Oncogene (2011) 30, 3477–3488; doi:10.1038/onc.2011.160; published online 16 May 2011

Raf kinases in cancer–roles and therapeutic opportunities

G Maurer1, B Tarkowski1 and M Baccarini1

1Max F Perutz Laboratories, Center for Molecular Biology, University of Vienna, Vienna, Austria

Correspondence: Professor M Baccarini, Max F Perutz Laboratories, Center for Molecular Biology, University of Vienna, Dr Bohr Gasse 9, Vienna A-1030, Austria. E-mail:

Received 15 February 2011; Revised 3 April 2011; Accepted 3 April 2011; Published online 16 May 2011.



Raf are conserved, ubiquitous serine/protein kinases discovered as the cellular elements hijacked by transforming retroviruses. The three mammalian RAF proteins (A, B and CRAF) can be activated by the human oncogene RAS, downstream from which they exert both kinase-dependent and kinase-independent, tumor-promoting functions. The kinase-dependent functions are mediated chiefly by the MEK/ERK pathway, whose activation is associated with proliferation in a broad range of human tumors. Almost 10 years ago, activating BRAF mutations were discovered in a subset of human tumors, and in the past year treatment with small-molecule RAF inhibitors has yielded unprecedented response rates in melanoma patients. Thus, Raf qualifies as an excellent molecular target for anticancer therapy. This review focuses on the role of BRAF and CRAF in different aspects of carcinogenesis, on the success of molecular therapies targeting Raf and the challenges they present.


Raf; Ras; ERK pathway; hallmarks of cancer; kinase inhibitors


Raf proteins and their effectors

The first member of the Raf family, C-Raf-1 (also known as Raf-1), was identified in a oncogene capture experiment in which its catalytic domain was found fused to the retroviral Gag protein, resulting in the constitutive activation of the serine/threonine kinase activity of C-Raf (Rapp et al., 1983; Moelling et al., 1984); 4 years later, B-Raf was discovered in a similar experiment (Marx et al., 1988). Within 10 years of its discovery, C-Raf was identified both as an interaction partner and activator of mitogen-activated protein kinase (MAPK)/ERK kinase (MEK), the dual-specificity kinase responsible for activation of extracellular signal-regulated kinase (ERK), and an effector of Ras, which was reported to recruit C-Raf to the membrane and stimulate its activation by mechanisms, which, roughly 18 years later, are still incompletely understood. Both the history and the regulation of Raf have been reviewed recently (Wellbrock et al., 2004; Niault and Baccarini, 2010). Suffice it to say here that a wealth of studies have led to a widely accepted model in which Raf activation primarily consists in the relief of the inhibition imposed on the Raf catalytic domain by an N-terminal regulatory domain, featuring both a Ras-binding domain and a cysteine-rich domain responsible for interaction with the kinase domain and for Raf autoinhibition (Figure 1a). This basic mechanism applies to all three Raf proteins (A-Raf, B-Raf and C-Raf), although both A-Raf and C-Raf need additional steps, such as phosphorylation of activating residues and dephosphorylation of negative regulatory residues, to reach maximal activation. Thus, B-Raf is the family member most easily activated by Ras (Wellbrock et al., 2004; Niault and Baccarini, 2010). In addition, the basal kinase activity of B-Raf is higher than that of C-Raf and, likely, A-Raf (Pritchard et al., 1995; Emuss et al., 2005). This provides a potential rationale for the frequent mutational activation of BRAF (for example by the prominent BRAFV600E mutation; (Davies et al., 2002)), but not CRAF or ARAF, observed in human tumors. A major advance of the past few years was the discovery that Raf kinases can homo- and heterodimerize (Garnett et al., 2005; Rushworth et al., 2006), and that, in fact, the structure of an active Raf kinase is that of a side-to-side dimer in which only one partner must have catalytic activity (Rajakulendran et al., 2009). Dimerization is enhanced by Ras (Weber et al., 2001) and is subject to negative feedback regulation by ERK (Rushworth et al., 2006; Ritt et al., 2010) (Figure 1b).

Figure 1.
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The structure of Raf and interactions within the Ras/Raf/MEK/ERK pathway. (a) A schematic view of Raf. All three Raf proteins consist of a regulatory and a kinase domain. In quiescent cells, the interaction between these two domains inhibits catalytic activity. The cysteine-rich domain (CRD) is necessary for this inhibition (indicated by the red blunt arrow), which is relieved by the binding of Ras to the Ras-binding domain (RBD). A-Raf and C-Raf need additional steps for full-fledged activation. (b) Regulation of the ERK pathway. Raf kinases can be activated by homo- and heterodimerization. Dimerization is induced by Ras and can occur in different combinations including not only the Raf kinases but also the pseudokinase KSR. Phosphorylation of Raf residues by activated ERK counteracts dimerization, allowing negative feedback control of the pathway (red blunt arrow). RKIP is an inhibitory protein whose expression is often lost in cancer and which can regulate pathway output at the level of Raf as well as MEK activation. ERK, extracellular signal-regulated kinase; MEK, MAPK/ERK kinase; RKIP, Raf kinase-inhibitory protein.

Full figure and legend (75K)

In the Raf/Mek/Erk pathway, dimerization can be used to exert tight temporal control of the signal, in cases in which one dimer subunit is more prone to negative feedback regulation than the other (C-Raf<B-Raf (Dougherty et al., 2005; Ritt et al., 2010) and Mek1<Mek2 (Catalanotti et al., 2009); reviewed by Wimmer and Baccarini (2010)). A further level of control is exerted by the interaction with inhibitory proteins (Kolch, 2005). In the context of cancer, the most relevant of these is the Raf kinase-inhibitory protein, RKIP (Zeng et al., 2008) (Figure 1b). In addition, a high degree of spatial control is provided by the interaction of pathway components with scaffolds that direct them to distinct subcellular compartments (Kolch, 2005; McKay and Morrison, 2007).

Overexpression of full-length Raf or the truncated catalytic domain leads to the activation of the ERK pathway and increases proliferation in cultured cells and in vivo. Thus, MEK/ERK is undoubtedly a target of activated Raf in tumorigenesis. In the case of C-Raf, other targets potentially contributing to cell transformation have been proposed, such as the nuclear factor-κB pathway (Baumann et al., 2000), Rb (Kinkade et al., 2008) and BAD (Polzien et al., 2009), all reviewed by Niault and Baccarini (2010). In addition, C-Raf can inhibit apoptosis by binding to, and inhibiting, the stress-induced kinase ASK-1 (Chen et al., 2001) and the homolog of Drosophila's Hippo, the MST-2 kinase (O’Neill et al., 2004; Matallanas et al., 2007); and finally, C-Raf interferes, by direct binding, with the activity of the cytoskeleton-based Rho effector Rok-α (also known as ROCK2), resulting in defects in cell migration, apoptosis and differentiation (Ehrenreiter et al., 2005, 2009; Piazzolla et al., 2005) (Figure 2).

Figure 2.
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Functions of Raf. Gene ablation experiments have shown that B-Raf is essential for MEK–ERK activation in most systems. A-Raf and C-Raf heterodimerize with B-Raf and can participate in ERK activation (double-headed arrows). It is unclear whether A-Raf has functions outside the MEK/ERK pathway; C-Raf, however, can promote nuclear factor-κB activation and can inhibit (blunt-headed arrow) signal transducers involved in motility (Rok-α), apoptosis (ASK-1 and MST-2), proliferation and angiogenesis (Rb). ERK, extracellular signal-regulated kinase; MEK, MAPK/ERK kinase.

Full figure and legend (48K)


Raf and the hallmarks of cancer

Six hallmarks of cancer, describing the acquired cell-autonomous capabilities of a cancer cell, were outlined in a legendary review by Hanahan and Weinberg (2000) more than 10 years ago. More recently, the list has been revised to include other features of cancer cells related to their interaction with the environment, such as avoidance of immunosurveillance (Dunn et al., 2004; Smyth et al., 2006; Zitvogel et al., 2006) and the stress phenotypes of cancer (Luo et al., 2009), as well as genomic instability (Negrini et al., 2010).

In the following section, we will highlight the contribution of Raf and of the Raf-dependent pathways to the hallmarks and states of cancer (Figure 3).

Figure 3.
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Contribution of Raf to the hallmarks and phenotypes of cancer. The hallmarks of cancer are depicted in black and the stress phenotypes associated with cancer in dark gray (adapted from Negrini et al. (2010)). The contributions of various Raf isoforms to each hallmark/phenotype and the downstream pathway mediating them are indicated. Raf* represents activated B-Raf or C-Raf; the red arrows indicate kinase-dependent functions of Raf; the red blunt arrows represent kinase-dependent inhibition of downstream pathways and the green blunt arrows represent kinase-independent inhibition processes. For clarity, only the hallmarks/phenotypes in which Raf has been implicated are depicted; mitotic stress and proteotoxic stress have been omitted.

Full figure and legend (233K)

Genomic instability is a feature of almost all human cancers (Negrini et al., 2010). In hereditary cancers, germline mutations in caretaker genes (DNA-repair genes and mitotic checkpoint genes) promote tumor development by increasing the mutational rate and leading to chromosomal instability. In sporadic cancer, the caretaker genes are not mutational targets, and chromosomal instability is rather a consequence of the DNA replication stress induced by the activation of oncogenes, notably Ras.

Germline Raf mutations do not appear to contribute to cancer; instead, mutation in both BRAF and CRAF have been found in human genetic syndromes defined as ‘Rasopathies’ because they are caused by mutations in components of the Ras/ERK pathway (Tidyman and Rauen, 2009). The observed mutations cause activation of BRAF or CRAF, but the two kinases are not interchangeable in this context: mutations in the regulatory domain of CRAF are associated with the development of Noonan Syndrome, also caused by mutations in SOS1 and KRAS, and Leopard syndrome. By contrast, BRAF mutations are associated with Cardio-facio-cutaneous syndrome (CFC), also initiated by activating mutations of MEK (reviewed by Tidyman and Rauen, 2009).

