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 mechanism of MYB transcriptional regulation by MLL-AF9 oncoprotein

Acute leukaemias express high levels of MYB which are required for the initiation and maintenance of the disease. Inhibition of MYB expression or activity has been shown to suppress MLL-fusion oncoprotein-induced acute myeloid leukaemias (AML), which are among the most aggressive forms of AML, and indeed MYB transcription has been reported to be regulated by the MLL-AF9 oncoprotein. This highlights the importance of understanding the mechanism of MYB transcriptional regulation in these leukaemias. Here we have demonstrated that the MLL-AF9 fusion protein regulates MYB transcription directly at the promoter region, in part by recruiting the transcriptional regulator kinase CDK9, and CDK9 inhibition effectively suppresses MYB expression as well as cell proliferation. However, MYB regulation by MLL-AF9 does not require H3K79 methylation mediated by the methyltransferase DOT1L, which has also been shown to be a key mediator of MLL-AF9 leukemogenicity. The identification of specific, essential and druggable transcriptional regulators may enable effective targeting of MYB expression, which in turn could potentially lead to new therapeutic approaches for acute myeloid leukaemia with MLL-AF9.

A hyperactive transcriptional state marks genome reactivation at the mitosis-G1 transition

Hsiung et al. tracked Pol II occupancy genome-wide in mammalian cells progressing from mitosis through late G1. During the earliest rounds of transcription at the mitosis–G1 transition, ∼50% of active genes and distal enhancers exhibit a spike in transcription, exceeding levels observed later in G1 phase. The transcriptional spike occurs heterogeneously and propagates to cell-to-cell differences in mature mRNA expression.

During mitosis, RNA polymerase II (Pol II) and many transcription factors dissociate from chromatin, and transcription ceases globally. Transcription is known to restart in bulk by telophase, but whether de novo transcription at the mitosis–G1 transition is in any way distinct from later in interphase remains unknown. We tracked Pol II occupancy genome-wide in mammalian cells progressing from mitosis through late G1. Unexpectedly, during the earliest rounds of transcription at the mitosis–G1 transition, ∼50% of active genes and distal enhancers exhibit a spike in transcription, exceeding levels observed later in G1 phase. Enhancer–promoter chromatin contacts are depleted during mitosis and restored rapidly upon G1 entry but do not spike. Of the chromatin-associated features examined, histone H3 Lys27 acetylation levels at individual loci in mitosis best predict the mitosis–G1 transcriptional spike. Single-molecule RNA imaging supports that the mitosis–G1 transcriptional spike can constitute the maximum transcriptional activity per DNA copy throughout the cell division cycle. The transcriptional spike occurs heterogeneously and propagates to cell-to-cell differences in mature mRNA expression. Our results raise the possibility that passage through the mitosis–G1 transition might predispose cells to diverge in gene expression states.

MYB elongation is regulated by the nucleic acid binding of NFκB p50 to the intronic stem-loop region

MYB transcriptional elongation is regulated by an attenuator sequence within intron 1 that has been proposed to encode a RNA stem loop (SLR) followed by a polyU tract. We report that NFκBp50 can bind the SLR polyU RNA and promote MYB transcriptional elongation together with NFκBp65. We identified a conserved lysine-rich motif within the Rel homology domain (RHD) of NFκBp50, mutation of which abrogated the interaction of NFκBp50 with the SLR polyU and impaired NFκBp50 mediated MYB elongation. We observed that the TAR RNA-binding region of Tat is homologous to the NFκBp50 RHD lysine-rich motif, a finding consistent with HIV Tat acting as an effector of MYB transcriptional elongation in an SLR dependent manner. Furthermore, we identify the DNA binding activity of NFκBp50 as a key component required for the SLR polyU mediated regulation of MYB. Collectively these results suggest that the MYB SLR polyU provides a platform for proteins to regulate MYB and reveals novel nucleic acid binding properties of NFκBp50 required for MYB regulation.

A novel G-quadruplex-forming GGA repeat region in the c-myb promoter is a critical regulator of promoter activity

The c-myb promoter contains multiple GGA repeats beginning 17 bp downstream of the transcription initiation site. GGA repeats have been previously shown to form unusual DNA structures in solution. Results from chemical footprinting, circular dichroism and RNA and DNA polymerase arrest assays on oligonucleotides representing the GGA repeat region of the c-myb promoter demonstrate that the element is able to form tetrad:heptad:heptad:tetrad (T:H:H:T) G-quadruplex structures by stacking two tetrad:heptad G-quadruplexes formed by two of the three (GGA)(4) repeats. Deletion of one or two (GGA)(4) motifs destabilizes this secondary structure and increases c-myb promoter activity, indicating that the G-quadruplexes formed in the c-myb GGA repeat region may act as a negative regulator of the c-myb promoter. Complete deletion of the c-myb GGA repeat region abolishes c-myb promoter activity, indicating dual roles of the c-myb GGA repeat element as both a transcriptional repressor and an activator. Furthermore, we demonstrated that Myc-associated zinc finger protein (MAZ) represses c-myb promoter activity and binds to the c-myb T:H:H:T G-quadruplexes. Our findings show that the T:H:H:T G-quadruplex-forming region in the c-myb promoter is a critical cis-acting element and may repress c-myb promoter activity through MAZ interaction with G-quadruplexes in the c-myb promoter.

