Monocyte–vascular smooth muscle cell interaction enhances nitric oxide production

Abstract

Objective: The adhesive interaction of monocytes and vascular smooth muscle cells (VSMCs) has been suggested to be a regulatory signal in the cellular activation that is involved in the pathogenesis of atherosclerosis. We investigated the effects of monocyte–VSMC interaction on inducible nitric oxide (NO) synthase expression. Methods: NO production by the cultured cells was determined by measuring the nitrite content of the culture media using the Griess reagent. The expression of inducible NO synthase protein was assayed by Western blotting. Results: Interleukin-1β (IL-1β) induced nitrite production by VSMCs in a time-dependent manner. The addition of the mouse monocyte cell line J774 to IL-1β-stimulated VSMCs further increased nitrite production in a monocyte number-dependent manner. Enhanced nitrite production by coculture was accompanied by increased inducible NO synthase protein accumulation. Addition of tumor necrosis factor-α (TNF-α) also enhanced IL-1β-induced nitrite production by VSMCs, but TNF-α showed no effect in the presence of monocytes. Coculture of monocytes and VSMCs in the presence of IL-1β secreted substantial amounts of TNF-α. The production of nitrite by coculture was markedly inhibited by an anti-TNF-α antibody. Conclusions: The present study revealed that direct cell-to-cell interaction between monocytes and VSMCs enhances NO production, suggesting an important role for their interaction in the pathogenesis of atherosclerosis.