A critical role for Mnt in Myc-driven T-cell proliferation and oncogenesis

Single-cell mapping identifies MSI+ cells as a common origin for diverse subtypes of pancreatic cancer

Identifying the cells from which cancers arise is critical for understanding the molecular underpinnings of tumor evolution. To determine whether stem/progenitor cells can serve as cells of origin, we created a Msi2-CreERT2 knock-in mouse. When crossed to CAG-LSL-MycT58A mice, Msi2-CreERT2 mice developed multiple pancreatic cancer subtypes: ductal, acinar, adenosquamous, and rare anaplastic tumors. Combining single-cell genomics with computational analysis of developmental states and lineage trajectories, we demonstrate that MYC preferentially triggers transformation of the most immature MSI2+ pancreas cells into multi-lineage pre-cancer cells. These pre-cancer cells subsequently diverge to establish pancreatic cancer subtypes by activating distinct transcriptional programs and large-scale genomic changes, and enforced expression of specific signals like Ras can redirect subtype specification. This study shows that multiple pancreatic cancer subtypes can arise from a common pool of MSI2+ cells and provides a powerful model to understand and control the programs that shape divergent fates in pancreatic cancer.

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 "Superoncogene" Myc at the Crossroad between Metabolism and Gene Expression in Glioblastoma Multiforme

The concept of the Myc (c-myc, n-myc, l-myc) oncogene as a canonical, DNA-bound transcription factor has consistently changed over the past few years. Indeed, Myc controls gene expression programs at multiple levels: directly binding chromatin and recruiting transcriptional coregulators; modulating the activity of RNA polymerases (RNAPs); and drawing chromatin topology. Therefore, it is evident that Myc deregulation in cancer is a dramatic event. Glioblastoma multiforme (GBM) is the most lethal, still incurable, brain cancer in adults, and it is characterized in most cases by Myc deregulation. Metabolic rewiring typically occurs in cancer cells, and GBM undergoes profound metabolic changes to supply increased energy demand. In nontransformed cells, Myc tightly controls metabolic pathways to maintain cellular homeostasis. Consistently, in Myc-overexpressing cancer cells, including GBM cells, these highly controlled metabolic routes are affected by enhanced Myc activity and show substantial alterations. On the other hand, deregulated cancer metabolism impacts Myc expression and function, placing Myc at the intersection between metabolic pathway activation and gene expression. In this review paper, we summarize the available information on GBM metabolism with a specific focus on the control of the Myc oncogene that, in turn, rules the activation of metabolic signals, ensuring GBM growth.

Demystifying the Druggability of the MYC Family of Oncogenes

The MYC family of oncogenes (MYC, MYCN, and MYCL) encodes a basic helix-loop-helix leucine zipper (bHLHLZ) transcriptional regulator that is responsible for moving the cell through the restriction point. Through the HLHZIP domain, MYC heterodimerizes with the bHLHLZ protein MAX, which enables this MYC-MAX complex to bind to E-box regulatory DNA elements thereby controlling transcription of a large group of genes and their proteins. Translationally, MYC is one of the foremost oncogenic targets, and deregulation of expression of the MYC family gene/proteins occurs in over half of all human tumors and is recognized as a hallmark of cancer initiation and maintenance. Additionally, unexpected roles for this oncoprotein have been found in cancers that nominally have a non-MYC etiology. Although MYC is rarely mutated, its gain of function in cancer results from overexpression or from amplification. Moreover, MYC is a pleiotropic transcription factor possessing broad pathogenic prominence making it a coveted cancer target. A widely held notion within the biomedical research community is that the reliable modulation of MYC represents a tremendous therapeutic opportunity given its role in directly potentiating oncogenesis. However, the MYC-MAX heterodimer interaction contains a large surface area with a lack of well-defined binding sites creating the perception that targeting of MYC-MAX is forbidding. Here, we discuss the biochemistry behind MYC and MYC-MAX as it relates to cancer progression associated with these transcription factors. We also discuss the notion that MYC should no longer be regarded as undruggable, providing examples that a therapeutic window is achievable despite global MYC inhibition.

