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Activation of type II alveolar epithelial cells during acute endotoxemia
Vasanthi R Sunil1, Agnieszka J Connor, Yan Guo
1Department of Toxicology and Pharmacology, Rutgers University, 170 Frelinghuysen Road, Piscataway, New Jersey 08854-8020, USA.
Abstract:
Lung injury induced by acute endotoxemia is associated with increased generation of inflammatory mediators such as nitric oxide and eicosanoids, which have been implicated in the pathophysiological process. Although production of these mediators by alveolar macrophages (AM) has been characterized, the response of type II cells is unknown and was assessed in the present studies. Acute endotoxemia caused a rapid (within 1 h) and prolonged (up to 48 h) induction of nitric oxide synthase-2 (NOS-2) in type II cells but a delayed response in AM (12-24 h). In both cell types, this was associated with increased nitric oxide production. Although type II cells, and to a lesser extent AM, constitutively expressed cyclooxygenase-2, acute endotoxemia did not alter this activity. Endotoxin administration had no effect on mitogen-activated protein kinase or protein kinase B-alpha (PKB-alpha) expression. However, increases in phosphoinositide 3-kinase and phospho-PKB-alpha were observed in type II cells. The finding that this was delayed for 12-24 h suggests that these proteins do not play a significant role in the regulation of NOS-2 in this model. After endotoxin administration to rats, a rapid (within 1-2 h) activation of nuclear factor-kappaB was observed. This response was transient in type II cells but was sustained in AM. Interferon regulatory factor-1 (IRF-1) was also activated rapidly in type II cells. In contrast, IRF-1 activation was delayed in AM. These data demonstrate that type II cells, like AM, are highly responsive during acute endotoxemia and may contribute to pulmonary inflammation.
Insights
Acute endotoxemia rapidly activates nitric oxide synthase-2 (NOS-2) in lung type II cells, contributing to pulmonary inflammation. This contrasts with the delayed response in alveolar macrophages (AM).
Area of Science:
- Pulmonary immunology
- Cellular inflammation
- Molecular mechanisms of lung injury
Background:
- Acute endotoxemia triggers lung injury via inflammatory mediators like nitric oxide.
- Alveolar macrophages (AM) role in mediator production is known, but type II cell response is unclear.
- Understanding type II cell responses is crucial for elucidating endotoxemia-induced lung inflammation.
Purpose of the Study:
- To investigate the inflammatory response of lung type II cells during acute endotoxemia.
- To compare the activation kinetics of inflammatory pathways in type II cells versus AM.
- To assess the role of specific signaling molecules in endotoxemia-induced lung inflammation.
Main Methods:
- Induction of acute endotoxemia in a rat model.
- Analysis of nitric oxide synthase-2 (NOS-2) expression and activity in isolated type II cells and AM.
- Measurement of cyclooxygenase-2 (COX-2), mitogen-activated protein kinase (MAPK), and phosphoinositide 3-kinase (PI3K)/protein kinase B-alpha (PKB-alpha) signaling.
- Assessment of nuclear factor-kappaB (NF-κB) and interferon regulatory factor-1 (IRF-1) activation.
Main Results:
- Type II cells showed rapid (1h) and sustained (48h) NOS-2 induction and nitric oxide production.
- AM exhibited a delayed NOS-2 response (12-24h) compared to type II cells.
- NF-κB activation was rapid but transient in type II cells, sustained in AM.
- IRF-1 activation was rapid in type II cells and delayed in AM.
- PI3K and phospho-PKB-alpha increased in type II cells, but their delayed kinetics suggest no role in NOS-2 regulation.
Conclusions:
- Lung type II cells are highly responsive to acute endotoxemia, similar to AM.
- Type II cells contribute significantly to pulmonary inflammation during endotoxemia.
- Distinct temporal activation patterns of inflammatory pathways exist between type II cells and AM.