Identification of a c-myb attenuator-binding factor

Expression of the c-myb proto-oncogene is developmentally regulated at the level of transcription elongation. In pre-B cells, complete c-myb transcripts are produced, whereas transcripts are attenuated near or within a 300-base pair (bp) interval of the first c-myb intron in mature cells. Hypothesizing that transcription attenuation results from a protein complex that physically impedes the progress of RNA polymerase II through the intron, we used electrophoretic mobility shift assays (EMSA) to search for DNA-binding activities that correlated with downregulation of c-myb transcription. We identified a stage-specific DNA binding activity, termed ABF, present in mature B cells but not in pre-B cells. ABF binds to a 15-bp DNA element located within a 300-bp BstEII-XbaI fragment. DMSO-treatment of murine erythroleukemia cells results in rapid downregulation of c-myb transcription and upregulation of ABF DNA binding activity. Thus, ABF binding activity correlates with downregulation of c-myb transcription in two systems. Preliminary biochemical characterization of ABF from mature B cells demonstrates that its primary DNA-binding component is a 64-kDa-protein. We hypothesize that this factor may represent a member of the transcriptional attenuation complex.

Constitutive c-Myb amino-terminal phosphorylation and DNA binding activity uncoupled during entry and passage through the cell cycle

The c-myb gene encodes a transcription factor that is central to hematopoietic cell growth. Phosphorylation of c-Myb by casein kinase 2 (CK2) at serines 11 and 12 has been variously implicated in the regulation of DNA binding. However, it is unclear when c-Myb phosphorylation at serines 11 and 12 occurs during the cell cycle and how this is regulated. We generated specific antisera that recognize phosphoserines 11 and 12 of c-Myb. C-Myb protein levels, extent of CK2 phosphorylation and DNA binding were then monitored following mitogenic stimulus and passage through the cell cycle in normal peripheral T-cells and the T leukemia cell line CCRF-CEM. We found that endogenous c-Myb is constitutively phosphorylated at serines 11 and 12. The amount of phosphorylated c-Myb correlates with DNA binding activity in cycling CEM cells but not upon entry of T-cells into the cell cycle. Exogenous expression of c-Myb with substitutions of serines 11 and 12 with glutamic acid or alanine had no effect on the transactivation of a c-Myb responsive reporter. These data strongly suggest that c-Myb is constitutively phosphorylated on serines 11 and 12 by CK2 or like activity and is not regulated during the cell cycle.

Signal transduction and post-transcriptional gene expression

Traditionally, growth factor-coupled signaling to the nucleus has been thought to be primarily directed toward transcriptional regulation. However, there are now increasing indications from a diversity of experimental systems that other aspects of RNA processing, including translation, lifetime and stability, and splicing are under strict growth factor control. In this review, we present the emerging evidence for growth factor signaling pathways that impact on these different RNA processing events. Particularly noteworthy is the realization that growth factor signaling through Ras can effect the regulation of two RNA cap-binding proteins, the cytosolic eIF-4E complex, which is necessary for initiating translation, and the nuclear cap-binding complex, the CBC, which plays a role in cap-dependent pre-mRNA splicing, U snRNA export and 3'-end processing. This, taken together with other findings that demonstrate the ability of stress response pathways and the small G protein, Cdc42, to activate the CBC, raises some interesting possibilities regarding how signaling to the two cellular RNA cap-binding protein complexes may coordinate the growth-coupled regulation of gene expression at the level of RNA processing.

Intron 1 rather than 5' flanking sequence mediates cell type-specific expression of c-myb at level of transcription elongation

Previous studies have shown that expression of steady-state c-myb mRNA was regulated primarily by a block in intron 1 during transcription elongation. This study shows that the block site maps approximately 1700 bp from the start of the intron. Studies based on a reporter construct containing c-myb flanking region and intron 1 suggest that the flanking region is not important in the regulation of the cell type-specific expression of c-myb. RNA splicing of intron 1 may enhance the expression in a non-cell type-specific manner. A conserved intron domain comprising the block site is required for defining this site, but this function of the domain is independent of cell type. The cell type-specific regulation of c-myb transcription elongation is mediated by a 5' intron sequence. A mechanism for down regulation of c-myb gene expression by the block to transcription elongation has been proposed.

