Endothelial cell production of nitrogen oxides in response to interferon gamma in combination with tumor necrosis...

Clinical studies using biological response modifiers in cancer therapy have shown that the major dose-limiting toxic effects are hypotension and diffuse microvascular leakage. The cause and pathophysiology of this hypotension remains unknown. Previous experiments have demonstrated that a number of cell types, including endothelial cells, neutrophils, and macrophages, can secrete a potent hypotensive agent--endothelium-derived relaxing factor, which has recently been identified as nitric oxide. In this study, we tested interferon gamma, tumor necrosis factor, interleukin-1, interleukin-2, muramyl dipeptide, and endotoxin for their effects on production of nitrogen oxides by endothelial cells. Interferon gamma, in combination with tumor necrosis factor, interleukin-1 (IL-1), or endotoxin, induced murine brain endothelial cells to secrete nitrites (20-45 microM within 48 hr), which are breakdown products of nitric oxide. Nitrite production was blocked by incubation of endothelial cells in medium without L-arginine, a substrate for nitric-oxide synthase. Accumulation of nitrites was also inhibited by addition of NG-monomethyl-L-arginine (L-NMMA), which acts as a competitive inhibitor of this enzyme. The inhibitory effects of L-NMMA were reversed by addition of excess L-arginine. These results suggest (a) that endothelial cells produce nitric oxide in response to immunomodulators and (b) that endothelial cell-derived nitric oxide plays a role in the development of hypotension in patients treated with tumor necrosis factor or interleukins. Furthermore, administration of substrate analogues such as L-NMMA may favorably alter the toxicity associated with these immunomodulators and result in a higher maximum tolerated dose, with subsequent improvement in the antitumor activity.