Superoxide limits cyclosporine-A-induced formation of peroxynitrite in endothelial cells(2).

Peroxynitrite (ONOO(-)) is a potent oxidant formed by the nonenzymatic reaction between superoxide anion (O(2)(-)) and nitric oxide (NO) in a one-to-one stoichiometry. Accumulated evidence suggests that endothelial dysfunction coincides with an enhanced NO* synthase expression and O(2)(-) production, facilitating ONOO(-) formation. In vivo, formation of ONOO(-) has been associated with atherosclerosis and vascular aging. The immunosuppressor Cyclosporine A (CsA) has been associated to human endothelial dysfunction and accelerated atherosclerosis. We have previously shown that CsA induced a transcriptionally mediated increase of the eNOS gene expression and that CsA induced the formation of nitric oxide, O(2)(-), and ONOO(-) in vascular endothelial cells. In this work, we evaluate the CsA-induced relative amounts of formation of O(2)(-) and NO, providing data consistent with a role of O(2)(-), and not NO, as the limiting factor in the CsA-dependent intracellular formation of ONOO(-) in vascular endothelial cells. Furthermore, when endothelial cells were treated with CsA in a situation of increased generation of superoxide such as that provided by high glucose levels, a further increase in the formation of peroxynitrite was detected. The temporal availability of O(2)(*-) for peroxynitrite formation may thus become critical in the pathophysiological scenarios where reactive nitrogen intermediates are operative.