The importance of metal ions for the formation and isolation of insulin-like growth factor-binding protein 3–transferrin (IGFBP-3–Tf) complexes, and the analysis of their physiological involvement

Abstract

Insulin-like growth factor (IGF) binding protein 3 (IGFBP-3) is the most abundant IGFBP in the circulation and its main role is to regulate the amount of free, receptor-reactive IGFs that are involved in a broad range of metabolic and mitogenic activities. IGFBP-3 interacts with several other biomolecules forming complexes that might have IGF-dependent or independent functions. Transferrin (Tf) forms complexes with IGFBP-3, but the physiological significance of this interaction remains unclear. Tf is an iron-transporting protein, and is able to carry other metal ions as well. IGFBP-3, on the other hand, possesses one metal-binding domain. As both proteins have the ability to bind metals, the formation of the IGFBP-3–Tf complexes may be expected to influence the metabolism of metal ions and/or mitogenic/metabolic roles in which IGFBP-3 is involved. The aim of this study was to quantify the amount of complexes in the circulation, to investigate the importance of a specific metal ion for their formation and isolation, and to search for the possible physiological place of the IGFBP-3–Tf complexes. The effects of the following ions were analyzed: Fe³⁺, Cu²⁺, Ni²⁺, Mg²⁺ and Zn²⁺. Results have shown that the amount of IGFBP-3 involved in the complex formation with Tf in healthy persons was 5.4 ± 1.02 nM and iron ions were substantial for their formation and isolation without employment of harsh conditions. The amount of IGFBP-3–Tf complexes was further determined in persons with iron over-load, patients with iron-deficiency anemia and patients with colorectal carcinoma-associated anemia. The amount of complexes was in direct correlation with the concentrations of iron and IGFBP-3. In the case of the colorectal carcinoma-associated anemia, the amount of complexes was inversely correlated with the concentration of ferritin. IGFBP-3, thus, appears to be a member of a network of iron-binding proteins that participates in cell signaling that involves iron-associated response.