The role of osteoprotegerin (OPG) in fibrosis: its potential as a biomarker and/or biological target for the treatment of fibrotic diseases.

Fibrosis is defined by excessive formation and accumulation of extracellular matrix proteins, produced by myofibroblasts, that supersedes normal wound healing responses to injury and results in progressive architectural remodelling. Fibrosis is often detected in advanced disease stages when an organ is already severely damaged and can no longer function properly. Therefore, there is an urgent need for reliable and easily detectable markers to identify and monitor fibrosis onset and progression as early as possible; this will greatly facilitate the development of novel therapeutic strategies. Osteoprotegerin (OPG), a well-known regulator of bone extracellular matrix and most studied for its role in regulating bone mass, is expressed in various organs and functions as a decoy for receptor activator of nuclear factor kappa-B ligand (RANKL) and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Recently, OPG has been linked to fibrosis and fibrogenesis, and has been included in a panel of markers to diagnose liver fibrosis. Multiple studies now suggest that OPG may be a general biomarker suitable for detection of fibrosis and/or monitoring the impact of fibrosis treatment. This review summarizes our current understanding of the role of OPG in fibrosis and will discuss its potential as a biomarker and/or novel therapeutic target for fibrosis.

microRNA-143-3p regulates odontogenic differentiation of human dental pulp stem cells through regulation of the osteoprotegerin-RANK ligand pathway by targeting RANK.

NEW FINDINGS: What is the central question of this study? What is the role of miR-143-3p during human dental pulp stem cell (hDPSC) differentiation. What is the main finding and its importance? miR-143-3p negatively regulates receptor activator of nuclear factor-κB (RANK). RANK ligand (RANKL) binds to RANK and stimulates the development of osteoclasts. Osteoprotegerin (OPG) inhibits the interaction between RANK and RANKL. The OPG-RANKL signalling pathway regulates odontogenic differentiation of hDPSCs. ABSTRACT: Human dental pulp stem cells (hDPSCs) are capable of differentiating into odontoblast-like cells, which secrete reparative dentin after injury, in which the role of microRNA-143-3p (miR-143-3p) has been identified. Therefore, we investigated the mechanism by which miR-143-3p influences odontoblastic differentiation of hDPSCs. The relationship between miR-143-3p and receptor activator of nuclear factor-κB (RANK) was initially identified by bioinformatics prediction and further verified by dual luciferase reporter gene assay. Gain- and loss-of-function analysis with miR-143-3p mimic and miR-143-3p inhibitor was subsequently conducted. Dentin sialophosphoprotein (DSPP), bone sialoprotein (BSP), alkaline phosphatase (ALP), osteocalcin (OCN) and osteopontin (OPN) mRNA levels were then evaluated by RT-qPCR. Osteoprotegerin (OPG), RANK ligand (RANKL), nuclear factor-κB (NF-κB) p65 protein levels and the extent of NF-κB p65 phosphorylation were examined by western blot analysis. Alizarin red staining was performed to assess the mineralization of hDPSCs. Cell apoptosis and cell cycle distribution were determined using flow cytometry. During odontoblastic differentiation of hDPSC, miR-143-3p had high expression, but RANK expression was low. miR-143-3p was found to target RANK, and its inhibition enhanced mineralization and hDPSC apoptosis, while blocking cell cycle entry. At the same time, miR-143-3p inhibition elevated the extent of NF-κB p65 phosphorylation, as well as the expression of RANK, RANKL, DSPP, BSP, ALP, OCN and OPN, while decreasing the OPG level. Silencing RANK had opposite effects on these markers. miR-143-3p regulates odontoblastic differentiation of hDPSCs via the OPG-RANKL pathway that targets RANK. The elucidation of the mechanisms of odontogenic differentiation of hDPSCs may contribute to the development of effective dental pulp repair therapies for the clinical setting.

Muscle weakness and selective muscle atrophy in osteoprotegerin-deficient mice.

