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.
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