Diabetic complications: from oxidative stress to inflammatory cardiovascular disorders

Insititut d’Investigacions Biomèdiques August Pi
i Sunyer (IDIBAPS)
Barcelona, SPAIN

by A. Ceriello, Spain

Evidence implicates hyperglycemia-derived oxygen free radicals as mediators of diabetic complications. However, intervention studies with classic antioxidants, such as vitamin E, have failed to demonstrate any beneficial effect. Recent studies demonstrate that a single hyperglycemiainduced process of superoxide overproduction by the mitochondrial electron- transport chain seems to be the first and key event in the activation of all the other pathways involved in the pathogenesis of diabetic complications. These include increased polyol pathway flux, advanced glycation end product formation, and hexosamine pathway flux, and activation of protein kinase C. These processes result in acute endothelial dysfunction in diabetic blood vessels that, convincingly, also contributes to the development of diabetic complications. While waiting for more focused tools, we will have to use other options, particularly the oral hypoglycemic agent gliclazide, which reduces glycemia while exerting an antioxidant effect.

Medicographia. 2011;33:29-34 (see French abstract on page 34)

In the last few decades the occurrence of type 2 diabetes mellitus has rapidly increased internationally, and it has been estimated that the number of diabetic patients will more than double within 15 years.1 As type 2 diabetes is mainly characterized by the development of increased cardiovascular disease (CVD) morbidity and mortality, it has been suggested that diabetes could be considered a CVD.1 However, diabetes is also characterized by dramatic microangiopathic complications, such as retinopathy, nephropathy, and neuropathy.1

Recent evidence suggests that glucose overload may damage cells through oxidative stress.2 This is currently the basis of the “unifying hypothesis,” in which hyperglycemia- induced oxidative stress may account for the pathogenesis of all diabetic complications.2

The central role of oxidative stress in the pathogenesis of diabetic complications

It has been suggested that four key biochemical changes induced by hyperglycemia—( i) increased flux through the polyol pathway (in which glucose is reduced to sorbitol, lowering levels of both reduced nicotinamide adenine dinucleotide phosphate [NADPH] and reduced glutathione); (ii) increased formation of advanced glycation end products (AGEs); (iii) activation of protein kinase C (PKC) (with effects ranging from vascular occlusion to expression of proinflammatory genes); and (iv) in- creased shunting of excess glucose through the hexosamine pathway (mediating increased transcription of genes for inflammatory cytokines)— are all activated by a common mechanism: overproduction of superoxide radicals.2

Excess plasma glucose drives excess production of electron donors (mainly NADH/H+) from the tricarboxylic acid cycle; in turn, this surfeit results in the transfer of single electrons (instead of the usual electron pairs) to oxygen, producing superoxide radicals and other reactive oxygen species (instead of the usual end product, H2O). The superoxide anion itself inhibits the key glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase, and consequently, glucose and glycolytic intermediates spill into the polyol and hexosamine pathways, as well as additional pathways that culminate in PKC activation and intracellular AGE formation (Figure 1).

Figure 1
Figure 1. Potential mechanism by which hyperglycemia-induced mitochondrial superoxide overproduction activates four pathways of hyperglycemic damage.

Excess superoxide partially inhibits the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH), thereby diverting upstream metabolites from glycolysis into pathways of glucose overutilization. This results inincreased flux of dihydroxyacetone phosphate (DHAP) to diacylglycerol (DAG), an activator of protein kinase C (PKC), and of triose phosphates to methylglyoxal, the main intracellular advanced glycation end product (AGE) precursor. Increased flux of fructose-6-phosphate to uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) increases modification of proteins by O-linked N-acetylglucosamine (GlcNAc) and increased glucose flux through the polyol pathway consumes nicotinamide dinucleotide phosphate (NADPH) and depletes glutathione.
Abbreviations: AGE, advanced glycation end product; DAG, diacylglycerol; DHAP, dihydroxyacetone phosphate; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GFAT, glutamine: fructose-6-phosphate aminotransferase; Gln, glutamine; Glu, glutamate; NAD(P), nicotinamide dinucleotide (phosphate); P, phosphate; PKC, protein kinase C; UDP-GlcNAc, uridine diphosphate-N-acetylglucosamine.

