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Scavenger receptors, which include various classes, play an important role in atherogenesis by mediating the unrestricted uptake of modified lipoproteins, resulting in the massive accumulation of cholesteryl esters. Because macrophage-derived foam cells are considered to be an important feature in early atherogenesis, we investigated the role of scavenger receptor class A (SR-A) overexpression, especially on macrophages in lipoprotein metabolism and atherosclerosis. Bone marrow from human SR-A (MSR1)-overexpressing mice was transplanted into irradiated low density lipoprotein receptor knockout [LDLR(-/-)] mice. The transplantation resulted in an increase in total serum cholesterol (approximately 15 to 25%), especially in the VLDL fraction, when compared with LDLR(-/-) mice that were transplanted with bone marrow of wild-type littermates. Quantification of atherosclerotic lesions in the mice that were fed a 'Western-type' diet for 3 months revealed that there were no differences in mean lesion area between LDLR(-/-) mice transplanted with MSR1 overexpressing and wild-type littermate bone marrow, despite increased scavenger receptor activity in vitro. The presence or absence of the LDLR in the transplanted bone marrow did not influence these results. In conclusion, introduction of MSR1-overexpressing bone marrow in LDLR(-/-) mice via bone marrow transplantation resulted in a slight increase in lipoprotein levels, but had no effect on the atherosclerotic lesion area, despite increased scavenger receptor activity in vitro. - Herijgers, N., M. P. J. de Winther, M. Van Eck, L. M. Havekes, M. H. Hofker, P. M. Hoogerbrugge, and T. J. C. Van Berkel. Effect of human scavenger receptor class A overexpression in bone marrow-derived cells on lipoprotein metabolism and atherosclerosis in low density lipoprotein receptor knockout mice. Chemicals/CAS: acetyl-LDL; Cell Adhesion Molecules; Lipoproteins; Lipoproteins, LDL; MSR1 protein, human; Msr1 protein, mouse; Receptors, Immunologic; Receptors, LDL; Receptors, Scavenger; Scavenger Receptors, Class A

Objective. Sulfasalazine (SSZ) is a commonly used drug in the treatment of inflammatory diseases such as rheumatoid arthritis and Crohn's disease. In both diseases, the proinflammatory cytokine tumor necrosis factor α (TNFα) plays a prominent role. In these studies, we investigated the mechanism by which SSZ inhibits TNFα expression in macrophages and macrophage-like cell lines. Methods. Monocyte-derived macrophages and several macrophage-like cell lines were exposed to SSZ in vitro, and the effect on TNFα expression was monitored by reverse transcriptase-polymerase chain reaction and Western blot analysis. In addition, the effects of SSZ in vivo were examined by intraperitoneally injecting mice with SSZ, after which peritoneal cells were harvested and examined using various staining methods. Results. Preincubation of macrophages with SSZ, but not with methotrexate, inhibited lipopolysaccharide (LPS)-induced TNFα expression. Inhibition of TNFα expression by SSZ coincided with the induction of apoptosis, as judged by the appearance of morphologic changes typical of apoptosis, such as nuclear condensation and fragmentation. Induction of apoptosis by SSZ was confirmed by TUNEL analysis and by the detection of cleaved U1-70K, a substrate of caspase 3. Intraperitoneal injections of SSZ in mice resulted in the induction of apoptosis of peritoneal cells within a few hours. SSZ-induced cleavage of the U1-70K protein was inhibited by Zn2+ and by specific inhibitors of caspases 3 and 8, but not caspases 1 and 9. Interestingly, the reduced expression of LPS-induced TNFα in the presence of SSZ was restored by inhibition of caspase 8. Conclusion. Inhibition of TNFα expression in macrophages by SSZ is due to the induction of apoptosis and involves the activation of caspase 8. Chemicals/CAS: Adenosine, 58-61-7; Anti-Inflammatory Agents, Non-Steroidal; Caspases, EC 3.4.22.-; Cysteine Proteinase Inhibitors; Methotrexate, 59-05-2; Sulfasalazine, 599-79-1; Tumor Necrosis Factor-alpha

In the arterial wall, scavenger receptor class A (SRA) is implicated in pathological lipid deposition. In contrast, in the liver, SRA is suggested to remove modified lipoproteins from the circulation, thereby protecting the body from their pathological action. The role of SRA on bone marrow-derived cells in lipid metabolism and atherogenesis was assessed in vivo by transplantation of bone marrow cells overexpressing human SRA (MSR1) to apoE-deficient mice. In vitro studies with peritoneal macrophages from the transplanted mice showed that macrophage scavenger receptor function, as measured by cell association and degradation studies with acetylated LDL, was ≃3-fold increased on overexpression of MSR1 in bone marrow-derived cells as compared with control mice. Despite the increased macrophage scavenger receptor function in vitro, no significant effect of MSR1 overexpression in bone marrow-derived cells on the in vivo atherosclerotic lesion development was found. In addition to arterial wall macrophages, liver sinusoidal Kupffer cells also overexpress MSR1 after bone marrow transplantation, which may scavenge atherogenic particles more efficiently from the blood compartment. Introduction of bone marrow cells overexpressing human MSR1 in apoE-deficient mice induced a significant reduction in serum cholesterol levels of ≃20% (P < 0.001, 2-way ANOVA) as the result of a decrease in VLDL cholesterol. It is suggested that the reduction in VLDL cholesterol levels is due to increased clearance of modified lipoproteins by the overexpressed MSR 1 in Kupffer cells of the liver, thereby protecting the arterial wall against the proatherogenic action of modified lipoproteins. Chemicals/CAS: acetyl-LDL; Apolipoproteins E; Cholesterol, VLDL; Lipoproteins, LDL; Membrane Proteins; oxidized low density lipoprotein; Receptors, Immunologic; Receptors, Lipoprotein; Receptors, Scavenger; Scarb1 protein, mouse; Scavenger Receptors, Class A; Scavenger Receptors, Class B; Triglycerides

