Sialomucin complex in the rat respiratory tract: a model for its role in epithelial protection

The pulmonary epithelium has a multitude of specialized functions, which depend on regulated growth and differentiation of several cell types. One such function is the synthesis and secretion of mucins, which offer the epithelium protection from and a means for removal of noxious environmental factors. Sialomucin complex (SMC) is a heterodimeric glycoprotein consisting of a mucin subunit (ASGP-1, ascites sialoglycoprotein-1) and a transmembrane protein (ASGP-2) with two epidermal-growth-factor-like domains. SMC was originally discovered in a highly metastatic rat mammary adenocarcinoma and has been implicated in metastasis and in the protection of the tumour cells from natural killer cells. It can also act as a ligand for the receptor tyrosine kinase 185^neu, suggesting that it is bifunctional as well as heterodimeric. SMC is expressed on the epithelium of rat conducting airways, with the highest levels occurring in the proximal trachea and progressively decreasing into the bronchioles. Airway SMC consists of two forms: a soluble form that lacks the C-terminal cytoplasmic and transmembrane domains and accounts for about 70% of the total, and a membrane-associated form that has the C-terminal domains. Immunocytochemical analyses show that SMC is predominantly present on the apical surfaces of the airway epithelium, but not in goblet cells. Soluble form can be removed from the trachea by rinsing, suggesting that a fraction of the protein is adsorbed to the apical surface. Based on these results, we propose a protective mechanism in which membrane and soluble forms of SMC are produced by airway luminal epithelial cells to provide a cell-associated epithelial glycoprotein barrier that also serves as an interface with flowing mucus. In support of this mechanism, we demonstrated secretion of soluble SMC by primary cultures of tracheal epithelial cells. This model suggests that SMC is a critical element in the protective barrier of the airway epithelium.