Induction of surfactant protein (SP-A) biosynthesis and SP-A mRNA in activated type II cells during acute silicosis in rats

The synthesis of the major surfactant protein, SP-A, was studied in activated alveolar type II cells isolated from the lungs of rats exposed to silica by intratracheal instillation. Exposure of rats to silica resulted in large increases in the levels of disaturated phosphatidylcholine and SP-A in the extracellular and intracellular surfactant compartments. Isolated type II cells were used to determine if the observed increases in SP-A were associated with increased SP-A synthesis. Type II cells were isolated by a combination of elastase digestion, centrifugal elutriation, and differential adherence on IgG-coated petri dishes. Type II cells from silica-treated lungs were separated into two populations, designated type IIA and type IIB. The type IIB, or activated population, consisted of type II cells that were larger than normal type II cells and, in addition, contained larger and more numerous lamellar bodies than normal type II cells. Type IIB cells contained 4.3-fold higher levels of SP-A compared to normal type II cells. SP-A synthesis was measured by incubating freshly isolated cells with [35S]Translabel (70% [35S]methionine, 15% [35S]cysteine) for up to 4 h in methionine-free medium, followed by immunoprecipitation of newly synthesized protein. The rate of SP-A synthesis was increased approximately 6.7-fold in the activated type II cells. Analysis of the newly synthesized protein by one-dimensional SDS-PAGE indicated three intracellular forms of SP-A with molecular weights of approximately 26,000, 30,000, and 34,000. In type II cells from control rats, the 34-kD protein accounted for approximately 93% of the newly synthesized SP-A after 4 h of incubation; only a small amount of radioactivity was associated with the lower molecular weight species. The increased biosynthesis of SP-A in the activated type II cells was associated with a 7.3-fold increase in the level of SP-A mRNA. These results indicate that the content and synthesis of SP-A are both highly elevated in activated type II cells and that these increases may be due to increased levels of SP-A mRNA.

Cis-acting sequences from a human surfactant protein gene confer pulmonary-specific gene expression in transgenic mice

Pulmonary surfactant is produced in late gestation by developing type II epithelial cells lining the alveolar epithelium of the lung. Lack of surfactant at birth is associated with respiratory distress syndrome in premature infants. Surfactant protein C (SP-C) is a highly hydrophobic peptide isolated from pulmonary tissue that enhances the biophysical activity of surfactant phospholipids. Like surfactant phospholipid, SP-C is produced by epithelial cells in the distal respiratory epithelium, and its expression increases during the latter part of gestation. A chimeric gene containing 3.6 kilobases of the promoter and 5'-flanking sequences of the human SP-C gene was used to express diphtheria toxin A. The SP-C-diphtheria toxin A fusion gene was injected into fertilized mouse eggs to produce transgenic mice. Affected mice developed respiratory failure in the immediate postnatal period. Morphologic analysis of lungs from affected pups showed variable but severe cellular injury confined to pulmonary tissues. Ultrastructural changes consistent with cell death and injury were prominent in the distal respiratory epithelium. Proximal components of the tracheobronchial tree were not severely affected. Transgenic animals were of normal size at birth, and structural abnormalities were not detected in nonpulmonary tissues. Lung-specific diphtheria toxin A expression controlled by the human SP-C gene injured type II epithelial cells and caused extensive necrosis of the distal respiratory epithelium. The absence of type I epithelial cells in the most severely affected transgenic animals supports the concept that developing type II cells serve as precursors to type I epithelial cells.

Lamellar bodies of cultured human fetal lung: content of surfactant protein A (SP-A), surface film formation and structural transformation in vitro

Lamellar bodies were isolated from dexamethasone and T3-treated explant cultures of human fetal lung, using sucrose density-gradient centrifugation. We examined their content of surfactant apoprotein A (SP-A), and their ability to form surface films and to undergo structural transformation in vitro. SP-A measured by ELISA composed less than 2% of total protein within lamellar bodies; this represented, as a minimum estimate, a 2-12-fold enrichment over homogenate. One- and two-dimensional gel electrophoresis also suggested that SP-A was a minor protein component of lamellar bodies. Adsorption of lamellar bodies to an air/water interface was moderately rapid, but accelerated dramatically upon addition of exogenous SP-A in ratios of 1:2-16 (SP-A:phospholipid, w/w). Similar adsorption patterns were seen for lamellar bodies from fresh adult rat and rabbit lung. Lamellar bodies incubated under conditions that promote formation of tubular myelin underwent structural rearrangement only in the presence of exogenous SP-A, with extensive formation of multilamellate whorls of lipid bilayers (but no classical tubular myelin lattices). We conclude that lamellar bodies are enriched in SP-A, but have insufficient content of SP-A for structural transformation to tubular myelin and rapid surface film formation in vitro.

