Hyperoxia inhibits fetal rat lung fibroblast proliferation and expression of procollagens

ERK1/2 Signaling Pathway Activated by EGF Promotes Proliferation, Transdifferentiation, and Migration of Cultured Primary Newborn Rat Lung Fibroblasts

Background

Bronchopulmonary dysplasia (BPD) is a common and serious complication in premature infants. Lung fibroblasts (LFs) are present in the extracellular matrix and participate in pulmonary development in response to BPD. The aim of this study was to investigate the effect of extracellular signal-regulated kinase (ERK) on LFs cultured from newborn rats. Material and Methods. Primary LFs were isolated and treated with epidermal growth factor (EGF, 20 ng/mL) in the presence or absence of an ERK inhibitor, PD98059 (10 μmol/L). Phosphorylated ERK1/2 (p-ERK1/2) protein levels were determined using immunocytochemistry, western blotting, and real-time reverse transcription quantitative (RT–q)PCR. LF proliferation was examined by flow cytometry and a cell counting kit-8 assay. LF transdifferentiation was examined by protein and mRNA expression of α-smooth muscle actin (α-SMA) by immunocytochemistry, western blotting, and RT–qPCR. LF migration was examined by the transwell method.

Results

Phosphorylated ERK1/2, which was activated by EGF, promoted LF proliferation by accelerating cell-cycle progression from the G1 to S phase. After treatment with PD98059, the expression of p-ERK1/2 in LFs, cellular proliferation, and the percentage of cells in S phase were significantly decreased. Phosphorylated ERK1/2 also promoted the differentiation of LFs into myofibroblasts through increased α-SMA synthesis and migration.

Conclusion

The activation of ERK promotes proliferation, transdifferentiation, and migration of lung fibroblasts from newborn rats.

Moderate hyperoxia induces senescence in developing human lung fibroblasts

Hyperoxia exposure in premature infants increases the risk of subsequent lung diseases, such as asthma and bronchopulmonary dysplasia. Fibroblasts help maintain bronchial and alveolar integrity. Thus, understanding mechanisms by which hyperoxia influences fibroblasts is critical. Cellular senescence is increasingly recognized as important to the pathophysiology of multiple diseases. We hypothesized that clinically relevant moderate hyperoxia (<50% O₂) induces senescence in developing fibroblasts. Using primary human fetal lung fibroblasts, we investigated effects of 40% O₂ on senescence, endoplasmic reticulum (ER) stress, and autophagy pathways. Fibroblasts were exposed to 21% or 40% O₂ for 7 days with etoposide as a positive control to induce senescence, evaluated by morphological changes, β-galactosidase activity, and DNA damage markers. Senescence-associated secretory phenotype (SASP) profile of inflammatory and profibrotic markers was further assessed. Hyperoxia decreased proliferation but increased cell size. SA-β-gal activity and DNA damage response, cell cycle arrest in G₂/M phase, and marked upregulation of phosphorylated p53 and p21 were noted. Reduced autophagy was noted with hyperoxia. mRNA expression of proinflammatory and profibrotic factors (TNF-α, IL-1, IL-8, MMP3) was elevated by hyperoxia or etoposide. Hyperoxia increased several SASP factors (PAI-1, IL1-α, IL1-β, IL-6, LAP, TNF-α). The secretome of senescent fibroblasts promoted extracellular matrix formation by naïve fibroblasts. Overall, we demonstrate that moderate hyperoxia enhances senescence in primary human fetal lung fibroblasts with reduced autophagy but not enhanced ER stress. The resulting SASP is profibrotic and may contribute to abnormal repair in the lung following hyperoxia.

