We compared apoptosis in mouse thymocytes following exposure to low doses of high linear energy transfer (LET), 62.5-MeV (p-->Be+) fast neutrons and low LET, 4-MeV photons by flow cytometric analysis of hypodiploid cells. The incidence of apoptotic cell death rose steeply at very low radiation doses reaching a plateau of 3 Gy. Both the time course and the radiation dose-response curves were similar for high and low LET radiation modalities. The relative biological effectiveness (RBE) of 1.0 for apoptosis in the mouse thymocyte system contrasts with the much higher value typically seen in many classical systems of clonogenic cell survival and tissue response. This difference suggests that while radiation-induced apoptosis may contribute significantly to loss of susceptible cells at doses of < or = 2 Gy, it appears to have a questionable role in determining the relative intrinsic radiosensitivity of mammalian cells to high and low LET irradiation at clinically relevant levels of cell kill.

Summary We have previously reported a correlation between high endogenous expression of the protein product of the RAF-1 protooncogene, intrinsic cellular radiosensitivity and rapid exit from a G/M delay induced by 2 Gy of y-irradiation. Rafl is a positive serine/threonine kinase signal transduction factor that relays signals from the cell membrane to the MAP kinase system further downstream and is believed to be involved in an ionizing radiation signal transduction pathway modulating the G /S checkpoint. We therefore extended our flow cytometric studies to investigate relationships between radiosensitivity, endogenous expression of the Rafl protein and perturbation of cell cycle checkpoints, leading to alterations in the G1, S and G/M populations after 2 Gy of y-irradiation. Differences in intrinsic radiosensitivity after modulation of the GW/S checkpoint have generally been understood to involve p53 function up to the present time. A role for dominant oncogenes in control of G1/S transit in radiation-treated cells has not been identified previously. Here, we show in 12 human in vitro cancer cell lines that late G1 accumulation after 2 Gy of radiation is related to both Rafl expression (r = 0.91, P = 0.0001) and the radiosensitivity parameter SF2 (r = -0.71, P = 0.009).