Hepatocyte nuclear factor 1 mediates the cell density associated inductionof the alpha-fetoprotein gene

The alpha-fetoprotein gene (AFP) is highly expressed in proliferating hepat ocytes. The relationship between cell density and AFP gene expression was i nvestigated in Hep3B and McA-RH8994 hepatoma cell lines. We found that the expression of the AFP gene was induced when the hepatoma cell number increa sed. However. the fold induction in cell number did not directly reflect th e 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 gene s coincided in both synchronized and non-synchronized hepatoma cells indica ting that cell cycle also played a role in AFP gene regulation. To investig ate the molecular mechanism underlying AFP gene induction, the expression o f the hepatocyte nuclear factor 1 (HNF1) and HNF4 genes, encoding important trans-acting factors for the AFP and HNF1 genes, respectively, were examin ed. The amount of HNF1 bound to the two HNF1 binding sites located on the A FP gene promoter was enhanced as cell density increased. However, cell dens ity did not change the binding level of HNF4. By transient transfection, th e promoter of the AFP gene, containing the two HNF1 binding sites, could be induced when cell number increased. Furthermore, the expression of the alb umin gene, another HNF1 target gent, was also regulated by cell density. In contrast, non-HNF1 target genes such as c-myc, c-jun, and beta-actin were not affected. Taken together, these data indicate that cell density, cell-c ell contact. and cell cycle play important roles in AFP gene regulation. Th e cell density-associated AFP gene induction is mediated through an increas e in post-transcriptional regulation of HNF1. (C) 1999 Elsevier Science Ire land Ltd. All rights reserved.