CRAF inhibition induces apoptosis in melanoma cells with non-V600E BRAF mutations

Blockade of the MEK/ERK signalling cascade by AS703026, a novel selective MEK1/2 inhibitor, induces pleiotropic anti-myeloma activity in vitro and in vivo

This study investigated the cytotoxicity and mechanism of action of AS703026, a novel, selective, orally bioavailable MEK1/2 inhibitor, in human multiple myeloma (MM). AS703026 inhibited growth and survival of MM cells and cytokine-induced osteoclast differentiation more potently (9- to 10-fold) than AZD6244. Inhibition of proliferation induced by AS703026 was mediated by G0-G1 cell cycle arrest and was accompanied by reduction of MAF oncogene expression. AS703026 further induced apoptosis via caspase 3 and Poly ADP ribose polymerase (PARP) cleavage in MM cells, both in the presence or absence of bone marrow stromal cells (BMSCs). Importantly, AS703026 sensitized MM cells to a broad spectrum of conventional (dexamethasone, melphalan), novel or emerging (lenalidomide, perifosine, bortezomib, rapamycin) anti-MM therapies. Significant tumour growth reduction in AS703026- vs. vehicle-treated mice bearing H929 MM xenograft tumours correlated with downregulated pERK1/2, induced PARP cleavage, and decreased microvessels in vivo. Moreover, AS703026 (<200 nmol/l) was cytotoxic against the majority of tumour cells tested from patients with relapsed and refractory MM (84%), regardless of mutational status of RAS and BRAF genes. Importantly, BMSC-induced viability of MM patient cells was similarly blocked within the same dose range. Our results therefore support clinical evaluation of AS703026, alone or in combination with other anti-MM agents, to improve patient outcome.

The brothers RAF

Targeted molecular therapies for cancer treatment have shown promise, but also have limitations. In this issue, Heidorn et al. (2010) find that a class of targeted molecular therapies with clinical effectiveness against one melanoma subtype may have adverse clinical effects in another.

Understanding melanoma signaling networks as the basis for molecular targeted therapy

Despite years of research, there has been little improvement in survival for patients with disseminated melanoma. Recent work has identified mutations in BRAF and NRAS, leading to constitutive mitogen-activated protein kinase (MAPK) pathway as well as constitutive activity in the phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) pathway, as being critical events in melanoma growth and progression. In the current review, we discuss how these complex mutational and signaling profiles can be understood using a network biology approach, and suggest how an understanding of the key signaling nodes involved in progression and survival will lead to improvements in melanoma therapy.

Targets of Raf in tumorigenesis

Some 25 years ago, Raf was discovered as the transforming principle shared by a murine sarcoma and an avian carcinoma virus. Thus, Raf and tumorigenesis have been connected from the very beginning. Ten years later, the work of many groups instated Raf as the link between Ras, the oncogene most frequently mutated in human cancers, and the mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK/ERK) module, which with its manifold substrates can contribute to different aspects of carcinogenesis. Finally, the discovery of activating B-Raf mutations in a subset of human cancers, notably melanomas, conclusively established Raf as a major player in tumor development. Recent studies in animal models now show that endogenous C-Raf is essential for the development and maintenance of Ras-induced epidermal tumors. Surprisingly, the role of C-Raf in this case is not that of an mitogen-activated protein kinase activator, but rather that of an endogenous inhibitor of Rho signaling, expanding the range of tumor-related Raf targets. This review focuses on old and new targets of Raf in tumorigenesis.

Sorafenib

Sorafenib (BAY 43-9006, Nexavar) is a novel oral kinase inhibitor that targets multiple tyrosine kinases in vivo and in vitro. Main targets are receptor tyrosine kinase pathways frequently deregulated in cancer such as the raf-ras pathway, vascular endothelial growth factor (VEGF) pathway, and FMS-like tyrosine kinase 3 (FLT3). Sorafenib was approved by the FDA in fast track for advanced renal cell cancer and hepatocellular cancer and shows good clinical activity in thyroid cancer. Multiple clinical trials are undertaken to further investigate the role of sorafenib alone or in combination for the treatment of various tumor entities.

Geldanamycin and its anti-cancer activities

Geldanamycin is a benzoquinone ansamycin antibiotic that manifests anti-cancer activity through the inhibition of HSP90-chaperone function. The HSP90 molecular chaperone is expressed at high levels in a wide variety of human cancers including melanoma, leukemia, and cancers in colon, prostate, lung, and breast. In cancer cells dependent upon mutated and/or over-expressed oncogene proteins, HSP90 is thought to have a critical role in regulating the stability, folding, and activity of HSP90-associated proteins, so-called "client proteins". These client proteins include the growth-stimulating proteins and kinases that support malignant transformation. Recently, oncogenic activating BRAF mutants have been identified in variety of cancers where constitutive activation of the MEK/ERK MAPK signaling pathway is the key for tumorigenesis, and they have been shown to be client proteins for HSP90. Accordingly, HSP90 inhibition can suppress certain cancer-causing client proteins and therefore represents an important therapeutic target. The molecular mechanism underlying the anti-cancer effect of HSP90 inhibition is complicated. Geldanamycin and its derivatives have been shown to induce the depletion of mutationally-activated BRAF through several mechanisms. In this review, we will describe the HSP90-inhibitory mechanism, focusing on recent progress in understanding HSP90 chaperone structure-function relationships, the identification of new HSP90 client proteins and the development of HSP90 inhibitors for clinical applications.

