How MAP kinases are regulated

A highly specific inhibitor of human p38 MAP kinase binds in the ATP pocket

The crystal structure of human p38 mitogen-activated protein (MAP) kinase in complex with a potent and highly specific pyridinyl-imidazole inhibitor has been determined at 2.0 A resolution. The structure of the kinase, which is in its unphosphorylated state, is similar to that of the closely-related ERK2. The inhibitor molecule is bound in the ATP pocket. A hydrogen bond is made between the pyridyl nitrogen of the inhibitor and the main chain amido nitrogen of residue 109, analogous to the interaction from the N1 atom of ATP. The crystal structure provides possible explanations for the specificity of this class of inhibitors. Other protein kinase inhibitors may achieve their specificity through a similar mechanism. The structure also reveals a possible second binding site for this inhibitor, with currently unknown function.

A mitogen-activated protein kinase (MAP-kinase) cascade is stimulated by platelet activating factor (PAF) in corneal epithelium

PURPOSE

The mitogen-activated protein kinases (MAPK) are a family of important proteins that respond to a variety of receptor-mediated stimuli and can link events occurring at the cell membrane with changes in the nucleus. In this study we investigate the effect of platelet activating factor (PAF), a lipid mediator formed in the cornea after injury, on the activation of a MAPK cascade in the rabbit corneal epithelium.

METHODS

Rabbit corneas were incubated with or without 500 nM PAF. PAF antagonists BN50730 or 50727 (10 microM) were added 10 min before PAF and the epithelium scraped and homogenated. To determine the enzymatic activity of MAPK and MAPK-kinase (MEK1 and MEK2), a 100,000 x g cytosolic fraction was used directly, fractionated by DE-52 cellulose or immunoprecipitated with antibodies. Activities of MAPK and MEK were assayed in the presence of myelin basic protein (MBP) as substrate (for MAPK) activity or inactive extracellular-signal regulated protein kinase (ERK2 or MAPK). Western blot analysis was performed using anti-ERK2, anti-MEK1, and anti-MEK2 antibodies.

RESULTS

Corneal tissue expresses ERK2 or MAPK, and both MEK1 and MEK2, the immediate upstream regulators of MAPK. PAF produces a rapid activation of MEK, as measured by in vitro kinase assays using either inactive ERK2 as substrate or a MAPK fraction obtained by DE-52 chromatography. There was a subsequent activation of MAPK, the maximal activity of which occurs 15 min after stimulation by PAF. PAF antagonists blocked the MEK/MAPK cascade, suggesting that the activation was by a receptor-mediated mechanism.

CONCLUSIONS

The evidence presented here, that a MAPK cascade is rapidly activated by PAF in the corneal epithelium, suggests that this signal transduction mechanism can be involved in the increased expression of collagenase and other protease genes, as well as in the activation of phospholipase A2, events that occur in the corneal epithelium after PAF stimuli.

Dual effect of lysophosphatidic acid on proliferation of glomerular mesangial cells

Among the variety of factors able to contribute to mesangial hypertrophy by altering mesangial cell growth, lysophosphatidic acid (LPA) is the focus of increasing attention. It is produced in plasma following platelet activation, as well as by mesangial cells stimulated by secretory phospholipase A2. As mitogenic/antimitogenic properties of LPA are already described in a variety of cells, knowledge of its specific actions on mesangial cells is of potential interest regarding the pathophysiology of glomerulus damage in situ. We tested the effect of LPA on cultured rat mesangial cell growth. At 10 to 20 microM, LPA stimulated thymidine incorporation as well as phosphorylation of mitogen-activated protein kinases (MAP-kinases) p42-p44 in dose- and time-dependent manner, which demonstrated a positive effect on cell proliferation. However, higher concentrations of LPA (100 microM) were unable to stimulate thymidine incorporation and partly inhibited the proliferative effect as well as p42-p44 phosphorylation evoked by serum. Finally, whereas lysophosphatidylcholine (10 to 20 microM) was lytic for mesangial cells, no cell lysis could be detected at the highest concentrations of LPA. Taken together, these results suggest that LPA exerts a dual effect on mesangial cell proliferation, which could be due to activation of distinct specific signaling pathways, in dose-dependent fashion. Specific actions of LPA able to modify mesangial cell proliferation in a positive or negative manner are also likely to influence the pathophysiological processes involved in the progression of glomerulosclerosis in the kidney.

Sodium salicylate induces apoptosis via p38 mitogen-activated protein kinase but inhibits tumor necrosis factor-induced c-Jun N-terminal kinase/stress-activated protein kinase activation

In a previous study, we demonstrated that sodium salicylate (NaSal) selectively inhibits tumor necrosis factor (TNF)-induced activation of the p42 and p44 mitogen-activated protein kinases (MAPKs) (known as extracellular signal-regulated kinases). Here we show that in normal human FS-4 fibroblasts NaSal inhibits TNF-induced activation of another member of the MAPK family, the c-Jun N-terminal kinase/stress-activated protein kinase. c-Jun N-terminal kinase activation induced by interleukin 1 or epidermal growth factor was less strongly inhibited by NaSal. Unexpectedly, treatment of FS-4 cells with NaSal alone produced a strong activation of p38 MAPK and cell death by apoptosis. NaSal-induced apoptosis was blocked by the selective p38 MAPK inhibitor SB-203580, indicating that p38 MAPK serves as a mediator of NaSal-induced apoptosis in human fibroblasts. Activation of p38 MAPK and the resulting induction of apoptosis may be important in the demonstrated antineoplastic actions of nonsteroidal anti-inflammatory drugs.

Mitogen-activated protein kinase pathways

Nearly all cell surface receptors utilize one or more of the mitogen-activated protein kinase cascades in their repertoire of signal transduction mechanisms. Recent advances in the study of such cascades include the cloning of genes encoding novel members of the cascades, further definition of the roles of the cascades in responses to extracellular signals, and examination of cross-talk between different cascades.

TAK1 mediates the ceramide signaling to stress-activated protein kinase/c-Jun N-terminal kinase

Ceramide has been proposed as a second messenger molecule implicated in a variety of biological processes. It has recently been reported that ceramide activates stress-activated protein kinase (SAPK, also known as c-Jun NH2-terminal kinase JNK), a subfamily member of mitogen-activated protein kinase superfamily molecules and that the ceramide/SAPK/JNK signaling pathway is required for stress-induced apoptosis. However, the molecular mechanism by which ceramide induces SAPK/JNK activation is unknown. Here we show that TAK1, a member of the mitogen-activated protein kinase kinase kinase family, is activated by treatment of cells with agents and stresses that induce an increase in ceramide. Ceramide itself stimulated the kinase activity of TAK1. Expression of a constitutively active form of TAK1 resulted in activation of SAPK/JNK and SEK1/MKK4, a direct activator of SAPK/JNK. Furthermore, expression of a kinase-negative form of TAK1 interfered with the activation of SAPK/JNK induced by ceramide. These results indicate that TAK1 may function as a mediator of ceramide signaling to SAPK/JNK activation.

