β-Cell Glucose Toxicity, Lipotoxicity, and Chronic Oxidative Stress in Type 2 Diabetes

Many clinical studies suggest that patients with type 2 diabetes are subjected to chronic oxidative stress (1–6). Sophisticated laboratory techniques, including high-performance liquid chromatography, gas chromatography/mass spectrometry, and immunostaining of pancreas biopsies, have been used. Pro-oxidants and markers for oxidative tissue damage, such as 8-hydroxy-deoxyguanine, 4-hydroxy-2-nonenal (HNE) proteins, 8-epi-prostaglandin F_2α, hydroperoxides, and oxidation of DNA bases, have been reported to be elevated in serum, plasma, white blood cells, and pancreas biopsies of patients with type 2 diabetes (Table 1). Compared with nondiabetic control subjects, these markers showed changes that have ranged to fivefold above normal. Glutathione (GSH), the primary intracellular antioxidant, has been studied extensively. Murakami et al. (7) reported that erythrocytes from diabetic patients contain low levels of the reduced form of GSH, high levels of the oxidized form (GSSG), and a 51% reduction in the GSH/GSSG ratio. Sharma et al. (8) also reported that type 2 diabetic patients in poor glycemic control had depressed erythrocyte GSH levels. After treatment with sulfonylureas decreased HbA_1c levels, the erythrocyte GSH concentration increased twofold, almost reaching the values found in a nondiabetic control group. Similar data were reported by Yoshida et al. (9), who in addition reported that the activity of the rate-limiting enzyme of GSH synthesis, γ-glutamylcysteine synthase, increased with improved glycemic control. This group also reported that erythrocyte thiol transport levels were significantly and inversely correlated with HbA_1c. Paolisso et al. (10) reported that intravenous infusion of GSH in type 2 diabetic patients improved insulin secretion and glucose tolerance during oral glucose tolerance tests.

Many investigators have used animal models in their research to assess the proposition that deteriorating glycemic control in type 2 diabetes causes oxidative stress on β-cells. For example, Ihara et al. (11) measured levels of 8-hydroxy-deoxyguanosine and 4-hydroxy-2-nonenal–modified proteins by immunohistochemical methods in β-cells of Goto-Kakizaki (GK) rats. They reported higher levels of these oxidative stress markers in GK rats compared with Wistar rats. Moreover, a diet rich in sucrose increased these two markers in GK rats, whereas inclusion of the hypoglycemic agent voglibose in the diet decreased marker levels. Tanaka et al. (12) reported elevated plasma concentrations of these two markers in Zucker diabetic fatty (ZDF) rats and that treatment prevented the increase. However, demonstration that glucose itself is capable of generating reactive oxygen species (ROS) in β-cells is essential to the hypothesis that glucose-induced oxidative stress is a mechanism for glucose toxicity. Potential glucose-related pathways through which ROS can be formed include autoxidation, oxidative phosphorylation, glycosylation, and the glucosamine pathways (13–17). Wolff and colleagues (13,14) were among the first to suggest that autoxidation of glucose could yield ROS. They showed that glyceraldehyde in the presence of heavy metals could form a radical anion via enolization. In the presence of oxygen, superoxide was generated, which in turn formed hydrogen peroxide via superoxide dismutase. In the presence of heavy metals, hydrogen peroxide generates highly toxic hydroxyl radicals. Evidence that this sequence of events might occur specifically in β-cells was provided by Ihara et al. (11), as mentioned above. Studies by Tanaka et al. used the peroxide-sensitive probe carboxyl dichlorodihydrofluorescein diacetate and flow cytometry to examine the consequences of exposing human isolated islets to 30 mmol/l glucose for 72 h (18). Compared with 5.6 mmol/l glucose, 30 mmol/l glucose caused an increase in intracellular peroxides (Fig. 1). The same phenomenon was observed in isolated rat islets, but only after pretreatment with buthionine sulfoximine, an agent that inhibits intracellular synthesis of GSH.

