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The expression of activated RAS oncogenes has been shown to increase radioresistance in a number of cell lines. The pathways by which RAS leads to radioresistance, however, are unknown. RAS activates several signal transduction pathways, with the RAF-MAP2K-MAP kinase pathway perhaps the best studied. MAP kinase has also been shown to be activated by radiation through this pathway. Given the important role of MAP kinase in multiple signaling events, we asked if radioresistance induced by RAS was mediated through the activation of MAPK. Cells of two human bladder carcinoma cell lines were used, one with a mutated oncogenic HRAS (T24) and other with a wild-type HRAS (RT4). The surviving fraction after exposure to 2 Gy of radiation (SF2) for the T24 cell lines was found to be 0.62, whereas that for RT4 cells was 0.40. Treatment with the farnesyl transferase inhibitor (FTI) L744,832, which inhibits RAS processing and activity, decreased the SF2 of T24 cells to 0.29, whereas the SF2 of RT4 cells remained unchanged after FTI treatment, thus demonstrating the importance of RAS activation to the radiosensitivity of cells with mutated RAS. MAP kinase activation was found to be constitutive and dependent on RAS in T24 cells, while it was inducible by radiation and was independent of RAS in RT4 cells. Treatment of both cell lines with the MAP2K inhibitor PD98059 inhibited MAPK activation; however, inhibiting MAPK activation had no effect on radiation survival of T24 or RT4 cells. These data indicate that MAPK activation does not contribute to RAS-induced radioresistance in this system.

Exposure of mammalian cells to X rays results in prolongation of the cell cycle, including delays or arrests in G1, S and G2 phase. While G1-phase arrest occurs only in cells with wild-type p53 function, a G2-phase delay occurs in all cells regardless of p53 status. In this review, we summarize what is known about cell cycle progression through G2 and M phase and discuss the experimental findings that implicate different mechanisms in the G2-phase delay. Finally, we consider the possibility that G2-phase arrest plays a role in cell survival after irradiation.

The irradiation of cells results in delayed progression through the G2 phase of the cell cycle. Treatment of irradiated HeLa cells with caffeine greatly reduces the G2-phase delay, while caffeine does not alter progression of cells through the cell cycle in unirradiated cells. In this report we demonstrate that treatment of HeLa cells with the kinase inhibitor staurosporine, but not with the inhibitor H7, also results in a reduction of the G2-phase arrest after irradiation. Cell cycle progression in unirradiated cells is unaffected by 4.4 nM (2 ng/ml) staurosporine, which releases the radiation-induced G2-phase arrest. In HeLa cells, the G2-phase delay after irradiation in S phase is accompanied by decreased expression of cyclin B1 mRNA. Coincident with the reduction in G2-phase delay, we observed an increase in cyclin B1 mRNA accumulation in irradiated, staurosporine-treated cells compared to cells treated with irradiation alone. Caffeine treatment of irradiated HeLa cells also resulted in an elevation in the levels of cyclin B1 message. These results support the hypothesis that diminished cyclin B1 mRNA levels influence G2-phase arrest to some degree. The findings that both staurosporine and caffeine treatments reverse the depression in cyclin B1 expression suggest that these two compounds may act on a common pathway of cell cycle control in response to radiation injury.

Many studies have demonstrated the effect of oncogene transfection on the radiation sensitivity of primary rat embryo fibroblasts. Our results indicate that transformation by H-ras plus v-myc oncogenes confers radiation resistance to a much greater extent than transformation by either gene alone. We have further shown that the radioresistant phenotype is accompanied by a prolonged G2-phase delay. This is consistent with the hypothesis that the extent of this delay is an important determinant of radiation sensitivity. The study of cyclin expression during the progression of cells through G2 and M phase after irradiation has also revealed several possible mechanisms for induction of G2-phase arrest. Control of cyclin B levels was seen both at the mRNA level as evidenced by decreased cyclin B mRNA expression after irradiation in the S phase, and at the protein level as demonstrated after irradiation in G2 phase. It remains to be determined how these mechanisms might be differentially regulated in radioresistant and sensitive cells.

The levels of cyclin B mRNA and protein rise rapidly in G2 + M phase, and fall at the end of mitosis. The studies described here were initiated to determine the effects of ionizing radiation on the level of cyclin B bearing in mind that the division delay induced by ionizing radiation might be influenced by the expression of cyclin B. After irradiation in S phase, the cyclin B mRNA in HeLa cells was measured as the cells proceeded through the cell cycle. Instead of the usual rise, after irradiation cyclin B mRNA levels remained low during the G2 delay. After irradiation in G2 phase, cyclin B mRNA was readily detectable although at slightly lower levels than in the controls. However, cyclin B protein was markedly decreased in amount.

Cell cycle perturbation after irradiation was studied in five cell lines transfected with oncogenes. Two immortalized, radio-sensitive cell lines with D0s of 1.06 and 1.08 Gy were compared to three radioresistant cell lines with D0s of 1.68-2.17 Gy. The sensitive cell lines were transfected with the v-myc or c-myc oncogenes, the resistant cell lines with the v-myc plus H-ras oncogenes. Exponentially growing populations were exposed to 5, 10, or 15 Gy of orthovoltage radiation. The percentage of cells in each phase of the cell cycle was determined at various times after irradiation using flow cytometry. All cell lines underwent a dose-dependent arrest in G2 phase after irradiation, but the resistant cell lines underwent a significantly longer arrest in G2 phase after irradiation than did the sensitive cell lines. In conjunction with other results from our laboratories, we suggest that this difference in G2 arrest may be the basis for the increased resistance of cells transfected with oncogenes to irradiation.