ERK5 signalling pathway is essential for fluid shear stress-induced COX-2 gene expression in MC3T3-E1 osteoblast

Piezo1-mediated fluid shear stress promotes OPG and inhibits RANKL via NOTCH3 in MLO-Y4 osteocytes

Piezo1, a mechanosensitive ion channel, participates in a variety of biological processes in maintaining bone homeostasis. As the most abundant cells in bones of the mammals, osteocytes play an essential role in bone formation, remodeling, and bone mass maintenance. Here, by exposing MLO-Y4 osteocytes to the fluid shear stress (FSS) microenvironment, we explored the effect of Piezo1-mediated FSS on the expression of the molecules critical to the process of bone formation and resorption, Receptor Activator of Nuclear Factor-Kappa-B Ligand (RANKL) and Osteoprotegerin (OPG). It was found that 9 dyne/cm² loading for 30 minutes showed an upregulation trend on Piezo1 when MLO-Y4 osteocytes were exposed to an FSS microenvironment. FSS promotes the expression of OPG and inhibits the expression of RANKL. The blocker of Piezo1, GsMTx4, downregulates the effect of FSS on the expression of these two molecules. In addition, NOTCH3 was involved in this process. Thus, the results demonstrated that Piezo1-mediated FSS promotes the expression of OPG and inhibits the expression of RANKL via NOTCH3 in MLO-Y4 osteocytes.

Targeting apoptosis to manage acquired resistance to third generation EGFR inhibitors

A significant clinical challenge in lung cancer treatment is management of the inevitable acquired resistance to third-generation epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (EGFR-TKIs), such as osimertinib, which have shown remarkable success in the treatment of advanced NSCLC with EGFR activating mutations, in order to achieve maximal response duration or treatment remission. Apoptosis is a major type of programmed cell death tightly associated with cancer development and treatment. Evasion of apoptosis is considered a key hallmark of cancer and acquisition of apoptosis resistance is accordingly a key mechanism of drug acquired resistance in cancer therapy. It has been clearly shown that effective induction of apoptosis is a key mechanism for third generation EGFR-TKIs, particularly osimertinib, to exert their therapeutic efficacies and the development of resistance to apoptosis is tightly associated with the emergence of acquired resistance. Hence, restoration of cell sensitivity to undergo apoptosis using various means promises an effective strategy for the management of acquired resistance to third generation EGFR-TKIs.

Effects of Mechanical Stress Stimulation on Function and Expression Mechanism of Osteoblasts

Osteoclasts and osteoblasts play a major role in bone tissue homeostasis. The homeostasis and integrity of bone tissue are maintained by ensuring a balance between osteoclastic and osteogenic activities. The remodeling of bone tissue is a continuous ongoing process. Osteoclasts mainly play a role in bone resorption, whereas osteoblasts are mainly involved in bone remodeling processes, such as bone cell formation, mineralization, and secretion. These cell types balance and restrict each other to maintain bone tissue metabolism. Bone tissue is very sensitive to mechanical stress stimulation. Unloading and loading of mechanical stress are closely related to the differentiation and formation of osteoclasts and bone resorption function as well as the differentiation and formation of osteoblasts and bone formation function. Consequently, mechanical stress exerts an important influence on the bone microenvironment and bone metabolism. This review focuses on the effects of different forms of mechanical stress stimulation (including gravity, continuously compressive pressure, tensile strain, and fluid shear stress) on osteoclast and osteoblast function and expression mechanism. This article highlights the involvement of osteoclasts and osteoblasts in activating different mechanical transduction pathways and reports changings in their differentiation, formation, and functional mechanism induced by the application of different types of mechanical stress to bone tissue. This review could provide new ideas for further microscopic studies of bone health, disease, and tissue damage reconstruction.

