Human M1 macrophages secrete glutamine: a role for the transporters ASCT2 and SNAT5

Previous results indicated that LPS treatment of human macrophages is associated with the induction of glutamate-ammonia-ligase (GLUL), the enzyme that synthesizes glutamine (Gln), although the effects on cell Gln level were not assessed by those studies. Here, we show that M1 polarization of human THP-1 macrophage-like cells with LPS and interferon γ (IFNγ) induces not only GLUL but also the exchange Gln transporter ASCT2 and the bidirectional carrier SNAT5. Consequently, Gln influx markedly increases, with ASCT2 and SNAT5 inhibitors suppressing the effect. Notwithstanding these changes, cell Gln is comparable in human M0 and M1 macrophages. However, under Gln-free conditions, Gln efflux is threefold faster in M1 macrophages. With the same approach, we have demonstrated that cell glutamate (Glu) mostly derives from extracellular Gln. Furthermore, Glu efflux is markedly higher in M1 macrophages, consistent with the induction of the Glu-cystine exchanger xCT. Induction of GLUL, SNAT5, and xCT, along with SNAT5-dependent stimulation of Gln transport, are clearly detectable also in primary human macrophages, derived from monocytes of peripheral blood and M1-polarized with LPS/IFNγ. These data indicate that, through a stimulation of Gln and Glu efflux, M1-polarized human macrophages can increase the concentration of both amino acids in the extracellular microenvironment.NEW & NOTEWORTHY M1 polarization of human macrophages promotes the induction of glutamate-ammonia-ligase (GLUL) and of the Gln transporters SNAT5 and ASCT2, leading to a faster efflux of the amino acid into the extracellular compartment. Under the same conditions, induction of the transporter xCT mediates the increased efflux of glutamate. Therefore, M1 macrophages can increase the extracellular concentrations of both glutamine and glutamate, potentially influencing the metabolic and functional behavior of other cell populations present in the microenvironment.