Rapid inactivation of NOS-I by lipopolysaccharide plus interferon-γ-induced tyrosine phosphorylation

Human astrocytoma T67 cells constitutively express a neuronal NO synthase (NOS-I) and, following administration of lipopolysaccharide (LPS) plus interferon-gamma (IFN gamma), an inducible NOS isoform (NOS-II). Previous results indicated that a treatment of T67 cells with the combination of LPS plus IFN gamma, by affecting NOS-I activity, also inhibited NO production in a very short time. Here, we report that under basal conditions, a NOS-I protein of about 150 kDa was weakly and partially tyrosine-phosphorylated, as verified by immunoprecipitation and Western blotting. Furthermore, LPS plus IFN gamma increased the tyrosine phosphorylation of NOS-I, with a concomitant inhibition of its enzyme activity. The same effect was observed in the presence of vanadate, an inhibitor of phosphotyrosine-specific phosphatases. On the contrary, genistein, an inhibitor of protein-tyrosine kinases, reduced tyrosine phosphorylation of NOS-I, enhancing its enzyme activity. Finally, using reverse transcriptase-polymerase chain reaction, we have observed that a suboptimal induction of NOS-II mRNA expression in T67 cells was enhanced by vanadate (or L-NAME) and inhibited by genistein. Because exogenous NO has been found to suppress NOS-II expression, the decrease of NO production that we have obtained from the inactivation of NOS-I by LPS/IFN gamma-induced tyrosine phosphorylation provides the best conditions for NOS-II expression in human astrocytoma T67 cells.