Expression, activity, and subcellular localization of the Yin Yang 1 transcription factor in Xenopus oocytes and embryos

YY1 Oligomerization Is Regulated by Its OPB Domain and Competes with Its Regulation of Oncoproteins

Simple Summary

YY1 regulates various cancer-related genes and activates key oncoproteins. In this study, we discovered that the oncoprotein binding (OPB) domain of YY1 is both necessary and stimulatory to its oligomerization. The hydrophobic residues, especially F219, in the OPB are essential to YY1 intermolecular interaction. Strikingly, the mutations of the hydrophobic residues showed better ability than wild-type YY1 in promote breast cancer cell proliferation and migration. Our further study revealed that YY1 proteins with mutated hydrophobic residues in the OPB domain showed improved binding affinity to EZH2. Overall, our data support the model of a mutually exclusive process between oligomerization of YY1 and its regulation of the oncoproteins EZH2, AKT and MDM2.

Abstract

Yin Yang 1 (YY1) plays an oncogenic role through regulating the expression of various cancer-related genes and activating key oncoproteins. Previous research reported that YY1 protein formed dimers or oligomers without definite biological implications. In this study, we first demonstrated the oncoprotein binding (OPB) and zinc finger (ZF) domains of YY1 as the regions involved in its intermolecular interactions. ZFs are well-known for protein dimerization, so we focused on the OPB domain. After mutating three hydrophobic residues in the OPB to alanines, we discovered that YY1(F219A) and YY1(3A), three residues simultaneously replaced by alanines, were defective of intermolecular interaction. Meanwhile, the OPB peptide could robustly facilitate YY1 protein oligomerization. When expressed in breast cancer cells with concurrent endogenous YY1 knockdown, YY1(F219A) and (3A) mutants showed better capacity than wt in promoting cell proliferation and migration, while their interactions with EZH2, AKT and MDM2 showed differential alterations, especially with improved EZH2 binding affinity. Our study revealed a crucial role of the OPB domain in facilitating YY1 oligomerization and suggested a mutually exclusive regulation between YY1-mediated enhancer formation and its activities in promoting oncoproteins.

YY1 regulates cancer cell immune resistance by modulating PD-L1 expression

Recent advances in the treatment of various cancers have resulted in the adaptation of several novel immunotherapeutic strategies. Notably, the recent intervention through immune checkpoint inhibitors has resulted in significant clinical responses and prolongation of survival in patients with several therapy-resistant cancers (melanoma, lung, bladder, etc.). This intervention was mediated by various antibodies directed against inhibitory receptors expressed on cytotoxic T-cells or against corresponding ligands expressed on tumor cells and other cells in the tumor microenvironment (TME). However, the clinical responses were only observed in a subset of the treated patients; it was not clear why the remaining patients did not respond to checkpoint inhibitor therapies. One hypothesis stated that the levels of PD-L1 expression correlated with poor clinical responses to cell-mediated anti-tumor immunotherapy. Hence, exploring the underlying mechanisms that regulate PD-L1 expression on tumor cells is one approach to target such mechanisms to reduce PD-L1 expression and, therefore, sensitize the resistant tumor cells to respond to PD-1/PD-L1 antibody treatments. Various investigations revealed that the overexpression of the transcription factor Yin Yang 1 (YY1) in most cancers is involved in the regulation of tumor cells' resistance to cell-mediated immunotherapies. We, therefore, hypothesized that the role of YY1 in cancer immune resistance may be correlated with PD-L1 overexpression on cancer cells. This hypothesis was investigated and analysis of the reported literature revealed that several signaling crosstalk pathways exist between the regulations of both YY1 and PD-L1 expressions. Such pathways include p53, miR34a, STAT3, NF-kB, PI3K/AKT/mTOR, c-Myc, and COX-2. Noteworthy, many clinical and pre-clinical drugs have been utilized to target these above pathways in various cancers independent of their roles in the regulation of PD-L1 expression. Therefore, the direct inhibition of YY1 and/or the use of the above targeted drugs in combination with checkpoint inhibitors should result in enhancing the cell-mediated anti-tumor cell response and also reverse the resistance observed with the use of checkpoint inhibitors alone.

c-Abl phosphorylation of Yin Yang 1's conserved tyrosine 254 in the spacer region modulates its transcriptional activity