p38 mitogen-activated protein kinase down-regulates nitric oxide and up-regulates prostaglandin E2 biosynthesis stimulated by interleukin-1beta

The inflammatory cytokine interleukin 1beta (IL-1beta) induces both cyclooxygenase-2 (Cox-2) and the inducible nitric-oxide synthase (iNOS) with increases in the release of prostaglandins (PGs) and nitric oxide (NO) from glomerular mesangial cells. However, the intracellular signaling mechanisms by which IL-1beta induces iNOS and Cox-2 expression is obscure. Our current studies demonstrate that IL-1beta produces a rapid increase in p38 mitogen-activated protein kinase (MAPK) phosphorylation and activation. Serum starvation and SC68376, a drug which selectively inhibits p38 MAPK in mesangial cells, were used to investigate whether p38 MAPK contributes to the signaling mechanism of IL-1beta induction of NO and PG synthesis. Serum starvation and SC68376 selectively inhibited IL-1beta-induced activation of p38 MAPK. Both SC68376 and serum starvation enhanced NO biosynthesis by increasing iNOS mRNA expression, protein expression, and nitrite production. In contrast, both SC68376 and serum starvation suppressed PG release by inhibiting Cox-2 mRNA, protein expression, and PGE2 synthesis. These data demonstrate that IL-1beta phosphorylates and activates p38 MAPK in mesangial cells. The activation of p38 MAPK may provide a crucial signaling mechanism, which mediates the up-regulation of PG synthesis and the down-regulation of NO biosynthesis induced by IL-1beta.

Identification of two essential phosphorylated threonine residues in the catalytic domain of Mekk1. Indirect activation by Pak3 and protein kinase C

The 78-kDa protein kinase Mekk1 plays an important role in the stress response pathway that involves the activation of downstream kinases Sek1 and stress-activated protein kinase/c-Jun NH2-terminal kinase. Conserved serine and threonine residues located between the kinase subdomains VII and VIII of many protein kinases are phosphorylated for maximal kinase activation. Two threonine residues within this region in Mekk1 at positions 560 and 572, but not the serine at 557, were shown to be essential for catalytic activity in this study. When these threonine residues were replaced with alanine, there was a significant loss in phosphotransferase activity toward the primary substrate, Sek1, and a large decrease in autophosphorylation activity. Site-directed mutagenesis demonstrated that these threonine residues cannot be replaced with either serine or glutamic acid for preservation of phosphotransferase activity. Further examination of the Mekk1 mutants isolated from 32P-labeled transfected COS cells showed that Thr-560 and Thr-572 were indeed phosphorylated after two-dimensional tryptic-chymotryptic phosphopeptide analysis. Additional determinants in the NH2-terminal domain of Mekk1 also play a role in the regulation of Mekk1 activity. Although Pak3 and PKC can activate Mekk1 in vivo, this interaction is indirect and independent, since there was no direct phosphorylation of Mekk1 by Pak3 or PKC or of Pak3 by PKC, respectively.

Differential effects of a mitogen-activated protein kinase kinase inhibitor on human neutrophil responses to chemotactic factors

Chemotactic factors, i.e., an N-formyl peptide, C5a, interleukin-8, and leukotriene B4, induced neutrophils to activate mitogen-activated protein (MAP) kinases, as defined by the tyrosine phosphorylation and decrease in electrophoretic mobility of immunodetected 44-, 42-, and 40-kDa proteins. PD 98059, an inhibitor of MAP kinase kinase activation, blocked these changes. The drug likewise blocked neutrophil chemotaxis but did not alter superoxide anion production and paradoxically enhanced degranulation responses to the stimuli. The MAP kinase pathway appears to have a highly selective role in mediating motility but not other cellular responses.

NIK is a new Ste20-related kinase that binds NCK and MEKK1 and activates the SAPK/JNK cascade via a conserved regulatory domain

Nck, an adaptor protein composed of one SH2 and three SH3 domains, is a common target for a variety of cell surface receptors. We have identified a novel mammalian serine/threonine kinase that interacts with the SH3 domains of Nck, termed Nck Interacting Kinase (NIK). This kinase is most homologous to the Sterile 20 (Ste20) family of protein kinases. Of the members of this family, GCK and MSST1 are most similar to NIK in that they bind neither Cdc42 nor Rac and contain an N-terminal kinase domain with a putative C-terminal regulatory domain. Transient overexpression of NIK specifically activates the stress-activated protein kinase (SAPK) pathway. Both the kinase domain and C-terminal regulatory region of NIK are required for full activation of SAPK. NIK likely functions upstream of MEKK1 to activate this pathway; a dominant-negative MEK kinase 1 (MEKK1) blocks activation of SAPK by NIK. MEKK1 and NIK also associate in cells and this interaction is mediated by regulatory domains on both proteins. Two other members of this kinase family, GCK and HPK1, contain C-terminal regulatory domains with homology to that of NIK. These findings indicate that the C-terminal domain of these proteins encodes a new protein domain family and suggests that this domain couples these kinases to the SAPK pathway, possibly by interacting with MEKK1 or related kinases.

Differential effects of protein kinase C, Ras, and Raf-1 kinase on the induction of the cardiac B-type natriuretic peptide gene through a critical promoter-proximal M-CAT element

The cardiac genes for the A- and B-type natriuretic peptides (ANP and BNP) are coordinately induced by growth promoters, such as alpha1-adrenergic receptor agonists (e.g. phenylephrine (PE)). Although inducible elements in the ANP gene have been identified, responsible elements in the BNP gene are unknown. In this study, reporter constructs transfected into neonatal rat ventricular myocytes showed that in the context of 2.5 kilobase pairs of native BNP 5'-flanking sequences, a 2-base pair mutation in a promoter-proximal M-CAT site (CATTCT) disrupted basal and PE-inducible transcription by more than 98%. Expression of constitutively active forms of Ras, Raf-1 kinase, and protein kinase C, all of which are activated by PE in cardiac myocytes, strongly stimulated BNP reporter expression. Isolated M-CAT elements conferred PE, protein kinase C, and Ras inducibility to a minimal BNP promoter, however, they did not confer Raf-1 inducibility. These results show that M-CAT elements can serve as targets for Ras-dependent, Raf-1-independent pathways, implying the involvement of c-Jun N-terminal kinase and/or p38 mitogen-activated protein kinases, but not extracellular signal-regulated protein kinase/mitogen-activated protein kinase. Moreover, the essential M-CAT element distinguishes the BNP gene from the ANP gene, which utilizes serum response elements and an Sp1-like sequence.

