Inducible nitric oxide synthase colocalizes with signs of lipid oxidation/peroxidation in human atherosclerotic plaques

Advanced human atherosclerotic plaques are characterized by the abundant presence of the autofluorescent non-soluble lipid pigment ceroid, consisting of oxidized lipoproteins. The aim of the present study was to examine the topographical and cellular distribution of inducible nitric oxide synthase (iNOS or NOS II) within different stages of atherosclerosis and its colocalization with ceroid deposits and nitrotyrosine. Different stages of atherosclerosis were studied by immunohistochemistry on whole-mount longitudinal sections of carotid endarterectomy specimens. In the adaptive intimal thickening the predominant cell type were smooth muscle cells. The fatty streaks contained both smooth muscle cells and macrophages with an extremely low NOS II immunoreactivity. The advanced atherosclerotic plaques however, showed a very dense infiltration by macrophages, of which a subpopulation expressed NOS II as a vesicular immunoreactivity in their cytoplasm. These were mainly present around the necrotic core, in association with ceroid accumulation and nitrotyrosine. Fluorescence quenching microscopy showed the presence of NOS II on autofluorescent ceroid vesicles in the macrophages. Large extracellular ceroid granules were not NOS II immunoreactive. NOS II mRNA was detected by RT-PCR and the protein by Western blot in the plaque tissue but not in mammary arteries used as controls. Ceroid, nitrotyrosine and NOS II colocalized in late stages of atherosclerosis and were found around the necrotic core in the plaque. This could suggest that NOS II expression in macrophages is involved in oxidation and peroxidation of lipids, leading to ceroid formation.