raf family serine/threonine protein kinases in mitogen signal transduction

A-RAF kinase functions in ARF6 regulated endocytic membrane traffic

BACKGROUND

RAF kinases direct ERK MAPK signaling to distinct subcellular compartments in response to growth factor stimulation.

METHODOLOGY/PRINCIPAL FINDINGS

Of the three mammalian isoforms A-RAF is special in that one of its two lipid binding domains mediates a unique pattern of membrane localization. Specific membrane binding is retained by an N-terminal fragment (AR149) that corresponds to a naturally occurring splice variant termed DA-RAF2. AR149 colocalizes with ARF6 on tubular endosomes and has a dominant negative effect on endocytic trafficking. Moreover actin polymerization of yeast and mammalian cells is abolished. AR149/DA-RAF2 does not affect the internalization step of endocytosis, but trafficking to the recycling compartment.

CONCLUSIONS/SIGNIFICANCE

A-RAF induced ERK activation is required for this step by activating ARF6, as A-RAF depletion or inhibition of the A-RAF controlled MEK-ERK cascade blocks recycling. These data led to a new model for A-RAF function in endocytic trafficking.

BAG1 promotes axonal outgrowth and regeneration in vivo via Raf-1 and reduction of ROCK activity

Improved survival of injured neurons and the inhibition of repulsive environmental signalling are prerequisites for functional regeneration. BAG1 (Bcl-2-associated athanogene-1) is an Hsp70/Hsc70-binding protein, which has been shown to suppress apoptosis and enhance neuronal differentiation. We investigated BAG1 as a therapeutic molecule in the lesioned visual system in vivo. Using an adeno-associated viral vector, BAG1 (AAV.BAG1) was expressed in retinal ganglion cells (RGC) and then tested in models of optic nerve axotomy and optic nerve crush. BAG1 significantly increased RGC survival as compared to adeno-associated viral vector enhanced green fluorescent protein (AAV.EGFP) treated controls and this was independently confirmed in transgenic mice over-expressing BAG1 in neurons. The numbers and lengths of regenerating axons after optic nerve crush were also significantly increased in the AAV.BAG1 group. In pRGC cultures, BAG1-over-expression resulted in a approximately 3-fold increase in neurite length and growth cone surface. Interestingly, BAG1 induced an intracellular translocation of Raf-1 and ROCK2 and ROCK activity was decreased in a Raf-1-dependent manner by BAG1-over-expression. In summary, we show that BAG1 acts in a dual role by inhibition of lesion-induced apoptosis and interaction with the inhibitory ROCK signalling cascade. BAG1 is therefore a promising molecule to be further examined as a putative therapeutic tool in neurorestorative strategies.

Key molecules involved in receptor-mediated lymphocyte activation

This unit, along with Unit 11.9B, provides a summary of our current knowledge about various signaling pathways critical to the function of immune cells. Here, our understanding of T cell receptor (TCR)- and B cell receptor (BCR)-mediated signaling is summarized. A schematic representation of immunologically relevant cytokine receptors and the Janus Family Kinases (JAKs) that is activated through these receptors is provided, along with details about molecules involved in interleukin 2 mediated signal transduction.

Role of receptor tyrosine kinases during Drosophila development

In vertebrates, a tyrosine kinase activity has been identified as an integral component of growth factor receptors and the products of proto-oncogenes. Many of these receptor tyrosine kinases (RTKs) appear to play a key role in the regulation of cell growth. Recent analyses of several Drosophila genes encoding putative RTKs indicate that this class of proteins also plays an important role in decisions about cell fate that depend on cellular interactions during development. The sevenless RTK mediates the position-dependent specification of a particular photoreceptor cell type (R7) in the eye. The local specification of R7 cells requires a functional tyrosine kinase domain of the sevenless protein but does not depend on the spatially restricted expression of the sevenless gene. The Drosophila EGF receptor homologue serves multiple functions during development, some of which are clearly unrelated to regulation of cell growth. Finally, the torso gene encodes an RTK required for the specification of the terminal regions of the Drosophila larva. A number of other genes have been genetically identified that appear to function in the same developmental processes upstream or downstream of these three RTKs. These loci are excellent candidates for genes encoding other components of the signalling pathways, such as ligands or substrates of the RTKs.

Raf kinases: Oncogenesis and drug discovery

Raf kinase signaling has been thoroughly investigated over the last 20 years. A-Raf, B-Raf and C-Raf, the 3 mammalian members of the Raf family, are involved in a variety of cellular processes such as growth, proliferation, survival, differentiation and transformation. The detection of B-RAF mutations in a wide variety of human cancers, the description of wildtype and mutant B-RAF as tumor antigens in melanoma and the promising outcome of clinical trials evaluating the Raf inhibitor Nexavar (Sorafenib, BAY 43-9006) have sparked a broad interest in the scientific community. After a short historical detour and an introduction into Raf kinase signaling, we are going to discuss here recent outcomes of Raf kinase research with respect to tumor formation and give an overview on current efforts to develop anticancer therapies interfering with aberrant Raf kinase signaling.

The Drosophila MOS ortholog is not essential for meiosis

In metazoan oocytes, a metaphase arrest coordinates the completion of meiosis with fertilization. Vertebrate mos maintains the metaphase II arrest of mature oocytes and prevents DNA replication between the meiotic divisions. We identified a Drosophila homolog of mos and showed it to be the mos ortholog by two additional criteria. The dmos transcripts are present in Drosophila oocytes but not embryos, and injection of dmos into Xenopus embryos blocks mitosis and elevates active MAPK levels. In Drosophila, MAPK is activated in oocytes, consistent with a role in meiosis. We generated deletions of dmos and found that, as in vertebrates, dmos is responsible for the majority of MAPK activation. Unexpectedly, the oocytes that do mature complete meiosis normally and produce fertilized embryos that develop, although there is a reduction in female fertility and loss of some oocytes by apoptosis. Therefore, Drosophila contains a mos ortholog that activates a MAPK cascade during oogenesis and is nonessential for meiosis. This could be because there are redundant pathways regulating meiosis, because residual, low levels of active MAPK are sufficient, or because active MAPK is dispensable for meiosis in Drosophila. These results highlight the complexity of meiotic regulation that evolved to ensure accurate control over the reproductive process.

