v-Raf/v-Myc synergism in abrogation of IL-3 dependence: v-Raf suppresses apoptosis.
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Animals↗
Biomedical subjects
U R Rapp
Publications and source records attributed to U R Rapp.
The role of Raf-1 phosphorylation in signal transduction.
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Amino Acid Sequence↗
Serum-, TPA-, and Ras-induced expression from Ap-1/Ets-driven promoters requires Raf-1 kinase.
Raf-1 serine-threonine protein kinase has the hallmarks of a critical switch that connects growth factor receptor activation at the cell membrane with transcriptional events in the nucleus. We show by use of Raf-1 dominant-negative mutants that Raf-1 is required for serum-, TPA-, and Ras-induced expression from the oncogene-responsive element in the polyomavirus enhancer. The minimal region of Raf-1 that displays this dominant-negative phenotype (Raf-C4) contains a cysteine finger motif. Raf-C4 appears to function by titrating out a Raf-1-activating factor that is induced by Ras following serum or TPA treatment of NIH-3T3 cells. In addition, we show that Raf-1 and Ras cooperate in trans-activation through the oncogene-responsive element and that the cysteine-rich region is necessary for this effect.
3T3 Cells↗
Oncoprotein-mediated signalling cascade stimulates c-Jun activity by phosphorylation of serines 63 and 73.
In resting cells, c-Jun is phosphorylated on five sites. Three of these sites reside next to its DNA binding domain and negatively regulate DNA binding. In response to expression of oncogenic Ha-Ras, phosphorylation of these sites decreases, while phosphorylation of two other sites within c-Jun's activation domain is greatly enhanced. Phosphorylation of these residues, serines 63 and 73, stimulates the transactivation function of c-Jun and is required for oncogenic cooperation with Ha-Ras. We now show that the same changes in c-Jun phosphorylation are elicited by a variety of transforming oncoproteins with distinct biochemical activities. These oncoproteins, v-Sis, v-Src, Ha-Ras, and Raf-1, participate in a signal transduction pathway that leads to increased phosphorylation of serines 63 and 73 on c-Jun. While oncogenic Ha-Ras is a constitutive stimulator of c-Jun activity and phosphorylation, the normal c-Ha-Ras protein is a serum-dependent modulator of c-Jun's activity. c-Jun is therefore a downstream target for a phosphorylation cascade involved in cell proliferation and transformation.
3T3 Cells↗
95-kilodalton B-Raf serine/threonine kinase: identification of the protein and its major autophosphorylation site.
B-Raf, a member of the Raf family of serine/threonine kinases, is expressed primarily in the brain and in the nervous system. In this study, the biochemical properties of the B-Raf protein were investigated in nerve growth factor (NGF)-responsive cell lines and in brain tissues. B-Raf was identified by using phosphopeptide mapping analysis and cDNA analysis as a 95-kDa protein which is primarily localized in the cytosol. NGF rapidly stimulated both serine and threonine phosphorylation in vivo and autophosphorylation activity in vitro of the B-Raf protein. In PC12 cells, B-Raf autokinase activity was induced by both differentiation factors and mitogens, with maximal activity observed after 5 min of factor addition. B-Raf kinase activity was also observed following NGF treatment of SH-SY5Y neuroblastoma cells and in adult mouse brain and hippocampus. Induction of B-Raf kinase activity in NGF-treated PC12 cells required expression of kinase-active trk receptors. Exogenous substrates or a peptide containing the autophosphorylation site became phosphorylated when added to immune complex kinase assays and reduced the in vitro autophosphorylation activity of B-Raf, suggesting that in vitro autophosphorylation sites and exogenous substrates compete for active sites of the B-Raf kinase. Finally, the major in vitro autophosphorylation site of B-Raf was identified as threonine 372 in the conserved region 2 domain. A threonine residue is present at similar positions in all three mammalian Raf family members and may represent a regulatory site for these proteins.
Amino Acid Sequence↗
Ras controls coupling of growth factor receptors and protein kinase C in the membrane to Raf-1 and B-Raf protein serine kinases in the cytosol.
A dominant negative mutant of Ras, M17 Ras, was used to study the role of Ras in receptor coupling of Raf-1 and B-Raf protein serine/threonine kinases (PSKs). We found that mutant Ras blocks serum- and 12-O-tetradecanoyl phorbol 13-acetate-induced activation of Raf-1 kinase in NIH3T3 cells and Raf-1 as well as B-Raf PSK stimulation by nerve growth factor (NGF) in PC12 pheochromocytoma cells. Mitogen stimulation of Raf kinase was measured by determination of Raf hyperphosphorylation and activity towards exogenous substrates and both of these events were inhibited in cells expressing M17 Ras. In contrast, tyrosine phosphorylation of a direct substrate of activated tyrosine kinase receptors, phospholipase C-gamma 1 (PLC-gamma 1), was unaffected. These data indicate that tyrosine phosphorylation of PLC-gamma 1 is not sufficient for growth induction in NIH3T3 cells and that Ras mediates signal transfer from activated membrane receptors to Raf kinases in the cytosol. As activated Raf induced differentiation in PC12 cells expressing M17 Ras we conclude that Raf kinase activation may be sufficient to account for this aspect of NGF function.
