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

Our previous studies have shown that both increase in Ca²⁺ levels and activation of protein kinase C (PKC) are required for monocyte-mediated O₂⁻ production and low density lipoprotein (LDL) peroxidation. Phosphoinositide-specific phospholipase C (phosphoinositidase C or PIC) is believed to mediate release of intracellular Ca²⁺ through InsP₃ 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 InsP₃ 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 InsP₃ formation but did not alter LDL lipid peroxidation nor O₂⁻ 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 InsP₃. The activity of this pathway, however, is not required for cell-mediated O₂⁻ production, PKC activation or LDL oxidation