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

Different nuclear signals are activated by the B cell receptor during positive versus negative signaling

It is not known how immunogenic versus tolerogenic cellular responses are signaled by receptors such as the B cell antigen receptor (BCR). Here we compare BCR signaling in naive cells that respond positively to foreign antigen and self-tolerant cells that respond negatively to self-antigen. In naive cells, foreign antigen triggered a large biphasic calcium response and activated nuclear signals through NF-AT, NF-kappa B, JNK, and ERK/pp90rsk. In tolerant B cells, self-antigen stimulated low calcium oscillations and activated NF-AT and ERK/pp90rsk but not NF-kappa B or JNK. Self-reactive B cells lacking the phosphatase CD45 did not exhibit calcium oscillations or ERK/pp90rsk activation, nor did they repond negatively to self-antigen. These data reveal striking biochemical differences in BCR signaling to the nucleus during positive selection by foreign antigens and negative selection by self-antigens.

Interleukin 8-stimulated phosphatidylinositol-3-kinase activity regulates the migration of human neutrophils independent of extracellular signal-regulated kinase and p38 mitogen-activated protein kinases

Chemoattractants and chemokines, such as interleukin 8 (IL-8), are defined by their ability to induce directed cell migration of responsive cells. The signal transduction pathway(s) leading to cell migration remain ill defined. We demonstrate that phosphatidylinositol-3-kinase (PI3K) activity, as determined by inhibition using wortmannin and LY294002, is required for IL-8-induced cell migration of human neutrophils. Recently we reported that IL-8 caused the activation of the Ras/Raf/extracellular signal-regulated kinase (ERK) pathway in human neutrophils and that this activation was dependent on PI3K activity. The regulation of cell migration by IL-8 is independent of ERK kinase and ERK activation since the ERK kinase inhibitor PD098059 had no effect on IL-8-induced cell migration of human neutrophils. Additionally, activation of p38-mitogen-activated protein kinase is insufficient and activation of c-Jun N-terminal kinase is unnecessary to induce cell migration of human neutrophils. Therefore, regulation of neutrophil migration appears to be largely independent of the activation of the mitogen-activated protein kinases. The data argue that PI3K activity plays a central role in multiple signal transduction pathways within the human neutrophil leading to distinct cell functions.

Cloning of a novel mitogen-activated protein kinase kinase kinase, MEKK4, that selectively regulates the c-Jun amino terminal kinase pathway

Mitogen-activated protein kinases (MAPKs) are components of sequential kinase cascades that are activated in response to a variety of extracellular signals. Members of the MAPK family include the extracellular response kinases (ERKs or p42/44(MAPK)), the c-Jun amino-terminal kinases (JNKs), and the p38/Hog 1 protein kinases. MAPKs are phosphorylated and activated by MAPK kinases (MKKs or MEKs), which in turn are phosphorylated and activated by MKK/MEK kinases (Raf and MKKK/MEKKs). We have isolated two cDNAs encoding splice variants of a novel MEK kinase, MEKK4. The MEKK4 mRNA is widely expressed in mouse tissues and encodes for a protein of approximately 180 kDa. The MEKK4 carboxyl-terminal catalytic domain is approximately 55% homologous to the catalytic domains of MEKKs 1, 2, and 3. The amino-terminal region of MEKK4 has little sequence homology to the previously cloned MEKK proteins. MEKK4 specifically activates the JNK pathway but not ERKs or p38, distinguishing it from MEKKs 1, 2 and 3, which are capable of activating the ERK pathway. MEKK4 is localized in a perinuclear, vesicular compartment similar to the Golgi. MEKK4 binds to Cdc42 and Rac; kinase-inactive mutants of MEKK4 block Cdc42/Rac stimulation of the JNK pathway. MEKK4 has a putative pleckstrin homology domain and a proline-rich motif, suggesting specific regulatory functions different from those of the previously characterized MEKKs.

Activation of the transcription factor MEF2C by the MAP kinase p38 in inflammation

For cells of the innate immune system to mount a host defence response to infection, they must recognize products of microbial pathogens such as lipopolysaccharide (LPS), the endotoxin secreted by Gram-negative bacteria. These cellular responses require intracellular signalling pathways, such as the four MAP kinase (MAPK) pathways. In mammalian cells the MAPK p38 is thought to play an important role in the regulation of cellular responses during infection through its effects on the expression of proinflammatory molecules. One means of understanding the role of p38 in these responses is to identify proteins with functions regulated by p38-catalysed phosphorylation. Here we demonstrate a link between the p38 pathway and a member of the myocyte-enhancer factor 2 (MEF2) group of transcription factors. We found that in monocytic cells, LPS increases the transactivation activity of MEF2C through p38-catalysed phosphorylation. One consequence of MEF2C activation is increased c-jun gene transcription. Our results show that p38 may influence host defence and inflammation by maintaining the balance of c-Jun protein consumed during infection.

The structure of mitogen-activated protein kinase p38 at 2.1-A resolution

The structure of mitogen-activated protein (MAP) kinase p38 has been solved at 2.1-A to an R factor of 21.0%, making p38 the second low activity MAP kinase solved to date. Although p38 is topologically similar to the MAP kinase ERK2, the phosphorylation Lip (a regulatory loop near the active site) adopts a different fold in p38. The peptide substrate binding site and the ATP binding site are also different from those of ERK2. The results explain why MAP kinases are specific for different activating enzymes, substrates, and inhibitors. A model presented for substrate and activator interactions has implications for the evolution of protein kinase cascades.

Hyperosmotic stress as a stimulant for proinflammatory cytokine production

The synthesis of proinflammatory cytokines involves members of the mitogen-activated protein (MAP) kinase stress pathway, particularly p38 MAP kinase and c-jun NH2-terminal kinase. In this report we used hyperosmotic stress to study changes in steady-state mRNA levels and synthesis of proinflammatory cytokines in freshly obtained human peripheral blood mononuclear cells (PBMC) in vitro. There was no evidence of interleukin (IL)-8 gene expression in freshly obtained human blood despite 30 cycles of amplification of reverse-transcribed mRNA using the polymerase chain reaction. In contrast, exposure of PBMC to hyperosmotic conditions (330-410 mOsM) by the addition of NaCl to tissue culture medium induced gene expression for IL-1 alpha, IL-1 beta, and IL-8. Routine tissue culture medium is hyperosmotic (305 mOsM) compared to human plasma (280-295 mOsM), but decreasing the osmolarity to the physiological range resulted in a 50% reduction in baseline IL-8 synthesis (P < 0.001). Although hyperosmotically induced accumulation of steady-state mRNA levels for IL-1 alpha and IL-1 beta increased 50- and 7-fold over control, respectively, these were poorly translated into each respective cytokine. However, in PBMC stimulated by hyperosmotic stress, the addition of femtomolar concentrations of bacterial lipopolysaccharide, IL-1, or 1% normal human serum resulted in a synergistic synthesis (at least twice that expected) of IL-1 alpha, IL-1 beta, TNF-alpha, and IL-8.

MAP kinase is required for the spindle assembly checkpoint but is dispensable for the normal M phase entry and exit in Xenopus egg cell cycle extracts

In Xenopus laevis egg cell cycle extracts that mimic early embryonic cell cycles, activation of MAP kinase and MAP kinase kinase occurs in M phase, slightly behind that of maturation promoting factor. To examine the possible role of MAP kinase in the in vitro cell cycle, we depleted the extracts of MAP kinase by using anti-Xenopus MAP kinase antibody. Like in the mock-treated extracts, the periodic activation and deactivation of MPF occurred normally in the MAP kinase-depleted extracts, suggesting that MAP kinase is dispensable for the normal M phase entry and exit in vitro. It has recently been reported that microtubule depolymerization by nocodazole treatment can block exit from mitosis in the extracts if enough sperm nuclei are present, and that the addition of MAP kinase-specific phosphatase MKP-1 overcomes this spindle assembly checkpoint, suggesting the involvement of MAP kinase in the checkpoint signal transduction. We show here that the spindle assembly checkpoint mechanism cannot operate in the MAP kinase-depleted extracts. But, adding recombinant Xenopus MAP kinase to the MAP kinase-depleted extracts restored the spindle assembly checkpoint. These results indicate unambiguously that classical MAP kinase is required for the spindle assembly checkpoint in the cell cycle extracts. In addition, we show that strong activation of MAP kinase by the addition of a constitutively active MAP kinase kinase kinase in the absence of sperm nuclei and nocodazole, induced mitotic arrest in the extracts. Therefore, activation of MAP kinase alone is sufficient for inducing the mitotic arrest in vitro.

