HMGB1 is a bone-active cytokine

PTH receptor signalling, osteocytes and bone disease induced by diabetes mellitus

Basic, translational and clinical research over the past few decades has provided new understanding on the mechanisms by which activation of the receptor of parathyroid hormone (parathyroid hormone 1 receptor (PTH1R)) regulates bone physiology and pathophysiology. A fundamental change in the field emerged upon the recognition that osteocytes, which are permanent residents of bone and the most abundant cells in bone, are targets of the actions of natural and synthetic ligands of PTH1R (parathyroid hormone and abaloparatide, respectively), and that these cells drive essential actions related to bone remodelling. Among the numerous genes regulated by PTH1R in osteocytes, SOST (which encodes sclerostin, the WNT signalling antagonist and inhibitor of bone formation) has a critical role in bone homeostasis and changes in its expression are associated with several bone pathologies. The bone fragility syndrome induced by diabetes mellitus is accompanied by increased osteocyte apoptosis and changes in the expression of osteocytic genes, including SOST. This Review will discuss advances in our knowledge of the role of osteocytes in PTH1R signalling and the new opportunities to restore bone health in diabetes mellitus by targeting the osteocytic PTH1R-sclerostin axis.

The immunologic phenotype of thrombi is associated with future vascular events after cerebral infarction

BACKGROUND

Thrombi retrieved from patients with acute ischemic stroke may contain prognostic information.

OBJECTIVE

To investigate the relationship between the immunologic phenotype of thrombi and future vascular events in patients with a stroke.

METHODS

This study included patients with acute ischemic stroke who underwent endovascular thrombectomy at Chung-Ang University Hospital in Seoul, Korea, between February 2017 and January 2020. Laboratory and histological variables were compared between patients with and without recurrent vascular events (RVEs). Kaplan-Meier analysis followed by the Cox proportional hazards model was used to identify factors related to RVE. Receiver operating characteristic (ROC) analysis was conducted to evaluate the performance of the immunologic score by combining immunohistochemical phenotypes to predict RVE.

RESULTS

A total of 46 patients were included in the study with 13 RVEs (mean±SD age, 72.8±11.3 years; 26 (56.5%) men). Thrombi with a lower percentage of programmed death ligand-1 expression (HR=11.64; 95% CI 1.60 to 84.82) and a higher number of citrullinated histone H3 positive cells (HR=4.19; 95% CI 0.81 to 21.75) were associated with RVE. The presence of high-mobility group box 1 positive cell was associated with reduced risk of RVE, but the association was lost after adjustment for stroke severity. The immunologic score, which consists of the three immunohistochemical phenotypes, showed good performance in predicting RVE (area under the ROC curve, 0.858; 95% CI 0.758 to 0.958).

CONCLUSIONS

The immunological phenotype of thrombi could provide prognostic information after stroke.

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.

Ghost messages: cell death signals spread

Cell death is a mystery in various forms. Whichever type of cell death, this is always accompanied by active or passive molecules release. The recent years marked the renaissance of the study of these molecules showing they can signal to and communicate with recipient cells and regulate physio- or pathological events. This review summarizes the defined forms of messages cells could spread while dying, the effects of these signals on the target tissue/cells, and how these types of communications regulate physio- or pathological processes. By doing so, this review hopes to identify major unresolved questions in the field, formulate new hypothesis worthy of further investigation, and when possible, provide references for the search of novel diagnostic/therapeutics agents. Video abstract.

The osteocyte as a signaling cell

Osteocytes, former osteoblasts encapsulated by mineralized bone matrix, are far from being passive and metabolically inactive bone cells. Instead, osteocytes are multifunctional and dynamic cells capable of integrating hormonal and mechanical signals and transmitting them to effector cells in bone and in distant tissues. Osteocytes are a major source of molecules that regulate bone homeostasis by integrating both mechanical cues and hormonal signals that coordinate the differentiation and function of osteoclasts and osteoblasts. Osteocyte function is altered in both rare and common bone diseases, suggesting that osteocyte dysfunction is directly involved in the pathophysiology of several disorders affecting the skeleton. Advances in osteocyte biology initiated the development of novel therapeutics interfering with osteocyte-secreted molecules. Moreover, osteocytes are targets and key distributors of biological signals mediating the beneficial effects of several bone therapeutics used in the clinic. Here we review the most recent discoveries in osteocyte biology demonstrating that osteocytes regulate bone homeostasis and bone marrow fat via paracrine signaling, influence body composition and energy metabolism via endocrine signaling, and contribute to the damaging effects of diabetes mellitus and hematologic and metastatic cancers in the skeleton.

lncRNA MIAT/HMGB1 Axis Is Involved in Cisplatin Resistance via Regulating IL6-Mediated Activation of the JAK2/STAT3 Pathway in Nasopharyngeal Carcinoma

Cisplatin-based chemotherapy and radiotherapy are the main first-line treatment strategies for nasopharyngeal carcinoma (NPC) patients. Unfortunately, resistance is a major obstacle in the clinical management of NPC patients. We prove that the expression level of high-mobility group box 1 (HMGB1) is dramatically increased in resistant NPC cells than that in sensitive cells. HMGB1 induces the expression and secretion of IL6, which leads to constitutive autocrine activation of the JAK2/STAT3 pathway and eventually contributes to chemoresistance in NPC cells. Long non-coding RNAs (lncRNAs) have been identified as key regulators involved in drug resistance. In this study, using GO analysis of the biological process and differential expression analysis, we find 12 significantly altered IncRNAs in NPC cell lines, which may be involved in regulating gene expression. Furthermore, we determine that elevated lncRNA MIAT level upregulates HMGB1 expression, contributing to cisplatin resistance in NPC cells. We find that the deficiency of the lncRNA MIAT/HMGB1 axis, inhibition of JAK2/STAT3, or neutralization of IL6 by antibodies significantly re-sensitizes resistant NPC cells to cisplatin in resistant NPC cells. Moreover, we provide the in vivo evidence that the deficiency of HMGB1 reduces cisplatin-resistant tumor growth. Most importantly, we provide clinical evidence showing that the expression level of the lncRNA MIAT/HMGB1/IL6 axis is elevated in resistant NPC tumors, which is highly correlated with poor clinical outcome. Our findings identify a novel chemoresistance mechanism regulated by the lncRNA MIAT/HMGB1/IL6 axis, which indicates the possibilities for lncRNA MIAT, HMGB1, and IL6 as biomarkers for chemoresistance and targets for developing novel strategies to overcome resistance in NPC patients.

Local administration of HMGB-1 promotes bone regeneration on the critical-sized mandibular defects in rabbits

Reconstruction of a critical-sized osseous defect is challenging in maxillofacial surgery. Despite novel treatments and advances in supportive therapies, severe complications including infection, nonunion, and malunion can still occur. Here, we aimed to assess the use of a beta-tricalcium phosphate (β-TCP) scaffold loaded with high mobility group box-1 protein (HMGB-1) as a novel critical-sized bone defect treatment in rabbits. The study was performed on 15 specific pathogen-free New Zealand rabbits divided into three groups: Group A had an osseous defect filled with a β-TCP scaffold loaded with phosphate-buffered saline (PBS) (100 µL/scaffold), the defect in group B was filled with recombinant human bone morphogenetic protein 2 (rhBMP-2) (10 µg/100 µL), and the defect in group C was loaded with HMGB-1 (10 µg/100 µL). Micro-computed tomography (CT) examination demonstrated that group C (HMGB-1) showed the highest new bone volume ratio, with a mean value of 66.5%, followed by the group B (rhBMP-2) (31.0%), and group A (Control) (7.1%). Histological examination of the HMGB-1 treated group showed a vast area covered by lamellar and woven bone surrounding the β-TCP granule remnants. These results suggest that HMGB-1 could be an effective alternative molecule for bone regeneration in critical-sized mandibular bone defects.

