Interferon- (IFN-) is a multi-functional cytokine that elicits an anti-fibrotic activity in a variety of organs. In this study, we investigated the potential role and mechanism of IFN- in modulating the fibrogenic action of TGF-1 in tubular epithelial cells. Incubation of human proximal tubular epithelial (HKC) cells with IFN- inhibited TGF-1-mediated -smooth muscle actin (-SMA) expression. IFN- also abolished TGF-1-induced fibronectin and plasminogen activator inhibitor-1 (PAI-1) expression. To explore the mechanisms by which INF- inhibits TGF-1 action, the signaling pathways that are critical for mediating the anti-fibrotic activity of IFN- were studied. Stimulation of HKC cells with IFN- triggered a sustained activation of the extracellular signal-regulated protein kinase-1 and -2 (Erk-1/2) and signal transducer and activator of transcription-3 (Stat3). Blockade of Erk-1/2 activation with Mek1 inhibitor abolished the inhibitory effect of IFN- on SMA expression, whereas inhibition of Stat3 activation had no influence. Constitutive activation of Erk-1/2 by ectopic expression of activated Mek1 mimicked IFN- and suppressed TGF-1-mediated -SMA expression. Interestingly, inhibition of Stat3 activation abolished the ability of IFN- to attenuate TGF-1-mediated PAI-1 and fibronectin expression in HKC cells. These findings indicate that IFN- is capable of antagonizing the fibrogenic actions of TGF-1 in renal tubular epithelial cells. The anti-fibrotic action of IFN- appears to be mediated through a coordinated activation of both Erk-1/2 and Stat3 signal pathways in a mutually independent fashion.