Role of the outward delayed rectifier K+ current in ceramide-induced caspase activation and apoptosis in cultured cortical neurons

We studied the novel hypothesis that an up-modulation of channels for outward delayed rectifier K+ current (I-K) plays a key role in ceramide-induced neuronal apoptosis. Exposure for 6-10 h to the membrane-permeable C-2-ceramide (25 mu M) or to sphingomyelinase (0.2 unit/ml), but not to the inactive ceramide analogue C-2-dihydroceramide (25 mu M), enhanced the whole-cell I-K current without affecting the transient A-type K+ current and increased caspase activity, followed by neuronal apoptosis 24 h after exposure onset. Tetraethylammonium (TEA) or 4-chloro-N,N-diethyl-N-heptylbenzenebutanaminium tosylate (clofilium), at concentrations inhibiting I-K, attenuated the C-2-ceramide-induced caspase-3-like activation as well as neuronal apoptosis. Raising extracellular K+ to 25 mM similarly blocked the C-2-ceramide-induced cell death; the neuroprotection by 25 mM K+ or TEA was not eliminated by blocking voltage-gated Ca2+ channels. An inhibitor of tyrosine kinases, herbimycin A (10 nM) or lavendustin A (0.1-1 mu M), suppressed I-K enhancement and/or apoptosis induced by C-2-ceramide. It is suggested that ceramide-induced I-K current enhancement is mediated by tyrosine phosphorylation and plays a critical role in neuronal apoptosis.