Mitogen-activated protein kinase activation resulting from selective oncogene expression in NIH 3T3 and rat 1a cells

40 Years of RAS-A Historic Overview

It has been over forty years since the isolation of the first human oncogene (HRAS), a crucial milestone in cancer research made possible through the combined efforts of a few selected research groups at the beginning of the 1980s. Those initial discoveries led to a quantitative leap in our understanding of cancer biology and set up the onset of the field of molecular oncology. The following four decades of RAS research have produced a huge pool of new knowledge about the RAS family of small GTPases, including how they regulate signaling pathways controlling many cellular physiological processes, or how oncogenic mutations trigger pathological conditions, including developmental syndromes or many cancer types. However, despite the extensive body of available basic knowledge, specific effective treatments for RAS-driven cancers are still lacking. Hopefully, recent advances involving the discovery of novel pockets on the RAS surface as well as highly specific small-molecule inhibitors able to block its interaction with effectors and/or activators may lead to the development of new, effective treatments for cancer. This review intends to provide a quick, summarized historical overview of the main milestones in RAS research spanning from the initial discovery of the viral RAS oncogenes in rodent tumors to the latest attempts at targeting RAS oncogenes in various human cancers.

Liposomal delivery of nucleic acid-based anticancer therapeutics: BP-100-1.01

INTRODUCTION

Antisense oligonucleotides, siRNA, anti-microRNA are designed to selectively bind to target mRNAs, and silence disease-causing or -associated proteins. The clinical development of nucleic acid drugs has been limited by their poor bioavailability.

AREAS COVERED

This review article examines the strategies that have been utilized to improve the bioavailability of nucleic acids. The chemical modifications made to nucleic acids that have improved their resistance against nuclease degradation are briefly discussed. The design of cationic and neutral lipid nanoparticles that enable the systemic delivery of nucleic acids in vivo is reviewed, and the proof-of-concept evidence that intravenous administration of nucleic acids incorporated into lipid nanoparticles leads to decreased expression of target genes in humans. Preclinical results of the neutral BP-100-1.01 nanoparticle are highlighted.

EXPERT OPINION

To further improve the clinical potential of nucleic acid cancer drugs, we predict research on the next generation of lipid nanoparticles will focus on: i) enhancing nucleic acid delivery to poorly vascularized tumors, as well as tumors behind the blood-brain barrier; and ii) improving the accessibility of nucleic acids to the cytoplasm by enhancing endosomal escape of nucleic acids and/or reducing exocytosis of nucleic acids to the external milieu.

Ras history: The saga continues

Although the roots of Ras sprouted from the rich history of retrovirus research, it was the discovery of mutationally activated RAS genes in human cancer in 1982 that stimulated an intensive research effort to understand Ras protein structure, biochemistry and biology. While the ultimate goal has been developing anti-Ras drugs for cancer treatment, discoveries from Ras have laid the foundation for three broad areas of science. First, they focused studies on the origins of cancer to the molecular level, with the subsequent discovery of genes mutated in cancer that now number in the thousands. Second, elucidation of the biochemical mechanisms by which Ras facilitates signal transduction established many of our fundamental concepts of how a normal cell orchestrates responses to extracellular cues. Third, Ras proteins are also founding members of a large superfamily of small GTPases that regulate all key cellular processes and established the versatile role of small GTP-binding proteins in biology. We highlight some of the key findings of the last 28 years.

Induction of differentiation of human leukemia cells by combinations of COX inhibitors and 1,25-dihydroxyvitamin D3 involves Raf1 but not Erk 1/2 signaling

Differentiation therapy of cancer is being explored as a potential modality for treatment of myeloid leukemia, and derivatives of vitamin D are gaining prominence as agents for this form of therapy. Cyclooxygenase (COX) inhibitors have been reported to enhance 1,25-dihydroxyvitamin D(3) (1,25D)-induced monocytic differentiation of promyeloblastic HL60 cells, but the mechanisms of this effect are not fully elucidated, and whether this potentiation can occur in other types of myeloid leukemia is not known. We found that combination treatment with 1,25D and non-specific COX inhibitors acetyl salicylic acid (ASA) or indomethacin can robustly potentiate differentiation of other types of human leukemia cells, i.e., U937, THP-1, and that ASA +/- 1,25D is effective in primary AML cultures. Increased cell differentiation is paralleled by arrest of the cells in the G(1) phase of the cell cycle, and by increased phosphorylation of Raf1 and p90RSK1 proteins. However, there is no evidence that this increase in phosphorylation of Raf1 is transmitted through the ERK module of the MAPK signaling cascade. Transfection of small interfering (si) RNA to Raf1 decreased differentiation of U937 cells induced by a combination of ASA or indomethacin with 1,25D. However, phosphorylation levels of ERK1/2, though not of p90RSK, were increased when P-Raf1 levels were decreased by the siRNA, suggesting that in this system the ERK module does not function in the conventional manner. Identification of the strong antiproliferative activity of ASA/1,25D combinations associated with monocytic differentiation has implications for cancer chemoprevention in individuals who have a predisposition to myeloid leukemia.

Independent regulation of Rap1 and mitogen-activated protein kinase by the alpha chain of Go

Receptors coupled to G(i/o) proteins stimulate the mitogen-activated protein kinase (MAPK) cascade. The intracellular pathways linking the alpha chains of these G proteins to MAPK activation are not completely understood. One of the signaling molecules which has been suggested to act downstream of Galpha(i/o) is the small G protein Rap1. We investigated the role of Rap1 in MAPK stimulation by Galpha(o) in Chinese hamster ovary (CHO) cells. Our previous results have shown that in this cell system activated Galpha(o) strongly potentiates the MAPK response to the epidermal growth factor (EGF) receptor. Rap1 regulation was examined in cells transfected with Rap1 and wild-type Galpha(o) or the activated mutant Galpha(o)-Q205L. Immunocytochemical analysis detected both Rap1 and the Galpha(o) subunit at the plasma membrane as well as on perinuclear cytoplasmic vesicles. Expression of wild-type Galpha(o) had no significant effect on the levels of activated Rap1. In contrast, Galpha(o)-Q205L virtually abolished the activation of Rap1 induced by EGF. Further experiments showed that MAPK stimulation by EGF was greatly inhibited by expression of activated Rap1, suggesting that Rap1 inhibition could mediate the effect of Galpha(o) on the MAPK cascade. However, Galpha(o)-Q205L efficiently inhibited the activation of Rap1 induced by fibroblast growth factor (FGF). We have previously found that the ability of FGF to activate MAPK is not modified by Galpha(o). In addition, expression of the GAP protein RAP1GAPII blocked Rap1 activation without affecting EGF- or FGF-dependent MAPK stimulation. These findings provide evidence for independent regulation of Rap1 and MAPK by the G(o )alpha chain.

Activation of the Y1 receptor by neuropeptide Y regulates the growth of prostate cancer cells

This study deals with the role of neuropeptide Y (NPY) in the regulation of cell proliferation. NPY is expressed in the normal and tumoral prostate, but no data on its possible role in prostate cancer (PCa) progression are available. Therefore, we evaluated the direct effect of NPY on the growth of the human PCa cell lines LNCaP (androgen dependent) and DU145 and PC3 (androgen independent). All PCa cell lines expressed Y1-R gene and protein. NPY treatment reduced the proliferation of LNCaP and DU145 cells and increased that of PC3 cells. The Y1-R antagonist BIBP3226 abolished such effects, suggesting a mandatory role of Y1-R in this process. LNCaP cells showed elevated constitutive levels of phosphorylated ERK1/2, which were not affected by NPY. In DU145 cells, NPY stimulated a long-lasting ERK1/2 activation, whereas, in PC3 cells, this effect was rapid and transient and required activation of protein kinase C. Moreover, in both cell lines, pretreatment with BIBP3226 prevented the NPY-induced ERK1/2 phosphorylation, further supporting Y1-R involvement. NPY treatment reduced forskolin-stimulated cAMP accumulation only in PC3 cells and did not change intracellular calcium concentration in any PCa cell line. These data indicate that NPY may directly regulate PCa cell growth via Y1-R. The direction of this effect appears to be related to the time kinetics of MAPK activation, i.e. long-lasting vs. transient, and to the clone-specific involvement of other intracellular signals. These findings suggest that NPY-related mechanisms might play a relevant role in the progression of PCa, at both androgen dependent and independent stages.

