Partial purification of a mitogen-activated protein kinase kinase activator from bovine brain. Identification as B-Raf or a B-Raf-associated activity

BRAFnon-V600E more frequently co-occurs with IDH1/2 mutations in adult patients with gliomas than in patients harboring BRAFV600E but without a survival advantage

BACKGROUND

The effects of BRAF^non-V600E and BRAF^V600E on the outcomes and the molecular characteristics of adult glioma patients are unknown and need to be explored, although BRAF^V600E has been extensively studied in pediatric glioma.

METHODS

Co-occurring mutations and copy number alterations of associated genes in the MAPK and p53 pathways were investigated using data from The Cancer Genome Atlas (TCGA) public database retrieved by cBioPortal. The prognosis of available adult glioma cohorts with BRAF^V600E and BRAF^non-V600E mutations were also investigated.

RESULTS

Ninety patients with BRAF^V600E or BRAF^non-V600E were enrolled in this study, and data from 52 nonredundant patients were investigated. Glioblastoma multiform was the most common cancer type, with BRAF ^non-V600E and BRAF^V600E. TP53 (56.00% vs. 7.41%), IDH1/2 (36.00% vs. 3.70%), and ATRX (32.00% vs. 7.41%) exhibited more mutations in BRAF^non-V600E than in BRAF^V600E, and TP53 was an independent risk factor (56.00% vs. 7.41%). Both BRAF^non-V600E and BRAF^V600E frequently overlapped with CDKN2A/2B homozygous deletions (HDs), but there was no significant difference. Survival analysis showed no difference between the BRAF ^non-V600E and BRAF^V600E cohorts, even after excluding the survival benefit of IDH1/2 mutations and considering the BRAF^non-V600E mutations in the glycine-rich loop (G-loop) and in the activation segment. The estimated mean survival of patients with BRAF^non-V600E & IDH1/2^WT with mutations in the G-loop groups was the shortest.

CONCLUSIONS

BRAF^non-V600E exhibited a stronger association with IDH1/2 mutations than BRAF^V600E, but no survival advantage was found.

Acquired Resistance to Clinical Cancer Therapy: A Twist in Physiological Signaling

Although modern therapeutic strategies have brought significant progress to cancer care in the last 30 years, drug resistance to targeted monotherapies has emerged as a major challenge. Aberrant regulation of multiple physiological signaling pathways indispensable for developmental and metabolic homeostasis, such as hyperactivation of pro-survival signaling axes, loss of suppressive regulations, and impaired functionalities of the immune system, have been extensively investigated aiming to understand the diversity of molecular mechanisms that underlie cancer development and progression. In this review, we intend to discuss the molecular mechanisms of how conventional physiological signal transduction confers to acquired drug resistance in cancer patients. We will particularly focus on protooncogenic receptor kinase inhibition-elicited tumor cell adaptation through two major core downstream signaling cascades, the PI3K/Akt and MAPK pathways. These pathways are crucial for cell growth and differentiation and are frequently hyperactivated during tumorigenesis. In addition, we also emphasize the emerging roles of the deregulated host immune system that may actively promote cancer progression and attenuate immunosurveillance in cancer therapies. Understanding these mechanisms may help to develop more effective therapeutic strategies that are able to keep the tumor in check and even possibly turn cancer into a chronic disease.

B-RAF and its novel negative regulator reticulocalbin 1 (RCN1) modulates cardiomyocyte hypertrophy

AIM

Activation of the kinase RAF and its downstream targets leads to cardiomyocyte hypertrophy. It has been hypothesized that B-RAF might be the main activator of MEK in various cell types. Therefore, the aim of this study was to investigate the role of B-RAF and its modulating factors in cardiomyocyte hypertrophy.

METHODS AND RESULTS

Neonatal rat cardiomyocytes were pre-treated with and without the specific B-RAF inhibitor SB590885 and then stimulated with phenylephrine to induce hypertrophy. Inhibition of B-RAF completely impeded the hypertrophic response and led to a significant reduction of MEK1/2 phosphorylation. By applying a eukaryotic cDNA expression screen, based on a dual-luciferase reporter assay for B-RAF activity measurement, we identified RCN1 as a new negative modulator of B-RAF activity. Adenovirus-mediated overexpression of reticulocalbin 1 (RCN1) completely impeded phenylephrine-induced hypertrophy and led to significantly reduced MEK1/2 phosphorylation. Conversely, adenoviral knockdown of RCN1 with a specific synthetic miRNA induced cardiomyocyte hypertrophy and significantly increased MEK1/2 phosphorylation.

CONCLUSIONS

In summary, our results show that the inhibition of B-RAF abolishes cardiomyocyte hypertrophy and we identified RCN1 as novel negative modulator of cardiomyocyte hypertrophy by inhibition of the mitogen-activated protein kinase signalling cascade. Our results show that B-RAF kinase activity is essential for cardiac hypertrophy and RCN1, its newly identified negative regulator, abolishes hypertrophic response of cardiomyocytes in vitro.

Characterization of the B-Raf interactome in mouse hippocampal neuronal cells

B-Raf links a variety of extracellular stimuli downstream of cell surface receptors, constituting a determining factor in the ability of neurons to activate ERK. A detailed study of the B-Raf interactome is necessary to clarify the intricacy of B-Raf-dependent signal transduction. We used a mouse hippocampal cell line (HT22) that expresses B-Raf at high levels, to identify B-Raf associated proteins under endogenous expression conditions, avoiding artificial interactions from overexpression studies. We used stringent procedures to co-immunoprecipitate proteins that specifically associate with endogenous B-Raf with the help of gel electrophoresis separation and off-line LC-MALDI-MS/MS proteomic analysis. Our stringent protein identification criteria allowed confident identification of B-Raf interacting proteins under non-stimulating conditions. The presence of previously reported B-Raf interactors among the list of proteins identified confirms the quality of proteomic data. We identified tubulin and actin as B-Raf interactors for the first time, among structural and accessory proteins of cell cytoskeleton, molecular chaperones (Hsc70, GRP78), and cellular components involved in aspects of mRNA metabolism and translation. Interactions were validated in HT22 cells and in the neuronal cell line Neuro-2a providing further evidence that the identified proteins are B-Raf interactors, which constitute a basis for understanding MAPK pathway regulation in neurons.

The crystal structure of BRAF in complex with an organoruthenium inhibitor reveals a mechanism for inhibition of an active form of BRAF kinase

Substitution mutations in the BRAF serine/threonine kinase are found in a variety of human cancers. Such mutations occur in approximately 70% of human malignant melanomas, and a single hyperactivating V600E mutation is found in the activation segment of the kinase domain and accounts for more than 90% of these mutations. Given this correlation, the molecular mechanism for BRAF regulation as well as oncogenic activation has attracted considerable interest, and activated forms of BRAF, such as BRAF(V600E), have become attractive targets for small molecule inhibition. Here we report on the identification and subsequent optimization of a potent BRAF inhibitor, CS292, based on an organometallic kinase inhibitor scaffold. A cocrystal structure of CS292 in complex with the BRAF kinase domain reveals that CS292 binds to the ATP binding pocket of the kinase and is an ATP competitive inhibitor. The structure of the kinase-inhibitor complex also demonstrates that CS292 binds to BRAF in an active conformation and suggests a mechanism for regulation of BRAF by phosphorylation and BRAF(V600E) oncogene-induced activation. The structure of CS292 bound to the active form of the BRAF kinase also provides a novel scaffold for the design of BRAF(V600E) oncogene selective BRAF inhibitors for therapeutic application.

