Retinoid orphan nuclear receptor alpha (RORα) suppresses the epithelial-mesenchymal transition (EMT) by directly repressing Snail transcription

Retinoid orphan nuclear receptor alpha (ROR alpha) is a member of the orphan nuclear factor family and regulates gene expression by binding to ROR response elements (ROREs). ROR alpha has been identified as a potential tumor suppressor; however, how downregulation of ROR alpha promotes cancer progression is not fully understood. Here, we showed that protein levels of ROR alpha were downregulated during the Snail-, Twist-, or transforming growth factor-beta-induced epithelial-mesenchymal transition (EMT). We found that silencing of ROR alpha induced expression of mesenchymal markers in MCF10A cells, accompanied by enhanced cell invasion, migration, and mammosphere formation. Furthermore, ectopic expression of ROR alpha suppressed transforming growth factor-beta-induced EMT processes in MCF10A and HMLE cells. These results indicate that downregulation of ROR alpha is crucial for the induction of EMT in mammary epithelial cells. By analyzing gene expression profiles in control and ROR alpha-expressing cells, we also identified Snail, a key regulator of EMT, as a potential target of ROR alpha. We show that RORa expression significantly inhibits Snail transcription in breast cancer cells. Chromatin immunoprecipitation analysis demonstrated that ROR alpha bound to the ROREs in promoter region of SNAI1 gene, and using the luciferase reporter assay, we showed that binding to the ROREs was critical for ROR alpha to repress Snail transcription. Finally, rescue experiments substantiated that Snail mediates ROR alpha function in suppressing EMT and mammosphere formation. These results reveal a novel function of ROR alpha in suppressing EMT and identify Snail as a direct target of ROR alpha in mammary epithelial cells.