ERK5 activation is essential for osteoclast differentiation

Bone and Extracellular Signal-Related Kinase 5 (ERK5)

Bones are vital for anchoring muscles, tendons, and ligaments, serving as a fundamental element of the human skeletal structure. However, our understanding of bone development mechanisms and the maintenance of bone homeostasis is still limited. Extracellular signal-related kinase 5 (ERK5), a recently identified member of the mitogen-activated protein kinase (MAPK) family, plays a critical role in the pathogenesis and progression of various diseases, especially neoplasms. Recent studies have highlighted ERK5's significant role in both bone development and bone-associated pathologies. This review offers a detailed examination of the latest research on ERK5 in different tissues and diseases, with a particular focus on its implications for bone health. It also examines therapeutic strategies and future research avenues targeting ERK5.

Ethanol extract of Cyathulae Radix inhibits osteoclast differentiation and bone loss

Cyathulae Radix, a traditional Chinese medicine and a common vegetable, boasts a history spanning millennia. It enhances bone density, boosts metabolism, and effectively alleviates osteoporosis-induced pain. Despite its historical use, the molecular mechanisms behind Cyathulae Radix's impact on osteoporosis remain unexplored. In this study, we investigated the effects and mechanisms of Cyathulae Radix ethanol extract (CEE) in inhibiting osteoporosis and osteoclastogenesis. Eight-week-old female mice underwent ovariectomy and were treated with CEE for eight weeks. Micro-computed tomography (micro-CT) assessed histomorphometric parameters, bone tissue staining observed distal femur histomorphology, and three-point bending tests evaluated tibia mechanical properties. Enzyme-linked immunosorbent assay (ELISA) measured serum estradiol (E2), receptor activator for nuclear factor B ligand (RANKL), and osteoprotegerin (OPG) levels. Osteoclastogenesis-related markers were analyzed via Western blotting (WB) and quantitative real-time polymerase chain reaction (qRT-PCR). Additionally, CEE effects on RANKL-induced osteoclast formation and bone resorption were investigated in vitro using tartrate-resistant acid phosphatase (TRAP) staining, qRT-PCR, and WB assay. Compared with the ovariectomy (OVX) group, CEE treatment enhanced trabecular bone density, maximal load-bearing capacity, and various histomorphometric parameters. Serum E2 and OPG levels significantly increased, while Receptor activator of nuclear factor-κB (RANK) decreased in the CEE group. CEE downregulated matrix metallopeptidase 9 (MMP-9), Cathepsin K (CTSK), and TRAP gene and protein expression. In bone marrow macrophages (BMMs), CEE reduced mature osteoclasts, bone resorption pit areas, and MMP-9, CTSK, and TRAP expression during osteoclast differentiation. Compared with DMSO treatment, CEE markedly inhibited RANK, TNF receptor associated factor 6 (TRAF6), Proto-oncogene c-Fos (c-Fos), Nuclear factor of activated T-cells cytoplasmic 1 (NFATc1) expressions, and Extracellular regulated protein kinases (ERK), c-Jun N-terminal kinase (JNK), NF-kappa B-p65 (p65) phosphorylation in osteoclasts. In conclusion, CEE significantly inhibits OVX-induced osteoporosis and RANKL-induced osteoclastogenesis, potentially through modulating the Estrogen Receptor (ER)/RANK/NFATc1 signaling pathway.

Extracellular Signal-Regulated Kinases Play Essential but Contrasting Roles in Osteoclast Differentiation

Bone homeostasis is regulated by the balanced actions of osteoblasts that form the bone and osteoclasts (OCs) that resorb the bone. Bone-resorbing OCs are differentiated from hematopoietic monocyte/macrophage lineage cells, whereas osteoblasts are derived from mesenchymal progenitors. OC differentiation is induced by two key cytokines, macrophage colony-stimulating factor (M-CSF), a factor essential for the proliferation and survival of the OCs, and receptor activator of nuclear factor kappa-B ligand (RANKL), a factor for responsible for the differentiation of the OCs. Mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinases (ERKs), p38, and c-Jun N-terminal kinases, play an essential role in regulating the proliferation, differentiation, and function of OCs. ERKs have been known to play a critical role in the differentiation and activation of OCs. In most cases, ERKs positively regulate OC differentiation and function. However, several reports present conflicting conclusions. Interestingly, the inhibition of OC differentiation by ERK1/2 is observed only in OCs differentiated from RAW 264.7 cells. Therefore, in this review, we summarize the current understanding of the conflicting actions of ERK1/2 in OC differentiation.

