BIOCHEMICAL EVENTS DURING INITIATION OF RAT HEPATOCARCINOGENESIS

Carcinogenesis is a multistep, multistage process that begins with irr eversible, but heritable damage to a single cell. The partial hepatect omy/diethylnitrosamine (DEN) model of rat hepatocarcinogenesis has bee n well characterized and many aspects of the stage of initiation are k nown. Recently, it has been suggested that hepatocytes expressing the placental isozyme of glutathione S-transferase (PGST) may be one popul ation of initiated cells. Male Fischer rats were subjected to a 70% pa rtial hepatectomy and at the peak of cell proliferation 24 h later wer e administered either the solvent trioctanoin, or 10 mg DEN/kg. The ra ts were administered 100 mg bromodeoxyuridine (BrdU)/kg 1 h prior to d eath at various times after DEN administration. Since initiation of th e carcinogenesis process requires the division of cells containing DNA damage to induce mutations, we examined the concentration of alkylate d adducts and the labeling index at various times after DEN administra tion. In addition, the time course of hepatic PGST expression was dete rmined concurrent with the adduct concentration and labeling index. Du ring the first day after DEN or solvent administration to a rat subjec ted to a 70% partial hepatectomy, a diurnal variation in labeling inde x was observed. A recovery to postsurgical labeling index levels was d emonstrated for both the solvent- and DEN-treated groups by 7 days. Th e concentration of three promutagenic lesions was maximal at 6 h after DEN administration. The detectable level of the O(6)EG adduct was neg ligible by 24 h after DEN administration, while the two O-alkylpyrimid ines, O(2)ET and O(4)ET, were retained for much longer periods. Single hepatocytes expressing PGST were observed by 2 days after DEN adminis tration, while small foci of PGST-expressing hepatocytes could be reli ably detected by 2 weeks. Two phases of PGST expression in single hepa tocytes were observed. The first phase was maximal at day 3 and comple te by day 6, while the second reached a plateau by day 8 and was maint ained for the 28 days of the study. The presence of the three O-alkyla tion adducts during a time of enhanced cellular proliferation suggests that all three promutagenic adducts may contribute to the initiation that results in the partial hepatectomy/DEN model of rat hepatocarcino genesis.