Effects of hyperoxia on transdifferentiation of primary cultured typeII alveolar epithelial cells from premature rats

ILC2 influence the differentiation of alveolar type II epithelial cells in bronchopulmonary dysplasia mice

Bronchopulmonary dysplasia, a common complication of premature infants, is mainly characterized by blocked alveolarization. Proverbially, the injury of alveolar type II epithelial cells is regarded as the pathologic basis of occurrence and development of bronchopulmonary dysplasia. In the case of alveolar epithelial damage, alveolar type II epithelial cells can also differentiate to alveolar type I epithelial cells as progenitor cells. During bronchopulmonary dysplasia, the differentiation of alveolar type II epithelial cells becomes abnormal. Group 2 innate lymphoid cells can produce type 2 cytokines in response to a variety of stimuli, including the epithelial cytokines IL-25, IL-33, and thymic stromal lymphopoietin. Previous studies have shown that group 2 innate lymphoid cells can inhibit the alveolarization process of bronchopulmonary dysplasia by secreting IL-13. However, whether group 2 innate lymphoid cells can affect the differentiation of alveolar type II epithelial cells in the pathologic process of bronchopulmonary dysplasia remains unclear. In this study, we have shown that IL-13 secreted by group 2 innate lymphoid cells increased during bronchopulmonary dysplasia, which was related to the release of large amounts of IL-33 by impaired alveolar type II epithelial cells. This led to abnormal differentiation of alveolar type II epithelial cells, reduced differentiation to alveolar type I epithelial cells, and increased transdifferentiation to mesenchymal cells through the epithelial-mesenchymal transition. Taken together, our study provides a complementary understanding of the development of bronchopulmonary dysplasia and highlights a novel immune mechanism in the pathogenesis of bronchopulmonary dysplasia.

Calcitonin Gene-Related Peptide Attenuates Hyperoxia-Induced Oxidative Damage in Alveolar Epithelial Type II Cells Through Regulating Viability and Transdifferentiation

As a stem cell of alveolar epithelium, the physiological status of alveolar epithelium type II cells (AECII) after hyperoxia exposure is closely related to the occurrence of hyperoxia-induced lung injury and the restoration of normal morphological function of damaged alveolar epithelium. However, the relevant mechanisms involved are not very clear. Therefore, this study aimed to explore the effect of calcitonin gene-related peptide (CGRP) on AECII exposed to hyperoxia and its potential mechanisms. The AECII viability was detected using MTT assay. The malondialdehyde (MDA) level and superoxide dismutase (SOD) activity were detected by spectrophotometry. The transdifferentiation capacity of AECII was evaluated by flow cytometry. The expression levels of Notch1, Hes, HERP, and AECII markers were detected using immunohistochemistry and/or RT-qPCR or immunofluorescence. ELISA was used for the determination of inflammatory markers. The results showed that CGRP significantly promoted cell viability, and markedly suppressed hyperoxia-induced transdifferentiation of AECII; these biological alterations were coincided with decreased MDA level, increased SOD activity, and activated Notch signaling pathway (upregulated expression levels of Notch1, Hes, and HERP). Notably, the in vitro effects of CGRP on Notch signaling pathway were further investigated in animal model, and the HE staining results showed that CGRP reduced in vivo oxidative injury and inflammation in hyperoxia-treated AECII through the promotion of structural and functional regeneration, accompanied by elevated Notch1 expression and activated Notch signaling cascade as shown by immunohistochemistry and QPCR, respectively. Immunohistochemistry of APQ-5 and SPC indicated that CGRP reversed the transdifferentiation of AECIIs in vivo. Our current results were consistent across both in vitro and in vivo settings, and provide a new direction for the prevention and treatment of bronchopulmonary dysplasia (BPD).

Mesenchymal stem cell-derived extracellular vesicles suppress hyperoxia-induced transdifferentiation of rat alveolar type 2 epithelial cells

