PLDLS-dependent interaction of E1A with CtBP: regulation of CtBP nuclear localization and transcriptional functions

C-Terminal Binding Protein: Regulator between Viral Infection and Tumorigenesis

C-terminal binding protein (CtBP), a transcriptional co-repressor, significantly influences cellular signaling, impacting various biological processes including cell proliferation, differentiation, apoptosis, and immune responses. The CtBP family comprises two highly conserved proteins, CtBP1 and CtBP2, which have been shown to play critical roles in both tumorigenesis and the regulation of viral infections. Elevated CtBP expression is noted in various tumor tissues, promoting tumorigenesis, invasiveness, and metastasis through multiple pathways. Additionally, CtBP's role in viral infections varies, exhibiting differing or even opposing effects depending on the virus. This review synthesizes the advances in CtBP's function research in viral infections and virus-associated tumorigenesis, offering new insights into potential antiviral and anticancer strategies.

Identification and characterization of a new potent inhibitor targeting CtBP1/BARS in melanoma cells

BACKGROUND

The C-terminal-binding protein 1/brefeldin A ADP-ribosylation substrate (CtBP1/BARS) acts both as an oncogenic transcriptional co-repressor and as a fission inducing protein required for membrane trafficking and Golgi complex partitioning during mitosis, hence for mitotic entry. CtBP1/BARS overexpression, in multiple cancers, has pro-tumorigenic functions regulating gene networks associated with "cancer hallmarks" and malignant behavior including: increased cell survival, proliferation, migration/invasion, epithelial-mesenchymal transition (EMT). Structurally, CtBP1/BARS belongs to the hydroxyacid-dehydrogenase family and possesses a NAD(H)-binding Rossmann fold, which, depending on ligands bound, controls the oligomerization of CtBP1/BARS and, in turn, its cellular functions. Here, we proposed to target the CtBP1/BARS Rossmann fold with small molecules as selective inhibitors of mitotic entry and pro-tumoral transcriptional activities.

METHODS

Structured-based screening of drug databases at different development stages was applied to discover novel ligands targeting the Rossmann fold. Among these identified ligands, N-(3,4-dichlorophenyl)-4-{[(4-nitrophenyl)carbamoyl]amino}benzenesulfonamide, called Comp.11, was selected for further analysis. Fluorescence spectroscopy, isothermal calorimetry, computational modelling and site-directed mutagenesis were employed to define the binding of Comp.11 to the Rossmann fold. Effects of Comp.11 on the oligomerization state, protein partners binding and pro-tumoral activities were evaluated by size-exclusion chromatography, pull-down, membrane transport and mitotic entry assays, Flow cytometry, quantitative real-time PCR, motility/invasion, and colony assays in A375MM and B16F10 melanoma cell lines. Effects of Comp.11 on tumor growth in vivo were analyzed in mouse tumor model.

RESULTS

We identify Comp.11 as a new, potent and selective inhibitor of CtBP1/BARS (but not CtBP2). Comp.11 directly binds to the CtBP1/BARS Rossmann fold affecting the oligomerization state of the protein (unlike other known CtBPs inhibitors), which, in turn, hinders interactions with relevant partners, resulting in the inhibition of both CtBP1/BARS cellular functions: i) membrane fission, with block of mitotic entry and cellular secretion; and ii) transcriptional pro-tumoral effects with significantly hampered proliferation, EMT, migration/invasion, and colony-forming capabilities. The combination of these effects impairs melanoma tumor growth in mouse models.  CONCLUSIONS: This study identifies a potent and selective inhibitor of CtBP1/BARS active in cellular and melanoma animal models revealing new opportunities to study the role of CtBP1/BARS in tumor biology and to develop novel melanoma treatments.

Dachshund and C-terminal Binding Protein bind directly during Drosophila eye development

The transcription factor Dachshund (Dac) and the transcriptional co-regulator C-terminal Binding Protein (CtBP) were identified as the retinal determination factors during Drosophila eye development . A previous study established that Dac and CtBP interact genetically during eye development. Co-immunoprecipitation assays suggested that both molecules interact in the Drosophila larval eye-antennal disc. Our present study shows that Dac and CtBP bind each other directly, as determined by GST pull-down assays. Thus, our results demonstrate the molecular mechanism of Dac and CtBP interaction and suggest the direct binding of these two transcription regulators in the cells of the eye disc promotes the Drosophila eye specification.

