Kyoto, Japan -- Macrophages are white blood cells that serve in the first line of defense against infections. When they encounter harmful pathogens or bacteria, macrophages undergo metabolic reprogramming: significant metabolic changes that facilitate the production of inflammatory molecules.
Metabolic reprogramming is largely regulated by hypoxia inducible factor 1α -- or HIF-1α -- a key transcription factor in cellular metabolism. HIF-1α can be activated by both inflammatory stimuli and hypoxia -- oxygen deprivation -- yet the mechanisms behind its activation are still poorly understood. This motivated a team of researchers at Kyoto University to investigate the forces behind this critical immune response.
"Our team has long been fascinated by how macrophages reprogram their metabolism to fight infections," says first author Shota Yasukura.
The scientists began by performing parallel genome-wide CRISPR screenings of both inflammatory and hypoxic conditions, comparing the factors regulating HIF-1α activation under both. This led to the discovery that SNAT2, an amino acid transporter, suppresses HIF-1α expressly under inflammatory stimulation. Subsequent RNA-sequencing and metabolic analyses uncovered that deleting this protein enhances HIF-1α activity, promoting metabolic reprogramming in macrophages.
Building on these discoveries, the team performed metabolome analysis and observed that inflammatory stimulation promotes the accumulation of small amino acids in macrophages, but that a lack of SNAT2 inhibits this response, instead activating the osmotic stress response and thus promoting metabolic reprogramming. Finally, to test these discoveries, the researchers generated macrophage-specific SNAT2-deficient mice and infected them with Listeria monocytogenes, leading to a discovery that, compared to wild mice, proliferation of the infection was suppressed in SNAT2-deficient mice.
These results reveal that the amino acids transported into macrophages by SNAT2 maintain intracellular osmotic balance to suppress metabolic reprogramming and inflammatory activation. This in turn suggests that amino acid insufficiency could induce macrophage inflammatory activation.
"Uncovering this connection reshapes our understanding of how amino acid metabolism influences macrophage activation, and represents an important step toward understanding how macrophage metabolism is regulated during inflammatory responses," says co-corresponding author Masanori Yoshinaga.
This discovery expands our understanding of amino acids beyond their conventional functions as nutrients and the building blocks of proteins, highlighting previously undiscovered roles in regulation.
These findings also point to the possibility of controlling amino acid metabolism to facilitate the adaptation of macrophages to osmotic stress in inflamed tissue, which has therapeutic potential in treating inflammatory disorders. The team next plans to investigate the regulatory mechanisms behind SNAT2 expression and amino acid modulation of osmotic homeostasis.
