The epidermal growth factor receptor network in type 2 pneumocytes exposed to hyperoxia in vitro

Hyperoxia is a well-characterized model of injury and repair of the lung. Type 1 cell damage is followed by type 2 cell proliferation and differentiation which restore normal structure and function. The epidermal growth factor receptor (EGFR) network is known to be a potent modulator of epithelial cell growth. Here we examine the EGFR network on isolated rat type 2 cells and SV40T-T2, a type 2 cell line, under normoxic conditions, after 24 and 48 h of in vitro hyperoxia, and after 24 h of normoxic recovery. EGF induces tyrosine phosphorylation of EGFRs in type 2 cells and SV40T-T2 cells, which decreases with hyperoxia and increases above normoxic levels in recovering cells, suggesting biphasic changes in receptor number or function with injury. The EGFR appears to be stimulated in an autocrine fashion in these cells. There is decreased DNA synthesis and proliferation in SV40T-T2 and isolated type 2 cells treated with tyrphostin B56, a specific EGFR inhibitor. Pretreatment with suramin, which binds to growth factor, results in increased EGFR tyrosine phosphorylation after stimulation, suggesting disruption of normal autocrine receptor downregulation. We have also identified transforming growth factor-alpha (TGF-alpha) in conditioned media (CM) from normoxic and hyperoxic SV40T-T2 and type 2 cells. Finally, we show increased EGF bioactivity in both bronchoalveolar lavage (BAL) from hyperoxic rats and CM from hyperoxic cells compared with normoxic controls. These findings support an integral role for an autocrine EGFR network in the type 2 cell response to injury.