Background
Atherosclerotic cardiovascular disease (ASCVD) remains the leading cause of morbidity and mortality worldwide [
1]. Although primarily a disease of adults, youth with obesity show evidence of subclinical ASCVD [
1‐
3] which places them at increased risk as adults for coronary heart disease [
2] and stroke [
4]. The mechanisms by which obesity confers cardiovascular risk are not fully understood, but inflammation within visceral adipose tissue (VAT) is thought to be contributory [
5,
6]. Further, the impact of excess adipose tissue on distal sites such as arterial wall monocytes/macrophages, direct participants in ASCVD, are also thought to contribute to disease pathogenesis [
7].
Development of ASCVD is characterized by macrophage lipid overload leading to the formation of foam cells, and factors that accelerate this process are deemed atherogenic [
8]. Macrophage cholesterol homeostasis is a delicate balance between influx, endogenous synthesis, esterification and hydrolysis, and efflux [
9,
10]. Reduction in cholesterol efflux from macrophages is inversely related to carotid intima-media thickness, elevating the likelihood of the development of ASCVD [
11]. Like other systemic inflammatory conditions psoriasis [
12] and rheumatoid arthritis [
10], obesity is a risk factor for ASCVD, but the mechanistic link between excess adiposity and ASCVD remains poorly understood [
6].
In an effort to determine how adipose tissue affects distant cells and tissues, we identified adipocyte-derived exosomes as a potential link between obesity and its comorbidities [
13‐
15]. Extracellular vesicles (EVs) are microvesicles that allow intercellular communication, carrying signaling molecules such as proteins and nucleic acids, including functional mRNA and microRNA [
16]. We previously showed that adipocyte-derived EV microRNA content is pathologically altered by obesity and reversed by weight-loss surgery [
13,
15]. A growing line of evidence from animal studies show that exosome-like vesicles released from adipose tissue carry the majority of circulating microRNAs [
17] and are capable of pro-atherogenic effects [
18].
Therefore, we sought determine the relationship between macrophage cholesterol efflux capacity and circulating adipocyte-derived EV microRNAs. We also sought to determine if exposure to VAT EVs regulated macrophage cholesterol efflux and cholesterol efflux gene expression in vitro. We hypothesized that exosomal microRNAs targeting established cholesterol efflux genes (ABCA1, ABCG1, LXRA, CPY27A1, and PPARγ) would be associated with cholesterol efflux capacity. Further, we hypothesized exposure to VAT EVs from patients with obesity would reduce macrophage cholesterol efflux capacity and cholesterol gene expression in vitro.
Discussion
In this study we show, for the first time, significant alterations in cholesterol efflux capacity in adolescents throughout the range of BMI, a relationship between six circulating adipocyte-derived EVs microRNAs targeting ABCA1 and cholesterol efflux capacity, and in vitro alterations of cholesterol efflux in THP-1 macrophages exposed to VAT adipocyte-derive EVs acquired from human subjects. These results suggest that adipocyte-derived EVs, and their microRNA content, may play a critical role in the early pathological development of ASCVD.
ASCVD remains the leading cause of morbidity and mortality worldwide [
1]. Although primarily a disease of adults, youth with obesity show evidence of subclinical ASCVD [
2‐
4], which places them at increased risk as adults for coronary heart disease [
3] and stroke [
5]. Primary prevention of ASCVD would be informed by better understanding of the early pathologic events in youth with obesity. One of the hallmarks of ASCVD is macrophage cholesterol efflux [
11,
12,
21‐
23,
25] impairment which leads to intracellular accumulation of modified LDL and subsequent generation of plaque-forming lipid-rich foam cells [
6]. This is the first study demonstrating a wide range of cholesterol efflux capacity in adolescents throughout the BMI continuum (BMI range for study: 22–70 kg/m
2). By using cluster analysis, we show that differences in efflux capacity are not related to differences in BMI, systemic inflammation (GlycA), or insulin resistance (LPIR). Furthermore, these changes are occurring before any clinically detectable changes in traditional lipid parameters would suggest concern. The MOD (significant) and Low (non-significant) efflux capacity group did show higher total cholesterol, LDL, and LDL particle concentrations as compared to the High efflux capacity groups which may indicate alterations in efflux capacity are impacting circulating lipid profiles.
Adipose tissue can be considered a metabolic organ capable of communicating with cell types relevant to ASCVD, including macrophages [
26]. More recently adipocyte-derived EVs have become of significant interest as a potential mechanism linking adipose tissue communication with other peripheral tissues. In obese mice, adipocyte-derived EVs contribute to the development of insulin resistance via activation of adipose-resident macrophages and secretion of pro-inflammatory cytokines that can result in insulin resistance [
24]. Furthermore, they have been linked to macrophage polarization, foam cell formation, and aortic plaque deposits [
18]. Thus, the effect of adipocyte-derived EVs on macrophage foam cell formation is an emerging area of interest, though the mechanism through which they cause disturbances is not well understood.
