ATP binding cassette (ABC) transporters represent a large and diverse family of proteins that transport specific substrates across a membrane. The importance of these transporters is illustrated by ...the finding that inactivating mutations within 17 different family members are known to lead to specific human diseases. Clinical data from humans and/or studies with mice lacking functional transporters indicate that ABCA1, ABCG1, ABCG4, ABCG5 and ABCG8 are involved in cholesterol and/or phospholipid transport. This review discusses the multiple mechanisms that control cellular sterol homeostasis, including the roles of microRNAs, nuclear and cell surface receptors and ABC transporters, with particular emphasis on recent findings that have provided insights into the role(s) of ABCG1. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945–2010).
► Cellular cholesterol homeostasis is a tightly regulated process. ► Mutations in many proteins involved in cholesterol homeostasis result in disease. ► Specific role and substrate of ABCG1 are still unknown. ► This review explores new insights into ABCG1 structure and function.
To determine whether activation of farnesoid X receptor (FXR) alters cellular and plasma cholesterol homeostasis as a result of regulation of Srebp-2 and miR-33.
Chromatin immunoprecipitation ...sequencing data identified an FXR response element within intron 10 of the Srebp-2 gene. Consistent with this observation, treatment of mice with FXR-specific agonists (GSK2324 or GW4064) rapidly increased hepatic levels of Srebp-2 mRNA, precursor sterol response element binding protein 2 (pSREBP-2) protein, and miR-33. Furthermore, miR-33 targets, that include ABCA1 (ATP binding cassette transporter A1), NSF (N-ethylmaleimide-sensitive factor), and CPT1 (carnitine palmitoyltransferase 1), were all reduced in GSK2324-treated mice. In contrast, neither nuclear SREBP-2 protein (nSREBP-2) nor SREBP-2 target genes were induced after FXR activation. The inability to process pSREBP-2 to nSREBP-2 is likely a consequence of the induction of insulin INSIG-2A (induced gene 2A) by FXR agonists. Finally, we show that FXR-dependent induction of both Srebp-2 and miR-33 is ablated in Scap(-/-) mice that lack nuclear SREBP-2.
We demonstrate that the activation of FXR uncouples the expression of nuclear SREBP-2 and miR-33, and the regulation of their respective target genes. Further, we conclude that the FXR agonist-dependent increase in miR-33 requires transcription of the Srebp-2 gene.
OBJECTIVE—In a recent article in Arteriosclerosis, Thrombosis, and Vascular Biology, it was reported that ATP-binding cassette transporter G1 (ABCG1) containing leucine at position 550 (ABCG1-L550) ...was localized to the plasma membrane, whereas ABCG1-P550 (proline at position 550) was intracellular. Because the published data on the subcellular localization of ABCG1 are controversial, we performed additional experiments to determine the importance of leucine or proline at amino acid 550.
APPROACH AND RESULTS—We transfected multiple cell lines (CHO-K1, Cos-7, and HEK293) with untagged or FLAG-tagged ABCG1 containing either leucine or proline at position 550. Immunofluorescence studies demonstrated that in all cases, ABCG1 localized to intracellular endosomal vesicles. We also show that both ABCG1-L550 and ABCG1-P550 are equally active in both promoting the efflux of cellular cholesterol to exogenous high-density lipoprotein and in inducing the activity of sterol regulatory element–binding protein-2, presumably as a result of redistributing intracellular sterols away from the endoplasmic reticulum. Importantly, we treated nontransfected primary peritoneal macrophages with a liver X receptor agonist and demonstrate, using immunofluorescence, that although endogenous ABCG1 localizes to intracellular endosomes, none was detectable at the cell surface/plasma membrane.
CONCLUSIONS—ABCG1, irrespective of either a leucine or proline at position 550, is an intracellular protein that localizes to vesicles of the endosomal pathway where it functions to mobilize sterols away from the endoplasmic reticulum and out of the cell.
Abstract
Background and Objective
There is increasing interest in the role of lipids in processes that modulate lung fibrosis with evidence of lipid deposition in idiopathic pulmonary fibrosis (IPF) ...histological specimens. The aim of this study was to identify measurable markers of pulmonary lipid that may have utility as IPF biomarkers.
Study Design and Methods
IPF and control lung biopsy specimens were analysed using a unbiased lipidomic approach. Pulmonary fat attenuation volume (PFAV) was assessed on chest CT images (CT
PFAV
) with 3D semi‐automated lung density software. Aerated lung was semi‐automatically segmented and CT
PFAV
calculated using a Hounsfield‐unit (−40 to −200HU) threshold range expressed as a percentage of total lung volume. CT
PFAV
was compared to pulmonary function, serum lipids and qualitative CT fibrosis scores.
Results
There was a significant increase in total lipid content on histological analysis of IPF lung tissue (23.16 nmol/mg) compared to controls (18.66 mol/mg,
p
= 0.0317). The median CT
PFAV
in IPF was higher than controls (1.34% vs. 0.72%,
p
< 0.001) and CT
PFAV
correlated significantly with DLCO% predicted (
R
2
= 0.356,
p
< 0.0001) and FVC% predicted (
R
2
= 0.407,
p
< 0.0001) in patients with IPF. CT
PFAV
correlated with CT features of fibrosis; higher CT
PFAV
was associated with >10% reticulation (1.6% vs. 0.94%,
p
= 0.0017) and >10% honeycombing (1.87% vs. 1.12%,
p
= 0.0003). CT
PFAV
showed no correlation with serum lipids.
Conclusion
CT
PFAV
is an easily quantifiable non‐invasive measure of pulmonary lipids. In this pilot study, CT
PFAV
correlates with pulmonary function and radiological features of IPF and could function as a potential biomarker for IPF disease severity assessment.
