The activation of mitogen-activated protein kinases (MAPKs) is critically 
involved in inflammatory events through mediation of the production of various 
inflammatory cytokines. The Tpl2 (tumor progression locus 2)-MEK (MAPK/ERK 
kinase)-ERK (extracellular signal-regulated kinase) signaling pathway plays an 
essential role in the production of tumor necrosis factor alpha (TNFalpha) in 
macrophages stimulated with lipopolysaccharide (LPS). Here, we studied the 
molecular mechanisms of Tpl2-mediated TNFalpha production using a potent Tpl2 
kinase inhibitor, 1,7-naphtyridine-3-carbonitrile, and LPS-stimulated RAW264.7 
cells. This inhibitor was effective in suppressing the in vitro Tpl2 kinase 
activity, and caused a significant reduction in TNFalpha production via specific 
suppression of the phosphorylation of MEK and ERK but not that of p38 and c-Jun 
N-terminal kinase (JNK). A p38 inhibitor, SB203580, also inhibited the TNFalpha 
production dose-dependently. Although the TNFalpha mRNA level was not altered by 
either inhibitor, the Tpl2 inhibitor increased the nuclear TNFalpha mRNA level, 
while decreasing that in the cytoplasm. Tip-associated protein (TAP), a key 
molecule in the nucleocytoplasmic transport of TNFalpha mRNA, was up-regulated 
by LPS, but this increase was impaired by the Tpl2 inhibitor. In all cases, 
SB203580 was without effect in the presence of LPS. These results suggest that 
the LPS-induced TNFalpha production via the Tpl2-MEK-ERK signaling pathway is 
regulated by changing the TAP level at the nucleocytoplasmic transport level. 
These results improve understanding of TNFalpha regulatory mechanisms and might 
provide a new therapeutic strategy against inflammatory diseases.