C-terminal-binding protein corepresses epithelial and proapoptotic gene expression programs

Master corepressor inactivation through multivalent SLiM-induced polymerization mediated by the oncogene suppressor RAI2

While the elucidation of regulatory mechanisms of folded proteins is facilitated due to their amenability to high-resolution structural characterization, investigation of these mechanisms in disordered proteins is more challenging due to their structural heterogeneity, which can be captured by a variety of biophysical approaches. Here, we used the transcriptional master corepressor CtBP, which binds the putative metastasis suppressor RAI2 through repetitive SLiMs, as a model system. Using cryo-electron microscopy embedded in an integrative structural biology approach, we show that RAI2 unexpectedly induces CtBP polymerization through filaments of stacked tetrameric CtBP layers. These filaments lead to RAI2-mediated CtBP nuclear foci and relieve its corepressor function in RAI2-expressing cancer cells. The impact of RAI2-mediated CtBP loss-of-function is illustrated by the analysis of a diverse cohort of prostate cancer patients, which reveals a substantial decrease in RAI2 in advanced treatment-resistant cancer subtypes. As RAI2-like SLiM motifs are found in a wide range of organisms, including pathogenic viruses, our findings serve as a paradigm for diverse functional effects through multivalent interaction-mediated polymerization by disordered proteins in healthy and diseased conditions.

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.

Unveiling the metabolic landscape of pulmonary hypertension: insights from metabolomics

Pulmonary hypertension (PH) is regarded as cardiovascular disease with an extremely poor prognosis, primarily due to irreversible vascular remodeling. Despite decades of research progress, the absence of definitive curative therapies remains a critical challenge, leading to high mortality rates. Recent studies have shown that serious metabolic disorders generally exist in PH animal models and patients of PH, which may be the cause or results of the disease. It is imperative for future research to identify critical biomarkers of metabolic dysfunction in PH pathophysiology and to uncover metabolic targets that could enhance diagnostic and therapeutic strategies. Metabolomics offers a powerful tool for the comprehensive qualitative and quantitative analysis of metabolites within specific organisms or cells. On the basis of the findings of the metabolomics research on PH, this review summarizes the latest research progress on metabolic pathways involved in processes such as amino acid metabolism, carbohydrate metabolism, lipid metabolism, and nucleotide metabolism in the context of PH.

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.

Transcriptional corepressor activity of CtBP1 is regulated by ISG15 modification

C-terminal binding protein 1 (CtBP1) is a critical transcriptional corepressor of many tumor suppressor genes and plays diverse roles in the progression of cancers. The transcriptional repression function of CtBP1 is mediated by recruiting histone-modifying enzymes, such as histone deacetylases and histone methyltransferases, to target genes by binding with DNA-interacting factors. Several post-translational modifications of CtBP1 have been identified, including ubiquitination, phosphorylation, and SUMOylation. This paper reports that CtBP1 is conjugated by ISG15. Endogenous CtBP1 was modified by ISG15 after interferon-α treatment in HeLa cells. The ISGylation process of CtBP1 was regulated by deISGylation enzyme USP18 and ISG15 E3 ligase EFP. Interestingly, CtBP1 ISGylation affected the binding affinity between CtBP1 and some components of CtBP1-associated transcriptional complexes. HDAC1 and LSD1 bound more efficiently to ISG15-conjugated CtBP1 than non-conjugated CtBP1. On the other hand, binding between CtBP1 and HDAC4 was unaffected by ISG15 modification. Furthermore, ISG15 modification enhanced the transcriptional repression activity of CtBP1 on several target genes related to EMT and apoptosis. These findings suggest that the ISG15 modification of CtBP1 modulates the function and activity of CtBP1 and that CtBP1 ISGylation may provide a new insight for CtBP1-mediated cancers.

Dual role of Snail1 as transcriptional repressor and activator

Snail1 transcriptional factor plays a key role in the control of epithelial to mesenchymal transition, a process that remodels tumor cells increasing their invasion and chemo-resistance as well as reprograms their metabolism and provides stemness properties. During this transition, Snail1 acts as a transcriptional repressor and, as growing evidences have demonstrated, also as a direct activator of mesenchymal genes. In this review, I describe the different proteins that interact with Snail1 and are responsible for these two different functions on gene expression; I focus on the transcriptional factors that associate to Snail1 in their target promoters, both activated and repressed. I also present working models for Snail1 action both as repressor and activator and raise some issues that still need to be investigated.

A regulatory role for the unstructured C-terminal domain of the CtBP transcriptional corepressor

The C-terminal binding protein (CtBP) is a transcriptional corepressor that plays critical roles in development, tumorigenesis, and cell fate. CtBP proteins are structurally similar to alpha hydroxyacid dehydrogenases and feature a prominent intrinsically disordered region in the C terminus. In the mammalian system, CtBP proteins lacking the C-terminal domain (CTD) are able to function as transcriptional regulators and oligomerize, putting into question the significance of this unstructured domain for gene regulation. Yet, the presence of an unstructured CTD of ∼100 residues, including some short motifs, is conserved across Bilateria, indicating the importance of maintaining this domain over evolutionary time. To uncover the significance of the CtBP CTD, we functionally tested naturally occurring Drosophila isoforms of CtBP that possess or lack the CTD, namely CtBP(L) and CtBP(S). We used the CRISPRi system to recruit dCas9-CtBP(L) and dCas9-CtBP(S) to endogenous promoters to directly compare their transcriptional impacts in vivo. Interestingly, CtBP(S) was able to significantly repress transcription of the Mpp6 promoter, while CtBP(L) was much weaker, suggesting that the long CTD may modulate CtBP's repression activity. In contrast, in cell culture, the isoforms behaved similarly on a transfected Mpp6 reporter gene. The context-specific differences in activity of these two developmentally regulated isoforms suggests that the CTD may help provide a spectrum of repression activity suitable for developmental programs.

A regulatory role for the unstructured C-terminal domain of the CtBP transcriptional corepressor

The C-terminal Binding Protein (CtBP) is a transcriptional corepressor that plays critical roles in development, tumorigenesis, and cell fate. CtBP proteins are structurally similar to alpha hydroxyacid dehydrogenases and feature a prominent intrinsically disordered region in the C-terminus. In the mammalian system, CtBP proteins lacking the C-terminal Domain (CTD) are able to function as transcriptional regulators and oligomerize, putting into question the significance of this unstructured domain for gene regulation. Yet, the presence of an unstructured CTD of ~100 residues, including some short motifs, is conserved across Bilateria, indicating the importance of maintaining this domain over evolutionary time. To uncover the significance of the CtBP CTD, we functionally tested naturally occurring Drosophila isoforms of CtBP that possess or lack the CTD, namely CtBP(L) and CtBP(S). We used the CRISPRi system to recruit dCas9-CtBP(L) and dCas9-CtBP(S) to endogenous promoters to directly compare their transcriptional impacts in vivo. Interestingly, CtBP(S) was able to significantly repress transcription of the Mpp6 promoter, while CtBP(L) was much weaker, suggesting that the long CTD may modulate CtBP's repression activity. In contrast, in cell culture, the isoforms behaved similarly on a transfected Mpp6 reporter gene. The context-specific differences in activity of these two developmentally-regulated isoforms suggests that the CTD may help provide a spectrum of repression activity suitable for developmental programs.

CtBP Neuroprotective Role in Toxin-Based Parkinson's Disease Models: From Expression Pattern to Dopaminergic Survival

C-terminal binding proteins (CtBP) are transcriptional co-repressors regulating gene expression. CtBP promote neuronal survival through repression of pro-apoptotic genes, and may represent relevant targets for neurodegenerative disorders, such as Parkinson's disease (PD). Nevertheless, evidence of the role of CtBP1 and CtBP2 in neurodegeneration are scarce. Herein, we showed that CtBP1 and CtBP2 are expressed in neurons, dopaminergic neurons, astrocytes, and microglia in the substantia nigra (SN) and striatum of adult mice. Old mice showed a lower expression of CtBP1 in the SN and higher expression of CtPB2 in the SN and striatum compared with adult mice. In vivo models for PD (paraquat, MPTP, 6-OHDA) showed increased expression of CtBP1 in the SN and striatum while CtBP2 expression was increased in the striatum of paraquat-treated rats only. Moreover, an increased expression of both CtBP was found in a dopaminergic cell line (N27) exposed to 6-OHDA. In the 6-OHDA PD model, we found a dual effect using an unspecific ligand of CtBP, the 4-methylthio 2-oxobutyric acid (MTOB): higher concentrations (e.g. 2500 µM, 1000 µM) inhibited dopaminergic survival, while at 250 μM it counteracted cell death. In vitro, this latter protective role was absent after the siRNA silencing of CtBP1 or CtBP2. Altogether, this is the first report exploring the cellular and regional expression pattern of CtBP in the nigrostriatal pathway and the neuroprotective role in PD toxin-based models. CtBP could counteract dopaminergic cell death in the 6-OHDA PD model and, therefore, CtBP function and therapeutic potential in PD should be further explored.

