Journal of Biological Chemistry
Volume 271, Issue 45, 8 November 1996, Pages 28128-28137
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Cell Biology and Metabolism
Synthesis and Intracellular Trafficking of Muc-1 and Mucins by Polarized Mouse Uterine Epithelial Cells*

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Mucins function as a protective layer rendering the apical surface of epithelial cells nonadhesive to a variety of microorganisms and macromolecules. 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. K., and Carson, D. D. (1995) Endocrinology 136, 3639-3647). In the present study, the kinetics of Muc-1 assembly, cell surface expression, release, and degradation were examined in polarized mouse UEC in vitro. Mucins were identified as the predominant glycoconjugates synthesized, apically expressed, and vectorially released in both wild-type and Muc-1 null mice. When mucins were released, greater than 95% were directed to the apical compartment. Approximately half of the cell-associated mucins lost during a 24-h period were found in the apical compartment. Vectorial biotinylation detected apically disposed, cell-surface mucin and indicated that at least 34% of these mucins are released apically within 24 h. This suggests that release of mucin ectodomains is part of the mechanism of mucin removal from the apical cell surface of UEC. The half-lives of total cell-associated mucins and Muc-1 were 19.5 ± 1 and 16.5 ± 0.8 h, respectively. Muc-1 represented approximately 10% of the [3H]glucosamine-labeled, cell-associated mucins. Studies of the kinetics of intracellular transport of Muc-1 indicated transit times of 21 ± 15 min from the rough endoplasmic reticulum to Golgi apparatus and 111 ± 28 min from the Golgi apparatus to the cell surface. Collectively, these studies provide the first comprehensive description of Muc-1 and mucin maturation, metabolism, and release by polarized cells, as well as defining a major metabolic fate for mucins expressed by UEC. Normal metabolic processing appears to be sufficient to account for the removal of Muc-1 protein during the transition of UEC to a receptive state.

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*

These studies were supported by National Institutes of Health Grant HD-29963 (awarded to D. D. C.) as part of the National Cooperative Program for Markers of Uterine Receptivity for nonhuman Blastocyst Implantation. The University of Texas M.D. Anderson Cancer Center Core Facilities for peptide synthesis and antibody generation are supported by National Institutes of Health-NCI Grant CA-16672. The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “dvertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.