SCHMIDT, K.E.*; LEAL, R.; DESAI, P.; KELLEY, K.M.: Proposed Growth-Inhibitory Role of a Membrane-Associated Insulin-like Growth Factor-Binding Protein (IGFBP) in an Iguanid Heart Cell

A heart cell line derived from Iguana iguana expresses a single IGFBP that is present as both a secreted 30 kDa protein as well as bound on the cell membranes. Competition binding assays for the soluble iguana IGFBP demonstrate that the protein binds ¹²⁵I-IGF-I in a highly specific manner, with 50% inhibition of ¹²⁵I-IGF-I binding between 0.1 –0.5 ng/ml of added unlabeled IGF-I. Affinity crosslinking of ¹²⁵I-IGF-I to cell monolayers demonstrates that this IGFBP is also bound to the cell membranes at levels that rival that of the type-I IGF receptor. Addition of excess unlabeled IGF-I at concentrations above 10 ng/ml results in the removal of membrane-bound IGFBP, upon which a corresponding increase in its levels in the medium are detected. This “membrane-removal” effect of IGF-I occurs rapidly, within 5 min of peptide addition. When the cells are precultured for 15 min in medium containing 100 ng/ml IGF-I, followed by replacement of fresh identical medium (i.e., IGFBP is removed from membrane and the medium containing released IGFBP discarded), the mitogenic effect of IGF-I is significantly enhanced as compared with cells in which the IGFBP remains throughout. This has promoted the hypothesis that this IGFBP serves to inhibit IGF-I actions in the iguana heart cell, using a unique mechanism by which IGF-I binding to abundant cell membrane-associated IGFBP is followed by release of the IGF/IGFBP complex from proximity to the cellular type-I IGF receptor. [Support in part by NSF grant IBN-9600783 & NIH grant GM50089]