The Cynosure of CtBP: Evolution of a Bilaterian Transcriptional Corepressor

Evolution of sequence-specific transcription factors clearly drives lineage-specific innovations, but less is known about how changes in the central transcriptional machinery may contribute to evolutionary transformations. In particular, transcriptional regulators are rich in intrinsically disordered regions that appear to be magnets for evolutionary innovation. The C-terminal Binding Protein (CtBP) is a transcriptional corepressor derived from an ancestral lineage of alpha hydroxyacid dehydrogenases; it is found in mammals and invertebrates, and features a core NAD-binding domain as well as an unstructured C-terminus (CTD) of unknown function. CtBP can act on promoters and enhancers to repress transcription through chromatin-linked mechanisms. Our comparative phylogenetic study shows that CtBP is a bilaterian innovation whose CTD of about 100 residues is present in almost all orthologs. CtBP CTDs contain conserved blocks of residues and retain a predicted disordered property, despite having variations in the primary sequence. Interestingly, the structure of the C-terminus has undergone radical transformation independently in certain lineages including flatworms and nematodes. Also contributing to CTD diversity is the production of myriad alternative RNA splicing products, including the production of "short" tailless forms of CtBP in Drosophila. Additional diversity stems from multiple gene duplications in vertebrates, where up to five CtBP orthologs have been observed. Vertebrate lineages show fewer major modifications in the unstructured CTD, possibly because gene regulatory constraints of the vertebrate body plan place specific constraints on this domain. Our study highlights the rich regulatory potential of this previously unstudied domain of a central transcriptional regulator.

Metabolic modulation of CtBP dimeric status impacts the repression of DNA damage repair genes and the platinum sensitivity of ovarian cancer

Platinum drug-based chemotherapy plays a dominant role in OC (ovarian cancer) treatment. The expression of DNA damage repair (DDR) genes is critical in distinguishing drug-sensitive and drug-refractory patients, as well as in the development of drug resistance in long-term treated patients. CtBP is a highly expressed oncogene in OC and was found to repress DDR genes expression in our previous study. In the present study, the formation of CtBP dimers in live cells was studied, and the functional differences between monomeric and oligomeric CtBP were explored by CHIP-seq and RNA-seq. Besides, the dynamics of CtBP dimer formation in response to the metabolic modulation were investigated by the protein fragment complementation (PCA) assays. We show that dimerized CtBP, but not the dimerization-defective mutant, binds to and represses DDR gene expression in OC cells. Treatment of the mice tumors grown from engrafted OC cells by cisplatin disclosed that high-level CtBP expression promotes the CtBP dimerization and increases the therapeutic effect of cisplatin. Moreover, the CtBP dimerization is responsive to the intracellular metabolic status as represented by the free NADH abundance. Metformin was found to increase the dimerization of CtBP and potentiate the therapeutic effect of cisplatin in a CtBP dimerization-dependent manner. Our data suggest that the CtBP dimerization status is a potential biomarker to predict platinum drug sensitivity in patients with ovarian cancer and a target of metformin to improve the therapeutic effect of platinum drugs in OC treatment.

N6-methyladenosine-mediated SH3BP5-AS1 upregulation promotes GEM chemoresistance in pancreatic cancer by activating the Wnt signaling pathway

Background

Pancreatic cancer (PC) is highly malignant. Chemotherapy is the main treatment strategy, especially for patients with advanced PC. However, chemoresistance has always been a frequently encountered bottleneck. Hence, there is an urgent need to enhance the sensitivity of PC to gemcitabine (GEM).

Results

We demonstrated that SH3BP5-AS1 was significantly upregulated in GEM-resistant PC and predicted a poorer prognosis. SH3BP5-AS1 stability was regulated by ALKBH5/IGF2BP1-mediated m6A modification. Loss of SH3BP5-AS1 reduced PC cell migration and invasion and enhanced the sensitivity of PC to GEM, as confirmed by gain- and loss-of-function assays in vitro and in vivo. Bioinformatics analysis revealed that SH3BP5-AS1 acted as a ceRNA against miR-139-5p and directly targeted CTBP1, affecting the biological behavior of PC cells. The mechanistic studies revealed that the upregulation of SH3BP5-AS1 increased CTBP1 expression by directly activating the Wnt signaling pathway, promoting GEM resistance.

Conclusions

This study revealed that SH3BP5-AS1 activated Wnt signaling pathway by sponging miR-139-5p, upregulating CTBP1 expression, and contributing to the sensitivity of PC cells to GEM. SH3BP5-AS1 might be a potential target for PC therapy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13062-022-00347-5.

NADH/NAD+ binding and linked tetrameric assembly of the oncogenic transcription factors CtBP1 and CtBP2

The activation of oncogenic C-terminal binding Protein (CtBP) transcriptional activity is coupled with NAD(H) binding and homo-oligomeric assembly, although the level of CtBP assembly and nucleotide binding affinity continues to be debated. Here, we apply biophysical techniques to address these fundamental issues for CtBP1 and CtBP2. Our ultracentrifugation results unambiguously demonstrate that CtBP assembles into tetramers in the presence of saturating NAD⁺ or NADH with tetramer to dimer dissociation constants about 100 nm. Isothermal titration calorimetry measurements of NAD(H) binding to CtBP show dissociation constants between 30 and 500 nm, depending on the nucleotide and paralog. Given cellular levels of NAD⁺ , CtBP is likely to be fully saturated with NAD under physiological concentrations suggesting that CtBP is unable to act as a sensor for NADH levels.

Circular RNA circHERC4 as a novel oncogenic driver to promote tumor metastasis via the miR-556-5p/CTBP2/E-cadherin axis in colorectal cancer

BACKGROUND

The main cause of death in colorectal cancer patients is metastasis. Accumulating evidences suggest that circRNA plays pivotal roles in cancer initiation and development. However, the underlying molecular mechanisms of circRNAs that orchestrate cancer metastasis remain vague and need further clarification.

METHODS

Two paired CRC and adjacent normal tissues were used to screen the upregulated circRNAs by circRNA-seq; then, cell invasion assay was applied to confirm the functional invasion-related circRNAs. According to the above methods, circHERC4 (hsa_circ_0007113) was selected for further research. Next, we investigated the clinical significance of circHERC4 in a large cohort of patients with CRC. The oncogenic activity of circHERC4 was investigated in both CRC cell lines and animal xenograft studies. Finally, we explored the molecular mechanisms underlying circHERC4 as a malignant driver.

RESULTS

We demonstrated that circHERC4 was aberrantly elevated in CRC tissues (P < 0.001), and was positively associated with lymph node metastasis and advanced tumor grade (P < 0.01). Notably, the expression of circHERC4 was associated with worse survival in patients with CRC. Silencing of circHERC4 significantly inhibited the proliferation and migration of two highly aggressive CRC cell lines and reduced liver and lung metastasis in vivo. Mechanistically, we revealed that circHERC4 inactivated the tumor suppressor, miR-556-5p, leading to the activation of CTBP2/E-cadherin pathway which promotes tumor metastasis in CRC.

CONCLUSIONS

CircHERC4 exerts critical roles in promoting tumor aggressiveness through miR-556-5p/CTBP2/E-cadherin pathway and is a prognostic biomarker of the disease, suggesting that circHERC4 may serve as an exploitable therapeutic target for patients with CRC.

CircIMMP2L promotes esophageal squamous cell carcinoma malignant progression via CtBP1 nuclear retention dependent epigenetic modification

BACKGROUND

Esophageal squamous cell carcinoma (ESCC) is one of the most aggressive cancers. The two major lethal causes of ESCC are diagnosis at an advanced stage and lymph node metastasis (LNM). Circular RNAs (circRNAs) play critical regulatory roles in cancer progression, though, largely through unclear mechanisms. However, the character of circRNAs in the malignant progression of ESCC remains unclear.

