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Analyzing the mechanisms of interferon-induced apoptosis using CrmA and hepatitis C virus NS5A
H J Ezelle1, S Balachandran, F Sicheri
1Department of Microbiology and Immunology, University of Miami School of Medicine, Miami, Florida 33136, USA.
Abstract:
The dsRNA-dependent protein kinase, PKR, is a key component of interferon (IFN)-mediated anti-viral action and is frequently inhibited by many viruses following infection of the cell. Recently, we have demonstrated that IFN and PKR can sensitize cells to apoptosis predominantly through the FADD/caspase-8 pathway (S. Balachandran, P. C. Roberts, T. Kipperman, K. N. Bhalla, R. W. Compans, D. R. Archer, and G. N. Barber. (2000b) J. Virol. 74, 1513-1523). Given these findings, it is thus plausible that rather than specifically target IFN-inducible genes such as PKR, viruses could also subvert the mechanisms of IFN action, in part, at locations that could block the apoptotic cascade. To explore this possibility, we analyzed whether the poxvirus caspase-8 inhibitor, CrmA, was able to inhibit IFN or PKR/dsRNA-mediated apoptosis. Our findings indicated that CrmA could indeed inhibit apoptosis induced by both viral infection and dsRNA without blocking PKR activity or inhibiting IFN signaling. In contrast HCV-encoded NS5A, a putative inhibitor of PKR, did not appear to inhibit cell death mediated by a number of apoptotic stimuli, including IFN, TRAIL, and etoposide. Our data imply that viral-encoded inhibitors of apoptosis, such as CrmA, can block the innate arms of the immune response, including IFN-mediated apoptosis, and therefore potentially constitute an alternative family of inhibitors of IFN action in the cell.
Insights
Viruses can evade the immune system by blocking interferon-mediated apoptosis. Poxvirus CrmA inhibits this cell death pathway, while HCV NS5A does not, suggesting viral inhibitors target innate immune responses.
Area of Science:
- Immunology
- Virology
- Molecular Biology
Background:
- Interferon (IFN) signaling activates the dsRNA-dependent protein kinase (PKR) to mediate anti-viral defense.
- PKR and IFN can induce apoptosis via the FADD/caspase-8 pathway.
- Viruses often inhibit PKR or other IFN-inducible genes to evade immune responses.
Purpose of the Study:
- To investigate if viral apoptosis inhibitors can interfere with IFN or PKR/dsRNA-mediated apoptosis.
- To determine if CrmA, a poxvirus caspase-8 inhibitor, affects IFN or PKR/dsRNA-induced cell death.
- To assess the effect of Hepatitis C virus (HCV)-encoded NS5A on various apoptotic stimuli.
Main Methods:
- Treatment of cells with dsRNA, IFN, viral infection, TRAIL, and etoposide.
- Analysis of apoptosis induction and PKR activity.
- Assessment of CrmA and NS5A's inhibitory effects on apoptosis and IFN signaling.
Main Results:
- CrmA inhibited apoptosis induced by viral infection and dsRNA without affecting PKR activity or IFN signaling.
- HCV NS5A did not inhibit cell death induced by IFN, TRAIL, or etoposide.
- These findings suggest CrmA targets the apoptotic cascade downstream of PKR activation.
Conclusions:
- Viral inhibitors of apoptosis, like CrmA, can subvert innate immune responses by blocking IFN-mediated apoptosis.
- These viral proteins represent a distinct class of IFN antagonists.
- Targeting apoptotic pathways offers viruses an alternative strategy to inhibit IFN action.