RORα Suppresses Breast Tumor Invasion by Inducing SEMA3F Expression

Inactivation of tumor suppressors and inhibitory microenvironmental factors is necessary for breast cancer invasion; therefore, identifying those suppressors and factors is crucial not only to advancing our knowledge of breast cancer, but also to discovering potential therapeutic targets. By analyzing gene expression profiles of polarized and disorganized human mammary epithelial cells in a physiologically relevant three-dimensional (3D) culture system, we identified retinoid orphan nuclear receptor alpha (ROR alpha) as a transcription regulator of semaphorin 3F (SEMA3F), a suppressive microenvironmental factor. We showed that expression of ROR alpha was downregulated in human breast cancer tissue and cell lines, and that reduced mRNA levels of ROR alpha and SEMA3F correlated with poor prognosis. Restoring ROR alpha expression reprogrammed breast cancer cells to form non-invasiveness structures in 3D culture and inhibited tumor growth in nude mice, accompanied by enhanced SEMA3F expression. Inactivation of ROR alpha in nonmalignant human mammary epithelial cells inhibited SEMA3F transcription and impaired polarized acinar morphogenesis. Using chromatin immunoprecipitation and luciferase reporter assays, we showed that transcription of SEMA3F is directly regulated by ROR alpha. Knockdown of SEMA3F in ROR alpha-expressing cancer cells rescued the aggressive 3D phenotypes and tumor invasion. These findings indicate that ROR alpha is a potential tumor suppressor and inhibits tumor invasion by inducing suppressive cell microenvironment. Cancer Res; 72(7); 1728-39. (C)2012 AACR.