Mitogen-activated protein kinase p38 and MK2, MK3 and MK5: ménage à trois or ménage à quatre?

Computational and Functional Analysis of Structural Features in the ZAKα Kinase

The kinase ZAKα acts as the proximal sensor of translational impairment and ribotoxic stress, which results in the activation of the MAP kinases p38 and JNK. Despite recent insights into the functions and binding partners of individual protein domains in ZAKα, the mechanisms by which ZAKα binds ribosomes and becomes activated have remained elusive. Here, we highlight a short, thrice-repeated, and positively charged peptide motif as critical for the ribotoxic stress-sensing function of the Sensor (S) domain of ZAKα. We use this insight to demonstrate that the mutation of the SAM domain uncouples ZAKα activity from ribosome binding. Finally, we use 3D structural comparison to identify and functionally characterize an additional folded domain in ZAKα with structural homology to YEATS domains. These insights allow us to formulate a model for ribosome-templated ZAKα activation based on the re-organization of interactions between modular protein domains. In sum, our work both advances our understanding of the protein domains and 3D architecture of the ZAKα kinase and furthers our understanding of how the ribotoxic stress response is activated.

The TLK1-MK5 Axis Regulates Motility, Invasion, and Metastasis of Prostate Cancer Cells

Background: Metastatic dissemination of prostate cancer (PCa) accounts for the majority of PCa-related deaths. However, the exact mechanism of PCa cell spread is still unknown. We uncovered a novel interaction between two unrelated promotility factors, tousled-like kinase 1 (TLK1) and MAPK-activated protein kinase 5 (MK5), that initiates a signaling cascade promoting metastasis. In PCa, TLK1−MK5 signaling might be crucial, as androgen deprivation therapy (ADT) leads to increased expression of both TLK1 and MK5 in metastatic patients, but in this work, we directly investigated the motility, invasive, and metastatic capacity of PCa cells following impairment of the TLK1 > MK5 axis. Results: We conducted scratch wound repair and transwell invasion assays with LNCaP and PC3 cells to determine if TLK1 and MK5 can regulate motility and invasion. Both genetic depletion and pharmacologic inhibition of TLK1 and MK5 resulted in reduced migration and invasion through a Matrigel plug. We further elucidated the potential mechanisms underlying these effects and found that this is likely due to the reorganization of the actin fibers at lamellipodia and the focal adhesions network, in conjunction with increased expression of some MMPs that can affect penetration through the ECM. PC3, a highly metastatic cell line when assayed in xenografts, was further tested in a tail-vein injection/lung metastasis model, and we showed that, following inoculation, treatment with GLPG0259 (MK5 specific inhibitor) or J54 (TLK1 inhibitor) resulted in the lung tumor nodules being greatly diminished in number, and for J54, also in size. Conclusion: Our data support that the TLK1−MK5 axis is functionally involved in driving PCa cell metastasis and clinical aggressiveness; hence, disruption of this axis may inhibit the metastatic capacity of PCa.

Mechanistic and functional extrapolation of SET and MYND domain-containing protein 2 to pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal neoplasms worldwide and represents the vast majority of pancreatic cancer cases. Understanding the molecular pathogenesis and the underlying mechanisms involved in the initiation, maintenance, and progression of PDAC is an urgent need, which may lead to the development of novel therapeutic strategies against this deadly cancer. Here, we review the role of SET and MYND domain-containing protein 2 (SMYD2) in initiating and maintaining PDAC development through methylating multiple tumor suppressors and oncogenic proteins. Given the broad substrate specificity of SMYD2 and its involvement in diverse oncogenic signaling pathways in many other cancers, the mechanistic extrapolation of SMYD2 from these cancers to PDAC may allow for developing new hypotheses about the mechanisms driving PDAC tumor growth and metastasis, supporting a proposition that targeting SMYD2 could be a powerful strategy for the prevention and treatment of PDAC.

Costly immunometabolic remodelling in disused muscle buildup through physical exercise

The mechanisms underlying the immunometabolic disturbances during skeletal muscle atrophy caused by a plethora of circumstances ranging from hospitalization to spaceflight missions remain unknown. Here, we outline the possible pathways that might be dysregulated in such conditions and assess the potential of physical exercise to mitigate and promote the recovery of muscle morphology, metabolism and function after intervals of disuse. Studies applying exercise to attenuate disuse-induced muscle atrophy have shown a pivotal role of circulating myokines in the activation of anabolic signalling pathways. These muscle-derived factors induce accretion of contractile proteins in the myofibers, and at the same time decrease protein breakdown and loss. Regular exercise plays a crucial role in re-establishing adequate immunometabolism and increasing the migration and presence in the muscle of macrophages with an anti-inflammatory phenotype (M2) and T regulatory cells (Tregs) after disease-induced muscle loss. Additionally, the switch in metabolic pathways (glycolysis to oxidative phosphorylation [OXPHOS]) is important for achieving rapid metabolic homeostasis during muscle regeneration. In this review, we discuss the molecular aspects of the immunometabolic response elicited by exercise during skeletal muscle regeneration. There is not, nevertheless, consensus on a single optimal intensity of exercise required to improve muscle strength, mass and functional capacity owing to the wide range of exercise protocols studied so far. Despite the absence of agreement on the specific strategy, physical exercise appears as a powerful complementary strategy to attenuate the harmful effects of muscle disuse in different scenarios.

Cynanchum auriculatum Royle ex Wight., Cynanchum bungei Decne. and Cynanchum wilfordii (Maxim.) Hemsl.: Current Research and Prospects

Cynanchum auriculatum Royle ex Wight. (CA), Cynanchum bungei Decne. (CB) and Cynanchum wilfordii (Maxim.) Hemsl. (CW) are three close species belonging to the Asclepiadaceous family, and their dry roots as the bioactive part have been revealed to exhibit anti-tumor, neuroprotection, organ protection, reducing liver lipid and blood lipid, immunomodulatory, anti-inflammatory, and other activities. Until 2021, phytochemistry investigations have uncovered 232 compounds isolated from three species, which could be classified into C21-steroids, acetophenones, terpenoids, and alkaloids. In this review, the morphology characteristics, species identification, and the relationship of botany, extraction, and the separation of chemical constituents, along with the molecular mechanism and pharmacokinetics of bioactive constituents of three species, are summarized for the first time, and their phytochemistry, pharmacology, and clinical safety are also updated. Moreover, the direction and limitation of current research on three species is also discussed.

SIRT1 provides new pharmacological targets for polydatin through its role as a metabolic sensor

The SIRT family of proteins constitutes highly conserved deacetylases with diverse and extensive functions. These proteins have specific biological functions, including regulation of transcription, cell cycle, cell differentiation, apoptosis, stress, metabolism, and genomic stability. Polydatin is a monocrystalline compound isolated from a Chinese herb, Polygonum cuspidatum. The pharmacological mechanisms of polydatin are mostly unclear but involve members of the SIRT protein family, among which SIRT1 plays a vital role. Polydatin is usually considered a potential SIRT1 activator. This review summarizes the signaling mechanism of polydatin involving SIRT1 and discusses the roles of related signal molecules such as PGC-1α, Nrf2, p38-MAPK, NLPR3 inflammasome, and p53. Further, we describe the metabolic regulation of related biological macromolecules and demonstrate that SIRT1, as a metabolic sensor, may act as a new pharmacological target for polydatin.

Characterization of TGF-β by Induced Oxidative Stress in Human Trabecular Meshwork Cells

Oxidative stress generated by reactive oxygen species (ROS) plays a critical role in the pathomechanism of glaucoma, which is a multifactorial blinding disease that may cause irreversible damage within human trabecular meshwork cells (HTMCs). It is known that the transforming growth factor-β (TGF-β) signaling pathway is an important component of oxidative stress-induced damage related to extracellular matrix (ECM) fibrosis and activates cell antioxidative mechanisms. To elucidate the dual potential roles and regulatory mechanisms of TGF-β in effects on HTMCs, we established an in vitro oxidative model using hydrogen peroxide (H₂O₂) and further focused on TGF-β-related oxidative stress pathways and the related signal transduction. Via a series of cell functional qualitative analyses to detect related protein level alterations and cell fibrosis status, we illustrated the role of TGF-β1 and TGF-β2 in oxidative stress-induced injury by shTGF-β1 and shTGF-β2 knockdown or added recombinant human TGF-β1 protein (rhTGF-β1). The results of protein level showed that p38 MAPK, TGF-β, and its related SMAD family were activated after H₂O₂ stimulation. Cell functional assays showed that HTMCs with H₂O₂ exposure duration had a more irregular actin architecture compared to normal TM cells. Data with rhTGF-β1 (1 ng/mL) pretreatment reduced the cell apoptosis rate and amount of reactive oxygen species (ROS), while it also enhanced survival. Furthermore, TGF-β1 and TGF-β2 in terms of antioxidant signaling were related to the activation of collagen I and laminin, which are fibrosis-response proteins. Succinctly, our study demonstrated that low concentrations of TGF-β1 (1 ng/mL) preserves HTMCs from free radical-mediated injury by p-p38 MAPK level and p-AKT signaling balance, presenting a signaling transduction mechanism of TGF-β1 in HTMC oxidative stress-related therapies.

