that inhibit release or biogenesis of extracellular vesicles. We used a Receptor activator of nuclear factor kappa-Β ligand-induced osteoclastogenesis assay to evaluate the biological effect of different fractions of extracellular vesicles obtained through centrifugation of medium from hematopoietic stem cells. Osteoclast formation was reduced by microvesicles (10 000× g) obtained after low-intensity loading and induced by exosomes (100 000× g) obtained after high-intensity loading. These osteoclast-modulating effects could be diminished or eliminated by depletion of extracellular vesicles from the conditioned medium, inhibition of general extracellular vesicle release, inhibition of microvesicle biogenesis (low intensity), inhibition of ESCRT-independent exosome biogenesis (high intensity), as well as by inhibition of dynamin-dependent vesicle uptake in osteoclast progenitor cells. Taken together, the intensity of mechanical loading affects the release of extracellular vesicles and change their osteoclast-modulating effect.

Osteoclast-specific Plastin 3 knockout in mice fail to develop osteoporosis despite dramatic increased osteoclast resorption activity

PLS3 loss-of-function mutations in humans and mice cause X-linked primary osteoporosis. However, it remains largely unknown how PLS3 mutations cause osteoporosis and which function PLS3 plays in bone homeostasis. A recent study showed that ubiquitous Pls3 KO in mice results in osteoporosis. Mainly osteoclasts were impacted in their function However, it has not been proven if osteoclasts are the major cell type affected and responsible for osteoporosis development in ubiquitous Pls3 KO mice. Here, we generated osteoclast-specific Pls3 KO mice. Additionally, we developed a novel polyclonal PLS3 antibody that showed specific PLS3 loss in immunofluorescence staining of osteoclasts in contrast to previously available antibodies against PLS3, which failed to show PLS3 specificity in mouse cells. Moreover, we demonstrate that osteoclast-specific Pls3 KO causes dramatic increase in resorptive activity of osteoclasts in vitro. Despite these findings, osteoclast-specific Pls3 KO in vivo failed to cause any osteoporotic phenotype in mice as proven by micro-CT and three-point bending test. This demonstrates that the pathomechanism of PLS3-associated osteoporosis is highly complex and cannot be reproduced in a system singularly focused on one cell type. Thus, the loss of PLS3 in alternative bone cell types might contributes to the osteoporosis phenotype in ubiquitous Pls3 KO mice.

Lymphatic platelet thrombosis limits bone repair by precluding lymphatic transporting DAMPs

Lymphatic vessels (LVs) interdigitated with blood vessels, travel and form an extensive transport network in the musculoskeletal system. Blood vessels in bone regulate osteogenesis and hematopoiesis, however, whether LVs in bone affect fracture healing is unclear. Here, by near infrared indocyanine green lymphatic imaging (NIR-ICG), we examined lymphatic draining function at the tibial fracture sites and found lymphatic drainage insufficiency (LDI) occurred as early as two weeks after fracture. Sufficient lymphatic drainage facilitates fracture healing. In addition, we identified that lymphatic platelet thrombosis (LPT) blocks the draining lymphoid sinus and LVs, caused LDI and then inhibited fracture healing, which can be rescued by a pharmacological approach. Moreover, unblocked lymphatic drainage decreased neutrophils and increased M2-like macrophages of hematoma niche to support osteoblast (OB) survival and bone marrow-derived mesenchymal stem cell (BMSC) proliferation via transporting damage-associated molecular patterns (DAMPs). These findings demonstrate that LPT limits bone regeneration by blocking lymphatic drainage from transporting DAMPs. Together, these findings represent a novel way forward in the treatment of bone repair.

All the might of the osteocyte: emerging roles in chronic kidney disease

Osteocytes are the most abundant type of bone cell and play crucial roles in bone health. Osteocytes sense mechanical stress and orchestrate osteoblasts and osteoclasts to maintain bone density and strength. Beyond this, osteocytes have also emerged as key regulators of organ crosstalk, and they function as endocrine organs via their roles in secreting factors that mediate signaling within their neighboring bone cells and in distant tissues. As such, osteocyte dysfunction has been associated with the bone abnormalities seen across a spectrum of chronic kidney disease. Specifically, dysregulated osteocyte morphology and signaling have been observed in the earliest stages of chronic kidney disease and have been suggested to contribute to kidney disease progression. More important, US Food and Drug Administration-approved inhibitors of osteocytic secreted proteins, such as fibroblast growth factor 23 and sclerostin, have been used to treat bone diseases. The present mini review highlights new research that links dysfunctional osteocytes to the pathogenesis of chronic kidney disease mineral and bone disorder.

ROS-mediated mitophagy and necroptosis regulate osteocytes death caused by TCP particles in MLO-Y4 cells

Our previous data have revealed TCP particles caused cell death of osteocytes, comprising over 95 % of all bone cells, which contribute to periprosthetic osteolysis, joint loosening and implant failure, but its mechanisms are not fully understood. Here, we reported that TCP particles inhibited cell viability of osteocytes MLO-Y4, and caused cell death. TCP particles caused mitochondrial impairment and increased expressions of LC-3 II, Parkin and PINK 1, accompanied by the elevation of autophagy flux and intracellular acidic components, the accumulation of LC-3II, PINK1 and Parkin in damaged mitochondria, and p62 reduction. The increased LC-3II expression and cell death extent were significantly enhanced by the autophagy inhibitor Baf A1, compared with Baf A1 (or TCP particles) alone, indicating that TCP particles increase autophagic flux and lead to cell even death of MLO-Y4 cells, closely associated with mitophagy. Furthermore, TCP particles induced propidium iodide (PI) uptake and the phosphorylation of RIP1, RIP3 and MLKL, thereby increasing necroptosis in MLO-Y4 cells. The pro-necroptotic effect was alleviated by the RIP1 inhibitor Nec-1 or the MLKL inhibitor NSA. Additionally, TCP particles promoted the production of intracellular reactive oxygen species (ROS) and mitochondrial ROS (mtROS), and increased TXNIP expression, but decreased protein levels of TRX1, Nrf2, HO-1 and NQO1, leading to oxidative stress. The ROS scavenger NAC remarkably reversed mitophagy and necroptosis caused by TCP particles, suggesting that ROS is responsible for mitophagy and necroptosis. Collectively, ROS-mediated mitophagy and necroptosis regulate osteocytes death caused by TCP particles in MLO-Y4 cells, which enhances osteoclastogenesis and periprosthetic osteolysis.

Bone on-a-chip: a 3D dendritic network in a screening platform for osteocyte-targeted drugs

Age-related musculoskeletal disorders, including osteoporosis, are frequent and associated with long lasting morbidity, in turn significantly impacting on healthcare system sustainability. There is therefore a compelling need to develop reliable preclinical models of disease and drug screening to validate novel drugs possibly on a personalized basis, without the need ofin vivoassay. In the context of bone tissue, although the osteocyte (Oc) network is a well-recognized therapeutic target, currentin vitropreclinical models are unable to mimic its physiologically relevant and highly complex structure. To this purpose, several features are needed, including an osteomimetic extracellular matrix, dynamic perfusion, and mechanical cues (e.g. shear stress) combined with a three-dimensional (3D) culture of Oc. Here we describe, for the first time, a high throughput microfluidic platform based on 96-miniaturized chips for large-scale preclinical evaluation to predict drug efficacy. We bioengineered a commercial microfluidic device that allows real-time visualization and equipped with multi-chips by the development and injection of a highly stiff bone-like 3D matrix, made of a blend of collagen-enriched natural hydrogels loaded with hydroxyapatite nanocrystals. The microchannel, filled with the ostemimetic matrix and Oc, is subjected to passive perfusion and shear stress. We used scanning electron microscopy for preliminary material characterization. Confocal microscopy and fluorescent microbeads were used after material injection into the microchannels to detect volume changes and the distribution of cell-sized objects within the hydrogel. The formation of a 3D dendritic network of Oc was monitored by measuring cell viability, evaluating phenotyping markers (connexin43, integrin alpha V/CD51, sclerostin), quantification of dendrites, and responsiveness to an anabolic drug. The platform is expected to accelerate the development of new drug aimed at modulating the survival and function of osteocytes.

