Noncanonical function of MEKK2 and MEK5 PB1 domains for coordinated extracellular signal-regulated kinase 5 and c-Jun N-terminal kinase signaling

Mechanisms underlying sensing of cellular stress signals by mammalian MAP3 kinases

Cellular homeostasis is continuously challenged by environmental cues and cellular stress conditions. In their defense, cells need to mount appropriate stress responses that, dependent on the cellular context, signaling intensity, and duration, may have diverse outcomes. The stress- and mitogen-activated protein kinase (SAPK/MAPK) system consists of well-characterized signaling cascades that sense and transduce an array of different stress stimuli into biological responses. However, the physical and chemical nature of stress signals and how these are sensed by individual upstream MAP kinase kinase kinases (MAP3Ks) remain largely ambiguous. Here, we review the existing knowledge of how individual members of the large and diverse group of MAP3Ks sense specific stress signals through largely non-redundant mechanisms. We emphasize the large knowledge gaps in assigning function and stress signals for individual MAP3K family members and touch on the potential of targeting this class of proteins for clinical benefit.

Clinical Significance and Regulation of ERK5 Expression and Function in Cancer

Extracellular signal-regulated kinase 5 (ERK5) is a unique kinase among MAPKs family members, given its large structure characterized by the presence of a unique C-terminal domain. Despite increasing data demonstrating the relevance of the ERK5 pathway in the growth, survival, and differentiation of normal cells, ERK5 has recently attracted the attention of several research groups given its relevance in inflammatory disorders and cancer. Accumulating evidence reported its role in tumor initiation and progression. In this review, we explore the gene expression profile of ERK5 among cancers correlated with its clinical impact, as well as the prognostic value of ERK5 and pERK5 expression levels in tumors. We also summarize the importance of ERK5 in the maintenance of a cancer stem-like phenotype and explore the major known contributions of ERK5 in the tumor-associated microenvironment. Moreover, although several questions are still open concerning ERK5 molecular regulation, different ERK5 isoforms derived from the alternative splicing process are also described, highlighting the potential clinical relevance of targeting ERK5 pathways.

The MEK5/ERK5 Pathway in Health and Disease

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

Discovery of a Gatekeeper Residue in the C-Terminal Tail of the Extracellular Signal-Regulated Protein Kinase 5 (ERK5)

The extracellular signal-regulated protein kinase 5 (ERK5) is a non-redundant mitogen-activated protein kinase (MAPK) that exhibits a unique C-terminal extension which comprises distinct structural and functional properties. Here, we sought to elucidate the significance of phosphoacceptor sites in the C-terminal transactivation domain of ERK5. We have found that Thr732 acted as a functional gatekeeper residue controlling C-terminal-mediated nuclear translocation and transcriptional enhancement. Consistently, using a non-bias quantitative mass spectrometry approach, we demonstrated that phosphorylation at Thr732 conferred selectivity for binding interactions of ERK5 with proteins related to chromatin and RNA biology, whereas a number of metabolic regulators were associated with full-length wild type ERK5. Additionally, our proteomic analysis revealed that phosphorylation of the Ser730-Glu-Thr732-Pro motif could occur independently of dual phosphorylation at Thr218-Glu-Tyr220 in the activation loop. Collectively, our results firmly establish the significance of C-terminal phosphorylation in regulating ERK5 function. The post-translational modification of ERK5 on its C-terminal tail might be of particular relevance in cancer cells where ERK5 has be found to be hyperphosphoryated.

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

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

Silencing of the MEKK2/MEKK3 Pathway Protects against Spinal Cord Injury via the Hedgehog Pathway and the JNK Pathway