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.

Effect of Acupuncture on the p38 Signaling Pathway in Several Nervous System Diseases: A Systematic Review

Acupuncture is clinically used to treat various diseases and exerts positive local and systemic effects in several nervous system diseases. Advanced molecular and clinical studies have continually attempted to decipher the mechanisms underlying these effects of acupuncture. While a growing understanding of the pathophysiology underlying several nervous system diseases shows it to be related to inflammation and impair cell regeneration after ischemic events, the relationship between the therapeutic mechanism of acupuncture and the p38 MAPK signal pathway has yet to be elucidated. This review discusses the latest advancements in the identification of the effect of acupuncture on the p38 signaling pathway in several nervous system diseases. We electronically searched databases including PubMed, Embase, and the Cochrane Library from their inception to April 2020, using the following keywords alone or in various combinations: "acupuncture", "p38 MAPK pathway", "signaling", "stress response", "inflammation", "immune", "pain", "analgesic", "cerebral ischemic injury", "epilepsy", "Alzheimer's disease", "Parkinson's disease", "dementia", "degenerative", and "homeostasis". Manual acupuncture and electroacupuncture confer positive therapeutic effects by regulating proinflammatory cytokines, ion channels, scaffold proteins, and transcription factors including TRPV1/4, Nav, BDNF, and NADMR1; consequently, p38 regulates various phenomena including cell communication, remodeling, regeneration, and gene expression. In this review article, we found the most common acupoints for the relief of nervous system disorders including GV20, GV14, ST36, ST37, and LI4. Acupuncture exhibits dual regulatory functions of activating or inhibiting different p38 MAPK pathways, contributing to an overall improvement of clinical symptoms and function in several nervous system diseases.

Cox2-mediated PGE2 production via p38/JNK-c-fos signaling inhibits cell apoptosis in 3D floating culture clumps of mesenchymal stem cell/extracellular matrix complexes

Mesenchymal stem cells (MSCs), a class of adult stem cells, have attracted scientific and medical attention due to their self-renewing properties, multipotency, and trophic factor production. Although MSCs were originally studied on classical two-dimensional (2D) plastic plates, extensive scientific efforts have developed three-dimensional (3D) MSC culture systems, including MSCs spheroids and organoids that can mimic physical conditions. Moreover, we have recently developed 3D culture clumps of MSCs/extracellular matrix (ECM) complexes (C-MSCs) for novel bone regenerative cell therapy. Of note, even though it is widely accepted that cell detachment from the culture plate causes cell apoptosis, so called anoikis, these 3D MSCs constructs can be maintained in floating culture conditions. Currently, it is unclear why 3D floating-cultured MSCs constructs can escape from anoikis. To answer this question, the present study explored trophic factor production in 3D floating-cultured C-MSCs that play a cytoprotective role against anoikis and clarified the underlying molecular mechanism in vitro. Compared with cells cultured on 2D plastic plates, PGE2 production mediated by COX2 was significantly increased, and its inhibition drastically induced cell apoptosis in 3D floating-cultured C-MSCs. In the process of C-MSCs preparation, detachment of the cell sheet from culture plate activated the p38/JNK-c-Fos signaling pathway. Moreover, blockage of this signaling by chemical inhibitors abrogated COX2/PGE2 expressions and induced severe apoptosis. These results demonstrated that cell detachment facilitates cytoprotective COX2-mediated PGE2 synthesis via p38/JNK-c-Fos signaling, revealing a possible mechanism that allows resistance against anoikis in floating-cultured 3D MSCs constructs.

Integrating long noncoding RNAs and mRNAs expression profiles of response to Plasmodiophora brassicae infection in Pakchoi (Brassica campestris ssp. chinensis Makino)

The biotrophic protist Plasmodiophora brassicae causes serious damage to Brassicaceae crops grown worldwide. However, the molecular mechanism of the Brassica rapa response remains has not been determined. Long noncoding RNA and mRNA expression profiles in response to Plasmodiophora brassicae infection were investigated using RNA-seq on the Chinese cabbage inbred line C22 infected with P. brassicae. Approximately 5,193 mRNAs were significantly differentially expressed, among which 1,345 were upregulated and 3,848 were downregulated. The GO enrichment analysis shows that most of these mRNAs are related to the defense response. Meanwhile, 114 significantly differentially expressed lncRNAs were identified, including 31 upregulated and 83 downregulated. Furthermore, a total of 2,344 interaction relationships were detected between 1,725 mRNAs and 103 lncRNAs with a correlation coefficient greater than 0.8. We also found 15 P. brassicaerelated mRNAs and 16 lncRNA interactions within the correlation network. The functional annotation showed that 15 mRNAs belong to defense response proteins (66.67%), protein phosphorylation (13.33%), root hair cell differentiation (13.33%) and regulation of salicylic acid biosynthetic process (6.67%). KEGG annotation showed that the vast majority of these genes are involved in the biosynthesis of secondary metabolism pathways and plant-pathogen interactions. These results provide a new perspective on lncRNA-mRNA network function and help to elucidate the molecular mechanism of P. brassicae infection.

Cardiac Fibroblast p38 MAPK: A Critical Regulator of Myocardial Remodeling

The cardiac fibroblast is a remarkably versatile cell type that coordinates inflammatory, fibrotic and hypertrophic responses in the heart through a complex array of intracellular and intercellular signaling mechanisms. One important signaling node that has been identified involves p38 MAPK; a family of kinases activated in response to stress and inflammatory stimuli that modulates multiple aspects of cardiac fibroblast function, including inflammatory responses, myofibroblast differentiation, extracellular matrix turnover and the paracrine induction of cardiomyocyte hypertrophy. This review explores the emerging importance of the p38 MAPK pathway in cardiac fibroblasts, describes the molecular mechanisms by which it regulates the expression of key genes, and highlights its potential as a therapeutic target for reducing adverse myocardial remodeling.

Prions activate a p38 MAPK synaptotoxic signaling pathway

Synaptic degeneration is one of the earliest pathological correlates of prion disease, and it is a major determinant of the progression of clinical symptoms. However, the cellular and molecular mechanisms underlying prion synaptotoxicity are poorly understood. Previously, we described an experimental system in which treatment of cultured hippocampal neurons with purified PrP^Sc, the infectious form of the prion protein, induces rapid retraction of dendritic spines, an effect that is entirely dependent on expression of endogenous PrP^C by the target neurons. Here, we use this system to dissect pharmacologically the underlying cellular and molecular mechanisms. We show that PrP^Sc initiates a stepwise synaptotoxic signaling cascade that includes activation of NMDA receptors, calcium influx, stimulation of p38 MAPK and several downstream kinases, and collapse of the actin cytoskeleton within dendritic spines. Synaptic degeneration is restricted to excitatory synapses, spares presynaptic structures, and results in decrements in functional synaptic transmission. Pharmacological inhibition of any one of the steps in the signaling cascade, as well as expression of a dominant-negative form of p38 MAPK, block PrP^Sc-induced spine degeneration. Moreover, p38 MAPK inhibitors actually reverse the degenerative process after it has already begun. We also show that, while PrP^C mediates the synaptotoxic effects of both PrP^Sc and the Alzheimer’s Aβ peptide in this system, the two species activate distinct signaling pathways. Taken together, our results provide powerful insights into the biology of prion neurotoxicity, they identify new, druggable therapeutic targets, and they allow comparison of prion synaptotoxic pathways with those involved in other neurodegenerative diseases.

