Myc and Max: a putative transcriptional complex in search of a cellular target

Structural basis for effects of CpA modifications on C/EBPβ binding of DNA

CCAAT/enhancer binding proteins (C/EBPs) regulate gene expression in a variety of cells/tissues/organs, during a range of developmental stages, under both physiological and pathological conditions. C/EBP-related transcription factors have a consensus binding specificity of 5'-TTG-CG-CAA-3', with a central CpG/CpG and two outer CpA/TpG dinucleotides. Methylation of the CpG and CpA sites generates a DNA element with every pyrimidine having a methyl group in the 5-carbon position (thymine or 5-methylcytosine (5mC)). To understand the effects of both CpG and CpA modification on a centrally-important transcription factor, we show that C/EBPβ binds the methylated 8-bp element with modestly-increased (2.4-fold) binding affinity relative to the unmodified cognate sequence, while cytosine hydroxymethylation (particularly at the CpA sites) substantially decreased binding affinity (36-fold). The structure of C/EBPβ DNA binding domain in complex with methylated DNA revealed that the methyl groups of the 5mCpA/TpG make van der Waals contacts with Val285 in C/EBPβ. Arg289 recognizes the central 5mCpG by forming a methyl-Arg-G triad, and its conformation is constrained by Val285 and the 5mCpG methyl group. We substituted Val285 with Ala (V285A) in an Ala-Val dipeptide, to mimic the conserved Ala-Ala in many members of the basic leucine-zipper family of transcription factors, important in gene regulation, cell proliferation and oncogenesis. The V285A variant demonstrated a 90-fold binding preference for methylated DNA (particularly 5mCpA methylation) over the unmodified sequence. The smaller side chain of Ala285 permits Arg289 to adopt two alternative conformations, to interact in a similar fashion with either the central 5mCpG or the TpG of the opposite strand. Significantly, the best-studied cis-regulatory elements in RNA polymerase II promoters and enhancers have variable sequences corresponding to the central CpG or reduced to a single G:C base pair, but retain a conserved outer CpA sequence. Our analyses suggest an important modification-dependent CpA recognition by basic leucine-zipper transcription factors.

MAX is an epigenetic sensor of 5-carboxylcytosine and is altered in multiple myeloma

The oncogenic transcription factor MYC and its binding partner MAX regulate gene expression by binding to DNA at enhancer-box (E-box) elements 5΄-CACGTG-3΄. In mammalian genomes, the central E-box CpG has the potential to be methylated at the 5-position of cytosine (5mC), or to undergo further oxidation to the 5-hydroxymethyl (5hmC), 5-formyl (5fC), or 5-carboxyl (5caC) forms. We find that MAX exhibits the greatest affinity for a 5caC or unmodified C-containing E-box, and much reduced affinities for the corresponding 5mC, 5hmC or 5fC forms. Crystallization of MAX with a 5caC modified E-box oligonucleotide revealed that MAX Arg36 recognizes 5caC using a 5caC–Arg–Guanine triad, with the next nearest residue to the carboxylate group being Arg60. In an analysis of >800 primary multiple myelomas, MAX alterations occurred at a frequency of ∼3%, more than half of which were single nucleotide substitutions affecting a basic clamp-like interface important for DNA interaction. Among these, arginines 35, 36 and 60 were the most frequently altered. In vitro binding studies showed that whereas mutation of Arg36 (R36W) or Arg35 (R35H/L) completely abolished DNA binding, mutation of Arg60 (R60Q) significantly reduced DNA binding, but retained a preference for the 5caC modified E-box. Interestingly, MAX alterations define a subset of myeloma patients with lower MYC expression and a better overall prognosis. Together these data indicate that MAX can act as a direct epigenetic sensor of E-box cytosine modification states and that local CpG modification and MAX variants converge to modulate the MAX-MYC transcriptional network.

The lncRNA-MYC regulatory network in cancer

Long non-coding RNAs (lncRNAs) have been widely studied in recent years, and accumulating evidence identified lncRNAs as crucial regulators of various biological processes, including cell cycle progression, chromatin remodeling, gene transcription, and posttranscriptional processing. In addition, the fact that lncRNAs interact with the MYC gene family in human carcinomas has been discovered. This review summarizes the latest progress on the investigation of lncRNAs and MYC, particularly focusing on the interplay between lncRNAs and MYC in cancer to reveal the significance of lncRNA-MYC network in regulating initiation, development, and metastasis of tumors. Further research and collection of clinical data would provide a better understanding of lncRNA-MYC network in cancer diagnosis and treatment.

ALK is a MYCN target gene and regulates cell migration and invasion in neuroblastoma

Human anaplastic lymphoma kinase (ALK) has been identified as an oncogene that is mutated or amplified in NBLs. To obtain a better understanding of the molecular events associated with ALK in the pathogenesis of NBL, it is necessary to clarify how ALK gene contributes to NBL progression. In the present study, we found that ALK expression was significantly high in NBL clinical samples with amplified MYCN (n = 126, P < 0.01) and in developing tumors of MYCN-transgenic mice. Indeed, promoter analysis revealed that ALK is a direct transcriptional target of MYCN. Overexpression and knockdown of ALK demonstrated its function in cell proliferation, migration and invasion. Moreover, treatment with an ALK inhibitor, TAE-684, efficiently suppressed such biological effects in MYCN amplified cells and tumor growth of the xenograft in mice. Our present findings explore the fundamental understanding of ALK in order to develop novel therapeutic tools by targeting ALK for aggressive NBL treatment.

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.

