Transcriptional and posttranscriptional control of c-myc gene expression in WEHI 231 cells

Dynamic modeling of folliculogenesis signaling pathways in the presence of miRNAs expression

Background

TEK signaling plays a very important role in folliculogenesis. It activates Ras/ERK/MYC, PI3K/AKT/mTORC1 and ovarian steroidogenesis activation pathways. These are the main pathways for cell growth, differentiation, migration, adhesion, proliferation, survival and protein synthesis.

Results

TEK signaling on each of the two important pathways where levels of pERK, pMYC, pAkt, pMCL1 and pEIF4EBP1 are increased in dominant follicles and pMYC is decreased in dominant follicles. Over activation of ERK and MYC which are the main cell growth and proliferation and over activation of Akt, MCl1, mTORC1 and EIF4EBP1 which are the main cell survival and protein synthesis factors act as promoting factors for folliculogenesis. In case of over expression of hsa-miR-30d-3p and hsa-miR-451a, MYC activity level is considerably increased in subordinate follicles. Our simulation results show that in the presence of has-miR-548v and bta-miR-22-3p, downstream factors of pathways are inhibited.

Conclusions

Our work offers insight into the design of natural biological procedures and makes predictions that can guide further experimental studies on folliculogenesis pathways. Moreover, it defines a simple signal processing unit that may be useful for engineering synthetic biology and genes circuits to carry out cell-based computation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13048-017-0371-y) contains supplementary material, which is available to authorized users.

Toll-like receptor agonists induce apoptosis in mouse B-cell lymphoma cells by altering NF-κB activation

Toll-like receptor 9 (TLR9) recognizes microbial DNA containing unmethylated cytosyl guanosyl (CpG) sequences, induces innate immune responses, and facilitates antigen-specific adaptive immunity. Recent studies report that in addition to stimulating innate immunity, TLR9 ligands induce apoptosis of TLR9 expressing cancer cells. To understand the mechanism of TLR9-induced apoptosis, we compared the effects of CpG containing oligodeoxynucleotides (CpG ODN) on a mouse B-cell lymphoma line, CH27, with those on mouse splenic B cells. CpG ODN inhibited constitutive proliferation and induced apoptosis in the CH27 B-cell lymphoma line. In contrast, CpG ODN-treated primary B cells were stimulated to proliferate and were rescued from spontaneous apoptosis. The induction of apoptosis required the ODNs to contain the CpG motif and the expression of TLR9 in lymphoma B cells. A decrease in Bcl-xl expression and an increase in Fas and Fas ligand expression accompanied lymphoma B-cell apoptosis. Treatment with the Fas ligand-neutralizing antibody inhibited CpG ODN-induced apoptosis. CpG ODN triggered a transient NF-κB activation in the B-cell lymphoma cell line, which constitutively expresses a high level of c-Myc, while CpG ODN induced sustained increases in NF-κB activation and c-Myc expression in primary B cells. Furthermore, an NF-κB inhibitor inhibited the proliferation of the CH27 B-cell lymphoma line. Our data suggest that the differential responses of lymphoma and primary B cells to CpG ODN are the result of differences in NF-κB activation. The impaired NF-κB activation in the CpG ODN-treated B-cell lymphoma cell line alters the balance between NF-κB and c-Myc, which induces Fas/Fas ligand-dependent apoptosis.

Sensing and integration of Erk and PI3K signals by Myc

The transcription factor Myc plays a central role in regulating cell-fate decisions, including proliferation, growth, and apoptosis. To maintain a normal cell physiology, it is critical that the control of Myc dynamics is precisely orchestrated. Recent studies suggest that such control of Myc can be achieved at the post-translational level via protein stability modulation. Myc is regulated by two Ras effector pathways: the extracellular signal-regulated kinase (Erk) and phosphatidylinositol 3-kinase (PI3K) pathways. To gain quantitative insight into Myc dynamics, we have developed a mathematical model to analyze post-translational regulation of Myc via sequential phosphorylation by Erk and PI3K. Our results suggest that Myc integrates Erk and PI3K signals to result in various cellular responses by differential stability control of Myc protein isoforms. Such signal integration confers a flexible dynamic range for the system output, governed by stability change. In addition, signal integration may require saturation of the input signals, leading to sensitive signal integration to the temporal features of the input signals, insensitive response to their amplitudes, and resistance to input fluctuations. We further propose that these characteristics of the protein stability control module in Myc may be commonly utilized in various cell types and classes of proteins.