Bone and muscle are tightly coupled and form a functional unit under normal conditions. The receptor-activator of nuclear factor κB/receptor-activator of nuclear factor κB ligand/osteoprotegerin (RANK/RANKL/OPG) triad plays a crucial role in bone remodeling. RANKL inhibition by OPG prevents osteoporosis. In contrast, the absence of OPG results in elevated serum RANKL and early onset osteoporosis. However, the impacts of OPG deletion on muscle structure and function are unknown. Our results showed that 1-, 3- and 5-month-old Opg-/- mice have reduced tibial and femoral bone biomechanical properties and higher levels of circulating RANKL. OPG-deficient mice displayed reduced locomotor activity and signs of muscle weakness at 5 months of age. Furthermore, OPG deficiency did not affect the skeletal muscles in 1- and 3-month-old mice. However, it impaired fast-twitch EDL but not slow-twitch Sol muscles in 5-month-old Opg-/- mice. Moreover, 5-month-old Opg-/- mice exhibited selective atrophy of fast-twitch-type IIb myofibers, with increased expression of atrophic proteins such as NF-kB, atrogin-1 and MuRF-1. We used an in vitro model to show that RANKL-stimulated C2C12 myotubes significantly increased the expression of NF-kB, atrogin-1 and MuRF-1. A 2-month anti-RANKL treatment starting at 3 months of age in Opg-/- mice improved voluntary activity, the ex vivo maximum specific force (sP0) of EDL muscles, and whole limb grip force performance and rescued the biomechanical properties of bone. In conclusion, the deletion of OPG and the disruption of the RANKL/OPG balance induced osteoporosis as well as the selective weakness and atrophy of the powerful fast-twitch IIb myofibers, which was partly alleviated by an anti-RANKL treatment.

Fluid Shear Stress Increases Osteocyte and Inhibits Osteoclasts via Downregulating Receptor-Activator of Nuclear Factor κB (RANK)/Osteoprotegerin Expression in Myeloma Microenvironment.

BACKGROUND The aim of this study was to determine the effects of myeloma cells exposed to fluid shear stress on osteocytes and osteoclasts, and clarify the potential underlying mechanisms. MATERIAL AND METHODS A flow and a non-flow model were established using a flow fluid chamber. The myeloma cell line U266 and murine osteocytic MLO-Y4 cells were cultured in vitro. The osteocytes and osteoclasts were examined under a microscope. Osteoclasts were stained for tartrate-resistant acid phosphatase (TRAP) activity. RANKL and osteoprotegerin (OPG) gene expression were detected using reverse transcription-quantitative polymerase chain reaction. RESULTS Compared with the controls, Y4 cells cultured with U266 culture supernatant showed altered morphology, fewer osteocytes, increased RANKL gene expression, a higher RANKL/OPG gene ratio, and a greater number of TRAP-positive osteoclasts (P<0.05 for all). Compared to the no-flow model, the flow model showed a higher number of Y4 cells, increased OPG gene expression, decreased RANKL gene expression, a lower RANKL/OPG gene ratio, and fewer TRAP-positive osteoclasts (P<0.05 for all). CONCLUSIONS Our study revealed that fluid shear stress ameliorated the inhibitory effects of myeloma cells on osteocyte growth and inhibited osteoclast proliferation by means of decreasing RANKL/OPG gene expression. This may have clinical implications in patients with multiple myeloma in that mechanical loading with low-intensity vibration or mild exercise may prevent the progression of myeloma bone disease.

The Role of Osteoprotegerin in Vascular Calcification and Bone Metabolism: The Basis for Developing New Therapeutics.

Osteoporosis (OP) and cardiovascular diseases (CVD) are both important causes of mortality and morbidity in aging patients. There are common mechanisms underlying the regulation of bone remodeling and the development of smooth muscle calcification; a temporal relationship exists between osteoporosis and the imbalance of mineral metabolism in the vessels. Vascular calcification appears regulated by mechanisms that include both inductive and inhibitory processes. Multiple factors are implicated in both bone and vascular metabolism. Among these factors, the superfamily of tumor necrosis factor (TNF) receptors including osteoprotegerin (OPG) and its ligands has been established. OPG is a soluble decoy receptor for receptor activator of nuclear factor-kB ligand (RANKL) and TNF-related apoptosis-inducing ligand (TRAIL). OPG binds to RANKL and TRAIL, and inhibits the association with their receptors, which have been labeled as the receptor activator of NF-kB (RANK). Sustained release of OPG from vascular endothelial cells (ECs) has been demonstrated in response to inflammatory proteins and cytokines, suggesting that OPG/RANKL/RANK system plays a modulatory role in vascular injury and inflammation. For the development of potential therapeutic strategies targeting vascular calcification, critical consideration of the implications for bone metabolism must be taken into account to prevent potentially detrimental effects to bone metabolism.

