Insulin-like growth factor(IGF)-1 and IGF-2, may be important regulatory molecules in the CNS. Possible origins of IGFs in brain include either de novo synthesis or transport of circulating IGFs from blood into brain via receptor mediated transcytosis mechanisms at the brain capillary endothelial wall, ie, the blood-brain barrier (BBB). In the present studies, isolated human brain capillaries are used as an in vitro model system of the human BBB and the characteristics of IGF-1 or IGF-2 binding to this preparation were assessed. The total binding of IGF-2 at 37°C exceeded 130% per mg protein and was threefold greater than the total binding for IGF-1. However, at 37°C nonsaturable binding equaled total binding, suggesting that endocytosis is rate limiting at physiologic temperatures. Binding studies performed at 4°C slowed endocytosis to a greater extent than membrane binding, and specific binding of either IGF-1 or IGF-2 was detectable. Scatchard plots for either peptide were linear and the molar dissociation constant of IGF-1 and IGF-2 binding was 2.1 ± 0.4 and 1.1. ± 0.1 nmol/L, respectively. Superphysiologic concentrations of porcine insulin inhibited the binding of both IGF-1 (ED50 = 2 μg/mL) and IGF-2 (ED50 = 0.5 μg/mL). Affinity cross linking of 125I-IGF-1, 125I-IGF-2, and 125I-insulin to isolated human brain capillaries was performed using disuccinimidylsuberate (DSS). These studies revealed a 141 kd binding site for both IGF-1 and IGF-2, and a 133 kd binding site for insulin. No high molecular weight binding site for IGF-2 was detected. In conclusion, these studies suggest that a type 1 IGF receptor is present on isolated human brain capillaries that binds both IGFs, with a slightly higher affinity for IGF-2. The threefold greater activity of this receptor for IGF-2 relative to IFG-1 parallels the much higher concentration of IGF-2 in human brain and CSF (relative to IGF-1). These results are consistent with the hypothesis that the human BBB IGF receptor is a transport system for the circulating peptides, particularly for IGF-2.
ASJC Scopus subject areas
- Endocrinology, Diabetes and Metabolism