Identification of a lipocalin in mucosal glands of the human tracheobronchial tree and its enhanced secretion in cystic fibrosis.

ABSTRACT

Members of the lipocalin protein family are characterized by their ability to bind small hydrophobic molecules. Some of them are known to be produced by various glands and secretory cells. Under certain conditions, these proteins would be ideally suited for clearance of lipophilic, potentially harmful substances and might also act as protection factors in airway secretions. We therefore used RT-PCR analysis with a set of oligonucleotide primers deduced from conserved regions of lipocalin members to identify specific RNA isolated from human trachea. With two of these oligonucleotide primers, a positive result was obtained. Sequencing of the RT-PCR products revealed that the DNA fragments were identical to the lipocalin 1 (LCN1) encoding cDNA. LCN1 is an unusual lipocalin member that binds a variety of lipophilic compounds and exhibits cysteine proteinase inhibitor and antimicrobial activities. The local production and topographic distribution of LCN1 in the human tracheobronchial tree was then investigated by immunoperoxidase staining on thin-layer sections using a specific antiserum. LCN1 was detectable in the acini of serous mucosal glands and sometimes within the glandular lumen, suggesting excretion of the protein. The latter finding was tested and verified by Western blot analysis of bronchial secretions of healthy individuals. Furthermore, the results of SDS-PAGE and Western blot analysis of bronchial secretions from patients with cystic fibrosis (CF), which are usually characterized by an increase of airway lipids, suggested that LCN1 secretion was enhanced. Northern blot analysis of RNA from normal trachea and RNA isolated from tracheal biopsies of patients with CF indicated that induced secretion was due to an up-regulated expression of the LCN1 gene. Thus, our investigations present the first clear evidence that LCN1 is induced in infection or inflammation and support the idea that this lipocalin functions as a physiologic protection factor of epithelia in vivo.