Adaptation to increased insulin demand is mediated by β-cell proliferation and neogenesis among other mechanisms. Although it is known that pancreatic β-cells can arise from ductal progenitors, these observations have been limited mostly to the neonatal period. We have recently reported that the duct is a source of insulin secreting cells in adult insulin resistant states. To further explore the signaling pathways underlying the dynamic β-cell reserve during insulin resistance we undertook human islet and duct transplantations under the kidney capsule of immunodeficient NOD SCID gamma (NSG) mouse models that were either pregnant, insulin resistant or had insulin resistance superimposed upon pregnancy (pregnancy+insulin resistance), followed by single-nucleus RNA-sequencing (snRNA-seq) on snap-frozen graft samples. We observed an upregulation of proliferation markers (e.g., NEAT1), expression of islet endocrine cell markers (e.g., GCG and PPY) as well as mature β-cell markers (e.g., INS), in transplanted human duct grafts in response to high insulin demand. We also noted downregulation of ductal cell identity genes (e.g., KRT19 and ONECUT2) coupled with upregulation of β-cell development and insulin signaling pathways. These results indicate that subsets of ductal cells are able to gain β-cell identity and reflect a form of compensation during the adaptation to insulin resistance in both physiological and pathological states.

Diabetic patients show elevated plasma IL18 concentrations. IL18 has two receptors: the IL18 receptor (IL18r) and the Na-Cl co-transporter (NCC). Here, we report that IL18 is expressed on islet α cells, NCC on β cells, and IL18r on acinar cells in human and mouse pancreases. The deficiency of these receptors reduces islet size, β cell proliferation, and insulin secretion but increases β cell apoptosis and exocrine macrophage accumulation after diet-induced glucose intolerance or streptozotocin-induced hyperglycemia. Together with the glucagon-like peptide-1 (GLP1), IL18 uses the NCC and GLP1 receptors on β cells to trigger β cell development and insulin secretion. IL18 also uses the IL18r on acinar cells to block hyperglycemic pancreas macrophage expansion. The β cell-selective depletion of the NCC or acinar-cell-selective IL18r depletion reduces glucose tolerance and insulin sensitivity with impaired β cell proliferation, enhanced β cell apoptosis and macrophage expansion, and inflammation in mouse hyperglycemic pancreas. IL18 uses NCC, GLP1r, and IL18r to maintain islet β cell function and homeostasis.

Uncoupling protein 2 (UCP2), a mitochondrial protein, is known to be upregulated in pancreatic islets of patients with type 2 diabetes (T2DM); however, the pathological significance of this increase in UCP2 expression is unclear. In this study, we highlight the molecular link between the increase in UCP2 expression in β-cells and β-cell failure by using genetically engineered mice and human islets. β-cell-specific UCP2-overexpressing transgenic mice (βUCP2Tg) exhibited glucose intolerance and a reduction in insulin secretion. Decreased mitochondrial function and increased aldolase B (AldB) expression through oxidative-stress-mediated pathway were observed in βUCP2Tg islets. AldB, a glycolytic enzyme, was associated with reduced insulin secretion via mitochondrial dysfunction and impaired calcium release from the endoplasmic reticulum (ER). Taken together, our findings provide a new mechanism of β-cell dysfunction by UCP2 and AldB. Targeting the UCP2/AldB axis is a promising approach for the recovery of β-cell function.

Inflammation has profound but poorly understood effects on metabolism, especially in the context of obesity and nonalcoholic fatty liver disease (NAFLD). Here, we report that hepatic interferon regulatory factor 3 (IRF3) is a direct transcriptional regulator of glucose homeostasis through induction of Ppp2r1b, a component of serine/threonine phosphatase PP2A, and subsequent suppression of glucose production. Global ablation of IRF3 in mice on a high-fat diet protected against both steatosis and dysglycemia, whereas hepatocyte-specific loss of IRF3 affects only dysglycemia. Integration of the IRF3-dependent transcriptome and cistrome in mouse hepatocytes identifies Ppp2r1b as a direct IRF3 target responsible for mediating its metabolic actions on glucose homeostasis. IRF3-mediated induction of Ppp2r1b amplified PP2A activity, with subsequent dephosphorylation of AMPKα and AKT. Furthermore, suppression of hepatic Irf3 expression with antisense oligonucleotides reversed obesity-induced insulin resistance and restored glucose homeostasis in obese mice. Obese humans with NAFLD displayed enhanced activation of liver IRF3, with reversion after bariatric surgery. Hepatic PPP2R1B expression correlated with HgbA1C and was elevated in obese humans with impaired fasting glucose. We therefore identify the hepatic IRF3-PPP2R1B axis as a causal link between obesity-induced inflammation and dysglycemia and suggest an approach for limiting the metabolic dysfunction accompanying obesity-associated NAFLD.

Growing evidence indicates an important link between gut microbiota, obesity, and metabolic syndrome. Alterations in exocrine pancreatic function are also widely present in patients with diabetes and obesity. To examine this interaction, C57BL/6J mice were fed either a chow diet, high-fat diet (HFD) or HFD plus oral vancomycin or metronidazole to modify the gut microbiome. HFD alone leads to a 40% increase in pancreas weight, decreased glucagon-like peptide-1 and peptide YY levels, and increased glucose-dependent insulinotropic peptide in the plasma. Quantitative proteomics identified 138 host proteins in fecal samples of these mice, of which 32 were significantly changed by HFD. The most significant of these were the pancreatic enzymes. These changes in amylase and elastase were reversed by antibiotic treatment. These alterations could be reproduced by transferring gut microbiota from donor C57BL/6J mice to germ-free. By contrast, antibiotics had no effect on pancreatic size or exocrine function in C57BL/6J mice fed a chow diet. Further, one week vancomycin administration significantly increased amylase and elastase levels in obese prediabetic men. Thus, the alterations in gut microbiota in obesity can alter pancreatic growth, exocrine function and gut endocrine function, and may contribute to the alterations observed in patients with obesity and diabetes.

AIMS: Insulin potentiates glucose-stimulated insulin secretion. These effects are attenuated in beta cell-specific insulin receptor knockout mice and insulin resistant humans. This investigation examines whether short duration insulin exposure regulates beta cell responsiveness to arginine, a non-glucose secretagogue, in healthy humans.

MATERIALS AND METHODS: Arginine-stimulated insulin secretion was studied in 10 healthy humans. In each subject arginine was administered as a bolus followed by continuous infusion on two occasions one month apart, after sham/saline or hyperinsulinemic-isoglycemic clamp, respectively providing low and high insulin pre-exposure conditions. Arginine-stimulated insulin secretion was measured by C-peptide deconvolution, and by a selective immunogenic (DAKO) assay for direct measurement of endogenous but not exogenous insulin.

RESULTS: Pre-exposure to exogenous insulin augmented arginine-stimulated insulin secretion. The effect was seen acutely following arginine bolus (endogenous DAKO insulin incremental AUC240-255min 311.6 ± 208.1 (post-insulin exposure) versus 120.6 ± 42.2 μU/ml•min (sham/saline) (t-test P = 0.021)), as well as in response to continuous arginine infusion (DAKO insulin incremental AUC260-290min 1095.3 ± 592.1 (sham/saline) versus 564.8 ± 207.1 μU/ml•min (high insulin)(P = 0.009)). Findings were similar when beta cell response was assessed using C-peptide, insulin secretion rates by deconvolution, and the C-peptide to glucose ratio.

CONCLUSIONS: We demonstrate a physiologic role of insulin in regulation of the beta cell secretory response to arginine.