Bronchopulmonary dysplasia (BPD) remains the most important respiratory morbidity of preterm infants with few effective preventive strategies. Administration of mesenchymal stem cells (MSC) was considered effective to prevent BPD via paracrine extracellular vesicles (EVs), while appropriate regimens of MSC-EVs and the mechanism remain unclear. Therefore, we established a hyperoxia-induced rat BPD model, and examined the effect of early intraperitoneal MSC-EVs with different doses on BPD. We found that MSC-EVs ameliorated hyperoxia-induced lung injury in a dose-dependent manner, and high dose MSC-EVs ameliorated alveolar simplification and fibrosis. Also, MSC-EVs showed its beneficial effects on vascular growth and pulmonary hypertension. Primary AT2 cells were observed to transdifferentiate into AT1 cells when exposure to hyperoxia in vitro. Administration of MSC-EVs at the first-day culture significantly delayed the transdifferentiation of AT2 cells induced by hyperoxia. We further found that exposure to hyperoxia led to elevated expression of WNT5a mRNA and protein, a key agent in AT2 transdifferentiation, while MSC-EVs administration decreased it. Further study is warranted that MSC-EVs may delay the transdifferentiation of AT2 cells via WNT5a. These studies provide key preclinical evidence of MSC-EVs therapeutics on BPD and highlight the effect of MSC-EVs on suppressing the transdifferentiation of AT2 cells and its possible mechanism through downregulation of WNT5a.

The function role of ubiquitin proteasome pathway in the ER stress-induced AECII apoptosis during hyperoxia exposure

Background

Bronchopulmonary dysplasia (BPD) is a major cause of mortality and morbidity in premature infants, characterized by alveolar dysplasia and pulmonary microvascular remodeling. In the present study, we have investigated the functional roles of ubiquitin proteasome pathway (UPP) in BPD, and its relationship with endoplasmic reticulum stress (ERS) mediated type II alveolar epithelial cell (AECII) apoptosis.

Methods

A hyperoxia-induced BPD rat model was constructed and the pathologic changes of lung tissues were evaluated by hematoxylin–eosin staining. Cell apoptosis and protein expression were determined by TUNEL assay and Western blotting, respectively. Further reagent kit with specific fluorescent substrate was utilized to measure the activity of 20 s proteasome. Meanwhile, AECII were cultured in vitro and exposed to hyperoxia. AECII apoptosis were measured by flow cytometry. In contrast, MG132 treatment was induced to explore UPP during hyperoxia exposure on AECII apoptosis and ERS sensors expression.

Results

A significant increase in apoptosis and total ubiquitinated proteins expression were observed in BPD rats and AECII culture, and the change of UPP was associated with ERS. In order to confirm the role of UPP in AECII apoptosis of BPD, AECII cells were treated by MG132 with the concentration of 10 μmol/L under hyperoxia exposure. We found that the proteins expression of glucose-regulated protein 78 (GRP-78), PKR-like ER kinase (PERK), activating transcription factor 4 (ATF4), activating transcription factor 6 (ATF6) and C/EBP homologous protein (CHOP), as well as AECII apoptosis were increased following MG132 treatment. Furthermore, the relatively up-regulated in the levels of total ubiquitinated proteins expression and 20 s proteasome activity were correlated with increased ERS sensors expression.

Conclusions

Our findings indicate that UPP may participate in the ERS-induced AECII apoptosis under hyperoxia condition.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12890-021-01751-9.

Autophagy inducers restore impaired autophagy, reduce apoptosis, and attenuate blunted alveolarization in hyperoxia-exposed newborn rats

AIM

Autophagy is a common process during development. Abnormal autophagy can impact cell apoptosis. Previous studies have shown that apoptosis is present during bronchopulmonary dysplasia (BPD). However, there is no consensus on the level of coexisting autophagy. This study was designed to investigate the role of autophagy and the effects of autophagy inducers in a BPD model.

METHOD

A total of 100 newborn Sprague-Dawley rats were randomly assigned to model and control groups. BPD models were established by hyperoxic induction(FiO₂ 0.80). Some of them were treated with autophagy-inducing agents.

RESULT

As compared to the control group, more autophagic bodies were found within Type II alveolar epithelial cells (AT-II cells) under transmission electron microscopy (TEM) in the model group at 3 d . These autophagic bodies were also accompanied by apoptotic bodies and expression of both bodies peaked at 7 d. As shown by TdT-mediated dUTP nick end labeling (TUNEL), there were more apoptotic cells in the model group than in the control group. Protein expression levels of LC3B-II, p62, Lamp1, and cleaved Caspase-3 increased with increased hyperoxic exposure time. No significant differences were observed in the mRNA expression levels of LC3B, p62, and Lamp1. After introducing an autophagy inducer, either rapamycin or lithium chloride, the radial alveolar count (RAC) value of BPD model group increased as compared with placebo group, the thickness of alveolar septum decreased, while apoptosis decreased.

