Alfred Boettcher

Apolipoprotein A-1 interaction with plasma membrane lipid rafts controls cholesterol export from macrophages

Cholesterol efflux to apolipoprotein A‐1 (apoA‐1) from cholesterol‐loaded macrophages is an important anti‐atherosclerotic mechanism in reverse cholesterol transport. We recently provided kinetic evidence for two distinct pathways for cholesterol efflux to apoA‐1 [Gaus et al. (2001) Biochemistry 40, 9363]. Cholesterol efflux from two membrane pools occurs sequentially with different kinetics; a small pool rapidly effluxed over the first hour, followed by progressive release from a major, slow efflux pool over several hours. In the present study, we propose that the rapid and slow cholesterol efflux pools represent cholesterol derived from lipid raft and nonraft domains of the plasma membrane, respectively. We provide direct evidence that apoA‐1 binds to both lipid raft and nonraft domains of the macrophage plasma membrane. Conditions that selectively deplete plasma membrane lipid raft cholesterol, such as incorporation of 7‐ketocholesterol or rapid exposure to cyclodextrins, block apoA‐1 binding to these domains but also inhibit cholesterol efflux from the major, slow pool. We propose that cholesterol exported to apoA‐1 from this major slow efflux pool derives from nonraft regions of the plasma membrane but that the interaction of apoA‐1 with lipid rafts is necessary to stimulate this efflux.

The carboxyterminus of the ATP-binding cassette transporter A1 interacts with a β2-syntrophin/utrophin complex

Recent work identified ABCA1 as the major regulator of plasma HDL-cholesterol responsible for the removal of excess choline-phospholipids and cholesterol from peripheral cells and tissues. ABCA1 function may depend on the association with heteromeric proteins and to identify these candidates a human liver yeast two-hybrid library was screened with the carboxyterminal 144 amino acids of ABCA1. Beta2-syntrophin was found to interact with ABCA1 and the C-terminal five amino acids of ABCA1 proned to represent a perfect tail for binding to syntrophin PDZ domains. Immunoprecipitation further confirmed the association of ABCA1 and beta2-syntrophin and in addition utrophin, known to couple beta2-syntrophin and its PDZ ligands to the F-actin cytoskeleton, was identified as a constituent of this complex. ABCA1 in the plasmamembrane of human macrophages was found to be partially associated with Lubrol rafts and effluxed choline-phospholipids involve these microdomains. Beta2-syntrophin does not colocalize in these rafts indicating that beta2-syntrophin may participate in the retaining of ABCA1 in cytoplasmic vesicles and for the targeting of ABCA1 to plasmamembrane microdomains when ABCA1 is released from beta2-syntrophin.

Synthetic Farnesoid X Receptor Agonists Induce High-Density Lipoprotein-Mediated Transhepatic Cholesterol Efflux in Mice and Monkeys and Prevent Atherosclerosis in Cholesteryl Ester Transfer Protein Transgenic Low-Density Lipoprotein Receptor (−/−) Mice

Farnesoid X receptor (FXR), a bile acid-activated nuclear hormone receptor, plays an important role in the regulation of cholesterol and more specifically high-density lipoprotein (HDL) homeostasis. Activation of FXR is reported to lead to both pro- and anti-atherosclerotic effects. In the present study we analyzed the impact of different FXR agonists on cholesterol homeostasis, plasma lipoprotein profiles, and transhepatic cholesterol efflux in C57BL/6J mice and cynomolgus monkeys and atherosclerosis development in cholesteryl ester transfer protein transgenic (CETPtg) low-density lipoprotein receptor (LDLR) (−/−) mice. In C57BL/6J mice on a high-fat diet the synthetic FXR agonists isopropyl 3-(3,4-difluorobenzoyl)-1,1-dimethyl-1,2,3,6-tetrahydroazepino[4,5-b]indole-5-carboxylate (FXR-450) and 4-[2-[2-chloro-4-[[5-cyclopropyl-3-(2,6-dichlorophenyl)-4-isoxazolyl]methoxy]phenyl]cyclopropyl]benzoic acid (PX20606) demonstrated potent plasma cholesterol-lowering activity that affected all lipoprotein species, whereas 3-[2-[2-chloro-4-[[3-(2,6-dichlorophenyl)-5-(1-methylethyl)-4-isoxazolyl]methoxy]phenyl]ethenyl]benzoic acid (GW4064) and 6-ethyl chenodeoxycholic acid (6-ECDCA) showed only limited effects. In FXR wild-type mice, but not FXR(−/−) mice, the more efficacious FXR agonists increased fecal cholesterol excretion and reduced intestinal cholesterol (re)uptake. In CETPtg-LDLR(−/−) mice PX20606 potently lowered total cholesterol and, despite the observed HDL cholesterol (HDLc) reduction, caused a highly significant decrease in atherosclerotic plaque size. In normolipidemic cynomolgus monkeys PX20606 and 6-ECDCA both reduced total cholesterol, and PX20606 specifically lowered HDL2c but not HDL3c or apolipoprotein A1. That pharmacological FXR activation specifically affects this cholesterol-rich HDL2 subclass is a new and highly interesting finding and sheds new light on FXR-dependent HDLc lowering, which has been perceived as a major limitation for the clinical development of FXR agonists.

High density lipoprotein modulates platelet function.

Platelet activation by atherogenic lipoproteins can be antagonized by high density lipoprotein (HDL), probably via interaction with the ATP‐binding cassette transporter A1 (ABCA1).