Modelling cell guidance and curvature control in evolving biological tissues

Tissue geometry is an important influence on the evolution of many biological tissues. The local curvature of an evolving tissue induces tissue crowding or spreading, which leads to differential tissue growth rates, and to changes in cellular tension, which can influence cell behaviour. Here, we investigate how directed cell motion interacts with curvature control in evolving biological tissues. Directed cell motion is involved in the generation of angled tissue growth and anisotropic tissue material properties, such as tissue fibre orientation. We develop a new cell-based mathematical model of tissue growth that includes both curvature control and cell guidance mechanisms to investigate their interplay. The model is based on conservation principles applied to the density of tissue synthesising cells at or near the tissue's moving boundary. The resulting mathematical model is a partial differential equation for cell density on a moving boundary, which is solved numerically using a hybrid front-tracking method called the cell-based particle method. The inclusion of directed cell motion allows us to model new types of biological growth, where tangential cell motion is important for the evolution of the interface, or for the generation of anisotropic tissue properties. We illustrate such situations by applying the model to simulate both the resorption and infilling components of the bone remodelling process, and to simulate root hair growth. We also provide user-friendly MATLAB code to implement the algorithms.

Glucocorticoid-induced autophagy and apoptosis in bone

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

High-mobility group box 1 released by traumatic occlusion accelerates bone resorption in the root furcation area in mice

BACKGROUND AND OBJECTIVE

Traumatic occlusion can cause bone resorption without bacterial infection. Although bone resorption in periodontitis has been relatively well studied, little is known about bone resorption by traumatic occlusion. High-mobility group box 1 (HMGB1) is released from damaged tissue and has been recently shown to promote bone resorption in a murine periodontitis model and may also promote bone resorption by traumatic occlusion. The present study aimed to examine whether HMGB1 accelerates bone resorption by traumatic occlusion in mice.

MATERIALS AND METHODS

Occlusal trauma was induced in the lower left first molar of mice by bonding a wire to the upper left first molar, and bone resorption and osteoclast formation were evaluated histochemically. The expression of HMGB1, Toll-like receptor 4 (TLR4; the receptor for HMGB1), and receptor activator of NFκB ligand (RANKL; an essential osteoclast differentiation factor) was evaluated immunohistologically. In addition, mice were administrated with an anti-HMGB1-neutralizing antibody to analyze the role of HMGB1.

RESULTS

Bone resorption and osteoclast formation gradually increased until day 5 at the furcation area after the application of traumatic occlusion. Expression of HMGB1 was observed at the furcation area on day 1, but was attenuated by day 3. Expression of RANKL gradually increased until day 3, but was attenuated by day 5. Administration of anti-HMGB1 antibody significantly reduced the number of osteoclasts and the expression of RANKL and TLR4 at the furcation area.

CONCLUSION

Release of HMGB1 in the root furcation area accelerated bone resorption by up-regulating RANKL and TLR4 expression in mice with traumatic occlusion.

Biomarkers of vascular disease in diabetes: the adipose-immune system cross talk

Experimental and clinical studies aimed at investigating the mechanism(s) underlying vascular complications of diabetes indicate that a great number of molecules are involved in the pathogenesis of these complications. Most of these molecules are inflammatory mediators or markers generated by immune or adipose tissue. Some of them, i.e. resistin and sortilin, have been shown to be involved in the cross talk between adipocytes and inflammatory cells. This interaction is an attractive area of research, particularly in type 2 diabetes and obesity. Other proteins, such as adiponectin and visfatin, appear to be more promising as possible vascular markers. In addition, some molecules involved in calcium/phosphorus metabolism, such as klotho and FGF23, have an involvement in the pathogenesis of diabetic vasculopathy, which appears to be dependent on the degree of vascular impairment. Inflammatory markers are a promising tool for treatment decisions while measuring plasma levels of adipokines, sortilin, Klotho and FGF23 in adequately sized longitudinal studies is expected to allow a more precise characterization of diabetic vascular disease and the optimal use of personalized treatment strategies.

Alarmins in Osteoporosis, RAGE, IL-1, and IL-33 Pathways: A Literature Review

Alarmins are endogenous mediators released by cells following insults or cell death to alert the host’s innate immune system of a situation of danger or harm. Many of these, such as high-mobility group box-1 and 2 (HMGB1, HMGB2) and S100 (calgranulin proteins), act through RAGE (receptor for advanced glycation end products), whereas the IL-1 and IL-33 cytokines bind the IL-1 receptors type I and II, and the cellular receptor ST2, respectively. The alarmin family and their signal pathways share many similarities of cellular and tissue localization, functions, and involvement in various physiological processes and inflammatory diseases including osteoporosis. The aim of the review was to evaluate the role of alarmins in osteoporosis. A bibliographic search of the published scientific literature regarding the role of alarmins in osteoporosis was organized independently by two researchers in the following scientific databases: Pubmed, Scopus, and Web of Science. The keywords used were combined as follows: “alarmins and osteoporosis”, “RAGE and osteoporosis”, “HMGB1 and osteoporosis”, “IL-1 and osteoporosis”, “IL 33 and osteopororsis”, “S100s protein and osteoporosis”. The information was summarized and organized in the present review. We highlight the emerging roles of alarmins in various bone remodeling processes involved in the onset and development of osteoporosis, as well as their potential role as biomarkers of osteoporosis severity and progression. Findings of the research suggest a potential use of alarmins as pharmacological targets in future therapeutic strategies aimed at preventing bone loss and fragility fractures induced by aging and inflammatory diseases.

Pro-inflammatory Cytokines: Cellular and Molecular Drug Targets for Glucocorticoid-induced-osteoporosis via Osteocyte

Glucocorticoids are widely used to treat varieties of allergic and autoimmune diseases, however, long-term application results in glucocorticoid-induced osteoporosis (GIOP). Inflammatory cytokines: tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) play important regulatory roles in bone metabolism, but their roles in GIOP remain largely unknown. Osteocytes can modulate the formation and function of both osteoblasts and osteoclasts, directly via gap junctions, or indirectly by transferring molecule signaling. Apoptotic osteocytes release RANKL, HMGB1 and pro-inflammatory cytokines to stimulate osteoclastogenesis. Moreover, osteocytes can secrete FGF23 to regulate bone metabolism. Exposure to high levels of GCs can drive osteocyte apoptosis and influence gap junctions, leading to bone loss. GCs treatment is regarded to produce more FGF23 to inhibit bone mineralization. GCs also disrupt the vascular to decrease osteocyte feasibility and mineral appositional rate, resulting in a decline in bone strength. Apoptotic bodies from osteocytes induced by GCs treatment can enhance production of TNF-α and IL-6. On the other hand, TNF-α and IL-6 show synergistic effects by altering osteocytes signaling towards osteoclasts and osteoblasts. In addition, TNF-α can induce osteocyte apoptosis and attribute to a worsened bone quality in GCs. IL-6 and osteocytes may interact with each other. Therefore, we hypothesize that GCs regulate osteocyteogenesis through TNF-α and IL-6, which are highly expressed around osteocyte undergoing apoptosis. In the present review, we summarized the roles of osteocytes in regulating osteoblasts and osteoclasts. Furthermore, the mechanism of GCs altered relationship between osteocytes and osteoblasts/osteoclasts. In addition, we discussed the roles of TNF-α and IL-6 in GIOP by modulating osteocytes. Lastly, we discussed the possibility of using pro-inflammatory signaling pathway as therapeutic targets to develop drugs for GIOP.