Assembling the Puzzle Pieces. Insights for in Vitro Bone Remodeling

As a highly dynamic organ, bone changes during throughout a person's life. This process is referred to as 'bone remodeling' and it involves two stages - a well-balanced osteoclastic bone resorption and an osteoblastic bone formation. Under normal physiological conditions bone remodeling is highly regulated that ensures tight coupling between bone formation and resorption, and its disruption results in a bone metabolic disorder, most commonly osteoporosis. Though osteoporosis is one of the most prevalent skeletal ailments that affect women and men aged over 40 of all races and ethnicities, currently there are few, if any safe and effective therapeutic interventions available. Developing state-of-the-art cellular systems for bone remodeling and osteoporosis can provide important insights into the cellular and molecular mechanisms involved in skeletal homeostasis and advise better therapies for patients. This review describes osteoblastogenesis and osteoclastogenesis as two vital processes for producing mature, active bone cells in the context of interactions between cells and the bone matrix. In addition, it considers current approaches in bone tissue engineering, pointing out cell sources, core factors and matrices used in scientific practice for modeling bone diseases and testing drugs. Finally, it focuses on the challenges that bone regenerative medicine is currently facing.

Jintiange proteins promote osteogenesis and inhibit apoptosis of osteoblasts by enhancing autophagy via PI3K/AKT and ER stress pathways

ETHNOPHARMACOLOGICAL RELEVANCE

Tiger bone, which had long been used in traditional Chinese medicine, had the action of removing wind and alleviating pain, strengthening the sinews and bones, and often used to treat bone impediment, and atrophic debility of bones in TCM clinical practice. As a substitute of natural bone tiger, artificial tiger bone Jintiange (JTG), has been approved by the State Food and Drug Administration of China for relief the symptom of osteoporosis, such as lumbago and back pain, lassitude in loin and legs, flaccidity and weakness legs, and walk with difficulty based on TCM theory. JTG has similar chemical profile to natural tiger bone, and contains mineral substance, peptides and proteins, and has been shown to protect bone loss in ovariectomized mice and exert the regulatory effects on osteoblast and osteoclast activities. But how the peptides and proteins in JTG modulate bone formation remains unclear.

AIM

To investigate the stimulating effects of JTG proteins on osteogenesis and explore the possible underlying mechanisms.

MATERIALS AND METHODS

JTG proteins were prepared from JTG Capsules by extracting calcium, phosphorus and other inorganic elements using SEP-PaktC18 desalting column. MC3T3-E1 cells were treated with JTG proteins to evaluate their effects and explore the underlying mechanisms. Osteoblast proliferation was detected by CCK-8 method. ALP activity was detected using a relevant assay kit, and bone mineralized nodules were stained with alizarin red-Tris-HCl solution. Cell apoptosis was analyzed by flow cytometry. Autophagy was observed by MDC staining, and autophagosomes were observed by TEM. Nuclear translocations of LC3 and CHOP were detected by immunofluorescence and observed under a laser confocal microscope. The expression of key proteins related to osteogenesis, apoptosis, autophagy and PI3K/AKT and ER stress pathways was analyzed by Western Blot analysis.

RESULTS

JTG proteins improved osteogenesis as evidenced by the alteration of proliferation, differentiation and mineralization of MC3T3-E1 osteoblasts, inhibited their apoptosis, and enhanced autophagosome formation and autophagy. They also regulated the expression of key proteins of PI3K/AKT and ER stress pathways. In addition, PI3K/AKT and ER stress pathway inhibitors could reverse the regulatory effects of JTG proteins on osteogenesis, apoptosis, autophagy and PI3K/AKT and ER stress pathways.

CONCLUSION

JTG proteins increased the osteogenesis and inhibited osteoblast apoptosis by enhancing autophagy via PI3K/AKT and ER stress signaling pathways.

Osteocytes Exposed to Titanium Particles Inhibit Osteoblastic Cell Differentiation via Connexin 43

Periprosthetic osteolysis (PPO) induced by wear particles is the most severe complication of total joint replacement; however, the mechanism behind PPO remains elusive. Previous studies have shown that osteocytes play important roles in wear-particle-induced osteolysis. In this study, we investigated the effects of connexin 43 (Cx43) on the regulation of osteocyte-to-osteoblast differentiation. We established an in vivo murine model of calvarial osteolysis induced by titanium (Ti) particles. The osteolysis characteristic and osteogenesis markers in the osteocyte-selective Cx43 (CKO)-deficient and wild-type (WT) mice were observed. The calvarial osteolysis induced by Ti particles was partially attenuated in CKO mice. The expression of β-catenin and osteogenesis markers increased significantly in CKO mice. In vitro, the osteocytic cell line MLO-Y4 was treated with Ti particles. The co-culturing of MLO-Y4 cells with MC3T3-E1 osteoblastic cells was used to observe the effects of Ti-treated osteocytes on osteoblast differentiation. When Cx43 of MLO-Y4 cells was silenced or overexpressed, β-catenin was detected. Additionally, co-immunoprecipitation detection of Cx43 and β-catenin binding in MLO-Y4 cells and MC3T3-E1 cells was performed. Finally, β-catenin expression in MC3T3-E1 cells and osteoblast differentiation were evaluated after 18α-glycyrrhetinic acid (18α-GA) was used to block the intercellular communication of Cx43 between MLO-Y4 and MC3T3-E1 cells. Ti particles increased Cx43 expression and decreased β-catenin expression in MLO-Y4 cells. The silencing of Cx43 increased the β-catenin expression, and the over-expression of Cx43 decreased the β-catenin expression. In the co-culture model, Ti treatment of MLO-Y4 cells inhibited the osteoblastic differentiation of MC3T3-E1 cells and Cx43 silencing in MLO-Y4 cells attenuated the inhibitory effects on osteoblastic differentiation. With Cx43 silencing in the MLO-Y4 cells, the MC3T3-E1 cells, co-cultured alongside MLO-Y4, displayed decreased Cx43 expression, increased β-catenin expression, activation of Runx2, and promotion of osteoblastic differentiation in vitro co-culture. Finally, Cx43 expression was found to be negatively correlated to the activity of the Wnt signaling pathway, mostly through the Cx43 binding of β-catenin from its translocation to the nucleus. The results of our study suggest that Ti particles increased Cx43 expression in osteocytes and that osteocytes may participate in the regulation of osteoblast function via the Cx43 during PPO.

Targeting strategies for bone diseases: signaling pathways and clinical studies

Since the proposal of Paul Ehrlich's magic bullet concept over 100 years ago, tremendous advances have occurred in targeted therapy. From the initial selective antibody, antitoxin to targeted drug delivery that emerged in the past decades, more precise therapeutic efficacy is realized in specific pathological sites of clinical diseases. As a highly pyknotic mineralized tissue with lessened blood flow, bone is characterized by a complex remodeling and homeostatic regulation mechanism, which makes drug therapy for skeletal diseases more challenging than other tissues. Bone-targeted therapy has been considered a promising therapeutic approach for handling such drawbacks. With the deepening understanding of bone biology, improvements in some established bone-targeted drugs and novel therapeutic targets for drugs and deliveries have emerged on the horizon. In this review, we provide a panoramic summary of recent advances in therapeutic strategies based on bone targeting. We highlight targeting strategies based on bone structure and remodeling biology. For bone-targeted therapeutic agents, in addition to improvements of the classic denosumab, romosozumab, and PTH1R ligands, potential regulation of the remodeling process targeting other key membrane expressions, cellular crosstalk, and gene expression, of all bone cells has been exploited. For bone-targeted drug delivery, different delivery strategies targeting bone matrix, bone marrow, and specific bone cells are summarized with a comparison between different targeting ligands. Ultimately, this review will summarize recent advances in the clinical translation of bone-targeted therapies and provide a perspective on the challenges for the application of bone-targeted therapy in the clinic and future trends in this area.