C-Raf is associated with disease progression and cell proliferation in a subset of melanomas

PURPOSE

Raf-kinases include three major isoforms. Although the role of B-Raf in melanoma is well established, little is known about C-Raf. We studied effects of C-Raf knockdown in vitro and assessed expression of C-Raf in a large cohort of melanomas and nevi.

EXPERIMENTAL DESIGN

Using specific siRNAs, we knocked down C-Raf expression, and determined the effect on viability, MAP extracellular signal-regulated kinase (ERK)/ERK kinase signaling, and apoptosis in seven melanoma cell lines. We determined the IC(50) of the C-Raf inhibitors sorafenib and GW5074, and studied the effects of GW5074 on cell signaling. Using an automated method to measure in situ protein expression, we quantified C-Raf expression in 263 nevi and 523 melanomas.

RESULTS

C-Raf was knocked down in three cell lines with detectable phospho-C-Raf, resulting in decreased viability in two of the three (YULAC and YUROB). This resulted in decreased Bcl-2 expression and phospho-Bad cleavage, without affecting phospho-MEK and phospho-ERK. Sensitivity to sorafenib and GW5074 varied. GW5074 inhibited mitogen-activated protein kinase signaling without Bcl-2 and phospho-Bad down-regulation. C-Raf was highly expressed in melanomas compared with nevi (P < 0.0001), and no nevi had high C-Raf expression. C-Raf expression was higher in metastatic than primary specimens (P = 0.0225).

CONCLUSIONS

C-Raf siRNA knock-down results in decreased viability of YULAC (B-Raf(V600K)) and YUROB (B-Raf(WT)) melanoma cells, likely mediated by Bcl-2 inhibition rather than mitogen-activated protein kinase inhibition. Cotargeting C-Raf and parallel pathways might be an effective therapeutic approach for melanoma. C-Raf expression is up-regulated in a subset of melanomas but not in nevi, suggesting that it might be a valuable diagnostic marker and therapeutic target.

Drug targeting of oncogenic pathways in melanoma

Melanoma continues to be one of the most aggressive and morbid malignancies once metastatic. Overall survival for advanced unresectable melanoma has not changed over the past several decades. However, the presence of some long-term survivors of metastatic melanoma highlights the heterogeneity of this disease and the potential for improved outcomes. Current research is uncovering the molecular and genetic scaffolding of normal and aberrant cell function. The known oncogenic pathways in melanoma and the attempts to develop therapy for them are discussed. The targeting of certain cellular processes, downstream of the common genetic alterations, for which the issues of target and drug validation are somewhat distinct, are also highlighted.

Molecular-targeted therapy in malignant melanoma

Malignant melanoma is a deadly disease in which standard treatment options have remained remarkably static over the past 30 years. Recent discoveries have expanded the understanding of the molecular processes critical to melanomagenesis. During this same time period, therapeutic agents have been developed that target these processes, leading to an explosion of preclinical research. Several agents that have shown promise in the preclinical setting have now entered clinical trials. To date, the success of these molecularly targeted approaches as single agents has been limited. Although more encouraging results have been seen when these agents have been used in combination with cytotoxic therapy, the specific contribution of the targeted agents to the observed anti-tumor effects remains to be established in randomized controlled Phase III trials. This article presents a review of the limitations of current therapy, a description of key pathogenic mechanisms for which inhibitors exist and a summary of therapeutic trials of molecularly targeted agents in this disease.

Combined targeting of BRAF and CRAF or BRAF and PI3K effector pathways is required for efficacy in NRAS mutant tumors

Background

Oncogenic RAS is a highly validated cancer target. Attempts at targeting RAS directly have so far not succeeded in the clinic. Understanding downstream RAS-effectors that mediate oncogenesis in a RAS mutant setting will help tailor treatments that use RAS-effector inhibitors either alone or in combination to target RAS-driven tumors.

Methodology/Principal Findings

In this study, we have investigated the sufficiency of targeting RAS-effectors, RAF, MEK and PI3-Kinase either alone or in combination in RAS mutant lines, using an inducible shRNA in vivo mouse model system. We find that in colon cancer cells harboring a KRAS^G13D mutant allele, knocking down KRAS alone or the RAFs in combination or the RAF effectors, MEK1 and MEK2, together is effective in delaying tumor growth in vivo. In melanoma cells harboring an NRAS^Q61L or NRAS^Q61K mutant allele, we find that targeting NRAS alone or both BRAF and CRAF in combination or both BRAF and PIK3CA together showed efficacy.

Conclusion/Significance

Our data indicates that targeting oncogenic NRAS-driven melanomas require decrease in both pERK and pAKT downstream of RAS-effectors for efficacy. This can be achieved by either targeting both BRAF and CRAF or BRAF and PIK3CA simultaneously in NRAS mutant tumor cells.

Genetic subgrouping of melanoma reveals new opportunities for targeted therapy

The discovery of activating oncogenic BRAF V600E mutations in the majority of melanomas has not yet been translated into more effective therapy. The failure of agents may be due to lack of sufficiently targeted therapeutics, but is more likely based on the activation of multiple oncogenic pathways in melanomas in addition to the mitogen-activated protein kinase signaling pathway. In contrast, there are groups of melanomas that instead rely on either c-KIT or CRAF signaling that may be amenable to single-agent targeted therapy. In the current review, we discuss how knowledge about these new melanoma subgroups may lead to improved strategies for treating melanomas harboring BRAF V600E mutations.