Expression of a type I insulin-like growth factor receptor with low affinity for insulin-like growth factor II.
We investigated the binding properties of the type I insulin-like growth factor (IGF) receptor expressed in NIH-3T3 fibroblasts transfected with a human type I receptor cDNA. Cell surface receptors bound IGF-I with KD = 1 nM as predicted. Although recent studies have suggested that IGF-I and IGF-II bind to type I receptors with near-equal affinity, the receptors in this system bound IGF-II with much lower affinity (KD = 15-20 nM). When type I receptors from the transfected cells were solubilized and immunopurified, however, both 125I-IGF-I and 125I-IGF-II bound to the purified receptors with extremely high and relatively similar affinities (KD = 8 and 17 pM respectively). Thus the immunopurified receptors had higher affinity but lower specificity for the two ligands. The monoclonal antibody alpha IR-3 effectively inhibited IGF-I binding to cell surface receptors (75 +/- 10%), but did not inhibit IGF-II binding. In the purified receptor assay, alpha IR-3 also inhibited IGF-I binding more effectively than IGF-II binding (38 +/- 7% versus 10 +/- 4%). We conclude that the products of this cDNA can account for the binding patterns that we previously observed in receptors immunopurified from human placenta. The differential effect of alpha IR-3 on IGF-I versus IGF-II raises the possibility that these homologous growth factors bind to immunologically distinct epitopes on the type I receptor.
Germain-Lee
E L
EL
Division of Pediatric Endocrinology, CMSC 3-110, Johns Hopkins University School of Medicine, Baltimore, MD 21205.
Janicot
M
M
Lammers
R
R
Ullrich
A
A
Casella
S J
SJ
D016475:3T3 Cells; D000818:Animals; D000911:Antibodies, Monoclonal / Q000378:metabolism; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D007335:Insulin-Like Growth Factor II / Q000378:metabolism*; D051379:Mice; D011956:Receptors, Cell Surface / Q000096:biosynthesis* / Q000235:genetics / Q000276:immunology / Q000378:metabolism; D017451:Receptors, Somatomedin; D014162:Transfection
D000911:Antibodies, Monoclonal; D011956:Receptors, Cell Surface; D017451:Receptors, Somatomedin; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II