MNT suppresses T cell apoptosis via BIM and is critical for T lymphomagenesis

The importance of c-MYC in regulating lymphopoiesis and promoting lymphomagenesis is well-established. Far less appreciated is the vital supporting role of MYC's relative MNT. Using Rag1Cre-mediated Mnt deletion in lymphoid progenitor cells, we show here that, during normal T cell development, MNT loss enhances apoptosis, at least in part by elevating expression of the pro-apoptotic BH3-only protein BIM. Moreover, using T lymphoma-prone VavP-MYC transgenic mice, we show that Mnt deletion reduces the pool of pre-malignant MYC-driven T lymphoid cells and abrogates thymic T lymphomagenesis. In addition, we establish that Mnt deletion prevents T lymphoma development in γ-irradiated mice, most likely by enhancing apoptosis of T lymphoid cells repopulating the depleted thymus. Taken together with our recent demonstration that MNT is vital for the survival of MYC-driven pre-malignant and malignant B lymphoid cells, these results suggest that MNT represents an important new drug target for both T and B lymphoid malignancies.

T-cell Dysfunction upon Expression of MYC with Altered Phosphorylation at Threonine 58 and Serine 62

As a transcription factor that promotes cell growth, proliferation, and apoptosis, c-MYC (MYC) expression in the cell is tightly controlled. Disruption of oncogenic signaling pathways in human cancers can increase MYC protein stability, due to altered phosphorylation ratios at two highly conserved sites, Threonine 58 (T58) and Serine 62 (S62). The T58 to Alanine mutant (T58A) of MYC mimics the stabilized, S62 phosphorylated, and highly oncogenic form of MYC. The S62A mutant is also stabilized, lacks phosphorylation at both Serine 62 and Threonine 58, and has been shown to be nontransforming in vitro. However, several regulatory proteins are reported to associate with MYC lacking phosphorylation at S62 and T58, and the role this form of MYC plays in MYC transcriptional output and in vivo oncogenic function is understudied. We generated conditional c-Myc knock-in mice in which the expression of wild-type MYC (MYCWT), the T58A mutant (MYCT58A), or the S62A mutant (MYCS62A) with or without expression of endogenous Myc is controlled by the T-cell-specific Lck-Cre recombinase. MYCT58A expressing mice developed clonal T-cell lymphomas with 100% penetrance and conditional knock-out of endogenous Myc accelerated this lymphomagenesis. In contrast, MYCS62A mice developed clonal T-cell lymphomas at a much lower penetrance, and the loss of endogenous MYC reduced the penetrance while increasing the appearance of a non-transgene driven B-cell lymphoma with splenomegaly. Together, our study highlights the importance of regulated phosphorylation of MYC at T58 and S62 for T-cell transformation.

IMPLICATIONS

Dysregulation of phosphorylation at conserved T58 and S62 residues of MYC differentially affects T-cell development and lymphomagenesis.

FTO Alleviates CdCl2-Induced Apoptosis and Oxidative Stress via the AKT/Nrf2 Pathway in Bovine Granulosa Cells

Cadmium (Cd) is a common environmental heavy metal contaminant of reproduction toxicity. Cd accumulation in animals leads to the damage of granulosa cells. However, its mechanism needs to be elucidated. This research found that treating granulosa cells with Cd resulted in reduced cell viability. The flow cytometry results showed that Cd increased the degree of apoptosis and level of superoxide anion (O₂^-) in granulosa cells. Further analysis showed that Cd treatment resulted in reduced expression levels of nuclear factor erythroid 2-related factor-2 (Nrf2), superoxide dismutase (SOD), catalase (CAT) and NAD(P)H: quinone oxidoreductase 1 (NQO1), and an increased expression level of malondialdehyde (MDA); the expression levels of Bcl-2 associated X (Bax) and caspase-3 increased, whereas that of B-cell lymphoma 2 (Bcl-2) decreased. Changes in m⁶A methylation-related enzymes were noted with Cd-induced damage to granulosa cells. The results of transcriptome and MeRIP sequencing revealed that the AKT pathway participated in Cd-induced damage in granulosa cells, and the MAX network transcriptional repressor (MNT) may be a potential target gene of fat mass and obesity-associated protein (FTO). FTO and YTH domain family member 2 (YTHDF2) regulated MNT expression through m⁶A modification. FTO overexpression alleviated Cd-induced apoptosis and oxidative stress through the activation of the AKT/Nrf2 pathway; this process could be reversed using siMNT. Overall, these findings associated m⁶A with Cd-induced damage to granulosa cells and provided insights into Cd-induced granulosa cell cytotoxicity from a new perspective centered on m⁶A modification.

