Stability of the Hepatocyte Nuclear Factor 6 Transcription Factor Requires Acetylation by the CREB-binding Protein Coactivator

Abstract

We previously demonstrated that the formation of complexes between the DNA binding domains of the hepatocyte nuclear factor 6 (HNF6) and Forkhead Box a2 (Foxa2) transcription factors resulted in synergistic transcriptional activation of a Foxa2 target promoter. This Foxa2.HNF6 transcriptional synergy was mediated by the recruitment of CREB-binding protein (CBP) coactivator through the HNF6 Cut-Homeodomain sequences. Although the HNF6 DNA binding domain sequences are sufficient to recruit CBP coactivator for HNF6.Foxa2 transcriptional synergy, paradoxically these HNF6 Cut-Homeodomain sequences were unable to stimulate the transcription of an HNF6-dependent reporter gene. Here, we investigated whether the CBP coactivator protein played a different role in regulating HNF6 transcriptional activity. We showed that acetylation of the HNF6 protein by CBP increased both HNF6 protein stability and its ability to stimulate transcription of the glucose transporter 2 promoter. Mutation of the HNF6 Cut domain lysine 339 residue to an arginine residue abrogated CBP acetylation, which is required for HNF6 protein stability. Furthermore, the HNF6 K339R mutant protein, which failed to accumulate detected protein levels, was transcriptionally inactive and could not be stabilized by inhibiting the ubiquitin proteasome pathway. Finally, increased HNF6 protein levels stabilized the Foxa2 protein, presumably through the formation of the Foxa2.HNF6 complex. These studies show for the first time that HNF6 protein stability is controlled by CBP acetylation and provides a novel mechanism by which the activity of the CBP coactivator may regulate steady levels of two distinct liver-enriched transcription factors.