The c-myc story: where we've been, where we seem to be going

Effect of a c-MYC Gene Polymorphism (g.3350G>C) on Meat Quality Traits in Berkshire

c-MYC (v-myelocytomatosis viral oncogene homologue) is a transcription factor that plays important role in many biological process including cell growth and differentiation, such as myogenesis and adipogenesis. In this study, we aimed to detect MYC gene polymorphisms, their genotype frequencies and to determine associations between these polymorphisms and meat quality traits in Berkshire pigs. We identified a single nucleotide polymorphism (SNP) in intron 2 of MYC gene by Sanger sequencing, i.e., g.3350G>C (rs321898326), that is only found in Berkshire pigs, but not in other breeds including Duroc, Landrace, and Yorkshire pigs that were used in this study. Genotypes of total 378 Berkshire pigs (138 sows and 240 boars) were determined using Hha I restriction enzyme digestion after polymerase chain reaction. Observed allele frequencies of GG, GC, and CC genotypes were 0.399, 0.508, and 0.093 respectively. Statistical analysis indicated that the g.3350G>C polymorphism was significantly associated with pH45min and cooking loss (p<0.05), suggesting that g.3350G>C SNP can be used for pre-selection of pH45min and cooking loss traits in Berkshire pigs.

Molecular characterisation and expression profiling of the ENO1 gene in the ovarian follicle of the Sichuan white goose

The ENO1 gene encodes a multifunctional enzyme that has been identified as a key component of the glycolytic pathway. Our previous studies demonstrated that ENO1 gene expression was higher in the ovaries of laying geese compared with prelaying geese. However, the molecular characterisation and expression profiling of the ENO1 gene in geese tissues and ovarian follicles remain to be determined. In this study, ENO1 cDNA (1,445 bp long) of the Sichuan white goose was cloned and characterised. The ORF of ENO1 cDNA is 1,305 bp in length and encodes a 434 amino acid protein with a molecular weight of 47.27 kDa. ENO1 expression in all of the examined tissues was the highest in spleen and the lowest in breast muscle. High expression of ENO1 appeared in the kidney, liver, adrenal gland, and retina. With increasing follicle growth, ENO1 gene expression began to decrease from the small white follicle to F5, which was followed by a sharp increase in expression in F4 and then a gradual decrease in expression from F3 to F1. Furthermore, in the postovulatory follicles (POF), the levels of ENO1 gene expression decreased gradually from POF1 to POF4. In conclusion, the ENO1 transcript was widely distributed in various tissues of the Sichuan white goose, but ENO1 expression was tissue-specific. Furthermore, the results of the ENO1 expression profiling of ovarian follicles suggest that ENO1 may play an important dual role in the progress of follicular development, where ENO1 acts as a glycolytic enzyme and also mediates apoptosis.

Hypoxia induces differential translation of enolase/MBP-1

Background

Hypoxic microenvironments in tumors contribute to transformation, which may alter metabolism, growth, and therapeutic responsiveness. The α-enolase gene encodes both a glycolytic enzyme (α-enolase) and a DNA-binding tumor suppressor protein, c-myc binding protein (MBP-1). These divergent α-enolase gene products play central roles in glucose metabolism and growth regulation and their differential regulation may be critical for tumor adaptation to hypoxia. We have previously shown that MBP-1 and its binding to the c-myc P₂ promoter regulates the metabolic and cellular growth changes that occur in response to altered exogenous glucose concentrations.

Results

To examine the regulation of α-enolase and MBP-1 by a hypoxic microenvironment in breast cancer, MCF-7 cells were grown in low, physiologic, or high glucose under 1% oxygen. Our results demonstrate that adaptation to hypoxia involves attenuation of MBP-1 translation and loss of MBP-1-mediated regulation of c-myc transcription, evidenced by decreased MBP-1 binding to the c-myc P₂ promoter. This allows for a robust increase in c-myc expression, "early c-myc response", which stimulates aerobic glycolysis resulting in tumor acclimation to oxidative stress. Increased α-enolase mRNA and preferential translation/post-translational modification may also allow for acclimatization to low oxygen, particularly under low glucose concentrations.

Conclusions

These results demonstrate that malignant cells adapt to hypoxia by modulating α-enolase/MBP-1 levels and suggest a mechanism for tumor cell induction of the hyperglycolytic state. This important "feedback" mechanism may help transformed cells to escape the apoptotic cascade, allowing for survival during limited glucose and oxygen availability.

