Characterization of the Somatomedin-C/Insulin-Like Growth Factor I (SM-C/IGF-I) Receptor on Cultured Human Fibroblast Monolayers: Regulation of Receptor Concentrations by SM-C/IGF-I and Insulin*

Abstract

We have characterized 125I-labeled somatomedin-C/insulin-like growth factor 1 ([125I]SM-C/IGF-I) binding to cultured human fibroblast monolayers and have investigated the modulation of receptor concentrations by SM-C/IGF-I and insulin. Specific binding of [125I]SM-C/IGF-I to confluent monolayers at 15 C reached a steady state in 2.5 h and averaged 11.37 ± 0.71% (mean ± SEM)/1.5 × 106 cells. At 30 C, binding occurred more rapidly, peaking at 30 min, but averaged only 4%. Binding was reversible at both temperatures, and degradation of radioligand by fibroblast monolayers was negligible. Half-maximal inhibition of [125I]SM-C/IGF-I binding was observed at SM-C/IGF-I concentrations of 7 ng/ml. Compared to SM-C/IGF-I, the relative potencies of other hormones was: IGF-II, 1:10; multiplication-stimulating activity, 1:17, porcine insulin, 1:300; porcine proinsulin, 1:1000; and human GH nonreactive. Scatchard analysis was consistent with a single class of receptors, with a Ka of 1.07 × 109 M−1 and 20,000 sites/cell...