Histochemical evidence for inducible nitric oxide synthase in advanced butnon-ruptured human atherosclerotic carotid arteries

In response to cytokine stimulation, the inducible isoform of the nitric ox ide synthase (iNOS) produces large amounts of nitric oxide with potential c onsequences in the pathophysiology of atherosclerosis. Previous investigati ons have demonstrated the presence of iNOS in human atherosclerotic lesions . The goal of this study was to evaluate the occurrence of the expression o f iNOS in ruptured versus non-ruptured human carotid atherosclerotic plaque s. Using plastic-embedded sections, we performed in situ hybridization and immunohistochemistry on very advanced atherosclerotic lesions type V (non-r uptured) and type VI (ruptured) from 12 atheromatous carotid arteries from endarterectomy and six non-atherosclerotic internal mammary arteries from a orto-coronary bypass. Only one internal mammary artery expressed iNOS in th e endothelium. In contrast, iNOS mRNA and protein were repeatedly expressed in advanced lesions type V in 5/7 cases, particularly in inflammatory regi ons. Specific cell markers identified iNOS-positive cells as macrophages an d T-lymphocytes but also as smooth muscle cells and endothelial cells adjac ent to these inflammatory regions. Nitration of protein tyrosines was not a lways associated to iNOS expression but more likely to the presence of infl ammatory cells. In complicated lesions type VI, the occurrence of iNOS mRNA and protein expression diminished drastically (1/5 cases). Combined expres sion of iNOS mRNA and protein is frequently found in advanced but non-ruptu red human atherosclerotic carotid lesions while it becomes rare after the p laque has ruptured. These findings suggest that iNOS could be an active par ticipant in the plaque rupture event.