Oncogenic Runx1-Myc axis in p53-deficient thymic lymphoma

p53 deficiency and Myc dysregulation are frequently associated with cancer. However, the molecular mechanisms linking these two major oncogenic events are poorly understood. Using an osteosarcoma model caused by p53 loss, we have recently shown that Runx3 aberrantly upregulates Myc via mR1, a Runx consensus site in the Myc promoter. Here, we focus on thymic lymphoma, a major tumour type caused by germline p53 deletion in mice, and examine whether the oncogenic Runx-Myc axis plays a notable role in the development of p53-deficient lymphoma. Mice lacking p53 specifically in thymocytes (LP mice) mostly succumbed to thymic lymphoma. Runx1 and Myc were upregulated in LP mouse lymphoma compared with the normal thymus. Depletion of Runx1 or Myc prolonged the lifespan of LP mice and suppressed lymphoma development. In lymphoma cells isolated from LP mice, knockdown of Runx1 led to Myc suppression, weakening their tumour forming ability in immunocompromised mice. The mR1 locus was enriched by both Runx1 and H3K27ac, an active chromatin marker. LP mice with mutated mR1 had a longer lifespan and a lower incidence of lymphoma. Treatment with AI-10-104, a Runx inhibitor, improved the survival of LP mice. These results suggest that Myc upregulation by Runx1 is a key event in p53-deficient thymic lymphoma development and provide a clinical rationale for targeting the Runx family in p53-deficient malignancies.

Normal and Neoplastic Growth Suppression by the Extended Myc Network

Among the first discovered and most prominent cellular oncogenes is MYC, which encodes a bHLH-ZIP transcription factor (Myc) that both activates and suppresses numerous genes involved in proliferation, energy production, metabolism and translation. Myc belongs to a small group of bHLH-ZIP transcriptional regulators (the Myc Network) that includes its obligate heterodimerization partner Max and six "Mxd proteins" (Mxd1-4, Mnt and Mga), each of which heterodimerizes with Max and largely opposes Myc's functions. More recently, a second group of bHLH-ZIP proteins (the Mlx Network) has emerged that bears many parallels with the Myc Network. It is comprised of the Myc-like factors ChREBP and MondoA, which, in association with the Max-like member Mlx, regulate smaller and more functionally restricted repertoires of target genes, some of which are shared with Myc. Opposing ChREBP and MondoA are heterodimers comprised of Mlx and Mxd1, Mxd4 and Mnt, which also structurally and operationally link the two Networks. We discuss here the functions of these "Extended Myc Network" members, with particular emphasis on their roles in suppressing normal and neoplastic growth. These roles are complex due to the temporal- and tissue-restricted expression of Extended Myc Network proteins in normal cells, their regulation of both common and unique target genes and, in some cases, their functional redundancy.

The Multiple Faces of MNT and Its Role as a MYC Modulator

MNT is a crucial modulator of MYC, controls several cellular functions, and is activated in most human cancers. It is the largest, most divergent, and most ubiquitously expressed protein of the MXD family. MNT was first described as a MYC antagonist and tumor suppressor. Indeed, 10% of human tumors present deletions of one MNT allele. However, some reports show that MNT functions in cooperation with MYC by maintaining cell proliferation, promoting tumor cell survival, and supporting MYC-driven tumorigenesis in cellular and animal models. Although MAX was originally considered MNT's obligate partner, our recent findings demonstrate that MNT also works independently. MNT forms homodimers and interacts with proteins both outside and inside of the proximal MYC network. These complexes are involved in a wide array of cellular processes, from transcriptional repression via SIN3 to the modulation of metabolism through MLX as well as immunity and apoptosis via REL. In this review, we discuss the present knowledge of MNT with a special focus on its interactome, which sheds light on the complex and essential role of MNT in cell biology.

