Raf-1: a kinase currently without a cause but not lacking in effects

Impacts of SARS-CoV-2 on brain renin angiotensin system related signaling and its subsequent complications on brain: A theoretical perspective

Cellular ACE2 (cACE2), a vital component of the renin-angiotensin system (RAS), possesses catalytic activity to maintain AngII and Ang 1-7 balance, which is necessary to prevent harmful effects of AngII/AT2R and promote protective pathways of Ang (1-7)/MasR and Ang (1-7)/AT2R. Hemostasis of the brain-RAS is essential for maintaining normal central nervous system (CNS) function. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a viral disease that causes multi-organ dysfunction. SARS-CoV-2 mainly uses cACE2 to enter the cells and cause its downregulation. This, in turn, prevents the conversion of Ang II to Ang (1-7) and disrupts the normal balance of brain-RAS. Brain-RAS disturbances give rise to one of the pathological pathways in which SARS-CoV-2 suppresses neuroprotective pathways and induces inflammatory cytokines and reactive oxygen species. Finally, these impairments lead to neuroinflammation, neuronal injury, and neurological complications. In conclusion, the influence of RAS on various processes within the brain has significant implications for the neurological manifestations associated with COVID-19. These effects include sensory disturbances, such as olfactory and gustatory dysfunctions, as well as cerebrovascular and brain stem-related disorders, all of which are intertwined with disruptions in the RAS homeostasis of the brain.

1,3-Dichloroadamantyl-Containing Ureas as Potential Triple Inhibitors of Soluble Epoxide Hydrolase, p38 MAPK and c-Raf

Soluble epoxide hydrolase (sEH) is an enzyme involved in the metabolism of bioactive lipid signaling molecules. sEH converts epoxyeicosatrienoic acids (EET) to virtually inactive dihydroxyeicosatrienoic acids (DHET). The first acids are "medicinal" molecules, the second increase the inflammatory infiltration of cells. Mitogen-activated protein kinases (p38 MAPKs) are key protein kinases involved in the production of inflammatory mediators, including tumor necrosis factor-α (TNF-α) and cyclooxygenase-2 (COX-2). p38 MAPK signaling plays an important role in the regulation of cellular processes, especially inflammation. The proto-oncogenic serine/threonine protein kinase Raf (c-Raf) is a major component of the mitogen-activated protein kinase (MAPK) pathway: ERK1/2 signaling. Normal cellular Raf genes can also mutate and become oncogenes, overloading the activity of MEK1/2 and ERK1/2. The development of multitarget inhibitors is a promising strategy for the treatment of socially dangerous diseases. We synthesized 1,3-disubstituted ureas and diureas containing a dichloroadamantyl moiety. The results of computational methods show that soluble epoxide hydrolase inhibitors can act on two more targets in different signaling pathways of mitogen-activated protein kinases p38 MAPK and c-Raf. The two chlorine atoms in the adamantyl moiety may provide additional Cl-π interactions in the active site of human sEH. Molecular dynamics studies have shown that the stability of ligand-protein complexes largely depends on the "spacer effect." The compound containing a bridge between the chloroadamantyl fragment and the ureide group forms more stable ligand-protein complexes with sEH and p38 MAPK, which indicates a better conformational ability of the molecule in the active sites of these targets. In turn, a compound containing two chlorine atoms forms a more stable complex with c-Raf, probably due to the presence of additional halogen bonds of chlorine atoms with amino acid residues.

Abundance of Non-Polarized Lung Macrophages with Poor Phagocytic Function in Chronic Obstructive Pulmonary Disease (COPD)

Lung macrophages are the key immune effector cells in the pathogenesis of Chronic Obstructive Pulmonary Disease (COPD). Several studies have shown an increase in their numbers in bronchoalveolar lavage fluid (BAL) of subjects with COPD compared to controls, suggesting a pathogenic role in disease initiation and progression. Although reduced lung macrophage phagocytic ability has been previously shown in COPD, the relationship between lung macrophages' phenotypic characteristics and functional properties in COPD is still unclear. (1) Methods: Macrophages harvested from bronchoalveolar lavage (BAL) fluid of subjects with and without COPD (GOLD grades, I-III) were immuno-phenotyped, and their function and gene expression profiles were assessed using targeted assays. (2) Results: BAL macrophages from 18 COPD and 10 (non-COPD) control subjects were evaluated. The majority of macrophages from COPD subjects were non-polarized (negative for both M1 and M2 markers; 77.9%) in contrast to controls (23.9%; p < 0.001). The percentages of these non-polarized macrophages strongly correlated with the severity of COPD (p = 0.006) and current smoking status (p = 0.008). Non-polarized macrophages demonstrated poor phagocytic function in both the control (p = 0.02) and COPD (p < 0.001) subjects. Non-polarized macrophages demonstrated impaired ability to phagocytose Staphylococcus aureus (p < 0.001). They also demonstrated reduced gene expression for CD163, CD40, CCL13 and C1QA&B, which are involved in pathogen recognition and processing and showed an increased gene expression for CXCR4, RAF1, amphiregulin and MAP3K5, which are all involved in promoting the inflammatory response. (3) Conclusions: COPD is associated with an abundance of non-polarized airway macrophages that is related to the severity of COPD. These non-polarized macrophages are predominantly responsible for the poor phagocytic capacity of lung macrophages in COPD, having reduced capacity for pathogen recognition and processing. This could be a key risk factor for COPD exacerbation and could contribute to disease progression.

Molecular Landscape and Treatment Options for Patients with Metastatic Colorectal Cancer

Over the last 20 years, median survival for patients with metastatic colorectal cancer (CRC) has remarkably improved from about 12 to over 30 months, mainly because of the development of new agents and patient selection using predictive biomarkers. However, the identification of the most effective treatment for an individual patient is still a challenge. Molecular profiling of CRC has made great progress, but it is limited by tumor heterogeneity and absence of driver mutation. However, RAS, BRAF, and microsatellite instability are validated biomarker recommended by NCCN and ESMO. In this review, we discuss recent advances and future developments.

