Expression of myb, myc and fos proto-oncogenes during the differentiation of a murine myeloid leukaemia

Uniform response of c-raf expression to differentiation induction and inhibition of proliferation in a rat rhabdomyosarcoma cell line

The clonal rat rhabdomyosarcoma cell line BA-HAN-1C is composed of proliferating mononuclear cells, some of which spontaneously fuse to terminally differentiated myotube-like giant cells. Both the induction of differentiation by retinoic acid (RA) and by sodium butyrate (NaBut), as well as the inhibition of proliferation by fetal calf serum (FCS)-depleted medium uniformly resulted in the same effects. There was a significant (p less than 0.001) inhibition of proliferation and induction of cellular differentiation, as evidenced by a significant (p less than 0.05) increase in creatine kinase activity. Furthermore, after exposure to RA-supplemented or FCS-depleted medium, a significant (p less than 0.001) increase in the number of myotube-like giant cells was observed. These effects were preceded by a uniform enhancement of c-raf mRNA expression, which became evident 6 h after exposure to RA, NaBut and FCS-depleted media. C-raf mRNA expression persisted at an elevated level throughout the observation period of 5 days after exposure to RA or NaBut, whereas the increased expression of c-raf mRNA observed after FCS-depletion declined near to the basal level after only 24 h. Furthermore, a transient c-fos mRNA expression was observed 15 and 30 min after exposure to RA-supplemented and FCS-depleted medium but not after exposure to NaBut. The present results suggest a possible role of c-raf in the regulation of differentiation and proliferation of this cell line. Since all our experiments with RA, NaBut and FCS-depletion resulted in an early peak of c-raf mRNA expression, it is suggested that this early peak may be sufficient to trigger events crucial for differentiation and proliferation of BA-HAN-1C tumor cells.

Expression of the M-CSF receptor is controlled posttranscriptionally by the dominant actions of GM-CSF or multi-CSF

We have isolated a murine myeloid precursor cell line (FDC-P1/MAC) that simultaneously expresses receptors for multi-CSF, GM-CSF, and M-CSF (c-fms protooncogene). FDC-P1/MAC cells express high levels of c-fms mRNA and protein when grown in M-CSF, whereas growth in multi-CSF or GM-CSF caused a dramatic reduction of c-fms glycoprotein and mRNA. Nuclear run-off assays demonstrated that c-fms transcription was not growth factor dependent and the regulation occurred posttranscriptionally. Factor switching experiments have shown that both multi-CSF and GM-CSF act dominantly and in a factor concentration dependent manner to suppress c-fms expression. In vitro agar assays of bone marrow cells grown in the presence of GM-CSF and M-CSF, individually and in combination, support the concept that GM-CSF can act dominantly to prevent monocyte/macrophage development. These results suggest that GM-CSF and multi-CSF can suppress development along the monocyte/macrophage lineage and offer a simple stochastic mechanism governing myeloid lineage restriction.

Activation of the interferon system during myogenesis in vitro

Differentiation of skeletal muscle involves withdrawal of myoblasts from cell replication, fusion to form multinucleated myotubes, coordinate appearance of a variety of muscle-specific proteins and the disappearance of a set of other proteins responsible for cell growth. The possible activation of the interferon (IFN) system in this process was studied. Thus, the activity of two IFN-induced enzymes known to be part of the system-(2'-5') oligoadenylate synthetase (2-5A synthetase) and double-stranded RNA-activated protein kinase as well as the expression of 2-5A synthetase coding genes were examined during myogenesis. It is demonstrated that the activity of the enzymes is transiently increased in cultured myoblasts, reaching a peak activity on the 3rd day in culture and then declining to a basal level. This peak activity precedes both cell fusion and the appearance of muscle-specific proteins--acetylcholine receptors (AChR) and creatine kinase. The same kinetics of 2-5A synthetase activity was evident in myoblasts from chick, rat or mouse origin. The enzymatic product appears to be primarily the trimer form of 2-5A, rather than a set of oligomers observed in enzymatic reactions performed on IFN-treated cells, including muscle cultures. The kinetics of 2-5A synthetase gene expression revealed that the largest amount of specific RNA transcripts appeared on the 1st day after seeding, followed by a reduction thereafter. In addition, a decrease was also observed in expression of c-myc, a cell-growth-associated protooncogene. However, an increase towards the 2nd day of both AChR and myosin light chain gene expression was evident, indicating selective regulation of gene expression during myogenesis.

