Cyclooxygenase-2, not microsomal prostaglandin E synthase-1, is the mechanism for interleukin-1ß-induced prostaglandin E2 production and inhibition of insulin secretion in pancreatic islets.

Arachidonic acid is converted to prostaglandin E(2) (PGE(2)) by a sequential enzymatic reaction performed by two isoenzyme groups, cyclooxygenases (COX-1 and COX-2) and terminal prostaglandin E synthases (cPGES, mPGES-1, and mPGES-2). mPGES-1 is widely considered to be the final enzyme regulating COX-2-dependent PGE(2) synthesis. These generalizations have been based in most part on experiments utilizing gene expression analyses of cell lines and tumor tissue. To assess the relevance of these generalizations to a native mammalian tissue, we used isolated human and rodent pancreatic islets to examine interleukin (IL)-1ß-induced PGE(2) production, because PGE(2) has been shown to mediate IL-1ß inhibition of islet function. Rat islets constitutively expressed mRNAs of COX-1, COX-2, cPGES, and mPGES-1. As expected, IL-1ß increased mRNA levels for COX-2 and mPGES-1, but not for COX-1 or cPGES. Basal protein levels of COX-1, cPGES, and mPGES-2 were readily detected in whole cell extracts but were not regulated by IL-1ß. IL-1ß increased protein levels of COX-2, but unexpectedly mPGES-1 protein levels were low and unaffected. In microsomal extracts, mPGES-1 protein was barely detectable in rat islets but clearly present in human islets; however, in neither case did IL-1ß increase mPGES-1 protein levels. To further assess the importance of mPGES-1 to IL-1ß regulation of an islet physiologic response, glucose-stimulated insulin secretion was examined in isolated islets of WT and mPGES-1-deficient mice. IL-1ß inhibited glucose-stimulated insulin secretion equally in both WT and mPGES-1(-/-) islets, indicating that COX-2, not mPGES-1, mediates IL-1ß-induced PGE(2) production and subsequent inhibition of insulin secretion.

Zinc, not insulin, regulates the rat alpha-cell response to hypoglycemia in vivo.

The intra-islet insulin hypothesis proposes that the decrement in beta-cell insulin secretion during hypoglycemia provides an activation signal for alpha-cells to release glucagon. A more recent hypothesis proposes that zinc atoms suppress glucagon secretion via their ability to open alpha-cell ATP-sensitive K(+) channels. Since insulin binds zinc, and zinc is co-secreted with insulin, we tested whether decreased zinc delivery to the alpha-cell activates glucagon secretion. In streptozotocin-induced diabetic Wistar rats, we observed that switching off intrapancreatic artery insulin infusions in vivo during hypoglycemia greatly improved glucagon secretion (area under the curve [AUC]: control group 240 +/- 261 and experimental group 4,346 +/- 1,259 pg x ml(-1) x 90 min(-1); n = 5, P < 0.02). Switching off pancreatic artery infusions of zinc chloride during hypoglycemia also improved the glucagon response (AUC: control group 817 +/- 107 and experimental group 3,445 +/- 573 pg x ml(-1) x 90 min(-1); n = 6, P < 0.01). However, switching off zinc-free insulin infusions had no effect. Studies of glucose uptake in muscle and liver cell lines verified that the zinc-free insulin was biologically active. We conclude that zinc atoms, not the insulin molecule itself, provide the switch-off signal from the beta-cell to the alpha-cell to initiate glucagon secretion during hypoglycemia.

Pancreatic islet beta-cell and oxidative stress: the importance of glutathione peroxidase.

Pancreatic beta-cell function continuously deteriorates in type 2 diabetes despite optimal treatment regimens, which has been attributed to hyperglycemia itself via formation of excess levels of reactive oxygen species (ROS). Glutathione peroxidase GPx), by virtue of its ability to catabolize both H(2)O(2) and lipid peroxides, is uniquely positioned to protect tissues from ROS. The level of this antioxidant in beta cells is extremely low and overexpression of GPx in islets provides enhanced protection against oxidative stress. This suggests that GPx mimetics may represent a valuable ancillary treatment that could add a novel layer of protection for the beta-cell.

Chronic oxidative stress as a mechanism for glucose toxicity of the beta cell in type 2 diabetes.

