Mechanisms of translational regulation in the pancreatic β cell stress response

Abstract

The islet beta cell is unique in its ability to synthesize and secrete insulin for use in the body. A number of factors including proinflammatory cytokines, free fatty acids, and islet amyloid are known to cause beta cell stress. These factors lead to lipotoxic, inflammatory, and ER stress in the beta cell, contributing to beta cell dysfunction and death, and diabetes. While transcriptional responses to beta cell stress are well appreciated, relatively little is known regarding translational responses in the stressed beta cell. To study translation, I established conditions in vitro with MIN6 cells and mouse islets that mimicked UPR conditions seen in diabetes. Cell extracts were then subjected to polyribosome profiling to monitor changes to mRNA occupancy by ribosomes. Chronic exposure of beta cells to proinflammatory cytokines (IL-1 beta, TNF-alpha, IFN-gamma), or to the saturated free fatty acid palmitate, led to changes in global beta cell translation consistent with attenuation of translation initiation, which is a hallmark of ER stress. In addition to changes in global translation, I observed transcript specific regulation of ribosomal occupancy in beta cells. Similar to other privileged mRNAs (Atf4, Chop), Pdx1 mRNA remained partitioned in actively translating polyribosomes during the UPR, whereas the mRNA encoding a proinsulin processing enzyme (Cpe) partitioned into inactively translating monoribosomes. Bicistronic luciferase reporter analyses revealed that the distal portion of the 5’ untranslated region of mouse Pdx1 (between bp –105 to –280) contained elements that promoted translation under both normal and UPR conditions. In contrast to regulation of translation initiation, deoxyhypusine synthase (DHS) and eukaryotic translation initiation factor 5A (eIF5A) are required for efficient translation elongation of specific stress relevant messages in the beta cell including Nos2. Further, p38 signaling appears to promote translational elongation via DHS in the islet beta cell. Together, these data represent new insights into stress induced translational regulation in the beta cell. Mechanisms of differential mRNA translation in response to beta cell stress may play a key role in maintenance of islet beta cell function in the setting of diabetes.