Extracellular signal-regulated kinase 7 (ERK7), a novel ERK with a C-terminal domain that regulates its activity, its cellular localization, and cell growth

Functional consequences of mutations in CDKL5, an X-linked gene involved in infantile spasms and mental retardation

Mutations in the X-linked cyclin-dependent kinase-like 5 (CDKL5) gene have been identified in patients with Rett syndrome, West syndrome, and X-linked infantile spasms sharing the common features of generally intractable early seizures and mental retardation. Disease-causing mutations are distributed in both the catalytic domain and in the large COOH terminus. In this report, we examine the functional consequences of some Rett mutations of CDKL5 together with some synthetically designed derivatives useful to underline the functional domains of the protein. The mutated CDKL5 derivatives have been subjected to in vitro kinase assays and analyzed for phosphorylation of the TEY (Thr-Glu-Tyr) motif within the activation loop, their subcellular localization, and the capacity of CDKL5 to interact with itself. Whereas wild-type CDKL5 autophosphorylates and mediates the phosphorylation of the methyl-CpG-binding protein 2 (MeCP2) in vitro, Rett-mutated proteins show both impaired and increased catalytic activity suggesting that a tight regulation of CDKL5 is required for correct brain functions. Furthermore, we show that CDKL5 can self-associate and mediate the phosphorylation of its own TEY (Thr-Glu-Tyr) motif. Eventually, we show that the COOH terminus regulates CDKL5 properties; in particular, it negatively influences the catalytic activity and is required for its proper sub-nuclear localization. We propose a model in which CDKL5 phosphorylation is required for its entrance into the nucleus whereas a portion of the COOH-terminal domain is responsible for a stable residency in this cellular compartment probably through protein-protein interactions.

Characterization and expression patterns of the MAPK family in zebrafish

The mitogen activated protein kinases (MAPK) family pathway is conserved in evolution through the plant and animal kingdoms. These proteins have been implicated in diverse cellular processes including cell growth, proliferation, differentiation, survival and development. In this study we annotated and cloned members of the zebrafish MAPK gene-family, containing the ERK, JNK and p38 subfamilies. Their sequences were compared to orthologs of other vertebrates (human, mouse and rat) and the temporal and spatial expression levels of the zebrafish mapk genes were determined during early zebrafish development. Semi-quantitative reverse transcriptase-PCR analysis revealed that most mapk genes are expressed throughout zebrafish development. Erk2,3 and p38a were expressed at a constant level throughout zebrafish embryogenesis, whereas erk1,4,5,6,7 and p38b showed specific temporal expression patterns. The spatial expression patterns were obtained by whole mount in situ hybridization at 24 h post fertilization (hpf) and 48 hpf embryos. The expression patterns were localized in specific regions at both stages and were tightly regulated during embryogenesis. For p38b, no staining was detected at 24 and 48 hpf. However, its expression was demonstrated at blastula-stage. Together, we identified the zebrafish orthologs of the zebrafish MAPK gene family and determined their specific spatial and temporal expression and distribution patterns during zebrafish embryogenesis.

Functions of the MAPK family in vertebrate-development

The mitogen activated protein kinase (MAPK) family, consisting of the extracellular signal regulated protein kinase, c-Jun amino terminal MAPK and p38 subfamilies, is conserved in evolution throughout the plant and animal kingdoms. These proteins have been implicated in diverse cellular processes including cell growth, migration, proliferation, differentiation, survival and development. Gene-targeting approaches in mice, chickens, frogs and zebrafish revealed crucial roles of MAPK in vertebrate development. Gene-disruption or -silencing often lead to lethal effects, therefore the zebrafish ex utero development provides an excellent in vivo model to study the function of MAPK in early embryogenesis. In this review, we summarize the current understanding of the MAPK family function in vertebrate-development and place this into the perspective of possibilities for future research.

The ERK cascade: a prototype of MAPK signaling

Sequential activation of protein kinases within the mitogen-activated protein kinase (MAPK) cascades is a common mechanism of signal transduction in many cellular processes. Four such cascades have been elucidated thus far, and named according to their MAPK tier component as the ERK1/2, JNK, p38MAPK, and ERK5 cascades. These cascades cooperate in transmitting various extracellular signals, and thus control cellular processes such as proliferation, differentiation, development, stress response, and apoptosis. Here we describe the classic ERK1/2 cascade, and concentrate mainly on the properties of MEK1/2 and ERK1/2, including their mode of regulation and their role in various cellular processes and in oncogenesis. This cascade may serve as a prototype of the other MAPK cascades, and the study of this cascade is likely to contribute to the understanding of mitogenic and other processes in many cell lines and tissues.

ERK8 down-regulates transactivation of the glucocorticoid receptor through Hic-5

Extracellular signal-regulated kinase 8 (ERK8) is the most recently identified member of the ERK subfamily of MAPKs. Although other members of the ERK subfamily are established regulators of signaling pathways involved in cell growth and/or differentiation, less is known about ERK8. To understand the cellular function of ERK8, a yeast two-hybrid screen of a human lung library was performed to identify binding partners. One binding partner identified was Hic-5 (also known as ARA55), a multiple LIM domain containing protein implicated in focal adhesion signaling and the regulation of specific nuclear receptors, including the androgen receptor and the glucocorticoid receptor (GR). Co-immunoprecipitation experiments in mammalian cells confirmed the interaction between Hic-5 and both ERK8 and its rodent ortholog ERK7. The C-terminal region of ERK8 was not required for the interaction. Although the LIM3 and LIM4 domains of Hic-5 were sufficient and required for this interaction, the specific zinc finger motifs in these domains were not. Transcriptional activation reporter assays revealed that ERK8 can negatively regulate transcriptional co-activation of androgen receptor and GRalpha by Hic-5 in a kinase-independent manner. Knockdown of endogenous ERK8 in human airway epithelial cells enhanced dexamethasone-stimulated transcriptional activity of endogenous GR. Transcriptional regulation of GRalpha and interaction with its ligand binding domain by ERK8 were dependent on the presence of Hic-5. These results provide the first physiological function for human ERK8 as a negative regulator of human GRalpha, acting through Hic-5, and suggest a broader role for ERK8 in the regulation of nuclear receptors beyond estrogen receptor alpha.

Characterization of the reversible phosphorylation and activation of ERK8

ERK8 (extracellular-signal-regulated protein kinase 8) expressed in Escherichia coli or insect cells was catalytically active and phosphorylated at both residues of the Thr-Glu-Tyr motif. Dephosphorylation of the threonine residue by PP2A (protein serine/threonine phosphatase 2A) decreased ERK8 activity by over 95% in vitro, whereas complete dephosphorylation of the tyrosine residue by PTP1B (protein tyrosine phosphatase 1B) decreased activity by only 15-20%. Wild-type ERK8 expressed in HEK-293 cells was over 100-fold less active than the enzyme expressed in bacteria or insect cells, but activity could be increased by exposure to hydrogen peroxide, by incubation with the protein serine/threonine phosphatase inhibitor okadaic acid, or more weakly by osmotic shock. In unstimulated cells, ERK8 was monophosphorylated at Tyr-177, and exposure to hydrogen peroxide induced the appearance of ERK8 that was dually phosphorylated at both Thr-175 and Tyr-177. IGF-1 (insulin-like growth factor 1), EGF (epidermal growth factor), PMA or anisomycin had little effect on activity. In HEK-293 cells, phosphorylation of the Thr-Glu-Tyr motif of ERK8 was prevented by Ro 318220, a potent inhibitor of ERK8 in vitro. The catalytically inactive mutants ERK8[D154A] and ERK8[K42A] were not phosphorylated in HEK-293 cells or E. coli, whether or not the cells had been incubated with protein phosphatase inhibitors or exposed to hydrogen peroxide. Our results suggest that the activity of ERK8 in transfected HEK-293 cells depends on the relative rates of ERK8 autophosphorylation and dephosphorylation by one or more members of the PPP family of protein serine/threonine phosphatases. The major residue in myelin basic protein phosphorylated by ERK8 (Ser-126) was distinct from that phosphorylated by ERK2 (Thr-97), demonstrating that, although ERK8 is a proline-directed protein kinase, its specificity is distinct from ERK1/ERK2.

