Hyperoxia increases ventilator-induced lung injury via mitogen-activated protein kinases: a prospective, controlled animal experiment

Hyperoxic acute lung injury and ventilator-induced/associated lung injury: new insights into intracellular signaling pathways

In patients with acute respiratory distress syndrome (ARDS), supportive therapy with mechanical ventilation and oxygen is often life saving. Further acute lung injury however, is an unfortunate consequence of oxygen therapy as well as mechanical injury secondary to ventilator induced/associated lung injury (VI/ALI). In this issue of Critical Care, Li et al. expand on the intra-cellular signaling pathways regulating interactions between injury cascades resulting from hyperoxia and high tidal volume ventilation. The findings, suggest that interference or cooperation of different signals may have critical consequences as evidenced by indices of increased lung inflammation, microvascular permeability, and lung epithelial apoptotic cell death.

Mitogen-activated protein kinases regulate susceptibility to ventilator-induced lung injury

Background

Mechanical ventilation causes ventilator-induced lung injury in animals and humans. Mitogen-activated protein kinases have been implicated in ventilator-induced lung injury though their functional significance remains incomplete. We characterize the role of p38 mitogen-activated protein kinase/mitogen activated protein kinase kinase-3 and c-Jun-NH₂-terminal kinase-1 in ventilator-induced lung injury and investigate novel independent mechanisms contributing to lung injury during mechanical ventilation.

Methodology and Principle Findings

C57/BL6 wild-type mice and mice genetically deleted for mitogen-activated protein kinase kinase-3 (mkk-3^−/−) or c-Jun-NH₂-terminal kinase-1 (jnk1^−/−) were ventilated, and lung injury parameters were assessed. We demonstrate that mkk3^−/− or jnk1^−/− mice displayed significantly reduced inflammatory lung injury and apoptosis relative to wild-type mice. Since jnk1^−/− mice were highly resistant to ventilator-induced lung injury, we performed comprehensive gene expression profiling of ventilated wild-type or jnk1^−/− mice to identify novel candidate genes which may play critical roles in the pathogenesis of ventilator-induced lung injury. Microarray analysis revealed many novel genes differentially expressed by ventilation including matrix metalloproteinase-8 (MMP8) and GADD45α. Functional characterization of MMP8 revealed that mmp8^−/− mice were sensitized to ventilator-induced lung injury with increased lung vascular permeability.

Conclusions

We demonstrate that mitogen-activated protein kinase pathways mediate inflammatory lung injury during ventilator-induced lung injury. C-Jun-NH₂-terminal kinase was also involved in alveolo-capillary leakage and edema formation, whereas MMP8 inhibited alveolo-capillary protein leakage.

Involvement of epithelial cell apoptosis in interstitial lung diseases

Lung epithelium is the primary site of lung damage in interstitial lung diseases. Although there are various initiating factors, the terminal stages are characterized by pulmonary fibrosis. Conventional therapy consisting of glucocorticoids or immunosuppressive drugs is usually ineffective. Epithelial cell apoptosis have been considered to be initial events in interstitial lung diseases. The death receptor-mediated signaling pathway directly induces caspase activation and apoptosis. Other stresses induce the release of cytochrome from mitochondria and caspase activation. Endoplasmic reticulum stress also induces apoptosis. Epithelial cell death is followed by remodeling processes, which consist of epithelial and fibroblast activation, cytokine production, activation of the coagulation pathway, neoangiogenesis, re-epithelialization and fibrosis. Epithelial and mesenchymal interaction plays important roles in these processes. Further understanding of apoptosis signaling may lead to effective strategies against devastating lung diseases. We review the role of epithelial cell apoptosis in the molecular mechanisms of pulmonary fibrosis.

Redox imbalance and ventilator-induced lung injury

Mechanical ventilation (MV) is an indispensable therapy in the care of critically ill patients with acute lung injury and the acute respiratory distress syndrome; however, it is also known to further lung injury in certain conditions of mechanical stress, leading to ventilator-induced lung injury (VILI). The mechanisms by which conventional MV exacerbates lung injury and inflammation are of considerable clinical significance. Redox imbalance has been postulated, among other mechanisms, to enhance/perpetuate susceptibility to VILI. A better understanding of these pathologic mechanisms will help not only in alleviating the side effects of mechanical forces but also in the development of new therapeutic strategies. Here, we review the relevance of oxidative stress in VILI from human studies as well as cellular and mouse models of mechanical stress. Potential therapeutic avenues for the treatment of VILI with exogenous administration of antioxidants also are discussed.