MICRONUCLEUS INDUCTION IN 10 HUMAN TUMOR-CELLS AFTER HIGH-DOSE AND LOW-DOSE RADIATION

A number of data measuring survival of animal or human cells to low LE T ionizing radiation have demonstrated that these cells may be hyperse nsitive to doses below 1 Gy, possibly due to the absence of an inducib le repair mechanism, which is observed at higher doses. The production of micronuclei (MN) in cells exposed to ionizing radiation reflects g enotoxic damage. Moreover, the micronucleus assay is sensitive to low radiation doses. We have exposed 10 human tumour cell lines to doses r anging between 0.12 and 4 Gy. Using cytochalasin B to block the cells in a binucleate phase, we have scored the fraction of binucleate cells (BNC) expressing MN, as well as the number of MN per BNC, as a functi on of gamma-ray dose. Experimental points were fitted with a binomial equation. Doses from 1 to 4 Gy were fitted separately from those below 1 Gy, and the initial slopes after both fits were compared. Taken tog ether, the initial slopes of MN induction after low-dose (LD) irradiat ion were not different from those after high-dose (HD) irradiation, On ly in one cell line was a significant increase in MN production detect ed after LD irradiation. This cell line had the shallowest linear term after HD irradiation. It appeared that the likeliness of expressing h ypersensitivity at LD was correlated with the quadratic term of MN ind uction at HD, which does not contradict an inducible repair hypothesis . However, the failure of observation of a significant hypersensitivit y at LD for nine cell lines, and the high variability of response at L D suggests that this occasional effect may be influenced by other fact ors as well.