The BH3-only protein Bim(L) overrides Bcl-2-mediated apoptosis resistance in melanoma cells

Melanoma cells are characterized by apoptosis deficiency coinciding with reduced expression of the proapoptotic Bcl-2 protein Bim. An adenoviral vector was constructed with the BimL cDNA controlled by an inducible promoter. Highly efficient apoptosis induction and abrogated cell proliferation was seen in melanoma cells upon BimL overexpression. Loss of mitochondrial membrane potential, release of mitochondrial apoptogenic factors and caspase-9 processing indicated the activation of mitochondrial apoptosis pathways. BimL activated both Bax and Bak, as shown by siRNA knockdown and activation-specific antibodies. Of note, BimL overrode the apoptosis blockade by Bcl-2 overexpression or by Bax/Bak single knockdown. The high efficacy correlated to BimL interaction with all antiapoptotic Bcl-2 family members in melanoma cells, shown by co-immunoprecipitation analyses for Bcl-2, Bcl-xL, Mcl-1 and Bcl-w. Thus, BimL reveals an outstanding proapoptotic potential in melanoma cells, and strategies for its re-expression appear of interest. These have been reported for B-Raf inhibitors, and their efficacy may be partly attributed to BimL.