Developmental and hormonal regulation of surfactant protein C (SP-C) gene expression in fetal lung. Role of transcription and mRNA stability

Methylprednisolone pulse treatment improves ProSP-C trafficking in twins with SFTPC mutation: An isoform story?

Mutations in the gene encoding surfactant protein C (SP-C) cause interstitial lung disease (ILD), and glucocorticosteroid (GC) treatment is the most recognized therapy in children. We aimed to decipher the mechanisms behind successful GC treatment in twins carrying a BRICHOS c.566G > A (p.Cys189Tyr) mutation in the SP-C gene (SFTPC). METHODS: The twins underwent bronchoscopy before and after GC treatment and immunoblotting analysis of SP-C proprotein (proSP-C) and SP-C mature in bronchoalveolar fluid (BALF). Total RNA was extracted and analysed using quantitative real-time PCR assays. In A549 cells, the processing of mutated protein C189Y was studied by immunofluorescence and immunoblotting after heterologous expression of eukaryotic vectors containing wild type or C189Y mutant cDNA. RESULTS: Before treatment, BALF analysis identified an alteration of the proSP-C maturation process. Functional study of C189Y mutation in alveolar A549 cells showed that pro-SP-C^C189Y was retained within the endoplasmic reticulum together with ABCA3. After 5 months of GC treatment with clinical benefit, the BALF analysis showed an improvement of proSP-C processing. SFTPC mRNA analysis in twins revealed a decrease in the expression of total SFTPC mRNA and a change in its splicing, leading to the expression of a second shorter proSP-C isoform. In A549 cells, the processing and the stability of this shorter wild-type proSP-C isoform was similar to that of the longer isoform, but the half-life of the mutated shorter isoform was decreased. These results suggest a direct effect of GC on proSP-C metabolism through reducing the SFTPC mRNA level and favouring the expression of a less stable protein isoform.

Organic dust inhibits surfactant protein expression by reducing thyroid transcription factor-1 levels in human lung epithelial cells

Exposure to organic dust is a risk factor for the development of respiratory diseases. Surfactant proteins (SP) reduce alveolar surface tension and modulate innate immune responses to control lung inflammation. Therefore, changes in SP levels could contribute to the development of organic-dust-induced respiratory diseases. Because information on the effects of organic dust on SP levels is lacking, we studied the effects of dust from a poultry farm on SP expression. We found that dust extract reduced SP-A and SP-B mRNA and protein levels in H441 human lung epithelial cells by inhibiting their promoter activities, but did not have any effect on SP-D protein levels. Dust extract also reduced SP-A and SP-C levels in primary human alveolar epithelial cells. The inhibitory effects were not due to LPS or protease activities present in dust extract or mediated via oxidative stress, but were dependent on a heat-labile factor(s). Thyroid transcription factor-1, a key transcriptional activator of SP expression, was reduced in dust-extract-treated cells, indicating that its down-regulation mediates inhibition of SP levels. Our study implies that down-regulation of SP levels by organic dust could contribute to the development of lung inflammation and respiratory diseases in humans.

Posttranscriptional control of airway inflammation

Acute inflammation in the lungs is a vital protective response, efficiently and swiftly eliminating inciters of tissue injury. However, in respiratory diseases characterized by chronic inflammation, such as chronic obstructive pulmonary disease and asthma, enhanced expression of inflammatory mediators leads to tissue damage and impaired lung function. Although transcription is an essential first step in the induction of proinflammatory genes, tight regulation of inflammation requires more rapid, flexible responses. Increasing evidence shows that such responses are achieved by posttranscriptional mechanisms directly affecting mRNA stability and translation initiation. RNA-binding proteins, microRNAs, and long noncoding RNAs interact with messenger RNA and each other to impact the stability and/or translation of mRNAs implicated in lung inflammation. Recent research has shown that these biological processes play a central role in the pathogenesis of several important pulmonary conditions. This review will highlight several posttranscriptional control mechanisms that influence lung inflammation and the known associations of derangements in these mechanisms with common respiratory diseases. WIREs RNA 2018, 9:e1455. doi: 10.1002/wrna.1455 This article is categorized under: RNA in Disease and Development > RNA in Disease RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA Turnover and Surveillance > Regulation of RNA Stability.

