Expression of a transfected human c-myc oncogene inhibits differentiation of a mouse erythroleukaemia cell line

The Friend-virus-derived mouse erythroleukaemia (MEL) cell lines represent transformed early erythroid precursors that can be induced to differentiate into more mature erythroid cells by a variety of agents including dimethyl sulphoxide (DMSO). There is a latent period of 12 hours after inducer is added, when 80-90% of the cells become irreversibly committed to the differentiation programme, undergoing several rounds of cell division before permanently ceasing to replicate. After DMSO induction, a biphasic decline in steady-state levels of c-myc and c-myb messenger RNAs occurs. Following the initial decrease in c-myc mRNA expression, the subsequent increase occurs in, and is restricted to, the G1 phase of the cell cycle. We sought to determine whether the down-regulation is a necessary step in chemically induced differentiation. Experiments reported here indicate that expression in MEL cells of a transfected human c-myc gene inhibits the terminal differentiation process.

Identification and characterization of the NMYC gene product in human neuroblastoma cells by monoclonal antibodies with defined specificities

Increased N-myc (now designated NMYC in human gene nomenclature) gene expression has been detected at the transcriptional level in certain types of neoplasms. As yet, the N-myc gene product has not been identified. To detect and characterize the N-myc gene product, we have developed monoclonal antibodies against the putative N-myc gene product made in Escherichia coli as a fusion protein. The antibodies that recognize the N-myc-specific regions were selected on the basis of their reactivities to different portions of the fusion protein. These monoclonal antibodies detect a pair of closely migrating polypeptides of 60 and 63 kDa in nuclear fractions of human neuroblastoma cells. The relative levels of the polypeptides are roughly proportional to the level of N-myc transcripts present in a panel of neuroblastoma lines. These two polypeptides have a half-life of approximately equal to 35 min, and they are indistinguishable from each other by their epitopic profiles.

The distribution of the c-myc oncogene product in malignant lymphomas and various normal tissues as demonstrated by immunocytochemistry

The expression of c-myc was studied in 51 malignant lymphomas and in a variety of normal tissues by immunocytochemistry using monoclonal antibodies raised to different synthetic peptides and reacting monospecifically with the c-myc product (p62c-myc). The c-myc product was detected in only a minority of malignant lymphomas principally those containing cells with immunoblastic characteristics, and was predominantly localised to the cytoplasm. In normal lymphoid tissues only plasma cells and histiocytes were found to have immunoreactivity. In non-lymphoid normal tissues, however, the c-myc product was distributed widely. Marked differences in its intracellular distribution were apparent in different tissues. These findings suggest that the relationship of p62c-myc expression to cell division may be more complex than previously suggested by in vitro studies, and raise the possibility that it may have other functions within the cell.

Consequences of widespread deregulation of the c-myc gene in transgenic mice: multiple neoplasms and normal development

We have constructed a transgenic mouse strain in which a mammary tumor virus LTR/c-myc fusion gene is anomalously expressed in a wide variety of tissues. The deregulated c-myc transgene, now glucocorticoid inducible, contributes to an increased incidence of a variety of tumors, including those of testicular, breast, lymphocytic (B cell and T cell), and mast cell origin. The deregulated gene does not, however, otherwise disturb cell proliferation, nor does it interfere with normal development in these animals. Moreover, since not all tissues that express the transgene develop neoplasms, these results begin to define the transforming spectrum of the c-myc oncogene. They also extend to several organ systems the notion that elements in addition to an activated c-myc gene are required to induce malignancy in the living organism.

Identification and characterization of the protein encoded by the human N-myc oncogene

The human N-myc gene is related to the c-myc proto-oncogene, and has been shown to have transforming potential in vitro. Many studies have reported amplification of N-myc in human neuroblastoma and retinoblastoma cell lines. In primary tumors, amplification of the gene was found to correlate directly with behavior of the tumor. Specific restriction fragments of a partial complementary DNA clone of N-myc from LA-N-5 human neuroblastoma cells were placed into a bacterial expression vector for the purpose of producing antigens representative of the N-myc protein. Rabbits immunized with these antigens produced antisera that recognized a protein of 62-64 kilodaltons in neuroblastoma cells. By several criteria, this protein appears to be part of the same proto-oncogene family as the c-myc protein. Moreover, the antisera to fragments of this protein were capable of histochemically identifying malignant cells in clinical specimens.

