Chronic Oxidative Stress as a Central Mechanism for Glucose Toxicity in Pancreatic Islet Beta Cells in Diabetes

Glucose in chronic excess causes toxic effects on structure and function of organs, including the pancreatic islet. Multiple biochemical pathways and mechanisms of action for glucose toxicity have been suggested. These include glucose autoxidation, protein kinase C activation, methylglyoxal formation and glycation, hexosamine metabolism, sorbitol formation, and oxidative phosphorylation. There are many potential mechanisms whereby excess glucose metabolites traveling along these pathways might cause beta cell damage. However, all these pathways have in common the formation of reactive oxygen species that, in excess and over time, cause chronic oxidative stress, which in turn causes defective insulin gene expression and insulin secretion as well as increased apoptosis. This minireview provides an overview of these mechanisms, as well as a consideration of whether antioxidant strategies might be used to protect further deterioration of the beta cell after the onset of diabetes and hyperglycemia. Diabetes mellitus is a disease characterized by hyperglycemia and is caused by absolute or relative insulin deficiency, sometimes associated with insulin resistance. It has multiple etiologies and segregates into two major forms. Type 1 diabetes is an autoimmune disease in which the patient's own immune system reacts against islet antigens and destroys the beta cell. Type 2 diabetes is a polygenic syndrome with multiple etiologies rather than a single specific disease. As the hyperglycemia of diabetes becomes chronic, the sugar that normally serves as substrate, fuel, and signal takes on the darker role of toxin. Chronic hyperglycemia is the proximate cause of retinopathy, kidney failure, neuropathies, and macrovascular disease in diabetes. The beta cell in type 2 diabetes is also adversely affected by chronic hyperglycemia and, in this sense, is also a target for secondary complications. As hyperglycemia worsens, the beta cell steadily undergoes deterioration, secretes less and less insulin, and becomes a participant in a downward spiral of loss of function. This relentless deterioration in cell function caused by constant exposure to supraphysiologic concentrations of glucose is termed glucose toxicity. In physiologic concentrations, endogenous reactive oxygen species (ROS) 1The abbreviations used are: ROS, reactive oxygen species; GAPDH, glyceraldehyde 3-phosphate dehydrogenase; AGE, advanced glycation end product; PKC, protein kinase C; TGF, transforming growth factor; GFAT, glutamine:fructose-6-phosphate aminotransferase; SOD, superoxide dismutase; ZDF, Zucker diabetic fatty; JNK, c-Jun NH2-terminal kinase; γ-GCL, γ-glutamylcysteine ligase. help to maintain homeostasis. However, when ROS accumulate in excess for prolonged periods of time, they cause chronic oxidative stress and adverse effects. This is particularly relevant and dangerous for the islet, which is among those tissues that have the lowest levels of intrinsic antioxidant defenses. Multiple biochemical pathways and mechanisms of action have been implicated in the deleterious effects of chronic hyperglycemia and oxidative stress on the function of vascular, retinal, and renal tissues. Considerably less work has been performed using islet tissue. At least six pathways are emphasized in the literature as being major contributors of ROS. Each will be considered briefly. Glyceraldehyde Autoxidation—Glyceraldehyde 3-phosphate is a phosphorylation product formed from glucose during anaerobic glycolysis. The partner product, dihydroxyacetone phosphate, also contributes to intracellular glyceraldehyde concentrations via enzymatic conversion by triose-phosphate isomerase. Thereafter, glyceraldehyde 3-phosphate is oxidized by glyceraldehyde-phosphate dehydrogenase (GAPDH). Continuance of glycolysis yields pyruvate, which enters the mitochondria where it is oxidized to acetyl-CoA, and the processes of the tricarboxylic acid cycle and oxidative phosphorylation