An acid-stable insulin-like growth factor (IGF)-binding protein from pig serum inhibits binding of IGF-I and IGF-II to vascular endothelial cells

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Abstract
The effects of a porcine insulin-like growth factor (IGF)-binding protein on binding of IGF-I and IGF-II to porcine aortic endothelial cells (PAEC) were determined. Binding of 125I-labelled IGF-I and -II to IGF receptors was inhibited by IGF-binding protein. IGF-binding protein inhibited binding of IGF-I and -II in a dose-dependent manner with half-maximal inhibition occurring at 5·43 and 108 μg/l respectively. A125I-labelled IGF-I–IGF-binding protein complex, formed by incubating 125I-labelled IGF-I with IGF-binding protein overnight at 4 °C, did not effectively bind to endothelial IGF receptors. Addition of IGF-binding protein to PAEC previously incubated with IGF-I caused a marked dissociation of bound IGF-I (47% dissociation within 12 h). These results indicate that the acid-stable IGF-binding protein which appears to be a part of the 150 kDa GH-dependent binding protein, blocks binding of IGF-I and -II by the IGF receptors and appears to exhibit a higher affinity for IGF-I than the endothelial type-I IGF receptor. The ramifications of this latter point with respect to transfer of circulating IGFs (bound to their IGF-binding proteins) across the vascular endothelium are not clear. Journal of Endocrinology (1989) 120, 231–236

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