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

Stimulation of the stress-activated mitogen-activated protein kinase subfamilies in perfused heart. p38/RK mitogen-activated protein kinases and c-Jun N-terminal kinases are activated by ischemia/reperfusion

It has recently been recognized that cellular stresses activate certain members of the mitogen-activated protein kinase (MAPK) superfamily. One role of these "stress-activated" MAPKs is to increase the transactivating activity of the transcription factors c-Jun, Elk1, and ATF2. These findings may be particularly relevant to hearts that have been exposed to pathological stresses. Using the isolated perfused rat heart, we show that global ischemia does not activate the 42- and 44-kD extracellular signal-regulated (protein) kinase (ERK) subfamily of MAPKs but rather stimulates a 38-kD activator of MAPK-activated protein kinase-2 (MAPKAPK2). This activation is maintained during reperfusion. The molecular characteristics of this protein kinase suggest that it is a member of the p38/reactivating kinase (RK) group of stress-activated MAPKs. In contrast, stress-activated MAPKs of the c-Jun N-terminal kinase (JNK/SAPKs) subfamily are not activated by ischemia alone but are activated by reperfusion following ischemia. Furthermore, transfection of ventricular myocytes with activated protein kinases (MEKK1 and SEK1) that may be involved in the upstream activation of JNK/ SAPKs induces increases in myocyte size and transcriptional changes typical of the hypertrophic response. We speculate that activation of multiple parallel MAPK pathways may be important in the responses of hearts to cellular stresses.

Characterization of the structure and function of a new mitogen-activated protein kinase (p38beta)

Mitogen-activated protein (MAP) kinase cascades represent one of the major signal systems used by eukaryotic cells to transduce extracellular signals into cellular responses. Four MAP kinase subgroups have been identified in humans: ERK, JNK (SAPK), ERK5 (BMK), and p38. Here we characterize a new MAP kinase, p38beta. p38beta is a 372-amino acid protein most closely related to p38. It contains a TGY dual phosphorylation site, which is required for its kinase activity. Like p38, p38beta is activated by proinflammatory cytokines and environmental stress. A comparison of events associated with the activation of p38beta and p38 revealed differences, most notably in the preferred activation of p38beta by MAP kinase kinase 6 (MKK6), whereas p38 was activated nearly equally by MKK3, MKK4, and MKK6. Moreover, in vitro and in vivo experiments showed a strong substrate preference by p38beta for activating transcription factor 2 (ATF2). Enhancement of ATF2-dependent gene expression by p38beta was approximately20-fold greater than that of p38 and other MAP kinases tested. The data reported here suggest that while closely related, p38beta and p38 may be regulated by differing mechanisms and may exert their actions on separate downstream targets.

5-Oxo-eicosanoids and hematopoietic cytokines cooperate in stimulating neutrophil function and the mitogen-activated protein kinase pathway

The newly defined eicosatetraenoates (ETEs), 5-oxoETE and 5-oxo-15(OH)-ETE, share structural motifs, synthetic origins, and bioactions with leukotriene B4 (LTB4). All three eicosanoids stimulate Ca2+ transients and chemotaxis in human neutrophils (PMN). However, unlike LTB4, 5-oxoETE and 5-oxo-15(OH)-ETE alone cause little degranulation and no superoxide anion production. However, we show herein that, in PMN pretreated with granulocyte-macrophage or granulocyte colony-stimulating factor (GM-CSF or G-CSF), the oxoETEs become potent activators of the last responses. The oxoETEs also induce translocation of secretory vesicles from the cytosol to the plasmalemma, an effect not requiring cytokine priming. To study the mechanism of PMN activation in response to the eicosanoids, we examined the activation of mitogen-activated protein kinase (MAPK) and cytosolic phospholipase A2 (cPLA2). PMN expressed three proteins (40, 42, and 44 kDa) that reacted with anti-MAPK antibodies. The oxoETEs, LTB4, GM-CSF, and G-CSF all stimulated PMN to activate the MAPKs and cPLA2, as defined by shifts in these proteins' electrophoretic mobility and tyrosine phosphorylation of the MAPKs. However, the speed and duration of the MAPK response varied markedly depending on the stimulus. 5-OxoETE caused a very rapid and transient activation of MAPK. In contrast, the response to the cytokines was rather slow and persistent. PMN pretreated with GM-CSF demonstrated a dramatic increase in the extent of MAPK tyrosine phosphorylation and electrophoretic mobility shift in response to 5-oxoETE. Similarly, 5-oxoETE induced PMN to release some preincorporated [14C]arachidonic acid, while GM-CSF greatly enhanced the extent of this release. Thus, the synergism exhibited by these agents is prominent at the level of MAPK stimulation and phospholipid deacylation. Pertussis toxin, but not Ca2+ depletion, inhibited MAPK responses to 5-oxoETE and LTB4, indicating that responses to both agents are coupled through G proteins but not dependent upon Ca2+ transients. 15-OxoETE and 15(OH)-ETE were inactive while 5-oxo-15(OH)-ETE and 5(OH)-ETE had 3- and 10-fold less potency than 5-oxoETE, indicating a rather strict structural specificity for the 5-keto group. LY 255283, a LTB4 antagonist, blocked the responses to LTB4 but not to 5-oxoETE. Therefore, the oxoETEs do not appear to operate through the LTB4 receptor. In summary, the oxoETEs are potent activators of PMN that share some but not all activities with LTB4. The response to the oxoETEs is greatly enhanced by pretreatment with cytokines, indicating that combinations of these mediators may be very important in the pathogenesis of inflammation.

Dual effect of 1 alpha,25-dihydroxyvitamin D3 on hsp28 and PKC beta gene expression in phorbol ester-resistant human myeloid HL-525 leukemic cells

We investigated the effect of 1 alpha-25-dihydroxyvitamin D3 [1,25-(OH)2D3] on the expression of the 28-kDa heat shock protein gene (hsp28) and the protein kinase C beta gene (PKC beta) in the human myeloid HL-60 leukemic cell variant HL-525, which is resistant to phorbol ester-induced macrophage differentiation. Northern and western blot analysis showed little or no hsp28 gene expression in the HL-60 cell variant, HL-205, which is susceptible to such differentiation, while a relatively high basal level of hsp28 gene expression was observed in the HL-525 cells. However, both cell lines demonstrated heat shock-induced expression of this gene. During treatment with 50-300 nM 1,25-(OH)2D3, a marked reduction of hsp28 gene expression along with an induction of PKC beta gene expression was observed in HL-525 cells. A gel mobility-shift assay demonstrated that the 1,25-(OH)2D3-induced alteration of hsp28 gene expression was associated with decreased binding activity to the vitamin D3 receptor-vitamin D3 response element (VDR-VDRE), whereas binding to the heat shock transcription factor-heat shock element (HSF-HSE) was not altered. Our results suggest that the dual effect of 1,25-(OH)2D3 on hsp28 and PKC beta gene expression is due to the different sequence composition of the vitamin D response element in the promoter region as well as an accessory factor for each gene or that 1,25-(OH)2D3 increases PKC beta gene expression, which, in turn, negatively regulates the expression of the hsp28 gene or vice versa.

