Estradiol regulation of insulin-like growth factor-I expression in osteoblastic cells: Evidence for transcriptional control

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 α₁(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, 17β-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-1β-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.