Nestin is a potential mediator of malignancy in human neuroblastoma cells

Amplification of the N-myc proto-oncogene signifies aggressive behavior in human neuroblastoma. Likewise, overexpression of the intermediate filament nestin, a neuroectodermal stem cell marker, is linked to increased aggressiveness in several nervous system tumors. We investigated the interaction of these two proteins in human neuroblastoma cells. Neuroblastic cell variants with high levels of N-Myc protein have significantly higher nestin protein levels than non-amplified cell lines, suggesting that the transcription factor N-Myc may regulate nestin expression. Stable transfection of a nestin antisense sequence into neuroblastic, N-myc-amplified, LA1-55n cells results in a 2-fold reduction in nestin protein without altering N-Myc expression. However, cell functions attributed to N-Myc (growth rate, anchorage-independent growth, and motility) all decrease significantly. Transfection studies that modulate N-Myc levels also result in commensurate changes in nestin mRNA and protein amounts as well as in cell proliferation and motility. Thus, nestin appears to be downstream of and regulated by N-Myc. Gel mobility shift assays show that N-Myc binds specifically to E-box sequences in the regulatory second intron of the nestin gene and nuclear run-off studies show that increases in N-Myc protein up-regulate nestin transcription rate. Subcellular fractionation and immunoblot studies indicate that nestin is present in the nucleus as well as in the cytoplasm of neuroblastoma cell lines. Finally, DNA cross-linking experiments show that nestin binds DNA in N-myc-amplified N-type cell lines. Thus, nestin may be one mediator of N-myc-associated tumor aggressiveness of human neuroblastoma.

Transcriptional regulation of the ornithine decarboxylase gene by c-Myc/Max/Mad network and retinoblastoma protein interacting with c-Myc

c-Myc is an oncogenic transcription factor involved in the regulation of cell proliferation, differentiation and apoptosis. The direct targets of c-Myc mediating these various processes are slowly being unravelled. This study indicates that the ornithine decarboxylase (ODC) gene is a physiological transcriptional target of c-Myc in association with induction of cell proliferation and transformation, but not with induction of apoptosis. In addition to the two conserved CACGTG c-Myc-binding sites in the first intron, the CATGTG motif in the 5'-flanking region of the murine odc is also shown to be a functional c-Myc response element. odc is thus a c-Myc target with three binding sites a distance apart. Transient transfection studies with different c-Myc, Max and Mad constructs in COS-7 cells showed that the balance between c-Myc/Max, Max/Max and Max/Mad complexes is crucial for the regulation, resulting in either transactivation or transrepression of an ODC-CAT reporter gene. Transcription of both ODC-CAT and endogenous odc was strongly induced in HeLa cells expressing tetracycline-regulated c-Myc, concomitant with c-Myc promoting the S-phase entry of the cells. Transformation of NIH3T3 cells by c-Ha-ras-(Val12) oncogene was reversed by expression of transcriptionally inactive c-Myc, which was associated with repression of ODC-CAT expression. Further, the c-Myc-induced transactivation of ODC-CAT in COS-7 cells was suppressed by co-expression of the retinoblastoma tumor suppresser pRb, evidently as a result of pRb directly or indirectly interacting with c-Myc. Importantly, the endogenous c-Myc and pRb proteins were also found to associate in Colo 320HSR cells under physiological conditions. These results suggest that c-Myc and pRb can interact in vivo, and may in part control some aspects of cell proliferation and transformation through modulation of odc expression.

Inhibition of the terminal stages of adipocyte differentiation by cMyc

The nuclear oncoprotein Myc is a pivotal regulator of several important biological processes, including cellular proliferation, differentiation, and apoptosis. Deregulated Myc expression is incompatible with terminal differentiation in a variety of cell types, including adipocytes. To understand how Myc inhibits adipogenesis, we analyzed the effect of Myc on the expression of genes characteristic of distinct phases of the hormonally induced adipogenic differentiation program in 3T3-L1 preadipocytes. We show that the early regulators, C/EBPbeta and C/EBPdelta, are induced normally in response to hormone in 3T3-L1 preadipocytes constitutively expressing Myc, but that expression of the downstream regulators, C/EBPalpha and PPARgamma2, and later markers of differentiation is suppressed. These data demonstrate that Myc specifically inhibits the terminal stages of the adipogenic program and suggest that Myc may act by blocking C/EBPbeta- and C/EBPdelta-directed activation of C/EBPalpha and PPARgamma2 expression, although the precise molecular mechanism is not understood. Surprisingly, a serum component(s) could override the Myc-induced differentiation block, suggesting that the ability of a cell to undergo terminal differentiation is governed by the action of both positive and negative factors. Since differentiation and proliferation are mutually exclusive events, this has important implications since it may be possible to force malignant cells along a differentiation pathway, thereby curbing their proliferative potential.

Effects of EGF and TGFbeta1 on c-myc gene expression and DNA synthesis in embryonic hamster palate mesenchymal cells

Previous work has shown that cell proliferation is a major contributor to the early palate morphogenesis in mammals. The present study was undertaken to examine the effect of EGF, TGFbeta1 and their combination on proliferation (measured by DNA synthesis) and on the expression of a growth related proto-oncogene, c-myc, in embryonic hamster palate mesenchymal cells (HPMC). Vertically developing hamster palatal shelves were dissected on day 11 of gestation, and trypsinized, and primary cultures were grown in DMEM + 10% serum at 37 degrees C and 5% CO2. Following appropriate growth factor treatment of HPMC, DNA synthesis was measured by scintillation counting and extracted RNA was subjected to Northern blot analysis. In serum-starved, pre-confuent cultures treated with EGF (20 ng/ml), DNA synthesis was stimulated in the presence of 2.5% serum. In contrast, treatment of HPMC with TGFbeta1 (10 ng/ml) in the presence or absence of EGF/serum for 24 hr, or HPMC pre-treatment with TGFbeta1 (30 min) followed by EGF/serum (24 hr), resulted in an arrest of DNA synthesis. Northern blot analysis of RNA extracted from HPMC showed that as serum-starved, growth-arrested cells progressed through G0 to G1 phase of the cell cycle, following EGF treatment, c-myc was expressed by 1 hr and declined thereafter. In contrast, TGFbeta1 did not support expression of c-myc. Following pre- or co-treatment with TGFbeta1, the EGF +/- serum-induced expression of c-myc was seen between 1 and 6 hr. It appears that EGF-induced expression of c-myc may be involved in advancing the HPMC in G1, and thus may contribute to the onset of DNA synthesis in HPMC. Since co- or pre-treatment with TGFbeta1 did not inhibit EGF/serum induced expression of c-myc, it is possible that growth arresting effect of TGFbeta1 may not be exerted directly through inhibition or blockage of c-myc expression.

