Activation of B-Raf kinase requires phosphorylation of the conserved residues Thr598 and Ser601

Direct recruitment of ERK cascade components to inducible genes is regulated by heterogeneous nuclear ribonucleoprotein (hnRNP) K

Components of the ERK cascade are recruited to genes, but it remains unknown how they are regulated at these sites. The RNA-binding protein heterogeneous nuclear ribonucleoprotein (hnRNP) K interacts with kinases and is found along genes including the mitogen-inducible early response gene EGR-1. Here, we used chromatin immunoprecipitations to study co-recruitment of hnRNP K and ERK cascade activity along the EGR-1 gene. These measurements revealed that the spatiotemporal binding patterns of ERK cascade transducers (GRB2, SOS, B-Raf, MEK, and ERK) at the EGR-1 locus resemble both hnRNP K and RNA polymerase II (Pol II). Inhibition of EGR-1 transcription with either serum-responsive factor knockdown or 5,6-dichloro-1-β-D-ribofuranosylbenzimidazole altered recruitment of all of the above ERK cascade components along this locus that mirrored the changes in Pol II and hnRNP K profiles. siRNA knockdown of hnRNP K decreased the levels of active MEK and ERK at the EGR-1, changes associated with decreased levels of elongating pre-mRNA and less efficient splicing. The hnRNP K dependence and pattern of ERK cascade activation at the c-MYC locus were different from at EGR-1. Ribonucleoprotein immunoprecipitations revealed that hnRNP K was associated with the EGR-1 but not c-MYC mRNAs. These data suggest a model where Pol II transcription-driven recruitment of hnRNP K along the EGR-1 locus compartmentalizes activation of the ERK cascade at these genes, events that regulate synthesis of mature mRNA.

Direct recruitment of insulin receptor and ERK signaling cascade to insulin-inducible gene loci

OBJECTIVE

Insulin receptor (IR) translocates to the nucleus, but its recruitment to gene loci has not been demonstrated. Here, we tested the hypothesis that IR and its downstream mitogenic transducers are corecruited to two prototypic insulin-inducible genes: early growth response 1 (egr-1), involved in mitogenic response, and glucokinase (Gck), encoding a key metabolic enzyme.

RESEARCH DESIGN AND METHODS

We used RNA and chromatin from insulin-treated rat hepatic tumor cell line expressing human insulin receptor (HTC-IR) and livers from lean and insulin-resistant ob/ob glucose-fed mice in quantitative RT-PCR and chromatin immunoprecipitation studies to determine gene expression levels and associated recruitment of RNA polymerase II (Pol II), insulin receptor, and cognate signaling proteins to gene loci, respectively.

RESULTS

Insulin-induced egr-1 mRNA in HTC-IR cells was associated with corecruitment of IR signaling cascade (IR, SOS, Grb2, B-Raf, MEK, and ERK) to this gene. Recruitment profiles of phosphorylated IR, B-Raf, MEK, and Erk along egr-1 transcribed region were similar to those of elongating Pol II. Glucose-feeding increased Gck mRNA expression in livers of lean but not ob/ob mice. In lean mice, there was glucose feeding-induced recruitment of IR and its transducers to Gck gene synchronized with elongating Pol II. In sharp contrast, in glucose-fed ob/ob mice, the Gck recruitment patterns of active MEK/Erk, IR, and Pol II were asynchronous.

CONCLUSIONS

IR and its signal transducers recruited to genes coupled to elongating Pol II may play a role in maintaining productive mRNA synthesis of target genes. These studies suggest a possibility that impaired Pol II processivity along genes bearing aberrant levels of IR/signal transducers is a previously unrecognized facet of insulin resistance.

The Role of B-RAF Mutations in Melanoma and the Induction of EMT via Dysregulation of the NF-κB/Snail/RKIP/PTEN Circuit

Melanoma is a highly metastatic cancer, and there are no current therapeutic modalities to treat this deadly malignant disease once it has metastasized. Melanoma cancers exhibit B-RAF mutations in up to 70% of cases. B-RAF mutations are responsible, in large part, for the constitutive hyperactivation of survival/antiapoptotic pathways such as the MAPK, NF-κB, and PI3K/AKT. These hyperactivated pathways regulate the expression of genes targeting the initiation of the metastatic cascade, namely, the epithelial to mesenchymal transition (EMT). EMT is the result of the expression of mesenchymal gene products such as fibronectin, vimentin, and metalloproteinases and the invasion and inhibition of E-cadherin. The above pathways cross-talk and regulate each other's activities and functions. For instance, the NF-κB pathway directly regulates EMT through the transcription of gene products involved in EMT and indirectly through the transcriptional up-regulation of the metastasis inducer Snail. Snail, in turn, suppresses the expression of the metastasis suppressor gene product Raf kinase inhibitor protein RKIP (inhibits the MAPK and the NF-κB pathways) as well as PTEN (inhibits the PI3K/AKT pathway). The role of B-RAF mutations in melanoma and their direct role in the induction of EMT are not clear. This review discusses the hypothesis that B-RAF mutations are involved in the dysregulation of the NF-κB/Snail/RKIP/PTEN circuit and in both the induction of EMT and metastasis. The therapeutic implications of the dysregulation of the above circuit by B-RAF mutations are such that they offer novel targets for therapeutic interventions in the treatment of EMT and metastasis.

BRAF mutation in papillary thyroid carcinoma: pathogenic role and clinical implications

Papillary thyroid cancer (PTC) is the most common endocrine malignancy, accounting for 85-90% of all thyroid cancers. Genetic alternations involving the mitogen-activated protein kinase (MAPK) pathway are frequently demonstrated in PTC, such as RET/PTC, RAS, and B-type Raf kinase (BRAF) mutations. Over 90% of BRAF mutations are T1799A, resulting in a BRAF(V600E) mutation. BRAF(V600E) is present in approximately 50% of PTC and also found in aggressive histologic variants and PTC-derived anaplastic thyroid cancer, but is rare in follicular variants, and not found in follicular thyroid cancer. The tumorigenic role of BRAF(V600E) in the development of PTC was documented in thyroid-targeted BRAF(V600E) transgenic mice, and rat thyroid cells overexpressed with BRAF(V600E) suggested that BRAF(V600E) is an initiator of tumorigenesis and is required for tumor progression in PTC. Most clinical studies have demonstrated an association of BRAF(V600E) mutation with aggressive clinicopathologic characteristics and high tumor recurrence, although the results are controversial. The association is also observed in patients with papillary thyroid microcarcinomas and low-risk PTC. As a highly specific and unique mutation in PTC, testing for BRAF(V600E) in fine-needle aspiration specimens has been shown to refine the diagnostic accuracy of PTC in indeterminate cytology. Preoperative BRAF(V600E) analysis in low-risk patients may provide important value for prognostication, and these patients might benefit from receiving more intensive management and frequent follow-up. BRAF-targeted therapies have been developed to treat various human cancers including advanced thyroid cancers. Preclinical results are encouraging, but the anticancer effects of clinical trials are disappointing. Studies of multi-kinase inhibitors and/or combination with other regimens are underway in the treatment of advanced thyroid cancers. In this article, we review the pathogenesis of PTC, and the clinical implications of BRAF(V600E) mutation in the diagnosis, prognosis and potential targeted therapeutic strategies for thyroid cancers.