Effect of a reconstituted basement membrane on expression of surfactant apoproteins in cultured adult rat alveolar type II cells

Pulmonary surfactant, which is composed of phospholipids and three lung-specific apoproteins, is synthesized and secreted by alveolar type II cells. Previous work from this laboratory (Biochim. Biophys. Acta 1987; 931:143-156) has shown that cell-extracellular matrix interactions and cuboidal cell shape affect both the ultrastructural appearance and pattern of phospholipids synthesized by cultured rat type II cells. In the present study, we have examined the effects of cell-matrix interactions and cell shape on the ability of adult rat type II cells to express the surfactant apoproteins in culture. Isolated adult rat type II cells were cultured for 2, 4, and 8 days on either tissue culture plastic, on an extract of the Engelbreth-Holm-Swarm (EHS) tumor, or on laminin-coated plastic dishes. Expression of surfactant proteins A, B, and C (SP-A, SP-B, and SP-C) was evaluated by Northern analysis using specific rat cDNA probes for these mRNAs. SP-A content was determined by enzyme-linked immunosorbent assay using a polyclonal antibody raised against rat SP-A purified from lavage. Type II cells cultured on plastic dishes assumed an attenuated morphology soon after being placed in culture. Except for an occasional positive signal on day 2 of culture, these cells were uniformly negative for the presence of mRNA for SP-A, SP-B, or SP-C. Type II cells cultured on plastic did not contain SP-A. In contrast, type II cells cultured on EHS gels formed three-dimensional aggregates on the surface of the substratum; these aggregates were composed of polarized cells that had their apical surfaces directed inward. Type II cells cultured on this substratum showed a positive signal for mRNA for all three surfactant proteins; the abundance of these mRNAs, however, was significantly below that seen in freshly isolated type II cells. While the abundance of mRNA for SP-A and SP-B steadily increased with time in culture under these conditions, the abundance of SP-C mRNA decreased, suggesting that SP-C is regulated independently of SP-A and SP-B. These cultures were also positive for SP-A content, which increased with increasing time in culture. Type II cells cultured on laminin-coated dishes initially spread more slowly across the culture surface than cells on plastic, but were extremely attenuated by day 8 in culture. These cells contained neither SP-A, nor mRNA for any of the three surfactant proteins.(ABSTRACT TRUNCATED AT 400 WORDS)

Interferon-gamma and synthesis of surfactant components by cultured human fetal lung

We examined the effects of interferon-gamma (IFN-gamma) on development of the surfactant system in alveolar epithelial cells of fetal lung. Explants of second-trimester human fetal lung were cultured for 1 to 6 days in serum-free medium containing recombinant human IFN-gamma (0.03 to 30 ng/ml) and/or dexamethasone (10 or 100 nM). Treatment for 3 days with IFN-gamma alone, dexamethasone alone, and IFN plus dexamethasone increased the content of surfactant protein A (SP-A, 28 to 36 kD) by approximately 3-, 2.5-, and 10-fold, respectively. The biphasic response pattern of SP-A to dexamethasone (stimulation initially and inhibition with continued culture) was not altered by the presence of IFN-gamma. IFN-gamma also stimulated accumulation of SP-A mRNA (2.7-fold at 24 h) but did not affect the levels of mRNAs for surfactant protein B (18 kD) and surfactant protein C (5 kD). To assess the effect of IFN-gamma on synthesis of surfactant lipids, we determined the content of phosphatidylcholine, the rate of labeled choline incorporation into phosphatidylcholine, saturation of newly synthesized phosphatidylcholine, and the activity of fatty acid synthetase, a glucocorticoid-inducible enzyme. Treatment of explants for 5 days with IFN-gamma had no effect on these parameters. Studies by light and electron microscopy revealed little difference between control and IFN-treated explants with regard to cell viability and epithelial cell differentiation. We conclude that IFN-gamma has a selective stimulatory effect on SP-A among surfactant components.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogeny of surfactant apoproteins in the rat

Content of the 26-38-kD surfactant apoprotein (SP-A) was determined in lung homogenates from fetal (17-21 d gestation), postnatal (1-28 d of life), and adult male and female rats by a double sandwich ELISA. Expression of mRNA for SP-A as well as the hydrophobic apoproteins, SP-B and SP-C, were also determined in lung homogenates from fetal and adult rats of both sexes by Northern blot analysis. SP-A was undetectable in fetal lungs on d 17 (day of birth = d 22) and barely detectable on d 18. On d 19 there was a 3- to 4-fold increase in SP-A content above d 18 levels. Between d 19 and 21 SP-A content significantly increased another 6- to 9-fold. SP-A content on the day of birth was not significantly different from that seen on gestational d 21. SP-A content decreased 35-40% between the day of birth and postnatal d 7. After the second postnatal week SP-A content gradually increased, reaching adult levels after d 28. No sex differences in SP-A content were observed during fetal or postnatal lung maturation. SP-A mRNA was first detected in fetal lungs on d 18 and increased in relative abundance until d 21, but remained below adult levels. Developmental changes in fetal lung SP-A content closely paralleled changes in fetal expression of SP-A mRNA. SP-B mRNA was also first detected on d 18, then increased in relative abundance to adult levels by d 20. SP-C mRNA was clearly detectable on d 17, then increased in relative abundance to adult levels by d 20-21. Unlike surfactant phospholipids, there are no apparent sex differences in the expression of any of the surfactant apoproteins during late gestation. The differences observed during fetal lung maturation in the time of onset and changes in relative abundance among the three apoprotein mRNA imply that their genes may be differentially regulated in the developing rat lung.

