Evidence for secretion of high molecular weight mucins by canine tracheal epithelial cells in primary culture: effects of select secretagogues in mucin secretion

Promoter of the canine tracheobronchial mucin gene

The mucin gene is up-regulated in diseases such as cystic fibrosis (CF) and asthma. To understand the mechanisms involved in transcriptional regulation of mucin gene expression we have characterized the region of the mucin gene up-stream of the transcriptional start site and analysed the cis-acting elements required for mucin promoter activity. We isolated clones from a dog genomic library containing the promoter region for the tracheobronchial mucin gene (TBM). The authenticity of the promoter was tested by nucleotide sequencing, primer extension analysis, electrophoretic mobility shift assay (EMSA) and reporter gene expression analysis. The canine TBM promoter is different from housekeeping gene promoters (as it is not rich in GC content and contains TATA- and CAAT-like sequences) and different from that of regulatory genes (because it contains many TATA- and CAAT-like sequences and multiple transcriptional initiation sites). Reporter gene analysis using canine TBM promoter-chloramphenicol acetyltransferase (CAT) fusion plasmids established the regions responsible for promoter activity and verified the positions of the major mucin transcriptional initiation sites. Reporter gene analysis also established that a region of the canine TBM promoter and first exon containing all of the transcriptional initiation sites is more active in mucin expressing cells (e.g. CT1 cells-immortalized canine tracheal epithelial cells, human CFT1 cells-immortalized tracheal epithelial cells from a CF subject, or HBE1 cells-immortalized tracheal epithelial cells from non-CF subject) than in mucin non-expressing cells (COS7, 3T3), suggesting cell specificity. The promoter region contained cAMP response element (CRE) sequences, and the TBM gene transcription was enhanced when cAMP analogs were added to transfected cells. EMSA indicated the presence of at least two DNA binding proteins in CT1 cells. This is the first report describing the characterization of a TBM gene promoter. The information obtained in the present studies will be valuable in understanding mucin gene regulation in normal and pathological conditions.

Molecular cloning of the carboxy terminus of a canine tracheobronchial mucin

A cDNA library constructed from canine tracheal mRNA was screened with polyclonal antiserum specific to canine tracheal apomucin (CTM-A). Eight antibody reactive clones were isolated and purified to clonality. One of the clones, designated pCTM-A, had a 1.7 kb insert and included a single open reading frame with a poly (A)+ tail. The amino acid composition of the encoded protein was consistent with that expected for CTM-A. The fusion protein produced by cloning the 1.7 kb insert in the pMALc expression vector reacted with the purified anti-apomucin CTM-A antibody. Also, polyclonal antibodies raised to the purified protein product encoded by pCTM-A reacted with deglycosylated CTM-A confirming that this clone does indeed code for apomucin CTM-A. This is the first report of a cDNA encoding the C-terminus of a canine tracheal mucin.