Characterization of a novel Foxa (hepatocyte nuclear factor-3) site in the glucagon promoter that is conserved between rodents and humans

SK Sharma, U Leinemann, R Ratke, E Oetjen… - Biochemical …, 2005 - portlandpress.com
SK Sharma, U Leinemann, R Ratke, E Oetjen, R Blume, C Dickel, W Knepel
Biochemical Journal, 2005portlandpress.com
The pancreatic islet hormone glucagon stimulates hepatic glucose production and thus
maintains blood glucose levels in the fasting state. Transcription factors of the Foxa [Fox (f
orkhead b ox) subclass A; also known as HNF-3 (hepatocyte nuclear factor-3)] family are
required for cell-specific activation of the glucagon gene in pancreatic islet α-cells. However,
their action on the glucagon gene is poorly understood. In the present study, comparative
sequence analysis and molecular characterization using protein–DNA binding and transient …
The pancreatic islet hormone glucagon stimulates hepatic glucose production and thus maintains blood glucose levels in the fasting state. Transcription factors of the Foxa [Fox (forkhead box) subclass A; also known as HNF-3 (hepatocyte nuclear factor-3)] family are required for cell-specific activation of the glucagon gene in pancreatic islet α-cells. However, their action on the glucagon gene is poorly understood. In the present study, comparative sequence analysis and molecular characterization using protein–DNA binding and transient transfection assays revealed that the well-characterized Foxa-binding site in the G2 enhancer element of the rat glucagon gene is not conserved in humans and that the human G2 sequence lacks basal enhancer activity. A novel Foxa site was identified that is conserved in rats, mice and humans. It mediates activation of the glucagon gene by Foxa proteins and confers cell-specific promoter activity in glucagon-producing pancreatic islet α-cell lines. In contrast with previously identified Foxa-binding sites in the glucagon promoter, which bind nuclear Foxa2, the novel Foxa site was found to bind preferentially Foxa1 in nuclear extracts of a glucagon-producing pancreatic islet α-cell line, offering a mechanism that explains the decrease in glucagon gene expression in Foxa1-deficient mice. This site is located just upstream of the TATA box (between −30 and −50), suggesting a role for Foxa proteins in addition to direct transcriptional activation, such as a role in opening the chromatin at the start site of transcription of the glucagon gene.
portlandpress.com