CtBP2 interacts with TGIF to promote the progression of esophageal squamous cell cancer through the Wnt/β‑catenin pathway

C-terminal-binding protein 2 (CtBP2), a transcriptional co-repressor, plays a main role in tumorigenesis and in the development of multiple tumors. Transforming growth interacting factor (TGIF) is involved in a number of cellular signal transduction pathways and is related to tumor occurrence and development. In the present study, the proteins interacting with CtBP2 were identified and the mechanisms underlying the biological activity of CtBP2 in esophageal squamous cell carcinoma (ESCC) were investigated. The Search Tool for the Retrieval of Interacting Genes (STRING) database was used to search for known proteins interacting with CtBP2, and co-immunoprecipitation (Co-IP) assay was performed to validate the interactions. Reverse transcription-quantitative PCR (RT-qPCR), immunohistochemistry (IHC) and western blot analysis were performed to examine the expression levels of CtBP2 and TGIF in ESCC. The correlation between CtBP2 and TGIF was analyzed using Gene Expression Profiling Interactive Analysis (GEPIA) by Pearson's correlation analysis, and the co-localization of CtBP2 with TGIF in the ECA109 cells was identified using immunofluorescence staining. XAV939 treatment, CCK-8, 5-ethynyl-2′-deoxyuridine (EdU) staining, wound healing and Transwell assays were performed to investigate the signaling pathways involved in the biological activity of CtBP2 in ECA109 cells. According to the results obtained from STRING and Co-IP analysis, an interaction between CtBP2 and TGIF was indicated, and these proteins were co-localized in the nucleus. CtBP2 and TGIF mRNA and protein expression levels were robustly and simultaneously increased in both ESCC tissues and cell lines. There was a direct correlation between CtBP2 and TGIF expression levels in ESCC tissues, and both were significantly associated with metastasis and survival. The TGIF and CtBP2 expression levels were significantly increased or decreased simultaneously, in ECA109 cells transfected with LV-CtBP2 or sh-CtBP2, and vice versa. According to the results of CCK-8 assay, EdU staining and Transwell assay, CtBP2 promoted the proliferation, migration and invasion of ECA109 cells through the Wnt/β-catenin pathway. On the whole, the present study demonstrates that CtBP2 interacts with TGIF and promotes the malignant progression of ESCC through the Wnt/β-catenin pathway.

ZEB2, the Mowat-Wilson Syndrome Transcription Factor: Confirmations, Novel Functions, and Continuing Surprises

After its publication in 1999 as a DNA-binding and SMAD-binding transcription factor (TF) that co-determines cell fate in amphibian embryos, ZEB2 was from 2003 studied by embryologists mainly by documenting the consequences of conditional, cell-type specific Zeb2 knockout (cKO) in mice. In between, it was further identified as causal gene causing Mowat-Wilson Syndrome (MOWS) and novel regulator of epithelial-mesenchymal transition (EMT). ZEB2's functions and action mechanisms in mouse embryos were first addressed in its main sites of expression, with focus on those that helped to explain neurodevelopmental and neural crest defects seen in MOWS patients. By doing so, ZEB2 was identified in the forebrain as the first TF that determined timing of neuro-/gliogenesis, and thereby also the extent of different layers of the cortex, in a cell non-autonomous fashion, i.e., by its cell-intrinsic control within neurons of neuron-to-progenitor paracrine signaling. Transcriptomics-based phenotyping of Zeb2 mutant mouse cells have identified large sets of intact-ZEB2 dependent genes, and the cKO approaches also moved to post-natal brain development and diverse other systems in adult mice, including hematopoiesis and various cell types of the immune system. These new studies start to highlight the important adult roles of ZEB2 in cell-cell communication, including after challenge, e.g., in the infarcted heart and fibrotic liver. Such studies may further evolve towards those documenting the roles of ZEB2 in cell-based repair of injured tissue and organs, downstream of actions of diverse growth factors, which recapitulate developmental signaling principles in the injured sites. Evident questions are about ZEB2's direct target genes, its various partners, and ZEB2 as a candidate modifier gene, e.g., in other (neuro)developmental disorders, but also the accurate transcriptional and epigenetic regulation of its mRNA expression sites and levels. Other questions start to address ZEB2's function as a niche-controlling regulatory TF of also other cell types, in part by its modulation of growth factor responses (e.g., TGFβ/BMP, Wnt, Notch). Furthermore, growing numbers of mapped missense as well as protein non-coding mutations in MOWS patients are becoming available and inspire the design of new animal model and pluripotent stem cell-based systems. This review attempts to summarize in detail, albeit without discussing ZEB2's role in cancer, hematopoiesis, and its emerging roles in the immune system, how intense ZEB2 research has arrived at this exciting intersection.