Elevated expression of proto-oncogenes during interleukin-5-induced growth and differentiation of murine B lineage cells

Interleukin 5 (IL-5), a lymphokine produced by helper T cells, is involved in the regulation of growth and differentiation of B cells and other hematopoietic cells. To elucidate IL-5-mediated intracellular mechanisms, we have established IL-5-dependent and -independent murine early B cell lines, J6 and MJ88-1, respectively, and examined the effect of IL-5 on the expression of proto-oncogenes during proliferation. Two- to 3.5-fold increases in the levels of c-myb, c-myc, c-fos, and c-fms mRNA were observed in J6 cells, compared with those in MJ88-1 cells. Further, a role of IL-5 in the proto-oncogene expression during differentiation was examined by using thymidine-treated murine B-cell chronic leukemia BCL1-B20 cells with growth arrest. After 4-day culture, the amount of IgM secreted from BCL1-B20 cells was augmented 4-6 fold in the presence of IL-5. Although expression of c-myb, c-fos, and c-fms mRNA did not change, only c-myc mRNA expression was elevated within 30 min of stimulation with IL-5 and reached a maximal level by 1 hr. Addition of phorbol 12-myristate 13-acetate (PMA) or IL-4 to the culture of BCL1-B20 cells inhibited both the IL-5-mediated augmentation of IgM secretion and the elevated expression of c-myc mRNA. These findings suggest that the IL-5 signal may be associated with the up-regulation of c-myc expression.

c-fos expression precedes osteogenic differentiation of cartilage cells in vitro

We have investigated the temporal pattern of expression of c-fos in cartilage cells in mouse mandibular condyles. During in vitro cultivation, the progenitor cells in this organ differentiate to osteoblasts, and hypertrophic chondrocytes start to show features indicative of osteogenic differentiation. Prior to these processes we observed two distinct patterns of c-fos expression. High, transient c-fos expression was found in the entire tissue within 30 min of culture. This type of c-fos expression appeared to result from mechanical forces applied during dissection. The second type of c-fos expression appeared in individual cells in the zone of hypertrophic chondrocytes. A varying number of formerly quiescent chondrocytes expressed high levels of c-fos mRNA after between 30 min and 10 d in culture, with a peak in the number of cells between days 1 and 3. c-fos expression in these cartilage cells was followed by DNA replication and expression of genes typifying osteoblastic differentiation. After 7 d in culture, groups of cells with the typical ultrastructural features of osteoblasts, and surrounded by an osteoid-like matrix, were observed in single chondrocyte-type lacunae, suggesting division of chondrocytes and differentiation to osteoblasts. The data suggest that c-fos may play a crucial role in the perturbation of determined pathways of skeletoblast differentiation and in the regulation of endochondral bone formation.

Characterization of fos-induced osteogenic tumours and tumour-derived murine cell lines

Osteogenic tumours from c-fos (MT-c-fos-LTR)-transgenic mice and from mice infected with the v-fos-bearing FBR murine osteosarcoma virus (FBR MSV) showed close morphological and neoplastic similarities. Fos mRNA expression was elevated in both types of tumours, and expression of several genes characteristic of differentiated bone cells was either lower, enhanced, or not detectable in comparison to that in normal bone. Tumour-derived cell lines showed variable levels of exogenous fos expression; bone-cell-specific genes were similarly expressed in both primary tumours and tumour-derived cell lines. Upon transplantation the tumour cells formed fibrosarcomas, some of which contained areas of focal osseochondrous differentiation. Non-tumorigenic cell lines established from bone tissue of normal and MT-c-fos-LTR transgenic mice showed osteoblastic characteristics, whereas no parathyroid hormone (PTH) response was observed in transgenic tumour cell lines in spite of high alkaline phosphatase activity. These data indicate that deregulated fos expression interferes with terminal osteogenic differentiation in v-fos- and c-fos-induced bone tumours.

The cellular myb oncogene is amplified, rearranged and activated in human glioblastoma cell lines

The human c-myb gene which encodes a DNA binding protein and which is rarely amplified in neoplastic cells was found to be altered in four human glioblastoma cell lines. It exists in multiple copies in 2 out of 4 cases studied. The degree of amplification as determined by densitometry was about 10-fold, a rearrangement within the coding region and an enhanced gene activity of c-myb were noted. The observation of c-myb oncogene amplification and activity in glioblastoma cell lines presents the first report of this effect in human brain tumor cells.

c-fos proto-oncogene expression in astrocytes associated with differentiation or proliferation but not depolarization

Expression of the c-fos proto-oncogene in rat neocortical astrocytes in culture was examined using Northern blotting and immunocytochemistry. Marked induction of c-fos mRNA in astrocytes was observed after treatment with epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), dibutyryl cyclic AMP (db-cAMP), and phorbol diester (TPA; 12-O-tetra-decanoylphorbol 13-acetate), which are known to induce the proliferation or differentiation of astrocytes. Increase of c-fos protein immunoreactivity (IR) was obtained after treatment with fetal calf serum, EGF, bFGF, db-cAMP and TPA. High concentrations of calcium ionophore A23187, which were lethal to cultured astrocytes, also increased c-fos protein-IR. Treatment with lower concentrations of calcium ionophore (which slightly increase Ca2+ uptake), high K+ and nerve growth factor had no detectable effect on c-fos expression. These results show that depolarization does not induce c-fos in astrocytes and suggest that c-fos may play a role in differentiation and proliferation of astrocytes.

