Insulin–Insulin-like Growth Factors Hybrids as Molecular Probes of Hormone:Receptor Binding Specificity

Insulin, insulin-like growth factors 1 and 2 (IGF-1 and -2, respectively), and their receptors (IR and IGF-1R) are the key elements of a complex hormonal system that is essential for the development and functioning of humans. The C and D domains of IGFs (absent in insulin) likely play important roles in the differential binding of IGF-1 and -2 to IGF-1R and to the isoforms of IR (IR-A and IR-B) and specific activation of these receptors. Here, we attempted to probe the impact of IGF-1 and IGF-2 D domains (D<sub>I</sub> and D<sub>II</sub>, respectively) and the IGF-2 C domain (C<sub>II</sub>) on the receptor specificity of these hormones. For this, we made two types of insulin hybrid analogues: (i) with the C-terminus of the insulin A chain extended by the amino acids from the D<sub>I</sub> and D<sub>II</sub> domains and (ii) with the C-terminus of the insulin B chain extended by some amino acids derived from the C<sub>II</sub> domain. The receptor binding affinities of these analogues and their receptor autophosphorylation potentials were characterized. Our results indicate that the D<sub>I</sub> domain has a more negative impact than the D<sub>II</sub> domain does on binding to IR, and that the D<sub>I</sub> domain Pro-Leu-Lys residues are important factors for a different IR-A versus IR-B binding affinity of IGF-1. We also showed that the additions of amino acids that partially “mimic” the C<sub>II</sub> domain, to the C-terminus of the insulin B chain, change the binding and autophosphorylation specificity of insulin in favor of the “metabolic” IR-B isoform. This opens new venues for rational enhancement of insulin IR-B specificity by modifications beyond the C-terminus of its B chain.