Focal impairment of growth in hepatocellular neoplasms of C57BL/6 mice: a possible explanation for the strain's resistance to hepatocarcinogenesis.

C57BL/6 mice are relatively more resistant to hepatocarcinogens than C3H and C57BL/6 x C3H F1 (hereafter called B6C3F1) mice; however, the basis for this strain-dependent difference is not clear. In order to address this issue, male C57BL/6 mice were treated i.p. with 5 mg/kg diethylnitrosamine when 15 days old and the histological features of induced hepatocellular neoplasms were assessed at intervals over the ensuing 80 weeks. In many respects the natural history of the tumors was similar to that previously reported for hepatic neoplasms in B6C3F1 mice; they invaded hepatic vein branches while still microscopic (18 weeks after diethylnitrosamine) and developed into metastasizing trabecular carcinomas by 80 weeks. However, the tumors in the C57BL/6 mice were unique in their early focal development of cells containing inclusions of secretory protein within the endoplasmic reticulum. At 12 weeks after diethylnitrosamine, more than 90% of the neoplasms contained inclusions. Over the ensuing months, the inclusions increased in size and number and the regions containing them became more sharply defined. In mice killed 48 weeks after carcinogen, the extent of inclusion formation was correlated with [3H]thymidine labeling indices. Mean labeling indices were higher in the inclusion-poor than in the inclusion-rich areas: 5.4% versus 1.5% for the 36 neoplasms smaller than 1 mm in diameter and 14% versus 6% for the 18 neoplasms larger than 1 mm. Thus, we suggest that the focal slowing of hepatocellular tumor growth in C57BL/6 mice contributes to the strain's apparent resistance to hepatocarcinogenicity. However, the impairment does not appear to block tumor progression and the ultimate development of trabecular carcinomas. In addition, the data indicate that, independent of the presence or absence of inclusions, larger tumors have higher labeling indices, and presumably higher growth rates, than smaller ones. While the reason for the association between cytoplasmic inclusions and the low labeling indices is not known at present, at the very least the inclusions serve as unique markers for the growth retardation.