Analysis of the proteome of human airway epithelial secretions

Abstract

BackgroundAirway surface liquid, often referred to as mucus, is a thin layer of fluid covering the luminal surface that plays an important defensive role against foreign particles and chemicals entering the lungs. Airway mucus contains various macromolecules, the most abundant being mucin glycoproteins, which contribute to its defensive function. Airway epithelial cells cultured in vitro secrete mucins and nonmucin proteins from their apical surface that mimics mucus production in vivo. The current study was undertaken to identify the polypeptide constituents of human airway epithelial cell secretions to gain a better understanding of the protein composition of respiratory mucus.ResultsFifty-five proteins were identified in the high molecular weight fraction of apical secretions collected from in vitro cultures of well-differentiated primary human airway epithelial cells and isolated under physiological conditions. Among these were MUC1, MUC4, MUC5B, and MUC16 mucins. By proteomic analysis, the nonmucin proteins could be classified as inflammatory, anti-inflammatory, anti-oxidative, and/or anti-microbial.ConclusionsBecause the majority of the nonmucin proteins possess molecular weights less than that selected for analysis, it is theoretically possible that they may associate with the high molecular weight and negatively charged mucins to form a highly ordered structural organization that is likely to be important for maintaining the proper defensive function of airway mucus.

Airway Mucins: “Aircraft Carriers” in the Apical Surface Fluid

“…Later, Ali et al [19] identified 56 proteins in a "mucin" fraction isolated under physiological conditions from primary human tracheobronchial epithelial cells grown in air/liquid interface, supporting the previous report [11] that airway mucins are tightly associated with various molecules. Proteomic analysis of the 56 proteins included not only mucins but also many functionally active proteins, including antimicrobial, anti-proteolytic, anti-oxidative, and anti-inflammatory proteins [19]. This finding is highly significant because it had been thought that the complex structure of "mucins" itself was responsible for their multifaceted properties that are necessary for host defense against inhaled harmful substances, including anti-microbial, anti-proteolytic, and anti-oxidative activities [20].…”

“…Later, Ali et al [19] identified 56 proteins in a "mucin" fraction isolated under physiological conditions from primary human tracheobronchial epithelial cells grown in air/liquid interface, supporting the previous report [11] that airway mucins are tightly associated with various molecules. Proteomic analysis of the 56 proteins included not only mucins but also many functionally active proteins, including antimicrobial, anti-proteolytic, anti-oxidative, and anti-inflammatory proteins [19].…”

NEU1 Sialidase Regulates Membrane-tethered Mucin (MUC1) Ectodomain Adhesiveness for Pseudomonas aeruginosa and Decoy Receptor Release

“…8, A and B). Again, given the dilutional effect inherent to the BAL procedure, combined with the known sequestration of a fraction of MUC1-ED within the fluid lining the bronchoalveolar compartment (38), these data likely underestimate the true levels of shed MUC1-ED in the BALF from P. aeruginosa-colonized patients.…”

Analytical constraints for the analysis of human cell line secretomes by shotgun proteomics

“…Recent shotgun proteomics reports revealed large catalogues of exoproteomes of several bacteria [11,12] and fungi [13] comprising a large list of items unsuspected from prior 2D-gel based studies. Regarding human proteomes, a large panel of secretomes have been described from myeloid cells [14], dendritic cells [15], stem cells [16], endothelial cells [17], astrocytes [18], and airway epithelial cells [19]. The description of specific cancer biomarkers being the aim of most secretome studies, more than 28 reports presented the catalogue of secreted proteins from cancer cell lines [5].…”