Constitutive expression of a c-myb cDNA blocks Friend murine erythroleukemia cell differentiation

Regulation of the T-cell receptor delta enhancer by functional cooperation between c-Myb and core-binding factors

A T-cell-specific transcriptional enhancer lies within the J delta 3-C delta intron of the human T-cell receptor (TCR) delta gene. The 30-bp minimal enhancer element denoted delta E3 carries a core sequence (TGTGGTTT) that binds a T-cell-specific factor, and that is necessary but not sufficient for transcriptional activation. Here we demonstrate that the transcription factor c-Myb regulates TCR delta enhancer activity through a binding site in delta E3 that is adjacent to the core site. Both v-Myb and c-Myb bind specifically to delta E3. The Myb site is necessary for enhancer activity, because a mutation that eliminates Myb binding abolishes transcriptional activation by the delta E3 element and by the 370-bp TCR delta enhancer. Transfection of cells with a c-Myb expression construct upregulates delta E3 enhancer activity, whereas treatment of cells with an antisense c-myb oligonucleotide inhibits delta E3 enhancer activity. Since intact Myb and core sites are both required for delta E3 function, our data argue that c-Myb and core binding factors must cooperate to mediate transcriptional activation through delta E3. Efficient cooperation depends on the relative positioning of the Myb and core sites, since only one of two overlapping Myb sites within delta E3 is functional and alterations of the distance between this site and the core site disrupt enhancer activity. Cooperative regulation by c-Myb and core-binding factors is likely to play an important role in the control of gene expression during T-cell development.

Oncogenic truncation of the first repeat of c-Myb decreases DNA binding in vitro and in vivo

Oncogenic activation of c-Myb in both avian and murine systems often involves N-terminal truncation. In particular, the first of three DNA-binding repeats in c-Myb has been largely deleted during the genesis of the v-myb oncogenes of avian myeloblastosis virus and E26 avian leukemia virus. This finding suggests that the first DNA-binding repeat may have an important role in cell growth control. We demonstrate that truncation of the first DNA-binding repeat of c-Myb is sufficient for myeloid transformation in culture, but deletion of the N-terminal phosphorylation site and adjacent acidic region is not. Truncation of the first repeat decreases the ability of a Myb-VP16 fusion protein to trans activate the promoter of a Myb-inducible gene (mim-1) involved in differentiation. Moreover, truncation of the first repeat decreases the ability of the Myb protein to bind DNA both in vivo and in vitro. These results suggest that N-terminal mutants of c-Myb may transform by regulating only a subset of those genes normally regulated by c-Myb.

Regulation of the T-cell receptor delta enhancer by functional cooperation between c-Myb and core-binding factors

A T-cell-specific transcriptional enhancer lies within the J delta 3-C delta intron of the human T-cell receptor (TCR) delta gene. The 30-bp minimal enhancer element denoted delta E3 carries a core sequence (TGTGGTTT) that binds a T-cell-specific factor, and that is necessary but not sufficient for transcriptional activation. Here we demonstrate that the transcription factor c-Myb regulates TCR delta enhancer activity through a binding site in delta E3 that is adjacent to the core site. Both v-Myb and c-Myb bind specifically to delta E3. The Myb site is necessary for enhancer activity, because a mutation that eliminates Myb binding abolishes transcriptional activation by the delta E3 element and by the 370-bp TCR delta enhancer. Transfection of cells with a c-Myb expression construct upregulates delta E3 enhancer activity, whereas treatment of cells with an antisense c-myb oligonucleotide inhibits delta E3 enhancer activity. Since intact Myb and core sites are both required for delta E3 function, our data argue that c-Myb and core binding factors must cooperate to mediate transcriptional activation through delta E3. Efficient cooperation depends on the relative positioning of the Myb and core sites, since only one of two overlapping Myb sites within delta E3 is functional and alterations of the distance between this site and the core site disrupt enhancer activity. Cooperative regulation by c-Myb and core-binding factors is likely to play an important role in the control of gene expression during T-cell development.

Oncogenic truncation of the first repeat of c-Myb decreases DNA binding in vitro and in vivo

Oncogenic activation of c-Myb in both avian and murine systems often involves N-terminal truncation. In particular, the first of three DNA-binding repeats in c-Myb has been largely deleted during the genesis of the v-myb oncogenes of avian myeloblastosis virus and E26 avian leukemia virus. This finding suggests that the first DNA-binding repeat may have an important role in cell growth control. We demonstrate that truncation of the first DNA-binding repeat of c-Myb is sufficient for myeloid transformation in culture, but deletion of the N-terminal phosphorylation site and adjacent acidic region is not. Truncation of the first repeat decreases the ability of a Myb-VP16 fusion protein to trans activate the promoter of a Myb-inducible gene (mim-1) involved in differentiation. Moreover, truncation of the first repeat decreases the ability of the Myb protein to bind DNA both in vivo and in vitro. These results suggest that N-terminal mutants of c-Myb may transform by regulating only a subset of those genes normally regulated by c-Myb.

Subcellular localization of Bcr, Abl, and Bcr-Abl proteins in normal and leukemic cells and correlation of expression with myeloid differentiation

We used specific antisera and immunohistochemical methods to investigate the subcellular localization and expression of Bcr, Abl, and Bcr-Abl proteins in leukemic cell lines and in fresh human leukemic and normal samples at various stages of myeloid differentiation. Earlier studies of the subcellular localization of transfected murine type IV c-Abl protein in fibroblasts have shown that this molecule resides largely in the nucleus, whereas transforming deletion variants are localized exclusively in the cytoplasm. Here, we demonstrate that the murine type IV c-Abl protein is also found in the nucleus when overexpressed in a mouse hematopoietic cell line. However, in both normal and leukemic human hematopoietic cells, c-Abl is discerned predominantly in the cytoplasm, with nuclear staining present, albeit at a lower level. In contrast, normal endogenous Bcr protein, as well as the aberrant p210BCR-ABL and p190BCR-ABL proteins consistently localize to the cytoplasm in both cell lines and fresh cells. The results with p210BCR-ABL were confirmed in a unique Ph1-positive chronic myelogenous leukemia (CML) cell line, KBM5, which lacks the normal chromosome 9 and hence the normal c-Abl product. Because the p210BCR-ABL protein appears cytoplasmic in both chronic phase and blast crisis CML cells, as does the p190BCR-ABL in Ph1-positive acute leukemia, a change in subcellular location of Bcr-Abl proteins between cytoplasm and nucleus cannot explain the different spectrum of leukemias associated with p210 and p190, nor the transition from the chronic to the acute leukemia phenotype seen in CML. Further analysis of fresh CML and normal hematopoietic bone marrow cells reveals that p210BCR-ABL, as well as the normal Bcr and Abl proteins, are expressed primarily in the early stages of myeloid maturation, and that levels of expression are reduced significantly as the cells mature to polymorphonuclear leukocytes. Similarly, a decrease in Bcr and Abl levels occurs in HL-60 cells induced by DMSO to undergo granulocytic differentiation. The action of p210BCR-ABL and its normal counterparts may, therefore, take place during the earlier stages of myeloid development.

