Radiation-induced signalling: pathways, RAS proteins, receptors and paracrine factors

The mechanisms by which cellular radiosensitivity is regulated are not fully understood. Initial portions of the presentation will describe what is known about the mechanisms of ionizing radiation-induced signaling in human tumor cells. The RAS oncogene family of GTP binding proteins is known to enhance radiation resistance; however no studies have been performed in isogenic carcinoma cells to determine the relative effects of RAS isoform function on radiosensitivity. By using HCT116 cells (which express a single allele of activated K-RAS D13), HCT116 cells genetically deleted for expression of K-RAS D13, and HCT116 cells deleted for expression of K-RAS D13 stably transfected with HRAS V12, we have explored the role of K-RAS and HRAS in human carcinoma cell radiosensitivity. In isogenic HCT116 cells we determined that radiation preferentially activated the ERK1/2 pathway over the PI3K/AKT pathway whereas in HCT116 cells expressing H-RAS V12 radiation preferentially activated PI3K/AKT signaling over that of ERK1/2. In agreement with these findings, radiation resistance mediated by K-RAS D13 was dependent on ERK1/2 signaling whereas resistance mediated by H-RAS V12 was mediated via paracrine signaling from heregulin to the PI3K / AKT pathway. Although H-RAS V12 promoted PI3K translocation into the plasma membrane, the actions of the paracrine factor heregulin in promoting AKT activation and radioresistance were essential to H-RAS V12 radioresistance. The use of clinically relevant inhibitors of AKT and PDK-1 also radiosensitized cells expressing H-RAS V12 to a greater extent than in cells expressing K-RAS D13. Thus H-RAS and K-RAS generate qualitative and quantitative differences in cellular radiation resistance