Type 2 diabetes is characterized by a relentless decline in pancreatic islet beta cell function and worsening hyperglycemia despite optimal medical treatment. Our central hypothesis is that residual hyperglycemia, especially after meals, generates reactive oxygen species (ROS), which in turn causes chronic oxidative stress on the beta cell. This hypothesis is supported by several observations. Exposure of isolated islets to high glucose concentrations induces increases in intracellular peroxide levels. The beta cell has very low intrinsic levels of antioxidant proteins and activities and thus is very vulnerable to ROS. Treatment with antioxidants protects animal models of type 2 diabetes against complete development of phenotypic hyperglycemia. The molecular mechanisms responsible for the glucose toxic effect on beta cell function involves disappearance of two important regulators of insulin promoter activity, PDX-1 and MafA. Antioxidant treatment in vitro prevents disappearance of these two transcription factors and normalizes insulin gene expression. These observations suggest that the ancillary treatment with antioxidants may improve outcomes of standard therapy of type 2 diabetes in humans.

Diabetes, glucose toxicity, and oxidative stress: A case of double jeopardy for the pancreatic islet beta cell.

Diabetes is commonly referred to in terms of type 1 and type 2. Both forms involve pancreatic islet beta-cell abnormalities, characterized by death in type 1 and accelerated apoptosis in type 2. The resultant chronic hyperglycemia leads to chronic oxidative stress for all tissues because glucose in abnormally high concentrations forms reactive oxygen species. It has been repeatedly emphasized that this can lead to oxidative damage in the classical secondary targets of diabetes, such as eyes, kidneys, nerves, and blood vessels. However, it has been much less appreciated that the beta cell itself is also a prime target, a case of double jeopardy. This situation is all the more pernicious because islets contain among the lowest levels of antioxidant enzyme activities compared to other tissues. This adverse effect of high glucose concentrations is referred to as glucose toxicity. A major manifestation of glucose toxicity in the beta cell is defective insulin gene expression, diminished insulin content, and defective insulin secretion. The molecular mechanisms involve the development of decreased levels of two very important insulin promoter transcription factors, PDX-1 and MafA. Studies with animal models of type 2 diabetes have established that pharmacologic protection against oxidative stress ameliorates the severity of diabetes progression. Translational research with humans is now under way to ascertain whether this protection can be provided to patients experiencing inadequate glycemic control.

Prevention of oxidative stress by adenoviral overexpression of glutathione-related enzymes in pancreatic islets.

Chronic exposure to supraphysiologic glucose concentrations causes functional damage to cells and tissues, a process known as glucose toxicity. Recent research indicates that one important mechanism for glucose toxicity is oxidative stress. Glucose has been shown to form reactive oxygen species through several metabolic pathways. The pancreatic islet is distinguished by its relatively low antioxidant enzyme content and activity, which render it especially susceptible to oxidative stress. Adenoviral overexpression of glutathione peroxidase as well as gamma-glutamylcysteine ligase have been shown to protect the islet against oxidative stress. Antioxidants have been shown to brake the worsening of diabetes by improving beta cell function in animal models. These observations suggest that enhancing antioxidant defense mechanisms in pancreatic islets may be a valuable pharmacologic approach to managing diabetes.

Inactivation of specific ß cell transcription factors in type 2 diabetes.

Type 2 diabetes (T2DM) commonly arises from islet ß cell failure and insulin resistance. Here, we examined the sensitivity of key islet-enriched transcription factors to oxidative stress, a condition associated with ß cell dysfunction in both type 1 diabetes (T1DM) and T2DM. Hydrogen peroxide treatment of ß cell lines induced cytoplasmic translocation of MAFA and NKX6.1. In parallel, the ability of nuclear PDX1 to bind endogenous target gene promoters was also dramatically reduced, whereas the activity of other key ß cell transcriptional regulators was unaffected. MAFA levels were reduced, followed by a reduction in NKX6.1 upon development of hyperglycemia in db/db mice, a T2DM model. Transgenic expression of the glutathione peroxidase-1 antioxidant enzyme (GPX1) in db/db islet ß cells restored nuclear MAFA, nuclear NKX6.1, and ß cell function in vivo. Notably, the selective decrease in MAFA, NKX6.1, and PDX1 expression was found in human T2DM islets. MAFB, a MAFA-related transcription factor expressed in human ß cells, was also severely compromised. We propose that MAFA, MAFB, NKX6.1, and PDX1 activity provides a gauge of islet ß cell function, with loss of MAFA (and/or MAFB) representing an early indicator of ß cell inactivity and the subsequent deficit of more impactful NKX6.1 (and/or PDX1) resulting in overt dysfunction associated with T2DM.