Differential transcriptional activation by v-myb and c-myb in animal cells and Saccharomyces cerevisiae

The v-myb oncogene and its cellular homolog c-myb encode sequence-specific DNA-binding proteins which regulate transcription from promoters containing Myb-binding sites in animal cells. We have developed a Saccharomyces cerevisiae system to assay transcriptional activation by v-Myb and c-Myb. In yeast strains containing integrated reporter genes, activation was strictly dependent upon both the Myb DNA-binding domain and the Myb recognition element. BAS1, an endogenous Myb-related yeast protein, was not required for transactivation by animal Myb proteins and by itself had no detectable effect on a Myb reporter gene. Deletion analyses demonstrated that a domain of v-Myb C terminal to the previously mapped Myb transcriptional activation domain was required for transactivation in animal cells but not in S. cerevisiae. The same domain is also required for the efficient transformation of myeloid cells by v-Myb. In contrast to results in animal cells, in S. cerevisiae the full-length c-Myb was a much stronger transactivator than a protein bearing the oncogenic N- and C-terminal truncations of v-Myb. These results imply that negative regulation of c-Myb by its own termini requires an additional animal cell protein or small molecule that is not present in S. cerevisiae.

Degeneration of skeletal and cardiac muscles in c-myb transgenic mice

In order to reveal cellular processes sensitive to abnormal c-myb expression in vivo, transgenic mice were produced by introducing the c-myb nuclear proto-oncogene under the ubiquitous transcriptional regulatory unit of the cytoplasmic beta-actin gene. Expression of c-myb in thymus did not cause apparent abnormality, but the mice unexpectedly developed degenerative abnormalities in skeletal and cardiac muscles; this occurred predominantly in males. Expression of c-myb in skeletal muscle was correlated with an inflammation of muscle and was accompanied by vacuolar degeneration of muscle fibres, their regeneration, and lymphocyte infiltration. The identical pathological progression in cardiac muscle was associated with cardiomegaly.

Differential transcriptional activation by v-myb and c-myb in animal cells and Saccharomyces cerevisiae

The v-myb oncogene and its cellular homolog c-myb encode sequence-specific DNA-binding proteins which regulate transcription from promoters containing Myb-binding sites in animal cells. We have developed a Saccharomyces cerevisiae system to assay transcriptional activation by v-Myb and c-Myb. In yeast strains containing integrated reporter genes, activation was strictly dependent upon both the Myb DNA-binding domain and the Myb recognition element. BAS1, an endogenous Myb-related yeast protein, was not required for transactivation by animal Myb proteins and by itself had no detectable effect on a Myb reporter gene. Deletion analyses demonstrated that a domain of v-Myb C terminal to the previously mapped Myb transcriptional activation domain was required for transactivation in animal cells but not in S. cerevisiae. The same domain is also required for the efficient transformation of myeloid cells by v-Myb. In contrast to results in animal cells, in S. cerevisiae the full-length c-Myb was a much stronger transactivator than a protein bearing the oncogenic N- and C-terminal truncations of v-Myb. These results imply that negative regulation of c-Myb by its own termini requires an additional animal cell protein or small molecule that is not present in S. cerevisiae.

Induction of H3.3 replacement histone mRNAs during the precommitment period of murine erythroleukemia cell differentiation

Differential hybridization to a cDNA library made from the mRNA of differentiating mouse erythroleukemia (MEL) cells has been used to identify sequences that are induced during the early stages of MEL cell differentiation. One of the differentially expressed genes identified encodes the H3.3 histone subtype. We show here that the three polyadenylated mRNAs produced from the H3.3B gene, as well as the single mRNA produced from the related H3.3A gene, are coordinately induced during the first few hours of MEL cell differentiation and subsequently down regulated as cells undergo terminal differentiation. Nuclear run-on transcription experiments indicate that the accumulation and decay of these mRNAs are controlled at the post-transcriptional level. Unlike the polyadenylated mRNAs of two H1 histone genes that exhibit similar kinetics of induction and decay controlled by c-myc, induction of the H3.3 mRNAs is unaffected by deregulated expression of c-myc.

Only the DNA binding and transactivation domains of c-Myb are required to block terminal differentiation of murine erythroleukemia cells

The c-Myb protein is a transcription factor with an apparent but poorly defined role in hematopoietic cell growth and differentiation. The DNA binding and several transcriptional regulatory domains of the c-Myb protein have been defined by transient transfections into nonhematopoietic cell lines. Although the relationship between these domains and transformation has been studied, little is known about the function of these domains during hematopoietic maturation. Up-regulation of stably transfected c-myb in murine erythroleukemia (MEL) cells blocks terminal differentiation when MEL cells are induced to differentiate with N,N'-hexamethylene bisacetamide. To determine which functional domains of c-Myb are necessary and sufficient to block differentiation, mutated c-myb constructs under the control of a murine metallothionein promoter were transfected into C19 MEL cells, and stable clonal cell lines were established. The ability of Myb mutants to block differentiation paralleled their ability to transactivate transcription of a reporter gene containing Myb-responsive elements, by transient transfection into a lymphoid cell line. The smallest c-Myb mutant able to block differentiation consisted of the DNA binding domain juxtaposed to the transactivation domain. Therefore, the DNA binding domain and the transactivation domain are necessary and sufficient for c-Myb to block differentiation in MEL cells.

