Protein synthesis inhibitors reveal differential regulation of mitogen-activated protein kinase and stress-activated protein kinase pathways that converge on Elk-1

The ETS-domain transcription factor family

During recent years, several significant discoveries have been made concerning the function of ETS-domain transcription factors. This family of transcription factors was originally defined on the basis of the conserved primary sequence of their DNA-binding domains. The ETS DNA-binding domain is also conserved at the structural level and is a divergent member of the winged helix-turn-helix superfamily of DNA binding proteins. This sequence conservation is reflected by their overlapping DNA-binding specificities based on the central GGAA/T motif. In addition to DNA-protein interactions, protein-protein interactions with partner proteins often play major roles in targeting ETS-domain proteins to specific promoters. Several such partner proteins have been identified. ETS-domain proteins function as either transcriptional activators or repressors and their activities are often regulated by signal transduction pathways, including the MAP kinase pathways. Specific links between such pathways and ETS-domain proteins have been established in several different experimental systems. ETS-domain transcription factors regulate a diverse array of biological functions including mammalian haematopoiesis and Drosophila eye development. In vertebrates, many ETS-domain proteins regulate embryonic and adult haematopoiesis. Deregulation of ETS-domain protein activity often leads to tumorigenesis. Future work will uncover further details of how these transcription factors work at the molecular level to regulate specific biological processes.

Acetylcholine agonists stimulate mitogen-activated protein kinase in oligodendrocyte progenitors by muscarinic receptors

Oligodendrocytes, the myelin-producing cells of the central nervous system, express muscarinic acetylcholine receptors (mAChR). Activation of this neurotransmitter receptor by the stable acetylcholine analog carbachol (CCh) triggers transducing events, modulating c-fos expression and cellular proliferation. To elucidate the signal transduction pathways involved in the transmission of these cellular events, we examined the ability of CCh to activate mitogen-activated protein kinase (MAPK) in primary cultures of oligodendrocyte progenitors prepared from newborn rat brain. CCh produced a concentration- and time-dependent increase in MAPK activity (predominantly the p42mapk or ERK2) as determined by in-gel MBP kinase assays. Using the non-selective muscarinic antagonist atropine we determined that MAPK-activation by CCH is mediated by muscarinic receptors. In the presence of PD098059, a specific inhibitor of MAPK kinase (MEK), MAPK activity was blocked. Similarly, the presence of extracellular calcium was required for CCh-mediated MAPK activation. To further elucidate the mechanisms involved in MAPK activation by CCh, the role of PKC was studied. In cells in which protein kinase had been downregulated by chronic treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA), the effect of carbachol on MAPK activation was maintained. In contrast, the response to CCh was blocked by the PKC inhibitors H7 and bisindolylmaleimide GF109203X. Our results suggest that MAPK is implicated in the transmission of the signal for mACh receptors and involves a TPA-insensitive PKC pathway. Further work is required to define the upstream and downstream events which result in CCh-mediated MAPK activation and proliferation of oligodendrocyte progenitors.

Genistein analogues: effects on epidermal growth factor receptor tyrosine kinase and on stress-activated pathways

Two genistein analogues (MD831 and MD833) have been synthesized and analyzed for their biological properties and their mechanism of action im comparison to genistein either in vitro or in intact cells. We showed that, in vitro, one of these compounds (MD831) inhibits the tyrosine kinase activity associated with the epidermal growth factor receptor (EGFR) as efficiently as genistein. However, treatment of A431 cells with these compounds did not result in any significant modification of EGFR tyrosine phosphorylation. Extracellular-signal regulated kinase (ERK) phosphorylation in cells stimulated by EGF was enhanced in the presence of MD831, whereas the other compounds, genistein and MD833, were able to activate the c-jun N-terminal kinase (JNK). This study showed that two structurally related compounds could elicit markedly different pharmacological effects on two signalling pathways, one involved in the mitogenic response and the other in the stress response. Such compounds may be useful to characterize signalling events involved in cell response to physiological stimuli.

Regulation of a c-Jun amino-terminal kinase/stress-activated protein kinase cascade by a sodium-dependent signal transduction pathway

Palytoxin is a novel skin tumor promoter that does not activate protein kinase C. Previous studies demonstrated that palytoxin stimulates a sodium-dependent signaling pathway that activates the c-Jun NH2-terminal kinase/stress-activated protein kinase (JNK) in Swiss 3T3 fibroblasts. In this study we show that a JNK kinase known as the stress-activated protein kinase/extracellular signal-regulated kinase-1 (SEK1) plays an important role in the regulation of JNK by palytoxin. We found that palytoxin stimulates the sustained activation of both JNK and SEK1 in COS7 and HeLa cells. Transiently expressed SEK1 isolated from palytoxin-treated cells can phosphorylate and activate JNK, which, in turn, can phosphorylate c-Jun. Furthermore, expression of a dominant negative mutant of SEK1 blocks activation of JNK by palytoxin. Sodium appears to play an important role in the regulation of JNK and SEK1 by palytoxin. Activation of JNK and SEK1 by palytoxin, but not anisomycin, requires extracellular sodium. Complementary studies showed that the sodium ionophore gramicidin can mimic palytoxin by regulating JNK and SEK1 through a sodium-dependent mechanism. Collectively, these results demonstrate that palytoxin stimulates a sodium-dependent signaling pathway that activates the SEK1/JNK/c-Jun protein kinase cascade.

Hypoxia induces c-fos transcription via a mitogen-activated protein kinase-dependent pathway

Hypoxia is a pathophysiological condition that occurs during injury, ischemia, and stroke. It is characterized by a decrease of reactive oxygen intermediates and a change of the intracellular redox level. In tumors hypoxia is regarded as a trigger for enhanced growth and metastasis. Here we report that in HeLa cells, hypoxic conditions induce the transcriptional activation of c-fos transcription via the serum response element. Mutations in the binding site for the ternary complex factor Elk-1 and the serum response factor abolished this induction, indicating that a ternary complex at the serum response element is necessary for the induction of the c-fos gene under hypoxia. The transcription factor Elk-1 was covalently modified by phosphorylation in response to hypoxia. Furthermore this hyperphosphorylation of Elk-1, the activation of mitogen-activated protein kinase (MAPK), and the induction of c-fos transcripts were blocked by PD98059, a specific inhibitor of mitogen-activated protein kinase kinase/extracellular signal-regulated protein kinase kinase 1. An in vitro kinase assay with Elk-1 as substrate showed that MAPK is activated under hypoxia. The activation of MAPK corresponds temporally with the phosphorylation and activation of Elk-1. Thus, a decrease of the intracellular reactive oxygen intermediate level by hypoxia induces c-fos via the MAPK pathway. These results suggest that the intracellular redox levels may be directly coupled to tumor growth, invasion, and metastasis via Elk-1-dependent induction of c-Fos controlled genes.

Contact Between Human Bone Marrow Stromal Cells and B Lymphocytes Enhances Very Late Antigen-4/Vascular Cell Adhesion Molecule-1–Independent Tyrosine Phosphorylation of Focal Adhesion Kinase, Paxillin, and ERK2 in Stromal Cells

Contact with bone marrow stromal cells is crucial for the normal growth and development of B-cell precursors. We have previously shown that human bone marrow stromal cell tyrosine kinase activity can be activated by direct contact with B-lymphoid cells (J Immunol 155:2359, 1995). In the present study, we show that increased tyrosine phosphorylation of focal adhesion kinase, paxillin, and extracellular-related kinase 2 (or p42 MAP kinase) accounted for the major changes occurring in stromal cell tyrosine phosphorylation after 5 to 10 minutes of contact with the RAMOS B-lymphoma cell line. Although adhesion of B-cell precursors to stromal cells is primarily mediated by very late antigen-4 (VLA-4) and vascular cell adhesion molecule-1 (VCAM-1), VLA-4–deficient and adhesion-deficient RAMOS cells were equally capable of stimulating stromal cell tyrosine phosphorylation. Similar changes in the tyrosine phosphorylation pattern of stromal cells were induced by contact with normal human B-cell precursors and several other B-lineage cell lines. After 5 to 30 minutes of contact with stromal cells, no change in protein tyrosine phosphorylation was detected in RAMOS or normal human B-cell precursors removed from stromal cells. Pretreatment of stromal cells with cytochalasin D abrogated contact-mediated enhancement of stromal cell tyrosine phosphorylation, suggesting that an intact cytoskeleton was essential. These results suggest that B-cell contact activates stromal cell signaling cascades that regulate cytoskeletal organization and transcription, independent of the interaction mediated by VLA-4 and VCAM-1.

