Docosahexaenoic acid induces increases in [Ca2+]i via inositol 1,4,5-triphosphate production and activates protein kinase Cγ and -δ via phosphatidylserine binding site:: Implication in apoptosis in U937 cells

We investigated, in monocytic leukemia U937 cells, the effects of docosahexaenoic acid (DHA; 22:6 n-3) on calcium signaling and determined the implication of phospholipase C (PLC) and protein kinase C (PKC) in this pathway. DHA induced dose-dependent increases in [Ca2+](i), which were contributed by intracellular pool, via the production of inositol-1,4,5-triphosphate (IP3) and store-operated Ca2+ (SOC) influx, via opening of Ca2+ release-activated Ca2+ (CRAC) channels. Chemical inhibition of PLC, PKC gamma, and PKC delta, but not of PKC beta I/II, PKC alpha, or PKC beta I, significantly diminished DHA-induced increases in [Ca2+](i). In vitro PKC assays revealed that DHA induced a similar to 2-fold increase in PKC gamma and -delta activities, which were temporally correlated with the DHA-induced increases in [Ca2+](i). In cell-free assays, DHA, but not other structural analogs of fatty acids, activated these PKC isoforms. Competition experiments revealed that DHA-induced activation of both the PKCs was dose-dependently inhibited by phosphatidylserine (PS). Furthermore, DHA induced apoptosis via reactive oxygen species (ROS) production, followed by caspase-3 activation. Chemical inhibition of PKC gamma/delta and of SOC/CRAC channels significantly attenuated both DHA-stimulated ROS production and caspase-3 activity. Our study suggests that DHA-induced activation of PLC/IP3 pathway and activation of PKC gamma/delta, via its action on PS binding site, may be involved in apoptosis in U937 cells.