The phosphorylation specificity of B-RAF WT, B-RAF D594V, B-RAF V600E and B-RAF K601E kinases: an in silico study

Phosphorylation of the B-RAF kinase is an essential process in tumour induction and maintenance in several cancers. Herein the phosphorylation specificity of the activation segment of the wild type B-RAF kinase and the B-RAF(D594V), B-RAF(V600E) and B-RAF(K601E) mutants was examined by molecular dynamics (MD) simulations and GRID molecular interaction field analysis. According to our analysis, Thr599 and Ser602 were the only residues in the activation segment in B-RAF(WT) that were well exposed to ATP binding, which is in agreement with the experimental results, and provide a molecular basis of the observed phosphorylation. The phosphorylation specificity was altered significantly for each of the three different mutants studied due to the large conformational changes and subsequent alterations in the electrostatic forces between several residues for each of these mutants. Thus the analysis revealed limited phosphorylation potential of the non-active B-RAF(D594V) mutant and several potential ATP binding sites were identified for the highly active B-RAF(V600E) mutant. The Lys601 residue, which is specific to RAF and not present in the activation segment of other similar kinases, was identified to potentially be of major importance to the observed differences in the phosphorylation specificity of the mutants. Our results indicate that Lys601 might be a specific ATP coordinating residue, contributing to the B-RAF phosphorylation specificity. The overall results can be helpful for the understanding of the B-RAF phosphorylation processes on a molecular level.

Application of a novel [3+2] cycloaddition reaction to prepare substituted imidazoles and their use in the design of potent DFG-out allosteric B-Raf inhibitors

B-Raf protein kinase, which is a key signaling molecule in the RAS-RAF-MEK-ERK signaling pathway, plays an important role in many cancers. The B-Raf V600E mutation represents the most frequent oncogenic kinase mutation known and is responsible for increased kinase activity in approximately 7% of all human cancers, establishing B-Raf as an important therapeutic target for inhibition. Through the use of an iterative program that utilized a chemocentric approach and a rational structure based design, we have developed novel, potent, and specific DFG-out allosteric inhibitors of B-Raf kinase. Here, we present efficient and versatile chemistry that utilizes a key one pot, [3+2] cycloaddition reaction to obtain highly substituted imidazoles and their application in the design of allosteric B-Raf inhibitors. Inhibitors based on this scaffold display subnanomolar potency and a favorable kinase profile.

Multi-determinants analysis of molecular alterations for predicting clinical benefit to EGFR-targeted monoclonal antibodies in colorectal cancer

Background

KRAS mutations occur in 35–45% of metastatic colorectal cancers (mCRC) and preclude responsiveness to EGFR-targeted therapy with cetuximab or panitumumab. However, less than 20% patients displaying wild-type KRAS tumors achieve objective response. Alterations in other effectors downstream of the EGFR, such as BRAF, and deregulation of the PIK3CA/PTEN pathway have independently been found to give rise to resistance. We present a comprehensive analysis of KRAS, BRAF, PIK3CA mutations, and PTEN expression in mCRC patients treated with cetuximab or panitumumab, with the aim of clarifying the relative contribution of these molecular alterations to resistance.

Methodology/Principal Findings

We retrospectively analyzed objective tumor response, progression-free (PFS) and overall survival (OS) together with the mutational status of KRAS, BRAF, PIK3CA and expression of PTEN in 132 tumors from cetuximab or panitumumab treated mCRC patients. Among the 106 non-responsive patients, 74 (70%) had tumors with at least one molecular alteration in the four markers. The probability of response was 51% (22/43) among patients with no alterations, 4% (2/47) among patients with 1 alteration, and 0% (0/24) for patients with ≥2 alterations (p<0.0001). Accordingly, PFS and OS were increasingly worse for patients with tumors harboring none, 1, or ≥2 molecular alteration(s) (p<0.001).

Conclusions/Significance

When expression of PTEN and mutations of KRAS, BRAF and PIK3CA are concomitantly ascertained, up to 70% of mCRC patients unlikely to respond to anti-EGFR therapies can be identified. We propose to define as ‘quadruple negative’, the CRCs lacking alterations in KRAS, BRAF, PTEN and PIK3CA. Comprehensive molecular dissection of the EGFR signaling pathways should be considered to select mCRC patients for cetuximab- or panitumumab-based therapies.

Applications of genomics in melanoma oncogene discovery

The identification of recurrent alterations in the melanoma genome has provided key insights into the biology of melanoma genesis and progression. These discoveries have come about as a result of the systematic deployment and integration of diverse genomic technologies, including DNA sequencing, chromosomal copy number analysis, and gene expression profiling. Here, the discoveries of several key melanoma oncogenes affecting critical cell pathways are described and the role played by evolving genomics technologies in melanoma oncogene discovery is examined. These advances are being exploited to improve prognosis and treatment of melanoma patients through the development of genome-based diagnostic and targeted therapeutic avenues.

An in silico study of the molecular basis of B-RAF activation and conformational stability

BACKGROUND

B-RAF kinase plays an important role both in tumour induction and maintenance in several cancers and it is an attractive new drug target. However, the structural basis of the B-RAF activation is still not well understood.

RESULTS

In this study we suggest a novel molecular basis of B-RAF activation based on molecular dynamics (MD) simulations of B-RAFWT and the B-RAFV600E, B-RAFK601E and B-RAFD594V mutants. A strong hydrogen bond network was identified in B-RAFWT in which the interactions between Lys601 and the well known catalytic residues Lys483, Glu501 and Asp594 play an important role. It was found that several mutations, which directly or indirectly destabilized the interactions between these residues within this network, contributed to the changes in B-RAF activity.

CONCLUSION

Our results showed that the above mechanisms lead to the disruption of the electrostatic interactions between the A-loop and the alphaC-helix in the activating mutants, which presumably contribute to the flipping of the activation segment to an active form. Conversely, in the B-RAFD594V mutant that has impaired kinase activity, and in B-RAFWT these interactions were strong and stabilized the kinase inactive form.

