Abstract LB-211: beta1 integrin targeting results in radiosensitization by controlling DNA repair mechanisms and chromatin organization

Abstract

Abstract Integrin-mediated cell-extracellular matrix (ECM) interactions confer cancer cell resistance to therapy. Our own data and data of others generated in three-dimensional (3D) ECM-based cell cultures and in vivo provided evidence that particularly beta1 integrin targeting potently reduces this resistance involving downstream integrin signaling mediators like focal adhesion kinase, cortactin and JNK1. However, it remains unknown whether beta1 integrin inhibition affects the repair of radiation-induced DNA double strand breaks (DSB) and their distribution in eu- and heterochromatic DNA regions. Here, we show that 3D growth (cell culture, xenograft models) increased the amount of heterochromatin in human head and neck squamous cell (HNSCC) and lung carcinoma cell lines relative to 2D, induced higher radioresistance, reduced the number of chromosomal aberrations and gammaH2AX/53BP1-positive foci, which was accompanied by a shift of the euchromatin-to-heterochromatin DSB distribution from a 2:1 (2D) to a 1:1 (3D). Upon beta1 integrin inhibition using the inhibitory monoclonal antibody AIIB2 or siRNA, cells grown in 3D or as xenografts in nude mice were radiosensitized or showed a significant growth delay, respectively. The disruption of beta1 integrin-mediated cell-ECM interactions induced an increase in heterochromatin levels indicated by HP1alpha and H3K9me3 elevation. In line with these findings was a strong impairment of both ATM and DNA-PK and their associated downstream signaling pathways (e.g. Chk2, Mre11, Nbs1, Rad50) in beta1 integrin inhibited, irradiated cells relative to IgG isotype controls. In contrast to unchanged number of heterochromatic foci identified in stably transfected HP1alpha cells, kinetic DSB measurements revealed an increased number of euchromatic foci upon beta1 integrin blocking, which evidently showed strongly perturbed DSB repair. In conclusion, our data generated in 3D cell culture and tumor xenograft models suggest a regulatory function of beta1 integrin signaling in the repair of radiogenic DSBs and the organization of chromatin. In addition to our understanding of the initial steps deactivated upon beta1 integrin blocking, these findings bridge membrane bound integrin function to nuclear DNA repair processes, thus providing further insights into how integrins confer cancer cell radioresistance and why integrin targeting is potentially useful as adjuvant to conventional radio- and radiochemotherapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-211. doi:1538-7445.AM2012-LB-211