Dynamic changes in C-Raf phosphorylation and 14-3-3 protein binding in response to growth factor stimulation: differential roles of 14-3-3 protein binding sites

Phosphorylation events play a crucial role in Raf activation. Phosphorylation of serines 259 and 621 in C-Raf and serines 364 and 728 in B-Raf has been suggested to be critical for association with 14-3-3 proteins. To study the functional consequences of Raf phosphorylations at these positions, we developed and characterized phosphospecific antibodies directed against 14-3-3 binding epitopes: a monoclonal phosphospecific antibody (6B4) directed against pS621 and a polyclonal antibody specific for B-Raf-pS364 epitope. Although 6B4 detected both C- and B-Raf in Western blots, it specifically recognizes the native form of C-Raf but not B-Raf. Contrary to B-Raf, a kinase-dead mutant of C-Raf was found to be only poorly phosphorylated in the Ser-621 position. Moreover, serine 259 to alanine mutation prevented the Ser-621 phosphorylation suggesting an interdependence between these two 14-3-3 binding domains. Direct C-Raf.14-3-3 binding studies with purified proteins combined with competition assays revealed that the 14-3-3 binding domain surrounding pS621 represents the high affinity binding site, whereas the pS259 epitope mediates lower affinity binding. Raf isozymes differ in their 14-3-3 association rates. The time course of endogenous C-Raf activation in mammalian cells by nerve growth factor (NGF) has been examined using both phosphospecific antibodies directed against 14-3-3 binding sites (6B4 and anti-pS259) as well as phosphospecific antibodies directed against the activation domain (anti-pS338 and anti-pY340/pY341). Time course of Ser-621 phosphorylation, in contrast to Ser-259 phosphorylation, exhibited unexpected pattern reaching maximal phosphorylation within 30 s of NGF stimulation. Phosphorylation of tyrosine 340/341 reached maximal levels subsequent to Ser-621 phosphorylation and was coincident with emergence of kinase activity. Taken together, we found substantial differences between C-Raf.14-3-3 binding epitopes pS259 and pS621 and visualized for the first time the sequence of the essential C-Raf phosphorylation events in mammalian cells in response to growth factor stimulation.

Mutations in conserved regions 1, 2, and 3 of Raf-1 that activate transforming activity

To investigate the role of Raf-1 in v-Ha-ras transformation, we have isolated and characterized a number of Raf-1 mutants that display increased transforming activity in Rat2 fibroblasts. A dipeptide deletion (Delta144-145) in the cysteine-rich domain (CRD) of conserved region (CR) 1 increased the interaction between Raf-1 and v-Ha-ras effector loop mutants in the yeast two-hybrid system, supporting the proposal that the CRD serves as a secondary ras-binding domain. Many activating mutations were located in CR2. Two representative CR2 mutants (Delta250-258 and S257L) displayed increased interaction with v-Ha-ras effector loop mutants and with mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase (MEK) 1 in the two-hybrid system. One novel mutation in CR3 was recovered; G361S affected the third glycine of the GXGXXG protein kinase motif involved in ATP binding. Expression of G361S Raf-1 in Rat2 fibroblasts activated MEK and ERK. The CR1, CR2, and CR3 activating mutations, when combined in cis, cooperated in transforming Rat2 fibroblasts. Conversely, Raf-1 transforming activity was decreased when the S257L or G361S mutation was combined in cis with the R89E substitution, which disrupts ras-Raf interaction. This mutant analysis provides additional information about the distinct functions of individual Raf-1 regions and documents a novel genetic mechanism for activating an oncogenic kinase.

Anti-(Raf-1) RNA aptamers that inhibit Ras-induced Raf-1 activation

RNA aptamers with affinity for the Ras-binding domain (RBD) of Raf-1 were isolated from a degenerate pool by in vitro selection. These aptamers efficiently inhibited the Ras interaction with the Raf-1 RBD, and also inhibited Ras-induced Raf-1 activation in a cell-free system. The RNA aptamer with the most potent inhibitory effect specifically inhibited the Ras-Raf-1 interaction and had no affinity for the RBD of the RGL protein, a homolog of the Ral GDP dissociation stimulator. Although the aptamer was capable of binding to the B-Raf RBD, the RNA did not inhibit the interaction between Ras and the B-Raf RBD. Enzymatic and chemical probing experiments indicated that the aptamer was folded into a pseudoknot structure, and some loop regions of the pseudoknot were located at the binding interface for the Raf-1 RBD.

Drosophila-raf acts to elaborate dorsoventral pattern in the ectoderm of developing embryos

In the early Drosophila embryo the activity of the EGF-receptor (Egfr) is required to instruct cells to adopt a ventral neuroectodermal fate. Using a gain-of-function mutation we showed that D-raf acts to transmit this and other late-acting embryonic Egfr signals. A novel role for D-raf was also identified in lateral cell development using partial loss-of-function D-raf mutations. Thus, we provide evidence that zygotic D-raf acts to specify cell fates in two distinct pathways that generate dorsoventral pattern within the ectoderm. These functional requirements for D-raf activity occur subsequent to its maternal role in organizing the anterioposterior axis. The consequences of eliminating key D-raf regulatory domains and specific serine residues in the transmission of Egfr and lateral epidermal signals were also addressed here.

Signal transduction pathway for anterior-posterior development in Drosophila

In Drosophila, the establishment of embryonic polarity along the anterior-posterior axis of the egg is determined by the activity of maternal gene products that accumulate during oogenesis. Amongst these are the Bicoid, the Nanos, and the terminal class gene products, some of which are oncoproteins involved in signal transduction for the formation of terminal structures in the embryo. Several signal transduction pathways have been described in Drosophila, and this review explores the potential of oncogene studies using one of those pathways - the terminal class signal transduction pathway - to better understand the cellular mechanisms of proto-oncogenes that mediate cellular responses in vertebrates including humans.

Activation of mitogen-activated protein kinase is necessary but not sufficient for proliferation of human thyroid epithelial cells induced by mutant Ras

Given the high frequency of ras oncogene activation in several common human cancers, its signal pathways are an important target for novel therapy. For practical reasons, however, these have been studied mainly in the context of transformation of established fibroblast cell lines, whereas ras acts at an earlier stage in human tumorigenesis and predominantly on epithelial cells. Here we have developed a more directly relevant model - human primary thyroid epithelial cells - which are a major target of naturally-occurring Ras mutation, and in which expression of mutant Ras in culture induces clonal expansion without morphological transformation, closely reproducing the phenotype of the corresponding tumour in vivo. Transient or stable expression of mutant H-ras (by scrapeloading or retroviral infection) at levels which stimulated proliferation induced sustained activation and translocation of MAP kinase (MAPK) in these cells. Inhibition of the MAPK pathway at the level of MAPKK, by expression of a dominant-negative mutant or by the pharmacological inhibitor PD98059, efficiently blocked the proliferative response. Conversely, selective activation of MAPK by a constitutively-active MAPKK1 mutant failed to mimic the action of Ras and, although this was achievable with activated Raf, micro-injection of anti-ras antibodies showed that this still required endogenous wild-type Ras function. In contrast to recent results obtained with a rodent thyroid cell line (WRT), therefore, activation of the MAPK pathway is necessary, but not sufficient, for the proliferogenic action of mutant Ras on primary human thyroid cells. These data emphasize the unreliability of extrapolation from cell lines and establish the feasibility of using a more representative human epithelial model for Ras signalling studies.

