Effect of mitogenic or regenerative cell proliferation on lacz mutant frequency in the liver of Muta™Mice treated with 5,9-dimethyldibenzo[c,g]carbazole

The purpose of this work was to investigate the impact of cell proliferation on liver mutagenesis. The genotoxic hepatocarcinogen 5,9-dimethyldibenzo[c,g]carbazole (DMDBC) was administered to lacZ transgenic Muta™Mice at a nonhepatotoxic dose of 10 mg/kg, which induces only a slight increase in the liver lacZ mutant frequency (MF). To determine if cell proliferation stimuli enhanced DMDBC mutagenicity, MF was analyzed in mice first receiving DMDBC 10 mg/kg, then ~2 weeks later, either carbon tetrachloride (CCl4, a cytotoxic agent inducing regenerative cell proliferation) or phenobarbital (PB, a mitogenic agent inducing direct hyperplasia). In preliminary studies, the extent of cell proliferation induced by CCl4, PB and DMDBC was determined in non-transgenic CD2F1 mice by means of 5-bromodeoxyuridine labeling. The labeling index was significantly increased after CCl4 and PB, while no change was detected with DMDBC. MF was then determined in Muta™Mice 28 days after initial DMDBC treatment. No increase in MF was detected in mice receiving CCl4 or PB alone. A 2- to 3-fold increase in MF was detected in mice treated with 10 mg/kg DMDBC alone. In contrast, MF was markedly increased in mice receiving DMDBC followed by proliferative treatment (15-fold with CCl4 and 25-fold with PB). These results demonstrate that expression of DMDBCinduced mutations in mouse liver largely depends on the induction of cell proliferation (by a cytotoxic or mitogenic stimulus) and illustrate that Muta™Mouse is a valuable tool to investigate the early events of liver carcinogenesis.