Contact Between Human Bone Marrow Stromal Cells and B Lymphocytes Enhances Very Late Antigen-4/Vascular Cell Adhesion Molecule-1–Independent Tyrosine Phosphorylation of Focal Adhesion Kinase, Paxillin, and ERK2 in Stromal Cells

Contact with bone marrow stromal cells is crucial for the normal growth and development of B-cell precursors. We have previously shown that human bone marrow stromal cell tyrosine kinase activity can be activated by direct contact with B-lymphoid cells (J Immunol 155:2359, 1995). In the present study, we show that increased tyrosine phosphorylation of focal adhesion kinase, paxillin, and extracellular-related kinase 2 (or p42 MAP kinase) accounted for the major changes occurring in stromal cell tyrosine phosphorylation after 5 to 10 minutes of contact with the RAMOS B-lymphoma cell line. Although adhesion of B-cell precursors to stromal cells is primarily mediated by very late antigen-4 (VLA-4) and vascular cell adhesion molecule-1 (VCAM-1), VLA-4–deficient and adhesion-deficient RAMOS cells were equally capable of stimulating stromal cell tyrosine phosphorylation. Similar changes in the tyrosine phosphorylation pattern of stromal cells were induced by contact with normal human B-cell precursors and several other B-lineage cell lines. After 5 to 30 minutes of contact with stromal cells, no change in protein tyrosine phosphorylation was detected in RAMOS or normal human B-cell precursors removed from stromal cells. Pretreatment of stromal cells with cytochalasin D abrogated contact-mediated enhancement of stromal cell tyrosine phosphorylation, suggesting that an intact cytoskeleton was essential. These results suggest that B-cell contact activates stromal cell signaling cascades that regulate cytoskeletal organization and transcription, independent of the interaction mediated by VLA-4 and VCAM-1.

The level of intracellular glutathione is a key regulator for the induction of stress-activated signal transduction pathways including Jun N-terminal protein kinases and p38 kinase by alkylating agents

Monofunctional alkylating agents like methyl methanesulfonate (MMS) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) are potent inducers of cellular stress leading to chromosomal aberrations, point mutations, and cell killing. We show that these agents induce a specific cellular stress response program which includes the activation of Jun N-terminal kinases/stress-activated protein kinases (JNK/SAPKs), p38 mitogen-activated protein kinase, and the upstream kinase SEK1/MKK4 and which depends on the reaction mechanism of the alkylating agent in question. Similar to another inducer of cellular stress, UV irradiation, damage of nuclear DNA by alkylation is not involved in the MMS-induced response. However, in contrast to UV and other inducers of the JNK/SAPKs and p38 pathways, activation of growth factor and G-protein-coupled receptors does not play a role in the MMS response. We identified the intracellular glutathione (GSH) level as critical for JNK/SAPK activation by MMS: enhancing the GSH level by pretreatment of the cells with GSH or N-acetylcysteine inhibits, whereas depletion of the cellular GSH pool causes hyperinduction of JNK/SAPK activity by MMS. In light of the JNK/SAPK-dependent induction of c-jun and c-fos transcription, and the Jun/Fos-induced transcription of xenobiotic-metabolizing enzymes, these data provide a potential critical role of JNK/SAPK and p38 in the induction of a cellular defense program against cytotoxic xenobiotics such as MMS.

Specific activation of a c-Jun NH2-terminal kinase isoform and induction of neurite outgrowth in PC-12 cells by staurosporine

Staurosporine, a protein kinase inhibitor, is known to mimic the effect of nerve growth factor (NGF) in promoting neurite outgrowth. To elucidate the mechanism by which staurosporine induces neurite outgrowth in PC-12 cells, we performed an in-gel kinase assay using myelin basic protein as a substrate, and found that staurosporine induced the activation of a kinase with an apparent molecular mass of 57 kDa. The dose of staurosporine required to activate this kinase was consistent with that required to induce neurite outgrowth. Interestingly, the staurosporine-activated kinase was immunoprecipitated by anti-c-Jun NH2-terminal kinase (JNK) isoforms antibody, but not by anti-JNK1-specific antibody or anti-ERK1 antibody, raising the possibility that this kinase is a novel JNK isoform. The substrate specificity of the kinase was distinct from those of osmotic shock-activated JNKs and NGF-activated ERK1. The kinase phosphorylates transcription factors including c-Jun, Elk-1, and ATF2, as well as myelin basic protein, suggesting that it plays a role in gene induction. Furthermore, staurosporine induced immediate-early genes including Nur77 and fos, but not jun. The activation of the staurosporine-activated kinase, as well as the induction of neurite outgrowth, did not require Ras function, while Ras was required for the activation of ERKs and neurite outgrowth induced by NGF. Taken together, these results indicate staurosporine specifically activates a JNK isoform, which may contribute to biological activities including neurite outgrowth.

TGFbeta regulation of mitogen-activated protein kinases in human breast cancer cells

We demonstrate herein the ability of transforming growth factor-beta-2 (TGFbeta2) to potently activate extracellular signal-regulated kinase 2 (ERK2) in the highly TGFbeta-sensitive breast cancer cell (BCC) line Hs578T. The ERK2 isoform was activated by 3-fold within 5 min of TGFbeta2 addition to Hs578T cells. However, TGFbeta2 only slightly activated ERK2 (1.5-fold) in the partially TGFbeta-responsive BCC line MDA-MB-23 1. The magnitude of the difference in activation of ERK2 by TGFbeta2 in the two cell lines paralleled the difference in the IC50 values for TGFbeta inhibition of DNA synthesis; the IC50 value in the MDA-MB-231 cells was 32-fold greater than that in the Hs578T cells. Further, our data demonstrate that TGFbeta2 activated the stress-activated protein kinase/Jun N-terminal kinase (SAPK/JNK) type of mitogen-activated protein kinases (MAPKs); maximal induction levels were 2.5-fold above basal values and were attained at 30 min after TGFbeta2 treatment. Transient co-transfection of a luciferase reporter construct (3TP-Lux) containing three AP-1 sites and the plasminogen activator inhibitor-1 (PAI-1) promoter, in conjunction with a construct that directs expression of a dominant-negative mutant ERK2 (TAYF) protein, did not block the ability of TGFbeta to induce AP-1 or PAI-1 activity. In contrast, TAYF ERK2 was able to block EGF and insulin-induced 3TP-Lux-reporter activity. These results indicate that in these BCCs, the activation of ERK2 by TGFbeta is more tightly linked to the ability of TGFbeta to inhibit DNA synthesis than to the ability to stimulate promoter regions important for TGFbeta production and control of the extracellular matrix. In addition, this is the first demonstration that TGFbeta can activate the SAPK/JNK type of MAPK in TGFbeta-sensitive human BCCs.