Bench to Bedside: Animal Models of Radiation Induced Musculoskeletal Toxicity

Ionizing radiation is a critical aspect of current cancer therapy. While classically mature bone was thought to be relatively radio-resistant, more recent data have shown this to not be the case. Radiation therapy (RT)-induced bone loss leading to fracture is a source of substantial morbidity. The mechanisms of RT likely involve multiple pathways, including changes in angiogenesis and bone vasculature, osteoblast damage/suppression, and increased osteoclast activity. The majority of bone loss appears to occur rapidly after exposure to ionizing RT, with significant changes in cortical thickness being detectable on computed tomography (CT) within three to four months. Additionally, there is a dose–response relationship. Cortical thinning is especially notable in areas of bone that receive >40 gray (Gy). Methods to mitigate toxicity due to RT-induced bone loss is an area of active investigation. There is an accruing clinical trial investigating the use of risderonate, a bisphosphonate, to prevent rib bone loss in patients undergoing lung stereotactic body radiation therapy (SBRT). Additionally, several other promising therapeutic/preventative approaches are being explored in preclinical studies, including parathyroid hormone (PTH), amifostine, and mechanical loading of irradiated bones.

Calvarial Versus Long Bone: Implications for Tailoring Skeletal Tissue Engineering

Tissue-engineered graft substitutes have shown great potential to treat large bone defects. While we usually assume that therapeutic approaches developed for appendicular bone healing could be similarly translated for application in craniofacial reconstruction and vice versa, this is not necessarily accurate. In addition to those more well-known healing-associated factors, such as age, lifestyle (e.g., nutrition and smoking), preexisting disease (e.g., diabetes), medication, and poor blood supply, the developmental origins and surrounding tissue of the wound sites can largely affect the fracture healing outcome as well as designed treatments. Therefore, the strategies developed for long bone fracture repair might not be suitable or directly applicable to skull bone repair. In this review, we discuss aspects of development, healing process, structure, and tissue engineering considerations between calvarial and long bones to assist in designing the tailored bone repair strategies. Impact Statement We summarized, in this review, the existing body of knowledge between long bone and calvarial bone with regard to their development and healing, tissue structure, and consideration of current tissue engineering strategies. By highlighting their similarities and differences, we propose that tailored tissue engineering strategies, such as scaffold features, growth factor usage, and the source of cells for tissue or region-specific bone repair, are necessary to ensure an optimized healing outcome.

Regulatory Effects and Interactions of the Wnt and OPG-RANKL-RANK Signaling at the Bone-Cartilage Interface in Osteoarthritis

Cartilage and the bordering subchondral bone form a functionally active regulatory interface with a prominent role in osteoarthritis pathways. The Wnt and the OPG-RANKL-RANK signaling systems, as key mediators, interact in subchondral bone remodeling. Osteoarthritic osteoblasts polarize into two distinct phenotypes: a low secretory and an activated, pro-inflammatory and anti-resorptive subclass producing high quantities of IL-6, PGE2, and osteoprotegerin, but low levels of RANKL, thus acting as putative effectors of subchondral bone sclerosis. Wnt agonists, Wnt5a, Wisp-1 initiate excessive bone remodeling, while Wnt3a and 5a simultaneously cause loss of proteoglycans and phenotype shift in chondrocytes, with decreased expression of COL2A, aggrecan, and Sox-9. Sclerostin, a Wnt antagonist possesses a protective effect for the cartilage, while DKK-1 inhibits VEGF, suspending neoangiogenesis in the subchondral bone. Experimental conditions mimicking abnormal mechanical load, the pro-inflammatory milieu, but also a decreased OPG/RANKL ratio in the cartilage, trigger chondrocyte apoptosis and loss of the matrix via degradative matrix metalloproteinases, like MMP-13 or MMP-9. Hypoxia, an important cofactor exerts a dual role, promoting matrix synthesis via HIF-1α, a Wnt silencer, but turning on HIF-2α that enhances VEGF and MMP-13, along with aberrant collagen expression and extracellular matrix deterioration in the presence of pro-inflammatory cytokines.

Omnipresence of inflammasome activities in inflammatory bone diseases

The inflammasomes are intracellular protein complexes that are assembled in response to a variety of perturbations including infections and injuries. Failure of the inflammasomes to rapidly clear the insults or restore tissue homeostasis can result in chronic inflammation. Recurring inflammation is also provoked by mutations that cause the constitutive assembly of the components of these protein platforms. Evidence suggests that chronic inflammation is a shared mechanism in bone loss associated with aging, dysregulated metabolism, autoinflammatory, and autoimmune diseases. Mechanistically, inflammatory mediators promote bone resorption while suppressing bone formation, an imbalance which over time leads to bone loss and increased fracture risk. Thus, while acute inflammation is important for the maintenance of bone integrity, its chronic state damages this tissue. In this review, we discuss the role of the inflammasomes in inflammation-induced osteolysis.

Advanced Glycation End Products (AGEs), Receptor for AGEs, Diabetes, and Bone: Review of the Literature

Diabetes compromises bone cell metabolism and function, resulting in increased risk of fragility fracture. Advanced glycation end products (AGEs) interact with the receptor for AGEs (RAGE) and can make a meaningful contribution to bone cell metabolism and/or alter function. Searches in PubMed using the key words "advanced glycation end-product," "RAGE," "sRAGE," "bone," and "diabetes" were made to explain some of the clinical outcomes of diabetes in bone metabolism through the AGE-RAGE signaling pathway. All published clinical studies were included in tables. The AGE-RAGE signaling pathway participates in diabetic complications, including diabetic osteopathy. Some clinical results in diabetic patients, such as reduced bone density, suppressed bone turnover markers, and bone quality impairment, could be potentially due to AGE-RAGE signaling consequences. However, the AGE-RAGE signaling pathway has some helpful roles in the bone, including an increase in osteogenic function. Soluble RAGE (sRAGE), as a ligand decoy, may increase in either conditions of RAGE production or destruction, and then it cannot always reflect the AGE-RAGE signaling. Recombinant sRAGE can block the AGE-RAGE signaling pathway but is associated with some limitations, such as accessibility to AGEs, an increase in other RAGE ligands, and a long half-life (24 hours), which is associated with losing the beneficial effect of AGE/RAGE. As a result, sRAGE is not a helpful marker to assess activity of the RAGE signaling pathway. The recombinant sRAGE cannot be translated into clinical practice due to its limitations.

