Activation of the MAP kinase pathway by the protein kinase raf

The dual-specificity CLK kinase induces neuronal differentiation of PC12 cells

CLK is a dual-specificity protein kinase capable of phosphorylating serine, threonine, and tyrosine residues. We have investigated the action of CLK by establishing stable PC12 cell lines capable of inducibly expressing CLK. Expression of CLK in stably transfected PC12 cells mimicked a number of nerve growth factor (NGF)-dependent events, including the morphological differentiation of these cells and the elaboration of neurites. Moreover, CLK expression enhanced the rate of NGF-mediated neurite outgrowth of these cells, indicating that CLK expression and NGF treatment activate similar signal transduction pathways. CLK expression, unlike NGF, was not able to promote PC12 cell survival in serum-free media, demonstrating that CLK only partially recapitulated the actions of NGF on these cells and that the biochemical pathways necessary for morphological differentiation can be stimulated without also stimulating those necessary for survival. Induction of CLK expression also resulted in the selective activation of protein kinases that are components of growth factor-stimulated signal transduction cascades, including ERK1, ERK2, pp90RSK, and S6PKII. Induction of CLK expression, however, did not stimulate pp70S6K or Fos kinase, two NGF-sensitive protein kinases. These data indicate that CLK action mediates the morphological differentiation of these cells through its capacity to independently stimulate signal transduction pathways normally employed by NGF.

Mechanism of inhibition of Raf-1 by protein kinase A

The cytoplasmic Raf-1 kinase is essential for mitogenic signalling by growth factors, which couple to tyrosine kinases, and by tumor-promoting phorbol esters such as 12-O-tetradecanoylphorbol-13-acetate, which activate protein kinase C (PKC). Signalling by the Raf-1 kinase can be blocked by activation of the cyclic AMP (cAMP)-dependent protein kinase A (PKA). The molecular mechanism of this inhibition is not precisely known but has been suggested to involve attenuation of Raf-1 binding to Ras. Using purified proteins, we show that in addition to weakening the interaction of Raf-1 with Ras, PKA can inhibit Raf-1 function directly via phosphorylation of the Raf-1 kinase domain. Phosphorylation by PKA interferes with the activation of Raf-1 by either PKC alpha or the tyrosine kinase Lck and even can downregulate the kinase activity of Raf-1 previously activated by PKC alpha or amino-terminal truncation. This type of inhibition can be dissociated from the ability of Raf-1 to associate with Ras, since (i) the isolated Raf-1 kinase domain, which lacks the Ras binding domain, is still susceptible to inhibition by PKA, (ii) phosphorylation of Raf-1 by PKC alpha alleviates the PKA-induced reduction of Ras binding but does not prevent the downregulation of Raf-1 kinase activity by PKA and (iii) cAMP agonists antagonize transformation by v-Raf, which is Ras independent.

The Ras/Raf signaling pathway is required for progression of mouse embryos through the two-cell stage

We have used microinjection of antisense oligonucleotides, monoclonal antibody, and the dominant negative Ras N-17 mutant to interfere with Ras expression and function in mouse oocytes and early embryos. Microinjection of either ras antisense oligonucleotides or anti-Ras monoclonal antibody Y13-259 did not affect normal progression of oocytes through meiosis and arrest at metaphase II. However, microinjection of fertilized eggs with constructs expressing Ras N-17 inhibited subsequent development through the two-cell stage. The inhibitory effect of Ras N-17 was overcome by simultaneous injection of a plasmid expressing an active raf oncogene, indicating that it resulted from interference with the Ras/Raf signaling pathway. In contrast to the inhibition of two-cell embryo development resulting from microinjection of pronuclear stage eggs, microinjection of late two-cell embryos with Ras N-17 expression constructs did not affect subsequent cleavages and development to morulae and blastocysts. It thus appears that the Ras/Raf signaling pathway, presumably activated by autocrine growth factor stimulation, is specifically required at the two-cell stage, which is the time of transition between maternal and embryonic gene expression in mouse embryos.

The mitogen-activated protein kinase cascade is activated by B-Raf in response to nerve growth factor through interaction with p21ras

Nerve growth factor (NGF) activates the mitogen-activated protein (MAP) kinase cascade through a p21ras-dependent signal transduction pathway in PC12 cells. The linkage between p21ras and MEK1 was investigated to identify those elements which participate in the regulation of MEK1 activity. We have screened for MEK activators using a coupled assay in which the MAP kinase cascade has been reconstituted in vitro. We report that we have detected a single NGF-stimulated MEK-activating activity which has been identified as B-Raf. PC12 cells express both B-Raf and c-Raf1; however, the MEK-activating activity was found only in fractions containing B-Raf. c-Raf1-containing fractions did not exhibit a MEK-activating activity. Gel filtration analysis revealed that the B-Raf eluted with an apparent M(r) of 250,000 to 300,000, indicating that it is present within a stable complex with other unidentified proteins. Immunoprecipitation with B-Raf-specific antisera quantitatively precipitated all MEK activator activity from these fractions. We also demonstrate that B-Raf, as well as c-Raf1, directly interacted with activated p21ras immobilized on silica beads. NGF treatment of the cells had no effect on the ability of B-Raf or c-Raf1 to bind to activated p21ras. These data indicate that this interaction was not dependent upon the activation state of these enzymes; however, MEK kinase activity was found to be associated with p21ras following incubation with NGF-treated samples at levels higher than those obtained from unstimulated cells. These data provide direct evidence that NGF-stimulated B-Raf is responsible for the activation of the MAP kinase cascade in PC12 cells, whereas c-Raf1 activity was not found to function within this pathway.

