Liver Tumor Promotion by .BETA.-Naphthoflavone, a Strong CYP 1A1/2 Inducer, in a 28 Week Two-stage Rat Hepatocarcinogenesis Model Using Diethylnitrosamine as an Initiator.

Abstract

β-naphthoflavone (BNF) is a non-genotoxic, strong inducer of cytochrome P450 (CYP) 1A but not CYP 2B. In order to determine its liver tumor promotion potential, groups of male F344 rats were initiated with a single intraperitoneal injection of 200 mg/kg of diethylnitrosamine (DEN) and starting two weeks later, given 1% BNF (n=12) or non-supplemented diet (n=13) for 26 weeks. All animals were subjected to two-thirds partial hepatectomy at week 3. At sacrifice, absolute and relative liver weights in the DEN + BNF group showed statistically significant increase as compared to the DEN-alone group. The incidence of hepatocellular adenomas and numbers of such tumors and altered hepatocellular foci, positive for glutathione S-transferase placental form (GST-P), were also elevated. However, five animals given BNF alone without DEN initiation exhibited no focal hepatocellular lesions including GST-P positive foci. The numbers and areas of connexin 32 (Cx32)-positive spots per hepatocyte in centrilobular areas, and within altered hepatocellular foci and hepatocellular adenomas of the DEN + BNF group were significantly lower than those of the DEN-alone group. Immunohistochemically, CYP 1A1/2 was stained diffusely in the livers of the DEN + BNF group, but hepatocellular adenomas and some altered hepatocellular foci were generally negative. Proliferating cell nuclear antigen (PCNA) labeling indices of surrounding hepatocytes in the DEN + BNF group was about twice that of DEN-alone group, but the difference did not achive statistical significance. In the altered hepatocellular foci and adenomas of DEN + BNF group, the PCNA labeling indices were significantly higher than those of surrounding hepatocytes. These results indicate that BNF, a strong CYP 1A but not CYP 2B inducer, promote hepatocarcinogenesis initiated by DEN.