PULMONARY SURFACTANT PROTEIN-A (SP-A) IS EXPRESSED BY EPITHELIAL-CELLS OF SMALL AND LARGE-INTESTINE

Surfactant protein A (SP-A) is the most abundant protein associated wi th phospholipids in pulmonary surfactant, There are several lines of e vidence that pulmonary and gastrointestinal epithelium produce closely related surface-active materials, although the presence of SP-A in ga strointestinal tract has so far not been reported, Indirect immunofluo rescence experiments using different antibodies raised against rat pul monary SP-A showed that some jejunal and colonic but not gastric epith elial cells positively stained for SP-A. Analysis of the proteins in c ell lysates from rat small intestine and colon studied by Western blot revealed several immunoreactive bands, including the characteristic t riplet of 26-, 32-, and 38-kDa monomeric proteins, less strongly label ed than in lung cells, and higher molecular mass forms of 66 and 120 k Da also present in lung cells, The 66- and 120-kDa bands displayed the expected isoelectric pH of SP-A after two-dimensional electrophoresis . Alkylation induced conversion of the 120-kDa form (almost completely ) and the 66-kDa form (partly) into the 26-38-kDa monomeric species, T he presence of SP-A mRNA in rat stomach, small intestine, and colon wa s then searched for by conventional cDNA/reverse transcriptase-polymer ase chain reaction, Products of appropriate size (372 base pairs) iden tical to that of pulmonary tissue were amplified in small intestine an d colon but not in stomach or in other tissues used as controls. Cloni ng and sequencing of rat colon SP-A cDNA revealed the same sequence as the one reported for rat lung SP-A Furthermore, analysis of the trans criptional initiation site of SP-A gene in colon by anchored-polymeras e chain reaction showed that transcription was initiated at the same s ite in both colon and lung. These data, which demonstrate that small i ntestine and colon express SP-A constitutively and that this protein i s present in some epithelial cells, extend the concept of intestinal s urfactant and underline its close relationships to pulmonary surfactan t.