H4 histone messenger RNA decay in cell-free extracts initiates at or near the 3' terminus and proceeds 3' to 5'

The relative decay of four human messenger RNAs, gamma globin, delta globin, c-myc and H4 histone, were compared in a cell-free system. Under appropriate conditions, they are degraded in vitro in approximately the same relative order as in vivo: histone faster than c-myc and delta globin faster than gamma globin. Degradation of polysome-associated H4 histone mRNA and of deproteinized histone mRNA begins at or near the 3' terminus. At least a portion of the mRNA then continues to be degraded in a 3' to 5' direction. Discrete 3'-terminal degradation hold-up points are observed, suggesting that 3' to 5' degradation occurs non-uniformly. Cycloheximide and puromycin inhibit protein synthesis but do not affect the rate or directionality of histone mRNA decay in vitro. We conclude that the rate-limiting step in H4 histone mRNA decay occurs at or near the 3' terminus and that at least a portion of the mRNA molecule is subsequently degraded 3' to 5', probably via a processive exonuclease.

Multiple growth-associated nuclear proteins immunoprecipitated by antisera raised against human c-myc peptide antigens

Different antisera raised against various regions of the human c-myc protein were used to identify four human c-myc proteins with apparent molecular masses in sodium dodecyl sulfate-polyacrylamide gels ranging from 64 to 68 kilodaltons (phosphoproteins pp64 and pp67 and nonphosphorylated proteins p65 and p68). pp64 and p65 were the major detectable c-myc proteins, and pp67 and p68 were minor but specific components of the immunoprecipitates. The c-myc proteins were all localized in the cell nucleus. Accumulation of [35S]methionine-labeled p65 was observed after pulse-labeling and chase, suggesting that the stable p65 c-myc protein is generated posttranslationally from short-lived precursors. pp64, pp67, and p68 possessed short half-lives and may therefore be precursors of the stable p65. Confirmation of the nuclear localization of the human c-myc proteins was obtained by immunofluorescent staining. The human c-myc proteins were revealed as a pattern of punctate nuclear staining with, particularly for p65, nucleolar enhancement that left an unstained annulus surrounding the nucleolus.

Nucleotide sequence of the human N-myc gene

Human neuroblastomas frequently display amplification and augmented expression of a gene known as N-myc because of its similarity to the protooncogene c-myc. It has therefore been proposed that N-myc is itself a protooncogene, and subsequent tests have shown that N-myc and c-myc have similar biological activities in cell culture. We have now detailed the kinship between N-myc and c-myc by determining the nucleotide sequence of human N-myc and deducing the amino acid sequence of the protein encoded by the gene. The topography of N-myc is strikingly similar to that of c-myc: both genes contain three exons of similar lengths; the coding elements of both genes are located in the second and third exons; and both genes have unusually long 5' untranslated regions in their mRNAs, with features that raise the possibility that expression of the genes may be subject to similar controls of translation. The resemblance between the proteins encoded by N-myc and c-myc sustains previous suspicions that the genes encode related functions.

Kinetics of cellular oncogene expression in mouse lymphocytes. I. Expression of c-myc and c-ras-Ha in T lymphocytes induced by various mitogens

Murine splenic T lymphocytes display maximal cellular myc gene (c-myc) expression already 3 h after concanavalin A stimulation and subsequent down-regulation before the onset of DNA synthesis. Stimulation by leucoagglutinin in the presence or absence of interleukin 2 leads to only low initial levels of c-myc-specific RNA which, however, increase later on. A similar pattern of c-myc expression is shown by the Lyt-2+ T cell subpopulation stimulated with either concanavalin A or leucoagglutinin in the presence of interleukin 2. Although [3H]thymidine incorporation was identical, the leucoagglutinin-stimulated Lyt-2+ T cells were void of any demonstrable c-myc-specific RNA at 3 h post-stimulation. Thus, the kinetics of c-myc expression in mouse T lymphocytes are not at all uniform, but depend on the mitogen and the subpopulation. In contrast, levels of c-ras-Ha-specific RNA were always low at early times, always increased towards the onset of DNA synthesis and down-regulation was not observed.

