Transcriptional regulation of human pulmonary surfactant proteins SP-B and SP-C by glucocorticoids

The Glucocorticoid Receptor: Isoforms, Functions, and Contribution to Glucocorticoid Sensitivity

Glucocorticoids exert pleiotropic effects on all tissues to regulate cellular and metabolic homeostasis. Synthetic forms are used therapeutically in a wide range of conditions for their anti-inflammatory benefits, at the cost of dose and duration-dependent side effects. Significant variability occurs between tissues, disease states, and individuals with regard to both the beneficial and deleterious effects. The glucocorticoid receptor (GR) is the site of action for these hormones and a vast body of work has been conducted understanding its function. Traditionally, it was thought that the anti-inflammatory benefits of glucocorticoids were mediated by transrepression of pro-inflammatory transcription factors, while the adverse metabolic effects resulted from direct transactivation. This canonical understanding of the GR function has been brought into question over the past 2 decades with advances in the resolution of scientific techniques, and the discovery of multiple isoforms of the receptor present in most tissues. Here we review the structure and function of the GR, the nature of the receptor isoforms, and the contribution of the receptor to glucocorticoid sensitivity, or resistance in health and disease.

Glucocorticoids and Their Receptor Isoforms: Roles in Female Reproduction, Pregnancy, and Foetal Development

Alterations in the hypothalamic-pituitary-adrenal (HPA) axis and associated changes in circulating levels of glucocorticoids are integral to an organism's response to stressful stimuli. Glucocorticoids acting via glucocorticoid receptors (GRs) play a role in fertility, reproduction, placental function, and foetal development. GRs are ubiquitously expressed throughout the female reproductive system and regulate normal reproductive function. Stress-induced glucocorticoids have been shown to inhibit reproduction and affect female gonadal function by suppressing the hypothalamic-pituitary-gonadal (HPG) axis at each level. Furthermore, during pregnancy, a mother's exposure to prenatal stress or external glucocorticoids can result in long-lasting alterations to the foetal HPA and neuroendocrine function. Several GR isoforms generated via alternative splicing or translation initiation from the GR gene have been identified in the mammalian ovary and uterus. The GR isoforms identified include the splice variants, GRα and GRβ, and GRγ and GR-P. Glucocorticoids can exert both stimulatory and inhibitory effects and both pro- and anti-inflammatory functions in the ovary, in vitro. In the placenta, thirteen GR isoforms have been identified in humans, guinea pigs, sheep, rats, and mice, indicating they are conserved across species and may be important in mediating a differential response to stress. Distinctive responses to glucocorticoids, differential birth outcomes in pregnancy complications, and sex-based variations in the response to stress could all potentially be dependent on a particular GR expression pattern. This comprehensive review provides an overview of the structure and function of the GR in relation to female fertility and reproduction and discusses the changes in the GR and glucocorticoid signalling during pregnancy. To generate this overview, an extensive non-systematic literature search was conducted across multiple databases, including PubMed, Web of Science, and Google Scholar, with a focus on original research articles, meta-analyses, and previous review papers addressing the subject. This review integrates the current understanding of GR variants and their roles in glucocorticoid signalling, reproduction, placental function, and foetal growth.

The proportions of term or late preterm births after exposure to early antenatal corticosteroids, and outcomes: systematic review and meta-analysis of 1.6 million infants

OBJECTIVE

To systematically review the proportions of infants with early exposure to antenatal corticosteroids but born at term or late preterm, and short term and long term outcomes.

DESIGN

Systematic review and meta-analyses.

DATA SOURCES

Eight databases searched from 1 January 2000 to 1 February 2023, reflecting recent perinatal care, and references of screened articles.

ELIGIBILITY CRITERIA FOR SELECTING STUDIES

Randomised controlled trials and population based cohort studies with data on infants with early exposure to antenatal corticosteroids (<34 weeks) but born at term (≥37 weeks), late preterm (34-36 weeks), or term/late preterm combined.

DATA EXTRACTION AND SYNTHESIS

Two reviewers independently screened titles, abstracts, and full text articles and assessed risk of bias (Cochrane risk of bias tool for randomised controlled trials and Newcastle-Ottawa scale for population based studies). Reviewers extracted data on populations, exposure to antenatal corticosteroids, and outcomes. The authors analysed randomised and cohort data separately, using random effects meta-analyses.

MAIN OUTCOME MEASURES

The primary outcome was the proportion of infants with early exposure to antenatal corticosteroids but born at term. Secondary outcomes included the proportions of infants born late preterm or term/late preterm combined after early exposure to antenatal corticosteroids and short term and long term outcomes versus non-exposure for the three gestational time points (term, late preterm, term/late preterm combined).

RESULTS

Of 14 799 records, the reviewers screened 8815 non-duplicate titles and abstracts and assessed 713 full text articles. Seven randomised controlled trials and 10 population based cohort studies (1.6 million infants total) were included. In randomised controlled trials and population based data, ∼40% of infants with early exposure to antenatal corticosteroids were born at term (low or very low certainty). Among children born at term, early exposure to antenatal corticosteroids versus no exposure was associated with increased risks of admission to neonatal intensive care (adjusted odds ratio 1.49, 95% confidence interval 1.19 to 1.86, one study, 5330 infants, very low certainty; unadjusted relative risk 1.69, 95% confidence interval 1.51 to 1.89, three studies, 1 176 022 infants, I2=58%, τ2=0.01, low certainty), intubation (unadjusted relative risk 2.59, 1.39 to 4.81, absolute effect 7 more per 1000, 95% confidence interval from 2 more to 16 more, one study, 8076 infants, very low certainty, one study, 8076 infants, very low certainty), reduced head circumference (adjusted mean difference -0.21, 95% confidence interval -0.29 to -0.13, one study, 183 325 infants, low certainty), and any long term neurodevelopmental or behavioural disorder in population based studies (eg, any neurodevelopmental or behavioural disorder in children born at term, adjusted hazard ratio 1.47, 95% confidence interval 1.36 to 1.60, one study, 641 487 children, low certainty).

CONCLUSIONS

About 40% of infants exposed to early antenatal corticosteroids were born at term, with associated adverse short term and long term outcomes (low or very low certainty), highlighting the need for caution when considering antenatal corticosteroids.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO CRD42022360079.

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.

Neonatal and Maternal Outcomes of Lower Versus Standard Doses of Antenatal Corticosteroids for Women at Risk of Preterm Delivery: A Systematic Review of Randomized Controlled Trials

OBJECTIVE

Our objective was to systematically review randomized and quasi-randomized trials on the neonatal and maternal effects of lower doses of antenatal corticosteroids (<24 mg of betamethasone or dexamethasone) compared with standard double doses of antenatal corticosteroids (24 mg of betamethasone or dexamethasone) administered to women at risk of preterm delivery.

DATA SOURCES

Medline, Embase, CINAHL, Web of Science, Cochrane CENTRAL, ClinicalTrials.gov, the WHO International Clinical Trials Registry Platform, and the Australia New Zealand Clinical Trials Registry were searched from inception to December 8, 2019.

STUDY SELECTION

A total of 2401 titles, abstracts, and protocols were independently screened by two reviewers, and subsequently 113 full-text articles were reviewed.

DATA EXTRACTION

Our primary outcomes were perinatal death and severe respiratory distress syndrome.

DATA SYNTHESIS

We identified one large in-progress trial comparing 11.4 mg versus 22.8 mg betamethasone and one published randomized controlled trial that compared a lower dose of dexamethasone (16 mg) to a standard dose of betamethasone (24 mg). The only relevant data from the published trial suggests minor changes in fetal heart rate variability between baseline and 24- to 48-hour follow-up between the two groups. Data for other outcomes had to be excluded due to the administration of weekly courses of antenatal corticosteroids.