The effect of the fungal metabolite radicicol analog A on mRNA degradation

The AU-rich element (ARE) is a stability determinant found in the 3' UTR of a number of short-lived mRNAs. The best characterized ARE is the Shaw-Kamen (SK) box or AUUUA motif. Previously, a fungal metabolite, radicicol analog A (RAA), was shown to destabilize SK box-containing mRNAs based on 16 mRNAs examined [T. Kastelic et al., Cytokine 8 (1996) 751-761]. Using serial analysis of gene expression (SAGE) to examine the global effect of RAA on mRNA expression in interferon-gamma/lipopolysaccharide-stimulated THP-1 human monocytes, we observed that the expression level of greater than 99% of the SAGE tags was unchanged by RAA treatment and only 34 of the 17,608 unique tags annotated were reduced (p< or =0.0001). RAA destabilized approximately half of the down-regulated transcripts. Whereas all the destabilized mRNAs possessed at least one SK box, for transcripts not destabilized but nonetheless down-regulated, RAA appears to function by a SK box-independent mechanism not currently understood.

Deregulation in trans or c-myc expression in immortalized human urothelial cells and in T24 bladder carcinoma cells

The expression of a number of cellular oncogenes was investigated in human urothelial cell lines with different in vitro growth properties. Constitutively elevated levels of expression of c-myc RNA were found in Hu609, an immortalized, nontumorigenic cell line that was derived from normal urothelium, and in the bladder carcinoma cell line T24. Potential mechanisms that might underlie deregulation of c-myc expression in these cells were investigated. It was found that the c-myc gene was apparently intact and not amplified in Hu609 and T24. No increased stability of c-myc RNA was detected. A c-myc-CAT fusion construct containing 2.5 kb of normal c-myc 5' sequences showed levels of expression that paralleled the overexpression of the endogenous gene, indicating that the high constitutive levels of c-myc expression were due, at least in part, to alterations in the activities of cellular trans-acting transcriptional regulators.

Basic fibroblast growth factor decreases elastin gene transcription in aortic smooth muscle cells

The extracellular matrix (ECM) protein elastin plays an essential role in the cardiovascular system by imparting elasticity to blood vessel wall. In this study, we examined the effect of basic fibroblast growth factor (bFGF) on the expression of elastin in aortic smooth muscle cells (SMC) to gain insight into events associated with cardiovascular diseases. The results show that bFGF treatment of SMC causes a significant decrease in elastin mRNA and secreted tropoelastin levels. Nuclear run-on analyses demonstrate that the downregulation is due to a decrease in the level of elastin gene transcription. Transient transfections of SMC with wild-type and mutated elastin gene promoter/chloramphenicol acetyl transferase (CAT) constructs show that a previously identified activator protein-1-cAMP response element (AP1/CRE) (-564 to -558-bp) within the elastin promoter mediates the bFGF-dependent downregulation of elastin gene transcription in SMC. Addition of bFGF to SMC activates the extracellular signal-regulated kinases 1/2 (ERK1/2) resulting in their translocation into the nucleus and subsequent induction of Fra-1. The addition of PD-98059, an inhibitor of ERK1/2 kinase, abrogates the bFGF-dependent decrease of elastin mRNA in SMC. The described inhibitory effect of bFGF on elastin gene expression in SMC may significantly contribute to the inefficient repair of elastin in early stages of vascular wall injury.

Repression of transcription of the p27(Kip1) cyclin-dependent kinase inhibitor gene by c-Myc

Upon engagement of the B Cell Receptor (BCR) of WEHI 231 immature B cells, a drop in c-Myc expression is followed by activation of the cyclin-dependent kinase inhibitor (CKI) p27(Kip1), which induces growth arrest and apoptosis. Here, we report inverse patterns of p27 and c-Myc protein expression follow BCR engagement. We present evidence demonstrating, for the first time, that the p27(Kip1) gene is a target of transcriptional repression by c-Myc. Specifically, the changes in p27 protein levels correlated with changes in p27 mRNA levels, and gene transcription. Induction of p27 promoter activity followed BCR engagement of WEHI 231 cells, and this induction could be repressed upon co-transfection of a c-Myc expression vector. Inhibition of the TATA-less p27 promoter by c-Myc was also observed in Jurkat T cells, vascular smooth muscle, and Hs578T breast cancer cells, extending the observation beyond immune cells. Consistent with a putative Inr element CCAGACC (where +1 is underlined) at the start site of transcription in the p27 promoter, deletion of Myc homology box II reduced the extent of repression. Furthermore, enhanced repression was observed upon transfection of the c-Myc 'super-repressor', with mutation of Phe115 to Leu. The sequences mediating transcriptional activity and c-Myc repression were mapped to bp -20 to +20 of the p27 gene. Finally, binding of Max was shown to facilitate c-Myc binding and repression of p27 promoter activity. Overall, these studies identify the p27 CKI gene as a new target whereby c-Myc can control cell proliferation, survival and neoplastic transformation.

