Activation loop phosphorylation of the atypical MAP kinases ERK3 and ERK4 is required for binding, activation and cytoplasmic relocalization of MK5

A review of non-classical MAPK family member, MAPK4: A pivotal player in cancer development and therapeutic intervention

Mitogen-Activated Protein Kinases (MAPKs) are serine/threonine protein kinases that play a crucial role in transmitting extracellular signals to the intracellular environment, influencing a wide range of cellular processes including proliferation, differentiation, apoptosis, metabolic activities, immune function and stress response. MAPK4, a non-classical MAPK, is frequently overexpressed in various malignancies, including prostate, breast, cervix, thyroid, and gliomas. It orchestrates cell proliferation, migration, and apoptosis via the AKT/mTOR and/or PDK1 signaling pathways, thus facilitating tumor cell growth. Furthermore, MAPK4 expression is closely associated with the effectiveness of specific inhibitors like PI3K and PARP1, and also correlate with the survival rates of cancer patients. Increasing evidence highlights MAPK4's involvement in the tumor microenvironment, modulating immune response and inflammation-related diseases. This review comprehensively explores the structure, function, and oncogenic role of MAPK4, providing a deeper understanding of its activation and mechanisms of action in tumorigenesis, which might be helpful for the development of innovative therapeutic strategies for cancer management.

ERK3 is involved in regulating cardiac fibroblast function

ERK3/MAPK6 activates MAP kinase-activated protein kinase (MK)-5 in selected cell types. Male MK5 haplodeficient mice show reduced hypertrophy and attenuated increase in Col1a1 mRNA in response to increased cardiac afterload. In addition, MK5 deficiency impairs cardiac fibroblast function. This study determined the effect of reduced ERK3 on cardiac hypertrophy following transverse aortic constriction (TAC) and fibroblast biology in male mice. Three weeks post-surgery, ERK3, but not ERK4 or p38α, co-immunoprecipitated with MK5 from both sham and TAC heart lysates. The increase in left ventricular mass and myocyte diameter was lower in TAC-ERK3+/- than TAC-ERK3+/+ hearts, whereas ERK3 haploinsufficiency did not alter systolic or diastolic function. Furthermore, the TAC-induced increase in Col1a1 mRNA abundance was diminished in ERK3+/- hearts. ERK3 immunoreactivity was detected in atrial and ventricular fibroblasts but not myocytes. In both quiescent fibroblasts and "activated" myofibroblasts isolated from adult mouse heart, siRNA-mediated knockdown of ERK3 reduced the TGF-β-induced increase in Col1a1 mRNA. In addition, intracellular type 1 collagen immunoreactivity was reduced following ERK3 depletion in quiescent fibroblasts but not myofibroblasts. Finally, knocking down ERK3 impaired motility in both atrial and ventricular myofibroblasts. These results suggest that ERK3 plays an important role in multiple aspects of cardiac fibroblast biology.

Role of the Atypical MAPK ERK3 in Cancer Growth and Progression

Extracellular signal-regulated kinase 3 (ERK3) is an atypical mitogen-activated protein kinase (MAPK) whose structural and regulatory features are distinct from those of conventional MAPKs, such as ERK1/2. Since its identification in 1991, the regulation, substrates and functions of ERK3 have remained largely unknown. However, recent years have witnessed a wealth of new findings about ERK3 signaling. Several important biological functions for ERK3 have been revealed, including its role in neuronal morphogenesis, inflammation, metabolism, endothelial cell tube formation and epithelial architecture. In addition, ERK3 has been recently shown to play important roles in cancer cell proliferation, migration, invasion and chemoresistance in multiple types of cancers. Furthermore, accumulating studies have uncovered various molecular mechanisms by which the expression level, protein stability and activity of ERK3 are regulated. In particular, several post-translational modifications (PTMs), including ubiquitination, hydroxylation and phosphorylation, have been shown to regulate the stability and activity of ERK3 protein. In this review, we discuss recent findings regarding biochemical and cellular functions of ERK3, with a main focus on its roles in cancers, as well as the molecular mechanisms of regulating its expression and activity.

Phosphoproteomic analysis identifies supervillin as an ERK3 substrate regulating cytokinesis and cell ploidy

Extracellular signal-regulated kinase 3 (ERK3) is a poorly characterized member of the mitogen-activated protein (MAP) kinase family. Functional analysis of the ERK3 signaling pathway has been hampered by a lack of knowledge about the substrates and downstream effectors of the kinase. Here, we used large-scale quantitative phosphoproteomics and targeted gene silencing to identify direct ERK3 substrates and gain insight into its cellular functions. Detailed validation of one candidate substrate identified the gelsolin/villin family member supervillin (SVIL) as a bona fide ERK3 substrate. We show that ERK3 phosphorylates SVIL on Ser245 to regulate myosin II activation and cytokinesis completion in dividing cells. Depletion of SVIL or ERK3 leads to increased cytokinesis failure and multinucleation, a phenotype rescued by wild type SVIL but not by the non-phosphorylatable S245A mutant. Our results unveil a new function of the atypical MAP kinase ERK3 in cell division and the regulation of cell ploidy.

Targeting the RAS/RAF/MAPK pathway for cancer therapy: from mechanism to clinical studies

Metastatic dissemination of solid tumors, a leading cause of cancer-related mortality, underscores the urgent need for enhanced insights into the molecular and cellular mechanisms underlying metastasis, chemoresistance, and the mechanistic backgrounds of individuals whose cancers are prone to migration. The most prevalent signaling cascade governed by multi-kinase inhibitors is the mitogen-activated protein kinase (MAPK) pathway, encompassing the RAS-RAF-MAPK kinase (MEK)-extracellular signal-related kinase (ERK) pathway. RAF kinase is a primary mediator of the MAPK pathway, responsible for the sequential activation of downstream targets, such as MEK and the transcription factor ERK, which control numerous cellular and physiological processes, including organism development, cell cycle control, cell proliferation and differentiation, cell survival, and death. Defects in this signaling cascade are associated with diseases such as cancer. RAF inhibitors (RAFi) combined with MEK blockers represent an FDA-approved therapeutic strategy for numerous RAF-mutant cancers, including melanoma, non-small cell lung carcinoma, and thyroid cancer. However, the development of therapy resistance by cancer cells remains an important barrier. Autophagy, an intracellular lysosome-dependent catabolic recycling process, plays a critical role in the development of RAFi resistance in cancer. Thus, targeting RAF and autophagy could be novel treatment strategies for RAF-mutant cancers. In this review, we delve deeper into the mechanistic insights surrounding RAF kinase signaling in tumorigenesis and RAFi-resistance. Furthermore, we explore and discuss the ongoing development of next-generation RAF inhibitors with enhanced therapeutic profiles. Additionally, this review sheds light on the functional interplay between RAF-targeted therapies and autophagy in cancer.

