Extracellular Matrix Remodeling Regulates Glucose Metabolism through TXNIP Destabilization.

The metabolic state of a cell is influenced by cell-extrinsic factors, including nutrient availability and growth factor signaling. Here, we present extracellular matrix (ECM) remodeling as another fundamental node of cell-extrinsic metabolic regulation. Unbiased analysis of glycolytic drivers identified the hyaluronan-mediated motility receptor as being among the most highly correlated with glycolysis in cancer. Confirming a mechanistic link between the ECM component hyaluronan and metabolism, treatment of cells and xenografts with hyaluronidase triggers a robust increase in glycolysis. This is largely achieved through rapid receptor tyrosine kinase-mediated induction of the mRNA decay factor ZFP36, which targets TXNIP transcripts for degradation. Because TXNIP promotes internalization of the glucose transporter GLUT1, its acute decline enriches GLUT1 at the plasma membrane. Functionally, induction of glycolysis by hyaluronidase is required for concomitant acceleration of cell migration. This interconnection between ECM remodeling and metabolism is exhibited in dynamic tissue states, including tumorigenesis and embryogenesis.

TXNIP loss expands Myc-dependent transcriptional programs by increasing Myc genomic binding.

The c-Myc protooncogene places a demand on glucose uptake to drive glucose-dependent biosynthetic pathways. To meet this demand, c-Myc protein (Myc henceforth) drives the expression of glucose transporters, glycolytic enzymes, and represses the expression of thioredoxin interacting protein (TXNIP), which is a potent negative regulator of glucose uptake. A Mychigh/TXNIPlow gene signature is clinically significant as it correlates with poor clinical prognosis in triple-negative breast cancer (TNBC) but not in other subtypes of breast cancer, suggesting a functional relationship between Myc and TXNIP. To better understand how TXNIP contributes to the aggressive behavior of TNBC, we generated TXNIP null MDA-MB-231 (231:TKO) cells for our study. We show that TXNIP loss drives a transcriptional program that resembles those driven by Myc and increases global Myc genome occupancy. TXNIP loss allows Myc to invade the promoters and enhancers of target genes that are potentially relevant to cell transformation. Together, these findings suggest that TXNIP is a broad repressor of Myc genomic binding. The increase in Myc genomic binding in the 231:TKO cells expands the Myc-dependent transcriptome we identified in parental MDA-MB-231 cells. This expansion of Myc-dependent transcription following TXNIP loss occurs without an apparent increase in Myc's intrinsic capacity to activate transcription and without increasing Myc levels. Together, our findings suggest that TXNIP loss mimics Myc overexpression, connecting Myc genomic binding and transcriptional programs to the nutrient and progrowth signals that control TXNIP expression.

The glucose-sensing transcription factor MLX balances metabolism and stress to suppress apoptosis and maintain spermatogenesis.

Male germ cell (GC) production is a metabolically driven and apoptosis-prone process. Here, we show that the glucose-sensing transcription factor (TF) MAX-Like protein X (MLX) and its binding partner MondoA are both required for male fertility in the mouse, as well as survival of human tumor cells derived from the male germ line. Loss of Mlx results in altered metabolism as well as activation of multiple stress pathways and GC apoptosis in the testes. This is concomitant with dysregulation of the expression of male-specific GC transcripts and proteins. Our genomic and functional analyses identify loci directly bound by MLX involved in these processes, including metabolic targets, obligate components of male-specific GC development, and apoptotic effectors. These in vivo and in vitro studies implicate MLX and other members of the proximal MYC network, such as MNT, in regulation of metabolism and differentiation, as well as in suppression of intrinsic and extrinsic death signaling pathways in both spermatogenesis and male germ cell tumors (MGCTs).

Metabolic reprogramming in triple-negative breast cancer through Myc suppression of TXNIP.

