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-dichloro-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.