Spinal cord injury (SCI) is a devastating medical condition, often accompanied by motor and sensory dysfunction. The Hedgehog (Hh) pathway has a protective role in pathological injury after SCI. However, the specific mechanism remains unclear. The present study aimed to confirm the effects of the mitogen-activated protein kinase kinase-2 (MEKK2)/MEKK3/JNK/Hh pathway on SCI. SCI rat models were established and then inoculated with plasmids overexpressing MEKK2/MEKK3 or with small interfering RNA (siRNA) against MEKK2/MEKK3. The expression of MEKK2 and -3 was detected in dorsal root ganglia (DRG) cells. The motor function of hindlimbs, the expression of the c-Jun N-terminal kinase (JNK)- and Hh-pathway-related genes, and the level of neurofilament-200 (NF-200) and glial fibrillary acidic protein (GFAP) were measured. MEKK2 and -3 were expressed at a high level in DRG cells. The silencing of MEKK2/MEKK3 in rats caused an increase in the expression of glioma-associated oncogene homolog-1 (Gli-1), Nestin, smoothened (Smo), and Sonic Hedgehog (Shh). The Basso, Beattie, and Bresnahan (BBB) rating and the level of NF-200 protein also increased. However, the expression of monocyte chemoattractant protein-1 (MCP-1), macrophage inflammatory protein-1β (MIP-1β), MIP-3α, p-JNK/JNK, and p-c-Jun/c-Jun and the level of GFAP were reduced. Downregulation of MEKK2/MEKK3 ameliorated the symptoms of SCI by promoting neural progenitor cell differentiation via activating the Hh pathway and disrupting the JNK pathway. The findings in this study reveal a potential biomarker for SCI treatment.

Oncogenic signaling of MEK5-ERK5

Mitogen-activated protein kinases (MAPKs) regulate diverse cellular processes including proliferation, cell survival, differentiation, and apoptosis. While conventional MAPK constituents have well-defined roles in oncogenesis, the MEK5 pathway has only recently emerged in cancer research. In this review, we consider the MEK5 signaling cascade, focusing specifically on its involvement in drug resistance and regulation of aggressive cancer phenotypes. Moreover, we explore the role of MEK5/ERK5 in tumorigenesis and metastatic progression, discussing the discrepancies in preclinical studies and assessing its viability as a therapeutic target for anti-cancer agents.

Cell Signaling and Stress Responses

Stress-signaling pathways are evolutionarily conserved and play an important role in the maintenance of homeostasis. These pathways are also critical for adaptation to new cellular environments. The endoplasmic reticulum (ER) unfolded protein response (UPR) is activated by biosynthetic stress and leads to a compensatory increase in ER function. The JNK and p38 MAPK signaling pathways control adaptive responses to intracellular and extracellular stresses, including environmental changes such as UV light, heat, and hyperosmotic conditions, and exposure to inflammatory cytokines. Metabolic stress caused by a high-fat diet represents an example of a stimulus that coordinately activates both the UPR and JNK/p38 signaling pathways. Chronic activation of these stress-response pathways ultimately causes metabolic changes associated with obesity and altered insulin sensitivity. Stress-signaling pathways, therefore, represent potential targets for therapeutic intervention in the metabolic stress response and other disease processes.

Structure and dynamics of the MKK7-JNK signaling complex

Signaling specificity in the mitogen-activated protein kinase (MAPK) pathways is controlled by disordered domains of the MAPK kinases (MKKs) that specifically bind to their cognate MAPKs via linear docking motifs. MKK7 activates the c-Jun N-terminal kinase (JNK) pathway and is the only MKK containing three motifs within its regulatory domain. Here, we characterize the conformational behavior and interaction mechanism of the MKK7 regulatory domain. Using NMR spectroscopy, we develop an atomic resolution ensemble description of MKK7, revealing highly diverse intrinsic conformational propensities of the three docking sites, suggesting that prerecognition sampling of the bound-state conformation is not prerequisite for binding. Although the different sites exhibit similar affinities for JNK1, interaction kinetics differ considerably. Importantly, we determine the crystal structure of JNK1 in complex with the second docking site of MKK7, revealing two different binding modes of the docking motif correlating with observations from NMR exchange spectroscopy. Our results provide unique insight into how signaling specificity is regulated by linear motifs and, in general, into the role of conformational disorder in MAPK signaling.