Krüppel-like factor 5 mediates the transforming activity of oncogenic H-Ras

Previous studies indicate that Krüppel-like factor 5 (KLF5), also known as intestinal-enriched Krüppel-like factor (IKLF), is a positive regulator of cell proliferation and gives rise to a transformed phenotype when overexpressed. Here we demonstrate that levels of KLF5 transcript and protein are significantly elevated in oncogenic H-Ras-transformed NIH3T3 cells. These cells display an accelerated rate of proliferation in both serum-containing and serum-deprived media and form anchorage-independent colonies in soft agar assays. H-Ras-transformed cells also contain elevated mitogen-activated protein kinase (MAPK) activity. When treated with inhibitors of MEK (MAPK kinase), H-Ras-transformed cells lose their growth advantage and no longer form colonies. Significantly, levels of KLF5 transcript and protein are substantially reduced in H-Ras-transformed cells treated with MEK inhibitors. Moreover, inhibition of KLF5 expression in H-Ras-transformed cells with KLF5-specific small interfering RNA (siRNA) leads to a decreased rate of proliferation and a significant reduction in colony formation. H-Ras-transformed cells also contain elevated levels of Egr1 that are diminished by MEK inhibitors. Inhibition of Egr1 by siRNA results in a reduced level of KLF5, indicating that Egr1 mediates the inductive action of MAPK on KLF5. Lastly, KLF5 activates expression of cyclin D1. These findings indicate that the increased expression of KLF5 in H-Ras-transformed cells is secondary to increased MAPK activity from H-Ras overexpression and that the elevated level of KLF5 is in part responsible for the proproliferative and transforming activities of oncogenic H-Ras.

Annexin 1 regulates cell proliferation by disruption of cell morphology and inhibition of cyclin D1 expression through sustained activation of the ERK1/2 MAPK signal

Cellular proliferation is controlled by the integration and coordination of extracellular signals. This study explores the role of the protein annexin 1 (ANXA1) in the regulation of such events. We show that ANXA1 has a cell-type independent, anti-proliferative function through sustained activation of the ERK signaling cascade. Moreover, ANXA1 reduces proliferation by ERK-mediated disruption of the actin cytoskeleton and ablation of cyclin D1 protein expression and not by ERK-mediated induction of the cyclin-dependent kinase, CDK2, inhibitor p21(cip/waf). Finally, ANXA1 regulates the ERK pathway at a proximal location, by SH2 domain-independent association with the adapter protein Grb-2. In summary, overexpression of ANXA1 mediates the disruption of normal cell morphology and inhibits cyclin D1 expression, therefore reducing cell proliferation through proximal modulation of the ERK signal transduction pathway.

The effects of laminin-1 on spiral ganglion neurons are dependent on the MEK/ERK signaling pathway and are partially independent of Ras

Laminin-1 (LN) is expressed along the route of neural growth from spiral ganglion (SG) neurons towards the developing organ of Corti, and has been shown to enhance neurite outgrowth from SG neurons in vitro. Signal transduction pathways linking LN signaling at the cell membrane to the cell nucleus can involve a variety of signaling molecules. Data from other systems suggest the potential involvement of the small G protein Ras, and the mitogen-activated protein kinases (MAPKs) Erk and/or p38. To assess these possibilities, the length and number of processes extending from SG explants cultured on LN-coated surfaces were evaluated after treatment with the Ras inhibitor FTI-277, the p38 inhibitor SB203580 and MAPK kinase (MEK) inhibitor U0126, which operates immediately upstream of the Erk MAPK. Treatment with the Ras inhibitor at levels known to inhibit the H- and N-Ras isoforms had no effect, while FTI-277 levels known to inhibit K-Ras reduced only neurite length. Suppression of MEK resulted in a decrease of both parameters, while incubation with the p38 inhibitor had no effect. The results of this study suggest that MEK plays a central role in LN signaling in SG neurites. While K-Ras signaling may participate in MEK-dependent increases in neurite length, the MEK-dependent increase in neurite number appears to be activated by a different intracellular pathway.

Transforming growth factor-beta-mediated mast cell migration depends on mitogen-activated protein kinase activity

Transforming growth factor-beta (TGF-beta) isoforms regulate numerous cellular functions through binding to receptors with intrinsic serine/threonine kinase activity that transduce the intracellular signals via activation of Smad proteins. In this study, we examined the signalling pathways involved in TGF-beta1-mediated growth inhibition and migration in a human mast cell line, HMC-1. TGF-beta1 evoked optimal migration at 40 fM, whereas maximal growth inhibition was obtained at 400 pM. Protein tyrosine kinase inhibitors completely inhibited TGF-beta1-mediated migration, without affecting the antimitogenic response. Smad2 was phosphorylated upon TGF-beta1 treatment, both in the absence and presence of genistein. The mitogen-induced extracellular kinase (MEK) inhibitor, PD98059, blocked the migratory response without affecting growth inhibition. In contrast, the p38 MAP kinase inhibitor, SB203580, had no significant effect on either migration or growth inhibition. These results indicate that different signalling pathways mediate TGF-beta1-induced migration and growth inhibition in HMC-1 cells, where the migration involves MEK activity.

Mammalian mitogen-activated protein kinase signal transduction pathways activated by stress and inflammation

The molecular details of mammalian stress-activated signal transduction pathways have only begun to be dissected. This, despite the fact that the impact of these pathways on the pathology of chronic inflammation, heart disease, stroke, the debilitating effects of diabetes mellitus, and the side effects of cancer therapy, not to mention embryonic development, innate and acquired immunity, is profound. Cardiovascular disease and diabetes alone represent the most significant health care problems in the developed world. Thus it is not surprising that understanding these pathways has attracted wide interest, and in the past 10 years, dramatic progress has been made. Accordingly, it is now becoming possible to envisage the transition of these findings to the development of novel treatment strategies. This review focuses on the biochemical components and regulation of mammalian stress-regulated mitogen-activated protein kinase (MAPK) pathways. The nuclear factor-kappa B pathway, a second stress signaling paradigm, has been the subject of several excellent recent reviews (258, 260).

The Raf-1 kinase associates with vimentin kinases and regulates the structure of vimentin filaments

Using immobilized GST-Raf-1 as bait, we have isolated the intermediate filament protein vimentin as a Raf-1-associated protein. Vimentin coimmunoprecipitated and colocalized with Raf-1 in fibroblasts. Vimentin was not a Raf-1 substrate, but was phosphorylated by Raf-1-associated vimentin kinases. We provide evidence for at least two Raf-1-associated vimentin kinases and identified one as casein kinase 2. They are regulated by Raf-1, since the activation status of Raf-1 correlated with the phosphorylation of vimentin. Vimentin phosphorylation by Raf-1 preparations interfered with its polymerization in vitro. A subset of tryptic vimentin phosphopeptides induced by Raf-1 in vitro matched the vimentin phosphopeptides isolated from v-raf-transfected cells labeled with orthophosphoric acid, indicating that Raf-1 also induces vimentin phosphorylation in intact cells. In NIH 3T3 fibroblasts, the selective activation of an estrogen-regulated Raf-1 mutant induced a rearrangement and depolymerization of the reticular vimentin scaffold similar to the changes elicited by serum treatment. The rearrangement of the vimentin network occurred independently of the MEK/ERK pathway. These data identify a new branch point in Raf-1 signaling, which links Raf-1 to changes in the cytoskeletal architecture.