Transcriptional elongation of c-myb is regulated by NF-kappaB (p50/RelB)

High levels of c-myb expression are necessary for the proliferation of hematopoietic precursor cells whereas down-regulation of c-myb is required for terminal differentiation; this down-regulation occurs through a conditional block to transcriptional elongation in intron I. We previously observed that cAMP analogs prevented the late down-regulation of c-myb during hexamethylene bisacetamide (HMBA)-induced differentiation of murine erythroleukemia (MEL) cells and blocked differentiation; this correlated with the induction of NF-kappaB (p50/RelB) complexes which were shown to bind to NF-kappaB recognition sites flanking the transcriptional pause site of c-myb. We now selected stably-transfected MEL cells which overexpressed p50, RelB or both at levels similar to those induced by cAMP to determine whether these NF-kappaB proteins regulate c-myb expression in intact cells. We demonstrate that transcriptionally active NF-kappaB (p50/RelB) complexes, but not p50 or RelB alone, prevented the early and late down-regulation of c-myb mRNA and increased c-myb transcriptional elongation in HMBA-induced MEL cells. The increase in c-myb expression was sufficient to block erythroid differentiation and allow continuous proliferation of cells in the presence of HMBA. Steady-state c-myb mRNA levels in untreated cells were not affected by overexpression of NF-kappaB, suggesting that p50/RelB specifically modulated the efficiency of transcriptional attenuation during MEL cell differentiation.

c-Myc and Mxi1 immunoreactivities in the calcifying areas of the epiphyseal-plate cartilage matrix of growing rats

We looked for the protooncogene protein, c-Myc, its dimerization partner, Max, and the repressors of its transactivation activity, Mad1 and Mxi1, in the epiphyseal-plate cartilage matrix of growing rats by immunocytochemistry in the electron microscope. c-Myc and Mxi1 immunoreactivities were found in the calcifying areas of the cartilage matrix only. There was no immunolabeling in response to anti-Max or anti-Mad1 antibodies. Mxi1 immunoreactivity was mainly in the early calcifying areas, in the calcification front and ahead of it, whereas c-Myc immunoreactivity was essentially in the incompletely calcified regions of the matrix. The two immunolabelings occurred mainly over the large type II collagen fibrils of the cartilage matrix and over the thin filaments connecting them. c-Myc and Mxi1 immunoreactivities were rarely found along the dark cristallites. There was no immunolabeling associated with the matrix vesicles, or in their immediate surroundings. The data suggest that the protooncogene proteins, c-Myc and Mxi1, could be implicated in the calcification involving type II collagen fibrils of the epiphyseal-plate cartilage. The absence of Max immunoreactivity from the calcifying cartilage matrix raises the question of whether there are other c-Myc- and Mxi1-dimerization partners.

Novel transcriptional mechanisms are involved in regulating preproenkephalin gene expression in vivo

For the dissection of the temporal and spatial patterns of cell- and tissue-specific gene expression an understanding of the contributing regulating mechanisms is required. We now confirm that there are novel mechanisms regulating preproenkephalin gene expression in basal as well as cholinergic agonist treated rats. Moreover, we demonstrate that these novel transcriptional mechanisms are consistent with RNA intragenic elongation pausing, alternate promoter usage, and small sense and antisense RNA transcription from the preproenkephalin gene locus. We report that while basal striatal and olfactory bulb proenkephalin RNA transcripts are initiated from the "normal" proximal promoter, in cerebellum de novo RNA transcription appears to be initiated from the distal so-called "germ-cell" promoter. Furthermore, "normally" initiated olfactory bulb proenkephalin RNA transcripts appear to be down-regulated by the time the RNA polymerase II complex reaches the first preproenkephalin intron, in a way that is consistent with RNA elongation pausing. As the pattern of small sense and antisense transcripts found associated with this gene's expression is tissue-specific, we suggest that they may also play a role in regulating gene expression. The understanding of this gene's regulation should have widespread importance, not only to those interested in opioid gene expression, but also to those interested in gene regulation, in general.

Identification of the major positive regulators of c-myb expression in hematopoietic cells of different lineages

The c-myb gene is primarily expressed in hematopoietic cells, and it is overexpressed in many leukemias. The regulation of its expression is of critical importance in hematopoietic cells. We identified the major positive regulatory sites in the 5'-flanking sequence of the human c-myb gene, and we found that the positive regulators differed in cells of different lineages. In the Molt-4 T-cell line, two Ets-like binding sites were required for the expression of c-myb. The 5' site played a minor role in the regulation of c-myb expression, and we demonstrated that a protein of 67 kDa bound to this site. Antibodies against Ets proteins showed no cross-reactivity with this protein. We showed that Ets-1 bound to the 3'-regulatory site in the c-myb promoter by electrophoretic mobility shift assay and antibody studies. Both of these Ets-like binding sites were nonfunctional in the DHL-9 B-cell line and the K562 myeloid cell line. We identified a novel transcription factor of 50.5 kDa that was required for expression of c-myb in these cell lines.

