IGFBP‐3 Forms Multimers in Physiologic Conditions

Abstract

Insulin‐like growth factors 1 and 2 (IGF‐1 and IGF‐2) are peptide hormones similar in molecular structure to insulin, and they regulate a variety of cellular activities, including metabolism, proliferation, and growth. IGF actions are tightly modulated by a family of proteins called insulin‐like growth factor‐binding proteins (IGFBPs), of which IGFBP‐1 through 6 have been characterized. Most IGFBPs in the blood originate from the liver, but they are also expressed in many other tissues. IGFBP‐3 and ‐5 are the most abundant IGFBPs in the circulation. They form a ternary complex with IGFs and a third protein termed acid‐labile subunit.It has been suggested that IGFBP‐3 and ‐5 can form homomultimers, possibly through disulfide bond exchange, but this effect has been regarded mainly as an artifact. In this work, we show that IGFBP‐3 multimers can occur in physiologic conditions. We demonstrate by surface plasmon resonance that the affinity of the IGFBP‐3 self‐association reaction is in the low nanomolar range. Additionally, we demonstrate by native gel electrophoresis, that IGFBP‐3 multimers of various sizes occur at physiologic concentrations and in several physiologic settings. Moreover, we show that most of these species are SDS‐stable. Our work provides evidence of a previously unrecognized complexity in the IGF‐ 1/IGFBP‐3 system.