CONCLUSION

Reduced autophagy resulting from blocked autophagy flow may be a key link in the pathogenesis of BPD. By enhancing repressed autophagy, apoptosis could be reduced and alveolar development improved.

Decreased ZONAB expression promotes excessive transdifferentiation of alveolar epithelial cells in hyperoxia-induced bronchopulmonary dysplasia

Previous studies by our group have confirmed excessive transdifferentiation of alveolar epithelial cells (AECs) in a hyperoxia‑induced bronchopulmonary dysplasia (BPD) model, but the underlying mechanism have remained elusive. The transcription factor zonula occludens 1‑associated nucleic acid binding protein (ZONAB) has the biological functions of inhibition of epithelial cell differentiation and promotion of epithelial cell proliferation. The aim of the present study was to explore the regulatory effect of ZONAB on the transdifferentiation and proliferation of AECs in a model of hyperoxia‑induced lung injury. Newborn Wistar rats were randomly allocated to a model group (inhalation of 85% O2) or a control group (inhalation of normal air), and ZONAB expression in lung tissues was detected at different time‑points. Type II AECs (AEC II) isolated from normal newborn rats were primarily cultured under an atmosphere of 85 or 21% O2, and ZONAB expression in the cells was examined. The primary cells were further transfected with ZONAB plasmid or small interfering (si)RNA and then exposed to hyperoxia, and the indicators for transdifferentiation and proliferation were measured. The present study indicated that ZONAB expression in AEC II of the BPD rats was significantly decreased from 7 days of exposure to hyperoxia onwards. In the AEC II isolated from normal neonatal rats, ZONAB expression in the model group was also reduced compared with that in the control group. After transfection with the plasmid pCMV6‑ZONAB, the expression of aquaporin 5 (type I alveolar epithelial cell marker) decreased and the expression of surfactant protein C (AEC II marker), proliferating‑cell nuclear antigen and cyclin D1 increased, which was opposite to the effects of ZONAB siRNA. Transfection with pCMV6‑ZONAB also alleviated excessive transdifferentiation and inhibited proliferation of AEC II induced by hyperoxia treatment. These results suggest that ZONAB expression in AEC II decreases under hyperoxia conditions, which promotes transdifferentiation and inhibits proliferation of AECs. This may, at least in part, be the underlying mechanism of lung epithelial injury in the hyperoxia-induced BPD model.

Characterization of air-liquid interface culture of A549 alveolar epithelial cells

Alveolar epithelia play an essential role in maintaining the integrity and homeostasis of lungs, in which alveolar epithelial type II cells (AECII) are a cell type with stem cell potential for epithelial injury repair and regeneration. However, mechanisms behind the physiological and pathological roles of alveolar epithelia in human lungs remain largely unknown, partially owing to the difficulty of isolation and culture of primary human AECII cells. In the present study, we aimed to characterize alveolar epithelia generated from A549 lung adenocarcinoma cells that were cultured in an air-liquid interface (ALI) state. Morphological analysis demonstrated that A549 cells could reconstitute epithelial layers in ALI cultures as evaluated by histochemistry staining and electronic microscopy. Immunofluorescent staining further revealed an expression of alveolar epithelial type I cell (AECI) markers aquaporin-5 protein (AQP-5), and AECII cell marker surfactant protein C (SPC) in subpopulations of ALI cultured cells. Importantly, molecular analysis further revealed the expression of AQP-5, SPC, thyroid transcription factor-1, zonula occludens-1 and Mucin 5B in A549 ALI cultures as determined by both immunoblotting and quantitative RT-PCR assay. These results suggest that the ALI culture of A549 cells can partially mimic the property of alveolar epithelia, which may be a feasible and alternative model for investigating roles and mechanisms of alveolar epithelia in vitro.

Wnt5a reverses the inhibitory effect of hyperoxia on transdifferentiation of alveolar epithelial type II cells to type I cells