Basic mechanisms of transcript elongation and its regulation

Ternary complexes of DNA-dependent RNA polymerase with its DNA template and nascent transcript are central intermediates in transcription. In recent years, several unusual biochemical reactions have been discovered that affect the progression of RNA polymerase in ternary complexes through various transcription units. These reactions can be signaled intrinsically, by nucleic acid sequences and the RNA polymerase, or extrinsically, by protein or other regulatory factors. These factors can affect any of these processes, including promoter proximal and promoter distal pausing in both prokaryotes and eukaryotes, and therefore play a central role in regulation of gene expression. In eukaryotic systems, at least two of these factors appear to be related to cellular transformation and human cancers. New models for the structure of ternary complexes, and for the mechanism by which they move along DNA, provide plausible explanations for novel biochemical reactions that have been observed. These models predict that RNA polymerase moves along DNA without the constant possibility of dissociation and consequent termination. A further prediction of these models is that the polymerase can move in a discontinuous or inchworm-like manner. Many direct predictions of these models have been confirmed. However, one feature of RNA chain elongation not predicted by the model is that the DNA sequence can determine whether the enzyme moves discontinuously or monotonically. In at least two cases, the encounter between the RNA polymerase and a DNA block to elongation appears to specifically induce a discontinuous mode of synthesis. These findings provide important new insights into the RNA chain elongation process and offer the prospect of understanding many significant biological regulatory systems at the molecular level.

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

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

Induction of differentiation and down-regulation of c-myb gene expression in ML-1 human myeloblastic leukemia cells by the clinically effective anti-leukemia agent meisoindigo

Meisoindigo, a second generation derivative of indirubin, is an effective chemotherapeutic agent with very low toxicity used in the treatment of chronic myeloid leukemia. To determine the nature of this activity, the effect of a nontoxic concentration (0.72 micrograms/mL) of this compound on ML-1 human myeloblastic leukemic cells was examined. At such a concentration, differentiation induction was found to be the most pronounced drug effect. During the 3-day drug incubation period, the viable cell number remained essentially constant, with approximately 48% of the cells demonstrating a mature phenotype with increased acid phosphatase activity and nitroblue tetrazolium dye reduction. As observed with other DNA-specific agents, induction of ML-1 differentiation by meisoindigo was accompanied by the down-regulation of c-myb gene expression. These data suggest that induction of leukemic cell differentiation associated with decreased c-myb expression may be one of the mechanisms of the antitumor action of meisoindigo.

Interferon-independent activation of (2′-5′) oligoadenylate synthetase in Friend erythroleukemia cell variants exposed to HMBA

To provide evidence for the implication of interferon (IFN)-induced proteins in the regulation of cell growth during differentiation, the activation of (2′-5′) oligoadenylate synthetase (2-5A synthetase) as well as of PKR, two IFN-induced proteins, during differentiation of Friend erythroleukemia cells, was studied. Two cell variants were used. The first (FL) was completely susceptible to hexamethylene bis-acetamide (HMBA)-treatment and responded in both growth-retardation and hemoglobin synthesis. The second (R1) failed to synthesize hemoglobin in response to HMBA although cell growth was still inhibited. In both cell variants, 2–5A synthetase enzyme activity was induced in a similar fashion, reaching a peak at 26 hours after treatment with HMBA. However, the down regulation of activity thereafter was not identical in both cases. In R1 cells, the reduction was much slower compared to FL cells. A similar pattern was observed with the appearance of the 43 kDa isoform of 2–5A synthetase in immunoblots. An analysis of 2–5A synthetase gene expression revealed the presence of 1.7 kb transcripts which peaked at 16 hours after HMBA-treatment in both cell variants. Again, the down-regulation in expression was slower in R1 than in FL cells. Addition of anti-murin alpha/beta-IFN antibodies did not reduce the level of either 2–5A synthetase expression or enzyme activity in either cell variant. Interestingly, the presence of antibodies also did not affect the pattern of pRb phosphorylation in the cell variants exposed to HMBA. In both cell variants, an increase in the amount of the phosphorylated form (ppRb) was observed in immunoblots after 4 hours. This form was gradually transformed to the underphosphorylated molecule (pRb) with time in culture, even in the presence of antibodies. This further substantiates the notion that IFN-induced regulation of pRb phosphorylation is mediated by IFN-induced proteins. The basal level of either expression or ezymatic activity of PKR detected in untreated FL or R1 cells, was relatively high. Treatment with HMBA did not result in further induction of PKR in either cell variant.

c-myb intron I protein binding and association with transcriptional activity in leukemic cells

Specific binding of nuclear proteins to the region of transcriptional attenuation has been shown to modulate the expression of c-myb, a nuclear proto-oncogene preferentially expressed in lympho-hematopoietic cells. Here, it plays an important role in processes of differentiation and proliferation. The mechanism that regulates c-myb expression is not yet fully understood. The block of transcriptional elongation which has been mapped to a 1 kb region within murine intron 1 may represent one regulatory pathway. The DNA sequences containing the transcriptional pause site are well conserved between murine and human species, thus Implying similar transcription-control strategies. We compared the binding potential of nuclear extracts (from human fibroblasts and MOLT4 as well as murine NIH3T3- and 70Z/3B- cell lines) to oligonucleotide sequences previously shown to be target binding sites in the murine system. One complex containing a 70 D protein was found to be associated specifically with transcriptionally active leukemia cells. We performed transient expression studies with a CAT reporter construct containing this putative enhancer sequence and yielded significant CAT activity. We identified further a putative 20 kD repressor protein in transcriptionally silent cells and demonstrated that c-Jun is part of an ubiquitously present complex. Our results confirm the participation of intron 1 in transcriptional regulation of the c-myb gene (in mouse and human) and implicate multiple and complex regulatory mechanisms of activation during myelomonocytic differentiation and leukemic cell growth control.

