Rosiglitazone, a peroxisome proliferator-activated receptor-gamma agonist, prevents hyperoxia-induced neonatal rat lung injury in vivo

[Pulmonary lipofibroblasts in adults and alveolar regeneration in emphysema]

Lipofibroblasts form a sub-population of fibroblasts located in the mesenchymal alveolar stem cell niche. They show close proximity with alveolar epithelial type 2 cells and play a key role in alveolar development and lung homeostasis. Their role in various diseases such as acute respiratory distress syndrome, pulmonary fibrosis and emphysema is progressively better understood. Through the activation of signaling pathways such as PPARg lipofibroblasts may help to induce endogenous alveolar regeneration.

PPAR Gamma Receptor: A Novel Target to Improve Morbidity in Preterm Babies

Worldwide, three-quarters of a million babies are born extremely preterm (<28 weeks gestation) with devastating outcomes: 20% die in the newborn period, a further 35% develop bronchopulmonary dysplasia (BPD), and 10% suffer from cerebral palsy. Pioglitazone, a Peroxisome Proliferator Activated Receptor Gamma (PPARγ) agonist, may reduce the incidence of BPD and improve neurodevelopment in extreme preterm babies. Pioglitazone exerts an anti-inflammatory action mediated through Nuclear Factor-kappa B repression. PPARγ signalling is underactive in preterm babies as adiponectin remains low during the neonatal period. In newborn animal models, pioglitazone has been shown to be protective against BPD, necrotising enterocolitis, and lipopolysaccharide-induced brain injury. Single Nucleotide Polymorphisms of PPARγ are associated with inhibited preterm brain development and impaired neurodevelopment. Pioglitazone was well tolerated by the foetus in reproductive toxicology experiments. Bladder cancer, bone fractures, and macular oedema, seen rarely in adults, may be avoided with a short treatment course. The other effects of pioglitazone, including improved glycaemic control and lipid metabolism, may provide added benefit in the context of prematurity. Currently, there is no formulation of pioglitazone suitable for administration to preterm babies. A liquid formulation of pioglitazone needs to be developed before clinical trials. The potential benefits are likely to outweigh any anticipated safety concerns.

Pulmonary phenotypes of bronchopulmonary dysplasia in the preterm infant

Bronchopulmonary dysplasia (BPD) remains the most common complication of premature birth, imposing a significant and potentially life-long burden on patients and their families. Despite advances in our understanding of the mechanisms that contribute to patterns of lung injury and dysfunctional repair, current therapeutic strategies remain non-specific with limited success. Contemporary definitions of BPD continue to rely on clinician prescribed respiratory support requirements at specific time points. While these criteria may be helpful in broadly identifying infants at higher risk of adverse outcomes, they do not offer any precise information regarding the degree to which each compartment of the lung is affected. In this review we will outline the different pulmonary phenotypes of BPD and discuss important features in the pathogenesis, clinical presentation, and management of these frequently overlapping scenarios.

Postnatal growth restriction impairs rat lung structure and function

The negative impact of nutritional deficits in the development of bronchopulmonary dysplasia is well recognized, yet mechanisms by which nutrition alters lung outcomes and nutritional strategies that optimize development and protect the lung remain elusive. Here, we use a rat model to assess the isolated effects of postnatal nutrition on lung structural development without concomitant lung injury. We hypothesize that postnatal growth restriction (PGR) impairs lung structure and function, critical mediators of lung development, and fatty acid profiles at postnatal day 21 in the rat. Rat pups were cross-fostered at birth to rat dams with litter sizes of 8 (control) or 16 (PGR). Lung structure and function, as well as serum and lung tissue fatty acids, and lung molecular mediators of development, were measured. Male and female PGR rat pups had thicker airspace walls, decreased lung compliance, and increased tissue damping. Male rats also had increased lung elastance, increased lung elastin protein abundance, and lysol oxidase expression, and increased elastic fiber deposition. Female rat lungs had increased conducting airway resistance and reduced levels of docosahexaenoic acid in lung tissue. We conclude that PGR impairs lung structure and function in both male and female rats, with sex-divergent changes in lung molecular mediators of development.

Time-resolved transcriptomic profiling of the developing rabbit's lungs: impact of premature birth and implications for modelling bronchopulmonary dysplasia

BACKGROUND

Premature birth, perinatal inflammation, and life-saving therapies such as postnatal oxygen and mechanical ventilation are strongly associated with the development of bronchopulmonary dysplasia (BPD); these risk factors, alone or combined, cause lung inflammation and alter programmed molecular patterns of normal lung development. The current knowledge on the molecular regulation of lung development mainly derives from mechanistic studies conducted in newborn rodents exposed to postnatal hyperoxia, which have been proven useful but have some limitations.

METHODS

Here, we used the rabbit model of BPD as a cost-effective alternative model that mirrors human lung development and, in addition, enables investigating the impact of premature birth per se on the pathophysiology of BPD without further perinatal insults (e.g., hyperoxia, LPS-induced inflammation). First, we characterized the rabbit's normal lung development along the distinct stages (i.e., pseudoglandular, canalicular, saccular, and alveolar phases) using histological, transcriptomic and proteomic analyses. Then, the impact of premature birth was investigated, comparing the sequential transcriptomic profiles of preterm rabbits obtained at different time intervals during their first week of postnatal life with those from age-matched term pups.

RESULTS

Histological findings showed stage-specific morphological features of the developing rabbit's lung and validated the selected time intervals for the transcriptomic profiling. Cell cycle and embryo development, oxidative phosphorylation, and WNT signaling, among others, showed high gene expression in the pseudoglandular phase. Autophagy, epithelial morphogenesis, response to transforming growth factor β, angiogenesis, epithelium/endothelial cells development, and epithelium/endothelial cells migration pathways appeared upregulated from the 28th day of gestation (early saccular phase), which represents the starting point of the premature rabbit model. Premature birth caused a significant dysregulation of the inflammatory response. TNF-responsive, NF-κB regulated genes were significantly upregulated at premature delivery and triggered downstream inflammatory pathways such as leukocyte activation and cytokine signaling, which persisted upregulated during the first week of life. Preterm birth also dysregulated relevant pathways for normal lung development, such as blood vessel morphogenesis and epithelial-mesenchymal transition.

CONCLUSION

These findings establish the 28-day gestation premature rabbit as a suitable model for mechanistic and pharmacological studies in the context of BPD.

Alteration in branching morphogenesis via YAP/TAZ in fibroblasts of fetal lungs in an LPS-induced inflammation model

BACKGROUND

Chorioamnionitis is a common cause of preterm birth and leads to serious complications in newborns. The objective of this study was to investigate the role of the Hippo signaling pathway in lung branching morphogenesis under a lipopolysaccharide (LPS)-induced inflammation model.

MATERIALS AND METHODS

IMR-90 cells and ex vivo fetal lungs were treated with 0, 10, 30, or 50 μg/ml LPS for 24 and 72 h. Supernatant levels of lactate dehydrogenase (LDH), interleukin (IL)-6, IL-8, Chemokine (C-X-C motif) ligand 1(CXCL1), branching and the surface area ratio, Yes-associated protein (YAP), transcription coactivator with PDZ-binding motif (TAZ), fibroblast growth factor 10 (FGF10), fibroblast growth factor receptor II (FGFR2), SRY-box transcription factor 2 (SOX2), SOX9, and sirtuin 1 (SIRT1) levels were examined. Differentially expressed genes in fetal lungs after LPS treatment were identified by RNA-sequencing.

RESULTS

LPS at 50 μg/ml increased IL-6 and IL-8 in IMR-90 cells and increased IL-6, CXCL1 and LDH in fetal lungs. The branching ratio significantly increased by LPS at 30 μg/ml compared to the control but the increased level had decreased by 50 μg/ml LPS exposure. Exposure to 50 μg/ml LPS increased phosphorylated (p)-YAP, p-YAP/YAP, and p-TAZ/TAZ in IMR-90 cells, whereas 50 μg/ml LPS decreased FGF10 and SOX2. Consistently, p-YAP/YAP and p-TAZ/TAZ were increased in fibronectin⁺ cells of fetal lungs. Moreover, results of RNA-sequencing in fetal lungs showed that SMAD, FGF, IκB phosphorylation, tissue remodeling and homeostasis was involved in branching morphogenesis following exposure to 50 μg/ml LPS. The p-SIRT1/SIRT1 ratio increased in IMR-90 cells by LPS treatment.

CONCLUSIONS

This study showed that regulation of the Hippo pathway in fibroblasts of fetal lungs was involved in branching morphogenesis under an inflammatory disease such as chorioamnionitis.

PGC-1α activity and mitochondrial dysfunction in preterm infants

Extremely low gestational age neonates (ELGANs) are born in a relatively hyperoxic environment with weak antioxidant defenses, placing them at high risk for mitochondrial dysfunction affecting multiple organ systems including the nervous, respiratory, ocular, and gastrointestinal systems. The brain and lungs are highly affected by mitochondrial dysfunction and dysregulation in the neonate, causing white matter injury (WMI) and bronchopulmonary dysplasia (BPD), respectively. Adequate mitochondrial function is important in providing sufficient energy for organ development as it relates to alveolarization and axonal myelination and decreasing oxidative stress via reactive oxygen species (ROS) and reactive nitrogen species (RNS) detoxification. Peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1α) is a master regulator of mitochondrial biogenesis and function. Since mitochondrial dysfunction is at the root of WMI and BPD pathobiology, exploring therapies that can regulate PGC-1α activity may be beneficial. This review article describes several promising therapeutic agents that can mitigate mitochondrial dysfunction through direct and indirect activation and upregulation of the PGC-1α pathway. Metformin, resveratrol, omega 3 fatty acids, montelukast, L-citrulline, and adiponectin are promising candidates that require further pre-clinical and clinical studies to understand their efficacy in decreasing the burden of disease from WMI and BPD in preterm infants.

