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)GLUL, the enzyme that synthesizes glutamine, 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 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 three-fold faster in M1 macrophages. With the same approach, we have demonstrated that cell glutamate 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.