NAD-linked mechanisms of gene de-repression and a novel role for CtBP in persistent adenovirus infection of lymphocytes

BACKGROUND

Adenovirus (AdV) infection is ubiquitous in the human population and causes acute infection in the respiratory and gastrointestinal tracts. In addition to lytic infections in epithelial cells, AdV can persist in a latent form in mucosal lymphocytes, and nearly 80% of children contain viral DNA in the lymphocytes of their tonsils and adenoids. Reactivation of latent AdV is thought to be the source of deadly viremia in pediatric transplant patients. Adenovirus latency and reactivation in lymphocytes is not well studied, though immune cell activation has been reported to promote productive infection from latency. Lymphocyte activation induces global changes in cellular gene expression along with robust changes in metabolic state. The ratio of free cytosolic NAD+/NADH can impact gene expression via modulation of transcriptional repressor complexes. The NAD-dependent transcriptional co-repressor C-terminal Binding Protein (CtBP) was discovered 25 years ago due to its high affinity binding to AdV E1A proteins, however, the role of this interaction in the viral life cycle remains unclear.

METHODS

The dynamics of persistently- and lytically-infected cells are evaluated. RT-qPCR is used to evaluate AdV gene expression following lymphocyte activation, treatment with nicotinamide, or disruption of CtBP-E1A binding.

RESULTS

PMA and ionomycin stimulation shifts the NAD+/NADH ratio in lymphocytic cell lines and upregulates viral gene expression. Direct modulation of NAD+/NADH by nicotinamide treatment also upregulates early and late viral transcripts in persistently-infected cells. We found differential expression of the NAD-dependent CtBP protein homologs between lymphocytes and epithelial cells, and inhibition of CtBP complexes upregulates AdV E1A expression in T lymphocyte cell lines but not in lytically-infected epithelial cells.

CONCLUSIONS

Our data provide novel insight into factors that can regulate AdV infections in activated human lymphocytes and reveal that modulation of cellular NAD+/NADH can de-repress adenovirus gene expression in persistently-infected lymphocytes. In contrast, disrupting the NAD-dependent CtBP repressor complex interaction with PxDLS-containing binding partners paradoxically alters AdV gene expression. Our findings also indicate that CtBP activities on viral gene expression may be distinct from those occurring upon metabolic alterations in cellular NAD+/NADH ratios or those occurring after lymphocyte activation.

C-Terminus of E1A Binding Protein 1 Stimulates Malignant Phenotype in Human Hepatocellular Carcinoma

<strong>BACKGROUND</strong> The C-terminus of E1A binding proteins (CTBPs) has recently been shown to stimulate tumorigenesis in several human tissues by participating in cell signal transduction. However, to date, the expression profile of CTBP isoforms in hepatocellular carcinoma (HCC) and the impact of CTBPs on HCC cell phenotype have not been fully explored. <strong>MATERIAL AND METHODS</strong> The expression level of CTBP1 was investigated in various HCC cell lines and HCC tissues by RT-qPCR, Western blotting, and immunohistochemistry assays. The phosphatidylinositol-3-kinase (PI3K) inhibitor LY294002 was utilized to treat hepatic astrocyte cells, and the impact of CTBP1 on proliferation and metastasis of hepatic astrocytes and HCC cells was accessed by CCK-8, clone-forming, Transwell chamber, and cell scratch assays. <strong>RESULTS</strong> Increased expression of CTBP1 was observed in HCC tissues and was a predictor of poor prognosis in HCC patients. CTBP1 modified proliferation and migratory activity of HCC cells via the PI3K/protein kinase B (Akt) signaling pathway in hepatic astrocytes. Moreover, genetic loss of CTBP1 significantly reduced the metastatic activity of HCC cells <i>in vitro</i>. <strong>CONCLUSIONS</strong> Our data suggest that the loss of CTBP1 suppresses cell proliferative and invasive activity of HCC cells via the PI3K/Akt pathway.

