Hepatocyte nuclear factor 1 mediates the cell density associated induction of the α-fetoprotein gene

Abstract

The α-fetoprotein gene (AFP) is highly expressed in proliferating hepatocytes. The relationship between cell density and AFP gene expression was investigated in Hep3B and McA-RH8994 hepatoma cell lines. We found that the expression of the AFP gene was induced when the hepatoma cell number increased. However, the fold induction in cell number did not directly reflect the fold increase in AFP mRNA suggesting the induction was not controlled by cell number exclusively. AFP mRNA was also induced when quiescent hepatoma cells entered into the cell cycle. The induction of the AFP and cyclin genes coincided in both synchronized and non-synchronized hepatoma cells indicating that cell cycle also played a role in AFP gene regulation. To investigate the molecular mechanism underlying AFP gene induction, the expression of the hepatocyte nuclear factor 1 (HNF1) and HNF4 genes, encoding important trans-acting factors for the AFP and HNF1 genes, respectively, were examined. The amount of HNF1 bound to the two HNF1 binding sites located on the AFP gene promoter was enhanced as cell density increased. However, cell density did not change the binding level of HNF4. By transient transfection, the promoter of the AFP gene, containing the two HNF1 binding sites, could be induced when cell number increased. Furthermore, the expression of the albumin gene, another HNF1 target gene, was also regulated by cell density. In contrast, non-HNF1 target genes such as c-myc, c-jun, and β-actin were not affected. Taken together, these data indicate that cell density, cell–cell contact, and cell cycle play important roles in AFP gene regulation. The cell density-associated AFP gene induction is mediated through an increase in post-transcriptional regulation of HNF1.