Role of endogenous ROS production in impaired metabolism-secretion coupling of diabetic pancreatic β cells

Excessive Intake of High-Fructose Corn Syrup Drinks Induces Impaired Glucose Tolerance

The number of patients with diabetes was approximately 463 million worldwide in 2019, with almost 57.6% of this population concentrated in Asia. Asians often develop type 2 diabetes (T2D), even if they are underweight and consume a smaller amount of food. Soft drinks contain large amounts of sweeteners, such as high-fructose corn syrup (HFCS). Excessive intake of HFCS drinks is considered to be one of the causes of T2D. In the present study, we investigated the effect of excessive consumption of HFCS-water on glucose tolerance and obesity under conditions of controlled caloric intake using a mouse model. Three-week-old male ICR mice were divided into two groups and given free access to 10% HFCS-water or deionized water. The caloric intake was adjusted to be the same in both groups using a standard rodent diet. The excess HFCS-water intake did not lead to obesity, but led to impaired glucose tolerance (IGT) due to insulin-secretion defect. It affected glucose and fructose metabolism; for example, it decreased the expression of glucokinases, ketohexokinase, and glucose transporter 2 in the pancreas. These results suggest that excessive consumption of HFCS drinks, such as soft drinks, without a proper diet, induces nonobese IGT due to insulin-secretion defect.

Bitter melon fruit extract enhances intracellular ATP production and insulin secretion from rat pancreatic β-cells

Bitter melon (Momordica charantia L.) has been shown to have various health-promoting activities, including antidiabetic and hypoglycaemic effects. Improvement in insulin sensitivity and increase in glucose utilisation in peripheral tissues have been reported, but the effect on insulin secretion from pancreatic β-cells remains unclear. In this study, we investigated the effect of bitter melon fruit on insulin secretion from β-cells and the underlying mechanism. The green fruit of bitter melon was freeze-dried and extracted with methanol. The bitter melon fruit extract (BMFE) was fractionated using ethyl acetate (fraction A), n-butanol (fraction B) and water (fraction C). Insulin secretory capacity from INS-1 rat insulinoma cell line and rat pancreatic islets, as well as glucose tolerance in rats by oral glucose tolerance test (OGTT), was measured using BMFE and fractions. ATP production in β-cells was also examined. BMFE augmented insulin secretion from INS-1 cells in a dose-dependent manner. The significant augmentation of insulin secretion was independent of the glucose dose. Fraction A (i.e. hydrophobic fraction), but not fractions B and C, augmented insulin secretion significantly at the same level as that by BMFE. This finding was also observed in pancreatic islets. In OGTT, BMFE and fraction A decreased blood glucose levels and increased serum insulin levels after glucose loading. The decrease in blood glucose levels was also observed in streptozotocin-induced diabetic rats. In addition, BMFE and fraction A increased the ATP content in β-cells. We concluded that hydrophobic components of BMFE increase ATP production and augment insulin secretion from β-cells, consequently decreasing blood glucose levels.

GSTO1 regulates insulin biosynthesis in pancreatic β cells

Insulin biosynthesis and secretion by pancreatic β cells are critical for the maintenance of blood glucose homeostasis. Here, we show that the expression of glutathione S-transferase omega-1 (GSTO1) is upregulated in the primary islet cells of diabetic Goto-Kakizaki (GK) rats. Knocking out GSTO1 upregulated insulin transcripts and increased the insulin content in both INS-1 cells and primary islet cells. In contrast, overexpression of GSTO1 reduced the insulin content. Furthermore, knocking out GSTO1 increased the expression of pancreatic duodenal homeobox-1 (PDX1) at both the transcription and protein levels. These results indicate that GSTO1 may be involved in the regulation of insulin biosynthesis by modulating the transcriptional expression of PDX1.

Novel mechanism of impaired metabolism-secretion coupling in β-cells: Loss of cytosolic adenosine triphosphate by leakage

Zhang et al. recently proposed a new mechanism of metabolism‐secretion coupling impairment in diabetic β‐cells involving the loss of cytosolic adenosine triphosphate by leakage through plasma membrane. Hyperglycemia increases mistargeting expression of the adenosine triphosphate‐conducting mitochondrial outer membrane voltage‐dependent anion channel‐1 on the plasma membrane leading to adenosine triphosphate depletion. The interaction between reactive oxygen species overproduction and voltage‐dependent anion channel‐1 induction is an interesting issue to be resolved.

