Macrophage enrichment with the isoflavan glabridin inhibits NADPH oxidase-induced cell-mediated oxidation of low density lipoprotein - A possible role for protein kinase C

Macrophage-mediated oxidation of low density lipoprotein (LDL) is considere d to be of major importance in early atherogenesis; therefore, intervention means to inhibit this process are being extensively studied. In the presen t study, we questioned the ability of the isoflavan glabridin (from licoric e) to accumulate in macrophages and to affect cell-mediated oxidation of LD L, We first performed in vitro studies, using mouse peritoneal macrophages (MPMs) and the J-774 A.1 macrophage-like cell line. Both cells accumulated up to 1.5 mu g of glabridin/mg of cell protein after 2 h of incubation, and this process was time- and glabridin dose-dependent. In parallel, in glabr idin-enriched cells, macrophage-mediated oxidation of LDL was inhibited by up to 80% in comparison with control cells. Glabridin inhibited superoxide release from MPMs in response to phorbol 12-myristate 13-acetate, or to LDL when added together with copper ions, by up to 60%, Translocation of P-47, a cytosolic component of NADPH oxidase to the plasma membrane was substant ially inhibited, In glabridin-enriched macrophages, protein kinase C activi ty reduced by similar to 70%. All of the above effects of glabridin require d the presence of the two hydroxyl groups on the flavonoid's B phenol ring, In order to assess the physiological significance of these results, we nex t performed in vivo studies, using the atherosclerotic apolipoprotein E-def icient (E-0) mice. MPMs harvested from glabridin-treated E-0 mice (20 mu g/ mouse/day for a period of 6 weeks) demonstrated reduced capability to oxidi ze LDL by 80% in comparison with placebo-treated mice. This latter phenomen on was associated with a reduction in the lesion oxysterols and a 50% reduc tion in the aortic lesion size. We thus conclude that glabridin accumulation in macrophages is associated w ith reduced cell-mediated oxidation of LDL and decreased activation of the NADPH oxidase system. These phenomena could be responsible for the attenuat ion of atherosclerosis in E-0 mice, induced by glabridin.