REGULATION OF ALVEOLAR TYPE-II CELL-DIFFERENTIATION AND PROLIFERATIONIN ADULT-RAT LUNG EXPLANTS

Alveolar type II cells produce pulmonary surfactant and serve as the s tem cell of the alveolar epithelium by proliferating and transforming into type I cells. The study of the differentiated function and prolif erative capacity of type II cells in response to injury in vivo has be en hindered by the complexity of the systemic response to injury. In v itro studies have in turn been limited by the impaired proliferative p otential and loss of markers of differentiation in isolated type II ce lls maintained in culture. We describe an in vitro system in which typ e II cells proliferate spontaneously and simultaneously maintain diffe rentiated characteristics. Other investigators have maintained slices of adult lung in culture after agarose infusion for up to 9 wk. To fur ther develop this model for the study of epithelial cell differentiati on and proliferation, we assessed epithelial differentiation, prolifer ative capacity, and regulation of cell-specific gene expression in sli ce explants of agarose-infused rat lungs. We prepared 1-mm-thick expla nts and maintained them in culture for up to 2 wk. Maintenance of diff erentiation was confirmed morphologically by light and electron micros copy, by the accumulation of epithelial cell-specific surfactant prote ins, and by phospholipid analysis. Proliferative capacity was assessed by measuring [H-3]thymidine incorporation in alveolar and small airwa y cells at baseline and in response to growth stimuli. Type II cell pr oliferation was inhibited in a dose-dependent manner by glucocorticoid s. Glucocorticoids regulated RNA levels in explants in a manner simila r to that seen in vivo and in fetal lung explants. The alveolar epithe lium in adult lung slice explants maintains differentiated function an d the ability to proliferate, thereby providing a useful system for th e study of distal airway and alveolar cell homeostasis and response to injury.