ERK3 and DGKζ interact to modulate cell motility in lung cancer cells

Extracellular signal-regulated kinase 3 (ERK3) promotes cell migration and tumor metastasis in multiple cancer types, including lung cancer. The extracellular-regulated kinase 3 protein has a unique structure. In addition to the N-terminal kinase domain, ERK3 includes a central conserved in extracellular-regulated kinase 3 and ERK4 (C34) domain and an extended C-terminus. However, relatively little is known regarding the role(s) of the C34 domain. A yeast two-hybrid assay using extracellular-regulated kinase 3 as bait identified diacylglycerol kinase ζ (DGKζ) as a binding partner. DGKζ was shown to promote migration and invasion in some cancer cell types, but its role in lung cancer cells is yet to be described. The interaction of extracellular-regulated kinase 3 and DGKζ was confirmed by co-immunoprecipitation and in vitro binding assays, consistent with their co-localization at the periphery of lung cancer cells. The C34 domain of ERK3 was sufficient for binding to DGKζ, while extracellular-regulated kinase 3 bound to the N-terminal and C1 domains of DGKζ. Surprisingly, in contrast to extracellular-regulated kinase 3, DGKζ suppresses lung cancer cell migration, suggesting DGKζ might inhibit ERK3-mediated cell motility. Indeed, co-overexpression of exogenous DGKζ and extracellular-regulated kinase 3 completely blocked the ability of ERK3 to promote cell migration, but DGKζ did not affect the migration of cells with stable ERK3 knockdown. Furthermore, DGKζ had little effect on cell migration induced by overexpression of an ERK3 mutant missing the C34 domain, suggesting DGKζ requires this domain to prevent ERK3-mediated increase in cell migration. In summary, this study has identified DGKζ as a new binding partner and negative regulator of extracellular-regulated kinase 3 in controlling lung cancer cell migration.

Evolution of mitogen-activated protein kinase family and their immune function in Apostichopus japonicus

The mitogen-activated protein kinase family plays an important role in cell differentiation, growth, proliferation, and survival. However, the current research on the mitogen-activated protein kinase (MAPK) family in invertebrates is limited to the individual gene, and the analysis has not been conducted at the family level. In the present study, echinoderm MAPK family was identified by genomic screening, and five members, including three ERK subfamily members, one c-Jun N-terminal kinase (JNK) subfamily, and one p38-MAPK member were detected. Phylogenetic analysis showed that three MAPK subfamilies were separated into three separated clusters, and ERK subfamily appeared earlier than the other two subfamilies. Synteny analysis revealed that the p38 subfamily might be derived from the continuous gene duplication events of MAPK14 subfamily in invertebrates, which displayed genome expansion via gene duplication in vertebrates. The role of MAPK family in echinoderm immune defense was determined by investigating the expression profiles of MAPKs in Vibrio splendidus-challenged Apostichopus japonicus and LPS-exposed coelomocytes. The result showed that five MAPK members displayed induced expression profiles both in vitro and in vivo, and the peak expression was detected at different time points. Our study provides new insights into the evolutionary history of the MAPK family and show the similar immune function among MAPK members.

Cyclo-glycylproline attenuates hydrogen peroxide-induced cellular damage mediated by the MDM2-p53 pathway in human neural stem cells

Cyclo-glycylproline (cGP), a cyclic dipeptide containing a condensation bond between glycine and proline, is produced by the cyclization of the N-terminal tripeptide of insulin-like growth factor-1. Previous studies have shown that cGP administration exerts a neuroprotective effect and enhances the regenerative ability in rats with ischemic brain injury. The efficacy of cGP is medicated by regulating the bioavailability of insulin-like growth factor-1 (IGF-1), however, the molecular mechanisms underlying the neuroprotective effects of cGP on brain damage remains to be elucidated. In the current study, we investigated the cGP-mediated molecular mechanism in human fetal neural stem cells (hfNSCs) exposed to oxidative stress, which is a key factor affecting the development of several brain diseases, including traumatic brain injury and Parkinson's disease. We found that cGP treatment attenuated oxidative stress-induced cell death in cultured hfNSCs in a dose-dependent manner. Transcriptome analysis revealed that under oxidative stress conditions, p53-mediated signaling was activated, accompanied by upregulation of mouse double minute 2 homolog (MDM2), a p53-specific E3 ubiquitin ligase, in cGP-treated hfNSCs. By using a comprehensive protein phosphorylation array, we found that cGP induced the activation of Akt signaling pathway, which enhanced the expression of MDM2, in hfNSCs exposed to oxidative stress. Moreover, the MDM2 inhibitor nutlin-3 inhibited the protective effect of cGP on oxidative stress-induced cell death and apoptosis. Therefore, cGP attenuates oxidative stress-induced cell death mediated by the interplay between IGF-1 signaling and the MDM2-p53 pathway in human NSCs. We revealed the molecular mechanism underlying cGP-induced neuroprotective properties in a model of brain damage.

On the Therapeutic Potential of ERK4 in Triple-Negative Breast Cancer

ERK3 and ERK4 define a distinct and understudied subfamily of mitogen-activated protein kinases (MAPKs). Little is known about the physiological roles of these atypical MAPKs and their association with human diseases. Interestingly, accumulating evidence points towards a role for ERK3 and ERK4 signaling in the initiation and progression of various types of cancer. Notably, a recent study reported that ERK4 is expressed in a subset of triple-negative breast cancer (TNBC) cell lines and that this expression is critical for AKT activation and for sustaining TNBC cell proliferation in vitro and tumor growth in mice. The authors also showed that depletion of ERK4 sensitizes TNBC cells to phosphatidylinositol-3-kinase (PI3K) inhibitors. They concluded that ERK4 is a promising therapeutic target for TNBC and has potential for combination therapy with PI3K inhibitors. Here, we raise concerns about the cellular models and experimental approaches used in this study, which compromise the conclusions on the oncogenic role of ERK4 in TNBC.

Identification of Senescence-Related Subtypes, the Development of a Prognosis Model, and Characterization of Immune Infiltration and Gut Microbiota in Colorectal Cancer

Cellular senescence is associated with tumorigenesis, and the subtype and prognostic signatures of senescence-related genes (SRGs) in the tumor microenvironment (TME) and gut microbiota have not been fully determined. Analysis of 91 SRGs obtained from the GSEA and MSigDB, and mRNA sequencing of genes in the Gene Expression Omnibus (GEO) and The Cancer Genome Atlas (TCGA) databases enabled the identification of two distinct molecular types of colorectal cancer (CRC). Patient samples were clustered into two subtypes, with Kaplan-Meier survival analyses showing significant differences in patient survival between the two subtypes. Cluster C2 was associated with patient clinicopathological features, high immune score, high abundance of immune infiltrating cells and somewhat high abundance of bacteria. A risk model based on eight SRGs showed that a low risk score was characterized by inhibition of immune activity and was indicative of better prognosis in patients with CRC. In combination with clinical characteristics, risk score was found to be an independent prognostic predictor of survival in patients with CRC. In conclusion, the present study showed that senescence-related subtypes and a signature consisting of eight SRGs were associated with CRC patient prognosis, as well as with immune cell infiltration and gut microbiota. These findings may enable better prediction of CRC patient prognosis and facilitate individualized treatments.

MAPK-Activated Protein Kinases: Servant or Partner?

