Characterization of the IGF axis in a rat liver-derived epithelial cell line.

We have used the techniques of chemical cross-linking, Western blotting and immunoprecipitation-ligand blotting to demonstrate that insulin-like growth factor binding protein-2 (IGFBP-2) is associated with plasma membranes of an epithelial cell line derived from rat liver as well as being secreted into the medium by these cells. We demonstrate that these cells secrete IGF-I, but not IGF-II into serum free medium. Evidence from signalling, cell proliferation and cross-linking experiments indicate that these cells also express cell surface IGF-I receptors. Dose-response experiments indicate an enhanced biological activity of the IGF-I analogue des (1-3) IGF-I compared to wild-type IGF-I in both acute signalling experiments and longer-term (24 h) mitogenic assays. As this IGF-I analogue has lower affinity for IGFBPs, we believe that in this cell culture system, activity of IGF-I may be attenuated in the long and short term by the accumulation of IGFBP-2 in conditioned medium and by the presence of IGFBP-2 associated with the cell membrane and/or ECM.