Increased Six1 expression is associated with poor prognosis in patients with osteosarcoma

Targeting sine oculis homeoprotein 1 (SIX1): A review of oncogenic roles and potential natural product therapeutics

Sine oculis homeoprotein 1 (SIX1), a prominent representative of the homeodomain transcription factors within the SIX family, has attracted significant interest owing to its role in tumorigenesis, cancer progression, and prognostic assessments. Initially recognized for its pivotal role in embryonic development, SIX1 has emerged as a resurgent factor across a diverse set of mammalian cancers. Over the past two decades, numerous investigations have emphasized SIX1's dual significance as a developmental regulator and central player in oncogenic processes. A mounting body of evidence links SIX1 to the initiation of diverse cancers, encompassing enhanced cellular metabolism and advancement. This review provides an overview of the multifaceted roles of SIX1 in both normal development and oncogenic processes, emphasizing its importance as a possible therapeutic target and prognostic marker. Additionally, this review discusses the natural product agents that inhibit various pro-oncogenic mechanisms associated with SIX1.

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 and EWS/FLI1 co-regulate an anti-metastatic gene network in Ewing Sarcoma

Ewing sarcoma (ES), which is characterized by the presence of oncogenic fusion proteins such as EWS/FLI1, is an aggressive pediatric malignancy with a high rate of early dissemination and poor outcome after distant spread. Here we demonstrate that the SIX1 homeoprotein, which enhances metastasis in most tumor types, suppresses ES metastasis by co-regulating EWS/FLI1 target genes. Like EWS/FLI1, SIX1 promotes cell growth/transformation, yet dramatically inhibits migration and invasion, as well as metastasis in vivo. We show that EWS/FLI1 promotes SIX1 protein expression, and that the two proteins share genome-wide binding profiles and transcriptional regulatory targets, including many metastasis-associated genes such as integrins, which they co-regulate. We further show that SIX1 downregulation of integrins is critical to its ability to inhibit invasion, a key characteristic of metastatic cells. These data demonstrate an unexpected anti-metastatic function for SIX1, through coordinate gene regulation with the key oncoprotein in ES, EWS/FLI1.

The Significance of SIX1 as a Prognostic Biomarker for Survival Outcome in Various Cancer Patients: A Systematic Review and Meta-Analysis

Sine Oculis Homeobox Homolog 1 (SIX1) is reported to promote cancer initiation and progression in many preclinical models and is demonstrated in human cancer tissues. However, the correlation between SIX1 and cancer patients’ prognosis has not yet been systematically evaluated. Therefore, we performed a systematic review and meta-analysis in various human cancer types and extracted some data from TCGA datasets for further verification and perfection. We constructed 27 studies and estimated the association between SIX1 expression in various cancer patients’ overall survival and verified with TCGA datasets. Twenty-seven studies with 4899 patients are include in the analysis of overall, and disease-free survival, most of them were retrospective. The pooled hazard ratios (HRs) for overall and disease-free survival in high SIX1 expression patients were 1.54 (95% CI: 1.32-1.80, P<0.00001) and 1.83 (95% CI: 1.31-2.55, P=0.0004) respectively. On subgroup analysis classified in cancer type, high SIX1 expression was associated with poor overall survival in patients with hepatocellular carcinoma (HR 1.50; 95% CI: 1.17-1.93, P =0.001), breast cancer (HR 1.31; 95% CI: 1.10-1.55, P =0.002) and esophageal squamous cell carcinoma (HR 1.89; 95% CI: 1.42-2.52, P<0.0001). Next, we utilized TCGA online datasets, and the consistent results were verified in various cancer types. SIX1 expression indicated its potential to serve as a cancer biomarker and deliver prognostic information in various cancer patients. More works still need to improve the understandings of SIX1 expression and prognosis in different cancer types.

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.

The SIX Family of Transcription Factors: Common Themes Integrating Developmental and Cancer Biology

The sine oculis (SIX) family of transcription factors are key regulators of developmental processes during embryogenesis. Members of this family control gene expression to promote self-renewal of progenitor cell populations and govern mechanisms of cell differentiation. When the function of SIX genes becomes disrupted, distinct congenital defects develops both in animal models and humans. In addition to the embryonic setting, members of the SIX family have been found to be critical regulators of tumorigenesis, promoting cell proliferation, epithelial-to-mesenchymal transition, and metastasis. Research in both the fields of developmental biology and cancer research have provided an extensive understanding of SIX family transcription factor functions. Here we review recent progress in elucidating the role of SIX family genes in congenital disease as well as in the promotion of cancer. Common themes arise when comparing SIX transcription factor function during embryonic and cancer development. We highlight the complementary nature of these two fields and how knowledge in one area can open new aspects of experimentation in the other.

