Negative regulation by protein tyrosine phosphatase of IFN-γ-dependent expression of inducible nitric oxide synthase

Treatment of cultured peritoneal macrophages with IFN-gamma resulted in tyrosine phosphorylation of I kappa B alpha and I kappa B beta, NF-kappa B activation, and expression of inducible NO synthase (iNOS). Since tyrosine phosphorylation of I kappa B alpha is sufficient to activate NF-kappa B in Jurkat cells, macrophages were treated with the protein tyrosine phosphatase inhibitor peroxovanadate (POV), which elicited an intense tyrosine phosphorylation of both I kappa B, However, this phosphorylation failed to activate NF-kappa B, Treatment with POV of macrophages stimulated with IFN-gamma or LPS potentiated the degradation of I kappa B alpha and I kappa B beta, the activation of NF-kappa B, and the expression of iNOS. Analysis of the iNOS gene promoter activity corresponding to the 5'-flanking region indicated that POV potentiates the cooperation between IFN-gamma-activated transcription factors and NF-kappa B. These results indicate that tyrosine phosphorylation of I kappa B is not sufficient to activate NF-kappa B in macrophages and propose a negative role for protein tyrosine phosphatase in the expression of iNOS in response to IFN-gamma.