The RAS signal transduction pathway and its role in radiation sensitivity.

RAS has been shown to increase radiation resistance. Upstream and downstream pathways from RAS could thus be targets for manipulation of radiosensitivity. EGFR expression and AKT phosphorylation are also associated with the response to radiation. A retrospective study evaluating EGFR and AKT in patients treated with multimodality therapy found a significant association between P-AKT and treatment failure. Moreover, these data are strengthened by in vitro studies showing that inhibition of EGFR, RAS, PI3K, and AKT radiosensitized cancer cell lines. We have previously shown that PI3K is a mediator of RAS-induced radiation resistance. We now suggest that EGFR, which is upstream of PI3K, may also mediate resistance through a common pathway. In addition to EGFR and RAS, PTEN can also regulate the PI3K pathway. Identifying a common signal for EGFR, RAS, or PTEN that results in radiation resistance may uncover targets for developing molecular-based radiosensitization protocols for tumors resistant to radiation and thus improve local control.