Smad7 is induced by CD40 and protects WEHI 231 B-lymphocytes from transforming growth factor-beta -induced growth inhibition and apoptosis

Transforming growth factor-beta (TGF-beta) is a potent inducer of apoptosis in B-lymphocytes and is essential for immune regulation and maintenance of self-tolerance. Here we show that concomitant signaling through CD40 sustains proliferation and rescues the premature B cell line WEHI 231 from both TGF-beta-induced and anti-IgM-induced apoptosis. The anti-apoptotic effect of CD40 is associated with the transcriptional activation of the inhibitory Smad7 protein. The transactivation of Smad7 by CD40 is NFkappaB-dependent in that pharmacological inhibitors of this pathway, N-tosyl-l-phenylalanine chloromethyl ketone and pyrrolidine dithiocarbamate, abrogate CD40-induced Smad7 expression. Ectopic overexpression of Smad7 inhibited Smad2 activation, TGF-beta-mediated growth inhibition, and apoptosis in WEHI 231 cells. Consistent with this result, dominant negative interference with Smad2 and Smad3 function also inhibited TGF-beta-induced apoptosis. The inhibitory effects of Smad7 overexpression were specific to TGF-beta-induced apoptosis and were without effect on anti-IgM-induced cell death. These results suggest a mechanism of suppression of TGF-beta-induced apoptosis by CD40, mediated through activation of NF-kappaB and, consequently, induction of Smad7 expression.

A-myb rescues murine B-cell lymphomas from IgM-receptor–mediated apoptosis through c-myctranscriptional regulation

A-myb is a member of the myb family of transcription factors, which regulates proliferation, differentiation, and apoptosis of hematopoietic cells. A-Myb expression is normally restricted to the proliferating B-cell centroblasts and transgenic mice overexpressing A-myb displayed enhanced hyperplasia of the lymph nodes. Because A-Myb is highly expressed in several subtypes of human B-cell neoplasias, we sought to determine whether the A-myb gene promoted proliferation and survival of B lymphocytes, using the WEHI 231 and CH33 murine B-cell lymphomas as models. Here, we show that ectopic expression of A-mybrescues WEHI 231 and CH33 cells from growth arrest and apoptosis induced by anti-IgM treatment. Previously, we demonstrated an essential role of the c-myc gene in promoting cell survival of WEHI 231 cells in response to a variety of apoptotic stimuli. Furthermore, we and others have shown that the c-myc gene is potently transactivated by A-Myb in several cell types. Thus, we sought to determine whether c-Myc would mediate the A-Myb antiapoptotic effect in B cells. Here we show that ectopic expression of A-myb leads to maintenance of c-myc expression, and that expression of antisense c-myc RNA ablates A-Myb–mediated survival signals. Thus, these findings strongly implicate the A-myb gene in the regulation of B-cell survival and confirm the c-myc gene as one of the downstream targets of A-myb in these cells. Overall, our observation suggests that A-mybexpression may be relevant to the pathology of human B-cell neoplasias.

A-myb rescues murine B-cell lymphomas from IgM-receptor–mediated apoptosis through c-myctranscriptional regulation

A-myb is a member of the myb family of transcription factors, which regulates proliferation, differentiation, and apoptosis of hematopoietic cells. A-Myb expression is normally restricted to the proliferating B-cell centroblasts and transgenic mice overexpressing A-myb displayed enhanced hyperplasia of the lymph nodes. Because A-Myb is highly expressed in several subtypes of human B-cell neoplasias, we sought to determine whether the A-myb gene promoted proliferation and survival of B lymphocytes, using the WEHI 231 and CH33 murine B-cell lymphomas as models. Here, we show that ectopic expression of A-mybrescues WEHI 231 and CH33 cells from growth arrest and apoptosis induced by anti-IgM treatment. Previously, we demonstrated an essential role of the c-myc gene in promoting cell survival of WEHI 231 cells in response to a variety of apoptotic stimuli. Furthermore, we and others have shown that the c-myc gene is potently transactivated by A-Myb in several cell types. Thus, we sought to determine whether c-Myc would mediate the A-Myb antiapoptotic effect in B cells. Here we show that ectopic expression of A-myb leads to maintenance of c-myc expression, and that expression of antisense c-myc RNA ablates A-Myb–mediated survival signals. Thus, these findings strongly implicate the A-myb gene in the regulation of B-cell survival and confirm the c-myc gene as one of the downstream targets of A-myb in these cells. Overall, our observation suggests that A-mybexpression may be relevant to the pathology of human B-cell neoplasias.

Structural determinants for post-transcriptional stabilization of lactate dehydrogenase A mRNA by the protein kinase C signal pathway

Activation of protein kinase C (PKC) and protein kinase A (PKA) in rat C6 glioma cells increases the half-life of short-lived lactate dehydrogenase (LDH)-A mRNA about 5- and 8-fold, respectively. PKA and PKC act synergistically and prolong LDH-A mRNA half-life more than 21-fold. Similar effects were observed after transfection and transcription of a globin/lactate dehydrogenase minigene consisting of a beta-globin expression vector in which the 3'-untranslated region (UTR) of beta-globin had been replaced with the LDH-A 3'-UTR. Synergism was only obtained by transcription of minigenes containing the entire 3'-UTR and did not occur when truncated 3'-UTR fragments were analyzed. Additional mutational analyses showed that a 20-nucleotide region, named PKC-stabilizing region (PCSR), is responsible for mediating the stabilizing effect of PKC. Previous studies (Tian, D., Huang, D., Short, S., Short, M. L., and Jungmann, R. A. (1998) J. Biol. Chem. 273, 24861-24866) have demonstrated the existence of a cAMP-stabilizing region in LDH-A 3'-UTR. Sequence analysis of PCSR identified a 13-nucleotide AU-rich region that is common to both cAMP-stabilizing region and PCSR. These studies identify a specific PKC-responsive stabilizing element and indicate that interaction of PKA and PKC results in a potentiating effect on LDH-A mRNA stabilization.