Repression of the c-myb gene by WT1 protein in T and B cell lines

The c-myb gene is primarily expressed in immature hematopoietic cells, and it is overexpressed in many leukemias. We have investigated the role of negative regulatory sites in the c-myb promoter in the Molt-4 T cell line and in the DHL-9 B cell line. A potential binding site for either the EGR-1 or WT1 protein was identified by in vivo footprinting in the 5'-flanking region of c-myb in a region of negative regulatory activity in T cells. We showed by electrophoretic mobility shift assay and electrophoretic mobility shift assay Western that WT1, EGR-1, and Sp1 bound to this site. A mutation of this site which prevented protein binding increased the activity of the c-myb promoter by 2.5-fold. In the DHL-9 B cell line, this site was nonfunctional; however, we found a potential EGF-1/WT1 site located more 3' in a region of negative regulatory activity. We showed that WT1, EGR-1, and Sp1 bound to this site, and that mutation of this site increased the activity of the c-myb promoter by 3.2-fold. Cotransfection of a WT1 expression vector repressed the activity of the c-myb promoter in both cell lines, and this repression was relieved when the EGR-1/WT1 sites were removed. Cotransfection of either an EGR-1 or Sp1 expression vector had no significant effect on the activity of the c-myb promoter. We conclude that WT1 is a negative regulator of c-myb expression in both T and B cell lines.

Regulation of pre-mRNA processing by src

BACKGROUND

Changes in gene expression in response to external signals provide a key mechanisms for the regulation of higher eukaryotic cell functions. The importance of transcriptional control in the response of cells to growth factors and cytokines has been extensively documented, but gene expression has also been shown to be controlled at other levels, such as the stability of mRNA in the cytoplasm, its localization and translation. By contrast to transcriptional control, little is known of the contribution of pre-mRNA nuclear processing to the regulation of gene expression, as most of our knowledge of pre-mRNA processing in vivo is indirect, being inferred from comparisons of transcription rates and levels of mRNA accumulation.

RESULTS

In this study, we have used as a model the well-characterized maturation pathway of transcripts of the cytokine, tumour necrosis factor beta (TNF beta). We have used the murine TNF beta gene as a reporter for pre-mRNA processing, using a co-transfection approach to investigate whether overproduction of proteins involved in signal transduction influences the processing of TNF beta transcripts. Although transfection of both activated ras and src genes led to an increase in RNA accumulation in the nuclear and cytoplasmic compartments, as expected from their transactivation of the TNF beta expression vector, only src induced a modification of RNA processing. Comparison of several modes of src activation indicated that two distinct effects of src on pre-mRNA processing can be coupled: one involves slowing down splicing and the other allows the export of partially spliced transcripts. These effects can be observed not only on the three introns of TNF beta but also on transcripts from a beta globin expression vector.

DISCUSSION

We have characterized how the processing of transcripts of TNF beta and beta globin is regulated by the signal transduction pathway that includes the Src protein, establishing that external signals have the capacity to regulate gene expression at a post-transcriptional level within the nucleus. Src seems to act on a general mechanism of splicing and/or mRNA transport, but its biologically relevant targets are likely to be restricted to genes for which either alternative processing pathways are in competition, or the kinetics of splicing is critical. This regulation could reflect a modulation by Src of the activity of components of the splicing and transport machineries, but could also involve RNA-binding proteins, which have been shown to interact with Src.

Members of the nuclear factor kappa B family transactivate the murine c-myb gene

Expression of the c-myb proto-oncogene is primarily detected in normal tissue and tumor cell lines of immature hematopoietic origin, and the down-regulation of c-myb expression is associated with hematopoietic maturation. Cell lines that represent mature, differentiated hematopoietic cell types contain 10-100-fold less c-myb mRNA than immature hematopoietic cell types. Differences in steady-state c-myb mRNA levels appear to be primarily maintained by a conditional block to transcription elongation that occurs in the first intron of the gene. The block to transcription elongation has been mapped, using nuclear run-on analysis, to a region of DNA sequence that is highly conserved between mouse and man. Two sets of DNA-protein interactions, flanking the site of the block to transcription elongation, were detected that exhibited DNA-binding activities that strongly correlated with low steady-state c-myb mRNA levels. Several criteria demonstrated that members of the nuclear factor kappa B (NF-kappa B) family of transcription factors were involved in the DNA-protein interactions identified in these two sets. Surprisingly, cotransfection experiments demonstrated that coexpression of members of the NF-kappa B family, specifically p50 with p65 and p65 with c-Rel, transactivated a c-myb/chloramphenicol acetyltransferase reporter construct that contained 5'-flanking sequences, exon I, intron I, and exon II of the c-myb gene. Transactivation by these heterodimer combinations was dependent on regions of the c-myb first intron containing the NF-kappa B-binding sites. These findings suggest that NF-kappa B family members may be involved in either modifying the efficiency of transcription attenuation or acting as an enhancer-like activity to increase transcription initiation. Thus, the regulation of c-myb transcription may be quite complex, and members of the NF-kappa B family likely play an important role in this regulation.