Anti-Inflammatory Properties of the Enaminone E121 in the Dextran Sulfate Sodium (DSS) Colitis Model

BACKGROUND

Enaminones are synthetic compounds with an established role in the prevention of various forms of seizures. Recent evidence suggests potent anti-tussive, bronchodilation and anti-inflammatory properties. Pre-treatment with particularly E121 compound resulted in a decrease in leukocyte recruitment in the ovalbumin induced-model of asthma, immune cell proliferation and cytokine release in vitro. We hypothesize that E121 might serve as a therapeutic potential in intestinal inflammation through modulating immune cell functions.

METHODS

Colitis was induced by daily dextran sulfate sodium (DSS) administration for 5 days, and its severity was determined by gross and histological assessments. The plasma level of various cytokines was measured using flow cytometry-based assay. The colonic expression/ phosphorylation level of various molecules was determined by immunofluorescence and western blotting. The effects of E121 treatment on in vitro neutrophil chemotaxis (under-agarose assay), superoxide release (luminol oxidation assay) and apoptosis (annexin V/7AAD) were also determined.

RESULTS

DSS-induced colitis in mice was significantly reduced by daily E121 treatment (30-100 mg/kg) at gross and histological levels. This effect was due to modulated plasma levels of interleukin (IL-2) and colonic expression levels of various signaling molecules and proteins involved in apoptosis. In vitro neutrophil survival, chemotaxis, and superoxide release were also reduced by E121 treatment.

CONCLUSION

Our results indicate important anti-inflammatory actions of E121 in the pathogenesis of IBD.

Role of Raf kinase inhibitor protein in Helicobacter pylori-mediated signaling in gastric cancer

Helicobacter pylori is a helical bacterium that colonizes the stomach in over half of the world's population. Infection with this bacterium has been linked to peptic ulcer disease and gastric cancer. The bacterium has been shown to affect regulatory pathways in its host cells through specific virulence factors that control gene expression. Infection with H. pylori increases levels of phosphorylation of Raf kinase inhibitor protein (pRKIP) in gastric adenocarcinoma (AGS) cells in vitro and in vivo. We investigated the role of H. pylori in the phosphorylation of RKIP as a possible mechanism to downregulate pro-survival signals in gastric adenocarcinoma. pRKIP induces RKIP transcriptional activity, which serves to induce apoptosis of damaged cells to prevent further tumorigenesis. Infection of wild type and RKIP knockout mice with H. pylori for 2 months further confirmed roles of RKIP and pRKIP in the prevention of gastric cancer progression. Loss of RKIP in AGS cells results in increased expression of the Cag A virulence factor after H. pylori infection and RKIP overexpression inhibits H. pylori-mediated STAT3 phosphorylation and STAT3 and NF-κB transcriptional activity. We examined the role of mTOR (mammalian target of rapamycin) after H. pylori infection on the phosphorylation of RKIP. Cells treated with rapamycin, an inhibitor of mTOR, displayed less expression of pRKIP after H. pylori infection. Microarray antibody analysis was conducted on wild-type and RKIP-knockdown AGS cells and showed that in the absence of RKIP, there was increased expression of pro-tumorigenic proteins such as EGFR, Raf-1, and MAPKs. Although further work is needed to confirm the interaction of RKIP and mTOR in AGS cells as a result of H. pylori infection, we hypothesize that H. pylori-mediated induction of pro-survival signaling in gastric epithelial cells induces a feedback response through the activation of RKIP. The phosphorylated, or active, form of RKIP is important in protecting gastric epithelial cells from tumorigenesis after H. pylori infection.

MicroRNA-455 inhibits proliferation and invasion of colorectal cancer by targeting RAF proto-oncogene serine/threonine-protein kinase

Colorectal cancer (CRC, also known as colon cancer, rectal cancer, or bowel cancer) is the second leading cause of cancer mortality in the Western world. MicroRNAs (miRNAs) are a class of small (18-25 nucleotides long) noncoding RNAs with important posttranscriptional regulatory functions. miRNAs play important roles in various physiological and pathological processes including carcinogenesis in various solid cancers including CRC. In order to investigate the roles that miRNAs played in CRC, the expression of human miRNAs (in 20 normal adjacent tissue samples and 20 colon cancer samples) was examined in this study. miR-455, miR-484, and miR-101 were significantly downregulated in colon cancer samples. And overexpression of miR-455 significantly inhibited the proliferation and the invasion of SW480, but had no effect on apoptosis. PCR and Western blot showed that overexpression of miR-455 decreased protein expression of RAF proto-oncogene serine/threonine-protein kinase (RAF1) but had no effect on mRNA level. Luciferase assay indicated that miR-455 regulated RAF1 expression directly. Moreover, overexpression of RAF1 partially reversed the inhibitory effect of miR-455 on the growth and the invasion of SW480. The data indicated that miR-455 regulates the proliferation and invasion of colorectal cancer cells, at least in part, by downregulating RAF1, a direct target of miR-455. Collectively, our study demonstrated that miR-455-RAF1 may represent a new potential therapeutic target for colorectal carcinoma treatment.

Cellular mechanisms of the 5-HT7 receptor-mediated signaling

Serotonin (5-hydroxytryptamine or 5-HT) is an important neurotransmitter regulating a wide range of physiological and pathological functions via activation of heterogeneously expressed 5-HT receptors. The 5-HT₇ receptor is one of the most recently described members of the 5-HT receptor family. Functionally, 5-HT₇ receptor is associated with a number of physiological and pathological responses, including serotonin-induced phase shifting of the circadian rhythm, control of memory as well as locomotor and exploratory activity. A large body of evidence indicates involvement of the 5-HT₇ receptor in anxiety and depression, and recent studies suggest that 5-HT₇ receptor can be highly relevant for the treatment of major depressive disorders. The 5-HT₇ receptor is coupled to the stimulatory G_s-protein, and receptor stimulation results in activation of adenylyl cyclase (AC) leading to a rise of cAMP concentration. In addition, this receptor is coupled to the G₁₂-protein to activate small GTPases of the Rho family. This review focuses on molecular mechanisms responsible for the 5-HT₇ receptor-mediated signaling. We provide detailed overview of signaling cascades controlled and regulated by the 5-HT₇ receptor and discuss the functional impact of 5-HT₇ receptor for the regulation of different cellular and subcellular processes.