Cot protooncoprotein activates the dual specificity kinases MEK-1 and SEK-1 and induces differentiation of PC12 cells

Mitogenic signals initiated at the plasma membrane are transmitted to the nucleus through an intricate signalling network. We identified the protooncoprotein Cot as a new component of mitogenic signalling cascades, which activates both the classic cytoplasmic cascade and the SAPK stress pathway. Wildtype and activated Cot phosphorylate and activate MEK-1 and SEK-1 in vitro. These findings are consistent with the sequence homology between Cot and the rat gene Tpl-2. Expression of oncogenic Cot in 293, NIH3T3 and PC12 cells leads to in vivo phosphorylation of endogenous c-Jun and Erk-1/2 suggesting that the serine/threonine kinase Cot functions beside c-Raf-1 and Mos as a direct activator of MEK-1. Furthermore, we have examined the biological effects of Cot on the phenotype of fibroblastic and neuronal cells. In order to test a potential c-Raf-1 dependency of Cot transformation, the effect of oncogenic Cot on Raf revertant CHP25 cells was determined. Cot could restore the transformed phenotype indicating that Cot transformation is not dependent on active c-Raf-1 and that Cot is not a target for the putative Raf inhibitor, which is presumably active in the revertant cell line. Expression of oncogenic versions of Raf as well as v-Mos leads to differentiation of PC12 cells. Cot also induces neurite outgrowth of PC12 cells. These data are consistent with the role of Cot in the classic mitogenic cascade and suggest that the simultaneously activated JNK/SAPK stress pathway has no antagonistic effects in this context.

Dissociation between genes activated in long-term potentiation and in spatial learning in the rat

We have compared changes in mRNA of three genes, zif268, raf B, and syntaxin 1 B, following the unilateral induction of long-term potentiation (LTP) in rats previously trained in a water maze, and in behaviourally naive animals. mRNA of all three genes was enhanced in the potentiated dentate gyrus of naive animals 3 h after the induction of LTP. Training did not affect expression of mRNA for zif268 or for syntaxin 1 B. Expression of raf B was enhanced by training, and in trained animals the LTP-associated increase in expression of raf B was occluded. These results suggest that LTP and spatial training engage a common pathway utilizing an increase in mRNA for raf B, and demonstrate a dissociation between LTP and spatial learning with respect to expression of zif268 and syntaxin 1B.

Studies of perinuclear and nuclear translocation of the Raf-1 protein in rodent fibroblasts

Raf-1, A-Raf and B-Raf comprise a small family of highly conserved serine/threonine protein kinases, whose activities play a fundamental role in the control of proliferation and differentiation. The best studied family member, Raf-1, is expressed ubiquitously and constitutively, and its activity is regulated by post-translational mechanisms. Raf-1 can be activated by many signals that include growth factors, tumor promoters, inflammatory cytokines, calcium mobilization, DNA damaging agents, and oxygen radicals. Ras-mediated translocation of Raf-1 to the plasma membrane is a crucial step in its activation process, and is thought to facilitate phosphorylation by membrane-bound kinases. Raf-1 has also been reported to undergo intracellular redistribution following its activation: to the perinuclear space in murine NIH3T3 cells and rat hepatic Ito cells, and into the nucleus in gerbil hippocampal pyramidal cells and human MO7 leukemia cells. In contrast to the translocation to the plasma membrane, the perinuclear and/or nuclear translocation of Raf-1 has not been investigated in detail. In this paper, we report an examination of the subcellular localization of endogenous Raf-1 in a fibroblastic cell line (Rat-1) commonly used in transformation assays. Using the methods of cellular fractionation as well as in situ immunofluorescence, we show that no detectable movement of Raf-1 to the perinuclear or nuclear space can be observed. Tethering of activated Raf to the plasma membrane does not interfere with its transforming activity.

Hepatitis B virus X protein induces RNA polymerase III-dependent gene transcription and increases cellular TATA-binding protein by activating the Ras signaling pathway

Our previous studies have shown that the hepatitis B virus protein, X, activates all three classes of RNA polymerase III (pol III)-dependent promoters by increasing the cellular level of TATA-binding protein (TBP) (H.-D. Wang et al., Mol. Cell. Biol. 15:6720-6728, 1995), a limiting transcription component (A. Trivedi et al., Mol. Cell. Biol. 16:6909-6916, 1996). We have investigated whether these X-mediated events are dependent on the activation of the Ras/Raf-1 signaling pathway. Transient expression of a dominant-negative mutant Ras gene (Ras-ala15) in a Drosophila S-2 stable cell line expressing X (X-S2), or incubation of the cells with a Ras farnesylation inhibitor, specifically blocked both the X-dependent activation of a cotransfected tRNA gene and the increase in cellular TBP levels. Transient expression of a constitutively activated form of Ras (Ras-val12) in control S2 cells produced both an increase in tRNA gene transcription and an increase in cellular TBP levels. These events are not cell type specific since X-mediated gene induction was also shown to be dependent on Ras activation in a stable rat 1A cell line expressing X. Furthermore, increases in RNA pol III-dependent gene activity and TBP levels could be restored in X-S2 cells expressing Ras-ala15 by coexpressing a constitutively activated form of Raf-1. These events are serum dependent, and when the cells are serum deprived, the X-mediated effects are augmented. Together, these results demonstrate that the X-mediated induction of RNA pol III-dependent genes and increase in TBP are both dependent on the activation of the Ras/Raf-1 signaling cascade. In addition, these studies define two new and important consequences mediated by the activation of the Ras signal transduction pathway: an increase in the central transcription factor, TBP, and the induction of RNA pol III-dependent gene activity.

c-Raf kinase binds to N-terminal domain of c-Myc

We have demonstrated that the 50 N-terminal amino acids of c-Myc bind a kinase activity, which phosphorylates Myc in vitro predominantly on Thr8. We also have shown that c-Raf, a widely known Ser/Thr kinase, involved in the Ras signaling pathway, binds to the same portion of c-Myc in vitro. In addition we were able to precipitate native c-Myc/Raf complex from various cell lysates. Physical interaction of Myc and Raf may potentially be a part of their well-known functional cooperation.