High mobility group box 1 protein regulates osteoclastogenesis through direct actions on osteocytes and osteoclasts in vitro

Old age and Cx43 deletion in osteocytes are associated with increased osteocyte apoptosis and osteoclastogenesis. We previously demonstrated that apoptotic osteocytes release elevated concentrations of the proinflammatory cytokine, high mobility group box 1 protein (HMGB1) and apoptotic osteocyte conditioned media (CM) promotes osteoclast differentiation. Further, prevention of osteocyte apoptosis blocks osteoclast differentiation and attenuates the extracellular release of HMGB1 and RANKL. Moreover, sequestration of HMGB1, in turn, reduces RANKL production/release by MLO-Y4 osteocytic cells silenced for Cx43 (Cx43^def ), highlighting the possibility that HMGB1 promotes apoptotic osteocyte-induced osteoclastogenesis. However, the role of HMGB1 signaling in osteocytes has not been well studied. Further, the mechanisms underlying its release and the receptor(s) responsible for its actions is not clear. We now report that a neutralizing HMGB1 antibody reduces osteoclast formation in RANKL/M-CSF treated bone marrow cells. In bone marrow macrophages (BMMs), toll-like receptor 4 (TLR4) inhibition with LPS-RS, but not receptor for advanced glycation end products (RAGE) inhibition with Azeliragon attenuated osteoclast differentiation. Further, inhibition of RAGE but not of TLR4 in osteoclast precursors reduced osteoclast number, suggesting that HGMB1 produced by osteoclasts directly affects differentiation by activating TLR4 in BMMs and RAGE in preosteoclasts. Our findings also suggest that increased osteoclastogenesis induced by apoptotic osteocytes CM is not mediated through HMGB1/RAGE activation and that direct HMGB1 actions in osteocytes stimulate pro-osteoclastogenic signal release from Cx43^def osteocytes. Based on these findings, we propose that HMGB1 exerts dual effects on osteoclasts, directly by inducing differentiation through TLR4 and RAGE activation and indirectly by increasing pro-osteoclastogenic cytokine secretion from osteocytes.

The Emerging Role of Osteocytes in Cancer in Bone

Advances in the last decade have established the osteocyte, the most abundant cell in bone, as a dynamic and multifunctional cell capable of controlling bone homeostasis by regulating the function of both osteoblasts and osteoclasts. In addition, accumulating evidence demonstrates that osteocyte function is altered in several skeletal disorders, and targeting osteocytes and their derived factors improves skeletal health. Despite the remarkable progress in our understanding of osteocyte biology, there has been a paucity of information regarding the role of osteocytes in the progression of cancer in bone. Exciting, recent discoveries suggest that tumor cells communicate with osteocytes to generate a microenvironment that supports the growth and survival of cancer cells and stimulates bone destruction. This review features these novel findings and discussions regarding the impact of chemotherapy on osteocyte function and the potential of targeting osteocytes for the treatment of cancer in bone. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

RAGE Signaling in Skeletal Biology

PURPOSE OF REVIEW

The receptor for advanced glycation end products (RAGE) and several of its ligands have been implicated in the onset and progression of pathologies associated with aging, chronic inflammation, and cellular stress. In particular, the role of RAGE and its ligands in bone tissue during both physiological and pathological conditions has been investigated. However, the extent to which RAGE signaling regulates bone homeostasis and disease onset remains unclear. Further, RAGE effects in the different bone cells and whether these effects are cell-type specific is unknown. The objective of the current review is to describe the literature over RAGE signaling in skeletal biology as well as discuss the clinical potential of RAGE as a diagnostic and/or therapeutic target in bone disease.

RECENT FINDINGS

The role of RAGE and its ligands during skeletal homeostasis, tissue repair, and disease onset/progression is beginning to be uncovered. For example, detrimental effects of the RAGE ligands, advanced glycation end products (AGEs), have been identified for osteoblast viability/activity, while others have observed that low level AGE exposure stimulates osteoblast autophagy, which subsequently promotes viability and function. Similar findings have been reported with HMGB1, another RAGE ligand, in which high levels of the ligand are associated with osteoblast/osteocyte apoptosis, whereas low level/short-term administration stimulates osteoblast differentiation/bone formation and promotes fracture healing. Additionally, elevated levels of several RAGE ligands (AGEs, HMGB1, S100 proteins) induce osteoblast/osteocyte apoptosis and stimulate cytokine production, which is associated with increased osteoclast differentiation/activity. Conversely, direct RAGE-ligand exposure in osteoclasts may have inhibitory effects. These observations support a conclusion that elevated bone resorption observed in conditions of high circulating ligands and RAGE expression are due to actions on osteoblasts/osteocytes rather than direct actions on osteoclasts, although additional work is required to substantiate the observations. Recent studies have demonstrated that RAGE and its ligands play an important physiological role in the regulation of skeletal development, homeostasis, and repair/regeneration. Conversely, elevated levels of RAGE and its ligands are clearly related with various diseases associated with increased bone loss and fragility. However, despite the recent advancements in the field, many questions regarding RAGE and its ligands in skeletal biology remain unanswered.

Bone function, dysfunction and its role in diseases including critical illness

The skeleton is one of the largest organs in the human body. In addition to its conventional functions such as support, movement and protection, the skeleton also contributes to whole body homeostasis and maintenance of multiple important non-bone organs/systems (extraskeletal functions). Both conventional and extraskeletal functions of the skeleton are defined as bone function. Bone-derived factors (BDFs) are key players regulating bone function. In some pathophysiological situations, including diseases affecting bone and/or other organs/systems, the disorders of bone itself and the subsequently impaired functions of extraskeletal organs/systems caused by abnormal bone (impaired extraskeletal functions of bone) are defined as bone dysfunction. In critical illness, which is a health status characterized by the dysfunction or severe damage of one or multiple important organs or systems, the skeleton shows rapid bone loss resulting from bone hyper-resorption and impaired osteoblast function. In addition, the dysfunctions of the skeleton itself are also closely related to the severity and prognosis of critical illness. Therefore, we propose that there is bone dysfunction in critical illness. Some methods to inhibit osteoclast activity or promote osteoblast function by the treatment of bisphosphonates or PTH1-34 benefit the outcome of critical illness, which indicates that enhancing bone function may be a potential novel strategy to improve prognosis of diseases including critical illness.

miR-193a inhibits osteogenic differentiation of bone marrow-derived stroma cell via targeting HMGB1

BACKGROUND

miR-193a has been shown to be involved in a variety of biological processes, including cell proliferation, differentiation, and apoptosis. However, little is known about how miR-193a regulates osteogenic differentiation.

METHODS

We employed RT-qPCR to determine the level of miR-193a and mRNA level of HMGB1 and osteoblast-specific markers (Runx2, ALP, OSX, OCN). Besides, westernblot was used to probe protein level of phosphorylated MAPK family members and β-catenin. Bioinformatic analysis was used to predict the putative binding sequence of miR-193a to the 3'-UTR of HMGB1 and we confirmed this result by dual luciferase reporter assay. Alizarin red staining assay (ARS) and alkaline phosphatase activity (ALP) were performed to detect osteogenic differentiation.

