Insulin biosynthesis and beta cell survival regulated by Pdx1 targets

Type 2 diabetes is the product of impaired insulin secretion and β cell apoptosis in the context of insulin resistance. Mutations in the transcription factor Pancreatic duodenal homeobox 1 (Pdx1) cause early onset diabetes as well as type 2 diabetes. We performed chromatin immunoprecipitation sequencing (ChIPSeq) to characterize genome-wide Pdx1 occupancy in vivo. Pdx1 occupancy in human and mouse islets showed distinct profiles with limited conservation of binding regions, in contrast there was substantial conservation at the gene level with enrichment for genes with functions in diabetes, gene expression, and cell death. ChIPSeq analysis identified Pdx1 binding motifs and motifs that may be bound by neighboring factors. EMSA studies showed that probes containing the de novo motif derived from sequences in the type 2 diabetes susceptibility gene, Tcf7l2, were bound by Pdx1-Pbx1a heterodimers. We compared occupied genes to gene expression profiling of Pdx1+/- islets to identify direct targets that may contribute to the impaired insulin secretion, reduced insulin content, and β cell death that occur in the setting of Pdx1 deficiency. Pdx1 was found to occupy and differentially regulate the endoplasmic reticulum (ER) oxidoreductase, Ero1lβ, which is involved in disulfide bond formation and abundantly expressed in the β cell. Pdx1 was found to occupy Ero1lβ in mouse insulinoma MIN6 cells and Ero1lβ transcript was reduced in Pdx1 deficient β cells. Silencing of Ero1lβ in MIN6 cells decreased insulin content and increased susceptibility to ER stress induced cell death. Ero1lβ was identified as a direct target of Pdx1 that may contribute to the impaired insulin secretion and increased β cell apoptosis observed in Pdx1 deficient models. Pdx1 regulates a broad array of genes in the β cell that affect insulin secretion and cell death, and investigation of these genes may uncover therapeutic targets to enhance β cell function and survival in patients with type 2 diabetes