Head and neck cancer cell radiosensitization upon dual targeting of c-Abl and beta1-integrin

Integrin-mediated cell adhesion to extracellular matrix (ECM) critically contributes to cancer cell therapy resistance and DNA double strand break (DSB) repair. c-Abl tyrosine kinase has been linked to both of these processes. Based on our previous findings indicating c-Abl hyperphosphorylation on tyrosine (Y) 412 and threonine (T) 735 upon beta1-integrin knockdown, we hypothesized c-Abl tyrosine kinase as an important mediator of beta1-integrin signaling for radioresistance. In a panel of 8 cell lines from different solid cancer types grown in 3D laminin-rich ECM cultures, we targeted beta1 integrin with AIIB2 (mAb) and c-Abl with Imatinib with and without X-ray irradiation and subsequently examined clonogenic survival, residual DSBs, protein expression and -phosphorylation. Single or combined treatment with AIIB2 and Imatinib resulted in cell line-dependent cytotoxicity. Intriguingly, a subgroup in this cell line panel responding to AIIB2/Imatinib treatment with higher radiosensitization relative to both single treatments was identified. Likewise, AIIB2/Imatinib co-treatment elicited a significantly higher number of residual DSBs than controls, which was accompanied by decreased Ku70 expression and enhanced ATM S1981 phosphorylation. Mechanistically, c-Abl was located in the beta1-integrin/JNK signaling axis and abrogated AIIB2/Imatinib-related radiosensitization when exogenously overexpressed in either wildtype or constitutively activated form. Our data generated in more physiological 3D cancer cell culture models indicate c-Abl as important determinant of radioresistance and DNA repair downstream of beta1-integrin. For solid cancers, c-Abl phosphorylation status might be an indicator for reasonable Imatinib application as adjuvant for conventional radio(chemo)therapy.