A MAP kinase targeted by endotoxin and hyperosmolarity in mammalian cells

Activation of yeast PBS2 MAPKK by MAPKKKs or by binding of an SH3-containing osmosensor

The role of mitogen-activated protein (MAP) kinase cascades in integrating distinct upstream signals was studied in yeast. Mutants that were not able to activate PBS2 MAP kinase kinase (MAPKK; Pbs2p) at high osmolarity were characterized. Pbs2p was activated by two independent signals that emanated from distinct cell-surface osmosensors. Pbs2p was activated by MAP kinase kinase kinases (MAPKKKs) Ssk2p and Ssk22p that are under the control of the SLN1-SSK1 two-component osmosensor. Alternatively, Pbs2p was activated by a mechanism that involves the binding of its amino terminal proline-rich motif to the Src homology 3 (SH3) domain of a putative transmembrane osmosensor Sho1p.

The regulation of tyrosine kinase signalling pathways by growth factor and G-protein-coupled receptors

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Mechanical stress activates protein kinase cascade of phosphorylation in neonatal rat cardiac myocytes

We have previously shown that stretching cardiac myocytes evokes activation of protein kinase C (PKC), mitogen-activated protein kinases (MAPKs), and 90-kD ribosomal S6 kinase (p90rsk). To clarify the signal transduction pathways from external mechanical stress to nuclear gene expression in stretch-induced cardiac hypertrophy, we have elucidated protein kinase cascade of phosphorylation by examining the time course of activation of MAP kinase kinase kinases (MAPKKKs), MAP kinase kinase (MAPKK), MAPKs, and p90rsk in neonatal rat cardiac myocytes. Mechanical stretch transiently increased the activity of MAPKKKs. An increase in MAPKKKs activity was first detected at 1 min and maximal activation was observed at 2 min after stretch. The activity of MAPKK was increased by stretch from 1-2 min, with a peak at 5 min after stretch. In addition, MAPKs and p90rsk were maximally activated at 8 min and at 10 approximately 30 min after stretch, respectively. Raf-1 kinase (Raf-1) and (MAPK/extracellular signal-regulated kinase) kinase kinase (MEKK), both of which have MAPKKK activity, were also activated by stretching cardiac myocytes for 2 min. The angiotensin II receptor antagonist partially suppressed activation of Raf-1 and MAPKs by stretch. The stretch-induced hypertrophic responses such as activation of Raf-1 and MAPKs and an increase in amino acid uptake was partially dependent on PKC, while a PKC inhibitor completely abolished MAPK activation by angiotensin II. These results suggest that mechanical stress activates the protein kinase cascade of phosphorylation in cardiac myocytes in the order of Raf-1 and MEKK, MAPKK, MAPKs and p90rsk, and that angiotensin II, which may be secreted from stretched myocytes, may be partly involved in stretch-induced hypertrophic responses by activating PKC.

Activation of extracellular signal-regulated kinases Erk-1 and Erk-2 by cell swelling in H4IIE hepatoma cells

Hepatic metabolism and gene expression are among the factors controlled by the cellular hydration state, which changes within minutes in response to aniso-osmotic environments, cumulative substrate uptake, oxidative stress and under the influence of hormones such as insulin. The signalling events coupling cell-volume changes to altered cell function were studied in H4IIE rat hepatoma cells. Hypo-osmotic cell swelling resulted within 1 min in a tyrosine kinase-mediated activation of the extracellular signal-regulated protein kinases Erk-1 and Erk-2, which was independent of protein kinase C and cytosolic calcium. Activation of mitogen-activated protein kinases was followed by an increased phosphorylation of c-Jun, which may explain our recently reported finding of an about 5-fold increase in c-jun mRNA level in response to cell swelling. Pretreatment of cells with pertussis or cholera toxin abolished the swelling-induced activation of Erk-1 and Erk-2, suggesting the involvement of G-proteins. Thus, a signal-transduction pathway resembling growth factor signalling is activated already by osmotic water shifts across the plasma membrane, thereby providing a new perspective for adaption of cell function to alterations of the environment.

Transcriptional control by protein phosphorylation: signal transmission from the cell surface to the nucleus

Two general mechanisms have evolved for the rapid and accurate transmission of signals from cell-surface receptors to the nucleus, both involving protein phosphorylation. One mechanism depends on the regulated translocation of activated protein kinases from the cytoplasm to the nucleus, where they phosphorylate target transcription factors. In the second mechanism, transcription factors are kept in a latent state in the cytoplasm and are translocated into the nucleus upon activation.

The protein kinase C-activated MAP kinase pathway of Saccharomyces cerevisiae mediates a novel aspect of the heat shock response

The PKC1 gene of budding yeast encodes a homolog of the alpha, beta, and gamma isoforms of mammalian PKC that is proposed to regulate a MAPK-activation pathway. Mutants in this pathway undergo cell lysis resulting from a deficiency in cell wall construction when they attempt to grow at elevated temperatures. We show that the PKC1-regulated pathway is important for induced thermotolerance and that the MPK1 protein kinase (the MAPK of this pathway) is strongly activated by mild heat shock. This activation is sustained during growth at high temperature and is dependent on the function of pathway components proposed to function upstream of MPK1, including PKC1. Expression of genes under the control of known heat shock-inducible promoter elements (HSEs and STREs) was not compromised in PKC1 pathway mutants, indicating that this pathway mediates a novel aspect of the yeast heat shock response. We propose that the heat-induced signal for pathway activation is generated in response to weakness in the cell wall created during growth under thermal stress, perhaps as a result of increased membrane fluidity. Evidence is presented that the mechanism by which the cell detects this weakness is by measuring stretch of the plasma membrane.

