Effect of intermittent hypoxia or hyperoxia on lung development in preterm rat neonates during constant oxygen therapy

Oxygen and mechanical stretch in the developing lung: risk factors for neonatal and pediatric lung disease

Chronic airway diseases, such as wheezing and asthma, remain significant sources of morbidity and mortality in the pediatric population. This is especially true for preterm infants who are impacted both by immature pulmonary development as well as disproportionate exposure to perinatal insults that may increase the risk of developing airway disease. Chronic pediatric airway disease is characterized by alterations in airway structure (remodeling) and function (increased airway hyperresponsiveness), similar to adult asthma. One of the most common perinatal risk factors for development of airway disease is respiratory support in the form of supplemental oxygen, mechanical ventilation, and/or CPAP. While clinical practice currently seeks to minimize oxygen exposure to decrease the risk of bronchopulmonary dysplasia (BPD), there is mounting evidence that lower levels of oxygen may carry risk for development of chronic airway, rather than alveolar disease. In addition, stretch exposure due to mechanical ventilation or CPAP may also play a role in development of chronic airway disease. Here, we summarize the current knowledge of the impact of perinatal oxygen and mechanical respiratory support on the development of chronic pediatric lung disease, with particular focus on pediatric airway disease. We further highlight mechanisms that could be explored as potential targets for novel therapies in the pediatric population.

Hormonal, metabolic, and angiogenic responses to all-out sprint interval exercise under systemic hyperoxia

OBJECTIVE

Hyperoxic gas inhalation during exercise may negatively affect all-out sprint interval exercise (SIE)-induced hormonal, metabolic, and angiogenic responses. We investigated the effects of acute all-out SIE under systemic hyperoxia on hormonal, metabolic, and angiogenic responses.

DESIGN

This was a randomised-crossover trial. Ten healthy males (mean ± standard error of age = 23.1 ± 0.9 years; height = 171.0 ± 1.6 cm; body mass = 66.2 ± 2.0 kg; body mass index = 22.6 ± 0.5 kg/m²) completed the following two experimental regimens: 1) SIE under normoxia and 2) SIE under systemic hyperoxia (FiO₂ = 60%). The subjects performed four bouts of 30-s maximal cycling efforts with 4 min recovery between efforts. The circulating levels of hormonal (growth hormone, epinephrine, and norepinephrine), metabolic (glucose, free fatty acid, and lactate), and angiogenic (vascular endothelial growth factor, matrix metalloproteinase-2 and -9, and endostatin) markers were measured before and at 0 (immediately after the regimen), 30, and 120 min after both regimens.

RESULTS

In response to both SIE regimens, the peak and mean power outputs gradually decreased over the intermittent exercise session compared with those in the first bout (p < 0.01) with no significant differences between the regimens. Both regimens significantly increased the circulating concentrations of all hormonal, metabolic, and angiogenic markers (p < 0.01). However, there were no significant differences in the levels of these markers in response to the two regimens at any time point (p > 0.05).

CONCLUSION

These findings suggest that acute systemic hyperoxia does not influence the hormonal, metabolic, and angiogenic responses to all-out SIE.

The CD146-HIF-1α axis regulates epithelial cell migration and alveolar maturation in a mouse model of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is the most common challenge in preterm neonates. Retardation of alveolar development characterizes the pulmonary pathology in BPD. In the present study, we explored the roles of the CD146-HIF-1α axis in BPD. We demonstrated that the levels of reactive oxygen species (ROS) and soluble CD146 (sCD1146) were increased in the peripheral blood of preterm neonates with BPD. In alveolar epithelial cells, hyperoxia promoted the expression of HIF-1α and CD146, which reinforced each other. In a mouse model of BPD, by exposing pups to 65% hyperoxia, HIF-1α and CD146 were increased in the pulmonary tissues. Mechanistically, CD146 hindered the migration of alveolar epithelial cells; in contrast, movement was significantly enhanced in CD146-knockout alveolar epithelial cells. As expected, CD146-knockout ameliorated alveolarization and improved BPD disease severity. Taken together, our findings imply that the CD146-HIF-1α axis contributes to alveolarization and that CD146 may be a novel candidate in BPD therapy.

Oxygen Toxicity to the Immature Lung-Part I: Pathomechanistic Understanding and Preclinical Perspectives

In utero, the fetus and its lungs develop in a hypoxic environment, where HIF-1α and VEGFA signaling constitute major determinants of further development. Disruption of this homeostasis after preterm delivery and extrauterine exposure to high fractions of oxygen are among the key events leading to bronchopulmonary dysplasia (BPD). Reactive oxygen species (ROS) production constitutes the initial driver of pulmonary inflammation and cell death, altered gene expression, and vasoconstriction, leading to the distortion of further lung development. From preclinical studies mainly performed on rodents over the past two decades, the deleterious effects of oxygen toxicity and the injurious insults and downstream cascades arising from ROS production are well recognized. This article provides a concise overview of disease drivers and different therapeutic approaches that have been successfully tested within experimental models. Despite current studies, clinical researchers are still faced with an unmet clinical need, and many of these strategies have not proven to be equally effective in clinical trials. In light of this challenge, adapting experimental models to the complexity of the clinical situation and pursuing new directions constitute appropriate actions to overcome this dilemma. Our review intends to stimulate research activities towards the understanding of an important issue of immature lung injury.

