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 acti vity that determines the postreceptor virus-host tropism. Despite the detailed study of this enhancer our knowledge of the mechanisms underl ying this behavior is very limited. Here we report that the hepatocyte nuclear factor 3 (HNF3) is al least in part responsible for the liver -specific activity of the enhancer. We demonstrate that recombinant HN F3 alpha binds the enhancer at two sites with different affinity. Tran sfection 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 th e enhancer is active only in liver extracts but not in extracts prepar ed from HeLa cells. Furthermore, the latter extract cannot be activate d by addition of recombinant HNF3 alpha. A similar behavior is manifes ted in transfected cells and, here again, the inactive enhancer is not activated by cotransfected HNF3 beta and alpha. Collectively, our stu dy shows that HNF3 activators are required but not sufficient for full activation of the HBV enhancer and there is a need for additional liv er-specific activators or coactivators. (C) 1995 Academic Press, Inc.