Molecular basis of MAP kinase regulation

Nuclear P38: Roles in Physiological and Pathological Processes and Regulation of Nuclear Translocation

The p38 mitogen-activated protein kinase (p38MAPK, termed here p38) cascade is a central signaling pathway that transmits stress and other signals to various intracellular targets in the cytoplasm and nucleus. More than 150 substrates of p38α/β have been identified, and this number is likely to increase. The phosphorylation of these substrates initiates or regulates a large number of cellular processes including transcription, translation, RNA processing and cell cycle progression, as well as degradation and the nuclear translocation of various proteins. Being such a central signaling cascade, its dysregulation is associated with many pathologies, particularly inflammation and cancer. One of the hallmarks of p38α/β signaling is its stimulated nuclear translocation, which occurs shortly after extracellular stimulation. Although p38α/β do not contain nuclear localization or nuclear export signals, they rapidly and robustly translocate to the nucleus, and they are exported back to the cytoplasm within minutes to hours. Here, we describe the physiological and pathological roles of p38α/β phosphorylation, concentrating mainly on the ill-reviewed regulation of p38α/β substrate degradation and nuclear translocation. In addition, we provide information on the p38α/β ’s substrates, concentrating mainly on the nuclear targets and their role in p38α/β functions. Finally, we also provide information on the mechanisms of nuclear p38α/β translocation and its use as a therapeutic target for p38α/β-dependent diseases.

A cryptic tubulin-binding domain links MEKK1 to curved tubulin protomers

The protein kinase MEKK1 activates stress-signaling pathways in response to various cellular stressors, including chemotherapies that disrupt dynamics of the tubulin cytoskeleton. We show that MEKK1 contains a previously uncharacterized domain that can preferentially bind to the curved tubulin heterodimer—which is found in soluble tubulin and at sites of microtubule assembly and disassembly. Mutations that interfere with MEKK1−tubulin binding disrupt microtubule networks in migrating cells and are enriched in patient-derived tumor sequences. These results suggest that MEKK1−tubulin binding may be relevant to cancer progression, and the efficacy of microtubule-disrupting chemotherapies that require the activity of MEKK1.

The MEKK1 protein is a pivotal kinase activator of responses to cellular stress. Activation of MEKK1 can trigger various responses, including mitogen-activated protein (MAP) kinases, NF-κB signaling, or cell migration. Notably, MEKK1 activity is triggered by microtubule-targeting chemotherapies, among other stressors. Here we show that MEKK1 contains a previously unidentified tumor overexpressed gene (TOG) domain. The MEKK1 TOG domain binds to tubulin heterodimers—a canonical function of TOG domains—but is unusual in that it appears alone rather than as part of a multi-TOG array, and has structural features distinct from previously characterized TOG domains. MEKK1 TOG demonstrates a clear preference for binding curved tubulin heterodimers, which exist in soluble tubulin and at sites of microtubule polymerization and depolymerization. Mutations disrupting tubulin binding decrease microtubule density at the leading edge of polarized cells, suggesting that tubulin binding may play a role in MEKK1 activity at the cellular periphery. We also show that MEKK1 mutations at the tubulin-binding interface of the TOG domain recur in patient-derived tumor sequences, suggesting selective enrichment of tumor cells with disrupted MEKK1–microtubule association. Together, these findings provide a direct link between the MEKK1 protein and tubulin, which is likely to be relevant to cancer cell migration and response to microtubule-modulating therapies.

Evolutionarily conserved transcription factors drive the oxidative stress response in Drosophila

As organisms are constantly exposed to the damaging effects of oxidative stress through both environmental exposure and internal metabolic processes, they have evolved a variety of mechanisms to cope with this stress. One such mechanism is the highly conserved p38 MAPK (p38K) pathway, which is known to be post-translationally activated in response to oxidative stress, resulting in the activation of downstream antioxidant targets. However, little is known about the role of p38K transcriptional regulation in response to oxidative stress. Therefore, we analyzed the p38K gene family across the genus Drosophila to identify conserved regulatory elements. We found that oxidative stress exposure results in increased p38K protein levels in multiple Drosophila species and is associated with increased oxidative stress resistance. We also found that the p38Kb genomic locus includes conserved AP-1 and lola-PT transcription factor consensus binding sites. Accordingly, over-expression of these transcription factors in D. melanogaster is sufficient to induce transcription of p38Kb and enhances resistance to oxidative stress. We further found that the presence of a putative lola-PT binding site in the p38Kb locus of a given species is predictive of the species' survival in response to oxidative stress. Through our comparative genomics approach, we have identified biologically relevant putative transcription factor binding sites that regulate the expression of p38Kb and are associated with resistance to oxidative stress. These findings reveal a novel mode of regulation for p38K genes and suggest that transcription may play as important a role in p38K-mediated stress responses as post-translational modifications.

ERK signalling: a master regulator of cell behaviour, life and fate

The proteins extracellular signal-regulated kinase 1 (ERK1) and ERK2 are the downstream components of a phosphorelay pathway that conveys growth and mitogenic signals largely channelled by the small RAS GTPases. By phosphorylating widely diverse substrates, ERK proteins govern a variety of evolutionarily conserved cellular processes in metazoans, the dysregulation of which contributes to the cause of distinct human diseases. The mechanisms underlying the regulation of ERK1 and ERK2, their mode of action and their impact on the development and homeostasis of various organisms have been the focus of much attention for nearly three decades. In this Review, we discuss the current understanding of this important class of kinases. We begin with a brief overview of the structure, regulation, substrate recognition and subcellular localization of ERK1 and ERK2. We then systematically discuss how ERK signalling regulates six fundamental cellular processes in response to extracellular cues. These processes are cell proliferation, cell survival, cell growth, cell metabolism, cell migration and cell differentiation.

Pseudophosphatase MK-STYX: the atypical member of the MAP kinase phosphatases

The regulation of the phosphorylation of mitogen-activated protein kinases (MAPKs) is essential for cellular processes such as proliferation, differentiation, survival, and death. Mutations within the MAPK signaling cascades are implicated in diseases such as cancer, neurodegenerative disorders, arthritis, obesity, and diabetes. MAPK phosphorylation is controlled by an intricate balance between MAPK kinases (enzymes that add phosphate groups) and MAPK phosphatases (MKPs) (enzymes that remove phosphate groups). MKPs are complex negative regulators of the MAPK pathway that control the amplitude and spatiotemporal regulation of MAPKs. MK-STYX (MAPK phosphoserine/threonine/tyrosine-binding protein) is a member of the MKP subfamily, which lacks the critical histidine and nucleophilic cysteine residues in the active site required for catalysis. MK-STYX does not influence the phosphorylation status of MAPK, but even so it adds to the complexity of signal transduction cascades as a signaling regulator. This review highlights the function of MK-STYX, providing insight into MK-STYX as a signal regulating molecule in the stress response, HDAC 6 dynamics, apoptosis, and neurite differentiation.

Shikonin Inhibits Cancer Through P21 Upregulation and Apoptosis Induction

Shikonin is a natural naphthoquinone compound and has demonstrated potent anti-cancer activities; however, the underlying molecular mechanisms remained elusive. Here we report that Shikonin inhibited the growth of a wide range of human cancer cell lines, illustrating a broad anticancer effect. Mechanistically, we show that Shikonin arrested the cell cycle at the G2/M phase, inhibited the ERK-dependent cell growth signal, and induced cell death in both P53 wild type and mutant cancer cells, which collectively contributed to the growth inhibitory effect of Shikonin. A pan-apoptosis inhibitor largely suppressed Shikonin-induced cell death, suggesting an important role of apoptosis in this process. Intriguingly, Shikonin also activated autophagy and inhibition of autophagy by depleting critical autophagic genes further increased Shikonin-induced cell death, indicating a protective role of autophagy. In uncovering the molecular mechanisms underlying these effects of Shikonin, we found that Shikonin induced a robust upregulation of P21 independent of the P53 status, upregulated autophagy genes, as well as inhibited expression of genes required for cell growth. Using mouse tumor models, we confirmed the strong anticancer effect of Shikonin in vivo. Together, our data reveal a broad range of pharmacological functions of Shikonin, involving simultaneous growth inhibition, cell cycle arrest, autophagy activation and apoptosis induction through regulating expression of critical genes involved in these pathways. Our study may facilitate the development of Shikonin in cancer therapy as a single agent or in combination with other anticancer therapies.