Protein kinase cascades activated by stress and inflammatory cytokines

Signal transduction pathways constructed around a core module of three consecutive protein kinases, the most distal being a member of the extracellular signal-regulated kinase (ERK) family, are ubiquitous among eukaryotes. Recent work has defined two cascades activated preferentially by the inflammatory cytokines TNF-alpha and IL-1-beta, as well as by a wide variety of cellular stresses such as UV and ionizing radiation, hyperosmolarity, heat stress, oxidative stress, etc. One pathway converges on the ERK subfamily known as the "stress activated' protein kinases (SAPKs, also termed Jun N-terminal kinases, JNKs), whereas the second pathway recruits the p38 kinases. Upstream inputs are diverse, and include small GTPases (primarily Rac and Cdc42; secondarily Ras) acting through mammalian homologs of the yeast Ste20 kinase, other kinase subfamilies (e.g. GC kinase) and ceramide, a putative second messenger for certain TNF-alpha actions. These two cascades signal cell cycle delay, cellular repair or apoptosis in most cells, as well as activation of immune and reticuloendothelial cells.

Altered expression of glomerular heat shock protein 27 in experimental nephrotic syndrome

Although nephrotic syndrome is a very common kidney disease, little is known about the molecular changes occurring within glomerular capillary loops during development of disease. The characteristic histologic change is retraction (effacement) of the distal "foot" processes of glomerular epithelial cells (GEC) which surround the capillary loops. The GEC foot processes are an essential part of the kidney's filtration barrier, and their structure is regulated primarily by actin microfilaments, cytoskeletal proteins present in high concentrations in foot processes. Actin polymerization has been reported to be regulated via phosphorylation of the low molecular weight heat shock protein, hsp27. We localized hsp27 within normal rat GECs using immunofluorescence and immunoelectron microscopy. Induction of nephrotic syndrome and GEC foot process effacement using the puromycin aminonucleoside rat model resulted in significant increases in: (a) renal cortical hsp27 mRNA expression (826 +/- 233%, x +/- SEM, P < 0.01 vs. control); (b) glomerular hsp27 protein expression (87 +/- 2%, P < 0.001 vs. control); and (c) glomerular hsp27 phosphorylation (101 +/- 32%, P < 0.05 vs. control). These findings support the hypothesis that hsp27, by regulating GEC foot process actin polymerization, may be important in maintaining normal foot process structure, and regulating pathophysiologic GEC cytoskeletal changes during development of nephrotic syndrome.

Identification of MAP kinase domains by redirecting stress signals into growth factor responses

Mitogen-activated protein kinase (MAPK) cascades, termed MAPK modules, channel extracellular signals into specific cellular responses. Chimeric molecules were constructed between p38 and p44 MAPKs, which transduce stress and growth factor signals, respectively. A discrete region of 40 residues located in the amono-terminal p38MAPK lobe directed the specificity of response to extracellular signals, whereas the p44MAPK chimera, expressed in vivo, redirected stress signals into early mitogenic responses, demonstrating the functional independence of these domains.

Interleukin-1 signal transduction

Interleukin-1 (IL-1) is primarily an inflammatory cytokine, although it is capable of mediating a wide variety of effects on many different cell types. Nearly every known signal transduction pathway has been reported to be activated in response to IL-1. However, the significance of many of these signaling events is unclear, due to the use of different and sometimes unique cell lines in studying IL-1-initiated signal transduction. Complicating matters further is the lack of association in many studies between identified IL-1-induced signals and subsequent biological responses. In this article, we review what is known about IL-1 receptor signaling and, whenever possible, correlate signaling events to biological responses.

Mitogen-activated protein kinase cascades and regulation of gene expression

Mitogen-activated protein kinases (MAPKs) are a group of serine/threonine specific, proline directed, protein kinases which are activated by a wide spectrum of extracellular stimuli. MAPK activation is achieved through kinase cascades, which include a MAPK kinase (MAPKK or MEK) and a MAPKK/MEK kinase (MAPKKK/MEKK). These cascades serve as information relays, connecting cell-surface receptors to specific transcription factors and other regulatory proteins, thus allowing extracellular signals to regulate the expression of specific genes. Genetic and biochemical analyses have revealed many tiers in the regulation of the activities of MAPKs, as well as different routes that lead to the activation of an individual MAPK. An emerging topic of great interest is the basis for specificity in the activation of individual MAPKs and their ability to recognize their substrates.

Phosphorylation of actin-binding protein 280 by growth factors is mediated by p90 ribosomal protein S6 kinase

Although Ras-related small GTPases are believed to control cell proliferation and motility through activation of protein kinase cascades, little is known about the intracellular protein targets of activated kinases. Here we show that the p90 ribosomal S6 kinase 2 (RSK2) phosphorylates actin-binding protein (ABP-280) in intact rat 3Y1 fibroblasts. Growth factors such as fetal calf serum, epidermal growth factor, phorbol 12-myristate 13-acetate, and lysophosphatidic acid stimulate the phosphorylation of serine residues in ABP-280 in quiescent 3Y1 cells. Extracts from 3Y1 cells prepared after stimulation by lysophosphatidic acid, fetal calf serum, and epidermal growth factor retain activated protein kinase activity(s) toward ABP-280 in vitro. ABP kinase activities in lysates from lysophosphatidic acid-stimulated 3Y1 cells can be fractionated by MonoQ anion exchange column chromatography into three peaks having ABP kinase activities. One (ABP kinase peak 1) coelutes with the peak of RSK2 as judged by immunoblotting and S6 peptide kinase assays. Two-dimensional phosphopeptide maps show RSK2 phosphorylated ABP-280 to be phosphorylated at the same site(s) as those stimulated by growth factors in vivo. Incubation of ABP kinase peak 1 fractionated from unstimulated cells with activated ERK2 activates latent ABP kinase activity. These results show RSK2 to phosphorylate ABP-280 in vivo.