Triple-negative breast cancers (TNBCs) are aggressive and lack targeted therapies. Understanding how nutrients are used in TNBCs may provide new targets for therapeutic intervention. We demonstrate that the transcription factor c-Myc drives glucose metabolism in TNBC cells but does so by a previously unappreciated mechanism that involves direct repression of thioredoxin-interacting protein (TXNIP). TXNIP is a potent negative regulator of glucose uptake, aerobic glycolysis, and glycolytic gene expression; thus its repression by c-Myc provides an alternate route to c-Myc-driven glucose metabolism. c-Myc reduces TXNIP gene expression by binding to an E-box-containing region in the TXNIP promoter, possibly competing with the related transcription factor MondoA. TXNIP suppression increases glucose uptake and drives a dependence on glycolysis. Ectopic TXNIP expression decreases glucose uptake, reduces cell proliferation, and increases apoptosis. Supporting the biological significance of the reciprocal relationship between c-Myc and TXNIP, a Mychigh/TXNIPlow gene signature correlates with decreased overall survival and decreased metastasis-free survival in breast cancer. The correlation between the Mychigh/TXNIPlow gene signature and poor clinical outcome is evident only in TNBC, not in other breast cancer subclasses. Mutation of TP53, which is a defining molecular feature of TNBC, enhances the correlation between the Mychigh/TXNIPlow gene signature and death from breast cancer. Because Myc drives nutrient utilization and TXNIP restricts glucose availability, we propose that the Mychigh/TXNIPlow gene signature coordinates nutrient utilization with nutrient availability. Further, our data suggest that loss of the p53 tumor suppressor cooperates with Mychigh/TXNIPlow-driven metabolic dysregulation to drive the aggressive clinical behavior of TNBC.

Interactions between Myc and MondoA transcription factors in metabolism and tumourigenesis.

Metabolic reprogramming towards aerobic glycolysis is a common feature of transformed cells and can be driven by a network of transcription factors. It is well established that c-Myc and hypoxia-inducible factor-1α (HIF-1α) contribute to metabolic reprogramming by driving the expression of glycolytic target genes. More recently, the c-Myc-related transcription factor MondoA has been shown to restrict glucose uptake and aerobic glycolysis via its induction of thioredoxin-interacting protein (TXNIP). Three recent studies demonstrate that complex and cancer type-specific interactions between c-Myc, MondoA and HIF-1α underlie metabolism, tumourigenesis and drug response. In triple-negative breast cancer, c-Myc blocks MondoA-dependent activation of TXNIP to stimulate aerobic glycolysis. In contrast, in neuroblastoma, N-Myc requires MondoA for metabolic reprogramming and tumourigenesis. Finally, the therapeutic response of BRAF(V600E) melanoma cells to vemurafenib requires downregulation of c-Myc and HIF-1α and upregulation of MondoA-TXNIP, and the subsequent reprogramming away from aerobic glycolysis. In this minireview we highlight the findings in these three studies and present a working model to explain why c-Myc and MondoA function cooperatively in some cancers and antagonistically in others.

MondoA senses adenine nucleotides: transcriptional induction of thioredoxin-interacting protein.

The MondoA-Mlx transcription complex plays a pivotal role in glucose homoeostasis by activating target gene expression in response to G6P (glucose 6-phosphate), the first reaction intermediate in glycolysis. TXNIP (thioredoxin-interacting protein) is a direct and glucose-responsive target of MondoA that triggers a negative-feedback loop by restricting glucose uptake when G6P levels increase. We show in the present study that TXNIP expression is also activated by AICAR (5-amino-4-imidazolecarboxamide ribofuranoside) and adenosine. Using pharmacological inhibitors and genetic knockdowns of purine metabolic enzymes, we establish that TXNIP induction by AICAR and adenosine requires their cellular uptake and metabolism to adenine nucleotides. AICAR induction of TXNIP depended on MondoA, but was independent of AMPK (AMP-activated protein kinase) activation and calcium. The findings of the present study have two important implications. First, in addition to activating AMPK, AICAR may have AMPK-independent effects on gene expression by regulating MondoA-Mlx activity following its flux into the adenine nucleotide pool. Secondly, MondoA-Mlx complexes sense elevated levels of G6P and adenine nucleotides to trigger a TXNIP-dependent feedback inhibition of glycolysis. We propose that this mechanism serves as a checkpoint to restore metabolic homoeostasis.