Structural analysis of alpha-enolase. Mapping the functional domains involved in down-regulation of the c-myc protooncogene

Myc-binding protein-1 (MBP-1) is a 37-kDa protein with sequence homology to the 3' portion of the alpha-enolase gene. alpha-Enolase is a 48-kDa protein, which plays a critical role in the glycolytic pathway. MBP-1 binds to the c-myc P2 promoter and down-regulates c-myc expression. We have investigated the role of alpha-enolase in regulation of the c-myc protooncogene. RNase protection assay shows that alpha-enolase is transcribed into a single RNA species in HeLa cells. A start codon, 400 base pairs downstream of the alpha-enolase ATG, corresponds to the MBP-1 ATG, suggesting that MBP-1 is an alternative translation initiation product of the alpha-enolase RNA. Domain mapping was performed using constructs containing truncations of the alpha-enolase gene. In vitro binding to the c-myc gene was abolished after deletion of the N-terminal portion of alpha-enolase. In order to determine the relationship between DNA binding activity and transcription inhibition, we performed co-transfection assays in HeLa cells. These studies confirmed that an N-terminal deletion of alpha-enolase is unable to down-regulate c-myc promoter activity. Our data suggest that alpha-enolase plays an important role in regulation of c-myc promoter activity in the form of an alternative translation product MBP-1, which is distinct from its role as a glycolytic enzyme.

Increased p27Kip1 Cyclin-Dependent Kinase Inhibitor Gene Expression Following Anti-IgM Treatment Promotes Apoptosis of WEHI 231 B Cells

Engagement of the B cell receptor of WEHI 231 immature B cells leads sequentially to a drop in c-Myc, to induction of the cyclin-dependent kinase inhibitor p27Kip1, and finally to apoptosis. Recently we demonstrated that the drop in c-Myc expression promotes cell death, whereas the induction of p27 has been shown to lead to growth arrest. In this paper, we demonstrate that increased p27 expression also promotes apoptosis of WEHI 231 B cells. The rescue of WEHI 231 cells by CD40 ligand engagement of its receptor prevented the increase in p27 induction. Inhibition of p27-ablated apoptosis induced upon expression of antisense c-myc RNA. Furthermore, specific induction of p27 gene expression resulted in apoptosis of WEHI 231 cells. Lastly, inhibition of expression of c-Myc, upon induction of an antisense c-myc RNA vector, was sufficient to induce increased p27 levels and apoptosis. Thus, these findings define a signaling pathway during B cell receptor engagement in which the drop in c-Myc levels leads to an increase in p27 levels that promotes apoptosis.

Protein kinase A-regulated instability site in the 3'-untranslated region of lactate dehydrogenase-A subunit mRNA

Expression of the lactate dehydrogenase A subunit (LDH-A) gene can be controlled by transcriptional as well as posttranscriptional mechanisms. In rat C6 glioma cells, LDH-A mRNA is stabilized by activation and synergistic interaction of protein kinases A and C. In the present study, we aimed to identify the sequence domain which determines and regulates mRNA stability/instability by protein kinase A and focused our attention on the 3'-untranslated region (3'-UTR) of LDH-A mRNA. We have constructed various chimeric globin/lactate dehydrogenase (ldh) genes linked to the c-fos promoter and stably transfected them into rat C6 glioma cells. After their transfection, we determined the half-life of transcribed chimeric globin/ldh mRNAs. The results showed that at least three sequence domains within the LDH-A 3'-UTR consisting of nucleotides 1286-1351, 1453-1471, and 1471-1502 are responsible for the relatively rapid rate of LDH-A mRNA turnover in the cytoplasm. Whereas chimeric globin/ldh mRNAs containing the base sequences 1286-1351 and 1453-1471 were not stabilized by (Sp)-cAMPS, an activator of protein kinase A, instability caused by the 1471-1502 domain was significantly reversed. Additional deletion and mutational analyses demonstrated that the 3'-UTR fragment consisting of the 22 bases 1478-1499 is a critical determinant for the (Sp)-cAMPS-mediated LDH-A mRNA stabilizing activity. Because of its functional characteristics, we named the 22-base region "cAMP-stabilizing region."

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.

