Neuregulin-1 and human epidermal growth factor receptors 2 and 3 play a role in human lung development in vitro

The human epidermal growth factor receptor (HER) family consists of four distinct receptors: HER1 (epidermal growth factor receptor), HER2, HER3, and HER4. Their specific activating ligands are collectively known as neuregulins (NRG). We hypothesized that one member of the NRG family, NRG-1, and the HER family would play a role in fetal lung development. To test this hypothesis, we defined NRG-1 and HER gene expression in mid-trimester human fetal lung tissue. HER2 and HER3 messenger RNA and protein were detected in the fetal lung, but HER4 expression was not detected. Immunohistochemical staining of fetal lung tissue localized HER2 and HER3 protein to the developing lung epithelium. NRG-1 expression was not found in freshly isolated human fetal lung, but it was observed in fetal lung explants after 2 d of explant culture. Immunohistochemistry of cultured human fetal lung explants revealed that NRG-1 protein was also expressed in pulmonary epithelial cells. Exposing human fetal lung to recombinant NRG-1 activated the HER receptor complex as measured by approximately 4-fold increases in receptor phosphotyrosine content. In addition, NRG-1 increased explant epithelial cell volume density approximately 2-fold (P < 0. 03); increased epithelial cell proliferation approximately 2-fold, as determined by bromodeoxyuridine labeling (P = 0.002); and reduced surfactant protein-A (SP-A) levels by 53% (P < 0.05). These data are consistent with an autocrine regulatory process mediated by NRG-1 activation of HER2/HER3 heterodimers expressed on developing human fetal lung epithelial cells. Receptor activation results in increased lung epithelial cell proliferation and volume density, and decreased SP-A production, a marker of type II pneumocyte differentiation.

Intracellular localization of processing events in human surfactant protein B biosynthesis

Surfactant protein B (SP-B) is essential to the function of pulmonary surfactant and to alveolar type 2 cell phenotype. Human SP-B is the 79-amino acid product of extensive post-translational processing of a 381-amino acid preproprotein. Processing involves modification of the primary translation product from 39 to 42 kDa and at least 3 subsequent proteolytic cleavages to produce the mature 8-kDa SP-B. To examine the intracellular sites of SP-B processing, we carried out immunofluorescence cytochemistry and inhibitor studies on human fetal lung in explant culture and isolated type 2 cells in monolayer culture using polyclonal antibodies to human SP-B(8) (Phe(201)-Met(279)) and specific epitopes within the N- (NFProx, Ser(145)-Leu(160); NFlank Gln(186)-Gln(200)) and C-terminal (CFlank, Gly(284)-Ser(304)) propeptides of pro-SP-B. Fluorescence immunocytochemistry using epitope-specific antisera showed colocalization of pro-SP-B with the endoplasmic reticulum resident protein BiP. The 25-kDa intermediate was partially endo H-sensitive, colocalized with the medial Golgi resident protein MG160, and shifted into the endoplasmic reticulum in the presence of brefeldin A, which interferes with anterograde transport from endoplasmic reticulum to Golgi. The 9-kDa intermediate colocalized in part with MG160 but not with Lamp-1, a transmembrane protein resident in late endosomes and lamellar bodies. Brefeldin A induced a loss of colocalization between MG160 and NFlank, shifting NFlank immunostaining to a juxtanuclear tubular array. In pulse-chase studies, brefeldin A blocked all processing of 42-kDa pro-SP-B whereas similar studies using monensin blocked the final N-terminal processing event of 9 to 8 kDa SP-B. We conclude that: 1) the first enzymatic cleavage of pro-SP-B to the 25-kDa intermediate is in the brefeldin A-sensitive, medial Golgi; 2) cleavage of the 25-kDa intermediate to a 9-kDa form is a trans-Golgi event that is slowed but not blocked by monensin; 3) the final cleavage of 9 to 8 kDa SP-B is a monensin-sensitive, post-Golgi event occurring prior to transfer of SP-B to lamellar bodies.

Activity of growth factors in the IL-6 group in the differentiation of human lung adenocarcinoma

The role of the interleukin-6 (IL-6) group of cytokines in differentiation of two lung adenocarcinoma cell lines has been examined using induction of alkaline phosphatase and expression of surfactant protein A. Oncostatin M was the most active and potent for alkaline phosphatase in A549 cells, with IL-6 having similar activity but less potency. Neither cytokine induced alkaline phosphatase in NCI-H441 cells, although induction was obtained with lung fibroblast-conditioned medium. Surfactant protein A was induced in NCI-H441 cells by conditioned medium and dexamethasone and, to a much lesser extent, by oncostatin M or IL-6. Induction of alkaline phosphatase and surfactant protein A were both dexamethasone-dependent, though some induction of surfactant protein A was obtained with interferon-alpha in the absence of dexamethasone. The activity present in lung fibroblast-conditioned medium suggests paracrine control, but this appears not to be due to oncostatin M or IL-6 as disabling antibodies to either cytokine were not inhibitory, and, although alkaline phosphatase was induced in A549 by both cytokines, it was only induced by conditioned medium in NCI-H441 cells. Furthermore, surfactant protein A was induced in H441 by conditioned medium to a much greater extent than by oncostatin M or IL-6. These data demonstrate that cytokines of the IL-6 group have potential as differentiation inducers in lung adenocarcinoma cells and that there is an equivalent paracrine factor(s) in lung fibroblast conditioned medium. As the production of this factor by fibroblasts is not enhanced by glucocorticoid, although the response of the target cell is, it would appear to be distinct from the fibrocyte pneumocyte factor previously described by Post et al 1984.

Retinoic acid stimulation of the human surfactant protein B promoter is thyroid transcription factor 1 site-dependent

Surfactant B (SP-B) is a 79-amino acid peptide critical to postnatal respiratory adaptation. Expression of SP-B by respiratory epithelial cells is regulated by developmental and hormonal influences at the level of gene transcription. Previous studies supported the role of retinoic acids (RA) and their receptors (RARs) in SP-B gene transcription. In the present study, RARalpha was detected in mouse alveolar type II epithelial cells where SP-B is synthesized and processed. Deletion and site-specific mutagenesis analysis identified clustered retinoic acid-responsive element sites in the 5'-flanking enhancer region of the hSP-B gene that bound RARalpha proteins. RAR coactivators ACTR, SRC-1, and transcriptional intermediary factor 2 (TIF2) stimulated human (h) SP-B promoter activity in a dose-dependent fashion in pulmonary adenocarcinoma H441 cells. In addition, an RAR-associated protein, CREB-binding protein (CBP), potentiated the effects of RAR on hSP-B promoter activity in H441 cells. Importantly, RA stimulation of the hSP-B promoter depends on tissue-specific thyroid transcription factor (TTF-1) DNA-binding sites. TTF-1 protein synergistically stimulated the hSP-B promoter with RARalpha, CBP, and nuclear receptor coactivators in H441 cells. In addition, TTF-1 interacted directly with RARalpha and TIF2 in the mammalian two-hybrid system. These findings support a model in which RAR/retinoid X receptor, TTF-1, coactivators, and CBP form a transcription activation complex in the upstream enhancer region of the hSP-B gene.

Effect of hyperoxia on surfactant protein gene expression in hypoplastic lung in nitrofen-induced diaphragmatic hernia in rats

The hypoplastic lung in congenital diaphragmatic hernia (CDH) has both a quantitative and qualitative reduction in surfactant. Recently, the role of oxygen (O2) as a regulator of pulmonary surfactant-associated protein (SP) gene expression has been reported. The mRNA level of SP has been demonstrated to be increased in the lungs of animals exposed to hyperoxia. The aim of this study was to investigate SP mRNA expression in hypoplastic CDH lung in rats during mechanical ventilation in order to determine the effect of O2 on SP synthesis in CDH. A CDH model was induced in pregnant rats following administration of nitrofen. The newborn rats with CDH and controls were intubated and ventilated. Ventilation was continued for 6 h under 100% oxygen. Reverse-transcription polymerase chain reaction (RT-PCR) was performed to evaluate the relative amounts of mRNA expression of SP-A, SP-B, SP-C, and SP-D. Relative amounts of SP-A, SP-B, and SP-D mRNA expression in CDH lung were significantly decreased compared to controls at birth and 6 h after ventilation. There was no significant difference in SP-C mRNA expression between CDH animals and controls. Upregulated mRNA expression of SP-A, SP-B, and SP-D in lungs of control animals at 6 h after ventilation suggests that oxygenation accelerates postnatal SP synthesis in normal lungs. The inability of O2 to increase SP mRNA expression in hypoplastic CDH lung suggests that the hypoplastic lung is not responsive to increased oxygenation for the synthesis of SP.