The MAP kinase ERK5/MAPK7 is a downstream effector of oxytocin signaling in myometrial cells

The hormone oxytocin (OT) has pleiotropic activities both in the central nervous system as well as in peripheral tissues, including uterotonic effects on the myometrium during parturition. OT effects are mediated by a single transmembrane receptor, belonging to the GPCR (G protein-coupled receptor) superfamily and coupled primarily to Gq- and Gi-containing heterotrimeric G proteins. Upon receptor stimulation, one well-studied downstream effect is activation of the ERK1/2 MAP (mitogen-activated protein) kinase, and studies have shown that induction of COX-2 by OT in the myometrium required ERK1/2 activity. Many studies investigating the role of ERK1/2 in myometrial tissue were based on the use of chemical inhibitors that, to varying degrees, also inhibited ERK5/MAPK7. Here we report that OT activates ERK5 in a human myometrial cell line in a dose- and time-dependent manner through the activation of Gi/o heterotrimers. Using complementary approaches, we demonstrate that OT-induced COX-2 induction and the concomitant release of PGF2α into the media are primarily ERK5-dependent and to a much lesser extent ERK1/2-dependent. Moreover, in contrast to ERK1/2 activation, ERK5 activation is downstream of Gi/o activation. Here, we also found that ERK5 impacted both basal and to a lesser extent, OT-mediated myometrial cell contraction in vitro. Finally, tracking both ERK1/2 and ERK5 activity during different stages of gestation in rat myometrium, we showed that they followed distinct patterns starting at the onset of labor corresponding to the highest COX-2 expression levels. Overall, our results reveal an important, hitherto unrecognized role for ERK5 in myometrial cell contraction involving induction of COX-2. This novel pathway is likely to play an important role in supporting uterine contractions during parturition.

Inhibition of MEK5/ERK5 signaling overcomes acquired resistance to the third generation EGFR inhibitor, osimertinib, via enhancing Bim-dependent apoptosis

The promising therapeutic efficacy of the third generation EGFR inhibitor, osimertinib (AZD9291), for the treatment of patients with EGFR-mutant non-small cell lung cancer (NSCLC) has been demonstrated in the clinic both as first-line and second line therapy. However, inevitable acquired resistance limits its long-term benefit to patients and is thus a significant clinical challenge. The current study focuses on studying the potential role of targeting MEK5-ERK5 signaling in overcoming acquired resistance to osimertinib. Osimertinib and other third generation EGFR inhibitors exerted a rapid and sustained suppressive effect on ERK5 phosphorylation primarily in EGFR-mutant NSCLC cell lines and lost this activity in some osimertinib-resistant cell lines. Osimertinib combined with either ERK5 or MEK5 inhibitors synergistically decreased the survival of osimertinib-resistant cell lines with enhanced induction of apoptosis primarily via augmenting Bim expression. Moreover, the combination effectively inhibited the growth of osimertinib-resistant xenografts in vivo. Together, these findings suggest the potential role of MEK5-ERK5 signaling in modulating development of acquired resistance to osimertinib and value of targeting this signaling as a potential strategy in overcoming acquired resistance to osimertinib and possibly other third generation EGFR inhibitors.

ERK5 negatively regulates Kruppel-like factor 4 and promotes osteogenic lineage cell proliferation in response to MEK5 overexpression or fluid shear stress

Aim of the study: Fluid shear stress (FSS) plays a critical role in osteoblast proliferation via extracellular signal-regulated kinase 5 (ERK5). Kruppel-like factor 4 (KLF4) knockout robustly enhances bone formation due to increased osteoblast differentiation and mineralization. However, the effect of KLF4 on osteoblast proliferation is unresolved. Therefore, the aim of our study was to investigate the effect of KLF4 on osteogenic lineage cell proliferation and the relationship between KLF4 and ERK5. Materials and methods: MC3T3-E1 cells were treated with FSS and/or KLF4 siRNA, cell viability was accessed by Edu labeling and CCK-8 assay, and proliferative gene expression were assessed by PCR array. Bone marrow stromal cells (BMSCs) were infected with adenovirus expressing KLF4 and/or constitutively active MEK5, cell viability was evaluated using crystal violet staining, colony formation assay, and cell WST1 assay. The levels of KLF4 and ERK5 phosphorylation were identified through qRT-PCR and western blot, respectively. Results: KLF4 expression was significantly down-regulated by FSS exposure, however, this was reversed by ERK5 siRNA. KLF4 overexpression inhibited colony formation efficiency and cell viability in BMSCs. Adenoviruses expressing constitutively active MEK5 increased ERK5 phosphorylation, which inhibited KLF4 expression, and promoted BMSC proliferation. FSS-induced osteoblast proliferation also involved elevation of Cyclin B2 and Cdc14b as well as repressed expression of P27. Conclusions: KLF4 negatively regulates osteogenic lineage cell proliferation, and ERK5 negatively regulates KLF4 expression and promotes osteogenic lineage cell proliferation.

