Upstream stimulatory factor regulates expression of the cell cycle-dependent cyclin B1 gene promoter

CCAAT binding factor (CBF) binding mediates cell cycle activation of topoisomerase IIalpha. Conventional CBF activation domains are not required

To understand the role of the CCAAT binding factor (CBF) in transcription during the cell cycle, we studied the mouse topoisomerase II alpha (topo II alpha) promoter, which is activated during the late S and G(2)/M phases of the cell cycle and contains multiple CBF binding sites. Mutational analysis of the promoter shows that CBF binding to an inverted orientation of the CCAAT motif in the topo II alpha promoter, but not to a direct orientation, is required for transcription activation during the cell cycle. In contrast, analysis of the promoter in an in vitro reconstituted transcription system shows that CBF activates transcription of the topo II alpha promoter irrespective of the orientation of the CBF binding sites. This analysis demonstrates that only one of the three transcription start sites of the topo II alpha promoter is stimulated by CBF, indicating that transcription activation by CBF is dependent on basal promoter structure. Interestingly, mutations of the start site that abolish CBF-dependent transcription activation in vitro do not inhibit activation of the promoter during the cell cycle. Consistent with this observation, expression of a truncated CBF-B subunit lacking a transcription activation domain, which inhibits activity of a collagen promoter, does not affect activity of the topo II alpha promoter in fibroblast cells. In contrast, expression of an allele-specific CBF-B mutant that binds high affinity to a mutant CBF binding site containing a CCAAC motif revives transcription activation of an inactive mutant topo II alpha promoter containing CCAAC during the cell cycle. Altogether, this study indicates that CBF binding, but not conventional CBF activation domains, are required for activation of the topo II alpha promoter during the cell cycle. Considering these results together with results of another recent study, we hypothesize that binding of CBF that disrupts the nucleosomal structure in the topo II alpha promoter is a major function of CBF by which it regulates the cell cycle-dependent transcription of the topo II alpha promoter and possibly many other cell cycle-regulated promoters containing CBF binding sites.

Repression of IRF-4 target genes in human T cell leukemia virus-1 infection

The human T cell leukemia/lymphotropic virus-1 (HTLV-I) is the etiologic agent of adult T cell leukemia (ATL), an aggressive and fatal leukemia of CD4+ T lymphocytes. Interferon regulatory factor-4 (IRF-4) was shown previously to be constitutively expressed in T cells infected with HTLV-1. In this study, we investigated the role of IRF-4 gene regulation in the context of HTLV-1 infection using gene array technology and IRF-4 expressing T cells. Many potential IRF-4 regulated genes were identified, the vast majority of which were repressed by IRF-4 expression. Cyclin B1, a G2-M checkpoint protein identified as an IRF-4 repressed gene in the array, was further characterized in the context of HTLV-1 infection. All HTLV-1 infected cell lines and ATL patient lymphocytes demonstrated a dramatic decrease in cyclin B1 levels; subsequent analysis of the cyclin B1 promoter identified two sites important in IRF-4 binding and repression of cyclin B1 expression. Furthermore, IRF-4-mediated repression of cyclin B1 led to a significant decrease in CDC2 kinase activity in HTLV-1 infected T cells. IRF-4 expression in HTLV-1 infected T cells also downregulated other genes implicated in the mitotic checkpoint as well as genes involved in actin cytoskeletal rearrangement, DNA repair, apoptosis, metastasis and immune recognition. Several of the identified genes are dysregulated in ATL and may provide important mechanistic information concerning pathways critical to the emergence of ATL.

USF-1 and USF-2 trans-repress IL-1beta-induced iNOS transcription in mesangial cells

Transcriptional activation of the inducible nitric oxide synthase (iNOS) gene requires multiple interactions of cis elements and trans-acting factors. Previous in vivo footprinting studies (Goldring CE, Reveneau S, Algarte M, and Jeannin JF. Nucleic Acids Res 24: 1682-1687, 1996) of the murine iNOS gene demonstrated lipopolysaccharide-inducible protection of guanines in the region -904/-883, which includes an E-box motif. In this report, by using site-directed mutagenesis of the -893/-888 E-box and correlating functional assays of the mutated iNOS promoter with upstream stimulatory factor (USF) DNA-binding activities, we demonstrate that the -893/-888 E-box motif is functionally required for iNOS regulation in murine mesangial cells and that USFs are in vivo components of the iNOS transcriptional response complex. Mutation of the E-box sequence augmented the iNOS response to interleukin-1beta (IL-1beta) in transiently transfected mesangial cells. Gel mobility shift assays demonstrated that USFs cannot bind to the -893/-888 E-box promoter region when the E-box is mutated. Cotransfection of USF-1 and USF-2 expression vectors with iNOS promoter-luciferase reporter constructs suppressed IL-1beta-simulated iNOS promoter activity. Cotransfection of dominant-negative USF-2 mutants lacking the DNA binding domain or cis-element decoys containing concatamers of the -904/-883 region augmented IL-1beta stimulation of iNOS promoter activity. Gel mobility shift assays showed that only USF-1 and USF-2 supershifted the USF protein-DNA complexes. These results demonstrated that USF binding to the E-box at -893/-888 serves to trans-repress basal expression and IL-1beta induction of the iNOS promoter.

Epithelial monolayer wounding stimulates binding of USF-1 to an E-box motif in the plasminogen activator inhibitor type 1 gene

Several proteases and their co-expressed inhibitors modulate the interdependent processes of cell migration and matrix proteolysis during wound repair. Transcription of the gene encoding plasminogen activator inhibitor type 1 (PAI-1), a serine protease inhibitor important in the control of barrier proteolysis and cell-to-matrix adhesion, is spatially-temporally regulated following epithelial denudation injury in vitro as well as in vivo. Using a well-defined culture model of acute epidermal wounding and reepithelialization, PAI-1 mRNA/protein synthesis was induced early after monolayer scraping and restricted to cells comprising the motile cohort. PAI-1 levels in locomoting cells remained elevated (relative to the distal, contact-inhibited monolayer regions) throughout the time course of trauma repair. Targeted PAI-1 downregulation by transfection of antisense PAI-1 expression constructs significantly impaired keratinocyte migration and monolayer scrape wound closure. Injury-induced PAI-1 transcription closely paralleled growth state-dependent controls on the PAI-1 gene. An E-box motif (CACGTG) in the PAI-1 proximal promoter (located at nucleotides -160 to -165), previously shown to be necessary for serum-induced PAI-1 expression, was bound by nuclear factors from wound-stimulated but not quiescent, contact-inhibited, keratinocytes. UV crosslinking approaches to identify E-box-binding factors coupled with deoxyoligonucleotide affinity chromatography and gel retardation assays confirmed at least one major E-box-binding protein in both serum- and wound-activated cells to be USF-1, a member of the helix-loop-helix family of transcription factors. An intact hexanucleotide E-box motif was necessary and sufficient for USF-1 binding using nuclear extracts from both serum- and wound-simulated cells. Two species of immunoreactive USF-1 were identified by western blotting of total cellular lysates that corresponded to the previously characterized phosphorylated and non-phosphorylated forms of the protein. USF-1 isolated by PAI-1 promoter-DNA affinity chromatography was almost exclusively phosphorylated. Only a fraction of the total cellular USF-1 in proliferating cultures, by comparison, was phosphorylated at any given time. PAI-1 E-box binding activity, assessed by probe mobility shift criteria, increased within 2 hours of monolayer scrape injury, a time frame consistent with wound-stimulated increases in PAI-1 transcription. Relative to intact cultures, scrape site-juxtaposed cells had significantly greater cytoplasmic and nuclear USF-1 immunoreactivity correlating with the specific in situ-restricted expression of PAI-1 transcripts/protein in the wound-edge cohort. USF-1 immunocytochemical staining declined significantly with increasing distance from the denudation site. These data are the first to indicate that binding of USF-1 to its target motif can be induced by 'tissue' injury in vitro and implicate USF-1 as a transcriptional regulator of genes (e.g. PAI-1) involved in wound repair.

