Surfactant proteins in the digestive tract, mesentery, and other organs: evolutionary significance

For years, the so-called surfactant proteins (SPs) that were discovered in the phospholipid-rich material designated pulmonary surfactant, were consid ered to be lung-specific. The fact that surfactant-like materials composed of phospholipids are secreted by a number of other organs recently prompted several groups to search for SP expression in these organs also. The hydro philic proteins SP-A and SP-D and their transcripts have been found in a nu mber of tissues, including gastric and intestinal mucosae, mesothelial tiss ues (mesentery, peritoneum, and pleural), synovial cells, Eustachian tube a nd sinus. and possibly in salivary glands, pancreas, and urinary tract. By contrast, the hydrophobic proteins SP-B and SP-C actually appear to be expr essed in lung epithelium only. SP-A and SP-D belong to the innate defence s ystem against pathogens and play a role as opsonins for facilitating phagoc ytosis. Their expression appears as a general feature of organs exposed to pathogens because they present an interface with the external milieu. Altho ugh this function has thus far been investigated in the lung only through t he gene-targeting approach, increased expression of SP-A in the infected mi ddle ear and of SP-D in the Helicobacter-infected antrum argues for such a function also in other organs. In organs that are not exposed to external p athogens, their role is likely to exert anti-inflammatory and immunomodulat ory functions, as suggested by increased SP-A immunoreactivity in rheumatoi d disease. SP-A and SP-B have been found in association with phospholipids in the lung of all air-breathing vertebrates, including the most primitive forms represented by lungfish. which implies that the surfactant system had a single evolutionary origin. Immunochemical proximity of the proteins amo ng vertebrates indicates considerable conservation during evolution. Moreov er, the finding of an SP-A-like protein in intestine and swim bladder of ac tinopterygian fish implies that the ancestral form of the protein was alrea dy present before the emergence of lung structures. (C) 2001 Elsevier Scien ce Inc. Ail rights reserved.