Inhibition of Ras-induced DNA synthesis by expression of the phosphatase MKP-1

Mitogen-activated protein kinases (MAP kinases) are common components of signaling pathways induced by diverse growth stimuli. Although the guanidine nucleotide-binding Ras proteins are known to be upstream activators of MAP kinases, the extent to which MAP kinases directly contribute to the mitogenic effect of Ras is as yet undefined. In this study, inhibition of MAP kinases by the MAP kinase phosphatase MKP-1 blocked the induction of DNA synthesis in quiescent rat embryonic fibroblast REF-52 cells by an activated mutant of Ras, V12Ras. These results suggest an essential role for activation of MAP kinases in the transition from the quiescent to the DNA replication phase of the eukaryotic cell cycle.

Activation of Raf-1 by 14-3-3 proteins

The protein Raf-1, a key mediator of mitogenesis and differentiation, associates with p21ras (refs 1-3). However, the regulation of the serine/threonine kinase activity of Raf-1 is still not understood. Using the yeast two-hybrid system, we identified two structurally related proteins that interact with the aminoterminal region of Raf-1. These proteins, 14-3-3 zeta (PLA2) and 14-3-3 beta (HS1), are members of the 14-3-3 family of proteins. Expression of 14-3-3 proteins in Xenopus oocytes enhanced Raf-1 activity and promoted Raf-1-dependent oocyte maturation. A dominant negative mutant of Raf-1 blocked the effects of 14-3-3 protein.

Tpl-2 acts in concert with Ras and Raf-1 to activate mitogen-activated protein kinase

Mitogenic signals initiated at the plasma membrane by extracellular factors acting on receptor tyrosine kinases or G protein-coupled receptors are transmitted to the nucleus through an intricate signaling network. Components of this network participate, upon stimulation, in a complex array of phosphorylation-dependent protein-protein interactions which leads to the formation of transient multimolecular complexes. Complexes containing products of the protooncogenes ras and raf-1 and the protein kinase MEK-1 activate the mitogen-activated protein kinases (MAPKs), which play a central role in the integration of different mitogenic signals by directly phosphorylating cytoplasmic and nuclear targets. In this report we present evidence that the kinase encoded by the tumor progression locus 2 gene (Tpl-2) contributes to the activation of the MAPK cascade. MAPK activation induced by the Tpl-2 protein is blocked by dominant negative mutants of Ras and Raf-1, whereas a kinase-deficient Tpl-2 mutant down-regulates mitogenic signals induced by v-Ha-Ras or v-Raf. These data suggest that Tpl-2 activates the MAPK cascade, perhaps through its participation in the assembly of Ras/Raf-1-containing multimolecular complexes.

Biological function of PDGF-induced PI-3 kinase activity: its role in alpha PDGF receptor-mediated mitogenic signaling

The tyrosine phosphorylation sites in the human alpha PDGF receptor (alpha PDGFR) required for association with PI-3 kinase have been identified as tyrosines 731 and 742. Mutation of either tyrosine substantially reduced PDGF-induced PI-3 kinase activity but did not impair the receptor-mediated mitogenic response. We sought to determine whether PDGF-induced PI-3 kinase activity could be further ablated so as to exclude a low threshold requirement for PDGFR signal transduction. Thus, we mutated both tyrosine 731 and 742 and expressed the double mutant (Y731F/Y742F) in 32D hematopoietic cells. In such transfectants, PDGF induced no detectable receptor-associated or anti-P-Tyr recoverable PI-3 kinase activity. Under the same conditions, neither mobility shift of raf-1 nor tyrosine phosphorylation of either PLC gamma or MAP kinase was impaired. 32D transfectants expressing the double mutant showed wild-type alpha PDGFR levels of mitogenic and chemotactic responses to PDGF. To examine the effect of the double mutation in cells that normally respond to PDGF, we generated chimeras in which the cytoplasmic domains of wild-type alpha PDGFR, Y731F, and Y731F/Y742F were linked to the extracellular domain of colony-stimulating factor-1 (CSF-1) receptor (fms). After introduction of the chimeric receptors into mouse NIH/3T3 fibroblasts, the ability of CSF-1 to stimulate growth of these transfectants was examined. Our data show that all these chimeric receptors exhibited similar abilities to mediate CSF-1-stimulated cell growth. These findings lead us to conclude that PDGF-induced PI-3 kinase activity is not required for PDGF-stimulated mitogenic pathway in both NIH/3T3 fibroblasts and 32D hematopoietic cells.

The mitogen-activated protein kinase cascade is activated by B-Raf in response to nerve growth factor through interaction with p21ras

Nerve growth factor (NGF) activates the mitogen-activated protein (MAP) kinase cascade through a p21ras-dependent signal transduction pathway in PC12 cells. The linkage between p21ras and MEK1 was investigated to identify those elements which participate in the regulation of MEK1 activity. We have screened for MEK activators using a coupled assay in which the MAP kinase cascade has been reconstituted in vitro. We report that we have detected a single NGF-stimulated MEK-activating activity which has been identified as B-Raf. PC12 cells express both B-Raf and c-Raf1; however, the MEK-activating activity was found only in fractions containing B-Raf. c-Raf1-containing fractions did not exhibit a MEK-activating activity. Gel filtration analysis revealed that the B-Raf eluted with an apparent M(r) of 250,000 to 300,000, indicating that it is present within a stable complex with other unidentified proteins. Immunoprecipitation with B-Raf-specific antisera quantitatively precipitated all MEK activator activity from these fractions. We also demonstrate that B-Raf, as well as c-Raf1, directly interacted with activated p21ras immobilized on silica beads. NGF treatment of the cells had no effect on the ability of B-Raf or c-Raf1 to bind to activated p21ras. These data indicate that this interaction was not dependent upon the activation state of these enzymes; however, MEK kinase activity was found to be associated with p21ras following incubation with NGF-treated samples at levels higher than those obtained from unstimulated cells. These data provide direct evidence that NGF-stimulated B-Raf is responsible for the activation of the MAP kinase cascade in PC12 cells, whereas c-Raf1 activity was not found to function within this pathway.