Ebselen treatment prevents islet apoptosis, maintains intranuclear Pdx-1 and MafA levels, and preserves ß-cell mass and function in ZDF rats.

We reported earlier that ß-cell-specific overexpression of glutathione peroxidase (GPx)-1 significantly ameliorated hyperglycemia in diabetic db/db mice and prevented glucotoxicity-induced deterioration of ß-cell mass and function. We have now ascertained whether early treatment of Zucker diabetic fatty (ZDF) rats with ebselen, an oral GPx mimetic, will prevent ß-cell deterioration. No other antihyperglycemic treatment was given. Ebselen ameliorated fasting hyperglycemia, sustained nonfasting insulin levels, lowered nonfasting glucose levels, and lowered HbA1c levels with no effects on body weight. Ebselen doubled ß-cell mass, prevented apoptosis, prevented expression of oxidative stress markers, and enhanced intranuclear localization of pancreatic and duodenal homeobox (Pdx)-1 and v-maf musculoaponeurotic fibrosarcoma oncogene family, protein A (MafA), two critical insulin transcription factors. Minimal ß-cell replication was observed in both groups. These findings indicate that prevention of oxidative stress is the mechanism whereby ebselen prevents apoptosis and preserves intranuclear Pdx-1 and MafA, which, in turn, is a likely explanation for the beneficial effects of ebselen on ß-cell mass and function. Since ebselen is an oral antioxidant currently used in clinical trials, it is a novel therapeutic candidate to ameliorate fasting hyperglycemia and further deterioration of ß-cell mass and function in humans undergoing the onset of type 2 diabetes.

ATP-sensitive K+ channel mediates the zinc switch-off signal for glucagon response during glucose deprivation.

OBJECTIVE: The intraislet insulin hypothesis proposes that glucagon secretion during hypoglycemia is triggered by a decrease in intraislet insulin secretion. A more recent hypothesis based on in vivo data from hypoglycemic rats is that it is the decrease in zinc cosecreted with insulin from beta-cells, rather than the decrease in insulin itself, that signals glucagon secretion from alpha-cells during hypoglycemia. These studies were designed to determine whether closure of the alpha-cell ATP-sensitive K(+) channel (K(ATP) channel) is the mechanism through which the zinc switch-off signal triggers glucagon secretion during glucose deprivation. RESEARCH DESIGN AND METHODS: All studies were performed using perifused isolated islets. RESULTS: In control experiments, the expected glucagon response to an endogenous insulin switch-off signal during glucose deprivation was observed in wild-type mouse islets. In experiments with streptozotocin-treated wild-type islets, a glucagon response to an exogenous zinc switch-off signal was observed during glucose deprivation. However, this glucagon response to the zinc switch-off signal during glucose deprivation was not seen in the presence of nifedipine, diazoxide, or tolbutamide or if K(ATP) channel knockout mouse islets were used. All islets had intact glucagon responses to epinephrine. CONCLUSIONS: These data demonstrate that closure of K(ATP) channels and consequent opening of calcium channels is the mechanism through which the zinc switch-off signal triggers glucagon secretion during glucose deprivation.

beta-Cell-specific overexpression of glutathione peroxidase preserves intranuclear MafA and reverses diabetes in db/db mice.

Chronic hyperglycemia causes oxidative stress, which contributes to damage in various tissues and cells, including pancreatic beta-cells. The expression levels of antioxidant enzymes in the islet are low compared with other tissues, rendering the beta-cell more susceptible to damage caused by hyperglycemia. The aim of this study was to investigate whether increasing levels of endogenous glutathione peroxidase-1 (GPx-1), specifically in beta-cells, can protect them against the adverse effects of chronic hyperglycemia and assess mechanisms that may be involved. C57BLKS/J mice overexpressing the antioxidant enzyme GPx-1 only in pancreatic beta-cells were generated. The biological effectiveness of the overexpressed GPx-1 transgene was documented when beta-cells of transgenic mice were protected from streptozotocin. The transgene was then introgressed into the beta-cells of db/db mice. Without use of hypoglycemic agents, hyperglycemia in db/db-GPx(+) mice was initially ameliorated compared with db/db-GPx(-) animals and then substantially reversed by 20 wk of age. beta-Cell volume and insulin granulation and immunostaining were greater in db/db-GPx(+) animals compared with db/db-GPx(-) animals. Importantly, the loss of intranuclear musculoaponeurotic fibrosarcoma oncogene homolog A (MafA) that was observed in nontransgenic db/db mice was prevented by GPx-1 overexpression, making this a likely mechanism for the improved glycemic control. These studies demonstrate that enhancement of intrinsic antioxidant defenses of the beta-cell protects it against deterioration during hyperglycemia.

