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2.  UCP2 Regulates the Glucagon Response to Fasting and Starvation 
Diabetes  2013;62(5):1623-1633.
Glucagon is important for maintaining euglycemia during fasting/starvation, and abnormal glucagon secretion is associated with type 1 and type 2 diabetes; however, the mechanisms of hypoglycemia-induced glucagon secretion are poorly understood. We previously demonstrated that global deletion of mitochondrial uncoupling protein 2 (UCP2−/−) in mice impaired glucagon secretion from isolated islets. Therefore, UCP2 may contribute to the regulation of hypoglycemia-induced glucagon secretion, which is supported by our current finding that UCP2 expression is increased in nutrient-deprived murine and human islets. Further to this, we created α-cell–specific UCP2 knockout (UCP2AKO) mice, which we used to demonstrate that blood glucose recovery in response to hypoglycemia is impaired owing to attenuated glucagon secretion. UCP2-deleted α-cells have higher levels of intracellular reactive oxygen species (ROS) due to enhanced mitochondrial coupling, which translated into defective stimulus/secretion coupling. The effects of UCP2 deletion were mimicked by the UCP2 inhibitor genipin on both murine and human islets and also by application of exogenous ROS, confirming that changes in oxidative status and electrical activity directly reduce glucagon secretion. Therefore, α-cell UCP2 deletion perturbs the fasting/hypoglycemic glucagon response and shows that UCP2 is necessary for normal α-cell glucose sensing and the maintenance of euglycemia.
PMCID: PMC3636632  PMID: 23434936
3.  Correction: A Novel High-Throughput Assay for Islet Respiration Reveals Uncoupling of Rodent and Human Islets 
PLoS ONE  2013;8(12):10.1371/annotation/f21efac9-2906-4a2f-ad22-befcb7714dd0.
PMCID: PMC3855804
4.  β-Cell Uncoupling Protein 2 Regulates Reactive Oxygen Species Production, Which Influences Both Insulin and Glucagon Secretion 
Diabetes  2011;60(11):2710-2719.
The role of uncoupling protein 2 (UCP2) in pancreatic β-cells is highly debated, partly because of the broad tissue distribution of UCP2 and thus limitations of whole-body UCP2 knockout mouse models. To investigate the function of UCP2 in the β-cell, β-cell–specific UCP2 knockout mice (UCP2BKO) were generated and characterized.
UCP2BKO mice were generated by crossing loxUCP2 mice with mice expressing rat insulin promoter-driven Cre recombinase. Several in vitro and in vivo parameters were measured, including respiration rate, mitochondrial membrane potential, islet ATP content, reactive oxygen species (ROS) levels, glucose-stimulated insulin secretion (GSIS), glucagon secretion, glucose and insulin tolerance, and plasma hormone levels.
UCP2BKO β-cells displayed mildly increased glucose-induced mitochondrial membrane hyperpolarization but unchanged rates of uncoupled respiration and islet ATP content. UCP2BKO islets had elevated intracellular ROS levels that associated with enhanced GSIS. Surprisingly, UCP2BKO mice were glucose-intolerant, showing greater α-cell area, higher islet glucagon content, and aberrant ROS-dependent glucagon secretion under high glucose conditions.
Using a novel β-cell–specific UCP2KO mouse model, we have shed light on UCP2 function in primary β-cells. UCP2 does not behave as a classical metabolic uncoupler in the β-cell, but has a more prominent role in the regulation of intracellular ROS levels that contribute to GSIS amplification. In addition, β-cell UCP2 contributes to the regulation of intraislet ROS signals that mediate changes in α-cell morphology and glucagon secretion.
PMCID: PMC3198081  PMID: 21984579
5.  A Novel High-Throughput Assay for Islet Respiration Reveals Uncoupling of Rodent and Human Islets 
PLoS ONE  2012;7(5):e33023.
