Transforming growth factor-alpha enhances alveolar epithelial cell repair in a new in vitro model

Alveolar epithelial type II cells are essential for regenerating an intact alveolar barrier after destruction of type I cells in vivo. The first objective of these experimental studies was to develop an in vitro model to quantify alveolar epithelial cell wound repair. The second objective was to investigate mechanisms of alveolar epithelial cell wound healing by studying the effects of serum and transforming growth factor-alpha (TGF-alpha) on wound closure. Primary cultures of rat alveolar type II cells were prepared by standard methods and grown to form confluent monolayers in 48 h. Then a wound was made by denuding an area (mean initial area of 2.1 +/- 0.6 mm2) of the monolayer. Re-epithelialization of the denuded area over time in the presence or absence of serum was measured using quantitative measurements from time-lapse video microscopy. The half time of wound healing was significantly enhanced in the presence of serum compared with serum-free conditions (2.4 +/- 0.2 vs. 17.4 +/- 0.8 h, P < 0.001). We then tested the hypothesis that TGF-alpha is an important growth factor for stimulating wound repair of alveolar epithelial cells. Exogenous addition of TGF-alpha in serum-free medium resulted in a significantly more rapid wound closure, and, furthermore, the addition of a monoclonal antibody to TGF-alpha in the presence of serum significantly decreased fourfold the rate of wound closure. Measurement of internuclear cell distance confirmed that both cell motility and cell spreading were responsible for closure of the wound. These data demonstrate that 1) the mechanisms of alveolar cell repair can be studied in vitro and that 2) TGF-alpha is a potent growth factor that enhances in vitro alveolar epithelial cell wound closure.