The dual-specificity CLK kinase induces neuronal differentiation of PC12 cells

CLK is a dual-specificity protein kinase capable of phosphorylating serine, threonine, and tyrosine residues. We have investigated the action of CLK by establishing stable PC12 cell lines capable of inducibly expressing CLK. Expression of CLK in stably transfected PC12 cells mimicked a number of nerve growth factor (NGF)-dependent events, including the morphological differentiation of these cells and the elaboration of neurites. Moreover, CLK expression enhanced the rate of NGF-mediated neurite outgrowth of these cells, indicating that CLK expression and NGF treatment activate similar signal transduction pathways. CLK expression, unlike NGF, was not able to promote PC12 cell survival in serum-free media, demonstrating that CLK only partially recapitulated the actions of NGF on these cells and that the biochemical pathways necessary for morphological differentiation can be stimulated without also stimulating those necessary for survival. Induction of CLK expression also resulted in the selective activation of protein kinases that are components of growth factor-stimulated signal transduction cascades, including ERK1, ERK2, pp90RSK, and S6PKII. Induction of CLK expression, however, did not stimulate pp70S6K or Fos kinase, two NGF-sensitive protein kinases. These data indicate that CLK action mediates the morphological differentiation of these cells through its capacity to independently stimulate signal transduction pathways normally employed by NGF.

Mechanism of inhibition of Raf-1 by protein kinase A

The cytoplasmic Raf-1 kinase is essential for mitogenic signalling by growth factors, which couple to tyrosine kinases, and by tumor-promoting phorbol esters such as 12-O-tetradecanoylphorbol-13-acetate, which activate protein kinase C (PKC). Signalling by the Raf-1 kinase can be blocked by activation of the cyclic AMP (cAMP)-dependent protein kinase A (PKA). The molecular mechanism of this inhibition is not precisely known but has been suggested to involve attenuation of Raf-1 binding to Ras. Using purified proteins, we show that in addition to weakening the interaction of Raf-1 with Ras, PKA can inhibit Raf-1 function directly via phosphorylation of the Raf-1 kinase domain. Phosphorylation by PKA interferes with the activation of Raf-1 by either PKC alpha or the tyrosine kinase Lck and even can downregulate the kinase activity of Raf-1 previously activated by PKC alpha or amino-terminal truncation. This type of inhibition can be dissociated from the ability of Raf-1 to associate with Ras, since (i) the isolated Raf-1 kinase domain, which lacks the Ras binding domain, is still susceptible to inhibition by PKA, (ii) phosphorylation of Raf-1 by PKC alpha alleviates the PKA-induced reduction of Ras binding but does not prevent the downregulation of Raf-1 kinase activity by PKA and (iii) cAMP agonists antagonize transformation by v-Raf, which is Ras independent.

The Ras/Raf signaling pathway is required for progression of mouse embryos through the two-cell stage

We have used microinjection of antisense oligonucleotides, monoclonal antibody, and the dominant negative Ras N-17 mutant to interfere with Ras expression and function in mouse oocytes and early embryos. Microinjection of either ras antisense oligonucleotides or anti-Ras monoclonal antibody Y13-259 did not affect normal progression of oocytes through meiosis and arrest at metaphase II. However, microinjection of fertilized eggs with constructs expressing Ras N-17 inhibited subsequent development through the two-cell stage. The inhibitory effect of Ras N-17 was overcome by simultaneous injection of a plasmid expressing an active raf oncogene, indicating that it resulted from interference with the Ras/Raf signaling pathway. In contrast to the inhibition of two-cell embryo development resulting from microinjection of pronuclear stage eggs, microinjection of late two-cell embryos with Ras N-17 expression constructs did not affect subsequent cleavages and development to morulae and blastocysts. It thus appears that the Ras/Raf signaling pathway, presumably activated by autocrine growth factor stimulation, is specifically required at the two-cell stage, which is the time of transition between maternal and embryonic gene expression in mouse embryos.

The role of protein kinase C in the regulation of extracellular signal-regulated kinase by the T cell antigen receptor

The aim of this study was to explore the role of protein kinase C (PKC) in the activation of mitogen-activated protein kinases (MAPK) in T lymphocytes. The MAPK extracellular signal-regulated kinase-2 (ERK2) is activated in response to phorbol esters which stimulate PKC, by transient expression of a constitutively active ras mutant by cell activation via the G protein-coupled type 1 muscarinic acetylcholine receptor (HM1R) or in response to triggering of the T cell antigen receptor (TCR). The relative contribution of PKC to TCR and HM1R regulation of ERK2 was explored by examining the effects of a PKC inhibitor (Ro 31-8425) on ERK2 activation. The data demonstrate that phorbol ester and HM1R regulation of ERK2 was prevented by the PKC inhibitor, but that the inhibitor had no effect on ERK2 activation induced by expression of a constitutively active ras mutant p21v-Ha-ras. Furthermore, the TCR stimulates both PKC and p21ras but TCR regulation of ERK2 was only weakly suppressed by the PKC inhibitor. These data indicate that PKC has a potential but not a predominant role in TCR regulation of ERK2.

The sevenmaker gain-of-function mutation in p42 MAP kinase leads to enhanced signalling and reduced sensitivity to dual specificity phosphatase action

A mammalian mutant MAP kinase, D319N ERK2, analogous to Drosophila melanogaster sevenmaker (rlsem) gain-of-function mutation was shown to have an increased sensitivity to low levels of signalling in vivo. However, the mutation does not lead to an elevated basal kinase activity and still requires activation by MAP kinase kinase (MAPKK) as does wild type ERK2. This increased responsiveness seen in vivo is not due to an increased ability to phosphorylate substrates but appears to reflect a reduced sensitivity to a MAP kinase phosphatase CL100.

