Hypoxia Increases LDL Oxidation and Expression of 15-Lipoxygenase-2 in Human Macrophages

Abstract

Objective-Macrophage-mediated oxidation of low-density lipoprotein (LDL) by enzymes, such as the lipoxygenases, is considered of major importance for the formation of oxidized LDL during atherogenesis. Macrophages have been identified in hypoxic areas in atherosclerotic plaques. Methods and Results-To investigate the role of hypoxia in macrophage-mediated LDL oxidation, we incubated human monocyte-derived macrophages with LDL under normoxic (21% O 2 ) or hypoxic (0% O 2 ) conditions. The results showed that hypoxic macrophages oxidized LDL to a significantly higher extent than normoxic cells. Interestingly, the mRNA and protein expression of 15-lipoxygenase-2 (15-LOX-2) as well as the activity of this enzyme are elevated in macrophages incubated at hypoxia. Both the unspliced 15-LOX-2 and the spliced variant 15-LOX-2sv-a are found in macrophages. In addition, 15-LOX-2 was identified in carotid plaques in some macrophage-rich areas but was only expressed at low levels in nondiseased arteries. Conclusions-In summary, these observations show for the first time that 15-LOX-2 is expressed in hypoxic macrophages and in atherosclerotic plaques and suggest that 15-LOX-2 may be one of the factors involved in macrophage-mediated LDL oxidation at hypoxia. A n early phenomenon in atherosclerosis is the retention, oxidation, and accumulation of low-density lipoprotein (LDL) in the vessel wall. 1,2 Oxidized LDL (oxLDL), one of the key players in atherogenesis, attracts monocytes to the vessel wall where they differentiate into macrophages. 3,4 Oxidation of LDL mediated by macrophages is considered to be of major importance for the formation of oxLDL within the atherosclerotic plaque. Enzymes involved in this process are 15-lipoxygenase (15-LOX), 5,6 myeloperoxidase (MPO), 7 and NADPH oxidase. 8 Macrophages in the arterial wall take up oxLDL through scavenger receptors and accumulate oxLDL as cholesterol esters, which results in foam cell formation.The thickness of the arterial wall increases as the atherosclerotic plaque develops. This leads to an impaired diffusion, which results in oxygen and nutrient deficiency in the deeper portion of the arterial intima and in atherosclerotic plaques. Simultaneously, oxygen consumption by cells within the plaque rises, 9,10 which could be because of the increased number of energy-consuming foam cells. 10 In healthy tissues, oxygen tension is 20 to 70 mm Hg (2.5% to 9% O 2 ). However, in diseased tissue, eg, in atherosclerotic plaques, inadequate perfusion may reduce O 2 tension to below 10 mm Hg (Ͻ1% O 2 ) in some regions. 11 Results from our laboratory have previously shown that areas of hypoxia occur within atherosclerotic plaques in cholesterol-fed rabbits. 12 Hypoxia may lead to retention of macrophages in these areas, because it has been shown that macrophage migration is reduced by hypoxia. 13 The role of hypoxia in the development of atherosclerotic plaques is not known. In this study, we have explored the effect of hypoxia on macrophage-mediated LDL oxidation and the expression o...

Atherosclerotic Plaque Progression and Vulnerability to Rupture

“…9 In vitro studies have shown that low-density lipoprotein (LDL) uptake by macrophages is facilitated by a 2-step oxidation process, beginning with mild oxidation of lipid 10,11 followed by apolipoprotein B oxidation, a modification required for scavenger receptor recognition, which is unaffected by the cholesterol content of the cell. 12 Cultured macrophages exposed to oxidized LDL are richer in free cholesterol than cholesterol esters.…”

Hypoxia Regulates Macrophage Functions in Inflammation

“…Rydberg et al (19) showed that macrophages subjected to hypoxia in vitro up-regulate the expression of lipooxygenase-2, an enzyme implicated in low-density lipoprotein oxidation. In addition, hypoxia also up-regulates very low-density lipoprotein (VLDL) receptors on macrophages, and high levels of VLDL receptors are expressed by plaque macrophages in vivo (20,21).…”

Participation of ATP7A in macrophage mediated oxidation of LDL

“…Macrophages present in arterial lesions are thought to be the principal mediators of LDL oxidation in the arterial wall [see review in ( 11 )]. Macrophage can modulate LDL oxidation through extracellular [ceruloplasmin ( 12 ), paraoxonase-1 ( 13 ), and myeloperoxidase ( 14 )] and intracellular ( 15,16 ), NADPH oxidase ( 17 ), LDL receptor-related protein 1 (LRP1) ( 18 ), and cytosolic phospholipase A 2 ␣ (cPLA 2 ␣ ) ( 19 )] mechanisms. Moreover, LDL oxidation by macrophages was recently reported to occur in the intracellular compartment within lysosomes ( 20 ).…”