C-terminal of E1A binding protein 1 enhances the migration of gastric epithelial cells and has a clinicopathologic significance in human gastric carcinoma

Background

Recent studies have claimed that the C-terminal of E1A binding proteins (CtBPs) influence tumorigenesis through participating in cell signal transduction in various human tumors. However, the detailed expression profiles of CtBP isoforms in human gastric cancer (GC) and the molecular mechanisms of CtBP involvement in tumor cell phenotypes warrant further investigation.

Materials and methods

The expression of CtBPs in GC cell lines and a human gastric epithelial cell line were explored via RT-qPCR and Western blotting assays. Moreover, the expression profiles of CtBPs in GC and histologically noncancerous tissues were explored by immunohistochemistry. To explore the effects of CtBP1 on the metastatic phenotype in GC, gastric epithelial cells were transfected with a eukaryotic expression plasmid to overexpress CTBP1, and the endogenous CtBP1 or JAK1 in GC cells was silenced through an RNA interference (RNAi) method. These transfections were validated via Western blotting, and the activation state of the JAK1/Stat3 signaling pathway was also explored via Western blotting. Furthermore, the malignant phenotype of GC cells was evaluated via a Cell Counting Kit-8 (CCK8) assay, colony formation assay, transwell assay, and wound-healing experiment.

Results

Our data revealed that the expression of CtBP1, but not CTBP2, was upregulated in 102 GC tissue samples compared with 98 noncancerous tissue samples, and the elevated expression level of CtBP1 was notably associated with distant metastasis. CTBP1 modulated cell migration and invasion through the JAK1/Stat3 signaling pathway in gastric epithelial cells. In addition, genetic silence of CtBP1 expression in GC cells notably constrained cell proliferation, invasion and migration abilities through inhibiting the activation of the JAK1/Stat3 pathway in GC cells.

Conclusion

Our data reveal that the knockout of CtBP1 notably constrains distant metastasis in GC through the JAK1/Stat3 pathway, suggesting that targeting CtBP1 is a practical anti-tumor approach to restrain tumor progression in GC.

C-terminal of E1A binding protein 2 promotes the malignancy of osteosarcoma cells via JAK1/Stat3 signaling

Previous studies have demonstrated that the C-terminal of E1A binding proteins (CtBPs) influences tumorigenesis by participating in cell signal transduction in various human malignancies. However, the detailed expression patterns of CtBP isoforms in human osteosarcoma (OS) and the molecular mechanisms of CtBP involvement in tumor cell phenotypes requires further investigation. In the present study, the expression patterns of CtBP2 in OS cells and tissues were explored by immunohistochemistry. Fetal osteoblast cells were transfected with a eukaryotic expression plasmid to overexpress CtBP2, and the endogenous CtBP2 in OS cells was silenced via a short hairpin RNA. These transfections were validated and the phosphorylation levels of the JAK1/Stat3 signaling pathway were explored via western blotting. Furthermore, the malignant phenotype of OS cells was evaluated via a Cell Counting Kit-8 assay, cell colony formation assay, cell migration assay and scratch wound healing assay. The results revealed that the expression of CtBP2, but not CtBP1, was upregulated in OS tissue samples and the elevated expression level of CtBP2 was notably associated with distant metastasis. CtBP2 was demonstrated to modulate cell migration and invasion via JAK1/Stat3 signaling pathway in fetal osteoblast cells. In addition, genetic silencing of CtBP2 expression in OS cells notably reduced cell migration abilities and the phosphorylation of the JAK1/Stat3 pathway. In summary, the present studies revealed that the loss of CtBP2 constrained distant metastasis through the JAK1/Stat3 pathway in OS, suggesting that targeting CtBP2 may be a practical anti-tumor approach to prevent OS tumor progression.

The Adenovirus E1A C Terminus Suppresses a Delayed Antiviral Response and Modulates RAS Signaling

The N-terminal half of adenovirus e1a assembles multimeric complexes with host proteins that repress innate immune responses and force host cells into S-phase. In contrast, the functions of e1a's C-terminal interactions with FOXK, DCAF7, and CtBP are unknown. We found that these interactions modulate RAS signaling, and that a single e1a molecule must bind all three of these host proteins to suppress activation of a subset of IFN-stimulated genes (ISGs). These ISGs were otherwise induced in primary respiratory epithelial cells at 12 hr p.i. This delayed activation of ISGs required IRF3 and coincided with an ∼10-fold increase in IRF3 from protein stabilization. The induced IRF3 bound to chromatin and localized to the promoters of activated ISGs. While IRF3, STAT1/2, and IRF9 all greatly increased in concentration, there were no corresponding mRNA increases, suggesting that e1a regulates the stabilities of these key activators of innate immune responses, as shown directly for IRF3.