Diosmin mitigates dexamethasone-induced osteoporosis in vivo: Role of Runx2, RANKL/OPG, and oxidative stress

Secondary osteoporosis is commonly caused by long-term intake of glucocorticoids (GCs), such as dexamethasone (DEX). Diosmin, a natural substance with potent antioxidant and anti-inflammatory properties, is clinically used for treating some vascular disorders. The current work targeted exploring the protective properties of diosmin to counteract DEX-induced osteoporosis in vivo. Rats were administered DEX (7 mg/kg) once weekly for 5 weeks, and in the second week, vehicle or diosmin (50 or 100 mg/kg/day) for the next four weeks. Femur bone tissues were collected and processed for histological and biochemical examinations. The study findings showed that diosmin alleviated the histological bone impairments caused by DEX. In addition, diosmin upregulated the expression of Runt-related transcription factor 2 (Runx2) and phosphorylated protein kinase B (p-AKT) and the mRNA transcripts of Wingless (Wnt) and osteocalcin. Furthermore, diosmin counteracted the rise in the mRNA levels of receptor activator of nuclear factor-kB ligand (RANKL) and the reduction in osteoprotegerin (OPG), both were induced by DEX. Diosmin restored the oxidant/antioxidant equilibrium and exerted significant antiapoptotic activity. The aforementioned effects were more pronounced at the dose level of 100 mg/kg. Collectively, diosmin has proven to protect rats against DEX-induced osteoporosis by augmenting osteoblast and bone development while hindering osteoclast and bone resorption. Our findings could be used as a stand for recommending supplementation of diosmin for patients chronically using GCs.

Osteocyte apoptosis and cellular micropetrosis signify skeletal aging in type 1 diabetes

Bone fragility is a profound complication of type 1 diabetes mellitus (T1DM), increasing patient morbidity. Within the mineralized bone matrix, osteocytes build a mechanosensitive network that orchestrates bone remodeling; thus, osteocyte viability is crucial for maintaining bone homeostasis. In human cortical bone specimens from individuals with T1DM, we found signs of accelerated osteocyte apoptosis and local mineralization of osteocyte lacunae (micropetrosis) compared with samples from age-matched controls. Such morphological changes were seen in the relatively young osteonal bone matrix on the periosteal side, and micropetrosis coincided with microdamage accumulation, implying that T1DM drives local skeletal aging and thereby impairs the biomechanical competence of the bone tissue. The consequent dysfunction of the osteocyte network hampers bone remodeling and decreases bone repair mechanisms, potentially contributing to the enhanced fracture risk seen in individuals with T1DM. STATEMENT OF SIGNIFICANCE: Type 1 diabetes mellitus (T1DM) is a chronic autoimmune disease that causes hyperglycemia. Increased bone fragility is one of the complications associated with T1DM. Our latest study on T1DM-affected human cortical bone identified the viability of osteocytes, the primary bone cells, as a potentially critical factor in T1DM-bone disease. We linked T1DM with increased osteocyte apoptosis and local accumulation of mineralized lacunar spaces and microdamage. Such structural changes in bone tissue suggest that T1DM speeds up the adverse effects of aging, leading to the premature death of osteocytes and potentially contributing to diabetes-related bone fragility.

Quercetin potentiates the anti-osteoporotic effects of alendronate through modulation of autophagy and apoptosis mechanisms in ovariectomy-induced bone loss rat model

BACKGROUND

Osteoporosis is a bone disease leading to bone fracture and affects 200 million women worldwide. Autophagy and apoptosis are two fundamental mechanisms that are involved in the development of osteoporosis. In this study we aim to investigate the combined effects of quercetin and alendronate on the markers of osteoporosis, autophagy, and apoptosis in the bone of ovariectomized rats.

METHODS AND RESULTS

Fifty adult female Sprague-Dawley rats were ovariectomized and treated with alendronate alone (5 µg/kg/day) or alendronate (5 µg/kg/day) in combination with quercetin (15 mg/kg/day) for 12 weeks. Then, ELISA, stereological tests, Real-time PCR analysis, and immunofluorescence assay were used to measure the markers of osteoporosis, autophagy, and apoptosis in the serum and tibia of rats. The serum osteocalcin was significantly decreased in ovariectomized rats that received quercetin and alendronate compared with alendronate only. Stereological data showed that except for osteoclasts, the total trabecular volume, bone weight, bone volume, osteocyte, and osteoblast numbers were increased in an ovariectomized group that was treated with quercetin and alendronate compared with alendronate alone. Except for Bcl2, the autophagy markers (Beclin-1 and LC3B) and Caspase-3 were significantly downregulated in ovariectomized rats that received quercetin and alendronate compared with those treated with alendronate alone.

CONCLUSION

Our results show that quercetin enhances the anti-osteoporotic effects of alendronate, possibly through the regulation of autophagy and apoptosis mechanisms. These findings suggest that the combination of quercetin and alendronate could be a useful therapeutic strategy in the treatment of osteoporosis in postmenopausal women.

Oxidative Stress and Inflammation in Osteoporosis: Molecular Mechanisms Involved and the Relationship with microRNAs

Osteoporosis is characterized by the alteration of bone homeostasis due to an imbalance between osteoclastic bone resorption and osteoblastic bone formation. Estrogen deficiency causes bone loss and postmenopausal osteoporosis, the pathogenesis of which also involves oxidative stress, inflammatory processes, and the dysregulation of the expression of microRNAs (miRNAs) that control gene expression at post-transcriptional levels. Oxidative stress, due to an increase in reactive oxygen species (ROS), proinflammatory mediators and altered levels of miRNAs enhance osteoclastogenesis and reduce osteoblastogenesis through mechanisms involving the activation of MAPK and transcription factors. The present review summarizes the principal molecular mechanisms involved in the role of ROS and proinflammatory cytokines on osteoporosis. Moreover, it highlights the interplay among altered miRNA levels, oxidative stress, and an inflammatory state. In fact, ROS, by activating the transcriptional factors, can affect miRNA expression, and miRNAs can regulate ROS production and inflammatory processes. Therefore, the present review should help in identifying targets for the development of new therapeutic approaches to osteoporotic treatment and improve the quality of life of patients.

Effects of joint immobilization and treadmill exercise on marrow adipose tissue and trabecular bone after anterior cruciate ligament reconstruction in the rat proximal tibial epiphysis

Marrow adipose tissue (MAT) adversely affects bone metabolism under certain conditions. Although mechanical stress is an important factor in regulating MAT and bone mass, how stress from different rehabilitation protocols after anterior cruciate ligament (ACL) reconstruction affects trabecular bone and MAT is unclear. We aimed to examine the effects of joint immobilization and treadmill exercise on trabecular bone and MAT after ACL reconstruction. Rats received unilateral knee ACL transection and reconstruction surgery. After surgery, rats were reared without intervention, with joint immobilization, or with treadmill exercise (12 m/min, 60 min/day, six days/week), with untreated rats as controls. At two or four weeks after starting experiments, we examined histological changes in trabecular bone and MAT in the proximal tibial epiphysis. After ACL reconstruction, there were no significant changes in trabecular bone area and MAT area at both time points. Joint immobilization after ACL reconstruction resulted in reduced trabecular bone area and MAT accumulation due to adipocyte hyperplasia and hypertrophy within four weeks. Treadmill exercise after ACL reconstruction did not affect any parameters in trabecular bone and MAT. We detected a moderate negative correlation between trabecular bone area and MAT area. Therefore, MAT accumulation induced by joint immobilization may contribute, at least in part, to reducing trabecular bone area. To minimize trabecular bone loss and MAT accumulation, joint immobilization after ACL reconstruction should be minimized. Exercise after ACL reconstruction did not alter trabecular bone and MAT.

Oxidative Stress and Natural Antioxidants in Osteoporosis: Novel Preventive and Therapeutic Approaches

This review reports in detail the cellular and molecular mechanisms which regulate the bone remodeling process in relation to oxidative stress (OS), inflammatory factors, and estrogen deficiency. OS is considered an important pathogenic factor of osteoporosis, inducing osteocyte apoptosis and varying levels of specific factors, such as receptor activator κB ligand (RANKL), sclerostin, and, according to recent evidence, fibroblast growth factor 23, with consequent impairment of bone remodeling and high bone resorption. Bone loss increases the risk of fragility fractures, and the most commonly used treatments are antiresorptive drugs, followed by anabolic drugs or those with a double effect. In addition, recent data show that natural antioxidants contained in the diet are efficient in preventing and reducing the negative effects of OS on bone remodeling and osteocytes through the involvement of sirtuin type 1 enzyme. Indeed, osteocytes and some of their molecular factors are considered potential biological targets on which antioxidants can act to prevent and reduce bone loss, as well as to promote bone anabolic and regenerative processes by restoring physiological bone remodeling. Several data suggest including antioxidants in novel therapeutic approaches to develop better management strategies for the prevention and treatment of osteoporosis and OS-related bone diseases. In particular, anthocyanins, as well as resveratrol, lycopene, oleuropein, some vitamins, and thiol antioxidants, could have protective and therapeutic anti-osteoporotic effects.