Research advances in the role of MAPK cascade in regulation of cell growth, immunity, inflammation, and cancer

Mitogen-activated protein kinase (MAPK) cascade system is one of the highly conserved signal systems in eukaryotic cells, which participates in the regulation of many biological processes. Under the stimulation of different signals (such as cytokines, neurotransmitters, and hormones), MAPK cascade activates downstream targets and controls a variety of cellular processes, including growth, immunity, inflammation, and stress response. In different cells, the effects of MAPK cascade on cells vary with the stimuli and the duration of stimulation. MAPK cascade induces Th differentiation and participates in T cell receptor signal pathway and B cell receptor signal pathway. MAPK cascades regulate various cellular activities related to the occurrence and development of cancer. A thorough and systematic understanding of the specific regulatory effects of MAPK cascade on various cellular processes will provide theoretical guidance for treating various diseases.

The role of MEK1/2 and MEK5 in melatonin-mediated actions on osteoblastogenesis, osteoclastogenesis, bone microarchitecture, biomechanics, and bone formation

Melatonin, the primary hormone involved in circadian entrainment, plays a significant role in bone physiology. This study aimed to assess the role of MEK1/2 and MEK5 in melatonin-mediated actions in mouse and human mesenchymal stem cells (MSCs) and on bone using small-molecule inhibitors and CRISPR/Cas9 knockout approaches. Consistent with in vitro studies performed in mMSCs and hMSCs, nightly (25 mg/kg, i.p., 45 days) injections with PD184352 (MEK1/2 inhibitor) or Bix02189 (MEK5 inhibitor) or SC-1-151 (MEK1/2/5 inhibitor) demonstrated that MEK1/2 and MEK5 were the primary drivers underlying melatonin's actions on bone density, microarchitecture (i.e., trabecular number, separation, and connectivity density), and bone mechanical properties (i.e., ultimate stress) through increases in osteogenic (RUNX2, BMP-2, FRA-1, OPG) expression and decreases in PPARγ. Furthermore, CRISPR/Cas9 knockout of MEK1 or MEK5 in mMSCs seeded on PLGA scaffolds and placed into critical-size calvarial defects in Balb(c) mice (male and female) revealed that treatment with melatonin (15 mg/L; p.o., nightly, 90 days) mediates sex-specific actions of MEK1 and MEK5 in new bone formation. This study is the first to demonstrate a role for MEK1/2 and MEK5 in modulating melatonin-mediated actions on bone formation in vivo and in a sex-specific manner.

Epigallocatechin gallate attenuates tumor necrosis factor (TNF)-α-induced inhibition of osteoblastic differentiation by up-regulating lncRNA TUG1 in osteoporosis

Promoting osteoblast proliferation and differentiation contributes to the prevention and clinical treatment of osteoporosis. This study was to investigate the effect and mechanism of epigallocatechin gallate (EGCG) on tumor necrosis factor (TNF)-α-caused inhibition of osteoblastic differentiation. First, we cultured mouse embryo osteoblast precursor cells (MC3T3-E1) and induced by TNF-α (0, 2.5, 5, 10 ng/mL). The results revealed that TNF-α significantly inhibited the proliferation, ALP activity and mineralized nodule formation of MC3T3-E1 cells and promoted apoptosis. However, EGCG pretreatment significantly alleviated the inhibitory effect of TNF-α on MC3T3-E1. In addition, TNF-α significantly downregulated the expression of lncRNA TUG1 in MC3T3-E1, while EGCG upregulated the expression of lncRNA TUG1. After overexpression of lncRNA TUG1 in TNF-α-induced MC3T3-E1 cells, it could show similar effects as EGCG. However, interference with lncRNA TUG1 expression diminished the protective effect of EGCG on TNF-α-induced MC3T3-E1 cells. Finally, we found that EGCG inhibited TNF-α-induced activation of the Hippo/YAP signaling pathway, and that low expression of lncRNA TUG1 suppressed this effect. In conclusion, EGCG could suppress Hippo/YAP pathway activity by up-regulating lncRNA TUG1, ultimately improving TNF-α-caused inhibition of osteoblastic differentiation.

