Pdx-1 modulates endoplasmic reticulum calcium homeostasis in the islet β cell via transcriptional enhancement of SERCA2b

Abstract

Diabetes mellitus affects an estimated 285 million people worldwide, and a central component of diabetes pathophysiology is diminished pancreatic islet beta cell function resulting in the inability to manage blood glucose effectively. The beta cell is a highly specialized metabolic factory that possesses a number of specialized characteristics, chief among these a highly developed endoplasmic reticulum (ER). The sarco endoplasmic reticulum Ca2+ ATPase 2b (SERCA2b) pump maintains a steep Ca2+ gradient between the cytosol and ER lumen, and while the Pancreatic and duodenal homeobox 1 (Pdx-1) transcription factor is known to play an indispensable role in beta cell development and function, recent data also implicate Pdx-1 in the maintenance of ER health. Our data demonstrates that a decrease of beta cell Pdx-1 occurs in parallel with decreased SERCA2b expression in models of diabetes, while in silico analysis of the SERCA2b promoter reveals multiple putative Pdx-1 binding sites. We hypothesized that Pdx-1 loss under inflammatory and diabetic conditions leads to decreased SERCA2b with concomitant alterations in ER health. To test this, siRNA-mediated knockdown of Pdx-1 was performed in INS-1 cells. Results revealed reduced SERCA2b expression and decreased ER Ca2+, which was measured using an ER-targeted D4ER adenovirus and fluorescence lifetime imaging microscopy. Co-transfection of human Pdx-1 with a reporter fused to the human SERCA2 promoter increased luciferase activity three-fold relative to the empty vector control, and direct binding of Pdx-1 to the proximal SERCA2 promoter was confirmed by chromatin immunoprecipitation. To determine whether restoration of SERCA2b could rescue ER stress induced by Pdx-1 loss, Pdx1+/- mice were fed high fat diet for 8 weeks. Isolated islets from these mice demonstrated increased expression of spliced Xbp1, signifying ER stress, while subsequent SERCA2b overexpression in isolated islets reduced spliced Xbp1 levels to that of wild-type controls. These results identify SERCA2b as a direct transcriptional target of Pdx-1 and define a novel role for altered ER Ca2+ regulation in Pdx-1 deficient states.