Effects of S21403 (mitiglinide) on postprandial generation of oxidative stress and inflammation in type 2 diabetic patients

AIM/HYPOTHESIS

Evidence suggests that postprandial hyperglycaemia may be a cardiovascular risk factor in diabetes. Oxidative stress and inflammation are involved in the pathogenesis of diabetic complications and previous studies have shown increased oxidative stress and inflammation in the postprandial phase in diabetic patients. The aim of the present study was to evaluate whether controlling postprandial hyperglycaemia with S21403 (mitiglinide) is accompanied by a reduced generation of oxidative stress and inflammation.

SUBJECTS AND METHODS

Forty type 2 diabetic patients participated in the study. Two different breakfast-tests were performed in each patient, with placebo or S21403. Plasma nitrotyrosine, plasma malondialdehyde (MDA), oxidised LDL (oxLDL), plasma total radical-trapping antioxidant parameter (TRAP), IL-6, IL-18, TNF-alpha, plasma glucose and insulin were measured.

RESULTS

After the administration of S21403, 40 mg, a rapid stimulation of insulin secretion was observed, accompanied by a reduction of postprandial hyperglycaemia. With S21403, a significant decrease of either nitrotyrosine, MDA and oxLDL levels, and a preservation of plasma TRAP compared with placebo was found. Significant decreases of IL-6, IL-18 and TNF-alpha were also observed with S21403 compared with placebo.

CONCLUSIONS/INTERPRETATION

This study shows that controlling postprandial hyperglycaemia with S21403 significantly improves the cluster of oxidative stress and inflammation markers that are increased in the postprandial state in diabetic patients.

Effect of irbesartan on nitrotyrosine generation in non-hypertensive diabetic patients

AIMS/HYPOTHESIS

Oxidative stress is involved in the pathogenesis of microangiopathic and macroangiopathic diabetic complications. The results of recent trials suggest that type 1 angiotensin II (AT-1) receptor blockers may prevent or delay nephropathy and cardiovascular disease in diabetic patients, independently of their anti-hypertensive action. There is evidence that AT-1 receptor blockers can work as intracellular antioxidants. This study investigated whether the AT-1 receptor blocker irbesartan is able to reduce nitrotyrosine formation in non-hypertensive diabetic patients under fasting conditions and during acute hyperglycaemia.

METHODS

A total of 40 non-hypertensive, non-microalbuminuric Type 2 diabetic patients and 20 healthy, normotensive subjects were recruited for this study. Diabetic patients followed a randomised, double-blind, placebo-controlled, crossover protocol, taking either irbesartan (150 mg orally, twice daily) or placebo for 60 days. Fasting glucose and nitrotyrosine were measured at baseline and at the end of each treatment period. An OGTT was also performed at the same time intervals, during which plasma glucose and nitrotyrosine levels were monitored.

RESULTS

Compared with baseline measurements, treatment with irbesartan (0.57+/-0.4 vs 0.35+/-0.3 micromol/l, p<0.01) but not placebo (0.58+/-0.3 vs 0.59+/-0.2 micromol/l) significantly reduced fasting nitrotyrosine levels. Irbesartan also significantly reduced nitrotyrosine formation during the OGTT.

CONCLUSIONS/INTERPRETATION

. This study demonstrates that irbesartan reduces plasma levels of nitrotyrosine in diabetic patients and is effective in counterbalancing nitrotyrosine formation during acute hyperglycaemia. Our results may help to elucidate how AT-1 receptor blockers exert their beneficial effect independently of their BP-lowering activity.

Intermittent high glucose enhances ICAM-1, VCAM-1, E-selectin and interleukin-6 expression in human umbilical endothelial cells in culture: the role of poly(ADP-ribose) polymerase

It has been previously reported that endothelial cells exposed to constant high concentrations of glucose upregulate the expression of adhesion molecules. Moreover, it has been suggested that this phenomenon is related to generation of oxidative stress. It has also been suggested that oxidative injuries, related to high glucose, induce the activation of the enzyme poly ADP ribose polymerase (PARP), which can promote the expression of adhesion molecules and the generation of inflammation. Recent in-vivo and in-vitro evidence suggests that oscillation of glucose may play an autonomous and direct role in favoring the development of cardiovascular complications in diabetes. In this study we have investigated the effects of constantly high and intermittently high glucose on nitrotyrosine formation (a marker of nitrosative stress) and adhesion molecule (ICAM-1, VCAM-1 and E-selectin), as well as on interleukin (IL)-6 expression in human umbilical vein endothelial cells, either in the presence or in the absence of PJ34, a potent inhibitor of PARP. We found that oscillating glucose was more effective in triggering the generation of nitrotyrosine and inducing the expression of adhesion molecules and IL-6 than stable high glucose. Pharmacological inhibition of PARP suppressed both nitrotyrosine formation, adhesion molecule expression and IL-6 to the levels seen in the normal glucose conditions. Thus, PARP activation appears to be involved in both promoting nitrosative stress and upregulating adhesion molecules and inflammation in endothelial cells exposed to oscillating high glucose conditions.

