Reciprocal binding of xanthine oxidase and myeloperoxidase with ApoA‐I correlates with HDL anti‐ and pro‐inflammatory properties

Abstract

Recent studies show myeloperoxidase (MPO) binds to apolipoprotein AI (apoA-I) and catalyzes oxidative modification (nitration and chlorination) of apoA-I. Xanthine oxidase (XO), an enzymatic source of O2−, causes endothelial dysfunction in chronic states of inflammation. We hypothesize that HDL binds XO as a means of protecting endothelial cell function. Pull-down assays of apoA-I were performed from the plasma of C57BL/6 mice and LDLr knockout mice (Western Diet, 6 weeks) that were treated with and without apoA-I mimetic peptide D-4F (1mg/kg), then immunoblotted for XO, MPO and nitrotyrosine (3NT) on/in apoA-I. Pro-inflammatory HDL levels were determined by the 2′,7′-dichloro-fluorescin diacetate (DCF) fluorescence assay. D-4F significantly decreased both 3NT content within apoA-I (~40%, n=6, p<0.05) and MPO associated with apoA-I (~30%, n=6, p<0.05) in the plasma of the hypercholesterolemic (HC) mice. D-4F did not disrupt MPO-apoA-I interactions in vitro. Neither did D-4F have any effect on total plasma MPO levels in HC mice in vivo. D-4F did however significantly decrease pro-inflammatory HDL levels in these mice (~38%, n=6, p<0.05). Moreover, D-4F treatments increased XO association with apoA-I by ~20% (n=6, p<0.05). These data are the first to show that apoA-I in anti-inflammatory HDL binds XO. Our findings support the notion that oxidative modification of apoA-I by MPO turns HDL into a pro-inflammatory lipoprotein. One of the mechanisms by which HDL may protect vascular endothelial cell function is by apoA-I scavenging XO. Our findings show XO and MPO share a reciprocal interaction with apoA-I that appears to define HDL’s anti- and pro-inflammatory properties.