CONCLUSIONS

Randomized trial data comparing lower doses of antenatal corticosteroids to standard double doses are scarce. Given concerns regarding current antenatal corticosteroids dosing patterns, there is an urgent need for randomized controlled trials examining lower versus standard double doses of antenatal corticosteroids.

Vitamin D deficiency and its effect on respiratory distress syndrome in premature infants: results from a prospective study in a tertiary care centre

BACKGROUND

The positive effects of steroids on lung development are well known, and 1,25-dihydroxy vitamin D₃ has been shown to exert positive effects on fetal lung development.

OBJECTIVE

We aimed to investigate the relationship between 25-hydroxyvitamin D [25(OH)D] levels and respiratory distress syndrome (RDS) in premature infants.

METHODS

Infants aged ≤32 gestational weeks who were admitted to the neonatal intensive care unit (NICU) during 1 year were enrolled in this prospective study. 25(OH)D levels were obtained at the time of admission to NICU. Patients were divided into three groups according to their 25(OH)D levels: severe (group 1), moderate (group 2), and mild (group 3) 25(OH)D deficiencies.

RESULTS

The study comprised 72 patients; of them, RDS was observed in 49 and not observed in 23 patients. The mean 25(OH)D levels were significantly lower in RDS patients (p=0.04). Multivariate analysis showed that patients with higher 25(OH)D levels can be preventive for the development of RDS (odds ratio 0.89; 95% confidence interval 0.8-0.99; p=0.04).

CONCLUSION

Our study revealed that 25(OH)D deficiency is an independent risk factor for RDS in premature infants. However, further studies are necessary to explore the association between 25(OH)D deficiency and RDS.

The window of improved neonatal respiratory compliance after rescue antenatal steroids

OBJECTIVE

To evaluate whether premature infants delivered ≤7 days after rescue antenatal steroid treatment (ideal treatment) have increased passive respiratory compliance compared to those delivered >7 days after treatment (remote treatment).

METHODS

Secondary analysis of a randomized trial of rescue antenatal steroids on respiratory compliance. Infants in the treatment group were stratified by the interval between rescue antenatal steroids and delivery. We then compared the respiratory compliance in the ideal vs. remote groups.

RESULTS

Forty-four women (56 infants) received rescue antenatal steroids. Forty-nine infants had evaluable respiratory compliance measurements, with 27 (GA 30.1 weeks, BW 1362 g) "ideally" treated, and 22 (GA 33.8 weeks, BW 2248 g) "remotely" treated. Respiratory compliance was significantly higher for the ideal compared to the remote group (1.32 vs. 1.06 mL/cm H2O/kg; p = 0.037).

CONCLUSION

Infants treated with rescue antenatal steroids have a significantly higher respiratory compliance if delivery occurs within 7 days after treatment.

Antiinflammatory Effects of Budesonide in Human Fetal Lung

Lung inflammation in premature infants contributes to the development of bronchopulmonary dysplasia (BPD), a chronic lung disease with long-term sequelae. Pilot studies administering budesonide suspended in surfactant have found reduced BPD without the apparent adverse effects that occur with systemic dexamethasone therapy. Our objective was to determine budesonide potency, stability, and antiinflammatory effects in human fetal lung. We cultured explants of second-trimester fetal lung with budesonide or dexamethasone and used microscopy, immunoassays, RNA sequencing, liquid chromatography/tandem mass spectrometry, and pulsating bubble surfactometry. Budesonide suppressed secreted chemokines IL-8 and CCL2 (MCP-1) within 4 hours, reaching a 90% decrease at 12 hours, which was fully reversed 72 hours after removal of the steroid. Half-maximal effects occurred at 0.04-0.05 nM, representing a fivefold greater potency than for dexamethasone. Budesonide significantly induced 3.6% and repressed 2.8% of 14,500 sequenced mRNAs by 1.6- to 95-fold, including 119 genes that contribute to the glucocorticoid inflammatory transcriptome; some are known targets of nuclear factor-κB. By global proteomics, 22 secreted inflammatory proteins were hormonally regulated. Two glucocorticoid-regulated genes of interest because of their association with lung disease are CHI3L1 and IL1RL1. Budesonide retained activity in the presence of surfactant and did not alter its surface properties. There was some formation of palmitate-budesonide in lung tissue but no detectable metabolism to inactive 16α-hydroxy prednisolone. We concluded that budesonide is a potent and stable antiinflammatory glucocorticoid in human fetal lung in vitro, supporting a beneficial antiinflammatory response to lung-targeted budesonide:surfactant treatment of infants for the prevention of BPD.

Association of vitamin D deficiency with acute lower respiratory tract infections in newborns

OBJECTIVE

To determine the association between serum 25-hydroxy vitamin D [25(OH)D] levels and acute respiratory tract infections (ALRTI) in newborns.

STUDY DESIGN

The study group consisted of 30 term newborns with ALRTI who were admitted to our neonatal intensive care unit. Controls were 30 healthy newborns with the same age as the study group. Newborns and their mothers were tested for serum 25(OH)D levels, with a low level defined as ≤15 ng/mL.

RESULTS

The groups were similar in gestational week, birthweight, postnatal age and gender. Forty-three of the 60 infants (including study and control) had low 25(OH)D levels. The median 25(OH)D levels were lower [9.5 ng/mL (IQR = 7.9-12.2)] in the study group than those of the control group [15.5 ng/mL (IQR: 12-18)] (p = 0.0001). The median serum 25(OH)D levels in the mothers of the study group were also lower than those in the mothers of the control group [11.6 ng/mL (IQR = 9.4-15.8) and 17.3 ng/mL (IQR = 13.7-20.6), respectively] (p = 0.0001).

CONCLUSION

Lower blood 25(OH)D levels might be associated with increased risk of ALRTI in term newborn babies. Appropriate vitamin D supplementation during pregnancy and early childhood may enhance newborns' respiratory health.

Antenatal glucocorticoids: where are we after forty years?

Since their introduction more than forty years ago, antenatal glucocorticoids have become a cornerstone in the management of preterm birth and have been responsible for substantial reductions in neonatal mortality and morbidity. Clinical trials conducted over the past decade have shown that these benefits may be increased further through administration of repeat doses of antenatal glucocorticoids in women at ongoing risk of preterm and in those undergoing elective cesarean at term. At the same time, a growing body of experimental animal evidence and observational data in humans has linked fetal overexposure to maternal glucocorticoids with increased risk of cardiovascular, metabolic and other disorders in later life. Despite these concerns, and somewhat surprisingly, there has been little evidence to date from randomized trials of longer-term harm from clinical doses of synthetic glucocorticoids. However, with wider clinical application of antenatal glucocorticoid therapy there has been greater need to consider the potential for later adverse effects. This paper reviews current evidence for the short- and long-term health effects of antenatal glucocorticoids and discusses the apparent discrepancy between data from randomized clinical trials and other studies.

