Surfactant/SP-B [1B9]

ProSP-B is glycosylated in the Golgi apparatus and undergoes carboxy- and amino-terminal proteolysis by a cathepsin D-like protease. Pulmonary surfactant is a complex mixture of phospholipids and proteins that is secreted from type II cells in alveoli and reduces the surface tension at the alveolar air-liquid interface, providing alveolar stability necessary for normal ventillation. Four distinct proteins isolated from pulmonary surfactant are termed surfactant proteins A, B, C, and D. SP-A (28-36kDa) and SP-D (43kDa) are collagenous carbohydrate-binding proteins, whereas SP-B (8-9kDa) and SP-C (4kDa) are non-collagenous hydrophobic proteins. SP-B is expressed in pulmonary adenocarcinomas with acinar, papillary, bronchioloalveolar, and solid growth patterns. Squamous cell and large cell carcinomas of the lung and nonpulmonary adenocarcinomas do not express SP-B.