Mechanisms of Ras-mediated radioresistance

The Ras oncogene causes resistance to ionizing radiation (IR), limiting the effectiveness of radiotherapy. It was shown previously that blocking Ras function allows for sensitization of cells to IR. To determine the mechanisms by which Ras mediates radioresistance, we have carefully analyzed Ras signaling in the context of IR.; In this body of work we present evidence that Ras utilizes multiple signaling pathways to increase the radioresistance of RIE-1 cells. We have shown that Ras activates signaling through the PI3-K survival pathway to increase radioresistance and that blocking this pathway radiosensitizes cells. Ras also signals through its effector Raf to increase radioresistance, but intriguingly this signaling is independent of MEK, the best characterized Raf effector. Thus, we are searching for a non-MEK Raf effector that is important for Ras-mediated radioresistance. While JNK, p38 MAPK, or NF-κB are important for Ras transformation, these molecules are not important for Ras-mediated radioresistance. Ras signaling through PI3-K and Raf together does not reconstitute all of the resistance conferred by Ras, leading us to search for other effectors important for radioresistance.; We found that Ras also utilizes autocrine signaling through the EGFR to increase radioresistance of RIE-1 cells. Ras-transformation increases the expression of the EGFR ligand TGFα, which then stimulates EGFR and increases survival following IR. Blocking EGFR with an EGFR kinase inhibitor radiosensitizes Ras-transformed cells. That an oncogene can drive autocrine signaling to increase radioresistance is an exciting new finding and opens up new possibilities for radiosensitizing tumor cells.; Ras has long been thought to function exclusively at the plasma membrane. Recent findings have shown that Ras is also localized at the endoplasmic reticulum and Golgi apparatus, and that Ras signaling from these locations has physiological significance. We found that Ras exclusively localized to endomembranes can increase radioresistance nearly as well as normally localized Ras, which is found both at the plasma membrane and the endomembranes. Each of these Ras-dependent signaling mechanisms that play a role in radioresistance is a potential therapeutic target to be blocked in concert with IR in radiotherapy.