Radiation-induced activation of a common variant of EGFR confers enhanced radioresistance.

ABSTRACT

BACKGROUND AND PURPOSE: The type-III EGFR variant (EGFRvIII) is known to promote enhanced tumorigenicity. We have previously defined the importance of EGFRvIII in cellular radiation responses using Chinese hamster ovary cells (CHO). In the current study, we have extended our investigations of EGFRvIII to human tumor cells in vitro and in vivo and further verified the important role of EGFRvIII in modulating radiosensitivity. MATERIAL AND METHODS: The cell lines MDA-MB-231, U-87 MG, A-431 and U-373 MG were used. Adenoviral (Ad) vectors were produced to overexpress EGFRvIII in vitro or in xenograft tumors in vivo. The EGFR, EGFRvIII expression and tyrosine phosphorylation (Tyr-P) levels were quantified by Western blotting. The relative radiosensitivities were assessed in vitro by standard colony formation and in vivo by tumor growth delay assays. RESULTS: The presence of EGFRvIII was verified in all xenograft tumors tested with no detectable expression in the corresponding cells under in vitro culture conditions. MDA-MB-231 xenograft tumors demonstrated EGFRvIII expression levels, which were 1.9-fold higher relative to EGFRwt compared to a 14.5-fold higher Tyr-P. Ionizing radiation of these tumors at 4 Gy induced an average 3.2-fold increase in EGFRvIII Tyr-P. EGFRvIII expression in U-373 MG cells significantly enhanced survival after 4Gy, which was completely abolished by dominant-negative EGFR-CD533. Finally, the ability of EGFRvIII to accelerate tumor growth during irradiation was confirmed in vivo. CONCLUSION: EGFRvIII is frequently expressed in a variety of different tumor types and can confer significant radioresistance, thus further providing evidence for EGFRvIII as an additional important target in our approaches to radiosensitize malignant solid tumors.