Stored platelets alter glycerophospholipid and sphingolipid species, which are differentially transferred to newly released extracellular vesicles.

BACKGROUND: Stored platelet concentrates (PLCs) for transfusion develop a platelet storage lesion (PSL), resulting in decreased platelet (PLT) viability and function. The processes leading to PSL have not been described in detail and no data describe molecular changes occurring in all three components of stored PLCs: PLTs, PLC extracellular vesicles (PLC‐EVs), and plasma.

Lipidomic analysis of platelet senescence

BACKGROUND: A hallmark of platelet (PLT) storage lesion is the loss of PLT lipids. Due to technical limitations a detailed lipidomic analysis of plateletpheresis products during storage was so far not available.

A flow cytometric screening test for detergent-resistant surface antigens in monocytes.

Rafts resemble cholesterol‐ and glycosphingolipid‐enriched, liquid‐ordered plasma membrane microdomains, showing resistance to nonionic detergents, and are involved in various cellular processes. In the present study, we have tested surface antigens on resting and lipopolysaccharide (LPS)‐stimulated human peripheral blood monocytes for their detergent resistance (i.e. raft‐association), by flow cytometry. Constitutive (CD14, CD32, CD55), or LPS‐induced (CD81) raft‐association, and detergent solubility (i.e. exclusion of rafts) (CD71) of monocyte antigens in the presence of 0.01% Triton X‐100 are clearly demonstrated. Flow cytometric detergent insolubility is a powerful tool for rapid screening the raft‐association of monocyte antigens in a whole‐blood assay.

Lipid abnormalities in alpha/beta2-syntrophin null mice are independent from ABCA1

The syntrophins alpha (SNTA) and beta 2 (SNTB2) are molecular adaptor proteins shown to stabilize ABCA1, an essential regulator of HDL cholesterol. Furthermore, SNTB2 is involved in glucose stimulated insulin release. Hyperglycemia and dyslipidemia are characteristic features of the metabolic syndrome, a serious public health problem with rising prevalence. Therefore, it is important to understand the role of the syntrophins herein. Mice deficient for both syntrophins (SNTA/B2-/-) have normal insulin and glucose tolerance, hepatic ABCA1 protein and cholesterol. When challenged with a HFD, wild type and SNTA/B2-/- mice have similar weight gain, adiposity, serum and liver triglycerides. Hepatic ABCA1, serum insulin and insulin sensitivity are normal while glucose tolerance is impaired. Liver cholesterol is reduced, and expression of SREBP2 and HMG-CoA-R is increased in the knockout mice. Scavenger receptor-BI (SR-BI) protein is strongly diminished in the liver of SNTA/B2-/- mice while SR-BI binding protein NHERF1 is not changed and PDZK1 is even induced. Knock-down of SNTA, SNTB2 or both has no effect on hepatocyte SR-BI and PDZK1 proteins. Further, SR-BI levels are not reduced in brown adipose tissue of SNTA/B2-/- mice excluding that syntrophins directly stabilize SR-BI. SR-BI stability is regulated by MAPK and phosphorylated ERK2 is induced in the liver of the knock-out mice. Blockage of ERK activity upregulates hepatocyte SR-BI showing that increased MAPK activity contributes to low SR-BI. Sphingomyelin which is well described to regulate cholesterol metabolism is reduced in the liver and serum of the knock-out mice while the size of serum lipoproteins is not affected. Current data exclude a major function of these syntrophins in ABCA1 activity and insulin release but suggest a role in regulating glucose uptake, ERK and SR-BI levels, and sphingomyelin metabolism in obesity.

Transcriptomic profiling of platelet senescence and platelet extracellular vesicles

Platelets (PLTs) are derived from megakaryocytes during PLT shedding. Senescent or activated PLTs are expanded in vascular and neurological diseases and release PLT extracellular vesicles (PL‐EVs). A systematic analysis of regular messenger RNA (mRNA) and small RNA composition in PLTs and PL‐EVs during in vitro PLT senescence has not yet been published.

Keratinocyte ATP binding cassette transporter expression is regulated by ultraviolet light

Many ATP binding cassette (ABC) transporters are important regulators of lipid homeostasis and have been implicated in keratinocyte lipid transport. Ultraviolet (UV) light exposure is a known epidermal stressor, which amongst other effects causes lipid alterations and defective lamellar body biogenesis. To elucidate the background of these lipid changes we studied the effect of UVB light on ABC transporter expression. The effect of UVB treatment on the levels of 47 known human ABC transporter mRNAs was analyzed in normal human epidermal keratinocytes. Immunoblots and promoter assays were carried out for ABCA1 and ABCG1. The mRNA levels of cholesterol transport regulators ABCA1 and ABCG1 were markedly downregulated by UVB, parallel to the lamellar ichthyosis related glucosylceramide transporter ABCA12 and the suspected sphingosine-1-phosphate and cholesterol sulfate transporter ABCC1. The long but not the short alternative splice variant of the ABCF2 was found to be markedly upregulated rapidly after UVB irradiation. Immunoblot confirmed ABCA1 and ABCG1 protein downregulation, and luciferase assays showed suppression of their promoters by UVB. These proteins mostly transport lipids, which account for the integrity of the epidermal barrier; therefore our findings on the UVB regulation of ABC transporters may explain the appearance of barrier dysfunction after UVB exposure.