Conclusion
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.
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.