REGULATION OF INSULIN-LIKE-GROWTH-FACTOR-I (IGF-I) BIOACTIVITY IN-VIVO - FURTHER CHARACTERIZATION OF AN IGF-I-ENHANCING ANTIBODY

We have previously demonstrated the ability of a polyclonal antibody r aised against human insulin-like growth factor I (IGF-I) to potentiate , rather than inhibit, the growth-promoting activity of IGF-I. The ant i-IGF-I Ig had a modest affinity for IGF-I, protected IGF-I from degra dation, and reduced the IGF-I clearance rate while allowing efficient transfer of peptide from the circulation, leading to the suggestion th at the antiserum might be behaving in an analogous manner to enhancing IGF-binding proteins (IGFBPs). The purpose of these studies was to in vestigate further the characteristics of this antiserum as a means of assessing the importance of IGF-I associated with circulating high mol wt IGFBPs to serve as a bioavailable reservoir of IGF-I peptide. 1) E pitope scanning using sequential and overlapping peptides spanning the entire length of IGF-I revealed one major linear region of anti-IGF-I Ig binding to IGF-I comprising the C-terminal region of the C-domain and the N-terminal region of the A-domain (Arg(36)-Ile(43)), a region not associated with receptor or IGFBP binding. 2) The fact that the an tibody could potentiate IGF-I whether administered as a preincubated c omplex or separately indicated that complex formation could occur in t he presence of IGFBPs in vivo. 3) The ability of the antibody to atten uate the acute hypoglycemic actions of IGF-I and LR(3)IGF-I was assess ed by pretreating dwarf rats with either anti-IGF-I Ig or nonimmune Ig ; 1 h after sc administration of peptide, plasma glucose levels decrea sed by about 4 mM (P < 0.001) in rats pretreated with nonimmune Ig. Th e duration of hypoglycemia was more prolonged in the LR(3)IGF-I-treate d rats (P < 0.01). Neither IGF-I or LR(3)IGF-I induced any decrease in circulating glucose concentrations in the rats pretreated with the an ti-IGF-I Ig, suggesting that the antibody gave protection against inap propriate acute IGF-induced hypoglycemia. 4) The potentiating effects of the anti-IGF-I Ig on the anabolic actions of IGF-I and LR(3)IGF-I w ere compared in dwarf mice. The anti-IGF-I Ig potentiated the increase in whole body weight gain induced by IGF-I by over a-fold (P < 0.001) , but did not change the anabolic action of LR(3)IGF-I despite its abi lity to double circulating levels of both IGF peptides. It is, therefo re, possible that part of the mechanism of action of the anti-IGF-I Ig involves transfer of IGF-I to smaller mol wt binding proteins. These data confirm the potential of IGFBP-associated IGF-I to act as a reser voir of peptide and to regulate IGF-I activity in vivo.