Growth factor-induced transcription via the serum response element is inhibited by cyclic adenosine 3',5'-monophosphate in MCF-7 breast cancer cells

The effect of increased intracellular cAMP on MCF-7 breast cancer cell growth was examined by treating cells with either forskolin, an activator of adenylate cyclase, or 8-[4-chlorophenylthio]-cAMP (8-CPT-cAMP), a cAMP analog. Compared to cells maintained in control medium, treatment with either 1 or 10 microM forskolin decreased cell growth by 17% and 68%, respectively, whereas treatment with 250 microM 8-CPT-cAMP decreased cell growth by 29%. To determine whether this effect of cAMP on cell growth was mediated by inhibition of the activity of extracellular signal-regulated kinases 1 and 2 (ERK1 and -2), two mitogen-activated protein kinases, the effect of cAMP on growth factor-induced ERK activity in MCF-7 cells was examined. Treatment with either insulin-like growth factor I (IGF-I) or epidermal growth factor (EGF) for 10 min stimulated a 4- to 8-fold increase in ERK1 and -2 activity. This effect of IGF-I and EGF was not inhibited by increased intracellular cAMP generated by pretreatment of the cells with 10 microM forskolin. Similarly, 10 microM forskolin had no effect on IGF-I- or EGF-induced ERK activity in cells treated with growth factor for 30 min. To determine whether cAMP inhibits other growth factor-mediated effects, its effect on the activity of the serum response element (SRE), a DNA promoter element whose activity is regulated by a variety of growth-promoting events, was examined. For these assays, MCF-7 cells were transiently transfected with pTK81-SRE-Luc, a luciferase fusion gene that contains the SRE cloned 5' to a minimal thymidine kinase promoter and the luciferase gene. Treatment with either IGF-I or EGF increased pTK81-SRE-Luc activity in a dose-dependent fashion. Pretreatment of cells with 10 microM forskolin decreased IGF-I- and EGF-stimulated luciferase activity by approximately 75%. An intermediate effect was observed using 1 microM forskolin. When intracellular cAMP levels were increased using 8-CPT-cAMP, similar results were obtained. SRE activity is dependent upon the activation by phosphorylation of a ternary complex factor; included among the ternary complex factors is Elk-1. When MCF-7 cells were cotransfected with a vector that expresses a Gal4/Elk-1 fusion protein and UAS-TK-Luc, a plasmid that contains two Gal4 DNA recognition sites cloned 5' to a thymidine kinase promoter and the luciferase gene, treatment with forskolin partially inhibited the activation of Elk-1 by IGF-I and EGF. These data demonstrate that in MCF-7 breast cancer cells, cAMP has no effect on IGF-I- or EGF-induced ERK activity, but it inhibits growth factor-induced transcription. Taken together with the effects of cAMP on IGF-I- and EGF-induced Elk-1 activation, these data suggest that the effect of cAMP on SRE activity occurs distal to ERK activation, possibly via inhibition of an ERK-independent pathway. Finally, these data indicate that the effect of increased intracellular cAMP on breast cancer growth may be mediated through inhibition of specific growth factor-induced effects, including gene transcription.

The effect of tumor necrosis factor-alpha and cAMP on induction of AP-1 activity in MA-10 tumor Leydig cells

The immunostimulant tumor necrosis factor-alpha (TNF alpha), produced by monocytes/macrophages in response to inflammatory disorders, regulates gene expression in part through induction of mitogen-activated protein kinases (MAPKs), including the stress-activated protein kinase (SAPK) (c-Jun N-terminal kinase [JNK]) and the extracellular signal-regulated kinases (ERKs). In testicular Leydig cells, the induction of steroidogenesis by cAMP is inhibited by TNF alpha. To examine the potential mechanisms governing the mutual inhibition between cAMP and TNF alpha in Leydig cells, the intracellular signaling pathways that contribute to AP-1-dependent gene expression were examined in the mouse MA-10 Leydig cell line. TNF alpha induced SAPK activity sixfold at 15 min, and the PKC inhibitor calphostin C reduced the induction of SAPK by 30%. cAMP induced SAPK activity twofold but reduced TNF alpha-induced SAPK activity. ERK activity was inhibited by both cAMP and TNFa. TNFa increased c-Jun protein, but only weakly induced FOS proteins (c-Fos, FosB, Fra-1, and Fra-2) whereas cAMP increased the abundance of several FOS proteins (c-Fos, FosB, Fra-1, and Fra-2), with little effect on c-Jun levels. AP-1 binding activity, assessed using electrophoretic mobility shift assays, was increased twofold by TNF alpha and fivefold by cAMP. Cyclic AMP alone induced AP-1-responsive reporter (p3TPLUX) activity threefold after 2 h with peak effect of 4-fold at 4 hr. AP-1 reporter was not induced by TNF alpha alone but in the presence of cAMP, TNF alpha induced AP-1 reporter activity 12-fold. In conclusion, TNF alpha and cAMP induce distinct components that separately contribute to the modulation of AP-1 activity in MA-10 cells.

Ribotoxic stress response: activation of the stress-activated protein kinase JNK1 by inhibitors of the peptidyl transferase reaction and by sequence-specific RNA damage to the alpha-sarcin/ricin loop in the 28S rRNA

Inhibition of protein synthesis per se does not potentiate the stress-activated protein kinases (SAPKs; also known as cJun NH2-terminal kinases [JNKs]). The protein synthesis inhibitor anisomycin, however, is a potent activator of SAPKs/JNKs. The mechanism of this activation is unknown. We provide evidence that in order to activate SAPK/JNK1, anisomycin requires ribosomes that are translationally active at the time of contact with the drug, suggesting a ribosomal origin of the anisomycin-induced signaling to SAPK/JNK1. In support of this notion, we have found that aminohexose pyrimidine nucleoside antibiotics, which bind to the same region in the 28S rRNA that is the target site for anisomycin, are also potent activators of SAPK/JNK1. Binding of an antibiotic to the 28S rRNA interferes with the functioning of the molecule by altering the structural interactions of critical regions. We hypothesized, therefore, that such alterations in the 28S rRNA may act as recognition signals to activate SAPK/JNK1. To test this hypothesis, we made use of two ribotoxic enzymes, ricin A chain and alpha-sarcin, both of which catalyze sequence-specific RNA damage in the 28S rRNA. Consistent with our hypothesis, ricin A chain and alpha-sarcin were strong agonists of SAPK/JNK1 and of its activator SEK1/MKK4 and induced the expression of the immediate-early genes c-fos and c-jun. As in the case of anisomycin, ribosomes that were active at the time of exposure to ricin A chain or alpha-sarcin were able to initiate signal transduction from the damaged 28S rRNA to SAPK/JNK1 while inactive ribosomes were not.

Role of p38 and JNK mitogen-activated protein kinases in the activation of ternary complex factors

The transcription factors Elk-1 and SAP-1 bind together with serum response factor to the serum response element present in the c-fos promoter and mediate increased gene expression. The ERK, JNK, and p38 groups of mitogen-activated protein (MAP) kinases phosphorylate and activate Elk-1 in response to a variety of extracellular stimuli. In contrast, SAP-1 is activated by ERK and p38 MAP kinases but not by JNK. The proinflammatory cytokine interleukin-1 (IL-1) activates JNK and p38 MAP kinases and induces the transcriptional activity of Elk-1 and SAP-1. These effects of IL-1 appear to be mediated by Rho family GTPases. To examine the relative roles of the JNK and p38 MAP kinase pathways, we examined the effects of IL-1 on CHO and NIH 3T3 cells. Studies of NIH 3T3 cells demonstrated that both the JNK and p38 MAP kinases are required for IL-1-stimulated Elk-1 transcriptional activity, while only p38 MAP kinase contributes to IL-1-induced activation of SAP-1. In contrast, studies of CHO cells demonstrated that JNK (but not the p38 MAP kinase) is required for IL-1-stimulated Elk-1-dependent gene expression and that neither JNK nor p38 MAP kinase is required for IL-1 signaling to SAP-1. We conclude that (i) distinct MAP kinase signal transduction pathways mediate IL-1 signaling to ternary complex transcription factors (TCFs) in different cell types and (ii) individual TCFs show different responses to the JNK and p38 signaling pathways. The differential utilization of TCF proteins and MAP kinase signaling pathways represents a potential mechanism for the determination of cell-type-specific responses to extracellular stimuli.

