Abstract 2705: Novel functions of the homeoprotein SIX2 in mediating anchorage independence and metastasis in breast cancer.

Abstract

Abstract Homeobox genes encode for transcription factors that are master regulators of embryogenesis, and their misexpression has been implicated in multiple cancers. The role of the homeoprotein Six2 in developing kidney has been well demonstrated; however, its role in cancer progression is largely unknown. Here, we demonstrate, for the first time, that Six2 is causally involved in mammary tumor progression. When Six2 is knocked down (KD) in the 66cl4 mammary carcinoma cells, lung metastasis is significantly decreased compared to control KD; however, Six2 KD conferred no significant effect on growth of the primary tumor or on tumor-associated angiogenesis/lymphangiogenesis, in contrast to its closely related family member, Six1, which has been implicated in all the aforementioned properties; suggesting that Six2 may participate in later stages of the metastatic cascade. Expression of Six2 in the 4TO7 mammary carcinoma cell line (a cell line that is syngeneic with 66cl4, but expresses very low levels of endogenous Six2) led to changes in cell morphology, increased growth in soft agar, increased resistance to anoikis, and significantly enhanced lung metastasis in Balb/c mice. To determine the mechanism by which Six2 mediates metastasis, microarray analysis was performed on 4TO7-control and Six2 expressing cells. Interestingly, genes which have been implicated in lung metastasis (MMP, ANGPTL4, VCAM1) are significantly up-regulated in Six2 overexpressing 4TO7 cells; while the epithelial marker, E-Cadherin, is dramatically decreased. Finally, analysis of SIX2 expression from public microarray datasets indicates that SIX2 is increased in breast cancers compared to normal breast tissue. In addition, high expression of SIX2 correlates with poor prognosis (distant metastasis free survival, overall survival and relapse free survival) in 1881 human breast tumors examined using the GOBO (Gene Expression-Based Outcome for Breast Cancer Online) database, and upon further investigation we found that SIX2 expression is particularly associated with poor prognosis in luminal A, normal-like and ER-positive breast tumors. Together, our studies define a novel role of Six2 in breast cancer metastasis. Citation Format: Chu-An Wang, Paul Jedlicka, Vadym Zaberezhnyy, Aik-Choon Tan, Heide Ford. Novel functions of the homeoprotein SIX2 in mediating anchorage independence and metastasis in breast cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2705. doi:10.1158/1538-7445.AM2013-2705