Induction and Repair of DNA Strand Breaks in X-irradiated Proliferating and Quiescent CHO Cells

Abstract

Induction and repair of DNA strand breaks was studied in X-irradiated proliferating and quiescent CHO cells using the alkaline unwinding technique. The results showed that induction of strand breaks is identical for both states of proliferation, whereas repair is different. The decrease in the number of DNA strand breaks with incubation time at 37 degrees C is best described by a sum of three exponential components I, II and III. The half-times of component I were similar (tau I,p = 1.73 min versus tau I,q = 1.66 min) whereas strand breaks comprising component II were repaired slightly faster (tau II,p = 17.0 min versus tau II,q = 14.2 min) and those comprising component III were repaired significantly faster (tau III,p = 218 min versus tau III,q = 113 min) in quiescent as compared with proliferating cells. In contrast, the initial fractions, f, of the three components were closely similar for both states of growth (proliferating cells: fI = 0.69, fII = 0.25, fIII = 0.06; quiescent cells: fI = 0.65, fII = 0.29, fIII = 0.06). Radiosensitivity as assayed by colony formation was found to be lower for quiescent cells than for proliferating cells. By fitting the survival data to the linear-quadratic equation, -ln(S/S0) = alpha D + beta D2, the ratios alpha p/alpha q = 1.7 and beta p/beta q = 1.2 were obtained, which are similar to the ratios of the half-times of components III and II (tau III,p/tau III,q = 1.9; tau II,p/tau II,q = 1.2). This result indicates that the DNA damage represented by components II and III might be involved in cell killing.