p300 interacts with the nuclear proto-oncoprotein SYT as part of the active control of cell adhesion

Adhesome Receptor Clustering is Accompanied by the Colocalization of the Associated Genes in the Cell Nucleus

Proteins on the cell membrane cluster to respond to extracellular signals; for example, adhesion proteins cluster to enhance extracellular matrix sensing; or T-cell receptors cluster to enhance antigen sensing. Importantly, the maturation of such receptor clusters requires transcriptional control to adapt and reinforce the extracellular signal sensing. However, it has been unclear how such efficient clustering mechanisms are encoded at the level of the genes that code for these receptor proteins. Using the adhesome as an example, we show that genes that code for adhesome receptor proteins are spatially co-localized and co-regulated within the cell nucleus. Towards this, we use Hi-C maps combined with RNA-seq data of adherent cells to map the correspondence between adhesome receptor proteins and their associated genes. Interestingly, we find that the transcription factors that regulate these genes are also co-localized with the adhesome gene loci, thereby potentially facilitating a transcriptional reinforcement of the extracellular matrix sensing machinery. Collectively, our results highlight an important layer of transcriptional control of cellular signal sensing.

Discovery of berberine analogs as potent and highly selective p300/CBP HAT inhibitors

The protein p300 is a positive regulator of cancer progression and is related to many human pathological conditions. To find effective p300/CBP HAT inhibitors, we screened an internal compound library and identified berberine as a lead compound. Next, we designed, synthesized, and screened a series of novel berberine analogs, and discovered that analog 5d was a potent and highly selective p300/CBP HAT inhibitor with IC50 values of 0.070 μM and 1.755 μM for p300 and CBP, respectively. Western blotting further proved that 5d specifically decreased H3K18Ac and interfere with the function of histone acetyltransferase. Although 5d had only a moderate inhibitory effect on the MDA-MB-231 cell line, 5d suppressed the growth of 4T1 tumor growth in mice with a tumor weight inhibition ratio (TWI) of 39.7%. Further, liposomes-encapsulated 5d increased its inhibition of tumor growth to 57.8 % TWI. In addition, 5d has no obvious toxicity to the main organ of mice and the pharmacokinetic study confirmed that 5d has good absorption properties in vivo.

Synovial Sarcoma: A Complex Disease with Multifaceted Signaling and Epigenetic Landscapes

PURPOSE OF REVIEW

Aside from a characteristic SS18-SSX translocation identified in almost all cases, no genetic anomalies have been reliably isolated yet to drive the pathogenesis of synovial sarcoma. In the following review, we explore the structural units of wild-type SS18 and SSX, particularly as they relate to the transcriptional alterations and cellular pathway changes imposed by SS18-SSX.

RECENT FINDINGS

Native SS18 and SSX contribute recognizable domains to the SS18-SSX chimeric proteins, which inflict transcriptional and epigenetic changes through selective protein interactions involving the SWI/SNF and Polycomb chromatin remodeling complexes. Multiple oncogenic and developmental pathways become altered, collectively reprogramming the cellular origin of synovial sarcoma and promoting its malignant transformation. Synovial sarcoma is characterized by complex epigenetic and signaling landscapes. Identifying the operational pathways and concomitant genetic changes induced by SS18-SSX fusions could help develop tailored therapeutic strategies to ultimately improve disease control and patient survivorship.

Impact of Wnt/β-Catenin Inhibition on Cell Proliferation through CDC25A Downregulation in Soft Tissue Sarcomas

Simple Summary

Growing evidence suggests that Wnt signaling may be crucial for tumorigenesis and progression of soft tissue sarcomas (STS). Inhibitors of this pathway are currently in clinical trials or pre-clinical studies in order to validate its utility in different neoplasia. One of this inhibitors, PRI-724, is showing promising results for advanced pancreatic adenocarcinoma or ovarian cancer. We found that PRI-724 is able to suppress cell viability/proliferation and to increase cell death rates of soft tissue sarcomas cells in vitro. CDC25A, a target gene of Wnt signaling pathway, is essential for STS proliferation because its downregulation via siRNA was able to mimic the effect of PRT-724 on cell cycle arrest and evaluation of NCBI/GenBank data confirmed its overexpression in STS patients’ samples. Moreover, in vitro administration of PRI-724 along with standard STS chemotherapeutic drugs improved the efficacy of chemotherapy, suggesting that Wnt inhibition could be a promising new therapeutic strategy in STS.

Abstract

The Wnt signaling pathway is an important cellular mechanism for regulating differentiation processes as well as cell cycle events, and different inhibitors of this pathway, for example, PRI-724, are showing promising results in clinical trials for treatment of advanced pancreatic adenocarcinoma or ovarian cancer. Growing evidence suggests that Wnt signaling may also be crucial for tumorigenesis and progression of soft tissue sarcomas (STS), a malignant neoplasm with few therapeutic options at an advanced state. Our study with several STS cell lines and primary cultures shows that inhibition of Wnt/β-catenin signaling with PRI-724 is able to suppress cell viability/proliferation and to increase cell death rates. TCF/β-catenin-mediated transcriptional activity is decreased in treated cells, leading to downregulation of its target genes CCND1 and CDC25A. The latter was critical because its downregulation via siRNA was able to mimic the effect of PRI-724 on cell cycle arrest and cell death induction. An evaluation of NCBI/GenBank data confirmed that CDC25A mRNA is elevated in STS patients. Importantly, PRI-724 in combination with standard STS chemotherapeutics doxorubicin or trabectedin enhanced their antitumoral effect in a synergistic manner according to isobolographic analysis, suggesting that Wnt inhibition through PRI-724 could be a beneficial combination regime in patients with advanced STS.

Dissecting myogenin-mediated retinoid X receptor signaling in myogenic differentiation

Deciphering the molecular mechanisms underpinning myoblast differentiation is a critical step in developing the best strategy to promote muscle regeneration in patients suffering from muscle-related diseases. We have previously established that a rexinoid x receptor (RXR)-selective agonist, bexarotene, enhances the differentiation and fusion of myoblasts through a direct regulation of MyoD expression, coupled with an augmentation of myogenin protein. Here, we found that RXR signaling associates with the distribution of myogenin at poised enhancers and a distinct E-box motif. We also found an association of myogenin with rexinoid-responsive gene expression and identified an epigenetic signature related to histone acetyltransferase p300. Moreover, RXR signaling augments residue-specific histone acetylation at enhancers co-occupied by p300 and myogenin. Thus, genomic distribution of transcriptional regulators is an important designate for identifying novel targets as well as developing therapeutics that modulate epigenetic landscape in a selective manner to promote muscle regeneration.

Khilji et al analyse how myogenin distribution changes in differentiating C2C12 myoblasts using ChIP-sequencing. They find that rexinoid x receptor signalling promotes myogenin localisation to poised enhancers and the association of myogenin with p300-mediated histone acetylation, providing insights into epigenetic mechanism of myogenic differentiation.

HDAC2 Regulates Site-Specific Acetylation of MDM2 and Its Ubiquitination Signaling in Tumor Suppression

Histone deacetylases (HDACs) are promising targets for cancer therapy, although their individual actions remain incompletely understood. Here, we identify a role for HDAC2 in the regulation of MDM2 acetylation at previously uncharacterized lysines. Upon inactivation of HDAC2, this acetylation creates a structural signal in the lysine-rich domain of MDM2 to prevent the recognition and degradation of its downstream substrate, MCL-1 ubiquitin ligase E3 (MULE). This mechanism further reveals a therapeutic connection between the MULE ubiquitin ligase function and tumor suppression. Specifically, we show that HDAC inhibitor treatment promotes the accumulation of MULE, which diminishes the t(X; 18) translocation-associated synovial sarcomagenesis by directly targeting the fusion product SS18-SSX for degradation. These results uncover a new HDAC2-dependent pathway that integrates reversible acetylation signaling to the anticancer ubiquitin response.

