The effect of exogenous insulin-like growth factor-I (IGF-I) on the reproductive performance of female rats, and on serum concentrations of endogenous IGF-I and IGF-I binding proteins

ABSTRACT Incubation of 125I-labelled insulin-like growth factor-I (IGF-I) with rat plasma at 4 °C led to the transfer of approximately half the radioactivity to 150 kDa and smaller complexes with IGF-binding proteins. The extent of association was greater with labelled IGF-II and essentially absent with the truncated IGF-I analogue, des(1–3)IGF-I. A greater degree of binding of IGF peptides with binding proteins occurred after i.v. injection of the tracers into rats, but most of the des(1–3)IGF-I radioactivity remained free. Measurement of the total plasma clearances showed the rapid removal of des(1–3)IGF-I compared with IGF-I and IGF-II; the mean clearances were 4·59, 1·20 and 1·34 ml/min per kg respectively. The mean steadystate volume of distribution was larger for des(1–3)IGF-I than for IGF-I and IGF-II (461, 167 and 181 ml/kg respectively), probably because of the differences in plasma protein binding. With all tracers, radioactivity appeared in the kidneys to a greater extent than in other organs. The amount of radioactivity found in the adrenals, brain, skin, stomach, duodenum, ileum plus jejunum and colon was in rank order, des(1–3)IGF-I > IGF-I > IGF-II. Since this ranking is the opposite of the abilities of the three IGF peptides to form complexes with plasma binding proteins, we propose that the plasma binding proteins inhibit the transfer of the growth factors to their tissue sites of action. Moreover, we suggest that IGF analogues that are cleared rapidly from blood may have greater biological potencies in vivo. Journal of Endocrinology (1991) 128, 197–204

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Phage display-derived ligands provide structural insight into insulin-like growth factor I function

Spectroscopy An International Journal ◽

10.1155/2004/142593 ◽

2004 ◽

Vol 18 (2) ◽

pp. 237-249

Author(s):

Nicholas J. Skelton ◽

Michelle L. Schaffer ◽

Kurt Deshayes ◽

Tamas Blandl ◽

Steven Runyon ◽

...

Keyword(s):

Growth Factor ◽

Phage Display ◽

Binding Proteins ◽

Cell Surface Receptor ◽

Triple Resonance ◽

Triple Resonance Nmr ◽

Insulin Like Growth Factor ◽

Factor I ◽

Igf I ◽

Growth Factor I

Insulin–like growth factor–I (IGF–I) is a central mediator of cell growth, differentiation and metabolism. Structural characterization of the protein has been hampered by a combination of internal dynamics and self–association that prevent crystallization and produce broad NMR resonances. To better characterize the functions of IGF–I, we have used phage display to identify peptides that antagonize the binding of IGF–I to its plasma binding proteins (IGFBPs) and cell–surface receptor (IGF–R). Interestingly, binding of peptide improves dramatically the quality of the NMR resonances of IGF–I, and enables the use of triple–resonance NMR methods to characterize the complexes. One such peptide, designated IGF–F1–1, has been studied in detail. In the complex, the peptide retains the same loop–helix motif seen in the free state whilst IGF–I contains three helices, as has been seen previously in low–resolution structures in the absence of ligand. The peptide binds at a hydrophobic patch between helix 1 and 3, a site identified previously by mutagenesis as a contact site for IGFBP1. Thus, antagonism of IGFBP1 binding exhibited by the peptide occurs by a simple steric occlusion mechanism. Antagonism of IGF–R binding may also be explained by a similar mechanism if receptor binding occurs by a two–site process, as has been postulated for insulin binding to its receptor. Comparisons with crystallographic structures determined for IGF–I in other complexes suggest that the region around helix 1 of IGF–I is conformationally conserved whereas the region around helix 3 adopts several different ligand–induced conformations. The ligand–induced structural variability of helix 3 appears to be a common feature across the insulin super–family. In the case of IGF–I, exchange between such conformations may be the source of the dynamic nature of free IGF–I, and likely has functional significance for the ability of IGF–I to recognize two signaling receptors and six binding proteins with high affinity.

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