Simian virus 40 small t antigen cooperates with mitogen-activated kinases to stimulate AP-1 activity

The simian virus 40 small tumor antigen (small t) specifically interacts with protein phosphatase type 2A (PP2A) in vivo and alters its catalytic activity in vitro. Among the substrates for PP2A in vitro are the activated forms of MEK and ERK kinases. Dephosphorylation of the activating phosphorylation sites on MEK and ERKs by PP2A in vitro results in a decrease in their respective kinase activities. Recently, it has been shown that overexpression of small t in CV-1 cells results in an inhibition of PP2A activity toward MEK and ERK2 and a constitutive upregulation of MEK and ERK2 activity. Previously, we have observed that overexpression of either ERK1, MEK1, or a constitutively active truncated form of c-Raf-1 (BXB) is insufficient to activate AP-1 in REF52 fibroblasts. We therefore examined whether overexpression of small t either alone or in conjunction with ERK1, MEK1, or BXB could activate AP-1. We found that coexpression of small t and either ERK1, MEK1, or BXB resulted in an increase in AP-1 activity, whereas expression of either small t or any of the kinases alone did not have any effect. Similarly, coexpression of small t and ERK1 activated serum response element-regulated promoters. Coexpression of kinase-deficient mutants of ERK1 and ERK2 inhibited the activation of AP-1 caused by expression of small t and either MEK1 or BXB. Coexpression of an interfering MEK, which inhibited AP-1 activation by small t and BXB, did not inhibit the activation of AP-1 caused by small t and ERK1. In contrast to REF52 cells, we observed that overexpression of either small or ERK1 alone in CV-1 cells was sufficient to stimulate AP-1 activity and that this stimulation was not enhanced by expression of small t and ERK1 together. These results show that the effects of small t on immediate-early gene expression depend on the cell type examined and suggest that the mitogen-activated protein kinase activation pathway is distinctly regulated in different cell types.

Stimulatory effects of yeast and mammalian 14-3-3 proteins on the Raf protein kinase

Intracellular signaling from receptor tyrosine kinases in mammalian cells results in activation of a signal cascade that includes the guanine nucleotide-binding protein Ras and the protein kinases Raf, MEK [mitogen-activated protein kinase (MAPK) or extracellular signal-regulated kinase (ERK) kinase], and MAPK. MAPK activation that is dependent on the coupling of Ras and Raf was reconstituted in yeast. Yeast genes were isolated that, when overexpressed, enhanced the function of Raf. One of them is identical to BMH1, which encodes a protein similar to members of the mammalian 14-3-3 family. Bacterially synthesized mammalian 14-3-3 protein stimulated the activity of Raf prepared from yeast cells expressing c-Raf-1. Thus, the 14-3-3 protein may participate in or be required for activation of Raf.

Binding of 14-3-3 proteins to the protein kinase Raf and effects on its activation

To identify proteins that may participate in the activation of the protein kinase Raf, proteins that interact with Raf were selected in a two-hybrid screen. Two members of the 14-3-3 protein family were isolated that interacted with both the amino terminal regulatory regions of Raf and the kinase domain of Raf, but did not compete with the guanine nucleotide-binding protein Ras for binding to Raf. 14-3-3 proteins associated with Raf in mammalian cells and accompanied Raf to the membrane in the presence of activated Ras. In yeast cells expressing Raf and MEK, mammalian 14-3-3 beta or 14-3-3 zeta activated Raf to a similar extent as did expression of Ras. Therefore, 14-3-3 proteins may participate in or be required for the regulation of Raf function. These findings suggest a role for 14-3-3 proteins in Raf-mediated signal transduction.

A temperature-sensitive MEK mutation demonstrates the conservation of the signaling pathways activated by receptor tyrosine kinases

MEK, a dual specificity threonine/tyrosine kinase, has been postulated to be a convergent point for signaling from receptor protein tyrosine kinases (RTKs) and G-protein-coupled receptors. In contrast to yeast and mammalian cells where several MEKs have been isolated, only one Drosophila MEK (D-Mek) has been characterized to date. Previous studies have shown that D-Mek acts in the Torso RTK signaling pathway. To demonstrate that D-Mek also operates downstream of other RTKs, we generated a temperature-sensitive allele of D-mek (D-mekts) by site-directed mutagenesis based on the amino acid change of a yeast cdc2ts mutation. Using D-mekts, we show that in addition to its role in Torso signaling, D-Mek operates in the Sevenless and in the Drosophila epidermal growth factor RTK pathways. Because loss-of-function mutations in D-mek and the upstream receptors give rise to similar phenotypes, it suggests that D-mek is the only MEK activated by Drosophila RTKs. In addition, we demonstrate that different RTK pathways respond differently to alteration in D-Mek activity.

Constitutive activation of Mek1 by mutation of serine phosphorylation sites

A variety of extracellular signals lead to the phosphorylation and activation of mitogen-activated protein kinases (MAP kinases). An activator of MAP kinases, Mek1, phosphorylates MAP kinases at threonine and tyrosine residues and is itself phosphorylated at serine-218 and -222 by the protooncogene product Raf-1. By introducing negatively charged residues that may mimic the effect of phosphorylation at positions 218 and 222, we have activated the capacity of Mek1 to phosphorylate MAP kinase by > 100-fold. The most effective activation by a single substitution resulted from the introduction of aspartate at position 218, whereas the introduction of either aspartate or glutamate at position 222 was ineffective. Expression of the activated Mek1 phosphorylation-site mutants in COS-7 cells led to the activation of MAP kinase in the cells and resulted in an increase in the mass of the transfected COS-7 cell population, suggesting an important role of Mek1 in the transduction of mitogenic signals.

Mitogen-activated protein kinases mediate changes in gene expression, but not cytoskeletal organization associated with cardiac muscle cell hypertrophy

Shortly after birth, cardiac myocytes lose the ability to divide, and, in adult animals, heart muscle grows by a process of cellular hypertrophy where each individual cell gets larger. We have previously shown that activated Ras protein can induce markers of the hypertrophic phenotype, including atrial natriuretic factor (ANF) expression and organization of contractile proteins, and that Ras is at least partially required for the hypertrophic effect of phenylephrine. In the present study, we examine the requirement for the mitogen-activated protein kinases (MAP kinases) in the hypertrophic response induced by phenylephrine. We find that phenylephrine treatment results in the activation of the MAP kinases and that this activity is required for transactivation of the fos, ANF, and MLH promoters. However, inhibition of MAP kinases does not prevent phenylephrine-induced organization of actin. These results suggest that the signal transduction pathways leading to different hypertrophic responses diverge upstream of the MAP kinases but possibly downstream of Ras.