Adaptive metabolic response to 4 weeks of sugar-sweetened beverage consumption in healthy, lightly active individuals and chronic high glucose availability in primary human myotubes.

PURPOSE: Chronic sugar-sweetened beverage (SSB) consumption is associated with obesity and type 2 diabetes mellitus (T2DM). Hyperglycaemia contributes to metabolic alterations observed in T2DM, such as reduced oxidative capacity and elevated glycolytic and lipogenic enzyme expressions in skeletal muscle tissue. We aimed to investigate the metabolic alterations induced by SSB supplementation in healthy individuals and to compare these with the effects of chronic hyperglycaemia on primary muscle cell cultures. METHODS: Lightly active, healthy, lean subjects (n = 11) with sporadic soft drink consumption underwent a 4-week SSB supplementation (140 ± 15 g/day, ~2 g glucose/kg body weight/day, glucose syrup). Before and after the intervention, body composition, respiratory exchange ratio (RER), insulin sensitivity, muscle metabolic gene and protein expression were assessed. Adaptive responses to hyperglycaemia (7 days, 15 mM) were tested in primary human myotubes. RESULTS: SSB supplementation increased fat mass (+1.0 kg, P < 0.05), fasting RER (+0.12, P < 0.05), fasting glucose (+0.3 mmol/L, P < 0.05) and muscle GAPDH mRNA expressions (+0.94 AU, P < 0.05). PGC1α mRNA was reduced (-0.20 AU, P < 0.05). Trends were found for insulin resistance (+0.16 mU/L, P = 0.09), and MondoA protein levels (+1.58 AU, P = 0.08). Primary myotubes showed elevations in GAPDH, ACC, MondoA and TXNIP protein expressions (P < 0.05). CONCLUSION: Four weeks of SSB supplementation in healthy individuals shifted substrate metabolism towards carbohydrates, increasing glycolytic and lipogenic gene expression and reducing mitochondrial markers. Glucose-sensing protein MondoA might contribute to this shift, although further in vivo evidence is needed to corroborate this.

Bioluminescence Assay of Lysine Deacylase Sirtuin Activity.

Lysine acylation can direct protein function, localization, and interactions. Sirtuins deacylate lysine towards maintaining cellular homeostasis, and their aberrant expression contributes to the pathogenesis of multiple pathological conditions, including cancer. Measuring sirtuins' activity is essential to exploring their potential as therapeutic targets, but accurate quantification is challenging. We developed 'SIRTify', a high-sensitivity assay for measuring sirtuin activity in vitro and in vivo. SIRTify is based on a split-version of the NanoLuc® luciferase consisting of a truncated, catalytically inactive N-terminal moiety (LgBiT) that complements with a high-affinity C-terminal peptide (p86) to form active luciferase. Acylation of two lysines within p86 disrupts binding to LgBiT and abates luminescence. Deacylation by sirtuins reestablishes p86 and restores binding, generating a luminescence signal proportional to sirtuin activity. Measurements accurately reflect reported sirtuin specificity for lysine acylations and confirm the effects of sirtuin modulators. SIRTify effectively quantifies lysine deacylation dynamics and may be adaptable to monitoring additional post-translational modifications.

MondoA senses non-glucose sugars: regulation of thioredoxin-interacting protein (TXNIP) and the hexose transport curb.

Glucose is required for cell growth and proliferation. The MondoA·Mlx transcription factor is glucose-responsive and accumulates in the nucleus by sensing glucose 6-phosphate. One direct and glucose-induced target of MondoA·Mlx complexes is thioredoxin-interacting protein (TXNIP). TXNIP is a potent negative regulator of glucose uptake, and hence its regulation by MondoA·Mlx triggers a feedback loop that restricts glucose uptake. This feedback loop is similar to the "hexose transport curb" first described almost 30 years ago. We show here that MondoA responds to the non-glucose hexoses, allose, 3-O-methylglucose, and glucosamine by accumulating in the nucleus and activating TXNIP transcription. The metabolic inhibitor 3-bromopyruvate blocks the transcriptional response to allose and 3-O-methylglucose, indicating that their metabolism, or a parallel pathway, is required to stimulate MondoA activity. Our dissection of the hexosamine biosynthetic pathway suggests that in addition to sensing glucose 6-phosphate, MondoA can also sense glucosamine 6-phosphate. Analysis of glucose uptake in wild-type, MondoA-null, or TXNIP-null murine embryonic fibroblasts indicates a role for the MondoA-TXNIP regulatory circuit in the hexose transport curb, although other redundant pathways also contribute.