Only the DNA binding and transactivation domains of c-Myb are required to block terminal differentiation of murine erythroleukemia cells

The c-Myb protein is a transcription factor with an apparent but poorly defined role in hematopoietic cell growth and differentiation. The DNA binding and several transcriptional regulatory domains of the c-Myb protein have been defined by transient transfections into nonhematopoietic cell lines. Although the relationship between these domains and transformation has been studied, little is known about the function of these domains during hematopoietic maturation. Up-regulation of stably transfected c-myb in murine erythroleukemia (MEL) cells blocks terminal differentiation when MEL cells are induced to differentiate with N,N'-hexamethylene bisacetamide. To determine which functional domains of c-Myb are necessary and sufficient to block differentiation, mutated c-myb constructs under the control of a murine metallothionein promoter were transfected into C19 MEL cells, and stable clonal cell lines were established. The ability of Myb mutants to block differentiation paralleled their ability to transactivate transcription of a reporter gene containing Myb-responsive elements, by transient transfection into a lymphoid cell line. The smallest c-Myb mutant able to block differentiation consisted of the DNA binding domain juxtaposed to the transactivation domain. Therefore, the DNA binding domain and the transactivation domain are necessary and sufficient for c-Myb to block differentiation in MEL cells.

Mechanism of c-myc regulation by c-Myb in different cell lineages

Activation of the murine c-myc promoter by murine c-Myb protein was examined in several cell lines by using a transient expression system in which Myb expression vectors activate the c-myc promoter linked to a chloramphenicol acetyltransferase reporter gene or a genomic beta-globin gene. S1 nuclease protection analyses confirmed that the induction of c-myc by c-Myb was transcriptional and affected both P1 and P2 start sites in a murine T-cell line, EL4, and a myelomonocytic line, WEHI-3. Mutational analyses of the c-myc promoter revealed that two distinct regions could confer Myb responsiveness in two T-cell lines, a distal site upstream of P1 and a proximal site within the first noncoding exon. In contrast, only the proximal site was required for other cell lineages examined. Five separate Myb-binding sites were located in this proximal site and found to be important for c-Myb trans activation. DNA binding was necessary for c-myc activation, as shown by the loss of function associated with mutation of Myb's DNA-binding domain and by trans-dominant repressor activity of the DNA binding, trans-activation-defective mutant. The involvement of additional protein factors was addressed by inhibiting protein synthesis with cycloheximide in a conditional expression system in which the activity of presynthesized Myb was under the control of estrogen. These experiments indicate that de novo synthesis of additional proteins was not necessary for c-myc trans activation. Together these data reveal two cell lineage-dependent pathways by which c-Myb regulates c-myc; however, both pathways are mechanistically indistinguishable in that direct DNA binding by Myb is required for activating c-myc whereas neither de novo protein synthesis nor other labile proteins are necessary.

Mechanism of c-myc regulation by c-Myb in different cell lineages

Activation of the murine c-myc promoter by murine c-Myb protein was examined in several cell lines by using a transient expression system in which Myb expression vectors activate the c-myc promoter linked to a chloramphenicol acetyltransferase reporter gene or a genomic beta-globin gene. S1 nuclease protection analyses confirmed that the induction of c-myc by c-Myb was transcriptional and affected both P1 and P2 start sites in a murine T-cell line, EL4, and a myelomonocytic line, WEHI-3. Mutational analyses of the c-myc promoter revealed that two distinct regions could confer Myb responsiveness in two T-cell lines, a distal site upstream of P1 and a proximal site within the first noncoding exon. In contrast, only the proximal site was required for other cell lineages examined. Five separate Myb-binding sites were located in this proximal site and found to be important for c-Myb trans activation. DNA binding was necessary for c-myc activation, as shown by the loss of function associated with mutation of Myb's DNA-binding domain and by trans-dominant repressor activity of the DNA binding, trans-activation-defective mutant. The involvement of additional protein factors was addressed by inhibiting protein synthesis with cycloheximide in a conditional expression system in which the activity of presynthesized Myb was under the control of estrogen. These experiments indicate that de novo synthesis of additional proteins was not necessary for c-myc trans activation. Together these data reveal two cell lineage-dependent pathways by which c-Myb regulates c-myc; however, both pathways are mechanistically indistinguishable in that direct DNA binding by Myb is required for activating c-myc whereas neither de novo protein synthesis nor other labile proteins are necessary.

v-myb blocks granulocyte colony-stimulating factor-induced myeloid cell differentiation but not proliferation

To understand the effects of v-myb expression on mammalian hematopoietic cell differentiation, we have constructed a retroviral vector which can efficiently express v-myb gene product in mammalian cells. Infection of interleukin-3-dependent murine progenitor cell line 32D Cl3, which undergoes terminal differentiation to mature granulocytes in the presence of granulocyte colony-stimulating factor (GCSF), with this recombinant retrovirus does not abrogate its requirement of interleukin-3 for growth. However, expression of v-myb in these cells blocks their ability to differentiate in response to GCSF. Instead, the v-myb-infected cells proliferate indefinitely in the presence of GCSF. 32D Cl3 cells infected with empty vector carrying only the neomycin resistance gene responded to the addition of GCSF in a manner identical to that of the uninfected cells and underwent terminal differentiation into granulocytes. These results suggest that oncogenic forms of myb gene bring about transformation by blocking the differentiation signal derived by cytokines while promoting the proliferative signal transduction pathways.

v-myb blocks granulocyte colony-stimulating factor-induced myeloid cell differentiation but not proliferation

To understand the effects of v-myb expression on mammalian hematopoietic cell differentiation, we have constructed a retroviral vector which can efficiently express v-myb gene product in mammalian cells. Infection of interleukin-3-dependent murine progenitor cell line 32D Cl3, which undergoes terminal differentiation to mature granulocytes in the presence of granulocyte colony-stimulating factor (GCSF), with this recombinant retrovirus does not abrogate its requirement of interleukin-3 for growth. However, expression of v-myb in these cells blocks their ability to differentiate in response to GCSF. Instead, the v-myb-infected cells proliferate indefinitely in the presence of GCSF. 32D Cl3 cells infected with empty vector carrying only the neomycin resistance gene responded to the addition of GCSF in a manner identical to that of the uninfected cells and underwent terminal differentiation into granulocytes. These results suggest that oncogenic forms of myb gene bring about transformation by blocking the differentiation signal derived by cytokines while promoting the proliferative signal transduction pathways.