RESULTS

miR-193a was downregulated in OM (osteogenic medium)induced hBMSC. More interestingly, we found that miR-193a mimic attenuated matrix mineralization and alkaline phosphatase activity, whereas miR-193a inhibitor exerted the opposite phenotypes. Mechanistically, we observed that miR-193a played an inhibitory role in expression of osteoblast-specific markers and activation of MAPK and Wnt signaling pathways. Bioinformatic analysis and dual luciferase assay demonstrated that miR-193a directly targeted 3'-UTR of HMGB1. Furthermore, we overexpressed HMGB1 in miR-193a overexpressed hBMSC to establish that HMGB1 acted as downstream target of miR-193a-inhibited osteogenic differentiation.

CONCLUSIONS

Here, we reveal miR-193a plays a suppressive role in osteogenic differentiation of hBMSC via targeting HMGB1. These findings provide a novel mechanism underlying osteogenic differentiation and offer therapeutical strategy for bone formation.

High Mobility Group Box 1 Promotes Aortic Calcification in Chronic Kidney Disease via the Wnt/β-Catenin Pathway

Vascular calcification (VC) is common in chronic kidney disease (CKD), where cardiovascular mortality remains the leading cause of death. Here, we examined the role of high-mobility group box1 (HMGB1), a nuclear DNA-binding protein involved in inflammation, in aortic calcification and renal dysfunction induced by high phosphate in a mouse model of CKD induced by 5/6 nephrectomy. HMGB1 and kidney function markers were measured by ELISA in the serum of CKD patients and in CKD mice. Sections of the aortas of mice were analyzed by immunofluorescence and Alizarin red staining, and protein lysates were generated to analyze the expression of related proteins in response to silencing of HMGB1 or β-catenin by western blotting. Our results showed that serum HMGB1 levels were significantly higher in CKD patients than in healthy controls and related to disease stage. High phosphate promoted the translocation of HMGB1 from the nucleus to the cytosol and aortic calcification in CKD mice in vivo, whereas HMGB1 knockdown ameliorated part of renal and vascular function. β-catenin silencing reversed high phosphate-induced calcification and restored renal marker levels. Taken together, our results suggest that HMGB1 is involved in VC associated with CKD via a mechanism involving the β-catenin.

NELL-1, HMGB1, and CCN2 Enhance Migration and Vasculogenesis, But Not Osteogenic Differentiation Compared to BMP2

Currently, autografts still represent the gold standard treatment for the repair of large bone defects. However, these are associated with donor-site morbidity and increased pain, cost, and recovery time. The ideal therapy would use biomaterials combined with bone growth factors to induce and instruct bone defect repair without the need to harvest patient tissue. In this line, bone morphogenetic proteins (BMPs) have been the most extensively used agents for clinical bone repair, but at supraphysiological doses that are not without risk. Because of the need to eliminate the risks of BMP2 use in vivo, we assessed the ability of three putative osteogenic factors, nel-like molecule type 1 (NELL-1), high mobility group box 1 (HMGB1), and CCN2, to enhance the essential processes for bone defect repair in vitro and compared them to BMP2. Although it has been reported that NELL-1, HMGB1, and CCN2 play a role in bone formation, less is known about the contribution of these proteins to the different events involved, such as cell migration, osteogenesis, and vasculogenesis. In this study, we investigated the effects of different doses of NELL-1, HMGB, CCN2, and BMP2 on these three processes as a model for the recruitment and differentiation of resident cells in the in vivo bone defect repair situation, using cells of human origin. Our data demonstrated that NELL-1, HMGB1, and CCN2 significantly induced mesenchymal stem cell migration (from 1.58-fold increase compared to control), but BMP2 did not. Interestingly, only BMP2 increased osteogenesis in marrow stromal cells, whereas it inhibited osteogenesis in preosteoblasts. Moreover, the four proteins studied promoted significantly endothelial cell migration, reaching a maximum of 2.4-fold increase compared to control, and induced formation of tube-like structures. NELL-1, HMGB1, and CCN2 had these effects at relatively low doses compared to BMP2. This work indicates that NELL-1, HMGB1, and CCN2 might enhance bone defect healing via the recruitment of endogenous cells and induction of vascularization and act via different processes than BMP2.

MicroRNA-505 is downregulated in human osteosarcoma and regulates cell proliferation, migration and invasion

Recent studies have demonstrated that microRNAs (miRNAs/miRs) are involved in osteosarcoma tumorigenesis, progression, invasion and metastasis. For example, miR-505 plays important roles in human carcinogenesis; however, its exact function in osteosarcoma remains unclear. MicroRNA profiles of osteosarcoma and normal tissues were obtained by miRNA microarray assays, which were validated by quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR). Then, high-mobility group box 1 (HMGB1) expression was evaluated by qRT-PCR and western blot analysis. The correlation between miR-505 and HMGB1 was analyzed by Pearson correlation. In vitro, the biological functions of miR-505 were examined by wound healing, MTT and Transwell assays and western blot analysis in MG63 cells transfected with miRNA mimics or empty vector. Luciferase assay was utilized to assess whether HMGB1 is a target of miR-505. miRNA microarrays revealed 26 aberrant miRNAs in osteosarcoma tissues; miR-505 showed the most pronounced decrease (P<0.01), which was significantly associated with TNM stage and metastasis status (P<0.05). In addition, HMGB1 was highly expressed in osteosarcoma tissues (P<0.01), with a significantly negative correlation with miR-505 (r=−0.6679, P<0.001). Furthermore, miR-505 inhibited proliferation, migration and invasion abilities of MG63 cells (P<0.01). Moreover, luciferase activity of the HMGB1-3′-UTR plasmid was suppressed following miR-505 binding (P<0.01). Finally, HMGB1 overexpression partly reversed the effects of miR-505 on MG63 cells. In conclusion, miR-505 levels are decreased in osteosarcoma tissues, and reduced miR-505 expression is significantly associated with poorer clinical prognosis in patients with osteosarcomas. miR-505 inhibits osteosarcoma cell proliferation, migration and invasion by regulating HMGB1.

Serum high mobility group box-1 and osteoprotegerin levels are associated with peripheral arterial disease and critical limb ischemia in type 2 diabetic subjects

BACKGROUND

High mobility group box-1 (HMGB-1) is a nuclear protein also acting as inflammatory mediator, whilst osteoprotegerin (OPG), member of tumor necrosis factor receptor superfamily, is indicated as marker of vascular calcification. Peripheral artery disease (PAD) and type 2 diabetes (T2D) are clinical conditions characterized by elevated serum inflammatory markers and vascular calcification enhancement. The aim of this study was to investigate the potential role of HMGB-1, OPG and several inflammatory mediators such as C-reactive protein (HsCRP), tumor necrosis factor-alpha and interleukin-6 (IL-6) on the presence and severity of peripheral artery disease in patients with T2D.

METHODS

In this retrospective observational study, we have analyzed HMGB-1, OPG and inflammatory cytokines serum levels in 1393 type 2 diabetic patients with PAD and without PAD (WPAD).

