MafK overexpression in pancreatic b-cells caused impairment of glucose-stimulated insulin secretion Homare Shimohata a , Keigyou Yoh b , Naoki Morito a , Hitoshi Shimano c , Takashi Kudo a , Satoru Takahashi a, * a Department of Anatomy and Embryology, Biomolecular and Integrated Medical Sciences, Graduate School of Comprehensive Human Sciences, University of Tsukuba, Ibaraki, Japan b Pathophysiology of Renal Diseases, Medical Sciences for Control of Pathological Processes, Graduate School of Comprehensive Human Sciences, University of Tsukuba, Ibaraki, Japan c Department of Internal Medicine, Metabolism and Endocrinology, Graduate School of Comprehensive Human Sciences, University of Tsukuba, Ibaraki, Japan Received 17 May 2006 Available online 8 June 2006 Abstract MafA is a member of large Maf transcription factors and activates the insulin gene in pancreatic b-cells. Other large Maf transcription factors, MafB and c-Maf, also express and activate insulin transcription in b-cells. However, the functional relationship between MafA and other Maf proteins in b-cells has not been established. In order to suppress the function of large Maf proteins, we generated trans- genic mice overexpressing MafK, which act as dominant negative protein in pancreatic b-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. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Maf; Transcription factor; Transgenic mouse; Insulin; Diabetes mellitus Introduction The level of blood glucose after meals is regulated by the polypeptide hormone insulin, which is secreted by pancre- atic b-cells. Impaired insulin secretion is a major cause of type 2 diabetes, a common life style-related disease. There are various factors that contribute to insulin secretion, although the most important factor is glucose [1]. In addi- tion to the stimulation of insulin secretion, glucose also enhances insulin gene transcription. The effect of glucose on insulin gene expression is regulated by many cis-acting elements in the insulin promoter that are recognized by several transcription factors [2]. Pancreatic duodenal homeo- box factor-1 (PDX-1) is one of these trans-acting factors [3–5]. It binds to the A3 elements of the insulin promoter and it is involved in both insulin gene expression and pan- creas development. The transcription factor Beta2 binds to E1 elements and also plays an important role in insulin production and islet development [6,7]. The RIPE3b1/C1 element is a critical cis-acting sequence for insulin gene expression and is bound by the RIPE3b1 transcription acti- vator [8]. The gene encoding the RIPE3b1 activator has recently been cloned and identified as MafA [9–11]. The maf gene was originally identified in the genome of the avi- an musculoaponeurotic fibrosarcoma virus, AS42 [12]. Maf family proteins contain a C-terminal basic leucine zipper domain (b-Zip domain) that binds to specific DNA 0006-291X/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.bbrc.2006.05.184 * Corresponding author. Fax: +81 29 853 6965. E-mail address: satoruta@md.tsukuba.ac.jp (S. Takahashi). www.elsevier.com/locate/ybbrc Biochemical and Biophysical Research Communications 346 (2006) 671–680 BBRC