Specificity and prognostic validation of a polyclonal antibody to detect Six1 homeoprotein in ovarian cancer

All eyes on Eya: A unique transcriptional co-activator and phosphatase in cancer

The Eya family of proteins (consisting of Eyas1-4 in mammals) play vital roles in embryogenesis by regulating processes such as proliferation, migration/invasion, cellular survival and pluripotency/plasticity of epithelial and mesenchymal states. Eya proteins carry out such diverse functions through a unique combination of transcriptional co-factor, Tyr phosphatase, and PP2A/B55α-mediated Ser/Thr phosphatase activities. Since their initial discovery, re-expression of Eyas has been observed in numerous tumor types, where they are known to promote tumor progression through a combination of their transcriptional and enzymatic activities. Eya proteins thus reinstate developmental processes during malignancy and represent a compelling class of therapeutic targets for inhibiting tumor progression.

SIX1 reprograms myogenic transcription factors to maintain the rhabdomyosarcoma undifferentiated state

Rhabdomyosarcoma (RMS) is a pediatric muscle sarcoma characterized by expression of the myogenic lineage transcription factors (TFs) MYOD1 and MYOG. Despite high expression of these TFs, RMS cells fail to terminally differentiate, suggesting the presence of factors that alter their functions. Here, we demonstrate that the developmental TF SIX1 is highly expressed in RMS and critical for maintaining a muscle progenitor-like state. SIX1 loss induces differentiation of RMS cells into myotube-like cells and impedes tumor growth in vivo. We show that SIX1 maintains the RMS undifferentiated state by controlling enhancer activity and MYOD1 occupancy at loci more permissive to tumor growth over muscle differentiation. Finally, we demonstrate that a gene signature derived from SIX1 loss correlates with differentiation status and predicts RMS progression in human disease. Our findings demonstrate a master regulatory role of SIX1 in repression of RMS differentiation via genome-wide alterations in MYOD1 and MYOG-mediated transcription.

Role of SIX1, EYA2, and E-cadherin in ovarian carcinoma. Evidence on epithelial-mesenchymal transition from an immunohistochemical study

This study aims to investigate the expression of SIX1, EYA2, and E-cadherin in ovarian cancer (OC). It was conducted on 97 cases of surface epithelial tumors (SEOTs). Immunohistochemistry (IHC) staining for the three markers was applied to archival paraffin-embedded sections. Results of semi-quantitative scoring were statistically compared, correlated with clinic-pathologic parameters, response to therapy and with patient survival. RESULTS: There was a significant association of SIX1 expression in the intratumoral stroma (ITS) with malignant cases (P < 0.0001). There was a significant direct correlation between tumour cell expression of SIX1 and EYA2 (P = 0.03) and an inverse correlation between SIX1 and E-cadherin (P = 0.03). Additionally, there were direct correlations between SIX1 expression and larger tumour size (P = 0.05), high mitosis (P < 0.0001), and advanced FIGO stage (P = 0.06), and between EYA2 expression and LN metastasis (P = 0.02), and low apoptotic index (P = 0.007). Only SIX1 expression in ITS affected the patient survival by univariate analysis (P = 0.004). CONCLUSIONS: SIX1/EYA2 complex may have a poor prognostic role in OC. SIX1 expression in ITS may be used as a predictive marker of stromal invasion in ovarian borderline tumors and could affect patients' survival in OC. SIX1, EYA2, and E-cadherin may constitute a pathway that could be targeted to stop the progression of SEOTs.

Regulation of cancer stem cell properties by SIX1, a member of the PAX-SIX-EYA-DACH network

