Displacement of BrdUrd-induced YY1 by serum response factor activates skeletal alpha-actin transcription in embryonic myoblasts

Angiotensin II-induced stimulation of smooth muscle alpha-actin expression by serum response factor and the homeodomain transcription factor MHox

The objective of the present study was to examine the molecular mechanisms whereby angiotensin II (Ang II) stimulates smooth muscle (SM) alpha-actin expression in rat aortic smooth muscle cells (SMCs). Nuclear run-on analysis and transfection studies indicated that the effects of Ang II on SM alpha-actin were mediated at least in part at the transcriptional level. Transfection of various rat SM alpha-actin promoter/chloramphenicol acetyltransferase (CAT) constructs into SMCs demonstrated that the first 155 bp of the SM alpha-actin promoter was sufficient to confer maximal Ang II responsiveness, conferring an approximately 4-fold increase in reporter activities in these SMCs compared with vehicle-treated SMCs. Mutation of either of two highly conserved CArG elements, designated A (-62) and B (-112), completely abolished Ang II-induced increases in reporter activity, whereas mutation of a homeodomain-like binding sequence at -145 (ATTA) reduced reporter activity by half. Results of EMSAs showed that nuclear extracts from Ang II-treated SMCs exhibited enhanced binding activity of serum response factor (SRF) to the CArG elements and of a homeodomain factor, MHox, to the ATTA element. Northern analyses showed that Ang II also stimulated marked increases in MHox mRNA levels. Western analyses demonstrated that Ang II-induced increases in SRF binding were not due to increased SRF protein expression. Recombinant MHox markedly enhanced binding activity of SRF in EMSAs. Finally, MHox overexpression transactivated a SM alpha-actin promoter/CAT reporter construct by approximately 3.5-fold in transient cotransfection studies. These results provide evidence for involvement of a homeodomain transcription factor, MHox, in Ang II-mediated stimulation of SM alpha-actin via a CArG/SRF-dependent mechanism.

Skeletal and cardiac alpha-actin isoforms exhibit unanticipated temporal and tissue-specific gene expression patterns in developing avian limbs and embryos

The initial expression of skeletal muscle structural genes typically occurs after myogenic determination factor gene expression. We investigated this temporal relationship via a reverse transcription-polymerase chain reaction (RT-PCR) analysis of skeletal and cardiac alpha-actin (s- and c-actin) mRNA during chick limb development. c-actin transcripts were first detected at the beginning of muscle cytodifferentiation in stage 24/25 limbs, shortly after the initial appearance of MyoD and myogenin mRNAs, and were not detected in nonmyogenic tissues. In contrast, s-actin mRNA was detected in limbs at stage 15-16, periods when myf5 and MRF4 but not MyoD and myogenin transcripts are present (Lin-Jones, J., and Hauschka, S. D., Dev. Biol. 174, 407-422, 1996). While s-actin mRNA was not detected in E7 neural retina and was at variable levels in stage 9-15 neural tube, significant levels were consistently detected in mesodermal tissues which contribute nonmyogenic cells to the limb: stage 9-12 lateral plate and distal portions of stage 25/26 limbs. s-actin transcripts detected in the earliest limbs could thus be in myogenic and/or nonmyogenic cells. These data indicate that while c-actin expression is activated at the onset of limb muscle cytodifferentiation, s-actin expression occurs much earlier, as well as in some nonmyogenic tissues. Whether the precocious expression of s-actin plays a functional role in muscle development remains to be determined.

Creation of an In vivo cytosensor using engineered mesangial cells. Automatic sensing of glomerular inflammation controls transgene activity

Automatic control over exogenous gene expression in response to the activity of disease is a crucial hurdle for gene transfer-based therapies. Towards achieving this goal, we created a "cytosensor" that perceives local inflammatory states and subsequently regulates foreign gene expression. alpha-Smooth muscle actin is known to be expressed in glomerular mesangial cells exclusively in pathologic situations. CArG box element, the crucial regulatory sequence of the alpha-smooth muscle actin promoter, was used as a sensor for glomerular inflammation. Rat mesangial cells were stably transfected with an expression plasmid that introduces a beta-galactosidase gene under the control of CArG box elements. In vitro, the established cells expressed beta-galactosidase exclusively after stimulation with serum. To examine whether the cells are able to automatically control transgene activity in vivo, serum-stimulated or unstimulated cells were transferred into normal rat glomeruli or glomeruli subjected to anti-Thy 1 glomerulonephritis. When stimulated cells were transferred into the normal glomeruli, beta-galactosidase expression was switched off in vivo within 3 d. In contrast, when unstimulated cells were transferred into the nephritic glomeruli, transgene expression was substantially induced. These data indicate the feasibility of using the CArG box element as a molecular sensor for glomerular injury. In the context of advanced forms of gene therapy, this approach provides a novel concept for automatic regulation of local transgene expression where the transgene is required to be activated during inflammation and deactivated when the inflammation has subsided.

Structure and alternative promoters of the mouse glutamic acid decarboxylase 67 gene

gamma-Aminobutyric acid is synthesized by glutamic acid decarboxylase (GAD), which has two forms, GAD65 and GAD67. Genomic clones coding mouse GAD67 (mGAD67) have been isolated. The restriction map of the overlapping clones covers a region of more than 45 kb of genomic DNA. The mGAD67 gene contains 16 translated exons in addition to an exon which is preferentially expressed in foetal brain. The rapid amplification of 5'-cDNA ends showed that mGAD67 gene transcripts have two different 5'-untranslated regions. Analysis of the genomic clones encompassing the 5'-exons revealed that the two transcripts arose from a single gene by alternative splicing using two different donor sites and a common acceptor. The exons were found 1.5 and 0.6 kb upstream of exon 1. The corresponding promoter regions of these exons have a number of putative regulatory elements, including Sp1- and Krox-24-binding sites. Analysis of mGAD67 transcripts demonstrated that each of the 5'-untranslated exons was expressed in mouse brain. In contrast, exon 0A, but not exon 0B, was expressed in mouse testis and pancreas. These results suggest that these transcripts may be regulated under the control of independent promoters.

Adenovirus E1A inhibits cardiac myocyte-specific gene expression through its amino terminus

Adenovirus E1A oncoproteins inhibit muscle-specific gene expression and myogenic differentiation by suppressing the transcriptional activating functions of basic helix-loop-helix proteins. As one approach to identifying cardiac-specific gene regulatory proteins, we analyzed the functional regions of E1A proteins that are required for muscle gene repression in cardiac cells. Myocyte-specific promoters, including the alpha-actins and alpha-myosin heavy chain, were selectively and potently inhibited (>90%) by E1A, while the ubiquitously expressed beta-actin promoter was only partially ( approximately 30%) repressed; endogenous gene expression was also affected. Distinct E1A protein binding sites mediated repression of muscle-specific and ubiquitous actin promoters. E1A-mediated inhibition of beta-actin required both an intact binding site for the tumor repressor proteins pRb and p107 and a second E1A domain (residues 15-35). In contrast, cardiac-specific promoter repression required the E1A amino-terminal residues 2-36. The proximal skeletal actin promoter (3' to base pair -153) was a target for repression by E1A. Although E1A binding to p300 was not required for inhibition of either promoter, co-expression of p300 partially reversed E1A-mediated transcriptional repression. We conclude that cardiac-specific and general promoter inhibition by E1A occurs by distinct mechanisms and that cardiac-specific gene expression is modulated by cellular factors interacting with the E1A p300/CBP-binding domain.