Spatial and temporal changes in signal transduction pathways during LTP

Following LTP induction in freely moving rats, in situ hybridization revealed discrete changes in the expression of one isoform in each of four families of serine/threonine kinases constitutively expressed in the dentate gyrus of the hippocampus. Expression of the alpha isoform of CaMKII showed a transient increase over the soma and a more persistent increase over the dendritic field of dentate granule cells. Of the PKC isoforms, only gamma PKC was up-regulated substantially 2 hr after LTP induction, declining to control levels 48 hr later. An increase in the expression of mRNA for ERK2 and raf-B was seen at 24 hr only. These results show that, during the maintenance phase of LTP in the hippocampus, there are selective increases in the expression of serine/threonine kinases and that these increases have specific and characteristic temporal and spatial profiles.

Simian virus 40 small t antigen cooperates with mitogen-activated kinases to stimulate AP-1 activity

The simian virus 40 small tumor antigen (small t) specifically interacts with protein phosphatase type 2A (PP2A) in vivo and alters its catalytic activity in vitro. Among the substrates for PP2A in vitro are the activated forms of MEK and ERK kinases. Dephosphorylation of the activating phosphorylation sites on MEK and ERKs by PP2A in vitro results in a decrease in their respective kinase activities. Recently, it has been shown that overexpression of small t in CV-1 cells results in an inhibition of PP2A activity toward MEK and ERK2 and a constitutive upregulation of MEK and ERK2 activity. Previously, we have observed that overexpression of either ERK1, MEK1, or a constitutively active truncated form of c-Raf-1 (BXB) is insufficient to activate AP-1 in REF52 fibroblasts. We therefore examined whether overexpression of small t either alone or in conjunction with ERK1, MEK1, or BXB could activate AP-1. We found that coexpression of small t and either ERK1, MEK1, or BXB resulted in an increase in AP-1 activity, whereas expression of either small t or any of the kinases alone did not have any effect. Similarly, coexpression of small t and ERK1 activated serum response element-regulated promoters. Coexpression of kinase-deficient mutants of ERK1 and ERK2 inhibited the activation of AP-1 caused by expression of small t and either MEK1 or BXB. Coexpression of an interfering MEK, which inhibited AP-1 activation by small t and BXB, did not inhibit the activation of AP-1 caused by small t and ERK1. In contrast to REF52 cells, we observed that overexpression of either small or ERK1 alone in CV-1 cells was sufficient to stimulate AP-1 activity and that this stimulation was not enhanced by expression of small t and ERK1 together. These results show that the effects of small t on immediate-early gene expression depend on the cell type examined and suggest that the mitogen-activated protein kinase activation pathway is distinctly regulated in different cell types.

Induction of clonal anergy by oral administration of staphylococcal enterotoxin B

The staphylococcal enterotoxin is a major cause of food poisoning. The bacterial substance stimulates T cells expressing specific V beta T cell receptors (TcR) and is termed "the superantigen". We have previously demonstrated that intravenous injection of staphylococcal enterotoxin B (SEB) induces functional unresponsiveness (anergy) of reactive T cells as well as a partial deletion by activation-induced programmed cell death. In the present study, we examined the effect of oral administration of SEB in mice. Our results indicate that spleen T cells from SEB-primed mice are hyporesponsive to SEB stimulation in vitro, but the response to SEA was normal. V beta 8+ T cells purified from SEB-primed mice did not respond to stimulation of TcR. This SEB-specific unresponsiveness could not be reversed by exogenous interleukin-2, but was partially reversed by phorbol 12-myristate 13-acetate. Activation of mitogen-activated protein kinase during TcR-mediated stimulation was significantly inhibited in anergic T cells. Although the mechanisms of oral tolerance are not well understood, these results show that oral administration of SEB induce clonal anergy in peripheral T cells.

Monocyte deactivation by interleukin 10 via inhibition of tyrosine kinase activity and the Ras signaling pathway

Activation of monocytes by bacterial lipopolysaccharides (LPSs) is a central component in the pathogenesis of septic shock syndrome. Interleukin 10 (IL-10) is a potent monocyte-deactivating factor and transcriptionally inhibits LPS-induced expression of proinflammatory mediators. The intracellular signaling pathways of LPS have been only partially characterized and mechanisms of IL-10 signaling remain unknown. We show that LPS activates the protein tyrosine kinase (PTK) p56lyn and that this is associated with tyrosine phosphorylation of the protooncogene product Vav. These events are completely blocked by the tyrosine kinase inhibitor herbimycin A. LPS also increases Ras activation in monocytes. LPS-triggered phosphorylation of mitogen-activated protein kinase is a downstream activation event that is also reduced by herbimycin A. Analysis of the IL-10 effects shows that it completely inhibits the p56lyn tyrosine kinase activation and all other subsequent events in this pathway including Ras activation. The IL-10 effects are selective since it reduced PTK-dependent cytokine mRNA expression but not the PTK independent induction of c-jun and c-fos mRNA in LPS-activated monocytes. These results identify the Ras signaling pathway as a component of intracellular signaling in LPS-stimulated monocytes and define early events in this response as targets of monocyte deactivation by IL-10.

Phosphatidylinositol-3-OH kinase as a direct target of Ras

Ras (p21ras) interacts directly with the catalytic subunit of phosphatidylinositol-3-OH kinase in a GTP-dependent manner through the Ras effector site. In vivo, dominant negative Ras mutant N17 inhibits growth factor induced production of 3' phosphorylated phosphoinositides in PC12 cells, and transfection of Ras, but not Raf, into COS cells results in a large elevation in the level of these lipids. Therefore Ras can probably regulate phosphatidylinositol-3-OH kinase, providing a point of divergence in signalling pathways downstream of Ras.