The mitogenic and myogenic actions of insulin-like growth factors utilize distinct signaling pathways

It is well established that mitogens inhibit differentiation of skeletal muscle cells, but the insulin-like growth factors (IGFs), acting through a single receptor, stimulate both proliferation and differentiation of myoblasts. Although the IGF-I mitogenic signaling pathway has been extensively studied in other cell types, little is known about the signaling pathway leading to differentiation in skeletal muscle. By using specific inhibitors of the IGF signal transduction pathway, we have begun to define the signaling intermediates mediating the two responses to IGFs. We found that PD098059, an inhibitor of mitogen-activated protein (MAP) kinase kinase activation, inhibited IGF-stimulated proliferation of L6A1 myoblasts and the events associated with it, such as phosphorylation of the MAP kinases and elevation of c-fos mRNA and cyclin D protein. Surprisingly, PD098059 caused a dramatic enhancement of differentiation, evident both at a morphological (fusion of myoblasts into myotubes) and biochemical level (elevation of myogenin and p21 cyclin-dependent kinase inhibitor expression, as well as creatine kinase activity). In sharp contrast, LY294002, an inhibitor of phosphatidylinositol 3-kinase, and rapamycin, an inhibitor of the activation of p70 S6 kinase (p70(S6k)), completely abolished IGF stimulation of L6A1 differentiation. We found that p70(S6k) activity increased substantially during differentiation, and this increase was further enhanced by PD098059. Our results demonstrate that the MAP kinase pathway plays a primary role in the mitogenic response and is inhibitory to the myogenic response in L6A1 myoblasts, while activation of the phosphatidylinositol 3-kinase/p70(S6k) pathway is essential for IGF-stimulated differentiation. Thus, it appears that signaling from the IGF-I receptor utilizes two distinct pathways leading either to proliferation or differentiation.

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.

Stimulation of the mitogen-activated protein kinase via the A2A-adenosine receptor in primary human endothelial cells

Adenosine exerts a mitogenic effect on human endothelial cells via stimulation of the A2A-adenosine receptor. This effect can also be elicited by the beta2-adrenergic receptor but is not mimicked by elevation of intracellular cAMP levels. In the present work, we report that stimulation of the A2A-adenosine receptor and of the beta2-adrenergic receptor activates mitogen-activated protein kinase (MAP kinase) in human endothelial cells based on the following criteria: adenosine analogues and beta-adrenergic agonists cause an (i) increase in tyrosine phosphorylation of the p42 isoform and to a lesser extent of the p44 isoform of MAP kinase and (ii) stimulate the phosphorylation of myelin basic protein by MAP kinase; (iii) this is accompanied by a redistribution of the enzyme to the perinuclear region. Pretreatment of the cells with cholera toxin (to down-regulate Gsalpha) abolishes activation of MAP kinase by isoproterenol but not that induced by adenosine analogues. In addition, MAP kinase stimulation via the A2A-adenosine receptor is neither impaired following pretreatment of the cells with pertussis toxin (to block Gi-dependent pathways) nor affected by GF109203X (1 microM; to inhibit typical protein kinase C isoforms) nor by a monoclonal antibody, which blocks epidermal growth factor-dependent signaling. In contrast, MAP kinase activation is blocked by PD 098059, an inhibitor of MAP kinase kinase 1 (MEK1) activation, which also blunts the A2A-adenosine receptor-mediated increase in [3H]thymidine incorporation. Activation of the A2A-adenosine receptor is associated with increased levels of GTP-bound p21(ras). Thus, our experiments define stimulation of MAP kinase as the candidate cellular target mediating the mitogenic action of the A2A-adenosine receptor on primary human endothelial cells; the signaling pathway operates via p21(ras) and MEK1 but is independent of Gi, Gs, and the typical protein kinase C isoforms. This implies an additional G protein which links this prototypical Gs-coupled receptor to the MAP kinase cascade.

Ras-dependent activation of c-Jun N-terminal kinase/stress-activated protein kinase in response to interleukin-3 stimulation in hematopoietic BaF3 cells

Activation of the c-Jun N-terminal kinase (JNK)/stress-activated protein kinase pathway in response to stimulation of the interleukin (IL)-3 or granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor was examined in mouse hematopoietic BaF3-derived cell lines (BaF3-N6 and -V2 cells). Significant increase in the activity of JNK1 was observed within 30 min following IL-3 or GM-CSF stimulation at physiological concentrations. Dominant-negative Ras(S17N), which is conditionally expressed in the presence of isopropyl-1-thio-beta-D-galactoside in BaF3-N6 cells, prevented the IL-3 stimulation of JNK1, whereas anisomycin-induced JNK1 activation was unaffected. Furthermore, a deletion mutant of the common beta subunit for IL-3 and GM-CSF receptors that consists of only the membrane-proximal region, including box 1 and box 2 motifs, was incapable of facilitating JNK1 activity as well as Ras activation. These results provide evidence that Ras is required for IL-3-stimulated JNK1 activation. We also examined if constitutively active Ras(G12V) alone could stimulate JNK1 activity by using the inducible expression system. Isopropyl-1-thio-beta-D-galactoside induction of Ras(G12V) in the BaF3-V2 cell line caused no significant increase in JNK1 activity, which could be activated by IL-3 or anisomycin. On the contrary, the extracellular signal-regulated kinase/mitogen-activated protein kinase pathway was fully activated following Ras(G12V) induction. Together with these results, it seems likely that the Ras protein is indispensable for the IL-3 stimulation of JNK1 although Ras activation by itself is insufficient for JNK1 activation.

Phosphorylation of protein kinase C-alpha on serine 657 controls the accumulation of active enzyme and contributes to its phosphatase-resistant state

Serine 657 in protein kinase C-alpha (PKCalpha) is a site of phosphorylation on expression of the recombinant protein in mammalian cells. To define the function of this phosphorylation, PKCalpha species with mutations of this site were investigated. The alanine mutant, S657A PKCalpha, displayed slow phosphate accumulation in pulse-chase experiments, indicating a rate-limiting role in the initial phase of phosphorylation. Consistent with this, the aspartic acid mutant, S657D PKCalpha, showed an increased rate of phosphate accumulation. Both the S657D and S657A PKCalpha mutants were slow to accumulate as fully phosphorylated forms during a second phase of phosphorylation. This latter property is shown to correlate with an increased phosphatase sensitivity and decreased protein kinase activity for these two PKCalpha mutants. It is further shown that once fully phosphorylated, the S657D PKCalpha mutant displays WT PKCalpha properties with respect to thermal stability and phosphatase sensitivity in vitro and in vivo; in contrast, the S657A PKCalpha mutant remains sensitive. The properties of the Ser-657 site PKCalpha mutants define functional roles for this phosphorylation in both the accumulation of phosphate on PKCalpha as well as in its agonist-induced dephosphorylation. These results are discussed in the context of a working model of PKCalpha behavior, providing insight into the workings of other kinases with equivalent sites of phosphorylation.