Circadian Transcription. Thinking outside the E-Box

The E-Box is a widely used DNA control element. Despite its brevity and broad distribution the E-Box is a remarkably versatile sequence that affects many different genetic programs, including proliferation, differentiation, tissue-specific responses, and cell death. The circadian clock is one of the latest pathways shown to employ this element. In this context, E-Boxes are likely to play a key role in establishing the robust waves of gene expression characteristic of circadian transcription. The regulatory flexibility of the E-Box hinges on the sequence ambiguity allowed at its core, the strong influence of the surrounding sequences, and the recruitment of spatially and temporally regulated E-Box-binding factors. Therefore, understanding how a particular E-Box can accomplish a specific task entails the identification and systematic analysis of these cis- and trans-acting E-Box modifiers. In the present study we compared the E-Box-containing minimal promoters of vasopressin and cyclin B1, two genes that can respond to the transcriptional oscillators driving the circadian clock and cell cycle, respectively. Results of this comparison will help elucidate the manner in which discreet DNA modules associate to either augment or restrain the activation of potential circadian E-Boxes in response to competing regulatory signals.

Characterization of cis-acting element involved in cell cycle phase-independent activation of Arath;CycB1;1 transcription and identification of putative regulatory proteins

Progression through the cell cycle is driven by cyclin-dependent kinases (CDKs) whose activity is controlled by regulatory subunits called cyclins. The expression of cyclins is subject to numerous controls at multiple levels, not least at the level of transcription. As a first step to unravel the mechanisms that regulate expression of B-cyclins in plants, we undertook the identification of the required promoter elements of the Arath;CycB1;1 gene. A detailed analysis of different promoter fragments consisted in analysing their ability to mediate cell cycle-dependent transcriptional oscillations of the gus reporter gene in transformed BY-2 cell lines. We showed that different promoter regions took part in transcriptional activation. Furthermore, 202 bp upstream of the ATG were sufficient to induce M-phase-specific expression. This region contains an 18 bp sequence including a Myb-binding core (AACGG) which is able to activate reporter gene without leading to M-phase-specific expression. Electrophoretic mobility shift assays showed that this 18 bp sequence specifically binds protein complexes from Arabidopsis cell suspension enriched either in G1 or G2 phase. Furthermore, the Myb core, AACGG, was characterized as necessary for the binding of proteins. DNA affinity purification of the complexes bound to the 18 bp sequence allowed the isolation of three different complexes and two proteins from these complexes were identified by mass spectrometry analyses. A new putative Myb transcription factor and a hypothetical protein, HYP containing with a leucine zipper and Myc-type dimerization domains were identified. When over-expressed in plants, HYP factor is able to trans-activate the expression of gus reporter gene downstream from the -202 promoter fragment as well as the endogenous CycB1;1 gene.

Role of USF1 phosphorylation on cardiac alpha-myosin heavy chain promoter activity

Contractile activity of the cardiac myocyte is required for maintaining cell mass and phenotype, including expression of the cardiac-specific alpha-myosin heavy chain (alpha-MHC) gene. An E-box hemodynamic response element (HME) located at position -47 within the alpha-MHC promoter is both necessary and sufficient to confer contractile responsiveness to the gene and has been shown to bind upstream stimulatory factor-1 (USF1). When studied in spontaneously contracting cardiac myocytes, there is enhanced binding of USF1 to the HME compared with quiescent cells, which correlates with a threefold increase in alpha-MHC promoter activity. A molecular mechanism by which contractile function modulates alpha-MHC transcriptional activity may involve signaling via phosphorylation of USF1. The present studies showed that purified rat USF1 was phosphorylated in vitro by protein kinase C (PKC) and cAMP-dependent protein kinase (PKA) but not casein kinase II. Phosphorylated USF1 by either PKC or PKA had increased DNA binding activity to the HME. PKC-mediated phosphorylation also leads to the formation of USF1 multimers as assessed by gel shift assay. Analysis of in vivo phosphorylated nuclear proteins from cultured ventricular myocytes showed that USF1 was phosphorylated, and resolution by two-dimensional gel electrophoresis identified at least two distinct phosphorylated USF1 molecules. These results suggest that endogenous kinases can covalently modify USF1 and provide a potential molecular mechanism by which the contractile stimulus mediates changes in myocyte gene transcription.

Cell cycle specific changes in the human cyclin B1 gene regulatory region as revealed by response to trichostatin A

The human cyclin Bl gene is cell cycle regulated with maximal activity during G(2)/M. We examined the role of histone deacetylation in cyclin Bl regulation using the histone deacetylase inhibitor trichostatin A (TSA). TSA treatment (100 ng/ml) of NIH3T3 cells containing the luciferase reporter construct pCycB(-287)-LUC caused an increase in promoter activity in G(0) and G(1) but no significant change in G(2). Removal of upstream sequences including an E-box and Sp1 site eliminated the TSA induced increase in G(0) and G(1), and caused a decrease in promoter activity during S and G(2). Promoter activity increased only 2-fold following TSA treatment of G(0) cells containing the construct pCycB(MUT-E-Box)-LUC with an E-box mutation, and a decrease in activity was detected during G(2). We conclude that histone deacetylation contributes to the repression of cyclin B1 expression in G(0) and G(1), and that this mechanism requires, in part, the E-box. TSA reduction of cyclin B1 promoter activity in G(2), however, involves sequences within the first 119 bp. A working model for cyclin B1 regulation is provided.

Relationship between expression of genes involved in cell cycle control and apoptosis in diffuse large B cell lymphoma: a preferential survivin-cyclin B link

Accumulating evidence suggests that lack of balance between proliferation and apoptosis may lead to clonal expansion and cancer emergence. In diffuse large B cell lymphoma (DLBCL), survivin expression by tumor cells has been recently described as a poor prognostic marker. We assessed the relationship between survivin gene up-regulation and several other factors involved in either cell cycle or apoptosis control. The expression of 34 genes from 27 cases of DLBCL with typical IPI factor-related poor prognostic outcome was analyzed by RNase protection assay. Using non-neoplastic tissues and low grade lymphomas as control, survivin expression was high in 80% of the cases without significant relation to patient overall survival (P = 0.64). However, the expression of several genes encoding for cell cycle inhibitors, cyclins, Bcl-2 or IAP family factors was significantly associated with the survivin up-regulation. Gene expression profiling showed that both survivin and cyclin B expression can define two subgroups of DLBCL: the previously described germinal center-like and activated B-like lymphomas, determined by protein expression analysis. We also identified a preferential survivin-cyclin B relationship (P = 0.017), suggesting that cyclin B over-expression, when linked to survivin over-expression in aggressive forms of lymphoma, might demonstrate a specific G2/M transition promotion.

Mxi1 inhibits the proliferation of U87 glioma cells through down-regulation of cyclin B1 gene expression

Mxi1 is a Mad family member that plays a role in cell proliferation and differentiation. To test the role of Mxi1 on tumorigenesis of glioma cells we transfected a CMV-driven MXI1 cDNA in U87 human glioblastoma cells. Two clones were isolated expressing MXI1 levels 18- and 3.5-fold higher than wild-type U87 cells (clone U87.Mxi1.14 and U87.Mxi1.22, respectively). In vivo, U87.Mxi1.14 cells were not tumorigenic in nude mice and delayed development of tumours was observed with U87.Mxi1.22 cells. In vitro, the proliferation rate was partially and strongly inhibited in U87.Mxi1.22 and U87.Mxi1.14 cells respectively. The cell cycle analysis revealed a relevant accumulation of U87.Mxi1.14 cells in the G(2)/M phase. Interestingly, the expression of cyclin B1 was inhibited to about 60% in U87.Mxi1.14 cells. This inhibition occurs at the transcriptional level and depends, at least in part, on the E-box present on the cyclin B1 promoter. Consistent with this, the endogenous Mxi1 binds this E-box in vitro. Thus, our findings indicate that Mxi1 can act as a tumour suppressor in human glioblastomas through a molecular mechanism involving the transcriptional down-regulation of cyclin B1 gene expression.

The cyclin B1 gene is actively transcribed during mitosis in HeLa cells

In mammalian cells, the expression level of the cyclin B1 gene plays a critical role in the progression through mitosis. Here we demonstrate that the transcriptional activity of the human cyclin B1 promoter, as well as the rate of gene transcription, is high during mitosis. Indeed, the cyclin B1 promoter maintains an open chromatin configuration at the mitotic stage. Consistent with this, we show that the cyclin B1 promoter is occupied and bound to NF-Y during mitosis in vivo. Our results provide the first example of RNA polymerase II-dependent transcription during mitosis in mammalian cells.

