Trophoblast cell surface antigen-2 phosphorylation triggered by binding of galectin-3 drives metastasis through down-regulation of E-cadherin.

The expression of trophoblast cell surface antigen-2 (Trop-2) is enhanced in many tumor tissues and is correlated with increased malignancy and poor survival of patients with cancer. Previously, we demonstrated that the Ser-322 residue of Trop-2 is phosphorylated by protein kinase Cα (PKCα) and PKCδ. Here, we demonstrate that phosphomimetic Trop-2 expressing cells have markedly decreased E-cadherin mRNA and protein levels. Consistently, mRNA and protein of the E-cadherin-repressing transcription factors zinc finger E-Box binding homeobox 1 (ZEB1) were elevated, suggesting transcriptional regulation of E-cadherin expression. The binding of galectin-3 to Trop-2 enhanced the phosphorylation and subsequent cleavage of Trop-2, followed by intracellular signaling by the resultant C-terminal fragment. Binding of β-catenin/transcription factor 4 (TCF4) along with the C-terminal fragment of Trop-2 to the ZEB1 promoter upregulated ZEB1 expression. Of note, siRNA-mediated knockdown of β-catenin and TCF4 increased the expression of E-cadherin through ZEB1 downregulation. Knockdown of Trop-2 in MCF-7 cells and DU145 cells resulted in downregulation of ZEB1 and subsequent upregulation of E-cadherin. Furthermore, wild-type and phosphomimetic Trop-2 but not phosphorylation-blocked Trop-2 were detected in the liver and/or lung of some nude mice bearing primary tumors inoculated intraperitoneally or subcutaneously with wild-type or mutated Trop-2 expressing cells, suggesting that Trop-2 phosphorylation, plays an important role in tumor cell mobility in vivo, too. Together with our previous finding of Trop-2 dependent regulation of claudin-7, we suggest that the Trop-2-mediated cascade involves concurrent derangement of both tight and adherence junctions, which may drive metastasis of epithelial tumor cells.