Role of nitrosative stress in the pathogenesis of diabetic vascular dysfunction

Abstract

Here we overview the role of reactive nitrogen species (nitrosative stress) and associated pathways in the pathogenesis of diabetic vascular complications. Increased extracellular glucose concentration, a principal feature of diabetes mellitus, induces a dysregulation of reactive oxygen and nitrogen generating pathways. These processes lead to a loss of the vascular endothelium to produce biologically active nitric oxide (NO), which impairs vascular relaxations. Mitochondria play a crucial role in this process: endothelial cells placed in increase extracellular glucose respond with a marked increase in mitochondrial superoxide formation. Superoxide, when combining with NO generated by the endothelial cells (produced by the endothelial isoform of NO synthase), leads to the formation of peroxynitrite, a cytotoxic oxidant. Reactive oxygen and nitrogen species trigger endothelial cell dysfunction through a multitude of mechanisms including substrate depletion and uncoupling of endothelial isoform of NO synthase. Another pathomechanism involves DNA strand breakage and activation of the nuclear enzyme poly(ADP-ribose) polymerase (PARP). PARP-mediated poly(ADP-ribosyl)ation and inhibition of glyceraldehyde-3-phosphate dehydrogenase importantly contributes to the development of diabetic vascular complications: it induces activation of multiple pathways of injury including activation of nuclear factor kappa B, activation of protein kinase C and generation of intracellular advanced glycation end products. Reactive species generation and PARP play key roles in the pathogenesis of 'glucose memory' and in the development of injury in endothelial cells exposed to alternating high/low glucose concentrations.

Figure 1

Selected peroxynitrite and poly(ADP-ribose) polymerase (PARP)-dependent pathways involved in the pathogenesis of diabetic complications in endothelial cells exposed to the cytotoxic effects of increased extracellular glucose. Increased glucose leads to increased mitochondrial formation of reactive oxygen species (ROS), such as superoxide, which, when reacting with nitric oxide (NO), produces peroxynitrite. Peroxynitrite induces cellular damage through depletion of the co-factor of endothelial isoform of nitric oxide synthase (eNOS), tetrahydrobiopterin (BH4), as well as multiple additional pathways not outlined in this scheme (see Table 1). One of the pathways of peroxynitrite-mediated injury involves DNA strand breakage, activation of the nuclear enzyme PARP, poly(ADP-ribosyl)ation and inhibition of GAPDH (glyceraldehyde-3-phosphate dehydrogenase), and activation of multiple pathways of diabetic complications including the polyol pathway, the advanced glycation end products (AGE) pathway, the protein kinase C (PKC) pathway and the hexosamine pathway. PARP activation can also lead to intracellular NAD⁺ and NADPH depletion (the latter being an essential co-factor of eNOS), and it can also up-regulate various pro-inflammatory pathways leading to pathological modifications in adhesion molecule expression, angiogenesis and other processes.