The molecular mechanism by which FoxO1 regulates islet vascularization and compensative beta cell proliferation

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Genetic studies revealed that the ablation of insulin/IGF-1 signaling in the pancreas causes diabetes. FoxO1 is a downstream transcription factor of insulin/IGF-1 signaling. We previously reported that FoxO1 haploinsufficiency restored . cell mass and rescued diabetes in IRS2 knockout mice. However, it is still unclear whether FoxO1 dysregulation in the pancreas could be the cause of diabetes. To test this hypothesis, we generated transgenic mice overexpressing constitutively active FoxO1 specifically in the pancreas (TG). TG mice had impaired glucose tolerance and some of them indeed developed diabetes due to the reduction of . cell mass, which is associated with decreased Pdx1 and MafA in . cells. We also observed that TG mice have islet hypervascularities due to increased VEGF-A expression in . cells. We performed chromatin immunoprecipitation (ChIP) assays and showed that FoxO1 binds to the VEGF-A promoter. We also showed in luciferase assays that FoxO1 regulates VEGF-A transcription in . cells. When FoxO1 is over expressed by adenovirus, VEGF-A mRNA level was increased in .TC3 cells. Despite severe reduction of . cells, plasma insulin levels and blood glucose levels as well as glucose tolerance were marginally impaired. We suppose this is due to increased VEGF-A expression and increased islets vascularity in TG mice. We propose that FoxO1 in pancreas plays important roles in the regulation of glucose metabolism.