Underlying Mechanisms of the Anti-inflammatory and Antioxidant Effects of Quercetin in Human Aortic Endothelial Cells : Implications for Vascular Inflammation and Atherosclerosis

Graduation date: 2016Access restricted to the OSU Community, at author's request, from June 11, 2016 - December 11, 2016Atherosclerosis is the underlying cause of ischemic heart disease and stroke and is the leading cause of death worldwide, especially in developed countries. As an inflammatory disease of arteries in a hyperlipidemic milieu, expression of adhesion molecules, such as endothelial-leukocyte adhesion molecule-1 (E-selectin) and intercellular adhesion molecule-1 (ICAM-1), on endothelial cell surfaces is critical for the initiation and progression of atherosclerosis. Expression of cellular adhesion molecules has been reported to be inhibited by quercetin, a dietary flavonoid, but the underlying mechanisms of quercetin’s effects are still not completely understood. In this study, we found that lipopolysaccharide (LPS)-induced mRNA and protein expression of E-selectin and ICAM-1 were inhibited by quercetin in a dose-dependent manner in human aortic endothelial cells (HAEC). Levels of reactive oxygen species (ROS) in HAEC were also significantly reduced by quercetin treatment. Further study revealed that quercetin treatment time- and dose-dependently induced nuclear factor erythroid 2-related factor 2 (Nrf2) activation, increased mRNA and protein levels of heme oxygenase 1 (HO-1), NAD(P)H: quinone oxidoreductase 1 (NQO1) and glutamate-cysteine ligase (GCL) catalytic (GCLC) and regulatory (GCLM) subunits. The p38 mitogen-activated protein kinase (p38) inhibitor, SB203580, significantly attenuated quercetin-induced activation of Nrf2 and expression of antioxidant enzymes. However, quercetin had no effect on Kelch-like ECH-associated protein 1 (Keap1) protein levels. \ud In addition to increasing expression of GCL, the rate-limiting enzyme in glutathione synthesis, quercetin treatment also elevated glutathione reductase (GR) protein level and enzymatic activity and substantially altered glutathione status in HAEC. A moderate reduction followed by a complete recovery of reduced glutathione (GSH) was observed after quercetin treatment, as well as an up to 50% decline in oxidized glutathione (GSSG), resulting in an up to 70% increase in the GSH/GSSG ratio. Quercetin also induced the expression of multidrug resistance protein 1 (MRP1) and formation of quercetin-glutathione conjugates. Formation and cellular export of these quercetin-glutathione conjugates, together with induction of GCL and GR, likely account for the quercetin-induced alterations of glutathione status in HAEC. \ud Together, my results suggest that quercetin induces activation of Nrf2 and, subsequently, induction of HO-1, NQO1, GCL, GR and MRP-1, in part, via activation of p38. Induction of antioxidant enzymes and alterations in intracellular glutathione status readily explain the antioxidant effects of quercetin against basal and LPS-induced oxidant formation in HAEC, whereas the anti-inflammatory effects of quercetin on LPS-induced adhesion molecule expression are likely due to increased expression of HO-1 and may occur through antioxidant-independent mechanisms, such as HO-1 mediated inhibition of the transcription factor activator protein-1 (AP-1)