Structure and expression of the murine N-myc gene

We have demonstrated that the entire murine N-myc gene and the sequences necessary for its expression in human neuroblastoma cells are contained within a 7.4-kilobase murine genomic clone. The complete nucleotide sequence of this gene reveals a number of striking similarities and differences when compared to the related c-myc gene including the following: (i) each gene contains three exons of which the first encodes a long 5'-untranslated leader sequence; (ii) the coding regions of the N- and c-myc genes share regions of substantial nucleic acid homology, the putative N-myc protein shares substantial homology with the c-myc protein; (iii) as with c-myc, extensive nucleotide sequence homology exists between the untranslated regions of the human and murine N-myc gene transcripts; however, the N-myc and c-myc untranslated regions are totally divergent; (iv) the N-myc transcriptional promoter differs from that of c-myc and is more related to the promoter of the simian virus 40. We discuss these findings in the context of previously defined similarities and differences in the potential functional and regulatory aspects of these two myc-family members.

Human small-cell lung cancers show amplification and expression of the N-myc gene

We have found that 6 of 31 independently derived human small-cell lung cancer (SCLC) cell lines have 5- to 170-fold amplified N-myc gene sequences. The amplification is seen with probes from two separate exons of N-myc, which are homologous to either the second or the third exon of the c-myc gene. Amplified N-myc sequences were found in a tumor cell line started prior to chemotherapy, in SCLC tumor samples harvested directly from tumor metastases at autopsy, and from a resected primary lung cancer. Several N-myc-amplified tumor cell lines also exhibited N-myc hybridizing fragments not in the germ-line position. In one patient's tumor, an additional amplified N-myc DNA fragment was observed and this fragment was heterogenously distributed in liver metastases. In contrast to SCLC with neuroendocrine properties, no non-small-cell lung cancer lines examined were found to have N-myc amplification. Fragments encoding two N-myc exons also detect increased amounts of a 3.1-kilobase N-myc mRNA in N-myc-amplified SCLC lines and in one cell line that does not show N-myc gene amplification. Both DNA and RNA hybridization experiments show that in any one SCLC cell line, only one myc-related gene is amplified and expressed. We conclude that N-myc amplification is both common and potentially significant in the tumorigenesis or tumor progression of SCLC.

Interaction between Raf and Myc oncogenes in transformation in vivo and in vitro

3611 MSV, a raf-oncogene-transducing murine retrovirus, induces fibrosarcomas and erythroid hyperplasia in newborn mice after a latency of 4-8 wk. In contrast, new recombinant murine retroviruses carrying the myc oncogene (J-3, J-5 construct viruses) do not induce tumors before greater than 9 wk. A combination of both oncogenes in an infectious murine retrovirus (J-2) induces hematopoietic neoplasms in addition to less prominent fibrosarcomas and pancreatic adenocarcinoma 1-3 wk after inoculation. The hematologic neoplasms consist of immunoblastic lymphomas of T and B cell lineage and erythroblastosis. If animals were inoculated with a variant of the J-3 virus, which induces altered foci in cultures of NIH 3T3 cells, carcinoma developed in the pancreas with a 2-6 mo latency. In parallel to the synergistic action of both oncogenes on hematopoietic cells in vivo, we find that raf-oncogene-induced transformation of bone marrow cells in culture is enhanced by the addition of myc, which by itself does not transform these cells when grown in standard media. We conclude that concomitant expression of raf and myc oncogenes in hematopoietic and epithelial cells alters their respective transforming activities. The contribution of v-myc in this synergism was examined by use of a series of recombinant murine retroviruses capable of expressing the avian v-myc to study the effect of altered myc expression on hematopoietic/lymphoid cells. With either interleukin 3- or interleukin 2-dependent cell lines, introduction of the recombinant viruses abrogated the requirement for IL 3 or IL 2 for growth, and associated with this was the suppression of c-myc expression. The findings suggest that myc is a component in the signal transduction pathway for IL 3 and IL 2 and support an autoregulatory mechanism of c-myc expression. In contrast to v-myc, expression of v-raf in primary lymphoid/hematopoietic cells has an immortalizing function without abrogating the requirement for IL 3 for growth. This suggests that v-raf and v-myc affect different components of growth regulation, as, for example, commitment (v-myc) and cell cycle progression (v-raf).