Identification of SHCBP1 as a novel downstream target gene of SS18-SSX1 and its functional analysis in progression of synovial sarcoma

The SS18-SSX1 fusion gene has been shown to play important roles in the development of synovial sarcoma (SS), but the underlying molecular mechanisms and its downstream target genes are still not clear. Here SHC SH2-domain binding protein 1 (SHCBP1) was identified and validated to be a novel downstream target gene of SS18-SSX1 by using microarray assay, quantitative real-time (qPCR) and western blot. Expression of SHCBP1 was firstly confirmed in SS cell line and SS tissues. The effects of SHCBP1 overexpression or knockdown on SS cell proliferation and tumorigenicity were then studied by cell proliferation, DNA replication, colony formation, flow cytometric assays, and its in vivo tumorigenesis was determined in the nude mice. Meanwhile, the related signaling pathways of SHCBP1 were also examined in SS cells. The results indicated that SHCBP1 was significantly increased in SS cells and SS tissues compared with adjacent noncancerous tissues. The expression of SHCBP1 was demonstrated to be positively correlated with the SS18-SSX1 level. Overexpression and ablation of SHCBP1 promoted and inhibited, respectively, the proliferation and tumorigenicity of SS cells in vitro. SHCBP1 knockdown also significantly inhibited SS cell growth in nude mice, and lowered the MAPK/ERK and PI3K/AKT/mTOR signaling pathways and cyclin D1 expression. Our findings disclose that SHCBP1 is a novel downstream target gene of SS18-SSX1, and demonstrate that the oncogene SS18-SSX1 promotes tumorigenesis by increasing the expression of SHCBP1, which normally acts as a tumor promoting factor.

The genomic and transcriptomic landscape of anaplastic thyroid cancer: implications for therapy

BACKGROUND

Anaplastic thyroid carcinoma is the most undifferentiated form of thyroid cancer and one of the deadliest of all adult solid malignancies. Here we report the first genomic and transcriptomic profile of anaplastic thyroid cancer including those of several unique cell lines and outline novel potential drivers of malignancy and targets of therapy.

METHODS

We describe whole genomic and transcriptomic profiles of 1 primary anaplastic thyroid tumor and 3 authenticated cell lines. Those profiles augmented by the transcriptomes of 4 additional and unique cell lines were compared to 58 pairs of papillary thyroid carcinoma and matched normal tissue transcriptomes from The Cancer Genome Atlas study.

RESULTS

The most prevalent mutations were those of TP53 and BRAF; repeated alterations of the epigenetic machinery such as frame-shift deletions of HDAC10 and EP300, loss of SMARCA2 and fusions of MECP2, BCL11A and SS18 were observed. Sequence data displayed aneuploidy and large regions of copy loss and gain in all genomes. Common regions of gain were however evident encompassing chromosomes 5p and 20q. We found novel anaplastic gene fusions including MKRN1-BRAF, FGFR2-OGDH and SS18-SLC5A11, all expressed in-frame fusions involving a known proto-oncogene. Comparison of the anaplastic thyroid cancer expression datasets with the papillary thyroid cancer and normal thyroid tissue transcriptomes suggested several known drug targets such as FGFRs, VEGFRs, KIT and RET to have lower expression levels in anaplastic specimens compared with both papillary thyroid cancers and normal tissues, confirming the observed lack of response to therapies targeting these pathways. Further integrative data analysis identified the mTOR signaling pathway as a potential therapeutic target in this disease.

CONCLUSIONS

Anaplastic thyroid carcinoma possessed heterogeneous and unique profiles revealing the significance of detailed molecular profiling of individual tumors and the treatment of each as a unique entity; the cell line sequence data promises to facilitate the more accurate and intentional drug screening studies for anaplastic thyroid cancer.

Lysine acetyltransfer supports platelet function

BACKGROUND AND OBJECTIVES

The reversible acetylation of protein lysine ε-amino groups, catalyzed by lysine acetyltransferases and deacetylases, serves as a molecular switch in the orchestration of diverse cellular activities. Here, we aimed to investigate the role of lysine acetyltransfer in platelet function.

METHODS AND RESULTS

Proteomics methods identified 552 acetyllysine (acK) modifications on 273 platelet proteins that serve as candidate substrates for lysine acetyltransferases. Bioinformatics analyses of the identified acK-modified platelet proteins supported roles for the lysine acetyltransferase p300 in the regulation of actin-mediated platelet processes. Biochemical experiments showed that platelets express p300, which is activated in an Src kinase-dependent manner upon platelet stimulation with the platelet glycoprotein VI agonist collagen-related peptide (CRP). Inhibition of platelet p300 abrogated CRP-stimulated lysine acetylation of actin, filamin, and cortactin, as well as F-actin polymerization, integrin activation, and platelet aggregation. Super-resolution visualization of platelet actin-rich adhesion structures revealed abundant acK protein colocalized with platelet actin cytoskeletal proteins. Inhibition of p300 blocked platelet filopodium formation and the spreading of platelets on fibrinogen and collagen surfaces. In whole blood, p300 inhibition prevented the formation of platelet aggregates under shear, suggesting a physiologic role for lysine acetyltransferase activity in platelet function.

CONCLUSION

Together, our findings reveal lysine acetyltransfer to be a potential regulator of platelet actin dynamics, and potential roles for lysine acetylation in the molecular coordination of platelet activation and function.

Reprogramming of mesenchymal stem cells by oncogenes

Mesenchymal stem cells (MSCs) originate from embryonic mesoderm and give rise to the multiple lineages of connective tissues. Transformed MSCs develop into aggressive sarcomas, some of which are initiated by specific chromosomal translocations that generate fusion proteins with potent oncogenic properties. The sarcoma oncogenes typically prime MSCs through aberrant reprogramming. They dictate commitment to a specific lineage but prevent mature differentiation, thus locking the cells in a state of proliferative precursors. Deregulated expression of lineage-specific transcription factors and controllers of chromatin structure play a central role in MSC reprogramming and sarcoma pathogenesis. This suggests that reversing the epigenetic aberrancies created by the sarcoma oncogenes with differentiation-related reagents holds great promise as a beneficial addition to sarcoma therapies.

Truncated SSX protein suppresses synovial sarcoma cell proliferation by inhibiting the localization of SS18-SSX fusion protein

Synovial sarcoma is a relatively rare high-grade soft tissue sarcoma that often develops in the limbs of young people and induces the lung and the lymph node metastasis resulting in poor prognosis. In patients with synovial sarcoma, specific chromosomal translocation of t(X; 18) (p11.2;q11.2) is observed, and SS18-SSX fusion protein expressed by this translocation is reported to be associated with pathogenesis. However, role of the fusion protein in the pathogenesis of synovial sarcoma has not yet been completely clarified. In this study, we focused on the localization patterns of SS18-SSX fusion protein. We constructed expression plasmids coding for the full length SS18-SSX, the truncated SS18 moiety (tSS18) and the truncated SSX moiety (tSSX) of SS18-SSX, tagged with fluorescent proteins. These plasmids were transfected in synovial sarcoma SYO-1 cells and we observed the expression of these proteins using a fluorescence microscope. The SS18-SSX fusion protein showed a characteristic speckle pattern in the nucleus. However, when SS18-SSX was co-expressed with tSSX, localization of SS18-SSX changed from speckle patterns to the diffused pattern similar to the localization pattern of tSSX and SSX. Furthermore, cell proliferation and colony formation of synovial sarcoma SYO-1 and YaFuSS cells were suppressed by exogenous tSSX expression. Our results suggest that the characteristic speckle localization pattern of SS18-SSX is strongly involved in the tumorigenesis through the SSX moiety of the SS18-SSX fusion protein. These findings could be applied to further understand the pathogenic mechanisms, and towards the development of molecular targeting approach for synovial sarcoma.

