A high-throughput image-based screen for the identification of Bax/Bak-independent caspase activators against drug-resistant cancer cells

Despite the use of new generation target specific drugs or combination treatments, drug-resistance caused by defective apoptosis signaling remains a major challenge in cancer treatment. A common apoptotic defect in drug-resistant tumor is the failure of cancer cells to undergo Bax/Bak-dependent mitochondrial permeabilization due to impaired signaling of Bcl-2 family proteins. Therefore, Bax and Bak-independent caspase-activating compounds appear to be effective in killing such tumor cells. An image-based cellular platform of caspase sensors in Bax and Bak deficient background allowed us to identify several potential Bax/Bak-independent caspase-activating compounds from a limited high-throughput compound screening. FRET-based caspase sensor probe targeted at the nucleus enabled accurate and automated segmentation, yielding a Z-value of 0.72. Some of the positive hits showed promising activity against drug-resistant human cancer cells expressing high levels of Bcl-2 or Bcl-xL. Using this approach, we describe thiolutin, CD437 and TPEN as the most potentially valuable drug candidates for addressing drug-resistance caused by aberrant expression of Bcl-2 family proteins in tumor cells. The screen also enables the quantification of multiparameter apoptotic events along with caspase activation in HTS manner in live mode, allowing characterization of non-classical apoptosis signaling.