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PubMedDate not supplied

IFN-gamma production from human Th1 cells is controlled by Raf kinase.

Raf kinase is an important intracellular mediator in T cell signalling and may be crucial for the proliferation of this inflammatory cell. In order to elucidate its effect on cytokine production by human T cells in response to T cell receptor activation, experiments were carried out on human T cell clones using antisense (AS) oligodeoxynucleotides (ODN) to inhibit the expression of Raf kinase. AS ODN to Raf were shown to have a significant effect on a human Th1-like T cell clone, inhibiting antiCD3-induced IFN-gamma secretion by 76%, whereas no inhibitory effect was observed on IL-5 or IL-4 production by a Th2-like clone. IL-2 secretion from both clones was also not affected by the Raf AS ODN. In all cases, a reduction in Raf kinase within the cell was demonstrated by Western blot. Our results clearly demonstrate the importance of Raf kinase in the production of IFN-gamma from Th1 cells, but also show the lack of effect of this intracellular mediator on cytokine (IL-5, IL-4) release from Th2 cells.

Humans↗
PubMed1994

The ins and outs of Raf kinases.

Raf kinases are signal-integrating enzymes that have the ability to switch tyrosine kinase signalling to serine/threonine phosphorylation and connect growth factor receptors with transcription factors. The connection involves a cascade of protein kinases that is essential for cellular proliferation and differentiation of species ranging from worms to humans. This cascade also mediates transformation by most oncogenes.

Animals↗
PubMed2005

Raf kinase inhibitory protein regulates Raf-1 but not B-Raf kinase activation.

Raf kinase inhibitory protein (RKIP; also known as phosphatidylethanolamine-binding protein or PEBP) is a modulator of the Raf/MAPK signaling cascade and a suppressor of metastatic cancer. Here, we show that RKIP inhibits MAPK by regulating Raf-1 activation; specifically, RKIP acts subsequent to Raf-1 membrane recruitment, prevents association of Raf-1 and p21-activated kinase (PAK), and blocks phosphorylation of the Raf-1 kinase domain by PAK and Src family kinases. Mutation of the PAK and Src phosphorylation sites on Raf-1 to aspartate, a phosphate mimic, prevented RKIP association with or inhibition of Raf-1 signaling. Interestingly, although RKIP can interact with B-Raf, RKIP depletion had no effect on activation of B-Raf. Because c-Raf-1 and B-Raf are both required for maximal MAPK stimulation by epidermal growth factor in neuronal and epithelial cell lines, we determined whether RKIP significantly affects MAPK signaling. In fact, RKIP depletion increased not only the amplitude but also the sensitivity of MAPK and DNA synthesis to epidermal growth factor stimulation by up to an order of magnitude. These results indicate that selective modulation of c-Raf-1 but not B-Raf activation by RKIP can limit the dynamic range of the MAPK signaling response to growth factors and may play a critical role in growth and development.

Animals↗
PubMed1993

1,25-Dihydroxyvitamin D3 activates Raf kinase and Raf perinuclear translocation via a protein kinase C-dependent pathway.

1,25-Dihydroxyvitamin D3's (D3) potential mitogenic mechanism of action was pursued in cultured rat hepatic Ito cells, a fibrogenic effector cell which proliferates in vivo during liver injury and fibrogenesis. D3 stimulated Ito cell DNA synthesis and potentiated platelet-derived growth factor-induced mitogenesis. D3's enhancement of [3H]thymidine incorporation was associated with nuclear Egr expression. Recent studies have causally linked the activated proto-oncogene c-Raf with downstream Egr induction. The serine-threonine kinase Raf protein is phosphorylation-activated by a large array of agonists including plasma membrane and cytoplasmic tyrosine kinases but has not previously been associated with the steroid superfamily of mediators. To consider potential prenuclear acute pathways of D3-induced stimulation, the activation of Raf was examined following D3 exposure. D3 induced Raf activation as assessed via (a) enhanced Raf phosphorylation following in vivo 32P labeling, (b) enhanced kinase function utilizing exogenous histone 1 protein as substrate, and (c) the shift in Raf physical localization changing from a diffuse cytoplasmic distribution to a perinuclear domain. A similar activation of Raf kinase was found in 3T3 cells exposed to D3 with enhanced histone phosphorylation detectable within 1 min following stimulation. The proximal cascade leading to Raf kinase activation may involve a protein kinase activity was severely attenuated by stimulated kinase activity was severely attenuated by previous phorbol ester treatment for 20 h or staurosporine pretreatment.