Regulation of megakaryocyte phenotype in human erythroleukemia cells

Induction of human erythroleukemia (HEL) cells with nanomolar tumor-promoting phorbol myristate acetate (PMA) diesters results in the synchronous acquisition of multiple markers of the megakaryocyte phenotype. Induced cells markedly increase their content of cytoplasm and show features of morphological maturation. At the ultrastructural level, PMA-treated cells show increases in cytoplasm, nuclear lobulation and nucleolar content, and free ribosomes. Limited numbers of cells also express alpha-granules and nascent demarcation membrane systems. Functionally, PMA-stimulated HEL cells express increased amounts of the megakaryocyte/platelet proteins: glycoprotein IIb/IIIa, platelet factor 4, von Willebrand factor, glycoprotein Ib, and thrombospondin. No changes are observed in antigenic markers of the erythroid (glycophorin A) or macrophage lineages (MO-1 or MO-2). The increases in antigenic expression are rapid, reaching maximum levels within 3-4 d under serum-free conditions. Treatment with PMA also abruptly (within 1-2 d) inhibits cellular division in these cells. Washout studies indicate that phorbols exert their effect within 18-24 h, the approximate cell cycle time for these cells. Consistent with proliferative arrest, c-myc proto-oncogene transcripts begin to decline within 8 h of PMA treatment, although transcripts of c-myb are unaffected. Importantly, megakaryocyte differentiation is associated with endomitotic DNA synthesis (i.e., continued DNA synthesis in the absence of mitosis and cytokinesis), with HEL cells reaching a DNA content of 3-12 times that of unstimulated cells. Endomitosis is coordinately regulated with changes in antigenic expression and cell size such that those cells having the highest DNA content are the largest and also express the greatest levels of antigen.

Interleukin 3 alone does not support the proliferation of bone marrow cells from A/J mice: a novel system for studying the synergistic activities of IL-3

The number of colonies produced by bone marrow cells in response to interleukin 3 (IL-3) in soft agar cultures varies according to the strain of the donor mice. A/J, AKR, A.TH and A.TL bone marrow cells are particularly hyporesponsive, producing only occasional colonies in the presence of IL-3. Bone marrow cells from all strains of mice, including A/J, produce distinctively large colonies in response to the combination of IL-3 and macrophage colony stimulating factor (M-CSF). In cultures of A/J bone marrow cells, the synergy between IL-3 and M-CSF is further reflected in an increase in both the number and the variety of colonies produced. The increase in colony numbers may be due to the priming of a population of A/J colony-forming-cells (CFCs) by IL-3, enabling them to respond to M-CSF. In support of this notion, IL-3 enhanced the level of c-fms (M-CSF receptor) messenger RNA in cultures of A/J bone marrow cells. It is also possible that a subpopulation of CFCs requires both IL-3 and M-CSF as co-mitogens. The A/J strain provides a novel system for studying the mechanisms involved in the interaction between IL-3 and M-CSF in haemopoiesis.

Peptide growth factors and cell cycle control

Research on mammalian cell cycle control focuses on the points discussed below. Peptide growth factors are multifunctional regulators of growth and differentiation that act by autocrine and paracrine mechanisms. Gene transcription changes are key steps in the control of the G0 in equilibrium with G1----S transition of the cell cycle. Both peptide growth factors and classical tropic hormones, are capable of rapidly modulating transcription through the induction of genes (fos/jun) that encode nuclear transregulator proteins.

Stage-related proliferative activity determines c-myb functional requirements during normal human hematopoiesis

To determine if MYB protein is preferentially required during specific stages of normal human hematopoiesis we incubated normal marrow mononuclear cells (MNC) with c-myb antisense oligodeoxynucleotides. Treated cells were cultured in semisolid medium under conditions designed to favor the growth of specific progenitor cell types. Compared with untreated controls, granulocyte-macrophage (GM) CFU-derived colonies decreased 77% when driven by recombinant human (rH) IL-3, and 85% when stimulated by rH GM colony-stimulating factor (CSF); erythroid burst-forming unit (BFU-E)- and CFU-E-derived colonies decreased 48 and 78%, respectively. In contrast, numbers of G-CSF-stimulated granulocyte colonies derived from antisense treated MNC were unchanged from controls, though the numbers of cells composing these colonies decreased approximately 90%. Similar results were obtained when MY10+ cells were exposed to c-myb antisense oligomers. When compared with untreated controls, numbers of CFU-GM and BFU-E colonies derived from MY10+ cells were unchanged, but the numbers of cells composing these colonies were reduced approximately 75 and greater than 90%, respectively, in comparison with controls. c-myc sense and antisense oligomers were without significant effect in these assays. Using the reverse transcription-polymerase chain reaction, c-myb mRNA was detected in developing hematopoietic cells on days 0-8. At day 14 c-myb expression was no longer detectable using this technique. These results suggest that c-myb is required for proliferation of intermediate-late myeloid and erythroid progenitors, but is less important for lineage commitment and early progenitor cell amplification.