Organization and myogenic restricted expression of the murine serum response factor gene. A role for autoregulation

Serum response factor (SRF), a member of an ancient family of DNA-binding proteins, is generally assumed to be a ubiquitous transcription factor involved in regulating growth factor-responsive genes. However, avian SRF was recently shown (Croissant, J. D., Kim, J.-H., Eichele, G., Goering, L., Lough, J., Prywes, R., and Schwartz, R. J. (1996) Dev. Biol. 177, 250-264) to be preferentially expressed in myogenic lineages and is required for regulating post-replicative muscle gene expression. Given the central importance of SRF for the muscle tissue-restricted expression of the striated alpha-actin gene family, we wanted to determine how SRF might contribute to this muscle-restricted expression. Here we have characterized the murine SRF genomic locus, which has seven exons interrupted by six introns, with the entire locus spanning 11 kilobases. Murine SRF transcripts were processed to two 3'-untranslated region polyadenylation signals, yielding 4.5- and 2.5-kilobase mRNA species. Murine SRF mRNA levels were the highest in adult skeletal and cardiac muscle, but barely detected in liver, lung, and spleen tissues. During early mouse development, in situ hybridization analysis revealed enrichment of SRF transcripts in the myotomal portion of somites, the myocardium of the heart, and the smooth muscle media of vessels of mouse embryos. Likewise, murine SRF promoter activity was tissue-restricted, being 80-fold greater in primary skeletal myoblasts than in liver-derived HepG2 cells. In addition, SRF promoter activity increased 6-fold when myoblasts withdrew from the cell cycle and fused into differentiated myotubes. A 310-base pair promoter fragment depended upon multiple intact serum response elements in combination with Sp1 sites for maximal myogenic restricted activity. Furthermore, cotransfected SRF expression vector stimulated SRF promoter transcription, whereas dominant-negative SRF mutants blocked SRF promoter activity, demonstrating a positive role for an SRF-dependent autoregulatory loop.

Multiple mechanisms of transcriptional repression by YY1

The four C-terminal GLI-Krüppel type zinc fingers of YY1 have been identified as a transcriptional repression domain. Previous reports have proposed DNA-bending and activator-quenching mechanisms for this zinc finger-mediated repression. In addition, previous work indicated that p300 and CBP might be involved in YY1-mediated repression. We have analyzed these possible models for the zinc finger-mediated repression. The role of each zinc finger in the repression and DNA-binding functions was determined by using a structure-and-function approach. We show that zinc finger 2 of YY1 plays a central role in both DNA binding and transcriptional repression. However, a survey of a panel of YY1 mutants indicates that these two functions can be separated, which argues against the DNA-bending model for repression. We show that the physical interaction between YY1 and p300, a coactivator for CREB, is not sufficient for repression of CREB-mediated transcription. Our studies indicate that YY1 functions as an activator-specific repressor. Repression of CTF-1-directed transcription may be accomplished through direct physical interaction between YY1 and this activator. In contrast, physical interaction is not necessary for YY1 to repress Sp1- and CREB-mediated transcription. Rather, the repression likely reflects an ability of YY1 to interfere with communication between these activators and their targets within the general transcription machinery. Taken together, our results suggest that YY1 employs multiple mechanisms to achieve activator-specific repression.

Structure and expression of the gene encoding secretor-type galactoside 2-alpha-L-fucosyltransferase (FUT2)

The expression and secretion of ABO antigens in epithelial cells of glands are controlled by secretor-type alpha (1,2)fucosyltransferase activity. We have examined the expression of the secretor-type alpha(1,2)fucosyltransferase gene (FUT2) and a pseudogene of FUT2 (Sec1) in several tumor cell lines by northern blot and/or reverse-transcription-PCR (RT-PCR) analyses. Transcripts of FUT2 were found in total RNA from ovarian, gastric and colonic cancer cell lines but not from six leukemic cell lines, including erythroleukemic HEL cells, by RT-PCR. On the other hand, RT-PCR indicated that Sec1 was expressed in all these tumor cells, including all hematopoietic cells studied. Northern blot analysis indicated that FUT2 transcripts with a similar size (3.3 kb) were expressed in cancer cell lines. Rapid amplification of cDNA ends suggested that the entire FUT2 cDNA is 3.1-kb long and has two Alu repetitive elements in its 3' untranslated region, including an inverted repeat. The mRNA, therefore, may form a large stem-and-loop structure (1.2 kb). Each stem contains about 300 bases, the loop contains 640 bases, and the percentage of complementary nucleotide sequences in the stem region is 85%. The presence of a large stem-and-loop structure in the 3' untranslated region may regulate the level of the FUT2 transcript by affecting the stability of the mRNA.

A competitive mechanism of CArG element regulation by YY1 and SRF: implications for assessment of Phox1/MHox transcription factor interactions at CArG elements

In the promoters of many immediate early genes, including c-fos, CArG DNA regulatory elements mediate basal constituitive expression and rapid and transient serum induction. CArG boxes also occur in the promoters of muscle-specific genes, including skeletal alpha-actin, where it confers muscle-specific expression. These elements are regulated, at least in part, by the ubiquitous transcription factors serum response factor (SRF) and YY1. The homeobox transcription factor Phox1/MHox has also been implicated in regulation of the c-fos CArG element and is thought to function by facilitating SRF binding to DNA. Here, we provide in vitro and in vivo evidence that the mechanism of YY1 repression of CArG elements results from competition with SRF for overlapping binding sites. We describe in detail the binding sites of YY1 and SRF through serial point mutations of the skeletal alpha-actin proximal CArG element and identify a mutation that dramatically reduces YY1 binding but retains normal SRF binding. YY1 competes with SRF for binding to wild-type CArG elements, but not to this point mutant in vitro. This mutant is sufficient for muscle-specific expression in vivo but is much less sensitive to repression by YY1 overexpression. We utilized the YY1/SRF competition to address the role of Phox1 at these elements. Phox1 overexpression did not diminish YY1-mediated repression, suggesting that transcriptional activation by Phox1 does not result from enhanced SRF binding to these elements. These methods may prove to be useful for assessing interactions between other CArG element regulatory factors.

A transforming growth factor beta (TGFbeta) control element drives TGFbeta-induced stimulation of smooth muscle alpha-actin gene expression in concert with two CArG elements

The goal of the present study was to determine the molecular mechanism whereby transforming growth factor beta (TGFbeta) increases smooth muscle (SM) alpha-actin expression. Confluent, growth-arrested rat aortic smooth muscle cells (SMC) were transiently transfected with various SM alpha-actin promoter/chloramphenicol acetyltransferase deletion mutants and stimulated with TGFbeta (2.5 ng/ml). Results demonstrated that the first 125 base pairs of the SM alpha-actin promoter were sufficient to confer TGFbeta responsiveness. Three cis elements were shown to be required for TGFbeta inducibility: two highly conserved CArG boxes, designated A (-62) and B (-112) and a novel TGFbeta control element (TCE) (-42). Mutation of any one of these elements completely abolished TGFbeta-induced reporter activity. Results of electrophoretic mobility shift assays demonstrated that nuclear extracts from TGFbeta-treated SMC enhanced binding activity of serum response factor to the CArG elements and binding of an as yet unidentified factor to the TCE. Northern analysis showed that TGFbeta also stimulated transcription of two other SM (SM myosin heavy chain) differentiation marker genes, SM myosin heavy chain and h1 calponin, whose promoters also contained a TCE-like element. In summary, we identified a TGFbeta response element in the SM alpha-actin promoter that may contribute to coordinate regulation of expression of multiple cell-type specific proteins during SMC differentiation.

Structural organization and promoter analysis of the bovine cytochrome c oxidase subunit VIIc gene. A functional role for YY1

Cytochrome c oxidase (COX) subunit VIIc is one of the nuclear encoded subunits of the 13-subunit holoenzyme that carries out the terminal step in the electron transport chain. We have isolated the gene for this subunit, previously shown to be ubiquitously expressed from a single copy gene in the genome, and show that 167 base pairs of DNA surrounding the transcriptional start site contain the minimal promoter of this gene. This basal promoter contains two YY1 sites and at least one site for NRF-2, which show binding to their cognate factors. Mutation of both YY1 sites eliminates most of the promoter activity. Mutation at the upstream YY1 site significantly reduces the efficiency of transcript initiation at the major start site and thus plays the dominant role in COX7C regulation. COX7C is, thus, the second nuclear gene of COX that is regulated by YY1, suggesting that it is a third common factor, along with NRF-1 and NRF-2, to be associated with COX gene regulation.