METHODS

The circRNA microarray was used to explore the circRNAs that were differentially expressed between ESCC and paired adjacent normal tissues. The function of circIMMP2L was validated by gain or loss of function assays. Pull-down, RNA immunoprecipitation assays were used to demonstrate the biological mechanism of circIMMP2L. Tissue microarray (TMA), specimen, and paired plasma were investigated to evaluate the clinical significance of circIMMP2L.

RESULTS

CircIMMP2L, commonly upregulated in tumor and plasma from advanced-stage ESCC patients and LNM patients, predicts poorer patient survival. CircIMMP2L was also found to be a significant indicator for LNM, even in the T1 stage of ESCC. CircIMMP2L depletion suppressed the malignant progression of ESCC both in vitro and in vivo. Mechanistically, cytoplasmic circIMMP2L interacted with CtBP1 and facilitated the nuclear retention of CtBP1 in a CtBP2-independent manner. Moreover, circIMMP2L promoted the interaction of CtBP1 with HDAC1 in the nucleus, which is essential for epigenetic remodeling and transcriptional suppression of E-cadherin and p21.

CONCLUSIONS

These findings demonstrated that circIMMP2L promotes the malignant progression of ESCC mediated by CtBP1 nuclear retention and is a robust biomarker for the diagnosis, prognosis, and LNM in ESCC. Further, the findings extend our knowledge about the mechanism of circRNA regulation of gene transcription through epigenetics.

CtBP1/2 differentially regulate genomic stability and DNA repair pathway in high-grade serous ovarian cancer cell

The C-terminal binding proteins (CtBPs), CtBP1 and CtBP2, are transcriptional co-repressor that interacts with multiple transcriptional factors to modulate the stability of chromatin. CtBP proteins were identified with overexpression in the high-grade serous ovarian carcinoma (HGSOC). However, little is known about CtBP proteins’ regulatory roles in genomic stability and DNA repair in HGSOC. In this study, we combined whole-transcriptome analysis with multiple research methods to investigate the role of CtBP1/2 in genomic stability. Several key functional pathways were significantly enriched through whole transcription profile analysis of CtBP1/2 knockdown SKOV3 cells, including DNA damage repair, apoptosis, and cell cycle. CtBP1/2 knockdown induced cancer cell apoptosis, increased genetic instability, and enhanced the sensitivity to DNA damage agents, such as γ-irradiation and chemotherapy drug (Carboplatin and etoposide). The results of DNA fiber assay revealed that CtBP1/2 contribute differentially to the integrity of DNA replication track and stability of DNA replication recovery. CtBP1 protects the integrity of stalled forks under metabolic stress condition during prolonged periods of replication, whereas CtBP2 acts a dominant role in stability of DNA replication recovery. Furthermore, CtBP1/2 knockdown shifted the DSBs repair pathway from homologous recombination (HR) to non-homologous end joining (NHEJ) and activated DNA-PK in SKOV3 cells. Interesting, blast through TCGA tumor cases, patients with CtBP2 genetic alternation had a significantly longer overall survival time than unaltered patients. Together, these results revealed that CtBP1/2 play a different regulatory role in genomic stability and DSBs repair pathway bias in serous ovarian cancer cells. It is possible to generate novel potential targeted therapy strategy and translational application for serous ovarian carcinoma patients with a predictable better clinical outcome.

The transrepression and transactivation roles of CtBPs in the pathogenesis of different diseases

Gene transcription is strictly controlled by transcriptional complexes, which are assemblies of transcription factors, transcriptional regulators, and co-regulators. Mammalian genomes encode two C-terminal-binding proteins (CtBPs), CtBP1 and CtBP2, which are both well-known transcriptional corepressors of oncogenic processes. Their overexpression in tumors is associated with malignant behavior, such as uncontrolled cell proliferation, migration, and invasion, as well as with an increase in the epithelial-mesenchymal transition. CtBPs coordinate with other transcriptional regulators, such as histone deacetylases (HDACs) and histone acetyltransferases (p300 and CBP [CREBP-binding protein]) that contain the PXDLS motif, and with transcription factors to assemble transcriptional complexes that dock onto the promoters of genes to initiate gene transcription. Emerging evidence suggests that CtBPs function as both corepressors and coactivators in different biological processes ranging from apoptosis to inflammation and osteogenesis. Therapeutic targeting of CtBPs or the interactions required to form transcriptional complexes has also shown promising effects in preventing disease progression. This review summarizes the most recent progress in the study of CtBP functions and therapeutic inhibitors in different biological processes. This knowledge may enable a better understanding of the complexity of the roles of CtBPs, while providing new insights into therapeutic strategies that target CtBPs.

Tête-à-tête with CtBP dimers

Jecrois et al. (2020) use cryoelectron microscopy to illuminate the tetrameric conformation of the CtBP2 transcriptional corepressor, a protein frequently overexpressed in human cancers. The in vivo functional characterization of tetramer-destabilizing mutants indicates that tetramerization is a physiologically important process, critical for CtBP control of gene regulation and cell migration.

Deciphering molecular mechanisms of metastasis: novel insights into targets and therapeutics

BACKGROUND

The transition of a primary tumour to metastatic progression is driven by dynamic molecular changes, including genetic and epigenetic alterations. The metastatic cascade involves bidirectional interactions among extracellular and intracellular components leading to disintegration of cellular junctions, cytoskeleton reorganization and epithelial to mesenchymal transition. These events promote metastasis by reprogramming the primary cancer cell's molecular framework, enabling them to cause local invasion, anchorage-independent survival, cell death and immune resistance, extravasation and colonization of distant organs. Metastasis follows a site-specific pattern that is still poorly understood at the molecular level. Although various drugs have been tested clinically across different metastatic cancer types, it has remained difficult to develop efficacious therapeutics due to complex molecular layers involved in metastasis as well as experimental limitations.

CONCLUSIONS

In this review, a systemic evaluation of the molecular mechanisms of metastasis is outlined and the potential molecular components and their status as therapeutic targets and the associated pre-clinical and clinical agents available or under investigations are discussed. Integrative methods like pan-cancer data analysis, which can provide clinical insights into both targets and treatment decisions and help in the identification of crucial components driving metastasis such as mutational profiles, gene signatures, associated pathways, site specificities and disease-gene phenotypes, are discussed. A multi-level data integration of the metastasis signatures across multiple primary and metastatic cancer types may facilitate the development of precision medicine and open up new opportunities for future therapies.

NAD(H) phosphates mediate tetramer assembly of human C-terminal binding protein (CtBP)

C-terminal binding proteins (CtBPs) are cotranscriptional factors that play key roles in cell fate. We have previously shown that NAD(H) promotes the assembly of similar tetramers from either human CtBP1 and CtBP2 and that CtBP2 tetramer destabilizing mutants are defective for oncogenic activity. To assist structure-based design efforts for compounds that disrupt CtBP tetramerization, it is essential to understand how NAD(H) triggers tetramer assembly. Here, we investigate the moieties within NAD(H) that are responsible for triggering tetramer formation. Using multiangle light scattering (MALS), we show that ADP is able to promote tetramer formation of both CtBP1 and CtBP2, whereas AMP promotes tetramer assembly of CtBP1, but not CtBP2. Other NAD(H) moieties that lack the adenosine phosphate, including adenosine and those incorporating nicotinamide, all fail to promote tetramer assembly. Our crystal structures of CtBP1 with AMP reveal participation of the adenosine phosphate in the tetrameric interface, pinpointing its central role in NAD(H)-linked assembly. CtBP1 and CtBP2 have overlapping but unique roles, suggesting that a detailed understanding of their unique structural properties might have utility in the design of paralog-specific inhibitors. We investigated the different responses to AMP through a series of site-directed mutants at 13 positions. These mutations reveal a central role for a hinge segment, which we term the 120s hinge that connects the substrate with coenzyme-binding domains and influences nucleotide binding and tetramer assembly. Our results provide insight into suitable pockets to explore in structure-based drug design to interfere with cotranscriptional activity of CtBP in cancer.

