HEPARIN-BINDING EGF-LIKE GROWTH-FACTOR IS A MITOGEN FOR RAT ALVEOLAR TYPE-II CELLS

Alveolar type II cells proliferate and differentiate into type I epith elial cells to restore the alveolar epithelium after lung injury. Sinc e mitogens that bind the epidermal growth factor (EGF), EGF, receptor and transforming growth factor alpha (TGF alpha) have been shown to st imulate type II cell proliferation, studies were undertaken to determi ne whether the recently described protein, heparin-binding EGF-like gr owth factor (HB-EGF), was a mitogen for rat alveolar type II cells in primary culture. In addition, since HB-EGF is produced by macrophages, it was of interest to determine whether mitogenic activity for type I I cells present in macrophage conditioned medium was due to HB-EGF. Ra t and human recombinant HB-EGF stimulated thymidine incorporation into rat type II cells in a concentration-dependent manner up to 10-50 ng/ ml then became inhibitory. The nuclear labeling index of type II cells increased from 2% to 16% with 10 ng/ml HB-EGF. However, HB-EGF induce d only a small increase in cell number after 48 h and did not support low-density proliferation of alveolar type II cells. Conditioned mediu m from the human monocytic cell line, U937, stimulated type II cell DN A synthesis, and stimulatory activity could be partially purified by S -sepharose and heparin-sepharose chromatography. The growth-promoting activity from U937 cells that bound to heparin-sepharose was inhibited by a neutralizing antibody to human HB-EGF. Immunoblot analysis of ac tive fractions also verified the presence of HB-EGF. However, the neut ralizing antibody to rat HB-EGF did not inhibit mitogenic activity for type II cells found in rat bronchoalveolar lavage fluid. HB-EGF mRNA was found to be expressed in human alveolar macrophages to similar lev els as differentiated U937 cells but was not detected in rat alveolar macrophages by Northern analysis of total mRNA. There was no differenc e in the level of HB-EGF mRNA expression in human alveolar macrophages from patients with interstitial lung disease compared with macrophage s from normal subjects. The results demonstrate that HB-EGF is a mitog en for rat alveolar type II cells but appears to show species-specific differences with regard to its production by macrophages.