Sequential modification of serines 621 and 624 in the Raf-1 carboxyl terminus produces alterations in its electrophoretic mobility

The Raf-1 serine/threonine protein kinase plays a central role in many of the mitogenic signaling pathways regulating cell growth and differentiation. The regulation of Raf-1 is complex, and involves protein-protein interactions as well as changes in the phosphorylation state of Raf-1 that are accompanied by alterations in its electrophoretic mobility. We have previously shown that a 33-kDa COOH-terminal, kinase-inactive fragment of Raf-1 underwent a mobility shift in response to the stimulation of cells with serum or phorbol esters. Here we demonstrate that treatment of NIH 3T3 cells or Sf9 cells with hydrogen peroxide (H2O2) also induces the mobility shift of the kinase-inactive Raf-1 fragment. A series of deletion mutants of the Raf-1 COOH terminus were analyzed, and the region required for the mobility shift was localized to a 78-amino acid fragment (residues 566-643). Metabolic labeling revealed that the slower migrating forms of the 33-kDa and of the smaller fragment contained phosphorus. Mutation of a previously characterized phosphorylation site, serine 621, to alanine prevented the mobility shift as well as phosphate incorporation or Src and Ras-dependent kinase activation in Sf9 cells when this mutation was engineered into the full-length Raf-1. Mutation of 621 to aspartate yielded a protein that existed in both the shifted and unshifted forms, demonstrating that a negative charge at 621 was necessary, but not sufficient, for the mobility shift to occur; however, its full-length form was still resistant to activation in the Sf9 system. Additional mutation of nearby serine 624 to alanine blocked the shift, implicating this residue as the site of the second of a two-step modification process leading to the slower migrating form. Co-expression of the 33-kDa fragment with an activated form of mitogen-activated protein kinase kinase in NIH 3T3 led to the appearance of the shifted form in a serum-independent manner. These results demonstrate that a mitogen-activated protein kinase kinase-induced event involving modification of serines 621 and 624 leads to the mobility shift of Raf-1.

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

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

Bcl-2 interacting protein, BAG-1, binds to and activates the kinase Raf-1

The Bcl-2 protein blocks programmed cell death (apoptosis) through an unknown mechanism. Previously we identified a Bcl-2 interacting protein BAG-1 that enhances the anti-apoptotic effects of Bcl-2. Like BAG-1, the serine/threonine protein kinase Raf-1 also can functionally cooperate with Bcl-2 in suppressing apoptosis. Here we show that Raf-1 and BAG-1 specifically interact in vitro and in yeast two-hybrid assays. Raf-1 and BAG-1 can also be coimmunoprecipitated from mammalian cells and from insect cells infected with recombinant baculoviruses encoding these proteins. Furthermore, bacterially-produced BAG-1 protein can increase the kinase activity of Raf-1 in vitro. BAG-1 also activates this mammalian kinase in yeast. These observations suggest that the Bcl-2 binding protein BAG-1 joins Ras and 14-3-3 proteins as potential activators of the kinase Raf-1.

The effect of c-raf antisense oligonucleotides on growth factor-induced proliferation of hematopoietic cells

While it is well established that Raf-1 kinase is activated by phosphorylation in growth factor-dependent hematopoietic cell lines stimulated with a variety of hematopoietic growth factors, little is known about the biological effects of Raf-1 activation on normal hematopoietic cells. Therefore, we examined the requirement for Raf-1 in growth factor-regulated proliferation and differentiation of hematopoietic cells using c-faf antisense oligonucleotide. Raf-1 required for the proliferation of growth factor dependent cell lines stimulated by IL-2, IL-3, G-CSF, GM-CSF and EPO that bind to the hematopoietin class of receptors. Raf-1 is also required for the proliferation of cell lines stimulated by growth factors that use the tyrosine kinase containing receptor class, including SLF and CSF-1. In addition, Raf-1 is also required for IL-6, LIF- and OSM-induced proliferation whose receptors share the gp 130 subunit. In contrast to previous results which demonstrated that IL-4 could not activate Raf-1 kinase, c-raf antisense oligonucleotides also inhibited IL-4-induced proliferation of T cell and myeloid cell lines. Using normal hematopoietic cells, c-raf antisense oligonucleotides completely suppressed the colony formation of murine hematopoietic progenitors in response to single growth factors, such as IL-3, CSF-1 or GM-CSF. Further, c-raf antisense oligonucleotides inhibited the growth of murine progenitors stimulated with synergistic combinations of growth factors (required for primitive progenitor growth) including two, three and four factor combinations. In comparison to murine hematopoietic cells, c-raf antisense oligonucleotides also inhibited both IL-3 and GM-CSF-induced colony formation of CD 34+ purified human progenitors. In addition, Raf-1 is required for the synergistic response of CD 34+ human bone marrow progenitors to multiple cytokines; however, this effect was only observed when additional antisense oligonucleotides were added to the cultures at day 7 of a 14 day assay. Finally, Raf-1 is required for the synergistic response of human Mo-7e cells and of normal human fetal liver cells to five factor combinations. Thus, Raf-1 is required to transduce growth factor-induced proliferative signals in factor-dependent progenitor cells lines for all known classes of hematopoietic growth factor receptors, and is required for the growth of normal murine and human bone marrow-derived progenitors.

RAS signalling is abnormal in a c-raf1 MEK1 double mutant

A mutant rat cell clone that suppresses the transformation defects of RAS effector loop substitutions is heterozygous for mutations in c-raf1 and MEK1. The mutant cells can be transformed by many otherwise defective RAS effector mutants, including RAS genes with the effector regions of distantly related GTPases, even though the encoded RAS proteins do not interact with either the mutant or wild-type RAF in Saccharomyces cerevisiae. While the significance of the c-raf1 mutation is unclear, the MEK1 mutation increases MEK1 activity and leads to activation of mitogen-activated protein kinase. The mutant MEK1 is coupled to the epidermal growth factor pathway but exhibits decreased physical interaction with RAF. When overexpressed, the MEK1 mutation is transforming and causes hyperphosphorylation of RAF. Signalling from RAS to MEK1 may be mediated by something other than RAF alone, but signalling through MEK1 is probably sufficient for RAS transformation.

