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The biological potencies of recombinant human insulin-like growth factor-I (IGF-I) and two of its analogues were examined for hydrogen peroxide release by neutrophils and blastogenesis by mononuclear cells. The binding affinities of these peptides for bovine serum IGF-binding proteins (IGFBPs) and IGF-I receptors on bovine neutrophils and mononuclear cells were also investigated. Relative to control treatment containing no IGF-I, preincubation of neutrophils with 12·5 µg/l of IGF-I, des(1–3)IGF-I (an analogue of human IGF-I lacking the N-terminal tripeptide Gly-Pro-Glu) and long R³ IGF-I (an analogue of human IGF-I with arginine replacing glutamate at position 3 of human IGF-I and the N-terminal extension Met-Phe-Pro-Ala-Met-ProLeu-Ser-Ser-Leu-Phe-Val-Asn) increased the release of H₂O₂ by 65%, 64% and 32% respectively. However, the difference in stimulating the release of H₂O₂ between long R³ IGF-I and other two (IGF-I and des(1–3)IGF-I) was reduced at a dosage of 100 µg/l. In the absence or presence of 2·5% fetal calf serum (FCS), 100 µg/l of IGF-I, des(1–3)IGF-I but not long R³ IGF-I significantly stimulated thymidine incorporation into mononuclear cells. In addition, des(1–3)IGF-I was more potent than IGF-I in stimulating thymidine incorporation into mononuclear cells in the presence of 2·5% FCS. IGF-I displaced ¹²⁵I-labelled IGF-I binding to serum IGFBPs with half-maximal inhibitory concentrations of approximately 1·5 nmol/1, while des(1–3)IGF-I and long R³ IGF-I only inhibited binding by 20% and 6% respectively, even at a concentration of 35 nmol/l. Similar affinities for IGF-I receptors on neutrophils and mononuclear cells were shown for IGF-I and des(1–3)IGF-I. Conversely, much lower affinities for these receptors were demonstrated for long R³ IGF-I. These results suggest that the biological activities of IGF-I and its analogues in cells of the bovine immune system depend on their binding characteristics both to receptors and to binding proteins.

Journal of Endocrinology (1993) 139, 259–265

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