Effect of dexamethasone, triiodothyronine and dimethyl-isopropyl-thyronine on lung maturation of the fetal rat lung

Our purpose was to elucidate why clinical studies have up to now failed to demonstrate a positive effect of TRH combined with glucocorticosteroids on fetal lung maturity. Morphological and biochemical lung maturation were determined by electron microscopy, choline incorporation, and surfactant-protein-A m-RNA synthesis in rat lung organoid cultures after exposure with a series of concentrations of dexamethasone, triiodothyronine, and dimethyl-isopropyl-thyronine. Thyroid hormones improved morphogenesis of lung histotypic structures but had a negative effect on surfactant synthesis whereas glucocorticosteroids had a positive effect on the surfactant synthesis but a negative effect on morphogenesis. The combination of both substances even had the most negative effect on morphogenesis. Since morphogenesis of lung histotypic structures is prerequisite for surfactant synthesis and secretion, we hypothesize that a sequential treatment of thyroid hormones to improve morphogenesis followed by the application of glucocorticosteroids might be an option to improve neonatal lung function.

The MAL gene is expressed in primary mediastinal large B-cell lymphoma

Primary mediastinal large B-cell lymphoma (PMBL) appears to be a distinct clinicopathologic entity among diffuse large B-cell lymphomas (DLBLs). To find molecular alterations associated with this disease, we compared the mRNAs expressed in 3 PMBLs and 3 peripheral DLBLs by differential display-reverse transcription (DDRT) and identified a mRNA specifically expressed in PMBLs. Sequence analysis showed that this mRNA is encoded by the MAL gene, the expression of which was shown to be restricted to the T-cell lineage during hematopoiesis. MAL gene expression was demonstrated by Northern blot and reverse transcription-polymerase chain reaction (RT-PCR) in 8 of 12 PMBLs. However, there was little or no MAL gene expression in 8 peripheral DLBLs. Immunohistochemical analysis evidenced expression of MAL protein in tumoral B cells restricted to the PMBL subtype. Finally, Southern blot studies did not demonstrate rearrangement of the MAL gene. Altogether, our results indicate that MAL expression is recurrent in PMBLs, providing further evidence that PMBL represents a distinct entity among DLBLs. Because MAL protein is located in detergent-insoluble glycolipid-enriched membrane (GEM) domains involved in lymphocyte signal transduction, abnormal expression of MAL protein in the B-lymphoid lineage may have significant implications in PMBL lymphomagenesis.

Granulocyte-macrophage colony-stimulating factor increases surfactant phospholipid in premature rabbits

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a hematopoietic cytokine that is low in airway specimens from immature lungs at birth. In adult mice, an absence of GM-CSF causes excessive accumulation of alveolar surfactant due to a lack of catabolism. Our aim was to investigate whether recombinant human GM-CSF (rhGM-CSF) affects the pool sizes or the turnover of disaturated phosphatidylcholine (DPC) in preterm (gestation 29 d) rabbits at birth and in term rabbits, age 3 d. 3H-labeled dipalmitoyl phosphatidylcholine, 14C-acetate, and either rhGM-CSF (125 or 25 microg/kg body weight) or placebo were given intratracheally. Thereafter, the intra- and extracellular surfactant fractions were isolated and quantified for DPC and radioactivity. In preterm animals, GM-CSF increased dose-dependently within 24 h both the pool sizes of surfactant DPC and the 3H,14C-labeling of surfactant DPC (p < 0.05). The expression of surfactant protein B mRNA was unaffected, whereas surfactant protein B in bronchoalveolar lavage increased. The number of cells in the whole lung, the type II alveolar epithelial cells, and the lavageable alveolar macrophages were unaffected. At term, rhGM-CSF increased the turnover but did not affect the pool sizes of surfactant DPC. Intraperitoneal rhGM-CSF increased blood eosinophils but had no effect on surfactant DPC. Depending on the degree of lung maturity, GM-CSF in the alveolar space may either up-regulate the pool size or increase the turnover of surfactant phospholipid after birth.

Transcriptional regulation of the murine surfactant protein-A gene by B-Myb

Surfactant protein A (SP-A) is selectively synthesized in subsets of cells lining the respiratory epithelium, where its expression is regulated by various transcription factors including thyroid transcription factor-1 (TTF-1). Cell-specific transcription of the mouse SP-A promoter is mediated by binding of TTF-1 at four distinct cis-active sites located in the 5'-flanking region of the gene. Mutation of TTF-1-binding sites (TBE) 1, 3, and 4 in combination markedly decreased transcriptional activity of SP-A promoter-chloramphenicol acetyltransferase constructs containing SP-A gene sequences from -256 to +45. In contrast, the same mutations enhanced transcriptional activity in constructs containing additional 5' SP-A sequences from -399 to +45 suggesting that cis-acting elements within the region -399 to -256 influence effects of TTF-1 on SP-A promoter activity. A consensus Myb-binding site was identified within the region, located at positions -380 to -371 in the mouse gene. Mutation of the Myb-binding site decreased activity of SP-A promoter constructs in MLE-15 cells. MLE-15 cells, a cell line expressing SP-A mRNA, also expressed B-Myb. B-Myb bound to the MBS in the SP-A gene as assessed by electrophoretic mobility shift assay. While co-transfection of HeLa cells with a B-Myb expression plasmid activated the transfected SP-A promoter about 3-fold, co-transfection of B-myb with cyclin A and cdk-2, to enhance phosphorylation of B-Myb, increased transcriptional activity of SP-A constructs approximately 20-fold. Taken together, the data support activation of SP-A gene promoter activity by B-Myb which acts at a cis-acting element in the SP-A gene.

Antagonistic effects of pyrrolidine dithiocarbamate and N-acetyl-L-cysteine on surfactant protein A and B mRNAs

Pulmonary surfactant, a mixture of phospholipids and specific associated proteins, reduces surface tension at the air-liquid interface of the lung and protects the large epithelial surface of the lung from infectious organisms. Surfactant proteins, SP-A and SP-B, are required for normal surfactant function. In the current work, increased levels of oxidized glutathione (GSSG) are demonstrated at doses of pyrrolidine dithiocarbamate (PDTC) which decrease SP-A and SP-B mRNAs, suggesting that cellular oxidation reduces surfactant protein expression. Similarly, reduction of SP-A and SP-B mRNA levels following accumulation of GSSG induced by glutathione reductase inhibitor 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU), supports the hypothesis that surfactant protein synthesis is reduced in response to oxidation of pulmonary epithelial glutathione. Concurrent induction of apolipoprotein J (apoJ) mRNA by PDTC demonstrates the selectivity of pulmonary gene regulation by the dithiocarbamate. In contrast, the glutathione precursor N-acetyl-l-cysteine (NAC) prevented PDTC-dependent increase in GSSG/GSH ratio, inhibition of SP-A and -B mRNAs, and induction of apoJ. Insufficiency of SP-A and -B, which occurs in inflammatory lung diseases, may result from the exposure of the pulmonary epithelium to oxidant stress and may be reversed by the antioxidant NAC.

Surfactant protein B (SP-B) -/- mice are rescued by restoration of SP-B expression in alveolar type II cells but not Clara cells

Surfactant protein B (SP-B) mRNA and protein are restricted to alveolar Type II and Clara cells in the respiratory epithelium. In order to investigate the function of SP-B in these distinct cell types, transgenic mice were generated in which SP-B expression was selectively restored in Type II cells or Clara cells of SP-B -/- mice. The 4.8-kilobase murine SP-C promoter was used to generate 3 transgenic lines which expressed human SP-B in Type II cells (mSP-C/hSP-B). Likewise, the 2.3-kilobase murine CCSP promoter was used to generate two transgenic lines which expressed human SP-B in Clara cells (mCCSP/hSP-B). mSP-C/hSP-B and mCCSP/hSP-B transgenic mice were subsequently bred to SP-B +/- mice in order to selectively express SP-B in Type II cells or Clara cells of SP-B -/- mice. Selective restoration of SP-B expression in Type II cells completely rescued the neonatal lethal phenotype in SP-B -/- mice. Expression of SP-B in some, but not all Type II cells of SP-B -/- mice, allowed postnatal survival, but resulted in significantly altered lung architecture and function. Selective restoration of SP-B expression in Clara cells of SP-B -/- mice resulted in respiratory dysfunction and invariable neonatal death, related to the complete absence of mature SP-B peptide in these mice. These results indicate that expression and processing of the SP-B proprotein to the mature peptide in Type II cells is absolutely required for lung function in vivo and that SP-B expression in Clara cells cannot substitute for this function.

