Nucleotide sequence to the v-myc oncogene of avian retrovirus MC29

MYC Oncogene: A Druggable Target for Treating Cancers with Natural Products

Various diseases, including cancers, age-associated disorders, and acute liver failure, have been linked to the oncogene, MYC. Animal testing and clinical trials have shown that sustained tumor volume reduction can be achieved when MYC is inactivated, and different combinations of therapeutic agents including MYC inhibitors are currently being developed. In this review, we first provide a summary of the multiple biological functions of the MYC oncoprotein in cancer treatment, highlighting that the equilibrium points of the MYC/MAX, MIZ1/MYC/MAX, and MAD (MNT)/MAX complexes have further potential in cancer treatment that could be used to restrain MYC oncogene expression and its functions in tumorigenesis. We also discuss the multifunctional capacity of MYC in various cellular cancer processes, including its influences on immune response, metabolism, cell cycle, apoptosis, autophagy, pyroptosis, metastasis, angiogenesis, multidrug resistance, and intestinal flora. Moreover, we summarize the MYC therapy patent landscape and emphasize the potential of MYC as a druggable target, using herbal medicine modulators. Finally, we describe pending challenges and future perspectives in biomedical research, involving the development of therapeutic approaches to modulate MYC or its targeted genes. Patients with cancers driven by MYC signaling may benefit from therapies targeting these pathways, which could delay cancerous growth and recover antitumor immune responses.

Normal and Neoplastic Growth Suppression by the Extended Myc Network

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

BET and Aurora Kinase A inhibitors synergize against MYCN-positive human glioblastoma cells

Glioblastoma multiforme (GBM) is the most common primary malignant brain tumor in adults. Patients usually undergo surgery followed by aggressive radio- and chemotherapy with the alkylating agent temozolomide (TMZ). Still, median survival is only 12–15 months after diagnosis. Many human cancers including GBMs demonstrate addiction to MYC transcription factor signaling and can become susceptible to inhibition of MYC downstream genes. JQ1 is an effective inhibitor of BET Bromodomains, a class of epigenetic readers regulating expression of downstream MYC targets. Here, we show that BET inhibition decreases viability of patient-derived GBM cell lines. We propose a distinct expression signature of MYCN-elevated GBM cells that correlates with significant sensitivity to BET inhibition. In tumors showing JQ1 sensitivity, we found enrichment of pathways regulating cell cycle, DNA damage response and repair. As DNA repair leads to acquired chemoresistance to TMZ, JQ1 treatment in combination with TMZ synergistically inhibited proliferation of MYCN-elevated cells. Bioinformatic analyses further showed that the expression of MYCN correlates with Aurora Kinase A levels and Aurora Kinase inhibitors indeed showed synergistic efficacy in combination with BET inhibition. Collectively, our data suggest that BET inhibitors could potentiate the efficacy of either TMZ or Aurora Kinase inhibitors in GBM treatment.

Myc activity is required for maintenance of the neuromesodermal progenitor signalling network and for segmentation clock gene oscillations in mouse

The Myc transcriptional regulators are implicated in a range of cellular functions, including proliferation, cell cycle progression, metabolism and pluripotency maintenance. Here, we investigated the expression, regulation and function of the Myc family during mouse embryonic axis elongation and segmentation. Expression of both cMyc (Myc - Mouse Genome Informatics) and MycN in the domains in which neuromesodermal progenitors (NMPs) and underlying caudal pre-somitic mesoderm (cPSM) cells reside is coincident with WNT and FGF signals, factors known to maintain progenitors in an undifferentiated state. Pharmacological inhibition of Myc activity downregulates expression of WNT/FGF components. In turn, we find that cMyc expression is WNT, FGF and Notch protein regulated, placing it centrally in the signalling circuit that operates in the tail end that both sustains progenitors and drives maturation of the PSM into somites. Interfering with Myc function in the PSM, where it displays oscillatory expression, delays the timing of segmentation clock oscillations and thus of somite formation. In summary, we identify Myc as a component that links NMP maintenance and PSM maturation during the body axis elongation stages of mouse embryogenesis.

Inactivation of MYC reverses tumorigenesis

The MYC proto-oncogene is an essential regulator of many normal biological programmes. MYC, when activated as an oncogene, has been implicated in the pathogenesis of most types of human cancers. MYC overexpression in normal cells is restrained from causing cancer through multiple genetically and epigenetically controlled checkpoint mechanisms, including proliferative arrest, apoptosis and cellular senescence. When pathologically activated in the correct epigenetic and genetic contexts, MYC bypasses these mechanisms and drives many of the 'hallmark' features of cancer, including uncontrolled tumour growth associated with DNA replication and transcription, cellular proliferation and growth, protein synthesis and altered cellular metabolism. MYC also dictates tumour cell fate by enforcing self-renewal and by abrogating cellular senescence and differentiation programmes. Moreover, MYC influences the tumour microenvironment, including activating angiogenesis and suppressing the host immune response. Provocatively, brief or even partial suppression of MYC back to its physiological levels of activation can lead to the restoration of intrinsic checkpoint mechanisms, resulting in acute and sustained tumour regression associated with tumour cells undergoing proliferative arrest, differentiation, senescence and apoptosis, as well as remodelling of the tumour microenvironment, recruitment of an immune response and shutdown of angiogenesis. Hence, tumours appear to be addicted to the MYC oncogene because of both tumour cell intrinsic and host-dependent mechanisms. MYC is important for the regulation of both the initiation and maintenance of tumorigenesis.