A kinase associated with chromatin that can be activated by ligand-p185c-Neu or epidermal growth factor-receptor interactions

Some growth factors transduce positive growth signals, while others can act as growth inhibitors. Nuclear signaling events of previously quiescent cells stimulated with various growth factors have been studied by isolating the complexed chromatin-associated proteins and chromatin-associated proteins. Signals from the plasma membrane are integrated within the cells and quickly transduced to the nucleus. It is clear that several growth factors, such as epidermal growth factor, transforming growth factor alpha (but not transforming growth factor beta), and platelet-derived growth factor, utilize similar intracellular signaling biochemistries to modulate nucleosomal characteristics. The very rapid and consistent phosphorylation of nuclear p33, p54, and low molecular mass proteins in the range of 15-18 kDa after growth factor stimulation implies that there is a coordination and integration of the cellular signaling processes. Additionally, phosphorylation of p33 and some low molecular mass histones has been found to occur within 5 min of growth factor treatment and to reach a maximum by 30 min. In this study, we report that Neu receptor activating factor also utilizes the same signaling mechanism and causes p33 to become phosphorylated. In addition, both the tumor promoter okadaic acid (which inhibits protein phosphatases 1 and 2A) and phorbol ester (phorbol 12-tetradecanoate 13-acetate) stimulate phosphorylation of p33, p54, and low molecular mass histones. However, transforming growth factor beta, which is a growth inhibitor for fibroblasts, fails to increase p33 phosphorylation. In general, p33 phosphorylation patterns correspond to positive and negative mitogenic signal transduction. p33 isolated from the complexed chromatin-associated protein fraction appears to be a kinase, or tightly associated with a kinase, and shares antigenicity with the cell division cycle-dependent Cdk2 kinase as determined by antibody-dependent analysis. The rapid phosphorylation of nucleosomal proteins may influence sets of early genes needed for the induction and progression of the cell cycle.

Functional mapping of the N-terminal regulatory domain in the human Raf-1 protein kinase

Raf-1 is a serine/threonine kinase poised at a key relay point in mitogenic signal transduction pathways from the cell surface to the nucleus. Activation of the transforming potential of Raf-1 has been associated with N-terminal truncation and/or fusion to other proteins, suggesting that the Raf-1 N-terminal half harbors a negative regulatory domain. Seven internal deletion mutants that together scan the entire N-terminal half of human Raf-1 protein were generated to map functional regions in this regulatory domain. Effects of the deletion mutations on kinase activity of Raf-1 were evaluated using a baculovirus/insect cell overexpression system and an in vitro kinase assay with the known physiological substrate of Raf-1, mitogen-activated protein kinase kinase. Deletion of amino acids 276-323 in the unique sequence between conserved regions 2 and 3 leads to modest elevation of Raf-1 basal kinase activity, whereas deletion of amino acids 133-180 in conserved region 1 results in diminished kinase activity. Surprisingly, none of the Raf-1 N-terminal deletion mutants, including a truncated version that is transforming in rodent fibroblasts, exhibits greatly increased levels of basal kinase activity. In addition, while activation of Raf-1 kinase by Ras requires sequences in conserved region 1, only the C-terminal half containing the kinase domain of Raf-1 is required for activation by Src. These findings demonstrate that N-terminal deletions in Raf-1 do not necessarily result in constitutively elevated basal kinase activity and that the N-terminal regulatory domain is completely dispensable for Raf-1 activation by Src.

14-3-3 is not essential for Raf-1 function: identification of Raf-1 proteins that are biologically activated in a 14-3-3- and Ras-independent manner

Recent reports have demonstrated the in vivo association of Raf-1 with members of the 14-3-3 protein family. To address the significance of the Raf-1-14-3-3 interaction, we investigated the enzymatic activity and biological function of Raf-1 in the presence and absence of associated 14-3-3. The interaction between these two molecules was disrupted in vivo and in vitro with a combination of molecular and biochemical techniques. Biochemical studies demonstrated that the enzymatic activities of Raf-1 were equivalent in the presence and absence of 14-3-3. Furthermore, mixing of purified Raf-1 and 14-3-3 in vitro was not sufficient to activate Raf-1. With a molecular approach, Cys-165 and Cys-168 as well as Ser-259 were identified as residues of Raf-1 required for the interaction with 14-3-3. Cys-165 and Cys-168 are located within the conserved cysteine-rich region of the CR1 domain, and Ser-259 is a conserved site of serine phosphorylation found within the CR2 domain. Mutation of either Cys-165 and Cys-168 or Ser-259 prevented the stable interaction of Raf-1 with 14-3-3 in vivo. Consistent with the model in which a site of serine phosphorylation is involved in the Raf-1-14-3-3 interaction, dephosphorylated Raf-1 was unable to associate with 14-3-3 in vitro. Phosphorylation may represent a general mechanism mediating 14-3-3 binding, because dephosphorylation of the Bcr kinase (known to interact with 14-3-3) also eliminated its association with 14-3-3. Finally, mutant Raf-1 proteins unable to stably interact with 14-3-3 exhibited enhanced enzymatic activity in human 293 cells and Xenopus oocytes and were biologically activated, as demonstrated by their ability to induced meiotic maturation of Xenopus oocytes. However, in contrast to wild-type Raf-1, activation of these mutants was independent of Ras. Our results therefore indicate that interaction with 14-3-3 is not essential for Raf-1 function.

Raf1 interaction with Cdc25 phosphatase ties mitogenic signal transduction to cell cycle activation

The Ras and Raf1 proto-oncogenes transduce extracellular signals that promote cell growth. Cdc25 phosphatases activate the cell division cycle by dephosphorylation of critical threonine and tyrosine residues within the cyclin-dependent kinases. We show here that Cdc25 phosphatase associates with raf1 in somatic mammalian cells and in meiotic frog oocytes. Furthermore, Cdc25 phosphatase can be activated in vitro in a Raf1-dependent manner. We suggest that activation of the cell cycle by the Ras/Raf1 pathways might be mediated in part by Cdc25.

The MEK kinase activity of the catalytic domain of RAF-1 is regulated independently of Ras binding in T cells

Deletion of the amino-terminal domain of Raf-1, which contains the Ras-binding region, results in the constitutive activation of the liberated Raf-1 catalytic domain in fibroblast cell lines. We demonstrate that the MEK kinase activity of the isolated Raf-1 catalytic domain, Raf-BXB, is not constitutively active, but is regulated in Jurkat T cells. Raf-BXB is activated by engaging the antigen receptor-CD3 complex, or treating cells with phorbol myristate acetate or okadaic acid. Increasing intracellular cAMP inhibits Raf-1 activation stimulated by phorbol myristate acetate, but not the activation of Raf-BXB. Serine 621, but not serine 499, is essential for Raf-BXB MEK kinase activity. Because Raf-BXB does not bind Ras, the data establishes a Ras-independent signal in directly regulating the activity of the Raf-1 catalytic domain.

Raf-1 protein expression in human breast cancer cells

BACKGROUND

The Raf-1 kinase, a 72-kDa cytoplasmic serine-threonine kinase, plays a central role as a second messenger in signal transduction. After ligand binding to a variety of transmembrane tyrosine kinase growth factor receptors including epidermal growth factor (EGF) receptor, the 72-kDa kinase is activated through phosphorylation to a 74-kDa phosphoprotein. The Raf-1 kinase is constitutively activated in many transformed cells either directly, by mutations within its amino-terminus regulatory region, or indirectly, due to overstimulation by autocrine growth factors or activated proximal oncogenes. The role of Raf-1 kinase in breast cancer has not been studied.