Antitumor efficacy of the novel RAF inhibitor GDC-0879 is predicted by BRAFV600E mutational status and sustained extracellular signal-regulated kinase/mitogen-activated protein kinase pathway suppression

Oncogenic activation of the BRAF serine/threonine kinase has been associated with initiation and maintenance of melanoma tumors. As such, development of pharmacologic agents to target RAF proteins or their effector kinases is an area of intense investigation. Here we report the biological properties of GDC-0879, a highly selective, potent, and orally bioavailable RAF small-molecule inhibitor. We used extracellular signal-regulated kinase (ERK)-1/2 and mitogen-activated protein kinase/ERK kinase (MEK)-1/2 phosphorylation as biomarkers to explore the relationship between tumor outcome and pharmacodynamic inhibition of the RAF-MEK-ERK pathway. In GDC-0879-treated mice, both cell line- and patient-derived BRAF(V600E) tumors exhibited stronger and more sustained pharmacodynamic inhibition (>90% for 8 hours) and improved survival compared with mutant KRAS-expressing tumors. Despite the involvement of activated RAF signaling in RAS-induced tumorigenesis, decreased time to progression was observed for some KRAS-mutant tumors following GDC-0879 administration. Moreover, striking differences were noted for RAF and MEK inhibition across a panel of 130 tumor cell lines. Whereas GDC-0879-mediated efficacy was associated strictly with BRAF(V600E) status, MEK inhibition also attenuated proliferation and tumor growth of cell lines expressing wild-type BRAF (81% KRAS mutant, 38% KRAS wild type). The responsiveness of BRAF(V600E) melanoma cells to GDC-0879 could be dramatically altered by pharmacologic and genetic modulation of phosphatidylinositol 3-kinase pathway activity. These data suggest that GDC-0879-induced signaling changes are dependent on the point of oncogenic activation within the RAS network. Taken together, these studies increase our understanding of the molecular determinants for antitumor efficacy resulting from RAF pathway inhibition and have implications for therapeutic intervention in the clinic.

Detection and molecular characterization of a novel BRAF activated domain mutation in follicular variant of papillary thyroid carcinoma

To assess the mutational status of BRAF in FVPTC, we directly sequenced the genomic DNA of 30 primary FVPTC samples. BRAF mutations were found in only 4 (13%) tumors. We also identified a previously unknown (novel) mutation in the activation kinase domain of the BRAF (A598V), replacing alanine with valine. Functional analysis showed that this mutation led to the up-regulation of the BRAF kinase activity and its downstream signaling factors. The effect of this mutation on the structural formation of the protein is highlighted. Our results confirm the infrequency of BRAF (V600E) mutation in FVPTC and identify a novel (A598V) mutation of this gene.

Role of B-Raf(V600E) in differentiated thyroid cancer and preclinical validation of compounds against B-Raf(V600E)

B-Raf(V600E), an oncogenic protein kinase, is the most frequent genetic alteration in papillary thyroid carcinomas (PTC). PTC represents 80-90% of all thyroid cancers and over the past five years, more than 200 manuscripts have been published about the relationship between "B-Raf(V600E) and thyroid cancer". B-Raf(V600E) genetically arises from a transversion point mutation (valine-to-glutamate substitution at amino acid residue-600, V600E) and leads to over activation of the mitogen-activated protein kinases (MAPK) signaling pathway. The MAPK pathway is essential for transmitting proliferation signals generated by cell surface receptors and cytoplasmic signaling elements to the nucleus. In many cancers, including thyroid cancer, B-Raf(V600E) appears to play a crucial role in cell proliferation, survival and de-differentiation. In thyroid cancer, the V600E mutation occurs with greater frequently in aggressive subtypes of PTC, and in individuals that present at advanced stages of disease with extra-thyroidal extension and/or lymph node metastases. B-Raf(V600E) is considered a marker of aggressive disease in both PTC (>1 cm) and micro-PTC (</=1 cm), and interestingly, is associated with both loss of I-131 avidity and PTC recurrence. Though treatment of patients with thyroid cancer is usually successful and most patients are rendered disease-free, to date there are no effective therapies for patients with invasive, non-radioiodine sensitive tumors or metastatic disease. In this article we will review the relation between B-Raf(V600E) and PTC, as well as both non-selective and selective pharmacological agents currently under investigation for treatment of B-Raf(V600E) positive PTC.

A novel complex BRAF mutation detected in a solid variant of papillary thyroid carcinoma

BRAF gene mutations are identified in about 45% of papillary thyroid carcinomas (PTC) and represent the most common genetic event in this tumor. Here, we report a case of PTC, solid variant, with a complex BRAF mutation that involves one nucleotide substitution, C1796T, and a CTT triplet insertion, 1798_1799insCTT, located on the same allele. This mutation leads to the replacement of a threonine with an isoleucine, T599I, and replacement of a valine with an alanine and a leucine, V600delinsAL. This mutation was identified both in the preoperative fine needle aspirate sample and in the surgical specimen after total thyroidectomy. Other rare BRAF mutations in PTC are reviewed.

Wild-type BRAF is required for response to panitumumab or cetuximab in metastatic colorectal cancer

PURPOSE Cetuximab or panitumumab are effective in 10% to 20% unselected metastatic colorectal cancer (CRC) patients. KRAS mutations account for approximately 30% to 40% patients who are not responsive. The serine-threonine kinase BRAF is the principal effector of KRAS. We hypothesized that, in KRAS wild-type patients, BRAF mutations could have a predictive/prognostic value. PATIENTS AND METHODS We retrospectively analyzed objective tumor responses, time to progression, overall survival (OS), and the mutational status of KRAS and BRAF in 113 tumors from cetuximab- or panitumumab-treated metastatic CRC patients. The effect of the BRAF V600E mutation on cetuximab or panitumumab response was also assessed using cellular models of CRC. Results KRAS mutations were present in 30% of the patients and were associated with resistance to cetuximab or panitumumab (P = .011). The BRAF V600E mutation was detected in 11 of 79 patients who had wild-type KRAS. None of the BRAF-mutated patients responded to treatment, whereas none of the responders carried BRAF mutations (P = .029). BRAF-mutated patients had significantly shorter progression-free survival (P = .011) and OS (P < .0001) than wild-type patients. In CRC cells, the introduction of BRAF V600E allele impaired the therapeutic effect of cetuximab or panitumumab. Treatment with the BRAF inhibitor sorafenib restored sensitivity to panitumumab or cetuximab of CRC cells carrying the V600E allele. CONCLUSION BRAF wild-type is required for response to panitumumab or cetuximab and could be used to select patients who are eligible for the treatment. Double-hit therapies aimed at simultaneous inhibition of epidermal growth factor receptor and BRAF warrant exploration in CRC patients carrying the V600E oncogenic mutation.

Characterization of Ser338 phosphorylation for Raf-1 activation

Raf kinases are essential for regulating cell proliferation, survival, and tumorigenesis. However, the mechanisms by which Raf is activated are still incompletely understood. Phosphorylation plays a critical role in Raf activation in response to mitogens. The present study characterizes phosphorylation of Ser338, a crucial event for Raf-1 activation. Here we report that mutation of Lys375 to Met diminishes phosphorylation of Ser338 on both wild type Raf-1 in cells treated with epidermal growth factor (EGF) or 12-O-tetradecanoylphorbol-13-acetate (TPA) and a constitutively active mutant in which Tyr340/Tyr341 are replaced by 2 aspartic acids, a conserved substitution present in natural B-Raf. The loss of Ser338 phosphorylation in these Raf mutants is not engendered by a mutation-induced conformational change, inasmuch as mutation of another site (Ser471 to Ala) in the activation segment also abolishes Ser338 phosphorylation, whereas both the kinase-dead mutants of Raf-1 are phosphorylated well by active Pak1. Furthermore, our data demonstrate that EGF-stimulated phosphorylation of Ser338 is inhibited by Sorafenib, a Raf kinase inhibitor, but not by the MEK inhibitor U0126. Interestingly, a kinase-dead mutation and Sorafenib also markedly reduce phosphorylation of Ser445 on B-Raf, a site equivalent to Raf-1 Ser338. Finally, our data reveal that Ser338 is phosphorylated on inactive Raf-1 by an active mutant of Raf-1 when they are dimerized in cells and that artificial dimerization of Raf-1 causes Ser338 phosphorylation, accompanied by activation of ERK1/2. Altogether, our data suggest that Ser338 on Raf-1 is autophosphorylated in response to mitogens.