Targeting the Wnt pathway in synovial sarcoma models

Synovial sarcoma is an aggressive soft-tissue malignancy of children and young adults, with no effective systemic therapies. Its specific oncogene, SYT-SSX (SS18-SSX), drives sarcoma initiation and development. The exact mechanism of SYT-SSX oncogenic function remains unknown. In an SYT-SSX2 transgenic model, we show that a constitutive Wnt/β-catenin signal is aberrantly activated by SYT-SSX2, and inhibition of Wnt signaling through the genetic loss of β-catenin blocks synovial sarcoma tumor formation. In a combination of cell-based and synovial sarcoma tumor xenograft models, we show that inhibition of the Wnt cascade through coreceptor blockade and the use of small-molecule CK1α activators arrests synovial sarcoma tumor growth. We find that upregulation of the Wnt/β-catenin cascade by SYT-SSX2 correlates with its nuclear reprogramming function. These studies reveal the central role of Wnt/β-catenin signaling in SYT-SSX2-induced sarcoma genesis, and open new venues for the development of effective synovial sarcoma curative agents.

SIGNIFICANCE

Synovial sarcoma is an aggressive soft-tissue cancer that afflicts children and young adults, and for which there is no effective treatment. The current studies provide critical insight into our understanding of the pathogenesis of SYT–SSX-dependent synovial sarcoma and pave the way for the development of effective therapeutic agents for the treatment of the disease in humans.

Hypoxic adaptation engages the CBP/CREST-induced coactivator complex of Creb-HIF-1α in transactivating murine neuroblastic glucose transporter

We have shown in vitro a hypoxia-induced time-dependent increase in facilitative glucose transporter isoform 3 (GLUT3) expression in N2A murine neuroblasts. This increase in GLUT3 expression is partially reliant on a transcriptional increase noted in actinomycin D and cycloheximide pretreatment experiments. Transient transfection assays in N2A neuroblasts using murine glut3-luciferase reporter constructs mapped the hypoxia-induced enhancer activities to -857- to -573-bp and -203- to -177-bp regions. Hypoxia-exposed N2A nuclear extracts demonstrated an increase in HIF-1α and p-Creb binding to HRE (-828 to -824 bp) and AP-1 (-187 to -180 bp) cis-elements, respectively, in electromobility shift and supershift assays, which was confirmed by chromatin immunoprecipitation assays. In addition, the interaction of CBP with Creb and HIF-1α and CREST with CBP in hypoxia was detected by coimmunoprecipitation. Furthermore, small interference (si)RNA targeting Creb in these cells decreased endogenous Creb concentrations that reduced by twofold hypoxia-induced glut3 gene transcription. Thus, in N2A neuroblasts, phosphorylated HIF-1α and Creb mediated the hypoxia-induced increase in glut3 transcription. Coactivation by the Ca⁺⁺-dependent CREST and CBP proteins may enhance cross-talk between p-Creb-AP-1 and HIF-1α/HRE of the glut3 gene. Collectively, these processes can facilitate an adaptive response to hypoxic energy depletion targeted at enhancing glucose transport and minimizing injury while fueling the proliferative potential of neuroblasts.

Genome-wide recruitment to Polycomb-modified chromatin and activity regulation of the synovial sarcoma oncogene SYT-SSX2

Background

SYT-SSX is the oncogene associated with synovial sarcoma (SS), a stem cell disease. SYT-SSX is thought to be responsible for sarcoma initiation and development. It interacts with components of Polycomb and SWI/SNF complexes, the two epigenetic controllers that maintain the heritable status of differentiation-specific genes in the stem/progenitor cell. Through these associations SYT-SSX is thought to alter gene expression programs by epigenetic mechanisms. Recently, we reported that SYT-SSX2 reprograms mesenchymal stem cells and myoblasts by dictating their commitment to the neural lineage while disrupting their normal differentiation. This reprogramming was due to the direct occupancy of proneural genes by the SYT-SSX2 nuclear complex. To gain a clear understanding of SYT-SSX2 control of gene expression networks, we conducted a thorough genome-wide analysis to determine the mechanism of its recruitment and identify signature sets of epigenetic markers that would predict its targeting and transcriptional activity.

Results

SYT-SSX2 was recruited to distinct loci across all chromosomes, and an overwhelming number of Polycomb-modified sites enriched with the trimethylated histone H3 on lysine 27 (H3K27me3) formed the main recruiting module for SYT-SSX2. Not all SYT-SSX2/H3K27me3-occupied genes had altered expression, denoting the requirement for additional signals upon oncogene binding. Differential binding and epigenetic patterns distinguished upregulated and downregulated genes. Most activated genes had SYT-SSX2 sites enriched with H3K27me3 within their body or near their transcription start site (TSS) whereas a majority of downregulated genes were characterized by SYT-SSX2/H3K27me3-rich regions at long-range, or by modifications associated with transcription activation within the gene body or near the TSS. Hierarchical and functional clustering identified H3K27me3 as the dominant epigenetic marker associated with SYT-SSX2 binding and gene expression. Notably, this analysis revealed a cluster of upregulated neuronal genes densely covered by H3K27me3, consistent with programming toward the neural lineage by SYT-SSX2 observed previously.

Conclusions

The data analysis revealed that Polycomb complexes or their modified chromatin and their stably silenced differentiation programs seem to be the main target for SYT-SSX2, suggesting that their perturbation is at the center of tumorigenesis driven by the oncogene. Further research into this mechanism is crucial to the full understanding of SS biology.

Downstream and intermediate interactions of synovial sarcoma-associated fusion oncoproteins and their implication for targeted therapy

Synovial sarcoma (SS), an aggressive type of soft tissue tumor, occurs mostly in adolescents and young adults. The origin and molecular mechanism of the development of SS remain only partially known. Over 90% of SS cases are characterized by the t(X;18)(p11.2;q11.2) translocation, which results mainly in the formation of SS18-SSX1 or SS18-SSX2 fusion genes. In recent years, several reports describing direct and indirect interactions of SS18-SSX1/SSX2 oncoproteins have been published. These reports suggest that the fusion proteins particularly affect the cell growth, cell proliferation, TP53 pathway, and chromatin remodeling mechanisms, contributing to SS oncogenesis. Additional research efforts are required to fully explore the protein-protein interactions of SS18-SSX oncoproteins and the pathways that are regulated by these partnerships for the development of effective targeted therapy.

SYT-SSX1 (synovial sarcoma translocated) regulates PIASy ligase activity to cause overexpression of NCOA3 protein

Chromosomal translocations are a major source of genetic abnormalities causally linked to certain malignancies. Synovial sarcoma is an aggressive soft tissue tumor characterized by a chromosomal translocation between chromosome 18 and X, generating oncoproteins such as SYT-SSX1 and SYT-SSX2. The molecular mechanism underlying the oncogenic potential of SYT-SSX1/2 is not clear. Here we show that SYT-SSX1 leads to up-regulation of NCOA3, a protein critical for the formation of various cancers. The increase of NCOA3 is essential for SYT-SSX1-mediated synovial sarcoma formation. SYT-SSX1 does so by increasing the sumoylation of NCOA3 through interaction with a SUMO E3 ligase, PIASy, as well as the sumoylation of NEMO. NEMO has also been shown to physically interact with NCOA3. Increased sumoylation of NCOA3 leads to its increased steady state level and nuclear localization. Our findings represent the first example that an oncoprotein directly regulates substrate modification by a SUMO E3 ligase, and leads to overexpression of a protein essential for tumor formation. Such a mechanistic finding provides an opportunity to design specific therapeutic interventions to treat synovial sarcoma.

