Epidermal Growth Factor Receptor as a Genetic Therapy Target for Carcinoma Cell Radiosensitization

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Abstract
Background: Exposure of human cancer cells to ionizing radiation activates the epidermal growth factor receptor (EGFR), which, in turn, mediates a cytoprotective response that reduces the cells' sensitivity to ionizing radiation. Overexpression of a dominant-negative EGFR mutant, EGFR-CD533, disrupts the cytoprotective response by preventing radiation-induced activation of the receptor and its downstream effectors. To investigate whether gene therapy with EGFR-CD533 has the potential to increase tumor cell radiosensitivity, we introduced an adenoviral vector containing EGFR-CD533 into xenograft tumors in nude mice and evaluated the tumor response to ionizing radiation. Methods: Xenograft tumors established from the human mammary carcinoma cell line MDA-MB-231 were transduced via infusion with the adenoviral vector Ad-EGFR-CD533 or a control vector containing the β-galactosidase gene, Ad-LacZ. The transduced tumors were then exposed to radiation in the therapeutic dose range, and radiation-induced EGFR activation was assessed by examining the tyrosine phosphorylation of immunoprecipitated EGFR. Radiosensitization was determined in vitro by colony-formation assays. All statistical tests were two-sided. Results: The transduction efficiency of MDA-MB-231 tumors by Ad-LacZ was 44%. Expression of EGFR-CD533 in tumors reduced radiation-induced EGFR activation by 2.94-fold (95% confidence interval [CI] = 2.23 to 4.14). The radiosensitivity of Ad-EGFR-CD533-transduced tumors was statistically significantly higher (46%; P<.001) than that of Ad-LacZ-transduced tumors, yielding a dose-enhancement ratio of 1.85 (95% CI = 1.54 to 2.51). Conclusions: Transduction of MDA-MB-231 xenograft tumors with Ad-EGFR-CD533 conferred a dominant-negative EGFR phenotype and induced tumor radiosensitization. Therefore, disruption of EGFR function through overexpression of EGFR-CD533 may hold promise as a gene therapeutic approach to enhance the sensitivity of tumor cells to ionizing radiation.