The role of TLE1 in synovial sarcoma

Phenethyl isothiocyanate inhibits metastasis potential of non-small cell lung cancer cells through FTO mediated TLE1 m6A modification

N6-methyladenosine (m6A) modification is a prevalent RNA epigenetic modification, which plays a crucial role in tumor progression including metastasis. Isothiocyanates (ITCs) are natural compounds and inhibit the tumorigenesis of various cancers. Our previous studies show that ITCs inhibit the proliferation and metastasis of non-small cell lung cancer (NSCLC) cells, and have synergistic effects with chemotherapy drugs. In this study, we investigated the molecular mechanisms underlying the inhibitory effects of ITCs on cancer cell metastasis. We showed that phenethyl isothiocyanate (PEITC) dose-dependently inhibited the cell viability of both NSCLC cell lines H1299 and H226 with IC50 values of 17.6 and 15.2 μM, respectively. Furthermore, PEITC dose-dependently inhibited the invasion and migration of H1299 and H226 cells. We demonstrated that PEITC treatment dose-dependently increased m6A methylation levels and inhibited the expression of the m6A demethylase fat mass and obesity-associated protein (FTO) in H1299 and H226 cells. Knockdown of FTO significantly increased m6A methylation in H1299 and H226 cells, impaired their abilities of invasion and migration in vitro, and enhanced the inhibition of PEITC on tumor growth in vivo. Overexpression of FTO promoted the migration of NSCLC cells, and also mitigated the inhibitory effect of PEITC on migration of NSCLC cells. Furthermore, we found that FTO regulated the mRNA m6A modification of a transcriptional co-repressor Transducin-Like Enhancer of split-1 (TLE1) and further affected its stability and expression. TCGA database analysis revealed TLE1 was upregulated in NSCLC tissues compared to normal tissues, which might be correlated with the metastasis status. Moreover, we showed that PEITC suppressed the migration of NSCLC cells by inhibiting TLE1 expression and downstream Akt/NF-κB pathway. This study reveals a novel mechanism underlying ITC's inhibitory effect on metastasis of lung cancer cells, and provided valuable information for developing new therapeutics for lung cancer by targeting m6A methylation.

The prognostic role of the Transducin-like Enhancer of split protein family in lung adenocarcinoma

Background

Lung cancer claims more lives than any other cancer worldwide. Lung adenocarcinoma (LUAD) accounts for approximately 40% of all lung cancers. Members of the Transducin-like Enhancer of split (TLE) protein family repress transcription through multiple mechanisms; however, their prognostic value in LUAD is still unclear.

Methods

A dataset from The Cancer Genome Atlas was used to analyze the relationship between the expression of TLE family members and outcomes of LUAD. The expression of TLE family members in 59 normal and 513 tumor samples in the TCGA dataset was selected. For paired analysis, 57 normal and 57 tumor paired tissues were selected. Gene Ontology (GO) term and Reactome pathway enrichment analyses of the TLE family members were performed. Progression-free survival (PFS) and overall survival (OS) served as endpoints in this study. All statistical analyses were performed with R 3.6.0.

Results

The expression levels of TLE family proteins differed between 59 normal and 513 tumor samples. High TLE1 and low TLE2 levels were associated with poor progression-free and OS (all P<0.050). Multivariate analysis demonstrated that high TLE1 expression and low TLE2 expression were independent risk factors for a poor outcome in LUAD. Moreover, the combined expression of these two proteins was a good tool for prognostication.

Conclusions

High TLE1 expression and low TLE2 are independent adverse prognostic factors in LUAD and can serve as prognostic biomarkers.

Diagnostic Value of TLE1 in Synovial Sarcoma: A Systematic Review and Meta-Analysis

BACKGROUND

Synovial sarcoma can present morphologically in multiple forms, including biphasic and monophasic subtypes. As a result, the histological diagnosis can sometimes be challenging. Transducin-Like Enhancer 1 (TLE1) is a transcriptional corepressor that normally is involved in embryogenesis and hematopoiesis but is also expressed in certain tumors. This systematic review examines the potential role of TLE1 as a diagnostic biomarker for the synovial sarcoma. Materials and Methods. A literature review and meta-analysis were conducted using the electronic databases Pubmed, the Cochrane Library, and Google Scholar. Thirteen studies met our eligibility criteria and were selected for in-depth analysis.