We focused on adipocyte-derived exosomal microRNAs for multiple reasons: (1) the accumulating evidence for the role of microRNAs in ASCVD [
27]; (2) due to our previous work indicating a high amount of small non-coding RNAs in adipocyte-derived EVs as compared to other genetic and molecular material [
13] and; (3) that adipose tissue is a significant source of circulating microRNAs [
17]. This led us to hypothesize that adipocyte-derived EVs microRNAs would target mRNAs involved in macrophage cholesterol efflux. In our subset of adolescents with and without obesity we identified six adipocyte-derived EVs microRNAs (Fig.
2a–f, all targeting ABCA1, to be significantly related to cholesterol efflux capacity. ABCA1 is a well-studied regulator of macrophage cholesterol efflux, working to prevent excess intracellular cholesterol accumulation [
28]. Further work is required to understand the role of these microRNA’s role, individually and in concert, in regulating ABCA1 expression in macrophages and resultant changes in macrophage cholesterol efflux.
To extend the work of adipocyte-derived exosomes in animal and cell models, we sought to establish that visceral adipocyte-derived EVs, isolated from VAT of our adolescent cohort, impair macrophage cholesterol efflux in THP-1 cells. THP-1 human monocytic leukemia cells were chosen for the study because they share many properties with normal human monocytes, including expression of scavenger receptors and cholesterol transport proteins, and are a well-accepted model for ASCVD [
29]. Our present study is the first study to utilize human samples and supports a role for adipocyte-derived EV in cholesterol efflux impairment. Exposure of THP-1 macrophages to exosomes isolated from VAT from obese subjects significantly increased Dil-oxLDL retention and resulted in decreased cholesterol efflux in a dose-dependent manner. Furthermore, we show an EVs dose-dependent alteration of macrophage cholesterol efflux genes ABCA1, CD36, CYP27A1, and LXRA. Together these experiments help extend the animal work [
18,
24] and provide the first evidence that EVs from human adipose tissue result in the dysregulation of cholesterol efflux in vitro.
Contrary to our original hypothesis, we do not show an effect of obesity on THP-1 macrophage cholesterol efflux. This is a similar finding to that of Xie et al. [
18] who showed similar effects of VAT EVs from wild type mice and mice fed a high fat diet. Given our findings of circulating EV microRNAs targeting ABCA1, we suspect that EVs, in part, exert their pro-atherogenic effect through transfer of microRNAs. However, our in vitro experiments cannot rule out other potential exosomal mechanisms such as macrophage polarization or protein signaling [
18]. More studies testing various conditions are needed to fully elucidate how adipose-derived EVs regulate macrophage function or interact with other molecules, such as ox-LDL, to influence macrophage function. We also limited our studies to only using THP-1 cells, which is a limitation that future studies should address by using multiple cell lines, including primary monocyte derived macrophages. Further studies exploring the role of specific exosomal microRNAs are needed to help elucidate the connection between circulating EVs microRNAs, macrophage behavior, and macrophage cholesterol efflux. More studies using EVs isolated from human adipose tissue, as well as other significant sources of EVs such as platelets and skeletal muscle, are needed as human obesity is a multifactorial and heterogenous condition not easily captured in animal models.
Obesity, and specifically the accumulation of visceral adipose tissue, is a significant risk factor in the development of chronic cardiometabolic and increased cardiovascular risk profiles [
3,
4]. However, the molecular link between visceral adipose tissue and peripheral tissue dysfunction is still poorly understood. More recent thinking has moved away from focusing solely on the quantity of adiposity, but instead understanding the molecular changes in adipose tissue that may drive these multifactorial diseases [
6]. Our group has focused on adipocyte-derived EVs and previously demonstrated obesity-driven changes in adipocyte-derived EVs microRNAs and changes following bariatric weight-loss surgery [
13,
15]. MicroRNAs, and specifically microRNAs packaged in EVs, are ideal for tissue crosstalk due to the stable nature of microRNAs and the cellular access the lipid vesicle provides [
17,
27]. Furthermore, adipocyte-derived EV microRNAs offer a potential biomarker to determine the molecular nature of the adiposity and risk for developing cardiovascular disease and comorbidities. Our data on the relationship between adipocyte-derived EV microRNAs and cholesterol efflux capacity, as well as the in vitro alterations of macrophage cholesterol efflux, offer potential starting points for further mechanistic and longitudinal studies.
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