YY1 represses beta-casein gene expression by preventing the formation of a lactation-associated complex

The Why of YY1: Mechanisms of Transcriptional Regulation by Yin Yang 1

First described in 1991, Yin Yang 1 (YY1) is a transcription factor that is ubiquitously expressed throughout mammalian cells. It regulates both transcriptional activation and repression, in a seemingly context-dependent manner. YY1 has a well-established role in the development of the central nervous system, where it is involved in neurogenesis and maintenance of homeostasis in the developing brain. In neurodevelopmental and neurodegenerative disease, the crucial role of YY1 in cellular processes in the central nervous system is further underscored. In this mini-review, we discuss the various mechanisms leading to the transcriptional activating and repressing roles of YY1, including its role as a traditional transcription factor, its interactions with cofactors and chromatin modifiers, the role of YY1 in the non-coding genome and 3D chromatin organization and the possible implications of the phase-separation mechanism on YY1 function. We provide examples on how these processes can be involved in normal development and how alterations can lead to various diseases.

Rel/NF-kappaB represses bcl-2 transcription in pro-B lymphocytes

The mechanisms controlling programmed cell death (PCD) during early B cell development are not well understood. Members of both the Bcl-2 family of apoptosis-related proteins and the nuclear factor-kappa B/Rel (NF-kappaB/Rel) family of transcription factors are expressed differentially during B cell development. To date, however, no direct interactions between these two families have been demonstrated. The FL5.12 cell line represents a model for progenitor B cell development. Such cells reproducibly undergo PCD upon IL-3 withdrawal. The signal to enter the apoptotic pathway is mediated by a shift in the ratio of Bcl-2:Bax. While bax levels remain constant, bcl-2 transcription rate, steady-state mRNA, and protein levels decrease. Analysis of the bcl-2 promoter reveals 3 kappaB sites functionally able to bind kappaB factors from FL5.12 nuclear extracts. Cotransfection studies demonstrate that NF-kappaB factors can repress bcl-2 transcription and that site-directed mutagenesis of the kappaB motifs abolishes this repression. These studies suggest that NF-kappaB mediates PCD in pro-B cells through transcriptional repression of the survival gene bcl-2, thus shifting the bcl-2:bax ratio in favor of death-promoting complexes.

Mammary cell-activating factor regulates the hormone-independent transcription of the early lactation protein (ELP) gene in a marsupial

The regulation of the tammar wallaby (Macropus eugenii) early lactation protein (ELP) gene is complex. ELP is responsive to the lactogenic hormones; insulin (I), hydrocortisone (HC) and prolactin (PRL) in mammary gland explants but could not be induced with lactogenic hormones in tammar primary mammary gland cells, nor in KIM-2 conditionally immortalised murine mammary epithelial cells. Similarly, ELP promoter constructs transiently-transfected into human embryonic kidney (HEK293T) cells constitutively expressing the prolactin receptor (PRLR) and Signal Transducer and Activator of Transcription (STAT)5A were unresponsive to prolactin, unlike the rat and mouse β-casein (CSN2) promoter constructs. Identification of the minimal promoter required for the hormone-independent transcription of tammar ELP in HEK293Ts and comparative analysis of the proximal promoters of marsupial ELP and the orthologous eutherian colostrum trypsin inhibitor (CTI) gene suggests that mammary cell-activating factor (MAF), an E26 transformation-specific (ETS) factor, may bind to an AGGAAG motif and activate tammar ELP.

The β-casein in camels: molecular characterization of the CSN2 gene, promoter analysis and genetic variability

The β-casein is the most abundant protein in camel milk and its encoding gene (CSN2) is considered in other species a 'major' gene for the presence of alleles associated to different level of expression. In the present paper, we report for the first time the characterization of the nucleotide sequence of the whole β-casein-encoding gene (CSN2) plus 2,141 bp at the 5'-flanking region in Camelus dromedarius. The promoter region and the complete cDNA are also provided for the first time in Camelus bactrianus. The gene is spread over 7.8 kb and consists of 9 exons varying in length from 24 bp (exon 5) to 519 bp (exon 7), and 8 introns from 95 bp (intron 5) to 1,950 bp (intron 1). The composite response element (CoRE) region was identified in the promoter, whereas the presence of mature microRNA sequences improves the knowledge on the factors putatively involved in the gene regulation. A total of 46 polymorphic sites have been detected. The transition g.2126A>G falls within the TATA-box of dromedary CSN2 promoter with a putative influence on the transcription factor binding activity. The frequency of the G allele is 0.35 in a population of 180 she-camels belonging to 4 different ecotypes. In the same population, a conservative SNP (g.4175C>A) was found at the codon 7 of the signal peptide, whereas a comparative analysis with a cDNA sequence available in the database evidenced a missense SNP (g.4180T(Leu)>G(Arg)) at exon 2. Four SNPs were found in the bactrian camel. The SNP c.666G>A is responsible for the amino acid change Met(201)→Ile and it represents the first missense allele at the β-casein in camels. Finally, five interspersed repeated elements were identified at intronic level, whereas the presence of putative bio-functional peptides belonging to ACE-inhibitor and anti-oxidative families confirms the potential protective role of the camel milk for the human nutrition.

Gene expression analysis of protein synthesis pathways in bovine mammary epithelial cells purified from milk during lactation and short-term restricted feeding

The objective of the study was to investigate selected key regulatory pathways of milk protein biosynthesis in primary bovine mammary epithelial cells (MECs) of dairy cows during the first 155 days of lactation. In addition, cows were exposed to feed restriction for a short period (FR) during different stages of lactation (week 4 and 21 pp) to study adjustment processes of molecular protein biosynthesis to metabolic challenge. Morning milk samples from twenty-four Holstein-Friesian cows were collected throughout the experimental period (n = 10 per animal). MEC from raw milk were purified using an immunomagnetic separation technique and used for real-time quantitative PCR analyses. As was seen in transcript abundances of all major milk proteins, mRNA levels of E74-like factor 5 (ELF5), an enhancer of signal transducer and activator of transcription (STAT) action, concomitantly decreased towards mid-lactation. Expression of ELF5 as well as of all milk protein genes showed a similar increase during FR in early lactation. Occasional changes in expression could be seen in other Janus kinase (JAK)/STAT factors and in mammalian target of rapamycin (mTOR) pathway elements. Amino acid transfer and glucose transporter and the β-casein expression were also partially affected. In conclusion, our findings suggest a pivotal role of the transcription factor ELF5 in milk protein mRNA expression with complementary JAK/STAT and mTOR signalling for the regulation of protein biosynthesis in the bovine mammary gland.