RESULTS

The mean sensitivity and specificity of TLE1 in detecting synovial sarcoma were 94% (95% CI 91%-97%) and 81% (95% CI 72%-91%), respectively, when all studies were aggregated together. The mean positive predictive value (PPV) of TLE1 was 75% (95% CI 62%-87%), whereas the negative predictive value (NPV) was 96% (95% CI 93%-98%).

CONCLUSION

TLE1 is a sensitive and specific marker for synovial sarcoma that can aid in its diagnosis. Due to its involvement in several relevant signaling pathways, TLE1 might have direct relevance to the pathophysiology of the disease.

TLE1 expression fails to distinguish between synovial sarcoma, atypical fibroxanthoma, and dermatofibrosarcoma protuberans

Transducin-like enhancer of split 1 (TLE1) belongs to the Groucho/TLE/Grg family. It functions as a transcriptional corepressor and is widely used as a biomarker of synovial sarcoma (SS). Within the skin, atypical fibroxanthoma (AFX) and dermatofibrosarcoma protuberans (DFSP) often enter the histopathologic differential diagnosis. TLE1 expression has not been evaluated in these neoplasms. We examined archived tissues sections from the surgical pathology files from 10 adult patients diagnosed with AFX and 10 adult patients diagnosed with DFSP. We found nuclear staining in 10 of 10 AFX and 2 of 10 DFSP. We also noticed three patterns of staining in AFX: predominantly spindle component, predominantly epithelioid component, or mixed pattern of both epithelioid and spindle components. The group with the predominantly spindle pattern expressed the strongest nuclear TLE1 staining. In the DFSP group, one lesion demonstrated staining of epithelioid cells, with strong, diffuse nuclear TLE 1 expression, and the second lesion stained only the spindled cells, with weak nuclear TLE1 marking. In conclusion, TLE1, while a sensitive marker for SS, is not specific. A wide range of cutaneous spindle cell neoplasms also express TLE1. AFX and DFSP should be added to this list. TLE1 might be added to a diagnostic panel in this differential diagnosis.

Cascaded Wx: A Novel Prognosis-Related Feature Selection Framework in Human Lung Adenocarcinoma Transcriptomes

Artificial neural network-based analysis has recently been used to predict clinical outcomes in patients with solid cancers, including lung cancer. However, the majority of algorithms were not originally developed to identify genes associated with patients' prognoses. To address this issue, we developed a novel prognosis-related feature selection framework called Cascaded Wx (CWx). The CWx framework ranks features according to the survival of a given cohort by training neural networks with three different high- and low-risk groups in a cascaded fashion. We showed that this approach accurately identified features that best identify the patients' prognoses, compared to other feature selection algorithms, including the Cox proportional hazards and Coxnet models, when applied to The Cancer Genome Atlas lung adenocarcinoma (LUAD) transcriptome data. The prognostic potential of the top 100 genes identified by CWx outperformed or was comparable to those identified by the other methods as assessed by the concordance index (c-index). In addition, the top 100 genes identified by CWx were found to be associated with the Wnt signaling pathway, providing biologically relevant evidence for the value of these genes in predicting the prognosis of patients with LUAD. Further analyses of other cancer types showed that the genes identified by CWx had the highest prognostic values according to the c-index. Collectively, the CWx framework will potentially be of great use to prognosis-related biomarker discoveries in a variety of diseases.

