Mechanical Induction of BMP-7 in Osteocyte Blocks Glucocorticoid-Induced Apoptosis Through PI3K/AKT/GSK3β Pathway

Fluid shear stress-mediated Piezo1 alleviates osteocyte apoptosis by activating the PI3K/Akt pathway

Glucocorticoid-induced osteoporosis serves as a primary cause for secondary osteoporosis and fragility fractures, representing the most prevalent adverse reaction associated with prolonged glucocorticoid use. In this study, to elucidate the impact and underlying mechanisms of fluid shear stress (FSS)-mediated Piezo1 on dexamethasone (Dex)-induced apoptosis, we respectively applied Dex treatment for 6 h, FSS at 9 dyne/cm2 for 30 min, Yoda1 treatment for 2 h, and Piezo1 siRNA transfection to intervene in MLO-Y4 osteocytes. Western blot analysis was used to assess the expression of Cleaved Caspase-3, Bax, Bcl-2, and proteins associated with the PI3K/Akt pathway. Additionally, qRT-PCR was utilized to quantify the mRNA expression levels of these molecules. Hoechst 33258 staining and flow cytometry were utilized to evaluate the apoptosis levels. The results indicate that FSS at 9 dyne/cm2 for 30 min significantly upregulates Piezo1 in osteocytes. Following Dex-induced apoptosis, the phosphorylation levels of PI3K and Akt are markedly suppressed. FSS-mediated Piezo1 exerts a protective effect against Dex-induced apoptosis by activating the PI3K/Akt pathway. Additionally, downregulating the expression of Piezo1 in osteocytes using siRNA exacerbates Dex-induced apoptosis. To further demonstrate the role of the PI3K/Akt signaling pathway, after intervention with the PI3K pathway inhibitor, the activation of the PI3K/Akt pathway by FSS-mediated Piezo1 in osteocytes was significantly inhibited, reversing the anti-apoptotic effect. This study indicates that under FSS, Piezo1 in MLO-Y4 osteocytes is significantly upregulated, providing protection against Dex-induced apoptosis through the activation of the PI3K/Akt pathway.

Dexamethasone Induced Osteocyte Apoptosis in Steroid-Induced Femoral Head Osteonecrosis through ROS-Mediated Oxidative Stress

OBJECTIVE

Glucocorticoid (GC) overuse is strongly associated with steroid-induced osteonecrosis of the femoral head (SINFH). However, the underlying mechanism of SINFH remains unclear. This study aims to investigate the effect of dexamethasone (Dex)-induced oxidative stress on osteocyte apoptosis and the underlying mechanisms.

METHODS

Ten patients with SINFH and 10 patients with developmental dysplasia of the hips (DDH) were enrolled in our study. Sixty rats were randomly assigned to the Control, Dex, Dex + N-Acetyl-L-cysteine (NAC), Dex + Dibenziodolium chloride (DPI), NAC, and DPI groups. Magnetic resonance imaging (MRI) was used to examine edema in the femoral head of rats. Histopathological staining was performed to assess osteonecrosis. Immunofluorescence staining with TUNEL and 8-OHdG was conducted to evaluate osteocyte apoptosis and oxidative damage. Immunohistochemical staining was carried out to detect the expression of NOX1, NOX2, and NOX4. Viability and apoptosis of MLO-Y4 cells were measured using the CCK-8 assay and TUNEL staining. 8-OHdG staining was conducted to detect oxidative stress. 2',7'-Dichlorodihydrofluorescein diacetate (DCFH-DA) staining was performed to measure reactive oxygen species (ROS). The expression of NOX1, NOX2, and NOX4 in MLO-Y4 cells was analyzed by Western blotting. Multiple comparisons were performed using one-way analysis of variance (ANOVA).

RESULTS

In patients and the rat model, hematoxylin-eosin (HE) staining revealed a significantly higher rate of empty lacunae in the SINFH group than in the DDH group. Immunofluorescence staining indicated a significant increase in TUNEL-positive cells and 8-OHdG-positive cells in the SINFH group compared to the DDH group. Immunohistochemical staining demonstrated a significant increase in the expression of NOX1, NOX2, and NOX4 proteins in SINFH patients compared to DDH patients. Moreover, immunohistochemical staining showed a significant increase in the proportion of NOX2-positive cells compared to the Control group in the femoral head of rats. In vitro, Dex significantly inhibited the viability of osteocyte cells and induced apoptosis. After Dex treatment, the intracellular ROS level increased. However, Dex treatment did not alter the expression of NOX proteins in vitro. Additionally, NAC and DPI inhibited the generation of intracellular ROS and partially alleviated osteocyte apoptosis in vivo and in vitro.