Tumor necrosis factor alpha-induced activation of c-jun N-terminal kinase is mediated by TRAF2

Tumor necrosis factor alpha (TNF alpha) a pro-inflammatory cytokine is an endogenous mediator of septic shock, inflammation, anti-viral responses and apoptotic cell death. TNF alpha elicits its complex biological responses through the individual or cooperative action of two TNF receptors of mol. wt 55 kDa (TNF-RI) and mol. wt 75 kDa (TNF-RII). To determine signaling events specific for TNF-RII we fused the extracellular domain of the mouse CD4 antigen to the intracellular domain of TNF-RII. Crosslinking of the chimeric receptor using anti-CD4 antibodies initiates exclusively TNF-RII-mediated signals. Our findings show that: (i) TNF-RII is able to activate two members of the MAP kinase family: extracellular regulated kinase (ERK) and c-jun N-terminal kinase (JNK); (ii) TRAF2, a molecule that binds TNF-RII and associates indirectly with TNF-RI, is sufficient to activate JNK upon overexpression; (iii) dominant-negative TRAF2 blocks TNF alpha-mediated JNK activation and (iv) TRAF2 signals the activation of JNK and NF-kappaB through different pathways. Our findings suggest that TNF alpha-mediated JNK activation in fibroblasts is independent of the cell death pathway and that TRAF2 occupies a key role in TNF receptor signaling to JNK.

CREB is activated by UVC through a p38/HOG-1-dependent protein kinase

Changes in environmental conditions such as the addition of growth factors or irradiation of cells in culture first affect immediate response genes. We have shown previously that short wavelength UV irradiation (UVC) elicits massive activation of several growth factor receptor-dependent pathways. At the level of the immediate response gene c-fos, these pathways activate the transcription factor complex serum response factor (SRF)-p62TCF which mediates part of the UV-induced transcriptional response. These studies have, however, suggested that more that one pathway is required for full UV responsiveness of c-fos. Using appropriate promoter mutations and dominant-negative cAMP response element (CRE)-binding protein (CREB), we now find that UVC-induced transcriptional activation depends also on the CRE at position -60 of the c-fos promoter and on the functionality of a CREB. Upon UV irradiation, CREB and ATF-1 are phosphorylated at serines 133 and 63, respectively, preceded by and dependent on activation of p38/RK/HOG-1 and of a p38/RK/HOG-1-dependent p108 CREB kinase. Although p90RSK1 and MAPKAP kinase 2 are also activated by UV, p90RSK1 does not, at least not decisively, participate in this signalling pathway to CREB and ATF-1 as it is not p38/RK/HOG-1 dependent, and CREB is a poor substrate for MAPKAP kinase 2 in vitro. On the basis of resistance to the growth factor receptor inhibitor suramin and of several types of cross-refractoriness experiments, the UVC-induced CREB/ATF-1 phosphorylation represents an as yet unrecognized route of UVC-induced signal transduction, independent of suramin-inhibitable growth factor receptors and different from the Erk 1,2-p62TCF pathway.

Common and distinct intracellular signaling pathways in human neutrophils utilized by platelet activating factor and FMLP

Stimulation of human neutrophils with chemoattractants FMLP or platelet activating factor (PAF) results in different but overlapping functional responses. We questioned whether these differences might reflect patterns of intracellular signal transduction. Stimulation with either PAF or FMLP resulted in equivalent phosphorylation and activation of the mitogen-activated protein kinase (MAPk) homologue 38-kD murine MAP kinase homologous to HOG-1 (p38) MAPk. Neither FMLP nor PAF activated c-jun NH2-terminal MAPk (JNKs). Under identical conditions, FMLP but not PAF, resulted in significant p42/44 (ERK) MAPk activation. Both FMLP and PAF activated MAP kinase kinase-3 (MKK3), a known activator of p38 MAPk. Both MAP ERK kinase kinase-1 (MEKK1) and Raf are activated strongly by FMLP, but minimally by PAF. Pertussis toxin blocked FMLP-induced activation of the p42/44 (ERK) MAPk cascade, but not that of p38 MAPk. A specific p38 MAPk inhibitor (SK&F 86002) blocked superoxide anion production in response to FMLP and reduced adhesion and chemotaxis in response to PAF or FMLP. These results demonstrate distinct patterns of intracellular signaling for two chemoattractants and suggest that selective activation of intracellular signaling cascades may underlie different patterns of functional responses.

Rac regulation of transformation, gene expression, and actin organization by multiple, PAK-independent pathways

Rac1 and RhoA are members of the Rho family of Ras-related proteins and function as regulators of actin cytoskeletal organization, gene expression, and cell cycle progression. Constitutive activation of Rac1 and RhoA causes tumorigenic transformation of NIH 3T3 cells, and their functions may be required for full Ras transformation. The effectors by which Rac1 and RhoA mediate these diverse activities, as well as the interrelationship between these events, remain poorly understood. Rac1 is distinct from RhoA in its ability to bind and activate the p65 PAK serine/threonine kinase, to induce lamellipodia and membrane ruffling, and to activate the c-Jun NH2-terminal kinase (JNK). To assess the role of PAK in Rac1 function, we identified effector domain mutants of Rac1 and Rac1-RhoA chimeric proteins that no longer bound PAK. Surprisingly, PAK binding was dispensable for Rac1-induced transformation and lamellipodium formation, as well as activation of JNK, p38, and serum response factor (SRF). However, the ability of Rac1 to bind to and activate PAK correlated with its ability to stimulate transcription from the cyclin D1 promoter. Furthermore, Rac1 activation of JNK or SRF, or induction of lamellipodia, was neither necessary nor sufficient for Rac1 transforming activity. Finally, the signaling pathways that mediate Rac1 activation of SRF or JNK were distinct from those that mediate Rac1 induction of lamellipodia. Taken together, these observations suggest that Rac1 regulates at least four distinct effector-mediated functions and that multiple pathways may contribute to Rac1-induced cellular transformation.

Hypertonic saline activates protein tyrosine kinases and mitogen-activated protein kinase p38 in T-cells

OBJECTIVES

In previous in vitro studies, we have found that hypertonic saline (HTS) can augment T-cell proliferation and restore the function of suppressed T-cells. Our animal models have shown that HTS resuscitation reverses immunosuppression after hemorrhage and reduces mortality from sepsis. In the present study, we investigated if and how HTS may influence T-cell signaling and function on a subcellular level.

DESIGN

Human peripheral blood mononuclear cells (PBMC) were used to determine the effect of HTS on T-cell interleukin 2 (IL-2) production and proliferation. Human Jurkat T-cells were used to study the effects of HTS on T-cell signal transduction, IL-2 mRNA transcription, and IL-2 expression.

MATERIAL AND METHODS

The effect of HTS on T-cell proliferation and IL-2 production was measured with PBMC and Jurkat T-cells. IL-2 mRNA transcription in HTS-treated Jurkat cells was measured by reverse transcriptase polymerase chain reaction. HTS-induced protein tyrosine phosphorylation in Jurkat T-cells was determined by immunoblotting with anti-phosphotyrosine antibodies. Expression in Jurkat cells of the mitogen-activated protein kinase p38 (MAPK p38), a signal transduction protein that is activated by osmotic stress, was determined by immunoblotting with anti-MAPK p38 antibodies. HTS-induced MAPK p38 activation in Jurkat cells was measured with an immune-complex kinase assay using ATF-2 as a substrate.

MEASUREMENTS AND MAIN RESULTS

Proliferation of activated human PBMC increased significantly upon addition of HTS to the culture medium. This effect of HTS was paralleled by enhanced IL-2 production of activated PBMC and Jurkat cells and IL-2 mRNA transcription of Jurkat cells. HTS exposure of Jurkat cells caused tyrosine phosphorylation of a number of cellular proteins. We found that Jurkat T-cells expressed MAPK p38 and that it was activated in the presence of HTS. All these effects of HTS on T-cell signaling and function were observed at NaCl concentrations that were within physiologically relevant levels (20-100 mmol/L hypertonicity).

