Insulin-like Growth Factor I Stimulates Proliferation and Fas-Mediated Apoptosis of Human Osteoblasts

Abstract

In vitrostudies have shown that insulin-like growth factor I (IGF-I) is a potent growth factor for osteoblasts, although both bone formation and resorption are upregulated by IGF-Iin vivo.To understand the difference in the action of IGF-I observedin vitroandin vivoexperiments, we examined the effect of IGF-I on the proliferation and Fas-mediated apoptosis of human osteoblastsin vitro.Human osteoblastic cell line MG63 and human primary osteoblast-like cells obtained from biopsy specimens were used as human osteoblasts. Cells were cultured with or without various concentrations of IGF-I followed by determination of the proliferative response and Fas-mediated apoptosis. IGF-I dose dependently stimulated the proliferation of cultured human osteoblasts. Both Fas expression and the degree of anti-Fas IgM-induced apoptosis of human osteoblasts was also augmented by IGF-I. Furthermore, the cytotoxicity of Fas ligand (FasL) cDNA transformants against human osteoblasts was increased when IGF-I-stimulated osteoblasts were used as target cells, indicating that stimulation of IGF-I increased functional Fas expression on human osteoblasts as well as their proliferation. The addition of DEVD-CHO, a specific inhibitor of CPP32, to the culture resulted in a significant inhibition of Fas-mediated apoptosis of both unstimulated and IGF-I-stimulated osteoblasts, although it did not affect the proliferative response or Fas expression. Our data suggest that activation of CPP32 is necessary for Fas-mediated apoptosis of human osteoblasts, and treatment of IGF-I increased this signaling pathway. In contrast, regulation of proliferation and Fas expression of the cells were probably not affected by CPP32 activation. Our results suggest that IGF-I acts on cultured human osteoblasts by increasing their proliferation and induction of Fas-mediated apoptosis by neighbouring FasL+cells such as osteoclasts, thus probably functioning as a local coupling factor in the bonein vivo,stimulating both bone formation and resorption.