Oxygen Toxicity to the Immature Lung-Part II: The Unmet Clinical Need for Causal Therapy

Oxygen toxicity continues to be one of the inevitable injuries to the immature lung. Reactive oxygen species (ROS) production is the initial step leading to lung injury and, subsequently, the development of bronchopulmonary dysplasia (BPD). Today, BPD remains the most important disease burden following preterm delivery and results in life-long restrictions in lung function and further important health sequelae. Despite the tremendous progress in the pathomechanistic understanding derived from preclinical models, the clinical needs for preventive or curative therapies remain unmet. This review summarizes the clinical progress on guiding oxygen delivery to the preterm infant and elaborates future directions of research that need to take into account both hyperoxia and hypoxia as ROS sources and BPD drivers. Many strategies have been tested within clinical trials based on the mechanistic understanding of ROS actions, but most have failed to prove efficacy. The majority of these studies were tested in an era before the latest modes of non-invasive respiratory support and surfactant application were introduced or were not appropriately powered. A comprehensive re-evaluation of enzymatic, antioxidant, and anti-inflammatory therapies to prevent ROS injury is therefore indispensable. Strategies will only succeed if they are applied in a timely and vigorous manner and with the appropriate outcome measures.

Intermittent Hypoxia-Hyperoxia and Oxidative Stress in Developing Human Airway Smooth Muscle

Premature infants are frequently and intermittently administered supplemental oxygen during hypoxic episodes, resulting in cycles of intermittent hypoxia and hyperoxia. The relatively hypoxic in utero environment is important for lung development while hyperoxia during the neonatal period is recognized as detrimental towards the development of diseases such as bronchopulmonary dysplasia and bronchial asthma. Understanding early mechanisms that link hypoxic, hyperoxic, and intermittent hypoxic-hyperoxic exposures to altered airway structure and function are key to developing advanced therapeutic approaches in the clinic. Changes in oxygen availability can be detrimental to cellular function and contribute to oxidative damage. Here, we sought to determine the effect of oxygen on mitochondria in human fetal airway smooth muscle cells exposed to either 5% O₂, 21% O₂, 40% O₂, or cycles of 5% and 40% O₂ (intermittent hypoxia-hyperoxia). Reactive oxygen species production, altered mitochondrial morphology, and changes in mitochondrial respiration were assessed in the context of the antioxidant N-acetylcysteine. Our findings show developing airway smooth muscle is differentially responsive to hypoxic, hyperoxic, or intermittent hypoxic-hyperoxic exposure in terms of mitochondrial structure and function. Cycling O₂ decreased mitochondrial branching and branch length similar to hypoxia and hyperoxia in the presence of antioxidants. Additionally, hypoxia decreased overall mitochondrial respiration while the addition of antioxidants increased respiration in normoxic and O₂-cycling conditions. These studies show the necessity of balancing oxidative damage and antioxidant defense systems in the developing airway.

Involvement of nuclear factor erythroid 2‑related factor 2 in neonatal intestinal interleukin‑17D expression in hyperoxia

Interleukin 17D (IL-17D) plays an important role in host defense against inflammation and infection. In the present study, the role of nuclear factor erythroid 2-related factor 2 (Nrf2) in regulating the production of IL-17D was investigated under hyperoxia. For this purpose, neonatal rats were randomized into two groups; the model group was exposed to hyperoxia (80-85% O₂), while the control group was maintained under normoxic conditions (21% O₂). Small intestine tissue was collected on postnatal days 3, 7, 10 and 14. IL-17D expression was detected by immunofluorescence, immunohistochemistry and western blotting. The levels of Nrf2 and kelch-like ECH-associated protein 1 (keap1) were detected by immunohistochemistry and western blotting. Results showed that IL-17D expression in intestine epithelial cells increased steadily, reaching a peak on day 7, and decreased gradually on days 10 and 14 under hyperoxia. Nrf2 expression was consis-tent with IL-17D, and it was positively correlated with IL-17D. However, on postnatal days 10 and 14, the number of CD4⁺ T cells and CD19⁺ B cells expressing IL-17D was increased, and positive cells of the model group were significantly more than that of the control group. Keap1 levels were lower at the early stage. In conclusion, the expression levels of intestinal IL-17D and Nrf2 were altered simultaneously following neonatal rat development in hyperoxia, indicating that Nrf2 may be involved in regulating the expression of IL-17D in intestinal epithelial cells. Moreover, IL-17D in intestinal epithelial cells may play a unique immunological role during hyperoxia.