Protein kinases as targets for antimalarial intervention: Kinomics, structure-based design, transmission-blockade, and targeting host cell enzymes

The surge of interest in protein kinases as targets for chemotherapeutic intervention in a number of diseases such as cancer and neurodegenerative disorders has stimulated research aimed at determining whether enzymes of this class might also be considered as targets in the context of diseases caused by parasitic protists. Here, we present an overview of recent developments in this field, concentrating (i) on the benefits gained from the availability of genomic databases for a number of parasitic protozoa, (ii) on the emerging field of structure-aided design of inhibitors targeting protein kinases of parasitic protists, (iii) on the concept known as transmission-blockade, whereby kinases implicated in the development of the parasite in their arthropod vector might be targeted to interfere with disease transmission, and (iv) on the possibility of controlling parasitic diseases through the inhibition of host cell protein kinases that are required for the establishment of infection by the parasites.

Mitogen-activated protein kinases, adherens junction dynamics, and spermatogenesis: a review of recent data

Mitogen-activated protein kinases (MAPKs) are important regulators of many cellular processes. In mammalian testes, these kinases are involved in controlling cell division, differentiation, survival and death, and are therefore critical to spermatogenesis. Recent studies have also illustrated their involvement in junction restructuring in the seminiferous epithelium, especially at the ectoplasmic specialization (ES), a testis-specific adherens junction (AJ) type. ES contributes to the adhesion between Sertoli cells at the blood-testis barrier, as well as between Sertoli and developing spermatids (step 9 and beyond) at the adluminal compartment. MAPKs regulate AJ dynamics in the testis via their effects on the turnover of junction-associated protein complexes, the production of proteases and protease inhibitors, and the cytoskeleton structure. In this review, roles of the three major MAPK members, namely extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 MAPK, in ES dynamics are critically discussed. An integrated model of how these three MAPKs regulate adhesion function in the seminiferous epithelium is also presented. This model will serve as the framework for future investigation in the field.

Down-regulation of c-Fos/c-Jun AP-1 dimer activity by sumoylation

The inducible transcriptional complex AP-1, composed of c-Fos and c-Jun proteins, is crucial for cell adaptation to many environmental changes. While its mechanisms of activation have been extensively studied, how its activity is restrained is poorly understood. We report here that lysine 265 of c-Fos is conjugated by the peptidic posttranslational modifiers SUMO-1, SUMO-2, and SUMO-3 and that c-Jun can be sumoylated on lysine 257 as well as on the previously described lysine 229. Sumoylation of c-Fos preferentially occurs in the context of c-Jun/c-Fos heterodimers. Using nonsumoylatable mutants of c-Fos and c-Jun as well as a chimeric protein mimicking sumoylated c-Fos, we show that sumoylation entails lower AP-1 transactivation activity. Interestingly, single sumoylation at any of the three acceptor sites of the c-Fos/c-Jun dimer is sufficient to substantially reduce transcription activation. The lower activity of sumoylated c-Fos is not due to inhibition of protein entry into the nucleus, accelerated turnover, and intrinsic inability to dimerize or to bind to DNA. Instead, cell fractionation experiments suggest that decreased transcriptional activity of sumoylated c-Fos is associated with specific intranuclear distribution. Interestingly, the phosphorylation of threonine 232 observed upon expression of oncogenically activated Ha-Ras is known to superactivate c-Fos transcriptional activity. We show here that it also inhibits c-Fos sumoylation, revealing a functional antagonism between two posttranslational modifications, each occurring within a different moiety of a bipartite transactivation domain of c-Fos. Finally we report that the sumoylation of c-Fos is a dynamic process that can be reversed via multiple mechanisms. This supports the idea that this modification does not constitute a final inactivation step that necessarily precedes protein degradation.

Activation of a nuclear Cdc2-related kinase within a mitogen-activated protein kinase-like TDY motif by autophosphorylation and cyclin-dependent protein kinase-activating kinase

Male germ cell-associated kinase (MAK) and intestinal cell kinase (ICK) are nuclear Cdc2-related kinases with nearly identical N-terminal catalytic domains and more divergent C-terminal noncatalytic domains. The catalytic domain is also related to mitogen-activated protein kinases (MAPKs) and contains a corresponding TDY motif. Nuclear localization of ICK requires subdomain XI and interactions of the conserved Arg-272, but not kinase activity or, surprisingly, any of the noncatalytic domain. Further, nuclear localization of ICK is required for its activation. ICK is activated by dual phosphorylation of the TDY motif. Phosphorylation of Tyr-159 in the TDY motif requires ICK autokinase activity but confers only basal kinase activity. Full activation requires additional phosphorylation of Thr-157 in the TDY motif. Coexpression of ICK with constitutively active MEK1 or MEK5 fails to increase ICK phosphorylation or activity, suggesting that MEKs are not involved. ICK and MAK are related to Ime2p in budding yeast, and cyclin-dependent protein kinase-activating kinase Cak1p has been placed genetically upstream of Ime2p. Recombinant Cak1p phosphorylates Thr-157 in the TDY motif of recombinant ICK and activates its activity in vitro. Coexpression of ICK with wild-type CAK1 but not kinase-inactive CAK1 in cells also increases ICK phosphorylation and activity. Our studies establish ICK as the prototype for a new group of MAPK-like kinases requiring dual phosphorylation at TDY motifs.

Activation-loop autophosphorylation is mediated by a novel transitional intermediate form of DYRKs

Autophosphorylation of a critical residue in the activation loop of several protein kinases is an essential maturation event required for full enzyme activity. However, the molecular mechanism by which this happens is unknown. We addressed this question for two dual-specificity tyrosine-phosphorylation-regulated protein kinases (DYRKs), as they autophosphorylate their activation loop on an essential tyrosine but phosphorylate their substrates on serine and threonine. Here we demonstrate that autophosphorylation of the critical activation-loop tyrosine is intramolecular and mediated by the nascent kinase passing through a transitory intermediate form. This DYRK intermediate differs in residue and substrate specificity, as well as sensitivity to small-molecule inhibitors, compared with its mature counterpart. The intermediate's characteristics are lost upon completion of translation, making the critical tyrosine autophosphorylation a "one-off" inceptive event. This mechanism is likely to be shared with other kinases.

