Interferon-α enhances TRAIL-mediated apoptosis by up-regulating caspase-8 transcription in human hepatoma cells

Background/Aims: IFN alpha is an approved treatment option for patients chronically infected with the hepatitis B and C viruses. Additionally, there is an indication for tumor therapy. The exact mechanisms underlying the antiviral and antitumor effects of IFN alpha are not completely understood. In this study, we investigated if the pro-apoptotic factor caspase-8 is a target gene of IFN alpha signalling. Methods: Huh7 hepatoma cells were used for measuring caspase-8 promoter activity in luciferase reporter assays after IFN alpha stimulation. Caspase-8 expression was monitored by RT-PCR, immunoblotting and measurement of enzymatic activity. Functional caspase-8 promoter elements were identified in gelshift assays and by site directed mutagenesis. Caspase-8 was inhibited using siRNA. Results: IFN alpha treatment induced caspase-8 promoter activity and mRNA expression. We identified a unique promoter element mediating the IFN alpha-dependent increase in caspase-8 transcription. Up-regulation of caspase-8 expression by IFN alpha had no impact on the rate of apoptosis per se. However, co-stimulation with IFN alpha doubled TRAIL-mediated apoptosis and enzymatic caspase-8 activity. The synergistic effect of TRAIL and IFN alpha could be blocked by inhibiting caspase-8 expression. Conclusions: We demonstrate that caspase-8 is a target gene of IFN alpha and provide evidence showing that IFN alpha treatment sensitizes cells for apoptosis via enhanced caspase-8 transcription. (C) 2005 European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved.