Phosphorylation of Xenopus mitogen-activated protein (MAP) kinase kinase by MAP kinase kinase kinase and MAP kinase

Fine-tuning OsCPK18/OsCPK4 activity via genome editing of phosphorylation motif improves rice yield and immunity

Plants have evolved complex signalling networks to regulate growth and defence responses under an ever-changing environment. However, the molecular mechanisms underlying the growth-defence tradeoff are largely unclear. We previously reported that rice CALCIUM-DEPENDENT PROTEIN KINASE 18 (OsCPK18) and MITOGEN-ACTIVATED PROTEIN KINASE 5 (OsMPK5) mutually phosphorylate each other and that OsCPK18 phosphorylates and positively regulates OsMPK5 to suppress rice immunity. In this study, we found that OsCPK18 and its paralog OsCPK4 positively regulate plant height and yield-related traits. Further analysis reveals that OsCPK18 and OsMPK5 synergistically regulate defence-related genes but differentially regulate development-related genes. In vitro and in vivo kinase assays demonstrated that OsMPK5 phosphorylates C-terminal threonine (T505) and serine (S512) residues of OsCPK18 and OsCPK4, respectively. The kinase activity of OsCPK18^T505D , in which T505 was replaced by aspartic acid to mimic T505 phosphorylation, displayed less calcium sensitivity than that of wild-type OsCPK18. Interestingly, editing the MAPK phosphorylation motif in OsCPK18 and its paralog OsCPK4, which deprives OsMPK5-mediated phosphorylation but retains calcium-dependent activation of kinase activity, simultaneously increases rice yields and immunity. This editing event also changed the last seven amino acid residues of OsCPK18 and attenuated its binding with OsMPK5. This study presents a new regulatory circuit that fine tunes the growth-defence tradeoff by modulating OsCPK18/4 activity and suggests that CRISPR/Cas9-mediated engineering phosphorylation pathways could simultaneously improve crop yield and immunity.

Bystander effects induced by chemicals and ionizing radiation: evaluation of changes in gene expression of downstream MAPK targets

Radiation-induced bystander effects have been evaluated extensively, including the involvement of the mitogen-activated protein kinase (MAPK) pathways. However, few studies have examined the ability of chemicals to induce bystander effects, and the molecular mechanisms involved in chemical bystander effects have not been investigated. We have previously demonstrated the ability of mitomycin C (MMC) and phleomycin (PHL) to induce bystander effects in normal human lymphoblastoid cells. Here, we demonstrate changes in the expression of MAPK target genes following bystander exposure to MMC or PHL or ionizing radiation. The expression changes of 18 genes, which code for proteins that are downstream targets of MAPK proteins, were evaluated at various time points following direct or bystander exposure to MMC, PHL and ionizing radiation. The 18 genes were analysed as groups belonging to one of the seven possible combinations of the three MAPK pathways. We observed statistically significant changes in expression of several genes following exposure to each agent. However, when the expression changes were analysed in the bystander cells alone, significant increases in expression of MAPK target genes were observed for MMC- and radiation-induced bystander effects but not for PHL. PHL is an acknowledged radiomimetic agent; however, in the present study, PHL responses did not resemble those of radiation. These results provide evidence for bystander-induced changes in MAPK proteins and downstream targets and suggest that the bystander effects are a part of a general stress response.

The subcellular localization of MEK and ERK--a novel nuclear translocation signal (NTS) paves a way to the nucleus

The ERK cascade is a central signaling pathway that regulates a large number of intracellular processes including proliferation, differentiation, development and also survival or apoptosis. The induction of so many distinct and even opposing cellular processes raises the question as to how the signaling specificity of the cascade is regulated. In the past few years, subcellular localization of components of the ERK cascade was shown to play an important role in specificity determination. Here we describe the dynamic subcellular localization of Raf kinases, MEKs, and particularly ERKs, which translocate into the nucleus during many cellular processes to induce transcription. We also describe in details the recent identification of a novel nuclear translocation mechanism for ERKs, which is based on a nuclear translocation sequence (NTS) within their kinase insert domain (KID). Phosphorylation of this domain, mainly upon stimulation, allows ERKs to interact with the nuclear importing protein - importin7, which mediates the penetration of the interacting ERKs into the nucleus via nuclear pores. Interestingly, the NTS is not specific to ERKs, and seems to be a general signal for regulating nuclear accumulation of various proteins, including MEKs, upon their stimulation. Better understanding of this mechanism may clarify the role of the massive nuclear translocation of many regulatory proteins shortly after their stimulation.

Identification and characterization of a general nuclear translocation signal in signaling proteins

Upon stimulation, many proteins translocate into the nucleus in order to regulate a variety of cellular processes. The mechanism underlying the translocation is not clear since many of these proteins lack a canonical nuclear localization signal (NLS). We searched for an alternative mechanism in extracellular signal-regulated kinase (ERK)-2 and identified a 3 amino acid domain (SPS) that is phosphorylated upon stimulation to induce nuclear translocation of ERK2. A 19 amino acid stretch containing this phosphorylated domain inserts nondiffusible proteins to the nucleus autonomously. The phosphorylated SPS acts by binding to importin7 and the release from nuclear pore proteins. This allows its functioning both in passive and active ERK transports. A similar domain appears in many cytonuclear shuttling proteins, and we found that phosphorylation of similar sequences in SMAD3 or MEK1 also induces their nuclear accumulation. Therefore, our findings show that this phosphorylated domain acts as a general nuclear translocation signal (NTS).

Mechanisms Regulating Oocyte Meiotic Resumption: Roles of Mitogen-Activated Protein Kinase

Oocyte meiotic maturation is one of the important physiological requirements for species survival. However, little is known about the detailed events occurring during this process. A number of studies have demonstrated that MAPK plays a pivotal role in the regulation of meiotic cell cycle progression in oocytes, but controversial findings have been reported in both lower vertebrates and mammals. In this review, we summarized the roles of MAPK cascade and related signal pathways in oocyte meiotic reinitiation in both lower vertebrates and mammals. We also tried to reconcile the paradoxical results and highlight the new findings concerning the function of MAPK in both oocytes and the surrounding follicular somatic cells. The unresolved questions and future research directions regarding the role of MAPK in meiotic resumption are addressed.

