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 18F-FDG PET, has been proposed as a non-invasive 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 MLPS) or alternative (M2) polarization, respectively. Glucose uptake was measured using 3H-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 3H-deoxyglucose uptake. Enhanced glucose uptake by MLPS 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. 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 MLPS, 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 18F-FDG PET.

miR-125b controls monocyte adaptation to inflammation through mitochondrial metabolism and dynamics

“…Profound metabolic reprogramming also occurs under macrophage polarizing conditions, with glycolysis being a hallmark of the inflamed state that occurs in M1 macrophages, and OXPHOS occurring in immunoregulatory M2 macrophages. 44 M1 polarization uses anaerobic glycolysis to generate large quantities of ATP and to maintain a high mitochondrial membrane potential, despite the complete inhibition of respiration, allowing cells to adapt functions quickly and cope with the hypoxic tissue environment while providing a defense mechanism against cell death induced by endogenous reactive oxygen species. In contrast, M2 macrophages preferentially use OXPHOS metabolism to provide sustained ATP supply for tissue remodeling and repair and immunomodulation.…”

“…All these genes provide a likely mechanism for the THP-1 cells that occurs after increased miR-125b expression, because they are indicative of the M1-type polarization known to induce distinct genetic and metabolic profiles, switching metabolism from oxidative phosphorylation (OXPHOS) to glycolysis and triggering specific proinflammatory cytokines. 44 To induce classic (M1) polarization, PMA-differentiated THP-1 macrophages (Mf) were incubated in the presence of IFN-g plus LPS. 45 Polarization was first confirmed using a panel of established markers ( Figure 4B) and then confirmed by real-time monitoring of the mitochondrial energy metabolism ( Figure 4C).…”

Human Coronary Thrombus Formation Is Associated With Degree of Plaque Disruption and Expression of Tissue Factor and Hexokinase II

“…12 Glucose uptake and HK-II expression, however, are increased in macrophages stimulated with LPS but not in those with the M1 and M2 phenotypes. 11 This discrepancy might be due to different cell sources and cell culture protocols. Both TF and HK-II were detected in M1 and M2 macrophages from symptomatic coronary arteries.…”

NOTCH reprograms mitochondrial metabolism for proinflammatory macrophage activation

“…ATP generation through glycolysis is 19-fold less efficient than that by glucose oxidation. Although the rate of ATP production by glycolysis can be approximately 100 times faster than the rate of oxidation (59), we and others (30,60) only observed a 2-fold increase in glucose utilization in M1 Macs. Thus, the high velocity of the glycolysis cannot adequately support required ATP, supporting the notion that metabolic shift to glycolysis is energetically unfavorable for the phagocytic activity (61).…”