Hepatitis B virus enhancer binds and is activated by the Hepatocyte nuclear factor 3

The enhancer of hepatitis B (HBV) virus displays a liver-specific activity that determines the postreceptor virus-host tropism. Despite the detailed study of this enhancer our knowledge of the mechanisms underlying this behavior is very limited. Here we report that the hepatocyte nuclear factor 3 (HNF3) is at least in part responsible for the liver-specific activity of the enhancer. We demonstrate that recombinant HNF3 alpha binds the enhancer at two sites with different affinity. Transfection studies have demonstrated that the enhancer is active only in liver cells and that integrity of the HNF3 binding sites is important for its full activity. In vitro transcription assays revealed that the enhancer is active only in liver extracts but not in extracts prepared from HeLa cells. Furthermore, the latter extract cannot be activated by addition of recombinant HNF3 alpha. A similar behavior is manifested in transfected cells and, here again, the inactive enhancer is not activated by cotransfected HNF3 beta and alpha. Collectively, our study shows that HNF3 activators are required but not sufficient for full activation of the HBV enhancer and there is a need for additional liver-specific activators or coactivators.