Activation of B-Raf and regulation of the mitogen-activated protein kinase pathway by the G(o) alpha chain

Many receptors coupled to the pertussis toxin-sensitive G(i/o) proteins stimulate the mitogen-activated protein kinase (MAPK) pathway. The role of the alpha chains of these G proteins in MAPK activation is poorly understood. We investigated the ability of Galpha(o) to regulate MAPK activity by transient expression of the activated mutant Galpha(o)-Q205L in Chinese hamster ovary cells. Galpha(o)-Q205L was not sufficient to activate MAPK but greatly enhanced the response to the epidermal growth factor (EGF) receptor. This effect was not associated with changes in the state of tyrosine phosphorylation of the EGF receptor. Galpha(o)-Q205L also potentiated MAPK stimulation by activated Ras. In Chinese hamster ovary cells, EGF receptors activate B-Raf but not Raf-1 or A-Raf. We found that expression of activated Galpha(o) stimulated B-Raf activity independently of the activation of the EGF receptor or Ras. Inactivation of protein kinase C and inhibition of phosphatidylinositol-3 kinase abolished both B-Raf activation and EGF receptor-dependent MAPK stimulation by Galpha(o). Moreover, Galpha(o)-Q205L failed to affect MAPK activation by fibroblast growth factor receptors, which stimulate Raf-1 and A-Raf but not B-Raf activity. These results suggest that Galpha(o) can regulate the MAPK pathway by activating B-Raf through a mechanism that requires a concomitant signal from tyrosine kinase receptors or Ras to efficiently stimulate MAPK activity. Further experiments showed that receptor-mediated activation of Galpha(o) caused a B-Raf response similar to that observed after expression of the mutant subunit. The finding that Galpha(o) induces Ras-independent and protein kinase C- and phosphatidylinositol-3 kinase-dependent activation of B-Raf and conditionally stimulates MAPK activity provides direct evidence for intracellular signals connecting this G protein subunit to the MAPK pathway.

Negative regulation of Par-4 by oncogenic Ras is essential for cellular transformation

Oncogenic variants of the cellular Ras protein are often associated with different types of human cancers. However, the mechanisms by which oncogenic Ras induces transformation are not fully established. Expression of the transcriptional repressor Par-4 was down-regulated by oncogenic Ras via the Raf-MEK-ERK pathway. Restoration of Par-4 levels by abrogation of the Raf-MEK-ERK pathway with the MEK-inhibitor PD98059 or by ectopic Par-4, that acted to inhibit ERK expression and activation, was sufficient to suppress oncogenic Ras-induced transformation. These findings identify Par-4 as a novel target that has to be down-modulated by oncogenic Ras for successful transformation.

The topography and subcellular distribution of mitogen-activated protein kinase kinase1 (MEK1) in adult rat brain and differentiating PC12 cells

Mitogen-activated protein kinase signal transduction pathway involved in the regulation of proliferation and differentiation of various mammalian cells consists of a sequential activation of three protein kinases, Raf, mitogen-activated protein kinase kinase, and mitogen-activated protein kinase. These kinases are highly expressed in brain and play an important role in neuronal signalling. In this study, to further characterize mitogen-activated protein kinase signalling pathway in brain, we have elucidated the topography and subcellular distribution of mitogen-activated protein kinase kinasel in adult rat brain and differentiating PC12 cells. Our immunohistochemical data indicate that mitogen-activated protein kinase kinase1 is widely distributed throughout the brain and expressed prominently in cortex, hippocampus, brainstem, hypothalamus and cerebellum. In these areas of brain mitogen-activated protein kinase kinasel is exclusively neuronal in origin and is localized within perikarya and dendrites. Confocal microscopy data has determined that a portion of mitogen-activated protein kinase kinase1 in rat brain is co-localized with microtubules. This co-localization was observed only within neuritic shaft and cilia of ventricular ependymal cells. In nerve growth factor-induced differentiating PC12 cells, mitogen-activated protein kinase kinase1 displays co-localization with microtubules within proximal regions of neuritic shafts and their junctions with the cell somas. From bovine brain extract, mitogen-activated protein kinase kinasel co-purifies with microtubules. In vitro kinase assay detected mitogen-activated protein kinase kinase1 activity within purified microtubules. These observations indicate that mitogen-activated protein kinase kinase1 is associated with microtubules within some specialized compartments of the brain and microtubule-associated mitogen-activated protein kinase kinase1 is catalytically active.

Platelet-derived growth factor stimulation of mitogen-activated protein kinases and cyclin D1 promoter activity in cultured airway smooth-muscle cells. Role of Ras

We hypothesized that in bovine tracheal myocytes, growth factor treatment induces transcription from the cyclin D1 promoter that is dependent on the activation of both Ras and extracellular signal-related kinase (ERK). We found that platelet-derived growth factor (PDGF) treatment induced substantial activation of ERK2 that was blocked by expression of a dominant-negative Ha-Ras. Further, expression of a constitutively active Ha-Ras induced substantial ERK2 activity, consistent with the notion that Ras is required and sufficient for ERK activation. PDGF treatment induced only modest activation of the Jun amino terminal kinase-1 (JNK1) and p38 mitogen-activated protein kinases (MAPKs). Active Ras induced similar responses, implying that complete activation of the JNK and p38 pathways requires additional or alternative upstream signaling intermediates besides Ras. In contrast, expression of a constitutively active Rac1, an alternative guanosine triphosphatase involved in intracellular signaling, produced a high level of JNK1 activation, suggesting that Rac1 is an important upstream activator of JNK in this system. Active Ras and MAPK/ ERK kinase-1 (MEK1) (the upstream activator of ERK) each induced cyclin D1 promoter activity, whereas active stress-activated protein kinase/ERK kinase-1 (SEK1), an upstream activator of JNK, did not. Finally, the synthetic MEK inhibitor PD98059 blocked Ras-induced cyclin D1 promoter activity. Together, these data suggest that in bovine tracheal myocytes: (1) activation of MAPK by PDGF is dependent on Ras; (2) active Ras is sufficient for ERK activation but is insufficient for maximal activation of JNK or p38; (3) activation of Rac1 is sufficient for maximal JNK activation; and (4) Ras, MEK, and ERK constitute a distinct pathway to cyclin D1 transcriptional activation.

Partial characterization of a novel mitogen-activated protein kinase/extracellular signal-regulated kinase activator in airway smooth-muscle cells

We demonstrated previously that in bovine tracheal myocytes, pretreatment with either forskolin or histamine significantly reduces both platelet-derived growth factor (PDGF)- and epidermal growth factor- induced Raf-1 activation but fails to inhibit extracellular signal-regulated kinase (ERK) activation substantially, evidence of a Raf-1-independent ERK activation pathway. To identify Raf-1-independent upstream signaling intermediates of mitogen-activated protein kinase/ERK kinase-1 (MEK1), the dual-function kinase required and sufficient for ERK activation in these cells, lysates from forskolin and PDGF-treated bovine tracheal myocytes were resolved using ion exchange chromatography. Kinase activity for MEK1 was assessed by in vitro phosphorylation assay. In all experiments, the major peak of MEK1 phosphorylation activity was detected in fractions 18 through 26 (80 to 160 mM NaCl), with the peak fraction eluting at a NaCl concentration of 140 mM. The ability of these fractions to activate MEK1 was confirmed by examining the phosphorylation of myelin basic protein, a known substrate for ERKs, in the presence of functional MEK1 and ERK1. Fractions containing kinase activity were also probed with antibodies against MEK kinase-1, Raf-1, A-Raf, B-Raf, Mos, and Tpl-2. None of these proteins was detected in fractions containing peak kinase activity, suggesting the presence of a novel PDGF-stimulated, forskolin-insensitive MEK1 kinase. Further separation of fractions holding peak MEK phosphorylation activity by gel filtration suggested an apparent molecular mass of 40 to 45 kD. We conclude that PDGF-induced activation of MEK1 in bovine tracheal myocytes is mediated at least in part by a novel kinase.