Effects of the antisense v-myb' expression on K562 human leukemia cell proliferation and differentiation

Recombinant plasmids containing v-myb' (803 bp fragment of the 3' end of v-myb) were constructed to induce sense or antisense v-myb' RNA expression with dexamethasone in human cells. These plasmids were used as a tool for the investigation of the role of c-myb gene in human leukemia cell proliferation and differentiation. They were transfected by electroporation into the K562 human leukemia cell line derived from a patient with chronic myelogenous leukemia in blastic crisis. After induction of transcription by dexamethasone, the plasmid with antisense v-myb' repressed the expression of p75c-myb from the endogenous c-myb gene of K562 cells. It also reduced the proliferation rate of K562 cells to 50% of the control level, and induced these K562 cells to express the myelomonocytic differentiation cell surface marker CD13 and increased NBT reducing activity. The plasmid with sense v-myb' did not have an effect on p75c-myb expression, the proliferation of K562 cells or the expression of myelomonocytic differentiation phenotypes. These observations suggest that antisense v-myb' RNA represses p75c-myb expression and that a decrease of p75c-myb suppresses K562 cell proliferation and induces its differentiation towards the myelomonocytic lineage.

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.

Dissociation of cellular proliferation and c-myc expression by buttercup extract

Buttercup extract (BE), an extract of the buttercup plant (Zanthoriza simplicissima), inhibits RNA and DNA synthesis by HL-60 promyelocytic leukemia cells. Exposure of these cells to 3% BE for 48 hours results in dramatic inhibition of RNA synthesis without loss of cell viability. The effect of BE is partially reversible over 12-24 hours with the level of RNA synthesis returning nearly to control levels during this time period. DNA synthesis is also reversibly inhibited by exposure to BE. Despite the inhibition of RNA synthesis in HL-60 cells, there is no decrease in the level of c-myc mRNA, even at high BE concentrations. The level of gene-specific mRNA for the c-Ha-ras, c-fms, and c-mos genes in these cells also remained constant during exposure to BE. Ribosomal RNA is not degraded during 24 hours of BE treatment in vitro, suggesting that BE does not maintain the relative mRNA level for these genes by selective degradation of other RNA species. The inhibition of RNA and DNA synthesis by BE without a corresponding alteration in the level of expression of the c-myc gene suggests that this agent dissociates c-myc expression and cellular proliferation in these cells.

Adenovirus E1A-mediated negative control of genes activated during F9 differentiation

The phenotype of a differentiated cell results from the expression of a unique set of genes in that cell. The differentiation of F9 teratocarcinoma cells in response to retinoic acid and cyclic AMP is an excellent example of this process, as the appearance of several gene products during the course of the differentiation process has been documented. In principle, the activation of gene expression could be due to the appearance of positive-acting factors, the loss of negative-acting factors, or a combination of both. Since F9 cells have been shown to express a cellular E1A analog whereas differentiated F9 cells do not, and it is known that the viral E1A gene exerts a negative effect on transcription of both viral and cellular genes, we determined whether the cellular genes activated during F9 cell differentiation are subject to E1A negative control. We found that infection of differentiated F9 cells with wild-type adenovirus resulted in a decline in the levels of collagen type IV mRNA and plasminogen activator mRNA, both of which are induced by differentiation. At least for the collagen gene, this phenomenon appears to involve a transcriptional repression.

Multiparametric Evaluation of Retinoic Acid-Induced Terminal Differentiation of Blastoid Cells from Acute Non-Lymphocytic Leukemia Patients in Vitro

Recent findings that retinoic acid (RA) induces terminal granulocytic differentiation of the human promyelocytic leukemia cell line HL-60 in vitro and blast cell maturation in patients suffering from acute non-lymphocytic leukemia (ANLL) prompted an investigation on the ability of this agent to induce terminal maturation in blast cells from ANLL patients in vitro. We tested the ability of RA at 3×10–6 M, 3×10–7 M and 3×108– M concentrations to induce differentiation in blastold cells from 16 patients with ANLL using cytochemical and cytologic parameters, in addition to cytofluorometric methods. Leukemic cells in primary culture from all the patients underwent cytochemical and biochemical changes after treatment with RA. However, the extent of differentiation-positive cell clones (D+ clones) varied from patient to patient. Morphologic maturation was observed in a significant number of bone marrow samples. Leukocyte alkaline phosphatase and NBT reduction ability of cells, which are biochemical markers of granulocytic differentiation, were also significantly increased with a simultaneous decrease in DNA and RNA synthesis (which was estimated using a Phywe ICP-11 impulse flow cytometer).

Expression of c-myb in embryonal carcinoma cells and embryonal stem cells

Mouse c-myb has been implicated in the regulation of differentiation and proliferation of haematopoietic cells. Analysis of the chromatin structure of the promoter region of c-myb in embryonal carcinoma (EC) cells and embryonal stem (ES) cells reveals a DNAse I-hypersensitive site coincident with a site found in c-myb-expressing haematopoietic cells, but absent in murine fibroblasts (which do not express c-myb). EC and ES cells were found to express c-myb mRNA, albeit at a level lower than found in haematopoietic cells. Differentiation of ES cells into embryoid bodies resulted in an elevated level of c-myb expression.

