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.