Abstract B48: Oncogenic Ras reprograms metabolism by suppressing TXNIP translation
Authors: Ye Zhizhou , Ayer Donald E. 
Published: May 2017

Salt Lake City, UT.

Background: Oncogenic Ras promotes aerobic glycolysis and glutaminolysis to supply central carbon metabolism. Ras-transformed cells also upregulate autophagy and internalization of extracellular proteins and lipids to fuel their enhanced metabolic needs. Ras-mediated metabolic reprogramming generates sufficient ATP and macromolecule precursors to sustain rapid growth and proliferation.

Thioredoxin interacting protein (TXNIP) is a potent inhibitor of glucose uptake and aerobic glycolysis. It also promotes β-oxidation, apoptosis, and mediates cell cycle arrest. Consistent with these anti-proliferative functions, TXNIP is strongly downregulated in a variety of human cancers. Further, low TXNIP expression predicts poor clinical outcomes in breast cancer patients. TXNIP transcription and translation are tightly regulated in response to various intracellular and extracellular cues. Our lab previously discovered that TXNIP is acutely downregulated during the G0/G1 transition. Activation of the PI3K and MAPK pathways downregulate TXNIP transcription and translation, respectively. Interestingly, constitutively active RasG12V also suppresses TXNIP transcription and translation.

We hypothesize that TXNIP coordinates cell growth by functioning as a metabolic checkpoint. In transformed cells, persistent engagement of growth factor signaling pathways suppresses TXNIP to allow metabolic reprogramming for unchecked cell growth. Understanding the mechanism(s) by which TXNIP expression is inhibited by oncogenic signaling will shed light on novel therapeutic targets for Ras-driven cancers.

Results: In this study, we investigated the mechanism by which constitutively active RasG12V (Ras henceforth) suppresses TXNIP expression in mouse embryonic fibroblasts. Our results demonstrated that Ras is a potent suppressor of TXNIP mRNA and protein levels. By expressing TXNIP under a constitutive promoter, we discovered that Ras downregulates TXNIP protein level independent of its mRNA downregulation. Further, that repression resulted from inhibition of protein synthesis, rather than increased protein turnover. Polysome profiling experiments revealed that Ras inhibits translation elongation of TXNIP mRNA. Consistent with this result, the TXNIP coding region (CDS) was sufficient for Ras to stall translation elongation. Ras did not suppress TXNIP translation when it was encoded by a message with optimized codon usage. This finding suggests that poor codon fitness of the TXNIP CDS is required for its translational inhibition by Ras.

Conclusions: Our studies established that oncogenic Ras represses TXNIP expression by inhibiting translation elongation of the TXNIP mRNA. Given the prevalence of Ras activation and TXNIP downregulation in cancers, our findings suggest a novel route by which Ras-driven cancers rewire their metabolism to support their aggressive growth.

Because Ras activation upregulates global translation, our experiments suggest that the translation elongation machinery discriminates among mRNAs based on codon usage. Specifically, in non-transformed cells, the translation machinery can translate a spectrum of mRNAs with different codon adaptiveness. By contrast, Ras transformation places an increased demand on the translational machinery, resulting in a bias towards mRNAs with optimal codon usage over mRNAs like TXNIP with relatively poor codon usage.

Citation Format: Zhizhou Ye, Donald E. Ayer. Oncogenic Ras reprograms metabolism by suppressing TXNIP translation. [abstract]. In: Proceedings of the AACR Special Conference on Translational Control of Cancer: A New Frontier in Cancer Biology and Therapy; 2016 Oct 27-30; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2017;77(6 Suppl):Abstract nr B48.