Are MAP kinases drug targets? Yes, but difficult ones

Pharmaceutical companies are facing an increasing interest in new target identification and validation. In particular, extensive efforts are being made in the field of protein kinase inhibitors research and development, and the past ten years of effort in this field have altered our perception of the potential of kinases as drug targets. Therefore, in the drug discovery process, the selection of relevant, susceptible protein kinase targets combined with searches for leads and candidates have become a crucial approach. The success of recent launches of protein kinase inhibitors (Gleevec, Imatinib, Sutent, Iressa, Nexavar, Sprycel) gave another push to this field. Numerous other kinase inhibitors are currently undergoing clinical trials or clinical development. Some questions are nevertheless unanswered, mostly related to the great number of known kinases in the human genome, to their similarity with each other, to the existence of functionally redundant kinases for specific pathways, and also because the connection between particular pathways and diseases is not always clear. The review is leading the reader through a panoramic view of protein kinase inhibition with a major focus on MAPK, successful examples and clinical candidates.

Identification of RIP1 kinase as a specific cellular target of necrostatins

Necroptosis is a cellular mechanism of necrotic cell death induced by apoptotic stimuli in the form of death domain receptor engagement by their respective ligands under conditions where apoptotic execution is prevented. Although it occurs under regulated conditions, necroptotic cell death is characterized by the same morphological features as unregulated necrotic death. Here we report that necrostatin-1, a previously identified small-molecule inhibitor of necroptosis, is a selective allosteric inhibitor of the death domain receptor-associated adaptor kinase RIP1 in vitro. We show that RIP1 is the primary cellular target responsible for the antinecroptosis activity of necrostatin-1. In addition, we show that two other necrostatins, necrostatin-3 and necrostatin-5, also target the RIP1 kinase step in the necroptosis pathway, but through mechanisms distinct from that of necrostatin-1. Overall, our data establish necrostatins as the first-in-class inhibitors of RIP1 kinase, the key upstream kinase involved in the activation of necroptosis.

Positive regulation of A-RAF by phosphorylation of isoform-specific hinge segment and identification of novel phosphorylation sites

In mammals the RAF family of serine/threonine kinases consists of three members, A-, B-, and C-RAF. Activation of RAF kinases involves a complex series of phosphorylations. Although the most prominent phosphorylation sites of B- and C-RAF are well characterized, little is known about regulatory phosphorylation of A-RAF. Using mass spectrometry, we identified here a number of novel in vivo phosphorylation sites in A-RAF. In particular, we found that Ser-432 participates in MEK binding and is indispensable for A-RAF signaling. On the other hand, phosphorylation within the activation segment does not contribute to epidermal growth factor-mediated activation. Furthermore, we show that the potential 14-3-3 binding domains in A-RAF are phosphorylated independently of its activation status. Of importance, we identified a novel regulatory domain in A-RAF (referred to as IH-segment) positioned between amino acids 248 and 267 that contains seven putative phosphorylation sites. Three of these sites, serines 257, 262, and 264, regulate A-RAF activation in a stimulatory manner. The spatial model of the A-RAF fragment, including residues between Ser-246 and Glu-277, revealed a switch of charge at the molecular surface of the IH-region upon phosphorylation, suggesting a mechanism in which the high accumulation of negative charges may lead to an electrostatic destabilization of protein-membrane interaction resulting in depletion of A-RAF from the plasma membrane. Together, we provide here for the first time a detailed analysis of in vivo A-RAF phosphorylation status and demonstrate that regulation of A-RAF by phosphorylation exhibits unique features compared with B- and C-RAF.

Necrotic cell death and 'necrostatins': now we can control cellular explosion

The receptor-interacting protein 1 (RIP1) kinase activity is necessary for death-receptor-induced necrotic cell death. Recently, it has been demonstrated that 'necrostatins' efficiently block tumor necrosis factor-induced necrotic cell death through the inhibition of RIP1 kinase activity. This discovery supports the concept that receptor-induced necrosis, just like apoptosis, is a controlled cellular process. In addition, necrostatins are becoming important tools for evaluating the contribution of necrotic cell death in experimental disease models.

Characterization of a novel complex BRAF mutation in a follicular variant papillary thyroid carcinoma

INTRODUCTION

Activating mutations of the BRAF oncogene are frequently detected in papillary thyroid carcinoma (PTC) and have been associated with a worse prognosis. The amino acid substitution V600E accounts for 90% of all oncogenic BRAF mutations and is typically detected in classic PTCs, whereas other less frequent BRAF mutations seem to be associated with other PTC histotypes.

CASE

Screening for activating BRAF mutations in a series of 83 PTCs identified the most common V600E mutation in 39 cases (histologically, 38 classic PTCs and 1 sclerosing variant PTC) and a complex in-frame mutation involving amino acids V600-S605 in a stage III multicentric follicular variant PTC, occurring in a 50-year-old female patient, who was affected by hypothyroidism in autoimmune thyroiditis and had a family history of PTC and autoimmune thyroiditis. Since the identified BRAF mutation was novel in the literature, bioinformatic modeling was performed to predict its impact on BRAF activity. Although the mutation resulted in loss of a phosphorylation site in the activation loop of BRAF, it was predicted to increase BRAF kinase activity by mimicking an activating phosphorylation.

CONCLUSIONS

This study, which reports a new BRAF mutation, highlights the usefulness of bioinformatic modeling in the prediction of functional effects of new mutations and indicates that mutation-specific screening tests might miss some rare BRAF mutations. These facts should be taken into consideration in the molecular diagnosis of thyroid cancer and in the design of therapeutic protocols based on inhibitors of the BRAF pathway.

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.

Relations between the mitogen-activated protein kinase and the cAMP-dependent protein kinase pathways: comradeship and hostility

Inter- and intracellular communications and responses to environmental changes are pivotal for the orchestrated and harmonious operation of multi-cellular organisms. These well-tuned functions in living organisms are mediated by the action of signal transduction pathways, which are responsible for receiving a signal, transmitting and amplifying it, and eliciting the appropriate cellular responses. Mammalian cells posses numerous signal transduction pathways that, rather than acting in solitude, interconnect with each other, a phenomenon referred to as cross-talk. This allows cells to regulate the distribution, duration, intensity and specificity of the response. The cAMP/cAMP-dependent protein kinase (PKA) pathway and the mitogen-activated protein kinase (MAPK) cascades modulate common processes in the cell and multiple levels of cross-talk between these signalling pathways have been described. The first- and best-characterized interconnections are the PKA-dependent inhibition of the MAPKs ERK1/2 mediated by RAF-1, and PKA-induced activation of ERK1/2 interceded through B-RAF. Recently, novel interactions between components of these pathways and new mechanisms for cross-talk have been elucidated. This review discusses both known and novel interactions between compounds of the cAMP/PKA and MAPKs signalling pathways in mammalian cells.