Cell cycle analysis of p53-induced cell death in murine erythroleukemia cells

A temperature-sensitive mutant of murine p53 (p53Val-135) was transfected by electroporation into murine erythroleukemia cells (DP16-1) lacking endogenous expression of p53. While the transfected cells grew normally in the presence of mutant p53 (37.5 degrees C), wild-type p53 (32.5 degrees C) was associated with a rapid loss of cell viability. Genomic DNA extracted at 32.5 degrees C was seen to be fragmented into a characteristic ladder consistent with cell death due to apoptosis. Following synchronization by density arrest, transfected cells released into G1 at 32.5 degrees C were found to lose viability more rapidly than did randomly growing cultures. Following release into G1, cells became irreversibly committed to cell death after 4 h at 32.5 degrees C. Commitment to cell death correlated with the first appearance of fragmented DNA. Synchronized cells allowed to pass out of G1 prior to being placed at 32.5 degrees C continued to cycle until subsequently arrested in G1; loss of viability occurred following G1 arrest. In contrast to cells in G1, cells cultured at 32.5 degrees C for prolonged periods during S phase and G2/M, and then returned to 37.5 degrees C, did not become committed to cell death. G1 arrest at 37.5 degrees C, utilizing either mimosine or isoleucine deprivation, does not lead to rapid cell death. Upon transfer to 32.5 degrees C, these G1 synchronized cell populations quickly lost viability. Cells that were kept density arrested at 32.5 degrees C (G0) lost viability at a much slower rate than did cells released into G1. Taken together, these results indicate that wild-type p53 induces cell death in murine erythroleukemia cells and that this effect occurs predominantly in the G1 phase of actively cycling cells.

Cell cycle analysis of p53-induced cell death in murine erythroleukemia cells

A temperature-sensitive mutant of murine p53 (p53Val-135) was transfected by electroporation into murine erythroleukemia cells (DP16-1) lacking endogenous expression of p53. While the transfected cells grew normally in the presence of mutant p53 (37.5 degrees C), wild-type p53 (32.5 degrees C) was associated with a rapid loss of cell viability. Genomic DNA extracted at 32.5 degrees C was seen to be fragmented into a characteristic ladder consistent with cell death due to apoptosis. Following synchronization by density arrest, transfected cells released into G1 at 32.5 degrees C were found to lose viability more rapidly than did randomly growing cultures. Following release into G1, cells became irreversibly committed to cell death after 4 h at 32.5 degrees C. Commitment to cell death correlated with the first appearance of fragmented DNA. Synchronized cells allowed to pass out of G1 prior to being placed at 32.5 degrees C continued to cycle until subsequently arrested in G1; loss of viability occurred following G1 arrest. In contrast to cells in G1, cells cultured at 32.5 degrees C for prolonged periods during S phase and G2/M, and then returned to 37.5 degrees C, did not become committed to cell death. G1 arrest at 37.5 degrees C, utilizing either mimosine or isoleucine deprivation, does not lead to rapid cell death. Upon transfer to 32.5 degrees C, these G1 synchronized cell populations quickly lost viability. Cells that were kept density arrested at 32.5 degrees C (G0) lost viability at a much slower rate than did cells released into G1. Taken together, these results indicate that wild-type p53 induces cell death in murine erythroleukemia cells and that this effect occurs predominantly in the G1 phase of actively cycling cells.

Deregulated c-myb disrupts interleukin-6- or leukemia inhibitory factor-induced myeloid differentiation prior to c-myc: role in leukemogenesis

The c-myb proto-oncogene is abundantly expressed in tissues of hematopoietic origin, and changes in endogenous c-myb genes have been implicated in both human and murine hematopoietic tumors. c-myb encodes a DNA-binding protein capable of trans-activating the c-myc promoter. Suppression of both of these proto-oncogenes was shown to occur upon induction of terminal differentiation but not upon induction of growth inhibition in myeloid leukemia cells. Myeloblastic leukemia M1 cells that can be induced for terminal differentiation with the physiological hematopoietic inducers interleukin-6 and leukemia inhibitory factor were genetically manipulated to constitutively express a c-myb transgene. By using immediate-early to late genetic and morphological markers, it was shown that continuous expression of c-myb disrupts the genetic program of myeloid differentiation at a very early stage, which precedes the block previously shown to be exerted by deregulated c-myc, thereby indicating that the c-myb block is not mediated via deregulation of c-myc. Enforced c-myb expression also prevents the loss in leukemogenicity of M1 cells normally induced by interleukin-6 or leukemia inhibitory factor. Any changes which have taken place, including induction of myeloid differentiation primary response genes, eventually are reversed. Also, it was shown that suppression of c-myb, essential for terminal differentiation, is not intrinsic to growth inhibition. Taken together, these findings show that c-myb plays a key regulatory role in myeloid differentiation and substantiate the notion that deregulated expression of c-myb can play an important role in leukemogenicity.

Deregulated c-myb disrupts interleukin-6- or leukemia inhibitory factor-induced myeloid differentiation prior to c-myc: role in leukemogenesis

The c-myb proto-oncogene is abundantly expressed in tissues of hematopoietic origin, and changes in endogenous c-myb genes have been implicated in both human and murine hematopoietic tumors. c-myb encodes a DNA-binding protein capable of trans-activating the c-myc promoter. Suppression of both of these proto-oncogenes was shown to occur upon induction of terminal differentiation but not upon induction of growth inhibition in myeloid leukemia cells. Myeloblastic leukemia M1 cells that can be induced for terminal differentiation with the physiological hematopoietic inducers interleukin-6 and leukemia inhibitory factor were genetically manipulated to constitutively express a c-myb transgene. By using immediate-early to late genetic and morphological markers, it was shown that continuous expression of c-myb disrupts the genetic program of myeloid differentiation at a very early stage, which precedes the block previously shown to be exerted by deregulated c-myc, thereby indicating that the c-myb block is not mediated via deregulation of c-myc. Enforced c-myb expression also prevents the loss in leukemogenicity of M1 cells normally induced by interleukin-6 or leukemia inhibitory factor. Any changes which have taken place, including induction of myeloid differentiation primary response genes, eventually are reversed. Also, it was shown that suppression of c-myb, essential for terminal differentiation, is not intrinsic to growth inhibition. Taken together, these findings show that c-myb plays a key regulatory role in myeloid differentiation and substantiate the notion that deregulated expression of c-myb can play an important role in leukemogenicity.