CONCLUSIONS

In T-cells, HTS triggers a signaling pathway that includes increased tyrosine phosphorylation of several cellular proteins and activation of MAPK p38. HTS alone does not result in IL-2 mRNA transcription, IL-2 expression, or T-cell proliferation. However, in combination with other stimuli, HTS augments T-cell IL-2 expression and proliferation. We speculate that HTS could "resuscitate" suppressed T-cells in trauma patients by circumvention of, or substituting for, blocked signaling pathways.

Oxidative stress-induced actin reorganization mediated by the p38 mitogen-activated protein kinase/heat shock protein 27 pathway in vascular endothelial cells

Vascular endothelial cells are constantly in contact with oxyradicals and must be especially well equipped to resist their toxic effects and generate appropriate physiological responses. Despite the importance of oxyradicals in the physiopathology of the vascular endothelium, the mechanisms regulating the oxidative response of endothelial cells are poorly understood. In the present study, we observed that H2O2 in concentrations that induced severe fragmentation of F-actin in fibroblasts rather induced a reorganization of F-actin in primary cultures of human umbilical vein endothelial cells (HUVECs) that was characterized by the accumulation of stress fibers, the recruitment of vinculin to focal adhesions, and the loss of membrane ruffles, H2O2 also induced in these cells a strong (10- to 14-fold) activation of the p38 mitogen-activated protein (MAP) kinase, which resulted in activation of MAP kinase-activated protein kinase-2/3 and phosphorylation of the F-actin polymerization modulator, heat shock protein 27 (HSP27). The MAP kinases extracellular-regulated kinase, and c-Jun N-terminal kinase/stress-activated protein kinase were only slightly increased by these treatments. Inhibiting p38 activity with the highly specific inhibitor SB203580 blocked the H2O2-induced endothelial microfilament responses. Moreover, fibroblasts acquired an endothelium-like SB203580-sensitive actin response when HSP27 concentration was increased by gene transfection to the same high level as found in HUVECs. The results indicate that activation of p38 MAP kinase in cells such as endothelial cells, which naturally express high level of HSP27, plays a central role in modulating microfilament responses to oxidative stress. Consequently, the p38 MAP kinase pathway may participate in the several oxyradical-activated functions of the endothelium that are associated with reorganization of microfilament network.

Mediation by arachidonic acid metabolites of the H2O2-induced stimulation of mitogen-activated protein kinases (extracellular-signal-regulated kinase and c-Jun NH2-terminal kinase)

Reactive oxygen species modulate major cellular functions by mechanisms which are still poorly understood. Recently, H2O2 has been reported to stimulate the activity of the mitogen-activated protein kinases (MAPKs) ERK and JNK, and the expression of the proto-oncogenes c-fos and c-jun. As their expression is enhanced by H2O2 in astrocytes, we studied whether these MAPKs were stimulated by H2O2 in primary cultured astrocytes. The result was positive, a maximum of stimulation being reached with 200 microM H2O2 (0.3 pmol H2O2/cell) for both ERK and JNK. ERK was previously reported to stimulate cytosolic phospholipase A2 phosphorylation and activity. H2O2 stimulated the release of arachidonic acid in astrocytes, as already reported in other cell types. We found also that cPLA2 phosphorylation was increased by H2O2. Moreover, the stimulation by H2O2 of ERK and JNK was decreased by phospholipase A2 activity inhibitors. When astrocytes were incubated first with eicosatetraynoic acid, a structural analogue competing in arachidonic acid metabolism, the stimulation of JNK by H2O was also inhibited, suggesting the involvement of arachidonic acid metabolites. Cyclooxygenase or cytochrome P450 monooxygenase inhibitors failed in decreasing the MAPK stimulation by H2O2, whereas lipoxygenase inhibitors completely abolished that of JNK. Mitogenicity has been reported to be stimulated by H2O2 in other cell types. Although ERK was strongly and durably stimulated by 200 microM H2O2 in astrocytes, at the same extent as by mitogenic growth factors, basal thymidine incorporation rate was decreased by more than 80% after 12-15 h. Moreover, the stimulation of thymidine incorporation induced by basic fibroblast growth factor was transiently abolished by H2O2. Furthermore, H2O2 likely induced the expression of CL100/PAC1/MKP-1, a dual specificity phosphatase which has been implicated in ERK and JNK inactivation in the nucleus. Finally, the prior treatment of astrocytes with MK886, a 5-lipoxygenase-activating protein inhibitor, prevented JNK from stimulation, but did not prevent thymidine incorporation from inhibition, both induced by H2O2. These results strongly suggest an involvement of arachidonic acid and/or its metabolites in the stimulation of both ERK and JNK following the oxidative stress evoked by H2O2, which induced a cell cycle arrest probably independent of the stimulation of JNK.

Molecular cloning and functional characterization of a novel mitogen-activated protein kinase phosphatase, MKP-4

Extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK), and p38/RK/CSBP (p38) mitogen-activated protein (MAP) kinases are target enzymes activated by a wide range of cell-surface stimuli. Recently, a distinct class of dual specificity phosphatase has been shown to reverse activation of MAP kinases by dephosphorylating critical tyrosine and threonine residues. By searching the expressed sequence tag data base (dbEST) for homologues of known dual specificity phosphatases, we identified a novel partial human sequence for which we isolated a full-length cDNA (termed MKP-4). The deduced amino acid sequence of MKP-4 is most similar to MKP-X/PYST2 (61% identity) and MKP-3/PYST1 (57% identity), includes two N-terminal CH2 domains homologous to the cell cycle regulator Cdc25 phosphatase, and contains the extended active site sequence motif VXVHCXAGXSRSXTX3AYLM (where X is any amino acid) conserved in dual specificity phosphatases. MKP-4 produced in Escherichia coli catalyzes vanadate-sensitive breakdown of p-nitrophenyl phosphate as well as in vitro inactivation of purified ERK2. When expressed in COS-7 cells, MKP-4 blocks activation of MAP kinases with the selectivity ERK > p38 = JNK/SAPK. This cellular specificity is similar to MKP-3/PYST1, although distinct from hVH-5/M3-6 (JNK/SAPK = p38 >>> ERK). Northern analysis reveals a highly restricted tissue distribution with a single MKP-4 mRNA species of approximately 2.5 kilobases detected only in placenta, kidney, and embryonic liver. Immunocytochemical analysis showed MKP-4 to be present within cytosol although punctate nuclear staining co-localizing with promyelocytic protein was also observed in a subpopulation (10-20%) of cells. Chromosomal localization by analysis of DNAs from human/rodent somatic cell hybrids and a panel of radiation hybrids assign the human gene for MKP-4 to Xq28. The identification and characterization of MKP-4 highlights the emergence of an expanding family of structurally homologous dual specificity phosphatases possessing distinct MAP kinase specificity and subcellular localization as well as diverse patterns of tissue expression.

Transcriptional activation by peroxisome proliferator-activated receptor gamma is inhibited by phosphorylation at a consensus mitogen-activated protein kinase site

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) regulates transcription in response to prostanoid and thiazolidinedione ligands and promotes adipocyte differentiation. The amino-terminal A/B domain of this receptor contains a consensus mitogen-activated protein kinase site in a region common to PPARgamma1 and -gamma2 isoforms. The A/B domain of human PPARgamma1 was phosphorylated in vivo, and this was abolished either by mutation of serine 84 to alanine (S84A) or coexpression of a phosphoprotein phosphatase. In vitro, this domain was phosphorylated by ERK2 and JNK, and this was markedly reduced in the S84A mutant. A wild type Gal4-PPARgamma(A/B) chimera exhibited weak constitutive transcriptional activity. Remarkably, this was significantly enhanced in the S84A mutant fusion. Ligand-dependent activation by full-length mouse PPARgamma2 was also augmented by mutation of the homologous serine in the A/B domain to alanine. The nonphosphorylatable form of PPARgamma was also more adipogenic. Thus, phosphorylation of a mitogen-activated protein kinase site in the A/B region of PPARgamma inhibits both ligand-independent and ligand-dependent transactivation functions. This observation provides a potential mechanism whereby transcriptional activation by PPARgamma may be modulated by growth factor or cytokine-stimulated signal transduction pathways involved in adipogenesis.

