Transforming growth factor-β-mediated tumor necrosis factor-related apoptosis-inducing ligand expression and apoptosis in hepatoma cells requires functional cooperation between smad proteins and activator protein-1

Abstract

Transforming growth factor-β (TGF-β) has been shown to induce apoptotic cell death in normal and transformed hepatocytes. We recently identified tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) as an important mediator of TGF-β-induced apoptosis in hepatoma cells. In this study, we have further explored the mechanism by which TGF-β up-regulates TRAIL expression. The 5′-flanking region of the TRAIL gene was isolated and characterized. Deletion mutants of the 5′-untranslated region of the TRAIL gene revealed a region comprising nucleotides -1950 to -1100 responsible for TRAIL induction following treatment with TGF-β. Within this region, we have identified an activator protein-1 (AP-1) site indispensable for TGF-β-mediated induction of TRAIL. Activation of this AP-1 site is mediated by a JunD·FosB heterodimer. Expression of DNSmad4, DNJunD, or DNFosB significantly impairs TGF-β-mediated activation of the TRAIL promoter. Furthermore, with tRNA interference targeting Smad4, junD, FosB, we could abolish TRAIL expression and, subsequently, TGF-β-induced TRAIL-mediated apoptosis in hepatoma cells. Our results reveal a new AP-1 site within the TRAIL promoter functionally involved in TGF-β-induced TRAIL expression and apoptosis in hepatomas and thus provide evidence for the underlying mechanism by which TGF-β might regulate cell death in liver cancer. Copyright © 2008 American Association for Cancer Research.