Ancient MAPK ERK7 is regulated by an unusual inhibitory scaffold required for Toxoplasma apical complex biogenesis

Apicomplexan parasites include organisms that cause widespread and devastating human diseases such as malaria, cryptosporidiosis, and toxoplasmosis. These parasites are named for a structure, called the “apical complex,” that organizes their invasion and secretory machinery. We found that two proteins, apical cap protein 9 (AC9) and an enzyme called ERK7, work together to facilitate apical complex assembly. Intriguingly, ERK7 is an ancient molecule that is found throughout Eukaryota, though its regulation and function are poorly understood. AC9 is a scaffold that concentrates ERK7 at the base of the developing apical complex. In addition, AC9 binding likely confers substrate selectivity upon ERK7. This simple competitive regulatory model may be a powerful but largely overlooked mechanism throughout biology.

Apicomplexan parasites use a specialized cilium structure called the apical complex to organize their secretory organelles and invasion machinery. The apical complex is integrally associated with both the parasite plasma membrane and an intermediate filament cytoskeleton called the inner-membrane complex (IMC). While the apical complex is essential to the parasitic lifestyle, little is known about the regulation of apical complex biogenesis. Here, we identify AC9 (apical cap protein 9), a largely intrinsically disordered component of the Toxoplasma gondii IMC, as essential for apical complex development, and therefore for host cell invasion and egress. Parasites lacking AC9 fail to successfully assemble the tubulin-rich core of their apical complex, called the conoid. We use proximity biotinylation to identify the AC9 interaction network, which includes the kinase extracellular signal-regulated kinase 7 (ERK7). Like AC9, ERK7 is required for apical complex biogenesis. We demonstrate that AC9 directly binds ERK7 through a conserved C-terminal motif and that this interaction is essential for ERK7 localization and function at the apical cap. The crystal structure of the ERK7–AC9 complex reveals that AC9 is not only a scaffold but also inhibits ERK7 through an unusual set of contacts that displaces nucleotide from the kinase active site. ERK7 is an ancient and autoactivating member of the mitogen-activated kinase (MAPK) family and its regulation is poorly understood in all organisms. We propose that AC9 dually regulates ERK7 by scaffolding and concentrating it at its site of action while maintaining it in an “off” state until the specific binding of a true substrate.

The bacterial metalloprotease NleD selectively cleaves mitogen-activated protein kinases that have high flexibility in their activation loop

Microbial pathogens often target the host mitogen-activated protein kinase (MAPK) network to suppress host immune responses. We previously identified a bacterial type III secretion system effector, termed NleD, a metalloprotease that inactivates MAPKs by specifically cleaving their activation loop. Here, we show that NleDs form a growing family of virulence factors harbored by human and plant pathogens as well as insect symbionts. These NleDs disable specifically Jun N-terminal kinases (JNKs) and p38s that are required for host immune response, whereas extracellular signal-regulated kinase (ERK), which is essential for host cell viability, remains intact. We investigated the mechanism that makes ERK resistant to NleD cleavage. Biochemical and structural analyses revealed that NleD exclusively targets activation loops with high conformational flexibility. Accordingly, NleD cleaved the flexible loops of JNK and p38 but not the rigid loop of ERK. Our findings elucidate a compelling mechanism of native substrate proteolysis that is promoted by entropy-driven specificity. We propose that such entropy-based selectivity is a general attribute of proteolytic enzymes.

Isoform-specific functions of c-Jun N-terminal kinase 1 and 2 in lung ischemia-reperfusion injury through the c-Jun/activator protein-1 pathway

BACKGROUND

c-Jun N-terminal kinase 1 (JNK1) and JNK2 regulate distinct pathological processes in lung diseases. Here we discriminated the respective roles of these kinases in lung transplantation-induced ischemia-reperfusion injury (IRI).

METHODS

Rat pulmonary microvascular endothelial cells were transfected with JNK1 small-interfering RNA (siRNA) and JNK2 siRNA and then subjected to in vitro IRI. For the isoform confirmed to aggravate IRI, the delivery of short-hairpin RNA (shRNA) plasmid was performed by intratracheal administration 48 hours before transplantation into donor rats. After a 3-hour reperfusion, the samples were collected.

RESULTS

JNK1 siRNA decreased but JNK2 siRNA increased JNK phosphorylation and activity, phosphorylated and total c-Jun, and activator protein-1 activity. Although JNK1 siRNA decreased apoptosis and the levels of malondialdehyde, interleukin (IL)-1, IL-6, and tumor necrosis factor (TNF-α), it increased the levels of superoxide dismutase, S-phase percentage, and cyclin D1; JNK2 siRNA had a converse effect. JNK1 siRNA decreased the level of lactate dehydrogenase and increased the levels of VE-cadherin, nitric oxide, phosphorylated nitric oxide synthase, and cell viability; JNK2 si RNA had a converse effect. Compared with the control group, the JNK1 shRNA group exhibited a higher lung oxygenation index and lower lung apoptosis index, injury score, wet weight:dry weight ratio, and levels of IL-1, IL-6, and TNF-α.

CONCLUSIONS

JNK1 aggravated, but JNK2 alleviated, IRI through differential regulation of the JNK1 pathway in in vitro ischemia-reperfusion. JNK1 silence attenuated lung graft dysfunction by inhibiting inflammation and apoptosis. These findings provide a theoretical basis for devising therapeutic strategies against IRI after lung transplantation.

Protective effect of ginsenoside Rg5 against kidney injury via inhibition of NLRP3 inflammasome activation and the MAPK signaling pathway in high-fat diet/streptozotocin-induced diabetic mice

Diabetic nephropathy (DN) is a common and serious complication of diabetes and causes kidney failure. Ginsenoside Rg5 (Rg5) is an important monomer in the main protopanaxadiol component of black ginseng. Rg5 has exhibited some beneficial biological effects, such as anti-cancer, neuroprotection, and anti-depression, but the effect of Rg5 on DN and its potential mechanism remains unclear. The aim of this study is to investigate the effect of Rg5 on kidney injury of C57BL/6 diabetic mice induced by high-fat diet and streptozotocin. After treatment with different concentration of Rg5 (30 and 60 mg kg^-1·d^-1) for 6 consecutive weeks, the fasting blood glucose, insulin levels, serum creatinine, serum urea, and serum UA in Rg5-treated DN mice were significantly reduced, while the renal histopathology was remarkably improved, compared with untreated DN mice. Moreover, ROS production, oxidative stress markers (MDA, SOD, and GSH-PX), Nox4 and TXNIP expressions of kidney in DN mice were significantly reduced after Rg5 treatment. Additionally, the expression levels of the NLRP3 inflammasome (NLRP3, ASC, and Caspase-1) and the inflammatory cytokines IL-1β and IL-18 were significantly inhibited, and the expression of NF-kB and the phosphorylation of p38 MAPK were also decreased with Rg5 treatment compared with no treatment in DN mice. Together, our results indicate that Rg5 attenuated renal injury in diabetic mice by inhibiting oxidative stress and NLRP3 inflammasome activation to reduce inflammatory responses, indicating that Rg5 is a potential compound to prevent or control diabetic renal injury.