Proto-oncogene activity in the testis of the lizard, Podarcis s. sicula, during the annual reproductive cycle

Since proto-oncogenes play a central role in the regulation of cellular growth and differentiation, localization of MYC, FOS, and JUN proteins has been studied in the testis of the lizard, Podarcis s. sicula, during the annual reproductive cycle by immunocytochemistry using antisera against c-myc, c-fos, and c-jun products. MYC was localized in the nuclei of spermatogonia (SPG), I and II spermatocytes (SPC), and spermatids (SPT). Strong immunoreactivity was detected in Sertoli cells just prior to the onset of the early spring spermatogenic wave coinciding with the androgen peak. FOS protein was present in the nuclei of SPG and SPC. In SPG an exclusive nuclear localization was seen during the active spermatogenic period (February-March and September). A perinuclear localization was observed during other months. Immunoreactivity in Sertoli cells was also observed during the periods of active spermatogenesis. JUN protein was localized in the cytoplasm of SPG as well as in I and II SPC and was detected in the nuclei of I and II SPC during April and October when spermatogenic waves occur. These data suggest that proto-oncogene activities have regulatory roles in the spermatogenesis of the lizard.

Modeling signal transduction in normal and cancer cells using complex adaptive systems

As a first approximation, organisms can be defined by the complement of cell types that they possess. Each cell type is defined by its specific collection of signal transduction pathways. While many pathways are common to most cell types (e.g. glycolysis), others are specific to a particular cell type and serve to characterized that cell. Many diseases, including cancer, are characterized by aberrations in general and specific signal-transduction pathways. These pathways are generally intricate and not easily modeled. The formalism of complex adaptive system theory, however, provides the tools by which these pathways can be investigated. By modeling signal-transduction pathways from the viewpoint of complex adaptive systems, a deeper understanding of their intricacies may result. This could eventually lead to novel methods of therapeutic intervention in diseases that arise from aberrant signal transduction.

Mad proteins contain a dominant transcription repression domain

Transcription repression by the basic region-helix-loop-helix-zipper (bHLHZip) protein Mad1 requires DNA binding as a ternary complex with Max and mSin3A or mSin3B, the mammalian orthologs of the Saccharomyces cerevisiae transcriptional corepressor SIN3. The interaction between Mad1 and mSin3 is mediated by three potential amphipathic alpha-helices: one in the N terminus of Mad (mSin interaction domain, or SID) and two within the second paired amphipathic helix domain (PAH2) of mSin3A. Mutations that alter the structure of the SID inhibit in vitro interaction between Mad and mSin3 and inactivate Mad's transcriptional repression activity. Here we show that a 35-residue region containing the SID represents a dominant repression domain whose activity can be transferred to a heterologous DNA binding region. A fusion protein comprising the Mad1 SID linked to a Ga14 DNA binding domain mediates repression of minimal as well as complex promoters dependent on Ga14 DNA binding sites. In addition, the SID represses the transcriptional activity of linked VP16 and c-Myc transactivation domains. When fused to a full-length c-Myc protein, the Mad1 SID specifically represses both c-Myc's transcriptional and transforming activities. Fusions between the GAL DNA binding domain and full-length mSin3 were also capable of repression. We show that the association between Mad1 and mSin3 is not only dependent on the helical SID but is also dependent on both putative helices of the mSin3 PAH2 region, suggesting that stable interaction requires all three helices. Our results indicate that the SID is necessary and sufficient for transcriptional repression mediated by the Mad protein family and that SID repression is dominant over several distinct transcriptional activators.

Confocal laser scanning microscopy of oncogene localization in rainbow trout cell lines derived from normal and tumor tissue

We examined the localization and expression of the nuclear oncoprotein c-myc and the cytoplasmic membrane-associated oncoprotein c-ras in rainbow trout cell lines derived from both normal and tumor tissue in order to question whether c-myc and ras oncoprotein immunostaining was increased in cells derived from tumors compared to cells derived from normal tissue. Cell lines examined were derived from normal rainbow trout gonadal cells (RTG-2), a rainbow trout hepatoma (RTH-149), and a rainbow trout mesothelioma (RTM). Protein products of c-ras and c-myc were visualized in these 3 cell lines by employing fluorescein-labeled anti-mouse pan-ras and c-myc antibodies. The RTG-2 cells were used in this study as normal, control cells, and they exhibited little pan-ras and c-myc staining. The RTH-149 cell line (a tumorigenic cell line) exhibited positive pan-ras staining in regions of the membrane and cell cytoplasm. Localization of c-myc staining to perinuclear regions was punctate in RTH-149 cells. RTM cells (also a tumorigenic cell line) displayed a ras staining localization similar to the pattern seen in RTH-149 cells. RTM cells exhibit a diffuse perinuclear staining and, thus, display a more ubiquitous localization of c-myc than RTH-149 cells. Northern blot analysis indicated that c-myc expression was highest in RTM cells, whereas RTG-2 cells and RTH-149 cells expressed similar lower levels of c-myc expression. We were unable to detect significant ras expression in any of the cell lines by Northern blot analysis. In summary, the cell line derived from normal tissue, the RTG-2 cells, displayed little ras and c-myc immunostaining, whereas the cell lines derived from tumorigenic tissue, RTH and RTM cells, displayed increased immunostaining for c-myc and ras proteins.