Increased synthesis and mRNA of surfactant protein A in oxygen-exposed rats

Surfactant protein A (SP-A) is an abundant glycoprotein in surfactant that is synthesized and secreted by alveolar type II cells and likely has important roles in mediating surfactant function and metabolism. In the present study, we demonstrate that exposure to 85% oxygen increased alveolar lavage and lung SP-A, and that these increases were related to increased SP-A synthesis and mRNA. Adult rats were exposed to room air or to 85% oxygen for 3, 5, or 7 days. Continuous exposure to hyperoxia progressively increased SP-A content, with a 20-fold increase in alveolar lavage and a 10-fold increase in lung SP-A content observed after 7 days. SP-A-specific mRNA increased in the lungs of rats exposed to oxygen, occurring with a time course similar to the increase in tissue SP-A. SP-A mRNA was increased 7-fold after 7 days of oxygen exposure. Synthesis of SP-A was increased 2- to 3-fold and secretion was increased 6- to 7-fold by type II epithelial cells isolated from oxygen-exposed rats. We conclude that exposure to hyperoxia increased lung and alveolar SP-A pool sizes. Increased expression of SP-A was related, at least in part, to increased SP-A mRNA and increased SP-A synthesis and secretion by type II epithelial cells.

Processing of hydrophobic pulmonary surfactant protein B in rat type II cells

The amino acid sequence of surfactant protein B (SP-B), derived from human genomic and cDNA sequences, indicates that the active polypeptide is contained within the sequence of a preproprotein of 381 residues. Synthesis of mature SP-B, which requires proteolytic processing at both the NH2- and COOH-termini of the proprotein, was studied in primary cultures of rat alveolar type II epithelial cells. Type II cells were pulse labeled with [35S]methionine-cysteine for 15-30 min and chased for 0-18 h. SP-B proprotein (Mr = 42,000) accumulated in the medium at early time points but declined at later time points suggesting extracellular proteolysis of the proprotein. In contrast, surfactant protein A (SP-A), another surfactant protein, continued to accumulate extracellularly during this time period. A proteolytic fragment of SP-B (Mr = 25,000) accumulated in the medium with a slightly slower time course, consistent with extracellular proteolysis of the proprotein. Intracellular processing of SP-B was also detected. SP-B polypeptides of Mr 8,000 and 12,000 were detected intracellularly and in the medium at late time points. These forms of SP-B (Mr = 8,000 and 12,000) comigrated in two-dimensional isoelectric-focusing sodium dodecyl sulfate-polyacrylamide gel electrophoresis with mature SP-B isolated from rat alveolar lavage fluid. The mature form of SP-B (Mr = 8,000), but not the Mr 42,000 and 25,000 SP-B precursors, was also detected in lamellar bodies isolated from rat lung homogenates. These experiments demonstrate complete proteolytic processing of prepro-SP-B to the alveolar Mr 8,000 form by type II epithelial cells in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Postnatal stimulation of rat surfactant protein A synthesis by dexamethasone

The effects of postnatal dexamethasone treatment in vivo on the synthesis of surfactant protein A (SP-A) were examined at the protein and RNA levels. Rats ranging from 1 day old to adult were injected with 200 micrograms of dexamethasone/kg body wt or with vehicle alone and were killed 24 h after injection. One portion of the lung was metabolically labeled with [35S]methionine, the proteins immunoprecipitated using an antiserum to SP-A, and analyzed electrophoretically. Both newly synthesized intracellular and secreted SP-A levels were increased by dexamethasone, reaching averages of 2.3 and 4.5 times control values, respectively. Another portion of the lung tissue was used for RNA analysis. SP-A mRNA levels were also elevated an average of 1.4 times control values by hormone treatment. Dose-response experiments using 16-day-old pups showed that both total SP-A, as measured by enzyme-linked immunosorbent assay, and total SP-A mRNA levels were elevated with dexamethasone treatment, reaching maximal stimulation at 2 mg. We conclude that postnatal dexamethasone treatment in vivo results in increased levels of both newly synthesized SP-A and SP-A mRNA, suggesting that pretranslational events may in part contribute to this process.