In addition to the mutations identified in Noonan and Leopard syndrome, two weakly transforming germline mutations in the kinase domain of CRAF have been described in patients with therapy-related acute myeloid leukemia, which arises from concomitant loss of the Raf-inhibitory protein, RKIP (Zebisch et al., 2006, 2009). In general, the frequency of mutational changes of CRAF in human cancers is low (1%; However, amplification of CRAF and other members of the ERK pathway have been observed during hormone escape in androgen-independent prostate cancer (Edwards et al., 2003), and both CRAF amplifications (4%) and deletions (2.2%) are strongly associated with tumor progression and an overall poorer survival in bladder cancer (Simon et al., 2001). Similarly, activation of the ERK pathway owing to BRAF gene duplication or mutation has emerged as a mechanism in the pathogenesis of low-grade astrocytomas (Pfister et al., 2008). Besides alterations in copy number, chromosomal translocations involving CRAF are found in certain human cancer sub-types such as stomach cancer (Shimizu et al., 1986) and pilocytic astrocytomas (Jones et al., 2009). The latter tumors also harbor chromosomal translocations involving BRAF activation (Jones et al., 2008); although seldom, such alterations have also been observed in nevi (Dessars et al., 2007) and radiation-induced thyroid cancer (Ciampi et al., 2005). In all cases, the alterations lead to constitutive RAF activation through loss of the autoinhibitory N-terminal domain.

More recently, chromosomal translocations yielding gene fusion transcripts containing the C-terminal kinase domain of CRAF or BRAF have been identified at low frequency in prostate cancer, gastric cancer and melanoma. Both fusion proteins promoted MEK/ERK-dependent cell proliferation, migration and anchorage-independent growth in human prostate cells, but whereas expression of the BRAF fusion protein in NIH 3T3 cells induced tumor formation in nude mice, the CRAF fusion protein failed to do so (Palanisamy et al., 2010), implying crucial signaling differences between the BRAF and the CRAF fusion proteins. Interestingly, prostate cancer also harbored the reciprocal CRAF fusion, containing the CRAF-regulatory domain; this protein, however, has not been investigated in detail.

Besides being the target of chromosomal rearrangements, RAF has also been implicated in the induction of genomic instability. Two types of mutations have been associated with increased genomic instability thus far: BRAFV600E, the activating mutation observed with the highest frequency in melanoma and other cancers, induces genomic instability in a thyroid cell line (Mitsutake et al., 2005); in addition, expression of B-RafD594A, a transforming B-Raf mutant with impaired MEK kinase activity, can promote aneuploidy in a C-Raf-dependent, MEK-independent manner in mouse splenocytes and embryonic fibroblasts (Kamata et al., 2010).

C-Raf, too, has been implicated in promoting genomic instability, albeit indirectly. A balance between C-Raf and RKIP, the Raf inhibitor often lost in breast, prostate and melanoma tumors (Granovsky and Rosner, 2008), is necessary to guarantee fidelity of chromosome segregation. Loss of RKIP or C-Raf overexpression lowers the activity of the Aurora-B kinase, allowing cells to bypass the spindle assembly checkpoint and potentially resulting in genomic instability (Eves et al., 2006).

Self-sufficiency in proliferative signals is a crucial step on the road to transformation. In healthy tissues, soluble mitogenic growth factors are produced by one cell type and stimulate the proliferation of another. Many cancer cells are able to produce and respond to their own growth factors, resulting in a positive feedback signaling loop (autocrine stimulation), which makes them independent from their tissue environment. These proliferative signals include production of growth factors, overexpression/constitutive activation of growth factor receptors and alterations in downstream signaling cascades. The first two changes are likely to activate the Raf pathway, although the degree to which the resulting proliferation may depend on it may vary. In the context of alterations in signaling components, apart from mutational activation of RAF itself, the most direct connection is that between RAF and the members of the RAS gene family, which are activated in 33% of human cancers, particularly in those of epithelial origin (

Several RAF mutations driving the proliferation of cancer cells have been described. The most frequent BRAF mutation, BRAFV600E, causes constitutive activation of the kinase as well as insensitivity to negative feedback mechanisms (Davies et al., 2002; Pratilas et al., 2009). In addition, less frequent BRAF mutations have been described that can stimulate the MEK/ERK pathway by activating wild-type CRAF in the context of a heterodimer (Davies et al., 2002; Garnett et al., 2005; Kamata et al., 2010). Mutations in CRAF itself are extremely rare, but overexpression has been reported at high frequency in subsets of human cancers, including hepatocellular carcinoma and squamous cell carcinoma of the head and neck (Riva et al., 1995; Hwang et al., 2004). CRAF overexpression is regarded as an early tumor marker for human lung adenocarcinoma (Cekanova et al., 2007); consistent with this, lung-restricted overexpression of full-length CRAF or of its truncated kinase domain causes the MEK-dependent formation of lung adenomas (Kerkhoff et al., 2000; Kramer et al., 2004). Similarly, elevated BRAF and CRAF expression and kinase activity have been observed in human glioblastomas, and a constitutive active CRAF mutant contributes to glioma formation in mice (Lyustikman et al., 2008). Cumulatively, these results imply that most alterations in Raf drive proliferation through stimulation of the MEK/ERK pathway. In addition to the dominant role played by BRAF oncogenic mutants, endogenous, wild-type BRAF mediates ERK activation and proliferation in uveal melanoma cells lacking RAS/RAF mutations (Calipel et al., 2006); conversely, CRAF, but not BRAF, is required for these process downstream from mutated NRAS in melanoma cell lines (Dumaz et al., 2006) or from mutated KRAS in non-small cell lung cancer cell lines (Takezawa et al., 2009). Studies in cultured cells and in vivo studies suggest that autocrine/paracrine factors resulting from ERK activation play a role in the self-sufficiency of cells harboring activating Raf mutations (Troppmair et al., 1998; Schulze et al., 2001, 2004; Vale et al., 2001), generating a feed-forward loop and promoting the concomitant activation of parallel proliferative pathways.

In addition to generating their own proliferative signals, either in a cell-autonomous or in a paracrine manner, cancer cells must develop insensitivity to antiproliferative signals that maintain tissue homeostasis. A crucial inducer of antiproliferative signals is transforming growth factor-β (TGFβ) (Seoane, 2008). Many tumors disable TGFβ signaling by downregulation or mutation of the TGFβ receptor, or through inactivation of its downstream targets SMAD4, p15INK4B and the retinoblastoma protein Rb (Hanahan and Weinberg, 2000). Activation of Raf and ERK induces TGFβ production but at the same time protects cells from differentiation and apoptosis (Lehmann et al., 2000; Park et al., 2000; Schulze et al., 2001, 2004; Wang et al., 2004; Riesco-Eizaguirre et al., 2009), enabling them to draw on the pro-tumorigenic effects of this cytokine such as promotion of proliferation, invasiveness, radioresistance and immunosuppression. In addition, C-Raf interacts with, and phosphorylates, Rb. This interaction results in the recruitment of the Rb/C-Raf complex to proliferative promoters and increases E2F1-dependent transcriptional activity, counteracting the antiproliferative function of Rb (Wang et al., 1998; Dasgupta et al., 2006; Kinkade et al., 2008).

Induction of differentiation is a powerful obstacle to proliferative signals. As long as the initiated stem cells or early progenitor cells giving rise to a tumor have not lost sensitivity to differentiating signals, these can be exploited in therapy. A particularly good illustration of this is the introduction of a combination of chemotherapy and differentiation therapy, which has revolutionized the treatment of leukemia (Wang and Chen, 2008). Recently, we have shown that endogenous C-Raf is essential to maintain an undifferentiated status in Ras-driven epidermal tumors. Conditional ablation of C-Raf results in rapid regression of established tumors through MEK/ERK-independent activation of a differentiation program induced by hyper-activation of the cytoskeleton-based kinase Rok-α (Ehrenreiter et al., 2009). These data show that Ras-driven tumors are addicted to non-oncogenic C-Raf, and offer proof of principle that differentiation (co)therapy may be feasible in solid tumors.

Confirming the importance of Raf in the maintenance of an undifferentiated state, recent work has shown that amplification of CRAF leads to the ERK-dependent activation of β-catenin, and to the expansion of breast tumor-initiating cells in culture and cancer progression in xenografts (Chang et al., 2011). Thus, both MEK/ERK-dependent and -independent mechanisms can contribute to the maintenance of an undifferentiated state in tumor cells.

Evasion of senescence and apoptosis

From the above, it is clear that overexpression of full-length RAF or the truncated catalytic domain leads to the activation of the ERK pathway in cultured cells and in vivo. In both situations, strong activation of the pathway correlates with the induction of senescence, which has to be bypassed before hyper-proliferation ensues (Sewing et al., 1997; Woods et al., 1997; Ravi et al., 1998; Zhu et al., 1998; Roper et al., 2001). Thus, senescence is the Achilles’ heel of the Ras/Raf/Erk pathway. Possible bypass mechanisms include direct regulation of the activity of the B-RafV600E mutant, to lower it to a level that would not induce senescence. In this context, candidates are Akt3, which can decrease the activity by phosphorylating negative-regulatory residues on BRAF (Cheung et al., 2008), and endogenous C-Raf, which has been shown to restrain B-RafV600E activity in the context of a heterodimer (Karreth et al., 2009).