Glutathione peroxidase protein expression and activity in human islets isolated for transplantation.

BACKGROUND: Overexpression of antioxidant enzymes has been reported to protect rodent beta cells from oxidative stress. However, very little is known about protein expression and activity of antioxidant enzymes in human islets. METHOD/RESULTS: Human islet protein levels by Western analysis and enzymatic activity for the key antioxidant enzymes superoxide dismutases (SODs), catalase, and glutathione peroxidase-1 (GPx) were examined. Enzyme protein expression and activity were in the order SODs > catalase > GPx. Human islet GPx protein expression was significantly less than that found for catalase (p < 0.0001) and levels of GPx activity were virtually undetectable. As glucose and estrogens have been proposed to alter antioxidant enzyme levels, we examined islet data from male and female donors separately and under varying glucose concentrations. We found significantly less (p < 0.001) GPx protein expression in islets from females compared to males, but no significant regulation by glucose in either gender. CONCLUSIONS: Human islets have very low protein and activity levels for GPx, the essential enzyme for protection against excessive levels of intracellular lipid peroxides. GPx mimetics may be especially valuable in providing human islets with the broadest spectrum of protection against oxidative stress during isolation and transplantation.

Regulation of the insulin gene by glucose and fatty acids.

The insulin gene is expressed almost exclusively in pancreatic beta-cells. Metabolic regulation of insulin gene expression enables the beta-cell to maintain adequate stores of intracellular insulin to sustain the secretory demand. Glucose is the major physiologic regulator of insulin gene expression; it coordinately controls the recruitment of transcription factors [e.g., pancreatic/duodenal homeobox-1 (PDX-1), mammalian homologue of avian MafA/L-Maf (MafA), Beta2/Neuro D (B2), the rate of transcription, and the stability of insulin mRNA. However, chronically elevated levels of glucose (glucotoxicity) and lipids (lipotoxicity) also contribute to the worsening of beta-cell function in type 2 diabetes, in part via inhibition of insulin gene expression. The mechanisms of glucotoxicity, which involve decreased binding activities of PDX-1 and MafA and increased activity of C/EBPbeta, are mediated by high-glucose-induced generation of oxidative stress. On the other hand, lipotoxicity is mediated by de novo ceramide synthesis and involves inhibition of PDX-1 nuclear translocation and MafA gene expression. Glucotoxicity and lipotoxicity have common targets, which makes their combination particularly harmful to insulin gene expression and beta-cell function in type 2 diabetes.

Oxidative stress-mediated, post-translational loss of MafA protein as a contributing mechanism to loss of insulin gene expression in glucotoxic beta cells.

Glucose toxicity in pancreatic islet beta cells causes loss of insulin gene expression, content, and secretion due to loss of binding of transcription factors, most notably PDX-1 and RIPE-3b1 activator, to the promoter region of the insulin gene. Recently, RIPE-3b1 activator was cloned and identified as the mammalian homologue of avian MafA/Maf-L (MafA). This enabled us to carry out more extensive studies of the role of MafA in glucotoxicity than were hitherto possible. Northern analysis of glucotoxic HIT-T15 cells revealed normal amounts of MafA mRNA, but Western analysis demonstrated a 97 +/- 1% reduction in MafA protein (p < 0.0001). The proteasome is a likely site for MafA degradation as lactacystin, an irreversible proteasome inhibitor, caused an accumulation of MafA protein. Antioxidants have previously been shown to prevent the adverse effects of glucose toxicity on beta cell function both in vivo and in vitro. In the current study, chronic culturing of HIT-T15 cells with the antioxidant N-acetylcysteine (NAC) prevented loss of MafA protein (late passage = 18.9 +/- 10.4% of early passage, p < 0.001; late passage with NAC = 68.7 +/- 19.7% of early passage, p = not significant) and loss of DNA binding (late passage = 63.7 +/- 9% of early passage, p < 0.02; late passage with NAC = 116 +/- 10% of early passage, p = not significant). Additionally, transient transfection of PDX-1 or MafA cDNA into glucotoxic cells increased PDX-1 and MafA protein levels and individually increased insulin promoter activity (untreated = 34%, PDX-1 = 70%, MafA = 78%; percentage of activity of early passage cells), whereas the combined transfection of MafA and PDX-1 completely restored insulin promoter activity. This recovery of promoter activity following transient transfection had no effect on endogenous insulin mRNA. However, adenoviral infection of MafA and PDX-1 significantly increased endogenous insulin mRNA levels by 93% (121 +/- 9 versus 233 +/- 18 density light units; n = 5, p < 0.001). We conclude that the absence of MafA protein from beta cells via chronic oxidative stress contributes importantly to the loss of endogenous insulin gene expression as glucose toxicity develops.