Reversion of Ras- and phosphatidylcholine-hydrolyzing phospholipase C-mediated transformation of NIH 3T3 cells by a dominant interfering mutant of protein kinase C lambda is accompanied by the loss of constitutive nuclear mitogen-activated protein kinase/extracellular signal-regulated kinase activity

The transformed phenotype of v-Ras- or Bacillus cereus phosphatidylcholine-hydrolyzing phospholipase C (PC-PLC)-expressing NIH 3T3 cells is reverted by expressing a kinase-defective mutant of protein kinase C lambda (lambdaPKC). We report here that extracellular signal-regulated kinase (ERK)-1 and -2 are constitutively activated in v-Ras- and PC-PLC-transformed cells in the absence of added growth factors. Interestingly, the activated ERKs were exclusively localized to the cell nucleus. Consistently, the transactivating potential of the C-terminal domain of Elk-1, which is activated upon ERK-mediated phosphorylation, was strongly induced in serum-starved cells expressing v-Ras or PC-PLC. Reversion of v-Ras- or PC-PLC-induced transformation by expression of dominant negative lambdaPKC abolished the nuclear ERK activation suggesting lambdaPKC as a novel, direct or indirect, activator of mitogen-activated protein kinase/ERK kinase in response to activated Ras or elevated levels of phosphatidylcholine-derived diacylglycerol. Transient transfection experiments confirmed that lambdaPKC acts downstream of Ras but upstream of mitogen-activated protein kinase/ERK kinase. We found both the v-Ras- and PC-PLC-transformed cells to be insensitive to stimulation with platelet-derived growth factor (PDGF). No detectable receptor level, autophosphorylation, or superinduction of DNA synthesis could be observed in response to treatment with PDGF. Reversion of the transformed cell lines by expression of dominant negative lambdaPKC restored the receptor level and the ability to respond to PDGF in terms of receptor autophosphorylation, ERK activation, and induction of DNA synthesis.

Mitogen-activated protein kinases activate the serine/threonine kinases Mnk1 and Mnk2

Mitogen-activated protein (MAP) kinases bind tightly to many of their physiologically relevant substrates. We have identified a new subfamily of murine serine/threonine kinases, whose members, MAP kinase-interacting kinase 1 (Mnk1) and Mnk2, bind tightly to the growth factor-regulated MAP kinases, Erk1 and Erk2. MNK1, but not Mnk2, also binds strongly to the stress-activated kinase, p38. MNK1 complexes more strongly with inactive than active Erk, implying that Mnk and Erk may dissociate after mitogen stimulation. Erk and p38 phosphorylate MNK1 and Mnk2, which stimulates their in vitro kinase activity toward a substrate, eukaryotic initiation factor-4E (eIF-4E). Initiation factor eIF-4E is a regulatory phosphoprotein whose phosphorylation is increased by insulin in an Erk-dependent manner. In vitro, MNK1 rapidly phosphorylates eIF-4E at the physiologically relevant site, Ser209. In cells, Mnk1 is post-translationally modified and enzymatically activated in response to treatment with either peptide growth factors, phorbol esters, anisomycin or UV. Mitogen- and stress-mediated MNK1 activation is blocked by inhibitors of MAP kinase kinase 1 (Mkk1) and p38, demonstrating that Mnk1 is downstream of multiple MAP kinases. MNK1 may define a convergence point between the growth factor-activated and one of the stress-activated protein kinase cascades and is a candidate to phosphorylate eIF-4E in cells.

Restoration of the transient outward potassium current by noradrenaline in chagasic canine epicardium

1. The transient outward potassium current (Ito) is reduced in canine epicardial myocytes during the acute stage of infection with Trypanosoma cruzi (Chagas' disease). Sympathetic nerve terminals are also destroyed during the acute stage of this disease. To test whether the reduction of Ito is related to the absence of sympathetic innervation, acutely infected isolated epicardial myocytes were exposed in vitro to the sympathetic neurotransmitter noradrenaline (NA) and the effects of NA exposure on Ito were determined. 2. Continuous exposure to NA (1.0 microM) for 0-6 h had no effect on Ito density, whereas exposure to NA for 24 h significantly increased Ito density. Ito was also restored 24 h after a 1 h exposure to NA. Cell capacitance was not significantly affected by NA. 3. The alpha1-adrenergic receptor antagonist prazosin (0.1 microM) blocked the effects of NA on Ito, but the beta-adrenergic receptor antagonist propranolol (20 microM) did not. The beta-adrenergic receptor agonist isoprenaline (1 microM) had no effect on Ito. 4. Restoration of Ito by NA was prevented by pretreatment with neomycin (100 microM), a phospholipase C inhibitor, but not by pretreatment with 100-400 ng ml(-1) pertussis toxin (PTX). 5. The protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (0.1 microM) mimicked the effect of NA on Ito, whereas the inactive analogue 4alpha-phorbol (20 microM) had no effect on Ito. Pretreatment with bisindolylmaleimide (0.1 microM), a specific PKC inhibitor, completely blocked the effect of NA on Ito. 6. Thus, NA restores Ito in chagasic canine epicardial myocytes. The induction of Ito by NA appears to result from alpha1-adrenergic stimulation of PKC via a PTX-insensitive signalling cascade. These results suggest that the reduction of Ito in chagasic myocytes during the acute stage of Chagas' disease may reflect the lack of the trophic effects of sympathetic innervation.

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.

Convergence of MAP kinase pathways on the ternary complex factor Sap-1a

The serum response element (SRE), which is pivotal for transcriptional up-regulation of the c-fos protooncogene, is constitutively occupied by a protein complex comprising the serum response factor and a ternary complex factor (TCF). Phosphorylation of the TCFs Elk-1 and Sap-1a by the ERK and JNK subclasses of MAP kinases triggers c-fos transcription. We demonstrate here that Elk-1 is barely activated by a third subclass of MAP kinases (p38), most likely because the critical residues Ser383 and Ser389 are poorly phosphorylated by p38 MAP kinase. In contrast, the TCF Sap-1a is efficiently phosphorylated by p38 MAP kinase in vitro and in vivo on the homologous residues Ser381 and Ser387. Mutation of these sites to alanine severely reduces c-fos SRE-dependent transcription mediated by Sap-1a and p38 MAP kinase. Thus, Sap-1a may be an important target for mitogens, stress and apoptotic signals to elicit a nuclear response. However, signaling from p38 MAP kinase to Sap-1a or from Sap-1a to the basal transcription machinery does not occur in all cell types nor at promoters other than the c-fos SRE, which may ensure the specificity of signaling.

