Uptake by J774 macrophages of very-low-density lipoproteins isolated from apoE-deficient mice is mediated by a distinct receptor and stimulated by lipoprotein lipase

Pathophysiology of triglyceride-rich lipoproteins in atherothrombosis: cellular aspects

Elevated plasma levels of triglyceride-rich lipoproteins (TGRLP), including very low-density lipoproteins (VLDL), chylomicrons, and their remnants, are now acknowledged as risk factors for cardiovascular disease. Interactions of TGRLP with lipoprotein receptors on monocytes, macrophages, and endothelial cells may be mechanistically linked to this risk. Triglyceride-rich lipoproteins from hypertriglyceridemic (HTG) subjects have the abnormal ability to bind to low-denisty lipoprotein receptors via apoE, and plasma chylomicrons from all subjects bind to a new, distinct receptor for apoB48 that is expressed specifically by monocytes, macrophages, and endothelial cells. Receptor binding and uptake of TGRLP by these cells are likely mechanisms involved in the formation of lipid-filled, macrophage-derived "foam cells" of atherosclerotic lesions and for defective fibrinolysis due to endothelial dysfunction. Recognition of the atherothrombogenic potential of TGRLP may lead to improved interventions to lessen or prevent the often fatal sequelae of coronary atherosclerosis and thrombosis associated with elevated plasma triglyceride levels.

Formation of ceramide-enriched domains in lipid particles enhances the binding of apolipoprotein E

We investigated the interaction between apolipoprotein E (apoE) and ceramide (CER)-enriched domains on the particles, by using lipid emulsions containing sphingomyelin (SM) or CER as model particles of lipoproteins. The sphingomyelinase (SMase)-induced aggregation of emulsion particles was prevented by apoE. CER increased the amount of apoE bound to emulsion particles. The confocal images of CER-containing large emulsions with two fluorescent probes showed three-dimensional microdomains enriched in CER. SMase also induced the formation of CER-enriched domains. We propose apoE prefers to bind on CER-enriched domains exposed on particle surface, and thus inhibits the aggregation or fusion of the particles.

Peroxisome proliferator-activated receptor (PPAR) agonists decrease lipoprotein lipase secretion and glycated LDL uptake by human macrophages

Lipoprotein lipase (LPL) acts independently of its function as triglyceride hydrolase by stimulating macrophage binding and uptake of native, oxidized and glycated LDL. Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors expressed in monocyte/macrophages, where they control cholesterol homeostasis. Here we study the role of PPARs in the regulation of LPL expression and activity in human monocytes and macrophages. Incubation of human monocytes or macrophages with PPARalpha or PPARgamma ligands increases LPL mRNA and intracellular protein levels. By contrast, PPAR activators decrease secreted LPL mass and enzyme activity in differentiated macrophages. These actions of PPAR activators are associated with a reduced uptake of glycated LDL and could influence atherosclerosis development associated with diabetes.

The LDL receptor is the major pathway for beta-VLDL uptake by mouse peritoneal macrophages

In order to determine the contribution of the low density lipoprotein receptor (LDL-R) to the removal of apoB-containing native lipoproteins by macrophages, we compared the uptake of beta-VLDL in peritoneal macrophages (MPM) from wild type mice and mice lacking the LDL-R. The d<1.006 g/ml lipoproteins obtained from apoE deficient mice fed a high fat diet were poorly degraded by macrophages and caused only a slight formation of CE in macrophages from both types of mice. On the other hand, d<1.006 g/ml lipoproteins obtained from LDL-R deficient mice fed a high fat diet, beta-VLDL with apoE, were avidly taken up by and markedly stimulated CE formation in wild type macrophages, but not in macrophages lacking the LDL-R. The degradation of 125I-labeled-apoE-containing beta-VLDL by wild type MPM was poorly inhibited by unlabeled human LDL, and beta-VLDL without apoE had no effects. In conclusion, we propose that the in vitro uptake of native apoE-enriched lipoproteins by murine macrophages is primarily mediated by the LDL receptor and not by other apoE-recognizing receptor systems such as: the LDL receptor related protein, the VLDL receptor or the triglyceride-rich lipoprotein receptor.

Macrophage specific overexpression of the human macrophage scavenger receptor in transgenic mice, using a 180-kb yeast artificial chromosome, leads to enhanced foam cell formation of isolated peritoneal macrophages

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.