B-cell tolerance to the B-cell receptor variable regions

An enormous number of B cells with different B-cell receptors (BCRs) are continuously produced in the bone marrow. BCRs are further diversified during the germinal center reaction. Due to extensive recirculation, B cells with mutually binding BCR are likely to meet in lymphoid organs. We have addressed possible outcomes of such an encounter in vitro. B lymphoma cells were transfected with complementary BCR, one transfectant expressing an Idiotype⁺ (Id⁺) BCR and the other an anti-Id BCR. To exclude confounding effects of secreted Ig, the transfected B lymphoma cells only expressed membrane IgD. Coincubation of paired Id⁺/anti-Id lymphoma cells results in conjugate formation, signaling, activation of Caspase 3/7, and apoptosis of at least one of the two cells in the pair. Our data provide suggestive evidence for a mechanism whereby the B-cell compartment is partly purged of B cells with complementary BCRs.

Noonan syndrome associated with both a new Jnk-activating familial SOS1 and a de novo RAF1 mutations

Noonan syndrome is a genetic condition characterized by congenital heart defects, short stature, and characteristic facial features. Familial or de novo mutations in PTPN11, RAF1, SOS1, KRAS, and NRAS are responsible for 60-75% of the cases, thus, additional genes are expected to be involved in the pathogenesis. In addition, the genotype-phenotype correlation has been hindered by the highly variable expressivity of the disease. For all these reasons, expanding the genotyped and clinically evaluated case numbers will benefit the clinical community. A mutation analysis has been performed on RAF1, SOS1, and GRB2, in 24 patients previously found to be negative for PTPN11 and KRAS mutations. We identified four mutations in SOS1 and one in RAF1, while no GRB2 variants have been found. Interestingly, the RAF1 mutation was present in a patient also carrying a newly identified p.R497Q familial SOS1 mutation, segregating with a typical Noonan Syndrome SOS1 cutaneous phenotype. Functional analysis demonstrated that the R497Q SOS1 mutation leads to Jnk activation, but has no effect on the Ras effector Erk1. We propose that this variant might contribute to the onset of the peculiar ectodermal traits displayed by the propositus amidst the more classical Noonan syndrome presentation. To our knowledge, this is the first reported case of a patient harboring mutations in two genes, with an involvement of both Ras and Rac1 pathways, indicating that SOS1 may have a role of modifier gene that might contribute the variable expressivity of the disease, evidencing a genotype-phenotype correlation in the family.

Oridonin-Induced A431 Cell Apoptosis Partially Through Blockage of the Ras/Raf/ERK Signal Pathway

We have reported that oridonin, a diterpenoid isolated from the plant Rabdosia rubescens, had apoptosis-inducing activities in many cell lines (e.g., human melanoma A375-S2, human cervical cancer HeLa, human breast adenocarcinoma MCF-7, and murine fibrosarcoma L929). In this study, we further investigated signaling events involved in oridonin-induced apoptosis in human epidermoid carcinoma A431 cells. It was found that the total tyrosine kinase activity was inhibited and the protein expressions of epidermal growth factor receptor (EGFR) and phosphorylated EGFR were decreased in oridonin-induced A431 cell apoptosis. Expression of EGFR downstream effector proteins, Grb2, Ras, Raf-1, and extracellular signal-regulated kinase (ERK), was also downregulated by oridonin. Moreover, the oridonin-induced apoptosis was augmented by the Ras inhibitor manumycin A, Raf-1 inhibitor GW5074, or ERK inhibitor PD98059, suggesting that inactivation of Ras, Raf, or ERK participates in oridonin-induced apoptosis. Taken together, oridonin-induced apoptosis in A431 cells might be through blocking EGFR and its downstream Ras/Raf/ERK signal pathway.

Effects of Raf-1 siRNA on human cerebral microvascular endothelial cells: A potential therapeutic strategy for inhibition of tumor angiogenesis

The serine/threonine kinase Raf-1 is involved in the regulation of tumor cell survival, proliferation and metastasis formation, and has therefore emerged as a promising target for cancer therapy. In addition, Raf-1 activity mediates proliferation of endothelial cells thereby promoting angiogenesis and invasive growth of various tumors, including highly vascularized malignant glioblastoma. The aim of this study was to evaluate the effects of small inhibitory RNA (siRNA) directed against Raf-1 on viability, proliferation and motility in glioma cells and cerebral endothelial cells. Half-quantitative RT-PCR and Western blotting revealed efficient siRNA-mediated Raf-1 down regulation in glioma cells (U373, U251) and in human cerebral microvascular endothelial cells (HCMEC). Surprisingly, Raf-1 gene silencing failed to affect cell survival, proliferation or migration activity in the glioblastoma cell lines. In HCMEC, however, pronounced decrease of cell survival and significant inhibition of tube formation was achieved by Raf-1 siRNA compared to non-functional siRNA or vehicle controls. In conclusion, Raf-1 silencing appears as a potential therapeutic strategy to inhibit brain tumor angiogenesis and thereby outgrowth of highly vascularized glioblastoma multiforme, whereas direct cytotoxic effects of Raf-1 knockdown in tumor cells may vary.

Choline phosphate potentiates sphingosine-1-phosphate-induced Raf-1 kinase activation dependent of Ras--phosphatidylinositol-3-kinase pathway

In NIH3T3 cells, sphingosine-1-phosphate (S1P) caused a significant increase of Raf-1 kinase activity as early as 2 min. Interestingly, choline phosphate (ChoP) produced synergistic increase of S1P-stimulated Raf-1 kinase activation in the presence of ATP while showing additive effect in the absence of ATP. However, Raf-1 kinase activation induced by S1P decreased in the presence of ATP when applied alone. The overexpression of N-terminal fragment of Raf-1 (RfI) to inhibit Raf--Ras interaction caused the inhibition of S1P-induced Raf-1 kinase activation. Also, wortmannin, phosphatidylinositol-3-kinase (PI3K) inhibitor, exhibited inhibitory effects on S1P-induced activation of Raf-1 kinase. In addition, we demonstrated that the chemical antioxidant, N-acetylcysteine attenuated Raf-1 activation induced by S1P, suggesting that H(2)O(2) may be required for the signalling pathway leading to Raf-1 activation. This H(2)O(2)-induced Raf-1 kinase activation was also blocked by inhibition of Ras--PI3K signalling pathway using alpha-hydroxyfarnesylphosphonic acid and wortmannin. Taken together, these results indicate that S1P-induced Raf-1 kinase activation is mediated by H(2)O(2) stimulation of Ras--PI3K pathway, and is enhanced by ChoP in the presence of ATP.

