Insulin-Like Growth Factor I Has Independent Effects on Bone Matrix Formation and Cell Replication*

TLDR

The study used cultured 21‑day‑old fetal rat calvariae and histomorphometric autoradiography to quantify IGF‑I and insulin effects on bone matrix synthesis and cell replication over 96 h. IGF‑I markedly stimulated bone matrix synthesis (45–50 % increase) and cell proliferation (up to 8‑fold in osteoprogenitors), whereas insulin produced similar matrix effects but weaker cell labeling; blocking DNA synthesis with hydroxyurea eliminated IGF‑I’s proliferative effect but only partially reduced matrix formation, indicating IGF‑I’s matrix action is partly independent of DNA synthesis. Endocrinology 122: 254–260 (1988).

Abstract

The effects of insulin-like growth factor-I (IGFI) and insulin on bone matrix synthesis and bone cell replication were studied in cultured 21-day-old fetal rat calvariae. Histomorphometry techniques were developed to measure the incorporation of [2,3-3H]proline and [methyl-3H]thymidine into bone matrix and bone cell nuclei, respectively, using autoradiographs of sagittal sections of calvariae cultured with IGF-I, insulin, or vehicle for up to 96 h. To confirm an effect on bone formation, IGF-I was also studied for its effects on [3H]proline incorporation into collagenase-digestible protein (CDP) and noncollagen protein and on [3H]thymidine incorporation into acid-precipitable material (DNA). IGF-I at 10-9-10-7 M significantly increased the rate of bone matrix apposition and CDP after 24 h by 45-50% and increased cell labeling by 8-fold in the osteoprogenitor cell zone, by 4-fold in the osteoblast cell zone, and by 2-fold in the periosteal fibroblast zone. Insulin at 10-9-10-6 M also increased matrix apposition rate and CDP by 40-50%, but increased cell labeling by 2-fold only at a concentration of 10-7 M or higher and then only in the osteoprogenitor cell zone. When hydroxyurea was added to IGF-I-treated bones, the effects of IGF-I on DNA synthesis were abolished, but the increase in bone matrix apposition induced by IGF-I was only partly diminished. In conclusion, IGF-I stimulates matrix synthesis in calvariae, an effect that is partly, although not completely, dependent on its stimulatory effect on DNA synthesis. (Endocrinology122: 254–260, 1988)