Transdifferentiation of alveolar epithelial type II cells (AECIIs) to type I cells (AECIs) is critical for reestablishment and maintenance of an intact alveolar epithelium. However, this process is frequently destroyed by hyperoxia treatment, which is commonly used in respiratory distress syndrome therapy in preterm infants. Wnt5a is considered to participate in this physiopathologic process, but the clear mechanisms still need to be further investigated. In this study, preterm rats and primary rat AECIIs were exposed to hyperoxia. Hematoxylin and eosin staining was used to examine the histological changes of the lungs. Real-time PCR and western blotting were used to examine Wnt5a expression and biomarkers of AECII and AECI expression. Immunohistochemistry and immunofluorescence were also used to determine the expression and location of selected biomarkers. Furthermore, AECIIs transfected with Wnt5a gene and exogenous Wnt5a were used to examine whether Wnt5a contributes to the transdifferentiation of AECIIs to AECIs. Results showed that hyperoxia inhibited the transdifferentiation of AECIIs to AECIs in vitro, which is represented by biomarkers of two types of cell that remained unchanged. In addition, Wnt5a protein expression was found to be decreased after hyperoxia exposure in vitro and in vivo. Furthermore, both the overexpression of Wnt5a and exogenous Wnt5a addition blocked the inhibitory effect of hyperoxia in vitro. In conclusion, our results suggest that the transdifferentiation of AECIIs to AECIs is impaired by hyperoxia, and this process may be associated with Wnt5a downregulation. Targeting Wnt5a may have the potential for the therapy of lung injury in preterm infants induced by hyperoxia.

rIL-10 enhances IL-10 signalling proteins in foetal alveolar type II cells exposed to hyperoxia

Although the mechanisms by which hyperoxia promotes bronchopulmonary dysplasia are not fully defined, the inability to maintain optimal interleukin (IL)-10 levels in response to injury secondary to hyperoxia seems to play an important role. We previously defined that hyperoxia decreased IL-10 production and pre-treatment with recombinant IL-10 (rIL-10) protected these cells from injury. The objectives of these studies were to investigate the responses of IL-10 receptors (IL-10Rs) and IL-10 signalling proteins (IL-10SPs) in hyperoxic foetal alveolar type II cells (FATIICs) with and without rIL-10. FATIICs were isolated on embryonic day 19 and exposed to 65%-oxygen for 24 hrs. Cells in room air were used as controls. IL-10Rs protein and mRNA were analysed by ELISA and qRT-PCR, respectively. IL-10SPs were assessed by Western blot using phospho-specific antibodies. IL-10Rs protein and mRNA increased significantly in FATIICs during hyperoxia, but JAK1 and TYK2 phosphorylation showed the opposite pattern. To evaluate the impact of IL-8 (shown previously to be increased) and the role of IL-10Rs, IL-10SPs were reanalysed in IL-8-added normoxic cells and in the IL-10Rs' siRNA-treated hyperoxic cells. The IL-10Rs' siRNA-treated hyperoxic cells and IL-8-added normoxic cells showed the same pattern in IL10SPs with the hyproxic cells. And pre-treatment with rIL-10 prior to hyperoxia exposure increased phosphorylated IL-10SPs, compared to the rIL-10-untreated hyperoxic cells. These studies suggest that JAK1 and TYK2 were significantly suppressed during hyperoxia, where IL-8 may play a role, and rIL-10 may have an effect on reverting the suppressed JAK1 and TYK2 in FATIICs exposed to hyperoxia.

Hyperoxia stimulates the transdifferentiation of type II alveolar epithelial cells in newborn rats

Supplemental oxygen treatment in preterm infants may cause bronchopulmonary dysplasia (BPD), which is characterized by alveolar simplification and vascular disorganization. Despite type II alveolar epithelial cell (AEC II) damage being reported previously, we found no decrease in the AEC II-specific marker, surfactant protein C (SP-C), in the BPD model in our previous study. We thus speculated that AEC II injury is not a unique mechanism of BPD-related pulmonary epithelial repair dysfunction and that abnormal transdifferentiation can exist. Newborn rats were randomly assigned to model (85% oxygen inhalation) and control groups (room air inhalation). Expressions of AEC I (aquaporin 5, T1α) and AEC II markers (SP-C, SP-B) were detected at three levels: 1) in intact lung tissue, 2) in AEC II isolated from rats in the two groups, and 3) in AEC II isolated from newborn rats, which were further cultured under either hyperoxic or normoxic conditions. In the model group, increased AEC I was observed at both the tissue and cell level, and markedly increased transdifferentiation was observed by immunofluorescent double staining. Transmission electron microscopy revealed morphological changes in alveolar epithelium such as damaged AECs, a fused air-blood barrier structure, and opened tight junctions in the model group. These findings indicate that transdifferentiation of AECs is not suppressed but rather is increased under hyperoxic treatment by compensation; however, such repair during injury cannot offset pulmonary epithelial air exchange and barrier dysfunction caused by structural damage to AECs.