Skeletal muscle mitoflashes, pH, and the role of uncoupling protein-3

Mitochondrial reactive oxygen species (ROS) are important cellular signaling molecules, but can cause oxidative damage if not kept within tolerable limits. An important proximal form of ROS in mitochondria is superoxide. Its production is thought to occur in regulated stochastic bursts, but current methods using mitochondrial targeted cpYFP to assess superoxide flashes are confounded by changes in pH. Accordingly, these flashes are generally referred to as 'mitoflashes'. Here we provide regulatory insights into mitoflashes and pH fluctuations in skeletal muscle, and the role of uncoupling protein-3 (UCP3). Using quantitative confocal microscopy of mitoflashes in intact muscle fibers, we show that the mitoflash magnitude significantly correlates with the degree of mitochondrial inner membrane depolarization and ablation of UCP3 did not affect this correlation. We assessed the effects of the absence of UCP3 on mitoflash activity in intact skeletal muscle fibers, and found no effects on mitoflash frequency, amplitude or duration, with a slight reduction in the average size of mitoflashes. We further investigated the regulation of pH flashes (pHlashes, presumably a component of mitoflash) by UCP3 using mitochondrial targeted SypHer (mt-SypHer) in skeletal muscle fibers. The frequency of pHlashes was significantly reduced in the absence of UCP3, without changes in other flash properties. ROS scavenger, tiron, did not alter pHlash frequency in either WT or UCP3KO mice. High resolution respirometry revealed that in the absence of UCP3 there is impaired proton leak and Complex I-driven respiration and maximal coupled respiration. Total cellular production of hydrogen peroxide (H₂O₂) as detected by Amplex-UltraRed was unaffected. Altogether, we demonstrate a correlation between mitochondrial membrane potential and mitoflash magnitude in skeletal muscle fibers that is independent of UCP3, and a role for UCP3 in the control of pHlash frequency and of proton leak- and Complex I coupled-respiration in skeletal muscle fibers. The differential regulation of mitoflashes and pHlashes by UCP3 and tiron also indicate that the two events, though may be related, are not identical events.

KEAP1/NRF2 axis regulates H2O2-induced apoptosis of pancreatic β-cells

In human pancreatic β-cells, oxidative stress and cellular injures can be induced by H₂O₂ treatment. The KEAP1/NRF2 axis is a key antioxidant signaling pathway. The present study attempted to elucidate the mechanism by which the KEAP1/NRF2 axis mediates oxidative stress-induced death in pancreatic β-cells. Our data showed that H₂O₂ treatment obviously induced the apoptosis of β-cells. Further experiments demonstrated that KEAP1 expression was downregulated in H₂O₂-treated pancreatic β-cells and this change correlated with increase in the cellular abundance and nuclear translocation of NRF2. The restoration of KEAP1 expression in cells resulted in a recovery of cell proliferation and inhibition of apoptosis. Furthermore, we found that KEAP1 overexpression negatively regulated the abundance of NRF2, subsequently causing decreased antioxidant response element activation. This led to HO-1 protein downregulation in H₂O₂-treated human pancreatic β-cells, which was also observed in NRF2-silenced β-cells. Conversely, the silencing of KEAP1 led to NRF2 upregulation and inhibited ARE and HO-1 signaling in pancreatic β-cells. The increase in the abundance of NRF2 following treatment with H₂O₂ drastically elevated the production of BAX, FAS, FAS-L, CASP-3, and CASP-9, and this change was reversed by KEAP1 overexpression or NRF2 silencing. Taken together, H₂O₂ treatment activated KEAP1/NRF2 signaling to promote the production of pro-apoptotic factors and consequently led to the apoptosis of human pancreatic β-cells.