The post-prandial state in Type 2 diabetes and endothelial dysfunction: effects of insulin aspart

OBJECTIVE

Recently, much attention has been focused on the possibility that the post-prandial state may be a cardiovascular risk factor in diabetes. The aim of the present study was to evaluate whether the post-prandial state is associated with endothelial dysfunction in patients with diabetes and to explore the effect on this aspect of managing post-prandial hyperglycaemia by insulin aspart.

RESEARCH DESIGN AND METHODS

Twenty-three patients with Type 2 diabetes and 10 normal controls were recruited. In the diabetic patients two different tests were performed in each subject: a standard meal preceded by subcutaneous injection of soluble insulin (0.15 U/kg body weight) or of short-acting insulin aspart (0.15 U/kg body weight). These tests were designed to achieve different levels of post-prandial hyperglycaemia. Controls received a single standard meal test. Immediately before, and 1, 2, 4 and 6 h after each meal, blood glucose, triglycerides, free fatty acids and flow-mediated vasodilation were measured.

RESULTS

Compared with regular insulin, insulin aspart significantly reduced the area under the curve for post-prandial hyperglycaemia (58.3 +/- 17.6 vs. 68.1 +/- 17.7; P<0.04), and preserved flow-mediated vasodilation, which was decreased in the post-prandial state (39.4 +/- 2.9 vs. 34.1 +/- 2.2; P<0.01). Triglyceride and free fatty acid levels were not differentially affected by the treatment. In normal controls the meal did not affect flow-mediated vasodilation.

CONCLUSION

This study shows that the post-prandial state is accompanied by endothelial dysfunction in Type 2 diabetic patients and that insulin aspart improved endothelial function.

High-fructose diet decreases catalase mRNA levels in rat tissues

Insulin resistance and hyperinsulinemia have recently been identified as independent determinants of several risk factors for cardiovascular disease. The generation of reactive oxygen species (ROS) may play an important role as a final common mediator by which glucose and insulin resistance might contribute to development of cardiovascular disease and hypertension. The aim of the present study was to evaluate changes on mRNA expression of antioxidant enzymes [catalase, Cu-Zn superoxide dismutase (Cu-ZnSOD), MnSOD], blood pressure and metabolic parameters in insulin resistance that follow feeding normotensive Wistar rats a high-fructose-enriched diet. In our investigation 26 normal male Wistar rats were fed a high-fructose diet for 2 weeks (no.=14) or normal chow to serve as a control group (no.=12). In vivo insulin resistance was verified in a subgroup of control and fructose-fed rats by the euglycemic hyperinsulinemic clamp technique at 2 different insulin infusion rates, 29 (submaximal stimulation) and 290 (maximal stimulation) pmol/kg/min respectively. The glucose infusion rate (GIR) was not significantly different in the two groups during the submaximal infusion of insulin (1.4 +/- 0.8 mmol/kg/min in fructose-fed rats vs 1.6 +/- 0.7 mmol/kg/min in control rats, NS) while in fructose-fed rats it was significantly lower (-29.8%) than in control rats during maximal infusion of insulin (2.6 +/- 0.3 mmol/kg/min vs 3.7 +/- 0.3 mmol/kg/min, p<0.05). Fructose feeding markedly reduced the expression of catalase mRNA and Cu-ZnSOD mRNA in the liver, catalase mRNA in the heart (p<0.05). A tendency of fructose feeding to reduce the expression of antioxidant enzymes in skeletal muscle and adipose tissue was also observed (NS). Fructose feeding also increased plasma uric acid (119.9 +/- 30.4 vs 42.1 +/- 10 pmol/l, p<0.05) and systemic blood pressure (128 +/- 4 vs 109 +/- 5 mmHg, p<0.05) respect to control animals. No significant changes were observed in plasma levels of glycemia and tryglycerides. Our study suggests that in non-hyperglycemic, fructose-fed insulin-resistant rats the expression of catalase is inhibited in liver and heart. This condition might lead to higher susceptibility to oxidative stress in insulin resistance. However, an adaptive cellular response to maintain the effectiveness of intracellular signaling pathways mediated by insulin-activated hydrogen peroxide generating systems may also be hypothesized.

Intermittent high glucose enhances apoptosis in human umbilical vein endothelial cells in culture

To explore the effect of fluctuating glucose on endothelial cells, human umbilical vein endothelial cells were incubated for 14 days in media containing different glucose concentrations: 5 mmol/l, 20 mmol/l, or a daily alternating 5 or 20 mmol/l glucose. Apoptosis was studied by different methods: viability assay, cell cycle analysis, DNA fragmentation, and morphological analysis. Furthermore, the levels of Bcl-2 and Bax, well known proteins involved in apoptosis, were evaluated. Stable high glucose induced apoptosis in human umbilical vein endothelial cells, a phenomenon accompanied by a significant decrease of Bcl-2 and a simultaneous increase of Bax expression. However, apoptosis was enhanced in human umbilical vein endothelial cells exposed to intermittent, rather than constant, high glucose concentration. In this condition, Bcl-2 was not detectable, whereas Bax expression was significantly enhanced. These findings suggest that variability in glycemic control could be more deleterious to endothelial cells than a constant high concentration of glucose.

