Human polynucleotide phosphorylase (hPNPaseold-35) is a type I IFN-inducible 3′,5′ exoribonuclease that mediates mRNA degradation. In melanoma cells, slow and sustained overexpression of hPNPaseold-35 induces G1 cell cycle arrest ultimately culminating in apoptosis, whereas rapid overexpression of hPNPaseold-35 directly promotes apoptosis without cell cycle changes. These observations imply that inhibition of cell cycle progression and induction of apoptosis by hPNPaseold-35 involve multiple intracellular targets and signaling pathways. We now provide evidence that the apoptosis-inducing activity of hPNPaseold-35 is mediated by activation of double-stranded RNA-dependent protein kinase (PKR). Activation of PKR by hPNPaseold-35 precedes phosphorylation of eukaryotic initiation factor-2α and induction of growth arrest and DNA damage-inducible gene 153 (GADD153) that culminates in the shutdown of protein synthesis and apoptosis. Activation of PKR by hPNPaseold-35 also instigates down-regulation of the antiapoptotic protein Bcl-xL. A dominant-negative inhibitor of PKR, as well as GADD153 antisense or bcl-x L overexpression, effectively inhibits apoptosis induction by hPNPaseold-35. These studies elucidate a novel pathway by which an evolutionary conserved RNA-metabolizing enzyme, hPNPaseold-35 , regulates cell growth and viability.
ASJC Scopus subject areas
- Cancer Research