Reduced expression of SMARCB1/INI1 protein in synovial sarcoma

Synovial sarcoma is classified as a tumor of uncertain differentiation, and some synovial sarcomas have rhabdoid cells. In previous studies, all malignant rhabdoid tumors and renal medullary carcinomas, some extraskeletal myxoid chondrosarcomas, almost all epithelioid sarcomas and half of epithelioid malignant peripheral nerve sheath tumors showed a loss of SMARCB1/INI1 protein expression in tumor cells and all of these tumors are also known to have rhabdoid cells. We analyzed the immunohistochemical and mRNA expression of SMARCB1/INI1 in 95 synovial sarcomas (73 monophasic fibrous type, 18 biphasic type and 4 poorly differentiated type) and 30 spindle cell sarcomas (3 adult fibrosarcomas, 7 fibrosarcomas arising in dermatofibrosarcoma protuberans, 10 leiomyosarcomas and 10 malignant peripheral nerve sheath tumors) resembling monophasic fibrous synovial sarcoma. The results have shown that 66 of the 95 synovial sarcoma cases (69%) had reduced SMARCB1/INI1 protein expression, whereas the remaining 29 cases (31%) and all 30 spindle cell sarcomas showed preserved this protein expression. No case with a complete loss of SMARCB1/INI1 protein expression was recognized. The median values of SMARCB1/INI1 mRNA expression in non-tumor skeletal muscle and synovial sarcoma with reduced protein expression were 12.86 and 134.01, respectively, and a statistically significant difference was detected between these two groups (P=0.0000004). However, there was no statistically significant difference of prognosis between the synovial sarcoma group with reduced and that with preserved SMARCB1/INI1 protein expression (P=0.46). Therefore, it was suggested that there is a post-transcriptional SMARCB1/INI1 regulatory mechanism in the tumor cells of synovial sarcoma.

Targeting Wnt signaling: can we safely eradicate cancer stem cells?

The Wnt signaling pathways have been conserved throughout evolution and regulate cell proliferation, morphology, motility, and fate during embryonic development. These pathways also play important roles throughout adult life to maintain homeostasis of tissues including skin, blood, intestine, and brain by regulating somatic stem cells and their niches. Aberrant regulation of the Wnt pathway leads to neoplastic proliferation in these same tissues. It has been suggested that Wnt signaling is also involved in the regulation of cancer stem cells (CSC), because there are many similarities in the signaling pathways that regulate normal adult stem cells and CSC. In this Perspective, we have focused on the Wnt/beta-catenin signaling pathway, which is the most intensively studied and best characterized Wnt signaling pathway. We provide an overview on the function of the Wnt/beta-catenin signaling pathway in CSC, and the possibility of the development of novel therapeutics to target this pathway.

Epigenetic features of human mesenchymal stem cells determine their permissiveness for induction of relevant transcriptional changes by SYT-SSX1

BACKGROUND

A characteristic SYT-SSX fusion gene resulting from the chromosomal translocation t(X;18)(p11;q11) is detectable in almost all synovial sarcomas, a malignant soft tissue tumor widely believed to originate from as yet unidentified pluripotent stem cells. The resulting fusion protein has no DNA binding motifs but possesses protein-protein interaction domains that are believed to mediate association with chromatin remodeling complexes. Despite recent advances in the identification of molecules that interact with SYT-SSX and with the corresponding wild type SYT and SSX proteins, the mechanisms whereby the SYT-SSX might contribute to neoplastic transformation remain unclear. Epigenetic deregulation has been suggested to be one possible mechanism.

METHODOLOGY/PRINCIPAL FINDINGS

We addressed the effect of SYT/SSX expression on the transcriptome of four independent isolates of primary human bone marrow mesenchymal stem cells (hMSC). We observed transcriptional changes similar to the gene expression signature of synovial sarcoma, principally involving genes whose regulation is linked to epigenetic factors, including imprinted genes, genes with transcription start sites within a CpG island and chromatin related genes. Single population analysis revealed hMSC isolate-specific transcriptional changes involving genes that are important for biological functions of stem cells as well as genes that are considered to be molecular markers of synovial sarcoma including IGF2, EPHRINS, and BCL2. Methylation status analysis of sequences at the H19/IGF2 imprinted locus indicated that distinct epigenetic features characterize hMSC populations and condition the transcriptional effects of SYT-SSX expression.

CONCLUSIONS/SIGNIFICANCE

Our observations suggest that epigenetic features may define the cellular microenvironment in which SYT-SSX displays its functional effects.

The Arabidopsis GRF-INTERACTING FACTOR gene family performs an overlapping function in determining organ size as well as multiple developmental properties

Previously, the GRF-INTERACTING FACTOR1 (GIF1)/ANGUSTIFOLIA3 (AN3) transcription coactivator gene, a member of a small gene family comprising three genes, was characterized as a positive regulator of cell proliferation in lateral organs, such as leaves and flowers, of Arabidopsis (Arabidopsis thaliana). As yet, it remains unclear how GIF1/AN3 affects the cell proliferation process. In this study, we demonstrate that the other members of the GIF gene family, GIF2 and GIF3, are also required for cell proliferation and lateral organ growth, as gif1, gif2, and gif3 mutations cause a synergistic reduction in cell numbers, leading to small lateral organs. Furthermore, GIF1, GIF2, and GIF3 overexpression complemented a cell proliferation defect of the gif1 mutant and significantly increased lateral organ growth of wild-type plants as well, indicating that members of the GIF gene family are functionally redundant. Kinematic analysis on leaf growth revealed that the gif triple mutant as well as other strong gif mutants developed leaf primordia with fewer cells, which was due to the low rate of cell proliferation, eventually resulting in earlier exit from the proliferative phase of organ growth. The low proliferative activity of primordial leaves was accompanied by decreased expression of cell cycle-regulating genes, indicating that GIF genes may act upstream of cell cycle regulators. Analysis of gif double and triple mutants clarified a previously undescribed role of the GIF gene family: gif mutants had small vegetative shoot apical meristems, which was correlated with the development of small leaf primordia. gif triple mutants also displayed defective structures of floral organs. Taken together, our results suggest that the GIF gene family plays important roles in the control of cell proliferation via cell cycle regulation and in other developmental properties that are associated with shoot apical meristem function.

Cytosolic SYT/SS18 isoforms are actin-associated proteins that function in matrix-specific adhesion

SYT (SYnovial sarcoma Translocated gene or SS18) is widely produced as two isoforms, SYT/L and SYT/S, that are thought to function in the nucleus as transcriptional coactivators. Using isoform-specific antibodies, we detected a sizable pool of SYT isoforms in the cytosol where the proteins were organized into filamentous arrays. Actin and actin-associated proteins co-immunoprecipitated with SYT isoforms, which also co-sedimented and co-localized with the actin cytoskeleton in cultured cells and tissues. The association of SYT with actin bundles was extensive yet stopped short of the distal ends at focal adhesions. Disruption of the actin cytoskeleton also led to a breakdown of the filamentous organization of SYT isoforms in the cytosol. RNAi ablation of SYT/L alone or both isoforms markedly impaired formation of stress fibers and focal adhesions but did not affect formation of cortical actin bundles. Furthermore, ablation of SYT led to markedly impaired adhesion and spreading on fibronectin and laminin-111 but not on collagen types I or IV. These findings indicate that cytoplasmic SYT isoforms interact with actin filaments and function in the ability cells to bind and react to specific extracellular matrices.

