The cellular oncogenes c-myc, c-myb and c-erb are transcribed in defined types of avian hematopoietic cells

The possible role of normal chicken cellular sequences c-erb, c-myb and c-myc, together referred to as c-onc genes and related to the oncogenes of defective avian acute leukemia retroviruses (DLVs), was investigated by determining the accumulation of c-onc RNA in different avian cells an cell lines. Levels of c-myc and in some instances c-myb RNA are elevated in immature hematopoietic cells or cell lines from various lineages but more mature hematopoietic cells, as well as non-hematopoietic cells, contain only low levels. In contrast, the level of c-erb RNA is generally low, but high in a small number of normal bone marrow cells. The results indicate that the cellular homologues of the viral oncogenes are differentially expressed during hematopoiesis. They also indicate that the hypothesis that DLV target cells express their homologous c-onc genes might hold for c-erb, but is not valid in its simple form for c-myc and c-myb.     

c-myc gene expression in human cells is controlled by glucose

The c-myc oncogene is implicated in normal growth and differentiation processes. Human cell lines IM9 and HepG2 stably cultured at "low" glucose concentrations (5.5 mM) show c-myc mRNA levels 3-4 times higher than cells cultured at "high" glucose concentrations (25 nM). D-fructose (a metabolizable exose) substitutes for D-glucose in reducing c-myc expression while 3-ortho-methylglucose (a non metabolizable exose) is uneffective. c-myc expression is up-regulated (by PMA) or down-regulated (by dexamethasone and long-term exposure to FCS) in human cells cultured at "low" glucose but not in cells cultured at "high" glucose. We previously demonstrated that insulin receptor gene expression in human cell lines in enhanced by glucose. Therefore, glucose controls in an opposite way the expression of two genes important in the regulation of eukaryotic cell growth and differentiation.     

Proteins encoded by the human c-myc oncogene: differential expression in neoplastic cells.

To examine myc protein products in the wide variety of human tumor cells having alterations of the c-myc locus, we have prepared an antiserum against a synthetic peptide corresponding to the predicted C-terminal sequence of the human c-myc protein. This antiserum (anti-hu-myc 12C) specifically precipitated two proteins of 64 and 67 kilodaltons in quantities ranging from low levels in normal fibroblasts to 10-fold-higher levels in Epstein-Barr virus-immortalized and Burkitt's lymphoma cell lines, to 20- to 60-fold-higher levels in cell lines having amplified c-myc. The p64 and p67 proteins were found to be highly related by partial V8 proteolytic mapping, and both were demonstrated to be encoded by the c-myc oncogene, using hybrid-selected translation of myc-specific RNA. In addition, the p64 protein was specifically precipitated from cells transfected with a translocated c-myc gene. Both p64 and p67 were found to be nuclear phosphoproteins with extremely short half-lives. In tumor cell lines having alterations at the c-myc locus due to amplification or translocation, we observed a significant change in the expression of p64 relative to p67 when compared with normal or Epstein-Bar virus-immortalized cells.     

Clustered somatic mutations in and around first exon of non-rearranged c-myc in Burkitt lymphoma with t(8;22) translocation.

We have examined the restriction map of the c-myc gene in 15 BL cell lines carrying the variant t(8;22) translocation in which c-myc is known to remain on chromosome 8. Using 3 restriction enzymes cutting outside the c-myc domain (EcoRI, BamHI, HindIII), we found no evidence for a c-myc/Ig lambda rearrangement in 14 BL cell lines. In the last one, BL 37, the 3' flanking region was rearranged corresponding to the already identified breakpoint located 400 pb downstream from the c-myc gene (9). Using 4 restriction enzymes cutting inside the c-myc gene (PvuII, PstI, SacI, HincII) we looked for discrete abnormalities within the gene limits, and we found in 9 BL cell lines several abolished and created sites, compatible with multiple independent somatic mutations. They are significantly clustered in the 5' non coding region, with a striking prevalence at the end of exon 1. The role of mutations in the non-coding first exon region for the deregulation of c-myc expression is discussed.     

Role of c-myc and other genes in interleukin 2 regulated CT6 T lymphocytes and their malignant variants

The cloned murine cytotoxic T cell line CT6 solely requires interleukin 2 (IL 2) for viability and cell cycle progression. Treatment of G arrested cultures of CT6 cells with recombinant IL 2 induces the rapid sequential expression of the nuclear proto-oncogenes c-fos, c-myc, and c-myb but does not affect the expression of several cytosolic or membrane-associated proto-oncogenes. A comparison of early genes induced by growth factor treatment of quiescent NIH/3T3 fibroblasts and CT6 cells demonstrated that only c-fos and c-myc induction is shared in the two different lineages. Factor-independent lines derived from CT6 cells show no mitogenic response to IL 2, yet binding of IL 2 with its receptor in the cells was capable of inducing the expression of c-fos and c-myc. In factor-independent cell lines, c-myc was uniformly expressed at high constitutive levels, suggesting that c-myc abrogates growth factor requirements of these cells. The levels of c-myc expression in the factor-independent lines was not due to an autocrine production of IL 2 but may be a consequence of constitutively activated IL 2 receptors.     

c-myc gene expression and interleukin-2 receptor levels in cloned human CD2+,CD3+ and CD2+,CD3- lymphocytes

The levels of c-myc mRNA and interleukin-2 receptors (IL-2 Rec) were studied in human peripheral blood lymphocytes (PBL); mature CD2+,CD3+ T cell clones and CD2+,CD3- natural killer (NK) cell clones, and CD2+,CD3+ and CD2-,CD3- T lymphoma cell lines. A transient induction of the expression of c-myc and IL-2 Rec was observed in PBL after activation with phytohemagglutinin (PHA). Expression of c-myc and IL-2 Rec was also found in the CD2+,CD3+ and CD2+,CD3- clones. The CD2+,CD3+ showed higher levels of c-myc mRNA and IL-2 Rec than the CD2+,CD3- clones. In three T lymphoma cell lines constitutively high levels of c-myc mRNA but no IL-2 Rec were found. Only in JURKAT (CD2+,CD3+), c-myc mRNA levels could be further enhanced by PHA. These results suggest that in the presence of PHA, expression of c-myc and IL-2 Rec is induced via the CD3 receptor, and in the absence of PHA and/or the CD3 receptor alternative routes of induction are involved.     

