In a separate study, we identified PGE2 as a potent inhibitor of TGF-beta1induced epithelial-mesenchymal transition (EMT) in cultured Madin-Darby canine kidney (MDCK) cells (Zhang A, Wang M-H, Dong Z, and Yang T. Am J Physiol Renal Physiol 291: F1323-F1331, 2006). This finding prompted us to examine the roles of other prostanoids: PGD2, PGF(2alpha), PGI2, and thromboxane A2 (TXA2). Treatment with 10 ng/ml TGF-beta1 for 3 days induced EMT as reflected by conversion to the spindle-like morphology, loss of E-cadherin, and activation of alpha-smooth muscle actin (alpha-SMA). Treatment with PGD2 remarkably preserved the epithelial-like morphology, restored the expression of E-cadherin, and abolished the activation of alpha-SMA. In contrast, PGF(2alpha), carbocyclic thromboxane A2, PGI2 and its stable analog beraprost were without an effect. MDCK cells expressed DP1 and DP2 receptors; however, the effect of PGD2 was neither prevented by DP1 antagonist BW-A868C or DP2 antagonist BAY-u3405 nor was mimicked by DP1 agonist BW-245C. cAMP-elevating agents forskolin and 8-Br-cAMP blocked EMT. However, cAMP blockers H89 and Rp-cAMP failed to block the effect of PGD2. PGD2 did not seem to act via its metabolites as 15-deoxy-Delta(12,14)-prostaglandin J2 (15d-PGJ2) levels in the medium following incubation with 3 microM PGD2 were well below the values predicted from the cross activity of the assay. Exposure to TGF-beta1 induced a threefold increase in reactive oxygen species production that was completely abolished by PGD2. We conclude that 1) PGD2, but not PGI2, PGF(2alpha), and TXA2 inhibit EMT, 2) PGD2 inhibits EMT independently of DP1 and DP2 receptors, and 3) PGD2 exhibits antioxidant property which may, in part, account for the antifibrotic action of this PG.