Adipocyte ATP-binding cassette G1 promotes triglyceride storage, fat mass growth, and human obesity

The role of the ATP-binding cassette G1 (ABCG1) transporter in human pathophysiology is still largely unknown. Indeed, beyond its role in mediating free cholesterol efflux to HDL, the ABCG1 transporter equally promotes lipid accumulation in a triglyceride (TG)-rich environment through regulation of the bioavailability of lipoprotein lipase (LPL). Because both ABCG1 and LPL are expressed in adipose tissue, we hypothesized that ABCG1 is implicated in adipocyte TG storage and therefore could be a major actor in adipose tissue fat accumulation. Silencing of Abcg1 expression by RNA interference in 3T3-L1 preadipocytes compromised LPL-dependent TG accumulation during the initial phase of differentiation. Generation of stable Abcg1 knockdown 3T3-L1 adipocytes revealed that Abcg1 deficiency reduces TG storage and diminishes lipid droplet size through inhibition of Pparγ expression. Strikingly, local inhibition of adipocyte Abcg1 in adipose tissue from mice fed a high-fat diet led to a rapid decrease of adiposity and weight gain. Analysis of two frequent ABCG1 single nucleotide polymorphisms (rs1893590 [A/C] and rs1378577 [T/G]) in morbidly obese individuals indicated that elevated ABCG1 expression in adipose tissue was associated with increased PPARγ expression and adiposity concomitant to increased fat mass and BMI (haplotype AT>GC). The critical role of ABCG1 in obesity was further confirmed in independent populations of severe obese and diabetic obese individuals. This study identifies for the first time a major role of adipocyte ABCG1 in adiposity and fat mass growth and suggests that adipose ABCG1 might represent a potential therapeutic target in obesity.

Abstract 230: Stimulation of Cholesterol Efflux from Human Macrophage by Liver X Receptor Agonists is a 2-Step Mechanism Requiring ARL7 and ABCA1

Nuclear Liver X Receptors activation by synthetic agonists was proven to be atheroprotective in mice; an effect likely based on stimulation of cellular cholesterol efflux from arterial macrophages. However, mechanisms involved in free cholesterol efflux from mouse macrophages appear distinct from those operating in human macrophages.

The objective of this study was to decipher precise cellular mechanisms controlling free cholesterol efflux from human macrophages upon LXR stimulation.

In THP-1 and human monocyte-derived macrophages (HMDM), treatment with the LXR agonist GW3965 efficiently induced ARL7 expression (6-fold, p<0.05), an effect associated with an increased amount of plasma membrane free cholesterol available for efflux (+25%, p<0.05) and a higher lipid rafts formation (+10%, p<0.05). Both effects were abolished in ARL7 Knockdown (KD) macrophages, leading to a lack of stimulation of cholesterol efflux by GW3965.

Specific targeting of each LXR isoforms, LXRα and LXRβ, by RNAi revealed that LXRα silencing in THP-1 and HMDM reduced significantly expression of cholesterol transporters ABCA1, ABCG1 and receptor SR-BI/Cla-1 mRNA levels, as well as free cholesterol efflux to apoA1 (-30%, p<0.05) and to HDL (-20%, p<0.05) upon stimulation with LXR, whereas LXRβ silencing has no impact. Interestingly, stimulation of cholesterol efflux to HDL by GW3965 was significantly reduced (-50%, p<0.05) in ABCA1 KD THP-1 macrophages; those cells being incapable to promote cholesterol efflux to apoA1. However, silencing of ABCG1 or SR-B1/Cla-1 had no impact on cholesterol efflux to HDL from either control or ABCA1 KD THP-1 macrophages treated or not with LXR agonist. By contrast stimulation of cholesterol efflux to HDL by GW3965 was completely abolished in LXRα/ABCA1 double KD macrophages, highlighting the major contribution of ABCA1 in cholesterol efflux from human macrophage.

We conclude that LXR-mediated stimulation of cholesterol efflux from human macrophages is a two-steps mechanism. First, LXR activation promotes ARL7-dependent free cholesterol transport to plasma membrane, mostly in lipid raft domains. Then, membrane free cholesterol is exported to apoA1 and HDL acceptors through ABCA1; this latter step being controlled selectively by LXRα.

Human ATP-binding cassette G1 controls macrophage lipoprotein lipase bioavailability and promotes foam cell formation

OBJECTIVE

The physiological function of the ATP-binding cassette G1 (ABCG1) transporter in humans is not yet elucidated, as no genetic disease caused by ABCG1 mutations has been documented. The goal of our study was, therefore, to investigate the potential role(s) of ABCG1 in lipid metabolism in humans.

METHODS AND RESULTS

Here we report that among the 104 polymorphisms present in the ABCG1 gene, the analysis of the frequent functional rs1893590 and rs1378577 single nucleotide polymorphisms located in the regulatory region of ABCG1 in the Regression Growth Evaluation Statin Study population revealed that both ABCG1 single nucleotide polymorphisms were significantly associated with plasma lipoprotein lipase (LPL) activity. Moreover, we observed that plasma LPL activity was modestly reduced in Abcg1(-/-) mice as compared with control mice. Adipose tissue and skeletal muscle are the major tissues accounting for levels and activity of plasma LPL in the body. However, beyond its lipolytic action in the plasma compartment, LPL was also described to act locally at the cellular level. Thus, macrophage LPL was reported to promote foam cell formation and atherosclerosis in vivo. Analysis of the relationship between ABCG1 and LPL in macrophages revealed that the knockdown of ABCG1 expression (ABCG1 knockdown) in primary cultures of human monocyte-derived macrophages using small interfering RNAs led to a marked reduction of both the secretion and activity of LPL. Indeed, LPL was trapped at the cell surface of ABCG1 knockdown human monocyte-derived macrophages, likely in cholesterol-rich domains, thereby reducing the bioavailability and activity of LPL. As a consequence, LPL-mediated lipid accumulation in human macrophage foam cells in the presence of triglyceride-rich lipoproteins was abolished when ABCG1 expression was repressed.

CONCLUSIONS

We presently report that ABCG1 controls LPL activity and promotes lipid accumulation in human macrophages in the presence of triglyceride-rich lipoproteins, thereby suggesting a potential deleterious role of macrophage ABCG1 in metabolic situations associated with high levels of circulating triglyceride-rich lipoproteins together with the presence of macrophages in the arterial wall.