Yin Yang 1 (YY1) is a multifunctional transcription factor that can activate or repress transcription depending on the promotor and/or the co-factors recruited. YY1 is phosphorylated in various signaling pathways and is critical for different biological functions including embryogenesis, apoptosis, proliferation, cell-cycle regulation and tumorigenesis. Here we report that YY1 is a substrate for c-Abl kinase phosphorylation at conserved residue Y254 in the spacer region. Pharmacological inhibition of c-Abl kinase by imatinib, nilotinib and GZD824, knock-down of c-Abl using siRNA, and the use of c-Abl kinase-dead drastically reduces tyrosine phosphorylation of YY1. Both radioactive and non-radioactive in vitro kinase assays, as well as co-immunoprecipitation in different cell lines, show that the target of c-Abl phosphorylation is tyrosine residue 254. c-Abl phosphorylation has little effect on YY1 DNA binding ability or cellular localization in asynchronous cells. However, functional studies reveal that c-Abl mediated phosphorylation of YY1 regulates YY1's transcriptional ability in vivo. In conclusion, we demonstrate the novel role of c-Abl kinase in regulation of YY1's transcriptional activity, linking YY1 regulation with c-Abl tyrosine kinase signaling pathways.

Conserved RNA binding activity of a Yin-Yang 1 homologue in the ova of the purple sea urchin Strongylocentrotus purpuratus

Yin-Yang 1 (YY1) is a highly conserved transcription factor possessing RNA-binding activity. A putative YY1 homologue was previously identified in the developmental model organism Strongylocentrotus purpuratus (the purple sea urchin) by genomic sequencing. We identified a high degree of sequence similarity with YY1 homologues of vertebrate origin which shared 100% protein sequence identity over the DNA- and RNA-binding zinc-finger region with high similarity in the N-terminal transcriptional activation domain. SpYY1 demonstrated identical DNA- and RNA-binding characteristics between Xenopus laevis and S. purpuratus indicating that it maintains similar functional and biochemical properties across widely divergent deuterostome species. SpYY1 binds to the consensus YY1 DNA element, and also to U-rich RNA sequences. Although we detected SpYY1 RNA-binding activity in ova lysates and observed cytoplasmic localization, SpYY1 was not associated with maternal mRNA in ova. SpYY1 expressed in Xenopus oocytes was excluded from the nucleus and associated with maternally expressed cytoplasmic mRNA molecules. These data demonstrate the existence of an YY1 homologue in S. purpuratus with similar structural and biochemical features to those of the well-studied vertebrate YY1; however, the data reveal major differences in the biological role of YY1 in the regulation of maternally expressed mRNA in the two species.

Methylation of transcription factor YY2 regulates its transcriptional activity and cell proliferation

Yin Yang 1 (YY1) is a multifunctional DNA-binding transcription factor shown to be critical in a variety of biological processes, and its activity and function have been shown to be regulated by multitude of mechanisms, which include but are not limited to post-translational modifications (PTMs), its associated proteins and cellular localization. YY2, the paralog of YY1 in mouse and human, has been proposed to function redundantly or oppositely in a context-specific manner compared with YY1. Despite its functional importance, how YY2’s DNA-binding activity and function are regulated, particularly by PTMs, remains completely unknown. Here we report the first PTM with functional characterization on YY2, namely lysine 247 monomethylation (K247me1), which was found to be dynamically regulated by SET7/9 and LSD1 both in vitro and in cultured cells. Functional study revealed that SET7/9-mediated YY2 methylation regulated its DNA-binding activity in vitro and in association with chromatin examined by chromatin immunoprecipitation coupled with sequencing (ChIP-seq) in cultured cells. Knockout of YY2, SET7/9 or LSD1 by CRISPR (clustered, regularly interspaced, short palindromic repeats)/Cas9-mediated gene editing followed by RNA sequencing (RNA-seq) revealed that a subset of genes was positively regulated by YY2 and SET7/9, but negatively regulated by LSD1, which were enriched with genes involved in cell proliferation regulation. Importantly, YY2-regulated gene transcription, cell proliferation and tumor growth were dependent, at least partially, on YY2 K247 methylation. Finally, somatic mutations on YY2 found in cancer, which are in close proximity to K247, altered its methylation, DNA-binding activity and gene transcription it controls. Our findings revealed the first PTM with functional implications imposed on YY2 protein, and linked YY2 methylation with its biological functions.