Down-regulation of C-terminal binding protein 2 (CtBP2) inhibits proliferation, migration, and invasion of human SHSY5Y cells in vitro

Neuroblastoma is the most common extracranial solid tumor in children and is responsible for ∼15% of pediatric cancer deaths. CtBP2 is a member of the CtBP family of proteins that functions as a transcription regulator and has been demonstrated to interact with the C-terminus of the adenoviral E1A oncoprotein. In this study, the expression of CtBP2 in the human neuroblastoma cell line SHSY5Y was down-regulated using lentiviral-mediated RNA interference. Down-regulation of CtBP2 inhibited the expression of c-myc, MMP2, and MMP9 proteins. Moreover, low expression of CtBP2 resulted in inhibited cell growth, proliferation, migration, and invasion, and the cell cycle was arrested at G2/M-phase. These results indicate that CtBP2 may be a potential target to suppress tumorigenesis in neuroblastoma.

CtBP2 is an independent prognostic marker that promotes GLI1 induced epithelial-mesenchymal transition in hepatocellular carcinoma

C-terminal binding protein 2 (CtBP2) is a transcriptional co-repressor that promotes cancer cell migration and invasion by inhibiting multiple tumor suppressor genes that contribute to cell mobility and adhesion. In this investigation, we showed thatCtBP2 expression was increased significantly in HCC tissues when compared to matched normal adjacent liver tissues. We also showed that CtBP2 expression is associated with worse HCC patient prognosis after liver resection. CtBP2 over-expression induced epithelial-mesenchymal transition (EMT) in Huh7 cells and, correspondingly, silencing CtBP2 suppressed EMT in MHCC97H cells. ChIP assays revealed that GLI1 increased CtBP2 transcription by directly binding its promoter. Furthermore, interaction of CtBP2 and Snail Family Zinc Finger 1 (SNAI1), both of which were found to be positively regulated by GLI1, was confirmed by Co-IP assay. SNAI1 knockdown revealed that SNAI1 was essential for CtBP2 induction of the EMT phenotype of HCC cells, and CtBP2 knockdown reversed GLI1-SNAI1 driven EMT in Huh7 cells. Finally, in vivo experiments demonstrated that enhanced CtBP2expression promoted HCC xenograft growth and induced EMT. In conclusion, CtBP2 may serve as a prognostic marker for post liver resection HCC and may play a role during GLI1-driven EMT as a transcriptional co-repressor of SNAI1.

Wnt/β-Catenin Mediates AICAR Effect to Increase GATA3 Expression and Inhibit Adipogenesis

A better understanding of the mechanism and manipulation of the tightly regulated cellular differentiation process of adipogenesis may contribute to a reduction in obesity and diabetes. Multiple transcription factors and signaling pathways are involved in the regulation of adipogenesis. Here, we report that the AMP-activated protein kinase activator, 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) can activate AMPK in preadipocytes and thus increase the expression of GATA3, an anti-adipogenic factor. However, AICAR-increased GATA3 is mediated by the stimulation of Wnt/β-catenin signaling in preadipocytes. Mechanistically, AICAR-activated AMPK inhibits GSK3β through a phosphorylation process that stabilizes β-catenin. This stabilized β-catenin then translocates into nucleus where it interacts with T-cell factors (TCF), leading to the increased β-catenin/TCF transcriptional activity that induces GATA3 expression. In addition, AICAR also relieves the repressing effect of the C-terminal-binding protein (CtBP) co-repressor by diverting CtBP away from the β-catenin·TCF complex at the GATA3 promoter. The anti-adipogenic effect of GATA3 and AICAR is consistently attenuated by the disruption of Wnt/β-catenin signaling. Furthermore, GATA3 suppresses key adipogenic regulators by binding to the promoters of these regulators, such as the peroxisome proliferator-activated receptor-γ (PPARγ) gene, and the disruption of Wnt/β-catenin signaling reduces the GATA3 binding at the PPARγ promoter. In differentiated adipocytes, GATA3 expression inhibition is facilitated by the down-regulation of β-catenin levels, the reduction in β-catenin binding, and the increase in CtBP binding at the GATA3 promoter. Our findings shed light on the molecular mechanism of adipogenesis by suggesting that different regulation pathways and adipogenic regulators collectively modulate adipocyte differentiation through cross-talk.