Expression and activity of L-Myc in normal mouse development

To determine the role of L-Myc in normal mammalian development and its functional relationship to other members of the Myc family, we determined the normal patterns of L-myc gene expression in the developing mouse by RNA in situ hybridization and assessed the phenotypic impact of L-Myc deficiency produced through standard gene targeting methodology. L-myc transcripts were detected in the developing kidney and lung as well as in both the proliferative and the differentiative zones of the brain and neural tube. Despite significant expression of L-myc in developing mouse tissue, homozygous null L-myc mice were found to be viable, reproductively competent, and represented in expected frequencies from heterozygous matings. A detailed histological survey of embryonic and adult tissues, characterization of an embryonic neuronal marker, and measurement of cellular proliferation in situ did not reveal any congenital abnormalities. The lack of an apparent phenotype associated with L-Myc deficiency indicates that L-Myc is dispensable for gross morphological development and argues against a unique role for L-Myc in early central nervous system development as had been previously suggested. Although overlapping expression patterns among myc family members raise the possibility of complementation of L-Myc deficiency by other Myc oncoproteins, compensatory changes in the levels of c- and/or N-myc transcripts were not detected in homozygous null L-myc mice.

An amino-terminal domain of Mxi1 mediates anti-Myc oncogenic activity and interacts with a homolog of the yeast transcriptional repressor SIN3

Documented interactions among members of the Myc superfamily support a yin-yang model for the regulation of Myc-responsive genes in which transactivation-competent Myc-Max heterodimers are opposed by repressive Mxi1-Max or Mad-Max complexes. Analysis of mouse mxi1 has led to the identification of two mxi1 transcript forms possessing open reading frames that differ in their capacity to encode a short amino-terminal alpha-helical domain. The presence of this segment dramatically augments the suppressive potential of Mxi1 and allows for association with a mammalian protein that is structurally homologous to the yeast transcriptional repressor SIN3. These findings provide a mechanistic basis for the antagonistic actions of Mxi1 on Myc activity that appears to be mediated in part through the recruitment of a putative transcriptional repressor.

Analysis of the DNA-binding and dimerization activities of Neurospora crassa transcription factor NUC-1

NUC-1, a positive regulatory protein of Neurospora crassa, controls the expression of several unlinked target genes involved in phosphorus acquisition. The carboxy-terminal end of the NUC-1 protein has sequence similarity to the helix-loop-helix family of transcription factors. Bacterially expressed and in vitro-synthesized proteins, which consist of the carboxy-terminal portion of NUC-1, bind specifically to upstream sequences of two of its target genes, pho2+ and pho-4+. These upstream sequences contain the core sequence, CACGTG, a target for many helix-loop-helix proteins. A large loop region (47 amino acids) separates the helix I and helix II domains. Mutations and deletion within the loop region did not interfere with the in vitro or in vivo functions of the protein. Immediately carboxy-proximal to the helix II domain, the NUC-1 protein contains an atypical zipper domain which is essential for function. This domain consists of a heptad repeat of alanine and methionine rather than leucine residues. Analysis of mutant NUC-1 proteins suggests that the helix II and the zipper domains are essential for the protein dimerization, whereas the basic and the helix I domains are involved in DNA binding. The helix I domain, even though likely to participate in dimer formation while NUC-1 is bound to DNA, is not essential for in vitro dimerization.

Analysis of the DNA-binding and dimerization activities of Neurospora crassa transcription factor NUC-1

NUC-1, a positive regulatory protein of Neurospora crassa, controls the expression of several unlinked target genes involved in phosphorus acquisition. The carboxy-terminal end of the NUC-1 protein has sequence similarity to the helix-loop-helix family of transcription factors. Bacterially expressed and in vitro-synthesized proteins, which consist of the carboxy-terminal portion of NUC-1, bind specifically to upstream sequences of two of its target genes, pho2+ and pho-4+. These upstream sequences contain the core sequence, CACGTG, a target for many helix-loop-helix proteins. A large loop region (47 amino acids) separates the helix I and helix II domains. Mutations and deletion within the loop region did not interfere with the in vitro or in vivo functions of the protein. Immediately carboxy-proximal to the helix II domain, the NUC-1 protein contains an atypical zipper domain which is essential for function. This domain consists of a heptad repeat of alanine and methionine rather than leucine residues. Analysis of mutant NUC-1 proteins suggests that the helix II and the zipper domains are essential for the protein dimerization, whereas the basic and the helix I domains are involved in DNA binding. The helix I domain, even though likely to participate in dimer formation while NUC-1 is bound to DNA, is not essential for in vitro dimerization.

The TYE7 gene of Saccharomyces cerevisiae encodes a putative bHLH-LZ transcription factor required for Ty1-mediated gene expression

In Saccharomyces cerevisiae, expression of a gene adjacent to the retrotransposon Ty1 is often mediated by Ty-internal sequences. We have identified novel mutants, tye7, which are affected in Ty1-mediated expression of ADH2 through a Ty1 sequence distal to the 5' long terminal repeat sequence. The TYE7 gene has been isolated and characterized. It encodes a 33 kDa protein whose N-terminal third is extremely rich in serine residues (28%). Within its C-terminal sequence, a remarkable similarity to Myc and Max proteins can be found. Thus, TYE7 is a potential member of the basic region/helix-loop-helix/leucine-zipper protein family. TYE7 function is not essential for growth. It may primarily function as a transcriptional activator in Ty1-mediated gene expression, as has been confirmed by the activation of reporter gene expression by a LexA-TYE7 hybrid protein. ADH2 activation by defined Ty1 derivatives revealed that TYE7 acts positively through the more distal Ty1 enhancer element (region D), and negatively in a region between A (the 5' proximal enhancer element) and D.