Characterization of the swelling-induced alkalinization of endocytotic vesicles in fluorescein isothiocyanate-dextran-loaded rat hepatocytes

Short-term cultivated rat hepatocytes were allowed to endocytose fluorescein isothiocyanate (FITC)-coupled dextran and the apparent vesicular pH (pHves) was measured by single-cell fluorescence. After 2 h of exposure to FITC-dextran, the apparent pH in the vesicular compartments accessible to endocytosed FITC-dextran was 6.01 +/- 0.05 (n = 39) in normo-osmotic media. Hypo-osmotic exposure increased, whereas hyper-osmotic exposure decreased apparent pHves. by 0.18 +/- 0.02 (n = 26) and 0.12 +/- 0.01 (n = 23) respectively. Incubation of the cells with unlabelled dextran for 2h before a 2-h FITC-dextran exposure had no effect on apparent pHves and its osmosensitivity. When, however, hepatocytes were exposed to unlabelled dextran for 5 h after a 2 h exposure to FITC-dextran, in order to allow transport of endocytosed FITC-dextran to late endocytotic/lysosomal compartments, apparent pHves. decreased to 5.38 +/- 0.04 (n = 12) and the apparent pH in the vesicular compartment containing the dye was no longer sensitive to aniso-osmotic exposure. These findings indicate that the osomosensitivity of pHves. is apparently restricted to early endocytotic compartments. Aniso-osmotic regulation of apparent pHves. in freshly FITC-loaded hepatocytes was not accompanied by aniso-osmolarity-induced changes of the cytosolic free calcium concentration, and neither vasopressin nor extracellular ATP, which provoked a marked Ca2+ signal, affected apparent pHves. Dibutyryl-cyclic AMP (cAMP) or vanadate (0.5 mmol/l) were without effect on apparent pHves. and its osmosensitivity. However, pertussis toxin-treatment or genistein (but not daidzein) or the erbstatin analogue methyl 2,5-dihydroxycinnamate fully abolished the osmo-sensitivity of apparent pHves., but did not affect apparent pHves. It is concluded that regulation of pHves. by cell volume occurs in early endocytotic compartments, but probably not in lysosomes, and is mediated by a G-protein and tyrosine kinase-dependent, but Ca2+- and cAMP-independent mechanism.

Selective activation of the JNK signaling cascade and c-Jun transcriptional activity by the small GTPases Rac and Cdc42Hs

The Rho subfamily of GTPases is involved in control of cell morphology in mammals and yeast. The mammalian Rac and Cdc42 proteins control formation of lamellipodia and filopodia, respectively. These proteins also activate MAP kinase (MAPK) cascades that regulate gene expression. Constitutively activated forms of Rac and Cdc42Hs are efficient activators of a cascade leading to JNK and p38/Mpk2 activation. RhoA did not exhibit this activity, and none of the proteins activated the ERK subgroup of MAPKs. JNK, but not ERK, activation was also observed in response to Dbl, an oncoprotein that acts as a nucleotide exchange factor for Cdc42Hs. Results with dominant interfering alleles place Rac1 as an intermediate between Ha-Ras and MEKK in the signaling cascade leading from growth factor receptors and v-Src to JNK activation. JNK and p38 activation are likely to contribute to the biological effects of Rac, Cdc42Hs, and Dbl on cell growth and proliferation.

The small GTP-binding proteins Rac1 and Cdc42 regulate the activity of the JNK/SAPK signaling pathway

c-Jun amino-terminal kinases (JNKs) and mitogen-activated protein kinases (MAPKs) are closely related; however, they are independently regulated by a variety of environmental stimuli. Although molecules linking growth factor receptors to MAPKs have been recently identified, little is known about pathways controlling JNK activation. Here, we show that in COS-7 cells, activated Ras effectively stimulates MAPK but poorly induces JNK activity. In contrast, mutationally activated Rac1 and Cdc42 GTPases potently activate JNK without affecting MAPK, and oncogenic guanine nucleotide exchange factors for these Rho-like proteins selectively stimulate JNK activity. Furthermore, expression of inhibitory molecules for Rho-related GTPases and dominant negative mutants of Rac1 and Cdc42 block JNK activation by oncogenic exchange factors or after induction by inflammatory cytokines and growth factors. Taken together, these findings strongly support a critical role for Rac1 and Cdc42 in controlling the JNK signaling pathway.

