Avoidance of therapy-induced apoptosis is a hallmark of acquired resistance towards radiotherapy. Thus, breaking resistance still challenges modern cancer therapy. The Bcl-2 protein family is known for its regulatory role in apoptosis signaling, making Bcl-2, Mcl-1 and Bcl-xL promising targets. This study evaluates the effects of highly specific inhibitors for Bcl-xL (WEHI-539), Bcl-2 (ABT-199) and Mcl-1 (S63845) as radiosensitizers. Covering a broad spectrum of solid tumors, Non-Small-Cell Lung Cancer (NSCLC), Head and Neck Squamous Cell Carcinoma (HNSCC) and synovial sarcoma cell lines were exposed to fractionated radiation as standard therapy with or without Bcl-2 protein inhibition. Protein expression was detected by Western blot and cell death was assessed by flow cytometry measuring apoptosis. In contrast to NSCLC, a high level of Bcl-xL and its upregulation during radiotherapy indicated radioresistance in HNSCC and synovial sarcoma. Radioresistant cell lines across all entities benefited synergistically from combined therapy with Bcl-xL inhibition and fractionated radiation. In NSCLC cell lines, Mcl-1 inhibition significantly augmented radiotherapy independent of the expression level. Our data suggest that among antiapoptotic Bcl-2 proteins, targeting Bcl-xL may break resistance to radiation in HNSCC, synovial sarcoma and NSCLC in vitro. In NSCLC, Mcl-1 might be a promising target that needs further investigation.

Intrahepatic, perihilar, and distal cholangiocarcinoma (iCCA, pCCA, dCCA) are highly malignant tumours with increasing mortality rates due to therapy resistances. Among the mechanisms mediating resistance, overexpression of anti‐apoptotic Bcl‐2 proteins (Bcl‐2, Bcl‐xL, Mcl‐1) is particularly important. In this study, we investigated whether antiapoptotic protein patterns are prognostically relevant and potential therapeutic targets in CCA. Bcl‐2 proteins were analysed in a pan‐cancer cohort from the NCT/DKFZ/DKTK MASTER registry trial (n = 1140, CCA n = 72) via RNA‐sequencing and transcriptome‐based protein activity interference revealing high ranks of CCA for Bcl‐xL and Mcl‐1. Expression of Bcl‐xL, Mcl‐1, and Bcl‐2 was assessed in human CCA tissue and cell lines compared with cholangiocytes by immunohistochemistry, immunoblotting, and quantitative‐RT‐PCR. Immunohistochemistry confirmed the upregulation of Bcl‐xL and Mcl‐1 in iCCA tissues. Cell death of CCA cell lines upon treatment with specific small molecule inhibitors of Bcl‐xL (Wehi‐539), of Mcl‐1 (S63845), and Bcl‐2 (ABT‐199), either alone, in combination with each other or together with chemotherapeutics was assessed by flow cytometry. Targeting Bcl‐xL induced cell death and augmented the effect of chemotherapy in CCA cells. Combined inhibition of Bcl‐xL and Mcl‐1 led to a synergistic increase in cell death in CCA cell lines. Correlation between Bcl‐2 protein expression and survival was analysed within three independent patient cohorts from cancer centers in Germany comprising 656 CCA cases indicating a prognostic value of Bcl‐xL in CCA depending on the CCA subtype. Collectively, these observations identify Bcl‐xL as a key protein in cell death resistance of CCA and may pave the way for clinical application.