Upregulating carnitine palmitoyltransferase 1 attenuates hyperoxia-induced endothelial cell dysfunction and persistent lung injury

BACKGROUND

Bronchopulmonary dysplasia (BPD) is a chronic lung disease in premature infants that may cause long-term lung dysfunction. Accumulating evidence supports the vascular hypothesis of BPD, in which lung endothelial cell dysfunction drives this disease. We recently reported that endothelial carnitine palmitoyltransferase 1a (Cpt1a) is reduced by hyperoxia, and that endothelial cell-specific Cpt1a knockout mice are more susceptible to developing hyperoxia-induced injury than wild type mice. Whether Cpt1a upregulation attenuates hyperoxia-induced endothelial cell dysfunction and lung injury remains unknown. We hypothesized that upregulation of Cpt1a by baicalin or L-carnitine ameliorates hyperoxia-induced endothelial cell dysfunction and persistent lung injury.

METHODS

Lung endothelial cells or newborn mice (< 12 h old) were treated with baicalin or L-carnitine after hyperoxia (50% and 95% O2) followed by air recovery.

RESULTS

We found that incubation with L-carnitine (40 and 80 mg/L) and baicalin (22.5 and 45 mg/L) reduced hyperoxia-induced apoptosis, impaired cell migration and angiogenesis in cultured lung endothelial cells. This was associated with increased Cpt1a gene expression. In mice, neonatal hyperoxia caused persistent alveolar and vascular simplification in a concentration-dependent manner. Treatment with L-carnitine (150 and 300 mg/kg) and baicalin (50 and 100 mg/kg) attenuated neonatal hyperoxia-induced alveolar and vascular simplification in adult mice. These effects were diminished in endothelial cell-specific Cpt1a knockout mice.

CONCLUSIONS

Upregulating Cpt1a by baicalin or L-carnitine ameliorates hyperoxia-induced lung endothelial cell dysfunction, and persistent alveolar and vascular simplification. These findings provide potential therapeutic avenues for using L-carnitine and baicalin as Cpt1a upregulators to prevent persistent lung injury in premature infants with BPD.

Daily Intraperitoneal Administration of Rosiglitazone Does Not Improve Lung Function or Alveolarization in Preterm Rabbits Exposed to Hyperoxia

Thiazolidinediones (TZDs) are potent PPARγ agonists that have been shown to attenuate alveolar simplification after prolonged hyperoxia in term rodent models of bronchopulmonary dysplasia. However, the pulmonary outcomes of postnatal TZDs have not been investigated in preterm animal models. Here, we first investigated the PPARγ selectivity, epithelial permeability, and lung tissue binding of three types of TZDs in vitro (rosiglitazone (RGZ), pioglitazone, and DRF-2546), followed by an in vivo study in preterm rabbits exposed to hyperoxia (95% oxygen) to investigate the pharmacokinetics and the pulmonary outcomes of daily RGZ administration. In addition, blood lipids and a comparative lung proteomics analysis were also performed on Day 7. All TZDs showed high epithelial permeability through Caco-2 monolayers and high plasma and lung tissue binding; however, RGZ showed the highest affinity for PPARγ. The pharmacokinetic profiling of RGZ (1 mg/kg) revealed an equivalent biodistribution after either intratracheal or intraperitoneal administration, with detectable levels in lungs and plasma after 24 h. However, daily RGZ doses of 1 mg/kg did not improve lung function in preterm rabbits exposed to hyperoxia, and daily 10 mg/kg doses were even associated with a significant lung function worsening, which could be partially explained by the upregulation of lung inflammation and lipid metabolism pathways revealed by the proteomic analysis. Notably, daily postnatal RGZ produced an aberrant modulation of serum lipids, particularly in rabbit pups treated with the 10 mg/kg dose. In conclusion, daily postnatal RGZ did not improve lung function and caused dyslipidemia in preterm rabbits exposed to hyperoxia.

Maladaptive functional changes in alveolar fibroblasts due to perinatal hyperoxia impair epithelial differentiation

Infants born prematurely worldwide have up to a 50% chance of developing bronchopulmonary dysplasia (BPD), a clinical morbidity characterized by dysregulated lung alveolarization and microvascular development. It is known that PDGFR alpha-positive (PDGFRA+) fibroblasts are critical for alveolarization and that PDGFRA+ fibroblasts are reduced in BPD. A better understanding of fibroblast heterogeneity and functional activation status during pathogenesis is required to develop mesenchymal population-targeted therapies for BPD. In this study, we utilized a neonatal hyperoxia mouse model (90% O2 postnatal days 0-7, PN0-PN7) and performed studies on sorted PDGFRA+ cells during injury and room air recovery. After hyperoxia injury, PDGFRA+ matrix and myofibroblasts decreased and PDGFRA+ lipofibroblasts increased by transcriptional signature and population size. PDGFRA+ matrix and myofibroblasts recovered during repair (PN10). After 7 days of in vivo hyperoxia, PDGFRA+ sorted fibroblasts had reduced contractility in vitro, reflecting loss of myofibroblast commitment. Organoids made with PN7 PDGFRA+ fibroblasts from hyperoxia in mice exhibited reduced alveolar type 1 cell differentiation, suggesting reduced alveolar niche-supporting PDGFRA+ matrix fibroblast function. Pathway analysis predicted reduced WNT signaling in hyperoxia fibroblasts. In alveolar organoids from hyperoxia-exposed fibroblasts, WNT activation by CHIR increased the size and number of alveolar organoids and enhanced alveolar type 2 cell differentiation.

PPARγ activation in late gestation does not promote surfactant maturation in the fetal sheep lung

Respiratory distress syndrome results from inadequate functional pulmonary surfactant and is a significant cause of mortality in preterm infants. Surfactant is essential for regulating alveolar interfacial surface tension, and its synthesis by Type II alveolar epithelial cells is stimulated by leptin produced by pulmonary lipofibroblasts upon activation by peroxisome proliferator-activated receptor γ (PPARγ). As it is unknown whether PPARγ stimulation or direct leptin administration can stimulate surfactant synthesis before birth, we examined the effect of continuous fetal administration of either the PPARγ agonist, rosiglitazone (RGZ; Study 1) or leptin (Study 2) on surfactant protein maturation in the late gestation fetal sheep lung. We measured mRNA expression of genes involved in surfactant maturation and showed that RGZ treatment reduced mRNA expression of LPCAT1 (surfactant phospholipid synthesis) and LAMP3 (marker for lamellar bodies), but did not alter mRNA expression of PPARγ, surfactant proteins (SFTP-A, -B, -C, and -D), PCYT1A (surfactant phospholipid synthesis), ABCA3 (phospholipid transportation), or the PPARγ target genes SPHK-1 and PAI-1. Leptin infusion significantly increased the expression of PPARγ and IGF2 and decreased the expression of SFTP-B. However, mRNA expression of the majority of genes involved in surfactant synthesis was not affected. These results suggest a potential decreased capacity for surfactant phospholipid and protein production in the fetal lung after RGZ and leptin administration, respectively. Therefore, targeting PPARγ may not be a feasible mechanistic approach to promote lung maturation.

Impact of maternal late gestation undernutrition on surfactant maturation, pulmonary blood flow and oxygen delivery measured by magnetic resonance imaging in the sheep fetus

Restriction of fetal substrate supply has an adverse effect on surfactant maturation in the lung and thus affects the transition from in utero placental oxygenation to pulmonary ventilation ex utero. The effects on surfactant maturation are mediated by alteration in mechanisms regulating surfactant protein and phospholipid synthesis. This study aimed to determine the effects of late gestation maternal undernutrition (LGUN) and LGUN plus fetal glucose infusion (LGUN+G) compared to Control on surfactant maturation and lung development, and the relationship with pulmonary blood flow and oxygen delivery ( D O 2 ) measured by magnetic resonance imaging (MRI) with molecules that regulate lung development. LGUN from 115 to 140 days' gestation significantly decreased fetal body weight, which was normalized by glucose infusion. LGUN and LGUN+G resulted in decreased fetal plasma glucose concentration, with no change in fetal arterial P O 2 compared to control. There was no effect of LGUN and LGUN+G on the mRNA expression of surfactant proteins (SFTP) and genes regulating surfactant maturation in the fetal lung. However, blood flow in the main pulmonary artery was significantly increased in LGUN, despite no change in blood flow in the left or right pulmonary artery and D O 2 to the fetal lung. There was a negative relationship between left pulmonary artery flow and D O 2 to the left lung with SFTP-B and GLUT1 mRNA expression, while their relationship with VEGFR2 was positive. These results suggest that increased pulmonary blood flow measured by MRI may have an adverse effect on surfactant maturation during fetal lung development. KEY POINTS: Maternal undernutrition during gestation alters fetal lung development by impacting surfactant maturation. However, the direction of change remains controversial. We examined the effects of maternal late gestation maternal undernutrition (LGUN) on maternal and fetal outcomes, signalling pathways involved in fetal lung development, pulmonary haemodynamics and oxygen delivery in sheep using a combination of molecular and magnetic resonance imaging (MRI) techniques. LGUN decreased fetal plasma glucose concentration without affecting arterial P O 2 . Surfactant maturation was not affected; however, main pulmonary artery blood flow was significantly increased in the LGUN fetuses. This is the first study to explore the relationship between in utero MRI measures of pulmonary haemodynamics and lung development. Across all treatment groups, left pulmonary artery blood flow and oxygen delivery were negatively correlated with surfactant protein B mRNA and protein expression in late gestation.