PIK3CA, KRAS, and BRAF mutations in intraductal papillary mucinous neoplasm/carcinoma (IPMN/C) of the pancreas

BACKGROUND AND AIMS

Recent studies have reported high frequencies of somatic mutations in the phosphoinositide-3-kinase catalytic-alpha (PIK3CA) gene in various human tumors. Three hot-spot mutations in the exons 9 and 20 have been proven to activate the Akt signalling pathway. The Raf/MEK/ERK (mitogen-activated protein kinase) signal transduction is an important mediator of a number of cellular fates including growth, proliferation, and survival. The BRAF gene is activated by oncogenic RAS, leading to cooperative effects in cells responding to growth factor signals. Here we evaluate the mutational status of PIK3CA, KRAS, and BRAF in intraductal papillary mucinous neoplasm/carcinoma (IPMN/IPMNC) of the pancreas.

MATERIALS AND METHODS

Exons 1, 4, 5, 6, 7, 9, 12, 18, and 20 of PIK3CA, exons 1 of KRAS, and exons 5, 11, and 15 of BRAF were analyzed in 36 IPMN/IPMC and two mucinous cystadenoma specimens by direct genomic DNA sequencing.

RESULTS

We identified four somatic missense mutations of PIK3CA within the 36 IPMN/IPMC specimens (11%). One of the four mutations, H1047R, has been previously reported to be a hot-spot mutation. Furthermore, we found 17 (47%) KRAS mutations in exon 1 and one missense mutation (2.7%) in exon 15 of BRAF.

CONCLUSION

This data is the first report of PIK3CA mutation in pancreatic cancer and it appears to be the first oncogene to be mutated in IPMN/IPMC but not in conventional ductal adenocarcinoma of the pancreas. Our data provide evidence that PIK3CA and BRAF contribute to the tumorigenesis of IPMN/IPMC, but at a lower frequency than KRAS.

Na/H exchange regulatory factor 1, a novel AKT-associating protein, regulates extracellular signal-regulated kinase signaling through a B-Raf-mediated pathway

Na/H exchange regulatory factor 1 (NHERF1) is a scaffolding protein that regulates signaling and trafficking of several G protein-coupled receptors (GPCRs), including the parathyroid hormone receptor (PTH1R). GPCRs activate extracellular signal-regulated kinase (ERK)1/2 through different mechanisms. Here, we characterized NHERF1 regulation of PTH1R-stimulated ERK1/2. Parathyroid hormone (PTH) stimulated ERK1/2 phosphorylation by a protein kinase A (PKA)-dependent, but protein kinase C-, cyclic adenosine 5'-monophosphate-, and Rap1-independent pathway in Chinese hamster ovary cells stably transfected with the PTH1R and engineered to express NHERF1 under the control of tetracycline. NHERF1 blocked PTH-induced ERK1/2 phosphorylation downstream of PKA. This suggested that NHERF1 inhibitory effects on ERK1/2 occur at a postreceptor locus. Forskolin activated ERK1/2, and this effect was blocked by NHERF1. NHERF1 interacted with AKT and inhibited ERK1/2 activation by decreasing the stimulatory effect of 14-3-3 binding to B-Raf, while increasing the inhibitory influence of AKT negative regulation on ERK1/2 activation. This novel regulatory mechanism provides a new model by which cytoplasmic adapter proteins modulate ERK1/2 activation through a receptor-independent mechanism involving B-Raf.

Human cutaneous melanoma; a review of NRAS and BRAF mutation frequencies in relation to histogenetic subclass and body site

A majority of cutaneous melanomas show activating mutations in the NRAS or BRAF proto-oncogenes, components of the Ras-Raf-Mek-Erk signal transduction pathway. Consistent data demonstrate the early appearance, in a mutually exclusive manner, of these mutations. The purpose of this paper is to summarize the literature on NRAS and BRAF activating mutations in melanoma tumors with respect to available data on histogenetic classification as well as body site and presumed UV-exposure. Common alterations of the signal transducing network seem to represent molecular hallmarks of cutaneous melanomas and therefore should continue to strongly stimulate design and testing of targeted molecular interventions.

Unique N-region Determines Low Basal Activity and Limited Inducibility of A-RAF Kinase

In mammals the RAF family of serine/threonine kinases consists of three members, A-, B-, and C-RAF. A prominent feature of RAF isoforms regards differences in basal and inducible kinase activities. To elucidate the nature of these differences, we studied the role of the nonconserved residues within the N-region (Negative-charge regulatory region). The nonconserved amino acids in positions -3 and +1 relative to the highly conserved serine 299 in A-RAF and serine 338 in C-RAF have so far not been considered as regulatory residues. Here we demonstrate the essential role of these residues in the RAF activation process. Substitution of tyrosine 296 in A-RAF to arginine led to a constitutively active kinase. In contrast, substitution of glycine 300 by serine (mimicking B- and C-RAF) acts in an inhibitory manner. Consistent with these data, the introduction of glycine in the analogous position of C-RAF (S339G mutant) led to a constitutively active C-RAF kinase. Based on the three-dimensional structure of the catalytic domain of B-RAF and using the sequences of the N-regions of A- and C-RAF, we searched by molecular modeling for the putative contact points between these two moieties. A tight interaction between the N-region residue serine 339 of C-RAF and arginine 398 of the catalytic domain was identified and proposed to inhibit the kinase activity of RAF proteins, because abrogation of this interaction contributes to RAF activation. Furthermore, tyrosine 296 in A-RAF favors a spatial orientation of the N-region segment, which enables a tighter contact to the catalytic domain, whereas a glutamine residue at this position in C-RAF abrogates this interaction. Considering this observation, we suggest that tyrosine 296, which is unique for A-RAF, is a major determinant of the low activating potency of this RAF isoform.

BRAF(E600) in benign and malignant human tumours

Of the RAF family of protein kinases, BRAF is the only member to be frequently activated by mutation in cancer. A single amino acid substitution (V600E) accounts for the vast majority and results in constitutive activation of BRAF kinase function. Its expression is required to maintain the proliferative and oncogenic characteristics of BRAF(E600)-expressing human tumour cells. Although BRAF(E600) acts as an oncogene in the context of additional genetic lesions, in primary cells it appears to be associated rather with transient stimulation of proliferation. Eventually, BRAF(E600) signalling triggers cell cycle arrest with the hallmarks of cellular senescence, as is illustrated by several recent studies in cultured cells, animal models and benign human lesions. In this review, we will discuss recent advances in our understanding of the role of BRAF(E600) in benign and malignant human tumours and the implications for therapeutic intervention.