Conserved and divergent expression patterns of the proteolipid protein gene family in the amphibian central nervous system

The recent discovery of a proteolipid protein gene family has revealed that its members are in fact widely distributed and are not exclusively associated with myelination. To date, three different gene products, DMalpha/DM-20/PLP, DMbeta/M6a, and DMgamma/M6b, have been isolated from certain primitive fish species, mouse, and human central nervous system (CNS). We cloned Xenopus laevis orthologues of DMbeta/M6a and DMgamma/M6b and investigated the expression patterns of these gene transcripts as well as that of PLP in developing Xenopus CNS. As is the case in shark and mouse, the mRNA encoding the major myelin integral protein, PLP, is first detected at stage 42/43 in tadpoles and is exclusively found in morphologically recognizable oligodendrocytes throughout the brain, while DMbeta mRNA is solely expressed in young presumptive neurons in the gray matter. There exist two distinct DMgamma mRNAs and, in contrast to these evolutionarily conserved expression patterns, DMgamma mRNAs distribute uniquely within the ventricular zone in young tadpoles (stage 25) through maturity. Furthermore, both DMbeta and DMgamma are expressed in the developing retina, and their distributions are different from one other. In Xenopus CNS, therefore, the expression patterns of three proteolipid proteins, PLP, DMbeta, and DMgamma, are distinct from each other, implying very different roles for their protein products within the cell populations in which they are expressed.

Glucocorticoid activity in the fetal spontaneously hypertensive rat

Fetal exposure to high concentrations of corticosteroids in the rat is associated with elevated blood pressure in postnatal life. In this study we have investigated indicators of corticosteroid activity in fetal spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) in order to determine whether fetal corticosteroid exposure is increased in the SHR. Placental 11beta-hydroxysteroid dehydrogenase (11beta-HSD) activity, which prevents maternal steroids from crossing the placenta, was not impaired in the SHR. Concentrations of amniotic fluid corticosterone were significantly decreased in the SHR compared with the WKY at fetal Day 20, but were not significantly different on fetal Days 16 or 22. This suggests that rather than increased exposure to corticosteroids in the SHR fetus corticosteroid exposure may be reduced. Expression of lung surfactant protein A (Sp-A), a gene induced in late gestation by corticosteroids, was decreased in the SHR. In addition, differences in amniotic fluid electrolyte concentrations were observed which may reflect delayed renal maturation in the fetal SHR. These data suggest that the SHR fetus is exposed to low concentrations of corticosteroids and that the late gestation rise in fetal corticosteroid may be delayed in the SHR.

Surfactant protein B deficiency: clinical, histological and molecular evaluation

Congenital alveolar proteinosis due to surfactant protein B deficiency is an inherited disease which results in severe respiratory failure in term infants soon after birth. The pathophysiologic basis of this disease is now known to be an inability to synthesise adequate quantities of normally functioning surfactant protein B. We report a male infant with fatal respiratory failure of neonatal onset, and histopathological features typical of those seen in congenital alveolar proteinosis. Molecular analysis of genomic DNA revealed two mutations, the 'common' 121ins2 mutation in exon 4, and a novel 2bp frameshift mutation in exon 5. We believe this is the first Australian case of surfactant protein B deficiency confirmed by molecular analysis.

Production of surfactant protein C in the baculovirus expression system: the information required for correct folding and palmitoylation of SP-C is contained within the mature sequence

Surfactant protein C (SP-C) is synthesized in the alveolar type II cells of the lung as a 21 kDa propeptide which is proteolytically processed to a 4.2 kDa mature active form. The main function of this extremely hydrophobic protein is to enhance lipid insertion into the air/liquid interface in the lung upon inhalation. This is necessary to maintain a relatively low surface tension at this interface during breathing. In this report we describe the production of mature human SP-C in the baculovirus expression system. The recombinant protein contains a secondary structure with a high alpha-helical content (73%), comparable to native SP-C, as determined by circular dichroism and attenuated total reflection Fourier transform infrared analysis. The expressed protein is a mixture of dipalmitoylated (15%) and non-palmitoylated SP-C. This suggests that the information required for palmitoylation is contained within the sequence of the mature protein. The activity of the protein to insert phospholipids into a preformed monolayer of lipids at an air/liquid interface was determined with a captive bubble surfactometer. Recombinant SP-C significantly reduced the surface tension at the air/liquid interface during dynamic expansion and compression. We conclude that correctly folded, dipalmitoylated and active SP-C can be expressed in the baculovirus expression system. Our results may facilitate investigations into the relation between structure and function of SP-C and into protein palmitoylation in general.

Glucose decreases steady state mRNA content of hydrophobic surfactant proteins B and C in fetal rat lung explants

The streptozotocin-induced diabetic (STZ-DB) rat model is associated with fetal hyperglycemia, but with low to normal plasma insulin concentration. Because surfactant protein (SP) mRNA content in fetal rat lung is decreased in STZ-DB pregnancy, we investigated the effect of increasing concentrations of glucose on SP gene expression in lung organ cultures. SP mRNA content (SP-A, SP-B, SP-C) was assessed by Northern blot analysis in fetal day 20 lung explants (term = 22 days) cultured for 44 hours in medium containing 10, 25, 50, or 100 mM glucose. Our findings were (1) No consistent alteration in SP-A mRNA content was observed at different glucose concentrations (P > .05); (2) SP-B and SP-C mRNA content were reduced in a dose-dependent manner when glucose concentration was increased from 10 mM to 100 mM. The mRNA content, compared to 10 mM glucose, decreased to 50-60% at 25 mM glucose, to 20-25% at 50 mM glucose, and to lower than 10% at 100 mM glucose (P < .01). These findings indicate that the decrease in SP-B and SP-C mRNA in fetuses of STZ-DB rats may be, in part, due to a direct effect of hyperglycemia, whereas the decrease in SP-A mRNA content in STZ-DB rats appears to be due to other effects of diabetes in pregnancy.

Interaction of NK lysin, a peptide produced by cytolytic lymphocytes, with endotoxin

NK lysin is a 9-kDa polypeptide that was originally isolated from porcine intestinal tissue based on its antibacterial activity. It is produced by cytolytic lymphocytes and is cytolytic against a number of different types of tumor cells. Here we report the binding of NK lysin to lipopolysaccharide (LPS) and its anti-LPS activity. NK lysin binds to matrix-coated LPS from Escherichia coli, Pseudomonas aeruginosa, and different strains of Salmonella enterica. Lipid A and polymyxin B inhibited the binding, demonstrating a preferential interaction of NK lysin with the lipid part of LPS. Chromium-labeled lymphoma cells were lysed by NK lysin, and LPS dose-dependently inhibited the cytolysis at equimolar amounts. In the same manner, NK lysin inhibited certain LPS-stimulated effects on mouse bone marrow cells as well as LPS binding to mouse granulocytes. These results suggest that NK lysin may be a another natural LPS-binding protein from lymphocytes that may participate in the endogenous defense response associated with elevated concentrations of LPS.

Trafficking of newly synthesized surfactant protein A in isolated rat alveolar type II cells

We examined the synthesis, transport, and localization of surfactant protein A (SP-A) in primary cultures of alveolar type II cells. In type II cells maintained in culture for 6 h, 39% of the SP-A pool detected with an enzyme-linked immunosorbent assay (ELISA) was found in lamellar bodies (LBs). After 24 h in culture, 53% of the cellular SP-A pool was found in LBs. The absolute amount of SP-A in the LB compartment was almost identical at 6 and 24 h of culture. In contrast to the results obtained with ELISA, 35S labeling of newly synthesized SP-A revealed that less than 7% of the cellular SP-A pool was in LBs at either 6 or 24 h of culture. In the 6-h cultures, 17% of the total (i.e., cells and media) [35S]SP-A pool was extracellular. In the 24-h cultures, 70% of the [35S]SP-A pool was extracellular. The secretion of [35S]SP-A was blocked by brefeldin A at all times. When medium containing newly secreted [35S]SP-A was incubated with alveolar type II cells maintained in culture for 24 h, the protein was taken up and incorporated into the LB fraction. More than 80% of the internalized SP-A was associated with the LB compartment after a 6 h incubation. The uptake of [35S]SP-A was blocked at 4 degrees C and was promoted by addition of unlabeled SP-A at 37 degrees C. These findings support a pathway of extracellular routing of SP-A prior to its accumulation in LBs in cultured type II cells.

