c-myc mRNA levels in the cell cycle change in mouse erythroleukemia cells following inducer treatment

Staurosporine induces a neuronal phenotype in SH-SY5Y human neuroblastoma cells that resembles that induced by the phorbol ester 12-O-tetradecanoyl phorbol-13 acetate TPA)

Treatment of SH-SY5Y human neuroblastoma cells with the protein kinase inhibitor staurosporine, induced both morphological and functional differentiation in these cells. The effects of staurosporine were comparable to those induced by the protein kinase C (PKC) activator, 12-O-tetradecanoyl phorbol 13-acetate (TPA), with respect to induction of neuronal differentiation, i.e. neurite outgrowth, inhibition of DNA synthesis, induction and down-regulation of c-myc protein expression, induction of mRNA for both neuropeptide Y (NPY) and growth associated protein 43 (GAP-43) and stimulation of tyrosine hydroxylase expression. Staurosporine failed to translocate PKC to the membrane fraction or to stimulate phosphorylation of the endogenous PKC substrate M(r) 80,000 (p80). Instead, staurosporine inhibited TPA-induced phosphorylation of p80.

S-phase-specific expression of the Mad3 gene in proliferating and differentiating cells

The Myc/Max/Mad transcription factor network plays a central role in the control of cellular proliferation, differentiation and apoptosis. In order to elucidate the biological function of Mad3, we have analysed the precise temporal patterns of Mad3 mRNA expression during the cell cycle and differentiation in cultured cells. We show that Mad3 is induced at the G1/S transition in proliferating cells; expression persists throughout S-phase, and then declines as cells pass through G2 and mitosis. The expression pattern of Mad3 is coincident with that of Cdc2 throughout the cell cycle. In contrast, the expression of Mad3 during differentiation of cultured mouse erythroleukemia cells shows two transient peaks of induction. The first of these occurs at the onset of differentiation, and does not correlate with the S-phase of the cell cycle, whereas the second is coincident with the S-phase burst that precedes the terminal stages of differentiation. Our results therefore suggest that Mad3 serves a cell-cycle-related function in both proliferating and differentiating cells, and that it may also have a distinct role at various stages of differentiation.

Regulation of c-myc mRNA decay in vitro by a phorbol ester-inducible, ribosome-associated component in differentiating megakaryoblasts

The K562 leukemia cell line is bipotential for erythroid and megakaryoblastic differentiation. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) activates a genetic program of gene expression in these cells leading to their differentiation into megakaryoblasts, a platelet precursor. Thus, K562 cells offer a means to examine early changes in gene expression necessary for megakaryoblastic commitment and differentiation. An essential requirement for differentiation of many hematopoietic cell types is the down-regulation of c-myc expression, because its constitutive expression blocks differentiation. TPA-induced differentiation of K562 cells causes rapid down-regulation of c-myc expression, due in part to an mRNA decay rate that is 4-fold faster compared with dividing cells. A cell-free mRNA decay system reconstitutes TPA-induced destabilization of c-myc mRNA, but it requires at least two components for reconstitution. One component fractionates to the post-ribosomal supernatant from either untreated or treated cells. This component is sensitive to cycloheximide and micrococcal nuclease. The other component is polysome-associated and is induced or activated by TPA. Although in dividing cells c-myc mRNA decays via a sequential pathway involving removal of the poly(A) tract followed by degradation of the mRNA body, TPA activates a deadenylation-independent pathway. The cell-free mRNA decay system reconstitutes this alternate decay pathway as well.

Fli-1, an Ets-related transcription factor, regulates erythropoietin-induced erythroid proliferation and differentiation: evidence for direct transcriptional repression of the Rb gene during differentiation

Erythropoietin (Epo) is a major regulator of erythropoiesis that alters the survival, proliferation, and differentiation of erythroid progenitor cells. The mechanism by which these events are regulated has not yet been determined. Using HB60, a newly established erythroblastic cell line, we show here that Epo-induced terminal erythroid differentiation is associated with a transient downregulation in the expression of the Ets-related transcription factor Fli-1. Constitutive expression of Fli-1 in HB60 cells, similar to retroviral insertional activation of Fli-1 observed in Friend murine leukemia virus (F-MuLV)-induced erythroleukemia, blocks Epo-induced differentiation while promoting Epo-induced proliferation. These results suggest that Fli-1 modulates the response of erythroid cells to Epo. To understand the mechanism by which Fli-1 regulates erythropoiesis, we searched for downstream target genes whose expression is regulated by this transcription factor. Here we show that the retinoblastoma (Rb) gene, which was previously shown to be involved in the development of mature erythrocytes, contains a Fli-1 consensus binding site within its promoter. Fli-1 binds to this cryptic Ets consensus site within the Rb promoter and transcriptionally represses Rb expression. Both the expression level and the phosphorylation status of Rb are consistent with the response of HB60 cells to Epo-induced terminal differentiation. We suggest that the negative regulation of Rb by Fli-1 could be one of the critical determinants in erythroid progenitor cell differentiation that is specifically deregulated during F-MuLV-induced erythroleukemia.