IQGAP1 modulates activation of B-Raf

IQGAP1 modulates several cellular functions, including cell-cell adhesion, transcription, cytoskeletal architecture, and selected signaling pathways. We previously documented that IQGAP1 binds ERK and MAPK kinase (MEK) and regulates EGF-stimulated MEK and ERK activity. Here we characterize the interaction between IQGAP1 and B-Raf, the molecule immediately upstream of MEK in the Ras/MAPK signaling cascade. B-Raf binds directly to IQGAP1 in vitro and coimmunoprecipitates with IQGAP1 from cell lysates. Importantly, IQGAP1 modulates B-Raf function. EGF is unable to stimulate B-Raf activity in IQGAP1-null cells and in cells transfected with an IQGAP1 mutant construct that is unable to bind B-Raf. Interestingly, binding to IQGAP1 significantly enhances B-Raf activity in vitro. Our data identify a previously unrecognized interaction between IQGAP1 and B-Raf and suggest that IQGAP1 is a scaffold necessary for activation of B-Raf by EGF.

Raf kinases: function, regulation and role in human cancer

The Ras-Raf-MAPK pathway regulates diverse physiological processes by transmitting signals from membrane based receptors to various nuclear, cytoplasmic and membrane-bound targets, coordinating a large variety of cellular responses. Function of Raf family kinases has been shown to play a role during organism development, cell cycle regulation, cell proliferation and differentiation, cell survival and apoptosis and many other cellular and physiological processes. Aberrations along the Ras-Raf-MAPK pathway play an integral role in various biological processes concerning human health and disease. Overexpression or activation of the pathway components is a common indicator in proliferative diseases such as cancer and contributes to tumor initiation, progression and metastasis. In this review, we focus on the physiological roles of Raf kinases in normal and disease conditions, specifically cancer, and the current thoughts on Raf regulation.

KSR and CNK: two scaffolds regulating RAS-mediated RAF activation

The RAS-RAF-MEK-extracellular-regulated kinase (RAS/ERK) pathway is a major intracellular route used by metazoan cells to channel to downstream targets a diverse array of signals, including those controlling cell proliferation and survival. Recent findings suggest that the pathway is assembled by specific scaffolding proteins that in turn regulate the efficiency, the location and/or the duration of signal transmission. Here, through the angle of studies conducted in Drosophila and C. elegans, we present two such proteins, the kinase suppressor of RAS (KSR) and connector enhancer of KSR (CNK) scaffolds, and highlight their implication in a novel mechanism regulating RAS-mediated RAF activation. Based on recent findings, we discuss the possibility that KSR, a RAF-like protein, does not solely act as a scaffold, but directly induces RAF catalytic function by a kinase-independent mechanism apparently shared by RAF-like proteins.

Key signaling pathways and targets in lung cancer therapy

Despite the use of chemotherapy, radiation therapy, and surgery, the overall outcome for lung cancer continues to be disappointing. In order to make a difference in the treatment of lung cancer, novel therapeutics need to be developed. The molecular mechanisms of carcinogenesis in lung cancer are complex and involve multiple oncogenes, tumor suppressor genes, receptor tyrosine kinases, cytoplasmic enzymes, and tumor interstitial elements, among other cellular proteins. In this review, the authors discuss key signaling pathways and molecular targets in the treatment of lung cancer. Through understanding molecular targets and the utilization of specific inhibitors, hopefully, a dramatic impact will be made in the biology and therapy of lung cancer.

Raf kinase signaling functions in sensory neuron differentiation and axon growth in vivo

To define the role of the Raf serine/threonine kinases in nervous system development, we conditionally targeted B-Raf and C-Raf, two of the three known mammalian Raf homologs, using a mouse line expressing Cre recombinase driven by a nestin promoter. Targeting of B-Raf, but not C-Raf, markedly attenuated baseline phosphorylation of Erk in neural tissues and led to growth retardation. Conditional elimination of B-Raf in dorsal root ganglion (DRG) neurons did not interfere with survival, but instead caused marked reduction in expression of the glial cell line-derived neurotrophic factor receptor Ret at postnatal stages, associated with a profound reduction in levels of transcription factor CBF-beta. Elimination of both alleles of Braf, which encodes B-Raf, and one allele of Raf1, which encodes C-Raf, affected DRG neuron maturation as well as proprioceptive axon projection toward the ventral horn in the spinal cord. Finally, conditional elimination of all Braf and Raf1 alleles strongly reduced neurotrophin-dependent axon growth in vitro as well as cutaneous axon terminal arborization in vivo. We conclude that Raf function is crucial for several aspects of DRG neuron development, including differentiation and axon growth.

Minimally invasive biomarkers for detection and staging of colorectal cancer

Colorectal cancer (CRC) is the third most prevalent cancer, with approximately one million cases estimated world wide in 2006, resulting in more than 500,000 deaths. Approximately 40% of colorectal cancers are diagnosed with localized disease, which have approximately a 90% five-year survival rate. However, the prognosis worsens with advancing stage, and only 5% of patients diagnosed with distant metastases survive 5 years. Therefore, the need for early detection is clear. Currently, guaiac-based fecal-occult blood testing (gFOBT) in large population-based trials has been shown to reduce CRC-related mortality, but the sensitivity for detecting early disease is low. DNA-based fecal screening appears to have the potential to significantly increase sensitivity without decreasing specificity. This review will focus primarily on newer molecular-based biomarkers and their utility in screening large populations for CRC, predicting treatment efficacy and providing prognostic information related to survival and disease progression.

BRAF and KRAS gene mutations in intraductal papillary mucinous neoplasm/carcinoma (IPMN/IPMC) of the pancreas

The Raf/MEK/ERK (MAPK) signal transduction is an important mediator of a number of cellular fates including growth, proliferation, and survival. The BRAF gene is activated by oncogenic RAS, leading to cooperative effects in cells responding to growth factor signals. Our study was performed to elucidate a possible role of BRAF in the development of IPMN (Intraductal Papillary Mucinous Neoplasm) and IPMC (Intraductal Papillary Mucinous Carcinoma) of the pancreas. Mutations of BRAF and KRAS were evaluated in 36 IPMN/IPMC samples and two mucinous cystadenomas by direct genomic sequencing. Exons 1 for KRAS, and 5, 11, and 15 for BRAF were examined. Totally we identified 17 (47%) KRAS mutations in exon 1, codon 12 and one missense mutation (2.7%) within exon 15 of BRAF. The mutations appear to be somatic since the same alterations were not detected in the corresponding normal tissues. Our data provide evidence that oncogenic properties of BRAF contribute to the tumorigenesis of IPMN/IPMC, but at a lower frequency than KRAS.

The integration of signaling by multiprotein complexes containing Raf kinases

In vivo, eukaryotic cells are subjected simultaneously to a broad array of signals ranging from mitogens and inflammatory inputs to environmental stresses and developmental cues. The combinatorial nature of cellular signaling necessitates that a cell integrate its signal transduction pathways so as to implement rapidly and efficiently an appropriate suite of responses. Emerging evidence indicates that, over the course of evolution, cells have developed multiprotein signaling complexes, or "signalosomes" that mediate the coordinate regulation of different signaling pathways. Such molecular signal integration contrasts with the classical notion of signaling complexes assembled by scaffold proteins-entities that function to segregate specific pathways from one another. This review will focus on two signal integrating multiprotein complexes that involve Raf family kinases: the MLK3-B-Raf-Raf-1 complex and the Raf-1-Mst-2 complex.