ING2 PHD domain links histone H3 lysine 4 methylation to active gene repression.

Dynamic regulation of diverse nuclear processes is intimately linked to covalent modifications of chromatin. Much attention has focused on methylation at lysine 4 of histone H3 (H3K4), owing to its association with euchromatic genomic regions. H3K4 can be mono-, di- or tri-methylated. Trimethylated H3K4 (H3K4me3) is preferentially detected at active genes, and is proposed to promote gene expression through recognition by transcription-activating effector molecules. Here we identify a novel class of methylated H3K4 effector domains--the PHD domains of the ING (for inhibitor of growth) family of tumour suppressor proteins. The ING PHD domains are specific and highly robust binding modules for H3K4me3 and H3K4me2. ING2, a native subunit of a repressive mSin3a-HDAC1 histone deacetylase complex, binds with high affinity to the trimethylated species. In response to DNA damage, recognition of H3K4me3 by the ING2 PHD domain stabilizes the mSin3a-HDAC1 complex at the promoters of proliferation genes. This pathway constitutes a new mechanism by which H3K4me3 functions in active gene repression. Furthermore, ING2 modulates cellular responses to genotoxic insults, and these functions are critically dependent on ING2 interaction with H3K4me3. Together, our findings establish a pivotal role for trimethylation of H3K4 in gene repression and, potentially, tumour suppressor mechanisms.

Glutamine-dependent anapleurosis dictates glucose uptake and cell growth by regulating MondoA transcriptional activity.

Glucose and glutamine are abundant nutrients required for cell growth, yet how cells sense and adapt to changes in their levels is not well understood. The MondoA transcription factor forms a heterocomplex with its obligate partner Mlx to regulate approximately 75% of glucose-dependent transcription. By mediating glucose-induced activation of thioredoxin-interacting protein (TXNIP), MondoA:Mlx complexes directly repress glucose uptake. We show here that glutamine inhibits transcriptional activation of TXNIP by triggering the recruitment of a histone deacetylase-dependent corepressor to the amino terminus of MondoA. Therefore, in the presence of both glucose and glutamine, TXNIP expression is low, which favors glucose uptake and aerobic glycolysis; the Warburg effect. Consistent with MondoA functioning upstream of TXNIP, MondoA knockdown reduces TXNIP expression, elevates glucose uptake and stimulates cell proliferation. Although glutamine has many intracellular fates, a cell permeable analog of a tricarboxylic acid cycle (TCA) intermediate, alpha-ketoglutarate, also blocks the transcriptional activity of MondoA at the TXNIP promoter and stimulates glucose uptake. Together our data suggest that glutamine-dependent mitochondrial anapleurosis dictates glucose uptake and aerobic glycolysis by blocking MondoA:Mlx-dependent transcriptional activation of TXNIP. We propose that this previously unappreciated coordination between glutamine and glucose utilization defines a metabolic checkpoint that restricts cell growth when subthreshold levels of these essential nutrients are available.

Age-associated decline of MondoA drives cellular senescence through impaired autophagy and mitochondrial homeostasis.

