Stabilization of myc proto-oncogene proteins during Friend murine erythroleukemia cell differentiation

Glucocorticoids, oxysterols, and cAMP with glucocorticoids each cause apoptosis of CEM cells and suppress c-myc

In clones of the CEM human acute lymphoblastic leukemic cell line, glucocorticoids, oxysterols and activators of the cAMP pathway acting synergistically with glucocorticoids, each can cause apoptotic cell death. Morphologically and kinetically, these deaths resemble one another. The kinetics are striking: in each case, after addition of the lethal compound(s), an interval of approximately 24 h follows, during which cell growth continues unabated. During this "prodromal" period, removal of the apoptotic agent leaves the cells fully viable. We hypothesize that a sequence of biochemical events occurs during the prodrome which eventually results in the triggering of the full apoptotic response as evidenced by the activation of caspases and DNA fragmentation. At some point, the process is irreversible and proceeds relatively rapidly to cell death. Suppression of c-Myc seems a universal early event evoked by each of these lethal compounds or combinations, and we conclude that the negative regulation of this proto-oncogene is an important aspect of the critical pre-apoptotic events in these cells.

Anti-IgM-mediated regulation of c-myc and its possible relationship to apoptosis

Anti-IgM treatment of Burkitt's lymphoma cells is followed by either growth arrest or induction of apoptosis. In this study we have explored the role of c-myc in these events. Our results in Ramos cells indicate the following. (a) The decline in c-myc mRNA occurs at about 4 h; inhibition of about 80% being observed. (b) The stability of c-myc message is involved since the half-life of c-myc mRNA is decreased from about 30 min in untreated cells to about 15 min following treatment with anti-IgM. In the presence of cycloheximide, a protein synthesis inhibitor, the half-life is increased to about 50 min and was unaltered by treatment with anti-IgM. (c) By contrast, nuclear run-on experiments indicated no change in transcription rates for c-myc message due to treatment with anti-IgM. (d) A decrease in c-myc causes apoptosis since specific repression of c-myc with antisense oligonucleotides decreases the levels of c-Myc, inhibits growth rate, decreases viability, and induces apoptosis. (e) Anti-CD40 inhibition of apoptosis occurs without alteration in anti-IgM-induced down-regulation of c-myc mRNA, suggesting that it acts distally to c-myc down-regulation. Other cell lines were also investigated. In Epstein-Barr virus (EBV)-positive cell lines (Daudi, Raji, and Namalwa), anti-IgM treatment for 24 h results in growth inhibition without induction of apoptosis. In EBV-negative cell lines (ST486 and CA46, as well as Ramos), a more heterogeneous pattern of responses to anti-IgM are observed. Ramos and ST486 cells both show growth inhibition and apoptosis upon anti-IgM treatment; CA46 cells shown only growth inhibition but not apoptosis. Anti-IgM causes a decline in c-myc mRNA levels in all of these lines, as well as in c-Myc protein level in the two lines investigated, Daudi and Ramos, regardless of apoptosis. Addition of antisense c-myc oligonucleotides to the cells reduced growth in both Daudi and Ramos cells lines, however it resulted in substantial apoptosis only in Ramos cells. These results suggest that anti-IgM destabilizes c-myc mRNA by a process that involves mRNA turnover, rather than transcription rates. However anti-IgM exerts differential effects in EBV-positive and EBV-negative cell lines. EBV-positive cells are uniformly resistant to apoptosis, while EBV-negative cell lines show a tendency to apoptosis but with exceptions. Growth inhibition can be uncoupled from apoptosis in EBV-positive cell lines, but not in those EBV-negative cell lines prone to apoptosis. Furthermore, down-regulation of c-myc message correlates with growth inhibition in these cells, but is an insufficient link to apoptosis. By contrast inhibition of apoptosis by anti-CD40 occurs even though c-myc mRNA is decreased.

c-Myc antigens in the mammalian enteric nervous system

We have investigated the presence of c-Myc-like antigens in the enteric nervous system of the guinea-pig, rat, dog, sheep, monkey and human. c-Myc-like immunoreactivity was demonstrated by immunohistochemistry in the enteric nervous system of all animals tested, with one or more monoclonal antibodies raised against peptide sequences found in the human c-Myc protein. While in most cases the labelling was nuclear, cytoplasmic labelling was also observed. In the guinea-pig enteric nervous system, c-Myc-like immunoreactivity detected by two different antibodies remained detectable for up to 4 h in the presence of cycloheximide. The size and density of labelled nuclei in the ileal submucous plexus were consistent with exclusive neuronal labelling by one antibody and neuronal plus glial labelling by the other. Double-labelling with antiserum directed against vasoactive intestinal peptide revealed a subset of c-Myc-immunoreactive neurons that also contain this neuropeptide. Anti-c-Myc antibodies specifically immunoprecipitated proteins from guinea-pig myenteric plexus-longitudinal muscle preparations whose sizes were consistent with previous observations for c-Myc antigens and whose distribution was consistent with synthesis in the myenteric plexus. We conclude that c-Myc proteins are expressed in mammalian enteric neurons and that they have characteristics similar to those of c-Myc proteins in other nonproliferative cells.

