Identification of ras and ras-related low-molecular-mass GTP-binding proteins associated with rat lung lamellar bodies

Proteomic analysis of lamellar bodies isolated from rat lungs

Background

Lamellar bodies are lysosome-related secretory granules and store lung surfactant in alveolar type II cells. To better understand the mechanisms of surfactant secretion, we carried out proteomic analyses of lamellar bodies isolated from rat lungs.

Results

With peptide mass fingerprinting by Matrix Assisted Laser Desorption/Ionization – Time of Flight mass spectrometry, 44 proteins were identified with high confidence. These proteins fell into diverse functional categories: surfactant-related, membrane trafficking, calcium binding, signal transduction, cell structure, ion channels, protein processing and miscellaneous. Selected proteins were verified by Western blot and immunohistochemistry.

Conclusion

This proteomic profiling of lamellar bodies provides a basis for further investigations of functional roles of the identified proteins in lamellar body biogenesis and surfactant secretion.

Phospholipid metabolism in lung surfactant

Pulmonary surfactant is a mixture of lipids, mostly phospholipids, and proteins that allows for breathing with minimal effort. The current chapter discusses the metabolism of the phospholipids of this material. Surfactant phospholipids are synthesized in the type II epithelial cells of the lung. The lipids and surfactant proteins are assembled in intracellular storage organelles, called lamellar bodies, and are subsequently secreted into the alveolar space. Within this extracellular space surfactant undergoes several transformations. First the lamellar bodies unravel to form a highly organized lattice-like lipid:protein structure tubular myelin. Second, the organized structures, in particular tubular myelin, adsorb to form a lipid at the air-liquid interface of the alveoli. It is, in fact, this surface tension reducing film that is responsible for the physiological role of surfactant, to prevent lung collapse and allow ease of inflation. Third, the surface film is converted to a small vesicular form. Finally, these small vesicles are taken-up by the type II cells for recycling and degradation and by alveolar macrophages for degradation.

Synexin and GTP increase surfactant secretion in permeabilized alveolar type II cells

We have previously suggested that synexin (annexin VII), a Ca(2+)-dependent phospholipid binding protein, may have a role in surfactant secretion, since it promotes membrane fusion between isolated lamellar bodies (the surfactant-containing organelles) and plasma membranes. In this study, we investigated whether exogenous synexin can augment surfactant phosphatidylcholine (PC) secretion in synexin-deficient lung epithelial type II cells. Isolated rat type II cells were cultured for 20-22 h with [(3)H]choline to label cellular PC. The cells were then treated with beta-escin, which forms pores in the cell membrane and releases cytoplasmic proteins including synexin. These cells, however, retained lamellar bodies. The permeabilized type II cells were evaluated for PC secretion during a 30-min incubation. Compared with PC secretion under basal conditions, the presence of Ca(2+) (up to 10 microM) did not increase PC secretion. In the presence of 1 microM Ca(2+), synexin increased PC secretion in a concentration-dependent manner, which reached a maximum at approximately 5 microg/ml synexin. The secretagogue effect of synexin was abolished when synexin was inactivated by heat treatment (30 min at 65 degrees C) or by treatment with synexin antibodies. GTP or its nonhydrolyzable analog beta:gamma-imidoguanosine-5'-triphosphate also increased PC secretion in permeabilized type II cells. The PC secretion was further increased in an additive manner when a maximally effective concentration of synexin was added in the presence of 1 mM GTP, suggesting that GTP acts by a synexin-independent mechanism to increase membrane fusion. Thus our results support a direct role for synexin in surfactant secretion. Our study also suggests that membrane fusion during surfactant secretion may be mediated by two independent mechanisms.

Lamellar body membrane turnover is stimulated by secretagogues

Lamellar bodies are specialized cellular organelles used for storage of surfactant by alveolar type II cells of the lung. We utilized monoclonal antibody (MAb) 3C9, which recognizes an integral lamellar body-limiting membrane protein of 180 kDa, to follow lamellar body trafficking. (125)I-labeled MAb 3C9 bound to the surface of type II cells and was internalized by the cells in a time- and concentration-dependent manner that was inhibitable by excess unlabeled antibody. The internalized antibody remained undegraded over a 4-h time period. The L2 rat lung cell line that does not have lamellar bodies did not bind iodinated 3C9. Exposure of type II cells to the secretagogues ATP, phorbol 12-myristate 13-acetate, and cAMP resulted in a 1.5- to 2-fold enhancement of binding and uptake of MAb 3C9. Calphostin C inhibited phorbol 12-myristate 13-acetate-stimulated phospholipid secretion and also reduced binding and uptake of MAb 3C9 by type II cells. Treatment of type II cells with phenylarsine oxide to obstruct clathrin-mediated endocytosis had no effect on the internalization of MAb 3C9 while markedly blocking the uptake of surfactant protein A and transferrin. An actin-mediated process was important for lamellar body membrane uptake because incubation with cytochalasin D partially inhibited MAb 3C9 incorporation by type II cells. These studies are compatible with enhanced lamellar body membrane turnover associated with surfactant secretion and indicate that this process can be monitored by the trafficking of the antigen reporter MAb 3C9.

