Insulin resistance and β cell dysfunction contribute to the pathogenesis of type 2 diabetes. Unlike insulin resistance, β cell dysfunction remains difficult to predict and monitor, because of the inaccessibility of the endocrine pancreas, the integrated relationship with insulin sensitivity, and the paracrine effects of incretins. The goal of our study was to survey the plasma response to a metabolic challenge in order to identify factors predictive of β cell dysfunction. To this end, we combined (i) the power of unbiased iTRAQ (isobaric tag for relative and absolute quantification) mass spectrometry with (ii) direct sampling of the portal vein following an intravenous glucose/arginine challenge (IVGATT) in (iii) mice with a genetic β cell defect. By so doing, we excluded the effects of peripheral insulin sensitivity as well as those of incretins on β cells, and focused on the first phase of insulin secretion to capture the early pathophysiology of β cell dysfunction. We compared plasma protein profiles with ex vivo islet secretome and transcriptome analyses. We detected changes to 418 plasma proteins in vivo, and detected changes to 262 proteins ex vivo The impairment of insulin secretion was associated with greater overall changes in the plasma response to IVGATT, possibly reflecting metabolic instability. Reduced levels of proteins regulating redox state and neuronal stress markers, as well as increased levels of coagulation factors, antedated the loss of insulin secretion in diabetic mice. These results suggest that a reduced complement of antioxidants in response to a mixed secretagogue challenge is an early correlate of future β cell failure.

Objective

Diabetes is characterized by pancreatic β-cell dedifferentiation. Dedifferentiating β cells inappropriately metabolize lipids over carbohydrates and exhibit impaired mitochondrial oxidative phosphorylation. However, the mechanism linking the β-cell's response to an adverse metabolic environment with impaired mitochondrial function remains unclear.

Methods

Here we report that the oxidoreductase cytochrome b5 reductase 3 (Cyb5r3) links FoxO1 signaling to β-cell stimulus/secretion coupling by regulating mitochondrial function, reactive oxygen species generation, and nicotinamide actin dysfunction (NAD)/reduced nicotinamide actin dysfunction (NADH) ratios.

Results

The expression of Cyb5r3 is decreased in FoxO1-deficient β cells. Mice with β-cell-specific deletion of Cyb5r3 have impaired insulin secretion, resulting in glucose intolerance and diet-induced hyperglycemia. Cyb5r3-deficient β cells have a blunted respiratory response to glucose and display extensive mitochondrial and secretory granule abnormalities, consistent with altered differentiation. Moreover, FoxO1 is unable to maintain expression of key differentiation markers in Cyb5r3-deficient β cells, suggesting that Cyb5r3 is required for FoxO1-dependent lineage stability.

Conclusions

The findings highlight a pathway linking FoxO1 to mitochondrial dysfunction that can mediate β-cell failure.