Mechanisms of transcriptional regulation in the maintenance of β cell function

Abstract

The islet β cell is central to the maintenance of glucose homeostasis as the β cell is solely responsible for the synthesis of Insulin. Therefore, better understanding of the molecular mechanisms governing β cell function is crucial to designing therapies for diabetes. Pdx1, the master transcription factor of the β cell, is required for the synthesis of proteins that maintain optimal β cell function such as Insulin and glucose transporter type 2. Previous studies showed that Pdx1 interacts with the lysine methyltransferase Set7/9, relaxing chromatin and increasing transcription. Because Set7/9 also methylates non-histone proteins, I hypothesized that Set7/9-mediated methylation of Pdx1 increases its transcriptional activity. I showed that recombinant and cellular Pdx1 protein is methylated at two lysine residues, Lys123 and Lys131. Lys131 is involved in Set7/9 mediated augmented transactivation of Pdx1 target genes. Furthermore, β cell-specific Set7/9 knockout mice displayed glucose intolerance and impaired insulin secretion, accompanied by a reduction in the expression of Pdx1 target genes. Our results indicate a previously unappreciated role for Set7/9 in the maintenance of Pdx1 activity and β cell function. β cell function is regulated on both the transcriptional and translational levels. β cell function is central to the development of type 1 diabetes, a disease wherein the β cell is destroyed by immune cells. Although the immune system is considered the primary instigator of the disease, recent studies suggest that defective β cells may initiate the autoimmune response. I tested the hypothesis that improving β cell function would reduce immune infiltration of the islet in the NOD mouse, a mouse model of spontaneous type 1 diabetes. Prediabetic NOD mice treated with pioglitazone, a drug that improves β cell function, displayed an improvement in β cell function, a reduction in β cell death, accompanied by reductions in β cell autoimmunity, indicating that β cell dysfunction assists in the development of type 1 diabetes. Therefore, understanding the molecular mechanisms involved in β cell function is essential for the development of therapies for diabetes.