Immortalization by c-myc, H-ras, and Ela oncogenes induces differential cellular gene expression and growth factor responses.

Early-passage rat kidney cells were immortalized or rescued from senescence with three different oncogenes: viral promoter-driven c-myc, H-ras (Val-12), and adenovirus type 5 E1a. The normal c-myc and H-ras (Gly-12) were unable to immortalize cells under similar conditions. Quantitation of RNA in the ras-immortalized lines demonstrated that the H-ras oncogene was expressed at a level equivalent to that of the normal H-ras gene in established human or rat cell lines. Cell lines immortalized by different oncogenes were found to have distinct growth responses to individual growth factors in a short-term assay. E1a-immortalized cells were largely independent of serum growth factors, whereas c-myc-immortalized cells responded to serum better than to epidermal growth factor and insulin. H-ras-immortalized cells responded significantly to insulin alone and gave a maximal response to epidermal growth factor and insulin. Several cellular genes associated with platelet-derived growth factor stimulation, including c-myc, were expressed at high levels in the H-ras-immortalized cells, and c-myc expression was deregulated, suggesting that the H-ras oncogene has provided a "competence" function. H-ras-immortalized cells could not be morphologically transformed by secondary transfection with a long terminal repeat-c-myc oncogene, but secondary transfection of the same cells with H-ras (Val-12) produced morphologically transformed colonies that had 20- to 40-fold higher levels of H-ras oncogene expression. Thus, transformation in this system is dependent on high levels of H-ras oncogene expression rather than on the presence of activated H-ras and c-myc oncogenes in the same cell.     

Co-transfection of normal NIH/3T3 DNA and retroval LTR sequences: a novel strategy for the detection of potential c-onc genes.

Morphologically transformed, tumorigenic cell lines were obtained after co-transfecting normal NIH/3T3 DNA and cloned 3'-long terminal repeat sequences of Moloney leukemia virus (Mo-LTR) onto NIH/3T3 recipient cells. In four such cell lines the malignant phenotype was found to be associated with single and specific Mo-LTR integration sites that were retained after serial passages through NIH/3T3 and rat 208F cells, indicating that Mo-LTR sequences are linked to the activated oncogenes. In one of these clones the activated transforming gene was identified as c-raf, the cellular homologue of a recently described retroviral oncogene. This finding not only demonstrates that the mouse c-raf gene can be activated to exhibit an oncogenic potential but also that the approach chosen in this study is suitable for the detection of potential c-onc genes. In contrast to this clone, the activated transforming genes in other cell lines appear to be different from 19 previously isolated v-onc and c-onc genes. These results demonstrate the potential of the established transformation system for the detection and isolation of previously unidentified c-onc genes.     

Expressions of the c-Ha-ras and c-myc genes in rat liver tumors

Expressions of the c-Ha-ras and c-myc genes were studied by Northern blotting of total RNA from primary tumors and non-tumorous parts of the liver of rats given diet containing 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB) and from established rat hepatoma cell lines. The expression of the c-Ha-ras gene was found to be high in the primary tumors, non-tumorous parts of 3'-Me-DAB-treated livers and hepatoma cell lines. In contrast, the c-myc gene was expressed at a high level only in primary tumors and hepatoma cell lines. During 3'-Me-DAB treatment, the c-Ha-ras mRNA level in the liver increased by day 5 and then remained high. Increase in expression of the c-Ha-ras gene in regenerating liver was confirmed. These findings suggest that increase in expression of the c-Ha-ras gene is related to proliferation of hepatocytes, whereas expression of the c-myc gene is associated with hepatocarcinogenesis.     

Expression of p53, bcl-2, bax, bcl-x2 and c-myc in radiation-induced apoptosis in Burkitt's lymphoma cells

Apoptosis, a form of physiological cell death, is a genetically determined program essential for normal development and maintenance of tissues, which has been linked to a variety of gene products. We have examined the susceptibility to radiation-induced apoptosis of cell lines derived from the human B cell tumour, Burkitt's lymphoma (BL), displaying a variety of phenotypic characteristics and expressing genes implicated in apoptosis at different levels. The susceptibility to apoptosis following gamma radiation varied significantly amongst the lines. Cell lines with wild type p53 were susceptible to radiation-induced apoptosis but two of five BL lines with only mutant p53 allele also displayed similar susceptibility. Some BL cell lines that expressed bcl-2 at levels comparable with Epstein-Barr virus (EBV) transformed normal B cells were highly susceptible to gamma radiation-induced apoptosis, whereas others expressing low levels were resistant. When these lines were analysed for bax and bcl-X(L) expression again no correlation was observed with susceptibility or resistance to apoptosis. Two BL cell lines having deregulated expression of c-myc were resistant to the induction of apoptosis while two others which had regulated c-myc expression were susceptible. Thus the status of p53, c-myc, bcl-2, bcl-X(L) and bax is not sufficiently informative in BL lines to predict susceptibility to radiation-induced apoptosis.