Making the head: Caspases in life and death

The term apoptosis, as a way of programmed cell death, was coined a half century ago and since its discovery the process has been extensively investigated. The anatomy and physiology of the head are complex and thus apoptosis has mostly been followed in separate structures, tissues or cell types. This review aims to provide a comprehensive overview of recent knowledge concerning apoptosis-related molecules involved in the development of structures of head with a particular focus on caspases, cysteine proteases having a key position in apoptotic pathways. Since many classical apoptosis-related molecules, including caspases, are emerging in several non-apoptotic processes, these were also considered. The largest organ of the head region is the brain and its development has been extensively investigated, including the roles of apoptosis and related molecules. Neurogenesis research also includes sensory organs such as the eye and ear, efferent nervous system and associated muscles and glands. Caspases have been also associated with normal function of the skin and hair follicles. Regarding mineralised tissues within craniofacial morphogenesis, apoptosis in bones has been of interest along with palate fusion and tooth development. Finally, the role of apoptosis and caspases in angiogenesis, necessary for any tissue/organ development and maintenance/homeostasis, are discussed. Additionally, this review points to abnormalities of development resulting from improper expression/activation of apoptosis-related molecules.

Endocrine sequelae of hematopoietic stem cell transplantation: Effects on mineral homeostasis and bone metabolism

Hematopoietic stem cell transplantation (HSCT) is an established therapeutic strategy for the treatment of malignant (leukemia and lymphoma) and non-malignant (thalassemia, anemia, and immunodeficiency) hematopoietic diseases. Thanks to the improvement in patient care and the development of more tolerable conditioning treatments, which has extended the applicability of therapy to the elderly, a growing number of patients have successfully benefited from HSCT therapy and, more importantly, HSCT transplant-related mortality has consistently reduced in recent years. However, concomitantly to long term patient survival, a growing incidence of late HSCT-related sequelae has been reported, being variably associated with negative effects on quality of life of patients and having a non-negligible impact on healthcare systems. The most predominantly observed HSCT-caused complications are chronic alterations of the endocrine system and metabolism, which endanger post-operative quality of life and increase morbidity and mortality of transplanted patients. Here, we specifically review the current knowledge on HSCT-derived side-effects on the perturbation of mineral metabolism; in particular, the homeostasis of calcium, focusing on current reports regarding osteoporosis and recurrent renal dysfunctions that have been observed in a percentage of HSC-transplanted patients. Possible secondary implications of conditioning treatments for HSCT on the physiology of the parathyroid glands and calcium homeostasis, alone or in association with HSCT-caused renal and bone defects, are critically discussed as well.

Cell death regulation: A new way for natural products to treat osteoporosis

Osteoporosis is a common metabolic bone disease that results from the imbalance of homeostasis within the bone. Intra-bone homeostasis is dependent on a precise dynamic balance between bone resorption by osteoclasts and bone formation by mesenchymal lineage osteoblasts, which comprises a series of complex and highly standardized steps. Programmed cell death (PCD) (e.g., apoptosis, autophagy, ferroptosis, pyroptosis, and necroptosis) is a cell death process that involves a cascade of gene expression events with tight structures. These events play a certain role in regulating bone metabolism by determining the fate of bone cells. Moreover, existing research has suggested that natural products derived from a wide variety of dietary components and medicinal plants modulate the PCDs based on different mechanisms, which show great potential for the prevention and treatment of osteoporosis, thus revealing the emergence of more acceptable complementary and alternative drugs with lower costs, fewer side effects and more long-term application. Accordingly, this review summarizes the common types of PCDs in the field of osteoporosis. Moreover, from the perspective of targeting PCDs, this review also discussed the roles of currently reported natural products in the treatment of osteoporosis and the involved mechanisms. Based on this, this review provides more insights into new molecular mechanisms of osteoporosis and provides a reference for developing more natural anti-osteoporosis drugs in the future.

Iron overload induced osteocytes apoptosis and led to bone loss in Hepcidin-/- mice through increasing sclerostin and RANKL/OPG

BACKGROUND

Numerous studies have demonstrated that iron overload is a risk factor of osteoporosis. However, there has been no systematic and in-depth studies on the effect of iron overload on osteocytes and its role in iron overload-induced bone loss. Therefore, to address this problem, we carried out in vitro and in vivo studies using MLO-Y4 osteocyte-like cells and Hepcidin^-/- mice as iron overload models.

METHODS

(1) MLO-Y4 cells were treated with ferric ammonium citrate (FAC). Intracellular reactive oxygen species (ROS) levels and apoptosis of MLO-Y4 cells were determined by flow cytometry. Western blotting was performed to evaluate the effect of FAC on the expression of sclerostin and RANKL/OPG. (2) The conditioned medium of MLO-Y4 cells after treatment with FAC was collected and used to treat pre-osteoblasts and monocytes. Alkaline phosphatase (ALP) staining and alizarin red (AR) staining were used to evaluate osteogenic differentiation capacity, and tartrate-resistant acid phosphatase (TRAP) staining was performed to demonstrate osteoclast differentiation capacity. (3) In vivo studies included a wild type mouse, Hepcidin^-/- mice, Hepcidin^-/- mice + deferoxamine (DFO), and Hepcidin^-/- mice + N-actyl-l-cysteine (NAC) group. Micro-CT was performed to evaluate the bone mineral density (BMD), bone volume, and bone micro-architecture of the mice, and three bending tests were used to assess bone strength. Histological analysis was used to detect alterations in bone turnover. TUNEL staining and scanning electron microscopy (SEM) were performed to evaluate the apoptosis and morphology of osteocytes. Immunohistochemical staining and Western blotting were used to determine alterations in sclerostin and RANKL/OPG expression levels in mice.

RESULTS

(1) FAC increased intracellular ROS and apoptosis in MLO-Y4 cells, while FAC enhanced the expression of sclerostin and RANKL/OPG in MLO-Y4 cells. (2) Conditioned medium of MLO-Y4 cells inhibited the osteogenic capacity of osteoblasts while stimulating osteoclast differentiation. (3) By increasing oxidative stress, iron overload promotes the apoptosis of osteocytes and undermines the morphology of osteocytes in Hepcidin^-/- mice, further increasing the expression levels of sclerostin and RANKL/OPG in osteocytes, which is considered to be the causative factor for reduced bone formation and enhanced bone resorption. DFO administration reduced iron levels, and NAC treatment decreased oxidative stress in Hepcidin^-/- mice. Therefore, DFO or NAC treatment rescued the decrease in BMD, bone volume, and bone strength and attenuated the deterioration of bone architecture in Hepcidin^-/- mice by attenuating the effect of iron overload on osteocytes.

CONCLUSION

Osteocyte apoptosis due to increased ROS and resultant sclerostin and RANKL/OPG expression alteration was the main reason for bone loss in Hepcidin^-/- mice. Osteocytes are the main targets for the prevention and treatment of iron overload-induced osteoporosis.