Evidence for multiple activators for stress-activated protein kinase/c-Jun amino-terminal kinases. Existence of novel activators

Stress-activated protein kinases (SAPKs) or c-Jun amino-terminal kinases (JNKs), which belong to a subgroup of the mitogen-activated protein kinase (MAPK) superfamily, are activated in response to a variety of stresses in mammalian cells. An activity to activate a recombinant rat SAPK alpha was detected in extracts obtained from rat fibroblastic 3Y1 cells exposed to hyperosmolar media and was resolved into unadsorbed and adsorbed fractions on Q-Sepharose chromatography. The adsorbed activity was identified as XMEK2/SEK1/MKK4 by using several anti-XMEK2 antibodies. Thus, a 45-kDa protein that was recognized specifically by these anti-XMEK2 antibodies co-eluted with the SAPK alpha activating activity during chromatography on Q-Sepharose and Superose 6, and the activity could be immunoprecipitated by the antibodies from these fractions. The unadsorbed activity, whose level was much greater than that of the adsorbed activity, did not contain XMEK2/SEK1/MKK4 and was also activated in a time-dependent manner by osmotic shock. This activity was further resolved into several peaks during chromatography on heparin-Sepharose and hydroxylapatite. Most of these peaks eluted separately from major peaks of a kinase activity toward p38/MPK2, another subgroup of the MAPK superfamily, whereas the activated XMEK2/SEK1/MKK4 could phosphorylate p38/MPK2 efficiently. These results indicate the existence of multiple activators for SAPK/JNK; one is XMEK2/SEK1/MKK4, and the others are previously undescribed factors.

A peptide, ALTTE, within the fimbrial subunit protein from Porphyromonas gingivalis, induces production of interleukin 6, gene expression and protein phosphorylation in human peripheral blood mononuclear cells

Porphyromonas gingivalis 381 fimbriae and a synthetic peptide composed of residues 69-73 (ALTTE) of the fimbrial subunit protein, FP381(69-73), function in the induction of interleukin 6 (IL-6) production, IL-6 mRNA expression, and tyrosine and serine/threonine phosphorylation of several proteins in human peripheral blood mononuclear cells (PBMC). Herbimycin A and H-7, inhibitors of tyrosine kinases and protein kinase C (PKC), markedly inhibited IL-6 production, gene expression, and tyrosine and serine/threonine phosphorylation of proteins. An inactive analog of synthetic peptide replaced alanine to glycine at position 69 in FP381(69-73), GLTTE, exhibited an antagonistic effect on the IL-6 production induced by the fimbriae. These results suggest that the peptide ALTTE functions as an agent in inflammatory reactions and immune responses in the inflamed gingival and periodontal tissues, in which the participation of protein phosphorylation by tyrosine kinases and PKC in signal transduction may be considered.

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.

Components of a new human protein kinase signal transduction pathway

We have identified two components of a new protein kinase signaling cascade, MAPK/ERK kinase 5 (MEK5) and extracellular signal-regulated kinase 5 (ERK5). The MEK5 cDNA was isolated by degenerate PCR and encodes a 444-amino acid protein, which has approximately 40% identity to known MEKs. ERK5 was identified by a specific interaction with the MEK5 mutants S311A/T315A and K195M in the yeast two-hybrid system. The proteins were found to interact in an in vitro binding assay as well. ERK5 did not interact with MEK1 or MEK2. ERK5 is predicted to contain 815 amino acids and is approximately twice the size of all known ERKs. The C terminus of ERK5 has sequences which suggest that it may be targeted to the cytoskeleton. Sequences located in the N terminus of MEK5 may be important in coupling GTPase signaling molecules to the MEK5 protein kinase cascade. Both MEK5 and ERK5 are expressed in many adult tissue and are abundant in heart and skeletal muscle. A recombinant GST-ERK5 kinase domain displays autophosphorylation on Ser/Thr and Tyr residues.

Direct interaction between Ras and the kinase domain of mitogen-activated protein kinase kinase kinase (MEKK1)

Mitogen-activated protein kinase kinase kinase (MEKK1) is a serine-threonine kinase that regulates sequential protein kinase pathways involving stress-activated protein kinases and mitogen-activated protein kinases. MEKK1 is activated in response to growth factor stimulation of cells and by expression of activated Ras. We demonstrate that the kinase domain of MEKK1 (MEKKCOOH) binds to GST-RasV12 in a GTP-dependent manner. Purified bacterially expressed MEKKCOOH binds to GST-RasV12(GTP gamma S) (GTP gamma S is guanosine 5'-3-O-(thio)triphosphate), demonstrating a direct interaction of the two proteins. A Ras effector domain peptide blocks the binding of MEKKCOOH to GST-RasV12(GTP gamma S). MEKKCOOH complexed with GST-RasV12(GTP gamma S) is capable of phosphorylating MEK1. These findings indicate that MEKK1 directly binds Ras.GTP. Thus, Ras interacts with protein kinases of both the Raf and MEKK families.