Loss of MGA repression mediated by an atypical polycomb complex promotes tumor progression and invasiveness

MGA, a transcription factor and member of the MYC network, is mutated or deleted in a broad spectrum of malignancies. As a critical test of a tumor suppressive role, we inactivated Mga in two mouse models of non-small cell lung cancer using a CRISPR-based approach. MGA loss significantly accelerated tumor growth in both models and led to de-repression of non-canonical Polycomb ncPRC1.6 targets, including genes involved in metastasis and meiosis. Moreover, MGA deletion in human lung adenocarcinoma lines augmented invasive capabilities. We further show that MGA-MAX, E2F6, and L3MBTL2 co-occupy thousands of promoters and that MGA stabilizes these ncPRC1.6 subunits. Lastly, we report that MGA loss also induces a pro-growth effect in human colon organoids. Our studies establish MGA as a bona fide tumor suppressor in vivo and suggest a tumor suppressive mechanism in adenocarcinomas resulting from widespread transcriptional attenuation of MYC and E2F target genes mediated by MGA-MAX associated with a non-canonical Polycomb complex.

miR-139-5p Regulates the Proliferation of Acute Promyelocytic Leukemia Cells by Targeting MNT

Acute promyelocytic leukemia (APL) patients with progressive leukocytosis are more likely to have various complications and poor outcomes. However, the regulatory roles of microRNAs in the leukocytosis of APL have not been clarified. Our study aims to evaluate the effects of miRNAs on leukocytosis during induction therapy of APL patients and explore its potential mechanisms. During induction treatment, patients with white blood cell count higher than 10 × 10⁹/L were divided into leukocytosis group and others were nonleukocytosis group. Using microarray assays, we found that miR-139-5p was significantly downregulated in the leukocytosis group. Elevated expression of miR-139-5p inhibited the proliferation of NB4 cells by arresting the cell cycle and inducing apoptosis. We further identified that MNT was a target of miR-139-5p. miR-139-5p significantly inhibited the proliferation, invasion, and migration function of NB4 cells through targeting MNT. Strategies for regulating miR-139-5p or MNT expression might provide new therapeutic approaches for progressive leukocytosis in APL.

A novel role of MNT as a negative regulator of REL and the NF-κB pathway

MNT, a transcription factor of the MXD family, is an important modulator of the oncoprotein MYC. Both MNT and MYC are basic-helix-loop-helix proteins that heterodimerize with MAX in a mutually exclusive manner, and bind to E-boxes within regulatory regions of their target genes. While MYC generally activates transcription, MNT represses it. However, the molecular interactions involving MNT as a transcriptional regulator beyond the binding to MAX remain unexplored. Here we demonstrate a novel MAX-independent protein interaction between MNT and REL, the oncogenic member of the NF-κB family. REL participates in important biological processes and it is altered in a variety of tumors. REL is a transcription factor that remains inactive in the cytoplasm in an inhibitory complex with IκB and translocates to the nucleus when the NF-κB pathway is activated. In the present manuscript, we show that MNT knockdown triggers REL translocation into the nucleus and thus the activation of the NF-κB pathway. Meanwhile, MNT overexpression results in the repression of IκBα, a bona fide REL target. Both MNT and REL bind to the IκBα gene on the first exon, suggesting its regulation as an MNT-REL complex. Altogether our data indicate that MNT acts as a repressor of the NF-κB pathway by two mechanisms: (1) retention of REL in the cytoplasm by MNT interaction, and (2) MNT-driven repression of REL-target genes through an MNT-REL complex. These results widen our knowledge about MNT biological roles and reveal a novel connection between the MYC/MXD and NF-κB pathways, two of the most prominent pathways in cancer.

Detection of Post-translational Modifications on MYC

Detection of post-translational modifications in c-Myc is an invaluable tool in assessing Myc status, particularly in cancer. However, it can be challenging to detect these modifications. The evaluation of phosphorylation status of c-Myc can also be challenging with the current commercially available phosphorylation sensitive antibodies. Here, we describe protocols for the immunoprecipitation of endogenous c-Myc to probe for phosphorylation status, as well as the detection of ubiquitination and SUMOylation on c-Myc. We will also discuss the challenges of detecting phosphorylated c-Myc in formalin-fixed paraffin-embedded tissues by immunofluorescence and describe a protocol using a new rat monoclonal antibody we have generated suitable for this purpose.