However, superoxide overproduction is also accompanied by increased nitric oxide (NO) generation, due to the uncoupled state of endothelial nitric oxide synthase (eNOS) and inducible nitric oxide synthase (iNOS),3 a phenomenon favoring the formation of the strong oxidant peroxynitrite, which in turn damages DNA.3 DNA damage is an obligatory stimulus for the activation of the nuclear enzyme poly(adenosine diphosphate [ADP]-ribose) polymerase (PARP).4 PARP activation in turn depletes the intracellular concentration of its substrate NAD+, slowing the rate of glycolysis, electron transport, and adenosine triphosphate formation, and producing ADP-ribosylation of the glyceraldehyde-3-phosphate dehydrogenase.4 These processes result in endothelial dysfunction (Figure 2).

These pathways have been confirmed by at least one study on the perfusion for 2 hours of isolated rat hearts with solutions of 11.1 mmol/L glucose, 33.3 mmol/L glucose, or 33.1 mmol/L glucose plus glutathione. In the hearts perfused with high glucose concentrations, coronary perfusion pressure increased significantly; there was a 40% increase in NO levels and an upregulation of iNOS, but a 300% increase in the production of superoxide species; nitrotyrosine and cardiac cell apoptosis also increased significantly.5 All these effects were substantially prevented by glutathione, which effectively removes reactive oxygen species, including peroxynitrite.5

However, more recently, evidence from in vitro studies suggests that marked fluctuations in glucose levels, as seen in diabetic patients, have consequences that are even more deleterious than those of continuous high glucose levels, and that oxidative stress is convincingly involved. For example, in cultures of human umbilical vein endothelial cells, levels of nitrotyrosine (a marker of oxidative stress), intercellular adhesion molecule 1 (ICAM-1), vascular cellular adhesion molecule 1, E-selectin, interleukin 6 (IL-6), and 8-hydroxydeoxyguanosine (a marker of oxidative damage of DNA) all increased after incubation in a medium containing 20 mmol glucose compared with incubation in a 5 mmol glucose medium, but alternating the two media caused even greater increases.6-8 In addition, intermittent hyperglycemic conditions increased rates of cellular apoptosis, and stimulated the expression of caspase 3 (a proapoptotic protein), but decreased Bcl2 (an antiapoptotic protein). These effects were abolished by adding superoxide dismutase (SOD), which scavenges free radicals, or inhibitors of the mitochondrial electron-transport chain, suggesting that overproduction of free radicals in the mitochondria mediates the apoptotic effects of increased glucose concentrations and fluctuations.9

Figure 2
Figure 2. Intracellular hyperglycemia induces overproduction of superoxide
at the mitochondrial level.

Overproduction of superoxide is the first and key event in the activation of all other pathways involved in the pathogenesis of diabetic complications, such as polyol pathway flux, increased AGE formation, and increased hexosamine pathway flux. Gliclazide may contribute to reduce
the risk for complications because it shows, simultaneously, a glycemia-lowering effect and
antioxidant action.
Abbreviation: AGE, advanced glycation end product.

Oxidative stress in diabetes: in vivo evidence

The response-to-injury hypothesis of atherosclerosis states that the initial damage affects the arterial endothelium, in terms of endothelial dysfunction. Notably, today’s evidence confirms that endothelial dysfunction, associated with oxidative stress, predicts CVD.10 Indeed, studies show that high glucose concentrations induce endothelial dysfunction in diabetic as well as normal subjects.3 The role of free radical generation in producing hyperglycemia-dependent endothelial dysfunction is also suggested by studies showing that the acute effects of hyperglycemia are counterbalanced by antioxidants.11,12

Numerous studies have also noted the effect of hyperglycemia- induced oxidative stress on inflammation. A study in which insulin secretion was blocked, and subjects were maintained at plasma glucose levels of 15 mmol/L for 5 hours, found that levels of IL-6, tumor necrosis factor α (TNF-α), and the proinflammatory cytokine IL-18 rose significantly and returned to baseline within 3 hours in the control group.13 However, patients with impaired glucose tolerance had significantly higher TNF-α and IL-6 levels at baseline, and cytokine levels reached substantially higher peaks and stayed elevated for considerably longer than in the control subjects.13 All changes in plasma cytokine levels were abolished by infusion of the antioxidant glutathione, consistent with the hypothesis that hyperglycemia, especially in the form of spikes, is linked to immune activation via an oxidative mechanism.13 Another study matching diabetic patients and healthy controls found increases in circulating ICAM-1 in both groups during an oral glucose tolerance test (OGTT); these increases were also abolished by glutathione.14 Glutathione administered without a glucose load decreased circulating ICAM-1 levels in the diabetic group, but not in the control group, again suggesting that hyperglycemia increases ICAM-1 levels via an oxidative mechanism.14

