Functional characterization and regulation of liver transcription factors hepatocyte nuclear factor 3

Hepatocyte nuclear factor 3 (HNF-3) proteins are liver-enriched transcription factors. In combination with other liver-enriched or ubiquitous transcription factors, HNF-3 coordinately regulate the expression of many liver genes, including the gene encoding transthyretin (TTR). HNF-3 proteins play an important role in endoderm differentiation and hepatogenesis.;HNF-3 proteins are phosphorylated in human hepatoma HepG2 cells and can be phosphorylated in vitro by purified kinases. The DNA-binding domain of HNF-3 is located in the center of HNF-3. Amino acid residues that are essential for DNA-binding were determined by site-directed mutagenesis. HNF-3 DNA-binding domain also contains nuclear localization signals. The amino-terminal transactivation domain of HNF-3 was located between amino acids 14 and 93. The secondary structure of this activation domain is important for its function.;We identified an AP-1/HNF-3 composite site in the TTR promoter which is essential for both the basal and phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) induced expression of TTR in HepG2 cells. HNF-3 and AP-1 proteins compete for the binding and activation of the composite site. TPA treatment causes a rapid and dramatic induction in AP-1 activity while reduces the DNA-binding activity as well as mRNA levels of HNF-3;Studies of HNF-3 and TTR expression during the acute-phase response suggest that the dramatic reduction in HNF-3 and the inability of Jun-Jun homodimers to efficiently activate the composite site may be the cause of the decrease in TTR expression during the acute phase response. HNF-3 and TTR expression change only slightly during liver regeneration, suggesting that the down-regulation of HNF-3 by TPA in HepG2 cells may be a result of activation of the signalling pathways used by acute-phase cytokines rather than those activated during liver regeneration.