MYC activation is a hallmark of cancer initiation and maintenance

The MYC proto-oncogene has been implicated in the pathogenesis of most types of human tumors. MYC activation alone in many normal cells is restrained from causing tumorigenesis through multiple genetic and epigenetically controlled checkpoint mechanisms, including proliferative arrest, apoptosis, and cellular senescence. When pathologically activated in a permissive epigenetic and/or genetic context, MYC bypasses these mechanisms, enforcing many of the "hallmark" features of cancer, including relentless tumor growth associated with DNA replication and transcription, cellular proliferation and growth, protein synthesis, and altered cellular metabolism. MYC mandates tumor cell fate, by inducing stemness and blocking cellular senescence and differentiation. Additionally, MYC orchestrates changes in the tumor microenvironment, including the activation of angiogenesis and suppression of the host immune response. Provocatively, brief or even partial suppression of MYC back to its physiological levels of activation can result in the restoration of intrinsic checkpoint mechanisms, resulting in acute and sustained tumor regression, associated with tumor cells undergoing proliferative arrest, differentiation, senescence, and apoptosis, as well as remodeling of the tumor microenvironment, recruitment of an immune response, and shutdown of angiogenesis. Hence, tumors appear to be "addicted" to MYC because of both tumor cell-intrinsic, cell-autonomous and host-dependent, immune cell-dependent mechanisms. Both the trajectory and persistence of many human cancers require sustained MYC activation. Multiscale mathematical modeling may be useful to predict when tumors will be addicted to MYC. MYC is a hallmark molecular feature of both the initiation and maintenance of tumorigenesis.

Retroviral oncogenes: a historical primer

Retroviruses are the original source of oncogenes. The discovery and characterization of these genes was made possible by the introduction of quantitative cell biological and molecular techniques for the study of tumour viruses. Key features of all retroviral oncogenes were first identified in src, the oncogene of Rous sarcoma virus. These include non-involvement in viral replication, coding for a single protein and cellular origin. The MYC, RAS and ERBB oncogenes quickly followed SRC, and these together with PI3K are now recognized as crucial driving forces in human cancer.

Retroviral insertion mutagenesis in mice as a comparative oncogenomics tool to identify disease genes in human leukemia

Retroviral insertion mutagenesis has recently received much attention because of its adverse effects in the application of retroviral vector-based gene therapy, resulting in leukemia in certain patients. At the same time, retroviral mutagenesis in mice is being considered a powerful forward genetic strategy to identify disease genes involved in cancer. The publication of the mouse genome sequence and the development of high-throughput genomic approaches have given a further boost to this rapidly evolving field. The increasing numbers of new potential oncogenes identified in retroviral screens have given a valuable basis for a better understanding of cancer related pathways in mice. Important challenges that now lie ahead of us are (i) to determine the relevance and causal relationship of these genes with various types of human cancer (ii) to develop strategies to identify tumor suppressor genes on a large scale, (iii) to place the disease genes into regulatory networks to better understand their role in the complex pathogenesis of cancer, and (iv) to determine their value for diagnosis refinement and therapeutic target intervention in human disease. In this review, we will give a brief update of the current state-of-the-art and thoughts concerning these issues. We will specifically focus on the value of employing retroviral insertion mutagenesis in mice and gene expression profiling in man in the context of acute myeloid leukemia.

Mechanism of telomerase activation by v-Rel and its contribution to transformation

Telomerase is activated during the transformation of lymphoid cells and fibroblasts by v-Rel, the oncogenic member of the Rel/NF-kappaB family of transcription factors. v-Rel-transformed cell lines have longer telomeres than untransformed chicken lymphoid cells and have high levels of telomerase activity. v-Rel-mediated activation of telomerase is achieved by multiple mechanisms. The expression of the gene encoding the catalytic subunit of telomerase (TERT) was directly upregulated by v-Rel. Moreover, the expression of v-Rel altered the ratio of alternatively spliced and full-length TERT transcripts in favor of the full-length forms. The activation of telomerase by v-Rel in lymphocytes was also accompanied by inactivation of nuclear inhibitors. The inhibition of telomerase activity in v-Rel-transformed cell lines led to apoptosis within 24 h. The expression of v-Rel in a macrophage cell line resulted in elevated levels of reactive oxygen species (ROS), increased telomerase activity, and increased sensitivity to telomerase inhibitors. In contrast, the ectopic expression of TERT decreased the extent of apoptosis induced by ROS. The activation of telomerase by v-Rel may, therefore, partially protect the transformed cells from apoptosis induced by ROS.

Myc pathways provoking cell suicide and cancer

A paradox for the cancer biology field has been the revelation that oncogenes, once thought to simply provide advantages to a cancer cell, actually put it at dire risk of cell suicide. Myc is the quintessential oncogene in this respect, as in normal cells it is required for cell cycle traverse, whereas in cancers it is overexpressed and functions as the angiogenic switch. Nonetheless, Myc overexpression kills normal cells dead in their tracks. Here we review Myc-induced pathways that contribute to the apoptotic response. Molecular analysis of Myc-induced tumors has established that some of these apoptotic pathways are essential checkpoints that guard the cell from cancer, as they are selectively bypassed during tumorigenesis. The precise mechanism(s) by which Myc targets these pathways are largely unresolved, but we propose that they involve crosstalk and feedback regulatory loops between arbiters of cell death.

Transforming ability of Gag-Myc fusion proteins correlates with Gag-Myc protein stability and transcriptional repression

Avian retroviruses that have transduced c-myc are useful tools to study the conditions necessary for cellular transformation. FH3, one such retrovirus which encodes a Gag-Myc fusion protein, is not transforming in quail embryonic fibroblasts, but a late variant of FH3 that arose after passaging FH3-infected cells is transforming. Mutational analysis of FH3 revealed that the presence of a portion of the retroviral protease in FH3 inhibited transformation and that this inhibition was transferable to a more highly transforming retrovirus, MC29. Transforming and non-transforming FH3-derived and MC29-derived Gag-Myc proteins were used to further explore characteristics of Myc necessary for transformation. Gag-Myc proteins which were transforming were found to be the most stable in the cell. To distinguish whether transactivation and/or repression is correlated to transformation, the various Gag-Myc fusion proteins were tested for their ability to activate or repress c-Myc targets. Results indicated that a correlation exists between transforming Gag-Myc proteins and their ability to repress, whereas all Gag-Myc proteins could transactivate, regardless of their ability to transform. Taken together, these results suggest that protein stabilization of Myc and repression of target genes by Myc are important for cellular transformation.

