Sphingosine Induces Apoptosis in Hippocampal Neurons and Astrocytes by Activating Caspase-3/-9 via A Mitochondrial Pathway Linked to SDK/14-3-3 Protein/Bax/Cytochrome c

The present study examined sphingosine-induced apoptosis in cultured rat hippocampal neurons and astrocytes. Sphingosine induced apoptosis in a concentration (1-100 mu M)-dependent manner, that is inhibited by the PKC-delta inhibitor rottlerin, and a similar effect was obtained with the sphingosine kinase inhibitors, to raise intracellular sphingosine concentrations. Sphingosine increased presence of sphingosine-dependent protein kinase (SDK), and the effect was suppressed by rottlerin. Sphingosine increased phosphorylated 14-3-3 protein, thereby transforming the protein from a dimeric structure into a monomeric structure. Sphingosine accumulated Bax in the mitochondria and stimulated cytochrome c release into the cytosol, and those effects were inhibited by rottlerin. Sphingosine disrupted mitochondrial membrane potentials, that was abolished by silencing the PKC-delta-targeted gene. Moreover, sphingosine activated caspase-9 and the effector caspase-3 in a PKC-delta-dependent manner. Taken together, the results of the present study indicate that sphingosine activates SDK, produced through proteolytic processing of an active form of PKC-delta, to phosphorylate 14-3-3 protein and transform into a monomeric structure, causing Bax dissociation from 14-3-3 protein and accumulation in the mitochondria, which perturbs mitochondrial membrane potentials allowing cytochrome c release into the cytosol, to activate caspase-9 and the effector caspase-3, responsible for apoptosis in hippocampal neurons and astrocytes. J. Cell. Physiol. 226: 2329-2337, 2011. (C) 2010 Wiley-Liss, Inc.