Regulation of surfactant protein gene transcription

Surfactant protein concentrations are precisely maintained during fetal development and postnatally controlled, at least in part, by the regulation of gene transcription and/or mRNA stability. Together, these mechanisms contribute to the unique temporal-spatial distribution of surfactant protein synthesis that is characteristic of the mammalian lung. Surfactant proteins A, B and C are expressed primarily in subsets of respiratory epithelial cells, wherein their expression is modified by developmental, physiological, humoral and inflammatory stimuli. Cell specific and humoral regulation of surfactant protein transcription is determined by the interactions of a number of nuclear transcription proteins that function in combination, by binding to cis-acting elements located in the 5' regulatory regions of each of the surfactant protein genes. The unique combination of distinct and shared cis-acting elements and transcriptional proteins serves to modulate surfactant protein synthesis in the lung. The present review will summarize efforts to identify the mechanisms contributing to the regulation of surfactant protein gene transcription in the lung, focusing to the nuclear transcription factor, TTF-1 (or thyroid transcription factor-1), a member of the Nkchi2 family of nuclear transcription proteins. A complete review of regulatory aspects of surfactant homeostasis is beyond the scope of the present summary.

TGF-beta1 inhibits surfactant component expression and epithelial cell maturation in cultured human fetal lung

Transforming growth factor-beta1 (TGF-beta1) is a multifunctional cytokine shown to play a critical role in organ morphogenesis, development, growth regulation, cellular differentiation, gene expression, and tissue remodeling after injury. We examined the effect of exogenously administered TGF-beta1 on the expression of surfactant proteins (SPs) and lipids, fatty acid synthetase, and ultrastructural morphology in human fetal lung cultured for 5 days with and without dexamethasone (10 nM). Expression of the type II cell-specific marker surfactant proprotein C (proSP-C), studied by [35S]Met incorporation and immunoprecipitation, increased sevenfold with dexamethasone treatment. TGF-beta1 (0.1-100 ng/ml) in the presence of dexamethasone inhibited 21-kDa proSP-C expression in a dose-dependent manner (maximal inhibition 31% of control level at 100 ng/ml). There was no change in [35S]Met incorporation into total protein in any of the treatment groups vs. the control group. In immunoblotting experiments, TGF-beta1 blocked culture-induced accumulation of SP-A and SP-B. Under the same conditions, TGF-beta1 reduced mRNA content for SP-A, SP-B, and SP-C to 20, 38, and 41%, respectively, of matched control groups but did not affect levels of beta-actin mRNA. SP transcription rates after 24 h of exposure to TGF-beta1 were reduced to a similar extent (20-50% of control level). In both control and dexamethasone-treated explants, TGF-beta1 (10 ng/ml) also decreased fatty acid synthetase mRNA, protein, and enzyme activity and the rate of [3H]choline incorporation into phosphatidylcholine. By electron microscopy, well-differentiated type II cells lining potential air spaces were present in explants cultured with dexamethasone, whereas exposure to TGF-beta1 with or without dexamethasone resulted in epithelial cells lacking lamellar bodies. We conclude that exogenous TGF-beta1 disrupts culture-induced maturation of fetal lung epithelial cells and inhibits expression of surfactant components through effects on gene transcription.

Insulin inhibits surfactant protein A and B gene expression in the H441 cell line

Fetuses of mothers with uncontrolled gestational diabetes have an increased risk of developing neonatal respiratory distress syndrome and are frequently hyperinsulinemic, thus it has been proposed that high levels of insulin delay fetal lung maturation. We have shown previously that insulin inhibits the accumulation of mRNA for the surfactant-associated proteins A and B (SP-A and SP-B) in human fetal lung explants maintained in vitro. To test the hypothesis that the inhibitory effects of insulin on the surfactant proteins are the result of a direct action of insulin on the lung epithelial cell, we evaluated the effects of insulin in the H441 cell line, a human pulmonary adenocarcinoma cell line that expresses SP-A and SP-B mRNA. We observed that insulin treatment for 48 h decreased SP-A mRNA and protein levels in a concentration-dependent manner when compared to controls. The inhibitory effect of insulin on SP-A mRNA levels was apparent as early as after 4 h of exposure. SP-B mRNA levels were also significantly decreased by insulin in a concentration-dependent manner. Insulin, at 2.5 microg/ml, inhibited SP-A gene transcription by approx. 67%, and inhibited SP-B gene transcription by about 32%. There was no significant effect of insulin on SP-A or SP-B mRNA stability. Thus, we have observed a pattern of insulin inhibition of SP-A and SP-B gene expression in the H441 lung epithelial cell line similar to that previously observed in human fetal lung explants, which are comprised of both epithelial and mesenchymal cells. Our findings provide further evidence that insulin may delay fetal lung maturation by inhibiting SP-A and SP-B gene expression. Furthermore, our findings suggest that the inhibitory effects of insulin are, at least partially, the result of a direct action on the lung epithelial cell.

Intravenous keratinocyte growth factor protects against experimental pulmonary injury

Keratinocyte growth factor (KGF) administered by intratracheal instillation is well documented to stimulate the proliferation of alveolar and bronchial cells. In the present study, intravenous KGF was also shown to stimulate the proliferation of alveolar and bronchial cells in mice and rats, although to a lesser degree than intratracheal KGF. Despite the decreased potency of intravenous KGF on pulmonary cell 5-bromo-2'-deoxyuridine incorporation compared with intratracheal KGF, intravenous KGF was very effective in preventing experimental bleomycin-induced pulmonary dysfunction, weight loss, and mortality in either mice or rats and experimental hyperoxia-induced mortality in mice. The effectiveness of intravenous administration of KGF in preventing lung injury suggests that the mechanisms of the protective effect of KGF may involve more than pulmonary cell proliferation and also suggests the potential use of systemic KGF for clinical trials in settings of pulmonary injury.

Human surfactant proteins A1 and A2 are differentially regulated during development and by soluble factors

An RT-PCR method for the relative quantitation of the mRNAs for human surfactant protein (SP) A1 and SP-A2 was developed, verified, and then utilized to determine the relative levels of these mRNAs in fetal and adult lung samples in vivo, as well as in cultured human fetal lung explants and H441 cells. For the cultured tissue and cells, we assessed the effects of a variety of soluble factors known to modulate total SP-A. Comprehensive analysis revealed many significant findings, including the following: both mRNAs were expressed as early as 15 wk of gestation; throughout midgestation, SP-A1 was present at higher levels than SP-A2, with an average ratio of 30:1. In the adult lung, SP-A1 mRNA was present at lower levels than SP-A2, with a ratio of 0.4:1, whereas in H441 cells, the ratio was 0.85:1. In fetal lung cultured for 4 days, both mRNAs increased, with a greater increase in SP-A2 (97-fold) than in SP-A1 (15-fold), resulting in a final ratio of 4:1. Differential regulation was demonstrated for 8-(4-chlorophenylthio)-cAMP, interferon (IFN)-gamma, tumor necrosis factor-alpha, and transforming growth factor (TGF)-beta in the human fetal lung explant system, with SP-A2 being more affected, and for IFN-gamma and TGF-beta in the H441 cells, where SP-A1 showed greater regulation. Of the soluble factors tested, IFN-gamma and TGF-beta had the most potent and consistent effects in both systems.