Specific interaction between casein kinase 2 and the nucleolar protein Nopp140

Casein kinase 2 (CK2) is a multifunctional second messenger-independent protein serine/threonine kinase that phosphorylates many different proteins. To understand the function and regulation of this enzyme, biochemical methods were used to search for CK2-interacting proteins. Using immobilized glutathione S-transferase fusion proteins of CK2, the nucleolar protein Nopp140 was identified as a CK2-associated protein. It was found that Nopp140 binds primarily to the CK2 regulatory subunit, beta. The possible in vivo association of Nopp140 with CK2 was also suggested from a coimmunoprecipitation experiment in which Nopp140 was detected in immunoprecipitates of CK2 prepared from cell extracts. Further studies using an overlay technique with radiolabeled CK2 as a probe revealed a direct CK2-Nopp140 interaction. Using deletion mutants of CK2beta subunits, the binding region of the CK2beta subunit to Nopp140 has been mapped. It was found that the NH2-terminal 20 amino acids of CK2beta are involved. Since Nopp140 has been identified as a nuclear localization sequence-binding protein and has been shown to shuttle between the cytoplasm and the nucleus, the finding of a CK2-Nopp140 interaction could shed light on our understanding of the function and regulation of CK2 and Nopp140.

Extracellular matrix is required for MAP kinase activation and proliferation of rat glomerular epithelial cells

This study examined the role of extracellular matrix (ECM) in the regulation of glomerular epithelial cell (GEC) proliferation. Epidermal growth factor (EGF) stimulated proliferation of GEC when the cells were adherent to collagen matrices, but not plastic substratum. Significant and prolonged EGF receptor (R) tyrosine autophosphorylation (which reflects EGF-R kinase activation) was induced by EGF only in GEC adherent to collagen. In addition, EGF stimulated the activity and tyrosine phosphorylation of p42 mitogen-activated protein (MAP) kinase (ERK2) in collagen-adherent GEC, but not in cells on plastic. An inhibitor of the p-42 MAP kinase pathway, PD98059, blocked EGF-induced MAP kinase activity and proliferation. Thus, adhesion to ECM enables EGF to induce proliferation of GEC, by facilitating activation of EGF-R and the p42 MAP kinase pathway. Signals from ECM to growth factor receptor tyrosine kinases may regulate cell turnover in the glomerulus under normal conditions and during immune glomerular injury.

Activation of signal transduction kinases by tamoxifen

PURPOSE

To study the signal transduction mechanisms of tamoxifen via the activation of MAPKs, JNK and ERK in order to understand its regulation of gene expression.

METHODS

The effects of tamoxifen (TAM) on the activation of serine/threonine mitogen-activated protein kinase (MAPK, p42/ERK2) and the stress-activated protein kinases (p46 SAPK or c-Jun N-terminal kinase, JNK1) were evaluated using a human cervical epitheloid carcinoma HeLa cell line.

RESULTS

TAM activated both JNK1 and ERK2 activities in a time- and dose-dependent manner in HeLa cells. The activation of JNK1 was enhanced when the cells were pretreated with prooxidant H2O2.

CONCLUSIONS

These studies show that TAM activates the signal transduction kinases, JNK1 and ERK2, which may play important roles in the regulation of gene expression by TAM.

Regulation of actin filament dynamics by p38 map kinase-mediated phosphorylation of heat shock protein 27

We have studied the contribution of the individual kinases of the MAP (mitogen-activated protein) kinase family, including ERK (extracellular-signal regulated kinase), JNK/SAPK (c-JUN NH2-terminal kinase/stress-activated protein kinase) and p38, to activation of the HSP27 (heat shock protein 27) kinase MAPKAP kinase-2/3 and to HSP27 phosphorylation in Chinese hamster CCL39 cells stimulated by either growth factors, cytokines or stressing agents. In vitro assays using fractionated cell extracts or immunoprecipitates indicated that only fractions containing ERK or p38, and not those containing JNK/SAPK, had the capacity to activate MAPKAP kinase-2/3. In vivo, however, it appeared that only p38 is an upstream activator of HSP27 phosphorylation after both stress or growth factor stimulation: expression of an interfering mutant of ras, which blocked the activation of ERK by both types of inducers, had no effect on HSP27 phosphorylation and p38 activation; and the cell-permeant specific inhibitor of 038, SB203580, blocked MAPKAP-kinase2/3 activation and HSP27 phosphorylation. HSP27 has been suggested to have a phosphorylation-activated homeostatic function at the actin cytoskeleton level. This raises the possibility that p38 might be directly involved in mediating actin responses to external stimuli. Accordingly, we observed that a prior activation of p38 increased the stability of the actin microfilaments in cells exposed to cytochalasin D. The effect was dependent on the expression of HSP27 and was totally annihilated by blocking the p38 activity with SB203580. The results provide strong support to the idea that activation of p38 during adverse environmental conditions serves a homeostatic function aimed at regulating actin dynamics that would otherwise be destabilized during stress. Its activation during normal agonist stimulation may constitute an additional actin signaling pathway, the importance of which depends on the level of expression of HSP27.

Differential expression and activation of MAP kinases in dedifferentiated MDCK-focus cells

Mitogen-activated protein kinases (MAPK) play a key role in the regulation of cellular processes such as cell growth, cell differentiation, and apoptosis. However, the specific function of single isoforms of the MAPK family in renal epithelial cell differentiation and/or proliferation has not been investigated so far. We now report stable reduction of extracellular signal-regulated kinase 1 (ERK1) protein expression and lack of serum-induced ERK1 activation in alkali-dedifferentiated Madin-Darby canine kidney-C7 focus (MDCK-C7F) cells compared with their parental epithelial MDCK-C7 cells. The changes in ERK1 protein expression and activation were accompanied by a small rise in c-jun NH2-terminal kinase 1 (JNK1) protein expression but slightly decreased basal and anisomycin-stimulated JNK1 activity. In contrast, ERK2 phosphorylation, as assessed by using an antibody which detects phosphorylated tyrosine 204 of both ERK1 and ERK2, as well as enzymatic ERK2 activity, was substantially increased in untreated and fetal calf serum-treated MDCK-C7F cells, although ERK2 protein expression remained unchanged. Differential expression and activation of ERK1, ERK2, and JNK1 were accompanied by an inhibition of serum-induced MDCK-C7F cell proliferation. Together, our results demonstrate an association between changes in the activation of certain MAPK and alkali-induced stable MDCK-C7 cell dedifferentiation. Moreover, these data provide evidence for distinct signaling functions of ERK1 and ERK2 in these cells.

Akt signaling: linking membrane events to life and death decisions

Many growth factors exert their effects on cells by simulating intracellular kinases, which add phosphate groups to proteins. Two reports on p. 661 and p. 665 of this week's issue implicate the oncogenic kinase Akt in IGF-1-mediated neuronal survival (Dudek et al .) and show how lipids can regulate the activity of Akt (Franke et al .). In his Perspective, Hemmings discusses these results and several other recent reports about Akt and its unique mode of activation by phospholipids.

