Macrophage-mediated oxidation of
low density lipoprotein (
LDL) is considered to be of major importance in early
atherogenesis; therefore, intervention means to inhibit this process are being extensively studied. In the present study, we questioned the ability of the isoflavan
glabridin (from licorice) to accumulate in macrophages and to affect cell-mediated oxidation of
LDL. 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 micrograms of
glabridin/mg of cell
protein after 2 h of incubation, and this process was time- and
glabridin dose-dependent. In parallel, in
glabridin-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 substantially inhibited. In
glabridin-enriched macrophages,
protein kinase C activity reduced by approximately 70%. All of the above effects of
glabridin required 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 next performed in vivo studies, using the atherosclerotic
apolipoprotein E-deficient (E0) mice.
MPMs harvested from
glabridin-treated E0 mice (20 micrograms/mouse/day for a period of 6 weeks) demonstrated reduced capability to oxidize
LDL by 80% in comparison with placebo-treated mice. This latter phenomenon was associated with a reduction in the lesion
oxysterols and a 50% reduction in the aortic lesion size. We thus conclude that
glabridin accumulation in macrophages is associated with reduced cell-mediated oxidation of
LDL and decreased activation of the
NADPH oxidase system. These phenomena could be responsible for the attenuation of
atherosclerosis in E0 mice, induced by
glabridin.