Myc oncogene expression and nude mouse tumorigenicity and metastasis formation are higher in alveolar than embryonal rhabdomyosarcoma cell lines

Accumulated clinical evidence suggests that alveolar rhabdomyosarcoma (ARMS) is more aggressive than embryonal rhabdomyosarcoma (ERMS). Here, we study six childhood rhabdomyosarcoma cell lines, three ERMS and three ARMS. We have assayed the ability of the tumor cells to grow in culture and in nude mice as well as their propensity for pulmonary metastasis formation by tail vein injection. We also compared levels of c- and N-myc oncogene expression and DNA copy number. We find no correlation of histologic tumor type (i.e. ERMS versus ARMS) with growth rate in culture, but we do find suggestive correlations of histologic type with tumorigenicity (mean tumor diameter in millimeters at 6 wk: ARMS 30, ERMS 10; p1 = 0.1) and metastasis formation (ARMS 12, ERMS 0; p1 = 0.1). These properties also correlate with uniform greater overexpression of c-myc in ARMS (mean 39.3-fold, range 16-83) compared with ERMS (mean 5.3, range 4-8) (p1 = 0.05, control fibroblasts = 1). Although c-myc was often amplified in vitro (four of six lines), there was no correlation with histologic type (2/3 ARMS, 2/3 ERMS). These data on rhabdomyosarcoma cell lines derived from verified ERMS and ARMS tumors support the impression from previous clinicopathologic observations that ARMS is a more malignant form of rhabdomyosarcoma than ERMS.

Decreased expression of c-myc oncoprotein by peripheral blood mononuclear cells in thalassaemia patients receiving desferrioxamine

Desferrioxamine (DFX) is an iron chelation agent widely used in the treatment of transfusional iron overload in patients with thalassaemia major and other severe refractory anaemias. DFX has been shown to induce inhibition of DNA synthesis and apoptosis in vitro; however, the molecular targets of DFX action are not well known. The c-myc proto-oncogene is involved in a number of cellular processes including proliferation, differentiation and apoptotic cell death. We have examined the expression of c-myc in peripheral blood mononuclear cells from 71 patients with homozygous beta-thalassaemia in regular transfusion and iron chelation therapy with DFX, 5 non-transfusion, non-chelation-dependent thalassaemic patients, and 15 healthy volunteers using an APAAP immunocytochemical method. We have found that mononuclear cells from thalassaemic patients receiving DFX express significantly lower levels of c-myc protein compared to control healthy volunteers or thalassaemics receiving no DFX. In vitro treatment of HL60 or K562 leukaemic cells with 100 microliters DFX also induced a rapid decrease in c-myc mRNA and protein levels, followed by apoptosis and inhibition of DNA synthesis. These effects were blocked by simultaneous addition of ferric chloride. Our data suggest that deprivation of cellular iron induces downregulation of c-myc expression in vitro and in vivo and may influence haemopoietic cell growth and survival.

Structure-activity relationship of 17 structural analogues of n-butyric acid upon c-myc expression

Terminal differentiation of hematopoietic cells in vivo and in vitro is almost invariably accompanied by down-regulated expression of the c-myc proto-oncogene. Constitutive expression of c-myc in tumor cells inhibits terminal differentiation and maintains proliferation. In Burkitt's lymphoma, chromosomal translocations cause a deregulation of the c-myc gene through fusion of this locus with one of the immunoglobulin gene loci. However, the down-regulation of c-myc by n-butyric acid, a potent inducer of differentiation, is also observed in BL cells. Unlike other inducers of differentiation such as dimethylsulfoxide or hexamethylenebisacetamide, which down-regulate c-myc expression, albeit transiently, n-butyric acid causes a continuous, transcriptional shut-off. Because of the possible therapeutic implication of this finding, we have assayed structural analogues of n-butyric acid for their effect on c-myc expression in Burkitt's lymphoma cells. Of the analogues tested, 12 were active and 5 were inactive. Only unbranched fatty acids with 4 and 5 carbon atoms showed activity, a 4-carbon chain being optimal. 3-chloropropionic acid had maximal activity at a 3-fold lower concentration than n-butyric acid (1 mM versus 3 mM). The corresponding ester-analogues were equally effective. Those analogues found capable of down-regulating c-myc in Burkitt's lymphoma cells were similarly effective in their ability to induce terminal differentiation in murine erythroleukemia cells.

Among 17 inducers of differentiation only sodium butyrate causes a permanent down-regulation of c-myc in Burkitt's lymphoma

De-regulation of c-myc by chromosomal translocation is one crucial step for the development of Burkitt's lymphoma. The de-regulation is caused through juxtaposition of c-myc with one of the 3 immunoglobulin loci. We have reported earlier that treatment of Burkitt's lymphoma cells with n-butyrate causes transcriptional down-regulation of c-myc expression. Because of the possible therapeutic implication of this result, we looked for other compounds which, on the one hand, might be applicable in vivo and, on the other hand, might cause down-regulation of c-myc expression in Burkitt's lymphoma cells. Since n-butyrate is known to induce differentiation, we have examined other differentiation inducers of different chemical nature for their ability to reduce c-myc expression in Burkitt's lymphoma cells. Many of the substances tested caused down-regulation of c-myc expression, which, however, was transient except for n-butyrate. Three types of compounds proved to be particularly active: polar planar compounds (e.g., dimethylsulfoxide), heterocyclic compounds (e.g., hypoxanthine), and short-chain fatty acids (e.g., n-butyric acid). The action of n-butyrate on c-myc suppression was exceptional not only in not being transient, but also in being allele-specific: it down-regulated the translocated allele without affecting the normal one. Medium transfer experiments revealed that neither degradation of the active compound nor an intracellular resistance mechanism can fully account for the reversibility of c-myc down-regulation after treatment with the transiently acting polar planar compounds.