A serum response factor-dependent transcriptional regulatory program identifies distinct smooth muscle cell sublineages

The SM22alpha promoter has been used as a model system to define the molecular mechanisms that regulate smooth muscle cell (SMC) specific gene expression during mammalian development. The SM22alpha gene is expressed exclusively in vascular and visceral SMCs during postnatal development and is transiently expressed in the heart and somites during embryogenesis. Analysis of the SM22alpha promoter in transgenic mice revealed that 280 bp of 5' flanking sequence is sufficient to restrict expression of the lacZ reporter gene to arterial SMCs and the myotomal component of the somites. DNase I footprint and electrophoretic mobility shift analyses revealed that the SM22alpha promoter contains six nuclear protein binding sites (designated smooth muscle elements [SMEs] -1 to -6, respectively), two of which bind serum response factor (SRF) (SME-1 and SME-4). Mutational analyses demonstrated that a two-nucleotide substitution that selectively eliminates SRF binding to SME-4 decreases SM22alpha promoter activity in arterial SMCs by approximately 90%. Moreover, mutations that abolish binding of SRF to SME-1 and SME-4 or mutations that eliminate each SME-3 binding activity totally abolished SM22alpha promoter activity in the arterial SMCs and somites of transgenic mice. Finally, we have shown that a multimerized copy of SME-4 (bp -190 to -110) when linked to the minimal SM22alpha promoter (bp -90 to +41) is necessary and sufficient to direct high-level transcription in an SMC lineage-restricted fashion. Taken together, these data demonstrate that distinct transcriptional regulatory programs control SM22alpha gene expression in arterial versus visceral SMCs. Moreover, these data are consistent with a model in which combinatorial interactions between SRF and other transcription factors that bind to SME-4 (and that bind directly to SRF) activate transcription of the SM22alpha gene in arterial SMCs.

Regulation of murine cytochrome oxidase Vb gene expression in different tissues and during myogenesis. Role of a YY-1 factor-binding negative enhancer

The mouse cytochrome oxidase (COX) Vb promoter contains three sequence motifs with partial or full consensus for YY-1 and GTG factor binding and a CArG box, located between positions -480 and -390. Individually, all three motifs stimulated transcription of the TKCAT promoter, and bound distinctly different proteins from the liver and differentiated C2C12 nuclear extracts. Collectively, these motifs, together with the downstream flanking sequence, -378 to -320, suppressed the transcription activity of heterologous promoters, thymidine kinase-chloramphenicol acetyltransferase (TKCAT) and COXIV/CAT. The transcription activities of both TKCAT and COXIV/CAT constructs were induced 3-4-fold during induced myogenesis of C2C12 cells. The downstream CArG-like motif binds transcription factor YY-1, while the upstream YY-1-like motif binds to a yet unidentified factor. Co-expression with intact YY-1, but not that lacking the DNA binding domain suppressed the transcriptional activity. Mutations targeted to the CArG-like motif abolished the suppressive effect of the negative enhancer and the inducibility of the promoter during myogenic differentiation. Our results suggest that the activity of the negative enhancer may determine the level of expression of the COX Vb gene in different tissues.

Yeast two-hybrid cloning of a novel zinc finger protein that interacts with the multifunctional transcription factor YY1

Muscle-restricted transcription of sarcomeric actin genes is negatively controlled by the zinc finger protein YY1, which is down-regulated at the protein level during myogenic differentiation. To identify cellular proteins that might mediate the function/stability of YY1 in muscle cells, we screened an adult human muscle cDNA library using the yeast two-hybrid cloning system. We report the isolation and characterization of a novel protein termed YAF2 (YY1- associated factor 2) that interacts with YY1. The YAF2 cDNA encodes a 180 amino acid basic protein (pI 10.5) containing a single N-terminal C2-X10-C2 zinc finger. Lysine clusters are present that may function as a nuclear localization signal. Domain mapping analysis shows that the first and second zinc fingers of YY1 are targeted for YAF2 protein interaction. In contrast to the down-regulation of YY1, YAF2 message levels increase during in vitro differentiation of both rat skeletal and cardiac muscle cells. YAF2 appears to have a promyogenic regulatory role, since overexpression of YAF2 in C2 myoblasts stimulates myogenic promoter activity normally restricted by YY1. Co-transfection of YY1 reverses the stimulatory effect of YAF2. YAF2 also greatly potentiates proteolytic cleavage of YY1 by the calcium- activated protease m-calpain. The isolation of YAF2 may help in understanding the mechanisms through which inhibitors of myogenic transcription may be antagonized or eliminated by proteolysis during muscle development.

Characterization of the transcriptional regulator YY1. The bipartite transactivation domain is independent of interaction with the TATA box-binding protein, transcription factor IIB, TAFII55, or cAMP-responsive element-binding protein (CPB)-binding protein

YY1 is a multifunctional transcription factor implicated in both positive and negative regulation of gene expression as well as in initiation of transcription. We show that YY1 is ubiquitously expressed in growing, differentiated, and growth-arrested cells. The protein is phosphorylated and has a half-life of 3.5 h. To define functional domains, we have generated a large panel of YY1 mutant proteins. These were used to define precisely the DNA-binding domain, the region responsible for nuclear localization, and the transactivation domain. The two acidic domains at the N terminus each provide about half of the transcriptional activating activity. Furthermore, the spacer region between the Gly/Ala-rich and zinc finger domains has accessory function in transactivation. YY1 has been shown previously to bind to TAFII55, TATA box-binding protein, transcription factor IIB, and p300. In addition, we identified cAMP-responsive element-binding protein (CBP)-binding protein as a YY1 binding partner. Surprisingly, these proteins did not bind to the domains involved in transactivation, but rather to the zinc finger and Gly/Ala-rich domains of YY1. Thus, these proteins do not explain the transcriptional activating activity of YY1, but rather may be involved in repression or in initiation.

Binding of YY1 to a site overlapping a weak TATA box is essential for transcription from the uteroglobin promoter in endometrial cells

The gene for rabbit uteroglobin codes for a small calcium-, steroid-, and biphenyl metabolite-binding homodimeric protein which is expressed in a variety of epithelial cell types such as Clara cells (lung) and the glandular and luminal cells of the endometrium. One important region mediating its efficient transcription in a human endometrium-derived cell line, Ishikawa, is centered around a noncanonical TATA box. Two factors, TATA core factor (TCF), expressed in cell lines derived from uteroglobin-expressing tissues, and the ubiquitously expressed TATA palindrome factor, bind to the DNA major groove at two adjacent sites within this region. Here, we report the identification of the TATA palindrome factor as the transcription/initiation factor YY1 by microsequencing of the biochemically purified factor from HeLa cells. The binding site for YY1 within the uteroglobin gene is unique in its sequence and its location overlapping a weak TATA box (TACA). Binding of YY1 was required for efficient transcription in TCF-positive Ishikawa cells, which responded only weakly to a change of TACA to TATA, although in vitro binding affinity for the TATA-box-binding protein increased by 1 order of magnitude. In contrast, in CV-1 cells, lacking TCF, binding of YY1 was not required for transcription in the context of a wild-type TACA box, whereas a change from TACA to TATA led to significantly increased reporter gene expression. DNA binding data exclude a role of YY1 in stabilizing the interaction of the TATA-box-binding protein with the uteroglobin promoter. We conclude that cell lines derived from uteroglobin-expressing tissues overcome the weak TATA box with the help of auxiliary factors, one of them being YY1.

