Cristina Sena

Endothelial dysfunction — A major mediator of diabetic vascular disease

The vascular endothelium is a multifunctional organ and is critically involved in modulating vascular tone and structure. Endothelial cells produce a wide range of factors that also regulate cellular adhesion, thromboresistance, smooth muscle cell proliferation, and vessel wall inflammation. Thus, endothelial function is important for the homeostasis of the body and its dysfunction is associated with several pathophysiological conditions, including atherosclerosis, hypertension and diabetes. Patients with diabetes invariably show an impairment of endothelium-dependent vasodilation. Therefore, understanding and treating endothelial dysfunction is a major focus in the prevention of vascular complications associated with all forms of diabetes mellitus. The mechanisms of endothelial dysfunction in diabetes may point to new management strategies for the prevention of cardiovascular disease in diabetes. This review will focus on the mechanisms and therapeutics that specifically target endothelial dysfunction in the context of a diabetic setting. Mechanisms including altered glucose metabolism, impaired insulin signaling, low-grade inflammatory state, and increased reactive oxygen species generation will be discussed. The importance of developing new pharmacological approaches that upregulate endothelium-derived nitric oxide synthesis and target key vascular ROS-producing enzymes will be highlighted and new strategies that might prove clinically relevant in preventing the development and/or retarding the progression of diabetes associated vascular complications.

Methylglyoxal promotes oxidative stress and endothelial dysfunction.

Modern diets can cause modern diseases. Research has linked a metabolite of sugar, methylglyoxal (MG), to the development of diabetic complications, but the exact mechanism has not been fully elucidated. The present study was designed to investigate whether MG could directly influence endothelial function, oxidative stress and inflammation in Wistar and Goto-Kakizaki (GK) rats, an animal model of type 2 diabetes. Wistar and GK rats treated with MG in the drinking water for 3 months were compared with the respective control rats. The effects of MG were investigated on NO-dependent vasorelaxation in isolated rat aortic arteries from the different groups. Insulin resistance, NO bioavailability, glycation, a pro-inflammatory biomarker monocyte chemoattractant protein-1 (MCP-1) and vascular oxidative stress were also evaluated. Methylglyoxal treated Wistar rats significantly reduced the efficacy of NO-dependent vasorelaxation (p<0.001). This impairment was accompanied by a three fold increase in the oxidative stress marker nitrotyrosine. Advanced glycation endproducts (AGEs) formation was significantly increased as well as MCP-1 and the expression of the receptor for AGEs (RAGE). NO bioavailability was significantly attenuated and accompanied by an increase in superoxide anion immunofluorescence. Methylglyoxal treated GK rats significantly aggravated endothelial dysfunction, oxidative stress, AGEs accumulation and diminished NO bioavailability when compared with control GK rats. These results indicate that methylglyoxal induced endothelial dysfunction in normal Wistar rats and aggravated the endothelial dysfunction present in GK rats. The mechanism is at least in part by increasing oxidative stress and/or AGEs formation with a concomitant increment of inflammation and a decrement in NO bioavailability. The present study provides further evidence for methylglyoxal as one of the causative factors in the pathogenesis of atherosclerosis and development of macrovascular diabetic complication.

Metformin restores endothelial function in aorta of diabetic rats

The effects of metformin, an antidiabetic agent that improves insulin sensitivity, on endothelial function have not been fully elucidated. This study was designed to assess the effect of metformin on impaired endothelial function, oxidative stress, inflammation and advanced glycation end products formation in type 2 diabetes mellitus.

Effects of α‐lipoic acid on endothelial function in aged diabetic and high‐fat fed rats

This study was conducted to investigate the effects of α‐lipoic acid (α‐LA) on endothelial function in diabetic and high‐fat fed animal models and elucidate the potential mechanism underlying the benefits of α‐LA.

Methylglyoxal, obesity, and diabetes

Methylglyoxal (MG) is a highly reactive compound derived mainly from glucose and fructose metabolism. This metabolite has been implicated in diabetic complications as it is a strong AGE precursor. Furthermore, recent studies suggested a role for MG in insulin resistance and beta-cell dysfunction. Although several drugs have been developed in the recent years to scavenge MG and inhibit AGE formation, we are still far from having an effective strategy to prevent MG-induced mechanisms. This review summarizes the mechanisms of MG formation, detoxification, and action. Furthermore, we review the current knowledge about its implication on the pathophysiology and complications of obesity and diabetes.