The v-myc oncogene

v-myc is the viral homolog of c-myc transduced by several acute transforming retroviruses, many of which encode this gene as a Gag-Myc fusion protein. The v-myc oncogene can transform several lineages of mammalian and avian cells either alone or in cooperation with other oncogenes. While the Gag portion of the Gag-Myc fusion protein and the nuclear localization signal each appear to be dispensable for transformation, the N- and C-termini of the Myc sequence have been found to be essential for transformation. All v-myc genes contain point mutations which seem to confer a greater potency to v-myc in the process of transformation, proliferation, and apoptosis. In v-myc-transformed myelomonocytic cells, secondary events occur, such as the expression of colony stimulating factor-1 (CSF-1) which play a critical role in immortalization and subsequent tumor progression. Inhibition of the autocrine loop of CSF-1 was found to induce apoptosis in the immortalized cells. While overexpression of v-Myc blocks terminal differentiation of hematopoietic cells, this is not sufficient to block the differentiation of certain neural and skeletal muscle cells. Recent developments on the effects of v-myc on cell growth, transformation, differentiation and apoptosis are discussed in this review.

Viral Myc oncoproteins in infected fibroblasts down-modulate thrombospondin-1, a possible tumor suppressor gene

We are interested in identifying the transcriptional targets of the Myc oncoproteins. To this end, we have fused Myc of the MC29 retrovirus with the rat glucocorticoid receptor. This chimeric protein requires dexamethasone to undergo nuclear translocation and achieve an active conformation. We employed a differential hybridization approach to identify mRNAs that are induced or repressed in infected avian fibroblasts in response to dexamethasone. This screen yielded one mRNA underrepresented in the dexamethasone-treated cells. In Myc-transformed cell clones, its level decreases 6-fold as early as 4 h and more than 30-fold after 32 h of exposure to the hormone. This mRNA was also down-regulated by recombinant Myc retroviruses in rodent fibroblasts, including those refractory to transformation. Sequence analysis revealed that it is homologous to the 3' untranslated regions of the mammalian thrombospondin-1 genes. Using an anti-thrombospondin antibody, we confirmed that rodent cells overexpressing Myc produce very small amounts of this protein. Also, they do not support efficient expression of a reporter gene driven by the thrombospondin-1 promoter. Thus, thrombospondin-1 is a bona fide target of Myc. Moreover, its silencing might pertain to the transforming activity of Myc, since in several systems thrombospondin-1 exhibits tumor suppressor properties, presumably due to its negative effect on neovascularization.

Discrimination between different E-box-binding proteins at an endogenous target gene of c-myc

c-myc plans a key role in regulating mammalian cell proliferation and apoptosis. The gene codes for a transcription factor, Myc, that belongs to the helix-loop-helix/leucine zipper (HLH/LZ) family of proteins. Myc heterodimerizes with a partner protein termed Max; the heterodimeric complex binds to CAC(G/A)TG (E-box) sequences and activates transcription from these sites. However, several other HLH/LZ proteins, including USF and TFE-3, bind to and trans-activate from the same element, yet have no documented effect on cell proliferation or apoptosis. Therefore, it is likely that mechanisms exist that discriminate between these proteins for activation of natural target genes of Myc. We now show that trans-activation from the E-box in the rat prothymosin-alpha intron enhancer is indeed specific for Myc, and identify both the distance from the start site of transcription and a second E-box element adjacent to that recognized by Myc as critical determinants of specificity. Surprisingly, transcription activation domains required for Myc to activate from this distal enhancer position differ from previously mapped domains and closely correlate with those domains essential for transformation. As observed in transformation assays, Myc and Max strongly synergize in activation from a distal enhancer position. Our data suggest that trans-activation from the prothymosin intron enhancer is a faithful reflection of the transforming properties of the Myc protein.

Tyrosine kinase inhibition prevents deformation-stimulated vascular smooth muscle growth

The goal of this study was to determine the role of tyrosine phosphorylation in transducing deformation-stimulated vascular smooth muscle growth. Rat aorta-derived vascular smooth muscle cells were cultured on flexible silicone elastomer membranes and subjected to cyclic deformation (15 cycles per minute, deformed 2 seconds, relaxed 2 seconds). Deformation significantly increased proto-oncogene expression, [3H]thymidine incorporation, [3H]leucine incorporation, and cell number. Time course studies showed an 8-hour lag between initiation of cell deformation and onset of [3H]thymidine incorporation, with peak levels achieved after 18 to 24 hours. Western analysis of protein blots from deformed cells (10 minutes) demonstrated increased levels of phosphotyrosine-containing proteins having molecular weights of 110 to 130 and 70 to 80 kD. Deformation-stimulated tyrosine phosphorylation was prevented by the tyrosine kinase inhibitor Herbimycin A. Tyrosine kinase inhibition also prevented deformation-stimulated vascular smooth muscle cell growth as measured by [3H]thymidine incorporation. Cyclic deformation stimulates vascular smooth muscle proliferation through activation of tyrosine kinases. Inhibition of tyrosine phosphorylation is an effective means of preventing deformation-induced vascular smooth muscle growth in vitro.

L-myc and N-myc in hematopoietic malignancies

The myc proto-oncogenes encode nuclear DNA-binding phosphoproteins which regulate cell proliferation and differentiation. The c-myc gene is implicated in hematopoietic malignancies on the basis of its frequent deregulation in naturally occurring leukemias and lymphomas. Recent evidence suggests that also the N-myc and L-myc genes may have a role in normal and malignant hematopoiesis. N-myc and to a certain degree L-myc can substitute for c-myc in transformation assays in vitro, and their overexpression can block the differentiation of leukemia cell lines. Immunoglobulin heavy chain enhancer (IgH) -driven overexpression of N-myc or L-myc genes cause lymphatic and myeloid tumors, respectively, in transgenic mice. Furthermore, the L-myc and N-myc genes are expressed in several human leukemias and leukemia cell lines, L-myc predominantly in myeloid and N-myc both in myeloid and in some lymphoid leukemias. All N/L-myc positive leukemias and leukemia cell lines coexpress the c-myc gene, thus exemplifying a lack of negative cross-regulation between the different myc genes in leukemia cells. Taken together, these data suggest that L-myc and N-myc may participate in the growth regulation of hematopoietic cells.