Staurosporine inhibits phorbol 12-myristate 13-acetate- and insulin-stimulated translocation of GLUT1 and GLUT4 glucose transporters in rat adipose cells

Staurosporine, a widely used protein kinase C inhibitor, completely inhibited both phorbol 12-myristate 13-acetate (PMA)- and insulin-stimulated glucose transport activity in isolated rat adipocytes. The inhibition was non-competitive and was attributed to a blockade of the PMA- and insulin-induced translocation of both GLUT1 and GLUT4 glucose transporters. The PMA-stimulated glucose transport activity was more sensitive to inhibition by staurosporine than was insulin-stimulated transport activity (PMA, IC50 = 1.1 +/- 0.1 microM; insulin, IC50 = 6.4 +/- 0.7 microM; P < 0.05, n = 3). At 1 microM staurosporine the insulin-sensitivity was decreased, i.e. EC50 increased from 0.12 nM to 5.4 nM, but the maximum response to insulin and the time course for stimulation were unaffected. At 6 microM staurosporine the insulin-sensitivity was further decreased, the maximal stimulation was decreased by 25%, and the apparent half-time for stimulation was extended from 2.5 min in control cells to 9.4 min. Staurosporine (30 microM) was able to block insulin's ability to stimulate glucose transport, whether added before or after insulin, by a mechanism that did not alter the rate of GLUT4 internalization. In intact adipose cells, staurosporine (30 microM) induced a slight (30%) decrease in the maximal insulin-induced receptor autophosphorylation and a similar decrease in the tyrosine phosphorylation of pp60 and pp160 (insulin-receptor substrate-1: 'IRS-1'), but was without effect on insulin binding to its receptor. Conversely, staurosporine induced a concentration-dependent inhibition of the constitutively tyrosine-phosphorylated (pp120) protein and of an insulin-stimulated protein pp53 in the cytosol. The locus of staurosporine's action appears to be distal from the initial insulin-receptor signalling, at a step that regulates the specific translocation of the glucose transporters to the plasma membranes.

Transformation of mammalian cells by constitutively active MAP kinase kinase

Mitogen-activated protein (MAP) kinase kinase (MAPKK) activates MAP kinase in a signal transduction pathway that mediates cellular responses to growth and differentiation factors. Oncogenes such as ras, src, raf, and mos have been proposed to transform cells by prolonging the activated state of MAPKK and of components downstream in the signaling pathway. To test this hypothesis, constitutively active MAPKK mutants were designed that had basal activities up to 400 times greater than that of the unphosphorylated wild-type kinase. Expression of these mutants in mammalian cells activated AP-1-regulated transcription. The cells formed transformed foci, grew efficiently in soft agar, and were highly tumorigenic in nude mice. These findings indicate that constitutive activation of MAPKK is sufficient to promote cell transformation.

Activation of the mitogen-activated protein kinase cascade by tyrphostin (RG 50864)

Tyrphostins are synthetic compounds which have been described as in vitro inhibitors of epidermal growth factor (EGF)-receptor tyrosine kinase activity. In NIH3T3 cells, stimulation of EGF-receptor tyrosine kinase leads to an increase of intracellular protein phosphorylations, among them the phosphorylation of mitogen-activated protein (MAP) kinase and the S6 kinases p90rsk and p70S6K. Phosphorylation of these proteins, either on tyrosine or serine/threonine residues or on both residues increases their protein kinase activity. Unexpectedly, treatment of NIH3T3 cells with both tyrphostin (RG 50864) and EGF results in an increase in the level of tyrosine phosphorylation of the MAP kinase. During this treatment, we also observed an increase in MAP kinase and S6 kinase p90rsk activities. Tyrphostin treatment diminishes the level of c-fos mRNA but has no effect on c-myc mRNA expression nor on S6 kinase p70S6K activity. Mitogenic signalling induced by EGF in NIH3T3 cells was blocked by tyrphostin, suggesting that the target(s) for this event may be elements downstream from the MAP kinase or independent of this signal transduction.

Complexes between STE5 and components of the pheromone-responsive mitogen-activated protein kinase module

We present genetic evidence for complex formation of STE5 and the STE11, STE7, and FUS3 protein kinases, the pheromone-responsive mitogen-activated protein kinase module of Saccharomyces cerevisiae. Interaction between STE5 and STE11 is not dependent on STE7, and interaction between STE5 and STE7 does not require STE11. The N-terminal regulatory domain of STE11 is both necessary and sufficient for interaction with STE5. Interaction between STE7 and STE11 is bridged by STE5, suggesting the formation of a multiprotein complex. We also demonstrate biochemical interaction between STE5 and STE11 by using a combination of bacterially expressed fusion proteins and extracts prepared from yeast. Our results suggest that STE5 is a scaffolding protein that facilitates interactions between components of the pheromone-responsive mitogen-activated protein kinase module. We further propose that such scaffolding proteins serve to inhibit cross-talk between functionally unrelated mitogen-activated protein kinase modules within the same cell.

EGF triggers neuronal differentiation of PC12 cells that overexpress the EGF receptor

BACKGROUND

Mitogen-activated protein (MAP) kinase is the central component of a signal transduction pathway that is activated by growth factors interacting with receptors that have protein tyrosine kinase activity. The stimulation of PC12 phaeochromocytoma cells with nerve growth factor leads to the sustained activation and nuclear translocation of the p42 and p44 isoforms of MAP kinase and induces the differentiation of these chromaffin cells to a sympathetic-neuron-like phenotype. In contrast, stimulation with epidermal growth factor induces a transient activation of p42 and p44 MAP kinases without pronounced nuclear translocation and does not trigger cell differentiation. We have examined whether the differential activation of MAP kinases forms the basis of the differential response of the cells to the two factors.

RESULTS

By overexpressing either wild-type or mutant receptors for epidermal growth factor in PC12 cells, we found that p42 and p44 MAP kinase activity remains elevated for longer in cells that overexpress receptors than in untransfected cells. Epidermal growth factor promotes both a striking nuclear translocation of p42 MAP kinase and the differentiation of the overexpressing cells.