A novel C/EBP beta-YY1 complex controls the cell-type-specific activity of the human papillomavirus type 18 upstream regulatory region

The human papillomavirus type 18 (HPV-18) upstream regulatory region (URR) controls viral gene transcription in a cell-type-specific manner. The HPV-18 URR is active in HeLa cells but inactive in HepG2 cells. The activating activity of YY1 in HeLa cells is dependent on its functional interactions with the switch region which is critical for the HPV-18 URR activity in HeLa cells. Here, we show that a protein complex composed of C/EBP beta and YY1 binds the switch region which is detected only in HeLa cells, not in HepG2 cells. Transfection of C/EBP beta into HepG2 cells restored the formation of the C/EBP beta-YY1-switch region complex, accompanied by increased transcription directed by the HPV-18 URR. Mutations in the switch region that abolished the complex formation also abrogated C/EBP beta-induced transcriptional activation. This provides a strong correlation between the binding of the C/EBP beta-YY1 complex to the switch region and cell-type-specific URR activity. Taken together, we have identified a novel C/EBP beta-YY1 complex that binds the switch region and contributes to cell-type-specific HPV-18 URR activity.

Recruitment of the tinman homolog Nkx-2.5 by serum response factor activates cardiac alpha-actin gene transcription

We recently showed that the cardiogenic homeodomain factor Nkx-2.5 served as a positive acting accessory factor for serum response factor (SRF) and that together they provided strong transcriptional activation of the cardiac alpha-actin promoter, depending upon intact serum response elements (SREs) (C. Y. Chen, J. Croissant, M. Majesky, S. Topouz, T. McQuinn, M. J. Frankovsky, and R. J. Schwartz, Dev. Genet. 19:119-130, 1996). As shown here, Nkx-2.5 and SRF collaborated to activate the endogenous murine cardiac alpha-actin gene in 10T1/2 fibroblasts by a mechanism in which SRF recruited Nkx-2.5 to the alpha-actin promoter. Activation of a truncated promoter consisting of the proximal alpha-actin SRE1 occurred even when Nkx-2.5 DNA-binding activity was blocked by a point mutation in the third helix of its homeodomain. Investigation of protein-protein interactions showed that Nkx-2.5 was bound to SRF in the absence of DNA in soluble protein complexes retrieved from cardiac myocyte nuclei but could also be detected in coassociated binding complexes on the proximal SRE1. Recruitment of Nkx-2.5 to an SRE depended upon SRF DNA-binding activity and was blocked by the dominant negative SRFpm1 mutant, which allowed for dimerization of SRF monomers but prevented DNA binding. Interactive regions shared by Nkx-2.5 and SRF were mapped to N-terminal/helix I and helix II/helix III regions of the Nkx-2.5 homeodomain and to the N-terminal extension of the MADS box. Our study suggests that physical association between Nkx-2.5 and SRF is one way that cardiac specified genes are activated in cardiac cell lineages.

YY1 and c-Myc associate in vivo in a manner that depends on c-Myc levels

The c-Myc oncoprotein has previously been shown to associate with transcription regulator YY1 and to inhibit its activity. We show herein that endogenous c-Myc and YY1 associate in vivo and that changes in c-Myc levels, which accompany mitogenic stimulation or differentiation of cultured cells, affect the ratio of free to c-Myc-associated YY1. We have also investigated the mechanism by which association with c-Myc inhibits YY1's ability to regulate transcription. c-Myc does not block binding of YY1 to DNA. However, protein association studies suggest that c-Myc interferes with the ability of YY1 to contact basal transcription proteins TATA-binding protein and TFIIB.

YY1 represses human papillomavirus type 16 transcription by quenching AP-1 activity

YY1 is a multifunctional transcription factor that has been shown to regulate the expression of a number of cellular and viral genes, including the human papillomavirus (HPV) oncogenes E6 and E7. In this study, we have analyzed the YY1-mediated repression of the HPV type 16 (HPV-16) E6-E7 promoter. A systematic analysis to identify YY1 sites present in the HPV-16 long control region showed that of 30 potential YY1 binding motifs, 24 bound purified recombinant YY1 protein, but only 10 of these were able to bind YY1 when nuclear extracts of HeLa cells were used. Of these, only a cluster of five sites, located in the vicinity of an AP-1 motif, were found to be responsible for repressing the HPV-16 P97 promoter. All five sites were required for repression, the mutation of any one site giving rise to a four- to sixfold increase in transcriptional activity. The target for YY1-mediated repression was identified as being a highly conserved AP-1 site, and we propose that AP-1 may represent a common target for YY1 repression. We also provide data demonstrating that YY1 can bind the transcriptional coactivator CREB-binding protein and propose a potentially novel mechanism by which YY1 represses AP-1 activity as a result of this YY1-CREB-binding protein interaction.

The nuclear factor YY1 suppresses the human gamma interferon promoter through two mechanisms: inhibition of AP1 binding and activation of a silencer element

Our group has previously reported that the nuclear factor Yin-Yang 1 (YY1), a ubiquitous DNA-binding protein, is able to interact with a silencer element (BE) in the gamma interferon (IFN-gamma) promoter region. In this study, we demonstrated that YY1 can directly inhibit the activity of the IFN-gamma promoter by interacting with multiple sites in the promoter. In cotransfection assays, a YY1 expression vector significantly inhibited IFN-gamma promoter activity. Mutation of the YY1 binding site in the native IFN-gamma promoter was associated with an increase in the IFN-gamma promoter activity. Analysis of the DNA sequences of the IFN-gamma promoter revealed a second functional YY1 binding site (BED) that overlaps with an AP1 binding site. In this element, AP1 enhancer activity was suppressed by YY1. Since the nuclear level of YY1 does not change upon cell activation, our data support a model that the nuclear factor YY1 acts to suppress basal IFN-gamma transcription by interacting with the promoter at multiple DNA binding sites. This repression can occur through two mechanisms: (i) cooperation with an as-yet-unidentified AP2-like repressor protein and (ii) competition for DNA binding with the transactivating factor AP1.

Interleukin-1beta is a negative transcriptional regulator of alpha1-adrenergic induced gene expression in cultured cardiac myocytes

We recently reported that interleukin-1beta (IL-1beta) induces a novel form of cardiac myocyte hypertrophy characterized by an increase in protein content but an absence of the fetal program of skeletal alpha-actin or beta-myosin heavy chain (beta-MHC) gene expression (Palmer, J. N., Hartogensis, W. E., Patten, M., Fortuin, F. D., and Long, C. S. (1995) J. Clin. Invest. 95, 2555-2564). Because of the apparent disparity between this myocardial phenotype and that seen with other hypertrophic agents in culture, such as catecholamines, we investigated the effect of IL-1beta on alpha1-induced cardiomyocyte hypertrophy. Although there was no augmentation in total protein when IL-1beta and phenylephrine were given simultaneously, IL-1beta attenuated the increase in contractile protein mRNAs (skeletal alpha-actin and beta-MHC) in response to phenylephrine. Transient transfection studies with skeletal alpha-actin and beta-MHC promoter constructs linked to the chloramphenicol acetyltransferase (CAT)-reporter gene indicate that repression occurred at the level of gene transcription. In view of the previously reported activity of the zinc finger protein YY1 in the negative regulation of the skeletal alpha-actin promoter in cardiomyocytes (MacLellan, W. R., Lee, T. C., Schwartz, R. J., and Schneider, M. D. (1994) J. Biol. Chem. 269, 16754-16760), we investigated the potential role of this factor in the IL-1beta-mediated effects. Using transient transfection, we found that a mutation in the YY1 binding site of the skeletal alpha-actin promoter abolished the inhibitory effect of IL-1beta. We further found that the 127-base pair fragment of the skeletal alpha-actin promoter required for the IL-1beta effect is also required for inhibition by the overexpression of YY1 in the myocytes. Furthermore, increased levels of YY1 protein are found in IL-1beta treated myocytes. Taken together these results suggest that the repression of contractile protein gene transcription by IL-1beta may be due, at least in part, to activation of the negative transcription factor YY1.

