Inhibiting the DNA damage repair of HNSCC cells in combination with normo-fractionated radiotherapy influences clonogenicity, senescence and expression of NK cell activation markers

Treatment of head and neck squamous cell carcinomas (HNSCC) remains challenging with regards to radioresistance, particularly of Human Papilloma Virus (HPV)-negative tumors. Several new approaches are currently under pre-clinical and clinical investigation. Combination of radiotherapy (RT) and kinase inhibitors of the DNA damage repair system (DDRi), targeting Ataxia Telangiectasia Mutated (ATM) or ATM and Rad3-related (ATR), are promising, but the consequences on tumor cell phenotype are still scarce. We used AZD0156, an ATM inhibitor, and VE-822, an ATR inhibitor, in combination with normo-fractionated RT to treat two HPV-positive and two HPV-negative HNSCC cell lines. Generally, an effective reduction of clonogenicity was detected in tumor cells treated with a combination of RT + DDRi. Inhibiting ATM in combination with RT changed the cellular morphology, enhanced β-Gal activity and intensified secretion of senescence-associated cytokines. As senescent cells are naturally targeted by NK cells, we next analyzed the release of the cytokines IL-6 and IL-8 and found them to be differently regulated by the inhibitors. In co-culture with NK cells, an upregulation of activation markers on NK cells was observed, particularly after contact with RT + ATMi-treated HPV-negative HNSCC cells. We conclude that ATM inhibitor-related induction of senescence in HNSCC cells shapes the tumor micro-environment in way that NK cell phenotype is changed.