17-beta estradiol elicits an autocrine leiomyoma cell proliferation: evidence for a stimulation of protein kinase-dependent pathway

The mechanism by which estradiol (E2) acts on cell proliferation is still unclear. In this paper, we report the results of a series of experiments in an attempt to elucidate the effector pathway(s) involved in coupling the E2 receptors binding to cellular growth response in leiomyoma cells (LSMC). Under conditions of E2-dependent growth, E2 treatment of LSMC triggers rapid and transient activation of the MAP-kinase pathway. Interestingly, we demonstrate that the early downstream signal transduction events determined by E2-stimulation in quiescent LSMC, including the rapid protein tyrosine phosphorylation of a subset of intracellular proteins, such GAP, PI-3-K, and PLCgamma, and the concomitant activation of ancillary protein kinases, are related to E2-induced PDGF secretion. Moreover, we identify the PDGF, alone or in association with other growth factors, as the main growth factor involved in the proliferation response of LSMC to E2 stimulation. The addition of neutralizing antibodies anti-PDGF was able to inhibit the mitogenic activity present in LSMC conditioned media samples. On the other hand, E2 did not affect the constitutive expression as well as the ligand affinity of PDGF receptors on LSMC plasmamembrane. Cell treatment with the antiestrogen ICI 182780 correlate both with a perturbation of E2-induced transductional circuit and with the disappearance of the mitogenic factor, PDGF, in LSMC conditioned media; the latter therefore, represents the main autocrine mediator of cell growth modulation, upregulated by E2 and down-regulated by antiestrogenic compound. Our experiments suggest that growth factor secretion is an initial and integral part of the signaling events mediated by the estradiol receptors, not related, at least in part, to E2 transcriptional modulation.

Biochemical characterization and localization of the dual specificity kinase CLK1

CLK1 was one of the first identified dual specificity kinases and is the founding member of the ‘LAMMER’ family of kinases. We have established the substrate site specificity of CLK1. We report here that truncation of the N terminus of CLK1 resulted in a dramatic increase in CLK1 enzymatic activity, indicating that the N terminus acts as a negative regulatory domain. The N-terminal truncation resulted in a 45-fold increase in V(max), suggesting that this domain does not contain a pseudo-substrate motif, but may act to conformationally constrain the catalytic activity of CLK1. Tyrosine phosphorylation has been proposed to be critical for CLK1 activity, however, CLK1 activity was unaffected by exposure to tyrosine phosphatases. Treatment of CLK1 with the serine/threonine specific phosphatase PP2A, resulted in a 2- to 6-fold increase in enzymatic activity. Incubation of CLK1 with tyrosine phosphatases in combination with PP2A abolished CLK1 activity. These data suggest that CLK1 is regulated by three distinct mechanisms that serve to both positively and negatively regulate CLK1 activity. CLK1 activity is positively regulated by phosphorylation on either tyrosine residues or serine/threonine residues, and is negatively regulated by steric constraints mediated by the N-terminal domain, as well as, by phosphorylation on a subset of serine/threonine residues within the catalytic domain. CLK1 mRNA is expressed at low levels in all tissues and cell lines examined. The full-length and truncated splice forms are expressed at roughly equivalent levels in most tissues. The ratio of the two splice variants of CLK1 can be altered by treatment with cycloheximide. CLK1 protein expression is limited to a small subset of highly localized neuronal populations in the rat brain. Contrary to previous studies using overexpression systems, we show that CLK1 protein is primarily found in the cytoplasm of these cells, with only a small fraction localized to the nucleus.

Nerve growth factor-stimulated B-Raf catalytic activity is refractory to inhibition by cAMP-dependent protein kinase

The cAMP-dependent protein kinase (PKA) exhibits both inhibitory and stimulatory effects upon growth factor signaling mediated by the mitogen-activated protein kinase signaling pathway. PKA has been demonstrated to inhibit Raf-1-mediated cellular proliferation. PKA can both prevent Ras-dependent Raf-1 activation and directly inhibit Raf-1 catalytic activity. In contrast to the inhibitory effect of PKA on Raf-1-dependent processes, PKA potentiates nerve growth factor-stimulated PC12 cell differentiation, a B-Raf mediated process. This potentiation, rather than inhibition, of PC12 cell differentiation is curious in light of the ability of PKA to inhibit Raf-1 catalytic activity. The kinase domains of Raf-1 and B-Raf are highly conserved, and it has been predicted that B-Raf catalytic activity would also be inhibited by PKA. In this study we examined the ability of PKA to regulate the kinase activity of the B-raf proto-oncogene. We report that nerve growth factor-stimulated B-Raf activity is not inhibited by PKA. By contrast, an N-terminally truncated, constitutively active form of B-Raf is inhibited by PKA both in vitro and in transfected PC12 cells. These results suggest that the N-terminal regulatory domain interferes with the ability of PKA to modulate B-Raf catalytic activity and provide an explanation for the observed resistance of B-Raf-dependent processes to PKA inhibition.

The p21-Ras signal transduction pathway and growth regulation in human high-grade gliomas

Deregulated p21-Ras function, as a result of mutation, overexpression or growth factor-induced overactivation, contributes to at least 30% of human cancer. This article reviews the potential role of the p21-Ras family of GTPases in the regulation of growth of high-grade gliomas and describes how targeting this oncoprotein clinically may provide a novel strategy to counteract glioma proliferation. The application of strategies directed at selectively opposing the deregulated signal transduction pathway of high-grade gliomas may be of potential therapeutic benefit and may offer a whole new arsenal of antineoplastic agents to be included in the multimodal treatment of these challenging neoplasms.