More direct evidence for the central role of oxidative stress is derived from clinical studies that measured markers. For example, among 20 diabetic patients, either a low-carbohydrate or a high-carbohydrate meal increased levels of plasma glucose, insulin, triglycerides, and malondialdehyde (a marker for lipid peroxidation), and decreased nonsterified fatty acids and the total radical-trapping antioxidant parameter (TRAP), a global measure of antioxidant capacity in the plasma.15 However, the high-carbohydrate meal (designed to produce higher postprandial glucose levels) increased glucose and malondialdehyde more, decreased TRAP significantly more, and rendered low-density lipoprotein more susceptible to oxidation than the low-carbohydrate meal.15 The decrease in TRAP highlights the fact that oxidative stress may also ensue from the failure of normal antioxidant defenses: the same group found that during the OGTT, TRAP was reduced from baseline in both well-controlled, nonsmoking diabetic subjects and healthy age-matched subjects, as were levels of protein-bound thiol (-SH) groups, vitamins C and E, and uric acid.15

As aforementioned, a superoxide anion combines with NO to produce a peroxynitrite ion; this species is capable of peroxidating lipoproteins and damaging DNA, which then activates the nuclear enzyme poly(ADP-ribose) polymerase, depleting intracellular NAD+ and (among other effects) causing acute endothelial dysfunction.3 In one study involving 12 healthy subjects, infusion of L-arginine (to supply NO) reversed hyperglycemia-induced increases in systolic and diastolic blood pressure, heart rate, plasma catecholamine levels, ADPinduced platelet aggregation, and blood viscosity.16 However, infusing NG-monomethyl-L-arginine, which inhibits the synthesis of endogenous NO, produced effects that were very similar to those produced by hyperglycemia. Thus, decreased NO availability may be one mechanism by which hyperglycemia induces hemodynamic and rheological changes in blood.16 It has been shown, however, that unlike normal con- trols, patients with diabetes have significantly elevated fasting nitrotyrosine levels, as well as postprandial increases after eating a standard mixed meal; the effect was significantly normalized by insulin aspart (which targets postprandial glucose), but not by regular insulin.17

Finally, consistent with the recent emerging role of glucose fluctuations, a new study confirms that in type 2 diabetes, diurnal glucose fluctuations are the most powerful predictors of oxidative stress generation.18

New perspectives: oxidative stress and hyperglycemia- induced “metabolic memory”

Large randomized studies have established that early intensive glycemic control reduces the risk of diabetic complications, both micro- and macrovascular.19 Moreover, epidemiological and prospective data support the idea that early metabolic control has a long-term influence on clinical outcomes.19 This phenomenon has recently been defined as “metabolic memory.”19 Potential mechanisms for propagating this “memory” are the nonenzymatic glycation of cellular proteins and lipids and an excess of cellular reactive oxygen and nitrogen species, in particular those that originated at the level of glycated mitochondrial proteins, perhaps acting in concert with one another to maintain stress signaling.19

_ Experimental evidence supporting the concept of “metabolic memory” and its possible link with oxidative stress
Several years ago, there were preliminary reports of the possibility that “hyperglycemic memory” for hyperproduction of fibronectin and collagen in endothelial cells persists after glucose normalization.20 Using the same design, ie, 14 days in high concentration glucose followed by 7 days of culture in normal concentration glucose, it has been shown that the overproduction of free radicals in endothelial cells persists after normalization of glucose concentration, and this is accompanied by a prolongation of the induction of PKC-β, NAD(P)H oxidase, Bax, collagen, and fibronectin, in addition to 3-nitrotyrosine.21 This suggests that oxidative stress may be involved in the “metabolic memory” effect.