RESULTS

HMGB-1 (7.89 ± 15.23 ng/mL), OPG (6.54 ± 7.76 pmol/L), HsCRP (15.6 ± 14.4 mg/L) and IL-6 (56.1 ± 28.6 pg/mL) serum levels were significantly higher in patients with PAD than in those WPAD (3.02 ± 8.12 ng/mL, P ˂ 0.001; 2.98 ± 2.01 pmol/L, P < 0.001; 7.05 ± 4.4 mg/L, P < 0.001; 37.5 ± 20.2 pg/mL, P < 0.001 respectively). Moreover HMGB-1 (P < 0.001), OPG (P < 0.001), HsCRP (P < 0.001) and IL-6 (P < 0.001) serum levels were positively correlated with clinical severity of PAD. HMGB-1 (adjusted OR 12.32; 95% CI 3.56-23.54, P = 0.023) and OPG (adjusted OR 3.53; 95% CI 1.54-6.15, P = 0.019) resulted independent determinants of PAD in patients with T2D after adjusting for the conventional cardiovascular risk factor and established inflammatory mediators.

CONCLUSIONS

In T2D patients HMGB-1 and OPG serum levels are higher in patients affected by PAD and independently associated with its occurrence and clinical severity.

Disruption of the Cx43/miR21 pathway leads to osteocyte apoptosis and increased osteoclastogenesis with aging

Skeletal aging results in apoptosis of osteocytes, cells embedded in bone that control the generation/function of bone forming and resorbing cells. Aging also decreases connexin43 (Cx43) expression in bone; and osteocytic Cx43 deletion partially mimics the skeletal phenotype of old mice. Particularly, aging and Cx43 deletion increase osteocyte apoptosis, and osteoclast number and bone resorption on endocortical bone surfaces. We examined herein the molecular signaling events responsible for osteocyte apoptosis and osteoclast recruitment triggered by aging and Cx43 deficiency. Cx43‐silenced MLO‐Y4 osteocytic (Cx43^def) cells undergo spontaneous cell death in culture through caspase‐3 activation and exhibit increased levels of apoptosis‐related genes, and only transfection of Cx43 constructs able to form gap junction channels reverses Cx43^def cell death. Cx43^def cells and bones from old mice exhibit reduced levels of the pro‐survival microRNA miR21 and, consistently, increased levels of the miR21 target phosphatase and tensin homolog (PTEN) and reduced phosphorylated Akt, whereas PTEN inhibition reduces Cx43^def cell apoptosis. miR21 reduction is sufficient to induce apoptosis of Cx43‐expressing cells and miR21 deletion in miR21^fl/fl bones increases apoptosis‐related gene expression, whereas a miR21 mimic prevents Cx43^def cell apoptosis, demonstrating that miR21 lies downstream of Cx43. Cx43^def cells release more osteoclastogenic cytokines [receptor activator of NFκB ligand (RANKL)/high‐mobility group box‐1 (HMGB1)], and caspase‐3 inhibition prevents RANKL/HMGB1 release and the increased osteoclastogenesis induced by conditioned media from Cx43^def cells, which is blocked by antagonizing HMGB1‐RAGE interaction. These findings identify a novel Cx43/miR21/HMGB1/RANKL pathway involved in preventing osteocyte apoptosis that also controls osteoclast formation/recruitment and is impaired with aging.

The Role of the Molecular Clock in Skeletal Muscle and What It Is Teaching Us About Muscle-Bone Crosstalk

PURPOSE OF REVIEW

This review summarizes what has been learned about the interaction between skeletal muscle and bone from mouse models in which BMAL1, a core molecular clock protein has been deleted. Additionally, we highlight several genes which change following loss of BMAL1. The protein products from these genes are secreted from muscle and have a known effect on bone homeostasis.

RECENT FINDINGS

Circadian rhythms have been implicated in regulating systems homeostasis through a series of transcriptional-translational feedback loops termed the molecular clock. Recently, skeletal muscle-specific disruption of the molecular clock has been shown to disrupt skeletal muscle metabolism. Additionally, loss of circadian rhythms only in adult muscle has an effect on other tissue systems including bone. Our finding that the expression of a subset of skeletal muscle-secreted proteins changes following BMAL1 knockout combined with the current knowledge of muscle-bone crosstalk suggests that skeletal muscle circadian rhythms are important for maintenance of musculoskeletal homeostasis. Future research on this topic may be important for understanding the role of the skeletal muscle molecular clock in a number of diseases such as sarcopenia and osteoporosis.

Loss of menin in osteoblast lineage affects osteocyte-osteoclast crosstalk causing osteoporosis

During osteoporosis bone formation by osteoblasts is reduced and/or bone resorption by osteoclasts is enhanced. Currently, only a few factors have been identified in the regulation of bone integrity by osteoblast-derived osteocytes. In this study, we show that specific disruption of menin, encoded by multiple endocrine neoplasia type 1 (Men1), in osteoblasts and osteocytes caused osteoporosis despite the preservation of osteoblast differentiation and the bone formation rate. Instead, an increase in osteoclast numbers and bone resorption was detected that persisted even when the deletion of Men1 was restricted to osteocytes. We demonstrate that isolated Men1-deficient osteocytes expressed numerous soluble mediators, such as C-X-C motif chemokine 10 (CXCL10), and that CXCL10-mediated osteoclastogenesis was reduced by CXCL10-neutralizing antibodies. Collectively, our data reveal a novel role for Men1 in osteocyte–osteoclast crosstalk by controlling osteoclastogenesis through the action of soluble factors. A role for Men1 in maintaining bone integrity and thereby preventing osteoporosis is proposed.

HMGB1 Induces Secretion of Matrix Vesicles by Macrophages to Enhance Ectopic Mineralization

Numerous clinical conditions have been linked to ectopic mineralization (EM). This process of pathological biomineralization is complex and not fully elucidated, but thought to be started within matrix vesicles (MVs). We hypothesized that high mobility group box 1 (HMGB1), a cytokine associated with biomineralizing process under physiological and pathological conditions, induces EM via promoting MVs secretion from macrophages. In this study, we found that HMGB1 significantly promoted secretion of MVs from macrophages and subsequently led to mineral deposition in elevated Ca/Pi medium in vitro. Transmission electron microscopy of calcifying MVs showed formation of hydroxyapatite crystals in the vesicle interior. Subcutaneous injection into mice with MVs derived from HMGB1-treated cells showed a greater potential to initiate regional mineralization. Mechanistic experiments revealed that HMGB1 activated neutral sphingomyelinase2 (nSMase2) that involved the receptor for advanced glycation end products (RAGE) and p38 MAPK (upstream of nSMase2). Inhibition of nSMase2 with GW4869 or p38 MAPK with SB-239063 prevented MVs secretion and mineral deposition. Collectively, HMGB1 induces MVs secretion from macrophages at least in part, via the RAGE/p38 MAPK/nSMase2 signaling pathway. Our findings thus reveal a novel mechanism by which HMGB1 induces ectopic mineralization.