Vitamin D deficiency and diabetes

Vitamin D deficiency has been linked to the onset of diabetes. This review summarizes the role of Vitamin D in maintaining the normal release of insulin by the pancreatic beta cells (β-cells). Diabetes is initiated by the onset of insulin resistance. The β-cells can overcome this resistance by releasing more insulin, thus preventing hyperglycaemia. However, as this hyperactivity increases, the β-cells experience excessive Ca2+ and reactive oxygen species (ROS) signalling that results in cell death and the onset of diabetes. Vitamin D deficiency contributes to both the initial insulin resistance and the subsequent onset of diabetes caused by β-cell death. Vitamin D acts to reduce inflammation, which is a major process in inducing insulin resistance. Vitamin D maintains the normal resting levels of both Ca2+ and ROS that are elevated in the β-cells during diabetes. Vitamin D also has a very significant role in maintaining the epigenome. Epigenetic alterations are a feature of diabetes by which many diabetes-related genes are inactivated by hypermethylation. Vitamin D acts to prevent such hypermethylation by increasing the expression of the DNA demethylases that prevent hypermethylation of multiple gene promoter regions of many diabetes-related genes. What is remarkable is just how many cellular processes are maintained by Vitamin D. When Vitamin D is deficient, many of these processes begin to decline and this sets the stage for the onset of diseases such as diabetes.

The Nrf2/Keap1/ARE Pathway and Oxidative Stress as a Therapeutic Target in Type II Diabetes Mellitus

Despite improvements in awareness and treatment of type II diabetes mellitus (TIIDM), this disease remains a major source of morbidity and mortality worldwide, and prevalence continues to rise. Oxidative damage caused by free radicals has long been known to contribute to the pathogenesis and progression of TIIDM and its complications. Only recently, however, has the role of the Nrf2/Keap1/ARE master antioxidant pathway in diabetic dysfunction begun to be elucidated. There is accumulating evidence that this pathway is implicated in diabetic damage to the pancreas, heart, and skin, among other cell types and tissues. Animal studies and clinical trials have shown promising results suggesting that activation of this pathway can delay or reverse some of these impairments in TIIDM. In this review, we outline the role of oxidative damage and the Nrf2/Keap1/ARE pathway in TIIDM, focusing on current and future efforts to utilize this relationship as a therapeutic target for prevention, prognosis, and treatment of TIID.

C-peptide antioxidant adaptive pathways in β cells and diabetes

In this review, we present findings that support autocrine cell protection by C-peptide in the context of clinical studies of type 1 diabetes (T1D), which universally measure C-peptide serum levels as a surrogate for β cell functional mass. Over the last decade, evidence has accumulated that supports models in which C-peptide, cosecreted with insulin by pancreatic β cells, acts on peripheral targets including the vascular endothelium to reduce oxidative stress and apoptosis subsequent to exposure to diabetic insults. In parallel, as assays have become more sensitive, C-peptide has been detected in the circulation of most subjects with T1D where higher C-peptide levels are associated with fewer and slower development of diabetic microvascular complications, consistent with antioxidant protection by C-peptide. Clinical trials investigating C-peptide-replacement therapy effects have demonstrated amelioration of T1D nephropathy and neuropathy. Recently, the antioxidant action of C-peptide was extended to the β cells secreting it, that is an autocrine mechanism. Autocrine protection has major implications for the treatment of diabetes because the more C-peptide secreted, the more protection provided to the same β cells resulting in a slower decay in β cell functional mass over the time course of disease. Why β cells evolved to cosecrete an antioxidant C-peptide hormone together with the glycaemia-lowering insulin hormone is explored in the context of proposed evolutionary advantages of physiologically transient oxidative stress and insulin resistance as an adaptation for survival through times of fuel scarcity. The importance of recognizing autocrine C-peptide protection of functional β cell mass in observational clinical studies, and its therapeutic implications in interventional C-peptide-replacement studies, will be discussed.

Zinc Up-Regulates Insulin Secretion from β Cell-Like Cells Derived from Stem Cells from Human Exfoliated Deciduous Tooth (SHED)

Stem cells from human exfoliated deciduous tooth (SHED) offer several advantages over other stem cell sources. Using SHED, we examined the roles of zinc and the zinc uptake transporter ZIP8 (Zrt- and irt-like protein 8) while inducing SHED into insulin secreting β cell-like stem cells (i.e., SHED-β cells). We observed that ZIP8 expression increased as SHED differentiated into SHED-β cells, and that zinc supplementation at day 10 increased the levels of most pancreatic β cell markers-particularly Insulin and glucose transporter 2 (GLUT2). We confirmed that SHED-β cells produce insulin successfully. In addition, we note that zinc supplementation significantly increases insulin secretion with a significant elevation of ZIP8 transporters in SHED-β cells. We conclude that SHED can be converted into insulin-secreting β cell-like cells as zinc concentration in the cytosol is elevated. Insulin production by SHED-β cells can be regulated via modulation of zinc concentration in the media as ZIP8 expression in the SHED-β cells increases.