A model system for nonhomologous recombination between retroviral and cellular RNA

A current model for the generation of transforming retroviruses proposes that read-through RNAs, containing both viral and cellular sequences, are copackaged with viral genomic RNA. It is, however, possible that a cellular mRNA is occasionally encapsidated into a retroviral particle, even though viral packaging sequences are absent. We have generated recombinant proviruses following copackaging of an avian leukosis viral genomic RNA and a neo-containing RNA completely devoid of retroviral sequences. In these studies, we used the packaging cell line SE21Q1b, which has the unique ability to randomly package cellular mRNA into retroviral particles. We describe 10 recombinants obtained following copackaging of nonhomologous RNAs. Our data show that recombination is not occurring at the DNA level in the parental SE21Q1b cells but is occurring at the RNA level, during reverse transcription. These data further suggest that reverse transcriptase can preferentially jump between templates at short stretches of homology in otherwise unrelated RNAs. We conclude that retroviral sequences are not required for packaged mRNA to be reverse transcribed and to be included in integrated proviruses.

Cloning and functional characterization of early B-cell factor, a regulator of lymphocyte-specific gene expression

Early B-cell factor (EBF) was identified previously as a tissue-specific and differentiation stage-specific DNA-binding protein that participates in the regulation of the pre-B and B lymphocyte-specific mb-1 gene. Partial amino acid sequences obtained from purified EBF were used to isolate cDNA clones, which by multiple criteria encode EBF. The recombinant polypeptide formed sequence-specific complexes with the EBF-binding site in the mb-1 promoter. The cDNA hybridized to multiple transcripts in pre-B and B-cell lines, but transcripts were not detected at significant levels in plasmacytoma, T-cell, and nonlymphoid cell lines. Expression of recombinant EBF in transfected nonlymphoid cells strongly activated transcription from reporter plasmids containing functional EBF-binding sites. Analysis of DNA binding by deletion mutants of EBF identified an amino-terminal cysteine-rich DNA-binding domain lacking obvious sequence similarity to known transcription factors. DNA-binding assays with cotranslated wild-type and truncated forms of EBF indicated that the protein interacts with its site as a homodimer. Deletions delineated a carboxy-terminal dimerization region containing two repeats of 15 amino acids that show similarity with the dimerization domains of basic-helix-loop-helix proteins. Together, these data suggest that EBF represents a novel regulator of B lymphocyte-specific gene expression.

A novel oncogene, v-ryk, encoding a truncated receptor tyrosine kinase is transduced into the RPL30 virus without loss of viral sequences

The RPL viruses are acute oncogenic avian retroviruses isolated from chicken tumors. We carried out a genetic analysis of three of the viruses, RPL25, RPL28, and RPL30. While RPL25 and RPL28 were shown to contain the erbB oncogene, RPL30 appeared to contain a novel protein tyrosine kinase oncogene. This gene, v-ryk, was cloned and sequenced. The v-ryk oncogene contains a 1.39-kb nonretroviral sequence that includes a tyrosine kinase domain which was inserted into the viral envelope protein gp37-coding region and fused in frame with upstream gp37 to generate a P69gp37-ryk fusion oncoprotein. Unlike that of other acutely transforming retroviruses, transduction of the v-ryk gene into RPL30 did not result in deletion of viral sequences. Sequence analysis suggested that v-Ryk is more homologous to receptor-type tyrosine kinases than to nonreceptor-type kinases. By reconstitution of a virus from its cDNA, the v-ryk oncogene has been shown to be fully responsible for the transforming activity of the RPL30 virus. Antibodies specific to v-Ryk immunoprecipitated the v-Ryk oncoprotein from cells transformed by the RPL30 virus. The v-Ryk protein was shown to be first synthesized as a 150-kDa precursor and then cleaved into the mature 69-kDa gp37-Ryk fusion protein, both parts of which were found to be localized to the membrane fraction. As expected from the sequence of v-Ryk, immunoprecipitates of v-Ryk from RPL30-transformed cells were found to display a protein tyrosine kinase activity in vitro, and the levels of tyrosine-phosphorylated proteins are elevated in v-ryk-transformed cells.

Mutations within the 5' half of the avian retrovirus MC29 v-myc gene alter or abolish transformation of chicken embryo fibroblasts and macrophages

Avian myelocytomatosis virus MC29 induces a wide variety of neoplastic diseases in infected birds and transforms cells of the macrophage lineage as well as fibroblasts and epithelial cells. A biological and biochemical analysis, carried out on a series of in-frame insertion and deletion mutations within the gag-myc gene of MC29, revealed several mutations within the 5' portion of the v-myc gene that encode proteins either completely defective for transformation or compromised in their ability to transform chicken embryo fibroblasts but not macrophages. Mutations within the 3' end of the v-myc gene which disrupt sequences encoding the basic/helix-loop-helix region were defective for transformation of both fibroblasts and macrophages. Eight variants were cloned into the replication-competent avian expression vector RCAS. Analysis of cells infected with transformation-defective, replication-competent viruses confirmed the expression of functionally defective Myc proteins. Further, expression of the transformation defective variant dl91-137 in chicken fibroblasts inhibited subsequent transformation by wild-type MC29. The results reported herein support the hypothesis that Myc proteins function as regulators of transcription in a variety of cell types and clearly point out the necessity of putative regulatory domains within the amino-terminal half of the Myc protein.