Typically, however, senescence is disabled when tumor suppressors such as p16INK4a, p19ARF, p53 or PTEN are lost (Fedorov et al., 2003; Michaloglou et al., 2005; Goel et al., 2006, 2009; Gray-Schopfer et al., 2006; Dankort et al., 2007, 2009; Lyustikman et al., 2008; Dhomen et al., 2009; Yu et al., 2009; Carragher et al., 2010), or cooperating proto-oncogenes such as c-myc or Rac1b are expressed (Matos et al., 2008; Zhuang et al., 2008).

While RAF activation does not contribute to senescence evasion, it does have multiple, in part isoform-specific, roles in counteracting apoptosis. Downstream from activated Raf and Ras, but also from other oncogenes, the ERK pathway restrains apoptosis by regulating the expression and/or the activity of BCL-2 family members (Balmanno and Cook, 2009). In addition, MEK-independent pro-survival mechanisms, such as activation of MEKK1 and the nuclear factor-κB pathway (Baumann et al., 2000) and inactivation of the BH3-only BCL-2 family member BAD (Polzien et al., 2009), have been proposed for C-Raf (all reviewed by Niault and Baccarini, 2010). Reinforcing the connection between C-Raf and the Bcl2 family, Bcl2 deletion hinders the development of lung adenomas induced by the truncated, oncogenic form of C-Raf (Fedorov et al., 2002). In addition, endogenous C-Raf can restrain apoptosis in a kinase-independent manner by binding to, and inhibiting, the stress-induced, mitochondria-based kinase ASK-1 (Chen et al., 2001) as well as the homolog of Drosophila's Hippo, the MST-2 kinase (O’Neill et al., 2004; Matallanas et al., 2007), and by regulating Fas trafficking through its interaction with the cytoskeleton-based kinase Rok-α (Piazzolla et al., 2005). By conferring a survival advantage, any of these events might potentially promote tumorigenesis, although their significance in this context has not yet been shown in vivo.

Limitless replicative potential can be achieved through avoidance of telomere shortening, which causes a DNA-damage response mediated by p53 and p21, and finally senescence. This senescent program, induced by telomere attrition, differs from the fast, oncogene-induced proliferation barrier observed, for instance, in BRAFV600E-expressing premalignant nevi (Michaloglou et al., 2005; Gray-Schopfer et al., 2006). An 85–90% portion of all cancer cells escape telomere attrition by upregulating telomerase, a reverse transcriptase that restores telomeric repeats after every cell division (Chan and Blackburn, 2004). Ets transcription factors, well-established targets of activated ERK, can stimulate the transcriptional activation of the telomerase catalytic subunit gene downstream from oncogenic growth factor receptor, Ras and Raf (Goueli and Janknecht, 2004; Dwyer et al., 2007), thereby potentially antagonizing telomere shortening and supporting the replicative potential of the mutated cells.

Sustained angiogenesis is absolutely required for growth of solid tumors beyond a size of about 3mm3. Tumor cells are able to initiate an angiogenic shift toward angiogenic initiating signals (for example, vascular endothelial growth factor (VEGF) and fibroblast growth factor-1 and 2 and suppress inhibitory signals (thrombospondin-1 and interferon-β). The impact of Raf on angiogenesis in vivo has been established by the delivery of a kinase-dead C-Raf construct to the tumor-associated vasculature in mice. The kinase-dead protein induced the apoptosis of both endothelial and tumor cells, leading to tumor regression (Hood et al., 2002). C-Raf can promote endothelial cell survival by either MEK-dependent or -independent pathways, including ASK-1 inhibition (Alavi et al., 2003, 2007). In addition, selective disruption of the interaction between C-Raf and Rb inhibits the development of tumor-associated microvessels and suppresses the growth of tumor xenografts (Dasgupta et al., 2004; Kinkade et al., 2008). Thus, several C-Raf-dependent pathways can contribute to angiogenesis. By contrast, induction of angiogenesis by B-RAFV600E, involving expression of hypoxia-inducible factor-1α (Kumar et al., 2007) and VEGF (Sharma et al., 2005, 2006; Sumimoto et al., 2006), is entirely MEK-dependent. Conversely, conditional ablation of endogenous B-Raf prevents the angiogenic switch in a mouse model of pancreatic islet carcinoma driven by loss of function of the tumor suppressors p53 and Rb. B-Raf-deficient tumor cells proliferate normally despite decreased ERK activation, but produce insufficient amounts of the proangiogenic factors VEGF and TGFβ, resulting in reduced blood vessel density and tumor proliferation, and delayed tumor progression (Sobczak et al., 2008).

Tissue invasion and metastasis depends upon all the other hallmarks acquired during the process of tumor formation as well as on changes in proteins tethering cells to their surroundings. Changes in the expression of cell–cell adhesion molecules and/or in the binding specificities of integrins, as well as upregulation and activation of extracellular proteases, result in the ability of cancer cells to invade and colonize new terrain. Raf can influence invasion at several levels. First, activated Raf is involved in the production of TGFβ, which promotes invasion and metastasis (Lehmann et al., 2000; Sobczak et al., 2008; Riesco-Eizaguirre et al., 2009), as well as the epithelial–mesenchymal transition that precedes invasion in response to this factor (Janda et al., 2002). Second, both B-Raf and C-Raf have essential, if opposite, roles in cell contractility and migration: B-Raf increasing Rho-dependent contractility and opposing migration in an ERK-dependent manner (Pritchard et al., 2004), and C-Raf reducing contractility and increasing migration by inhibiting the Rho effector Rok-α (Ehrenreiter et al., 2005). In addition, B-RAFV600E/MEK/ERK are responsible for upregulation of several proteins involved in migration, and support integrin signaling, inducing melanoma cell invasion and metastases (Liang et al., 2007; Klein et al., 2008; Argast et al., 2009; Old et al., 2009). The B-RafV600E/MEK/ERK axis can also increase melanoma cell contractility and invasion by repressing the gene coding for a cGMP-specific phospshodiesterase, PDE5A (Arozarena et al., 2010). The resulting increase in the cGMP pool causes a raise in intracellular Ca++ and ultimately increased contractility, which boosts the rounded, bleb-associated mode of motility adopted during invasion (Sahai and Marshall, 2003).

PDE5A expression was found to be lower in metastasis-derived patient material than in primary tumors. As ERK is likely activated in both samples, it would have been interesting to know at which level is PDE5A expression further regulated, and what are the secondary events leading to full-fledged downregulation in metastasis. One possibility here are changes in tissue architecture, which in itself can exert strong antiproliferative effects and counteract invasion. Cadherin-based cell–cell adhesion, for instance, efficiently counteracts tumor proliferation, angiogenesis and metastasis in CRAF-driven lung adenomas (Ceteci et al., 2007). Additional regulators of tissue architecture are matrix metalloproteases, which are often overexpressed in cancer (Kessenbrock et al., 2010). Increased expression of one of these enzymes, matrix metalloprotease-9, by the Raf/MEK/ERK pathway in three-dimensional breast tissue cultures leads to a remodeling of the microenvironment, which induces loss of tissue polarity and re-initiation of proliferation (Beliveau et al., 2010).

In keeping with a role for the Raf/MEK/ERK pathway in invasion, the Raf-inhibitor protein RKIP has been identified as a suppressor of metastasis in many cancers (Granovsky and Rosner, 2008). Recently, a pathway has been discovered in which RKIP, through inhibition of the Raf/MEK/ERK module, increases the processing of the let-7 miRNA. This, in turn, inhibits the chromatin-remodeling factor HMGA2, which contributes to the expression of several metastasis-promoting genes (Dangi-Garimella et al., 2009).

To colonize a new site, tumor cells must extravasate from the blood vessels. In melanoma cells, BRAFV600E promotes this process by causing the production of both tumor- and microenvironment-derived interleukin-8. This cytokine recruits polymorphonuclear leukocytes, which bind to melanoma cells ultimately facilitating their trans-endothelial passage (Liang et al., 2007).

Avoidance of immunosurveillance enables tumors to evade recognition and destruction by the immune system. Cancers escape surveillance by selecting for non-immunogenic tumor cells, such as those that have downregulated human leukocyte antigen class-I molecules and/or have become resistant to cytotoxic T-lymphocyte-induced killing (immunoselection). Alternatively, tumors can actively repress immune cells in various ways, creating an immune-privileged environment (immunosubversion) (reviewed by Zitvogel et al. (2006)). BRAFV600E, for instance, mediates immunosubversion by inducing the cytokines interleukin-10 and interleukin-6 (Sumimoto et al., 2006), and contributes to immune evasion by inducing an MEK/ERK-dependent decrease in the expression of melanoma differentiation antigens, which is recognized by antigen-specific T-lymphocytes (immunoselection). Unlike MEK inhibitors, treatment with a BRAF-specific inhibitor leaves the function of T-lymphocytes intact, raising hopes that such inhibitors might bypass immunoevasion (Boni et al., 2010).

Implementing the hallmark events described above comes at a high stress cost for tumor cells. Recently, five stress phenotypes of cancer have been defined (Luo et al., 2009): (1) DNA damage, resulting from telomere shortening, replication stress and oncogene activation, and from mutations of DNA-repair and DNA-damage checkpoint genes; (2) mitotic stress, a consequence of chromosomal instability; (3) proteotoxic stress, caused by accumulation of misfolded proteins; (4) oxidative stress mediated by the generation of reactive oxygen species within a cancer cell and (5) metabolic stress (Kroemer and Pouyssegur, 2008), a consequence of enhanced aerobic glycolysis used by cancer cells for energy production.