3T3 Cells↗
B-raf and a B-raf pseudogene are located on 7q in man.
The B-raf gene was localized to human chromosome region 7p11-7qter by Southern blot analysis of its segregation in a panel of rodent-human hybrids, using a B-raf cDNA clone. Chromosomal in situ hybridization refined the localization to 7q33-36. Analysis of genomic B-raf clones identified a B-raf pseudogene in addition to the active gene. Based on the chromosomal mapping data we conclude that the pseudogene is located near the active gene.
Base Sequence↗
The phosphorylation and activation of B-raf in PC12 cells stimulated by nerve growth factor.
Treatment of PC12 cells with nerve growth factor does not alter the levels of B-raf mRNA, but does induce rapid phosphorylation of B-raf proteins. Phosphorylation was observed after 1.5 min and reached a maximum by 10-15 min. B-raf protein was phosphorylated almost exclusively on serine residues; no tyrosine phosphorylation was detected. Nerve growth factor-induced phosphorylation was not affected by depletion of protein kinase C or by removal of extracellular calcium but was inhibited by K-252a. Concomitant with the increase in serine phosphorylation, nerve growth factor treatment also increased the serine/threonine kinase activity of B-raf protein within 1-2 min.
Amino Acid Sequence↗
Retinoic acid and transforming growth factor beta differentially inhibit platelet-derived-growth-factor-induced Ito-cell activation.
Sinusoidal Ito cells (stellate or fat-storing cells) undergo excessive cellular proliferation before the establishment and progression of hepatic fibrosis and cirrhosis. Retinoic acid and transforming growth factor beta (TGF beta) both inhibit Ito-cell [3H]thymidine incorporation in serum-containing media. Serum-induced mitogenicity was dependent on platelet-derived growth factor (PDGF). Additionally, pre-treatment of Ito cells with retinoic acid and TGF beta blocked PDGF-induced cell proliferation. TGF beta, but not retinoic acid, diminished PDGF-receptor and smooth-muscle alpha-actin abundance.
Actins↗
Erythropoietin induces Raf-1 activation and Raf-1 is required for erythropoietin-mediated proliferation.
Erythropoietin mediates the rapid phosphorylation of Raf-1 in the murine cell lines HCD-57 and FDC-P1/ER, which proliferate in response to this cytokine. Phosphorylation occurs at both serine and tyrosine residues and as such is similar to the Raf-1 phosphorylation seen after interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor, and interleukin-2 stimulation in other murine cell lines. Such data suggest that these growth factors may share a common mechanism(s) of Raf-1 phosphorylation. Furthermore, in association with Raf-1 phosphorylation, erythropoietin induces a 2-3-fold increase in Raf-1 kinase activity as measured in immune complex kinase assays in vitro. Finally, a c-raf antisense oligodeoxyribonucleotide, which specifically decreases intracellular Raf-1 levels, also substantially inhibits both erythropoietin and IL-3-directed DNA synthesis. Together, these results provide evidence that activated Raf-1 is a necessary component of erythropoietin and IL-3 growth signaling pathways.
Animals↗
Evidence for one or more Raf-1 kinase kinase(s) activated by insulin and polypeptide growth factors.
The protein product of the Raf-1 proto-oncogene is a protein serine/threonine kinase that is activated after stimulation of cells with insulin and other mitogens. To investigate the mechanism of this activation, we used purified Raf-1 expressed in E. coli as a substrate for a putative Raf-1 protein kinase kinase. In three different insulin-sensitive cell types, insulin activated Raf-1 kinase kinase activity in crude cytosolic cellular fractions. The insulin stimulation of this activity was evident as early as 2 min after exposure to insulin, maximal at 5-8 min, and inapparent at 15 min. Phosphoamino acid analysis of phosphorylated Raf-1 revealed that serine was the primary phosphate acceptor for the insulin-activated kinase or kinases; small amounts of phosphothreonine were also detected. The insulin effect occurred in cells depleted of protein kinase C, and in extracts depleted of endogenous Raf-1 kinase by immunodepletion; these data argue against protein kinase C or Raf-1 kinase itself being the insulin-stimulated activity. The insulin-activated kinase or kinases phosphorylated the Raf-1 protein on multiple sites in vitro, as evidenced by tryptic mapping; at least some of these appeared to overlap with sites phosphorylated in response to serum in intact cells. Several other mitogens and growth factors stimulated Raf-1 kinase kinase activity, including epidermal growth factor, platelet-derived growth factor, fibroblast growth factor, serum, and phorbol 12-myristate 13-acetate. This insulin- and mitogen-stimulated Raf-1 kinase kinase activity may play a role in mediating the phosphorylation and possibly the activation of the Raf-1 kinase by insulin and other growth factors.