[Progress of research on the relationship between calcitonin gene-related peptide and RANK/RANKL/OPG system in the bone reconstruction].

OBJECTIVE: To summarize the research progress on the calcitonin gene-related peptide (CGRP) and receptor activator of nuclear factor κB (RANK)/receptor activator of nuclear factor κB ligand (RANKL)/osteoprotegerin (OPG) system during bone reconstruction to provide theoretical basis for further research on the prevention and treatment of bone-related diseases. METHODS: The relevant research results at home and abroad in recent years were analyzed and summarized. RESULTS: CGRP and RANK/RANKL/OPG system play important regulatory roles in the bone reconstruction. CONCLUSION: At present, the research on the mechanism of CGRP and RANK/RANKL/OPG system in bone reconstruction is insufficient. Therefore, it is necessary to study further on the process and interrelation of CGRP and RANK/RANKL/OPG system in bone reconstruction to confirm their mechanism, which will bring new ideas and methods for the treatment of bone related diseases in clinic.

[Association between osteoporosis and atherosclerosis in dyslipidemia.]

Age-related osteoporosis and atherosclerosis is promoted by life style-related diseases such as dyslipidemia and diabetes mellitus. Common factors pathophysiologically involved in both osteoporosis and vascular calcification include senescent cells and osteoprotegerin(OPG). Dyslipidemia may impair both osteoclast and osteoblast function,thereby causing osteoporosis. Statins may have favorable effect on bone. Some anti-osteoporotic medications have also been suggested to show protective effect from atherosclerosis.

Dynamics of Bone Cell Interactions and Differential Responses to PTH and Antibody-Based Therapies.

We propose a mathematical model describing the dynamics of osteoblasts and osteoclasts in bone remodeling. The goal of this work is to develop an integrated modeling framework for bone remodeling and bone cell signaling dynamics that could be used to explore qualitatively combination treatments for osteoporosis in humans. The model has been calibrated using 57 checks from the literature. Specific global optimization methods based on qualitative objectives have been developed to perform the model calibration. We also added pharmacokinetics representations of three drugs to the model, which are teriparatide (PTH(1-34)), denosumab (a RANKL antibody) and romosozumab (a sclerostin antibody), achieving excellent goodness-of-fit of human clinical data. The model reproduces the paradoxical effects of PTH on the bone mass, where continuous administration of PTH results in bone loss but intermittent administration of PTH leads to bone gain, thus proposing an explanation of this phenomenon. We used the model to simulate different categories of osteoporosis. The main attributes of each disease are qualitatively well captured by the model, for example changes in bone turnover in the disease states. We explored dosing regimens for each disease based on the combination of denosumab and romosozumab, identifying adequate ratios and doses of both drugs for subpopulations of patients in function of categories of osteoporosis and the degree of severity of the disease.

Notch Signaling Pathway in Apical Periodontitis: Correlation with Bone Resorption Regulators and Proinflammatory Cytokines.

INTRODUCTION: The exact mechanisms of periapical bone resorption have not been fully elucidated. This study aimed to analyze the expression of Notch signaling molecules (Notch2, Jagged1, and Hey1) and proinflammatory cytokines (tumor necrosis factor alpha [TNF-α], interleukin [IL]-1ß, and IL-6) in human apical periodontitis lesions with different receptor activator of nuclear factor kappa B ligand (RANKL)/osteoprotegerin (OPG) ratios and determine their potential correlation. METHODS: The study group consisted of 50 periapical lesions collected in conjunction with apicoectomy. The relative gene expression of the investigated molecules (Notch2, Jagged1, Hey1, RANKL, OPG, TNF-α, IL-1ß, and IL-6) in all tissue samples was analyzed using reverse transcriptase real-time polymerase chain reaction. The Student t test, Mann-Whitney U test, and Spearman correlation were used for statistical analysis. RESULTS: Based on the RANKL/OPG ratio, periapical lesions were either RANKL predominant (RANKL > OPG, n = 33) or OPG predominant (RANKL < OPG, n = 17). Symptomatic lesions occurred more frequently in RANKL-predominant compared with OPG-predominant lesions (24 vs 7, P = .029). Notch2, Jagged1, Hey1, and TNF-α were significantly overexpressed in lesions with predominant RANKL compared with lesions with predominant OPG (P = .001, P = .001, P = .027, and P = .016, respectively). Significant correlations were observed between the investigated genes in periapical lesions. CONCLUSIONS: Notch signaling appeared to be activated in periapical inflammation. An increase in Notch2, Jagged1, Hey1, and TNF-α expression in RANKL-predominant periapical lesions corroborates their joined involvement in extensive periapical bone resorption.