Transactivation of erythroid transcription factor GATA-1 by a myb-ets-containing retrovirus

ME26 virus is a recombinant mouse retrovirus construct homologous to the avian E26 virus. Both encode a 135-kDa gag-myb-ets fusion protein which is localized in the nucleus. We have recently shown that ME26 virus can induce erythropoietin (Epo) responsiveness in hematopoietic cells. Mice infected with ME26 virus develop a hyperplasia of Epo-dependent hematopoietic precursor cells from which permanent cell lines can be established. In vitro, ME26 virus specifically induces Epo responsiveness in the interleukin-3-dependent myeloid cell line FDC-P2 by enhancing expression of the Epo receptor (EpoR). In the present study we demonstrate that ME26 virus infection of FDC-P2 cells also results in enhanced expression of beta-globin and the erythroid-specific transcription factor GATA-1, a protein which can transactivate both the EpoR promoter and globin genes. In addition, these cells exhibit a down-regulation of c-myb expression similar to that seen in differentiating erythroid cells. To determine the molecular basis for activation of erythroid genes in ME26 virus-infected cells, we carried out transient expression assays with DNA constructs of either the EpoR promoter of the GATA-1 promoter linked to reporter genes. Our results indicate that while ME26 virus did not directly enhance expression from the EpoR promoter, both it and its avian parent, E26, transactivated the GATA-1 promoter. Furthermore, ME26 virus cooperates with the GATA-1 protein to enhance expression of the EpoR gene. We propose that the mechanism by which ME26 virus induces erythroleukemia involves transactivation of the GATA-1 gene, thus positively regulating the expression of the EpoR and leading to the proliferation of a unique population of Epo-responsive cells. By specifically inducing Epo responsiveness in hematopoietic cells via transactivation of a transcription factor, ME26 virus utilizes a novel mechanism for retrovirus pathogenesis.

Expression of the CD4 gene requires a Myb transcription factor

We have analyzed the control of developmental expression of the CD4 gene, which encodes an important recognition molecule and differentiation antigen on T cells. We have determined that the CD4 promoter alone functions at high levels in the CD4+ CD8- mature T cell but not at the early CD4+ CD8+ stage of T-cell development. In addition, the CD4 promoter functions only in T lymphocytes; thus, the stage and tissue specificity of the CD4 gene is mediated in part by its promoter. We have determined that a Myb transcription factor binds to the CD4 promoter and is critical for full promoter function. Thus, Myb plays an important role in the expression of T-cell-specific developmentally regulated genes.

Expression of the CD4 gene requires a Myb transcription factor

We have analyzed the control of developmental expression of the CD4 gene, which encodes an important recognition molecule and differentiation antigen on T cells. We have determined that the CD4 promoter alone functions at high levels in the CD4+ CD8- mature T cell but not at the early CD4+ CD8+ stage of T-cell development. In addition, the CD4 promoter functions only in T lymphocytes; thus, the stage and tissue specificity of the CD4 gene is mediated in part by its promoter. We have determined that a Myb transcription factor binds to the CD4 promoter and is critical for full promoter function. Thus, Myb plays an important role in the expression of T-cell-specific developmentally regulated genes.

Positive autoregulation of c-myb expression via Myb binding sites in the 5' flanking region of the human c-myb gene

The nuclear proto-oncogene c-myb is preferentially expressed in lymphohematopoietic cells, in which it plays an important role in the processes of differentiation and proliferation. The mechanism(s) that regulates c-myb expression is not fully understood, although in mouse cells a regulatory mechanism involves a transcriptional block in the first intron. To analyze the contribution of the 5' flanking sequences in regulating the expression of the human c-myb gene, we isolated a genomic clone containing extensive 5' flanking sequences, the first exon, and a large portion of the first intron. Sequence analysis of a subcloned 1.3-kb BamHI insert corresponding to 687 nucleotides of the 5' flanking sequence, the entire first exon, and 300 nucleotides of the first intron revealed the presence of closely spaced putative Myb binding sites within a segment extending from nucleotides -616 to -575 upstream from the cap site. A 165-bp segment containing these putative Myb binding sites was linked to a human thymidine kinase (TK) cDNA driven by a low-activity proliferating cell nuclear antigen promoter and cotransfected into TK- ts13 cells with a plasmid in which a full-length human c-myb cDNA is driven by the early simian virus 40 promoter; Myb inducibility of TK mRNA expression was observed both in transient expression assays and in stable transformants. The highest level of inducibility was detected when the 165-bp fragment was placed 138 bp upstream of the proliferating cell nuclear antigen promoter-TK cDNA reporter unit or 3' of the TK cDNA. Mutation of the putative Myb binding sites greatly reduced c-myb transactivation of TK mRNA expression and specifically reduced the binding of in vitro-translated Myb protein at those sites. Finally, c-myb transactivated TK mRNA expression driven by a segment of the authentic c-myb 5' flanking region containing the Myb binding sites. These data suggest that human c-myb maintains high levels of Myb protein in cells that require this gene product for proliferation and/or differentiation by an autoregulatory mechanism involving Myb binding sites in the 5' flanking region.

Positive autoregulation of c-myb expression via Myb binding sites in the 5' flanking region of the human c-myb gene

The nuclear proto-oncogene c-myb is preferentially expressed in lymphohematopoietic cells, in which it plays an important role in the processes of differentiation and proliferation. The mechanism(s) that regulates c-myb expression is not fully understood, although in mouse cells a regulatory mechanism involves a transcriptional block in the first intron. To analyze the contribution of the 5' flanking sequences in regulating the expression of the human c-myb gene, we isolated a genomic clone containing extensive 5' flanking sequences, the first exon, and a large portion of the first intron. Sequence analysis of a subcloned 1.3-kb BamHI insert corresponding to 687 nucleotides of the 5' flanking sequence, the entire first exon, and 300 nucleotides of the first intron revealed the presence of closely spaced putative Myb binding sites within a segment extending from nucleotides -616 to -575 upstream from the cap site. A 165-bp segment containing these putative Myb binding sites was linked to a human thymidine kinase (TK) cDNA driven by a low-activity proliferating cell nuclear antigen promoter and cotransfected into TK- ts13 cells with a plasmid in which a full-length human c-myb cDNA is driven by the early simian virus 40 promoter; Myb inducibility of TK mRNA expression was observed both in transient expression assays and in stable transformants. The highest level of inducibility was detected when the 165-bp fragment was placed 138 bp upstream of the proliferating cell nuclear antigen promoter-TK cDNA reporter unit or 3' of the TK cDNA. Mutation of the putative Myb binding sites greatly reduced c-myb transactivation of TK mRNA expression and specifically reduced the binding of in vitro-translated Myb protein at those sites. Finally, c-myb transactivated TK mRNA expression driven by a segment of the authentic c-myb 5' flanking region containing the Myb binding sites. These data suggest that human c-myb maintains high levels of Myb protein in cells that require this gene product for proliferation and/or differentiation by an autoregulatory mechanism involving Myb binding sites in the 5' flanking region.

