Detecting Apoptotic Blocks and Sensitvity to ABT-737 and Conventional Chemotherapy Via BH3 Profiling.

Cancer cells require blocks in apoptosis to survive despite many abnormalities that should trigger apoptosis. Here we demonstrate that BH3 profiling is an effective tool for the systematic investigation of the types of blocks in apoptosis for which cancer cells select. BH3 profiling can segregate lymphoma cell lines into three different classes based on the type of apoptotic block each exhibits. In one class, cells are kept alive via expression of anti-apoptotic proteins of the BCL-2 family. When BCL-2 itself is detected by BH3 profiling as the protein keeping the cell alive, the cell is sensitive to ABT-737, a BCL-2 antagonist compound. Sensitivity to ABT-737 correlates highly with abundance of pro-apoptotic BIM complexed to BCL-2. Knockdown of BIM reduces sensitivity to ABT-737. We describe such cancer cells that maintain survival based on tonic sequestration of large amounts of pro-death molecules by anti-apoptotic proteins like BCL-2 as “primed for death”. “Primed” lymphoma cell lines are more sensitive than “unprimed” cancer cells to conventional agents killing via apoptosis like vincristine, adriamycin, and etoposide. It is important to note that, in general, non-malignant cells are unlikely to be “primed”. Hence “priming” may be an important determinant of the therapeutic window between normal and malignant cells. These results suggest that the degree to which a cell is “primed” can influence sensitivity to chemotherapy agents targeting distinct cellular targets. Therefore, BH3 profiling is a tool potentially useful not only for predicting sensitivity to agents targeting antiapoptotic BCL-2 proteins, but also to conventional cytotoxic agents that use the mitochondrial apoptotic pathway to kill.