HIF-1 inhibits mitochondrial biogenesis and cellular respiration in VHL-deficient renal cell carcinoma by repression of C-MYC activity.

Many cancer cells are characterized by increased glycolysis and decreased respiration, even under aerobic conditions. The molecular mechanisms underlying this metabolic reprogramming are unclear. Here we show that hypoxia-inducible factor 1 (HIF-1) negatively regulates mitochondrial biogenesis and O(2) consumption in renal carcinoma cells lacking the von Hippel-Lindau tumor suppressor (VHL). HIF-1 mediates these effects by inhibiting C-MYC activity via two mechanisms. First, HIF-1 binds to and activates transcription of the MXI1 gene, which encodes a repressor of C-MYC transcriptional activity. Second, HIF-1 promotes MXI-1-independent, proteasome-dependent degradation of C-MYC. We demonstrate that transcription of the gene encoding the coactivator PGC-1beta is C-MYC dependent and that loss of PGC-1beta expression is a major factor contributing to reduced respiration in VHL-deficient renal carcinoma cells.

c-Myc suppression of miR-23a/b enhances mitochondrial glutaminase expression and glutamine metabolism.

Altered glucose metabolism in cancer cells is termed the Warburg effect, which describes the propensity of most cancer cells to take up glucose avidly and convert it primarily to lactate, despite available oxygen. Notwithstanding the renewed interest in the Warburg effect, cancer cells also depend on continued mitochondrial function for metabolism, specifically glutaminolysis that catabolizes glutamine to generate ATP and lactate. Glutamine, which is highly transported into proliferating cells, is a major source of energy and nitrogen for biosynthesis, and a carbon substrate for anabolic processes in cancer cells, but the regulation of glutamine metabolism is not well understood. Here we report that the c-Myc (hereafter referred to as Myc) oncogenic transcription factor, which is known to regulate microRNAs and stimulate cell proliferation, transcriptionally represses miR-23a and miR-23b, resulting in greater expression of their target protein, mitochondrial glutaminase, in human P-493 B lymphoma cells and PC3 prostate cancer cells. This leads to upregulation of glutamine catabolism. Glutaminase converts glutamine to glutamate, which is further catabolized through the tricarboxylic acid cycle for the production of ATP or serves as substrate for glutathione synthesis. The unique means by which Myc regulates glutaminase uncovers a previously unsuspected link between Myc regulation of miRNAs, glutamine metabolism, and energy and reactive oxygen species homeostasis.

Alterations in nucleolar structure and gene expression programs in prostatic neoplasia are driven by the MYC oncogene.

Increased nucleolar size and number are hallmark features of many cancers. In prostate cancer, nucleolar enlargement and increased numbers are some of the earliest morphological changes associated with development of premalignant prostate intraepithelial neoplasia (PIN) lesions and invasive adenocarcinomas. However, the molecular mechanisms that induce nucleolar alterations in PIN and prostate cancer remain largely unknown. We verify that activation of the MYC oncogene, which is overexpressed in most human PIN and prostatic adenocarcinomas, leads to formation of enlarged nucleoli and increased nucleolar number in prostate luminal epithelial cells in vivo. In prostate cancer cells in vitro, MYC expression is needed for maintenance of nucleolar number, and a nucleolar program of gene expression. To begin to decipher the functional relevance of this transcriptional program in prostate cancer, we examined FBL (encoding fibrillarin), a MYC target gene, and report that fibrillarin is required for proliferation, clonogenic survival, and proper ribosomal RNA accumulation/processing in human prostate cancer cells. Further, fibrillarin is overexpressed in PIN lesions induced by MYC overexpression in the mouse prostate, and in human clinical prostate adenocarcinoma and PIN lesions, where its expression correlates with MYC levels. These studies demonstrate that overexpression of the MYC oncogene increases nucleolar number and size and a nucleolar program of gene expression in prostate epithelial cells, thus providing a molecular mechanism responsible for hallmark nucleolar alterations in prostatic neoplasia.

c-Myc-regulated microRNAs modulate E2F1 expression.