Inhibition of an erythroid differentiation switch by the helix-loop-helix protein Id1

The Id proteins function as negative regulators of basic-helix-loop-helix transcription factors, which play important roles in determination of cell lineage and in tissue-specific differentiation. Down-regulation of Id1 mRNA is associated with dimethyl sulfoxide-induced terminal differentiation of mouse erythroleukemia cells. To examine the significance of Id1 down-regulation in erythroid differentiation, we generated stable mouse erythroleukemia cell lines that constitutively express a "marked" form of the murine Id1 gene. Terminal erythroid differentiation was inhibited in these lines, as indicated by a block in activation of the erythroid-specific genes alpha-globin, beta-globin, and band 3 and continued proliferation in the presence of dimethyl sulfoxide. Interestingly, this block occurred even in the presence of normal levels of the lineage-specific transcription factors GATA-1, NF-E2, and EKLF. Constitutive expression of Id1 did not interfere with DNase I hypersensitivity at site HS2 of the locus control region, expression of the erythropoietin receptor gene, or down-regulation of the endogenous Id1 or c-myc genes. The differentiation block is reversible in these lines and can be rescued by fusion with human erythroleukemia cells. These findings suggest that in vivo, Id1 functions as an antagonist of terminal erythroid differentiation.

A mammalian origin of bidirectional DNA replication within the Chinese hamster RPS14 locus

Two complementary experimental approaches have been used to identify a chromosomal origin of bidirectional DNA replication within or immediately downstream of the Chinese hamster ribosomal protein S14 gene (RPS14). The replication origin, designated oriS14, maps within a 1.6- to 2.0-kbp region of RPS14 that includes the gene's third and fourth introns, exons IV plus V, and approximately 500 bp of proximal downstream flanking DNA. The nucleic acid sequence encoding oriS14 closely resembles the other mammalian chromosomal replication origins whose primary structures are known. It contains DNA binding sites for a large number of transcription factors, replication proteins, and mammalian oncogenes as well as several dinucleotide repeat motifs, an AT-rich region, and a sequence that is likely to bend the DNA. In contrast to the other well-characterized mammalian replication origins, which are autosomal and therefore carried as two copies per somatic cell, oriS14 is encoded by single-copy DNA within a hemizygous segment of chromosome 2q in CHO-K1 cells. Also, other known mammalian replication origins are situated in nontranscribed, intergenic DNA, whereas the DNA sequence encoding oriS14 substantially overlaps the transcribed portion of a constitutively expressed housekeeping gene.

A mammalian origin of bidirectional DNA replication within the Chinese hamster RPS14 locus

Two complementary experimental approaches have been used to identify a chromosomal origin of bidirectional DNA replication within or immediately downstream of the Chinese hamster ribosomal protein S14 gene (RPS14). The replication origin, designated oriS14, maps within a 1.6- to 2.0-kbp region of RPS14 that includes the gene's third and fourth introns, exons IV plus V, and approximately 500 bp of proximal downstream flanking DNA. The nucleic acid sequence encoding oriS14 closely resembles the other mammalian chromosomal replication origins whose primary structures are known. It contains DNA binding sites for a large number of transcription factors, replication proteins, and mammalian oncogenes as well as several dinucleotide repeat motifs, an AT-rich region, and a sequence that is likely to bend the DNA. In contrast to the other well-characterized mammalian replication origins, which are autosomal and therefore carried as two copies per somatic cell, oriS14 is encoded by single-copy DNA within a hemizygous segment of chromosome 2q in CHO-K1 cells. Also, other known mammalian replication origins are situated in nontranscribed, intergenic DNA, whereas the DNA sequence encoding oriS14 substantially overlaps the transcribed portion of a constitutively expressed housekeeping gene.

Changes of proliferative activity and phenotypes in spontaneous differentiation of a colon cancer cell line

We examined the alterations of proliferative activity and c-myc expression of a colon cancer cell line (Caco-2) during its spontaneous differentiation. Caco-2 cells were cultured in various types of media and the degree of differentiation was monitored in terms of dome formation in cell monolayers and expression of alkaline phosphatase (ALP) activity. In Caco-2 cells cultured with Eagle's minimum essential medium (EMEM) containing 10% fetal calf serum (FCS), dome formation was demonstrated and ALP activity was markedly increased after the cells reached confluence. Five-fold reduction of c-myc mRNA and a marked decrease in S-phase cells were observed in the differentiated cells. These changes were not induced in FCS-free EMEM. The addition of insulin and transferrin to FCS-free EMEM did not induce cell differentiation or reduction of c-myc mRNA expression. When Caco-2 cells were cultured with three different serum-free media, the induction of dome formation and the increase of ALP activity were observed to varying degrees. Expression of c-myc mRNA in the cells cultured with one serum-free medium decreased to a level similar to that in fully differentiated cells cultured with EMEM containing 10% FCS. These results suggest that a spontaneous switch from a proliferative state with high c-myc expression to differentiated phenotype occurs after cells reach confluence and depends on the culture conditions.