begin. One alternative to this classic pathway of glucose metabolism is the less familiar route of glyceraldehyde autoxidation (Fig. 1, pathway 1). The potential relevance of this pathway to diabetes mellitus was pointed out by Wolff and Dean (1Wolff S.P. Dean R.T. Biochem. J. 1987; 245: 243-250Crossref PubMed Scopus (1151) Google Scholar), who emphasized that autoxidation of α-hydroxyaldehydes generates hydrogen peroxide (H2O2) and α-ketoaldehydes. In the presence of redox active metals, H2O2 can form the highly toxic hydroxyl radical. This pathway, therefore, forms two potentially toxic substances, α-ketoaldehydes, which contribute to glycosylation-related protein chromophore development, and the hydroxyl radical, a reactive oxygen species that can cause mutagenic alterations in DNA. Although glyceraldehyde is characteristically thought of as an insulin secretagogue, when present in excess it may also inhibit insulin secretion (2Hellman B. Idahl L.A. Lernmark A. Sehlin J. Taljedal I.B. Arch. Biochem. Biophys. 1974; 162: 448-457Crossref PubMed Scopus (75) Google Scholar). Long term exposure to high glucose concentrations decreases GAPDH activity in islets (3Sakai K. Matsumoto K. Nishikawa T. Suefuji M. Nakamaru K. Hirashima Y. Kawashima J. Shirotani T. Ichinose K. Brownlee M. Araki E. Biochem. Biophys. Res. Commun. 2003; 300: 216-222Crossref PubMed Scopus (283) Google Scholar), which favors excess glyceraldehyde accumulation. Exposure of endothelial cells to 30 mm glucose caused GAPDH inhibition (4Du X. Matsumura T. Edelstein D. Rossetti L. Zsengeller Z. Szabo C. Brownlee M. J. Clin. Invest. 2003; 112: 1049-1057Crossref PubMed Scopus (504) Google Scholar) through the mechanism of ROS-activated poly(ADP-ribosyl)ation of GAPDH by poly(ADP-ribose) polymerase. This in turn was associated with intracellular advanced glycation end product (AGE) formation and activation of PKC, the hexosamine pathway, and NF-κB. PKC Activation—Dihydroxyacetone can undergo reduction to glycerol 3-phosphate and acylation and thereby increase de novo synthesis of diacylglycerol, which activates protein kinase C, of which there at least 11 isoforms (Fig. 1, pathway 2). Activation of PKC has many biochemical consequences that relate to microvascular disease in diabetes. PKC activation is associated with increases in TGF-β1, vascular endothelial growth factor, endothelin-1, NAD(P)H oxidase, NF-κB, and ROS (5Brownlee M. Nature. 2001; 414: 813-820Crossref PubMed Scopus (7128) Google Scholar, 6Inoguchi T. Battan R. Handler E. Sportsman J.R. Heath W. King G.L. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 11059-11063Crossref PubMed Scopus (704) Google Scholar, 7Ishii H. Jirousek M.R. Koya D. Takagi C. Xia P. Clermont A. Bursell S.E. Kern T.S. Ballas L.M. Heath W.F. Stramm L.E. Feener E.P. King G.L. Science. 1996; 272: 728-731Crossref PubMed Scopus (1091) Google Scholar). Methylglyoxal, Glycation, and Sorbitol—Three reactive intracellular dicarbonyls (glyoxal, methylglyoxal, and 3-deoxyglucosone) form AGEs by reacting with amino groups on intracellular and extracellular proteins (8Thornalley P.J. Langborg A. Minhas H.S. Biochem. J. 1999; 344: 109-116Crossref PubMed Scopus (1026) Google Scholar) (Fig. 1, pathways 3 and 4). AGEs play important roles in the pathogenesis of secondary complications of diabetes, especially with regard to microvascular disease in the retina, nerves, and kidney and likely islets. When GAPDH-mediated catabolism of glyceraldehyde-3-P is impaired, such as in the presence of high glucose concentrations, accumulation of glyceraldehyde-3-P and dihydroxyacetone favors formation of methylglyoxal. Additionally, increased flux along the polyol pathway as a result of hyperglycemia results in aldose reductase-mediated NADPH-dependent reduction of glucose to form sorbitol. Oxidation of sorbitol by NAD+ increases the cytosolic NADH:NAD+ ratio, which tends to inhibit GAPDH activity. This can lead to increased levels of triose phosphates, methylglyoxal, and diacylglycerol. This chain of events is also associated with consumption of NAD+ by activated poly(ADP-ribose) polymerase, which can be activated by hyperglycemia via increased production of ROS and DNA strand breaks (4Du X. Matsumura T. Edelstein D. Rossetti L. Zsengeller Z. Szabo C. Brownlee M. J. Clin. Invest. 