Inhibition of Cellular Processing of Surfactant Protein C by Drugs Affecting Intracellular pH Gradients*

Abstract

Surfactant protein C (SP-C) is a hydrophobic protein synthesized and secreted exclusively by alveolar type II cells through proteolysis of a 21-kDa propeptide (SP-C₂₁) to produce the 3.7-kDa surface active form. Previous studies from this laboratory have demonstrated that early processing of proSP-C involves extensive intracellular proteolysis of the COOH terminus of proSP-C₂₁ in subcellular compartments, which include the acidic type II cell-specific subcellular organelle, the lamellar body. (Beers, M. F., Kim, C. Y., Dodia, C., and Fisher, A. B. (1994) J. Biol. Chem. 269, 20318-20328). The role of intracellular pH gradients in SP-C processing was studied in freshly isolated rat type II cells. Using vital fluorescence microscopy, the pH indicator acridine orange (AO) identified intense fluorescence staining of acidic cytoplasmic vesicles within fresh type II cells. The AO vesicular staining pattern was similar in cells labeled with the lamellar body marker phosphine 3R and the phospholipid dye nile red. AO fluorescence was quenched by the addition of a membrane-permeable weak base, methylamine. Immunoprecipitation of cell lysates with anti-proSP-C antisera following pulse-chase labeling (0–2 h) with ³⁵S-Translabel demonstrated rapid synthesis of ³⁵S-proSP-C₂₁ with a time-dependent appearance of 16- and 6-kDa intermediates (SP-C₁₆ and SP-C₆). Tricine polyacrylamide gel electrophoresis analysis of organic extracts of cell lysates showed time-dependent appearance of mature SP-C_3.7. The addition of 5 mM methylamine significantly blocked the post-translational processing of proSP-C resulting in disruption of normal precursor-product relationships and inhibition of SP-C_3.7 formation. Methylamine-treated cells exhibited slow accumulation of SP-C₁₆ and SP-C₆, a persistence of SP-C₂₁, and an absence of SP-C_3.7 for the duration of the chase period. The lysosomotropic agent chloroquine, the proton ionophore monensin, and bafilomycin A₁, a specific vacuolar H⁺-ATPase inhibitor, each caused inhibition of proSP-C processing in a similar manner. These results demonstrate that normal post-translational proteolysis of proSP-C occurs in acidic intracellular compartments, which include the lamellar body, and that complete processing to SP-C_3.7 is dependent upon maintenance of transmembrane pH gradients by a vacuolar H⁺-ATPase.