Paraoxonase 1 (PON1) enhances HDL-mediated macrophage cholesterol efflux via the ABCA1 transporter in association with increased HDL binding to the cells: a possible role for lysophosphatidylcholine

Abstract

We investigated the role of HDL-associated paraoxonase 1 (PON1) in HDL-mediated macrophage cholesterol efflux by using HDL derived from wild type mice (Control-HDL), from human PON1-transgenic mice (HDL-PON1Tg) or from PON1-knockout mice (HDL-PON1⁰). Cholesterol efflux from mouse peritoneal macrophages (MPM) or from J774 A.1 macrophage cell line by HDL-PON1Tg, was significantly increased (by 60%) compared to HDL-PON1⁰. We demonstrated that this PON1 effect was associated with an increased HDL binding to the cells, as the binding of HDL-PON1Tg (or HDL-PON1⁰ that was enriched with PON1) was increased by 50% compared to that of HDL-PON1⁰. Using either a cAMP analogue, to increase ABCA1 receptor expression, or rabbit anti-mouse SR-BI specific antibody to block the SR-BI receptor, PON1 stimulation of HDL binding and of HDL-mediated macrophage cholesterol efflux, were both found to involve the ABCA1 transporter. Studies with PON1 specific inhibitors revealed that PON1 activity was required for its stimulation of HDL-mediated macrophage cholesterol efflux. Upon incubation of macrophages with Control-HDL or with HDL-PON1Tg, macrophage lysophosphatidylcholine (LPC) content was increased by 3.7- and 7.5-fold, respectively. Such an LPC enrichment of macrophages resulted in up to 60% increased HDL binding to the cells, and a 41% increased HDL-mediated cholesterol efflux. Similarly, macrophage loading with LPC (by either adding LPC, or PON1 or phospholipase A₂) significantly increased apolipoprotein A-I (apoA-I) mediated cholesterol efflux by 104, 65 and 56%, respectively, in ABCA1 overexpressing macrophages. We conclude that HDL-associated PON1 may contribute to the attenuation of atherosclerosis development by its ability to act on macrophage phospholipids, to form LPC, in turn, stimulates HDL binding and HDL-mediated macrophage cholesterol efflux via the ABCA1 transporter.

Introduction

Serum paraoxonase 1 (PON1) is an HDL-associated esterase [1] and its activity is inversely related to the risk of cardiovascular diseases [2], [3]. The role of PON1 in atherosclerosis was demonstrated in studies using mice lacking PON1 [4], [5], [6], or in mice overexpressing PON1 [7], [8]. These studies, as well as in vitro studies, showed that PON1 can protect against oxidative stress [5], [6], [8], [9], [10], [11], [12], [13] by hydrolyzing specific oxidized lipids in lipoproteins [9], [10], [12] and in atherosclerotic lesions [13], including macrophages [6], [14]. Macrophage foam cell formation is the hallmark of early atherogenesis [15], and cellular cholesterol accumulation is dependent on the balance between cholesterol influx and efflux. PON1 contributes to a decrement in macrophage cholesterol accumulation by inhibiting cholesterol biosynthesis [16] and the uptake of oxidized LDL via the scavenger receptor CD-36 [14]. A possible role for PON1 in cholesterol efflux may be related to its association in serum with the HDL particles. HDL was shown to remove excess cholesterol from arterial cells, including macrophage foam cells [17], [18], [19], [20], via the scavenger receptor B1 (SR-BI, [19], [20], [21], [22]) and the ATP binding cassette transporter A1 (ABCA1, [19], [20], [23], [24]). In the present study, we questioned the contribution of HDL-associated PON1 to HDL-mediated macrophage cholesterol efflux and the possible mechanism for such a phenomenon.