The ethylene hormone response in Arabidopsis: a eukaryotic two-component signaling system

The simple gas ethylene affects numerous physiological processes in the growth and development of higher plants. With the use of molecular genetic approaches, we are beginning to learn how plants perceive ethylene and how this signal is transduced. Components of ethylene signal transduction are defined by ethylene response mutants in Arabidopsis thaliana. The genes corresponding to two of these mutants, etr1 and etr1, have been cloned. The ETR1 gene encodes a homolog of two-component regulators that are known almost exclusively in prokaryotes. The two-component regulators in prokaryotes are involved in the perception and transduction of a wide range of environmental signals leading to adaptive responses. The CTR1 gene encodes a homolog of the Raf family of serine/threonine protein kinases. Raf is part of a mitogen-activated protein kinase cascade known to regulate cell growth and development in mammals, worms, and flies. The ethylene response pathway may, therefore, exemplify a conserved protein kinase cascade regulated by a two-component system. The dominance of all known mutant alleles of ETR1 may be due to either constitutive activation of the ETR1 protein or dominant interference of wild-type activity. The discovery of Arabidopsis genes encoding proteins related to ETR1 suggests that the failure to recover recessive etr1 mutant alleles may be due to the presence of redundant genes.

Activation of the MAP kinase homologue RK requires the phosphorylation of Thr-180 and Tyr-182 and both residues are phosphorylated in chemically stressed KB cells

A MAP kinase homologue, termed the reactivating kinase (RK), lies in a signalling pathway which mediates cellular responses to stress. Here we demonstrate that the stress-induced activation of the RK in human KB cells is accompanied by the phosphorylation of Thr-180 and Tyr-182, and that the phosphorylation of both residues is required for the activation of this enzyme.

SB 203580 is a specific inhibitor of a MAP kinase homologue which is stimulated by cellular stresses and interleukin-1

A class of pyridinyl imidazoles inhibit the MAP kinase homologue, termed here reactivating kinase (RK) [Lee et al. (1994) Nature 372, 739-746]. We now show that one of these compounds (SB 203580) inhibits RK in vitro (IC50 = 0.6 microM), suppresses the activation of MAPKAP kinase-2 and prevents the phosphorylation of heat shock protein (HSP) 27 in response to interleukin-1, cellular stresses and bacterial endotoxin in vivo. These results establish that MAPKAP kinase-2 is a physiological RK substrate, and that HSP27 is phosphorylated by MAPKAP kinase-2 in vivo. The specificity of SB 203580 was indicated by its failure to inhibit 12 other protein kinases in vitro, and by its lack of effect on the activation of RK kinase and other MAP kinase cascades in vivo. We suggest that SB 203580 will be useful for identifying other physiological roles and targets of RK and MAPKAP kinase-2.

The ethylene signal transduction pathway in plants

Ethylene (C2H4), the chemically simplest plant hormone, is among the best-characterized plant growth regulators. It participates in a variety of stress responses and developmental processes. Genetic studies in Arabidopsis have defined a number of genes in the ethylene signal transduction pathway. Isolation of two of these genes has revealed that plants sense this gas through a combination of proteins that resemble both prokaryotic and eukaryotic signaling proteins. Ethylene signaling components are likely conserved for responses as diverse as cell elongation, cell fate patterning in the root epidermis, and fruit ripening. Genetic manipulation of these genes will provide agriculture with new tools to prevent or modify ethylene responses in a variety of plants.

Molecular dissection of the paired helical filament

Abundant neurofibrillary tangles, neuropil threads and plaque neurites constitute the neurofibrillary pathology of Alzheimer's disease. They form in the nerve cells that undergo degeneration in the disease where their regional distribution correlates with the degree of dementia. Each lesion contains the paired helical filament (PHF) as its major fibrous component. Recent work has shown that PHFs are composed of the microtubule-associated protein tau in a hyperphosphorylated state. PHF-tau is hyperphosphorylated on six adult brain tau isoforms. As a consequence, tau is unable to bind to microtubules and is believed to self-assemble into the PHF. Current evidence suggests that protein kinases or protein phosphatases with a specificity for serine/threonine-proline residues play an important role in the hyperphosphorylation of tau. Candidate protein kinases include mitogen-activated protein kinase, glycogen synthase kinase-3 and cyclin-dependent kinase 5, whereas the trimeric form of protein phosphatase 2A is a candidate phosphatase.

Hyperosmolality inhibits bicarbonate absorption in rat medullary thick ascending limb via a protein-tyrosine kinase-dependent pathway

In the rat medullary thick ascending limb (MTAL), hyperosmolality inhibits transepithelial HCO3- absorption (JHCO3-) by inhibiting apical membrane Na+/H+ exchange. To examine signaling mechanisms involved in this regulatory response, MTALs were isolated and perfused in vitro with 25 mM HCO3- solutions (290 mosmol/kg H2O). Osmolality was increased in lumen and bath solutions by addition of 300 mM mannitol or 75 mM NaCl. Addition of mannitol reduced JHCO3- by 60% and addition of NaCl reduced JHCO3- by 50%. With the protein tyrosine kinase (PTK) inhibitor genistein (7 microM) or herbimycin A (1 microM) in the bath, addition of mannitol reduced JHCO3- only by 11% and addition of NaCl reduced JHCO3- only by 15%. Staurosporine (10(-7) M) or forskolin (10(-6) M) in the bath had no effect on inhibition of JHCO3- by hypertonic NaCl. Genistein had no effect on inhibition of JHCO3- by vasopressin (a cyclic AMP-dependent process) or stimulation of JHCO3- by prostaglandin E2 (a protein kinase C-dependent process). Under isosmotic conditions, addition of genistein or herbimycin A to the bath increased JHCO3- by 30% through stimulation of apical membrane Na+/H+ exchange. Addition of the tyrosine phosphatase inhibitor molybdate (50 microM) to the bath reproduced the inhibition of JHCO3- observed with hyperosmolality. These data indicate that 1) the effect of hyperosmolality to inhibit MTAL HCO3- absorption through inhibition of apical membrane Na+/H+ exchange is mediated via a PTK-dependent pathway that functions independent of regulation by cyclic AMP and protein kinase C, and 2) a constitutive PTK activity inhibits apical membrane Na+/H+ exchange and HCO3- absorption under isosmotic conditions. Our results suggest that tyrosine phosphorylation is a critical step in inhibition of the apical Na+/H+ exchanger isoform NHE-3 by hyperosmolality.