Mitogen-activated protein kinase (MAPK)-activated protein kinases (MAPKAPKs) are defined by their exclusive activation by MAPKs. They can be activated by classical and atypical MAPKs that have been stimulated by mitogens and various stresses. Genetic deletions of MAPKAPKs and availability of highly specific small-molecule inhibitors have continuously increased our functional understanding of these kinases. MAPKAPKs cooperate in the regulation of gene expression at the level of transcription; RNA processing, export, and stability; and protein synthesis. The diversity of stimuli for MAPK activation, the cross talk between the different MAPKs and MAPKAPKs, and the specific substrate pattern of MAPKAPKs orchestrate immediate-early and inflammatory responses in space and time and ensure proper control of cell growth, differentiation, and cell behavior. Hence, MAPKAPKs are promising targets for cancer therapy and treatments for conditions of acute and chronic inflammation, such as cytokine storms and rheumatoid arthritis. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

ERK3-MK5 signaling regulates myogenic differentiation and muscle regeneration by promoting FoxO3 degradation

The physiological functions and downstream effectors of the atypical mitogen-activated protein kinase extracellular signal-regulated kinase 3 (ERK3) remain to be characterized. We recently reported that mice expressing catalytically-inactive ERK3 (Mapk6^KD/KD ) exhibit a reduced postnatal growth rate as compared to control mice. Here, we show that genetic inactivation of ERK3 impairs postnatal skeletal muscle growth and adult muscle regeneration after injury. Loss of MAPK-activated protein kinase 5 (MK5) phenocopies the muscle phenotypes of Mapk6^KD/KD mice. At the cellular level, genetic or pharmacological inactivation of ERK3 or MK5 induces precocious differentiation of C2C12 or primary myoblasts, concomitant with MyoD activation. Reciprocally, ectopic expression of activated MK5 inhibits myogenic differentiation. Mechanistically, we show that MK5 directly phosphorylates FoxO3, promoting its degradation and reducing its association with MyoD. Depletion of FoxO3 rescues in part the premature differentiation of C2C12 myoblasts observed upon inactivation of ERK3 or MK5. Our findings reveal that ERK3 and its substrate MK5 act in a linear signaling pathway to control postnatal myogenic differentiation.

TLK1-mediated MK5-S354 phosphorylation drives prostate cancer cell motility and may signify distinct pathologies

Metastases account for the majority of prostate cancer (PCa) deaths, and targeting them is a major goal of systemic therapy. We identified a novel interaction between two kinases: tousled‐like kinase 1 (TLK1) and MAP kinase‐activated protein kinase 5 (MK5) that promotes PCa spread. In PCa progression, TLK1–MK5 signalling appears to increase following antiandrogen treatment and in metastatic castration‐resistant prostate cancer (mCRPC) patients. Determinations of motility rates (2D and 3D) of different TLK1‐ and MK5‐perturbed cells, including knockout (KO) and knockdown (KD), as well as the use of specific inhibitors, showed the importance of these two proteins for in vitro dissemination. We established that TLK1 phosphorylates MK5 on three residues (S160, S354 and S386), resulting in MK5 activation, and additionally, mobility shifts of MK5 also supported its phosphorylation by TLK1 in transfected HEK 293 cells. Expression of MK5‐S354A or kinase‐dead MK5 in MK5‐depleted mouse embryonic fibroblast (MEF) cells failed to restore their motility compared with that of wild‐type (WT) MK5‐rescued MK5^−/− MEF cells. A pMK5‐S354 antiserum was used to establish this site as an authentic TLK1 target in androgen‐sensitive human prostate adenocarcinoma (LNCaP) cells, and was used in immunohistochemistry (IHC) studies of age‐related PCa sections from TRAMP (transgenic adenocarcinoma of the mouse prostate) mice and to probe a human tissue microarray (TMA), which revealed pMK5‐S354 level is correlated with disease progression (Gleason score and nodal metastases). In addition, The Cancer Genome Atlas (TCGA) analyses of PCa expression and genome‐wide association study (GWAS) relations identify TLK1 and MK5 as potential drivers of advanced PCa and as markers of mCRPC. Our work suggests that TLK1–MK5 signalling is functionally involved in driving PCa cell motility and clinical features of aggressiveness; hence, disruption of this axis may inhibit the metastatic spread of PCa.

FBXW7-mediated ERK3 degradation regulates the proliferation of lung cancer cells

Extracellular signal-regulated kinase 3 (ERK3) is an atypical member of the mitogen-activated protein kinase (MAPK) family, members of which play essential roles in diverse cellular processes during carcinogenesis, including cell proliferation, differentiation, migration, and invasion. Unlike other MAPKs, ERK3 is an unstable protein with a short half-life. Although deubiquitination of ERK3 has been suggested to regulate the activity, its ubiquitination has not been described in the literature. Here, we report that FBXW7 (F-box and WD repeat domain-containing 7) acts as a ubiquitination E3 ligase for ERK3. Mammalian two-hybrid assay and immunoprecipitation results demonstrated that ERK3 is a novel binding partner of FBXW7. Furthermore, complex formation between ERK3 and the S-phase kinase-associated protein 1 (SKP1)-cullin 1-F-box protein (SCF) E3 ligase resulted in the destabilization of ERK3 via a ubiquitination-mediated proteasomal degradation pathway, and FBXW7 depletion restored ERK3 protein levels by inhibiting this ubiquitination. The interaction between ERK3 and FBXW7 was driven by binding between the C34D of ERK3, especially at Thr417 and Thr421, and the WD40 domain of FBXW7. A double mutant of ERK3 (Thr417 and Thr421 to alanine) abrogated FBXW7-mediated ubiquitination. Importantly, ERK3 knockdown inhibited the proliferation of lung cancer cells by regulating the G₁/S-phase transition of the cell cycle. These results show that FBXW7-mediated ERK3 destabilization suppresses lung cancer cell proliferation in vitro.

MAPK4 promotes triple negative breast cancer growth and reduces tumor sensitivity to PI3K blockade

About 15–20% of breast cancer (BCa) is triple-negative BCa (TNBC), a devastating disease with limited therapeutic options. Aberrations in the PI3K/PTEN signaling pathway are common in TNBC. However, the therapeutic impact of PI3K inhibitors in TNBC has been limited and the mechanism(s) underlying this lack of efficacy remain elusive. Here, we demonstrate that a large subset of TNBC expresses significant levels of MAPK4, and this expression is critical for driving AKT activation independent of PI3K and promoting TNBC cell and xenograft growth. The ability of MAPK4 to bypass PI3K for AKT activation potentially provides a direct mechanism regulating tumor sensitivity to PI3K inhibition. Accordingly, repressing MAPK4 greatly sensitizes TNBC cells and xenografts to PI3K blockade. Altogether, we conclude that high MAPK4 expression defines a large subset or subtype of TNBC responsive to MAPK4 blockage. Targeting MAPK4 in this subset/subtype of TNBC both represses growth and sensitizes tumors to PI3K blockade.

PI3K inhibitors have limited efficacy in triple negative breast cancer (TNBC). Here, the authors show that MAPK4 activates AKT independent of PI3K and thus promotes tumour growth in a subset of TNBC and that MAPK4 inhibition sensitizes to PI3K blockade in these tumours.’