CBF/NF-Y functions both in nucleosomal disruption and transcription activation of the chromatin-assembled topoisomerase IIalpha promoter. Transcription activation by CBF/NF-Y in chromatin is dependent on the promoter structure

To understand the role of CCAAT-binding factor (CBF) in transcription in the context of chromatin-assembled DNA, we used regularly spaced nucleosomal DNA using topoisomerase IIalpha (topo IIalpha) and alpha2(1) collagen promoter templates, which were subsequently reconstituted in an in vitro transcription reaction. Binding of CBF to the nucleosomal wild-type topo IIalpha promoter containing four CBF-binding sites disrupted the regular nucleosomal structure not only in the promoter region containing the CBF-binding sites but also in the downstream region over the transcription start site. In contrast, no nucleosome disruption was observed in a mutant topo IIalpha promoter containing mutations in all CBF-binding sites. Interestingly, CBF also activated transcription from nucleosomal wild-type topo IIalpha promoter. In this experiment, a promoter containing one wild-type CBF-binding site was activated very weakly, whereas the promoter containing mutations in all sites was not activated by CBF. A truncated CBF that lacked the glutamine-rich domains did not activate transcription from nucleosomal wild-type topo IIalpha promoter but disrupted the nucleosomal structure about as much as did the binding of full-length CBF. Two nucleosomal mouse alpha2(1) collagen promoter DNAs, one containing a single and the other containing four CBF- binding sites, were also reconstituted in an in vitro transcription reaction. None of the nucleosomal collagen promoters was activated by CBF. However, both of these collagen promoters were activated by CBF when the transcription reaction was performed using naked DNA templates. Binding of CBF to the nucleosomal collagen promoter containing four binding sites disrupted the nucleosomal structure, similarly as observed in the topo IIalpha promoter. Altogether this study indicates that CBF-mediated nucleosomal disruption occurred independently of transcription activation. It also suggests that specific promoter structure may play a role in the CBF-mediated transcription activation of nucleosomal topo IIalpha promoter template.

Forkhead transcription factors contribute to execution of the mitotic programme in mammals

Cell cycle progression is a process that is tightly controlled by internal and external signals. Environmental cues, such as those provided by growth factors, activate early signals that promote cell cycle entry. Cells that have progressed past the restriction point become independent of growth factors, and cell cycle progression is then controlled endogenously. The phosphatidylinositol 3OH kinase (PI(3)K)/protein kinase B (PKB) pathway must be activated in G1 to inactivate forkhead transcription factors (FKH-TFs) and allow cell cycle entry. Here we show that subsequent attenuation of the PI(3)K/PKB pathway is required to allow transcriptional activation of FKH-TF in G2. FKH-TF activity in G2 controls mammalian cell cycle termination, as interference with FKH transcriptional activation by disrupting PI(3)K/PKB downregulation, or by expressing a transcriptionally inactive FKH mutant, induces cell accumulation in G2/M, defective cytokinesis, and delayed transition from M to G1 of the cell cycle. We demonstrate that FKH-TFs regulate expression of mitotic genes such as cyclin B and polo-like kinase (Plk). Our results support the important role of forkhead in the control of mammalian cell cycle completion, and suggest that efficient execution of the mitotic programme depends on downregulation of PI(3)K/PKB and consequent induction of FKH transcriptional activity.

Increased levels of forkhead box M1B transcription factor in transgenic mouse hepatocytes prevent age-related proliferation defects in regenerating liver

The forkhead box (Fox) family of transcription factors share homology in the winged helix/forkhead DNA-binding domain and play important roles in regulating cellular proliferation, differentiation, longevity, and cellular transformation. Forkhead box M1B (FoxM1B) is a ubiquitously expressed member of the Fox transcription factor family whose expression is restricted to proliferating cells and that mediates hepatocyte entry into DNA synthesis and mitosis during liver regeneration. Recent cDNA microarray studies indicated that age-related defects in cellular proliferation are associated with diminished expression of the FoxM1B transcription factor. Here, we show that increased levels of FoxM1B in regenerating liver of old transgenic mice restore the sharp peaks in hepatocyte DNA replication and mitosis that are the hallmarks of young regenerating mouse liver. Restoration of the young regenerating liver phenotype is associated with increased expression of numerous cell cycle regulatory genes that include cyclin D1, cyclin A2, cyclin F, cyclin B1, cyclin B2, Cdc25B, and p55cdc. Cotransfection assays in the human hepatoma HepG2 cell line demonstrated that FoxM1B protein stimulated expression of both the cyclin B1 and cyclin D1 promoters, suggesting that these cyclin genes are a direct FoxM1B transcriptional target. These results suggest that FoxM1B controls the transcriptional network of genes that are essential for cell division and exit from mitosis. Our results indicate that reduced expression of the FoxM1B transcription factor contributes to the decline in cellular proliferation observed in the aging process.

G2/M-phase-specific transcription during the plant cell cycle is mediated by c-Myb-like transcription factors

Plant B-type cyclin genes are expressed specifically in late G2- and M-phases during the cell cycle. Their promoters contain a common cis-acting element, called the MSA (M-specific activator) element, that is necessary and sufficient for periodic promoter activation. This motif also is present in the tobacco kinesin-like protein gene NACK1, which is expressed with timing similar to that of B-type cyclin genes. In this study, we show that G2/M-phase-specific activation of the NACK1 promoter also is regulated by the MSA element, suggesting that a defined set of G2/M-phase-specific genes are coregulated by an MSA-mediated mechanism. In a search for MSA binding factors by yeast one-hybrid screening, we identified three different Myb-like proteins that interact specifically with the MSA sequence. Unlike the majority of plant Myb-like proteins, these Myb proteins, NtmybA1, NtmybA2, and NtmybB, have three imperfect repeats in the DNA binding domain, as in animal c-Myb proteins. During the cell cycle, the level of NtmybB mRNA did not change significantly, whereas the levels of NtmybA1 and A2 mRNAs fluctuated and peaked at M-phase, when B-type cyclin genes were maximally induced. In transient expression assays, NtmybA1 and A2 activated the MSA-containing promoters, whereas NtmybB repressed them. Furthermore, expression of NtmybB repressed the transcriptional activation mediated by NtmybA2. Our data show that a group of plant Myb proteins that are structurally similar to animal c-Myb proteins have unexpected roles in G2/M-phase by modulating the expression of B-type cyclin genes and may regulate a suite of coexpressed genes.

G2/M-phase-specific transcription during the plant cell cycle is mediated by c-Myb-like transcription factors

Plant B-type cyclin genes are expressed specifically in late G2- and M-phases during the cell cycle. Their promoters contain a common cis-acting element, called the MSA (M-specific activator) element, that is necessary and sufficient for periodic promoter activation. This motif also is present in the tobacco kinesin-like protein gene NACK1, which is expressed with timing similar to that of B-type cyclin genes. In this study, we show that G2/M-phase-specific activation of the NACK1 promoter also is regulated by the MSA element, suggesting that a defined set of G2/M-phase-specific genes are coregulated by an MSA-mediated mechanism. In a search for MSA binding factors by yeast one-hybrid screening, we identified three different Myb-like proteins that interact specifically with the MSA sequence. Unlike the majority of plant Myb-like proteins, these Myb proteins, NtmybA1, NtmybA2, and NtmybB, have three imperfect repeats in the DNA binding domain, as in animal c-Myb proteins. During the cell cycle, the level of NtmybB mRNA did not change significantly, whereas the levels of NtmybA1 and A2 mRNAs fluctuated and peaked at M-phase, when B-type cyclin genes were maximally induced. In transient expression assays, NtmybA1 and A2 activated the MSA-containing promoters, whereas NtmybB repressed them. Furthermore, expression of NtmybB repressed the transcriptional activation mediated by NtmybA2. Our data show that a group of plant Myb proteins that are structurally similar to animal c-Myb proteins have unexpected roles in G2/M-phase by modulating the expression of B-type cyclin genes and may regulate a suite of coexpressed genes.