Strain dependency of cell-type specificity and onset of lymphoma development in Emu-myc transgenic mice

c-myc is a nuclear proto-oncogene that, when activated, induces malignancies in a variety of tissues. Most murine plasmacytomas and human Burkitt's lymphomas have been shown to carry a chromosomal translocation involving c-myc and immunoglobulin genes. To study genetic or epigenetic factors that affect myc-induced lymphoid cell tumors, we previously introduced the Emu-myc delta gene lacking its own promoter and first exon into two inbred strains of mice, C57BL/6 and C3H/HeJ. We observed three characteristic features in our transgenic mice. First, T cell lymphoma predominated in the C3H background. Second, both pre-B and B cell lymphoma developed at equal frequency in C57BL/6 transgenic mice. Third, the average age of onset is earlier than that reported by other investigators. To test whether these characteristics are due either to the lack of the promoter region and first exon of the c-myc gene in the construct or to the genetic background of the mice, we introduced Emu-myc gene containing the complete c-myc gene into fertilized eggs of C57BL/6 and C3H/HeJ mice. The cell-type specificity, differentiation-stage specificity and the average age at onset of lymphoma development were not affected by the transgene construct.

gag as well as myc sequences contribute to the transforming phenotype of the avian retrovirus FH3

The avian retrovirus FH3, like MC29 and CMII, encodes a Gag-Myc fusion protein. However, the FH3-encoded protein is larger, about 145 kDa, and contains almost the entire retroviral gag gene. In contrast to the other gag-myc avian retroviruses, FH3 fails to transform fibroblasts in vitro, although macrophages are transformed both in vitro and in vivo (C. Chen, B. J. Biegalke, R. N. Eisenman, and M. L. Linial, J. Virol. 63:5092-5100, 1989). We have used the polymerase chain reaction technique to obtain a molecular clone of FH3. Sequence analysis of the FH3 myc oncogene revealed a single proline----histidine change (position 223) relative to c-myc. However, substitution of the FH3 myc sequence with the chicken c-myc sequence did not alter the transformation potential of the virus. Hence, overexpression of the proto-oncogene as a Gag-Myc retroviral protein is sufficient for macrophage, but not fibroblast, transformation. After passage of FH3 in fibroblast cultures, a virus (FH3L) that is capable of rapidly transforming fibroblasts appears. The Gag-Myc protein encoded by FH3L is smaller (ca. 130 kDa) than that encoded by the original viral stock (FH3E). Sequencing of an FH3L molecular clone revealed a 212-amino-acid deletion within the Gag portion. Using FH3E/FH3L recombinants, we have demonstrated that the ability of encoded viruses to transform fibroblasts directly correlates with the presence of this deletion. Moreover, the addition of the Gag sequence deleted from FH3L to the MC29 oncoprotein significantly reduces its transforming activity as measured by focus assay. These data suggest that the C-terminal segment of Gag attenuates the oncogenic potential of Gag-Myc fusion proteins.

Depletion of c-myc with specific antisense sequences reverses the transformed phenotype in ras oncogene-transformed NIH 3T3 cells

ras oncogene-transformed NIH 3T3 cells expressing glucocorticoid-inducible antisense c-myc cDNA transcripts at levels sufficient to deplete c-myc protein lost their transformed morphology and the ability to grow in soft agar; their ability to form tumors in nude mice was also impaired. These changes were dependent on the continuous expression of the antisense sequences. No major effects on plating efficiencies, growth rates in monolayer culture, or immortalization were observed in the revertant cells, indicating that the observed effects were not a toxic consequence of c-myc protein depletion. Transfection with the same vector expressing c-myc in the sense orientation or other control vectors had no effect on transformation. These results suggest that a certain minimum level of expression of c-myc is required for the maintenance of ras transformation in NIH 3T3 cells.

Depletion of c-myc with specific antisense sequences reverses the transformed phenotype in ras oncogene-transformed NIH 3T3 cells

ras oncogene-transformed NIH 3T3 cells expressing glucocorticoid-inducible antisense c-myc cDNA transcripts at levels sufficient to deplete c-myc protein lost their transformed morphology and the ability to grow in soft agar; their ability to form tumors in nude mice was also impaired. These changes were dependent on the continuous expression of the antisense sequences. No major effects on plating efficiencies, growth rates in monolayer culture, or immortalization were observed in the revertant cells, indicating that the observed effects were not a toxic consequence of c-myc protein depletion. Transfection with the same vector expressing c-myc in the sense orientation or other control vectors had no effect on transformation. These results suggest that a certain minimum level of expression of c-myc is required for the maintenance of ras transformation in NIH 3T3 cells.

Nuclear colocalization of cellular and viral myc proteins with HSP70 in myc-overexpressing cells

The c-myc oncogene and its viral counterpart v-myc encode phosphoproteins which have been located within cell nuclei, excluding nucleoli. We have expressed the c-myc gene under the simian virus 40 early promoter and studied the distribution of its protein product in transient expression assays in COS, HeLa, and 293 cells. We found three distinct patterns of c-myc immunofluorescence in the transfected cells: one-third of the c-myc-positive cells displayed a diffuse nuclear distribution, and in two-thirds of the cells the c-myc fluorescence was accumulated either in small amorphous or in large multilobed phase-dense nuclear structures. Unexpectedly, these structures also stained for the HSP70 heat shock protein in both heat-shocked and untreated cells. Our results indicate that both transient and stable overexpression of either the c-myc or v-myc protein induces translocation of the endogenous HSP70 protein from the cytoplasm to the nucleus, where it becomes sequestered in structures containing the myc protein. Interestingly, the closely related N-myc protein does not stimulate substantial nuclear expression of the HSP70 protein. Studies with chimeric myc proteins revealed that polypeptide sequences encoded by the second exon of c-myc are involved in colocalization with HSP70.

