Alveolar epithelial type II cell apoptosis in vivo during resolution of keratinocyte growth factor-induced hyperplasia in the rat

Keratinocyte growth factor (KGF) induces rapid and transient hyperplasia of alveolar epithelial type II cells. We sought to determine components of th e apoptotic process involved in the resolution of this hyperplasia and the fate of the apoptotic cells. Rats received intrabronchial instillation of 5 mg KCF/kg body weight or diluent. Lungs were fixed 1, 2, 3, 5, and 7 days later. Apoptosis was identified by TdT-mediated dUTP nick-end labeling (TUN EL), double-labeling for TUNEL and the type II cell marker MNF116, and elec tron microscopy. Fas, FasL, Bax, Bcl-2, and pro- and active caspase-3 were studied by immunohistochemistry. Changes were quantified by stereology. Cel l type specificity was investigated by immunofluorescence double staining. Type II cells exhibited Fas, FasL, Bcl-2, and procaspase-3 irrespective of treatment and time. Immunoelectron microscopy revealed Fas at the apical ty pe II cell membrane. Bax staining was prominent in controls (45-95% of type II cell surface fraction), markedly decreased during hyperplasia at days 2 (20-40%) and 3 (0-10%), and reappeared at day 7 (25-45%) when apoptosis wa s prominent. Remnants of apoptotic type II cells were incorporated in membr ane-bound vacuoles of type II cell neighbors as well as alveolar macrophage s. The results indicate that type II cells can enter the Fas/FasL/caspase-3 pathway regulated by Bax and Bcl-2. High Bcl-2:Bax levels favor type II ce ll survival and a low rate of apoptosis during hyperplasia. Low Bcl-2:Bax l evels favor type II cell apoptosis during resolution. Because of time-depen dent changes that occur within a short time, the KGF-treated rat lung provi des a useful in vivo model to investigate apoptosis in the context of tissu e remodeling and repair.