Prion diseases are a group of fatal neurodegenerative disorders that includes Creutzfeldt-Jakob disease and kuru in humans, and bovine spongiform encephalopathy in cattle. The infectious agent, or prion, that transmits these diseases is a naked protein molecule, the prion protein (PrP), which is an altered form of a normal, cellular protein. Although a great deal is known about how prions propagate themselves and transmit infection, the process by which they actually cause neurons to degenerate has remained mysterious. Here, we have used a specialized neuronal culture system to dissect the cellular and molecular mechanisms by which prions damage synapses, the structures that connect nerve cells and that play a crucial role in learning, memory, and neurological disease. Our results define a stepwise molecular pathway underlying prion synaptic toxicity, which involves activation of glutamate neurotransmitter receptors, influx of calcium ions into the neuron, and stimulation of specific mitogen-activated protein kinases, which attach phosphate groups to proteins to regulate their activity. We demonstrate that specific drugs, as well as a dominant-negative kinase mutant, block these steps and thereby prevent the synaptic degeneration produced by prions. Our results provide new insights into the pathogenesis of prion diseases, they uncover new drug targets for treating these diseases, and they allow us to compare prion diseases to other, more common neurodegenerative disorders like Alzheimer’s disease.

MiR-137 inhibited inflammatory response and apoptosis after spinal cord injury via targeting of MK2

Spinal cord injuries are common and troublesome disorder, which is mediated by various signal pathways and mechanisms. MK2 is also involved in numerous inflammatory diseases including spinal cord injury. The role of microRNA-137 (miR-137) and its detailed working mechanism in spinal cord injuries remain unclear. In the present study, we found that an elevated MK2 but a decreased miR-137 was expressed in serum specimens of patients with spinal cord injury and in hydrogen peroxide-treated C8-D1A and C8-B4 cells. Meanwhile, we suggested that upregulation of miR-137 could inhibit the expression of TNF-α and IL-6, two markers of inflammatory response after SCI, and apoptosis in hydrogen peroxide-treated C8-D1A and C8-B4 cells. Furthermore, we verified that MK2 was a direct target of miR-137 thorough a constructed luciferase assay. Even further, we elucidated that miR-137 could suppress the inflammatory response and apoptosis via negative regulation of MK2. Finally, through an animal model trial performed using mice, we demonstrated the protective effect of how miR-137 works on inflammatory response and apoptosis after spinal cord injury. Considering all the forementioned, our findings revealed that miR-137 inhibited inflammatory response and apoptosis after spinal cord injury via the targeting of MK2. The outcomes of the present study might indicate a new target in molecular treatment of SCI.

Pharmacological and genetic inhibition of downstream targets of p38 MAPK in experimental nephrotic syndrome

Nie X, Chanley MA, Pengal R, Thomas DB, Agrawal S, Smoyer WE. Pharmacological and genetic inhibition of downstream targets of p38 MAPK in experimental nephrotic syndrome. Am J Physiol Renal Physiol 314: F602-F613, 2018. First published November 29, 2017; doi: 10.1152/ajprenal.00207.2017 .-The p38 MAPK pathway plays a crucial role in various glomerulopathies, with activation being associated with disease and inhibition being associated with disease amelioration. We hypothesized that the downstream targets of p38 MAPK, MAPK-activated protein kinase 2 and/or 3 (MK2 and/or MK3), play an important role in mediating injury in experimental nephrotic syndrome via their actions on their downstream substrates heat shock protein B1 (HSPB1) and cyclooxygenase-2 (COX-2). To test this hypothesis, the effects of both pharmacological and genetic inhibition of MK2 and MK3 were examined in mouse adriamycin (ADR) and rat puromycin aminonucleoside (PAN) nephropathy models. MK2^-/-, MK3^-/-, and MK2^-/-MK3^-/- mice were generated in the Sv129 background and subjected to ADR-induced nephropathy. MK2 and MK3 protein expression was completely abrogated in the respective knockout genotypes, and massive proteinuria and renal histopathological changes developed after ADR treatment. Furthermore, renal cortical HSPB1 was induced in all four genotypes by day 21, but HSPB1 was activated only in the wild-type and MK3^-/- mice. Expression of the stress proteins HSPB8 and glucose-regulated protein 78 (GRP78) remained unaltered across all genotypes. Finally, while MK2 and/or MK3-knockout downregulated the proinflammatory enzyme COX-2, ADR significantly induced renal cortical COX-2 only in MK2^-/- mice. Additionally, pharmacological MK2 inhibition with PF-318 during PAN-induced nephropathy did not result in significant proteinuria reduction in rats. Together, these data suggest that while the inhibition of MK2 and/or MK3 regulates the renal stress response, our currently available approaches are not yet able to safely and effectively reduce proteinuria in experimental nephrotic syndrome and that other p38MAPK downstream targets should also be considered to improve the future treatment of glomerular disease.

MK5: A novel regulator of cardiac fibroblast function?

MAP kinase-activated protein kinases (MKs), protein serine/threonine kinases downstream of the MAPKs, regulate a number of biological functions. MK5 was initially identified as a substrate for p38 MAPK but subsequent studies revealed that MK5 activity is regulated by atypical MAPKs ERK3 and ERK4. However, the roles of these MAPKs in activating MK5 remain controversial. The interactome and physiological function of MK5 are just beginning to be understood. Here, we provide an overview of the structure-function of MK5 including recent progress in determining its role in cardiac structure and function. The cardiac phenotype of MK5 haplodeficient mice, and the effect of reduced MK5 expression on cardiac remodeling, is also discussed. © 2017 IUBMB Life, 69(10):785-794, 2017.

pERK-dependent defective TCR-mediated activation of CD4+ T cells in end-stage renal disease patients

Background

Patients with end-stage renal disease (ESRD) have an impaired immune response with a prematurely aged T-cell system. Mitogen-activated protein kinases (MAPKs) including extracellular signal-regulated kinase (ERK) and p38, regulate diverse cellular programs by transferring extracellular signals into an intracellular response. T cell receptor (TCR)-induced phosphorylation of ERK (pERK) may show an age-associated decline, which can be reversed by inhibiting dual specific phosphatase (DUSP) 6, a cytoplasmic phosphatase with substrate specificity to dephosphorylate pERK. The aim of this study was to assess whether ESRD affects TCR-mediated signaling and explore possibilities for intervening in ESRD-associated defective T-cell mediated immunity.

Results

An age-associated decline in TCR-induced pERK-levels was observed in the different CD4⁺ (P < 0.05), but not CD8⁺, T-cell subsets from healthy individuals (HI). Interestingly, pERK-levels of CD4⁺ T-cell subsets from young ESRD patients were in between young and elderly HI. A differentiation-associated decline in TCR-induced ERK and p38 phosphorylation was observed in T cells, although TCR-induced p38 phosphorylation was not significantly affected by age and/or ESRD. Frequencies of TCR-induced CD69-expressing CD4⁺ T cells declined with age and were positively associated with pERK. In addition, an age-associated tendency of increased expression of DUSP6 was observed in CD4⁺ T cells of HI and DUSP6 expression in young ESRD patients was similar to old HI. Inhibition of DUSP6 significantly increased TCR-induced pERK-levels of CD4⁺ T cells in young and elderly ESRD patients, and elderly HI.

Conclusions

TCR-mediated phosphorylation of ERK is affected in young ESRD patients consistent with the concept of premature immunological T cell ageing. Inhibition of DUSP6 specific for pERK might be a potential intervention enhancing T-cell mediated immunity in ESRD patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s12979-017-0096-1) contains supplementary material, which is available to authorized users.

Metformin potentiates the effect of arsenic trioxide suppressing intrahepatic cholangiocarcinoma: roles of p38 MAPK, ERK3, and mTORC1

Background

Arsenic trioxide (ATO) is commonly used in the treatment of acute promyelocytic leukemia (APL), but does not benefit patients with solid tumors. When combined with other agents or radiation, ATO showed treatment benefits with manageable toxicity. Previously, we reported that metformin amplified the inhibitory effect of ATO on intrahepatic cholangiocarcinoma (ICC) cells more significantly than other agents. Here, we investigated the chemotherapeutic sensitization effect of metformin in ATO-based treatment in ICC in vitro and in vivo and explored the underlying mechanisms.