In silico and in vitro screening of small molecule Inhibitors against SYT-SSX1 fusion protein in synovial sarcoma

Synovial sarcoma (SS) is characterized by a tumour specific chromosomal translocation t(X;18) (p11;q11) which results in the formation of SYT-SSX1 fusion protein. This fusion protein represents a clear therapeutic target and molecules specifically targeting SYT-SSX1 fusion protein are currently not available. In this study, SYT-SSX1 fusion protein sequence was retrieved from Uniprot and 3D structure was generated using I-TASSER modeling program. A structure based computational screening approach has been employed using Glide docking software to identify potential SYT-SSX1 small molecule inhibitors that bind to the junction region of the fusion protein. The obtained inhibitors were further filtered based on the docking score and ADME/T properties. Ten best fit compounds were chosen for in vitro studies. The anti-proliferative activities of these 10 compounds were screened in Yamato, ASKA (carries SYT-SSX1 fusion protein) and other sarcoma cell lines such as A673, 143B to understand the specificity of inhibition of the chosen compounds. The in vitro activity was compared against HEK293 cell lines. The compound 5-fluoro-3-(1-phenyl-1H-tetraazol-5-yl)-1H-indole (FPTI) was found to be selectively cytotoxic in synovial sarcoma cell lines (Yamato and ASKA) and this compound also showed insignificant anti proliferative activity on other cell lines. Further, target gene expression study confirmed that FPTI treatment down-regulated SYT-SSX1 and modulated its downstream target genes. Cell cycle analysis revealed the involvement of an apoptotic mechanism of cell death. Further experimental validations may elucidate the therapeutic potentials of FPTI against SYT-SSX1 fusion protein.

Transducing-Like Enhancer of Split 1: A Potential Immunohistochemical Marker for Glomus Tumor

Glomus tumors (GTs) are rare, perivascular soft tissue tumors. Although GTs are usually found in the subcutaneous tissue, they may be detected in extracutaneous sites and mucosal areas. Transducing-like enhancer of split 1 (TLE1) is a highly useful immunohistochemical marker, which basically helps in differential diagnosis of synovial sarcoma. Based on a coincidental detection of TLE1 in one GT case, we studied 26 additional GT cases to establish the importance and distribution of TLE1 in GTs. Of 24 subcutaneous GTs, 22 (91.6%) were positive for TLE1 antibody and the remaining 2 mucosal GTs were negative. Of the 22 positive cases, 10 showed strong nuclear positivity. There was no difference between the subtypes of the GTs. Although TLE1 expression is significantly correlated to SS18 (SYT) rearrangements in synovial sarcomas, the fluorescence in situ hybridization analyses of the GTs showed no evidence of translocation involving this locus. TLE1 is a potential immunohistochemical marker for GTs, but further studies are required to confirm this finding.

Anoikis and EMT: Lethal "Liaisons" during Cancer Progression

Anoikis is a unique mode of apoptotic cell death that occurs consequentially to insufficient cell-matrix interactions. Resistance to anoikis is a critical contributor to tumor invasion and metastasis. The phenomenon is regulated by integrins, which upon engagement with components of the extracellular matrix (ECM) form adhesion complexes and the actin cytoskeleton drives the formation of cell protrusions used to adhere to ECM, directing cell migration. The epithelial-mesenchymal transition (EMT) confers stem cell properties and leads to acquisition of a migratory and invasive phenotype by causing adherens junction breakdown and circumventing anoikis in the tumor microenvironment. The investigation of drug discovery platforms for apoptosis-driven therapeutics identified several novel agents with antitumor action via reversing resistance to anoikis, inhibiting survival pathways and impacting the EMT landscape in human cancer. In this review, we discuss current evidence on the contribution of the anoikis phenomenon functionally linked to EMT to cancer metastasis and the therapeutic value of antitumor drugs that selectively reverse anoikis resistance and/or EMT to impair tumor progression toward the development/optimization of apoptosis-driven therapeutic targeting of metastatic disease.

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.

