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 g ene is mediated by DNA-binding proteins that interact with the proximal GL promoter element, which contains several AT-rich domains. The homeodomain t ranscription factors brain-4, pax-6, and cdx-2 have been shown to bind to t hese sites and to transactivate glucagon gene expression. In the present st udy, we investigated the interaction of cdx-2 and pax-6 with p300, a co-act ivator coupled to the basal transcription machinery. In transient transfect ion-expression experiments, we found that the transactivating effects of cd x-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 inte ractions of both pax-6 and cdx-2 with the N-terminal C/H1 domain of p300. p ax-6 and cdx-2 also directly interacted with one another at the protein lev el. pax-6, bound to its DNA recognition site in the glucagon G1 promoter el ement, 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 implic ated in tissue-specific glucagon gene expression with the basal transcripti onal machinery.