Translocation (3;5)(p26;q13) in a patient with chronic T-cell lymphoproliferative disorder

A 67-year-old patient with large granular lymphocyte (LGL) leukemia is described. At fluorescence-activated cell sorting (FACS) analysis of the peripheral blood, the lymphocytes were positive for CD3, CD4, CD5, CD29, CD45RA, CD57, and TCR alpha/beta and negative for CD7, CD8, CD16, CD56, CD19, CD22, and TCR gamma/delta. Bone marrow histology and immunohistochemistry did not reveal any lymphocyte infiltration. Cytogenetic examination of peripheral blood cultures showed a clone with the karyotype 46,XY,t(3;5)(p26;q13). Molecular analysis revealed rearrangement of the gamma-T-cell-receptor chain. The region 3p25-3p26 which harbors the von Hippel-Lindau tumor suppressor gene and the RAF1 oncogene has been rearranged in a few cases of T-cell leukemia. The translocation in this case has not yet been described and may reflect an alternative mechanism in the pathogenesis of these disorders.

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.

Raf-induced proliferation or cell cycle arrest is determined by the level of Raf activity with arrest mediated by p21Cip1

The Raf family of protein kinases display differences in their abilities to promote the entry of quiescent NIH 3T3 cells into the S phase of the cell cycle. Although conditional activation of deltaA-Raf:ER promoted cell cycle progression, activation of deltaRaf-1:ER and deltaB-Raf:ER elicited a G1 arrest that was not overcome by exogenously added growth factors. Activation of all three deltaRaf:ER kinases led to elevated expression of cyclin D1 and cyclin E and reduced expression of p27Kip1. However, activation of deltaB-Raf:ER and deltaRaf-1:ER induced the expression of p21Cip1, whereas activation of deltaA-Raf:ER did not. A catalytically potentiated form of deltaA-Raf:ER, generated by point mutation, strongly induced p21Cip1 expression and elicited cell cycle arrest similarly to deltaB-Raf:ER and deltaRaf-1:ER. These data suggested that the strength and duration of signaling by Raf kinases might influence the biological outcome of activation of this pathway. By titration of deltaB-Raf:ER activity we demonstrated that low levels of Raf activity led to activation of cyclin D1-cdk4 and cyclin E-cdk2 complexes and to cell cycle progression whereas higher Raf activity elicited cell cycle arrest correlating with p21Cip1 induction and inhibition of cyclin-cdk activity. Using green fluorescent protein-tagged forms of deltaRaf-1:ER in primary mouse embryo fibroblasts (MEFs) we demonstrated that p21Cip1 was induced by Raf in a p53-independent manner, leading to cell cycle arrest. By contrast, activation of Raf in p21Cip1(-/-) MEFs led to a robust mitogenic response that was similar to that observed in response to platelet-derived growth factor. These data indicate that, depending on the level of kinase activity, Raf can elicit either cell cycle progression or cell cycle arrest in mouse fibroblasts. The ability of Raf to elicit cell cycle arrest is strongly associated with its ability to induce the expression of the cyclin-dependent kinase inhibitor p21Cip1 in a manner that bears analogy to alpha-factor arrest in Saccharomyces cerevisiae. These data are consistent with a role for Raf kinases in both proliferation and differentiation of mammalian cells.

A factor with a zinc- and phorbol ester-binding domain is necessary for endosome fusion

An inhibitory effect of several zinc chelators on endosome fusion reconstituted in an in vitro system has been recently reported (A. Aballay et al., 1995, Biochem. J. 312, 919-923). The factor that requires zinc for its activity is still unknown. Since the regulatory domain of protein kinase C (PKC) contains cysteine-rich motifs which coordinate zinc, we suspected that PKC or a PKC-like protein might be that factor. To test this hypothesis, we studied the effect of calphostin C, a specific inhibitor of PKC that interacts with the cysteine-rich motif, and PMA (phorbol 12-myristate 13-acetate), an activator of several PKC isoforms that bind to the same region, on endosome fusion. Calphostin C inhibited endosome fusion in a zinc-regulated manner, whereas PMA enhanced endosome fusion. Moreover, fusion was strongly stimulated when both PMA and zinc were added together to zinc-depleted fusion reactions. Inhibitors of the catalytic domain of PKC had no effect on the assay suggesting that the kinase activity is not required. In contrast, a glutathione S-transferase fusion protein containing a cysteine-rich region of the regulatory domain of PKCgamma inhibited endosome fusion in a PMA-dependent manner. Western blot analysis demonstrated the presence of proteins containing PKC-like cysteine-rich regions that are released from endosomal fractions by zinc chelators. These results indicate that the previously proposed zinc-dependent factor required for endosome fusion could be either a PKC isoform or a protein containing the phorbol ester-binding domain of PKC.

Expression of mixed lineage kinase-1 in pancreatic beta-cell lines at different stages of maturation and during embryonic pancreas development

Events controlling differentiation to insulin-secreting beta-cells in the pancreas are not well understood, although beta-cells are thought to arise from pluripotent ductal precursor cells. To search for signaling proteins that might be involved in beta-cell maturation, we analyzed protein kinase expression in two developmentally and functionally distinct pancreatic beta-cell lines, RIN-5AH and RIN-A12, by reverse transcriptase polymerase chain reaction. A number of tyrosine and serine/threonine kinases were identified in both lines. One protein kinase, mixed lineage kinase-1 (MLK-1), was expressed at both the RNA and protein levels in RIN-5AH cells, which display an immature beta-cell phenotype, but was not detected in the more mature RIN-A12 cells. Furthermore, levels of MLK-1 mRNA and protein were increased after brief stimulation of RIN-5AH cells with either the differentiation inducer, sodium butyrate, or with serum after serum starvation. These increases in expression were independent of phenotypic markers such as insulin secretion or surface expression of major histocompatibility class I- and A2B5-reactive ganglioside. In addition, increases in MLK-1 expression in the stimulated RIN-5AH cells were accompanied by phosphorylation of MLK-1 on serine but not tyrosine. Antisense oligonucleotides to two distinct regions of MLK-1 caused RIN-5AH cells, but not RIN-A12 cells, to adopt a highly undifferentiated morphology, with a reduction in DNA synthesis and MLK-1 protein levels and elevated glucagon mRNA levels, but with no effect on insulin mRNA. In an immunohistochemical survey of embryonic mouse tissues, we found that temporal expression of MLK-1 was regulated in a tissue-specific manner. In the embryonic pancreas, MLK-1 expression was evident in ductal cells from day 13 to 16 but was not detected in late stage gestation or neonatal pancreas. These data suggest that MLK-1 is regulated in immature pancreatic beta-cells and their ductal precursors at the level of functional maturity and may therefore play a role in beta-cell development.