Differential binding of nuclear factors to the intron 1 sequences containing the transcriptional pause site correlates with c-myb expression

The molecular mechanisms that modulate c-myb mRNA levels in hematopoietic cells appear to involve premature termination of transcription in the first intron of the gene. We have examined the DNA-protein interactions within the first intron of the c-myb gene and identified a 1.0-kilobase region that could be responsible for its transcriptional regulation. Using the mobility-shift assay, we show a direct correlation between the extent of sequence-specific protein binding to intron 1 DNA fragments, and c-myb mRNA levels in different cell types. During dimethyl sulfoxide-induced differentiation of mouse erythroleukemic cells, there was a dramatic decrease in these nuclear factors that correlated with the decrease in the levels of c-myb mRNA. Nucleotide sequence analysis and DNase I footprinting revealed the presence of putative regulatory elements that are implicated in the binding of these nuclear factors. We propose that binding of nuclear factors to the site of transcriptional pause could play an important role in the regulation of c-myb transcription.

Transcriptional trans-repression by the c-myb proto-oncogene product

We report that the c-myb protein binds to another site, MBS-II, in the SV40 enhancer with low affinity. In co-transfection experiments with a c-myb expression plasmid, tandem repeats of the sequence containing the MBS-II site induced c-myb-dependent transcriptional repression. Results of mutational analyses of the sequence around the MBS-II site suggested that the c-myb protein represses transcription by competing with another trans-activator. These results indicate that c-myb protein can regulate transcription not only positively but also negatively.

Efficient retroviral transfer of a mouse c-myc construct into HL60

We introduced an LTR-driven mouse c-myc second and third exon, Tn5Neo gene construct into the inducible human leukemia line HL60 using an amphotropic retroviral vector system. Over 90% of the cells became neo-resistant and the transfected myc gene was transcribed in several neomycin resistant clones. Making use of the simultaneous presence of the different myc genes in the same cell, we compared expression of the corresponding mRNAs after differentiation and their decay mechanisms.

Specific expression of nuclear proto-oncogenes before entry into meiotic prophase of spermatogenesis

The expression of proto-oncogenes representative of several functional categories has been investigated during development of mouse male germ cells. The c-raf proto-oncogene and three members of the c-ras gene family were expressed in mitotically active stem cells, throughout the prophase of meiosis and to varying extents in post-meiotic cell types. In contrast, the nuclear proto-oncogenes c-fos, c-jun, and c-myc were specifically expressed at high levels in type B spermatogonia. High levels of c-myc and c-jun RNAs were also detected in spermatocytes early in the prophase of meiosis. The type B spermatogonia represent the last mitotic cell division before entry into meiotic prophase; therefore, these nuclear proto-oncogenes may be involved in altering programs of gene expression at this developmental transition.

Transcription of the chicken myb proto-oncogene starts within a CpG island

The nucleotide sequence of an 8.2-kb DNA fragment from the 5' proximal part of the chicken myb proto-oncogene spanning 1761 nucleotides upstream and 6436 nucleotides downstream from a presumed c-myb initiation codon was determined. A 3.3-kb G + C-rich region found in this sequence had also other features characterizing CpG islands, i.e. no CpG underrepresentation and lack of CpG methylation. In haematopoietic tissues c-myb mRNA synthesis starts in two major regions of the CpG island, namely 98 to 108 and 143 to 145 nucleotides upstream from the c-myb initiation codon. These two regions are in or close to the 124-bp evolutionarily conserved element located in the middle part of the CpG island. No alternative splicing of the 5' end of c-myb mRNA suggested earlier (1,2) was observed. The c-myb promoter contains neither TATA nor CAAT box-like structures at the usual positions. Instead, numerous potential Sp1 factor binding sites were found both upstream and downstream from the transcription initiation sites. Moreover, consensus v-myb protein DNA-binding sites were revealed in the promoter region and in sequences downstream from it.

Transcriptional activation by the v-myb oncogene and its cellular progenitor, c-myb

The v-myb oncogene, like its cellular progenitor c-myb, encodes a short-lived nuclear protein involved in processes affecting growth and differentiation in a number of cell types. Fusion proteins, in which v-myb sequences are linked to the DNA binding domain of the yeast transcriptional activator GAL4, can activate transcription from a reporter gene linked in cis to a GAL4 binding site. The domain of v-myb responsible for transcriptional activation is located between residues 204 and 254, and is both necessary and sufficient for activation. Intact v-myb and c-myb proteins can also activate transcription, via a myb binding site linked in cis to a reporter gene. A v-myb protein bearing a deletion in the activator domain is no longer capable of stimulating transcription.

Adenovirus E1A-mediated negative control of genes activated during F9 differentiation

The phenotype of a differentiated cell results from the expression of a unique set of genes in that cell. The differentiation of F9 teratocarcinoma cells in response to retinoic acid and cyclic AMP is an excellent example of this process, as the appearance of several gene products during the course of the differentiation process has been documented. In principle, the activation of gene expression could be due to the appearance of positive-acting factors, the loss of negative-acting factors, or a combination of both. Since F9 cells have been shown to express a cellular E1A analog whereas differentiated F9 cells do not, and it is known that the viral E1A gene exerts a negative effect on transcription of both viral and cellular genes, we determined whether the cellular genes activated during F9 cell differentiation are subject to E1A negative control. We found that infection of differentiated F9 cells with wild-type adenovirus resulted in a decline in the levels of collagen type IV mRNA and plasminogen activator mRNA, both of which are induced by differentiation. At least for the collagen gene, this phenomenon appears to involve a transcriptional repression.

