Downregulation of cystine transporter x_c^- by irinotecan in human head and neck cancer FaDu xenografts

Background: The purpose of this study was: (1) to document the critical requirement of cystine for growth of human tumor cells in vitro, and (2) to determine the effect of the anticancer agent irinotecan on the cystine transporter x_c^- in head and neck FaDu xenografts. Methods: Cell growth was measured by sulforhodamine B assay. xCT protein, glutathione (GSH) and DNA damage were determined using Western blot, spectrophotometry, and immunohistochemistry, respectively. Results: Depletion of cystine from the medium inhibited tumor cell growth. Treatment of FaDu tumor with a therapeutic dose of irinotecan resulted in depression of xCT protein levels, leading to tumor growth retardation and downregulation of GSH with increased reactive oxygen species (ROS). The accumulation of ROS correlated with increased DNA damage as evidenced by increased H2AX. Conclusion: Depression of xCT protein by irinotecan resulted in downregulation of GSH and increase in ROS, which could be the other possible mechanisms of DNA damage by irinotecan.