Thyroid hormone affects Schwann cell and oligodendrocyte gene expression at the glial transition zone of the VIIIth nerve prior to cochlea function

All cranial nerves, as well as the VIIIth nerve which invades the cochlea, have a proximal end in which myelin is formed by Schwann cells and a distal end which is surrounded by oligodendrocytes. The question which arises in this context is whether peripheral and central parts of these nerves myelinate simultaneously or subsequently and whether the myelination of either of the parts occurs simultaneously at the onset of the cochlea function and under the control of neuronal activity. In the present paper, we examined the relative time course of the myelinogenesis of the distal part of the VIIIth nerve by analyzing the expression of peripheral protein P0, proteolipid protein and myelin basic protein. To our surprise, we observed that the expression of myelin markers in the peripheral and central part of the intradural part of the VIIIth nerve started simultaneously, from postnatal day 2 onwards, long before the onset of cochlea function. The expression rapidly achieved saturation levels on the approach to postnatal day 12, the day on which the cochlea function commenced. Because of its importance for the neuronal and morphological maturation of the cochlea during this time, an additional role of thyroid hormone in cochlear myelinogenesis was considered. Indeed, it transpires that this hormone ensures the rapid accomplishment of glial gene expression, not only in the central but also in the peripheral part of the cochlea. Furthermore, an analysis of the thyroid hormone receptors, TRaplha and TRbeta, indicates that TRbeta is necessary for myelinogenesis of the VIIIth nerve. Rapid thyroid hormone-dependent saturation of myelin marker gene expression in Schwann cells and oligodendrocytes of the VIIIth nerve may guarantee nerve conduction and synchronized impulse transmission at the onset of hearing. The thyroid hormone-dependent commencement of nerve conduction is discussed in connection with the patterning refinement of central auditory pathways and the acquisition of deafness.

Surfactant protein B processing in human fetal lung

Surfactant protein B (SP-B8), 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-B8 (Phe201-Met279) and to specific epitopes within the NH2- and COOH-terminal propeptide domains (Ser145-Leu160, Gln186-Gln200, and Gly284-Ser304). Western blot analysis revealed a novel intermediate at approximately 9 kDa, representing mature SP-B8, with a residual NH2-terminal peptide of approximately 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-B8 increased, resulting in a net increase in tissue SP-B8. Dexamethasone did not affect the rate of proSP-B processing but markedly enhanced the rate of SP-B8 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.

Regulation and function of pulmonary surfactant protein B

Pulmonary surfactant, a complex mixture of phospholipids and specific associated proteins, reduces the surface tension at the air-liquid interface of the distal conducting airways and gas exchanging alveoli of the lung. Lipids, primarily neutral and phospholipids, compose approximately 90% of the surfactant complex. The remaining 10% of surfactant is composed of at least three surfactant-specific proteins, designated surfactant protein A (SP-A), SP-B, and SP-C. These proteins contribute to the formation, stabilization, and function of organized surfactant structures. This article briefly reviews the normal composition and function of pulmonary surfactant and specifically reviews the structure, function, and regulation of surfactant protein B (SP-B). The recent identification of neonates with refractory respiratory failure due to a genetic absence of SP-B and the study of transgenic mice in which SP-B gene expression has been ablated highlight the importance of the protein to surfactant function, synthesis, and metabolism and to the maintenance of lung function. Gene reconstitution experiments in vitro and in SP-B-deficient transgenic mice suggest specific functions for the amino and carboxyl terminal domains of the protein. SP-B deficiency is a potential target for gene therapy in human patients.

Retinoic acid-receptor activation of SP-B gene transcription in respiratory epithelial cells

Retinoids are known to play important roles in organ development of the lung. Retinoids exert their activity by modulating the expression of numerous genes, generally influencing gene transcription, in target cells. In the present work, the mechanism by which retinoic acid (RA) regulates surfactant protein (SP) B expression was assessed in vitro. RA (9-cis-RA) enhanced SP-B mRNA in pulmonary adenocarcinoma cells (H441 cells) and increased transcriptional activity of the SP-B promoter in both H441 and mouse lung epithelial cells (MLE-15). Cotransfection of H441 cells with retinoid nuclear receptor (RAR)-alpha, -beta, and -gamma and retinoid X receptor (RXR)-gamma further increased the response of the SP-B promoter to RA. Treatment of H441 cells with RA increased immunostaining for the SP-B proprotein and increased the number of cells in which the SP-B proprotein was detected. An RA responsive element mediating RA stimulation of the human SP-B promoter was identified. RAR-alpha and -gamma and RXR-alpha but not RAR-beta or RXR-beta and -gamma were detected by immunohistochemical analysis of H441 cells. RA, by activating RAR activity, stimulated the transcription and synthesis of SP-B in pulmonary adenocarcinoma cells.

[Expression of surfactant protein SP-A, SP-B, and SP-C mRNA in lungs of rats with bleomycin-induced pulmonary fibrosis]

OBJECTIVE

The expressions of surfactant protein (SP)SP-A, SP-B, and SP-CmRNA in lungs of rats with bleomycin-induced pulmonary fibrosis were studied.

METHOD

A single dose of bleomycin(BLM) was intratracheal injected to induce pulmonary fibrosis of rats. Animals were killed at day 3, 7, 14 and 28 after BLM administration. The total RNA was extracted from the lung tissue. The expressions of SPsmRNA were analyzed with Northern blot.

RESULT

The number of alveolar type II epithelial cells increased in BLM-administered rats. The expressions of SP-A, SP-B and SP-CmRNA decreased at day 3 after BLM administration and decreased maximally at day 7, and then began to increase at day 14 and significantly increased at day 28, though they were still below the control levels.

CONCLUSION

The results show that the changes of expressions of SP-A, SP-B and SP-CmRNA occur during the development of bleomycin-induced pulmonary fibrosis in rats and they may play a role in the pathogenesis of lung fibrosis.

The A1A0 ATPase from Methanosarcina mazei: cloning of the 5' end of the aha operon encoding the membrane domain and expression of the proteolipid in a membrane-bound form in Escherichia coli

Three additional ATPase genes, clustered in the order ahaH, ahaI, and ahaK, were found upstream of the previously characterized genes ahaECFABDG coding for the archaeal A1A0 ATPase from Methanosarcina mazei. ahaH, the first gene in the cluster, is preceded by a conserved promoter sequence. Northern blot analysis revealed that the clusters ahaHIK and ahaECFABDG are transcribed as one message. AhaH is a hydrophilic polypeptide and is similar to peptides of previously unassigned function encoded by genes preceding postulated ATPase genes in Methanobacterium thermoautotrophicum and Methanococcus jannaschii. AhaI has a two-domain structure with a hydrophilic domain of 39 kDa and a hydrophobic domain with seven predicted transmembrane alpha helices. It is similar to the 100-kDa polypeptide of V1V0 ATPases and is therefore suggested to participate in proton transport. AhaK is a hydrophobic polypeptide with two predicted transmembrane alpha helices and, on the basis of sequence comparisons and immunological studies, is identified as the proteolipid, a polypeptide which is essential for proton translocation. However, it is only one-half and one-third the size of the proteolipids from M. thermoautotrophicum and M. jannaschii, respectively. ahaK is expressed in Escherichia coli, and it is incorporated into the cytoplasmic membrane despite the different chemical natures of lipids from archaea and bacteria. This is the first report on the expression and incorporation into E. coli lipids of a membrane integral enzyme from a methanogens, which will facilitate analysis of the structure and function of the membrane domain of the methanoarchaeal ATPase.

Inflammatory and epithelial responses in mouse strains that differ in sensitivity to hyperoxic injury

The pulmonary response to various toxicants including bleomycin, ozone, ionizing radiation, and hyperoxia is highly variable among mouse strains. The current study tests the hypothesis that at a similar stage of injury, regardless of strain, expression of inflammatory cytokine and epithelial marker genes would be similar, indicating a common pathway of injury progression. Three strains of mice, C57B1/6J, 129/J, and C3H/HeJ, ranging from sensitive to resistant, were exposed to > 95% O2 for varying times. Ribonuclease protection was used to quantify changes in cytokine mRNA. Despite differences in the kinetics, each strain demonstrated similar hyperoxia-induced changes in the abundance of interleukin (IL)-6, IL-1 beta, IL-3, and tumor neucrosis factor (TNF)-alpha. For each strain, death was accompanied by similar increases in cytokine mRNAs above steady-state control levels. Other inflammatory cytokines, including IL-1 alpha, IL-4, and interferon (IFN)-gamma, were unaltered in all strains at all times. In situ hybridization analysis of the epithelial markers, surfactant protein B (SPB), and clara cell secretory protein (CCSP) at the time of proinflammatory induction showed a similar pattern of expression in all strains. Increased SPB was detected in bronchiolar epithelium, while the number of type II cells expressing this message declined. Both the number of cells expressing CCSP as well as abundance per cell declined. These results suggest that although differences in acute sensitivity to hyperoxia exist between mouse strains, once initiated, acute epithelial cell injury and associated inflammatory changes follow the same pattern in all strains.

