Inhaled nitric oxide increases surfactant protein gene expression in the intact lamb

Chronic lung disease in the preterm infant. Lessons learned from animal models

Neonatal chronic lung disease, also known as bronchopulmonary dysplasia (BPD), is the most common complication of premature birth, affecting up to 30% of very low birth weight infants. Improved medical care has allowed for the survival of the most premature infants and has significantly changed the pathology of BPD from a disease marked by severe lung injury to the "new" form characterized by alveolar hypoplasia and impaired vascular development. However, increased patient survival has led to a paucity of pathologic specimens available from infants with BPD. This, combined with the lack of a system to model alveolarization in vitro, has resulted in a great need for animal models that mimic key features of the disease. To this end, a number of animal models have been created by exposing the immature lung to injuries induced by hyperoxia, mechanical stretch, and inflammation and most recently by the genetic modification of mice. These animal studies have 1) allowed insight into the mechanisms that determine alveolar growth, 2) delineated factors central to the pathogenesis of neonatal chronic lung disease, and 3) informed the development of new therapies. In this review, we summarize the key findings and limitations of the most common animal models of BPD and discuss how knowledge obtained from these studies has informed clinical care. Future studies should aim to provide a more complete understanding of the pathways that preserve and repair alveolar growth during injury, which might be translated into novel strategies to treat lung diseases in infants and adults.

Pulmonary Collectins in Diagnosis and Prevention of Lung Diseases

Pulmonary surfactant is a complex mixture of lipids and proteins, and is synthesized and secreted by alveolar type II epithelial cells and bronchiolar Clara cells. It acts to keep alveoli from collapsing during the expiratory phase of the respiratory cycle. After its secretion, lung surfactant forms a lattice structure on the alveolar surface, known as tubular myelin. Surfactant proteins (SP)-A, B, C and D make up to 10% of the total surfactant. SP-B and SPC are relatively small hydrophobic proteins, and are involved in the reduction of surface-tension at the air-liquid interface. SP-A and SP-D, on the other hand, are large oligomeric, hydrophilic proteins that belong to the collagenous Ca²⁺-dependent C-type lectin family (known as “Collectins”), and play an important role in host defense and in the recycling and transport of lung surfactant (Awasthi 2010) (Fig. 43.1). In particular, there is increasing evidence that surfactant-associated proteins A and -D (SP-A and SP-D, respectively) contribute to the host defense against inhaled microorganisms (see 10.1007/978-3-7091-1065_24 and 10.1007/978-3-7091-1065_25). Based on their ability to recognize pathogens and to regulate the host defense, SP-A and SP-D have been recently categorized as “Secretory Pathogen Recognition Receptors”. While SP-A and SP-D were first identified in the lung; the expression of these proteins has also been observed at other mucosal surfaces, such as lacrimal glands, gastrointestinal mucosa, genitourinary epithelium and periodontal surfaces. SP-A is the most prominent among four proteins in the pulmonary surfactant-system. The expression of SP-A is complexly regulated on the transcriptional and the chromosomal level. SP-A is a major player in the pulmonary cytokine-network and moreover has been described to act in the pulmonary host defense. This chapter gives an overview on the understanding of role of SP-A and SP-D in for human pulmonary disorders and points out the importance for pathology-orientated research to further elucidate the role of these molecules in adult lung diseases. As an outlook, it will become an issue of pulmonary pathology which might provide promising perspectives for applications in research, diagnosis and therapy (Awasthi 2010).

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.

Opposing regulation of human alveolar type II cell differentiation by nitric oxide and hyperoxia

Clinical trials demonstrated decreasing rates of bronchopulmonary dysplasia in preterm infants with hypoxic respiratory failure treated with inhaled nitric oxide (iNO). However, the molecular and biochemical effects of iNO on developing human fetal lungs remain vastly unknown. By using a well-characterized model of human fetal alveolar type II cells, we assessed the effects of iNO and hyperoxia, independently and concurrently, on NO-cGMP signaling pathway and differentiation. Exposure to iNO increased cGMP levels by 40-fold after 3 d and by 8-fold after 5 d despite constant expression of phosphodiesterase-5 (PDE5). The levels of cGMP declined significantly on exposure to iNO and hyperoxia at 3 and 5 d, although expression of soluble guanylyl cyclase (sGC) was sustained. Surfactant proteins B and C (SP-B, SP-C) and thyroid transcription factor (TTF)-1 mRNA levels increased in cells exposed to iNO in normoxia but not on exposure to iNO plus hyperoxia. Collectively, these data indicate an increase in type II cell markers when undifferentiated lung epithelial cells are exposed to iNO in room air. However, hyperoxia overrides these potentially beneficial effects of iNO despite sustained expression of sGC.

