Mutations in the hepatocyte nuclear factor (HNF) 4 alpha gene cause a form of maturity-onset diabetes of the young (MODY1), which is a monogenic form of type 2 diabetes characterized by impaired insulin secretion by pancreatic beta-cells. HNF4 alpha is a transcription factor expressed in the liver, kidney, intestine, and pancreatic islet. Multiple splice variants of the HNF4 alpha gene have been identified and an isoform of HNF4 alpha 8, an N-terminal splice variant, is expressed in pancreatic beta-cells. However, expression levels of HNF4 alpha protein in pancreatic beta-cells and the transcriptional activity of HNF4 alpha 8 are not yet understood. In the present study, we investigated the expression of HNF4 alpha in beta-cells and examined its functional properties. Western blotting and immunohistochemical analysis revealed that the expression of HNF4 alpha protein in pancreatic islets and INS-1 cells was much lower than in the liver. A reporter gene assay showed that the transactivation potential of HNF4 alpha 8 was significantly weaker than that of HNF4 alpha 2, which is a major isoform in the liver, suggesting that the total level of HNF4 alpha activity is very weak in pancreatic beta-cells. We also showed that the N-terminal A/B region of HNF4 alpha 8 possessed no activation function and C-terminal F region negatively regulated the transcriptional activity of HNF4 alpha 8. The information presented here would be helpful for the better understanding of MODY1/HNF4 alpha diabetes. (c) 2005 Elsevier Inc. All rights reserved.