We have previously identified a Foxo1 binding protein, FCoR, which suppresses Foxo1 activity by increasing acetylation. FCoR is expressed in pancreatic α- and β-cells. FCoR expression is decreased in high-fat fed mice and in Leprdb/db mice, and FcorKO mice exhibited decreased insulin secretion, increased α-cell mass and β to α-cell conversion, suggesting FCoR involvement in the pathophysiology of diabetes. In FcorKO islets, nucleus localization of Foxo1 was increased, and Foxo1 acetylation was significantly decreased compared to WT. These data suggest that FCoR regulates acetylation and activity of Foxo1 in islets. The qPCR in isolated islets from FcorKO showed increased Arx, Mafb and Gcg expression compared to WT, supporting the phenotype of increased α-cell mass. Interestingly, expression level of Mafa and Neurod1, which are the β-cell markers and target genes of Foxo1, were also significantly increased, confirming activation of Foxo1 in the absence of FCoR. FCoR suppresses the expression of Aristaless related homeobox (Arx), which is essential for α-cell differentiation, via increased methylation of the CG rich promoter region of Arx. One of the epigenetic modifications of histone, the tri-methylation at H3K4 (H3K4me3), is an important marker of Arx expression in α-cells. Chromatin immunoprecipitation assay revealed that H3K4me3 was significantly decreased in αTC cells transduced with Flag-FCoR compared to LacZ-transduced-cells, suggesting that FCoR suppresses Arx expression through epigenetic regulation involving both DNA and histone methylation. Our data show that FCoR regulates Foxo1 activity in islets, and that FCoR regulates pancreatic α- and β-cell identity by epigenetic regulation of DNA and histone methylation.
N. Kodani: Research Support; Self; Lilly Diabetes, Novartis Pharmaceuticals Corporation. M. Kobayashi: None. O. Kikuchi: None. T. Kitamura: None. H. Itoh: None. J. Nakae: None.