HEPATOCYTE NUCLEAR FACTORS IN LIVER REGENERATION

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The liver is one of the few adult organs capable of completely regenerating itself in response to cellular injury from toxins, viral infections or tissue removal. The long term goal of this proposal is to understand the role of proliferation- and hepatocyte-specific transcription factors mediating replication during liver regeneration. Transcriptional regulation of hepatocyte-specific gene expression relies on combinatorial interaction of multiple DNA binding sites by several distinct hepatocyte nucleus factors (HNF). Regeneration of the liver following injury involves a balance between hepatocyte proliferation plus sustained expression of hepatocyte-specific genes required for homeostasis and simultaneously diminishing the transcription of cell- specific genes that may interfere with hepatocyte replication. Although the mechanisms involved in this transcriptional decline remain uncharacterized, the cut-homeodomain HNF-6 protein is likely to be involved because it is one of the few hepatocyte-specific transcription factors whose expression is reduced during liver regeneration. Furthermore, a number of potential HNF-6 target genes decline during liver regeneration, several of which may inhibit hepatocyte replication. A transgenic mouse line will be generated in which decreased HFN-6 expression during liver regeneration can be prevented. We will use these mice to test the hypothesis that undiminished HFN-6 expression is inhibitory to liver regeneration. The immediate early gene expression that is induced during liver regeneration is known to mediate hepatocyte progression into the G1 stage of the cell-cycle. However, the transcriptional mechanisms mediating the later stages of hepatocyte replication are not completely understood. To further examine transcriptional processes underlying cell cycle progression during liver regeneration, we will alter hepatocyte expression of the winged helix HNF-3/fork head homolog-11 (HFH-11) gene. HFH-11 is a candidate transcription factor involved in cell cycle progression because its expression is transiently reactivated during the period of hepatocyte proliferation, and putative HFH-11 binding sites are present in cell- cycle regulatory genes such as c-myc, c-myb, TGFalpha, cyclin B1 and cyclin D1. We will generate mice possessing a conditional targeted HFH- 11 gene disruption in adult hepatocytes and liver regeneration studies with these mice will allow us to test the hypothesis that HFH-11 expression is necessary for hepatocyte replication. Gain of function studies in transgenic mice are proposed to examine whether premature hepatocyte expression of HFH-11 during liver regeneration results in altered cell cycle kinetics of proliferating hepatocytes. Assessment of these transgenic mice for development of liver nodules in a liver tumor promotion model will allow us to investigate whether continuous hepatocyte expression of HFH-11 increases the incidence of hepatocellular carcinoma. An understanding of transcriptional mechanisms which mediate liver regeneration may allow us to identify regulatory pathways that are potentially disrupted during liver fibrosis and cirrhosis.