MicroRNAs (miRNAs) are 21-23 nucleotide RNA molecules that regulate the stability or translational efficiency of target messenger RNAs. miRNAs have diverse functions, including the regulation of cellular differentiation, proliferation and apoptosis. Although strict tissue- and developmental-stage-specific expression is critical for appropriate miRNA function, mammalian transcription factors that regulate miRNAs have not yet been identified. The proto-oncogene c-MYC encodes a transcription factor that regulates cell proliferation, growth and apoptosis. Dysregulated expression or function of c-Myc is one of the most common abnormalities in human malignancy. Here we show that c-Myc activates expression of a cluster of six miRNAs on human chromosome 13. Chromatin immunoprecipation experiments show that c-Myc binds directly to this locus. The transcription factor E2F1 is an additional target of c-Myc that promotes cell cycle progression. We find that expression of E2F1 is negatively regulated by two miRNAs in this cluster, miR-17-5p and miR-20a. These findings expand the known classes of transcripts within the c-Myc target gene network, and reveal a mechanism through which c-Myc simultaneously activates E2F1 transcription and limits its translation, allowing a tightly controlled proliferative signal.

Major role for mRNA stability in shaping the kinetics of gene induction.

BACKGROUND: mRNA levels in cells are determined by the relative rates of RNA production and degradation. Yet, to date, most analyses of gene expression profiles were focused on mechanisms which regulate transcription, while the role of mRNA stability in modulating transcriptional networks was to a large extent overlooked. In particular, kinetic waves in transcriptional responses are usually interpreted as resulting from sequential activation of transcription factors. RESULTS: In this study, we examined on a global scale the role of mRNA stability in shaping the kinetics of gene response. Analyzing numerous expression datasets we revealed a striking global anti-correlation between rapidity of induction and mRNA stability, fitting the prediction of a kinetic mathematical model. In contrast, the relationship between kinetics and stability was less significant when gene suppression was analyzed. Frequently, mRNAs that are stable under standard conditions were very rapidly down-regulated following stimulation. Such effect cannot be explained even by a complete shut-off of transcription, and therefore indicates intense modulation of RNA stability. CONCLUSION: Taken together, our results demonstrate the key role of mRNA stability in determining induction kinetics in mammalian transcriptional networks.

Activation of transferrin receptor 1 by c-Myc enhances cellular proliferation and tumorigenesis.

Overexpression of transferrin receptor 1 (TFRC1), a major mediator of iron uptake in mammalian cells, is a common feature of human malignancies. Therapeutic strategies designed to interfere with tumor iron metabolism have targeted TFRC1. The c-Myc oncogenic transcription factor stimulates proliferation and growth by activating thousands of target genes. Here we demonstrate that TFRC1 is a critical downstream target of c-Myc. Using in vitro and in vivo models of B-cell lymphoma, we show that TFRC1 expression is activated by c-Myc. Chromatin immunoprecipitation experiments reveal that c-Myc directly binds a conserved region of TFRC1. In light of these findings, we sought to determine whether TFRC1 is required for c-Myc-mediated cellular proliferation and cell size control. TFRC1 inhibition decreases cellular proliferation and results in G1 arrest without affecting cell size. Consistent with these findings, expression profiling reveals that TFRC1 depletion alters expression of genes that regulate the cell cycle. Furthermore, enforced TFRC1 expression confers a growth advantage to cells and significantly enhances the rate of c-Myc-mediated tumor formation in vivo. These findings provide a molecular basis for increased TFRC1 expression in human tumors, illuminate the role of TFRC1 in the c-Myc target gene network, and support strategies that target TFRC1 for cancer therapy.

Myc stimulates nuclearly encoded mitochondrial genes and mitochondrial biogenesis.

Although several genes involved in mitochondrial function are direct Myc targets, the role of Myc in mitochondrial biogenesis has not been directly established. We determined the effects of ectopic Myc expression or the loss of Myc on mitochondrial biogenesis. Induction of Myc in P493-6 cells resulted in increased oxygen consumption and mitochondrial mass and function. Conversely, compared to wild-type Myc fibroblasts, Myc null rat fibroblasts have diminished mitochondrial mass and decreased number of normal mitochondria. Reconstitution of Myc expression in Myc null fibroblasts partially restored mitochondrial mass and function and normal-appearing mitochondria. Concordantly, we also observed in primary hepatocytes that acute deletion of floxed murine Myc by Cre recombinase resulted in diminished mitochondrial mass in primary hepatocytes. Our microarray analysis of genes responsive to Myc in human P493-6 B lymphocytes supports a role for Myc in mitochondrial biogenesis, since genes involved in mitochondrial structure and function are overrepresented among the Myc-induced genes. In addition to the known direct binding of Myc to many genes involved in mitochondrial structure and function, we found that Myc binds the TFAM gene, which encodes a key transcriptional regulator and mitochondrial DNA replication factor, both in P493-6 lymphocytes with high ectopic MYC expression and in serum-stimulated primary human 2091 fibroblasts with induced endogenous MYC. These observations support a pivotal role for Myc in regulating mitochondrial biogenesis.

