Inhomogeneity of the nucleus to 125IUdR cytotoxicity

Synchronized suspension cultures of Chinese hamster ovary (CHO) cells were used to determine the lethal effects produced by the decay of 125I incorporated into different subfractions of the nuclear genome. Such a shift in nuclear incorporation pattern was achieved by using the drug aphidicolin, which inhibits 95% of all nuclear DNA synthesis, is nontoxic to cells in a colony-forming assay, and does not modify the radiation response of CHO cells to X irradiation. In addition to shifting incorporation of 125I to only 5% of the nuclear genome, both nuclease digestions to characterize the molecular location of 125I and electron microscope autoradiography show an inhomogeneous distribution of sites of 125I incorporation in the presence of 5 micrograms/ml aphidicolin. These data in combination with survival curves of CHO cells labeled with 125I-iododeoxyuridine (125IUdR) either with or without aphidicolin showed a dramatic change in the survival response (DO: 30 decays/cell and 96 decays/cell, respectively). It is concluded, therefore, that the nucleus is not a homogeneous target for radiation-induced cell death because when subfractions of the nuclear genome are labeled, radically different levels in cell survival are obtained.