Stabilization of c-myc protein in human glioma cells

The regulation of c-myc protein, product of c-myc/genes, was studied in four glioma cell lines by Northern blot, pulse-chase dot blot, immunoblot and immunoprecipitation analyses. Northern blot analysis revealed no overexpression of c-myc transcript, and pulse-chase dot blot analysis showed normal turnover rate of c-myc transcript, suggestive of no evidence of aberrant regulation of c-myc at post-transcriptional level. The synthesis levels of c-myc protein were shown by immunoprecipitation and closely associated with the c-myc transcript levels demonstrated by Northern blot, suggestive of no evidence of aberrant translational control of c-myc, whereas they were dissociated from the accumulation levels of c-myc protein shown by immunoblot, suggestive of an evidence of aberrant regulation of c-myc at post-translational level. The mean (+/- standard deviation) half-lives of c-myc protein in four glioma cell lines were calculated from the pulse-chase immunoprecipitation analysis, and being 98 +/- 8 to 143 +/- 11 min, were about four- to sixfold longer than normal. In surgical specimens, the immunostain of c-myc protein was not found in normal astrocytes but localized heterogenously in nuclei of reactive astrocytes and glioma cells, and increased in stained cell number in proportion to malignancy. Although this study was limited to four glioma cell lines, it suggests that the c-myc protein in glioma cells may be accumulated due to its prolonged half-life contributing to an uncontrolled proliferation.

c-Myc interferes with the commitment to differentiation of murine erythroleukemia cells at a reversible point

When murine erythroleukemia (MEL) cells, containing the transferred rat c-myc gene under the control of human metallothionein II gene promoter, are induced to differentiate with dimethyl sulfoxide, the level of differentiation is dependent on the c-Myc level, which is modulated by the addition of Zn ions. In this work, we examined the point of inhibition of differentiation by elevated levels of c-Myc in one (clone 38-2) of the typical transformants. Commitment assay indicated that elevated levels of c-Myc interfere with entry of the transformant into the commitment event, but when c-myc expression was reduced by removing Zn ions from the medium, the cells could reenter the commitment program. However, once the cells were committed, such cells could not return to the uncommitted state. In addition, time-dependent expression of two erythroid specific genes was inhibited by elevated levels of c-Myc in time-dependent manner. These results suggest that c-Myc modulates MEL cell differentiation at a reversible point of commitment.

Changes in proteinase activities during the differentiation of murine erythroleukemia cells

Changes in intracellular proteinase activities were examined during DMSO-induced differentiation of murine erythroleukemia cells. Suc-APA-MCA hydrolytic activity was significantly decreased, and apparent ATP-dependent multicatalytic proteinase activity was also decreased with MEL cell differentiation. Cathepsin B and L activity was mainly present in the microsomal fraction of control cells, but a part of this activity had shifted to the lysosomal fraction of differentiated cells. With the translocation of cathepsin B from the microsomal to the lysosomal fraction, the pro-enzyme form of cathepsin B was converted into the mature enzyme. These results suggest that the lysosomal pathway contributes to the degradation of specific proteins with cell differentiation.

Expression of synaptophysin and neuron-specific enolase during neuronal differentiation in vitro: effects of dimethyl sulfoxide

Neural development in dissociated cell cultures of fetal rat brain can be expected to depend on synaptic interactions between cultured neurons. Therefore, an attempt was made to obtain a quantitative measure of the time course of synaptogenesis in such a culture system by assessing the level of the secretory vesicle-associated protein synaptophysin (p38). The developmental schedule of p38 was compared to that of neuron-specific enolase (NSE), an established marker of neuronal differentiation. Cultures were raised from dissociated 14 day-old fetal rat diencephalon. In cultures grown for 1-2 days in vitro (DIV), p38-immunoreactivity was preferentially located in neuronal perikarya. After 10-16 DIV, neurons in culture had formed a dense neuritic network, and almost all of the p38-immunoreactivity occurred in the form of fine punctate deposits associated with neuronal processes that often outlined neuronal cell bodies in a basket-like fashion. Electron-microscopic immunocytochemistry proved the punctate deposits to be presynaptic elements, mostly in the form of axonal varicosities. Quantitative immunoblotting showed that levels of p38 increased from the start of cultivation to DIV 4, stayed fairly constant from DIV 4 to DIV 8, and rose again steeply to peak at DIV 12. In contrast, levels of NSE rose continuously up to DIV 12. After DIV 12, levels of both p38 and NSE fell again. Treatment of cultures with dimethyl sulfoxide (DMSO), an agent known to induce differentiation in various normal and malignant cell types, resulted in a significant increase of p38 levels and in a decrease of NSE levels. The amount of p38 continued to increase beyond DIV 12, whereas NSE diminished after having reached a maximum at DIV 12.(ABSTRACT TRUNCATED AT 250 WORDS)

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.