Recent studies demonstrate that oxidative protein folding in the ER is regulated via a metabolic redox relay through the supply of NAPDH, glutathione, and thioredoxin [23,50,51,52]. As previously discussed, thioredoxin and glutathione can assist ER resident PDIs in forming native disulfide bonds and reducing non-native bonds, which has been referred to as redox buffering [23]. Importantly, recycling oxidized glutathione and thioredoxin to their reduced states is catalyzed by cytosolic NADPH-dependent reductases, glutathione reductase (GSR1) and TXNRD1, respectively. In turn, the terminal reductant, NADPH, has been shown by multiple studies to precisely fluctuate with changes in β-cell glucose metabolism [66,67,68,69]. These observations directly link metabolic activity in the β-cell with available redox donors to support ER oxidative protein folding.

Several enzymes contribute to NADPH reduction in β-cells, including cytosolic isocitrate dehydrogenase-1 (IDH1), mitochondrial IDH2, and glucose-6-phosphate dehydrogenase (G6PDH). These enzymes are required for insulin secretion, but not mitochondrial function [69,70,71,72,73]. Additional NADPH-generating enzymes involved in one-carbon folate metabolism can also influence β-cell function, including methylenetetrahydrofolate dehydrogenase 2 (MTHFD2) and aldehyde dehydrogenase 1 family member L2 (ALDH1L2), yet their contribution to the NADPH pool is less clear [74]. While malic enzymes (four isoforms) can also generate NADPH, their loss does not affect glucose homeostasis or insulin secretion [75], suggesting only a minor role in β-cell function.

In support of the metabolic redox relay, suppressing NADPH cycling in mouse islets via IDH1 knockdown disrupted ER redox and impaired proinsulin trafficking [23]. This impairment was reversible using the reducing agent, DTT, highlighting the importance of NADPH-derived reductants in ER redox homeostasis [23]. Furthermore, the mitochondrial-directed antioxidant, MitoQ, restored NADPH cycling and ER redox balance in diabetic β-cell models [23]. While the role of other NADPH-producing enzymes discussed above in β-cell ER redox control remains uncertain, IDH2 suppression in hepatocytes has also been linked to ER redox defects [51].

The pathways linking metabolic and ER functions in β-cells are important for three reasons. First, glucose metabolism is primarily directed through mitochondrial oxidative phosphorylation to generate adenosine trisphosphate (ATP) along with signaling intermediates known to regulate insulin exocytosis [2]. Absence of lactate dehydrogenases (LDH A, B, C) and the monocarboxylate carrier (SLC16A1) to export lactate prevents the β-cell from utilizing anaerobic glycolysis as an alternate mechanism for energy and substrate production [76]. As a result, β-cells depend on mitochondria for both ATP production and maintaining cellular redox balance. Second, the β-cell is uniquely adapted to sense physiological fluctuations in glucose because of the K_m of the glucose transporters, GLUT1 and GLUT2, and glucokinase, which operate within the physiological range of blood glucose, 4.4–6.1 mM [2,77,78]. Consequently, β-cell metabolism, including redox cycles, oscillates with increased activity during the fed state and decreased activity in the fasted state [66,67,68,69]. Moreover, glucose metabolism not only triggers insulin secretion, but also stimulates INS mRNA translation [79,80,81]. Thus, the regulation of multiple tiers of the insulin biosynthetic and exocytic pathways are efficiently coordinated by metabolic cycles [1]. Finally, mitochondrial dysfunction and declined secretory capacity are hallmarks of β-cell dysfunction in T2D; however, their mechanistic connection has only recently been established [23]. Depletion of cellular ATP stores can certainly strain ER functions (e.g., protein folding, Ca²⁺ uptake, ERAD), but in addition, redox imbalances also adversely affect oxidative protein folding (e.g., disulfide bond formation). Indeed, restoring ER redox balance with exogenous reductants or hydrogen peroxide scavengers can promote proinsulin trafficking even when mitochondrial function is compromised [23]. Similarly, glucokinase inhibition rapidly led to ER hyperoxidation, which can be reversed by supplementation with cellular reductant [23]. Thus, loss of mitochondrial function and the concomitant depletion of the redox relay can negatively impact proinsulin folding in the ER and thereby limit insulin production.