High expression of SIX1 is an independent predictor of poor prognosis in endometrial cancer

Objective: The overexpression of transcription factor Sine oculis homeobox 1 (SIX1) is discovered in various malignant tumors and has been known to be closely associated with tumorigenesis, progression and prognosis. This study aims to determine the role of SIX1 in endometrial cancer (EC). Methods: In this study, we analyzed the SIX1 expression profile and the correlation with the corresponding clinical characteristics of EC samples from the Cancer Genome Atlas (TCGA), Gene Expression Omnibus (GEO) and Clinical Proteomic Tumor Analysis Consortium (CPTAC) databases. Wilcoxon signed-rank test was applied to analyze the difference between tumor group and control group. The potential biological processes or signaling pathways related to SIX1 activity in EC was also assessed. Results: The results showed that SIX1 was overexpressed in EC tissues compared to normal tissues (P=2.029e-15, P=6.25e-6). The SIX1 level was correlated with tumor grade (P=2.91e-4), peritoneal cytology (P=0.005), and the subsequent tumor surgery (P=1.169e-4). SIX1 expression was negatively associated with overall survival rate (P=4.241e-4, P=0.000241) and served as an independent factor that affected EC overall survival rate (P=0.005063), similar to other factors such as age, Figo stage, and tumor (T) stage. SIX1 participates in cancer pathogenesis through gene regulation that involves PI3K/AKT/MTOR signaling, mitotic spindle, G2M checkpoint, E2F targets, NOTCH signaling, glycolysis, cholesterol homeostasis, DNA repair and early estrogen response. Conclusions: Our data demonstrate that SIX1 is overexpressed in EC and associated with adverse clinicopathological outcomes, which can function as an independent factor for EC prognosis.

miR-23a-3p/SIX1 regulates glucose uptake and proliferation through GLUT3 in head and neck squamous cell carcinomas

SIX1 overexpression has been reported in several cancers. However, its involvement in head and neck squamous cell carcinoma (HNSCC) remains unclear. In this study we investigated the clinical significance and biological roles of SIX1 in HNSCC. SIX1 expression was upregulated in HNSCC and correlated with TNM stage and nodal metastasis. Analysis of TCGA dataset demonstrated that high SIX1 expression correlated with poor patient prognosis. Overexpression of SIX1 in the Fadu cell line upregulated cell proliferation, colony formation, glucose uptake and ATP production. In contrast, SIX1 depletion in the Detroit562 cell line downregulated cell proliferation, colony formation, glucose uptake and ATP production. We analyzed a series of genes involved in glucose metabolism and found that SIX1 overexpression upregulated GLUT3, an important glucose transporter, at both mRNA and protein levels. Using the TRANSFAC database, we found that SIX1 had potential binding sites on the GLUT3 promoter, which was validated by chromatin immunoprecipitation (ChIP) assays. Next, we focused on miR-23a-3p, which could target SIX1 in HNSCC cells. The miR-23a-3p mimic downregulated SIX1 expression while the miR-23a-3p inhibitor upregulated SIX1 expression. The binding of miR-23a-3p to the 3'-UTR of SIX1 was confirmed using the luciferase reporter assay. Analysis of TCGA dataset showed a negative correlation between the miR-23a-3p and SIX1. Furthermore, the miR-23a-3p mimic inhibited cell proliferation, ATP production and glucose uptake, which could be rescued by transfection with the SIX1 plasmid. In summary, our study demonstrated that SIX1 facilitated HNSCC cell growth through regulation of GLUT3 and glucose uptake. miR-23a-3p targeted the SIX1/GLUT3 axis and suppressed glucose uptake and proliferation in HNSCC.