The pancreatic beta cell is unique in its response to nutrient by increased fuel oxidation. Recent studies have demonstrated that oxygen consumption rate (OCR) may be a valuable predictor of islet quality and long term nutrient responsiveness. To date, high-throughput and user-friendly assays for islet respiration are lacking. The aim of this study was to develop such an assay and to examine bioenergetic efficiency of rodent and human islets.
Methodology/Principal Findings
The XF24 respirometer platform was adapted to islets by the development of a 24-well plate specifically designed to confine islets. The islet plate generated data with low inter-well variability and enabled stable measurement of oxygen consumption for hours. The F1F0 ATP synthase blocker oligomycin was used to assess uncoupling while rotenone together with myxothiazol/antimycin was used to measure the level of non-mitochondrial respiration. The use of oligomycin in islets was validated by reversing its effect in the presence of the uncoupler FCCP. Respiratory leak averaged to 59% and 49% of basal OCR in islets from C57Bl6/J and FVB/N mice, respectively. In comparison, respiratory leak of INS-1 cells and C2C12 myotubes was measured to 38% and 23% respectively. Islets from a cohort of human donors showed a respiratory leak of 38%, significantly lower than mouse islets.
The assay for islet respiration presented here provides a novel tool that can be used to study islet mitochondrial function in a relatively high-throughput manner. The data obtained in this study shows that rodent islets are less bioenergetically efficient than human islets as well as INS1 cells.
PMCID: PMC3351473  PMID: 22606219
6.  Nobiletin Attenuates VLDL Overproduction, Dyslipidemia, and Atherosclerosis in Mice With Diet-Induced Insulin Resistance 
Diabetes  2011;60(5):1446-1457.
Increased plasma concentrations of apolipoprotein B100 often present in patients with insulin resistance and confer increased risk for the development of atherosclerosis. Naturally occurring polyphenolic compounds including flavonoids have antiatherogenic properties. The aim of the current study was to evaluate the effect of the polymethoxylated flavonoid nobiletin on lipoprotein secretion in cultured human hepatoma cells (HepG2) and in a mouse model of insulin resistance and atherosclerosis.
Lipoprotein secretion was determined in HepG2 cells incubated with nobiletin or insulin. mRNA abundance was evaluated by quantitative real-time PCR, and Western blotting was used to demonstrate activation of cell signaling pathways. In LDL receptor–deficient mice (Ldlr−/−) fed a Western diet supplemented with nobiletin, metabolic parameters, gene expression, fatty acid oxidation, glucose homeostasis, and energy expenditure were documented. Atherosclerosis was quantitated by histological analysis.
In HepG2 cells, activation of mitogen-activated protein kinase-extracellular signal–related kinase signaling by nobiletin or insulin increased LDLR and decreased MTP and DGAT1/2 mRNA, resulting in marked inhibition of apoB100 secretion. Nobiletin, unlike insulin, did not induce phosphorylation of the insulin receptor or insulin receptor substrate-1 and did not stimulate lipogenesis. In fat-fed Ldlr−/− mice, nobiletin attenuated dyslipidemia through a reduction in VLDL-triglyceride (TG) secretion. Nobiletin prevented hepatic TG accumulation, increased expression of Pgc1α and Cpt1α, and enhanced fatty acid β-oxidation. Nobiletin did not activate any peroxisome proliferator–activated receptor (PPAR), indicating that the metabolic effects were PPAR independent. Nobiletin increased hepatic and peripheral insulin sensitivity and glucose tolerance and dramatically attenuated atherosclerosis in the aortic sinus.
Nobiletin provides insight into treatments for dyslipidemia and atherosclerosis associated with insulin-resistant states.
PMCID: PMC3292317  PMID: 21471511
7.  Molecular and Metabolic Evidence for Mitochondrial Defects Associated With β-Cell Dysfunction in a Mouse Model of Type 2 Diabetes 
Diabetes  2009;59(2):448-459.