Structure and expression of the Drosophila ubiquitin-80-amino-acid fusion-protein gene

In the fruitfly Drosophila, as in all eukaryotes examined so far, some ubiquitin-coding sequences appear fused to unrelated open reading frames. Two of these fusion genes have been previously described (the homologues of UBI1-UBI2 and UBI4 in yeast), and we report here the organization and expression of a third one, the DUb80 gene (the homologue of UBI3 in yeast). This gene encodes a ubiquitin monomer fused to an 80-amino-acid extension which is homologous with the ribosomal protein encoded by the UB13 gene. The 5' regulatory region of DUb80 shares common features with another ubiquitin fusion gene, DUb52, and with the ribosomal protein genes of Drosophila, Xenopus and mouse. We also find helix-loop-helix protein-binding sequences (E-boxes). The DUb80 gene is transcribed to a 0.9 kb mRNA which is particularly abundant under conditions of high protein synthesis, such as in ovaries and exponentially growing cells.

c-Myc binds to 5' flanking sequence motifs of the dihydrofolate reductase gene in cellular extracts: role in proliferation

The dihydrofolate reductase is a key enzyme of the folate metabolism which supplies the cell with dTTPs for DNA synthesis. Using cellular extracts, we demonstrate the formation of c-Myc/Max heterodimers at the dihydrofolate reductase (DHFR) 5' flanking CANNTG (E-box) motifs. The presence of these complexes correlates with c-Myc levels and active cellular proliferation.

Molecular cloning and characterization of rat brain 2',3'-cyclic nucleotide 3'-phosphodiesterase isoform 2

We have isolated a cDNA coding for the larger isoform of the rat brain 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP2), a protein associated with myelination in the central nervous system (CNS). The complete 420 amino acid sequence was deduced from the nucleotide sequence of the cDNA. Sequence comparisons show that rat CNP shares 96% homology with mouse, 84% with bovine, and 86% with human CNP. Errors in the published sequence of rat CNP1 have now been corrected. Comparisons with other proteins reveal several interesting conserved motifs, including two leucine repeat heptads, and two consensus motifs for phosphorylation in the N-terminal domain of CNP2.

Suppression of Myc, but not E1a, transformation activity by Max-associated proteins, Mad and Mxi1

Mad and Mxi1, two members of the Myc-related basic-region helix-loop-helix/leucine-zipper family of proteins, associate directly with Max to form sequence-specific DNA binding heterodimers that are transactivation-incompetent. Mad-Max complexes have been shown to exert a strong repressive effect on Myc-induced transactivation, perhaps through the competitive occupation of common promoter binding sites also recognized by active Myc-Max heterodimers. To place these recent biochemical observations in a biological context, mad and mxi1 expression vectors were tested for their ability to influence Myc transformation activity in the rat embryo fibroblast cooperation assay. Addition of an equimolar amount of mad or mxi1 expression vector to mouse c-myc/ras cotransfections resulted in a dramatic reduction in both the number of foci generated and the severity of the malignant phenotype. Myc-specific suppression by Mad and Mxi1 was demonstrated by their ability to affect c- and N-myc-, but not ela-, induced transformation. In contrast, mad and mxi1 expression constructs bearing deletions in the basic region exerted only mild repressive effects on Myc transformation activity, suggesting that occupation of common DNA binding sites by transactivation-incompetent Mad-Max or Mxi1-Max complexes appears to play a more dominant role in this suppression than titration of limited intracellular pools of Max away from active Myc-Max complexes. Thus, these biological data support a current model for regulation of Myc function in which relative intracellular levels of Mad and Mxi1 in comparison to those of Myc may determine the degree of activation of Myc-responsive growth pathways.

Myc-Max-Mad: a transcription factor network controlling cell cycle progression, differentiation and death

The Myc oncoprotein dimerizes with its partner, Max, to bind DNA, activate transcription, and promote cell proliferation, as well as programmed cell death. Max also forms homodimers or heterodimers with its alternative partners, Mad and Mxi-1. These complexes behave as antagonists of Myc/Max through competition for common DNA targets, and perhaps permit cellular differentiation.

Distinct DNA binding preferences for the c-Myc/Max and Max/Max dimers

The transcription factor c-Myc and its dimerisation partner Max are members of the basic/helix-loop-helix/leucine-zipper (bHLH-Z) family and bind to the DNA core sequence CACGTG. Using a site-selection protocol, we determined the complete 12 base pair consensus binding sites of c-Myc/Max (RACCACGTGGTY) and Max/Max (RANCACGTGNTY) dimers. We find that the c-Myc/Max dimer fails to bind the core when it is flanked by a 5'T or a 3'A, while the Max/Max dimer readily binds such sequences. Furthermore we show that inappropriate flanking sequences preclude transactivation by c-Myc in vivo. In conclusion, Max/Max dimers are less discriminatory than c-Myc/Max and may regulate other genes in addition to c-Myc/Max targets.