Structure-Enabled Discovery of a Stapled Peptide Inhibitor to Target the Oncogenic Transcriptional Repressor TLE1

TLE1 is an oncogenic transcriptional co-repressor that exerts its repressive effects through binding of transcription factors. Inhibition of this protein-protein interaction represents a putative cancer target, but no small-molecule inhibitors have been published for this challenging interface. Herein, the structure-enabled design and synthesis of a constrained peptide inhibitor of TLE1 is reported. The design features the introduction of a four-carbon-atom linker into the peptide epitope found in many TLE1 binding partners. A concise synthetic route to a proof-of-concept peptide, cycFWRPW, has been developed. Biophysical testing by isothermal titration calorimetry and thermal shift assays showed that, although the constrained peptide bound potently, it had an approximately five-fold higher Kd than that of the unconstrained peptide. The co-crystal structure suggested that the reduced affinity was likely to be due to a small shift of one side chain, relative to the otherwise well-conserved conformation of the acyclic peptide. This work describes a constrained peptide inhibitor that may serve as the basis for improved inhibitors.

Clinicopathologic and Prognostic Significance of Transducin-Like Enhancer of Split 1 Protein Expression in Invasive Breast Cancer

Purpose

Transducin-like enhancer of split 1 (TLE1) is a member of the TLE family of transcriptional co-repressors that control the transcription of a wide range of genes. We investigated the prognostic significance of TLE1 protein expression in breast cancers by using immunohistochemistry and explored the relationship of TLE1 with clinicopathological parameters.

Methods

Immunohistochemistry was performed on 456 cases of breast cancer tiled on tissue microarrays. The relationship between TLE1 expression in normal breast specimens and ductal carcinoma in situ (DCIS) was also analyzed.

Results

TLE1 was highly expressed in 57 of 456 (12.5%) carcinoma samples. TLE1 was more frequently expressed in DCIS and invasive breast cancers than in normal breast tissue (p=0.002). High expression of TLE1 significantly correlated with negative lymph node (LN) metastasis (p=0.007), high histologic grade (p<0.001), estrogen receptor negativity (p<0.001), progesterone receptor negativity (p<0.001), human epidermal growth factor receptor 2 (HER2) positivity (p<0.001), and high Ki-67 proliferation index (p<0.001). Based on intrinsic subtypes, high TLE1 expression was strongly associated with HER2+ and triple-negative breast cancers (TNBC) (p<0.001). Survival analysis demonstrated no significant association between TLE1 expression and disease-free survival (DFS) (p=0.167) or overall survival (OS) (p=0.286). In subgroup analyses, no correlation was found between TLE1 expression and DFS or OS according to LN status or intrinsic subtype.

Conclusion

High TLE1 expression is significantly associated with the HER2+ and TNBC subtypes. This is the first study documenting immunohistochemical expression of TLE1 in invasive breast cancer and its association with clinicopathological parameters, prognosis, and intrinsic subtype.

TLE1 function and therapeutic potential in cancer

Groucho (Gro)/Transducin-like enhancer of split (TLE) family proteins act as co-repressors of many transcription factors, and are involved in key signaling pathways. TLE1 negatively regulates inflammation and has potential roles in various diseases, including cancer. Previous studies suggest TLE1 could be used as a diagnostic marker and is a possible therapeutic target in various malignancies. It is therefore important to elucidate the mechanisms underlying TLE1 function during cancer initiation and metastasis. In this review, we highlight the functions of TLE1 in cancer and explore targeted approaches for cancer diagnosis and treatment. In particular, we discuss the TLE1 function in pancreatic cancer.

The Anoikis Effector Bit1 Inhibits EMT through Attenuation of TLE1-Mediated Repression of E-Cadherin in Lung Cancer Cells

