Radiosensitivity of Tumor Cell Lines after Pretreatment with the EGFR Tyrosine Kinase Inhibitor ZD1839 (Iressa®)

Abstract

The epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor ZD1839 (Iressa) reduces survival and augments radiation response of certain tumor cells. The aim of this study was to identify cellular events that are associated with the modulation of radiosensitivity by ZD1839. Three tumor cell lines (A549, H596, FaDu) were exposed to ionizing radiation, treatment with ZD1839, and combined treatment. Clonogenic cell survival was determined by colony assays, EGFR and transforming growth factor-(TGF-)alpha expression by quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR), cell cycle distribution and apoptosis by flow cytometry. In A549 and H596 cells ZD1839 had little effect on clonogenic growth, but survival curves revealed a radiosensitizing effect of 5 microM ZD1839 on A549 cells. Both cell lines expressed moderate amounts of EGFR mRNA and very low levels of TGF-alpha mRNA. FaDu cells expressed relatively high amounts of EGFR and TGF-alpha transcripts and showed marked inhibition of clonogenic growth, reduction of S-phase cells, and induction of apoptosis after treatment with 1 microM ZD1839 and combined treatment. Surprisingly, the subpopulation of FaDu cells surviving ZD1839 pretreatment was more radioresistant. Exposure to ZD1839 caused a decrease in EGFR mRNA expression in A549 cells, no change in H596, and even an increase in FaDu cells. The sensitivity to ZD1839 correlated with the EGFR expression level, an inhibition of cell proliferation, and induction of apoptosis in the cell lines analyzed. A radiosensitizing effect of ZD1839 was associated with downregulation of EGFR mRNA expression.