CONCLUSION

This study demonstrates that GC promotes apoptosis of osteocyte cells through ROS-induced oxidative stress. Furthermore, we found that the increased expression of NOXs induced by GC serves as an important source of ROS generation.

Enhancement of Bone Regeneration Through the Converse Piezoelectric Effect, A Novel Approach for Applying Mechanical Stimulation

Serious bone injuries have devastating effects on the lives of patients including limiting working ability and high cost. Orthopedic implants can aid in healing injuries to an extent that exceeds the natural regenerative capabilities of bone to repair fractures or large bone defects. Autografts and allografts are the standard implants used, but disadvantages such as donor site complications, a limited quantity of transplantable bone, and high costs have led to an increased demand for synthetic bone graft substitutes. However, replicating the complex physiological properties of biological bone, much less recapitulating its complex tissue functions, is challenging. Extensive efforts to design biocompatible implants that mimic the natural healing processes in bone have led to the investigation of piezoelectric smart materials because the bone has natural piezoelectric properties. Piezoelectric materials facilitate bone regeneration either by accumulating electric charge in response to mechanical stress, which mimics bioelectric signals through the direct piezoelectric effect or by providing mechanical stimulation in response to electrical stimulation through the converse piezoelectric effect. Although both effects are beneficial, the converse piezoelectric effect can address bone atrophy from stress shielding and immobility by improving the mechanical response of a healing defect. Mechanical stimulation has a positive impact on bone regeneration by activating cellular pathways that increase bone formation and decrease bone resorption. This review will highlight the potential of the converse piezoelectric effect to enhance bone regeneration by discussing the activation of beneficial cellular pathways, the properties of piezoelectric biomaterials, and the potential for the more effective administration of the converse piezoelectric effect using wireless control.

Does Physical Exercise Always Improve Bone Quality in Rats?

For decades, the osteogenic effect from different physical activities on bone in rodents remained uncertain. This literature review presents for the first time the effects on five exercise models (treadmill running, wheel running, swimming, resistance training and vibration modes) in three different experimental rat groups (males, females, osteopenic) on bone quality. The bone parameters presented are bone mineral density, micro-architectural and mechanical properties, and osteoblast/osteocyte and osteoclast parameters. This review shows that physical activities have a positive effect (65% of the results) on bone status, but we clearly observed a difference amongst the different protocols. Even if treadmill running is the most used protocol, the resistance training constitutes the first exercise model in term of osteogenic effects (87% of the whole results obtained on this model). The less osteogenic model is the vibration mode procedure (31%). It clearly appears that the gender plays a role on the bone response to swimming and wheel running exercises. Besides, we did not observe negative results in the osteopenic population with impact training, wheel running and vibration activities. Moreover, about osteoblast/osteocyte parameters, we conclude that high impact and resistance exercise (such jumps and tower climbing) seems to increase bone formation more than running or aerobic exercise. Among the different protocols, literature has shown that the treadmill running procedure mainly induces osteogenic effects on the viability of the osteocyte lineage in both males and females or ovariectomized rats; running in voluntary wheels contributes to a negative effect on bone metabolism in older male models; whole-body vertical vibration is not an osteogenic exercise in female and ovariectomized rats; whereas swimming provides controversial results in female models. For osteoclast parameters only, running in a voluntary wheel for old males, the treadmill running program at high intensity in ovariectomized rats, and the swimming program in a specific ovariectomy condition have detrimental consequences.

Osteocyte-Mediated Translation of Mechanical Stimuli to Cellular Signaling and Its Role in Bone and Non-bone-Related Clinical Complications

PURPOSE OF REVIEW

Osteocytes comprise > 95% of the cellular component in bone tissue and produce a wide range of cytokines and cellular signaling molecules in response to mechanical stimuli. In this review, we aimed to summarize the molecular mechanisms involved in the osteocyte-mediated translation of mechanical stimuli to cellular signaling, and discuss their role in skeletal (bone) diseases and extra-skeletal (non-bone) clinical complications.