Oxidative stress has been considered by many as an explanation for the tissue damage that accompanies chronic hyperglycemia. However, most of this research has been conducted in non-islet tissue. This is ironic because the islet has the lowest intrinsic antioxidant capacity compared with other metabolic tissues such as liver, kidney, skeletal muscle, and fat. Grankvist et al. (19) reported that islets contain relatively small activities for the major antioxidant enzymes Cu/Zn superoxide dismutase, Mn superoxide dismutase, catalase, and glutathione peroxidase (GPx). This observation was more recently reinforced by Tiedge et al. (20) (Table 2), who made the interesting observation that high glucose concentrations caused cellular stress in islets (as indicated by a rise in the heme oxygenase-1 protein), but did not induce higher activities of the antioxidant enzymes. This suggests that the islet is among the most vulnerable of tissues during times of oxidative stress.

Many investigators have recognized the therapeutic implications of the adverse consequences on the β-cell from chronic oxidative stress in diabetes. This has led to many reports of experiments designed to assess whether antioxidant drugs can be used to protect against oxidative stress in models of type 1 and type 2 diabetes. For example, Mendola et al. (21) reported that nicotinamide and desferrioxamine partially protected islets from immune destruction during low-dose streptozotocin-induced insulitis—a process in which hydroxyl radicals play an important role. Nomikos et al. (22) demonstrated that in vivo treatment with superoxide dismutase and catalase protected islet tissue from oxygen metabolites induced by transplantation of islet allografts. Metformin or troglitazone, both of which have antioxidant properties, prevent hyperglycemia in the ZDF rat (23,24). Fukui et al. (25) examined lipid hydroperoxide concentrations and superoxide dismutase activity and determined that troglitazone prevented increased levels of lipid hydroperoxide and superoxide dismutase activity in the OLETF rat, an animal model of type 2 diabetes. Sano et al. (26) used electron spin resonance to determine levels of ROS in vivo in Wistar rats that received streptozotocin. They found that the spin clearance rate significantly correlated with malondialdehyde levels, a marker for lipid peroxidation, and that treatment with insulin decreased the spin clearance rate in diabetic rats. They concluded that the diabetic state enhances the generation of ROS in vivo and that glycemic control can reduce oxidative stress. Ho et al. (27) used electron spin resonance to document that alloxan generates ROS in the pancreas and that the antioxidant drug n-acetylcysteine (NAC) inhibits NF-κB activation and reduces hyperglycemia. Kaneto et al. (28) also demonstrated protective effects of NAC in C57BL/KsJ-db/db mice. As these mice developed hyperglycemia, their insulin content and insulin gene expression decreased. NAC treatment was associated with preserved insulin content and insulin mRNA as well as increased amounts of PDX-1, an important insulin gene transcription factor. The use of a weaker antioxidant, vitamin E, led to controversial results. Tajiri and Grill (29) found that vitamin E failed to restore glucose-induced insulin secretion in rat pancreatic islets transplanted under the kidney capsule in streptozotocin-induced diabetic rats. On the other hand, Ihara et al. (11), who had previously noted increased immunostaining for 8-OH-deoxyglucose and HNE-modified proteins in GK islets, found that vitamin E had beneficial effects on glycemic control in GK rats, which was accompanied by significant improvement in insulin secretion and lower levels of HbA_1c (30). In our studies with ZDF rats, animals were treated with the antioxidant NAC, the combined antioxidant and nitric oxide synthase inhibitor aminoguanidine, the selective nitric oxide synthase inhibitor s-methylisothiourea, or placebo (12). Treatment was begun at 6 weeks of age and carried through 12 weeks of age. Progressive worsening of hyperglycemia, glucose tolerance, insulin secretion, PDX-1 binding to the insulin promoter, and insulin gene expression were associated with the increase of oxidative stress markers. In contrast, the ZDF animals treated with NAC or aminoguanidine, but not s-methylisothiourea or placebo, developed much less hyperglycemia and had preserved PDX-1 binding, insulin gene expression, and nearly normal insulin secretion and glucose tolerance. These findings support chronic oxidative stress as a potential mechanism of action for glucose toxicity and point away from nitric oxide species as a candidate ROS in this animal model. In this context, it is interesting to recall that troglitazone, a drug that prevents glucotoxic decreases in insulin RNA levels in the ZDF animal (23,24), is also an antioxidant.

