Foxa1 and Foxa2 Regulate α-Cell Differentiation, Glucagon Biosynthesis, and Secretion

The Forkhead box A transcription factors are major regulators of glucose homeostasis. They show both distinct and redundant roles during pancreas development and in adult mouse beta-cells. In vivo ablation studies have revealed critical implications of Foxa1 on glucagon biosynthesis and requirement of Foxa2 in alpha-cell terminal differentiation. In order to examine the respective role of these factors in mature alpha-cells, we used small interfering RNA (siRNA) directed against Foxa1 and Foxa2 in rat primary pancreatic alpha-cells and rodent alpha-cell lines leading to marked decreases in Foxa1 and Foxa2 mRNA levels and proteins. Both Foxa1 and Foxa2 control glucagon gene expression specifically through the G2 element. Although we found that Foxa2 controls the expression of the glucagon, MafB, Pou3f4, Pcsk2, Nkx2.2, Kir6.2, and Sur1 genes, Foxa1 only regulates glucagon gene expression. Interestingly, the Isl1 and Gipr genes were not controlled by either Foxa1 or Foxa2 alone but by their combination. Foxa1 and Foxa2 directly activate and bind the promoter region the Nkx2.2, Kir6.2 and Sur1, Gipr, Isl1, and Pou3f4 genes. We also demonstrated that glucagon secretion is affected by the combined effects of Foxa1 and Foxa2 but not by either one alone. Our results indicate that Foxa1 and Foxa2 control glucagon biosynthesis and secretion as well as alpha-cell differentiation with both common and unique target genes.