β-Catenin, a Sox2 binding partner, regulates the DNA binding and transcriptional activity of Sox2 in breast cancer cells

Sox2, an embryonic stem cell marker, has been recently implicated in the pathogenesis of breast cancer (BC). Using liquid chromatography-mass spectrometry and co-immunoprecipitation, we identified beta-catenin as a Sox2 binding partner in MCF7 cells. The interaction between Sox2 and beta-catenin was substantially different between the two cell subsets separated based on their differential responsiveness to a Sox2 reporter. Specifically, while beta-catenin binds to Sox2 in the nuclear fraction of cells showing reporter-responsiveness (i.e. RR cells), this interaction was not detectable in those that were reporter-unresponsive (i.e. RU cells). In RR but not in RU cells, siRNA knockdown of beta-catenin significantly upregulated the Sox2 transcriptional activity, enhanced its DNA binding and increased the expression of its target genes. Correlating with these findings, while inhibition of beta-catenin significantly downregulated the mammosphere formation efficiency in RU cells, this treatment paradoxically increased that of RR cells. To conclude, we identified that beta-catenin is an important binding partner of Sox2 and a regulator of its transcriptional activity in a small subset of BC cells. The interaction between Sox2 and beta-catenin provides a novel mechanism underlying the functional dichotomy of BC cells, which carries potential therapeutic implications. (C) 2013 Elsevier Inc All rights reserved.