FAT1 expression and function in chronic liver disease and hepatocellular carcinoma (2014)

FAT1 is a member of the atypical cadherin FAT subfamily. The first identified member of this family was Drosophila Fat which has been regarded as a tumor suppressor because of its implication in tissue growth. In vertebrates FAT subfamily consists of 4 members, FAT1, -2, -3 and FAT-J (or FAT4). Only FAT1 has so far been studied more intensively. FAT1 is expressed in a wide range of tissues and a homozygous knockout for FAT1 in mice was perinatal lethal. In vitro studies indicate that it is involved in cell polarity and migration. Cancer research revealed deregulated FAT1 expression. In some tumors as breast cancer FAT1 is over-expressed while FAT1 expression is reduced or deleted in others as astrocytic tumors or cholangiocarcinoma. In the present work, we first analyzed the role of FAT1 in chronic liver disease. We found FAT1 upregulation in a murine model of non-alcoholic steatohepatitis (NASH) and could confirm this result in human tissue from NASH patients. Moreover, we detected induced FAT1 expression in further murine models of chronic liver injury. Thus, bile duct ligation (BDL) as well as hepatotoxic thioacetamide (TAA) administration induced hepatic upregulation of FAT1 expression. Furthermore, FAT1 expression was significantly heightened in cirrhotic human liver tissue. Activated hepatic stellate cells (HSC) were identified as the main cellular source of FAT1 in chronic liver disease. Suppression of FAT1 by siRNA inhibited NFκB activation and thereto proinflammatory gene expression, and reduced apoptosis resistance in activated HSC. Next, we investigated FAT1 expression and function in hepatocellular carcinoma (HCC). FAT1 expression was significantly upregulated in HCC cell lines as well as in HCC tumor tissue compared to primary human hepatocytes (PHH) and non-tumorous liver tissue. In human HCC tissues strong FAT1 expression correlated with tumor stage and proliferation rate. Stable suppression of FAT1 by shRNA transfection reduced proliferation as well as migratory potential of HCC cells. Moreover, cells with suppressed FAT1 expression demonstrated higher susceptibility towards apoptosis, induced by serum starvation. To evaluate the role of FAT1 in HCC in vivo we injected stable FAT1 suppressed cells into nude mice. We observed delayed tumor onset and more apoptotic cells in tumors derived from FAT1 suppressed cell clones. Finally, we searched for mechanisms responsible for FAT1 upregulation in HCC. We found that hepatocyte growth factor (HGF), a factor secreted by activated HSCs, induced FAT1 expression in vitro. FAT1 expression was further increased in HCC cells under hypoxia, and this induction was strongly repressed by inhibition of hypoxia inducible factor 1 alpha (HIF1α). Hypoxia caused significantly reduced levels of the methyl group donor S-adenosylmethionine (SAM), and SAM supplementation inhibited the hypoxia induced FAT1 expression in HCC cells. Conversely, demethylating agents induced FAT1 expression in HCC cells, and expression analysis in 24 human HCC tissues showed a significant correlation between FAT1 and MAT2A, which is known to critically affect SAM levels in HCC. In conclusion, increased FAT1 expression in chronic liver disease and HCC functionally promotes the course of disease and thereto may be a new therapeutic target for the prevention and treatment of hepatic fibrosis in chronic liver disease and hepatocellular carcinoma.