Detection of antibodies to the antigens involving differentiation of myeloid cells in sera from patients with systemic lupus erythematosus

The sera from 66 patients with systemic lupus erythematosus (SLE) were examined by the immunoblotting method to detect antibodies to the antigens on the cultured myeloid cell lines, fresh monocytes and granulocytes, and were compared with the sera from 26 healthy subjects sex- and roughly age-matched to the patients. The sera from SLE patients demonstrated antibodies to many antigens on myeloid cells at high frequencies, compared with healthy subjects. A high reactivity with similar patterns was demonstrated with K562, KG-1 and HL60 cells, while reactivity to U937, monocytes and granulocytes was rather low. In particular, the sera from SLE patients were found to contain the antibody to the antigens with Mr of 60K on K562, KG-1, and HL60 cells, which are known to express a good amount of c-myc products. However, the sera from healthy subjects demonstrated hardly any antibody to the 60K antigen on HL60 cells. After an incubation of HL60 cells with TPA or vitamin D3 to induce their monocytic differentiation, the SLE sera became able to detect the 55K antigen on the differentiated HL60 cells, while the 60K antigen turned to undetectable or only faintly detected. These findings suggested that the 60K antigen on HL60 cells may be related to a gene product involving cell growth or differentiation, such as c-myc protein. Actually, polyclonal antibody to myc-specific peptide could identify the 60K antigen as one of the cellular products of HL60.(ABSTRACT TRUNCATED AT 250 WORDS)

Establishment and characterization of mouse leukemia cell lines L615K and L7212K derived from transplantable murine leukemias maintained in China

Two culture cell lines L615K and L7212K were established from transplanted murine leukemias L615 and L7212, which had been established and maintained in China for years. Based on morphological, immunological and gene rearrangement analyses, L7212K cells are considered to be of T-cell origin while L615K cells might be immature T-cells. Immunofluorescence assays of viable leukemia cells and fluorescence focus assays of their culture supernate for infectious viruses suggested that recombinant mink cell focus-inducing viruses were significantly involved in both leukemic cell lines. Chromosome analysis of the L615K cells revealed a translocation t(12;17) which probably involved the c-fos locus on chromosome 12, since the DNA rearrangement of c-fos was demonstrated by Southern blot analysis with Hind III, and c-fos has been assigned to this chromosome. Although the expression of this gene was not detected by RNA Northern blot analysis, c-myc was slightly expressed in both L615K and L7212K cells.

An oligomer complementary to c-myb-encoded mRNA inhibits proliferation of human myeloid leukemia cell lines

To study the role of the protooncogene c-myb in regulating myeloid leukemia cell proliferation and differentiation, we exposed cells of the human leukemia lines HL-60, ML-3, KG-1, and KG-1a to an oligodeoxynucleotide complementary to an 18-base-pair (bp) sequence of c-myb-encoded mRNA. This treatment resulted in a significant decrease in cell proliferation in all of the lines, which was most marked in HL-60 cells. After 5 days in culture, in several separate experiments with different oligomer preparations, 75% growth inhibition was observed in c-myb antisense treated cells in comparison to untreated HL-60 cells. Two c-myb antisense oligomers of identical length with either 2- or 4-bp mismatches had no effect on cell growth nor did an 18-bp c-myb sense or myeloperoxidase antisense oligomer. The effect of a c-myc antisense oligomer (18 bp) on the growth of HL-60, KG-1, and KG-1a cells was also studied. This oligomer had much less inhibitory effect on cell proliferation than did the c-myb antisense sequence. Interestingly, although c-myc antisense treatment induced maturation of HL-60 cells while it inhibited cell proliferation, such an effect was not noted in c-myb antisense treated cells. These studies indicate that the nuclear protein encoded by the c-myb protooncogene is required for maintenance of proliferation in certain leukemia cell lines. In compared to c-myc protein suggest that, at least in HL-60 cells, c-myc amplification or N-ras activation may not be sufficient to maintain the leukemic growth in the absence of c-myb protein. These findings support the hypothesis that development and maintenance of a malignant phenotype requires a multiplicity of interrelated genetic events.

Elevated c-fos expression inhibits differentiation of L6 rat myoblasts

Expression of c-fos is induced by a number of signals in several cell systems. Although the exact function of the c-fos product is unknown, it has been implicated to be of importance for both cell growth and differentiation (Verma and Sassone-Corsi, 1987). To analyze how c-fos expression relates to in vitro myogenic differentiation, the kinetics of c-fos mRNA expression during spontaneous in vitro differentiation of L6J1 myoblasts was examined; c-fos transcripts were most abundant at day 4 of the differentiation process. Multinucleated myotubes and expression of alpha-actin and myosin heavy chain (MHC) mRNA appeared later, at day 6 or 7, and increased to maximal levels after 10 days in culture. To analyze further the relation between c-fos expression and L6J1 myogenic differentiation, L6J1 myoblasts were transfected with expression vectors containing the murine c-fos gene driven by a metallothionein promoter. The growth rate of c-fos-transfected L6J1 cells did not differ from that of control cells. However, formation of myotubes was significantly reduced in c-fos-transfected L6J1 cultures compared with neo-transfected controls. Myotube formation and expression of the myogenic markers alpha-actin and MHC were reduced in subclones expressing high levels of c-fos, but not in subclones with lower levels of c-fos expression. These results indicate that a marked elevation of c-fos expression at least partially inhibits L6J1 myogenic differentiation.

