Concomitant induction of inducible nitric oxide synthase (iNOS) and generation of TNF-α in LPS-stimulated macrophages raised the possibility that iNOS was involved in TNF-α expression. Consistently, pharmacological blockade or knockout of iNOS reduced LPS-mediated TNF-α secretion. Interestingly, SNAP (a NO donor) or 8-br-cGMP (a cGMP analogue) provoked the production of TNF-α in both wild type and iNOS null macrophages. Previous reports have indicated the participation of MEK and ERK in increasing the transcription of tnfα gene. Consistently, LPS-, SNAP-, and 8-br-cGMP-mediated activation of ERK as well as induction of TNF-α were greatly suppressed in macrophages treated with PD98059 (the MEK inhibitor). The strong association between ERK activation and TNF-α secretion in macrophages devoid of iNOS exposed to LPS, SNAP and 8-br-cGMP indicated that via activation of ERK, iNOS was required for LPS-elicited TNF-α generation.
