Abstract 488: cSrc is a radioresistance molecule in cancer radiotherapy: cSrc-siRNA transfection augments tumor cell radiosensitivity in vitro and chemical inhibition of Src kinases increases tumor response to radiation

Background: cSrc is often over-expressed in cancer cells including head and neck squamous cell carcinoma (HNSCC) and is implicated in promoting cell survival, proliferation, angiogenesis, cell adhesion and motility. Its role in tumor resistance to cytotoxic agents, such as radiation, is less clear. Present study investigated (1) the relationship between the level of activated cSrc expression and the degree of cell radioresistance and (2) the role of cSrc in the cellular response to radiation, using both in vitro and in vivo models. Materials and methods: Ten HNSCC lines (FaDu, HN-5, UMSCC-1, HN-30, MDA1386, UMSCC-17B, UMSCC-47, Sqccy-1, SN2 and PCI15B) were assessed for their sensitivity to ionizing radiation by clonogenic survival and for expression levels of active Src kinases using phospho-kinase antibody array kit (Proteome ProfilerTM, R&D Systems, Inc., Minneapolis, MN). cSrc expression levels were validated by Western blot. cSrc-siRNA was transfected by electroporation to suppress cSrc expression, after which the cells were assessed for radiosensitivity. We also tested the effect of dasatinib, an inhibitor of Src kinases, on in vitro cell radiosensitivity, and on the growth of tumor xenografts in nude mice. Results: HNSCC lines exhibited a wide range of both radiosensitivity and levels of phosphorylated Src family kinases. There was a highly positive correlation (r=0.883) between SF2 values (cell survival at 2 Gy) and the degree of basal phosphorylated cSrc kinase expression. As the latter increased the ability of cells to survive radiation, increased. Inhibition of cSrc expression by cSrc-siRNA in 2 cell lines tested (FaDu and UMSCC1) resulted in a significant increase in their radiosensitivity, suggesting a causal relationship between cSrc and cell radioresistance. Inhibition of cSrc by dasatinib also resulted in increase of radiosensitivity of 3 cell lines tested (FaDu, HN-5, UMSCC-1). Additionally, dasatinib enhanced in vivo tumor response to radiation as shown by a marked increase in radiation-induced growth delay of FaDu xenografts. Conclusions: Our data showed that (1) a strong correlation exists between the expression level of cSrc and cell radiosensitivity of HNSCCs, (2) inhibition of cSrc expression by siRNA can render tumor cells more radiosensitive, implicating a causal relationship between cSrc and radioresistance, and (3) dasatinib enhanced both the in vitro tumor cell radiosensitivity and in vivo tumor response to radiation. These results suggest that targeting cSrc signaling network may have potential to improve the efficacy of radiotherapy. Supported in part by Bristol-Myers Squibb Company Sponsored Agreement (PI: U. Raju), NIH PO-1 Grant CA-06294 (PI: KK. Ang) Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 488.