Small molecule ERK5 kinase inhibitors paradoxically activate ERK5 signalling: be careful what you wish for…

ERK5 is a protein kinase that also contains a nuclear localisation signal and a transcriptional transactivation domain. Inhibition of ERK5 has therapeutic potential in cancer and inflammation and this has prompted the development of ERK5 kinase inhibitors (ERK5i). However, few ERK5i programmes have taken account of the ERK5 transactivation domain. We have recently shown that the binding of small molecule ERK5i to the ERK5 kinase domain stimulates nuclear localisation and paradoxical activation of its transactivation domain. Other kinase inhibitors paradoxically activate their intended kinase target, in some cases leading to severe physiological consequences highlighting the importance of mitigating these effects. Here, we review the assays used to monitor ERK5 activities (kinase and transcriptional) in cells, the challenges faced in development of small molecule inhibitors to the ERK5 pathway, and classify the molecular mechanisms of paradoxical activation of protein kinases by kinase inhibitors.

The MEK5/ERK5 Pathway in Health and Disease

The MEK5/ERK5 mitogen-activated protein kinases (MAPK) cascade is a unique signaling module activated by both mitogens and stress stimuli, including cytokines, fluid shear stress, high osmolarity, and oxidative stress. Physiologically, it is mainly known as a mechanoreceptive pathway in the endothelium, where it transduces the various vasoprotective effects of laminar blood flow. However, it also maintains integrity in other tissues exposed to mechanical stress, including bone, cartilage, and muscle, where it exerts a key function as a survival and differentiation pathway. Beyond its diverse physiological roles, the MEK5/ERK5 pathway has also been implicated in various diseases, including cancer, where it has recently emerged as a major escape route, sustaining tumor cell survival and proliferation under drug stress. In addition, MEK5/ERK5 dysfunction may foster cardiovascular diseases such as atherosclerosis. Here, we highlight the importance of the MEK5/ERK5 pathway in health and disease, focusing on its role as a protective cascade in mechanical stress-exposed healthy tissues and its function as a therapy resistance pathway in cancers. We discuss the perspective of targeting this cascade for cancer treatment and weigh its chances and potential risks when considering its emerging role as a protective stress response pathway.

Signaling pathways in human osteoclasts differentiation: ERK1/2 as a key player

Little is known about the signaling pathways involved in the differentiation of human osteoclasts. The present study evaluated the roles of the Ras/PI3K/Akt/mTOR, Ras/Raf/MEK1/2/ERK1/2, calcium-PKC, and p38 signaling pathways in human osteoclast differentiation. Mononuclear cells were isolated from the peripheral blood of control persons and patients with neurofibromatosis 1 (NF1), and the cells were differentiated into osteoclasts in the presence of signaling pathway inhibitors. Osteoclast differentiation was assessed using tartrate-resistant acid phosphatase 5B. Inhibition of most signaling pathways with chemical inhibitors decreased the number of human osteoclasts and disrupted F-actin ring formation, while the inhibition of p38 resulted in an increased number of osteoclasts, which is a finding contradictory to previous murine studies. However, the p38 inhibition did not increase the bone resorption capacity of the cells. Ras-inhibitor FTS increased osteoclastogenesis in samples from control persons, but an inhibitory effect was observed in NF1 samples. Inhibition of MEK, PI3K, and mTOR reduced markedly the number of NF1-deficient osteoclasts, but no effect was observed in control samples. Western blot analyses showed that the changes in the phosphorylation of ERK1/2 correlated with the number of osteoclasts. Our results highlight the fact that osteoclastogenesis is regulated by multiple interacting signaling pathways and emphasize that murine and human findings related to osteoclastogenesis are not necessarily equivalent.

