Eukaryotic translation initiation factor 2A protects pancreatic beta cells during endoplasmic reticulum stress while rescuing translation inhibition

Abstract

The endoplasmic reticulum (ER) stress-induced unfolded protein response (UPR) helps decide cell survival in diabetes. The alternative eukaryotic initiation factor 2A (EIF2A) has been proposed to mediate EIF2S1-independent translation during cellular stress and viral infection, but its role in cells is unknown. EIF2A abundance is high in human and mouse islets relative to other tissues, and both thapsigargin and palmitate significantly increased EIF2A mRNA and EIF2A protein levels in MIN6 cells, mouse islets and human islets. Knockdowns of EIF2A, the related factor EIF2D, or both EIF2A and EIF2D, were not sufficient to cause apoptosis. On the other hand, transient or stable EIF2A over-expression protected MIN6 cells, primary mouse islets, and human islets from ER stress-induced, caspase-3-dependent apoptosis. Mechanistically, EIF2A overexpression decreased ERN1 (also known as IRE1) expression in thapsigargin-treated MIN6 cells or human islets. In vivo, cell specific EIF2A viral overexpression reduced ER stress, improved insulin secretion, and abrogated hyperglycemia in Ins2Akita/WT mice. EIF2A overexpression significantly increased expression of genes involved in protein translation and reduced expression of pro-apoptotic genes (e.g. ALDH1A3). Remarkably, the decrease in global protein synthesis during UPR was prevented by EIF2A, despite ER stress-induced EIF2S1 phosphorylation. The protective effects of EIF2A were additive to those of ISRIB, a drug that counteracts the effects of EIF2S1 phosphorylation. Cells overexpressing EIF2A showed higher expression of translation factor EIF2B5, which may contribute to the lack of translational inhibition in these cells. We conclude that EIF2A is a novel target for cell protection and the circumvention of EIF2S1-mediated translational repression.

Sex differences in islet stress responses support female β cell resilience

“…While estrogen affects insulin biosynthesis via ERα [ 117 ], future studies will need to determine whether estrogen also allows female islets to restore protein synthesis to basal levels faster than male islets following ER stress. We currently lack this knowledge, as most studies on UPR-mediated recovery from protein translation repression use single- and mixed-sex animal groups, or cultured cells [ [119] , [120] , [121] , [122] , [123] , [124] ].…”

Sex differences in islet stress responses support female beta cell resilience

“…Ideally, this type of study would also monitor the activity of pathways known to regulate protein synthesis repression during ER stress. For example, while we did not detect any changes in levels of phosphorylated eIF2α (also known as Eif2s1), which is known to mediate UPR-induced protein synthesis repression (75), our chosen timepoints did not overlap with the rapid changes in phospho-eIF2α following ER stress published in other studies (100,101). A more detailed time course will therefore be necessary to assess p-eIF2α levels during ER stress in both sexes, and to test a role for phospho-eIF2α in mediating differences in protein synthesis repression.…”

To initiate or not to initiate: A critical assessment of eIF2A, eIF2D, and MCT‐1·DENR to deliver initiator tRNA to ribosomes

“…Uncharacteristic of a translation initiation factor, several studies have provided evidence that eIF2A is non-cytosolic ( Kim et al, 2011 ; Panzhinskiy et al, 2021 ; Thul et al, 2017 ; Uhlen et al, 2010 ); however, no two reports are in agreement. Unfortunately, eIF2A is not included in the Yeast GFP Fusion Localization Database ( Huh et al, 2003 ).…”

“…To our knowledge, whether this localization pattern is due to an alternate function of eIF2A, such as ribosome assembly, is not reported. Second, Panzhinskiy et al demonstrated that eIF2A co-localizes with the endoplasmic reticulum (ER) in MIN6 cells during normal physiological conditions ( Panzhinskiy et al, 2021 ). ER localization during normal conditions is uncharacteristic of a translation initiation factor since translation initiation occurs in the cytosol.…”

“…Whether eIF2A shuttling occurs in other cell types and the mechanism was not reported. Enigmatically, eIF2A has been reported to also co-localize with the ER in MIN6 cells ( Panzhinskiy et al, 2021 ) and have localization pattern resembling mitochondria in U2OS and A-431 cells. It is not clear how eIF2A could function in translation initiation if localized or sequestered away from the translation machinery in the cytosol.…”