The inability of pancreatic β-cells to appropriately respond to glucose and secrete insulin are primary defects associated with β-cell failure in type 2 diabetes. Mitochondrial dysfunction has been implicated as a key factor in the development of type 2 diabetes; however, a link between mitochondrial dysfunction and defective insulin secretion is unclear.
We investigated the changes in islet mitochondrial function and morphology during progression from insulin resistance (3 weeks old), immediately before hyperglycemia (5 weeks old), and after diabetes onset (10 weeks old) in transgenic MKR mice compared with controls. The molecular and protein changes at 10 weeks were determined using microarray and iTRAQ proteomic screens.
At 3 weeks, MKR mice were hyperinsulinemic but normoglycemic and β-cells showed negligible mitochondrial or morphological changes. At 5 weeks, MKR islets displayed abrogated hyperpolarization of mitochondrial membrane potential (ΔΨm), reduced mitochondrial Ca2+ uptake, slightly enlarged mitochondria, and reduced glucose-stimulated insulin secretion. By 10 weeks, MKR mice were hyperglycemic and hyperinsulinemic and β-cells contained swollen mitochondria with disordered cristae. β-Cells displayed impaired stimulus-secretion coupling including reduced hyperpolarization of ΔΨm, impaired Ca2+-signaling, and reduced glucose-stimulated ATP/ADP and insulin release. Furthermore, decreased cytochrome c oxidase–dependent oxygen consumption and signs of oxidative stress were observed in diabetic islets. Protein profiling of diabetic islets revealed that 36 mitochondrial proteins were differentially expressed, including inner membrane proteins of the electron transport chain.
We provide novel evidence for a critical role of defective mitochondrial oxidative phosphorylation and morphology in the pathology of insulin resistance–induced β-cell failure.
PMCID: PMC2809957  PMID: 19903739
8.  Naringenin Prevents Dyslipidemia, Apolipoprotein B Overproduction, and Hyperinsulinemia in LDL Receptor–Null Mice With Diet-Induced Insulin Resistance 
Diabetes  2009;58(10):2198-2210.
The global epidemic of metabolic syndrome and its complications demands rapid evaluation of new and accessible interventions. Insulin resistance is the central biochemical disturbance in the metabolic syndrome. The citrus-derived flavonoid, naringenin, has lipid-lowering properties and inhibits VLDL secretion from cultured hepatocytes in a manner resembling insulin. We evaluated whether naringenin regulates lipoprotein production and insulin sensitivity in the context of insulin resistance in vivo.
LDL receptor–null (Ldlr−/−) mice fed a high-fat (Western) diet (42% calories from fat and 0.05% cholesterol) become dyslipidemic, insulin and glucose intolerant, and obese. Four groups of mice (standard diet, Western, and Western plus 1% or 3% wt/wt naringenin) were fed ad libitum for 4 weeks. VLDL production and parameters of insulin and glucose tolerance were determined.
We report that naringenin treatment of Ldlr−/− mice fed a Western diet corrected VLDL overproduction, ameliorated hepatic steatosis, and attenuated dyslipidemia without affecting caloric intake or fat absorption. Naringenin 1) increased hepatic fatty acid oxidation through a peroxisome proliferator–activated receptor (PPAR) γ coactivator 1α/PPARα-mediated transcription program; 2) prevented sterol regulatory element–binding protein 1c–mediated lipogenesis in both liver and muscle by reducing fasting hyperinsulinemia; 3) decreased hepatic cholesterol and cholesterol ester synthesis; 4) reduced both VLDL-derived and endogenously synthesized fatty acids, preventing muscle triglyceride accumulation; and 5) improved overall insulin sensitivity and glucose tolerance.
Thus, naringenin, through its correction of many of the metabolic disturbances linked to insulin resistance, represents a promising therapeutic approach for metabolic syndrome.
PMCID: PMC2750228  PMID: 19592617

Results 1-8 (8)