Hafnium (IV) oxide obtained by atomic layer deposition (ALD) technology promotes early osteogenesis via activation of Runx2-OPN-mir21A axis while inhibits osteoclasts activity

BACKGROUND

Due to increasing aging of population prevalence of age-related disorders including osteoporosis is rapidly growing. Due to health and economic impact of the disease, there is an urgent need to develop techniques supporting bone metabolism and bone regeneration after fracture. Due to imbalance between bone forming and bone resorbing cells, the healing process of osteoporotic bone is problematic and prolonged. Thus searching for agents able to restore the homeostasis between these cells is strongly desirable.

RESULTS

In the present study, using ALD technology, we obtained homogeneous, amorphous layer of hafnium (IV) oxide (HfO2). Considering the specific growth rate (1.9Å/cycle) for the selected process at the temperature of 90 °C, we performed the 100 nm deposition process, which was confirmed by measuring film thickness using reflectometry. Then biological properties of the layer were investigated with pre-osteoblast (MC3T3), pre-osteoclasts (4B12) and macrophages (RAW 264.7) using immunofluorescence and RT-qPCR. We have shown, that HfO2 (i) enhance osteogenesis, (ii) reduce osteoclastogenesis (iii) do not elicit immune response and (iv) exert anti-inflammatory effects.

CONCLUSION

HfO2 layer can be applied to cover the surface of metallic biomaterials in order to enhance the healing process of osteoporotic bone fracture.

The Role of miR-21 in Osteoblasts-Osteoclasts Coupling In Vitro

MiR-21 is being gradually more and more recognized as a molecule regulating bone tissue homeostasis. However, its function is not fully understood due to the dual role of miR-21 on bone-forming and bone-resorbing cells. In this study, we investigated the impact of miR-21 inhibition on pre-osteoblastic cells differentiation and paracrine signaling towards pre-osteoclasts using indirect co-culture model of mouse pre-osteoblast (MC3T3) and pre-osteoclast (4B12) cell lines. The inhibition of miR-21 in MC3T3 cells (MC3T3_inh21) modulated expression of genes encoding osteogenic markers including collagen type I (Coll-1), osteocalcin (Ocl), osteopontin (Opn), and runt-related transcription factor 2 (Runx-2). Inhibition of miR-21 in osteogenic cultures of MC3T3 also inflected the synthesis of OPN protein which is essential for proper mineralization of extracellular matrix (ECM) and anchoring osteoclasts to the bones. Furthermore, it was shown that in osteoblasts miR-21 regulates expression of factors that are vital for survival of pre-osteoclast, such as receptor activator of nuclear factor κB ligand (RANKL). The pre-osteoclast cultured with MC3T3_inh21 cells was characterized by lowered expression of several markers associated with osteoclasts' differentiation, foremost tartrate-resistant acid phosphatase (Trap) but also receptor activator of nuclear factor-κB ligand (Rank), cathepsin K (Ctsk), carbonic anhydrase II (CaII), and matrix metalloproteinase (Mmp-9). Collectively, our data indicate that the inhibition of miR-21 in MC3T3 cells impairs the differentiation and ECM mineralization as well as influences paracrine signaling leading to decreased viability of pre-osteoclasts.

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.

Pathogenesis of Osteoporosis

Osteoporosis is a condition where bone resorption exceeds bone formation leading to degeneration. With an aging population, the prevalence of osteoporosis is on the rise. Although advances in the field have made progress in targeting the mechanisms of the disease, the efficacy of current treatments remains limited and is complicated by unexpected side effects. Therefore, to overcome this treatment gap, new approaches are needed to identify and elucidate the cellular mechanisms mediating the pathogenesis of osteoporosis, which requires a strong understanding of bone biology. This chapter will focus on bone cells (osteoclasts, osteoblasts, and osteocytes) and their role in the bone turnover process in normal physiology and in pathology. With regard to osteoclast function, the regulators and underpinning signaling pathways leading to bone resorption will be discussed. Decreased osteoblastogenesis also contributes to bone deterioration with aging and osteoporosis; hence the factors and signaling pathways mediating osteoblast formation and function will be examined. Osteocytes are mature osteoblasts embedded in bone matrix and act as endocrine cells; their role in bone health and pathology will also be reviewed. In addition, this chapter will explore the emerging role of adipocytes in bone biology and the implications of increased bone marrow fat infiltration with aging on bone degeneration. In conclusion, a greater understanding of the pathogenesis of osteoporosis is of utmost importance in order to develop more effective treatments for osteoporosis and other bone diseases.

