Wolfgang Drobnik

The gene encoding ATP-binding cassette transporter 1 is mutated in Tangier disease.

Tangier disease (TD) is an autosomal recessive disorder of lipid metabolism. It is characterized by absence of plasma high-density lipoprotein (HDL) and deposition of cholesteryl esters in the reticulo-endothelial system with splenomegaly and enlargement of tonsils and lymph nodes. Although low HDL cholesterol is associated with an increased risk for coronary artery disease, this condition is not consistently found in TD pedigrees. Metabolic studies in TD patients have revealed a rapid catabolism of HDL and its precursors. In contrast to normal mononuclear phagocytes (MNP), MNP from TD individuals degrade internalized HDL in unusual lysosomes, indicating a defect in cellular lipid metabolism. HDL-mediated cholesterol efflux and intracellular lipid trafficking and turnover are abnormal in TD fibroblasts, which have a reduced in vitro growth rate. The TD locus has been mapped to chromosome 9q31 (ref. 9). Here we present evidence that TD is caused by mutations in ABC1, encoding a member of the ATP-binding cassette (ABC) transporter family, located on chromosome 9q22–31 (ref. 10). We have analysed five kindreds with TD and identified seven different mutations, including three that are expected to impair the function of the gene product. The identification of ABC1 as the TD locus has implications for the understanding of cellular HDL metabolism and reverse cholesterol transport, and its association with premature cardiovascular disease.

Transport of lipids from golgi to plasma membrane is defective in tangier disease patients and Abc1-deficient mice.

Mutations in the gene encoding ATP-binding cassette transporter 1 ( ABC1) have been reported in Tangier disease (TD), an autosomal recessive disorder that is characterized by almost complete absence of plasma high-density lipoprotein (HDL), deposition of cholesteryl esters in the reticulo-endothelial system (RES) and aberrant cellular lipid trafficking. We demonstrate here that mice with a targeted inactivation of Abc1 display morphologic abnormalities and perturbations in their lipoprotein metabolism concordant with TD. ABC1 is expressed on the plasma membrane and the Golgi complex, mediates apo-AI associated export of cholesterol and phospholipids from the cell, and is regulated by cholesterol flux. Structural and functional abnormalities in caveolar processing and the trans-Golgi secretory pathway of cells lacking functional ABC1 indicate that lipid export processes involving vesicular budding between the Golgi and the plasma membrane are severely disturbed.

Lipopolysaccharide and ceramide docking to CD14 provokes ligand‐specific receptor clustering in rafts

The glycosylphosphatidylinositol‐anchored receptor CD14 plays a major role in the inflammatory response of monocytes to lipopolysaccharide. Here, we describe that ceramide, a constituent of atherogenic lipoproteins, binds to CD14 and induces clustering of CD14 to co‐receptors in rafts. In resting cells, CD14 was associated with CD55, the Fcγ‐receptors CD32 and CD64 and the pentaspan CD47. Ceramide further recruited the complement receptor 3 (CD11b/CD18) and CD36 into proximity of CD14. Lipopolysaccharide, in addition, induced co‐clustering with Toll‐like receptor 4, Fcγ‐RIIIa (CD16a) and the tetraspanin CD81 while CD47 was dissociated. The different receptor complexes may be linked to ligand‐specific cellular responses initiated by CD14.

Apo AI/ABCA1-dependent and HDL3-mediated lipid efflux from compositionally distinct cholesterol-based microdomains.

