Metabolic adaptation of tumor cells under chronic acidosis: a shift towards reductive glutamine metabolism driven by the SIRT1/HIF2α axis

Draoui, Nihed [UCL] Polet, Florence [UCL] Pinto, Adan [UCL] Drozak, Xavier [UCL] Riant, Olivier [UCL] Feron, Olivier [UCL]

Mechanisms underlying cancer progression are strongly influenced by the physico-chemical properties of the tumor microenvironment. In particular, tumor cells must adapt to survive under low pO2 and low pH. Although the impact of hypoxia on tumor metabolism is well described, little is known on how tumor cells adapt their metabolism to acidosis. To address this question, we exposed tumor cells derived from various tissues to low pH conditions (pH 6.5) for several weeks. We first found that after this acidic acclimation, tumor cells ended up proliferating at the same rate as parent cells maintained at pH 7.4. We then documented that chronic low pH exposure triggered the metabolic reprogramming of tumor cells from the preferential use of glucose towards glutamine metabolism. The use of glutamine as a major fuel of the TCA cycle in low pH-adapted tumor cells was proven using a variety of techniques including measurements of [U13C5]-glutamine incorporation in metabolites (determined by GC-MS) and of cellular bioenergetics parameters using the SeahorseTM technology. The metabolic switch was mediated by SIRT1, a protein deacetylase activated by the increased pool of NAD+ in low pH-adapted cells, through two distinct mechanisms. First, free acetate could act as a counteranion to export excess protons out of the cells via MCT1 and thereby maintain the intracellular pH in a physiological range. Second, SIRT1 stimulated the activity of HIF2α thereby supporting the glutamine metabolism via the upregulation of the glutamine transporter SLC1A5 and enzymes supporting the reductive glutamine metabolism including IDH1. The 'glutamine-addicted' phenotype was also proven to be reversible when acclimated cells were again cultured under physiological pH, thereby excluding a clonal selection of tumor cells in response to the acidic conditions. Finally, pharmacological inhibition of either glutamine metabolism (using the glutaminase inhibitor BPTES) or SIRT1 deacetylase activity preferentially killed low pH-adapted cancer cells in vitro (vs. parent cells) and delayed the growth of corresponding tumor xenografts in vivo. Altogether, these data indicate that a major metabolic shift from glucose to glutamine metabolism is induced in tumor cells chronically exposed to an acidic environment and importantly makes them particularly suited for dedicated pharmacological treatments.