Function of YY1 in Long-Distance DNA Interactions

During B cell development, long-distance DNA interactions are needed for V(D)J somatic rearrangement of the immunoglobulin (Ig) loci to produce functional Ig genes, and for class switch recombination (CSR) needed for antibody maturation. The tissue-specificity and developmental timing of these mechanisms is a subject of active investigation. A small number of factors are implicated in controlling Ig locus long-distance interactions including Pax5, Yin Yang 1 (YY1), EZH2, IKAROS, CTCF, cohesin, and condensin proteins. Here we will focus on the role of YY1 in controlling these mechanisms. YY1 is a multifunctional transcription factor involved in transcriptional activation and repression, X chromosome inactivation, Polycomb Group (PcG) protein DNA recruitment, and recruitment of proteins required for epigenetic modifications (acetylation, deacetylation, methylation, ubiquitination, sumoylation, etc.). YY1 conditional knock-out indicated that YY1 is required for B cell development, at least in part, by controlling long-distance DNA interactions at the immunoglobulin heavy chain and Igκ loci. Our recent data show that YY1 is also required for CSR. The mechanisms implicated in YY1 control of long-distance DNA interactions include controlling non-coding antisense RNA transcripts, recruitment of PcG proteins to DNA, and interaction with complexes involved in long-distance DNA interactions including the cohesin and condensin complexes. Though common rearrangement mechanisms operate at all Ig loci, their distinct temporal activation along with the ubiquitous nature of YY1 poses challenges for determining the specific mechanisms of YY1 function in these processes, and their regulation at the tissue-specific and B cell stage-specific level. The large numbers of post-translational modifications that control YY1 functions are possible candidates for regulation.

Transcriptional repression of Bim by a novel YY1-RelA complex is essential for the survival and growth of Multiple Myeloma

Multiple Myeloma (MM) is an incurable plasma cell cancer that is caused by several chromosomal translocations and gene deletions. Although deregulation of several signaling pathways including the Nuclear Factor-Kappa B (NF-κB) pathway has been reported in MM, the molecular requirement and the crosstalk between NF-κB and its target genes in MM cell survival has been largely unclear. Here, we report that Yin Yang1 (YY1), a target gene for NF-κB, is hyperexpressed in most MM tumor cells obtained from human patients, exhibits constitutive nuclear localization, and is essential for survival of MM cells. Mechanistically, we report a novel YY1-RelA complex formation, which is essential to transcriptionally repress a proapoptotic gene Bim. In line with this, depletion of YY1 or RelA resulted in elevated levels of Bim and apoptosis. Moreover, both YY1 and RelA are recruited to the Bim promoter and are required to repress the Bim promoter. Importantly, depletion of YY1 or RelA almost completely impaired the colony forming ability of MM progenitor cells suggesting that both RelA and YY1 are essential for the survival and growth of MM progenitor cells. Moreover, depletion of either YY1 or RelA completely inhibited MM tumor growth in xenograft models for human myeloma. Thus, a novel RelA-YY1 transcriptional repression complex is an attractive drug target in MM.

Cys2His2 zinc finger protein family: classification, functions, and major members

Cys2His2 (C2H2)-type zinc fingers are widespread DNA binding motifs in eukaryotic transcription factors. Zinc fingers are short protein motifs composed of two or three β-layers and one α-helix. Two cysteine and two histidine residues located in certain positions bind zinc to stabilize the structure. Four other amino acid residues localized in specific positions in the N-terminal region of the α-helix participate in DNA binding by interacting with hydrogen donors and acceptors exposed in the DNA major groove. The number of zinc fingers in a single protein can vary over a wide range, thus enabling variability of target DNA sequences. Besides DNA binding, zinc fingers can also provide protein-protein and RNA-protein interactions. For the most part, proteins containing the C2H2-type zinc fingers are trans regulators of gene expression that play an important role in cellular processes such as development, differentiation, and suppression of malignant cell transformation (oncosuppression).

Two-timing zinc finger transcription factors liaising with RNA

Classical zinc fingers (ZFs) are one of the most common protein domains in higher eukaryotes and have been known for almost 30 years to act as sequence-specific DNA-binding domains. This knowledge has come, however, from the study of a small number of archetypal proteins, and a larger picture is beginning to emerge that ZF functions are far more diverse than originally suspected. Here, we review the evidence that a subset of ZF proteins live double lives, binding to both DNA and RNA targets and frequenting both the cytoplasm and the nucleus. This duality can create an important additional level of gene regulation that serves to connect transcriptional and post-transcriptional control.