SRF and TEF-1 control of chicken skeletal alpha-actin gene during slow-muscle hypertrophy

The purpose of this study was to delineate the alpha-actin regulatory elements and transcription factors that are responsible for conferring stretch-overload responsiveness during hypertrophy of the anterior latissimus dorsi (ALD) muscle of young chickens by weighting one wing. Minimal promoter constructs were evaluated by direct injection into the ALD, which demonstrated that both serum response element 1 (SRE1) and the transcriptional enhancer factor 1 (TEF-1) elements were sufficient for increased expression during stretch overload. A mutated SRE1 prevented expression in both basal and stretched ALD muscles, whereas a mutated TEF-1 element reduced actin promoter function in both control and stretched muscles. The serum response factor (SRF)-SRE1 binding complex demonstrated faster migration in mobility shift assays from day 3-and day 6-stretched ALD nuclear extracts relative to their control. TEF-1 binding was qualitatively increased in stretched extracts at day 3 but not day 6 of stretch overload. Skeletal alpha-actin mRNA accumulated from day 3 to day 6 of stretch overload. These data demonstrate that SRE1 is necessary and sufficient for stretch-overload responsiveness from the skeletal alpha-actin promoter and that the SRF-SRE1 binding complex migrates faster in stretched nuclear extracts of hypertrophied relative to control extracts from intact ALD muscles of chickens.

Physical interaction between the mitogen-responsive serum response factor and myogenic basic-helix-loop-helix proteins

Terminal differentiation of muscle cells results in opposite effects on gene promoters: muscle-specific promoters, which are repressed during active proliferation of myoblasts, are turned on, whereas at least some proliferation-associated promoters, such as c-fos, which are active during cell division, are turned off. MyoD and myogenin, transcription factors from the basic-helix-loop-helix (bHLH) family, are involved in both processes, up-regulating muscle genes and down-regulating c-fos. On the other hand, the serum response factor (SRF) is involved in the activation of muscle-specific genes, such as c-fos, as well as in the up-regulation of a subset of genes that are responsive to mitogens. Upon terminal differentiation, the activity of these various transcription factors could be modulated by the formation of distinct protein-protein complexes. Here, we have investigated the hypothesis that the function of SRF and/or MyoD and myogenin could be modulated by a physical association between these transcription factors. We show that myogenin from differentiating myoblasts specifically binds to SRF. In vitro analysis, using the glutathione S-transferase pull-down assay, indicates that SRF-myogenin interactions occur only with myogenin-E12 heterodimers and not with isolated myogenin. A physical interaction between myogenin, E12, and SRF could also be demonstrated in vivo using a triple-hybrid approach in yeast. Glutathione S-transferase pull-down analysis of various mutants of the proteins demonstrated that the bHLH domain of myogenin and that of E12 were necessary and sufficient for the interaction to be observed. Specific binding to SRF was also seen with MyoD. In contrast, Id, a natural inhibitor of myogenic bHLH proteins, did not bind SRF in any of the situations tested. These data suggest that SRF, on one hand, and myogenic bHLH, on the other, could modulate each other's activity through the formation of a heterotrimeric complex.

Transcriptional analysis of the 5'-noncoding region of the human involucrin gene

Human involucrin whose gene transcription is directed by a 2456-nucleotide (nt) 5'-noncoding region is a structural component of the epithelial cornified layer. Transient transfection assays demonstrated that this region is transcriptionally active in multiplying keratinocytes and is enhanced by 2 mM CaCl2 treatment. Calcium-independent transcriptional activity and the interaction with the AP-1 transcriptional factor was located on the proximal part (nt -159 to -1) of the 5'-noncoding region. However, CaCl2 responsiveness was mapped to a distal 1185-nt fragment (nt -2456 to -1272). Moreover, this fragment potentiated the Herpes simplex thymidine kinase promoter in normal keratinocytes and is responsive to calcium treatment in a cell type-specific manner. Interestingly, the absence of a 491-nt fragment located between the two enhancer domains (nt -651 to -160) resulted in transcriptional activation in multiplying keratinocytes. This fragment interacts with AP-1 and the YY1 transcriptional silencer. It is concluded that human involucrin 5'-noncoding region contains at least three regulatory domains, a distal CaCl2-responsive enhancer, a putative transcriptional silencer (that interacts with AP-1 and YY1), and a proximal enhancer/promoter (that interacts with AP-1). Thus, this study demonstrates the presence of particular transcriptional factors can potentially regulate the human involucrin expression.

Nuclear factor YY1 activates the mammalian F0F1 ATP synthase alpha-subunit gene

Analysis of the promoters of the bovine and human nuclear-encoded mitochondrial F0F1 ATP synthase alpha-subunit genes (ATPA) has identified several positive cis-acting regulatory regions that are important for basal promoter activity in human HeLa cells. We have previously determined that the binding of a protein factor, termed ATPF1, to an E-box sequence (CANNTG) located within one of these cis-acting regions is critical for transcriptional activation of the ATPA gene. In this article, we describe a second positive cis-acting regulatory element of the ATPA gene that is important for expression of the ATPA gene. We show that this cis-acting element also contains a binding site for a protein present in HeLa cells. On the basis of electrophoretic mobility shift patterns, oligonucleotide competition assays, and immunological cross-reactivity, we conclude that this protein factor is Yin-Yang 1 (YY1). Experiments carried out to examine the functional role of YY1 within the context of the ATPA promoter demonstrated that YY1 acts as a positive regulator of the ATPA gene. For example, when the YY1 binding site of the ATPA promoter was placed upstream of a reporter gene it was found to activate transcription in transient transfection assays. In addition, disruption of the YY1 binding site in the ATPA gene resulted in a loss of transcriptional activity. Furthermore, in cotransfection experiments overexpression of YY1 in trans was found to activate transcription of ATPA promoter-CAT constructs. Thus, at least two positive trans-acting regulatory factors, ATPF1 and YY1, are important for expression of the bovine and human F0F1 ATP synthase alpha-subunit genes.

Characterization of the human granulocyte-macrophage colony-stimulating factor gene promoter: an AP1 complex and an Sp1-related complex transactivate the promoter activity that is suppressed by a YY1 complex

It is well documented that a repeated CATT element in the human granulocyte-macrophage colony-stimulating factor (GM-CSF) gene promoter is required for promoter activity. However, the transcription factors that are able to transactivate this enhancer element remain unidentified. Recently, we have found that nuclear factor YY1 can interact with the enhancer element. Here, we report that in addition to YY1, two other nuclear factors have been identified in the DNA-protein complexes formed by the CATT oligonucleotide and the Jurkat T-cell nuclear protein. One of these factors is AP1, and the other one is an Sp1-related protein. Results from transient transfection of Jurkat T cells have revealed that formation of both AP1 and the Sp1-related complex is required for the full enhancer activity of the CATT element. This result is supported by cotransfection of a c-jun expression vector and mutational analysis of the AP1 site or the Sp1-related protein binding site. In contrast, formation of the YY1 complex suppresses enhancer activity, since deletion of the YY1 complex induces an augmentation of the enhancer activity and overexpression of YY1 results in an attenuation of the enhancer activity. Results from the mechanism study have revealed that YY1 is able to inhibit transactivation mediated by either AP1 or the Sp1-related protein, and YY1 suppressive activity is DNA binding dependent. Taken together, these data support the ideas that AP1 and the Sp1-related nuclear protein are required for transactivation of the human GM-CSF gene promoter and that YY1 can suppress transactivation of the promoter even under inducible conditions.