Evaluation of myc E-box phylogenetic footprints in glycolytic genes by chromatin immunoprecipitation assays.

Prediction of gene regulatory sequences using phylogenetic footprinting has advanced considerably but lacks experimental validation. Here, we report whether transcription factor binding sites predicted by dot plotting or web-based Trafac analysis could be validated by chromatin immunoprecipitation assays. MYC overexpression enhances glycolysis without hypoxia and hence may contribute to altered tumor metabolism. Because the full spectrum of glycolytic genes directly regulated by Myc is not known, we chose Myc as a model transcription factor to determine whether it binds target glycolytic genes that have conserved canonical Myc binding sites or E boxes (5'-CACGTG-3'). Conserved canonical E boxes in ENO1, HK2, and LDHA occur in 31- to 111-bp islands with high interspecies sequence identity (>65%). Trafac analysis revealed another region in ENO1 that corresponds to a murine region with a noncanonical E box. Myc bound all these conserved regions well in the human P493-6 B lymphocytes. We also determined whether Myc could bind nonconserved canonical E boxes found in the remaining human glycolytic genes. Myc bound PFKM, but it did not significantly bind GPI, PGK1, and PKM2. Binding to BPGM, PGAM2, and PKLR was not detected. Both GAPD and TPI1 do not have conserved E boxes but are induced and bound by Myc through regions with noncanonical E boxes. Our results indicate that Myc binds well to conserved canonical E boxes, but not nonconserved E boxes. However, the binding of Myc to unpredicted genomic regions with noncanonical E boxes reveals a limitation of phylogenetic footprinting. In aggregate, these observations indicate that Myc is an important regulator of glycolytic genes, suggesting that MYC plays a key role in a switch to glycolytic metabolism during cell proliferation or tumorigenesis.

In silico identification of transcriptional regulators associated with c-Myc.

The development of powerful experimental strategies for functional genomics and accompanying computational tools has brought major advances in the delineation of transcriptional networks in organisms ranging from yeast to human. Regulation of transcription of eukaryotic genes is to a large extent combinatorial. Here, we used an in silico approach to identify transcription factors (TFs) that form recurring regulatory modules with c-Myc, a protein encoded by an oncogene that is frequently disregulated in human malignancies. A recent study identified, on a genomic scale, human genes whose promoters are bound by c-Myc and its heterodimer partner Max in Burkitt's lymphoma cells. Using computational methods, we identified nine TFs whose binding-site signatures are highly overrepresented in this promoter set of c-Myc targets, pointing to possible functional links between these TFs and c-Myc. Binding sites of most of these TFs are also enriched on the set of mouse homolog promoters, suggesting functional conservation. Among the enriched TFs, there are several regulators known to control cell cycle progression. Another TF in this set, EGR-1, is rapidly activated by numerous stress challenges and plays a central role in angiogenesis. Experimental investigation confirmed that c-Myc and EGR-1 bind together on several target promoters. The approach applied here is general and demonstrates how computational analysis of functional genomics experiments can identify novel modules in complex networks of transcriptional regulation.

RADOM, an efficient in vivo method for assembling designed DNA fragments up to 10 kb long in Saccharomyces cerevisiae.

We describe rapid assembly of DNA overlapping multifragments (RADOM), an improved assembly method via homologous recombination in Saccharomyces cerevisiae, which combines assembly in yeasto with blue/white screening in Escherichia coli. We show that RADOM can successfully assemble ∼3 and ∼10 kb DNA fragments that are highly similar to the yeast genome rapidly and accurately. This method was tested in the Build-A-Genome course by undergraduate students, where 125 ∼3 kb "minichunks" from the synthetic yeast genome project Sc2.0 were assembled. Here, 122 out of 125 minichunks achieved insertions with correct sizes, and 102 minichunks were sequenced verified. As this method reduces the time-consuming and labor-intensive efforts of yeast assembly by improving the screening efficiency for correct assemblies, it may find routine applications in the construction of DNA fragments, especially in hierarchical assembly projects.

Cell-type independent MYC target genes reveal a primordial signature involved in biomass accumulation.