The upstream stimulatory factor-2a inhibits plasminogen activator inhibitor-1 gene expression by binding to a promoter element adjacent to the hypoxia-inducible factor-1 binding site

Plasminogen activator inhibitor-1 (PAI-1) expression is induced by hypoxia (8% O(2)) via the PAI-1 promoter region -175/-159 containing a hypoxia response element (HRE-2) binding the hypoxia-inducible factor-1 (HIF-1) and an adjacent response element (HRE-1) binding a so far unknown factor. The aim of the present study was to identify this factor and to investigate its role in the regulation of PAI-1 expression. It was found by supershift assays that the upstream stimulatory factor-2a (USF-2a) bound mainly to the HRE-1 of the PAI-1 promoter and to a lesser extent to HRE-2. Overexpression of USF-2a inhibited PAI-1 messenger RNA and protein expression and activated L-type pyruvate kinase expression in primary rat hepatocytes under normoxia and hypoxia. Luciferase (Luc) gene constructs driven by 766 and 276 base pairs of the 5'-flanking region of the PAI-1 gene were transfected into primary hepatocytes together with expression vectors encoding wild-type USF-2a and a USF-2a mutant lacking DNA binding and dimerization activity (DeltaHU2a). Cotransfection of the wild-type USF-2a vector reduced Luc activity by about 8-fold, whereas cotransfection of DeltaHU2a did not influence Luc activity. Mutation of the HRE-1 (-175/-168) in the PAI-1 promoter Luc constructs decreased USF-dependent inhibition of Luc activity. Mutation of the HRE-2 (-165/-158) was less effective. Cotransfection of a HIF-1alpha vector could compete for the binding of USF at HRE-2. These results indicated that the balance between 2 transcriptional factors, HIF-1 and USF-2a, which can bind adjacent HRE sites, appears to be involved in the regulation of PAI-1 expression in many clinical conditions.

Induction of the BRCA2 promoter by nuclear factor-kappa B

BRCA2 is a tumor suppressor gene that has been implicated in response to DNA damage, cell cycle control, and transcription. BRCA2 has been found to be overexpressed in many breast tumors, suggesting that altered expression of the BRCA2 gene may contribute to breast tumorigenesis. To determine how BRCA2 is overexpressed in tumors, we investigated the transcriptional regulation of the BRCA2 promoter. Deletion mapping of the BRCA2 promoter identified three regions associated with 3-fold activation or repression and one upstream stimulatory factor binding site associated with 20-fold activation. Gel shift and cotransfection studies verified the role of USF in regulation of BRCA2 transcription. Analysis of the -144 to -59 region associated with 3-fold activation identified a putative NFkappaB binding site. Cotransfection of the p65 and p50 subunits of NFkappaB up-regulated the BRCA2 promoter 16-fold in a luciferase reporter assay, whereas mutations in the binding site ablated the effect. Gel shift and supershift assays with anti-p65 and -p50 antibodies demonstrated that NFkappaB binds specifically to the NFkappaB site. In addition, ectopic expression of NFkappaB resulted in increased levels of endogeneous BRCA2 expression. Thus, NFkappaB and USF regulate BRCA2 expression through the BRCA2 promoter.

Cell cycle-dependent regulation of TIAP/m-survivin expression

TIAP, a murine homologue of human survivin, is a member of the inhibitor of apoptosis (IAP) family and is specifically expressed at G2/M phase of the cell cycle. To elucidate regulatory mechanisms of the cycle-dependent expression, we have analyzed the promoter region of TIAP/mouse survivin (m-survivin). The 5'-flanking region of the TIAP/m-survivin gene contained a TATA-less promoter, two AP2 sites, three NF-kB sites, one Sp1 site, many cell cycle-dependent elements (CDEs) and one cell cycle gene homology region (CHR). Primer extension and 5'-rapid amplification of cDNA ends identified one transcription start site at position -100 upstream of the ATG start site (+1). TIAP/m-survivin promoter-luciferase analysis identified a minimal promoter region within the most proximal -271 bp upstream of the ATG start site, and the region between -410 and -272 was critical for the enhancer activity. The combination between the CHR at -51 and the CDE at -57 is also essential for the cell cycle-dependent expression. Mutation of the CDE/CHR element and the enhancer elements may cause disordered expression of TIAP/m-survivin to affect cell survival and oncogenesis.

Genomic organization and promoter characterization of the rat cyclin B1 gene

Cyclin B1 is a key regulatory protein involved in cellular mitosis. We have cloned 1.8kb of DNA sequence upstream of the rat cyclin B1 gene translation start site from Rattus norvegicus liver genomic DNA and a commercial rat testis genomic library. The mRNA transcription start point (tsp) was determined by primer extension and mRNA end ligation followed by RT-PCR across the ligated 3' and 5' ends. An authentic tsp was confirmed approximately 100bp upstream of the translation start site. A second potential tsp was also detected approximately 32bp downstream from the first. RT-PCR analysis of rat liver poly(A)(+) RNA using 5'-derived oligonucleotide primers indicated that the 5' end sequence was present in both the 1.6 and 2. 4kb rat liver cyclin B1 mRNA species. Like many other cyclin promoters, there was no apparent TATA box upstream of the transcription initiation sites. However, computer analysis of the promoter region identified a group of consensus transcription factor binding sites, some of which are also reported in other cyclin promoters. These include those for p53, p21, Ap-1, Ap-2, Ets-1, CAATT, E-Box and Yi. We also performed luciferase reporter assays using a set of promoter deletion constructs in human HuH-7 hepatoma and HeLa carcinoma cell lines. Our results suggest that an E-Box and/or CCAAT binding sites are important for transcription, and that there may be negative regulatory elements present between 1800 and 1100bp upstream of the translation start site.

Factors controlling cyclin B expression

Cyclins control the transition between the phases of the eukaryotic cell cycle as regulatory subunits of the cyclin-dependent kinases (CDKs). Phase-specific activation of the CDK is in part regulated by phase-specific expression of their cyclin component. In most eukaryotic cells including higher plant, B-type cyclin genes are expressed specifically at G2/M phase during the cell cycle. Promoters from yeast, plant and animal B-type cyclin genes are all activated in a cell cycle-regulated manner. In yeast, a transcription factor, Mcm1, in cooperation with an uncloned factor SFF, regulates the cell cycle-dependent promoter activation of mitotic B-type cyclin genes, CLB1 and CLB2. Activity of the human cyclin B1 promoter is regulated by a complex mechanism involving multiple cis-acting elements, none of which are sufficient for G2/M-specific promoter activation. In contrast, plants employ a simple mechanism for cell cycle-regulated promoter activation of B-type cyclin genes. Plant B-type cyclin gene promoters contain a common cis-acting element, called the MSA element, which is necessary and sufficient for the phase-specific promoter activation. MSA-like sequences are also found in the promoters of G2/M-specific genes encoding kinesin-like proteins, suggesting that a defined set of G2/M-specific genes are co-regulated by a common MSA-mediated mechanism in plants. Thus, the molecular mechanisms regulating B-type cyclin gene expression are evolutionarily divergent, and the MSA-mediated mechanism seems to be specific to plants. The consensus sequence of the MSA element resembles the binding sites of animal Myb transcription factors. A set of our data suggest the possibility that plant Myb may have unexpected roles in G2/M by inducing B-type cyclin genes, together with other cell cycle-related genes in plants.

Two yeast forkhead genes regulate the cell cycle and pseudohyphal growth

There are about 800 genes in Saccharomyces cerevisiae whose transcription is cell-cycle regulated. Some of these form clusters of co-regulated genes. The 'CLB2' cluster contains 33 genes whose transcription peaks early in mitosis, including CLB1, CLB2, SWI5, ACE2, CDC5, CDC20 and other genes important for mitosis. Here we find that the genes in this cluster lose their cell cycle regulation in a mutant that lacks two forkhead transcription factors, Fkh1 and Fkh2. Fkh2 protein is associated with the promoters of CLB2, SWI5 and other genes of the cluster. These results indicate that Fkh proteins are transcription factors for the CLB2 cluster. The fkh1 fkh2 mutant also displays aberrant regulation of the 'SIC1' cluster, whose member genes are expressed in the M-G1 interval and are involved in mitotic exit. This aberrant regulation may be due to aberrant expression of the transcription factors Swi5 and Ace2, which are members of the CLB2 cluster and controllers of the SIC1 cluster. Thus, a cascade of transcription factors operates late in the cell cycle. Finally, the fkh1 fkh2 mutant displays a constitutive pseudohyphal morphology, indicating that Fkh1 and Fkh2 may help control the switch to this mode of growth.