The proto-oncogene c-myc is involved in cell differentiation as well as cell proliferation: studies on growth plate chondrocytes in situ

A combination of immunocytochemistry and microdensitometry has been used to localize and quantify the expression of the proto-oncogene c-myc within chondrocytes of the proximal growth plates of rat and chick long bones. Although the c-myc protein was localized in all chondrocytes of the growth plate of both species the most intense staining was restricted to the proliferating and differentiating chondrocytes. These were identified by their ability to synthesize DNA (bromodeoxyuridine positive) and the presence of alkaline phosphatase activity, respectively. Species differences did exist with the c-myc concentration of the chick proliferating and differentiating chondrocytes being higher (128% and 240%, respectively) than the respective chondrocytes of the rat. The higher c-myc concentration in the chick proliferating chondrocytes paralleled the differences in the bromodeoxyuridine labelling index between the two species. In the rat, the concentration of c-myc protein present in the differentiating chondrocytes was 74% higher than in the respective proliferating chondrocytes, while in the chick it was 146% higher. The data not only provides further evidence for a role of the c-myc protein in cell proliferation but also suggests involvement of this protein in chondrocyte differentiation and/or hypertrophy.

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.

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.

Differentiation of mouse erythroleukemia cells is blocked by late up-regulation of a c-myb transgene

During chemically induced differentiation of Friend virus-infected mouse erythroleukemia (MEL) cell lines, there is a biphasic down-regulation of the c-myb proto-oncogene. A plasmid containing a murine c-myb cDNA controlled by a mouse metallothionein I promoter was transfected into the C19 MEL cell line. For six transfected clones, it was found that expression of the exogenous c-myb mRNA could be up-regulated by the addition of 120 microM ZnCl2 and that the N,N'-hexamethylenebisacetamide-induced differentiation of these transfectants was inhibited in proportion to the level of exogenous c-myb mRNA expression. By adding or removing ZnCl2 at different times during the induction process, it was possible to show that up-regulation of exogenous c-myb limited to the first 2 days of induction had little or no effect on differentiation. In contrast, continuous expression of exogenous c-myb beginning at any time during the period of induction blocked further differentiation. These results suggest that during HMBA induction of MEL cells, the early down-regulation of c-myb mRNA is not necessary for terminal differentiation, whereas the down-regulation of c-myb at a later time is necessary.

Differentiation of mouse erythroleukemia cells is blocked by late up-regulation of a c-myb transgene

During chemically induced differentiation of Friend virus-infected mouse erythroleukemia (MEL) cell lines, there is a biphasic down-regulation of the c-myb proto-oncogene. A plasmid containing a murine c-myb cDNA controlled by a mouse metallothionein I promoter was transfected into the C19 MEL cell line. For six transfected clones, it was found that expression of the exogenous c-myb mRNA could be up-regulated by the addition of 120 microM ZnCl2 and that the N,N'-hexamethylenebisacetamide-induced differentiation of these transfectants was inhibited in proportion to the level of exogenous c-myb mRNA expression. By adding or removing ZnCl2 at different times during the induction process, it was possible to show that up-regulation of exogenous c-myb limited to the first 2 days of induction had little or no effect on differentiation. In contrast, continuous expression of exogenous c-myb beginning at any time during the period of induction blocked further differentiation. These results suggest that during HMBA induction of MEL cells, the early down-regulation of c-myb mRNA is not necessary for terminal differentiation, whereas the down-regulation of c-myb at a later time is necessary.

Rapid induction of polyadenylated H1 histone mRNAs in mouse erythroleukemia cells is regulated by c-myc

Chemically induced differentiation of murine erythroleukemia cells is a multistep process involving a precommitment period in which exposure to inducer leads to cells that are irreversibly committed to terminal differentiation. Certain changes in the expression of cellular proto-oncogenes are an important feature of the precommitment phase. We have identified two H1 histone genes that are rapidly induced during this period. Unlike most histone genes, these two H1 genes encode polyadenylated mRNAs with long 3' untranslated regions. To investigate the relationship between induction of the H1 mRNAs and changes in proto-oncogene expression, we studied two independent series of mouse erythroleukemia cell lines that are inhibited from differentiating because of deregulated expression of transfected copies of c-myc or c-myb. The results showed that induction of the H1 mRNAs was negatively regulated by c-myc. The two H1 histone genes are among the first examples of specific cellular genes that are regulated by c-myc. The timing of their induction suggests that they may play an important role in achieving commitment to terminal differentiation.

Rapid induction of polyadenylated H1 histone mRNAs in mouse erythroleukemia cells is regulated by c-myc

Chemically induced differentiation of murine erythroleukemia cells is a multistep process involving a precommitment period in which exposure to inducer leads to cells that are irreversibly committed to terminal differentiation. Certain changes in the expression of cellular proto-oncogenes are an important feature of the precommitment phase. We have identified two H1 histone genes that are rapidly induced during this period. Unlike most histone genes, these two H1 genes encode polyadenylated mRNAs with long 3' untranslated regions. To investigate the relationship between induction of the H1 mRNAs and changes in proto-oncogene expression, we studied two independent series of mouse erythroleukemia cell lines that are inhibited from differentiating because of deregulated expression of transfected copies of c-myc or c-myb. The results showed that induction of the H1 mRNAs was negatively regulated by c-myc. The two H1 histone genes are among the first examples of specific cellular genes that are regulated by c-myc. The timing of their induction suggests that they may play an important role in achieving commitment to terminal differentiation.