The contribution of adhesion signaling to lactogenesis

The mammary gland undergoes hormonally controlled cycles of pubertal maturation, pregnancy, lactation, and involution, and these processes rely on complex signaling mechanisms, many of which are controlled by cell-cell and cell-matrix adhesion. The adhesion of epithelial cells to the extracellular matrix initiates signaling mechanisms that have an impact on cell proliferation, survival, and differentiation throughout lactation. The control of integrin expression on the mammary epithelial cells, the composition of the extracellular matrix and the presence of secreted matricellular proteins all contribute to essential adhesion signaling during lactogenesis. In vitro and in vivo studies, including the results from genetically engineered mice, have shed light on the regulation of these processes at the cell and tissue level and have led to increased understanding of the essential signaling components that are regulated in temporal and cell specific manner during lactogenesis. Recent studies suggest that a secreted matricellular protein, CTGF/CCN2, may play a role in lactogenic differentiation through binding to β1 integrin complexes, enhancing the production of extracellular matrix components and contributions to cell adhesion signaling.

The epigenetic landscape of mammary gland development and functional differentiation

Most of the development and functional differentiation in the mammary gland occur after birth. Epigenetics is defined as the stable alterations in gene expression potential that arise during development and proliferation. Epigenetic changes are mediated at the biochemical level by the chromatin conformation initiated by DNA methylation, histone variants, post-translational modifications of histones, non-histone chromatin proteins, and non-coding RNAs. Epigenetics plays a key role in development. However, very little is known about its role in the developing mammary gland or how it might integrate the many signalling pathways involved in mammary gland development and function that have been discovered during the past few decades. An inverse relationship between marks of closed (DNA methylation) or open chromatin (DnaseI hypersensitivity, certain histone modifications) and milk protein gene expression has been documented. Recent studies have shown that during development and functional differentiation, both global and local chromatin changes occur. Locally, chromatin at distal regulatory elements and promoters of milk protein genes gains a more open conformation. Furthermore, changes occur both in looping between regulatory elements and attachment to nuclear matrix. These changes are induced by developmental signals and environmental conditions. Additionally, distinct epigenetic patterns have been identified in mammary gland stem and progenitor cell sub-populations. Together, these findings suggest that epigenetics plays a role in mammary development and function. With the new tools for epigenomics developed in recent years, we now can begin to establish a framework for the role of epigenetics in mammary gland development and disease.

Prostaglandin F2alpha suppresses rat steroidogenic acute regulatory protein expression via induction of Yin Yang 1 protein and recruitment of histone deacetylase 1 protein

Prostaglandin F2alpha (PGF2alpha) plays a pivotal role in ovarian luteolysis by inhibiting the expression of steroidogenic acute regulatory (StAR) protein, leading to a decrease in intracellular cholesterol transport and luteal steroid production. Previously we have demonstrated that the transcription factor Yin Yang 1 (YY1) bound to three regions in the StAR promoter in vitro and repressed promoter activity. This study further defined the YY1-mediated PGF2alpha effect on the inhibition of StAR protein expression through YY1 interaction with a single region in the StAR promoter in vivo. PGF2alpha consistently suppressed StAR mRNA and protein expression in cultured luteal cells in a dose-dependent manner. PGF2alpha also enhanced YY1 protein expression and binding to its cis-element in a time-dependent pattern that preceded the decline in StAR protein levels. The StAR promoter region bound by YY1 was also associated with histone deacetylase 1 (HDAC1). PGF2alpha treatment promoted HDAC1 binding to and suppressed the histone H3 acetylation in this region. On the contrary, YY1 knockdown decreased HDAC1 binding, increased histone H3 acetylation, enhanced StAR protein expression, and negated PGF2alpha effect on StAR protein expression. Luciferase assays showed that YY1 overexpression inhibited StAR promoter activity and the addition of a HDAC inhibitor, trichostatin A, abrogated the effect of YY1. Trichostatin A-treated luteal cells displayed increased StAR protein expression. These data indicate that PGF2alpha enhances a direct YY1/StAR promoter interaction and the recruitment of HDAC1 to the promoter, thereby preventing transcriptional activation of the StAR gene.

Progesterone Receptor Repression of Prolactin/Signal Transducer and Activator of Transcription 5-Mediated Transcription of the β-Casein Gene in Mammary Epithelial Cells

Prolactin (PRL) and glucocorticoids act synergistically to stimulate transcription of the beta-casein milk protein gene. Signal transducer and activator of transcription 5 (Stat5) mediates PRL-dependent trans-activation, and glucocorticoid potentiation occurs through cross talk between glucocorticoid receptor (GR) and Stat5 at the beta-casein promoter. In the mouse, progesterone withdrawal leads to terminal differentiation and secretory activation of the mammary gland at parturition, indicating progesterone's role in repressing milk protein gene expression during pregnancy. To investigate the mechanism of the inhibitory action of progesterone, experiments were performed with cell culture systems reconstituted to express progesterone receptor (PR), the PRL receptor/Stat5 signaling pathway, and GR, enabling evaluation of PR, GR, and Stat5 interactions at the beta-casein promoter. With COS-1, normal murine mammary gland, HC-11, and primary mammary epithelial cells, progestin-PR directly repressed the PRL receptor/Stat5a signaling pathway's mediation of PRL-induced beta-casein transcription. Progestin-PR also inhibited glucocorticoid-GR enhancement of PRL induced trans-activation of beta-casein. Inhibition depended on a functional PR DNA binding domain and specific PR-DNA interactions at the beta-casein promoter. Chromatin immunoprecipitation assays in HC-11 cells revealed recruitment of PR and Stat5a to the beta-casein promoter by progestin or PRL, respectively. Recruitment was disrupted by cotreatment with progestin and PRL, suggesting a mutual interference between activated PR and Stat5a. Without PRL, progestin-PR also recruited Stat5a to the beta-casein promoter, suggesting that recruitment of an unactivated form of Stat5a may contribute to inhibition of beta-casein by progesterone. These results define a negative cross talk between PR and Stat5a/GR that may contribute to the physiological role of progesterone to repress lactogenic hormone induction of the beta-casein gene in the mammary gland during pregnancy.

