Association between Hepatocyte Nuclear Factor 6 (HNF-6) and FoxA2 DNA Binding Domains Stimulates FoxA2 Transcriptional Activity but Inhibits HNF-6 DNA Binding

In previous studies we used transgenic mice or recombinant adenovirus infection to increase hepatic expression of forkhead box A2 (FoxA2, previously called hepatocyte nuclear factor 3β [HNF-3β]), which caused diminished hepatocyte glycogen levels and reduced expression of glucose homeostasis genes. Because this diminished expression of FoxA2 target genes was associated with reduced levels of the Cut-Homeodomain HNF-6 transcription factor, we conducted the present study to determine whether there is a functional interaction between HNF-6 and FoxA2. Human hepatoma (HepG2) cotransfection assays demonstrated that HNF-6 synergistically stimulated FoxA2 but not FoxA1 or FoxA3 transcriptional activity, and protein-binding assays showed that this protein interaction required the HNF-6 Cut-Homeodomain and FoxA2 winged-helix DNA binding domains. Furthermore, we show that the HNF-6 Cut-Homeodomain sequences were sufficient to synergistically stimulate FoxA2 transcriptional activation by recruiting the p300/CBP coactivator proteins. This was supported by the fact that FoxA2 transcriptional synergy with HNF-6 was dependent on retention of the HNF-6 Cut domain LXXLL sequence, which mediated recruitment of the p300/CBP proteins. Moreover, cotransfection and DNA binding assays demonstrated that increased FoxA2 levels caused a decrease in HNF-6 transcriptional activation of the glucose transporter 2 (Glut-2) promoter by interfering with the binding of HNF-6 to its target DNA sequence. These data suggest that at a FoxA-specific site, HNF-6 serves as a coactivator protein to enhance FoxA2 transcription, whereas at an HNF-6-specific site, FoxA2 represses HNF-6 transcription by inhibiting HNF-6 DNA binding activity. This is the first reported example of a liver-enriched transcription factor (HNF-6) functioning as a coactivator protein to potentiate the transcriptional activity of another liver factor, FoxA2.