Protein Phosphorylation Dynamics: Unexplored Because of Current Methodological Limitations: Dynamics of Processive Phosphorylation

The study of intrinsic phosphorylation dynamics and kinetics in the context of complex protein architecture in vivo has been challenging: Method limitations have prevented significant advances in the understanding of the highly variable turnover of phosphate groups, synergy, and cooperativity between P-sites. However, over the last decade, powerful analytical technologies have been developed to determine the full catalog of the phosphoproteome for many species. The curated databases of phospho sites found by mass spectrometry analysis and the computationally predicted sites based on the linear sequence of kinase motifs are valuable tools. They allow investigation of the complexity of phosphorylation in vivo, albeit with strong discrepancies between different methods. A series of hypothetical scenarios on combinatorial processive phosphorylation is proposed that are likely unverifiable with current methodologies. These proposed a priori postulates could be considered as possible extensions of the known schemes of the activation/inhibition signaling process in vivo.

Dysregulation of Dual-Specificity Phosphatases by Epstein-Barr Virus LMP1 and Its Impact on Lymphoblastoid Cell Line Survival

The strongest evidence of the oncogenicity of Epstein-Barr virus (EBV) in vitro is its ability to immortalize human primary B lymphocytes into lymphoblastoid cell lines (LCLs). Yet the underlying mechanisms explaining how the virus tempers the growth program of the host cells have not been fully elucidated. The mitogen-activated protein kinases (MAPKs) are implicated in many cellular processes and are constitutively activated in LCLs. We questioned the expression and regulation of the dual-specificity phosphatases (DUSPs), the main negative regulator of MAPKs, during EBV infection and immortalization. Thirteen DUSPs, including 10 typical and 3 atypical types of DUSPs, were tested. Most of them were downregulated after EBV infection. Here, a role of viral oncogene latent membrane protein 1 (LMP1) in limiting DUSP6 and DUSP8 expression was identified. Using MAPK inhibitors, we found that LMP1 activates extracellular signal-regulated kinase (ERK) or p38 to repress the expression of DUSP6 and DUSP8, with corresponding substrate specificity. Morphologically, overexpression of DUSP6 and DUSP8 attenuates the ability of EBV-immortalized LCL cells to clump together. Mechanistically, apoptosis induced by restoring DUSP6 and DUSP8 in LCLs indicated a novel mechanism for LMP1 to provide a survival signal during EBV immortalization. Collectively, this report provides the first description of the interplay between EBV genes and DUSPs and contributes considerably to the interpretation of MAPK regulation in EBV immortalization.IMPORTANCE Infections by the ubiquitous Epstein-Barr virus (EBV) are associated with a wide spectrum of lymphomas and carcinomas. It has been well documented that activation levels of MAPKs are found in cancer cells to translate various external or intrinsic stimuli into cellular responses. Physiologically, the dual-specificity phosphates (DUSPs) exhibit great ability in regulating MAPK activities with respect to their capability of dephosphorylating MAPKs. In this study, we found that DUSPs were generally downregulated after EBV infection. EBV oncogenic latent membrane protein 1 (LMP1) suppressed DUSP6 and DUSP8 expression via MAPK pathway. In this way, LMP1-mediated MAPK activation was a continuous process. Furthermore, DUSP downregulation was found to contribute greatly to prevent apoptosis of EBV-infected cells. To sum up, this report sheds light on a novel molecular mechanism explaining how EBV maintains the unlimited proliferation status of the immortalized cells and provides a new link to understand EBV-induced B cell survival.

The evolutionarily conserved MAPK/Erk signaling promotes ancestral T-cell immunity in fish via c-Myc-mediated glycolysis

The mitogen-activated protein kinase (MAPK) cascade is an ancient and evolutionarily conserved signaling pathway involved in numerous physiological processes. Despite great advances in understanding MAPK-mediated regulation of adaptive immune responses in mammals, its contribution to T-cell immunity in early vertebrates remains unclear. Herein, we used Nile tilapia (Oreochromis niloticus) to investigate the regulatory roles of MAPK/extracellular signal-regulated kinase (Erk) signaling in ancestral T-cell immunity of jawed fish. We found that Nile tilapia possesses an evolutionarily conserved MAPK/Erk axis that is activated through a classical three-tier kinase cascade, involving sequential phosphorylation of RAF proto-oncogene serine/threonine-protein kinase (Raf), MAPK/Erk kinase 1/2 (Mek1/2), and Erk1/2. In Nile tilapia, MAPK/Erk signaling participates in adaptive immune responses during bacterial infection. Upon T-cell activation, the MAPK/Erk axis is robustly activated, and MAPK/Erk blockade by specific inhibitors severely impairs T-cell activation. Furthermore, signals from MAPK/Erk were indispensable for primordial T cells to proliferate and exert their effector functions. Mechanistically, activation of the MAPK/Erk axis promoted glycolysis via induction of the transcriptional regulator proto-oncogene c-Myc (c-Myc), to ensure the proper activation and proliferation of fish T cells. Our results reveal the regulatory mechanisms of MAPK/Erk signaling in T-cell immunity in fish and highlight a close link between immune signals and metabolic programs. We propose that regulation of T-cell immunity by MAPK/Erk is a basic and sophisticated strategy that evolved before the emergence of the tetrapod lineage. These findings shed light on the evolution of the adaptive immune system.

JAK-STAT-dependent regulation of scavenger receptors in LPS-activated murine macrophages

In atherosclerosis progression, atherosclerotic plaques develop upon accumulated foam cells derived from macrophages that take up modified low-density lipoprotein (LDL). CD36 and CD204 are the principal scavenger receptors responsible for the uptake of modified LDL. Lipopolysaccharide (LPS) exacerbates atherosclerosis by enhancing the expression of scavenger receptors and thus increasing the uptake of modified LDL into macrophages. However, the signaling pathways that mediate LPS and scavenger receptor expression have not been fully elucidated. We used mouse bone marrow-derived macrophages and investigated the effects of LPS in vitro. LPS enhanced the phosphorylation of extracellular signal-regulated kinase (ERK) and signal transducer and activator of transcription-1 (STAT-1). Inhibitors of the mitogen-activated protein kinase (MAPK)/ERK kinase (MEK) pathway (U0126 and PD0325901) suppressed the uptake of acetylated-LDL (Ac-LDL) and the expression of CD204 but not CD36 in LPS-activated macrophages. Inhibitors of the Janus tyrosine kinase (JAK)-STAT pathway (ruxolitinib and tofacitinib) suppressed the uptake of Ac-LDL and the expression of both CD36 and CD204 in LPS-activated macrophages. We next injected LPS into the peritoneal cavity of mice and analyzed the effects of LPS. MEK inhibitor U0126 suppressed the uptake of Ac-LDL and the expression of CD204 but not CD36 in LPS-activated macrophages. JAK inhibitor ruxolitinib suppressed the uptake of Ac-LDL and the expression of both CD36 and CD204 in LPS-activated macrophages. These results suggest that scavenger receptors in LPS-activated mouse macrophages are regulated through a JAK-STAT-dependent pathway. Although further evaluation is necessary, JAK-STAT inhibition could be useful in atherosclerosis therapy, at least for atherosclerosis exacerbated by LPS.

Inhibitory Effects of Myricetin on Lipopolysaccharide-Induced Neuroinflammation

Microglial activation elicits an immune response by producing proinflammatory modulators and cytokines that cause neurodegeneration. Therefore, a plausible strategy to prevent neurodegeneration is to inhibit neuroinflammation caused by microglial activation. Myricetin, a natural flavanol, induces neuroprotective effects by inhibiting inflammation and oxidative stress. However, whether myricetin inhibits lipopolysaccharide (LPS)-induced neuroinflammation in hippocampus and cortex regions is not known. To test this, we examined the effects of myricetin on LPS-induced neuroinflammation in a microglial BV2 cell line. We found that myricetin significantly downregulated several markers of the neuroinflammatory response in LPS-induced activated microglia, including inducible nitric oxide (NO) synthase (iNOS), cyclooxygenase-2 (COX-2), and proinflammatory modulators and cytokines such as prostaglandin E2 (PGE₂), interleukin-1β (IL-1β), and tumor necrosis factor-α (TNF-α). Moreover, myricetin suppressed the expression of c-Jun NH2-terminal kinase (JNK), p38 MAPK, and extracellular signal-regulated kinase (ERK), which are components of the mitogen-activated protein kinase (MAPK) signaling pathway. Furthermore, myricetin inhibited LPS-induced macrophages and microglial activation in the hippocampus and cortex of mice. Based on our results, we suggest that myricetin inhibits neuroinflammation in BV2 microglia by inhibiting the MAPK signaling pathway and the production of proinflammatory modulators and cytokines. Therefore, this could potentially be used for the treatment of neuroinflammatory diseases.