Development and survival of MYC-driven lymphomas require the MYC antagonist MNT to curb MYC-induced apoptosis

Deregulated overexpression of MYC is implicated in the development and malignant progression of most (∼70%) human tumors. MYC drives cell growth and proliferation, but also, at high levels, promotes apoptosis. Here, we report that the proliferative capacity of MYC-driven normal and neoplastic B lymphoid cells depends on MNT, a MYC-related transcriptional repressor. Our genetic data establish that MNT synergizes with MYC by suppressing MYC-driven apoptosis, and that it does so primarily by reducing the level of pro-apoptotic BIM. In Eμ-Myc mice, which model the MYC/IGH chromosome translocation in Burkitt's lymphoma, homozygous Mnt deletion greatly reduced lymphoma incidence by enhancing apoptosis and markedly decreasing premalignant B lymphoid cell populations. Strikingly, by inducing Mnt deletion within transplanted fully malignant Eμ-Myc lymphoma cells, we significantly extended transplant recipient survival. The dependency of lymphomas on MNT for survival suggests that drugs inhibiting MNT could significantly boost therapy of MYC-driven tumors by enhancing intrinsic MYC-driven apoptosis.

The MNT transcription factor autoregulates its expression and supports proliferation in MYC-associated factor X (MAX)-deficient cells

The MAX network transcriptional repressor (MNT) is an MXD family transcription factor of the basic helix-loop-helix (bHLH) family. MNT dimerizes with another transcriptional regulator, MYC-associated factor X (MAX), and down-regulates genes by binding to E-boxes. MAX also dimerizes with MYC, an oncogenic bHLH transcription factor. Upon E-box binding, the MYC-MAX dimer activates gene expression. MNT also binds to the MAX dimerization protein MLX (MLX), and MNT-MLX and MNT-MAX dimers co-exist. However, all MNT functions have been attributed to MNT-MAX dimers, and no functions of the MNT-MLX dimer have been described. MNT's biological role has been linked to its function as a MYC oncogene modulator, but little is known about its regulation. We show here that MNT localizes to the nucleus of MAX-expressing cells and that MNT-MAX dimers bind and repress the MNT promoter, an effect that depends on one of the two E-boxes on this promoter. In MAX-deficient cells, MNT was overexpressed and redistributed to the cytoplasm. Interestingly, MNT was required for cell proliferation even in the absence of MAX. We show that in MAX-deficient cells, MNT binds to MLX, but also forms homodimers. RNA-sequencing experiments revealed that MNT regulates the expression of several genes even in the absence of MAX, with many of these genes being involved in cell cycle regulation and DNA repair. Of note, MNT-MNT homodimers regulated the transcription of some genes involved in cell proliferation. The tight regulation of MNT and its functionality even without MAX suggest a major role for MNT in cell proliferation.

Misregulation of Drosophila Myc Disrupts Circadian Behavior and Metabolism

Drosophila Myc (dMyc) is highly conserved and functions as a transcription factor similar to mammalian Myc. We previously found that oncogenic Myc disrupts the molecular clock in cancer cells. Here, we demonstrate that misregulation of dMyc expression affects Drosophila circadian behavior. dMyc overexpression results in a high percentage of arrhythmic flies, concomitant with increases in the expression of clock genes cyc, tim, cry, and cwo. Conversely, flies with hypomorphic mutations in dMyc exhibit considerable arrhythmia, which can be rescued by loss of dMnt, a suppressor of dMyc activity. Metabolic profiling of fly heads revealed that loss of dMyc and its overexpression alter steady-state metabolite levels and have opposing effects on histidine, the histamine precursor, which is rescued in dMyc mutants by ablation of dMnt and could contribute to effects of dMyc on locomotor behavior. Our results demonstrate a role of dMyc in modulating Drosophila circadian clock, behavior, and metabolism.

