SYNTHESIS AND INTRACELLULAR TRAFFICKING OF MUC-1 AND MUCINS BY POLARIZED MOUSE UTERINE EPITHELIAL-CELLS

Mucins function as a protective layer rendering the apical surface of epithelial cells nonadhesive to a variety of microorganisms and macrom olecules. Muc-1 is a transmembrane mucin expressed at the apical cell surface of mouse uterine epithelial cells (UEC) that disappears as UEC become receptive for embryo implantation (Surveyor, G. A., Gendler, S . J., Pemberton, L., Das, S. K. Chakraborty, I., Julian, J., Pimental, R. A., Wegner, C. W., Dey, S. H., and Carson, D. D. (1995) Endocrinol ogy 136, 3639-3647). In the present study, the kinetics of Muc-1 assem bly, cell surface expression, release, and degradation were examined i n polarized mouse UEC in vitro. Mucins were identified as the predomin ant glycoconjugates synthesized, apically expressed, and vectorially r eleased in both wild-type and Muc-1 null mice. When mucins were releas ed, greater than 95% were directed to the apical compartment. Approxim ately half of the cell-associated mucins lost during a 24-h period wer e found in the apical compartment. Vectorial biotinylation detected ap ically disposed, cell-surface mucin and indicated that at least 34% of these mucins are released apically within 24 h. This suggests that re lease of mucin ectodomains is part of the mechanism of mucin removal f rom the apical cell surface of UEC. The half-lives of total cell-assoc iated mucins and Muc-1 were 19.5 +/- 1 and 16.5 +/- 0.8 h, respectivel y. Muc-1 represented approximately 10% of the [H-3]glucosamine-labeled , cell-associated mucins. Studies of the kinetics of intracellular tra nsport of Muc-1 indicated transit times of 21 +/- 15 min from the roug h endoplasmic reticulum to Golgi apparatus and 111 +/- 28 min from the Golgi apparatus to the cell surface. Collectively, these studies prov ide the first comprehensive description of Muc-1 and mucin maturation, metabolism, and release by polarized cells, as well as defining a maj or metabolic fate for mucins expressed by UEC. Normal metabolic proces sing appears to be sufficient to account for the removal of Muc-1 prot ein during the transition of UEC to a receptive state.