Deprivation of a single amino acid induces protein synthesis-dependent increases in c-jun, c-myc, and ornithine decarboxylase mRNAs in Chinese hamster ovary cells

Genes of higher eucaryotic cells are considered to show only a limited response to nutritional stress. Here we show, however, that omission of a single essential amino acid from the medium caused a marked rise in the mRNA levels of c-myc, c-jun, junB and c-fos oncogenes and ornithine decarboxylase (ODC) in CHO cells. There was no general accumulation of mRNAs in amino acid-starved cells, since the gamma-actin, beta-tubulin, protein kinase C, RNA polymerase II, and glyceraldehyde-3-phosphate dehydrogenase mRNAs and the total poly(A)+ mRNA were not increased. The levels of c-myc, ODC, and c-jun mRNAs were elevated more by amino acid starvation than by inhibition of protein synthesis with cycloheximide, which is known to increase the levels of these mRNAs. Importantly, however, cycloheximide present during amino acid starvation reduced the rise in the levels of the mRNAs down to the level obtained with cycloheximide alone. This implies that protein synthesis is required for the accumulation of c-myc, ODC, and c-jun mRNAs in amino acid-deprived cells. The junB and c-fos mRNAs, instead, were increased to the same extent or less by amino acid starvation than by cycloheximide treatment. The accumulation of the c-myc mRNA in amino acid-starved cells was due to both stabilization of the mRNA and increase of its transcription. The rise in the c-jun mRNA level seemed to be caused merely by stabilization of the mRNA. Further, despite the inhibition of general protein synthesis, amino acid starvation led to an increase in the synthesis of c-myc polypeptide. The results suggest that mammalian cells have a specific mechanism for registering shortages of amino acids in order to make adjustments compatible with cellular growth.

Nobel Lecture. Retroviruses and oncogenes II

The relationship between retroviral genes and oncogenes is described.

Deprivation of a single amino acid induces protein synthesis-dependent increases in c-jun, c-myc, and ornithine decarboxylase mRNAs in Chinese hamster ovary cells

Genes of higher eucaryotic cells are considered to show only a limited response to nutritional stress. Here we show, however, that omission of a single essential amino acid from the medium caused a marked rise in the mRNA levels of c-myc, c-jun, junB and c-fos oncogenes and ornithine decarboxylase (ODC) in CHO cells. There was no general accumulation of mRNAs in amino acid-starved cells, since the gamma-actin, beta-tubulin, protein kinase C, RNA polymerase II, and glyceraldehyde-3-phosphate dehydrogenase mRNAs and the total poly(A)+ mRNA were not increased. The levels of c-myc, ODC, and c-jun mRNAs were elevated more by amino acid starvation than by inhibition of protein synthesis with cycloheximide, which is known to increase the levels of these mRNAs. Importantly, however, cycloheximide present during amino acid starvation reduced the rise in the levels of the mRNAs down to the level obtained with cycloheximide alone. This implies that protein synthesis is required for the accumulation of c-myc, ODC, and c-jun mRNAs in amino acid-deprived cells. The junB and c-fos mRNAs, instead, were increased to the same extent or less by amino acid starvation than by cycloheximide treatment. The accumulation of the c-myc mRNA in amino acid-starved cells was due to both stabilization of the mRNA and increase of its transcription. The rise in the c-jun mRNA level seemed to be caused merely by stabilization of the mRNA. Further, despite the inhibition of general protein synthesis, amino acid starvation led to an increase in the synthesis of c-myc polypeptide. The results suggest that mammalian cells have a specific mechanism for registering shortages of amino acids in order to make adjustments compatible with cellular growth.

Failure to detect viral genomic sequences of three viruses (herpes simplex, simian virus 40 and adenovirus) in human and rat brain tumors

Little is known about oncogenesis in brain tumors. Viruses are thought to be involved in some neurological diseases, the presence of subfractions of viral DNA has been reported in various circumstances and the oncogenicity of some viruses has been demonstrated in animal experiments. The discovery of homologies between retroviral oncogenes and normal cellular genes (proto-oncogenes) has stimulated once again the search for viral responsibility in oncogenesis. Having a large bank of tumor material available, we systematically examined 39 brain tumors using Southern blot hybridization with DNAs of three viruses, known to be involved in neurological diseases: herpes simplex virus (HSV), simian virus 40 (SV40) and adenovirus type 2 (Ad2). We detected no homology between the DNAs of the examined material and the viral DNA probes. We compare these negative results with those of other published studies and discuss the experimental conditions, with special reference to the possibility of non-specific hybridization, which could account for the positive results reported. The present negative results could be interpreted either as absence of involvement of the three investigated viruses in brain tumor oncogenesis, or an indirect involvement through a hit-and-run mechanism or a highly dispersed state of the viral sequences among the host genome, which would prevent hybridization with the probe, as it has been supposed to be the case during the latency phase of herpes virus.

Expression of the avian gag-myc oncogene in Saccharomyces cerevisiae

The gag-myc oncogenic sequence of the avian retrovirus MC29 was first inserted in a multicopy expression vector allowing its expression in Saccharomyces cerevisiae. The oncogene transcripts were detected in yeast by Northern blot hybridization and gag-myc proteins were revealed by immunoprecipitation. On solid medium, the average size of gag-myc transformant colonies was smaller than control. In liquid cultures, the gag-myc strains had a doubling time of 4.7 h compared with 3.1 h in the controls. In one of the recipient strains, and after an initial transient period of 5 days, the gag-myc transformants became physiologically indistinguishable from control. In another recipient strain, the slow-growth phenotype is permanent. Plasmid instability is increased in gag-myc transformants. When a single copy of the gag-myc gene was inserted in a yeast chromosome, no phenotype was observed, showing that slow growth is the consequence of plasmid loss.

FH3, a v-myc avian retrovirus with limited transforming ability

We have isolated a new acute avian transforming virus which contains the oncogene myc. This virus, designated FH3, was isolated after injection of a 10-day-old chick embryo with avian leukosis virus. While FH3 shares many properties with other v-myc-containing avian retroviruses, it also has several unique properties. The primary target for transformation in vitro is chicken macrophages; infection of chicken fibroblasts does not lead to complete morphological transformation. FH3 also exhibits a limited host range, in that Japanese quail macrophages and fibroblasts are infected but are not completely transformed. FH3 induces in vivo a limited tumor type if injected into 10-day-old chick embryos; only a cranial myelocytoma, which does not appear to be metastatic, can be detected. The v-myc gene of FH3 is expressed predominantly as a P145 Gag-Myc protein which is encoded by a ca. 8-kilobase genomic RNA. This FH3-encoded polyprotein is localized in the nucleus of all infected cells, whether or not they are transformed.