The MYC transcription factor network: balancing metabolism, proliferation and oncogenesis

Transcription factor networks have evolved in order to control, coordinate, and separate, the functions of distinct network modules spatially and temporally. In this review we focus on the MYC network (also known as the MAX-MLX Network), a highly conserved super-family of related basic-helix-loop-helix-zipper (bHLHZ) proteins that functions to integrate extracellular and intracellular signals and modulate global gene expression. Importantly the MYC network has been shown to be deeply involved in a broad spectrum of human and other animal cancers. Here we summarize molecular and biological properties of the network modules with emphasis on functional interactions among network members. We suggest that these network interactions serve to modulate growth and metabolism at the transcriptional level in order to balance nutrient demand with supply, to maintain growth homeostasis, and to influence cell fate. Moreover, oncogenic activation of MYC and/or loss of a MYC antagonist, results in an imbalance in the activity of the network as a whole, leading to tumor initiation, progression and maintenance.

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.

Mnt modulates Myc-driven lymphomagenesis

The transcriptional represser Mnt is a functional antagonist of the proto-oncoprotein Myc. Both Mnt and Myc utilise Max as an obligate partner for DNA binding, and Myc/Max and Mnt/Max complexes compete for occupancy at E-box DNA sequences in promoter regions. We have previously shown in transgenic mouse models that the phenotype and kinetics of onset of haemopoietic tumours varies with the level of Myc expression. We reasoned that a decrease in the level of Mnt would increase the functional level of Myc and accelerate Myc-driven tumorigenesis. We tested the impact of reduced Mnt in three models of myc transgenic mice and in p53+/- mice. To our surprise, mnt heterozygosity actually slowed Myc-driven tumorigenesis in vavP-MYC10 and Eμ-myc mice, suggesting that Mnt facilitates Myc-driven oncogenesis. To explore the underlying cause of the delay in tumour development, we enumerated Myc-driven cell populations in healthy young vavP-MYC10 and Eμ-myc mice, expecting that the reduced rate of leukaemogenesis in mnt heterozygous mice would be reflected in a reduced number of preleukaemic cells, due to increased apoptosis or reduced proliferation or both. However, no differences were apparent. Furthermore, when mnt+/+ and mnt+/- pre-B cells from healthy young Eμ-myc mice were compared in vitro, no differences were seen in their sensitivity to apoptosis or in cell size or cell cycling. Moreover, the frequencies of apoptotic, senescent and proliferating cells were comparable in vivo in mnt+/- and mnt+/+ Eμ-myc lymphomas. Thus, although mnt heterozygosity clearly slowed lymphomagenesis in vavP-MYC10 and Eμ-myc mice, the change(s) in cellular properties responsible for this effect remain to be identified.

MNT and Emerging Concepts of MNT-MYC Antagonism

MYC family proteins play fundamental roles in stem and progenitor cell homeostasis, morphogenesis and cancer. As expected for proteins that profoundly affect the fate of cells, the activities of MYC are regulated at a multitude of levels. One mechanism with the potential to broadly affect the activities of MYC is transcriptional antagonism by a group of MYC-related transcriptional repressors. From this group, the protein MNT has emerged as having perhaps the most far-reaching impact on MYC activities. In this review, we discuss the current understanding of MNT, its regulation and how, as a MYC antagonist, it functions both as a tumor suppressor and facilitator of MYC-driven proliferation and oncogenesis.

MicroRNA-210 and its theranostic potential

INTRODUCTION

MicroRNAs (miRNAs) are a set of small single-stranded noncoding RNAs with diverse biological functions. As a prototypical hypoxamir, human microRNA-210 (hsa-miR-210) is one of the most widely studied miRNAs thus far. In addition to its involvement in sophisticated regulation of numerous biological processes, miR-210 has also been shown to be associated with the development of different human diseases including various types of cancers, cardiovascular and cerebrovascular diseases, and immunological diseases. Given its multi-faceted functions, miR-210 may serve as a novel and promising theranostic target for prevention and treatment of diseases. Areas covered: This review aims to provide a comprehensive overview of miR-210, the regulation of its expression, biological functions and molecular mechanisms, with particular emphasis on its diagnostic and therapeutic potential. Expert opinion: Although the exact roles of miR-210 in various diseases have not been fully clarified, targeting miR-210 may be a promising therapeutic strategy. Further investigations are also needed to facilitate therapeutic-clinical applications of miR-210 in human diseases.