Analysis of signaling protein kinases in human colon or colorectal carcinomas

Extracellular signal-related kinase (ERK) and c-Jun N-terminal kinase (JNK) mitogen-activated protein (MAP) kinases are highly activated in an in vivo rat model of colorectal carcinogenesis. In addition, other protein kinases such as c-Src and c-Yes have been shown to be up-regulated in some human colon cancers. To evaluate the activity of these kinases in human colorectal carcinomas, we examined colon cancers and adjacent normal intestinal mucosa from 11 patients. Moderate increases in ERK and JNK activities, in addition to up-regulation of c-Src, p125FAK, and tyrosine-phosphorylated proteins, were observed in a subset of the colorectal carcinomas. There was a significant correlation found between levels of c-Src, p125FAK, and tyrosine-phosphorylated proteins, as well as between c-Src protein levels and JNK activity. This is the first report that examines several different kinases as markers to characterize colorectal cancers in the same carcinoma sample, allowing the determination of correlations between markers in the same tumors.

Raf and fibroblast growth factor phosphorylate Elk1 and activate the serum response element of the immediate early gene pip92 by mitogen-activated protein kinase-independent as well as -dependent signaling pathways

Previous studies have shown that a mitogen activated protein (MAP) kinase (MEK)-independent signaling pathway is required by activated Raf or fibroblast-derived growth factor (FGF) for the differentiation of rat hippocampal neuronal H19-7 cells. We now demonstrate that both Raf and FGF similarly induce prolonged transcription and translation of the immediate early gene pip92 in the absence of activation of the MAP kinases (MAPKs) ERK1 and ERK2. To determine the mechanism by which this occurs and to identify novel Raf-activated signaling pathways, we investigated the induction of the pip92 promoter by both FGF and an estradiol-activated Raf-1-estrogen receptor fusion protein (deltaRaf-1:ER) in H19-7 cells. Deletion analysis of the pip92 promoter indicated that activation by the MAPK-independent pathway occurs primarily within the region containing a serum response element (SRE). Further analysis of the SRE by using a heterologous thymidine kinase promoter showed that both an Ets and CArG-like site are required. Elk1, which binds to the Ets site, was phosphorylated both in vitro and in vivo by the MAPK-independent pathway, and phosphorylation of an Elk1-GAL4 fusion protein by this pathway was sufficient for transactivation. Finally, at least two Elk1 kinases were fractionated by gel filtration, and analysis by an in-gel kinase assay revealed at least three novel Raf-activated Elk1 kinases. These results indicate that both FGF and Raf activate MAPK-independent kinases that can stimulate Elk1 phosphorylation and immediate early gene transcription.

Importance of MEK in Neutrophil Microbicidal Responsiveness

Exposure of neutrophils to inflammatory stimuli such as the chemoattractant FMLP leads to activation of responses including cell motility, the oxidative burst, and secretion of proteolytic enzymes. A signaling cascade involving sequential activation of Raf-1, mitogen-activated protein kinase (MEK), and extracellular signal regulated kinase (ERK) is also rapidly activated after agonist exposure. The temporal relationship between these events suggests that the kinases may be involved in triggering the effector functions, but direct evidence of a causal relationship is lacking. To assess the role of the MEK/ERK pathway in the activation of neutrophil responses, we studied the effects of PD098059, a potent and selective inhibitor of MEK. Preincubation of human neutrophils with 50 μM PD098059 almost completely (>90%) inhibited the FMLP-induced activation of MEK-1 and MEK-2, the isoforms expressed by neutrophils. This dose of PD098059 virtually abrogated chemoattractant-induced tyrosine phosphorylation and activation of ERK-1 and ERK-2, implying that MEKs are the predominant upstream activators of these mitogen-activated protein kinases. Pretreatment of neutrophils with the MEK antagonist inhibited the oxidative burst substantially and phagocytosis only moderately. In addition, PD098059 antagonized the delay of apoptosis induced by exposure to granulocyte-macrophage CSF. However, the effects of PD098059 were selective, as it failed to inhibit other responses, including chemoattractant-induced exocytosis of primary and secondary granules, polymerization of F-actin, chemotaxis, or activation of phospholipase A2. We conclude that MEK and ERK contribute to the activation of the oxidative burst and phagocytosis, and participate in cytokine regulation of apoptosis.

Repression of mitogen-activated protein kinases ERK1/ERK2 activity by a protein tyrosine phosphatase in rat fibroblasts transformed by upstream oncoproteins

The observation that mitogen-activated protein (MAP) kinases ERK1 and ERK2 are constitutively activated in a number of oncogene-transformed cell lines has led to the hypothesis that prolonged activation of these enzymes is required for the transformation process. To investigate this question, we have examined the regulation of the ERK pathway in Rat1 fibroblasts transformed with activated c-Raf-1 (Raf22W), v-Ha-Ras, and v-Src. Expression of these oncoproteins had no effect on the enzymatic activity of ERK1 and ERK2 in either serum-starved or exponentially growing cells. Moreover, the stimulatory effect of serum on ERK1/ERK2 activity was substantially reduced or abrogated in these cells; this impairment was associated with a strong attenuation of c-fos gene induction. In contrast, expression of Raf22w, v-Ha-Ras, or v-Src resulted in the constitutive activation of the upstream kinases MEK1 and MEK2. Treatment of the cells with vanadate completely restored the activation of ERK1/ERK2 in oncogene-transformed cells, suggesting the involvement of a vanadate-sensitive tyrosine phosphatase. Northern blot analysis of VH1-like dual-specificity MAP kinase phosphatases did not reveal any significant difference in the mRNA expression pattern of these genes between parental and transformed Rat1 cells. Phosphoamino acid analysis indicated that ERK1 is phosphorylated on threonine, but not on tyrosine, in oncogene-transformed cells and that vanadate treatment restores tyrosine phosphorylation. We conclude from these results that ERK1/ERK2 activity is repressed by a single-specificity tyrosine phosphatase in oncogene-transformed rat fibroblasts.

Mitogen-activated protein kinase activation is not necessary for, but antagonizes, 3T3-L1 adipocytic differentiation

In 3T3-L1 fibroblasts, Ras proteins mediate both insulin-induced differentiation to adipocytes and its activation of cytosolic serine/threonine kinases, including Raf-1 kinase, mitogen-activated protein kinase (MAPK), and Rsk. Here, we report that insulin- and Ras-induced activation of MAPK is not required for the differentiation process and in fact antagonizes it. The treatment of 3T3-L1 preadipocytes with MEK-specific inhibitor PD98059 blocked insulin- and Ras-induced MAPK activation but had no effect on or slightly enhanced adipocytic differentiation. Tumor necrosis factor alpha (TNF-alpha), an inhibitor of insulin-stimulated adipogenesis, activated MAPK in 3T3-L1 cells. PD98059 treatment blocked MAPK activation by TNF-alpha and reversed the blockade of adipogenesis mediated by low (1 ng/ml) TNF-alpha concentrations. 3T3-L1 transfectants containing hyperactivated MEK1 or overexpressed MAPK displayed impaired adipocytic differentiation. PD98059 treatment also reversed the blockade of differentiation in MEK1 transfectants. These results indicate that MAPK does not promote but can contribute to inhibition of the process of adipocytic differentiation of 3T3-L1 cells.

Tamoxifen inhibits prolactin signal transduction in ER - NOG-8 mammary epithelial cells

Tamoxifen (TAM), an antiestrogen, also acts as an antilactogen in mammary cells. In the present study we analyze the effect of TAM on the signal transduction pathway for prolactin (Prl). TAM bound specifically to NOG-8, an estrogen receptor-negative mammary cell line. Within 5 min of Prl treatment, raf-1, MEK and MAP kinase were induced 2-3-fold over the control level. TAM completely inhibited this Prl-induced activation of kinases as well as Prl binding and cell growth. These results indicate the potential role of TAM as an antilactogen in Prl responsive systems.

Mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways are not sufficient for insulin-like growth factor I-induced mitogenesis and tumorigenesis

Insulin-like growth factor-I (IGF-I) and insulin are known to activate a signaling cascade involving ras --> kappa raf-1 --> mitogen-activated protein (MAP) kinase kinase (MEK) --> p42/p44 MAP kinase (Erk-1 and -2). Recent reports suggest that activation of this ras/MAP kinase pathway is involved in mitogenesis and c-fos transcription but is not required for insulin action on metabolic processes such as glycogen synthesis, lipogenesis, and GLUT-4-mediated glucose transport. Previously we and others have demonstrated that substitution of both tyrosines at positions 1250 and 1251 in the carboxy-terminal region of the human IGF-I receptor has relatively small effects on receptor and endogenous substrate phosphorylation but completely abrogated the ability of these cells to form tumors in nude mice or proliferate in response to IGF-I in culture. Replacement of the tyrosine at position 1316 also did not affect the kinase activity of the receptor with respect to autophosphorylation or phosphorylation of endogenous substrates but did reduce the ability of the receptor to mediate mitogenic or tumorigenic signals. To further characterize the role of these tyrosines in IGF-I receptor function, we have used three distinct approaches to examine the ras/MAP kinase pathway in IGF-I-induced mitogenesis and tumorigenesis in NIH-3T3 cells overexpressing wild-type and mutated IGF-I receptors: 1) tyrosine phosphorylation of the MAP kinases Erk-1 and -2; 2), mobility shifts indicative of MAP kinase phosphorylation; and 3) in vitro MAP kinase activation. We have also examined IGF-I-induced phosphatidylinositol (PI) 3-kinase activation in the same cell lines. By each method we show that the IGF-I-induced MAP kinase phosphorylation/activation and PI 3-kinase activation, are not different between cells overexpressing wild-type IGF-I receptors and cells carrying IGF-I receptors having tyrosine motifs replaced at positions 1250 and 1251. We conclude that mitogenic and tumorigenic signals involving tyrosine residues in the C-terminal domain of the IGF-I-receptor include pathways other than the MAP kinase and PI 3-kinase pathways.

Cell cycle arrest mediated by the MEK/mitogen-activated protein kinase pathway

The mitogen-activated protein kinase (MAPK) cascade plays a crucial role in the transduction of extracellular signals into responses governing growth and differentiation. The effects of a specific inhibitor of the MAPK kinase (MEK)/MAPK pathway (PD98059) on nerve growth factor (NGF)-induced growth arrest and inhibition of cell cycle-dependent kinases (CDKs) have been examined. Treatment of NIH 3T3 cells expressing TRKA with PD98059 dramatically reversed the complete inhibition of growth of these cells caused by NGF. PD98059 also blocked the ability of NGF to inhibit the activities of CDK4 and CDK2, while partially preventing NGF induction of p21Cip1/WAF1. To independently evaluate the involvement of the MEK/MAPK pathway in growth arrest, an inducible activated form of the Raf-1 protooncogene (delta RAF-1:ER) was expressed in these cells. Activation of delta RAF-1:ER resulted in a prolonged increase in MAPK activity and growth arrest of these cells, with concomitant induction of p21Cip1/WAF1 and inhibition of CDK2 activity. These effects of delta RAF-1:ER activation were all reversed by treatment of cells with PD98059. These data indicate that in addition to functioning as a positive effector of growth, stimulation of the MEK/MAPK pathway can result in an inhibition of CDK activity and cell cycle arrest.

Synergy between the Mos/mitogen-activated protein kinase pathway and loss of p53 function in transformation and chromosome instability

Constitutive activation of mitogen-activated protein kinase (MAPK) is a property common to many oncoproteins, including Mos, Ras, and Raf, and is essential for their transforming activities. We have shown that high levels of expression of the Mos/MAPK pathway in Swiss 3T3 fibroblast cause cells in S phase to undergo apoptosis, while cells in G1 irreversibly growth arrest. Interestingly, cells in G2 and M phases also arrest at a G1-like checkpoint after proceeding through mitosis. These cells fail to undergo cytokinesis and are binucleated. Thus, constitutive overexpression of Mos and MAPK cannot be tolerated, and fibroblasts transformed by Mos express only low levels of the mos oncogene product. Here, we show that p53 plays a key role in preventing oncogene-mediated activation of MAPK. In the absence of p53 (p53-/-), the growth arrest normally observed in wild-type p53 (p53+/+) mouse embryo fibroblasts (MEFs) is markedly reduced. The mos transformation efficiency in p53-/- MEFs is two to three orders of magnitude higher than that in p53+/+ cells, and p53-/- cells tolerate > 10-fold higher levels of both Mos and activated MAPK. Moreover, we show that, like Mos, both v-ras and v-raf oncogene products induce apoptosis in p53+/+ MEFs. These oncogenes also display a high transforming activity in p53-/- MEFs, as does a gain-of-function MAPK kinase mutant (MEK*). Thus, the p53-dependent checkpoint pathway is responsive to oncogene-mediated MAPK activation in inducing irreversible G1 growth arrest and apoptosis. Moreover, we show that the chromosome instability induced by the loss of p53 is greatly enhanced by the constitutive activation of the Mos/MAPK pathway.

Mek1 phosphorylation site mutants activate Raf-1 in NIH 3T3 cells

MAP (mitogen-activated protein) kinases are activated by a family of dual specificity kinases called Meks (MAP kinase/Erk kinase). Mek1 can be activated by Raf by phosphorylation on serine 218 and serine 222. Mutation of these sites to acidic residues leads to constitutively active Mek1 in some cases. When fibroblast lines were infected with high titer retroviral stocks carrying these Mek1 genes, the resultant transformation and morphological changes correlated with the kinase activity of the respective Mek1 enzymes. Although [Asp218]- and [Asp218,Asp222]Mek immunoprecipitated from clonal cell lines could phosphorylate kinase-inactive Erk1 equally well in vitro, the endogenous MAP kinase activity was 5-7-fold greater in [Asp218]Mek1-infected clonal lines, and did not correlate with the degree of transformation. Analysis of the Erk1 pathway revealed Raf-1 activation, which correlated qualitatively with the MAP kinase activity seen in the [Asp218]- and [Asp218,Asp222]Mek1-infected clonal cell lines. Expression of dominant negative Ras did not affect the elevated Raf-1 activity observed in these cells, however. These data suggest that Mek1 phosphorylation site mutants activate Raf-1 and MAP kinase by a Ras-independent pathway and that the mechanism by which transformation occurs may utilize pathways that are MAP kinase-independent.

Involvement of the protein tyrosine phosphatase SHP-1 in Ras-mediated activation of the mitogen-activated protein kinase pathway

Ubiquitously expressed SH2-containing tyrosine phosphatases interact physically with tyrosine kinase receptors or their substrates and relay positive mitogenic signals via the activation of the Ras-mitogen-activated protein kinase (MAPK) pathway. Conversely, the structurally related phosphatase SHP-1 is predominantly expressed in hemopoietic cells and becomes tyrosine phosphorylated upon colony-stimulating factor 1 treatment of macrophages without associating with the colony-stimulating factor 1 receptor tyrosine kinase. Mice lacking functional SHP-1 (me/me and me(v)/me(v)) develop systemic autoimmune disease with accumulation of macrophages, suggesting that SHP-1 may be a negative regulator of hemopoietic cell growth. By using macrophages expressing dominant negative Ras and the me(v)/me(v) mouse mutant, we show that SHP-1 is activated in the course of mitogenic signal transduction in a Ras-dependent manner and that its activity is necessary for the Ras-dependent activation of the MAPK pathway but not of the Raf-1 kinase. Consistent with a role for SHP-1 as an intermediate between Ras and the MEK-MAPK pathway, Ras-independent activation of the latter kinases by bacterial lipopolysaccharide occurred normally in me(v)/me(v) cells. Our results sharply accentuate the diversity of signal transduction in mammalian cells, in which the same signaling intermediates can be rearranged to form different pathways.

