The sensitivity of tumor cells to the killing effects of ionizing radiation is thought to be one of the major determinants of curability in patients treated with radiation therapy. Physiological factors such as hypoxia, blood supply, pH and nutritional status are thought to be important, but it is now recognized that intrinsic radioresistance determined by genetic factors may also play a role. The purpose of this proposal is to explore the molecular genetics of the role of oncogenes in the induction of radioresistance in cultured mammalian cells. Primary rat embryo cells (REC) have been chosen as a model system in which the effects on radiation resistance of the Hras oncogene could be studied on a uniform genetic background. These cells offer several useful advantages. The cells prior to transformation are diploid and because they have been in culture only for a few passages prior to transformation with the oncogene it is unlikely that any pre-existing mutation affecting radiation response could be present. Additionally, the use of REC permits the study of the effects of synergism between oncogenes on the induction of the radioresistant phenotype. The results show that the activated Hras oncogene induces radiation resistance in primary rat cells after transformation, but that the effect of the oncogene by itself is small. However, the myc oncogene, which has no effect on radiation resistance by itself, has a synergistic effect on the induction of radiation resistance by Hras. Radiation resistance induced by Hras plus myc is characterized by an increase in the slope of the curve at high doses but there is also a large effect within the shoulder region of the radiation survival curve. This proposal addresses whether the radioresistant phenotype after oncogene transfection represent selection of a pre-existing cellular phenotype or induction of a new phenotype. The relationship of oncogene expression to the phenotype will be examined. This proposal involves isolotion of cloned, stable cell lines which differ in their radiobiological properties. These will be used as recipients for constructs which will allow the modulation of oncogene expression. This will be done by placing the oncogenes under the control of inducible promoters and by using antisense vectors to suppress the expression of transfected oncogenes. The karyotypic changes induced by oncogene transfection will be correlated with the radioresistant phenotype. The species specificity of oncogene induced radioresistance will be explored.