2003; 112: 1049-1057Crossref PubMed Scopus (504) Google Scholar). Above and beyond the damage that reactive dicarbonyls can cause through enhancement of glycation and the formation of AGEs, the Maillard reaction between carbohydrates and proteins also generates ROS (9Wells-Knecht K.J. Zyzak D.V. Litchfield J.E. Thorpe S.R. Baynes J.W. Biochemistry. 1995; 34: 3702-3709Crossref PubMed Scopus (562) Google Scholar). Thus, hyperglycemia simultaneously enhances both glycative and oxidative stress, which together synergistically contribute to the development of diabetic complications. Hexosamine Pathway—In states of excess intracellular glucose, fructose 6-phosphate via glutamine:fructose-6-phosphate aminotransferase (GFAT) can form glucosamine 6-phosphate and then UDP-N-acetylglucosamine, which supports proteoglycan synthesis and the formation of (O)-linked glycoproteins (Fig. 1, pathway 5). This pathway has been shown to be related to increases in transcription of TGF-α, TGF-β1, and PAI-1 and has been implicated in insulin resistance (5Brownlee M. Nature. 2001; 414: 813-820Crossref PubMed Scopus (7128) Google Scholar, 10Marshall S. Bacote V. Traxinger R.R. J. Biol. Chem. 1991; 266: 4706-4712Abstract Full Text PDF PubMed Google Scholar). Glucosamine infusions in rodents and in humans have been associated with interference with glucose sensing by the beta cell and with insulin sensitivity (11Monauni T. Zenti M.G. Cretti A. Daniels M.C. Targher G. Caruso B. Caputo M. McClain D. Del Prato S. Giaccari A. Muggeo M. Bonora E. Bonadonna R.C. Diabetes. 2000; 49: 926-935Crossref PubMed Scopus (123) Google Scholar). Adenovirus-mediated overexpression of GFAT was reported to impair glucose-stimulated insulin secretion and to reduce expression levels of the insulin, GLUT2, and glucokinase genes (12Kaneto H. Xu G. Song K.H. Suzuma K. Bonner-Weir S. Sharma A. Weir G.C. J. Biol. Chem. 2001; 276: 31099-31104Abstract Full Text Full Text PDF PubMed Scopus (258) Google Scholar). The DNA binding activity of PDX-1, a critical transcription factor for these genes, was also markedly reduced. In these experiments glucosamine was found to increase hydrogen peroxide levels, and the antioxidant n-acetylcysteine prevented the adverse effects of GFAT overexpression. Oxidative Phosphorylation—High glucose concentrations increase the mitochondrial proton gradient as a result of overproduction of electron donors by the tricarboxylic acid cycle, which in turn increase production of mitochondrial superoxide (13Du X.L. Edelstein D. Rossetti L. Fantus I.G. Goldberg H. Ziyadeh F. Wu J. Brownlee M. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 12222-12226Crossref PubMed Scopus (900) Google Scholar) (Fig. 1, pathway 6). In these experiments, inhibition by Mn-SOD or UCP-1 of hyperglycemia-induced overproduction of mitochondrial superoxide prevented the increases in polyol pathway flux, intracellular AGE formation, PKC activation, and hexosamine pathway activity in endothelial cells. High glucose concentrations were shown to increase mitochondrial superoxide production, proton leak, lower ATP levels, and impaired glucose-induced insulin secretion in islets from wild type but not from UCP-2-knock-out animals (14Krauss S. Zhang C.Y. Scorrano L. Dalgaard L.T. St-Pierre J. Grey S.T. Lowell B.B. J. Clin. Invest. 2003; 112: 1831-1842Crossref PubMed Scopus (334) Google Scholar), suggesting that superoxide-mediated activation of UCP-2 could play a role in type 2 diabetes. It has also been reported that a 2 mm glyceraldehyde concentration in 24-h islet incubations increased ROS levels and inhibited insulin secretion, effects that were abrogated by n-acetylcysteine (15Takahashi H. Tran P.O.T. Leroy E. Tanaka Y. Robertson R.P. J. Biol. Chem. 2004; 279: 37316-37323Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar). However, in these studies neither inhibitors of mitochondrial oxidative phosphorylation nor adenovirus overexpression of Mn-SOD prevented the ability of glyceraldehyde to increase islet reactive oxygen species levels. 