Inducible Nitric Oxide: An Autoregulatory Feedback Inhibitor of Vascular Inflammation

Abstract

Inducible nitric oxide (iNO) is produced at sites of vascular inflammation by resident and nonresident vascular wall cells, but its role in the inflammatory process is not known. In this study, we show that a novel function of iNO is to terminate inflammatory processes. We find that iNO produced by murine macrophage-like cells, RAW264.7, can inhibit cytokine-induced endothelial cell activation in a separated and mixed endothelial-RAW264.7 coculture system. Both iNO production and endothelial VCAM-1 expression were induced simultaneously with bacterial LPS and murine-specific IFN-γ. Inhibition of iNO synthase (iNOS) activity with Nω-monomethyl-l-arginine in endothelial-RAW264.7 cocultures, stimulated with murine-specific IFN-γ and LPS, decreased iNO production by 86%, augmented VCAM-1 and iNOS expression in endothelial and RAW264.7 cells, respectively, and increased monocyte adhesion to the endothelial cell surface. Transient transfection studies using various VCAM-1 promoter constructs demonstrated that inhibitory effects of iNO on VCAM-1 gene transcription were mediated, in part, by inhibitory effects of iNO on κB cis-acting elements. Immunofluorescence studies using an Ab to the RelA (p65) subunit of nuclear factor-κB revealed that iNO inhibited the activation of nuclear factor-κB. These studies indicate that iNO attenuates iNOS expression in macrophages and inhibits monocyte adhesion to endothelial cells, and suggest that endogenously derived iNO may be an important autoregulatory inhibitor of vascular inflammation.