A novel role for Cdk1/cyclin B in regulating B-raf activation at mitosis

MAPK activity is important during mitosis for spindle assembly and maintenance of the spindle checkpoint arrest. We previously identified B-Raf as a critical activator of the MAPK cascade during mitosis in Xenopus egg extracts and showed that B-Raf activation is regulated in an M-phase-dependent manner. The mechanism that mediates B-Raf activation at mitosis has not been elucidated. Interestingly, activation of 95-kDa B-Raf at mitosis does not require phosphorylation of Thr-599 and Ser-602 residues (Thr-633 and Ser-636 in Xenopus B-Raf), previously shown to be essential for B-Raf activation by Ras. Instead, we provide evidence for Cdk1/cyclin B in mediating mitotic activation of B-Raf. In particular, Cdk1/cyclin B complexes associate with B-Raf at mitosis in Xenopus egg extracts and contribute to its phosphorylation. Mutagenesis and in vitro kinase assays demonstrated that Cdk1/cyclin B directly phosphorylates B-Raf at Serine-144, which is part of a conserved Cdk1 preferential consensus site (S(144)PQK). Importantly, phosphorylation of Ser-144 is absolutely required for mitotic activation of B-Raf and subsequent activation of the MAPK cascade. However, substitution of a phospho-mimicking amino acid at Ser-144 failed to produce a constitutive active B-Raf indicating that, in addition of Ser-144 phosphorylation, other regulatory events may be needed to activate B-Raf at mitosis. Taken together, our data reveal a novel cell cycle mechanism for activating the B-Raf/MEK/MAPK cascade.

Identification of BRAF as a new interactor of PLCε1, the protein mutated in nephrotic syndrome type 3

Steroid-resistant nephrotic syndrome is a malfunction of the kidney glomerular filter that leads to proteinuria, hypoalbuminemia, edema, and renal failure. Recently, we identified recessive mutations in the phospholipase C epsilon 1 gene ( PLCE1) as a new cause of early-onset nephrotic syndrome and demonstrated interaction of PLCε1 with IQGAP1. To further elucidate the mechanism by which PLCE1 mutations cause nephrotic syndrome, we sought to identify new protein interaction partners of PLCε1. We utilized information from the genetic interaction network of C. elegans. It relates the PLCE1 ortholog ( plc-1) to the C. elegans ortholog ( lin-45) of human BRAF ( v-raf murine sarcoma viral oncogene homolog B1). We hypothesized that this may indicate a functional protein-protein interaction. Using GST pull down of HEK293T cell lysates in vitro and coimmunoprecipation of mouse kidney lysates in vivo, we show that BRAF interacts with PLCε1. By immunohistochemistry in rat kidney, we demonstrate that both proteins are coexpressed and colocalize in developing and mature glomerular podocytes, reporting for the first time the expression of BRAF in the glomerular podocyte.

Differential mRNA distribution of components of the ERK/MAPK signalling cascade in the adult mouse brain

The mitogen-activated protein kinases (MAPKs), also called extracellular signal-regulated kinases (ERKs), are a group of serine/threonine terminal protein kinases activated downstream of a pleiotrophy of transmembrane receptors. Main intracellular components of the MAPK signalling pathway are the RAF, MEK, and ERK proteins, which work in a cascade of activator and effector proteins. They regulate many fundamental cellular functions, including cell proliferation, cell survival, and cell differentiation by transducing extracellular signals to cytoplasmic and nuclear effectors. To reveal more details about possible activation cascades in this pathway, the present study gives a complete description of the differential expression of Braf, Mek1, Mek2, Mek5, Erk1, Erk2, Erk3, and Erk5 in the adult murine brain by way of in situ hybridization analysis. In this study, we found that each gene is widely expressed in the whole brain, except for Mek2, but each displays a very distinct expression pattern, leading to distinct interactions of the MAPK components within different regions. Most notably we found that 1) Braf and Erk3 are coexpressed in the hippocampus proper, confirming a possible functional interaction; 2) in most forebrain areas, Mek5 and Erk5 are coexpressed; and 3) in the neurogenic regions of the brain, namely, the olfactory bulb and the dentate gyrus, Braf is absent, indicating that other activator proteins have to take over its function. Despite these differences, our results show widespread coexpression of the pathway components, thereby confirming the hypothesis of redundant functions among several MEK and ERK proteins in some regions of the brain.

No evidence of ARAF, CRAF and MET mutations in BRAFT1799A negative human papillary thyroid carcinoma

The MAPK signaling pathway plays a crucial role in tumorgenesis and cell proliferation in human papillary thyroid carcinoma (PTC). Ret/PTC rearrangements, RAS and BRAF mutations, the main non-overlapping genetic alterations all leading to MAPK cascade activation, are cumulatively identified in 60-80% of PTCs. In approximately one-fourth of the cases, oncogenic background potentially contributing to MAPK activation in PTC might be different. We therefore attempted to evaluate the mutational status of genes encoding other members of RAF family known to act upstream of MAPKs, ARAF and CRAF (RAF-1). In addition we also analyzed the MET gene that encodes hepatocyte growth factor/scatter factor receptor overexpressed in most of PTCs and a MAPK cascade contributor. In 129 Japanese patients with PTC, BRAF(T1799A) was detected in 65 cases (50.4%), and the remaining 64 tumor specimens were subjected to mutation analysis of kinase domains of ARAF, CRAF and MET genes, and hotspots of K- and N-RAS genes. No ARAF, CRAF, MET, K- and N-RAS mutations were revealed. Based on these observations, we concluded that despite the fact that ARAF, CRAF and MET are actively expressed, alterations of these genes are rare in PTC and unlikely to play a perceptible role in the molecular pathogenesis of this type of human malignancy.

B-Raf and C-Raf are required for Ras-stimulated p42 MAP kinase activation in Xenopus egg extracts

During mitosis, a select pool of MEK1 and p42/p44 MAPK becomes activated at the kinetochores and spindle poles, without substantial activation of the bulk of the cytoplasmic p42/p44 MAPK. Recently, we set out to identify the MAP kinase kinase kinase (MAPKKK) responsible for this mitotic activation, using cyclin-treated Xenopus egg extracts as a model system, and presented evidence that Mos was the relevant MAPKKK . However, a second MAPKKK distinct from Mos was readily detectable as well. Here, we partially purify this second MAPKKK and identify it as B-Raf. No changes in the activity of B-Raf were detectable during progesterone-induced oocyte maturation, after egg fertilization, or during the early embryonic cell cycle, arguing against a role for B-Raf in the mitotic activation of MEK1 and p42 MAPK. Ras proteins can bring about activation of MEK1 and p42 MAPK in extracts, and Ras may contribute to signaling from the classical progesterone receptor during oocyte maturation and from receptor tyrosine kinases during early embryogenesis. We found that both B-Raf and C-Raf, but not Mos, are required for Ras-induced MEK1 and p42 MAPK activation. These data indicate that two upstream stimuli, active Ras and active Cdc2, utilize different MAPKKKs to activate MEK1 and p42 MAPK.

