Sphingosine kinase expression regulates apoptosis and caspase activation in PC12 cells

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Abstract
Sphingosine‐1‐phosphate (SPP), a bioactive sphingolipid metabolite, suppresses apoptosis of many types of cells, including rat pheochromocytoma PC12 cells. Elucidating the molecular mechanism of action of SPP is complicated by many factors, including uptake and metabolism, as well as activation of specific G‐protein‐coupled SPP receptors, known as the endothelial differentiation gene‐1 (EDG‐1) family. In this study, we overexpressed type 1 sphingosine kinase (SPHK1), the enzyme that converts sphingosine to SPP, in order to examine more directly the role of intracellularly generated SPP in neuronal survival. Enforced expression of SPHK1 in PC12 cells resulted in significant increases in kinase activity, with corresponding increases in intracellular SPP levels and concomitant decreases in both sphingosine and ceramide, and marked suppression of apoptosis induced by trophic factor withdrawal or by C2‐ceramide. NGF, which protects PC12 cells from serum withdrawal‐induced apoptosis, also stimulated SPHK1 activity. Surprisingly, overexpression of SPHK1 had no effect on activation of two known NGF‐stimulated survival pathways, extracellular signal regulated kinase ERK 1/2 and Akt. However, trophic withdrawal‐induced activation of the stress activated protein kinase, c‐Jun amino terminal kinase (SAPK/JNK), and activation of the executionary caspases 2, 3 and 7, were markedly suppressed. Moreover, this abrogation of caspase activation, which was prevented by the SPHK inhibitor N,N‐dimethylsphingosine, was not affected by pertussis toxin treatment, indicating that the cytoprotective effect was likely not mediated by binding of SPP to cell surface Gi‐coupled SPP receptors. In agreement, there was no detectable release of SPP into the culture medium, even after substantially increasing cellular SPP levels by NGF or sphingosine treatment. In contrast to PC12 cells, C6 astroglioma cells secreted SPP, suggesting that SPP might be one of a multitude of known neurotrophic factors produced and secreted by glial cells. Collectively, our results indicate that SPHK/SPP may play an important role in neuronal survival by regulating activation of SAPKs and caspases.