[Human osteoprotegerin inhibits osteoclasts and promotes hydroxyapatite to repair the mandibular defects in ovariectomized rats].

OBJECTIVE: This study aims to investigate the effect of human osteoprotegerin (hOPG) gene-modified rat bone marrow mesenchymal stem cells (rBMSCs) combined with hydroxyapatite (HA) scaffolds on the repair of mandibular defects in ovariectomized rats. METHODS: rBMSCs were transfected with adenovirus carrying pDC316-hOPG-EGFP. The expression of hOPG and the inhibition of osteoclast function were detected by Western blot and bone-grinding experiment respectively. The model of mandibular bone defect in rats with osteoporosis was established; HA, untransfected rBMSCs-conjugated HA, and transfected rBMSCs-conjugated HA scaffolds were implanted into the mandibular bone defects. After six weeks, tartrateresistant acid phosphatase staining and hematoxylin-eosin staining were used to observe the number of osteoclasts and repair of bone defect. RESULTS: Adenovirus carrying hOPG gene in vitro were successfully transfected into rBMSCs. The hOPG with anti-osteoclast activity was expressed by hOPG-rBMSCs, and rBMSCs expressing hOPG combined with HA scaffolds promoted mandibular defect repair. CONCLUSIONS: rBMSCs transfected with hOPG gene inhibited the function of osteoclasts both in vitro and in vivo, and transfected rBMSCs combined with HA scaffolds promoted the repair of mandibular defects in rats with osteoporosis.

The Role of Osteoprotegerin as a Cardioprotective Versus Reactive Inflammatory Marker: the Chicken or the Egg Paradox

Cardiovascular disease is one of the most frequent causes of mortality and morbidity worldwide. Several variables have been identified as risk factors for cardiovascular disease. Recently, the role of receptor activator of nuclear factor kappa B, receptor activator of nuclear factor kappa B ligand, and the osteoprotegerin system has been recognized as more important in the pathogenesis of cardiovascular disease. Besides their roles in the regulation of bone resorption, these molecules have been reported to be associated with the pathophysiology of cardiovascular disease. There are conflicting data regarding the impact of osteoprotegerin, a glycoprotein with a regulatory role in the cardiovascular system. The aim of this review is to discuss the current knowledge and the role of osteoprotegerin in cardiovascular disease.

Osteoprotegerin deficiency causes morphological and quantitative damage in epithelial rests of Malassez.

Epithelial rests of Malassez (ERM), the only odontogenic epithelial structures in periodontal tissue, are proposed to correlate with root resorption, but the detailed mechanism remains unclear. Osteoprotegerin (OPG), the main inhibitor of osteoclastogenesis, plays a pivotal role in inhibiting root resorption, and ERM cells express OPG mRNA in vitro. Thus, in this study, we aimed to clarify OPG expression in ERM in vivo and to explore the role of OPG in ERM to determine whether ERM are associated with root resorption via OPG. We established Opg-knockout (Opg-KO) mice and detected the OPG expression in ERM by immunohistochemical staining in 4-, 6-, 10-, 26- and 52-week-old mice. The ERM of wild-type (WT) mice and Opg-KO mice were evaluated histologically at 4, 10 and 26 weeks of age. Orthodontic root resorption models were established, maxillae were collected after 4 weeks, and ERM were analysed by histomorphometric analysis. In our study, OPG displayed sustained expression in ERM, and OPG deficiency caused the destruction of ERM, characterized by irregular morphology and reduced numbers. Moreover, after orthodontic treatment, the loss of OPG severely damaged ERM, aggravating root resorption. Together, our results demonstrated that ERM expressed the OPG protein in vivo and that OPG deficiency resulted in morphological and quantitative damage to ERM. Furthermore, ERM may be associated with root resorption via OPG, thus helping to explain the mechanism underlying root resorption.

Cardiovascular Autonomic Dysfunction: Link Between Multiple Sclerosis Osteoporosis and Neurodegeneration.

