TGF-alpha perturbs surfactant homeostasis in vivo

Disaturated-phosphatidylcholine and surfactant protein-B turnover in human acute lung injury and in control patients

Background

Patients with Adult Respiratory Distress Syndrome (ARDS) and Acute Lung Injury (ALI) have low concentrations of disaturated-phosphatidylcholine and surfactant protein-B in bronchoalveolar lavage fluid. No information is available on their turnover.

Objectives

To analyze disaturated-phosphatidylcholine and surfactant protein-B turnover in patients with ARDS/ALI and in human adults with normal lungs (controls).

Methods

²H₂O as precursor of disaturated-phosphatidylcholine-palmitate and 1¹³C-Leucine as precursor of surfactant protein-B were administered intravenously to 12 patients with ARDS/ALI and to 8 controls. Disaturated-phosphatidylcholine and surfactant protein-B were isolated from serial tracheal aspirates, and their fractional synthetic rate was derived from the ²H and ¹³C enrichment curves, obtained by gas chromatography mass spectrometry. Disaturated-phosphatidylcholine, surfactant protein-B, and protein concentrations in tracheal aspirates were also measured.

Results

1) Surfactant protein-B turned over at faster rate than disaturated-phosphatidylcholine both in ARDS/ALI patients and in controls. 2) In patients with ARDS/ALI the fractional synthesis rate of disaturated-phosphatidylcholine was 3.1 times higher than in controls (p < 0.01), while the fractional synthesis rate of surfactant protein-B was not different. 3) In ARDS/ALI patients the concentrations of disaturated-phosphatidylcholine and surfactant protein-B in tracheal aspirates were markedly and significantly reduced (17% and 40% of the control values respectively).

Conclusions

1) Disaturated-phosphatidylcholine and surfactant protein-B have a different turnover both in healthy and diseased lungs. 2) In ARDS/ALI the synthesis of these two surfactant components may be differently regulated.

Signaling pathways in the epithelial origins of pulmonary fibrosis

Pulmonary fibrosis complicates a number of disease processes and leads to substantial morbidity and mortality. Idiopathic pulmonary fibrosis (IPF) is perhaps the most pernicious and enigmatic form of the greater problem of lung fibrogenesis with a median survival of three years from diagnosis in affected patients. In this review, we will focus on the pathology of IPF as a model of pulmonary fibrotic processes, review possible cellular mechanisms, review current treatment approaches and review two transgenic mouse models of lung fibrosis to provide insight into processes that cause lung fibrosis. We will also summarize the potential utility of signaling pathway inhibitors as a future treatment in pulmonary fibrosis. Finally, we will present data demonstrating a minimal contribution of epithelial-mesenchymal transition in the development of fibrotic lesions in the transforming growth factor-alpha transgenic model of lung fibrosis.

Pulmonary surfactant surface tension influences alveolar capillary shape and oxygenation

Alveolar capillaries are located in close proximity to the alveolar epithelium and beneath the surfactant film. We hypothesized that the shape of alveolar capillaries and accompanying oxygenation are influenced by surfactant surface tension in the alveolus. To prove our hypothesis, surfactant surface tension was regulated by conditional expression of surfactant protein (SP)-B in Sftpb(-/-) mice, thereby inhibiting surface tension-lowering properties of surfactant in vivo within 24 hours after depletion of Sftpb. Minimum surface tension of isolated surfactant was increased and oxygen saturation was significantly reduced after 2 days of SP-B deficiency in association with deformation of alveolar capillaries. Intravascularly injected 3.2-mum-diameter microbeads through jugular vein were retained within narrowed pulmonary capillaries after reduction of SP-B. Ultrastructure studies demonstrated that the capillary protrusion typical of the normal alveolar-capillary unit was reduced in size, consistent with altered pulmonary blood flow. Pulmonary hypertension and intrapulmonary shunting are commonly associated with surfactant deficiency and dysfunction in neonates and adults with respiratory distress syndromes. Increased surfactant surface tension caused by reduction in SP-B induced narrowing of alveolar capillaries and oxygen desaturation, demonstrating an important role of surface tension-lowering properties of surfactant in the regulation of pulmonary vascular perfusion.

Improved lung growth and function through hypoxia-inducible factor in primate chronic lung disease of prematurity

Bronchopulmonary dysplasia (BPD), a chronic lung disease affecting preterm neonates, is associated with significant childhood and adult health problems. Histopathologic features of BPD include impaired vascular and distal airway development. We previously showed that activation of hypoxia-inducible factors (HIFs) by inhibition of prolyl hydroxylase domain-containing proteins (PHDs) is feasible and that it stimulates vascular endothelial growth factor (VEGF)-dependent angiogenesis in vitro. We tested the hypothesis that enhancement of angiogenesis by activation of HIFs improves lung growth and function in prematurely born neonates in vivo. Preterm baboons (125 day+14 day pro re nata O2 model, corresponding to 27 human gestational weeks) were treated for 14 days with intravenous (i.v.) FG-4095, a PHD inhibitor. Notably, 77% of diminished total alveolar surface area in untreated controls was recovered by FG-4095 treatment. Functional significance of the structural changes was indicated by improved oxygenation and lung compliance in FG-4095-treated newborns. Surfactant proteins B and C and saturated phosphatidylcholine were unchanged. Incidence of spontaneous ductus arteriosus closure was increased, likely contributing to lower ratio of pulmonary to systemic blood flow in FG-4095 group. These findings indicate that HIF stimulation by PHD inhibition ameliorates pathological and physiological consequences of BPD.

Gene expression profiling identifies regulatory pathways involved in the late stage of rat fetal lung development

Fetal lung development is a complex biological process that involves temporal and spatial regulations of many genes. To understand the molecular mechanisms of this process, we investigated gene expression profiles of fetal lungs on gestational days 18, 19, 20, and 21, as well as newborn and adult rat lungs. For this analysis, we used an in-house rat DNA microarray containing 6,000 known genes and 4,000 expressed sequence tags (ESTs). Of these, 1,512 genes passed the statistical significance analysis of microarray (SAM) test; an at least twofold change was shown for 583 genes (402 known genes and 181 ESTs) between at least two time points. K-means cluster analysis revealed seven major expression patterns. In one of the clusters, gene expression increased from day 18 to day 20 and then decreased. In this cluster, which contained 10 known genes and 5 ESTs, 8 genes are associated with development. These genes can be integrated into regulatory pathways, including growth factors, plasma membrane receptors, adhesion molecules, intracellular signaling molecules, and transcription factors. Real-time PCR analysis of these 10 genes showed an 88% consistency with the microarray data. The mRNA of LIM homeodomain protein 3a (Lhx3), a transcription factor, was enriched in fetal type II cells. In contrast, pleiotrophin, a growth factor, had a much higher expression in fetal lung tissues than in fetal type II cells. Immunohistochemistry revealed that Lhx3 was localized in fetal lung epithelial cells and pleiotrophin in the mesenchymal cells adjacent to the developing epithelium and blood vessel. Using GenMAPP, we identified four regulatory pathways: transforming growth factor-beta signaling, inflammatory response, cell cycle, and G protein signaling. We also identified two metabolic pathways: glycolysis-gluconeogenesis and proteasome degradation. Our results may provide new insights into the complex regulatory pathways that control fetal lung development.