Roles of Olfactomedin 1 in Muscle and Bone Alterations Induced by Gravity Change in Mice

Microgravity causes both muscle and bone loss. Although we previously revealed that gravity change influences muscle and bone through the vestibular system in mice, its detailed mechanism has not been elucidated. In this study, we investigated the roles of olfactomedin 1 (OLFM1), whose expression was upregulated during hypergravity in the soleus muscle, in mouse bone cells. Vestibular lesion significantly blunted OLFM1 expression in the soleus muscle and serum OLFM1 levels enhanced by hypergravity in mice. Moreover, a phosphatidylinositol 3-kinase inhibitor antagonized shear stress-enhanced OLFM1 expression in C2C12 myotubes. As for the effects of OLFM1 on bone cells, OLFM1 inhibited osteoclast formation from mouse bone marrow cells and mouse preosteoclastic RAW264.7 cells. Moreover, OLFM1 suppressed RANKL expression and nuclear factor-κB signaling in mouse osteoblasts. Serum OLFM1 levels were positively related to OLFM1 mRNA levels in the soleus muscle and trabecular bone mineral density of mice. In conclusion, we first showed that OLFM1 suppresses osteoclast formation and RANKL expression in mouse cells.

Fluid shear stress induces Runx-2 expression via upregulation of PIEZO1 in MC3T3-E1 cells

Mechanically induced biological responses in bone cells involve a complex biophysical process. Although various mechanosensors have been identified, the precise mechanotransduction pathway remains poorly understood. PIEZO1 is a newly discovered mechanically activated ion channel in bone cells. This study aimed to explore the involvement of PIEZO1 in mechanical loading (fluid shear stress)-induced signaling cascades that control osteogenesis. The results showed that fluid shear stress increased PIEZO1 expression in MC3T3-E1 cells. The fluid shear stress elicited the key osteoblastic gene Runx-2 expression; however, PIEZO1 silencing using small interference RNA blocked these effects. The AKT/GSK-3β/β-catenin pathway was activated in this process. PIEZO1 silencing impaired mechanically induced activation of the AKT/GSK-3β/β-catenin pathway. Therefore, the results demonstrated that MC3T3-E1 osteoblasts required PIEZO1 to adapt to the external mechanical fluid shear stress, thereby inducing osteoblastic Runx-2 gene expression, partly through the AKT/GSK-3β/β-catenin pathway.

Fluid Shear Stress Suppresses Osteoclast Differentiation in RAW264.7 Cells through Extracellular Signal-Regulated Kinase 5 (ERK5) Signaling Pathway

Background

Although extracellular signal-regulated kinase 5 (ERK5) is known to be critical for osteoclast differentiation, there are few studies on how fluid shear stress (FSS) regulates osteoclast differentiation through the ERK5 signaling pathway. We examined the expression of nuclear factor of activated T cells c1 (NFATc1) in RAW264.7 cells and its downstream factors, including cathepsin K (CTSK), tartrate-resistant acid phosphatase (TRAP), matrix metalloproteinases-9 (MMP-9) and their relationship with ERK5.

Material/Methods

RAW264.7 cells were treated with RANKL, XMD8-92 (ERK5 inhibitor), and then loaded onto 12 dyn/cm² FSS for 4 days. Endpoints measured were osteoclast differentiation, bone resorption, and TRAP activity. Cell viability was detected by using the Cell Counting Kit-8 (CCK-8) assay. Western blot was used to analyze protein expression of phosphorylated-ERK5 (p-ERK5), NFATc1, CTSK, TRAP, and MMP-9.

Results

FSS inhibited osteoclast differentiation and expression of NFATc1, CTSK, TRAP, and MMP-9; cell viability was not affected. ERK5 expression increased by FSS but not by RANKL, and it was blocked by XMD8-92. Furthermore, FSS suppressed osteoclast differentiation in RAW264.7 cells through ERK5 pathway.

Conclusions

Our findings demonstrated that FSS inhibited osteoclast differentiation in RAW264.7 cells via the ERK5 pathway through reduced NFATc1 expression and its downstream factors MMP-9, CTSK, and TRAP.