METHODS

To investigate the role of Raf-1 kinase expression and its activation in breast cancer, we studied three human breast cancer cell lines expressing varying amounts of EGF receptor to determine the level of Raf-1 protein and the proportion expressed in the higher molecular weight form. Effects of serum starvation and stimulation with EGF on the Raf-1 protein were studied in T47D, BT474, and MDA-MB231 cells by precipitation of cell lysates with an anti-Raf-1 antibody followed by immunoblotting. [3H]Thymidine incorporation by these cells after EGF stimulation was also determined as a measure of DNA synthesis.

RESULTS

In all three breast cancer cell lines studied, the Raf-1 protein was identified in a 70- and a 74-kDa form. The level of Raf-1 was similar in all three cell lines and appeared unrelated to EGF receptor expression on the cell surface. The majority of the protein was found in the 74-kDa form even after serum starvation. A minor shift from the lower to higher molecular weight form of Raf-1 was apparent in cells treated with EGF, and increased [3H] thymidine incorporation could be demonstrated in two of the cell lines after EGF stimulation.

CONCLUSION

Baseline expression of the 74-kDa or activated form of the Raf-1 kinase appeared to be elevated in the breast cancer cells studied, indicating constitutive activation. Further investigation into the role of Raf-1 protein in the pathogenesis of breast cancer is indicated.

Hepatitis B virus HBx protein activates Ras-GTP complex formation and establishes a Ras, Raf, MAP kinase signaling cascade

Hepatitis B virus produces a small (154-amino acid) transcriptional transactivating protein, HBx, which is required for viral infection and has been implicated in virus-mediated liver oncogenesis. However, the molecular mechanism for HBx activity and its possible influence on cell proliferation have remained obscure. A number of studies suggest that HBx may stimulate transcription by indirectly activating transcription factors, possibly by influencing cell signaling pathways. We now present biochemical evidence that HBx activates Ras and rapidly induces a cytoplasmic signaling cascade linking Ras, Raf, and mitogen-activated protein kinase (MAP kinase), leading to transcriptional transactivation. HBx strongly elevates levels of GTP-bound Ras, activated and phosphorylated Raf, and tyrosine-phosphorylated and activated MAP kinase. Transactivation of transcription factor AP-1 by HBx is blocked by inhibition of Ras or Raf activities but not by inhibition of Ca(2+)- and diacylglycerol-dependent protein kinase C. HBx was also found to stimulate DNA synthesis in serum-starved cells. The hepatitis B virus HBx protein therefore stimulates Ras-GTP complex formation and promotes downstream signaling through Raf and MAP kinases, and may influence cell proliferation.

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 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.

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.

FMLP activates Ras and Raf in human neutrophils. Potential role in activation of MAP kinase

Chemoattractants bind to seven transmembrane-spanning, G-protein-linked receptors on polymorphonuclear leukocytes (neutrophils) and induce a variety of functional responses, including activation of microtubule-associated protein (MAP) kinase. Although the pathways by which MAP kinases are activated in neutrophils are unknown, we hypothesized that activation of the Ras/Raf pathway leading to activation of MAP/ERK kinase (MEK) would be induced by the chemoattractant f-met-leu-phe. Human neutrophils exposed to 10 nM FMLP for 30 s exhibited an MAP kinase kinase activity coeluting with MEK-1. Immunoprecipitation of Raf-1 kinase after stimulation with FMLP revealed an activity that phosphorylated MEK, was detectable at 30 s, and peaked at 2-3 min. Immunoprecipitation of Ras from both intact neutrophils labeled with [32P]orthophosphate and electropermeabilized neutrophils incubated with [32P]GTP was used to determine that FMLP treatment was associated with activation of Ras. Activation of both Ras and Raf was inhibited by treatment of neutrophils with pertussis toxin, indicating predominant linkage to the Gi2 protein. Although phorbol esters activated Raf, activation induced by FMLP appeared independent of protein kinase C, further suggesting that Gi2 was linked to Ras and Raf independent of phospholipase C and protein kinase C. Dibutyryl cAMP, which inhibits many neutrophil functional responses, blocked the activation of Raf by FMLP, suggesting that interruption of the Raf/MAP kinase pathway influences neutrophil responses to chemoattractants. These data suggest that Gi2-mediated receptor regulation of the Ras/Raf/MAP kinase pathway is a primary response to chemoattractants.

Expression of the raf protooncogene in glial cells of the adult rat cerebral cortex, brain stem and spinal cord

The raf protooncogenes encode cytoplasmic serine/threonine-specific protein kinases which can be activated from different growth factor receptors by phosphorylation. Our previous immunohistochemical studies proved that raf kinases are present in neurons of the mammalian central nervous system. The present study describes the immunohistochemical localization of raf kinases in glia-like cells of the rat cerebral cortex, spinal cord and brain stem. Small glia-like cells measuring 8-12 microns were observed in the neocortex, the entorhinal and prepiriform allocortical areas and the subcortical white matter. In the hippocampus, the immunoreactive cells were most numerous in the fimbria, the alveus and the molecular layer of the dentate fascia. Ultrastructural studies following preembedding immunohistochemistry revealed that in the cerebral cortex only astrocytes contained raf-protein-like immunoreactivity. Our immunofluorescence studies showed, that the white matter of the spinal cord, the pyramids of the medulla and the basis of the pons contained small glia-like cells, too. No electron microscopic investigations were performed, but the location (white matter tracts) and size (6-12 microns) of these cells suggested their glial nature. On the basis of data from other cell systems we expect that raf kinases participate in growth factor- and cytokine-regulated glial functions of the mammalian central nervous system.

The proto-oncogene C-raf-1 is highly expressed only in the hypertrophic zone of the growth plate

Proto-oncogene c-raf-1, the cellular homologue of the acutely transforming oncogene v-raf, has a central role in the signal transduction pathways. The growth plate, due to its non-overlapping zones of chondrocyte maturation, provides a physiological in situ model for investigating the role of c-raf-1 in proliferation and differentiation of chondrocytes. In this study, Northern blotting was first performed to examine the expression of mRNA for c-raf-1 in the embryonic chick tibial growth plate. It revealed that the normal levels of c-raf-1 mRNA were associated with the whole growth plate. We then investigated the localization of c-raf-1 mRNA and c-raf-1 protein in the growth plate by in situ hybridization and immunohistochemistry in order to determine whether c-raf-1 is involved in chondrocyte maturation. Our results showed that c-raf-1 mRNA and c-raf-1 protein were detected only in the hypertrophic zone. The data suggest involvement of this proto-oncogene in chondrocyte differentiation and/or hypertrophy rather than in proliferation.