Human synovial sarcoma proto-oncogene Syt is essential for early embryonic development through the regulation of cell migration

SYT-SSX protein, resulted from chromosomal translocation, causes synovial sarcoma, which is a malignant tumor accounting for 10% of soft tissue sarcoma. However, biological functions of SYT (synovial sarcoma translocation), also known as SS18, are largely unclear, whereas it has been proven that Syt-null mice die at early stages of embryonic development. Here, we generated Syt-deficient mice and confirmed the reported phenotypes, including growth retardation, open neural tube and haplo-insufficient lethality, and therefore, there is no doubt that Syt is essential for embryonic development. However, placental defects, described in the earlier report, were rarely seen in our mice and we frequently observed cardiac defect in Syt-deficient mice. As the mechanisms responsible for embryonic lethality seem to be complicate, we performed additional experiments. By using primary cultured embryonic fibroblasts, we showed that Syt(-/-) MEFs deregulate actin organization and suppressed cell migration. These observations suggest that Syt may contribute to the signaling pathway important for various cellular functions in vivo and in vitro, and we propose that Syt-deficient MEFs would be a powerful means to understand the biological roles of SYT in vitro.

A calcium-dependent switch in a CREST-BRG1 complex regulates activity-dependent gene expression

CREST plays a critical role in activity-dependent development, but its mechanism of action is not well understood. Here, we show that a CREST-BRG1 complex regulates promoter activation by orchestrating a calcium-dependent release of a repressor complex and a recruitment of an activator complex. In resting neurons, transcription of the c-fos promoter is inhibited by BRG1-dependent recruitment of a phospho-Rb-HDAC repressor complex. Upon calcium influx, Rb becomes dephosphorylated at serine 795 by calcineurin, which leads to release of the repressor complex. At the same time, there is increased recruitment of CBP to the promoter by a CREST-dependent mechanism, which leads to transcriptional activation. The CREST-BRG1 also binds to the NR2B promoter, and activity-dependent induction of NR2B expression involves a release of HDAC1 and recruitment of CBP, suggesting that this mechanism may be generally involved in regulating calcium-dependent transcription of neuronal genes.

The synovial sarcoma-associated SYT-SSX2 oncogene antagonizes the polycomb complex protein Bmi1

This study demonstrates deregulation of polycomb activity by the synovial sarcoma-associated SYT-SSX2 oncogene, also known as SS18-SSX2. Synovial sarcoma is a soft tissue cancer associated with a recurrent t(X:18) translocation event that generates one of two fusion proteins, SYT-SSX1 or SYT-SSX2. The role of the translocation products in this disease is poorly understood. We present evidence that the SYT-SSX2 fusion protein interacts with the polycomb repressive complex and modulates its gene silencing activity. SYT-SSX2 causes destabilization of the polycomb subunit Bmi1, resulting in impairment of polycomb-associated histone H2A ubiquitination and reactivation of polycomb target genes. Silencing by polycomb complexes plays a vital role in numerous physiological processes. In recent years, numerous reports have implicated gain of polycomb silencing function in several cancers. This study provides evidence that, in the appropriate context, expression of the SYT-SSX2 oncogene leads to loss of polycomb function. It challenges the notion that cancer is solely associated with an increase in polycomb function and suggests that any imbalance in polycomb activity could drive the cell toward oncogenesis. These findings provide a mechanism by which the SYT-SSX2 chimera may contribute to synovial sarcoma pathogenesis.

Chemical regulation of epigenetic modifications: opportunities for new cancer therapy

Epigenetics is concerned about heritable changes in gene expression without alteration of the coding sequence. Epigenetic modification of chromatin includes methylation of genomic DNA as well as post-translational modification of chromatin-associated proteins, in particular, histones. The spectrum of histone and non-histone modifications ranges from the addition of relatively small groups such as methyl, acetyl and phosphoryl groups to the attachment of larger moieties such as poly(ADP-ribose) and small proteins ubiquitin or small ubiquitin-like modifier (SUMO). The combinatorial nature of DNA methylation and histone modifications constitutes a significant pathway of epigenetic regulation and considerably extends the information potential of the genetic code. Chromatin modification has emerged as a new fundamental mechanism for gene transcriptional activity control associated with many cellular processes like proliferation, growth, and differentiation. Also it is increasingly recognized that epigenetic modifications constitute important regulatory mechanisms for the pathogenesis of malignant transformations. We review here the recent progress in the development of chemical inhibitors/activators that target different chromatin modifying enzymes. Such potent natural or synthetic modulators can be utilized to establish the quantitative contributions of epigenetic modifications in DNA regulated pathways including transcription, replication, recombination and repair, as well as provide leads for developing new cancer therapeutics.

The proto-oncoprotein SYT (SS18) controls ATP release and regulates cyst formation by polarized MDCK cells

The SYT proto-oncoprotein (also known as SS18) is a gene expression regulator conserved across species. Although its biological function is still unknown, the importance of SYT as a housekeeping protein is illustrated by the lethal phenotype of SYT-null embryos. Notably, SYT is a component of the synovial sarcoma-associated translocation product, the SYT-SSX oncogene. SYT was previously reported as a mediator of cell adhesion. In the present study we show that SYT possesses distinct domains that control MDCK cyst formation in three-dimensional collagen cultures. While the carboxy-half of SYT, the QPGY domain, is required for cyst growth, the amino-terminal region appears to exert on this process a regulatory effect. Further analysis suggested that the purinergic G protein-coupled P2Y receptor signaling is involved in SYT-induced cystogenesis. Activation of this cascade is due to facilitation of ATP release in the extracellular space of polarized MDCK cells by SYT. These studies allow us to begin to understand the vital role of SYT in controlling epithelial morphogenesis and might explain the lethality of its loss in the developing embryo.

Synovial sarcoma: from genetics to genetic-based animal modeling

Synovial sarcomas are highly aggressive mesenchymal cancers that show modest response to conventional cytotoxic chemotherapy, suggesting a definite need for improved biotargeted agents. Progress has been hampered by the lack of insight into pathogenesis of this deadly disease. The presence of a specific diagnostic t(X;18) translocation leading to expression of the unique SYT-SSX fusion protein in effectively all cases of synovial sarcoma suggests a role in the etiology. Other nonspecific anomalies such as overexpression of Bcl-2, HER-2/neu, and EGFR have been reported, but their role in the pathogenesis remains unclear. Using gene targeting, we recently generated mice conditionally expressing the human SYT-SSX2 fusion gene from mouse endogenous ROSA26 promoter in chosen tissue types in the presence of Cre recombinase. These mice develop synovial sarcoma when SYT-SSX2 is expressed within myoblasts, thereby identifying a source of this enigmatic tumor and establishing a mouse model of this disease that recapitulates the clinical, histologic, immunohistochemical, and transcriptional profile of human synovial sarcomas. We review the genetics of synovial sarcoma and discuss the usefulness of genetics-based mouse models as a valuable research tool in the hunt for key molecular determinants of this lethal disease as well as a preclinical platform for designing and evaluating novel treatment strategies.

The (epi)genetics of human synovial sarcoma

Human synovial sarcomas are aggressive soft tissue tumors with relatively high rates of recurrences and metastases. They display a variable response to common treatment protocols such as radiation and chemotherapy. For the development of novel diagnostic, prognostic, and therapeutic approaches, detailed information on the molecular mechanisms underlying the development of these tumors is of imperative importance. Fusion of the SS18 and (one of the) SSX genes is a molecular hallmark of human synovial sarcomas. The SS18 and SSX genes encode nuclear proteins that exhibit opposite transcription regulatory activities, likely through epigenetic mechanisms. The SS18 protein functions as a transcriptional coactivator and interacts directly with members of the epigenetic chromatin remodeling and modification machineries. In contrast, the SSX proteins function as transcriptional corepressors and are associated with several Polycomb group proteins. Since the domains involved in these apparently opposite transcription regulatory activities are retained in the SS18-SSX fusion proteins, we hypothesize that these fusion proteins function as "activator-repressors" of transcription. The implications of this model for human synovial sarcoma development and future treatment are discussed.