Effect of c-myc gene expression on early inducible reactions required for erythroid differentiation in vitro

By employing cell fusion between two genetically marked mouse erythroleukemia (MEL) cells in which an artificially introduced c-myc gene had been placed under the control of human metallothionein promoter, we investigated the mechanism of the suppressive action of c-myc gene expression in erythroid differentiation. The results indicated that the expression of the c-myc gene blocked the induction of dimethyl sulfoxide-inducible activity, one of the two early activities required for triggering the differentiation.

An in vivo analysis of c-myc and c-fos expression during terminal erythroid differentiation in mouse spleen progenitors

Using thiamphenicol and scheduled bleeding, we were able to induce an adequate number of erythroid stem cells (CFU-e) in mice in order to conduct an in vivo study of the changing expression of c-myc and c-fos oncogenes during erythropoiesis. Results indicated that c-myc and c-fos are active in erythropoiesis and have a similar pattern of expression. A large decrease in expression of both c-myc and c-fos occurs when erythroid cells begin the biochemical transition into mature phenotypes, i.e., when RNA synthesis is down-regulated.

Effect of verapamil on cell cycle transit and c-myc gene expression in normal and malignant murine cells

Verapamil, the prototype calcium channel blocker, reversibly inhibits cell proliferation in many normal and tumour cell lines (Schmidt et al., Cancer Res., 48, 3617, 1988). We have found that two closely related cell lines - B16 murine melanoma cells and B10.BR normal murine melanocytes growing in culture - behave differently in the presence of verapamil, and we are now utilising these two related cell lines to help elucidate the molecular basis of verapamil's antiproliferative effect. In this study, we studied cell cycle phase distribution and c-myc gene expression in both cell lines in the absence of verapamil, during incubation with verapamil and after the cells were washed free of verapamil. Our studies show that 100 microM verapamil rapidly blocks DNA synthesis in melanocytes but not in B16 cells. Similarly, incubation with verapamil for 6-24 h results in a decreased c-myc signal in melanocytes, but a transient increase in c-myc expression in B16 cells. After verapamil is washed from the cells following a 24-h incubation with drug, c-myc expression increases in melanocytes as they begin again to proliferate, but decreases in B16 cells as they begin to die. Our disparate results with these cell lines suggest that c-myc gene expression, regardless of its known involvement in growth control, is not the immediate target for verapamil's inhibitory action.

Effect of c-myc gene expression on early inducible reactions required for erythroid differentiation in vitro

By employing cell fusion between two genetically marked mouse erythroleukemia (MEL) cells in which an artificially introduced c-myc gene had been placed under the control of human metallothionein promoter, we investigated the mechanism of the suppressive action of c-myc gene expression in erythroid differentiation. The results indicated that the expression of the c-myc gene blocked the induction of dimethyl sulfoxide-inducible activity, one of the two early activities required for triggering the differentiation.

Poly(A) shortening and degradation of the 3' A+U-rich sequences of human c-myc mRNA in a cell-free system

The early steps in the degradation of human c-myc mRNA were investigated, using a previously described cell-free mRNA decay system. The first detectable step was poly(A) shortening, which generated a pool of oligoadenylated mRNA molecules. In contrast, the poly(A) of a stable mRNA, gamma globin, was not excised, even after prolonged incubation. The second step, degradation of oligoadenylated c-myc mRNA, generated decay products whose 3' termini were located within the A+U-rich portion of the 3' untranslated region. These products disappeared soon after they were formed, consistent with rapid degradation of the 3' region. In contrast, the 5' region, corresponding approximately to c-myc exon 1, was stable in vitro. The data indicate a sequential degradation pathway in which 3' region cleavages occur only after most or all of the poly(A) is removed. To account for rapid deadenylation, we suggest that the c-myc poly(A)-poly(A)-binding protein complex is readily dissociated, generating a protein-depleted poly(A) tract that is no longer resistant to nucleases.

Alterations in c-myc expression in relation to maturational status of human colon carcinoma cells

Previous work from this laboratory established a large bank of human colon carcinoma cell lines in culture and classified them with respect to growth regulatory phenotypes based on several biological and biochemical characteristics. In the present report, Northern analysis indicates that addition of the maturational agent N,N-dimethylformamide (DMF) (1.0%) to proliferating HCT 116 and MOSER cells resulted in a repression of c-myc proto-oncogene expression; retinoic acid (1.0 microM) was less effective in this regard. Repression of c-myc expression by DMF was observed in MOSER and HCT 116 cells, whether it was added to proliferating or late log phase cultures, and was associated with a corresponding reduction in cellular proliferation. The reduction in c-myc expression by DMF did not require new protein synthesis and occurred within a few minutes after its addition, resulting in a 70% reduction within approximately 2 hr. Previous work from this laboratory indicated that transforming growth factor-beta (TGF-beta) elicited alterations in MOSER cells similar to those observed following DMF treatment. The present report demonstrates that proliferating, but not late log phase, MOSER cells responded to TGF-beta with a repression of c-myc expression. Similarly, an inhibition of cellular proliferation was only observed when TGF-beta (10 ng/ml) was added to proliferating cells. Collectively, these findings indicate that repression of c-myc expression is associated with diminished cellular proliferation and the induction of a more benign phenotype in human colon carcinoma cells. Furthermore, this report is the first demonstration of a c-myc response to TGF-beta in an epithelial cell line.