Amplification of steroid-mediated SP-B expression by physiological levels of caffeine

Factors positively influencing surfactant homeostasis in general and surfactant protein B (SP-B) expression in particular are considered of clinical importance regarding an improvement of lung function in preterm infants. The objective of this study was to identify effects of physiological levels of caffeine on glucocorticoid-mediated SP-B expression in vitro and in vivo. Levels of SP-B and pepsinogen C were quantified by quantitative real-time RT-PCR or immunoblotting in NCI-H441 cells daily exposed to caffeine and/or dexamethasone (DEX). In vivo, SP-B expression was analyzed in bronchoalveolar lavage (BAL) of preterm sheep exposed to antenatal DEX and/or postnatal caffeine. If DEX and caffeine were continuously present, SP-B mRNA and protein levels were increased for up to 6 days after induction (P < 0.05). Additionally, caffeine enhanced SP-B mRNA expression in DEX-pretreated cells (P < 0.05). Moreover, caffeine amplified DEX-induced pepsinogen C mRNA expression (P < 0.05). After short-term treatment with caffeine in vivo, only slightly higher SP-B levels could be detected in BAL of preterm sheep following antenatal DEX, combined with an increase of arterial oxygen partial pressure (P < 0.01). Our data demonstrated that the continuous presence of caffeine in vitro is able to amplify DEX-mediated SP-B expression. In contrast, short-term improvement of lung function in vivo is likely to be independent of altered SP-B transcription and translation. An impact of caffeine on release of surfactant reservoirs from lamellar bodies could, however, quickly affect SP-B content in BAL, which has to be further investigated. Our findings indicate that caffeine is able to amplify main effects of glucocorticoids that result from changes in surfactant production, maturation, and release.

Sequences of a hairpin structure in the 3'-untranslated region mediate regulation of human pulmonary surfactant protein B mRNA stability

The ability of pulmonary surfactant to reduce alveolar surface tension requires adequate expression of surfactant protein B (SP-B). Dexamethasone (DEX, 10(-7) M) increases human SP-B mRNA stability by a mechanism that requires a 126-nt-long segment (the 7.6S region) of the 3'-untranslated region (3'-UTR). The objective of this study was to identify sequences in the 7.6S region that mediate regulation of SP-B mRNA stability. The 7.6S region was found to be sufficient for DEX-mediated stabilization of mRNA. Sequential substitution mutagenesis of the 7.6S region indicates that a 90-nt region is required for DEX-mediated stabilization and maintenance of intrinsic stability. In this region, one 30-nt-long element (002), predicted to form a stem-loop structure, is sufficient for DEX-mediated stabilization of mRNA and intrinsic mRNA stability. Cytosolic proteins specifically bind element 002, and binding activity is unaffected whether proteins are isolated from cells incubated in the absence or presence of DEX. While loop sequences of element 002 have no role in regulation of SP-B mRNA stability, the proximal stem sequences are required for DEX-mediated stabilization and specific binding of proteins. Mutation of the sequences that comprise the proximal or distal arm of the stem negates the destabilizing activity of element 002 on intrinsic SP-B mRNA stability. These results indicate that cytosolic proteins bind a single hairpin structure that mediates intrinsic and hormonal regulation of SP-B mRNA stability via mechanisms that involve sequences of the stems of the hairpin structure.

Intra-amniotic LPS and antenatal betamethasone: inflammation and maturation in preterm lamb lungs

The proinflammatory stimulus of chorioamnionitis is commonly associated with preterm delivery. Women at risk of preterm delivery receive antenatal glucocorticoids to functionally mature the fetal lung. However, the effects of the combined exposures of chorioamnionitis and antenatal glucocorticoids on the fetus are poorly understood. Time-mated ewes with singleton fetuses received an intra-amniotic injection of lipopolysaccharide (LPS) either preceding or following maternal intramuscular betamethasone 7 or 14 days before delivery, and the fetuses were delivered at 120 days gestational age (GA) (term = 150 days GA). Gestation matched controls received intra-amniotic and maternal intramuscular saline. Compared with saline controls, intra-amniotic LPS increased inflammatory cells in the bronchoalveolar lavage and myeloperoxidase, Toll-like receptor 2 and 4 mRNA, PU.1, CD3, and Foxp3-positive cells in the fetal lung. LPS-induced lung maturation measured as increased airway surfactant and improved lung gas volumes. Intra-amniotic LPS-induced inflammation persisted until 14 days after exposure. Betamethasone treatment alone induced modest lung maturation but, when administered before intra-amniotic LPS, suppressed lung inflammation. Interestingly, betamethasone treatment after LPS did not counteract inflammation but enhanced lung maturation. We conclude that the order of exposures of intra-amniotic LPS or maternal betamethasone had large effects on fetal lung inflammation and maturation.

A role for mineralocorticoid receptors in the physiology of the ovine fetus: effects on ACTH and lung liquid composition

In the human and ovine fetus, the presence of 11β-hydroxysteroid dehydrogenase 1 allows cortisol and other corticosteroids to act at mineralocorticoid receptors (MRs) in lung and brain. To test the physiologic role of MRs in the late gestation fetus, fetal lambs were infused with a specific MR antagonist for 12 h. Infusion of the MR antagonist significantly increased plasma ACTH and cortisol concentrations. Infusion of the MR antagonist also significantly increased fetal Pco2 and hematocrit, and decreased fetal pH, but did not alter fetal heart rate or blood pressure. Infusion of the MR antagonist altered the ratio of Na⁺ to K⁺ in lung fluid but did not alter the rate of production of lung liquid or the expression of the epithelial sodium channel α or of the Na,K ATPaseα1 in lung. These results suggest that corticosteroids act at MR to regulate ACTH and blood volume and modulate lung fluid composition in the fetus, but basal levels of corticosteroids do not alter lung liquid production rate through effects on MR.

Glucocorticoid regulation of human pulmonary surfactant protein-B (SP-B) mRNA stability is independent of activated glucocorticoid receptor

Adequate expression of surfactant protein-B (SP-B) is critical in the function of pulmonary surfactant to reduce alveolar surface tension. Expression of SP-B mRNA is restricted to specific lung-airway epithelial cells, and human SP-B mRNA stability is increased in the presence of the synthetic glucocorticoid dexamethasone (DEX). Although the mechanism of SP-B mRNA stabilization by DEX is unknown, studies suggest involvement of the glucocorticoid receptor (GR). We developed a dual-cistronic plasmid-based expression assay in which steady-state levels of SP-B mRNA, determined by Northern analysis, reproducibly reflect changes in SP-B mRNA stability. Using this assay, we found that steady-state levels of SP-B mRNA increased greater than twofold in transfected human-airway epithelial cells (A549) incubated with DEX (10(-7) M). DEX-mediated changes in SP-B mRNA levels required the presence of the SP-B mRNA 3'-untranslated region but did not require ongoing protein synthesis. The effect of DEX on SP-B mRNA levels was dose dependent, with maximal effect at 10(-7) M. DEX increased levels of SP-B mRNA in cells lacking GR, and the presence of the GR antagonist RU486 did not interfere with the effect of DEX. Surprisingly, other steroid hormones (progesterone, estradiol, and vitamin D; 10(-7) M) significantly increased SP-B mRNA levels, suggesting a common pathway of steroid hormone action on SP-B mRNA stability. These results indicate that the effect of DEX to increase SP-B mRNA stability is independent of activated GR and suggests that the mechanism is mediated by posttranscriptional or nongenomic effects of glucocorticoids.

In vitroquantification of dexamethasone-induced surfactant protein B expression in human lung cells

OBJECTIVE

To determine whether the effect of a single 48-h exposure to dexamethasone in human lung cells is limited to 7-8 days.

STUDY DESIGN

We used the NCI-H441 cell line, in which stability can be maintained beyond 7 days. The outcome was the stimulatory effect of dexamethasone on surfactant protein B (SP-B) gene transcription as expressed by SP-B mRNA accumulation. The experiment was conducted five times, in parallel with control. SP-B mRNA was determined at baseline, 48 h after dexamethasone exposure, and at 48-h intervals thereafter, up to 14 days, by quantitative reverse transcription polymerase chain reaction. Comparisons were made by the Mann-Whitney test.