The functions of key oncogenic transcription factors independent of context have not been fully delineated despite our richer understanding of the genetic alterations in human cancers. The MYC oncogene, which produces the Myc transcription factor, is frequently altered in human cancer and is a major regulatory hub for many cancers. In this regard, we sought to unravel the primordial signature of Myc function by using high-throughput genomic approaches to identify the cell-type independent core Myc target gene signature. Using a model of human B lymphoma cells bearing inducible MYC, we identified a stringent set of direct Myc target genes via chromatin immunoprecipitation (ChIP), global nuclear run-on assay, and changes in mRNA levels. We also identified direct Myc targets in human embryonic stem cells (ESCs). We further document that a Myc core signature (MCS) set of target genes is shared in mouse and human ESCs as well as in four other human cancer cell types. Remarkably, the expression of the MCS correlates with MYC expression in a cell-type independent manner across 8,129 microarray samples, which include 312 cell and tissue types. Furthermore, the expression of the MCS is elevated in vivo in Eµ-Myc transgenic murine lymphoma cells as compared with premalignant or normal B lymphocytes. Expression of the MCS in human B cell lymphomas, acute leukemia, lung cancers or Ewing sarcomas has the highest correlation with MYC expression. Annotation of this gene signature reveals Myc's primordial function in RNA processing, ribosome biogenesis and biomass accumulation as its key roles in cancer and stem cells.

Global regulation of nucleotide biosynthetic genes by c-Myc.

BACKGROUND: The c-Myc transcription factor is a master regulator and integrates cell proliferation, cell growth and metabolism through activating thousands of target genes. Our identification of direct c-Myc target genes by chromatin immunoprecipitation (ChIP) coupled with pair-end ditag sequencing analysis (ChIP-PET) revealed that nucleotide metabolic genes are enriched among c-Myc targets, but the role of Myc in regulating nucleotide metabolic genes has not been comprehensively delineated. METHODOLOGY/PRINCIPAL FINDINGS: Here, we report that the majority of genes in human purine and pyrimidine biosynthesis pathway were induced and directly bound by c-Myc in the P493-6 human Burkitt's lymphoma model cell line. The majority of these genes were also responsive to the ligand-activated Myc-estrogen receptor fusion protein, Myc-ER, in a Myc null rat fibroblast cell line, HO.15 MYC-ER. Furthermore, these targets are also responsive to Myc activation in transgenic mouse livers in vivo. To determine the functional significance of c-Myc regulation of nucleotide metabolism, we sought to determine the effect of loss of function of direct Myc targets inosine monophosphate dehydrogenases (IMPDH1 and IMPDH2) on c-Myc-induced cell growth and proliferation. In this regard, we used a specific IMPDH inhibitor mycophenolic acid (MPA) and found that MPA dramatically inhibits c-Myc-induced P493-6 cell proliferation through S-phase arrest and apoptosis. CONCLUSIONS/SIGNIFICANCE: Taken together, these results demonstrate the direct induction of nucleotide metabolic genes by c-Myc in multiple systems. Our finding of an S-phase arrest in cells with diminished IMPDH activity suggests that nucleotide pool balance is essential for c-Myc's orchestration of DNA replication, such that uncoupling of these two processes create DNA replication stress and apoptosis.

The c-Myc target gene Rcl (C6orf108) encodes a novel enzyme, deoxynucleoside 5'-monophosphate N-glycosidase.

RCL is a c-Myc target with tumorigenic potential. Genome annotation predicted that RCL belonged to the N-deoxyribosyltransferase family. However, its putative relationship to this class of enzymes did not lead to its precise biochemical function. The purified native or N-terminal His-tagged recombinant rat RCL protein expressed in Escherichia coli exhibits the same enzyme activity, deoxynucleoside 5'-monophosphate N-glycosidase, never before described. dGMP appears to be the best substrate. RCL opens a new route in the nucleotide catabolic pathways by cleaving the N-glycosidic bond of deoxynucleoside 5'-monophosphates to yield two reaction products, deoxyribose 5-phosphate and purine or pyrimidine base. Biochemical studies show marked differences in the terms of the structure and catalytic mechanism between RCL and of its closest enzyme family neighbor, N-deoxyribosyltransferase. The reaction products of this novel enzyme activity have been implicated in purine or pyrimidine salvage, glycolysis, and angiogenesis, and hence are all highly relevant for tumorigenesis.

The c-Myc target gene network.

For more than a decade, numerous studies have suggested that the c-Myc oncogenic protein is likely to broadly influence the composition of the transcriptome. However, the evidence required to support this notion was made available only recently, much to the anticipation of an eagerly awaiting field. In the past 5 years, many high-throughput screens based on microarray gene expression profiling, serial analysis of gene expression (SAGE), chromatin immunoprecipitation (ChIP) followed by genomic array analysis, and Myc-methylase chimeric proteins have generated a wealth of information regarding Myc responsive and target genes. From these studies, the c-Myc target gene network is estimated to comprise about 15% of all genes from flies to humans. Both genomic and functional analyses of c-Myc targets suggest that while c-Myc behaves as a global regulator of transcription, groups of genes involved in cell cycle regulation, metabolism, ribosome biogenesis, protein synthesis, and mitochondrial function are over-represented in the c-Myc target gene network. c-Myc also consistently represses genes involved in cell growth arrest and cell adhesion. The overexpression of c-Myc predisposes cells to apoptosis under nutrient or growth factor deprivation conditions, although the critical sets of genes involved remain elusive. Despite tremendous advances, the downstream target genes that distinguish between physiologic and tumorigenic functions of c-Myc remain to be delineated.