Animals↗
Phorbol ester and bryostatin differentially regulate the hydrolysis of phosphatidylethanolamine in Ha-ras- and raf-oncogene-transformed NIH 3T3 cells.
Previously it was reported that transformation of NIH 3T3 fibroblast by the Ha-ras, v-src, v-fms, and A-raf oncogenes decreased the stimulatory effects of phorbol 12-myristate 13-acetate (PMA; 'TPA'), an activator of protein kinase C (PKC), on the phosphorylation of an endogenous 80 kDa substrate and on 86Rb uptake [Wolfman, Wingrove, Blackshear & Macara (1987) J. Biol. Chem. 262, 16546-16552], as well as on sphingomyelin synthesis [Kiss, Rapp & Anderson (1988) FEBS Lett. 240, 221-226]. Here, we investigated how transformation affects the PMA-stimulated hydrolysis of phosphatidylethanolamine (PtdEtn), a recently characterized mechanism which may contribute to the generation of the second messengers phosphatidic acid and 1,2-diacylglycerol. The effects of PMA were compared with those of bryostatin, a non-tumour-promoter activator of PKC. Transformation of NIH 3T3 cells with Ha-ras, v-raf, or A-raf enhanced the stimulatory effect of PMA on the phospholipase D-mediated hydrolysis of PtdEtn. On the other hand, the effects of bryostatin on PtdEtn hydrolysis were only slightly increased, if at all, in cells transformed with these oncogenes. In crude membrane preparations isolated from these transformed cells, PMA, but not bryostatin, enhanced the combined stimulatory effects of ATP and the GTP analogue guanosine 5'-[gamma-thio]triphosphate on phospholipase D-mediated PtdEtn hydrolysis. The PKC inhibitor 1-(5-isoquinolinesulphonyl)-2-methylpiperazine inhibited the stimulatory effect of PMA only in intact cells. These results indicate that transformation of cells by certain oncogenes differentially affects phospholipase D-mediated hydrolysis of PtdEtn induced by PMA and bryostatin, suggesting that the action of PMA might involve two different mechanisms.
Adenosine Triphosphate↗
Immunohistochemical localization of raf protein kinase in dendritic spines and spine apparatuses of the rat cerebral cortex.
The ultrastructural localization of raf protein (product of the raf protooncogene) in the neocortex, pyriform cortex and hippocampus of the rat has been investigated by means of pre-embedding immunohistochemistry. Specificity of the antiserum was tested with Western blotting. Besides the immunoreactivity of the dendrites, remarkably strong immunostaining of the dendritic spines and spine apparatuses was noted in each of the investigated areas. The postsynaptic densities were also stained. Since raf proteins are serine/threonine-specific protein kinases, our findings could be important steps toward the understanding of dendritic spine plasticity.
Animals↗
Raf-1 protein kinase is required for growth of induced NIH/3T3 cells.
Many growth factors regulate the cytoplasmic Raf-1 protein kinase, consistent with its having a central role in transduction of growth signals. The kinase is ubiquitously expressed and can promote proliferation, presumably in a manner dependent on growth-factor receptors and membrane-associated oncogenes. We have now examined the dependence of serum- and TPA (12-O-tetradecanoylphorbol-13-acetate)-regulated NIH/3T3 cell growth on RAF-1 kinase to determine whether Raf-1 is essential for receptor signalling. We inhibited Raf-1 function by expressing c-raf-1 antisense RNA or kinase-defective c-raf-1 mutants. Antisense RNA for c-raf-1 interferes with proliferation of normal NIH/3T3 cells and reverts raf-transformed cells. In revertant cells, DNA replication induced by serum or TPA was eliminated or reduced proportionately to the reduction in Raf protein levels. Expression of a kinase-defective Raf-1 mutant (craf301) or a regulatory domain fragment (HCR) inhibited serum-induced NIH/3T3-cell proliferation and raf transformation even more efficiently. Inhibition by antisense RNA or craf301 blocked proliferation and transformation by Ki- and Ha-ras oncogenes. We conclude that raf functions as an essential signal transducer downstream of serum growth factor receptors, protein kinase C and ras.