Integration of prolactin and glucocorticoid signaling at the beta-casein promoter and enhancer by ordered recruitment of specific transcription factors and chromatin modifiers

Lactogenic hormone regulation of beta-casein gene expression in mammary epithelial cells provides an excellent system in which to perform kinetic studies of chromatin remodeling and transcriptional activation. Using HC11 cells as a model, we have investigated the effects of prolactin (Prl) and glucocorticoids both singly and in combination at different time points after hormone treatment. Using chromatin immunoprecipitation analysis, we have determined the dynamics of assembly and disassembly of signal transducer and activator of transcription 5, glucocorticoid receptor, CCAAT enhancer binding protein beta, and Ying Yang-1 at the hormonally activated beta-casein proximal promoter as well as the distal mouse beta-casein enhancer located approximately -6 kb upstream of the transcription start site. Prl alone resulted in a rapid recruitment of both signal transducer and activator of transcription 5 and histone deacetylase 1 to the beta-casein promoter and enhancer, and reciprocally the dissociation of Ying Yang-1 from the proximal promoter. In addition, we have examined the recruitment of coactivator p300 and determined chromatin acetylation status as a function of hormonal treatment. Finally, we have established the time course of RNA polymerase II and phospho-RNA polymerase II accumulation at the beta-casein promoter and enhancer after stimulation with hydrocortisone and Prl. Although glucocorticoids alone led to a rapid increase in histone H3 acetylation, treatment with both hormones was required for stable association of p300 and phospho-RNA polymerase II at both the promoter and enhancer. Collectively, these data suggest a model for the assembly of a multiprotein complex that helps to define how the signaling pathways controlled by these lactogenic hormones are integrated to regulate beta-casein gene expression.

Transcription factor YY1: structure, function, and therapeutic implications in cancer biology

The ubiquitous transcription factor Yin Yang 1 (YY1) is known to have a fundamental role in normal biologic processes such as embryogenesis, differentiation, replication, and cellular proliferation. YY1 exerts its effects on genes involved in these processes via its ability to initiate, activate, or repress transcription depending upon the context in which it binds. Mechanisms of action include direct activation or repression, indirect activation or repression via cofactor recruitment, or activation or repression by disruption of binding sites or conformational DNA changes. YY1 activity is regulated by transcription factors and cytoplasmic proteins that have been shown to abrogate or completely inhibit YY1-mediated activation or repression; however, these mechanisms have not yet been fully elucidated. Since expression and function of YY1 are known to be intimately associated with progression through phases of the cell cycle, the physiologic significance of YY1 activity has recently been applied to models of tumor biology. The majority of the data are consistent with the hypothesis that YY1 overexpression and/or activation is associated with unchecked cellular proliferation, resistance to apoptotic stimuli, tumorigenesis and metastatic potential. Studies involving hematopoetic tumors, epithelial-based tumors, endocrine organ malignancies, hepatocellular carcinoma, and retinoblastoma support this hypothesis. Molecular mechanisms that have been investigated include YY1-mediated downregulation of p53 activity, interference with poly-ADP-ribose polymerase, alteration in c-myc and nuclear factor-kappa B (NF-kappaB) expression, regulation of death genes and gene products, and differential YY1 binding in the presence of inflammatory mediators. Further, recent findings implicate YY1 in the regulation of tumor cell resistance to chemotherapeutics and immune-mediated apoptotic stimuli. Taken together, these findings provide strong support of the hypothesis that YY1, in addition to its regulatory roles in normal biologic processes, may possess the potential to act as an initiator of tumorigenesis and may thus serve as both a diagnostic and prognostic tumor marker; furthermore, it may provide an effective target for antitumor chemotherapy and/or immunotherapy.

The liver-enriched inhibitory protein isoform of CCAAT/enhancer-binding protein beta, but not nuclear factor-kappaB, mediates the transcriptional inhibition of beta-casein by tumor necrosis factor-alpha

TNF-alpha is a physiological regulator of mammary gland development that stimulates the growth of both normal and malignant mammary epithelial cells in primary culture and inhibits functional differentiation. To understand how TNF exerts its effects, the current study examined the mechanism by which TNF down-regulates expression of the beta-casein and whey acidic protein (WAP) genes. TNF treatment markedly decreased activity of the beta-casein and WAP promoters in transiently transfected HC11 mammary epithelial cells. Overexpression of the nuclear factor-kappaB (NFkappaB) p50 and/or p65 proteins increased the transcriptional activity of the beta-casein and WAP promoters in HC11 cells, suggesting that the inhibitory effect of TNF on transcription of these genes is not mediated by NFkappaB. This was further confirmed in experiments in which an NFkappaB super-repressor was overexpressed, and by deletion of an NFkappaB binding site in the beta-casein promoter. In contrast, we found that TNF induced both nuclear expression and the DNA-binding activity of liver-enriched inhibitory protein (LIP) isoform of CCAAT/enhancer-binding protein beta. Moreover, cotransfection of LIP and beta-casein expression vectors showed that LIP suppressed the transcriptional activity of the beta-casein promoter. Together, these results suggest that LIP plays a critical role in mediating TNF-induced down-regulation of the beta-casein gene.

Diminished milk synthesis in upstream stimulatory factor 2 null mice is associated with decreased circulating oxytocin and decreased mammary gland expression of eukaryotic initiation factors 4E and 4G

Previous studies have suggested that upstream stimulatory factors (USFs) regulate genes involved with cell cycle progression. Because of the relationship of USFs to an important oncogene in breast cancer, c-myc, we chose to determine the importance of USF to normal mammary gland development in the mouse. Expression of USF in the mammary gland throughout development demonstrated only modest changes. Mutation of the Usf2 gene was associated with reduced fertility in females, but had no effect on prepartum mammary gland development. However, lactation performance in Usf2-/- females was only half of that observed in Usf2+/+ females, and both lactose and nitrogen were decreased in milk from Usf2-/- dams. This decrease was associated with diminished mammary tissue wet weight and luminal area by d 9 of lactation and with a decreased protein-DNA ratio. This decrease was associated with reduced abundance of the eukaryotic initiation factors eIF4E and eIF4G. Blood oxytocin concentrations on d 9 postpartum were also lower in Usf2-/- mice than Usf2+/+ mice. In contrast, the mutation had no effect on blood prolactin concentrations, mammary cell proliferation or apoptosis, mammary tissue oxytocin receptors, or milk protein gene expression. The mutation had only modest effects on maternal behavior. These data support the idea that USF is important to physiological processes necessary for the establishment and maintenance of normal lactation and suggest that USF-2 may impact lactation through both systemic and mammary cell-specific mechanisms.