Global mapping of c-Myc binding sites and target gene networks in human B cells.

The protooncogene MYC encodes the c-Myc transcription factor that regulates cell growth, cell proliferation, cell cycle, and apoptosis. Although deregulation of MYC contributes to tumorigenesis, it is still unclear what direct Myc-induced transcriptomes promote cell transformation. Here we provide a snapshot of genome-wide, unbiased characterization of direct Myc binding targets in a model of human B lymphoid tumor using ChIP coupled with pair-end ditag sequencing analysis (ChIP-PET). Myc potentially occupies > 4,000 genomic loci with the majority near proximal promoter regions associated frequently with CpG islands. Using gene expression profiles with ChIP-PET, we identified 668 direct Myc-regulated gene targets, including 48 transcription factors, indicating that Myc is a central transcriptional hub in growth and proliferation control. This first global genomic view of Myc binding sites yields insights of transcriptional circuitries and cis regulatory modules involving Myc and provides a substantial framework for our understanding of mechanisms of Myc-induced tumorigenesis.

The c-Myc target gene PRDX3 is required for mitochondrial homeostasis and neoplastic transformation.

Deregulated expression of the c-Myc transcription factor is found in a wide variety of human tumors. Because of this significant role in oncogenesis, considerable effort has been devoted to elucidating the molecular program initiated by deregulated c-myc expression. The primary transforming activity of Myc is thought to arise through transcriptional regulation of numerous target genes. Thus far, Myc target genes involved in mitochondrial function have not been characterized in depth. Here, we describe a nuclear c-Myc target gene, PRDX3, which encodes a mitochondrial protein of the peroxiredoxin gene family. Expression of PRDX3 is induced by the mycER system and is reduced in c-myc(-/-) cells. Chromatin immunoprecipitation analysis spanning the entire PRDX3 genomic sequence reveals that Myc binds preferentially to a 930-bp region surrounding exon 1. We show that PRDX3 is required for Myc-mediated proliferation, transformation, and apoptosis after glucose withdrawal. Results using mitochondria-specific fluorescent probes demonstrate that PRDX3 is essential for maintaining mitochondrial mass and membrane potential in transformed rat and human cells. These data provide evidence that PRDX3 is a c-Myc target gene that is required to maintain normal mitochondrial function.

A strategy for identifying transcription factor binding sites reveals two classes of genomic c-Myc target sites.

Defining the hardwiring of transcription factors to their cognate genomic binding sites is essential for our understanding of biological processes. We used scanning chromatin immunoprecipitation to identify in vivo binding regions (E boxes) for c-Myc in three target genes as a model system. Along with other c-Myc target genes that have been validated by chromatin immunoprecipitation, we used the publicly available genomic sequences to determine whether experimentally derived in vivo binding sites might be predictable from nonexonic sequence conservation across species. Our studies revealed two classes of target genomic binding sites. Although the majority of target genes studied [class I: B23 (NPM1), CAD, CDK4, cyclin D2, ID2, LDH-A, MNT, PTMa, ODC, NM23B, nucleolin, prohibitin, SHMT1, and SHMT2] demonstrate significant sequence conservation of the E boxes and flanking regions, several genes (cyclin B1, JPO1, and PRDX3) belong to a second class (class II) that does not display sequence conservation at and around the site of c-Myc binding. On the basis of our model, we propose a strategy for predicting transcription factor binding sites using phylogenetic sequence comparisons, which will select potential class I target genes among the many emerging candidates from DNA-microarray studies for experimental validation by chromatin immunoprecipitation.

An integrated database of genes responsive to the Myc oncogenic transcription factor: identification of direct genomic targets.

We report a database of genes responsive to the Myc oncogenic transcription factor. The database Myc Target Gene prioritizes candidate target genes according to experimental evidence and clusters responsive genes into functional groups. We coupled the prioritization of target genes with phylogenetic sequence comparisons to predict c-Myc target binding sites, which are in turn validated by chromatin immunoprecipitation assays. This database is essential for the understanding of the genetic regulatory networks underlying the genesis of cancers.