Biochanin A protects against PM2.5-induced acute pulmonary cell injury by interacting with the target protein MEK5

Epidemiological studies have shown that exposure to ambient fine particulate matter (PM_2.5) is associated with an increased risk for cardiopulmonary diseases. The MEK5/ERK5 and NF-κB signaling pathways are closely related to the regulation of acute pulmonary cell injury (APCI) and may play an important role in the underlying pathophysiological mechanisms. Related studies have shown that Biochanin A (BCA) effectively interferes with APCI, but the underlying mechanism through which this occurs is not fully understood. Previously, based on proteomic and bioinformatic research, we found the indispensable role of MEK5 in mediating remission effects of BCA against PM_2.5-induced lung toxicity. Therefore, using A549 adenocarcinoma human alveolar basal epithelial cells (A549 cells), we combined western blot and qRT-PCR to study the protective signaling pathways induced by BCA, indicating that MEK5/ERK5 and NF-κB are both involved in mediating APCI in response to PM_2.5, and MEK5/ERK5 positively activated NF-κB and its downstream cellular regulatory factors. BCA significantly suppressed PM_2.5-induced upregulation of MEK5/ERK5 expression and phosphorylation and activation of NF-κB. Furthermore, due to the specificity of the MEK5/ERK5 protein structure, the binding sites and binding patterns of BCA and MEK5 were analyzed using molecular docking correlation techniques, which showed that there are stable hydrogen bonds between BCA and the PB1 domain of MEK5 as well as its kinase domain. BCA forms a stable complex with MEK5, which has potential effects on MEKK2/3-MEK5-ERK5 ternary interactions, p62/αPKC-mediated NF-κB regulation, and inhibition of MEK5 target protein phosphorylation. Therefore, our study suggests that MEK5 is an important regulator of intracellular signaling of APCI in response to PM_2.5 exposure. BCA may exert anti-APCI activity by targeting MEK5 to inhibit activation of the MEK5/ERK5/NF-κB signaling pathway.

Conditional Activation of NF-κB Inducing Kinase (NIK) in the Osteolineage Enhances Both Basal and Loading-Induced Bone Formation

Studies from global loss-of-function mutants suggest that alternative NF-κB downstream of NF-κB inducing kinase (NIK) is a cell-intrinsic negative regulator of osteogenesis. However, the interpretation of the osteoblast and/or osteocyte contribution to the bone phenotype is complicated by simultaneous osteoclast defects in these models. Therefore, we turned to a transgenic mouse model to investigate the direct role of NIK in the osteolineage. Osx-Cre;NT3 animals (NT3-Cre +), which bear a constitutively active NIK allele (NT3) driven by Osx-Cre, were compared with their Cre-negative, Control (Ctrl) littermates. NT3-Cre + mice had elevated serum P1NP and CTX levels. Despite this high turnover state, µCT showed that constitutive activation of NIK resulted in a net increase in basal bone mass in both cortical and cancellous compartments. Furthermore, NT3-Cre + mice exhibited a greater anabolic response following mechanical loading compared with controls. We next performed RNA-Seq on nonloaded and loaded tibias to elucidate possible mechanisms underlying the increased bone anabolism seen in NT3-Cre + mice. Hierarchical clustering revealed two main transcriptional programs: one loading-responsive and the other NT3 transgene-driven. Gene ontology (GO) analysis indicated a distinct upregulation of receptor, kinase, and growth factor activities including Wnts, as well as a calcium-response signature in NT3-Cre + limbs. The promoters of these GO-term associated genes, including many known to be bone-anabolic, were highly enriched for multiple κB recognition elements (κB-RE) relative to the background frequency in the genome. The loading response in NT3-Cre + mice substantially overlapped (>90%) with Ctrl. Surprisingly, control animals had 10-fold more DEGs in response to loading. However, most top DEGs shared between genotypes had a high incidence of multiple κB-RE in their promoters. Therefore, both transcriptional programs (loading-responsive and NT3 transgene-driven) are modulated by NF-κB. Our studies uncover a previously unrecognized role for NF-κB in the promotion of both basal and mechanically stimulated bone formation. © 2019 American Society for Bone and Mineral Research.