DNA binding sites for the transcriptional activator/repressor YY1

YY1 is ubiquitously expressed zinc finger DNA binding protein. It can act as a transcriptional repressor or activator and, when binding at the initiator element, as a component of the basal transcription complex. Binding sites for YY1 have been reported in a wide variety of promoters and they exhibit substantial diversity in their sequence. To better understand how YY1 interacts with DNA and to be able to predict the presence of YY1 sites in a more comprehensive fashion, we have selected YY1 binding sites from a random pool of oligonucleotides. The sites display considerable heterogeneity, but contain a conserved 5'-CAT-3' core flanked by variable regions, generating the consensus 5'-(C/g/a)(G/t)(C/t/a)CATN(T/a)(T/g/c)-3', where the upper case letters represent the preferred base. This high degree of flexibility in DNA recognition can be predicted by modeling the interaction of the four YY1 zinc fingers with DNA and a detailed model for this interaction is presented and discussed.

YY1 facilitates the association of serum response factor with the c-fos serum response element

YY1 is a multifunctional transcription factor that acts as an activator or repressor in different contexts. YY1 binds to multiple sites in the mouse c-fos promoter, inducing at each site a sharp DNA bend. Binding of YY1 to a site situated between the cyclic AMP response element (CRE) and the TATA box bends the DNA in a way that interferes with the interaction of proteins bound at the CRE and TATA elements, resulting in repression of transcription. Here, we show that binding of YY1 to a different site in the c-fos promoter has a different result. Binding of YY1 to the c-fos serum response element (SRE) enhances the binding of serum response factor (SRF). This enhancement requires the binding of YY1 to SRE DNA. YY1 and SRF can cooccupy the SRE at least transiently. In the region of overlapping contact, YY1 contacts DNA in the major groove, while SRF contacts DNA in the minor groove. YY1 also enhances the association of SRF with the SRE in transfected insect cells. Thus, although YY1 induces similar structural changes in DNA at different binding sites, it can have distinct local effects on protein-DNA and protein-protein interactions. These data support a general role for YY1 in the building of highly organized promoter complexes.

Multiple Tandemly Repeated Binding Sites for the YY1 Repressor and Transcription Factors AP-1 and SP-1 Are Clustered within Intron-1 of the Gene Encoding the IE110 Transactivator of Herpes simplex Virus Type 1

Expression of the IE110 (ICP0) transactivator protein of HSV appears to be critical for reactivation from the latent state and occurs at immediate-early times during the lytic cycle under the control of an upstream divergent enhancer-promoter region that contains multiple Oct and Sp-1 binding sites overlapping with VP16 response elements. Surprisingly, the large 800-bp first intron of the HSV-1 IE110 gene also proved to have a complex repetitive organization encompassing multiple transcription factor binding sites within four distinct domains. DNaseI footprinting studies revealed that 13 of 17 copies of a 15-bp repeated element represented high-affinity binding sites for the cellular YY1 repressor protein. Between 4 and 7 of these sites are direct tandem repeats and the rest are interpersed with three repeated AT-rich motifs and a dyad symmetry region containing two strong AP-1 binding sites and an adjacent SP-1 binding site on each arm. Several of the YY1 sites also bound weakly to SRF. The intron also contains four clustered purine/pyrimidine tracts of between 16 and 23 bp long. Both the AP-1/AP-2/SP-1 dyad protein binding region and, to a lesser extent, the YY1 tandem-repeat cluster conferred responsiveness to TPA when placed upstream of a heterologous promoter in transient expression assays. The functional significance of the HSV-1 IE110 intron region is unknown as yet, but the novel arrangement of tandemly repeated YY1 sites has the potential to produce structural bending and transcriptional attenuation effects. Interestingly, few of these transcription factor binding motifs are conserved in the equivalent IE110 intron of HSV-2, and the domain appears to represent a unique alternative control region that is specific for HSV-1. Copyright 1995 S. Karger AG, Basel

A skeletal muscle-specific enhancer regulated by factors binding to E and CArG boxes is present in the promoter of the mouse myosin light-chain 1A gene

The mouse myosin light-chain 1A (MLC1A) gene, expressed in the atria of the adult heart, is one of the first muscle genes to be activated when skeletal as well as cardiac muscles form in the embryo. It is also transcribed in skeletal muscle cell lines at the onset of differentiation. Transient transfection assays of mouse skeletal muscle cell lines with DNA constructs containing MLC1A promoter fragments fused to the chloramphenicol acetyltransferase (CAT) gene show that the first 630 bp of the promoter is sufficient to direct expression of the reporter gene during myotube formation. Two E boxes located at bp -76 and -519 are necessary for this regulation. MyoD and myogenin proteins bind to them as heterodimers with E12 protein and, moreover, transactivate them in cotransfection experiments with the MLC1A promoter in nonmuscle cells. Interestingly, the effect of mutating each E box is less striking in primary cultures than in the C2 or Sol8 muscle cell line. A DNA fragment from bp -36 to -597 confers tissue- and stage-specific activity to the herpes simplex virus thymidine kinase promoter in both orientations, showing that the skeletal muscle-specific regulation of the MLC1A gene is under the control of a muscle-specific enhancer which extends into the proximal promoter region. At bp -89 is a diverged CArG box, CC(A/T)6AG, which binds the serum response factor (SRF) in myotube nuclear extracts, as does the wild-type sequence, CC(A/T)6GG. Both types of CArG box also bind a novel myotube-enriched complex which has contact points with the AT-rich part of the CArG box and adjacent 3' nucleotides. Mutations within the CArG box distinguish between the binding of this complex and binding of SRF; only SRF binding is directly involved in the specific regulation of the MLC1A gene in skeletal muscle cell lines.

Transcriptional repression of the c-fos gene by YY1 is mediated by a direct interaction with ATF/CREB

Transcriptional activation of the mouse c-fos gene by the adenovirus 243-amino-acid E1A protein requires a binding site for transcription factor YY1 located at -54 of the c-fos promoter. YY1 normally represses transcription of c-fos, and this repression depends on the presence of a cyclic AMP (cAMP) response element located immediately upstream of the -54 YY1 DNA-binding site. This finding suggested that the mechanism of transcriptional repression by YY1 might involve a direct interaction with members of the ATF/CREB family of transcription factors. In vitro and in vivo binding assays were used to demonstrate that YY1 can interact with ATF/CREB proteins, including CREB, ATF-2, ATFa1, ATFa2, and ATFa3. Structure-function analyses of YY1 and ATFa2 revealed that the C-terminal zinc finger domain of YY1 is necessary and sufficient for binding to ATFa2 and that the basic-leucine zipper region of ATFa2 is necessary and sufficient for binding to YY1. Overexpression of YY1 in HeLa cells resulted in repression of a mutant c-fos chloramphenicol acetyltransferase reporter that lacked binding sites for YY1, suggesting that repression can be triggered through protein-protein interactions with ATF/CREB family members. Consistent with this finding, repression was relieved upon removal of the upstream cAMP response element. These data support a model in which YY1 binds simultaneously to its own DNA-binding site in the c-fos promoter and also to adjacent DNA-bound ATF/CREB proteins in order to effect repression. They further suggest that the ATF/CREB-YY1 complex serves as a target for the adenovirus 243-amino-acid E1A protein.