Regulation of Mitogen-Activated Protein Kinase Signaling Pathways by the Ubiquitin-Proteasome System and Its Pharmacological Potential

Mitogen-activated protein kinase (MAPK) cascades are evolutionarily conserved signaling pathways that play essential roles in transducing extracellular environmental signals into diverse cellular responses to maintain homeostasis. These pathways are classically organized into an architecture of three sequentially acting protein kinases: a MAPK kinase kinase that phosphorylates and activates a MAPK kinase, which in turn phosphorylates and activates the effector MAPK. The activity of MAPKs is tightly regulated by phosphorylation of their activation loop, which can be modulated by positive and negative feedback mechanisms to control the amplitude and duration of the signal. The signaling outcomes of MAPK pathways are further regulated by interactions of MAPKs with scaffolding and regulatory proteins. Accumulating evidence indicates that, in addition to these mechanisms, MAPK signaling is commonly regulated by ubiquitin-proteasome system (UPS)-mediated control of the stability and abundance of MAPK pathway components. Notably, the biologic activity of some MAPKs appears to be regulated mainly at the level of protein turnover. Recent studies have started to explore the potential of targeted protein degradation as a powerful strategy to investigate the biologic functions of individual MAPK pathway components and as a new therapeutic approach to overcome resistance to current small-molecule kinase inhibitors. Here, we comprehensively review the mechanisms, physiologic importance, and pharmacological potential of UPS-mediated protein degradation in the control of MAPK signaling. SIGNIFICANCE STATEMENT: Accumulating evidence highlights the importance of targeted protein degradation by the ubiquitin-proteasome system in regulating and fine-tuning the signaling output of mitogen-activated protein kinase (MAPK) pathways. Manipulating protein levels of MAPK cascade components may provide a novel approach for the development of selective pharmacological tools and therapeutics.

Conditional ERK3 overexpression cooperates with PTEN deletion to promote lung adenocarcinoma formation in mice

ERK3, officially known as mitogen‐activated protein kinase 6 (MAPK6), is a poorly studied mitogen‐activated protein kinase (MAPK). Recent studies have revealed the upregulation of ERK3 expression in cancer and suggest an important role for ERK3 in promoting cancer cell growth and invasion in some cancers, in particular lung cancer. However, it is unknown whether ERK3 plays a role in spontaneous tumorigenesis in vivo. To determine the role of ERK3 in lung tumorigenesis, we created a conditional ERK3 transgenic mouse line in which ERK3 transgene expression is controlled by Cre recombinase. By crossing these transgenic mice with a mouse line harboring a lung tissue–specific Cre recombinase transgene driven by a club cell secretory protein gene promoter (CCSP‐iCre), we have found that conditional ERK3 overexpression cooperates with phosphatase and tensin homolog (PTEN) deletion to induce the formation of lung adenocarcinomas (LUADs). Mechanistically, ERK3 overexpression stimulates activating phosphorylations of erb‐b2 receptor tyrosine kinases 2 and 3 (ERBB2 and ERBB3) by upregulating Sp1 transcription factor (SP1)–mediated gene transcription of neuregulin 1 (NRG1), a potent ligand for ERBB2/ERBB3. Our study has revealed a bona fide tumor‐promoting role for ERK3 using genetically engineered mouse models. Together with previous findings showing the roles of ERK3 in cultured cells and in a xenograft lung tumor model, our findings corroborate that ERK3 acts as an oncoprotein in promoting LUAD development and progression.

Crystal Structure and Inhibitor Identifications Reveal Targeting Opportunity for the Atypical MAPK Kinase ERK3

Extracellular signal-regulated kinase 3 (ERK3), known also as mitogen-activated protein kinase 6 (MAPK6), is an atypical member of MAPK kinase family, which has been poorly studied. Little is known regarding its function in biological processes, yet this atypical kinase has been suggested to play important roles in the migration and invasiveness of certain cancers. The lack of tools, such as a selective inhibitor, hampers the study of ERK3 biology. Here, we report the crystal structure of the kinase domain of this atypical MAPK kinase, providing molecular insights into its distinct ATP binding pocket compared to the classical MAPK ERK2, explaining differences in their inhibitor binding properties. Medium-scale small molecule screening identified a number of inhibitors, several of which unexpectedly exhibited remarkably high inhibitory potencies. The crystal structure of CLK1 in complex with CAF052, one of the most potent inhibitors identified for ERK3, revealed typical type-I binding mode of the inhibitor, which by structural comparison could likely be maintained in ERK3. Together with the presented structural insights, these diverse chemical scaffolds displaying both reversible and irreversible modes of action, will serve as a starting point for the development of selective inhibitors for ERK3, which will be beneficial for elucidating the important functions of this understudied kinase.

Long noncoding RNA MAPKAPK5-AS1 promotes colorectal cancer progression by cis-regulating the nearby gene MK5 and acting as a let-7f-1-3p sponge

BACKGROUND

Long noncoding RNAs (lncRNAs) are considered critical regulators in cancers; however, the clinical significance and mechanisms of MAPKAPK5-AS1 (hereinafter referred to as MK5-AS1) in colorectal cancer (CRC) remain mostly unknown.

METHODS

In this study, quantitative real-time PCR (qPCR) and western blotting were utilized to detect the levels of MK5-AS1, let-7f-1-3p and MK5 (MAPK activated protein kinase 5) in CRC tissues and cell lines. The biological functions of MK5-AS1, let-7f-1-3p and MK5 in CRC cells were explored using Cell Counting Kit-8 (CCK8), colony formation and transwell assays. The potential mechanisms of MK5-AS1 were evaluated by RNA pull-down, RNA immunoprecipitation (RIP), dual luciferase reporter assay, chromatin immunoprecipitation (ChIP) and bioinformatics analysis. The effects of MK5-AS1 and MK5 on CRC were investigated by a xenotransplantation model.

RESULTS

We confirmed that MK5-AS1 was significantly increased in CRC tissues. Knockdown of MK5-AS1 suppressed cell migration and invasion in vitro and inhibited lung metastasis in mice. Mechanistically, MK5-AS1 regulated SNAI1 expression by sponging let-7f-1-3p and cis-regulated the adjacent gene MK5. Moreover, MK5-AS1 recruited RBM4 and eIF4A1 to promote the translation of MK5. Our study verified that MK5 promoted the phosphorylation of c-Jun, which activated the transcription of SNAI1 by directly binding to its promoter.

CONCLUSIONS

MK5-AS1 cis-regulated the nearby gene MK5 and acted as a let-7f-1-3p sponge, playing a vital role in CRC tumorigenesis. This study could provide novel insights into molecular therapeutic targets of CRC.

Phosphorylation of steroid receptor coactivator-3 (SRC-3) at serine 857 is regulated by the p38MAPK-MK2 axis and affects NF-κB-mediated transcription

Steroid receptor coactivator-3 (SRC-3) regulates the activity of both nuclear hormone receptors and a number of key transcription factors. It is implicated in the regulation of cell proliferation, inflammation and in the progression of several common cancers including breast, colorectal and lung tumors. Phosphorylation is an important regulatory event controlling the activities of SRC-3. Serine 857 is the most studied phospho-acceptor site, and its modification has been reported to be important for SRC-3-dependent tumor progression. In this study, we show that the stress-responsive p38MAPK-MK2 signaling pathway controls the phosphorylation of SRC-3 at S857 in a wide range of human cancer cells. Activation of the p38MAPK-MK2 pathway results in the nuclear translocation of SRC-3, where it contributes to the transactivation of NF-kB and thus regulation of IL-6 transcription. The identification of the p38MAPK-MK2 signaling axis as a key regulator of SRC-3 phosphorylation and activity opens up new possibilities for the development and testing of novel therapeutic strategies to control both proliferative and metastatic tumor growth.