Differential effect of shear stress on extracellular signal-regulated kinase and N-terminal Jun kinase in endothelial cells. Gi2- and Gbeta/gamma-dependent signaling pathways

Shear stress differentially regulates production of many vasoactive factors at the level of gene expression in endothelial cells that may be mediated by mitogen-activated protein kinases, including extracellular signal-regulated kinase (ERK) and N-terminal Jun kinase (JNK). Here we show, using bovine aortic endothelial cells (BAEC), that shear stress differentially regulates ERK and JNK by mechanisms involving Gi2 and pertussis toxin (PTx)-insensitive G-protein-dependent pathways, respectively. Shear activated ERK with a rapid, biphasic time course (maximum by 5 min and basal by 30-min shear exposure) and force dependence (minimum and maximum at 1 and 10 dyn/cm2 shear stress, respectively). PTx treatment prevented shear-dependent activation of ERK1/2, consistent with a Gi-dependent mechanism. In contrast, JNK activity was maximally turned on by a threshold level of shear force (0.5 dyn/cm2 or higher) with a much slower and prolonged time course (requiring at least 30 min to 4 h) than that of ERK. Also, PTx had no effect on shear-dependent activation of JNK. To further define the shear-sensitive ERK and JNK pathways, vectors expressing hemagglutinin epitope-tagged ERK (HA-ERK) or HA-JNK were co-transfected with other vectors by using adenovirus-polylysine in BAEC. Expression of the mutant (alpha)i2(G203), antisense G(alpha)i2 and a dominant negative Ras (N17Ras) prevented shear-dependent activation of HA-ERK, while that of (alpha)i2(G204) and antisense (alpha)i3 did not. Expression of a Gbeta/gamma scavenger, the carboxyl terminus of beta-adrenergic receptor kinase (betaARK-ct), and N17Ras inhibited shear-dependent activation of HA-JNK. Treatment of BAEC with genistein prevented shear-dependent activation of ERK and JNK, indicating the essential role of tyrosine kinase(s) in both ERK and JNK pathways. These results provide evidence that 1) Gi2-protein, Ras, and tyrosine kinase(s) are upstream regulators of shear-dependent activation of ERK and 2) that shear-dependent activation of JNK is regulated by mechanisms involving Gbeta/gamma, Ras, and tyrosine kinase(s).

Regulation of muscarinic M2 receptors

The molecular mechanisms involved in the regulation of muscarinic receptor gene expression are poorly understood. In an effort to gain a better understanding of the regulation of M2 receptors, we have investigated homologous and heterologous regulation of M2 muscarinic receptor protein and gene expression in human embryonic lung fibroblasts (HEL 299 cells). HEL 299 cells constitutively express m2 receptors, with no evidence of other muscarinic receptor subtypes. We have shown that M2 receptors in these cells can be down-regulated by muscarinic and beta2-adrenergic receptor agonists. Unlike the down-regulation mediated by muscarinic and beta-adrenergic stimulation, activation of PKC with PDBu was mediated through changes in m2 muscarinic receptor mRNA through reduced gene transcription. Because of the inflammatory nature of asthma, we have focused on delineating the interactions between cytokines and M2 receptors in an attempt to define potential endogenous modulators of M2 receptor expression. We have shown that the multi-functional cytokine, transforming growth factor beta1 (TGF-beta1), which is involved in several inflammatory conditions induces desensitization and down-regulation of M2 muscarinic receptor protein and gene expression that was mediated through a reduction in the rate of m2 receptor gene transcription. Other cytokines of interest are tumor necrosis factor alpha (TNF-alpha) and interleukin 1beta (IL-1beta) which are elevated in asthma. We have demonstrated that TNF-alpha and IL-1beta synergise to induce down-regulation of M2 muscarinic receptor protein and mRNA which was associated with functional desensitization of the receptor protein. The M2 receptor mRNA down-regulation appeared to be mediated through a reduction in the rate of m2 receptor gene transcription which may be dependent on the transcription and translation of unknown protein factor(s). Moreover, a role of PKA and ceramide pathways in M2 receptor regulation is suggested. Collectively, our work provides a mechanistic explanation of previous reports indicating altered function of M2 receptors in asthma. Ours results also suggest that the expression of this receptor subtype may be under the control of a cytokine network at the airways.

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.

Suppression of nerve growth factor-induced neuronal differentiation of PC12 cells. N-acetylcysteine uncouples the signal transduction from ras to the mitogen-activated protein kinase cascade

The cellular redox state is thought to play an important role in a wide variety cellular signaling pathways. Here, we investigated the involvement of redox regulation in the nerve growth factor (NGF) signaling pathway and neuronal differentiation in PC12 cells. N-acetyl-L-cysteine (NAC), which acts as a reductant in cells both by its direct reducing activity and by increasing the synthesis of the cellular antioxidant glutathione, inhibited neuronal differentiation induced by NGF or by the expression of oncogenic ras in PC12 cells. NAC suppressed NGF-induced c-fos gene expression and AP-1 activation. These results suggest that neuronal differentiation and NGF signaling are subject to regulation by the cellular redox state. NAC also suppressed the NGF-induced activation of mitogen-activated protein kinases (MAPKs) and decreased the amount of tyrosine phosphorylation of MAPKs. The suppression of MAPK by NAC was independent of glutathione synthesis. In parallel with the suppression of MAPK, the activation of MAPK kinase kinase activity was also suppressed in the presence of NAC. In contrast, NGF-induced activation of Ras was not inhibited by NAC. The inhibitory effect of NAC on the MAPK cascade was independent of transcription and translation. Thus, NAC suppresses NGF-induced neuronal differentiation by uncoupling the signal transduction from Ras to the MAP kinase cascade in PC12 cells.

Molecular cloning and characterization of a novel protein kinase with a catalytic domain homologous to mitogen-activated protein kinase kinase kinase

Mitogen-activated protein kinase (MAPK) signaling cascades include MAPK or extracellular signal-regulated kinase (ERK), MAPK kinase (MKK or MEK), and MAPK kinase kinase (MAPKKK or MEKK). MAPKK kinase/MEKK phosphorylates and activates its downstream protein kinase, MAPK kinase/MEK, which in turn activates MAPK. We report herein the isolation of a cDNA encoding a novel protein kinase designated MAPKKK5 from a human macrophage library. The nucleotide sequence predicts that MAPKKK5 encodes an open reading frame of 1374 amino acids with all 11 kinase subdomains. The putative catalytic domain of MAPKKK5 shows significant sequence homology to the kinase domains of the MAPKKK/MEKK level protein kinases from mouse MEKK2 and -3, Drosophila melanogaster PK92B, Saccharomyces cerevisiae STE11, and Schizosaccharomyces pombe BYR2. Northern blot analysis showed that MAPKKK5 transcript is abundantly expressed in human heart and pancreas. When transiently expressed in COS and 293 cells, MAPKKK5 markedly activated c-Jun N-terminal kinase or stress-activated protein kinase, but not MAPK/ERK. Furthermore, MAPKKK5 that was immunoprecipitated from transfected 293 cells was able to phosphorylate and activate MKK4 in vitro, suggesting that MAPKKK5 may be an upstream activator of MKK4 in the c-Jun N-terminal kinase pathway.