Surfactant function and composition in premature infants treated with inhaled nitric oxide

OBJECTIVES

We hypothesized that inhaled nitric oxide treatment of premature infants at risk for bronchopulmonary dysplasia would not adversely affect endogenous surfactant function or composition.

METHODS

As part of the Nitric Oxide Chronic Lung Disease Trial of inhaled nitric oxide, we examined surfactant in a subpopulation of enrolled infants. Tracheal aspirate fluid was collected at specified intervals from 99 infants with birth weights <1250 g who received inhaled nitric oxide (20 ppm, weaned to 2 ppm) or placebo gas for 24 days. Large-aggregate surfactant was analyzed for surface activity with a pulsating bubble surfactometer and for surfactant protein contents with an immunoassay.

RESULTS

At baseline, before administration of study gas, surfactant function and composition were comparable in the 2 groups, and there was a positive correlation between minimum surface tension and severity of lung disease for all infants. Over the first 4 days of treatment, minimum surface tension increased in placebo-treated infants and decreased in inhaled nitric oxide-treated infants. There were no significant differences between groups in recovery of large-aggregate surfactant or contents of surfactant protein A, surfactant protein B, surfactant protein C, or total protein, normalized to phospholipid.

CONCLUSIONS

We conclude that inhaled nitric oxide treatment for premature infants at risk of bronchopulmonary dysplasia does not alter surfactant recovery or protein composition and may improve surfactant function transiently.

Nitric oxide attenuates epithelial-mesenchymal transition in alveolar epithelial cells

Patients with interstitial lung diseases, such as idiopathic pulmonary fibrosis (IPF) and bronchopulmonary dysplasia (BPD), suffer from lung fibrosis secondary to myofibroblast-mediated excessive ECM deposition and destruction of lung architecture. Transforming growth factor (TGF)-β1 induces epithelial-mesenchymal transition (EMT) of alveolar epithelial cells (AEC) to myofibroblasts both in vitro and in vivo. Inhaled nitric oxide (NO) attenuates ECM accumulation, enhances lung growth, and decreases alveolar myofibroblast number in experimental models. We therefore hypothesized that NO attenuates TGF-β1-induced EMT in cultured AEC. Studies of the capacity for endogenous NO production in AEC revealed that endothelial nitric oxide synthase (eNOS) and inducible nitric oxide synthase (iNOS) are expressed and active in AEC. Total NOS activity was 1.3 pmol·mg protein−1·min−1 with 67% derived from eNOS. TGF-β1 (50 pM) suppressed eNOS expression by more than 60% and activity by 83% but did not affect iNOS expression or activity. Inhibition of endogenous NOS with l-NAME led to spontaneous EMT, manifested by increased α-smooth muscle actin (α-SMA) expression and a fibroblast-like morphology. Provision of exogenous NO to TGF-β1-treated AEC decreased stress fiber-associated α-SMA expression and decreased collagen I expression by 80%. NO-treated AEC also retained an epithelial morphology and expressed increased lamellar protein, E-cadherin, and pro-surfactant protein B compared with those treated with TGF-β alone. These findings indicate that NO serves a critical role in preserving an epithelial phenotype and in attenuating EMT in AEC. NO-mediated regulation of AEC fate may have important implications in the pathophysiology and treatment of diseases such as IPF and BPD.

Inhaled nitric oxide attenuates hyperoxic and inflammatory injury without alteration of phosphatidylcholine synthesis in rat lungs

We hypothesized that inhaled nitric oxide (iNO), a selective vasodilator for pulmonary hypertension, may exacerbate hyperoxia-related lung inflammatory injury by alteration of phosphatidylcholine (PC) synthesis in mature lungs. Healthy adult rats were allocated to 4 groups and exposed to: 95% oxygen, or 20ppm iNO, or both (ONO), or room air, all for 48h. (3)H-choline chloride was injected i.v. at 10min, 8, 16, and 24h prior to the end of 48h exposure and the animal lungs were processed. In oxygen group, oxidative damage and inflammation were significantly induced compared to the room air group. In ONO group there were significantly elevated glutathione, attenuated malondialdehyde, myeloperoxidase, and wet-to-dry lung weight ratio in lung parenchyma, decreased white cell counts and vascular-to-alveolar leakage of albumin in bronchoalveolar lavage fluid. In both oxygen and ONO groups both total phospholipids and surfactant protein-A were significantly increased compared with the room air group. Newly synthesized (3)H-PC was low in the lungs of NO group but high over time in both oxygen and ONO groups. Morphologically, lung injury was mild in ONO, but moderate in both oxygen and NO groups. We conclude that iNO alleviated oxidative damage and inflammation, and reduced alveolar leakage in hyperoxic injury of the mature lungs. Hyperoxia enhanced production of surfactant, whereas iNO did not attenuate this effect.