Abundance of cyclin B1 regulates γ-radiation–induced apoptosis

γ-Radiation is a potent inducer of apoptosis. There are multiple pathways regulating DNA damage-induced apoptosis, and we set out to identify novel mechanisms regulating γ-radiation–induced apoptosis in hematopoietic cells. In this report, we present data implicating the cyclin B1 protein as a regulator of apoptotic fate following DNA damage. Cyclin B1 is the regulatory subunit of the cdc2 serine/threonine kinase, and accumulation of cyclin B1 in late G2 phase of the cell cycle is a prerequisite for mitotic initiation in mammalian cells. We find that abundance of the cyclin B1 protein rapidly increases in several mouse and human hematopoietic cells (Ramos, DP16, HL60, thymocytes) undergoing γ-radiation–induced apoptosis. Cyclin B1 accumulation occurs in all phases of the cell cycle. Antisense inhibition of cyclin B1 accumulation decreases apoptosis, and ectopic cyclin B1 expression is sufficient to induce apoptosis. These observations are consistent with the idea that cyclin B1 is both necessary and sufficient for γ-radiation-induced apoptosis.

Abundance of cyclin B1 regulates γ-radiation–induced apoptosis

γ-Radiation is a potent inducer of apoptosis. There are multiple pathways regulating DNA damage-induced apoptosis, and we set out to identify novel mechanisms regulating γ-radiation–induced apoptosis in hematopoietic cells. In this report, we present data implicating the cyclin B1 protein as a regulator of apoptotic fate following DNA damage. Cyclin B1 is the regulatory subunit of the cdc2 serine/threonine kinase, and accumulation of cyclin B1 in late G2 phase of the cell cycle is a prerequisite for mitotic initiation in mammalian cells. We find that abundance of the cyclin B1 protein rapidly increases in several mouse and human hematopoietic cells (Ramos, DP16, HL60, thymocytes) undergoing γ-radiation–induced apoptosis. Cyclin B1 accumulation occurs in all phases of the cell cycle. Antisense inhibition of cyclin B1 accumulation decreases apoptosis, and ectopic cyclin B1 expression is sufficient to induce apoptosis. These observations are consistent with the idea that cyclin B1 is both necessary and sufficient for γ-radiation-induced apoptosis.

The mammalian Fizzy and Fizzy-related genes are regulated at the transcriptional and post-transcriptional levels

The cyclosome pathway of ubiquitin-mediated proteolysis plays an essential role in cell cycle control. The multisubunit cyclosome is regulated by transient interactions with Fizzy (Fzy) and Fizzy-related (Fzr) genes. We report here that both Fzy and Fzr are transcribed in a cell cycle specific but distinct manner. Fzy transcription starts after the restriction point in late G1 and ceases upon cell division. Fzr transcription also ceases upon cell division but resumes already in mid G1, before the restriction point, and takes place also in G0. Fzr has further a striking cell cycle specific pattern of mRNA stability. During most of the cell cycle its message is fairly stable, however upon exit from mitosis it is rapidly degraded. This result is puzzling because Fzr is essential for cyclosome activity in G1, and points to a complex pattern of Fzr regulation.

Transcriptional up-regulation of the cyclin D2 gene and acquisition of new cyclin-dependent kinase partners in human T-cell leukemia virus type 1-infected cells

Human T-cell leukemia virus type 1 (HTLV-1) is the etiologic agent for adult T-cell leukemia/lymphoma (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis. Tax(1) is a 40-kDa phosphoprotein, predominantly localized in the nucleus of the host cell, which functions to transactivate both viral and cellular promoters. It seems likely that HTLV-1, through expression of the viral regulatory protein Tax(1), provides some initial alteration in cell metabolism predisposing the development of ATL. Here, we demonstrate that HTLV-1 infection in T-cell lines and patient samples causes overexpression of an early G(1) cyclin, cyclin D2. The transcriptional up-regulation of the cyclin D2 gene is due to activation of Tax on the cyclin D2 gene. More important, we find that overexpression of cyclin D2 is accompanied by acquisition of new partners such as cyclin-dependent kinase 2 (cdk2), cdk4, and cdk6 in infected cells. This is in contrast to uninfected T cells, where cyclin D2 associates only with cdk6. Functional effects of these cyclin-cdk complexes in infected cells are shown by hyperphosphorylation of Rb and histone H1, indicators of active progression into S phase as well as changes in cellular chromatin and transcription machinery. These studies link HTLV-1 infection with changes of cellular cyclin gene expression, hence providing clues to development of T-cell leukemia.

DNA-binding activity of the transcription factor upstream stimulatory factor 1 (USF-1) is regulated by cyclin-dependent phosphorylation

The ubiquitous transcription factor upstream stimulatory factor (USF) 1 is a member of the bzHLH (leucine zipper-basic-helix-loop-helix) family, which is structurally related to the Myc family of proteins. It plays a role in the regulation of many genes, including the cyclin B1 gene, which is active during the G2/M and M phases of the cell cycle and may also play a role in the regulation of cellular proliferation. We show that the affinity of recombinant USF-1 for DNA is greatly increased by treatment with active cyclin A2-p34(cdc2) or cyclin B1-p34(cdc2) complexes and that its interaction with DNA is dependent on p34(cdc2)-mediated phosphorylation. We have localized the phosphorylation site(s) to a region that lies outside the minimal DNA-binding domain but overlaps with the previously identified USF-specific region. Deletion studies of USF-1 suggest that amino acids 143-197 regulate DNA-binding activity in a phosphorylation-dependent manner.

Isolation and initial characterization of the BRCA2 promoter

The hereditary breast cancer susceptibility gene, BRCA2, is considered to be a tumor suppressor gene that may be involved in the cellular response to DNA damage. The transcript for this gene is cell cycle regulated with mRNA levels reaching a peak just before the onset of DNA synthesis. In order to define the mechanisms by which BRCA2 is transcriptionally regulated, we have begun to study upstream regulatory sequences. In this report, we define a minimal promoter region that has strong activity in human breast epithelial cells. Deletions of this sequence narrowed the strong basal activity to a region extending from -66 to +129 with respect to the BRCA2 transcriptional start site. This sequence demonstrated cell cycle regulated activity with kinetics similar to the endogenous transcript. Examination of the sequence revealed several consensus binding sites for transcription factors including an E-box, E2F and Ets recognition motifs. Electrohoretic mobility shift assays revealed specific protein binding to two sequences upstream of the start site; the palindromic E-box and an Ets/E2F site. Site-directed mutagenesis of either of these sites reduced both the basal activity in log phase cells and the cell cycle regulated activity of the promoter. Mutational inactivation of both sites within the same construct effectively eliminated promoter activity. Antibodies to candidate transcription factors used in super shift experiments revealed specific interactions between the BRCA2 promoter and the basic region/helix - loop - helix containing USF-1 and 2 proteins and Elf-1, an Ets domain protein. Binding of these factors depended upon the presence of intact recognition sequences. The USF factors were shown to bind predominantly as a heterodimeric complex of USF-1 and 2 while Elf-1 bound the promoter when it was not occupied by USF. Co-transfection studies with USF proteins and the varicella zoster IE62 protein provide evidence for the involvement of endogenous and exogenous USF in the activation of the BRCA2 promoter. We propose that interactions between USF-1, USF-2 and Elf-1 play an important role in the transcriptional regulation of the BRCA2 gene.

Human transcription factor hTAF(II)150 (CIF150) is involved in transcriptional regulation of cell cycle progression

Here we present evidence that CIF150 (hTAF(II)150), the human homolog of Drosophila TAF(II)150, plays an important and selective role in establishing gene expression patterns necessary for progression through the cell cycle. Gel filtration experiments demonstrate that CIF150 (hTAF(II)150) seems to be less tightly associated with human transcription factor IID than hTAF(II)130 is associated with hTAF(II)250. The transient functional knockout of CIF150 (hTAF(II)150) protein led to cell cycle arrest at the G(2)/M transition in mammalian cell lines. PCR display analysis with the RNA derived from CIF150-depleted cells indicated that CIF150 (hTAF(II)150) is required for the transcription of only a subset of RNA polymerase II genes. CIF150 (hTAF(II)150) directly stimulated cyclin B1 and cyclin A transcription in cotransfection assays and in vitro assays, suggesting that the expression of these genes is dependent on CIF150 (hTAF(II)150) function. We defined a CIF150 (hTAF(II)150) consensus binding site and demonstrated that a CIF150-responsive cis element is present in the cyclin B1 core promoter. These results suggest that one function of CIF150 (hTAF(II)150) is to select specific RNA polymerase II core promoter elements involved in cell cycle progression.

