Arsenite is a well-known human carcinogen that especially targets skin. The tumor progression locus 2 (Tpl2) gene encodes a serine/threonine protein kinase that is overexpressed in various cancer cells. However, the relevance of Tpl2in arsenite-induced carcinogenesis and the underlying mechanisms remain to be explored. We show that arsenite increased Tpl2kinase activity and its phosphorylation in mouse epidermal JB6 P+ cells in a dose- and time-dependent manner. Exposure to arsenite resulted in a marked induction of cyclooxygenase-2( COX-2) and prostaglandin E2 (PGE2), important mediators of inflammation and tumor promotion. Treatment with a Tpl2kinase inhibitor or Tpl2short hairpin RNA suppressed COX-2e xpression and PGE2 production induced by arsenite treatment, suggesting that Tpl2is critical in arsenite-induced carcinogenesis. We also found that arsenite-induced phosphorylation of extracellular signal-regulated kinases (ERK) or c-Jun NH 2-terminal kinases (JNK) was markedly suppressed by Tpl2kinase inhibitor or Tpl2short hairpin RNA. Inhibition of arsenite-induced ERK or JNK signaling using a pharmacologic inhibitor of ERK or JNK substantially blocked COX-2express ion. Furthermore, inhibition of Tpl2r educed the arsenite-induced promoter activity of NF-κB and activator protein-1 (AP-1), indicating that NF-KB and AP-1 are downstream transducers of arsenite-triggered Tpl2. Our results show that Tpl2 plays a key role in arseniteinduced COX-2expre ssion and PGE2 production and further elucidate the role of Tpl2in arsenite signals that activate ERK/JNK and NF-κB/AP-1 in JB6 P+ cells.