“Hydrophobic” Surfactant Apoproteins and Augmentation of Phospholipid Recycling

Structural and metabolic consequences of liposome-lipoprotein interactions

The two major proposed uses for liposomes, i.e., drug delivery and mobilization of peripheral deposits of cholesterol, each impose requirements and restrictions on liposomal structure, particularly as it affects interactions with lipoproteins. This chapter focuses on the role of lipoproteins and apolipoproteins in (1) disrupting membrane structure and causing the leakage of liposomal contents by inducing disc formation and (2) marking liposomes for whole-particle uptake by receptors involved in lipoprotein metabolism. Control of membrane stability and whole-particle half-life can be achieved by several strategies, such as membrane stiffening, shielding the membrane surface, and increasing the dose or predosing with "empty" liposomes. The rationales and applicabilities of these strategies are discussed in the contexts of liposomes as drug delivery vehicles and as antiatherogenic particles. Directions for further basic and applied research are also presented.

The stimulation of cellular phospholipid uptake by surfactant apoproteins

Surfactant recycling by alveolar cells is influenced by the surfactant apoproteins SP-A, -B and -C (Wright, J.R. and Dobbs, L.G. (1991) Annu. Rev. Physiol. 53, 395-414). Alveolar macrophages and type II cells, but not lung fibroblasts, were reported to accumulate surfactant phospholipid in the presence of SP-A in low calcium medium, although high affinity binding of SP-A to alveolar macrophages and type II cells showed an absolute requirement for mM calcium. SP-B, one of two very hydrophobic surfactant proteins, stimulated phospholipid uptake by type II cells and Chinese hamster lung fibroblasts suspended in Dulbecco's minimum essential medium containing mM Ca2+. We postulated that calcium influences cellular phospholipid uptake stimulated by SP-A or SP-B. We used isolated rat alveolar and peritoneal macrophages and Vero cells, an African Green Monkey kidney fibroblast cell line, and studied the effect of calcium concentrations ranging from 2 microM to 2 mM on cellular uptake of liposomes containing 3H-labeled phosphatidylcholine. For alveolar and peritoneal macrophages, increasing calcium concentration enhanced SP-A stimulation of phospholipid uptake. SP-A did not stimulate phosphatidylcholine uptake by Vero cells. SP-B stimulated phosphatidylcholine uptake by alveolar and peritoneal macrophages and Vero cells independent of the calcium concentration. These studies demonstrate that the enhancement of phospholipid uptake in alveolar and peritoneal macrophages by SP-A, but not SP-B is augmented by calcium.

The C-terminal domain of the pulmonary surfactant protein C precursor contains signals for intracellular targeting

We have studied the intracellular transport of the pulmonary surfactant SP-C precursor in vitro and in vivo. In order to monitor the route of the SP-C precursor, we constructed various C-terminally truncated forms of SP-C, which were tagged with a sequence derived from the C-terminus of the human c-myc gene (aa 409-419). Expression of these constructs under the control of the SV40 enhancer and the huMT-II promoter in stably transformed Chinese hamster ovary (CHO) cells revealed that the complete precursor molecule is localized mostly in vesicular structures, probably of lysosomal origin. The truncated precursor lacking the last 22 amino acids at the C-terminus (SP-C/Ctag), however, was restricted to the endoplasmic reticulum as shown by immunofluorescence, using antibodies directed against the tag-sequence, the lysosomal enzyme cathepsin D, the enzyme disulfide isomerase, and the Golgi zone. The intracellular localization was substantiated by subcellular fractionation analysis, suggesting that the last 22 amino acids are necessary for intracellular targeting. Furthermore, Triton X-114 extractions from CHO cells revealed a modification of the SP-C precursor. In vitro translation and pulse-chase experiments in the absence or presence of microsomes showed that the modification occurs post-translationally and in a time-dependent manner. Membrane association studies using an SP-C precursor lacking the mature peptide indicated that the modification is of hydrophobic nature but not a thioester-linked fatty acid.

