Role of the Outward Delayed Rectifier K+ Current in Ceramide‐Induced Caspase Activation and Apoptosis in Cultured Cortical Neurons

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Abstract
: We studied the novel hypothesis that an upmodulation of channels for outward delayed rectifier K+ current (Iκ) plays a key role in ceramide‐induced neuronal apoptosis. Exposure for 6‐10 h to the membrane‐permeable C2‐ceramide (25 μM) or to sphingomyelinase (0.2 unit/ml), but not to the inactive ceramide analogue C2‐dihydroceramide (25 μM), enhanced the whole‐cell Iκ 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 Iκ, attenuated the C2‐ceramide‐induced caspase‐3‐like activation as well as neuronal apoptosis. Raising extracellular K+ to 25 mM similarly blocked the C2‐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 μM), suppressed Iκ enhancement and/or apoptosis induced by C2‐ceramide. It is suggested that ceramide‐induced Iκ current enhancement is mediated by tyrosine phosphorylation and plays a critical role in neuronal apoptosis.