Transcriptional mechanisms and protein kinase signaling mediate organic dust induction of IL-8 expression in lung epithelial and THP-1 cells

Exposure to the agricultural work environment is a risk factor for the development of respiratory symptoms and chronic lung diseases. Inflammation is an important contributor to the pathogenesis of tissue injury and disease. Cellular and molecular mechanisms mediating lung inflammatory responses to agricultural dust are not yet fully understood. We studied the effects of poultry dust extract on molecular regulation of interleukin-8 (IL-8), a proinflammatory cytokine, in A549 and Beas2B lung epithelial and THP-1 monocytic cells. Our findings indicate that poultry dust extract potently induces IL-8 levels by increasing IL-8 gene transcription without altering IL-8 mRNA stability. Increase in IL-8 promoter activity was due to enhanced binding of activator protein 1 and NF-κB. IL-8 induction was associated with protein kinase C (PKC) and mitogen-activated protein kinase (MAPK) activation and inhibited by PKC and MAPK inhibitors. IL-8 increase was not inhibited by polymyxin B or l-nitroarginine methyl ester, indicating lack of involvement of lipopolysaccharide and nitric oxide in the induction. Lung epithelial and THP-1 cells share common mechanisms for induction of IL-8 levels. Our findings identify key roles for transcriptional mechanisms and protein kinase signaling pathways for IL-8 induction and provide insights into the mechanisms regulating lung inflammatory responses to organic dust exposure.

Early secreted antigenic target of 6 kDa (ESAT-6) protein of Mycobacterium tuberculosis induces interleukin-8 (IL-8) expression in lung epithelial cells via protein kinase signaling and reactive oxygen species

Early secreted antigenic target of 6 kDa (ESAT-6) of Mycobacterium tuberculosis is critical for the virulence and pathogenicity of M. tuberculosis. IL-8, a major chemotactic cytokine for neutrophils and T lymphocytes, plays important roles in the development of lung injury. To further understand the role of ESAT-6 in lung pathology associated with tuberculosis development, we studied the effects of ESAT-6 on the regulation of IL-8 expression in lung epithelial cells. ESAT-6 induced IL-8 expression by increasing IL-8 gene transcription and mRNA stability. ESAT-6 induction of IL-8 promoter activity was dependent on nuclear factor-κB (NF-κB) and activator protein-1 (AP-1) binding and sensitive to pharmacological inhibition of PKC and ERK and p38 MAPK pathways. ESAT-6 activated ERK and p38 MAPK phosphorylation and rapidly induced reactive oxygen species (ROS) production. Dimethylthiourea but not mannitol inhibited IL-8 induction by ESAT-6, further supporting the involvement of ROS in the induction of IL-8 expression. Exposure of mice to ESAT-6 induced localized inflammatory cell aggregate formation with characteristics of early granuloma concomitant with increased keratinocyte chemoattractant CXCL1 staining in bronchiolar and alveolar type II epithelial cells and alveolar macrophages. Our studies have identified a signal transduction pathway involving ROS, PKC, ERK, and p38 MAPKs and NF-κB and AP-1 in the ESAT-6 induction of IL-8 expression in lung epithelial cells. This has important implications for the understanding of lung innate immune responses to tuberculosis and the pathogenesis of lung injury in tuberculosis.

Transcriptional regulation of SP-B gene expression by nitric oxide in H441 lung epithelial cells

Surfactant protein B (SP-B) is essential for the surface tension-lowering function of pulmonary surfactant. Surfactant dysfunction and reduced SP-B levels are associated with elevated nitric oxide (NO) in inflammatory lung diseases, such as acute respiratory distress syndrome. We previously found that NO donors decreased SP-B expression in H441 and MLE-12 lung epithelial cells by reducing SP-B promoter activity. In this study, we determined the roles of DNA elements and interacting transcription factors necessary for NO inhibition of SP-B promoter activity in H441 cells. We found that the NO donor diethylenetriamine-nitric oxide adduct (DETA-NO) decreased SP-B promoter thyroid transcription factor 1 (TTF-1), hepatocyte nuclear factor 3 (HNF-3), and Sp1 binding activities but increased activator protein 1 (AP-1) binding activity. DETA-NO decreased TTF-1, but not Sp1, levels, suggesting that reduced TTF-1 expression contributes to reduced TTF-1 binding activity. Lack of effect on Sp1 levels suggested that DETA-NO inhibits Sp1 binding activity per se. Overexpression of Sp1, but not TTF-1, blocked DETA-NO inhibition of SP-B promoter activity. DETA-NO inhibited SP-B promoter induction by exogenous TTF-1 without altering TTF-1 levels. DETA-NO decreased TTF-1 mRNA levels and gene transcription rate, indicating that DETA-NO inhibits TTF-1 expression at the transcriptional level. We conclude that NO inhibits SP-B promoter by decreasing TTF-1, Sp1, and HNF-3 binding activities and increasing AP-1 binding activity. NO inhibits TTF-1 levels and activity to decrease SP-B expression. NO inhibition of SP-B expression could be a mechanism by which surfactant dysfunction occurs in inflammatory lung diseases.