Cryo-EM structure of CtBP2 confirms tetrameric architecture

C-terminal binding proteins 1 and 2 (CtBP1 and CtBP2) are transcriptional regulators that activate or repress many genes involved in cellular development, apoptosis, and metastasis. NADH-dependent CtBP activation has been implicated in multiple types of cancer and poor patient prognosis. Central to understanding activation of CtBP in oncogenesis is uncovering how NADH triggers protein assembly, what level of assembly occurs, and if oncogenic activity depends upon such assembly. Here, we present the cryoelectron microscopic structures of two different constructs of CtBP2 corroborating that the native state of CtBP2 in the presence of NADH is tetrameric. The physiological relevance of the observed tetramer was demonstrated in cell culture, showing that CtBP tetramer-destabilizing mutants are defective for cell migration, transcriptional repression of E-cadherin, and activation of TIAM1. Together with our cryoelectron microscopy studies, these results highlight the tetramer as the functional oligomeric form of CtBP2.

Stabilization of C-terminal binding protein 2 by cellular inhibitor of apoptosis protein 1 via BIR domains without E3 ligase activity

C-terminal binding protein 2 (CtBP2) is a transcriptional co-repressor that regulates many genes involved in normal cellular events. Because CtBP2 overexpression has been implicated in various human cancers, its protein levels must be precisely regulated. Previously, we reported that CtBP1 and CtBP1-mediated transcriptional repression are regulated by X-linked inhibitor of apoptosis protein (XIAP). In the present study, we sought to investigate whether CtBP2 is also regulated by XIAP or any other human IAP. We found that cIAP1 interacts with CtBP2 via through BIR domains to regulates the steady-state levels of CtBP2 protein in the nucleus. The levels of CtBP2 were gradually increased upon cIAP1 overexpression and downregulated upon cIAP1 depletion. Interestingly, the RING domain of cIAP1 responsible for E3 ligase activity was not required for this regulation. Finally, the levels of CtBP2 modulated by cIAP1 affected the transcription of CtBP2 target genes and subsequent cell migration. Taken together, our data demonstrate a novel function of cIAP1 which involves protecting CtBP2 from degradation to stabilize its steady-state level. These results suggest that cIAP1 might be a useful target in strategies aiming to downregulate the steady-state level of CtBP2 protein in treating human cancers.

C-terminal binding protein-2 is a prognostic marker for lung adenocarcinomas

C-terminal binding protein-2 (CtBP2) a transcriptional corepressor, has been reported to involve in tumorigenesis and progression and predict a poor prognosis in several human cancers. However, few studies on CtBP2 in lung cancer tissues have been performed. In the present study, we first explored the CtBP2 gene expression profile from the the cancer genome atlas (TCGA) datasets, then western blot analysis and immunohistochemistry were performed to investigate and verified whether lung adenocarcinoma (LUAD) tissues exhibit deregulated CtBP2 expression. We evaluated the correlations between CtBP2 expression and the clinicopathological characteristics, and Kaplan-Meier survival analyses were performed to estimate the effect of CtBP2 expression on prognosis of LUAD patients. The results revealed that CtBP2 expression was significantly upregulated in LUAD tissues compared with normal lung tissues. Furthermore, increasing CtBP2 expression in LUAD was significantly associated with tumor differentiation (P = .028), tumor node metastasis (TNM) stage (P = .042). CtBP2 expression was significantly correlated with LUAD patients' survival (P = .028). In conclusion, the present study revealed that CtBP2 protein is a novel prognostic marker for LUAD. A further large-scale study is needed to confirm the present results.

Glycolysis, via NADH-dependent dimerisation of CtBPs, regulates hypoxia-induced expression of CAIX and stem-like breast cancer cell survival

Adaptive responses to hypoxia are mediated by the hypoxia-inducible factor (HIF) family of transcription factors. These responses include the upregulation of glycolysis to maintain ATP production. This also generates acidic metabolites, which require HIF-induced carbonic anhydrase IX (CAIX) for their neutralisation. C-terminal binding proteins (CtBPs) are coregulators of gene transcription and couple glycolysis with gene transcription due to their regulation by the glycolytic coenzyme NADH. Here, we find that experimental manipulation of glycolysis and CtBP function in breast cancer cells through multiple complementary approaches supports a hypothesis whereby the expression of known HIF-inducible genes, and CAIX in particular, adapts to available glucose in the microenvironment through a mechanism involving CtBPs. This novel pathway promotes the survival of stem cell-like cancer (SCLC) cells in hypoxia.

Stem cell-like breast cancer cells with acquired resistance to metformin are sensitive to inhibitors of NADH-dependent CtBP dimerization

Altered flux through major metabolic pathways is a hallmark of cancer cells and provides opportunities for therapy. Stem cell-like cancer (SCLC) cells can cause metastasis and therapy resistance. They possess metabolic plasticity, theoretically enabling resistance to therapies targeting a specific metabolic state. The C-terminal binding protein (CtBP) transcriptional regulators are potential therapeutic targets in highly glycolytic cancer cells, as they are activated by the glycolytic coenzyme nicotinamide adenine dinucleotide (NADH). However, SCLC cells commonly exist in an oxidative state with low rates of glycolysis. Metformin inhibits complex I of the mitochondrial electron transport chain; it can kill oxidative SCLC cells and has anti-cancer activity in patients. SCLC cells can acquire resistance to metformin through increased glycolysis. Given the potential for long-term metformin therapy, we have studied acquired metformin resistance in cells from the claudin-low subtype of breast cancer. Cells cultured for 8 weeks in sub-IC50 metformin concentration proliferated comparably to untreated cells and exhibited higher rates of glucose uptake. SCLC cells were enriched in metformin-adapted cultures. These SCLC cells acquired sensitivity to multiple methods of inhibition of CtBP function, including a cyclic peptide inhibitor of NADH-induced CtBP dimerization. Single-cell mRNA sequencing identified a reprogramming of epithelial-mesenchymal and stem cell gene expression in the metformin-adapted SCLC cells. These SCLC cells demonstrated an acquired dependency on one of these genes, Tenascin C. Thus, in addition to acquisition of sensitivity to glycolysis-targeting therapeutic strategies, the reprograming of gene expression in the metformin-adapted SCLC cells renders them sensitive to potential therapeutic approaches not directly linked to cell metabolism.

CtBP determines ovarian cancer cell fate through repression of death receptors

C-terminal binding protein 2 (CtBP2) is elevated in epithelial ovarian cancer, especially in the aggressive and highly lethal subtype, high-grade serous ovarian cancer (HGSOC). However, whether HGSOC tumor progression is dependent on CtBP2 or its paralog CtBP1, is not well understood. Here we report that CtBP1/2 repress HGSOC cell apoptosis through silencing of death receptors (DRs) 4/5. CtBP1 or 2 knockdown upregulated DR4/5 expression, and triggered autonomous apoptosis via caspase 8 activation, but dependent on cell-type context. Activation of DR4/5 by CtBP1/2 loss also sensitized HGSOC cell susceptibility to the proapoptotic DR4/5 ligand TRAIL. Consistent with its function as transcription corepressor, CtBP1/2 bound to the promoter regions of DR4/5 and repressed DR4/5 expression, presumably through recruitment to a repressive transcription regulatory complex. We also found that CtBP1 and 2 were both required for repression of DR4/5. Collectively, this study identifies CtBP1 and 2 as potent repressors of DR4/5 expression and activity, and supports the targeting of CtBP as a promising therapeutic strategy for HGSOC.

Epigenetic re-wiring of breast cancer by pharmacological targeting of C-terminal binding protein

The C-terminal binding protein (CtBP) is an NADH-dependent dimeric family of nuclear proteins that scaffold interactions between transcriptional regulators and chromatin-modifying complexes. Its association with poor survival in several cancers implicates CtBP as a promising target for pharmacological intervention. We employed computer-assisted drug design to search for CtBP inhibitors, using quantitative structure-activity relationship (QSAR) modeling and docking. Functional screening of these drugs identified 4 compounds with low toxicity and high water solubility. Micro molar concentrations of these CtBP inhibitors produces significant de-repression of epigenetically silenced pro-epithelial genes, preferentially in the triple-negative breast cancer cell line MDA-MB-231. This epigenetic reprogramming occurs through eviction of CtBP from gene promoters; disrupted recruitment of chromatin-modifying protein complexes containing LSD1, and HDAC1; and re-wiring of activating histone marks at targeted genes. In functional assays, CtBP inhibition disrupts CtBP dimerization, decreases cell migration, abolishes cellular invasion, and improves DNA repair. Combinatorial use of CtBP inhibitors with the LSD1 inhibitor pargyline has synergistic influence. Finally, integrated correlation of gene expression in breast cancer patients with nuclear levels of CtBP1 and LSD1, reveals new potential therapeutic vulnerabilities. These findings implicate a broad role for this class of compounds in strategies for epigenetically targeted therapeutic intervention.