2 mm glyceraldehyde has been reported to increase intraislet glyceraldehyde concentrations to a level similar to that achieved with 20 mm glucose (16Taniguchi S. Okinaka M. Tanigawa K. Miwa I. J. Biochem. (Tokyo). 2000; 127: 289-295Crossref PubMed Scopus (21) Google Scholar). Thus, when the glycolytic pathway is swamped by glucose, it seems likely that both mitochondrial and non-mitochondrial pathways contribute ROS to the glucotoxic process that impairs beta cell function. At the root of the relentless decline in beta cell function found in the glucotoxic state is abnormal insulin gene expression as well as decreases in insulin content and insulin secretion. These deleterious effects of glucose toxicity also converge with the adverse consequences of lipotoxicity, both of which can cause increased islet apoptosis. Effects of Chronic Oxidative Stress on Insulin Gene Expression—Chronic exposure of the beta cell to supraphysiologic concentrations of glucose causes defective insulin gene expression accompanied by marked decreases in insulin content and abnormal insulin secretion (17Robertson R.P. Zhang H.J. Pyzdrowski K.L. Walseth T.F. J. Clin. Invest. 1992; 90: 320-325Crossref PubMed Scopus (195) Google Scholar). The defect in insulin gene expression is due to the loss of at least two critical proteins that activate the insulin promoter (Fig. 2). One is PDX-1 (18Olson L.K. Redmon J.B. Towle H.C. Robertson R.P. J. Clin. Invest. 1993; 92: 514-519Crossref PubMed Scopus (178) Google Scholar, 19Olson L.K. Sharma A. Peshavaria M. Wright C.V. Towle H.C. Robertson R.P. Stein R. Proc. Natl. Acad. Sci. U. S. 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To differentiate in vivo between hyperlipidemia and hyperglycemia as the cause of islet triglyceride accumulation and defective insulin gene expression in ZDF rats, animals were treated with either the lipid-lowering drug bezafibrate or the blood glucose-lowering agent phlorizin (37Harmon J.S. Gleason C.E. Tanaka Y. Poitout V. Robertson R.P. Diabetes. 2001; 50: 2481-2486Crossref PubMed Scopus (120) Google Scholar). Islet triglyceride content was decreased and insulin gene expression was preserved by phlorizin but not by bezafibrate that islet triglyceride accumulation requires hyperglycemia. studies the mechanism of action whereby palmitate inhibits insulin gene expression that the palmitate-induced in insulin gene expression was associated with inhibition of insulin promoter activity and increased levels of intracellular C.L. B. Poitout V. J. Biol. Chem. 2003; Full Text Full Text PDF PubMed Scopus Google Scholar). was found when islets were in mm glucose as with islets in mm glucose or 11 mm mm glucose M. M. L. M. Z. C. L. R. P. Bertuzzi F. G. P. R. G. F. Diabetes. 2001; 50: PubMed Scopus Google Scholar). The gene was by these conditions was reduced. The genes and were in the islets in high glucose of beta cell from an increase in beta cell was reported to be an important to the of the diabetic state in ZDF D. T. Diabetes. 2001; 50: PubMed Scopus Google Scholar). of has been reported to lead to (34Unger R.H. LeRoith D. Olefsky J. Taylor S. Diabetes Mellitus: a Fundamental and Clinical Text. 3rd Ed. Lippincott Williams & Wilkins, Inc., Philadelphia2004: 141-149Google Scholar). However, in other studies de novo synthesis was not for palmitate-induced inhibitors of de novo synthesis not palmitate-induced and palmitate-induced occurred in cells in de novo synthesis J.E. J. Biol. Chem. 2001; 276: Full Text Full Text PDF PubMed Scopus Google Scholar). These experiments also that palmitate-induced causes of ROS and that this is inhibited by which simultaneously activity. This a role for ROS in the of by The of this minireview is that of high glucose levels after onset of either type 1 or type 2 diabetes has secondary adverse effects on the beta cell not that glucose toxicity is the cause of either disease. In the of type 1 diabetes it has been that increased glucose levels are associated with increased beta cell of K. P. F. J. Kaiser N. J. Clin. Invest. 