Extracellular-Signal Regulated Kinase: A Central Molecule Driving Epithelial-Mesenchymal Transition in Cancer

Epithelial-mesenchymal transition (EMT) is a reversible cellular process, characterized by changes in gene expression and activation of proteins, favoring the trans-differentiation of the epithelial phenotype to a mesenchymal phenotype. This process increases cell migration and invasion of tumor cells, progression of the cell cycle, and resistance to apoptosis and chemotherapy, all of which support tumor progression. One of the signaling pathways involved in tumor progression is the MAPK pathway. Within this family, the ERK subfamily of proteins is known for its contributions to EMT. The ERK subfamily is divided into typical (ERK 1/2/5), and atypical (ERK 3/4/7/8) members. These kinases are overexpressed and hyperactive in various types of cancer. They regulate diverse cellular processes such as proliferation, migration, metastasis, resistance to chemotherapy, and EMT. In this context, in vitro and in vivo assays, as well as studies in human patients, have shown that ERK favors the expression, function, and subcellular relocalization of various proteins that regulate EMT, thus promoting tumor progression. In this review, we discuss the mechanistic roles of the ERK subfamily members in EMT and tumor progression in diverse biological systems.

Germ Line Deletion Reveals a Nonessential Role of Atypical Mitogen-Activated Protein Kinase 6/Extracellular Signal-Regulated Kinase 3

Mitogen-activated protein kinase 6/extracellular signal-regulated kinase 3 (MAPK6/ERK3) is an atypical member of the MAPKs. An essential role has been suggested by the perinatal lethal phenotype of ERK3 knockout mice carrying a lacZ insertion in exon 2 due to pulmonary dysfunction and by defects in function, activation, and positive selection of T cells. To study the role of ERK3 in vivo, we generated mice carrying a conditional Erk3 allele with exon 3 flanked by loxP sites. Loss of ERK3 protein was validated after deletion of Erk3 in the female germ line using zona pellucida 3 (Zp3)-cre and a clear reduction of the protein kinase MK5 is detected, providing the first evidence for the existence of the ERK3/MK5 signaling complex in vivo In contrast to the previously reported Erk3 knockout phenotype, these mice are viable and fertile and do not display pulmonary hypoplasia, acute respiratory failure, abnormal T-cell development, reduction of thymocyte numbers, or altered T-cell selection. Hence, ERK3 is dispensable for pulmonary and T-cell functions. The perinatal lethality and lung and T-cell defects of the previous ERK3 knockout mice are likely due to ERK3-unrelated effects of the inserted lacZ-neomycin resistance cassette. The knockout mouse of the closely related atypical MAPK ERK4/MAPK4 is also normal, suggesting redundant functions of both protein kinases.

Reevaluation of the Role of Extracellular Signal-Regulated Kinase 3 in Perinatal Survival and Postnatal Growth Using New Genetically Engineered Mouse Models

The physiological functions of the atypical mitogen-activated protein kinase extracellular signal-regulated kinase 3 (ERK3) remain poorly characterized. Previous analysis of mice with a targeted insertion of the lacZ reporter in the Mapk6 locus (Mapk6^lacZ ) showed that inactivation of ERK3 in Mapk6^lacZ mice leads to perinatal lethality associated with intrauterine growth restriction, defective lung maturation, and neuromuscular anomalies. To further explore the role of ERK3 in physiology and disease, we generated novel mouse models expressing a catalytically inactive (Mapk6^KD ) or conditional (Mapk6^Δ ) allele of ERK3. Surprisingly, we found that mice devoid of ERK3 kinase activity or expression survive the perinatal period without any observable lung or neuromuscular phenotype. ERK3 mutant mice reached adulthood, were fertile, and showed no apparent health problem. However, analysis of growth curves revealed that ERK3 kinase activity is necessary for optimal postnatal growth. To gain insight into the genetic basis underlying the discrepancy in phenotypes of different Mapk6 mutant mouse models, we analyzed the regulation of genes flanking the Mapk6 locus by quantitative PCR. We found that the expression of several Mapk6 neighboring genes is deregulated in Mapk6^lacZ mice but not in Mapk6^KD or Mapk6^Δ mutant mice. Our genetic analysis suggests that off-target effects of the targeting construct on local gene expression are responsible for the perinatal lethality phenotype of Mapk6^lacZ mutant mice.

Activation loop phosphorylation of ERK3 is important for its kinase activity and ability to promote lung cancer cell invasiveness

ERK3 is an atypical mitogen-activated protein kinase (MAPK) that has recently gained interest for its role in promoting cancer cell migration and invasion. However, the molecular regulation of ERK3 functions in cancer cells is largely unknown. ERK3 has a single phospho-acceptor site (Ser¹⁸⁹) in its activation motif rather than the TXY conserved in conventional MAPKs such as ERK1/2. Although dual phosphorylation of the TXY motif is known to be critical for the activation of conventional MAPKs, the role of Ser¹⁸⁹ phosphorylation in ERK3 activity and its function in cancer cells remain elusive. In this study, we revealed that activation loop phosphorylation is important for ERK3 in promoting cancer cell invasiveness, as the S189A mutation greatly decreased the ability of ERK3 to promote migration and invasion of lung cancer cells. Interestingly, a catalytically inactive ERK3 mutant was still capable of increasing migration and invasion, although to a lesser extent compared with WT ERK3, suggesting that ERK3 promotes cancer cell invasiveness by both kinase-dependent and kinase-independent mechanisms. To elucidate how the S189A mutation reduces the invasiveness-promoting ability of ERK3, we tested its effect on the kinase activity of ERK3 toward steroid receptor coactivator 3 (SRC3), a recently identified substrate of ERK3 critical for cancer cell invasiveness. Compared with ERK3, ERK3-S189A exhibited a dramatic decrease in kinase activity toward SRC3 and a concomitantly reduced ability to stimulate matrix metalloproteinase expression. Taken together, our study unravels the importance of Ser¹⁸⁹ phosphorylation for intramolecular regulation of ERK3 kinase activity and invasiveness-promoting ability in lung cancer cells.

L290P/V mutations increase ERK3's cytoplasmic localization and migration/invasion-promoting capability in cancer cells

Protein kinases are frequently mutated in human cancers, which leads to altered signaling pathways and contributes to tumor growth and progression. ERK3 is an atypical mitogen-activated protein kinase (MAPK) containing an S-E-G activation motif rather than the conserved T-X-Y motif in conventional MAPKs such as ERK1/2. Recent studies have revealed important roles for ERK3 in cancers. ERK3 promotes cancer cell migration/invasion and tumor metastasis, and its expression is upregulated in multiple cancers. Little is known, however, regarding ERK3 mutations in cancers. In the present study, we functionally and mechanistically characterized ERK3 L290P/V mutations, which are located within ERK3’s kinase domain, and are shown to exist in several cancers including lung cancer and colon cancer. We found that in comparison with wild type ERK3, both L290P and L290V mutants have greatly increased activity in promoting cancer cell migration and invasion, but have little impact on ERK3’s role in cell proliferation. Mechanistically, while they have no clear effect on kinase activity, L290P/V mutations enhance ERK3’s cytoplasmic localization by increasing the interaction with the nuclear export factor CRM1. Our findings suggest that L290P/V mutations of ERK3 may confer increased invasiveness to cancers.

