Abstract 800: ESI-09 and HJC0197, known EPAC inhibitors, sensitize lung cancer cells to glucose starvation by uncoupling mitochondrial electron transport, leading to bioenergetic crisis
Authors: Iwai Yuki , Kikuchi Ryota , Nakamura Hiroyuki , Koyama Nobuyuki , Hagiwara Koichi , Aoshiba Kazutetsu 
Published: August 2019

Abstract
Cancer cells utilize aerobic glycolysis and proliferate continuously beyond the capacity of their blood supply, leading to microenvironmental stresses such as hypoxia, nutrient (glucose) deprivation and extracellular acidosis. We have previously showed that extracellular acidosis prevents glucose starvation-induced death of lung cancer cells by reducing glycolytic energy production and de novo protein/RNA synthesis that consume ATP (AACR 2018). This acidosis-dependent, glucose starvation-resistant, and ATP-saving phenotype is thought to serve as an adaptive response to glucose deprivation in cancer cells to survive an energy-restricted tumor microenvironment. In the present study, we screened small-molecule inhibitors to explore therapeutic reagents that can exert cytotoxicity against the glucose starvation-resistant phenotype in the lung cancer cell lines, A549 and H1299 cells. Among the various inhibitors tested, we found that ESI-09 and HJC0197, known as inhibitors of the exchanger protein directly activated by cAMP (EPAC), reduced the cellular ATP levels and survival under glucose deprivation stress under both acidic (pH 6.8) and neutral (pH 7.4) conditions. Interestingly, the ATP-reducing effect of ESI-09 and HJC0197 was not due to inhibition of the EPAC function, because the effect cannot be reproduced with other EPAC inhibitors, such as CE3F4, EPAC5376753 and ESI-05. Rather, the effect of ESI-09 and HJC0197 was attributed to their previously unknown action of uncoupling the mitochondrial electron transport chain (ETC). We found that ESI-09 and HJC0197 induced mitochondrial proton leak, causing ATP deficiency and cell death via the following three mechanisms: 1) decreased mitochondrial ATP production due to ETC uncoupling, 2) increased ATP consumption resulting from reversal of F0/F1-ATPsynthase/ATPase to maintain the mitochondrial membrane potential, and 3) glucose deficiency as a result of increased glucose consumption due to a compensatory stimulation of glycolytic ATP production. The energy-saving phenotype of cancer cells is thought to serve as an adaptive strategy that allows cell survival in an energy-restricted, acidic tumor microenvironment. Our results suggest that ESI-09 and HJC0197 disrupt bioenergetic tumor metabolism for energy homeostasis in both acidic and neutral microenvironment, and may exert a therapeutic effect against the starvation-resistant phenotype of cancer cells.
The complete chemical structures of ESI-09 and HJC0197 will be divulged at the time of the presentation at the meeting.
Citation Format: Yuki Iwai, Ryota Kikuchi, Hiroyuki Nakamura, Nobuyuki Koyama, Koichi Hagiwara, Kazutetsu Aoshiba. ESI-09 and HJC0197, known EPAC inhibitors, sensitize lung cancer cells to glucose starvation by uncoupling mitochondrial electron transport, leading to bioenergetic crisis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 800.