Essential role of B-Raf in ERK activation during extraembryonic development

The kinases of the Raf family have been intensively studied as activators of the mitogen-activated protein kinase kinase/extra-cellular signal-regulated kinase (ERK) module in regulated and deregulated proliferation. Genetic evidence that Raf is required for ERK activation in vivo has been obtained in lower organisms, which express only one Raf kinase, but was hitherto lacking in mammals, which express more than one Raf kinase. Ablation of the two best studied Raf kinases, B-Raf and Raf-1, is lethal at midgestation in mice, hampering the detailed study of the essential functions of these proteins. Here, we have combined conventional and conditional gene ablation to show that B-Raf is essential for ERK activation and for vascular development in the placenta. B-Raf-deficient placentae show complete absence of phosphorylated ERK and strongly reduced HIF-1alpha and VEGF levels, whereas all these parameters are normal in Raf-1-deficient placentae. In addition, neither ERK phosphorylation nor development are affected in B-raf-deficient embryos that are born alive obtained by epiblast-restricted gene inactivation. The data demonstrate that B-Raf plays a nonredundant role in ERK activation during extraembyronic mammalian development in vivo.

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.

Raf: A Strategic Target for Therapeutic Development Against Cancer

The mitogen-activated protein kinase (MAPK) signaling pathway plays a critical role in transmitting proliferative signals generated by cell surface receptors and cytoplasmic signaling elements to the nucleus. Several important signaling elements of the MAPK pathway, particularly Ras and Raf, are encoded by oncogenes, and as such, their structures and functions can be modified, rendering them constitutively active. Because the MAPK pathway is dysregulated in a notable proportion of human malignancies, many of its aberrant and critical components represent strategic targets for therapeutic development against cancer. Raf, which is an essential serine/threonine kinase constituent of the MAPK pathway and a downstream effector of the central signal transduction mediator Ras, is activated in a wide range of human malignancies by aberrant signaling upstream of the protein (eg, growth factor receptors and mutant Ras) and activating mutations of the protein itself, both of which confer a proliferative advantage. Three isoforms of Raf have been identified, and therapeutics targeting Raf, including small-molecule inhibitors and antisense oligodeoxyribonucleotides (ASON), are undergoing clinical evaluation. The outcomes of these investigations may have far-reaching implications in the management of many types of human cancer. This review outlines the structure and diverse functions of Raf, the rationale for targeting Raf as a therapeutic strategy against cancer, and the present status of various therapeutic approaches including ASONs and small molecules, particularly sorafenib (BAY 43-9006).

Integrating signals between cAMP and the RAS/RAF/MEK/ERK signalling pathways. Based on the anniversary prize of the Gesellschaft für Biochemie und Molekularbiologie Lecture delivered on 5 July 2003 at the Special FEBS Meeting in Brussels

One of the hallmarks of cAMP is its ability to inhibit proliferation in many cell types, but stimulate proliferation in others. Clearly cAMP has cell type specific effects and the outcome on proliferation is largely attributed to crosstalk from cAMP to the RAS/RAF/mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK) kinase (MEK)/ERK pathway. We review the crosstalk between these two ancient and conserved pathways, describing the molecular mechanisms underlying the interactions between these pathways and discussing their possible biological importance.

Raf-1 serine 338 phosphorylation plays a key role in adhesion-dependent activation of extracellular signal-regulated kinase by epidermal growth factor

Activation of the extracellular signal-regulated kinase (ERK) 1/2 cascade by polypeptide growth factors is tightly coupled to adhesion to extracellular matrix in nontransformed cells. Raf-1, the initial kinase in this cascade, is intricately regulated by phosphorylation, localization, and molecular interactions. We investigated the complex interactions between Raf-1, protein kinase A (PKA), and p21-activated kinase (PAK) to determine their roles in the adhesion dependence of signaling from epidermal growth factor (EGF) to ERK. We conclude that Raf-1 phosphorylation on serine 338 (S338) is a critical step that is inhibited in suspended cells. Restoration of phosphorylation at S338, either by expression of highly active PAK or by expression of an S338 phospho-mimetic Raf-1 mutation, led to a partial rescue of ERK activation in suspended cells. Raf-1 inhibition in suspension was not due to excessive negative regulation on inhibitory sites S43 and S259, as these serines were largely dephosphorylated in suspended cells. Finally, strong phosphorylation of Raf-1 S338 provided resistance to PKA-mediated inhibition of ERK activation. Phosphorylation at Raf-1 S43 and S259 by PKA only weakly inhibited EGF activation of Raf-1 and ERK when cells maintained high Raf-1 S338 phosphorylation.

B-Raf and Raf-1 are regulated by distinct autoregulatory mechanisms

B-Raf is a key regulator of the ERK pathway and is mutationally activated in two-thirds of human melanomas. In this work, we have investigated the activation mechanism of B-Raf and characterized the roles of Ras and of B-Raf phosphorylation in this regulation. Raf-1 is regulated by an N-terminal autoinhibitory domain whose actions are blocked by interaction with Ras and subsequent phosphorylation of Ser(338). We observed that B-Raf also contains an N-terminal autoinhibitory domain and that the interaction of this domain with the catalytic domain was inhibited by binding to active H-Ras. However, unlike Raf-1, the phosphorylation of B-Raf at Ser(445) was constitutive and was only moderately increased by expression of constitutively active H-Ras or constitutively active PAK1. Ser(445) phosphorylation is important to the B-Raf activation mechanism, however, because mutation of this site to alanine increased the affinity of the regulatory domain for the catalytic domain and increased autoinhibition. Similarly, expression of constitutively active PAK1 also decreased auto-inhibition. B-Raf autoinhibition was negatively regulated by acidic substitutions at phosphorylation sites within the activation loop of B-Raf and by the oncogenic substitution V599E. However, these substitutions did not affect the ability of the regulatory domain to co-immunoprecipitate with the catalytic domain. These data demonstrate that B-Raf activity is autoregulated, that constitutive phosphorylation of Ser(445) primes B-Raf for activation, and that a key feature of phosphorylation within the activation loop or of oncogenic mutations within this region is to block autoinhibition.

The RAF proteins take centre stage

Since their discovery over 20 years ago, the RAF proteins have been intensely studied. For most of that time, the focus of the field has been the C-RAF isoform and its role as an effector of the RAS proteins. However, a report that implicates B-RAF in human cancer has highlighted the importance of all members of this protein kinase family and recent studies have uncovered intriguing new data relating to their complex regulation and biological functions.