Alterations of phospholipid concentration and species composition of the intestinal mucus barrier in ulcerative colitis: a clue to pathogenesis

BACKGROUND

Phospholipids are essential for the normal function of the intestinal mucus barrier. The objective of this study was to systematically investigate phospholipids in the intestinal mucus of humans suffering from inflammatory bowel diseases, where a barrier defect is strongly supposed to be pathogenetic.

METHODS

Optimal mucus recovery was first validated in healthy mice and the method was then transferred to the endoscopic acquisition of ileal and colonic mucus from 21 patients with ulcerative colitis (UC), 10 patients with Crohn's disease (CD), and 29 healthy controls. Nano-electrospray ionization tandem mass spectrometry (ESI-MS/MS) was used to determine phosphatidylcholine (PC), lysophosphatidylcholine (LPC), and sphingomyelin (SM) in lipid extracts of mucus specimens.

RESULTS

Human and rodent mucus contained very similar phospholipid species. In the ileal and colonic mucus from patients suffering from UC, the concentration of PC was highly significantly lower (607 +/- 147 pmol/100 microg protein and 745 +/- 148 pmol/100 microg protein) compared to that of patients with CD (3223 +/- 1519 pmol/100 microg protein and 2450 +/- 431 pmol/100 microg protein) and to controls (3870 +/- 760 pmol/100 microg protein and 2790 +/- 354 pmol/100 microg protein); overall, P = 0.0002 for ileal specimens and P < 0.0001 for colonic specimens. Independent of disease activity, patients suffering from UC showed an increased saturation grade of PC fatty acid residues and a higher LPC-to-PC ratio.

CONCLUSIONS

The intestinal mucus barrier of patients with UC is significantly altered concerning its phospholipid concentration and species composition. These alterations may be very important for the pathogenesis of this disease and underline new therapeutic strategies.

The role of sphingosine 1-phosphate in the TNF-alpha induction of IL-8 gene expression in lung epithelial cells

Tumor necrosis factor-alpha (TNF-alpha) is an important cytokine involved in the pathogenesis of inflammatory diseases of the lung. Interleukin-8 (IL-8), a C-X-C chemokine, is induced by TNF-alpha and initiates injury by acting as a chemoattractant for neutrophils and other immune cells. Although sphingolipids such as ceramide and sphingosine 1-phosphate (S1-P) have been shown to serve as signaling molecules in the TNF-alpha inflammatory response, their role in the TNF-alpha induction of IL-8 gene expression in lung epithelial cells is not known. We investigated the role of sphingolipids in the TNF-alpha induction of IL-8 gene expression in H441 lung epithelial cells. We found that TNF-alpha induced IL-8 mRNA levels by increasing gene transcription, and the stability of IL-8 mRNA was not affected. Exogenous S1-P but not ceramide or sphingosine increased IL-8 mRNA levels and IL-8 secretion. Dimethylsphingosine, an inhibitor of sphingosine kinase, partially inhibited TNF-alpha induction of IL-8 mRNA levels indicating the importance of intracellular increases in S1-P in the IL-8 induction. S1-P induction of IL-8 mRNA was due to an increase in gene transcription, and the stability of IL-8 mRNA was not affected. S1-P induction of IL-8 mRNA was associated with an increase in the binding activity of AP-1 but the activities of NF-kappaB and NF IL-6 were unchanged. S1-P induced the phosphorylation of ERK, p38 and JNK MAPKs. Pharmacological inhibitors of ERK and p38 but not JNK partly inhibited S1-P induction of IL-8 mRNA levels. These data show that increases in the intracellular S1-P partly mediate TNF-alpha induction of IL-8 gene expression in H441 lung epithelial cells via ERK and p38 MAPK signaling pathways and increased AP-1 DNA binding.

