Nutrient metabolism regulates insulin granule formation in the pancreatic islet β-cell via ER redox homeostasis

Abstract

Defects in the β-cells secretion system are well-described in Type 2 diabetes (T2D), including reduced insulin storage and impaired proinsulin processing; however, the cellular mechanisms underlying these secretory defects and the contribution of chronic hyperglycemia to this process remain poorly understood. In this study, we provide evidence that oxidative protein folding in the endoplasmic reticulum (ER) is perturbed in models of β-cell dysfunction, leading to delays in proinsulin trafficking and insulin granule formation. Using an in situ fluorescent pulse-chase labeling strategy and APEX2-based proximity labeling, we demonstrate that enriched interactions of proinsulin with ER oxidoreductases coincides with a delay in proinsulin ER export. Furthermore, our data show that proinsulin ER export can be regulated by metabolically-derived NADPH and reducing equivalent (glutathione) availability. Together, these data highlight an emerging role for nutrient metabolism and mitochondrial dysfunction in the maladaptive remodeling of the β-cells secretory pathway during the decline of β-cell function in T2D.

Nutrient Regulation of Pancreatic Islet β-Cell Secretory Capacity and Insulin Production

“…ATP and other metabolic signals generated by glucose oxidation directly trigger and amplify insulin granule exocytosis through membrane depolarization and insulin granule recruitment [ 48 , 49 , 50 ]. In addition, the mitochondrial metabolism of glucose and other fuels can directly enhance preproinsulin mRNA translation [ 13 , 26 , 27 ], while metabolism-derived redox shuttles support disulfide bond formation and isomerization to ensure successful folding of nascent proinsulin polypeptides [ 122 , 129 ]. Similarly, Golgi export functions may be dependent on nutrient signals that can either accelerate or pause the assembly of secretory granule cargo into the budding granule to favor an excess of insulin granule production relative to insulin release [ 189 , 191 ].…”

“…Thus, in addition to metabolic regulation of insulin secretion, the oxidative protein folding capacity of the β-cell’s ER may also be metabolically responsive and rely on reducing equivlaents to support increased proinsulin biosynthesis ( Figure 4 ). Indeed, the loss of the primary NADPH-producing enzyme in β-cells, cytosolic isocitrate dehydrogenase 1 (Idh1), not only decreases available GSH and impairs insulin secretion [ 11 , 55 , 57 ] but also diminishes insulin granule formation [ 129 ]. While it remains to be determined if Idh1 loss results in a direct effect on ER oxidative protein folding capacity, proinsulin trafficking and insulin granule formation can be restored by the addition of cellular reducing equivalents.…”

“…Mitochondrial dysfunction resulting in alterations to glucose metabolism, including decreased NADPH and GSH production, are hallmarks of T2D β-cells [ 29 , 35 , 55 ], supporting the idea that the availability of redox equivalents may be limited by impaired metabolic capacity and/or increased usage for neutralizing reactive oxygen species. In contrast, antioxidant treatment using butylated hydroxyanisole or supplementation with reducing agents can improve proinsulin folding and promote insulin granule formation in diabetes models [ 91 , 114 , 129 ]. Thus, while the hyperglycemia observed in T2D β-cells may signal increased proinsulin synthesis, metabolic dysfunction fails to provide the necessary redox shuttles to support ER functions.…”

“…Multiple studies in various cell types, including β-cells, PC12 pheochromocytomas, and adrenal chromaffin cells, showed that VGF directly interacts with CgB [ 75 , 153 ]. Interactions between proinsulin, VGF, and CgB were also recently demonstrated through ascorbate peroxidase 2 (APEX2) proximity labeling [ 129 ]. Based on the recently proposed model of liquid phase separation for protein segregation in the TGN, highly disordered regions of CgB may function as a “cargo sponge” through multivalent interactions with client proteins, such as proinsulin, and promote phase separation from other soluble proteins not destined for the insulin secretory granule [ 149 , 151 ].…”

Pathways Linking Nicotinamide Adenine Dinucleotide Phosphate Production to Endoplasmic Reticulum Protein Oxidation and Stress

“…Thus, beta cells might have a greater requirement for ER reducing power. In support of this idea, proinsulin maturation was impaired in beta cells with a hyperoxidizing ER ( Rohli et al, 2021 ). Therefore, it is conceivable that different cell types, such as hepatocytes and beta cells, have distinct needs for oxidation versus reduction, dependent not only on the types of proteins that a given cell type has to secrete, but also on cellular conditions such as nutrient fluxes that impact the relative composition of the secretome at any given time.…”