In vitro translation, post-translational processing and secretion of pulmonary surfactant protein B precursors

Surfactant proteolipid SP-B is a hydrophobic protein of Mr = 8000 identified in organic solvent extracts of pulmonary surfactant. Analysis of the human SP-B RNA predicts that the active surfactant peptide is derived by proteolysis of an Mr = 40,000 precursor. In the present work, characteristics of synthesis, secretion and processing of SP-B were demonstrated in a pulmonary adenocarcinoma cell line by immunoprecipitation of radiolabelled precursors. Treatment of cells with tunicamycin resulted in synthesis and secretion of unglycosylated proSP-B of Mr = 39,000. Immunoprecipitation of protein produced by in vitro translation of human lung poly(A)+ RNA detected an Mr = 40,000 protein; the size discrepancy is likely related to cleavage of a leader signal sequence. Endoglycosidase-H-sensitive precursors of Mr = 41,000-43,000, pI = 5.1-5.4 were the first isoforms detected within the cells and were processed to endoglycosidase-H-resistant isoforms and secreted. Neuraminidase and endoglycosidase-F-sensitive forms of proSP-B were first detected in the media at 60 min as Mr = 42-46,000 isoforms with pI = 4.6-5.1. Proteolytically processed isoforms of proSP-B were detected primarily in the media and were generated by cleavage of an amino-terminal Mr = 16,000 peptide resulting in Mr = 27,000-33,000 isoforms (pH = 5.6-6.8). The Mr = 27,000-33,000 isoforms were sensitive to neuraminidase, resulting in isoforms with pH = 6.0-6.8. Digestion of the Mr = 27,000-33,000 peptide with endoglycosidase-F resulted in isoforms of Mr = 23,000, pH = 6.0-6.8. The endoglycosidase-F-resistant peptide of Mr = 16,000, pI = 4.2-4.4 was identified with an antiserum generated against synthetic peptides derived from the amino-terminal domain, as deduced from the SP-B DNA sequence. Further proteolytic processing of the Mr = 27,000-33,000 isoforms to the Mr = 8000 peptide detected in surfactant was not observed in this cell line. Thus, in the H441-4 cells (a cell line with morphologic features of Clara cells), SP-B is synthesized as a preproprotein which undergoes cleavage of a signal sequence and addition of asparagine-linked carbohydrate; proSP-B is secreted by processes which are independent of glycosylation. SP-B peptides of Mr = 27,000-33,000 and Mr = 16,000, representing carboxy and amino-terminal domains, accumulate in the media.

Glucocorticoids both stimulate and inhibit production of pulmonary surfactant protein A in fetal human lung

Pulmonary surfactant is a mixture of phospholipids and proteins which stabilizes lung alveoli and prevents respiratory failure. The surfactant-associated protein of Mr = 28,000-36,000 (SP-A) influences the structure, function (film formation), and metabolism of surfactant. We have characterized glucocorticoid regulation of SP-A and SP-A mRNA in explants of fetal human lung. The time course of response to dexamethasone was biphasic, with early stimulation and later inhibition of SP-A accumulation. Maximal induction of SP-A occurred with 3-10 nM dexamethasone and approximately 300 nM cortisol for 72 hr, and stimulation diminished at higher concentrations. SP-A mRNA accumulation was maximally stimulated at 24-48 hr of exposure to dexamethasone (10 nM) and was generally inhibited by 4-6 days. Stimulation was also observed with cortisone and corticosterone but not with sex steroids, suggesting a receptor-mediated process. When explants were exposed to cortisol for only 24 hr, SP-A content was transiently increased above the level in continuously treated tissue and subsequently was similar to control. The content of SP-A and its mRNA was also increased by dibromo-cAMP, terbutaline, and forskolin, and effects were approximately additive with those of dexamethasone. However, elevated in tracellular cAMP did not alter the biphasic time course or dose-response patterns of dexamethasone. We propose that glucocorticoids have both stimulatory and inhibitory effects on SP-A gene expression. This biphasic regulation is not consistent with generalized toxic effects, product-feedback inhibition, or receptor down-regulation, and it appears to be specific for SP-A among the various surfactant components.

Hyaline membrane disease and surfactant protein, SAP-35, in diabetes in pregnancy

Surfactant-associated protein of Mr 28,000 to 35,000 (SAP-35) is an abundant glycoprotein present in the alveolus of the lung, which imparts both structural organization to surfactant phospholipids and provides regulatory information controlling surfactant phospholipid secretion and metabolism. SAP-35 expression is enhanced by 3'-5'-cyclic adenosine monophosphate and epidermal growth factor during perinatal differentiation of type II epithelial cells. Its synthesis and RNA are also controlled by a variety of inhibitory factors, which include transforming growth factor and insulin. Glucocorticoids both enhance and inhibit SAP-35 expression in fetal lung explants. There is evidence that fetal hyperinsulinemia or hyperglycemia, or both, inhibit the morphologic differentiation of the type II epithelial cell in association with decreased phospholipid surfactant synthesis or secretion. Insulin is also a potent inhibitor of SAP-35 expression in fetal lung tissue, and decreased SAP-35 was previously noted in amniotic fluid of patients with diabetes during pregnancy. Recent progress in the management of diabetes in pregnancy, characterized by more rigorous metabolic control, has decreased the risk of hyaline membrane disease for the infant of the diabetic mother and is associated with normal levels of SAP-35 in amniotic fluid. Hyaline membrane disease remains a major cause of morbidity in infants of diabetic mothers but may also reflect a higher incidence of premature delivery, cesarean section, and asphyxia at delivery as well as inhibition of pulmonary surfactant phospholipid synthesis or expression of the surfactant protein SAP-35.