CtBP maintains cancer cell growth and metabolic homeostasis via regulating SIRT4

Cancer cells rely on glycolysis to maintain high levels of anabolism. However, the metabolism of glucose via glycolysis in cancer cells is frequently incomplete and results in the accumulation of acidic metabolites such as pyruvate and lactate. Thus, the cells have to develop strategies to alleviate the intracellular acidification and maintain the pH stability. We report here that glutamine consumption by cancer cells has an important role in releasing the acidification pressure associated with cancer cell growth. We found that the ammonia produced during glutaminolysis, a dominant glutamine metabolism pathway, is critical to resist the cytoplasmic acidification brought by the incomplete glycolysis. In addition, C-terminal-binding protein (CtBP) was found to have an essential role in promoting glutaminolysis by directly repressing the expression of SIRT4, a repressor of glutaminolysis by enzymatically modifying glutamate dehydrogenase in mitochondria, in cancer cells. The loss of CtBP in cancer cells resulted in the increased apoptosis due to intracellular acidification and the ablation of cancer cell metabolic homeostasis represented by decreased glutamine consumption, oxidative phosphorylation and ATP synthesis. Importantly, the immunohistochemistry staining showed that there was excessive expression of CtBP in tumor samples from breast cancer patients compared with surrounding non-tumor tissues, whereas SIRT4 expression in tumor tissues was abolished compared with the non-tumor tissues, suggesting CtBP-repressed SIRT4 expression contributes to the tumor growth. Therefore, our data suggest that the synergistically metabolism of glucose and glutamine in cancer cells contributes to both pH homeostasis and cell growth. At last, application of CtBP inhibitor induced the acidification and apoptosis of breast cancer cells and inhibited glutaminolysis in engrafted tumors, suggesting that CtBP can be potential therapeutic target of cancer treatment.

CtBPs sense microenvironmental oxygen levels to regulate neural stem cell state

Bone morphogenetic proteins (BMPs) secreted by the dorsal neural tube and overlying ectoderm are key signals for the specification of the roof plate and dorsal interneuron populations. However, the signals that confer nonneurogenic character to the roof plate region are largely unknown. We report that the roof plate region shows elevated oxygen levels compared to neurogenic regions of the neural tube. These high oxygen levels are required for the expression of the antineuronal transcription factor Hes1 in the roof plate region. The transcriptional corepressor CtBP is a critical mediator of the oxygen-sensing response. High oxygen promotes a decrease in the CtBP occupancy of the promoter of Hes1. Furthermore, under conditions of high oxygen and BMP, CtBP associates with HES1 and represses neurogenesis. We propose that CtBP integrates signals originating from microenvironmental levels of oxygen and BMP to confer nonneurogenic character to the roof plate region.

Impact of EBV essential nuclear protein EBNA-3C on B-cell proliferation and apoptosis

For over 40 years, EBV infection has been implicated in the etiology of a variety of lymphoid malignancies with the exceptional ability to drive resting B cells to continuously proliferate by successfully overriding cellular apoptotic stimuli. EBV utilizes the normal physiology of B-cell differentiation to persist within the memory B-cell pool of the immunocompetent host and subsequently establishes a life-long latent infection. During latency, out of a subset of viral genes expressed, EBNA-3C is one of the essential antigens required for in vitro primary B-cell transformation. EBNA-3C acts as a transcriptional coregulator by interacting with various cellular and viral factors. For the last 10 years, we have been actively engaged in discerning the biological significance of these interactions and revealed that EBNA-3C primarily targets two important cellular pathways - cell cycle and apoptosis. This review aims to summarize our current knowledge on EBNA-3C-mediated functions and describe how EBNA-3C seizes these cellular pathways that eventually promote B-cell lymphomagenesis. A scrupulous understanding of the critical relationship between EBNA-3C and these cellular machineries will not only aid in elucidating EBV pathogenesis, but also largely facilitate the development of novel diagnostic, as well as therapeutic, strategies against a vast range of EBV-associated B-cell lymphomas.