The effect of reinstitution of good glucose control on hyperglycemia- induced increased oxidative stress and nitrative stress has also been previously evaluated in the retina of rats maintained with poor glucose control before initiation of good control.22 In diabetic rats, 2 or 6 months of poor control (glycated hemoglobin [HbA1c] >11.0%) was followed by 7months of good control (HbA1c <5.5%). Reinstitution of good control after 2 months of poor control inhibited elevations in retinal lipid peroxide and NO levels by approximately 50%, but failed to have any beneficial effects on nitrotyrosine formation. However, reversal of hyperglycemia after 6 months of poor control had no significant effect on retinal oxidative stress and NO levels. In the same rats, iNOS expression and nitrotyrosine levels remained elevated by >80% compared with normal rats or rats with good glucose control for the duration.22 In a similar study, caspase 3 activity in diabetic rats with poor control for 13 months was higher than in normal rats.23 Reinstitution of good glycemic control after 2 months of poor control partially normalized the hyperglycemia-induced activation of caspase 3 (to 140% of normal values), while reinstitution of good control after 6 months of poor control had no significant effect on the activation of caspase 3. In the same study, nuclear factor-κB (NFκB) activity was 2.5-fold higher in diabetic rats with poor glucose control than in normal rats.

Reinstitution of good control after 2 months of poor control partially reversed this increase, but good control after 6months of poor control had no effect. Initiation of good control soon after induction of diabetes in rats prevented activation of retinal caspase 3 and NFκB.23 Similar results are available for the kidney. Diabetic rats were maintained with good glycemic control (HbA1c = 5%) soon, or 6 months, after induction of diabetes, and were sacrificed after 13 months.24 For rats in which good control was initiated soon after the induction of diabetes, oxidative stress (as measured by levels of lipid peroxides, 8- hydroxy-2´-deoxyguanosine, and reduced glutathione) and NO levels in urine and renal cortex were no different from those observed in normal control rats, but when the reinstitution of good control was delayed for 6 months after induction of diabetes, oxidative stress and NO remained elevated in both urine and renal cortex.24 These data suggest that hyperglycemia- induced oxidative stress and NO, as well as activation of apoptosis and NFκB, can be prevented if good glycemic control is initiated very early, but are not easily reversed if poor control is maintained for longer durations. Therefore, these findings suggest the persistence of hyperglycemia-induced damage in such organs, even after glycemia normalization.

However, if excess reactive species are central to the development of hyperglycemia-related diabetic complications, could this excess explain the persistence of the risk of complications even when hyperglycemia is reduced or normalized?

The above reported studies suggest that long-lasting effects of hyperglycemia result in increased oxidative stress, while inhibiting oxidative stress has preliminarily been shown to reverse these effects.21 Mitochondrial overproduction of superoxide in hyperglycemia has been suggested as the “unifying hypothesis” for the development of diabetic complications.2 Therefore, it is reasonable to assume that mitochondria are also important players in propagating “metabolic memory.” Chronic hyperglycemia is thought to alter mitochondrial function through glycation of mitochondrial proteins.25 Levels of methylglyoxal, a highly-reactive alpha-dicarbonil byproduct of glycolysis, increase in diabetes.26 Methylglyoxal readily reacts with arginine, lysine, and sulfhydryl groups of proteins,26 in addition to nucleic acids,26 inducing the formation of a variety of structurally identified AGEs in both target cells and plasma.26 Methylglyoxal has an inhibitory effect on mitochondrial respiration and methylglyoxal-induced modifications are targeted to specific mitochondrial proteins.26 These premises are important because a recent study has described, for the first time, a direct relationship between formation of intracellular AGEs on mitochondrial proteins and the decline in mitochondrial function and excess formation of reactive species.25 Mitochondrial respiratory chain proteins that underwent glycation were prone to produce more superoxide, independent of the level of hyperglycemia. The glycation of mitochondrial proteins may be a contributing explanation for the phenomenon of “metabolic memory.” The glycation of mitochondrial proteins that overproduce free radicals, independent of actual glycemia, can also lead to a catastrophic cycle of mitochondrial DNA damage, as well as functional decline, cellular injury, further oxygen radical generation, and the continued activation of pathways involved in the pathogenesis of diabetic complications.27 Furthermore, mitochondrial proteins become damaged or posttranslationally modified as a consequence of a major change in a cell’s redox status.27 This may affect mitochondrially destined proteins that are imported into the mitochondrial outer membrane, inner membrane, or matrix space via specific import machinery transport components.27

In other words, it may be postulated that the cascade of events in “metabolic memory” is the same as that proposed by Brownlee2; the source of superoxide is still the mitochondria, but, in addition, the production of reactive species is unrelated to the presence of hyperglycemia; it depends on the level of glycation of mitochondrial proteins.