The MEK/ERK cascade: from signaling specificity to diverse functions

The ERK signaling cascade is a central MAPK pathway that plays a role in the regulation of various cellular processes such as proliferation, differentiation, development, learning, survival and, under some conditions, also apoptosis. The ability of this cascade to regulate so many distinct, and even opposing, cellular processes, raises the question of signaling specificity determination by this cascade. Here we describe mechanisms that cooperate to direct MEK-ERK signals to their appropriate downstream destinations. These include duration and strength of the signals, interaction with specific scaffolds, changes in subcellular localization, crosstalk with other signaling pathways, and presence of multiple components with distinct functions in each tier of the cascade. Since many of the mechanisms do not function properly in cancer cells, understanding them may shed light not only on the regulation of normal cell proliferation, but also on mechanisms of oncogenic transformation.

The extracellular signal-regulated kinase: multiple substrates regulate diverse cellular functions

The extracellular signal-regulated kinase (ERK) cascade is a central pathway that transmits signals from many extracellular agents to regulate cellular processes such as proliferation, differentiation and cell cycle progression. The signaling via the ERK cascade is mediated by sequential phosphorylation and activation of protein kinases in the different tiers of the cascade. Although the main core phosphorylation chain of the cascade includes Raf kinases, MEK1/2, ERK1/2 (ERKs) and RSKs, other alternatively spliced forms and distinct components exist in the different tiers, and participate in ERK signaling under specific conditions. These components enhance the complexity of the ERK cascade and thereby, enable the wide variety of functions that are regulated by it. Another factor that is important for the dissemination of ERKs' signals is the multiplicity of the cascade's substrates, which include transcription factors, protein kinases and phosphatases, cytoskeletal elements, regulators of apoptosis, and a variety of other signaling-related molecules. About 160 substrates have already been discovered for ERKs, and the list of these substrates, as well as the function and mechanism of activation of representative substrates, are described in the current review. Many of these substrates are localized in the nucleus, and seem to participate in the regulation of transcription upon stimulation. However, other substrates are found in the cytosol as well as in other cellular organelles, and those are responsible for processes such as translation, mitosis and apoptosis. Understanding of these processes may provide a full picture of the distinct, and even opposing cellular processes that are regulated by the ERK cascade.

B-Raf and C-Raf are required for Ras-stimulated p42 MAP kinase activation in Xenopus egg extracts

During mitosis, a select pool of MEK1 and p42/p44 MAPK becomes activated at the kinetochores and spindle poles, without substantial activation of the bulk of the cytoplasmic p42/p44 MAPK. Recently, we set out to identify the MAP kinase kinase kinase (MAPKKK) responsible for this mitotic activation, using cyclin-treated Xenopus egg extracts as a model system, and presented evidence that Mos was the relevant MAPKKK . However, a second MAPKKK distinct from Mos was readily detectable as well. Here, we partially purify this second MAPKKK and identify it as B-Raf. No changes in the activity of B-Raf were detectable during progesterone-induced oocyte maturation, after egg fertilization, or during the early embryonic cell cycle, arguing against a role for B-Raf in the mitotic activation of MEK1 and p42 MAPK. Ras proteins can bring about activation of MEK1 and p42 MAPK in extracts, and Ras may contribute to signaling from the classical progesterone receptor during oocyte maturation and from receptor tyrosine kinases during early embryogenesis. We found that both B-Raf and C-Raf, but not Mos, are required for Ras-induced MEK1 and p42 MAPK activation. These data indicate that two upstream stimuli, active Ras and active Cdc2, utilize different MAPKKKs to activate MEK1 and p42 MAPK.

Extracellular signal-regulated kinase activated by epidermal growth factor and cell adhesion interacts with and phosphorylates vinexin

Extracellular signal-regulated kinase 1/2 (ERK1/2) is activated by various extracellular stimuli including growth factors and cytokines and plays a pivotal role in regulating cell proliferation and differentiation by phosphorylating nuclear transcription factors. Recently, it was reported that activated ERK1/2 also concentrates at adhesion sites and regulates cell spreading and migration. Vinexin is a focal adhesion protein regulating both cell spreading and growth factor signaling. We show here that vinexin was directly phosphorylated by ERK1/2 upon stimulation with growth factors. ERK1/2 phosphorylated the linker region of vinexin between the second and third SH3 domains. Site-directed mutagenesis revealed that ERK2 mainly phosphorylated the serine 189 residue of vinexin beta. Furthermore, vinexin beta interacted with ERK1/2 both in vitro and in vivo. Vinexin interacted with the active but not inactive form of ERK1/2. A putative DEF (docking for ERK FXFP) domain located in the linker region of vinexin was required for the interaction with ERK1/2 and efficient phosphorylation of vinexin beta by ERK2. Finally, we showed that cell adhesion to fibronectin also induced the association of vinexin beta with ERK2 and the phosphorylation of vinexin beta. Furthermore, vinexin and ERK were co-localized to the periphery of cells during cell spreading on fibronectin. Together, these results suggest that vinexin is a novel substrate of ERK2 and may play roles in ERK-dependent cell regulation during cell spreading as well as in growth factor-induced responses.