RECENT FINDINGS

Two decades before, osteocytes were assumed as a dormant cells buried in bone matrix. In recent years, emerging evidences have shown that osteocytes are pivotal not only for bone homeostasis but also for vital organ functions such as muscle, kidney, and heart. Osteocyte mechanotransduction regulates osteoblast and osteoclast function and maintains bone homeostasis. Mechanical stimuli modulate the release of osteocyte-derived cytokines, signaling molecules, and extracellular cellular vesicles that regulate not only the surrounding bone cell function and bone homeostasis but also the distant organ function in a paracrine and endocrine fashion. Mechanical loading and unloading modulate the osteocytic release of NO, PGE₂, and ATPs that regulates multiple cellular signaling such as Wnt/β-catenin, RANKL/OPG, BMPs, PTH, IGF1, VEGF, sclerostin, and others. Therefore, the in-depth study of the molecular mechanism of osteocyte mechanotransduction could unravel therapeutic targets for various bone and non-bone-related clinical complications such as osteoporosis, sarcopenia, and cancer metastasis to bone.

Studies on Osteocytes in Their 3D Native Matrix Versus 2D In Vitro Models

PURPOSE OF REVIEW

Osteocytes are responsible for mechanosensing and mechanotransduction in bone and play a crucial role in bone homeostasis. They are embedded in a calcified collagenous matrix and connected with each other through the lacuno-canalicular network. Due to this specific native environment, it is a challenge to isolate primary osteocytes without losing their specific characteristics in vitro. This review summarizes the commonly used and recently established models to study the function of osteocytes in vitro.

RECENT FINDINGS

Osteocytes are mostly studied in monolayer culture, but recently, 3D models of osteocyte-like cells and primary osteocytes in vitro have been established as well. These models mimic the native environment of osteocytes and show superior osteocyte morphology and behavior, enabling the development of human disease models. Osteocyte-like cell lines as well as primary osteocytes isolated from bone are widely used to study the role of osteocytes in bone homeostasis. Both cells lines and primary cells are cultured in 2D-monolayer and 3D-models. The use of these models and their advantages and shortcomings are discussed in this review.

ANP32E, a Protein Involved in Steroid-Refractoriness in Ulcerative Colitis, Identified by a Systems Biology Approach

BACKGROUND AND AIMS

Steroid-refractoriness is a common and unpredictable phenomenon in ulcerative colitis [UC], but there are no conclusive studies on the molecular functions involved. We aimed to assess the mechanism of action related to steroid failure by integrating transcriptomic data from UC patients, and updated molecular data on UC and glucocorticoids.

METHODS

MicroRNA [miRNA] and mRNA expression were evaluated by sequencing and microarrays, respectively, from rectal biopsies of patients with moderately-to-severe active UC, obtained before and on the third day of steroid treatment. The differential results were integrated into the mathematical models generated by a systems biology approach.

RESULTS

This computational approach identified 18 proteins that stand out either by being associated with the mechanism of action or by providing a means to classify the patients according to steroid response. Their biological functions have been linked to inflammation, glucocorticoid-induced transcription and angiogenesis. All the selected proteins except ANP32E [a chaperone which has been linked to the exchange of H2A.z histone and promotes glucocorticoid receptor-induced transcription] had previously been related to UC and/or glucocorticoid-induced biological actions. Western blot and immunofluorescence assays confirmed the implication of this chaperone in steroid failure in patients with active UC.

CONCLUSIONS

A systems biology approach allowed us to identify a comprehensive mechanism of action of steroid-refractoriness, highlighting the key role of steroid-induced transcription and the potential implication of ANP32E in this phenomenon.