Aberrant cell lipid metabolism has been described in IPF but the underlying mechanisms are underexplored. In this study, chest CT is used to assess pulmonary fat attenuation volume (PFAV) in IPF. CT
PFAV
correlates with spirometry and IPF radiological markers. With further validation, CT
PFAV
could be used as a disease severity biomarker.
Dietary conjugated linoleic acids (CLA) have been reported to have a number of isomer-dependent effects on lipid metabolism including reduction in adipose tissue deposition, changes in plasma ...lipoprotein concentrations and hepatic lipid accumulation. The aim of this study was to compare the effect of individual CLA isomers against lipogenic and high 'Western' fat background diets. Golden Syrian hamsters were fed a high-carbohydrate rodent chow or chow supplemented with 17·25 % fat formulated to represent the type and amount of fatty acids found in a typical 'Western' diet (including 0·2 % cholesterol). Diets were further supplemented with 0·25 % (w/w) rapeseed oil, cis9, trans11 (c9,t11)-CLA or trans10, cis12 (t10,c12)-CLA. Neither isomer had a significant impact on plasma lipid or lipoprotein concentrations. The t10,c12-CLA isomer significantly reduced perirenal adipose tissue depot mass. While adipose tissue acetyl CoA carboxylase and fatty acid synthase mRNA concentrations (as measured by quantitative PCR) were unaffected by CLA, lipoprotein lipase mRNA was specifically reduced by t10,c12-CLA, on both background diets (P < 0·001). This was associated with a specific reduction of sterol regulatory element binding protein 1c expression in perirenal adipose tissue (P = 0·018). The isomers appear to have divergent effects on liver TAG content with c9,t11-CLA producing lower concentrations than t10,c12-CLA. We conclude that t10,c12-CLA modestly reduces adipose tissue deposition in the Golden Syrian hamster independently of background diet and this may possibly result from reduced uptake of lipoprotein fatty acids, as a consequence of reduced lipoprotein lipase gene expression.
OBJECTIVE—To generate Abcg1 Apoe mice to understand the mechanism and cell types involved in changes in atherosclerosis after loss of ABCG1.
METHODS AND RESULTS—ABCG1 is highly expressed in ...macrophages and endothelial cells, 2 cell types that play important roles in the development of atherosclerosis. Abcg1 Apoe and Apoe mice and recipient Apoe mice that had undergone transplantation with bone marrow from Apoe or Abcg1 Apoe mice were fed a Western diet for 12 or 16 weeks before quantification of atherosclerotic lesions. These studies demonstrated that loss of ABCG1 from all tissues, or from only hematopoietic cells, was associated with significantly smaller lesions that contained increased numbers of TUNEL- and cleaved caspase 3–positive apoptotic Abcg1 macrophages. We also identified specific oxysterols that accumulate in the brains and macrophages of the Abcg1 Apoe mice. These oxysterols promoted apoptosis and altered the expression of proapoptotic genes when added to macrophages in vitro.
CONCLUSION—Loss of ABCG1 from all tissues or from only hematopoietic cells results in smaller atherosclerotic lesions populated with increased apoptotic macrophages, by processes independent of ApoE. Specific oxysterols identified in tissues of Abcg1 Apoe mice may be critical because they induce macrophage apoptosis and the expression of proapoptotic genes.
RATIONALE:Mutations of the orphan transporter ABCC6 (ATP-binding cassette, subfamily C, member 6) cause the connective tissue disorder pseudoxanthoma elasticum. ABCC6 was thought to be located on the ...plasma membrane of liver and kidney cells.
OBJECTIVE:Mouse systems genetics and bioinformatics suggested that ABCC6 deficiency affects mitochondrial gene expression. We therefore tested whether ABCC6 associates with mitochondria.
METHODS AND RESULTS:We found ABCC6 in crude mitochondrial fractions and subsequently pinpointed its localization to the purified mitochondria-associated membrane fraction. Cell-surface biotinylation in hepatocytes confirmed that ABCC6 is intracellular. Abcc6-knockout mice demonstrated mitochondrial abnormalities and decreased respiration reserve capacity.
CONCLUSIONS:Our finding that ABCC6 localizes to the mitochondria-associated membrane has implications for its mechanism of action in normal and diseased states.
Poor quality of nutrition during fetal development is associated with adverse health outcomes in adult life. Epidemiological studies suggest that markers of fetal undernutrition are predictive of ...risk of the metabolic syndrome and CHD. Here we show that feeding a low-protein diet during pregnancy programmed the development of atherosclerosis in ApoE*3-Leiden mice. ApoE*3-Leiden mice carry a mutation of human ApoE*3 rendering them prone to atherosclerosis when fed a diet rich in cholesterol. It was noted that fetal exposure to protein restriction led to a greater degree of dyslipidaemia in mice when fed an atherogenic diet, with low-protein-exposed ApoE*3 mice having elevated total plasma cholesterol (34 % higher; P < 0·001) and TAG (39 % higher; P < 0·001) relative to offspring exposed to a control diet in utero. The low-protein group developed more severe atherosclerotic lesions within the aortic arch (2·61-fold greater lesion area; P < 0·001). Analysis of a targeted gene array suggested a potential role for members of the LDL receptor superfamily, along with similar programmed suppression of the mRNA expression of hepatic sterol regulatory element-binding protein-1c. This indicates that disordered lipid metabolism may play a role in the fetal programming of atherosclerosis in this model. Whereas earlier studies have shown early programming of cardiovascular risk factors, these results demonstrate for the first time that the interaction of prenatal undernutrition with a postnatal atherogenic diet increases the extent of atherosclerotic disease.
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