Chromosome condensation in pig oocytes: lack of a requirement for either cdc2 kinase or MAP kinase activity

In this study, butyrolactone I (BL I), a potent and specific inhibitor of cyclin-dependent kinases (cdk), is shown to inhibit germinal vesicle breakdown (GVBD) in pig oocytes. Oocytes treated with 100 microM BL I were arrested in the germinal vesicle (GV)-stage and displayed low activity of cdc2 kinase and MAP kinase. Nevertheless, chromosome condensation occurred and highly condensed bivalents were seen within an intact GV after a 24-hr culture in the presence of BL I. The inhibitory effect of BL I on MAP kinase activation during culture was likely mediated through a cdk-dependent pathway, since MAP kinase activity present in extracts derived from metaphase II eggs was not inhibited by BL I. The block of GVBD could be released by treating oocytes with okadaic acid (OA), an inhibitor of type 1 and 2A phosphatases; 82% of the oocytes treated with the combination of OA/BL I underwent GVBD, and MAP kinase became activated, while cdc2 kinase remained inhibited. These results suggest that both chromosome condensation and GVBD could occur without activation of cdc2 kinase, whereas an increase in MAP kinase activity may be a requisite for GVBD in pig oocytes in conditions when cdc2 kinase activation is blocked by BL I.

Butyrolactone I reversibly inhibits meiotic maturation of bovine oocytes,Without influencing chromosome condensation activity

In this study, butyrolactone I (BL I), a potent and specific inhibitor of cyclin-dependent kinases, was shown to block germinal vesicle (GV) breakdown (GVBD) in bovine oocytes in a concentration-dependent manner; GVBD was almost totally inhibited over the course of 24-48 h of culture when 100 microM BL I was included in tissue culture medium 199 containing either polyvinyl alcohol or BSA. Correlated with this inhibition was the failure of either p34(cdc2) kinase or mitogen-activated protein (MAP) kinase to become activated, and it was unlikely that BL I directly inhibited MAP kinase, since 100 microM BL I did not inhibit MAP kinase activity present in extracts obtained from metaphase II-arrested bovine eggs that possess high levels of MAP kinase activity. Nevertheless, the formation of highly condensed bivalents was observed in 78% of the BL I-treated GV-intact oocytes. This result suggests that chromosome condensation during first meiosis in bovine oocytes does not require the activity of either p34(cdc2) kinase or MAP kinase. Treatment of BL I-arrested oocytes with okadaic acid (OA) did not result in either the activation of p34(cdc2) kinase or MAP kinase, or inducement of GVBD. The BL I-induced block of GVBD for 24 h was reversible, and a subsequent 24-h culture resulted in 90% of oocytes reaching metaphase II with emission of the first polar body. Correlated with the progression to and arrest at metaphase II was the full activation of both p34(cdc2) and MAP kinases. The reversibility after 48 h of culture in BL I was partially decreased when compared to that achieved after an initial 24-h culture. Fertilization in vitro of these eggs resulted in a high incidence of both sperm penetration and pronucleus formation (88% and 70%, respectively).

Distinct Subcellular Localization and Substrate Specificity of Extracellular Signal-Regulated Kinase in B Cells upon Stimulation with IgM and CD40

We and others previously observed that IgM and CD40 stimulation in murine B cells resulted in activation of extracellular signal-regulated kinase (ERK), a subfamily of mitogen-activated protein kinase. The present study demonstrated that ERK was rapidly phosphorylated and translocated to the nucleus in murine B cells upon stimulation with CD40, whereas it was preferentially localized within the cytosol after stimulation with IgM, suggesting that signaling through CD40 and IgM differentially regulates ERK subcellular localization. Costimulation with CD40 and IgM (CD40/IgM) resulted in subcellular localization of ERK within the cytosol, supporting the notion that stimulation with IgM delivers the signal responsible for inhibition of ERK nuclear transport. Consistent with these observations, IgM and CD40/IgM stimulation resulted in activation of ribosomal S6 kinase, which is a cytoplasmic substrate for ERK, whereas CD40 stimulation had little effect on its activity. Disruption of the microtubule by colchicine in WEHI231 cells resulted in reduction of ERK activity in IgM signaling, but not in CD40 signaling, compatible with the notion that the microtubule network may hold cytoplasmic ERK activity mediated by IgM stimulation. These results support the notion that ERK could mediate different effector functions in B cells upon stimulation with IgM and CD40.

Basal extracellular signal-regulated kinase activity modulates cell-cell and cell-matrix interactions

Suppression of the basal extracellular signal-regulated kinase (ERK) activity in PC12 cells markedly altered their phenotype. Wild-type cells grew in a dissociated pattern adherent to the substrate. The stable expression of an ERK inhibitory mutant resulted in the formation of calcium-dependent aggregates which were less adherent to the substrate. Concomitantly, the cells reorganized their actin cytoskeleton and increased their expression of several adherens junction proteins, particularly cadherin. Metabolic labeling demonstrated an increased synthesis of cadherin and beta-catenin in these cells. Nontransfected PC12 cells and a ras-transformed MDCK cell line also formed aggregates and increased their expression of adherens junction proteins following treatment with the selective MEK inhibitor PD98059. A peptide containing the HAV cadherin recognition sequence attenuated the aggregation. These studies suggest that in PC12 and epithelial cells, ERKs are pivotally positioned to enhance substrate interactions when active or to release homotypic interactions when suppressed.

Molecular cloning and characterization of the human p44 mitogen-activated protein kinase gene

The complete genomic structure of the human p44(mapk) gene (HMGW-approved symbol PRKM3) has been determined. The gene covers 9 kb and is composed of nine exons and eight introns. This structure is identical to the previously reported mouse p44(mapk) gene, indicating a high degree of evolutionary conservation. A sequence differing by one nucleotide from the consensus TATA box is present 132 positions upstream of the main transcription initiation point. This point has been located 415 nucleotides upstream of the translation initiation codon ATG and perfectly meets the consensus criteria for an initiator element (Inr). Multiple consensus sequences for factors that regulate either basal transcription or gene expression during cell differentiation and proliferation can be found in the putative promoter region. Some of them, such as several G/C boxes located downstream from the transcription initiation point, are also present in the homologous mouse gene, where they were shown to be functional.