Structure and activity of the translocated c-myc in mouse plasmacytoma XRPC-24

In the mouse plasmacytoma XRPC-24 both c-mos and c-myc are rearranged. We cloned the rearranged c-myc and found that it was translocated to the immunoglobulin C alpha locus. The breakpoint is at the end of exon 1 in c-myc and approximately 0.5 kb upstream from exon 1 of C alpha. The cloned translocated c-myc linked to a strong transcriptional promoter can efficiently transform rat embryo fibroblasts when co-transfected with the activated Ha-ras. The transformed cells are tumorigenic in syngeneic rats.

Isolation of monoclonal antibodies specific for human c-myc proto-oncogene product

Six monoclonal antibodies have been isolated from mice immunized with synthetic peptide immunogens whose sequences are derived from that of the human c-myc gene product. Five of these antibodies precipitate p62c-myc from human cells, and three of these five also recognize the mouse c-myc gene product. None of the antibodies sees the chicken p110gag-myc protein. All six antibodies recognize immunoblotted p62c-myc. These reagents also provide the basis for an immunoblotting assay by which to quantitate p62c-myc in cells.

Expression of c-fos in parietal endoderm, amnion and differentiating F9 teratocarcinoma cells

The expression of the cellular proto-oncogene, c-fos, in extra-embryonic tissues of the mouse was investigated using a v-fos DNA probe and an affinity-purified antiserum raised against a C-terminal synthetic peptide. At 13.5 days of development, parietal endoderm--a tissue not previously studied using these methods--was found to express c-fos RNA at a higher level than the amnion or placenta. The previously reported dramatic increase in c-fos RNA levels in extra-embryonic membranes during gestation was found to be confined to the amnion. The antipeptide serum specifically recovered proteins with Mr values of 46,000 and 39,000 from extracts of parietal endoderm and amnion cells labelled for 15 min with 35S-methionine. On sodium-dodecyl-sulphate/polyacrylamide gel electrophoresis these proteins co-migrated with proteins immunoprecipitated using serum from rats inoculated with FBJ-MuSV-transformed cells (tumour-bearing rat serum). Pulse-chasing and 32P-labelling experiments showed that the protein with an Mr of 46,000 was rapidly converted into higher-molecular-weight phosphorylated derivatives. F9 teratocarcinoma stem cells differentiated into parietal-endoderm-like cells in response to treatment with retinoic acid and dibutyryl cyclic AMP. However, this differentiation was not accompanied by any large transient increase in c-fos RNA expression.

Growth-dependent synthesis of c-myc-encoded proteins: early stimulation by serum factors in synchronized mouse 3T3 cells