YY1 binds to and regulates cis-acting negative elements in the Epstein-Barr virus BZLF1 promoter

A 48-bp cis-acting negative element in the Epstein-Barr virus BZLF1 gene P1 promoter has been described previously. By DNase I footprinting experiments, two regions were identified as the protein-binding sites (previously designated site I and site II). In this report, the cellular transcription factor YY1 has been identified as a protein which binds to both of these elements, now designated ZIVA and ZIVB. Both ZIVA and ZIVB conferred cis-acting negative regulation on an enhancerless simian virus 40 promoter. In cotransfection experiments, overexpression of YY1 caused further repression of the enhancerless simian virus 40 promoter containing either the ZIVA or ZIVB element. Cotransfection of a plasmid expressing antisense to YY1 increased the expression of the heterologous promoter containing ZIVA but not ZIVB. In similar experiments carried out with the P1 promoter, overexpression of YY1 caused downregulation of P1 whereas antisense RNA to YY1 caused a slight increase in expression. Analyses of various P1 mutant constructions revealed additional YY1 sites downstream of ZIVB. Overexpression of YY1 also caused downregulation of a P1 mutant with no apparent YY1-binding sites. TPA treatment of Raji cells caused a temporal loss of YY1-binding activity but had no effect on the intracellular levels of YY1 protein. Serum induction of quiescent B cells also caused loss of YY1 binding to the ZIVB site, which was found to be a weak serum response element. In contrast, anti-immunoglobulin G treatment of Akata cells had no effect on either the YY1-binding activity or protein levels. The binding of YY1 to the cis-acting negative elements in infected B cells may play a pivotal role in the maintenance of Epstein-Barr virus latency.

Plasticity of vascular smooth muscle alpha-actin gene transcription. Characterization of multiple, single-, and double-strand specific DNA-binding proteins in myoblasts and fibroblasts

Transcriptional activity of the mouse vascular smooth muscle (VSM) alpha-actin promoter was governed by both cell type and developmental stage-specific mechanisms. A purine-rich motif (PrM) located as -181 to -176 in the promoter was absolutely required for activation in mouse AKR-2B embryonic fibroblasts and partially contributed to activation in undifferentiated mouse BC3H1 myoblasts. Transcriptional enhancer factor 1 recognized the PrM and cooperated with other promoter-binding proteins to regulate serum growth factor-dependent transcription in both myoblasts and fibroblasts. Two distinct protein factors (VAC-ssBF1 and VAC-ssBF2) also were identified that bound sequence-specifically to single-stranded oligonucleotide probes that spanned both the PrM and a closely positioned negative regulatory element. VAC-ssBF1 and BF2 binding activity was detected in undifferentiated myoblasts, embryonic fibroblasts, and several smooth muscle tissues in the mouse and human. A myoblast-specific protein (VAC-RF1) also was detected that bound double-stranded probes containing a CArG-like sequence that previously was shown to impart strong, cell type specific repression. The binding activity of transcription enhancer factor 1, VAC-RF1, and VAC-ssBF1 was significantly diminished when confluent BC3H1 myoblasts differentiated into myocytes and expressed VSM alpha-actin mRNA after exposure to serum-free medium. The results indicated that cell type-specific control of the VSM alpha-actin gene promoter required the participation of multiple DNA-binding proteins, including two that were enriched in smooth muscle and had preferential affinity for single-stranded DNA.

Functional interactions between YY1 and adenovirus E1A

YY1 is a C2H2-type zinc finger transcription factor that is a member of the human GLl-Kruppel family of proteins. YY1 represses transcription when bound upstream of transcription initiation sites. The repression can be relieved by adenovirus E1A and activation of target genes occurs. We have mapped the repression domain of YY1 to the C-terminal region, overlapping its DNA binding domain. We have also identified an activation domain within the first 69 amino acids of YY1. The YY1 C-terminal region is involved in physical interactions with E1A and is functionally necessary for YY1 to respond to E1A. This suggests that relief of YY1 repression by E1A involves YY1-E1A physical interactions. Although not involved in interactions with E1A, the N-terminal activation domain is also necessary for YY1 to respond to E1A. Presumably, under repressing conditions, the activation domain is masked by the conformation of YY1, but is released upon binding of E1A and is required to subsequently activate transcription. Consistent with this hypothesis, an ATF-2-YY1 chimeric protein containing the activation domain of ATF-2 and the C-terminal two-thirds of YY1 is still a potent repressor. Unlike the mutant YY1 lacking its own N-terminal activation domain, the chimeric protein is fully responsive to E1A.

Interaction with RAP74 subunit of TFIIF is required for transcriptional activation by serum response factor

A few general transcription factors, in particular TFIID and TFIIB, have been found to bind transcriptional activators. Here we show that the general transcription factor TFIIF is also a target for a transcriptional activator, namely serum response factor (SRF), which binds to the c-fos promoter. Using a yeast interaction assay, we find that SRF binds the RAP74 subunit of TFIIF and that SRF's transcriptional activation domain is the region involved in this binding. Further, RAP74's central charged cluster domain is required for binding to SRF's activation domain. Deletion of this domain impairs RAP74's ability to support SRF-activated transcription in vitro but has little effect on the protein's basal transcription activity or its ability to support SP1-activated transcription. The correlation of SRF-RAP74 binding with transcriptional activation suggests that RAP74 is a critical target for SRF-activated transcription.

Serum response element associated transcription factors in mouse embryos: serum response factor, YY1, and PEA3 factor

Many mammalian transcription factors, including human and mouse serum response factors (SRFs), are post-translationally modified with O-linked N-acetylglucosamine monosaccharides on multiple serine and/or threonine residues. Nuclear extracts were prepared from 9.5 to 19 days postcoitum mouse embryos and subsequently were fractionated by wheat germ agglutinin (WGA)-agarose affinity chromatography. SRF binds WGA-agarose and apparently is O-glycosylated. On the other hand, the low molecular weight serum response element (SRE)-binding proteins, including the previously named band I and band II factors, did not bind WGA-agarose. Furthermore, we showed that the fastest migrating complex contains the Yin-Yang 1 (YY1) factor. YY1 binds to the c-fos SRE and skeletal alpha-actin muscle regulatory element (MRE), but not the cardiac alpha-actin MRE. Nuclear extracts from NIH/3T3 fibroblasts contain similar, if not identical, SRE-binding complexes. Besides these SRE-binding factors, mouse PEA3-binding factor, presumably an ETS domain-containing protein, was found to bind SRF protein. This physical interaction, between SRF and ETS domain proteins, was shown to involve the DNA-binding domain-containing region of SRF and not the carboxyl-terminal transactivation domain.

A switch region determines the cell type-specific positive or negative action of YY1 on the activity of the human papillomavirus type 18 promoter

YY1 is a zinc finger transcription factor which acts as either a repressor or an activator dependent on the promoter context. YY1 is a potent activator of the genuine human papillomavirus type 18 (HPV-18) upstream regulatory region (URR) in HeLa cells, which are known for high-level expression of the HPV-18 early genes. The activating activity of YY1 is dependent on the presence of a newly identified switch region located upstream of the YY1 binding site. Deletion of this region causes YY1 to act as a repressor of HPV-18 promoter activity. In vivo footprinting of the HPV-18 URR and an in vitro electrophoretic mobility shift assay identified proteins binding to the switch region. Site-directed mutagenesis of the switch region and YY1 binding sites suggests that these two regions work in concert to yield high-level HPV-18 URR activity in HeLa cells but not in HepG2 cells, where HPV-18 is almost inactive. These data identified a novel mode of cell type-specific regulation of HPV-18 promoter activity by positive or negative action of YY1, determined by the switch region binding factor(s).

Identification of a DNA binding site for the nuclear factor YY1 in the human GM-CSF core promoter

It has been well documented that the repeated CATT(A/T) sequence, localized between -64 and -35 in the human GM-CSF promoter, is required for the promoter activity, and this region likely serves as a core recognition sequence for a cellular transcription factor. However, the transcription factor that interacts with this site was not identified. Here, we report that this element contains a binding site for the nuclear factor YY1, which has not been reported to play a role in the regulation of cytokine gene transcription. Results from transient transfection assays of the Jurkat T cell line revealed that this repeated CATT(A/T) element exhibited enhancer activity when linked to both the human IFN-gamma promoter and the TK promoter. Mutation of the YY1 binding site eliminated about 60% of the enhancer activity of the element. We have found that the YY1 binding site could form two specific DNA-protein complexes, A and B, with Jurkat nuclear proteins in the electrophoretic mobility shift assay and that the binding of these complexes correlates with the enhancer activity. UV cross-linking analysis revealed that the A complex is a multi-protein complex and in addition to YY1, other proteins are required for formation of the protein complex. Cotransfection assays with a YY1 expression vector revealed that overexpression of YY1 resulted in an inhibitory effect on the repeated CATT(A/T) element, indicating that in addition to YY1, cofactors also are required for the activator function of the A complex.

