Oxidized Low-Density Lipoprotein Stimulates Macrophage ¹⁸F-FDG Uptake via Hypoxia-Inducible Factor-1α Activation Through Nox2-Dependent Reactive Oxygen Species Generation

Abstract

For 18 F-FDG PET to be widely used to monitor atherosclerosis progression and therapeutic response, it is crucial to better understand how macrophage glucose metabolism is influenced by the atherosclerotic microenvironment and to elucidate the molecular mechanisms of this response. Oxidized low-density lipoprotein (oxLDL) is a key player in atherosclerotic inflammation that promotes macrophage recruitment, activation, and foam cell formation. We thus explored the effect of oxLDL on macrophage 18 F-FDG uptake and investigated the underlying molecular mechanism including the roles of hypoxia-inducible factor-1α (HIF-1α) and reactive oxygen species (ROS). Methods: RAW264.7 macrophages were stimulated with native LDL, oxLDL, or lipopolysaccharide. Cells were assessed for 18 F-FDG uptake, lactate production, membrane glucose transporter 1 (GLUT1) expression, and hexokinase activity. ROS generation, Nox expression, and HIF-1α activity were also measured. Results: oxLDL (20 μg/mL) induced a 17.5 ± 1.7-fold increase in macrophage 18 F-FDG uptake by 24 h, which was accompanied by increased lactate production, membrane GLUT1 expression, and hexokinase activity. oxLDL-stimulated 18 F-FDG uptake was completely blocked by inhibitors of Src or phosphoinositide 3-kinase. ROS generation was increased to 262.4% ± 17.9% of controls by oxLDL, and N-acetyl-L-cysteine completely abrogated both oxLDLinduced ROS production and 18 F-FDG uptake. oxLDL increased Nox2 expression, and nicotinamide adenine dinucleotide phosphate oxidase inhibition totally blocked increased ROS generation and 18 F-FDG uptake by oxLDL. Finally, there was a clear ROSdependent increase of HIF-1α accumulation by oxLDL, and silencing of HIF-1α completely abolished the metabolic effect of oxLDL. Conclusion: oxLDL is a strong stimulator of macrophage 18 F-FDG uptake and glycolysis through upregulation of GLUT1 and hexokinase. This metabolic response is mediated by Nox2-dependent ROS generation that promotes HIF-1α activation.

Differential Regulation of Macrophage Glucose Metabolism by Macrophage Colony-stimulating Factor and Granulocyte-Macrophage Colony-stimulating Factor: Implications for 18F FDG PET Imaging of Vessel Wall Inflammation

“…Macrophages have the capacity to effectively switch to glycolysis after exposure to hypoxia (14,17), bacterial products (9,13), or in response to certain other proinflammatory stimuli (eg, modified lipoproteins) (12,15). However, macrophages can efficiently use oxidative metabolism after exposure to the anti-inflammatory cytokine interleukin-4 to support their bioenergetic and metabolic needs (9,28).…”

“…Local production of M-CSF promotes the survival of foam cells and contributes to development of the early-stage, but not late-stage, atherosclerotic lesions (20,21 F-FDG PET studies (8)(9)(10)(11)(12)(13)(14). For example, glucose uptake and glycolysis are induced by bacterial-derived products, such as lipopolysaccharide (13), modified low-density lipoprotein (15,16), or hypoxia (17). However, the link between a proinflammatory macrophage phenotype and enhanced glucose uptake may be valid for only a number of stimuli and does not represent a universal phenomenon (9,14).…”

“…While broadly linked to the proinflammatory macrophage phenotype (12,13,17), our results indicate that glycolysis may increase in response to M-CSF despite its established M2-skewing effects. It is noteworthy that unlike the enhanced glycolysis observed in conventional proinflammatory M1 or hypoxic macrophages, which is associated with suppressed oxidative metabolism (9,(12)(13)(14)17), M-CSF induces a robust increase in both glycolysis and mitochondrial metabolism, which depicts another level of complexity in macrophage metabolic diversity (9,(12)(13)(14)(15)(16)(17)28). The functional consequences of such metabolic complexity in the regulation of various aspects of macrophage function (eg, cytokine production, phagocytosis, and cell migration) remain to be determined.…”

Protective effect of oleanolic acid on oxidized-low density lipoprotein induced endothelial cell apoptosis

“…A few studies have shown that HIF-1α played an essential role in the function of macrophages when challenged by ox-LDL and promoted the transformation of macrophages to foam cells (22,23). In leucocytes, which function as a key factor in the development and progression of atherosclerotic lesions, HIF-1α has been reported to have a direct effect on the cytokine profile (24).…”

Molecular imaging of atherosclerotic lesions by positron emission tomography - can it meet the expectations?

“…One study assessed 18-FDG uptake vs. histology (r = 0.521, p = 0.003) [32] and showed that the SUV was signifi cantly higher in symptomatic (1.75) vs. asymptomatic (1.43) patients. Oxidized (ox) LDL has been shown to increase F-18-FDG uptake and glycolytic metabolism by more than an order of magnitude via the upregulation of GLUT1 expression and hexokinase activity [33,34]. Furthermore, proinfl ammatory cytokines [35,36], interferon, monocyte colony stimulating factor and lipopolysaccharides [37] are signifi cant stimulators of FDG uptake in macrophages.…”