NKp44, a triggering receptor involved in tumor cell lysis by activated human natural killer cells, is a novel member of the immunoglobulin superfamily. (9/7876)

Surface receptors involved in natural killer (NK) cell triggering during the process of tumor cell lysis have recently been identified. Of these receptors, NKp44 is selectively expressed by IL-2- activated NK cells and may contribute to the increased efficiency of activated NK cells to mediate tumor cell lysis. Here we describe the molecular cloning of NKp44. Analysis of the cloned cDNA indicated that NKp44 is a novel transmembrane glycoprotein belonging to the Immunoglobulin superfamily characterized by a single extracellular V-type domain. The charged amino acid lysine in the transmembrane region may be involved in the association of NKp44 with the signal transducing molecule killer activating receptor-associated polypeptide (KARAP)/DAP12. These molecules were found to be crucial for the surface expression of NKp44. In agreement with data of NKp44 surface expression, the NKp44 transcripts were strictly confined to activated NK cells and to a minor subset of TCR-gamma/delta+ T lymphocytes. Unlike genes coding for other receptors involved in NK cell triggering or inhibition, the NKp44 gene is on human chromosome 6.  (+info)

The leukocyte Ig-like receptor (LIR)-1 for the cytomegalovirus UL18 protein displays a broad specificity for different HLA class I alleles: analysis of LIR-1 + NK cell clones. (10/7876)

Leukocyte Ig-like receptor (LIR)-1 is a member of the Ig superfamily which has been shown to bind the human cytomegalovirus MHC class I homologue UL-18 protein. In this study, we have analyzed the expression and function of LIR-1 in human NK cells. We show that LIR-1 is expressed by a subset of NK cells variable in size among different donors. When compared to the known HLA class I-specific NK receptors, the expression of LIR-1 was found to be partially overlapped with that of CD94-NKG2A or with that of killer inhibitory receptors (KIR) belonging to the Ig superfamily. The use of the soluble form of UL-18 molecule revealed, in double fluorescence analysis, a selective binding to LIR-1 + cells while no correlation was observed between expression of either KIR or CD94-NKG2A molecules and ability to bind UL18. We further determined whether LIR-1 could also function as receptor for HLA class I molecules. To this end, we assessed the capability of LIR-1 + NK cell clones of lysing HLA class I- target cells transfected with different class I alleles, including HLA-A, -B, -C and -G alleles. Data revealed that LIR-1 functions as a broad HLA class I-specific inhibitory receptor recognizing different alleles coded for by different HLA loci.  (+info)

Paradoxical effect on atherosclerosis of hormone-sensitive lipase overexpression in macrophages. (11/7876)

Foam cells formed from receptor-mediated uptake of lipoprotein cholesterol by macrophages in the arterial intima are critical in the initiation, progression, and stability of atherosclerotic lesions. Macrophages accumulate cholesterol when conditions favor esterification by acyl-CoA:cholesterol acyltransferase (ACAT) over cholesteryl-ester hydrolysis by a neutral cholesteryl-ester hydrolase, such as hormone-sensitive lipase (HSL), and subsequent cholesterol efflux mediated by extracellular acceptors. We recently made stable transfectants of a murine macrophage cell line, RAW 264.7, that overexpressed a rat HSL cDNA and had a 5-fold higher rate of cholesteryl-ester hydrolysis than control cells. The current study examined the effect of macrophage-specific HSL overexpression on susceptibility to diet-induced atherosclerosis in mice. A transgenic line overexpressing the rat HSL cDNA regulated with a macrophage-specific scavenger receptor promoter-enhancer was established by breeding with C57BL/6J mice. Transgenic peritoneal macrophages exhibited macrophage-specific 7-fold overexpression of HSL cholesterol esterase activity. Total plasma cholesterol levels in transgenic mice fed a chow diet were modestly elevated 16% compared to control littermates. After 14 weeks on a high-fat, high-cholesterol diet, total cholesterol increased 3-fold, with no difference between transgenics and controls. However, HSL overexpression resulted in thicker aortic fatty lesions that were 2.5-times larger in transgenic mice. HSL expression in the aortic lesions was shown by immunocytochemistry. Atherosclerosis was more advanced in transgenic mice exhibiting raised lesions involving the aortic wall, along with lipid accumulation in coronary arteries occurring only in transgenics. Thus, increasing cholesteryl-ester hydrolysis, without concomitantly decreasing ACAT activity or increasing cholesterol efflux, is not sufficient to protect against atherosclerosis. hormone-sensitive lipase overexpression in macrophages.  (+info)