The high prevalence of osteoporosis, observed in multiple sclerosis (MS) patients, has been attributed to reduced mobility and or the use of disease-modifying drugs. However, MS-impaired cardiovascular autonomic nervous system (ANS) function has the potential of reducing bone mass density (BMD) by altering the expression and/or function of the neuronal, systemic, and local mediators of bone remodeling. This review describes the complex regulation of bone homeostasis with a focus on MS, providing evidence that ANS dysfunction and low BMD are intertwined with MS inflammatory and neurodegenerative processes, and with other MS-related morbidities, including depression, fatigue, and migraine. Strategies for improving ANS function could reduce the prevalence of MS osteoporosis and slow the rate of MS progression, with a significant positive impact on patients' quality of life.

Mechanisms involved in bone resorption regulated by vitamin D.

Active forms of vitamin D enhance osteoclastogenesis in vitro and in vivo through the vitamin D receptor (VDR) in osteoblast-lineage cells consisting of osteoblasts and osteocytes. This pro-resorptive activity was evident basically with higher concentrations of active vitamin D than those expected in physiological conditions. Nevertheless, vitamin D compounds have been used in Japan for treating osteoporosis to increase bone mineral density (BMD). Of note, the increase in BMD by long-term treatment with pharmacological (=near-physiological) doses of vitamin D compounds was caused by the suppression of bone resorption. Therefore, whether vitamin D expresses pro-resorptive or anti-resorptive properties seems to be dependent on the treatment protocols. We established osteoblast lineage-specific and osteoclast-specific VDR conditional knockout (cKO) mice using Osterix-Cre transgenic mice and Cathepsin K-Cre knock-in mice, respectively. According to our observation using these cKO mouse lines, neither VDR in osteoblast-lineage cells nor that in osteoclasts played important roles for osteoclastogenesis and bone resorption at homeostasis. However, using our cKO lines, we observed that VDR in osteoblast-lineage cells, but not osteoclasts, was involved in the anti-resorptive properties of pharmacological doses of vitamin D compounds in vivo. Two different osteoblast-lineage VDR cKO mouse lines were reported. One is a VDR cKO mouse line using alpha 1, type I collagen (Col1a1)-Cre transgenic mice (here we call Col1a1-VDR-cKO mice) and the other is that using dentin matrix protein 1 (Dmp1)-Cre transgenic mice (Dmp1-VDR-cKO mice). Col1a1-VDR-cKO mice exhibited slightly increased bone mass due to lowered bone resorption. In contrast, Dmp1-VDR-cKO mice exhibited no difference in BMD in agreement with our results regarding Ob-VDR-cKO mice. Here we discuss contradictory results and multiple modes of actions of vitamin D in bone resorption in detail. (279 words).

Supraphysiological loading induces osteocyte-mediated osteoclastogenesis in a novel in vitro model for bone implant loosening.

We aimed to develop an in vitro model for bone implant loosening, allowing analysis of biophysical and biological parameters contributing to mechanical instability-induced osteoclast differentiation and peri-implant bone loss. MLO-Y4-osteocytes were mechanically stimulated for 1 h by fluid shear stress using regimes simulating: (i) supraphysiological loading in the peri-prosthetic interface (2.9 ± 2.9 Pa, 1 Hz, square wave); (ii) physiologic loading in the cortical bone (0.7 ± 0.7 Pa, 5 Hz, sinusoidal wave); and (iii) stress shielding. Cellular morphological parameters, membrane-bound RANKL expression, gene expression influencing osteoclast differentiation, nitric oxide release and caspase 3/7-activity were determined. Either Mouse bone marrow cells were cultured on top of loaded osteocytes or osteocyte-conditioned medium was added to bone marrow cells. Osteoclast differentiation was assessed after 6 days. We found that osteocytes subjected to supraphysiological loading showed similar morphology and caspase 3/7-activity compared to simulated physiological loading or stress shielding. Supraphysiological stimulation of osteocytes enhanced osteoclast differentiation by 1.9-fold compared to physiological loading when cell-to-cell contact was permitted. In addition, it enhanced the number of osteoclasts using conditioned medium by 1.7-fold, membrane-bound RANKL by 3.3-fold, and nitric oxide production by 3.2-fold. The stimulatory effect of supraphysiological loading on membrane-bound RANKL and nitric oxide production was higher than that achieved by stress shielding. In conclusion, the in vitro model developed recapitulated the catabolic biological situation in the peri-prosthetic interface during instability that is associated with osteoclast differentiation and enhanced RANKL expression. The model thus provides a platform for pre-clinical testing of pharmacological interventions with potential to stop instability-induced bone implant loosening. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1425-1434, 2018.