FOXM1c transactivates the human c-myc promoter directly via the two TATA boxes P1 and P2

FOXM1c transactivates the c-myc promoter via the P1 and P2 TATA boxes using a new mechanism. Whereas the P1 TATA box TATAATGC requires its sequence context to be FOXM1c responsive, the P2 TATA box TATAAAAG alone is sufficient to confer FOXM1c responsiveness to any minimal promoter. FOXM1c transactivates by binding to the TATA box as well as directly to TATA-binding protein, transcription factor IIB and transcription factor IIA. This new transactivation mechanism is clearly distinguished from the function of FOXM1c as a conventional transcription factor. The central domain of FOXM1c functions as an essential domain for activation via the TATA box, but as an inhibitory domain (retinoblastoma protein-independent transrepression domain and retinoblastoma protein-recruiting negative regulatory domain) for transactivation via conventional FOXM1c-binding sites. Each promoter with the P2 TATA box TATAAAAG is postulated to be transactivated by FOXM1c. This was demonstrated for the promoters of c-fos, hsp70 and histone H2B/a. A database search revealed almost 300 probable FOXM1c target genes, many of which function in proliferation and tumorigenesis. Accordingly, dominant-negative FOXM1c proteins reduced cell growth approximately threefold, demonstrating a proliferation-stimulating function for wild-type FOXM1c.

c-Myc induces chromosomal rearrangements through telomere and chromosome remodeling in the interphase nucleus

In previous work, we showed that telomeres of normal cells are organized within the 3D space of the interphase nucleus in a nonoverlapping and cell cycle-dependent manner. This order is distorted in tumor cell nuclei where telomeres are found in close association forming aggregates of various numbers and sizes. Here we show that c-Myc overexpression induces telomeric aggregations in the interphase nucleus. Directly proportional to the duration of c-Myc deregulation, we observe three or five cycles of telomeric aggregate formation in interphase nuclei. These cycles reflect the onset and propagation of breakage-bridge-fusion cycles that are initiated by end-to-end telomeric fusions of chromosomes. Subsequent to initial chromosomal breakages, new fusions follow and the breakage-bridge-fusion cycles continue. During this time, nonreciprocal translocations are generated. c-Myc-dependent remodeling of the organization of telomeres thus precedes the onset of genomic instability and subsequently leads to chromosomal rearrangements. Our findings reveal that c-Myc possesses the ability to structurally modify chromosomes through telomeric fusions, thereby reorganizing the genetic information.

Regulation of c-myc stability by selective stress conditions and by MEKK1 requires aa 127-189 of c-myc

C-myc availability is central for its ability to serve as a regulator of cell growth and death. Here we study the regulation of c-myc protein stability and identify domains of c-myc that are important for its stabilization in response to stress kinases activated following selective stress conditions. UV-irradiation elicited an increase in c-myc protein levels, which could be attenuated by inhibitors of stress kinases but also by actinomycin D-inhibitor of transcription. Inhibition of protein synthesis results in a noticeable decrease in c-myc levels, further pointing to the short half-life of the protein. However, in combination with tumor necrosis factor-alpha (TNF-alpha), cycloheximide efficiently increases steady-state levels of c-myc, suggesting that selective stress conditions are required to increase c-myc protein stability. Expression of MEKK1, an upstream regulator of protein kinases that has been implicated in mediating the response to diverse stress conditions, also results in an efficient increase in the half-life of c-myc protein. To map c-myc domains that are responsive to stress kinases, we monitored changes in the level of c-myc deletion mutants following MEKK1 expression. Of the seven c-myc deletion mutants analysed, the domain spanning amino acids 127-189 was found to be required for MEKK1-dependent increase in c-myc stability. In all, the present study identifies a novel domain that is important for the regulation of c-myc stability by stress kinases in response to selective stress conditions.

Transcriptional consequences of topoisomerase inhibition

In principle, the generation, transmission, and dissipation of supercoiling forces are determined by the arrangement of the physical barriers defining topological boundaries and the disposition of enzymes creating (polymerases and helicases, etc.) or releasing (topoisomerases) torsional strain in DNA. These features are likely to be characteristic for individual genes. By using topoisomerase inhibitors to alter the balance between supercoiling forces in vivo, we monitored changes in the basal transcriptional activity and DNA conformation for several genes. Every gene examined displayed an individualized profile in response to inhibition of topoisomerase I or II. The expression changes elicited by camptothecin (topoisomerase I inhibitor) or adriamycin (topoisomerase II inhibitor) were not equivalent. Camptothecin generally caused transcription complexes to stall in the midst of transcription units, while provoking little response at promoters. Adriamycin, in contrast, caused dramatic changes at or near promoters and prevented transcription. The response to topoisomerase inhibition was also context dependent, differing between chromosomal or episomal c-myc promoters. In addition to being well-characterized DNA-damaging agents, topoisomerase inhibitors may evoke a biological response determined in part from transcriptional effects. The results have ramifications for the use of these drugs as antineoplastic agents.

