CELLULAR AND MOLECULAR BIOLOGY OF THE HEPATIC STEM CELL COMPARTMENT

点击翻译按钮获取中文摘要

The research is focused on defining the cellular and molecular biology of the hepatic and pancreatic stem cell compartments in normal and neoplastic organs (see also Project No. Z01CP05262). We have isolated and characterized several primitive epithelial cell lines from adult rat liver (RLE) and pancreas (RPE) that share many cellular markers with the hepatic "oval" cells in vivo. The "oval" cells are thought to be a progeny of the hepatic stem cell compartment. The RLE cells, in contrast to differentiated hepatocytes, express cytokeratins 14 and 8 as well as connexin 43, and these markers are also transiently expressed in oval cells in vivo. The step-wise process of spontaneous transformation of RLE cells in vitro coincides with a differentiation switch from the primitive epithelial phenotype characterized by the expression of cytokeratin 14, connexin 43 and vimentin to a hepatocytic differentiation phase characterized by expression of cytokeratins 8 and 18, connexin 32, albumin and `-fetoprotein. Furthermore, transplantation of the transformed RLE cells gives rise to differentiated hepatocellular carcinomas. Increased expression of c-myc and TGF-alpha genes was also strongly associated with transformation of RLE cells in vitro, and we observed similar association of c-myc and TGF-alpha upregulation with hepatic tumor formation in vivo. We have examined the interaction and the possible causative effects of the c-myc and TGF-alpha genes upregulation in hepatic neoplasia by generating transgenic mice carrying both c-myc and TGF-alpha as transgenes. The myc transgene is targeted and constitutively expressed in the liver, whereas the TGF-alpha transgene is controlled by an inducible promoter. Induction of the TGF-alpha transgene in these double transgenic animals results in hepatic tumor formation within two months after activation of the TGF-alpha gene. In contrast, formation of liver tumors in single TGF-alpha transgenic mice or in the double transgenic mice without activation of the TGF-` transgene takes over a year. These results indicate that interaction of nuclear oncogenes such as c-myc and growth factors such as TGF-alpha may play a pivotal role in tumor formation in the liver.