Protein truncation is required for the activation of the c-myb proto-oncogene

The protein product of the v-myb oncogene of avian myeloblastosis virus, v-Myb, differs from its normal cellular counterpart, c-Myb, by (i) expression under the control of a strong viral long terminal repeat, (ii) truncation of both its amino and carboxyl termini, (iii) replacement of these termini by virally encoded residues, and (iv) substitution of 11 amino acid residues. We had previously shown that neither the virally encoded termini nor the amino acid substitutions are required for transformation by v-Myb. We have now constructed avian retroviruses that express full-length or singly truncated forms of c-Myb and have tested them for the transformation of chicken bone marrow cells. We conclude that truncation of either the amino or carboxyl terminus of c-Myb is sufficient for transformation. In contrast, the overexpression of full-length c-Myb does not result in transformation. We have also shown that the amino acid substitutions of v-Myb by themselves are not sufficient for the activation of c-Myb. Rather, the presence of either the normal amino or carboxyl terminus of c-Myb can suppress transformation when fused to v-Myb. Cells transformed by c-Myb proteins truncated at either their amino or carboxyl terminus appear to be granulated promyelocytes that express the Mim-1 protein. Cells transformed by a doubly truncated c-Myb protein are not granulated but do express the Mim-1 protein, in contrast to monoblasts transformed by v-Myb that neither contain granules nor express Mim-1. These results suggest that various alterations of c-Myb itself may determine the lineage of differentiating hematopoietic cells.

Protein truncation is required for the activation of the c-myb proto-oncogene

The protein product of the v-myb oncogene of avian myeloblastosis virus, v-Myb, differs from its normal cellular counterpart, c-Myb, by (i) expression under the control of a strong viral long terminal repeat, (ii) truncation of both its amino and carboxyl termini, (iii) replacement of these termini by virally encoded residues, and (iv) substitution of 11 amino acid residues. We had previously shown that neither the virally encoded termini nor the amino acid substitutions are required for transformation by v-Myb. We have now constructed avian retroviruses that express full-length or singly truncated forms of c-Myb and have tested them for the transformation of chicken bone marrow cells. We conclude that truncation of either the amino or carboxyl terminus of c-Myb is sufficient for transformation. In contrast, the overexpression of full-length c-Myb does not result in transformation. We have also shown that the amino acid substitutions of v-Myb by themselves are not sufficient for the activation of c-Myb. Rather, the presence of either the normal amino or carboxyl terminus of c-Myb can suppress transformation when fused to v-Myb. Cells transformed by c-Myb proteins truncated at either their amino or carboxyl terminus appear to be granulated promyelocytes that express the Mim-1 protein. Cells transformed by a doubly truncated c-Myb protein are not granulated but do express the Mim-1 protein, in contrast to monoblasts transformed by v-Myb that neither contain granules nor express Mim-1. These results suggest that various alterations of c-Myb itself may determine the lineage of differentiating hematopoietic cells.

Nucleotide preferences in sequence-specific recognition of DNA by c-myb protein

Using a binding site selection procedure, we have found that sequence-specific DNA-binding by the mouse c-myb protein involves recognition of nucleotides outside of the previously identified hexanucleotide motif. Oligonucleotides containing a random nucleotide core were immunoprecipitated in association with c-Myb, amplified by the Polymerase Chain Reaction and cloned in plasmids prior to sequencing. By alignment of sequences it was apparent that additional preferences existed at each of three bases immediately 5' of the hexanucleotide consensus, allowing an extension of the preferred binding site to YGRCVGTTR. The contributions of these 5' nucleotides to binding affinity was established in bandshift analyses with oligonucleotides containing single base substitutions; in particular, it was found that replacement of the preferred guanine at position -2 with any other base greatly reduced c-Myb binding. We found that the protein encoded by the related B-myb gene bound the preferred c-Myb site with similar affinity; however, B-Myb and c-Myb showed distinct preferences for the identity of the nucleotide at position -1 relative to the hexanucleotide consensus. This study demonstrates that the c-Myb DNA-binding site is more extensive than recognised hitherto and points to similar but distinct nucleotide preferences in recognition of DNA by related Myb proteins.

Normal and leukemic hematopoietic cells manifest differential sensitivity to inhibitory effects of c-myb antisense oligodeoxynucleotides: an in vitro study relevant to bone marrow purging

The c-myb protooncogene is preferentially expressed in hematopoietic cells, and its encoded protein, Myb, is required for hematopoietic cell proliferation. To analyze the relative Myb dependence of normal and leukemic human hematopoietic progenitor cells, normal bone marrow cells, several types of leukemic blast cells, and 1:1 mixtures of normal and leukemic cells were cultured in the presence of c-myb sense or antisense oligodeoxynucleotides; cell viability and cloning efficiency were then assessed. c-myb sense oligomers had negligible effects on normal and leukemic cells. In contrast, c-myb antisense oligomers strongly inhibited or completely abolished clonogenic growth of a T-cell leukemia line, 78% (18 of 23) of primary acute myelogenous leukemia cases examined, and 4 of 5 primary chronic myelogenous leukemia (CML) cases in blast crisis. In three of the latter patients, polymerase chain reaction analysis of a 1:1 mixture of c-myb antisense-treated normal and CML cells revealed a complete absence of bcr-abl expression, suggesting that the CML clonogenic units had been completely eliminated from the cultures. At antisense doses that inhibited leukemic cell growth, normal hematopoietic progenitor cells survived. Thus, normal and leukemic hematopoietic cells show differential sensitivity to the toxic effects of c-myb antisense DNA. Perturbation of c-myb function with antisense oligodeoxynucleotides might eventually form the basis for a molecular approach to leukemia therapy, perhaps most immediately as ex vivo bone marrow purging agents.