Effect of hyperoxia on the viability and proliferation of the primary type II alveolar epithelial cells

To observe the effect of hyperoxia on the growth of type II alveolar epithelial cells (AEC II). The lungs of 19-day gestation fetal rats were primary cultured and the AEC II were purified by differential adhesion method. The cells were divided into control (normoxia) group and hyperoxia group. The cell growth, cell viability, cell apoptosis, and cell cycle were examined at 2, 4, 6, and 8 days of normoxia or hyperoxia exposure. The number of cells in hyperoxia-exposed group significantly decreased as compared to those of air control group. Number of cells in hyperoxia group was the highest at day 2 of exposure and gradually decreased with time. The viability of cells exposed to hyperoxia was substantially reduced compared with cells exposed to air. Percentage of cells in G1 phase and S phase in hyperoxia group increased gradually with increase in exposure duration and significant differences were seen at day 4 and day 6 compared with either the preceding time points and also with corresponding air-exposed cells. The percentage of both early apoptotic cells (Annexin-V(+)/PI(-)) and late apoptotic cells and necrotic cells (Annexin-V(+)/PI(+)) increased significantly in cells exposed to hyperoxia compared with cells exposed to air. Hyperoxia inhibits proliferation, viability and growth of AEC II and promotes apoptosis.

Expression of SP-C and Ki67 in lungs of preterm infants dying from respiratory distress syndrome

This study aimed at exploring the expression of Surfactant protein-C (SP-C) and Ki67 in autopsy lung tissues of premature infants dying from respiratory distress syndrome (RDS) who were exposed to mechanical ventilation and elevated oxygen concentrations. The possible influence of pulmonary surfactant (PS) on the expression of SP-C and Ki67 was also investigated. Thirty preterm infants were selected who were histologically and clinically diagnosed as RDS. Preterm infants with RDS were divided into 4 groups, according to the time of death: infants ventilated for 1–3 days, 4–8 days, 9–16 days and >6 days. Five premature infants died within 1 day after delivery for non- pulmonary reasons served as controls. The expression of SP-C and Ki67 in lungs was detected by immunohistochemistry. Compared with the control group, the expression of SP-C and Ki67 in RDS infants decreased significantly after 1–3 days of ventilation, but increased after 4 days and reached peak value after 9–16 days. No significant difference in the expression of SP-C and Ki67 was found between infants treated with PS and those without. Thus our results suggest SP-C and Ki67 may have participated in the pulmonary pathological process in ventilated/oxygen treated preterm infants with RDS, and exogenous surfactant had no effect on the expression of SP-C and Ki67 in the lungs of ventilated/oxygen treated preterm infants with RDS.

Regulation and function of the two-pore-domain (K2P) potassium channel Trek-1 in alveolar epithelial cells

Hyperoxia can lead to a myriad of deleterious effects in the lung including epithelial damage and diffuse inflammation. The specific mechanisms by which hyperoxia promotes these pathological changes are not completely understood. Activation of ion channels has been proposed as one of the mechanisms required for cell activation and mediator secretion. The two-pore-domain K(+) channel (K2P) Trek-1 has recently been described in lung epithelial cells, but its function remains elusive. In this study we hypothesized that hyperoxia affects expression of Trek-1 in alveolar epithelial cells and that Trek-1 is involved in regulation of cell proliferation and cytokine secretion. We found gene expression of several K2P channels in mouse alveolar epithelial cells (MLE-12), and expression of Trek-1 was significantly downregulated in cultured cells and lungs of mice exposed to hyperoxia. Similarly, proliferation cell nuclear antigen (PCNA) and Cyclin D1 expression were downregulated by exposure to hyperoxia. We developed an MLE-12 cell line deficient in Trek-1 expression using shRNA and found that Trek-1 deficiency resulted in increased cell proliferation and upregulation of PCNA but not Cyclin D1. Furthermore, IL-6 and regulated on activation normal T-expressed and presumably secreted (RANTES) secretion was decreased in Trek-1-deficient cells, whereas release of monocyte chemoattractant protein-1 was increased. Release of KC/IL-8 was not affected by Trek-1 deficiency. Overall, deficiency of Trek-1 had a more pronounced effect on mediator secretion than exposure to hyperoxia. This is the first report suggesting that the K(+) channel Trek-1 could be involved in regulation of alveolar epithelial cell proliferation and cytokine secretion, but a direct association with hyperoxia-induced changes in Trek-1 levels remains elusive.