Genome-wide profiles of CtBP link metabolism with genome stability and epithelial reprogramming in breast cancer

The C-terminal binding protein (CtBP) is a NADH-dependent transcriptional repressor that links carbohydrate metabolism to epigenetic regulation by recruiting diverse histone modifying complexes to chromatin. Here, global profiling of CtBP in breast cancer cells reveals that it drives epithelial to mesenchymal transition, stem cell pathways, and genome instability. CtBP expression induces mesenchymal and stem cell-like features while CtBP depletion or caloric restriction reverses gene repression and increases DNA repair. Multiple members of the CtBP-targeted gene network are selectively down-regulated in aggressive breast cancer subtypes. Differential expression of CtBP-targeted genes predicts poor clinical outcome in breast cancer patients, and elevated levels of CtBP in patient tumors predict shorter median survival. Finally, both CtBP promoter targeting and gene repression can be reversed by small molecule inhibition. These findings define broad roles for CtBP in breast cancer biology and suggest novel chromatin-based strategies for pharmacologic and metabolic intervention in cancer.

CtBP1 is involved in epithelial-mesenchymal transition and is a potential therapeutic target for hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a highly invasive type of cancer. Metastasis is the leading cause of mortality of advanced HCC patients. In the metastasis cascade, cancer cells undergo epithelial-mesenchymal transition resulting in the loss of cell‑to‑cell adhesion, migration and invasion into the stroma. Loss of E-cadherin expression is a key molecular event in epithelial-mesenchymal transition through several regulatory mechanisms including epigenetic modification, regulation by inhibitory transcriptional factors and deletion of chromosome 16q24 locus. C-terminal binding protein 1 (CtBP1) functions as a corepressor binding to several transcriptional factors and suppresses E-cadherin expression. We found that CtBP1 was upregulated in HCC when compared with paired normal liver tissues and was inversely correlated with E-cadherin expression in HCC by immunohistochemical assay using tissue array. Western blot analysis confirmed the results of the immunohistochemical assays. When CtBP1 was knocked down by siRNA in HepG2 cells (a human HCC cell line), E-cadherin was upregulated and the invasive ability of HepG2 cells was inhibited. In addition, following CtBP1 knockdown, the cell viability was decreased along with increased apoptosis rather than cell cycle arrest. These data suggest a pivotal role of CtBP1 in EMT of HCC, and its potential as a therapeutic target in human disease.

RREB-1 is a transcriptional repressor of HLA-G

The nonclassical HLA-G is a molecule specifically involved in immune tolerance with highly restricted tissue distribution in healthy conditions. Yet it is overexpressed in numerous tumors and in allografts with better acceptance. Major mechanisms involved in regulation of HLA-G transcription are still poorly described. Thus, to characterize these mechanisms we have developed a specific proteomic approach to identify proteins that bind differentially to the HLA-G gene promoter by promoter pull-down assay followed by spectrometry mass analysis. Among specific binding factors, we focused on RREB-1, a ras-responsive element binding protein 1. We demonstrated that RREB-1 represses HLA-G transcriptional activity and binds three ras response elements within the HLA-G promoter. RREB-1 protein, specifically in HLA-G-negative cells, interacts with subunits of CtBP complex implicated in chromatin remodeling. This demonstration is the first of a repressor factor of HLA-G transcriptional activity taking part in HLA-G repression by epigenetic mechanisms.

Interaction of ZEB and histone deacetylase with the PLDLS-binding cleft region of monomeric C-terminal binding protein 2

Background

Proteins of the C-terminal binding protein (CtBP) family, CtBP1 and CtBP2 are closely related transcriptional regulators that are coded by two different gene loci in the vertebrate genomes. They perform redundant and unique functions during animal development. CtBP proteins mediate their transcriptional function through interaction with various DNA-binding repressors that contain PLDLS-like motifs and chromatin modifying enzymes, such as class I histone deacetylases (HDAC) that do not contain such motifs. The N-terminal region of CtBP1/2 forms a hydrophobic cleft and is involved in interaction with both PLDLS-containing factors and non-PLDLS factors. CtBP proteins function as dimers to mediate transcriptional repression and dimerization is modulated by specific binding to NAD/NADH.