Methods

ICC cell lines (CCLP-1, RBE, and HCCC-9810) were treated with metformin and/or ATO; the anti-proliferation effect was evaluated by cell viability, cell apoptosis, cell cycle, and intracellular-reactive oxygen species (ROS) assays. The in vivo efficacy was determined in nude mice with CCLP-1 xenografts. The active status of AMPK/p38 MAPK and mTORC1 pathways was detected by western blot. In addition, an antibody array was used screening more than 200 molecules clustered in 12 cancer-related pathways in CCLP-1 cells treated with metformin and/or ATO. Methods of genetic modulation and pharmacology were further used to demonstrate the relationship of the molecule. Seventy-three tumor samples from ICC patients were used to detect the expression of ERK3 by immunohistochemistry. The correlation between ERK3 and the clinical information of ICC patients were further analyzed.

Results

Metformin and ATO synergistically inhibited proliferation of ICC cells by promoting cell apoptosis, inducing G0/G1 cell cycle arrest, and increasing intracellular ROS. Combined treatment with metformin and ATO efficiently reduced ICC growth in an ICC xenograft model. Mechanistically, the antibody array revealed that ERK3 exhibited the highest variation in CCLP-1 cells after treatment with metformin and ATO. Results of western blot confirm that metformin and ATO cooperated to inhibit mTORC1, activate AMP-activated protein kinase (AMPK), and upregulate ERK3. Metformin abrogated the activation of p38 MAPK induced by ATO, and this activity was partially dependent on AMPK activation. Inactivation of p38 MAPK by SB203580 or specific short interfering RNA (siRNA) promoted the inactivation of mTORC1 in ICC cells treated with metformin and ATO. Activation of p38 MAPK may be responsible for resistance to ATO in ICC. The relationship between p38 MAPK and ERK3 was not defined by our findings. Finally, AMPK is a newfound positive regulator of ERK3. Overexpression of EKR3 in ICC cells inhibited cell proliferation through inactivation of mTORC1. ERK3 expression is associated with a better prognosis in ICC patients.

Conclusions

Metformin sensitizes arsenic trioxide to suppress intrahepatic cholangiocarcinoma via the regulation of AMPK/p38 MAPK-ERK3/mTORC1 pathways. ERK3 is a newfound potential prognostic predictor and a tumor suppressor in ICC.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-017-0424-0) contains supplementary material, which is available to authorized users.

The Extracellular-Regulated Kinase Effector Lk6 is Required for Glutamate Receptor Localization at the Drosophila Neuromuscular Junction

The proper localization and synthesis of postsynaptic glutamate receptors are essential for synaptic plasticity. Synaptic translation initiation is thought to occur via the target of rapamycin (TOR) and mitogen-activated protein kinase signal-integrating kinase (Mnk) signaling pathways, which is downstream of extracellular-regulated kinase (ERK). We used the model glutamatergic synapse, the Drosophila neuromuscular junction, to better understand the roles of the Mnk and TOR signaling pathways in synapse development. These synapses contain non-NMDA receptors that are most similar to AMPA receptors. Our data show that Lk6, the Drosophila homolog of Mnk1 and Mnk2, is required in either presynaptic neurons or postsynaptic muscle for the proper localization of the GluRIIA glutamate receptor subunit. Lk6 may signal through eukaryotic initiation factor (eIF) 4E to regulate the synaptic levels of GluRIIA as either interfering with eIF4E binding to eIF4G or expression of a nonphosphorylatable isoform of eIF4E resulted in a significant reduction in GluRIIA at the synapse. We also find that Lk6 and TOR may independently regulate synaptic levels of GluRIIA.

The p38 mitogen-activated protein kinase signaling pathway is involved in regulating low-density lipoprotein receptor-related protein 1-mediated β-amyloid protein internalization in mouse brain

Alzheimer's disease (AD) is one of the most common neurodegenerative diseases. Recently, increasing evidence suggests that intracellular β-amyloid protein (Aβ) alone plays a pivotal role in the progression of AD. Therefore, understanding the signaling pathway and proteins that control Aβ internalization may provide new insight for regulating Aβ levels. In the present study, the regulation of Aβ internalization by p38 mitogen-activated protein kinases (MAPK) through low-density lipoprotein receptor-related protein 1 (LRP1) was analyzed in vivo. The data derived from this investigation revealed that Aβ1-42 were internalized by neurons and astrocytes in mouse brain, and were largely deposited in mitochondria and lysosomes, with some also being found in the endoplasmic reticulum. Aβ1-42-LRP1 complex was formed during Aβ1-42 internalization, and the p38 MAPK signaling pathway was activated by Aβ1-42 via LRP1. Aβ1-42 and LRP1 were co- localized in the cells of parietal cortex and hippocampus. Furthermore, the level of LRP1-mRNA and LRP1 protein involved in Aβ1-42 internalization in mouse brain. The results of this investigation demonstrated that Aβ1-42 induced an LRP1-dependent pathway that related to the activation of p38 MAPK resulting in internalization of Aβ1-42. These results provide evidence supporting a key role for the p38 MAPK signaling pathway which is involved in the regulation of Aβ1-42 internalization in the parietal cortex and hippocampus of mouse through LRP1 in vivo.

A dominant mutation in MAPKAPK3, an actor of p38 signaling pathway, causes a new retinal dystrophy involving Bruch's membrane and retinal pigment epithelium

Inherited retinal dystrophies are clinically and genetically heterogeneous with significant number of cases remaining genetically unresolved. We studied a large family from the West Indies islands with a peculiar retinal disease, the Martinique crinkled retinal pigment epitheliopathy that begins around the age of 30 with retinal pigment epithelium (RPE) and Bruch's membrane changes resembling a dry desert land and ends with a retinitis pigmentosa. Whole-exome sequencing identified a heterozygous c.518T>C (p.Leu173Pro) mutation in MAPKAPK3 that segregates with the disease in 14 affected and 28 unaffected siblings from three generations. This unknown variant is predicted to be damaging by bioinformatic predictive tools and the mutated protein to be non-functional by crystal structure analysis. MAPKAPK3 is a serine/threonine protein kinase of the p38 signaling pathway that is activated by a variety of stress stimuli and is implicated in cellular responses and gene regulation. In contrast to other tissues, MAPKAPK3 is highly expressed in the RPE, suggesting a crucial role for retinal physiology. Expression of the mutated allele in HEK cells revealed a mislocalization of the protein in the cytoplasm, leading to cytoskeleton alteration and cytodieresis inhibition. In Mapkapk3-/- mice, Bruch's membrane is irregular with both abnormal thickened and thinned portions. In conclusion, we identified the first pathogenic mutation in MAPKAPK3 associated with a retinal disease. These findings shed new lights on Bruch's membrane/RPE pathophysiology and will open studies of this signaling pathway in diseases with RPE and Bruch's membrane alterations, such as age-related macular degeneration.

MK3 modulation affects BMI1-dependent and independent cell cycle check-points

Although the MK3 gene was originally found deleted in some cancers, it is highly expressed in others. The relevance of MK3 for oncogenesis is currently not clear. We recently reported that MK3 controls ERK activity via a negative feedback mechanism. This prompted us to investigate a potential role for MK3 in cell proliferation. We here show that overexpression of MK3 induces a proliferative arrest in normal diploid human fibroblasts, characterized by enhanced expression of replication stress- and senescence-associated markers. Surprisingly, MK3 depletion evokes similar senescence characteristics in the fibroblast model. We previously identified MK3 as a binding partner of Polycomb Repressive Complex 1 (PRC1) proteins. In the current study we show that MK3 overexpression results in reduced cellular EZH2 levels and concomitant loss of epigenetic H3K27me3-marking and PRC1/chromatin-occupation at the CDKN2A/INK4A locus. In agreement with this, the PRC1 oncoprotein BMI1, but not the PCR2 protein EZH2, bypasses MK3-induced senescence in fibroblasts and suppresses P16^INK4A expression. In contrast, BMI1 does not rescue the MK3 loss-of-function phenotype, suggesting the involvement of multiple different checkpoints in gain and loss of MK3 function. Notably, MK3 ablation enhances proliferation in two different cancer cells. Finally, the fibroblast model was used to evaluate the effect of potential tumorigenic MK3 driver-mutations on cell proliferation and M/SAPK signaling imbalance. Taken together, our findings support a role for MK3 in control of proliferation and replicative life-span, in part through concerted action with BMI1, and suggest that the effect of MK3 modulation or mutation on M/SAPK signaling and, ultimately, proliferation, is cell context-dependent.

