Membrane-associated insulin-like growth factor (IGF) binding structures in placental cells

The biological activities of IGF-I and ?II are mediated mainly by the type 1 IGF receptor (IGF 1R) and controlled by their interaction with soluble proteins, the IGF binding proteins (IGFBPs). Although there is a growing body of evidence that some IGFBPs may be cell surface-bound, published data concerning cell association of IGFBP-1 are scarce and none of them concern placental cells. The cell membranes used in this study were isolated from term human placenta. Detergent-solubilized membranes were shown to contain two types of IGF binding structures that were separated by gel filtration on a Sephadex G-100 column. Proteins in the first peak were eluted at V0(Mr>100 kD) and they bound IGF-I with greater specificity and affinity than IGF-II and insulin. Most likely, they represented the IGF 1R. Small proteins (Mr?45 kD) were eluted with the membrane proteins in the second maximum. They were able to bind IGF-I and IGF-II, but not insulin. The identity of these proteins was shown to be IGFBP-1 on the basis of their reaction with specific anti-IGFBP-1 antibodies. To the best of our knowledge, the existence of IGFBP-1 associated with human placental cell membranes has not been reported in the literature before. Colocalisation of IGFBP-1 with IGF 1R in cell membranes could provide efficient modulation of IGF 1R receptor-ligand interactions.