] ratio may promote the expression of cyclooxygenase-2 (COX-2), a critical enzyme in the synthesis of prostaglandins. Given antifibrotic effects of prostaglandins through activation of protein kinase A (PKA), we examined if cardiac glycosides stimulate COX-2 expression in human lung fibroblasts and how they affect myofibroblast differentiation. Ouabain stimulated a profound COX-2 expression and a sustained PKA activation, which was blocked by COX-2 inhibitor or by COX-2 knockdown. Ouabain-induced COX-2 expression and PKA activation were abolished by the inhibitor of the Na ϩ /Ca 2ϩ exchanger, KB-R4943. Ouabain inhibited transforming growth factor- (TGF-)-induced Rho activation, stress fiber formation, serum response factor activation, and the expression of smooth muscle ␣-actin, collagen-1, and fibronectin. These effects were recapitulated by an increase in intracellular [Na ϩ ]/[K ϩ ] ratio through the treatment of cells with K ϩ -free medium or with digoxin. Although inhibition of COX-2 or of the Na ϩ /Ca 2ϩ exchanger blocked ouabain-induced PKA activation, this failed to reverse the inhibition of TGF--induced Rho activation or myofibroblast differentiation by ouabain. Together, these data demonstrate that ouabain, through the increase in intracellular [Na ϩ ]/[K ϩ ] ratio, drives the induction of COX-2 expression and PKA activation, which is accompanied by a decreased Rho activation and myofibroblast differentiation in response to TGF-. However, COX-2 expression and PKA activation are not sufficient for inhibition of the fibrotic effects of TGF- by ouabain, suggesting that additional mechanisms must exist. ouabain; digoxin; intracellular sodium-to-potassium ion ratio; cyclooxygenase-2 IDIOPATHIC PULMONARY FIBROSIS (IPF) is a progressive fatal disease characterized by parenchymal fibrosis and structural distortion of the lungs. Age-adjusted mortality due to pulmonary fibrosis is increasing, and it poses a vexing clinical challenge given the lack of efficacious therapy. IPF is thought to be a disorder of abnormal wound healing (54), in which the initial trigger to the fibrotic response is injury to the alveolar epithelial cell, followed by an exuberant nonresolving woundhealing response (50). Injury of alveolar epithelial cells is thought to result in the elaboration of a fibrinous provisional matrix and activation of several proinflammatory, procoagulant, and profibrotic mediators, of which transforming growth factor-1 (TGF-1) is the most established (47). Fibroblasts, under stimulation of TGF-1, respond by altering their gene expression profile with de novo expression of cytoskeletal and contractile proteins normally found within smooth muscle (SM) cells, modified focal adhesion complexes, and components of the extracellular matrix (18,27). These SM-like fibroblasts are called myofibroblasts and display a phenotype that is in an intermediate state between fibroblasts and SM cells (10,19). Several cytoskeletal and SM proteins are expressed in myofibroblasts, including SM ␣-actin, the most established m...