Osteopenia and fractures associated with long-term therapy with MEK inhibitors

Targeted therapies have markedly improved the survival of patients with melanoma. We report the case of two patients with advanced melanoma controlled by long-term MEK inhibitor or combination of BRAF and MEK inhibitors, who developed fractures related to severe osteopenia. A 48-year-old woman was treated by pimasertib after the failure of two lines of chemotherapy, and a 42-year-old man was treated by an association of BRAFi (dabrafenib) and MEKi (trametinib) after the failure of one line of chemotherapy. During follow-up, both complained of buttock pain, revealing primary fractures of the pelvis and lumbar vertebra. In both patients, none had osteoporosis risk factors; DEXA scan revealed osteopenia, and analysis ruled out metastatic bone lesion or secondary osteoporosis. Zoledronic acid, cholecalciferol (vitamin D3), oral calcium, and pain killers were introduced, leading to no further bone event. Numerous pathways are involved in the homeostasis of bone turnover, and the effect of tyrosine kinase inhibitors on those pathways is not well known yet. The absence of usual causes of osteoporosis or metastatic bone lesion and kinetics of symptoms lead us to suggest that MEK inhibitors were responsible for the development of osteoporosis. To the best of our knowledge, this is the first report of fractures associated with osteopenia in patients treated with MEKi. Long-term survival owing to new targeted treatment could be associated with yet underestimated adverse effects such as osteopenia/osteoporosis that could impair patient's quality of life and should be investigated.

Roles of Mitogen-Activated Protein Kinases in Osteoclast Biology

Bone undergoes continuous remodeling, which is homeostatically regulated by concerted communication between bone-forming osteoblasts and bone-degrading osteoclasts. Multinucleated giant osteoclasts are the only specialized cells that degrade or resorb the organic and inorganic bone components. They secrete proteases (e.g., cathepsin K) that degrade the organic collagenous matrix and establish localized acidosis at the bone-resorbing site through proton-pumping to facilitate the dissolution of inorganic mineral. Osteoporosis, the most common bone disease, is caused by excessive bone resorption, highlighting the crucial role of osteoclasts in intact bone remodeling. Signaling mediated by mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38, has been recognized to be critical for normal osteoclast differentiation and activation. Various exogenous (e.g., toll-like receptor agonists) and endogenous (e.g., growth factors and inflammatory cytokines) stimuli contribute to determining whether MAPKs positively or negatively regulate osteoclast adhesion, migration, fusion and survival, and osteoclastic bone resorption. In this review, we delineate the unique roles of MAPKs in osteoclast metabolism and provide an overview of the upstream regulators that activate or inhibit MAPKs and their downstream targets. Furthermore, we discuss the current knowledge about the differential kinetics of ERK, JNK, and p38, and the crosstalk between MAPKs in osteoclast metabolism.

Biological effects of melatonin on osteoblast/osteoclast cocultures, bone, and quality of life: Implications of a role for MT2 melatonin receptors, MEK1/2, and MEK5 in melatonin-mediated osteoblastogenesis