Resident interstitial lung fibroblasts and their role in alveolar stem cell niche development, homeostasis, injury, and regeneration

Developing, regenerating, and repairing a lung all require interstitial resident fibroblasts (iReFs) to direct the behavior of the epithelial stem cell niche. During lung development, distal lung fibroblasts, in the form of matrix-, myo-, and lipofibroblasts, form the extra cellular matrix (ECM), create tensile strength, and support distal epithelial differentiation, respectively. During de novo septation in a murine pneumonectomy lung regeneration model, developmental processes are reactivated within the iReFs, indicating progenitor function well into adulthood. In contrast to the regenerative activation of fibroblasts upon acute injury, chronic injury results in fibrotic activation. In murine lung fibrosis models, fibroblasts can pathologically differentiate into lineages beyond their normal commitment during homeostasis. In lung injury, recently defined alveolar niche cells support the expansion of alveolar epithelial progenitors to regenerate the epithelium. In human fibrotic lung diseases like bronchopulmonary dysplasia (BPD), idiopathic pulmonary fibrosis (IPF), and chronic obstructive pulmonary disease (COPD), dynamic changes in matrix-, myo-, lipofibroblasts, and alveolar niche cells suggest differential requirements for injury pathogenesis and repair. In this review, we summarize the role of alveolar fibroblasts and their activation stage in alveolar septation and regeneration and incorporate them into the context of human lung disease, discussing fibroblast activation stages and how they contribute to BPD, IPF, and COPD.

Identification of a FGF18-expressing alveolar myofibroblast that is developmentally cleared during alveologenesis

Alveologenesis is an essential developmental process that increases the surface area of the lung through the formation of septal ridges. In the mouse, septation occurs postnatally and is thought to require the alveolar myofibroblast (AMF). Though abundant during alveologenesis, markers for AMFs are minimally detected in the adult. After septation, the alveolar walls thin to allow efficient gas exchange. Both loss of AMFs or retention and differentiation into another cell type during septal thinning have been proposed. Using a novel Fgf18:CreERT2 allele to lineage trace AMFs, we demonstrate that most AMFs are developmentally cleared during alveologenesis. Lung mesenchyme also contains other poorly described cell types, including alveolar lipofibroblasts (ALF). We show that Gli1:CreERT2 marks both AMFs as well as ALFs, and lineage tracing shows that ALFs are retained in adult alveoli while AMFs are lost. We further show that multiple immune cell populations contain lineage-labeled particles, suggesting a phagocytic role in the clearance of AMFs. The demonstration that the AMF lineage is depleted during septal thinning through a phagocytic process provides a mechanism for the clearance of a transient developmental cell population.

Persistent Pulmonary Hypertension of the Newborn: Pathophysiological Mechanisms and Novel Therapeutic Approaches

Persistent pulmonary hypertension of the newborn (PPHN) is one of the main causes of neonatal morbidity and mortality. It is characterized by sustained elevation of pulmonary vascular resistance (PVR), preventing an increase in pulmonary blood flow after birth. The affected neonates fail to establish blood oxygenation, precipitating severe respiratory distress, hypoxemia, and eventually death. Inhaled nitric oxide (iNO), the only approved pulmonary vasodilator for PPHN, constitutes, alongside supportive therapy, the basis of its treatment. However, nearly 40% of infants are iNO resistant. The cornerstones of increased PVR in PPHN are pulmonary vasoconstriction and vascular remodeling. A better understanding of PPHN pathophysiology may enlighten targeted and more effective therapies. Sildenafil, prostaglandins, milrinone, and bosentan, acting as vasodilators, besides glucocorticoids, playing a role on reducing inflammation, have all shown potential beneficial effects on newborns with PPHN. Furthermore, experimental evidence in PPHN animal models supports prospective use of emergent therapies, such as soluble guanylyl cyclase (sGC) activators/stimulators, l-citrulline, Rho-kinase inhibitors, peroxisome proliferator-activated receptor-γ (PPAR-γ) agonists, recombinant superoxide dismutase (rhSOD), tetrahydrobiopterin (BH4) analogs, ω-3 long-chain polyunsaturated fatty acids (LC-PUFAs), 5-HT2A receptor antagonists, and recombinant human vascular endothelial growth factor (rhVEGF). This review focuses on current knowledge on alternative and novel pathways involved in PPHN pathogenesis, as well as recent progress regarding experimental and clinical evidence on potential therapeutic approaches for PPHN.

Bronchopulmonary Dysplasia: Crosstalk Between PPARγ, WNT/β-Catenin and TGF-β Pathways; The Potential Therapeutic Role of PPARγ Agonists

Bronchopulmonary dysplasia (BPD) is a serious pulmonary disease which occurs in preterm infants. Mortality remains high due to a lack of effective treatment, despite significant progress in neonatal resuscitation. In BPD, a persistently high level of canonical WNT/β-catenin pathway activity at the canalicular stage disturbs the pulmonary maturation at the saccular and alveolar stages. The excessive thickness of the alveolar wall impairs the normal diffusion of oxygen and carbon dioxide, leading to hypoxia. Transforming growth factor (TGF-β) up-regulates canonical WNT signaling and inhibits the peroxysome proliferator activated receptor gamma (PPARγ). This profile is observed in BPD, especially in animal models. Following a premature birth, hypoxia activates the canonical WNT/TGF-β axis at the expense of PPARγ. This gives rise to the differentiation of fibroblasts into myofibroblasts, which can lead to pulmonary fibrosis that impairs the respiratory function after birth, during childhood and even adulthood. Potential therapeutic treatment could target the inhibition of the canonical WNT/TGF-β pathway and the stimulation of PPARγ activity, in particular by the administration of nebulized PPARγ agonists.

Modulators of inflammation in Bronchopulmonary Dysplasia

Over 50 years after its first description, Bronchopulmonary Dysplasia (BPD) remains a devastating pulmonary complication in preterm infants with respiratory failure and develops in 30-50% of infants less than 1000-gram birth weight. It is thought to involve ventilator- and oxygen-induced damage to an immature lung that results in an inflammatory response and ends in aberrant lung development with dysregulated angiogenesis and alveolarization. Significant morbidity and mortality are associated with this most common chronic lung disease of childhood. Thus, any therapies that decrease the incidence or severity of this condition would have significant impact on morbidity, mortality, human costs, and healthcare expenditure. It is clear that an inflammatory response and the elaboration of growth factors and cytokines are associated with the development of BPD. Numerous approaches to control the inflammatory process leading to the development of BPD have been attempted. This review will examine the anti-inflammatory approaches that are established or hold promise for the prevention or treatment of BPD.

Understanding alveolarization to induce lung regeneration

Background

Gas exchange represents the key physiological function of the lung, and is dependent upon proper formation of the delicate alveolar structure. Malformation or destruction of the alveolar gas-exchange regions are key histopathological hallmarks of diseases such as bronchopulmonary dysplasia (BPD), chronic obstructive pulmonary disease (COPD), and pulmonary fibrosis; all of which are characterized by perturbations to the alveolo-capillary barrier structure. Impaired gas-exchange is the primary initial consequence of these perturbations, resulting in severe clinical symptoms, reduced quality of life, and death. The pronounced morbidity and mortality associated with malformation or destruction of alveoli underscores a pressing need for new therapeutic concepts. The re-induction of alveolarization in diseased lungs is a new and exciting concept in a regenerative medicine approach to manage pulmonary diseases that are characterized by an absence of alveoli.

Main text

Mechanisms of alveolarization first need to be understood, to identify pathways and mediators that may be exploited to drive the induction of alveolarization in the diseased lung. With this in mind, a variety of candidate cell-types, pathways, and molecular mediators have recently been identified. Using lineage tracing approaches and lung injury models, new progenitor cells for epithelial and mesenchymal cell types – as well as cell lineages which are able to acquire stem cell properties – have been discovered. However, the underlying mechanisms that orchestrate the complex process of lung alveolar septation remain largely unknown.

Conclusion

While important progress has been made, further characterization of the contributing cell-types, the cell type-specific molecular signatures, and the time-dependent chemical and mechanical processes in the developing, adult and diseased lung is needed in order to implement a regenerative therapeutic approach for pulmonary diseases.

A novel mouse Cre-driver line targeting Perilipin 2-expressing cells in the neonatal lung

Pulmonary diseases such as chronic obstructive pulmonary disease, lung fibrosis, and bronchopulmonary dysplasia are characterized by the destruction or malformation of the alveolar regions of the lung. The underlying pathomechanisms at play are an area of intense interest since these mechanisms may reveal pathways suitable for interventions to drive reparative processes. Lipid-laden fibroblasts (lipofibroblasts) express the Perilipin 2 (Plin2) gene-product, PLIN2, commonly called adipose-differentiation related protein (ADRP). These cells are also thought to play a role in alveolarization and repair after injury to the alveolus. Progress in defining the functional contribution of lipofibroblasts to alveolar generation and repair is hampered by a lack of in vivo tools. The present study reports the generation of an inducible mouse Cre-driver line to target cells of the ADRP lineage. Robust Cre-mediated recombination in this mouse line was detected in mesenchymal cells of the postnatal lung, and in additional organs including the heart, liver, and spleen. The generation and validation of this valuable new tool to genetically target, manipulate, and trace cells of the ADRP lineage is critical for assessing the functional contribution of lipofibroblasts to lung development and repair.

Glucocorticoids Retain Bipotent Fibroblast Progenitors during Alveolar Septation in Mice

Glucocorticoids have been widely used and exert pleiotropic effects on alveolar structure and function, but do not improve the long-term clinical outcomes for patients with bronchopulmonary dysplasia, emphysema, or interstitial lung diseases. Treatments that foster alveolar regeneration could substantially improve the long-term outcomes for such patients. One approach to alveolar regeneration is to stimulate and guide intrinsic alveolar progenitors along developmental pathways used during secondary septation. Other investigators and we have identified platelet-derived growth factor receptor-α-expressing fibroblast subpopulations that are alternatively skewed toward myofibroblast or lipofibroblast phenotypes. In this study, we administered either the glucocorticoid receptor agonist dexamethasone (Dex) or the antagonist mifepristone to mice during the first postnatal week and evaluated their effects on cellular proliferation and adoption of α-smooth muscle actin and lipid droplets (markers of the myofibroblast and lipofibroblast phenotypes, respectively). We observed that Dex increased the relative abundance of fibroblasts with progenitor characteristics, i.e., containing both α-smooth muscle actin and lipid droplets, uncoupling protein-1 (a marker of brown and beige adipocytes), delta-like ligand-1, and stem cell antigen-1. Dex enhanced signaling through the Smad1/5 pathway, which increased uncoupling protein-1 in a lung fibroblast progenitor cell line. We conclude that glucocorticoid receptor manipulation can sustain fibroblast plasticity, and posit that targeting downstream glucocorticoid responsive pathways could steer fibroblast progenitors along more desirable regenerative pathways.