The CtBP1-p300-FOXO3a transcriptional complex represses the expression of the apoptotic regulators Bax and Bim in human osteosarcoma cells

C-terminal binding protein 1 (CtBP1), a well-known transcriptional corepressor, functions as an oncogene in multiple cancer types, including osteosarcoma, by modulating the transcription of many tumor suppressors, such as cadherin 1 (CDH1), phosphatase and tensin homolog (PTEN), Bcl2-associated X (Bax), Bcl-2-interacting mediator (Bim), and cyclin-dependent kinase inhibitor 1A (CDKN1A). However, it is still unclear how CtBP1 regulates the expression of these downstream targets. Here, we identified that CtBP1 is overexpressed in osteosarcoma cells and found that CtBP1 directly interacts with the transcription factor forkhead box O3 (FOXO3a) and the histone acetyltransferase p300 in vivo and in vitro. Through microarray analysis, we found that CtBP1 negatively regulates FOXO3a levels. In contrast to the CtBP1 level, the FOXO3a expression level was found to be significantly reduced in osteosarcoma cells. Knockdown of CtBP1 or overexpression of FOXO3a in U2OS cells resulted in different gene expression patterns, and the former caused upregulation of CtBP1 downstream target genes such as CDH1, PTEN, Bax, Bim, and CDKN1A, whereas the latter caused upregulation of Bax and Bim, but not CDH1, PTEN, and CDKN1A. Further analysis indicated that the CtBP1-p300-FOXO3a transcriptional complex specifically binds to the promoters of Bax and Bim. Inhibition of CtBP1 by the constitutive expression of Pep1-E1A^WT peptide in U2OS and OSA cells reversed oncogenic phenotypes, including colony formation, cellular proliferation, and migration, and limited tumor growth in vivo. Together our results demonstrated that the CtBP1-p300-FOXO3a transcriptional complex represses the expression of the apoptotic regulators Bax and Bim in human osteosarcoma cells and that targeting CtBP1-mediated transcriptional events might be a potential therapeutic strategy for the osteosarcoma treatment.

A pathogenic CtBP1 missense mutation causes altered cofactor binding and transcriptional activity

We previously reported a pathogenic de novo p.R342W mutation in the transcriptional corepressor CTBP1 in four independent patients with neurodevelopmental disabilities [1]. Here, we report the clinical phenotypes of seven additional individuals with the same recurrent de novo CTBP1 mutation. Within this cohort, we identified consistent CtBP1-related phenotypes of intellectual disability, ataxia, hypotonia, and tooth enamel defects present in most patients. The R342W mutation in CtBP1 is located within a region implicated in a high affinity-binding cleft for CtBP-interacting proteins. Unbiased proteomic analysis demonstrated reduced interaction of several chromatin-modifying factors with the CtBP1 W342 mutant. Genome-wide transcriptome analysis in human glioblastoma cell lines expressing -CtBP1 R342 (wt) or W342 mutation revealed changes in the expression profiles of genes controlling multiple cellular processes. Patient-derived dermal fibroblasts were found to be more sensitive to apoptosis during acute glucose deprivation compared to controls. Glucose deprivation strongly activated the BH3-only pro-apoptotic gene NOXA, suggesting a link between enhanced cell death and NOXA expression in patient fibroblasts. Our results suggest that context-dependent relief of transcriptional repression of the CtBP1 mutant W342 allele may contribute to deregulation of apoptosis in target tissues of patients leading to neurodevelopmental phenotypes.

Glycolysis Regulates Human Embryonic Stem Cell Self-Renewal under Hypoxia through HIF-2α and the Glycolytic Sensors CTBPs

Glycolysis and hypoxia are key regulators of human embryonic stem cell (hESC) self-renewal, but how changes in metabolism affect gene expression is poorly understood. C-terminal binding proteins (CTBPs) are glycolytic sensors that through NADH binding link the metabolic state of the cell to its gene expression, by acting as transcriptional corepressors, or coactivators. However, the role of CTBPs in hESCs has not previously been investigated. A direct interaction between hypoxia-inducible factor 2α (HIF-2α) and the CTBP proximal promoters in hESCs cultured only under hypoxia was demonstrated. Decreasing the rate of flux through glycolysis in hESCs maintained under hypoxia resulted in a reduction of CTBPs, OCT4, SOX2, and NANOG, but also in the expression of HIF-2α. Silencing CTBP expression resulted in the loss of pluripotency marker expression demonstrating that CTBPs are involved in hESC maintenance. These data suggest that under hypoxia, glycolysis regulates self-renewal through HIF-2α and the induction of the metabolic sensors CTBPs.

An intestinal stem cell niche in Apc mutated neoplasia targetable by CtBP inhibition

C-terminal binding protein 2 (CtBP2) drives intestinal polyposis in the Apc^min mouse model of human Familial Adenomatous Polyposis. As CtBP2 is targetable by an inhibitor of its dehydrogenase domain, understanding CtBP2’s role in adenoma formation is necessary to optimize CtBP-targeted therapies in Apc mutated human neoplasia. Tumor initiating cell (TIC) populations were substantially decreased in Apc^minCtbp2^+/- intestinal epithelia. Moreover, normally nuclear Ctbp2 was mislocalized to the cytoplasm of intestinal crypt stem cells in Ctbp2^+/- mice, both Apc^min and wildtype, correlating with low/absent CD133 expression in those cells, and possibly explaining the lower burden of polyps in Apc^min Ctbp2^+/- mice. The CtBP inhibitor 4-chloro-hydroxyimino phenylpyruvate (4-Cl-HIPP) also robustly downregulated TIC populations and significantly decreased intestinal polyposis in Apc^min mice. We have therefore demonstrated a critical link between polyposis, intestinal TIC’s and Ctbp2 gene dosage or activity, supporting continued efforts targeting CtBP in the treatment or prevention of Apc mutated neoplasia.

Assembly of human C-terminal binding protein (CtBP) into tetramers

C-terminal binding protein 1 (CtBP1) and CtBP2 are transcriptional coregulators that repress numerous cellular processes, such as apoptosis, by binding transcription factors and recruiting chromatin-remodeling enzymes to gene promoters. The NAD(H)-linked oligomerization of human CtBP is coupled to its co-transcriptional activity, which is implicated in cancer progression. However, the biologically relevant level of CtBP assembly has not been firmly established; nor has the stereochemical arrangement of the subunits above that of a dimer. Here, multi-angle light scattering (MALS) data established the NAD⁺- and NADH-dependent assembly of CtBP1 and CtBP2 into tetramers. An examination of subunit interactions within CtBP1 and CtBP2 crystal lattices revealed that both share a very similar tetrameric arrangement resulting from assembly of two dimeric pairs, with specific interactions probably being sensitive to NAD(H) binding. Creating a series of mutants of both CtBP1 and CtBP2, we tested the hypothesis that the crystallographically observed interdimer pairing stabilizes the solution tetramer. MALS data confirmed that these mutants disrupt both CtBP1 and CtBP2 tetramers, with the dimer generally remaining intact, providing the first stereochemical models for tetrameric assemblies of CtBP1 and CtBP2. The crystal structure of a subtle destabilizing mutant suggested that small structural perturbations of the hinge region linking the substrate- and NAD-binding domains are sufficient to weaken the CtBP1 tetramer. These results strongly suggest that the tetramer is important in CtBP function, and the series of CtBP mutants reported here can be used to investigate the physiological role of the tetramer.

The Role of CtBP1 in Oncogenic Processes and Its Potential as a Therapeutic Target

Transcriptional corepressor proteins have emerged as an important facet of cancer etiology. These corepressor proteins are often altered by loss- or gain-of-function mutations, leading to transcriptional imbalance. Thus, research directed at expanding our current understanding of transcriptional corepressors could impact the future development of new cancer diagnostics, prognostics, and therapies. In this review, our current understanding of the CtBP corepressors, and their role in both development and disease, is discussed in detail. Importantly, the role of CtBP1 overexpression in adult tissues in promoting the progression of multiple cancer types through their ability to modulate the transcription of developmental genes ectopically is explored. CtBP1 overexpression is known to be protumorigenic and affects the regulation of gene networks associated with "cancer hallmarks" and malignant behavior, including increased cell survival, proliferation, migration, invasion, and the epithelial-mesenchymal transition. As a transcriptional regulator of broad developmental processes capable of promoting malignant growth in adult tissues, therapeutically targeting the CtBP1 corepressor has the potential to be an effective method for the treatment of diverse tumor types. Although efforts to develop CtBP1 inhibitors are still in the early stages, the current progress and the future perspectives of therapeutically targeting this transcriptional corepressor are also discussed. Mol Cancer Ther; 16(6); 981-90. ©2017 AACR.