2002; PubMed Scopus Google Scholar), which are In a model of type 2 diabetes, high glucose concentrations increased intracellular peroxide levels in islets Y. Tran P.O. Harmon J. Robertson R.P. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: PubMed Scopus Google Scholar). This the state of antioxidant the islet and whether of these might be an to the of diabetes and hyperglycemia on the beta cell. against Chronic Oxidative of elevated levels of oxidative stress in patients with type 2 diabetes are the islet is among the least well tissues in of intrinsic antioxidant enzyme including and K. Taljedal I.B. Biochem. J. PubMed Scopus Google Scholar, M. S. J. S. Diabetes. 1997; PubMed Google Scholar). In gene expression of γ-glutamylcysteine the enzyme for is well in islets. In our laboratory the levels of mRNA are with those found in and than those found in and T. S. M. E. C. C. T. J. E. T. J. S. Harmon, and R. P. Robertson, unpublished results. Long term exposure to high glucose conditions decreases expression in as well as and this is associated with a in levels L.A. P. D. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar, Y. R. Invest. Sci. 1999; Google Scholar). Thus, type 2 diabetes mellitus is associated with elevated of chronic oxidative stress, pancreatic islets levels of antioxidant gene and elevated glucose levels the enzyme for Effects of in of superoxide have been reported to against diabetes K. S. Taljedal I.B. Nature. PubMed Scopus Google Scholar). animals superoxide have beta cell to oxidative diabetes J. R. E. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). of antioxidant in beta cell against and combinatorial rather than single overexpression of antioxidant is M. S. R. S. Diabetes. 1998; 47: PubMed Scopus Google Scholar). overexpression of for the beta cell against and hydrogen peroxide B. Biol. 1999; PubMed Scopus Google Scholar). overexpression of and superoxide has been shown to protect islets C. L. F. J. J. P. S. 1998; PubMed Scopus Google Scholar, C. F. J. P. 2000; PubMed Scopus Google Scholar) and a beta cell A. Tran R. J. Clin. Invest. 1998; 101: PubMed Scopus Google Scholar) against oxidative In our laboratory overexpression of Y. Tran P.O. Harmon J. Robertson R.P. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: PubMed Scopus Google Scholar) and have been shown to protect islets against the adverse consequences of on insulin gene insulin and insulin secretion. by against Oxidative antioxidant have been as against beta cell oxidative against oxidative stress and diabetes in ZDF and Y. Gleason C.E. Tran P.O. Harmon J.S. Robertson R.P. Proc. Natl. Acad. Sci. U. S. A. 1999; PubMed Scopus Google Scholar, H. Y. J. T. Y. Y. T. Y. Y. M. Diabetes. 1999; 48: PubMed Scopus Google Scholar). In both this drug preserved insulin content and insulin gene expression as well as PDX-1 binding to the insulin The and have antioxidant and hyperglycemia in the ZDF rat (30Harmon J.S. Gleason C.E. Tanaka Y. Oseid E.A. Hunter-Berger K.K. Robertson R.P. Diabetes. 1999; 48: 1995-2000Crossref PubMed Scopus (87) Google Scholar, S. J. W. J. 1996; PubMed Google Scholar). has effects on in Y. Y. S. K. N. T. A. T. A. H. Y. 2000; PubMed Scopus Google Scholar). a used used in the of type 2 diabetes, has been shown to protect pancreatic beta cells from damage by hydrogen peroxide K. K. T. Y. H. H. E. T. K. H. Biochem. Biophys. Res. Commun. 2003; PubMed Scopus Google Scholar). These findings suggest that with may a to the of type 2 diabetes. One potential mechanism for glucose toxicity is the formation of excess ROS levels, which takes multiple mitochondrial and non-mitochondrial pathways. The islet is especially to ROS of intrinsic level of antioxidant glucose and ROS levels can cause decreased insulin gene expression via loss of the transcription PDX-1 and MafA and can also of apoptosis. This the for antioxidant as an in the of diabetes.