Hemopoietic growth factors with the exception of interleukin-4 activate the p38 mitogen-activated protein kinase pathway

The mammalian mitogen-activated protein (MAP) kinase homologue p38 has been shown to be activated by pro-inflammatory cytokines as well as physical and chemical stresses. We now show that a variety of hemopoietic growth factors, including Steel locus factor, colony stimulating factor-1, granulocyte/macrophage-colony stimulating factor, and interleukin-3, activate p38 MAP kinase and the downstream kinase MAPKAP kinase-2. Furthermore, although these growth factors activate both p38 MAP kinase and Erk MAP kinases, we demonstrate using a specific inhibitor of p38 MAP kinase, SB 203580, that p38 MAP kinase activity was required for MAP kinase-activated protein kinase-2 activation. Conversely p38 MAP kinase was shown not to be required for in vivo activation of p90(rsk), known to be downstream of the Erk MAP kinases. Interleukin-4 was unique among the hemopoietic growth factors we examined in failing to induce activation of either p38 MAP kinase or MAP kinase-activated protein kinase-2. These findings demonstrate that the activation of p38 MAP kinase is involved not only in responses to stresses but also in signaling by growth factors that regulate the normal development and function of cells of the immune system.

MEKKs, GCKs, MLKs, PAKs, TAKs, and tpls: upstream regulators of the c-Jun amino-terminal kinases?

Regulation of the mitogen-activated protein kinase (MAPK) family members - which include the extracellular response kinases (ERKs), p38/HOG1, and the c-Jun amino-terminal kinases (JNKs) - plays a central role in mediating the effects of diverse stimuli encompassing cytokines, hormones, growth factors and stresses such as osmotic imbalance, heat shock, inhibition of protein synthesis and irradiation. A rapidly increasing number of kinases that activate the JNK pathways has been described recently, including the MAPK/ERK kinase kinases, p21-activated kinases, germinal center kinase, mixed lineage kinases, tumor progression locus 2, and TGF-beta-activated kinase. Thus, regulation of the JNK pathway provides an interesting example of how many different stimuli can converge into regulating pathways critical for the determination of cell fate.

Phosphorylation of protein-tyrosine phosphatase PTP-1B on identical sites suggests activation of a common signaling pathway during mitosis and stress response in mammalian cells

PTP-1B is a widely expressed non-transmembrane tyrosine-specific phosphatase. Previous studies indicated that, at mitosis, PTP-1B undergoes phosphorylation on two sites, 352Ser-Pro-Leu-Asn and 386Ser-Pro-Ala-Lys. Although the Ser-386 site can be phosphorylated by Cyclin B/Cdc2 in vitro, the kinase for the Ser-352 site is unknown. We have found that these phosphorylation events are not unique to normal mitosis. Instead, treatment with many, but not all, stress stimuli, in particular osmotic shock and certain phosphatase and protein synthesis inhibitors, leads to phosphorylation of PTP-1B. Tryptic phosphopeptide and mutant analysis reveals that, as in mitosis, stress-induced PTP-1B phosphorylation involves both Ser-352 and Ser-386. Activation of the proline-directed kinases Erk1/2, JNKs, and p38 was neither necessary nor sufficient for stress-induced PTP-1B phosphorylation. Our data suggest the existence of a novel mitogen-activated protein kinase pathway in mammalian cells, which is activated at mitosis and in response to osmotic shock and other stresses and results in PTP-1B phosphorylation. This pathway may be similar to the recently described Spc1/Sty1 pathway in Schizosaccharomyces pombe.

Tumor necrosis factor alpha-induced E-selectin expression is activated by the nuclear factor-kappaB and c-JUN N-terminal kinase/p38 mitogen-activated protein kinase pathways

E-selectin expression by endothelium is crucial for leukocyte recruitment during inflammatory responses. Transcriptional regulation of the E-selectin promoter by tumor necrosis factor alpha (TNFalpha) requires multiple nuclear factor-kappaB (NF-kappaB) binding sites and a cAMP-responsive element/activating transcription factor-like binding site designated positive domain II (PDII). Here we characterize the role of the stress-activated family of mitogen-activated protein (MAP) kinases in induced expression of this adhesion molecule. By UV cross-linking and immunoprecipitation, we demonstrated that a heterodimer of transcription factors ATF-2 and c-JUN is constitutively bound to the PDII site. TNFalpha stimulation of endothelial cells induces transient phosphorylation of both ATF-2 and c-JUN and induces marked activation of the c-JUN N-terminal kinase (JNK1) and p38 but not extracellular signal-regulated kinase (ERK1). JNK and p38 are constitutively present in the nucleus, and DNA-bound c-JUN and ATF-2 are stably contacted by JNK and p38, respectively. MAP/ERK kinase kinase 1 (MEKK1), an upstream activator of MAP kinases, increases E-selectin promoter transcription and requires an intact PDII site for maximal induction. MEKK1 can also activate NF-kappaB -dependent gene expression. The effects of dominant interfering forms of the JNK/p38 signaling pathway demonstrate that activation of these kinases is critical for cytokine-induced E-selectin gene expression. Thus, TNFalpha activates two signaling pathways, NF-kappaB and JNK/p38, which are both required for maximal expression of E-selectin.

The effects of IL-1 on mitogen-activated protein kinases in rabbit articular chondrocytes

IL-1-activated chondrocytes express a large number of genes which contribute to cartilage degradation. The signaling pathways activated in response to IL-1 in these cells are not well-defined. We examined the effects of IL-1 and other stimuli on the mitogen activated protein kinase (MAPK) pathways in rabbit articular chondrocytes. We demonstrate that IL-1 activates three MAPKs, ERK, JNK and p38, in a time and dose-dependent manner. Activation is maximal by 15 minutes and returns to baseline levels by 1 hour. Maximal activation of ERK and p38 occurs with 1 ng/ml IL-1 whereas activation of JNK requires 10-fold higher levels. In contrast to IL-1, the PKC activator, PDBu preferentially activates ERK while TNF alpha preferentially activates JNK. LPS and TGF beta fail to stimulate any of the kinases examined. These results suggest that activation of the various MAPK pathways is important in the response of chondrocytes to IL-1, cytokines and growth factors.

Chemotactic peptide N-formyl-met-leu-phe activation of p38 mitogen-activated protein kinase (MAPK) and MAPK-activated protein kinase-2 in human neutrophils

Activation of polymorphonuclear leukocytes (PMN) by chemotactic peptides initiates a series of functional responses that serve to eliminate pathogens. The intermediate steps that link engagement of the chemoattractant receptor to the microbicidal responses involve protein kinases that have yet to be identified. In this study we detected in human PMN the presence of p38 mitogen-activated protein kinase (MAPK), which became rapidly tyrosine phosphorylated and activated in response to the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP). Pretreatment of PMN with wortmannin, a phosphatidylinositol 3-kinase inhibitor, or bis-indolylmaleimide, a protein kinase C antagonist, resulted in partial inhibition of p38 phosphorylation upon fMLP stimulation. Similarly, phosphorylation of p38 was only partially inhibited when the fMLP-induced cytosolic calcium transient was prevented. Stimulation of PMN by the chemoattractant also resulted in the rapid phosphorylation and activation of MAPK-activated protein kinase-2 (MAPKAPK-2), which was completely inhibited by the specific p38 inhibitor, SB203580. The physical interaction of p38 with MAPKAPK-2 was studied by coimmunoprecipitation. These two kinases were found to be associated in unstimulated PMN but dissociated upon activation of the cells by fMLP. Together these findings demonstrate the activation of p38 by chemotactic peptides in human PMN by a process involving phosphatidylinositol 3-kinase, protein kinase C, and calcium. p38, in turn, is an upstream activator of MAPKAPK-2.

