Active N-Ras and B-Raf Inhibit Anoikis by Downregulating Bim Expression in Melanocytic Cells

B-Raf and N-Ras proteins are often activated in melanoma, yet their roles in producing inherent survival signals are not fully understood. In this study, we investigated how N-RASQ61K and B-RAFV600E contribute to melanoma's resistance to apoptosis induced by detachment from the extracellular matrix (anoikis). We found that expression of constitutively active N-RASQ61K and B-RAFV600E downregulated the proapoptotic Bim protein in an immortalized melanocyte cell line. Bim is one of the main proapoptotic mediators of anoikis. Western blot analysis showed that detachment increased Bim expression in melanocytes, and Annexin V staining indicated that detachment induced cell death significantly in melanocytes. Blocking Bim expression by using RNAi vectors or by expressing N-RASQ61K significantly inhibited anoikis in melanocytes. In summary, this report indicates that N-RASQ61K and B-RAFV600E contribute to melanoma's resistance to apoptosis in part by downregulating Bim expression, suggesting that Bim is a possible treatment target for overriding melanoma's inherent defenses against cell death.