The role of osteoprotegerin in the crosstalk between vessels and bone: Its potential utility as a marker of cardiometabolic diseases.

Among the numerous molecules that are being studied for their potential utility as biomarkers of cardiovascular diseases, much interest has been shown in the superfamily of tumor necrosis factor (TNF) receptors. Members of this family include osteoprotegerin (OPG) and its ligands, which are receptor activators of nuclear factor κB ligand (RANKL) and TNF-related apoptosis-inducing ligand (TRAIL). These signals may be expressed and regulated, and their functions could be involved in several physiological and pathological processes. The relationship between bone regulatory proteins and vascular biology has attracted attention, and it has been suggested that OPG may mediate vascular calcification and cardiometabolic diseases. OPG is steadily released from vascular endothelial cells in response to inflammatory stimuli, suggesting that it plays a modulatory role in vascular injury, inflammation, and atherosclerosis. Vascular calcification, a hallmark of atherosclerosis, is similar to bone remodeling. It is an actively regulated mechanism that includes both inductive and inhibitory processes. There is a temporal link between the development of osteoporosis and vascular calcification, which is particularly marked in post-menopausal women and the elderly. The precise nature of the link between bone metabolism, vascular calcification and cardiovascular disease is largely unknown but increasing evidence suggests that the triad of RANK/RANKL/OPG may be important in the initiation of various diseases. An increased release of OPG is associated with increased cardiovascular risk and it is suggested that increased OPG levels resulting from vascular damage correspond to a protective mechanism. Circulating OPG levels could be used as independent biomarkers of cardiovascular disease in patients with acute or chronic cardiometabolic disease and thus an improved prognosis.

Acute ketamine administration corrects abnormal inflammatory bone markers in major depressive disorder.

Patients with major depressive disorder (MDD) have clinically relevant, significant decreases in bone mineral density (BMD). We sought to determine if predictive markers of bone inflammation-the osteoprotegerin (OPG)-RANK-RANKL system or osteopontin (OPN)-play a role in the bone abnormalities associated with MDD and, if so, whether ketamine treatment corrected the abnormalities. The OPG-RANK-RANKL system plays the principal role in determining the balance between bone resorption and bone formation. RANKL is the osteoclast differentiating factor and diminishes BMD. OPG is a decoy receptor for RANKL, thereby increasing BMD. OPN is the bone glue that acts as a scaffold between bone tissues matrix composition to bind them together and is an important component of bone strength and fracture resistance. Twenty-eight medication-free inpatients with treatment-resistant MDD and 16 healthy controls (HCs) participated in the study. Peripheral bone marker levels and their responses to IV ketamine infusion in MDD patients and HCs were measured at four time points: at baseline, and post-infusion at 230 min, Day 1, and Day 3. Patients with MDD had significant decreases in baseline OPG/RANKL ratio and in plasma OPN levels. Ketamine significantly increased both the OPG/RANKL ratio and plasma OPN levels, and significantly decreased RANKL levels. Bone marker levels in HCs remained unaltered. We conclude that the OPG-RANK-RANKL system and the OPN system play important roles in the serious bone abnormalities associated with MDD. These data suggest that, in addition to its antidepressant effects, ketamine also has a salutary effect on a major medical complication of depressive illness.

Effects of Cyclosporine, Tacrolimus, and Rapamycin on Osteoblasts.