Neurofibrillary tangles of Alzheimer's disease brains contain 14-3-3 proteins

The localisation of 14-3-3 proteins compared to that of tau and ubiquitin-protein conjugates in sections of hippocampus from Alzheimer's disease (AD) brains was examined by immunohistochemistry. In all cases (n = 10), anti-14-3-3 stained a proportion of neurofibrillary tangles (NFT). In general, NFT stained by anti-14-3-3 were smaller than those stained by anti-tau or anti-ubiquitin-protein conjugates and were more confined to the neuronal cell body. Occasionally, cortical Lewy bodies in cases of Lewy body dementia were also found to be 14-3-3-positive. Since 14-3-3 proteins are central to MAP kinase signalling, the results support the proposal that this pathway is in part responsible for the hyperphosphorylation of tau, which leads to the formation of the paired helical filaments seen in AD brains.

Mitogen-activated protein kinase (MAPK) in cardiac tissues

Mitogen-activated protein kinase (MAPK) has recently emerged as a prominent role player in intracellular signalling in the ventricular myocyte with attention being focussed on its possible role in the development of ventricular hypertrophy. It is becoming clear that MAPK is also active in other cells of cardiac origin such as cardiac fibroblasts and possible functions of this signalling pathway in the heart have yet to be explored. In this report the mammalian MAPK pathway is briefly outlined, before reviewing current knowledge of the MAPK pathway in cardiac tissue (ventricular myocytes, vascular smooth muscle cells and cardiac fibroblasts). New data is also presented on the presence and activity of MAPK in two additional cardiac celltypes namely atrial myocytes and vascular endothelial cells from the coronary microcirculation.

Identification of mitogen-activated protein (MAP) kinase-activated protein kinase-3, a novel substrate of CSBP p38 MAP kinase

CSBP p38 is a mitogen-activated protein kinase that is activated in response to stress, endotoxin, interleukin 1, and tumor necrosis factor. Using a catalytically inactive mutant (D168A) of human CSBP2 as the bait in a yeast two-hybrid screen, we have identified and cloned a novel kinase which shares approximately 70% amino acid identity to mitogen-activated protein kinase-activated protein kinase (MAPKAP kinase)-2, and thus was designated MAPKAP kinase-3. The binding of CSBP to MAPKAP kinase-3 was confirmed in vitro by the precipitation of epitope-tagged CSBP1, CSBP2, and CSBP2(D168A) and endogenous CSBP from mammalian cells by a bacterially expressed GST-MAPKAP kinase-3 fusion protein and in vivo by co-precipitation of the epitope-tagged proteins co-expressed in HeLa cells. MAPKAP kinase-3 was phosphorylated by both CSBP1 and CSBP2 and was then able to phosphorylate HSP27 in vitro. Treatment of HeLa cells with sorbitol or TNF resulted in activation of CSBP and MAPKAP kinase-3 and activation of MAPKAP kinase-3 could be blocked by preincubation of cells with SB203580, a specific inhibitor of CSBP kinase activity. These data suggest that MAPKAP kinase-3 is activated by stress and cytokines and is a novel substrate of CSBP both in vitro and in vivo.

SAP kinase-3, a new member of the family of mammalian stress-activated protein kinases

Stress-activated protein kinases are MAP kinase homologues that are activated by cellular stresses, bacterial endotoxin and inflammatory cytokines. They are activated by a dual threonine/tyrosine phosphorylation within a TPY sequence in the case of stress-activated protein kinase-1 and its isoforms (also called JNKs) or a TGY sequence in the case of stress-activated protein kinase-2 and its isoforms (also called p38, p40, RK, CSBPs, XMpk2 and Mxi2). Here we report the cloning and sequencing of a new protein kinase from rat with a TGY sequence in the activation domain. This stress-activated protein kinase-3 is 60% identical to mouse stress-activated protein kinase-2 and 45% identical to HOG1 from Saccharomyces cerevisiae. Transcripts encoding stress-activated protein kinase-3 are widely expressed, with high levels in skeletal muscle.

src-related tyrosine kinases regulate transcriptional activation of the interstitial collagenase gene, MMP-1, in interleukin-1-stimulated synovial fibroblasts

OBJECTIVE

To characterize tyrosine kinases that contribute to the transcription of interstitial collagenase.

METHODS

Four thousand six hundred fourteen basepairs of the rabbit collagenase promoter region were cloned and sequenced. Plasmids containing collagenase promoter fragments linked to the luciferase reporter gene were transiently transfected into primary rabbit synovial fibroblasts. Regulation of gene activation by inflammatory mediators and tyrosine kinase inhibitors was assessed. To identify specific tyrosine kinases that contribute to collagenase gene expression, v-src was transiently expressed in rabbit synovial fibroblasts along with collagenase promoter constructs, and basal and interleukin-(IL-l)-induced collagenase transcription was assayed.

RESULT

An inhibitor of src-related tyrosine kinases, herbimycin A, inhibited increases of collagenase messenger RNA in IL-1- and phorbol myristate acetate-treated fibroblasts. Transcriptional activation of collagenase by IL-1 was also inhibited by herbimycin A. Expression of v-src in synovial fibroblasts enhanced basal and IL-1-inducible transcription.

CONCLUSION

Activation of collagenase transcription by inflammatory mediators involves activation of an src-related tyrosine kinase.

Micro-trap phosphorylation assay of mitogen-activated protein (MAP) kinases to detect their activation by lipopolysaccharides

We designed a microplate-based assay method for mitogen-activated protein (MAP) kinase. Using anion-exchanger resin, MAP kinases from murine macrophages were partially purified in 96-well plates. The activities of these purified enzymes correlated well with those detected in heretofore used assays. The micro-trap phosphorylation assay has advantages over conventional methods (immunoprecipitation, Western blotting for the detection of mobility shift, or kinase detection assay in myelin basic protein (MBP)-containing gel), in terms of sensitivity, economy and rapid execution for hundreds of samples. Using micro-trap phosphorylation assay, it was demonstrated that MAP kinase activities in macrophages were persistently increased by lipopolysaccharide (LPS) stimulation, and this activation was inhibited by polymyxin B or tyrosine kinase inhibitors. This method is expected to give a wide range of application, such as determining effects of drug inhibitors or antisense oligonucleotides on MAP kinases, or measuring the various protein kinases after specificity controls were done.