Differentiation of human leukaemic cell lines and fresh leukaemia cells by low dose Ara-C: monitoring by expression of c-myc and c-fos oncogenes

The effects of low dose Ara-C on cellular oncogene expression were measured in HL-60 and U-937 cell lines and in primary cultures from leukaemic patients. Expression of c-myc was completely abolished in U-937 and greatly reduced in HL-60 after a 3 day exposure to the drug, whereas specific c-fos transcripts were increased. In fresh myeloid leukaemia samples, growth and DNA synthesis were reduced as in the two cell lines. Conversely, changes compatible with the induction of differentiation along the myelomonocytic pathway were much less pronounced than in cell lines treated with the same dose of Ara-C. Cells from one patient did not show any appreciable morphological change. The same sample displayed a greatly reduced expression of c-myc accompanied by a concurrent 10-fold increased expression of c-fos. The data suggest that the action of low dose Ara-C on oncogene expression is comparable to that of other differentiation-inducing agents that display both cytostatic and maturation promoting effects. Evaluation of cellular oncogene expression may therefore represent a useful tool for monitoring effects of low dose Ara-C on leukaemia cells.

Poly(A) shortening and degradation of the 3' A+U-rich sequences of human c-myc mRNA in a cell-free system

The early steps in the degradation of human c-myc mRNA were investigated, using a previously described cell-free mRNA decay system. The first detectable step was poly(A) shortening, which generated a pool of oligoadenylated mRNA molecules. In contrast, the poly(A) of a stable mRNA, gamma globin, was not excised, even after prolonged incubation. The second step, degradation of oligoadenylated c-myc mRNA, generated decay products whose 3' termini were located within the A+U-rich portion of the 3' untranslated region. These products disappeared soon after they were formed, consistent with rapid degradation of the 3' region. In contrast, the 5' region, corresponding approximately to c-myc exon 1, was stable in vitro. The data indicate a sequential degradation pathway in which 3' region cleavages occur only after most or all of the poly(A) is removed. To account for rapid deadenylation, we suggest that the c-myc poly(A)-poly(A)-binding protein complex is readily dissociated, generating a protein-depleted poly(A) tract that is no longer resistant to nucleases.

Expression of cellular protooncogenes in the mouse male germ line: a distinctive 2.4-kilobase pim-1 transcript is expressed in haploid postmeiotic cells

We report that a 2.4-kilobase (kb) pim-1 transcript is expressed in the germ cells of mouse testis. Analysis of purified populations of spermatogenic cell types indicates that the 2.4-kb transcript is selectively expressed in haploid postmeiotic early spermatids. The evidence for a developmentally regulated expression of pim-1 in haploid spermatids suggests a possible developmental role for this protooncogene product. The 2.4-kb pim-1 transcript present in postmeiotic cells differs in size from the 2.8-kb transcript usually detected in somatic tissues. Similar testis-specific transcripts have been seen for mos and abl genes. These data suggest specificity in transcription or processing of certain genes in haploid male germ cells. We have also analyzed other representative protooncogenes, including examples of protein kinases, the ras family, and the "nuclear" protooncogenes. The results indicate that additional protooncogenes are preferentially expressed in either meiotic pachytene cells or postmeiotic early spermatids. These findings suggest a differential regulation of gene expression in these two developmental stages of germ cells. In particular, analysis of expression of the three members of the ras gene family indicates a distinct temporal differential regulation in the expression of the Harvey, Kirsten, and N-ras genes in these germ cells.

Hepatitis B virus DNA detection by in situ hybridization in human hepatocellular carcinoma

The distribution of hepatitis B virus (HBV) DNA in tumor tissue sections from six Korean patients with HBsAg positive hepatocellular carcinoma (HCC) was examined by in situ hybridization using a biotin-labeled recombinant, cloned HBV DNA probe. All patients tested were positive for both HBeAg and anti-HBc in their sera. HBV DNA was distributed abundantly in the cytoplasm and rarely in the nuclei of tumor cells. The validity of the in situ hybridization assay was confirmed by the dot blotting technique using a ³²P-labeled HBV DNA probe obtained by nick translation. In conclusion, it is speculated that integration of HBV DNA into host DNA as well as persistant amplified replication of the HBV DNA within the hepatocytes is linked etiologically to the development of human hepatocellular carcinoma.

Messenger RNAs coding for enzymes of polyamine biosynthesis are induced during the G0-G1 transition but not during traverse of the normal G1 phase

The events occurring during emergence of cells from quiescence ("G0") are not necessarily identical to those in the G1 phase of continuously dividing cells. Cellular levels of the mRNAs coding for ornithine decarboxylase (ODC) and S-adenosyl-methionine decarboxylase (SDC), key enzymes in polyamine synthesis, increased maximally within 5 h after addition of serum to resting 3T3 cells, following a kinetic course similar to that of c-myc mRNA. In a pure early G1 population of cells, prepared by centrifugal elutriation of growing fibroblasts, the levels of ODC and SDC mRNAs were not significantly lower than in other phases of the cell cycle and approximated serum-induced levels rather than the reduced values found in serum-starved cells. Thus, we conclude that the mRNAs coding for the polyamine biosynthetic enzymes, like c-myc, are growth controlled, but not regulated during traverse of a normal cell cycle.