Identification of substrates and regulators of the mitogen-activated protein kinase ERK5 using chimeric protein kinases

Extracellular signal-regulated protein kinase 5 (ERK5) is a recently discovered orphan mitogen-activated protein kinase for which no substrates or strong activators have been described. Two ERK5 chimeras were created as a novel approach to discover its substrates and upstream regulators. One chimeric protein contained the N-terminal domain of the ERK5 catalytic core (subdomains I-IV) and the C-terminal domain of the ERK2 catalytic core (subdomains V-XI). This chimera was highly responsive to stimuli that regulate ERK2 in vitro and in cells. A second chimeric protein consisted of the N-terminal domain of ERK2 (subdomains I-IV) and the C-terminal domain of the ERK5 catalytic core (subdomains V-XI). This chimera was activated in bacteria by coexpression with a constitutively active mutant of MEK1. Using the activated chimera, we identified three in vitro substrates of ERK5. Assaying ERK5 activity in immune complexes with one of these substrates, c-Myc, we determined that the ERK5 catalytic domain is activated by V12 H-Ras and to a lesser extent by phorbol ester but not by constitutively active mutants of Raf-1. Thus, ERK5 is a target of a novel Ras effector pathway that may communicate with c-Myc.

Norepinephrine stimulates mitogen-activated protein kinase activity in GT1-1 gonadotropin-releasing hormone neuronal cell lines

The GT1-1 GnRH neuronal cell lines exhibit highly differentiated properties of GnRH neurons. We have used GT1-1 cells to study the roles of norepinephrine (NE), membrane depolarization, calcium influx, and phorbol esters in the regulation of mitogen-activated protein (MAP) kinase. NE, which is known to stimulate the release of GnRH, induced MAP kinase activity, the tyrosine phosphorylation of MAP kinase, and MAP kinase kinase activity. Forskolin led to activation of MAP kinase comparable with that induced by NE, and a selective inhibitor of cAMP-dependent protein kinase, H8, attenuated the NE-induced activation of MAP kinase. On the other hand, elimination of extracellular calcium by EGTA completely blocked NE-induced tyrosine phosphorylation of MAP kinase, and a selective inhibitor of calcium/calmodulin-dependent protein kinase, KN-62, attenuated the NE-induced activation of MAP kinase. Furthermore, depolarization of GT1-1 cells with 75 mM KCl, 10 microM BayK 8644, or 1 microM calcium ionophore (A23187) induced rapid tyrosine phosphorylation of MAP kinase. The omission of calcium from the extracellular medium completely abolished these effects of tyrosine phosphorylation of MAP kinase. Phorbol 12-myristate 13-acetate (PMA) also induced MAP kinase activity, but pretreatment of the cultured cells with PMA to down-regulate protein kinase C did not abolish the activation of MAP kinase by NE. In addition, although phosphorylation of Raf-1 kinase was stimulated by PMA, this phosphorylation was not induced by either NE or A23187. These results demonstrate that NE activates MAP kinase directly in GT1-1 cells, and that the effect of NE is mediated by increase in the cAMP level and by calcium influx, but not by PMA-sensitive protein kinase C or Raf-1 kinase.

Mechanistic studies of the dual phosphorylation of mitogen-activated protein kinase

Previous work on the responses of mitogen-activated protein (MAP) kinase cascade components in a Xenopus oocyte extract system demonstrated that p42 MAP kinase (MAPK) exhibits a sharp, sigmoidal stimulus/response curve, rather than a more typical hyperbolic curve. One plausible explanation for this behavior requires the assumption that MAP kinase kinase (MAPKK) carries out its dual phosphorylation of p42 MAPK by a distributive mechanism, where MAPKK dissociates from MAPK between the first and second phosphorylations, rather than a processive mechanism, where MAPKK carries out both phosphorylations before dissociating. Here we have investigated the mechanism through which a constitutively active form of human MAPKK-1 (denoted MAPKK-1 R4F or MAPKK-1*) phosphorylates Xenopus p42 MAPK in vitro. We found that the amount of monophosphorylated MAPK formed during the phosphorylation reaction exceeded the amount of MAPKK-1* present, which would not be possible if the phosphorylation occurred exclusively by a processive mechanism. The monophosphorylated MAPK was phosphorylated predominantly on tyrosine, but a small proportion was phosphorylated on threonine, indicating that the first phosphorylation is usually, but not invariably, the tyrosine phosphorylation. We also found that the rate at which pulse-labeled monophosphorylated MAPK became bisphosphorylated depended on the MAPKK-1* concentration, behavior that is predicted by the distributive model but incompatible with the processive model. These findings indicate that MAPKK-1* phosphorylates p42 MAPK by a two-collision, distributive mechanism rather than a single-collision, processive mechanism, and provide a mechanistic basis for understanding how MAP kinase can convert graded inputs into switch-like outputs.

Signaling through intercellular adhesion molecule 1 (ICAM-1) in a B cell lymphoma line. The activation of Lyn tyrosine kinase and the mitogen-activated protein kinase pathway

Intercellular adhesion molecule 1 (ICAM-1) (CD54) is an adhesion molecule of the immunoglobulin superfamily. The interaction between ICAM-1 on B lymphocytes and leukocyte function-associated antigen 1 on T cells plays a major role in several aspects of the immune response, including T-dependent B cell activation. While it was originally believed that ICAM-1 played a purely adhesive role, recent evidence suggests that it can itself transduce biochemical signals. We demonstrate that cross-linking of ICAM-1 results in the up-regulation of class II major histocompatibility complex, and we investigate the biochemical mechanism for the signaling role of ICAM-1. We show that cross-linking of ICAM-1 on the B lymphoma line A20 induces an increase in tyrosine phosphorylation of several cellular proteins, including the Src family kinase p53/p56(lyn). In vitro kinase assays showed that Lyn kinase was activated within 1 min after ICAM-1 cross-linking. In addition, ICAM-1 cross-linking resulted in activation of Raf-1 and mitogen-activated protein kinases, as determined by gel mobility shift. Activation of these kinases may represent important components in the cascade of signals that link ICAM-1 to various ICAM-1-elicited cellular responses. These data confirm the important role of ICAM-1 as a signaling molecule in B cell activation.

