Decreased Expression of Bcl‐x Protein during Hepatocarcinogenesis Induced Exogenously and Endogenously in Rats

Dysregulations of apoptosis have been widely recognized as important events in multi‐stage carcinogenesis. Bcl‐x, a member of the Bcl‐2 family, is known to act as a regulator of apoptosis. The present study was conducted to assess the role of altered Bcl‐x protein expression in exogenous and endogenous hepatocarcinogenesis in rats. In the short‐term exogenous models, male Fischer 344 rats, 6 weeks old, were given a single intraperitoneal injection of diethylnitrosamine (DEN) at a dose of 200 mg/kg body weight, partially hepatectomized at the end of week 3, administered phenobarbital at a concentration of 0.05% from the end of week 2 for 6 weeks, and sacrificed. In the livers, glutathione S‐transferase (GST‐P)‐positive, putative preneoplastic lesions were induced, and Bcl‐x protein expression was decreased in 24.7% of such lesions. The incidence of GST‐P‐positive lesions with decreased Bcl‐x increased depending on the size of the lesions; 18.9%, 32.4% and 86.5% in the lesions smaller than 0.03, between 0.03 and 0.3, and larger than 0.3 mm(2), respectively. In GST‐P‐positive lesions larger than 0.3 mm(2), both apoptosis induction and cell proliferation activity were enhanced when Bcl‐x protein expression was decreased. In the long‐term exogenous models, rats were given 10 mg/kg of DEN, partially hepatectomized 4 h after treatment, administered 0.5 mg/kg of colchicine at the end of days 1 and 3, subjected to a selection procedure, and sacrificed at the end of week 45. Hepatocellular carcinomas were induced with the decreased Bcl‐x protein expression. In the endogenous model, rats were fed a choline‐deficient, l‐amino acid‐defined diet for 16 or 80 weeks and sacrificed. Bcl‐x protein expression was decreased both in GST‐P‐positive lesions and hepatocellular carcinoma. These results suggest that this decrease of Bcl‐x protein might serve as an indicator of the advanced form of preneoplastic lesions, and that this decrease could also be associated with a potential to progress into carcinoma in both exogenous and endogenous hepatocarcinogenesis of rats.