Calcium-dependent immediate-early gene induction in lymphocytes is negatively regulated by p21Ha-ras

The induction of immediate-early (IE) response genes, such as egr-1, c-fos, and c-jun, occurs rapidly after the activation of T lymphocytes. The process of activation involves calcium mobilization, activation of protein kinase C (PKC), and phosphorylation of tyrosine kinases. p21(ras), a guanine nucleotide binding factor, mediates T-cell signal transduction through PKC-dependent and PKC-independent pathways. The involvement of p21(ras) in the regulation of calcium-dependent signals has been suggested through analysis of its role in the activation of NF-AT. We have investigated the inductions of the IE genes in response to calcium signals in Jurkat cells (in the presence of activated p21(ras)) and their correlated consequences. The expression of activated p21(ras) negatively regulated the induction of IE genes by calcium ionophore. This inhibition of calcium-activated IE gene induction was reversed by treatment with cyclosporin A, suggesting the involvement of calcineurin in this regulation. A later result of inhibition of this activation pathway by p21(ras) was down-regulation of the activity of the transcription factor AP-1 and subsequent coordinate reductions in IL-2 gene expression and protein production. These results suggest that p2l(ras) is an essential mediator in generating not only positive but also negative modulatory mechanisms controlling the competence of T cells in response to inductive stimulations.

What's new in ras genes? Physiological role of ras genes in signal transduction and significance of ras gene activation in tumorigenesis

Ras gene mutations have been found with variable prevalence in different tumor types. While during the past decade a lot of information has been accumulated on the frequency of ras oncogene activation in tumors, the last two years brought considerable progress in elucidating molecular mechanisms of signal transduction for which cellular ras proteins are key elements. They transmit signals from upstream tyrosine kinases to downstream serine/threonine kinases ultimately leading to changes of gene expression cytoskeletal architecture, cell-to-cell interactions and metabolism. These signalling pathways are of interest for the physiological regulation of proliferation and differentiation in normal, as well as in cancer tissue. Mutational activation of cellular ras genes to transforming oncogenes is thought to promote cell growth even in the absence of extracellular stimuli, and may thereby contribute to the initiation and/or progression of tumors.

Raf, but not MEK or ERK, is sufficient for differentiation of hippocampal neuronal cells

To elucidate signal transduction pathways leading to neuronal differentiation, we have investigated a conditionally immortalized cell line from rat hippocampal neurons (H19-7) that express a temperature sensitive simian virus 40 large T antigen. Treatment of H19-7 cells with the differentiating agent basic fibroblast growth factor at 39 degrees C, the nonpermissive temperature for T function, resulted in the activation of c-Raf-1, MEK, and mitogen-activated protein (MAP) kinases (ERK1 and -2). To evaluate the role of Raf-1 in neuronal cell differentiation, we stably transfected H19-7 cells with v-raf or an oncogenic human Raf-1-estrogen receptor fusion gene (deltaRaf-1:ER). deltaRaf-1:ER transfectants in the presence of estradiol for 1 to 2 days expressed a differentiation phenotype only at the nonpermissive temperature. However, extended exposure of the deltaRaf-1:ER transfectants to estradiol or stable expression of the v-raf construct yielded cells that extended processes at the permissive as well as the nonpermissive temperature, suggesting that cells expressing the large T antigen are capable of responding to the Raf differentiation signal. deltaRaf-1:ER, MEK, and MAP kinase activities in the deltaRaf-1:ER cells were elevated constitutively for up to 36 h of estradiol treatment at the permissive temperature. At the nonpermissive temperature, MEK and ERKs were activated to a significantly lesser extent, suggesting that prolonged MAP kinase activation may not be sufficient for differentiation. To test this possibility, H19-7 cells were transfected or microinjected with constitutively activated MEK. The results indicate that prolonged activation of MEK or MAP kinases (ERK1 and -2) is not sufficient for differentiation of H19-7 neuronal cells and raise the possibility that an alternative signaling pathway is required for differentiation of H19-7 cells by Raf.

G alpha 13 stimulates Na+-H+ exchange through distinct Cdc42-dependent and RhoA-dependent pathways

Activity of the ubiquitously expressed Na+-H+ exchanger subtype NHE1 is stimulated upon activation of receptor tyrosine kinases and G protein-coupled receptors. The intracellular signaling pathways mediating receptor regulation of the exchanger, however, are poorly understood. Using transient expression of dominant interfering and constitutively active alleles in CCL39 fibroblasts, we determined that the GTPases Ha-Ras and Galpha 13 stimulate NHE1 through distinct signaling cascades. Exchange activity stimulated by constitutively active RasV12 occurs through a Rafl- and mitogen-activated protein kinase kinase/extracellular signal-regulated kinase kinase (MEK)-dependent mechanism. Constitutively active Galpha 13QL, recently shown to stimulate the Jun kinase cascade, activates NHE1 through a Cdc42- and MEK kinase (MEKK1)-dependent mechanism that is independent of Rac1. Constitutively active Rac1V12 does stimulate NHE1 through a MEKK1-dependent mechanism, but dominant interfering Rac1N17 does not inhibit Galpha 13QL-mediated or constitutively active Cdc42V12-mediated stimulation of the exchanger. Conversely, Cdc42NI7 does not inhibit Rac1V12 activation of NHE1, suggesting that Rae I and Cdc42 independently regulate a MEKK1-dependent activation of the exchanger. Rapid (<10 min) stimulation of NHE1 with a Ga13/Gaz chimera also was inhibited by a kinase-inactive MEKK. Galpha 13QL, but not RasV12, also stimulates NHE1 through a RhoA-dependent pathway that is independent of MEKK, and microinjection of mutationally active Galpha 13 results in a Rho phenotype of increased stress fiber formation. These findings indicate a new target for Rho-like proteins: the regulation of H+ ex- change and intracellular pH. Our findings also suggest that a MEKK cascade diverges to regulate effectors other than transcription factors.

Activation of mitogen-activated protein kinase by heat shock treatment in Drosophila

Heat shock treatment of Drosophila melanogaster tissue culture cells causes increased tyrosine phosphorylation of several 44 kDa proteins, which are identified as Drosophila mitogen-activated protein (MAP) kinases. Tyrosine phosphorylation occurs within 5 min, and is maintained at high levels during heat shock. It decreases to basal levels during recovery, concurrent with the repression of heat shock transcription and heat-shock-protein synthesis. The increased MAP kinase tyrosine phosphorylation is parallelled by increased MAP kinase activity. At least two MAP kinases, DmERK-A and DmERK-B, are identified whose tyrosine phosphorylation increases during heat shock. Thus MAP kinase activation is an immediate early response to heat shock, and its increased activity is maintained throughout heat shock treatment. Protracted MAP kinase activation may contribute to heat shock transcription factor phosphorylation and the numerous metabolic alterations that constitute the heat-shock response.

Hypertrophic agonists stimulate the activities of the protein kinases c-Raf and A-Raf in cultured ventricular myocytes

We detected expression of two Raf isoforms, c-Raf and A-Raf, in neonatal rat heart. Both isoforms phosphorylated, activated, and formed complexes with mitogen-activated protein kinase kinase 1 in vitro. However, these isoforms were differentially activated by hypertrophic stimuli such as peptide growth factors, endothelin-1 (ET1), or 12-O-tetradecanoylphorbol-13-acetate (TPA) that activate the mitogen-activated protein kinase cascade. Exposure of cultured ventricular myocytes to acidic fibroblast growth factor activated c-Raf but not A-Raf. In contrast, TPA produced a sustained activation of A-Raf and only transiently activated c-Raf. ET1 transiently activated both isoforms. TPA and ET1 were the most potent activators of c-Raf and A-Raf. Both utilized protein kinase C-dependent pathways, but stimulation by ET1 was also partially sensitive to pertussis toxin pretreatment. cRaf was inhibited by activation of cAMP-dependent protein kinase although A-Raf was less affected. Fetal calf serum, phenylephrine, and carbachol were less potent activators of c-Raf and A-Raf. These results demonstrate that A-Raf and c-Raf are differentially regulated and that A-Raf may be an important mediator of mitogen-activated protein kinase cascade activation when cAMP is elevated.

