Surfactant protein B processing in human fetal lung

Surfactant protein B (SP-B-8), an 8-kDa hydrophobic protein essential for surfactant and normal lung function, is produced from the intracellular processing of preproSP-B. To characterize SP-B processing in human type 2 cells, we used human fetal lung in explant culture and polyclonal antibodies to human SP-BB (Phe(201)-Met(279)) and to specific epitopes within the NH2- and COOH-terminal propeptide domains (Ser(145)-Leu(160), Gln(186)-Gln(200) and Gly(284)-Ser(304)). Western blot analysis revealed a novel intermediate at similar to 9 kDa, representing mature SP-B-8, with a residual NH2-terminal peptide of similar to 10 amino acids. Pulse-chase studies showed a precursor-product relationship between the 9- and 8-kDa forms. During differentiation of type 2 cells in explant culture, the rate of proSP-B conversion to 25-kDa intermediate remained constant, whereas the rate of 25-kDa intermediate conversion to SP-B-8 increased, resulting in a net increase in tissue SP-B-8. Dexamethasone did not affect the rate of proSP-B processing but markedly enhanced the rate of SP-B-8 accumulation. We conclude that NH2-terminal propeptide cleavage of proSP-B is a multistep process and that more distal processing events are rate limiting and both developmentally and hormonally regulated.