Ubiquitin chains in the ladder of MAPK signaling

With a better understanding of the cellular stress response, it has become evident that catalytic modules consisting of kinases that mediate the activation of downstream effector components are subject to multiple layers of regulation. Such regulatory mechanisms are not limited to those involving scaffold proteins or protein phosphatases, and they appear to include a growing number of modifications by ubiquitin and ubiquitin-like proteins. The role of ubiquitin in the regulation of mitogen-activated protein kinase (MAPK) emerges as a paradigm for understanding the role of ubiquitination in regulating other signal transduction pathways. Ubiquitination influences signal diversification and limits the duration of the signal through its role in the assembly of protein kinase complexes, subcellular localization, and the actual degradation of the kinase or its substrate. This review summarizes our current understanding of the roles of ubiquitin in regulating MAPK signaling.

Phosphorylation of c-Fos by members of the p38 MAPK family. Role in the AP-1 response to UV light

Exposure to sources of UV radiation, such as sunlight, induces a number of cellular alterations that are highly dependent on its ability to affect gene expression. Among them, the rapid activation of genes coding for two subfamilies of proto-oncoproteins, Fos and Jun, which constitute the AP-1 transcription factor, plays a key role in the subsequent regulation of expression of genes involved in DNA repair, cell proliferation, cell cycle arrest, death by apoptosis, and tissue and extracellular matrix remodeling proteases. Besides being regulated at the transcriptional level, Jun and Fos transcriptional activities are also regulated by phosphorylation as a result of the activation of intracellular signaling cascades. In this regard, the phosphorylation of c-Jun by UV-induced JNK has been readily documented, whereas a role for Fos proteins in UV-mediated responses and the identification of Fos-activating kinases has remained elusive. Here we identify p38 MAPKs as proteins that can associate with c-Fos and phosphorylate its transactivation domain both in vitro and in vivo. This phosphorylation is transduced into changes in its transcriptional ability as p38-activated c-Fos enhances AP1-driven gene expression. Our findings indicate that as a consequence of the activation of stress pathways induced by UV light, endogenous c-Fos becomes a substrate of p38 MAPKs and, for the first time, provide evidence that support a critical role for p38 MAPKs in mediating stress-induced c-Fos phosphorylation and gene transcription activation. Using a specific pharmacological inhibitor for p38alpha and -beta, we found that most likely these two isoforms mediate UV-induced c-Fos phosphorylation in vivo. Thus, these newly described pathways act concomitantly with the activation of c-Jun by JNK/MAPKs, thereby contributing to the complexity of AP1-driven gene transcription regulation.

A novel ERK-like, CRK-like protein kinase that modulates growth in Trypanosoma brucei via an autoregulatory C-terminal extension

The protozoan parasite Trypanosoma brucei undergoes a complex developmental cycle coordinated with cell cycle control. These processes in eukaryotes are frequently regulated through mitogen-activated protein kinases (MAPKs) and cyclin-dependent protein kinases (CDKs), respectively. We have discovered a novel protein kinase which shares features of both ERK-type MAPKs and CDKs (T. brucei ERK-like, CDK-like protein kinase). This molecule, named TbECK1, is similar to the unusual mammalian KKIAMRE protein kinase family. Moreover, TbECK1 possesses a long C-terminal extension reminiscent of those found in mammalian ERK5, ERK7 and ERK8. Expression analyses demonstrate that TbECK1 is constitutively expressed during the trypanosome life cycle at both RNA and protein level. In transgenic parasites we demonstrate that expression of a mutant of TbECK1 that lacks the C-terminal extension produces a slow growth phenotype, associated with the appearance of cells with aberrant karyotypes. Using this as an assay we further demonstrate that the phenotype is dependent upon the potential for catalytic activity of TbECK1 and on the integrity of at least one of the phosphorylable amino acids in its phosphorylation lip. C-terminal extensions are a common feature of kinetoplastid protein kinases. Our results demonstrate for the first time that this domain has a regulatory function.

Counting on mitogen-activated protein kinases—ERKs 3, 4, 5, 6, 7 and 8

Signal transduction pathways in eukaryotic cells integrate diverse extracellular signals, and regulate complex biological responses such as growth, differentiation and death. One group of proline-directed Ser/Thr protein kinases, the mitogen-activated protein kinases (MAPKs), plays a central role in these signalling pathways. Much attention has focused in recent years on three subfamilies of MAPKs, the extracellular signal regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs) and the p38 MAPKs. However, the ERK family is broader than the ERK1 and ERK2 proteins that have been the subject of most studies in this area. Here we overview the work on ERKs 3 to 8, emphasising where possible their biological activities as well as distinctive biochemical properties. It is clear from these studies that these additional ERKs show similarities to ERK1 and ERK2, but with some interesting differences that challenge the paradigm of the archetypical ERK1/2 MAPK pathway.

Scaffolding by ERK3 regulates MK5 in development

Extracellular-regulated kinase 3 (ERK3, MAPK6) is an atypical member of the ERKs, lacking the threonine and tyrosine residues in the activation loop, carrying a unique C-terminal extension and being mainly regulated by its own protein stability and/or by autophosphorylation. Here we show that ERK3 specifically interacts with the MAPK-activated protein kinase 5 (MK5 or PRAK) in vitro and in vivo. Expression of ERK3 in mammalian cells leads to nuclear-cytoplasmic translocation and activation of MK5 and to phosphorylation of both ERK3 and MK5. Remarkably, activation of MK5 is independent of ERK3 enzymatic activity, but depends on its own catalytic activity as well as on a region in the C-terminal extension of ERK3. In mouse embryonic development, mRNA expression patterns of ERK3 and MK5 suggest spatiotemporal coexpression of both kinases. Deletion of MK5 leads to strong reduction of ERK3 protein levels and embryonic lethality at about stage E11, where ERK3 expression in wild-type mice is maximum, indicating a role of this signalling module in development.

p38 MAP kinase: a convergence point in cancer therapy

Recent studies show that activation of p38 mitogen-activated protein kinase (MAPK) results in cancer cell apoptosis initiated by retinoids, cisplatin and other chemotherapeutic agents. The observation that divergent therapies act through a common signal transduction pathway raises the possibility of developing new anti-cancer agents that lack the side-effects caused by events upstream of p38 MAPK. Here, we review p38-MAPK-mediated tumor cell apoptosis and implications for cancer therapeutics.

PfPK7, an atypical MEK-related protein kinase, reflects the absence of classical three-component MAPK pathways in the human malaria parasite Plasmodium falciparum

Two members of the mitogen-activated protein kinase (MAPK) family have been previously characterized in Plasmodium falciparum, but in vitro attempts at identifying MAP kinase kinase (MAPKK) homologues have failed. Here we report the characterization of a novel plasmodial protein kinase, PfPK7, whose top scores in blastp analysis belong to the MAPKK3/6 subgroup of MAPKKs. However, homology to MAPKKs is restricted to regions of the C-terminal lobe of the kinase domain, whereas the N-terminal region is closer to fungal protein kinase A enzymes (PKA, members of the AGC group of protein kinases). Hence, PfPK7 is a 'composite' enzyme displaying regions of similarity to more than one protein kinase family, similar to a few other plasmodial protein kinases. PfPK7 is expressed in several developmental stages of the parasite, both in the mosquito vector and in the human host. Recombinant PfPK7 displayed kinase activity towards a variety of substrates, but was unable to phosphorylate the two P. falciparum MAPK homologues in vitro, and was insensitive to PKA and MEK inhibitors. Together with the absence of a typical MAPKK activation site in its T-loop, this suggests that PfPK7 is not a MAPKK orthologue, despite the fact that this enzyme is the most 'MAPKK-like' enzyme encoded in the P. falciparum genome. This is consistent with recent observations that the plasmodial MAPKs are not true orthologues of the ERK1/2, p38 or JNK MAPKs, and strengthens the evidence that classical three-component module-dependent MAPK signalling pathways do not operate in malaria parasites, a feature that has not been described in any other eukaryote.