Secondary structure and limited proteolysis give experimental evidence that the precursor of pulmonary surfactant protein B contains three saposin-like domains

The 42 kDa precursor of surfactant protein B generates the 79 residue mature SP-B polypeptide, which belongs to the family of saposin-like proteins and has unique functional roles in pulmonary surfactant. From sequence comparisons it has been suggested that proSP-B, in addition to SP-B, contains two saposin-like domains, but their existence has until now not been experimentally verified. The 381 residue human proSP-B was now fused to an N-terminal poly-His tag, expressed in Escherichia coli, and purified from inclusion bodies by resolubilisation with 2.5% (w/v) SDS and, after removal of SDS, subsequent metal affinity chromatography. Recombinant proSP-B thus obtained exhibits about 35% alpha-helical structure in sodium phosphate buffer and is proteolytically cleaved preferentially between the three saposin-like domains. These results experimentally support that proSP contains, in addition to SP-B, two further saposin-like domains.

Regulation of surfactant proteins A and B by TNF-alpha and phorbol ester independent of NF-kappa B

Acute lung inflammation is complicated by altered pulmonary surfactant phospholipid and protein composition. The proinflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) and the phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA) inhibit expression of surfactant-associated proteins A and B (SP-A and SP-B), both important for normal surfactant function. The transcription factor nuclear factor-kappa B (NF-kappa B) frequently mediates regulation of gene expression by TPA and TNF-alpha. In the present study, electrophoretic mobility shift assays (EM-SAs) and pyrrolidine dithiocarbamate (PDTC), an inhibitor of NF-kappa B activation, were utilized to determine the role of NF-kappa B activation in TPA and TNF-alpha inhibition of the surfactant proteins in NCI-H441 cells. Pentoxifylline (PTX), which inhibits TNF-alpha cellular effects without preventing NF-kappa B activation, was also tested. By EMSA, TPA and TNF-alpha increased nuclear NF-kappa B binding activity in temporally distinct patterns. PDTC decreased TPA- and TNF-alpha-induced NF-kappa B binding activity but did not limit their inhibition of SP-A and SP-B mRNAs. PDTC independently decreased both SP-A and SP-B mRNAs. PTX partially reversed TNF-alpha-but not TPA-mediated inhibition of SP-A and SP-B mRNAs without altering NF-kappa B binding. The effects of PDTC and PTX on NF-kappa B and the surfactant proteins suggest that NF-kappa B activation does not mediate TPA or TNF-alpha inhibition of SP-A and SP-B mRNA accumulation.

Effects of soluble factors and extracellular matrix on DNA synthesis and surfactant gene expression in primary cultures of rat alveolar type II cells

Proliferation and differentiation of epithelial cells are thought to be regulated by soluble factors in extracellular fluid and insoluble components of the extracellular matrix. We have examined the combined effects of soluble factors and an extracellular matrix (EHS matrix) on DNA synthesis, cell proliferation, and surfactant protein gene expression in primary cultures of alveolar type II epithelial cells. Cells on EHS matrix cultured in DMEM containing insulin, cholera toxin, EGF, aFGF, 5% rat serum, and 15-fold concentrated bronchoalveolar lavage fluid (D-GM) formed larger aggregates than cells cultured on the same substratum in DMEM containing 5% rat serum (D-5). Cells cultured in D-GM on EHS matrix incorporated more [3H]-thymidine than cells on the same substratum in D-5, with an eight-fold increase seen on day 4 of culture. This increase in [3H]-thymidine incorporation was accompanied by a labeling index of greater than 65% of the cells. Cell counts showed that exposure of type II cells on EHS matrix to D-GM resulted in increased cell number on day 4 of culture. [3H]-thymidine autoradiography combined with immunostaining with anti-cytokeratin, anti-SP-A, and anti-vimentin antibodies demonstrated that the proliferating cells were epithelial cells that contained SP-A. Type II cells cultured on plastic in D-GM also showed increased [3H]-thymidine incorporation compared to cells cultured in D-5. The level of [3H]-thymidine incorporation by cells on plastic, however, was significantly less than that seen in cells cultured in the same medium on EHS matrix. Type II cells cultured on EHS matrix in D-GM had a decreased abundance of mRNAs for SP-A and SP-C than cells cultured on EHS matrix in D-5 as determined by Northern analysis. This inhibition was reversed by switching from D-GM to D-5 on day 4 and culturing the cells for an additional 4 days. In contrast, SP-B mRNA was increased in response to D-GM. This increase was not reversed by switching from D-GM to D-5 on day 4. These results suggest that the interaction of soluble factors and extracellular matrix components has a strong influence on type II cell proliferation, which were partially associated with the reversible inhibition of lung tissue-specific protein mRNAs. Their dynamic interplay among the type II cell, the extracellular matrix, and growth factors may determine multicellular functions and play an important role in normal lung development and in the repair of the lung epithelium following injury.

Effect of maternal/fetal vitamin A deficiency on fetal rat lung surfactant protein expression and the response to prenatal dexamethasone

The purpose of this work was to determine whether maternal/fetal vitamin A deficiency in vivo had an effect on fetal lung surfactant protein expression and its response to antenatal maternal dexamethasone (DEX). Weanling female rats at 21 d (30-35 g) were fed control (C) (4 mg of vitamin A/kg of diet) or a vitamin A-deficient (D) (0.06 of mg vitamin A/kg) diet. These females were mated, and at selected pregnancy dates fetal and maternal tissues were obtained. Control mothers had liver retinyl palmitate (RP) concentrations of 246 +/- 32 nmol/g of wet weight; those in the D group had 6.1 +/- 2.9 nmol/g of wet weight. Control fetal liver RP was 12-fold higher and control fetal lung RP was 3-fold higher than in the D group (liver: 18.5 +/- 0.4 nmol/g versus 1.5 +/- 0.25 nmol/g; lung: 1.8 +/- 0.98 nmol/g versus 0.6 +/- 0.2 nmol/g). Neither fetal lung surfactant protein (SP)-C mRNA nor SP-A mRNA was affected by vitamin A deficiency. In a second experiment, pregnant rats from both C and D groups were injected with either DEX (1 mg/kg) or an equal volume of saline on d 15-17, and killed on d 18. DEX increased fetal lung SP-C mRNA 2-fold over the level found in the saline-injected group (saline, 1.0 +/- 0.2 versus DEX, 2.1 +/- 0.2, p < 0.02). This increase in SP-C mRNA also occurred in fetal lungs from the D group (saline, 1.8 +/- 0.4 versus DEX 3.7 +/- 0.2, p < 0.01). Retinoic acid receptor-beta mRNA, which responds to vitamin A levels and DEX in many systems, was lower in fetal lungs of the D group that had been treated with DEX. We conclude that fetal rat lung development, as measured by SP-C mRNA and SP-A mRNA, and the SP-C mRNA response to DEX, was not affected by vitamin A deficiency.

Influence of the cytoskeleton on surfactant protein gene expression in cultured rat alveolar type II cells

We have investigated the role of the cytoskeleton in surfactant protein gene expression. Cytochalasin D (CD), colchicine (Col), or nocodazole (Noco) were tested on primary cultures of adult rat alveolar type II cells. Treatment with any of the drugs did not result in dramatic cell shape changes, but ultrastructural examination revealed that the cytoplasm of cells treated with CD was markedly disorganized; cells treated with Col did not exhibit such changes. Treatment with any of the three drugs resulted in a reduction in surfactant protein (SP) mRNAs. These decreases were not the result of cell toxicity, since overall protein synthesis was unimpaired by drug treatment. Washing the cells followed by an additional 2 days of culture resulted in a reaccumulation of SP mRNAs in CD-treated cells but not in Col-treated cells. Washing of Noco-treated cultures resulted in partial recovery. SP mRNA stability was estimated in the presence or absence of cytoskeleton-disrupting drugs. Disruption of either microfilaments or microtubules significantly affected the half-lives of mRNAs for SP-A, SP-B, and SP-C. These data support a role for the cytoskeleton in the maintenance of type II cell differentiation and suggest that the role of the cytoskeleton is at least in part to stabilize SP mRNAs.