The PAX-SIX-EYA-DACH network (PSEDN) is a central developmental transcriptional regulatory network from Drosophila to humans. The PSEDN is comprised of four conserved protein families; including paired box (PAX), sine oculis (SIX), eyes absent (EYA), and dachshund (DACH). Aberrant expression of PSEDN members, particularly SIX1, has been observed in multiple human cancers, where SIX1 expression correlates with increased aggressiveness and poor prognosis. In conjunction with its transcriptional activator EYA, the SIX1 transcription factor increases cancer stem cell (CSC) numbers and induces epithelial-mesenchymal transition (EMT). SIX1 promotes multiple hallmarks and enabling characteristics of cancer via regulation of cell proliferation, senescence, apoptosis, genome stability, and energy metabolism. SIX1 also influences the tumor microenvironment, enhancing recruitment of tumor-associated macrophages and stimulating angiogenesis, to promote tumor development and progression. EYA proteins are multifunctional, possessing a transcriptional activation domain and tyrosine phosphatase activity, that each contributes to cancer stem cell properties. DACH proteins function as tumor suppressors in solid cancers, opposing the actions of SIX-EYA and reducing CSC prevalence. Multiple mechanisms can lead to increased SIX1 expression, including loss of SIX1-targeting tumor suppressor microRNAs (miRs), whose expression correlates inversely with SIX1 expression in cancer patient samples. In this review, we discuss the major mechanisms by which SIX1 confers CSC and EMT features and other important cancer cell characteristics. The roles of EYA and DACH in CSCs and cancer progression are briefly highlighted. Finally, we summarize the clinical significance of SIX1 in cancer to emphasize the potential therapeutic benefits of effective strategies to disrupt PSEDN protein interactions and functions.

microRNA-488 inhibits chemoresistance of ovarian cancer cells by targeting Six1 and mitochondrial function

Dysregulation of miR-488 has been implicated in several human cancers. In this study, we aim to explore its expression and biological function in ovarian cancers. We found miR-488 expression was downregulated in ovarian cancer tissues. Using CCK8 and colony formation assay showed that miR-488 inhibited SKOV3 cell proliferation and colony formation, with downregulation of cyclin D1 and cyclin E protein. While miR-488 inhibitor promoted OVCAR3 cell growth and colony formation. Cell viability and Annexin V/PI staining showed that miR-488 downregulated cell survival and increased apoptosis rate when treated with cisplatin and paclitaxel. Further experiments using MitoTracker and JC-1 staining indicated that miR-488 regulated mitochondrial fission/fusion balance and inhibited mitochondrial membrane potential, with p-Drp1, Drp1 and Fis1 downregulation. Luciferase reporter assay showed that Six1 is a target of miR-488. We also found a negative association between Six1 and miR-488 in ovarian cancer tissues. In addition, Six1 overexpression induced mitochondrial fission and increased mitochondrial potential, with upregulation of Drp1 signaling. Six1 depletion showed the opposite effects. Restoration of Six1 in SKOV3 cells rescued decreased p-Drp1 and Drp1 expression induced by miR-488 mimic. Six1 plasmid also reversed the effects of miR-488 on chemoresistance and apoptosis. Taken together, the present study showed that, by targeting Six1, miR-488 inhibits chemoresistance of ovarian cancer cells through regulation of mitochondrial function.

The Eya phosphatase: Its unique role in cancer

The Eya proteins were originally identified as essential transcriptional co-activators of the Six family of homeoproteins. Subsequently, the highly conserved C-terminal domains of the Eya proteins were discovered to act as a Mg²⁺-dependent Tyr phosphatases, making Eyas the first transcriptional activators to harbor intrinsic phosphatase activity. Only two direct targets of the Eya Tyr phosphatase have been identified: H2AX, whose dephosphorylation directs cells to the DNA repair instead of the apoptotic pathway upon DNA damage, and ERβ, whose dephosphorylation inhibits its anti-tumor transcriptional activity. The Eya Tyr phosphatase mediates breast cancer cell transformation, migration, invasion, as well as metastasis, through targets not yet identified. Intriguingly, the N-terminal domain of Eya contains a separate Ser/Thr phosphatase activity implicated in innate immunity and in regulating c-Myc stability. Thus, Eya proteins are highly complex, containing two separable phosphatase domains and a transcriptional activation domain, thereby influencing tumor progression through multiple mechanisms.

Pax2/Pax8-defined subdomains and the occurrence of apoptosis in the posterior placodal area of mice

