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 nativ e low density lipoprotein (LDL) and transform it to a cytotoxin. We al so found that intracellular Ca2+ levels are integrally involved in lip id oxidation of LDL by activated monocytes. In these studies, we inves tigated the protein kinase C (PKC) signaling pathway for its contribut ion to the process of monocyte oxidation of LDL lipids. We found subst antial protein phosphorylation induced upon monocyte activation. Pharm acologic inhibition of PKC activity with the PKC inhibitors H-7 (1-100 mu M), calphostin C (1-10 mu M), and GF109203X (0.1-10 mu M) caused a dose-dependent inhibition of cellular protein phosphorylation, includ ing that of several previously identified PKC substrates. These inhibi tors 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 contras t, inhibition of cAMP dependent protein kinase activity altered neithe r monocyte-mediated LDL lipid oxidation nor O-2(-) production by activ ated monocytes. Delaying the addition of PKC inhibitors until after th e peak production of O-2(-) which occurs during the respiratory burst, still resulted in inhibition bf 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 dow n-regulate PKC activity and also used antisense oligonucleotides as sp ecific PKC inhibitors. Results of both types of studies support the co nclusion that PKC activity is required for activated monocytes to oxid ize LDL lipids. Thus, PKC activation in this system is essential, one critical pathway regulated by PKC activity is the production of O-2(-) and continued PKC activity is required for optimal oxidation of LDL L ipids.