Neuroprotective action of cycloheximide involves induction of bcl-2 and antioxidant pathways

The ability of the protein synthesis inhibitor cycloheximide (CHX) to prevent neuronal death in different paradigms has been interpreted to indicate that the cell death process requires synthesis of "killer" proteins. On the other hand, data indicate that neurotrophic factors protect neurons in the same death paradigms by inducing expression of neuroprotective gene products. We now provide evidence that in embryonic rat hippocampal cell cultures, CHX protects neurons against oxidative insults by a mechanism involving induction of neuroprotective gene products including the antiapoptotic gene bcl-2 and antioxidant enzymes. Neuronal survival after exposure to glutamate, FeSO4, and amyloid beta-peptide was increased in cultures pretreated with CHX at concentrations of 50-500 nM; higher and lower concentrations were ineffective. Neuroprotective concentrations of CHX caused only a moderate (20-40%) reduction in overall protein synthesis, and induced an increase in c-fos, c-jun, and bcl-2 mRNAs and protein levels as determined by reverse transcription-PCR analysis and immunocytochemistry, respectively. At neuroprotective CHX concentrations, levels of c-fos heteronuclear RNA increased in parallel with c-fos mRNA, indicating that CHX acts by inducing transcription. Neuroprotective concentrations of CHX suppressed accumulation of H2O2 induced by FeSO4, suggesting activation of antioxidant pathways. Treatment of cultures with an antisense oligodeoxynucleotide directed against bcl-2 mRNA decreased Bcl-2 protein levels and significantly reduced the neuroprotective action of CHX, suggesting that induction of Bcl-2 expression was mechanistically involved in the neuroprotective actions of CHX. In addition, activity levels of the antioxidant enzymes Cu/Zn-superoxide dismutase, Mn-superoxide dismutase, and catalase were significantly increased in cultures exposed to neuroprotective levels of CHX. Our data suggest that low concentrations of CHX can promote neuron survival by inducing increased levels of gene products that function in antioxidant pathways, a neuroprotective mechanism similar to that used by neurotrophic factors.

Double-stranded RNA is a trigger for apoptosis in vaccinia virus-infected cells

The vaccinia virus E3L gene codes for double-stranded RNA (dsRNA) binding proteins which can prevent activation of the dsRNA-dependent, interferon-induced protein kinase PKR. Activated PKR has been shown to induce apoptosis in HeLa cells. HeLa cells infected with vaccinia virus with the E3L gene deleted have also been shown to undergo apoptosis, whereas HeLa cells infected with wild-type vaccinia virus do not. In this report, using virus recombinants expressing mutant E3L products or alternative dsRNA binding proteins, we show that suppression of induction of apoptosis correlates with functional binding of proteins to dsRNA. Infection of HeLa cells with ts23, which leads to synthesis of increased dsRNA at restrictive temperature, induced apoptosis at restrictive but not permissive temperatures. Treatment of cells with cytosine arabinoside, which blocks the late buildup of dsRNA in vaccinia virus-infected cells, prevented induction of apoptosis by vaccinia virus with E3L deleted. Cells transfected with dsRNA in the absence of virus infection also underwent apoptosis. These results suggest that dsRNA is a trigger that can initiate a suicide response in virus-infected and perhaps uninfected cells.

Antioxidants as well as oxidants activate c-fos via Ras-dependent activation of extracellular-signal-regulated kinase 2 and Elk-1

Intracellular reactive oxygen intermediate (ROI) levels play an important role in numerous physiological and pathophysiological conditions. Apart from causing oxidative stress and damage, ROI changes differentially activate gene expression. However the proto-oncogene encoding the AP-1 transcription factor subunit c-Fos is induced by both prooxidants and antioxidants. Here, the transcription factor Elk-1 is identified as being responsible for c-fos serum response element (SRE) induction in response to changes in the cellular redox status induced by treatment with either the oxidant H2O2 or various structurally unrelated antioxidants. A temporal correlation is observed between changes in the phosphorylation status of Elk-1 and the activation of the mitogen-activated protein kinase 2 (ERK2) in response to cellular redox changes. Correspondingly, the transcriptional response of the SRE to redox fluctuations is attenuated upon mutation of critical ERK2 target residues within the Elk-1 transactivation domain to alanine. Signals elicited by antagonistic intracellular redox changes converge at or above the level of Ras or an effector of Ras, leading to similar activation of c-fos transcription, since an [N17]Ras mutant interfered with redox signaling. Hence components of signaling pathways are revealed to be shared by mitogenic and redox-dependent stimuli.

Mitogen-activated protein kinase phosphatases inactivate stress-activated protein kinase pathways in vivo

The c-Jun N-terminal protein kinases (JNKs), also called stress-activated protein kinases, are members of the growing family of serine/threonine kinases in the mitogen-activated protein (MAP) kinase superfamily. Like other MAP kinases, JNKs are activated via phosphorylation on adjacent threonine and tyrosine residues and can be inactivated by a unique family of dual specificity phosphatases, called MAP kinase phosphatases (MKPs). MKPs are encoded by immediate early genes and induced in response to environmental stressors and growth factor stimulation. Two prevalent isoforms of MKP, MKP1 and MKP2, are co-expressed in a wide variety of cell types. In this study, we examined the actions of MKP1 and MKP2 on JNK1 and JNK2. JNK1 phosphorylation and activation was inhibited by expression of both MKP1 and MKP2, although MKP1 selectivity toward JNK1 appeared significantly higher than that of MKP2. In contrast, JNK2 activity was inhibited by either phosphatase to similar degrees. Both MKP1 and MKP2 were highly effective at blocking the activation of the physiological target of JNK activation, the transcription factor c-Jun. In PC12 cells, MKP1 and MKP2 are transcriptionally induced following stimulation by nerve growth factor. In these cells, UV light-evoked JNK activation was reduced by pretreatment with nerve growth factor. Therefore, JNKs may be selective targets of MKP action in certain cells.

Activation of the Sap-1a transcription factor by the c-Jun N-terminal kinase (JNK) mitogen-activated protein kinase

Ternary complex factors (TCFs) bind to the serum response element in the c-fos promoter and mediate its activation by many extracellular stimuli. Some of these stimuli activate the ERK subclass of mitogen-activated protein kinases (MAPKs) that target the TCF Sap-1a. We show that Sap-1a is also phosphorylated by the stress-activated JNK subclass of MAPKs leading to stimulation of both c-fos serum response element and E74-site-dependent transcription in RK13 cells. Several JNK-1 phosphorylation sites were mapped within Sap-1a, and mutation of these sites affected the transactivation mediated by Sap-1a and JNK-1. The impact of these phosphorylation sites varied at different promoters and was dependent on whether Sap-1a was stimulated by ERK-1 or JNK-1. Additionally, a comparison of Sap-1a with another TCF, Elk-1, revealed that these proteins behaved differently to stimulation by ERK-1 and JNK-1. Furthermore, activation of Sap-1a by JNK-1 was inhibited by the p38(MAPK) in RK13 cells, possibly by competition for a common upstream activator. Altogether, our data suggest that Sap-1a plays an important role in the nuclear response elicited by cellular stress.

Activation of stress-activated protein kinase-3 (SAPK3) by cytokines and cellular stresses is mediated via SAPKK3 (MKK6); comparison of the specificities of SAPK3 and SAPK2 (RK/p38)

Stress-activated protein kinase-3 (SAPK3), a recently described MAP kinase family member with a wide-spread tissue distribution, was transfected into several mammalian cell lines and shown to be activated in response to cellular stresses, interleukin-1 (IL-1) and tumour necrosis factor (TNF) in a similar manner to SAPK1 (also termed JNK) and SAPK2 (also termed p38, RK, CSBP and Mxi2). SAPK3 and SAPK2 were activated at similar rates in vitro by SAPKK3 (also termed MKK6), and SAPKK3 was the only activator of SAPK3 that was induced when KB or 293 cells were exposed to cellular stresses or stimulated with IL-1 or TNF. Co-transfection with SAPKK3 induced SAPK3 activity and greatly enhanced activation in response to osmotic shock. These experiments indicate that SAPKK3 mediates the activation of SAPK3 in several mammalian cells. SAPK3 and SAPK2 phosphorylated a number of proteins at similar rates, including the transcription factors ATF2, Elk-1 and SAP1, but SAPK3 was far less effective than SAPK2 in activating MAPKAP kinase-2 and MAPKAP kinase-3. Unlike SAPK2, SAPK3 was not inhibited by the drug SB 203580. SAPK3 phosphorylated ATF2 at Thr69, Thr71 and Ser90, the same residues phosphorylated by SAPK1, whereas SAPK2 only phosphorylated Thr69 and Thr71. Our results suggest that cellular functions previously attributed to SAPK1 and/or SAPK2 may be mediated by SAPK3.