The present work aims to improve our understanding of the causes and functions of apoptosis during the morphogenesis of epibranchial placodes in mice. Schematic maps helped to compare the spatiotemporal sequence of apoptotic events with the protein expression patterns of general (Six1) and specific placodal markers (Pax2, Pax8). Our findings challenge the view that, in mammals, all three epibranchial placodes spring from the original posterior placodal area (PPA) of presomite or early somite embryos. Instead, close-meshed analysis of the Pax2/Pax8 expression patterns demonstrates the stepwise emergence of two subdomains which both belong to the gradually expanding PPA, and which largely give rise to the otic placode and epibranchial placode 1 (anterior subdomain), or to the caudal epibranchial placodes (posterior subdomain). Our observations reinforce previous doubts raised on the PPA progeny of early somite Xenopus embryos (Schlosser and Ahrens, Dev Biol 271:439-466, 2004). They also demonstrate that partly different Pax2/Pax8 codes accompany epibranchial placode development in Xenopus laevis and mice. In mice, interplacodal apoptosis assists in the establishment of the two PPA subdomains and, subsequently, of individualized placodes by predominantly eliminating Six1⁺ placodal precursor cells. Onset of interplacodal and intraplacodal large-scale apoptosis is almost always preceded and/or paralleled by Pax2/Pax8 expression minima in the very same region. Future work will demand the use of knock-out mice and whole embryo culture to experimentally test, whether the combined action of differentially expressed Pax2 and Pax8 genes exerts antiapoptotic effects in the mammalian PPA.

Pro-survival effects by NF-κB, Akt and ERK(1/2) and anti-apoptosis actions by Six1 disrupt apoptotic functions of TRAIL-Dr4/5 pathway in ovarian cancer

Apoptotic signaling provoked by death receptors, DR4 and DR5, are generally considered to promote cell death and chemosensitivity in multiple cancers, but this view is being thrown into doubt with recent findings that up-regulated DR4 and DR5 in advanced stages of ovarian cancer are associated with the poor prognosis. For this conflict, two reasonable explanations have been proposed: one is that DR4 and DR5 not exclusively mediate apoptotic pathway, but also favor survival signal; another is that apoptotic signals by DR4 and DR5 are disrupted by some regulators. This study identified these two speculations in TRAIL-resistant (SKOV-3ip1 and A2780) or sensitive (OVCAR-3) ovarian cancer cells. Activation of DR4 and DR5 using their specific ligand, TRAIL, activated pro-survival factors including NF-κB, Akt and ERK(1/2) in ovarian cancer SKOV-3ip1 and A2780 cells. Pharmacological inhibition of their activities potentiated TRAIL cytotoxicity, reducing cell viability and increasing apoptosis. Six1, a homeobox transcription factor, had higher expression in SKOV-3ip1 and A2780 cells than in OVCAR-3 cells. Silencing Six1 raised levels of apoptotic factors including cleaved Bid, caspase-8 and caspase-3, and overrode the TRAIL-resistance. Co-treatment with Six1 knockdown and peptidyl O-glycosyltransferase 14 overexpression showed additive effects on apoptosis signal, leading to increased apoptosis in SKOV-3ip1 and A2780 cells. This study demonstrated that pro-survival effects by NF-κB, Akt and ERK(1/2) and anti-apoptosis actions by Six1 disrupt apoptotic functions of TRAIL-Dr4/5 pathway in ovarian cancer, which may explain why up-regulated DR4 and DR5 in ovarian cancer are associated with poor prognosis and low survival ratio of the patients.

miR-30b regulates migration and invasion of human colorectal cancer via SIX1

CRC (colorectal cancer) is one of the most malignant tumours in both developing and developed countries. It is estimated that 60% of CRC patients have liver metastasis. In the present study, we show that miR-30b is an important regulator in human CRC migration and invasion, which are vital steps in CRC liver metastasis. miR-30b was significantly down-regulated in primary CRC specimens compared with normal tissues. Furthermore, miR-30b was much lower in liver metastasis tissues than in CRCs. We validated SIX1 (SIX homeobox 1), a member of the SIX homeodomain family of transcription factors and an EMT (epithelial-mesenchymal transition)-promoting gene, as the direct target of miR-30b. Forced expression of miR-30b inhibited CRC cell migration and invasion in vitro via its target gene SIX1. Furthermore, an inverse correlation between expression of SIX1 and miR-30b has been observed both in primary CRC specimens and liver metastasis. Taken together, miR-30b plays an important role in mediating metastatic related behaviour in CRC. miR-30b may serve as a potential diagnostic marker and therapeutic target for patients with CRC in the future.

