Normal human diploid cell strains were transfected with an activated Ha-ras oncogene (EJ ras) and/or SV40 T-antigen. Clones expressing SV40 T-antigen alone or in combination with ras protein p21 were significantly radioresistant <tex-math>$(D_{0}=1.68-2.73\ {\rm Gy})$</tex-math> compared with their parent cells or clones transfected with the neo gene only <tex-math>$(D_{0}=1.20-1.35\ {\rm Gy})$</tex-math>. This radioresistant phenotype persisted in posterisis, immortalized cell lines. Cell cycle perturbations after X irradiations were studied in four immortalized, radioresistant cell lines transfected with EJ-ras plus SV40 T or SV40 T alone as well as in two nontransfected parental cell strains. Exponentially growing cells were exposed to various doses of X radiation and the distributions within the cell cycle were determined by flow cytometry. The time of onset and duration of division delay were also measured by cell counting. All cells underwent a dose-dependent G2 arrest; the duration of this division delay was proportional to radiation dose. The radioresistant cell lines had a longer arrest in G2 phase of the cell cycle compared to that of the parental cell strains. These data suggest that a prolonged cell cycle delay may be one of the factors involved in the radioresistance acquired by transfection of human diploid cells with SV40 T-antigen.

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