The synovial sarcoma SYT-SSX2 oncogene remodels the cytoskeleton through activation of the ephrin pathway

Synovial sarcoma is a soft tissue cancer associated with a recurrent t(X:18) translocation that generates one of two fusion proteins, SYT-SSX1 or SYT-SSX2. In this study, we demonstrate that SYT-SSX2 is a unique oncogene. Rather than confer enhanced proliferation on its target cells, SYT-SSX2 instead causes a profound alteration of their architecture. This aberrant morphology included elongation of the cell body and formation of neurite-like extensions. We also observed that cells transduced with SYT-SSX2 often repulsed one another. Notably, cell repulsion is a known component of ephrin signaling. Further analysis of SYT-SSX2-infected cells revealed significant increases in the expression and activation of Eph/ephrin pathway components. On blockade of EphB2 signaling SYT-SSX2 infectants demonstrated significant reversion of the aberrant cytoskeletal phenotype. In addition, we discovered, in parallel, that SYT-SSX2 induced stabilization of the microtubule network accompanied by accumulation of detyrosinated Glu tubulin and nocodazole resistance. Glu tubulin regulation was independent of ephrin signaling. The clinical relevance of these studies was confirmed by abundant expression of both EphB2 and Glu tubulin in SYT-SSX2-positive synovial sarcoma tissues. These results indicate that SYT-SSX2 exerts part of its oncogenic effect by altering cytoskeletal architecture in an Eph-dependent manner and cytoskeletal stability through a concurrent and distinct pathway.

The C terminus of the synovial sarcoma-associated SSX proteins interacts with the LIM homeobox protein LHX4

As a result of the synovial sarcoma-associated t(X;18) translocation, the SS18 gene on chromosome 18 is fused to either one of the three closely related SSX genes on the X chromosome. The SS18 protein is thought to act as a transcriptional co-activator, whereas the SSX proteins are thought to act as transcriptional corepressors. The main SSX-repression domain is located in its C terminus, a domain that is retained in the respective SS18-SSX fusion proteins. Both the SS18 and SSX proteins lack DNA-binding domains. Previously, we found that the SS18 and SS18-SSX fusion proteins may be tethered to DNA targets via the SS18-interacting protein AF10. Here, we set out to isolate proteins that interact with the SSX C-terminal repression domain using a yeast two-hybrid interaction trap. Of the positive clones isolated, two corresponded to the LIM homeobox protein LHX4, a DNA-binding protein that is involved in transcription regulation. An endogenous interaction was subsequently established in mammalian cells via colocalization and coimmunoprecipitation of the respective proteins. Interestingly, the LHX4 gene was previously found to be deregulated in various human leukemias. In addition, it was previously found that LIM homeobox proteins may bind to and activate the glycoprotein-alpha (CGA) promoter. Using LHX4 chromatin immunoprecipitation and CGA-promoter assays, we found that endogenous LHX4 binds to the CGA promoter and that LHX4-mediated CGA activation is enhanced by the SS18-SSX protein, but not by the SSX protein. Taken together, we conclude that this novel protein - protein interaction may have direct consequences for the (de)regulation of SSX and/or SS18-SSX target genes and, thus, for the development of human synovial sarcomas.

Sarcomas: genetics, signalling, and cellular origins. Part 2: TET-independent fusion proteins and receptor tyrosine kinase mutations

Although the mechanisms that underlie sarcoma development are still poorly understood, the identification of non-random chromosomal translocations and receptor tyrosine kinase mutations associated with defined sarcoma types has provided new insight into the pathogenesis of these tumours. In Part 1 of the review (J Pathol 2007;213:4-20), we addressed sarcomas that express fusion genes containing TET gene family products. Part 2 of the review summarizes our current understanding of the implications of fusion genes that do not contain TET family members in sarcoma development, as well as that of specific mutations in genes encoding receptor tyrosine kinases (RTKs). The final section will serve as a summary of both reviews and will attempt to provide a synthesis of some of the emerging principles of sarcomagenesis.

The SYT-SSX fusion protein down-regulates the cell proliferation regulator COM1 in t(x;18) synovial sarcoma

Chromosomal translocations are frequently associated with soft-tissue sarcomas. Fusion proteins generated by such translocations often play critical roles in tumorigenesis. Therefore, it is important to understand the function of the fusion protein to develop therapeutic interventions. The t(X;18)(p11.2;q11.2) translocation found in synovial sarcomas results in a fusion between the SYT gene on chromosome 18 and an SSX gene on the X chromosome. Although SYT-SSX fusion proteins appear to trigger synovial sarcoma development, little is known about the downstream targets of SYT-SSX. We found that the SYT-SSX fusion protein produces a dominant-negative function for SYT, which is a transcriptional coactivator. We then analyzed the gene expression profiles of SYT-SSX1-expressing HeLa cells using oligonucleotide microarrays and found that the SYT-SSX1 fusion protein directly down-regulated the expression of COM1, a regulator of cell proliferation. COM1 was found to be expressed at relatively low levels in synovial sarcoma tissues and cell lines. We then investigated the impact of conditional COM1 expression in the synovial sarcoma cell line. Increased COM1 expression resulted in induced apoptosis and in reduced cell growth and colony formation activity. Our results suggested that restoration of COM1 expression may be of therapeutic benefit in synovial sarcoma.

The Synovial Sarcoma–Associated SS18-SSX2 Fusion Protein Induces Epigenetic Gene (De)Regulation

Fusion of the SS18 and either one of the SSX genes is a hallmark of human synovial sarcoma. The SS18 and SSX genes encode nuclear proteins that exhibit opposite transcriptional activities. The SS18 protein functions as a transcriptional coactivator and is associated with the SWI/SNF complex, whereas the SSX proteins function as transcriptional corepressors and are associated with the polycomb complex. The domains involved in these opposite transcriptional activities are retained in the SS18-SSX fusion proteins. Here, we set out to determine the direct transcriptional consequences of conditional SS18-SSX2 fusion protein expression using complementary DNA microarray-based profiling. By doing so, we identified several clusters of SS18-SSX2-responsive genes, including a group of genes involved in cholesterol synthesis, which is a general characteristic of malignancy. In addition, we identified a group of SS18-SSX2-responsive genes known to be specifically deregulated in primary synovial sarcomas, including IGF2 and CD44. Furthermore, we observed an uncoupling of EGR1, JUNB, and WNT signaling in response to SS18-SSX2 expression, suggesting that the SWI/SNF-associated coactivation functions of the SS18 moiety are impaired. Finally, we found that SS18-SSX2 expression affects histone modifications in the CD44 and IGF2 promoters and DNA methylation levels in the IGF2 imprinting control region. Together, we conclude that the SS18-SSX2 fusion protein may act as a so-called transcriptional "activator-repressor," which induces downstream target gene deregulation through epigenetic mechanisms. Our results may have implications for both the development and clinical management of synovial sarcomas.