Identification of microRNAs that Regulate the MAPK Pathway in Human Cumulus Cells from PCOS Women with Insulin Resistance

Polycystic ovary syndrome (PCOS) is one of the most common gynaecological endocrine disorders, and more than 60% of PCOS patients have varying degrees of insulin resistance (IR). The regulatory role of microRNAs (miRNAs) at post-transcriptional levels in human cumulus cells relating to IR in PCOS remains unclear. In this case-control study, 26 PCOS patients with IR (PCOS-IR) and 24 patients without IR (PCOS-control) were enrolled. We determined the differentially expressed miRNA and mRNA using next-generation sequencing technology, and these miRNAs and mRNAs were validated by quantitative real-time polymerase chain reaction (PCR). These miRNA regulating pathways (e.g., MAPK pathway) were analysed by bioinformatics analysis, and the Rap1b was demonstrated to be targeted by miR-612 based on quantitative real-time PCR, western blot and luciferase activity assay. A total of 59 known miRNAs and 617 differentially expressed genes were identified that differentially expressed between PCOS-IR and PCOS-control cumulus cells. Moreover, the potential regulating roles of miRNAs and their targeting genes in pathophysiology of IR and PCOS were analysed by gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway annotation, and several key processes were enriched, such as MAPK activity. Furthermore, Rap1b, a regulator of the MAPK pathway, was demonstrated to be suppressed directly by miR-612 in PCOS-IR cumulus cells based on negative expression correlation validation, dual luciferase activity assay and reduction of Rap1b expression after miR-612 mimics transfection. Our results suggested that miRNAs and their targeted pathways in ovarian cumulus cells may play important roles in the aetiology and pathophysiology of PCOS with IR.

Leveraging New Definitions of the LxVP SLiM To Discover Novel Calcineurin Regulators and Substrates

The Phosphoprotein Phosphatase Calcineurin (CN, PP2B, PP3) recognizes and binds to two short linear motifs (SLiMs), PxIxIT and LxVP, in its regulators and substrates. These interactions enable CN function in many key biological processes. The identification of SLiMs is difficult because of their short, degenerate sequence and often low binding affinity. Here we combine Structure Based Shape Complementarity (SBSC) analysis and proteome-wide affinity purification-mass spectrometry to identify PxIxIT and LxVP containing CN interactors to expand and thereby redefine the LxVP motif. We find that the new πφ-LxVx primary sequence defines an ensemble of binding competent confirmations and thus the binding on-rate, making it difficult to predict the LxVP binding strength from its sequence. Our analysis confirms existing and, more importantly, identifies novel CN interactors, substrates, and thus biological functions of CN.

Modulating multi-functional ERK complexes by covalent targeting of a recruitment site in vivo

Recently, the targeting of ERK with ATP-competitive inhibitors has emerged as a potential clinical strategy to overcome acquired resistance to BRAF and MEK inhibitor combination therapies. In this study, we investigate an alternative strategy of targeting the D-recruitment site (DRS) of ERK. The DRS is a conserved region that lies distal to the active site and mediates ERK-protein interactions. We demonstrate that the small molecule BI-78D3 binds to the DRS of ERK2 and forms a covalent adduct with a conserved cysteine residue (C159) within the pocket and disrupts signaling in vivo. BI-78D3 does not covalently modify p38MAPK, JNK or ERK5. BI-78D3 promotes apoptosis in BRAF inhibitor-naive and resistant melanoma cells containing a BRAF V600E mutation. These studies provide the basis for designing modulators of protein-protein interactions involving ERK, with the potential to impact ERK signaling dynamics and to induce cell cycle arrest and apoptosis in ERK-dependent cancers.

Mapping low-affinity/high-specificity peptide-protein interactions using ligand-footprinting mass spectrometry

Short linear peptide motifs that are intracellular ligands of folded proteins are a modular, incompletely understood molecular interaction language in signaling systems. Such motifs, which frequently occur in intrinsically disordered protein regions, often bind partner proteins with modest affinity and are difficult to study with conventional structural biology methods. We developed LiF-MS (ligand-footprinting mass spectrometry), a method to map peptide binding sites on folded protein domains that allows consideration of their dynamic disorder, and used it to analyze a set of D-motif peptide-mitogen-activated protein kinase (MAPK) associations to validate the approach and define unknown binding structures. LiF-MS peptide ligands carry a short-lived, indiscriminately reactive cleavable crosslinker that marks contacts close to ligand binding sites with high specificity. Each marked amino acid provides an independent constraint for a set of directed peptide-protein docking simulations, which are analyzed by agglomerative hierarchical clustering. We found that LiF-MS provides accurate ab initio identification of ligand binding surfaces and a view of potential binding ensembles of a set of D-motif peptide-MAPK associations. Our analysis provides an MKK4-JNK1 structural model, which has thus far been crystallographically unattainable, a potential alternate binding mode for part of the NFAT4-JNK interaction, and evidence of bidirectional association of MKK4 peptide with ERK2. Overall, we find that LiF-MS is an effective noncrystallographic way to understand how short linear motifs associate with specific sites on folded protein domains at the level of individual amino acids.

DUSP8 phosphatase: structure, functions, expression regulation and the role in human diseases

Dual-specificity phosphatases (DUSPs) are a subset of protein tyrosine phosphatases (PTPs), many of which dephosphorylate the residues of phosphor-serine/threonine and phosphor-tyrosine on mitogen-activated protein kinases (MAPKs), and hence are also referred to as MAPK phosphatases (MKPs). Homologue of Vaccinia virus H1 phosphatase gene clone 5 (HVH-5), also known as DUSP8, is a unique member of the DUSPs family of phosphatases. Accumulating evidence has shown that DUSP8 plays an important role in phosphorylation-mediated signal transduction of MAPK signaling ranging from cell oxidative stress response, cell apoptosis and various human diseases. It is generally believed that DUSP8 exhibits significant dephosphorylation activity against JNK, however, with the deepening of research, plenty of new literature reports that DUSP8 also has effective dephosphorylation activity on p38 MAPK and ERKs, successfully affects the transduction of MAPKs pathway, indicating that DUSP8 presents a unknown diversity of DUSPs family on distinct corresponding dephosphorylated substrates in different biological events. Therefore, the in-depth study of DUSP8 not only throws a new light on the multi-biological function of DUSPs, but also is much valuable for the reveal of complex pathobiology of clinical diseases. In this review, we provide a detail overview of DUSP8 phosphatase structure, biological function and expression regulation, as well as its role in related clinical human diseases, which might be help for the understanding of biological function of DUSP8 and the development of prevention, diagnosis and therapeutics in related human diseases.

Preparation of Phosphorylated Proteins for NMR Spectroscopy

Phosphorylation is a ubiquitous posttranslational modification that is essential for the regulation of many cellular processes. The human genome consists of more than 200,000 phosphorylation sites, whose phosphorylation is tightly controlled by ≥500 kinases and ~200 phosphatases. Given the large number of phosphorylation sites and the key role phosphorylation plays in regulating cellular processes, it is essential to characterize the impact of phosphorylation on substrate structure, dynamics, and function. However, a major challenge is the large-scale production of phosphorylated proteins in vitro for these structural, functional, and dynamic studies. Here, we describe an efficient protocol used routinely in our laboratory for the production of phosphorylated proteins. We also describe the methods used for identifying, characterizing, and separating the resulting phosphorylated proteins for subsequent studies.