Structure and function of MK5/PRAK: the loner among the mitogen-activated protein kinase-activated protein kinases

Mitogen-activated protein kinase (MAPK) pathways are important signal transduction pathways that control pivotal cellular processes including proliferation, differentiation, survival, apoptosis, gene regulation, and motility. MAPK pathways consist of a relay of consecutive phosphorylation events exerted by MAPK kinase kinases, MAPK kinases, and MAPKs. Conventional MAPKs are characterized by a conserved Thr-X-Tyr motif in the activation loop of the kinase domain, while atypical MAPKs lack this motif and do not seem to be organized into the classical three-tiered kinase cascade. One functional group of conventional and atypical MAPK substrates consists of protein kinases known as MAPK-activated protein kinases. Eleven mammalian MAPK-activated protein kinases have been identified, and they are divided into five subgroups: the ribosomal-S6-kinases RSK1-4, the MAPK-interacting kinases MNK1 and 2, the mitogen- and stress-activated kinases MSK1 and 2, the MAPK-activated protein kinases MK2 and 3, and the MAPK-activated protein kinase MK5 (also referred to as PRAK). MK5/PRAK is the only MAPK-activated protein kinase that is a substrate for both conventional and atypical MAPK, while all other MAPKAPKs are exclusively phosphorylated by conventional MAPKs. This review focuses on the structure, activation, substrates, functions, and possible implications of MK5/PRAK in malignant and nonmalignant diseases.

Comparative molecular dynamics simulations of mitogen-activated protein kinase-activated protein kinase 5

The mitogen-activated protein kinase-activated protein kinase MK5 is a substrate of the mitogen-activated protein kinases p38, ERK3 and ERK4. Cell culture and animal studies have demonstrated that MK5 is involved in tumour suppression and promotion, embryogenesis, anxiety, cell motility and cell cycle regulation. In the present study, homology models of MK5 were used for molecular dynamics (MD) simulations of: (1) MK5 alone; (2) MK5 in complex with an inhibitor; and (3) MK5 in complex with the interaction partner p38α. The calculations showed that the inhibitor occupied the active site and disrupted the intramolecular network of amino acids. However, intramolecular interactions consistent with an inactive protein kinase fold were not formed. MD with p38α showed that not only the p38 docking region, but also amino acids in the activation segment, αH helix, P-loop, regulatory phosphorylation region and the C-terminal of MK5 may be involved in forming a very stable MK5-p38α complex, and that p38α binding decreases the residual fluctuation of the MK5 model. Electrostatic Potential Surface (EPS) calculations of MK5 and p38α showed that electrostatic interactions are important for recognition and binding.

Homology modeling and ligand docking of Mitogen-activated protein kinase-activated protein kinase 5 (MK5)

BACKGROUND

Mitogen-activated protein kinase-activated protein kinase 5 (MK5) is involved in one of the major signaling pathways in cells, the mitogen-activated protein kinase pathway. MK5 was discovered in 1998 by the groups of Houng Ni and Ligou New, and was found to be highly conserved throughout the vertebrates. Studies, both in vivo and in vitro, have shown that it is implicated in tumor suppression as well as tumor promotion, embryogenesis, anxiety, locomotion, cell motility and cell cycle regulation.

METHODS

In order to obtain a molecular model of MK5 that can be used as a working tool for development of chemical probes, three MK5 models were constructed and refined based on three different known crystal structures of the closely related MKs; MK2 [PDB: 2OZA and PDB: 3M2W] and MK3 [PDB: 3FHR]. The main purpose of the present MK5 molecular modeling study was to identify the best suited template for making a MK5 model. The ability of the generated models to effectively discriminate between known inhibitors and decoys was analyzed using receiver operating characteristic (ROC) curves.

RESULTS

According to the ROC curve analyzes, the refined model based on 3FHR was most effective in discrimination between known inhibitors and decoys.

CONCLUSIONS

The 3FHR-based MK5 model may serve as a working tool for development of chemical probes using computer aided drug design. The biological function of MK5 still remains elusive, but its role as a possible drug target may be elucidated in the near future.

MK5 activates Rag transcription via Foxo1 in developing B cells

The kinase MK5 phosphorylates and activates Foxo1 at serine 215, and this modification is required for Foxo1 to induce Rag transcription.

Foxo1 is a critical, direct regulator of Rag (recombination activating gene) transcription during B cell development and is thus essential for the generation of a diverse repertoire of antigen receptors. Although Foxo1 regulation has been widely studied in many cell types, pathways regulating Foxo1 in B cells have not been fully elucidated. By screening a panel of Foxo1 mutants, we identified serine 215 on Foxo1 as a novel phosphorylation site that is essential for the activation of Rag transcription. Mutation of S215 strongly attenuated transactivation of Rag but did not affect most other Foxo1 target genes. We show that MK5, a MAPK-activated protein kinase, is a previously unidentified upstream regulator of Foxo1. MK5 was necessary and sufficient to activate Rag transcription in transformed and primary pro–B cells. Together, our experiments show that MK5 positively regulates Rag transcription via phosphorylation of Foxo1 in developing B cells.

Crosstalk between p38 and Smad3 through TGF-β1 in JEG-3 choriocarcinoma cells

Choriocarcinoma is a highly malignant trophoblastic tumor related to pregnancy that often occurs with a complete hydatidiform mole. It grows quickly and can also widely metastasize to other organs or tissues through both the venous and lymphatic systems. The transforming growth factor-β1 (TGF-β1) belongs to a growth factor superfamily and has been suggested to play a critical role in regulating the genesis and development of choriocarcinoma through a variety of Smad-independent pathways, including the p38 MAPK pathway. Previous studies indicated that TGF-β can activate the p38 MAPK pathway. In this study, we investigated Smad and p38 MAPK signaling in JEG-3 choriocarcinoma cells using p38 MAPK inhibitor and TGF-β receptor inhibitor. Immunofluorescence and western blot assays were used to detect the proteins in Smad and p38 MAPK pathways. Our data demonstrated that TGF-β can activate Smad3 and induce Smad3 translocation into the nucleus in JEG-3 cells. Blockade of the TGF-β pathway significantly reduced the expression levels of p38 and phospho-p38. p38 MAPK inhibitors (SB 203580) can attenuate TGF-β1-induced Smad3 expression and suppress the activation of smad3. These findings indicate crosstalk between p38 and smad3 through TGF-β1 in choriocarcinoma cells.

Molecular modeling of p38α mitogen-activated protein kinase inhibitors through 3D-QSAR and molecular dynamics simulations

The p38 mitogen-activated protein kinase (MAPK) signaling pathway plays an essential role in inflammation and other physiological processes. Because specific inhibitors of p38α and p38β MAPK block the production of the major inflammatory cytokines and other proteins, p38α and p38β MAPK represent promising targets for the treatment of inflammation. In this work, a series of p38α inhibitors based on the structural scaffold of 4-benzoyl-5-aminopyrazole were analyzed using a combination of molecular modeling techniques. We generated three-dimensional quantitative structure-activity relationship (3D-QSAR) models for both comparative molecular field analysis (CoMFA) and comparative molecular similarity index analysis (CoMSIA) to highlight the structural requirements for p38 MAPK inhibition. Furthermore, we employed molecular dynamics (MD) simulations and the MM/GBSA method to compare the binding modes and binding free energies of a potent and selective compound interacting with p38α, p38β, p38γ, and p38δ MAPK in detail. Contour maps generated via 3D-QSAR analysis identified several key interactions that were also indicated through MD simulations. The binding free energies calculated via the MM/GBSA method were strongly correlated with experimentally observed biological activities and explained the selective inhibition of p38α and p38β, but not p38γ and p38δ detected here. On the basis of the obtained results, we provide insights regarding the development of novel potent p38α MAPK inhibitors.

The Role of Mitogen-Activated Protein Kinase-Activated Protein Kinases (MAPKAPKs) in Inflammation

Mitogen-activated protein kinase (MAPK) pathways are implicated in several cellular processes including proliferation, differentiation, apoptosis, cell survival, cell motility, metabolism, stress response and inflammation. MAPK pathways transmit and convert a plethora of extracellular signals by three consecutive phosphorylation events involving a MAPK kinase kinase, a MAPK kinase, and a MAPK. In turn MAPKs phosphorylate substrates, including other protein kinases referred to as MAPK-activated protein kinases (MAPKAPKs). Eleven mammalian MAPKAPKs have been identified: ribosomal-S6-kinases (RSK1-4), mitogen- and stress-activated kinases (MSK1-2), MAPK-interacting kinases (MNK1-2), MAPKAPK-2 (MK2), MAPKAPK-3 (MK3), and MAPKAPK-5 (MK5). The role of these MAPKAPKs in inflammation will be reviewed.

