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

Stress-induced transcriptional activation

Living cells, both prokaryotic and eukaryotic, employ specific sensory and signalling systems to obtain and transmit information from their environment in order to adjust cellular metabolism, growth, and development to environmental alterations. Among external factors that trigger such molecular communications are nutrients, ions, drugs and other compounds, and physical parameters such as temperature and pressure. One could consider stress imposed on cells as any disturbance of the normal growth condition and even as any deviation from optimal growth circumstances. It may be worthwhile to distinguish specific and general stress circumstances. Reasoning from this angle, the extensively studied response to heat stress on the one hand is a specific response of cells challenged with supra-optimal temperatures. This response makes use of the sophisticated chaperoning mechanisms playing a role during normal protein folding and turnover. The response is aimed primarily at protection and repair of cellular components and partly at acquisition of heat tolerance. In addition, heat stress conditions induce a general response, in common with other metabolically adverse circumstances leading to physiological perturbations, such as oxidative stress or osmostress. Furthermore, it is obvious that limitation of essential nutrients, such as glucose or amino acids for yeasts, leads to such a metabolic response. The purpose of the general response may be to promote rapid recovery from the stressful condition and resumption of normal growth. This review focuses on the changes in gene expression that occur when cells are challenged by stress, with major emphasis on the transcription factors involved, their cognate promoter elements, and the modulation of their activity upon stress signal transduction. With respect to heat shock-induced changes, a wealth of information on both prokaryotic and eukaryotic organisms, including yeasts, is available. As far as the concept of the general (metabolic) stress response is concerned, major attention will be paid to Saccharomyces cerevisiae.

Modulation of actin dynamics during stress and physiological stimulation by a signaling pathway involving p38 MAP kinase and heat-shock protein 27

HSP27, like other proteins of the heat-shock protein family, accumulates to high levels after exposure of cells to a short period of hyperthermia and contributes to the development of a transient state of thermoresistance. In vitro, HSP27 behaves as an actin cap-binding protein and can inhibit actin polymerization. In vivo, the protective function of HSP27 is exerted mainly at the level of the microfilaments and appears as an extension of a normal function of the protein. This function is regulated by phosphorylation in a mitogen- and stress-sensitive signaling pathway involving the newly characterized p38 MAP kinase. The phosphorylation-modulated function of HSP27 can contribute to agonist-induced reorganization of the actin cytoskeleton and, in the case of stress activation, provides an actin-based adaptive response of cells to the new environmental conditions.

Pyp1 and Pyp2 PTPases dephosphorylate an osmosensing MAP kinase controlling cell size at division in fission yeast

Simultaneous inactivation of pyp1 and pyp2 PTPases in fission yeast leads to aberrant cell morphology and growth arrest. Spontaneous recessive mutations that bypass the requirement for pyp1 and pyp2 and reside in two complementation groups were isolated, sty1 and sty2. sty1- and sty2- mutant cells are substantially delayed in the timing of mitotic initiation. We have isolated the sty1 gene, which encodes a MAP kinase that is closely related to a subfamily of MAP kinases regulated by osmotic stress including Saccharomyces cervisiae HOG1 and human CSBP1. We find that sty2 is allelic to the wis1 MAP kinase kinase and that delta sty1 and delta wis1 cells are unable to grow in high osmolarity medium. Osmotic stress induces both tyrosine phosphorylation of Sty1 and a reduction in cell size at division. Pyp2 associates with and tyrosine dephosphorylates Sty1 in vitro. We find that wis1-dependent induction of pyp2 mRNA is responsible for tyrosine dephosphorylation of Sty1 in vivo on prolonged exposure to osmotic stress. We conclude that Pyp1 and Pyp2 are tyrosine-specific MAP kinase phosphatases that inactivate an osmoregulated MAP kinase, Sty1, which acts downstream of the Wis1 MAP kinase kinase to control cell size at division in fission yeast.

A synthetic inhibitor of the mitogen-activated protein kinase cascade

Treatment of cells with a variety of growth factors triggers a phosphorylation cascade that leads to activation of mitogen-activated protein kinases (MAPKs, also called extracellular signal-regulated kinases, or ERKs). We have identified a synthetic inhibitor of the MAPK pathway. PD 098059 [2-(2'-amino-3'-methoxyphenyl)-oxanaphthalen-4-one] selectively inhibited the MAPK-activating enzyme, MAPK/ERK kinase (MEK), without significant inhibitory activity of MAPK itself. Inhibition of MEK by PD 098059 prevented activation of MAPK and subsequent phosphorylation of MAPK substrates both in vitro and in intact cells. Moreover, PD 098059 inhibited stimulation of cell growth and reversed the phenotype of ras-transformed BALB 3T3 mouse fibroblasts and rat kidney cells. These results indicate that the MAPK pathway is essential for growth and maintenance of the ras-transformed phenotype. Further, PD 098059 is an invaluable tool that will help elucidate the role of the MAPK cascade in a variety of biological settings.