Negative modulation of membrane localization of the Raf-1 protein kinase by hyperphosphorylation

The serine/threonine-specific protein kinase Raf-1 plays a key role in mitogenic signal transduction by coupling Ras to the mitogen-activated protein (MAP) kinase cascade. Ras-mediated translocation to the plasma membrane represents a crucial step in the process of serum-stimulated Raf-1 kinase activation. The exact role of the multisite phosphorylation in Raf regulation, however, is not clear. We have previously reported that the mobility shift-associated hyperphosphorylation of Raf correlates with a reduction of serum-stimulated Raf kinase activity (Wartmann, M., and Davis, R. J. (1994) J. Biol. Chem. 269, 6695-6701). Here we show that incubation of serum-starved CHO cells with D609, a purported inhibitor of phosphatidylcholine-specific phospholipase C, also results in a mobility shift of Raf-1 that is due to hyperphosphorylation on sites identical to those observed following mitogen stimulation. Subcellular fractionation analyses revealed that D609-induced mobility shift-associated hyperphosphorylation was paralleled by a decreased membrane association of Raf-1. Similar results were obtained in an in vitro reconstitution system. Furthermore, PD98059, a specific inhibitor of activation of the MAP kinase kinase MEK, prevented D609-induced Raf hyperphosphorylation and restored the amount of membrane-bound Raf to control levels. Taken together, these data suggest that mobility shift-associated hyperphosphorylation of Raf-1, by virtue of reducing the amount of plasma membrane-bound Raf-1, represents a negative feedback mechanism contributing to the desensitization of the MAP kinase signaling cascade.

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.

The Ras-Raf pathway is activated in human immunodeficiency virus-infected monocytes and particpates in the activation of NF-kappa B

Persistent human immunodeficiency virus (HIV) infection of human monocytes and macrophages increases I kappa B alpha degradation, resulting in the activation of NF-kappa B, a key transcription factor in the regulation of the HIV long terminal repeat. The signal transduction pathways leading to NF-kappa B activation in cells of the monocytic lineage, especially those regulated by HIV infection, and their relevance in regulating viral persistence remain unknown. Both p21ras and its downstream Raf-1 kinase participate in the transduction of signals initiated from a variety of cell surface receptors and in the regulation of transcription factors. We have studied whether the Ras-Raf pathway is functional and participates in HIV-mediated NF-kappa B activation in monocytic cells. Constitutively active p21ras (v-H-Ras) activated NF- kappa B-dependent transcription and induces the nuclear translocation of a bona fide p65/p50 heterodimer by targeting I kappa B alpha. In addition, the constitutively active form of Raf (RafBXB) also increases the NF-kappa B-dependent transcriptional activity. Because of the similarity between HIV and Ras-Raf-induced NF-kappa B activation in monocytic cells, we next tested whether HIV-induced NF-kappa B activation was mediated by the Ras-Raf signal transduction pathway. Negative dominant forms of both Ras (Ras N17) and Raf (Raf 301) decreased the HIV- but not lipopolysaccharide-dependent NF-kappa B activation in U937 cells. Moreover, Raf-1 kinase activity was greater in HIV-infected than uninfected monocytic cells in in vitro kinase assays. Altogether, these results indicate that the Ras-Raf pathway is unregulated in HIV monocytic cells and participates in the virus-induced activation of NF-kappa B.

Thrombin receptor activation elicits rapid protein tyrosine phosphorylation and stimulation of the raf-1/MAP kinase pathway preceding delayed mitogenesis in cultured rat aortic smooth muscle cells: evidence for an obligate autocrine mechanism promoting cell proliferation induced by G-protein-coupled receptor agonist

Treatment of quiescent rat aortic smooth muscle cells with either alpha-thrombin or a thrombin receptor-derived agonist peptide (SFLLRNP) resulted in pronounced increases in [3H]thymidine incorporation that were concentration dependent and reached a maximum of approximately 15-fold above serum-starved controls. However, in contrast to FBS, PDGF-BB, or basic fibroblast growth factor (bFGF), that initiated DNA synthesis promptly after 16-19 h, thymidine incorporation in response to thrombin was delayed by an additional 3-6 h. Delayed mitogenesis correlated with the appearance of a potent mitogenic activity in conditioned media samples obtained from thrombin-stimulated rat aortic smooth muscle cells, as assayed using Swiss 3T3 fibroblasts. This activity was not inhibited by neutralizing antibodies directed against PDGF or bFGF. Furthermore, in the Swiss 3T3 cells, simple addition of either alpha-thrombin or SFLLRNP failed to elicit a significant mitogenic response. In signal transduction studies, both thrombin and SFLLRNP treatment led to rapid tyrosine phosphorylation of proteins with apparent molecular masses of 42, 44, 75, 120, and 190 kD, respectively, as assessed by antiphosphotyrosine immunoblotting. The overall pattern of protein tyrosine phosphorylation was distinct from that observed after PDGF-BB addition. Activation of Raf-1 and the mitogen-activated protein (MAP) kinases p44mapk and p42mapk was also observed. However, the time course and duration of Raf-1/MAP kinase activation after thrombin stimulation were similar to those elicited by PDGF-BB. Taken together, our results indicate that thrombin-stimulated vascular smooth muscle proliferation is delayed and requires the de novo expression of one or more autocrine mitogens. In addition, the rapid induction of discrete intracellular signaling mechanisms by thrombin, including the Raf-1/MAP kinase pathway, appears to be insufficient alone to promote vascular smooth muscle cell mitogenesis.