3T3 Cells↗
PubMed2005

Raf kinase inhibitors in oncology.

The importance of the MAP kinase pathway, which includes the kinases Raf, MEK1/2, and ERK1/2, for the proliferation and survival of tumor cells recently increased with the discovery of activating BRAF mutations in human tumors. Therefore, in addition to a role in controlling tumors with Ras mutations and activated growth factor receptors, inhibitors of Raf kinase may harbor therapeutic potential in tumors carrying a BRAF oncogene. A variety of agents have been discovered to interfere with Raf kinase, including antisense oligonucleotides and small molecules. These inhibitors prevent the expression of Raf protein, block Ras/Raf interaction, or obstruct its kinase activity. Raf inhibitors that are currently undergoing clinical evaluation show promising signs of anti-cancer efficacy with a very tolerable safety profile. Clinically most advanced is the Raf inhibitor BAY 43-9006, which recently entered phase III clinical testing. This review addresses the rationale for targeting Raf kinase and the current status of various pharmacological approaches.

Animals↗
PubMed2003

Mechanisms of regulating the Raf kinase family.

The MAP Kinase pathway is a key signalling mechanism that regulates many cellular functions such as cell growth, transformation and apoptosis. One of the essential components of this pathway is the serine/threonine kinase, Raf. Raf (MAPKK kinase, MAPKKK) relays the extracellular signal from the receptor/Ras complex to a cascade of cytosolic kinases by phosphorylating and activating MAPK/ERK kinase (MEK; MAPK kinase, MAPKK) that phosphorylates and activates extracellular signal regulated kinase (ERK; mitogen-activated protein kinase, MAPK), which phosphorylates various cytoplasmic and nuclear proteins. Regulation of both Ras and Raf is crucial in the proper maintenance of cell growth as oncogenic mutations in these genes lead to high transforming activity. Ras is mutated in 30% of all human cancers and B-Raf is mutated in 60% of malignant melanomas. The mechanisms that regulate the small GTPase Ras as well as the downstream kinases MEK and extracellular signal regulated kinase (ERK) are well understood. However, the regulation of Raf is complex and involves the integration of other signalling pathways as well as intramolecular interactions, phosphorylation, dephosphorylation and protein-protein interactions. From studies using mammalian isoforms of Raf, as well as C. elegans lin45-Raf, common patterns and unique differences of regulation have emerged. This review will summarize recent findings on the regulation of Raf kinase.

Animals↗
PubMed2006

ERK MAP kinase signaling in post-mortem brain of suicide subjects: differential regulation of upstream Raf kinases Raf-1 and B-Raf.

The Raf kinases Raf-1 and B-Raf are upstream activators of the extracellular signal-regulated kinase (ERK)-signaling pathway and therefore participates in many physiological functions in brain, including neuronal survival and synaptic plasticity. Previously, we observed that activation of ERK-1/2, the downstream component of ERK signaling, is significantly reduced in post-mortem brain of suicide victims. The present study was undertaken to further examine whether suicide brain is also associated with abnormalities in upstream molecules in ERK signaling. The study was performed in prefrontal cortex (PFC) and hippocampus obtained from 28 suicide victims and 21 normal controls. mRNA levels of Raf-1, B-Raf, and cyclophilin were measured by quantitative RT-PCR. Protein levels of Raf-1 and B-Raf were determined by Western blot, whereas their catalytic activities were determined by immunoprecipitation and enzymatic assays. It was observed that the catalytic activity of B-Raf was significantly reduced in PFC and hippocampus of suicide subjects. This decrease was associated with a decrease in its protein, but not mRNA, level. On the other hand, catalytic activity, and mRNA and protein levels, of Raf-1 were not altered in post-mortem brain of suicide subjects. The observed changes were not related to confounding variables; however, Raf-1 showed a negative correlation with age. Also, the changes in B-Raf were present in all suicide subjects, irrespective of psychiatric diagnosis. Our results of selective reduction in catalytic activity and expression of B-Raf but not Raf-1 suggest that B-Raf may be playing an important role in altered ERK signaling in brain of suicide subjects, and thus in the pathophysiology of suicide.