Mutation of B-Raf in human choroidal melanoma cells mediates cell proliferation and transformation through the MEK/ERK pathway

The BRAF gene, encoding a mitogen-activated protein kinase kinase kinase, is mutated in several human cancers, with the highest incidence occurring in cutaneous melanoma. The activating V599E mutation accounted for 80% of all mutations detected in cutaneous melanoma cell lines. Reconstitution experiments have shown that this mutation increases ectopically expressed B-Raf kinase activity and induces NIH3T3 cell transformation. Here we used tumor-derived cell lines to characterize the activity of endogenous mutated B-Raf protein and assess its specific role in transformation. We show that three cell lines (OCM-1, MKT-BR, and SP-6.5) derived from human choroidal melanoma, the most frequent primary ocular neoplasm in humans, express B-Raf containing the V599E mutation. These melanoma cells showed a 10-fold increase in endogenous B-RafV599E kinase activity and a constitutive activation of the MEK/ERK pathway that is independent of Ras. This, as well as melanoma cell proliferation, was strongly diminished by siRNA-mediated depletion of the mutant B-Raf protein. Moreover, blocking B-RafV599E-induced ERK activation by different experimental approaches significantly reduced cell proliferation and anchorage-independent growth of melanoma cells. Finally, quantitative immunoblot analysis allowed us to identify signaling and cell cycle proteins that are differentially expressed between normal melanocytes and melanoma cells. Although the expression of signaling molecules was not sensitive to U0126 in melanoma cells, the expression of a cluster of cell cycle proteins remained regulated by the B-RafV599E/MEK/ERK pathway. Our results pinpoint this pathway as an important component in choroidal melanoma cell lines.

Raf proteins and cancer: B-Raf is identified as a mutational target

A recent report has shown that activating mutations in the BRAF gene are present in a large percentage of human malignant melanomas and in a proportion of colon cancers. The vast majority of these mutations represent a single nucleotide change of T-A at nucleotide 1796 resulting in a valine to glutamic acid change at residue 599 within the activation segment of B-Raf. This exciting new discovery is the first time that a direct association between any RAF gene and human cancer has been reported. Raf proteins are also indirectly associated with cancer as effectors of activated Ras proteins, oncogenic forms of which are present in approximately one-third of all human cancers. BRAF and RAS mutations are rarely both present in the same cancers but the cancer types with BRAF mutations are similar to those with RAS mutations. This has been taken as evidence that the inappropriate regulation of the downstream ERKs (the p42/p44 MAP kinases) is a major contributing factor in the development of these cancers. Recent studies in mice with targeted mutations of the raf genes have confirmed that B-Raf is a far stronger activator of ERKs than its better studied Raf-1 homologue, even in cell types in which the protein is barely expressed. The explanation for this lies in a number of key differences in the regulation of B-Raf and Raf-1 activity. Constitutive phosphorylation of serine 445 of B-Raf leads to this protein having a higher basal kinase activity than Raf-1. Phosphorylation of threonine 598 and serine 601 within the activation loop of B-Raf at the plasma membrane also regulates its activity. The V599E mutation is thought to mimic these phosphorylations, resulting in a protein with high activity, leading to constitutive ERK activation. B-Raf now provides a critical new target to which drugs for treating malignant melanoma can be developed and, with this in mind, it is now important to gain clear insight into the biochemical properties of this relatively little characterised protein.

Rheb is in a high activation state and inhibits B-Raf kinase in mammalian cells

Rheb (Ras homolog enriched in brain) is a member of the Ras family of proteins, and is in the immediate Ras/Rap/Ral subfamily. We found in three different mammalian cell lines that Rheb was highly activated, to levels much higher than for Ras or Rap 1, and that Rheb's activation state was unaffected by changes in growth conditions. Rheb's high activation was not secondary to unique glycine to arginine, or glycine to serine substitutions at positions 14 and 15, corresponding to Ras residues 12 and 13, since Rheb R14G and R14G, S15G mutants had similarly high activation levels as wild type Rheb. These data are consistent with earlier work which showed that purified Rheb has similar GTPase activity as Ras, and suggest a relative intracellular deficiency of Rheb GTPase activating proteins (GAPs) compared to Rheb activators. Further evidence for relatively low intracellular GAP activity was that increased Rheb expression led to a marked increase in Rheb activation. Rheb, like Ras and Rap1, bound B-Raf kinase, but in contrast to Ras and Rap 1, Rheb inhibited B-Raf kinase activity and prevented B-Raf-dependent activation of the transcription factor Elk-1. Thus, Rheb appears to be a unique member of the Ras/Rap/Ral subfamily, and in mammalian systems may serve to regulate B-Raf kinase activity.

Novel activation of STAT5b in response to epidermal growth factor

Of the seven signal transducers and activators of transcription that have been identified, STATs 1, 3, and 5a/5b can be activated not only by a multitude of cytokines but also by some growth factors. The data presented here demonstrate that, in contrast to activation by the cytokine, growth hormone (GH), the activation of STAT5b by the growth factor, epidermal growth factor (EGF), requires overexpression of the EGF receptor (EGFR). We have shown that EGF activates STAT5b not only in a HEK293 cell model in which the EGFR is stably overexpressed but also in the MDA-MB468 breast cancer cell line. Furthermore, EGF (but not GH) is able to activate tyrosine phosphorylation of a Tyr-699 mutant of STAT5b. Using metabolic labeling studies as well as site-directed mutagenesis, we have identified three novel EGF-induced tyrosine phosphorylation sites, Tyr-725, Tyr-740, and Tyr-743. Luciferase assays using a STAT5-specific DNA sequence demonstrate that, although Tyr-699 is absolutely required for transcriptional activation, tyrosines 725, 740, and 743 may be involved in a negative regulation of transcription. Because overexpression of the EGFR is common in many cancers, including advanced breast cancer, characterization of EGF-induced STAT5b may have direct implications in therapeutic applications.

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.

Role of phosphoinositide 3-kinase and endocytosis in nerve growth factor-induced extracellular signal-regulated kinase activation via Ras and Rap1

Neurotrophins promote multiple actions on neuronal cells including cell survival and differentiation. The best-studied neurotrophin, nerve growth factor (NGF), is a major survival factor in sympathetic and sensory neurons and promotes differentiation in a well-studied model system, PC12 cells. To mediate these actions, NGF binds to the TrkA receptor to trigger intracellular signaling cascades. Two kinases whose activities mediate these processes include the mitogen-activated protein (MAP) kinase (or extracellular signal-regulated kinase [ERK]) and phosphoinositide 3-kinase (PI3-K). To examine potential interactions between the ERK and PI3-K pathways, we studied the requirement of PI3-K for NGF activation of the ERK signaling cascade in dorsal root ganglion cells and PC12 cells. We show that PI3-K is required for TrkA internalization and participates in NGF signaling to ERKs via distinct actions on the small G proteins Ras and Rap1. In PC12 cells, NGF activates Ras and Rap1 to elicit the rapid and sustained activation of ERKs respectively. We show here that Rap1 activation requires both TrkA internalization and PI3-K, whereas Ras activation requires neither TrkA internalization nor PI3-K. Both inhibitors of PI3-K and inhibitors of endocytosis prevent GTP loading of Rap1 and block sustained ERK activation by NGF. PI3-K and endocytosis may also regulate ERK signaling at a second site downstream of Ras, since both rapid ERK activation and the Ras-dependent activation of the MAP kinase kinase kinase B-Raf are blocked by inhibition of either PI3-K or endocytosis. The results of this study suggest that PI3-K may be required for the signals initiated by TrkA internalization and demonstrate that specific endocytic events may distinguish ERK signaling via Rap1 and Ras.

