Double-stranded RNA-dependent protein kinase (PKR) is a stress-responsive kinase that induces NFκB-mediated resistance against mercury cytotoxicity

The interferon-inducible, double-stranded (ds)RNA-dependent protein kinase (PKR) plays a major role in antiviral defense mechanisms where it down-regulates translation via phosphorylation of eukaryotic translation initiation factor 2 alpha. PKR is also involved in the activation of nuclear factor kappa B (NF kappa B) through activation of the I kappa B kinase complex. Activation of PKR can occur in the absence of dsRNA and in such case is controlled by intracellular regulators like the PKR-activating protein (PACT), the PKR inhibitor p58(IPK), or heat-shock proteins (Hsp). These regulators are activated by stress stimuli, supporting a role for PKR in response to stress; however the final outcome of PKR. activation in stress situations is unclear. We present here evidence that expression and activation of PKR contributes to all increased cellular resistance to mercury cytotoxicity. In two cell lines constitutively expressing PKR (THP-1 and Molt-3), treatment with the PKR inhibitor 2-aminopurine increases their sensitivity to mercury. In contrast, Ramos cells, which do not constitutively express PKR, present all increased resistance to mercury when PKR expression is induced by polylC or interferon-beta treatment. This protective effect is inhibited by 2-aminopurine. We also show that exposure of Ramos cells to mercury leads to the induction of Hsp70. Treatment of cells with Hsp70 or NF kappa B inhibitors suppresses the PKR-dependent protection. We propose a model where PKR, modulated by Hsp70, activates a NF kappa B-mediated protective pathway. Because the cytotoxicity of mercury is primarily due to the generation of reactive oxygen species, our results suggest a more general function of PKR in the mechanisms of cellular response to oxidative stress. (c) 2005 Elsevier Inc. All rights reserved.