The dual specificity mitogen-activated protein kinase phosphatase-1 and -2 are induced by the p42/p44MAPK cascade

Mitogen-activated protein (MAP) kinase phosphatase-1 (MKP-1) and MKP-2 are two members of a recently described family of dual specificity phosphatases that are capable of dephosphorylating p42/p44MAPK. Overexpression of MKP-1 or MKP-2 inhibits MAP kinase-dependent intracellular signaling events and fibroblast proliferation. By using specific antibodies that recognize endogenous MKP-1 and MKP-2 in CCL39 cells, we show that MKP-1 and MKP-2 are not expressed in quiescent cells, but are rapidly induced following serum addition, with protein detectable as early as 30 min (MKP-1) or 60 min (MKP-2). Serum induction of MKP-1 and MKP-2 is sustained, with protein detectable up to 14 h after serum addition. Induction of MKP-1 and, to a lesser extent, MKP-2 temporally correlates with p42/p44MAPK inactivation. To analyze the contribution of the MAP kinase cascade to MKP-1 and MKP-2 induction, we examined CCL39 cells transformed with either v-ras or a constitutively active direct upstream activator of MAP kinase, mitogen-activated protein kinase kinase-1 (MKK-1; MKK-1(SD/SD) mutant). In both cell models, MKP-1 and MKP-2 are constitutively expressed, with MKP-2 being prevalent. In addition, in CCL39 cells expressing an estradiol-inducible deltaRaf-1::ER chimera, activation of Raf alone is sufficient to induce MKP-1 and MKP-2. The role of the MAP kinase cascade in MKP induction was highlighted by the MKK-1 inhibitor PD 098059, which blunted both the activation of p42/p44MAPK and the induction of MKP-1 and MKP-2. However, the MAP kinase cascade is not absolutely required for the induction of MKP-1, as this phosphatase, but not MKP-2, was induced to detectable levels by agents that stimulate protein kinases A and C. Thus, activation of the p42/p44MAPK cascade promotes the induction of MKP-1 and MKP-2, which may then attenuate p42/p44MAPK-dependent events in an inhibitory feedback loop.

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.

The p38 and ERK MAP kinase pathways cooperate to activate Ternary Complex Factors and c-fos transcription in response to UV light

We investigated the activation of c-fos transcription following UV irradiation, a 'stress' stimulus. In both HeLa TK- and NIH 3T3 cells the Serum Response Element is required for efficient UV-induced c-fos transcription, and in HeLa TK- cells the Ternary Complex Factor (TCF) binding site contributes substantially to activation. Consistent with this, UV irradiation activates LexA-TCF fusion proteins more strongly in HeLa TK- than in NIH 3T3 cells. The TCF C-termini of the TCFs are substrates for UV-induced MAP kinases: both the Elk-1 and SAP-1a C-termini are efficiently phosphorylated by the p38 MAPK, but only the Elk-1 C-terminus is a good substrate for the SAPK/JNKs. The specificity and activation kinetics of TCF C-terminal kinases, and the susceptibility of transcriptional activation by LexA-TCF fusion proteins to specific inhibitors of different MAPK pathways, show that both the ERK and p38 MAPK pathways contribute to TCF activation in response to UV irradiation. Activity of both these pathways is also required for the response of the c-fos gene itself to UV stimulation.

Regulation of interleukin-2 transcription by inducible stable expression of dominant negative and dominant active mitogen-activated protein kinase kinase kinase in jurkat T cells. Evidence for the importance of Ras in a pathway that is controlled by dual receptor stimulation

Engagement of the T cell receptor induces the activation of several mitogen-activated protein kinase modules, including the extracellular signal-regulated kinase and c-Jun N-terminal kinase (JNK) cascades. Whereas extracellular signal-regulated kinase is activated by T cell receptor/CD3 ligation alone, activation of JNK requires co-stimulation by the CD28 receptor. Activation of MEKK-1, which acts as a mitogen-activated protein kinase kinase kinase in the JNK pathway, was also induced by CD3 plus CD28 (CD3/CD28) ligation in Jurkat cells. To study the significance of the JNK cascade in T lymphocytes, we established stable Jurkat cell lines that inducibly express dominant active (DA) or dominant negative (DN) MEKK-1. Whereas expression of DA-MEKK-1 resulted in the constitutive activation of JNK along with the transcriptional activation of the minimal interleukin-2 (IL-2) promoter, DN-MEKK-1 inhibited JNK responsiveness during CD3/CD28 co-stimulation. In addition to inhibiting CD3/CD28-induced IL-2 mRNA expression, DN-MEKK-1 abrogated the transcriptional activation of the IL-2 promoter and the distal nuclear factor of activated T cells (NFAT)-activating protein 1 (AP-1) response element in that promoter. A c-Jun mutant lacking activation sites for JNK also interfered with the activation of the distal NFAT/AP-1 complex, suggesting that the JNK pathway functions by controlling AP-1 response elements in the IL-2 promoter. Using inducible stable expression of DA- and DN-Ras in Jurkat cells, we found that Ras regulates JNK activation in these cells. Our results suggest that the dual ligation of CD3 and CD28 in T cells triggers a cascade of events that involve Ras, the JNK cascade, and one or more AP-1 response elements in the IL-2 promoter.

Expression of mitogen-activated protein kinase phosphatase-1 in the early phases of human epithelial carcinogenesis

Many mitogens and human oncogenes activate extracellular regulated kinases (ERKs), which in turn convey proliferation signals. ERKs or mitogen-activated protein (MAP) kinases are inactivated in vitro by MAP kinase phosphatases (MKPs). The gene encoding one of these MKPs, MKP-1, is a serum-inducible gene and is transcriptionally activated by mitogenic signals in cultured cells. As MKP-1 has been shown to block DNA synthesis by inhibiting ERKs when expressed at elevated levels in cultured cells, it has been suggested that it may act as a tumor suppressor. MKP-1 mRNA and MAP kinase (ERK-1 and -2) protein expression was assessed in 164 human epithelial tumors of diverse tissue origin by in situ hybridization and immunohistochemistry. MKP-1 was overexpressed in the early phases of prostate, colon, and bladder carcinogenesis, with progressive loss of expression with higher histological grade and in metastases. In contrast, breast carcinomas showed significant MKP-1 expression even when poorly differentiated or in late stages of the disease. MKP-1, ERK-1, and ERK-2 were co-expressed in most tumors examined. In a subset of 15 tumors, ERK-1 enzymatic activity as well as structural alterations that might be responsible for loss of function of MKP-1 during tumor progression, were examined. ERK-1 enzymatic activity was found to be elevated despite MKP-1 overexpression. No loss of 5q35-ter (containing the MKP-1 locus) was detected by polymerase chain reaction in metastases compared with primary tumors. Finally, no mutations were found in the catalytic domain of MKP-1. These data indicate that MKP-1 is an early marker for a wide range of human epithelial tumors and suggest that MKP-1 does not behave as a tumor suppressor in epithelial tumors.