Protein kinases of the human malaria parasite Plasmodium falciparum: the kinome of a divergent eukaryote

BACKGROUND

Malaria, caused by the parasitic protist Plasmodium falciparum, represents a major public health problem in the developing world. The P. falciparum genome has been sequenced, which provides new opportunities for the identification of novel drug targets. Eukaryotic protein kinases (ePKs) form a large family of enzymes with crucial roles in most cellular processes; hence malarial ePKS represent potential drug targets. We report an exhaustive analysis of the P. falciparum genomic database (PlasmoDB) aimed at identifying and classifying all ePKs in this organism.

RESULTS

Using a variety of bioinformatics tools, we identified 65 malarial ePK sequences and constructed a phylogenetic tree to position these sequences relative to the seven established ePK groups. Predominant features of the tree were: (i) that several malarial sequences did not cluster within any of the known ePK groups; (ii) that the CMGC group, whose members are usually involved in the control of cell proliferation, had the highest number of malarial ePKs; and (iii) that no malarial ePK clustered with the tyrosine kinase (TyrK) or STE groups, pointing to the absence of three-component MAPK modules in the parasite. A novel family of 20 ePK-related sequences was identified and called FIKK, on the basis of a conserved amino acid motif. The FIKK family seems restricted to Apicomplexa, with 20 members in P. falciparum and just one member in some other Apicomplexan species.

CONCLUSION

The considerable phylogenetic distance between Apicomplexa and other Eukaryotes is reflected by profound divergences between the kinome of malaria parasites and that of yeast or mammalian cells.

Identification of mitogen-activated protein kinase homologues from Leishmania mexicana

Mitogen-activated protein kinases are key-regulatory elements in the differentiation, proliferation, apoptosis and stress response of eukaryotic cells. Our recent identification of a mitogen-activated protein kinase homologue in Leishmania mexicana which is essential for the proliferation of the amastigote stage of the parasite living in the parasitophorous vacuole of the infected macrophage prompted us to screen the genome of L. mexicana for additional mitogen-activated protein kinase homologues using degenerate oligonucleotide primers in a polymerase chain reaction amplification approach. We cloned and sequenced the genes for eight new mitogen-activated protein kinase homologues which were subsequently shown to be present in one copy per haploid genome. The mRNA levels of the kinases varied significantly in pro- and amastigote life stages of the parasite. We used the structural information of the p38 stress-activated protein kinase, which belongs to the family of mitogen-activated protein kinases, for the alignment of the deduced proteins and the verification of the predicted secondary structure elements. All new mitogen-activated protein kinases reveal the typical 12 subdomain primary structure, the conserved residues characterising serine/threonine protein kinases and the characteristic TXY motif in the phosphorylation lip. Typical features of some of the molecules are amino acid insertions between the subdomains and long carboxy-terminal amino acid extensions carrying putative src-homology 3-binding motifs.

Mitogen-activated protein kinases in apoptosis regulation

Cells are continuously exposed to a variety of environmental stresses and have to decide 'to be or not to be' depending on the types and strength of stress. Among the many signaling pathways that respond to stress, mitogen-activated protein kinase (MAPK) family members are crucial for the maintenance of cells. Three subfamilies of MAPKs have been identified: extracellular signal-regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs), and p38-MAPKs. It has been originally shown that ERKs are important for cell survival, whereas JNKs and p38-MAPKs were deemed stress responsive and thus involved in apoptosis. However, the regulation of apoptosis by MAPKs is more complex than initially thought and often controversial. In this review, we discuss MAPKs in apoptosis regulation with attention to mouse genetic models and critically point out the multiple roles of MAPKs.

ERK7 expression and kinase activity is regulated by the ubiquitin-proteosome pathway

ERK7 is a unique member of the extracellular signal-regulated kinase (ERK) subfamily of MAP kinases. Although ERK7 shares a TEY motif in the activation loop of the kinase, it displays constitutive activation, nuclear localization, and growth inhibitory properties that are regulated by its C-terminal domain. Because ERK7 is expressed at low levels compared with ERK2 and its activity is dependent upon its expression level, we investigated the mechanism by which ERK7 expression is regulated. We now show that ERK7 expression is regulated by ubiquitination and rapid proteosomal turnover. Furthermore, both the kinase domain and the C-terminal tail are independently degraded at a rate comparable with that of the intact protein. Analysis of a series of chimeras between ERK2 and ERK7 reveal that the N-terminal 20 amino acids of the kinase domain are a primary determinant of ERK7 degradation. Fusion of the N-terminal 20 amino acids is both necessary and sufficient to cause proteolytic degradation of both ERK2 and green fluorescent protein. Finally, ERK7 is stabilized by an N-terminal mutant of Cullin-1 suggesting that ERK7 is ubiquitinated by the Skip1-Cullin-F box complex. These results indicate that ERK7 is a highly regulated enzyme whose cellular expression and kinase activation level is tightly controlled by the ubiquitin-proteosome pathway.

Src-dependent ERK5 and Src/EGFR-dependent ERK1/2 activation is required for cell proliferation by asbestos

Crocidolite asbestos elicits oxidative stress and cell proliferation, but the signaling cascades linked to these outcomes are unclear. To determine the role of mitogen-activated protein kinases (MAPK) in asbestos-induced cell signaling, we evaluated the effects of crocidolite asbestos, EGF and H2O2, on MAPK activation in murine lung epithelial cells (C10 line). In contrast to rapid and transient activation of extracellular signal-regulated kinase 5 (ERK5) by EGF or H2O2, asbestos caused protracted oxidant-dependent ERK5 activation that was inhibited by an Src kinase inhibitor (PP2), but not by an inhibitor of epidermal growth factor receptor (EGFR) phosphorylation (AG1478). ERK1/2 activation by asbestos was inhibited by either PP2 or AG1478. To confirm the involvement of Src in ERK1/2 and ERK5 activation, a dominant-negative Src construct was used. These experiments showed that Src was essential for ERK1/2 and also ERK5 phosphorylation by asbestos. Time frame studies indicated immediate activation of Src by asbestos fibers, whereas EGFR phosphorylation occurred subsequently. Data suggest that asbestos causes activation of ERK5 through an EGFR-independent pathway, whereas ERK1/2 activation is dependent on Src through a mechanism involving phosphorylation of the EGFR. Furthermore, Src, ERK1/2 and ERK5 activation are essential for cell proliferation by asbestos. The use of a dominant-negative ERK5 construct caused selective downregulation of c-jun expression, whereas inhibition of Src by PP2 or MEK1 by PD98059 caused decreases in c-fos, fra-1 and c-jun expression in asbestos-exposed C10 cells. These observations may have broad relevance to cell proliferation by carcinogenic mineral fibers and oxidants.