Accumulation of senescent cells affects organismal aging and the prevalence of age-associated disease. Emerging evidence suggests that activation of autophagy protects against age-associated diseases and promotes longevity, but the roles and regulatory mechanisms of autophagy in cellular senescence are not well understood. Here, we identify the transcription factor, MondoA, as a regulator of cellular senescence, autophagy, and mitochondrial homeostasis. MondoA protects against cellular senescence by activating autophagy partly through the suppression of an autophagy-negative regulator, Rubicon. In addition, we identify peroxiredoxin 3 (Prdx3) as another downstream regulator of MondoA essential for mitochondrial homeostasis and autophagy. Rubicon and Prdx3 work independently to regulate senescence. Furthermore, we find that MondoA knockout mice have exacerbated senescence during ischemic acute kidney injury (AKI), and a decrease of MondoA in the nucleus is correlated with human aging and ischemic AKI. Our results suggest that decline of MondoA worsens senescence and age-associated disease.

Glucose sensing by MondoA:Mlx complexes: a role for hexokinases and direct regulation of thioredoxin-interacting protein expression.

Glucose is a fundamental metabolite, yet how cells sense and respond to changes in extracellular glucose concentration is not completely understood. We recently reported that the MondoA:Mlx dimeric transcription factor directly regulates glycolysis. In this article, we consider whether MondoA:Mlx complexes have a broader role in sensing and responding to glucose status. In their latent state, MondoA:Mlx complexes localize to the outer mitochondrial membrane, yet shuttle between the mitochondria and the nucleus. We show that MondoA:Mlx complexes accumulate in the nucleus in response to glucose and 2-deoxyglucose (2-DG). Furthermore, nuclear localization of MondoA:Mlx depends on the enzymatic activity of hexokinases. These enzymes catalyze conversion of glucose to glucose-6-phosphate (G6P), which is the first step in the glycolytic pathway. Together, these findings suggest that MondoA:Mlx monitors intracellular G6P concentration and translocates to the nucleus when levels of this key metabolite increase. Transcriptional profiling experiments demonstrate that MondoA is required for >75% of the 2-DG-induced transcription signature. We identify thioredoxin-interacting protein (TXNIP) as a direct and glucose-regulated MondoA:Mlx transcriptional target. Furthermore, MondoA:Mlx complexes, via their regulation of TXNIP, are potent negative regulators of glucose uptake. These studies suggest a key role for MondoA:Mlx complexes in the adaptive transcriptional response to changes in extracellular glucose concentration and peripheral glucose uptake.

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

BACKGROUND: Protein synthesis is regulated by the availability of amino acids, the engagement of growth factor signaling pathways, and adenosine triphosphate (ATP) levels sufficient to support translation. Crosstalk between these inputs is extensive, yet other regulatory mechanisms remain to be characterized. For example, the translation initiation inhibitor rocaglamide A (RocA) induces thioredoxin-interacting protein (TXNIP). TXNIP is a negative regulator of glucose uptake; thus, its induction by RocA links translation to the availability of glucose. MondoA is the principal regulator of glucose-induced transcription, and its activity is triggered by the glycolytic intermediate, glucose 6-phosphate (G6P). MondoA responds to G6P generated by cytoplasmic glucose and mitochondrial ATP (mtATP), suggesting a critical role in the cellular response to these energy sources. TXNIP expression is entirely dependent on MondoA; therefore, we investigated how protein synthesis inhibitors impact its transcriptional activity. METHODS: We investigated how translation regulates MondoA activity using cell line models and loss-of-function approaches. We examined how protein synthesis inhibitors effect gene expression and metabolism using RNA-sequencing and metabolomics, respectively. The biological impact of RocA was evaluated using cell lines and patient-derived xenograft organoid (PDxO) models. RESULTS: We discovered that multiple protein synthesis inhibitors, including RocA, increase TXNIP expression in a manner that depends on MondoA, a functional electron transport chain and mtATP synthesis. Furthermore, RocA and cycloheximide increase mtATP and G6P levels, respectively, and TXNIP induction depends on interactions between the voltage-dependent anion channel (VDAC) and hexokinase (HK), which generates G6P. RocA treatment impacts the regulation of ~ 1200 genes, and ~ 250 of those genes are MondoA-dependent. RocA treatment is cytotoxic to triple negative breast cancer (TNBC) cell lines and shows preferential cytotoxicity against estrogen receptor negative (ER-) PDxO breast cancer models. Finally, RocA-driven cytotoxicity is partially dependent on MondoA or TXNIP. CONCLUSIONS: Our data suggest that protein synthesis inhibitors rewire metabolism, resulting in an increase in mtATP and G6P, the latter driving MondoA-dependent transcriptional activity. Further, MondoA is a critical component of the cellular transcriptional response to RocA. Our functional assays suggest that RocA or similar translation inhibitors may show efficacy against ER- breast tumors and that the levels of MondoA and TXNIP should be considered when exploring these potential treatment options.

