Matricellular proteins play a critical role in the development of tubulointerstitial fibrosis and renal disease progression. Connective tissue growth factor (CTGF/CCN2), a CCN family member of matricellular proteins, represents an important mediator during development of glomerular and tubulointerstitial fibrosis in progressive kidney disease. We have recently reported that oncostatin M (OSM) is a potent inhibitor of TGF-beta 1-induced CTGF expression in human proximal tubular cells (PTC). In the present study we examined the role of TGF-beta 1- and OSM-induced signaling mechanisms in the regulation of CTGF mRNA expression in human proximal tubular HK-2 cells. Utilizing siRNA-mediated gene silencing we found that TGF-beta 1-induced expression of CTGF mRNA after 2 h of stimulation at least partially depends on SMAD3 but not on SMAD2. In contrast to TGF-beta 1, OSM seems to exert a time-dependent dual effect on CTGF mRNA expression in these cells. While OSM led to a rapid and transient induction of CTGF mRNA expression between 15 min and 1 h of stimulation it markedly suppressed basal and TGF-beta 1-induced CTGF mRNA levels thereafter. Silencing of STAT1 or STAT3 attenuated basal CTGF mRNA levels indicating that both STAT isoforms may be involved in the regulation of basal CTGF mRNA expression. However, knockdown of STAT3 but not STAT1 prevented OSM-mediated suppression of basal and TGF-beta 1-induced upregulation of CTGF mRNA expression. Together these results suggest that the inhibitory effect of OSM on TGF-beta 1 -induced CTGF mRNA expression is mainly driven by STAT3, thereby providing a signaling mechanism whereby OSM may contribute to tubulointerstitial protection. (C) 2012 Elsevier Inc. All rights reserved.