Role for p38 mitogen-activated protein kinase in platelet aggregation caused by collagen or a thromboxane analogue

p38 mitogen-activated protein kinase (MAPK) was identified in platelets on the basis of (a) its reactivity with antibodies to C-terminal and N-terminal peptides, and (b) its ability to activate MAPK-activated protein kinase-2, which phosphorylates the small heat shock protein, hsp27. p38 MAPK was activated in platelets by collagen fibers, a collagen-related cross-linked peptide, thrombin, or the thromboxane analogue U46619. A highly specific inhibitor of p38 MAPK, a pyridinyl imidazole known as SB203580, inhibited the platelet enzyme in vitro (IC50 approximately 0.5 microM). At similar concentrations it also inhibited agonist-stimulated phosphorylation of hsp27 in platelets, and platelet aggregation and secretion induced by minimal aggregatory concentrations of collagen or U46619, but not thrombin. Inhibition of aggregation was overcome by increasing agonist dose. SB203580 might act by inhibiting thromboxane generation, but this was only inhibited by 10-20% at low agonist concentrations. p38 MAPK provides a crucial signal, which is necessary for aggregation caused by minimal concentrations of collagen fibers or U46619. Thrombin or high doses of these agonists generate signals that bypass the enzyme, or render the enzyme no longer rate-limiting.

Cellular stresses and cytokines activate multiple mitogen-activated-protein kinase kinase homologues in PC12 and KB cells

The identities of the upstream activators of the mitogen-activated protein (MAP) kinase homologues termed stress-activated-protein (SAP) kinase-1 (also known as JNK or SAPK) and SAP kinase-2 (also known as p38, RK and CSBP) were investigated in rat PC12 cells and human KB cells after exposure to cellular stresses and cytokines. In PC12 cells, the same two upstream activators, SAP kinase kinase-1 (SAPKK-1) and SAPKK-2 were activated after exposure to osmotic shock, ultraviolet irradiation or the protein synthesis inhibitor anisomycin, and more weakly in response to sodium arsenite. SAPKK-1 was capable of activating both SAP kinase-1 and SAP kinase-2 and was similar, if not identical, to the previously described MAP kinase kinase homologue MKK4, as judged by immunological criteria and by its ability to be activated by MEK kinase in vitro. In contrast, SAPKK-2 activated SAP kinase-2, but not SAP kinase-1 in vitro. In KB cells, five distinct upstream activators of SAP kinase-1 and SAP kinase-2 were induced, namely SAPKK-1, SAPKK-2, SAPKK-3, SAPKK-4 and SAPKK-5, whose appearance depended on the nature of the stimulus. SAPKK-3, which was strongly induced by every stimulus tested (osmotic shock, ultraviolet irradiation, anisomycin or IL-1), accounted for about 95% of the SAP kinase-2 activator activity in these cells, did not activate SAP kinase-1 and eluted from Mono S at a lower salt concentration than SAPKK-2. SAPKK-4 and SAPKK-5 were also eluted from Mono S at higher NaC1 concentrations than SAPKK-3 and these enzymes activated SAP kinase-1 but not SAP kinase-2. SAPKK-4 was the only SAP kinase-1 activator induced by interleukin-1 or ultraviolet irradiation, while two SAP kinase-1 activators, SAPKK-1 and SAPKK-5, were induced by osmotic shock or anisomycin. SAPKK-2, SAPKK-3, SAPKK-4 and SAPKK-5, were not activated by MEK kinase in vitro, were separable from the major activator(s) of p42 MAP kinase, and were not recognised by anti-MKK4 antibodies. At least two of these enzymes are likely to be novel MAP kinase kinase homologues. Our results demonstrate unexpected complexity in the upstream regulation of stress and cytokine-stimulated kinase cascades and indicate that the selection of the appropriate SAPKK varies with both the stimulus and the cell type.

3pK, a new mitogen-activated protein kinase-activated protein kinase located in the small cell lung cancer tumor suppressor gene region

NotI linking clones, localized to the human chromosome 3p21.3 region and homozygously deleted in small cell lung cancer cell lines NCI-H740 and NCI-H1450, were used to search for a putative tumor suppressor gene(s). One of these clones, NL1G210, detected a 2.5-kb mRNA in all examined human tissues, expression being especially high in the heart and skeletal muscle. Two overlapping cDNA clones containing the entire open reading frame were isolated from a human heart cDNA library and fully characterized. Computer analysis and a search of the GenBank database to reveal high sequence identity of the product of this gene to serine-threonine kinases, especially to mitogen-activated protein kinase-activated protein kinase 2, a recently described substrate of mitogen-activated kinases. Sequence identitiy was 72% at the nucleotide level and 75% at the amino acid level, strongly suggesting that this protein is a serine-threonine kinase. Here we demonstrate that the new gene, referred to as 3pK (for chromosome 3p kinase), in fact encodes a mitogen-activated protein kinase-regulated protein serine-threonine kinase with a novel substrate specificity.

Negative regulators of the interleukin-1 system: receptor antagonists and a decoy receptor

The IL-1 system includes 2 agonists, alpha and beta, processing and transport molecules, receptor antagonists, signalling receptor, a decoy receptor and an accessory molecule. Negative pathways of regulation include the antagonists, of which 3 isoforms have been cloned and the type II "decoy" receptor. Molecules that regulate inflammation and immunity coordinatively affect different components of the system. The complexity of the system and the existence of unique pathways of negative regulation, the antagonists and the decoy receptor, emphasize the need for a tight control of the production and action of IL-1.

MKK3- and MKK6-regulated gene expression is mediated by the p38 mitogen-activated protein kinase signal transduction pathway

The p38 mitogen-activated protein (MAP) kinase signal transduction pathway is activated by proinflammatory cytokines and environmental stress. The detection of p38 MAP kinase in the nucleus of activated cells suggests that p38 MAP kinase can mediate signaling to the nucleus. To test this hypothesis, we constructed expression vectors for activated MKK3 and MKK6, two MAP kinase kinases that phosphorylate and activate p38 MAP kinase. Expression of activated MKK3 and MKK6 in cultured cells caused a selective increase in p38 MAP kinase activity. Cotransfection experiments demonstrated that p38 MAP kinase activation causes increased reporter gene expression mediated by the transcription factors ATF2 and Elk-1. These data demonstrate that the nucleus is one target of the p38 MAP kinase signal transduction pathway.