YY1 controls immunoglobulin class switch recombination and nuclear activation-induced deaminase levels

Activation-induced deaminase (AID) is an enzyme required for class switch recombination (CSR) and somatic hypermutation (SHM), processes that ensure antibody maturation and expression of different immunoglobulin isotypes. AID function is tightly regulated by tissue- and stage-specific expression, nuclear localization, and protein stability. Transcription factor YY1 is crucial for early B cell development, but its function at late B cell stages is unknown. Here, we show that YY1 conditional knockout in activated splenic B cells interferes with CSR. Knockout of YY1 did not affect B cell proliferation, transcription of the AID and IgM genes, or levels of various switch region germ line transcripts. However, we show that YY1 physically interacts with AID and controls the accumulation of nuclear AID, at least in part, by increasing nuclear AID stability. We show for the first time that YY1 plays a novel role in CSR and controls nuclear AID protein levels.

Beta-arrestin1 regulates zebrafish hematopoiesis through binding to YY1 and relieving polycomb group repression

Beta-arrestin1 is a multifunctional protein critically involved in signal transduction. Recently, it is also identified as a nuclear transcriptional regulator, but the underlying mechanisms and physiological significance remain to be explored. Here, we identified beta-arrestin1 as an evolutionarily conserved protein essential for zebrafish development. Zebrafish embryos depleted of beta-arrestin1 displayed severe posterior defects and especially failed to undergo hematopoiesis. In addition, the expression of cdx4, a critical regulator of embryonic blood formation, and its downstream hox genes were downregulated by depletion of beta-arrestin1, while injection of cdx4, hoxa9a or hoxb4a mRNA rescued the hematopoietic defects. Further mechanistic studies revealed that beta-arrestin1 bound to and sequestered the polycomb group (PcG) recruiter YY1, and relieved PcG-mediated repression of cdx4-hox pathway, thus regulating hematopoietic lineage specification. Taken together, this study demonstrated a critical role of beta-arrestin1 during zebrafish primitive hematopoiesis, as well as an important regulator of PcG proteins and cdx4-hox pathway.

Regulation of the transcription factor YY1 in mitosis through phosphorylation of its DNA-binding domain

Yin-Yang 1 (YY1) is a ubiquitously expressed zinc finger transcription factor. It regulates a vast array of genes playing critical roles in development, differentiation, and cell cycle. Very little is known about the mechanisms that regulate the functions of YY1. It has long been proposed that YY1 is a phosphoprotein; however, a direct link between phosphorylation and the function of YY1 has never been proven. Investigation of the localization of YY1 during mitosis shows that it is distributed to the cytoplasm during prophase and remains excluded from DNA until early telophase. Immunostaining studies show that YY1 is distributed equally between daughter cells and rapidly associates with decondensing chromosomes in telophase, suggesting a role for YY1 in early marking of active and repressed genes. The exclusion of YY1 from DNA in prometaphase HeLa cells correlated with an increase in the phosphorylation of YY1 and loss of DNA-binding activity that can be reversed by dephosphorylation. We have mapped three phosphorylation sites on YY1 during mitosis and show that phosphorylation of two of these sites can abolish the DNA-binding activity of YY1. These results demonstrate a novel mechanism for the inactivation of YY1 through phosphorylation of its DNA-binding domain.

Biochemical characterization of Yin Yang 1-RNA complexes

YY1 (Yin Yang 1) is present in the Xenopus oocyte cytoplasm as a constituent of messenger ribonucleoprotein complexes (mRNPs). Association of YY1 with mRNPs requires direct RNA-binding activity. Previously, we have shown YY1 has a high affinity for U-rich RNA; however, potential interactions with plausible in vivo targets have not been investigated. Here we report a biochemical characterization of the YY1-RNA interaction including an investigation of the stability, potential 5'-methylguanosine affinity, and specificity for target RNAs. The formation of YY1-RNA complexes in vitro was highly resistant to thermal, ionic, and detergent disruption. The endogenous oocyte YY1-mRNA interactions were also found to be highly stable. Specific YY1-RNA interactions were observed with selected mRNA and 5S RNA probes. The affinity of YY1 for these substrates was within an order of magnitude of that for its cognate DNA element. Experiments aimed at determining the potential role of the 7-methylguanosine cap on RNA-binding reveal no significant difference in the affinity of YY1 for capped or uncapped mRNA. Taken together, the results show that the YY1-RNA interaction is highly stable, and that YY1 possesses the ability to interact with structurally divergent RNA substrates. These data are the first to specifically document the interaction between YY1 and potential in vivo targets.

The Yin and Yang of YY1 in the nervous system

The transcription factor Yin Yang 1 (YY1) is a multifunctional protein that can activate or repress gene expression depending on the cellular context. YY1 is ubiquitously expressed and highly conserved between species. However, its role varies in diverse cell types and includes proliferation, differentiation, and apoptosis. This review will focus on the function of YY1 in the nervous system including its role in neural development, neuronal function, developmental myelination, and neurological disease. The multiple functions of YY1 in distinct cell types are reviewed and the possible mechanisms underlying the cell specificity for these functions are discussed.

Photoreceptor-specific expression, light-dependent localization, and transcriptional targets of the zinc-finger protein Yin Yang 1 in the chicken retina

The zinc-finger transcription factor Yin Yang 1 (YY1) is a multifunctional protein that plays a critical role in embryonic development. Although it has been shown to play a role in eye development, its expression in the retina was not previously described. Here, we investigated YY1 expression in chicken tissues and we identified the neural retina as one of the tissues with highest YY1 protein levels. Immunohistochemical detection of YY1 in the retina revealed a clear-cut photoreceptor specificity and day/night differences in the cytoplasmic localization of the protein. YY1 was also present at high concentration in the nuclei of some photoreceptors. Gel-shift assays indicated YY1 bound to regulatory regions of several genes specifically expressed in photoreceptors. One of these genes, hydroxyindole-O-methyltransferase (EC 2.1.1.4), encodes the last enzyme of the melatonin synthesis pathway. Although over-expression of chicken YY1 was not sufficient to activate the chicken hydroxyindole-O-methyltransferase promoter in HEK293 cells, the YY1-binding site contained in this promoter was clearly required for full transcriptional activity in chicken embryonic retinal cells. These results suggest a role of YY1 in regulating the melatoninergic function of retinal photoreceptors.

Assembly of the Yin Yang 1 transcription factor into messenger ribonucleoprotein particles requires direct RNA binding activity

The early stages of vertebrate development depend heavily on control of maternally transcribed mRNAs that are stored for long periods in complexes termed messenger ribonucleoprotein particles (mRNPs) and utilized selectively following maturation and fertilization. The transcription factor Yin Yang 1 (YY1) is associated with cytoplasmic mRNPs in vertebrate oocytes; however, the mechanism by which any of the mRNP proteins associate with mRNA in the oocyte is unknown. Here we demonstrate the mechanism by which YY1 associates with mRNPs depends on its direct RNA binding activity. High affinity binding for U-rich single-stranded RNA and A:U RNA duplexes was observed in the nanomolar range, similar to the affinity for the cognate double-stranded DNA-binding element. Similar RNA binding affinity was observed with endogenous YY1 isolated from native mRNP complexes. In vivo expression experiments reveal epitope-tagged YY1 assembled into high molecular mass mRNPs, and assembly was blocked by microinjection of high affinity RNA substrate competitor. These findings present the first clues to how mRNPs assemble during early development.

Changes of maternal transcripts in oocytes from persistent follicles in cattle

A high incidence of early embryonic loss is associated with prolonged dominance of follicles. The objective of the present experiment was to determine if persistence of a follicle resulted in alterations in mRNA expression of important genes in the oocyte. Cows were assigned to four groups: growing follicles on day 6 (G0h) or day 8 (G48h) and persistent follicles on day 13 (P0h) or day 15 (P48h) of the estrous cycle (estrus = day 0). All cows were super-stimulated on day 1-4. Cows in G48h, P0h, and P48h groups received 25 mg prostaglandin (PG) F2alpha on day 6. Cows in P0h and P48h groups received progesterone from CIDR-B devices on day 5 through 13. Ovaries of cows in G0h, G48h, P0h, and P48h groups were removed on day 6, 8, 13, and 15, respectively. Oocytes were aspirated immediately after colpotomy and denuded of cumulus cells. Quantitative real-time PCR was used to measure the mRNA abundances of 10 selected genes important for early embryogenesis in oocytes obtained from growing and persistent follicles. Relative abundances of MSY2, PARN, and YY1 mRNA (P < 0.05) were significantly lower in oocytes from persistent than from growing follicles. Oocytes from persistent follicles, however, had greater abundances of PAP and eIF-4E transcripts (P < 0.05). The data indicate that persistence of a follicle leads to altered abundances of mRNA for genes important for regulation of transcription and protein translation in the oocyte, which could compromise development of early embryos in cows that ovulate a persistent follicle.

Multifunctional transcription factor YY1: a therapeutic target in human cancer?

The multifunctional transcription factor Yin Yang 1 (YY1) is a complex protein that has been shown to play pivotal roles in development, differentiation, cellular proliferation and apoptosis. It can act as a transcriptional repressor, an activator, or an initiator element binding protein that directs and initiates transcription of numerous cellular and viral genes. Because the expression and function of YY1 are known to be intimately associated with cell-cycle progression, the physiological significance of YY1 activity has recently been applied to models of cancer biology. Several lines of evidence imply that YY1 expression and/or activation is associated with tumourigenesis, in addition to its regulatory roles in normal biological processes. However, controversial results also raised and indicated that further studies are still needed to piece all of the seemingly contradictory data into a complete picture. On the basis of YY1 regulations and functions, novel drugs and specific treatment strategies may be developed with new therapeutic applications for tumour patients in the future.