Conditionally oncogenic forms of the A-Raf and B-Raf protein kinases display different biological and biochemical properties in NIH 3T3 cells

The protein kinase domains of mouse A-Raf and B-Raf were expressed as fusion proteins with the hormone binding domain of the human estrogen receptor in mammalian cells. In the absence of estradiol, 3T3 and rat1a cells expressing delta A-Raf:ER and delta B-Raf:ER were nontransformed, but upon the addition of estradiol the cells became oncogenically transformed. Morphological oncogenic transformation was more rapid and distinctive in cells expressing delta B-Raf:ER compared with cells expressing delta A-Raf:ER. Biochemical analysis of cells transformed by delta A-Raf:ER and delta B-Raf:ER revealed several interesting differences. The activation of delta B-Raf:ER consistently led to the rapid and robust activation of both MEK and p42/p44 MAP kinases. By contrast, the activation of delta A-Raf:ER led to a weak activation of MEK and the p42/p44 MAP kinases. The extent of activation of MEK in cells correlated with the ability of the different Raf kinases to phosphorylate and activate MEK1 in vitro. delta B-Raf:ER phosphorylated MEK1 approximately 10 times more efficiently than delta Raf-1:ER and at least 500 times more efficiently than delta A-Raf:ER under the conditions of the immune-complex kinase assays. These results were confirmed with epitope-tagged versions of the Raf kinase domains expressed in insect cells. The activation of all three delta Raf:ER proteins in 3T3 cells led to the hyperphosphorylation of the resident p74raf-1 and mSOS1 proteins, suggesting the possibility of "cross-talk" between the different Raf kinases and feedback regulation of intracellular signaling pathways. The activation of either delta B-Raf:ER or delta Raf-1:ER in quiescent 3T3 cells was insufficient to promote the entry of the cells into DNA synthesis. By contrast, the activation of delta A-Raf:ER in quiescent 3T3 cells was sufficient to promote the entry of the cells into S phase after prolonged exposure to beta-estradiol. The delta Raf:ER system has allowed us to reveal significant differences between the biological and biochemical properties of oncogenic forms of the Raf family of protein kinases. We anticipate that cells expressing these proteins and other estradiol-regulated protein kinases will be useful tools in future attempts to unravel the complex web of interactions involved in intracellular signal transduction pathways.

Mitogen-activated protein kinase activation requires two signal inputs from the human anaphylatoxin C5a receptor

The anaphylatoxin C5a receptor activates the Ras/Raf/mitogen-activated protein (MAP) kinase pathway in human neutrophils. The signal pathways involved in Ras/Raf/MAP kinase activation in response to C5a and other chemoattractant receptors is poorly understood. Stimulation of the C5a receptor expressed in HEK293 cells results in modest MAP kinase activation, which is inhibited by pertussis toxin-catalyzed ADP-ribosylation of G(i). Coexpression of the C5a receptor and the G16 alpha subunit (alpha 16) results in the G16-mediated activation of phospholipase C beta and a robust MAP kinase activation. Pertussis toxin treatment of C5a receptor/alpha 16-cotransfected cells inhibits C5a stimulation of MAP kinase activity approximately 60% relative to the control response. Similarly, the protein kinase C inhibitor, GF109203X inhibits activation of MAP kinase activation in C5a receptor/alpha 16-cotransfected cells by 60%; the protein kinase C inhibitor does not affect the modest C5a receptor response in the absence of alpha 16 expression. These results demonstrate that two independent signals are required for the maximal activation of MAP kinase by G protein-coupled receptors.

Rapid induction of heparin-binding epidermal growth factor/diphtheria toxin receptor expression by Raf and Ras oncogenes

We have used differential display PCR to search for mRNAs induced by delta Raf-1:ER, an estradiol-dependent form of Raf-1 kinase. Through this approach the gene encoding heparin-binding epidermal growth factor (HB-EGF) was identified as an immediate-early transcriptional target of oncogenic Raf kinases. Activation of delta Raf-1:ER and a conditional oncogenic form of B-Raf, delta B-RAF:ER, resulted in rapid and sustained induction of HB-EGF mRNA expression and secretion of mature HB-EGF from cells. Neutralizing anti-HB-EGF antisera prevented the delayed activation of the c-Jun amino-terminal kinases that is observed in cells transformed by delta Raf-1:ER. These results demonstrate that distinct signaling pathways can cross talk via the secretion of polypeptide growth factors. Furthermore, cells transformed by oncogenic Ras, which also induced HB-EGF expression, demonstrated a marked increase in sensitivity to the cytotoxic action of diphtheria toxin, for which the membrane anchored HB-EGF precursor acts as a cell-surface receptor.

Biochemical analysis of MEK activation in NIH3T3 fibroblasts. Identification of B-Raf and other activators

Numerous potential activators of MEK have been identified, including c-Raf-1, B-Raf, c-Mos, and a family of MEK kinases. However, little information gives insight into the activators actually utilized in vivo. To address this, we have used column chromatography and a coupled MEK activation assay to identify in NIH3T3 cells, two major MEK activators, and a third insulin-specific activator. The first MEK activator has an apparent M(r) of 40,000-50,000, was immunologically distinct from A-Raf, B-Raf, c-Raf-1, c-MEKK, c-Mos, MEK1, and MEK2, and was rapidly activated by serum, platelet-derived growth factor (PDGF), insulin, thrombin, and phorbol ester. The second MEK activator was identified as B-Raf. Activation of 93-95 kDa B-Raf was observed in column fractions and B-Raf immunoprecipitates from cytosolic and particulate fractions after stimulation with serum or PDGF, but not insulin. c-Raf-1 from cytosol did not exhibit MEK activator activity; however, c-Raf-1 immunoprecipitates from the particulate fraction revealed MEK activator activity that was enhanced after stimulation with PDGF or phorbol ester, but not serum or insulin. Both c-Mos and c-MEKK were present in NIH3T3 fibroblasts but did not show MEK activator activity. These data provide direct evidence that 93-95-kDa B-Raf isozymes and an unidentified 40-50-kDa MEK activator are major agonist-specific MEK activators in NIH3T3 fibroblasts.

Direct interaction and N-terminal phosphorylation of c-Jun by c-Mil/Raf

c-Mil is the avian homologue of the mammalian serine/threonine kinase c-Raf-1. c-Mil/Raf is a mediator of signal transduction leading to gene expression via the c-Jun DNA-binding site, AP-1. Here we show that c-Mil immunopurified from MC29-virus-transformed quail fibroblasts phosphorylates c-Jun in vitro near its N terminus (Ser-63 and -73). Furthermore, the viral oncogene product Gag-Mil of the avian wild-type retrovirus MH2 phosphorylates c-Jun in vitro. A contribution by other known kinases phosphorylating c-Jun, such as the mitogen-activated protein kinases (MAPKs) and the c-Jun N-terminal kinases, was excluded by control reactions. c-Raf-1 and c-Jun directly interact in vitro as shown by various immobilized glutathione S-transferase-Raf fusion proteins which specify the cysteine-rich region of c-Mil/Raf as the major N-terminal binding site. An additional minor binding site is located in the C-terminal region. The biological relevance of these results is demonstrated by coimmunoprecipitation of c-Jun and c-Mil from 32P-labeled MC29- and MH2-transformed fibroblasts as well as normal quail embryo fibroblasts, whereby c-Jun was identified by tryptic phosphopeptide analysis. The complexed c-Jun exhibits a decreased electrophoretic mobility corresponding to a more highly phosphorylated state. Cell fractionation analyses indicate that the c-Mil/c-Jun complex is located in the cytoplasm. The data demonstrate that c-Jun can be a direct target of the protein kinase c-Mil/Raf, suggesting an alternative pathway, which leads to c-Jun phosphorylation independent of the MAPKs and MAPK-related proteins.