Phosphorylation of the carboxyl-terminal transactivation domain of c-Fos by extracellular signal-regulated kinase mediates the transcriptional activation of AP-1 and cellular transformation induced by platelet-derived growth factor

Polypeptide growth factors, such as platelet-derived growth factor (PDGF), promote the reinitiation of DNA synthesis and cell growth through multiple intracellular signaling pathways that converge in the nucleus to regulate the activity of transcription factors, thereby controlling the expression of growth-promoting genes. Among them, the AP-1 (activating protein-1) family of transcription factors, including c-Fos and c-Jun family members, plays a key role, as AP-1 activity is potently activated by PDGF and is required to stimulate cell proliferation. However, the nature of the pathways connecting PDGF receptors to AP-1 is still poorly defined. In this study, we show that PDGF regulates AP-1 by stimulating the expression and function of c-Fos through extracellular signal-regulated kinase (ERK). The latter involves the direct phosphorylation by ERK of multiple residues in the carboxyl-terminal transactivation domain of c-Fos, which results in its increased transcriptional activity. Interestingly, the phosphorylation of c-Fos by ERK was required for the ability of PDGF and serum to stimulate the activity of c-Fos as well as AP-1-dependent transcription. Furthermore, we provide evidence that the ERK-dependent activation of c-Fos is an integral component of the mitogenic pathway by which PDGF regulates normal and aberrant cell growth.

Extracellular signal-regulated kinase 7, a regulator of hormone-dependent estrogen receptor destruction

Estrogen receptor alpha (ER alpha) degradation is regulated by ubiquitination, but the signaling pathways that modulate ER alpha turnover are unknown. We found that extracellular signal-regulated kinase 7 (ERK7) preferentially enhances the destruction of ER alpha but not the related androgen receptor. Loss of ERK7 was correlated with breast cancer progression, and all ER alpha-positive breast tumors had decreased ERK7 expression compared to that found in normal breast tissue. In human breast cells, a dominant-negative ERK7 mutant decreased the rate of endogenous ER alpha degradation >4-fold in the presence of hormone and potentiated estrogen responsiveness. ERK7 targets the ER alpha ligand-binding domain for destruction by enhancing its ubiquitination. Thus, ERK7 is a novel regulator of estrogen responsiveness through its control of ER alpha turnover.

Inhibition of the stem cell factor-induced migration of mast cells by dexamethasone

Mast cell accumulation can be causally related to several allergic inflammations. Previous work has demonstrated that glucocorticoids decreased tissue mast cell number, and stem cell factor (SCF)-induced migration of mast cells required p38 MAPK activation. In the present study we investigated the effects of dexamethasone on SCF-induced migration of rat peritoneal mast cells (RPMCs). SCF significantly induced the migration of RPMCs at 4 h. Dexamethasone dose-dependently inhibited SCF-induced migration of RPMCs (approximately 90.1% at 100 nM; P < 0.05). The MAPK p38 inhibitor SB203580 (20 microM) also inhibited the SCF-induced migration. The ability of SCF to enhance morphological alteration and filamentous actin formation was also abolished by treatment with dexamethasone. Dexamethasone inhibited SCF-induced p38 MAPK activation to near-basal levels and induced MAPK phosphatase-1 expression. In addition, SCF-induced inflammatory cytokine production was significantly inhibited by treatment with dexamethasone or SB203580 (P < 0.01). Our results show that dexamethasone potently regulates SCF-induced migration, p38 MAPK activation, and inflammatory cytokine production through the expression of MKP-1 protein in RPMCs. Such modulation may have functional consequences during dexamethasone treatment, especially mast cell-mediated allergic inflammation disorders.

Map kinase signaling pathways and hematologic malignancies

Mitogen-activated protein (Map) kinases are widely expressed serine-threonine kinases that mediate important regulatory signals in the cell. Three major groups of Map kinases exist: the p38 Map kinase family, the extracellular signal-regulated kinase (Erk) family, and the c-Jun NH2-terminal kinase (JNK) family. The members of the different Map kinase groups participate in the generation of various cellular responses, including gene transcription, induction of cell death or maintenance of cell survival, malignant transformation, and regulation of cell-cycle progression. Depending on the specific family isoform involved and the cellular context, Map kinase pathways can mediate signals that either promote or suppress the growth of malignant hematopoietic cells. Over the last few years, extensive work by several groups has established that Map kinase pathways play critical roles in the pathogenesis of various hematologic malignancies, providing new molecular targets for future therapeutic approaches. In this review, the involvement of various Map kinase pathways in the pathophysiology of hematologic malignances is summarized and the clinical implications of the recent advances in the field are discussed.

Differential interaction of the tyrosine phosphatases PTP-SL, STEP and HePTP with the mitogen-activated protein kinases ERK1/2 and p38alpha is determined by a kinase specificity sequence and influenced by reducing agents

The protein tyrosine phosphatases (PTPs) PTP-SL, STEP and HePTP are mitogen-activated protein kinase (MAPK) substrates and regulators that bind to MAPKs through a kinase-interaction motif (KIM) located in their non-catalytic regulatory domains. We have found that the binding of these PTPs to the MAPKs extracellular-signal-regulated kinase 1 and 2 (ERK1/2), and p38alpha is differentially determined by the KIM-adjacent C-terminal regions of the PTPs, which have been termed kinase-specificity sequences, and is influenced by reducing agents. Under control conditions, PTP-SL bound preferentially to ERK1/2, whereas STEP and HePTP bound preferentially to p38alpha. Under reducing conditions, the association of p38alpha with STEP or HePTP was impaired, whereas the association with PTP-SL was unaffected. On the other hand, the association of ERK1/2 with HePTP was increased under reducing conditions, whereas the association with STEP or PTP-SL was unaffected. In intact cells, PTP-SL and STEP distinctively regulated the kinase activity and the nuclear translocation of ERK1/2 and p38alpha. Our results suggest that intracellular redox conditions could modulate the activity and subcellular location of ERK1/2 and p38alpha by controlling their association with their regulatory PTPs.

Protein kinases and their involvement in the cellular responses to genotoxic stress

Cells are constantly subjected to genotoxic stress, and much has been learned regarding their response to this type of stress during the past year. In general, the cellular genotoxic response can be thought to occur in three stages: (1) damage sensing; (2) activation of signal transduction pathways; (3) biological consequences and attenuation of the response. The biological consequences, in particular, include cell cycle arrest and cell death. Although our understanding of the molecular mechanisms underlying cellular genotoxic stress responses remains incomplete, many cellular components have been identified over the years, including a group of protein kinases that appears to play a major role. Various DNA-damaging agents can activate these protein kinases, triggering a protein phosphorylation cascade that leads to the activation of transcription factors, and altering gene expression. In this review, the involvement of protein kinases, particularly the mitogen-activated protein kinases (MAPKs), at different stages of the genotoxic response is discussed.