The p42/p44 mitogen-activated protein kinase cascade is determinant in mediating activation of the Na+/H+ exchanger (NHE1 isoform) in response to growth factors

The ubiquitously expressed Na+/H+ exchanger NHE1 is the target of multiple signaling pathways, including those activated by tyrosine kinase receptors, G protein-coupled receptors, and integrins. The intracellular pathways leading to activation of NHE1 are poorly understood. To gain more insight into these activation pathways, we examined the role of mitogen-activated protein kinases (MAPKs) as potential mediators of NHE1 activation by extracellular stimuli such as growth factors and hyperosmotic stress. Whereas p44 MAPK does not appear to phosphorylate NHE1 in vitro, we found that inhibition of the p42/p44 MAPK signaling by expression of a dominant negative form of p44 MAPK, by expression of the MAP kinase phosphatase MKP-1, or by inhibition of MAPK kinase 1 (MKK1) with the PD 98059 compound reduced by 50-60% NHE1 activation in response to growth factors. This inhibitory effect also was observed in C-terminal NHE1 deletion mutants in which the major phosphorylation sites have been deleted. Furthermore, the use of a CCL39-derived cell line expressing an estradiol-regulated form of oncogenic Raf-1 (CCL39-deltaRaf-1:ER) revealed that the exclusive activation of the Raf --> MKK1 --> p42/p44 MAPK cascade was capable of inducing NHE1 activation to the same extent as potent growth factors like thrombin. Together, our findings demonstrate that the p42/p44 MAPK cascade plays a predominant role in the regulation of NHE1 by growth factors, an action that is mediated via accessory proteins that remain to be identified. In contrast, we found no evidence in favor of the contribution of any MAPK, p42/p44, p38 MAPKs, and Jun kinase, in NHE1 activation by osmotic stress.

Induction of apoptosis by ASK1, a mammalian MAPKKK that activates SAPK/JNK and p38 signaling pathways

Mitogen-activated protein (MAP) kinase cascades are activated in response to various extracellular stimuli, including growth factors and environmental stresses. A MAP kinase kinase kinase (MAPKKK), termed ASK1, was identified that activated two different subgroups of MAP kinase kinases (MAPKK), SEK1 (or MKK4) and MKK3/MAPKK6 (or MKK6), which in turn activated stress-activated protein kinase (SAPK, also known as JNK; c-Jun amino-terminal kinase) and p38 subgroups of MAP kinases, respectively. Overexpression of ASK1 induced apoptotic cell death, and ASK1 was activated in cells treated with tumor necrosis factor-alpha (TNF-alpha). Moreover, TNF-alpha-induced apoptosis was inhibited by a catalytically inactive form of ASK1. ASK1 may be a key element in the mechanism of stress- and cytokine-induced apoptosis.

Differential signaling by lymphocyte antigen receptors

Studies performed during the past several years make plain that ligand occupancy of antigen receptors need not necessarily provoke identical responses in all instances. For example, ligation of antigen receptors may stimulate a proliferative response, induce a state of unresponsiveness to subsequent stimulation (anergy), or induce apoptosis. How does a single type of transmembrane receptor induce these very heterogeneous cellular responses? In the following pages, we outline evidence supporting the view that the nature of the ligand/receptor interaction directs the physical recruitment of signaling pathways differentially inside the lymphocyte and hence defines the nature of the subsequent immune response. We begin by providing a functional categorization of antigen receptor components, considering the ways in which these components interact with the known set of signal transduction pathways, and then review the evidence suggesting that differential signaling through the TCR is achieved by qualitative differences in the effector pathways recruited by TCR, perhaps reflecting the time required to bring complicated signal transduction elements into proximity within the cell. The time-constant of the interaction between antigen and receptor in this way determines, at least in part, the nature of the resulting response. Finally, although our review focuses substantially on T cell receptor signaling, we have included a less detailed description of B cell receptor signaling as well, simply to emphasize the parallels that exist in these two closely related systems.

Crystallization and preliminary crystallographic analysis of recombinant human P38 MAP kinase

The recombinant human p38 MAP kinase has been expressed and purified from both Escherichia coli and SF9 cells, and has been crystallized in two forms by the hanging drop vapor diffusion method using PEG as precipitant. Both crystal forms belong to space group P2(1)2(1)2(1). The cell parameters for crystal form 1 are a = 65.2 A, b = 74.6 A and c = 78.1 A. Those for crystal form 2 are a = 58.3 A, b = 68.3 A and c = 87.9 A. Diffraction data to 2.0 A resolution have been collected on both forms.

UV-induced signal transduction

Irradiation of cells with wavelength ultraviolet (UVA, B and C) induces the transcription of many genes. The program overlaps with that induced by oxidants and alkylating agents and has both protective and other functions. Genes transcribed in response to UV irradiation include genes encoding transcription factors, proteases and viral proteins. While the transcription factor encoding genes is initiated in minutes after UV irradiation (immediate response genes) and depends exclusively on performed proteins, the transcription of protease encoding occurs only many hours after UV irradiation. Transcription factors controlling the activity of immediate response genes are activated by protein kinases belonging to the group of proline directed protein kinases immediately after UV irradiation. Experimental evidence suggests that these kinases are activated in UV irradiated cells through pathways which are used by growth factors. In fact, the first cellular reaction detectable in UV irradiated cells is the phosphorylation of several growth factor receptors at tyrosine residues. This phosphorylation does not depend on UV induced DNA damage, but is due to an inhibition of the activity of tyrosine phosphatases. In contrast, for late cellular reactions to UV, an obligatory role of DNA damage in transcribed regions of the genome can be demonstrated. Thus, UV is absorbed by several target molecules relevant for cellular signaling, and it appears that numerous signal transduction pathways are stimulated. The combined action of these pathways establishes the genetic program that determines the fate of UV irradiated cells.

Fas activation of the p38 mitogen-activated protein kinase signalling pathway requires ICE/CED-3 family proteases

The Fas receptor mediates a signalling cascade resulting in programmed cell death (apoptosis) within hours of receptor cross-linking. In this study Fas activated the stress-responsive mitogen-activated protein kinases, p38 and JNK, within 2 h in Jurkat T lymphocytes but not the mitogen-responsive kinase ERK1 or pp70S6k. Fas activation of p38 correlated temporally with the onset of apoptosis, and transfection of constitutively active MKK3 (glu), an upstream regulator of p38, potentiated Fas-induced cell death, suggesting a potential involvement of the MKK3/p38 activation pathway in Fas-mediated apoptosis. Fas has been shown to require ICE (interleukin-1 beta-converting enzyme) family proteases to induce apoptosis from studies utilizing the cowpox ICE inhibitor protein CrmA, the synthetic tetrapeptide ICE inhibitor YVAD-CMK, and the tripeptide pan-ICE inhibitor Z-VAD-FMK. In this study, crmA antagonized, and YVAD-CMK and Z-VAD-FMK completely inhibited, Fas activation of p38 kinase activity, demonstrating that Fas-dependent activation of p38 requires ICE/CED-3 family members and conversely that the MKK3/p38 activation cascade represents a downstream target for the ICE/CED-3 family proteases. Intriguingly, p38 activation by sorbitol and etoposide was resistant to YVAD-CMK and Z-VAD-FMK, suggesting the existence of an additional mechanism(s) of p38 regulation. The ICE/CED-3 family-p38 regulatory relationship described in the current work indicates that in addition to the previously described destructive cleavage of substrates such as poly(ADP ribose) polymerase, lamins, and topoisomerase, the apoptotic cysteine proteases also function to regulate stress kinase signalling cascades.

Trypsin stimulates proteinase-activated receptor-2-dependent and -independent activation of mitogen-activated protein kinases

We have examined protease-mediated activation of the mitogen-activated protein (MAP) kinase cascade in rat aortic smooth-muscle cells and bovine pulmonary arterial fibroblasts. Exposure of smooth-muscle cells to trypsin evoked rapid and transient activation of c-Raf-1, MAP kinase kinase 1 and 2 and MAP kinase that was sensitive to inhibition by soybean trypsin inhibitor. The actions of trypsin were closely mimicked by the proteinase-activated receptor 2 (PAR-2)-activating peptide sequence SLIGRL but not LSIGRL. Peak MAP kinase activation in response to both trypsin and SLIGRL was also dependent on concentration, with EC50 values of 12.1 +/- 3.4 nM and 62.5 +/- 4.5 microM respectively. Under conditions where MAP kinase activation by SLIGRL was completely desensitized by prior exposure of smooth-muscle cells to the peptide, trypsin-stimulated MAP kinase activity was markedly attenuated (78.9 +/- 15.1% desensitization), whereas the response to thrombin was only marginally affected (16.6 +/- 12.1% desensitization). Trypsin and SLIGRL also weakly stimulated the activation of the MAP kinase homologue p38 in smooth-muscle cells without any detectable activation of c-Jun N-terminal kinase. Strong activation of the MAP kinase cascade and modest activation of p38 by trypsin were also observed in fibroblasts, although in this cell type these effects were not mimicked by SLIGRL nor by the thrombin receptor-activating peptide SFLLRNPNDKYEPF. Reverse transcriptase-PCR analysis confirmed the presence of PAR-2 mRNA in smooth-muscle cells but not fibroblasts. Our results suggest that in vascular smooth-muscle cells, trypsin stimulates the activation of the MAP kinase cascade relatively selectively, in a manner consistent with an interaction with the recently described PAR-2. Activation of MAP kinase by trypsin in vascular fibroblasts, however, seems to be independent of PAR-2 and occurs by an undefined mechanism possibly involving novel receptor species.