JNK (c-Jun NH2-terminal kinase) is a target for antioxidants in T lymphocytes

AP-1 has been shown to behave as a redox-sensitive transcription factor that can be activated by both oxidant and antioxidant stimuli. However, the mechanisms involved in the activation of AP-1 by antioxidants are largely unknown. In this study we show that the structurally unrelated antioxidant agents pyrrolidine dithiocarbamate (PDTC), butylated hydroxyanisole, and Nacetylcysteine activated JNK (c-Jun NH2-terminal kinase) in Jurkat T cells. This activation differed substantially from that mediated by phorbol 12-myristate 13-acetate (PMA) and Ca2+ ionophore or produced by costimulation with antibodies against the T cell receptor-CD3 complex and to CD28. The activation of JNK by classical T cell stimuli was transient, whereas that mediated by PDTC and butylated hydroxyanisole (but not N-acetylcysteine) was sustained. The kinetics of JNK activation correlated with the expression of c-jun which was transient after stimulation with PMA plus ionophore and prolonged in response to PDTC, which also transiently induced c-fos. In addition, JNK activation by PMA plus ionophore was sensitive to inhibitors of signaling pathways involving Ca2+, protein kinase C, and tyrosine phosphorylation, which failed to inhibit the activation mediated by PDTC. Transfection of trans-dominant negative expression vectors of ras and raf, together with AP-1-dependent reporter constructs, as well as Western blot analysis using anti-ERK (extracellular signal-regulated kinase) antibodies, indicated that the Ras/Raf/ERK pathway did not appear to mediate the effect of the antioxidant. However, the combined treatment with PDTC and PMA, two agents that synergize on AP-1 activation, resulted in the persistent phosphorylation of ERK-2. In conclusion, our results identify JNK as a target of antioxidant agents which can be regulated differentially under oxidant and antioxidant conditions.

Selective response of ternary complex factor Sap1a to different mitogen-activated protein kinase subgroups

Mitogenic and stres signals results in the activation of extracellular signal-regulated kinases (ERKs) and stress-activated protein kinase/c-Jun N-terminal kinases (SAPK/JNKs), respectively, which are two subgroups of the mitogen-activated protein kinases. A nuclear target of mitogen-activated protein (MAP) kinases is the ternary complex factor Elk-1, which underlies its involvement in the regulation of c-fos gene expression by mitogenic and stress signals. A second ternary complex factor, Sap1a, is coexpressed with Elk-1 in several cell types and shares attributes of Elk-1, the significance of which is not clear. Here we show that Sap1a is phosphorylated efficiently by ERKs but not by SAPK/JNKs. Serum response factor-dependent ternary complex formation by Sap1a is stimulated by ERK phosphorylation but not by SAPK/JNKs. Moreover, Sap1a-mediated transcription is activated by mitogenic signals but not by cell stress. These results suggest that Sap1a and Elk-1 have distinct physiological functions.

Dual leucine zipper-bearing kinase (DLK) activates p46SAPK and p38mapk but not ERK2

Because the catalytic domain of dual leucine zipper-bearing kinase (DLK) bears sequence similarity to members of the mitogen-activated protein (MAP) kinase kinase kinase subfamily, this protein kinase was investigated for its ability to activate MAP kinase pathways. When transiently transfected and overexpressed in either COS 7 cells or NIH3T3 cells, wild type DLK potently activated p46(SAPK) (SAPK/JNK) but had no detectable effect in activating p42/44(MAPK). DLK also activated p38(mapk) when overexpressed in NIH3T3 cells. A catalytically inactive point mutant of DLK had no effect in these experiments. Consistent with its specificity in activating SAPK, DLK activated Elk-1 but not Sap1a-mediated transcription. In NIH3T3 cells, activation of SAPK by v-Src was markedly attenuated by coexpression of K185A, a catalytically inactive mutant of DLK, suggesting that this mutant could function in a dominant negative fashion in a pathway that leads from v-Src to SAPKs. In a series of co-transfection experiments, activation of p46(SAPK) by DLK was not inhibited by dominant negative mutants of Rac1 and Cdc42Hs, PAK65-R, or PAK65-A, but was attenuated by MEKK1(K432M). DLK(K185A) did not inhibit the ability of constitutively active MEKK1 to activate SAPK. Moreover, K185A significantly inhibited the activation of SAPK by constitutively active V-12 Rac1 and V-12 Cdc42Hs. These results suggest that DLK lies distal to Rac1 and/or Cdc42Hs but proximal to MEKK1 in a pathway leading from v-Src to SAPKs activation.

Urea-inducible Egr-1 transcription in renal inner medullary collecting duct (mIMCD3) cells is mediated by extracellular signal-regulated kinase activation

Urea (200-400 milliosmolar) activates transcription, translation of, and trans-activation by the immediate-early gene transcription factor Egr-1 in a renal epithelial cell-specific fashion. The effect at the transcriptional level has been attributed to multiple serum response elements and their adjacent Ets motifs located within the Egr-1 promoter. Elk-1, a principal ternary complex factor and Ets domain-containing protein, is a substrate of the extracellular signal-regulated kinase (ERK) mitogen-activated protein kinases. In the renal medullary mIMCD3 cell line, urea (200-400 milliosmolar) activated both ERK1 and ERK2 as determined by in-gel kinase assay and immune-complex kinase assay of epitope-tagged] ERK1 and ERK2. Importantly, urea did not affect abundance of either ERK. Urea-inducible Egr-1 transcription was a consequence of ERK activation because the ERK-specific inhibitor, PD98059, abrogated transcription from the murine Egr-1 promoter in a luciferase reported gene assay. In addition, activators of protein kinase A, including forskolin and 8-Br-cAMP, which are known to inhibit ERK-mediated events, also inhibited urea-inducible Egr-1 transcription. Furthermore, urea-inducible activation of the physiological ERK substrate and transcription factor, Elk-1, was demonstrated through transient cotransfection of a chimeric Elk-1/GAL4 expression plasmid and a GAL4-driven luciferase reporter plasmid. Taken together, these data indicate that, in mIMCD3 cells, urea activates ERKs and the ERK substrate, Elk-1, and that ERK inhibition abrogates urea-inducible Egr-1 transcription. These data are consistent with a model of urea-inducible renal medullary gene expression wherein sequential activation of ERKs and Elk-1 results in increased transcription of Egr-1 through serum response element/Ets motifs.

The Atf1 transcription factor is a target for the Sty1 stress-activated MAP kinase pathway in fission yeast

The atf1+ gene of Schizosaccharomyces pombe encodes a bZIP transcription factor with strong homology to the mammalian factor ATF-2. ATF-2 is regulated through phosphorylation in mammalian cells by the stress-activated mitogen-activated protein (MAP) kinases SAPK/JNK and p38. We show here that the fission yeast Atf1 factor is also regulated by a stress-activated kinase, Sty1. The Sty1 kinase is stimulated by a variety of different stress conditions including osmotic and oxidative stress and heat shock. Deletion of the atf1+ gene results in many, but not all, of the phenotypes associated with loss of Sty1, including sensitivity to environmental stress and inability to undergo sexual conjugation. Furthermore, we identify a number of target genes that are induced rapidly in a manner dependent upon both the Sty1 kinase and the Atf1 transcription factor. These genes include gpd1+, which is important for the response of cells to osmotic stress, the catalase gene lambda important for cells to combat oxidative stress, and pyp2+, which encodes a tyrosine-specific MAP kinase phosphatase. Induction of Pyp2 by Atf1 is direct in that it does not require de novo protein synthesis and results in a negative feedback loop that serves to control signaling through the Sty1/Wis1 pathway. We show that Atf1 associates stably and is phosphorylated by the Sty1 kinase in vitro. Taken together, these results indicate that the interaction between AM and Sty1 is direct. These findings highlight a remarkable level of conservation in transcriptional control by stress-activated MAP kinase pathways between fission yeast and mammalian cells.

