Hypoxia Converts Human Macrophages Into Triglyceride-Loaded Foam Cells

Abstract

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,...

Hypoxia Prolongs Monocyte/Macrophage Survival and Enhanced Glycolysis Is Associated with Their Maturation under Aerobic Conditions

“…5C); however, both studies found increased lactate production (Fig. 6B) (56). We noted at least at the gene level that, relative to the less differentiated monocytes isolated straight from blood, glycolytic enzymes were up-regulated in MDM to some extent even in normoxic conditions (Figs.…”

“…One of the macrophage lineage populations examined above and that is widely used as an in vitro model for human macrophages, namely MDM, does not require an exogenous CSF for survival under aerobic conditions and therefore is not suitable to study any prosurvival activity of hypoxia (46,55,56). Others have found that hypoxia increased Glut1 mRNA levels in MDM and speculated that this change could be important for macrophage survival in hypoxic diseased tissue (54).…”

Hypoxia causes triglyceride accumulation via HIF-1-mediated stimulation of lipin 1 expression

“…HIF-1 activates genes responsible for switching from oxidative to glycolytic metabolism such as the ones coding for glucose transporters, glycolytic enzymes, lactate dehydrogenase and pyruvate dehydrogenase kinase 1 (Majmundar et al, 2010). Furthermore, hypoxia has been shown to stimulate lipid storage and inhibit lipid catabolism in cultured cardiac myocytes and macrophages (Boström et al, 2006;Huss et al, 2001), but the involvement of HIFs in these processes was not investigated. More recent studies using transgenic mice have suggested that artificial or diet induced activation of liver HIF signalling can augment lipid accumulation by modifying hepatocyte lipid metabolism (Kim et al, 2006;Nath et al, 2011;Qu et al, 2011;Rankin et al, 2009).…”

Molecular Mechanisms and Physiological Significance of Organelle Interactions and Cooperation

“…HIF-induced transcription promotes angiogenesis, erythropoiesis, metastasis and metabolic reprogramming, such as shifting cell metabolism from oxidative phosphorylation to glycolysis. HIF activation due to hypoxia or loss of VHL function also reprograms lipid metabolism leading to lipid accumulation (Huss et al, 2001;Boström et al, 2006;Rankin et al, 2009;Kucejova et al, 2011;Qu et al, 2011;Walter et al, 2014).…”