Pbx1 is a co-factor for Cdx-2 in regulating proglucagon gene expression in pancreatic A cells

A number of Hox and Hox-like homeodomain (HD) proteins have been previously shown to utilize members of the TALE HD protein family as co-factors in regulating gene expression. The caudal HD protein Cdx-2 is a transactivator for the proglucagon gene, expressed in pancreatic A cells and intestinal endocrine L cells. We demonstrate here that co-transfection of the TALE homeobox gene Pbx1 enhanced the activation of Cdx-2 on the proglucagon promoter in either the pancreatic A cell line InR1-G9 or BHK fibroblasts. The activation was observed for proglucagon promoter constructs with or without the binding motifs for Pbx1. Furthermore, mutating the penta-peptide motif (binding motif for TALE HD proteins) on Cdx-2 substantially attenuated its activation on proglucagon promoter, but not on the sucrase-isomaltase gene (SI) promoter, or its own (Cdx-2) promoter; suggesting that Cdx-2 utilizes Pbx1 as a co-factor for regulating the expression of selected target genes. Physical interaction between Cdx-2 and Pbx1 was demonstrated by co-immunoprecipitation as well as GST fusion protein pull-down. We suggest that this study reveals a novel function for Pbx1 in pancreatic islet physiology: regulating proglucagon expression by serving as a co-factor for Cdx-2.