The promoter region of the human type-I-DNA-topoisomerase gene. Protein-binding sites and sequences involved in transcriptional regulation

We examined the promoter of the human type-I-DNA topoisomerase gene (hTOP1) for regions protected against DNase I digestion by nuclear proteins from HeLa or from adenovirus-transformed 293 cells. We identified ten protected DNA sequences within 580 bp of DNA upstream of the transcriptional-start sites and one additional site, which is located between the two clusters of transcriptional-start sites. Several of these protein-binding sites have significant similarities to recognition sequences of known transcription factors including factors Sp1, octamer transcription factor, cAMP-responsive-element-binding protein (CREB/ATF), NF-kappa B and members of the Myc-related family of basic/helix-loop-helix/leucine-zipper proteins. Other protein-binding sites show less or no similarities to known consensus sequences. We investigated the physiological significance of these protein-binding sites using a set of deletion and nucleotide-exchange mutants. We conclude that the expression of the hTOP1 gene is regulated by a complex network of negatively and positively acting transcription factors.

TATA-binding protein and the retinoblastoma gene product bind to overlapping epitopes on c-Myc and adenovirus E1A protein

Using a protein binding assay, we show that the amino-terminal 204 amino acids of the c-Myc protein interact directly with a key component of the basal transcription factor TFIID, the TATA box-binding protein (TBP). Essentially the same region of the c-Myc protein also binds the product of the retinoblastoma gene, the RB protein. c-Myc protein coimmunoprecipitates with TBP in lysates of mammalian cells, demonstrating that the proteins are also complexed in vivo. A short peptide that spans the RB binding site of the E7 protein of human papilloma virus type 16 interferes with the binding of c-Myc to TBP. The same peptide also blocks binding of adenovirus E1A protein to TBP, suggesting that c-Myc and E1A bind to RB and TBP through overlapping epitopes. Furthermore, we show that binding of RB to E1A prevents association of E1A with TBP. Our data suggest that one of the functions of RB and RB-like proteins is to prevent interaction of viral and cellular oncoproteins, such as c-Myc and E1A, with TBP.

Cell cycle regulation of the c-Myc transcriptional activation domain

The product of the c-myc gene (c-Myc) is a sequence-specific DNA-binding protein that has previously been demonstrated to be required for cell cycle progression. Here we report that the c-Myc DNA binding site confers cell cycle regulation to a reporter gene in Chinese hamster ovary cells. The observed transactivation was biphasic with a small increase in G1 and a marked increase during the S-to-G2/M transition of the cell cycle. This cell cycle regulation of transactivation potential is accounted for, in part, by regulatory phosphorylation of the c-Myc transactivation domain. Together, these data demonstrate that c-Myc may have an important role in the progression of cells through both the G1 and G2 phases of the cell cycle.

Regulation of c-myc expression by sodium butyrate in the colon carcinoma cell line Caco-2

The human colon carcinoma cell line Caco-2 spontaneously undergoes enterocytic differentiation in culture. We used sodium butyrate to modify differentiation and growth properties of this cell line and considered c-myc expression as a potential target. Degradation of normal c-myc mRNAs with a half-life of 20 min is not coupled to translation in this cell line, as determined by cycloheximide treatment. We show that butyrate reduces c-myc mRNA levels after a 30 min delay. Butyrate does not affect c-myc expression at the level of transcriptional initiation or elongation, as determined by run-on analysis, but at a post-transcriptional level. Cycloheximide blocks butyrate-dependent reduction of c-myc mRNA levels. Cross-linking experiments show that a 34 kDa protein binds specifically to the c-myc AU-rich instability determinant found in the 3'-untranslated region (ARE). Binding of this protein to the ARE is not modulated by butyrate or cycloheximide. These experiments suggest that butyrate induces a factor involved in c-myc mRNA degradation that differs from the known ARE-associated proteins. Post-transcriptional modification of gene expression could be one of the major targets for this anti-proliferative agent.

Cell cycle regulation of the c-Myc transcriptional activation domain

The product of the c-myc gene (c-Myc) is a sequence-specific DNA-binding protein that has previously been demonstrated to be required for cell cycle progression. Here we report that the c-Myc DNA binding site confers cell cycle regulation to a reporter gene in Chinese hamster ovary cells. The observed transactivation was biphasic with a small increase in G1 and a marked increase during the S-to-G2/M transition of the cell cycle. This cell cycle regulation of transactivation potential is accounted for, in part, by regulatory phosphorylation of the c-Myc transactivation domain. Together, these data demonstrate that c-Myc may have an important role in the progression of cells through both the G1 and G2 phases of the cell cycle.

Overproduction of v-Myc in the nucleus and its excess over Max are not required for avian fibroblast transformation

The cellular proto-oncogene c-myc can acquire transforming potential by a number of different means, including retroviral transduction. The transduced allele generally contains point mutations relative to c-myc and is overexpressed in infected cells, usually as a v-Gag-Myc fusion protein. Upon synthesis, v-Gag-Myc enters the nucleus, forms complexes with its heterodimeric partner Max, and in this complex binds to DNA in a sequence-specific manner. To delineate the role for each of these events in fibroblast transformation, we introduced several mutations into the myc gene of the avian retrovirus MC29. We observed that Gag-Myc with a mutated nuclear localization signal is confined predominantly in the cytoplasm and only about 5% of the protein could be detected in the nucleus (less than the amount of endogenous c-Myc). Consequently, only a small fraction of Max is associated with Myc. However, cells infected with this mutant exhibit a completely transformed phenotype in vitro, suggesting that production of enough v-Gag-Myc to tie up all cellular Max is not needed for transformation. While the nuclear localization signal is dispensable for transformation, minimal changes in the v-Gag-Myc DNA-binding domain completely abolish its transforming potential, consistent with a role of Myc as a transcriptional regulator. One of its potential targets might be the endogenous c-myc, which is repressed in wild-type MC29-infected cells. Our experiments with MC29 mutants demonstrate that c-myc down-regulation depends on the integrity of the v-Myc DNA-binding domain and occurs at the RNA level. Hence, it is conceivable that v-Gag-Myc, either directly or circuitously, regulates c-myc transcription.