HMGB1 Osteo-Modulatory Action on Osteosarcoma SaOS-2 Cell Line: An Integrated Study From Biochemical and -Omics Approaches

High mobility group box protein-1 (HMGB1) is released from cells under various pathological conditions and it plays a pivotal role as an alarmin signaling tissue damage. Little is known about the impact of HMGB1 in bone repair and remodeling. To this aim, we focused on HMGB1-induced effects on the in vitro osteoblast model SaOS-2. Cell proliferation was stimulated with a maximum at concentration of 2.5 nM, and such a dose also stimulated cell migration and scratch wound healing. We then characterized the modulatory effect of HMGB1 on bone biology, by using osteogenesis/mineralization assays, a PCR array, and the analysis of a series of osteogenic markers. We performed also a proteomic screening using SWATH-MS on SaOS-2 cell exposed to HMGB1 and we provide evidence for proteins modulated in HMGB1 exposed cells. Taken together, our data demonstrate that SaOS-2 cell proliferation, migration, and osteogenic differentiation were increased by HMGB1. We, therefore, propose that HMGB1 could be a potent bone-remodeling signal but the physiological meaning of this property remains to be more ascertained. J. Cell. Biochem. 117: 2559-2569, 2016. © 2016 Wiley Periodicals, Inc.

Pannexin-1 and P2X7-Receptor Are Required for Apoptotic Osteocytes in Fatigued Bone to Trigger RANKL Production in Neighboring Bystander Osteocytes

Osteocyte apoptosis is required to induce intracortical bone remodeling after microdamage in animal models, but how apoptotic osteocytes signal neighboring "bystander" cells to initiate the remodeling process is unknown. Apoptosis has been shown to open pannexin-1 (Panx1) channels to release adenosine diphosphate (ATP) as a "find-me" signal for phagocytic cells. To address whether apoptotic osteocytes use this signaling mechanism, we adapted the rat ulnar fatigue-loading model to reproducibly introduce microdamage into mouse cortical bone and measured subsequent changes in osteocyte apoptosis, receptor activator of NF-κB ligand (RANKL) expression and osteoclastic bone resorption in wild-type (WT; C57Bl/6) mice and in mice genetically deficient in Panx1 (Panx1KO). Mouse ulnar loading produced linear microcracks comparable in number and location to the rat model. WT mice showed increased osteocyte apoptosis and RANKL expression at microdamage sites at 3 days after loading and increased intracortical remodeling and endocortical tunneling at day 14. With fatigue, Panx1KO mice exhibited levels of microdamage and osteocyte apoptosis identical to WT mice. However, they did not upregulate RANKL in bystander osteocytes or initiate resorption. Panx1 interacts with P2X7 R in ATP release; thus, we examined P2X7 R-deficient mice and WT mice treated with P2X7 R antagonist Brilliant Blue G (BBG) to test the possible role of ATP as a find-me signal. P2X7 RKO mice failed to upregulate RANKL in osteocytes or induce resorption despite normally elevated osteocyte apoptosis after fatigue loading. Similarly, treatment of fatigued C57Bl/6 mice with BBG mimicked behavior of both Panx1KO and P2X7 RKO mice; BBG had no effect on osteocyte apoptosis in fatigued bone but completely prevented increases in bystander osteocyte RANKL expression and attenuated activation of resorption by more than 50%. These results indicate that activation of Panx1 and P2X7 R are required for apoptotic osteocytes in fatigued bone to trigger RANKL production in neighboring bystander osteocytes and implicate ATP as an essential signal mediating this process.

The Role of Osteocytes in Age-Related Bone Loss

The decrease in bone mass and strength during aging has multiple causes. Osteocytes are long-lived cells within the bone matrix that perform a variety of functions, including the control of bone remodeling. Because of their longevity, osteocytes are more likely than osteoclasts or osteoblasts to accumulate molecular damage over time. Osteocytes utilize quality-control pathways like autophagy to remove damaged organelles and macromolecules, and thereby maintain function. When the damage is excessive, cell death pathways such as apoptosis minimize the impact of potential osteocyte dysfunction on the skeleton. The goal of this review is to discuss how dysregulation of these pathways in osteocytes may contribute to the decline in bone mass and strength with age.

rhHMGB1 drives osteoblast migration in a TLR2/TLR4- and NF-κB-dependent manner

Osteoblast migration is significant in skeletal development. Recently, high mobility group box 1 protein (HMGB1) has been shown to highly expressed in cartilage to regulate endochondral ossification. Nevertheless, whether HMGB1 can modulate osteoblast proliferation and migration is poorly understood, as well as the intracellular signalling pathways that are involved in this process. Herein, we examined the effects of recombinant human HMGB1 (rhHMGB1) on the proliferation and migration of rat osteoblasts and investigated whether Toll-like receptor 2 (TLR2)- and TLR4-dependent signalling pathways are involved in the regulation of intracellular signalling. A transwell chamber assay was used to evaluate the migration of osteoblasts and the MTT assay was used to assess osteoblast proliferation. rhHMGB1 could significantly promote the migration of osteoblasts without inhibiting their proliferation. Meanwhile, rhHMGB1 can increase the nuclear translocation of nuclear factor-kappa B (NF-κB) p65. Specific siRNA constructs that target TLR2 or TLR4 could markedly inhibit HMGB1-induced migration of osteoblasts and HMGB1-enhanced activation of NF-κB. Collectively, HMGB1 could significantly enhance the migration of osteoblasts in vitro, and TLR2/TLR4-dependent NF-κB pathways are involved in HMGB1-induced osteoblast migration.

Osteocytes and Skeletal Pathophysiology

For many years, osteocytes have been the forgotten bone cells and considered as inactive spectators buried in the bone matrix. We now know that osteocytes detect and respond to mechanical and hormonal stimuli to coordinate bone resorption and bone formation. Osteocytes are currently considered a major source of molecules that regulate the activity of osteoclasts and osteoblasts, such as RANKL and sclerostin; and genetic and pharmacological manipulations of either molecule markedly affect bone homeostasis. Besides playing a role in physiological bone homeostasis, accumulating evidence supports the notion that dysregulation of osteocyte function and alteration of osteocyte life-span underlies the pathophysiology of skeletal disorders characterized by loss bone mass and increased bone fragility, as well as the damaging effects of cancer in bone. In this review, we highlight some of these investigations and discuss novel observations that demonstrate that osteocytes, far from being passive cells entombed in the bone, are critical for bone function and maintenance.

Biology of Bone Tissue: Structure, Function, and Factors That Influence Bone Cells

Bone tissue is continuously remodeled through the concerted actions of bone cells, which include bone resorption by osteoclasts and bone formation by osteoblasts, whereas osteocytes act as mechanosensors and orchestrators of the bone remodeling process. This process is under the control of local (e.g., growth factors and cytokines) and systemic (e.g., calcitonin and estrogens) factors that all together contribute for bone homeostasis. An imbalance between bone resorption and formation can result in bone diseases including osteoporosis. Recently, it has been recognized that, during bone remodeling, there are an intricate communication among bone cells. For instance, the coupling from bone resorption to bone formation is achieved by interaction between osteoclasts and osteoblasts. Moreover, osteocytes produce factors that influence osteoblast and osteoclast activities, whereas osteocyte apoptosis is followed by osteoclastic bone resorption. The increasing knowledge about the structure and functions of bone cells contributed to a better understanding of bone biology. It has been suggested that there is a complex communication between bone cells and other organs, indicating the dynamic nature of bone tissue. In this review, we discuss the current data about the structure and functions of bone cells and the factors that influence bone remodeling.