TGF beta 1 inhibits NF-kappa B/Rel activity inducing apoptosis of B cells: transcriptional activation of I kappa B alpha

TGF beta 1 treatment of B cell lymphomas decreases c-myc gene expression and induces apoptosis. Since we have demonstrated NF-kappa/Rel factors play a key role in transcriptional control of c-myc, we explored the effects of TGF beta1 on WEHI 231 immature B cells. A reduction in NF-kappa B/Rel activity followed TGF beta 1 treatment. In WEHI 231 and CH33 cells, we observed an increase in I kappa B alpha, a specific NF-kappa B/Rel inhibitor, due to transcriptional induction. Engagement of surface CD40 or ectopic c-Rel led to maintenance of NF-kappa B/Rel and c-Myc expression and protection of WEHI 231 cells from TGF beta 1-mediated apoptosis. Ectopic c-Myc expression overrode apoptosis induced by TGF beta 1. Thus, downmodulation of NF-kappa B/Rel reduces c-Myc expression, which leads to apoptosis in these immature B cell models of clonal deletion. The inhibition of NF-kappa B/Rel activity represents a novel TGF beta signaling mechanism.

mRNA stability in mammalian cells

This review concerns how cytoplasmic mRNA half-lives are regulated and how mRNA decay rates influence gene expression. mRNA stability influences gene expression in virtually all organisms, from bacteria to mammals, and the abundance of a particular mRNA can fluctuate manyfold following a change in the mRNA half-life, without any change in transcription. The processes that regulate mRNA half-lives can, in turn, affect how cells grow, differentiate, and respond to their environment. Three major questions are addressed. Which sequences in mRNAs determine their half-lives? Which enzymes degrade mRNAs? Which (trans-acting) factors regulate mRNA stability, and how do they function? The following specific topics are discussed: techniques for measuring eukaryotic mRNA stability and for calculating decay constants, mRNA decay pathways, mRNases, proteins that bind to sequences shared among many mRNAs [like poly(A)- and AU-rich-binding proteins] and proteins that bind to specific mRNAs (like the c-myc coding-region determinant-binding protein), how environmental factors like hormones and growth factors affect mRNA stability, and how translation and mRNA stability are linked. Some perspectives and predictions for future research directions are summarized at the end.

mRNA stability in mammalian cells

This review concerns how cytoplasmic mRNA half-lives are regulated and how mRNA decay rates influence gene expression. mRNA stability influences gene expression in virtually all organisms, from bacteria to mammals, and the abundance of a particular mRNA can fluctuate manyfold following a change in the mRNA half-life, without any change in transcription. The processes that regulate mRNA half-lives can, in turn, affect how cells grow, differentiate, and respond to their environment. Three major questions are addressed. Which sequences in mRNAs determine their half-lives? Which enzymes degrade mRNAs? Which (trans-acting) factors regulate mRNA stability, and how do they function? The following specific topics are discussed: techniques for measuring eukaryotic mRNA stability and for calculating decay constants, mRNA decay pathways, mRNases, proteins that bind to sequences shared among many mRNAs [like poly(A)- and AU-rich-binding proteins] and proteins that bind to specific mRNAs (like the c-myc coding-region determinant-binding protein), how environmental factors like hormones and growth factors affect mRNA stability, and how translation and mRNA stability are linked. Some perspectives and predictions for future research directions are summarized at the end.

The leukocyte integrin gene CD11c is transcriptionally regulated during monocyte differentiation

The leukocyte integrins, LFA-1, Mac-1 and p150,95, are heterodimeric proteins that consist of a distinct alpha and a common beta subunit. The beta subunit gene (CD18) is constitutively expressed on all leukocytes, however, the alpha subunit genes for LFA-1, Mac-1 and p150,95 (CD11a, CD11b and CD11c, respectively) show cell- and developmental stage-specific expression. We investigated the regulation of the CD11c gene in the promyeloblastic leukemic cell line, HL60, following differentiation along the monocytic pathway with phorbol 12-myristate 13-acetate (PMA). The steady-state level of CD11c mRNA increased markedly over 48 hr from the undetectable level present before differentiation. The half-life of CD11c MRNA in differentiated HL60 cells was not unusually long and similar to that of CD18 mRNA found in both undifferentiated and differentiated cells which suggested that altered mRNA stability did not account for the appearance of CD11c mRNA. Nuclear run-on analysis revealed that transcriptional activation during differentiation resulted in the appearance of CD11c mRNA. Inhibition of protein synthesis by cycloheximide in undifferentiated HL60 cells did not result in transcriptional activation of the CD11c gene. However, there was a significant increase (approximately eight-fold) in the steady-state level of CD18 mRNA which was not the result of transcriptional activation. Inhibition of protein synthesis in differentiated HL60 cells did not lead to significant changes in the steady-state levels of either CD11c or CD18 mRNAs. These findings indicated that the CD11c gene is regulated by transcriptional mechanisms which require prior protein synthesis. Transcriptional activation of the CD18 gene as a result of differentiation with PMA also requires protein synthesis. Further, in the absence of protein synthesis in undifferentiated HL60 cells, post-transcriptional mechanisms stabilize CD18 mRNA.