Furanoic Lipid F-6, A Novel Anti-Cancer Compound that Kills Cancer Cells by Suppressing Proliferation and Inducing Apoptosis

Identifying novel anti-cancer drugs is important for devising better cancer treatment options. In a series of studies designed to identify novel therapeutic compounds, we recently showed that a C-20 fatty acid (12,15-epoxy-13,14-dimethyleicosa-12,14-dienoic acid, a furanoic acid or F-6) present in the lipid fraction of the secretions of the Arabian Gulf catfish skin (Arius bilineatus Val.; AGCS) robustly induces neutrophil extracellular trap formation. Here, we demonstrate that a lipid mix (Ft-3) extracted from AGCS and F-6, a component of Ft-3, dose dependently kill two cancer cell lines (leukemic K-562 and breast MDA MB-231). Pure F-6 is approximately 3.5 to 16 times more effective than Ft-3 in killing these cancer cells, respectively. Multiplex assays and network analyses show that F-6 promotes the activation of MAPKs such as Erk, JNK, and p38, and specifically suppresses JNK-mediated c-Jun activation necessary for AP-1-mediated cell survival pathways. In both cell lines, F-6 suppresses PI3K-Akt-mTOR pathway specific proteins, indicating that cell proliferation and Akt-mediated protection of mitochondrial stability are compromised by this treatment. Western blot analyses of cleaved caspase 3 (cCasp3) and poly ADP ribose polymerase (PARP) confirmed that F-6 dose-dependently induced apoptosis in both of these cell lines. In 14-day cell recovery experiments, cells treated with increasing doses of F-6 and Ft-3 fail to recover after subsequent drug washout. In summary, this study demonstrates that C-20 furanoic acid F-6, suppresses cancer cell proliferation and promotes apoptotic cell death in leukemic and breast cancer cells, and prevents cell recovery. Therefore, F-6 is a potential anti-cancer drug candidate.

Targeting ERK beyond the boundaries of the kinase active site in melanoma

Extracellular signal-regulated kinase 1/2 (ERK1/2) constitute a point of convergence for complex signaling events that regulate essential cellular processes, including proliferation and survival. As such, dysregulation of the ERK signaling pathway is prevalent in many cancers. In the case of BRAF-V600E mutant melanoma, ERK inhibition has emerged as a viable clinical approach to abrogate signaling through the ERK pathway, even in cases where MEK and Raf inhibitor treatments fail to induce tumor regression due to resistance mechanisms. Several ERK inhibitors that target the active site of ERK have reached clinical trials, however, many critical ERK interactions occur at other potentially druggable sites on the protein. Here we discuss the role of ERK signaling in cell fate, in driving melanoma, and in resistance mechanisms to current BRAF-V600E melanoma treatments. We explore targeting ERK via a distinct site of protein-protein interaction, known as the D-recruitment site (DRS), as an alternative or supplementary mode of ERK pathway inhibition in BRAF-V600E melanoma. Targeting the DRS with inhibitors in melanoma has the potential to not only disrupt the catalytic apparatus of ERK but also its noncatalytic functions, which have significant impacts on spatiotemporal signaling dynamics and cell fate.

Structural Mechanism of the Arrestin-3/JNK3 Interaction

Arrestins, in addition to desensitizing GPCR-induced G protein activation, also mediate G protein-independent signaling by interacting with various signaling proteins. Among these, arrestins regulate MAPK signal transduction by scaffolding mitogen-activated protein kinase (MAPK) signaling components such as MAPKKK, MAPKK, and MAPK. In this study, we investigated the binding mode and interfaces between arrestin-3 and JNK3 using hydrogen/deuterium exchange mass spectrometry, ¹⁹F-NMR, and tryptophan-induced Atto 655 fluorescence-quenching techniques. Results suggested that the β1 strand of arrestin-3 is the major and potentially only interaction site with JNK3. The results also suggested that C-lobe regions near the activation loop of JNK3 form the potential binding interface, which is variable depending on the ATP binding status. Because the β1 strand of arrestin-3 is buried by the C-terminal strand in its basal state, C-terminal truncation (i.e., pre-activation) of arrestin-3 facilitates the arrestin-3/JNK3 interaction.

Mitogen-Activated Protein Kinase Phosphatases (MKPs) in Fungal Signaling: Conservation, Function, and Regulation

Mitogen-activated protein kinases (MAPKs) are key mediators of signaling in fungi, participating in the response to diverse stresses and in developmental processes. Since the precise regulation of MAPKs is fundamental for cell physiology, fungi bear dual specificity phosphatases (DUSPs) that act as MAP kinase phosphatases (MKPs). Whereas fungal MKPs share characteristic domains of this phosphatase subfamily, they also have specific interaction motifs and particular activation mechanisms, which, for example, allow some yeast MKPs, such as Saccharomyces cerevisiae Sdp1, to couple oxidative stress with substrate recognition. Model yeasts show that MKPs play a key role in the modulation of MAPK signaling flow. Mutants affected in S. cerevisiae Msg5 or in Schizosaccharomyces pombe Pmp1 display MAPK hyperactivation and specific phenotypes. MKPs from virulent fungi, such as Candida albicans Cpp1, Fusarium graminearum Msg5, and Pyricularia oryzae Pmp1, are relevant for pathogenicity. Apart from transcriptional regulation, MKPs can be post-transcriptionally regulated by RNA-binding proteins such as Rnc1, which stabilizes the S. pombePMP1 mRNA. P. oryzae Pmp1 activity and S. cerevisiae Msg5 stability are regulated by phosphorylation and ubiquitination, respectively. Therefore, fungi offer a platform to gain insight into the regulatory mechanisms that control MKPs.

PP-22 promotes autophagy and apoptosis in the nasopharyngeal carcinoma cell line CNE-2 by inducing endoplasmic reticulum stress, downregulating STAT3 signaling, and modulating the MAPK pathway

Paris polyphylla var. yunnanensis, named Chong Lou, is considered an antitumor substance. In this study, we investigated the effect of PP-22, a monomer purified from P. polyphylla var. yunnanensis, on the nasopharyngeal carcinoma cell line CNE-2 in vitro. The results showed that PP-22 could inhibit the proliferation of CNE-2 cells via the induction of apoptosis, with evidence of the characteristic morphological changes in the apoptosis in the nucleus and an increase in Annexin V-positive cells. In addition, we found that PP-22 could activate the p38 mitogen-activated protein kinase (MAPK) pathway and that this activation was reversed by SB203580, a specific inhibitor of the p38 MAPK pathway. In contrast, PP-22 promoted apoptosis via an intrinsic pathway, including the endoplasmic reticulum stress pathway, in a caspase-dependent manner. A further study showed that PP-22 also induced apoptosis by downregulating the signal transducers and activators of transcription 3 (STAT3) pathway, and the inhibitory effect was also confirmed by STAT3 small interfering RNA. In addition, PP-22 could promote autophagy by inhibiting the extracellular regulated protein kinases (ERK) pathway. And autophagy plays a protective role against apoptosis. Together, these data show that PP-22 promotes autophagy and apoptosis in the nasopharyngeal carcinoma CNE-2 cell line.

EZH2 plays a crucial role in ischemia/reperfusion-induced acute kidney injury by regulating p38 signaling

OBJECTIVE AND DESIGN

Renal ischemia-reperfusion (IR)-induced acute kidney injury (AKI) remains a major challenge in clinic. The histone methyltransferases enhancer of zest homolog-2 (EZH2) is associated with the development of renal injury. However, the molecular mechanism has not been fully elucidated.

MATERIALS

AKI in C57BL/6 mice was generated by renal IR.

TREATMENTS

The 3-deazaneplanocin A (DZNeP), a selective EZH2 inhibitor, or vehicle was administrated in mice after IR. HK-2 cells were exposed to hypoxia-reoxygenation (H/R) stress.