Heat shock proteins are differentially expressed in human gastrointestinal cancers

The heat shock proteins (Hsp) are stress-responsive genes present in all species; increases of Hsp can confer chemotherapeutic resistance to certain cancers. The purpose of this study was to determine Hsp expression in human gastric, pancreatic and colon cancers. Gastric (n = 3), pancreatic (n = 6) and colon (n = 8) cancers were extracted for RNA and protein, and Northern and Western blots performed. We found that hsp70 and hsp27 mRNA levels were differentially expressed in the gastrointestinal cancers; mRNA expression closely correlated with protein levels suggesting regulation at the level of transcription. In addition, Hsp90 and BiP proteins were constitutively expressed in the gastrointestinal cancers. We conclude that the Hsp are differentially expressed in human gastric, pancreatic and colon cancers; these increases in Hsp occur constitutively and are not the result of physiological or environmental stresses. Increases of Hsp expression in cancer cells may enhance resistance and account for the altered sensitivity of certain gastrointestinal cancers to chemotherapeutic agents.

Low affinity binding of interleukin-1 beta and intracellular signaling via NF-kappa B identify Fit-1 as a distant member of the interleukin-1 receptor family

The fit-1 gene gives rise to two different mRNA isoforms, which code for soluble (Fit-1S) and membrane-bound (Fit-1M) proteins related to the type I interleukin (IL)-1 receptor. To investigate IL-1 binding, we have synthesized and purified histidine-tagged polypeptides corresponding to Fit-1S and the extracellular domain of the type I IL-1 receptor using a vaccinia expression system. Fit-1S is shown to interact with IL-1 beta, but not with IL-1 alpha. However, Fit-1S binds IL-1 beta only with low affinity in contrast to the IL-1 receptor, suggesting that IL-1 beta is not a physiological ligand of Fit-1S. Moreover, expression of the membrane-bound protein Fit-1M in transiently transfected Jurkat cells did not result in activation of the transcription factor NF-kappa B following IL-1 beta treatment. However, a chimeric protein consisting of the extracellular domain of the type I IL-1 receptor and of the transmembrane and intracellular regions of Fit-1M stimulated NF-kappa B-dependent transcription as efficiently as the full-length type I IL-1 receptor. These data indicate that Fit-1M is a signaling molecule belonging to the IL-1 receptor family.

Conversion from oligomers to tetramers enhances autophosphorylation by lens alpha A-crystallin. Specificity between alpha A- and alpha B-crystallin subunits

Previously we showed that alpha-crystallins are autophosphorylated (Kantorow, M., and Piatigorsky, J. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 3112-3116). Here we report that addition of 1% deoxycholate converted alpha A-crystallin aggregates into 80-kDa tetramers which were 10-fold more active for autophosphorylation. Circular dichroism (CD) spectra of alpha-crystallin revealed little or no change in secondary and tertiary structures in 1% deoxycholate, alpha A2D, a truncated form of bovine alpha A that exists as a tetramer, was as active for autophosphorylation in the absence of deoxycholate as intact alpha A was in the presence of deoxycholate. At least one serine between amino acids 131 and 145 of bovine alpha A was autophosphorylated in peptide mapping experiments. Chicken alpha A-crystallin, which lacks the Ser-122 cAMP-dependent kinase site of bovine alpha A, was also autophosphorylated in the presence of deoxycholate. In contrast to alpha A-crystallin, autophosphorylation by alpha B-crystallin was not activated by deoxycholate despite its conversion to a tetrameric form, and alpha B was also more efficiently phosphorylated by cAMP-dependent kinase than alpha A. These data suggest metabolic differences between the alpha-crystallin subunits that may be related to specific expression of alpha A in the lens and ubiquitous expression of alpha B in numerous normal and diseased tissues.

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

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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 interleukin-1 axis and cutaneous inflammation

Since the discovery that epidermal cell-derived thymocyte-activating factor was identical to interleukin (IL)-1 alpha and -beta in 1986, these molecules have been implicated in the pathogenesis of skin diseases. In 1995, it has become clear that a group of gene products function to regulate the activity of IL-1. IL-1 alpha and mature 17-kD IL-1 beta (cleaved from precursor by IL-1 beta-converting enzyme) bind to the type 1 IL-1 receptor to transduce a signal. This process can be antagonized at the level of the receptor by two distinct forms of the IL-1 receptor antagonist, which bind to the type I receptor but do not transduce a signal. The process can also be antagonized at the level of the ligand by either cell-bound or soluble type 2 IL-1 receptor. This type 2 IL-1 receptor binds ligand but does not transduce a signal. Keratinocytes can make each of these variables in vitro, and the balance between agonists and antagonists dictates the biologic outcome of a putative IL-1-mediated event. Transgenic mice that overexpress each of these factors individually in epidermis will be useful for enhancing our understanding of the cutaneous biology of IL-1.

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.

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.

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.