Fluoride Exposure Provokes Mitochondria-Mediated Apoptosis and Increases Mitophagy in Osteocytes via Increasing ROS Production

Fluoride is a persistent environmental pollutant, and its excessive intake causes skeletal and dental fluorosis. However, few studies focused on the effects of fluoride on osteocytes, making up over 95% of all bone cells. This study aimed to investigate the effect of fluoride on osteocytes in vitro, as well as explore the underlying mechanisms. CCK-8, LDH assay, fluorescent probes, flow cytometry, and western blotting were performed to examine cell viability, apoptosis, mitochondria changes, reactive oxygen species (ROS) and mitochondrial ROS (mtROS), and protein expressions. Results showed that sodium fluoride (NaF) exposure (4, 8 mmol/L) for 24 h inhibited the cell viability of osteocytes MLO-Y4 and promoted G0/G1 phase arrest and increased cell apoptosis. NaF treatment remarkably caused mitochondria damage, loss of MMP, ATP decrease, Cyto c release, and Bax/Bcl-2 ratio increase and elevated the activity of caspase-9 and caspase-3. Furthermore, NaF significantly upregulated the expressions of LC-3II, PINK1, and Parkin and increased autophagy flux and the accumulation of acidic vacuoles, while the p62 level was downregulated. In addition, NaF exposure triggered the production of intracellular ROS and mtROS and increased malondialdehyde (MDA); but superoxide dismutase (SOD) activity and glutathione (GSH) content were decreased. The scavenger N-acetyl-L-cysteine (NAC) significantly reversed NaF-induced apoptosis and mitophagy, suggesting that ROS is responsible for the mitochondrial-mediated apoptosis and mitophagy induced by NaF exposure. These findings provide in vitro evidence that apoptosis and mitophagy are cellular mechanisms for the toxic effect of fluoride on osteocytes, thereby suggesting the potential role of osteocytes in skeletal and dental fluorosis.

Ablation of Discoidin Domain Receptor 1 Provokes an Osteopenic Phenotype by Regulating Osteoblast/Osteocyte Autophagy and Apoptosis

Discoidin domain receptor 1 (DDR1) is a collagen receptor that belongs to the receptor tyrosine kinase family. We have previously shown that DDR1 plays a crucial role during bone development, resulting in dwarfism and a short stature in osteoblast-specific knockout mice (OKO mice). However, the detailed pathophysiological effects of DDR1 on bone development throughout adulthood have remained unclear. This study aims to identify how DDR1 regulates osteoblast and osteocyte functions in vivo and in vitro during bone development in adulthood. The metabolic changes in bone tissues were analyzed using Micro-CT and immunohistochemistry staining (IHC) in vivo; the role of DDR1 in regulating osteoblasts was examined in MC3T3-E1 cells in vitro. The Micro-CT analysis results demonstrated that OKO mice showed a 10% reduction in bone-related parameters from 10 to 14 weeks old and a significant reduction in cortical thickness and diameter compared with flox/flox control mice (FF) mice. These results indicated that DDR1 knockout in OKO mice exhibiting significant bone loss provokes an osteopenic phenotype. The IHC staining revealed a significant decrease in osteogenesis-related genes, including RUNX2, osteocalcin, and osterix. We noted that DDR1 knockout significantly induced osteoblast/osteocyte apoptosis and markedly decreased autophagy activity in vivo. Additionally, the results of the gain- and loss-of-function of the DDR1 assay in MC3T3-E1 cells indicated that DDR1 can regulate the osteoblast differentiation through activating autophagy by regulating the phosphorylation of the mechanistic target of rapamycin (p-mTOR), light chain 3 (LC3), and beclin-1. In conclusion, our study highlights that the ablation of DDR1 results in cancellous bone loss by regulating osteoblast/osteocyte autophagy. These results suggest that DDR1 can act as a potential therapeutic target for managing cancellous bone loss.

Biphasic regulation of osteoblast development via the ERK MAPK-mTOR pathway

Emerging evidence supports that osteogenic differentiation of skeletal progenitors is a key determinant of overall bone formation and bone mass. Despite extensive studies showing the function of mitogen-activated protein kinases (MAPKs) in osteoblast differentiation, none of these studies show in vivo evidence of a role for MAPKs in osteoblast maturation subsequent to lineage commitment. Here, we describe how the extracellular signal-regulated kinase (ERK) pathway in osteoblasts controls bone formation by suppressing the mechanistic target of rapamycin (mTOR) pathway. We also show that, while ERK inhibition blocks the differentiation of osteogenic precursors when initiated at an early stage, ERK inhibition surprisingly promotes the later stages of osteoblast differentiation. Accordingly, inhibition of the ERK pathway using a small compound inhibitor or conditional deletion of the MAP2Ks Map2k1 (MEK1) and Map2k2 (MEK2), in mature osteoblasts and osteocytes, markedly increased bone formation due to augmented osteoblast differentiation. Mice with inducible deletion of the ERK pathway in mature osteoblasts also displayed similar phenotypes, demonstrating that this phenotype reflects continuous postnatal inhibition of late-stage osteoblast maturation. Mechanistically, ERK inhibition increases mitochondrial function and SGK1 phosphorylation via mTOR2 activation, which leads to osteoblast differentiation and production of angiogenic and osteogenic factors to promote bone formation. This phenotype was partially reversed by inhibiting mTOR. Our study uncovers a surprising dichotomy of ERK pathway functions in osteoblasts, whereby ERK activation promotes the early differentiation of osteoblast precursors, but inhibits the subsequent differentiation of committed osteoblasts via mTOR-mediated regulation of mitochondrial function and SGK1.

Polyphenols: Bioavailability, Microbiome Interactions and Cellular Effects on Health in Humans and Animals

Polyphenolic compounds have a variety of functions in plants including protecting them from a range of abiotic and biotic stresses such as pathogenic infections, ionising radiation and as signalling molecules. They are common constituents of human and animal diets, undergoing extensive metabolism by gut microbiota in many cases prior to entering circulation. They are linked to a range of positive health effects, including anti-oxidant, anti-inflammatory, antibiotic and disease-specific activities but the relationships between polyphenol bio-transformation products and their interactions in vivo are less well understood. Here we review the state of knowledge in this area, specifically what happens to dietary polyphenols after ingestion and how this is linked to health effects in humans and animals; paying particular attention to farm animals and pigs. We focus on the chemical transformation of polyphenols after ingestion, through microbial transformation, conjugation, absorption, entry into circulation and uptake by cells and tissues, focusing on recent findings in relation to bone. We review what is known about how these processes affect polyphenol bioactivity, highlighting gaps in knowledge. The implications of extending the use of polyphenols to treat specific pathogenic infections and other illnesses is explored.

Osteocytic cells exposed to titanium particles increase sclerostin expression and inhibit osteoblastic cell differentiation mostly via direct cell-to-cell contact

The mechanism underlying induction of periprosthetic osteolysis by wear particles remains unclear. In this study, cultured MLO‐Y4 osteocytic cells were exposed to different concentrations of titanium (Ti) particles. The results showed that Ti particles increased expression of the osteocytic marker SOST/sclerostin in a dose‐dependent manner, accelerated apoptosis of MLO‐Y4 cells, increased the expression of IL‐6, TNF‐α and connexin 43. SOST silence alleviated the increase of MLO‐Y4 cells apoptosis, decreased the expression of IL‐6, TNF‐α and connexin 43 caused by Ti particles. The different co‐culture systems of MLO‐Y4 cells with MC3T3‐E1 osteoblastic cells were further used to observe the effects of osteocytic cells' changes induced by Ti particles on osteoblastic cells. MLO‐Y4 cells treated with Ti particles inhibited dramatically differentiation of MC3T3‐E1 cells mostly through direct cell‐to‐cell contact. SOST silence attenuated the inhibition effects of Ti‐induced MLO‐Y4 on MC3T3‐E1 osteoblastic differentiation, which ALP level and mineralization of MC3T3‐E1 cells increased and the expression of ALP, OCN and Runx2 increased compared to the Ti‐treated group. Taken together, Ti particles had negative effects on MLO‐Y4 cells and the impact of Ti particles on osteocytic cells was extensive, which may further inhibit osteoblastic differentiation mostly through intercellular contact directly. SOST/sclerostin plays an important role in the process of mutual cell interaction. These findings may help to understand the effect of osteocytes in wear particle‐induced osteolysis.

Serum sclerostin levels in osteoporotic fracture patients

PURPOSE

Sclerostin inhibits bone formation and stimulates bone resorption. Previous studies found a positive association between bone density and serum sclerostin, but literature on sclerostin levels in osteoporotic fracture patients is scarce. The aim of the present study was to compare the serum sclerostin levels in osteoporotic and non-osteoporotic fracture patients and to assess the correlation of the sclerostin levels with bone mineral density and vitamin D status.

