NFκB mediates apoptosis through transcriptional activation of Fas (CD95) in adenoviral hepatitis

Abstract

NFκB is an essential survival factor in several physiological conditions such as embryonal liver development and liver regeneration. However, NFκB is also a main mediator of the cellular response to a variety of extracellular stress stimuli, and it has been shown that some viral- induced host cell apoptosis appears to be dependent on NFκB activation. The activation of NFκB upon viral infection may be a rapid way of initiating an innate immune response against the viral particles. We have assessed the role of NFkB during the early phase of adenoviral hepatitis in a nude mouse model using an adenoviral vector expressing a mutant form of IκBα. Administration of a LacZ-expressing adenoviral vector induces NFkB DNA and correlates with the up-regulation of Fas (CD95) mRNA, but not FasL (CD95L) mRNA, during the early phase of adenoviral hepatitis. The rapid increase in NFκB DNA binding after adenoviral infection of the liver could be very effectively inhibited by IκBα. Compared with the LacZ control virus, the IκBα-expressing adenoviral vector inhibits the increase of Fas (CD95) mRNA expression, in particular in the very early phase of the hepatitis. Reporter gene experiments in hepatoma cell lines with a Fas promoter-luciferase construct indicated that the repression of Fas (CD95) mRNA by IκBα was transcriptionally mediated. The functional relevance of the NFκB-dependent increase in Fas (CD95) transcription was assessed by caspase 3 assays and terminal dUTP nick-end labeling tests. Compared with the control, IκBα adenoviral infection resulted in reduced caspase 3 activity during the early phase of viral hepatitis and in a prevention of liver cell apoptosis 24 h after adenoviral administration. Therefore our study demonstrates a new pro- apoptotic function of NFκB in Fas (CD95)-mediated apoptosis of hepatocytes. Interestingly, NFκB mediates liver cell apoptosis upon viral infection even in a phase where tumor necrosis factor-α is already induced, as shown by the time curves of tumor necrosis factor-α serum levels. Therefore, the pro- or anti-apoptotic role of NFκB appears to be more determined by the nature of the death stimulus than by the origin of the tissue.