The mitochondrial Bcl-2 inhibitor of transcription 1 (Bit1) protein is part of an anoikis-regulating pathway that is selectively dependent on integrins. We previously demonstrated that the caspase-independent apoptotic effector Bit1 exerts tumor suppressive function in lung cancer in part by inhibiting anoikis resistance and anchorage-independent growth in vitro and tumorigenicity in vivo. Herein we show a novel function of Bit1 as an inhibitor cell migration and epithelial–mesenchymal transition (EMT) in the human lung adenocarcinoma A549 cell line. Suppression of endogenous Bit1 expression via siRNA and shRNA strategies promoted mesenchymal phenotypes, including enhanced fibroblastoid morphology and cell migratory potential with concomitant downregulation of the epithelial marker E-cadherin expression. Conversely, ectopic Bit1 expression in A549 cells promoted epithelial transition characterized by cuboidal-like epithelial cell phenotype, reduced cell motility, and upregulated E-cadherin expression. Specific downregulation of E-cadherin in Bit1-transfected cells was sufficient to block Bit1-mediated inhibition of cell motility while forced expression of E-cadherin alone attenuated the enhanced migration of Bit1 knockdown cells, indicating that E-cadherin is a downstream target of Bit1 in regulating cell motility. Furthermore, quantitative real-time PCR and reporter analyses revealed that Bit1 upregulates E-cadherin expression at the transcriptional level through the transcriptional regulator Amino-terminal Enhancer of Split (AES) protein. Importantly, the Bit1/AES pathway induction of E-cadherin expression involves inhibition of the TLE1-mediated repression of E-cadherin, by decreasing TLE1 corepressor occupancy at the E-cadherin promoter as revealed by chromatin immunoprecipitation assays. Consistent with its EMT inhibitory function, exogenous Bit1 expression significantly suppressed the formation of lung metastases of A549 cells in an in vivo experimental metastasis model. Taken together, our studies indicate Bit1 is an inhibitor of EMT and metastasis in lung cancer and hence can serve as a molecular target in curbing lung cancer aggressiveness.

Tle1 tumor suppressor negatively regulates inflammation in vivo and modulates NF-κB inflammatory pathway

Tle1 (transducin-like enhancer of split 1) is a corepressor that interacts with a variety of DNA-binding transcription factors and has been implicated in many cellular functions; however, physiological studies are limited. Tle1-deficient (Tle1(Δ/Δ)) mice, although grossly normal at birth, exhibit skin defects, lung hypoplasia, severe runting, poor body condition, and early mortality. Tle1(Δ/Δ) mice display a chronic inflammatory phenotype with increased expression of inflammatory cytokines and chemokines in the skin, lung, and intestine and increased circulatory IL-6 and G-CSF, along with a hematopoietic shift toward granulocyte macrophage progenitor and myeloid cells. Tle1(Δ/Δ) macrophages produce increased inflammatory cytokines in response to Toll-like receptor (TLR) agonists and lipopolysaccharides (LPS), and Tle1(Δ/Δ) mice display an enhanced inflammatory response to ear skin 12-O-tetradecanoylphorbol-13-acetate treatment. Loss of Tle1 not only results in increased phosphorylation and activation of proinflammatory NF-κB but also results in decreased Hes1 (hairy and enhancer of split-1), a negative regulator of inflammation in macrophages. Furthermore, Tle1(Δ/Δ) mice exhibit accelerated growth of B6-F10 melanoma xenografts. Our work provides the first in vivo evidence, to our knowledge, that TLE1 is a major counterregulator of inflammation with potential roles in a variety of inflammatory diseases and in cancer progression.

The Groucho/Transducin-like enhancer of split protein family in animal development

Corepressors are proteins that cannot bind DNA directly but repress transcription by interacting with partner proteins. The Groucho/Transducin-Like Enhancer of Split (TLE) are a conserved family of corepressor proteins present in animals ranging from invertebrates such as Drosophila to vertebrates such as mice and humans. Groucho/TLE proteins perform important functions throughout the life span of animals, interacting with several pathways and regulating fundamental processes such as metabolism. However, these proteins have especially crucial functions in animal development, where they are required in multiple tissues in a temporally regulated manner. In this review, we summarize the functions of the Groucho/TLE proteins during animal development, emphasizing on specific tissues where they play essential roles.