Cytoplasmic YY1 is associated with increased smooth muscle-specific gene expression: implications for neonatal pulmonary hypertension

Immediately after birth the adluminal vascular SMCs of the pulmonary elastic arteries undergo transient actin cytoskeletal remodeling as well as cellular de-differentiation and proliferation. Vascular smooth muscle phenotype is regulated by serum response factor, which is itself regulated in part by the negative regulator YY1. We therefore studied the subcellular localization of YY1 in arteries of normal newborn piglets and piglets affected by neonatal pulmonary hypertension. We found that YY1 localization changed during development and that expression of gamma-smooth muscle actin correlated with expression of cytoplasmic rather than nuclear YY1. Analysis of the regulation of YY1 localization in vitro demonstrated that polymerized gamma-actin sequestered EGFP-YY1 in the cytoplasm and that YY1 activation of c-myc promoter activity was inhibited by LIM kinase, which increases actin polymerization. Consistent with these data siRNA-mediated down-regulation of YY1 in C2C12 cells increased SM22-alpha expression and inhibited cell proliferation. Thus, actin polymerization controls subcellular YY1 localization, which contributes to vascular SMC proliferation and differentiation in normal pulmonary artery development. In the absence of actin depolymerization, YY1 does not relocate to the nucleus, and this lack of relocation may contribute to the pathobiology of pulmonary hypertension.

Caspase-dependent regulation and subcellular redistribution of the transcriptional modulator YY1 during apoptosis

The transcriptional regulator Yin Yang 1 (YY1) controls many aspects of cell behavior and is essential for development. We analyzed the fate of YY1 during apoptosis and studied the functional consequences. We observed that this factor is rapidly translocated into the cell nucleus in response to various apoptotic stimuli, including activation of Fas, stimulation by tumor necrosis factor, and staurosporine and etoposide treatment. Furthermore, YY1 is cleaved by caspases in vitro and in vivo at two distinct sites, IATD(12)G and DDSD(119)G, resulting in the deletion of the first 119 amino acids early in the apoptotic process. This activity generates an N-terminally truncated YY1 fragment (YY1Delta119) that has lost its transactivation domain but retains its DNA binding domain. Indeed, YY1Delta119 is no longer able to stimulate gene transcription but interacts with DNA. YY1Delta119 but not the wild-type protein or the caspase-resistant mutant YY1D12A/D119A enhances Fas-induced apoptosis, suggesting that YY1 is involved in a positive feedback loop during apoptosis. Our findings provide evidence for a new mode of regulation of YY1 and define a novel aspect of the involvement of YY1 in the apoptotic process.

GABP, HCF-1 and YY1 are involved in Rb gene expression during myogenesis

Muscle cell differentiation, or myogenesis, is a well-characterized process and involves the expression of specific sets of genes in an orderly manner. A prerequisite for myogenesis is the exit from the cell cycle, which is associated with the up-regulation of the tumor suppressor Rb. In this study, we set to investigate the regulatory mechanism of the Rb promoter that allows adequate up-regulation in differentiating myoblasts. We report that Rb expression is regulated by the transcription factors GABP, HCF-1 and YY1. Before induction of differentiation, Rb is expressed at a low level and GABP and YY1 are both present on the promoter. YY1, which exerts an inhibitory effect on Rb expression, is removed from the promoter as cells advance through myogenesis and translocates from the nucleus to the cytoplasm. On the other hand, upon induction of differentiation, the GABP cofactor HCF-1 is recruited to and coactivates the promoter with GABP. RNAi-mediated knock-down of HCF-1 results in inhibition of Rb up-regulation as well as myotube formation. These results indicate that the Rb promoter is subject to regulation by positive and negative factors and that this intricate activation mechanism is critical to allow the accurate Rb gene up-regulation observed during myogenesis.

