Copyright © 2006, European Society of Cardiology
Oxidized low-density lipoprotein depletes PKC
and attenuates reactive oxygen species formation in monocytes/macrophages
Institute of Biochemistry I, Faculty of Medicine, Johann Wolfgang Goethe-University, Theodor-Stern-Kai 7, 60590 Frankfurt, Germany
* Corresponding author. Tel.: +49 69 6301 7424; fax: +49 69 6301 4203. Email address: bruene{at}zbc.kgu.de
Objective: Preexposure of macrophages to oxidized low-density lipoprotein (oxLDL) attenuates formation of reactive oxygen species (ROS) upon stimulation with phorbol 12-myristate 13-acetate (PMA) or acute exposure to oxLDL. We examined whether attenuation of the oxidative burst is attributed to down-regulation of protein kinase C 
(PKC).
Methods and results Acute exposure of a mouse macrophage cell line (RAW 264.7) to both PMA and oxLDL provoked ROS generation that was blocked by the PKC/β1 inhibitor Go 6967. However, in RAW 264.7 macrophages preincubated with oxLDL, ROS formation in response to stimulation with oxLDL or PMA was reduced. Attenuated ROS production correlated with down-regulation of the amount of PKC protein in a time-dependent manner and was maximal at 8 h and concentrations of 50–100 µg/ml oxLDL. PMA, a well-established PKC activator, inhibited ROS formation as well when preincubated for 8 to 16 h. In cells stably overexpressing PKC-EGFP, we noticed that ROS formation remained intact upon pre-exposure of these cells to oxLDL. Moreover, in these cells endogenous but not overexpressed PKC-EGFP disappeared, further substantiating the importance of PKC in stimulating ROS production. In addition, we noticed a concentration-dependent ability of oxLDL to halt ROS production. Whereas 10 µg/ml oxLDL was insufficient in attenuating ROS formation over an 8-h incubation period in RAW 264.7 cells, 50 µg/ml oxLDL impaired ROS generation.
Conclusion These results indicate that attenuation of the oxidative burst in oxLDL-pretreated macrophages is closely associated with down-regulation of PKC, which is elicited in a dose- and time-dependent manner.
KEYWORDS Protein kinase C; Monocytes/macrophages; Oxygen radicals; Inflammation; Lipoproteins