Addressing the challenges of phenotyping pediatric pulmonary vascular disease

Pediatric pulmonary vascular disease (PVD) and pulmonary hypertension (PH) represent phenotypically and pathophysiologically diverse disease categories, contributing substantial morbidity and mortality to a complex array of pediatric conditions. Here, we review the multifactorial nature of pediatric PVD, with an emphasis on improved recognition, phenotyping, and endotyping strategies for pediatric PH. Novel tailored approaches to diagnosis and treatment in pediatric PVD, as well as the implications for long-term outcomes, are highlighted.

N-methyl-D-aspartate receptor activation mediates lung fibroblast proliferation and differentiation in hyperoxia-induced chronic lung disease in newborn rats

Background

Previous studies have suggested that endogenous glutamate and its N-methyl-D-aspartate receptors (NMDARs) play important roles in hyperoxia-induced acute lung injury in newborn rats. We hypothesized that NMDAR activation also participates in the development of chronic lung injury after withdrawal of hyperoxic conditions.

Methods

In order to rule out the anti-inflammatory effects of NMDAR inhibitor on acute lung injury, the efficacy of MK-801 was evaluated in vivo using newborn Sprague-Dawley rats treated starting 4 days after cessation of hyperoxia exposure (on postnatal day 8). The role of NMDAR activation in hyperoxia-induced lung fibroblast proliferation and differentiation was examined in vitro using primary cells derived from the lungs of 8-day-old Sprague-Dawley rats exposed to hyperoxic conditions.

Results

Hyperoxia for 3 days induced acute lung injury in newborn rats. The acute injury almost completely disappeared 4 days after cessation of hyperoxia exposure. However, pulmonary fibrosis, impaired alveolarization, and decreased pulmonary compliance were observed on postnatal days 15 and 22. MK-801 treatment during the recovery period was found to alleviate the chronic damage induced by hyperoxia. Four NMDAR 2 s were found to be upregulated in the lung fibroblasts of newborn rats exposed to hyperoxia. In addition, the proliferation and upregulation of alpha-smooth muscle actin and (pro) collagen I in lung fibroblasts were detected in hyperoxia-exposed rats. MK-801 inhibited these changes.

Conclusions

NMDAR activation mediated lung fibroblast proliferation and differentiation and played a role in the development of hyperoxia-induced chronic lung damage in newborn rats.

Human lung fibroblasts produce proresolving peroxisome proliferator-activated receptor-γ ligands in a cyclooxygenase-2-dependent manner

Human lung fibroblasts (HLFs) act as innate immune sentinel cells that amplify the inflammatory response to injurious stimuli. Here, we use targeted lipidomics to explore the hypothesis that HLFs also play an active role in the resolution of inflammation. We detected cyclooxygenase-2 (COX-2)-dependent production of both proinflammatory and proresolving prostaglandins (PGs) in conditioned culture medium from HLFs treated with a proinflammatory stimulus, IL-1β. Among the proresolving PGs in the HLF lipidome were several known ligands for peroxisome proliferator-activated receptor-γ (PPARγ), a transcription factor whose activation in the lung yields potent anti-inflammatory, antifibrotic, and proresolving effects. Next, we used a cell-based luciferase reporter to confirm the ability of HLF supernatants to activate PPARγ, demonstrating, for the first time, that primary HLFs activated with proinflammatory IL-1β or cigarette smoke extract produce functional PPARγ ligands; this phenomenon is temporally regulated, COX-2- and lipocalin-type PGD synthase-dependent, and enhanced by arachidonic acid supplementation. Finally, we used luciferase reporter assays to show that several of the PGs in the lipidome of activated HLFs independently activate PPARγ and/or inhibit NFκB. These results indicate that HLFs, as immune sentinels, regulate both proinflammatory and proresolving responses to injurious stimuli. This novel endogenous resolution pathway represents a new therapeutic target for globally important inflammatory diseases such as chronic obstructive pulmonary disease.

The Emergence of Physiology and Form: Natural Selection Revisited

Natural Selection describes how species have evolved differentially, but it is descriptive, non-mechanistic. What mechanisms does Nature use to accomplish this feat? One known way in which ancient natural forces affect development, phylogeny and physiology is through gravitational effects that have evolved as mechanotransduction, seen in the lung, kidney and bone, linking as molecular homologies to skin and brain. Tracing the ontogenetic and phylogenetic changes that have facilitated mechanotransduction identifies specific homologous cell-types and functional molecular markers for lung homeostasis that reveal how and why complex physiologic traits have evolved from the unicellular to the multicellular state. Such data are reinforced by their reverse-evolutionary patterns in chronic degenerative diseases. The physiologic responses of model organisms like Dictyostelium and yeast to gravity provide deep comparative molecular phenotypic homologies, revealing mammalian Target of Rapamycin (mTOR) as the final common pathway for vertical integration of vertebrate physiologic evolution; mTOR integrates calcium/lipid epistatic balance as both the proximate and ultimate positive selection pressure for vertebrate physiologic evolution. The commonality of all vertebrate structure-function relationships can be reduced to calcium/lipid homeostatic regulation as the fractal unit of vertebrate physiology, demonstrating the primacy of the unicellular state as the fundament of physiologic evolution.

Transplacental Administration of Rosiglitazone Attenuates Hyperoxic Lung Injury in a Preterm Rabbit Model

INTRODUCTION

Continuous improvements in perinatal care have allowed the survival of increasingly more prematurely born infants. The establishment of respiration in an extremely immature yet still developing lung results in chronic lung injury with significant mortality and morbidity. We experimentally evaluated a novel medical strategy to prevent hyperoxia-induced lung injury by prenatal rosiglitazone.

MATERIALS AND METHODS

Pregnant rabbits were injected with saline or rosiglitazone (3 mg/kg) 48 and 24 h prior to preterm delivery at 28 days of gestation (term = 31 days). The pups were held in normoxia (21% O2) or hyperoxia (>95% O2), and assessment was done at three different time points (1 h, 24 h and 7 days).

RESULTS

The administration of rosiglitazone resulted in a significant decrease in tissue damping (resistance) on day 7. Furthermore, significantly increased expression of vascular endothelial growth factor, fetal liver kinase 1 and surfactant protein B immediately after delivery was noted by immunohistochemistery. On day 7, there was a more mature lung parenchymal architecture in rosiglitazone-exposed pups.

DISCUSSION

In a preterm rabbit model, prenatal maternal administration of rosiglitazone attenuates neonatal hyperoxic lung injury and results in a more mature pulmonary parenchyma.

Transcriptome Analysis of the Preterm Rabbit Lung after Seven Days of Hyperoxic Exposure

The neonatal management of preterm born infants often results in damage to the developing lung and subsequent morbidity, referred to as bronchopulmonary dysplasia (BPD). Animal models may help in understanding the molecular processes involved in this condition and define therapeutic targets. Our goal was to identify molecular pathways using the earlier described preterm rabbit model of hyperoxia induced lung-injury. Transcriptome analysis by mRNA-sequencing was performed on lungs from preterm rabbit pups born at day 28 of gestation (term: 31 days) and kept in hyperoxia (95% O₂) for 7 days. Controls were preterm pups kept in normoxia. Transcriptomic data were analyzed using Array Studio and Ingenuity Pathway Analysis (IPA), in order to identify the central molecules responsible for the observed transcriptional changes. We detected 2217 significantly dysregulated transcripts following hyperoxia, of which 90% could be identified. Major pathophysiological dysregulations were found in inflammation, lung development, vascular development and reactive oxygen species (ROS) metabolism. To conclude, amongst the many dysregulated transcripts, major changes were found in the inflammatory, oxidative stress and lung developmental pathways. This information may be used for the generation of new treatment hypotheses for hyperoxia-induced lung injury and BPD.

Characterization of the platelet-derived growth factor receptor-α-positive cell lineage during murine late lung development

A reduced number of alveoli is the structural hallmark of diseases of the neonatal and adult lung, where alveoli either fail to develop (as in bronchopulmonary dysplasia), or are progressively destroyed (as in chronic obstructive pulmonary disease). To correct the loss of alveolar septa through therapeutic regeneration, the mechanisms of septa formation must first be understood. The present study characterized platelet-derived growth factor receptor-α-positive (PDGFRα(+)) cell populations during late lung development in mice. PDGFRα(+) cells (detected using a PDGFRα(GFP) reporter line) were noted around the proximal airways during the pseudoglandular stage. In the canalicular stage, PDGFRα(+) cells appeared in the more distal mesenchyme, and labeled α-smooth muscle actin-positive tip cells in the secondary crests and lipofibroblasts in the primary septa during alveolarization. Some PDGFRα(+) cells appeared in the mesenchyme of the adult lung. Over the course of late lung development, PDGFRα(+) cells consistently expressed collagen I, and transiently expressed markers of mesenchymal stem cells. With the use of both, a constitutive and a conditional PDGFRα(Cre) line, it was observed that PDGFRα(+) cells generated alveolar myofibroblasts including tip cells of the secondary crests, and lipofibroblasts. These lineages were committed before secondary septation. The present study provides new insights into the time-dependent commitment of the PDGFRα(+) cell lineage to lipofibroblasts and myofibroblasts during late lung development that is needed to better understand the cellular contribution to the process of alveolarization.