How does RAF contribute to overcoming stress? In the case of genotoxic stress, activated Ras and Raf can induce the expression of the mdm2 gene, leading to p53 degradation; at least in cells lacking the Mdm2 inhibitor p19ARF, this leads to reduced p53-dependent apoptosis following DNA damage (Ries et al., 2000). Downstream from p53, Ras or Raf activation by HB-EGF is responsible for the ERK-mediated induction of the cyclooxygenase-2 gene, which inhibits genotoxic stress-induced apoptosis (Han et al., 2002). Scatter Factor (hepatocyte growth factor), another growth factor that protects tumors from genotoxicity, uses Raf to signal survival through activation of nuclear factor-κB (Fan et al., 2007). In addition, an interplay between oncogenic Raf/ERK and ATM has been shown to promote homologous recombination repair in response to radiation (Golding et al., 2007), in line with previous reports showing a correlation between oncogenic Raf and radioresistance in tumors (Kasid et al., 1987, 1989, 1996). Finally, the RAF/MEK/ERK pathway protects multiple myeloma cells from DNA damage induced by treatment with a Chk1 inhibitor (Dai et al., 2008). These findings suggest that Raf and MEK inhibitors could be combined with cytostatic drugs or radiation in the therapy of cancer.

The relationships between Raf and oxidative stress are manifold: RAF/MEK/ERK activation can prevent the onset of oxidative stress in growth factor-deprived cells (Kuznetsov et al., 2008); on the other hand, generation of reactive oxygen species by derivatives of geldanamycin, a chemotherapeutic that inhibits the chaperone function of HSP90 and enforces the degradation of their client proteins, including Raf, is able to inhibit the activity of BRAFV600E (Fukuyo et al., 2008). Finally, a most interesting connection, relevant in terms of oncogene-selective therapy, has been reported recently between oncogenic activation of RAS and RAF, and the small-molecule drug erastin. Erastin causes mitochondrial dysfunction and oxidative cell death by activating voltage-dependent ion channels (voltage-dependent anion channels (VDACs)) on the mitochondria. In a panel of cancer cell lines of different origin, RAS and BRAF activation potentiated the lethality of the drug by inducing the expression of the VDACs (Yagoda et al., 2007). Thus, VDAC expression may represent a targetable weak spot in RAS- and RAF-driven tumors.

Metabolic stress, particularly lack of nutrients, imposes a number of metabolic checkpoints that cancer cells must bypass to continue proliferating under the dire conditions often found in the tumor microenvironment. BRAFV600E-expressing melanoma cells appear to solve this problem through ERK-mediated phosphorylation of the tumor suppressor and energy sensor LKB1. This phosphorylation, which occurs in the context of a physical complex including B-Raf V600E, prevents the LKB1-mediated activation of the AMP-activated protein kinase, which restricts protein synthesis (Zheng et al., 2009). The net result is normal operation despite nutrient shortage, and therefore a competitive advantage under conditions of metabolic stress.

RAF inhibitors–clinical success and challenges

From the above it is clear that Raf kinases are prime target for the design and application of molecule-target therapies of cancer, particularly melanoma. Several companies have generated Raf inhibitors currently in preclinical and clinical trials, and a drug specifically targeting BRAFV600E (PLX4032/RG7204; Plexxikon/Roche, Berkeley, CA, USA (Tsai et al., 2008; Joseph et al., 2010)) has recently produced dramatic results, with response rates of 70–80% as single agent in metastatic melanoma patients (Bollag et al., 2010; Flaherty et al., 2010). Similar results have been obtained with another ATP competitive BRAF inhibitor (GSK 2118436; GlaxoSmithKline, Brentford, UK (Kefford et al., 2010)). These drugs, which are currently being tested in clinical trials in patients affected by other solid tumors with BRAFV600E mutations, such as thyroid carcinomas or colon cancers (Arkenau et al., 2010; Puzanov et al., 2011), are reasonably well-tolerated. However, one intriguing and potentially worrying issue is the paradoxical increase in the proliferation and activation of the MEK–ERK pathway in cells not harboring the BRAFV600E mutation. The underlying mechanism is an allosteric effect of the drug, which enforces the dimerization of endogenous BRAF with CRAF or ARAF (Hatzivassiliou et al., 2010; Heidorn et al., 2010; Poulikakos et al., 2010). Within these dimers, only one active component is required for activation of the MEK–ERK pathway; therefore, at non-saturating concentrations, the inhibitors activate the pathway rather than disabling it, particularly in the presence of activated RAS, and it is possible that the rapid development of benign skin tumors in patients treated with RAF inhibitors might be fueled by such a mechanism (Degen et al., 2010; Robert et al., 2010) (Figure 4).

Figure 4.
Figure 4 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact or the author

RAF inhibitors: response, resistance and drug-related tumors. In normal cells, RAF activation drives ERK activation downstream from RAS. In melanoma, BRAF mutants (V600E) with high kinase activity drive ERK activation (red arrows) independently of RAS. Cancer cells harboring these mutants are sensitive to BRAF inhibitors, which blunt kinase activity and reduce ERK activation as well as proliferation (thin arrows). Unfortunately, however, resistance arises, by mechanisms involving activation of other MEK kinases, such as COT, but also upregulation of receptor tyrosine kinases (RTKs) and of other pathways downstream from RAS (purple arrows). Finally, drug-related epidermal tumors have been observed in patients treated with RAF inhibitors. They correlate with ERK activation, which results from the ability of the drug to promote RAF dimerization. If only one subunit of the dimer is bound to the inhibitor (for instance at non-saturating inhibitor concentrations, or in the case of inhibitors with fast off-rates), the other subunit is activated and is capable of phosphorylating MEK with high efficiency, generating a tonic signal leading to increased proliferation. Thus, RAF inhibitors can paradoxically function as ERK activators, and potentially induce the development of drug-related tumors. ERK, extracellular signal-regulated kinase; MEK, MAPK/ERK kinase.

Full figure and legend (102K)

More troublesome is the fact that melanoma cells develop chemoresistance by a number of different molecular mechanisms (reviewed by Poulikakos and Rosen, 2011), leading to relapse of drug-responsive disease. Unlike the case of imatinib resistance, often caused by mutations in the kinase domain of the target BCR-ABL (Weisberg et al., 2007), de novo mutations in B-RAF have not been observed in relapsing tumors. Rather, acquired resistance involved reactivation of the ERK pathway by switching to other MEK kinases (other RAF isoforms (Villanueva et al., 2010) or COT/Tpl2 (Johannessen et al., 2010)), or by activating mutations in NRAS (Nazarian et al., 2010), but also upregulation of receptor tyrosine kinases driving other pathways (Nazarian et al., 2010; Villanueva et al., 2010) (Figure 4). Therefore, overcoming melanoma resistance might require modulation of multiple pathways.



The study of the Raf pathways has been extremely rewarding. The first serine/threonine kinase oncogene discovered has proven an excellent target in single-agent therapy of the disease it is most frequently associated with; in turn, investigation of the mode of action of RAF inhibitors has shed light on the mechanism of regulation of the cellular Raf enzyme. Animal models continue to delineate essential functions of the pathway components and the discovery of protein–protein interactions within the pathway and cross-pathways provides further potential leads for novel therapeutic strategies. Almost 30 years after its discovery, Raf is still a fascinating topic for basic and clinical researchers, and will remain so for many years to come.


Conflict of interest

The authors declare no conflicts of interest.