Cell type-specific regulation of B-Raf kinase by cAMP and 14-3-3 proteins

Cyclic AMP can either activate or inhibit the mitogen-activated protein kinase (MAPK) pathway in different cell types; MAPK activation has been observed in B-Raf-expressing cells and has been attributed to Rap1 activation with subsequent B-Raf activation, whereas MAPK inhibition has been observed in cells lacking B-Raf and has been attributed to cAMP-dependent protein kinase (protein kinase A)-mediated phosphorylation and inhibition of Raf-1 kinase. We found that cAMP stimulated MAPK activity in CHO-K1 and PC12 cells but inhibited MAPK activity in C6 and NB2A cells. In all four cell types, cAMP activated Rap1, and the 95- and 68-kDa isoforms of B-Raf were expressed. cAMP activation or inhibition of MAPK correlated with activation or inhibition of endogenous and transfected B-Raf kinase. Although all cell types expressed similar amounts of 14-3-3 proteins, approximately 5-fold less 14-3-3 was associated with B-Raf in cells in which cAMP was inhibitory than in cells in which cAMP was stimulatory. We found that the cell type-specific inhibition of B-Raf could be completely prevented by overexpression of 14-3-3 isoforms, whereas expression of a dominant negative 14-3-3 mutant resulted in partial loss of B-Raf activity. Our data suggest that 14-3-3 bound to B-Raf protects the enzyme from protein kinase A-mediated inhibition; the amount of 14-3-3 associated with B-Raf may explain the tissue-specific effects of cAMP on B-Raf kinase activity.

Induction of cell cornification and enhanced squamous-cell marker SPRR1 gene expression by phorbol ester are regulated by different signaling pathways in human conducting airway epithelial cells

Phorbol ester is a strong inducer for both cell cornification and squamous-cell marker SPRR1 gene expression in conducting airway epithelial cells. However, the signaling pathways involved in the regulation of both events have not been completely elucidated. The current study focuses on the common and divergent pathways involved in the induction of these two activities by phorbol-13-myristate-12-acetate (PMA). Using a protein kinase (PK) C inhibitor, bisindolylmaleimide I, PMA-induced cell cornification and SPRR1 gene expression were abolished. Further, a PKC activator, indolactam V, induced cell cornification in the absence of PMA treatment. These results suggest a PKC-dependent signaling pathway for both gene induction and enhanced cell cornification by PMA. However, a mitogen-activated protein kinase-specific inhibitor, PD98059, could only block the gene induction event but failed to prevent cell cornification induced by PMA. These results suggest that diverse signaling pathways after PKC activation by PMA are involved in the regulation of these two events.

P(2Y) purinoceptor subtypes recruit different mek activators in astrocytes

Extracellular ATP can function as a glial trophic factor as well as a neuronal transmitter. In astrocytes, mitogenic signalling by ATP is mediated by metabotropic P(2Y) receptors that are linked to the extracellular signal regulated protein kinase (Erk) cascade, but the types of P(2Y) receptors expressed in astrocytes have not been defined and it is not known whether all P(2Y) receptor subtypes are coupled to Erk by identical or distinct signalling pathways. We found that the P(2Y) receptor agonists ATP, ADP, UTP and 2-methylthioATP (2MeSATP) activated Erk and its upstream activator MAP/Erk kinase (Mek). cRaf-1, the first kinase in the Erk cascade, was activated by 2MeSATP, ADP and UTP but, surprisingly, cRaf-1 was not stimulated by ATP. Furthermore, ATP did not activate B-Raf, the major isoform of Raf in the brain, nor other Mek activators such as Mek kinase 1 (MekK1) and MekK2/3. Reverse transcriptase-polymerase chain reaction (RT - PCR) studies using primer pairs for cloned rat P(2Y) receptors revealed that rat cortical astrocytes express P(2Y(1)), a receptor subtype stimulated by ATP and ADP and their 2MeS analogues, as well as P(2Y(2)) and P(2Y(4)), subtypes in rats for which ATP and UTP are equipotent. Transcripts for P(2Y(6)), a pyrimidine-preferring receptor, were not detected. ATP did not increase cyclic AMP levels, suggesting that P(2Y(11)), an ATP-preferring receptor, is not expressed or is not linked to adenylyl cyclase in rat cortical astrocytes. These signal transduction and RT - PCR experiments reveal differences in the activation of cRaf-1 by P(2Y) receptor agonists that are inconsistent with properties of the P(2Y(1)), P(2Y(2)) and P(2Y(4)) receptors shown to be expressed in astrocytes, i.e. ATP=UTP; ATP=2MeSATP, ADP. This suggests that the properties of the native P(2Y) receptors coupled to the Erk cascade differ from the recombinant P(2Y) receptors or that astrocytes express novel purine-preferring and pyrimidine-preferring receptors coupled to the ERK cascade.

Isotype-specific functions of Raf kinases

The family of Raf-protein kinases consisting of A-Raf, B-Raf, and c-Raf-1 is involved in cellular processes which regulate proliferation, differentiation, and apoptosis. Cell-culture experiments and the knockout of individual Raf genes suggested that the three Raf isoforms have overlapping and unique regulatory functions. However, it is not known how these isotype-specific functions of Raf kinases occur in the cell. Published data suggest that Raf proteins might differ in the regulation of their activation as well as in their ability to connect to downstream signaling pathways. Since Raf is part of a multiprotein complex and protein-protein interactions are important for Raf signaling, we propose that isotype-specific functions can be achieved by isotype-restricted protein binding. Recently we were able to identify candidates for such Raf-isoform-specific interaction partners.

Raf-1 and B-Raf proteins have similar regional distributions but differential subcellular localization in adult rat brain

The Raf kinases play an important and specific role in the activation of extracellular signal-regulated kinases (ERK) cascade. Beside its role in the control of proliferation and differentiation, the ERK cascade has also been implicated in neuron-specific functions. In order to gain clues on the function of Raf kinases in the adult central nervous system (CNS), we performed a comparative analysis of the distribution and subcellular localization of the different Raf kinases in rat brain with antibodies specific for the different Raf kinases. We show that B-Raf and Raf-1 proteins are present in most brain areas, whereas A-Raf is not detected. Interestingly, the two Raf proteins have an approximately similar pattern of distribution with a rostro-caudal decreasing gradient of expression. These two kinases are colocalized in neurons but they are differentially located in subcellular compartments. Raf-1 is localized mainly in the cytosolic fraction around the nucleus, whereas B-Raf is widely distributed in the cell bodies and in the neuritic processes. In addition, we demonstrated that numerous B-Raf isoforms are present in the brain. These isoforms have a differential pattern of distribution, some of them being ubiquitously expressed whereas others are localized to specific brain areas. These isoforms also have a clear differential subcellular localization, specially in Triton-insoluble fractions, but also in synaptosomal, membrane and cytosolic compartments. Altogether these results suggest that each Raf protein could have a distinct signalling regulatory function in the brain with regard to its subcellular localization.

