Molecular and Cellular Pathobiology The SIRT1/HIF2a Axis Drives Reductive Glutamine Metabolism under Chronic Acidosis and Alters Tumor Response to Therapy

Extracellular tumor acidosis largely results from an exacerbated glycolytic flux in cancer and cancerassociated cells. Conversely, little is known about how tumor cells adapt their metabolism to acidosis. Here, we demonstrate that long-term exposure of cancer cells to acidic pH leads to a metabolic reprogramming toward glutamine metabolism. This switch is triggered by the need to reduce the production of protons from glycolysis and further maintained by the NADþ-dependent increase in SIRT1 deacetylase activity to ensure intracellular pH homeostasis. A consecutive increase in HIF2a activity promotes the expression of various transporters and enzymes supporting the reductive and oxidative glutamine metabolism, whereas a reduction in functional HIF1a expression consolidates the inhibition of glycolysis. Finally, in vitro and in vivo experiments document that acidosis accounts for a net increase in tumor sensitivity to inhibitors of SIRT1 and glutaminase GLS1. These findings highlight the influence that tumor acidosis and metabolism exert on each other. Cancer Res; 74(19); 1–13. 2014 AACR. Introduction Acidosis and cycling hypoxia are two well-known physicochemical properties of the tumormicroenvironment (1, 2). The release of lactate (the end product of glycolysis) together with a proton by hypoxic tumor cells but also the hydratation of CO2 into bicarbonate and proton (by tumor cells that have access to O2) contribute to the acidification of the tumor microenvironment. Large amounts of lactate/Hþ are also released by tumor cells exhibiting aerobic glycolysis (the so-calledWarburg effect; refs. 3 and 4). Extracellular pH (pHe) has actually been determined in a wide variety of cancers to be significantly more acidic than in normal tissues, with values ranging from 5.9 to 7.2 (5, 6). Acidosis is known to contribute to the genetic instability of tumor cells (7) and to profoundly alter their transcriptomic profile (8), leading to phenotypes that are particularly suited for survival and growth in an acidic environment. A low pHe has for instance a wide impact on cancer progression by promoting tumor cell migration, invasion, and metastasis (9–11) and by stimulating angiogenesis (12–14). Although tumor metabolic peculiarities directly account for the acidification of the tumor microenvironment, whether acidosis may itself influence metabolism remains elusive. One may however reason that when the extracellular acidic stress increases or persists, reducing the source of Hþ by shifting the metabolic preference may also represent an option for tumor cells. Interestingly, although glucose metabolism is generally viewed as themain contributor to acidic tumor pHe, glutamine metabolism is less prone to Hþ production. Glutamine can for instance support anaplerosis/cataplerosis without the need for tricarboxylic acid (TCA) cycle to be coupled to OXPHOS and may thus supply tumor cells with ATP as well as a large variety of biosynthetic precursors and redox equivalents without producing carbonic acid (15–17). Although a few studies have addressed the effects of acute changes in pHe on tumor metabolism (8, 18), interpretation is usually complicated by the overall tumor cell death associated with the rapid exposure of tumor cells to an acidic pH. In this study, we postulated that long-term selection of tumor cells able to survive and proliferate under acidic conditions could offer more relevant models to get insights on the influence of acidosis on tumor metabolism. We found that despite a similar proliferation rate in tumor cells adapted to acidic pH and in parental cells (maintained at pH 7.4), a metabolic shift from a largely glycolytic metabolism toward the reductive glutamine metabolism was actually observed in response to chronic acidic conditions. The capacity of different tumor cells to develop resistance to the intracellular acidification was associated with an increase in SIRT1-driven protein deacetylation leading to a reduction in Pole of Pharmacology and Therapeutics (FATH), Institut de Recherche Exp erimentale et Clinique (IREC), Universit e catholique de Louvain, Brussels, Belgium. Molecules, Solids and Reactivity (MOST), Institute of Condensed Matter and Nanosciences (IMCN), Universit e catholique de Louvain, Louvain-la-Neuve, Belgium. Note: Supplementary data for this article are available at Cancer Research Online (http://cancerres.aacrjournals.org/). Corresponding Author: Olivier Feron, Pole of Pharmacology and Therapeutics (FATH), Institut de Recherche Exp erimentale et Clinique (IREC), UCLouvain, 53 Avenue E. Mounier, B1.53.09, B-1200 Brussels, Belgium. Phone: 32-2-7645264; Fax: 32-2-7645269; E-mail: [email protected] doi: 10.1158/0008-5472.CAN-14-0705 2014 American Association for Cancer Research. Cancer Research www.aacrjournals.org OF1 Research. on April 15, 2017. © 2014 American Association for Cancer cancerres.aacrjournals.org Downloaded from Published OnlineFirst August 1, 2014; DOI: 10.1158/0008-5472.CAN-14-0705