Association of the proliferation of lung fibroblasts with the ERK1/2 signaling pathway in neonatal rats with hyperoxia-induced lung fibrosis

Bronchopulmonary dysplasia (BPD) is a common, serious complication occurring in premature infants. Although clinical characteristics and pathologic changes are well described, the pathogenesis of alveolar dysplasia and interstitial fibrosis is less clear. Lung fibroblasts (LFs) are present in the extracellular matrix and serve essential roles during pulmonary epithelial injury and in response to fibrosis development in BPD. The current study investigated hyperoxia-induced proliferation of primary LFs in vitro and mechanisms that may be involved. Newborn rats were exposed to 90% oxygen, while control rats were kept in normal atmosphere. Primary LFs were isolated on postnatal day 3, 7 and 14. Hyperoxia-induced proliferation of LFs isolated on day 7 and 14 by accelerating the cell cycle progression from G1 to S phase. Collagen type I protein secretion and mRNA expression on day 7 and 14 were increased by hyperoxia compared with the controls. Hyperoxia significantly increased the phosphorylation of extracellular signal-regulated kinase (ERK) and significantly increased collagen type I expression compared with the room air control group. The findings indicated that an increase in LF proliferation in response to hyperoxia was associated with ERK1/2 phosphorylation. This mechanism may contribute to over-proliferation of LFs leading to disturbed formation of normal alveoli.

Primary culture of lung fibroblasts from hyperoxia-exposed rats and a proliferative characteristics study

Lung fibrosis is an ultimate consequence of bronchopulmonary dysplasia (BPD) which shows the excessive proliferation of lung fibroblasts (LFs). To find a better model for studying the role of LFs in hyperoxia-induced lung fibrosis at the cellular level, we isolated LFs from the lung tissue of hyperoxia- and normoxia-exposed rat lungs on postnatal days 7, 14 and 21 for primary culture to study their proliferative behavior. In the present study, the LF predominance was > 95% in our culture method. The LFs isolated from rats exposed to hyperoxia in vivo showed significantly greater proliferation than that from normoxia-exposed rats. Flow cytometry revealed that percentage of LFs in S and G2/M stage increased, and proportion in the G0/G1 stage declined at the same time. A greater presence of myofibroblasts in LFs isolated from rats exposed to hyperoxia compared with those exposed to normoxia. In addition, elevated collagen level, transforming growth factor-β and connective tissue growth factor protein expression in conditioned medium were also found in hyperoxia LFs. These data demonstrate that hyperoxia promotes LFs proliferation, myofibroblast transdifferentiation and collagen synthesis in a time-dependent manner. The primary culture of LFs from hyperoxia-exposed rats is a feasible method for studying the pathogenesis and treatment of lung fibrosis caused by BPD at the cellular level.

Inhibition of β-catenin signaling improves alveolarization and reduces pulmonary hypertension in experimental bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is the most common and serious chronic lung disease of preterm infants. The development of pulmonary hypertension (PH) significantly increases the mortality and morbidity of this disease. β-Catenin signaling plays an important role in tissue development and remodeling. Aberrant β-catenin signaling is associated with clinical and experiment models of BPD. To test the hypothesis that inhibition of β-catenin signaling is beneficial in promoting alveolar and vascular development and preventing PH in experimental BPD, we examined the effects of ICG001, a newly developed pharmacological inhibitor of β-catenin, in preventing hyperoxia-induced BPD in neonatal rats. Newborn rat pups were randomized at postnatal day (P)2 to room air (RA) + DMSO (placebo), RA + ICG001, 90% FiO2 (O2) + DMSO, or O2 + ICG001. ICG001 (10 mg/kg) or DMSO was given by daily intraperitoneal injection for 14 days during continuous exposure to RA or hyperoxia. Primary human pulmonary arterial smooth muscle cells (PASMCs) were cultured in RA or hyperoxia (95% O2) in the presence of DMSO or ICG001 for 24 to 72 hours. Treatment with ICG001 significantly increased alveolarization and reduced pulmonary vascular remodeling and PH during hyperoxia. Furthermore, administering ICG001 decreased PASMC proliferation and expression of extracellular matrix remodeling molecules in vitro under hyperoxia. Finally, these structural, cellular, and molecular effects of ICG001 were associated with down-regulation of multiple β-catenin target genes. These data indicate that β-catenin signaling mediates hyperoxia-induced alveolar impairment and PH in neonatal animals. Targeting β-catenin may provide a novel strategy to alleviate BPD in preterm infants.

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

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