The effect of monocyte/macrophage-derived wild-type mouse apolipoprotein E (apoE), human apoE3-Leiden, and human apoE2 on serum cholesterol levels and the development of atherosclerosis in apoE-deficient (apoe-/-) mice was investigated by using bone marrow transplantation (BMT). At 4 weeks after BMT, murine apoe+/+ bone marrow reduced serum cholesterol levels by 87% in apoe-/- mice, whereas macrophage-derived human apoE3-Leiden and human apoE2 induced a maximal, transient reduction of 35% and 48%, respectively. At 4 months after BMT, atherosclerosis was 23-fold (P<0.001) reduced in apoe+/+→apoe-/- mice, whereas no significant reduction in apoE3-Leiden.apoe- /-→apoe-/- and apoE2.apoe-/-→apoe-/- mice could be demonstrated. A highly significant decrease in serum cholesterol levels (78% reduction) and atherosclerosis (21-fold, P<0.001) was found in apoE3-Leiden.apoe-/- animals expressing high levels of apoE in multiple tissues, whereas apoE2 was ineffective even at high concentrations. Furthermore, in contrast to apoE- deficient macrophages, cholesterol efflux from apoE2 or apoE3-Leiden macrophages was not impaired. In conclusion, apoE3-Leiden as well as apoE2 are less effective in reducing cholesterol levels and atherosclerosis in apoe-/- animals, compared with apoe+/+, with apoE2<apoE3-Leiden<apoe+/+, irrespective of the observed adequate efflux of cholesterol from macrophages expressing apoE2 and apoE3-Leiden, indicating that normalization of cholesterol efflux by macrophages is not accompanied by measurable effects on lesion growth. Chemicals/CAS: Apolipoprotein E2; apolipoprotein E3 (Leidein); Apolipoprotein E3; Apolipoproteins E; Cholesterol, 57-88-5

BACKGROUND - Lipopolysaccharide (LPS), which is released from Gram-negative bacteria on multiplication or lysis, aggravates atherosclerosis in humans and rodents by inducing inflammation via toll-like receptors. Because apolipoprotein C-I (apoCI) enhances the LPS-induced inflammatory response in macrophages in vitro and in mice, we investigated the effect of endogenous apoCI expression on LPS-induced atherosclerosis in mice. METHODS AND RESULTS - Twelve-week-old apoeapoc1 and apoeapoc1 mice received weekly intraperitoneal injections of LPS (50 μg) or vehicle for a period of 10 weeks, and atherosclerosis development was assessed in the aortic root. LPS administration did not affect atherosclerotic lesion area in apoeapoc1 mice but increased it in apoeapoc1 mice. In fact, apoCI expression increased the LPS-induced atherosclerotic lesion area by 60% (P<0.05), concomitant with an increase in LPS-induced plasma levels of fibrinogen and E-selectin. This indicated that apoCI increased the LPS-induced inflammatory state, both systemically (ie, fibrinogen) and at the level of the vessel wall (ie, E-selectin). In addition, both macrophage-derived apoCI and HDL-associated apoCI increased the LPS-induced tumor necrosis factor-α response by macrophages in vitro. CONCLUSIONS - We conclude that apoCI is crucially involved in LPS-induced atherosclerosis in apoe mice, which mainly relates to an increased inflammatory response toward LPS. We anticipate that apoCI plasma levels contribute to accelerated atherosclerosis development in individuals who have chronic infection. © 2007 American Heart Association, Inc.

Macrophage scavenger receptors class A (MSR) are thought to play an important role in atherogenesis by mediating the unrestricted uptake of modified lipoproteins by macrophages in the vessel wall leading to foam cell formation. To investigate the in vivo role of the MSR in this process, a transgenic mouse model expressing both isoforms of the human MSR was generated. A 180-kb yeast artificial chromosome (YAC) containing the human MSR gene (MSR1) with 60- and 40-kb flanking sequence at the 5' and 3' end, respectively, was obtained by reducing the size of a 1050-kb YAC by homologous recombination. This 180-kb YAC was microinjected into mouse oocytes. In the resulting transgenic mice, high levels of mRNA for both type I and type II human MSR1 were detected in peritoneal macrophages and trace levels in other organs, known to contain macrophage-derived cells. Using an antibody against the human MSR, the Kupffer cells in the liver were shown to contain the MSR protein. In vivo clearance of acetyl-LDL was not changed in the MSR1-transgenic mice. However, in vitro studies using peritoneal macrophages from the transgenic mice showed a two-fold increased degradation of acetyl-LDL and cholesterolester accumulation concomitant with a four-fold increase in foam cell formation, as compared to wild-type macrophages. Thus, macrophage specific overexpression of the MSR may lead to increased foam cell formation, which is one of the initial and crucial steps in atherogenesis. Copyright (C) 1999 Elsevier Science Ireland Ltd.