Objective: We have previously shown that macrophages induce vascular smooth muscle cell (VSMC) apoptosis in vitro by cell-cell proximity and Fas-L/Fas interactions. Because NO is a short-range mediator, we tested whether NO mediates macrophage-induced VSMC apoptosis.
Methods and results: NO synthase (NOS) inhibitors markedly inhibited macrophage-induced apoptosis of carotid plaque VSMCs (apoptotic indices, 81+/-2.9% for control and 28.2+/-3.9% for N(G)-nitro-L-arginine methyl ester [L-NAME] treatment) and coronary medial VSMCs (apoptotic indices, 76+/-5.5% for control and 3.5+/-0.8% for L-NAME treatment). Inactive enantiomers were without effect (P>0.05). Cultured macrophages, but not VSMCs, expressed inducible NOS (but not neuronal NOS or endothelial NOS) concomitant with activation and secreted 1.51+/-0.3 fmol nitrite per cell, which was blocked by L-NAME (100 micro mol/L). Diethylene triamine nitric oxide (DETA/NO) and sodium nitroprusside (NO donors) induced VSMC cell-surface Fas and enhanced plaque VSMC apoptosis induced by agonistic anti-Fas antibody (apoptotic indices, 6.6+/-1.8% for control, 6.3+/-1.5% for DETA/NO, 26+/-1.8% for Fas, and 44+/-6.9% for Fas+DETA/NO). In isolated macrophages, NOS inhibitors reduced and NO donors increased surface Fas-L, indicating an NO-dependent autocrine enhancement of macrophage surface Fas-L.
Conclusions: Together, these data indicate that macrophage-derived NO is required for macrophage-induced VSMC apoptosis and that it acts by enhancing Fas-L/Fas interactions.