RESULTS

In conditions of our experiment, the inductive profile of SP-B mRNA after exposure to dexamethasone demonstrated maximal stimulation at 48 h (13-fold over control). Subsequently, there was a decline in mRNA, with return to near control levels by day 8, suggesting reversibility of dexamethasone action.

CONCLUSION

Our data support the view that the surfactant-inducing properties of corticosteroids are limited to 7-8 days.

Amniotic fluid concentration of surfactant proteins in intra-amniotic infection

OBJECTIVE

Pulmonary surfactant is a complex molecule of lipids and proteins synthesized and secreted by type II alveolar cells into the alveolar epithelial lining. Both lipid and protein components are essential for lung function in postnatal life. Infection is a well-established cause of preterm delivery, and several inflammatory cytokines play a role in the mechanisms of preterm parturition. An increased concentration of inflammatory cytokines in amniotic fluid or fetal plasma has been linked to the onset of preterm parturition and fetal/neonatal injury, including cerebral palsy and chronic lung disease. Experimental evidence indicates that inflammatory mediators also regulate surfactant protein synthesis, and histologic chorioamnionitis is associated with a decreased incidence of hyaline membrane disease in neonates. This study was conducted to determine if amniotic fluid concentrations of surfactant protein (SP)-A, SP-B, and SP-D change in patients with and without intra-amniotic infection (IAI).

MATERIALS AND METHODS

A case-control study was conducted to determine amniotic fluid concentrations of SP-A, SP-B, SP-D, and total protein in patients who had an amniocentesis performed between 18 and 34 weeks of gestation for the detection of IAI in patients with spontaneous preterm labor with intact membranes (n = 42) and cervical insufficiency prior to the application of cerclage (n = 6). Amniotic fluid samples were selected from a bank of biological specimens and included patients with (n = 16) and without (n = 32) IAI matched for gestational age at amniocentesis. Intra-amniotic infection was defined as a positive amniotic fluid culture for microorganisms. Each group was further subdivided according to a history of corticosteroid administration within 7 days prior to amniocentesis into the following subgroups: (1) patients without IAI who had received antenatal corticosteroids (n = 21), (2) patients with IAI who had received antenatal corticosteroids (n = 9), (3) patients without IAI who had not received antenatal corticosteroids (n = 11), and (4) patients with IAI who had not received antenatal corticosteroids (n = 7). Amniotic fluid was obtained by transabdominal amniocentesis. SP-A, SP-B, and SP-D concentrations in amniotic fluid were determined by enzyme-linked immunosorbent assay (ELISA). Non-parametric statistics were used for analysis.

RESULTS

Women with IAI had a higher median amniotic fluid concentration of SP-B and of SP-B/total protein, but not other SPs, than those without IAI (both p = 0.03). Among patients who had received antenatal corticosteroids, the median amniotic fluid concentration of SP-B and of SP-B/total protein was significantly higher in patients with IAI than in those without IAI (SP-B, IAI: median 148 ng/mL, range 37.3-809 ng/mL vs. without IAI: median 7.2 ng/mL, range 0-1035 ng/mL; p = 0.005 and SP-B/total protein, IAI: median 14.1 ng/mg, range 4.3-237.5 ng/mg vs. without IAI: median 1.45 ng/mg, range 0-79.5 ng/mg; p = 0.003). Among women who had not received antenatal corticosteroids, the median amniotic fluid concentrations of SP-B and of SP-B/total protein were not significantly different between patients with and without IAI (SP-B, IAI: median 4 ng/mL, range 0-31.4 ng/mL vs. without IAI: median 3.4 ng/mL, range 0-37 ng/mL; p = 0.8 and SP-B/total protein, IAI: median 0.55 ng/mg, range 0-6.96 ng/mg vs. without IAI: median 0.59 ng/mg, range 0-3.28 ng/mg; p = 0.9). The median amniotic fluid concentrations of SP-A, SP-A/total protein, SP-D, and SP-D/total protein were not significantly different between patients with and without IAI whether they received antenatal corticosteroids or not (all p > 0.05).

CONCLUSIONS

IAI was associated with an increased amniotic fluid concentration of SP-B in patients who received antenatal corticosteroids within 7 days prior to amniocentesis.

Is the fetal lung a mineralocorticoid receptor target organ? Induction of cortisol-regulated genes in the ovine fetal lung, kidney and small intestine

BACKGROUND

Lung, kidney and small intestine are involved in fetal volume regulation and amniotic fluid secretion and play a pivotal role in the transition from intrauterine to extrauterine life.

OBJECTIVE

This study was performed to determine the ontogeny of mineralocorticoid receptors (MR) and glucocorticoid receptors (GR), and of MR- and GR-regulated genes and proteins, serum and glucocorticoid-induced kinase (Sgk-1), epithelial sodium channel (ENaC alpha), and Na,K-ATPase alpha1.

METHODS

Lung, renal cortex and medulla, and small intestine were collected from fetuses at 80, 100, 120, 130 and 145 days' gestation and from day 1 and 7 neonatal lambs. Real-time PCR was performed to determine mRNA concentration for MR, GR, the 11 beta-hydroxysteroid dehydrogenases (11 beta-HSD1 and 2), Sgk-1, ENaC alpha, and Na,K-ATPase alpha1. Protein expression of ENaC alpha and Na,K-ATPase alpha1 in whole cell and membrane fractions was determined by immunoblotting.

RESULTS

Expression of corticosteroid-induced genes in renal cortex increases at term; in small intestine the induction occurs postnatally. In contrast, in lung expression of MR and GR mRNAs were greater at 100 days to term than postnatally and 11 beta-HSD1 peaked at 145 days; the corticosteroid-induced genes also increased prenatally: Sgk-1 and ENaC alpha increased by 120 days, peaking at 145 days, and Na,K-ATPase alpha1 was greatest at 130 days.

CONCLUSIONS

The expression of high levels of MR and 11 beta-HSD1 in preterm fetal lung suggest low endogenous fetal cortisol may exert actions at the high affinity MR in vivo, leading to increases in expression of sodium channels important in the regulation of lung liquid secretion and reabsorption.

Multiple courses of antenatal corticosteroids for preterm birth (MACS): a randomised controlled trial

BACKGROUND

One course of antenatal corticosteroids reduces the risk of respiratory distress syndrome and neonatal death. Weekly doses given to women who remain undelivered after a single course may have benefits (less respiratory morbidity) or cause harm (reduced growth in utero). We aimed to find out whether multiple courses of antenatal corticosteroids would reduce neonatal morbidity and mortality without adversely affecting fetal growth.

METHODS

1858 women at 25-32 weeks' gestation who remained undelivered 14-21 days after an initial course of antenatal corticosteroids and continued to be at high risk of preterm birth were randomly assigned to multiple courses of antenatal corticosteroids (n=937) or placebo (n=921), every 14 days until week 33 or delivery, whichever came first. The primary outcome was a composite of perinatal or neonatal mortality, severe respiratory distress syndrome, intraventricular haemorrhage (grade III or IV), periventricular leucomalacia, bronchopulmonary dysplasia, or necrotising enterocolitis. Analysis was by intention to treat. All patients and caregivers were unaware of the treatment given. This trial is registered as number ISRCTN2654148.

FINDINGS

Infants exposed to multiple courses of antenatal corticosteroids had similar morbidity and mortality to those exposed to placebo (150 [12.9%] vs 143 [12.5%]). Those receiving multiple doses of corticosteroids also weighed less at birth than those exposed to placebo (2216 g vs 2330 g, p=0.0026), were shorter (44.5 cm vs 45.4 cm, p<0.001), and had a smaller head circumference (31.1 cm vs 31.7 cm, p<0.001).

