Deregulation of E2F transcriptional control has been implicated in oncogenic transformation. Consistent with this idea, we recently demonstrated that during hepatocarcinogenesis in c-myc/TGF double transgenic mice, there is increased expression of E2F-1 and E2F-2, as well as induction of putative E2F target genes. Therefore, we generated transgenic mice expressing E2F-1 under the control of the albumin enhancer/promoter to test the hypothesis that E2F family members may contribute to liver tumor development. Overexpression of E2F-1 resulted in mild but persistent increases in cell proliferation and death during postnatal liver growth, and no increases in hepatic regenerative growth in response to partial hepatectomy. Nevertheless, from 2 months postnatally E2F-1 transgenic mice exhibited prominent hepatic histological abnormalities including preneoplastic foci adjacent to portal tracts and pericentral large cell dysplasia. From 6 to 8 months onwards, there was an abrupt increase in the number of neoplastic nodules ('adenomas') with 100% incidence by 10 months. Some adenomas showed evidence of malignant transformation, and 2 of 6 mice killed at 12 months showed trabecular hepatocellular carcinoma. Endogenous c-myc was up-regulated in the early stages of E2F-1 hepatocarcinogenesis, whereas p53 was overexpressed in the tumors, suggesting that both E2F-1 mediated-proliferation and apoptosis are operative but at different stages of hepatocarcinogenesis. In conclusion, E2F-1 overexpression in the liver causes dysplasia and tumors and suggests a cooperation between E2F-1 and c-myc oncogenes during liver oncogenesis.
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