Animals↗
Cerebral ischemia induces transient intracellular redistribution and intranuclear translocation of the raf proto-oncogene product in hippocampal pyramidal cells.
In this report we describe changes in the intracellular redistribution of raf serine/threonine protein kinase (product of the raf proto-oncogene family) in hippocampal neurons following cerebral ischemia in Mongolian gerbils. For immunohistochemical localization studies polyclonal antisera specific for each of the A, B, and Raf-1 isotypes of raf, as well as a pan-raf antisera, were employed. Of these, only sera recognizing B-raf, as well as the general v-raf (raised against the conserved C-terminal region) were positive, indicating that B-raf is the major isotype in this neuronal region. Three different ischemic models were used (repeated 3 times for two min and single 5 or 15 min occlusions, of the common carotid arteries) to demonstrate that ischemic insult causes redistribution of raf protein kinase into the cell nucleus of hippocampal neurons. Increased amounts of raf protein in the nuclei of pyramidal cells following ischemia was confirmed by Western blot analysis of isolated nuclear fractionations. Moreover, an elevation in the level of nuclear raf protein also was detected in the contralateral (i.e. non-occluded hemisphere) neurons of CA1 and CA3 subfields 4 days after the ischemic insult indicating a possible transsynaptic increase in the amount of raf protein along with redistribution. The intranuclear translocation of the immunoreactive material started from the perinucleolar rim and with time extended throughout the nucleus. Enhanced levels and altered redistribution of the raf polypeptide in the nuclei of pyramidal cells of the CA3 subfield appears to be reversible and returns to the normal level 12 days following the ischemic insult.(ABSTRACT TRUNCATED AT 250 WORDS)
Amino Acid Sequence↗
Induction of raf-1 protein immunoreactivity in guinea pig hippocampal slices during the in vitro maintenance.
The cellular raf-1 protooncogene encodes a cytoplasmic serine/threonine protein kinase; raf-1 is expressed not only in tumour cells, but also in normal brain tissue. However, the substrate of raf-1 kinase is unknown and its function is still hypothetical. In cell cultures, mitogen stimulation may alter its properties, including its kinase activity and subcellular distribution. The aims of the present study were: (1) light- and electron microscopic immunohistochemical localization of raf-1 kinase in hippocampal slices; (2) the follow-up of the possible changes in localization and detectability during the in vitro maintenance and low-frequency stimulation. Our results suggested the induction of neuronal raf-1 kinase during the in vitro maintenance. The possibilities of increased transcription and enzyme activation are discussed.
Animals↗
Mitochondrial protein p26 BCL2 reduces growth factor requirements of NIH3T3 fibroblasts.
The BCL2 (B cell lymphoma/leukemia-2) proto-oncogene encodes a 26-kDa protein that has been localized to the inner mitochondrial membrane and that has been shown to enhance the survival of some types of hematopoietic cells. Here we show that NIH3T3 fibroblasts stably transfected with a BCL2 expression plasmid exhibit reduced dependence on competence-inducing growth factors (platelet-derived growth factor, PDGF; epidermal growth factor, EGF) for initiation of DNA synthesis. The importance of BCL2 for growth factor-induced proliferation of these cells was further confirmed by the useage of BCL2 antisense oligodeoxynucleotides. The mechanisms by which overexpression of p26 BCL2 contributes to fibroblast proliferation are unknown, but do not involve alterations in: (a) the production of inositol triphosphates (IP3), (b) PDGF-induced transient elevations in cytosolic Ca2+ ions, or (c) the activity of protein kinase C enzymes in these transfected cells. The results imply that changes in mitochondrial functions play an important role in the early stages of the cell cycle that render 3T3 cells competent to respond to the serum progression factors that stimulate entry into S-phase.
Calcium↗
The Raf-1 serine-threonine kinase is a substrate for the p56lck protein tyrosine kinase in human T-cells.
The CD4 receptor subserves both adhesion and signal transduction functions on CD4+ T-lymphocytes. CD4 is physically associated with the src-related protein tyrosine kinase p56lck. Cell surface engagement of CD4 leads to enzymatic activation of the associated p56lck and the phosphorylation of T-cell proteins on tyrosine residues. We have identified a 72-74kD protein phosphorylated on tyrosine residues following activation of CD4-associated p56lck as the serine-threonine kinase Raf-1. The demonstration that Raf-1 is a substrate for the CD4/p56lck receptor system in normal cells suggests that receptor and nonreceptor classes of protein tyrosine kinases can independently engage functionally overlapping signal transduction pathways.
CD4-Positive T-Lymphocytes↗