Loss of cellular K+ mimics ribotoxic stress -: Inhibition of protein synthesis and activation of the stress kinases SEK1/MKK4, stress-activated protein kinase c-Jun NH2-terminal kinase 1, and p38/HOG1 by palytoxin

The tumor promoter palytoxin has been found to activate the stress-activated protein kinase/c-Jun NH2-terminal kinase 1 (SAPK/JNK1), and it also potentiates, as demonstrated here, the p38/HOG1 mitogen-activated protein kinase and the upstream activator of SAPK/JNK1, SEK1/MKK4. In search of possible mechanisms for both the cytotoxicity and the activation of stress kinases by palytoxin, we found that palytoxin is a potent inhibitor of cellular protein synthesis. The inhibition of translation by palytoxin does not result from its direct binding to the translational apparatus. We have previously demonstrated that ribotoxic stressors (Iordanov, M. S., Pribnow, D., Magun, J. L., Dinh, T.-H., Pearson, J. A., Chen, S. L.-Y., and Magun, B. E. (1997) Mol. Cell. Biol. 17, 3373-3381) signal the activation of SAPK/JNK1 by binding to or covalently modifying 28 S rRNA in ribosomes that are active at the time of exposure to the stressor, Palytoxin acted as a ribotoxic stressor, inasmuch as it required actively translating ribosomes at the time of exposure to activate SAPK/JNK1. Palytoxin has been shown to augment ion fluxes by binding to the Na+/K+-ATPase in the plasma membrane of cells. To determine whether altered fluxes of either Na+ or K+ could be responsible for the effects of palytoxin on translation and on activation of SAPK/JNK1, cells were exposed to palytoxin in modified culture medium in which a major portion of the Na+ was replaced by either K+ or by choline(+). The substitution of Na+ by K+ strongly inhibited the ability of palytoxin both to inhibit protein translation and to activate SAPK/JNK1, whereas the substitution of Na+ by choline(+) did not. These results suggest that palytoxin-induced efflux of cellular K+ mimics ribotoxic stress by provoking both translational inhibition and activation of protein kinases associated with cellular defense against stress.