CONCLUSIONS

Our results strongly suggest that the distinct effects of nerve growth factor and epidermal growth factor on PC12 cell differentiation can be explained by differences in the extent and duration of activation of p42 and p44 MAP kinases in response to the two factors, without invoking a signal transduction pathway specific to nerve growth factor.

Ras/MAP kinase-dependent and -independent signaling pathways target distinct ternary complex factors

Transcriptional activation of the immediate early genes c-fos and egr-1 by extracellular signals appears to be mediated by ternary complex factors (TCFs). In BAC-1 macrophages, growth factor stimulation leads to the retardation of protein-DNA complexes containing distinct TCFs. One TCF is recognized by Elk-1 antisera, whereas the other is immunologically related to SAP-1. The appearance and decay of hyperphosphorylated TCF/Elk-1-containing complexes after stimulation coincide with the activation of mitogen-activated protein kinase (MAPK) and the induction and repression of c-fos and egr-1, whereas modified TCF/SAP-1-containing complexes decay more slowly. Suppression of MAPK activation in macrophages and fibroblasts correlates with the failure to induce TCF/Elk-1 hyperphosphorylation without blocking TCF/SAP-1 modification. Accordingly the modified Elk-1 complex is generated in vitro by activated MAPK, whereas that of SAP-1 is not. Expression of a dominant-negative Ras mutant (RasAsn17) in BAC-1 cells does not affect CSF-1-induced TCF/SAP-1 modification while suppressing TCF/Elk-1 phosphorylation. Neither PKC down-regulation by TPA nor inhibition of Gi proteins by pertussis toxin pretreatment influences CSF-1-induced signaling to TCFs. These data indicate the existence of two separate signaling pathways for the modification of distinct TCFs: one dependent on Ras and MAPK and converging on TCF/Elk-1, and the other targeting TCF/SAP-1 independently of Ras and MAPK.

The mitogen activated protein kinase signal transduction pathway: from the cell surface to the nucleus

Activation of the mitogen activated protein kinase (MAPK) plays essential roles in many signal transduction pathways. MAPK has been demonstrated to phosphorylate and regulate numerous cellular proteins, including growth factor receptor, transcription factors, cytoskeletal proteins, phospholipase and other protein kinases. Activation of MAPK requires phosphorylation of both threonine and tyrosine residues, which are catalysed by a single protein kinase known as MAPK kinase or MEK. MEK itself is activated by phosphorylation on two conserved serine residues. Three distinct mammalian Ser/Thr kinases, including Raf, Mos and MEKK (for MEK kinase), have been demonstrated to phosphorylate and activate MEK. The MAP kinase cascade is highly conserved in all eukaryotes and involved in numerous cellular responses. Activation of MAPK is a transient event that is tightly regulated by both kinases and phosphatases. A growth factor induced dual specific phosphatase is likely to play an important role in MAPK regulation.

Fibroblast growth factor-induced increases in calcium currents in the PC12 pheochromocytoma cell line are tyrosine phosphorylation dependent

The PC12 rat pheochromocytoma cell line is widely used to study neuronal differentiation by growth factors. In response to nerve growth factor (NGF) and basic fibroblast growth factor (bFGF), PC12 cells differentiate into sympathetic-like neurons and become electrically excitable. Using whole cell patch-clamp recording, with barium as a charge carrier, we looked at the effects of bFGF on calcium channel expression as reflected by changes in barium current amplitudes normalized to cell membrane area. Similar to the effect reported for NGF, we show that 7 day treatment with bFGF increased the barium current approximately 4-fold. The largest contributor to the increase in barium current with bFGF treatment is a 6-fold increase in the high threshold voltage activated omega-conotoxin sensitive barium current. Smaller increases in current produced by bFGF treatment of PC12 cells are observed for the dihydropyridine sensitive and dihydropyridine/conotoxin insensitive currents. The bFGF-induced increases in barium currents are dependent on tyrosine phosphorylation, since the effects of bFGF are blocked by genistein, a tyrosine kinase inhibitor. This system will ultimately be useful in understanding the signaling pathways that control calcium channel expression in response to growth factors.

PC12 cells overexpressing the insulin receptor undergo insulin-dependent neuronal differentiation

BACKGROUND

Stimulation of phaeochromocytoma PC12 cells by nerve growth factor leads to growth arrest and neuronal differentiation, whereas insulin induces various metabolic responses such as metabolism of glucose and lipids. Moreover, both insulin and epidermal growth factor stimulate the proliferation of PC12 cells. In spite of their different biological effects, nerve growth factor, insulin and epidermal growth factor induce very similar early responses in PC12 cells. Stimulation with nerve growth factor leads to the sustained activation and nuclear translocation of mitogen-activated protein (MAP) kinase. By contrast, both insulin and epidermal growth factor induce the transient activation of MAP kinase, without pronounced nuclear translocation of the enzyme. We have investigated whether the differential activation of signaling pathway components can account for the distinct cellular responses to these different growth factors.

RESULTS

By overexpressing insulin receptors in PC12 cells, we observed insulin-dependent neurite outgrowth, similar to that induced by nerve growth factor in both non-transfected and overexpressing cells. Overexpression of insulin receptors in PC12 cells leads to a more pronounced, but similar pattern of insulin-induced tyrosine-phosphorylated proteins in PC12 cells, including enhanced recruitment of Grb2/Sos into a complex with either Shc or IRS1. MAP kinase activation in response to insulin stimulation of cells overexpressing the insulin receptor is similar to MAP kinase activation in response to NGF stimulation of parental or overexpressing PC12 cells: the activation is prolonged and nuclear translocation of the enzyme occurs.

CONCLUSION

The differential subcellular localization and duration of MAP kinase activation induced by insulin and NGF may explain the difference in the biological actions of these two factors on PC12 cells. Our results show that the strength of the signal generated by a receptor with tyrosine kinase activity can influence the downstream signaling pathway, leading to cell differentiation instead of cell proliferation.

