Modulation of selenoprotein P expression by TGF-β1 is mediated by Smad proteins

Selenoprotein P (SeP) is a selenium-rich plasma protein which accounts for more than 50% of total selenium in human plasma. In this study, the effect of TGF-beta (1) on the expression of SeP in the human liver cell line HepG2 was investigated. Western analysis revealed a dose-dependent reduction of SeP content in cell supernatant. RT-PCR analysis of SeP-mRNA expression demonstrated a marked inhibition and a reporter gene under control of the SeP promoter was negatively regulated by TGF-beta (1). Smad proteins are the transcriptional mediators of TGF-beta signaling. A putative Smad-binding element (SBE) is present in the SeP promoter. In electrophoretic-mobility-shift assays, TGF-beta (1) enhanced the binding of nuclear proteins to this SBE. Overexpression of Smad3 and 4 resulted in a downregulation of SeP-promoter activity whereas deletion of the SBE led to a loss of TGF-beta1 responsiveness. We conclude that SeP expression is modulated by the binding of Smad3/4 complexes to a functional SBE in the SeP promoter.