PURPOSE: One factor that can contribute to severe bone loss after transplantation is the direct action of immunosuppressants on bone cells. The aim of this work was to study the effects of cyclosporine (CsA), tacrolimus (FK-506), and rapamycin (RAPA) on the release of three local factors directly implicated in bone-remodeling regulation and apoptosis of human osteoblasts: interleukin (IL)-6, osteoprotegerin, and receptor activator of nuclear factor κß (RANKL). BASIC PROCEDURES: Human osteoblasts were obtained from five different patients who underwent orthopedic surgery. These cells were treated with what are considered to be a clinically high dose and an acceptable dose of each immunosuppressant-RAPA 50 ng/mL and 12 ng/mL, FK-506 20 ng/mL and 5 ng/mL, CsA 1000 ng/mL and 250 ng/mL-or vehicle. Apoptotic cell death was quantified using flow cytometry of DNA content in permeabilized, propidium iodide-stained cells. IL-6 was measured using enzyme-linked immunosorbent assay (ELISA; Quantikine Human IL6, R&D Systems, Minneapolis, Minn, United States). Messenger RNA (mRNA) expression of osteoprotegerin, RANKL, and IL-6 was measured using quantitative RT-PCR. MAIN FINDINGS: A significant increase in IL-6 (mRNA and released protein) was observed in the presence of FK-506 and RAPA. Addition of RAPA to the cultures of osteoblasts produced a significant increase in the OPG/RANKL ratio. A significant increase in osteoblast apoptosis was observed in the cells treated with FK-506 and RAPA 24 hours after the addition of immunosuppressants. CsA did not produce any significant changes in osteoblasts. PRINCIPAL CONCLUSIONS: These results suggest that an increase in osteoblast apoptosis by osteoblasts may be one of the mechanisms by which bone loss occurs after RAPA and FK-506 treatments.

Microfluidic co-culture platform for investigating osteocyte-osteoclast signalling during fluid shear stress mechanostimulation.

Bone cells exist in a complex environment where they are constantly exposed to numerous dynamic biochemical and mechanical stimuli. These stimuli regulate bone cells that are involved in various bone disorders, such as osteoporosis. Knowledge of how these stimuli affect bone cells have been utilised to develop various treatments, such as pharmaceuticals, hormone therapy, and exercise. To investigate the role that bone loading has on these disorders in vitro, bone cell mechanotransduction studies are typically performed using parallel plate flow chambers (PPFC). However, these chambers do not allow for dynamic cellular interactions among different cell populations to be investigated. We present a microfluidic approach that exposes different cell populations, which are located at physiologically relevant distances within adjacent channels, to different levels of fluid shear stress, and promotes cell-cell communication between the different channels. We employed this microfluidic system to assess mechanically regulated osteocyte-osteoclast communication. Osteoclast precursors (RAW264.7 cells) responded to cytokine gradients (e.g., RANKL, OPG, PGE-2) developed by both mechanically stimulated (fOCY) and unstimulated (nOCY) osteocyte-like MLO-Y4 cells simultaneously. Specifically, we observed increased osteoclast precursor cell densities and osteoclast differentiation towards nOCY. We also used this system to show an increased mechanoresponse of osteocytes when in co-culture with osteoclasts. We envision broad applicability of the presented approach for microfluidic perfusion co-culture of multiple cell types in the presence of fluid flow stimulation, and as a tool to investigate osteocyte mechanotransduction, as well as bone metastasis extravasation. This system could also be applied to any multi-cell population cross-talk studies that are typically performed using PPFCs (e.g. endothelial cells, smooth muscle cells, and fibroblasts).

Cementogenesis is inhibited under a mechanical static compressive force via Piezo1.

OBJECTIVE: To investigate whether Piezo1, a mechanotransduction gene mediates the cementogenic activity of cementoblasts under a static mechanical compressive force. MATERIALS AND METHODS: Murine cementoblasts (OCCM-30) were exposed to a 2.0 g/cm2 static compressive force for 3, 6, 12, and 24 hours. Then the expression profile of Piezo1 and the cementogenic activity markers osteoprotegerin (Opg), osteopontin (Opn), osteocalcin (Oc), and protein tyrosine phosphataselike member A (Ptpla) were analyzed. Opg, Opn, Oc, and Ptpla expression was further measured after using siRNA to knock down Piezo1. Real-time PCR, Western blot, and cell proliferation assays were performed according to standard procedures. RESULTS: After mechanical stimulation, cell morphology and proliferation did not change significantly. The expression of Piezo1, Opg, Opn, Oc, and Ptpla was significantly decreased, with a high positive correlation between Opg and Piezo1 expression. After Piezo1 knockdown, the expression of Opg, Opn, Oc, and Ptpla was further decreased under mechanical stimulation. CONCLUSIONS: Cementogenic activity was inhibited in OCCM-30 cells under static mechanical force, a process that was partially mediated by the decrease of Piezo1. This study provides a new viewpoint of the pathogenesis mechanism of orthodontically induced root resorption and repair.