Downregulation of the selectin ligand-producing fucosyltransferases Fuc-TIV and Fuc-TVII during foam cell formation in monocyte-derived macrophages

Glycosyltransferases, glycosylation and atherosclerosis

Cardiovascular diseases arising from atherosclerosis are currently the leading cause of mortality worldwide. Leukocyte recruitment is a key step for the successful initiation of atherosclerosis and occurs predominantly in the inflamed endothelium. Leukocyte recruitment is mediated by a group of adhesive molecules and chemokine receptors, which are often glycosylated protein. Recent studies demonstrated that post-translational glycosylation by glycosyltransferases is necessary for adhesive molecules and chemokine receptors activities. Several glycosyltransferases, such as α2,3-sialyltransferases IV, α1,3-fucosyltransferases IV and VII, core 2 β1,6-N-acetylglucosaminyltransferase-I, are considered to participate in the synthesis of glycosylation for adhesive molecules and chemokine receptors, and the initiation of atherosclerotic lesions. In this review, we will discuss new data concerning the roles of different glycosyltransferases in atherogenesis. The knowledge of glycosyltransferases in atherogenesis offers the opportunity to develop novel therapeutic strategies.

Expression of leukocyte adhesion-related glycosyltransferase genes in acute coronary syndrome patients

INTRODUCTION

Acute coronary syndrome (ACS) is caused by destabilization and rupture of atherosclerotic plaque in the coronary artery via mechanisms affecting leukocyte signaling, rolling, adhesion, extravasation and inflammation-promoting factors. The majority of cellular communication takes place on the membrane surface that is covered with glycoproteins and glycolipids synthesized by glycosyltransferases. The aim of this study was to determine the mRNA expression of leukocyte adhesion-related glycosyltransferases in patients during the onset and the chronic phase of ACS and to compare the expression with matching subjects without coronary disease.

SUBJECTS AND METHODS

The study included 26 ACS patients and 26 ACS-free matched-pair controls. Blood samples were collected at the time of hospital admittance and 8 days later. Expression analysis of six fucosyltransferases and six sialyltransferases was performed by a real-time polymerase chain reaction.

RESULTS

At the time of admittance ACS subjects had lower expression levels of FUT4, ST6GalNac4, ST6Gal1 and GM3 synthase (p < 0.05) than the control subjects, and moreover, after 8 days down-regulation of FUT7 and ST6GalNac3 was also observed (p < 0.05). When compared to the initial gene expression, after treatment and stabilization of ACS subjects, FUT7, ST6GalNac2 and ST6GalNac3 were down-regulated, whereas ST6GalNac1 was up-regulated. Expression levels of FUT7, ST6GalNac1, ST6GalNac2 and ST6GalNac3 were predicted by several drugs and medical history.

CONCLUSION

Expression of glycosyltransferase genes differs in ACS and control subjects. During the course of the ACS study we established further changes in gene expression levels. Medical history was predictive of gene expression levels while drugs were shown to modulate expression levels.

Redox regulation of protein damage in plasma

The presence and concentrations of modified proteins circulating in plasma depend on rates of protein synthesis, modification and clearance. In early studies, the proteins most frequently analysed for damage were those which were more abundant in plasma (e.g. albumin and immunoglobulins) which exist at up to 10 orders of magnitude higher concentrations than other plasma proteins e.g. cytokines. However, advances in analytical techniques using mass spectrometry and immuno-affinity purification methods, have facilitated analysis of less abundant, modified proteins and the nature of modifications at specific sites is now being characterised. The damaging reactive species that cause protein modifications in plasma principally arise from reactive oxygen species (ROS) produced by NADPH oxidases (NOX), nitric oxide synthases (NOS) and oxygenase activities; reactive nitrogen species (RNS) from myeloperoxidase (MPO) and NOS activities; and hypochlorous acid from MPO. Secondary damage to proteins may be caused by oxidized lipids and glucose autooxidation.

In this review, we focus on redox regulatory control of those enzymes and processes which control protein maturation during synthesis, produce reactive species, repair and remove damaged plasma proteins. We have highlighted the potential for alterations in the extracellular redox compartment to regulate intracellular redox state and, conversely, for intracellular oxidative stress to alter the cellular secretome and composition of extracellular vesicles. Through secreted, redox-active regulatory molecules, changes in redox state may be transmitted to distant sites.

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Loss of redox homeostasis may affect the secretome content and protein concentration, transmitting redox signals to distant cells through extracellular vesicles.

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Damaged glycoforms may arise from oxidants or aberrant biosynthetic regulation.

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Reactive species generation by NOX and NOS is controlled through redox regulation.