Oncogenes, Protein Tyrosine Kinases, and Signal Transduction

Many oncogenes encode protein tyrosine kinases (PTKs). Oncogenic mutations of these genes invariably result in constitutive activation of these PTKs. Autophosphorylation of the PTKs and tyrosine phosphorylation of their cellular substrates are essential events for transmission of the mitogenic signal into cells. The recent discovery of the characteristic amino acid sequences, of the src homology domains 2 and 3 (SH2 and SH3), and extensive studies on proteins containing the SH2 and SH3 domains have revealed that protein tyrosine-phosphorylation of PTKs provides phosphotyrosine sites for SH2 binding and allows extracellular signals to be relayed into the nucleus through a chain of protein-protein interactions mediated by the SH2 and SH3 domains. Studies on oncogenes, PTKs and SH2/SH3-containing proteins have made a tremendous contribution to our understanding of the mechanisms for the control of cell growth, oncogenesis, and signal transduction. This review is intended to provide an outline of the most recent progress in the study of signal transduction by PTKs. Copyright 1994 S. Karger AG, Basel

The mitogenic response of T cells to interleukin-2 requires Raf-1

The product of the c-raf-1 proto-oncogene, Raf-1, is known to encode a 74-kDa ubiquitously expressed cytoplasmic serine/threonine kinase. Various growth factors such as epidermal growth factor, acidic fibroblast growth factor, platelet-derived growth factor, insulin, granulocyte-macrophage colony-stimulating factor, interleukin (IL)-2, IL-3 and erythropoietin have been shown to induce phosphorylation of Raf-1, thereby activating Raf-1 kinase. Raf-1 is, thus, believed to play a role in coupling growth factor receptors to proliferation. We have examined the role of Raf-1 in the mitogenic response of human peripheral blood-derived IL-2 receptor expressing T cells to human recombinant IL-2 employing c-raf antisense (AS) oligodeoxyribonucleotide. Uptake studies of oligonucleotides indicated that incorporation of oligomers was maximal at 4 h and oligodeoxynucleotides remained stable in these cells for up to 24 h. Treatment of T cells with the AS oligodeoxyribonucleotide in intracellular duplex formation followed by efficient translation blockade of c-raf-1. In contrast, sense (S) and nonsense (NS) oligodeoxynucleotides failed to form intracellular duplexes and did not interfere with translation of c-raf-1, suggesting specific elimination of c-raf-1 by the AS oligomer. Proliferation of T cells ([3H]thymidine incorporation) following exposure to IL-2 was substantially reduced when the c-raf-1 AS oligodeoxyribonucleotide was added to cultures, while the mitogenic response to this factor remained almost unaffected in the presence of S and NS oligodeoxyribonucleotides.

Critical tyrosine residues regulate the enzymatic and biological activity of Raf-1 kinase

The serine/threonine kinase activity of the Raf-1 proto-oncogene product is stimulated by the activation of many tyrosine kinases, including growth factor receptors and pp60v-src. Recent studies of growth factor signal transduction pathways demonstrate that Raf-1 functions downstream of activated tyrosine kinases and p21ras and upstream of mitogen-activated protein kinase. However, coexpression of both activated tyrosine kinases and p21ras is required for maximal activation of Raf-1 in the baculovirus-Sf9 expression system. In this study, we investigated the role of tyrosine kinases and tyrosine phosphorylation in the regulation of Raf-1 activity. Using the baculovirus-Sf9 expression system, we identified Tyr-340 and Tyr-341 as the major tyrosine phosphorylation sites of Raf-1 when coexpressed with activated tyrosine kinases. Introduction of a negatively charged residue that may mimic the effect of phosphorylation at these sites activated the catalytic activity of Raf-1 and generated proteins that could transform BALB/3T3 cells and induce the meiotic maturation of Xenopus oocytes. In contrast, substitution of noncharged residues that were unable to be phosphorylated produced a protein that could not be enzymatically activated by tyrosine kinases and that could block the meiotic maturation of oocytes induced by components of the receptor tyrosine kinase pathway. These findings demonstrate that maturation of the tyrosine phosphorylation sites can dramatically alter the function of Raf-1. In addition, this is the first report that a transforming Raf-1 protein can be generated by a single amino acid substitution.

Oncogene activation of HIV-LTR-driven expression via the NF-kappa B binding sites

The Raf-1 proto-oncogene product is a highly regulated serine/threonine kinase that functions in signal transduction downstream from growth factor receptors and upstream from nuclear proto-oncogene products. Using a transient cotransfection assay we have found that activated Raf-1 activates expression from the HIV-LTR. Analysis of a series of 5' deletion and point mutations revealed the NF-kappa B motifs as the Raf-responsive element in the HIV-LTR. Moreover, Raf-BXB activated expression from heterologous promoters driven by the HIV NF-kappa B binding sites. In addition to Raf, we show that v-Src, v-H-Ras and v-Mos activate HIV-LTR expression through the NF-kappa B binding sites and v-H-Ras-induced HIV-LTR expression is mediated by Raf-1. These findings may have implications for the involvement of the cellular homologues of these oncogenes in the switch from latent to productive infection by HIV in response to T-cell activation.

Critical tyrosine residues regulate the enzymatic and biological activity of Raf-1 kinase

The serine/threonine kinase activity of the Raf-1 proto-oncogene product is stimulated by the activation of many tyrosine kinases, including growth factor receptors and pp60v-src. Recent studies of growth factor signal transduction pathways demonstrate that Raf-1 functions downstream of activated tyrosine kinases and p21ras and upstream of mitogen-activated protein kinase. However, coexpression of both activated tyrosine kinases and p21ras is required for maximal activation of Raf-1 in the baculovirus-Sf9 expression system. In this study, we investigated the role of tyrosine kinases and tyrosine phosphorylation in the regulation of Raf-1 activity. Using the baculovirus-Sf9 expression system, we identified Tyr-340 and Tyr-341 as the major tyrosine phosphorylation sites of Raf-1 when coexpressed with activated tyrosine kinases. Introduction of a negatively charged residue that may mimic the effect of phosphorylation at these sites activated the catalytic activity of Raf-1 and generated proteins that could transform BALB/3T3 cells and induce the meiotic maturation of Xenopus oocytes. In contrast, substitution of noncharged residues that were unable to be phosphorylated produced a protein that could not be enzymatically activated by tyrosine kinases and that could block the meiotic maturation of oocytes induced by components of the receptor tyrosine kinase pathway. These findings demonstrate that maturation of the tyrosine phosphorylation sites can dramatically alter the function of Raf-1. In addition, this is the first report that a transforming Raf-1 protein can be generated by a single amino acid substitution.