Insulin protects against amyloid β-peptide toxicity in brain mitochondria of diabetic rats

This study compared the status of brain mitochondria isolated from 12-week streptozotocin (STZ)-diabetic rats versus STZ-diabetic animals treated with insulin during a period of 4 weeks. Brain mitochondria isolated from 12-week citrate (vehicle)-treated rats were used as control. For that purpose, several mitochondrial parameters were evaluated: respiratory indexes (respiratory control ratio (RCR) and ADP/O ratio), transmembrane potential (DeltaPsim), repolarization lag phase, repolarization level, ATP, glutathione and coenzyme Q (CoQ) contents, production of H2O2, ATPase activity, and the capacity of mitochondria to accumulate Ca2+. Furthermore, the effect of Abeta1-40 was also analyzed. We observed that STZ-induced diabetes promoted a significant decrease in mitochondrial CoQ9, ATPase activity, and a lower capacity of mitochondria to accumulate Ca2+ when compared with control and insulin-treated diabetic rats. The presence of 4 microM Abeta1-40 induced a significant decrease in RCR in the three groups of rats. However, this peptide induced a significant increase in the repolarization lag phase and a significant decrease in the repolarization level in control and diabetic animals without insulin treatment. Furthermore, this peptide exacerbated significantly the production of H2O2 in STZ-diabetic rats, this effect being avoided by insulin treatment. Our data show that although diabetes induces some alterations in brain mitochondrial activity, those alterations do not interfere significantly with mitochondria functional efficiency. Similarly, insulin does not affect basal mitochondria function. However, in the presence of amyloid beta-peptide, insulin seems to prevent the decline in mitochondrial oxidative phosphorylation efficiency and avoids an increase in oxidative stress, improving or preserving the function of neurons under adverse conditions, such as Alzheimers disease.

CoQ10 therapy attenuates amyloid β-peptide toxicity in brain mitochondria isolated from aged diabetic rats

Using brain mitochondria isolated from 20-month-old diabetic Goto-Kakizaki rats, we evaluated the efficacy of CoQ10 treatment against mitochondrial dysfunction induced by Abeta1-40. For that purpose, several mitochondrial parameters were evaluated: respiratory indexes (RCR and ADP/O ratio), transmembrane potential (DeltaPsim), repolarization lag phase, repolarization and ATP levels and the capacity of mitochondria to produce hydrogen peroxide. We observed that 4 microM Abeta1-40 induced a significant decrease in the RCR and ATP content and a significant increase in hydrogen peroxide production. CoQ10 treatment attenuated the decrease in oxidative phosphorylation efficiency and avoided the increase in hydrogen peroxide production induced by the neurotoxic peptide. These results indicate that CoQ10 treatment counteracts brain mitochondrial alterations induced by Abeta1-40 suggesting that CoQ10 therapy can help to avoid a drastic energy deficiency that characterizes diabetes and Alzheimers disease pathophysiology.

Methylglyoxal‐induced imbalance in the ratio of vascular endothelial growth factor to angiopoietin 2 secreted by retinal pigment epithelial cells leads to endothelial dysfunction

Progressive microvascular complications are a main feature of diabetes and are associated with impairment of the angiogenic response. Methylglyoxal (MGO) has been implicated in the molecular events that lead to endothelial dysfunction in diabetes. In this study, we hypothesize that increased levels of MGO disrupt the ratio of vascular endothelial growth factor (VEGF) to angiopoietin 2 (Ang 2) secreted by retinal pigment epithelial (RPE) cells, which provides a key destabilizing signal that leads to apoptosis and decreased proliferation of retinal endothelial cells. Indeed, we show that MGO increases the levels of Ang 2 and dramatically decreases the levels of VEGF secreted by RPE cells in response to hypoxia. Downregulation of VEGF is likely to be related to decreased hypoxia‐inducible factor‐1α (HIF‐1α) protein levels and HIF‐1 transcriptional activity. Data further show that MGO‐induced imbalance in the VEGF/Ang II ratio significantly changes the levels of BAX and Bcl‐2 in endothelial cells. Moreover, this imbalance is accompanied by an increase in the activity of caspase‐3 and decreased proliferation of endothelial cells. Data obtained in cell culture systems are consistent with observations in retinas of diabetic animals, where increased availability of MGO is associated with changes in distribution and levels of HIF‐1α, VEGF and Ang 2 and increased microvascular permeability. In conclusion, the MGO‐induced imbalance in the VEGF/Ang 2 ratio secreted by retinal epithelial cells activates apoptosis and decreases proliferation of retinal endothelial cells, which are likely to contribute to endothelial dysfunction in diabetic retinopathy.