Synthesis of surfactant components by cultured type II cells from human lung

We examined the effect of monolayer culture on surfactant phospholipids and proteins of type II cells isolated from human adult and fetal lung. Type II cells were prepared from cultured explants of fetal lung (16-24 weeks gestation) and from adult surgical specimens. Cells were maintained for up to 6 days on plastic tissue culture dishes. Although incorporation of [methyl-3H]choline into phosphatidylcholine (PC) by fetal cells was similar on day 1 and day 5 of culture, saturation of PC fell from 35 to 26%. In addition, there was decreased distribution of labeled acetate into PC, whereas distribution into other phospholipids increased or did not change. The decrease in saturation of newly synthesized PC was not altered by triiodothyronine (T3) and dexamethasone treatment or by culture as mixed type II cell/fibroblast monolayers. The content of surfactant protein SP-A (28-36 kDa) in fetal cells, as measured by ELISA and immunofluorescence microscopy, rose during the first day and then fell to undetectable levels by the fifth. Synthesis of SP-A, as measured by [35S]methionine labeling and immunoprecipitation, was detectable on day 1 but not thereafter. Levels of mRNAs for SP-A and for the two lipophilic surfactant proteins SP-B (18 kDa) and SP-C (5 kDa) fell with half-times of maximally 24 h. In contrast, total protein synthesis measured by [35S]methionine incorporation increased and then plateaued. In adult cells, the content of SP-A and its mRNA decreased during culture, with time-courses similar to those for fetal cells. We conclude that in monolayer culture on plastic culture dishes, human type II cells lose their ability to synthesize both phospholipids and proteins of surfactant. The control of type II cell differentiation under these conditions appears to be at a pretranslational level.

Embryonic mouse lung morphogenesis and type II cytodifferentiation in serumless, chemically defined medium using prolonged in vitro cultures

The timing, position and mechanism(s) for determining type II cytodifferentiation during mammalian lung development are not known. To approach this problem, we have cultured Theiler stage 16 embryonic B10.A strain mouse lung primordia (12-days gestation, E12) in serumless, chemically defined medium in the presence or absence of dexamethasone (DEX) for periods up to 27 days in vitro. Morphogenesis and cytodifferentiation were evaluated by light and transmission electron microscopy and immunochemical techniques. Pulmonary surfactant-associated apoproteins (PSAP) were initially expressed by type II cells at 16.5-day gestation in vivo. DEX-supplementation to the culture medium resulted in the accelerated expression of PSAP; the apoprotein isoforms (A1, A2, and A3) produced in vitro were comparable to those synthesized during fetal and postnatal in vivo development by high resolution, two-dimensional gel electrophoresis coupled with immunoblot staining. Cultures without DEX produced PSAP A2 and A3 isoforms, but did not produce A1 (26-31 kDa, pI 5.2-5.3). DEX-treated cultures produced more lamellar bodies within type II cells than non-treated controls. The results demonstrate that long-term cultures of embryonic lung primordia express morphogenesis, cytodifferentiation and the synthesis and secretion of PSAP in the absence of exogenous hormones or growth factors. The data set further supports the hypothesis that morphogenesis and type II cytodifferentiation are regulated by autocrine and paracrine factors intrinsic to the embryonic lung developmental program and independent of exogenous hormone controls.

Requirement of the collagenous domain for carbohydrate processing and secretion of a surfactant protein, SP-A

Two distinct intracellular forms of surfactant protein Mr = 35,000 (SP-A) were demonstrated in both purified type II epithelial cells and rat lung in vivo. High-mannose precursors of Mr = 30,000 and 34,000 comprised a significant fraction of intracellular SP-A in vivo and in vitro. A second intracellular pool was demonstrated in lamellar body enriched fractions, which contained endoglycosidase-H resistant, sialylated forms of SP-A. Intracellular transport and secretion of SP-A was not altered by inhibitors of carbohydrate processing. However, incubation of type II cells with alpha,alpha'-dipyridyl or cis-4-hydroxy-L-proline, agents which disrupt triple-helix formation within collagenous peptide domains, inhibited sialylation, intracellular transport to the lamellar body fraction and secretion. In the presence of either alpha,alpha'-dipyridyl or cis-4-hydroxy-L-proline, high mannose precursors accumulated intracellularly and were not secreted after 16-18 h. Thus, high-mannose precursors in proximal intracellular pool(s) and sialylated forms in lamellar body-enriched fractions represent two major intracellular storage forms of SP-A in vitro and in vivo. SP-A is routed by processes dependent upon the hydroxylation of the collagenous domain of the polypeptide. Transport and secretion of SP-A are not dependent upon the addition or processing of asparagine-linked carbohydrate.