Scavenger receptor BI (SR-BI) mediates free cholesterol flux independently of HDL tethering to the cell surface. (12/7876)

In addition to its effect on high density lipoprotein (HDL) cholesteryl ester (CE) uptake, scavenger receptor BI (SR-BI) was recently reported to stimulate free cholesterol (FC) flux from Chinese hamster ovary (CHO) cells stably expressing mouse SR-BI, a novel function of SR-BI that may play a role in cholesterol removal from the vessel wall where the receptor can be found. It is possible that SR-BI stimulates flux simply by tethering acceptor HDL particles in close apposition to the cell surface thereby facilitating the movement of cholesterol between the plasma membrane and HDL. To test this, we used transiently transfected cells and compared the closely related class B scavenger receptors mouse SR-BI and rat CD36 for their ability to stimulate cholesterol efflux as both receptors bind HDL with high affinity. The results showed that, although acceptor binding to SR-BI may contribute to efflux to a modest extent, the major stimulation of FC efflux occurs independently of acceptor binding to cell surface receptors. Instead our data indicate that SR-BI mediates alterations to membrane FC domains which provoke enhanced bidirectional FC flux between cells and extracellular acceptors.  (+info)

Lower plasma levels and accelerated clearance of high density lipoprotein (HDL) and non-HDL cholesterol in scavenger receptor class B type I transgenic mice. (13/7876)

Recent studies have indicated that the scavenger receptor class B type I (SR-BI) may play an important role in the uptake of high density lipoprotein (HDL) cholesteryl ester in liver and steroidogenic tissues. To investigate the in vivo effects of liver-specific SR-BI overexpression on lipid metabolism, we created several lines of SR-BI transgenic mice with an SR-BI genomic construct where the SR-BI promoter region had been replaced by the apolipoprotein (apo)A-I promoter. The effect of constitutively increased SR-BI expression on plasma HDL and non-HDL lipoproteins and apolipoproteins was characterized. There was an inverse correlation between SR-BI expression and apoA-I and HDL cholesterol levels in transgenic mice fed either mouse chow or a diet high in fat and cholesterol. An unexpected finding in the SR-BI transgenic mice was the dramatic impact of the SR-BI transgene on non-HDL cholesterol and apoB whose levels were also inversely correlated with SR-BI expression. Consistent with the decrease in plasma HDL and non-HDL cholesterol was an accelerated clearance of HDL, non-HDL, and their major associated apolipoproteins in the transgenics compared with control animals. These in vivo studies of the effect of SR-BI overexpression on plasma lipoproteins support the previously proposed hypothesis that SR-BI accelerates the metabolism of HDL and also highlight the capacity of this receptor to participate in the metabolism of non-HDL lipoproteins.  (+info)

Apolipoprotein B stimulates formation of monocyte-macrophage surface-connected compartments and mediates uptake of low density lipoprotein-derived liposomes into these compartments. (14/7876)

