Effect of a single treatment with the alkylating carcinogens dimethylnitrosamine and methyl methanesulphonate on liver regenerating after partial hepatectomy III. Effect on DNA synthesis in vivo and on DNA polymerase activity assayed in vitro

Abstract

A single treatment with dimethylnitrosamine (DMN) but not with methyl methanesulphonate (MMS) induces liver cell carcinoma if given during the period of restorative hyperplasia following partial hepatectomy, a higher incidence of tumours being induced if the carcinogen is given during the period of DNA synthesis (24 h after the operation) than if given early in the prereplicative stage (at 6 h). To study the effect of treatment with DMN and with MMS on the regenerating liver, DNA replication was measured in vivo in partially hepatectomised animals treated with the methylating agents, and DNA polymerase activity was assayed in vitro. MMS given at 6 h produced a dose-dependent delay in the wave of DNA synthesis which normally follows partial hepatectomy. DMN at 6 h caused a dose-dependent inhibition of synthesis, restoration of total liver DNA occurring gradually during the succeeding week. In each case, the induction of DNA polymerase which normally takes place before the increase in DNA synthesis occurs was inhibited. The results suggest that damage to DNA inhibited induction of the enzymes necessary for DNA replication, and therefore extensive DNA synthesis could not occur until after the relevant damage had been repaired. The low level of synthesis on damaged DNA correlates with the fact that the incidence of tumours resulting from DMN administration at 6 h is low. When DMN or MMS were given 24 h after the operation, by which time DNA polymerase has already been induced, no evidence was obtained for a direct inhibition of the enzyme by the methylating agents. Therefore, as there is no need for further induction of essential enzymes, replication of DNA can occur before repair mechanisms have eliminated damage from the template DNA. The high incidence of tumours induced by DMN given at 24 h could be related to the replication of damaged DNA.