Induction of a herpesvirus saimiri small RNA AU binding factor (AUBF70) activity and lymphokine mRNAs by T cell mitogens

Herpesvirus saimiri (H. saimiri) can transform T lymphocytes and cause lymphoid tumors in rabbits and New World monkeys. H. saimiri-immortalized T cells express IL-2 and IL-4. The putative oncogenes of a group C strain of H. saimiri have been mapped to a region of the unique L-DNA which includes genes encoding four U-like small nuclear RNAs (HSUR1-HSUR4). Jurkat T cells express a 70 kD RNA binding factor (AUBF70) which binds HSUR2. Here we examined AUBF70 expression in resting and mitogen-stimulated human peripheral blood T cells and its sequence specificity and subcellular distribution. Band-shift assays demonstrated that resting human T cells express low amounts of AUBF70 which is induced by mitogen treatment. IL-2 and IL-4 mRNAs were co-induced with AUBF70 suggesting that AUBF70 is a positive regulator of lymphokine gene expression. Normal resting, mitogen-stimulated, and leukemic Jurkat T cells all express AUBF70 with virtually identical V8 proteolytic enzyme digestion patterns. Northern blots demonstrated that HSUR1 and HSUR2 are localized both in the nucleus and cytoplasm. HSUR2 accumulate in the cytoplasm in the presence of actinomycin D, which is consistent with re-transport of HSURs to the nucleus by (an) unstable factor(s). We hypothesize that HSUR1 and 2 transport AUBF70 from the cytoplasm to the nucleus; in the nucleus, AUBF70 binds and stabilizes lymphokine transcripts. Increased stability of lymphokine mRNAs could contribute to oncogenic transformation induced by H. saimiri.

Specific region of the c-myc promoter is responsive to electric and magnetic fields

The level of c-myc transcripts is increased in cells exposed to extremely low frequency (elf) electromagnetic (EM) fields at 60 Hz. The aim of the present experiments was to determine if regulatory regions upstream of the c-myc gene modulate the response to EM fields. DNA upstream of P1 of both mouse and human c-myc genes was transfected into cells as CAT constructs. The presence of DNA 5' to the human or mouse myc genes results in increased expression of CAT following 20 min exposures of cells to 60 Hz elf EM fields. Specific portions of the human upstream DNA were deleted and introduced into cells. The region responsive to EM fields is located between -353 and -1,257 relative to the P1 promoter.

Small RNA expression from the oncogenic region of a highly oncogenic strain of herpesvirus saimiri

Herpesvirus saimiri induces acute lymphomas and leukemias in primates and rabbits. Sequence divergence of the right end unique region of the genome classifies virus strains into three groups (A, B, and C), and previous studies have demonstrated correlation between DNA grouping and oncogenicity. In order to relate different oncogenicity to the underlying molecular mechanisms, we reported earlier the expression of a bicistronic mRNA from the oncogenic region in a highly oncogenic group C strain, and the present study is the first report on small RNA transcripts from the same region. The transcripts and 6.2 kbp on the oncogenic region were sequenced and characterized. We show that four U-type small RNAs are expressed in tumor cells transformed by this strain, in contrast to the seven small RNAs reported from a weakly oncogenic group A strain. Sequence comparisons between the two strains showed that the right end region of strain 484-77 of group C is about 1 kbp shorter. The conserved 5' AUUUA repeats of some small RNAs, and their proposed implication in lymphokine mRNA stabilization, are also discussed.

Analysis of topoisomerase II-mediated DNA cleavage of the c-myc gene during HL60 differentiation

We have investigated the effect of mAMSA, a potent topoisomerase II inhibitor, on the c-myc proto-oncogene of the acute promyelocytic leukemia HL60 cell line during its differentiation. When HL60 cells were induced by dimethylsulfoxide (DMSO) to terminally differentiate, a rapid drop in the level of c-myc mRNA was observed, followed by an arrest of cell proliferation. In contrast, the level of topoisomerase II mRNA was transiently increased with a maximum at 6 h after DMSO addition and was then completely abolished after 48 h, indicating that topoisomerase II is activated during the onset of HL60 differentiation. In exponentially growing cells, treatment by mAMSA results in the formation of topoisomerase II-mediated double strand DNA breaks in the c-myc gene at positions where topoisomerase II would normally nick and reseal the two strands. In HL60 cells treated with both mAMSA and DMSO, the sites in the c-myc gene at which mAMSA had induced cleavage were not altered. However, a DNA cleavage site located at the end of the first c-myc exon (position +3100), was strongly stimulated by mAMSA and DMSO treatment. This site fell within a DNase I hypersensitive region encompassing the MYC intron factor 1 (MIF1) binding site, where transcription elongation is normally blocked during differentiation. These data indicate that a change of topoisomerase II binding to critical regulatory region of the c-myc gene is associated with the downregulation of this gene during differentiation.