Mitogen-activated protein kinase: conservation of a three-kinase module from yeast to human

Mitogen-activated protein kinases (MAPK) are serine-threonine protein kinases that are activated by diverse stimuli ranging from cytokines, growth factors, neurotransmitters, hormones, cellular stress, and cell adherence. Mitogen-activated protein kinases are expressed in all eukaryotic cells. The basic assembly of MAPK pathways is a three-component module conserved from yeast to humans. The MAPK module includes three kinases that establish a sequential activation pathway comprising a MAPK kinase kinase (MKKK), MAPK kinase (MKK), and MAPK. Currently, there have been 14 MKKK, 7 MKK, and 12 MAPK identified in mammalian cells. The mammalian MAPK can be subdivided into five families: MAPKerk1/2, MAPKp38, MAPKjnk, MAPKerk3/4, and MAPKerk5. Each MAPK family has distinct biological functions. In Saccharomyces cerevisiae, there are five MAPK pathways involved in mating, cell wall remodelling, nutrient deprivation, and responses to stress stimuli such as osmolarity changes. Component members of the yeast pathways have conserved counterparts in mammalian cells. The number of different MKKK in MAPK modules allows for the diversity of inputs capable of activating MAPK pathways. In this review, we define all known MAPK module kinases from yeast to humans, what is known about their regulation, defined MAPK substrates, and the function of MAPK in cell physiology.

Modulation of kinase activity and oncogenic properties by alternative splicing reveals a novel regulatory mechanism for B-Raf

Members of the raf oncogene family encode serine/threonine protein kinases, which activate the mitogen-activated protein kinase kinase MEKs (MAPK or ERK kinases) through direct interaction and phosphorylation. Several recent studies have revealed interesting differences between two members of this family, Raf-1 and B-Raf, regarding their activation, regulation, and kinase activity. In particular, B-Raf was shown to display higher MEK kinase activity than Raf-1. By using both two-hybrid analysis and coimmunoprecipitation experiments, we demonstrate here that B-Raf also markedly differs from Raf-1 by a higher affinity for MEK. We previously reported that the B-raf gene encodes multiple protein isoforms resulting from complex alternative splicing of two exons (exons 8b and 10) located upstream of B-Raf kinase domain. In the present study, we show that these naturally occurring modifications within the protein sequence markedly modulate both the biochemical and oncogenic properties of B-Raf. The presence of exon 10 sequences enhances the affinity for MEK, the basal kinase activity, as well as the mitogenic and transforming properties of full-length B-Raf, whereas the presence of exon 8b sequences seems to have opposite effects. Therefore, alternative splicing represents a novel regulatory mechanism for a protein of the Raf family.

The Ras target AF-6 is a substrate of the fam deubiquitinating enzyme

The Ras target AF-6 has been shown to serve as one of the peripheral components of cell-cell adhesions, and is thought to participate in cell-cell adhesion regulation downstream of Ras. We here purified an AF-6-interacting protein with a molecular mass of approximately 220 kD (p220) to investigate the function of AF-6 at cell-cell adhesions. The peptide sequences of p220 were identical to the amino acid sequences of mouse Fam. Fam is homologous to a deubiquitinating enzyme in Drosophila, the product of the fat facets gene. Recent genetic analyses indicate that the deubiquitinating activity of the fat facets product plays a critical role in controlling the cell fate. We found that Fam accumulated at the cell-cell contact sites of MDCKII cells, but not at free ends of plasma membranes. Fam was partially colocalized with AF-6 and interacted with AF-6 in vivo and in vitro. We also showed that AF-6 was ubiquitinated in intact cells, and that Fam prevented the ubiquitination of AF-6.

Heptachlor and the mitogen-activated protein kinase module in human lymphocytes

The organochlorine pesticide heptachlor constitutes a potential health hazard because of its persistence in nature, its reported contamination in food and milk, and its possible carcinogenic effects. As a tumor promoter, heptachlor induces human myeloblastic leukemia cells to differentiate, and also down-regulates the tumor suppressor gene p53 in human immune cells. In this study, the heptachlor signaling pathway in human lymphocytes was studied. Addition of heptachlor to human CEM x174 lymphocytic cells reduced the cellular levels of MAP kinase (MAPK, mitogen-activated protein kinase) cascade proteins, including ERK1 (a 44-kDa MAPK), ERK2 (a 42-kDa MAPK), a 85-kDa and a 54-kDa MAP kinase, MEK1 (a 45-kDa ERK kinase) and MEKK (a 78-kDa MEK kinase). However, heptachlor treatment caused a marked increase in the expression of the activated (Thr- and Tyr-dually phosphorylated) ERK1 and ERK2 in the cells. These studies indicate that mitogen-activated protein kinases are important intermediates in the signal transduction pathway of immune cells upon heptachlor exposure, and the observation of stimulation of activated MAP kinases without a simultaneous accumulation of basal enzymes may suggest the involvement of a negative feedback control mechanism in the pathway.

Characterization of graf, the GTPase-activating protein for rho associated with focal adhesion kinase. Phosphorylation and possible regulation by mitogen-activated protein kinase

Graf is a GTPase-activating protein for Rho that interacts with focal adhesion kinase and co-localizes with the actin cytoskeleton (Hildebrand, J. D., Taylor, J. M. and Parsons, J. T. (1996) Mol. Cell. Biol. 16, 3169-3178). We examined the expression and regulation of Graf as a prelude to understanding the role of Graf in mediating signal transduction in vivo. We demonstrated that Graf is a ubiquitously expressed 95-kDa protein with high levels observed in heart and brain and cells derived from these tissues. Stimulation of PC12 cells with epidermal growth factor or nerve growth factor induced a phosphatase-reversible mobility shift upon gel electrophoresis, indicative of phosphorylation. In vitro, purified mitogen-activated protein (MAP) kinase catalyzed the phosphorylation of Graf on serine 510, suggesting that Graf phosphorylation may be mediated through MAP kinase signaling. In addition, the mutation of serine 510 to alanine inhibited the epidermal growth factor-induced mobility shift of mutant Graf protein in vivo, consistent with serine 510 being the site of in vivo phosphorylation. Based on these data we suggest that phosphorylation of Graf by MAP kinase or related kinases may be a mechanism by which growth factor signaling modulates Rho-mediated cytoskeletal changes in PC12 and perhaps other cells.

Role of MAP kinase in neurons

Extracellular stimuli such as neurotransmitters, neurotrophins, and growth factors in the brain regulate critical cellular events, including synaptic transmission, neuronal plasticity, morphological differentiation and survival. Although many such stimuli trigger Ser/Thr-kinase and tyrosine-kinase cascades, the extracellular signal-regulated kinases, ERK1 and ERK2, prototypic members of the mitogen-activated protein (MAP) kinase family, are most attractive candidates among protein kinases that mediate morphological differentiation and promote survival in neurons. ERK1 and ERK2 are abundant in the central nervous system (CNS) and are activated during various physiological and pathological events such as brain ischemia and epilepsy. In cultured hippocampal neurons, simulation of glutamate receptors can activate ERK signaling, for which elevation of intracellular Ca2+ is required. In addition, brain-derived neurotrophic factor and growth factors also induce the ERK signaling and here, receptor-coupled tyrosine kinase activation has an association. We describe herein intracellular cascades of ERK signaling through neurotransmitters and neurotrophic factors. Putative functional implications of ERK and other MAP-kinase family members in the central nervous system are give attention.

