Mitogen‐activated protein kinases regulate palytoxin‐induced calcium influx and cytotoxicity in cultured neurons

Background and purpose: Palytoxin (PLT) is a potent toxin that binds to the Na,K‐ATPase. Palytoxin is highly neurotoxic and increases the cytosolic calcium concentration ([Ca 2+ ] c ) while decreasing intracellular pH (pH i ) in neurons (Vale et al. , 2006; Vale‐Gonzalez et al. , 2007). It is also a tumour promoter that activates several protein kinases. Experimental approach: The role of different protein kinases in the effects of palytoxin on [Ca 2+ ] c , pH i and cytoxicity was investigated in cultured neurons. Key results: Palytoxin‐induced calcium load was not affected by inhibition of calcium‐dependent protein kinase C (PKC) isoforms but it was partially ameliorated by blockade of calcium‐independent PKC isozymes. Inhibition of the extracellular signal‐regulated kinase (ERK) 2 eliminated the palytoxin‐induced rise in calcium and intracellular acidification, whereas inhibition of MEK greatly attenuated the palytoxin effect on calcium without modifying the PLT‐evoked intracellular acidification. Blockade of c‐Jun N‐terminal protein kinases (JNK) somewhat decreased the palytoxin‐effect on calcium, whereas inhibition of the p38 mitogen activated protein kinases (MAPKs) delayed the onset of the palytoxin‐evoked rise in calcium and acidification. Furthermore, the cytotoxicity of palytoxin was completely blocked by inhibition of ERK 2 and partially prevented by inhibition of MEK. PLT increased phosphorylated ERK immunoreactivity in a concentration‐dependent manner. Conclusions and implications: MAPKs, specifically ERK 2, link palytoxin cytotoxicity with its effects on calcium homeostasis after inhibition of the Na,K‐ATPase. Binding of palytoxin to the Na,K‐ATPase would alter signal transduction pathways, even in non‐dividing cells, and this finding is related to the potent neurotoxicity of this marine toxin. British Journal of Pharmacology (2007) 152 , 256–266; doi: 10.1038/sj.bjp.0707389