Signal transduction pathways regulated by prolactin and Src result in different conformations of activated Stat5b

Stat5 is activated by a broad spectrum of cytokines, as well as non-receptor tyrosine kinases, such as Src. In this study, the DNA binding properties of the two closely related Stat5 proteins, Stat5a and Stat5b, induced either by prolactin (Prl) or by Src were analyzed by electrophoretic mobility shift assays using several different Stat5 binding sites. Src-induced Stat5b-DNA binding complexes consistently displayed a slightly faster mobility than those induced by Prl, as well as differences in their ability to be supershifted by anti-Stat5 antibodies. IP-Westerns performed using specific antibodies directed at the N and C termini of Stat5b suggested that depending on the activating stimulus, Stat5b exhibited different conformations, which influenced antibody accessibility at its C terminus. These conformational differences may in part be due to differential effects of Prl and Src on Stat5b tyrosine phosphorylation, since Src induced several additional sites of tyrosine phosphorylation of Stat5b at residues other than Tyr-699, including Tyr-724 and Tyr-679. The latter Tyr-679 is conserved in all mammalian Stat5bs, but is not present in Stat5a. A Stat 5bY679F mutant induced by Src kinase exhibited an altered pattern of nuclear localization as compared with wild-type Stat5b. Furthermore, this mutation inhibited v-Src-induced cyclin D1-luciferase reporter activity in transient transfection assays performed in Stat5a/b-deficient MEFs, suggesting that Tyr-679 phosphorylation may play a role in v-Src induced proliferation. Thus, depending on the signal transduction pathway responsible for activation, different conformations of activated Stat5 may result in selective biological responses.

Structure and regulation of the murine gamma-casein gene

The murine casein locus consists of five genes, which are coordinately regulated during mammary development. The levels of casein-specific mRNAs in mammary epithelial cells increase during the second half of pregnancy and remain high during lactation. The murine gamma-casein gene, which corresponds to the alphaS2-casein gene in ruminants, was isolated from a mouse bacterial artificial chromosome (BAC) library (strain 129SV). The gene contains 14 exons, which are distributed over 14 kb of DNA sequence. The expression pattern of the murine gamma-casein gene mimics that of the neighbouring beta-casein gene in terms of developmental induction in vivo. In cell culture, both the beta- and gamma-casein promoter are synergistically induced by prolactin and glucocorticoids. Glucocorticoid induction is critically dependent on prolactin-mediated activation of STAT5 in both promoters. Several consensus STAT5 binding sites were identified in the gamma-casein promoter, some of which may have an additive effect on prolactin induction. mRNA levels of gamma- and beta-casein are similar in lactating mammary tissue. However, promoter segments derived from the gamma-casein gene are significantly less active in cell culture than comparable fragments of the beta-casein promoter. Promoter hybrids between the gamma- and beta-casein promoters revealed that the critical sequences which are responsible for the different in vitro activity are located in a short promoter proximal region.

Autocrine human growth hormone (hGH) regulation of human mammary carcinoma cell gene expression. Identification of CHOP as a mediator of hGH-stimulated human mammary carcinoma cell survival

By use of cDNA array technology we have screened 588 genes to determine the effect of autocrine production of human growth hormone (hGH) on gene expression in human mammary carcinoma cells. We have used a previously described cellular model to study autocrine hGH function in which the hGH gene or a translation-deficient hGH gene was stably transfected into MCF-7 cells. Fifty two of the screened genes were regulated, either positively () or negatively (), by autocrine production of hGH. We have now characterized the role of one of the up-regulated genes, chop (gadd153), in the effect of autocrine production of hGH on mammary carcinoma cell number. The effect of autocrine production of hGH on the level of CHOP mRNA was exerted at the transcriptional level as autocrine hGH increased chloramphenicol acetyltransferase production from a reporter plasmid containing a 1-kilobase pair fragment of the chop promoter. The autocrine hGH-stimulated increase in CHOP mRNA also resulted in an increase in CHOP protein. As a consequence, autocrine hGH stimulation of CHOP-mediated transcriptional activation was increased. Stable transfection of human CHOP cDNA into mammary carcinoma cells demonstrated that CHOP functioned not as a mediator of hGH-stimulated mitogenesis but rather enhanced the protection from apoptosis afforded by hGH in a p38 MAPK-dependent manner. Thus transcriptional up-regulation of chop is one mechanism by which hGH regulates mammary carcinoma cell number.

YY1 Represses Vitamin D Receptor-Mediated 25-Hydroxyvitamin D3 24-Hydroxylase Transcription: Relief of Repression by CREB-Binding Protein

Ying Yang transcription factor (YY1) can repress or activate transcription. 25-Hydroxyvitamin D(3)-24-hydroxylase [24(OH)ase], an enzyme involved in the catabolism of 1,25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)], is up-regulated at the transcriptional level by 1,25-(OH)(2)D(3) to self-induce its deactivation. Here we report that YY1 can repress 1,25-(OH)(2)D(3)-induced 24(OH)ase transcription in CV1 cells transfected with vitamin D receptor (VDR) expression vector or in LLCPK(1) cells that contain VDR endogenously. With increasing amounts of YY1 DNA transfected (500 ng to 2 microg), ligand-dependent VDR activation of 24(OH)ase transcription was steadily repressed (maximum repression was 10-fold). Thus, YY1 may be a key modulator preventing activation at times that do not require the enzyme to be expressed. Relief of YY1 repression was observed in the presence of TFIIB or CBP (CREB binding protein) suggesting that YY1 may exert repression, in part, by sequestering TFIIB/CBP. Glutathione-S-transferase (GST) pull-down assays identified regions in the N and C termini of CBP that can bind YY1. In addition, the N-terminal region of CBP that interacts with YY1 can inhibit YY1 from binding to TFIIB. Thus, CBP may alleviate YY1-mediated repression, in part, by preventing YY1 from binding to TFIIB, which is required for VDR-mediated transcription. In summary, our results suggest that YY1 represses 24(OH)ase transcription, at least in part, by sequestering activator proteins involved in VDR-mediated transcription. In addition, our findings demonstrate a role for CBP in relief of repression of VDR-mediated transcription.

