Dual inhibition of anti-apoptotic proteins BCL-XL and MCL-1 enhances cytotoxicity of Nasopharyngeal carcinoma cells

One of the many strategies that Cancer cells evade death is through up-regulation of the Bcl-2 anti-apoptotic proteins. Hence, these proteins have become attractive therapeutic targets. Given that different cell populations rely on different anti-apoptotic proteins for survival, it is crucial to determine which proteins are important for Nasopharyngeal carcinoma (NPC) cell survival. Here we determined the survival requirements for the NPC cells using a combination of the CRISPR/Cas9 technique and selective BH3-mimetics. A human Apoptosis RT² Profiler PCR Array was first employed to profile the anti-apoptotic gene expressions in NPC cell lines HK-1 and C666-1. The HK-1 cells expressed all the anti-apoptotic genes (Mcl-1, Bfl-1, Bcl-2, Bcl-xL, and Bcl-W). Similarly, the C666-1 cells expressed all the anti-apoptotic genes except Bfl-1 (undetectable level). Notably, both cell lines highly expressed Mcl-1. Deletion of Mcl-1 sensitized the NPC cells to Bcl-xL selective inhibitor A-1331852, suggesting that Mcl-1 and Bcl-xL may be important for NPC cell survival. Co-inhibition of Mcl-1 and Bcl-2 with Mcl-1 selective inhibitor S63845 and Bcl-2 selective inhibitor ABT-199 inhibited NPC cell proliferation but the effect on cell viability was more profound with co-inhibition of Mcl-1 and Bcl-xL with S63845 and A-1331852, implying that Mcl-1 and Bcl-xL are crucial for NPC cell survival. Furthermore, co-inhibition of Mcl-1 and Bcl-xL inhibited the growth and invasion of NPC spheroids. Deletion of Bfl-1 sensitized NPC cells to A-1331852 suggesting that Bfl-1 may play a role in NPC cell survival. Taken together co-inhibition of Bcl-xL and Mcl-1/Bfl-1 could be potential treatment strategies for NPC.