Expression of c-myc proto-oncogene in normal human intestinal epithelium

We studied the expression of the human c-myc proto-oncogene in normal human colon epithelium by both in situ hybridization and immunohistochemistry. c-myc was found to be expressed uniformly throughout the entire thickness of the colon epithelium. The present findings do not support the contention that the c-myc proto-oncogene is primarily expressed in proliferating intestinal epithelial cell compartments.     

Probability that the commitment of murine erythroleukemia cell differentiation is determined by the c-myc level

During the commitment of mouse erythroleukemia cell differentiation, c-myc mRNA levels change dramatically. To examine the involvement of c-myc in the commitment of these cells, we have introduced the rat c-myc gene driven by inducible, heterologous (human metallothionein IIA) gene promoter into murine erythroleukemia cells and we have examined the ability of the transformed cells to undergo commitment to terminal differentiation. The induction of the exogenous c-myc gene expression inhibited the commitment of these cells. Time-dependent inhibition of the commitment was observed with the addition of zinc at an appropriate time after the induction with dimethyl sulfoxide. The result clearly indicated that late decline, not early decline, is required for the commitment. By examining the transformants expressing the exogenous c-myc mRNA at different levels, and the induction of the exogenous c-myc mRNA by varying the concentration of zinc, we demonstrated that the commitment may be determined by a stoichiometric amount of c-myc in the defined period. The data also suggest that the probability value for the commitment process occurring in a stochastic manner is well-correlated with the amount of c-myc mRNA.     

ADP-ribosyltransferase activity during the friend virus-induced murine erythroleukemia cell differentiation

A variety of chemical agents that are known to induce erythrodifferentiation in the Friend virus-induced murine erythroleukemia (MEL) cell have been suggested to mediate DNA cleavage in cultured cells prior to differentiation. The activation of the nuclear enzyme, ADP-ribosyltransferase, depends upon the presence of single strand breaks in DNA. If dimethyl sulfoxide (Me2SO) causes DNA breakage, it would be expected that the activity of ADP-ribosyltransferase would increase. A study of ADP-ribosyltransferase activity during cell growth indicates that both Me2SO-treated and untreated MEL cells exhibit a similar increase in the enzyme activity but the increase in Me2SO-treated cells is delayed by a few hours. When examined at comparable stages of growth, both treated and untreated cells show almost identical levels of enzyme activity. The present data thus do not support the contention that Me2SO induces DNA breakage in the MEL cells.     

Enhanced translation and increased turnover of c-myc proteins occur during differentiation of murine erythroleukemia cells.

To determine whether regulation of c-myc proteins occurs during the differentiation of murine erythroleukemia cells, we examined c-myc protein synthesis and accumulation throughout dimethyl sulfoxide (DMSO)- or hypoxanthine-induced differentiation. c-myc protein expression exhibited an overall biphasic reduction, with an initial concomitant decrease in c-myc RNA, protein synthesis, and protein accumulation early during the commitment phase. However, as the mRNA and protein levels recovered, c-myc protein synthesis levels dissociated from the levels of c-myc mRNA and protein accumulation. This dissociation appears to result from unusual translational and posttranslational regulation during differentiation. Translational enhancement was suggested by the observation that relatively high levels of c-myc proteins were synthesized from relatively moderate levels of c-myc RNA. This translational enhancement was not observed with c-myb. Under certain culture conditions, we also observed a change in the relative synthesis ratio of the two independently initiated c-myc proteins. Posttranslational regulation was evidenced by a dramatic postcommitment decrease in the accumulated c-myc protein levels despite relatively high levels of c-myc protein synthesis. This decrease corresponded with a twofold increase in the turnover of c-myc proteins. The consequence of this regulation was that the most substantial decrease in c-myc protein accumulation occurred during the postcommitment phase of differentiation. This result supports the hypothesis that the reduction in c-myc at relatively late times is most important for completion of murine erythroleukemia cell terminal differentiation.     

Terminal differentiation of human erythroleukemia cell line K562 induced by aphidicolin

To analyze the relationship between differentiation and DNA replication, the effect of aphidicolin, a specific inhibitor for DNA polymerase alpha, was measured with respect to erythroid differentiation and activities of DNA polymerases alpha, beta, and gamma. Five micromolar aphidicolin completely blocked the growth of K562 cells and caused 80% of cells to become hemoglobin positive after 5 days exposure. The cessation of K562 cell growth induced by aphidicolin was irreversible, whereas the inhibition of HeLa cell growth was completely reversible. The enzyme activity of DNA polymerase alpha of K562 cells showed a 50-110% increase with aphidicolin treatment as compared to control K562 cells; activities of DNA polymerases beta and gamma were not affected. These features sharply contrasted with the erythroid induction of the same cells by hemin, where cell growth was not suppressed and DNA polymerase alpha was not increased but rather decreased. The enzyme activity of DNA polymerase alpha remained high even after removal of aphidicolin from the culture medium. These results suggest that treatment with aphidicolin might induce an accumulation of protein factors for replication and/or differentiation, causing rapid cell differentiation of cells without cell division.     

