Estradiol Regulation of Insulin-Like Growth Factor-I Expression in Osteoblastic Cells: Evidence for Transcriptional Control

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Abstract
Insulin-like growth factor-I (IGF-I) has anabolic effects on skeletal tissues, acting as both a systemic hormone and an autocrine/paracrine regulator of cellular function. We have previously reported that estradiol (E2) stimulation of rat osteoblast proliferation in vitro was inhibited by IGF-I antibodies. We show here that E2, similar to IGF-I, also increases alpha 1(I) procollagen mRNA levels in primary cultures of rat calvarial osteoblasts. The E2 effect on collagen mRNA lags behind that produced by recombinant IGF-I by about 12 h and was also abolished in the presence of cycloheximide or by the addition of antibodies against IGF-I. Furthermore, 17 beta E2 induced a 2- to 2.5-fold elevation of the level of IGF-I mRNA within 2-4 h, which persisted thereafter. The E2 stimulation of IGF-I mRNA was not blocked by cycloheximide, suggesting that de novo protein synthesis of an intermediate protein was not required. The IGF-I mRNA half-life, estimated by treating the cells with the RNA polymerase inhibitor 5,6-dichloro-1 beta-D-ribofuranosylbenzimidazole, was about 7 h and was not altered by E2 treatment. On the other hand, nuclear run-on assays indicated that E2 increased the transcriptional activity of the IGF-I gene, and this effect was further enhanced in cells overexpressing E2 receptors after transient transfection. These findings suggest that IGF-I may serve as a mediator for the anabolic effects of E2 on bone, and that E2 stimulates IGF-I gene expression at least in part through transcriptional control.

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