Mitogen-activated protein kinase pathways mediated by ERK, JNK, and p38 protein kinases

Multicellular organisms have three well-characterized subfamilies of mitogen-activated protein kinases (MAPKs) that control a vast array of physiological processes. These enzymes are regulated by a characteristic phosphorelay system in which a series of three protein kinases phosphorylate and activate one another. The extracellular signal-regulated kinases (ERKs) function in the control of cell division, and inhibitors of these enzymes are being explored as anticancer agents. The c-Jun amino-terminal kinases (JNKs) are critical regulators of transcription, and JNK inhibitors may be effective in control of rheumatoid arthritis. The p38 MAPKs are activated by inflammatory cytokines and environmental stresses and may contribute to diseases like asthma and autoimmunity.

Ras-MAP kinase signaling pathways and control of cell proliferation: relevance to cancer therapy

The mitogen-activated protein (MAP) kinase pathways represent several families of signal transduction cascades that mediate information provided by extracellular stimuli. MAP kinase pathways regulate a wide range of physiological responses, including cell proliferation, apoptosis, cell differentiation, and tissue development. Constitutive activation of MAP kinase proteins in experimental models has been shown to cause cell transformation and is implicated in tumorigenesis. Of clinical importance, MAP kinase pathways are regulated by Ras G-proteins, which are found to be mutated and constitutively active in approximately 30% of all human cancers. Thus, a major goal in the treatment of cancer is the development of specific compounds that target Ras and critical downstream signaling proteins responsible for uncontrolled cell growth. A variety of biochemical, molecular, and structural approaches have been used to develop drug compounds that target signaling proteins important for MAP kinase pathway activation. These compounds have been useful tools for identifying the mechanisms of MAP kinase pathway signaling and hold promise for clinical use. This review will present an overview of the major proteins involved in Ras and MAP kinase signaling pathways and their function in regulating cell cycle events and proliferation. In addition, some of the relevant compounds that have been developed to inhibit the activities of these proteins and MAP kinase signaling are discussed.

ERK8, a new member of the mitogen-activated protein kinase family

The ERKs are a subfamily of the MAPKs that have been implicated in cell growth and differentiation. By using the rat ERK7 cDNA to screen a human multiple tissue cDNA library, we identified a new member of the ERK family, ERK8, that shares 69% amino acid sequence identity with ERK7. Northern analysis demonstrates that ERK8 is present in a number of tissues with maximal expression in the lung and kidney. Fluorescence in situ hybridization localized the ERK8 gene to chromosome 8, band q24.3. Expression of ERK8 in COS cells and bacteria indicates that, in contrast to constitutively active ERK7, ERK8 has minimal basal kinase activity and a unique substrate profile. ERK8, which contains two SH3-binding motifs in its C-terminal region, associates with the c-Src SH3 domain in vitro and co-immunoprecipitates with c-Src in vivo. Co-transfection with either v-Src or a constitutively active c-Src increases ERK8 activation indicating that ERK8 can be activated downstream of c-Src. ERK8 is also activated following serum stimulation, and the extent of this activation is reduced by pretreatment with the specific Src family inhibitor PP2. The ERK8 activation by serum or Src was not affected by the MEK inhibitor U0126 indicating that activation of ERK8 does not require MEK1, MEK2, or MEK5. Although most closely related to ERK7, the relatively low sequence identity, minimal basal activity, and different substrate profile identify ERK8 as a distinct member of the MAPK family that is activated by an Src-dependent signaling pathway.

Role of mitogen-activated protein kinases (MAPK) in cell injury and proliferation by environmental particulates

Cell signaling pathways may be initiated by environmental particulates by indirect mechanisms such as elaboration of reactive oxygen or nitrogen species (ROS/RNS) or directly upon contact of particulates with the plasma membrane and uptake by epithelial or mesothelial cells. Research in the last few years has mainly addressed cell signaling cascades leading to activation of the redox-sensitive transcription factors, nuclear factor kappa-B (NF-kappaB), and activator protein-1 (AP-1). The activation of these transcription factors may be linked to increases in gene expression controlling cell injury or apoptosis, proliferation and/or cell survival, and inflammatory cytokines. Here, we provide an overview of the MAPK signaling pathways and their activation by asbestos, specifically the role of ROS, receptor-dependent and independent activation via the epidermal growth factor receptor (EGFR), and strategies for proving causal relationships between these pathways and changes in epithelial cell phenotype linked to disease causation.

Coxsackievirus B3 replication is reduced by inhibition of the extracellular signal-regulated kinase (ERK) signaling pathway

Coxsackievirus B3 (CVB3) is the most common human pathogen for viral myocarditis. We have previously shown that the signaling protein p21(ras) GTPase-activating protein (RasGAP) is cleaved and that mitogen-activated protein kinases (MAPKs) ERK1/2 are activated in the late phase of CVB3 infection. However, the role of intracellular signaling pathways in CVB3-mediated myocarditis and the relative advantages of such pathways to host or virus remain largely unclear. In this study we extended our prior studies by examining the interaction between CVB3 replication and intracellular signaling pathways in HeLa cells. We observed that CVB3 infection induced a biphasic activation of ERK1/2, early transient activation versus late sustained activation, which were regulated by different mechanisms. Infection by UV-irradiated, inactivated virus capable of receptor binding and endocytosis triggered early ERK1/2 activation, but was insufficient to trigger late ERK1/2 activation. By using a general caspase inhibitor (zVAD.fmk) we further demonstrated that late ERK1/2 activation was not a result of CVB3-mediated caspase cleavage. Treatment of cells with U0126, a selective inhibitor of MAPK kinase (MEK), significantly inhibited CVB3 progeny release and decreased virus protein production. Furthermore, inhibition of ERK1/2 activation circumvented CVB3-induced apoptosis and viral protease-mediated RasGAP cleavage. Taken together, these data suggest that ERK1/2 activation is important for CVB3 replication and contributes to virus-mediated changes in host cells. Our findings demonstrate coxsackievirus takeover of a particular host signaling mechanism and uncover a prospective approach to stymie virus spread and preserve myocardial integrity.

Different domains of the mitogen-activated protein kinases ERK3 and ERK2 direct subcellular localization and upstream specificity in vivo

Extracellular signal-regulated kinase 3 (ERK3) is a member of the mitogen-activated protein (MAP) kinase family. ERK3 is most similar in its kinase catalytic domain to ERK2, yet it displays many unique properties. Among these, unlike ERK2, which translocates to the nucleus following activation, ERK3 is constitutively localized to the nucleus, despite the lack of a defined nuclear localization sequence. We created two chimeras between ERK2 and the catalytic domain of ERK3 (ERK3DeltaC), and some mutants of these chimeras, to examine the basis for the different behaviors of these two MAP kinase family members. We find the following: 1) the N-terminal folding domain of ERK3 functions in phosphoryl transfer reactions with the C-terminal folding domain of ERK2; 2) the C-terminal halves of ERK2 and ERK3DeltaC are primarily responsible for their subcellular localization in resting cells; and 3) the N-terminal folding domain of ERK2 is required for its activation in cells, its interaction with MEK1, and its accumulation in the nucleus.