The role of c-Jun N-terminal kinase (JNK) in apoptosis induced by ultraviolet C and gamma radiation. Duration of JNK activation may determine cell death and proliferation

c-Jun N-terminal kinases (JNKs) participate in cellular responses to mitogenic stimuli, environmental stresses, and apoptotic agents. The mechanisms by which JNK integrates with other signaling pathways and regulates the diverse cellular events are unclear. We found JNK, but not p38-mitogen-activated protein kinase (MAPK) or extracellular signal-regulated kinase 2, to be persistently activated in apoptosis induced by gamma radiation, UV-C, and anti-Fas treatment. Direct correlation was found between JNK activation and apoptosis induced by UV-C and gamma radiation; however, JNK induction and apoptosis induced by Fas signaling were not well correlated. Overexpression of activated JNK1 caused cell death in transfected cells, and the expression of a dominant-negative mutant of MAPK kinase 1 or JNK1 (but not a dominant-negative mutant of p38-MAPK or c-Raf) prevented the UV-C- and gamma radiation-induced cell death. The inductions of JNK in T-cell activation and apoptosis were distinguished by the different activation patterns, transient versus persistent, respectively. Co-treatment with a tyrosine phosphatase inhibitor (sodium orthovanadate) and T-cell activation signals (phorbol 12-myristate 13-acetate plus ionomycin) prolonged JNK induction, followed by T-cell apoptosis. Our data revealed the requirement of the JNK pathway in radiation-induced apoptosis and implicated the importance of the duration of JNK activation in determining the cell fates.

The MAP kinase cascades are activated during post-ischemic liver reperfusion

We have investigated the involvement of MAP kinase cascades in the response of the liver to post-ischemic reperfusion. Both JNKs and ERKs are activated but the duration and magnitude of the increase in their activities appear to be different. JNK activation is more marked but shorter than that of ERKs. The increase observed in the phosphotyrosine content of the 52 kDa Shc protein, accompanied by an increased amount of co-immunoprecipitated Grb2, and the activation of Raf-1 kinase provide evidence of the involvement of a Ras-Raf-dependent pathway, with a time course that is similar to that of ERK activation. The treatment of rats with IL-1 receptor antagonist modified all of the described effects, suggesting that IL-1 plays a role in the response of the liver to reperfusion.

3pK, a novel mitogen-activated protein (MAP) kinase-activated protein kinase, is targeted by three MAP kinase pathways

Recently we have identified a mitogen-activated protein kinase (MAPK)-activated protein kinase, named 3pK (G. Sithanandam, F. Latif, U. Smola, R. A. Bernal, F.-M. Duh, H. Li, I. Kuzmin, V. Wixler, L. Geil, S. Shresta, P. A. Lloyd, S. Bader, Y. Sekido, K. D. Tartof, V. I. Kashuba, E. R. Zabarovsky, M. Dean, G. Klein, B. Zbar, M. I. Lerman, J. D. Minna, U. R. Rapp, and A. Allikmets, Mol. Cell. Biol. 16:868-876, 1996). In vitro characterization of the kinase revealed that 3pK is activated by ERK. It was further shown that 3pK is phosphorylated in vivo after stimulation of cells with serum. However, the in vivo relevance of this observation in terms of involvement of the Raf/MEK/ERK cascade has not been established. Here we show that 3pK is activated in vivo by the growth inducers serum and tetradecanoyl phorbol acetate in promyelocytic HL60 cells and transiently transfected embryonic kidney 293 cells. Activation of 3pK was Raf dependent and was mediated by the Raf/MEK/ERK kinase cascade. 3pK was also shown to be activated after stress stimulation of cells. In vitro studies with recombinant proteins demonstrate that in addition to ERK, members of other subgroups of the MAPK family, namely, p38RK and Jun-N-terminal kinases/stress-activated protein kinases, were also able to phosphorylate and activate 3pK. Cotransfection experiments as well as the use of a specific inhibitor of p38RK showed that these in vitro upstream activators also function in vivo, identifying 3pK as the first kinase to be activated through all three MAPK cascades. Thus, 3pK is a novel convergence point of different MAPK pathways and could function as an integrative element of signaling in both mitogen and stress responses.

Role of the stress-activated/c-Jun NH2-terminal protein kinase pathway in the cellular response to adriamycin and other chemotherapeutic drugs

c-Jun NH2-terminal protein kinase (JNK), a member of the mitogen-activated protein kinase family, is activated in response to many stressful stimuli including heat shock, UV irradiation, protein synthesis inhibitors, and inflammatory cytokines. In this study, we investigated whether JNK plays a role in the cellular response to different drugs commonly used in cancer chemotherapy. Treatment of human KB-3 carcinoma cells with Adriamycin resulted in a time- and dose-dependent activation of JNK of up to 40-fold. Treatment with vinblastine or etoposide (VP-16) also activated JNK, with maximum increases of 6.5- and 4.3-fold, respectively. Consistent with these findings, increased c-Jun phosphorylation was observed after drug treatment of cells. In contrast, none of the drugs significantly activated the extracellular response kinase/mitogen-activated protein kinase pathway. Since these drugs are transport substrates for the MDR1 gene product, P-glycoprotein, JNK was assayed in two multidrug-resistant (MDR) KB cell lines, KB-A1 and KB-V1, selected for resistance to Adriamycin and vinblastine, respectively. Relative to KB-3 cells, basal JNK activity was increased 7-fold in KB-A1 cells and 4-fold in KB-V1 cells, with no change in JNK protein expression, indicating that JNK is present in a more highly activated form in the MDR cell lines. Under conditions optimal for JNK activation, Adriamycin, vinblastine, and VP-16 all induced MDR1 mRNA expression in KB-3 cells. Our findings suggest that JNK activation is an important component of the cellular response to several structurally and functionally distinct anticancer drugs and may also play a role in the MDR phenotype.

Activation of the HIV-1 long terminal repeat by cytokines and environmental stress requires an active CSBP/p38 MAP kinase

The human immunodeficiency virus, type 1 (HIV-1) promoter is known to be activated by proinflammatory cytokines and UV light. These stimuli also activate various members of the mitogen-activated protein kinase family, including JNK/SAPK and CSBP/p38. In HeLa cells containing an integrated HIV-1 long terminal repeat (LTR) -driven reporter, we now show that the specific p38 inhibitor, SB203580, inhibits activation of the HIV-1 LTR by interleukin-1, tumor necrosis factor, UV light, and osmotic stress. Inhibition was 70-90% in all but the case of tumor necrosis factor stimulation, where inhibition was 50%. Each of these stimuli activated p38, which was inhibited by SB203580 in vitro and in vivo with an IC50 (between 0.1 and 1 microM) similar to that required to inhibit transcription. In contrast, SB203580 had no effect on JNK, which was also activated by these stimuli. The NFkappaB sites in the HIV-1 LTR were required for a response to cytokines but not to UV, and SB203580 remained capable of inhibiting UV activation in the absence of the NFkappaB sites. Studies in which SB203580 was added at different times relative to UV stimulation suggested that the critical p38-mediated phosphorylation event occurred between 2 and 4 h after UV treatment. These data indicate that p38 is required for HIV-1 LTR activation but that the action of p38 is delayed, presumably due to substrate unavailability or inaccessibility.