Dimethyl sulfoxide-inducible cytoplasmic factor involved in erythroid differentiation in mouse erythroleukemia (Friend) cells

A previous report described an intracellular factor (differentiation-inducing factor I, or DIF-I) that seems to play a role in erythroid differentiation in mouse erythroleukemia (MEL) cells. We have detected another erythroid-inducing factor in cell-free extracts from dimethyl sulfoxide- or hexamethylenebis(acetamide)-treated MEL cells, which acts synergistically with DIF-I. The partially purified factor (termed DIF-II) triggered erythroid differentiation when introduced into undifferentiated MEL cells that had been potentiated by the induction of DIF-I. The activity in the extracts appeared in an inducible manner after addition of dimethyl sulfoxide or hexamethylenebis(acetamide), reached a maximum at 6 hr, and then rapidly decreased. The induction was inhibited by phorbol 12-myristate 13-acetate and also by cycloheximide. No induction was observed in a mutant MEL cell line defective in erythroid differentiation. These characteristics are consistent with the supposition that DIF-II is one of the putative dimethyl sulfoxide-inducible factors detected in previously reported cell-fusion and cytoplast-fusion experiments. The role of DIF-II in MEL-cell differentiation and in vitro differentiation in general is discussed.

Oncogene expression in myelopoiesis

Oncogenes are a class of genes hypothesized to be causally related to neoplasia. To date, specific oncogenes have been recognized chiefly by their ability to transform test cells to a neoplastic phenotype. This has been accomplished largely through mutational analysis of the genotype of retroviruses or through the analysis of tumor cell DNA by in vitro transfection of rodent fibroblasts. Oncogenes are believed to arise by some genetic alteration from normal cellular genes called proto-oncogenes. Although the normal function of most proto-oncogenes is unknown, it has been proposed that they may function as tissue-specific and temporally specific regulators of differentiation. The role of oncogenes in lymphoid malignancies has been extensively analyzed. Less is known about their role in myeloid leukemias and especially in normal myelopoiesis. Space limitations permit discussion of only salient features of a limited number of oncogenes; we have arbitrarily selected myc, myb, fos, fms, fes, sis, and abl.

Patterns of gene expression during plasmacytoid differentiation of chronic lymphocytic leukaemia cells

Chronic lymphocytic leukaemia (CLL) cells may be induced to undergo plasmacytoid differentiation in vitro in response to 12-O-tetradecanoyl phorbol acetate (TPA). We show here that plasmacytoid differentiation and the accompanying accumulation of Cmu immunoglobulin mRNA are preceded by a rapid transient increase in the expression of the proto-oncogenes, c-myc and c-fos. In terminally differentiated cells the level of c-fos mRNA returned to the original basal level whilst c-myc expression remained appreciably higher than in undifferentiated CLL cells. These data support a possible role for c-fos and c-myc in the programmed chain of events that occur during terminal differentiation of B-lymphocytes.

Myc family of cellular oncogenes

The myc family of cellular oncogenes contains three well-defined members: c-myc, N-myc and L-myc. Additional structural and functional evidence now suggests that other myc-family oncogenes exist. The overall structure and organization of the c-, N-, and L-myc genes and transcripts are very similar. Each gene contains three exons: encoding a long 5' untranslated leader and a long 3' untranslated region. The proteins encoded by these myc genes share several stretches of significant homology. The conservation of sequences at the carboxyterminus of the L-myc protein suggests that it is also a DNA-binding, nuclear-associated protein. Each myc gene will cooperate with an activated Ha-ras oncogene to cause transformation of primary rat embryo fibroblasts. Characteristics of several new myc-family members are described.

Changes in gene expression associated with induced differentiation of erythroleukemia: protooncogenes, globin genes, and cell division

Hexamethylenebisacetamide (HMBA)-induced differentiation of murine erythroleukemia cells (MELC) is a multistep process involving an early latent period during which a number of metabolic changes have been detected, but the cells are not yet committed irreversibly to differentiate. Commitment is defined as the capacity of MELC to go on to express the program of terminal cell division and gene expression (such as the accumulation of globin mRNA) upon removal of the HMBA from the culture. In the presence of HMBA, a small proportion of MELC are committed by 10-12 hr and greater than 90% by 48-60 hr. The present study shows that, during the initial 4 hr of culture, HMBA causes a marked decrease in c-myb and c-myc and an increase in c-fos mRNA levels. With continued culture, the decrease in c-myb and the increase in c-fos mRNA persists, while c-myc mRNA returns to control levels before the time that MELC begin to show irreversible differentiation. Dexamethasone, which blocks expression of HMBA-induced MELC differentiation, does not alter the early pattern of changes in protooncogene mRNA nor the sustained elevation of c-fos, but it does inhibit the continued suppression of c-myb allowing c-myb to return toward control levels. Hemin, which induces MELC to accumulate globins but does not initiate commitment to terminal cell division, does not alter these protooncogene mRNA levels. These studies suggest that, although the early decrease in c-myb and c-myc and increase in c-fos mRNAs may be involved in the multistep events leading to differentiation, the continued suppression of c-myb is critical for HMBA-induced MELC commitment to terminal cell division.

