The amino acid and oligopeptide transporter Solute carrier family 15 member A4 (SLC15A4), which resides in lysosomes and is preferentially expressed in immune cells, plays critical roles in the pathogenesis of lupus and colitis in murine models. Toll-like receptor (TLR)7/9- and nucleotide-binding oligomerization domain-containing protein 1 (NOD1)-mediated inflammatory responses require SLC15A4 function for regulating the mechanistic target of rapamycin complex 1 (mTORC1) or transporting L-Ala-γ-D-Glu-meso-diaminopimelic acid, IL-12: interleukin-12 (Tri-DAP), respectively. Here, we further investigated the mechanism of how SLC15A4 directs inflammatory responses. Proximity-dependent biotin identification revealed glycolysis as highly enriched gene ontology terms. Fluxome analyses in macrophages indicated that SLC15A4 loss causes insufficient biotransformation of pyruvate to the tricarboxylic acid cycle, while increasing glutaminolysis to the cycle. Furthermore, SLC15A4 was required for M1-prone metabolic change and inflammatory IL-12 cytokine productions after TLR9 stimulation. SLC15A4 could be in close proximity to AMP-activated protein kinase (AMPK) and mTOR, and SLC15A4 deficiency impaired TLR-mediated AMPK activation. Interestingly, SLC15A4-intact but not SLC15A4-deficient macrophages became resistant to fluctuations in environmental nutrient levels by limiting the use of the glutamine source; thus, SLC15A4 was critical for macrophage's respiratory homeostasis. Our findings reveal a mechanism of metabolic regulation in which an amino acid transporter acts as a gatekeeper that protects immune cells' ability to acquire an M1-prone metabolic phenotype in inflammatory tissues by mitigating metabolic stress.
Keywords: amino acid transporter; cytokine; immunometabolism; innate immune cell; macrophage.