Regulation of Raf-1 kinase activity by the 14-3-3 family of proteins

We have identified the beta (beta) isoform of the 14-3-3 family of proteins as an activator of the Raf-1 protein kinase. 14-3-3 was isolated in a yeast two-hybrid screen for Raf-1 kinase domain binding proteins. Purified bovine brain 14-3-3 interacted specifically with both c-Raf-1 and the isolated Raf-1 kinase domain. Association was sensitive to the activation status of Raf-1; 14-3-3 bound to unactivated Raf-1, but not Raf-1 activated by protein kinase C alpha or Ras and Lck. The significance of these interactions under physiological conditions was demonstrated by co-immunoprecipitation of Raf-1 and 14-3-3 from extracts of quiescent, but not mitogen-stimulated, NIH 3T3 cells. 14-3-3 was not a preferred Raf-1 substrate in vitro and did not significantly affect Raf-1 kinase activity in a purified system. However, in cell-free extracts 14-3-3 acted as a Ras-independent activator of both c-Raf-1 and the Raf-1 kinase domain. The same results were obtained in vivo using transfection assays; 14-3-3 enhanced both c-Raf-1- and Raf-1 kinase domain-stimulated expression of AP-1- and NF-kappa B-dependent reporter genes and accelerated Raf-1 kinase domain-triggered differentiation of PC12 cells. We conclude that 14-3-3 is a latent co-activator bound to unactivated Raf-1 in quiescent cells and mediates mitogen-triggered but Ras-independent regulatory effects aimed directly at the kinase domain.

Ras-dependent and -independent pathways target the mitogen-activated protein kinase network in macrophages

Mitogen-activated protein kinases (MAPKs) are activated upon a variety of extracellular stimuli in different cells. In macrophages, colony-stimulating factor 1 (CSF-1) stimulates proliferation, while bacterial lipopolysaccharide (LPS) inhibits cell growth and causes differentiation and activation. Both CSF-1 and LPS rapidly activate the MAPK network and induce the phosphorylation of two distinct ternary complex factors (TCFs), TCF/Elk and TCF/SAP. CSF-1, but not LPS, stimulated the formation of p21ras. GTP complexes. Expression of a dominant negative ras mutant reduced, but did not abolish, CSF-1-mediated stimulation of MEK and MAPK. In contrast, activation of the MEK kinase Raf-1 was Ras independent. Treatment with the phosphatidylcholine-specific phospholipase C inhibitor D609 suppressed LPS-mediated, but not CSF-1-mediated, activation of Raf-1, MEK, and MAPK. Similarly, down-regulation or inhibition of protein kinase C blocked MEK and MAPK induction by LPS but not that by CSF-1. Phorbol 12-myristate 13-acetate pretreatment led to the sustained activation of the Raf-1 kinase but not that of MEK and MAPK. Thus, activated Raf-1 alone does not support MEK/MAPK activation in macrophages. Phosphorylation of TCF/Elk but not that of TCF/SAP was blocked by all treatments that interfered with MAPK activation, implying that TCF/SAP was targeted by a MAPK-independent pathway. Therefore, CSF-1 and LPS target the MAPK network by two alternative pathways, both of which induce Raf-1 activation. The mitogenic pathway depends on Ras activity, while the differentiation signal relies on protein kinase C and phosphatidylcholine-specific phospholipase C activation.

Inhibition of platelet-derived growth factor- and epidermal growth factor-mediated mitogenesis and signaling in 3T3 cells expressing delta Raf-1:ER, an estradiol-regulated form of Raf-1

We have recently described the properties of delta Raf-1:ER, a fusion protein consisting of an oncogenic form of human Raf-1 and the hormone binding domain of the human estrogen receptor. In this study, we demonstrate that activation of delta Raf-1:ER in quiescent 3T3 cells (C2 cells), while sufficient to promote morphological oncogenic transformation, was insufficient to promote the entry of cells into DNA synthesis. Indeed, activation of delta Raf-1:ER potently inhibited the mitogenic response of cells to platelet-derived growth factor (PDGF) and epidermal growth factor (EGF) treatment. Addition of beta-estradiol to quiescent C2 cells led to rapid, sustained activation of delta Raf-1:ER and MEK but only two- to threefold activation of p42 mitogen-activating protein (MAP) kinase activity. Addition of PDGF or EGF to quiescent C2 cells in which delta Raf-1:ER was inactive led to rapid activation of Raf-1, MEK, and p42 MAP kinase activities, and entry of the cells into DNA synthesis. In contrast, when delta Raf-1:ER was activated in quiescent C2 cells prior to factor addition, there was a significant inhibition of certain aspects of the signaling response to subsequent treatment with PDGF or EGF. The expression and activation of PDGF receptors and the phosphorylation of p70S6K in response to PDGF treatment were unaffected by prior activation of delta Raf-1:ER. In contrast, PDGF-mediated activation of Raf-1 and p42 MAP kinases was significantly inhibited compared with that of controls. Interestingly, the mitogenic and signaling responses of quiescent C2 cells to stimulation with fetal bovine serum or phorbol myristate acetate were unaffected by prior activation of delta Raf-1:ER. It seems likely that at least two mechanisms contribute to the effects of delta Raf-1:ER in these cells. First, activation of delta Raf-1:ER appeared to uncouple the activation of Raf-1 from the activation of the PDGF receptor at the cell surface. This may be due to the fact that mSOS1 is constitutively phosphorylated as a consequence of the activation of delta Raf-1:ER. Second, quiescent C2 cells expressing activated delta Raf-1:ER appear to contain an inhibitor of the MAP kinase pathway that, because of its apparent sensitivity to sodium orthovanadate, may be a phosphotyrosine phosphatase. It is likely that the inhibitory effects of delta Raf-1:ER observed in these cells are a manifestation of the activation of some of the feedback inhibition pathways that normally modulate a cell's response to growth factors. 3T3 cells expressing delta Raf-1:ER will be a useful tool in unraveling the role of Raf-1 kinase activity in the regulation of such pathways.

Inhibition of platelet-derived growth factor- and epidermal growth factor-mediated mitogenesis and signaling in 3T3 cells expressing delta Raf-1:ER, an estradiol-regulated form of Raf-1

We have recently described the properties of delta Raf-1:ER, a fusion protein consisting of an oncogenic form of human Raf-1 and the hormone binding domain of the human estrogen receptor. In this study, we demonstrate that activation of delta Raf-1:ER in quiescent 3T3 cells (C2 cells), while sufficient to promote morphological oncogenic transformation, was insufficient to promote the entry of cells into DNA synthesis. Indeed, activation of delta Raf-1:ER potently inhibited the mitogenic response of cells to platelet-derived growth factor (PDGF) and epidermal growth factor (EGF) treatment. Addition of beta-estradiol to quiescent C2 cells led to rapid, sustained activation of delta Raf-1:ER and MEK but only two- to threefold activation of p42 mitogen-activating protein (MAP) kinase activity. Addition of PDGF or EGF to quiescent C2 cells in which delta Raf-1:ER was inactive led to rapid activation of Raf-1, MEK, and p42 MAP kinase activities, and entry of the cells into DNA synthesis. In contrast, when delta Raf-1:ER was activated in quiescent C2 cells prior to factor addition, there was a significant inhibition of certain aspects of the signaling response to subsequent treatment with PDGF or EGF. The expression and activation of PDGF receptors and the phosphorylation of p70S6K in response to PDGF treatment were unaffected by prior activation of delta Raf-1:ER. In contrast, PDGF-mediated activation of Raf-1 and p42 MAP kinases was significantly inhibited compared with that of controls. Interestingly, the mitogenic and signaling responses of quiescent C2 cells to stimulation with fetal bovine serum or phorbol myristate acetate were unaffected by prior activation of delta Raf-1:ER. It seems likely that at least two mechanisms contribute to the effects of delta Raf-1:ER in these cells. First, activation of delta Raf-1:ER appeared to uncouple the activation of Raf-1 from the activation of the PDGF receptor at the cell surface. This may be due to the fact that mSOS1 is constitutively phosphorylated as a consequence of the activation of delta Raf-1:ER. Second, quiescent C2 cells expressing activated delta Raf-1:ER appear to contain an inhibitor of the MAP kinase pathway that, because of its apparent sensitivity to sodium orthovanadate, may be a phosphotyrosine phosphatase. It is likely that the inhibitory effects of delta Raf-1:ER observed in these cells are a manifestation of the activation of some of the feedback inhibition pathways that normally modulate a cell's response to growth factors. 3T3 cells expressing delta Raf-1:ER will be a useful tool in unraveling the role of Raf-1 kinase activity in the regulation of such pathways.