Protein kinase C activity is required for lipid oxidation of low density lipoprotein by activated human monocytes

Our previous studies have shown that human monocytes can oxidize native low density lipoprotein (LDL) and transform it to a cytotoxin. We also found that intracellular Ca2+ levels are integrally involved in lipid oxidation of LDL by activated monocytes. In these studies, we investigated the protein kinase C (PKC) signaling pathway for its contribution to the process of monocyte oxidation of LDL lipids. We found substantial protein phosphorylation induced upon monocyte activation. Pharmacologic inhibition of PKC activity with the PKC inhibitors H-7 (1-100 microM), calphostin C (1-10 microM), and GF109203X (0.1-10 microM) caused a dose-dependent inhibition of cellular protein phosphorylation, including that of several previously identified PKC substrates. These inhibitors of PKC activity also substantially inhibited LDL lipid oxidation by activated monocytes. This inhibition was correlated with a profound suppression of superoxide anion production by these cells. In contrast, inhibition of cAMP-dependent protein kinase activity altered neither monocyte-mediated LDL lipid oxidation nor O2- production by activated monocytes. Delaying the addition of PKC inhibitors until after the peak production of O2-, which occurs during the respiratory burst, still resulted in inhibition of LDL lipid oxidation, suggesting roles for PKC in both early and late events. To corroborate these findings using other approaches, we used phorbol 12-myristate 13-acetate to down-regulate PKC activity and also used antisense oligonucleotides as specific PKC inhibitors. Results of both types of studies support the conclusion that PKC activity is required for activated monocytes to oxidize LDL lipids. Thus, PKC activation in this system is essential, one critical pathway regulated by PKC activity is the production of O2-, and continued PKC activity is required for optimal oxidation of LDL lipids.