Src regulates insulin secretion and glucose metabolism by influencing subcellular localization of glucokinase in pancreatic β-cells

Aims/Introduction

Src, a non‐receptor tyrosine kinase, regulates a wide range of cellular functions, and hyperactivity of Src is involved in impaired glucose metabolism in pancreatic β‐cells. However, the physiological role of Src in glucose metabolism in normal, unstressed β‐cells remains unclear. In the present study, we investigated the role of Src in insulin secretion and glucose metabolism.

Materials and Methods

Src was downregulated using small interfering ribonucleic acid in INS‐1 cells, and glucose‐induced insulin secretion, adenosine triphosphate content, intracellular calcium concentration, glucose utilization and glucokinase activity were measured. Expression levels of messenger ribonucleic acid and protein of glucokinase were examined by semiquantitative real‐time polymerase chain reaction and immunoblotting, respectively. Cells were fractionated by digitonin treatment, and subcellular localization of glucokinase was examined by immunoblotting. Interaction between glucokinase and neuronal nitric oxide synthase was estimated by immunoprecipitation.

Results

In Src downregulated INS‐1 cells, glucose‐induced insulin secretion was impaired, whereas insulin secretion induced by high K⁺ was not affected. Intracellular adenosine triphosphate content and elevation of intracellular calcium concentration by glucose stimulation were suppressed by Src downregulation. Src downregulation reduced glucose utilization in the presence of high glucose, which was accompanied by a reduction in glucokinase activity without affecting its expression. However, Src downregulation reduced glucokinase in soluble, cytoplasmic fraction, and increased it in pellet containing intaracellular organelles. In addition, interaction between glucokinase and neuronal nitric oxide synthase was facilitated by Src downregulation.

Conclusions

Src plays an important role in glucose‐induced insulin secretion in pancreatic β‐cells through maintaining subcellular localization and activity of glucokinase.

Glucagon-like peptide 1 protects INS-1E mitochondria against palmitate-mediated beta-cell dysfunction: a proteomic study

Proteomic analysis of the protein expression profiles of enriched mitochondrial preparations of rat INS-1E β cells treated with palmitate in the presence and in the absence of GLP-1.

Physiological effects and therapeutic potential of proinsulin C-peptide

Connecting Peptide, or C-peptide, is a product of the insulin prohormone, and is released with and in amounts equimolar to those of insulin. While it was once thought that C-peptide was biologically inert and had little biological significance beyond its role in the proper folding of insulin, it is now known that C-peptide binds specifically to the cell membranes of a variety of tissues and initiates specific intracellular signaling cascades that are pertussis toxin sensitive. Although it is now clear that C-peptide is a biologically active molecule, controversy still remains as to the physiological significance of the peptide. Interestingly, C-peptide appears to reverse the deleterious effects of high glucose in some tissues, including the kidney, the peripheral nerves, and the vasculature. C-peptide is thus a potential therapeutic agent for the treatment of diabetes-associated long-term complications. This review addresses the possible physiologically relevant roles of C-peptide in both normal and disease states and discusses the effects of the peptide on sensory nerve, renal, and vascular function. Furthermore, we highlight the intracellular effects of the peptide and present novel strategies for the determination of the C-peptide receptor(s). Finally, a hypothesis is offered concerning the relationship between C-peptide and the development of microvascular complications of diabetes.

Autocrine C-peptide mechanism underlying INS1 beta cell adaptation to oxidative stress

BACKGROUND

Excessive generation of reactive oxygen species (ROS) causing oxidative stress plays a major role in the pathogenesis of diabetes by inducing beta cell secretory dysfunction and apoptosis. Recent evidence has shown that C-peptide, produced by beta cells and co-secreted with insulin in the circulation of healthy individuals, decreases ROS and prevents apoptosis in dysfunctional vascular endothelial cells. In this study, we tested the hypothesis that an autocrine activity of C-peptide similarly decreases ROS when INS1 beta cells are exposed to stressful conditions of diabetes.