Interleukin 1-induced phosphorylation of MAD3, the major inhibitor of nuclear factor kappa B of HeLa cells. Interference in signalling by the proteinase inhibitors 3,4-dichloroisocoumarin and tosylphenylalanyl chloromethylketone

The regulation of the inhibitor of nuclear factor kappa B (I kappa B) by interleukin 1 (IL1) was investigated in HeLa cells. Two forms of I kappa B were resolved by ion-exchange chromatography. The major form (75%) was identified as MAD3 by specific antisera. IL1 generated rapidly (6 min) an electrophoretically retarded form of MAD3 that was stable in acid and was converted into the unmodified form by phosphatase 2A. It thus corresponded to a phosphorylation of the protein on serine or threonine. IL1 also caused the disappearance of MAD3 from the cells, which was complete 15 min after stimulation and coincided with a 46% reduction of cellular I kappa B activity. Newly-synthesized MAD3 accumulated to pre-stimulation levels between 60 and 90 min after stimulation and this coincided with the down-regulation of the phosphorylating activity. The serine proteinase inhibitors 3,4-dichloroisocoumarin (DCI) and tosylphenylalanyl chloromethylketone (TPCK) prevented phosphorylation and disappearance of MAD3. At the same concentrations (10-100 microM), they also increased basal phosphorylation of the small heat shock protein (hsp27) and prevented the IL1- and phorbol 12-myristate 13-acetate-induced increases of its phosphorylation. The inhibitors were thus interfering with protein kinases when blocking degradation of MAD3. Recombinant MAD3 phosphorylated in vitro by protein kinase C was not electrophoretically retarded, suggesting that MAD3 was phosphorylated by another kinase in IL1-stimulated cells. Our results suggest that the IL1-induced phosphorylation of MAD3 on serine or threonine leads to its degradation. DCI and TPCK blocked phosphorylation mechanisms and it could not be concluded that serine proteinases were involved in the breakdown of MAD3.

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.

Variation in the expression and/or phosphorylation of the human low molecular weight stress protein during in vitro cell differentiation

Members of the low molecular weight heat shock protein (hsp) family show regulated expression in both Drosophila and mice during development and differentiation. Here we have examined whether similar regulation of the single low molecular weight hsp (hsp 28) of humans exhibits differences in either its expression and/or phosphorylation during the course of in vitro differentiation of hematopoietic cells. In the promyelocytic leukemic cell line, HL-60, we show that early after commitment of the cells to a macrophage-like phenotype (via exposure to phorbol ester myristate, PMA) there occurs an accompanying increased phosphorylation of hsp 28. Over time and as the cells become terminally differentiated the levels of hsp 28 increase significantly. In contrast, cells stimulated to adopt a granulocyte-like phenotype (e.g. exposed to either dimethyl sulfoxide or retinoic acid) show no changes in either the phosphorylation or expression of hsp 28. Moreover, once differentiated the granulocyte-like cells no longer appear capable of phosphorylating hsp 28. Human K562 cells, in response to hemin, rapidly increase their expression and phosphorylation of hsp 28 during the course of their differentiation into erythroid-like cells. Addition of PMA to the K562 cells induces differentiation into a megakaryocyte-like phenotype but is not accompanied by changes in hsp 28 phosphorylation/expression. In the case of the HL-60 cells, differentiation toward the macrophage like lineage is accompanied by an increased adherence of the cells to their substratum and an apparent association of hsp 28 with the actin cytoskeleton.

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.

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.

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.

Stathmin is a major substrate for mitogen-activated protein kinase during heat shock and chemical stress in HeLa cells

Stathmin is a ubiquitous, highly conserved 19-kDa cytoplasmic protein whose expression and phosphorylation are regulated in relation to cell proliferation, differentiation or activation, in many biological systems. In this report, we show that stathmin undergoes major phosphorylation in HeLa cells submitted to heat or chemical stress. Heat-shock-induced stathmin phosphorylation was very rapid, as maximal incorporation of phosphate was observed at 5 min. Phosphorylation of stathmin might, therefore, occur as a very early step in the intracellular response to heat shock. The sites of phosphorylation of stathmin involved during the stress response were identified as mostly Ser25 and, to a lesser extent, Ser38. These sites are both followed by a proline residue, and known to be good substrates in vitro for mitogen-activated protein kinase (MAP-kinase) and p34cdc2 kinase, respectively. In lysates from heat-shocked cells, an increased stathmin-kinase activity, distinct from the histone-H1-kinase activity, was found to phosphorylate stathmin mostly on Ser25, the main site for MAP-kinase in vitro. This stathmin-kinase coeluted quantitatively with the stress-activated MAP-kinase from an FPLC MonoQ column. Furthermore, a stathmin kinase activity was precipitated from lysates of heat-shocked HeLa cells by an anti-(MAP-kinase) serum. Together, these results indicate that the phosphorylation of stathmin by MAP-kinase is likely to be a significant component of the signalling array controlling the cellular response to stress, and they further underline the general involvement of stathmin in intracellular signalling.

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.

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.

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.