title: Bioenergetic profiles diverge during macrophage polarization: implications for the interpretation of 18F-FDG PET imaging of atherosclerosis.
abstract: Conventional cardiovascular imaging is invaluable for the assessment of late sequelae of atherosclerosis, such as diminished perfusion reserve and luminal stenosis. Molecular imaging provides complementary information about plaque composition and ongoing biologic processes in the vessel wall, allowing the early diagnosis and risk stratification of patients. Detection of enhanced glucose uptake, using (18)F-FDG PET, has been proposed as a noninvasive approach to track macrophage activation as a critical event in the development and progression of atherosclerosis. In this study, we determined the impact of macrophage polarization on glucose metabolism and oxidative phosphorylation.
  
  
  METHODS
  Murine peritoneal macrophages were incubated in the presence of interferon-γ (IFN-γ) plus tumor necrosis factor-α (TNF-α), lipopolysaccharide (LPS), or interleukin-4 (IL-4) to induce classic (M1 and M(LPS)) or alternative (M2) polarization, respectively. Glucose uptake was measured using (3)H-deoxyglucose. Oxidative phosphorylation was evaluated using an extracellular flux analyzer. Mitochondrial DNA copy numbers were quantified by polymerase chain reaction. The expression of glucose transporter-1 (Glut-1), hexokinase-1 and -2 (Hk-1 and Hk-2, respectively), mitochondrial transcription factor-1 (Tfam), and cytochrome c oxidase subunit I (Cox-1) was determined by quantitative reverse transcription polymerase chain reaction.
  
  
  RESULTS
  Stimulation of macrophages by LPS, but not polarization with either IFN-γ plus TNF-α (M1) or IL-4 (M2), resulted in a 2.5-fold increase in (3)H-deoxyglucose uptake. Enhanced glucose uptake by M(LPS) macrophages paralleled the overexpression of rate-limiting proteins involved in transmembrane transport and intracellular trapping of glucose--that is, Glut-1, Hk-1, and Hk-2. Alternatively polarized M2 macrophages developed a markedly higher spare respiratory capacity than both nonpolarized and classically polarized M1 macrophages. M2 polarization was associated with a 4.6-fold increase in mitochondrial content of the cells, compared with nonpolarized macrophages. The expression of Tfam, a major regulator of mitochondrial biogenesis, and Cox-1, a critical component of respiratory chain, was significantly increased in M2 polarized macrophages.
  
  
  CONCLUSIONS
  Polarization of macrophages induces distinct metabolic profiles with respect to glycolysis versus oxidative phosphorylation, with alternatively polarized macrophages shifting to mitochondria as their main source of adenosine triphosphate. Only M(LPS), but not M1 or M2 polarized macrophages, showed increased glucose uptake, suggesting that glucose metabolism is regulated independent of the polarization state and macrophage polarization may not be detectable by (18)F-FDG PET.
authors.affiliation: Department of Radiology, University of Texas Health Science Center at San Antonio, San Antonio, Texas, USA.
authors.forename: Sina
authors.initials: S
authors.lastname: Tavakoli
authors.forename: Debora
authors.initials: D
authors.lastname: Zamora
authors.forename: Sarah
authors.initials: S
authors.lastname: Ullevig
authors.forename: Reto
authors.initials: R
authors.lastname: Asmis
mesh_terms: D000818:Animals; D050197:Atherosclerosis / Q000000981:diagnostic imaging* / Q000276:immunology* / Q000473:pathology; D016764:Cell Polarity; D002478:Cells, Cultured; D004734:Energy Metabolism; D019788:Fluorodeoxyglucose F18 / Q000493:pharmacokinetics*; D008264:Macrophages / Q000000981:diagnostic imaging* / Q000276:immunology*; D051379:Mice; D008810:Mice, Inbred C57BL; D049268:Positron-Emission Tomography / Q000379:methods*; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics
chemical_list: D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18
keywords: 18F-FDG; PET; glucose uptake; macrophage polarization; oxidative phosphorylation