Differential regulation of B-raf isoforms by phosphorylation and autoinhibitory mechanisms

The B-Raf proto-oncogene encodes several isoforms resulting from alternative splicing in the hinge region upstream of the kinase domain. The presence of exon 8b in the B2-Raf(8b) isoform and exon 9b in the B3-Raf(9b) isoform differentially regulates B-Raf by decreasing and increasing MEK activating and oncogenic activities, respectively. Using different cell systems, we investigated here the molecular basis of this regulation. We show that exons 8b and 9b interfere with the ability of the B-Raf N-terminal region to interact with and inhibit the C-terminal kinase domain, thus modulating the autoinhibition mechanism in an opposite manner. Exons 8b and 9b are flanked by two residues reported to down-regulate B-Raf activity upon phosphorylation. The S365A mutation increased the activity of all B-Raf isoforms, but the effect on B2-Raf(8b) was more pronounced. This was correlated to the high level of S365 phosphorylation in this isoform, whereas the B3-Raf(9b) isoform was poorly phosphorylated on this residue. In contrast, S429 was equally phosphorylated in all B-Raf isoforms, but the S429A mutation activated B2-Raf(8b), whereas it inhibited B3-Raf(9b). These results indicate that phosphorylation on both S365 and S429 participate in the differential regulation of B-Raf isoforms through distinct mechanisms. Finally, we show that autoinhibition and phosphorylation represent independent but convergent mechanisms accounting for B-Raf regulation by alternative splicing.

Role of the Mitogen-Activated Protein Kinase Signaling Pathway in the Regulation of Human Melanocytic Antigen Expression

Heterogeneous expression of melanocytic antigens occurs frequently in melanomas and represents a potent barrier to immunotherapy. We previously showed that coordinated losses of several melanocytic antigens are generally attributable to down-regulation of antigen gene expression rather than irreversible mutation. Treatment of melanoma cells with mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK) inhibitors blocks ERK activation and increases steady-state levels of mRNAs and corresponding protein expression for the melanocytic antigens Melan-A/MART-1, gp100, and tyrosinase. Although the degree of MEK inhibitor enhancement of antigen expression varied among different cell lines irrespective of their antigen expression status, all showed detectable responses. Notably, the antigen-enhancing effects of the MEK inhibitors could not be attributed to the master melanocytic regulator MITF-M. Because MAPK pathway activation via constitutively active mutant forms of BRAF is common in melanomas, correlation between BRAF function and antigen expression was investigated. No simple correlation of endogenous BRAF mutational status and antigen levels was observed, but transient overexpression of V600E BRAF increased ERK activation and reduced Melan-A/MART-1 levels in antigen-positive cell lines. These data indicate that whereas multiple factors may regulate antigen expression in melanomas, enhancement of MAPK signaling can act as a negative influence. Blocking such signaling with MEK inhibitors accordingly augments antigen levels, thereby enhancing Melan-A/MART-1-specific cytotoxic T-cell responses to antigen-negative cells following MEK inhibition treatment. Consequently, MAPK inhibition may assist targeting of melanomas for immunotherapy.

BRAF and NRAS mutations in melanoma and melanocytic nevi

In this report, we investigated BRAF/NRAS mutations in samples from a case-control study of melanoma and a series of benign melanocytic nevi. We evaluated potential associations between BRAF mutations and histopathologic and pigmentary characteristics of melanoma. Mutations in BRAF and NRAS were detected by sequencing microdissected/laser-captured DNA from 18 in-situ melanomas, 64 primary melanomas, and 51 nevi. Nevi showed the highest frequency of BRAF mutations (82%). BRAF mutations were identified in 29% of invasive melanomas and in only 5.6% of in-situ melanomas. Mutations in NRAS were found in 5.2% of primary melanomas, 5.9% of nevi and no NRAS mutations were seen in in-situ melanomas. A majority of the BRAF mutations observed in primary invasive melanoma were seen in superficial spreading melanoma (15/17), and melanomas with BRAF mutations were also more likely to be found on a body site that was likely to be exposed to intermittent sun exposure compared with chronic or no sun exposure (P=0.02). Tumors with BRAF mutations were also significantly more likely to occur in association with a contiguous nevus (odds ratio 3.49, 95% confidence interval 1.06-11.46), although a contiguous nevus was not found in all melanomas with a BRAF mutation. Our data support the evidence that the mitogen-activated protein kinase pathway is upregulated in a large percentage of melanocytic lesions, but these mutations are not sufficient for malignant transformation. We suggest that BRAF mutations contribute to benign melanocytic hyperplasia, but are likely to contribute to invasive melanoma only in conjunction with other mutations.

Rheb Inhibits C-Raf Activity and B-Raf/C-Raf Heterodimerization

The Ras-Raf-MEK signaling cascade is critical for normal development and is activated in many forms of cancer. We have recently shown that B-Raf kinase interacts with and is inhibited by Rheb, the target of the GTPase-activating domain of the tuberous sclerosis complex 2 gene product tuberin. Here, we demonstrate for the first time that activation of Rheb is associated with decreased B-Raf and C-Raf phosphorylation at residues Ser-446 and Ser-338, respectively, concomitant with a decrease in the activities of both kinases and decreased heterodimerization of B-Raf and C-Raf. Importantly, the impact of Rheb on B-Raf/C-Raf heterodimerization and kinase activity are rapamycin-insensitive, indicating that they are independent of Rheb activation of the mammalian target of rapamycin-Raptor complex. In addition, we found that Rheb inhibits the association of B-Raf with H-Ras. Taken together, these results support a central role of Rheb in the regulation of the Ras/B-Raf/C-Raf/MEK signaling network.

Assessing the utility of a mutational assay for B-RAF as an adjunct to conventional fine needle aspiration of the thyroid gland

Thyroid carcinoma is the most common endocrine malignancy; it is typified by a number of classical genomic insults, which tend to cluster with the discrete histologic subtypes. The most common of these is a mutation in B-RAF, which is present in approximately 44% (29% to 83%) of cases. In this review we have assessed the potential utility of a molecular test for somatically acquired mutations in B-RAF using thyroid malignancy as a model system according to 3 fundamental questions: would a test enhance our ability to distinguish benign from malignant, would a test unveil a risk factor not otherwise known, and would detecting a mutation enable a therapeutic option specific to those patients who carry the mutation?