Mitogen-activated protein kinases p42mapk and p44mapk are required for fibroblast proliferation

The mitogen-activated protein kinases (MAP kinases) p42mapk and p44mapk are serine/threonine kinases rapidly activated in cells stimulated with various extracellular signals by dual phosphorylation of tyrosine and threonine residues. They are thought to play a pivotal role in integrating and transmitting transmembrane signals required for growth and differentiation. Here we demonstrate that activation of these ubiquitously expressed MAP kinases is essential for growth. To specifically suppress MAP kinase activation in fibroblasts, we transiently expressed either the entire p44mapk antisense RNA or p44mapk kinase-deficient mutants (T192A or Y194F). As expected, and through independent mechanisms, both approaches strongly inhibited MAP kinase activation. The antisense reduced the expression of endogenous p42mapk and p44mapk by 90%, whereas overexpression of the T192A mutant inhibited growth factor activation of both endogenous MAP kinases by up to 70%. As a consequence, we found that the antisense as well as the T192A mutant of p44mapk inhibited growth factor-stimulated gene transcription (collagenase promoter assay with chloramphenicol acetyltransferase reporter) and cell growth. These effects were proportional to the extent of MAP kinase inhibition and reversed by coexpression of the wild-type p44mapk. Therefore we conclude that growth factor activation of p42mapk and p44mapk is an absolute requirement for triggering the proliferative response.

Growth factors induce nuclear translocation of MAP kinases (p42mapk and p44mapk) but not of their activator MAP kinase kinase (p45mapkk) in fibroblasts

Mitogen-activated protein kinases (p42mapk and p44mapk) are serine/threonine kinases that are activated rapidly in cells stimulated with various extracellular signals. This activation is mediated via MAP kinase kinase (p45mapkk), a dual specificity kinase which phosphorylates two key regulatory threonine and tyrosine residues of MAP kinases. We reported previously that the persistent phase of MAP kinase activation is essential for mitogenically stimulated cells to pass the "restriction point" of the cell cycle. Here, using specific polyclonal antibodies and transfection of epitope-tagged recombinant MAP kinases we demonstrate that these signaling protein kinases undergo distinct spatio-temporal localization in growth factor-stimulated cells. In G0-arrested hamster fibroblasts the activator p45mapkk and MAP kinases (p42mapk, p44mapk) are mainly cytoplasmic. Subsequent to mitogenic stimulation by serum or alpha-thrombin both MAP kinase isoforms translocate into the nucleus. This translocation is rapid (seen in 15 min), persistent (at least during the entire G1 period up to 6 h), reversible (by removal of the mitogenic stimulus) and apparently 'coupled' to the mitogenic potential; it does not occur in response to nonmitogenic agents such as alpha-thrombin-receptor synthetic peptides and phorbol esters that fail to activate MAP kinases persistently. When p42mapk and p44mapk are expressed stably at high levels, they are found in the nucleus of resting cells; this nuclear localization is also apparent with kinase-deficient mutants (p44mapk T192A or Y194F). In marked contrast the p45mapkk activator remains cytoplasmic even during prolonged growth factor stimulation and even after high expression levels achieved by transfection. We propose that the rapid and persistent nuclear transfer of p42mapk and p44mapk during the entire G0-G1 period is crucial for the function of these kinases in mediating the growth response.

Direct interaction of Ras and the amino-terminal region of Raf-1 in vitro

The Ras proteins are key regulators of the growth of eukaryotic cells, but their direct target enzymes, or 'effectors', are unknown. The protein encoded by the c-raf-1 proto-oncogene is thought to function downstream of p21ras because disruption of Raf blocks signalling by Ras in a number of systems. Here we report that the amino-terminal cysteine-rich regulatory region of p74c-raf-1 expressed as a glutathione-S-transferase (GST) fusion protein binds directly to Ras with relatively high affinity (50 nM). The binding is strictly dependent on the Ras protein being in the active GTP-bound conformation rather than the inactive GDP-bound state. Raf-GST interacts with wild-type and oncogenic Ras (Val 12) but fails to interact with a biologically inert effector mutant of Ras (Ala 38) and a dominant negative mutant (Asn 17). A peptide based on the effector region of Ras inhibits the interaction. Raf-GST acts as a potent competitive inhibitor of the GTPase-activating proteins p120GAP and neurofibromin. In addition, Raf itself displays weak GTPase-stimulating activity towards Ras. It is therefore likely that Raf is a direct effector of Ras.

Normal and oncogenic p21ras proteins bind to the amino-terminal regulatory domain of c-Raf-1

In higher eukaryotes, the Ras and Raf-1 proto-oncoproteins transduce growth and differentiation signals initiated by tyrosine kinases. The Ras polypeptide and the amino-terminal regulatory domain of Raf-1 (residues 1-257) are shown to interact, directly in vitro and in a yeast expression system. Raf-1 (1-257) binds GTP-Ras in preference to GDP-Ras, and inhibits Ras-GAP activity. Mutations in and around the Ras effector domain impair Ras binding to Raf-1 (1-257) and Ras transforming activity in parallel.

Raf-1 and p21v-ras cooperate in the activation of mitogen-activated protein kinase

Mitogen-activated protein (MAP) kinases Raf-1, pp60src, and p21ras all play important roles in the transfer of signals from the cell surface to the nucleus. We have used the baculovirus/Sf9 insect cell system to elucidate the regulatory relationships between pp60v-src, p21v-ras, MAP kinase (p44erk1/mapk), and Raf-1. In Sf9 cells, p44erk1/mapk is activated by coexpression with either v-Raf or a constitutively activated form of Raf-1 (Raf22W). In contrast, p44erk1/mapk is activated to only a limited extent by coexpression with either Raf-1 or p21v-ras alone. This activation of p44erk1/mapk is greatly enhanced by coexpression with both p21v-ras and Raf-1. Since we have previously shown that p21v-ras stimulates Raf-1 activity, the activation of p44erk1/mapk by p21v-ras may occur exclusively via a Raf-1-dependent pathway. However, a dominant-inhibitory mutant of Raf-1 (Raf301) does not block the activation of p44erk1/mapk by p21-v-ras. Further, pp60v-src, which activates Raf-1 at least as effectively as p21v-ras, fails to enhance p44erk1/mapk activity greatly when coexpressed with Raf-1. These data suggest that activation of p44erk1/mapk by p21v-ras may occur via both Raf-1-dependent and Raf-1-independent pathways.

C. elegans lin-45 raf gene participates in let-60 ras-stimulated vulval differentiation

Vulval differentiation in Caenorhabditis elegans is controlled by intercellular signalling mediated by a receptor tyrosine kinase and a ras gene product. The lin-45 gene encodes a homologue of the raf family of serine/threonine kinases and is necessary for vulval differentiation. The lin-45 raf gene product appears to act downstream of the ras protein in this pathway. A proto-oncogene-mediated signalling pathway may be a common feature of metazoan development.