1-Benzyl-3-cetyl-2-methylimidazolium iodide (NH125) Induces Phosphorylation of Eukaryotic Elongation Factor-2 (eEF2) A CAUTIONARY NOTE ON THE ANTICANCER MECHANISM OF AN eEF2 KINASE INHIBITOR

Eukaryotic elongation factor-2 kinase (eEF2K) relays growth and stress signals to protein synthesis through phosphorylation and inactivation of eukaryotic elongation factor 2 (eEF2). 1-Benzyl-3-cetyl-2-methylimidazolium iodide (NH125) is a widely accepted inhibitor of mammalian eEF2K and an efficacious anti-proliferation agent against different cancer cells. It implied that eEF2K could be an efficacious anticancer target. However, eEF2K siRNA was ineffective against cancer cells including those sensitive to NH125. To test if pharmacological intervention differs from siRNA interference, we identified a highly selective small molecule eEF2K inhibitor A-484954. Like siRNA, A-484954 had little effect on cancer cell growth. We carefully examined the effect of NH125 and A-484954 on phosphorylation of eEF2, the known cellular substrate of eEF2K. Surprisingly, NH125 increased eEF2 phosphorylation, whereas A-484954 inhibited the phosphorylation as expected for an eEF2K inhibitor. Both A-484954 and eEF2K siRNA inhibited eEF2K and reduced eEF2 phosphorylation with little effect on cancer cell growth. These data demonstrated clearly that the anticancer activity of NH125 was more correlated with induction of eEF2 phosphorylation than inhibition of eEF2K. Actually, induction of eEF2 phosphorylation was reported to correlate with inhibition of cancer cell growth. We compared several known inducers of eEF2 phosphorylation including AMPK-activators and an mTOR inhibitor. Interestingly, stronger induction of eEF2 phosphorylation correlated with more effective growth inhibition. We also explored signal transduction pathways leading to NH125-induced eEF2 phosphorylation. Preliminary data suggested that NH125-induced eEF2 phosphorylation was likely mediated through multiple pathways. These observations identified an opportunity for a new multipathway approach to anticancer therapies.