Synthesis of the c-myc gene product was measured during the entire cell cycle of subconfluent mouse 3T3 cells with an antibody raised against a human c-myc synthetic peptide. The antiserum recognized two mouse c-myc-encoded proteins with apparent molecular weights in sodium dodecyl sulfate-polyacrylamide gels of 62,000 and 60,000. Cell-derived p62 was compared with the mouse c-myc gene product synthesized in vitro. Immunoprecipitation, electrophoretic analyses, and peptide mapping provided evidence that p62 is encoded by the mouse c-myc gene. The rate of synthesis of the c-myc proteins was tightly coupled to the cellular growth state of nontransformed A31 3T3 cells, but not to that of their benzo(a)pyrene-transformed derivative (BPA31). Furthermore, the synthesis of the c-myc proteins was stimulated by the exposure of confluent, density-arrested A31 cells to platelet-derived growth factor or fibroblast growth factor. Tightly synchronized cell populations were obtained on the addition of serum factors to subconfluent, serum-deprived A31 cells, and c-myc expression could be monitored for more than one complete cell cycle. One hour after stimulation the steady-state level of the 2.2 kilobase c-myc transcript increased 30-fold relative to that of quiescent cells and decreased thereafter to the level observed during exponential growth. The rate of synthesis of c-myc-encoded proteins was determined by immunoprecipitation after a 2-h labeling period. After an initial sevenfold increase detectable 2 h after serum addition, the rate of synthesis remained constant throughout the rest of the cell cycle. No further changes associated with the late prereplicative period, S phase, G2, or mitosis could be demonstrated. Pulse-chase and long-term labeling experiments revealed different half-lives for the two c-myc-encoded proteins. The half-lives of the c-myc proteins, however, were independent of the cellular growth state. The sustained expression observed throughout the cell cycle suggests that the growth-related function of c-myc may be required during the G0-G1 transition and in all phases of the cycle of the growing cell.

Amplification and enhanced expression of the c-myc oncogene in mouse SEWA tumour cells

SEWA tumour cells are derived from an osteosarcoma induced in an A.SW mouse by infection with polyoma virus. Cytogenetic analyses have revealed three different characteristic chromosomal abnormalities diagnostic for the presence of amplified genes: 'double minutes' (DMs), homogeneously staining chromosomal regions (HSRs) and C-bandless chromosomes (CMs; for review see ref. 2). DMs may undergo fluctuation in number depending on the conditions in which the cells grow. Their number usually increases after injection of cells into a mouse and often is reduced to undetectable levels when the cells are explanted back into tissue culture; when the cells are re-introduced into the mouse, they again acquire multiple DMs. We show here that cells of SEWA lines carrying DMs, HSRs or CMs contain amplified copies of the proto-oncogene c-myc and enhanced levels of c-myc messenger RNA and c-myc protein. DMs or CMs are the sites of c-myc amplification in two different SEWA lines.

c-myc oncogene protein synthesis is independent of the cell cycle in human and avian cells

Several lines of evidence suggest a role for the myc oncogene in cell proliferation. Most recently, mitogenic stimulation of quiescent lymphoid, fibroblast and epithelial cells has been demonstrated to lead to a sharp increase in c-myc RNA levels. To determine how c-myc expression is linked to the cell proliferative cycle, we have used centrifugal elutriation to enrich for populations of avian and human cells at different stages of the cell cycle. Centrifugal elutriation is a counterflow centrifugation method that separates cells on the basis of volume, a parameter correlating well with progression through the cell cycle. Using myc-specific anti-peptide antibodies, we show here that the synthesis, half-life and modification of c-myc proteins are constant throughout the cell cycle of normal and transformed cells.

The protein encoded by the human proto-oncogene c-myc

The proto-oncogene c-myc may play a role in controlling the growth and division of normal cells, and abnormalities of the gene have been implicated in the genesis of a substantial variety of human tumors. To facilitate further study of these issues, we developed antisera that permit the identification and isolation of the protein encoded by the human and other mammalian versions of c-myc. We found that c-myc(human) gives rise to at least two phosphoproteins with apparent molecular weights of 62,000 [pp62c-myc(human), the major product] and 66,000 [pp66c-myc(human), produced in smaller quantities and possibly a modified version of the Mr 62,000 protein]. Both proteins have relatively short half-lives of approximately equal to 30 min. Mouse c-myc encodes similar proteins with molecular weights of 64,000 and 66,000. The use of cells transformed by DNA-mediated gene transfer sustained previous deductions that the entire coding domain of c-myc(human) is contained in the second and third exons of the gene and resolved previous ambiguities by showing that analogous exons specify the entire protein product of c-myc(chicken). Tumor cells containing amplification of c-myc(human) produce relatively large amounts of pp62/pp66c-myc(human). By contrast, translocations of c-myc found in cells derived from Burkitt lymphoma appear merely to sustain expression of c-myc(human) at levels found also in nontumorigenic lymphoblastoid cells, rather than to increase expression of the gene to manifestly abnormal levels.