Functional interactions among members of the MAX and MLX transcriptional network during oncogenesis

The transcription factor MYC and its related family members MYCN and MYCL have been implicated in the etiology of a wide spectrum of human cancers. Compared to other oncoproteins, such as RAS or SRC, MYC is unique because its protein coding region is rarely mutated. Instead, MYC's oncogenic properties are unleashed by regulatory mutations leading to unconstrained high levels of expression. Under both normal and pathological conditions MYC regulates multiple aspects of cellular physiology including proliferation, differentiation, apoptosis, growth and metabolism by controlling the expression of thousands of genes. How a single transcription factor exerts such broad effects remains a fascinating puzzle. Notably, MYC is part of a network of bHLHLZ proteins centered on the MYC heterodimeric partner MAX and its counterpart, the MAX-like protein MLX. This network includes MXD1-4, MNT, MGA, MONDOA and MONDOB proteins. With some exceptions, MXD proteins have been functionally linked to cell cycle arrest and differentiation, while MONDO proteins control cellular metabolism. Although the temporal expression patterns of many of these proteins can differ markedly they are frequently expressed simultaneously in the same cellular context, and potentially bind to the same, or similar DNA consensus sequence. Here we review the activities and interactions among these proteins and propose that the broad spectrum of phenotypes elicited by MYC deregulation is intimately connected to the functions and regulation of the other network members. Furthermore, we provide a meta-analysis of TCGA data suggesting that the coordinate regulation of the network is important in MYC driven tumorigenesis. This article is part of a Special Issue entitled: Myc proteins in cell biology and pathology.

Deregulated Myc requires MondoA/Mlx for metabolic reprogramming and tumorigenesis

Deregulated Myc transcriptionally reprograms cell metabolism to promote neoplasia. Here we show that oncogenic Myc requires the Myc superfamily member MondoA, a nutrient-sensing transcription factor, for tumorigenesis. Knockdown of MondoA, or its dimerization partner Mlx, blocks Myc-induced reprogramming of multiple metabolic pathways, resulting in apoptosis. Identification and knockdown of genes coregulated by Myc and MondoA have allowed us to define metabolic functions required by deregulated Myc and demonstrate a critical role for lipid biosynthesis in survival of Myc-driven cancer. Furthermore, overexpression of a subset of Myc and MondoA coregulated genes correlates with poor outcome of patients with diverse cancers. Coregulation of cancer metabolism by Myc and MondoA provides the potential for therapeutics aimed at inhibiting MondoA and its target genes.

Acetylation of the c-MYC oncoprotein is required for cooperation with the HTLV-1 p30(II) accessory protein and the induction of oncogenic cellular transformation by p30(II)/c-MYC

The human T-cell leukemia retrovirus type-1 (HTLV-1) p30(II) protein is a multifunctional latency-maintenance factor that negatively regulates viral gene expression and deregulates host signaling pathways involved in aberrant T-cell growth and proliferation. We have previously demonstrated that p30(II) interacts with the c-MYC oncoprotein and enhances c-MYC-dependent transcriptional and oncogenic functions. However, the molecular and biochemical events that mediate the cooperation between p30(II) and c-MYC remain to be completely understood. Herein we demonstrate that p30(II) induces lysine-acetylation of the c-MYC oncoprotein. Acetylation-defective c-MYC Lys→Arg substitution mutants are impaired for oncogenic transformation with p30(II) in c-myc(-/-) HO15.19 fibroblasts. Using dual-chromatin-immunoprecipitations (dual-ChIPs), we further demonstrate that p30(II) is present in c-MYC-containing nucleoprotein complexes in HTLV-1-transformed HuT-102 T-lymphocytes. Moreover, p30(II) inhibits apoptosis in proliferating cells expressing c-MYC under conditions of genotoxic stress. These findings suggest that c-MYC-acetylation is required for the cooperation between p30(II)/c-MYC which could promote proviral replication and contribute to HTLV-1-induced carcinogenesis.

