Differential Regulation of Macrophage Glucose Metabolism by M‐CSF and GM‐CSF: Implications for 18F‐FDG PET Imaging of Inflammation

Abstract

BackgroundThe high glycolytic capacity of macrophages allows for the utilization of 18F‐FDG in the detection of inflammation associated with various diseases. Microenvironmental signals (such as growth factors, cytokines and pathogen‐derived products) have profound effects on determining the activation state of macrophages, which in turn is delicately regulated through reprogramming of cell metabolism. Specifically, glycolysis has been suggested to enhance the pro‐inflammatory macrophage phenotype induced by pathogen‐derived stimuli. Here, we examined whether this phenomenon extends to endogenous microenvironmental activators by investigating the metabolic effects of two critical myeloid growth factors with opposing effects on the macrophage inflammatory state, i.e., macrophage colony‐stimulating factor (M‐CSF) and granulocyte‐macrophage colony‐stimulating factor (GM‐CSF).MethodsMurine peritoneal macrophages (Mϕ) were incubated ex vivo for two days in the absence (Mϕ0) or presence of M‐CSF (MϕM‐CSF) or GM‐CSF (MϕGM‐CSF). Glucose uptake was measured using 3H‐2‐deoxyglucose. Oxygen consumption rate (OCR) and extracellular acidification rate (ECAR), markers of oxidative and glycolytic metabolism, as well as mitochondrial content of the cells and lactate production were quantified. Expression of activation markers, glucose transporters and metabolic enzymes was measured by high‐throughput quantitative reverse‐transcription PCR.ResultsM‐CSF and GM‐CSF stimulated comparable levels of 3H‐2‐deoxyglucose uptake and induced similar expression levels of glucose transporter‐1 and ‐6 in Mϕ. Similarly, mitochondrial biogenesis and oxidative metabolism were induced to the same levels in MϕM‐CSF and MϕGM‐CSF. Unexpectedly, activation of glycolysis was more pronounced in MϕM‐CSF compared to MϕGM‐CSF, as evidenced by higher basal ECAR and ECAR‐to‐OCR ratio, as well as lactate production. A number of critical glycolytic enzymes were also differentially upregulated in MϕM‐CSF compared to MϕGM‐CSF.ConclusionDespite their pro‐inflammatory activation state, MϕGM‐CSF showed the same level of glucose uptake and oxidative metabolism as MϕM‐CSF, which are associated with inflammation resolution. Furthermore, MϕM‐CSF, not the pro‐inflammatory MϕGM‐CSF, demonstrate the higher glycolytic activity. These findings strongly suggest that the reported association between a pro‐inflammatory macrophage phenotype and increased glucose uptake and glycolysis is limited to certain microenvironmental signals, and does not represent a universal phenomenon, calling for a more prudent approach in the interpretation of 18F‐FDG imaging of inflammation.Support or Funding InformationThis work was supported by grants from Radiological Society of North America Grant to S.T. (RR1131) and the National Institute of Health (HL115858) to R.A.