Regulation of smooth muscle alpha-actin promoter by vasopressin and platelet-derived growth factor in rat aortic vascular smooth muscle cells

Vasoconstrictors such as arginine vasopressin (AVP) and angiotensin II (Ang II) have been shown to increase protein and mRNA levels of smooth muscle alpha-actin (SM-alpha-actin) in vascular smooth muscle cells. In the same cells, platelet-derived growth factor (PDGF) decreased SM-alpha-actin protein and mRNA. The rat SM-alpha-actin promoter that has recently been isolated contains two E-boxes and three CC(A/T)6GG (CArG) elements. To examine regulation of the SM-alpha-actin promoter, a 765-bp region of the rat SM-alpha-actin gene was ligated into chloramphenicol acetyltransferase (CAT)-containing vectors and transfected into rat aortic vascular smooth muscle cells. Stimulation of cells with either AVP or Ang II increased CAT activity 5- to 10-fold. PDGF was able to completely block the AVP-induced increase in CAT activity. To identify regions of the promoter responsible for both the AVP stimulation and PDGF inhibition of promoter activity, a series of truncation mutants were prepared and transfected into vascular smooth muscle cells. Truncation of both E-boxes and the most distal CArG element did not qualitatively alter either AVP-induced stimulation of CAT activity or PDGF inhibition. However, removal of the middle CArG element resulted in a loss of AVP stimulation. These studies indicate that the AVP-induced elevation and PDGF-induced inhibition of SM-alpha-actin levels in vascular smooth muscle cells are mediated at least in part through regulation of the SM-alpha-actin promoter. The critical region of the promoter mediating this effect involves at a minimum one of the CArG elements.

Novel clustering of Sp1 transcription factor binding sites at the transcription initiation site of the human muscle phosphofructokinase P1 promoter

The regulatory sequence elements of the human muscle phosphofructokinase (HPFKM) p1 promoter from -655 to +78 were cloned and characterized. In the human cervical carcinoma cell line, HeLa S3, the HPFKM type C RNA initiated from a single predominant transcription initiation site and the HPFKM p1 promoter displayed transcriptional activity in transient transfection assays. The HPFKM p1 promoter region was shown to possess eight binding sites for the Sp1 transcription factor by DNase I footprinting and gel retardation analysis. The functional importance of these interactions was examined by transient transfection analysis in Drosophila SL2 and HeLa S3 cells. This analysis demonstrated that the HPFKM p1 promoter sequence between +12 and +78 retained Sp1-dependent transcriptional activity in Drosophila SL2 cells and retained promoter activity in HeLa S3 cells. These results suggest that the Sp1 binding site (site 8 between +12 and +21) immediately adjacent to the transcription initiation site represents an important regulatory element of this promoter at least in the context of the minimal HPFKM p1 promoter. However mutagenesis of the Sp1 site 8 demonstrated that, in the context of a larger HPFKM p1 promoter region containing Sp1 sites 1 to 7, it now contributed very little to the total promoter activity. Therefore it appears the Sp1 sites in the HPFKM p1 promoter display functional redundancy.

The transcriptional regulator YY1 binds to the 5'-terminal region of B19 parvovirus and regulates P6 promoter activity

We performed a systematic study to identify cellular factors that bound to the terminal repeat region of B19 parvovirus. Using electrophoretic mobility shift assays, we detected one cellular factor which prominently bound to the repeat region. The factor was purified from K562 nuclear extract by specific DNA affinity column chromatography and identified as YY1, a multifunctional transcription factor. Of multiple possible YY1 binding sites in the upstream region of the P6 promoter, three showed specific strong binding. Transcription enhancement by YY1 was demonstrated in vitro by transient transfection assays. In studies using truncated and mutated versions of this region, YY1 activity was diminished by the alteration of any two of these three sites and abolished by the alteration of all three sites. Our results suggest that YY1 is a positive regulator of B19 parvovirus transcription.

Transcriptional activation of the alpha-1 acid glycoprotein gene by YY1 is mediated by its functional interaction with a negative transcription factor

Regulation of alpha-1 acid glycoprotein (AGP) gene expression involves both positive and negative transcription factors. We have previously identified two dominant factors: positive and negative transcription factors, AGP/EBP and factor B, respectively, involved in the transcription of AGP and other acute-phase response genes. In this report, we present evidence showing that the transcription of the AGP gene is positively regulated by a transcription factor, YY1. The activation of AGP gene by YY1 is mediated by a negative element B in the AGP promoter region. YY1 can also activate the B motif linked to a heterologous promoter. However, YY1 does not bind directly to the B motif per se. Rather, our data suggest that the activation of AGP gene by YY1 may be mediated by its functional interaction with factor B, which recognizes the B motif.

YY1 represses rat serum amyloid A1 gene transcription and is antagonized by NF-kappa B during acute-phase response

Serum amyloid A (SAA), one of the major acute-phase proteins, increases several hundredfold in concentration in plasma following acute inflammation, primarily as a result of a 200-fold increase in its transcriptional rate. Functional analysis of the rat SAA1 promoter has identified a 65-bp cytokine response unit (CRU; positions -135 to -71) that could confer cytokine responsiveness on a heterologous promoter. Within this CRU, two cis-regulatory elements, corresponding to NF-kappa B- and C/EBP-binding sites, were found to be functionally important and exerted synergistic effects on induced SAA1 expression. In this report, we show that a third transcription factor interacts with the CRU through a region located between the NF-kappa B- and C/EBP-binding sites. On the basis of its gel mobility shift patterns, ubiquitous binding activity, sequence specificity of DNA binding, zinc-dependent binding activity, and gel mobility supershift by specific antibodies, we concluded that this factor is identical to YY1. Methylation interference studies revealed that YY1 binding sequences overlapped with those of NF-kappa B, and gel mobility studies showed that NF-kappa binding to the CRU was effectively inhibited by YY1. Consistent with its presumed antagonistic role to NF-kappa B, YY1 exerted a negative effect on SAA1 expression, whereas disruption of its binding in the promoter elevated basal and cytokine-induced activities. Furthermore, overexpression of YY1 trans-repressed SAA1 promoter activity. Thus, our results demonstrate that SAA1 expression is tightly regulated by an on-off switch of activators and repressors, presumably to ensure that it is expressed only under appropriate physiological conditions.

Two pathways for serum regulation of the c-fos serum response element require specific sequence elements and a minimal domain of serum response factor

The c-fos serum response element (SRE) is necessary and sufficient for induction of the c-fos gene in response to serum and growth factors. This activation is dependent upon serum response factor (SRF), a transcriptional activator which binds the SRE. A factor, p62TCF, which binds in conjunction with SRF to the SRE and which is activated by mitogen-activated protein kinase, has also been implicated in c-fos regulation. By using a reporter gene system with weak SRE mutations that is dependent upon overexpression of SRF for serum induction, we have found that there are at least two pathways for serum induction that converge on the SRE. Loss of TCF binding by mutations in SRF and the SRE did not reduce serum induction of the reporter genes. We have found a pathway for serum induction that is sensitive to mutations in the A/T-containing central sequence of the SRE and which is independent of TCF. When this pathway was mutated, activation was dependent upon TCF binding, demonstrating that TCF can also function in serum induction. Both of the signalling pathways required a minimal domain of SRF. This domain, spanning SRF's DNA binding domain, was sufficient for serum induction when fused to a heterologous transcriptional activation domain.