The activities of MYC, MNT and the MAX-interactome in lymphocyte proliferation and oncogenesis

The MYC family of proteins plays essential roles in embryonic development and in oncogenesis. Efforts over the past 30 years to define the transcriptional activities of MYC and how MYC functions to promote proliferation have produced evolving models of MYC function. One picture that has emerged of MYC and its partner protein MAX is of a transcription factor complex with a seemingly unique ability to stimulate the transcription of genes that are epigenetically poised for transcription and to amplify the transcription of actively transcribed genes. During lymphocyte activation, MYC is upregulated and stimulates a pro-proliferative program in part through the upregulation of a wide variety of metabolic effector genes that facilitate cell growth and cell cycle progression. MYC upregulation simultaneously sensitizes cells to apoptosis and activated lymphocytes and lymphoma cells have pro-survival attributes that allow MYC-driven proliferation to prevail. For example, the MAX-interacting protein MNT is upregulated in activated lymphocytes and was found to protect lymphocytes from MYC-dependent apoptosis. Here we review the activities of MYC, MNT and other MAX interacting proteins in the setting of T and B cell activation and oncogenesis. This article is part of a Special Issue entitled: Myc proteins in cell biology and pathology.

Synthetic lethal screens as a means to understand and treat MYC-driven cancers

Although therapeutics against MYC could potentially be used against a wide range of human cancers, MYC-targeted therapies have proven difficult to develop. The convergence of breakthroughs in human genomics and in gene silencing using RNA interference (RNAi) have recently allowed functional interrogation of the genome and systematic identification of synthetic lethal interactions with hyperactive MYC. Here, we focus on the pathways that have emerged through RNAi screens and present evidence that a subset of genes showing synthetic lethality with MYC are significantly interconnected and linked to chromatin and transcriptional processes, as well as to DNA repair and cell cycle checkpoints. Other synthetic lethal interactions with MYC point to novel pathways and potentially broaden the repertoire of targeted therapies. The elucidation of MYC synthetic lethal interactions is still in its infancy, and how these interactions may be influenced by tissue-specific programs and by concurrent genetic change will require further investigation. Nevertheless, we predict that these studies may lead the way to novel therapeutic approaches and new insights into the role of MYC in cancer.

An overview of MYC and its interactome

This review is intended to provide a broad outline of the biological and molecular functions of MYC as well as of the larger protein network within which MYC operates. We present a view of MYC as a sensor that integrates multiple cellular signals to mediate a broad transcriptional response controlling many aspects of cell behavior. We also describe the larger transcriptional network linked to MYC with emphasis on the MXD family of MYC antagonists. Last, we discuss evidence that the network has evolved for millions of years, dating back to the emergence of animals.

Control of vertebrate development by MYC

The study of MYC has led to pivotal discoveries in cancer biology, induced pluripotency, and transcriptional regulation. In this review, continuing advances in our understanding of the function of MYC as a transcription factor and how its transcriptional activity controls normal vertebrate development and contributes to developmental disorders is discussed.

A repressor protein, Mnt, is a novel negative regulator of vascular smooth muscle cell hypertrophy by angiotensin II and neointimal hyperplasia by arterial injury

OBJECTIVE

The Max-interacting protein Mnt is a transcriptional repressor that can antagonize the transcriptional and proliferation-related activities of Myc. Here, we tested the hypothesis that Mnt is a negative regulator of pathological vascular remodeling.

METHODS

Adenovirus encoding Mnt or control GFP was infected to cultured rat vascular smooth muscle cells (VSMC) and carotid arteries after a balloon angioplasty.

RESULTS

In VSMC, adenoviral gene transfer of Mnt suppressed angiotensin II-induced protein expression of early growth response protein-1 (Egr1) and its promoter activation. Mnt adenovirus did not interfere with upstream signaling of angiotensin II. Angiotensin II-induced protein accumulation in VSMC was inhibited by Mnt adenovirus. Mnt adenovirus also inhibited platelet-derived growth factor-induced VSMC proliferation. Moreover, Mnt adenovirus prevented neointima formation in response to arterial injury. The adenoviral Mnt gene transfer also prevented Egr1 induction in neointima.

CONCLUSION

These data identify Mnt as a previously unrecognized negative regulator of pathological vascular remodeling.