Anti-apoptotic BCL-2 proteins govern cellular outcome following B-RAF(V600E) inhibition and can be targeted to reduce resistance

In theory, pharmacological inhibition of oncogenic signaling is an effective strategy to halt cellular proliferation, induce apoptosis, and eliminate cancer cells. In practice, drugs (e.g., PLX-4032) that inhibit oncogenes like B-RAF(V600E) provide relatively short-term success in patients, due to a combination of incomplete cellular responses and the development of resistance. To define the relationship between PLX-4032 induced responses and resistance, we interrogated the contributions of anti-apoptotic BCL-2 proteins in determining the fate of B-RAF(V600E) inhibited melanoma cells. While PLX-4032 eliminated B-RAF(V600E) signaling leading to marked cell cycle arrest, only a fraction of cells eventually underwent apoptosis. These data proposed two hypotheses regarding B-RAF(V600E) inhibition: (1) only a few cells generate a pro-apoptotic signal, or (2) all the cells generate a pro-apoptotic signal but the majority silences this pathway to ensure survival. Indeed, the latter hypothesis is supported by our observations as the addition of ABT-737, an inhibitor to anti-apoptotic BCL-2 proteins, revealed massive apoptosis following PLX-4032 exposure. B-RAF(V600E) inhibition alone sensitized cells to the mitochondrial pathway of apoptosis characterized by the rapid accumulation of BIM on the outer mitochondrial membrane, which could be functionally revealed by ABT-737 to promote apoptosis and loss of clonogenic survival. Furthermore, PLX-4032 resistant cells demonstrated collateral resistance to conventional chemotherapy; yet could be re-sensitized to PLX-4032 by BCL-2 family inhibition in vivo and conventional chemotherapies in vitro. Our data suggest that inhibiting anti-apoptotic BCL-2 proteins will enhance primary responses to PLX-4032, along with reducing the development of resistance to both targeted and conventional therapies.