METHODS

Reactive oxygen species and apoptosis were induced in INS1 beta cells pretreated with C-peptide by either 22 mM glucose or 100 μM hydrogen peroxide (H2 O2 ). To test C-peptide's autocrine activity, endogenous C-peptide secretion was inhibited by the KATP channel opener diazoxide and H2 O2 -induced ROS assayed after addition of either exogenous C-peptide or the secretagogue glibenclamide. In similar experiments, extracellular potassium, which depolarizes the membrane otherwise hyperpolarized by diazoxide, was used to induce endogenous C-peptide secretion. ROS was measured using the cell-permeant dye chloromethyl-2',7'-dichlorodihydrofluorescein diacetate (CM-H2 -DCFDA). Insulin secretion and apoptosis were assayed by enzyme-linked immunosorbent assay.

RESULTS

C-peptide significantly decreased high glucose-induced and H2 O2 -induced ROS and prevented apoptosis of INS1 beta cells. Diazoxide significantly increased H2 O2 -induced ROS, which was reversed by exogenous C-peptide or glibenclamide or potassium chloride.

CONCLUSIONS

These findings demonstrate an autocrine C-peptide mechanism in which C-peptide is bioactive on INS1 beta cells exposed to stressful conditions and might function as a natural antioxidant to limit beta cell dysfunction and loss contributing to diabetes.

The once-daily human glucagon-like peptide-1 analog, liraglutide, improves β-cell function in Japanese patients with type 2 diabetes

Aims/Introduction:  β‐cell function was evaluated by homeostasis model assessment of β‐cell function (HOMA‐B) index, proinsulin:insulin and proinsulin:C‐peptide ratios in adult, Japanese type 2 diabetes patients receiving liraglutide.

Materials and Methods:  Data from two randomized, controlled clinical trials (A and B) including 664 Japanese type 2 diabetes patients (mean values: glycated hemoglobin [HbA_1c] 8.61–9.32%; body mass index [BMI] 24.4–25.3 kg/m²) were analyzed. In two 24‐week trials, patients received liraglutide 0.9 mg (n = 268) or glibenclamide 2.5 mg (n = 132; trial A), or liraglutide 0.6, 0.9 mg (n = 176) or placebo (n = 88) added to previous sulfonylurea therapy (trial B).

Results:  Liraglutide was associated with improved glycemic control vs sulfonylurea monotherapy or placebo. In liraglutide‐treated groups in trials A and B, area under the curve (AUC) _{insulin 0–3 h} was improved (P < 0.001 for all) and the AUC_{insulin 0–3 h}:AUC_{glucose 0–3 h} ratio was increased (estimated treatment difference [liraglutide–comparator] 0.058 [0.036, 0.079]). HOMA‐B significantly increased with liraglutide relative to comparator in trial B (P < 0.05), but not in trial A. The reduction in fasting proinsulin:insulin ratio was 50% greater than in comparator groups.

Conclusions:  In Japanese type 2 diabetes patients, liraglutide was associated with effective glycemic control, restoration of prandial insulin response and indications of improved β‐cell function. This trial was registered with Clinicaltrials.gov (trial A: no. NCT00393718/JapicCTI‐060328 and trial B: no. NCT00395746/JapicCTI‐060324). (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2012.00193.x, 2012)

Reduction of reactive oxygen species ameliorates metabolism-secretion coupling in islets of diabetic GK rats by suppressing lactate overproduction

We previously demonstrated that impaired glucose-induced insulin secretion (IS) and ATP elevation in islets of Goto-Kakizaki (GK) rats, a nonobese model of diabetes, were significantly restored by 30-60-min suppression of endogenous reactive oxygen species (ROS) overproduction. In this study, we investigated the effect of a longer (12 h) suppression of ROS on metabolism-secretion coupling in β-cells by exposure to tempol, a superoxide (O2(-)) dismutase mimic, plus ebselen, a glutathione peroxidase mimic (TE treatment). In GK islets, both H2O2 and O2(-) were sufficiently reduced and glucose-induced IS and ATP elevation were improved by TE treatment. Glucose oxidation, an indicator of Krebs cycle velocity, also was improved by TE treatment at high glucose, whereas glucokinase activity, which determines glycolytic velocity, was not affected. Lactate production was markedly increased in GK islets, and TE treatment reduced lactate production and protein expression of lactate dehydrogenase and hypoxia-inducible factor 1α (HIF1α). These results indicate that the Warburg-like effect, which is characteristic of aerobic metabolism in cancer cells by which lactate is overproduced with reduced linking to mitochondria metabolism, plays an important role in impaired metabolism-secretion coupling in diabetic β-cells and suggest that ROS reduction can improve mitochondrial metabolism by suppressing lactate overproduction through the inhibition of HIF1α stabilization.