Fluid shear stress promotes osteoblast proliferation through the NFATc1-ERK5 pathway

PURPOSE

Extracellular-regulated kinase 5 (ERK5) is thought to regulate osteoblast proliferation. To further understand how ERK5 signaling regulates osteoblast proliferation induced by fluid shear stress (FSS), we examined some potential signaling targets associated with ERK5 in MC3T3-E1 cells.

METHODS

MC3T3-E1 cells were treated with XMD8-92 (an ERK5 inhibitor) or Cyclosporin A (CsA, a nuclear factor of activated T cells (NFAT) c1 inhibitor) and/or exposed to 12 dyn/cm² FSS. Phosphorylated-ERK5 (p-ERK5) and expression levels of NFATc1, ERK5, E2F2, and cyclin E1 were analyzed by western blot. The mRNA levels of genes associated with cell proliferation were analyzed by Polymerase Chain Reaction (PCR) array. Subcellular localization of p-ERK5 and NFATc1 were determined by immunofluorescence. Cell proliferation was evaluated by MTT assay.

RESULTS

NFATc1 expression was up-regulated by FSS. XMD8-92 only blocked ERK5 activation; however, CsA decreased NFATc1 and p-ERK5 levels, including after FSS stimulation. Exposure to NFATc1 inhibitor or ERK5 inhibitor resulted in decreased E2F2 and cyclin E1 expression and proliferation by proliferative MC3T3-E1 cells. Furthermore, immunofluorescence results illustrated that NFATc1 induced ERK5 phosphorylation, resulting in p-ERK5 translocation to the nucleus.

CONCLUSIONS

Our results reveal that NFATc1 acts as an intermediate to promote the phosphorylation of ERK5 induced by FSS. Moreover, activated NFATc1-ERK5 signaling up-regulates the expression of E2F2 and cyclin E1, which promote osteoblast proliferation.

Roles of Irisin in the Linkage from Muscle to Bone During Mechanical Unloading in Mice

Mechanical unloading induces disuse muscle atrophy and bone loss, but the details in mechanism involved in those pathophysiological conditions are not fully understood. Interaction between muscle and bone has been recently noted. Here, we investigated the roles of humoral factors linking muscle to bone during mechanical unloading using mice with hindlimb unloading (HU) and sciatic neurectomy (SNX). HU and SNX reduced muscle volume surrounding the tibia, tissue weights of soleus and gastrocnemius muscle, and trabecular bone mineral density (BMD) in the tibia of mice. Among humoral factors linking muscle to bone, HU and SNX reduced fibronectin type III domain-containing 5 (FNDC5) mRNA levels in the soleus muscle of mice. Simple regression analysis revealed that FNDC5 mRNA levels in the soleus muscle were positively related to trabecular BMD in the tibia of control and HU mice as well as sham and SNX mice. Moreover, FNDC5 mRNA levels were negatively correlated with receptor activator of nuclear factor-κB ligand (RANKL) mRNA levels in the tibia of control and HU mice. Irisin, a product of FNDC5, suppressed osteoclast formation from mouse bone marrow cells and RANKL mRNA levels in primary osteoblasts. FNDC5 mRNA levels elevated by fluid shear stress were antagonized by bone morphogenetic protein (BMP) and phosphatidylinositol 3-kinase (PI3K) signaling inhibitors in myoblastic C2C12 cells. In conclusion, the present study first showed that mechanical unloading reduces irisin expression in the skeletal muscle of mice presumably through BMP and PI3K pathways. Irisin might be involved in muscle/bone relationships regulated by mechanical stress in mice.

Physical stimulations and their osteogenesis-inducing mechanisms

Physical stimulations such as magnetic, electric and mechanical stimulation could enhance cell activity and promote bone formation in bone repair process via activating signal pathways, modulating ion channels, regulating bonerelated gene expressions, etc. In this paper, bioeffects of physical stimulations on cell activity, tissue growth and bone healing were systematically summarized, which especially focused on their osteogenesis-inducing mechanisms. Detailedly, magnetic stimulation could produce Hall effect which improved the permeability of cell membrane and promoted the migration of ions, especially accelerating the extracellular calcium ions to pass through cell membrane. Electric stimulation could induce inverse piezoelectric effect which generated electric signals, accordingly up-regulating intracellular calcium levels and growth factor synthesis. And mechanical stimulation could produce mechanical signals which were converted into corresponding biochemical signals, thus activating various signaling pathways on cell membrane and inducing a series of gene expressions. Besides, bioeffects of physical stimulations combined with bone scaffolds which fabricated using 3D printing technology on bone cells were discussed. The equipments of physical stimulation system were described. The opportunities and challenges of physical stimulations were also presented from the perspective of bone repair.

