Dai Nakae

Preventive Effect of 3‐Aminobenzamide on the Reduction of NAD Levels in Rat Liver Following Administration of Diethylnitrosamine

Nicotinamide adenine dinucleotide is utilized as the substrate of a chromatin‐bound enzyme, poly(ADP‐ribose) polymerase. The effects of diethylnitrosamine and/or 3‐aminobenzamide, a potent inhibitor of poly(ADP‐ribose) polymerase, on the cellular NAD levels in rat liver were investigated. 3‐Aminobenzamide (600 mg/kg) administered intraperitoneally was not detectable in the liver within 12 hr after administration; the inhibitor had a calculated half life of 90 min. Diethylnitrosamine reduced the NAD levels in rat liver in a dose‐dependent way. The NAD content reached a minimum level at 8 hr, returning to 78% of the control value after 48 hr. The reduction of the NAD levels caused by diethylnitrosamine was completely prevented when 3‐aminobenzamide was administered either simultaneously with diethylnitrosamine or 4 hr after diethylnitrosamine treatment. Furthermore, an immunohistochemical study showed that nuclear poly(ADP‐ribose) decreased 1 hr after the administration of 3‐aminobenzamide. These results suggest that inhibition of poly(ADF‐ribosyl)ation is involved in the initiation of liver carcinogenesis by diethylnitrosamine and 3‐aminobenzamide.

Possible Model of Liver Carcinogenesis Using Inhibitors of NAD+ ADP Ribosyl Transferase in Rats

The response of cellular NAD+ metabolism to DEN and/or ABA and the carcinogenesis of the liver initiated by DEN and ABA were studied in rats. The liver NAD+ level was depleted by an ip injection of 20 mg or 200 mg/kg body weight of DEN. ABA, administered ip at a dose of 600 mg/kg simultaneously with or 4 hours after DEN, prevented the depletion of NAD+ by DEN. These biochemical findings correlated with the changes of conspicuous intranuclear immunofluorescence of poly(ADP-ribose), which were studied by immunohistochemistry. When initiated by 20 mg/kg body weight DEN and 600 mg/kg ABA and then processed to selection pressure, the liver was found to be capable of developing hepatocellular carcinomas with or without PB promotion. These results suggest that the inhibition of poly(ADP-ribosylation) might lead to irreversible initiation of liver carcinogenesis by DEN in rats.

Involvement of Poly(ADP-Ribosylation) Reactions in Liver Carcinogenesis of Diethylnitrosamine in Rats

Our investigations of the possible involvement of poly(ADP-ribosylation) reaction in liver carcinogenesis were initiated by the evidence indicating that poly(ADP-ribose) participates in DNA damage and repair [1,2] since induction of DNA lesions by carcinogens and their repair have been considered to be closely related to carcinogenesis [3–6]. Previously, we reported enhancement of diethylnitrosamine (DEN) initiation of liver carcinogenesis by inhibitors of NAD+ ADP-ribosyl transferase in rats [7–9]. However, evidence for the direct involvement of the poly(ADP-ribosylation) reaction in the induction of liver carcinomas was lacking. In this paper, we describe a disturbance in NAD ADP-ribosyl transferase-dependent formation of homopolymers of repeating ADP-ribose units from NAD and possible inhibition of repair of DEN damaged DNA by ABA. The foci initiated by DEN and ABA were found to be capable of development into hepatocellular carcinomas without promotion by phenobarbital (PB).

Effect of 3-Aminobenzamide on the Induction of γ-Glutamyltranspeptidase Positive Foci in the Liver of Rats Treated with Various Chemical Carcinogens

Biochemical evidence of the participation of poly(ADP-ribose) in DNA damage and repair (1, 2) suggests the involvement of poly(ADP- ribosylation reactions in chemical carcinogenesis (3). We found that diethylnitrosamine (DEN) initiation of liver carcinogenesis was enhanced by inhibitors of ADP-ribosyl transferase (4, 5) and the foci initiated by DEN and 3-aminobenzamide (ABA) were found to be capable to develop into hepatocellular carcinomas without promotion by phenobarbitol (6). In the present experiment, we studied the effect of ABA on the induction of γ-glutamyltranspeptidase (GGT) positive foci in the liver of rats treated with various chemical carcinogens.

Additive Effect of Low-Dose Diethylnitrosamine on the Initiation of Liver Carcinogenesis in Rats

Simultaneous or exposure to low-dose carcinogens existing in the environment is an important causal factor in human carcinogenesis. It is gener- ally accepted that carcinogenesis is a multi-step process consisting of two qualitatively different stages, initiation and promotion1. Initiation is a transient “flash” phenomenon involving exposure to a carcinogen and may occur frequently in humans.

Carcinogenic effects of 3-di(hydroxymethyl)-amino-6-(5-nitro-2-furylethenyl)-1,2,4-triazine in the small intestine of Ishibashi and Wistar rats.

The carcinogenic effect of an oral administration of 3-di(hydroxymethyl)-amino-6-(5-nitro-2-furylethenyl)-1,2,4-triazin e (DHNT) was studied in two strains of rats. Ishibashi (IS) rats have a perpetual absorption disturbance while Wistar Imamichi (WI) rats have no such disturbance. Small intestinal carcinomas developed in 10 out of 15 (66.7%) IS rats and in all 8 (100%) Wistar rats given 3,500 ppm DHNT for 43 weeks. The total number of lesions in the small intestine was 24 in IS rats and 21 in WI rats. Histologically, the lesions consisted of 7 mucosal dysplasias, 4 adenomas and 12 tubular and 1 papillary adenocarcinoma in IS rats, and 3 mucosal dysplasias, 1 adenomas, and 15 tubular and 2 papillary adenocarcinomas in WI rats. The levels of plasma leucine aminopeptidase, copper and total protein were lower, while blood urea nitrogen was higher in both strains of rats given DHNT than in control rats.