HEPATOCARCINOGENESIS IN WOODCHUCK HEPATITIS-VIRUS E-MYC MICE - SUSTAINED CELL-PROLIFERATION AND BIPHASIC ACTIVATION OF INSULIN-LIKE GROWTH-FACTOR-II

Transgenic mice carrying the c-myc oncogene under control of woodchuck hepatitis virus (WHV) DNA sequences invariably develop hepatocellular carcinoma (HCC), despite a temporally limited expression of the trans gene in the neonatal liver. To better characterize the different steps of the tumorigenic process, we analyzed the liver expression of the c -myc transgene and several growth-related genes by in situ hybridizati on and Northern blotting. In parallel studies, proliferated changes we re investigated by detection of bromodeoxyuridine-positive S-phase nuc lei and apoptosis was evaluated by in situ nick end-labeling of DNA. D uring the neonatal period, high levels of c-myc messenger RNAs (mRNAs) were detected in all hepatocytes, and the expression of insulin-like growth factor II (IGF II) was frequently enhanced, correlating with in creased cell proliferation. Despite elevated expression of the p53 gen e, no change in liver cell apoptosis was observed. After weaning, c-my c transgene expression decreased to undetectable levels in all hepatoc ytes, whereas proliferation decreased but remained notably higher than in age-matched controls. The expression of c-fos, c-jun, and c-H-ras was highly variable during the preneoplastic period and in the tumors, with no consistent increase compared with controls. Resurgence of c-m yc transgene expression was evidenced in all cells from hyperplastic l esions and carcinomas, accompanied with frequent focal reactivation of IGF II. Thus the strong proliferative stimulus induced by the combine d effects of c-myc and IGF II in the neonatal liver might initiate a p rocess characterized by persistent, dysregulated hepatocyte proliferat ion, in turn greatly increasing the risk of hepatocellular transformat ion.