Reconstitution of novel signalling cascades responding to cellular stresses

Mammalian cells respond to their immediate environment by inducing signal transduction cascades that regulate metabolism, secretion and gene expression. Several of these signalling pathways are structurally and organizationally related insofar as they require activation of a protein-serine kinase via it's phosphorylation on tyrosine and threonine; the archetype being mitogen-activated protein kinase (MAPK) which responds primarily to mitogenic stimuli via Ras. In contrast, two more recently identified cascades are responsive to cellular stresses such as heat, inflammatory cytokines, ischaemia and metabolic poisons. The recent identification of the components of these pathways has allowed manipulation of the stress-responsive pathways and evaluation of their physiological roles. These studies reveal a high degree of independence between the pathways not apparent from in vitro studies. Manipulation of the pathways in vivo will likely result in novel therapies for inflammatory disease and reperfusion injury.

Feedback regulation of map kinase signal pathways

Ste7 is a MEK (MAPK/ERK kinase) family member that functions in the pheromone induced mating response pathway of Saccharomyces cerevisiae. We analysed the catalytic competence and in vivo function of Ste7 variants that have alterations of stimulatory and feedback phosphorylation sites. These analyses led us to unanticipated insights into two separate feedback mechanisms that impede the output of the mating response MAPK activation pathway.

Stress signal, mediated by a Hog1-like MAP kinase, controls sexual development in fission yeast

We identified the phh1+ gene that encodes a MAP kinase as the effector of Wis1 MAP kinase kinase in fission yeast, which is highly homologous with HOG1 of S. cerevisiae. Heterothalic phh1 dsiruptant is phenotypically indistinguishable from wis1 deletion mutant, both displaying the same extent of partial sterility and enhanced sensitivity to a variety of stress. In phh1 disruptant, nitrogen starvation-induced expression of ste11+, a key controller of sexual differentiation, is markedly diminished. Ectopic expression of ste11+ effectively restores fertility, but not stress resistance, to the phh1 disruptant. These data show that stress signal, mediated by a MAP kinase, is required for efficient start of sexual differentiation.

Differential activation of ERK, JNK/SAPK and P38/CSBP/RK map kinase family members during the cellular response to arsenite

Exposure of cells to either proliferative or stressful stimuli elicits a complex response involving one or more distinct phosphorylation cascades culminating in the activation of multiple members of the mitogen-activated protein kinase (MAPK) family, including extracellular signal regulated kinase (ERK), stress-activated c-Jun N-terminal kinase (JNK/SAPK), and p38/RK/CSBP protein kinase. While the pathways transducing mitogenic stimuli to these kinases are relatively well established, the early signalling events leading to their activation in response to stress are poorly understood. In the present study, we examined ERK, JNK/SAPK, and p38 activation in cells treated with the sulfhydryl-reactive agent sodium arsenite. Arsenite treatment potently activated both JNK/SAPK and p38, but only moderately activated ERK. Activation of all three kinases was prevented by the free radical scavenger N-Acetyl-L-cysteine, suggesting that an oxidative signal initiates the responses. Suramin, a growth factor receptor poison, significantly inhibited ERK activation by arsenite, but had little effect on either JNK/SAPK or p38 activity. In contrast, suramin inhibited the activation of all three kinases by short wavelength ultraviolet light (UVC) irradiation. In addition, comparative studies with wild-type PC12 cells and PC12 cells expressing a dominant negative Ras mutant allele indicated that arsenite activates ERK primarily through a Ras-dependent pathway(s), while activation of both JNK/SAPK and p38 occurs through a mechanism relatively independent of Ras. These results suggest that JNK/SAPK and p38 may share common upstream regulators distinct from those involved in ERK activation.

Heat-shock induced protein modifications and modulation of enzyme activities

Upon heat stress, the cell physiology is profoundly altered. The extent of the alterations depends on the severity of the stress and may lead to cell death. The heat shock response is an array of metabolic changes characterized by the impairment of major cellular functions and by an adaptative reprogramming of the cell metabolism. The enhanced synthesis of the HSPs is a spectacular manifestation of this reprogramming. Numerous post translational modifications of proteins occur in response to heat stress and can be related to altered cellular functions. Some proteins are heat-denatured and temporarily inactivated. Heat-denaturation is reversible, chaperones may contribute to the repair. The extent of heat-denaturation depends on the cell metabolism: (a) it is attenuated in thermotolerant cells or in cells overexpressing the appropriate chaperones (b) it is enhanced in energy-deprived cells. Covalent modifications may also rapidly alter protein function. Changes in protein glycosylation, methylation, acetylation, farnesylation, ubiquitination have been found to occur during stress. But protein phosphorylation is the most studied modification. Several protein kinase cascades are activated, among which the various mitogen activated protein kinase (MAP kinase) cascades which are also triggered by a wide range of stimuli. As a possible consequence, stress modifies the phosphorylation status and the activity of components from the transcriptional and translational apparatuses. The same kinases also target key enzymes of the cellular metabolism. Protein denaturation results in constitutive hsp titration, this titration is a signal to trigger the heat-shock gene transcription and to activate some of the protein kinase cascades.

Osmotic regulation of cytokine synthesis in vitro

These studies were undertaken to investigate the therapeutic mechanism of saturated solutions of KI, used to treat infectious and inflammatory diseases. The addition of 12-50 mM KI to cultured human peripheral blood mononuclear cells resulted in 319-395 mosM final solute concentration and induced interleukin (IL)-8 synthesis. Maximal IL-8 production was seen when 40 mM salt was added (375 mosM) and was equal to IL-8 induced by endotoxin or IL-1 alpha. However, there was no induction of IL-1 alpha, IL-1 beta, or tumor necrosis factor to account for the synthesis of IL-8; the effect of KI was not due to contaminating endotoxins. Hyperosmolar NaCl also induced IL-8 and increased steady-state levels of IL-8 mRNA similar to those induced by IL-1 alpha. IL-8 gene expression was elevated for 96 hr in peripheral blood mononuclear cells incubated with hyperosmolar NaCl. In human THP-1 macrophagic cells, osmotic stimulation with KI, NaI, or NaCl also induced IL-8 production. IL-1 signal transduction includes the phosphorylation of the p38 mitogen-activated protein kinase that is observed following osmotic stress. Using specific blockade of this kinase, a dose-response inhibition of hyperosmolar NaCl-induced IL-8 synthesis was observed, similar to that in cells stimulated with IL-1. Thus, these studies suggest that IL-1 and osmotic shock utilize the same mitogen-activated protein kinase for signal transduction and IL-8 synthesis.