METHODS

In this cross-sectional study, we included patients over 50 years, with an extremity fracture after low-energy trauma treated between 2012 and 2018, with biobank samples and available bone density measurements by Dual X-ray Absorption. Osteoporosis was diagnosed according the World Health Organisation criteria. Vitamin D deficiency was defined as a 25(OH)D concentration < 30 nmol/L. After defrosting biobank samples, serum sclerostin was measured using the human SOST (sclerostin) enzyme-linked immunosorbent assay kit. We prespecified a subgroup analysis including only female patients.

RESULTS

179 patients were included of whom 139(78%) were female. In 46 patients (25.7%), osteoporosis was diagnosed. Bone mineral density was positively associated with sclerostin levels (r = 0.17, p = 0.026) and patients with osteoporosis had a significantly lower serum sclerostin compared to non-osteoporotic fracture patients (mean 41.9 pmol/L vs 48.1 pmol/L; p = 0.03). This difference remained significant after correction for potential confounders. Similar results were found in the subgroup of female patients. No association between serum sclerostin and vitamin D deficiency was found.

CONCLUSION

Osteoporotic fracture patients had lower levels of sclerostin than non-osteoporotic fracture patients. Future research should focus on the use of sclerostin as biomarker for osteoporosis in fracture patients.

Localization of VEGF, TGF-β1, BMP-2, and Apoptosis Factors in Hypertrophic Nonunion of Human Tubular Bones

We studied localization of VEGF, TGF-β1, BMP-2, caspase-3, Bcl-2, and TNFα in the callus samples obtained from 5 patients (4 women and 1 man) aged 41-53 years during planned surgery for nonunion and pseudarthrosis of the clavicle (n=1), ulna (n=1), femur (n=1), and tibia (n=2) bones. Two control groups included material of hypertrophied callus (n=3) with consolidated fractures of long bones and samples of intact bones (n=3) obtained by postmortem autopsy of subjects without pathology of the musculoskeletal system. A nonuniform distribution of the studied markers was revealed. Active expression of VEGF was observed in fibroblast-like cells of the fibrous tissue, osteoblasts of the periosteum and osteons. Osteoblasts expressing BMP-2 were localized in the periosteum and the loose connective tissue of the Haversian canals. The number of immunopositive cells expressing TGF-β1 and TNFα in the callus exceeded that in the control and correlated with the expression of caspase-3 in fibroblast-like cells, osteoblasts, chondroblasts, and microvascular endotheliocytes. The results allow considering fracture nonunion as a result of overproduction of cytotoxic and proapoptotic factors in chronic inflammation and dysfunction of the expression of morphogenetic proteins. The morphochemical patterns of the studied markers open up prospects for the development of new methods of pharmacological correction of fracture repair.

Osteocytes in bone aging: Advances, challenges, and future perspectives

Osteocytes play a critical role in maintaining bone homeostasis and in regulating skeletal response to hormones and mechanical loading. Substantial evidence have demonstrated that osteocytes and their lacunae exhibit morphological changes in aged bone, indicating the underlying involvement of osteocytes in bone aging. Notably, recent studies have deciphered aged osteocytes to have characteristics such as impaired mechanosensitivity, accumulated cellular senescence, dysfunctional perilacunar/canalicular remodeling, and degenerated lacuna-canalicular network. However, detailed molecular mechanisms of osteocytes remain unclear. Nonetheless, osteocyte transcriptomes analyzed via advanced RNA sequencing (RNA-seq) techniques have identified several bone aging-related genes and signaling pathways, such as Wnt, Bmp/TGF, and Jak-STAT. Moreover, inflammation, immune dysfunction, energy shortage, and impaired hormone responses possibly affect osteocytes in age-related bone deterioration. In this review, we summarize the hallmarks of aging bone and osteocytes and discuss osteocytic mechanisms in age-related bone loss and impaired bone quality. Furthermore, we provide insights into the challenges faced and their possible solutions when investigating osteocyte transcriptomes. We also highlight that single-cell RNA-seq can decode transcriptomic messages in aged osteocytes; therefore, this technique can promote novel single cell-based investigations in osteocytes once a well-established standardized protocol specific for osteocytes is developed. Interestingly, improved understanding of osteocytic mechanisms have helped identify promising targets and effective therapies for aging-related osteoporosis and fragile fractures.

Targeting Sirt1, AMPK, Nrf2, CK2, and Soluble Guanylate Cyclase with Nutraceuticals: A Practical Strategy for Preserving Bone Mass

There is a vast pre-clinical literature suggesting that certain nutraceuticals have the potential to aid the preservation of bone mass in the context of estrogen withdrawal, glucocorticoid treatment, chronic inflammation, or aging. In an effort to bring some logical clarity to these findings, the signaling pathways regulating osteoblast, osteocyte, and osteoclast induction, activity, and survival are briefly reviewed in the present study. The focus is placed on the following factors: the mechanisms that induce and activate the RUNX2 transcription factor, a key driver of osteoblast differentiation and function; the promotion of autophagy and prevention of apoptosis in osteoblasts/osteoclasts; and the induction and activation of NFATc1, which promotes the expression of many proteins required for osteoclast-mediated osteolysis. This analysis suggests that the activation of sirtuin 1 (Sirt1), AMP-activated protein kinase (AMPK), the Nrf2 transcription factor, and soluble guanylate cyclase (sGC) can be expected to aid the maintenance of bone mass, whereas the inhibition of the serine kinase CK2 should also be protective in this regard. Fortuitously, nutraceuticals are available to address each of these targets. Sirt1 activation can be promoted with ferulic acid, N1-methylnicotinamide, melatonin, nicotinamide riboside, glucosamine, and thymoquinone. Berberine, such as the drug metformin, is a clinically useful activator of AMPK. Many agents, including lipoic acid, melatonin, thymoquinone, astaxanthin, and crucifera-derived sulforaphane, can promote Nrf2 activity. Pharmacological doses of biotin can directly stimulate sGC. Additionally, certain flavonols, notably quercetin, can inhibit CK2 in high nanomolar concentrations that may be clinically relevant. Many, though not all, of these agents have shown favorable effects on bone density and structure in rodent models of bone loss. Complex nutraceutical regimens providing a selection of these nutraceuticals in clinically meaningful doses may have an important potential for preserving bone health. Concurrent supplementation with taurine, N-acetylcysteine, vitamins D and K2, and minerals, including magnesium, zinc, and manganese, plus a diet naturally high in potassium, may also be helpful in this regard.

The effects of microgravity on bone structure and function

Humans are spending an increasing amount of time in space, where exposure to conditions of microgravity causes 1–2% bone loss per month in astronauts. Through data collected from astronauts, as well as animal and cellular experiments conducted in space, it is evident that microgravity induces skeletal deconditioning in weight-bearing bones. This review identifies contentions in current literature describing the effect of microgravity on non-weight-bearing bones, different bone compartments, as well as the skeletal recovery process in human and animal spaceflight data. Experiments in space are not readily available, and experimental designs are often limited due to logistical and technical reasons. This review introduces a plethora of on-ground research that elucidate the intricate process of bone loss, utilising technology that simulates microgravity. Observations from these studies are largely congruent to data obtained from spaceflight experiments, while offering more insights behind the molecular mechanisms leading to microgravity-induced bone loss. These insights are discussed herein, as well as how that knowledge has contributed to studies of current therapeutic agents. This review also points out discrepancies in existing data, highlighting knowledge gaps in our current understanding. Further dissection of the exact mechanisms of microgravity-induced bone loss will enable the development of more effective preventative and therapeutic measures to protect against bone loss, both in space and possibly on ground.

The Mechanosensory Role of Osteocytes and Implications for Bone Health and Disease States

Bone homeostasis is a dynamic equilibrium between bone-forming osteoblasts and bone-resorbing osteoclasts. This process is primarily controlled by the most abundant and mechanosensitive bone cells, osteocytes, that reside individually, within chambers of porous hydroxyapatite bone matrix. Recent studies have unveiled additional functional roles for osteocytes in directly contributing to local matrix regulation as well as systemic roles through endocrine functions by communicating with distant organs such as the kidney. Osteocyte function is governed largely by both biochemical signaling and the mechanical stimuli exerted on bone. Mechanical stimulation is required to maintain bone health whilst aging and reduced level of loading are known to result in bone loss. To date, both in vivo and in vitro approaches have been established to answer important questions such as the effect of mechanical stimuli, the mechanosensors involved, and the mechanosensitive signaling pathways in osteocytes. However, our understanding of osteocyte mechanotransduction has been limited due to the technical challenges of working with these cells since they are individually embedded within the hard hydroxyapatite bone matrix. This review highlights the current knowledge of the osteocyte functional role in maintaining bone health and the key regulatory pathways of these mechanosensitive cells. Finally, we elaborate on the current therapeutic opportunities offered by existing treatments and the potential for targeting osteocyte-directed signaling.