Sineoculis homeobox homolog 1 protein as an independent biomarker for gastric adenocarcinoma

Sine oculis homeobox homolog 1 (SIX1) protein is a member of the homeobox transcription factor family. Overexpression of SIX1 contributes to cancer progression and is associated with adverse outcomes in various cancer types including breast, ovarian, uterine cervical and liver. To investigate the clinicopathological significance of SIX1 protein expression in gastric adenocarcinomas (GAC), localization of the SIX1 protein was determined in MKN-1, a gastric cancer cell line, using immunofluorescence (IF) staining; SIX1 mRNA level was detected in fresh tissues of GAC and normal gastric mucosa using quantitative real-time polymerase chain reaction (qRT-PCR); and SIX1 protein expression was assessed in 163 GAC, 35 gastric dysplasia and 26 normal gastric mucosa using immunohistochemical (IHC) staining. Correlations between SIX1 protein expression and pathological parameters of GAC were analyzed using Chi-square tests, differences in survival curves were analyzed using log-rank tests, and multivariate survival analysis was performed using the Cox proportional hazards regression model. SIX1 protein showed a mainly cytoplasmic staining pattern in GAC using IF and IHC staining. The positive SIX1 protein expression rate was 80.4% in GAC, which was significantly higher than in either gastric dysplasia (45.7%) or normal gastric mucosa (26.9%) (P<0.01). qRT-PCR data also confirmed increased levels of SIX1 mRNA expression in GAC compared with the normal gastric mucosa in fresh tissues. In addition, the strongly positive SIX1 protein expression rate was significantly correlated with clinical stage, lymph node metastasis and serosal invasion of GAC (P<0.01 or P<0.05), while there was no association with gender, age, tumor size, Lauren classification or histological types of GAC. Notably, strongly positive signals were frequently observed in tumor blood vessels and/or lymphatic vessels. GAC patients with high expression of the SIX1 had shorter overall and disease-free survival rates than those with low SIX1 protein expression (P<0.01). Furthermore, using multivariate analysis, SIX1 protein expression was found to be an independent risk factor for survival in patients with GAC along with clinical stage and serosal invasion (P<0.01). In conclusion, SIX1 protein expression status may be an independent biomarker for prognostic evaluation of GAC.

MicroRNA-30a regulates zebrafish myogenesis through targeting the transcription factor Six1

Precise spatiotemporal regulation of the SIX1 homeoprotein is required to coordinate vital tissue development, including myogenesis. Whereas SIX1 is downregulated in most tissues following embryogenesis, it is re-expressed in numerous cancers, including tumors derived from muscle progenitors. Despite crucial roles in development and disease, the upstream regulation of SIX1 expression has remained elusive. Here, we identify the first direct mechanism for Six1 regulation in embryogenesis, through microRNA30a (miR30a)-mediated repression. In zebrafish somites, we show that miR30a and six1a and six1b (hereafter six1a/b) are expressed in an inverse temporal pattern. Overexpression of miR30a leads to a reduction in six1a/b levels, and results in increased apoptosis and altered somite morphology, which phenocopies six1a/b knockdown. Conversely, miR30a inhibition leads to increased Six1 expression and abnormal somite morphology, revealing a role for endogenous miR30a as a muscle-specific miRNA (myomiR). Importantly, restoration of six1a in miR30a-overexpressing embryos restores proper myogenesis. These data demonstrate a new role for miR30a at a key node in the myogenic regulatory gene network through controlling Six1 expression.

Sineoculis homeobox homolog 1 protein overexpression as an independent biomarker for pancreatic ductal adenocarcinoma

Sineoculis homeobox homolog 1 (SIX1) is a member of the SIX gene family. It is highly expressed in cancers derived from tissues that play a fundamental role during embryogenesis. Recent studies suggest that inappropriate expression of SIX1 can both initiate tumorigenesis and promote metastasis. To investigate the clinicopathological significance of SIX1 expression in pancreatic ductal adenocarcinoma (PDAC), and to further identify its role as a potential biomarker and therapeutic target in PDAC, 103 PDAC tissue samples and 45 normal pancreatic tissue samples were immunohistochemically stained for SIX1 protein. The localization of SIX1 protein was detected in Panc-1 cancer cells using immunofluorescence staining. Correlations between SIX1 overexpression and the clinicopathological features of pancreatic cancer were evaluated using Chi-square (χ(2)) tests, differences in survival curves were analyzed using log-rank tests, and multivariate survival analysis was performed using the Cox proportional hazard regression model. In results, SIX1 protein showed mainly cytoplasmic/perinuclear staining pattern in PDAC with immunohistochemistry. The strongly positive rate of SIX1 protein was 60.2% (62/103) in PDAC, which was significantly higher than normal pancreatic tissue (6.7%, 3/45). SIX1 overexpression was positively correlated with tumor size, TNM stage, lymph node metastasis, and grade of PDAC (P < 0.001). SIX1 high expression levels influenced overall survival rates in G1, G2, stage I-II and stage III-IV groups of PDAC; and high expression levels had significantly lower overall survival rates than SIX1 low expression levels. In conclusion, SIX1 emerged as a significant independent prognostic factor in PDAC. SIX1 overexpression appears to be associated with PDAC, and may be a potential biomarker for early diagnosis and prognostic evaluation of PDAC.