Ubiquitin-dependent regulation of MEKK2/3-MEK5-ERK5 signaling module by XIAP and cIAP1

Mitogen-activated protein kinases (MAPKs) are highly conserved protein kinase modules, and they control fundamental cellular processes. While the activation of MAPKs has been well studied, little is known on the mechanisms driving their inactivation. Here we uncover a role for ubiquitination in the inactivation of a MAPK module. Extracellular-signal-regulated kinase 5 (ERK5) is a unique, conserved member of the MAPK family and is activated in response to various stimuli through a three-tier cascade constituting MEK5 and MEKK2/3. We reveal an unexpected role for Inhibitors of Apoptosis Proteins (IAPs) in the inactivation of ERK5 pathway in a bimodal manner involving direct interaction and ubiquitination. XIAP directly interacts with MEKK2/3 and competes with PB1 domain-mediated binding to MEK5. XIAP and cIAP1 conjugate predominantly K63-linked ubiquitin chains to MEKK2 and MEKK3 which directly impede MEK5-ERK5 interaction in a trimeric complex leading to ERK5 inactivation. Consistently, loss of XIAP or cIAP1 by various strategies leads to hyperactivation of ERK5 in normal and tumorigenic cells. Loss of XIAP promotes differentiation of human primary skeletal myoblasts to myocytes in a MEKK2/3-ERK5-dependent manner. Our results reveal a novel, obligatory role for IAPs and ubiquitination in the physical and functional disassembly of ERK5-MAPK module and human muscle cell differentiation.

QUIRKY interacts with STRUBBELIG and PAL OF QUIRKY to regulate cell growth anisotropy during Arabidopsis gynoecium development

Organ morphogenesis largely relies on cell division and elongation, which need to be both coordinated between cells and orchestrated with cytoskeleton dynamics. However, components that bridge the biological signals and the effectors that define cell shape remain poorly described. We have addressed this issue through the functional characterisation of QUIRKY (QKY), previously isolated as being involved in the STRUBBELIG (SUB) genetic pathway that controls cell-cell communication and organ morphogenesis in Arabidopsis. QKY encodes a protein containing multiple C2 domains and transmembrane regions, and SUB encodes an atypical LRR-receptor-like kinase. We show that twisting of the gynoecium observed in qky results from the abnormal division pattern and anisotropic growth of clustered cells arranged sporadically along the gynoecium. Moreover, the cortical microtubule (CMT) network of these cells is disorganised. A cross to botero, a katanin mutant in which the normal orientation of CMTs and anisotropic cell expansion are impaired, strongly reduces silique deviation, reinforcing the hypothesis of a role for QKY in CMT-mediated cell growth anisotropy. We also show that QKY is localised at the plasma membrane and functions in a multiprotein complex that includes SUB and PAL OF QUIRKY (POQ), a previously uncharacterised PB1-domain-containing protein that localises both at the plasma membrane and in intracellular compartments. Our data indicate that QKY and its interactors play central roles linking together cell-cell communication and cellular growth.

Structural mechanism for the specific assembly and activation of the extracellular signal regulated kinase 5 (ERK5) module

Mitogen-activated protein kinase (MAPK) activation depends on a linear binding motif found in all MAPK kinases (MKK). In addition, the PB1 (Phox and Bem1) domain of MKK5 is required for extracellular signal regulated kinase 5 (ERK5) activation. We present the crystal structure of ERK5 in complex with an MKK5 construct comprised of the PB1 domain and the linear binding motif. We show that ERK5 has distinct protein-protein interaction surfaces compared with ERK2, which is the closest ERK5 paralog. The two MAPKs have characteristically different physiological functions and their distinct protein-protein interaction surface topography enables them to bind different sets of activators and substrates. Structural and biochemical characterization revealed that the MKK5 PB1 domain cooperates with the MAPK binding linear motif to achieve substrate specific binding, and it also enables co-recruitment of the upstream activating enzyme and the downstream substrate into one signaling competent complex. Studies on present day MAPKs and MKKs hint on the way protein kinase networks may evolve. In particular, they suggest how paralogous enzymes with similar catalytic properties could acquire novel signaling roles by merely changing the way they make physical links to other proteins.

MEK5/ERK5 pathway: the first fifteen years

While conventional MAP kinase pathways are one of the most highly studied signal transduction molecules, less is known about the MEK5 signaling pathway. This pathway has been shown to play a role in normal cell growth cycles, survival and differentiation. The MEK5 pathway is also believed to mediate the effects of a number of oncogenes. MEK5 is the upstream activator of ERK5 in many epithelial cells. Activation of the MEK-MAPK pathway is a frequent event in malignant tumor formation and contributes to chemoresistance and anti-apoptotic signaling. This pathway may be involved in a number of more aggressive, metastatic varieties of cancer due to its role in cell survival, proliferation and EMT transitioning. Further study of this pathway may lead to new prognostic factors and new drug targets to combat more aggressive forms of cancer.