Exercise to Mend Aged-tissue Crosstalk in Bone Targeting Osteoporosis & Osteoarthritis

Aging induces alterations in bone structure and strength through a multitude of processes, exacerbating common aging- related diseases like osteoporosis and osteoarthritis. Cellular hallmarks of aging are examined, as related to bone and the marrow microenvironment, and ways in which these might contribute to a variety of age-related perturbations in osteoblasts, osteocytes, marrow adipocytes, chondrocytes, osteoclasts, and their respective progenitors. Cellular senescence, stem cell exhaustion, mitochondrial dysfunction, epigenetic and intracellular communication changes are central pathways and recognized as associated and potentially causal in aging. We focus on these in musculoskeletal system and highlight knowledge gaps in the literature regarding cellular and tissue crosstalk in bone, cartilage, and the bone marrow niche. While senolytics have been utilized to target aging pathways, here we propose non-pharmacologic, exercise-based interventions as prospective "senolytics" against aging effects on the skeleton. Increased bone mass and delayed onset or progression of osteoporosis and osteoarthritis are some of the recognized benefits of regular exercise across the lifespan. Further investigation is needed to delineate how cellular indicators of aging manifest in bone and the marrow niche and how altered cellular and tissue crosstalk impact disease progression, as well as consideration of exercise as a therapeutic modality, as a means to enhance discovery of bone-targeted therapies.

Effect of Oxidative Stress-Induced Apoptosis on Active FGF23 Levels in MLO-Y4 Cells: The Protective Role of 17-β-Estradiol

The discovery that osteocytes secrete phosphaturic fibroblast growth factor 23 (FGF23) has defined bone as an endocrine organ. However, the autocrine and paracrine functions of FGF23 are still unknown. The present study focuses on the cellular and molecular mechanisms involved in the complex control of FGF23 production and local bone remodeling functions. FGF23 was assayed using ELISA kit in the presence or absence of 17β–estradiol in starved MLO-Y4 osteocytes. In these cells, a relationship between oxidative stress-induced apoptosis and up-regulation of active FGF23 levels due to MAP Kinases activation with involvement of the transcriptional factor (NF-kB) has been demonstrated. The active FGF23 increase can be due to up-regulation of its expression and post-transcriptional modifications. 17β–estradiol prevents the increase of FGF23 by inhibiting JNK and NF-kB activation, osteocyte apoptosis and by the down-regulation of osteoclastogenic factors, such as sclerostin. No alteration in the levels of dentin matrix protein 1, a FGF23 negative regulator, has been determined. The results of this study identify biological targets on which drugs and estrogen may act to control active FGF23 levels in oxidative stress-related bone and non-bone inflammatory diseases.

Spine Metastases in Immunocompromised Mice after Intracardiac Injection of MDA-MB-231-SCP2 Breast Cancer Cells

Simple Summary

Breast cancer cells typically metastasize to bone, where their interaction with bone remodeling cell types enhances metastatic outgrowth and osteolytic bone destruction. The respective knowledge is largely based on xenograft models, where human breast cancer cells are injected into immunocompromised mice. Importantly, however, whereas skeletal analyses in these animals are usually restricted to hindlimb bones, human skeletal metastases are far more frequent in the spine. Therefore, our study addressed the question, if breast cancer cells injected into immunocompromised mice would also metastasize to the spine, and if this process is influenced by the amount of trabecular bone. We injected an established breast cancer cell line into immunocompromised mice with or without a transgene causing severe osteoporosis. Importantly, we found more tumor cell clusters of different size in spine sections than in femora, but the presence of the transgene did not affect their spreading and metastatic outgrowth.

Abstract

Breast cancer cells frequently metastasize to bone, where their interaction with bone remodeling cell types enhances osteolytic bone destruction. Importantly, however, whereas skeletal analyses of xenograft models are usually restricted to hindlimb bones, human skeletal metastases are far more frequent in the spine, where trabecular bone mass is higher compared to femur or tibia. Here, we addressed whether breast cancer cells injected into immunocompromised mice metastasize to the spine and if this process is influenced by the amount of trabecular bone. We also took advantage of mice carrying the Col1a1-Krm2 transgene, which display severe osteoporosis. After crossing this transgene into the immunocompromised NSG background we injected MDA-MB-231-SCP2 breast cancer cells and analyzed their distribution three weeks thereafter. We identified more tumor cells and clusters of different size in spine sections than in femora, which allowed influences on bone remodeling cell types to be analyzed by comparing tumor-free to tumor-burdened areas. Unexpectedly, the Col1a1-Krm2 transgene did not affect spreading and metastatic outgrowth of MDA-MB-231-SCP2 cells, suggesting that bone tumor interactions are more relevant at later stages of metastatic progression.

Could Bone Biomarkers Predict Bone Turnover after Kidney Transplantation?—A Proof-of-Concept Study

Aim: Bone disease after kidney transplant (KT) results from multiple factors, including previous bone and mineral metabolism disturbances and effects of transplant-related medications. New biomolecules have been recently associated with the development and progression of the chronic kidney disease–associated bone and mineral disorder (CKD-MBD). These include sclerostin and the soluble receptor activator of nuclear factor-kB ligand (sRANKL). Methods: To better understand the role of biomarkers in post-transplant bone disease, this study was designed to prospectively evaluate and correlate results from the histomorphometric analysis of bone biopsies after KT with emerging serum biomarkers of the CKD-MBD: sclerostin, Dickkopf-related protein 1 (Dkk-1), sRANKL and osteo-protegerin (OPG). Results: Our data shows a significant increase in plasma levels of bioactive sclerostin after KT accompanied by a significant reduction in plasma levels of Dkk-1, suggesting a promotion of the inhibition of bone formation by osteoblasts through the activation of these inhibitors of the Wnt signaling pathway. In addition, we found a significant increase in plasma levels of free sRANKL after KT accompanied by a significant reduction in plasma levels of its decoy receptor OPG, suggesting an enhanced bone resorption by osteoclasts mediated by this mechanism. Conclusions: Taken together, these results suggest that the loss of bone volume observed after KT could be explain mainly by the inhibition of bone formation mediated by sclerostin accompanied by an enhanced bone resorption mediated by sRANKL.

Bisphenol A induces apoptosis and autophagy in murine osteocytes MLO-Y4: Involvement of ROS-mediated mTOR/ULK1 pathway

Bisphenol A (BPA) is a widely environmental endocrine disruptor. The accumulated BPA in humans is toxic to osteoblasts and osteoclasts, but few studies focused on the effects of BPA on osteocytes, the most abundant bone cell type, contributing to the development and metabolism of bone. Here, we reported that BPA (50, 100, 200 μmol/L) inhibited the cell viability of osteocytes MLO-Y4, promoted G0/G1 phase arrest and apoptosis in a dose-dependent manner. BPA treatment significantly increased the levels of autophagy-regulated proteins including Beclin-1 and LC3-II along with the decrease of p62, accompanied by the elevation of autophagy flux and the accumulation of acidic vacuoles, which was blocked by the autophagy inhibitor bafilomycin A1 (BafA1). Furthermore, BPA significantly inhibited the mammalian target of rapamycin (mTOR) and activated Unc-51 like autophagy activating kinase 1 (ULK1) signaling, leading to the decreased p-mTOR/mTOR ratio and the increased p-ULK1/ULK1 ratio. The mTOR activator MHY1485 (MHY) or the ULK1 inhibitor SBI-0206965 (SBI) prevented autophagy and enhanced apoptosis caused by BPA, respectively. In addition, BPA increased the levels of intracellular reactive oxygen species (ROS) and malondialdehyde (MDA) and decreased antioxidant enzymes nuclear factor E2-related factor 2 (Nrf2) and heme oxygenase-1 (HO-1) levels, resulting in oxidative stress. The ROS scavenger N-acetylcysteine (NAC) attenuated BPA-induced the mTOR/ULK1 pathway activation, apoptosis and autophagy. Collectively, ROS-mediated mTOR/ULK1 signaling is involved in BPA-induced apoptosis and autophagy in osteocytes MLO-Y4. Our data first provide in vitro evidence that apoptosis and autophagy as cellular mechanisms for the toxic effect of BPA on osteocytes, thereby advancing our understanding of the potential role of osteocytes in the adverse effect of BPA on bone health.

