EPIDERMAL GROWTH-FACTOR TRANSCRIPTION, TRANSLATION, AND SIGNAL TRANSDUCTION BY RAT TYPE-II PNEUMOCYTES IN CULTURE

Epidermal growth factor (EGF) is known to induce fetal lung maturation and its receptor is present in the lungs of several species.. Recently, EGF has been immunolocalized in type II pneumocytes in rat lung. We postulated that EGF is synthesized in type II pneumocytes and that, because of its position-restricted distribution within the alveolus, EGF might act as an autocrine regulator of type II pneumocyte function. Herein, we have tested the hypothesis using adult rat type II pneumocytes in primary culture. In situ hybridization, using an oligonucleotide probe corresponding to amino acid residues 1070 to 1081 of mouse EGF precursor, demonstrated the presence of EGF precursor mRNA. Upon S-200 Sephacryl gel chromatography of type II pneumocyte extracts, EGF-reactive protein eluted as a high-molecular-weight form (> 100 kD). EGF immunoreactivity was localized within type II pneumocytes in the periphery of groups of 10 to 15 cells in culture. The type II pneumocytes bound [I-125]EGF in a specific manner, indicating the presence of EGF receptors. Scatchard plots gave an apparent affinity constant (K(a)) of 1 x 10(9) liters/mol, and the number of receptors was estimated to be 4.8 x 10(11) mg protein (50 per cell). EGF receptor binding specificity was confirmed by the absence of an autoradiographic signal for cells incubated in the presence of a 100-fold excess concentration of transforming growth factor-alpha. Binding of [I-125]EGF could also be downregulated 95 % by incubation with 0.2 nM transforming growth factor-alpha. Transduction of the EGF signal in type II pneumocytes was indicated by a 2.6-fold increase in [H-3]thymidine incorporation into type II pneumocyte DNA in the presence of nanomolar concentrations of EGF, a 40% increase in phosphatidylinositol hydrolysis, and a 50% increase in surfactant phosphatidylcholine release. We conclude that rat type II pneumocytes in culture transcribe the EGF gene, translate a corresponding EGF polypeptide. and transduce the EGF signal. These data support the hypothesis that EGF has an autocrine function in type II pneumocytes.