Fluid shear stress promotes osteoblast proliferation via the Gαq-ERK5 signaling pathway

Fluid shear stress (FSS) is a ubiquitous mechanical stimulus that potently promotes osteoblast proliferation. Previously, we reported that extracellular signal-regulated kinase 5 (ERK5) is essential for FSS-induced osteoblast proliferation. However, the precise mechanism by which FSS promotes osteoblast proliferation via ERK5 activation is poorly understood. The aim of this study was to determine the critical role of Gαq in FSS-induced ERK5 phosphorylation and osteoblast proliferation, as well as the downstream targets of the Gαq-ERK5 pathway. MC3T3-E1 cells were transfected with 50 nM Gαq siRNA, treated with 5 mM XMD8-92 (a highly selective inhibitor of ERK5 activity), and/or exposed to FSS (12 dyn/cm(2)). Cell proliferation was evaluated using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The protein expression levels of Gαq, P-ERK5, ERK5, Cyclin B1, and CDK1 were analyzed by Western blot. Physiological FSS exposure for 60 min remarkably promoted MC3T3-E1 cell proliferation, however, this effect was suppressed by siRNA-mediated Gαq knockdown or inhibition of ERK5 activity by XMD8-92 treatment, suggesting that Gαq and ERK5 might modulate FSS-increased osteoblast proliferation. Furthermore, ERK5 phosphorylation was dramatically inhibited by Gαq siRNA. In addition, our study further revealed that FSS treatment of MC3T3-E1 cells for 60 min markedly upregulated the protein expression levels of Cyclin B1 and CDK1, and this increased expression was predominantly blocked by Gαq siRNA or XMD8-92 treatment. We propose that FSS acts on the Gαq-ERK5 signaling pathway to upregulate Cyclin B1 and CDK1 expression, thereby resulting in MC3T3-E1 cell proliferation. Thus, the Gαq-ERK5 signaling pathway may provide useful information regarding the treatment of bone metabolic disease.

Fluid shear stress suppresses TNF-α-induced apoptosis in MC3T3-E1 cells: Involvement of ERK5-AKT-FoxO3a-Bim/FasL signaling pathways

TNF-α is known to induce osteoblasts apoptosis, whereas mechanical stimulation has been shown to enhance osteoblast survival. In the present study, we found that mechanical stimulation in the form of fluid shear stress (FSS) suppresses TNF-α induced apoptosis in MC3T3-E1 cells. Extracellular signal-regulated kinase 5 (ERK5) is a member of the mitogen-activated protein kinase (MAPK) family that has been implicated in cell survival. We also demonstrated that FSS imposed by flow chamber in vitro leads to a markedly activation of ERK5, which was shown to be protective against TNF-α-induced apoptosis, whereas the transfection of siRNA against ERK5 (ERK5-siRNA) reversed the FSS-medicated anti-apoptotic effects. An initial FSS-mediated activation of ERK5 that phosphorylates AKT to increase its activity, and a following forkhead box O 3a (FoxO3a) was phosphorylated by activated AKT. Phosphorylated FoxO3a is sequestered in the cytoplasm, and prevents it from translocating to nucleus where it can increase the expression of FasL and Bim. The inhibition of AKT-FoxO3a signalings by a PI3K (PI3-kinase)/AKT inhibitor (LY294002) or the transfection of ERK5-siRNA led to the nuclear translocation of non-phosphorylated FoxO3a, and increased the protein expression of FasL and Bim. In addition, the activation of caspase-3 by TNF-α was significantly inhibited by aforementioned FSS-medicated mechanisms. In brief, the activation of ERK5-AKT-FoxO3a signaling pathways by FSS resulted in a decreased expression of FasL and Bim and an inhibition of caspase-3 activation, which exerts a protective effect that prevents osteoblasts from apoptosis.