INTERPRETATION

Multiple courses of antenatal corticosteroids, every 14 days, do not improve preterm-birth outcomes, and are associated with a decreased weight, length, and head circumference at birth. Therefore, this treatment schedule is not recommended.

FUNDING

Canadian Institutes of Health Research.

Genetic Abnormalities of Surfactant Metabolism

Pulmonary surfactant is the complex mixture of lipids and proteins needed to reduce alveolar surface tension at the air-liquid interface and prevent alveolar collapse at the end of expiration. It has been recognized for almost 50 years that a deficiency in surfactant production due to pulmonary immaturity is the principal cause of the respiratory distress syndrome (RDS) observed in prematurely born infants.1 Secondary surfactant deficiency due to injury to the cells involved in its production and functional inactivation of surfactant is also important in the pathophysiology of acute respiratory distress syndrome (ARDS) observed in older children and adults.2,3 In the past 15 years, it has been recognized that surfactant deficiency may result from genetic mechanisms involving mutations in genes encoding critical components of the surfactant system or proteins involved in surfactant metabolism.4,5 Although rare, these single gene disorders provide important insights into normal surfactant metabolism and into the genes in which frequently occurring allelic variants may be important in more common pulmonary diseases.

Regulation of membrane-associated mucins in the human corneal epithelial cells by dexamethasone

PURPOSE

To study the influence of dexamethasone on membrane-associated mucins produced by human corneal epithelial cells.

METHODS

Human corneal epithelial cells were cultured in medium supplemented with various concentrations of dexamethasone (ranging from 10 to 10 M). Reverse transcription-polymerase chain reaction (RT-PCR) and Western blot analysis using monoclonal antibodies specific for human MUC1 (HMFG-1), MUC4 (1G8), and MUC16 (OC125) were performed to evaluate the effect of dexamethasone on membrane-associated mucin expression. The effect of glucocorticoid receptor antagonist (RU38486) on dexamethasone-induced mucin expression was estimated.

RESULTS

RT-PCR revealed that MUC1 and MUC16 gene expression were upregulated 48 hours after addition of dexamethasone and that MUC4 gene expression was downregulated in the same condition. Western blot analysis showed that MUC1 and MUC16 proteins were increased after addition of dexamethasone. However, MUC4 was not detected by anti-MUC4 monoclonal antibody (1G8) for ASGP-2 under our conditions. Treatment with RU38486 inhibited the changes of MUC1, MUC4, and MUC16 by dexamethasone; thus, the effect of dexamethasone on mucin expression is mediated by glucocorticoid receptors.

CONCLUSIONS

This study shows that MUC1, MUC4, and MUC16 are regulated differently by dexamethasone in human corneal epithelial cells. External application of dexamethasone might affect the precorneal mucin.

Modeling glucocorticoid-mediated fetal lung maturation: II. Temporal patterns of gene expression in fetal rat lung

Our previous report described the temporal steroid patterns during pharmacokinetic (PK) studies with dexamethasone (DEX) where doses of six 1 micromol/kg injections were given during gestational ages 18 to 20 days in rats. DEX PK was used in conjunction with the endogenous corticosterone profile to understand the regulation of fetal lung pharmacodynamics (PD). Expression of the glucocorticoid receptor (GR) and surfactant proteins A and B mRNA were chosen as lung maturational markers. GR seemed to be insensitive to the circulating glucocorticoids, indicating that unlike the adult situation, GR was not under negative feedback control of its ligand. Surfactant protein B exhibited approximately 400-fold induction in control fetal lung during the last days of gestation, and the inductive effect was even greater in the treatment group. Surfactant protein A displayed approximately 100-fold induction in control fetal lung during late gestation. However, the treatment group exhibited biphasic stimulatory and inhibitory effects for surfactant protein A. The inhibitory effect indicated that the chosen dosing scheme for DEX was not an optimal regimen. These data were used to determine by simulation the DEX regimen that would reproduce the temporal pattern of lung maturation observed in control animals. PK/PD modeling indicated that maintaining steroid exposure at approximately twice the equilibrium dissociation constant for the steroid/receptor interaction should produce optimal stimulation of both surfactant proteins. The simulations illustrate that administering smaller quantities of steroids over extended periods that produce sustained steroid exposure might be the optimal approach for designing dose-sparing antenatal corticosteroid therapy.

Glucocorticoid Metabolism in the Human Fetal Lung: Implications for Lung Development and the Pulmonary Surfactant System

It has been nearly 35 years since Liggins and Howie first reported the benefits of antenatal glucocorticoid (GC) treatment to promote the maturation of the human fetal lung, and nearly that long since Pasqualini and colleagues demonstrated that the human fetal lung actively metabolizes GCs. Since that time, our understanding of the effects of GCs on fetal lung maturation and pulmonary surfactant production has increased dramatically. Similarly, characterization of the enzymes involved in GC metabolism has greatly expanded our understanding of GC signaling in target tissues. In man, the biologically active GC (cortisol) and the biologically inactive GC (cortisone) are interconverted by the tissue-specific expression of the type 1 and type 2 11beta-hydroxysteroid dehydrogenase enzymes (HSD1 and HSD2). Much of the research on GC metabolism in peripheral target tissues has focused on the role of HSD1 in amplifying the effects of GCs in liver and adipose tissue or on the role of HSD2 in blocking the effects of GCs in the kidney and placenta. In contrast, the role of GC metabolism in modulating the effects of GCs on fetal lung maturation and the pulmonary surfactant system in humans is less understood. The goal of this review article is to present a brief overview of the role of GCs in human fetal lung maturation and pulmonary surfactant production, and to familiarize the reader with the biochemistry of the metabolism of natural and synthetic GCs by the HSD enzymes. In addition, we will review data concerning the expression and activity of the HSD enzymes in the human fetal lung and contrast this to what is known about the HSD enzymes in the fetal rodent lung. Although rodents, rabbits, sheep, and several primates have been invaluable model systems for the study of fetal lung development, we have chosen to largely focus this review on human lung, since there are significant differences in GC metabolism between humans and other species.

Regulation of human pulmonary surfactant protein gene expression by 1α,25-dihydroxyvitamin D3

1α,25-Dihydroxyvitamin D3[1,25(OH)2D3] has been reported to stimulate lung maturity, alveolar type II cell differentiation, and pulmonary surfactant synthesis in rat lung. We hypothesized that 1,25(OH)2D3stimulates expression of surfactant protein-A (SP-A), SP-B, and SP-C in human fetal lung and type II cells. We found that immunoreactive vitamin D receptor was detectable in fetal lung tissue and type II cells only when incubated with 1,25(OH)2D3. 1,25(OH)2D3significantly decreased SP-A mRNA in human fetal lung tissue but did not significantly decrease SP-A protein in the tissue. In type II cells, 1,25(OH)2D3alone had no significant effect on SP-A mRNA or protein levels but reduced SP-A mRNA and protein in a dose-dependent manner when the cells were incubated with cAMP. SP-A mRNA levels in NCI-H441 cells, a nonciliated bronchiolar epithelial (Clara) cell line, were decreased in a dose-dependent manner in the absence or presence of cAMP. 1,25(OH)2D3had no significant effect on SP-B mRNA levels in lung tissue but increased SP-B mRNA and protein levels in type II cells incubated in the absence or presence of cAMP. Expression of SP-C mRNA was unaffected by 1,25(OH)2D3in lung tissue incubated ± cAMP. These results suggest that regulation of surfactant protein gene expression in human lung and type II cells by 1,25(OH)2D3is not coordinated; 1,25(OH)2D3decreases SP-A mRNA and protein levels in both fetal lung tissue and type II cells, increases SP-B mRNA and protein levels only in type II cells, and has no effect on SP-C mRNA levels.

