MondoA-Mlx Transcriptional Activity Is Limited by mTOR-MondoA Interaction

Abstract

Mammalian target of rapamycin (mTOR) integrates multiple signals, including nutrient status, growth factor availability, and stress, to regulate cellular and organismal growth. How mTOR regulates transcriptional programs in response to these diverse stimuli is poorly understood. MondoA and its obligate transcription partner Mlx are basic helix-loop-helix leucine zipper (bHLHZip) transcription factors that sense and execute a glucose-responsive transcriptional program. MondoA-Mlx complexes activate expression of thioredoxin-interacting protein (TXNIP), which is a potent inhibitor of cellular glucose uptake and aerobic glycolysis. Both mTOR and MondoA are central regulators of glucose metabolism, yet whether they interact physically or functionally is unknown. We show that inhibition of mTOR induces MondoA-dependent expression of TXNIP, coinciding with reduced glucose uptake. Mechanistically, mTOR binds to MondoA in the cytoplasm and prevents MondoA-Mlx complex formation, restricting MondoA's nuclear entry and reducing TXNIP expression. Further, we show that mTOR inhibitors and reactive oxygen species (ROS) regulate interaction between MondoA and mTOR in an opposing manner. Like mTOR's suppression of the MondoA-TXNIP axis, MondoA can also suppress mTOR complex 1 (mTORC1) activity via its direct transcriptional regulation of TXNIP. Collectively, these studies reveal a regulatory relationship between mTOR and the MondoA-TXNIP axis that we propose contributes to glucose homeostasis. Mammalian target of rapamycin (mTOR) plays a central role in cell growth and proliferation, and its dysregulation contributes to many diseases, including cancer and diabetes. mTOR is a Ser/Thr kinase that exists in two functionally distinct multiprotein complexes, rapamycin-sensitive mTOR complex 1 (mTORC1) and rapamycin-insensitive mTOR complex 2 (mTORC2). mTORC1 promotes protein synthesis by directly phosphorylating ribosomal S6 kinase (S6K) and eukaryotic translation initiation factor 4E (4E-BP1), whereas mTORC2 promotes cell survival and metabolism by phosphorylating Akt (1). Inhibition of mTOR affects global transcription, with genes driving anabolic or catabolic processes being generally repressed or activated, respectively (2-6). Because mTOR integrates progrowth and growth-suppressive signals, it is likely that mTOR regulates diverse transcription factors and chromatin modifiers to induce or repress gene expression; however, our knowledge of the transcription factors regulated by mTOR is limited (7,8).We focus on a family of transcription factors that respond to environmental and intracellular cues. MondoA and its dimerization partner Mlx are members of the basic helix-loop-helix leucine zipper (bHLHZip) family of transcription factors (9). MondoAMlx complexes shuttle between the cytosol and the nucleus; however, in response to high levels of glucose, they accumulate in the nucleus, bind the promoters of target genes, and regulate their expression. MondoA-Mlx complexes are important, perhaps the principal, regulators of glucose-in...

mTOR and HDAC Inhibitors Converge on the TXNIP/Thioredoxin Pathway to Cause Catastrophic Oxidative Stress and Regression of RAS-Driven Tumors

“…Interestingly, mTOR has been shown to negatively regulate MondoA, a basic helix-loop helix leucine zipper transcription factor that functions in an obligate heterodimer with Mlx. The MondoA-Mlx interaction is suppressed by mTOR activation (44), and is one of two transcriptional complexes known to regulate TXNIP expression (43,45). To determine whether MondoA-Mlx was regulating TXNIP transcription in this therapeutic context, MondoA expression was ablated using pooled siRNAs that target the gene that encodes MondoA, MLXIP (Fig.…”

Protein synthesis inhibitors stimulate MondoA transcriptional activity by driving an accumulation of glucose 6-phosphate

“…It is counter-intuitive that TXNIP protein would accumulate in the presence the translation initiation inhibitor RocA and following knockdown of EIF4E; however, TXNIP undergoes both cap-dependent and IRES-dependent translation (32). Our previous studies demonstrated that mTORC1 suppresses MondoA transcriptional activity and TXNIP expression by competing for its obligate transcriptional partner Mlx (19). Consistent with our previous findings, the mTORC1 inhibitor Torin increased TXNIP expression, but this increase was much more modest than that observed with CHX ( Figure 1H).…”

“…MondoA-dependent ( Figure 6A-B Cancer cells must coordinate the use and the availability of nutrients to support growth and division. TXNIP is a potent negative regulator of glucose uptake, in fact its loss or downregulation is sufficient to increase glucose uptake (19,40), suggesting that low TXNIP levels may be a common route to aerobic glycolysis common in cancer. Consistent with this hypothesis, TXNIP levels are generally lower in tumors compared to normal adjacent tissues (8), and a number of pro-growth/oncogenic pathways suppress TXNIP expression by a variety of mechanisms (19,20,35,(41)(42)(43).…”

Ras Suppresses TXNIP Expression by Restricting Ribosome Translocation

“…Thus, the MondoA-TXNIP axis constitutes a negative feedback loop that regulates glucose homeostasis (15, 25, 26). Our previous work established that the MondoA-TXNIP axis is downregulated by PI3K, mTOR or Myc activation, which contributes to their well characterized activities in driving aerobic glycolysis (20, 27, 28). TXNIP is also subject to post-translational regulation in response to signaling pathways.…”