beta1 integrins as novel co-regulators of DNA damage repair

Introduction: Resistance to cancer therapies is a major unsolved challenge. One responsible factor is integrin-mediated adhesion to extracellular matrix. Several studies identified targeting of beta1 integrin receptors as promising approach for radio- and chemosensitization of tumor cells. Although different prosurvival beta1 integrin-mediated signaling pathways were identified, it remains unclear whether they are critically involved in the repair of radiation-induced DNA double strand breaks (DSB). Therefore, we examined the impact of beta1 integrin targeting on DSB repair and describe a regulatory function of beta1 integrins for DNA-PK-dependent but not PARP-dependent non-homologous end-joining (NHEJ).
Materials and methods: To mimic physiological growth conditions in vitro, a 3D cell culture model based on laminin-rich extracellular matrix and tumor xenografts of human head and neck squamous cell carcinoma (HNSCC) cell lines were employed. beta1 integrin targeting was accomplished using the inhibitory monoclonal antibody AIIB2. AIIB2, X-ray irradiation, siRNA-mediated knockdown and inhibitor treatment (FAK, JNK, DNA-PK, PARP) were performed and residual DSB number, NHEJ activity, expression and phosphorylation of various DNA repair proteins as well as clonogenic survival were determined.
Results and discussion: Intriguingly, beta1 integrin targeting impaired the repair of radiogenic DSB (gammaH2AX/p53BP1, pDNA-PKcs T2609 foci) in vitro and in vivo, decreased NHEJ activity and reduced expression and phosphorylation of Ku70, Rad50, Nbs1 and pDNA-PKcs T2609. Further, we identified Ku70, Ku80 and DNA-PKcs but not PARP-1 to reside in the beta1 integrin signaling pathway. It was compelling to observe an additive radiosensitization of 3D grown HNSCC cell lines by dual AIIB2/Olaparib treatment relative to monotherapies. Moreover, FAK and JNK1 were identified as mediators of beta1 integrin-dependent DNA repair.
Conclusion: Here, we support beta1 integrins as promising cancer targets and highlight a regulatory role for beta1 integrins in the DNA-PK-dependent repair of radiation-induced DSB. Further studies are needed to understand the relevance of cell adhesion for nuclear processes and cancer cell therapy resistance.