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 ([ Ca2+](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 [ Ca2+] c, pHi 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.