Activation of dsRNA dependent protein kinase PKR in Karpas299 does not lead to cell death

Activated double-stranded RNA (dsRNA)-dependent protein kinase PKR is a potent growth inhibitory protein that is primarily activated in virally infected cells, inducing them to die. We have recently shown that PKR can be selectively activated in cancer cells, by in situ generation of dsRNA following introduction of antisense RNA complementary to an RNA expressed specifically in the cancer cell. The feasibility of this approach was demonstrated using a glioblastoma line that overexpresses a truncated form of the EGFR. PKR and its signaling pathway are not restricted to a given cell line; therefore, in principle, this dsRNA killing approach can be applied to any cancer that expresses unique RNA sequences. Nonetheless, applying this approach to Karpas299 cells, from a T-cell non-Hodgkin's lymphoma that harbors the NPM/ALK translocation, did not result in cell death, implying that PKR signaling pathway is repressed in this cell line. Indeed, the phosphorylation of elF2α by PKR was impaired in Karpas299 cells. Furthermore, levels of the cellular inhibitor p67 were elevated in these cells. Long antisense, as well as RNAi for p67, delivered into Karpas299 cells by adenoviruses, reduced p67 levels. The reduction in p67 levels led to increased phosphorylation of elF2α, and an additive effect was achieved by coinfection with NPM/ALK-AS encoding adenoviruses. Infection with these adenoviruses, however, did not promote growth inhibition. These findings imply that anti-apoptotic mechanisms counteract PKR signaling in this T-cell nonHodgkin's lymphoma.