  1. Alavi A, Hood JD, Frausto R, Stupack DG, Cheresh DA. (2003). Role of Raf in vascular protection from distinct apoptotic stimuli. Science 301: 94–96. | Article | PubMed | ISI | ChemPort |
  2. Alavi AS, Acevedo L, Min W, Cheresh DA. (2007). Chemoresistance of endothelial cells induced by basic fibroblast growth factor depends on Raf-1-mediated inhibition of the proapoptotic kinase, ASK1. Cancer Res 67: 2766–2772. | Article | PubMed | ISI | ChemPort |
  3. Argast GM, Croy CH, Couts KL, Zhang Z, Litman E, Chan DC et al. (2009). Plexin B1 is repressed by oncogenic B-Raf signaling and functions as a tumor suppressor in melanoma cells. Oncogene 28: 2697–2709. | Article | PubMed | ISI | ChemPort |
  4. Arkenau HT, Kefford R, Long GV. (2010). Targeting BRAF for patients with melanoma. Br J Cancer 104: 392–398. | Article | PubMed | ISI |
  5. Arozarena I, Sanchez-Laorden B, Packer L, Hidalgo-Carcedo C, Hayward R, Viros A et al. (2010). Oncogenic BRAF induces melanoma cell invasion by downregulating the cGMP-specific phosphodiesterase PDE5A. Cancer Cell 19: 45–57. | Article | ISI |
  6. Balmanno K, Cook SJ. (2009). Tumour cell survival signalling by the ERK1/2 pathway. Cell Death Differ 16: 368–377. | Article | PubMed | ISI | ChemPort |
  7. Baumann B, Weber CK, Troppmair J, Whiteside S, Israel A, Rapp UR et al. (2000). Raf induces NF-kappa B by membrane shuttle kinase MEKK1, a signaling pathway critical for transformation. Proc Natl Acad Sci USA 97: 4615–4620. | Article | PubMed | ChemPort |
  8. Beliveau A, Mott JD, Lo A, Chen EI, Koller AA, Yaswen P et al. (2010). Raf-induced MMP9 disrupts tissue architecture of human breast cells in three-dimensional culture and is necessary for tumor growth in vivo. Genes Dev 24: 2800–2811. | Article | PubMed | ISI |
  9. Bollag G, Hirth P, Tsai J, Zhang J, Ibrahim PN, Cho H et al. (2010). Clinical efficacy of a RAF inhibitor needs broad target blockade in BRAF-mutant melanoma. Nature 467: 596–599. | Article | PubMed | ISI | ChemPort |
  10. Boni A, Cogdill AP, Dang P, Udayakumar D, Njauw CN, Sloss CM et al. (2010). Selective BRAFV600E inhibition enhances T-cell recognition of melanoma without affecting lymphocyte function. Cancer Res 70: 5213–5219. | Article | PubMed | ISI | ChemPort |
  11. Calipel A, Mouriaux F, Glotin AL, Malecaze F, Faussat AM, Mascarelli F. (2006). Extracellular signal-regulated kinase-dependent proliferation is mediated through the protein kinase A/B-Raf pathway in human uveal melanoma cells. J Biol Chem 281: 9238–9250. | Article | PubMed | ISI | ChemPort |
  12. Carragher LA, Snell KR, Giblett SM, Aldridge VS, Patel B, Cook SJ et al. (2010). V600EBraf induces gastrointestinal crypt senescence and promotes tumour progression through enhanced CpG methylation of p16INK4a. EMBO Mol Med 2: 458–471. | Article | PubMed | ISI |
  13. Catalanotti F, Reyes G, Jesenberger V, Galabova-Kovacs G, de Matos Simoes R, Carugo O et al. (2009). A Mek1–Mek2 heterodimer determines the strength and duration of the Erk signal. Nat Struct Mol Biol 16: 294–303. | Article | PubMed | ISI |
  14. Cekanova M, Majidy M, Masi T, Al-Wadei HA, Schuller HM. (2007). Overexpressed Raf-1 and phosphorylated cyclic adenosine 3′-5′-monophosphatate response element-binding protein are early markers for lung adenocarcinoma. Cancer 109: 1164–1173. | Article | PubMed | ISI |
  15. Ceteci F, Ceteci S, Karreman C, Kramer BW, Asan E, Gotz R et al. (2007). Disruption of tumor cell adhesion promotes angiogenic switch and progression to micrometastasis in RAF-driven murine lung cancer. Cancer Cell 12: 145–159. | Article | PubMed | ISI | ChemPort |
  16. Chan SR, Blackburn EH. (2004). Telomeres and telomerase. Philos Trans R Soc Lond B Biol Sci 359: 109–121. | Article | PubMed | ISI | ChemPort |
  17. Chang CJ, Yang JY, Xia W, Chen CT, Xie X, Chao CH et al. (2011). EZH2 promotes expansion of breast tumor initiating cells through activation of RAF1–beta-catenin signaling. Cancer Cell 19: 86–100. | Article | PubMed | ISI |
  18. Chen J, Fujii K, Zhang L, Roberts T, Fu H. (2001). Raf-1 promotes cell survival by antagonizing apoptosis signal-regulating kinase 1 through a MEK–ERK independent mechanism. Proc Natl Acad Sci USA 98: 7783–7788. | Article | PubMed | ChemPort |
  19. Cheung M, Sharma A, Madhunapantula SV, Robertson GP. (2008). Akt3 and mutant V600E B-Raf cooperate to promote early melanoma development. Cancer Res 68: 3429–3439. | Article | PubMed | ISI | ChemPort |
  20. Ciampi R, Knauf JA, Kerler R, Gandhi M, Zhu Z, Nikiforova MN et al. (2005). Oncogenic AKAP9–BRAF fusion is a novel mechanism of MAPK pathway activation in thyroid cancer. J Clin Invest 115: 94–101. | Article | PubMed | ISI | ChemPort |
  21. Dai Y, Chen S, Pei XY, Almenara JA, Kramer LB, Venditti CA et al. (2008). Interruption of the Ras/MEK/ERK signaling cascade enhances Chk1 inhibitor-induced DNA damage in vitro and in vivo in human multiple myeloma cells. Blood 112: 2439–2449. | Article | PubMed | ISI | ChemPort |
  22. Dangi-Garimella S, Yun J, Eves EM, Newman M, Erkeland SJ, Hammond SM et al. (2009). Raf kinase inhibitory protein suppresses a metastasis signalling cascade involving LIN28 and let-7. EMBO J 28: 347–358. | Article | PubMed | ISI | ChemPort |
  23. Dankort D, Curley DP, Cartlidge RA, Nelson B, Karnezis AN, Damsky Jr WE et al. (2009). Braf(V600E) cooperates with Pten loss to induce metastatic melanoma. Nat Genet 41: 544–552. | Article | PubMed | ISI | ChemPort |
  24. Dankort D, Filenova E, Collado M, Serrano M, Jones K, McMahon M. (2007). A new mouse model to explore the initiation, progression, and therapy of BRAFV600E-induced lung tumors. Genes Dev 21: 379–384. | Article | PubMed | ISI | ChemPort |
  25. Dasgupta P, Rastogi S, Pillai S, Ordonez-Ercan D, Morris M, Haura E et al. (2006). Nicotine induces cell proliferation by beta-arrestin-mediated activation of Src and Rb–Raf-1 pathways. J Clin Invest 116: 2208–2217. | Article | PubMed | ISI |
  26. Dasgupta P, Sun J, Wang S, Fusaro G, Betts V, Padmanabhan J et al. (2004). Disruption of the Rb–Raf-1 interaction inhibits tumor growth and angiogenesis. Mol Cell Biol 24: 9527–9541. | Article | PubMed | ISI | ChemPort |
  27. Davies H, Bignell GR, Cox C, Stephens P, Edkins S, Clegg S et al. (2002). Mutations of the BRAF gene in human cancer. Nature 417: 949–954. | Article | PubMed | ISI | ChemPort |
  28. Degen A, Satzger I, Voelker B, Kapp A, Hauschild A, Gutzmer R. (2010). Does basal cell carcinoma belong to the spectrum of sorafenib-induced epithelial skin cancers. Dermatology 221: 193–196. | Article | PubMed | ISI |
  29. Dessars B, De Raeve LE, El Housni H, Debouck CJ, Sidon PJ, Morandini R et al. (2007). Chromosomal translocations as a mechanism of BRAF activation in two cases of large congenital melanocytic nevi. J Invest Dermatol 127: 1468–1470. | Article | PubMed | ISI | ChemPort |
  30. Dhomen N, Reis-Filho JS, da Rocha Dias S, Hayward R, Savage K, Delmas V et al. (2009). Oncogenic Braf induces melanocyte senescence and melanoma in mice. Cancer Cell 15: 294–303. | Article | PubMed | ISI | ChemPort |
  31. Dougherty MK, Muller J, Ritt DA, Zhou M, Zhou XZ, Copeland TD et al. (2005). Regulation of Raf-1 by direct feedback phosphorylation. Mol Cell 17: 215–224. | Article | PubMed | ISI | ChemPort |
  32. Dumaz N, Hayward R, Martin J, Ogilvie L, Hedley D, Curtin JA et al. (2006). In melanoma, RAS mutations are accompanied by switching signaling from BRAF to CRAF and disrupted cyclic AMP signaling. Cancer Res 66: 9483–9491. | Article | PubMed | ISI | ChemPort |
  33. Dunn GP, Old LJ, Schreiber RD. (2004). The three Es of cancer immunoediting. Annu Rev Immunol 22: 329–360. | Article | PubMed | ISI | ChemPort |
  34. Dwyer J, Li H, Xu D, Liu JP. (2007). Transcriptional regulation of telomerase activity: roles of the Ets transcription factor family. Ann NY Acad Sci 1114: 36–47. | Article | PubMed |
  35. Edwards J, Krishna NS, Witton CJ, Bartlett JM. (2003). Gene amplifications associated with the development of hormone-resistant prostate cancer. Clin Cancer Res 9: 5271–5281. | PubMed | ISI | ChemPort |
  36. Ehrenreiter K, Kern F, Velamoor V, Meissl K, Galabova-Kovacs G, Sibilia M et al. (2009). Raf-1 addiction in Ras-induced skin carcinogenesis. Cancer Cell 16: 149–160. | Article | PubMed | ISI | ChemPort |
  37. Ehrenreiter K, Piazzolla D, Velamoor V, Sobczak I, Small JV, Takeda J et al. (2005). Raf-1 regulates Rho signaling and cell migration. J Cell Biol 168: 955–964. | Article | PubMed | ISI | ChemPort |