Molecular characterization of the porcine surfactant, pulmonary-associated protein C gene

The surfactant, pulmonary-associated protein C (SFTPC) is a peptide secreted by the alveolar type II pneumocytes of the lung. We have characterized the porcine SFTPC gene at genomic, transcriptional, and protein levels. The porcine SFTPC is a single-copy gene on pig chromosome 14. Two transcripts were found in a newborn pig lung cDNA library: a full-length clone and a clone missing exon 5. cDNA sequence comparison revealed four synonymous and two nonsynonymous substitutions and in-frame insertions at the beginning of exon 5. Comparison of the SFTPC coding region between several mammals showed high levels of conservation. Northern blot studies showed lung-specific expression of the full-length SFTPC transcript, appearing in 50-day-old fetus and increasing during lung development. Both SFTPC transcripts were detected mainly in lung by real-time RT-PCR and they were significantly down-regulated in necrotic lungs of pigs infected with Actinobacillus pleuropneumoniae. Additionally, the protein levels were also decreased or absent in the necrotic tissue.

Nitric oxide increases IL-8 gene transcription and mRNA stability to enhance IL-8 gene expression in lung epithelial cells

Interleukin (IL)-8, a C-X-C chemokine, is a potent chemoattractant and an activator for neutrophils, T cells, and other immune cells. The airway and respiratory epithelia play important roles in the initiation and modulation of inflammatory responses via production of cytokines and surfactant. The association between elevated levels of nitric oxide (NO) and IL-8 in acute lung injury associated with sepsis, acute respiratory distress syndrome, respiratory syncytial virus infection in infants, and other inflammatory diseases suggested that NO may play important roles in the control of IL-8 gene expression in the lung. We investigated the role of NO in the control of IL-8 gene expression in H441 lung epithelial cells. We found that a variety of NO donors significantly induced IL-8 mRNA levels, and the increase in IL-8 mRNA was associated with an increase in IL-8 protein. NO induction of IL-8 mRNA was due to increases in IL-8 gene transcription and mRNA stability. NO induction of IL-8 mRNA levels was not inhibited by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one and KT-5823, inhibitors of soluble guanylate cyclase and protein kinase G, respectively, and 8-bromo-cGMP did not increase IL-8 mRNA levels. This indicated that NO induces IL-8 mRNA levels independently of changes in the intracellular cGMP levels. NO induction of IL-8 mRNA was significantly reduced by inhibitors of extracellular regulated kinase and protein kinase C. IL-8 induction by NO was also reduced by hydroxyl radical scavengers such as dimethyl sulfoxide and dimethylthiourea, indicating the involvement of hydroxyl radicals in the induction process. NO induction of IL-8 gene expression could be a significant contributing factor in the initiation and induction of inflammatory response in the respiratory epithelium.

Inflammatory and anti-inflammatory responsiveness of surfactant proteins in fetal and neonatal rabbit lung

Spontaneous preterm birth due to intrauterine infection is associated with increased concentrations of cytokines in amniotic fluid and in the airways at birth. Intra-amniotic IL-1 induces fetal lung maturity, consistent with the decrease in the incidence of respiratory distress syndrome (RDS) in intrauterine inflammation. On the other hand, antenatal corticosteroid decreases the incidence of RDS in infants born prematurely. The aim of the present study was to investigate the interaction between IL-1 and glucocorticoid in the expression of the surfactant proteins SP-A, -B, and -C. Lung explants from rabbit fetuses at 22 (immature), 27 (transitional), and 30 (mature) d of gestation (term, 30-31 d) and on d 1 after term birth were cultured with dexamethasone (Dx), IL-1alpha, or vehicle in the presence or absence of actinomycin D. According to the present results, IL-1alpha and Dx additively increased the expression of SP-A and SP-B on d 22. Later in gestation, SP-B and SP-C were suppressed by IL-1, whereas glucocorticoid tended to increase the expression of SP-B and SP-C and prevented the IL-1-induced suppression of SP. IL-1alpha and steroid interactively increased the stability of SP mRNA compared with the single agonist, possibly explaining the additive effects on the SP mRNA levels. The present results reveal beneficial additive effects of glucocorticoid and cytokine on lung surfactant. They may explain some of the acute beneficial effects of glucocorticoid therapy in chorioamnionitis before premature birth and in inflammatory lung disease after birth.