Lung mesenchymal stromal cells in development and disease: to serve and protect?

SIGNIFICANCE

Bronchopulmonary dysplasia (BPD) is a disease of the developing lung that afflicts extreme preterm infants in the neonatal intensive care unit. Follow-up studies into adulthood show that BPD is not merely a problem of the neonatal period, as it also may predispose to early-onset emphysema and poor lung function in later life.

RECENT ADVANCES

The increasing promise of bone marrow- or umbilical cord-derived mesenchymal stromal cells (MSCs) to repair neonatal and adult lung diseases may for the first time offer the chance to make substantial strides in improving the outcome of extreme premature infants at risk of developing BPD. As more knowledge has been obtained on MSCs over the past decades, it has become clear that each organ has its own reservoir of endogenous MSCs, including the lung.

CRITICAL ISSUES

We have only barely scratched the surface on what resident lung MSCs exactly are and what their role and function in lung development may be. Moreover, what happens to these putative repair cells in BPD when alveolar development goes awry and why do their counterparts from the bone marrow and umbilical cord succeed in restoring normal alveolar development when they themselves do not?

FUTURE DIRECTIONS

Much work remains to be carried out to validate lung MSCs, but with the high potential of MSC-based treatment for BPD and other lung diseases, a thorough understanding of the endogenous lung MSC will be pivotal to get to the bottom of these diseases.

The lung alveolar lipofibroblast: an evolutionary strategy against neonatal hyperoxic lung injury

SIGNIFICANCE

Oxygen, the main mode of support for premature infants with immature lungs, can cause toxicity by producing reactive oxygen species (ROS) that disrupt homeostasis; yet, these same molecules were entrained to promote vertebrate lung phylogeny. By providing a deeper understanding of this paradox, we propose physiologically rational strategies to prevent chronic lung disease (CLD) of prematurity.

RECENT ADVANCES

To prevent neonatal hyperoxic lung damage biologically, we have exploited the alveolar defense mechanism(s) that evolutionarily evolved to combat increased atmospheric oxygen during the vertebrate water to land transition.

CRITICAL ISSUES

Over the course of vertebrate lung evolution, ROS promoted the formation of lipofibroblasts, specialized adepithelial cells, which protect the alveoli against oxidant injury; peroxisome proliferator-activated receptor gamma (PPARγ), the master switch for lipofibroblast differentiation, prevents such oxidant lung injury, both by directly promoting mesodermal differentiation and its antioxidant defenses, and indirectly by stimulating the developmental epithelial-mesenchymal paracrine interactions that have physiologically determined lung surfactant production in accord with the lung's phylogenetic adaptation to atmospheric oxygen, preventing Respiratory Distress Syndrome at birth.

FUTURE DIRECTIONS

The molecular strategy (PPARγ agonists) to prevent CLD of prematurity, proposed by us, although seems to be robust, effective, and safe under experimental conditions, it awaits detailed pharmacokinetic and pharmacodynamic studies for its safe and effective clinical translation to human infants. Antioxid. Redox Signal. 21, 1893-1904. "I have procured air [oxygen]…between five and six times as good as the best common air that I have ever met with." -Joseph Priestley, 1775.

Pulmonary Hypertension in Preterm Infants with Bronchopulmonary Dysplasia

Bronchopulmonary dysplasia (BPD), the chronic lung disease of prematurity, is a significant contributor to perinatal morbidity and mortality. Premature birth disrupts pulmonary vascular growth and initiates a cascade of events that result in impaired gas exchange, abnormal vasoreactivity, and pulmonary vascular remodeling that may ultimately lead to pulmonary hypertension (PH). Even infants who appear to have mild BPD suffer from varying degrees of pulmonary vascular disease (PVD). Although recent studies have enhanced our understanding of the pathobiology of PVD and PH in BPD, much remains unknown with respect to how PH should be properly defined, as well as the most accurate methods for the diagnosis and treatment of PH in infants with BPD. This article will provide neonatologists and primary care providers, as well as pediatric cardiologists and pulmonologists, with a review of the pathophysiology of PH in preterm infants with BPD and a summary of current clinical recommendations for managing PH in this population.

Rosiglitazone, a peroxisome proliferator-activated receptor-γ agonist, restores alveolar and pulmonary vascular development in a rat model of bronchopulmonary dysplasia

PURPOSE

We tested whether rosiglitazone (RGZ), a peroxisome proliferator-activated receptor-γ agonist, can restore alveolar development and vascular growth in a rat model of bronchopulmonary dysplasia (BPD).

MATERIALS AND METHODS

A rat model of BPD was induced through intra-amniotic delivery of lipopolysaccharide (LPS) and postnatal hyperoxia (80% for 7 days). RGZ (3 mg/kg/d, i.p.) or vehicle was given daily to rat pups for 14 days. This model included four experimental groups: No BPD+vehicle (V), No BPD+RGZ, BPD+V, and BPD+RGZ. On D14, alveolarization, lung vascular density, and right ventricular hypertrophy (RVH) were evaluated.

RESULTS

Morphometric analysis revealed that the BPD+RGZ group had significantly smaller and more complex airspaces and larger alveolar surface area than the BPD+V group. The BPD+RGZ group had significantly greater pulmonary vascular density than the BPD+V group. Western blot analysis revealed that significantly decreased levels of vascular endothelial growth factor (VEGF) and its receptor VEGFR-2 by the combined exposure to intra-amniotic LPS and postnatal hyperoxia were restored by the RGZ treatment. RVH was significantly lesser in the BPD+RGZ group than in the BPD+V group.

CONCLUSION

These results suggest that RGZ can restore alveolar and pulmonary vascular development and lessen pulmonary hypertension in a rat model of BPD.

Peroxisome proliferator activated receptor-γ-Rho-kinase interactions contribute to vascular remodeling after chronic intrauterine pulmonary hypertension

Peroxisome proliferator-activated receptor-γ (PPARγ) and Rho-kinase (ROCK) regulate smooth muscle cell (SMC) proliferation and contribute to vascular remodeling in adult pulmonary hypertension. Whether these pathways interact to contribute to the development of vascular remodeling in persistent pulmonary hypertension of the newborn (PPHN) remains unknown. We hypothesized that ROCK-PPARγ interactions increase SMC proliferation resulting in vascular remodeling in experimental PPHN. Pulmonary artery SMCs (PASMCs) were harvested from fetal sheep after partial ligation of the ductus arteriosus in utero (PPHN) and controls. Cell counts were performed daily for 5 days with or without PPARγ agonists and ROCK inhibition. PPARγ and ROCK protein expression/activity were measured by Western blot in normal and PPHN PASMCs. We assessed PPARγ-ROCK interactions by studying the effect of ROCK activation on PPARγ activity and PPARγ inhibition (siRNA) on ROCK activity and PASMC proliferation. At baseline, PPHN PASMC cell number was increased by 38% above controls on day 5. ROCK protein expression/activity were increased by 25 and 34% and PPARγ protein/activity decreased by 40 and 50% in PPHN PASMC. ROCK inhibition and PPARγ activation restored PPHN PASMC growth to normal values. ROCK inhibition increased PPARγ activity by 50% in PPHN PASMC, restoring PPARγ activity to normal. In normal PASMCs, ROCK activation decreased PPARγ activity and PPARγ inhibition increased ROCK activity and cell proliferation, resulting in a PPHN hyperproliferative PASMC phenotype. PPARγ-ROCK interactions regulate SMC proliferation and contribute to increased PPHN PASMC proliferation and vascular remodeling in PPHN. Restoring normal PPARγ-ROCK signaling may prevent vascular remodeling and improve outcomes in PPHN.

Perinatal nicotine exposure induces asthma in second generation offspring

BACKGROUND

By altering specific developmental signaling pathways that are necessary for fetal lung development, perinatal nicotine exposure affects lung growth and differentiation, resulting in the offsprings' predisposition to childhood asthma; peroxisome proliferator-activated receptor gamma (PPARγ) agonists can inhibit this effect. However, whether the perinatal nicotine-induced asthma risk is restricted to nicotine-exposed offspring only; whether it can be transmitted to the next generation; and whether PPARγ agonists would have any effect on this process are not known.

METHODS

Time-mated Sprague Dawley rat dams received either placebo or nicotine (1 mg/kg, s.c.), once daily from day 6 of gestation to postnatal day (PND) 21. Following delivery, at PND21, generation 1 (F1) pups were either subjected to pulmonary function tests, or killed to obtain their lungs, tracheas, and gonads to determine the relevant protein markers (mesenchymal contractile proteins), global DNA methylation, histone 3 and 4 acetylation, and for tracheal tension studies. Some F1 animals were used as breeders to generate F2 pups, but without any exposure to nicotine in the F1 pregnancy. At PND21, F2 pups underwent studies similar to those performed on F1 pups.

RESULTS

Consistent with the asthma phenotype, nicotine affected lung function in both male and female F1 and F2 offspring (maximal 250% increase in total respiratory system resistance, and 84% maximal decrease in dynamic compliance following methacholine challenge; P < 0.01, nicotine versus control; P < 0.05, males versus females; and P > 0.05, F1 versus F2), but only affected tracheal constriction in males (51% maximal increase in tracheal constriction following acetylcholine challenge, P < 0.01, nicotine versus control; P < 0.0001, males versus females; P > 0.05, F1 versus F2); nicotine also increased the contractile protein content of whole lung (180% increase in fibronectin protein levels, P < 0.01, nicotine versus control, and P < 0.05, males versus females) and isolated lung fibroblasts (for example, 45% increase in fibronectin protein levels, P < 0.05, nicotine versus control), along with decreased PPARγ expression (30% decrease, P < 0.05, nicotine versus control), but only affected contractile proteins in the male trachea (P < 0.05, nicotine versus control, and P < 0.0001, males versus females). All of the nicotine-induced changes in the lung and gonad DNA methylation and histone 3 and 4 acetylation were normalized by the PPARγ agonist rosiglitazone except for the histone 4 acetylation in the lung.