Genome-wide pathway analysis reveals different signaling pathways between secreted lactoferrin and intracellular delta-lactoferrin

Human lactoferrin (LF) is a multifunctional protein involved in immunomodulation, cellular growth, and differentiation. In addition to its secreted form (sLF), an alternative form (ΔLF) lacking the signal sequence has been found to be downregulated in cancer. Although the signaling pathways mediated by LF have been studied in a few cell models, there have been no relevant systemic approaches. Therefore, this study was carried out to identify and compare signaling networks provoked by the two LF isoforms. For this, the two forms were overexpressed in HEK293 cells using the Flp-In T-Rex system, after which genome-wide expression analysis of 18,367 genes was conducted. Pathway analysis of the genes showing altered expression identified pathways which are responsible for cell survival and apoptosis. In addition, the pathways mediated by the two LF forms were within distantly related networks. GPCR, PI3K complex, and POU5F1, which are involved in receptor-mediated pathways, were centered in the sLF network, whereas RIF1, NOS3, and RNPS1, which are involved in intracellular signaling, were centered in the ΔLF network. These results suggest that structural differences between the LF isoforms, mainly glycosylation, determine the fate of LF signaling. Furthermore, these findings provide information relating to the role of ΔLF which is downregulated during carcinogenesis.

Crucial roles of the protein kinases MK2 and MK3 in a mouse model of glomerulonephritis

Elevated mitogen-activated protein kinase p38 (p38 MAPK) signaling has been implicated in various experimental and human glomerulopathies, and its inhibition has proven beneficial in animal models of these diseases. p38 MAPK signaling is partially mediated through MK2 and MK3, two phylogenetically related protein kinases that are its direct substrates. The current study was designed to determine the specific roles of MK2 and MK3 in a mouse model of acute proliferative glomerulonephritis, using mice with disrupted MK2 and/or MK3 genes. We found that the absence of MK3 alone worsened the disease course and increased mortality slightly compared to wild-type mice, whereas the absence of MK2 alone exhibited no significant effect. However, in an MK3-free background, the disease course depended on the presence of MK2 in a gene dosage-dependent manner, with double knock-out mice being most susceptible to disease induction. Histological and renal functional analyses confirmed kidney damage following disease induction. Because the renal stress response plays a crucial role in kidney physiology and disease, we analyzed the stress response pattern in this disease model. We found that renal cortices of diseased mice exhibited a pronounced and specific pattern of expression and/or phosphorylation of stress proteins and other indicators of the stress response (HSPB1, HSPB6, HSPB8, CHOP, eIF2α), partially in a MK2/MK3 genotype-specific manner, and without induction of a general stress response. Similarly, the expression and activation patterns of other protein kinases downstream of p38 MAPK (MNK1, MSK1) depended partially on the MK2/MK3 genotype in this disease model. In conclusion, MK2 and MK3 together play crucial roles in the regulation of the renal stress response and in the development of glomerulonephritis, which can potentially be exploited to develop novel therapeutic approaches to treat glomerular disease.

MAPK-activated protein kinase 2 contributes to Clostridium difficile-associated inflammation

Clostridium difficile infection (CDI) results in toxin-induced epithelial injury and marked intestinal inflammation. Fecal markers of intestinal inflammation correlate with CDI disease severity, but regulation of the inflammatory response is poorly understood. Previous studies demonstrated that C. difficile toxin TcdA activates p38 kinase in tissue culture cells and mouse ilium, resulting in interleukin-8 (IL-8) release. Here, we investigated the role of phosphorylated mitogen-activated protein kinase (MAPK)-activated protein kinase (MK2 kinase, pMK2), a key mediator of p38-dependent inflammation, in CDI. Exposure of cultured intestinal epithelial cells to the C. difficile toxins TcdA and TcdB resulted in p38-dependent MK2 activation. Toxin-induced IL-8 and GROα release required MK2 activity. We found that p38 and MK2 are activated in response to other actin-disrupting agents, suggesting that toxin-induced cytoskeleton disruption is the trigger for kinase-dependent cytokine response. Phosphorylated MK2 was detected in the intestines of C. difficile-infected hamsters and mice, demonstrating for the first time that the pathway is activated in infected animals. Furthermore, we found that elevated pMK2 correlated with the presence of toxigenic C. difficile among 100 patient stool samples submitted for C. difficile testing. In conclusion, we find that MK2 kinase is activated by TcdA and TcdB and regulates the expression of proinflammatory cytokines. Activation of p38-MK2 in infected animals and humans suggests that this pathway is a key driver of intestinal inflammation in patients with CDI.

A screen for selective killing of cells with chromosomal instability induced by a spindle checkpoint defect

BACKGROUND

The spindle assembly checkpoint is crucial for the maintenance of a stable chromosome number. Defects in the checkpoint lead to Chromosomal INstability (CIN), which is linked to the progression of tumors with poor clinical outcomes such as drug resistance and metastasis. As CIN is not found in normal cells, it offers a cancer-specific target for therapy, which may be particularly valuable because CIN is common in advanced tumours that are resistant to conventional therapy.

PRINCIPAL FINDINGS

Here we identify genes that are required for the viability of cells with a CIN phenotype. We have used RNAi knockdown of the spindle assembly checkpoint to induce CIN in Drosophila and then screened the set of kinase and phosphatase genes by RNAi knockdown to identify those that induce apoptosis only in the CIN cells. Genes identified include those involved in JNK signaling pathways and mitotic cytoskeletal regulation.

CONCLUSIONS/SIGNIFICANCE

The screen demonstrates that it is feasible to selectively kill cells with CIN induced by spindle checkpoint defects. It has identified candidates that are currently being pursued as cancer therapy targets (e.g. Nek2: NIMA related kinase 2), confirming that the screen is able to identify promising drug targets of clinical significance. In addition, several other candidates were identified that have no previous connection with mitosis or apoptosis. Further screening and detailed characterization of the candidates could potentially lead to the therapies that specifically target advanced cancers that exhibit CIN.

MK3 controls Polycomb target gene expression via negative feedback on ERK

Background

Gene-environment interactions are mediated by epigenetic mechanisms. Polycomb Group proteins constitute part of an epigenetic cellular transcriptional memory system that is subject to dynamic modulation during differentiation. Molecular insight in processes that control dynamic chromatin association and dissociation of Polycomb repressive complexes during and beyond development is limited. We recently showed that MK3 interacts with Polycomb repressive complex 1 (PRC1). The functional relevance of this interaction, however, remained poorly understood. MK3 is activated downstream of mitogen- and stress-activated protein kinases (M/SAPKs), all of which fulfill crucial roles during development. We here use activation of the immediate-early response gene ATF3, a bona fide PRC1 target gene, as a model to study how MK3 and its effector kinases MAPK/ERK and SAPK/P38 are involved in regulation of PRC1-dependent ATF3 transcription.

Results

Our current data show that mitogenic signaling through ERK, P38 and MK3 regulates ATF3 expression by PRC1/chromatin dissociation and epigenetic modulation. Mitogenic stimulation results in transient P38-dependent H3S28 phosphorylation and ERK-driven PRC1/chromatin dissociation at PRC1 targets. H3S28 phosphorylation by itself appears not sufficient to induce PRC1/chromatin dissociation, nor ATF3 transcription, as inhibition of MEK/ERK signaling blocks BMI1/chromatin dissociation and ATF3 expression, despite induced H3S28 phosphorylation. In addition, we establish that concomitant loss of local H3K27me3 promoter marking is not required for ATF3 activation. We identify pERK as a novel signaling-induced binding partner of PRC1, and provide evidence that MK3 controls ATF3 expression in cultured cells via negative regulatory feedback on M/SAPKs. Dramatically increased ectopic wing vein formation in the absence of Drosophila MK in a Drosophila ERK gain-of-function wing vein patterning model, supports the existence of MK-mediated negative feedback regulation on pERK.

Conclusion

We here identify and characterize important actors in a PRC1-dependent epigenetic signal/response mechanism, some of which appear to be nonspecific global responses, whereas others provide modular specificity. Our findings provide novel insight into a Polycomb-mediated epigenetic mechanism that dynamically controls gene transcription and support a direct link between PRC1 and cellular responses to changes in the microenvironment.

