Type 2 diabetes (T2D) is a metabolic disorder characterized by hyperglycemia and insulin resistance in which oxidative stress is thought to be a primary cause. Considering that mitochondria are the main source of ROS, we have set out to provide a general overview on how oxidative stress is generated and related to T2D. Enhanced generation of reactive oxygen species (ROS) and oxidative stress occurs in mitochondria as a consequence of an overload of glucose and oxidative phosphorylation. Endoplasmic reticulum (ER) stress plays an important role in oxidative stress, as it is also a source of ROS. The tight interconnection between both organelles through mitochondrial-associated membranes (MAMs) means that the ROS generated in mitochondria promote ER stress. Therefore, a state of stress and mitochondrial dysfunction are consequences of this vicious cycle. The implication of mitochondria in insulin release and the exposure of pancreatic β-cells to hyperglycemia make them especially susceptible to oxidative stress and mitochondrial dysfunction. In fact, crosstalk between both mechanisms is related with alterations in glucose homeostasis and can lead to the diabetes-associated insulin-resistance status. In the present review, we discuss the current knowledge of the relationship between oxidative stress, mitochondria, ER stress, inflammation, and lipotoxicity in T2D.

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Since metformin can exert beneficial vascular effects, we aimed at studying its effect on reactive oxygen species (ROS) production, antioxidant enzyme expression, levels of adhesion molecules, and leukocyte-endothelium interactions in the leukocytes from type 2 diabetic (T2D) patients. The study was carried out in 72 T2D patients (41 of whom were treated with metformin for at least 12 months at a dose of 1700 mg per day), and in 40 sex- and age-matched control subjects. Leukocytes from T2D patients exhibited enhanced levels of mitochondrial ROS and decreased mRNA levels of glutathione peroxidase 1 (gpx1) and sirtuin 3 (sirt3) with respect to controls, whereas metformin was shown to revert these effects. No changes were observed on total ROS production and the expression levels of superoxide dismutase 1 and catalase. Furthermore, increases in leukocyte-endothelial interactions and intercellular adhesion molecule-1 and P-selectin levels were found in T2D and were also restored in metformin-treated patients. Our findings raise the question of whether metformin could modulate the appearance of atherosclerosis in T2D patients and reduce vascular events by decreasing leukocyte oxidative stress through an increase in gpx1 and sirt3 expression, and undermining adhesion molecule levels and leukocyte-endothelium interactions. Antioxid. Redox Signal. 27, 1439-1445.