YY1 regulates the neural crest-associated slug gene in Xenopus laevis

slug gene expression is associated with the specification and migration of neural crest cells in the African clawed frog Xenopus laevis. We provide evidence that the protein Ying-Yang 1 (YY1) regulates the slug gene expression both indirectly and directly, via a YY1 cis-element in the slug promoter, during Xenopus development. The ability of the YY1 to bind this YY1 cis-element was confirmed by electromobility shift assays and reporter assays. YY1 was detected in the nuclei of ectodermal cells contemporaneously with the process of neural crest specification. The injection of anti-YY1 morpholino, which targeted both YY1alpha and YY1beta gene products, depleted YY1 expression below 20% and was lethal at gastrulation. Sublethal depletion of YY1 reduced the length of the anterior-posterior axis and severely inhibited the expression of the neural marker Nrp1 and of the slug gene. Overexpression of YY1 or mutation of the YY1 cis-element reduced the restricted spatial expression of the slug reporter gene in the neural ectoderm border and provoked its expression in the nonneural ectoderm. Chromatin immunoprecipitation indicated that endogenous YY1 interacts directly with the YY1 cis-element of the endogenous slug gene and with the slug gene reporter sequence injected into embryos. The results suggest that YY1 is essential for Xenopus development; is necessary for neural ectoderm differentiation, a prerequisite for neural crest specification; and restricts which cells can form neural crest mesenchyme through directly blocking slug gene activity.

Molecular Cloning and Functional Characterization of the Transcription Factor YY2

YY1 is a ubiquitous zinc finger transcription factor that binds to and regulates promoters and enhancers of many cellular and viral genes. Here we report the isolation of a human cDNA encoding a DNA sequence-specific binding protein with significant homology to the transcription factor YY1. A sequence analysis of this novel protein, YY2, revealed an overall 65% identity in the DNA sequence and a 56% identity in protein sequence compared with human YY1. The most pronounced similarity between YY1 and YY2 exists within the zinc finger regions of the two proteins, and consistent with this observation, YY2 can bind to and regulate some promoters known to be controlled by YY1. Similar to YY1, YY2 contains both transcriptional activation and repression functions. The finding of a protein with structure and function similar to YY1 provides a new opportunity to explore additional mechanisms by which YY1-responsive genes can be regulated and suggests that gene regulation by YY1 is far more complicated than previously assumed.

Transcriptional regulation of the cardiac-specificMLC2gene duringXenopusembryonic development

The mechanisms by which transcription factors, which are not themselves tissue restricted, establish cardiomyocyte-specific patterns of transcription in vivo are unknown. Nor do we understand how positional cues are integrated to provide regionally distinct domains of gene expression within the developing heart. We describe regulation of the Xenopus XMLC2 gene,which encodes a regulatory myosin light chain of the contractile apparatus in cardiac muscle. This gene is expressed from the onset of cardiac differentiation in the frog embryo and is expressed throughout all the myocardium, both before and after heart chamber formation. Using transgenesis in frog embryos, we have identified an 82 bp enhancer within the proximal promoter region of the gene that is necessary and sufficient for heart-specific expression of an XMLC2 transgene. This enhancer is composed of two GATA sites and a composite YY1/CArG-like site. We show that the low-affinity SRF site is essential for transgene expression and that cardiac-specific expression also requires the presence of at least one adjacent GATA site. The overlapping YY1 site within the enhancer appears to act primarily as a repressor of ectopic expression, although it may also have a positive role. Finally, we show that the frog MLC2 promoter drives pan myocardial expression of a transgene in mice, despite the more restricted patterns of expression of murine MLC2 genes. We speculate that a common regulatory mechanism may be responsible for pan-myocardial expression of XMLC2 in both the frog and mouse, modulation of which could have given rise to more restricted patterns of expression within the heart of higher vertebrates.

YY1AP, a novel co-activator of YY1

With the aim of identifying potential cellular proteins that mediate the transcriptional regulation of YY1, a HeLa cDNA library was screened using the yeast two-hybrid system. A previously unknown protein interacting with YY1 was identified and named YY1AP. By using the 5'-rapid amplification of cDNA ends technique, the full-length cDNA of YY1AP was cloned and sequenced. The cDNA was 2253 bp in length and encoded an open reading frame of 750 amino acids. The chromosomal gene was made up of 10 exons separated by nine introns and is localized on chromosome 1 (1q21.3). Northern blot analysis revealed that YY1AP is ubiquitously expressed in various human tissues and cancer cell lines. Co-immunoprecipitation and immunostaining of cells further indicated that YY1AP co-localizes with YY1 in the nucleus. Furthermore, YY1AP was shown to be capable of enhancing the transcriptional activation of an YY1 responsive promoter. Subsequent analysis by glutathione S-transferase pull-down assay showed that YY1AP contained two YY1 binding regions. The transactivation region of YY1AP would seem to be localized within the section of amino acids 260-345. It is proposed that YY1AP is a novel co-activator of YY1.