Expression of the c-myb and c-myc genes is regulated independently in differentiating mouse erythroleukemia cells by common processes of premature transcription arrest and increased mRNA turnover

The mechanisms that modulate c-myb mRNA levels in mouse erythroleukemia cells induced toward erythroid differentiation were compared with those that act on c-myc. Both genes exhibited regulation at the levels of premature transcription arrest and RNA turnover. However, these common processes allowed temporally distinct control of gene expression.

c-myc antisense transcripts accelerate differentiation and inhibit G1 progression in murine erythroleukemia cells

Friend murine erythroleukemia (F-MEL) cells were transfected with a plasmid bearing tandemly arranged mouse c-myc antisense and dihydrofolate reductase transcription units. Sixteen clones were isolated, each containing unrearranged c-myc sequences and expressing high levels of antisense transcripts. All antisense clones examined contained reduced amounts of cytoplasmic endogenous c-myc transcripts. The kinetics of reaccumulation of endogenous c-myc mRNA during a 24-h exposure to dimethyl sulfoxide (DMSO) were also retarded and the ultimate transcript levels attained were less than in control cells. Antisense clones grew as well as control F-MEL cells in medium containing 10% fetal calf serum but at only a half and a quarter of the control rates in media containing 5 and 2% serum, respectively. Antisense clones differentiated faster and to a greater degree than control cells following DMSO exposure. myc antisense transcript expression was increased by growing cells in methotrexate, which resulted in an enhanced response to DMSO. Fluorescence-activated cell sorter (FACS) analysis of cellular DNA content indicated that a greater fraction of antisense nuclei contained a G0/G1 2n DNA content following a 24-h exposure to DMSO. When density-arrested antisense clones were diluted into fresh medium to allow reentry into the cell cycle, they incorporated less [3H]thymidine than control cells. FACS analysis showed that this was because only a portion of the cell population was entering S phase. Whereas control cells did not increase in size following release from density arrested antisense cells contained a subpopulation which were initially smaller and which eventually attained the same size as control cells. Quiescent antisense cells thus comprise two populations, each arrested at a different point in G1. Dilutional replating allowed both populations to reenter the cell cycle. We propose a model which postulates that certain minimal myc levels are necessary for cells to traverse G1. Those with insufficient levels, due, for example, to antisense inhibition, are unable to completely traverse G1 during density arrest and synchronize at an earlier point than do control cells. This earlier point may be along the differentiation pathway and may account for the greater responsiveness of antisense cells to DMSO induction. This model postulates that F-MEL cells overexpressing myc fail to differentiate because myc levels are never sufficiently low enough to allow cells to enter the differentiation pathway.

c-AMP-induced c-fos expression in cells of melanocyte origin

The expression of the c-fos gene in murine cells of melanocyte origin in response to cAMP-elevating agents has been examined. Accumulation of c-fos mRNA at a high level as a consequence of these treatments precedes both proliferative and cytodifferentiative changes in non-tumorigenic or tumorigenic cell lines.

A FOS protein is present in a complex that binds a negative regulator of MYC

Regulation of the human proto-oncogene MYC apparently plays an important role in cellular proliferation and the genesis of diverse tumors. Transcription from MYC is governed principally by two promoters known as P1 and P2. Previously we have detected a negative regulator of these promoters upstream of MYC. We now report that this regulator comprises no more than 26 bp of DNA, with sequence that resembles the regulators of at least two other genes, and we describe nuclear factors that interact with the regulator. Nuclear extracts from human cells form three distinctive complexes with the negative regulator. One of these complexes includes the product of the proto-oncogene FOS or an antigenically related protein, and the FOS protein may, in turn, be associated with the product of the proto-oncogene JUN. Similarly, FOS and JUN proteins produced by translation in vitro bind cooperatively to the negative regulator. These results raise the possibility that FOS and JUN participate in the regulation of MYC.

Interleukin-2 induces a rapid increase in ornithine decarboxylase mRNA in a cloned murine T lymphocytic cell line

We recently observed a 25-fold increase in the activity of ornithine decarboxylase (ODC) 6 h after treatment of G1-synchronized CTLL-2 cells with interleukin-2 (IL-2). Here we show that the ODC mRNA content increased in parallel with the ODC activity during the first hours of stimulation with IL-2, resulting in a 25-fold increase at 6 h. Between 6 and 24 h the ODC mRNA content continued to increase steadily up to 50-fold, even after the ODC activity had returned to low basal levels. In the case of density-arrested CTLL-2 cells deprived of IL-2 for 16 h, the IL-2-mediated increase in ODC mRNA was 2-fold at 1 h and 5-fold at 8 h, irrespective of the capability of the cells to resume their cycle. There was no marked increase in the rate of transcription of the ODC gene, at least during the first 2 h of stimulation with IL-2. These findings suggest that the regulation of the ODC activity by IL-2 is a primary event in IL-2-induced cell proliferation and occurs at the post-transcriptional level, possibly by stabilizing the ODC mRNA and affecting the efficiency of translation of the messenger.

