Uncovering metabolic pathways in human alveolar macrophages in response to lipopolysaccharide.

Introduction: Alveolar macrophages (AMs) play an essential role in maintaining homeostasis in the lung and in innate immunity for host defense. To fuel inflammatory responses, AMs do not rely on glycolysis, but require oxidative phosphorylation. However, which nutrients AMs use to fuel their energy demand during inflammatory responses, is still unknown. The present study aimed to determine the contribution of three key metabolic pathways; fatty acid oxidation, glutaminolysis, and glycogenolysis, to the inflammatory response of AMs.

Methods: Primary AMs were isolated from healthy human volunteers and stimulated with lipopolysaccharide (LPS). After 24 hours, cells were subjected to analyses of metabolic flux, expression of genes involved in these metabolic pathways, and inflammatory cytokine secretion in the presence of metabolic inhibitors.

Results: The results of our study show that human AMs display expression of genes involved in fatty acid and glutamine metabolism and are capable of metabolizing oleic acid and glutamine during homeostasis, but do not use these metabolites to fuel the production of inflammatory cytokines. We demonstrate that AMs, while residing in a glucose-deprived environment, contain glycogen and use glycogenolysis to fuel inflammatory cytokine secretion, as reflected by reduced TNF, IL-1β and IL-6 levels in supernatant of LPS-stimulated AMs treated with the glycogenolysis inhibitor CP316819. Moreover, AMs display marked expression of genes involved in glycogenesis, including FBP1 and GYS.

Conclusions: Taken together, these results indicate that primary human AMs are equipped to use different nutrients to fuel their metabolic demands. Moreover, our findings suggest that glycogenolysis is critical for the inflammatory response of AMs.