Adult↗
PubMed2002

Extracellular signal regulated kinase (ERK)/mitogen activated protein kinase (MAPK)-independent functions of Raf kinases.

Raf comprises a family of three kinases, A-Raf, B-Raf and Raf-1, which are best known as key regulators of the MEK-MAPK/ERK cascade. This module is often perceived as a linear pathway in which ERK is the effector. However, recent advances have unveiled a role for Raf outside this established signalling unit. Current evidence, including gene-knockout studies in mice, suggests that there are ERK-independent functions of Raf kinases. Regulation of apoptosis is one area in which Raf may function independently of ERK, although its substrates remain to be identified. Other studies have suggested that Raf has kinase-independent functions and may act as a scaffold protein.

Animals↗
PubMed1993

Complementation of byr1 in fission yeast by mammalian MAP kinase kinase requires coexpression of Raf kinase.

Intracellular signalling from receptor tyrosine kinases in mammalian cells involves the activation of a signal cascade which includes p21ras and the protein kinases p74raf-1, MAP kinase kinase and MAP kinases. In the yeasts Schizosaccharomyces pombe and Saccharomyces cerevisiae the response to mating pheromones requires the Spk1 and KSS1/FUS3 kinases, which have sequence homology to vertebrate MAP kinases. The recent cloning of complementary DNAs for mammalian and frog MAP kinase kinases has shown that they are homologous to the S. pombe Byr1 (ref. 17) and S. cerevisiae STE7 (ref. 18) kinases, which have been proposed to function upstream of Spk1 and KSS1/FUS3, respectively. We have investigated whether these apparently similar kinase pathways are functionally conserved between vertebrates and S. pombe. We report here that expression of mammalian MAP kinase kinase alone fails to complement a byr1 mutant of S. pombe. When coexpressed with Raf kinase, however, MAP kinase kinase is activated by phosphorylation and the mating defect of the byr1 mutant is rescued. This suggests that the pathways are functionally homologous and that Raf kinase may directly phosphorylate and activate MAP kinase kinase.

Animals↗
PubMed2001

Phosphorylation by PKA of a site unique to B-Raf kinase.

The Raf kinases serve as central intermediates to relay signals from Ras to ERK. Cell-specific effects of these signals on growth, differentiation and survival can be observed due to the recruitment of different isoenzymes of the Raf family. The in vitro phosphorylation of a site unique to B-Raf (Ser429) has been proposed to be responsible for the negative regulation of the isoenzyme by Akt. Using phosphopetide mapping and site-directed mutagenesis we showed that Ser429 is phosphorylated upon cAMP elevation in PC12 cells and proposed that PKA is a major kinase phosphorylating the B-Raf-specific site in vivo.

Amino Acid Sequence↗
PubMed2005

Modulation of the MAP kinase signaling cascade by Raf kinase inhibitory protein.

Proteins like Raf kinase inhibitory protein (RKIP) that serve as modulators of signaling pathways, either by promoting or inhibiting the formation of productive signaling complexes through protein-protein interactions, have been demonstrated to play an increasingly important role in a number of cell types and organisms. These proteins have been implicated in development as well as the progression of cancer. RKIP is a particularly interesting regulator, as it is a highly conserved, ubiquitously expressed protein that has been shown to play a role in growth and differentiation in a number of organisms and can regulate multiple signaling pathways. RKIP is also the first MAP kinase signaling modulator to be identified as playing a role in cancer metastasis, and identification of the mechanism by which it regulates Raf-1 activation provides new targets for therapeutic intervention.