Expression of c-Myc in glucocorticoid-treated fibroblastic cells

Glucocorticoids inhibit proliferation of L929 fibroblastic cells in culture. Inhibition of proliferation is reversible and is not associated with changes in the plating efficiency of the cells. Flow cytometric analysis indicates that glucocorticoid-treated cells exhibit a decrease in the percentage of cells with DNA content > 2 N. Thymidine kinase expression is inhibited as cells with 2 N DNA content accumulate. These observations indicate that glucocorticoids arrest proliferation of L929 cells in the G1 phase of the cell cycle. The abundance of c-Myc mRNA does not decrease in glucocorticoid-treated cells, and c-Myc protein content in dexamethasone-treated cells is approximately the same as that detected in mid-log phase cells. Nuclear run-on transcription of c-Myc is not inhibited by glucocorticoids. These observations indicate that glucocorticoid regulation of fibroblastic cell proliferation does not involve inhibition of c-Myc transcription. Although regulation of c-Myc expression is central to the mechanism whereby glucocorticoids regulate proliferation of lymphoid cells, it is clear that different mechanisms must be involved in glucocorticoid regulation of fibroblastic cell proliferation.

Regulation of c-myb oncogene expression in immature and mature murine T cells

The c-myb oncogene encodes a nuclear binding protein which may play a major role in differentiation during early T cell development. However, the functionally important transcription regions in the GC promoter site have not been defined and the significance of the regulation of this promoter site in T cell differentiation has not been determined. Therefore, the promoter strength was determined by measurement of the CAT activity in cell extracts of EL-4 cells that were transfected with a CAT expression vector that contained cloned segments of the 5' myb gene. Stepwise removal of DNA sequences between -2300 bp and -346 bp upstream from the ATG initiation codon resulted in a gradual loss of 50% of CAT activity, whereas deletion of DNA sequences from -346 to -295 and -232 to -155 bp upstream from the ATG initiation codon eliminated promoter activity. On analysis of the CAT activity after transfection of various cell lines with these same constructs, it was found that the same two promoter regions were required for high CAT activity in all the cell lines, including murine cell lines which express the alpha/beta TCR and high levels of c-myb (BW5147), the alpha/beta TCR and low levels of c-myb (Yac-1), or the gamma/delta TCR (KN 12.1 and KN 2.4 T), a murine fibroblast T cell line (NIH-3T3), and a human epithelial cell line (HeLa). However, the CAT activity did not correlate with steady state levels of expression of the c-myb gene in the murine cell lines. Our data indicate that the c-myb oncogene promoter is constitutively expressed is highly dependent on a limited region of the 5' myb gene, requires two DNA elements for optimal activity, and is functional in diverse T cell lines.

Murine erythroleukemia cells contain two distinct GATA-binding proteins that have different patterns of expression during cellular differentiation

GATA-1 is a major transcription factor of the erythroid lineage that has been implicated in the induced expression of a variety of red cell-specific genes during terminal differentiation of murine erythroleukemia cells. Although the GATA-1 protein is present at nearly equal levels before and after differentiation of murine erythroleukemia cells, in this study it was found that in the early commitment stages of the differentiation program there is a transient decrease in the GATA-1 mRNA and DNA binding activity levels due to a temporary block in transcription of the gene. Moreover, using a whole cell extraction procedure it was discovered that murine erythroleukemia cells contain a second GATA binding activity (denoted GATA-rel) which appears to be distinct from the GATA-1 factor based on its non-reactivity to two GATA-1 antisera. This protein has a limited tissue specificity, as it could not be detected in extracts from CHO, NIH 3T3, or COS cells. Similarly to the GATA-1 DNA-binding activity, the GATA-rel activity decreased during the early stages of differentiation. However, unlike GATA-1, GATA-rel activity did not return to pre-induced levels at later times. These results suggest that changes in gene expression during erythroid terminal differentiation may involve an interplay on levels of different GATA-binding factors.

