title: Zinc partitions insulin-like growth factors (IGFs) from soluble IGF binding protein (IGFBP)-5 to the cell surface receptors of BC3H-1 muscle cells.
abstract: Zinc (Zn(2+)) is a multifunctional micronutrient. The list of functions for this micronutrient expanded with the recent discovery that Zn(2+) retains insulin-like growth factors binding proteins (IGFBPs) on the surface of cultured cells, lowers the affinity of cell-associated IGFBPs, and increases the affinity of the cell surface insulin-like growth factor (IGF)-type 1 receptor (IGF-1R). However, currently there is no information concerning the effect of Zn(2+) on soluble IGFBPs. In the current study, the soluble IGFBP-5 secreted by BC(3)H-1 cells is shown to bind approximately 50% more [(125)I]-IGF-II than [(125)I]-IGF-I at pH 7.4. Zn(2+) is shown to depress the binding of both IGF-I and IGF-II to soluble secreted IGFBP-5; [(125)I]-IGF-I binding is affected more so than [(125)I]-IGF-II binding. Zn(2+) acts by lowering the affinity (K(a)) of IGFBP-5 for the IGFs. Scatchard plots are non-linear indicating the presence of high and low affinity binding sites; Zn(2+) affects only binding to the high affinity site. In contrast, Zn(2+) increases the affinity by which either [(125)I]-IGF-I or [(125)I]-R(3)-IGF-I binds to the IGF-1R, but depresses [(125)I]-IGF-II binding to the IGF-type 2 receptor (IGF-2R) on BC(3)H-1 cells. By depressing the association of the IGFs with soluble IGFBPs, Zn(2+) is shown to repartition either [(125)I]-IGF-I or [(125)I]-IGF-II from soluble IGFBP-5 onto cell surface IGF receptors. Zn(2+) was active at physiological doses depressing IGF binding to IGFBP-5 and the IGF-2R at 15-20 microM. Hence, a novel mechanism is further characterized by which the trace micronutrient Zn(2+) could regulate IGF activity.
authors.affiliation: Department of Animal Sciences, Laboratory for Developmental Endocrinology, The University of Illinois, Urbana, Illinois 61801, USA. rmccuske@uiuc.edu
authors.forename: Robert H
authors.initials: RH
authors.lastname: McCusker
authors.forename: Jan
authors.initials: J
authors.lastname: Novakofski
mesh_terms: D000818:Animals; D001665:Binding Sites / Q000187:drug effects / Q000502:physiology; D002460:Cell Line; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D045604:Extracellular Fluid / Q000187:drug effects / Q000378:metabolism; D018975:Insulin-Like Growth Factor Binding Protein 5 / Q000187:drug effects / Q000378:metabolism*; D046911:Macromolecular Substances; D051379:Mice; D008958:Models, Molecular; D032342:Muscle Cells / Q000187:drug effects / Q000378:metabolism; D011485:Protein Binding / Q000187:drug effects / Q000502:physiology; D017434:Protein Structure, Tertiary / Q000502:physiology; D017526:Receptor, IGF Type 1 / Q000187:drug effects / Q000378:metabolism; D017527:Receptor, IGF Type 2 / Q000187:drug effects / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000187:drug effects / Q000378:metabolism*; D013002:Somatomedins / Q000187:drug effects / Q000378:metabolism*; D015032:Zinc / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D018975:Insulin-Like Growth Factor Binding Protein 5; D046911:Macromolecular Substances; D017527:Receptor, IGF Type 2; D011956:Receptors, Cell Surface; D013002:Somatomedins; D017526:Receptor, IGF Type 1; D015032:Zinc