D-Glyceraldehyde causes production of intracellular peroxide in pancreatic islets, oxidative stress, and defective beta cell function via non-mitochondrial pathways.

D-Glyceraldehyde (D-GLYC) is usually considered to be a stimulator of insulin secretion but theoretically can also form reactive oxygen species (ROS), which can inhibit beta cell function. We examined the time- and concentration-dependent effects of D-GLYC on insulin secretion, insulin content, and formation of ROS. We observed that a 2-h exposure to 0.05-2 mM D-GLYC potentiated glucose-stimulated insulin secretion (GSIS) in isolated Wistar rat islets but that higher concentrations inhibited GSIS. A 24-h exposure to 2 mm D-GLYC inhibited GSIS, decreased insulin content, and increased intracellular peroxide levels (2.14 +/- 0.31-fold increase, n = 4, p < 0.05). N-Acetylcysteine (10 mM) prevented the increase in intracellular peroxides and the adverse effects of d-GLYC on GSIS. In the presence of 11.1 but not 3.0 mm glucose, koningic acid (10 microM), a specific glyceraldehyde-3-phosphate dehydrogenase inhibitor, increased intracellular peroxide levels (1.88 +/- 0.30-fold increase, n = 9, p < 0.01) and inhibited GSIS (control GSIS = p < 0.001; koningic acid GSIS, not significant). To determine whether oxidative phosphorylation was the source of ROS formation, we cultured rat islets with mitochondrial inhibitors. Neither rotenone or myxothiazol prevented D-GLYC-induced increases in islet ROS. Adenoviral overexpression of manganese superoxide dismutase also failed to prevent the effect of D-GLYC to increase ROS levels. These observations indicate that exposure to excess D-GLYC increases reactive oxygen species in the islet via non-mitochondrial pathways and suggest the hypothesis that the oxidative stress associated with elevated D-GLYC levels could be a mechanism for glucose toxicity in beta cells exposed chronically to high glucose concentrations.

Adenoviral overexpression of the glutamylcysteine ligase catalytic subunit protects pancreatic islets against oxidative stress.

The catalytic subunit of glutamylcysteine ligase (GCLC) primarily regulates de novo synthesis of glutathione (GSH) in mammalian cells and is central to the antioxidant capacity of the cell. However, GCLC expression in pancreatic islets has not been previously examined. We designed experiments to ascertain whether GCLC is normally expressed in islets and whether it is up-regulated by interleukin-1 beta (IL-1 beta). GCLC expression levels were intermediate compared with other metabolic tissues (kidney, liver, muscle, fat, and lung). IL-1 beta up-regulated GCLC expression (10 ng/ml IL-1 beta, 3.76 +/- 0.86; 100 ng/ml IL-1 beta, 4.22 +/- 0.68-fold control) via the p38 form of mitogen-activated protein kinase and NF kappa B and also increased reactive oxygen species levels (10 ng/ml IL-1 beta, 5.41 +/- 1.8-fold control). This was accompanied by an increase in intraislet GSH/GSSG ratio (control, 7.1 +/- 0.1; 10 ng/ml IL-1 beta, 8.0 +/- 0.5; 100 ng/ml IL-1 beta, 8.2 +/- 0.5-fold control; p < 0.05). To determine whether overexpression of GCLC increases the antioxidant capacity of the islet and prevents the adverse effects of IL-1 beta on glucose-induced insulin secretion, islets were infected with an adenovirus encoding GCLC. IL-1 beta significantly decreased glucose-stimulated insulin secretion (control, 123.8 +/- 17.7; IL-1 beta, 40.2 +/- 3.9 microunits/ml insulin/islet). GCLC overexpression increased intraislet GSH levels and partially prevented the decrease in glucose-stimulated insulin secretion caused by IL-1 beta. These data provide the first report of GCLC expression in the islet and demonstrate that adenoviral overexpression of GCLC increases intracellular GSH levels and protects the beta cell from the adverse effects of IL-1 beta.