Localization of ALPHA₁‐fetoprotein and DNA‐synthesis in liver cell populations during experimental hepatocarcinogenesis in rats

Abstract

Abstract Six‐, 12‐ and 20‐week‐old rats were fed N‐nitrosomorpholine (NNM) at low concentrations (6 mg/kg/day) or high concentrations (20 mg/kg/day) for 6 or 12 weeks. Irrespective of the age of the rats, both NNM schedules resulted in development of hepatomas and during the early stages of hepatoma induction, liver histotoxic patterns depended only on the dose of carcinogen employed. Necrosis of hepatocytes and proliferation of small, oval‐shaped cells occurred when high doses of NNM were applied. Parallel to the proliferation of oval‐shaped cells, resurgence of alpha 1 ‐fetoprotein (AFP) in rat sera was observed and production of this protein was confined to the oval‐shaped cells as shown by immunoperoxidase staining. During proliferation of bile duct epithelium, induced by galactosamine injections, those cells could also stain for AFP, and proliferation of oval‐shaped cells concomitant with intracellular AFP staining resulted from restitution of heavily damaged liver. In pulse‐chase labelling experiments with 3 H‐thymidine, oval‐shaped cells were seen in continuous development towards hepatocytes, and distinct areas of hyperplastic appearance occurred. Normal hepatocytes and hyperplastic areas did not stain for AFP. Upon low‐dose feeding of NNM, no cellular AFP and no serum AFP were detected unless hepatoma cells had developed. At the stage of malignant conversion, distinct AFP‐staining nodules were localized which consisted of neoplastic hepatocytes. AFP‐staining and non‐AFP‐staining nodules were seen concomitantly in the same animal. AFP was localized only in neoplastic hepatocytes. Pulse‐labelling experiments with [ 3 H]thymidine showed the proliferative character of hepatoma cells from which the AFP‐staining population in particular was involved. No correlation was found between the presence of AFP and histological grading of hepatomas. The wide range of both serum AFP levels and their rising rates in individual rats indicated the highly heterogeneous character of the induced hepatomas with respect to AFP production.