Tumour promoting and suppressing roles of the atypical MAP kinase signalling pathway ERK3/4-MK5

Perturbed action of signal transduction pathways, including the mitogen-activated protein (MAP) kinase pathways, is one of the hallmarks of many cancers. While the implication of the typical MAP kinase pathways ERK1/2-MEK1/2, p38MAPK and JNK is well established, recent findings illustrate that the atypical MAP kinase ERK3/4-MK5 may also be involved in tumorigenic processes. Remarkably, the ERK3/4-MK5 pathway seems to possess anti-oncogenic as well as pro-oncogenic properties in cell culture and aninal models. This review summarizes the mutations in the genes encoding ERK3, ERK4 and MK5 that have been detected in different cancers, reports aberrant expression levels of these proteins in human tumours, and discusses the mechanisms by which this pathway can induce senescence, stimulate angiogenesis and invasiveness.

High-resolution protein interaction map of the Drosophila melanogaster p38 mitogen-activated protein kinases reveals limited functional redundancy

Functional redundancy is a pivotal mechanism that supports the robustness of biological systems at a molecular, cellular, and organismal level. The extensive prevalence of redundancy in molecular networks has been highlighted by recent systems biology studies; however, a detailed mechanistic understanding of redundant functions in specific signaling modules is often missing. We used affinity purification of protein complexes coupled to tandem mass spectrometry to generate a high-resolution protein interaction map of the three homologous p38 mitogen-activated protein kinases (MAPKs) in Drosophila and assessed the utility of such a map in defining the extent of common and unique functions. We found a correlation between the depth of integration of individual p38 kinases into the protein interaction network and their functional significance in cultured cells and in vivo. Based on these data, we propose a central role of p38b in the Drosophila p38 signaling module, with p38a and p38c playing more peripheral, auxiliary roles. We also present the first in vivo evidence demonstrating that an evolutionarily conserved complex of p38b with glycogen synthase links stress sensing to metabolic adaptation.

Septin 8 is an interaction partner and in vitro substrate of MK5

AIM: To identify novel substrates for the mitogen-activated protein kinase-activated protein kinase 5 (MK5).

METHODS: Yeast two-hybrid screening with MK5 as bait was used to identify novel possible interaction partners. The binding of putative partner was further examined by glutathione S-transferase (GST) pull-down, co-immunoprecipitation and fluorescence resonance energy transfer (FRET) analysis. In vitro kinase and peptide array assays were used to map MK5 phosphoacceptor sites on the new partner. Confocal microscopy was performed to study the subcellular localization of MK5 and its partners.

RESULTS: Septin 8 was identified as a novel interaction partner for MK5 by yeast two-hybrid screening. This interaction was confirmed by GST pull-down, co-immunoprecipitation and FRET analysis. Septin 5, which can form a complex with septin 8, did not interact with MK5. Serine residues 242 and 271 on septin 8 were identified as in vitro MK5 phosphorylation sites. MK5 and septin 8 co-localized in the perinuclear area and in cell protrusions. Moreover, both proteins co-localized with vesicle marker synaptophysin.

CONCLUSION: Septin 8 is a bona fide interaction partner and in vitro substrate for MK5. This interaction may be implicated in vesicle trafficking.

The extracellular signal-regulated kinase 3 (mitogen-activated protein kinase 6 [MAPK6])-MAPK-activated protein kinase 5 signaling complex regulates septin function and dendrite morphology

Mitogen-activated protein kinase-activated protein (MAPKAP) kinase 5 (MK5) deficiency is associated with reduced extracellular signal-regulated kinase 3 (ERK3) (mitogen-activated protein kinase 6) levels, hence we utilized the MK5 knockout mouse model to analyze the physiological functions of the ERK3/MK5 signaling module. MK5-deficient mice displayed impaired dendritic spine formation in mouse hippocampal neurons in vivo. We performed large-scale interaction screens to understand the neuronal functions of the ERK3/MK5 pathway and identified septin7 (Sept7) as a novel interacting partner of ERK3. ERK3/MK5/Sept7 form a ternary complex, which can phosphorylate the Sept7 regulators Binders of Rho GTPases (Borgs). In addition, the brain-specific nucleotide exchange factor kalirin-7 (Kal7) was identified as an MK5 interaction partner and substrate protein. In transfected primary neurons, Sept7-dependent dendrite development and spine formation are stimulated by the ERK3/MK5 module. Thus, the regulation of neuronal morphogenesis is proposed as the first physiological function of the ERK3/MK5 signaling module.

p38 MAPK activation and function following osmotic shock of erythrocytes

p38 protein kinase is activated by hyperosmotic shock, participates in the regulation of cell volume sensitive transport and metabolism and is involved in the regulation of various physiological functions including cell proliferation and apoptosis. Similar to apoptosis of nucleated cells, erythrocytes may undergo suicidal death or eryptosis, which is paralleled by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Triggers of eryptosis include hyperosmotic shock, which increases cytosolic Ca(2+) activity and ceramide formation. The present study explored whether p38 kinase is expressed in human erythrocytes, is activated by hyperosmotic shock and participates in the regulation of eryptosis. Western blotting was utilized to determine phosphorylation of p38 kinase, forward scatter to estimate cell volume, annexin V binding to depict phosphatidylserine exposure and Fluo3 fluorescence to estimate cytosolic Ca(2+) activity. As a result, erythrocytes express p38 kinase, which is phosphorylated upon osmotic shock (+550 mM sucrose). Osmotic shock decreased forward scatter, increased annexin V binding and increased Fluo3 fluorescence, all effects significantly blunted by the p38 kinase inhibitors SB203580 (2 μM) and p38 Inh III (1 μM). In conclusion, p38 kinase is expressed in erythrocytes and participates in the machinery triggering eryptosis following hyperosmotic shock.

High-throughput RNAi screening reveals novel regulators of telomerase

Telomerase is considered an attractive anticancer target on the basis of its common and specific activation in most human cancers. While direct telomerase inhibition is being explored as a therapeutic strategy, alternative strategies to target regulators of telomerase that could disrupt telomere maintenance and cancer cell proliferation are not yet available. Here, we report the findings of a high-throughput functional RNA interference screen to globally profile the contribution of kinases to telomerase activity (TA). This analysis identified a number of novel telomerase modulators, including ERK8 kinase, whose inhibition reduces TA and elicited characteristics of telomere dysfunction. Given that kinases represent attractive drug targets, we addressed the therapeutic implications of our findings, such as demonstrating how limiting TA via kinase blockade could sensitize cells to inhibition of the telomere-associated protein tankyrase. Taken together, our findings suggest novel combinatorial approaches to targeting telomere maintenance as a strategy for cancer therapy.

Physiological roles of mitogen-activated-protein-kinase-activated p38-regulated/activated protein kinase

Mitogen-activated protein kinases (MAPKs) are a family of proteins that constitute signaling pathways involved in processes that control gene expression, cell division, cell survival, apoptosis, metabolism, differentiation and motility. The MAPK pathways can be divided into conventional and atypical MAPK pathways. The first group converts a signal into a cellular response through a relay of three consecutive phosphorylation events exerted by MAPK kinase kinases, MAPK kinase, and MAPK. Atypical MAPK pathways are not organized into this three-tiered cascade. MAPK that belongs to both conventional and atypical MAPK pathways can phosphorylate both non-protein kinase substrates and other protein kinases. The latter are referred to as MAPK-activated protein kinases. This review focuses on one such MAPK-activated protein kinase, MAPK-activated protein kinase 5 (MK5) or p38-regulated/activated protein kinase (PRAK). This protein is highly conserved throughout the animal kingdom and seems to be the target of both conventional and atypical MAPK pathways. Recent findings on the regulation of the activity and subcellular localization, bona fide interaction partners and physiological roles of MK5/PRAK are discussed.