The Melatonin Osteoporosis Prevention Study (MOPS) demonstrated that nightly melatonin resulted in a time-dependent decrease in equilibrium ratios of serum osteoclasts and osteoblasts in perimenopausal women. This study examines mechanisms related to the ratios of osteoblasts and osteoclasts using coculture models (transwell or layered) of human mesenchymal stem cell (MSC) and human peripheral blood monocytes (PBMCs). Human MSC/PBMC cocultures exposed to melatonin in osteogenic (OS+) medium for 21 days induced osteoblast differentiation and mineralization; however, only in layered cocultures did melatonin inhibit osteoclastogenesis. Melatonin effects were mediated through MT2 melatonin receptors, MEK1/2, and MEK5. In layered but not transwell cocultures, melatonin increased OPG:RANKL ratios by inhibiting RANKL, suggesting that contact with osteoclasts during osteoblastogenesis inhibits RANKL secretion. Melatonin modulated expression of ERK1/2, ERK5, β1 integrin, GLUT4, and IRβ that was dependent upon the type of coculture; however, in both cultures, melatonin increased RUNX2 and decreased PPARγ expression, indicating a role for metabolic processes that control osteogenic vs adipogenic cell fates of MSCs. Furthermore, melatonin also has osteoblast-inducing effects on human adipose-derived MSCs. In vivo, one-year nightly melatonin (15 mg/L) given to neu female mice in their drinking water increased pErk1/2, pErk5, Runx2, and Opg and Rankl levels in bone consistent with melatonin's already reported bone-enhancing effects. Finally, analysis of daily logs from the MOPS demonstrated a significant improvement in mood and perhaps sleep quality in women receiving melatonin vs placebo. The osteoblast-inducing, bone-enhancing effects of melatonin and improvement in quality of life suggest that melatonin is a safe and effective bone loss therapy.

Analysis of Nkx3.1:Cre-driven Erk5 deletion reveals a profound spinal deformity which is linked to increased osteoclast activity

Extracellular signal-regulated protein kinase 5 (ERK5) has been implicated during development and carcinogenesis. Nkx3.1-mediated Cre expression is a useful strategy to genetically manipulate the mouse prostate. While grossly normal at birth, we observed an unexpected phenotype of spinal protrusion in Nkx3.1:Cre;Erk5 fl/fl (Erk5 fl/fl) mice by ~6-8 weeks of age. X-ray, histological and micro CT (µCT) analyses showed that 100% of male and female Erk5 fl/fl mice had a severely deformed curved thoracic spine, with an associated loss of trabecular bone volume. Although sex-specific differences were observed, histomorphometry measurements revealed that both bone resorption and bone formation parameters were increased in male Erk5 fl/fl mice compared to wild type (WT) littermates. Osteopenia occurs where the rate of bone resorption exceeds that of bone formation, so we investigated the role of the osteoclast compartment. We found that treatment of RANKL-stimulated primary bone marrow-derived macrophage (BMDM) cultures with small molecule ERK5 pathway inhibitors increased osteoclast numbers. Furthermore, osteoclast numbers and expression of osteoclast marker genes were increased in parallel with reduced Erk5 expression in cultures generated from Erk5 fl/fl mice compared to WT mice. Collectively, these results reveal a novel role for Erk5 during bone maturation and homeostasis in vivo.

Vomocytosis of live pathogens from macrophages is regulated by the atypical MAP kinase ERK5

Vomocytosis, or nonlytic extrusion, is a poorly understood process through which macrophages release live pathogens that they have failed to kill back into the extracellular environment. Vomocytosis is conserved across vertebrates and occurs with a diverse range of pathogens, but to date, the host signaling events that underpin expulsion remain entirely unknown. We use a targeted inhibitor screen to identify the MAP kinase ERK5 as a critical suppressor of vomocytosis. Pharmacological inhibition or genetic manipulation of ERK5 activity significantly raises vomocytosis rates in human macrophages, whereas stimulation of the ERK5 signaling pathway inhibits vomocytosis. Lastly, using a zebrafish model of cryptococcal disease, we show that reducing ERK5 activity in vivo stimulates vomocytosis and results in reduced dissemination of infection. ERK5 therefore represents the first host signaling regulator of vomocytosis to be identified and a potential target for the future development of vomocytosis-modulating therapies.