Supplementation of coenzyme Q10 and α-tocopherol lowers glycated hemoglobin level and lipid peroxidation in pancreas of diabetic rats

The importance of nutritional supplementation in diabetes remains an unresolved issue. The present study was undertaken to examine the effects of alpha-tocopherol and CoQ(10), powerful antioxidants, on metabolic control and on the pancreatic mitochondria of GK rats, a model of type 2 diabetes. We also evaluated the efficacy of these nutrients in preventing the diabetic pancreatic lesions observed in GK rats. Rats were divided into 4 groups, a control group of diabetic GK rats and 3 groups of GK rats administered with alpha-tocopherol and CoQ(10) alone or both in association, during 8 weeks. Fasting blood glucose levels were not significantly different between the groups, nor were blood glucose levels at 2 hours after a glucose load. HbA1c level was significantly reduced in the group supplemented with both antioxidants. Diabetes induced a decrease in coenzyme Q plasma levels that prevailed after treatment with antioxidants. In addition, the plasma alpha-tocopherol levels were higher after treatment with the antioxidants. An increment in some components of the antioxidant defense system was observed in pancreatic mitochondria of treated GK rats. Moreover, the antioxidants tested either alone or in association failed to prevent the pancreatic lesions in this animal model of type 2 diabetes. In conclusion, our results indicate that CoQ(10) and alpha-tocopherol decrease glycated HbA1c and pancreatic lipid peroxidation. These antioxidants increase some components of the antioxidant defense system but do not prevent pancreatic lesions. Thus, we cannot rule out the potential benefit of antioxidant treatments in type 2 diabetes in the prevention of their complications.

Insulin attenuates diabetes-related mitochondrial alterations: a comparative study.

This study evaluated and compared the effect of insulin treatment on the status of brain, heart and kidney mitochondria isolated from 12-week streptozotocin (STZ)-induced diabetic rats versus STZ-diabetic animals treated with insulin during a period of 4 weeks. Mitochondria isolated from 12-week citrate (vehicle)-treated rats were used as control. Several mitochondrial parameters were evaluated: respiratory indexes (state 3 and 4 of respiration, respiratory control and ADP/O ratios), transmembrane potential, depolarization and repolarization levels, ATP, glutathione and coenzyme Q contents, production of hydrogen peroxide, superoxide dismutase, glutathione peroxidase and glutathione reductase activities and the ability of mitochondria to accumulate calcium. We observed that diabetes promoted a significant decrease in kidney and brain mitochondrial coenzyme Q9 content while this parameter was increased in heart mitochondria. Furthermore, diabetes induced a significant increase in hydrogen peroxide production in kidney mitochondria this effect being accompanied by a significant increase in glutathione peroxidase and reductase activities. Furthermore, brain mitochondria isolated from diabetic animals presented a lower ATP content and ability to accumulate calcium. In contrast, heart and kidney mitochondria presented a slight higher capacity to accumulate calcium. Insulin treatment normalized the levels of coenzyme Q9 and glutathione peroxidase and reductase activities and increased ATP content and the ability to accumulate calcium. Altogether these results suggest that insulin treatment attenuates diabetes-induced mitochondrial alterations protecting against the increase in oxidative stress and improving oxidative phosphorylation efficiency. In this line, insulin therapy, besides its well-known importance in the maintenance of glycemic control, may help to protect against mitochondrial dysfunction associated to several age-related disorders such as diabetes.