Identification of a dual specificity kinase that activates the Jun kinases and p38-Mpk2

One Ras-dependent protein kinase cascade leading from growth factor receptors to the ERK (extracellular signal-regulated kinases) subgroup of mitogen-activated protein kinases (MAPKs) is dependent on the protein kinase Raf-1, which activates the MEK (MAPK or ERK kinase) dual specificity kinases. A second protein kinase cascade leading to activation of the Jun kinases (JNKs) is dependent on MEKK (MEK kinase). A dual-specificity kinase that activates JNK, named JNKK, was identified that functions between MEKK and JNK. JNKK activated the JNKs but did not activate the ERKs and was unresponsive to Raf-1 in transfected HeLa cells. JNKK also activated another MAPK, p38 (Mpk2; the mammalian homolog of HOG1 from yeast), whose activity is regulated similarly to that of the JNKs.

Effects of granulocyte-macrophage colony-stimulating factor and tumour necrosis factor-alpha on tyrosine phosphorylation and activation of mitogen-activated protein kinases in human neutrophils

The present study was undertaken to determine the identities and characteristics of proteins with molecular masses between 40 and 44 kDa whose tyrosine phosphorylation increases in human neutrophils following stimulation of these cells with tumour necrosis factor alpha (TNF-alpha) and granulocyte-macrophage colony-stimulating factor (GM-CSF). Immunoblotting results demonstrate that addition of GM-CSF to human neutrophils increases the tyrosine phosphorylation of two proteins with molecular masses of 42 and 44 kDa. However, the addition of TNF-alpha to neutrophils induces a time- and dose-dependent increase in tyrosine phosphorylation of a 40 kDa protein. Immunoprecipitation using specific mitogen-activated protein kinase (MAPK) isoform antibodies and an antibody which recognizes phosphotyrosine-containing proteins demonstrated that the 42 and 44 kDa proteins are isoforms of MAPKs. Utilizing an in situ gel kinase activity assay, GM-CSF increases the kinase activity of the 42 and 44 kDa proteins. Moreover, using immunoprecipitated p42 and p44 MAPK isoforms in this gel assay revealed activity associated with the p42 and p44 MAPK isoforms. Using the same in situ assay, TNF-alpha induces an increase in kinase activity of a 40-42 kDa protein. However, the 40 kDa protein whose phosphorylation on tyrosine residues increased in human neutrophils following stimulation with TNF-alpha is not a member of the known MAPK family, demonstrating the divergences in pathways utilized by GM-CSF and TNF-alpha. This 40 kDa protein may be related to the recently identified protein that becomes phosphorylated on tyrosine residues upon stimulation of the human epidermal carcinoma cell line KB by interleukin-1. In these cells the p40 protein is part of a protein kinase cascade which results in the phosphorylation of the small heat shock protein, hsp27.

Pro-inflammatory cytokines and environmental stress cause p38 mitogen-activated protein kinase activation by dual phosphorylation on tyrosine and threonine

Protein kinases activated by dual phosphorylation on Tyr and Thr (MAP kinases) can be grouped into two major classes: ERK and JNK. The ERK group regulates multiple targets in response to growth factors via a Ras-dependent mechanism. In contrast, JNK activates the transcription factor c-Jun in response to pro-inflammatory cytokines and exposure of cells to several forms of environmental stress. Recently, a novel mammalian protein kinase (p38) that shares sequence similarity with mitogen-activated protein (MAP) kinases was identified. Here, we demonstrate that p38, like JNK, is activated by treatment of cells with pro-inflammatory cytokines and environmental stress. The mechanism of p38 activation is mediated by dual phosphorylation on Thr-180 and Tyr-182. Immunofluorescence microscopy demonstrated that p38 MAP kinase is present in both the nucleus and cytoplasm of activated cells. Together, these data establish that p38 is a member of the mammalian MAP kinase group.