Negative regulation of YY1 transcription factor on the dynamin I gene promoter

Dynamin I is highly expressed in brain and plays a critical role in clathrin-mediated endocytosis and synaptic vesicle recycling. To elucidate the molecular mechanism by which expression of dynamin I is tissue-specifically regulated, we previously cloned and characterized the promoter of the mouse dynamin I gene and suggested that there is a negative regulatory element in this promoter region. In the present study, we showed that YY1 binds to this negative regulatory element located at -111 to -107 by using the EMSA and supershift analyses. Cotransfection experiment using an YY1 expression vector revealed that YY1 exerts a repressive role on the dynamin I gene promoter activity. These results demonstrate that transcription factor YY1 negatively regulates dynamin I expression via binding to the negative regulatory element.

Cooperative activation by GATA-4 and YY1 of the cardiac B-type natriuretic peptide promoter

YY1, a multifunctional protein essential for embryonic development, is a known repressor or activator of transcription. In cardiac and skeletal myocytes, YY1 has been described essentially as a negative regulator of muscle-specific genes. In this study, we report that YY1 is a transcriptional activator of the B-type natriuretic peptide (BNP) gene, which encodes one of the heart major secretory products. YY1 binds an element within the proximal cardiac BNP promoter, in close proximity to the high affinity binding sites for the zinc finger GATA proteins. We show that YY1 cooperates with GATA-4 to synergistically activate BNP transcription. Structure-function analysis revealed that the DNA binding domain of YY1 is sufficient for cooperative interaction with GATA-4, likely through corecruitment of the CREB-binding protein coactivator. The results suggest that YY1 and GATA factors are components of transcriptionally active complexes present in cardiac and other GATA-containing cells.

Synergistic and antagonistic interactions of transcription factors in the regulation of milk protein gene expression. Mechanisms of cross-talk between signalling pathways

The stage and tissue specific expression of milk protein genes in the mammary gland is controlled by modular response regions with multiple binding sites for distinct classes of transcription factors, which either co-operate or are antagonistic. In addition, the activity of some of these factors is individually control-led by diverse extracellular signals. A well studied paradigm for a synergistic co-operation is the activation of beta-casein gene transcription by prolactin and glucocorticoids mediated by the signal transducer and activator of transcription STAT5 and the glucocorticoid receptor (GR). As an example for an antagonistic interaction we can demonstrate inhibition of prolactin signalling by TNF-alpha, which is mediated by NF-kappa B. In both cases, the interactions occur at several levels: For GR and STAT5, the synergy is discussed to be promoted by protein-protein interactions. Furthermore, we can demonstrate a co-operation between GR and STAT5 in DNA binding by a mechanism, which is dependent on the integrity of the DNA binding domain of the GR and on the existence of half-palindromic GR binding sites in the hormone response region. Indirect effects of glucocorticoids by modulation of the expression of secondary genes are also important. They might account for the observed enhancement of prolactin induced tyrosine phosphorylation of STAT5 by glucocorticoids. For NF-kappa B and STAT5, one component of the antagonism is the inhibition of STAT5 tyrosine phosphorylation by activation of NF-kappa B. Another potential mechanism is the inhibition of DNA binding of STAT5 due to overlapping binding sites for STAT5 and NF-kappa B in the beta-casein gene promoter. Thus, synergistic and antagonistic interactions between GR, NF-kappa B, and STAT5 involve (a) cross-talk mechanisms influencing the activation of STAT5 and (b) promoter-dependent interactions modulating the DNA binding activity of the transcription factors.

Cooperative effects of STAT5 (signal transducer and activator of transcription 5) and C/EBPbeta (CCAAT/enhancer-binding protein-beta) on beta-casein gene transcription are mediated by the glucocorticoid receptor

Beta-casein gene transcription is controlled primarily by a composite response element (CoRE) that integrates signaling from the lactogenic hormones, PRL, insulin, and hydrocortisone, in mammary epithelial cells. This CoRE contains binding sites for STAT5 (signal transducer and activator of transcription 5) and C/EBPbeta (CCAAT/enhancer-binding protein-beta) and several half-sites for glucocorticoid receptor (GR). To examine how interactions among these three transcription factors might regulate beta-casein gene transcription, a COS cell reconstitution system was employed. Cooperative transactivation was observed when all three factors were expressed, but unexpectedly was not seen between STAT5 and C/EBPbeta in the absence of full-length, transcriptionally active GR. Cooperativity required the amino-terminal transactivation domain of C/EBPbeta, and neither C/EBPalpha nor C/EBPdelta was able to substitute for C/EBPbeta when cotransfected with STAT5 and GR. Different GR determinants were needed for transcriptional cooperation between STAT5 and GR as compared with those required for all three transcription factors. These studies provide some new insights into the mechanisms responsible for high level, tissue-specific expression conferred by the beta-casein CoRE.

Yin-yang 1 and glucocorticoid receptor participate in the Stat5-mediated growth hormone response of the serine protease inhibitor 2.1 gene

A growth hormone-inducible nuclear factor complex (GHINF), affinity-purified using the growth hormone response element (GHRE) from the promoter of rat serine protease inhibitor 2.1, was found to contain Stat5a and -5b, as well as additional components. The ubiquitous transcription factor yin-yang 1 (YY1) is present in GHINF. An antibody to YY1 inhibited the formation of the GHINF.GHRE complex in an electrophoretic mobility shift assay. Furthermore, Stat5 was co-immunoprecipitated from rat hepatic nuclear extracts with antibodies to YY1. An examination of the GHRE shows that, in addition to two gamma-activated sites, it contains a putative YY1 binding site between the two gamma-activated sites, overlapping them both. Mutation of this putative YY1 site results in a decrease of GHINF.GHRE complex formation in an electrophoretic mobility shift assay and a corresponding decrease in growth hormone (GH) response in functional assays. The glucocorticoid receptor was also present in GHINF, and Stat5 co-immunoprecipitates with glucocorticoid receptor in hepatic nuclear extracts from rats treated with GH. GH activation of serine protease inhibitor 2.1 requires the unique sequence of the GHRE encompassing the recognition sites of several transcription factors, and the interaction of these factors enhances the assembly of the transcription complex.