CtBP2 overexpression promotes tumor cell proliferation and invasion in gastric cancer and is associated with poor prognosis

C-terminal binding protein-2 (CtBP2), a transcriptional corepressor, has been reported to correlate with tumorigenesis and progression and predict a poor prognosis in several human cancers. However, few studies on CtBP2 in gastric cancer (GC) have been performed. In this research, we evaluated the correlations between CtBP2 expression and the clinicopathological characteristics, as well as prognosis of GC patients. The effects of silencing CtBP2 expression on GC cells biology activity were also assessed. The results showed that CtBP2 was overexpressed in GC tissues and closely correlated with poor differentiation, advanced tumor stage and poor prognosis in GC patients. CtBP2 induced epithelial-to-mesenchymal transition (EMT) and repressed PTEN to increase proliferation rate, migration, and invasion in GC cells. Silencing CtBP2 inhibited GC growth in nude mice model. In conclusion, CtBP2 is overexpressed in GC and may accelerate GC tumorigenesis and metastasis, which could represent an independent prognostic marker and promising therapeutic target for GC.

The miR-644a/CTBP1/p53 axis suppresses drug resistance by simultaneous inhibition of cell survival and epithelial-mesenchymal transition in breast cancer

Tumor cells develop drug resistance which leads to recurrence and distant metastasis. MicroRNAs are key regulators of tumor pathogenesis; however, little is known whether they can sensitize cells and block metastasis simultaneously. Here, we report miR-644a as a novel inhibitor of both cell survival and EMT whereby acting as pleiotropic therapy-sensitizer in breast cancer. We showed that both miR-644a expression and its gene signature are associated with tumor progression and distant metastasis-free survival. Mechanistically, miR-644a directly targets the transcriptional co-repressor C-Terminal Binding Protein 1 (CTBP1) whose knock-outs by the CRISPR-Cas9 system inhibit tumor growth, metastasis, and drug resistance, mimicking the phenotypes induced by miR-644a. Furthermore, downregulation of CTBP1 by miR-644a upregulates wild type- or mutant-p53 which acts as a 'molecular switch' between G1-arrest and apoptosis by inducing cyclin-dependent kinase inhibitor 1 (p21, CDKN1A, CIP1) or pro-apoptotic phorbol-12-myristate-13-acetate-induced protein 1 (Noxa, PMAIP1), respectively. Interestingly, an increase in mutant-p53 by either overexpression of miR-644a or downregulation of CTBP1 was enough to shift this balance in favor of apoptosis through upregulation of Noxa. Notably, p53-mutant patients, but not p53-wild type ones, with high CTBP1 have a shorter survival suggesting that CTBP1 could be a potential prognostic factor for breast cancer patients with p53 mutations. Overall, re-activation of the miR-644a/CTBP1/p53 axis may represent a new strategy for overcoming both therapy resistance and metastasis.

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.

TLE1 inhibits anoikis and promotes tumorigenicity in human lung cancer cells through ZEB1-mediated E-cadherin repression

The Transducin-like enhancer of split 1 (TLE1) corepressor protein is overexpressed in human lung tumors and is a putative lung-specific oncogene. However, the molecular mechanism underlying its oncogenic function remains to be delineated. Here, we report an important role of TLE1 in promoting lung tumorigenesis by a mechanism involving induction of anoikis resistance. Using the human lung adenocarcinoma A549 and immortalized bronchial epithelial BEAS-2B cell lines, we observed that TLE1 inhibits anoikis through transcriptional repression of E-cadherin gene. In support of E-cadherin as a downstream target of TLE1 to block anoikis, forced expression of E-cadherin attenuated TLE1-induced anoikis resistance while E-cadherin downregulation decreased the anoikis sensitivity of TLE1 knockdown cells. Furthermore, we determined that E-cadherin expression is transcriptionally induced upon loss of cell attachment and functions as an effector of anoikis. Loss of E-cadherin via the siRNA strategy or exogenous TLE1 expression was sufficient to attenuate anoikis in A549 and BEAS-2B cells. Importantly, we demonstrated that the ZEB1 transcriptional factor is required for TLE1-mediated E-cadherin repression and anoikis resistance. ZEB1 interacted with and recruited the TLE1 to the E-cadherin promoter to impose histone deacetylation and gene silencing. In vivo, TLE1 strongly promoted tumorigenicity of A549 cells in a ZEB1-dependent manner. Underscoring its role in anoikis insensitivity of lung cancer cells, the TLE1-mediated E-cadherin repression was negatively regulated by the tumor suppressor Bcl-2 inhibitor of transcription 1 (Bit1) to effect anoikis. These findings identify the ZEB1/TLE1/E-cadherin transcriptional mechanism as a novel pathway that promotes anoikis resistance and oncogenicity of lung cancer cells.

CtBP- an emerging oncogene and novel small molecule drug target: Advances in the understanding of its oncogenic action and identification of therapeutic inhibitors

C-terminal Binding Proteins (CtBP) 1 and 2 are oncogenic transcriptional co-regulators overexpressed in many cancer types, with their expression level correlating to worse prognostic outcomes and aggressive tumor features. CtBP negatively regulates the expression of many tumor suppressor genes, while coactivating genes that promote proliferation, epithelial-mesenchymal transition, and cancer stem cell self-renewal activity. In light of this evidence, the development of novel inhibitors that mitigate CtBP function may provide clinically actionable therapeutic tools. This review article focuses on the progress made in understanding CtBP structure, role in tumor progression, and discovery and development of CtBP inhibitors that target CtBP's dehydrogenase activity and other functions, with a focus on the theory and rationale behind the designs of current inhibitors. We provide insight into the future development and use of rational combination therapy that may further augment the efficacy of CtBP inhibitors, specifically addressing metastasis and cancer stem cell populations within tumors.

Transforming activity and therapeutic targeting of C-terminal-binding protein 2 in Apc-mutated neoplasia

Overexpression of the transcriptional coregulators C-terminal binding proteins 1 and 2 (CtBP) occurs in many human solid tumors and is associated with poor prognosis. CtBP modulates oncogenic gene expression programs and is an emerging drug target, but its oncogenic role is unclear. Consistent with oncogenic potential, exogenous CtBP2 transformed primary mouse and human cells to anchorage independence similarly to mutant H-Ras. To investigate CtBP’s contribution to in vivo tumorigenesis, Apc^min/+ mice, which succumb to massive intestinal polyposis, were bred to Ctbp2^+/− mice. CtBP interacts with Adenomatous Polyposis Coli (APC) protein, and is stabilized in both APC-mutated human colon cancers and Apc^min/+ intestinal polyps. Ctbp2 heterozygosity increased the median survival of Apc^min/+ mice from 21 to 48 weeks, and reduced polyp formation by 90%, with Ctbp2^+/− polyps exhibiting reduced levels of β-catenin and its oncogenic transcriptional target, cyclin D1. Ctbp’s potential as a therapeutic target was studied by treating Apc^min/+ mice with the CtBP small molecule inhibitors 4-methlythio-2-oxobutyric acid and 2-hydroxy-imino phenylpyruvic acid, both of which reduced polyposis by more than half compared with vehicle treatment. Phenocopying Ctbp2 deletion, both Ctbp inhibitors caused substantial decreases in the protein level of Ctbp2, as well its oncogenic partner β-catenin, and the effects of the inhibitors on CtBP and β-catenin levels could be modeled in an APC mutated human colon cancer cell line. CtBP2 is thus a druggable transforming oncoprotein critical for the evolution of neoplasia driven by Apc mutation.