MondoA-Mlx heterodimers are candidate sensors of cellular energy status: mitochondrial localization and direct regulation of glycolysis.

Transcription factors can be sequestered at specific organelles and translocate to the nucleus in response to changes in organellar homeostasis. MondoA is a basic helix-loop-helix leucine zipper transcriptional activator similar to Myc in function. However, unlike Myc, MondoA and its binding partner Mlx localize to the cytoplasm, suggesting tight regulation of their nuclear function. We show here that endogenous MondoA and Mlx associate with mitochondria in primary skeletal muscle cells and erythroblast K562 cells. Interaction between MondoA and the mitochondria is salt and protease sensitive, demonstrating that it associates with the outer mitochondrial membrane by binding a protein partner. Further, endogenous MondoA shuttles between the mitochondria and the nucleus, suggesting that it communicates between these two organelles. When nuclear, MondoA activates transcription of a broad spectrum of metabolic genes, including those for the glycolytic enzymes lactate dehydrogenase A, hexokinase II, and 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3. Regulation of these three targets is mediated by direct interaction with CACGTG sites in their promoters. Consistent with its regulation of glycolytic targets, MondoA is both necessary and sufficient for glycolysis. We propose that MondoA communicates information about the intracellular energy state between the mitochondria and the nucleus, resulting in transcriptional activation of glycolytic target genes.

Cellular acidosis triggers human MondoA transcriptional activity by driving mitochondrial ATP production.

Human MondoA requires glucose as well as other modulatory signals to function in transcription. One such signal is acidosis, which increases MondoA activity and also drives a protective gene signature in breast cancer. How low pH controls MondoA transcriptional activity is unknown. We found that low pH medium increases mitochondrial ATP (mtATP), which is subsequently exported from the mitochondrial matrix. Mitochondria-bound hexokinase transfers a phosphate from mtATP to cytoplasmic glucose to generate glucose-6-phosphate (G6P), which is an established MondoA activator. The outer mitochondrial membrane localization of MondoA suggests that it is positioned to coordinate the adaptive transcriptional response to a cell's most abundant energy sources, cytoplasmic glucose and mtATP. In response to acidosis, MondoA shows preferential binding to just two targets, TXNIP and its paralog ARRDC4. Because these transcriptional targets are suppressors of glucose uptake, we propose that MondoA is critical for restoring metabolic homeostasis in response to high energy charge.

MondoA drives muscle lipid accumulation and insulin resistance.

Obesity-related insulin resistance is associated with intramyocellular lipid accumulation in skeletal muscle. We hypothesized that in contrast to current dogma, this linkage is related to an upstream mechanism that coordinately regulates both processes. We demonstrate that the muscle-enriched transcription factor MondoA is glucose/fructose responsive in human skeletal myotubes and directs the transcription of genes in cellular metabolic pathways involved in diversion of energy substrate from a catabolic fate into nutrient storage pathways including fatty acid desaturation and elongation, triacylglyeride (TAG) biosynthesis, glycogen storage, and hexosamine biosynthesis. MondoA also reduces myocyte glucose uptake by suppressing insulin signaling. Mice with muscle-specific MondoA deficiency were partially protected from insulin resistance and muscle TAG accumulation in the context of diet-induced obesity. These results identify MondoA as a nutrient-regulated transcription factor that under normal physiological conditions serves a dynamic checkpoint function to prevent excess energy substrate flux into muscle catabolic pathways when myocyte nutrient balance is positive. However, in conditions of chronic caloric excess, this mechanism becomes persistently activated leading to progressive myocyte lipid storage and insulin resistance.

