Activation of PKC, superoxide anion production and LDL lipid peroxidation are not dependent on phosphoinositide-specific phospholipase C activity in U937 cells

Abstract

Our previous studies have shown that both increase in Ca 2+ levels and activation of protein kinase C (PKC) are required for monocyte-mediated O 2 − production and low density lipoprotein (LDL) peroxidation. Phosphoinositide-specific phospholipase C (phosphoinositidase C or PIC) is believed to mediate release of intracellular Ca 2+ through InsP 3 formation and activation of PKC through diacylglycerol (DAG). In these studies, we investigated the PIC pathway for its participation in monocytic cell-mediated lipid peroxidation of LDL. We found substantial InsP 3 formation in opsonized zymosan (ZOP)-activated U937-b cells, indicating the activation of PIC. Both inhibition of PIC by the PIC inhibitor U-73122 and reduction of the supply of the precursor lipid by lithium chloride suppressed InsP 3 formation but did not alter LDL lipid peroxidation nor O 2 − production by activated cells. Furthermore, we also found that suppression of PIC activity had no substantial inhibitory effect on PKC activity in ZOP-activated human monocytes. Our data suggest that PIC activity is induced upon cell activation resulting in increased levels of InsP 3. The activity of this pathway, however, is not required for cell-mediated O 2 − production, PKC activation or LDL oxidation