Cloning and expression of human deoxycytidine kinase cDNA

Deoxycytidine (dCyd) kinase is required for the phosphorylation of several deoxyribonucleosides and certain nucleoside analogs widely employed as antiviral and chemotherapeutic agents. Detailed analysis of this enzyme has been limited, however, by its low abundance and instability. Using oligonucleotides based on primary amino acid sequence derived from purified dCyd kinase, we have screened T-lymphoblast cDNA libraries and identified a cDNA sequence that encodes a 30.5-kDa protein corresponding to the subunit molecular mass of the purified protein. Expression of the cDNA in Escherichia coli results in a 40-fold increase in dCyd kinase activity over control levels. In dCyd kinase-deficient murine L cells, transfection with dCyd kinase cDNA in a mammalian expression vector produces a 400-fold increase over control in dCyd phosphorylating activity. The expressed enzyme has an apparent Km of 1.0 microM for dCyd and is also capable of phosphorylating dAdo and dGuo. Northern blot analysis reveals a single 2.8-kilobase mRNA expressed in T lymphoblasts at 5- to 10-fold higher levels than in B lymphoblasts, and decreased dCyd kinase mRNA levels are present in T-lymphoblast cell lines resistant to arabinofuranosylcytosine and dideoxycytidine. These findings document that this cDNA encodes the T-lymphoblast dCyd kinase responsible for the phosphorylation of dAdo and dGuo as well as dCyd and arabinofuranosylcytosine.

Functional analysis of c-Myb protein in T-lymphocytic cell lines shows that it trans-activates the c-myc promoter

The function of c-Myb protein was revealed by transfecting an expression vector containing the entire c-Myb protein-coding sequence into the murine CTLL-2 T-cell line. Expressions of high levels of c-Myb protein did not alter the expression of several T-cell markers, c-fos mRNA expression, responses to interleukin-2, and growth characteristics of these cells. Interestingly, expression of the c-myc gene was drastically increased in this clone. Further, the c-myb expression plasmid, but not a frameshift mutant of c-myb, enhanced the expression of a hybrid construct of c-myc promoter linked to a reporter gene by 8- to 14-fold. These results demonstrate a role of c-Myb protein in c-myc gene expression.

Functional analysis of c-Myb protein in T-lymphocytic cell lines shows that it trans-activates the c-myc promoter

The function of c-Myb protein was revealed by transfecting an expression vector containing the entire c-Myb protein-coding sequence into the murine CTLL-2 T-cell line. Expressions of high levels of c-Myb protein did not alter the expression of several T-cell markers, c-fos mRNA expression, responses to interleukin-2, and growth characteristics of these cells. Interestingly, expression of the c-myc gene was drastically increased in this clone. Further, the c-myb expression plasmid, but not a frameshift mutant of c-myb, enhanced the expression of a hybrid construct of c-myc promoter linked to a reporter gene by 8- to 14-fold. These results demonstrate a role of c-Myb protein in c-myc gene expression.

The leucine zipper of c-Myc is required for full inhibition of erythroleukemia differentiation

The leucine zipper motif has been observed in a number of proteins thought to function as eucaryotic transcription factors. Mutation of the leucine zipper interferes with protein dimerization and DNA binding. We examined the effect of point mutations in the leucine zipper of c-Myc on its ability to dimerize in vitro and to inhibit Friend murine erythroleukemia (F-MEL) differentiation. Glutaraldehyde cross-linking studies failed to provide evidence for homodimerization of in vitro-synthesized c-Myc protein, although it was readily demonstrated for c-Jun. Nevertheless, whereas transfected wild-type c-myc sequences strongly inhibited F-MEL differentiation, those with single or multiple mutations in the leucine zipper were only partially effective in this regard. Since the leucine zipper domain of c-Myc is essential for its cooperative effect in ras oncogene-mediated transformation, this study emphasizes the close relationship that exists between transformation and hematopoietic commitment and differentiation. c-Myc may produce its effects on F-MEL differentiation through leucine zipper-mediated heterodimeric associations rather than homodimeric ones.

The leucine zipper of c-Myc is required for full inhibition of erythroleukemia differentiation

The leucine zipper motif has been observed in a number of proteins thought to function as eucaryotic transcription factors. Mutation of the leucine zipper interferes with protein dimerization and DNA binding. We examined the effect of point mutations in the leucine zipper of c-Myc on its ability to dimerize in vitro and to inhibit Friend murine erythroleukemia (F-MEL) differentiation. Glutaraldehyde cross-linking studies failed to provide evidence for homodimerization of in vitro-synthesized c-Myc protein, although it was readily demonstrated for c-Jun. Nevertheless, whereas transfected wild-type c-myc sequences strongly inhibited F-MEL differentiation, those with single or multiple mutations in the leucine zipper were only partially effective in this regard. Since the leucine zipper domain of c-Myc is essential for its cooperative effect in ras oncogene-mediated transformation, this study emphasizes the close relationship that exists between transformation and hematopoietic commitment and differentiation. c-Myc may produce its effects on F-MEL differentiation through leucine zipper-mediated heterodimeric associations rather than homodimeric ones.

Inhibition of T-cell proliferation by a MYB antisense oligomer is accompanied by selective down-regulation of DNA polymerase alpha expression

We recently found that inhibition of MYB protein synthesis in human peripheral blood mononuclear cells (PBMC) exposed to human c-myb (designated MYB) antisense oligodeoxynucleotides prevents entry into S phase and cell proliferation. To determine the mechanism(s) by which down-regulation of human c-myb protein (MYB) synthesis interferes with DNA synthesis, we analyzed mRNA levels of DNA polymerase alpha and proliferating cell nuclear antigen (PCNA), transcripts of two genes required for DNA synthesis, in normal and leukemic T lymphocytes exposed to MYB antisense oligodeoxynucleotides. Expression of DNA polymerase alpha was inhibited both in normal T lymphocytes progressing from G0 to S phase and in exponentially growing CCRF-CEM leukemic cells, whereas expression of PCNA was inhibited only in mitogen-stimulated PBMC and remained essentially unaffected in the leukemia T-cell line. The functional link between expression of MYB and DNA polymerase alpha mRNAs was further demonstrated by analyzing DNA polymerase alpha mRNA levels in a temperature-sensitive (ts) fibroblast cell line (TK-ts13; TK is thymidine kinase) constitutively expressing human MYB mRNA driven by the simian virus 40 (SV40) promoter. In the MYB-expressing TK-ts13 cells, DNA polymerase alpha mRNA levels were unaffected following shift to the nonpermissive temperature of 39.6 degrees C, whereas in the parental line, DNA polymerase alpha mRNA levels were readily down-regulated. These findings indicate that the expression of MYB is related to that of DNA polymerase alpha in cells expressing MYB at high levels and suggest that there is a functional link between c-myb and DNA polymerase alpha mRNA expression during cell cycle progression of normal T lymphocytes.

