6495

SIX1

SIX homeobox 1;SIX1;SIX homeobox 1

homeobox protein SIX1|sine oculis homeobox homolog 1

14q23.1

protein-coding

Inappropriate activation of developmental pathways is a well-recognized tumor-promoting mechanism. Here we show that overexpression of the homeoprotein Six1, normally a developmentally restricted transcriptional regulator, increases TGF-beta signaling in human breast cancer cells and induces an epithelial-mesenchymal transition (EMT) that is in part dependent on its ability to increase TGF-beta signaling. TGF-beta signaling and EMT have been implicated in metastatic dissemination of carcinoma. Accordingly, we used spontaneous and experimental metastasis mouse models to demonstrate that Six1 overexpression promotes breast cancer metastasis. In addition, we show that, like its induction of EMT, Six1-induced experimental metastasis is dependent on its ability to activate TGF-beta signaling. Importantly, in human breast cancers Six1 correlated with nuclear Smad3 and thus increased TGF-beta signaling. Further, breast cancer patients whose tumors overexpressed Six1 had a shortened time to relapse and metastasis and an overall decrease in survival. Finally, we show that the effects of Six1 on tumor progression likely extend beyond breast cancer, since its overexpression correlated with adverse outcomes in numerous other cancers including brain, cervical, prostate, colon, kidney, and liver. Our findings indicate that Six1, acting through TGF-beta signaling and EMT, is a powerful and global promoter of cancer metastasis.

Six1 is a critical regulator of embryonic development that requires interaction with the Eya family of proteins (Eya1-4) to activate the transcription of genes involved in neurogenesis, myogenesis and nephrogenesis. Although expression of Six1 and Eya family members is predominantly observed in development, their overexpression is observed in numerous cancers. Importantly, both Six1 and Eya have independently been shown to mediate breast cancer metastasis, but whether they functionally interact during tumor progression has not been explored. Herein, we demonstrate that knockdown of Eya2 in MCF7 mammary carcinoma cells reverses the ability of Six1 to induce transforming growth factor-beta signaling, as well as to induce characteristics associated with epithelial-mesenchymal transition and cancer stem cells, suggesting that Six1 is dependent on Eya2 to mediate numerous pro-metastatic characteristics. The importance of the Six1-Eya interaction in human breast cancer is underscored by the finding that high levels of Six1 correlate with shortened time to relapse and metastasis as well as decreased survival only when co-expressed with high levels of Eya2. Overall, these data implicate Eya2 as a necessary co-factor for many of the metastasis promoting functions of Six1, suggesting that targeting the Six1-Eya interaction may inhibit breast cancer progression. As Six1 and Eya2 are not highly expressed in most adult tissues, the Six1-Eya interaction may be a valuable future therapeutic target whose inhibition would be expected to impair breast cancer progression while conferring limited side effects.

Sine oculis homeobox homolog 1 (SIX1) has been supposed to be correlated with the metastasis and poor prognosis of several malignancies. However, the effect of SIX1 on the metastatic phenotype of tumor cells and the underlying mechanisms were still unclear to date. Here we report that SIX1 can promote alpha5beta1-mediated metastatic capability of cervical cancer cells. SIX1 promoted the expression of alpha5beta1 integrin to enhance the adhesion capacity of tumor cells in vitro and tumor cell arrest in circulation in vivo. Moreover, higher expression of SIX1 in tumor cells resulted in the increased production of active MMP-2 and MMP-9, up-regulation of anti-apoptotic genes (BCL-XL and BCL2) and down-regulation of pro-apoptotic genes (BIM and BAX), thus promoting the invasive migration and anoikis-resistance of tumor cells. Importantly, blocking alpha5beta1 abrogated the regulatory effect of SIX1 on the expression of these genes, and also abolished the promotional effect of SIX1 on invasive capability of tumor cells. Furthermore, knock-down of alpha5 could abolish the promoting effect of SIX1 on the development of metastatic lesions in both experimental and spontaneous metastasis model. Therefore, by up-regulating alpha5beta1 expression, SIX1 not only promoted the adhesion capacity, but also augmented ECM-alpha5beta1-mediated regulation of gene expression to enhance the metastatic potential of cervical cancer cells. These results suggest that SIX1/alpha5beta1 might be considered as valuable marker for metastatic potential of cervical cancer cells, or a therapeutic target in cervical cancer treatment.

Six1 is a critical regulator of embryonic development that requires interaction with the Eya family of proteins (Eya1-4) to activate the transcription of genes involved in neurogenesis, myogenesis and nephrogenesis. Although expression of Six1 and Eya family members is predominantly observed in development, their overexpression is observed in numerous cancers. Importantly, both Six1 and Eya have independently been shown to mediate breast cancer metastasis, but whether they functionally interact during tumor progression has not been explored. Herein, we demonstrate that knockdown of Eya2 in MCF7 mammary carcinoma cells reverses the ability of Six1 to induce transforming growth factor-beta signaling, as well as to induce characteristics associated with epithelial-mesenchymal transition and cancer stem cells, suggesting that Six1 is dependent on Eya2 to mediate numerous pro-metastatic characteristics. The importance of the Six1-Eya interaction in human breast cancer is underscored by the finding that high levels of Six1 correlate with shortened time to relapse and metastasis as well as decreased survival only when co-expressed with high levels of Eya2. Overall, these data implicate Eya2 as a necessary co-factor for many of the metastasis promoting functions of Six1, suggesting that targeting the Six1-Eya interaction may inhibit breast cancer progression. As Six1 and Eya2 are not highly expressed in most adult tissues, the Six1-Eya interaction may be a valuable future therapeutic target whose inhibition would be expected to impair breast cancer progression while conferring limited side effects.

PURPOSE: To determine the prognostic value of the expression of Ezrin, CD44 and Six1 genes in osteosarcoma tissues of Chinese patients. METHODS: Fluorescent quantitative real-time PCR was applied to study the mRNA levels of Ezrin, CD44 and Six1 genes in 32 osteosarcoma patient samples and 10 adjacent normal tissues and MG63 osteosarcoma cell lines. The analysis of relationships between pulmonary metastasis and overall survival time were carried out based on the clinical data. RESULTS: mRNA levels of Ezrin and Six1 genes in osteosarcoma tissues were higher than those in adjacent normal tissues (P=0.015, 0.025). The mRNA levels of Ezrin, CD44 and Six1 genes were closely correlated with Enneking GTM clinical staging, while no correlations were demonstrated between the mRNA level of these genes with sex, age, location or pathological types. In addition, we demonstrated that the high mRNA level of Ezrin gene was related to shorter lung metastasis-free and overall survival time of the Chinese patients with osteosarcoma (P < 0.001). CONCLUSION: Our data suggest that Ezrin, but not CD44 and Six1, could be a prognostic factor and a predictor of potential lung metastasis in osteosarcoma. Further large sample studies need to be done to confirm the potential value of Ezrin as a new therapeutic target.