SOX4Induces Epithelial-Mesenchymal Transition In Human Mammary Epithelial Cells And Contributes To Breast Cancer Progression

Epithelial-mesenchymal transition （EMT） is a morphogenetic process in which cells losetheir epithelial characteristics and gain mesenchymal properties. EMT process involves aseries of changes: a change from characteristic cobblestone-like epithelial morphology to aspindle, fibroblast-like shape; a conversion of epithelial apical-basal polarity to front-backpolarity; a loss of the epithelial markers with concurrent acquisition of mesenchymal markers;and along with acquisition of migratory and invasive ability. EMT is a major mechanism fortissue remodeling during normal embryonic development. It is reported that EMT plays animportant role during wound healing, tissue regeneration and organ fibrosis. EMTs areclassified into Type1, Type2and Type3EMTs, the three different biological subtypes basedon the biological context in which they occur. Numerous observations of human tumours andexperimental animal models have provided convincing evidence for its physiologicalrelevance to both normal embryogenesis and carcinogenesis. Thus, similar to embryonicdevelopment, reactivation of an embryonic EMT program could be the underlying mechanismof tumor progression.Accumulating observations support the notion that EMT exerts a central role in tumorprogression. EMT programs have been associated with poor clinical outcome in multipletumor types. E-cadherin is the epithelial cell adhesion molecule. Functional loss of E-cadherinin epithelial cells has been considered a hallmark of EMT. Several developmentally importanttranscription factors have been reported to act as molecular triggers of the EMT program.These include direct transcriptional repressors of E-cadherin expression, such as Snail, Slug,ZEB1and ZEB2, whereas others, such as FOXC1, FOXC2, GSC and Twist, act less directlyon E-cadherin. Besides these transcriptional factors, multiple signaling pathways such asTGF-β, Wnt, Notch, Hedgehog and hypoxia can initiate an EMT programme. Emerging datasuggest extensive crosstalks among these signalling pathways and transcription factors,allowing them to form an intricate network that is responsible for establishing andmaintaining mesenchymal cell phenotypes. Recent studies have demonstrated that the EMTcan induce non-CSCs to enter into a CSC-like state. Triple-negative breast cancer （TNBC, ER-/PR-/HER2-） is a highly aggressive type of breast cancer. Recent studies have shown thatEMT significantly correlates with TNBC. Therefore, targeting the EMT program may be apromising therapeutic option for patients with TNBC.We here report that ectopic overexpression of SOX4in immortalized human mammaryepithelial cells was sufficient for acquisition of mesenchymal traits, enhanced cell migrationand invasion ability, along with mammary epithelial stem cell properties that defined by thepresence of a CD44^high/CD24^lowcell subpopulation. Furthermore, SOX4positively regulatedthe expression of known EMT inducers such as Snail, Twist and ZEB1, also activating theTGF-β pathway to contribute to EMT. SOX4itself was induced by TGF-β in mammaryepithelial cells and was required for TGF-β-induced EMT. Experiments using a mousexenograft model showed that SOX4cooperated with oncogenic Ras to promote tumorigenesisin vivo. Finally, we observed that SOX4was abnormally overexpressed and correlated withthe TNBC by using immunohistochemistry analysis of SOX4in clinical specimens of humanbreast cancer and by using the Oncomine analysis of the expression profile of SOX4in breastcancer microarray database.Data arising from this study find that SOX4is a novel EMT inducer and define animportant function for SOX4in the progression of breast cancer by orchestrating an EMTprogram. This study provides significant evidence for studying intricate regulating networkamong SOX4, EMT transcription factors and signalling pathways. Also, they implicate thepotential value of SOX4as a molecular marker for poor prognosis in human breast cancer.