Prednisolone treatment reduces the osteogenic effects of loading in mice

Glucocorticoid treatment, a major cause of drug-induced osteoporosis and fractures, is widely used to treat inflammatory conditions and diseases. By contrast, mechanical loading increases bone mass and decreases fracture risk. With these relationships in mind, we investigated whether mechanical loading interacts with GC treatment in bone. Three-month-old female C57BL/6 mice were treated with high-dose prednisolone (15 mg/60 day pellets/mouse) or vehicle for two weeks. During the treatment, right tibiae were subjected to short periods of cyclic compressive loading three times weekly, while left tibiae were used as physiologically loaded controls. The bones were analyzed using peripheral quantitative computed tomography, histomorphometry, real-time PCR, three-point bending and Fourier transform infrared micro-spectroscopy. Loading alone increased trabecular volumetric bone mineral density (vBMD), cortical thickness, cortical area, osteoblast-associated gene expression, osteocyte- and osteoclast number, and bone strength. Prednisolone alone decreased cortical area and thickness and osteoblast-associated gene expression. Importantly, prednisolone treatment decreased the load-induced increase in trabecular vBMD by 57% (p < 0.001) and expression of osteoblast-associated genes, while completely abolishing the load-induced increase in cortical area, cortical thickness, number of osteocytes and osteoclasts, and bone strength. When combined, loading and prednisolone decreased the collagen content. In conclusion, high-dose prednisolone treatment strongly inhibits the loading-induced increase in trabecular BMD, and abolishes the loading-induced increase in cortical bone mass. This phenomenon could be due to prednisolone inhibition of osteoblast differentiation and function.

OGT-related mitochondrial motility is associated with sex differences and exercise effects in depression induced by prenatal exposure to glucocorticoids

BACKGROUND

Prenatal exposure to glucocorticoids (GCs) has been found to trigger abnormal behaviors and deleterious neurological effects on offspring both in animals and in humans. The sex differences in depression have been replicated in numerous studies across cultures, persisting throughout the reproductive years. As an X-linked gene in rodents and in humans, O-GlcNAc transferase (OGT) may provide a novel perspective for the sex differences in depression.

METHODS

In the last third of pregnancy (gestational day 14-21), rats were subcutaneously administered either 0.13mg/kg dexamethasone-21-phosphate disodium salt (0.1mg/kg DEX) or vehicle (0.9% saline) once a day for 7 days. Adolescent (4 weeks) offspring were then trained in a swimming program or not.

RESULTS

Here we found that adult offspring rats exposed to DEX prenatally exhibited sex-specific depression-like behaviors, males being more vulnerable than females. Swimming exercise ameliorated the above-mentioned depressive syndromes, which may be a compensatory effect for male disadvantage suffering from prenatal stress. Furthermore, the effects of prenatal DEX exposure and swimming exercise on depression were associated with OGT-related mitochondrial motility, including PINK1/Parkin pathway and AKT/GSK3β pathway.

LIMITATIONS

Representative kymographs of mitochondrial motility were not detected and no causal effects were obtained by OGT gene overexpression or gene knockout in this study.

CONCLUSIONS

Our results provide a new perspective for better understanding sex differences and exercise effects in depression and may offer new mechanism-based therapeutic targets for depression.

TET3 Mediates Alterations in the Epigenetic Marker 5hmC and Akt pathway in Steroid-Associated Osteonecrosis

Steroid-associated osteonecrosis (SAON) is one of the common complications of clinical glucocorticoid (GC) administration, with osteocyte apoptosis appearing as the primary histopathological lesion. However, the precise mechanism underlying SAON remains unknown. Epigenetic modification may be a major cause of SAON. Recently, cumulative research revealed that Ten-Eleven Translocation (TET) proteins can catalyze the conversion of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC) and then alter the epigenetic state of DNA. Here, we report that TET3-5hmC was upregulated in the femoral head tissues of SAON patients and MLO-Y4 cells with dexamethasone (Dex) treatment. Knockdown of TET3 in MLO-Y4 cells decreased 5hmC enrichment and rescued Dex-induced apoptosis. Meanwhile, the local intramedullary injection of TET3 siRNA in Sprague-Dawley rats abrogated GC-induced osteocyte apoptosis, histopathological changes, abnormal MRI signals, and bone microstructure declines in the femoral head in vivo. Moreover, a hydroxymethylated DNA immunoprecipitation (hMeDIP)-chip analysis of Dex-treated osteocytes revealed 456 different 5hmC-enriched genes. The Akt pathway was found to mediate the functional effect of Dex-induced dynamic 5hmC change; this was further verified in clinical samples. The loss of TET3 in MLO-Y4 cells abrogated Dex-induced Akt signaling pathway inhibition. Therefore, our data for the first time identify the effect of TET3-5hmC on the Akt pathway and the necessity of this signaling cascade in SAON, identifying a new potential therapeutic target. © 2016 American Society for Bone and Mineral Research.