Gene overexpression experiments have been designed to test whether increased levels of antioxidant enzymes in pancreatic islets might protect against oxidative stress. Kubisch et al. (31) reported that a transgenic model with increased levels of Cu/Zn superoxide dismutase had enhanced tolerance of β-cells to oxidative stress induced by alloxan. In a later study, enhanced superoxide dismutase expression in transgenic mice was targeted specifically to pancreatic β-cells. In other experiments, the expression of the transgene significantly enhanced resistance to alloxan-induced diabetogenesis, which the authors concluded provided direct in vivo evidence that genetic variation in ROS metabolism can affect susceptibility to oxidative stress–mediated diabetogenesis (32). Benhamou et al. (33) demonstrated that overexpression of catalase in human islets protected against oxidative stress induced by xanthine oxidase-hypoxanthine. Similarly, Xu et al. (34) overexpressed catalase in mouse islets and found that it protected islet insulin secretion against hydrogen peroxide and significantly reduced the diabetogenic effect of streptozotocin in vivo.

The beneficial effects of antioxidant gene overexpression in protecting β-cell function in a ribose-related model of type 2 diabetes was recently reported by Tanaka et al. (18). These studies focused primarily on GSH as an intracellular antioxidant. Evidence to support the importance of GSH was found in experiments wherein buthionine sulfoximine was used to inhibit GSH synthesis. This drug potentiated the adverse consequences of ribose-induced ROS generation on rat islet β-cell insulin mRNA and glucose-induced insulin secretion. Adenoviral overexpression of GSH peroxidase protected the islets from ribose-induced formation of intracellular ROS and decreases in insulin gene expression and glucose-induced insulin secretion (Fig. 2). Therefore, there is ample evidence in animal models that type 2 diabetes is associated with increased markers of oxidative stress, that glucose can increase β-cell levels of ROS, that antioxidant treatment can ameliorate hyperglycemia, and that overexpression of antioxidant enzymes in the β-cell protects against increases in oxidants in its microenvironment.

Because type 2 diabetes in humans is frequently associated with obesity and hyperlipidemia as well as hyperglycemia, many investigators have examined whether high levels of free fatty acids or other lipids might be harmful to islet function. Indeed, there is ample evidence that fatty acids, which under normal circumstances are physiological fuels for the β-cell, become toxic when present at elevated concentrations for prolonged periods of time (35). Adverse effects of chronic exposure of the β-cell to elevated fatty acid concentrations include decreased glucose-induced insulin secretion (36–41), impaired insulin gene expression (42–45), and increased cell death (46–52).

The mechanisms by which fatty acids impair β-cell function are largely unknown. Initially, the possibility was raised that elevated fatty acid oxidation might decrease glucose oxidation (36,38), similar to the “glucose-fatty acid cycle” described in cardiac muscles (51). However, several observations argue against such a phenomenon being operative in β-cells. Experimental evidence suggests that the deleterious effects of fatty acids are mediated via lipid-derived intracytosolic metabolites rather than by fatty acid oxidation (52). Our hypothesis, initially proposed by Prentki and Corkey (53), is that in the presence of physiological glucose concentrations, excessive fatty acids are readily disposed of through mitochondrial β-oxidation. In contrast, when both fatty acids and glucose are elevated, accumulation of metabolites derived from fatty acid esterification impairs β-cell function. This model is supported by several observations. In vitro, prolonged exposure of isolated rat islets to palmitate inhibits insulin gene expression only in the presence of high glucose (44). This is accompanied by a glucose-dependent increase in neutral lipid synthesis (45). Further, forcing triglyceride synthesis in islets by overexpression of diacylglycerol acyltransferase inhibits glucose-induced insulin secretion after culture in high glucose, but not in low glucose (54). In vivo, the glucose-lowering agent phlorizin, but not the lipid-lowering agent bezafibrate, lowers islet triglyceride content and prevents the decrease in insulin mRNA levels in ZDF rats (Fig. 3) (55). Finally, high-fat feeding impairs glucose-induced insulin secretion in hyperglycemic GK rats, but not in normoglycemic Wistar rats, an effect prevented by normalization of blood glucose in GK rats (56). Overall, these findings support the hypothesis that lipotoxicity only occurs in the context of chronic hyperglycemia, consistent with the observation that most individuals with increased circulating lipid levels have normal β-cell function.

This work was supported by National Institutes of Health Grants R01 DK-38325 (to R.P.R.) and RO1 DK-58096 (to V.P.) and a Juvenile Diabetes Research Foundation advanced post-doctoral fellowship (to P.O.T.T.).