MafA is a member of large Maf transcription factors and activates the insulin gene in pancreatic P-cells. Other large Maf transcription factors, MafB and c-Maf. also express and activate insulin transcription in P-cells. However, the functional relationship between MafA and other Maf proteins in beta-cells has not been established. In order to suppress the function of large Maf proteins, we generated transgenic mice overexpressing MafK, which act as dominant negative protein in pancreatic P-cells. These mice showed hyperglycemia at a young age due to impairment of glucose-stimulated insulin secretion. Although the transgenic mice showed an abnormal response in the glucose tolerance test, hyperglycemia was restored in adulthood. Histological analysis revealed islet hypertrophy in adult transgenic mice. Finally, an electrophoretic gel mobility shift assay showed that the DNA-binding activity of endogenous MafA was significantly increased in the MafK transgenic mice. These results indicated that MafA may have relevance to compensatory response. (c) 2006 Elsevier Inc. All rights reserved.