Jun N-terminal kinase mediates activation of skeletal muscle glycogen synthase by insulin in vivo

Mitogen-activated protein kinases (MAPKs) represent a conserved family of Ser/Thr protein kinases with central roles in intracellular signaling. Activation of three prominent members of the MAPK family, i.e. extracellular response kinases (ERK), jun N-terminal kinase (JNK), and p38, was defined in vivo in order to establish their role, if any, in the cardinal response of skeletal muscle to insulin, the activation of glycogen synthesis. Insulin was found to activate ERK, JNK, and p38 in skeletal muscle. The time courses for activation of the three MAPKs by insulin, however, are distinctly different. Activation of JNK occurs most rapidly, within seconds. Activation of p38 by insulin follows that of JNK, within minutes. Activation of ERK occurs last, 4 min after administration of insulin. The temporal relationship between the activation of ERK, JNK, p38 and the downstream elements p90(rsk) and PP-1 in vivo suggest that JNK, but neither ERK nor p38 MAPKs, mediates insulin activation of glycogen synthase in vivo. Activation of JNK by anisomycin in vivo mimics activation of glycogen synthase by insulin. Challenge by anisomycin and insulin, in combination, are not additive, suggesting a common mode of glycogen synthase activation. The p90(rsk) isoform rapidly activated by insulin is identified as RSK3. In addition, RSK3 can be activated by JNK in vitro. Based upon these data a signal linkage map for activation of glycogen synthase in vivo in skeletal muscle can be constructed in which JNK mediates activation of glycogen synthase via RSK3.

Involvement of stress-activated protein kinase and p38 mitogen-activated protein kinase in mIgM-induced apoptosis of human B lymphocytes

Despite intensive efforts, the intracellular signaling pathways that mediate apoptosis remain unclear. The human B lymphoma cell line, B104, possesses characteristics that make it an attractive model for analysis of receptor-mediated apoptosis. Although these cells express both membrane IgM (mIgM) and membrane IgD (mIgD) crosslinking mIgM results in significant apoptosis while crosslinking mIgD does not. Our results show that crosslinking mIgM but not mIgD induced a delayed and sustained activation of the mitogen-activated protein kinase (MAPK) family members stress-activated protein kinase (SAPK) and p38 MAPK. The calcium ionophore ionomycin, which also induces apoptosis in B104 cells, stimulated a similar SAPK and p38 MAPK response. Cyclosporin A, a potent inhibitor of apoptosis induced by either mIgM or ionomycin, inhibited activation of both SAPK and p38 MAPK, suggesting that stimulation of these kinases may be required for induction of apoptosis. Collectively, our results indicate that SAPK and p38 MAPK may be downstream targets during mIgM-induced, calcium-mediated, apoptosis in human B lymphocytes.

Mammalian mitogen-activated protein kinase pathways are regulated through formation of specific kinase-activator complexes

Mammalian cells contain at least three signaling systems which are structurally related to the mitogen-activated protein kinase (MAPK) pathway. Growth factors acting through Ras primarily stimulate the Raf/MEK/MAPK cascade of protein kinases. In contrast, many stress-related signals such as heat shock, inflammatory cytokines, and hyperosmolarity induce the MEKK/SEK(MKK4)/SAPK(JNK) and/or the MKK3 or MKK6/p38(hog) pathways. Physiological agonists of these pathway types are either qualitatively or quantitatively distinct, suggesting few common proximal signaling elements, although past studies performed in vitro, or in cells using transient over-expression, reveal interaction between the components of all three pathways. These studies suggest a high degree of cross-talk apparently not seen in vivo. We have examined the possible molecular basis of the differing agonist profiles of these three MAPK pathways. We report preferential association between MAP kinases and their activators in eukaryotic cells. Furthermore, using the yeast 2-hybrid system, we show that association between these components can occur independent of additional eukaryotic proteins. We show that SAPK(JNK) or p38(hog) activation is specifically impaired by co-expression of cognate dominant negative MAP kinase kinase mutants, demonstrating functional specificity at this level. Further divergence and insulation of the stress pathways occurs proximal to the MAPK kinases since activation of the MAPK kinase kinase MEKK results in SAPK(JNK) activation but does not cause p38(hog) phosphorylation. Therefore, in intact cells, the three MAPK pathways may be independently regulated and their components show specificity in their interaction with cognate cascade members. The degree of intermolecular specificity suggests that mammalian MAPK signaling pathways may remain distinct without the need for specific scaffolding proteins to sequester components of individual pathways.

Initiation of acute phase response and synthesis of cytokines

A variety of injuries, such as bacterial infection or ischemic tissue necrosis, induce systemic acute phase reaction expressed as fever, leukocytosis, release of several hormones, activation of clotting, complement and kinin forming pathways, and drastic increase of synthesis of certain plasma proteins. The reaction is triggered by 'alarm molecules', including free radicals, which activate several stress-sensitive protein kinases (ERK, p38, JNK) in macrophages and other responsive cells. These kinases phosphorylate, usually in a multi-step cascade, transcription factors belonging primarily to C/EBP, NF-kappa B and AP-1 families. Active transcription factors after translocation to nucleus interact with responsive elements in the gene promoters of acute-phase cytokines: tumor necrosis factor-alpha, interleukin-1 and interleukin-6. Enhanced transcription of these genes is usually followed by rapid translation and precursor protein processing leading to the release of biologically active cytokines. Fine tuning of the acute phase response appears to be regulated at all stages: primary signals, kinase cascades, transcription factors, mRNA stability and translation, cytokine precursor processing, secretion and bioavailability. This makes possible designing of specific inhibitors of cytokine synthesis as potential therapeutic drugs.

Independent regulation of JNK/p38 mitogen-activated protein kinases by metabolic oxidative stress in the liver

The stress-activated protein kinases JNK and p38 mediate increased gene expression and are activated by environmental stresses and proinflammatory cytokines. Using an in vivo model in which oxidative stress is generated in the liver by intracellular metabolism, rapid protein-DNA complex formation on stress-activated AP-1 target genes was observed. Analysis of the induced binding complexes indicates that c-fos, c-jun, and ATF-2 were present, but also two additional jun family members, JunB and JunD. Activation of JNK precedes increased AP-1 DNA binding. Furthermore, JunB was shown to be a substrate for JNK, and phosphorylation requires the N-terminal activation domain. Unexpectedly, p38 activity was found to be constitutively active in the liver and was down-regulated through selective dephosphorylation following oxidative stress. One potential mechanism for p38 dephosphorylation is the rapid stress-induced activation of the phosphatase MKP-1, which has high affinity for phosphorylated p38 as a substrate. These data demonstrate that there are mechanisms for independent regulation of the JNK and p38 mitogen-activated protein kinase signal transduction pathways after metabolic oxidative stress in the liver.

A JNK signal transduction pathway that mediates morphogenesis and an immune response in Drosophila

The Drosophila MAP kinase DJNK is a homolog of the mammalian c-Jun amino-terminal kinase (JNK). Mutations in the DJNK gene correspond to the complementation group basket. DJNK is phosphorylated and activated by the Drosophila MAP kinase kinase HEP. Substrates of DJNK include the transcription factor DJun. DJNK participates in multiple physiological processes. Exposure to endotoxic lipopolysaccharide initiates an insect immune response and leads to DJNK activation. In addition, embryos lacking DJNK are defective in dorsal closure, a process in which the lateral epithelial cells migrate over the embryo and join at the dorsal midline. These data demonstrate that the DJNK signal transduction pathway mediates an immune response and morphogenesis in vivo.

p38 mitogen-activated protein kinase phosphorylates cytosolic phospholipase A2 (cPLA2) in thrombin-stimulated platelets. Evidence that proline-directed phosphorylation is not required for mobilization of arachidonic acid by cPLA2

The Ca2+-sensitive 85-kDa cytosolic phospholipase A2 (cPLA2) is responsible for thrombin-stimulated mobilization of arachidonic acid for the synthesis of thromboxane A2 in human platelets. We have previously shown that thrombin activates p38 kinase, a recently discovered new member of the mitogen-activated protein kinase family (Kramer, R. M., Roberts, E. F., Strifler, B. A., and Johnstone, E. M. (1995) J. Biol. Chem. 270, 27395-27398) and also induces phosphorylation of cPLA2, thereby increasing its intrinsic catalytic activity. In the present study we have examined the role of p38 kinase in the phosphorylation and activation of cPLA2 in stimulated platelets. We have observed that activation of p38 kinase accompanies receptor-mediated events in platelets and coincides with cPLA2 phosphorylation. Furthermore, in the presence of inhibitors of p38 kinase, the proline-directed phosphorylation of cPLA2 was completely blocked in platelets stimulated with the thrombin receptor agonist peptide SFLLRN and was suppressed during the early (up to 2 min) phase of platelet stimulation caused by thrombin. Unexpectedly, we found that prevention of proline-directed phosphorylation of cPLA2 in stimulated platelets did not attenuate its ability to release arachidonic acid from platelet phospholipids. We conclude that: 1) cPLA2 is a physiological target of p38 kinase; 2) p38 kinase is involved in the early phosphorylation of cPLA2 in stimulated platelets; and 3) proline-directed phosphorylation of cPLA2 is not required for its receptor-mediated activation.