Transin/stromelysin expression in the synovium of rats with experimental erosive arthritis. In situ localization and kinetics of expression of the transformation-associated metalloproteinase in euthymic and athymic Lewis rats

Transin is a neutral metalloproteinase initially isolated from malignantly transformed rat fibroblasts and subsequently shown to be homologous to human stromelysin. We performed Northern blot analysis on synovial tissue specimens from Lewis rats with proliferative and invasive streptococcal cell wall (SCW) arthritis. Transin mRNA was present in abundance, as was the mRNA of the c-myc oncogene, which is associated with cellular proliferation. Immunohistochemical staining of synovia from rats with chronic SCW arthritis showed high-level transin expression in the cells of the lining layer and underlying stroma, as well as in chondrocytes and osteoclasts in subchondral bone. Intense nuclear staining for the Myc oncoprotein was also detected with a cross-reactive antibody to v-Myc. Transin stained similarly in the early, rapid-onset, thymus-independent, acute phase of SCW arthritis. In the T cell-dependent adjuvant arthritis, transin expression was noted by day 4, 6 d before the influx of mononuclear cells and the onset of clinical disease. Athymic rats did not express transin. We concluded that transin is a marker of proliferative, invasive arthritis in rats and appears early in the course of disease development, but requires a competent immune system to sustain its expression in these model arthropathies.

The v-ski oncogene encodes a truncated set of c-ski coding exons with limited sequence and structural relatedness to v-myc

The nucleotide sequence of a biologically active v-ski gene from a cloned proviral segment shows that ski is a 1,312-base sequence embedded in the p19 region of the avian leukosis virus gag gene. The v-ski sequence contains a single open translational reading frame that encodes a polypeptide with a molecular mass of 49,000 daltons. The predicted amino acid sequence includes nuclear localization motifs that have been identified in other nuclear oncoproteins. It also contains a proline-rich region and a set of cysteine and histidine residues that could constitute a metal-binding domain. Two regions of the amino acid sequences of v-ski and v-myc are related, and the two proteins exhibit similar distributions of hydrophobic and hydrophilic amino acids. Cloned segments of the chicken c-ski proto-oncogene totaling 65 kilobases have been analyzed, and regions related to v-ski have been sequenced. The results indicate that v-ski is derived from at least five coding exons of c-ski, that it is correctly spliced, and that it is missing c-ski coding sequences at both its 5' and 3' ends. The c-ski and avian leukosis virus sequences that overlap the 5' virus/v-ski junction in Sloan-Kettering virus contain an 18-of-20-base sequence match that presumably played a role in the transduction of ski by facilitating virus/c-ski recombination.

Temporal order of gene replication in Chinese hamster ovary cells

To investigate the molecular basis of the regulatory mechanisms responsible for the orderly replication of the mammalian genome, we have developed an experimental system by which the replication order of various genes can be defined with relative ease and precision. Exponentially growing CHO-K1 cells were separated into populations representing various stages of the cell cycle by centrifugal elutriation and analyzed for cell cycle status flow cytometry. The replication of specific genes in each elutriated fraction was measured by labeling with 5-mercuri-dCTP and [3H]dTPP under conditions of optimal DNA synthesis after cell permeabilization with lysolecithin. Newly synthesized mercurated DNA from each elutriated fraction was purified by affinity chromatography on thiol-agarose and replicated with the large fragment of Escherichia coli DNA polymerase I by using [alpha-32P]dATP and random primers. The 32P-labeled DNA representative of various stages of the cell cycle was then hybridized with dot blots of plasmid DNA containing specific cloned genes. From these results, it was possible to deduce the nuclear DNA content at the time each specific gene replicated during S phase (C value). The C values of 29 genes, which included single-copy genes, multifamily genes, oncogenes, and repetitive sequences, were determined and found to be distributed over the entire S phase. Of the 28 genes studied, 19 had been examined by others using in vivo labeling techniques, with results which agreed with the replication pattern observed in this study. The replication times of nine other genes are described here for the first time. Our method of analysis is sensitive enough to determine the replication time of single-copy genes. The replication times of various genes and their levels of expression in exponentially growing CHO cells were compared. Although there was a general correlation between transcriptional activity and replication in the first half of S phase, examination of specific genes revealed a number of exceptions. Approximately 25% of total poly(A) RNA was transcribed from the late-replicating DNA.

Strain dependency of B and T lymphoma development in immunoglobulin heavy chain enhancer (E mu)-myc transgenic mice

The transgenic mice were produced by injecting eggs of B6 and C3H/HeJ mice with the human E mu-myc gene. Preferential development of B lymphomas was observed in the B6 transgenic mice, whereas the C3H/HeJ transgenic mice developed mostly T lymphomas. The phenotypic activation of B lineage cells but not of T lineage cells was detected in the prelymphomatous transgenic mice of both strains. The transgene was similarly expressed in B and T cells of the transgenic mice of both strains. These results suggest that a high incidence of T lymphomas in the C3H/HeJ transgenic mice may not be due to the preferential activation of or the preferential E mu-myc expression in T lymphocytes. When the bone marrow or fetal liver cells from the prelymphomatous transgenic mice of both strains were transferred into irradiated normal C3H/HeJ mice, most of the recipients developed T lymphomas. Moreover, even when irradiated B6 mice received the hematopoietic stem cells from the prelymphomatous B6 transgenic mice, the incidence of T lymphoma increased up to 50%. These findings suggest that B6 and C3H/HeJ mice might provide the environment that supports the development or growth of B and T lymphomas, respectively, and that such an environment could be modified by irradiation of the mice.