TLE1 promotes EMT in A549 lung cancer cells through suppression of E-cadherin

The Groucho transcriptional corepressor TLE1 protein has recently been shown to be a putative lung specific oncogene, but its underlying oncogenic activity in lung cancer has not been fully elucidated. In this report, we investigated whether TLE1 regulates lung cancer aggressiveness using the human lung adenocarcinoma cell line A549 as a model system. Through a combination of genetic approaches, we found that TLE1 potentiates epithelial-to-mesenchymal transition (EMT) in A549 cells in part through suppression of the tumor suppressor gene E-cadherin. Exogenous expression of TLE1 in A549 cells resulted in heightened EMT phenotypes (enhanced fibroblastoid morphology and increased cell migratory potential) and in molecular alterations characteristic of EMT (downregulation of the epithelial marker E-cadherin and upregulation of the mesenchymal marker Vimentin). Conversely, downregulation of endogenous TLE1 expression in these cells resulted in reversal of basal EMT characterized by a cuboidal-like epithelial cell phenotype, reduced cell motility, and upregulated E-cadherin expression. Mechanistic studies showed that TLE1 suppresses E-cadherin expression at the transcriptional level in part by recruiting histone deacetylase (HDAC) activity to the E-cadherin promoter. Consistently, the HDAC inhibitor TSA partially reversed the TLE1-induced E-cadherin downregulation and cell migration, suggesting a role for HDACs in TLE1-mediated transcriptional repression of E-cadherin and EMT function. These findings uncover a novel role of TLE1 in regulating EMT in A549 cells through its repressive effect on E-cadherin and provide a mechanism for TLE1 oncogenic activity in lung cancer.

The corepressor Tle4 is a novel regulator of murine hematopoiesis and bone development

Hematopoiesis is a complex process that relies on various cell types, signaling pathways, transcription factors and a specific niche. The integration of these various components is of critical importance to normal blood development, as deregulation of these may lead to bone marrow failure or malignancy. Tle4, a transcriptional corepressor, acts as a tumor suppressor gene in a subset of acute myeloid leukemia, yet little is known about its function in normal and malignant hematopoiesis or in mammalian development. We report here that Tle4 knockout mice are runted and die at around four weeks with defects in bone development and BM aplasia. By two weeks of age, Tle4 knockout mice exhibit leukocytopenia, B cell lymphopenia, and significant reductions in hematopoietic stem and progenitor cells. Tle4 deficient hematopoietic stem cells are intrinsically defective in B lymphopoiesis and exhaust upon stress, such as serial transplantation. In the absence of Tle4 there is a profound decrease in bone mineralization. In addition, Tle4 knockout stromal cells are defective at maintaining wild-type hematopoietic stem cell function in vitro. In summary, we illustrate a novel and essential role for Tle4 in the extrinsic and intrinsic regulation of hematopoiesis and in bone development.

The anoikis effector Bit1 displays tumor suppressive function in lung cancer cells

The mitochondrial Bit1 (Bcl-2 inhibitor of transcription 1) protein is a part of an apoptotic pathway that is uniquely regulated by integrin-mediated attachment. As an anoikis effector, Bit1 is released into the cytoplasm following loss of cell attachment and induces a caspase-independent form of apoptosis. Considering that anoikis resistance is a critical determinant of transformation, we hypothesized that cancer cells may circumvent the Bit1 apoptotic pathway to attain anchorage-independence and tumorigenic potential. Here, we provide the first evidence of the tumor suppressive effect of Bit1 through a mechanism involving anoikis induction in human lung adenocarcinoma derived A549 cells. Restitution of Bit1 in anoikis resistant A549 cells is sufficient to induce detachment induced-apoptosis despite defect in caspase activation and impairs their anchorage-independent growth. Conversely, stable downregulation of Bit1 in these cells significantly enhances their anoikis resistance and anchorage-independent growth. The Bit1 knockdown cells exhibit significantly enhanced tumorigenecity in vivo. It has been previously shown that the nuclear TLE1 corepressor is a putative oncogene in lung cancer, and we show here that TLE1 blocks Bit1 mediated anoikis in part by sequestering the pro-apoptotic partner of Bit1, the Amino-terminal Enhancer of Split (AES) protein, in the nucleus. Taken together, these findings suggest a tumor suppressive role of the caspase-independent anoikis effector Bit1 in lung cancer. Consistent with its role as a tumor suppressor, we have found that Bit1 is downregulated in human non-small cell lung cancer (NSCLC) tissues.