The extracellular nucleotide UTP is a potent inducer of hematopoietic stem cell migration

Homing and engraftment of hematopoietic stem cells (HSCs) to the bone marrow (BM) involve a complex interplay between chemokines, cytokines, and nonpeptide molecules. Extracellular nucleotides and their cognate P2 receptors are emerging as key factors of inflammation and related chemotactic responses. In this study, we investigated the activity of extracellular adenosine triphosphate (ATP) and uridine triphosphate (UTP) on CXCL12-stimulated CD34+ HSC chemotaxis. In vitro, UTP significantly improved HSC migration, inhibited cell membrane CXCR4 down-regulation by migrating CD34+ cells, and increased cell adhesion to fibronectin. In vivo, preincubation with UTP significantly enhanced the BM homing efficiency of human CD34+ cells in immunodeficient mice. Pertussis toxin blocked CXCL12- and UTP-dependent chemotactic responses, suggesting that G-protein alpha-subunits (Galphai) may provide a converging signal for CXCR4- and P2Y-activated transduction pathways. In addition, gene expression profiling of UTP- and CXCL12-treated CD34+ cells and in vitro inhibition assays demonstrated that Rho guanosine 5'-triphosphatase (GTPase) Rac2 and downstream effectors Rho GTPase-activated kinases 1 and 2 (ROCK1/2) are involved in UTP-promoted/CXCL12-dependent HSC migration. Our data suggest that UTP may physiologically modulate the homing of HSCs to the BM, in concert with CXCL12, via the activation of converging signaling pathways between CXCR4 and P2Y receptors, involving Galphai proteins and RhoGTPases.

The Transcriptional Activity of CITED1 Is Regulated by Phosphorylation in a Cell Cycle-dependent Manner

CITED1 is the founding member of the CITED family of cofactors that are involved in regulating a wide variety of CBP/p300-dependent transcriptional responses. In the present study, we show that the phosphorylation status of CITED1 changes during the cell cycle and affects its transcriptional cofactor activity. Tryptic mapping and mutagenesis studies identified five phosphorylated serine residues in CITED1. Phosphorylation of these residues did not affect CRM1-dependent nuclear export, but did decrease CITED1 binding to p300 and inhibited CITED1-dependent transactivation of Smad4 and p300. These results suggest that CITED1 functions as a cell cycle-dependent transcriptional cofactor whose activity is regulated by phosphorylation.

Targeted disruption of the synovial sarcoma-associated SS18 gene causes early embryonic lethality and affects PPARBP expression

The synovial sarcoma-associated protein SS18 (also known as SYT or SSXT) is thought to act as a transcriptional co-activator. This activity appears to be mediated through the SWI/SNF proteins BRG1 and INI1 and the histone acetyl transferase p300. Here, we report that disruption of the mouse Ss18 gene results in a recessive embryonic lethal phenotype, due to placental failure caused by impairment of placental vascularization and/or chorio-allantoic fusion. This phenotype resembles the p300 knockout phenotype, but is distinct from the Brg1 and Ini1 knockout phenotypes. Through expression profiling of knockout embryos, we observed altered expression of genes known to affect placental development, including the peroxisome proliferator-activated receptor-binding protein (Pparbp). Since Pparbp null mutant embryos display a similar, lethal phenotype with placental failure, we suggest that the functional and phenotypic co-linearities between Ss18 and p300 may also include the transcriptional co-activator Pparbp. Additional interbreeding of Ss18 and Ss18l1 (Crest) mutant mice indicates that these two functionally and structurally related genes may act synergistically during critical stages of embryonic development.

Functional analysis of bladder cancer-related protein gene: a putative cervical cancer tumor suppressor gene in cervical carcinoma

Our previous study has suggested thatthe bladder cancer-associated protein gene (BLCAP) was among the differentially expressed genes in cervical cancer. We confirm here that BLCAP is expressed in all noncancerous cervical tissues (10/10), but it is greatly lost in primary cervical cancer tissue (31/39). In order to further investigate the functional roles of BLCAP, we stably transfected BLCAP cDNA into HeLa cells. The HeLa cells expressing BLCAP show reduced cell growth and clone genicity compared to the vector-transfected cognate cells. BLCAP expression in HeLa cells leads to growth arrest and significantly enhanced apoptosis in vitro and reduced tumor formation in vivo. Thus, BLCAP might be a potential tumor suppressor gene in cervical carcinoma.

IGF2 is critical for tumorigenesis by synovial sarcoma oncoprotein SYT-SSX1

Synovial sarcoma is an aggressive soft tissue tumor characterized by a specific chromosomal translocation between chromosome 18 and X. This translocation can generate a fusion transcript encoding SYT-SSX1, a transforming oncoprotein. We present evidence that SYT-SSX1 induces insulin-like growth factor II expression in fibroblast cells. SYT-SSX2, a fusion also frequently found in synovial sarcoma, is necessary for maintaining Igf2 expression in the synovial sarcoma cell line, and the increased IGF2 synthesis protects cells from anoikis and is required for tumor formation in vivo. We also found a loss of imprinting (LOI) for Igf2 in a limited number of primary synovial sarcomas despite demethylation of CpG dinucleotides critical for maintaining imprinting. These findings suggest that inhibition of the IGF2/IGF1-R signaling pathway may represent a significant therapeutic modality for treating synovial sarcoma.

Common origin of the human synovial sarcoma associated SS18 and SS18L1 gene loci

The highly conserved synovial sarcoma associated protein SS18 is thought to act as a transcriptional co-activator through interactions with various proteins involved in (epigenetic) gene regulation. The SS18 SNH domain appears to act as a major interface for these protein-protein interactions. Previously, we used this SNH domain to identify SS18 paralogs (SS18L1 and SS18L2) and orthologs in various species. The functional significance of these SS18-like proteins is embodied by the observations that SS18L1 and SS18L2 can replace SS18 in its various protein-protein interactions, and that SS18L1 may act as a fusion partner of SSX in synovial sarcoma. In the current study, we performed a comprehensive comparison of SNH-containing loci in several distinct species. By doing so, we found that the vertebrate SS18 and SS18L1 genes map within co-linear DNA segments that may have evolved through a relatively recent genomic duplication event. An additional phylogenetic study indicated that the more divergent SS18L2 gene is most likely derived from an earlier gene duplication event, again in the vertebrate lineage.

The synovial sarcoma translocation protein SYT-SSX2 recruits β-catenin to the nucleus and associates with it in an active complex

Localization of beta-catenin in the cell is a key determinant in its decision to function as a critical mediator of cell adhesion at the surface or a transcription activator in the nucleus. SYT-SSX2 is the fusion product of the chromosomal translocation, t(X;18)(p11.2;q11.2), which occurs in synovial sarcoma, a soft tissue tumor. SYT-SSX2 is known to associate with chromatin remodeling complexes and is proposed to be involved in controlling gene expression. We report that SYT-SSX2 plays a direct role in beta-catenin regulation. When expressed in mammalian cells, SYT-SSX2-induced beta-catenin recruitment to the nucleus. Interestingly, known target genes of canonical Wnt were not activated as a result of SYT-SSX2 expression, nor was the nuclear localization of beta-catenin due to one of the signaling pathways normally implicated in this event. beta-Catenin accumulation in the nucleus led to the formation of a transcriptionally active nuclear complex that contained SYT-SSX2 and beta-catenin. More importantly, depletion of SYT-SSX2 in primary synovial sarcoma cells resulted in loss of nuclear beta-catenin signal and a significant decrease in its signaling activity. These results unravel a novel pathway in the control of beta-catenin cellular transport and strongly suggest that SYT-SSX2 contributes to tumor development, in part through beta-catenin signaling.