Molecular cloning of human p38 MAP kinase

Mitogen-activated protein (MAP) kinases are intracellular serine/threonine kinases activated by dual phosphorylation of adjacent threonine (T) and tyrosine (Y). A diverse number of extracellular signals induce activation of MAP kinases. Here we describe the cloning of a cDNA encoding human p38 MAP kinase (p38). The amino acid sequence of human p38 is 99.4% identical to mouse p38 [Han et al. (1994) Science 265, 808-11]. Like murine p38, the dual phosphorylation site of human p38 MAP kinase is characterized by a TGY sequence. Previous studies have described activation of p38 following exposure to products of microbial pathogens, physical-chemical stimuli and cytokines. The highly conserved nature of p38 suggests the importance of its function in regulating cellular responses.

Transforming G protein-coupled receptors potently activate JNK (SAPK). Evidence for a divergence from the tyrosine kinase signaling pathway

The expression of human muscarinic acetylcholine receptors (mAChRs) in NIH 3T3 cells has been used as a model for studying proliferative signaling through G protein-coupled receptors. In this biological system, the m1 class of mAChRs can effectively transduce mitogenic signals (Stephens, E.V., Kalinec, G., Brann, M.R., and Gutkind, J.S. (1993) Oncogene 8, 19-26) and induce malignant transformation if persistently activated (Gutkind, J.S., Novotny, E.A., Brann, M.R., and Robbins, K.C. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 4703-4708). Moreover, available evidence suggests that the m1-signaling pathway converges at the level of p21ras with that emerging from tyrosine kinase receptors (Crespo, P., Xu, N., Simonds, W.F., and Gutkind, J.S. (1994) Nature 369, 418-420). To explore nuclear events involved in growth regulation by G protein-coupled receptors in this setting, we compared the effect of platelet-derived growth factor (PDGF) and the cholinergic agonist, carbachol, on the expression of mRNA for members of the jun and fos family of nuclear proto-oncogenes. We found that activation of m1 receptors by carbachol induces the expression of a distinct set of nuclear transcription factors. In particular, carbachol caused a much greater induction of c-jun mRNA and AP-1 activity. These responses did not correlate with protein kinase C stimulation nor with the activation of mitogen-activated protein (MAP) kinases. Recently, it has been shown that a novel family of kinases structurally related to MAP kinases, stress-activated protein kinases, or Jun kinases (JNKs), phosphorylate in vivo the amino-terminal transactivating domain of the c-Jun protein, thereby increasing its transcriptional activity. In view of our results, this observation prompted us to ask whether m1 and PDGF can differentially activate JNKs. Here, we show that m1 mAChRs can induce a remarkable increase in JNK activity, which was temporally distinct from that of MAP kinase and was entirely protein kinase C independent. In contrast, PDGF failed to activate JNK in these cells, although it stimulated MAP kinase to an extent even greater than that for carbachol. These findings demonstrate that G protein-coupled receptors can signal through pathways leading to the activation of JNK, thus diverging at this level with those signaling routes utilized by tyrosine kinase receptors.

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.

Biochemical and biological analysis of Mek1 phosphorylation site mutants

Recently, we described the constitutive activation of Mek1 by mutation of its two serine phosphorylation sites. We have now characterized the biochemical properties of these Mek1 mutants and performed microinjection experiments to investigate the effect of an activated Mek on oocyte maturation. Single acidic substitution of either serine 218 or 222 activated Mek1 by 10-50 fold. The double acidic substitutions, [Asp218, Asp222] and [Asp218, Glu222], activated Mek1 over 6000-fold. The specific activity of the [Asp218, Asp222] and [Asp218, Glu222] Mek1 mutants, 29 nanomole phosphate per minute per milligram, is similar to that of wild-type Mek1 activated by Raf-1 in vitro. Although the mutants with double acidic substitutions could not be further activated by Raf-1, three of those with single acidic substitution were activated by Raf-1 to the specific activity of activated wild-type Mek1. Injection of the [Asp218, Asp222] Mek1 mutant into Xenopus oocytes activated both MAP kinase and histone H1 kinase and induced germinal vesicle breakdown, an effect that was only partially blocked by inhibition of protein synthesis. These data provide a measure of Mek's potential to influence cell functions and a quantitative basis to assess the biological effects of Mek1 mutants in a variety of circumstances.

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.

Does mitogen-activated-protein kinase have a role in insulin action? The cases for and against

The discovery of the mitogen-activated protein (MAP) kinase family of protein kinases has sparked off an intensive effort to elucidate their role in the regulation of many cellular processes. These protein kinases were originally identified based on their rapid activation by insulin. In this review we concentrate on examining the evidence for and against a role for the MAP kinases Erk-1 and Erk-2 in mediating the effects of insulin. While there is good evidence in favour of a direct role for MAP kinase in the growth-promoting effects of insulin and the regulation of Glut-1 and c-fos expression, and AP-1 transcriptional complex activity, this is by no means conclusive. MAP kinase may also play a role in the control of mRNA translation by insulin. On the other hand, the evidence suggests that MAP kinase is not sufficient for the acute regulation of glucose transport (Glut-4 translocation), glycogen synthesis, acetyl-CoA carboxylase or pyruvate dehydrogenase activity. The findings suggest that insulin may utilise at least three distinct signalling pathways which do not involve MAP kinase.