The osmoregulatory pathway represses mating pathway activity in Saccharomyces cerevisiae: isolation of a FUS3 mutant that is insensitive to the repression mechanism

Mitogen-activated protein (MAP) kinase cascades are conserved signal transduction pathways that are required for eukaryotic cells to respond to a variety of stimuli. Multiple MAP kinase pathways can function within a single cell type; therefore, mechanisms that insulate one MAP kinase pathway from adventitious activations by parallel pathways may exist. We have studied interactions between the mating pheromone response and the osmoregulatory (high-osmolarity glycerol response [HOG]) pathways in Saccharomyces cerevisiae which utilize the MAP kinases Fus3p and Hog1p, respectively. Inactivating mutations in HOG pathway kinases cause an increase in the phosphotyrosine content of Fus3p, greater expression of pheromone-responsive genes, and increased sensitivity to growth arrest by pheromone. Therefore, the HOG pathway represses mating pathway activity. In a HOG1+ strain, Fus3p phosphotyrosine increases modestly and transiently following an increase in the extracellular osmolarity; however, it increases to a greater extent and for a sustained duration in a hog1-delta strain. Thus, the HOG-mediated repression of mating pathway activity may insulate the mating pathway from activation by osmotic stress. A FUS3 allele whose gene product is resistant to the HOG-mediated repression of its phosphotyrosine content has been isolated. This mutant encodes an amino acid substitution in the highly conserved DPXDEP motif in subdomain XI. Other investigators have shown that the corresponding amino acid is also mutated in a gain-of-function allele of the MAP kinase encoded by the rolled locus in Drosophila melanogaster. These data suggest that the DPXDEP motif plays a role in the negative regulation of MAP kinases.

The fission yeast pmk1+ gene encodes a novel mitogen-activated protein kinase homolog which regulates cell integrity and functions coordinately with the protein kinase C pathway

We have isolated a gene, pmk1+, a third mitogen-activated protein kinase (MAPK) gene homolog from the fission yeast Schizosaccharomyces pombe. The predicted amino acid sequence shows the most homology (63 to 65% identity) to those of budding yeast Saccharomyces Mpk1 and Candida Mkc1. The Pmk1 protein contains phosphorylated tyrosines, and the level of tyrosine phosphorylation was increased in the dsp1 mutant which lacks an attenuating phosphatase for Pmk1. The level of tyrosine phosphorylation appears constant during hypotonic or heat shock treatment. The cells with pmk1 deleted (delta pmk1) are viable but show various defective phenotypes, including cell wall weakness, abnormal cell shape, a cytokinesis defect, and altered sensitivities to cations, such as hypersensitivity to potassium and resistance to sodium. Consistent with a high degree of conservation of amino acid sequence, multicopy plasmids containing the MPK1 gene rescued the defective phenotypes of the delta pmk1 mutant. The frog MAPK gene also suppressed the pmk1 disruptant. The results of genetic analysis indicated that Pmk1 lies on a novel MAPK pathway which does not overlap functionally with the other two MAPK pathways, the Spk1-dependent mating signal pathway and Sty1/Spc1/Phh1-dependent stress-sensing pathway. In Saccharomyces cerevisiae, Mpk1 is involved in cell wall integrity and functions downstream of the protein kinase C homolog. In contrast, in S. pombe, Pmk1 may not act in a linear manner with respect to fission yeast protein kinase C homologs. Interestingly, however, these two pathways are not independent; instead, they regulate cell integrity in a coordinate manner.

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.

Insulin stimulates mitogen-activated protein kinase by a Ras-independent pathway in 3T3-L1 adipocytes

To characterize tissue-specific differences in insulin signaling, we compared the mechanisms of mitogen-activated protein (MAP) kinase activation by insulin in the mitogenically active 3T3-L1 fibroblasts with the metabolically active 3T3-L1 adipocytes. In both cell lines, insulin significantly increased p21(ras).GTP loading (1.5-2-fold) and MAP kinase activity (5-8-fold). Inhibition of Ras farnesylation with lovastatin blocked activation of p21(ras) and Raf-1 kinase in both 3T3-L1 fibroblasts and 3T3-L1 adipocytes. In 3T3-L1 fibroblasts, this was accompanied by an inhibition of the stimulatory effect of insulin on MAP kinase. In contrast, in 3T3-L1 adipocytes, despite an inhibition of activation of p21(ras) and Raf-1 by lovastatin, insulin continued to stimulate MAP kinase activity. Fractionation of the cell lysates on the FPLC Mono-Q column revealed that lovastatin inhibited insulin stimulation of ERK2 (and, to a lesser extent, ERK1) in 3T3-L1 fibroblasts and had no effect on the insulin-stimulated ERK2 in 3T3-L1 adipocytes. These results demonstrate an important distinction between the mechanism of insulin signaling in the metabolically and mitogenically active cells. Insulin activates MAP kinase by the Ras-dependent pathway in the 3T3-L1 fibroblasts and by the Ras-independent pathway in the 3T3-L1 adipocytes.

c-Jun can recruit JNK to phosphorylate dimerization partners via specific docking interactions

Structurally related serine/threonine kinases recognize similar phosphoacceptor peptides in vitro yet in vivo, they phosphorylate distinct substrates. To understand the basis for this specificity, we studied the interaction between the Jun kinases (JNKs) and Jun proteins. JNKs phosphorylate c-Jun very efficiently, JunD less efficiently, but they do not phosphorylate JunB. Effective JNK substrates require a separate docking site and specificity-conferring residues flanking the phosphoacceptor. The docking site increases the efficiency and specificity of the phosphorylation reaction. JunB has a functional JNK docking site but lacks specificity-conferring residues. Insertion of such residues brings JunB under JNK control. JunD, by contrast, lacks a JNK docking site, but its phosphoacceptor peptide is identical to that of c-Jun. Substrates such as JunD can be phosphorylated by JNK through heterodimerization with docking competent partners. Therefore, heterodimerization can affect the recognition of transcription factors by signal-regulated protein kinases.

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.

Control of the translational regulators PHAS-I and PHAS-II by insulin and cAMP in 3T3-L1 adipocytes

The eukaryotic initiation factor 4E (eIF-4E)-binding proteins PHAS-I and PHAS-II were found to have overlapping but different patterns of expression in tissues. Both PHAS proteins were expressed in 3T3-L1 adipocytes, in which insulin stimulated their phosphorylation, promoted dissociation of PHAS.eIF-4E complexes, and decreased the ability of both to bind exogenous eIF-4E. The effects of insulin were attenuated by rapamycin and wortmannin, two agents that block activation of p70(S6K). Unlike PHAS-I, PHAS-II was readily phosphorylated by cAMP-dependent protein kinase in vitro; however, the effects of insulin on both PHAS proteins were attenuated by agents that increase intracellular cAMP, by cAMP derivatives, and by phosphodiesterase inhibitors. These agents also markedly inhibited the activation of p70(S6K). In summary, our results indicate that PHAS-I and -II are controlled by the mammalian target of rapamycin and p70(S6K) signaling pathway and that in 3T3-L1 adipocytes this pathway is inhibited by increased cAMP.