Molecular and cellular pathophysiology of autosomal recessive polycystic kidney disease (ARPKD)

Autosomal recessive polycystic kidney disease (ARPKD) belongs to a group of congenital hepatorenal fibrocystic syndromes characterized by dual renal and hepatic involvement of variable severity. Despite the wide clinical spectrum of ARPKD (MIM 263200), genetic linkage studies indicate that mutations at a single locus, PKHD1 (polycystic kidney and hepatic disease 1), located on human chromosome region 6p21.1-p12, are responsible for all phenotypes of ARPKD. Identification of cystic disease genes and their encoded proteins has provided investigators with critical tools to begin to unravel the molecular and cellular mechanisms of PKD. PKD cystic epithelia share common phenotypic abnormalities despite the different genetic mutations that underlie the disease. Recent studies have shown that many cyst-causing proteins are expressed in multimeric complexes at distinct subcellular locations within epithelia. This co-expression of cystoproteins suggests that cyst formation, regardless of the underlying disease gene, results from perturbations in convergent and/or integrated signal transduction pathways. To date, no specific therapies are in clinical use for ameliorating cyst growth in ARPKD. However, studies noted in this review suggest that therapeutic targeting of the cAMP and epidermal growth factor receptor (EGFR)-axis abnormalities in cystic epithelia may translate into effective therapies for ARPKD and, by analogy, autosomal dominant polycystic kidney disease (ADPKD). A particularly promising approach appears to be the targeting of downstream intermediates of both the cAMP and EGFR axis. This review focuses on ARPKD and presents a concise summary of the current understanding of the molecular genetics and cellular pathophysiology of this disease. It also highlights phenotypic and mechanistic similarities between ARPKD and ADPKD.

Functional analysis of the regulatory requirements of B-Raf and the B-Raf(V600E) oncoprotein

The BRAF(V600E) mutation is found in approximately 6% of human cancers and mimics the phosphorylation of the kinase domain activation segment. In wild-type B-Raf (B-Raf(wt)), activation segment phosphorylation is thought to cooperate with negative charges within the N-region for full activation. In contrast to Raf-1, the N-region of B-Raf is constitutively negatively charged owing to the presence of residues D447/D448 and the phosphorylation of S446. Therefore, it has been suggested that this hallmark predisposes B-Raf for oncogenic activation. In this study, we demonstrate that neutralizing mutations of these residues (in particular S446 and S447), or uncoupling of B-Raf from Ras-guanine 5'-triphosphate (GTP), strongly reduce the biological activity of B-Raf in a PC12 cell differentiation assay. We also confirm that S365 is a 14-3-3 binding site, and determine that mutation of this residue rescues the impaired biological activity of B-Raf proteins with a neutralized N-region, suggesting that the N-region opposes a 14-3-3-mediated transition into an inactive conformation. However, in the case of B-Raf(V600E), although complete N-region neutralization resulted in a 2.5-fold reduction in kinase activity in vitro, this oncoprotein strongly induced PC12 differentiation or transformation and epithelial-mesenchymal transition of MCF-10A cells regardless of its N-region charge. Furthermore, the biological activity of B-Raf(V600E) was independent of its ability to bind Ras-GTP. Our analysis identifies important regulatory differences between B-Raf(wt) and B-Raf(V600E) and suggests that B-Raf(V600E) cannot be inhibited by strategies aimed at blocking S446 phosphorylation or Ras activation.

A KSR/CNK complex mediated by HYP, a novel SAM domain-containing protein, regulates RAS-dependent RAF activation in Drosophila

RAF is a critical effector of the small GTPase RAS in normal and malignant cells. Despite intense scrutiny, the mechanism regulating RAF activation remains partially understood. Here, we show that the scaffold KSR (kinase suppressor of RAS), a RAF homolog known to assemble RAF/MEK/ERK complexes, induces RAF activation in Drosophila by a mechanism mediated by its kinase-like domain, but which is independent of its scaffolding property or putative kinase activity. Interestingly, we found that KSR is recruited to RAF prior to signal activation by the RAF-binding protein CNK (connector enhancer of KSR) in association with a novel SAM (sterile alpha motif) domain-containing protein, named Hyphen (HYP). Moreover, our data suggest that the interaction of KSR to CNK/HYP stimulates the RAS-dependent RAF-activating property of KSR. Together, these findings identify a novel protein complex that controls RAF activation and suggest that KSR does not only act as a scaffold for the MAPK (mitogen-activated protein kinase) module, but may also function as a RAF activator. By analogy to catalytically impaired, but conformationally active B-RAF oncogenic mutants, we discuss the possibility that KSR represents a natural allosteric inducer of RAF catalytic function.

The novel SAM domain protein Aveugle is required for Raf activation in the Drosophila EGF receptor signaling pathway

Activation of the Raf kinase by GTP-bound Ras is a poorly understood step in receptor tyrosine kinase signaling pathways. One such pathway, the epidermal growth factor receptor (EGFR) pathway, is critical for cell differentiation, survival, and cell cycle regulation in many systems, including the Drosophila eye. We have identified a mutation in a novel gene, aveugle, based on its requirement for normal photoreceptor differentiation. The phenotypes of aveugle mutant cells in the eye and wing imaginal discs resemble those caused by reduction of EGFR pathway function. We show that aveugle is required between ras and raf for EGFR signaling in the eye and for mitogen-activated protein kinase phosphorylation in cell culture. aveugle encodes a small protein with a sterile alpha motif (SAM) domain that can physically interact with the scaffold protein connector enhancer of Ksr (Cnk). We propose that Aveugle acts together with Cnk to promote Raf activation, perhaps by recruiting an activating kinase.

Mixed-lineage kinase 3 regulates B-Raf through maintenance of the B-Raf/Raf-1 complex and inhibition by the NF2 tumor suppressor protein

The Ras --> Raf --> MEK1/2 --> extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) pathway couples mitogenic signals to cell proliferation. B-Raf and Raf-1 function within an oligomer wherein they are regulated in part by mutual transactivation. The MAPK kinase kinase (MAP3K) mixed-lineage kinase 3 (MLK3) is required for mitogen activation of B-Raf and cell proliferation. Here we show that the kinase activity of MLK3 is not required for support of B-Raf activation. Instead, MLK3 is a component of the B-Raf/Raf-1 complex and is required for maintenance of the integrity of this complex. We show that the activation of ERK and the proliferation of human schwannoma cells bearing a loss-of-function mutation in the neurofibromatosis 2 (NF2) gene require MLK3. We find that merlin, the product of NF2, blunts the activation of both ERK and c-Jun N-terminal kinase (JNK). Finally, we demonstrate that merlin and MLK3 can interact in situ and that merlin can disrupt the interactions between B-Raf and Raf-1 or those between MLK3 and either B-Raf or Raf-1. Thus, MLK3 is part of a multiprotein complex and is required for ERK activation. The levels of this complex may be negatively regulated by merlin.

Oncogenic BRAF Is Required for Tumor Growth and Maintenance in Melanoma Models

The usual paradigm for developing kinase inhibitors in oncology is to use a high-affinity proof-of-concept inhibitor with acceptable metabolic properties for key target validation experiments. This approach requires substantial medicinal chemistry and can be confounded by drug toxicity and off-target activities of the test molecule. As a better alternative, we have developed inducible short-hairpin RNA xenograft models to examine the in vivo efficacy of inhibiting oncogenic BRAF. Our results show that tumor regression resulting from BRAF suppression is inducible, reversible, and tightly regulated in these models. Analysis of regressing tumors showed the primary mechanism of action for BRAF to be increased tumor cell proliferation and survival. In a metastatic melanoma model, conditional BRAF suppression slowed systemic tumor growth as determined by in vivo bioluminescence imaging. Taken together, gain-of-function BRAF signaling is strongly associated with in vivo tumorigenicity, confirming BRAF as an important target for small-molecule and RNA interference-based therapeutics.

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).