Differential suppression of glial nitric oxide synthase induction by structurally related tyrosine kinase inhibitors

Incubation of C6 astrocytoma cells with bacterial endotoxin (lipopolysaccharide; LPS) plus interferon-gamma (IFN-gamma), or with a combination of cytokines (TNF-alpha, IL1-beta, and IFN-gamma) leads to high levels of inducible nitric oxide synthase (iNOS) expression. Previous results demonstrated a requirement for tyrosine kinase (TK) activities for iNOS induction. In the present study, a set of structurally related TK inhibitors, the tyrphostins (TYRs), were used to characterize possible differences between LPS and cytokine iNOS induction. All TYRs tested suppressed both types of induction. However, dose-response curves revealed significant differences in the IC50 values obtained for some TYRs (T25 and T56), and significant differences in the IC50 potency rank order when comparing inhibition of LPS versus cytokine-dependent iNOS induction. These results are consistent with differential TK utilization by the LPS versus cytokine pathways of iNOS induction, and establish a basis for developing further selective inhibitors of iNOS expression.