Biochemical characterization of soluble Tn glycoproteins from malignant effusions of patients with carcinomas

The Tn determinant (GalNAc-O-Ser/Thr) is one of the most specific human tumor markers. In normal cells Tn is a cryptic structure in the peptide core of mucin type O-glycoproteins, and it is detected in an unmasked form in most human carcinomas evaluated by immunohistochemistry. Scarce data are available regarding the characteristics of soluble Tn bearing glycoproteins. We herein report the first comparative characterization of soluble Tn glycoproteins derived from different kinds of human tumors (breast, colon, gastric, ovarian and liver). Considerable heterogeneity was observed in the physicochemical properties of Tn soluble glycoproteins from all the tumor-associated effusions evaluated. In SDS-PAGE analysis Tn glycoproteins from liver and colon effusions migrated as a broad single major component (>500 kDa), while several components of >200 kDa were identified in samples from breast, ovarian, and gastric cancer. The results of perchloric acid (PCA) treatment and CsCl gradient ultracentrifugation indicated that the Tn glycoproteins in effusion fluids correspond predominantly to mucin-like glycoproteins. However, in samples from patients with colon and liver cancer, a fraction of Tn glycoproteins formed part of the immune complexes that precipitated in PCA, suggesting that the anti-Tn immune response in vivo could modify their physicochemical properties. The four apomucins evaluated (MUC1, MUC2, MUC5AC and MUC6) carried Tn epitopes in each of the effusions, indicating that soluble apomucin detection may reflect the abnormal expression of MUC genes inherent to these tumors. Taking together, these results indicate that apomucin expression profile is responsible, at least in part, for the high heterogeneity of soluble Tn glycoproteins, and suggest that the identification of Tn determinant on the different soluble apomucins could be useful for the development of new diagnostic tools as well as to evaluate the anti-tumor immune response in patients with cancer.