Regulation of milk protein gene expression

Studies using both transgenic mice and transfected mammary epithelial cells have established that composite response elements containing multiple binding sites for several transcription factors mediate the hormonal and developmental regulation of milk protein gene expression. Activation of signal transduction pathways by lactogenic hormones and cell-substratum interactions activate transcription factors and change chromatin structure and milk protein gene expression. The casein promoters have binding sites for signal transducers and activators of transcription 5, Yin Yang 1, CCAAT/enhancer binding protein, and the glucocorticoid receptor. The whey protein gene promoters have binding sites for nuclear factor I, as well as the glucocorticoid receptor and the signal transducers and activators of transcription 5. The functional importance of some of these factors in mammary gland development and milk protein gene expression has been elucidated by studying mice in which some of these factors have been deleted.

Differential effects of prolactin and src/abl kinases on the nuclear translocation of STAT5B and STAT5A

In this study, DNA binding and tyrosine phosphorylation of STAT5A and STAT5B were compared with their subcellular localization determined using indirect immunofluorescence microscopy. Following prolactin activation, both STAT5A and STAT5B were rapidly translocated into the nucleus and displayed a detergent-resistant, punctate nuclear staining pattern. Similar to prolactin induction, src activation resulted in tyrosine phosphorylation and DNA binding of both STAT5A and STAT5B. However, nuclear translocation of only STAT5B but not STAT5A was observed. This selective nuclear translocation appears to be mediated via the carboxyl-terminal sequences in STAT5B. Furthermore, overexpression of a dominant negative kinase-inactive mutant of JAK2 prevented prolactin-induced tyrosine phosphorylation and nuclear translocation of STAT5A and STAT5B but did not block src kinase activation and nuclear translocation of STAT5B. In co-transfection assays, prolactin-mediated activation but not src kinase-mediated activation of STAT5B resulted in the induction of a beta-casein promoter-driven reporter construct. These results suggest that STAT5 activation by src may occur by a mechanism distinct from that employed in cytokine activation of the JAK/STAT pathway, resulting in the selective nuclear translocation of STAT5B.

Two-dimensional gel electrophoresis analyses identify nucleophosmin as an estrogen regulated protein associated with acquired estrogen-independence in human breast cancer cells

We have used two-dimensional gel electrophoresis to identify proteins associated with estrogen-induced proliferation in MCF-7 breast cancer cells and their progression to estrogen-independent proliferation. We compared the total cellular proteins from MCF-7 cells and an estrogen independent derivative of the MCF-7 cells MCF-7/LCC1 (Brünner et al. Cancer Research 1993, 53, 283-290), each grown with and without estradiol. These comparisons reveal seven estrogen-regulated proteins. Three of these proteins (HI-1: 36 kDa/pI 4.5, HI-10: 40 kDa/pI 5.5 and HI-19: 62 kDa/pI 5.0) exhibit a 'progression-like' pattern, being induced by estradiol in MCF-7 cells and constitutively present/upregulated in the MCF-7/LCC1 growing without estradiol. HI-11 (65 kDa/pI 5.5) is strongly induced by estradiol in MCF-7 cells but constitutively downregulated and unresponsive to estradiol in the MCF-7/LCC1 cells. Two proteins exhibit a suppressor pattern and are downregulated by estradiol in the estrogen-dependent MCF-7 cells (HI-3: 44 kDa/pI 4.4 and HI-4: 56 kDa/ pI 5.2) and present in MCF-7/LCC1 cells growing without estradiol at levels comparable to that seen in estrogen-treated MCF-7 cells. One protein (HI-9: 68 kDa/pI 5.5) exhibits a marked estrogen regulated pI shift, rather than changes in abundance. We purified and sequenced the HI-10 protein, which we identified as the nucleolar protein, nucleophosmin (NPM). One- and two-dimensional Western blot analyses of MCF-7/LCC1 cell lysates confirmed that HI-10 is immunoreactive with an antinucleophosmin antibody. Western blotting also confirmed the estrogenic regulation of NPM seen in the initial two-dimensional gel electrophoresis studies. Thus, NPM is induced by estradiol in the MCF-7 cells and upregulated in the MCF-7/LCC1 cells growing without estrogen, clearly associating its expression with an acquired estrogen-independent phenotype. NPM has several potentially important roles in regulating cell function and signaling. It is a substrate for phosphorylation by p34cdc2 kinase, protein kinase C and nuclear kinase II, and a repressor of the transcriptional regulating activities of both the IRF-1 tumor suppressor protein and the YY1 transcription factor. Studies are currently underway to determine which of these NPM functions may be involved in the hormonal progression of breast cancer.

Identification and characterization of a novel phorbol ester-responsive DNA sequence in the 5'-flanking region of the human dopamine beta-hydroxylase gene

The phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), enhances transcription of many eukaryotic genes, including that for dopamine beta-hydroxylase (DBH). In the present study, we report identification and characterization of a novel sequence motif residing in the 5'-flanking region of the human DBH gene, which mediates transcriptional induction by TPA. Deletional analyses indicated the promoter region between -223 and -187 base pairs to be critical. Whereas this region does not contain any putative regulatory motifs with significant sequence homology to the AP-1 motif, extensive deletional and site-directed mutational analyses indicated that a sequence between -210 and -199 base pairs, 5'-ATCCGCCTGTCT-3', may represent a novel TPA-response element (TRE). In addition, alteration of the YY1-binding site decreased TPA-mediated induction of the DBH promoter activity, suggesting that contiguous cis-regulatory element(s) cooperate with this novel sequence motif. Furthermore, insertional mutation analyses between the YY1-binding site and the cyclic AMP-responsive element indicated that the stereospecificity of these motifs is important for intact transcriptional induction by TPA. Taken together, these data suggest that transcriptional up-regulation of the human DBH gene in response to TPA requires coordination of a novel TRE (human DBH TRE, hDTRE), cyclic AMP-responsive element, and the YY1-binding site.