We have investigated whether a raft heterogeneity exists in human monocyte‐derived macrophages and fibroblasts and whether these microdomains are modulated by lipid efflux. Triton X‐100 (Triton) or Lubrol WX (Lubrol) detergent‐resistant membranes from cholesterol‐loaded monocytes were associated with the following findings: (i) Lubrol‐DRM contained most of the cellular cholesterol and at least 75% of Triton‐detergent‐resistant membranes. (ii) ‘Lubrol rafts’, defined by their solubility in Triton but insolubility in Lubrol, were enriched in unsaturated phosphatidylcholine and showed a lower cholesterol to choline‐phospholipid ratio compared to Triton rafts. (iii) CD14 and CD55 were recovered in Triton‐ and Lubrol‐detergent‐resistant membranes, whereas CD11b was found exclusively in Triton DRM. ABCA1 implicated in apo AI‐mediated lipid efflux and CDC42 were partially localized in Lubrol‐ but not in Triton‐detergent‐resistant membranes. (iv) Apo AI preferentially depleted cholesterol and choline‐phospholipids from Lubrol rafts, whereas HDL3 additionally decreased the cholesterol content of Triton rafts. In fibroblasts, neither ABCA1 nor CDC42 was found in Lubrol rafts, and both apo AI and HDL3 reduced the lipid content in Lubrol‐ as well as in Triton‐detergent‐resistant membranes. In summary, we provide evidence for the existence of compositionally distinct membrane microdomains in human cells and their modulation by apo AI/ABCA1‐dependent and HDL3‐mediated lipid efflux.

Mice with targeted disruption of the fatty acid transport protein 4 (Fatp 4, Slc27a4) gene show features of lethal restrictive dermopathy

The fatty acid transport protein family is a group of evolutionarily conserved proteins that are involved in the cellular uptake and metabolism of long and very long chain fatty acids. However, little is known about their respective physiological roles. To analyze the functional significance of fatty acid transport protein 4 (Fatp4, Slc27a4), we generated mice with a targeted disruption of the Fatp4 gene. Fatp4-null mice displayed features of a neonatally lethal restrictive dermopathy. Their skin was characterized by hyperproliferative hyperkeratosis with a disturbed epidermal barrier, a flat dermal–epidermal junction, a reduced number of pilo-sebaceous structures, and a compact dermis. The rigid skin consistency resulted in an altered body shape with facial dysmorphia, generalized joint flexion contractures, and impaired movement including suckling and breathing deficiencies. Lipid analysis demonstrated a disturbed fatty acid composition of epidermal ceramides, in particular a decrease in the C26:0 and C26:0-OH fatty acid substitutes. These findings reveal a previously unknown, essential function of Fatp4 in the formation of the epidermal barrier.

The VP1 Unique Region of Parvovirus B19 and Its Constituent Phospholipase A2-Like Activity

ABSTRACT Parvovirus B19 is the causative agent of erythema infectiosum. In addition, parvovirus B19 infection may be associated with other disease manifestations, namely, thrombocytopenia or granulocytopenia, spontaneous abortion or hydrops fetalis in pregnant women, acute and chronic arthritis, and systemic lupus erythematosus. Based on sequence homology data, a phospholipase A2 motif has been identified in the VP1 unique region of parvovirus B19. (Y. Li et al., J. Gen. Virol. 82:2821-2825, 2001; Z. Zadori et al., Dev. Cell 1:291-302, 2001). We have established a new in vitro assay based on electrospray ionization tandem mass spectroscopy to show that phospholipase A2 activity is present in the VP1 unique region produced in Escherichia coli and in virus-like particles consisting of combinations of VP1 and VP2 proteins expressed by recombinant baculovirus. The enzyme activity of the VP1 unique region showed typical Ca2+ dependency and could be inhibited by manoalide and 4-bromophenacylbromide, which bind covalently to lysine and histidine residues, respectively, as part of the active center of the enzyme. By using subfragments, we demonstrated an association between the phospholipase A2-like activity and the carboxy-terminal domain of the VP1 unique region.

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 Zinc Finger Protein 202 (ZNF202) Is a Transcriptional Repressor of ATP Binding Cassette Transporter A1 (ABCA1) and ABCG1 Gene Expression and a Modulator of Cellular Lipid Efflux

