Role of apoptosis for mouse liver growth regulation and tumor promotion: comparative analysis of mice with high (C3H/He) and low (C57Bl/6J) cancer susceptibility.

Apoptosis constitutes one of the organisms defense lines against cancer. We investigated whether failure of apoptosis may be concurrently causative for the high cancer susceptibility in C3H/He as compared to C57BL/6J mice (low cancer susceptibility). First, in short-term in vivo experiments (7-21 days), mouse liver growth (C3H/He, C57BL/6J) was induced by administration of phenobarbital (PB; 2 days 500 ppm + 5 days 750 ppm via the food) or nafenopin (NAF; 7 days 500 ppm via the food), cessation of PB or NAF treatment served to initiate liver involution. Liver weight, DNA content, hepatocyte ploidy and apoptotic activity were studied as endpoints. Secondly, in a long-term study liver carcinogenesis was initiated by a single dose of N-nitrosodiethylamine (NDEA, 90 mg/kg b.w.) to 5-weeks-old C57Bl/6J and C3H/He mice. After 2 weeks, mice received either standard diet or a diet containing phenobarbital (PB, 90 mg/kg b.w.) for up to 90 weeks. Cell proliferation and apoptosis in normal liver tissue and (pre)neoplastic tissue was quantitatively analysed by histological means. The short term studies revealed that PB and NAF-induced mouse liver growth is essentially due to cell enlargement (hypertrophy). A moderate increase of liver DNA content was brought about by hepatocellular polyploidization; C3H/He mice exhibited the most pronounced ploidy shift, corresponding to their high cancer susceptibility. Upon cessation of PB or NAF treatment, regression of liver mass was neither associated with a loss of DNA nor an increase in apoptoses in the liver of C3H/He and C57Bl/6J mice; food restriction did not enforce the occurrence of apoptosis. Thus, the mouse strains did not differ with respect to the occurrence of apoptosis. In the long-term study, PB promoted liver tumor formation in all strains, exhibiting quantitative differences in growth kinetics of preneoplasia rather than a specific biological quality. Quantitative analysis of apoptosis in normal and (pre)neoplastic liver tissue of C3H/He and C57BL/6J mice revealed no clue to explain their different cancer susceptibility. Rather, cell proliferation seems to be the prevailing determinant of tumor promotion in the liver of both mouse strains.