Computer analysis of distribution of putative cis- and trans-regulatory elements in milk protein gene promoters

Multiple alignment of 28 milk protein gene promoters belonging to seven protein superfamilies is described. In these gene promoters three groups of common motifs were found: group I--specific for all milk protein gene promoters; group II--specific only for one gene superfamily; and group III--motifs shared by several gene superfamilies. Motifs of group I and III do not have any preferential location in the promoters, while group II motifs are located in the proximal part, from -36 to -224. Milk protein gene promoters were analysed for presence of putative binding sites for nine transcription factors important for the expression of this group of genes. The transcription factor binding sites for C/EBP, CTF/NF1, MAF and MGF were found in all promoters investigated. The set of putative transcription factor binding sites or response elements for GRE, IRE, PMF, STR and YY1 is unique for every gene superfamily.

Repression participates in mammary tissue-specific activation of the caprine beta-lactoglobulin promoter

Activation of the beta-lactoglobulin (BLG) gene promoter is restricted to overtly differentiated mammary tissue. To understand the mechanism underlying such tissue-specificity, activity of the caprine BLG promoter was analyzed comparatively in cultured mammary HC11 cells and non-mammary HeLa and CV-1 cells. The BLG promoter flanked by the 5'-regulatory sequence below -205 was strongly activated in the cells, regardless of the cell type. In non-mammary HeLa and CV-1 cells, this activation was repressed completely by the upper regulatory sequence. Weak repression was also observed in mammary HC11 cells kept non-confluent. As the mammary HC11 cells grew confluent and maintained the stabilized state, however, repression by the upper regulatory sequence was switched to activation. The repressive upstream flanking sequence was strongly recognized by the binding factors in non-mammary HeLa and CV-1 cells in an in vitro binding assay. Binding intensity and competition strength of the upstream regulatory regions were in a close correlation to their transcriptional repression activities in the cultured cells. The results suggest that the restricted activation of the caprine BLG promoter in differentiated mammary tissue is guaranteed by repression in non-mammary and undifferentiated mammary cells.

Upstream genomic sequence of the human connexin26 gene

Human connexin 26 (Cx26) has been considered to be a candidate suppressor gene in mammary epithelial cells. To gain insight into the transcriptional regulation of this gene, we have cloned and sequenced the 5' portion of the gene, which extends 4.8 kb upstream from the ATG translation start site. The 3' end of the non-coding exon 1 (160 bp) is located at 3149 bp upstream from the 5' end of exon 2. Comparison between the human Cx26 gene and the mouse gene reveals a highly conserved promoter region with 81% homology. In addition to six GC boxes and two GT boxes, a TTAAAA box is located at -24 to -19 bp upstream of the transcription start point. Analogous to the mouse beta-casein gene, the promoter region of the human Cx26 gene also contains a YY1-like binding site and a consensus mammary gland factor binding site.

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.

Structural organization and mammary-specific expression of the butyrophilin gene

Butyrophilin is a glycoprotein of the immunoglobulin superfamily that is secreted in association with the milk-fat-globule membrane from mammary epithelial cells. As a first step towards determining the possible function(s) of this protein in lactation, the mouse butyrophilin gene (Btn) has been cloned from a 129-ES cell genomic library. Over 14 kb of DNA was sequenced, including the entire transcriptional unit of the gene, and 4.6 kb and 1.1 kb of the 5' and 3' flanking region, respectively. In addition, the overall structure of the bovine gene (BTN) was determined by amplification of genomic DNA by the polymerase chain reaction. Both Btn and BTN comprise seven exons and six introns. The signal sequence and two immunoglobulin-like folds of the exoplasmic domain and the membrane anchor are encoded by separate exons, and the cytoplasmic domain is encoded by two short exons and a large terminal exon that also includes 3' untranslated sequence. The butyrophilin gene appears to have evolved from a subset of genes in the immunoglobulin superfamily and genes encoding the B30.2 domain, which is conserved in a family of zinc-finger proteins. Murine butyrophilin mRNA was detected specifically in the mammary gland by RNase protection analysis. Expression increased during the last half of pregnancy and was maximal during lactation. The 5' flanking region of Btn was analyzed for putative regulatory elements and is different from the promoters of other mammary-specific genes. Btn should be useful for determining the mechanisms underlying mammary-specific gene expression and potentially for the production of heterologous proteins in the milk of transgenic animals.

Characterization of functional domains within the multifunctional transcription factor, YY1

YY1 is a multifunctional transcription factor capable of either activation or repression of transcription. Using a series of mutant proteins, we have characterized domains responsible for activation or repression. We found that the YY1 transcriptional activation domain lies near the amino terminus and requires amino acids 16-29 and 80-100 for maximal activity. The region between residues 16 and 29 has the potential to form an acidic amphipathic helix, whereas residues between 80 and 100 are rich in proline and glutamine. The YY1 repression domain lies near the carboxyl terminus and is embedded within the YY1 zinc finger region necessary for binding to DNA. Deletion of YY1 amino acids, which include zinc fingers 3 and 4, abolishes repression. However, site-directed mutagenesis, progressive deletion, and internal deletion mutant analyses indicate that the normal structures of zinc fingers 3 and 4 are not required for repression.

Adenovirus E1A243 disrupts the ATF/CREB-YY1 complex at the mouse c-fos promoter

The adenovirus E1A243 protein can activate transcription of the mouse c-fos gene in a manner that depends on treatment of cells with inducers or analogs of cyclic AMP (cAMP). Activation requires conserved region 1 and the N-terminal domain of E1A243 and is mediated by a 22-bp E1A response element containing a cAMP response element (CRE) at -67 and a binding site for transcription factor YY1 at -54. In the absence of E1A243, YY1 represses CRE-dependent transcription of c-fos by physically interacting with ATF/CREB proteins bound to the -67 CRE. Here we present evidence that expression of E1A243 leads to relief of YY1-mediated repression by a disruption of the ATF/CREB-YY1 complex. Addition of E1A243 to in vitro binding assays prevented binding of ATF-2 to glutathione S-transferase-YY1. Similarly, expression of E1A243 in HeLa cells prevented the association of a YY1-VP16 fusion protein with endogenous ATF/CREB proteins bound to the -67 CRE of a transfected c-fosCAT reporter plasmid. In each case, the N-terminal domain of E1A243, which mediates a direct interaction with YY1, was responsible for disruption of the ATF/CREB-YY1 complex. On the basis of these and previously published results, we present a model for the synergistic transcriptional activation of the c-fos gene by E1A243 and cAMP.

