Proteomics of the upper airways : studies on a new lipopolysaccharide-binding protein, PLUNC

Sammanfattning: There is now significant interest in identifying, quantifying and characterizing the human proteome, and new powerful techniques (proteomics) have evolved to deal with this giant task. In the present study, proteomics have been applied for the first time to map the proteins of the upper airways. The protein contents of human nasal fluid (NLF) and saliva were analysed using two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) and the proteins were identified by peptide mass fingerprinting using matrix assisted laser desorptioniionization time of night mass spectrometry (MALDI-TOF MS) or by amino acid sequencing using electrospray ionization tandem mass spectrometry (ESI- MS/MS). More than 100 proteins were identified and protein maps of nasal fluid and saliva were thus established. Of particular interest was the identification of a new lipopolysaccharide (LPS)-binding protein, PLUNC (palate lung and nasal epithelial clone), which was shown to be the only protein in NLF that binds to LPS. PLUNC was characterized as multiple isoforms (Mr/p1: 27/5.1, 26/5.2, 25/5.3, 27.5/5.1, 27/5.2, 26/5.3, 25.1/5.5 and 24.8/5.4), and several of these isoforms were demonstrated to be sialylated. Notably, decreased levels of PLUNC were found in NLF of (i) smokers, (ii) epoxy workers with airway irritation, and (iii) patients with seasonal allergic rhinitis (SAR) during allergy season. In addition, the levels of von Ebner's gland protein, α1-antitrypsin, cystatin S, Clara cell protein 16 and lipocortin-1 were altered, either in smokers or SAR patients or both. One previously unidentified NLF protein was found in SAR patients during allergy season but not before season: this protein was identified as eosinophil lysophospholipase. Many of these proteins were post-translationally modified by glycosylation (PLUNC, α1-antitrypsin, von Ebner's gland protein), phosphorylation (cystatin S), acetylation (eosinophil lysophospholipase), or truncation (lipocortin-1). Altogether, these findings illustrate the potential use of proteomics for identifying new markers of upper airway inflammation and for revealing structural details of such markers. The findings also indicate that allergic inflammation in the nasal mucosa is associated with decreased nasal fluid levels of the endogenous proteinase inhibitors, cystatin S and von Ebner's gland protein, and of the new irritation marker, PLUNC. Further studies are required to explore the possibility that PLUNC plays an important part in microbial  recognition and that this function is impaired after exposure to airway irritants and during upper airway inflammation.