Overexpression of insulin receptor substrate-1 and hepatitis Bx genes causes premalignant alterations in the liver

Abstract

Activation of the insulin (IN)/insulin receptor substrate-1 (IRS-1)/mitogen-associated protein kinase (MAPK) and the Wnt/beta-catenin signaling cascades occurs frequently in hepatocellular carcinoma (HCC) associated with persistent viral infection. The aims of this study were to provide a chronic proliferative stimulus through IRS-1 in the context of hepatitis Bx (HBx) protein expression in transgenic mice and determine if constitutive expression of these genes is sufficient to cause hepatocyte dysplasia and cellular transformation. We generated transgenic mice in which the HBx (ATX), IRS-1, or both (ATX+/IRS-1) genes were expressed under a liver-specific promoter. We also assessed histology and oxidative damage as well as up-regulation of molecules related to these signal transduction cascades in the liver by quantitative reverse-transcriptase polymerase chain reaction. Whereas mice with a single transgene (ATX or IRS-1) did not develop tumors, ATX+/IRS-1+ double transgenic livers had increased frequency of hepatocellular dysplasia and developed HCC. All three transgenic lines had significantly increased insulin growth factor 1 (IGF-1), Wnt 1 and Wnt 3 mRNA levels, and evidence of DNA damage and oxidative stress. The ATX+/IRS+ double transgenic mice were distinguished by having the highest level of activation of Wnt 3 and Frizzled 7 and selectively increased expression of IGF-II, proliferating cell nuclear antigen, and aspartyl-(asparaginyl)-beta-hydroxylase, a gene associated with increased cell migration. These results suggest that continued expression of the ATX or IRS-1 transgenes can contribute to hepatocyte transformation but are not sufficient to trigger neoplastic changes in the liver. However, dual expression that activates both the IN/IRS-1/MAPK and Wnt/beta-catenin cascades is sufficient to cause dysplasia and HCC in a previously normal liver.