Protein kinase C modulates aryl hydrocarbon receptor nuclear translocator protein-mediated transactivation potential in a dimer context

Protein kinase C (PKC)- and protein kinase A (PKA)-mediated modulation of the transactivation potential of human aryl hydrocarbon receptor nuclear translocator (hARNT), a basic helix-loop-helix (bHLH)-PAS transcription factor, and the bHLH-ZIP transcription factors USF-1 (for upstream regulatory factor 1) and c-Myc were examined. An 81 nM dose of the PKC activator phorbol-12-myristate-13-acetate (PMA), shown here to specifically activate PKC in COS-1 cells, or a 1 nM dose of the PKA activator 8-bromoadenosine-3',5'-cyclic monophosphate (8-Br-cAMP) results in 2. 6- and 1.9-fold enhancements, respectively, in hARNT-mediated transactivation of the class B, E-box-driven reporter pMyc3E1bLuc relative to identically transfected, carrier solvent-treated COS-1 cells. In contrast, 81 nM PMA and 1 nM 8-Br-cAMP did not enhance transactivation of pMyc3E1bLuc-driven by USF-1 and c-Myc expression relative to identically transfected, carrier-treated COS-1 cells. Co-transfection of pcDNA3/ARNT-474-Flag, expressing a hARNT carboxyl-terminal transactivation domain deletion, and pMyc3E1bLuc does not result in induction of reporter activity, suggesting PMA's effects do not involve formation of unknown hARNT-protein heterodimers. Additionally, PMA had no effect on hARNT expression relative to Me2SO-treated cells. Metabolic 32P labeling of hARNT in cells treated with carrier solvent or 81 nM PMA demonstrates that PMA does not increase the overall phosphorylation level of hARNT. These results demonstrate, for the first time, that the transactivation potential of ARNT in a dimer context can be specifically modulated by PKC or PKA stimulation and that the bHLH-PAS and bHLH-ZIP transcription factors are differentially regulated by these pathways in COS-1 cells.

Regulation of cdc2 gene expression by the upstream stimulatory factors (USFs)

cdc2 gene expression is under the control of multiple factors. Although E2F/DP proteins have been reported to play a central role, they cannot account for all aspects of the fine modulation of cdc2 gene expression during cell cycle and embryonic development. To characterize the transcription factors that control cdc2 gene expression during nerve cell differentiation in avians, we have previously cloned the quail cdc2 gene promoter region. We had identified an octamer (CAGGTGGC) containing an E-box, which has important activity in regulating cdc2 transcription. Using in vivo genomic footprinting experiments, we show here that this motif, currently named IG, is the target of binding proteins at different stages of neuroretina development, confirming its importance as a regulatory response element for cdc2 gene expression. A subset of Helix-Loop-Helix family of transcription factors, known as Upstream Stimulatory Factors (USFs) specifically bind to this sequence as dimers. Moreover, our results indicate that USFs transactivate the promoter of cdc2 via the IG motif. These data may help to better understand the mechanisms that control cell division in differentiating nerve cells.

p53 regulates a G2 checkpoint through cyclin B1

The p53 tumor suppressor controls multiple cell cycle checkpoints regulating the mammalian response to DNA damage. To identify the mechanism by which p53 regulates G2, we have derived a human ovarian cell that undergoes p53-dependent G2 arrest at 32 degrees C. We have found that p53 prevents G2/M transition by decreasing intracellular levels of cyclin B1 protein and attenuating the activity of the cyclin B1 promoter. Cyclin B1 is the regulatory subunit of the cdc2 kinase and is a protein required for mitotic initiation. The ability of p53 to control mitotic initiation by regulating intracellular cyclin B1 levels suggests that the cyclin B-dependent G2 checkpoint has a role in preventing neoplastic transformation.

Cell-type-dependent activity of the ubiquitous transcription factor USF in cellular proliferation and transcriptional activation

USF1 and USF2 are basic helix-loop-helix transcription factors implicated in the control of cellular proliferation. In HeLa cells, the USF proteins are transcriptionally active and their overexpression causes marked growth inhibition. In contrast, USF overexpression had essentially no effect on the proliferation of the Saos-2 osteosarcoma cell line. USF1 and USF2 also lacked transcriptional activity in Saos-2 cells when assayed by transient cotransfection with USF-dependent reporter genes. Yet, there was no difference in the expression, subcellular localization, or DNA-binding activity of the USF proteins in HeLa and Saos-2 cells. Furthermore, Gal4-USF1 and Gal4-USF2 fusion proteins activated transcription similarly in both cell lines. Mutational analysis and domain swapping experiments revealed that the small, highly conserved USF-specific region (USR) was responsible for the inactivity of USF in Saos-2 cells. In HeLa, the USR serves a dual function. It acts as an autonomous transcriptional activation domain at promoters containing an initiator element and also induces a conformational change that is required for USF activity at promoters lacking an initiator. Taken together, these results suggest a model in which the transcriptional activity of the USF proteins, and consequently their antiproliferative activity, is tightly controlled by interaction with a specialized coactivator that recognizes the conserved USR domain and, in contrast to USF, is not ubiquitous. The activity of USF is therefore context dependent, and evidence for USF DNA-binding activity in particular cells is insufficient to indicate USF function in transcriptional activation and growth control.

Cell cycle-dependent usage of transcriptional start sites. A novel mechanism for regulation of cyclin B1

Cyclin B1 mRNA is expressed temporally throughout the cell cycle with peak expression in G2 and M phase. Both transcriptional and posttranscriptional controls are important for this cell cycle-dependent regulation of cyclin B1 mRNA. In this study, we observed that cyclin B1 has two major transcripts: (a) a constitutively expressed transcript, and (b) a cell cycle-regulated transcript expressed predominantly during G2-M phase. These different transcripts are due to alternative start sites. The constitutively expressed transcript starts 65 bases upstream from the cell cycle-regulated message. Changes in mRNA stability did not appear to control the expression of the cell cycle-specific transcript, but we were able to identify a 24-base pair region of the cyclin B1 promoter spanning the start site of the cell cycle-regulated transcript that was critical for its cell cycle-regulated promoter activity. This suggests that transcriptional regulation is responsible for controlling the presence of each message. The 24-base pair sequence required for cell cycle regulation was notable for containing the nucleotides GGCT repeated three times. The possibility that these two transcripts might be physiologically distinct was raised when the cell cycle-specific transcript was found to be translated more efficiently in vitro than the constitutively expressed transcript. These results characterize a novel mechanism for the regulation of cyclin B1 throughout the cell cycle that is dependent upon the use of different transcriptional start sites.

A role for Sp and helix-loop-helix transcription factors in the regulation of the human Id4 gene promoter activity

Id family helix-loop-helix (HLH) proteins are involved in the regulation of proliferation and differentiation of several cell types. To identify cis- and trans-acting factors that regulate Id4 gene expression, we have analyzed the promoter regulatory sequences of the human Id4 gene in transient transfections and gel mobility shift assays. We have identified two functional elements, both located downstream from the TATA motif, that control Id4 promoter activity. One element contains a consensus E-box, and we demonstrated that the protein complex binding to the E-box contains the bHLH-zip upstream stimulatory factor (USF) transcription factor. Enforced expression of USF1 leads to E-box-mediated stimulation of promoter activity. The E-box also mediated stimulatory effects of several bHLH transcription factors, and co-expression of Id4 blocked the stimulatory effect mediated by the bHLH factors. A second element is a GA motif, located downstream from the transcriptional start sites, mutation of which resulted in a 20-fold increase in transcriptional activity. Gel-shift analysis and transfections into Drosophila Schneider SL2 cells showed that the repressor element is recognized by both Sp1 and Sp3 factors. These data suggest that Id4 transcription control is highly complex, involving both negative and positive regulatory elements, including a novel inhibitory function exerted by Sp1 and Sp3 transcription factors.

