Tn antigen promotes breast cancer metastasis via impairment of CASC4

Breast cancer is one of the most serious and deadly cancers in women worldwide, with distant metastases being the leading cause of death. Tn antigen, a tumor-associated carbohydrate antigen, was frequently detected in breast cancer, but its exact role in breast cancer metastasis has not been well elucidated. Here we investigated the impact of Tn antigen expression on breast cancer metastasis and its underlying mechanisms. The expression of Tn antigen was induced in two breast cancer cell lines by deleting T-synthase or Cosmc, both of which are required for normal O-glycosylation. It showed that Tn-expressing cancer cells promoted epithelial-mesenchymal transition (EMT) and metastatic features as compared to Tn(-) control cells both in vitro and in vivo. Mechanistically, we found that cancer susceptibility candidate 4 (CASC4), a heavily O-glycosylated protein, was significantly downregulated in both Tn(+) cells. Overexpression of CASC4 suppressed Tn-induced activation of EMT and cancer metastasis via inhibition of Cdc42 signaling. Furthermore, we confirmed that O-glycosylation is essential for the functional role of CASC4 because defective O-glycosylated CASC4 (mutant CASC4, which lacks nine O-glycosylation sites) exerted marginal metastatic-suppressing effects in comparison with WT CASC4. Collectively, these data suggest that Tn-mediated aberrant O-glycosylation contributes to breast cancer metastasis via impairment of CASC4 expression and function.