Primary structure differences of human surfactant-associated proteins isolated from normal and proteinosis lung

The primary structures of human pulmonary surfactant-associated proteins SP-A, SP-B and SP-C isolated from lung lavage of patients with alveolar proteinosis exhibit significant differences from lung surfactant proteins isolated from lungs of healthy individuals. In contrast to SP-A from normal lungs, proteinosis SP-A was shown by SDS gel electrophoresis to contain large amounts of unreducibly cross-linked beta chains. Specific primary structure modifications of SP-C and SP-B proteins were established by direct molecular weight and structural analysis, using [252Cf]plasma desorption mass spectrometry (PD/MS) as the principal method. In comparison to normal lung surfactant SP-B, proteinosis SP-B showed a significantly increased molecular weight by approx. 500 Da for the unreduced protein dimer. SP-C proteins from normal lungs were identified to possess a bis-cysteinyl-5,6-(thioester)palmitoylated structure, and to contain a frayed N-terminus resulting in two sequences of 34 and 35 amino acid residues. In contrast, SP-C from proteinosis patients was modified by (i) partial or even complete removal of palmitate residues and (ii) additional N-terminal proteolytic degradation. These results indicate the presence of pathophysiological structure modifications, which are likely to occur in the alveolar space, and may lead to a reduced surfactant function.

Phospholipids co-isolated with rat surfactant protein C account for the apparent protein-enhanced uptake of liposomes into lung granular pneumocytes

Phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) were co-isolated with the low molecular weight rat surfactant-associated protein C (SP-C) of Mr approximately equal to 6,000. The contribution of these phospholipids to the incorporation of 3H-labeled phosphatidylcholine (PC) liposomes into rat alveolar type II cells stimulated by SP-C was examined. PG showed a concentration-dependent enhancement in the uptake of PC liposomes by the pneumocytes. PE alone had no effect but could inhibit the incorporation of liposomal PC stimulated by PG depending on the concentration of PG and the PG to PE ratio. SP-C augmented the cellular uptake of the PC liposomes only when the SP-C preparation had a protein to phospholipid ratio greater than 1 and a PG to PE ratio greater than 2. The results with the isolated SP-C could be reproduced using mixtures of PG and PE which reflected the phospholipid composition of the SP-C in the absence of SP-C protein. Thus, the ability of SP-C to stimulate liposomal PC uptake by rat type II cells could be accounted for by its phospholipid composition.

Current perspectives in assessment of fetal pulmonary surfactant status with amniotic fluid

In recent years, improvements in analytical methodology and clinical management of maternal-fetal diseases have altered the understanding of data from amniotic fluid analysis. Delays in phospholipid production or lung function are not currently reported in maternal diabetes. Fetal lung function following glucocorticoid therapy or premature membrane rupture is uncoupled from amniotic fluid phospholipid concentrations, which do not have the usual significance in these circumstances. Phosphatidylglycerol (PG) is present prior to the usual time it is detected by thin layer chromatography (TLC) methods, which vary in sensitivity for PG. Consequently, the significance of its "absence" is highly varied. These observations are discussed in light of the earlier methods and data, along with new perceptions of the functions of the individual phospholipids and apoproteins, the regulatory mechanism of surfactant production, and the relationship of amniotic fluid components to neonatal lung function.

No evidence for vitamin K-dependent carboxylation of canine surfactant apoproteins, 28-36 kDa

Recent research has shown that rat surfactant apoproteins (26-38 kDa) are vitamin K-dependent [Rannels, Gallaher, Wallin & Rannels (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 5952-5956]. We have investigated the effect of the vitamin K antagonist warfarin on this family of apoproteins in surfactant from dog lung. Our data suggest that warfarin does not interfere with synthesis and secretion of these proteins into dog lung surfactant. Abnormal surfactant apoproteins, produced in response to warfarin treatment of the dog, were also not found in lung surfactant. 4-Carboxyglutamic acid analysis of purified dog apoproteins also failed to detect the vitamin K-modification. When vitamin K-dependent 14C labelling of precursors of vitamin K-dependent proteins was carried out, fluorography of these precursors, when electrophoresed into SDS/polyacrylamide gels, revealed 14C-labelled proteins of apparent molecular mass 74, 46, 42, 34, 31 and 23 kDa. Antibodies produced against purified dog surfactant apoproteins recognized precursors of the surfactant apoproteins in lung microsomes but did not recognize any 14C-labelled carboxylase substrates. These precursors appeared on immunoblots with apparent molecular mass 29, 32, 33 and 50 kDa. Our data suggest that there are significant differences between this class of surfactant apoproteins in the rat and the dog.