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Cell surface and plasma thiol-oxidised proteins can be reduced and their activity modulated by thioredoxin, protein disulphide isomerase and reductases.

Pharmacological regulation of cholesterol efflux in human monocyte-derived macrophages in the absence of exogenous cholesterol acceptors

Cholesterol efflux from human monocyte-derived macrophages in the absence of exogenous acceptors has been described, but is unclear in mechanism. We investigated this process in relation to the expression of relevant genes, intracellular cholesterol storage and apoE secretion using drugs affecting different aspects of cholesterol metabolism. Both natural (22R-hydroxycholesterol/9-cis-retinoic acid) and synthetic (T0901317 and RO264456) LXR/RXR ligands increased ABCA1 and ABCG1 mRNAs in native macrophages and in cells loaded with acetylated LDL (acLDL). The ACAT inhibitor avasimibe increased only ABCG1 mRNA, whereas no treatment affected apoE mRNA. Avasimibe, progesterone, and natural but not synthetic LXR/RXR ligands prevented cholesterol esterification after acLDL-loading. Cholesterol efflux into acceptor-free medium was increased only by synthetic LXR/RXR ligands and avasimibe in acLDL-loaded cells. ApoE secretion was reduced by drugs affecting cholesterol trafficking but enhanced by LXR/RXR ligands. Incubation with an anti-apoE antibody virtually removed immunodetectable apoE from the medium, significantly increasing cholesterol storage and decreasing efflux. These findings indicate that in human macrophages spontaneous cholesterol efflux: (i) is not necessarily promoted by increasing intracellular free cholesterol, (ii) is increased by compounds that activate ABCA1 and, to a greater extent, ABCG1 and (iii) is only partially correlated with secretion of endogenous apoE, which acted as a cholesterol acceptor.

Cholesterol absorption inhibitor Ezetimibe blocks uptake of oxidized LDL in human macrophages

Ezetimibe belongs to a group of selective and very effective 2-azetidione cholesterol absorption inhibitors which act on the level of cholesterol entry into enterocytes. Recent data indicated that the drug prevents the formation of a heterocomplex consisting of annexin-2 and caveolin-l and leads to specific inhibition of an NPCILI-dependent cholesterol uptake pathway required for uptake of micellar cholesterol into enterocytes. Earlier studies have shown that caveolin-l and annexin-2 are also expressed in human macro-phages and we show in this study that human macrophages express NPC1L1. Moreover in human macrophages, Ezetimibe(SCH58235) and its analogue, SCH354909, are bound to specific cell surface receptors followed by endocytosis via the classical endocytic pathway. SCH58235 had no effect on uptake and/or processing of acetylated LDL (Ac-LDL). In contrast, the compound inhibited uptake of oxidized LDL (Ox-LDL) by -50% in a dose-dependent manner. SCH58235 blocked the lipid-induced induction of LXR/RXR target genes ABCAI, ABCGI, and apolipoprotein E distinctively more effectively in macrophages loaded with Ox-LDL than in those loaded with Ac-LDL. Based on these findings, we presume that the caveolin-l-, annexin-2-, and NPClLI-dependent cholesterol uptake system that is operating in enterocytes may also contribute to class B scavenger receptor-dependent uptake of Ox-LDL in human monocyte-derived macrophages.

Expression of the ATP-binding cassette transporter gene ABCG1 (ABC8) in Tangier disease

Several members of the ATP-binding cassette (ABC) transporter family are involved in cholesterol efflux from cells. A defect in one member, ABCA1, results in Tangier disease, a condition characterized by cholesterol accumulation in macrophages and virtual absence of mature circulating high-density lipoproteins. Expression of a second member, ABCG1, is increased by cholesterol-loading in human macrophages. We now show that ABCG1, which we identified by differential display RT-PCR in foamy macrophages, is overexpressed in macrophages from patients with Tangier disease compared to control macrophages. On examination by confocal laser scanning microscopy, ABCG1 was present in perinuclear structures within the cell. In addition, a combination of in situ hybridization and indirect immunofluorescence microscopy revealed that ABCG1 is expressed in foamy macrophages within the atherosclerotic plaque. These data indicate that not only ABCA1 but also ABCG1 may play a role in the cholesterol metabolism of macrophages in vitro and in the atherosclerotic plaque.

