Epithelial-to-mesenchymal transition (EMT) has emerged as a critical event in the pathogenesis of tubulointerstitial fibrosis. EMT is typically induced by transforming growth factor-β₁ (TGF-β₁) and inhibited by hepatocyte growth factor (HGF). The present study was undertaken to evaluate the potential role of cyclooxygenase (COX)-2-derived PGE₂ in regulation of EMT in cultured Madin-Darby canine kidney (MDCK) cells, in the setting of HGF treatment. Exposure to 50 ng/ml HGF significantly induced COX-2 protein expression and PGE₂ release, whereas other growth factors, including epidermal growth factor, the insulin-like growth factor I protein, platelet-derived growth factor-BB, and TGF-β₁, had no effects on COX-2 expression or PGE₂ release. COX-2 induction by HGF was preceded by activation of ERK1/2, and an ERK1/2-specific inhibitor, U-0126 (10 μM), completely abolished HGF-induced COX-2 expression. Exposure of MDCK cells to 10 ng/ml TGF-β₁ for 72 h induced EMT as evidenced by conversion to the spindle-like morphology, loss of E-cadherin, and activation of α-smooth muscle actin. In contrast, treatment with 1 μM PGE₂ completely blocked EMT, associated with a significant elevation of intracellular cAMP and complete blockade of TGF-β₁-induced oxidant production. cAMP-elevating agents, including 8-Br-cAMP, forskolin, and IBMX, inhibited EMT and associated oxidative stress induced by TGF-β₁, but inhibition of cAMP pathway with Rp-cAMP, the cAMP analog, and H89, the protein kinase A (PKA) inhibitor, did not block the effect of PGE₂. The effect of HGF on EMT was inhibited by ∼50% in the presence of a COX-2 inhibitor SC-58635 (10 μM). Therefore, our data suggest that PGE₂ inhibits EMT via inhibition of oxidant production and COX-2-derived PGE₂ partially accounts for the antifibrotic effect of HGF.