Asthma research: the real action is in children

Many adult diseases have their roots in infancy or even prenatally. If events that initiate these diseases, as opposed to those that propagate the disease state, are to be understood, then the difficult area of how ethically to research problems in infancy must be tackled. Furthermore, the predisposition to archetypally 'pure' adult problems such as chronic obstructive pulmonary disease, may lie antenatally, the effects being masked until the lung starts to age. An additional factor is that the success of paediatricians, for example in ensuring the survival of extremely premature, low birth weight infants leads to adult survivors with potentially a whole new morbidity. The first prerequisite to making progress is a sound understanding of the development of the normal lung and how adverse environmental and genetic influences, such as exposure to environmental tobacco smoke and maternal atopy, respectively, may affect growth. This paper focuses on three key areas: the implications of different pre-school wheezing phenotypes for adult disease; the importance of very early life events in cystic fibrosis; and the long term consequences of chronic lung disease of prematurity. Finally, the ethical principles that must underpin future research in pre-school children is discussed, as well as the means we might use to further our understanding of the relevant early disease processes.

Glucocorticoid regulation of the inflammatory response to injury

During the first half of the 20th century, physiologists were interested in the adrenal glands primarily because adrenalectomized animals failed to survive even mild degrees of systemic stress. It eventually became clear that hormones secreted by the adrenal cortex were critical for survival and, in this context, adrenal cortical hormones were widely considered to support or stimulate important responses to stress or injury. With the purification and manufacture of adrenal cortical hormones in the 1930s and 1940s, clinicians suddenly discovered the potent anti-inflammatory actions of glucocorticoids (GCs). This dramatic, and unexpected, discovery has dominated clinical and laboratory research into GC actions throughout the second half of the 20th century. More recent research is again reporting GC-induced stimulatory effects on a variety of inflammatory response components. These effects are usually observed at low GC concentrations, close to concentrations that are observed in vivo during basal, unstimulated states. For example, GC-mediated stimulation has been reported for the hepatic acute-phase response, for cytokine secretion, expression of cytokine/chemokine receptors, and for the pro-inflammatory mediator, macrophage migration inhibition factor. It seems clear that the long-held clinical view that GCs act solely as anti-inflammatory agents needs to be re-assessed. Varying doses of GCs do not lead simply to varying degrees of inflammation suppression, but rather GCs can exert a full range of effects from permissive to stimulatory to suppressive.

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.

Transcription of stem cell factor (SCF) is potentiated by glucocorticoids and interleukin‐1β through concerted regulation of a GRE‐like and an NF‐κB response element

Expression of stem cell factor SCF, a major mast cell growth factor, is potentiated shortly after co-treatment with interleukin (IL)-1beta and glucocorticoids. SCF promoter contains a GRE-like sequence and a putative kappaB site. We assessed the mechanisms of the regulation of SCF transcription in human lung fibroblasts in culture. Chromatin immunoprecipitation showed that co-treatment with IL-1beta and the glucocorticoid budesonide increased the SCF promoter occupancy by NF-kappaB and GR, as compared with IL-1beta and budesonide alone. In reporter gene assays, IL-1beta time-dependently increased the promoter activity, which was abolished by either pre-treatment with the MAP kinase inhibitors PD98059 (MEK) and SB203580 (p38), pre-treatment with the NF-kappaB inhibitor PDTC, or deletion of the kappaB site. Budesonide time-dependently decreased the promoter activity, an effect requiring the GRE-like element. Co-treatment with IL-1beta and budesonide potentiated the promoter activity at 30 min, an effect blocked by PD98059 and SB203580, PDTC, or deletion of the kappaB or GRE-like element. In conclusion, the GRE-like sequence mediating the repression of SCF expression, thus acting as a negative-responsive element, is turned into a positive element in an NF-kappaB site-dependent manner, indicating a concerted action of these two regulatory elements in the potentiation of SCF gene expression.

BR22, a 26 kDa thyroid transcription factor-1 associated protein (TAP26), is expressed in human lung cells

The current authors have previously identified BR22, a thyroid transcription factor (TTF)-1 associated protein 26 (TAP26), which interacts with TTF-1 to enhance human surfactant protein (SP)-B promoter activity in transfected 293 cells. However, the expression of TAP26 in the lung cells and its biological relevance to the SP-B production under physiological conditions were not examined. In this study, endogenous co-immunoprecipitation and in situ immunohistochemical staining techniques were employed to explore the presence of TAP26 and TTF-1 complex in the lung epithelial cells. The correlation of TAP26, TTF-1 and SP-B expression was inspected in H441 cells in the presence of dexamethasone, a known positive effector of the SP-B promoter. Monoclonal antibody (mAb) against TAP26 can co-immunoprecipitate both TAP26 and TTF-1 from H441 cells. Using this antibody in in situ staining of human lung sections, the data show that TAP26 is present in the lung alveolar epithelial cells. Reverse transcriptase-polymerase chain reaction and Western blot analyses of type-II cells as well as dexamethasone-treated H441 cells suggest that TAP26 expression is modulated coordinately with SP-B and TTF-1 in these cells. In summary, the current study demonstrates that thyroid transcription factor-1 associated protein 26 is an associated protein of thyroid transcription factor-1 in the lung alveolar epithelial cells where surfactant protein gene expressions take place in vivo.

Intratracheal administration of glucocorticoids using surfactant as a vehicle

1. Glucocorticoids are an effective treatment in the amelioration of chronic lung disease in neonates. However, systemic administration of glucocorticoids to neonates is associated with significant side-effects that preclude them as an early intervention to prevent onset of the condition. Conversely, local intratracheal administration of glucocorticoids may prevent inflammatory insult to the lungs without the development of systemic side-effects. We therefore investigated whether local intratracheal delivery of corticosteroids could be facilitated using surfactant as a vehicle. 2. Addition of dexamethasone to either diluted or commercial artificial surfactant, Survanta (Abbott Industries, Sydney, NSW, Australia), did not alter the surface properties of the surfactant. 3. After intratracheal instillation to rats, radiolabelled dexamethasone in Survanta was well distributed throughout all four lobes of the lungs. A concentration gradient of the steroid was observed between the root and the peripheral sections of all lobes. 4. Our results suggest that surfactant is an effective vehicle for intratracheal delivery of glucocorticoids. Moreover, we propose that prophylactic intratracheal administration of glucocorticoids administered shortly after birth may prevent inflammatory insult to the lungs and thereby reduce the likelihood of chronic lung disease developing.

Members of the C/EBP transcription factor family stimulate expression of the human and rat surfactant protein A (SP-A) genes

Members of the CCAAT enhancer binding protein (C/EBP) transcription factor family were detected in fetal lung of both human and rat. In rat lung, the level of C/EBPs increased with time of gestation, peaking around birth. In adult rat lung, C/EBPs were localized to the alveolar type II cells. The effect of C/EBPs on pulmonary surfactant protein A (SP-A), which is also expressed late in gestation, was investigated. In contrast to control plasmids, C/EBP delta expressing plasmids reversed the action of a transcriptional silencer just upstream of the rat SP-A promoter. In order to test the effect of C/EBPs on endogenous SP-A gene expression, cells that express SP-A were exposed to a phosphorothioate-substituted, double-stranded oligonucleotide matching the consensus C/EBP binding site (decoy oligonucleotide) at concentrations from 0.5 to 10 microM for 72 h. A mutant oligonucleotide with an 8-base pair (bp) substitution served as a control. The decoy oligonucleotide reduced SP-A mRNA as much as 75% compared to a mutant oligonucleotide both in the human lung cell line, NCI-H441, and in primary human fetal alveolar type II cells. The data indicate that C/EBPs facilitate SP-A gene expression, possibly by overcoming transcriptional silencing.