Animals↗
PubMed2003

Protein kinase C switches the Raf kinase inhibitor from Raf-1 to GRK-2.

Feedback inhibition is a fundamental principle in signal transduction allowing rapid adaptation to different stimuli. In mammalian cells, the major feedback inhibitor for G-protein-coupled receptors (GPCR) is G-protein-coupled receptor kinase 2 (GRK-2), which phosphorylates activated receptors, uncouples them from G proteins and initiates their internalization. The functions of GRK-2 are indispensable and need to be tightly controlled. Dysregulation promotes disorders such as hypertension or heart failure. In our search for a control mechanism for this vital kinase, here we show that the Raf kinase inhibitor protein (RKIP) is a physiological inhibitor of GRK-2. After stimulation of GPCR, RKIP dissociates from its known target, Raf-1 (refs 6-8), to associate with GRK-2 and block its activity. This switch is triggered by protein kinase C (PKC)-dependent phosphorylation of the RKIP on serine 153. The data delineate a new principle in signal transduction: by activating PKC, the incoming receptor signal is enhanced both by removing an inhibitor from Raf-1 and by blocking receptor internalization. A physiological role for this mechanism is shown in cardiomyocytes in which the downregulation of RKIP restrains beta-adrenergic signalling and contractile activity.

Androgen-Binding Protein↗
PubMed2000

Mechanism of suppression of the Raf/MEK/extracellular signal-regulated kinase pathway by the raf kinase inhibitor protein.

We have recently identified the Raf kinase inhibitor protein (RKIP) as a physiological endogenous inhibitor of the Raf-1/MEK/extracellular signal-regulated kinase (ERK) pathway. RKIP interfered with MEK phosphorylation and activation by Raf-1, resulting in the suppression of both Raf-1-induced transformation and AP-1-dependent transcription. Here we report the molecular mechanism of RKIP's inhibitory function. RKIP can form ternary complexes with Raf-1, MEK, and ERK. However, whereas MEK and ERK can simultaneously associate with RKIP, Raf-1 binding to RKIP and that of MEK are mutually exclusive. RKIP is able to dissociate a Raf-1-MEK complex and behaves as a competitive inhibitor of MEK phosphorylation. Mapping of the binding domains showed that MEK and Raf-1 bind to overlapping sites in RKIP, whereas MEK and RKIP associate with different domains in Raf-1, and Raf-1 and RKIP bind to different sites in MEK. Both the Raf-1 and the MEK binding sites in RKIP need to be destroyed in order to relieve RKIP-mediated suppression of the Raf-1/MEK/ERK pathway, indicating that binding of either Raf-1 or MEK is sufficient for inhibition. The properties of RKIP reveal the specific sequestration of interacting components as a novel motif in the cell's repertoire for the regulation of signaling pathways.

Alleles↗
PubMed2006

Raf kinase inhibitor protein regulation of raf and MAPK signaling.

The Raf kinase inhibitor protein (RKIP) belongs to an evolutionarily conserved family of phosphatidylethanolamine-binding proteins (PEBPs), which have important functions as inhibitors of kinase signaling pathways and metastasis. Most notably, RKIP can interrupt signaling through the Ras-Raf-MEK-ERK pathway by dissociating the interaction between Raf-1 and its substrate MEK, highlighting the importance of protein interactions as regulatory interfaces. Furthermore, RKIP was shown to inhibit IkappaB kinases (IKKs) interfering with the activation of nuclear factor kappa B (NFkappaB), and G-protein coupled receptor-kinase 2 (GRK2), impeding receptor downregulation and prolonging signaling. More recently, RKIP has emerged as an important suppressor of metastasis. Here, we review the functions of RKIP and present methods to detect and measure RKIP expression and activity in cells and tissues.

Amino Acid Sequence↗
PubMed2006

Raf kinases: oncogenesis and drug discovery.

