Alveolar Type II Cells: A Novel Target Of Simvastatin-induced Attenuation Of Lung Ischemia Reperfusion Injury

Ischemia-reperfusion injury induced pulmonary dysfunction is one of the major clinical problems in thoracic and cardiovascular surgery. Lung ischemia-reperfusion injury (LIRI) following surgical operations, such as lung transplantation, cardiopulmonary bypass and lobectomy with pulmonary artery reconstruction, usually leads to significant morbidity and mortality. LIRI is characterized by extensive lung edema and hypoxemia, which directly leads to non-specific cell breakdown and death of lung epithelial tissue. However the detailed pathophysiological process of LIRI still has not yet been fully studied. Thus, it would be of value to better understand the process for cardiothoracic surgeons in a clinically relevant animal model.The epithelium lining the alveoli of the lung is comprised of two morphologically and functionally distinct types of cells: alveolar type I (ATI) cells and type II (ATII) cells. The bulk of evidence indicates that ATII cells play a critical role in maintaining tissue homeostasis via the generation of surfactants, the transportation of fluid and ion, and even the restoration of epithelium. In addition, research in recent years has demonstrated that ATII cells are progenitor cells for the alveolar epithelium. In response to an acute lung injury, ATI cells are more vulnerable; meanwhile, hyperplastic ATII cells spread on the basement membrane and transdifferentiat into ATI cells. Therefore, enhancing ATII cells self-repair capacity may be a promising way to attenuate acute lung injury.Simvastain is a kind of HMG-CoA reductase inhibitor which has been widely used clinically to lowering lipid. The recent study found that except the traditional lipid-lowering effects, it also has the protection of experimental kidney, brain and heart and limb ischemia induced lung injury. In 2003, Naidu and his co-workers found that simvastatin has the capacity of protecting LIRI in a rat experiment for the first time, but its long-term effects of statin is still unknown. In addition, recent studies suggest that simvastatin in COPD model plays the role for promoting alveolar epithelial cells while the improvement of lung function. Whether statins has a similar effect in LIRI models remains unclear. Our study observed the dynamic changes of ATII cells in a rat long-term survival LIRI model. Meanwhile, we discussed the related mechanisms of simvastatin-induced attenuation of lung ischemia-reperfusion injury.Partâ… Establishment of a rat lung ischemia-reperfusion injury long-term survival model and the dynamic observation of the morphological and functional changes of alveolar type II cellsObjective To establish a simple and stable rat lung ischemia-reperfusion (LIRI) long-term survival model and observe the dynamic changes of the morphology and function of alveolar type II cells.Methods Male Sprague-Dawley rats were randomized into sham and ischemia-reperfusion (IR) groups. Animals of IR group underwent warm ischemia for 60 minutes by left pulmonary hilum occlusion. Injury was assessed by histological examination, wet/dry ratio (W/D ratio) and lung permeability index (LPI). Ultrastructure and stereological analysis were used to quantify the alterations of nuclei and lamellar bodies (LBs) of ATII cells.Results All the SD rats survived during the observation period. As compared with the sham group, wet/dry ratio and lung permeability index in the IR group significantly decreased at the early phase of LIRI and returned to normal in 7 days after reperfusion. Ultrastructure and stereological analysis of ATII cells also showed that LBs were remarkably impaired at the early phase of LIRI and recovered up to 7 days after reperfusion.Conclusions This model is simple, stable and reproducible. ATII cells demonstrated a self-repair capacity in a slow manner following the early phase of LIRI. Enhancing self-repair capacity of ATII cells may be a potential way of alleviating or curing LIRI.Part II Simvastatin Attenuates lung Ischemia-Reperfusion Injury via enhancing the proliferation ability of ATII cellsObjective To evaluate the protective effects on alveolar type II cells induced by simvastatin in a rat lung ischemia-reperfusion (LIRI) long-term survival model.Methods 144 healthy SD rats were randomly divided into 4 groups: the sham group (n=36 no hilar blocking); IR group (n=36, left hilar blocking); LSIM group(n=36, animals were orally treated with 0.5mg/kg/d Simvastatin 3 days prior to the surgery until the sacrificed day); HSIM group(n=36, animals were orally treated with 5mg/kg/d Simvastatin 3 days prior to the surgery until the sacrificed day).The lung tissue were collected at baseline before hilar occlusion and 1 hour after ischemia, 4 hours 1 day, 3 days and 7 days after reperfusion respectively. The indices were determined as follows: the myeloperoxidase (MPO) activity of lung tissue, pulmonary surfactant-C (SP-C) expression and the status of ATII cells'proliferation determined by SP-C/PCNA immunofluorensence double staining. Ultrastructure and stereological analysis was used to quantify the alterations of lamellar bodies (LBs) and profiles of ATII cells.Results The rat LIRI long-term survival model was successfully established. Compared with sham group, MPO activity, PaO2 and the SP-C mRNA level were significantly lower in LSIM and HSIM group at the time-points of 4 hours and 1 day after reperfusion (P<0.01 respectively). The number of SP-C/PCNA double positive ATII cells displayed that compared with sham group, the proliferation of ATII cells in HSIM and LSIM group were significantly increased at 1 day after reperfusion. Ultrastructure and stereological analysis of ATII cells also showed that LBs in LSIM and HSIM group were remarkably attenuated after reperfusion.Conclusions ATII cells might be a novel target of simvastatin-induced-attenuation of LIRI, in which enhancing the proliferation ability of ATII is involved as one of the important mechanisms.