Overproduction of v-Myc in the nucleus and its excess over Max are not required for avian fibroblast transformation

The cellular proto-oncogene c-myc can acquire transforming potential by a number of different means, including retroviral transduction. The transduced allele generally contains point mutations relative to c-myc and is overexpressed in infected cells, usually as a v-Gag-Myc fusion protein. Upon synthesis, v-Gag-Myc enters the nucleus, forms complexes with its heterodimeric partner Max, and in this complex binds to DNA in a sequence-specific manner. To delineate the role for each of these events in fibroblast transformation, we introduced several mutations into the myc gene of the avian retrovirus MC29. We observed that Gag-Myc with a mutated nuclear localization signal is confined predominantly in the cytoplasm and only about 5% of the protein could be detected in the nucleus (less than the amount of endogenous c-Myc). Consequently, only a small fraction of Max is associated with Myc. However, cells infected with this mutant exhibit a completely transformed phenotype in vitro, suggesting that production of enough v-Gag-Myc to tie up all cellular Max is not needed for transformation. While the nuclear localization signal is dispensable for transformation, minimal changes in the v-Gag-Myc DNA-binding domain completely abolish its transforming potential, consistent with a role of Myc as a transcriptional regulator. One of its potential targets might be the endogenous c-myc, which is repressed in wild-type MC29-infected cells. Our experiments with MC29 mutants demonstrate that c-myc down-regulation depends on the integrity of the v-Myc DNA-binding domain and occurs at the RNA level. Hence, it is conceivable that v-Gag-Myc, either directly or circuitously, regulates c-myc transcription.

Zebra fish myc family and max genes: differential expression and oncogenic activity throughout vertebrate evolution

To gain insight into the role of Myc family oncoproteins and their associated protein Max in vertebrate growth and development, we sought to identify homologs in the zebra fish (Brachydanio rerio). A combination of a polymerase chain reaction-based cloning strategy and low-stringency hybridization screening allowed for the isolation of zebra fish c-, N-, and L-myc and max genes; subsequent structural characterization showed a high degree of conservation in regions that encode motifs of known functional significance. On the functional level, zebra fish Max, like its mammalian counterpart, served to suppress the transformation activity of mouse c-Myc in rat embryo fibroblasts. In addition, the zebra fish c-myc gene proved capable of cooperating with an activated H-ras to effect the malignant transformation of mammalian cells, albeit with diminished potency compared with mouse c-myc. With respect to their roles in normal developing tissues, the differential temporal and spatial patterns of steady-state mRNA expression observed for each zebra fish myc family member suggest unique functions for L-myc in early embryogenesis, for N-myc in establishment and growth of early organ systems, and for c-myc in increasingly differentiated tissues. Furthermore, significant alterations in the steady-state expression of zebra fish myc family genes concomitant with relatively constant max expression support the emerging model of regulation of Myc function in cellular growth and differentiation.

Recognition by Max of its cognate DNA through a dimeric b/HLH/Z domain

The three-dimensional structure of the basic/helix-loop-helix/leucine zipper domain of the transcription factor Max complexed with DNA has been determined by X-ray crystallography at 2.9 A resolution. Max binds as a dimer to its recognition sequence CACGTG by direct contacts between the alpha-helical basic region and the major groove. This symmetric homodimer, a new protein fold, is a parallel, left-handed, four-helix bundle, with each monomer containing two alpha-helical segments separated by a loop. The two alpha-helical segments are composed of the basic region plus helix 1 and helix 2 plus the leucine repeat, respectively. As in GCN4, the leucine repeat forms a parallel coiled coil.

Zebra fish myc family and max genes: differential expression and oncogenic activity throughout vertebrate evolution

To gain insight into the role of Myc family oncoproteins and their associated protein Max in vertebrate growth and development, we sought to identify homologs in the zebra fish (Brachydanio rerio). A combination of a polymerase chain reaction-based cloning strategy and low-stringency hybridization screening allowed for the isolation of zebra fish c-, N-, and L-myc and max genes; subsequent structural characterization showed a high degree of conservation in regions that encode motifs of known functional significance. On the functional level, zebra fish Max, like its mammalian counterpart, served to suppress the transformation activity of mouse c-Myc in rat embryo fibroblasts. In addition, the zebra fish c-myc gene proved capable of cooperating with an activated H-ras to effect the malignant transformation of mammalian cells, albeit with diminished potency compared with mouse c-myc. With respect to their roles in normal developing tissues, the differential temporal and spatial patterns of steady-state mRNA expression observed for each zebra fish myc family member suggest unique functions for L-myc in early embryogenesis, for N-myc in establishment and growth of early organ systems, and for c-myc in increasingly differentiated tissues. Furthermore, significant alterations in the steady-state expression of zebra fish myc family genes concomitant with relatively constant max expression support the emerging model of regulation of Myc function in cellular growth and differentiation.

Comparative analysis of the expression and oncogenic activities of Xenopus c-, N-, and L-myc homologs

A polymerase chain reaction-based cloning strategy allowed for the isolation of two distinct Xenopus L-myc genes, as well as previously isolated xc- and xN-myc genes, thus demonstrating that these three well-defined members of the mammalian myc gene family are present in lower vertebrates as well. Comparison of the Xenopus and mammalian Myc families revealed a high degree of structural relatedness at the gene and protein levels; this homology was consistent with the ability of the xc-myc1 and xN-myc1 genes to function as oncogenes in primary mammalian cells. In contrast, the xL-myc1 gene was found to be incapable of transforming rat embryo fibroblast cells, and this inactivity may relate to localized but significant differences in its putative transactivation domain. Analysis of xc-, xN-, and xL-myc gene expression demonstrated that (i) all three genes were highly expressed during oogenesis and their transcripts accumulated as abundant maternal mRNAs, (ii) each gene exhibited a distinctive pattern of expression during embryogenesis and in adult tissues, and (iii) the xL-myc1 and xL-myc2 genes were coordinately expressed in the maternal and zygotic genomes. The markedly high expression of the Xenopus myc gene family in differentiated tissues, such as the central nervous system and kidney, contrasts sharply with the low levels observed in mammalian adult tissues. These differences may reflect unique functions of the Myc family proteins in processes specific to amphibians, such as tissue regeneration.