METHODS

Apoptosis was detected by TUNEL assay or flow cytometry. EZH2, caspase-3, p38, F4/80⁺ macrophages, and CD3⁺ T cells were examined by immunohistochemistry or Western blot. Tumor necrosis factor (TNF)-α, monocyte chemoattractant protein (MCP)-1, IL-6, and IL-18 were measured using RT-PCR.

RESULTS

Mice treated with DZNeP exhibited less severe renal dysfunction and tubular injury following IR. EZH2 inhibition decreased apoptotic cells while reducing activation of caspase-3 in kidneys under IR condition. Moreover, EZH2 inhibition impaired the recruitment of CD3⁺ T cells and F4/80⁺ cells in kidneys with IR. Administration of DZNeP suppressed the production of TNF-α, MCP-1, IL-6, and IL-18 in IR-treated kidneys. Of note, EZH2 inhibition reduced p38 phosphorylation in kidneys after IR. In H/R-treated HK-2 cells, DZNeP treatment or EZH2 knockdown reduced apoptosis. EZH2 inhibition inactivated p38 resulting in reduction of active caspase-3 and proinflammatory molecules. By contrast, EZH2 overexpression induced p38 phosphorylation, caspase-3 activation, and production of proinflammatory molecules, which was reversed by SB203580.

CONCLUSIONS

EZH2 plays a crucial role in IR-induced AKI via modulation of p38 signaling. Targeting EZH2/p38 signaling pathway may offer novel strategies to protect kidneys from acute kidney injury induced by ischemia-reperfusion.

Erianin protects against high glucose-induced oxidative injury in renal tubular epithelial cells

Erianin is the major bibenzyl compound found in Dendrobium chrysotoxum Lindl. The current study was designed to investigate the protective effects of erianin on high glucose-induced injury in cultured renal tubular epithelial cells (NRK-52E cells) and determine the possible mechanisms for its effects. NRK-52E cells were pretreated with erianin (5, 10, 25 or 50 nmol/L) for 1 h followed by further exposure to high glucose (30 mmol/L, HG) for 48 h. Erianin concentration dependently enhanced cell viability followed by HG treatment in NRK-52E cells. HG induced reactive oxygen species (ROS) generation, malondialdehyde production, and glutathione deficiency were recovered in NRK-52E cells pretreated with erianin. HG triggered cell apoptosis via the loss of mitochondrial membrane potential, depletion of adenosine triphosphate, upregulation of caspases 9 and 3, enhancement of cytochrome c release, and subsequent interruption of the Bax/Bcl-2 balance. These detrimental effects were ameliorated by erianin. HG also induced activation of p53, JNK, p38 mitogen-activated protein kinase (MAPK) and nuclear factor-κB (NF-κB) in NRK-52E cells, which were blocked by erianin. The results suggest that treatment NRK-52E cells with erianin halts HG-induced renal dysfunction through the suppression of the ROS/MAPK/NF-κB signaling pathways. Our findings provide novel therapeutic targets for diabetic nephropathy.

Gigantol has Protective Effects against High Glucose-Evoked Nephrotoxicity in Mouse Glomerulus Mesangial Cells by Suppressing ROS/MAPK/NF-κB Signaling Pathways

Gigantol is a bibenzyl compound derived from several medicinal orchids. This biologically active compound has shown promising therapeutic potential against diabetic cataracts, but whether this compound exerts beneficial effects on the other diabetic microvascular complications remains unclear. This study was carried out to examine effects of gigantol on high glucose-induced renal cell injury in cultured mouse kidney mesangial cells (MES-13). MES-13 cells were pretreated with gigantol (1, 5, 10 or 20 μmol/L) for 1 h followed by further exposure to high (33.3 mmol/L) glucose for 48 h. Gigantol concentration dependently enhanced cell viability followed by high glucose treatment in MES-13 cells. High glucose induced reactive oxygen species (ROS) generation, malondialdehyde production and glutathione deficiency were recoved in MES-13 cells pretreated with gigantol. High glucose triggered cell apoptosis via the the loss of mitochondrial membrane potential, depletion of adenosine triphosphate, upregulation of caspases 9 and 3, enhancement of cytochrome c release, and subsequent interruption of the Bax/Bcl-2 balance. These detrimental effects were ameliorated by gigantol. High glucose also induced activation of JNK, p38 mitogen-activated protein kinase (MAPK) and nuclear factor-κB (NF-κB) in MES-13 cells, which were blocked by gigantol. The results suggest that treatment MES-13 cells with gigantol halts high glucose-induced renal dysfunction through the suppression of the ROS/MAPK/NF-κB signaling pathways. Our data are of value to the understanding the mechanism for gigantol, and would benefit the study of drug development or food supplement for diabetes and nephropathy.

Nature of the Pre-Chemistry Ensemble in Mitogen-Activated Protein Kinases

In spite of the availability of a significant amount of structural detail on docking interactions involving mitogen-activated protein kinases (MAPKs) and their substrates, the mechanism by which the disordered phospho-acceptor on the substrate transiently interacts with the kinase catalytic elements and is phosphorylated, often with high efficiency, remains poorly understood. Here, this dynamic interaction is analyzed in the context of available biophysical and biochemical data for ERK2, an archetypal MAPK. A hypothesis about the nature of the ternary complex involving a MAPK, its substrate, and ATP immediately prior to the chemical step (the pre-chemistry complex) is proposed. It is postulated that the solution ensemble (the pre-chemistry ensemble) representing the pre-chemistry complex comprises several conformations that are linked by dynamics on multiple timescales. These individual conformations possess different intrinsic abilities to proceed through the chemical step. The overall rate of chemistry is therefore related to the microscopic nature of the pre-chemistry ensemble, its constituent conformational microstates, and their intrinsic abilities to yield a phosphorylated product. While characterizing these microstates within the pre-chemistry ensemble in atomic or near-atomic detail is an extremely challenging proposition, recent developments in hybrid methodologies that employ computational approaches driven by experimental data appear to provide the most promising path forward toward achieving this goal.

Antisense Oligonucleotides against miR-21 Inhibit the Growth and Metastasis of Colorectal Carcinoma via the DUSP8 Pathway

Accumulating research has documented that microRNA-21 (miR-21) plays an important role in the development of human colorectal carcinoma (CRC). Our recent work also showed that antisense oligonucleotides (ASOs) against miR-21 can impair the growth of CRC cells in vitro. However, the potential role of miR-21 in gene therapy against CRC remains to be fully elucidated. Here, we further observed the effect of ASOs against miR-21 on the growth and metastasis of CRC in vivo using a xenograft model of human CRC. We found that ASOs could effectively inhibit the growth and metastasis of CRC in vivo, accompanied by downregulated expression of miR-21 and reduced transduction of the AKT and ERK pathway. Mechanically, global gene expression analysis showed that the expression of DUSP8, a novel target of miR-21, was upregulated in tumor mass. Furthermore, overexpression of DUSP8 could remarkably suppress the proliferation and migration of CRC cells in vitro. Finally, downregulation of DUSP8 could abrogate the effects of ASOs against miR-21 on the proliferation and migration of CRC cells, as well as altered transduction of the AKT and ERK signaling pathway. Together, these data suggest that ASOs against miRNAs are an attractive and potential therapeutic for the treatment of human CRC and warrant further development.

Mechanisms shaping the role of ERK1/2 in cellular senescence (Review)

Senescence is a result of cellular stress and is a potential mechanism for regulating cancer. As a member of the mitogen-activated protein kinase family, ERK1/2 (extracellular signal-regulated protein kinase) has an important role in delivering extracellular signals to the nucleus, and these signals regulate the cell cycle, cell proliferation and cell development. Previous studies demonstrated that ERK1/2 is closely associated with cell aging; however other previous studies suggested that ERK1/2 exerts an opposite effect on aging models and target proteins, even within the same cell model. Recent studies demonstrated that the effect of ERK1/2 on aging is likely associated with its target proteins and regulators, negative feedback loops, phosphorylated ERK1/2 factors and ERK1/2 translocation from the cytoplasm to the nucleus. The present review aims to examine the mechanism of ERK1/2 and discuss its role in cellular outcomes and novel drug development.