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

Animals↗
PubMed2005

The biology of a prostate cancer metastasis suppressor protein: Raf kinase inhibitor protein.

Raf kinase inhibitor protein (RKIP) was originally identified as a protein that bound membrane phospholipids and was named phosphatidylethanolamine binding protein-2 (PEBP-2). RKIP was than identified as a protein that bound Raf and blocked its ability to phosphorylate MEK, thus earning its new name of RKIP. Subsequent to identification of its role in the Raf:MEK pathway, RKIP has been demonstrated to regulate several other signaling pathways including G-protein signaling and NF-kappaB signaling. Its involvement in several signaling pathways has engendered RKIP to contribute to several physiological processes including membrane biosynthesis, spermatogenesis, neural development, and apoptosis. RKIP is expressed in many tissues including brain, lung, and liver and thus, dysregulation of RKIP expression or function has potential to contribute to pathophysiology in these tissues. Loss of RKIP expression in prostate cancer cells confers a metastatic phenotype on them. Additionally, restoration of RKIP expression in a metastatic prostate cancer cell line does not effect primary tumor growth, but it does inhibit prostate cancer metastasis. These parameters identify RKIP as a metastasis suppressor gene. In this review, the biology and pathophysiology of RKIP is described.

Genes, Tumor Suppressor↗
PubMed1992

Activation of the MAP kinase pathway by the protein kinase raf.

Both MAP kinases and the protein kinase p74raf-1 are activated by many growth factors in a c-ras-dependent manner and by oncogenic p21ras. We were therefore interested in determining the relationship between MAP kinases and raf. The MAP kinase ERK2 is activated by expression of oncogenically activated raf, independently of cellular ras. Overexpressed p74raf-1 potentiates activation of ERK2 by EGF and TPA. MAP kinase kinase inactivated by phosphatase 2A treatment is phosphorylated and reactivated by incubation with p74raf-1 immunoprecipitated from phorbol ester-treated cells. We conclude that raf protein kinase is upstream of MAP kinases and is either a MAP kinase kinase kinase or a MAP kinase kinase kinase kinase.

Animals↗
PubMed2005

Identification of key residues in the A-Raf kinase important for phosphoinositide lipid binding specificity.

Raf kinases are involved in regulating cellular signal transduction pathways in response to a wide variety of external stimuli. Upstream signals generate activated Ras-GTP, important for the relocalization of Raf kinases to the membrane. Upon full activation, Raf kinases phosphorylate and activate downstream kinase in the mitogen-activated protein kinase (MAPK) signaling pathway. The Raf family of kinases has three members, Raf-1, B-Raf, and A-Raf. The ability of Raf-1 and B-Raf to bind phosphatidylserine (PS) and phosphatidic acid (PA) has been show to facilitate Raf membrane associations and regulate Raf kinase activity. We have characterized the lipid binding properties of A-Raf, as well as further characterized those of Raf-1. Both A-Raf and Raf-1 were found to bind to 3-, 4-, and 5-monophosphorylated phosphoinositides [PI(3)P, PI(4)P, and PI(5)P] as well as phosphatidylinositol 3,5-bisphosphate [PI(3,5)P(2)]. In addition, A-Raf also bound specifically to phosphatidylinositol 4,5- and 3,4-bisphosphates [PI(4,5)P(2) and PI(3,4)P(2)] and to PA. A mutational analysis of A-Raf localized the PI(4,5)P(2) binding site to two basic residues (K50 and R52) within the Ras binding domain. Additionally, an A-Raf mutant lacking the first 199 residues [i.e., the entire conserved region 1 (CR1) domain] bound the same phospholipids as full-length Raf-1. This suggests that a second region of A-Raf between amino acids 200 and 606 was responsible for interactions with the monophosphorylated PIs and PI(3,5)P(2). These results raise the possibility that Raf-1 and A-Raf bind to specific phosphoinositides as a mechanism to localize them to particular membrane microdomains rich in these phospholipids. Moreover, the differences in their lipid binding profiles could contribute to their proposed isoform-specific Raf functions.

Amino Acid Motifs↗