Comparison of Necroptosis With Apoptosis for OVX-Induced Osteoporosis

As one common kind of osteoporosis, postmenopausal osteoporosis (PMOP) is associated with the death and excessive loss of osteocytes. Estrogen deficiency of PMOP can cause osteocyte death by regulating necroptosis and apoptosis, but their roles in POMP have not been compared. In the present study, ovariectomy (OVX)-induced rat and murine long bone osteocyte Y4 (MLO-Y4) cells were used to compare the influence of necroptosis and apoptosis on osteocyte death and bone loss. Benzyloxycarbonyl-Val-Ala-Asp (zVAD) and necrostatin-1 (Nec-1) were used to specifically block cell apoptosis and necroptosis, respectively. OVX rats and MLO-Y4 cells were divided into zVAD group, Nec-1 group, zVAD + Nec-1 group, vehicle, and control group. The tibial plateaus of the rat model were harvested at 8 weeks after OVX and were analyzed by micro–computed tomography, transmission electron microscopy (TEM), the transferase dUTP nick end labeling assay, and western blot. The death of MLO-Y4 was stimulated by TNF-α and was measured by flow cytometry and TEM. The results found that necroptosis and apoptosis were both responsible for the death and excessive loss of osteocytes, as well as bone loss in OVX-induced osteoporosis, and furthermore necroptosis may generate greater impact on the death of osteocytes than apoptosis. Necroptotic death of osteocytes was mainly regulated by the receptor-interacting protein kinase 3 signaling pathway. Collectively, inhibition of necroptosis may produce better efficacy in reducing osteocyte loss than that of apoptosis, and combined blockade of necroptosis and apoptosis provide new insights into preventing and treating PMOP.

Bisphenol A induces pyroptotic cell death via ROS/NLRP3/Caspase-1 pathway in osteocytes MLO-Y4

Bisphenol A (BPA), a ubiquitous endocrine-disrupting chemical, is commonly used as a plasticizer to manufacture various food packaging materials. Evidence has demonstrated that BPA disturbed bone health. However, few studies focused on the effect of BPA on osteocytes, making up over 95% of all the bone cells. Here, we reported that BPA inhibited the cell viability of MLO-Y4 cells, and increased apoptosis in a dose-dependent manner. Furthermore, BPA up-regulated protein expressions of speck-like protein containing CARD (ASC), NLRP3, cleaved caspase-1 (Casp-1 p20) and cleaved gasdermin D (GSDMD-N), and increased the ratios of interleukin (IL)-1β/pro-IL-1β and IL-18/pro-IL-18 in MLO-Y4 cells. BPA enhanced levels of lactate dehydrogenase (LDH), IL-1β and IL-18 in culture supernatants. This pyroptotic death and the NLPR3 inflammasome activation were reversed by the caspase-1 inhibitor VX765 or the NLRP3 inflammasome inhibitor MCC950. Furthermore, BPA stimulated the production of intracellular reactive oxygen species (ROS), mitochondrial ROS (mtROS), elevated malondialdehyde (MDA) level and decreased superoxide dismutase (SOD) activity, which led to oxidative damage in MLO-Y4 cells. The ROS scavenger N-acetylcysteine (NAC) or the mitochondrial antioxidant Mito-TEMPO inhibited the NLPR3 inflammasome activation and pyroptotic death induced by BPA. Collectively, our data suggest that BPA causes pyroptotic death of osteocytes via ROS/NLRP3/Caspase-1 pathway.

Osteocytes and Weightlessness

PURPOSE OF REVIEW

Osteocytes are considered to be the cells responsible for mastering the remodeling process that follows the exposure to unloading conditions. Given the invasiveness of bone biopsies in humans, both rodents and in vitro culture systems are largely adopted as models for studies in space missions or in simulated microgravity conditions models on Earth.

RECENT FINDINGS

After a brief recall of the main changes in bone mass and osteoclastic and osteoblastic activities in space-related models, this review focuses on the potential role of osteocytes in directing these changes. The role of the best-known signalling molecules is questioned, in particular in relation to osteocyte apoptosis. The mechanotransduction actors identified in spatial conditions and the problems related to fluid flow and shear stress changes, probably enhanced by the alteration in fluid flow and lack of convection during spaceflight, are recalled and discussed.

Biologic and pathologic aspects of osteocytes in the setting of medication-related osteonecrosis of the jaw (MRONJ)

Medication-related osteonecrosis of the jaw (MRONJ) is a potentially severe, debilitating condition affecting patients with cancer and patients with osteoporosis who have been treated with powerful antiresorptives (pARs) or angiogenesis inhibitors (AgIs). Oral risk factors associated with the development of MRONJ include tooth extraction and inflammatory dental disease (e.g., periodontitis, periapical infection). In bone tissues, osteocytes play a bidirectional role in which they not only act as the "receiver" of systemic signals from blood vessels, such as hormones and drugs, or local signals from the mineralized matrix as it is deformed, but they also play a critical role as "transmitter" of signals to the cells that execute bone modeling and remodeling (osteoclasts, osteoblasts and lining cells). When the survival capacity of osteocytes is overwhelmed, they can die. Osteocyte death has been associated with several pathological conditions. Whereas the causes and mechanisms of osteocyte death have been studied in conditions like osteonecrosis of the femoral head (ONFH), few studies of the causes and mechanisms of osteocyte death have been done in MRONJ. The three forms of cell death that affect most of the different cells in the body (apoptosis, autophagy, and necrosis) have been recognized in osteocytes. Notably, necroptosis, a form of regulated cell death with "a necrotic cell death phenotype," has also been identified as a form of cell death in osteocytes under certain pathologic conditions. Improving the understanding of osteocyte death in MRONJ may be critical for preventing disease and developing treatment approaches. In this review, we intend to provide insight into the biology of osteocytes, cell death, in general, and osteocyte death, in particular, and discuss hypothetical mechanisms involved in osteocyte death associated with MRONJ.

Protection against Oxidative Stress-Induced Apoptosis by Fermented Sea Tangle (Laminaria japonica Aresch) in Osteoblastic MC3T3-E1 Cells through Activation of Nrf2 Signaling Pathway

The purpose of the present study was to explore the efficacy of fermented extract of sea tangle (Laminaria japonica Aresch, FST) with Lactobacillus brevis on DNA damage and apoptosis in hydrogen peroxide (H₂O₂)-stimulated osteoblastic MC3T3-E1 cells and clarify related signaling pathways. Our results showed that exposure to FST significantly improved cell viability, inhibited apoptosis, and suppressed the generation of reactive oxygen species (ROS) in H₂O₂-stimulated cells. In addition, H₂O₂ triggered DNA damage in MC3T3-E1 cells was markedly attenuated by FST pretreatment. Moreover, H₂O₂-induced mitochondrial dysfunctions associated with apoptotic events, including loss of mitochondrial membrane potential (MMP), decreased Bcl-2/Bcl-2 associated x-protein (Bax) ratio, and cytosolic release of cytochrome c, were reduced in the presence of FST. FST also diminished H₂O₂-induced activation of caspase-3, which was associated with the ability of FST to protect the degradation of poly (ADP-ribose) polymerase. Furthermore, FST notably enhanced nuclear translocation and phosphorylation of nuclear factor erythroid 2-related factor 2 (Nrf2) in the presence of H₂O₂ with concomitant upregulation of heme oxygenase-1 (HO-1) expression. However, artificial blockade of this pathway by the HO-1 inhibitor, zinc protoporphyrin IX, greatly abolished the protective effect of FST against H₂O₂-induced MC3T3-E1 cell injury. Taken together, these results demonstrate that FST could protect MC3T3-E1 cells from H₂O₂-induced damage by maintaining mitochondrial function while eliminating ROS along with activation of the Nrf2/HO-1 antioxidant pathway.