Much of the cholesterol that accumulates in atherosclerotic plaques is found within monocyte-macrophages transforming these cells into "foam cells." Native low density lipoprotein (LDL) does not cause foam cell formation. Treatment of LDL with cholesterol esterase converts LDL into cholesterol-rich liposomes having >90% cholesterol in unesterified form. Similar cholesterol-rich liposomes are found in early developing atherosclerotic plaques surrounding foam cells. We now show that cholesterol-rich liposomes produced from cholesterol esterase-treated LDL can cause human monocyte-macrophage foam cell formation inducing a 3-5-fold increase in macrophage cholesterol content of which >60% is esterified. Although cytochalasin D inhibited LDL liposome-induced macrophage cholesteryl ester accumulation, LDL liposomes did not enter macrophages by phagocytosis. Rather, the LDL liposomes induced and entered surface-connected compartments within the macrophages, a unique endocytic pathway in these cells that we call patocytosis. LDL liposome apoB rather than LDL liposome lipid mediated LDL liposome uptake by macrophages. This was shown by the findings that: 1) protease treatment of the LDL liposomes prevented macrophage cholesterol accumulation; 2) liposomes prepared from LDL lipid extracts did not cause macrophage cholesterol accumulation; and 3) purified apoB induced and accumulated within macrophage surface-connected compartments. Although apoB mediated the macrophage uptake of LDL liposomes, this uptake did not occur through LDL, LDL receptor-related protein, or scavenger receptors. Also, LDL liposome uptake was not sensitive to treatment of macrophages with trypsin or heparinase. Cholesterol esterase-mediated transformation of LDL into cholesterol-rich liposomes is an LDL modification that: 1) stimulates uptake of LDL cholesterol by apoB-dependent endocytosis into surface-connected compartments, and 2) causes human monocyte-macrophage foam cell formation.  (+info)

Analysis of macrophage scavenger receptor (SR-A) expression in human aortic atherosclerotic lesions. (15/7876)

The class A scavenger receptors (SR-As) are trimeric, integral membrane glycoproteins that exhibit unusually broad ligand-binding properties. A number of studies have suggested that these receptors may play an important role in host defense and in many macrophage-associated pathological processes, including atherosclerosis and Alzheimer's disease. The study of the expression and function of these receptors in human disease has been hampered by the lack of suitable antibodies recognizing human SR-A. This has generated questions regarding the nature of receptors responsible for scavenger receptor activity detected in a variety of cell types, including monocytes, macrophages, smooth muscle cells, and endothelial cells. To address these questions, we have produced high-titer antisera recognizing human SR-A by using mice deficient for SR-A (SR-A -/-). We show that SR-A -/- mice produce a significantly higher-titer immune response than do wild-type (SR-A +/+) littermates, with antisera of the former having a broad species reactivity and recognizing SR-A from humans, mice, and rabbits. The antisera recognize both type I and II SR-A in a wide range of immunological techniques. Using these antisera we show that the expression of SR-A protein is induced during monocyte to macrophage differentiation and that SR-A mediates 80% of the uptake of acetylated low density lipoprotein by human monocyte-derived macrophages. We also establish that human SR-A is expressed by tissue macrophages in liver and lung and by macrophage-derived foam cells within aortic atherosclerotic lesions, with little detectable expression by smooth muscle cells or aortic endothelium.  (+info)

Specific interaction of oxidized low-density lipoprotein with macrophage-derived foam cells isolated from rabbit atherosclerotic lesions. (16/7876)

Interaction of oxidized LDL (OxLDL) with macrophage-derived foam cells is one of the key events in the development and progression of atherosclerosis. To study this interaction, macrophage-derived foam cells were isolated from rabbit atherosclerotic lesions and the expression of scavenger receptors for OxLDL was examined. Atherosclerosis was induced in rabbits by denudation of the large arteries, followed by a hypercholesteremic diet. Macrophage-derived foam cells, characterized by immunostaining with an RAM-11 antibody (a macrophage marker), contained a high content of intracellular lipid. Maximal binding of radiolabeled OxLDL to isolated macrophage-derived foam cells (1652+/-235 ng 125I-OxLDL/mg of cell protein) was 20-fold higher compared with Bmax values of monocytes. Levels of association of OxLDL to macrophage-derived foam cells isolated from atherosclerotic lesions 12 weeks after denudation were >3-fold higher compared with the levels expressed by macrophage-derived foam cells isolated after 6 weeks. Association of 125I-OxLDL could be completely blocked by OxLDL, and partially by acetylated LDL and polyinosinic acid, indicating the presence of a specific binding site for OxLDL on macrophage-derived foam cells. The induction of scavenger receptors for OxLDL on macrophage-derived foam cells during the development of atherosclerosis, as described in this study, may facilitate the lipid accumulation in macrophage-derived foam cells, as observed in advanced atherosclerotic lesions.  (+info)