Protein kinase A acts at multiple points to inhibit Xenopus oocyte maturation

In Xenopus oocytes, initiation of maturation is dependent on reduction of cyclic AMP-dependent protein kinase (PKA) activity and the synthesis of the mos proto-oncogene product. Mos is required during meiosis I for the activation of both maturation-promoting factor (MPF) and mitogen-activated protein kinase (MAPK). Here we show that injection of the catalytic subunit of PKA (PKAc) prevented progesterone-induced synthesis of endogenous Mos as well as downstream MPF and MAPK activation. However, PKAc did not prevent injected soluble Mos product from activating MAPK. While MAPK is activated during Mos-PKAc coinjection, attendant MPF activation is blocked. Additionally, PKAc caused a potent block in the electrophoretic mobility shift of cdc25 that is associated with phosphatase activation. This inhibition of cdc25 activity was not reversed by progesterone, Mos, or MPF. We conclude that PKAc acts as a negative regulator at several points in meiotic maturation by preventing both Mos translation and MPF activation.

Inhibition of v-raf-dependent c-fos expression and transformation by a kinase-defective mutant of the mitogen-activated protein kinase Erk2

Receptor-bound growth factors elicit intracellular signals that lead to the phosphorylation and activation of numerous intracellular kinases and transcription factors with consequent changes in patterns of gene expression. Several oncogene products are able to mimic these signals, resulting in cell transformation and proliferation. For example, the introduction of oncogenic forms of Raf-1 kinase into fibroblasts induces transformation and leads to the constitutive expression of, among others, the c-fos proto-oncogene. Here it is shown that the elevation of c-fos promoter activity brought about by v-raf is mediated by TCF/Elk-1, which forms a ternary complex with SRF at the serum response element and is a substrate for mitogen-activating protein kinases in vitro. In NIH 3T3 fibroblasts, v-raf activates Erk2, and overexpression of an interfering mutant of Erk2 both blocks the ability of v-raf to activate the c-fos promoter and suppresses transformation. Mutation of individual mitogen-activating protein kinase phosphoacceptor sites in TCF/Elk-1 also compromises v-raf-activated expression of a Gal-Elk/Gal-chloramphenicol acetyltransferase reporter system. However, in at least one instance the introduction of glutamate, but not aspartate, at a phosphoacceptor site is compatible with activation. These results provide compelling evidence that phosphorylation of TCF/Elk-1 by Erk2 is a major link in the Raf-1 kinase-dependent signal transduction pathway that activates c-fos expression.

The role of Raf-1 in the regulation of extracellular signal-regulated kinase 2 by the T cell antigen receptor

Triggering of the T cell antigen receptor (TCR) complex activates the serine/threonine kinase Raf-1 whose function is necessary for TCR induction of the interleukin 2 gene. Raf-1 has been identified as a candidate mitogen-activated protein (MAP) kinase kinase kinase (MKKK) and thus has the potential to couple the TCR to the activation of the MAP kinases such as ERK2. In the present study, the role of Raf-1 in ERK2 regulation of ERK2 in T cells has been explored. A constitutively active Raf-1 kinase, v-raf, or a dominant inhibitory Raf-1 mutant were expressed transiently from the pEF BOS vector in Jurkat cells and the effects of these Raf-1 mutants on a coexpressed ERK2 reporter was assessed. The action of the constitutively active Raf-1 was to stimulate the ERK2 kinase, whereas the dominant negative version of Raf-1 inhibited the ERK2 activation induced by triggering of the TCR. These data indicate a role for Raf-1 in the regulation of ERK2 in T cells.

Protein kinase A acts at multiple points to inhibit Xenopus oocyte maturation

In Xenopus oocytes, initiation of maturation is dependent on reduction of cyclic AMP-dependent protein kinase (PKA) activity and the synthesis of the mos proto-oncogene product. Mos is required during meiosis I for the activation of both maturation-promoting factor (MPF) and mitogen-activated protein kinase (MAPK). Here we show that injection of the catalytic subunit of PKA (PKAc) prevented progesterone-induced synthesis of endogenous Mos as well as downstream MPF and MAPK activation. However, PKAc did not prevent injected soluble Mos product from activating MAPK. While MAPK is activated during Mos-PKAc coinjection, attendant MPF activation is blocked. Additionally, PKAc caused a potent block in the electrophoretic mobility shift of cdc25 that is associated with phosphatase activation. This inhibition of cdc25 activity was not reversed by progesterone, Mos, or MPF. We conclude that PKAc acts as a negative regulator at several points in meiotic maturation by preventing both Mos translation and MPF activation.

Cell signalling and the control of gene transcription

Recent discoveries have led to a better understanding of how signals generated by growth factors, cytokines and hormones elicit changes in gene expression in mammalian cells. Three general strategies of information pathways from the cell surface to the nucleus can be defined, all of which involve protein phosphorylation: (1) activation and translocation of cytoplasmic kinases to the nucleus, leading to changes in transcription-factor functions; (2) direct activation of latent cytoplasmic transcription factors by phosphorylation; and (3) release of transcription factors from cytoplasmic anchor proteins. These information conduits are illustrated by the mitogen-activated protein (MAP) kinase pathway, signal transducer and activator of transcription (Stat) proteins and nuclear factor-kappa B (NF-kappa B), respectively, and are discussed in this article by Dylan Edwards. Regulation of nucleo-cytoplasmic compartmentation emerges as a key aspect of signal transfer.

Raf meets Ras: completing the framework of a signal transduction pathway

The Ras oncoprotein, a GTP-activated molecular switch, interacts directly with the Raf oncoprotein to recruit the MAP kinases and their subordinates. In this way, a mitogenic signal initiated by tyrosine kinases is converted by Ras into a wave of regulatory phosphorylation on serine and threonine residues that, depending on its intensity and duration, and the variety of substrates available, results in cell differentiation or cell division.