Adipokine Chemerin Bridges Metabolic Dyslipidemia and Alveolar Bone Loss in Mice

Chemerin is an adipokine that regulates adipogenesis and metabolic functions of mature adipocytes mainly through the activation of chemokine-like receptor 1 (CMKLR1). Elevated levels of chemerin have been found in individuals with obesity, type 2 diabetes, and osteoporosis. This adipokine was identified as an inflammatory and metabolic syndrome marker. Considering that the association between metabolic syndrome and bone health remains unclear, the present study aimed to clarify the role of chemerin in the pathophysiology of bone loss induced by dyslipidemia, particularly modulating osteoclastogenesis. In vitro analyses showed a downregulation of CMKLR1 at the early stage of differentiation and a gradual increase at late stages. Strikingly, chemerin did not modify osteoclast differentiation markers or osteoclast formation; however, it increased the actin-ring formation and bone resorption activity in mature osteoclasts. The increased bone resorption activity induced by chemerin was effectively inhibited by CMKLR1 antagonist (CCX832). Chemerin boosting mature osteoclast activity involves ERK5 phosphorylation. Moreover, two models of dyslipidemia (high-fat diet [HFD]-treated C57/BL6 and db/db mice) exhibited significantly increased level of chemerin in the serum and gingival tissue. Morphometric analysis showed that HFD-treated and db/db mice exhibited increased alveolar bone loss compared to respective control mice, which was associated with an up-regulation of chemerin, CMKLR1 and cathepsin K mRNA expression in the gingival tissue. The treatment of db/db mice with CCX832 effectively inhibited bone loss. Antagonism of chemerin receptor also inhibited the expression of cathepsin K in the gingival tissue. Our results show that chemerin not only increases osteoclasts activity in vitro, but also that increased level of chemerin in dyslipidemic mice plays a critical role in bone homeostasis. © 2016 American Society for Bone and Mineral Research.

Curcumin reverses benzidine-induced epithelial-mesenchymal transition via suppression of ERK5/AP-1 in SV-40 immortalized human urothelial cells

Overexposure to benzidine has been manifested as an important cause of bladder cancer. However, the molecular mechanism of benzidine-induced malignancy is still insufficiently interpreted. Epithelial-mesenchymal transition (EMT) is a crucial pathophysiological process in embryonic development as well as initiation and development of epithelium-originated malignant tumors. The role of extracellular regulated protein kinase 5 (ERK5) in benzidine-meditated bladder cancer development has not been explored. In the present study, we explored the role of ERK5/AP-1 pathway in benzidine-induced EMT in human normal urothelial cells and the intervention effect of curcumin on bezidine-induced EMT. We found that benzidine-induced EMT in SV-40 immortalized human urothelial cells (SV-HUC-1) at low concentrations. We detected that ERK5/AP-1 pathway was notably activated. Specific ERK5 inhibitor, XMD8-92 was applied to determine the role of ERK5 in benzidine-induced EMT. Results indicated that XMD8-92 reversed the EMT process. Furthermore, curcumin effectively attenuated benzidine-induced urocystic EMT by suppressing ERK5/AP-1 pathway. In conclusion, the present study revealed the positive role of ERK5/AP-1 in benzidine-provoked urocystic EMT and the curcumin promising use in bladder cancer prevention and intervention via ERK5/AP-1 pathway.

TRAF1 Is Critical for DMBA/Solar UVR-Induced Skin Carcinogenesis

TRAF1 is a member of the TRAF protein family, which regulates the canonical and noncanonical NF-κB signaling cascades. Although aberrant TRAF1 expression in tumors has been reported, the role of TRAF1 remains elusive. Here, we report that TRAF1 is required for solar UV-induced skin carcinogenesis. Immunohistochemical analysis showed that TRAF1 expression is up-regulated in human actinic keratosis and squamous cell carcinoma. In vivo studies indicated that TRAF1 expression levels in mouse skin are induced by short-term solar UV irradiation, and a long-term skin carcinogenesis study showed that deletion of TRAF1 in mice results in a significant inhibition of skin tumor formation. Moreover, we show that TRAF1 is required for solar UV-induced extracellular signal-regulated kinase-5 (ERK5) phosphorylation and the expression of AP-1 family members (c-Fos/c-Jun). Mechanistic studies showed that TRAF1 expression enhances the ubiquitination of ERK5 on lysine 184, which is necessary for its kinase activity and AP-1 activation. Overall, our results suggest that TRAF1 mediates ERK5 activity by regulating the upstream effectors of ERK5 and also by modulating its ubiquitination status. Targeting TRAF1 function might lead to strategies for preventing and treating skin cancer.