Induction of Apoptosis by Double-Stranded-RNA-Dependent Protein Kinase (PKR) Involves the α Subunit of Eukaryotic Translation Initiation Factor 2 and NF-κB

Abstract

ABSTRACT The double-stranded (ds) RNA-dependent protein kinase (PKR) is a key mediator of antiviral effects of interferon (IFN) and an active player in apoptosis induced by different stimuli. The translation initiation factor eIF-2α (α subunit of eukaryotic translation initiation factor 2) and IκBα, the inhibitor of the transcription factor NF-κB, have been proposed as downstream mediators of PKR effects. To evaluate the involvement of NF-κB and eIF-2α in the induction of apoptosis by PKR, we have used vaccinia virus (VV) recombinants that inducibly express PKR concomitantly with a dominant negative mutant of eIF-2α or a repressor form of IκBα. We found that while expression of PKR by a VV vector resulted in extensive inhibition of protein synthesis and induction of apoptosis, coexpression of PKR with a dominant negative mutant of eIF-2α (Ser-51→Ala) reversed both the PKR-mediated translational block and PKR-induced apoptosis. Coexpression of PKR with a repressor form of IκBα (Ser-32,36-Ala) also leads to the inhibition of apoptosis by abolishing NF-κB induction, while translation remains blocked. Treating cells with two different proteasome inhibitors which block IκBα degradation, prevented PKR-induced apoptosis, supporting results from coexpression studies. Biochemical analysis and transient assays revealed that PKR expression by a VV vector induced NF-κB binding and transactivation. In addition, upregulation of Fas mRNA transcription occurred during PKR activation. Our findings provide direct evidence for the involvement of eIF-2α and NF-κB in the induction of apoptosis by PKR.