Tumors vary considerably in their radioresponsiveness and one important component of that variation is the intrinsic radiosensitivity of the tumor cells themselves. Oncogenes can alter the radiosensitivity of cells, but the mechanisms through which they affect the cells' response to radiation remain unknown. We have begun to explore whether oncogenes affect apoptosis after radiation. We have found that the oncogenes Hras and Myc when used to transform rat embryo cells (REC) lead to cell lines with substantially greater radioresistance than REC themselves or REC with either oncogene alone. We have shown that this difference is unrelated to the extent of double strand DNA breaks induced by radiation; nor is it related to the extent or kinetics of rejoining of these breaks. The radioresistant phenotype correlates with a longer G2 delay induced by radiation. We have recently shown that the resistant cells respond to radiation by depressing synthesis of one of the mitotic cyclins, cyclin B1, whereas, the sensitive cells show no effect of radiation on mitotic cyclin expression. The Myc transfected cells undergo apoptosis after serum withdrawal and after radiation. Apoptosis is seen to a much lesser degree in the cultures of the ras transfected cells after the same dose of radiation yet they still undergo apoptosis after serum withdrawal. Thus ras appears to protect the cells from radiation induced apoptosis, but does not protect the cells from apoptosis induced by serum withdrawal. A similar phenotype is seen in Hela cells which are also very resistant to radiation induced apoptosis and similarly have a prolonged radiation induced G2 delay with depressed cyclin B1 expression. The purpose of this application is to explore the hypothesis that the radiation induced G2 delay acts as a regulator of the apoptotic response. Since the transition through G2/M is regulated in part by and requires the expression of cyclin B1, we propose that the G2 delay and thus perhaps also apoptosis are regulated through cyclin B1 expression. We intend to investigate the contribution the oncogene makes to determining whether apoptosis occurs, both by manipulating oncogene expression or activity and by transfecting other genes which may counter the effects of the oncogenes on the apoptotic pathway. We will inhibit ras action using inhibitors of farnesylation. We will use antisense technology to alter the kinetics of mitotic cyclin expression and hence alter the timing of G2 in order to test the effect on apoptosis and radioresistance. Finally, we have evidence that the induction of a radiation induced G2 delay may be susceptible to pharmacological manipulation using caffeine and staurosporine. We wish to explore whether it is possible using such methods to alter the induction of apoptosis or of the radiosensitivity of the cells. These studies may have clinical utility if they open up new pathways which may be explored to alter the radiosensitivity of tumors or normal tissues.