An AGC kinase, PgAGC1 regulates virulence in the entomopathogenic oomycete Pythium guiyangense

Mosquitoes are the most important medical species by transmitting some of deadly infectious diseases to human. In recent years extensive studies of vector control have been focused on biological control agents due to the grave issue raised by continuous application of chemical compounds. Pythium guiyangense X.Q. Su was first isolated from infected larvae of Aedes albopictus in 2006 in China and it has been proven to be a promising mosquito control agent. However, the molecular mechanisms of this oomycete pathogenic to mosquitoes are still not clear. In this study, we identified a new gene from the genome of P. guiyangense, PgAGC1 that belongs to the AGC kinase group and we found that the transcriptional expression levels of this gene were significantly up-regulated during infection of mosquito Culex pipiens pallens. Disruption of the PgAGC1gene via genetic transformation methods affects colony growth and stress responses and results in reduced mortality and infection rates. All the evidence revealed that, besides its role in growth and stress resistance, PgAGC1 is putative determinants of P. guiyangense virulence. The results of this study become of particular importance in understanding the mechanisms of oomycete-mosquito interactions.

In silico-prediction of protein-protein interactions network about MAPKs and PP2Cs reveals a novel docking site variants in Brachypodium distachyon

Protein-protein interactions (PPIs) underlie the molecular mechanisms of most biological processes. Mitogen-activated protein kinases (MAPKs) can be dephosphorylated by MAPK-specific phosphatases such as PP2C, which are critical to transduce extracellular signals into adaptive and programmed responses. However, the experimental approaches for identifying PPIs are expensive, time-consuming, laborious and challenging. In response, many computational methods have been developed to predict PPIs. Yet, these methods have inherent disadvantages such as high false positive and negative results. Thus, it is crucial to develop in silico approaches for predicting PPIs efficiently and accurately. In this study, we identified PPIs among 16 BdMAPKs and 86 BdPP2Cs in B. distachyon using a novel docking approach. Further, we systematically investigated the docking site (D-site) of BdPP2C which plays a vital role for recognition and docking of BdMAPKs. D-site analysis revealed that there were 96 pairs of PPIs including all BdMAPKs and most BdPP2Cs, which indicated that BdPP2C may play roles in other signaling networks. Moreover, most BdPP2Cs have a D-site for BdMAPKs in our prediction results, which suggested that our method can effectively predict PPIs, as confirmed by their 3D structure. In addition, we validated this methodology with known Arabidopsis and yeast phosphatase-MAPK interactions from the STRING database. The results obtained provide a vital research resource for exploring an accurate network of PPIs between BdMAPKs and BdPP2Cs.

Roburic Acid Suppresses NO and IL-6 Production via Targeting NF-κB and MAPK Pathway in RAW264.7 Cells

In the present study, we investigated the anti-inflammatory effect of roburic acid on production of nitric oxide (NO) and interlukin-6 (IL-6) in lipopolysaccharide (LPS)-stimulated RAW264.7 macrophage cells. We found that roburic acid reduced production of NO and IL-6, and the expression of inducible nitric oxide synthases (iNOS). Meanwhile, phosphorylation of inhibitor of κBα (IκBα) and IκB kinase α/β (IKKα/β), as well as translocation of nuclear factor-κB (NF-κB) to the nucleus, was suppressed by roburic acid treatment. In addition, phosphorylation of mitogen-activated protein kinase (MAPKs) including p38 and c-Jun-NH₂-terminal kinase (JNK) was inhibited. Roburic acid exhibited inhibitory activities on production of NO and IL-6 via blocking IKK/IκB/NF-κB and MAPKs pathway, suggesting the potential application as a drug candidate for therapy of inflammatory diseases.

AQP4‑knockout alleviates the lipopolysaccharide‑induced inflammatory response in astrocytes via SPHK1/MAPK/AKT signaling

To date, aquaporin‑4 (AQP4) has been considered as a critical contributor to neuroinflammation, but little is known about the underlying mechanism. Previous studies have shown that a critical enzyme involved in the sphingomyelin cycle, sphingosine kinase 1 (SPHK1), is implicated in inflammatory processes and contributes to chronic neuroinflammation. The present study investigated the role of AQP4 in proinflammatory cytokine release from astrocytes, with an emphasis on the SPHK1/mitogen‑activated protein kinase (MAPK)/protein kinase B (AKT) pathway. Using primary cultures isolated from AQP4+/+ and AQP4‑/‑ embryos, the production of tumor necrosis factor‑α (TNF‑α)/interleukin‑6 (IL‑6) from astrocytes challenged by lipopolysaccharide (LPS) was compared. The results showed increased secretion of TNF‑α/IL‑6 in the two groups following LPS treatment, but a significantly lower level was observed in the AQP4‑/‑ group compared with that in the AQP4+/+ group. Although upregulation of SPHK1 was detected in the two genotypes, only a mild increase in SPHK1 was found in the AQP4‑/‑ genotype. The phosphorylation of MAPK/AKT was also confirmed to be attenuated in the AQP4‑/‑ group, suggesting decreased MAPK/AKT signaling over time in AQP4‑/‑ astrocytes. Overall, the study findings demonstrated that AQP4 deficiency alleviates proinflammatory cytokine release from astrocytes, in association with the SPHK1/MAPK/AKT pathway. This data improves our understanding of AQP4 in neuroinflammatory events, highlighting a novel profile of SPHK1 as a potential target for the treatment of CNS inflammation.

Crotoxin Isolated from Crotalus durissus terrificus Venom Modulates the Functional Activity of Dendritic Cells via Formyl Peptide Receptors

The Crotalus durissus terrificus rattlesnake venom, its main toxin, crotoxin (CTX), and its crotapotin (CA) and phospholipase A₂ (CB) subunits modulate the immune system. Formyl peptide receptors (FPRs) and lipoxin A₄ (LXA₄) are involved in CTX's effect on macrophages and neutrophils. Dendritic cells (DCs) are plasticity cells involved in the induction of adaptive immunity and tolerance maintenance. Therefore, we evaluated the effect of CTX, CA or CB on the maturation of DCs derived from murine bone marrow (BM). According to data, CTX and CB-but not CA-induced an increase of MHC-II, but not costimulatory molecules on DCs. Furthermore, CTX and CB inhibited the expression of costimulatory and MHC-II molecules, secretion of proinflammatory cytokines and NF-κBp65 and p38/ERK1/2-MAPK signaling pathways by LPS-incubated DCs. Differently, CTX and CB induced IL-10, PGE₂ and LXA₄ secretion in LPS-incubated DCs. Lower proliferation and IL-2 secretion were verified in coculture of CD3⁺ cells and DCs incubated with LPS plus CTX or CB compared with LPS-incubated DCs. The effect of CTX and CB on DCs was abolished in cultures incubated with a FPRs antagonist. Hence, CTX and CB exert a modulation on functional activity of DCs; we also checked the involvement the FPR family on cell activities.

