Dysfunctional, pro-inflammatory HDL directly upregulates monocyte PDGFRβ, chemotaxis and TNFα production

Abstract

Accelerated atherosclerosis is a major co-morbid condition in autoimmune diseases. Monocytes are the main immune cell involved in atherosclerosis initiation. We hypothesized that dysfunctional, pro-inflammatory HDL (piHDL), which occurs in approximately half of SLE patients, might directly influence monocyte gene expression and function. SLE subjects were stratified into three groups: 1) carotid artery plaque+piHDL+, 2) plaque-piHDL+, and 3) plaque-piHDL− (n = 18/group). PDGFRβ was upregulated in primary monocytes from plaque+piHDL+ patients and in THP-1 cells acutely treated in vitro with piHDL compared to normal HDL. THP-1 chemotaxis was enhanced after treatment with piHDL versus normal HDL. Abnormal migration was restored to normal levels by treatment with imatinib or an apoJ mimetic peptide. Increased piHDL-mediated TNFα protein levels were reduced with both inhibitors. Dysfunctional piHDL directly influences expression of a small number of transcripts and proteins, and piHDL inhibition through reducing piHDL oxidation or blocking PDGFRβ kinase activity restored normal monocyte chemotaxis.

Introduction

Systemic lupus erythematosus (SLE) patients have a 5-fold greater risk of myocardial infarction, occurring at a mean age of 49 (versus around 72 in healthy people), leading to a bimodal pattern of death where cardiovascular disease (CVD) is a leading cause of mortality 10 years after diagnosis of SLE [1]. The increase in cardiovascular events observed in SLE is due in part to traditional risk factors [1], [2], [3], [4], [5], but SLE itself is an independent factor predisposing to accelerated CVD. Young women (35–44 years old) with SLE are approximately 50 times more likely to have an MI versus age-matched control subjects [4]. Relative risk in a separate SLE cohort for MI or stroke was 10.1 and 7.9, respectively, even after controlling for Framingham factors [6]. Recent studies have also suggested that subclinical atherosclerosis, measured by ultrasound or CT of carotid or coronary arteries, is significantly increased in SLE patients compared to age-matched controls [4], [7], [8], [9].

High levels of plasma high density lipoprotein (HDL) are widely accepted as being protective against CVD, due to their promotion of reverse cholesterol transport (RCT) and reduction of oxidative species on low density lipoproteins [10], [11]. Our group has recently shown that a dysfunctional, pro-inflammatory form of HDL (piHDL) [12], [13] is present in 45% of SLE patients versus 4% of controls [9] and its presence highly correlates with the presence of carotid artery plaque, with an OR of 16.1 [14]. Autoantibodies that recognize apoA-I, the main protein component of HDL, are present in SLE patients [15] and recent data suggest that these autoantibodies could contribute to increased disease activity and organ damage through blocking the protective qualities of HDL [16], [17], [18]. Autoantibodies against oxidized LDL in humans and a mouse model with SLE-like characteristics have also been implicated in increased organ damage and risk for atherosclerosis [19], [20], [21].

Monocytes are the major immune cell involved in the initiation of atherosclerosis due to their integral roles at many stages of atherogenesis. Activated endothelial cells (EC) attract monocytes by secreting numerous chemokines, including monocyte chemoattractant protein-1 (MCP-1) and TNFα [22]. Monocytes then attach to EC and transmigrate into the arterial intima and secrete factors integral to the formation and ultimate destabilization of plaque [23]. Once in the subendothelial space, monocyte colony stimulating factor (M-CSF) drives monocytes to differentiate into macrophages that ultimately become cholesterol- and lipid-rich foam cells [23].

Studies on the contribution of monocytes to accelerated atherosclerosis and general disease progression in SLE have been limited: increased monocyte recruitment to aortic plaque were noted in a murine model of SLE [24], and TNF-receptor family mediated monocyte apoptosis in SLE-prone mice and humans has been linked to disease progression [25], [26].

CD36, a scavenger receptor for oxidized phospholipids implicated in atherogenesis, is upregulated in the human monocyte cell line THP-1 after treatment with SLE patient plasma [27]. In addition, HDL isolated from patients with antiphospholipid syndrome was less effective at inhibiting monocyte binding to cultured EC than HDL isolated from healthy controls [28]. Therefore, we hypothesized that SLE piHDL, in addition to its association with subclinical atherosclerosis, induces a pro-inflammatory phenotype through indirect or direct monocyte interactions.