Results

In this study, we have investigated the role of dimerization of CtBP2 in recruitment of PLDLS-motif cofactors and non-PLDLS cofactors. Our results indicate that mutations in CtBP2 that interfere with dimerization abolish CtBP2 interaction with most cellular factors, except the PLDLS-motif factor zinc-finger E-box binding homeobox (ZEB) and the non-PLDLS factor HDAC2. Unlike most PLDLS-containing CtBP-binding proteins, ZEB contains three PLDLS-like motifs and all three contribute to the interaction with the CtBP2 monomer. Despite the ability to interact with ZEB and HDAC, the CtBP2 monomer fails to mediate ZEB-dependent transcriptional repression. The lack of repression activity of the CtBP2 monomer is correlated with the competition between ZEB and HDAC for interaction with the CtBP2 monomer.

Conclusion

These results suggest a competition between the canonical PLDLS-motif factors such as E1A and non-PLDLS factor HDAC for interaction with CtBP. They also indicate that the affinity for the CtBP monomer may be determined by the number as well as amino acid sequence compositions of the PLDLS-like motifs. Our results are consistent with a model that the CtBP2 dimer may interact with a PLDLS-containing repressor through one monomer and recruit HDAC and other chromatin modifying enzymes through the second monomer in the CtBP2 dimer.

Adenovirus transforming protein E1A induces c-Myc in quiescent cells by a novel mechanism

Previously we showed that the E1A binding proteins p300 and CBP negatively regulate c-Myc in quiescent cells and that binding of E1A to p300 results in the induction of c-Myc and thereby induction of S phase. We demonstrated that p300 and HDAC3 cooperate with the transcription factor YY1 at an upstream YY1 binding site and repress the Myc promoter. Here we show that the small E1A protein induces c-Myc by interfering with the protein-protein interaction between p300, YY1, and HDAC3. Wild-type E1A but not the E1A mutants that do not bind to p300 interfered in recruitment of YY1, p300, and HDAC3 to the YY1 binding site. As E1A started to accumulate after infection, it transiently associated with promoter-bound p300. Subsequently, YY1, p300, and HDAC3 began to dissociate from the promoter. Later in infection, E1A dissociated from the promoter as well as p300, YY1, and HDAC3. Removal of HDAC3 from the promoter correlated with increased acetylation of Myc chromatin and induction. In vivo E1A stably associated with p300 and dissociated YY1 and HDAC3 from the trimolecular complex. In vitro protein-protein interaction studies indicated that E1A initially binds to the p300-YY1-HDAC3 complex, briefly associates with it, and then dissociates the complex, recapitulating somewhat the in vivo situation. Thus, E1A binding to the C-terminal region of p300 disrupts the important corepressor function provided by p300 in repressing c-Myc. Our results reveal a novel mechanism by which a viral oncoprotein activates c-Myc in quiescent cells and raise the possibility that the oncoproteins encoded by the small-DNA tumor viruses may use this mechanism to induce c-Myc, which may be critical for cell transformation.

Inhibition of transcriptional activation and cell proliferation activities of adenovirus E1A by the unique N-terminal domain of CtBP2

The 243-residue E1A protein of adenovirus induces cellular proliferation, at least partly by regulating the transcription of cellular genes. This E1A function requires E1A N-terminal region and conserved regions 1 and 2 (CR1 and CR2), which interact with histone acetyl transferases, p400 chromatin-modifying complex and the Rb family proteins. A PLDLS motif at the E1A C-terminal (CR4) region, interacts with the C-terminal binding proteins (CtBP1 and CtBP2), and antagonizes some E1A functions. In this report, we discovered that the transcriptional activation function of E1A was specifically repressed by a short N-terminal domain unique to CtBP2. The CtBP2-mediated repression of E1A transcriptional activation activity is independent of histone deacetylases, which can be recruited by CtBP1/2 proteins to inhibit transcription. Fusion of the CtBP2 N-terminal 20 residues to the E1A C-terminal region rendered E1A to be inactive in transcriptional activation without interfering with E1A's ability to interact with major cofactors such as pRb, p400 and p300. Substitution of the N-terminal domain of CtBP1 for the CtBP2 domain in E1A-CtBP2 fusion partially restored the transactivation activity of E1A. In a cell-proliferation model utilizing primary baby rat kidney cells and retrovirally expressed E1A, the ability of E1A to induce cellular proliferation was strongly inhibited when the CtBP2 N-terminal region was fused to E1A. These results are consistent with a hypothesis that CtBP2 may inhibit E1A induced cell proliferation by antagonizing the transcriptional activation function controlled by the N-terminal region of E1A.