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.

Deubiquitinating Enzyme USP20 Regulates Extracellular Signal-Regulated Kinase 3 Stability and Biological Activity

Extracellular signal-regulated kinase 3 (ERK3) is an atypical mitogen-activated protein kinase (MAPK) whose regulatory mechanisms and biological functions remain superficially understood. Contrary to most protein kinases, ERK3 is a highly unstable protein that is subject to dynamic regulation by the ubiquitin-proteasome system. However, the effectors that control ERK3 ubiquitination and degradation are unknown. In this study, we carried out an unbiased functional loss-of-function screen of the human deubiquitinating enzyme (DUB) family and identified ubiquitin-specific protease 20 (USP20) as a novel ERK3 regulator. USP20 interacts with and deubiquitinates ERK3 both in vitro and in intact cells. The overexpression of USP20 results in the stabilization and accumulation of the ERK3 protein, whereas USP20 depletion reduces the levels of ERK3. We found that the expression levels of ERK3 correlate with those of USP20 in various cellular contexts. Importantly, we show that USP20 regulates actin cytoskeleton organization and cell migration in a manner dependent on ERK3 expression. Our results identify USP20 as a bona fide regulator of ERK3 stability and physiological functions.

Regulation of atypical MAP kinases ERK3 and ERK4 by the phosphatase DUSP2

The atypical MAP kinases ERK3 and ERK4 are activated by phosphorylation of a serine residue lying within the activation loop signature sequence S-E-G. However, the regulation of ERK3 and ERK4 phosphorylation and activity is poorly understood. Here we report that the inducible nuclear dual-specificity MAP kinase phosphatase (MKP) DUSP2, a known regulator of the ERK and p38 MAPKs, is unique amongst the MKP family in being able to bind to both ERK3 and ERK4. This interaction is mediated by a conserved common docking (CD) domain within the carboxyl-terminal domains of ERK3 and ERK4 and the conserved kinase interaction motif (KIM) located within the non-catalytic amino terminus of DUSP2. This interaction is direct and results in the dephosphorylation of ERK3 and ERK4 and the stabilization of DUSP2. In the case of ERK4 its ability to stabilize DUSP2 requires its kinase activity. Finally, we demonstrate that expression of DUSP2 inhibits ERK3 and ERK4-mediated activation of its downstream substrate MK5. We conclude that the activity of DUSP2 is not restricted to the classical MAPK pathways and that DUSP2 can also regulate the atypical ERK3/4-MK5 signalling pathway in mammalian cells.

ERK3 regulates TDP2-mediated DNA damage response and chemoresistance in lung cancer cells

Posttranslational modifications (PTMs), such as phosphorylation and ubiquitination, play critical regulatory roles in the assembly of DNA damage response proteins on the DNA damage site and their activities in DNA damage repair. Tyrosyl DNA phosphodiesterase 2 (TDP2) repairs Topoisomerase 2 (Top2)-linked DNA damage, thereby protecting cancer cells against Top2 inhibitors-induced growth inhibition and cell death. The regulation of TDP2 activity by post-translational modifications in DNA repair, however, remains unclear. In the current study, we have found that ERK3, an atypical MAPK, phosphorylates TDP2 at S60 and regulates TDP2's phosphodiesterase activity, thereby cooperatively protecting lung cancer cells against Top2 inhibitors-induced DNA damage and growth inhibition. As such, our study revealed a post-translational regulation of TDP2 activity and discovered a new role of ERK3 in increasing cancer cells’ DNA damage response and chemoresistance to Top2 inhibitors.

Analysis of ERK3 intracellular localization: dynamic distribution during mitosis and apoptosis

Extracellular signal-regulated kinases (ERK) 1, 2 and 3 are involved in cell proliferation and differentiation, and apoptosis; although ERK1/2 have been widely studied, limited knowledge on ERK3 is available. The present work aimed at investigating ERK3 distribution during cell cycle and apoptosis in human tumor HeLa cells. The analysis performed by double immunofluorescence and immunoelectron microscopy revealed that during interphase ERK3 is mainly resident in the nucleoplasm in association with ribonuclear proteins involved in early pre-mRNA splicing, it undergoes cell cycle-dependent redistribution and, during apoptosis, it remains in the nucleus in the form of massive nuclear aggregates, then moves to the cytoplasm and is finally extruded.

The atypical MAPK ERK3 controls positive selection of thymocytes

Extracellular signal-regulated kinase 3 (ERK3 )is an atypical member of the mitogen-activated protein kinase (MAPK) family. We have previously shown that ERK3 is expressed during thymocyte differentiation and that its expression is induced in mature peripheral T cells following activation of ERK1/2 by T-cell receptor (TCR) signalling. Herein, we have investigated whether ERK3 expression is required for proper T-cell selection. Using a knock-in mouse model in which the coding sequence of ERK3 is replaced by the gene encoding for the β-galactosidase reporter, we show that ERK3 is expressed by double-positive (DP) thymocytes undergoing positive selection. In ERK3-deficient mice with a polyclonal TCR repertoire, we observe a decrease in positive selection. This reduction in positive selection was also observed when ERK3-deficient mice were backcrossed to class I- and class II-restricted TCR transgenic mice. Furthermore, the response of DP thymocytes to in vitro TCR stimulation was strongly reduced in ERK3-deficient mice. Together, these results show that ERK3 expression following TCR signalling is critical for proper thymic positive selection.

Activation of AtMPK9 through autophosphorylation that makes it independent of the canonical MAPK cascades

Mitogen-activated protein kinases (MAPKs) are part of conserved signal transduction modules in eukaryotes that are typically organized into three-tiered kinase cascades. The activation of MAPKs in these pathways is fully dependent on the bisphosphorylation of the TXY motif in the T-loop by the pertinent dual-specificity MAPK kinases (MAPKKs). The Arabidopsis mitogen-activated protein kinase 9 (AtMPK9) is a member of an atypical class of MAPKs. Representatives of this MAPK family have a TDY phosphoacceptor site, a long C-terminal extension and lack the common MAPKK-binding docking motif. In the present paper, we describe multiple in vitro and in vivo data showing that AtMPK9 is activated independently of any upstream MAPKKs but rather is activated through autophosphorylation. We mapped the autophosphorylation sites by MS to the TDY motif and to the C-terminal regulatory extension. We mutated the phosphoacceptor sites on the TDY, which confirmed the requirement for bisphorylation at this site for full kinase activity. Next, we demonstrated that the kinase-inactive mutant form of AtMPK9 is not trans-phosphorylated on the TDY site when mixed with an active AtMPK9, implying that the mechanism of the autocatalytic phosphorylation is intramolecular. Furthermore, we show that in vivo AtMPK9 is activated by salt and is regulated by okadaic acid-sensitive phosphatases. We conclude that the plant AtMPK9 shows similarities to the mammalian atypical MAPKs, such as extracellular-signal-regulated kinase (ERK) 7/8, in terms of an MAPKK-independent activation mechanism.