Localization of surfactant protein synthesis in human lung by in situ hybridization

In order to investigate the sites of synthesis of the pulmonary surfactant-associated proteins, we performed tissue in situ hybridization. We used frozen sections of human lung tissue and 35S-UTP-labeled cRNA probes to localize mRNAs for the 35 kDa surfactant-associated protein (PSP-A) and for the precursor of one of the hydrophobic, low molecular weight surfactant-associated proteins (PSP-B). We found that PSP-A mRNA is present only in the alveolar epithelial type II cells with alveolar macrophages, bronchiolar epithelium, and other cells of the interstitutium being negative. PSP-B mRNA is present in both alveolar type II cells and in some cells of the bronchiolar epithelium. Macrophages and other cells were negative. The data in this report demonstrate that: (1) type II pneumonocytes are capable of synthesizing both PSP-A and PSP-B, (2) some cells of the human bronchiolar epithelium contain PSP-B mRNA but not PSP-A, and (3) human alveolar macrophages do not synthesize either PSP-A or PSP-B.

Glucocorticoid enhances surfactant proteolipid Phe and pVal synthesis and RNA in fetal lung

Two newly described surfactant proteolipids (SPL), Phe and pVal, are produced by proteolytic processing of distinct precursors of Mr = 40,000 and 22,000, respectively. These proteins are structurally related and intimately associated with surfactant phospholipids. We now demonstrate the expression of both SPL(Phe) and SPL(pVal) in explants of human fetal lung from 16-24 weeks of gestation. Content, synthesis, and mRNA for the proteolipids were low prior to organ culture of fetal lung. Induction of synthesis of the proteolipids occurred rapidly in explant culture in the absence of exogenous hormones and was enhanced by addition of dexamethasone. Increased synthesis of the proteolipids was detected by enzyme-linked immunosorbent assay and by [35S]methionine incorporation into the glycosylated Mr = 40,000-43,000 SPL (Phe) precursor. The response to dexamethasone occurred rapidly and contrasted with effects of dexamethasone on the expression of surfactant-associated protein- (SAP) 35, a distinct surfactant glycoprotein. 8-Br-cAMP did not significantly increase proteolipid content but markedly increased synthesis of SAP-35 in identical cultures. Increased proteolipid content was associated with increased mRNA for each protein as determined by the Northern blot analysis. Proteolipid RNA was also increased by 8-Br-cAMP, however, not to the extent observed with the glucocorticoid. Immunohistochemical analysis of fetal lung with anti-proteolipid antiserum confirmed that the dexamethasone-enhanced synthesis of the proteins by Type II epithelial cells. The time and hormone dependence of the regulation of expression of both SPL(Phe) and SPL(pVal) precursors were distinct from that of SAP-35. Expression of the surfactant proteolipids increased during explant culture of human fetal lung and was further enhanced by glucocorticoid. Developmental and hormonal regulation of the surfactant proteolipids may be important factors in surfactant function at birth.

Dynamics of proteoliposome formation. Intermediate states during detergent dialysis

1. The intermediate structures formed during dialysis of mixtures of cholate, phospholipid and cytochrome c oxidase were analysed by gel chromatography and electron microscopy. Measurements of trapped phosphate and the degree of respiratory control were used to assess the integrity of the vesicular structures formed. Protein orientation in the bilayer was monitored by the accessibility of cytochrome c to cytochrome c oxidase. 2. The results indicate that proteoliposome formation by the detergent-dialysis procedure takes place in three distinct stages. In the first stage, cholate/phospholipid and cholate/phospholipid/protein micelles coexist in solution and grow in size as the detergent is slowly removed. At a detergent/phospholipid molar ratio of about 0.2, micelle fusion results in the formation of large bilayer aggregates permeable to both phosphate and cytochrome c. It is at this stage that cytochrome c oxidase is incorporated into the bilayer. In the final stage of dialysis the bilayer sheets fragment into small unilamellar vesicles. 3. The orientation of membrane protein in the final vesicles appears to be determined by the effect of protein conformation on the initial curvature of the bilayer sheets during the fragmentation process.

