Extracellular sphingosine 1-phosphate stimulates formation of ethanolamine from phosphatidylethanolamine: modulation of sphingosine 1-phosphate-induced mitogenesis by ethanolamine

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Abstract
In this work, we determined the effects of sphingosine 1-phosphate (S1P) on phospholipase D (PLD)-mediated hydrolysis of phosphatidylethanolamine (PtdEtn), and evaluated the effects of the water-soluble product ethanolamine on S1P-induced DNA synthesis in NIH 3T3 cells. In [14C]ethanolamine-labelled cells, S1P (0.5-5 μM) stimulated PLD-mediated hydrolysis of PtdEtn 1.5-2.1-fold. Down-regulation of protein kinase C by chronic (24 h) treatment of cells with 300 nM PMA, or pretreatments (10 min) with the cell-permeant calcium chelator 1,2-bis-(O-aminophenoxy)-ethane-N,N,Nʹ,Nʹ-tetra-acetic acid tetra-acetoxymethyl ester led to the inhibition of S1P-induced PtdEtn hydrolysis. S1P alone was a weak inducer of DNA synthesis, but its effects were enhanced by phosphocholine (PCho), insulin, ATP or PMA. Ethanolamine (5-100 μM) did not modify the mitogenic effect of S1P alone, whereas at 50-100 μM concentrations it actually enhanced the mitogenic effect of PCho via a mitogen-activated protein (MAP) kinase-independent mechanism. In contrast, 5-20 μM concentrations of ethanolamine, which correspond to normal blood ethanolamine levels in humans, strongly inhibited DNA synthesis induced by S1P plus PCho via a MAP kinase-dependent mechanism; importantly, less or no inhibition was observed with 50-100 μM concentrations of ethanolamine. At 5-50 μM concentrations, ethanolamine also inhibited the synergistic mitogenic effects of both S1P plus insulin (22-27% inhibition) and PCho plus ATP (45-73% inhibition) but not those of S1P plus PMA or S1P plus ATP. The results indicate that S1P stimulates PLD-mediated hydrolysis of PtdEtn by a mechanism that may involve a regulatory protein kinase C isoform. Increased formation of ethanolamine by PLD-mediated PtdEtn hydrolysis or by other means may be required for maximal stimulation of DNA synthesis by S1P in the presence of insulin, and particularly PCho.

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