CONCLUSIONS

Germline epigenetic marks imposed by exposure to nicotine during pregnancy can become permanently programmed and transferred through the germline to subsequent generations, a ground-breaking finding that shifts the current asthma paradigm, opening up many new avenues to explore.

Prenatal rosiglitazone administration to neonatal rat pups does not alter the adult metabolic phenotype

Prenatally administered rosiglitazone (RGZ) is effective in enhancing lung maturity; however, its long-term safety remains unknown. This study aimed to determine the effects of prenatally administered RGZ on the metabolic phenotype of adult rats. Methods. Pregnant Sprague-Dawley rat dams were administered either placebo or RGZ at embryonic days 18 and 19. Between 12 and 20 weeks of age, the rats underwent glucose and insulin tolerance tests and de novo fatty acid synthesis assays. The lungs, liver, skeletal muscle, and fat tissue were processed by Western hybridization for peroxisome proliferator-activated receptor (PPAR)γ, adipose differentiation-related protein (ADRP), and surfactant proteins B (SPB) and C (SPC). Plasma was assayed for triglycerides, cholesterol, insulin, glucagon, and troponin-I levels. Lungs were also morphometrically analyzed. Results. Insulin and glucose challenges, de novo fatty acid synthesis, and all serum assays revealed no differences among all groups. Western hybridization for PPARγ, ADRP, SPB, and SPC in lung, liver, muscle, and fat tissue showed equal levels. Histologic analyses showed a similar number of alveoli and septal thickness in all experimental groups. Conclusions. When administered prenatally, RGZ does not affect long-term fetal programming and may be safe for enhancing fetal lung maturation.

PPARγ Signaling Mediates the Evolution, Development, Homeostasis, and Repair of the Lung

Epithelial-mesenchymal interactions mediated by soluble growth factors determine the evolution of vertebrate lung physiology, including development, homeostasis, and repair. The final common pathway for all of these positively adaptive properties of the lung is the expression of epithelial parathyroid-hormone-related protein, and its binding to its receptor on the mesenchyme, inducing PPARγ expression by lipofibroblasts. Lipofibroblasts then produce leptin, which binds to alveolar type II cells, stimulating their production of surfactant, which is necessary for both evolutionary and physiologic adaptation to atmospheric oxygen from fish to man. A wide variety of molecular insults disrupt such highly evolved physiologic cell-cell interactions, ranging from overdistention to oxidants, infection, and nicotine, all of which predictably cause loss of mesenchymal peroxisome-proliferator-activated receptor gamma (PPARγ) expression and the transdifferentiation of lipofibroblasts to myofibroblasts, the signature cell type for lung fibrosis. By exploiting such deep cell-molecular functional homologies as targets for leveraging lung homeostasis, we have discovered that we can effectively prevent and/or reverse the deleterious effects of these pathogenic agents, demonstrating the utility of evolutionary biology for the prevention and treatment of chronic lung disease. By understanding mechanisms of health and disease as an evolutionary continuum rather than as dissociated processes, we can evolve predictive medicine.

Emerging PPARγ-Independent Role of PPARγ Ligands in Lung Diseases

Peroxisome proliferator activated receptor (PPAR)-γ is a nuclear hormone receptor that is activated by multiple agonists including thiazolidinediones, prostaglandins, and synthetic oleanolic acids. Many PPARγ ligands are under investigation as potential therapies for human diseases. These ligands modulate multiple cellular pathways via both PPARγ-dependent and PPARγ-independent mechanisms. Here, we review the role of PPARγ and PPARγ ligands in lung disease, with emphasis on PPARγ-independent effects. PPARγ ligands show great promise in moderating lung inflammation, as antiproliferative agents in combination to enhance standard chemotherapy in lung cancer and as treatments for pulmonary fibrosis, a progressive fatal disease with no effective therapy. Some of these effects occur when PPARγ is pharmaceutically antagonized or genetically PPARγ and are thus independent of classical PPARγ-dependent transcriptional control. Many PPARγ ligands demonstrate direct binding to transcription factors and other proteins, altering their function and contributing to PPARγ-independent inhibition of disease phenotypes. These PPARγ-independent mechanisms are of significant interest because they suggest new therapeutic uses for currently approved drugs and because they can be used as probes to identify novel proteins and pathways involved in the pathogenesis or treatment of disease, which can then be targeted for further investigation and drug development.

Dynamic regulation of platelet-derived growth factor receptor α expression in alveolar fibroblasts during realveolarization

Although the importance of platelet-derived growth factor receptor (PDGFR)-α signaling during normal alveogenesis is known, it is unclear whether this signaling pathway can regulate realveolarization in the adult lung. During alveolar development, PDGFR-α-expressing cells induce α smooth muscle actin (α-SMA) and differentiate to interstitial myofibroblasts. Fibroblast growth factor (FGF) signaling regulates myofibroblast differentiation during alveolarization, whereas peroxisome proliferator-activated receptor (PPAR)-γ activation antagonizes myofibroblast differentiation in lung fibrosis. Using left lung pneumonectomy, the roles of FGF and PPAR-γ signaling in differentiation of myofibroblasts from PDGFR-α-positive precursors during compensatory lung growth were assessed. FGF receptor (FGFR) signaling was inhibited by conditionally activating a soluble dominant-negative FGFR2 transgene. PPAR-γ signaling was activated by administration of rosiglitazone. Changes in α-SMA and PDGFR-α protein expression were assessed in PDGFR-α-green fluorescent protein (GFP) reporter mice using immunohistochemistry, flow cytometry, and real-time PCR. Immunohistochemistry and flow cytometry demonstrated that the cell ratio and expression levels of PDGFR-α-GFP changed dynamically during alveolar regeneration and that α-SMA expression was induced in a subset of PDGFR-α-GFP cells. Expression of a dominant-negative FGFR2 and administration of rosiglitazone inhibited induction of α-SMA in PDGFR-α-positive fibroblasts and formation of new septae. Changes in gene expression of epithelial and mesenchymal signaling molecules were assessed after left lobe pneumonectomy, and results demonstrated that inhibition of FGFR2 signaling and increase in PPAR-γ signaling altered the expression of Shh, FGF, Wnt, and Bmp4, genes that are also important for epithelial-mesenchymal crosstalk during early lung development. Our data demonstrate for the first time that a comparable epithelial-mesenchymal crosstalk regulates fibroblast phenotypes during alveolar septation.

Thy-1 signals through PPARγ to promote lipofibroblast differentiation in the developing lung

Thy-1 is a glycosylphosphytidylinositol-linked cell-surface glycoprotein present on a subset of lung fibroblasts, which plays an important role in postnatal alveolarization. In the present study, we define the role of Thy-1 in pulmonary lipofibroblast differentiation and in the regulation of lipid homeostasis via peroxisome proliferator-activated receptor-γ (PPARγ). Thy-1 was associated with interstitial cells containing lipid droplets in vivo. The transfection of Thy-1 into Thy-1 (-) fibroblasts increased triglyceride content, fatty-acid uptake, and the expression of the lipofibroblast marker adipocyte differentiation-related protein. Thy-1 (+) fibroblasts exhibited 2.4-fold higher PPARγ activity, and the inhibition or activation of PPARγ reduced and increased triglyceride content, respectively. Thy-1 (-) fibroblasts were not responsive to either of the PPARγ agonists ciglitazone or prostaglandin J(2), supporting the importance of Thy-1 in signaling via PPARγ. Thy-1 (+) fibroblasts expressed significantly higher concentrations of fatty-acid transporter protein-3 mRNA, and demonstrated higher rates of fatty-acid uptake and increased triglyceride content. The inhibition of fatty-acid transporter protein function reduced Thy-1 (+) fibroblast lipid content. The expression of Thy-1 in C57BL/6 lung fibroblasts increased during the neonatal period, coinciding with the onset of alveolarization. Thy-1 promoted lipofibroblast differentiation via the expression of PPARγ, stimulated lipid accumulation via fatty-acid esterification, and enhanced the fatty-acid uptake mediated by fatty-acid transporter proteins. Thy-1 is important in the regulation of lipofibroblast differentiation in the developing lung.

Aberrant signaling pathways of the lung mesenchyme and their contributions to the pathogenesis of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is a chronic lung disease in infants born extremely preterm, typically before 28 weeks' gestation, characterized by a prolonged need for supplemental oxygen or positive pressure ventilation beyond 36 weeks postmenstrual age. The limited number of autopsy samples available from infants with BPD in the postsurfactant era has revealed a reduced capacity for gas exchange resulting from simplification of the distal lung structure with fewer, larger alveoli because of a failure of normal lung alveolar septation and pulmonary microvascular development. The mechanisms responsible for alveolar simplification in BPD have not been fully elucidated, but mounting evidence suggests that aberrations in the cross-talk between growth factors of the lung mesenchyme and distal airspace epithelium have a key role. Animal models that recapitulate the human condition have expanded our knowledge of the pathology of BPD and have identified candidate matrix components and growth factors in the developing lung that are disrupted by conditions that predispose infants to BPD and interfere with normal vascular and alveolar morphogenesis. This review focuses on the deviations from normal lung development that define the pathophysiology of BPD and summarizes the various candidate mesenchyme-associated proteins and growth factors that have been identified as being disrupted in animal models of BPD. Finally, future areas of research to identify novel targets affected in arrested lung development and recovery are discussed.

