Isolation and Characterization of Insulin-Like Growth Factor I (Somatomedin-C) from Cultures of Fetal Rat Calvariae*

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Abstract
Cultured bones have been shown to secrete local regulators of bone remodeling, such as β2-microglobulin, transforming growth factor-β, and insulin-like growth factor (IGF), but the IGF secreted has not been characterized. In the present study, IGF from medium conditioned by 21-day-old fetal rat calvariae was isolated and characterized. IGF was purified using dialysis, gel filtration, and reverse phase HPLC. Amino acid composition was compatible with that of IGF I (somatomedin- C), and amino-terminal sequence analysis revealed homology with IGF-I. The concentration of IGF-I in the calvarial culture medium was 1 nM and was suppressed by cycloheximide. Calvaria- derived rat IGF I at 20 nM stimulated DNA and collagen synthesis by 42% and 26%, respectively, in monolayer cultures of osteoblast-rich rat parietal bone cells. This study indicates that locally produced IGF-I regulates bone formation in cultures of 21-day-old fetal rat calvariae. (Endocrinology122: 22–27, 1988)

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