Viral infection increases mitochondrial vulnerability and enhances TNF-induced apoptosis in hepatocytes

TNFR ligation can induce two diverse signalling events: Survival via NFkB signalling, or apoptosis, involving generation of ROS and caspase activation. The mechanisms that control this decision are still not completely understood. We identified viral infection to induce a deadly outcome of TNFR-signalling: In a novel CD8+ T-cell effector mechanism secreted TNF induces selective clearance of infected hepatocytes, whereas uninfected cells are protected.

To investigate the molecular mechanism(s) underlying the sensitisation of infected hepatocytes towards the apoptosis-inducing effect of TNF we use a model of viral hepatitis, where mice infected with hepatotropic adenovirus are challenged with TNF.

We found that viral infection overcomes early checkpoints regulating TNF apoptosis signalling. Only in infected hepatocytes an activation of the initiator caspase 8 could be detected, that subsequently induced Bid cleavage and culminated in effector caspase 3 activation. The dependency on caspase 9 activation revealed an essential involvement of mitochondrial apoptotic signalling. Viral infection directly interfered with mitochondria by an upregulation and increased mitochondrial translocation of the proapoptotic Bcl-2 family member Bax. Bax is responsible for pore formation in mitochondria and, indeed we found that mitochondria showed increased outer membrane permeability upon infection. The resulting release of mitochondrial proapoptotic mediators could further be increased through activated caspase 8, demonstrating an enhanced sensitivity of mitochondria towards apoptotic stimuli.

Taken together we could show that a switch in early events of TNFR signalling towards initiator caspase activation, as well as increased mitochondrial fragility contribute to a sensitization hepatocytes towards TNF-induced apoptosis upon viral infection enabling the new CTL effector function.