In vitro sulfation of pulmonary surfactant-associated protein-35

Surfactant-associated protein-35 consists of a group of phospholipid-associated proteins of 26-36 kDa isolated from pulmonary alveolar surfactant. In the rat, surfactant-associated protein-35 is synthesized from 26-kDa primary translation products which are cotranslationally acetylated and glycosylated to heterogeneous 30 and 34 kDa forms. High-mannose oligosaccharide-containing precursors of surfactant-associated protein-35 are processed in the rough endoplasmic reticulum and Golgi to complex-type oligosaccharides, resulting in a mature glycoprotein which exhibits extensive charge heterogeneity in two-dimensional isoelectric focusing SDS-polyacrylamide gel electrophoresis. Much of this charge heterogeneity is related to terminal sialylation of the two asparagine-linked oligosaccharides. In the present study, we report that surfactant-associated protein-35 is also sulfated. Sulfation of the 30 and 34 kDa forms of surfactant-associated protein-35 was clearly detected in primary cultures of rat Type II epithelial cells. These sulfated isoforms were sensitive to endoglycosidase F digestion, but resistant to neuraminidase, suggesting that sulfation occurred at oligosaccharide residues other than sialic acid. The lack of sulfation of the 26 kDa forms of surfactant-associated protein-35 and the resistance of the sulfated isoforms to endoglycosidase H digestion are consistent with Golgi-associated sulfation of the complex type oligosaccharides of surfactant-associated protein-35. Thus, sulfation is another component of the complex post-translational processing of surfactant-associated protein-35, which includes acetylation, hydroxylation, glycosylation, sialylation, sulfhydryl-dependent oligomerization and sulfation.

Differential effects of epidermal growth factor and transforming growth factor-beta on synthesis of Mr = 35,000 surfactant-associated protein in fetal lung

Differentiation of pulmonary Type II epithelial cells in late gestation is associated with the synthesis of pulmonary surfactant required for adaptation to air breathing at birth. In the present work, induction of synthesis of a Type II epithelial cell protein, surfactant-associated glycoprotein of Mr = 35,000 (SAP-35) was studied in human fetal lung tissue obtained at 15-24 weeks of gestation. SAP-35 content increased during organ culture in the absence of exogenous hormones. Epidermal growth factor or triiodothyronine stimulated the induction of SAP-35 synthesis during culture. Stimulation by epidermal growth factor (EGF) was detected as early as 2 days and persisted for up to 5 days in culture. Response to EGF was dose-dependent (0.01-10 ng/ml) and was associated with enhanced incorporation of [35S]methionine into immunoprecipitable SAP-35. Increased SAP-35 synthesis was associated with increased SAP-35 RNA as assessed by Northern blot and hybridization assays with human SAP-35 cDNA. Effects of EGF were comparable to the induction of SAP-35 synthesis by 8-bromo-cAMP. In contrast to the stimulatory effect of EGF and triiodothyronine, SAP-35 content was inhibited by transforming growth factor-beta. Both the stimulatory and inhibitory effects of these agents on SAP-35 content were associated with concomitant changes in SAP-35 synthesis. These findings demonstrate multihormonal control of SAP-35 expression and strongly implicate both EGF and transforming growth factor-beta in the regulation of surfactant apoprotein synthesis.

Induction of surfactant protein in fetal lung. Effects of cAMP and dexamethasone on SAP-35 RNA and synthesis

Synthesis of surfactant-associated glycoprotein of Mr = 30,000-35,000 (SAP-35) was induced in explant culture of human fetal lung obtained from 8 to 24 weeks of gestation. SAP-35 synthesis and content increased markedly during 1-5 days in organ culture in association with the morphologic maturation of Type II epithelial cells and the appearance of lamellar bodies. [35S] Methionine labeling of the explants and subsequent immunoprecipitation of SAP-35 demonstrated distinct high-mannose precursors and sialylated SAP-35 forms as early in culture as SAP-35 synthesis was detectable. The increase in SAP-35 synthesis was associated with increased SAP-35 RNA of 2.1 kilobases as assessed by hybridization assay with [32P]cDNA specific for human SAP-35. Specific SAP-35 RNA increased during organ culture and both SAP-35 content and SAP-35 RNA increased in the absence of exogenous hormones in 2% carbon-stripped fetal calf serum. SAP-35 content and synthesis was stimulated by 8-Br-cAMP. Addition of 100 microM 8-Br-cAMP, enhanced both the concentration of SAP-35 protein and the SAP-35 RNA as assessed by hybridization assay. In contrast, treatment of the explants with dexamethasone was associated with decreased SAP-35 protein synthesis, SAP-35 content, and decreased SAP-35 RNA levels compared to untreated explants. Inhibition by dexamethasone occurred at all gestational ages tested, was dose-dependent, and detectable within 24-48 h during organ culture. Dexamethasone significantly inhibited both basal and cAMP-induced SAP-35 synthesis. Induction of pulmonary surfactant protein (SAP-35) synthesis during organ culture of human fetal lung was associated with increased SAP-35 RNA. SAP-35 synthesis and SAP-35 RNA were inhibited by dexamethasone and enhanced cAMP.

Defective biosynthesis of proteolipid protein in Pelizaeus-Merzbacher disease

The brain of an 18-year-old patient with Pelizaeus-Merzbacher disease was examined by standard neuropathological and biochemical methods and by immunocytochemical and immunochemical techniques. Analysis revealed a lack of myelin-specific lipids, but showed a residual immunoreactivity for myelin basic protein, myelin-associated glycoprotein, and 2',3'-cyclic nucleotide-3'-phosphodiesterase. Examination by immunocytochemistry and enzyme-linked immunosorbent assay showed an absence of proteolipid apoprotein (lipophilin). The peripheral nervous system was normal. Pelizaeus-Merzbacher disease in humans shares many neuropathological and biochemical features with X-linked mutations in animals, e.g., the jimpy mouse and myelin-deficient rat. The specificity of this protein deficiency in Pelizaeus-Merzbacher disease gains additional support from the recent mapping of the lipophilin gene to the human X chromosome.

