Biochemical and morphologic studies of heterogeneous lobe responses in hepatocarcinogenesis

Experiments have demonstrated interlobe differences in the incidence of diethylnitrosamine (DEN)-induced hepatocellular carcinoma (HCCA), with a 100% incidence in the left and right median lobes and a 30% incidence in the right anterior lobe 20 weeks after exposure began. These tumor data provide a model to test the hypothesis that chemically induced neoplasia can be qualitatively and quantitatively related to promutagenic DNA damage and concurrent cell replication. Experiments were performed to measure O4-ethyldeoxythymidine (O4-EtdT) (a major pro-mutagenic lesion in hepatic DNA of rats exposed to DEN), N7-ethylguanine, cell replication and hepatocyte initiation using the induction of growth-selected gamma-glutamyl transferase-positive (GGT+) foci in the left and right median and right anterior hepatic lobes following 0, 3, 7, 14 or 28 days of DEN administration. Results demonstrated that O4-EtdT concentrations were consistently higher in the left and right median versus the right anterior hepatic lobes, while cell replication was transiently higher in the right median and right anterior lobes. Likewise, high numbers of GGT+ foci were observed in the left and right median lobes in DEN-exposed rats subjected to a Cayama-Farber growth selection protocol. Following administration of [14C]DEN, the distribution of radioactivity showed a marked left lobe preference in 4-week-old rats that had no prior exposure to DEN and in 8-week-old rats exposed to DEN for 4 weeks. This study suggests that interlobe differences in hepatocyte initiation and the incidence of HCCA may be due in part to differences in cell replication and in DNA alkylation resulting from differential DEN distribution and/or metabolism.