8P Targeting antiapoptotic Bcl-2 proteins with highly specific BH3 mimetics in solid tumors

Evasion of apoptosis is a hallmark of cancer survival and a reason for acquired resistance towards standard treatment, making it one of the challenges in modern cancer therapy. The antiapoptotic proteins Bcl-2, Bcl-xL and Mcl-1, might be key to break such a resistance. In this study we evaluated the effects of highly specific inhibitors for Bcl-xL (WEHI-539), Bcl-2 (ABT-199), and Mcl-1 (S63845) as well as dual inhibition of Bcl-xL/Bcl-2 (ABT-263) in commonly occurring solid tumors. Non-Small Cell Lung Cancer (NSCLC), Cholangiocarcinoma (CCA) and Breast Cancer cell lines were either exposed to fractionated photon radiation or chemotherapeutics (Epirubicin) as standard therapy with or without specific Bcl-2 protein inhibition. Protein expression was assessed via Western blots of cell lines. Effects on cell death were detected by flow cytometry measuring apoptosis. Dual Bcl-xL/Bcl-2 inhibition led to significantly higher cell death induction in combination with radiotherapy in NSCLC and with Epirubicin in triple-negative breast cancer. Sole inhibition of Bcl-xL caused an inferior but notable sensitization in both entities for standard therapy. In CCA, combination of fractionated photon beam radiation and specific Bcl-xL inhibition showed a significant increase of cell death in all four employed cell lines. In addition, the triple-negative breast cancer cell line benefited synergistically from combined therapy with Mcl-1 inhibition and Epirubicin. In NSCLC, no correlation between basal expression of Bcl-2 family proteins and response to therapy was found and proteins were not regulated upon irradiation. Upregulation of Mcl-1 may play a role in promoting radioresistance after specific Bcl-2 and Mcl-1 inhibition. Following Epirubicin treatment, breast cancer cell lines showed a downregulation of antiapoptotic Bcl-2 protein expressions. Our findings indicate that among antiapoptotic Bcl-2 proteins, targeting Bcl-xL might break resistance to radiation in NSCLC, CCA and breast cancer in vitro. Especially for breast cancer, Mcl-1 could also be a promising target that needs to be further investigated.