title: Enhanced insulin-like growth factor-I (IGF-I) cell association at reduced pH is dependent on IGF binding protein-3 (IGFBP-3) interaction.
abstract: The cellular microenvironment impacts how signals are transduced by cells and plays a key role in tissue homeostasis. Although pH is generally well regulated, there are a number of situations where acidosis occurs and our work addresses how low pH impacts cell association of insulin-like growth factor-I (IGF-I) in the presence of IGF binding protein-3 (IGFBP-3). We have previously shown that IGF-I cell binding was enhanced in the presence of IGFBP-3 at low pH and now show that this binding is IGFBP-mediated as it is inhibited by Y60L-IGF-I, a mutant with reduced affinity for the IGF receptor (IGF-IR), and unaffected by insulin, which binds but not IGFBPs. Using surface plasmon resonance (SPR), we show that direct binding between IGF-I and IGFBP-3 is pH sensitive. Despite this, the key step in the process appears to be IGFBP-3 cell surface association as Long-R(3)-IGF-I, a mutant with reduced affinity for IGFBPs, shows a similar increase in cell association at pH 5.8 in the presence of IGFBP-3 but does not exhibit pH-dependent binding by SPR. Further, analysis indicates a large increase in low-affinity binding sites for IGF-I in the presence of IGFBP-3 and an elimination of IGF-I enhanced binding when a non-cell associating mutant of IGFBP-3 is added in place of IGFBP-3. That the IGFBP-3-mediated binding localizes IGF-I away from IGF-IR is suggested by triton-solubility testing and indicates additional complexities to IGF-I regulation by IGFBP-3. Identifying the pH-dependent binding partner(s) for IGFBP-3 is a necessary next step in deciphering this process.
authors.affiliation: Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA. kfw@vt.edu
authors.forename: Kimberly
authors.initials: K
authors.lastname: Forsten-Williams
authors.forename: Theresa R
authors.initials: TR
authors.lastname: Cassino
authors.forename: Laura J
authors.initials: LJ
authors.lastname: Delo
authors.forename: Abigail D
authors.initials: AD
authors.lastname: Bellis
authors.forename: Anne S
authors.initials: AS
authors.lastname: Robinson
authors.forename: Thomas E
authors.initials: TE
authors.lastname: Ryan
mesh_terms: D000818:Animals; D001665:Binding Sites / Q000502:physiology; D002417:Cattle; D002449:Cell Aggregation / Q000502:physiology; D002460:Cell Line; D006863:Hydrogen-Ion Concentration; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000737:chemistry / Q000378:metabolism*; D007334:Insulin-Like Growth Factor I / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D007700:Kinetics; D046911:Macromolecular Substances / Q000378:metabolism; D009154:Mutation / Q000235:genetics; D011485:Protein Binding / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology; D012995:Solubility; D013499:Surface Properties
chemical_list: D018972:Insulin-Like Growth Factor Binding Protein 3; D046911:Macromolecular Substances; D007334:Insulin-Like Growth Factor I