Retinoic acid and dexamethasone affect RAR-beta and surfactant protein C mRNA in the MLE lung cell line

Lung development and surfactant biosynthesis are affected by retinoic acid (RA) and dexamethasone (Dex). Using a mouse lung epithelial cell line, we are exploring RA-Dex interactions through the study of RA and Dex effects on RA receptor (RAR) and surfactant protein (SP) C mRNA expression. RA increased expression of RAR-beta (5.5 times) and SP-C (2 times) mRNA, with maximal effects at 24 h and at 10(-6) M. The RA induction was not inhibited by cycloheximide, suggesting RA affects transcription. With added actinomycin D, RA did not affect the disappearance rate of RAR-beta mRNA, but SP-C mRNA degradation was slowed, indicating an effect on SP-C mRNA stability. Dex decreased RAR-beta and SP-C expression to 75 and 70% of control values, respectively, with greatest effects at 48 h and at 10(-7) M. There was no effect of Dex on either RAR-beta or SP-C mRNA disappearance with actinomycin D. However, cycloheximide prevented the effect of Dex. Despite Dex, RA increased both RAR-beta and SP-C mRNA. This work suggests that RA and Dex affect RAR-beta and SP-C genes by different mechanisms.

Nicotine stimulates branching and expression of SP-A and SP-C mRNAs in embryonic mouse lung culture

Although the effects of maternal smoking on fetal growth and viability are overwhelmingly negative, there is a paradoxical enhancement of lung maturation as evidenced, in part, by a lower incidence of respiratory distress syndrome in infants of smoking mothers. Other epidemiologic and experimental evidence further support the view that a tobacco smoke constituent, possibly nicotine, affects the development of the lung in utero. We are studying the direct effects of nicotine on murine lung development using a serumless organ culture system. We have found that embryonic lungs explanted at 11 days gestation showed a 32% increase in branching after 4 days in culture in the presence of 1 microM nicotine and 7- to 15-fold increases in mRNAs encoding surfactant proteins A and C after 11 days. The effect of nicotine exposure on surfactant gene expression is apparently mediated by nicotinic acetylcholine receptors because it was blocked by D-tubocurarine. The nicotine-induced stimulation of surfactant gene expression could, in part, account for the effect of maternal smoking on the incidence of respiratory distress syndrome.

Redefining the lipophilin family of proteolipid proteins

The past few years have seen a dramatic increase in our understanding, in molecular terms, of the involvement of the central nervous system proteolipid protein in myelinogenesis and X-linked genetic diseases. In addition, we have expanded our knowledge of the proteins that have been recruited into the vertebrate myelin membrane over the past 400 million years with the molecular cloning of several cDNAs encoding proteins which are homologous to the proteolipid protein gene. In searching for a name to distinguish these proteins from other "proteolipid" proteins of nonneural origin I propose that we resurrect the term "lipophilins" which describes a small family of unusually hydrophobic integral membrane proteins exhibiting identical topologies and similar physical properties. Two subgroups are distinguishable among the lipophilins based on the patterns of expression during development and the presence or absence of a small motif that is exposed to the extracellular space.

Expression of hydrophilic surfactant proteins by mesentery cells in rat and man

Human peritoneal dialysis effluent (PDE) contains a phosphatidylcholine-rich compound similar to the surfactant that lines lung alveoli. This material is secreted by mesothelial cells. Lung surfactant is also characterized by four proteins essential to its function. After having long been considered as lung-specific, some of them have been found in gastric and intestinal epithelial cells. To explore further the similarity between lung and peritoneal surfactants, we investigated whether mesothelial cells also produce surfactant proteins. We used rat transparent mesentery, human visceral peritoneum biopsies and PDE. Surfactant proteins were searched for after one- and two-dimensional SDS/PAGE and Western blotting. On a one-dimensional Western blot, bands at 38 and 66 kDa in rat mesentery, and at 38 and 66 kDa in human peritoneal mesothelial cells (in vivo and in vitro) and PDE, corresponded to monomeric and dimeric forms of lung surfactant protein A (SP-A). On two-dimensional Western blots, the 32 and 38 kDa spots in mesentery and PDE localized at the acidic pH appropriate to the SP-A monomer's isoelectric point. SP-D was also identified at the same 43 kDa molecular mass as in lung. SP-B was not detected in mesenteric samples. Expression of SP mRNA species was also assessed by reverse transcriptase-PCR, which was performed with specific primers of surfactant protein cDNA sequences. With primers of SP-A and SP-D, DNA fragments of the same size were amplified in lung and mesentery, indicating the presence of SP-A and SP-D mRNA species. These fragments were labelled by appropriate probes in a Southern blot. No amplification was obtained for SP-B. These results show that mesentery cells produce SP-A and SP-D, although they are of embryonic origin (mesodermal) and are different from those of the lung and digestive tract (endodermal) that secrete these surfactants.

Cloning of guinea pig surfactant protein A defines a distinct cellular distribution pattern within the lung

A full-length cDNA to guinea pig pulmonary surfactant protein (SP) A was cloned by screening a newborn guinea pig lung cDNA library with a human SP-A cDNA probe. The full-length guinea pig SP-A cDNA consists of 1,839 bp and is highly conserved at both nucleotide and amino acid sequence levels with those from other species. As expected, guinea pig SP-A mRNA is abundantly expressed in adolescent lung tissue and is undetectable in nonpulmonary tissues. In situ hybridization studies clearly show a unique cellular distribution pattern of SP-A mRNA within the guinea pig lung. SP-A mRNA expression is confined to cells of the alveolar epithelium with no expression in the bronchiolar epithelial cells, whereas SP-B mRNA is expressed in both alveolar and bronchiolar epithelial cell populations. This distinct expression pattern suggests that the guinea pig lung will be a useful model in which to study expression of transcription factors implicated in the regulation of SP genes.

GM-CSF enhances lung growth and causes alveolar type II epithelial cell hyperplasia in transgenic mice

The human surfactant protein (SP)-C gene promoter was used to direct expression of mouse granulocyte macrophage colony-stimulating factor (GM-CSF; SP-C-GM mice) in lung epithelial cells in GM-CSF-replete (GM+/+) or GM-CSF null mutant (GM-/-) mice. Lung weight and volume were significantly increased in SP-C-GM mice compared with GM+/+ or GM-/- control mice. Immunohistochemical staining demonstrated marked type II cell hyperplasia, and immunofluorescent labeling for proliferating cell nuclear antigen was increased in type II cells of SP-C-GM mice. Abundance of type II cells per mouse lung was increased three- to fourfold in SP-C-GM mice compared with GM+/+ and GM-/- mice. GM-CSF increased bromodeoxyuridine labeling of isolated type II cells in vitro. Type II cells, alveolar macrophages, and endothelial and bronchiolar epithelial cells were stained by antibodies to the GM-CSF receptor alpha-subunit in both GM+/+ mice and GM-CSF gene-targeted mice that are also homozygous for the SP-C-GM transgene. High levels of GM-CSF expression in type II cells of transgenic mice increased lung size and caused type II cell hyperplasia, demonstrating an unexpected role for the molecule in the regulation of type II cell proliferation and differentiation.

Pulmonary dysfunction in neonatal SP-B-deficient mice

Pulmonary function was assessed in newborn wild-type and homozygous and heterozygous surfactant protein B (SP-B)-deficient mice after birth. SP-B +/+ and SP-B+/- mice became well oxygenated and survived postnatally. Although lung compliance was decreased slightly in the SP-B+/- mice, lung volumes and compliances were decreased markedly in homozygous SP-B-/- mice. They died rapidly after birth, failing to inflate their lungs or oxygenate. SP-B proprotein was absent in the SP-B-/- mice and was reduced in the SP-B+/- mice, as assessed by Western analysis. Surfactant protein A, surfactant proprotein C, surfactant protein D, and surfactant phospholipid content in lungs from SP-B+/- and SP-B-/- mice were not altered. Lung saturated phosphatidylcholine and precursor incorporation into saturated phosphatidylcholine were not influenced by SP-B genotype. Intratracheal administration of perfluorocarbon resulted in lung expansion, oxygenation, and prolonged survival of SP-B-/- mice and in reduced lung compliance in SP-B+/+ and SP-B+/- mice. Lack of SP-B caused respiratory failure at birth, and decreased SP-B protein was associated with reduced lung compliance. These findings demonstrate the critical role of SP-B in perinatal adaptation to air breathing.