Uncovering a key to the process of metastasis in human cancers: a review of critical regulators of anoikis

PURPOSE

Anoikis ('homelessness' in Greek) is a form of apoptosis following the detachment of cells from the appropriate extracellular matrix (Chiarugi and Giannoni in Biochem Pharmacol 76:1352-1364, 2008). Resistance to anoikis is a critical mediator of metastasis in cancer by enabling cancer cells to survive during invasion and transport in the blood and lymph. Numerous regulators and mechanisms of anoikis in human cancer have been proposed to date. Consequently, the identification of key regulators of anoikis that can be targeted to at least partially restore anoikis sensitivity in cancer cells is important in the development of therapies to treat metastatic cancer.

METHODS

A literature search focusing on the regulators of anoikis in human cancer was performed on the Medline, Embase and Scopus databases.

RESULTS

Mcl-1, Cav-1, Bcl-(xL), cFLIP, 14-3-3ζ and Bit1 appear to regulate anoikis in human cancer by participating in the intrinsic apoptotic pathway, extrinsic apoptotic pathway or caspase-independent pathways. Mcl-1, Cav-1, Bcl-(xL), cFLIP and 14-3-3ζ are suppressors of anoikis, and their upregulation confers anoikis resistance to cancer cells. Bit1 is a promoter of anoikis and is downregulated to confer anoikis resistance in metastatic cancer.

CONCLUSION

Anoikis is a complex process involving the crosstalk between different signalling pathways. The dysregulated expression of key regulators of anoikis that participate in these signalling pathways promotes anoikis resistance in human cancer. These regulators of anoikis might therefore be the targets for developing therapies to overcome anoikis resistance in metastatic cancer.

Bit1 in anoikis resistance and tumor metastasis

Epithelial cells and most adherent normal cells rely on adhesion-dependent, integrin-mediated survival signals from the extracellular matrix (ECM) to survive. When these cells are deprived of adhesion to the ECM, they undergo a specific form of apoptosis termed "anoikis." In contrast, malignant cells have attained mechanisms to enable them to survive in the absence of adhesion and are considered anchorage-independent. This review will focus on the biological function of the Bcl2-inhibitor of transcription (Bit1) protein in the anoikis process, the underlying molecular mechanism of Bit1 apoptotic function, and its role in tumor metastasis.

TLE1 is an anoikis regulator and is downregulated by Bit1 in breast cancer cells

TLE1 is a Groucho-related transcriptional repressor protein that exerts survival and antiapoptotic function in several cellular systems and has been implicated in the pathogenesis of cancer. In the present study, we found that TLE1 is a regulator of anoikis in normal mammary epithelial and breast carcinoma cells. The induction of apoptosis following loss of cell attachment to the extracellular matrix (anoikis) in untransformed mammary epithelial MCF10A cells was associated with significant downregulation of TLE1 expression. Forced expression of exogenous TLE1 in these cells promoted resistance to anoikis. In breast cancer cells, TLE1 expression was significantly upregulated following detachment from the extracellular matrix. Genetic manipulation of TLE1 expression via overexpression and downregulation approaches indicated that TLE1 promotes the anoikis resistance and anchorage-independent growth of breast carcinoma cells. Mechanistically, we show that TLE1 inhibits the Bit1 anoikis pathway by reducing the formation of the proapoptotic Bit1-AES complex in part through sequestration of AES in the nucleus. The mitochondrial release of Bit1 during anoikis as well as exogenous expression of the cytoplasmic localized Bit1 or its cell death domain induced cytoplasmic translocation and degradation of nuclear TLE1 protein. These findings indicate a novel role for TLE1 in the maintenance of anoikis resistance in breast cancer cells. This conclusion is supported by an immunohistochemical analysis of a breast cancer tissue array illustrating that TLE1 is selectively upregulated in invasive breast tumors relative to noninvasive ductal carcinoma in situ and normal mammary epithelial tissues.