Contributions of the mitogen-activated protein (MAP) kinase backbone and phosphorylation loop to MEK specificity

To examine the specificity of MEKs for MAP kinase family members, we determined the abilities of several MEK isoforms to phosphorylate mutants of the MAP kinase ERK2 and the related kinase ERK3 which are modified in the phosphorylation loop. The ERK2 mutants included mutations of the two phosphorylation sites, mutations of the acidic residue between these two sites, and mutations that shorten the length of this loop. All mutants were tested for phosphorylation by six mammalian MEKs and compared with several wild type MAP kinases. MEK1 and MEK2 phosphorylate a majority of the ERK2 mutants. MEK2 but not MEK1 will phosphorylate ERK3. Alteration of the residue between the two phosphorylation sites neither dramatically affected the activity of MEK1 and MEK2 toward ERK2 nor conferred recognition by other MEKs. Likewise, reduction of the length of the phosphorylation loop only partially reduces recognition by MEK1 and MEK2 but does not promote recognition by other MEKs. Thus other yet to be identified factors must contribute to the specificity of MEK recognition of MAP kinases.

Selective activation of the mitogen-activated protein kinase subgroups c-Jun NH2 terminal kinase and p38 by IL-1 and TNF in human articular chondrocytes

Previous studies suggested that tyrosine kinase activation is an important signal transduction event in the IL-1 response of chondrocytes. The present study identifies the mitogen-activated protein (MAP) kinases extracellular signal-regulated kinase (ERK)-1 and ERK-2 as major tyrosine phosphorylated proteins in IL-1 stimulated chondrocytes. Kinase assays on immunoprecipitates with myelin basic protein as substrate showed that ERK-1 and ERK-2 activation was detectable within 5 min after IL-1 stimulation and decreased to baseline within 60 min. Analysis of other members of the MAP kinase family showed that chondrocytes also express c-Jun NH2 terminal kinase (JNK)-1, JNK-2, and p38 proteins. These kinases were time-dependently activated by IL-1. Among other chondrocyte activators tested, only TNF activated all three of the MAP kinase subgroups. JNK and p38 were not activated by any of the other cytokines and growth factors tested. However, ERK was also activated by PDGF, IGF-1, and IL-6. Phorbol 12-myristate 13-acetate, calcium ionophore, and cAMP analogues only increased ERK activity but had no significant effects on JNK or p38. These results suggest differential activation of MAP kinase subgroups by extracellular stimuli. ERK is activated in response to qualitatively diverse extracellular stimuli and various second messenger agonists. In contrast, JNK and p38 are only activated by IL-1 or TNF, suggesting that these kinases participate in the induction of the catabolic program in cartilage.

Induction of cyclin D1 by simian virus 40 small tumor antigen

Cell-cycle progression is mediated by a co-ordinated interaction between cyclin-dependent kinases and their target proteins including the pRB and E2F/DP-1 complexes. Immunoneutralization and antisense experiments have established that the abundance of cyclin D1, a regulatory subunit of the cyclin-dependent kinases, may be rate-limiting for G1 phase progression of the cell cycle. Simian virus 40 (SV40) small tumor (t) antigen is capable of promoting G1 phase progression and augments substantially the efficiency of SV40 transformation through several distinct domains. In these studies, small t antigen stimulated cyclin D1 promoter activity 7-fold, primarily through an AP-1 binding site at -954 with additional contributions from a CRE site at -57. The cyclin D1 AP-1 and CRE sites were sufficient for activation by small t antigen when linked to an heterologous promoter. Point mutations of small t antigen between residues 97-103 that reduced PP2A binding were partially defective in the induction of the cyclin D1 promoter. These mutations also reduced activation of MEK1 and two distinct members of the mitogen-activated protein kinase family, the ERKs (extracellular signal regulated kinases) and the SAPKs (stress-activated protein kinases), in transfected cells. Dominant negative mutants of either MEK1, ERK or SEK1, reduced small t-dependent induction of the cyclin D1 promoter. SV40 small t induction of the cyclin D1 promoter involves both the ERK and SAPK pathways that together may contribute to the proliferative and transformation enhancing activity of small t antigen.

Ischemic preconditioning triggers the activation of MAP kinases and MAPKAP kinase 2 in rat hearts

While much is known about the beneficial effects of myocardial stress adaptation, relatively less information is available about the adaptive mechanisms. To explore the signaling pathways of stress adaptation, isolated working rat hearts were divided into three groups. Group I was adapted to stress by conventional technique of repeated ischemia and reperfusion consisting of 5 min of ischemia followed by 10 min of reperfusion, repeated four times. Group II was treated with 100 microM of genistein, a tyrosine kinase inhibitor, followed by preconditioning as described for group I. The third group, perfused with buffer only for 60 min, served as control. All hearts were subjected to 30 min of ischemia followed by 30 min of reperfusion. The results of our study demonstrated better postischemic myocardial functions in the preconditioned hearts as evidenced by increased aortic flow, coronary flow, developed pressure and lesser amount of tissue injury as evidenced by the decreased creatine kinase release. The preconditioning effects were associated with enhancement of phospholipase D activity in the heart. The preconditioning effect was almost abolished by the genistein treatment which also prevented the enhancement of phospholipase D activities. Additionally, preconditioning of the rat hearts stimulated protein kinase C, MAP kinase, and MAPKAP kinase 2 activities which were inhibited by genistein. The results identifies for the first time tyrosine kinase-phospholipase D as potential signaling pathway for ischemic preconditioning, and implicates the involvement of multiple protein kinases in myocardial adaptation to ischemia.

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.

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.

Tyrosine phosphorylation is crucial for growth signaling by tissue inhibitors of metalloproteinases (TIMP-1 and TIMP-2)

[3H]Thymidine (TdR) incorporation by human osteosarcoma cell line MG-63 was significantly stimulated at as early as 3 h after the addition of either TIMP-1 or TIMP-2 alone. Maximum stimulation was attained at a concentration of either 20 ng/ml (0.71 nM) TIMP-1 or 1.0 ng/ml (46 pM) TIMP-2. Tyrosine kinase inhibitors such as genistein, erbstatin, and herbimycin A almost completely inhibited the [3H]TdR incorporation stimulated by either of the TIMPs. However, essentially no effect was observed with H-89, H-7, bisindolylmaleimide and K-252a. These inhibition studies suggest a crucial role for tyrosine kinase in the signal transduction of TIMPs. Phosphotyrosine-containing proteins were significantly elevated by the treatment with both TIMPs. We also found that either TIMP stimulated an increase in mitogen-activated protein (MAP) kinase activity, suggesting that MAP kinase plays a role in TIMP-dependent growth signaling.

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.