Differential activation of mitogen-activated protein kinase in response to basic fibroblast growth factor in skeletal muscle cells

In the MM14 mouse myoblast cell line, fibroblast growth factor (FGF) stimulates proliferation and represses differentiation. However, the intracellular signaling pathways used by FGF to affect these cellular processes are unknown. The predominant FGF receptor present on MM14 cells, FGFR1, is a receptor tyrosine kinase capable of activating the mitogen-activated protein kinase (MAPK) cascade in fibroblast and neuronal cell lines. To determine whether the FGF signal is mediated via the MAPK cascade in myoblasts, MM14 cells were stimulated with basic FGF (bFGF) and activities of the various kinases were measured. After withdrawal from serum and bFGF for 3 hr, bFGF stimulated MAPK kinase (MAPKK) activity, but MAPK and S6 peptide kinase activities were not detected. In contrast, when serum and bFGF were withdrawn for 10 hr, the activities of MAPKK, MAPK, and S6 peptide kinase were all stimulated by bFGF treatment. The inability of bFGF to stimulate MAPK after 3 hr of withdrawal may be due, in part, to the presence of a MAPK phosphatase activity that was detected in MM14 cell extracts. This dephosphorylating activity diminishes during commitment to terminal differentiation and is inhibited by sodium orthovanadate. Thus, the ability of bFGF to stimulate MAPK in MM14 cells is correlated with the loss of a MAPK phosphatase activity. These results show that although bFGF activates MAPKK in proliferating myoblasts, the mitogenic signal does not progress to the downstream kinases, providing a physiological example of an uncoupling of the MAPK cascade.

Specificity of receptor tyrosine kinase signaling: transient versus sustained extracellular signal-regulated kinase activation

A number of different intracellular signaling pathways have been shown to be activated by receptor tyrosine kinases. These activation events include the phosphoinositide 3-kinase, 70 kDa S6 kinase, mitogen-activated protein kinase (MAPK), phospholipase C-gamma, and the Jak/STAT pathways. The precise role of each of these pathways in cell signaling remains to be resolved, but studies on the differentiation of mammalian PC12 cells in tissue culture and the genetics of cell fate determination in Drosophila and Caenorhabditis suggest that the extracellular signal-regulated kinase (ERK-regulated) MAPK pathway may be sufficient for these cellular responses. Experiments with PC12 cells also suggest that the duration of ERK activation is critical for cell signaling decisions.

Characterization of 45-kDa/54-kDa HSP27 kinase, a stress-sensitive kinase which may activate the phosphorylation-dependent protective function of mammalian 27-kDa heat-shock protein HSP27

Heat-shock protein 27 (HSP27) is a major target of phosphorylation upon cell stimulation with a variety of agents and has been suggested to have a phosphorylation-regulated function at the level of actin filaments. Here we investigated comparatively the mechanisms of HSP27 phosphorylation by oxidative stresses, exposures to tumor necrosis factor (TNF), heat shock and growth factors. Extracts of Chinese hamster or human cells exposed to H2O2, xanthine/xanthine oxidase, menadione or TNF contained up to 15-fold more HSP27 kinase activity than comparable extracts obtained from control cells. Induction of HSP27 kinase activity by TNF or H2O2 was completely inhibited by first treating the cells with the antioxidant N-acetyl-L-cysteine, suggesting that generation of reactive oxygen metabolites was the key triggering element of this induction. In contrast, prior treatment with acetylcysteine had no or little effect on the induction by thrombin, serum and heat shock. The kinase activity in extracts of cells stimulated by heat shock, H2O2, sodium arsenite, TNF or growth factors was identified by in-gel renaturation and purified approximately 8000-fold by sequential chromatography. In all cases, the induced kinase activity was entirely associated with two polypeptides of 45 kDa and 54 kDa, identified as mitogen-activated-protein kinase-activated protein (MAPKAP) kinase-2 based on its reactivation in vitro by 42/44-kDa MAP kinases, its antigenic properties and its substrate specificity. The 45/54-kDa HSP27 kinase may play an important role in the cell response to oxidative stress. Overexpression of the wild-type HSP27 but not of a nonphosphorylatable form of human HSP27 in Chinese hamster cells conferred resistance to actin fragmentation by oxidative stress generated by H2O2. It is concluded that activation of the 45/54-kDa HSP27 kinase is a common mechanism of HSP27 phosphorylation to which converge both oxyradical-dependent and oxyradical-independent pathways and which may participate in a homeostatic response to stress at the level of actin microfilament.

Characterization of an autoinhibitory domain in human mitogen-activated protein kinase-activated protein kinase 2

Mitogen-activated protein (MAP) kinase-activated protein kinase 2, a Ser/Thr kinase, is phosphorylated and activated by MAP kinase. Sequence analysis of a clone isolated from the human HL-60 cell line revealed a 370-amino acid protein with a proline-rich N terminus, a highly conserved catalytic domain, and a C-terminal region containing a MAP kinase phosphorylation site. To better understand how the kinase is regulated, mutation analysis was used to map the functional domain(s). The wild type recombinant kinase had a low basal activity as detected by phosphorylation of a substrate peptide derived from the N terminus of glycogen synthase. Deletion of the proline-rich N terminus showed little effect on the basal activity. Deletion of the C terminus resulted in a marked increase in catalytic activity either with or without the pretreatment of the kinase by MAP kinase. Further analysis indicated that amino acid residues 339-353 in the C-terminal region were acting as an autoinhibitory domain. A synthetic peptide (RVLKEDKERWEDVK-amide) derived from this autoinhibitory domain inhibited the kinase activity in a concentration-dependent manner. These results suggest a regulatory model for the kinase.

