Abstract A49: MondoA-TXNIP acts as a metabolic checkpoint for coordinated cell growth
Authors: Ye Zhizhou 
Published: May 2016

Abstract
Background: Mammalian cell growth and proliferation is tightly controlled by a coordinated response to nutrient, growth factor, and intracellular energy status. AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) are prototypical metabolic or stress “sensors” that regulate cellular homeostasis in response to the changing environment. Our lab recently defined the MondoA-TXNIP axis as a metabolic checkpoint that responds to the hypermetabolic state common to transformed cells. MondoA is a basic helix loop helix leucine zipper transcription factor. TXNIP is a potent negative regulator of glucose uptake and a direct transcriptional target of MondoA. Upon glucose stimulation, the glycolytic intermediate glucose-6-phosphate (G6P) drives nuclear accumulation of MondoA where it binds the TXNIP promoter and activates its expression. We propose that MondoA's G6P-dependent induction of TXNIP constitutes a negative feedback mechanism that restores glucose homeostasis in response to high glycolytic flux. The MondoA-TXNIP checkpoint also acts as a sensor in response to other metabolic stresses such as lactic acidosis (LA), a characteristic feature of the tumor microenvironment. LA-induced TXNIP expression restricts glucose uptake and aerobic glycolysis. Underscoring the clinical significance of this finding, TXNIP is a critical component of a LA-induced gene signature that correlates with good clinical outcome in breast cancer. Like many metabolic checkpoints, the MondoA-TXNIP checkpoint is inactivated by common oncogenic lesions such as c-Myc, PI3K and activated Ras. Based on these findings, we propose that oncogene inactivation of the MondoA-TXNIP checkpoint is a common route to aerobic glycolysis, which is required for tumor progression.
Results: In this study, we investigated the mechanisms by which the MondoA-TXNIP checkpoint is activated by metabolic stress and suppressed by oncogenic signaling. We used Hank's balanced salt solution (HBSS) as a TXNIP inducing stress condition that recapitulates characteristics of the tumor microenvironment, e.g. low pH, low growth factors and nutrients. We used constitutively active RasG12V to inactivate the MondoA-TXNIP checkpoint.
Our results demonstrated that HBSS induced TXNIP transcription (primarily) and translation. The low pH of HBSS was responsible for the transcriptional upregulation through increased MondoA occupancy at the TXNIP promoter. The highly-conserved histidines at MondoA N-terminus acted as pH sensors to regulate MondoA nuclear accumulation and DNA binding. We mapped the secondary translational component of HBSS induction to the 5' and 3' untranslated regions of the TXNIP mRNA.
Serum stimulation and RasG12V suppressed TXNIP induction by HBSS. The PI3K-AKT and Ras-MEK-p42/44MAPK pathways were responsible for the transcriptional and translational downregulation, respectively. The TXNIP coding region was sufficient for translational repression and we showed that growth factor signaling stalled elongating ribosomes on the TXNIP message. Combined with the very short half-live of the TXNIP protein, these regulatory mechanisms ensure a rapid metabolic response to extra and intracellular growth signals.
Conclusions: Our studies establish that a mimic of the nutrient-poor microenvironment, i.e. HBSS, triggers the MondoA-TXNIP checkpoint in a pH-dependent manner with highly conserved histidines playing a critical role. Growth factor signaling pathways restrict transcription of the TXNIP gene (PI3K) and translation of TXNIP mRNA (Ras-MAPK). Given the prevalence of PI3K and Ras pathway dysregulation in cancer, our findings suggest that their repression of the MondoA-TXNIP checkpoint restricts the normal homeostatic response to microenvironmental signals.
Citation Format: Zhizhou Ye. MondoA-TXNIP acts as a metabolic checkpoint for coordinated cell growth. [abstract]. In: Proceedings of the AACR Special Conference: Metabolism and Cancer; Jun 7-10, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(1_Suppl):Abstract nr A49.