Enhancement of insulin-like growth factor I activity by novel antisera: Potential structure/function interactions

Insulin-like growth factor I(IGF-I) is essential for normal growth and deve lopment, regulating cell proliferation, differentiation, and survival. Litt le IGF-I exists in the free form; rather, it is bound to one of a family of six specific IGF-binding proteins (IGFBPs). Usually, IGFBPs have a high af finity for IGF-I and inhibit its activity. Intriguingly, some IGFBPs also p otentiate IGF-I action; the precise mechanism of this is unclear, but it is thought to include modification of the IGFBP to lower its affinity for IGF -I. We have previously generated a novel antihuman (h) IGF-I antiserum that , instead of inhibiting IGF-I activity, enhances it in vivo. As the enhanci ng anti-IGF-I antiserum and potentiating IGFBPs share several properties wi th regard to IGF action, the antibody may provide a model for examining the actions of enhancing IGFBPs. In this study we demonstrate that the antiser um can also enhance IGF-I activity in vitro, assessed as cell number of a b ovine fibroblast cell line, suggesting that its actions might not merely be confined to changing the kinetics of IGF-I clearance or degradation. Epito pe scanning using overlapping octamer and hexamer peptides spanning the ent ire sequence of IGF-I indicates that the enhancing antiserum recognizes a s pecific linear region spanning the C-terminal region of the C domain and th e proximal A domain (residues Ser(33) to Cys(47)), and that this recognitio n is not present in nonenhancing antisera. Further, this region is located on the opposite surface of IGF-I from putative type 1 receptor-binding resi dues, allowing the possibility that the antiserum might be able to modulate IGF-I receptor binding. Antibodies raised against a synthetic peptide corr esponding to Ser33 to Cys47 Of IGF-I also potentiated IGF-I activity in viv o. its TGF-I may be beneficial in various clinical conditions associated wi th catabolism or cell repair, we suggest that this potentiating anti-IGF-I antiserum has favorable properties that could form a basis for therapeutic strategy.