Etidronate inhibits human osteoblast apoptosis by inhibition of pro-apoptotic factor(s) produced by activated T cells

Humoral factors produced by activated T cells are thought to be Important i n the development of bone loss in patients with rheumatoid arthritis (RA). We investigated the inhibitory effect of etidronate disodium (EHDP) on apop tosis of human osteoblasts induced by supernatants from in vitro activated T cell cultures. Human osteoblastic cell line MG63 cells and human primary osteoblast-like cells were used in the present study as human osteoblasts. T cells were Incubated with interleukin-2 and further activated with 12-o-t etradecanoyl-phorbol 13-acetate and ionomycin, either in the presence or ab sence of EHDP. After we carried out the cultivation, we examined the cytoto xicity of cultured T cell supernatants toward MG63 cells and human primary osteoblast-like cells. Supernatants from activated but not resting T cell c ultures efficiently induced apoptosis of MG63 cells and primary osteoblast- like cells. Supernatants from activated T cell cultures, Incubated with EHD P, exhibited significantly less cytotoxicity than did supernatants incubate d in the absence of EHDP. In contrast, the cytotoxicity of activated T cell culture supernatants was not affected by direct treatment of human osteobl asts with EHDP. The concentration of soluble Fas ligand in activated T cell culture supernatants was actually increased by EHDP. However, EHDP did not influence soluble Fas and tumor necrosis factor-alpha concentrations in th e supernatant. Furthermore, treatment of human osteoblasts with EHDP did no t alter their expression of Bcl-2/Bcl-xL or their sensitivity to anti-Fas i mmunoglobulin M-induced apoptosis. Our results suggest that EHDP inhibits t he production of soluble factor that induces apoptosis of human osteoblasts and thus exhibits a protective action toward human osteoblast apoptosis in duced by activated T cell culture supernatants. Although the exact EHDP-reg ulated molecule that induces apoptosis of human osteoblasts is unknown at p resent, our study may explain part of the therapeutic action of bisphosphon ates in RA complicated by bone loss.