Regulation of protein synthesis and accumulation during murine erythroleukemia cell differentiation

We have examined the repertoire of cytoplasmic proteins present at different times during murine erythroleukemia (MEL) cell differentiation. Our laboratory has developed an improved differentiation system in which the use of rapidly inducing MEL subclones and culture conditions which stabilize terminally differentiated cells results in highly synchronous differentiation and the accumulation of large numbers of cells in the end stages of differentiation. Using two-dimensional gel electrophoresis, the proteins of MEL cell cytoplasm have been fractionated at different times of induction in the improved system. The protein composition of MEL cell cytoplasm changes dramatically during the differentiation program, in contrast to previously reported results. We observe patterns of changes that are consistent with alterations in the relative degradative rates as well as the relative synthetic rates of the different proteins. We find that the rate of incorporation of labeled amino acid into protein is reduced in induced cultures of MEL cells. We demonstrate that the contribution of uninduced cells to the protein patterns observed late in differentiation is minor in our system, and argue that the results previously obtained for differentiating MEL cells were influenced by the heterogeneity of the induced populations.     

DNA strand scission and ADP-ribosyltransferase activity during murine erythroleukemia cell differentiation

The accumulation of DNA strand breaks and activation of ADP-ribosyltransferase (ADPRT) have recently been associated with cellular differentiation. Murine erythroleukemia (MEL) cells undergo erythropoietic differentiation when exposed to dimethyl sulfoxide (Me2SO) and several studies have suggested that DNA strand scission induced by this agent is a prerequisite for expression of the differentiated phenotype. Me2SO induction of MEL cells has also been associated with increases in ADPRT activity in one study, but not in another. We have monitored the effects of Me2SO on DNA strand breaks in preformed and replicating MEL cell DNA. The results clearly demonstrate that DNA fragmentation is not detectable during Me2SO induction of MEL differentiation, even in the presence of 3-aminobenzamide, an inhibitor of ADPRT. Further, these results are consistent with an absence of detectable changes in both endogenous and total potential ADPRT activity during Me2SO-induced MEL differentiation. These findings would argue against Me2SO induction of DNA strand scission and ADPRT in MEL cells undergoing differentiation.     

Relationships between the cell cycle and the expression of c-myc and transferrin receptor genes during induced myeloid differentiation

We examined the relationship of cellular oncogene c-myc and transferrin receptor (TfR) gene expression to cell proliferation and cell cycle progression during myeloid differentiation in the HL-60 myeloid leukemia cell line. In order to determine levels of mRNA for these genes in HL-60 cells induced to differentiate along the myeloid pathway, RNA was isolated from HL-60 cells incubated with retinoic acid for 24 h and Northern blots were probed with labeled cDNAs for c-myc and TfR. c-myc mRNA decreased within 3 h of retinoic acid addition, and TfR mRNA decreased after 9 h; both mRNAs continued to decrease over 24 h. RNA was also isolated from HL-60 cells separated by centrifugal elutriation into cell cycle phases. TfR and c-myc cDNA probes hybridized equally to RNA from uninduced cells in all phases of the cell cycle. However, after 24 h incubation with the differentiation inducer retinoic acid, TfR mRNA was expressed substantially less in the G1 stage, whereas c-myc mRNA was still expressed equally in all cell cycle phases. These data indicate that, although TfR and c-myc expression are both associated with cell proliferation in the HL-60 line, TfR is down-regulated specifically in G1 upon induction of terminal differentiation whereas c-myc expression is disassociated from cell cycle control in these cells.     

Erythropoietin binding and induced differentiation of Rauscher erythroleukemia cell line red 5-1.5

We isolated from the Rauscher erythroleukemia cell line (Red 5), a subclone (Red 5-1.5), which contains erythropoietin (epo) binding sites and demonstrates an epo-dependent erythroid differentiation. One class of high affinity binding sites was detected with a Kd (+/- S.D.) of 0.43 +/- 0.09 nM and a mean density/cell of 1200 +/- 311. The cell-associated 125I-epo was displaced by nonlabeled epo but not by other hormones or factors. The 125I-epo binding to Red 5-1.5 cells was maximal within 3 h at 15 degrees C and 1 h at 37 degrees C and proportional to cell number. The addition of epo increased [3H] uridine incorporation into RNA by 6 h and [3H]thymidine incorporation into DNA by 60 h followed by 59Fe incorporation into protein, cell proliferation, and formation of hemoglobin-containing colonies. The incorporation of 59Fe into protein demonstrated a linear dose response (from 0.002 to 1.5 units of epo/ml) beginning 60 h after addition of the hormone to the cultures, and there was a dose-dependent increase (from 0.1 to 1.0 unit of epo/ml) in the formation of hemoglobin-containing colonies. We concluded that the binding of 125I-epo to Red 5-1.5 suggests the presence of specific epo receptors. The sequence of the epo-induced proliferation and differentiation events is similar to primary erythroid cultures but requires longer epo exposure. Receptor occupancy correlates with the induced biological response.     

A transfected L-myc gene can substitute for c-myc in blocking murine erythroleukemia differentiation.

We investigated the ability of the proto-oncogene L-myc to substitute for c-myc in blocking murine erythroleukemia differentiation. Murine erythroleukemia cells (line C19) were transfected with recombinant plasmids containing genomic and cDNA fragments of the L-myc gene driven by a Moloney murine leukemia virus long terminal repeat. Clones expressing constitutive high levels of L-myc failed to differentiate in response to the chemical inducer N,N'-hexamethylene bisacetamide (HMBA). The block to differentiation correlated with the level of L-myc expression. Furthermore, transfected clones grown in the presence of inducer for an extended period of time showed an increased level of L-myc expression. These results suggest that functional domains of the c-myc gene involved in differentiation are located in the discrete regions of homology between the c- and L-myc genes.