Abstract 2407: Extracellular acidosis inhibits glucose starvation-induced lung cancer cell death by suppressing overall metabolic rate
Authors: Iwai Yuki , Kikuchi Ryota , Watanabe Yasutaka , Koyama Nobuyuki , Hagiwara Koichi , Nakamura Hiroyuki , Aoshiba Kazutetsu 
Published: August 2018

Abstract
Bioenergetic tumor metabolism is essential to meet ATP requirements for energy homeostasis. Cancer cells utilize aerobic glycolysis and proliferate continuously beyond the capacity of blood supply. These phenotypic alterations lead to various microenvironmental stresses, such as hypoxia, nutrient (glucose) deprivation, and extracellular acidosis, to which cancer cells must metabolically adapt in order to survive and proliferate. However, compared to hypoxia, relatively little is known about the metabolic adaptations in response to glucose deprivation and acidosis. In the present study, we explored the effects of extracellular acidosis on cancer metabolism under glucose deprivation stress, as extracellular acidosis and insufficient glucose supply usually co-exist in the tumor microenvironment. To this aim, A549 lung cancer cells were treated with acidosis, glucose deprivation, or a combination. Then, cell survival, total cellular ATP turnover, relative glycolytic and mitochondrial dependency and capacity, and de novo rRNA and protein synthesis were determined. We found that acidosis (pH 6.8) inhibited ATP depletion and cell death induced by glucose deprivation. The cytoprotective effect of acidosis against glucose deprivation-induced cell death was mediated by two distinct mechanisms. First, acidosis shifted cellular ATP production from glycolysis to OXPHOS that became independent of glucose uptake but dependent on glutaminolysis and fatty acid β-oxidation. Second, acidosis reduced ATP consumption by inhibiting de novo rRNA and protein synthesis that utilizes ATP. In contrast, the protective effect of acidosis on glucose starvation-induced cell death was independent of AMPK, PI3K, ERK, and p38 MAPK. Based on this mechanistic knowledge of acidosis-dependent metabolic alterations, we tested the effect of the mitochondrial complex I inhibitor metformin on death of lung cancer cells. Metformin and a combination of rotenone and AICAR mimicking an action of metformin induced ATP depletion and cell death in glucose-free medium, but under acidosis the cytotoxic effect of metformin was significantly attenuated because overall ATP turnover was reduced under low pH. These results suggest that during acidosis, cells are forced to a reduced glycolytic energy production, but the decrease in protein synthesis and ATP requirement serves as an adaptive response to glucose deprivation stress. This acidosis-dependent, glucose starvation-resistant phenotype may provide an adaptive strategy in an energy-restricted tumor microenvironment. Our findings highlight the importance of considering the two factors, acidosis and nutrient starvation in combination to understand their contributions to tumor metabolism.
Citation Format: Yuki Iwai, Ryota Kikuchi, Yasutaka Watanabe, Nobuyuki Koyama, Koichi Hagiwara, Hiroyuki Nakamura, Kazutetsu Aoshiba. Extracellular acidosis inhibits glucose starvation-induced lung cancer cell death by suppressing overall metabolic rate [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2407.