α-Tocopherol Decreases Superoxide Anion Release in Human Monocytes Under Hyperglycemic Conditions Via Inhibition of Protein Kinase C-α

Abstract

Diabetes is a major risk factor for premature atherosclerosis, and oxidative stress appears to be an important mechanism. Previously, we showed that diabetic monocytes produce increased superoxide anion (O₂⁻), and α-tocopherol (AT) supplementation decreases this. The aim of this study was to elucidate the mechanism(s) of O₂⁻ release and inhibition by AT under hyperglycemic (HG) conditions in monocytes. O₂⁻ release, protein kinase C (PKC) activity, and translocation of PKC-α and -βII and p47phox were increased in THP-1 cells (human monocytic cell line) under HG (15 mmol/l glucose) conditions, whereas AT supplementation inhibited these changes. AT, NADPH oxidase inhibitors (apocynin and diphenyleneiodonium chloride [DPI]), and an inhibitor to PKC-α and other isoforms (2,2′,3,3′,4,4′-hexahydroxy-1,1′-biphenyl-6,6′-dimethanol dimethyl ether [HBDDE]) but not PKC-β II (LY379196) decreased O₂⁻ release and p47phox translocation. Antisense oligodeoxynucleotides to PKC-α and p47phox but not to PKC-βII inhibited HG-induced O₂⁻ release and p47phox translocation in THP-1 cells. Under HG conditions, reactive oxygen species release from monocytes was not inhibited by agents affecting mitochondrial metabolism but was inhibited in human endothelial cells. We conclude that under HG conditions, monocytic O₂⁻ release is dependent on NADPH oxidase activity but not the mitochondrial respiratory chain; HG-induced O₂⁻ release is triggered by PKC-α, and AT inhibits O₂⁻ release via inhibition of PKC-α.