Mechanism of reduced lung injury by high-frequency nasal ventilation in a preterm lamb model of neonatal chronic lung disease

The mechanism underlying the potentially beneficial effects of the "gentler" modes of ventilation on chronic lung disease (CLD) of the premature infant is not known. We have previously demonstrated that alveolar parathyroid hormone-related protein-peroxisome proliferator-activated receptorγ (PTHrP-PPARγ) signaling is critically important in alveolar formation, and this signaling pathway is disrupted in hyperoxia- and/or volutrauma-induced neonatal rat lung injury. Whether the same paradigm is also applicable to CLD, resulting from prolonged intermittent mandatory ventilation (IMV), and whether differential effects of the mode of ventilation on the PTHrP-PPARγ signaling pathway explain the potential benefits of the "gentler" modes of ventilation are not known. Using a well-established preterm lamb model of neonatal CLD, we tested the hypothesis that ventilatory support using high-frequency nasal ventilation (HFNV) promotes alveolar PTHrP-PPARγ signaling, whereas IMV inhibits it. Preterm lambs managed by HFNV or IMV for 21 d following preterm delivery at 132-d gestation were studied by Western hybridization and immunofluorescence labeling for key markers of alveolar homeostasis and injury/repair. In lambs managed by IMV, the abundance of key homeostatic alveolar epithelial-mesenchymal markers was reduced, whereas it was significantly increased in the HFNV group, providing a potential molecular mechanism by which "gentler" modes of ventilation reduce neonatal CLD.

Curcumin augments lung maturation, preventing neonatal lung injury by inhibiting TGF-β signaling

There is no effective intervention to prevent or treat bronchopulmonary dysplasia (BPD). Curcumin has potent antioxidant and anti-inflammatory properties, and it modulates signaling of peroxisome proliferator-activated receptor-γ (PPARγ), an important molecule in the pathobiology of BPD. However, its role in the prevention of BPD is not known. We determined 1) if curcumin enhances neonatal lung maturation, 2) if curcumin protects against hyperoxia-induced neonatal lung injury, and 3) if this protection is mediated by blocking TGF-β. Embryonic day 19 fetal rat lung fibroblasts were exposed to 21% or 95% O(2) for 24 h following 1 h of treatment with curcumin. Curcumin dose dependently accelerated e19 fibroblast differentiation [increased parathyroid hormone-related protein (PTHrP) receptor, PPARγ, and adipocyte differentiation-related protein (ADRP) levels and triolein uptake] and proliferation (increased thymidine incorporation). Pretreatment with curcumin blocked the hyperoxia-induced decrease (PPARγ and ADRP) and increase (α-smooth muscle actin and fibronectin) in markers of lung injury/repair, as well as the activation of TGF-β signaling. In a separate set of experiments, neonatal Sprague-Dawley rat pups were exposed to 21% or 95% O(2) for 7 days with or without intraperitoneal administration of curcumin. Analysis for markers of lung injury/repair [PTHrP receptor, PPARγ, ADRP, fibronectin, TGF-β receptor (activin receptor-like kinase 5), and Smad3] and lung morphology (radial alveolar count) demonstrated that curcumin effectively blocks TGF-β activation and hyperoxia-induced lung injury. Therefore, curcumin accelerates lung maturation by stimulating key alveolar epithelial-mesenchymal interactions and prevents hyperoxia-induced neonatal lung injury, possibly by blocking TGF-β activation, suggesting that it is a potential intervention against BPD.

Hypoxia-induced inhibition of lung development is attenuated by the peroxisome proliferator-activated receptor-γ agonist rosiglitazone

Hypoxia enhances transforming growth factor-β (TGF-β) signaling, inhibiting alveolar development and causing abnormal pulmonary arterial remodeling in the newborn lung. We hypothesized that, during chronic hypoxia, reduced peroxisome proliferator-activated receptor-γ (PPAR-γ) signaling may contribute to, or be caused by, excessive TGF-β signaling. To determine whether PPAR-γ was reduced during hypoxia, C57BL/6 mice were exposed to hypoxia from birth to 2 wk and evaluated for PPAR-γ mRNA and protein. To determine whether rosiglitazone (RGZ, a PPAR-γ agonist) supplementation attenuated the effects of hypoxia, mice were exposed to air or hypoxia from birth to 2 wk in combination with either RGZ or vehicle, and measurements of lung histology, function, parameters related to TGF-β signaling, and collagen content were made. To determine whether excessive TGF-β signaling reduced PPAR-γ, mice were exposed to air or hypoxia from birth to 2 wk in combination with either TGF-β-neutralizing antibody or vehicle, and PPAR-γ signaling was evaluated. We observed that hypoxia reduced PPAR-γ mRNA and protein, in association with impaired alveolarization, increased TGF-β signaling, reduced lung compliance, and increased collagen. RGZ increased PPAR-γ signaling, with improved lung development and compliance in association with reduced collagen and TGF-β signaling. However, no reduction was noted in hypoxia-induced pulmonary vascular remodeling. Inhibition of hypoxia-enhanced TGF-β signaling increased PPAR-γ signaling. These results suggest that hypoxia-induced inhibition of lung development is associated with a mutually antagonistic relationship between reduced PPAR-γ and increased TGF-β signaling. PPAR-γ agonists may be of potential therapeutic significance in attenuating TGF-β signaling and improving alveolar development.

IUGR differentially alters MeCP2 expression and H3K9Me3 of the PPARγ gene in male and female rat lungs during alveolarization

Intrauterine growth restriction (IUGR) increases the risk of postnatal lung disease, with males more affected. In rat lungs, IUGR impairs alveolarization in conjunction with altered expression of peroxisome proliferator-activated receptor gamma (PPARγ). In non-lung cells, PPARγ transcription is regulated in part by the epigenetic modifying enzyme, and the methyl CpG binding protein 2 (MeCP2). However, it is unknown if IUGR affects MeCP2 expression or its interaction with PPARγ in the rat lung during alveolarization. In this study, we hypothesized that the rat lung would be characterized by the presence of MeCP2 short and long mRNA transcripts, MeCP2 protein isoforms, and the MeCP2 regulatory micro RNA, miR132. We further hypothesized that IUGR would, in a gender-specific manner, alter the levels of MeCP2 components in association with changes in PPARγ mRNA, MeCP2 occupancy at the PPARγ promoters, and PPARγ histone 3 lysine 9 trimethylation (H3K9Me3). To test these hypotheses, we used a well-characterized rat model of uteroplacental insufficiency-induced IUGR. We demonstrated the presence of MeCP2 mRNA, protein, and miR132 in the rat lung throughout alveolarization. We also demonstrated that IUGR alters MeCP2 expression and its interaction with PPARγ in a gender-divergent manner. We conclude that IUGR induces gender-specific alterations in the epigenetic milieu in the rat lung. We speculate that in the IUGR rat lung, this altered epigenetic milieu may predispose to gender-specific alterations in alveolarization.

IUGR decreases elastin mRNA expression in the developing rat lung and alters elastin content and lung compliance in the mature rat lung

Complications of intrauterine growth restriction (IUGR) include increased pulmonary morbidities and impaired alveolar development. Normal alveolar development depends upon elastin expression and processing, as well as the formation and deposition of elastic fibers. This is true of the human and rat. In this study, we hypothesized that uteroplacental insufficiency (UPI)-induced IUGR decreases mRNA levels of elastin and genes required for elastin fiber synthesis and assembly, at birth (prealveolarization) and postnatal day 7 (midalveolarization) in the rat. We further hypothesized that this would be accompanied by reduced elastic fiber deposition and increased static compliance at postnatal day 21 (mature lung). We used a well characterized rat model of IUGR to test these hypotheses. IUGR decreases mRNA transcript levels of genes essential for elastic fiber formation, including elastin, at birth and day 7. In the day 21 lung, IUGR decreases elastic fiber deposition and increases static lung compliance. We conclude that IUGR decreases mRNA transcript levels of elastic fiber synthesis genes, before and during alveolarization leading to a reduced elastic fiber density and increased static lung compliance in the mature lung. We speculate that the mechanism by which IUGR predisposes to pulmonary disease may be via decreased lung elastic fiber deposition.

Fatty acid-binding proteins and peribronchial angiogenesis in bronchopulmonary dysplasia

Inflammation plays a key role in the pathogenesis of bronchopulmonary dysplasia (BPD). Fatty acid-binding proteins (FABPs) 4 and 5 regulate the inflammatory activity of macrophages. Whether FABPs 4 and 5 could play a role in the pathogenesis of BPD via the promotion of macrophage inflammatory activity is unknown. This study sought to examine whether the expression levels of FABP4 and FABP5 were altered in bronchoalveolar lavage fluid and lung tissue in a baboon model of BPD. This study also sought to characterize the cell types that express these proteins. Real-time PCR, immunoblotting, immunohistochemistry, and double immunofluorescence were used to examine the expression of FABPs in samples of BPD. Morphometric analysis was used to quantify FABP4-positive peribronchial blood vessels in lung sections. FABP4 was primarily expressed in macrophages in samples of BPD. In addition, FABP4 was expressed in the endothelial cells of blood vessels in peribronchial areas and the vasa vasorum, but not in the alveolar vasculature in samples of BPD. FABP4 concentrations were significantly increased in lungs and bronchoalveolar lavage fluid samples with BPD. An increased density of FABP4-positive peribronchial blood vessels was evident in both baboon and human BPD sections. FABP5 was expressed in several cell types, including alveolar epithelial cells and macrophages. FABP5 concentrations did not show any significant alterations in BPD. In conclusion, FABP4 but not FABP5 levels are increased in BPD. FABP4 is differentially expressed in endothelial cells of the bronchial microvasculature, which demonstrates a previously unrecognized expansion in BPD.

