Clathrin-mediated endocytosis of MUC1 is modulated by its glycosylation state

MUC1 isa mucin-like type 1 transmembrane protein associated with the apical surface of epithelial cells. In human tumors of epithelial origin MUC1 is overexpressed in an underglycosylated form with truncated O-glycans and acc umulates in intracellular compartments. To understand the basis for this al tered subcellular localization, we compared the synthesis and trafficking o f various glycosylated forms of MUC1 in normal (Chinese hamster ovary) cell s and glycosylation-defective (ldlD) cells that lack the epimerase to make UDP-Gal/GaNAc from UDP-Glc/GlcNAc. Although the MUC1 synthesized in ldlD ce lls was rapidly degraded, addition of GalNAc alone to the culture media res ulted in stabilization and near normal surface expression of MUC1 with trun cated but sialylated O-glycans. Interestingly, the initial rate of endocyto sis of this underglycosylated MUC1 was stimulated by twofold compared with fully glycosylated MUC1. However, the half-lives of the two forms were not different, indicating that trafficking to lysosomes was not affected. Both the normal and stimulated internalization of MUC1 could be blocked by hyper tonic media, a hallmark of clathrin-mediated endocytosis. MUC1 endocytosis was also blocked by expression of a dominant-negative mutant of dynamin-1 ( K44A), and MUC1 was observed in both clathrin-coated pits and vesicles by i mmunoelectron microscopy of ultrathin cryosections. Our data suggest that t he subcellular redistribution of MUC1 in tumor cells could be a direct resu lt of altered endocytic trafficking induced by its aberrant glycosylation; potential models are discussed. These results also implicate a new role for O-glycans on mucin-like membrane proteins entering the endocytic pathway t hrough clathrin-coated pits.