Dissection of TNF receptor 1 effector functions: JNK activation is not linked to apoptosis while NF-kappaB activation prevents cell death

Through its type 1 receptor (TNFR1), the cytokine TNF elicits an unusually wide range of biological responses, including inflammation, tumor necrosis, cell proliferation, differentiation, and apoptosis. We investigated how TNFR1 activates different effector functions; the protein kinase JNK, transcription factor NF-kappaB, and apoptosis. We found that the three responses are mediated through separate pathways. Recruitment of the signal transducer FADD to the TNFR1 complex mediates apoptosis but not NF-kappaB or JNK activation. Two other signal transducers, RIP and TRAF2, mediate both JNK and NF-kappaB activation. These two responses, however, diverge downstream to TRAF2. Most importantly, JNK activation is not involved in induction of apoptosis, while activation of NF-kappaB protects against TNF-induced apoptosis.

Crystal structure of p38 mitogen-activated protein kinase

p38 mitogen-activated protein kinase is activated by environmental stress and cytokines and plays a role in transcriptional regulation and inflammatory responses. The crystal structure of the apo, unphosphorylated form of p38 kinase has been solved at 2.3 A resolution. The fold and topology of p38 is similar to ERK2 (Zhang, F., Strand, A., Robbins, D., Cobb, M. H., and Goldsmith, E. J. (1994) Nature 367, 704-711). The relative orientation of the two domains of p38 kinase is different from that observed in the active form of cAMP-dependent protein kinase. The twist results in a misalignment of the active site of p38, suggesting that the orientation of the domains would have to change before catalysis could proceed. The residues that are phosphorylated upon activation of p38 are located on a surface loop that occupies the peptide binding channel. Occlusion of the active site by the loop, and misalignment of catalytic residues, may account for the low enzymatic activity of unphosphorylated p38 kinase.

Stimulation of stress-activated protein kinase and p38 HOG1 kinase in murine keratinocytes following photodynamic therapy with benzoporphyrin derivative

The activation state of the members of the mitogen-activated protein kinase family following photodynamic therapy (PDT) with benzoporphyrin derivative monoacid ring A was investigated using a naturally transformed murine keratinocyte cell line, Pam 212. PDT involves the use of photosensitizer molecules and a specific wavelength of visible light. The process of PDT generates singlet oxygen and other reactive oxygen intermediates (ROIs), and the cytotoxic effect of these ROIs is the basis for the use of PDT to treat cancer and psoriasis. PDT caused a strong dose- and time-dependent activation of both stress-activated protein kinase (SAPK) and p38 HOG1. The maximum activation of SAPK and p38 HOG1 occurred between 20 and 30 min following PDT treatment with 200 ng/ml benzoporphyrin derivative monoacid ring A and 2 J/cm2 of red light at 690 nm. In our system, PDT did not cause significant activation of extracellularly regulated kinase (ERK) 1 and ERK2. Under the same experimental conditions, ultraviolet light irradiation caused strong activation of SAPK and p38 HOG1 and minimum activation of ERK1 and ERK2 in Pam212 cells. A number of ROI scavengers were tested for their effect on PDT-induced SAPK and p38 HOG1 activation. Both L-histidine and N-acetyl-L-cysteine showed a significant inhibitory effect on PDT-induced SAPK and p38 HOG1 activation. This indicated that PDT-induced SAPK and p38 HOG1 activation may be partially mediated by ROI.

Purification and identification of a major activator for p38 from osmotically shocked cells. Activation of mitogen-activated protein kinase kinase 6 by osmotic shock, tumor necrosis factor-alpha, and H2O2

A stress-activated, serine/threonine kinase, p38 (also known as HOG1 or MPK2) belongs to a subgroup of mitogen-activated protein kinase (MAPK) superfamily molecules. An activity to activate p38 (p38 activator activity) as well as p38 activity itself were greatly stimulated by hyperosmolar media in mouse lymphoma L5178Y cells. The activator activity has been purified by sequential chromatography. A 36-kDa polypeptide that was coeluted with the activity in the final chromatography step was identified as MAPK kinase 6 (MAPKK6) by protein microsequencing analysis. Monoclonal and polyclonal antibodies raised against recombinant MAPKK6 recognized specifically the 36-kDa MAPKK6 protein but did not cross-react with MKK3 proteins. The use of these anti-MAPKK6 antibodies revealed that two major peaks of the p38 activator activity in the first chromatography step reside in the activated MAPKK6. Using a genetic screen in yeast, we isolated MKK3b, an alternatively spliced form of MKK3. Like MKK3 and MAPKK6, MKK3b was shown to be a specific activator for p38 and was activated by osmotic shock when expressed in COS7 cells. Immunoblotting analysis revealed that MAPKK6 is expressed highly in HeLa and KB cells and scarcely in PC12 cells, whereas MKK3 and MKK3b are expressed in all cells examined. Immunodepletion of MAPKK6 from the extracts obtained from L5178Y cells and KB cells exposed to hyperosmolar media depleted them of almost all of the p38 activator activity, indicating that MAPKK6 is a major activator for p38 in an osmosensing pathway in these cells. In addition, MAPKK6 was activated strongly by tumor necrosis factor-alpha, H2O2, and okadaic acid and moderately by cycloheximide in KB cells. Thus, there are at least three members of p38 activator, MKK3, MKK3b, and MAPKK6, and MAPKK6 may function as a major activator for p38 when expressed.

Ras-dependent and Ras-independent activation pathways for the stress-activated-protein-kinase cascade

We have previously shown that osmotic stress activates both the mitogen-activated protein kinase (MAPK) cascade and the stress-activated protein kinase (SAPK, also known as JNK) cascade in rat fibroblastic 3Y1 cells and rat PC12 cells. Here, we show that treatment of these cells with sodium arsenite, a chemical compound that mimics the effects of heat shock, or anisomycin, a protein synthesis inhibitor, induces activation of SAPKs potently. These chemical compounds also stimulated the activity of SEK1/MKK4/JNKK, SAPK activator, and the activity of MEKK, SEK1 activator. Expression of a dominant negative mutant of Ras blocked the anisomycin-induced activation of SAPK and SEK1, but did not affect markedly the arsenite-induced or heat shock-induced activation in PC12 cells. The osmotic-stress-induced activation of SAPK was insensitive to the expression of a dominant negative Ras, but was partly sensitive to down-regulation of protein kinase C. These results suggest the existence of Ras-dependent and Ras-independent activation pathways for the SAPK cascade triggered by environmental stresses including chemical stress in PC12 cells. Cell staining with a specific anti-SAPK serum showed that SAPKs were present in both the cytoplasm and the nucleus under normal conditions, and became located mainly in the nucleus after osmotic stress or ultraviolet treatment, suggesting the nuclear translocation of SAPKs.

Interleukin 1 activates jun N-terminal kinases JNK1 and JNK2 but not extracellular regulated MAP kinase (ERK) in human glomerular mesangial cells

Interleukin 1 (IL-1) potently activates human glomerular mesangial cells (HMC). In cytosolic extracts of IL-1-stimulated HMC or in anion exchange chromatography fractions we could not find any change in phosphorylation of myelin basic protein (MBP), a good substrate for extracellular regulated kinase (ERK). In contrast, IL-1 stimulated GST-jun kinase activity at least 10-fold. The jun kinase activity could be characterised as JNK1 and JNK2 at the protein and mRNA level. IL-1, TNF, UV light and osmotic stress, but not PMA, LPS, IL-3, IL-4, IL-6, IL-8, IL-10, IL-13, GM-CSF, PDGF, bFGF, TGF-beta and interferon-gamma were able to stimulate jun kinase activity in HMC, suggesting that jun kinase is selectively mediating signal transduction of the proinflammatory cytokines IL-1 and TNF as well as of cellular stress in HMC.