CHIP-dependent termination of MEKK2 regulates temporal ERK activation required for proper hyperosmotic response

The extracellular signal-regulated kinase (ERK) pathway is an important signalling pathway that regulates a large number of cellular processes, including proliferation, differentiation and gene expression. Hyperosmotic stress activates the ERK pathway, whereas little is known about the regulatory mechanisms and physiological functions of ERK activation in hyperosmotic response. Here, we show that MAPK/ERK kinase kinase 2 (MEKK2), a member of the MAPKKK family, mediated the specific and transient activation of ERK, which was required for the induction of aquaporin 1 (AQP1) and AQP5 gene expression in response to hyperosmotic stress. Moreover, we identified the E3 ubiquitin ligase carboxyl terminus of Hsc70-interacting protein (CHIP) as a binding partner of MEKK2. Depletion of CHIP by small-interference RNA or gene targeting attenuated the degradation of MEKK2 and prolonged the ERK activity. Interestingly, hyperosmolality-induced gene expression of AQP1 and AQP5 was suppressed by CHIP depletion and was reversed by inhibition of the prolonged phase of ERK activity. These findings show that transient activation of the ERK pathway, which depends not only on MEKK2 activation, but also on CHIP-dependent MEKK2 degradation, is crucial for proper gene expression in hyperosmotic stress response.

PB1 domain interaction of p62/sequestosome 1 and MEKK3 regulates NF-kappaB activation

p62/Sequestosome 1 is a scaffold protein involved in the regulation of autophagy, trafficking of proteins to the proteasome, and activation of NF-kappaB. p62 encodes an N-terminal PB1 domain in addition to the ZZ domain, TRAF6-binding domain, LC3 interaction region, and ubiquitin-associated domain, each critical for the physiological function of p62. PB1 domains have a beta-grasp topology where the front end of one PB1 domain binds the back end of a second PB1 domain. The p62 PB1 domain homodimerizes as well as heterodimerizes with other PB1 domains. The front end of the PB1 domain in p62 binds the PB1 domain of atypical protein kinases C, the MAPK kinase, MEK5, and the NBR1 protein. Other than its role in homodimerization, the rear end acidic cluster region of the p62 PB1 domain had no previous defined binding partners. Herein, we demonstrate that the rear end acidic cluster region of the p62 PB1 domain binds the front end basic region of the MAPK kinase kinase, MEKK3. p62 and MEKK3 co-localize in speckles or aggregates that are centers for organizing TRAF6-regulated NF-kappaB signaling and the assembly of polyubiquinated proteins sorting to sequestosomes and proteasomes. The p62-MEKK3 complex binds TRAF6, which regulates the ubiquitination of the IKK complex and NF-kappaB activation. p62 is required for the association of MEKK3 with TRAF6 and short hairpin RNA knockdown of p62 inhibits IL-1 and MEKK3 activation of NF-kappaB. The rear end acidic cluster of the p62 PB1 domain is used to organize cytosolic aggregates or speckles-associated TRAF6-p62-MEKK3 complex for control of NF-kappaB activation.

In vivo profiling endogenous interactions with knock-out in mammalian cells

To precisely identify and screen target-specific protein-protein interactions at the endogenous level, here we introduce a novel quantitative proteomic method we have termed in vivo Profiling Endogenous Interactions with Knock-out (iPEIK). In our design, mouse embryonic fibroblasts (MEFs) derived from target gene knockout (KO) mice can be stable isotope-tagged and serve as a target-free background to "light-up" the target protein-specific protein complex formed in the corresponding wild-type (WT) cells. In mass spectrometric analysis of the pairs of non-labeled versus heavy isotope-labeled peptide signals derived from WT versus KO cells, respectively, we then quantitatively measured the abundance differences of the proteins in the complex immunoprecipitated (IP) from the target-expressing WT versus target-absent KO cells, respectively. Those proteins detected with little or no presence in the cells of KO origin were determined as target-specific interacting partners. Further, dynamic interactors could be identified through different IP mixing schemes. Using iPEIK we identified multiple interacting partners both previously known and unknown to be associated with mitogen-activated protein kinase kinase kinase 2 (MEKK2). Because of the availability of a large library of knockout mice models with various target proteins of biological interests our method is generally applicable to screen any endogenous target-specific PPIs of physiological relevance.