Immortalization of human WI38 cells is associated with differential activation of the c-myc origins

To study the possible relationships between origin activities and cellular processes leading to malignancy, we used an isogenic system of human embryo lung fibroblast cells WI38 and a SV40-transformed variant, WI38 VA13 2RA (WI38(SV40)). We found that the activities of all initiation sites at the c-myc locus were approximately two-fold as high in WI38(SV40) cells as in WI38 cells. Thus, higher initiation frequency of origins at certain loci is induced with cell immortalization, one of the steps in the multi-step process leading to malignancy. We measured the activities of the four c-myc promoters P0, P1, P2, and P3 with nuclear runon assay in the two cell lines in order to detect potential individual promoter changes that may be also associated with immortalization by SV40 virus. The results show that the activities of the promoters P0, P1, and P3 did not significantly change, but the activity of the major promoter P2 in WI38(SV40) cells was about 7.5- to 8.0-fold as high as that in WI38 cells. The increased activity of promoter P2, although approximately 600 bp downstream of one of the major DNA replication initiation sites, had no preferential influence on the major sites of origin activity. Since the distribution of nascent strand abundance was not significantly altered, binding of transcription factors does not seem to facilitate the assembly of pre-replication complex (pre-RC) or otherwise preferentially alter the activities of the DNA replication proteins at this major initiation site.

Loss of FBP function arrests cellular proliferation and extinguishes c-myc expression

The c-myc regulatory region includes binding sites for a large set of transcription factors. The present studies demonstrate that in the absence of FBP [far upstream element (FUSE)-binding protein], which binds to the single-stranded FUSE, the remainder of the set fails to sustain endogenous c-myc expression. A dominant-negative FBP DNA-binding domain lacking effector activity or an antisense FBP RNA, expressed via replication-defective adenovirus vectors, arrested cellular proliferation and extinguished native c-myc transcription from the P1 and P2 promoters. The dominant-negative FBP initially augmented the single-stranded character of FUSE; however, once c-myc expression was abolished, melting at FUSE could no longer be supported. In contrast, with antisense FBP RNA, the single-stranded character of FUSE decreased monotonically as the transcription of endogenous c-myc declined. Because transcription is the major source of super-coiling in vivo, we propose that by binding torsionally strained DNA, FBP measures promoter activity directly. We also show that FUSE is predicted to behave as a torsion-regulated switch poised to regulate c-myc and to confer a higher order regulation on a large repertoire of factors.

Expression of the c-myc proto-oncogene is essential for HIV-1 infection in activated T cells

We previously found that activation of primary CD4+ T cells via both the T cell antigen receptor (TCR) and CD28 is required for HIV-1 DNA to be translocated from the cytoplasm to the nucleus. Here we report that expression of c-Myc protein in CD4+ T cells is induced only after such costimulation. In addition, cyclosporin A not only inhibits nuclear import of HIV-1 DNA but also inhibits expression of c-Myc protein. Because of these correlations, we tested whether c-Myc is necessary for nuclear import of HIV-1 DNA. Specific c-myc antisense, but not sense or non-sense, phosphorothioate oligodeoxynucleotides selectively induced the accumulation of two NH2-terminally truncated c-Myc proteins and abolished HIV-1 genome entry into host nuclei. Consequently, both virus replication and HIV-1-induced apoptotic cell death were inhibited. Synthesis of viral full-length DNA was not affected. Specific c-myc antisense oligonucleotide inhibited HIV-1 infection under conditions that did not affect cell cycle entry or proliferation. Thus, c-Myc appears to regulate HIV-1 DNA nuclear import via a mechanism distinct from those controlling entry into the cell cycle.

PRDI-BF1/Blimp-1 repression is mediated by corepressors of the Groucho family of proteins

The PRDI-BF1/Blimp-1 protein is a transcriptional repressor required for normal B-cell differentiation, and it has been implicated in the repression of beta-interferon (IFN-beta) and c-myc gene expression. Here, we show that PRDI-BF1 represses transcription of the IFN-beta promoter and of an artificial promoter through an active repression mechanism. We also identified a minimal repression domain in PRDI-BF1 that is sufficient for transcriptional repression when tethered to DNA as a Gal4 fusion protein. Remarkably, this repression domain interacts specifically with hGrg, TLE1, and TLE2 proteins, all of which are members of the Groucho family of transcriptional corepressors. In addition, the hGrg protein itself can function as a potent repressor when tethered to DNA through the Gal4 DNA-binding domain. We also find that the amino-terminal glutamine-rich domains of hGrg and TLE1 are sufficient to mediate dimerization of the two Groucho family proteins. Proteins containing only this domain can function as a dominant-negative inhibitor of PRDI-BF1 repression, and can significantly increase the IFN-beta promoter activity after virus induction. We conclude that PRDI-BF1/Blimp-1 represses transcription by recruiting a complex of Groucho family proteins to DNA, and suggest that such corepressor complexes are required for the postinduction repression of the IFN-beta promoter.