Distinctive inhibitory activity of docosahexaenoic acid against sphingosine-induced apoptosis

The effect of supplementation of docosahexaenoic acid (DHA) on the apoptosis of HL60 cells was examined using N-acetyl sphingosine (C2-ceramide) and sphingosine as apoptosis-inducing agents. Although C2-ceramide-induced apoptosis was not affected by DHA supplementation, sphingosine-induced apoptosis was reduced almost to the background level by preincubation with 10 microM DHA for 24 h. Among the fatty acids, only DHA appeared to be endowed with the ability to reduce sphingosine-induced apoptosis, whereas, other unsaturated fatty acids, such as arachidonic acid (AA) and eicosapentaenoic acid (EPA), did not show this activity. Incubation of HL60 with DHA within 6 h did not affect the apoptosis, suggesting that DHA probably expressed the inhibitory activity after modulation of the membrane fatty acid composition. DHA also attenuated the apoptosis induced by dimethylsphingosine and H-7, but not by calphostin C, indicating that enrichment of DHA in membranous phospholipid does not necessarily prevent all of the apoptosis associated with the inhibition of protein kinase C. The mechanism of the inhibition against sphingosine-induced apoptosis by DHA remains to be further explored. However, the inhibition of cytosolic phospholipase A2 (cPLA2) may be involved in the mechanism, because distinctive inhibitory activity of DHA against cPLA2 has been demonstrated [M. Shikano, Y. Masuzawa, K. Yazawa, K. Takayama, I. Kudo, K. Inoue, Biochim. Biophys. Acta, 1212, 1994, 211-216], and arachidonyl trifluoromethylketone, a specific inhibitor of cPLA2, attenuated the apoptosis induced by sphingosine.