Mitogen-activated protein kinase inhibition enhances the antitumor effects of sporamin in human pancreatic cancer cells

Sporamin, a sweet potato tuber storage protein, is a Kunitz-type trypsin inhibitor (TI) that has exhibited antitumor activity through poorly defined mechanisms in a number of types of tumor cells. The present study aimed to analyze the combined effects of sporamin and three mitogen-activated protein kinase (MAPK) inhibitors, PD98059, SP600125 and SB203580, on the pancreatic cancer cell line, PANC-1. Cell proliferation activity was assessed using a ³H-thymidine incorporation assay, and cell viability was analyzed using an MTT assay. Apoptosis was assayed by flow cytometry and fluorescence microscopy. Protein expression levels in PANC-1 cells were determined by western blotting. The results of this analysis demonstrated that sporamin induced a temporary increase in the phosphorylation of MAPKs, including phosphorylated extracellular signal regulated-kinase 1/2, phosphorylated c-Jun amino-terminal protein kinase 1/2 and phosphorylated p38-MAPK, in a concentration-dependent manner. However, treatment with MAPK inhibitors promoted the inhibition of cell proliferation and viability, and the induction of apoptosis in sporamin-treated PANC-1 cells. In conclusion, the present study demonstrated that MAPK inhibition enhanced the antitumor activity of sporamin in PANC-1 cells.

Down-regulation of UBC9 increases the sensitivity of hepatocellular carcinoma to doxorubicin

UBC9 is an E2-conjugating enzyme that is required for SUMOylation and has been implicated in regulating several critical cellular pathways. UBC9 is overexpressed in certain tumors, such as lung adenocarcinoma, ovarian carcinoma and melanoma, which implies that it has special clinical significance. However, the role of UBC9 in Hepatocellular carcinoma (HCC) drug responsiveness is not clear. In this study, we investigated the clinicopathological significance of UBC9 in HCC and investigated the mechanism of UBC9-mediated chemosensitivity to doxorubicin (DOX) in hepatocellular carcinoma cells. We found that relative to adjacent normal tissues, UBC9 was markedly overexpressed in HCC, which closely correlated with tumor size, tumor microsatellite formation, and tumor encapsulation. Our results also showed that down-regulation of UBC9 by shRNA reduced the expression of Bcl-2 and Bcl-xl and increased the expression of cleaved-Caspase3, which is a proapoptotic protein. These changes were associated with reduced apoptosis in response to DOX. Furthermore, we observed a mechanism involving modulation of the P38 and ERK1/2 signaling pathways. Together, our results indicate that down-regulation of UBC9 sensitizes cells to anticancer drugs, is possibly associated with the regulation of ERK1/2 and P38 activation and interacts with the intrinsic apoptosis pathway. Thus, knockdown of UBC9 may have a tumor suppressor effect and UBC9 could be a potential target for the treatment of HCC cancer.

Dynamic activation and regulation of the mitogen-activated protein kinase p38

Mitogen-activated protein kinases, which include p38, are essential for cell differentiation and autophagy. The current model for p38 activation involves activation-loop phosphorylation with subsequent substrate binding leading to substrate phosphorylation. Despite extensive efforts, the molecular mechanism of activation remains unclear. Here, using NMR spectroscopy, we show how the modulation of protein dynamics across timescales activates p38. We find that activation-loop phosphorylation does not change the average conformation of p38; rather it quenches the loop ps-ns dynamics. We then show that substrate binding to nonphosphorylated and phosphorylated p38 results in uniform µs-ms backbone dynamics at catalytically essential regions and across the entire molecule, respectively. Together, these results show that phosphorylation and substrate binding flatten the energy landscape of the protein, making essential elements of allostery and activation dynamically accessible. The high degree of structural conservation among ser/thr kinases suggests that elements of this mechanism may be conserved across the kinase family.

Tenascin‑C promotes the migration of bone marrow stem cells via toll‑like receptor 4‑mediated signaling pathways: MAPK, AKT and Wnt

There are currently limitations in stem cell therapy due to the low rate of homing and proliferation of cells following transplantation. The present study was designed to investigate the effects of Tenascin-C (TN-C) on bone marrow mesenchymal stem cells (BMSCs) and its underlying mechanisms. BMSCs were obtained from C57BL/6 mice. The survival and proliferation of BMSCs was analyzed by Cell Counting Kit-8 assay, migration was evaluated using the Transwell method, and differentiation was assessed by immunocytochemistry and immunofluorescence. In addition, the levels of proteins were detected by western blotting. High concentrations of TN-C promoted the migration of BMSCs. H₂O₂ at concentrations of 60–90 µmol/ml induced cell death in BMSCs, and thus, it was used to simulate oxidative stress in the microenvironment of acute myocardial infarction (AMI). High concentrations of TN-C were able to protect BMSCs from cell death, and promoted the migration of BMSCs (P<0.05). However, TAK-242 [the inhibitor of Toll-like receptor 4, (TLR4)] reduced the promoting effect of TN-C (P<0.05). By contrast, TN-C had no effect on the proliferation and differentiation of BMSCs. TN-C reduced the phosphorylation levels of p38 mitogen-activated protein kinase (MAPK), and increased the phosphorylation levels of Ser473 protein kinase B (AKT) and β-catenin, all of which were inhibited by TAK-242 (P<0.05). In the simulated AMI microenvironment, TN-C promoted the migration of BMSCs via TLR4-mediated signaling pathways, including MAPK, AKT and Wnt.

Role of granulosa cell mitogen-activated protein kinase 3/1 in gonadotropin-mediated meiotic resumption from diplotene arrest of mammalian oocytes

In mammals, preovulatory oocytes are encircled by several layers of granulosa cells (GCs) in follicular microenvironment. These follicular oocytes are arrested at diplotene arrest due to high level of cyclic nucleotides from encircling GCs. Pituitary gonadotropin acts at the level of encircling GCs and increases adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) and activates mitogen-activated protein kinase 3/1 (MAPK3/1) signaling pathway. The MAPK3/1 disrupts the gap junctions between encircling GCs and oocyte. The disruption of gap junctions interrupts the transfer of cyclic nucleotides to the oocyte that results a drop in intraoocyte cAMP level. A transient decrease in oocyte cAMP level triggers maturation promoting factor (MPF) destabilization. The destabilized MPF finally triggers meiotic resumption from diplotene arrest in follicular oocyte. Thus, MAPK3/1 from GCs origin plays important role in gonadotropin-mediated meiotic resumption from diplotene arrest in follicular oocyte of mammals.

Importance of ERK1/2 in Regulation of Protein Translation during Oocyte Meiosis

Although the involvement of the extracellular signal-regulated kinases 1 and 2 (ERK1/2) pathway in the regulation of cytostatic factor (CSF) activity; as well as in microtubules organization during meiotic maturation of oocytes; has already been described in detail; rather less attention has been paid to the role of ERK1/2 in the regulation of mRNA translation. However; important data on the role of ERK1/2 in translation during oocyte meiosis have been documented. This review focuses on recent findings regarding the regulation of translation and the role of ERK1/2 in this process in the meiotic cycle of mammalian oocytes. The specific role of ERK1/2 in the regulation of mammalian target of rapamycin (mTOR); eukaryotic translation initiation factor 4E (eIF4E) and cytoplasmic polyadenylation element binding protein 1 (CPEB1) activity is addressed along with additional focus on the other key players involved in protein translation.

Regulatory Mechanisms and Novel Therapeutic Targeting Strategies for Protein Tyrosine Phosphatases

An appropriate level of protein phosphorylation on tyrosine is essential for cells to react to extracellular stimuli and maintain cellular homeostasis. Faulty operation of signal pathways mediated by protein tyrosine phosphorylation causes numerous human diseases, which presents enormous opportunities for therapeutic intervention. While the importance of protein tyrosine kinases in orchestrating the tyrosine phosphorylation networks and in target-based drug discovery has long been recognized, the significance of protein tyrosine phosphatases (PTPs) in cellular signaling and disease biology has historically been underappreciated, due to a large extent to an erroneous assumption that they are largely constitutive and housekeeping enzymes. Here, we provide a comprehensive examination of a number of regulatory mechanisms, including redox modulation, allosteric regulation, and protein oligomerization, that control PTP activity. These regulatory mechanisms are integral to the myriad PTP-mediated biochemical events and reinforce the concept that PTPs are indispensable and specific modulators of cellular signaling. We also discuss how disruption of these PTP regulatory mechanisms can cause human diseases and how these diverse regulatory mechanisms can be exploited for novel therapeutic development.