Hypercholesterolemia and reduced HDL-C promote hematopoietic stem cell proliferation and monocytosis: studies in mice and FH children.

Previous studies have shown that mice with defects in cellular cholesterol efflux show hematopoietic stem cell (HSPC) and myeloid proliferation, contributing to atherogenesis. We hypothesized that the combination of hypercholesterolemia and defective cholesterol efflux would promote HSPC expansion and leukocytosis more prominently than either alone. We crossed Ldlr(-/-) with Apoa1(-/-) mice and found that compared to Ldlr(-/-) mice, Ldlr(-/-)/Apoa1(+/-) mice, with similar LDL-cholesterol levels but reduced HDL-cholesterol (HDL-C) levels, had expansion of HSPCs, monocytosis and neutrophilia.

Deficiency of ATP-binding cassette transporters A1 and G1 in macrophages increases inflammation and accelerates atherosclerosis in mice.

Rationale: Plasma high-density lipoprotein levels are inversely correlated with atherosclerosis. Although it is widely assumed that this is attributable to the ability of high-density lipoprotein to promote cholesterol efflux from macrophage foam cells, direct experimental support for this hypothesis is lacking.

Objective: To assess the role of macrophage cholesterol efflux pathways in atherogenesis.

ApoE regulates hematopoietic stem cell proliferation, monocytosis, and monocyte accumulation in atherosclerotic lesions in mice.

Leukocytosis is associated with increased cardiovascular disease risk in humans and develops in hypercholesterolemic atherosclerotic animal models. Leukocytosis is associated with the proliferation of hematopoietic stem and multipotential progenitor cells (HSPCs) in mice with deficiencies of the cholesterol efflux-promoting ABC transporters ABCA1 and ABCG1 in BM cells. Here, we have determined the role of endogenous apolipoprotein-mediated cholesterol efflux pathways in these processes.

Deletion of ABCA1 and ABCG1 impairs macrophage migration because of increased Rac1 signaling.

Rationale: Reduced plasma cholesterol and increased high-density lipoprotein (HDL) levels promote regression of atherosclerosis, in a process characterized by lipid unloading and emigration of macrophages from lesions. In contrast free cholesterol loading of macrophages leads to imbalanced Rac1/Rho activities and impaired chemotaxis.

Objective: To study the role of HDL and the ATP-binding cassette transporters ABCA1 and ABCG1 in modulating the chemotaxis of macrophages.

ATP-binding cassette transporters and HDL suppress hematopoietic stem cell proliferation.

Elevated leukocyte cell numbers (leukocytosis), and monocytes in particular, promote atherosclerosis; however, how they become increased is poorly understood. Mice deficient in the adenosine triphosphate-binding cassette (ABC) transporters ABCA1 and ABCG1, which promote cholesterol efflux from macrophages and suppress atherosclerosis in hypercholesterolemic mice, displayed leukocytosis, a transplantable myeloproliferative disorder, and a dramatic expansion of the stem and progenitor cell population containing Lin(-)Sca-1(+)Kit+ (LSK) in the bone marrow. Transplantation of Abca1(-/-) Abcg1(-/-) bone marrow into apolipoprotein A-1 transgenic mice with elevated levels of high-density lipoprotein (HDL) suppressed the LSK population, reduced leukocytosis, reversed the myeloproliferative disorder, and accelerated atherosclerosis.

Cholesterol efflux potential and antiinflammatory properties of high-density lipoprotein after treatment with niacin or anacetrapib.

Objective: To examine the effects of treatments with niacin or anacetrapib (an inhibitor of cholesteryl ester transfer protein) on the ability of high-density lipoprotein (HDL) to promote net cholesterol efflux and reduce toll-like receptor-mediated inflammation in macrophages.

Methods And Results: A total of 18 patients received niacin, 2 g/d, for 4 weeks; 20 patients received anacetrapib, 300 mg/d, for 8 weeks; and 2 groups (n=4 and n=5 patients) received placebo. HDL samples were isolated by polyethylene glycol precipitation or ultracentrifugation, tested for the ability to promote cholesterol efflux in cholesterol-loaded THP-I or mouse peritoneal macrophages, or used to pretreat macrophages, followed by lipopolysaccharide exposure.

ABCA1 and ABCG1 protect against oxidative stress-induced macrophage apoptosis during efferocytosis.

Rationale: Antiatherogenic effects of plasma high-density lipoprotein (HDL) include the ability to inhibit apoptosis of macrophage foam cells. The ATP-binding cassette transporters ABCA1 and ABCG1 have a major role in promoting cholesterol efflux from macrophages to apolipoprotein A-1 and HDL and are upregulated during the phagocytosis of apoptotic cells (efferocytosis).

Objective: The goal of this study was to determine the roles of ABCA1 and ABCG1 in preserving the viability of macrophages during efferocytosis.

