Protein kinase Cα regulates human monocyte O2•- production and low density lipoprotein lipid oxidation

Our previous studies have shown that human native low density lipoprotein (LDL) can be oxidized by activated human monocytes, In this process, both activation of protein kinase C (PKC) and induction of superoxide anion (O-2(radical anion)) production are required. PKC is a family of isoenzymes, and the functional roles of individual PKC isoenzymes are believed to differ based on subcellular location and distinct responses to regulatory signals. We have shown that the PKC isoenzyme that is required for both monocyte O-2(radical anion) production and oxidation of LDL is a member of the conventional PKC group of PKC isoenzymes (Li, Q., and Cathcart, M.K. (1994) J. Biol. Chem, 269, 17508-17515), The conventional PKC group includes PKC alpha, PKC beta I, PKC beta II, and PKC gamma. With the exception of PKC gamma, each of these isoenzymes was detected in human monocytes, in these studies, we investigated the requirement for select PKC isoenzymes in the process of monocyte-mediated LDL lipid oxidation, Our data indicate that PKC activity was rapidly induced upon monocyte activation with the majority of the activity residing in the membrane/particulate fraction. This enhanced PRC activity was sustained for up to 24 h after activation. PKC alpha, PKC beta I, and PKC beta II protein levels were induced upon monocyte activation, and PKC alpha and PKC beta II substantially shifted their location from the cytosol to the particulate/ membrane fraction. To distinguish between these isoenzymes for regulating monocyte O-2(radical anion) production and LDL oxidation, PKC alpha or PKC beta isoenzyme-specific antisense oligonucleotides were used to selectively suppress isoenzyme expression. We found that suppression of PKC alpha expression inhibited both monocyte-mediated O-2(radical anion) production and LDL lipid oxidation by activated human monocytes, In contrast, inhibition of PKC beta expression (including both PKC beta I and PKC beta II) did not affect O-2(radical anion) production or LDL lipid oxidation, Further studies demonstrated that the respiratory burst oxidase responsible for O-2(radical anion) production remained functionally intact in monocytes with depressed levels of PKC alpha because O-2(radical anion) production could be restored by treating the monocytes with arachidonic acid. Taken together, our data reveal that PKC alpha, and not PKC beta I or PKC beta II, is the predominant isoenzyme required for O-2(radical anion) production and maximal oxidation of LDL by activated human monocytes.