Abrogation of BRAFV600E-induced senescence by PI3K pathway activation contributes to melanomagenesis

Division of Molecular Oncology, The Netherlands Cancer Institute, 1066 CX Amsterdam, The Netherlands.
Genes & development (Impact Factor: 10.8). 05/2012; 26(10):1055-69. DOI: 10.1101/gad.187252.112
Source: PubMed


Human melanocytic nevi (moles) are benign lesions harboring activated oncogenes, including BRAF. Although this oncogene initially acts mitogenically, eventually, oncogene-induced senescence (OIS) ensues. Nevi can infrequently progress to melanomas, but the mechanistic relationship with OIS is unclear. We show here that PTEN depletion abrogates BRAF(V600E)-induced senescence in human fibroblasts and melanocytes. Correspondingly, in established murine BRAF(V600E)-driven nevi, acute shRNA-mediated depletion of PTEN prompted tumor progression. Furthermore, genetic analysis of laser-guided microdissected human contiguous nevus-melanoma specimens recurrently revealed identical mutations in BRAF or NRAS in adjacent benign and malignant melanocytes. The PI3K pathway was often activated through either decreased PTEN or increased AKT3 expression in melanomas relative to their adjacent nevi. Pharmacologic PI3K inhibition in melanoma cells suppressed proliferation and induced the senescence-associated tumor suppressor p15(INK4B). This treatment also eliminated subpopulations resistant to targeted BRAF(V600E) inhibition. Our findings suggest that a significant proportion of melanomas arise from nevi. Furthermore, these results demonstrate that PI3K pathway activation serves as a rate-limiting event in this setting, acting at least in part by abrogating OIS. The reactivation of senescence features and elimination of cells refractory to BRAF(V600E) inhibition by PI3K inhibition warrants further investigation into the therapeutic potential of simultaneously targeting these pathways in melanoma.

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Available from: Martin McMahon
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    • "They did not propose a pathway for this, but there are theoretical proliferative arrest pathways from PTEN both through FOXO and through AKT, ‐catenin and p16 (Figures 2, 3), although the latter would become irrelevant if p16 were already lost. One more candidate for helping to effect arrest in nevus cells is p15INK4B/CDKN2B, a paralogue of p16 that maps close to CDKN2A and is reported to be upregulated by BRAF V600E overexpression (Vredeveld et al., 2012). "
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    ABSTRACT: There are many links between cell senescence and the genetics of melanoma, meaning both familial susceptibility and somatic-genetic changes in sporadic melanoma. For example CDKN2A, the best-known melanoma susceptibility gene, encodes two effectors of cell senescence, while other familial melanoma genes are related to telomeres and their maintenance. This article aims to analyze our current knowledge of the genetic or epigenetic driver changes necessary to generate a cutaneous metastatic melanoma, the commonest order in which these occur and the relation of these changes to the biology and pathology of melanoma progression. Emphasis is laid on the role of cell senescence and the escape from senescence leading to cellular immortality, the ability to divide indefinitely.This article is protected by copyright. All rights reserved.
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    • "instance, are frequently positive for activating BRaf V600E mutations; however, these cells bear a senescent phenotype [75]. Abrogation of such oncogene-induced senescence by PI3K activation allows for melanoma formation [76]. Induction of cellular senescence therefore has been recognized as promising therapeutic approach to prevent the proliferation of cancer cells. "
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    • "In another context, mutant B-Raf induced cellular senescence rather than proliferation [23,24]. However, senescence can be overcome by phosphoinositide 3-kinase (PI3K)/AKT signaling [24] which is hyperactivated in RKO due to a PIK3CA mutation. By staining of senescence-associated β-galactosidase activity [25] we examined whether the differential proliferation rates observed upon serum deprivation were attributable to cellular senescence. "
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    ABSTRACT: Background Colorectal cancers carrying the B-Raf V600E-mutation are associated with a poor prognosis. The purpose of this study was to identify B-RafV600E-mediated traits of cancer cells in a genetic in vitro model and to assess the selective sensitization of B-RafV600E-mutant cancer cells towards therapeutic agents. Methods Somatic cell gene targeting was used to generate subclones of the colorectal cancer cell line RKO containing either wild-type or V600E-mutant B-Raf kinase. Cell-biologic analyses were performed in order to link cancer cell traits to the BRAF-mutant genotype. Subsequently, the corresponding tumor cell clones were characterized pharmacogenetically to identify therapeutic agents exhibiting selective sensitivity in B-RafV600E-mutant cells. Results Genetic targeting of mutant BRAF resulted in restoration of sensitivity to serum starvation-induced apoptosis and efficiently inhibited cell proliferation in the absence of growth factors. Among tested agents, the B-Raf inhibitor dabrafenib was found to induce a strong V600E-dependent shift in cell viability. In contrast, no differential sensitizing effect was observed for conventional chemotherapeutic agents (mitomycin C, oxaliplatin, paclitaxel, etoposide, 5-fluorouracil), nor for the targeted agents cetuximab, sorafenib, vemurafenib, RAF265, or for inhibition of PI3 kinase. Treatment with dabrafenib efficiently inhibited phosphorylation of the B-Raf downstream targets Mek 1/2 and Erk 1/2. Conclusion Mutant BRAF alleles mediate self-sufficiency of growth signals and serum starvation-induced resistance to apoptosis. Targeting of the BRAF mutation leads to a loss of these hallmarks of cancer. Dabrafenib selectively inhibits cell viability in B-RafV600E mutant cancer cells.
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