Nonoxidative modifications of lipoproteins in atherogenesis

The key initiating event in atherosclerosis is the retention of plasma lipoproteins in the subendothelial matrix. Subsequently, a series of biological responses to this retained material leads to specific molecular and cellular processes that promote lesion formation. There is considerable evidence that many of these biological responses, notably macrophage cholesteryl ester loading (foam cell formation), require subendothelial modification of the retained lipoproteins. Oxidation of lipoproteins is one such modification that likely occurs in vivo and promotes certain atherogenic events, but oxidation cannot explain all aspects of atherogenesis, including certain elements of macrophage foam cell formation. For this reason, there has been renewed interest in other modifications of lipoproteins that may be important in atherogenesis. This review addresses five such lipoprotein modifications, namely aggregation, glycation, immune complex formation, proteoglycan complex formation, and conversion to cholesterol-rich liposomes. The focus is on the evidence that these modifications occur in atherosclerotic lesions and on the potential role of these modified lipoproteins in atherogenesis, with an emphasis on macrophage foam cell formation.

Probucol selectively increases oxidation of atherogenic lipoproteins in cholesterol-fed mice and in Watanabe heritable hyperlipidemic rabbits

The anti-atherogenic and cholesterol-lowering drug probucol (0.5-1%) or quercetin (1%), a natural antioxidant, was given to cholesterol-fed (1.5%) mice for a period of 6 weeks and to Watanabe heritable hyperlipidemic (WHHL) rabbits for a period of 8 weeks to investigate the oxidative changes in plasma and lipoproteins. Oxidation was measured as the total amount of malondialdehyde (nmol MDA/g protein) by a very specific MDA-HPLC method. A large and significant increase in MDA was seen in LDL from probucol treated WHHL rabbits (1778.7+/-585.5 nmol/g vs. 394.4+/-144.5 nmol/g, P < 0.001) and cholesterol-fed mice (579.7 + 47.3 nmol/g vs. 408.1+/-85.8 nmol/g, P < 0.05) as compared to controls while LDL cholesterol was lowered (WHHL rabbits: P < 0.05; mice: P < 0.01). In WHHL rabbits VLDL oxidation was determined additionally, and also revealed a large increase in the probucol group (2102.7+/-1156.1 nmol/g vs. 455.0+/-207.8 nmol/g, P< 0.01). In contrast, the oxidation of plasma and HDL from probucol treated animals was not statistically significantly increased, implying that probucol mediates a selective oxidation of atherogenic cholesterol-transporting lipoproteins. Quercetin treated animals did not show increased oxidation of LDL (and VLDL in rabbits) and cholesterol levels were not decreased. Furthermore, no protective antioxidant effect of quercetin was seen. In conclusion, the results suggest that a prooxidant mechanism rather than antioxidative effects influences lipoprotein metabolism in these animals. It is hypothesized that the oxidation of lipoproteins might be a physiological mechanism performed by macrophages or other cells for uptake and degradation (by macrophages and liver) of excessive amounts of LDL or VLDL and that probucol oxidizes atherogenic lipoproteins and thereby leads to a decrease in cholesterol levels.

Transgenic mouse models to study the role of APOE in hyperlipidemia and atherosclerosis

Transgenic technologies have provided a series of very useful mouse models to study hyperlipidemia and atherosclerosis. Normally, mice carry cholesterol mainly in the high density lipoprotein (HDL) sized lipoproteins, and have low density lipoprotein (LDL) and very low density lipoprotein (VLDL) cholesterol levels. These low LDL and VLDL levels are due to the very rapid metabolism of remnant clearance in mice, which hamper metabolic studies. In addition, due to the lack of atherogenic lipoproteins, mice will not readily develop atherosclerosis. This situation has changed completely, because to date, most known genes in lipoprotein metabolism have been used in transgenesis to obtain mice in which genes have been silenced or overexpressed. These experiments have yielded many mouse strains with high plasma lipid levels and a greater susceptibility for developing atherosclerosis. One of the most widely used strains are knock-out mice deficient for apoE, which is one of the central players in VLDL metabolism. Subsequently, a wide variety of other transgenic studies involving APOE have been performed elucidating the role of apoE and apoE mutants in lipolysis, remnant clearance, cellular cholesterol efflux and atherogenesis. In addition, the APOE mouse models are excellent tools for the development of gene therapy for hyperlipidemias.