Transcription factors, transcriptional coregulators, and epigenetic modulation in the control of pulmonary vascular cell phenotype: therapeutic implications for pulmonary hypertension (2015 Grover Conference series)

Pulmonary hypertension (PH) is a complex and multifactorial disease involving genetic, epigenetic, and environmental factors. Numerous stimuli and pathological conditions facilitate severe vascular remodeling in PH by activation of a complex cascade of signaling pathways involving vascular cell proliferation, differentiation, and inflammation. Multiple signaling cascades modulate the activity of certain sequence-specific DNA-binding transcription factors (TFs) and coregulators that are critical for the transcriptional regulation of gene expression that facilitates PH-associated vascular cell phenotypes, as demonstrated by several studies summarized in this review. Past studies have largely focused on the role of the genetic component in the development of PH, while the presence of epigenetic alterations such as microRNAs, DNA methylation, histone levels, and histone deacetylases in PH is now also receiving increasing attention. Epigenetic regulation of chromatin structure is also recognized to influence gene expression in development or disease states. Therefore, a complete understanding of the mechanisms involved in altered gene expression in diseased cells is vital for the design of novel therapeutic strategies. Recent technological advances in DNA sequencing will provide a comprehensive improvement in our understanding of mechanisms involved in the development of PH. This review summarizes current concepts in TF and epigenetic control of cell phenotype in pulmonary vascular disease and discusses the current issues and possibilities in employing potential epigenetic or TF-based therapies for achieving complete reversal of PH.

Metabolic Reprogramming Regulates the Proliferative and Inflammatory Phenotype of Adventitial Fibroblasts in Pulmonary Hypertension Through the Transcriptional Corepressor C-Terminal Binding Protein-1

BACKGROUND

Changes in metabolism have been suggested to contribute to the aberrant phenotype of vascular wall cells, including fibroblasts, in pulmonary hypertension (PH). Here, we test the hypothesis that metabolic reprogramming to aerobic glycolysis is a critical adaptation of fibroblasts in the hypertensive vessel wall that drives proliferative and proinflammatory activation through a mechanism involving increased activity of the NADH-sensitive transcriptional corepressor C-terminal binding protein 1 (CtBP1).

METHODS

RNA sequencing, quantitative polymerase chain reaction,13C-nuclear magnetic resonance, fluorescence-lifetime imaging, mass spectrometry-based metabolomics, and tracing experiments with U-13C-glucose were used to assess glycolytic reprogramming and to measure the NADH/NAD+ ratio in bovine and human adventitial fibroblasts and mouse lung tissues. Immunohistochemistry was used to assess CtBP1 expression in the whole-lung tissues. CtBP1 siRNA and the pharmacological inhibitor 4-methylthio-2-oxobutyric acid (MTOB) were used to abrogate CtBP1 activity in cells and hypoxic mice.

RESULTS

We found that adventitial fibroblasts from calves with severe hypoxia-induced PH and humans with idiopathic pulmonary arterial hypertension (PH-Fibs) displayed aerobic glycolysis when cultured under normoxia, accompanied by increased free NADH and NADH/NAD+ ratios. Expression of the NADH sensor CtBP1 was increased in vivo and in vitro in fibroblasts within the pulmonary adventitia of humans with idiopathic pulmonary arterial hypertension and animals with PH and cultured PH-Fibs, respectively. Decreasing NADH pharmacologically with MTOB or genetically blocking CtBP1 with siRNA upregulated the cyclin-dependent genes (p15 and p21) and proapoptotic regulators (NOXA and PERP), attenuated proliferation, corrected the glycolytic reprogramming phenotype of PH-Fibs, and augmented transcription of the anti-inflammatory gene HMOX1. Chromatin immunoprecipitation analysis demonstrated that CtBP1 directly binds the HMOX1 promoter. Treatment of hypoxic mice with MTOB decreased glycolysis and expression of inflammatory genes, attenuated proliferation, and suppressed macrophage numbers and remodeling in the distal pulmonary vasculature.

CONCLUSIONS

CtBP1 is a critical factor linking changes in cell metabolism to cell phenotype in hypoxic and other forms of PH and a therapeutic target.

C-Terminal Binding Protein is Involved in Promoting to the Carcinogenesis of Human Glioma

C-terminal binding protein (CtBP) is responsible for regulating the pathogenesis of a lot of cancer types. However, whether CtBP1/2 is involved in regulating the growth and development of human glioma is still obscure. In the present study presented here, our results firstly reveal that CtBP1/2 deficiency, induced by siRNA interference, disrupts the functional integrity of the MRN complex that is responsible for DNA repair in human glioma cells. The dysfunction of the MRN complex further contributes to the up-regulation of ATM and Rad3-related kinase (ATR) and Chk1 signaling pathway, which inhibits cell cycle progression mediated by CDK2, preparing for the initiation of DNA repair. Under the condition of hypoxia, hypoxia-inducible factor-1α (HIF-1α) can be directly regulated by CDK2 on protein level, playing coordinately regulatory role in the carcinogenesis of human glioma cells. Overall, our findings reveal that CtBP1/2 is essential to promote to human glioma cell growth through maintaining the DNA stability regulated by the MRN/ATR/Chk1/CDK2/HIF-1α signaling pathway.

Classification of cancers based on copy number variation landscapes

Genomic alterations in DNA can cause human cancer. DNA copy number variants (CNV), as one of the types of DNA mutations, have been considered to be associated with various human cancers. CNVs vary in size from 1bp up to one complete chromosome arm. In order to understand the difference between different human cancers on CNVs, in this study, we developed a method to computationally classify six human cancer types by using only CNV level values. The CNVs of 23,082 genes were used as features to construct the classifier. Then the features are carefully selected by mRMR (minimum Redundancy Maximum Relevance Feature Selection) and IFS (Incremental Feature Selection) methods. An accuracy of over 0.75 was reached by using only the CNVs of 19 genes based on Dagging method in 10-fold cross validation. It was indicated that these 19 genes may play important roles in differentiating cancer types. We also analyzed the biological functions of several top genes within the 19 gene list. The statistical results and biological analysis of these genes from this work might further help understand different human cancer types and provide guidance for related validation experiments. This article is part of a Special Issue entitled "System Genetics" Guest Editor: Dr. Yudong Cai and Dr. Tao Huang.

Silencing of CtBP1 suppresses the migration in human glioma cells

Carboxyl-terminal binding protein 1 (CtBP1), up-regulated in various types of human cancers, has been functionally associated with proliferation, anti-apoptosis, and EMT in vitro studies. However, the functional significance of CtBP1 in the pathophysiology of glioma remains unknown. In the present study, we showed the expression of CtBP1 was markedly higher in glioma tissues compared with normal brain tissues by Western blot analysis. Immunohistochemical analysis revealed that CtBP1 mainly localized in the nucleus of glioma cells. Statistical analysis suggested the upregulation of CtBP1 was considerably correlated with the WHO grade (P < 0.05) and those patients with high CtBP1 levels exhibited shorter survival time (P < 0.01). Silencing CtBP1 by short hairpin RNAi caused an inhibition of cell migration. Moreover, knockdown of CtBP1 increases E-cadherin expression and decreases vimentin expression. These data uncovered that CtBP1 protein is a valuable marker of glioma pathogenic process and that CtBP1 can serve as a novel prognostic marker for glioma therapy.

Design, synthesis, and biological evaluation of substrate-competitive inhibitors of C-terminal Binding Protein (CtBP)

C-terminal Binding Protein (CtBP) is a transcriptional co-regulator that downregulates the expression of many tumor-suppressor genes. Utilizing a crystal structure of CtBP with its substrate 4-methylthio-2-oxobutyric acid (MTOB) and NAD(+) as a guide, we have designed, synthesized, and tested a series of small molecule inhibitors of CtBP. From our first round of compounds, we identified 2-(hydroxyimino)-3-phenylpropanoic acid as a potent CtBP inhibitor (IC50=0.24μM). A structure-activity relationship study of this compound further identified the 4-chloro- (IC50=0.18μM) and 3-chloro- (IC50=0.17μM) analogues as additional potent CtBP inhibitors. Evaluation of the hydroxyimine analogues in a short-term cell growth/viability assay showed that the 4-chloro- and 3-chloro-analogues are 2-fold and 4-fold more potent, respectively, than the MTOB control. A functional cellular assay using a CtBP-specific transcriptional readout revealed that the 4-chloro- and 3-chloro-hydroxyimine analogues were able to block CtBP transcriptional repression activity. This data suggests that substrate-competitive inhibition of CtBP dehydrogenase activity is a potential mechanism to reactivate tumor-suppressor gene expression as a therapeutic strategy for cancer.