Transcription Factor Trps1 Promotes Tubular Cell Proliferation after Ischemia-Reperfusion Injury through cAMP-Specific 3',5'-Cyclic Phosphodiesterase 4D and AKT

Trichorhinophalangeal 1 (Trps1) is a transcription factor essential for epithelial cell morphogenesis during kidney development, but the role of Trps1 in AKI induced by ischemia-reperfusion (I/R) remains unclear. Our study investigated Trps1 expression during kidney repair after acute I/R in rats and explored the molecular mechanisms by which Trps1 promotes renal tubular epithelial cell proliferation. Trps1 expression positively associated with the extent of renal repair after I/R injury. Compared with wild-type rats, rats with knockdown of Trps1 exhibited significantly delayed renal repair in the moderate I/R model, with lower GFR levels and more severe morphologic injury, whereas rats overexpressing Trps1 exhibited significantly accelerated renal repair after severe I/R injury. Additionally, knockdown of Trps1 inhibited and overexpression of Trps1 enhanced the proliferation of renal tubular epithelial cells in rats. Chromatin immunoprecipitation sequencing assays and RT-PCR revealed that Trps1 regulated cAMP-specific 3',5'-cyclic phosphodiesterase 4D (Pde4d) expression. Knockdown of Trps1 decreased the renal protein expression of Pde4d and phosphorylated Akt in rats, and dual luciferase analysis showed that Trps1 directly activated Pde4d transcription. Furthermore, knockdown of Pde4d or treatment with the phosphatidylinositol 3 kinase inhibitor wortmannin significantly inhibited Trps1-induced tubular cell proliferation in vitro Trps1 may promote tubular cell proliferation through the Pde4d/phosphatidylinositol 3 kinase/AKT signaling pathway, suggesting Trps1 as a potential therapeutic target for kidney repair after I/R injury.

An investigation of BMP-7 mediated alterations to BMP signalling components in human tenocyte-like cells

The incidence of tendon re-tears post-surgery is an ever present complication. It is suggested that the application of biological factors, such as bone morphogenetic protein 7 (BMP-7), can reduce complication rates by promoting tenogenic characteristics in in vitro studies. However, there remains a dearth of information in regards to the mechanisms of BMP-7 signalling in tenocytes. Using primary human tenocyte-like cells (hTLCs) from the supraspinatus tendon the BMP-7 signalling pathway was investigated: induction of the BMP associated Smad pathway and non-Smad pathways (AKT, p38, ERK1/2 and JNK); alterations in gene expression of BMP-7 associated receptors, Smad pathway components, Smad target gene (ID1) and tenogenic marker scleraxis. BMP-7 increases the expression of specific BMP associated receptors, BMPR-Ib and BMPR-II, and Smad8. Additionally, BMP-7 activates significantly Smad1/5/8 and slightly p38 pathways as indicated by an increase in phosphorylation and proven by inhibition experiments, where p-ERK1/2 and p-JNK pathways remain mainly unresponsive. Furthermore, BMP-7 increases the expression of the Smad target gene ID1, and the tendon specific transcription factor scleraxis. The study shows that tenocyte-like cells undergo primarily Smad8 and p38 signalling after BMP-7 stimulation. The up-regulation of tendon related marker genes and matrix proteins such as Smad8/9, scleraxis and collagen I might lead to positive effects of BMP-7 treatment for rotator cuff repair, without significant induction of osteogenic and chondrogenic markers.