  38. Emuss V, Garnett M, Mason C, Marais R. (2005). Mutations of C-RAF are rare in human cancer because C-RAF has a low basal kinase activity compared with B-RAF. Cancer Res 65: 9719–9726. | Article | PubMed | ISI | ChemPort |
  39. Eves EM, Shapiro P, Naik K, Klein UR, Trakul N, Rosner MR. (2006). Raf kinase inhibitory protein regulates aurora B kinase and the spindle checkpoint. Mol Cell 23: 561–574. | Article | PubMed | ISI | ChemPort |
  40. Fan S, Meng Q, Laterra JJ, Rosen EM. (2007). Ras effector pathways modulate scatter factor-stimulated NF-(kappa)B signaling and protection against DNA damage. Oncogene 26: 4774–4796. | Article | PubMed | ISI | ChemPort |
  41. Fedorov LM, Papadopoulos T, Tyrsin OY, Twardzik T, Gotz R, Rapp UR. (2003). Loss of p53 in craf-induced transgenic lung adenoma leads to tumor acceleration and phenotypic switch. Cancer Res 63: 2268–2277. | PubMed | ISI |
  42. Fedorov LM, Tyrsin OY, Papadopoulos T, Camarero G, Gotz R, Rapp UR. (2002). Bcl-2 determines susceptibility to induction of lung cancer by oncogenic CRaf. Cancer Res 62: 6297–6303. | PubMed | ISI |
  43. Flaherty KT, Puzanov I, Kim KB, Ribas A, McArthur GA, Sosman JA et al. (2010). Inhibition of mutated, activated BRAF in metastatic melanoma. N Engl J Med 363: 809–819. | Article | PubMed | ISI | ChemPort |
  44. Fukuyo Y, Inoue M, Nakajima T, Higashikubo R, Horikoshi NT, Hunt C et al. (2008). Oxidative stress plays a critical role in inactivating mutant BRAF by geldanamycin derivatives. Cancer Res 68: 6324–6330. | Article | PubMed | ISI |
  45. Garnett MJ, Rana S, Paterson H, Barford D, Marais R. (2005). Wild-type and mutant B-RAF activate C-RAF through distinct mechanisms involving heterodimerization. Mol Cell 20: 963–969. | Article | PubMed | ISI | ChemPort |
  46. Goel VK, Ibrahim N, Jiang G, Singhal M, Fee S, Flotte T et al. (2009). Melanocytic nevus-like hyperplasia and melanoma in transgenic BRAFV600E mice. Oncogene 28: 2289–2298. | Article | PubMed | ISI | ChemPort |
  47. Goel VK, Lazar AJ, Warneke CL, Redston MS, Haluska FG. (2006). Examination of mutations in BRAF, NRAS, and PTEN in primary cutaneous melanoma. J Invest Dermatol 126: 154–160. | Article | PubMed | ISI | ChemPort |
  48. Golding SE, Rosenberg E, Neill S, Dent P, Povirk LF, Valerie K. (2007). Extracellular signal-related kinase positively regulates ataxia telangiectasia mutated, homologous recombination repair, and the DNA damage response. Cancer Res 67: 1046–1053. | Article | PubMed | ISI | ChemPort |
  49. Goueli BS, Janknecht R. (2004). Upregulation of the catalytic telomerase subunit by the transcription factor ER81 and oncogenic HER2/Neu, Ras, or Raf. Mol Cell Biol 24: 25–35. | Article | PubMed | ISI | ChemPort |
  50. Granovsky AE, Rosner MR. (2008). Raf kinase inhibitory protein: a signal transduction modulator and metastasis suppressor. Cell Res 18: 452–457. | Article | PubMed | ISI | ChemPort |
  51. Gray-Schopfer VC, Cheong SC, Chong H, Chow J, Moss T, Abdel-Malek ZA et al. (2006). Cellular senescence in naevi and immortalisation in melanoma: a role for p16? Br J Cancer 95: 496–505. | Article | PubMed | ISI | ChemPort |
  52. Han JA, Kim JI, Ongusaha PP, Hwang DH, Ballou LR, Mahale A et al. (2002). p53-mediated induction of Cox-2 counteracts p53- or genotoxic stress-induced apoptosis. EMBO J 21: 5635–5644. | Article | PubMed | ISI | ChemPort |
  53. Hanahan D, Weinberg RA. (2000). The hallmarks of cancer. Cell 100: 57–70. | Article | PubMed | ISI | ChemPort |
  54. Hatzivassiliou G, Song K, Yen I, Brandhuber BJ, Anderson DJ, Alvarado R et al. (2010). RAF inhibitors prime wild-type RAF to activate the MAPK pathway and enhance growth. Nature 464: 431–435. | Article | PubMed | ISI | ChemPort |
  55. Heidorn SJ, Milagre C, Whittaker S, Nourry A, Niculescu-Duvas I, Dhomen N et al. (2010). Kinase-dead BRAF and oncogenic RAS cooperate to drive tumor progression through CRAF. Cell 140: 209–221. | Article | PubMed | ISI | ChemPort |
  56. Hood JD, Bednarski M, Frausto R, Guccione S, Reisfeld RA, Xiang R et al. (2002). Tumor regression by targeted gene delivery to the neovasculature. Science 296: 2404–2407. | Article | PubMed | ISI | ChemPort |
  57. Hwang YH, Choi JY, Kim S, Chung ES, Kim T, Koh SS et al. (2004). Overexpression of c-raf-1 proto-oncogene in liver cirrhosis and hepatocellular carcinoma. Hepatol Res 29: 113–121. | Article | PubMed | ISI | ChemPort |
  58. Janda E, Lehmann K, Killisch I, Jechlinger M, Herzig M, Downward J et al. (2002). Ras and TGF(beta) cooperatively regulate epithelial cell plasticity and metastasis: dissection of Ras signaling pathways. J Cell Biol 156: 299–313. | Article | PubMed | ISI | ChemPort |
  59. Johannessen CM, Boehm JS, Kim SY, Thomas SR, Wardwell L, Johnson LA et al. (2010). COT drives resistance to RAF inhibition through MAP kinase pathway reactivation. Nature 468: 968–972. | Article | PubMed | ISI | ChemPort |
  60. Jones DT, Kocialkowski S, Liu L, Pearson DM, Backlund LM, Ichimura K et al. (2008). Tandem duplication producing a novel oncogenic BRAF fusion gene defines the majority of pilocytic astrocytomas. Cancer Res 68: 8673–8677. | Article | PubMed | ISI | ChemPort |
  61. Jones DT, Kocialkowski S, Liu L, Pearson DM, Ichimura K, Collins VP. (2009). Oncogenic RAF1 rearrangement and a novel BRAF mutation as alternatives to KIAA1549:BRAF fusion in activating the MAPK pathway in pilocytic astrocytoma. Oncogene 28: 2119–2123. | Article | PubMed | ISI | ChemPort |
  62. Joseph EW, Pratilas CA, Poulikakos PI, Tadi M, Wang W, Taylor BS et al. (2010). The RAF inhibitor PLX4032 inhibits ERK signaling and tumor cell proliferation in a V600E BRAF-selective manner. Proc Natl Acad Sci USA 107: 14903–14908. | Article | PubMed |
  63. Kamata T, Hussain J, Giblett S, Hayward R, Marais R, Pritchard C. (2010). BRAF inactivation drives aneuploidy by deregulating CRAF. Cancer Res 70: 8475–8486. | Article | PubMed | ISI | ChemPort |
  64. Karreth FA, DeNicola GM, Winter SP, Tuveson DA. (2009). C-Raf inhibits MAPK activation and transformation by B-RafV600E. Mol Cell 36: 477–486. | Article | PubMed | ISI | ChemPort |
  65. Kasid U, Pfeifer A, Brennan T, Beckett M, Weichselbaum RR, Dritschilo A et al. (1989). Effect of antisense c-raf-1 on tumorigenicity and radiation sensitivity of a human squamous carcinoma. Science 243: 1354–1356. | Article | PubMed | ISI | ChemPort |
  66. Kasid U, Pfeifer A, Weichselbaum RR, Dritschilo A, Mark GE. (1987). The raf oncogene is associated with a radiation-resistant human laryngeal cancer. Science 237: 1039–1041. | Article | PubMed | ISI | ChemPort |
  67. Kasid U, Suy S, Dent P, Ray S, Whiteside TL, Sturgill TW. (1996). Activation of Raf by ionizing radiation. Nature 382: 813–816. | Article | PubMed | ISI | ChemPort |
  68. Kefford R, Arkenau H, Brown MP, Millward M, Infante JR, Long GV et al. (2010). Phase I/II study of GSK2118436, a selective inhibitor of oncogenic mutant BRAF kinase, in patients with metastatic melanoma and other solid tumors. ASCO Meeting Abstracts 28: 8503.
  69. Kerkhoff E, Fedorov LM, Siefken R, Walter AO, Papadopoulos T, Rapp UR. (2000). Lung-targeted expression of the c-Raf-1 kinase in transgenic mice exposes a novel oncogenic character of the wild-type protein. Cell Growth Differ 11: 185–190. | PubMed | ISI |
  70. Kessenbrock K, Plaks V, Werb Z. (2010). Matrix metalloproteinases: regulators of the Tumor Microenvironment. Cell 141: 52–67. | Article | PubMed | ISI |
  71. Kinkade R, Dasgupta P, Carie A, Pernazza D, Carless M, Pillai S et al. (2008). A small molecule disruptor of Rb/Raf-1 interaction inhibits cell proliferation, angiogenesis, and growth of human tumor xenografts in nude mice. Cancer Res 68: 3810–3818. | Article | PubMed | ISI | ChemPort |
  72. Klein RM, Spofford LS, Abel EV, Ortiz A, Aplin AE. (2008). B-RAF regulation of Rnd3 participates in actin cytoskeletal and focal adhesion organization. Mol Biol Cell 19: 498–508. | Article | PubMed | ISI |
  73. Kolch W. (2005). Coordinating ERK/MAPK signalling through scaffolds and inhibitors. Nat Rev Mol Cell Biol 6: 827–837. | Article | PubMed | ISI | ChemPort |
  74. Kramer BW, Gotz R, Rapp UR. (2004). Use of mitogenic cascade blockers for treatment of C-Raf induced lung adenoma in vivo: CI-1040 strongly reduces growth and improves lung structure. BMC Cancer 4: 24. | Article | PubMed |
  75. Kroemer G, Pouyssegur J. (2008). Tumor cell metabolism: cancer's Achilles’ heel. Cancer Cell 13: 472–482. | Article | PubMed | ISI | ChemPort |