The v-ski oncogene encodes a truncated set of c-ski coding exons with limited sequence and structural relatedness to v-myc

The nucleotide sequence of a biologically active v-ski gene from a cloned proviral segment shows that ski is a 1,312-base sequence embedded in the p19 region of the avian leukosis virus gag gene. The v-ski sequence contains a single open translational reading frame that encodes a polypeptide with a molecular mass of 49,000 daltons. The predicted amino acid sequence includes nuclear localization motifs that have been identified in other nuclear oncoproteins. It also contains a proline-rich region and a set of cysteine and histidine residues that could constitute a metal-binding domain. Two regions of the amino acid sequences of v-ski and v-myc are related, and the two proteins exhibit similar distributions of hydrophobic and hydrophilic amino acids. Cloned segments of the chicken c-ski proto-oncogene totaling 65 kilobases have been analyzed, and regions related to v-ski have been sequenced. The results indicate that v-ski is derived from at least five coding exons of c-ski, that it is correctly spliced, and that it is missing c-ski coding sequences at both its 5' and 3' ends. The c-ski and avian leukosis virus sequences that overlap the 5' virus/v-ski junction in Sloan-Kettering virus contain an 18-of-20-base sequence match that presumably played a role in the transduction of ski by facilitating virus/c-ski recombination.

Temporal order of gene replication in Chinese hamster ovary cells

To investigate the molecular basis of the regulatory mechanisms responsible for the orderly replication of the mammalian genome, we have developed an experimental system by which the replication order of various genes can be defined with relative ease and precision. Exponentially growing CHO-K1 cells were separated into populations representing various stages of the cell cycle by centrifugal elutriation and analyzed for cell cycle status flow cytometry. The replication of specific genes in each elutriated fraction was measured by labeling with 5-mercuri-dCTP and [3H]dTPP under conditions of optimal DNA synthesis after cell permeabilization with lysolecithin. Newly synthesized mercurated DNA from each elutriated fraction was purified by affinity chromatography on thiol-agarose and replicated with the large fragment of Escherichia coli DNA polymerase I by using [alpha-32P]dATP and random primers. The 32P-labeled DNA representative of various stages of the cell cycle was then hybridized with dot blots of plasmid DNA containing specific cloned genes. From these results, it was possible to deduce the nuclear DNA content at the time each specific gene replicated during S phase (C value). The C values of 29 genes, which included single-copy genes, multifamily genes, oncogenes, and repetitive sequences, were determined and found to be distributed over the entire S phase. Of the 28 genes studied, 19 had been examined by others using in vivo labeling techniques, with results which agreed with the replication pattern observed in this study. The replication times of nine other genes are described here for the first time. Our method of analysis is sensitive enough to determine the replication time of single-copy genes. The replication times of various genes and their levels of expression in exponentially growing CHO cells were compared. Although there was a general correlation between transcriptional activity and replication in the first half of S phase, examination of specific genes revealed a number of exceptions. Approximately 25% of total poly(A) RNA was transcribed from the late-replicating DNA.

Structure, origin, and transforming activity of feline leukemia virus-myc recombinant provirus FTT

A myc-containing recombinant feline leukemia provirus, designated FTT, was molecularly cloned from the cat T-cell lymphoma line F422. Its transforming activity, as well as the nucleotide sequence of the 3' 2.7 kilobases of FTT, including v-myc, was determined. The predicted v-myc protein differs from feline c-myc by three amino acid changes and is truncated by two amino acids at the carboxyl terminus. Comparison with feline leukemia virus (FeLV), feline c-myc, and other FeLV proviruses indicates that recombination junctions involved in the generation of FeLV-onc viruses occur at preferred locations within the virus. They usually follow or occur within the sequence ACCCC at 5' junctions and may result from homologous recombination between sequences of marked purine-pyrimidine strand bias, especially at 3' junctions. Some recombination sites also resemble recombinase recognition sequences utilized in immunoglobulin and T-cell receptor variable-region joining. Transfection of primary rat embryo fibroblasts and subsequent in vivo analysis revealed that morphologic and tumorigenic transformation require cotransfection of FTT with human EJ-ras DNA; neither gene alone is sufficient. FTT v-myc is expressed in these transformed rat cells as a 3.0-kilobase subgenomic RNA; however, in contrast to the depressed level of c-myc expression in v-myc-involved feline tumors, steady-state levels of rat c-myc RNA and protein are apparently unaltered.

The myc family of nuclear proto-oncogenes

Several members of the myc family of proto-oncogenes have been described, and some (c-, N-, and L-myc) have been characterized in considerable detail. They are united by a common gene structure and nucleotide homologies that were used to identify some of them initially. Their protein products also have scattered regions of amino acid identity or homology. Although the cellular activities of the various proteins are unknown, some members may play a role in regulating cell growth and differentiation. They share the ability to cooperate with an activated ras gene and cotransform embryonic rodent cells. In naturally occurring tumors, the members of the myc family of oncogenes appear to be activated by genetic changes (proviral insertion, chromosomal translocation, and gene amplification) that augment or otherwise disrupt normally regulated expression. The members of this family of genes differ markedly in their tissue specificity and developmental regulation of expression. This may account in part for the frequent appearance of activated c-myc genes in a wide variety of neoplasms and the limited appearance of activated N- and L-myc genes in tumors of embryonic or neural origin. The c-myc gene may be activated in tumors by a variety of mechanisms, whereas N- and L-myc appear to be activated only by gene amplification. Regulation of expression of the different myc genes also appears to occur by different mechanisms. Finally, the products of the different genes differ in may regions of the protein, and this divergence probably reflects their specific and individual functions.

Absence of missense mutations in activated c-myc genes in avian leukosis virus-induced B-cell lymphomas

We have determined the nucleotide sequences of two independent DNA clones which contained the activated c-myc genes from avian leukosis virus-induced B-cell lymphomas. Neither of these c-myc genes contained missense mutations. This strongly supports the notion that the c-myc proto-oncogene in avian leukosis virus-induced B-cell lymphomas can be oncogenically activated by altered expression of the gene without a change in the primary structure of the gene product.

Absence of missense mutations in activated c-myc genes in avian leukosis virus-induced B-cell lymphomas

We have determined the nucleotide sequences of two independent DNA clones which contained the activated c-myc genes from avian leukosis virus-induced B-cell lymphomas. Neither of these c-myc genes contained missense mutations. This strongly supports the notion that the c-myc proto-oncogene in avian leukosis virus-induced B-cell lymphomas can be oncogenically activated by altered expression of the gene without a change in the primary structure of the gene product.