Transformation by v-myb correlates with trans-activation of gene expression

The v-myb oncogene of avian myeloblastosis virus causes acute myelomonocytic leukemia in chickens and transforms avian myeloid cells in vitro. Its protein product p48v-myb is a nuclear, sequence-specific, DNA-binding protein which activates gene expression in transient DNA transfection studies. To investigate the relationship between transformation and trans-activation by v-myb, we constructed 15 in-frame linker insertion mutants. The 12 mutants which transformed myeloid cells also trans-activated gene expression, whereas the 3 mutants which did not transform also did not trans-activate. This implies that trans-activation is required for transformation by v-myb. One of the transformation-defective mutants localized to the cell nucleus but failed to bind DNA. The other two transformation-defective mutants localized to the cell nucleus and bound DNA but nevertheless failed to trans-activate. These latter mutants define two distinct domains of p48v-myb which control trans-activation by DNA-bound protein, one within the amino-terminal DNA-binding domain itself and one in a carboxyl-terminal domain which is not required for DNA binding.

Alternative splicing of RNAs transcribed from the human c-myb gene

An alternative splicing event in which a portion of the intron bounded by the vE6 and vE7 exons with v-myb homology is included as an additional 363-nucleotide coding exon (termed E6A or coding exon 9A) has been described for normal and tumor murine cells that express myb. We show here that this alternative splicing event is conserved in human c-myb transcripts. In addition, another novel exon (termed E7A or coding exon 10A) is identified in human c-myb mRNAs expressed in normal and tumor cells. Although the myb protein isoform encoded by murine E6A-containing mRNA is larger than the major c-myb protein, the predicted products of both forms of human alternatively spliced myb transcripts are 3'-truncated myb proteins that terminate in the alternative exons. These proteins are predicted to lack the same carboxy-terminal domains as the viral myb proteins encoded by avian myeloblastosis virus and E26 virus. The junction sequences that flank these exons closely resemble the consensus splice donor and splice acceptor sequences, yet the alternative transcripts are less abundant than is the major form of c-myb transcripts. The contribution that alternative splicing events in c-myb expression may make on c-myb function remains to be elucidated.

Alternative splicing of RNAs transcribed from the human c-myb gene

An alternative splicing event in which a portion of the intron bounded by the vE6 and vE7 exons with v-myb homology is included as an additional 363-nucleotide coding exon (termed E6A or coding exon 9A) has been described for normal and tumor murine cells that express myb. We show here that this alternative splicing event is conserved in human c-myb transcripts. In addition, another novel exon (termed E7A or coding exon 10A) is identified in human c-myb mRNAs expressed in normal and tumor cells. Although the myb protein isoform encoded by murine E6A-containing mRNA is larger than the major c-myb protein, the predicted products of both forms of human alternatively spliced myb transcripts are 3'-truncated myb proteins that terminate in the alternative exons. These proteins are predicted to lack the same carboxy-terminal domains as the viral myb proteins encoded by avian myeloblastosis virus and E26 virus. The junction sequences that flank these exons closely resemble the consensus splice donor and splice acceptor sequences, yet the alternative transcripts are less abundant than is the major form of c-myb transcripts. The contribution that alternative splicing events in c-myb expression may make on c-myb function remains to be elucidated.

Transformation by v-myb correlates with trans-activation of gene expression

The v-myb oncogene of avian myeloblastosis virus causes acute myelomonocytic leukemia in chickens and transforms avian myeloid cells in vitro. Its protein product p48v-myb is a nuclear, sequence-specific, DNA-binding protein which activates gene expression in transient DNA transfection studies. To investigate the relationship between transformation and trans-activation by v-myb, we constructed 15 in-frame linker insertion mutants. The 12 mutants which transformed myeloid cells also trans-activated gene expression, whereas the 3 mutants which did not transform also did not trans-activate. This implies that trans-activation is required for transformation by v-myb. One of the transformation-defective mutants localized to the cell nucleus but failed to bind DNA. The other two transformation-defective mutants localized to the cell nucleus and bound DNA but nevertheless failed to trans-activate. These latter mutants define two distinct domains of p48v-myb which control trans-activation by DNA-bound protein, one within the amino-terminal DNA-binding domain itself and one in a carboxyl-terminal domain which is not required for DNA binding.

trans activation of gene expression by v-myb

The v-myb oncogene causes acute myelomonocytic leukemia in chickens and transforms avian myeloid cells in vitro. Its product, p48v-myb, is a short-lived nuclear protein which binds DNA. We demonstrate that p48v-myb can function as a trans activator of gene expression in transient DNA transfection assays. trans activation requires the highly conserved amino-terminal DNA-binding domain and the less highly conserved carboxyl-terminal domain of p48v-myb, both of which are required for transformation. Multiple copies of a consensus sequence for DNA binding by p48v-myb inserted upstream of a herpes simplex virus thymidine kinase promoter are strongly stimulatory for transcriptional activation by a v-myb-VP16 fusion protein but not by p48v-myb itself, suggesting that the binding of p48v-myb to DNA may not be sufficient for trans activation.

trans activation of gene expression by v-myb

The v-myb oncogene causes acute myelomonocytic leukemia in chickens and transforms avian myeloid cells in vitro. Its product, p48v-myb, is a short-lived nuclear protein which binds DNA. We demonstrate that p48v-myb can function as a trans activator of gene expression in transient DNA transfection assays. trans activation requires the highly conserved amino-terminal DNA-binding domain and the less highly conserved carboxyl-terminal domain of p48v-myb, both of which are required for transformation. Multiple copies of a consensus sequence for DNA binding by p48v-myb inserted upstream of a herpes simplex virus thymidine kinase promoter are strongly stimulatory for transcriptional activation by a v-myb-VP16 fusion protein but not by p48v-myb itself, suggesting that the binding of p48v-myb to DNA may not be sufficient for trans activation.

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.

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.

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.

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.

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

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

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.

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

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

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.

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.