miR-489-3p/SIX1 Axis Regulates Melanoma Proliferation and Glycolytic Potential

Sine oculis homeobox 1 (SIX1), a key transcription factor for regulating aerobic glycolysis, participates in the occurrence of various cancer types. However, the role of SIX1 in melanoma and the upstream regulating mechanisms of SIX1 remain to be further investigated. MicroRNAs (miRNAs) have emerged as key regulators in tumorigenesis and progression. Here, we show that miR-489-3p suppresses SIX1 expression by directly targeting its 3′ untranslated region (3′ UTR) in melanoma cells. miR-489-3p suppressed melanoma cell proliferation, migration, and invasion through inhibition of SIX1. Mechanistically, by targeting SIX1, miR-489-3p dampens glycolysis, with decreased glucose uptake, lactate production, ATP generation, and extracellular acidification rate (ECAR), as well as an increased oxygen consumption rate (OCR). Importantly, glycolysis regulated by the miR-489-3p/SIX1 axis is critical for its regulation of melanoma growth and metastasis both in vitro and in vivo. In melanoma patients, miR-489-3p expression is negatively correlated with SIX1 expression. In addition, patients who had increased glucose uptake in tumors and with metastasis assessed by positron emission tomography (PET) scans showed decreased miR-489-3p expression and increased expression of SIX1. Collectively, our study demonstrates the importance of the miR-489-3p/SIX1 axis in melanoma, which can be a potential and a promising therapeutic target in melanoma.

MiR-150-5p regulates melanoma proliferation, invasion and metastasis via SIX1-mediated Warburg Effect

Aerobic glycolysis is a hallmark of cancer. Sine oculis homeobox 1 (SIX1), a key transcription factor in terms of regulating aerobic glycolysis (the Warburg Effect), plays a critical role in tumorigenesis of various cancer types, including breast cancer, liver cancer, and lung cancer. However, the upstream regulating mechanisms of SIX1 in melanoma remain to be determined. MicroRNAs (miRNAs) have emerged as key regulators in tumorigenesis and progression. Here, we initially showed that microRNA-150-5p (miR-150-5p) inhibits SIX1 expression by directly targeting its 3'-UTR in melanoma cells. miR-150-5p suppressed melanoma cell proliferation, migration, and invasion through inhibition of SIX1. Mechanistically, miR-150-5p dampens glycolysis by decreasing the glucose uptake, lactate production, ATP generation, and extracellular acidification rate (ECAR), and increasing oxygen consumption rate (OCR) by targeting SIX1. Importantly, glycolysis regulated by miR-150-5p/SIX1 axis is critical for its regulation of melanoma growth and metastasis both in vitro and in vivo. Collectively, our study demonstrates the importance of miR-150-5p/SIX1 axis in melanoma, which could be a promising therapeutic target in melanoma.

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.

SIX1 and DACH1 influence the proliferation and apoptosis of hepatocellular carcinoma through regulating p53

ABSTRACTS This research aimed to explore effects of SIX1 and DACH1 on hepatocellular carcinoma (HCC) cell proliferation, apoptosis and cell cycle. Fifty paired hepatocellular carcinoma tissues were screened for differentially expressed genes. SIX1 and DACH1 expressions were subjected to qRT-PCR and western blot in tumor tissues and cells. The knockdown efficiency of siRNAs and transfection efficiency of cDNAs and siRNAs were validated by qRT-PCR and western blot as well. Then colony formation assay and flow cytometry were applied to observe cell proliferation, cell apoptosis and cell cycle changes. Immunofluorescence co-localization and immunoprecipitation were used to analyze the interaction between proteins which was quantified using western blot. Effects of SIX1 and DACH1 on tumor growth and their expressions in tumors were confirmed in vitro in nude mice model. Results of these experiments showed that SIX1 was overexpressed while DACH1 was suppressed in HCC tissues and cells. The suppression of SIX1 and overexpression of DACH1 not only inhibited cell proliferation, but also induced cell apoptosis and arrested cell cycle in G2/M phase compared with control group. Results of immunofluorescence co-localization suggested that SIX1, p53 and DACH1 were significantly overlapped. Immunoprecipitation showed that DACH1 (marked with Flag tag) could pull down p53 and SIX1, but SIX1 (marked with His tag) could only pull down DACH1, which indicated that an indirect regulation between SIX1 and p53. Validated with western blot afterwards, DACH1 overexpression suppressed tumorigenesis in vivo by up-regulating p53 expression while SIX1 overexpression accelerated tumor growth by down-regulating p53 expression. Therefore, the decrease of SIX1 facilitated the expression of DACH1, thus activated the expression of p53 and suppressed the progression of HCC both in vitro and in vivo.