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Hepatic Nuclear Factor-3 (HNF-3 or Foxa2) Regulates Glucagon Gene Transcription by Binding to the G1 and G2 Promoter Elements

Unité de Diabétologie Clinique, Centre Médical Universitaire, 1211 Genève 4, Switzerland

Address all correspondence and requests for reprints to: Benoit Gauthier, Ph.D., Division de Biochimie Clinique, Centre Médical Universitaire, 1211 Genève 4, Switzerland. E-mail: benoit.gauthier{at}medecine.unige.ch

Glucagon gene expression in the endocrine pancreas is controlled by three islet-specific elements (G3, G2, and G4) and the -cell-specific element G1. Two proteins interacting with G1 have previously been identified as Pax6 and Cdx2/3. We identify here the third yet uncharacterized complex on G1 as hepatocyte nuclear factor 3 (HNF-3)ß, a member of the HNF-3/forkhead transcription family, which plays an important role in the development of endoderm-related organs. HNF-3 has been previously demonstrated to interact with the G2 element and to be crucial for glucagon gene expression; we thus define a second binding site for this transcription on the glucagon gene promoter. We demonstrate that both HNF-3 and -ß produced in heterologous cells can interact with similar affinities to either the G1 or G2 element. Pax6, which binds to an overlapping site on G1, exhibited a greater affinity as compared with HNF-3 or -ß. We show that both HNF-3ß and - can transactivate glucagon gene transcription through the G2 and G1 elements. However, HNF-3 via its transactivating domains specifically impaired Pax6-mediated transactivation of the glucagon promoter but had no effect on transactivation by Cdx2/3. We suggest that HNF-3 may play a dual role on glucagon gene transcription by 1) inhibiting the transactivation potential of Pax6 on the G1 and G3 elements and 2) direct activation through G1 and G2.

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