Physiological roles of mitogen-activated-protein-kinase-activated p38-regulated/activated protein kinase

Mitogen-activated protein kinases (MAPKs) are a family of proteins that constitute signaling pathways involved in processes that control gene expression, cell division, cell survival, apoptosis, metabolism, differentiation and motility. The MAPK pathways can be divided into conventional and atypical MAPK pathways. The first group converts a signal into a cellular response through a relay of three consecutive phosphorylation events exerted by MAPK kinase kinases, MAPK kinase, and MAPK. Atypical MAPK pathways are not organized into this three-tiered cascade. MAPK that belongs to both conventional and atypical MAPK pathways can phosphorylate both non-protein kinase substrates and other protein kinases. The latter are referred to as MAPK-activated protein kinases. This review focuses on one such MAPK-activated protein kinase, MAPK-activated protein kinase 5 (MK5) or p38-regulated/activated protein kinase (PRAK). This protein is highly conserved throughout the animal kingdom and seems to be the target of both conventional and atypical MAPK pathways. Recent findings on the regulation of the activity and subcellular localization, bona fide interaction partners and physiological roles of MK5/PRAK are discussed.

Inhibition of the protein kinase MK-2 protects podocytes from nephrotic syndrome-related injury

While mitogen-activated protein kinase (MAPK) activation has been implicated in the pathogenesis of various glomerular diseases, including nephrotic syndrome (NS), its specific role in podocyte injury is not known. We hypothesized that MK-2, a downstream substrate of p38 MAPK, mediates the adverse effects of this pathway and that inhibition of MK-2 would protect podocytes from NS-related injury. Using cultured podocytes, we analyzed 1) the roles of MK-2 and p38 MAPK in puromycin aminonucleoside (PAN)-induced podocyte injury; 2) the ability of specific MK-2 and p38 MAPK inhibitors to protect podocytes against injury; 3) the role of serum albumin, known to induce podocyte injury, in activating p38 MAPK/MK-2 signaling; and 4) the role of p38 MAPK/MK-2 signaling in the expression of Cox-2, an enzyme associated with podocyte injury. Treatment with protein kinase inhibitors specific for both MK-2 (C23, a pyrrolopyridine-type compound) or p38 MAPK (SB203580) reduced PAN-induced podocyte injury and actin cytoskeletal disruption. Both inhibitors reduced baseline podocyte p38 MAPK/MK-2 signaling, as measured by the degree of phosphorylation of HSPB1, a downstream substrate of MK-2, but exhibited disparate effects on upstream signaling. Serum albumin activated p38 MAPK/MK-2 signaling and induced Cox-2 expression, and these responses were blocked by both inhibitors. Given the critical importance of podocyte injury to both NS and other progressive glomerular diseases, these data suggest an important role for p38 MAPK/MK-2 signaling in podocyte injury and identify MK-2 inhibition as a promising potential therapeutic strategy to protect podocytes in various glomerular diseases.

Distinct roles of MK2 and MK5 in cAMP/PKA- and stress/p38MAPK-induced heat shock protein 27 phosphorylation

Background

Classical mammalian mitogen-activated protein kinase (MAPK) pathways consist of a cascade of three successive phosphorylation events resulting in the phosphorylation of a variety of substrates, including another class of protein kinases referred to as MAPK-activating protein kinases (MAPKAPKs). The MAPKAPKs MK2, MK3 and MK5 are closely related, but MK2 and MK3 are the major downstream targets of the p38^MAPK pathway, while MK5 can be activated by the atypical MAPK ERK3 and ERK4, protein kinase A (PKA), and maybe p38^MAPK. MK2, MK3, and MK5 can phosphorylate the common substrate small heat shock protein 27 (HSP27), a modification that regulates the role of HSP27 in actin polymerization. Both stress and cAMP elevating stimuli can cause F-actin remodeling, but whereas the in vivo role of p38^MAPK-MK2 in stress-triggered HSP27 phosphorylation and actin reorganization is well established, it is not known whether MK2 is involved in cAMP/PKA-induced F-actin rearrangements. On the other hand, MK5 can phosphorylate HSP27 and cause cytoskeletal changes in a cAMP/PKA-dependent manner, but its role as HSP27 kinase in stress-induced F-actin remodeling is disputed. Therefore, we wanted to investigate the implication of MK2 and MK5 in stress- and PKA-induced HSP27 phosphorylation.

Results

Using HEK293 cells, we show that MK2, MK3, and MK5 are expressed in these cells, but MK3 protein levels are very moderate. Stress- and cAMP-elevating stimuli, as well as ectopic expression of active MKK6 plus p38^MAPK or the catalytic subunit of PKA trigger HSP27 phosphorylation, and specific inhibitors of p38^MAPK and PKA prevent this phosphorylation. Depletion of MK2, but not MK3 and MK5 diminished stress-induced HSP27 phosphorylation, while only knockdown of MK5 reduced PKA-induced phosphoHSP27 levels. Stimulation of the p38^MAPK, but not the PKA pathway, caused activation of MK2.

Conclusion

Our results suggest that in HEK293 cells MK2 is the HSP27 kinase engaged in stress-induced, but not cAMP-induced phosphorylation of HSP27, while MK5 seems to be the sole MK to mediate HSP27 phosphorylation in response to stimulation of the PKA pathway. Thus, despite the same substrate specificity towards HSP27, MK2 and MK5 are implicated in different signaling pathways causing actin reorganization.

The role of specific mitogen-activated protein kinase signaling cascades in the regulation of steroidogenesis

Mitogen-activated protein kinases (MAPKs) comprise a family of serine/threonine kinases that are activated by a large variety of extracellular stimuli and play integral roles in controlling many cellular processes, from the cell surface to the nucleus. The MAPK family includes four distinct MAPK cascades, that is, extracellular signal-regulated kinase 1/2 (ERK1/2), p38 MAPK, c-Jun N-terminal kinase or stress-activated protein kinase, and ERK5. These MAPKs are essentially operated through three-tiered consecutive phosphorylation events catalyzed by a MAPK kinase kinase, a MAPK kinase, and a MAPK. MAPKs lie in protein kinase cascades. The MAPK signaling pathways have been demonstrated to be associated with events regulating the expression of the steroidogenic acute regulatory protein (StAR) and steroidogenesis in steroidogenic tissues. However, it has become clear that the regulation of MAPK-dependent StAR expression and steroid synthesis is a complex process and is context dependent. This paper summarizes the current level of understanding concerning the roles of the MAPK signaling cascades in the regulation of StAR expression and steroidogenesis in different steroidogenic cell models.

The MAPK cascades: signaling components, nuclear roles and mechanisms of nuclear translocation

The MAPK cascades are central signaling pathways that regulate a wide variety of stimulated cellular processes, including proliferation, differentiation, apoptosis and stress response. Therefore, dysregulation, or improper functioning of these cascades, is involved in the induction and progression of diseases such as cancer, diabetes, autoimmune diseases, and developmental abnormalities. Many of these physiological, and pathological functions are mediated by MAPK-dependent transcription of various regulatory genes. In order to induce transcription and the consequent functions, the signals transmitted via the cascades need to enter the nucleus, where they may modulate the activity of transcription factors and chromatin remodeling enzymes. In this review, we briefly cover the composition of the MAPK cascades, as well as their physiological and pathological functions. We describe, in more detail, many of the important nuclear activities of the MAPK cascades, and we elaborate on the mechanisms of ERK1/2 translocation into the nucleus, including the identification of their nuclear translocation sequence (NTS) binding to the shuttling protein importin7. Overall, the nuclear translocation of signaling components may emerge as an important regulatory layer in the induction of cellular processes, and therefore, may serve as targets for therapeutic intervention in signaling-related diseases such as cancer and diabetes. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Cross-talk between protein kinase A and the MAPK-activated protein kinases RSK1 and MK5

Typical mammalian mitogen-activated protein kinase (MAPK) pathways consist of a cascade of three consecutive phosphorylation events exerted by a MAPK kinase kinase (MAPKKK), a MAPK kinase (MAPKK), and finally a MAPK. MAPKs not only target non-protein kinase substrates, they can also phosphorylate other protein kinases designated as MAPK-activated protein kinases (MAPKAPK). The MAPKAPK family includes the ribosomal-S6-kinases (RSK1-4), the MAPK-interacting kinases (MNK1 and 2), the mitogen-and stress-activated kinases (MSK1 and 2), and the MAPKAPK (MK2, 3, and 5) subfamilies. Although several reports indicate extensive cross-talk between the MAPK and protein kinase A (PKA) pathways, evidence of a direct interaction at the level of the MAPKAPK only appeared recently. The MAPKAPKs RSK1 and MK5 can bind to PKA, but the features of these interactions are distinct. This review discusses the different characteristics of regulating the activity and subcellular localization of MK5 and RSK1 by PKA and the functional implications of these interactions.