Expression of a plasma membrane proteolipid during differentiation of neuronal and glial cells in primary culture

Plasma membrane proteolipid protein (PM-PLP) synthesis was examined in embryonic rat neurons and neonatal rat glial cells during differentiation in culture. Glial cultures were treated with 1 mM N6, O2, dibutyryl cyclic adenosine monophosphate (dbcAMP) following confluency to induce differentiation, which resulted in the elaboration of long cellular processes. However, no changes in the biosynthetic level of PM-PLP was observed during the differentiation of these cells. Neurons differentiated spontaneously in culture, forming cellular aggregates immediately following plating and elaborating a network of neurites over 7 days. The differentiation of neurons was accompanied by a seven-fold increase in PM-PLP synthesis with increases in biosynthetic increase in PM-PLP synthesis with increases in biosynthetic rate observed between days 1 and 3 and between days 3 and 7 in culture. Ultrastructural examination of neurons indicated that the Golgi apparatus was also developing during this period of time, with an increase in both the number of lamellae and generation of vesicles. The transport of PM-PLP to the plasma membrane was therefore examined in neurons at day 7 in culture by pulse labeling experiments with monensin and colchicine. Monensin (1 microM) was found to inhibit the appearance of radiolabeled PM-PLP in the plasma membrane by 63%, indicating that a functional Golgi apparatus is required for transport of PM-PLP to its target membrane. Colchicine (125 microM) also inhibited the appearance of newly synthesized PM-PLP in the plasma membrane by greater than 40%, suggesting that microtubules may also be required for PM-PLP transport to the plasma membrane.

Cloned proteolipid protein and myelin basic protein cDNA. Transcription of the two genes during myelination

cDNA clones of rat brain proteolipid protein (PLP), also named lipophilin, the major integral myelin membrane protein, and of myelin basic protein (MBP), the major extrinsic myelin protein, have been isolated from a rat brain cDNA library cloned into the PstI site of pBR322. Poly(A)+ RNA from actively myelinating 18-day-old rats has been reversely transcribed. Oligonucleotides synthesized according to the established amino-acid sequence of lipophilin and the nucleotide sequence of the small myelin basic protein of the N-terminal, the central and C-terminal region of their sequences were used as hybridization probes for screening. The largest insert in one of several lipophilin clones was 2,585 base pairs (bp) in length (pLp 1). It contained 521 bp of the C-terminal coding sequence and the complete 2,064 bp long non-coding 3' sequence. The myelin basic protein cDNA insert of clones pMBP5 and pMBP6 is 2,530 bp long and that of clones pMBP2 and pMBP3 640 bp. These clones were also characterized. pMBP2 was sequenced and used together with the lipophilin cDNA clones as hybridization probes to estimate the lipophilin and myelin basic protein mRNA levels of rat brain during the myelination period. The expression of the lipophilin and myelin basic protein genes during development of the myelin sheath appears to be strictly coordinated.

Synthesis of surfactant-associated protein, 35,000 daltons, in fetal lung

The major human pulmonary surfactant-associated protein of 35,000 daltons (Da) (SAP-35), consists of a group of related proteins of 27,000-36,000 Da, with isoelectric points ranging from pH 4.6 to 5.2. SAP-35 precursors were identified by immunoprecipitation of protein products of in vitro translation of normal adult human poly(A)+ mRNA with human SAP-35 antiserum. The translation products nearly comigrated with the most basic components of alveolar SAP-35 (mol mass = 24,500-27,000 Da). Processing of the primary translation products by canine pancreatic microsomal membranes increased their apparent molecular weight to 29,000-30,000-Da forms, which were sensitive to endoglycosidase F, suggesting the addition of asparagine-linked oligosaccharides to the molecules. A smaller protein of 24,500 Da was generated during treatment with canine microsomal membranes likely representing cleavage of a signal peptide. SAP-35 was not detected in explants of [35S]methionine-labeled fetal lung (20-24 wk gestation) after 1 day of culture or immunoprecipitates of in vitro translated poly(A)+ mRNA isolated from fetal human lung. However, after 3-5 days of organ culture, synthesis of SAP-35 was readily detected by immunoprecipitation of [35S] methionine-labeled tissue. Fully sialylated (neuraminidase-sensitive forms) comigrated with fully glycosylated SAP-35 isolated from human surfactant. High mannose (endoglycosidase H-sensitive precursors) were also synthesized by the organ cultures and were distinct from the secreted form in surfactant. Synthesis of surfactant-associated SAP-35 and its precursors was induced in association with morphological maturation of the type II epithelial cell during organ culture of human fetal lung.