Inhibition of Six1 promotes apoptosis, suppresses proliferation, and migration of osteosarcoma cells

Sineoculis homeobox homolog 1 (Six1) is one of the transcription factors that act as master regulators of development and is frequently dysregulated in cancers. However, the biological role of Six1 is not clear in osteosarcoma. To address the expression of Six1 in osteosarcoma cells, three osteosarcoma cell lines (U2OS, SaOS-2, and MG63) and a human osteoblastic cell line (hFOB1.19) were used to detect the expression of Six1 by quantitative real-time polymerase chain reaction and western blotting. The results showed that Six1 was upregulated in osteosarcoma cell lines compared to human osteoblastic cell line hFOB1.19. To investigate the role of Six1 in osteosarcoma cells, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, flow cytometry analysis, and transwell chamber assays were used to determine the effects of Six1 on the cell viability, cycle, apoptosis, and migration properties in U2OS cells. The results showed that Six1 could promote U2OS cell proliferation and migration, and suppress U2OS cell apoptosis. In addition, we investigated the effects of Six1 on the expression of following proteins (cyclin D1, caspase-3, and vascular endothelial growth factor-C (VEGF-C)). Results showed that Six1 could increase the expression of cyclin D1 and VEGF-C, and decrease the expression of caspase-3. All these data suggested that Six1 might be involved in the promotion of growth, proliferation, and migration of U2OS cells, as well as the inhibition of apoptosis of U2OS cells. These data might provide information for the prediction of osteosarcoma prognosis and potential targets for therapy of osteosarcoma.

Regulatory networks defining EMT during cancer initiation and progression

Epithelial to mesenchymal transition (EMT) is essential for driving plasticity during development, but is an unintentional behaviour of cells during cancer progression. The EMT-associated reprogramming of cells not only suggests that fundamental changes may occur to several regulatory networks but also that an intimate interplay exists between them. Disturbance of a controlled epithelial balance is triggered by altering several layers of regulation, including the transcriptional and translational machinery, expression of non-coding RNAs, alternative splicing and protein stability.

Identification and characterization of alternative exon usage linked glioblastoma multiforme survival

Background

Alternative exon usage (AEU) is an important component of gene regulation. Exon expression platforms allow the detection of associations between AEU and phenotypes such as cancer. Numerous studies have identified associations between gene expression and the brain cancer glioblastoma multiforme (GBM). The few consistent gene expression biomarkers of GBM that have been reported may be due to the limited consideration of AEU and the analytical approaches used. The objectives of this study were to develop a model that accounts for the variations in expression present between the exons within a gene and to identify AEU biomarkers of GBM survival.

Methods

The expression of exons corresponding to 25,403 genes was related to the survival of 250 individuals diagnosed with GBM in a training data set. Genes exhibiting AEU in the training data set were confirmed in an independent validation data set of 78 patients. A hierarchical mixed model that allows the consideration of covariation between exons within a gene and of the effect of the epidemiological characteristics of the patients was developed to identify associations between exon expression and patient survival. This general model describes all three possible scenarios: multi-exon genes with and without AEU, and single-exon genes.

Results

AEU associated with GBM survival was identified on 2477 genes (P-value < 5.0E-04 or FDR-adjusted P-value < 0.05). G-protein coupled receptor 98 (Gpr98) and epidermal growth factor (Egf) were among the genes exhibiting AEU with 30 and 9 exons associated with GBM survival, respectively. Pathways enriched among the AEU genes included focal adhesion, ECM-receptor interaction, ABC transporters and pathways in cancer. In addition, 24 multi-exon genes without AEU and 8 single-exon genes were associated with GBM survival (FDR-adjusted P-value < 0.05).

Conclusions

The inferred patterns of AEU were consistent with in silico AS models. The hierarchical model used offered a flexible and simple way to interpret and identify associations between survival that accommodates multi-exon genes with or without AEU and single exon genes. Our results indicate that differential expression of AEU could be used as biomarker for GBM and potentially other cancers.