Insulin-like growth factor I is a major fibroblast mitogen produced by primary cultures of human airway epithelial cells.

1. The ability of airway epithelial cells to produce insulin-like growth factor I may be important in the pathogenesis of subepithelial fibrosis observed in the airways of patients with asthma. We determined whether human airway epithelial cells are capable of producing polypeptide mediators that could induce fibroblast proliferative activity, in particular insulin-like growth factor I. 2. We examined 12 primary cultures of human airway epithelial cells grown to confluence on collagen gel-coated dishes. Using a colorimetric assay based on the uptake and subsequent release of Methylene Blue, increased proliferation of human fetal lung fibroblasts was detected in conditioned media from airway epithelial cells. The median stimulation of fibroblast proliferation was 49.9% (range 25.6-113.3%) above control values (observed at 1:2 dilution of media). 3. A neutralizing antiserum to insulin-like growth factor I partly inhibited fibroblast proliferation induced by epithelial cell conditioned media by 52.2% (49.9-109%; n = 5). 4. Radioimmunoassay for insulin-like growth factor I in conditioned media demonstrated a median concentration of 54.1 ng/ml (32.4-96.8 ng/ml). 5. Insulin-like growth factor I mRNA was detected in epithelial cell monolayers by Northern blot analysis using an insulin-like growth factor I cDNA probe. 6. The insulin-like growth factor I gene is expressed in cultured human airway epithelial cells, which also secrete insulin-like growth factor I protein. Insulin-like growth factor I also accounts for the major mitogenic activity for fibroblasts of cultured human epithelial cell conditioned media. Insulin-like growth factor I may function in a paracrine manner to modulate fibroblast behaviour and may be involved in airway processes, such as those occurring in asthma.