A novel cis-acting element in promoters of plant B-type cyclin genes activates M phase-specific transcription

Plant B-type cyclin genes are expressed late in the G2 and M phases of the cell cycle. Previously, we showed that the promoter of a Catharanthus roseus B-type cyclin, CYM, could direct M phase-specific transcription of a beta-glucuronidase reporter gene in synchronously dividing BY2 tobacco cells. In this study, we determined the regulatory elements contained within the CYM promoter by using a luciferase reporter gene. Mutational analysis showed that a 9-bp element is essential for M phase-specific promoter activity in synchronized BY2 cells. The CYM promoter contains three other sequences similar to this element. A gain-of-function assay demonstrated that when fused to a heterologous promoter, these elements are sufficient for M phase-specific expression; therefore, we named these elements M-specific activators (MSAs). We found MSA-like sequences in B-type cyclin promoters from tobacco, soybean, and Arabidopsis as well as in the promoters of two M phase-specific genes, NACK1 and NACK2, which encode tobacco kinesin-like proteins. Thus, MSA may be a common cis-acting promoter element that controls M phase-specific expression of cell cycle-related genes in plants.

The potent lipid mitogen sphingosylphosphocholine activates the DNA binding activity of upstream stimulating factor (USF), a basic helix-loop-helix-zipper protein

We previously demonstrated that the sphingolipid, sphingosylphosphocholine (SPC) increased DNA binding activity of AP-1 proteins accompanying cellular proliferation. Herein, the effects of SPC on DNA binding activity and transcription of the basic, helix-loop-helix, leucine zipper (bHLH-ZIP) proteins Myc, Max, and USF were investigated because they regulate genes involved in mitogenesis. E-box (CACGTG) DNA binding proteins were detected by electrophoretic mobility shift assays in nuclear extracts from Swiss 3T3 fibroblasts. The slowest migrating complex (complex I) increased within 1-3 min after treatment with SPC, remained elevated for 10 min, and increased again after 12 h. Complexes I and II contained USF-1 and USF-2 proteins, and complex I migrated similarly to recombinant USF-1 protein/DNA complex. Treatment of nuclear extracts with alkaline phosphatase decreased these complexes suggesting USF might be a phosphoprotein, post-translationally modified by SPC. max and usf-1 mRNA levels were unaffected by SPC treatment. In contrast, c-myc mRNA was rapidly elevated, reached maximum levels at 0.5-1 h, and showed an additional increase after 12 h, just preceding S phase. Thus, certain bHLH-ZIP transcription factors may be involved in cell growth regulation by SPC.

PU.1/Pip and Basic Helix Loop Helix Zipper Transcription Factors Interact With Binding Sites in the CD20 Promoter to Help Confer Lineage- and Stage-Specific Expression of CD20 in B Lymphocytes

CD20 is a B-lineage–specific gene expressed at the pre–B-cell stage of B-cell development that disappears on differentiation to plasma cells. As such, it serves as an excellent paradigm for the study of lineage and developmental stage-specific gene expression. Using in vivo footprinting we identified two sites in the promoter at −45 and −160 that were occupied only in CD20+ B cells. The −45 site is an E box that binds basic helix-loop-helix-zipper proteins whereas the −160 site is a composite PU.1 and Pip binding site. Transfection studies with reporter constructs and various expression vectors verified the importance of these sites. The composite PU.1 and Pip site likely accounts for both lineage and stage-specific expression of CD20 whereas the CD20 E box binding proteins enhance overall promoter activity and may link the promoter to a distant enhancer.

The basic helix-loop-helix-zipper transcription factor USF1 regulates expression of the surfactant protein-A gene

Expression of the rabbit pulmonary surfactant protein A (SP-A) gene is lung-specific, occurs primarily in type II cells, and is developmentally regulated. We previously identified two E-box-like enhancers, termed the distal binding element (DBE) and proximal binding element (PBE), in the 5'-flanking region of the rabbit SP-A gene. In the present study, the PBE was used to screen a rabbit fetal lung cDNA expression library; a cDNA insert was isolated which is highly similar in sequence to human upstream stimulatory factor 1 (hUSF1). By use of reverse transcription polymerase chain reaction, two isoforms of rabbit USF1 (rUSF1) mRNAs were identified in fetal rabbit lung and other tissues. The levels of rUSF1 mRNAs reach a peak in fetal rabbit lung at 23 days gestation, in concert with the time of initiation of SP-A gene transcription. Binding complexes of nuclear proteins obtained from fetal rabbit lung tissue and isolated type II cells with the DBE and PBE were supershifted by the addition of anti-rUSF1 IgG. Binding activity was enriched in type II cells compared with lung fibroblasts. Overexpression of rUSF1s in A549 adenocarcinoma cells positively regulated SP-A promoter activity of cotransfected reporter gene constructs. It is suggested that rUSF1s, which bind to two E-box elements in the SP-A gene 5'-flanking region, may serve a key role in the regulation of SP-A gene expression in pulmonary type II cells.

MyoD functions as a transcriptional repressor in proliferating myoblasts

The myogenic basic helix-loop-helix (myo-bHLH) proteins are a family of transcriptional regulators expressed in myoblasts and differentiated skeletal muscle. Ectopic expression of myo-bHLH regulators transdetermines some fibroblast cell lines into myoblasts, which exit the cell cycle and differentiate into skeletal muscle when cultured in low mitogen medium. While members of the myo-bHLH family have been shown to function as transcriptional activators in differentiating muscle, the molecular basis of their function in proliferating myoblasts has not been elucidated. In this report, we present evidence that MyoD functions as a transcriptional repressor in myoblasts. We show that transcription from a cyclin B1 promoter construct is repressed in proliferating myoblasts and that repression is mediated by a pair of MyoD binding sites. We also show that transcription from the cyclin B1 promoter is repressed in proliferating C3H10T1/2 cells by ectopic expression of MyoD. These results demonstrate that MyoD can repress transcription of specific genes in proliferating cells, a novel function that may be important to maintenance of the myogenic phenotype and to cell cycle regulation in myoblasts.

Binding of upstream stimulatory factor to an E-box in the 3'-flanking region stimulates alpha1(I) collagen gene transcription

Since several lines of evidence implicate the 3'-flanking region in regulating alpha1(I) collagen gene transcription, we analyzed 12. 4-kilobase pairs of 3'-flanking sequence of the murine alpha1(I) collagen gene for transcriptional elements. A region of the 3'-flanking region stimulated expression of the heterologous beta-globin gene promoter in an enhancer trap plasmid and of the alpha1(I) collagen gene promoter in a collagen-luciferase reporter gene construct when located 3' to the luciferase reporter gene. DNase I footprinting analysis demonstrated the presence of three regions where DNA binding proteins specifically interact within this 3'-stimulatory region. Inspection of the DNA sequence revealed a consensus E-box, a binding site for basic helix-loop-helix proteins, in one of the protein binding sites. Mobility shift assays demonstrated that upstream stimulatory factors (USF) USF-1 and USF-2 bind to this E-box. Mutating the E-box in the context of the 3'-flanking region confirmed that it contributes to the enhancement of transcriptional activity of the alpha1(I) collagen gene promoter. Mutations in all three protein binding sites abolished transcriptional activation by the 3'-flanking region, suggesting a complex interaction among the trans-acting factors in enhancing transcriptional activity. Thus, a region of the 3'-flanking region of the alpha1(I) collagen gene stimulates transcription of the alpha1(I) collagen gene promoter, and USF-1 and USF-2 contribute to this transcriptional stimulation.