Studies into the effect of the tyrosine kinase inhibitor herbimycin A on NF-kappa B activation in T lymphocytes. Evidence for covalent modification of the p50 subunit

The tyrosine kinase inhibitor herbimycin A was found to block NF-kappa B stimulation in response to interleukin-1 and phorbol 12-myristate 13-acetate in EL4.NOB-1 thymoma cells and phorbol 12-myristate 13-acetate in Jurkat T lymphoma cells. The effect appeared not to involve inhibition of tyrosine kinase activation as neither interleukin-1 nor phorbol 12-myristate 13-acetate induced major changes in tyrosine phosphorylation in EL4.NOB-1 or Jurkat cells, respectively. Herbimycin A did not interfere with I kappa B-alpha degradation, and in unstimulated cells, it modified NF-kappa B prior to chemical dissociation with sodium deoxycholate. Because herbimycin A is thiol-reactive, we suspected that the target was the p50 subunit of NF-kappa B, which has a key thiol at cysteine 62. Herbimycin A inhibited DNA binding when added to nuclear extracts prepared from stimulated cells, which were shown to contain high levels of p50. Incubation of herbimycin A with 2-mercaptoethanol attenuated the effect. Herbimycin A was also shown to react directly with p50, blocking its ability to bind to the NF-kappa B consensus sequence. However, a mutant form of p50 in which cysteine 62 was mutated to serine was insensitive to herbimycin A. Finally, we demonstrated that the compound inhibited the expression of interleukin-2 (an NF-kappa B-regulated gene) in EL4.NOB-1 cells. These data therefore suggest that herbimycin A inhibits NF-kappa B by modifying the p50 subunit on cysteine 62 in the NF-kappa B complex, which blocks DNA binding and NF-kappa B-driven gene expression. The results urge caution in the use of herbimycin A as a specific tyrosine kinase inhibitor and suggest that the development of agents that selectively modify p50 may have potential as a means of inhibiting NF-kappa B-dependent gene transcription.

Mechanism of action of the immunosuppressant rapamycin

Rapamycin has potent immunosuppressive properties reflecting its ability to disrupt cytokine signaling that promotes lymphocyte growth and differentiation. In IL-2-stimulated T cells, rapamycin impedes progression through the G1/S transition of the proliferation cycle, resulting in a mid-to-late G1 arrest. Two major biochemical alterations underlie this mode of action. The first one affects the phosphorylation/activation of the p70 S6 kinase (p70s6k), an early event of cytokine-induced mitogenic response. By inhibiting this enzyme, whose major substrate is the 40S ribosomal subunit S6 protein, rapamycin reduces the translation of certain mRNA encoding for ribosomal proteins and elongation factors, thereby decreasing protein synthesis. A second, later effect of rapamycin in IL-2-stimulated T cells is an inhibition of the enzymatic activity of the cyclin-dependent kinase cdk2-cyclin E complex, which functions as a crucial regulator of G1/S transition. This inhibition results from a prevention of the decline of the p27 cdk inhibitor, that normally follows IL-2 stimulation. To mediate these biochemical alterations, rapamycin needs to bind to intracellular proteins, termed FKBP, thereby forming a unique effector molecular complex. However, neither(p70s6k) inhibition, nor p27-induced cdk2-cyclin E inhibition are directly caused by the FKBP-rapamycin complex. Instead, this complex physically interacts with a novel protein, designated "mammalian target of rapamycin" (mTOR), which has sequence homology with the catalytic domain of phosphatidylinositol kinases and may therefore be itself a kinase. mTOR may act upstream of (p70s6K) and cdk2-cyclin E in a linear or bifurcated pathway of growth regulation. Molecular dissection of this pathway should further unravel cytokine-mediated signaling processes and help devise new immunosuppressants.

Activation of mitogen-activated protein kinase by heat shock treatment in Drosophila

Heat shock treatment of Drosophila melanogaster tissue culture cells causes increased tyrosine phosphorylation of several 44 kDa proteins, which are identified as Drosophila mitogen-activated protein (MAP) kinases. Tyrosine phosphorylation occurs within 5 min, and is maintained at high levels during heat shock. It decreases to basal levels during recovery, concurrent with the repression of heat shock transcription and heat-shock-protein synthesis. The increased MAP kinase tyrosine phosphorylation is parallelled by increased MAP kinase activity. At least two MAP kinases, DmERK-A and DmERK-B, are identified whose tyrosine phosphorylation increases during heat shock. Thus MAP kinase activation is an immediate early response to heat shock, and its increased activity is maintained throughout heat shock treatment. Protracted MAP kinase activation may contribute to heat shock transcription factor phosphorylation and the numerous metabolic alterations that constitute the heat-shock response.

Intracellular reactive oxygen species as apparent modulators of heat-shock protein 27 (hsp27) structural organization and phosphorylation in basal and tumour necrosis factor alpha-treated T47D human carcinoma cells

The small stress protein heat-shock protein 27 (hsp27) is an oligomeric phosphoprotein, constitutively expressed in most human cells, which enhances cellular resistance to tumour necrosis factor alpha (TNF alpha). This phenomenon correlates with dramatic changes in hsp27 cellular location, structural organization and phosphorylation. To gain a better understanding of the molecular mechanisms regulating these properties of hsp27, we investigated whether they were a consequence of the intracellular production of reactive oxygen species (ROS) generated by TNF alpha. Here, we report that, in T47D carcinoma cell lines, the rapid burst of intracellular ROS production and changes in hsp27 locale, structural organization and phosphoisoform composition induced by TNF alpha were abolished by the overexpression of the antioxidant enzyme seleno-glutathione peroxidase (GSHPx). These effects were greatly diminished when GSHPx-expressing cells were grown in the absence of selenium, a cofactor that is essential for seleno-GSHPx activity, indicating that they are directly linked to the increased GSHPx activity. Moreover, in growing T47D cells, GSHPx expression induced intracellular redistribution of hsp27 and decreased the phosphorylation of this protein without altering its pattern of oligomerization. In contrast, the heat-mediated phosphorylation of hsp27 was not altered by decreased intracellular ROS levels. Hence, in growing and TNF-treated cells, several hsp27 properties appear to be modulated by fluctuations in intracellular ROS levels.