How could oxidative stress be reduced with pharmacological intervention?

Antioxidant therapy may be of great value in diabetic patients. However, the classic antioxidants, like vitamins E and C, do not seem to be helpful. New insights into the mechanisms leading to the generation of oxidative stress indiabetes are now available. Presumably, these findings will lead to the discovery and evaluation of new antioxidant molecules, such as superoxide dismutase (SOD) and catalase mimetics, that may hopefully inhibit the mechanism leading to diabetic complications at an early stage. While waiting for these specific new compounds, it is reasonable to suggest that substances already available, such as statins, angiotensin-converting enzyme inhibitors, and angiotensin receptor blockers, should be used for their effectiveness as “causal and preventive” antioxidants (for an up-to-date review, see reference 28).

The availability of compounds that simultaneously decrease hyperglycemia, restore insulin secretion, and inhibit oxidative stress produced by high glucose is an interesting therapeutic prospect for the prevention of vascular complications of diabetes. Gliclazide, an oral hypoglycemic agent that belongs to the sulfonylurea class, has been demonstrated to be effective and safe in numerous clinical trials and in clinical practice. Several studies have demonstrated, both in vitro and in vivo, that gliclazide shows antioxidative potential, independent of its hyperglycemia-lowering effect.29 Gliclazide is a general free radical scavenger in vitro—in contrast with glibenclamide, which fails to produce any effect below a concentration of 25 ìg/mL (gliclazide induced strong concentration-dependent inhibition of free radical generation at therapeutic concentrations).29 Jennings et al confirmed these effects of gliclazide on oxidative stress in clinical conditions. They found that gliclazide-treated type 2 patients with retinopathy had a highly significant and sustained decrease in peroxidized lipids and an increase in erythrocyte SOD activity.30 Interestingly, glucose control did not differ between therapeutic groups, which supports the hypothesis that the effect results from the molecule gliclazide itself, rather than from a general improvement in metabolic control.

The antioxidative property of gliclazide convincingly impacts the vascular system in diabetes. Fava et al studied both the antioxidative potential of gliclazide in vivo and its effect on vascular reactivity.31 In this experiment, blood glucose control remained unchanged from baseline and similar in both groups, as patients were already being treated, which excludes any glucose-related “bias effect.” Thirty type 2 diabetic patients received glibenclamide or gliclazide in a 12-week, randomized, observer-blinded, parallel study. Blood pressure responses to an intravenous bolus of L-arginine were measured pre- and posttreatment. Gliclazide, but not glibenclamide, significantly reduced systolic and diastolic blood pressure in response to intravenous L-arginine. This provided the first demonstration that gliclazide significantly enhances NO-mediated vasodilatation and thus improves vascular reactivity in type 2 diabetic patients.

Finally, and this could be of great relevance, in order to avoid the development of diabetic complications, it has been shown that gliclazide can block the “metabolic memory” effect.32 In my opinion, all these effects33 may contribute to explaining why gliclazide prevented nephropathy in the Action in Diabetes and Vascular disease: PreterAx and DiamicroN MR Controlled Evaluation (ADVANCE).34 Conclusions

Our understanding of the molecular pathways activated inside the cell by hyperglycemia is growing, and evidence about the involvement of oxidative stress in the development of diabetic complications is becoming abundant, making the “unifying hypothesis” more persuasive every day. Against this background, the finding of unexpected protective effects of drugs intended for different uses or different pathologies has given us an intriguing opportunity to elucidate their underlying mechanisms, to tune up these “weapons” to be more and more effective, and to confirm the hypothesis formulated. These goals are becoming increasingly important due to the massive spread in diabetic pathology that is expected to occur in the coming years. _

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Keywords: hyperglycemia; free radicals; antioxidants; superoxide; pathogenesis; polyol; advanced glycosylation end product; hexosamine pathway