Characterization of endocytic compartments after holo-high density lipoprotein particle uptake in HepG2 cells.

Holo-high density lipoprotein (HDL) particle uptake, besides selective lipid uptake, constitutes an alternative pathway to regulate cellular cholesterol homeostasis. In the current study, the cellular path of holo-HDL particles was investigated in human liver carcinoma cells (HepG2) using combined light and electron microscopical methods. The apolipoprotein moiety of HDL was visualized with different markers: horseradish peroxidase, colloidal gold and the fluorochrome Alexa(568), used in fluorescence microscopy and after photooxidation correlatively at the ultrastructural level.

ABCG1 and HDL protect against endothelial dysfunction in mice fed a high-cholesterol diet.

Plasma HDL levels are inversely related to the incidence of atherosclerotic disease. Some of the atheroprotective effects of HDL are likely mediated via preservation of EC function. Whether the beneficial effects of HDL on ECs depend on its involvement in cholesterol efflux via the ATP-binding cassette transporters ABCA1 and ABCG1, which promote efflux of cholesterol and oxysterols from macrophages, has not been investigated.

Increased inflammatory gene expression in ABC transporter-deficient macrophages: free cholesterol accumulation, increased signaling via toll-like receptors, and neutrophil infiltration of atherosclerotic lesions.

Background: Two macrophage ABC transporters, ABCA1 and ABCG1, have a major role in promoting cholesterol efflux from macrophages. Peritoneal macrophages deficient in ABCA1, ABCG1, or both show enhanced expression of inflammatory and chemokine genes. This study was undertaken to elucidate the mechanisms and consequences of enhanced inflammatory gene expression in ABC transporter-deficient macrophages.

HDL, ABC transporters, and cholesterol efflux: implications for the treatment of atherosclerosis.

High-density lipoprotein (HDL) has been identified as a potential target in the treatment of atherosclerotic vascular disease. The failure of torcetrapib, an inhibitor of cholesteryl ester transfer protein (CETP) that markedly increased HDL levels in a clinical trial, has called into doubt the efficacy of HDL elevation. Recent analysis suggests that failure may have been caused by off-target toxicity and that HDL is functional and promotes regression of atherosclerosis.

SR-BI inhibits ABCG1-stimulated net cholesterol efflux from cells to plasma HDL.

This study compares the roles of ABCG1 and scavenger receptor class B type I (SR-BI) singly or together in promoting net cellular cholesterol efflux to plasma HDL containing active LCAT. In transfected cells, SR-BI promoted free cholesterol efflux to HDL, but this was offset by an increased uptake of HDL cholesteryl ester (CE) into cells, resulting in no net efflux. Coexpression of SR-BI with ABCG1 inhibited the ABCG1-mediated net cholesterol efflux to HDL, apparently by promoting the reuptake of CE from medium.

Cholesterol efflux via HDL resecretion occurs when cholesterol transport out of the lysosome is impaired.

Recently, we showed that holo HDL particle uptake and resecretion occur in physiologically relevant cell lines and that HDL uptake is mediated by scavenger receptor class B type I (SR-BI). Furthermore, we established that HDL resecretion is accompanied by [(3)H]cholesterol efflux. This study shows that HDL uptake and resecretion occur even when LDL uptake and cholesterol trafficking are disturbed.

A Chinese hamster ovarian cell line imports cholesterol by high density lipoprotein degradation.

Plasma high density lipoprotein (HDL) is inversely associated with the development of atherosclerosis. HDL exerts its atheroprotective role through involvement in reverse cholesterol transport in which HDL is loaded with cholesterol at the periphery and transports its lipid load back to the liver for disposal. In this pathway, HDL is not completely dismantled but only transfers its lipids to the cell.

SR-BI-mediated high density lipoprotein (HDL) endocytosis leads to HDL resecretion facilitating cholesterol efflux.

The high density lipoprotein (HDL) receptor, scavenger receptor class B, type I (SR-BI), mediates selective cholesteryl ester uptake from lipoproteins into liver and steroidogenic tissues but also cholesterol efflux from macrophages to HDL. Recently, we demonstrated the uptake of HDL particles in SR-BI overexpressing Chinese hamster ovarian cells (ldlA7-SRBI) using ultrasensitive microscopy. In this study we show that this uptake of entire HDL particles is followed by resecretion.

Visualization of the uptake of individual HDL particles in living cells via the scavenger receptor class B type I.

The scavenger receptor class B type I (SR-BI) plays an important role in mediating selective uptake of high-density lipoprotein (HDL)-derived cholesterol and cholesteryl ester in liver and steroidogenic tissues. The molecular mechanism by which this receptor mediates selective cholesteryl ester uptake remains still enigmatic. We applied ultrasensitive fluorescence microscopy to visualize the intracellular transport routes of HDL particles taken up via SR-BI in a Chinese hamster ovarian cell line.