Complications of atherosclerosis are the most common cause of death in Western societies. Among the many risk factors identified by epidemiological studies, only elevated levels of lipoproteins containing apolipoprotein (apo) B can drive the development of atherosclerosis in humans and experimental animals even in the absence of other risk factors. However, the mechanisms that lead to atherosclerosis are still poorly understood. We tested the hypothesis that the subendothelial retention of atherogenic apoB-containing lipoproteins is the initiating event in atherogenesis. The extracellular matrix of the subendothelium, particularly proteoglycans, is thought to play a major role in the retention of atherogenic lipoproteins. The interaction between atherogenic lipoproteins and proteoglycans involves an ionic interaction between basic amino acids in apoB100 and negatively charged sulphate groups on the proteoglycans. Here we present direct experimental evidence that the atherogenicity of apoB-containing low-density lipoproteins (LDL) is linked to their affinity for artery wall proteoglycans. Mice expressing proteoglycan-binding-defective LDL developed significantly less atherosclerosis than mice expressing wild-type control LDL. We conclude that subendothelial retention of apoB100-containing lipoprotein is an early step in atherogenesis.

Impaired cardiac function is associated with myocardial triglyceride accumulation, but it is not clear how the lipids accumulate or whether this accumulation is detrimental. Here we show that hypoxia/ischemia-induced accumulation of lipids in HL-1 cardiomyocytes and mouse hearts is dependent on expression of the VLDL receptor (VLDLR). Hypoxia-induced VLDLR expression in HL-1 cells was dependent on HIF-1α through its interaction with a hypoxia-responsive element in the Vldlr promoter, and VLDLR promoted the endocytosis of lipoproteins. Furthermore, VLDLR expression was higher in ischemic compared with nonischemic left ventricles from human hearts and was correlated with the total lipid droplet area in the cardiomyocytes. Importantly, Vldlr -/-mice showed improved survival and decreased infarct area following an induced myocardial infarction. ER stress, which leads to apoptosis, is known to be involved in ischemic heart disease. We found that ischemia-induced ER stress and apoptosis in mouse hearts were reduced in Vldlr -/-mice and in mice treated with antibodies specific for VLDLR. These findings suggest that VLDLR-induced lipid accumulation in the ischemic heart worsens survival by increasing ER stress and apoptosis.

Objectives-Atherosclerotic lesions have regions that are hypoxic. Because the lesion contains macrophages that are loaded with lipid, we investigated whether hypoxia can influence the accumulation of lipids in these cells. Methods and Results-Exposure of human macrophages to hypoxia for 24 hours resulted in an increased formation of cytosolic lipid droplets and an increased accumulation of triglycerides. Exposure of the macrophages to oxidized low-density lipoprotein (oxLDL) increased the accumulation of cytosolic lipid droplets because of an increase in cellular cholesterol esters. The accumulation of lipid droplets in oxLDL-treated cells was further increased after hypoxia, caused by an increased level of triglycerides. Expression analyses combined with immunoblot or RT-PCR demonstrated that hypoxia increased the expression of several genes that could promote the accumulation of lipid droplets. Hypoxia increased the mRNA and protein levels of adipocyte differentiation-related protein (ADRP). It is well known that an increased expression of ADRP increases the formation of lipid droplets. Hypoxia decreased the expression of enzymes involved in ␤-oxidation (acyl-coenzyme A synthetase and acyl-coenzyme A dehydrogenase) and increased the expression of stearoyl-coenzyme A desaturase, an important enzyme in the fatty acid biosynthesis. Moreover, exposure to hypoxia decreased the rate of ␤-oxidation, whereas the accumulation of triglycerides increased. The lipids are stored in cytosolic lipid droplets, 1 which have been suggested to consist of a core of neutral lipids (cholesterol esters or triglycerides) surrounded by a monolayer of amphipathic structures such as phospholipids and proteins. 2 The most well known of these proteins are the PAT proteins perilipin, adipocyte differentiation-related protein (ADRP), and tailinteracting protein 47. 2 ADRP is the predominant PAT protein of the lipid droplets in macrophages 3 and is present on newly formed droplets. 4 It been shown to strongly influence the formation of lipid droplets. 2,5 Phospholipase D1 (PLD1), which catalyzes the formation of phosphatidic acid, has been shown to have an important role in the assembly of lipid droplets. 6 The atherosclerotic lesion is characterized by regions of hypoxia. 7 The role of hypoxia in the development of the lesion is unknown. However, hypoxia has been shown to reduce macrophage migration. 8 Moreover, our previous results indicated that hypoxia resulted in an increased expression of 15-lipoxygenase-2 in macrophages, which correlated with an increased ability of the macrophage to participate in the oxidation of low-density lipoprotein (LDL). 9 Furthermore, hypoxia caused an increase in the secretion of interleukins. 10,11 Together, these observations suggest that the influence of hypoxia on macrophages is of fundamental importance for the inflammation that characterizes the atherosclerotic lesion. It was demonstrated recently that leukocytes respond to an inflammatory stimulus by the accumulation of cytosolic lipid droplets,...

RHO family proteins are important for the function of inflammatory cells. They are modified with a 20-carbon geranylgeranyl lipid in a process catalyzed by protein geranylgeranyltransferase type I (GGTase-I). Geranylgeranylation is viewed as essential for the membrane targeting and activity of RHO proteins. Consequently, inhibiting GGTase-I to interfere with RHO protein activity has been proposed as a strategy to treat inflammatory disorders. However, here we show that mice lacking GGTase-I in macrophages develop severe joint inflammation resembling erosive rheumatoid arthritis. The disease was initiated by the GGTase-I-deficient macrophages and was transplantable and reversible in bone marrow transplantation experiments. The cells accumulated high levels of active GTP-bound RAC1, CDC42, and RHOA, and RAC1 remained associated with the plasma membrane. Moreover, GGTase-I deficiency activated p38 and NF-κB and increased the production of proinflammatory cytokines. The results challenge the view that geranylgeranylation is essential for the activity and localization of RHO family proteins and suggest that reduced geranylgeranylation in macrophages can initiate erosive arthritis.

Oxysterols are biologically active molecules generated during oxidation of LDL. Several of these oxysterols were found in macrophage-derived foam cells from human atherosclerotic tissue (eg, 7-hydroxycholesterol, 7-ketocholesterol, 5-epoxycholesterol, and 25-hydroxycholesterol). A specific stimulation of interleukin-8 (IL-8) production by oxidized LDL (oxLDL) has been shown by other investigators. In foam cells from human atherosclerotic tissue, we found high levels of IL-8 (183.1 pg/10(6) cells) compared with monocytes (23.2 pg/10(6) cells) or monocyte-derived macrophages in culture (1.5 pg/10(6) cells). When monocytes and monocyte-derived macrophages, in vitro, were exposed to a series of different oxysterols, we found that all oxysterols tested had a tendency to stimulate IL-8 production but that 25-hydroxycholesterol was the most potent one. This stimulation of IL-8 production was time and dose dependent and could be blocked by cycloheximide. These results indicate that oxysterols in oxLDL may have a regulatory effect on IL-8 production. IL-8, a potent chemoattractant, may play a role in the recruitment of T lymphocytes and smooth muscle cells into the subendothelial space and may contribute to the formation of atherosclerotic lesions.