Regulation of Raf-1-dependent signaling during early Xenopus development

The Raf-1 gene product is activated in response to cellular stimulation by a variety of growth factors and hormones. Raf-1 activity has been implicated in both cellular differentiation and proliferation. We have examined the regulation of the Raf-1/MEK/MAP kinase (MAPK) pathway during embryonic development in the frog Xenopus laevis. We report that Raf-1, MEK, and MAPK activities are turned off following fertilization and remain undetectable up until blastula stages (stage 8), some 4 h later. Tight regulation of the Raf-1/MEK/MAPK pathway following fertilization is crucial for embryonic cell cycle progression. Inappropriate reactivation of MAPK activity by microinjection of oncogenic Raf-1 RNA results in metaphase cell cycle arrest and, consequently, embryonic lethality. Our findings demonstrate an absolute requirement, in vivo, for inactivation of the MAPK signaling pathway to allow normal cell cycle progression during the period of synchronous cell divisions which occur following fertilization. Further, we show that cytostatic factor effects are mediated through MEK and MAPK.

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.

The mouse B-raf gene encodes multiple protein isoforms with tissue-specific expression

The c-Rmil/B-raf proto-oncogene is a member of the mil/raf family encoding serine/threonine protein kinases shown to be involved in signal transduction from the membrane to the nucleus. We isolated from a mouse brain library B-raf cDNAs containing a previously unidentified 36-base pair alternatively spliced exon located between exons 8 and 9 and, therefore, designated exon 8b. Human and mouse B-raf mRNAs also contain the 120-base pair alternatively spliced exon 10 previously described in the avian c-Rmil gene. Independent splicing of these two exons, located between the conserved region 2 (CR2) and the catalytic domain (CR3) gives rise to mRNAs potentially encoding four distinct proteins. By using specific sera generated against different portions of B-Raf, we identified at least 10 protein isoforms in adult mouse tissues. Some isoforms, in the range of 69-72 kDa, are not recognized by antisera directed against peptides encoded by exons 1 and 2, indicating the existence of B-Raf proteins with two different NH2 extremities. The other isoforms, in the range of 79-99 kDa, contain the amino acids encoded by exons 1 and 2, by either or both of the alternatively spliced exons, and, possibly, by another of the unidentified exon. Analysis of B-raf mRNA expression by reverse transcriptase-polymerase chain reaction and immunocharacterization of B-Raf proteins in different tissues of the adult mouse showed a tissue-specific pattern of B-Raf isoforms expression. Interestingly, isoforms containing amino acids encoded by exon 10 are specifically expressed in neural tissues. Taken together, these results suggest that distinct B-Raf proteins could be involved, in a tissue-specific manner, in signal transduction pathways.

PkpA, a novel Phycomyces blakesleeanus serine/threonine protein kinase

This work reports the cloning and sequencing of pkpA, a gene of the filamentous fungus Phycomyces blakesleeanus, whose expression seems to be coupled to vegetative growth. This gene encodes a putative serine/threonine-specific protein kinase, whose sequence is related to that of the yeast protein STE20, involved in pheromone-response pathways, and to a number of MAPK kinase proteins. However, detailed analysis of the kinase sequence suggests that PkpA is a novel serine/threonine protein kinase that probably participates as an intermediate in an intracellular system controlling nuclear proliferation in P. blakesleeanus.

Signal transduction by cell adhesion receptors

Over the last few years, it has become clear that cell adhesion receptors function in signal transduction processes leading to the regulation of cell growth and differentiation. Signal transduction by both integrins and CAMs has been shown to involve activation of tyrosine kinases, while CAM signaling in neural cells involves G proteins as well. In the case of integrins, some of the downstream signaling events intersect with the Ras pathway, particularly the activation of MAP kinases. In fibroblasts, integrin mediated anchorage to the substratum regulates cell cycle traverse, while in epithelial cells, loss of anchorage can trigger programmed cell death. In many cell types, but particularly monocytic cells, integrin ligation has a profound impact on gene expression. Preliminary evidence also implicates CAMs and selectins in gene regulation. A consistent theme in signal transduction mediated by adhesion receptors concerns the role of the cytoskeleton. Integrin mediated signaling processes are interrupted by cytoskeletal disassembly. Identification of the APC and neurofibromatosis type 2 tumor suppressors suggest that cytoskeletal complexes also play a key role in signaling by cadherins and CD44, respectively. Thus, signaling by cell adhesion receptors may involve aspects that impinge on previously known signaling pathways including the RTK/Ras pathway and serpentine receptor/G protein pathways. However, novel aspects of signal transduction involving cytoskeletal assemblies may also be critical.

Developmental regulation of expression and activity of multiple forms of the Drosophila RAC protein kinase

We have characterized the Drosophila homologue of the proto-oncogenic RAC protein kinase (DRAC-PK). The DRAC-PK gene gives rise to two transcripts with the same coding potential, generated by the use of two different polyadenylation signals. Each transcript encodes two polypeptides because of the presence of a weaker initiator ACG codon, upstream from the major AUG, such that the larger protein contains an N-terminal extension. Like the human isoforms, DRAC-PKs possess a novel signaling region, the pleckstrin homology domain. DRAC-PK proteins have a similar expression pattern, being regulated both maternally and zygotically, and are expressed throughout Drosophila development. Antisera specific for recombinant DRAC-PK and for its C terminus detected two polypeptides of 66 and 85 kDa in Drosophila extracts. The antirecombinant antisera also recognized a polypeptide of 120 kDa from Drosophila, which apparently shared an epitope related to DRAC-PK sequences. The role of p120 appears to be restricted compared with that of DRAC-PK, since it was not detected in larvae or adult flies. There was no spatial restriction of DRAC-PK expression during embryogenesis, suggesting that localized activation might be a regulatory mechanism for its function. DRAC-PK possesses an intrinsic kinase activity that is approximately 8-fold higher in adult flies than in 0-3-h embryos undergoing rapid mitotic cycles.

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.