Collectins and cationic antimicrobial peptides of the respiratory epithelia

The respiratory epithelium is a primary site for the deposition of microorganisms that are acquired during inspiration. The innate immune system of the respiratory tract eliminates many of these potentially harmful agents preventing their colonization. Collectins and cationic antimicrobial peptides are antimicrobial components of the pulmonary innate immune system produced by respiratory epithelia, which have integral roles in host defense and inflammation in the lung. Synthesis and secretion of these molecules are regulated by the developmental stage, hormones, as well as many growth and immunoregulatory factors. The purpose of this review is to discuss antimicrobial innate immune elements within the respiratory tract of healthy and pneumonic lung with emphasis on hydrophilic surfactant proteins and beta-defensins.

Surfactant composition and function in a primate model of infant chronic lung disease: effects of inhaled nitric oxide

Bronchopulmonary dysplasia, or chronic lung disease (CLD), of premature infants involves injury from hyperoxia and mechanical ventilation to an immature lung. We examined surfactant and nitric oxide (NO), which are developmentally deficient in premature infants, in the baboon model of developing CLD. Fetuses were delivered at 125 d gestation and were managed for 14 d with ventilation and oxygen prn without (controls) or with inhaled NO at 5 ppm. Compared with term infants, premature control infants had reduced maximal lung volume, decreased tissue content of surfactant proteins SP-A, -B, and -C, abnormal lavage surfactant as assessed by pulsating bubble surfactometer, and a low concentration of SP-B/phospholipid. NO treatment significantly increased maximal lung volume and tissue SP-A and SP-C, reduced recovery of lavage surfactant by 33%, decreased the total protein:phospholipid ratio of surfactant by 50%, and had no effect on phospholipid composition or SP content except for SP-C (50%). In both treatment groups, levels of SP-B and SP-C in surfactant were negatively correlated with STmin, with a 5-fold greater SP efficiency for NO versus control animals. By contrast, lung volume and compliance were not correlated with surfactant function. We conclude that surfactant is often dysfunctional in developing CLD secondary to SP-B deficiency. NO treatment improves the apparent ability of hydrophobic SP to promote low surface tension, perhaps secondary to less protein inactivation of surfactant, and improves lung volume by a process unrelated to surfactant function.

Nitric oxide decreases surfactant protein gene expression in primary cultures of type II pneumocytes

Inhaled nitric oxide (NO) is a selective pulmonary vasodilator effective in treating persistent pulmonary hypertension in newborns and in infants following congenital heart disease surgery. Recently, multiple in vivo and in vitro studies have shown a negative effect of NO on surfactant activity as well as surfactant protein gene expression. Although the relationship between NO and surfactant has been studied previously, the data has been hard to interpret due to the model systems used. The objective of the current study was to characterize the effect of NO on surfactant protein gene expression in primary rat type II pneumocytes cultured on a substratum that promoted the maintenance of type II cell phenotype. Exposure to a NO donor, S-nitroso-N-acetylpenicillamine (SNAP), decreased surfactant protein (SP)-A, (SP)-B, and (SP)-C mRNA levels in type II pneumocytes in a time- and dose-dependent manner. The effect was mediated in part by an increase in endothelin-1 secretion and a decrease in the intracellular messenger, phosphorylated ERK1/2 mitogen-activated protein kinases (MAPK). Exposing type II pneumocytes to endothelin-1 receptor antagonists PD-156707 or bosentan before exposure to SNAP partially prevented the decrease in surfactant protein gene expression. The results showed that NO mediated the decrease in surfactant protein gene expression at least in part through an increase in endothelin-1 secretion and a decrease in phosphorylated ERK1/2 MAPKs.

Increased pulmonary blood flow does not alter surfactant protein gene expression in lambs within the first week of life

Neonates and infants with congenital heart disease with increased pulmonary blood flow suffer morbidity from poor oxygenation and decreased lung compliance. In a previous experiment involving 4-wk-old lambs with pulmonary hypertension secondary to increased pulmonary blood flow following an in utero placement of an aortopulmonary vascular graft, we found a decrease in surfactant protein (SP)-A gene expression as well as a decrease in SP-A and SP-B protein contents. To determine the timing of these changes, the objective of the present study was to characterize the effect of increased pulmonary blood flow and pulmonary hypertension on SP-A, -B, and -C gene expressions and protein contents within the first week of life. Of eight fetal lambs that underwent the in utero placement of the shunt, there was no difference in the expression of SP-A, -B, and -C mRNA levels or SP-A and -B protein contents compared with age-matched controls. The results showed that, in this model of congenital heart disease with pulmonary hypertension and increased pulmonary blood flow, the effect of the shunt on SP gene expression and protein content was not apparent within the first week of life.