Mitogen-activated protein kinase activity and microtubule organisation are altered by protein synthesis inhibition in maturing porcine oocytes

Previously we have shown that mitogen-activated protein (MAP) kinase activity abruptly increases at the first metaphase (M1) and remains significantly higher than that at the germinal vesicle (GV) stage until the second metaphase (M2) in porcine oocytes cultured in vitro. The present paper describes how the mechanism of the blockage of meiotic maturation by protein synthesis inhibition involves MAP kinase regulation. Cycloheximide arrested both germinal vesicle breakdown (GVBD) and the normal transition from M1 to M2. MAP kinase activation was also reduced in these maturation-inhibited oocytes. By using immunofluorescence microscopy with the monoclonal antibody raised against rat alpha-tubulin, we showed that cycloheximide caused morphological abnormality in a spindle at M1, but not at M2. All these results indicate that in porcine oocytes: (1) GV blockage by protein synthesis inhibition involves the suppression of both histone H1 kinase and MAP kinase activation, (2) during the transition from M1 to M2, maintenance of a normal metaphasic spindle and high MAP kinase activity require protein synthesis, and (3) once the M2 cytoskeletal structures have been completed, and/or after the 'critical period', cytostatic factor activity is independent of protein synthesis.

Characterization of ERK1 activation site mutants and the effect on recognition by MEK1 and MEK2

To discern MEK1 and MEK2 specificity for their substrate, extracellular signal-regulated kinase (ERK), site-directed mutagenesis was performed on the amino acid residues flanking the regulatory phosphorylation sites of ERK1. These ERK1 mutants were analyzed for the ability to act as a substrate for MEK1 and MEK2. Based on both phosphorylation and activation analyses, the mutants could be divided into four classes: 1) dramatically decreased phosphorylation and activation, 2) enhanced basal kinase activity, 3) preferentially enhanced phosphorylation of tyrosine and decreased phosphorylation of threonine, and 4) increased threonine phosphorylation with an increase in activation. In general, the residues proximal to the regulatory phosphorylation sites of ERK1 had greater influence on both phosphorylation and activation. This is consistent with the highly specific recognition of the ERK1 regulatory sites by MEK. Mutation of Arg-208 or Thr-207 to an alanine residue significantly altered the relative phosphorylation on Thr-202 and Tyr-204. The Arg-208 to alanine mutant increased the phosphorylation of Tyr-204 approximately 4-fold yet almost completely eliminated the phosphorylation on Thr-202. In contrast, mutation of Gly-199 to alanine resulted in an increased phosphorylation of Thr-202 relative to Tyr-204. This suggests that both Gly-199 and Arg-208 play important roles in determining the relative phosphorylation of Thr-202 and Tyr-204. Our results demonstrate that residues in the phosphorylation lip of ERK play an important role in the recognition and phosphorylation by MEK.

Hypoxia and hypoxia/reoxygenation activate Raf-1, mitogen-activated protein kinase kinase, mitogen-activated protein kinases, and S6 kinase in cultured rat cardiac myocytes

In response to hypoxia and reoxygenation, mammalian cells are known to express a variety of genes to adapt to these external stresses or lead to further cell damage. We investigated the intracellular signaling cascades in cultured rat cardiac myocytes subjected to hypoxia followed by reoxygenation (hypoxia/reoxygenation). Here, we show that both hypoxia and hypoxia/reoxygenation caused rapid activation of the mitogen-activated protein kinase kinase kinase (MAPKKK), activity of Raf-1. This was followed by the sequential activation of mitogen-activated protein kinase kinase (MAPKK), mitogen-activated protein (MAP) kinases, and S6 kinase (p90rsk). Furthermore, hypoxia caused hyperphosphorylation of Raf-1. The maximal hyperphosphorylation of Raf-1 appeared to be accompanied by a significant decrease in MAPKKK activity. These results strongly suggest the following: (1) Intracellular signals initiated by both hypoxia and hypoxia/reoxygenation converge on Raf-1 and activate its MAPKKK activity. Then, Raf1 activates downstream serine/threonine kinases including MAPKK, MAP kinases and p90rsk. (2) Raf-1 is not only located upstream from MAPKK and MAP kinases but also may be phosphorylated by MAP kinases directly or indirectly, and at least Raf-1 kinase activity may be downregulated by this feedback mechanism.

Involvement of signal transduction pathways in lung cancer biology

Pathways involved in the transduction of biological signals within cells overlap with those involved in oncogenesis. Previous studies have identified a number of discrete disturbances of some elements of these pathways in human lung cancer cells, by virtue of the overexpression or the mutation of certain key molecules. The sequence of biochemical events triggered by a mitogenic stimulus such as the exposure to bombesin-like peptides are being unravelled. The opportunity exists to identify additional changes involving regulatory proteins which may contribute to the regulation of these systems and which may function as suppressors of the malignant phenotype. Furthermore, the understanding of these pathways may identify targets for the pharmacological regulation of tumor cell response to mitogens which may be usable in the clinic.

Constitutive activation of the Saccharomyces cerevisiae mating response pathway by a MAP kinase kinase from Candida albicans

The HST7 gene of Candida albicans encodes a protein with structural similarity to MAP kinase kinases. Expression of this gene in Saccharomyces cerevisiae complements disruption of the Ste7 MAP kinase kinase required for both mating in haploid cells and pseudohyphal growth in diploids. However, Hst7 expression does not complement loss of either the Pbs2 (Hog4) MAP kinase kinase required for response to high osmolarity, or loss of the Mkk1 and Mkk2 MAP kinase kinases required for proper cell wall biosynthesis. Intriguingly, HST7 acts as a hyperactive allele of STE7; expression of Hst7 activates the mating pathway even in the absence of upstream signaling components including the Ste7 regulator Ste11, elevates the basal level of the pheromone-inducible FUS1 gene, and amplifies the pseudohyphal growth response in diploid cells. Thus Hst7 appears to be at least partially independent of upstream activators or regulators, but selective in its activity on downstream target MAP kinases. Creation of Hst7/Ste7 hybrid proteins revealed that the C-terminal two-thirds of Hst7, which contains the protein kinase domain, is sufficient to confer this partial independence of upstream activators.