Mitogen and stress response pathways: MAP kinase cascades and phosphatase regulation in mammals and yeast

Evolutionarily conserved from yeast to man, mitogen-activated protein kinase (MAPK) pathways respond to a variety of disparate signals which induce differentiation, proliferation, or changes in intracellular enzyme regulation. Recent advances have identified two new mammalian MAPK relatives, JNK1 and p38, and the pathways which are responsible for their activation.

Interleukin-1 signal transduction

Interleukin 1 (IL1) is a potent pro-inflammatory cytokine which has been implicated in the pathogenesis of chronic inflammatory disease. Despite much effect, the signal transduction pathway activated by IL1 has remained obscure. Recently, much attention has focussed on IL1 receptors and early events triggered by IL1 in cells, including activation of transcription factors and serine/threonine protein kinases. Two main types of IL1 receptors have been described, IL1RI and IL1RII. They appear to belong to a family of proteins which include most notably a Drosophila protein, Toll. Following receptor binding IL1 has been shown to increase protein phosphorylation in cells, and much effort has been made to identify the protein kinases responsible. Novel enzymes have been discovered, including a family of MAP kinase--like enzymes which are also activated by a range of stresses such as hypertonic stress and heat shock. Attention has also been focussed in the activation of the transcription factor NF kappa B, which is rapidly activated by IL1. This review will describe our current understanding of how IL1 activated cells and will particularly describe more recent work on IL1 receptors and early post-receptors events.

Opposing effects of ERK and JNK-p38 MAP kinases on apoptosis

Apoptosis plays an important role during neuronal development, and defects in apoptosis may underlie various neurodegenerative disorders. To characterize molecular mechanisms that regulate neuronal apoptosis, the contributions to cell death of mitogen-activated protein (MAP) kinase family members, including ERK (extracellular signal-regulated kinase), JNK (c-JUN NH2-terminal protein kinase), and p38, were examined after withdrawal of nerve growth factor (NGF) from rat PC-12 pheochromocytoma cells. NGF withdrawal led to sustained activation of the JNK and p38 enzymes and inhibition of ERKs. The effects of dominant-interfering or constitutively activated forms of various components of the JNK-p38 and ERK signaling pathways demonstrated that activation of JNK and p38 and concurrent inhibition of ERK are critical for induction of apoptosis in these cells. Therefore, the dynamic balance between growth factor-activated ERK and stress-activated JNK-p38 pathways may be important in determining whether a cell survives or undergoes apoptosis.

Fluorescence resonance energy transfer reveals interleukin (IL)-1-dependent aggregation of IL-1 type I receptors that correlates with receptor activation

Fluorescence resonance energy transfer (FRET) was used to investigate whether interleukin-1 (IL-1) causes the aggregation of IL-1 type I receptors (IL-1 RI) at the cell surface. For these experiments, a noncompetitive anti-IL1 RI monoclonal antibody, M5, was labeled separately with a donor probe, fluorescein isothiocyanate, or with an acceptor carbocyanine probe, Cy3. Donor-labeled M5 and acceptor-labeled M5 were simultaneously bound to transfected mouse IL-1 RI on either C-127 mouse mammary carcinoma cells or on Chinese hamster ovary (CHO)-K1 cells, and the ratio of acceptor emission at 590 nm to donor emission at 525 nm (excitation at 488 and 514 nm) was monitored with flow cytometry as an indicator of FRET. Addition of a saturating concentration of human IL-1 alpha at 22 degrees C causes a time-dependent increase in FRET for both cell lines that indicates IL-1-dependent self-association of IL-1 RI. Binding of the IL-1 receptor antagonist at 22 degrees C causes little or no FRET for both cell lines, indicating a correlation between receptor aggregation and the ability of the ligand to stimulate a functional response. When donor-labeled and acceptor-labeled Fab fragments of M5 are used to monitor FRET, IL-1 alpha causes efficient energy transfer in the CHO-K1 cells at 22 degrees C, but not at 4 degrees C. In contrast, IL-1 alpha causes much less FRET at 22 degrees C in C-127 cells when the M5 Fab fragments are used instead of the intact bivalent M5. In a striking parallel, IL-1 alpha-dependent activation of prostaglandin E2 production depends on the bivalent M5 antibody in the C-127 cells, but is independent of this monoclonal antibody in the CHO-K1 cells. These results provide a strong correlation between the ability of IL-1 to cause the aggregation of IL-1 RI and the stimulation of a functional response.

Thrombin induces activation of p38 MAP kinase in human platelets

In human platelets a proline-directed kinase distinct from the ERK MAP kinases is stimulated by both thrombin and the thrombin receptor agonist peptide SFLLRN and may be involved in the activation of Ca(2+)-dependent cytosolic phospholipase A2 (Kramer, R. M., Roberts, E. F., Hyslop, P. A., Utterback, B. G., Hui, K. Y., and Jakubowski, J.A. (1995) J. Biol. Chem. 270, 14816-14823). Here we show that this kinase is identical with or closely related to p38 (the mammalian homolog of HOG1 from yeast), a recently discovered protein kinase typically activated by inflammatory cytokines and environmental stress. Further, we demonstrate that activation of this kinase by thrombin is transient (with maximal stimulation at 1 min), is accompanied by tyrosine phosphorylation, and precedes the activation of the ERK kinases. This is the first report to show that p38 kinase is activated by thrombin and to suggest a role for this MAP kinase in the thrombin-mediated signaling events during platelet activation.

Constitutive activation of mitogen-activated protein kinase-activated protein kinase 2 by mutation of phosphorylation sites and an A-helix motif

A recently described downstream target of mitogen-activated protein kinases (MAPKs) is the MAPK-activated protein (MAPKAP) kinase 2 which has been shown to be responsible for small heat shock protein phosphorylation. We have analyzed the mechanism of MAPKAP kinase 2 activation by MAPK phosphorylation using a recombinant MAPKAP kinase 2-fusion protein, p44MAPK and p38/40MAPK in vitro and using an epitope-tagged MAPKAP kinase 2 in heat-shocked NIH 3T3 cells. It is demonstrated that, in addition to the known phosphorylation of the threonine residue carboxyl-terminal to the catalytic domain, Thr-317, activation of MAPKAP kinase 2 in vitro and in vivo is dependent on phosphorylation of a second threonine residue, Thr-205, which is located within the catalytic domain and which is highly conserved in several protein kinases. Constitutive activation of MAPKAP kinase 2 is obtained by replacement of both of these threonine residues by glutamic acid. A constitutively active form of MAPKAP kinase 2 is also obtained by deletion of a carboxyl-terminal region containing Thr-317 and the A-helix motif or by replacing the conserved residues of the A-helix. These data suggest a dual mechanism of MAPKAP kinase 2 activation by phosphorylation of Thr-205 inside the catalytic domain and by phosphorylation of Thr-317 outside the catalytic domain involving an autoinhibitory A-helix motif.