Purification and cDNA cloning of SAPKK3, the major activator of RK/p38 in stress- and cytokine-stimulated monocytes and epithelial cells

Two chromatographically distinct stress-activated protein kinase kinases (SAPKKs) have been identified in several mammalian cells, termed SAPKK2 and SAPKK3, which activate the MAP kinase family member RK/p38 but not JNK/SAPK in vitro. Here we demonstrate that SAPKK2 is identical or very closely related to the MAP kinase kinase family member MKK3. However, under our assay conditions, SAPKK3 was the major activator of RK/p38 detected in extracts prepared from stress- or interleukin-1-stimulated epithelial (KB) cells, from bacterial lipopolysaccharide and tumour necrosis factor alpha-stimulated THP1 monocytes or from rabbit skeletal muscle. The activated form of SAPKK3 was purified from muscle to near homogeneity, and tryptic peptide sequences were used to clone human and murine cDNAs encoding this enzyme. Human SAPKK3 comprised 334 amino acids and was 78% identical to MKK3. The murine and human SAPKK3 were 97% identical in their amino acid sequences. We also cloned a different murine cDNA that appears to encode a SAPKK3 protein truncated at the N-terminus. SAPKK3 is identical to the recently cloned MKK6.

Multiple p21ras effector pathways regulate nuclear factor of activated T cells

The transcription factor, Nuclear Factor of Activated T cells (NFAT) is a major target for p21ras and calcium signalling pathways in the IL-2 gene and is induced by p21ras signals acting in synergy with calcium/calcineurin signals. One p21ras effector pathway involves the MAP kinase ERK-2, and we have examined its role in NFAT regulation. Expression of dominant negative MAPKK-1 prevents NFAT induction. Constitutively active MAPKK-1 fully activates ERK-2 and the transcription factor Elk-1, but does not substitute for activated p21ras and synergize with calcium/calcineurin signals to induce NFAT. Expression of dominant negative N17Rac also prevents TCR and p21ras activation of NFAT, but without interfering with the ERK-2 pathway. The transcriptional activity of the NFAT binding site is mediated by a complex comprising a member of the NFAT group and AP-1 family proteins. The induction of AP-1 by p21ras also requires Rac-1 function. Activated Rac-1 could mimic activated p21ras to induce AP-1 but not to induce NFAT. Moreover, the combination of activated MAPKK-1 and Rac-1 could not substitute for activated p21ras and synergize with calcium signals to induce NFAT. Thus, p21ras regulation of NFAT in T cells requires the activity of multiple effector pathways including those regulated by MAPKK-1/ERK-2 and Rac-1.

Determinants of DNA-binding specificity of ETS-domain transcription factors

Several mechanisms are employed by members of transcription factor families to achieve sequence-specific DNA recognition. In this study, we have investigated how members of the ETS-domain transcription factor family achieve such specificity. We have used the ternary complex factor (TCF) subfamily as an example. ERK2 mitogen-activated protein kinase stimulates serum response factor-dependent and autonomous DNA binding by the TCFs Elk-1 and SAP-la. Phosphorylated Elk-1 and SAP-la exhibit specificities of DNA binding similar to those of their isolated ETS domains. The ETS domains of Elk-1 and SAP-la and SAP-2 exhibit related but distinct DNA-binding specificities. A single residue, D-69 (Elk-1) or V-68 (SAP-1), has been identified as the critical determinant for the differential binding specificities of Elk-1 and SAP-1a, and an additional residue, D-38 (Elk-1) or Q-37 (SAP-1), further modulates their DNA binding. Creation of mutations D38Q and D69V is sufficient to confer SAP-la DNA-binding specificity upon Elk-1 and thereby allow it to bind to a greater spectrum of sites. Molecular modelling indicates that these two residues (D-38 and D-69) are located away from the DNA-binding interface of Elk-1. Our data suggest a mechanism in which these residues modulate DNA binding by influencing the interaction of other residues with DNA.

Selective interaction of JNK protein kinase isoforms with transcription factors

The JNK protein kinase is a member of the MAP kinase group that is activated in response to dual phosphorylation on threonine and tyrosine. Ten JNK isoforms were identified in human brain by molecular cloning. These protein kinases correspond to alternatively spliced isoforms derived from the JNK1, JNK2 and JNK3 genes. The protein kinase activity of these JNK isoforms was measured using the transcription factors ATF2, Elk-1 and members of the Jun family as substrates. Treatment of cells with interleukin-1 (IL-1) caused activation of the JNK isoforms. This activation was blocked by expression of the MAP kinase phosphatase MKP-1. Comparison of the binding activity of the JNK isoforms demonstrated that the JNK proteins differ in their interaction with ATF2, Elk-1 and Jun transcription factors. Individual members of the JNK group may therefore selectively target specific transcription factors in vivo.

Cloning and characterization of MEK6, a novel member of the mitogen-activated protein kinase kinase cascade

Mitogen-activated protein kinases are members of a conserved cascade of kinases involved in many signal transduction pathways. They stimulate phosphorylation of transcription factors in response to extracellular signals such as growth factors, cytokines, ultraviolet light, and stress-inducing agents. A novel mitogen-activated protein kinase kinase, MEK6, was cloned and characterized. The complete MEK6 cDNA was isolated by polymerase chain reaction. It encodes a 334-amino acid protein with 82% identity to MKK3. MEK6 is highly expressed in skeletal muscle like many other members of this family, but in contrast to MKK3 its expression in leukocytes is very low. MEK6 is a member of the p38 kinase cascade and efficiently phosphorylates p38 but not c-Jun N-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) family members in direct kinase assays. Coupled kinase assays demonstrated that MEK6 induces phosphorylation of ATF2 by p38 but does not phosphorylate ATF2 directly. MEK6 is strongly activated by UV, anisomycin, and osmotic shock but not by phorbol esters, nerve growth factor, and epidermal growth factor. This separates MEK6 from the ERK subgroup of protein kinases. MEK6 is only a poor substrate for MEKK, a mitogen-activated protein kinase kinase kinase that efficiently phosphorylates the related family member JNKK.

Regulation of transcription by MAP kinase cascades

Kinases belonging to the mitogen-activated protein kinase (MAPK) family are used throughout evolution to control the cellular responses to external signals such as growth factors, nutrient status, stress or inductive signals. Many important substrates for MAPKs are transcription factors, and both the genetic and the biochemical links between MAPKs and transcription factors are becoming increasingly well understood.

SOS phosphorylation and disassociation of the Grb2-SOS complex by the ERK and JNK signaling pathways

Insulin activation of Ras is mediated by the plasma membrane targeting of the guanylnucleotide exchange factor SOS associated with the small adapter protein Grb2. SOS also lies in an insulin-stimulated feedback pathway in which the serine/threonine phosphorylation of SOS results in disassociation of the Grb2-SOS complex thereby limiting the extent of Ras activation. To examine the relative role of the mitogen-activated protein kinases in the feedback phosphorylation of SOS we determined the signaling specificity of insulin, osmotic shock, and anisomycin to activate the ERK (extracellular-signal regulated kinase) and JNK (c-Jun kinase) pathways. In Chinese hamster ovary cells expressing the human insulin receptor and murine 3T3L1 adipocytes, insulin specifically activated ERK with no significant effect on JNK, whereas anisomycin specifically activated JNK but was unable to activate ERK. In contrast, osmotic shock was equally effective in the activation of both kinase pathways. Insulin and osmotic shock, but not anisomycin, resulted in SOS phosphorylation and disassociation of the Grb2-SOS complex, demonstrating that the JNK pathway was not involved in the insulin-stimulated feedback uncoupling of the Grb2- SOS complex. Both the insulin and osmotic shock-induced activation of ERK was prevented by treatment of cells with the specific MEK inhibitor (PD98059). However, expression of dominant-interfering Ras (N17Ras) inhibited the insulin- but not osmotic shock-stimulated phosphorylation of ERK and SOS. These data demonstrate that activation of the ERK pathway, but not JNK, is responsible for the feedback phosphorylation and disassociation of the Grb2-SOS complex.

Signalling pathways: jack of all cascades

The transcription factors that bind the c-fos promoter element SRE are targeted by multiple, independent signalling cascades; the identities of these signalling pathways and their modes of activation are being elucidated.

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

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