Activation mechanism and function of the MAP kinase cascade

MAP kinase (MAPK) and its activator, MAP kinase kinase (MAPKK), are commonly activated by a variety of extracellular stimuli in mammalian cells and in the process of Xenopus oocyte maturation. In order to investigate the function of the MAPK cascade in oocyte maturation, we produced an anti-Xenopus MAPKK which specifically reacts with MAPKK in vitro. When this antibody was microinjected into immature oocytes, MAPK activation induced by progesterone was prevented. Surprisingly, H1 kinase activation and germinal vesicle breakdown were also inhibited in the oocytes injected with this antibody. These results suggest that the MAPK cascade plays an important role in the maturation promoting factor (MPF) activation during the oocyte maturation process. When this antibody together with Mos was microinjected into Xenopus two-cell embryos, the Mos-induced metaphase arrest (CSF arrest) was prevented. Thus, the MAPK cascade may mediate CSF arrest. During Xenopus early embryogenesis, a low but significant level of MAPK remains active. Injection of mRNA encoding a constitutively active MAPKK resulted in mesoderm induction in animal cap explants. In addition, fibroblast growth-factor (FGF)-induced mesoderm induction was inhibited by expressing CL100 (a MAP kinase phosphatase) in animal cap explants. Thus the MAPK cascade may be involved in the mesoderm induction of Xenopus embryos. The activation pathways and roles of the MAPKK/MAPK cascade in various signaling processes will be discussed.

Activation of two isoforms of mitogen-activated protein kinase kinase in response to epidermal growth factor and nerve growth factor

Mitogen-activated protein kinase kinase (MAPKK) is a dual-specificity protein kinase which phosphorylates and activates mitogen-activated protein kinase (MAPK). cDNAs encoding two isoforms of MAPKK, MAPKK1 and MAPKK2 (also known as MEK1 and MEK2), have been cloned in mammalian cells. To analyze the characteristics of MAPKK1 and MAPKK2 individually, we have produced specific anti-MAPKK serum against each isoform. MAPKK1 and MAPKK2 have apparent molecular masses of 45 kDa and 47 kDa, respectively, on SDS/polyacrylamide gel electrophoresis. In mouse tissues, MAPKK1 was highly enriched in brain, while MAPKK2 was present relatively evenly. In rat fibroblastic 3Y1 cells, epidermal growth factor (EGF) treatment induced activation of both MAPKK1 and MAPKK2. Immunoprecipitation experiments have shown that the time courses of activation and deactivation of both isoforms of MAPKK were superimposed. In PC12 cells, both MAPKK1 and MAPKK2 were activated in response to nerve growth factor (NGF) as well as EGF, and the time courses of activation and deactivation of both isoforms were indistinguishable from each other in the NGF-stimulated cells and also in the EGF-stimulated cells. Furthermore, localization of both MAPKK1 and MAPKK2 in the cytoplasm was unchanged in response to EGF and NGF. Thus, the same or quite similar mechanisms may operate in the regulation of the activation and deactivation of two isoforms of MAPKK, and both kinases might have redundant functions when expressed in the same cell.

Purification of a 12,020-dalton protein that enhances the activation of mitogen-activated protein (MAP) kinase by MAP kinase kinase

We have purified 3500-fold from rabbit skeletal muscle a 12,020-Da mitogen-activated protein kinase kinase (MEK)-enhancing factor (MEF) that stimulates both mitogen-activated protein kinase (MAPK) autophosphorylation and the rate (24-fold) at which the enzyme is phosphorylated by MEK in vitro. This was manifest by the finding that in the presence of MEF, molar equivalents of MEK to MAPK were sufficient to produce fully phosphorylated (2.1 +/- 0.4 mol/mol; S.D., n = 3) and activated MAPK. This contrasted with the 40:1 molar excess ratio of MEK to MAPK required to produce fully phosphorylated and activated MAPK in the absence of MEF. Phosphoamino acid analysis revealed that in the presence of MEF, phosphorylation of MAPK by MEK was ordered, with Tyr-185 phosphorylation preceding Thr-183 phosphorylation. However, the rate at which Thr-183 was phosphorylated relative to Tyr-185 was greatly increased. The finding that MEF stimulated MAPK autophosphorylation and increased its ability to be phosphorylated by MEK suggests a mechanism of action in which MEF interacts with MAPK to alter its conformation.

Increased expression and subcellular translocation of the mitogen activated protein kinase kinase and mitogen-activated protein kinase in Alzheimer's disease

The sequential activation of the mitogen-activated protein kinase kinase and its substrate, the mitogen-activated protein kinase is involved in a cascade of protein kinases which link a number of cell surface signals to intracellular changes in enzyme activity and gene expression. In vitro, mitogen-activated protein kinase is able to phosphorylate the microtubule-associated protein tau at Ser-Pro and Thr-Pro sites, thereby generating abnormally hyperphosphorylated tau species that are similar to paired helical filament-tau found in Alzheimer's disease. In the present study, we analysed the levels of immunoreactive mitogen-activated protein kinase kinase and mitogen-activated protein kinase in the temporal cortex (area 22) of patients with Alzheimer's disease by means of enzyme-linked immuno-sorbent assays and compared these changes with the content of abnormally phosphorylated paired helical filament-tau. The levels of immunochemically detected mitogen-activated protein kinase kinase and mitogen-activated protein kinase were both increased in Alzheimer's disease by between 35 and 40% compared with age-matched controls. Elevation of mitogen-activated protein kinase kinase was most pronounced during early stages of Alzheimer's disease and was inversely related to the tissue content of abnormally phosphorylated paired helical filament-tau. Pronounced immunoreactivity of mitogen-activated protein kinase kinase and mitogen-activated protein kinase was present in both tangle bearing neurons and unaffected neurons of the temporal cortex. Immunoreactive neurons were most often localized in the direct vicinity of neuritic plaques. In Alzheimer's disease, the subcellular distribution of mitogen-activated protein kinase kinase and mitogen-activated protein kinase showed a striking translocation from the cytoplasmic to the nuclear compartment. It is suggested that the activation of the mitogen-activated protein kinase cascade which appears to be an early feature of Alzheimer's disease might be critically involved in self-stimulating processes of neurodegeneration and aberrant repair under these conditions.