Phosphoregulatory tyrosine of Xenopus mitogen-activated protein kinase is out of the reach of the enzyme catalytic center after autophosphorylation. Biochemical evidence for conformational changes upon phosphorylation

Autophosphorylation of the recombinant mitogen-activated protein kinase (MAPK) from Xenopus laevis has been studied to detect the conformational changes in the region of regulatory phosphorylation upon enzyme activation. Slow autophosphorylation of Xenopus MAPK occurred predominantly on tyrosine, the major phosphoregulatory site of MAPKs, through an intramolecular mechanism and was accompanied by a low magnitude stimulation of the catalytic activity towards an exogenous substrate, myelin basic protein. Autophosphorylated but not unphosphorylated enzyme was shown to interact with the protein substrate. In contrast to the previously reported reversibility of many tyrosine kinase reactions, the tyrosine phosphorylation of Xenopus MAPK was found to be irreversible in the presence of high ADP concentrations, although ADP could competitively inhibit both autophosphorylation and myelin basic protein phosphorylation. We concluded, therefore, that the phosphoregulatory tyrosine is no more accessible to an intramolecular phosphotransferase reaction and is out of the reach of the enzyme catalytic center after phosphorylation. The conformational changes in the region of regulatory phosphorylation resulted in a reduced immunoprecipitation of autophosphorylated and MAPK-kinase-phosphorylated forms of the enzyme by a polyclonal antibody raised against a synthetic peptide corresponding to residues 173-197 of Xenopus MAPK which includes the sites of regulatory phosphorylation. The reduced recognition was not due to the phosphorylation itself, since the antibody efficiently immunoprecipitated SDS-denatured forms of the phosphorylated enzyme. The antibody was not a neutralizing antibody, allowing unphosphorylated MAPK to undergo autophosphorylation while in the immune complex. However, autophosphorylation caused a release of phosphorylated enzyme from the immune complex, suggesting that dramatic conformational changes, which could even overcome the antibody constraints, took place in the phosphoregulatory region of MAPK upon enzyme activation.

Differential activation of T cell cytokine production by the extracellular signal-regulated kinase (ERK) signaling pathway

Stimulation of T cells via the T cell receptor (TCR) activates a number of signaling pathways that are potentially involved in the elicitation of physiological responses, such as the production of cytokines. The extracellular signal-regulated kinases (ERK) are a group of molecules activated in response to TCR ligation, whose role in T cell cytokine production is controversial. In this study, we have asked whether ERK activation is coupled to the production of a number of T cell-derived cytokines, and whether particular cytokines are differentially affected by ERK activation. To address these questions, we have utilized a constitutively active version of the immediate upstream activator of both ERK1 and ERK2, mitogen-activated/extracellular signal-regulated kinase 1 (MEK1), to activate ERK signaling selectively in the absence of other TCR-activated signaling pathways. The effect of constitutive MEK/ERK activation on T cell cytokine production was measured by transiently co-transfecting newly activated mouse T cells with DNA encoding constitutively active MEK1 (CA-MEK1) and the human interleukin-2 (IL-2) receptor alpha chain (hCD25), purifying hCD25+ transfectants by flow-cytometric cell sorting, and measuring the production of IL-3, IL-4, interferon (IFN)-gamma and granulocyte/macrophage-colony-stimulating factor (GM-CSF) either in the presence or absence of ionomycin stimulation. Newly activated T cells were used in these experiments as they more closely resemble T cells activated in vivo than do transformed T cells or long-term established T cell clones. CA-MEK1 expression led to constitutive ERK activation, which acted synergystically with ionomycin treatment to stimulate cytokine production. Furthermore, these experiments revealed a hierarchy of cytokine responsiveness to MEK/ERK activation, such that the production of IL-3 was most affected, followed by GM-CSF, IFN-gamma, and IL-4.

Adriamycin activates c-jun N-terminal kinase in human leukemia cells: a relevance to apoptosis

We studied the activation of c-jun N-terminal kinase 1 (JNK 1) and extracellular signal-regulated kinases 1 and 2 (ERK 1/2) of mitogen-activated protein kinase (MAPK) family by adriamycin (ADR) in the human T cell leukemia line, H9. ADR caused an elevation of JNK1 activity at sublethal or lethal concentrations; however, at lower doses, ADR did not activate JNK1. The induction of JNK1 peaked at 4 h of treatment (about ten-fold over the control), and was sustained up to 5 h post-treatment. This induction preceded the onset of apoptosis, as determined by morphological features and internucleosomal degradation of DNA. Upon treatment of cells with JNK1-inducing doses, ADR caused an elevation of steady-state levels of c-jun and ATF3 mRNAs, as measured by RT-PCR. In contrast, the activity of ERK 1/2 remained unchanged throughout the treatments, indicating that members of MAPK family are differentially regulated in ADR-treated cells. A possible role of JNK1 activation in ADR-induced apoptosis is discussed.

Nerve growth factor-mediated activation of the mitogen-activated protein (MAP) kinase cascade involves a signaling complex containing B-Raf and HSP90

The nerve growth factor (NGF)-mediated activation of the mitogen-activated protein (MAP) kinase cascade is an obligatory step in the morphological differentiation of PC12 cells. Signal transduction through the MAP kinase cascade is dependent upon activation of p21(ras) which binds directly to Raf family protein kinases, mediating their association with the membrane and activation. PC12 cells express two Raf isoforms, c-Raf and B-Raf. The activation of the MAP kinase cascade in response to NGF is due principally to the action of B-Raf. NGF treatment of PC12 cells resulted in the enhanced phosphorylation of B-Raf and c-Raf, and both exhibit reduced electrophoretic mobilities following stimulation of the cells. The NGF-stimulated phosphorylation of B-Raf was correlated with its enzymatic activation as measured by the phosphorylation of its substrate MEK. However, c-Raf does not exhibit significant levels of activity. B-Raf was present as a component of a high molecular mass complex, which included the molecular chaperone, heat shock protein 90 (HSP90). Importantly, c-Raf did not participate in the formation of such complexes. The B-Raf containing HSP90 complexes were normally present in PC12 cells, and their assembly was not dependent upon NGF stimulation. These data suggest that the ability of B-Raf to activate the MAP kinase cascade is due to its association with a large signaling complex, which is likely to impart signaling pathway specificity.

Ultrasensitivity in the mitogen-activated protein kinase cascade

The mitogen-activated protein kinase (MAPK) cascade is a highly conserved series of three protein kinases implicated in diverse biological processes. Here we demonstrate that the cascade arrangement has unexpected consequences for the dynamics of MAPK signaling. We solved the rate equations for the cascade numerically and found that MAPK is predicted to behave like a highly cooperative enzyme, even though it was not assumed that any of the enzymes in the cascade were regulated cooperatively. Measurements of MAPK activation in Xenopus oocyte extracts confirmed this prediction. The stimulus/response curve of the MAPK was found to be as steep as that of a cooperative enzyme with a Hill coefficient of 4-5, well in excess of that of the classical allosteric protein hemoglobin. The shape of the MAPK stimulus/ response curve may make the cascade particularly appropriate for mediating processes like mitogenesis, cell fate induction, and oocyte maturation, where a cell switches from one discrete state to another.