Abstract
Diabetic complications are the leading cause of morbidity and mortality in diabetic patients. Elevated blood glucose contributes to the development of endothelial and vascular dysfunction, and, consequently, to diabetic micro- and macrovascular complications, because it increases the mitochondrial proton gradient and mitochondrial oxidant production. Therapeutic approaches designed to counteract glucose-induced mitochondrial reactive oxygen species (ROS) production in the vasculature are expected to show efficacy against all diabetic complications, but direct pharmacological targeting (scavenging) of mitochondrial oxidants remains challenging due to the high reactivity of some of these oxidant species. In a recent study, we have conducted a medium-throughput cell-based screening of a focused library of well-annotated pharmacologically active compounds and identified glucocorticoids as inhibitors of mitochondrial superoxide production in microvascular endothelial cells exposed to elevated extracellular glucose. The goal of the current study was to investigate the mechanism of glucocorticoids' action. Our findings show that glucocorticoids induce the expression of the mitochondrial UCP2 protein and decrease the mitochondrial potential. UCP2 silencing prevents the protective effect of the glucocorticoids on ROS production. UCP2 induction also increases the oxygen consumption and the “proton leak” in microvascular endothelial cells. Furthermore, glutamine supplementation augments the effect of glucocorticoids via further enhancing the expression of UCP2 at the translational level. We conclude that UCP2 induction represents a novel experimental therapeutic intervention in diabetic vascular complications. While direct repurposing of glucocorticoids may not be possible for the therapy of diabetic complications due to their significant side effects that develop during chronic administration, the UCP2 pathway may be therapeutically targetable by other, glucocorticoid-independent pharmacological means.
Highlights
Endothelial dysfunction has been implicated in the development of diabetic macrovascular and microvascular diseases [1]
We conducted further experiments to uncover the mechanism of action of glucocorticoid steroids, and here we report that steroids induce uncoupling protein 2 (UCP2) expression in microvascular endothelial cells and reduce the mitochondrial reactive oxygen species (ROS) generation by normalizing the mitochondrial membrane potential
To uncover the mechanism of action of glucocorticoid steroids against the glucose-induced mitochondrial ROS generation we characterized the major metabolic changes associated with the increased ROS production in the b.End3 cells exposed to hyperglycemia. b.End3 microvascular endothelial cells show a progressive increase in the mitochondrial MTT conversion but there is no change in the lactate dehydrogenase (LDH) activity when exposed to hyperglycemia (Fig 1A and 1B)
Summary
Endothelial dysfunction has been implicated in the development of diabetic macrovascular and microvascular diseases [1]. As a downstream effector of hyperglycemia oxidative stress is involved in the development of vascular dysfunction. The endogenous antioxidant system should counterbalance the ROS production in the vasculature, but it fails to do so in diabetes, leading to oxidative stress, which is considered a key step in the pathogenesis of endothelial dysfunction [4,5,6,7,8,9]. Inhibitors of the enzymatic sources of ROS generation [16,17,18,19,20,21,22] or the downstream effectors [23, 24] showed modest beneficial effect in preclinical studies confirming the involvement of these oxidative stress pathways. To develop novel therapies that can supplement current interventions, microvascular oxidative damage may be targeted
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