The Proto-oncoprotein SYT Interacts with SYT-interacting Protein/Co-activator Activator (SIP/CoAA), a Human Nuclear Receptor Co-activator with Similarity to EWS and TLS/FUS Family of Proteins

The proto-oncoprotein SYT is involved in the unique translocation t(X;18) found in synovial sarcoma SYT-SSX fusions. SYT has a conserved N-terminal domain (SNH domain) that interacts with the human paralog of Drosophila Brahma (hBRM) and Brahma-related gene 1 (BRG1) chromatin remodeling proteins and a C-terminal transactivating sequence rich in glutamine, proline, glycine, and tyrosine (QPGY domain). Here we reported the isolation of the ribonucleoprotein SYT-interacting protein/co-activator activator (SIP/CoAA), which specifically binds the QPGY domain of SYT and also the SYT-SSX2 translocation fusion. SIP/CoAA is a general nuclear co-activator and an RNA splicing modulator that contains two RNA recognition motifs and multiple hexapeptide repeats. We showed that the region consisting of the hexapeptide motif (YQ domain) is similar to the hexapeptide repeat domain found in EWS and in TLS/FUS family proteins. The YQ domain also resembles the QPGY region of SYT itself and like all these other domains acts as a transcriptional activator in reporter assays. Most interestingly, the last 84 amino acids adjacent to YQ down-modulate by 25-fold the YQ transactivation of the reporter gene, and both domains are important for SIP/CoAA binding to SYT. In addition, SYT acts together with SIP/CoAA in stimulating estrogen and glucocorticoid receptor-dependent transcriptional activation. Activation is hormone-dependent and requires functional hBRM and/or BRG1. The stimulation is strongly reduced if the N-terminal region of hBRM/BRG1 (amino acids 1-211) is deleted. This region encompasses the SNF11 binding domain (amino acids 156-211), which interacts specifically with SYT in vivo and in vitro.

Transcriptional co-activator activity of SYT is negatively regulated by BRM and Brg1

The t(X;18)(p11.2;q11.2) translocation found in synovial sarcomas results in the fusion of the SYT gene on chromosome 18 to the SSX gene on chromosome X. Although the SYT-SSX fusion proteins may trigger synovial sarcoma development, the biological functions of SYT, SSX and SYT-SSX genes are unclear. Transfections of Gal4 DNA binding domain fusion protein constructs demonstrate that SYT protein acts as a transcriptional co-activator at the C-terminal domain and that the activity is repressed through the N-terminus. The N-terminal 70 amino acids of SYT bind not only to BRM, but also to Brg1, both of which are subunits of SWI/SNF chromatin remodelling complexes. Here, we have investigated the functions of BRM and Brg1 on the repression of SYT activity. The negative regulation of SYT transcriptional co-activator activity is dependent on the ATP-hydrolysis of BRM and Brg1 in the protein complexes. This indicates that the SWI/SNF protein complexes regulate SYT activity using the chromatin remodelling activity.

Identification of a novel p300-specific-associating protein, PRS1 (phosphoribosylpyrophosphate synthetase subunit 1)

CBP [CREB (cAMP-response-element-binding protein)-binding protein] and p300 play critical roles in transcriptional co-activation, cell differentiation, proliferation and apoptosis. Multiple transcription factors associate with CBP/p300. With the exception of the SYT oncoprotein, no proteins have been identified that specifically associate with p300, but not CBP. In the present study, we isolated a novel p300-associated protein for which no interaction with CBP was observed by GST (glutathione S-transferase) pull-down assay using Jurkat cell lysates metabolically labelled with [35S]methionine. This protein bound the KIX (kinase-inducible) domain of p300. Following resolution by two-dimensional acrylamide gel electrophoresis, we identified the KIX-domain-bound protein by MS analysis as PRS1 (phosphoribosylpyrophosphate synthetase subunit 1), a protein essential for nucleoside biosynthesis. This is the first report to demonstrate the existence of a p300 KIX-domain-specific-interacting protein that does not interact with CBP. Thus p300 may play a role in the regulation of DNA synthesis through interactions with PRS1.

Synovial sarcoma translocation (SYT) encodes a nuclear receptor coactivator

We previously cloned and characterized a novel RNA-binding motif-containing coactivator, named coactivator activator (CoAA), as a thyroid hormone receptor-binding protein-interacting protein using a Sos-Ras yeast two-hybrid screening system. A database search revealed that CoAA is identical with synovial sarcoma translocation (SYT)-interacting protein. Thus, we hypothesized that SYT could also function as a coactivator. Subsequently, we isolated a cDNA encoding a larger isoform of SYT, SYT-long (SYT-L), from the brain and liver total RNA using RT-PCR. SYT-L possesses an additional 31 amino acids in its C terminus compared with SYT, suggesting that these two SYT isoforms may be expressed from two mRNAs produced by alternative splicing of a transcript from a single gene. By Northern blot analysis, we found that SYT-L mRNA is expressed in several human embryonic tissues, such as the brain, liver, and kidney. However, we could not detect SYT-L in adult tissues. Glutathione-S-transferase pull-down studies showed that SYT binds to the C-terminus of CoAA, but not to the coactivator modulator. Both isoforms of SYT function as transcriptional coactivators of nuclear hormone receptors in a ligand- and dose-dependent manner in CV-1, COS-1, and JEG-3 cells. However, the pattern of transactivation was different between SYT and SYT-L among these cells. SYT synergistically activates transcription with CoAA. In addition, SYT activates transcription through activator protein-1, suggesting that SYT may function as a general coactivator. These results indicate that SYT activates transcription, possibly through CoAA, to interact with the histone acetyltransferase complex.

Induction of p21WAF1/CIP1 by human synovial sarcoma-associated chimeric oncoprotein SYT-SSX1

Oncogenic protein provokes cell cycle arrest termed premature senescence. In this process Ras has been known to induce cyclin-dependent kinase inhibitor (CKI) p16(INK4A) in primary fibroblasts. Here, we present a novel finding that human chimeric oncoprotein SYT-SSX1 induces CKI p21(WAF1/CIP1) (p21) for suppression of cell growth. In human synovial sarcoma cell lines, the expression levels of p21 were high and the transcriptional activity of the p21 gene promoter was significantly elevated. The transient expression of SYT-SSX1-induced activation of the p21 gene promoter in human diploid fibroblasts. The N-terminus deletion form of SYT-SSX1, which failed to bind to hBRM one of the chromatin remodeling factors, preserved the p21 induction ability. This effect of SYT-SSX1 was similar in extent in both wild-type and p53-deficient HCT116 cell lines. Furthermore, the introduction of mutation in Sp1/Sp3 binding sites of the p21 gene promoter abolished the SYT-SSX1-induced transcriptional activity of its promoter. In SW13 cells, the stable expression of SYT-SSX1 suppressed cell growth in culture. These results suggest that SYT-SSX1 is able to induce p21 in a manner independent on hBRM and p53 but dependent on Sp1/Sp3.

The transcription factor AtGRF5 and the transcription coactivator AN3 regulate cell proliferation in leaf primordia of Arabidopsis thaliana

The development of the flat morphology of leaf blades is dependent on the control of cell proliferation as well as cell expansion. Each process has a polarity with respect to the longitudinal and transverse axes of the leaf blade. However, only a few regulatory components of these processes have been identified to date. We have characterized two genes from Arabidopsis thaliana: ANGUSTIFOLIA3 (AN3), which encodes a homolog of the human transcription coactivator SYT, and GROWTH-REGULATING FACTOR5 (AtGRF5), which encodes a putative transcription factor. AN3 is identical to GRF-INTERACTING FACTOR1 (AtGIF1). The an3 and atgrf5 mutants exhibit narrow-leaf phenotypes due to decreases in cell number. Conversely, cell proliferation in leaf primordia is enhanced and leaves grow larger than normal when AN3 or AtGRF5 is overexpressed. Both genes are expressed in leaf primordia, and in the yeast two-hybrid assay, the gene products were found to interact with each other through their N-terminal domains. These results suggest that AN3 and AtGRF5 act together and are required for the development of appropriate leaf size and shape through the promotion and/or maintenance of cell proliferation activity in leaf primordia.