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 development, regulating cell proliferation, differentiation, and survival. Little 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 affinity for IGF-I and inhibit its activity. Intriguingly, some IGFBPs also potentiate 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 enhancing anti-IGF-I antiserum and potentiating IGFBPs share several properties with regard to IGF action, the antibody may provide a model for examining the actions of enhancing IGFBPs. In this study we demonstrate that the antiserum can also enhance IGF-I activity in vitro, assessed as cell number of a bovine fibroblast cell line, suggesting that its actions might not merely be confined to changing the kinetics of IGF-I clearance or degradation. Epitope scanning using overlapping octamer and hexamer peptides spanning the entire sequence of IGF-I indicates that the enhancing antiserum recognizes a specific linear region spanning the C-terminal region of the C domain and the proximal A domain (residues Ser33 to Cys47), and that this recognition is not present in nonenhancing antisera. Further, this region is located on the opposite surface of IGF-I from putative type 1 receptor-binding residues, allowing the possibility that the antiserum might be able to modulate IGF-I receptor binding. Antibodies raised against a synthetic peptide corresponding to Ser33 to Cys47 of IGF-I also potentiated IGF-I activity in vivo. As IGF-I may be beneficial in various clinical conditions associated with catabolism or cell repair, we suggest that this potentiating anti-IGF-I antiserum has favorable properties that could form a basis for therapeutic strategy.
Pell
J M
JM
The Babraham Institute, Cambridge, United Kingdom. jenny.pell@bbsrc.ac.uk
Hill
R A
RA
Stewart
C E
CE
Weston
C R
CR
Flick-Smith
H C
HC
D000595:Amino Acid Sequence; D000818:Animals; D002417:Cattle; D002460:Cell Line; D003545:Cysteine; D004392:Dwarfism / Q000503:physiopathology; D000939:Epitopes / Q000032:analysis; D005347:Fibroblasts; D006801:Humans; D007106:Immune Sera / Q000494:pharmacology*; D007328:Insulin / Q000737:chemistry; D007334:Insulin-Like Growth Factor I / Q000737:chemistry* / Q000494:pharmacology* / Q000502:physiology; D007335:Insulin-Like Growth Factor II / Q000737:chemistry; D051379:Mice; D008817:Mice, Mutant Strains; D008958:Models, Molecular; D008969:Molecular Sequence Data; D019906:Nuclear Magnetic Resonance, Biomolecular; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry / Q000276:immunology / Q000494:pharmacology; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D012694:Serine; D014714:Vertebrates; D015430:Weight Gain / Q000187:drug effects
D000939:Epitopes; D007106:Immune Sera; D007328:Insulin; D011994:Recombinant Proteins; D012694:Serine; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II; D003545:Cysteine