Evidence for in vivo nicotine-induced alveolar interstitial fibroblast-to-myofibroblast transdifferentiation

Nicotine exposure alters normal homeostatic pulmonary epithelial-mesenchymal paracrine signaling pathways, resulting in alveolar interstitial fibroblast (AIF)-to-myofibroblast (MYF) transdifferentiation. Though this has been described under in vitro conditions, it is not known if the same phenomenon also takes place in vivo. A well-established rodent model of lung damage following perinatal nicotine exposure was used. By probing for the well-established markers of fibroblast differentiation (parathyroid hormone-related protein [PTHrP], peroxisome proliferator-activated receptor gamma [PPARgamma], adipocyte differentiation-related protein, alpha-smooth muscle actin, and fibronectin) at the mRNA, protein, and tissue levels, the authors provide the first in vivo evidence for nicotine-induced AIF-to-MYF transdifferentiation. In addition, these data also provide the first evidence for nicotine-induced up-regulation of Wnt signaling, accompanying the down-regulation of PTHrP/PPARgamma signaling in vivo following nicotine exposure during pregnancy. These data provide an integrated mechanism for in utero nicotine-induced lung damage and how it could permanently alter the "developmental program" of the developing lung by disrupting critically important epithelial-mesenchymal interactions. More importantly, these data are likely to provide specific interventions to augment the pulmonary mesenchymal lipogenic pathway to ameliorate nicotine-induced in utero lung injury.

Antenatally administered PPAR-gamma agonist rosiglitazone prevents hyperoxia-induced neonatal rat lung injury

The physiological development and homeostasis of the lung alveolus is determined by the expression of peroxisome proliferator-activated receptor-γ (PPAR-γ) by the interstitial lipofibroblast. We have recently shown (Dasgupta C et al., Am J Physiol Lung Cell Mol Physiol 296: L1031-L1041, 2009.) that PPAR-γ agonists administered postnatally accelerate lung maturation and prevent hyperoxia-induced lung injury. However, whether the same occurs antenatally is not known. The objective of this study was to test the hypothesis that the potent PPAR-γ agonist rosiglitazone (RGZ), administered antenatally, enhances fetal lung maturation and protects against hyperoxia-induced neonatal lung injury. Sprague-Dawley rat dams were administered either diluent or RGZ (3 mg/kg), at late gestation, to determine its effect on lung maturation and on hyperoxia (95% O(2) exposure for 24 h)-induced neonatal lung injury. The lungs were examined for the expression of specific markers of alveolar development (surfactant proteins A and B, cholinephosphate cytidylyltransferase-α, leptin receptor, triglyceride uptake, and [(3)H]choline incorporation into saturated phosphatidylcholine) and injury/repair, in particular, the markers of transforming growth factor-β signaling (activin receptor-like kinase-5, SMAD3, lymphoid enhancer factor-1, fibronectin, and calponin). Overall, antenatal RGZ accelerated lung maturation and blocked the inhibition of alveolar sacculation and septal wall thinning by hyperoxia. RGZ specifically stimulated the development of the alveolar epithelial type II cell, the lipofibroblast, and the vasculature. The increased expression of the transforming growth factor-β intermediates, such as SMAD3 and lymphoid enhancer factor-1, implicated in hyperoxic lung injury, was also blocked by antenatal RGZ treatment. In conclusion, PPAR-γ agonists can enhance fetal lung maturation and can effectively prevent hyperoxia-induced neonatal lung injury.

Nrf2-regulated PPAR{gamma} expression is critical to protection against acute lung injury in mice

RATIONALE

The NF-E2 related factor 2 (Nrf2)-antioxidant response element (ARE) pathway is essential for protection against oxidative injury and inflammation including hyperoxia-induced acute lung injury. Microarray expression profiling revealed that lung peroxisome proliferator activated receptor gamma (PPARgamma) induction is suppressed in hyperoxia-susceptible Nrf2-deficient (Nrf2(-/-)) mice compared with wild-type (Nrf2(+/+)) mice. PPARgamma has pleiotropic beneficial effects including antiinflammation in multiple tissues.

OBJECTIVES

We tested the hypothesis that PPARgamma is an important determinant of pulmonary responsivity to hyperoxia regulated by Nrf2.

METHODS

A computational bioinformatic method was applied to screen potential AREs in the Pparg promoter for Nrf2 binding. The functional role of a potential ARE was investigated by in vitro promoter analysis. A role for PPARgamma in hyperoxia-induced acute lung injury was determined by temporal silencing of PPARgamma via intranasal delivery of PPARgamma-specific interference RNA and by administration of a PPARgamma ligand 15-deoxy-Delta(12,14)-prostaglandin J(2) in mice.

MEASUREMENTS AND MAIN RESULTS

Deletion or site-directed mutagenesis of a potential ARE spanning -784/-764 sequence significantly attenuated hyperoxia-increased Pparg promoter activity in airway epithelial cells overexpressing Nrf2, indicating that the -784/-764 ARE is critical for Nrf2-regulated PPARgamma expression. Mice with decreased lung PPARgamma by specific interference RNA treatment had significantly augmented hyperoxia-induced pulmonary inflammation and injury. 15 Deoxy-Delta(12,14)-prostaglandin J(2) administration significantly reduced hyperoxia-induced lung inflammation and edema in Nrf2(+/+), but not in Nrf2(-/-) mice.

CONCLUSIONS

Results indicate for the first time that Nrf2-driven PPARgamma induction has an essential protective role in pulmonary oxidant injury. Our observations provide new insights into the therapeutic potential of PPARgamma in airway oxidative inflammatory disorders.

PDGF-Ralpha gene expression predicts proliferation, but PDGF-A suppresses transdifferentiation of neonatal mouse lung myofibroblasts

BACKGROUND

Platelet-derived growth factor A (PDGF-A) signals solely through PDGF-Ralpha, and is required for fibroblast proliferation and transdifferentiation (fibroblast to myofibroblast conversion) during alveolar development, because pdgfa-null mice lack both myofibroblasts and alveoli. However, these PDGF-A-mediated mechanisms remain incompletely defined. At postnatal days 4 and 12 (P4 and P12), using mouse lung fibroblasts, we examined (a) how PDGF-Ralpha correlates with ki67 (proliferation marker) or alpha-smooth muscle actin (alphaSMA, myofibroblast marker) expression, and (b) whether PDGF-A directly affects alphaSMA or modifies stimulation by transforming growth factor beta (TGFbeta).

METHODS

Using flow cytometry we examined PDGF-Ralpha, alphaSMA and Ki67 in mice which express green fluorescent protein (GFP) as a marker for PDGF-Ralpha expression. Using real-time RT-PCR we quantified alphaSMA mRNA in cultured Mlg neonatal mouse lung fibroblasts after treatment with PDGF-A, and/or TGFbeta.

RESULTS

The intensity of GFP-fluorescence enabled us to distinguish three groups of fibroblasts which exhibited absent, lower, or higher levels of PDGF-Ralpha. At P4, more of the higher than lower PDGF-Ralpha + fibroblasts contained Ki67 (Ki67+), and Ki67+ fibroblasts predominated in the alphaSMA + but not the alphaSMA- population. By P12, Ki67+ fibroblasts comprised a minority in both the PDGF-Ralpha + and alphaSMA+ populations. At P4, most Ki67+ fibroblasts were PDGF-Ralpha + and alphaSMA- whereas at P12, most Ki67+ fibroblasts were PDGF-Ralpha- and alphaSMA-. More of the PDGF-Ralpha + than - fibroblasts contained alphaSMA at both P4 and P12. In the lung, proximate alphaSMA was more abundant around nuclei in cells expressing high than low levels of PDGF-Ralpha at both P4 and P12. Nuclear SMAD 2/3 declined from P4 to P12 in PDGF-Ralpha-, but not in PDGF-Ralpha + cells. In Mlg fibroblasts, alphaSMA mRNA increased after exposure to TGFbeta, but declined after treatment with PDGF-A.

CONCLUSION

During both septal eruption (P4) and elongation (P12), alveolar PDGF-Ralpha may enhance the propensity of fibroblasts to transdifferentiate rather than directly stimulate alphaSMA, which preferentially localizes to non-proliferating fibroblasts. In accordance, PDGF-Ralpha more dominantly influences fibroblast proliferation at P4 than at P12. In the lung, TGFbeta may overshadow the antagonistic effects of PDGF-A/PDGF-Ralpha signaling, enhancing alphaSMA-abundance in PDGF-Ralpha-expressing fibroblasts.

Peroxisome proliferator-activated receptor-g agonist treatment increases septation and angiogenesis and decreases airway hyperresponsiveness in a model of experimental neonatal chronic lung disease

Chronic lung disease (CLD) affects premature newborns requiring supplemental oxygen and results in impaired lung development and subsequent airway hyperreactivity. We hypothesized that the maintenance of peroxisome proliferator-activated receptor gamma (PPARgamma) signaling is important for normal lung morphogenesis and treatment with PPARgamma agonists could protect against CLD and airway hyperreactivity (AHR) following chronic hyperoxic exposure. This was tested in an established hyperoxic murine model of experimental CLD. Newborn mice and mothers were exposed to room air (RA) or moderate hyperoxia (70% oxygen) for 10 days and fed a standard diet or chow impregnated with the PPARgamma agonist rosiglitazone (ROSI) for the duration of study. Following hyperoxic exposure (HE) animals were returned to RA until postnatal day (P) 13 or P41. The accumulation of ROSI in neonatal and adult tissue was confirmed by mass spectrometry. Analyses of body weight and lung histology were performed on P13 and P41 to localize and quantitate PPARgamma expression, determine alveolar and microvessel density, proliferation and alpha-smooth muscle actin (alpha-SMA) levels as a measure of myofibroblast differentiation. Microarray analyses were conducted on P13 to examine transcriptional changes in whole lung. Pulmonary function and airway responsiveness were analyzed at P55. ROSI treatment during HE preserved septation and vascular density. Key array results revealed ontogeny groups differentially affected by hyperoxia including cell cycle, angiogenesis, matrix, and muscle differentiation/contraction. These results were further confirmed by histological evaluation of myofibroblast and collagen accumulation. Late AHR to methacholine was present in mice following HE and attenuated with ROSI treatment. These findings suggest that rosiglitazone maintains downstream PPARgamma effects and may be beneficial in the prevention of severe CLD with AHR.