Antisense RNA and p53 regulation in induced murine cell differentiation

p53 expression is strongly modulated during the process of induced differentiation, at the same time as both cell cycle and genetic expression become modulated, giving rise to a commitment to terminal differentiation. We took advantage of two murine cell lines inducible for differentiation, an erythroleukemia and a melanoma cell line, to outline common features of the regulation of p53 expression during the differentiation process. We found that p53 mRNA decreased early after induced differentiation and that regulation was controlled at a posttranscriptional level. Our data showed that this regulation affects p53 pre-mRNA maturation. Because, in both systems used, actinomycin D treatment abolished the inducer-mediated decrease of p53 mRNA, we looked for induced RNAs potentially involved in this process. Using different parts of the p53 gene and flanking regions as probes, we identified three RNA species whose expression is modulated during induced differentiation. A first species is made of high molecular weight RNAs that accumulate in the nuclear compartment and seem to represent antisense transcripts of the p53 gene. A second species, 1.3-kb long, was found to accumulate in the nucleus of induced MEL cells and was homologous to a restricted part of the first intron of the p53 gene due to the presence of a B1 repetitive element in an antisense orientation with respect to the p53 pre-messenger RNA. Finally, a family of B2-containing RNAs was observed in both cytoplasmic and nuclear compartments. The variation in the amounts of sense and antisense RNAs, respectively, suggested an interesting speculative model for the maturation of B2-containing pre-messenger RNAs.

Detection of a transcriptional block in the first intron of the human c-myb gene

The levels of expression of the murine c-myb gene, like those of several other proto-oncogenes, can be controlled by a block of transcriptional elongation within the first intron of the gene. We have performed run-off experiments with double- and single-stranded probes on the myelomonocytic cell line U937, and show that this mechanism of transcriptional arrest is true also for the human c-myb gene and takes place within the first intron. Furthermore, we have sequenced the entire first intron of the human c-myb gene, and discuss the sequence structure in relation to its putative ability to arrest RNA polymerase II and its high degree of homology with the equivalent murine intron.

Expression of c-myc proto-oncogene during podophyllotoxin induced IW32 erythroleukemia cell differentiation

The effect of hemin or podophyllotoxin on the differentiation of the erythropoietin (epo)-producing IW32 erythroleukemia cells was investigated. Podophyllotoxin induced IW32 cells to differentiate, and hemin potentiated the differentiation. Hemin had no effect on cell proliferation whereas podophyllotoxin inhibited cell growth. c-myc mRNA levels decreased biphasically by hemin or podophyllotoxin, while the combined treatment of hemin plus podophyllotoxin did not result in the initial decrease in c-myc mRNA level. Our data suggested that down-regulation of c-myc expression was not a prerequisite of IW32 cell differentiation induced by hemin and podophyllotoxin combined.

Molecular genetic evidence for a differentiation-proliferation coupling during DMSO-induced myeloid maturation of HL-60 cells: role of the transcription elongation block in the c-myc gene

Proliferation-differentiation coupling was studied during dimethyl sulfoxide (DMSO)-induced myeloid maturation of HL-60 cells using transcription of the myeloperoxidase (MPO) and c-myc genes as indicators of differentiation and proliferation, respectively. Concomitant cell cycle kinetic analysis correlated the proliferation and transcription patterns. Transcription, cell cycle phases and rate of DNA synthesis were examined for up to 5 days of induction and, at 1-day intervals, analyzed during a 24-h reculture without the inducer. DMSO suppressed transcription of the c-myc and MPO genes with a t1/2 of 16 min and 7 h, respectively. The ability to recover transcription following reculture diminished with the progression of the induction and ultimately was lost; concomitantly, the cells irreversibly lost the capacity to divide. This indicated that the differentiation and proliferation processes are inseparable and that terminal differentiation accompanies irreversible proliferation arrest in HL-60 cells. We also studied the kinetics of the block to transcription elongation at the exon 1-intron 1 boundary of the c-myc gene. This block produces a 0.38 kb truncated transcript that is constitutively expressed in somatic cells (Re et al., Oncogene 5, 1247, 1990). During induction the level of the 0.38 kb RNA increased, while that of the complete c-myc mRNA decreased, indicating that this truncated RNA is generated instead of message through a monotonously initiated transcriptional process. Transcription initiation and synthesis of the 0.3 kb RNA persisted in terminally differentiated cells, suggesting a role for this RNA in non-proliferating cells.

Identification of an elastase-like activity, that decreased during the differentiation of MEL cells, as a prolyl endopeptidase

1. A Suc-APA-MCA hydrolytic activity was significantly decreased in murine erythroleukemia cells during DMSO-induced differentiation, but not in DMSO-resistant cells. 2. The Suc-APA-MCA hydrolytic enzyme was purified by ion exchange, adsorption, gel filtration and affinity chromotographies. The results of the chromatographies showed that only one enzyme hydrolyzed Suc-APA-MCA in MEL cells. 3. This enzyme is more sensitive to hydrolysis by Suc-GPLGP-MCA than Suc-APA-MCA at slightly acidic pH, and its activity is stimulated by 2-mercaptoethanol. 4. A cysteine proteinase inhibitor did not affect the activity, but a specific inhibitor of prolyl endopeptidase, Z-thioprothiazolidine, completely inhibited it. These results suggest that the Suc-APA-MCA hydrolytic enzyme is identical to a prolyl endopeptidase.