Comparative analysis of the expression and oncogenic activities of Xenopus c-, N-, and L-myc homologs

A polymerase chain reaction-based cloning strategy allowed for the isolation of two distinct Xenopus L-myc genes, as well as previously isolated xc- and xN-myc genes, thus demonstrating that these three well-defined members of the mammalian myc gene family are present in lower vertebrates as well. Comparison of the Xenopus and mammalian Myc families revealed a high degree of structural relatedness at the gene and protein levels; this homology was consistent with the ability of the xc-myc1 and xN-myc1 genes to function as oncogenes in primary mammalian cells. In contrast, the xL-myc1 gene was found to be incapable of transforming rat embryo fibroblast cells, and this inactivity may relate to localized but significant differences in its putative transactivation domain. Analysis of xc-, xN-, and xL-myc gene expression demonstrated that (i) all three genes were highly expressed during oogenesis and their transcripts accumulated as abundant maternal mRNAs, (ii) each gene exhibited a distinctive pattern of expression during embryogenesis and in adult tissues, and (iii) the xL-myc1 and xL-myc2 genes were coordinately expressed in the maternal and zygotic genomes. The markedly high expression of the Xenopus myc gene family in differentiated tissues, such as the central nervous system and kidney, contrasts sharply with the low levels observed in mammalian adult tissues. These differences may reflect unique functions of the Myc family proteins in processes specific to amphibians, such as tissue regeneration.

The role of c-myc in cell growth

Recent experiments have established that the c-myc oncogene encodes a sequence-specific DNA-binding protein that interacts with a specific intracellular partner, Max, and probably manifests its effects through transcriptional modulation. In addition, the range of biological functions attributed to expression of c-myc has grown to include not only transformation and mitogenesis but also cell death.

Pulsatile and steady flow induces c-fos expression in human endothelial cells

The effects of pulsatile and steady fluid flow on the mRNA levels of proto-oncogenes c-fos, c-jun, and c-myc in cultured human umbilical vein endothelial cells (HUVEC) were investigated. c-fos mRNA levels in stationary cultures were very low. A 1 Hz pulsatile flow with an average shear stress of 16 dynes/cm2 induced a dramatic increase of c-fos mRNA levels in HUVEC 0.5 h after the onset of flow, which declined rapidly to basal levels within 1 h. Steady flow with a similar shear stress also induced a transient increase of c-fos mRNA levels, but to a lesser extent. In addition, increased c-fos mRNA levels were observed when low shear (2-6 dynes/cm2) was replaced by high shear (16-33 dynes/cm2). Pulsatile and steady flow caused a slight increase of c-jun and c-myc mRNA levels. The role of pulsatility was also investigated in platelet-derived growth factor (PDGF) expression. Pulsatile flow induced a transient increase of PDGF A- and B-chain mRNA levels with peaks at 1.5-2 h. Pulsatile flow, which was more stimulatory in mediating c-fos expression, however, was less stimulatory than steady flow in mediating PDGF expression. By using various inhibitors, protein kinase C was found to be an important mediator in flow-induced c-fos expression, with the involvement of G proteins, phospholipase C, and intracellular calcium. Protein kinase C was previously shown as a possible major mediator in flow-induced PDGF expression which, at least partly, appeared to follow the induction mechanism of c-fos, suggesting a possible connection between c-fos and PDGF induction. However, the c-fos antisense treatment, which significantly inhibited c-fos transcription, failed to block the flow-induced PDGF expression, suggesting that flow-induced c-fos expression may not play an important role in the mechanism of flow-induced PDGF expression. The difference in the induction of c-fos and PDGF expression under pulsatile as compared to steady flow indicates that a complex, flow-mediated regulatory mechanism of gene expression exists in HUVEC. The increased expression of these proto-oncogenes mediated by flow may be important in regulating long-term cellular responses.

Myc family oncoproteins function through a common pathway to transform normal cells in culture: cross-interference by Max and trans-acting dominant mutants

The myc family of cellular oncogenes encodes three highly related nuclear phosphoproteins (c-Myc, N-Myc, and L-Myc) that are believed to function as sequence-specific transcription factors capable of regulating genes important in cellular growth and differentiation. Current evidence indicates that Myc family proteins exist as biologically active heterodimeric complexes in association with another helix-loop-helix leucine zipper phosphoprotein, Max. We have investigated the common and unique properties among the Myc family, as well as the physiological role of Max in the regulation of Myc family function. We demonstrate that trans-activation-incompetent mutants of one Myc family member can act in trans to dominantly suppress the cotransformation activities of all three Myc oncoproteins, indicating that the Myc family functions through common genetic elements in its cellular transformation pathways. Employing co-immunoprecipitation with either anti-Myc or anti-Max antibodies, we show that the transfected normal c-Myc, N-Myc, and L-Myc oncoproteins associate with the endogenous Max protein in REF transformants, indicating that the Max interaction represents at least one component common to Myc family function. In addition, we observed a striking reduction in Myc cotransformation activity when a Max expression construct was added to myc/ras co-transfections. We discuss these biological findings in the context of a proposed model for Myc/Max function and regulation in which Max serves as either an obligate partner in the Myc/Max transcriptional complex or as a repressor in the form of a transcriptionally inert Max/Max homodimer capable of occupying Myc/Max-responsive gene targets.