Human surfactant protein B promoter in transgenic mice: temporal, spatial, and stimulus-responsive regulation

Surfactant protein B (SP-B) is a developmentally and hormonally regulated lung protein that is required for normal surfactant function. We generated transgenic mice carrying the human SP-B promoter (-1,039/+431 bp) linked to chloramphenicol acetyltransferase (CAT). CAT activity was high in lung and immunoreactive protein localized to alveolar type II and bronchiolar epithelial cells. In addition, thyroid, trachea, and intestine demonstrated CAT activity, and each of these tissues also expressed low levels of SP-B mRNA. Developmental expression of CAT activity and SP-B mRNA in fetal lung were similar and both increased during explant culture. SP-B mRNA but not CAT activity decreased during culture of adult lung, and both were reduced by transforming growth factor (TGF)-beta(1). Treatment of adult mice with intratracheal bleomycin caused similar time-dependent decreases in lung SP-B mRNA and CAT activity. These findings indicate that the human SP-B promoter fragment directs tissue- and lung cell-specific transgene expression and contains cis-acting elements involved in regulated expression during development, fetal lung explant culture, and responsiveness to TGF-beta and bleomycin-induced lung injury.

The hypothalamic pituitary axis in the fetus and newborn

Glucocorticoid receptor activation in the fetal lung triggers maturation necessary for extra-uterine life. Antenatal treatment with betamethasone and dexamethasone has lowered severity of respiratory distress in very low birth weight infants, and dexamethasone given postnatally has resulted in short-term improvement in chronic lung disease. Recently, however, surfactant therapy has diminished the differential benefit of antenatal glucocorticoid treatment, and it has been difficult to show that postnatal dexamethasone therapy improves survival. Treated infants may have reduced weight gain, adrenal suppression, increased incidence of intestinal perforation and infection, and long-term developmental and metabolic problems. Recent data suggest that the fetal hypothalamic/pituitary/adrenal axis is active early and is precisely structured for an intricate sequence of specifically fetal developmental events, which may be deranged by dexamethasone therapy. We consider data suggesting that persistence of the fetal pattern in some premature infants constitutes adrenal insufficiency, and that therapy at stress replacement doses with less potent glucocorticoids might avoid side effects seen with traditional regimens.

Use of antenatal steroids to counteract the negative effects of tracheal occlusion in the fetal lamb model

Tracheal occlusion (TO) in fetal lambs induces pulmonary hyperplasia but has negative effects on type II cells. The purpose of this study was to determine whether antenatal steroids could reverse the adverse effects of TO on lung maturation in fetal lambs. Sixteen time-dated pregnant ewes (term, 145 d) and 24 of their fetuses were divided into six groups: 1) TO at 117 d gestation; 2) TO at 117 d with a single maternal intramuscular injection of 0.5 mg/kg betamethasone 24 h before delivery; 3) TO at 117 d and release of the occlusion 2 d before delivery; 4) TO and release of the occlusion with maternal steroids; 5) unoperated controls without antenatal steroid treatment; and 6) unoperated controls, littermates of groups 1-4, treated with antenatal steroids. All fetuses were killed at 137 d gestation. Outcome measurements consisted of lung weight-to-body weight ratio; lung morphometry determined by mean terminal bronchial density; and assessment of type II pneumocytes by in situ hybridization to the mRNA of surfactant proteins B and C. Lung weight-to-body weight ratio and mean terminal bronchial density were significantly different among groups with TO and controls, indicating increased lung growth and structural maturation. The density of type II pneumocytes was markedly decreased by TO. Release 2 d before sacrifice significantly increased the density and surfactant activity of type II pneumocytes, but to levels still far from controls. Steroids alone had an effect similar to release. An additive effect was noted with steroids and 2-d release resulting in type II cell density comparable to controls. After fetal TO, a single maternal intramuscular dose of 0.5 mg/kg of betamethasone 24 h before delivery allows partial recuperation of the type II pneumocytes, an effect that is potentiated by 2-d release.

Antenatal corticosteroids for preterm birth

A single course of antenatal corticosteroids is a rare example of a treatment that yields both a health benefit and a cost savings. This article reviews the history and background of antenatal corticosteroids, its use in clinical practice and the controversy today regarding the use of multiple courses of antenatal corticosteroids.

Maintenance of differentiated function of the surfactant system in human fetal lung type II epithelial cells cultured on plastic

We report a simplified culture system for human fetal lung type II cells that maintains surfactant expression. Type II cells isolated from explant cultures of hormone-treated lungs (18-22 wk gestation) by collagenase + trypsin digestion were cultured on plastic for 4 days in serum-free medium containing dexamethasone (Dex, 10 nM) + 8-bromo-cAMP (0.1 mM + isobutylmethylxanthine (0.1 mM) or were untreated (control). Surfactant protein (SP) mRNAs decreased markedly in control cells between days 1 and 4 of culture, but mRNA levels were high in treated cells on day) 4 (SP-A, SP-B, SP-C, SP-D; 600%, 100%, 85%, 130% of day 0 content, respectively). Dex or cAMP alone increased SP-B, SP-C, and SP-D mRNAs and together had additive effects. The greatest increase in SP-A mRNA occurred with cAMP alone. Treated cells processed pro-SP-B and pro-SP-C proteins to mature forms and had a higher rate of phosphatidylcholine (PC) synthesis (2-fold) and higher saturation of PC (approximately 34% versus 27%) than controls. Only treated cells maintained secretagogue-responsive phospholipid synthesis. By electron microscopy, the treated cells retained lamellar bodies and extensive microvilli. We conclude that Dex and cAMP additively stimulate expression of surfactant components in isolated fetal type II cells, providing a simplified culture system for investigation of surfactant-related, and perhaps other, type II cell functions.

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.

The effect in premature infants of prenatal corticosteroids on endogenous surfactant synthesis as measured with stable isotopes

Most in vitro studies show that prenatal administration of corticosteroids stimulates the synthesis of surfactant phosphatidylcholine (PC), but studies in animals are controversial. Whether prenatal corticosteroids stimulate surfactant PC synthesis in humans has not been studied. We studied endogenous surfactant PC synthesis in relation to prenatal corticosteroid treatment in 27 preterm infants with respiratory distress syndrome. Infants received a 24-h infusion of the stable isotope [U-(13)C]glucose, starting approximately 5 h after birth. We measured (13)C-incorporation into palmitic acid in surfactant PC from serial tracheal aspirates and in plasma triglycerides and phospholipids by isotope-ratio mass spectrometry. Premature infants had received either zero (n = 11), one (n = 4), or two doses (n = 12) of prenatal betamethasone (12 mg intramuscularly). The fractional synthesis rate (FSR) of surfactant PC from glucose was 1.7 +/- 0.3%/d without corticosteroid treatment, 2.9 +/- 1.4%/d with one dose of prenatal corticosteroid, and 5.8 +/- 1.3%/d after two doses of prenatal corticosteroid. Using multiple regression analysis, we found that the FSR of surfactant PC increased by 40% (confidence interval: 7 to 82%/d, p < 0.02) per dose of corticosteroid and doubled after two doses of corticosteroid. The (13)C-enrichment of plasma triglycerides and phospholipids was not increased by corticosteroid. These data show for the first time that prenatal corticosteroid treatment stimulates surfactant synthesis in the preterm infant.