Interferon gamma activation of Raf-1 is Jak1-dependent and p21ras-independent

Signal transduction through the interferongamma (IFNgamma) receptor involves the formation of a ligand-dependent multimolecular association of receptor chains (alpha and beta), Janus tyrosine kinases (Jak1 and Jak2), and the transcription factor (signal transducers and activators of transcription 1alpha (STAT1alpha)) in addition to activation of mitogen-activated protein kinases (MAPK). Interactions between components of the Jak/STAT cascade and the p21(ras)/Raf-1/MAPK cascade are unexplored. Treatment of HeLa cells with IFNgamma resulted in the rapid and transient activation of Raf-1 and MAPK. Parallel activation of cells resulted in essentially no enhancement of p21(ras) activation despite marked enhancement after treatment with epidermal growth factor. In HeLa (E1C3) and fibrosarcoma (U4A) cell lines, both of which are deficient in Jak1 kinase, Raf-1 activation by IFNgamma was absent. Reconstitution of Raf-1 activity was observed only with kinase active Jak1 in both cell lines. In COS cells, transient expression of wild type or kinase-inactive Jak1 coimmunoprecipitated with Raf-1, but activation of Raf-1 activity was only observed in cells expressing kinase-active Jak1. These observations suggest that a kinase-active Jak1 is required for IFNgamma activation of Raf-1 that is p21(ras)-independent.

The Ras target AF-6 interacts with ZO-1 and serves as a peripheral component of tight junctions in epithelial cells

The dynamic rearrangement of cell-cell junctions such as tight junctions and adherens junctions is a critical step in various cellular processes, including establishment of epithelial cell polarity and developmental patterning. Tight junctions are mediated by molecules such as occludin and its associated ZO-1 and ZO-2, and adherens junctions are mediated by adhesion molecules such as cadherin and its associated catenins. The transformation of epithelial cells by activated Ras results in the perturbation of cell-cell contacts. We previously identified the ALL-1 fusion partner from chromosome 6 (AF-6) as a Ras target. AF-6 has the PDZ domain, which is thought to localize AF-6 at the specialized sites of plasma membranes such as cell-cell contact sites. We investigated roles of Ras and AF-6 in the regulation of cell-cell contacts and found that AF-6 accumulated at the cell-cell contact sites of polarized MDCKII epithelial cells and had a distribution similar to that of ZO-1 but somewhat different from those of catenins. Immunoelectron microscopy revealed a close association between AF-6 and ZO-1 at the tight junctions of MDCKII cells. Native and recombinant AF-6 interacted with ZO-1 in vitro. ZO-1 interacted with the Ras-binding domain of AF-6, and this interaction was inhibited by activated Ras. AF-6 accumulated with ZO-1 at the cell-cell contact sites in cells lacking tight junctions such as Rat1 fibroblasts and PC12 rat pheochromocytoma cells. The overexpression of activated Ras in Rat1 cells resulted in the perturbation of cell-cell contacts, followed by a decrease of the accumulation of AF-6 and ZO-1 at the cell surface. These results indicate that AF-6 serves as one of the peripheral components of tight junctions in epithelial cells and cell-cell adhesions in nonepithelial cells, and that AF-6 may participate in the regulation of cell-cell contacts, including tight junctions, via direct interaction with ZO-1 downstream of Ras.

Induction and activation of mitogen-activated protein kinases of human lymphocytes as one of the signaling pathways of the immunomodulatory effects of morphine sulfate

Morphine sulfate causes immunomodulatory and immunosuppressive effects in human. In this study, the signaling pathway involved in these morphine effects was studied. Addition of morphine sulfate to human CEMx174 lymphocytic cells resulted in increased expression of mitogen-activated protein kinase cascade proteins. Morphine enhanced the cellular levels of ERK1 (44 kDa), ERK2 (42 kDa), a 54-kDa ERK, MEK1 (45 kDa), and MEKK (78 kDa). A time-dependent increase in the activated (Thr and Tyr dually phosphorylated) state of ERK1 and ERK2 was also observed. Naloxone, a morphine antagonist, reversed the observed morphine effects, implicating a micro opioid receptor-mediated process. These findings suggest that mitogen-activated protein kinases are important intermediates in signal transduction pathways initiated by morphine receptors in immune cells.

Characterization of the mitogen-activated protein kinase kinase 4 (MKK4)/c-Jun NH2-terminal kinase 1 and MKK3/p38 pathways regulated by MEK kinases 2 and 3. MEK kinase 3 activates MKK3 but does not cause activation of p38 kinase in vivo

We previously reported the isolation of cDNAs encoding two mammalian mitogen-activated protein kinase (MAPK)/extracellular-regulated kinase (ERK) kinase kinases, designated MEKK2 and MEKK3 (Blank, J.L., Gerwins, P., Elliott, E.M., Sather, S. and Johnson, G.L. (1996) J. Biol. Chem. 271, 5361-5368). In the present study, cotransfection experiments were used to examine the regulation by MEKK2 and MEKK3 of the dual specificity MAP kinase kinases, MKK3 and MKK4. MKK3 specifically phosphorylates and activates p38, whereas MKK4 phosphorylates and activates both p38 and JNK. Coexpression of MEKK2 or MEKK3 with MKK4 in COS-7 cells resulted in activation of MKK4, as assessed by enhanced autophosphorylation and by its ability to phosphorylate and activate recombinant JNK1 or p38 in vitro. MKK3 autophosphorylation and activation of p38 was also observed following coexpression of MKK3 with MEKK3, but not with MEKK2. Consistent with these observations, immunoprecipitated MEKK2 directly activated recombinant MKK4 in vitro but failed to activate MKK3. The sites of activating phosphorylation in MKK3 and MKK4 were identified within kinase subdomains VII and VIII. Replacement of Ser189 or Thr193 in MKK3 with Ala abolished autophosphorylation and activation of MKK3 by MEKK3. Analogous mutations in MKK4 indicated that Ser221 and, to a lesser extent, Thr225 were necessary for MKK4 activation by MEKK2 and MEKK3. These data indicate that MKK3 is preferentially activated by MEKK3, whereas MKK4 is activated both by MEKK2 and MEKK3. Consistent with these observations, MEKK2 and MEKK3 also activated JNK1 in vivo. However, MEKK3 failed to activate p38 when coexpressed in either the absence or presence of MKK3, indicating that MEKK3 is not coupled to p38 activation in vivo. These observations suggest that regulation of p38 and JNK1 pathways by MEKK3 may involve distinct mechanisms to prevent p38 activation but to allow JNK1 activation.