  76. Kumar SM, Yu H, Edwards R, Chen L, Kazianis S, Brafford P et al. (2007). Mutant V600E BRAF increases hypoxia inducible factor-1alpha expression in melanoma. Cancer Res 67: 3177–3184. | Article | PubMed | ISI | ChemPort |
  77. Kuznetsov AV, Smigelskaite J, Doblander C, Janakiraman M, Hermann M, Wurm M et al. (2008). Survival signaling by C-RAF: mitochondrial reactive oxygen species and Ca2+ are critical targets. Mol Cell Biol 28: 2304–2313. | Article | PubMed | ISI | ChemPort |
  78. Lehmann K, Janda E, Pierreux CE, Rytomaa M, Schulze A, McMahon M et al. (2000). Raf induces TGFbeta production while blocking its apoptotic but not invasive responses: a mechanism leading to increased malignancy in epithelial cells. Genes Dev 14: 2610–2622. | Article | PubMed | ISI | ChemPort |
  79. Liang S, Sharma A, Peng HH, Robertson G, Dong C. (2007). Targeting mutant (V600E) B-Raf in melanoma interrupts immunoediting of leukocyte functions and melanoma extravasation. Cancer Res 67: 5814–5820. | Article | PubMed | ISI |
  80. Luo J, Solimini NL, Elledge SJ. (2009). Principles of cancer therapy: oncogene and non-oncogene addiction. Cell 136: 823–837. | Article | PubMed | ISI | ChemPort |
  81. Lyustikman Y, Momota H, Pao W, Holland EC. (2008). Constitutive activation of Raf-1 induces glioma formation in mice. Neoplasia 10: 501–510. | PubMed | ISI | ChemPort |
  82. Marx M, Eychene A, Laugier D, Bechade C, Crisanti P, Dezelee P et al. (1988). A novel oncogene related to c-mil is transduced in chicken neuroretina cells induced to proliferate by infection with an avian lymphomatosis virus. EMBO J 7: 3369–3373. | PubMed | ISI | ChemPort |
  83. Matallanas D, Romano D, Yee K, Meissl K, Kucerova L, Piazzolla D et al. (2007). RASSF1A elicits apoptosis through an MST2 pathway directing proapoptotic transcription by the p73 tumor suppressor protein. Mol Cell 27: 962–975. | Article | PubMed | ISI | ChemPort |
  84. Matos P, Oliveira C, Velho S, Goncalves V, da Costa LT, Moyer MP et al. (2008). B-Raf(V600E) cooperates with alternative spliced Rac1b to sustain colorectal cancer cell survival. Gastroenterology 135: 899–906. | Article | PubMed | ISI | ChemPort |
  85. McKay MM, Morrison DK. (2007). Integrating signals from RTKs to ERK/MAPK. Oncogene 26: 3113–3121. | Article | PubMed | ISI | ChemPort |
  86. Michaloglou C, Vredeveld LC, Soengas MS, Denoyelle C, Kuilman T, van der Horst CM et al. (2005). BRAFE600-associated senescence-like cell cycle arrest of human naevi. Nature 436: 720–724. | Article | PubMed | ISI | ChemPort |
  87. Mitsutake N, Knauf JA, Mitsutake S, Mesa Jr C, Zhang L, Fagin JA. (2005). Conditional BRAFV600E expression induces DNA synthesis, apoptosis, dedifferentiation, and chromosomal instability in thyroid PCCL3 cells. Cancer Res 65: 2465–2473. | Article | PubMed | ISI | ChemPort |
  88. Moelling K, Heimann B, Beimling P, Rapp UR, Sander T. (1984). Serine- and threonine-specific protein kinase activities of purified gag-mil and gag-raf proteins. Nature 312: 558–561. | Article | PubMed | ISI | ChemPort |
  89. Nazarian R, Shi H, Wang Q, Kong X, Koya RC, Lee H et al. (2010). Melanomas acquire resistance to B-RAF(V600E) inhibition by RTK or N-RAS upregulation. Nature 468: 973–977. | Article | PubMed | ISI | ChemPort |
  90. Negrini S, Gorgoulis VG, Halazonetis TD. (2010). Genomic instability—an evolving hallmark of cancer. Nat Rev Mol Cell Biol 11: 220–228. | Article | PubMed | ISI | ChemPort |
  91. Niault TS, Baccarini M. (2010). Targets of Raf in tumorigenesis. Carcinogenesis 31: 1165–1174. | Article | PubMed | ISI |
  92. O'Neill E, Rushworth L, Baccarini M, Kolch W. (2004). Role of the kinase MST2 in suppression of apoptosis by the proto-oncogene product Raf-1. Science 306: 2267–2270. | Article | PubMed | ISI | ChemPort |
  93. Old WM, Shabb JB, Houel S, Wang H, Couts KL, Yen CY et al. (2009). Functional proteomics identifies targets of phosphorylation by B-Raf signaling in melanoma. Mol Cell 34: 115–131. | Article | PubMed | ISI | ChemPort |
  94. Palanisamy N, Ateeq B, Kalyana-Sundaram S, Pflueger D, Ramnarayanan K, Shankar S et al. (2010). Rearrangements of the RAF kinase pathway in prostate cancer, gastric cancer and melanoma. Nat Med 16: 793–798. | Article | PubMed | ISI | ChemPort |
  95. Park BJ, Park JI, Byun DS, Park JH, Chi SG. (2000). Mitogenic conversion of transforming growth factor-beta1 effect by oncogenic Ha-Ras-induced activation of the mitogen-activated protein kinase signaling pathway in human prostate cancer. Cancer Res 60: 3031–3038. | PubMed | ISI | ChemPort |
  96. Pfister S, Janzarik WG, Remke M, Ernst A, Werft W, Becker N et al. (2008). BRAF gene duplication constitutes a mechanism of MAPK pathway activation in low-grade astrocytomas. J Clin Invest 118: 1739–1749. | Article | PubMed | ISI | ChemPort |
  97. Piazzolla D, Meissl K, Kucerova L, Rubiolo C, Baccarini M. (2005). Raf-1 sets the threshold of Fas sensitivity by modulating Rok-{alpha} signaling. J Cell Biol 171: 1013–1022. | Article | PubMed | ISI | ChemPort |
  98. Polzien L, Baljuls A, Rennefahrt UE, Fischer A, Schmitz W, Zahedi RP et al. (2009). Identification of novel in vivo phosphorylation sites of the human proapoptotic protein BAD: pore-forming activity of BAD is regulated by phosphorylation. J Biol Chem 284: 28004–28020. | Article | PubMed | ISI |
  99. Poulikakos PI, Rosen N. (2011). Mutant BRAF melanomas—dependence and resistance. Cancer Cell 19: 11–15. | Article | PubMed | ISI |
  100. Poulikakos PI, Zhang C, Bollag G, Shokat KM, Rosen N. (2010). RAF inhibitors transactivate RAF dimers and ERK signalling in cells with wild-type BRAF. Nature 464: 427–430. | Article | PubMed | ISI | ChemPort |
  101. Pratilas CA, Taylor BS, Ye Q, Viale A, Sander C, Solit DB et al. (2009). (V600E)BRAF is associated with disabled feedback inhibition of RAF–MEK signaling and elevated transcriptional output of the pathway. Proc Natl Acad Sci USA 106: 4519–4524. | Article | PubMed | ChemPort |
  102. Pritchard CA, Hayes L, Wojnowski L, Zimmer A, Marais RM, Norman JC. (2004). B-Raf acts via the ROCKII/LIMK/cofilin pathway to maintain actin stress fibers in fibroblasts. Mol Cell Biol 24: 5937–5952. | Article | PubMed | ISI | ChemPort |
  103. Pritchard CA, Samuels ML, Bosch E, McMahon M. (1995). Conditionally oncogenic forms of the A-Raf and B-Raf protein kinases display different biological and biochemical properties in NIH 3T3 cells. Mol Cell Biol 15: 6430–6442. | PubMed | ISI | ChemPort |
  104. Puzanov I, Burnett P, Flaherty KT. (2011). Biological challenges of BRAF inhibitor therapy. Mol Oncol 5.: 116–123. | Article | PubMed | ISI |
  105. Rajakulendran T, Sahmi M, Lefrancois M, Sicheri F, Therrien M. (2009). A dimerization-dependent mechanism drives RAF catalytic activation. Nature 461: 542–545. | Article | PubMed | ISI | ChemPort |
  106. Rapp UR, Goldsborough MD, Mark GE, Bonner TI, Groffen J, Reynolds Jr FH et al. (1983). Structure and biological activity of v-raf, a unique oncogene transduced by a retrovirus. Proc Natl Acad Sci USA 80: 4218–4222. | Article | PubMed | ChemPort |
  107. Ravi RK, Weber E, McMahon M, Williams JR, Baylin S, Mal A et al. (1998). Activated Raf-1 causes growth arrest in human small cell lung cancer cells. J Clin Invest 101: 153–159. | Article | PubMed | ISI | ChemPort |
  108. Ries S, Biederer C, Woods D, Shifman O, Shirasawa S, Sasazuki T et al. (2000). Opposing effects of Ras on p53: transcriptional activation of mdm2 and induction of p19ARF. Cell 103: 321–330. | Article | PubMed | ISI | ChemPort |
  109. Riesco-Eizaguirre G, Rodriguez I, De la Vieja A, Costamagna E, Carrasco N, Nistal M et al. (2009). The BRAFV600E oncogene induces transforming growth factor beta secretion leading to sodium iodide symporter repression and increased malignancy in thyroid cancer. Cancer Res 69: 8317–8325. | Article | PubMed | ISI |
  110. Ritt DA, Monson DM, Specht SI, Morrison DK. (2010). Impact of feedback phosphorylation and Raf heterodimerization on normal and mutant B-Raf signaling. Mol Cell Biol 30: 806–819. | Article | PubMed | ISI | ChemPort |
  111. Riva C, Lavieille JP, Reyt E, Brambilla E, Lunardi J, Brambilla C. (1995). Differential c-myc, c-jun, c-raf and p53 expression in squamous cell carcinoma of the head and neck: implication in drug and radioresistance. Eur J Cancer B Oral Oncol 31: 384–391. | Article |
  112. Robert C, Arnault JP, Mateus C. (2010). RAF inhibition and induction of cutaneous squamous cell carcinoma. Curr Opin Oncol 23: 177–182. | Article | ISI |
  113. Roper E, Weinberg W, Watt FM, Land H. (2001). p19ARF-independent induction of p53 and cell cycle arrest by Raf in murine keratinocytes. EMBO Rep 2: 145–150. | Article | PubMed | ISI | ChemPort |