The osteocyte plays multiple roles in bone remodeling and mineral homeostasis

Osteocytes are the most abundant cells in bone and are the major orchestrators of bone remodeling and mineral homeostasis. They possess a specialized cellular morphology and a unique molecular feature. Osteocytes are a stellate shape with numerous long, slender dendritic processes. The osteocyte cell body resides in the bone matrix of the lacuna and the dendritic processes extend within the canaliculi to adjacent osteocytes and other cells on the bone surface. Osteocytes form extensive intercellular network to sense and respond to environmental mechanical stimulus by the lacunar-canalicular system and gap junction. Osteocytes are long-lived bone cells. They can undergo apoptosis, which may have specific regulatory effects on osteoclastic bone resorption. Osteocytes can secrete several molecules, including sclerostin, receptor activator of nuclear factor κB ligand and fibroblast growth factor 23 to regulate osteoblastic bone formation, osteoclastic bone resorption and mineral homeostasis. A deeper understanding of the complex mechanisms that mediate the control of osteoblast and osteoclast function by osteocytes may identify new osteocyte-derived molecules as potential pharmacological targets for treating osteoporosis and other skeletal diseases.

Expression of HMGB1 in the periodontal tissue subjected to orthodontic force application by Waldo's method in mice

Recent studies indicate that high mobility group box protein 1 (HMGB1) originating from periodontal ligament (PDL) cells can be a potential regulator in the process of orthodontic tooth movement and periodontal tissue remodeling. The aim of this study is to investigate HMGB1 expression in periodontal tissue during orthodontic tooth movement in mice according to Waldo's method. Six 7-week-old C57BL6 mice were used in these experiments. The elastic band was inserted into the teeth space between the right first and second maxillary molars. After 3 days of mechanical loading, mice were fixed with transcardial perfusion of 4 % paraformaldehyde in 0.1 M phosphate buffer (pH 7.4), and the maxillary was extracted for histochemical analyses. The histological examination revealed local PDL tear at the tension side and the formation of extensive cell-free hyaline zones at the compression side. The immunolocalization of HMGB1 was significantly presented at tension side of PDL, apical area and dental pulp, whereas at the compression side of PDL, the labeling of HMGB1 was almost undetectable as the presence of hyaline zone. Taken together, we concluded that the orthodontic tooth movement by Waldo's method leads to histological changes and HMGB1 expression pattern that differ from those of coil spring method, including PDL tear and extensive hyaline zone which may severely destroy periodontal tissue and in turn impede tooth movement.

Anabolic properties of high mobility group box protein-1 in human periodontal ligament cells in vitro

High mobility group box protein-1 (HMGB1) is mainly recognized as a chemoattractant for macrophages in the initial phase of host response to pathogenic stimuli. However, recent findings provide evidence for anabolic properties in terms of enhanced proliferation, migration, and support of wound healing capacity of mesenchymal cells suggesting a dual role of the cytokine in the regulation of immune response and subsequent regenerative processes. Here, we examined potential anabolic effects of HMGB1 on human periodontal ligament (PDL) cells in the regulation of periodontal remodelling, for example, during orthodontic tooth movement. Preconfluent human PDL cells (hPDL) were exposed to HMGB1 protein and the influence on proliferation, migration, osteogenic differentiation, and biomineralization was determined by MTS assay, real time PCR, immunofluorescence cytochemistry, ELISA, and von Kossa staining. HMGB1 protein increased hPDL cell proliferation, migration, osteoblastic marker gene expression, and protein production as well as mineralized nodule formation significantly. The present findings support the dual character of HMGB1 with anabolic therapeutic potential that might support the reestablishment of the structural and functional integrity of the periodontium following periodontal trauma such as orthodontic tooth movement.

Bone and the innate immune system

The immune system and bone are intimately linked with significant physical and functionally related interactions. The innate immune system functions as an immediate response system to initiate protections against local challenges such as pathogens and cellular damage. Bone is a very specific microenvironment, in which infectious attack is less common but repair and regeneration are ongoing and important functions. Thus, in the bone the primary goal of innate immune and bone interactions is to maintain tissue integrity. Innate immune signals are critical for removal of damaged and apoptotic cells and to stimulate normal tissue repair and regeneration. In this review we focus on the innate immune mechanisms that function to regulate bone homeostasis.

Apoptotic osteocytes and the control of targeted bone resorption

Studies from the 1950s and 1960s already recognize the fact that osteocytes, although long living cells, die, as evidenced by accumulation of osteocytic lacunae devoid of cells. More recently, it was demonstrated that these cells die by apoptosis. The rate of osteocyte apoptosis is regulated by the age of the bone, as well as by systemic hormones, local growth factors, cytokines, pharmacological agents, and mechanical forces. Apoptotic osteocytes, in turn, recruit osteoclasts to initiate targeted bone resorption. This results in the removal of "dead" bone and may improve the mechanical properties of the skeleton. However, the molecular regulators of osteocyte survival and targeted bone remodeling are not completely known. In this review, the current knowledge on the molecular mechanism that lead to osteocyte death or survival, and the signals that mediate targeted bone resorption is discussed.

HMGB1 localization during experimental periodontitis

AIM

This study sought to investigate the in vitro expression profile of high mobility group box 1 (HMGB1) in murine periodontal ligament fibroblasts (mPDL) stimulated with LPS or IL-1β and in vivo during ligature- or LPS-induced periodontitis in rats.

MATERIAL AND METHODS

For the in vivo study, 36 rats were divided into experimental and control groups, and biopsies were harvested at 7-30 d following disease induction. Bone loss and inflammation were evaluated. HMGB1 expression was assessed by immunohistochemistry, qPCR, and Western blot.

RESULTS

Significant increases in mPDL HMGB1 mRNA occurred at 4, 8, and 12 h with protein expression elevated by 24 h. HMGB1 mRNA expression in gingival tissues was significantly increased at 15 d in the LPS-PD model and at 7 and 15 d in the ligature model. Immunohistochemical staining revealed a significant increase in the number of HMGB1-positive cells during the experimental periods.

CONCLUSION

The results show that PDL cells produce HMGB1, which is increased and secreted extracellularly after inflammatory stimuli. In conclusion, this study demonstrates that HMGB1 may be associated with the onset and progression of periodontitis, suggesting that further studies should investigate the potential role of HMGB1 on periodontal tissue destruction.

Osteocyte-driven bone remodeling

Osteocytes, the most abundant cells in bone, have been long postulated to detect and respond to mechanical and hormonal stimuli and to coordinate the function of osteoblasts and osteoclasts. The discovery that the inhibitor of bone formation sclerostin is primarily expressed in osteocytes in bone and downregulated by anabolic stimuli provided a mechanism by which osteocytes influence the activity of osteoblasts. Advances of the last few years provided experimental evidence demonstrating that osteocytes also participate in the recruitment of osteoclasts and the initiation of bone remodeling. Apoptotic osteocytes trigger yet-to-be-identified signals that attract osteoclast precursors to specific areas of bone, which in turn differentiate to mature, bone-resorbing osteoclasts. Osteocytes are also the source of molecules that regulate the generation and activity of osteoclasts, such as OPG and RANKL; and genetic manipulations of the mouse genome leading to loss or gain of function or to altered expression of either molecule in osteocytes markedly affect bone resorption. This review highlights these investigations and discusses how the novel concept of osteocyte-driven bone resorption and formation impacts our understanding of the mechanisms by which current therapies control bone remodeling.