Regulation of Volume-Sensitive Osmolyte Efflux from Human SH-SY5Y Neuroblastoma Cells following Activation of Lysophospholipid Receptors

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Abstract
The ability of the lysophospholipids sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) to promote the release of the organic osmolyte taurine in response to hypoosmotic stress has been examined. Incubation of SH-SY5Y neuroblastoma cells under hypoosmotic conditions (230 mOsM) resulted in a time-dependent release of taurine that was markedly enhanced (3–7-fold) by the addition of micromolar concentrations of either S1P or LPA. At optimal concentrations, the effects of S1P and LPA on taurine efflux were additive and mediated via distinct receptors. Inclusion of 1,9-dideoxyfoskolin, 5-nitro-2-(3-phenylpropylamino benzoic acid, or 4-[(2-butyl-6,7-dicloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl)oxy]-butanoic acid blocked the ability of both lysophospholipids to enhance taurine release, indicating the mediation of a volume-sensitive organic osmolyte and anion channel. Both S1P and LPA elicited robust increases in intracellular calcium concentration that were attenuated by the removal of extracellular calcium, abolished by the depletion of intracellular calcium with thapsigargin, and were independent of phosphoinositide turnover. Taurine efflux mediated by S1P and LPA was unaffected by the removal of extracellular calcium but was attenuated by depletion of intracellular calcium (34–38%) and by inhibition of protein kinase C (PKC) with chelerythrine (38–72%). When intracellular calcium was depleted and PKC was inhibited, S1P- or LPA-stimulated taurine efflux was inhibited by 80%. Pretreatment of the cells with pertussis toxin, toxin B, or cytochalasin D had no effect on lysophospholipid-stimulated taurine efflux. The results indicate that both S1P and LPA receptors facilitate osmolyte release via a phospholipase C-independent mechanism that requires the availability of intracellular calcium and PKC activity.

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