Metabolic regulation of ER redox homeostasis maintains insulin granule formation in pancreatic β-cells
Creators: name: Rohli, Kristen Elizabeth
Publisher: name: University of Iowa
Year: 2025
Subjects: beta cell function
endoplasmic reticulum
mitochondria
redox
Insulin
Metabolism
Endocrinology

description: Pancreatic β-cells respond to glucose uptake by secreting the hormone insulin to control glycemia. However, over time β-cells lose the ability to secrete sufficient insulin to maintain blood glucose homeostasis. While the exact reason β-cells fail is not known, many factors including both genetic and environmental risk may contribute to this downfall of β-cell function. Prior to β-cell failure, β-cells attempt to compensate for hyperglycemia by secreting higher amounts of insulin to combat insulin resistance. For a subset of humans, β-cells cannot maintain the high rate of insulin secretion, and β-cell function declines. The triggering event initiating the transition from β-cell compensation to β-cell failure is not known but could offer new insights into treatment strategies to preserve β-cell health.
description: There are two well-described defects within T2D β-cells, including secretory dysfunction and decreased mitochondrial bioenergetics. Defects in the pancreatic β-cell’s secretion system include impaired proinsulin processing and a decrease in mature insulin-containing secretory granules; however, the cellular mechanisms underlying these defects remain poorly understood. To address this, we used an in situ fluorescent pulse-chase strategy called SNAPtag to investigate proinsulin trafficking in live β-cells. Using a rodent dietary model and cell culture model, we show that insulin granule formation is decreased under pre-diabetic and hyperglycemic conditions. Moreover, we link the defect in insulin granule formation to an early trafficking delay in endoplasmic reticulum (ER) export of proinsulin in a rodent genetic model of hyperglycemia. Using a ratiometric redox sensor, we show that the ER becomes hyperoxidized in β-cells from cell culture, rodent, and human models of hyperglycemia, and consequently, proinsulin ER export is delayed. However, addition of antioxidants restores ER export of proinsulin and insulin granule formation and partially restores β-cell function.
description: To understand the mechanism behind how ER function becomes compromised in T2D, we utilized redox sensors that to observe the flux of glutathione and NADPH, which play vital roles in cellular redox maintenance. Through these studies, we have identified glucose metabolism as a critical determinant in the redox homeostasis of the ER through the generation of redox donors. Using a β-cell-specific genetic rodent model of decreased mitochondrial bioenergetics, we show that loss of mitochondrial function correlates with diminished NADPH and glutathione flux, hyperoxidation of the ER lumen, delayed ER exit of proinsulin, and decreased insulin granule formation. Similarly, acute inhibition of mitochondrial metabolism in non-diabetic rodent β-cells, through treatment with mannoheptulose, decreases redox flux, hyperoxidizes the ER, and compromises proinsulin trafficking. Supplementing either model with antioxidants restores proinsulin maturation and partially increases β-cell function.
description: Our data demonstrate that β-cell ER redox homeostasis is supported by the metabolic supply of reductive redox donors. Limiting NADPH through genetic knockdown of cytosolic Idh1 disrupts insulin maturation in a similar manner to the models of mitochondrial dysfunction and display improved insulin granule formation following antioxidant treatment. Using shRNA-targeted knockdown of Thioredoxin reductase-1, we show that disruption of thioredoxin redox flux delays ER proinsulin export, whereas Txnip suppression restores ER redox and proinsulin trafficking. Taken together, we propose that β-cell ER redox homeostasis is buffered by cellular redox donor cycles, which are maintained through active glucose metabolism. Together, these data suggest a novel mechanism linking mitochondrial dysfunction and decreased secretory output through alterations in ER redox poise in T2D.