The zinc finger gene 202 (ZNF202) located within a hypoalphalipoproteinemia susceptibility locus on chromosome 11q23 is a transcriptional repressor of various genes involved in lipid metabolism. To provide further evidence for a functional linkage between ZNF202 and hypoalphalipoproteinemia, we investigated the effect of ZNF202 expression on ATP binding cassette transporter A1 (ABCA1) and ABCG1. ABCA1 is a key regulator of the plasma high density lipoprotein pool size, whereas ABCG1 is another mediator of cellular cholesterol and phospholipid efflux in human macrophage. We demonstrate here that the full-length ZNF202m1 isoform binds to GnT repeats within the promotors of ABCA1 (−229/−210) and ABCG1 (−572/−552). ZNF202m1 expression in HepG2 cells dose-dependently repressed the promotor activities of ABCA1 and ABCG1. This transcriptional effect required the presence of the SCAN domain in ZNF202 and the functional integrity of a TATA box at position −24 of ABCA1, whereas the presence of GnT binding motifs was nonessential. The state of ZNF202 SCAN domain oligomerization affected the ability of the adjacent ZNF202 Krüppel-associated box domain to recruit the transcriptional corepressor KAP1. Overexpression of ZNF202m1 in RAW264.7 macrophages prevented the induction of ABCA1 gene expression by 20(S)OH-cholesterol and 9-cis-retinoic acid, further substantiating the interference of ZNF202 in critical elements of transcriptional activation. Finally, HDL and apoAImediated lipid efflux was significantly reduced in RAW264.7 cells stably expressing ZNF202m1. In conclusion, we have identified ABCA1 and ABCG1 as target genes for ZNF202-mediated repression and thus, provide evidence for a functional linkage between ZNF202 and hypoalphalipoproteinemia.

Pharmacogenomics and pharmacogenetics of cholesterol-lowering therapy.

Abstract Cholesterol-lowering therapy is the central approach in the primary and secondary prevention of cardiovascular disease, the leading cause of death in industrialized countries. 3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) are currently the most potent and widely used cholesterol-lowering drugs. Large-scale clinical trials unequivocally demonstrated the efficacy of statin treatment in reducing the risk of cardiovascular events. In general, HMG-CoA reductase inhibitors are well tolerated, although in a minority of patients severe adverse effects like myopathy or rhabdomyolysis may develop. The incidence of this potentially life-threatening side effects increases with co-adminstration of drugs that are metabolized via the same pharmacokinetic pathways or at high-dose statin therapy. The recent focus on the pleiotropic effects of statins that are more frequently observed at higher doses and the conclusion drawn from the large statin trials that low-density lipoprotein (LDL)-cholesterol is “the lower the better”, may need careful consideration in individuals at risk of adverse drug reactions. On the other hand, not all patients respond to statin therapy with a reduction in coronary heart disease (CHD) risk. It is therefore of interest to develop diagnostic test systems, which would allow to identify patients at increased risk of adverse drug reactions or patients with a lack of therapeutic effect. Beside exogenous factors, genetic variability determines the response of an individual to drug therapy and the analysis of genetic variants affecting pharmacokinetic or pharmacodynamic aspects of drug therapy is the subject of pharmacogenomics. This review summarizes current knowledge of the pharmacology and the pharmacogenomics of statin therapy.

ATP-Binding Cassette Transporter A1 (ABCA1) in Macrophages: A Dual Function in Inflammation and Lipid Metabolism?

Activated lipid-laden macrophages in the vascular wall are key modulators of the inflammatory processes underlying atherosclerosis. We demonstrate here that the ATP-binding cassette (ABC) transporter ABCA1 is induced during differentiation of human monocytes into macrophages. ABCA1 has been implicated in macrophage interleukin-1β secretion and apoptosis. Moreover, ABCA1 mRNA and protein levels are strongly upregulated by uptake of modified LDL and downregulated by HDL3-mediated lipid efflux in macrophages. Mutation analysis in patients with the classical Tangier disease (TD), a monogenetic disorder characterized by hypersplenism, macrophage accumulation and deposition of cholesteryl esters in the reticuloendothelial system, low plasma HDL and premature atherosclerosis, revealed deleterious mutations in their ABCA1 gene. The localization pattern of the mutations within the ABCA1 protein appears to determine the tropism for either the reticuloendothelial system, as seen in the classical TD phenotype, or the artery wall, as in the case of HDL deficiency in the absence of splenomegaly. In a comprehensive analysis of the expression and regulation of all currently known human ABC transporters, we identified additional cholesterol-responsive genes that are induced during monocyte differentiation into macrophages. Our results indicate a dual regulatory function for ABCA1 in macrophage lipid metabolism and inflammation.