Lovastatin induces apoptosis in malignant mesothelioma cells

Malignant mesothelioma causes profound morbidity and nearly universal mortality that is refractory to conventional treatment with aggressive surgery, radiotherapy, or chemotherapy. We report that pharmacologic concentrations of lovastatin, a 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase inhibitor, induced apoptosis in human malignant mesothelioma cell lines. Mesothelioma cell viability was decreased in a dose-dependent manner by lovastatin (5 to 30 microM). These effects were not reversed by exogenous growth factors or cholesterol, but were reversed by addition of 100 microM mevalonate, confirming that lovastatin affected mesothelioma viability by inhibiting mevalonate synthesis. Lovastatin appeared to decrease mesothelioma viability by inducing apoptosis, as indicated by morphologic changes, histologic evidence of nuclear condensation and degeneration, and flow-cytometric analysis of DNA content. Lovastatin's effects on cell viability were partially reversed in the presence of farnesol, and treatment of mesothelioma cells with a specific farnesyl-protein transferase (FTP) inhibitor decreased cell viability and induced morphologic changes indistinguishable from those caused by lovastatin. In addition, lovastatin-treated cells showed translocation of ras guanosine triphosphate (GTP)-binding proteins from membrane to cytosolic fractions on Western blots, suggesting that lovastatin's effects on mesothelioma were mediated in part by disrupting acylation of GTP-binding proteins. Thus, lovastatin is a commercially available and clinically well-tolerated agent that reduces viability and induces apoptosis of mesothelioma cells, and may provide the basis for adjunctive treatments of patients with mesothelioma.

Lamellar bodies of rat alveolar type 2 cells have late endosomal marker proteins on their limiting membranes

Alveolar type 2 cells are known to take up surfactant phospholipids and proteins from the alveolar space and recycle them into secretory organelles via a receptor-mediated endocytic pathway. To clarify the intracellular route(s) through which materials ingested by the cells are processed, we examined the immunocytochemical localization of late endosomal and lysosomal membrane markers, rab 7 and lamp 1 proteins, within rat alveolar type 2 cells. The limiting membranes of lamellar bodies (LBs) showed positive immunoreactivity for both proteins, whereas multivesicular bodies (MVBs) exhibited positive immunoreactivity only for lamp 1 protein on free vesicles in the MVB lumen. From these findings, it is suggested that LBs are not only secretory granules, but also constitute one of the late endosomal compartments of the cells and that MVBs of this cell type may be targeted to cell organelle(s) other than lysosomes.

Expression and localization of two low molecular weight GTP-binding proteins, Rab8 and Rab10, by epitope tag

Small GTP-binding proteins of the YPT/SEC4/Rab family have been shown to play an essential role in intracellular membrane trafficking. In mammals, Rab8 and Rab10 are the two small GTP-binding proteins identified so far that are closest to SEC4, an essential gene product involved in post-Golgi constitutive secretion in the yeast Saccharomyces cerevisiae. To study the localization of Rab proteins, we have expressed the cDNAs with an influenza virus hemagglutinin (HA) epitope tag at the N terminus. The feasibility of this method was tested by using yeast SEC4. HA-tagged SEC4 functionally complemented a temperature-sensitive sec4 mutant similarly to wild-type SEC4, indicating that the modified protein retained functional integrity. Monoclonal antibody 12CA5, raised against the HA tag, was used to determine the expression and localization of HA-tagged proteins after transfection. In stably transfected CHO and Swiss 3T3 cells, HA-tagged Rab8 was localized to the cell periphery, with the highest concentration in the ruffling areas. In contrast, epitope-tagged Rab10 expressed in CHO and BHK cells was concentrated on membranes in the perinuclear region. By light microscopy, the staining partially overlapped with that of a Golgi marker, beta-COP. Thus, despite the high degree homology of Rab8 and Rab10 (66% identity), the two proteins are localized to distinct cellular compartments. This approach should provide a general tool for the analyses of other members of the YPT/SEC4/rab gene family.