Surfactant metabolic consequences of overexpression of GM-CSF in the epithelium of GM-CSF-deficient mice

Granulocyte macrophage colony-stimulating factor (GM-CSF) is a regulator of surfactant metabolism because GM-CSF-deficient mice have abnormally slow clearance and catabolism of saturated phosphatidylcholine (Sat PC) and surfactant protein (SP)-A in airspaces and lung tissue. Overexpression of GM-CSF only in respiratory epithelial cells of mice deficient in GM-CSF using the SP-C promotor (GM-/-,SP-C-GM+/+) resulted in increased type II cell numbers and normalization of alveolar Sat PC pool sizes. Metabolic measurements demonstrated that incorporation of radiolabeled choline and palmitate was increased more than twofold, but the amount of radiolabeled Sat PC that accumulated in airspaces relative to the amount incorporated was decreased by 50% relative to normal GM+/+ mice. The clearance of dipalmitoylphosphatidylcholine and SP-B from the airspaces was more rapid for GM-/-,SP-C-GM+/+ mice than for GM+/+ mice. Loss of Sat PC and SP-B from the lungs (alveolar plus lung tissue) was similar in the two strains of mice. The normal surfactant pools in the GM-/-,SP-C-GM+/+ mice were achieved by the net effects of increases in type II cell numbers, increased incorporation, decreased accumulation, and increased reuptake rates for surfactant components, demonstrating the multiple effects of GM-CSF on surfactant metabolism.

Expression and glucocorticoid regulation of surfactant protein C in human fetal lung

The hydrophobic surfactant protein C (SP-C) is known to modulate the biophysical properties of surfactant phospholipid. Although SP-C mRNA has been demonstrated in human fetal lung, there is limited information regarding developmental expression and processing of proSP-C protein. Two epitope-specific human proSP-C antisera, anti-hCPROSP-C (His59-Ser72) and anti-hCTERMSP-C (Gly162-Gly175), were generated to complement previously produced anti-NPROSP-C (Met10-Gln23) for the study of proSP-C expression in human fetal lung. Western blotting and immunocytochemistry detected expression of proSP-C protein by 12-16 wk of gestation. ProSP-C immunoreactivity of preculture lung, limited to expression of proSP-C21 in airway epithelial cells, was markedly enhanced by culture of lung explants in dexamethasone. To examine synthesis of proSP-C, homogenates from explants were labeled with 35S-Met/Cys for 0.5-4 h. Immunoprecipitation with anti-NPROSP-C detected 35S-proSP-C21 by 30 min and, after 2 h of labeling, there was a 15-fold increase in 35S-proSP-C21 in dexamethasone-treated lungs versus controls. Synthesis of proSP-C21 was followed by the appearance of a 24-kD form and smaller processing intermediates including 6-10-kD forms. Posttranslational processing of proSP-C21 was not observed in control explants. SP-C(6-10) were not recognized by either anti-CPROSP-C or anti-hCTERMSP-C. These results indicate that low level expression of proSP-C protein first occurs in epithelial cells early in the second trimester and that expression can be enhanced by dexamethasone. Initial posttranslational processing of human proSP-C involves modification of proSP-C21 to SP-C24 and subsequent proteolysis of C-terminal propeptide domains. We speculate that absence of low Mr intermediates in unstimulated second trimester fetal lung tissue reflects developmental and glucocorticoid dependent regulation of proSP-C21 synthesis and posttranslational processing.

TGF-beta 3 inhibits the increased gene expressions of pulmonary surfactant proteins induced by dexamethasone in fetal rat lung in vitro

OBJECTIVE

To determine the effects of transforming growth factor-beta 3 (TGF-beta 3) on expressions of surfactant proteins (SPs).

METHODS

Northern blot hybridization and reverse transcriptase polymerase chain reaction (RT-PCR) were employed to determine the SP-A, SP-B and SP-C mRNA levels of cultured lung explants and type II cells from fetal rat lungs.

RESULTS

TGF-beta 3 alone did not affect the expressions of SPs either in explants or in type II cells. However, TGF-beta 3 did inhibit the increase of SP-B, SP-C mRNA levels caused by dexamethasone (100 nmol/L), in a dose-dependent manner. Furthermore, TGF-beta 3 had no direct inhibition on the increase of SP-B, SP-C mRNA induced by dexamethasone in type II cells, but had an indirect inhibitory effect mediated by fibroblasts.

CONCLUSION

TGF-beta 3 plays an important inhibitory role in the development of pulmonary surfactant proteins. The present study may provide a possible explanation for the unideal effect of glucocorticoids in the prevention of neonatal respiratory distress syndrome.

Recombinant expression of the MAL proteolipid, a component of glycolipid-enriched membrane microdomains, induces the formation of vesicular structures in insect cells

The MAL proteolipid has been identified as a component of glycolipid-enriched membrane microdomains resistant to detergent solubilization in epithelial Madin-Darby canine cells, as well as in T lymphocytes and in myelin-forming cells. To study the function of the MAL proteolipid we have ectopically expressed a tagged form of MAL in both mammalian and insect cellular backgrounds. Immunofluorescence analysis in transiently transfected COS-7 cells showed the presence of MAL in large vesicular structures, and biochemical analysis identified MAL in the fraction of membranes resistant to Triton X-100 solubilization. Electron microscopic analysis showed that the expression of MAL in Sf21 cells morphologically resulted in the intracellular accumulation of large vesicles with a diameter from 200 to greater than 700 nm that were absent in uninfected or control infected cultures. Thus, ectopic expression of MAL in this heterologous expression system was sufficient to drive the formation of vesicles with a size similar to that of the vesicles detected in mammalian cells. These vesicles were clearly different from the caveolae-like vesicles induced by caveolin expression, as evidenced by co-infection experiments using a recombinant caveolin baculovirus. Taken together, these results suggest that the MAL proteolipid might play a role as a component of the machinery of vesiculation of glycolipid-enriched membranes.

Clearance of intra-amniotic lung surfactant: uptake and utilization by the fetal rabbit lung

To investigate the metabolism of intra-amniotic surfactant, surfactant containing double-labeled dipalmitoylphosphatidylcholine (DPPC) was injected in amniotic fluid on days 23-27 of gestation. Within 44 h, DPPC was distributed to the gastrointestinal tract (45.9%), fetal membranes and placenta (8.2%), fetal lung (6.6%), and liver (1.9%). DPPC uptake was higher in the upper than in the lower lung lobes. The mixture of phosphatidylglycerol and DPPC increased the uptake of DPPC that was not saturable (range 15-60 mg phospholipid). There was no detectable metabolism of DPPC taken up by the fetal lung. Surfactant protein A, originating from intra-amniotic heterplogous surfactant, was detected immunohistochemically in alveolar epithelium. Intra-amniotic surfactants did not affect the expression of surfactant protein mRNAs. Intra-amniotic surfactant (1,500-2,000 mg/kg on day 25.3) improved lung compliance of ventilated 27.0-day premature rabbits less than intratracheal surfactant at birth (75-100 mg/kg). Reutilization by the alveolar epithelium of surfactant secreted to future airspaces, airways, and amniotic fluid may be a mechanism that increases intracellular surfactant pool before birth.

Intraamniotic interleukin-1 accelerates surfactant protein synthesis in fetal rabbits and improves lung stability after premature birth

Intraamniotic infection is associated with increased IL-1 activity in amniotic fluid, increased incidence of preterm labor, and with decreased incidence of respiratory distress syndrome in infants born prematurely. We hypothesized that an elevated IL-1 in amniotic fluid promotes fetal lung maturation. On day 23 or 25 of gestation (term 31 d), either IL-1alpha (150 or 1,500 ng per fetus) or its antagonist IL-1 receptor antagonist (IL-1ra, 20 microg) was injected to the amniotic fluid sacs in one uterine horn, whereas the contralateral amniotic sacs were injected with vehicle. Within 40 h, IL-1alpha caused a dose-dependent increase in surfactant protein-A (SP-A) and SP-B mRNAs (maximally, fivefold), without affecting lung growth or increasing inflammatory cells in the lung. Both genders, and upper and lower lung lobes were similarly affected. IL-1ra did not modify SP-A, -B, or -C mRNA. IL-1 increased the intensity of staining of alveolar type II cells for SP-B, and the concentrations of SP-B, -A, and disaturated phosphatidylcholine in bronchoalveolar lavage. The dynamic lung compliance and the postventilatory expansion of lungs were increased two- to fourfold after IL-1alpha treatment. In fetal lung explants, IL-1alpha increased the expression of SP-A mRNA. IL-1 in amniotic fluid in preterm labor may promote lung maturation and thus be part of a host-defense mechanism that prepares the fetus for extrauterine life.