Respiratory failure in mice caused by a hybridoma making antibodies to the 15 kDa surfactant apoprotein

Hybridoma cells were obtained by fusing spleen cells from mice, immunized against the 15 kDa porcine surfactant apoprotein, with a myeloma cell line. Adult mice were inoculated intraperitoneally with this hybridoma; mice that were not inoculated or were inoculated with myeloma cells served as controls. Lung-thorax compliance was measured at various intervals after inoculation. The animals were then killed for histologic-morphometric evaluation of alveolar air expansion, inflammatory reaction in the pulmonary parenchyma, and intraalveolar edema. In the hybridoma group, the mice developed respiratory failure 9 days after inoculation, with markedly reduced lung-thorax compliance, lung congestion, alveolar collapse, hemorrhagic pulmonary edema, and hyaline membranes. Morphometric data from the same animals showed reduced volume density of alveolar air, and increased volume densities of intraalveolar "fluid" (edema) and tissue components. These lung lesions are similar to those in the adult respiratory distress syndrome.

Isolation of protein components from rat lung lamellar bodies

Lamellar bodies isolated from 10% (w/v) rat lung homogenates by discontinuous sucrose gradient centrifugation were shown to contain variable amounts of adhering proteins. These contaminating proteins could be removed by either Sepharose 4B gel filtration or precipitation of the crude preparation at pH 11.5. Both purification methods yielded membrane preparations with a phospholipid-to-protein ratio of 10.0 mumol/mg. Nearly complete separation of lamellar body phospholipid and protein could be achieved upon application of the purified membranes to DEAE-cellulose in the presence of 0.2% (v/v) Triton X-100. Phospholipid analyses showed that 83% of total lipid phosphorus was recovered in phosphatidylcholine. In phosphatidylethanolamine, phosphatidylglycerol, phosphatidylserine and phosphatidylinositol recoveries amounted to 4, 8, 2 and 2%, respectively. Molecular mass determinations of the isolated protein component of lamellar bodies by means of SDS polyacrylamide gel electrophoresis and staining with Coomassie brilliant blue revealed the presence of three protein bands with molecular masses of 64, 33 and 31 kDa. Upon staining with silver a 16 kDa protein was also visible. Sephadex G-100 gel filtration showed only one protein peak corresponding to a molecular mass of 64 kDa when protein was assayed with Coomassie brilliant blue.

Histochemical characterization of an antigen specific for the great alveolar cell in the mouse lung

Previous papers reported on a specific antigenic marker for the great alveolar (type-II) cell of the mouse lung and described its recognition by a specific rabbit anti-adult mouse lung serum. In the present study light- and electron-microscopical immunohistochemistry on fixed mouse lung sections showed the presence of the marker on the alveolar surface. The antigenic determinants recognized by the antibody were further characterized by immunoblotting and immunoprecipitation studies after in vitro translation of mouse lung messenger RNA. Immunoblots of a surfactant-enriched pellet of a bronchoalveolar lavage fraction of mouse lung showed that the antibody reacted with surfactant-associated proteins having apparent molecular weights of about 27,000, 32,000, and 38,000 daltons in SDS gels. Immunoblots of mouse-lung homogenate revealed the presence of 27,000, 30,000, 39,000, and 41,000 daltons proteins, presumably also surfactant-associated proteins. Immunoprecipitation after in vitro translation of mouse-lung mRNA showed specific reactivity only with a 12,000 dalton polypeptide, a component of the cell marker we were unable to relate to surfactant. Our findings indicate that the 12,000 dalton component of the antigenic marker for the great alveolar cell is a polypeptide whose synthesis is a lung-specific process and that the immunoreaction of the larger and surfactant-associated components is due to post-translational modifications.

In vitro translation of rat lung surfactant apoprotein mRNA

Rat pulmonary surfactant contains apoproteins of molecular weights 38,000, 32,000, 26,000 and 10,000-12,000. The structural and metabolic interrelationships of these proteins are not clear as yet. In order to investigate if they arise from a single or multiple precursor protein (s), we isolated total poly(A)RNA from rat lungs, performed its translation in vitro in the presence of [35s]-methionine and reticulocyte lysate, immunoprecipitated the translation products with anti-rat surfactant antibody, and analyzed them by SDS-polyacrylamide gel electrophoresis and autoradiography. A single translation product of molecular weight 35,000 was detected. Since the antibody used in the immunoprecipitation recognizes the 38,000, 32,000 and 26,000 dalton proteins, it is concluded that at least these three proteins arise from the 35,000 dalton precursor by post-translational modifications.