Enhancement of DEN initiation of liver carcinogenesis by inhibitors of NAD+ ADP ribosyl transferase in rats.

The effect of inhibitors of NAD+ ADP ribosyl transferase (ADPRT) on the early stage of liver carcinogenesis of diethylnitrosamine (DEN) was studied by estimating the number and size of gamma-glutamyltranspeptidase (gamma-GTP) positive foci assayed as markers of cell populations initiated by DEN in the rat liver. ADPRT inhibitors investigated were 3-aminobenzamide (ABA), 5-methylnicotinamide (MNAM), and thymidine. A single i.p. injection of ABA at greater than 150 mg/kg body weight (B.W.) enhanced dose-dependently the induction of gamma-GTP positive foci in rat liver initiated by 20 mg/kg B.W. of DEN. The magnitude of the effect was similar to that observed when partial hepatectomy (PH) was performed instead of ABA administration. Single i.p. injections of MNAM or thymidine at a dose of 600 mg/kg B.W. also enhanced the induction of foci in rat liver initiated by the 20 mg/kg dose of DEN. Based on the above results, ABA was used as a representative ADPRT inhibitor for clarifying the mechanisms underlying the effects. Administration of ABA at a dose of 600 mg/kg B.W. was effective in enhancing the induction of foci if given 1 day before DEN, simultaneously to DEN, and 1 day after DEN initiation but it was ineffective if it was given 3 days after DEN or thereafter. Liver cell necrosis was not detectable either by analysis of serum enzymes or histologically 1, 3, 5, 7 and 14 days after 20 mg/kg B.W. of DEN with or without administration of 600 mg/kg B.W. of ABA. No initiating activity was observed for ABA administered at doses of 600 and 1200 mg/kg B.W. as assayed by development of gamma-GTP positive foci. Long term ABA administration in the diet at concentrations of 0.05, 0.1 and 0.2% did not show any promoting activity for liver carcinogenesis initiated by DEN. Furthermore, chronic administration of 0.2% ABA in the diet did not result in detectable toxicity and/or carcinogenic effects. These results suggest that ADPRT and associated DNA repair plays an important role in the early initiating stage of liver carcinogenesis and provide the basis for a new experimental approach to the analysis of the mechanisms of chemical carcinogenesis and in establishing a more sensitive assay system for liver carcinogenesis in rats.