M1 muscarinic acetylcholine receptors activate extracellular signal-regulated kinase in CA1 pyramidal neurons in mouse hippocampal slices

Activation of extracellular signal-regulated kinases (ERK) is crucial for many neural functions, including learning, memory, and synaptic plasticity. As muscarinic acetylcholine receptors (mAChR) modulate many of the same higher brain functions as ERK, we examined mAChR-mediated ERK activation in mouse hippocampal slices. The cholinergic agonist carbachol caused an atropine-sensitive ERK activation in the dendrites and somata CA1 pyramidal neurons. To determine the responsible mAChR subtype, we combined pharmacologic and genetic approaches. Pretreatment with M1 antagonists inhibited ERK activation. Furthermore, mAChR-induced ERK activation was absent in slices from M1 knockout mice. ERK activation was normal in slices derived from other mAChR subtype knockouts (M2, M3, and M4), although these other subtypes are expressed in many of the same neurons. Thus, we demonstrate divergent functions for the different mAChR subtypes. We conclude that M1 is responsible for mAChR-mediated ERK activation, providing a mechanism by which M1 may modulate learning and memory.

Ten ERK-related proteins in three distinct classes associate with AP-1 proteins and/or AP-1 DNA

We have identified seven ERK-related proteins ("ERPs"), including ERK2, that are stably associated in vivo with AP-1 dimers composed of diverse Jun and Fos family proteins. These complexes have kinase activity. We designate them as "class I ERPs." We originally hypothesized that these ERPs associate with DNA along with AP-1 proteins. We devised a DNA affinity chromatography-based analytical assay for DNA binding, the "nucleotide affinity preincubation specificity test recognition" (NAPSTER) assay. In this assay, class I ERPs do not associate with AP-1 DNA. However, several new "class II" ERPs do associate with DNA. p41 and p44 are ERK1/2-related ERPs that lack kinase activity and associate along with AP-1 proteins with AP-1 DNA. Class I ERPs and their associated kinase activity thus appear to bind AP-1 dimers when they are not bound to DNA and then disengage and are replaced by class II ERPs to form higher order complexes when AP-1 dimers bind DNA. p97 is a class III ERP, related to ERK3, that associates with AP-1 DNA without AP-1 proteins. With the exception of ERK2, none of the 10 ERPs appear to be known mitogen-activated protein kinase superfamily members.

Specific association of a set of molecular chaperones including HSP90 and Cdc37 with MOK, a member of the mitogen-activated protein kinase superfamily

We have recently identified and cloned a novel member of mitogen-activated protein kinase superfamily protein, MOK (Miyata, Y., Akashi, M., and Nishida, E. (1999) Genes Cells 4, 299-309). To address its regulatory mechanisms, we searched for cellular proteins that specifically associate with MOK by co-immunoprecipitation experiments. Several cellular proteins including a major 90-kDa molecular chaperone HSP90 were found associated with MOK. Treatment of cells with geldanamycin, an HSP90-specific inhibitor, rapidly decreased the protein level of MOK, and the decrease was attributed to enhanced degradation of MOK through proteasome-dependent pathways. Our data suggest that the association with HSP90 may regulate intracellular protein stability and solubility of MOK. Experiments with a series of deletion mutants of MOK indicated that the region encompassing the protein kinase catalytic subdomains I-IV is required for HSP90 binding. Closely related protein kinases (male germ cell-associated kinase and male germ cell-associated kinase-related kinase) were also found to associate with HSP90, whereas conventional mitogen-activated protein kinases (extracellular signal-regulated kinase, p38, and c-Jun N-terminal kinase/stress-activated protein kinase) were not associated with HSP90. In addition, we found that other molecular chaperones including Cdc37, HSC70, HSP70, and HSP60 but not GRP94, FKBP52, or Hop were detected specifically in the MOK-HSP90 immunocomplexes. These results taken together suggest a role of a specific set of molecular chaperones in the stability of signal-transducing protein kinases.

ERK7 is an autoactivated member of the MAPK family

Extracellular signal-regulated kinase 7 (ERK7) shares significant sequence homology with other members of the ERK family of signal transduction proteins, including the signature TEY activation motif. However, ERK7 has several distinguishing characteristics. Unlike other ERKs, ERK7 has been shown to have significant constitutive activity in serum-starved cells, which is not increased further by extracellular stimuli that typically activate other members of the mitogen-activated protein kinase (MAPK) family. On the other hand, ERK7's activation state and kinase activity appear to be regulated by its ability to utilize ATP and the presence of its extended C-terminal region. In this study, we investigated the mechanism of ERK7 activation. The results suggest that 1) MAPK kinase (MEK) inhibitors do not suppress ERK7 kinase activity; 2) intramolecular autophosphorylation is sufficient for activation of ERK7 in the absence of an upstream MEK; and 3) multiple regions of the C-terminal domain of ERK7 regulate its kinase activity. Taken together, these results indicate that autophosphorylation is sufficient for ERK7 activation and that the C-terminal domain regulates its kinase activity through multiple interactions.

Distinct binding determinants for ERK2/p38alpha and JNK map kinases mediate catalytic activation and substrate selectivity of map kinase phosphatase-1

Mitogen-activated protein (MAP) kinase phosphatase 1 (MKP-1/CL100) is an inducible nuclear dual specificity protein phosphatase that can dephosphorylate and inactivate both mitogen- and stress-activated protein kinases in vitro and in vivo. However, the molecular mechanism responsible for the substrate selectivity of MKP-1 is unknown. In addition, it has been suggested that the signal transducers and activators of transcription 1 (STAT1) transcription factor is a physiological non-MAP kinase substrate for MKP-1. We have used the yeast two-hybrid assay to demonstrate that MKP-1 is able to interact selectively with the extracellular signal-regulated kinase 1/2 (ERK1/2), p38alpha, and c-Jun NH(2)-terminal kinase (JNK) MAP kinase isoforms. Furthermore, this binding is accompanied by catalytic activation of recombinant MKP-1 protein in vitro, and these end points show an absolute correlation with MKP-1 substrate selectivity in vivo. In contrast, MKP-1 does not interact with STAT1. Recombinant STAT1 does not cause catalytic activation of MKP-1; nor does MKP-1 block tyrosine phosphorylation of STAT1 in vivo. Both binding and catalytic activation of MKP-1 are abrogated by mutation of a conserved docking site in ERK2, p38alpha, and JNK1 MAP kinases. Within MKP-1, MAP kinase binding is mediated by the amino-terminal noncatalytic domain of the protein. However, mutation of a conserved cluster of positively charged residues within this domain abolishes the binding and activation of MKP-1 by ERK2 and p38alpha but not JNK1, indicating that there are distinct binding determinants for these MAP kinase isoforms. We conclude that the substrate selectivity of MKP-1 is determined by specific protein-protein interactions coupled with catalytic activation of the phosphatase and that these interactions are restricted to members of the MAP kinase family of enzymes.