Transfection of mouse erythroleukemia cells with myc sequences changes the rate of induced commitment to differentiate

We have examined the role of the c-myc protooncogene in chemically induced differentiation of mouse erythroleukemia (MEL) cells by transfecting the cells with recombinant plasmids in which c-myc coding sequences were cloned downstream from the mouse metallothionein I promoter in sense and antisense orientations. We previously showed that treatment of MEL cells with inducers of differentiation leads to a rapid (less than 2 hr) decrease in the level of c-myc mRNA. c-myc mRNA is then transiently restored to pretreatment levels approximately 12-18 hr later. These events occur prior to the detection of cells that are irreversibly committed to erythroid differentiation. MEL cell transfectants containing the plasmid with myc in the sense orientation express a chimeric MT-myc mRNA, which also decreases shortly after addition of inducer. However, these clones reexpress myc RNA more rapidly than the parental line and they also differentiate more rapidly. On the other hand, transfectants containing the plasmid with myc in the antisense orientation exhibited a delay in the reexpression of c-myc mRNA and were found to differentiate more slowly than parental cells. Thus, we find a correlation between the time at which myc RNA is reexpressed following inducer treatment and the rate of entry of cells into the terminal differentiation program.

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.

Effects of ornithine decarboxylase inhibition on c-myc expression during murine erythroleukemia cell proliferation and differentiation

The polyamines putrescine, spermidine, and spermine have been implicated in the regulation of cellular proliferation and differentiation. We have previously demonstrated that spermidine is required for proliferation of murine Friend erythroleukemia (MEL) cells. We have also shown that spermidine is required at a transcriptional level for induction of MEL globin synthesis. Since studies monitoring c-myc expression have suggested that this gene also plays a role in both growth and differentiation, we have monitored the effects of inhibiting ornithine decarboxylase (ODCase) activity and polyamine synthesis on levels of c-myc transcripts. The results demonstrate that the level of c-myc RNA is independent of ODCase inhibition and depletion of intracellular spermidine. More importantly, arrest of MEL proliferation is not associated with detectable changes in c-myc expression, while under these conditions there is a decline in ODCase transcripts. During induction of MEL differentiation with dimethyl sulfoxide (DMSO) and hexamethylene bisacetamide (HMBA), c-myc and ODCase undergo similar changes in patterns of expression. However, although spermidine is required for appearance of the differentiated MEL phenotype, depletion of this polyamine by ODCase inhibition had no detectable effect on the biphasic changes in c-myc RNA observed during MEL differentiation. Thus, these biphasic changes in c-myc expression are not sufficient for induction of the mature phenotype. Finally, these results would indicate that the regulation of c-myc expression during both proliferation and differentiation is independent of ODCase activity and inhibition of proliferation by spermidine depletion.

Sequential expression of protooncogenes during lectin-stimulated mitogenesis of normal human lymphocytes

The proliferation of non-neoplastic T lymphocytes is regulated, in part, by the coordinated expression of genes encoding T-cell growth factor (interleukin 2, IL2), IL2 receptors (IL2R), and transferrin receptors (TFR). In addition to growth factors and their receptors, protooncogenes may regulate lymphocyte proliferation. We used cloned cDNAs homologous to 21 different protooncogenes to screen for their expression at the mRNA level in human peripheral blood mononuclear cells (PBMC) stimulated with the mitogenic lectin phytohemagglutinin (PHA), and we compared the time course of accumulation of mRNAs for these protooncogenes to that of mRNAs for the IL2, IL2R, TFR, and histone H3 genes. mRNAs for c-abl, c-ets, c-yes, and N-ras were present in unstimulated PBMC. After stimulation of PBMC by PHA, we detected marked increases within 10 min in the levels of mRNA for c-fos and c-myc; within 6 hr for IL2 and IL2R mRNAs; within 14 hr for c-myb, p53, N-ras, and TFR mRNAs; and within 24-36 hr for H3 mRNA. Expression of c-abl, c-ets, and c-yes increased gradually following stimulation with PHA. None of the other protooncogenes tested was expressed in PBMC. Addition of the protein synthesis inhibitor cycloheximide, before the addition of PHA to cultures, abolished the PHA-induced accumulation of mRNAs for c-myb, N-ras, and TFR, but not of mRNAs for c-fos, c-myc, IL2, and IL2R. These data indicate that c-fos, c-myc, IL2, and IL2R belong to a group of genes expressed early, whereas c-myb, N-ras, and TFR belong to a group of genes expressed later in PHA-activated PBMC, and that the products of the c-fos and c-myc protooncogenes are not required for expression of IL2 or IL2R genes. Addition of purified IL2 augmented the expression of the later-expressed genes c-myb, p53, N-ras, and TFR in PHA-stimulated cultures of PBMC, as well as of the early genes c-myc and IL2R, but not of c-fos and IL2, thus suggesting that PHA and IL2 stimulate the expression of overlapping, but nonidentical, sets of genes in PBMC.

Constitutive c-myc oncogene expression blocks mouse erythroleukaemia cell differentiation but not commitment

Mouse erythroleukaemia cells (also called Friend cells) can be isolated from the spleen of certain strains of mice that have been infected with the Friend virus complex. The cells resemble proerythroblasts and, when exposed to dimethyl sulphoxide (DMSO) or a variety of other chemicals, can be induced to undergo a programme of differentiation which closely resembles the final stages of normal erythropoiesis. This includes the cessation of proliferation and large increases in the production of messenger RNA for both alpha- and beta-globin. In addition, DMSO induces a rapid (less than 2 h) decrease in c-myc mRNA levels. The c-myc oncogene is expressed in the majority of proliferating normal cells and altered expression of the gene has been implicated in the genesis of a wide variety of tumours. To study the influence of oncogene activation on differentiation, we have transfected viral-promoter-driven c-myc genes into mouse erythroleukaemia cells. Constitutive c-myc expression was found to block DMSO-induced differentiation.