Role of transcription factors in fetal lung development and surfactant protein gene expression

Branching morphogenesis of the lung and differentiation of specialized cell populations is dependent upon reciprocal interactions between epithelial cells derived from endoderm of embryonic foregut and surrounding mesenchymal cells. These interactions are mediated by elaboration and concerted actions of a variety of growth and differentiation factors binding to specific receptors. Such factors include members of the fibroblast growth factor family, sonic hedgehog, members of the transforming growth factor-beta family, epidermal growth factor, and members of the platelet-derived growth factor family. Hormones that increase cyclic AMP formation, glucocorticoids, and retinoids also play important roles in branching morphogenesis, alveolar development, and cellular differentiation. Expression of the genes encoding these morphogens and their receptors is controlled by a variety of transcription factors that also are highly regulated. Several of these transcription factors serve dual roles as regulators of genes involved in early lung development and in specialized functions of differentiated cells. Targeted null mutations of genes encoding many of these morphogens and transcription factors have provided important insight into their function during lung development. In this chapter, the cellular and molecular mechanisms that control lung development are considered, as well as those that regulate expression of the genes encoding the surfactant proteins.

Surfactant protein C in fetal and ventilated preterm rabbit lungs

The developing lung contains surfactant protein (SP) C mRNA levels comparable to term values before mature type II cells and alveolar surfactant lipids are detectable. Estimates of the amount of mature SP-C in the alveolar lavages of preterm lungs are not available. We used an antibody to a recombinant human SP-C to measure the amount of SP-C in alveolar lavages of preterm fetal rabbits, ventilated preterm rabbits, and term rabbits. The amounts of SP-C were compared with the amounts of saturated phosphatidylcholine (Sat PC). Median Sat PC amounts increased about 680-fold, and median SP-C values increased by over 5,000-fold in alveolar washes from 27 days gestation to term. There was no increase in Sat PC or SP-C with ventilation at 27 and 28 days gestation, but ventilation increased both Sat PC and SP-C at 29 days gestation. The molar percent of SP-C relative to Sat PC also increased with gestational age and with ventilation at 29 days gestation. proSP-C was abundant in a membrane fraction from lung tissue at 27 and 28 days gestation when minimal mature SP-C was detected in alveolar washes. At 29 days and at term, proSP-C decreased in membrane fractions. The preterm lung that is surfactant lipid deficient is also severely deficient in mature SP-C.

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.

Primary cell culture of human type II pneumonocytes: maintenance of a differentiated phenotype and transfection with recombinant adenoviruses

Studies of the regulation of surfactant lipoprotein metabolism and secretion and surfactant protein gene expression have been hampered by the lack of a cell culture system in which the phenotypic properties of type II cells are maintained. We have developed a primary culture system that facilitates the maintenance of a number of morphologic and biochemical properties of type II pneumonocytes for up to 2 wk. Cells were isolated by collagenase digestion of midgestation human fetal lung tissue that had been maintained in organ culture in the presence of dibutyryl cyclic AMP (Bt2cAMP) for 5 days. The isolated cells were enriched for epithelial components by treatment with DEAE-dextran, plated on an extracellular matrix (ECM) derived from Madin-Darby canine kidney (MDCK) cells, and incubated at an air/liquid interface in a minimal amount of culture medium containing Bt2cAMP. The cell cultures were comprised of islands of round epithelial-like cells containing numerous dense osmiophilic granules, surrounded by sparse spindle-shaped cells with the appearance of fibroblasts. Ultrastructural examination revealed that the osmiophilic granules had the appearance of lamellar bodies, the distinguishing feature of type II pneumonocytes. Additionally, the cultures maintained elevated levels of SP-A gene expression for up to 2 wk. The expression of mRNAs encoding SP-A, SP-B, and SP-C were regulated in the cultured cells by glucocorticoids and cyclic AMP in a manner similar to that observed in fetal lung tissue in organ culture. The differentiated phenotype was most apparent when the cells were cultured at an air/liquid interface. In order to utilize the cultured type II cells for study of the effects of overexpression of various proteins and for promoter analysis, it is of essence to transfect DNA constructs into these cells with high efficiency. Unfortunately, we found the cells to be refractory to efficient transfer of DNA using conventional methods (i.e., lipofection, electroporation, or calcium phosphate-mediated transfection). However, replication-defective recombinant human adenoviruses were found to provide a highly efficient means of introducing DNA into the type II pneumonocytes. Furthermore, we observed in type II cell-enriched cultures infected with recombinant adenoviruses containing the lacZ gene under control of a cytomegalovirus promoter, that beta-galactosidase was expressed uniformly in the islands of type II cells and surrounding fibroblasts. By contrast, in cultures infected with recombinant adenoviruses containing the human growth hormone (hGH) gene under control of the SP-A gene promoter and 5'-flanking region, hGH was expressed only in the type II cells. Thus, this culture system provides an excellent means for identifying genomic elements that mediate type II cell-specific gene expression.