Cyclic-AMP-induced c-fos expression and its relevance to differentiation of a transformed mast cell line

The effects of cyclic AMP on expression of the oncogenes c-myc, c-myb and c-fos in murine P815 mastocytoma cells were examined in relation to growth and differentiation. Induction of differentiation in mastocytoma cells by cyclic AMP was accompanied by a rapid increase in c-fos expression. Cyclic AMP induced stable expression of c-fos mRNA by increasing c-fos transcription 4-5-fold and slightly increasing the stability of c-fos mRNA. However, a high level of c-fos expression was not essential for differentiation of two temperature sensitive-mutant P815 cell lines, as c-fos mRNA did not increase in differentiating temperature-sensitive P815 cells. These results do not support an essential role for c-fos expression in the differentiation of mast cells. Although c-myc expression was lower after growth arrest by cyclic AMP, this decrease did not correlate with growth inhibition by cyclic AMP, since c-myc expression decreased only after cells had started to arrest in G1 phase.

Trans-activation by the c-myb proto-oncogene

We present evidence that the mouse c-myb proto-oncogene encodes a transcriptional trans-activator. Trans-activation was assayed by cotransfection into CV1 monkey kidney cells of a c-myb cDNA expression plasmid together with a reporter plasmid carrying the chloramphenicol acetyltransferase (CAT) gene under the control of a test promoter and enhancer. Cotransfection with the c-myb cDNA plasmid caused a 20-fold stimulation of transcription from the promoter of the mouse alpha 2(I) collagen gene linked to tandem repeats of the simian virus 40 (SV40) enhancer element. Using different promoters in combination with varying numbers of repeats of the SV40 enhancer element, it was shown that tandem repeats of the SV40 enhancer mediated the c-myb-induced activation of transcription. These results show that the mouse c-myb gene product either is itself or induces, an activator of transcription that recognizes specific sequences in the SV40 enhancer.

Detection of fos oncogene products by monoclonal antibody FO-120 in lymphoproliferative disorders

We investigated the expression of fos oncogene proteins in lymphoproliferative disorders, using a monoclonal antibody (FO-120) that was prepared against a synthetic oligopeptide of fos protein (amino acid sequence from 127 to 152). Although peripheral blood leukocytes were rarely positive for FO-120, they were transiently stained after lectin (PHA) stimulation. After culture with IL-2 for 1 or 2 weeks, less than 40% of the lymphocytes weakly reacted with FO-120, whereas strongly positive cells were detected in more than 70% of cells in half the T-cell lines established from preleukemic state of adult T-cell leukemia (pre-ATL) and all of ATL derived T-cell lines. All in vivo specimens of non-Hodgkin's malignant lymphomas, except for one case of T-cell lymphoma were also strongly positive. In addition, the extent of the antibody reactivity correlated with the histopathological grade of malignancy in B-cell lymphoma. The reactivity to most AILD-IBL lesions overlapped with that to T-lymphomas, and could be distinguished from that to reactive lesions. FO-120 appears to be a useful tool for detecting early neoplastic changes in lymphoproliferative disorders.

Oncogene expression in Rauscher murine leukemia virus induced erythroid, myeloid and lymphoid cell lines

A comparative study on the expression of nuclear and cytoplasmic oncogenes was carried out using the Northern blotting technique, in Rauscher virus induced primary leukemias and the more malignant transformed cell lines derived from them. The latter grow permanently in vitro. Hyperplastic spleens obtained from mice recovering from anemia were analysed as controls. In addition to the detection of mRNAs, Southern blotting was carried out to observe whether rearrangement or amplification of oncogenes had occurred. The results show that the nuclear oncogenes c-myc, c-myb and p53 are strongly expressed in leukemic tissue, whereas c-fos transcripts show a much weaker hybridization. The expression of two of these oncogenes, c-myc and c-myb was followed during differentiation in myeloid leukemic cells and showed a gradual decrease when compared with the actin gene, which is constitutively transcribed. A large number of cytoplasmic oncogenes is expressed in the leukemic cells lines, i.e. c-abl, c-fms, c-fes, c-src, c-ros, c-H-ras, c-K-ras and N-ras. Of these, transcripts coding for c-abl and c-src were absent in blast cells of acute erythroid leukemias. Transcripts coding for c-erb, c-mos and c-sis could also not be detected. A number of putative oncogenes which are reported to play a role in Moloney and Friend virus induced leukemias for instance pim-1, fis-1, fim-1 and fim-2 were also used for screening. Only expression of pim-1 in Rauscher virus induced myeloid leukemic cells and in primary acute erythroid leukemias could be observed. At the DNA level no rearrangement or amplification of any of the oncogenes investigated could be detected. The results show that a number of oncogenes are expressed simultaneously in the same leukemic tissue or cell lines. It therefore seems likely that the presence of transcripts of different oncogenes is associated with the progression of leukemia, but is not the primary cause of leukemogenesis or of the transformation of these cells into established cell lines.