Canine Tracheal Epithelial Cells Express the Type 1 Insulin-like Growth Factor Receptor and Proliferate in Response to Insulin-like Growth Factor I

Disaggregated airway epithelial cells replicate in serum-free media containing supraphysiologic concentrations of insulin. To examine the hypothesis that the type 1 insulin-like growth factor (IGF) receptor mediates the mitogenic action of insulin on these cells, we studied the mitogenic effects of IGF-I and insulin, and the expression of type 1 IGF receptors in primary cultures of adult canine tracheal epithelial cells. Isolated tracheal epithelial cells were grown in varying concentrations of IGF-I or insulin in Ham's F12 medium supplemented with transferrin, cholera toxin, and endothelial cell growth supplement. Both IGF-I and insulin increased DNA synthesis (measured as [3H]thymidine incorporation into DNA) and cell number in a concentration-dependent fashion, but IGF-I was at least 20 to 60 times more potent than insulin in its mitogenic effects. No additive or synergistic effect was observed with the simultaneous addition of IGF-I and insulin in maximally effective doses. A monoclonal antibody directed against the type 1 IGF receptor (αIR3) blocked the mitogenic activity of both IGF-I and insulin. Affinity labeling of type 1 IGF receptors by covalent cross-linking with disuccinimidyl suberate demonstrated the tracheal epithelial cell IGF-I binding moiety to have a relative molecular weight of 130,000 D. Binding of [125I]IGF-I to this protein was inhibited by low concentrations of IGF-I, relative to insulin, and by αIR3. An 11-kb transcript characteristic of mRNA for the type 1 IGF receptor was recognized in poly(A+) RNA derived from cultured canine tracheal epithelial cells. These data demonstrate, by functional, biochemical, and molecular methods, that cultured canine tracheal epithelial cells express type 1 IGF receptors which mediate their mitogenic response to IGF-I or supraphysiologic concentrations of insulin. These results also raise the possibility that IGF-I may have an important role in growth and differentiation of large airways epithelium.