Activation of mitogen-activated protein kinase during meiotic maturation in porcine oocytes

To investigate the involvement of mitogen-activated protein kinase (MAP kinase) in meiotic maturation of porcine oocytes, we assayed MAP kinase activity using myelin basic protein (MBP) as a substrate. MAP kinase activity was low during the germinal vesicle stage, 0-20 h of culture. An abrupt increase was observed at metaphase I (30 h of culture), and activity remained significantly higher than that at 0 h until 50 h of culture, with a transient slight decrease at the time of first polar body extrusion (40 h). Detection of the kinase activity by an in-gel phosphorylation assay confirmed that the 42 and 44 kDa MAP kinases were significantly activated in 45 h cultured oocytes but not in 0 h oocytes, and just slightly in 20 h oocytes. In immunoblotting, however, the 42 and 44 kDa bands were detected in 0, 20 and 45 h cultured oocytes. Furthermore, the signal strength of the two bands did not change during the period of culture, but shifted up to 45 h, indicating that the activation of MAP kinase depended not on the synthesis but on the phosphorylation of this enzyme. These results suggest that the activation of MAP kinase is involved in the regulation of meiotic maturation of porcine oocytes, and especially in the regulation after germinal vesicle breakdown.

Activation of protein kinase cascades by osmotic shock

Osmotic shock induces a variety of biochemical and physiological responses in vertebrate cells. By analyzing extracts obtained from rat 3Y1 fibroblastic cells exposed to hyper-osmolar media, we have found that mitogen-activated protein kinases (MAPKs) and stress-activated protein kinases (SAPKs, also known as JNKs) are both activated in response to osmotic shock. MAPKK1 (MEK1) was also activated markedly. Furthermore, Raf-1 and MEKK were activated strikingly by the osmotic shock. Activation of Raf-1 and MEKK in response to osmotic shock was detected also in PC12 cells, in which MEKK activation by the osmotic shock was much stronger than that by epidermal growth factor. Activation of SAPKs in PC12 cells by the osmotic shock was also more marked than that by epidermal growth factor. The activated MEKK phosphorylated not only MAPKKs but also XMEK2, which is distantly related to MAPKK. Recombinant wild-type XMEK2, but not kinase-negative XMEK2, was able to phosphorylate and activate recombinant SAPK alpha in vitro. In addition, this activity of XMEK2 was activated by the activated MEKK. These results suggest that the MAPK cascade consisting of Raf-1, MAPKK, and MAPK and the SAPK cascade consisting of MEKK, XMEK2, and SAPK are both activated in response to osmotic shock. Finally, it was found that XMEK2 is a good substrate for SAPK.

Prostaglandin F2 alpha stimulates formation of p21ras-GTP complex and mitogen-activated protein kinase in NIH-3T3 cells via Gq-protein-coupled pathway

Prostaglandin (PG) F2 alpha activated mitogen-activated protein (MAP) kinase and MAP kinase kinase in NIH-3T3 cells by a mechanism that was completely inhibited by protein kinase inhibitors, staurosporine (20 nM) or H-7 (20 microM), but was insensitive to pretreatment with islet-activating protein (100 ng/ml; 24 h) or 12-O-tetradecanoylphorbol 13-acetate (2.5 microM; 24 h). PGF2 alpha stimulation also led to a significant increase in Ras.GTP complex. Transfection of a cDNA encoding a constitutively active mutant of Gq alpha-subunit (Q209L) mimicked PGF2 alpha-induced MAP kinase activation, increase in Ras.GTP complex, and DNA synthesis in these cells, suggesting that activation of Gq mediates the PGF2 alpha-activation of Ras-MAP kinase pathway and mitogenesis in NIH-3T3 cells. These data provide a new insight into regulatory mechanisms of Ras-MAP kinase pathway through heterotrimeric G-protein-mediated pathways.

Biochemical analysis of MEK activation in NIH3T3 fibroblasts. Identification of B-Raf and other activators

Numerous potential activators of MEK have been identified, including c-Raf-1, B-Raf, c-Mos, and a family of MEK kinases. However, little information gives insight into the activators actually utilized in vivo. To address this, we have used column chromatography and a coupled MEK activation assay to identify in NIH3T3 cells, two major MEK activators, and a third insulin-specific activator. The first MEK activator has an apparent M(r) of 40,000-50,000, was immunologically distinct from A-Raf, B-Raf, c-Raf-1, c-MEKK, c-Mos, MEK1, and MEK2, and was rapidly activated by serum, platelet-derived growth factor (PDGF), insulin, thrombin, and phorbol ester. The second MEK activator was identified as B-Raf. Activation of 93-95 kDa B-Raf was observed in column fractions and B-Raf immunoprecipitates from cytosolic and particulate fractions after stimulation with serum or PDGF, but not insulin. c-Raf-1 from cytosol did not exhibit MEK activator activity; however, c-Raf-1 immunoprecipitates from the particulate fraction revealed MEK activator activity that was enhanced after stimulation with PDGF or phorbol ester, but not serum or insulin. Both c-Mos and c-MEKK were present in NIH3T3 fibroblasts but did not show MEK activator activity. These data provide direct evidence that 93-95-kDa B-Raf isozymes and an unidentified 40-50-kDa MEK activator are major agonist-specific MEK activators in NIH3T3 fibroblasts.