The Yin Yang-1 (YY1) protein undergoes a DNA-replication-associated switch in localization from the cytoplasm to the nucleus at the onset of S phase

The essential Yin Yang-1 gene (YY1) encodes a ubiquitous, conserved, multifunctional zinc-finger transcription factor in animals. The YY1 protein regulates initiation, activation, or repression of transcription from a variety of genes required for cell growth, development, differentiation, or tumor suppression, as well as from genes in some retroviruses and DNA viruses. Among the specific functions attributed to YY1 is a role in cell-cycle-specific upregulation of the replication-dependent histone genes. The YY1 protein binds to the histone alpha element, a regulatory sequence found in all replication-dependent histone genes. We therefore examined the abundance, DNA-binding activity and localization of the YY1 protein throughout the cell cycle in unperturbed, shake-off-synchronized Chinese hamster ovary and HeLa cells. We found that, whereas the DNA-binding activity of YY1 increased dramatically early in S phase, the YY1 mRNA and protein levels did not. YY1 changed subcellular distribution patterns during the cell cycle, from mainly cytoplasmic at G1 to mainly nuclear at early and middle S phase, then back to primarily cytoplasmic later in S phase. Nuclear accumulation of YY1 near the G1/S boundary coincided with both an increase in YY1 DNA-binding activity and the coordinate up-regulation of the replication-dependent histone genes. The DNA synthesis inhibitor aphidicolin caused a nearly complete loss of nuclear YY1, whereas addition of caffeine or 2-aminopurine to aphidicolin-treated cells restored both DNA synthesis and YY1 localization in the nucleus. These findings reveal a mechanism by which YY1 localization is coupled to DNA synthesis and responsive to cell-cycle signaling pathways. Taken together, our results provide insight into how YY1 might participate in the cell-cycle control over a variety of nuclear events required for cell division and proliferation.

Yin Yang 1, a vertebrate polycomb group gene, regulates antero-posterior neural patterning

Polycomb group (PcG) genes are required for the stable repression of the homeotic genes and other developmentally regulated genes. Yin Yang 1 (YY1), a vertebrate homolog of the Drosophila PcG pleiohomeotic (Pho), is a multifunctional protein that can act as a repressor or activator of transcription. Xenopus YY1 (XYY1) protein was localized in the central nervous system (CNS), particularly anterior neural tube of tailbud stage embryos. To elucidate the role of endogenous XYY1, loss-of-function studies were performed using XYY1 antisense morpholino oligonucleotide (XYY1 MO). Inhibition of XYY1 function resulted in embryos with antero-posterior axial patterning defects and reduction of head structures. XYY1 MO also reduced the expression of En2, a midbrain/hindbrain junction marker, which was rescued by co-injection of XYY1 mRNA. However, XYY1 MO-injection did not affect the expression of HoxB9, a spinal cord marker. These results suggest that YY1 controls antero-posterior patterning of the CNS during Xenopus embryonic development.

The Yin Yang 1 transcription factor associates with ribonucleoprotein (mRNP) complexes in the cytoplasm of Xenopus oocytes

Yin Yang 1 (YY1) is a multifunctional transcription factor that activates, represses, or initiates transcription of a diverse assortment of genes. Previous studies suggest a role for YY1 in cellular growth and differentiation, but its biological function during development of the vertebrate oocyte or embryo remains to be determined. We recently showed that YY1 is abundantly expressed throughout oogenesis and early embryonic stages of Xenopus, but it is sequestered in the cytoplasm and does not function directly in transcriptional regulation. In the present study we used a series of biochemical analyses to explore the potential function of YY1 in the oocyte cytoplasm. YY1 was isolated from oocyte lysates by oligo(dT)-cellulose chromatography, suggesting that it associates with maternally expressed mRNA in vivo. RNA mobility shift assays demonstrate that endogenous YY1 binds to labeled histone mRNA. Size exclusion chromatography of oocyte lysates revealed that YY1 exists in high molecular mass complexes in the range of 480 kDa. Destruction of endogenous RNA by RNase treatment of lysates, abolished the binding of YY1 to oligo(dT)-cellulose and resulted in redistribution from 480-kDa complexes to the monomeric form. Microinjection of RNase directly into the cytoplasm released YY1 from 480-kDa complexes and unmasked its DNA-binding activity, but did not promote translocation to the nucleus. These results provide evidence that YY1 is a component of ribonucleoprotein (mRNP) complexes in the Xenopus oocyte, indicating a novel function for YY1 in the storage or metabolism of maternal transcripts.