Scheduled and Unscheduled DNA Synthesis in Chick Embryo Liver Following X-irradiation and Treatment with DNA Repair Inhibitorsin Vivo

Abstract

Three hours following X-irradiation of chick embryos with doses of 4 and 8 Gy the in vitro incorporation of tritiated thymidine [( 3H]dT) into DNA (scheduled DNA synthesis, ss) of hepatocytes was reduced to about one-third. Within 24 h after the exposure, ss returned to control values. The return of ss to a normal rate could be strongly inhibited by 2',3'-dideoxythymidine (ddT), and to a lesser extent by 1-beta-D-arabinofuranosylcytosine (araC). In strong contrast to ss, the hydroxyurea (hu)-resistant [3H]dT incorporation (unscheduled DNA synthesis, us) showed a highly significant increase 24 h after treatment of the embryos with araC and/or X-irradiation. Autoradiographic studies revealed no change of total [3H]dT labelling frequency in the whole chick embryo liver 24 h after treatment with araC and/or X-irradiation, but a persistent depression of ss and a simultaneous increase of us. The histological discrimination between affected and non-affected areas argue for a stimulation of DNA synthesis as an antecedent of subsequent mitosis and reparative proliferation adjacent to cell necrosis. It is suggested that the fast recovery of ss in the 12-15-day-old chick embryo is due to an efficient DNA repair system for which DNA polymerase beta is important. The increase of hu-resistance may be an expression of an aberrant DNA synthesis.