Metabolic adaptation of tumor cells under chronic acidosis: role of the SIRT1/HIF2alpha axis to drive reductive glutamine metabolism

Corbet, Cyril [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, the influence of extracellular acidosis on the metabolic preferences of cancer cells is largely unknown. Still, tumor acidosis is known to affect important energy-consuming cellular functions including proliferation and invasion. In this study, we have therefore exposed tumor cells (derived from various tissues) to low pH conditions (pH 6.5) for several weeks. After acidic acclimation, we found that tumor cells proliferate at the same rate as parent cells maintained at pH 7.4. More interestingly, we documented that chronic low pH exposure triggers 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 measurement of [U13C5]-glutamine incorporation in metabolites (determined by GC-MS) and determination of the oxygen consumption rate (OCR) using a Seahorse microplate analyser. Mechanistic dissection of the low pH-driven phenotype led us to document that the metabolic switch was mediated by SIRT1, a NAD+-dependent protein deacetylase. We actually found that at acidic pH, SIRT1 promotes glutamine metabolism in a HIF2-dependent manner. By contrast, we observed a SIRT-1-mediated decrease in HIF1 signalling and associated glycolysis. These observations were repeated with the exact same conclusions using tumor cell lines of various origins (cervix SiHa, pharynx FaDu and colon HCT116). Also, the 'glutamine-addicted' phenotype was 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 observations 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.