Genomic sequence and structure of the human ABCG1 (ABC8) gene

The human ATP-binding cassette half transporter G1 (hABCG1) may play a role in cholesterol transport in macrophages. Using RACE assays we determined the structure of this gene. The hABCG1 gene spans more than 97 kb comprising 20 exons, 20 kb and 5 exons more than hitherto described. Four of the novel exons are upstream and one is downstream of previous exon 1, and they are predicted to encode at least five novel transcripts. We also detected two separate promoters, upstream of exons 1 and 5, respectively. The region 650 bp upstream of exon 1 was predicted to contain putative binding sites for SP1 and nuclear factor kappaB (NF-kappaB), but no sterol response elements (SREs) or retinoid X receptor (RXR) binding sites. The region 650 bp upstream of exon 5 contained 19 possible SP1 binding sites, one possible SRE, two possible NF-kappaB, and two putative RXR binding sites. Nevertheless, both promoters responded in macrophages to stimulation by hydroxycholesterol and retinoic acid.

Human alpha (1,3)-fucosyltransferase IV (FUTIV) gene expression is regulated by elk-1 in the U937 cell line

The alpha1,3-fucosyltransferase IV (FucTIV) encoded by its gene (FUTIV) is responsible for synthesis of Le(x) (Galbeta4[Fucalpha3]GlcNAcbeta3Galbeta1,R), which causes compaction in the morula stage of the preimplantation mouse embryo, as well as alpha1,3-fucosylation at multiple internal GlcNAc of unbranched poly-N-acetyllactosamine, termed "myeloglycan," the physiological epitope of E-selectin. Since myeloglycan-type structure is also expressed in various types of human cancer and may mediate E-selectin-dependent metastasis, expression of FUTIV is oncodevelopmentally regulated. The mechanisms controlling FUTIV expression remain to be clarified. In this report, we further characterize FUTIV gene structure and define a non-TATA box-dependent transcriptional start region just upstream from the translational start. FUTIV promoter/reporter fusion constructs defined a "full-length" promoter and highly active fragments in the macrophage-derived U937 and myeloid HL60 cell lines. One highly active fragment contains a consensus binding site for the Ets-1 transcription factor (Withers, D. A., and Hakomori, S. (1997) Glycoconj. J. 14, 764). Gel shift analysis shows specific binding to this site in nuclear extracts from U937 cells. Mutation of the Ets consensus site significantly reduces FUTIV promoter activity in both cell lines. Gel supershift and dominant negative cotransfection experiments identified the Ets family member Elk-1 as one component binding and regulating the FUTIV promoter in U937 cells. The significance of FUTIV regulation by Elk-1 is discussed.

Fucose in N-glycans: from plant to man

Fucosylated oligosaccharides occur throughout nature and many of them play a variety of roles in biology, especially in a number of recognition processes. As reviewed here, much of the recent emphasis in the study of the oligosaccharides in mammals has been on their potential medical importance, particularly in inflammation and cancer. Indeed, changes in fucosylation patterns due to different levels of expression of various fucosyltransferases can be used for diagnoses of some diseases and monitoring the success of therapies. In contrast, there are generally at present only limited data on fucosylation in non-mammalian organisms. Here, the state of current knowledge on the fucosylation abilities of plants, insects, snails, lower eukaryotes and prokaryotes will be summarised.

A cloned CD15s-negative variant of HL60 cells is deficient in expression of FUT7 and does not adhere to cytokine-stimulated endothelial cells

The initial steps of leukocyte adhesion depend on selectin/ligand interactions. Surface ligands on leukocytes are often modified by addition of the sialyl Lewis x (CD15s) determinant. Biosynthesis of CD15s is dependent upon alpha(2,3)sialyltransferases and alpha(1,3)fucosyltransferases. We report the isolation of an HL60 cell line variant, HL60A2, that no longer expresses CD15s. HL60A2 cells do not adhere to cytokine-stimulated endothelial cells. Enzymatic assays reveal that this cell line has normal alpha(2,3)sialyltransferase activity but is deficient in the alpha(1,3)fucosyltransferase responsible for biosynthesis of CD15s (FUT7). The fucosyltransferase that constructs the non-sialylated antigen, Lewis x (CD15), is expressed at high levels (FUT4). Transcript analyses show that FUT7 and FUT4 are inversely expressed in HL60 and variant cell lines. HL60A2 cells provide a tool to study the regulation of selectin ligands and corresponding human fucosyltransferase genes.

Leukocyte-endothelium adhesion molecules in atherosclerosis

The genetic manipulation of mice has provided an invaluable tool for studying the molecular mechanism(s) involved in atherosclerotic lesion development and maturation. The use of these new animal models has demonstrated that leukocyte-endothelium adhesion receptors play a significant part in promoting monocyte recruitment and consequently lesion growth. The next phase of investigation should test whether the inhibition of these adhesion receptors can reproduce the powerful anti-atherosclerotic effects seen in the adhesion receptor-deficient mice.