Homogeneous time-resolved fluorescence resonance energy transfer assay for measurement of Phox/Bem1p (PB1) domain heterodimerization

Twenty human proteins encode Phox/Bem1p (PB1) domains, which are involved in forming protein heterodimers. MEKK2, MEKK3, and MEK5 are 3 serine-threonine protein kinases that have PB1 domains. MEKK2, MEKK3, and MEK5 are the MAP3Ks and the MAP2K in the ERK5 mitogen-activated protein kinase (MAPK) signaling module. ERK5 is a critical MAPK for both development of the vasculature and vascular homeostasis in the adult, but no other MAPK has been shown to be critical in vascular maintenance in the adult animal. MEKK2 and MEKK3 are the only MAP3Ks shown to physically interact with and activate the MEK5-ERK5 signaling module. Interaction of MEKK2 or MEKK3 with MEK5 is mediated by heterodimerization of the MEKK2 (or MEKK3) PB1 and MEK5 PB1 domains. The authors have developed a homogeneous, time-resolved fluorescence resonance energy transfer (TR-FRET) assay to monitor PB1-PB1 domain heterodimerization. The assay uses a europium-chelate conjugated GST-MEK5 PB1 domain chimera, biotinylated MEKK2 PB1 domain, and streptavidin-Cy5. Interaction of the MEKK2 and MEK5 PB1 domains gives a robust FRET signal (Z' factor = 0.93), which is completely abrogated by mutation of 2 acidic residues (64D65E-->AA) within the MEK5 PB1 domain that causes loss of stable PB1-PB1 domain interaction. This assay can be used to study the specificity of PB1-PB1 domain interactions and to screen for molecules that can regulate MEKK2/MEKK3-MEK5 interactions. Disruption of PB1 domain interactions represents a novel approach for selectively regulating the ERK5 signaling pathway independent of kinase active site-directed adenosine triphosphate competitive inhibitors.

Structure and Function of the PB1 Domain, a Protein Interaction Module Conserved in Animals, Fungi, Amoebas, and Plants

Proteins containing the PB1 domain, a protein interaction module conserved in animals, fungi, amoebas, and plants, participate in diverse biological processes. The PB1 domains adopt a ubiquitin-like beta-grasp fold, containing two alpha helices and a mixed five-stranded beta sheet, and are classified into groups harboring an acidic OPCA motif (type I), the invariant lysine residue on the first beta strand (type II), or both (type I/II). The OPCA motif of a type I PB1 domain forms salt bridges with basic residues, especially the conserved lysine, of a type II PB1 domain, thereby mediating a specific PB1-PB1 heterodimerization, whereas additional contacts contribute to high affinity and specificity of the modular interaction. The canonical PB1 dimerization is required for the formation of complexes between p40(phox) and p67(phox) (for activation of the NADPH oxidase crucial for mammalian host defense), between the scaffold Bem1 and the guanine nucleotide exchange factor Cdc24 (for polarity establishment in yeasts), and between the polarity protein Par6 and atypical protein kinase C (for cell polarization in animal cells), as well as for the interaction between the mitogen-activated protein kinase kinase kinases MEKK2 or MEKK3 and the downstream target mitogen-activated protein kinase kinase MEK5 (for early cardiovascular development in mammals). PB1 domains can also mediate interactions with other protein domains. For example, an intramolecular interaction between the PB1 and PX domains of p40(phox) regulates phagosomal targeting of the microbicidal NADPH oxidase; the PB1 domain of MEK5 is likely responsible for binding to the downstream kinase ERK5, which lacks a PB1 domain; and the scaffold protein Nbr1 associates through a PB1-containing region with titin, a sarcomere protein without a PB1 domain. This Review describes various aspects of PB1 domains at the molecular and cellular levels.