PHOSPHORYLATION OF EUKARYOTIC TRANSLATION INITIATION-FACTOR-2 MEDIATES APOPTOSIS IN RESPONSE TO ACTIVATION OF THE DOUBLE-STRANDED RNA-DEPENDENT PROTEIN-KINASE

The interferon-inducible, double-stranded (ds) RNA-dependent serine/th reonine protein kinase (PKR) plays a role in viral pathogenesis, cell growth, and differentiation and is implicated as a tumor suppressor ge ne. Expression of a trans-dominant negative, catalytically inactive mu tant PKR protected NIH3T3 cells from apoptosis in response to either t reatment with tumor necrosis factor alpha (TNF alpha), serum deprivati on. In cells expressing mutant PKR, TNF alpha, but not dsRNA induced t ranscription from a nuclear factor kappa B-dependent promoter, demonst rating specificity for dsRNA in signaling through the PKR pathway. Ser um or platelet-derived growth factor addition to serum-deprived mutant PI(R-expressing cells induced transcription of the early response gen es c-fos and c-jun, indicating that the immediate early response signa ling was intact. Overexpression of wild-type PKR in a transient DNA tr ansfection system was sufficient to induce apoptosis. TNF alpha-induce d apoptosis correlated with increased phosphorylation of the alpha sub unit of eukaryotic translation initiation factor 2 (eIF-2 alpha), the primary physiological substrate of the PKR Furthermore, forced express ion of a nonphosphorylatable S51A mutant eIF-2 alpha partially protect ed cells from TNF alpha-induced apoptosis, and expression of a S51D mu tant eIF-2 alpha, a mutant that mimics phosphorylated eIF-2 alpha, was sufficient to induce apoptosis. Taken together, these studies identif y a novel requirement for PKR in stress-induced apoptosis that is medi ated through eIF-2 alpha phosphorylation.