Neither ERK nor JNK/SAPK MAP kinase subtypes are essential for histone H3/HMG-14 phosphorylation or c-fos and c-jun induction

The effects of EGF, TPA, UV radiation, okadaic acid and anisomycin on ERK and JNK/SAPK MAP kinase cascades have been compared with their ability to elicit histone H3/HMG-14 phosphorylation and induce c-fos and c-jun in C3H 10T1/2 cells. EGF and UV radiation activate both ERKs and JNK/SAPKs but to markedly different extents; EGF activates ERKs more strongly than JNK/SAPKs, whereas UV radiation activates JNK/SAPKs much more strongly than ERKs. Anisomycin and okadaic acid activate JNK/SAPKs but not ERKs, and conversely, TPA activates ERKs but not JNK/SAPKs. Nevertheless, all these agents elicit phosphorylation of ribosomal and pre-ribosomal S6, histone H3 and HMG-14, and the induction of c-fos and c-jun, showing that neither cascade is absolutely essential for these responses. We then analysed the relationship between ERKs, JNK/SAPKs and the transcription factors Elk-1 and c-Jun, implicated in controlling c-fos and c-jun, respectively. JNK/SAPKs bind to GST-cJun1-79, and ERKs, particularly ERK-2, to GST-Elk1(307–428); there is no cross-specificity of binding. Further, GST-Elk1(307–428) binds preferentially to active rather than inactive ERK-2. In vitro, JNK/SAPKs phosphorylate both GST-cJun1-79 and GST-Elk1(307–428), whereas ERKs phosphorylate GST-Elk1(307–428) but not GST-cJun1-79. Thus, neither ERKs nor JNK/SAPKs are absolutely essential for nuclear signalling and c-fos and c-jun induction. The data suggest either that activation of a single MAP kinase subtype is sufficient to elicit a complete nuclear response, or that other uncharacterised routes exist.

Detection of interleukin-1 signal transduction inhibitors: action of protein kinase inhibitors

The dysregulated production of IL-1 has been shown to play an important role in many pathological processes. Despite the apparent value of compounds able to inhibit either the secretion of IL-1 or its signal transduction pathway in a specific manner, there are no such compounds suitable for clinical use. A major problem in identifying novel and specific inhibitors of signal transduction is the lack of knowledge of the intracellular events which mediate the cellular actions of IL-1. In this study a simple cellular assay has been established to screen natural product and synthetic compound libraries for low molecular weight inhibitors of the cytokine signalling pathways of potential therapeutic value. In addition, we have studied the action of several known modulators of signal transduction on the actions of IL-1.

The activation of distinct mitogen-activated protein kinase cascades is required for the stimulation of 2-deoxyglucose uptake by interleukin-1 and insulin-like growth factor-1 in KB cells

The uptake of 2-deoxyglucose into KB cells was stimulated about 2-fold by interleukin-1 (IL1), anisomycin or insulin-like growth factor-1 (IGF1). Stimulation by IL1 and anisomycin was prevented by SB 203580, a specific inhibitor of the mitogen-activated protein (MAP) kinase homologue termed 're-activating kinase' [RK; also known as p38, p40 and CSBP (cytokine synthesis anti-inflammatory-drug-binding protein)], but was unaffected by PD 98059, a specific inhibitor of the activation of the classical MAP kinase pathway. In contrast, the stimulation of 2-deoxyglucose uptake by IGF1 was blocked by PD 98059 and unaffected by SB 203580. Consistent with these observations, IL1 and anisomycin were potent activators of MAP kinase-activated protein (MAPKAP) kinase-2, a physiological substrate of RK, whereas IGF1 was only a very weak activator of MAPKAP kinase-2. Conversely, IGF1 was a stronger activator of p42 MAP kinase than IL1 or anisomycin. These results imply that the activation of distinct MAP kinase pathways is required for the stimulation of glucose transport by IL1/anisomycin and IGF1 in KB cells, and suggest that the combined use of SB 203580 and PD 98059 is a powerful new approach to explore the roles of different MAP kinase cascades in cell regulation.

Activation of the SAPK pathway by the human STE20 homologue germinal centre kinase

Eukaryotic cells respond to different extracellular stimuli by recruiting homologous signalling pathways that use members of the MEKK, MEK and ERK families of protein kinases. The MEKK-->MEK-->ERK core pathways of Saccharomyces cerevisiae may themselves be regulated by members of the STE20 family of protein kinases. Here we report specific activation of the mammalian stress-activated protein kinase (SAPK) pathway by germinal centre kinase (GCK), a human STE20 homologue. SAPKs, members of the ERK family, are activated in situ by inflammatory stimuli, including tumour-necrosis factor (TNF) and interleukin-1, and phosphorylate and probably stimulate the transactivation function of c-Jun. Although GCK is found in many tissues, its expression in lymphoid follicles is restricted to the cells of the germinal centre, where it may participate in B-cell differentiation. Activation of the SAPK pathway by GCK illustrates further the striking conservation of eukaryotic signalling mechanisms and defines the first physiological function of a mammalian Ste20.

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.

Cloning and characterization of a human protein kinase with homology to Ste20

A human protein kinase (termed MST1) has been cloned and characterized. The MST1 catalytic domain is most homologous to Ste20 and other Ste20-like kinases (62-65% similar). MST1 is expressed ubiquitously, and the MST1 protein is present in all human cell lines examined. Biochemical characterization of MST1 catalytic activity demonstrates that it is a serine/threonine kinase, and that it can phosphorylate an exogenous substrate as well as itself in an in vitro kinase assay. Further characterization of the protein indicates MST1 activity increases approximately 3-4-fold upon treatment with PP2A, suggesting that MST1 is negatively regulated by phosphorylation. MST1 activity decreases approximately 2-fold upon treatment with epidermal growth factor; however, overexpression of MST1 does not affect extracellular signal-regulated kinase-1 and -2 activation. MST1 is unaffected by heat shock or high osmolarity, indicating that it is not involved in the stress-activated or high osmolarity glycerol signal transduction pathways. Thus MST1, although homologous to a member of a yeast MAPK cascade, is not involved in the regulation of a known mammalian MAPK pathway and potentially regulates a novel signaling cascade.