Glucagon gene transcription activation mediated by synergistic interactions of pax-6 and cdx-2 with the p300 co-activator

In the endocrine pancreas, alpha-cell-specific expression of the glucagon gene is mediated by DNA-binding proteins that interact with the proximal GL promoter element, which contains several AT-rich domains. The homeodomain transcription factors brain-4, pax-6, and cdx-2 have been shown to bind to these sites and to transactivate glucagon gene expression. In the present study, we investigated the interaction of cdx-2 and pax-6 with p300, a co-activator coupled to the basal transcription machinery. In transient transfection-expression experiments, we found that the transactivating effects of cdx-2 and pax-6 on the glucagon gene were greatly enhanced by the additional expression of p300. This enhancement was due to direct protein-protein interactions of both pax-6 and cdx-2 with the N-terminal C/H1 domain of p300. pax-6 and cdx-2 also directly interacted with one another at the protein level. pax-6, bound to its DNA recognition site in the glucagon G1 promoter element, tethered cdx-2 to the molecular complex of pax-g and p300. Further, we found that the presence of cdx-2 enhanced the interaction of pax-g with p300, thus establishing a molecular complex of transcription factors implicated in tissue-specific glucagon gene expression with the basal transcriptional machinery.