ИСТИНА

One of the most commonly exploited strategies in anticancer treatment is based on the induction of apoptotic cancer cell death. There are numerous agents which cause various types of cellular stresses and eventually lead to the mitochondrial outer membrane permeabilization (MOMP) and activation of intrinsic pathway of apoptosis. MOMP is tightly controlled by proteins of the Bcl-2 family, which consists of more than 20 members, which are either proapoptotic or antiapoptotic. Antiapoptotic proteins of the Bcl-2 family (such as Bcl-2, Bcl-xL, Mcl-1) represent attractive targets for anticancer therapy and, consistently, the so-called BH3-mimetics – inhibitors of antiapoptotic Bcl-2 proteins – are thoroughly investigated in both preclinical and clinical studies. Recently, the inhibitor of Bcl-2 Venetoclax was approved by FDA for clinical use, underlining the great potential of BH3-mimetics. Nowadays, at least three BH3-mimetics specifically targeting Mcl-1 are evaluated in clinical trials. In general, these compounds represent a powerful tool for induction of apoptosis. As BH3-mimetics act as direct regulators of MOMP, they are much more specific in triggering apoptosis in selected cell types compared to other agents, such as genotoxic drugs or microtubule poisons. At the same time, sensitivity to one or other BH3-mimetic varies between different tumor cells, which highlights the importance of biomarkers that will correlate to response to such drugs. Our studies have shown that among two BH3-mimetics targeting Mcl-1, A1210477 and S63845, the first one demonstrates lower efficacy compared to MCL1 knockdown, while S63845 is as potent as downregulation of Mcl-1 using siRNA. Apparently, differences in efficacy of these two compounds are due to diverse impact on activation of proapoptotic Bcl-2 family member Bak. Indeed, whereas A1210477 results in a moderate level of apoptosis in a Bak-independent fashion, S63845 induces higher level of apoptosis in a Bak-dependent manner. Next, using siRNA-mediated knockdown it was shown on different tumor cell lines (such as HeLa and H23) that S63845-induced apoptosis depends on level of Bak but not on Bax or Bim (other proapoptotic members of the Bcl-2 family). We have also generated cell lines, characterized by higher resistance to S63845 compared to parental cell lines. It was shown that acquired resistance to S63845 is accompanied by higher expression of prosurvival member of Bcl-2 family Bcl-xL. Finally, it was shown on a panel of cancer cell lines that lower levels of Bcl-xL correlate with better response to S63845. Taken together, these data demonstrate that Bak and Bcl-xL serve as biomarkers for sensitivity of cancer cells to BH3-mimetics targeting Mcl-1.