Prolactin-mediated gene activation in mammary epithelial cells

Mammary epithelial cells grow and develop with the onset of sexual maturity. In addition, lobular alveolar structures are formed during pregnancy, and quiescent differentiated cells secrete high levels of milk proteins after parturition. These events are governed by multiple hormones and growth factors and involve the sequential and synergistic action of functionally distinct signal transduction pathways. Milk protein genes have been analyzed and composite response elements have been identified in the promoter sequences. Transcription factors, which relay the hormonal signals, bind to these sequences. The factor that confers prolactin simulation to milk protein gene transcription has recently been identified. MGF/Stat5 is a latent transcription factor that becomes activated by a tyrosine-specific protein kinase, Jak2, associated with the prolactin receptor. Tyrosine phosphorylation converts the latent factor into one with DNA-binding and transcriptional activation potential. The regulation of MGF/Stat5 in vitro and in vivo indicates that it is a central component of the lactogenic hormone signaling pathway. Involvement of MGF/Stat5 in the signaling by other cytokines indicates that the same factor might be involved in regulation of growth-promoting genes, primarily in hematopoietic cells.

Activation of STAT factors by prolactin, interferon-gamma, growth hormones, and a tyrosine phosphatase inhibitor in rabbit primary mammary epithelial cells

In numerous studies on mammary epithelial cell lines multiple factors, added to the medium or contained in the serum, were required for casein gene expression. It has been shown in these systems that the mammary gland factor (MGF) is implicated in the activation of the beta-casein gene promoter. In the present study, we determined the relationship between known agents that affect casein gene expression and MGF activity using the properties of rabbit primary mammary epithelial cells to respond to PRL alone, when cultured in chemically defined medium. We demonstrate that MGF is rapidly activated by PRL alone or by human growth hormone, a natural ligand of many PRL receptors (PRL-Rs), in the cytoplasm and accumulated in the nucleus. The MGF activation by PRL occurred in the absence of endogenous extracellular matrix, a condition where casein synthesis is known to be markedly reduced. Different inhibitors of protein-tyrosine kinases, which have been shown to reduce casein mRNA synthesis, but not of protein kinase C, decrease the MGF activity. A tyrosine phosphatase inhibitor, sodium pervanadate, induced two GAS-binding complexes related to MGF and STAT1. Our data show that MGF is a latent cytoplasmic factor rapidly activated in mammary epithelial cells, by a mechanism involving a tyrosine kinase and a tyrosine phosphatase.

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

CCAAT/enhancer-binding protein isoforms beta and delta are expressed in mammary epithelial cells and bind to multiple sites in the beta-casein gene promoter

Lactogenic hormone-dependent expression of the rat beta-casein gene in mammary epithelial cells is controlled via a complex regulatory region in the promoter. The sequence between -176 and -82 is the minimal region to confer the response to glucocorticoid hormone and prolactin on a heterologous promoter. The response is further enhanced by the region between -282 and -176. DNase I footprinting experiments and electromobility shift assays revealed the presence of four binding sites for CCAAT/enhancer-binding protein (C/EBP) isoforms in the hormone response region between -220 and -132. In nuclear extracts from mammary epithelial cells, the prevalent C/EBP isoform binding to these sites is beta (C/EBP-beta). C/EBP-delta is also present in mammary epithelial cells, whereas C/EBP-alpha is not detectable. The C/EBP sites are located in close proximity to the previously characterized binding sites for the prolactin-inducible mammary gland factor/signal transducer and activator of transcription-5, the nuclear factor YY1, and the glucocorticoid receptor. The importance of the two proximal C/EBP binding sites at the 5' border of the minimal region was tested by mutational analysis. Mutations of each site were found to inhibit strongly both the basal and the lactogenic hormone-induced transcription of a beta-casein gene promoter chloramphenicol acetyltransferase construct. The results implicate C/EBPs as important regulators of beta-casein gene expression in the mammary epithelium.

Cyclophilin A and FKBP12 interact with YY1 and alter its transcriptional activity

YY1 is a zinc finger transcription factor with unusual structural and functional features. In a yeast two-hybrid screen, two cellular proteins, cyclophilin A (CyPA) and FK506-binding protein 12 (FKBP12), interacted with YY1. These interactions are specific and also occur in mammalian cells. Cyclosporin A and FK506 efficiently disrupt the YY1-CyPA and YY1-FKBP12 interactions. Overexpression of human CyPA and FKBP12 have different effects on YY1-regulated transcription with these effects being promoter-dependent. These results suggest that immunophilins may be mediators in the functional role of YY1.

Mediation of growth hormone-dependent transcriptional activation by mammary gland factor/Stat 5

Previous observations have shown that binding of growth hormone to its receptor leads to activation of transcription factors via a mechanism involving phosphorylation on tyrosine residues. In order to establish whether the prolactin-activated transcription factor Stat 5 (mammary gland factor) is also activated by growth hormone, nuclear extracts were prepared from COS-7 cells transiently expressing transfected Stat 5 and growth hormone receptor cDNA. Gel electrophoresis mobility shift analyses revealed the growth hormone-dependent presence of specific DNA-binding proteins in these extracts. The complexes formed could be supershifted by polyclonal anti-Stat 5 antiserum. In other experiments nuclear extracts from growth hormone-treated Chinese hamster ovary cells stably expressing transfected growth hormone receptor cDNA and liver from growth hormone-treated hypophysectomized rats were used for gel electrophoresis mobility shift analyses. These also revealed the presence of specific DNA-binding proteins sharing antigenic determinants with Stat 5. Stat 5 cDNA was shown to be capable of complementing the growth hormone-dependent activation of transcription of a reporter gene in the otherwise unresponsive COS-7 cell line. This complementation was dependent on the presence of Stat 5 tyrosine 694, suggesting a role for phosphorylation of this residue in growth hormone-dependent activation of DNA-binding and transcription.

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).