CXCR2 affects radiosensitization of HPV-negative head and neck squamous cell carcinoma cells by ABT-263

Background Radiation-induced senescence strongly contributes to therapy resistance of HPV-negative head and neck squamous cell carcinoma (HNSCC). It was shown, that the NF-κB-dependent arm of the senescence-associated secretory phenotype (SASP) triggers signaling pathways that strongly associate with and confer resistance towards irradiation. Beneath the SASP, another hallmark of senescence is the upregulation of anti-apoptotic proteins. The BH3-only mimetic ABT-263 was shown to successfully remove senescent cells. Within this study, we were interested, whether ABT-263 is capable of circumventing the therapy-induced resistance in HNSCC cells. Methods We treated the cell lines Cal33 and UPCI:SCC040 with a combination of ABT-263 and photons, followed by functional and mechanistic assays to study viability, apoptosis, senescence, secreted proteins, clonogenic survival and DNA repair. Results Functionally, ABT-263 induced apoptosis via impeding Bcl-xL and activating Bax, and lowered senescence levels after irradiation. Mechanistically, we observed cell line- and protein- specific alterations in the SASP, with a striking difference in expression of the CXCR2-receptor. Cal33 cells exhibited a strong downregulation of CXCR2, and these cells were radiosensitized by ABT-263, as observed by decreased viability and clonogenic survival. CXCR2 expression in UPCI:SCC040 cells was induced by the treatment and albeit viability was diminished, there was no effect on clonogenic survival. Interestingly, radiosensitization could be achieved with simultaneous inhibition of CXCR2 expression in this cell line. Moreover, the radiosensitizing effect is not conveyed by increased DNA damage as evidenced by γH2AX/53BP1 co-localization. Conclusions The results demonstrate a dominant position of the CXCR2 signaling pathway in the formation of radioresistance in HNSCC cells.