Expression cloning of oncogenes by retroviral transfer of cDNA libraries

a cDNA library transfer system based on retroviral vectors has been developed for expression cloning in mammalian cells. The use of retroviral vectors results in stable cDNA transfer efficiencies which are at least 100-fold higher than those achieved by transfection and therefore enables the transfer and functional screening of very large libraries. In our initial application of retroviral transfer of cDNA libraries, we have selected for cDNAs which induce oncogenic transformation of NIH 3T3 fibroblasts, as measured by loss of contact inhibition of proliferation. Nineteen different transforming cDNAs were isolated from a total of 300,000 transferred cDNA clones. Three of these cDNAs were derived from known oncogenes (raf-1, lck, and ect2), while nine others were derived from genes which had been cloned previously but were not known to have the ability to transform fibroblasts (beta-catenin, thrombin receptor, phospholipase C-gamma 2 and Spi-2 protease inhibitor genes). The Spi-2 cDNA was expressed in antisense orientation and therefore is likely to act as an inhibitor of an endogenous transformation suppressor. Seven novel cDNAs with transforming activities, including those for three new members of the CDC24 family of guanine nucleotide exchange factors, were also cloned from the retroviral cDNA libraries. Retroviral transfer of libraries should be generally useful for cloning cDNAs which confer selectable phenotypes on many different types of mammalian cells.

Characterization of a protein which binds phosphatidylinositol 3,4,5-trisphosphate and 4,5-bisphosphate

Protein(s) which bind polyphosphatidylinositol phosphates (PI 3,4,5-P3 and PI 4,5-P2) were identified in the wheat-germ agglutinin bound fraction of cells and tissues. The binding of this protein(s) to the phospholipid could be demonstrated in two ways, either by a shift in the migration of the lipid by size exclusion column chromatography or directly by binding to the protein after capture on wheat-germ agglutinin-coupled beads. Of the rat tissues tested (muscle, spleen, brain, heart, kidney and liver), the activity was highest in liver. The protein(s) was purified more than 5000-fold by sequential chromatography on columns of wheat-germ agglutinin, phosphocellulose, Blue-Sepharose, Mono Q and Superose 6. The peak of activity appeared to have a molecular weight on this latter column of approx. 240,000. The protein(s) bound PI 3,4,5-P3, PI 3,4-P2, and PI 3-P in the ratio of 4:2:1. The binding of 3-phosphorylated PI phosphates to the protein(s) was not significantly inhibited by 36 micrograms/ml of either phosphatidylinositol or phosphatidylcholine, but was inhibited 10% and 65% by 36 micrograms/ml of PI 4-P and PI 4,5-P2, respectively. Since these results suggested that the binding protein(s) could also bind PI 4,5-P2, binding of this lipid was directly tested and found to be comparable to that of PI 3,4,5-P3. These results suggest that this protein(s) could be involved in the signaling mechanism elicited by these polyphosphoinositides.

RAS and RAF-1 form a signalling complex with MEK-1 but not MEK-2

Recent studies have demonstrated the existence of a physical complex containing p21ras (RAS), p74raf-1 (RAF-1), and MEK-1. Although it is clear that formation of this complex depends on the activation state of RAS, it is not known whether this complex is regulated by the activation state of the cell and whether MEK-2 is also present in the complex. To analyze the regulation and specificity of this complex, we utilized immobilized RAS to probe lysates of cultured NIH 3T3 fibroblasts and analyzed the proteins complexing with RAS following serum starvation or stimulation. Complex formation among RAS, RAF-1, and MEK-1 was dependent only on RAS:GMP-PNP and not on cell stimulation. Incubations of lysates with immobilized RAS depleted all RAF-1 from the lysate but bound only a small fraction of cytosolic MEK-1, and further MEK-1 could bind immobilized RAS only if exogenous RAF-1 was added to the lysate. This indicates that binding of MEK-1 to RAS depends on the presence of RAF-1 or an equivalent protein. In contrast to MEK-1, MEK-2 was not detected in the RAS signalling complex. A proline-rich region of MEK-1 containing a phosphorylation site appears to be essential for signalling complex formation. Consistent with the preferential binding of MEK-1 to RAS:RAF-1, the basal activity of MEK-1 in v-ras-transformed cells was found to be elevated sixfold, whereas MEK-2 was elevated only twofold, suggesting that the RAS signalling pathway favors MEK-1 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.

RAS and RAF-1 form a signalling complex with MEK-1 but not MEK-2

Recent studies have demonstrated the existence of a physical complex containing p21ras (RAS), p74raf-1 (RAF-1), and MEK-1. Although it is clear that formation of this complex depends on the activation state of RAS, it is not known whether this complex is regulated by the activation state of the cell and whether MEK-2 is also present in the complex. To analyze the regulation and specificity of this complex, we utilized immobilized RAS to probe lysates of cultured NIH 3T3 fibroblasts and analyzed the proteins complexing with RAS following serum starvation or stimulation. Complex formation among RAS, RAF-1, and MEK-1 was dependent only on RAS:GMP-PNP and not on cell stimulation. Incubations of lysates with immobilized RAS depleted all RAF-1 from the lysate but bound only a small fraction of cytosolic MEK-1, and further MEK-1 could bind immobilized RAS only if exogenous RAF-1 was added to the lysate. This indicates that binding of MEK-1 to RAS depends on the presence of RAF-1 or an equivalent protein. In contrast to MEK-1, MEK-2 was not detected in the RAS signalling complex. A proline-rich region of MEK-1 containing a phosphorylation site appears to be essential for signalling complex formation. Consistent with the preferential binding of MEK-1 to RAS:RAF-1, the basal activity of MEK-1 in v-ras-transformed cells was found to be elevated sixfold, whereas MEK-2 was elevated only twofold, suggesting that the RAS signalling pathway favors MEK-1 activation.

Mechanism of inhibition of Raf-1 by protein kinase A

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

Mechanism of inhibition of Raf-1 by protein kinase A

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

Signal transduction and cell fate specification during Caenorhabditis elegans vulval development

A receptor tyrosine kinase/Ras signaling pathway controls the specification of vulval cell fates in Caenorhabditis elegans. Recently, C. elegans genes encoding proteins with similarity to mammalian Raf (lin-45), mitogen-activated protein kinase (mpk-1/sur-1), and an HNF-3 transcription factor (lin-31) have been identified and shown to act downstream of let-60 (ras) in this pathway. These genetically identified gene products bridge the gap between signal transduction at the plasma membrane and the control of cell fate specification in the nucleus.