Expression of the V(D)J recombinase gene RAG-1 is tightly regulated and involves both transcriptional and post-transcriptional controls

The V(D)J recombinase activating genes, RAG-1 and RAG-2, are coexpressed only in immature lymphocytes, and are sufficient and necessary for V(D)J recombination to occur in non-lymphoid cells. In order to examine control mechanisms operative in the regulation of RAG-1 and RAG-2, we have studied the pattern of expression of these genes in human pre-T cells, pre-B cells, and thymocytes treated with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA); an agent which mimics some of the lymphocyte maturation changes seen in vivo. The expression of RAG-1 and RAG-2 was tightly controlled in a rapid, yet very complex, manner with both positive and negative control elements operating. Treatment of immature lymphocytes with TPA caused the specific and rapid elimination of steady-state RAG-1 and RAG-2 RNA. Nuclear run-on assays showed that TPA completely repressed the transcription of RAG-1 within 30 min. In addition to repressing the transcription of RAG-1, TPA treatment caused the rapid and specific degradation of RAG-1 transcripts by decreasing the apparent half-life of RAG-1 mRNA more than two-fold. As judged by cycloheximide treatment of cells, the effects of TPA were not dependent on new protein synthesis. A labile transcriptional repressor, separate from the TPA-associated repression of transcription, was also active in cells transcribing RAG-1 and RAG-2 RNA. After depletion of this labile repressor by cycloheximide treatment, steady-state RAG-1 and RAG-2 RNA levels, and their transcription rates, were elevated four- to six-fold; but were still susceptible to elimination by TPA treatment. Treatment of pre-T CEM cells with interleukin-2, or theophylline (an agent that increases intracellular cAMP) resulted in a two-fold increase in RAG-1 RNA suggesting that lymphokines, either independently or through second messengers, may modulate RAG-1 and RAG-2 expression. The complex, rapid and precise regulation of RAG-1 and RAG-2 expression is consistent with the view that it is necessary for the cell to tightly regulate V(D)J recombinase levels; lower expression may result in inefficient recombination of Ig/TCR genes, whereas increased expression may lead to recombination errors that are deleterious to the cell.

Egr-1 mRNA expression is independent of regulatory proliferative responses in the immature B cell line WEHI-231

We have recently reported cellular growth arrest induced following crosslinking of surface IgM (sIgM) but not surface IgD (sIgD) in the WEHI-231 cell line, representative of the immature B cell stage, and its delta heavy chain (delta) transfectant. An initial report has indicated WEHI-231.7, a subclone of WEHI-231, failed to express Egr-1 mRNA following sIgM crosslinking, in contrast to significant up-regulation found in mature B lymphocytes. The implication for linkage between selective surface immunoglobulin (sIg) signal transduction, expression of immediate/early genes and control of cellular growth imposes an attractive model for induction of immature B cell tolerance. Our investigations examined the relationships between Egr-1 mRNA expression and growth regulation in WEHI-231, WEHI-231.7 and their respective delta-transfectants (WEHI-delta, WEHI-delta 7). We report sIgM and sIgD crosslinking leads to a rapid increase of Egr-1 mRNA expression in WEHI-231 and WEHI-delta but not in the subclone WEHI-231.7 and WEHI-delta 7. Nevertheless, both WEHI-231, WEHI-231.7 and their delta-transfectants demonstrate the ability to induce growth arrest following sIgM but not sIgD crosslinking. Furthermore, we found Egr-1 expression could be achieved by direct activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) circumventing the classical sIg activated phosphatidylinositol signal transduction pathway. Our results suggest Egr-1 expression does not directly participate in growth regulation of immature B cell clones but rather is a consequence of signal transduction through sIg.

Multiple rearrangements and activated expression of c-myc induced by woodchuck hepatitis virus integration in a primary liver tumour

Woodchuck hepatitis virus (WHV) is a small, partially double-stranded DNA virus. Like the related human hepatitis B virus (HBV), WHV induces acute and chronic hepatitis and hepatocellular carcinoma (HCC) in its natural host. WHV DNA integration into c-myc and N-myc, resulting in deregulated expression of these genes, has been described previously in woodchuck HCC. We have analysed a woodchuck liver tumour in which WHV DNA was integrated in the c-myc gene. The virus insertion provoked multiple alterations in one c-myc allele, probably involving secondary deletions and mutations. Integrated viral DNA, including promotor and enhancer sequences, acted as an insertional mutagen, leading to enhanced expression of heterogenous c-myc transcripts ranging from 7.2 to 14 kb in size, strikingly longer than normal 2.3-kb c-myc RNA. These results provide an additional example in which the oncogenic activation of a myc gene by cis-acting effect of WHV insertion may play a critical role in virus-induced woodchuck HCC.