Mithramycin selectively inhibits the transcriptional activity of a transfected human c-myc gene

The G-C specific DNA binding drug mithramycin selectively inhibits expression of the c-myc gene in a number of cell types. We have tested the ability of this agent to inhibit the expression of a transfected human c-myc gene in a murine fibroblast cell line. Expression of c-myc is inhibited in the first 24 hours of mithramycin exposure (in contrast to actin gene expression, which is unaffected). Nuclear runon transcription of c-myc by nuclei isolated from mithramycin treated cells is decreased, indicating inhibition of transcription initiation. However, treatment of isolated nuclei with mithramycin also results in decreased c-myc transcription. Thus, inhibition of c-myc expression by mithramycin in these cells appears to occur at the transcriptional level and is most likely mediated at both the transcription initiation and elongation level. This suggests that mithramycin selectively interacts with the G-C rich c-myc promoter, preventing formation of the c-myc transcription initiation complex.

Enhancement by theophylline of the butyrate-mediated induction of choriogonadotropin alpha-subunit in HeLa cells. II. Effect of both agents on mRNA turnover

In the accompanying paper it was demonstrated that among several methylxanthine phosphodiesterase inhibitors, only theophylline significantly increased production of the glycoprotein hormone alpha-subunit in HeLa cells, and that this action was synergistic with that of sodium butyrate. A correlation between alpha-subunit induction and cAMP concentrations was not evident. In this report we characterized the effect of these two drugs on the metabolism of alpha-subunit mRNA. Sodium butyrate decreased the apparent half-life of mRNAs encoding alpha-subunit, beta 2-microglobulin, and alpha-tubulin, as well as that of total poly(A)+ RNA and rRNA. Theophylline produced a two- to threefold increase in the apparent half-life of alpha-subunit mRNA but had no effect on the turnover of beta 2-microglobulin, alpha-tubulin, or total poly(A)+ mRNA. An inverse correlation was noted between the apparent half-life of the mRNA and the degree of destabilization elicited by butyrate. It is concluded that alpha-subunit induction by theophylline is in large part due to mRNA stabilization, and that the concerted effect of theophylline and butyrate results from inhibition by theophylline of the butyrate-mediated destabilization of alpha-subunit mRNA combined with the elevation in alpha-subunit gene transcription known to be produced by the fatty acid.

Demonstration in living cells of an intragenic negative regulatory element within the rodent c-fos gene

We studied c-fos gene expression in rat fibroblasts by microinjection of regulatory DNA sequences, such as the serum response element (SRE) present in c-fos promotor, in order to compete directly with such sequences for binding of putative regulatory factors. We show that an additional fos intragenic regulatory element (FIRE) is located at the end of exon 1. When coinjected with an SRE oligonucleotide, it induced c-fos expression in quiescent cells, whereas injection of SRE sequence alone failed to do so. Moreover, injection in quiescent cells of an SRE oligonucleotide together with a p-fos-lacZ construct containing the c-fos SRE as well as an in-frame insertion of FIRE resulted in a block to beta-galactosidase expression that can be relieved by coinjection of the FIRE sequence.

Changes in proteinase activities during the differentiation of murine erythroleukemia cells

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

Effects of dexamethasone on induction of monocytic differentiation in human U-937 cells by dimethylsulfoxide

The present studies demonstrate that dimethylsulfoxide (DMSO) treatment of human U-937 myelomonocytic leukemia cells is associated with induction of monocytic differentiation. The DMSO-induced U-937 monocytic phenotype was associated with 1) growth inhibition, 2) loss of clonogenic survival, 3) increases in alpha-naphthyl acetate esterase (NSE) staining, and 4) increases in cell surface expression of the monocyte marker Mac-1. DMSO treatment of U-937 cells was also associated with down-regulation of c-myc and c-myb gene expression as well as with increases in tumor necrosis factor (TNF) mRNA levels. The results further demonstrate that induction of U-937 monocytic differentiation by DMSO is accompanied by increases in phospholipase A2 activity. Moreover, this stimulation of phospholipase A2 was sensitive to dexamethasone. We therefore studied the effects of dexamethasone on DMSO-induced differentiation of U-937 cells. Although dexamethasone had no effect on growth inhibition or loss of clonogenic survival by DMSO, this glucocorticoid blocked increases in NSE staining and cell surface Mac-1 expression. Dexamethasone also had no effect on the down-regulation of c-myc and c-myb expression but blocked the reappearance of c-myb transcripts after 6 hr of DMSO treatment. Finally, dexamethasone inhibited DMSO-induced increases in TNF gene expression. Taken together, the results demonstrate that dexamethasone inhibits multiple characteristics, including the stimulation of phospholipase A2 activity, associated with DMSO-induced monocytic differentiation of U-937 cells.