SELECTIVE-INHIBITION BY DEXAMETHASONE OF INDUCTION OF NO SYNTHASE, BUT NOT OF INDUCTION OF L-ARGININE TRANSPORT, IN ACTIVATED MURINE MACROPHAGE J774 CELLS

1 Effects of dexamethasone on induction of nitric oxide (NO) synathase and L-arginine transport by lipopolysaccharide (LPS) were examined in a murine cultured macrophage cell line J774. Metabolism of L-arginine to L-Citrulline and subsequent changes in intracellular amino acids pools were correlated with changes in nitrite production. 2 Despite a high intracellular concentration of arginine in activated J774 cells, LPS (1 mug ml-1, 8 h) induced a 2.4 fold increase in arginine transport. Treatment of cells with cycloheximide (I mug ml-1) inhibited the time-dependent (I - 8 h) induction of NO synthase and arginine transport mediated by LPS. 3 Induction of NO synthase by LPS (I mug ml-1, 24 h) alone was accompanied by a marked increase in arginine utilisation leading to decreased intracellular arginine levels and elevated intracellular and extracellular L-Citrulline levels. These changes were further enhanced in the presence of interferon-gamma (IFN-gamma, 100 units ml-1, 24h). 4 Dexamethasone (I mum) abolished the increases in both nitrite and citrulline production induced by LPS alone but only partially reversed the combined effects of LPS and IFN-gamma. In contrast, treatment of cells with dexamethasone (I 0 mum) had no effect on the LPS-mediated induction of arginine transport or the decrease in intracellular arginine concentration. 5 We conclude that induction of arginine transporter activity in LPS-stimulated J774 cells involves de novo synthesis of carrier proteins, which increases transport of exogenous arginine during enhanced NO production. Moreover, the intracellular signalling pathways mediating induction of arginine transport and of NO synthase by LPS in activated macrophages diverge, since only the latter is sensitive to dexamethasone.