Enhanced insulin-like growth factor-I (IGF-I) cell association at reduced pH is dependent on IGF binding protein-3 (IGFBP-3) interaction

✍ Scribed by Kimberly Forsten-Williams; Theresa R. Cassino; Laura J. Delo; Abigail D. Bellis; Anne S. Robinson; Thomas E. Ryan

• Publisher: John Wiley and Sons
• Year: 2006
• Tongue: English
• Weight: 256 KB
• Volume: 210
• Category: Article
• ISSN: 0021-9541
• DOI: 10.1002/jcp.20849

✦ Synopsis

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. J. Cell. Physiol. 210: 298–308, 2007. © 2006 Wiley‐Liss, Inc.