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.

A protein kinase involved in the regulation of inflammatory cytokine biosynthesis

Production of interleukin-1 and tumour necrosis factor from stimulated human monocytes is inhibited by a new series of pyridinyl-imidazole compounds. Using radiolabelled and radio-photoaffinity-labelled chemical probes, the target of these compounds was identified as a pair of closely related mitogen-activated protein kinase homologues, termed CSBPs. Binding of the pyridinyl-imidazole compounds inhibited CSBP kinase activity and could be directly correlated with their ability to inhibit cytokine production, suggesting that the CSBPs are critical for cytokine production.

Role of SAPK/ERK kinase-1 in the stress-activated pathway regulating transcription factor c-Jun

The stress-activated protein kinases (SAPKs), which are distantly related to the MAP kinases, are the dominant c-Jun amino-terminal protein kinases activated in response to a variety of cellular stresses, including treatment with tumour-necrosis factor-alpha and interleukin-beta (refs 1, 2). SAPK phosphorylation of c-Jun probably activates the c-Jun transactivation function. SAPKs are part of a signal transduction cascade related to, but distinct from, the MAPK pathway. We have now identified a novel protein kinase, called SAPK/ERK kinase-1 (SEK1), which is structurally related to the MAP kinase kinases (MEKs). SEK1 is a potent activator of the SAPKs in vitro and in vivo. An inactive SEK1 mutant blocks SAPK activation by extracellular stimuli without interfering with the MAPK pathway. Although alternative mechanisms of SAPK activation may exist, as an immediate upstream activator of the SAPKs, SEK1 further defines a signalling cascade that couples cellular stress agonists to the c-Jun transcription factor.

Activation of stress-activated protein kinase by MEKK1 phosphorylation of its activator SEK1

A kinase distinct from the MEK activator Raf, termed MEK kinase-1 (MEKK), was originally identified by virtue of its homology to kinases involved in yeast mating signal cascades. Like Raf, MEKK is capable of activating MEK in vitro. High-level expression of MEKK in COS-7 cells or using vaccinia virus vectors also activates MEK and MAPK, indicating that MEKK and Raf provide alternative means of activating the MAPK signalling pathway. We have derived NIH3T3 cell sublines that can be induced to express active MEKK. Here we show that induction of MEKK does not result in the activation of MAPK, but instead stimulates the stress-activated protein kinases (SAPKs) which are identical to a Jun amino-terminal kinase. We find that MEKK regulates a new signalling cascade by phosphorylating an SAPK activator, SEK1 which in turn phosphorylates and activates SAPK.

MAP kinase pathways. Straight and narrow or tortuous and intersecting?

Stress and mitogens stimulate overlapping sets of MAP kinases in mammalian cells; MAP kinase pathways appear more distinct in yeast, but differences between the two systems may be less than is presently evident.

MAPKs: new JNK expands the group

Mitogen-activated protein kinases (MAPKs) are activated by dual phosphorylation on threonine and tyrosine in response to a wide array of extracellular stimuli. In the yeast Saccharomyces cerevisiae, a series of extracellular stimuli. In the yeast Saccharomyces cerevisiae, a series of MAPK signal transduction pathways have been demonstrated to control many cellular functions. By contrast, mammalian MAPKs are more poorly understood. However, recent studies have established important roles for three separate groups of mammalian MAPKs, which are characterized by distinct dual phosphorylation motifs. Together, these protein kinases mediate signal transduction in mammalian tissues and control many aspects of cellular physiology.

Histidine and aspartate phosphorylation: two-component systems and the limits of homology

Autophosphorylating histidine kinase and response-regulator domains constitute the building blocks of two-component signaling systems. These systems use a unique phosphotransfer chemistry to regulate many aspects of bacterial physiology. Homologous systems are now being found in eukaryotes. Despite their common mechanism of phosphotransfer, the two-component systems display an extensive diversity in the arrangement of their domains, and flexibility in their roles in different signal transduction circuits.

Interleukin-1 activates a novel protein kinase cascade that results in the phosphorylation of Hsp27

An IL-1-stimulated protein kinase cascade resulting in phosphorylation of the small heat shock protein hsp27 has been identified in KB cells. It is distinct from the p42 MAP kinase cascade. An upstream activator kinase phosphorylated a 40 kDa kinase (p40) upon threonine and tyrosine residues, which in turn phosphorylated a 50 kDa kinase (p50) upon threonine (and some serine) residues. p50 phosphorylated hsp27 upon serine. p40 and p50 were purified to near homogeneity. All three components were inactivated by protein phosphatase 2A, and p40 was inactivated by protein tyrosine phosphatase 1B. The substrate specificity of p40 differed from that of p42 and p54 MAP kinases. The upstream activator was not a MAP kinase kinase. p50 resembled MAPKAPK-2 and may be identical.