Role of nitric oxide in lysis of tumor cells by cytokine-activated endothelial cells.

The purpose of these studies was to determine whether nitric oxide produced by cytokine-activated murine lung vascular endothelial cells plays a role in their lytic destruction of M-5076 reticulum cell sarcoma. Vascular endothelial cells harvested from perfused lungs of mice were adapted to grow in culture. Cloned lines ascertained to be of endothelial origin were incubated in vitro with interferon gamma and tumor necrosis factor. Lysis of radiolabeled tumor cells and accumulation of nitrite in the culture medium were determined at several time points. The concentration of nitrite in the culture medium directly correlated with endothelial cell-mediated tumor cell lysis. Endothelial cells cultured in L-arginine-free medium did not produce significant tumor cell lysis nor accumulation of nitrite in the medium. Both tumor cell lysis and nitrite accumulation were observed when the deficient medium was reconstituted with L-arginine, suggesting that endothelial cell-mediated tumor lysis was dependent on L-arginine, a precursor of nitric oxide. Moreover, specific inhibition of nitric oxide synthesis by NG-methyl-L-arginine resulted in complete inhibition of endothelial cell-mediated lysis of the M-5076 reticulum cell sarcoma. Similarly, treatment of cytokine-activated endothelial cells with dexamethasone inhibited both target cell lysis and production of nitrite. Collectively, these results suggest that nitric oxide plays a major role in the lysis of tumor cells mediated by cytokine-activated endothelial cells.