Pulmonary surfactant metabolism in infants lacking surfactant protein B

Infants with inherited deficiency of pulmonary surfactant protein (SP) B develop respiratory failure at birth and die without lung transplantation. We examined aspects of surfactant metabolism in lung tissue and lavage fluid acquired at transplantation or postmortem from ten infants born at term with inherited deficiency of SP-B; comparison groups were infants with other forms of chronic lung disease (CLD) and normal infants. In pulse/chase labeling studies with cultured deficient tissue, no immunoprecipitable SP-B was observed and an approximately 6-kD form of SP-C accumulated that was only transiently present in CLD tissue. SP-B messenger RNA (mRNA) was approximately 8% of normal in deficient specimens, and some intact message was observed after, but not before, explant culture. Transcription rates for SP-B, assessed by nuclear run-on assay using probes for sequences both 5' and 3' of the common nonsense mutation (121ins2), were comparable in all lungs examined. The minimal surface tension achieved with lavage surfactant was similarly elevated in both deficient and CLD infants (26-31 mN/m) compared with normal infants (6 mN/m). Both SP-B-deficient and CLD infants had markedly decreased phosphatidylglycerol content of lavage and tissue compared with normal lung, whereas synthetic rates for phospholipids, including phosphatidylglycerol, were normal. We conclude that the mutated SP-B gene is transcribed normally but produces an unstable mRNA and that absence of SP-B protein blocks processing of SP-C. Chronic infant lung disease, of various etiologies, reduces surfactant function and apparently alters phosphatidylglycerol degradation.

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.

Transcriptional regulation of lung development: emergence of specificity

The lung is the product of a set of complex developmental interactions between two distinct tissues, the endodermally derived epithelium and the mesoderm. Each tissue contributes to lung development by fine-tuning the spatial and temporal pattern of gene expression for a distinct array of signaling molecules, transcriptional molecules and molecules related to the extracellular matrix. Morphoregulatory transcriptional factors such as NKX2.1 have the crucial role of connecting the cell-cell crosstalk to the activation or repression of gene expression through which processes such as cellular proliferation, migration, differentiation and apoptosis can be controlled. Although none of the factors participating in lung development are exclusively lung-specific, their unique combinations and interactions constitute the basis for emergence of lung structural and functional specificities. An understanding of the individual molecules and their unique interactions in the context of lung development is necessary for the construction of a morphogenetic map for this vital organ as well as for the development of rational and innovative approaches to congenital and induced lung disease.

Developmental and glucocorticoid regulation of surfactant protein mRNAs in preterm lambs

Glucocorticoid treatment increases content of surfactant protein (SP) A and SP-B in lung tissue and lavage fluid of preterm lambs. To investigate this process, we determined the ontogeny and glucocorticoid induction of SP mRNAs. In separate treatment protocols, each with its own controls, sheep were injected with betamethasone 15 h, 48 h, or weekly for 1-4 doses before preterm delivery. Using ovine SP cDNAs, we found an increase equal to or more than threefold in basal levels of all three SP mRNAs between 125 days and term. After betamethasone treatment, SP-B and SP-C mRNA levels increased by 15 h and all SP mRNAs were elevated after 24 h (>/=2-fold); mRNA levels in fetuses delivered 1-3 wk after betamethasone were not different from control. We conclude that in vivo betamethasone rapidly induces a coordinated increase in SP mRNAs, which is fully reversible within 7 days despite repetitive doses of betamethasone. Similar increases in mRNA and protein contents for SP-A and SP-B suggest that glucocorticoid regulation of these SPs in vivo is primarily pretranslational.

TGF-beta3-null mutation does not abrogate fetal lung maturation in vivo by glucocorticoids

Newborn transforming growth factor (TGF)-beta3-null mutant mice exhibit defects of palatogenesis and pulmonary development. Glucocorticoids, which play a central role in fetal lung maturation, have been postulated to mediate their stimulatory effects on tropoelastin mRNA expression through TGF-beta3 in cultured lung fibroblasts. In the present study, we analyzed the abnormally developed lungs in TGF-beta3-null mutant mice and compared the effects of glucocorticoids on gene expression and lung morphology between TGF-beta3 knockout and wild-type mice. Lungs of TGF-beta3-null mutant mice on embryonic day 18.5 did not form normal saccular structures and had a thick mesenchyme between terminal air spaces. Moreover, the number of surfactant protein C-positive cells was decreased in TGF-beta3-null mutant lungs. Interestingly, glucocorticoids were able to promote lung maturation and increased expression of both tropoelastin and fibronectin but decreased the relative number of surfactant protein C-positive cells in fetal lungs of both genotypes. This finding provides direct evidence that glucocorticoid signaling in the lung can use alternative pathways and can exert its effect without the presence of TGF-beta3.

Inhibition of hSP-B promoter in respiratory epithelial cells by a dominant negative retinoic acid receptor

Retinoic acid (RA) receptors (RARs) belong to the nuclear hormone receptor superfamily and play important roles in lung differentiation, growth, and gene regulation. Surfactant protein (SP) B is a small hydrophobic protein synthesized and secreted by respiratory epithelial cells in the lung. Expression of the SP-B gene is modulated at the transcriptional and posttranscriptional levels. In the present work, immunohistochemical staining revealed that RAR-alpha is present on day 14.5 of gestation in the fetal mouse lung. To assess whether RAR is required for SP-B gene transcription, a dominant negative mutant human (h) RAR-alpha403 was generated. The hRAR-alpha403 mutant was transcribed and translated into the truncated protein product by reticulocyte lysate in vitro. The mutant retained DNA binding activity in the presence of retinoid X receptor-gamma to an RA response element in the hSP-B promoter. When transiently transfected into pulmonary adenocarcinoma epithelial cells (H441 cells), the mutant hRAR-alpha403 was readily detected in the cell nucleus. Cotransfection of the mutant hRAR-alpha403 repressed activity of the hSP-B promoter and inhibited RA-induced surfactant proprotein B production in H441 cells, supporting the concept that RAR is required for hSP-B gene transcription in vitro.

Glucocorticoid deficiency delays stratum corneum maturation in the fetal mouse

The stratum corneum (SC) matures during late gestation in man and other mammals. Using the fetal rat as an experimental model, we have previously shown that glucocorticoids given in pharmacologic doses accelerate fetal SC maturation and barrier formation. To determine whether glucocorticoids are required for normal SC maturation, we examined the epidermal morphology of glucocorticoid-deficient (C-) murine pups, derived from matings of mice homozygous for null mutations of the corticotropin-releasing hormone alleles. In control pups on day 17.5 of gestation (term is 19.5 d), a multilayered SC was present and neutral lipid deposition in a membrane pattern was observed using Nile red fluorescence histochemistry. Ultrastructurally, mature lamellar unit structures predominate in the SC intercellular domains. In contrast, in C-pups only a single layer of SC was evident on day 17.5, and secreted lamellar material was not organized into mature lamellar structures. Furthermore, the expression of structural proteins necessary for cornified envelope formation, involucrin, loricrin, and filaggrin, and the activity of the lipid synthetic enzymes beta-glucocerebrosidase and steroid sulfatase, markers of barrier maturation, were reduced in day 17.5 C-pups. C-pups derived from pregnancies supplemented with physiologic amounts of cortisone, however, display normal SC ultrastructure on day 17.5 of gestation. Furthermore, at birth, both control and C-pups exhibit a multilayered SC replete with mature lamellar membrane structures. These data demonstrate that fetal glucocorticoid deficiency delays SC maturation, and suggests that normal levels of glucocorticoids are not absolutely required for SC development.

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.