Parallel signal processing among mammalian MAPKs

The intracellular signalling field is dominated by the mitogen-activated protein kinase (MAPK) cascade and its control, which involves the small GTPase Ras and sequential kinase activation. Until recently, ERK1 and ERK2 were the only cloned and well-characterized mammalian MAPKs; diverse ligand-stimulated, proline-directed protein phosphorylation events were attributed to these kinases. The recent discovery of two other MAPK subtypes, the JNK/SAPK subfamily and p38/RK (mammalian equivalents of HOG1 in yeast), reveals extreme complexity within the family and, most intriguingly, the existence in mammalian cells of parallel MAPK cascades that can be activated simultaneously.

Purification and characterization of REKS from Xenopus eggs. Identification of REKS as a Ras-dependent mitogen-activated protein kinase kinase kinase

We have previously identified a protein factor, named REKS (Ras-dependent Extracellular signal-regulated kinase/Mitogen-activated protein kinase kinase (MEK) Stimulator), which is necessary for Ras-dependent MEK activation. In this study, we attempted to highly purify and characterize REKS. We have highly purified REKS by successive column chromatographies using a cell-free assay system in which REKS activates recombinant extracellular signal-regulated kinase 2 through recombinant MEK in a guanosine 5'-O-(thiotriphosphate) (GTP gamma S)-Ki-Ras-dependent manner. REKS formed a stable complex with GTP gamma S-Ras; REKS was coimmunoprecipitated with GTP gamma S-Ki-Ras or GTP gamma S-Ha-Ras, but not with GDP-Ki-Ras or GDP-Ha-Ras by an anti-Ras antibody. REKS was absorbed to a GTP gamma S-glutathione S-transferase (GST)-Ha-Ras-coupled glutathione-agarose column but not to a GDP-GST-Ha-Ras-coupled glutathione-agarose column and was coeluted with GTP gamma S-GST-Ha-Ras by reduced glutathione. The minimum molecular mass of REKS was estimated to be about 98 kDa on SDS-polyacrylamide gel electrophoresis. REKS phosphorylated this 98-kDa protein as well as recombinant MEK. REKS was not recognized by any of the anti-c-Raf-1, anti-Mos, and anti-mSte11 antibodies. These results indicate that REKS is a Ras-dependent MEK kinase.

Independent human MAP-kinase signal transduction pathways defined by MEK and MKK isoforms

Mammalian mitogen-activated protein (MAP) kinases include extracellular signal-regulated protein kinase (ERK), c-Jun amino-terminal kinase (JNK), and p38 subgroups. These MAP kinase isoforms are activated by dual phosphorylation on threonine and tyrosine. Two human MAP kinase kinases (MKK3 and MKK4) were cloned that phosphorylate and activate p38 MAP kinase. These MKK isoforms did not activate the ERK subgroup of MAP kinases, but MKK4 did activate JNK. These data demonstrate that the activators of p38 (MKK3 and MKK4), JNK (MKK4), and ERK (MEK1 and MEK2) define independent MAP kinase signal transduction pathways.

The MAP kinase cascade: its role in Xenopus oocytes, eggs and embryos

Mitogen-activated protein kinase (MAPK) was originally identified as a serine/threonine kinase that is activated by mitogens. Now MAPK and its activator, MAPK kinase (MAPKK), are thought to function in a wide variety of intracellular signalling pathways from yeast to vertebrate. We describe here a brief summary of the dissection of the MAPK cascade and its possible functions, especially in Xenopus oocytes and embryos.

Stimulation by extracellular ATP and UTP of the mitogen-activated protein kinase cascade and proliferation of rat renal mesangial cells

1. Extracellular ATP and UTP have been reported to activate a nucleotide receptor that mediates phosphoinositide and phosphatidylcholine hydrolysis by phospholipases C and D, respectively. Here we report that ATP and UTP potently stimulate mesangial cell proliferation. 2. Both nucleotides stimulate phosphorylation and activation of mitogen-activated protein kinase and a biphasic phosphorylation of the up-stream mitogen-activated protein kinase kinase. 3. When added at 100 microM, ATP gamma S, UTP and ATP were the most potent activators of mitogen-activated protein kinase. beta gamma-imido-ATP was somewhat less active and ADP and 2-methylthio-ATP caused a weak induction of enzyme activity. Activation of mitogen-activated protein kinase by both ATP and UTP is dose-dependently attenuated by the P2-receptor antagonist, suramin. 4. The protein kinase C activator 12-0-tetradecanoylphorbol 13-acetate, but not the biologically inactive 4 alpha-phorbol 12,13-didecanoate, increased mitogen-activated protein kinase activity in mesangial cells, suggesting that protein kinase C may mediate nucleotide-induced stimulation of mitogen-activated protein kinase. 5. Down-regulation of protein kinase C -alpha and -delta isoenzymes by 4 h or 8 h treatment with phorbol ester partially inhibited ATP- and UTP-triggered mitogen-activated protein kinase activation. Moreover, a 24 h treatment of mesangial cells with phorbol ester, a regimen that also causes depletion of protein kinase C-epsilon did not further reduce the level of mitogen-activated protein kinase stimulation. 6. The specific protein kinase C inhibitor, CGP 41251, which displayed a selectivity for the Ca2+-dependent isoenzymes, as compared to the Ca2+-independent isoenzymes did not inhibit nucleotide stimulated mitogen-activated protein kinase phosphorylation, thus implicating the involvement of a Ca2+-independent protein kinase C isoform.7. In summary, these results suggest that ATP and UTP trigger the activation of the mitogen-activated protein kinase signalling cascade in mesangial cells and this may be responsible for the potent mitogenic activity of both nucleotides.