Ras Suppresses TXNIP Expression by Restricting Ribosome Translocation.

Oncogenic Ras upregulates aerobic glycolysis to meet the bioenergetic and biosynthetic demands of rapidly growing cells. In contrast, thioredoxin-interacting protein (TXNIP) is a potent inhibitor of glucose uptake and is frequently downregulated in human cancers. Our laboratory previously discovered that Ras activation suppresses TXNIP transcription and translation. In this study, we developed a system to study how Ras affects TXNIP translation in the absence of transcriptional effects. We show that whereas Ras drives a global increase in protein translation, it suppresses TXNIP protein synthesis by reducing the rate at which ribosomes transit the coding region of TXNIP mRNA. To investigate the underlying mechanism(s), we randomized or optimized the codons in the TXNIP message without altering the TXNIP primary amino acid sequence. Translation from these mRNA variants was still repressed by Ras, implying that mRNA secondary structure, microRNAs (miRNAs), RNA binding proteins, or codon usage does not contribute to the blockade of TXNIP synthesis. Rather, we show that the N terminus of the growing TXNIP polypeptide is the target for Ras-dependent translational repression. Our work demonstrates how Ras suppresses TXNIP translation elongation in the face of a global upregulation of protein synthesis and provides new insight into Ras-dependent metabolic reprogramming.

Pan-cancer Alterations of the MYC Oncogene and Its Proximal Network across the Cancer Genome Atlas.

Although the MYC oncogene has been implicated in cancer, a systematic assessment of alterations of MYC, related transcription factors, and co-regulatory proteins, forming the proximal MYC network (PMN), across human cancers is lacking. Using computational approaches, we define genomic and proteomic features associated with MYC and the PMN across the 33 cancers of The Cancer Genome Atlas. Pan-cancer, 28% of all samples had at least one of the MYC paralogs amplified. In contrast, the MYC antagonists MGA and MNT were the most frequently mutated or deleted members, proposing a role as tumor suppressors. MYC alterations were mutually exclusive with PIK3CA, PTEN, APC, or BRAF alterations, suggesting that MYC is a distinct oncogenic driver. Expression analysis revealed MYC-associated pathways in tumor subtypes, such as immune response and growth factor signaling; chromatin, translation, and DNA replication/repair were conserved pan-cancer. This analysis reveals insights into MYC biology and is a reference for biomarkers and therapeutics for cancers with alterations of MYC or the PMN.

Role for the mortality factors MORF4, MRGX, and MRG15 in transcriptional repression via associations with Pf1, mSin3A, and Transducin-Like Enhancer of Split.

mSin3A and Transducin-Like Enhancer of Split (TLE) are two histone deacetylase (HDAC)-containing corepressors that function to repress transcription at targeted genes. Pf1 is a plant homeodomain zinc finger protein that interacts with both mSin3A and TLE, suggesting that it coordinates their function. Here we show that mSin3A and TLE interact with members of the mortality factor (MORF) family of putative transcriptional regulators. This family comprises MORF on chromosome 4 (MORF4) and MORF-related genes on chromosomes X and 15 (MRGX and MRG15, respectively) and is proposed to contribute to cellular senescence. Consistent with a role in transcription, we demonstrate that Gal4 fusions to each MORF family member repress transcription from a Gal4-dependent luciferase reporter. By using both mapping experiments and a dominant negative form of TLE, we show that repression by MORFs requires associations with mSin3A and TLE. Therefore, common functions of the MORFs are likely elicited through the action of a MORF/mSin3A/TLE complex. While the MORFs may have common functions, MRG15, but not MRGX or MORF4, interacted with Pf1. Therefore, MRG15 may have functions that are distinct from those of MRGX and MORF4. Consistent with this hypothesis, Pf1 reduced transcriptional repression by Gal4-MRG15 but it had no effect on repression by MRGX and MORF4. Pf1 has independent binding sites for MRG15 and mSin3A. In addition, Pf1 and MRG15 bind different domains on mSin3A. Together, these data suggest that the unique functions of MRG15 are elicited through the action of an MRG15/Pf1/mSin3A complex.