Reduced activation and expression of ERK1/2 MAP kinase in the post-mortem brain of depressed suicide subjects

The extracellular regulated kinases (ERK) 1 and ERK2 are members of mitogen-activated protein (MAP) kinase family that play an important role in transducing extracellular signals to the nucleus and have been implicated in a broad spectrum of biological responses. To test the hypothesis that MAP kinases may be involved in depression, we examined the activation of p44/42 MAP kinase and expression of ERK1 and ERK2 in the post-mortem brain tissue obtained from non-psychiatric control subjects (n = 11) and age- and the post-mortem interval-matched depressed suicide subjects (n = 11). We observed that p44/42 MAP kinase activity was significantly decreased in the prefrontal cortical areas (Brodmann's areas 8, 9 and 10) and the hippocampus of depressed suicide subjects without any change in the cerebellum. This decrease was associated with a decrease in mRNA and protein levels of ERK1 and ERK2. In addition, the expression of MAP kinase phosphatase (MKP)2, a 'dual function' ERK1/2 phosphatase, was increased in the prefrontal cortex and hippocampus. These studies suggest that p44/42 MAP kinases are less activated in the post-mortem brain of depressed suicide subjects and this may be because of reduced expression of ERK1/2 and increased expression of MKP2. Given the role of MAP kinases in various physiological functions and gene expression, alterations in p44/42 MAP kinase activation and expression of ERK1/2 may contribute significantly to the pathophysiology of depressive disorders.

Molecular cloning of mouse ERK5/BMK1 splice variants and characterization of ERK5 functional domains

The mitogen-activated protein kinases (MAPKs) play important roles in regulation of cell growth and survival. Human MAPK 5 (ERK5) or Big MAP kinase 1 (BMK1) is a recently cloned member of the MAPK family. To identify ERK5-related kinases, we searched the GenBanktrade mark expressed sequence tag (EST) data base for mouse cDNAs with homology to human ERK5. A full-length mouse cDNA that was highly homologous to the human ERK5 was identified. Further analysis of ERK5 polymerase chain reaction products generated from mouse embryo cDNA yielded three mouse ERK5 cDNAs (mERK5a, mERK5b, and mERK5c). Sequence analysis showed that these cDNAs are alternative splice products of the mouse ERK5 gene. Interestingly, expressed mERK5b and mERK5c act as dominant negative inhibitors based on inhibition of mERK5a kinase activity and mERK5a-mediated MEF2C transactivation. However, the physiological significance of mERK5b and mERK5c is not fully understood. Further investigation using these mouse ERK5 splice variants and other constructed mutants identified functional roles of several regions of mERK5, which appear to be important for protein-protein interaction and intracellular localization. Specifically, we found that the long C-terminal tail, which contains a putative nuclear localization signal, is not required for activation and kinase activity but is responsible for the activation of nuclear transcription factor MEF2C due to nuclear targeting. In addition, the N-terminal domain spanning amino acids (aa) 1-77 is important for cytoplasmic targeting; the domain from aa 78 to 139 is required for association with the upstream kinase MEK5; and the domain from aa 140-406 is necessary for oligomerization. Taken together, these observations indicate that ERK5 is regulated by distinct mechanisms determined by its unique structure and presumably the presence of multiple splice variants.

Mitogen-activated protein (MAP) kinase pathways: regulation and physiological functions

Mitogen-activated protein (MAP) kinases comprise a family of ubiquitous proline-directed, protein-serine/threonine kinases, which participate in signal transduction pathways that control intracellular events including acute responses to hormones and major developmental changes in organisms. MAP kinases lie in protein kinase cascades. This review discusses the regulation and functions of mammalian MAP kinases. Nonenzymatic mechanisms that impact MAP kinase functions and findings from gene disruption studies are highlighted. Particular emphasis is on ERK1/2.

Mitogen-activated protein [MAP] kinase pathways in plants: versatile signaling tools

Mitogen-activated protein kinases (MAPKs) are important signaling tools in all eukaryotes, and function in mediating an enormous variety of external signals to appropriate cellular responses. MAPK pathways have been studied extensively in yeast and mammalian cells, and a large body of knowledge on their functioning has accumulated, which is summarized briefly. Plant MAPK pathways have attracted increasing interest, resulting in the isolation of a large number of different components of MAPK cascades. Studies on the functions of these components have revealed that MAPKs play important roles in the response to a broad variety of stresses, as well as in the signaling of most plant hormones and in developmental processes. Finally, the involvement of various plant phosphatases in the inactivation of MAPKs is discussed.

A novel mitogen-activated protein kinase is responsive to Raf and mediates growth factor specificity

The proto-oncogene Raf is a major regulator of growth and differentiation. Previous studies from a number of laboratories indicate that Raf activates a signaling pathway that is independent of the classic MEK1,2-ERK1,2 cascade. However, no other signaling cascade downstream of Raf has been identified. We describe a new member of the mitogen-activated protein kinase family, p97, an ERK5-related kinase that is activated and Raf associated when cells are stimulated by Raf. Furthermore, p97 is selectively responsive to different growth factors, providing a mechanism for specificity in cellular signaling. Thus, p97 is activated by the neurogenic factor fibroblast growth factor (FGF) but not the mitogenic factor epidermal growth factor (EGF) in neuronal cells. Conversely, the related kinase ERK5 is activated by EGF but not FGF. p97 phosphorylates transcription factors such as Elk-1 and Ets-2 but not MEF2C at transactivating sites, whereas ERK5 phosphorylates MEF2C but not Elk-1 or Ets-2. Finally, p97 is expressed in a number of cell types including primary neural and NIH 3T3 cells. Taken together, these results identify a new signaling pathway that is distinct from the classic Raf-MEK1,2-ERK1,2 kinase cascade and can be selectively stimulated by growth factors that produce discrete biological outcomes.

Extracellular signals and scores of phosphatases: all roads lead to MAP kinase

MAP kinases function as key signal integration points for a vast number of external stimuli that affect the life and death of cells and are therefore subject to rigorous regulation. Here we review the numerous protein phosphatases that directly counteract MAP kinase activation. To simplify the complexity, we attempt to integrate the information into a 'sequential phosphatase model' of MAP kinase regulation.

Signaling from G protein-coupled receptors to ERK5/Big MAPK 1 involves Galpha q and Galpha 12/13 families of heterotrimeric G proteins. Evidence for the existence of a novel Ras AND Rho-independent pathway

The regulation of gene expression by cell surface receptors often involves the stimulation of signaling pathways including one or more members of the MAPK superfamily of serine-threonine kinases. Upon their activation in the cytosol, MAPKs can translocate to the nucleus and affect the activity of a variety of transcription factors. Recently, it has been observed that a novel member of the MAPK superfamily, ERK5, can be potently activated by transforming G protein-coupled receptors (GPCRs) and that ERK5 participates in the regulation of c-jun expression through the activation of MEF2 transcription factors. How cell surface receptors, including GPCRs, stimulate ERK5 is still poorly understood. In this study, we have used transiently transfected COS-7 cells to begin delineating the biochemical route linking GPCRs to ERK5. We show that receptors that can couple to the G(q) and G(12/13) families of heterotrimeric G proteins, m1 and thrombin receptors, respectively, but not those coupled to G(i), such as m2 receptors, are able to regulate the activity of ERK5. To investigate which heterotrimeric G proteins signal to ERK5, we used a chimeric system by which Galpha(q)- and Galpha(13)-mediated signaling pathways can be conditionally activated upon ligand stimulation. Using this system, as well as the expression of activated forms of G protein subunits, we show that the Galpha(q) and Galpha(12/13) families of heterotrimeric G proteins, but not the Galpha(i), Galpha(s), and betagamma subunits, are able to regulate ERK5. Furthermore, we provide evidence that the stimulation of ERK5 by GPCRs involves a novel signaling pathway, which is distinct from those regulated by Ras and Rho GTPases.