Characterization of Signaling Pathways Associated with Pancreatic β-cell Adaptive Flexibility in Compensation of Obesity-linked Diabetes in db/db Mice*

An unbiased quantitative profiling of the protein expression, phosphorylation, and sialylation events that occur upon β-cell adaptive flexibility during the transition from hyperglycemia to euglycemia was assessed in islets from db/db and WT mice before and after β-cell rest. Our PTMomics data revealed new adaptive proteins, phosphosites, and SA N-linked glycosites involved in the regulation of proinsulin biosynthesis translation, protein folding, not ER stress, proinsulin processing, β-granule biogenesis, vesicular trafficking and the insulin secretory pathway, protein degradation, and endopeptidase activity, associated with β-cell adaptive flexibility. Graphical Abstract Highlights db/db β-cells restores appropriate insulin stores and normalize secretory function. Numerous changes in the phosphorylation and sialylation states by euglycemic rest. Restoration of numerous dysfunctional biological processes following euglycemic rest. β-cell adaptive flexibility may lead to improvement in endogenous β-cell function. The onset of obesity-linked type 2 diabetes (T2D) is marked by an eventual failure in pancreatic β-cell function and mass that is no longer able to compensate for the inherent insulin resistance and increased metabolic load intrinsic to obesity. However, in a commonly used model of T2D, the db/db mouse, β-cells have an inbuilt adaptive flexibility enabling them to effectively adjust insulin production rates relative to the metabolic demand. Pancreatic β-cells from these animals have markedly reduced intracellular insulin stores, yet high rates of (pro)insulin secretion, together with a substantial increase in proinsulin biosynthesis highlighted by expanded rough endoplasmic reticulum and Golgi apparatus. However, when the metabolic overload and/or hyperglycemia is normalized, β-cells from db/db mice quickly restore their insulin stores and normalize secretory function. This demonstrates the β-cell's adaptive flexibility and indicates that therapeutic approaches applied to encourage β-cell rest are capable of restoring endogenous β-cell function. However, mechanisms that regulate β-cell adaptive flexibility are essentially unknown. To gain deeper mechanistic insight into the molecular events underlying β-cell adaptive flexibility in db/db β-cells, we conducted a combined proteomic and post-translational modification specific proteomic (PTMomics) approach on islets from db/db mice and wild-type controls (WT) with or without prior exposure to normal glucose levels. We identified differential modifications of proteins involved in redox homeostasis, protein refolding, K48-linked deubiquitination, mRNA/protein export, focal adhesion, ERK1/2 signaling, and renin-angiotensin-aldosterone signaling, as well as sialyltransferase activity, associated with β-cell adaptive flexibility. These proteins are all related to proinsulin biosynthesis and processing, maturation of insulin secretory granules, and vesicular trafficking—core pathways involved in the adaptation of insulin production to meet metabolic demand. Collectively, this study outlines a novel and comprehensive global PTMome signaling map that highlights important molecular mechanisms related to the adaptive flexibility of β-cell function, providing improved insight into disease pathogenesis of T2D.