Adenosine monophosphate activated protein kinase (AMPK), a mediator of estradiol-induced apoptosis in long-term estrogen deprived breast cancer cells

Estrogens stimulate growth of hormone-dependent breast cancer but paradoxically induce tumor regress under certain circumstances. We have shown that long-term estrogen deprivation (LTED) enhances the sensitivity of hormone dependent breast cancer cells to estradiol (E2) so that physiological concentrations of estradiol induce apoptosis in these cells. E2-induced apoptosis involve both intrinsic and extrinsic pathways but precise mechanisms remain unclear. We found that exposure of LTED MCF-7 cells to E2 activated AMP activated protein kinase (AMPK). In contrast, E2 inhibited AMPK activation in wild type MCF-7 cells where E2 prevents apoptosis. As a result of AMPK activation, the transcriptional activity of FoxO3, a downstream factor of AMPK, was up-regulated in E2 treatment of LTED. Increased activity of FoxO3 was demonstrated by up-regulation of three FoxO3 target genes, Bim, Fas ligand (FasL), and Gadd45α. Among them, Bim and FasL mediate intrinsic and extrinsic apoptosis respectively and Gadd45α causes cell cycle arrest at the G2/M phase. To further confirm the role of AMPK in apoptosis, we used AMPK activator AICAR in wild type MCF-7 cells and examined apoptosis, proliferation and expression of Bim, FasL, and Gadd45α. The effects of AICAR on these parameters recapitulated those observed in E2-treated LTED cells. Activation of AMPK by AICAR also increased expression of Bax in MCF-7 cells and its localization to mitochondria, which is a required process for apoptosis. These results reveal that AMPK is an important factor mediating E2-induced apoptosis in LTED cells, which is implicative of therapeutic potential for relapsing breast cancer after hormone therapy.

CtBP2 Regulates TGFβ2-Induced Epithelial-Mesenchymal Transition Through Notch Signaling Pathway in Lens Epithelial Cells

BACKGROUND

Epithelial-mesenchymal transition (EMT) of human lens epithelial cells (LECs) contributes to posterior capsule opacification (PCO). C-terminal binding protein 2 (CtBP2) has been reported to be essential in EMT and embryonic development. However, the function of CtBP2 in EMT of LECs is unknown. The goal of this study was to investigate the role of CtBP2 through Notch signaling in transforming growth factor β2 (TGFβ2)-induced EMT in LECs.

METHODS

The human LEC line SRA01/04 was cultured in the presence of TGFβ2 for different periods of time or with γ-Secretase Inhibitor IX (DAPT), a specific inhibitor of Notch receptor cleavage, for 24 h, utilizing plasmid-based method. The levels of protein expression of CtBP2, EMT markers, and Notch signaling molecules were measured by Western bolts.

RESULTS

Treatment of SRA01/04 cells with TGFβ2 induced typical molecular changes of EMT and increased the expression of CtBP2 in a time-dependent manner. Similarly, the expressions of Jagged1 and Notch1 were increased after TGFβ2 treatment. Knockdown of CtBP2 by specific siRNA inhibited TGFβ2-induced changes of Connexin 43 (CX43), α-smooth muscle actin (α-SMA), Notch1, and Notch intracellular domain (NICD). In addition, treatment of LECs with ectopic expression of CtBP2 changed the expressions of CX43, α-SMA, Notch1, and NICD, but blockade of Notch pathway with DAPT inhibited CtBP2-induced changes of α-SMA and CX43.

CONCLUSION

Our data suggest that CtBP2 plays a critical role in TGFβ2-induced EMT via the Jagged/Notch signaling pathway in human LECs and may contribute to the development of PCO.

Inhibition of C-terminal binding protein attenuates transcription factor 4 signaling to selectively target colon cancer stem cells

Selective targeting of cancer stem cells (CSCs), implicated in tumor relapse, holds great promise in the treatment of colorectal cancer. Overexpression of C-terminal binding protein (CtBP), an NADH dependent transcriptional regulator, is often observed in colon cancer. Of note, TCF-4 signaling is also up-regulated in colonic CSCs. We hypothesized that CtBP, whose dehydrogenase activity is amenable to pharmacological inhibition by 4-methylthio-2-oxobutyric acid (MTOB), positively regulates TCF-4 signaling, leading to CSC growth and self-renewal. CSCs demonstrated significant upregulation of CtBP1 and CtBP2 levels (mRNA and protein) and activity partly due to increased NADH/NAD ratio, as well as increased TCF/LEF transcriptional activity, compared to respective controls. Depletion of CtBP2 inhibited, while its overexpression enhanced, CSC growth (1° spheroids) and self-renewal (2°/3° spheroids). Similarly, MTOB caused a robust inhibition of spheroid growth and self-renewal in a dose dependent manner. MTOB displayed significantly greater selectivity for growth inhibition in the spheroids, at least in part through induction of apoptosis, compared to monolayer controls. Moreover, MTOB inhibited basal as well as induced (by GSK-3β inhibitor) TCF/LEF activity while suppressing mRNA and protein levels of several β-catenin target genes (CD44, Snail, C-MYC and LGR5). Lastly, CtBP physically interacted with TCF-4, and this interaction was significantly inhibited in the presence of MTOB. The above findings point to a novel role of CtBPs in the promotion of CSC growth and self-renewal through direct regulation of TCF/LEF transcription. Moreover, small molecular inhibition of its function can selectively target CSCs, presenting a novel approach for treatment of colorectal cancer focused on targeting of CSCs.

C-terminal binding proteins: central players in development and disease

C-terminal binding proteins (CtBPs) were initially identified as binding partners for the E1A-transforming proteins. Although the invertebrate genome encodes one CtBP protein, two CtBPs (CtBP1 and CtBP2) are encoded by the vertebrate genome and perform both unique and duplicative functions. CtBP1 and CtBP2 are closely related and act as transcriptional corepressors when activated by nicotinamide adenine dinucleotide binding to their dehydrogenase domains. CtBPs exert transcriptional repression primarily via recruitment of a corepressor complex to DNA that consists of histone deacetylases (HDACs) and histone methyltransferases, although CtBPs can also repress transcription through HDAC-independent mechanisms. More recent studies have demonstrated a critical function for CtBPs in the transcriptional repression of pro-apoptotic genes such as Bax, Puma, Bik, and Noxa. Nonetheless, although recent efforts have characterized the essential involvement of CtBPs in promoting cellular survival, the dysregulation of CtBPs in both neurodegenerative disease and cancers remains to be fully elucidated.

Mechanism study of peptide GMBP1 and its receptor GRP78 in modulating gastric cancer MDR by iTRAQ-based proteomic analysis

Background

Multidrug resistance (MDR) is a major obstacle to the treatment of gastric cancer (GC). Using a phage display approach, we previously obtained the peptide GMBP1, which specifically binds to the surface of MDR gastric cancer cells and is subsequently internalized. Furthermore, GMBP1 was shown to have the potential to reverse the MDR phenotype of gastric cancer cells, and GRP78 was identified as the receptor for this peptide. The present study aimed to investigate the mechanism of peptide GMBP1 and its receptor GRP78 in modulating gastric cancer MDR.

Methods

Fluorescence-activated cell sorting (FACS) and immunofluorescence staining were used to investigate the subcellular location and mechanism of GMBP1 internalization. iTRAQ was used to identify the MDR-associated downstream targets of GMBP1. Differentially expressed proteins were identified in GMBP1-treated compared to untreated SGC7901/ADR and SGC7901/VCR cells. GO and KEGG pathway analyses of the differentially expressed proteins revealed the interconnection of these proteins, the majority of which are involved in MDR. Two differentially expressed proteins were selected and validated by western blotting.

Results

GMBP1 and its receptor GRP78 were found to be localized in the cytoplasm of GC cells, and GRP78 can mediate the internalization of GMBP1 into MDR cells through the transferrin-related pathway. In total, 3,752 and 3,749 proteins were affected in GMBP1-treated SGC7901/ADR and SGC7901/VCR cells, respectively, involving 38 and 79 KEGG pathways. Two differentially expressed proteins, CTBP2 and EIF4E, were selected and validated by western blotting.

Conclusion

This study explored the role and downstream mechanism of GMBP1 in GC MDR, providing insight into the role of endoplasmic reticulum stress protein GRP78 in the MDR of cancer cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1361-3) contains supplementary material, which is available to authorized users.