DIFFERENTIAL ACTIVATION OF MITOGEN-ACTIVATED PROTEIN-KINASES BY PALYTOXIN AND OUABAIN, 2 LIGANDS FOR THE NA-ATPASE(,K+)

We previously demonstrated that the marine toxin and skin tumor promot er palytoxin activates the stress-activated protein kinase/c-Jun N-ter minal kinase (JNK), but not the extracellular signal-regulated kinase (ERK), which is typically activated by mitogenic agents. JNK, ERK, and p38, another stress-activated protein kinase, are members of the mito gen-activated protein (MAP) kinase family of serine/threonine kinases, which coordinate the transmission of various signals through the cell . The Na+,K+-ATPase is the putative palytoxin receptor. Therefore, we hypothesized that the Na+,K+-ATPase inhibitor ouabain might also stimu late signaling pathways that activate MAP kinases. Using HeLa and COS7 cells, we found that, although there are similarities between the pro tein kinase cascades by which palytoxin and ouabain activate JNK, ther e are also significant differences between the activation of specific MAP kinases by palytoxin and ouabain. Transient expression of dominant negative mutants indicates that ouabain, like palytoxin, activates JN K through a protein kinase cascade that involves the JNK kinase SEK1 b ut does not require the GTPase Ras. Palytoxin activates JNK and p38 to a greater extent than ouabain. By contrast, ouabain activates ERK to a greater extent than palytoxin. Ouabain blocked palytoxin-stimulated activation of JNK and p38, but not anisomycin-stimulated activation of these kinases, supporting the conclusion that ouabain and palytoxin b ind to the same site on the Na+,K+-ATPase. These results suggest that the Na+,K+-ATPase can differentially mediate the activation of MAP kin ases by two diverse ligands, palytoxin and ouabain. (C) 1998 Academic Press.