Regulation of expression of the growth-state-related genes 2F1 and 2A9 during entry of quiescent smooth muscle cells into the cell cycle

Vascular smooth muscle cells (SMCs) play a key role in the development of major arteries. Furthermore, abnormal growth of vascular smooth muscle cells has been implicated in the progression of major diseases of the cardiovascular system. Here, we report detection in primary cultures of bovine vascular smooth muscle cells of mRNA for two growth-state-related genes, 2F1 and 2A9, which code for a mitochondrial ADP/ATP carrier and calcyclin, respectively, and on the characterization of their cell cycle expression. Cultures of exponentially growing smooth muscle cells were made quiescent by serum deprivation. Upon readdition of serum, cells entered the cell cycle synchronously; DNA synthesis began 12 h post-serum addition. Levels of 2F1 and 2A9 RNA were low in quiescent cells and increased between 2 and 4 h post-serum addition. No changes in the rates of transcription of the 2F1 or 2A9 genes were detected by nuclear run-off assays during the time course. Thus the regulation of changes in expression of 2F1 and 2A9 in early G1 is mediated post-transcriptionally.

Regulation of gene expression by tumor promoters

Tumor promoters change the program of genes expressed in cells in culture and in the multicellular organism. The growing list of genes that are induced or repressed includes protooncogenes, transcription factors, secreted proteases and viruses. Most of the regulation is at the level of transcription. Several of the cis-acting promoter elements mediating regulation, the transcription factors binding to these elements and their post-translational activation, as well as some of the initial steps of the interaction of cells with tumor promoters have been characterized. The components of the signal transduction chain to the nucleus are, however, still unknown. Mutant and inhibitor studies suggest that the activation or inactivation of certain genes constitute the basis for the development of the tumor promotion phenotype.

c-myc gene expression in human cells is controlled by glucose

The c-myc oncogene is implicated in normal growth and differentiation processes. Human cell lines IM9 and HepG2 stably cultured at "low" glucose concentrations (5.5 mM) show c-myc mRNA levels 3-4 times higher than cells cultured at "high" glucose concentrations (25 nM). D-fructose (a metabolizable exose) substitutes for D-glucose in reducing c-myc expression while 3-ortho-methylglucose (a non metabolizable exose) is uneffective. c-myc expression is up-regulated (by PMA) or down-regulated (by dexamethasone and long-term exposure to FCS) in human cells cultured at "low" glucose but not in cells cultured at "high" glucose. We previously demonstrated that insulin receptor gene expression in human cell lines in enhanced by glucose. Therefore, glucose controls in an opposite way the expression of two genes important in the regulation of eukaryotic cell growth and differentiation.

The myc family of nuclear proto-oncogenes

Several members of the myc family of proto-oncogenes have been described, and some (c-, N-, and L-myc) have been characterized in considerable detail. They are united by a common gene structure and nucleotide homologies that were used to identify some of them initially. Their protein products also have scattered regions of amino acid identity or homology. Although the cellular activities of the various proteins are unknown, some members may play a role in regulating cell growth and differentiation. They share the ability to cooperate with an activated ras gene and cotransform embryonic rodent cells. In naturally occurring tumors, the members of the myc family of oncogenes appear to be activated by genetic changes (proviral insertion, chromosomal translocation, and gene amplification) that augment or otherwise disrupt normally regulated expression. The members of this family of genes differ markedly in their tissue specificity and developmental regulation of expression. This may account in part for the frequent appearance of activated c-myc genes in a wide variety of neoplasms and the limited appearance of activated N- and L-myc genes in tumors of embryonic or neural origin. The c-myc gene may be activated in tumors by a variety of mechanisms, whereas N- and L-myc appear to be activated only by gene amplification. Regulation of expression of the different myc genes also appears to occur by different mechanisms. Finally, the products of the different genes differ in may regions of the protein, and this divergence probably reflects their specific and individual functions.

Oncogenes modulate cellular gene expression and repress glucocorticoid regulated gene transcription

The v-mos oncogene was subjected to the transcriptional control of the MMTV LTR and introduced by transfection into NIH 3T3 cells. The LTR v-mos gene was induced by the addition of glucocorticoid hormone to the growth medium of cells synchronized by culturing in 1.5% FCS for 36 h. The effects of p37 v-mos expression were monitored. The endogenous c-myc gene is induced as a consequence of p37 v-mos expression in a transient fashion, reaching a maximum of expression after 8 h. Induction of the c-myc gene was observed at the level of its transcriptional rate and at the level of mRNA concentration. Ornithine decarboxylase (ODC) mRNA was induced constitutively and high levels were found 8 and 25 h after v-mos induction. H4 histone mRNA is elevated at 25 h after hormone addition at a time when the mitogenic stimulus of v-mos causes DNA synthesis. The expression of actin mRNA is not affected by the v-mos oncogene. We have previously described a modulation of glucocorticoid dependent gene expression by oncogenes. In an extension of these observations the consequences of expression of the v-mos and the v-ras oncogenes were also studied in retrovirally infected NIH 3T3 cells. MMTV LTR constructs transfected into the infected cells could only be transiently induced by glucocorticoid hormone. The presence of the p37 v-mos and the p21 v-ras oncoproteins causes a repression of glucocorticoid hormone dependent gene transcription.