  114. Rushworth LK, Hindley AD, O'Neill E, Kolch W. (2006). Regulation and role of Raf-1/B-Raf heterodimerization. Mol Cell Biol 26: 2262–2272. | Article | PubMed | ISI | ChemPort |
  115. Sahai E, Marshall CJ. (2003). Differing modes of tumour cell invasion have distinct requirements for Rho/ROCK signalling and extracellular proteolysis. Nat Cell Biol 5: 711–719. | Article | PubMed | ISI | ChemPort |
  116. Schulze A, Lehmann K, Jefferies HB, McMahon M, Downward J. (2001). Analysis of the transcriptional program induced by Raf in epithelial cells. Genes Dev 15: 981–994. | Article | PubMed | ISI | ChemPort |
  117. Schulze A, Nicke B, Warne PH, Tomlinson S, Downward J. (2004). The transcriptional response to raf activation is almost completely dependent on mitogen-activated protein kinase kinase activity and shows a major autocrine component. Mol Biol Cell 15: 3450–3463. | Article | PubMed | ISI | ChemPort |
  118. Seoane J. (2008). The TGFBeta pathway as a therapeutic target in cancer. Clin Transl Oncol 10: 14–19. | Article | PubMed | ISI |
  119. Sewing A, Wiseman B, Lloyd AC, Land H. (1997). High-intensity Raf signal causes cell cycle arrest mediated by p21Cip1. Mol Cell Biol 17: 5588–5597. | PubMed | ISI | ChemPort |
  120. Sharma A, Tran MA, Liang S, Sharma AK, Amin S, Smith CD et al. (2006). Targeting mitogen-activated protein kinase/extracellular signal-regulated kinase kinase in the mutant (V600E) B-Raf signaling cascade effectively inhibits melanoma lung metastases. Cancer Res 66: 8200–8209. | Article | PubMed | ISI | ChemPort |
  121. Sharma A, Trivedi NR, Zimmerman MA, Tuveson DA, Smith CD, Robertson GP. (2005). Mutant V599EB-Raf regulates growth and vascular development of malignant melanoma tumors. Cancer Res 65: 2412–2421. | Article | PubMed | ISI | ChemPort |
  122. Shimizu K, Nakatsu Y, Nomoto S, Sekiguchi M. (1986). Structure of the activated c-raf-1 gene from human stomach cancer. Princess Takamatsu Symp 17: 85–91. | PubMed | ChemPort |
  123. Simon R, Richter J, Wagner U, Fijan A, Bruderer J, Schmid U et al. (2001). High-throughput tissue microarray analysis of 3p25 (RAF1) and 8p12 (FGFR1) copy number alterations in urinary bladder cancer. Cancer Res 61: 4514–4519. | PubMed | ISI | ChemPort |
  124. Smyth MJ, Dunn GP, Schreiber RD. (2006). Cancer immunosurveillance and immunoediting: the roles of immunity in suppressing tumor development and shaping tumor immunogenicity. Adv Immunol 90: 1–50. | Article | PubMed | ISI | ChemPort |
  125. Sobczak I, Galabova-Kovacs G, Sadzak I, Kren A, Christofori G, Baccarini M. (2008). B-Raf is required for ERK activation and tumor progression in a mouse model of pancreatic beta-cell carcinogenesis. Oncogene 27: 4779–4787. | Article | PubMed | ISI |
  126. Sumimoto H, Imabayashi F, Iwata T, Kawakami Y. (2006). The BRAF–MAPK signaling pathway is essential for cancer-immune evasion in human melanoma cells. J Exp Med 203: 1651–1656. | Article | PubMed | ISI | ChemPort |
  127. Takezawa K, Okamoto I, Yonesaka K, Hatashita E, Yamada Y, Fukuoka M et al. (2009). Sorafenib inhibits non-small cell lung cancer cell growth by targeting B-RAF in KRAS wild-type cells and C-RAF in KRAS mutant cells. Cancer Res 69: 6515–6521. | Article | PubMed | ISI | ChemPort |
  128. Tidyman WE, Rauen KA. (2009). The RASopathies: developmental syndromes of Ras/MAPK pathway dysregulation. Curr Opin Genet Dev 19: 230–236. | Article | PubMed | ISI | ChemPort |
  129. Troppmair J, Hartkamp J, Rapp UR. (1998). Activation of NF-kappa B by oncogenic Raf in HEK 293 cells occurs through autocrine recruitment of the stress kinase cascade. Oncogene 17: 685–690. | Article | PubMed | ISI | ChemPort |
  130. Tsai J, Lee JT, Wang W, Zhang J, Cho H, Mamo S et al. (2008). Discovery of a selective inhibitor of oncogenic B-Raf kinase with potent antimelanoma activity. Proc Natl Acad Sci USA 105: 3041–3046. | Article | PubMed |
  131. Vale T, Ngo TT, White MA, Lipsky PE. (2001). Raf-induced transformation requires an interleukin 1 autocrine loop. Cancer Res 61: 602–607. | PubMed | ISI |
  132. Villanueva J, Vultur A, Lee JT, Somasundaram R, Fukunaga-Kalabis M, Cipolla AK et al. (2010). Acquired resistance to BRAF inhibitors mediated by a RAF kinase switch in melanoma can be overcome by cotargeting MEK and IGF-1R/PI3K. Cancer Cell 18: 683–695. | Article | PubMed | ISI | ChemPort |
  133. Wang S, Ghosh RN, Chellappan SP. (1998). Raf-1 physically interacts with Rb and regulates its function: a link between mitogenic signaling and cell cycle regulation. Mol Cell Biol 18: 7487–7498. | PubMed | ISI | ChemPort |
  134. Wang X, Thomson SR, Starkey JD, Page JL, Ealy AD, Johnson SE. (2004). Transforming growth factor {beta}1 is upregulated by activated Raf in skeletal myoblasts but does not contribute to the differentiation-defective phenotype. J Biol Chem 279: 2528–2534. | Article | PubMed | ISI |
  135. Wang ZY, Chen Z. (2008). Acute promyelocytic leukemia: from highly fatal to highly curable. Blood 111: 2505–2515. | Article | PubMed | ISI | ChemPort |
  136. Weber CK, Slupsky JR, Kalmes HA, Rapp UR. (2001). Active Ras induces heterodimerization of cRaf and BRaf. Cancer Res 61: 3595–3598. | PubMed | ISI | ChemPort |
  137. Weisberg E, Manley PW, Cowan-Jacob SW, Hochhaus A, Griffin JD. (2007). Second generation inhibitors of BCR-ABL for the treatment of imatinib-resistant chronic myeloid leukaemia. Nat Rev Cancer 7: 345–356. | Article | PubMed | ISI | ChemPort |
  138. Wellbrock C, Karasarides M, Marais R. (2004). The RAF proteins take centre stage. Nat Rev Mol Cell Biol 5: 875–885. | Article | PubMed | ISI | ChemPort |
  139. Wimmer R, Baccarini M. (2010). Partner exchange: protein–protein interactions in the Raf pathway. Trends Biochem Sci 35: 660–668. | Article | PubMed | ISI |
  140. Woods D, Parry D, Cherwinski H, Bosch E, Lees E, McMahon M. (1997). Raf-induced proliferation or cell cycle arrest is determined by the level of Raf activity with arrest mediated by p21Cip1. Mol Cell Biol 17: 5598–5611. | PubMed | ISI | ChemPort |
  141. Yagoda N, von Rechenberg M, Zaganjor E, Bauer AJ, Yang WS, Fridman DJ et al. (2007). RAS–RAF–MEK-dependent oxidative cell death involving voltage-dependent anion channels. Nature 447: 864–868. | Article | PubMed | ISI | ChemPort |
  142. Yu H, McDaid R, Lee J, Possik P, Li L, Kumar SM et al. (2009). The role of BRAF mutation and p53 inactivation during transformation of a subpopulation of primary human melanocytes. Am J Pathol 174: 2367–2377. | Article | PubMed | ISI | ChemPort |
  143. Zebisch A, Haller M, Hiden K, Goebel T, Hoefler G, Troppmair J et al. (2009). Loss of RAF kinase inhibitor protein is a somatic event in the pathogenesis of therapy-related acute myeloid leukemias with C-RAF germline mutations. Leukemia 23: 1049–1053. | Article | PubMed | ISI |
  144. Zebisch A, Staber PB, Delavar A, Bodner C, Hiden K, Fischereder K et al. (2006). Two transforming C-RAF germline mutations identified in patients with therapy-related acute myeloid leukemia. Cancer Res 66: 3401–3408. | Article | PubMed | ISI | ChemPort |
  145. Zeng L, Imamoto A, Rosner MR. (2008). Raf kinase inhibitory protein (RKIP): a physiological regulator and future therapeutic target. Expert Opin Ther Targets 12: 1275–1287. | Article | PubMed | ISI |
  146. Zheng B, Jeong JH, Asara JM, Yuan Y-Y, Granter SR, Chin L et al. (2009). Oncogenic B-RAF negatively regulates the tumor suppressor LKB1 to promote melanoma cell proliferation. Mol Cell 33: 237–247. | Article | PubMed | ISI |
  147. Zhu J, Woods D, McMahon M, Bishop JM. (1998). Senescence of human fibroblasts induced by oncogenic Raf. Genes Dev 12: 2997–3007. | Article | PubMed | ISI | ChemPort |
  148. Zhuang D, Mannava S, Grachtchouk V, Tang WH, Patil S, Wawrzyniak JA et al. (2008). C-MYC overexpression is required for continuous suppression of oncogene-induced senescence in melanoma cells. Oncogene 27: 6623–6634. | Article | PubMed | ISI | ChemPort |
  149. Zitvogel L, Tesniere A, Kroemer G. (2006). Cancer despite immunosurveillance: immunoselection and immunosubversion. Nat Rev Immunol 6: 715–727. | Article | PubMed | ISI | ChemPort |


We thank all the members of the Baccarini lab for helpful discussions and apologize to all the colleagues whose work could not be cited in this review for reasons of space. The Baccarini lab is supported by the Austrian Scientific Research Fund (Grants P19530 and SFB 021) and the European Community (Grants INFLA-CARE and GROWTHSTOP).