The interplay between apoptosis and cellular senescence: Bcl-2 family proteins as targets for cancer therapy

Cell death by apoptosis and permanent cell cycle arrest by senescence serve as barriers to the development of cancer. Chemotherapeutic agents not only induce apoptosis, they can also induce senescence known as therapy-induced senescence (TIS). There are, however, controversies whether TIS improves or worsens therapeutic outcome. Unlike apoptosis, which permanently removes cancer cells, senescent cells are metabolically active, and can contribute to tumor progression and relapse. If senescent cells are not cleared by the immune system or if cancer cells escape senescence, they may acquire resistance to apoptotic stimuli and become highly aggressive. Thus, there have been significant efforts in developing senolytics, drugs that target these pro-survival molecules to eliminate senescent cells. The anti-apoptotic Bcl-2 family proteins not only protect against cell death by apoptosis, but they also allow senescent cells to survive. While combining senolytics with chemotherapeutic drugs is an attractive approach, there are also limitations. Moreover, members of the Bcl-2 family have distinct effects on apoptosis and senescence. The purpose of this review article is to discuss recent literatures on how members of the Bcl-2 family orchestrate the interplay between apoptosis and senescence, and the challenges and progress in targeting these Bcl-2 family proteins for cancer therapy.