Tumor necrosis factor receptors (Tnfr) in mouse fibroblasts deficient in Tnfr1 or Tnfr2 are signaling competent and activate the mitogen-activated protein kinase pathway with differential kinetics

To dissect tumor necrosis factor receptor (Tnfr)-1 (CD120a) and Tnfr2 (CD120b)-dependent signal transduction pathways, primary fibroblasts isolated from inguinal adipose tissue of wild type (wt), tnfr1(o), tnfr2(o), and tnfr1(o)/tnfr2(o) mice were studied. The mitogen-activated protein kinases Erk1 and Erk2 were found to be tyrosine-phosphorylated and activated by Tnf treatment in all wt, tnfr1(o), and tnfr2(o) fibroblasts; the activation was down-regulated 60 min after the start of steady state Tnf treatment. Distinct kinetics of Erk1 and Erk2 activation were detected; the Tnfr1-mediated activation of Erk1 and Erk2 started more slowly and persisted for more prolonged times as compared with Tnfr2 activation. Raf-1, Raf-B, Mek-1, Mek kinase, and p90(rsk) kinases were also shown to be activated independently in a distinct time-dependent pattern through the two Tnf receptors. In addition, both Tnfr1 and Tnfr2 mediated independently the activation of the transcription factor Ap-1 albeit with parallel activation kinetics. In contrast, Tnfr1 exclusively mediated activation of NF-kappaB and fibroblast proliferation; however, Tnfr2 enhanced proliferation triggered through Tnfr1. These findings indicate distinct but also overlapping roles of Tnfr1 and Tnfr2 in primary mouse fibroblasts and suggest different regulation mechanisms of signal transduction pathways under the control of both Tnf receptors.

Molecular cloning of mitogen-activated protein/ERK kinase kinases (MEKK) 2 and 3. Regulation of sequential phosphorylation pathways involving mitogen-activated protein kinase and c-Jun kinase

Mitogen-activated protein/ERK kinase kinases (MEKKs) phosphorylate and activate protein kinases which in turn phosphorylate and activate the p42/44 mitogen-activated protein kinase (MAPK), c-Jun/stress-activated protein kinases (JNKs), and p38/Hog1 kinase. We have isolated the cDNAs for two novel mammalian MEKKs (MEKK 2 and 3). MEKK 2 and 3 encode proteins of 69.7 and 71 kDa, respectively. The kinase domains encoded in the COOH-terminal moiety are 94% conserved; the NH2-terminal moieties are approximately 65% homologous, suggesting this region may encode sequences conferring differential regulation of the two kinases. Expression of MEKK 2 or 3 in HEK293 cells results in activation of p42/44MAPK and JNK but not of p38/Hog1 kinase. Immunoprecipitated MEKK 2 phosphorylated the MAP kinase kinases, MEK 1, and JNK kinase. Titration of MEKK 2 and 3 expression in transfection assays indicated that MEKK 2 preferentially activated JNK while MEKK 3 preferentially activated p42/44MAPK. These findings define a family of MEKK proteins capable of regulating sequential protein kinase pathways involving MAPK members.

Activation of brain B-Raf protein kinase by Rap1B small GTP-binding protein

Rap1 small GTP-binding protein has the same amino acid sequence at its effector domain as that of Ras. Rap1 has been shown to antagonize the Ras functions, such as the Ras-induced transformation of NIH 3T3 cells and the Ras-induced activation of the c-Raf-1 protein kinase-dependent mitogen-activated protein (MAP) kinase cascade in Rat-1 cells, whereas we have shown that Rap1 as well as Ras stimulates DNA synthesis in Swiss 3T3 cells. We have established a cell-free assay system in which Ras activates bovine brain B-Raf protein kinase. Here we have used this assay system and examined the effect of Rap1 on the B-Raf activity to phosphorylate recombinant MAP kinase kinase (MEK). Recombinant Rap1B stimulated the activity of B-Raf, which was partially purified from bovine brain and immunoprecipitated by an anti-B-Raf antibody. The GTP-bound form was active, but the GDP-bound form was inactive. The fully post-translationally lipid-modified form was active, but the unmodified form was nearly inactive. The maximum B-Raf activity stimulated by Rap1B was nearly the same as that stimulated by Ki-Ras. Rap1B enhanced the Ki-Ras-stimulated B-Raf activity in an additive manner. These results indicate that not only Ras but also Rap1 is involved in the activation of the B-Raf-dependent MAP kinase cascade.

Identification of AF-6 and canoe as putative targets for Ras

Ras (Ha-Ras, Ki-Ras, N-Ras) is implicated in the regulation of various cell functions such as gene expression and cell proliferation downstream from specific extracellular signals. Here, we partially purified a Ras-interacting protein with molecular mass of about 180 kDa (p180) from bovine brain membrane extract by glutathione S-transferase (GST)-Ha-Ras affinity column chromatography. This protein bound to the GTP gamma S (guanosine 5'-(3-O-thio)triphosphate, a nonhydrolyzable GTP analog).GST-Ha-Ras affinity column but not to those containing GDP.GST-Ha-Ras or GTP gamma S.GST-Ha-Ras with a mutation in the effector domain (Ha-RasA38). The amino acid sequences of the peptides derived from p180 were almost identical to those of human AF-6 that is identified as the fusion partner of the ALL-1 protein. The ALL-1/AF-6 chimeric protein is the critical product of the t (6:11) abnormality associated with some human leukemia. AF-6 has a GLGF/Dlg homology repeat (DHR) motif and shows a high degree of sequence similarity with Drosophila Canoe, which is assumed to function downstream from Notch in a common developmental pathway. The recombinant N-terminal domain of AF-6 and Canoe specifically interacted with GTP gamma S.GST-Ha-Ras. The known Ras target c-Raf-1 inhibited the interaction of AF-6 with GTP gamma S.GST-Ha-Ras. These results indicate that AF-6 and Canoe are putative targets for Ras.

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.

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.

Purification of a Ras-dependent mitogen-activated protein kinase kinase kinase from bovine brain cytosol and its identification as a complex of B-Raf and 14-3-3 proteins

We previously purified a protein factor, named REKS (Ras-dependent Extracellular Signal-regulated Kinase (ERK)/mitogen-activated protein kinase Kinase (MEK) Stimulator), from Xenopus eggs by use of a cell-free assay system in which recombinant GTP gamma S (guanosine 5'-(3-O-thio)triphosphate)-Ki-Ras activates recombinant MEK. By use of this assay system, we purified here bovine REKS to near homogeneity from the cytosol fraction of bovine brain by successive chromatographies of Mono S, Mono Q, GTP gamma S-glutathione S-transferase-Ha-Ras-coupled glutathione-agarose, and Mono Q columns. It was composed of three proteins with masses of about 95, 32, and 30 kDa as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The 95-, 32-, and 30-kDa proteins were identified by immunoblot analysis to be B-Raf protein kinase, 14-3-3 protein, and 14-3-3 protein, respectively. Moreover, the REKS activity was specifically immunoprecipitated by an anti-B-Raf antibody. Bovine REKS was activated by lipid-modified GTP gamma S-Ki-Ras far more effectively than by a lipid-unmodified one. Lipid-modified GDP-Ki-Ras was inactive. Exogenous addition of 14-3-3 proteins stimulated further the REKS activity both in the presence and absence of GTP gamma S-Ki-Ras. These results indicate that at least one of the direct targets of Ras is B-Raf complexed with 14-3-3 proteins in bovine brain.

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

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