Detection of nitrotyrosine in the diabetic plasma: evidence of oxidative stress

AIMS/HYPOTHESIS

Oxidative stress plays an important role in diabetic vascular complications. It has been shown that an imbalance in the ratio of nitric oxide: superoxide anion, because of a prevalence of superoxide anion, leads to an alteration in vascular reactivity. In this condition peroxynitrite production, resulting from the reaction between nitric oxide and superoxide, could increase. Peroxynitrite is responsible for nitration of tyrosine residues in proteins. Therefore, the presence of nitrotyrosine in plasma proteins is considered indirect evidence of peroxynitrite production. The aim of this study was to demonstrate the presence of nitrotyrosine in the plasma of patients with Type II (non-insulin-dependent) diabetes mellitus and to correlate its concentrations with the plasma concentrations of glucose and antioxidant defenses.

METHODS

A total of 40 Type II diabetic patients and 35 healthy subjects were enrolled, and glycaemia, plasma nitrotyrosine, total antioxidant parameter and glycated haemoglobin were measured. Nitrotyrosine was detected by ELISA with a detection limit of 10 nmol/l.

RESULTS

Nitrotyrosine was found in the plasma of all diabetic patients (means +/- SD = 0.251 +/- 0.141 micromol/l), whereas it was not detectable in the plasma of healthy control subjects. Nitrotyrosine plasma values were correlated with plasma glucose concentrations (r = 0.38, p < 0.02) but not with total antioxidant parameter or glycated haemoglobin. Total antioxidant parameter was reduced in diabetic patients (p < 0.01).

CONCLUSIONS

The presence of nitrotyrosine in the plasma of diabetic patients indicates that peroxynitrite is generated in diabetes, suggesting a possible involvement of peroxynitrite in the development of diabetic complications.

Defective intracellular antioxidant enzyme production in type 1 diabetic patients with nephropathy

There is an individual susceptibility to diabetic nephropathy, and oxidative stress is believed to play an important role in the pathogenesis of diabetic complications. Active oxygen species induce antioxidant enzyme expression in tissues, an effect considered to be a defensive mechanism. To test whether altered intracellular antioxidant enzyme production might explain the predisposition to diabetic nephropathy, we studied the effect of long-term (12 weeks) exposure to normal (5 mmol/l) or high (22 mmol/l) glucose concentrations on fibroblast antioxidant enzyme gene expression and protein activity in type 1 diabetic patients with and without nephropathy, nondiabetic nephropathic patients, and nondiabetic control subjects. Under conditions of normal glucose concentration in the culture media, CuZnSuperoxide-dismutase, MnSuperoxide-dismutase, catalase, and glutathione-peroxidase activity and mRNA expression were not different among the four groups. Under high-glucose conditions, CuZnSuperoxide-dismutase mRNA and activity increased similarly in all groups (P < 0.001 vs. basal), whereas MnSuperoxide-dismutase did not change. In contrast, catalase mRNA and activity as well as glutathione-peroxidase mRNA and activity increased in fibroblasts from type 1 diabetic patients without nephropathy (P < 0.001), in fibroblasts from nondiabetic nephropathic patients (P < 0.001), and in fibroblasts from nondiabetic control subjects (P < 0.001), but not in fibroblasts from type 1 diabetic patients with nephropathy. Exposure to high glucose concentrations significantly increased lipid peroxidation in cells, higher levels being found in cells from diabetic patients with nephropathy (P < 0.001). These data, while confirming that exposure to high glucose concentrations induces an antioxidant defense in skin fibroblasts from normal subjects, demonstrate a failure of this defensive mechanism in cells from type 1 diabetic patients with nephropathy, whereas skin fibroblasts from diabetic patients without complications or from nondiabetic nephropathic patients have an intact antioxidant response to glucose-induced oxidative stress.

Oxidative stress evaluation in diabetes

Over the recent years, researches have focused their attention on the pathologic role of free radicals in a variety of diseases, among which the most important are atherosclerosis, cancer, and diabetes. The set of intracellular and extracellular conditions that leads to chemical or metabolic generation of reactive species is termed "oxidative stress." The susceptibility to oxidative stress is a function of the overall balance between the factors that exert oxidative stress and those that exhibit antioxidant capability. There is currently great interest in the potential contribution of increased oxidative stress to the development of complications in diabetes mellitus. Direct measurement of oxidative stress in vivo is a very complex question, because free radicals are highly reactive, have a very short life, and are present in very low concentrations. Thus, indirect methods, used for measuring secondary products of oxidative stress, are rather unspecific and may give conflicting data. Nitrotyrosine detection in plasma and tissues may be a useful method to demonstrate peroxynitrite-mediated damage. The total radical-trapping potential (TRAP) in plasma represents a more reliable estimation of serum antioxidant capability than the measurement of each known antioxidant. The detection of increased levels of oxidation products in tissue and biological fluids is important to investigate the relation between free radical production and the development of pathology. This hypothesis suggests the possibility of a therapeutic intervention with antioxidant agents. The identification of a useful marker to assess the effect of antioxidants on oxidative stress seems to be mandatory.