Immortal, non-tumourigenic mouse mammary outgrowths express high levels of cyclin B1 and activation of cyclin B1/cdc2 kinase

Neoplastic transformation of mouse mammary epithelial cells is the result of several identifiable phenotypic changes which presumably require sequential genetic alterations. In our model system, mammary cells progress from a mortal state (virgin duct) to several morphologically distinct intermediate states. The intermediate states are distinct cell populations that are phenotypically identified as immortal, non-tumourigenic (i.e. EL11), weakly tumourigenic ductal/alveolar hyperplasia (i.e. EL12) and moderately tumourigenic alveolar hyperplasiaa (i.e. TM12) to invasive tumours (i.e. EL12T/TM12T). We have studied the changes in total cyclin A and B1 levels, cyclin A and B1 complexed to cdc2, cyclin B1cdc2 kinase activity and cyclin D proteins in EL11 and EL12 immortalized outgrowth lines. Results revealed increased levels in total cyclin B1 (> 5-fold), cyclin B1/cdc2 (3-4-fold) and cyclin B1/cdc2 kinase activity (2-3.5-fold) in EL11 and EL12 phenotypes when compared to control mammary gland (virgin). No changes in the levels of total cyclin A or cycln A associated to cdc2 were observed. Cyclin D1, D2 and D3 protein levels were low in the EL11 immortal ductal outgrowth. Exposure to hormones via a pituitary isograft stimulated the synthesis of cyclin D1 and D2 but not D3 associated to cdk4 as well as total cdk4 proteins. Bromodeoxyuridine (BrdUrd) labelling indices showed marked increases in immortal ductal outgrowths (EL11 and EL12) when compared to virgin, suggesting that epithelial cells are cycling in these cell populations. Even in the presence of hormone stimulation, EL11 outgrowths were not tumourigenic, suggesting that other events are necessary to drive the cells to a tumourigenic phenotype. The results suggest that increased levels of cyclin B1 and cyclin B1-cdc2 kinase activities are early events and may be an important marker for the immortalized phenotype.

Mouse USF1 gene cloning: comparative organization within the c-myc gene family

Upstream stimulatory factors (USF/MLTF) belong to the c-myc family of transcription factors. Through binding to target DNA as dimers, the ubiquitous USF proteins regulate a variety of genes. USF proteins are encoded by two genes, USF1 and USF2. Protein sequences of USF1 and 2 are highly homologous across species, suggesting functional conservation. To determine whether the genomic organization was conserved between USF1 and USF2, we isolated the murine USF1 gene and characterized its genomic structure. Both genes are similarly organized in 10 exons spanning over 10 kbp. By the 5'-rapid amplification of cDNA ends and S1 nuclease mapping methods, exon 1 was defined and the transcription initiation sites were mapped. The sequence of 8 kb of the gene, including 1.75 kb of 5'-flanking DNA, was determined. The promoter region is GC rich and lacks a typical TATA or CCAAT element. Strikingly, a comparison of the murine and human untranslated sequences reveals regions that exhibit greater than 73% sequence identity. A genomic alignment of the dimerization and DNA binding domains is presented for five genes of the c-myc family, suggesting a hypothetical common ancestor gene.

The proteolysis of mitotic cyclins in mammalian cells persists from the end of mitosis until the onset of S phase

We have studied how the cell cycle-specific oscillations of mitotic B-type cyclins are generated in mouse fibroblasts. A reporter enzyme comprising the N-terminus of a B-type cyclin fused to bacterial chloramphenicol acetyl transferase (CAT) was degraded at the end of mitosis like endogenous cyclins. Point mutations in the destruction box of this construct completely abolished its mitotic instability. When the destructible reporter was driven by the cyclin B2 promoter, CAT activity mimicked the oscillations in the level of the endogenous cyclin B2. These oscillations were largely conserved when the reporter was transcribed constitutively from the SV40 promoter. Pulse-chase experiments or addition of the proteasome inhibitors lactacystin and ALLN showed that cyclin synthesis continued after the end of mitosis. The destruction box-specific degradation of cyclins normally ceases at the onset of S phase, and is active in fibroblasts arrested in G0 and in differentiated C2 myoblasts. We were able to reproduce this proteolysis in vitro in extracts of synchronized cells. Extracts of G1 cells degraded cyclin B1 whereas p27Kip1 was stable, in contrast, cyclin B1 remained stable and p27Kip1 was degraded in extracts of S phase cells.

The basic region/helix-loop-helix/leucine repeat transcription factor USF interferes with Ras transformation

Upstream stimulatory factor (USF) is a transcription factor of the basic region/helix-loop-helix/leucine repeat family. It shares the same DNA-binding sequence as the myc oncogene. Based on the three-dimensional structures, its DNA-binding domain is structurally related to that of Max, the partner of Myc. In addition, USF can form heterodimers with a related factor, Fos-interacting protein/upstream stimulatory factor 2 (FIP/USF2), which has been shown to directly interact with Fos. In view of the provocative relationship of USF with other factors involved in cell proliferation, we investigated whether USF could also play a role in cellular growth control. In this study, we report that USF is not an oncogene, but interferes with Ras-driven transformation. This inhibitory effect is independent of USF transactivating domains, but requires its DNA-binding activity. However, the minimal USF DNA-binding domain does not display this inhibitory effect, and even slightly enhances Ras transformation. On the basis of these data, we propose that USF may play an important role in the control of cell growth and proliferation, through both binding to promoter sequences and specific protein/protein interactions.

Two Arabidopsis cyclin promoters mediate distinctive transcriptional oscillation in synchronized tobacco BY-2 cells

Cyclins are cell cycle regulators whose proteins oscillate dramatically during the cell cycle. Cyclin steady-state mRNA levels also fluctuate, and there are indications that both their rate of transcription and mRNA stability are under cell cycle control. Here, we demonstrate the transcriptional regulation of higher eukaryote cyclins throughout the whole cell cycle with a high temporal resolution. The promoters of two Arabidopsis cyclins, cyc3aAt and cyc1At, mediated transcriptional oscillation of the beta-glucuronidase (gus) reporter gene in stably transformed tobacco BY-2 cell lines. The rate of transcription driven by the cyc3aAt promoter was very low during G1, slowly increased during the S phase, peaked at the G2 phase and G2-to-M transition, and was down-regulated before early metaphase. In contrast, the rate of the cyc1At-related transcription increased upon exit of the S phase, peaked at the G2-to-M transition and during mitosis, and decreased upon exit from the M phase. This study indicates that transcription mechanisms that seem to be conserved among species play a significant role in regulating the mRNA abundance of the plant cyclins. Furthermore, the transcription patterns of cyc3aAt and cyc1At were coherent with their slightly higher sequence similarity to the A and B groups of animal cyclins, respectively, suggesting that they may fulfill comparable roles during the cell cycle.

Antiproliferative properties of the USF family of helix-loop-helix transcription factors

USF is a family of transcription factors characterized by a highly conserved basic-helix-loop-helix-leucine zipper (bHLH-zip) DNA-binding domain. Two different USF genes, termed USF1 and USF2, are ubiquitously expressed in both humans and mice. The USF1 and USF2 proteins contain highly divergent transcriptional activation domains but share extensive homologies in the bHLH-zip region and recognize the same CACGTG DNA motifs. Although the DNA-binding and transcriptional activities of these proteins have been characterized, the biological function of USF is not well understood. Here, focus- and colony-formation assays were used to investigate the potential involvement of USF in the regulation of cellular transformation and proliferation. Both USF1 and USF2 inhibited the transformation of rat embryo fibroblasts mediated by Ras and c-Myc, a bHLH-zip transcription factor that also binds CACGTG motifs. DNA binding was required but not fully sufficient for inhibition of Myc-dependent transformation by USF, since deletion mutants containing only the DNA-binding domains of USF1 or USF2 produced partial inhibition. While the effect of USF1 was selective for Myc-dependent transformation, wild-type USF2 exerted in addition a strong inhibition of E1A-mediated transformation and a strong suppression of HeLa cell colony formation. These results suggest that members of the USF family may serve as negative regulators of cellular proliferation in two ways, one by antagonizing the transforming function of Myc, the other through a more general growth-inhibitory effect.

Cell cycle-dependent regulation of the cyclin B1 promoter

Cyclin B1 mRNA expression varies through the cell cycle with its peak in G2/M. In cycling mammalian cells, its lowest level is in G1 with a steady increase in S until a level 50-fold greater than that in G1 is reached. In order to characterize the transcriptional component to this variation in expression, we cloned the upstream region 872 base pairs upstream from the start site of the cyclin B1 gene and have demonstrated that it confers cell cycle-dependent regulation onto two reporter genes, both chloramphenicol acetyltransferase and luciferase. Its activity was 25-fold greater in G2/M than in G1 in HeLa cells with intermediate activity in S. This cyclical activity could be seen with sequences encompassing only 90 base pairs upstream from the start site. Protein binding to this region was demonstrated using electrophoretic mobility shift assays, and the binding profiles appeared to vary depending upon the phase of the cycle in which the extracts are made. Thus, transcriptional control plays an important role in determining cyclin B1 mRNA levels, and cell cycle-dependent activity is regulated through interactions with the region 90 bases upstream from the start site.