Identification of new members of the MAPK gene family in plants shows diverse conserved domains and novel activation loop variants

Background

Mitogen Activated Protein Kinase (MAPK) signaling is of critical importance in plants and other eukaryotic organisms. The MAPK cascade plays an indispensible role in the growth and development of plants, as well as in biotic and abiotic stress responses. The MAPKs are constitute the most downstream module of the three tier MAPK cascade and are phosphorylated by upstream MAP kinase kinases (MAPKK), which are in turn are phosphorylated by MAP kinase kinase kinase (MAPKKK). The MAPKs play pivotal roles in regulation of many cytoplasmic and nuclear substrates, thus regulating several biological processes.

Results

A total of 589 MAPKs genes were identified from the genome wide analysis of 40 species. The sequence analysis has revealed the presence of several N- and C-terminal conserved domains. The MAPKs were previously believed to be characterized by the presence of TEY/TDY activation loop motifs. The present study showed that, in addition to presence of activation loop TEY/TDY motifs, MAPKs are also contain MEY, TEM, TQM, TRM, TVY, TSY, TEC and TQY activation loop motifs. Phylogenetic analysis of all predicted MAPKs were clustered into six different groups (group A, B, C, D, E and F), and all predicted MAPKs were assigned with specific names based on their orthology based evolutionary relationships with Arabidopsis or Oryza MAPKs.

Conclusion

We conducted global analysis of the MAPK gene family of plants from lower eukaryotes to higher eukaryotes and analyzed their genomic and evolutionary aspects. Our study showed the presence of several new activation loop motifs and diverse conserved domains in MAPKs. Advance study of newly identified activation loop motifs can provide further information regarding the downstream signaling cascade activated in response to a wide array of stress conditions, as well as plant growth and development.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1244-7) contains supplementary material, which is available to authorized users.

MAP Kinase Cascades in Antigen Receptor Signaling and Physiology

Mitogen-activated protein kinases (MAPKs) play roles in a cell type and context-dependent manner to convert extracellular stimuli to a variety of cellular responses, thereby directing cells to proliferation, differentiation, survival, apoptosis, and migration. Studies of genetically engineered mice or chemical inhibitors specific to each MAPK signaling pathway revealed that MAPKs have various, but non-redundant physiologically important roles among different families. MAPK cascades are obviously integrated in the B cell receptor signaling pathways as critical components to drive B cell-mediated immunity.

Up-regulated microRNA499a by hepatitis B virus induced hepatocellular carcinogenesis via targeting MAPK6

Emerging evidence showed miR499a could not only function as an oncogene but also as a tumor suppressor in various types of cancer, such as melanoma. However, whether miR499a was involved in hepatocarcinogenesis remains unknown. We previously reported that miR499a was up-regulated in HBV-mediated hepatocellular carcinoma (HCC). In this study, we found that HBV could induce the expression of miR499a by promoting its promoter activity. In addition, we reported that miR499a increased cell proliferation and cell migration of HCC cells. MAPK6 was further identified as a target of miR499a, which could also be down-regulated by HBV. Moreover, we demonstrated that MAPK6 could rescue the cell growth induced by miR499a and HBV. These findings indicated that miR499a might play an oncogene role by targeting MAPK6 in the development and progression of HBV-related HCC.

ERK3 promotes endothelial cell functions by upregulating SRC-3/SP1-mediated VEGFR2 expression

Despite a regain of interest recently in ERK3 kinase signaling, the molecular regulations of both ERK3 gene expression and protein kinase activity are still largely unknown. While it is shown that disruption of ERK3 gene causes neonatal lethality, cell type-specific functions of ERK3 signaling remain to be explored. In this study, we report that ERK3 gene expression is upregulated by cytokines through c-Jun in endothelial cells; c-Jun binds to the ERK3 gene and regulates its transcription. We further reveal a new role for ERK3 in regulating endothelial cell migration, proliferation and tube formation by upregulating SRC-3/SP-1-mediated VEGFR2 expression. The underlying molecular mechanism involves ERK3-stimulated formation of a transcriptional complex involving coactivator SRC-3, transcription factor SP-1 and the secondary coactivator CBP. Taken together, our study identified a molecular regulatory mechanism of ERK3 gene expression and revealed a previously unknown role of ERK3 in regulating endothelial cell functions.

Insulin-sensitizing and insulin-mimetic activities of Sarcopoterium spinosum extract

ETHNOPHARMACOLOGICAL RELEVANCE

Sarcopoterium spinosum is an abundant plant in Israel, used by Bedouin medicinal practitioners for the treatment of diabetes. In our previous study we validated the anti-diabetic activity of Sarcopoterium spinosum. The aim of this study was to further clarify its mechanism of action.

MATERIALS AND METHODS

In-vivo studies were performed on KK-a/y mice given the extract for 6 weeks. Insulin tolerance test was performed, and relative pancreatic islets area was measured. Mechanisms of action were investigated in L6 myotubes using protein array, Western blot analysis and confocal microscopy. Glucose uptake assays were performed in 3T3-L1 adipocytes.

RESULTS

Sarcopoterium spinosum extract reduced fasting blood glucose and improved insulin sensitivity in treated mice. Hypertrophic islets were detected in diabetic, but not in Sarcopoterium spinosum-treated mice. Sarcopoterium spinosum phosphorylated PTEN on ser380 and thr382/383, which are known inhibitory sites. PKB was not phosphorylated by Sarcopoterium spinosum, however, translocation of PKB from cytoplasm to the membrane and nucleus was detected. Target proteins of PKB were regulated by Sarcopoterium spinosum; GSK3β was phosphorylated and cytosolic localization of FoxO was increased. Glucose uptake was increased in a PI3K and AMPK-independent mechanism.

CONCLUSIONS

We suggest that Sarcopoterium spinosum inhibited PTEN and activated PKB by a mechanism which is independent of ser473 and thr308 phosphorylation. Other post translation modifications might be involved and should be analyzed further in order to understand this unique PKB activation. Identifying the active molecules in the extract, may lead to the development of new agents for the treatment of insulin resistance.

The catalytic activity of the mitogen-activated protein kinase extracellular signal-regulated kinase 3 is required to sustain CD4+ CD8+ thymocyte survival

Extracellular signal-regulated kinase 3 (ERK3) is an atypical member of the mitogen-activated protein kinase (MAPK) family whose function is largely unknown. Given the central role of MAPKs in T cell development, we hypothesized that ERK3 may regulate thymocyte development. Here we have shown that ERK3 deficiency leads to a 50% reduction in CD4(+) CD8(+) (DP) thymocyte number. Analysis of hematopoietic chimeras revealed that the reduction in DP thymocytes is intrinsic to hematopoietic cells. We found that early thymic progenitors seed the Erk3(-/-) thymus and can properly differentiate and proliferate to generate DP thymocytes. However, ERK3 deficiency results in a decrease in the DP thymocyte half-life, associated with a higher level of apoptosis. As a consequence, ERK3-deficient DP thymocytes are impaired in their ability to make successful secondary T cell receptor alpha (TCRα) gene rearrangement. Introduction of an already rearranged TCR transgene restores thymic cell number. We further show that knock-in of a catalytically inactive allele of Erk3 fails to rescue the loss of DP thymocytes. Our results uncover a unique role for ERK3, dependent on its kinase activity, during T cell development and show that this atypical MAPK is essential to sustain DP survival during RAG-mediated rearrangements.