Aluminum trichloride inhibits osteoblastic differentiation through inactivation of Wnt/β-catenin signaling pathway in rat osteoblasts

Exposure to aluminum (Al) suppresses bone formation. Osteoblastic differentiation plays a key role in the process of bone formation. However, the effect of Al on osteoblastic differentiation is still controversial, and the mechanism remains unclear. To investigate the effect of Al on osteoblastic differentiation and whether Wnt signaling pathway was involved in it, the primary rat osteoblasts were exposed to 1/40 IC50, 1/20 IC50 and 1/10 IC50 of aluminum trichloride (AlCl3) for 24h, respectively. The activity analysis of alkaline phosphate, qRT-PCR analysis of type I collagen, alkaline phosphate, Wnt3a and Dkk-1, Western blot analysis of p-GSK3β, GSK3β and β-catenin protein and Immunofluorescence staining for β-catenin suggested that AlCl3 inhibited osteoblastic differentiation and Wnt/β-catenin pathway. Moreover, we found exogenous Wnt3a application reversed the inhibitory effect of AlCl3 on osteoblastic differentiation, accompanied by activating the Wnt/β-catenin pathway. Taken together, these findings suggest that AlCl3 inhibites osteoblastic differentiation through inactivation of Wnt/β-catenin pathway in osteoblasts.

IL-6 and IGF-1 Signaling Within and Between Muscle and Bone: How Important is the mTOR Pathway for Bone Metabolism?

Insulin-like growth factor 1 (IGF-1) and interleukin 6 (IL-6) play an important role in the adaptation of both muscle and bone to mechanical stimuli. Here, we provide an overview of the functions of IL-6 and IGF-1 in bone and muscle metabolism, and the intracellular signaling pathways that are well known to mediate these functions. In particular, we discuss the Akt/mammalian target of rapamycin (mTOR) pathway which in skeletal muscle is known for its key role in regulating the rate of mRNA translation (protein synthesis). Since the role of the mTOR pathway in bone is explored to a much lesser extent, we discuss what is known about this pathway in bone and the potential role of this pathway in bone remodeling. We will also discuss the possible ways of influencing IGF-1 or IL-6 signaling by osteocytes and the clinical implications of pharmacological or nutritional modulation of the Akt/mTOR pathway.

BMP-7 enhances cell migration and αvβ3 integrin expression via a c-Src-dependent pathway in human chondrosarcoma cells

Bone morphogenic protein (BMP)-7 is a member of the transforming growth factor (TGF)-beta superfamily, which is originally identified based on its ability to induce cartilage and bone formation. In recent years, BMP-7 is also defined as a potent promoter of cell motility, invasion, and metastasis. However, there is little knowledge of the role of BMP-7 and its cellular function in chondrosarcoma cells. In the present study, we investigated the biological impact of BMP-7 on cell motility using transwell assay. In addition, the intracellular signaling pathways were also investigated by pharmacological and genetic approaches. Our results demonstrated that treatment with exogenous BMP-7 markedly increased cell migration by activating c-Src/PI3K/Akt/IKK/NF-κB signaling pathway, resulting in the transactivation of αvβ3 integrin expression. Indeed, abrogation of signaling activation, by chemical inhibition or expression of a kinase dead form of the protein attenuated BMP-7-induced expression of integrin αvβ3 and cell migration. These findings may provide a useful tool for diagnostic/prognostic purposes and even therapeutically in late-stage chondrosarcoma as an anti-metastatic agent.

CCL7 is a protective factor secreted by mechanically loaded osteocytes

In a search for factors up-regulated by mechanical strain in osteocytes, we discovered that chemokine (C-C motif) ligand 7 (CCL7), a chemotactic myokine, was highly expressed in MLO-Y4 osteocyte-like cells. Although MLO-Y4 cells secrete potent chemotactic factors for osteoclast precursors, CCL7 was not responsible for this activity. CCL7 was increased in osteocytes in response to tooth movement in vivo. Since mechanical loading plays a crucial role in maintaining osteocyte viability, CCL7 was tested for protective activity and found to be protective against cell death induced by dexamethasone and etoposide. CCL7 specific antibody partially, but in combination with indomethacin, completely abrogated the protective effects of fluid flow shear stress against dexamethasone-induced cell death. CCL7 activated the β-catenin pathway through phosphorylation of glycogen synthase kinase 3 (GSK-3), suggesting that this pathway is responsible for the observed protective effects. A related cytokine, CCL2, also produced by MLO-Y4 cells but not regulated by mechanical loading, proved to be more potent and protected against cell death induced by not only dexamethasone, but also by Tumor Necrosis Factor α (TNFα). Whereas osteocytes may produce CCL2 in constitutively low levels, a major function of mechanically induced CCL7 may be to selectively protect osteocytes in an autocrine manner against glucocorticoid-induced cell death.