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.

The non-classical MAP kinase ERK3 controls T cell activation

The classical mitogen-activated protein kinases (MAPKs) ERK1 and ERK2 are activated upon stimulation of cells with a broad range of extracellular signals (including antigens) allowing cellular responses to occur. ERK3 is an atypical member of the MAPK family with highest homology to ERK1/2. Therefore, we evaluated the role of ERK3 in mature T cell response. Mouse resting T cells do not transcribe ERK3 but its expression is induced in both CD4⁺ and CD8⁺ T cells following T cell receptor (TCR)-induced T cell activation. This induction of ERK3 expression in T lymphocytes requires activation of the classical MAPK ERK1 and ERK2. Moreover, ERK3 protein is phosphorylated and associates with MK5 in activated primary T cells. We show that ERK3-deficient T cells have a decreased proliferation rate and are impaired in cytokine secretion following in vitro stimulation with low dose of anti-CD3 antibodies. Our findings identify the atypical MAPK ERK3 as a new and important regulator of TCR-induced T cell activation.

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 is degraded in response to doxorubicin and negatively regulates doxorubicin-induced apoptosis in hepatocellular carcinoma cells

Hepatocellular carcinoma (HCC) is one of the most common malignant tumors worldwide. The mechanisms by which hepatoma cells resist apoptosis induced by doxorubicin are largely unknown. MAPKAPK5 (MK5), also named as p38-regulated/activated protein kinase (PRAK), has been identified as a crucial mediator of skin tumorigenesis in mouse and colon cancerogenesis in human. Here, we describe a novel role of MK5 in doxorubicin-induced apoptosis in human hepatoma cells. Expression of MK5 was highly upregulated in hepatoma cell lines. Doxorubicin rather than other chemotherapeutic drugs reduced MK5 protein level in a time- and concentration-dependent manner in hepatoma cells (HepG2 and Hep3B). We further showed that MK5 degradation induced by doxorubicin was via the 26S proteasome. Remarkably, stable overexpression of MK5 led to decreased cleavage of caspase-3 and PARP and attenuated doxorubicin-induced apoptosis, while stable knockdown of endogenous MK5 sensitized hepatoma cells to doxorubicin, which was coupled with increased cleavage of caspase-3 and PARP. Taken together, our results firstly demonstrate that MK5 is degraded in response to doxorubicin and negatively regulates doxorubicin-induced apoptosis, providing novel insights into the molecular mechanism of doxorubicin resistance in hepatoma cells.

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.

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.

ERK3 signals through SRC-3 coactivator to promote human lung cancer cell invasion

In contrast to the well-studied classic MAPKs, such as ERK1/2, little is known concerning the regulation and substrates of the atypical MAPK ERK3 signaling cascade and its function in cancer progression. Here, we report that ERK3 interacted with and phosphorylated steroid receptor coactivator 3 (SRC-3), an oncogenic protein overexpressed in multiple human cancers at serine 857 (S857). This ERK3-mediated phosphorylation at S857 was essential for interaction of SRC-3 with the ETS transcription factor PEA3, which promotes upregulation of MMP gene expression and proinvasive activity in lung cancer cells. Importantly, knockdown of ERK3 or SRC-3 inhibited the ability of lung cancer cells to invade and form tumors in the lung in a xenograft mouse model. In addition, ERK3 was found to be highly upregulated in human lung carcinomas. Our study identifies a previously unknown role for ERK3 in promoting lung cancer cell invasiveness by phosphorylating SRC-3 and regulating SRC-3 proinvasive activity by site-specific phosphorylation. As such, ERK3 protein kinase may be an attractive target for therapeutic treatment of invasive lung cancer.

IGF2BP1 promotes cell migration by regulating MK5 and PTEN signaling

In primary neurons, the oncofetal RNA-binding protein IGF2BP1 (IGF2 mRNA-binding protein 1) controls spatially restricted β-actin (ACTB) mRNA translation and modulates growth cone guidance. In cultured tumor-derived cells, IGF2BP1 was shown to regulate the formation of lamellipodia and invadopodia. However, how and via which target mRNAs IGF2BP1 controls the motility of tumor-derived cells has remained elusive. In this study, we reveal that IGF2BP1 promotes the velocity and directionality of tumor-derived cell migration by determining the cytoplasmic fate of two novel target mRNAs: MAPK4 and PTEN. Inhibition of MAPK4 mRNA translation by IGF2BP1 antagonizes MK5 activation and prevents phosphorylation of HSP27, which sequesters actin monomers available for F-actin polymerization. Consequently, HSP27-ACTB association is reduced, mobilizing cellular G-actin for polymerization in order to promote the velocity of cell migration. At the same time, stabilization of the PTEN mRNA by IGF2BP1 enhances PTEN expression and antagonizes PIP(3)-directed signaling. This enforces the directionality of cell migration in a RAC1-dependent manner by preventing additional lamellipodia from forming and sustaining cell polarization intrinsically. IGF2BP1 thus promotes the velocity and persistence of tumor cell migration by controlling the expression of signaling proteins. This fine-tunes and connects intracellular signaling networks in order to enhance actin dynamics and cell polarization.

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.

Activation and function of the MAPKs and their substrates, the MAPK-activated protein kinases

The mitogen-activated protein kinases (MAPKs) regulate diverse cellular programs by relaying extracellular signals to intracellular responses. In mammals, there are more than a dozen MAPK enzymes that coordinately regulate cell proliferation, differentiation, motility, and survival. The best known are the conventional MAPKs, which include the extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun amino-terminal kinases 1 to 3 (JNK1 to -3), p38 (α, β, γ, and δ), and ERK5 families. There are additional, atypical MAPK enzymes, including ERK3/4, ERK7/8, and Nemo-like kinase (NLK), which have distinct regulation and functions. Together, the MAPKs regulate a large number of substrates, including members of a family of protein Ser/Thr kinases termed MAPK-activated protein kinases (MAPKAPKs). The MAPKAPKs are related enzymes that respond to extracellular stimulation through direct MAPK-dependent activation loop phosphorylation and kinase activation. There are five MAPKAPK subfamilies: the p90 ribosomal S6 kinase (RSK), the mitogen- and stress-activated kinase (MSK), the MAPK-interacting kinase (MNK), the MAPK-activated protein kinase 2/3 (MK2/3), and MK5 (also known as p38-regulated/activated protein kinase [PRAK]). These enzymes have diverse biological functions, including regulation of nucleosome and gene expression, mRNA stability and translation, and cell proliferation and survival. Here we review the mechanisms of MAPKAPK activation by the different MAPKs and discuss their physiological roles based on established substrates and recent discoveries.