Effect of osteotropic agents on the expression of RANKL and OPG in Saos-2 cells

Abstract

Various osteotropic agents that influence bone resorption are known to act primarily via osteoblasts/stromal cells. Recently, receptor activator of nuclear factor-kB ligand (RANKL) and osteoprotegerin (OPG) have been suggested to be key molecules that regulate osteoclast differentiation and activation. RANKL induces osteoclastogenesis and activates mature osteoclasts while OPG acts as a physiologic inhibtor of RANKL. It is conceivable, therefore, that change in RANKL and OPG expression in osteoblasts/stromal cells affect their ability to support osteoclast formation, activity and survival. In this study, we examined the effects of several osteotropic agents on RANKL and OPG mRAN expression in Saos-2 human osteoblastic cells. Cells were exposed to parathyroid hormone (PYH, 10^-8M), 1,25-dihydroxyvitamin D_3 (1,25(OH)_2D_3, 10^-8M), dexamethasone (10^-8M), interleukin-1β (IL-1β, 5ng/ml), tumor necrosis factor-α (TNF-α, 5ng/ml), transforming growth factor-β (TNF-β, 5ng/ml), or insulin-like growth factor-I (IGF-I, 10ng/ml) for 2, 4, 8, and 24h, and mRNA levels were analyzed by semi-quantitative reverse transcription-polymerase chain reaction. All the tested osteotropic agents more or less regulated both RANKL and OPG mRNA level during the examined period. RaNKL/OPG ratio was up-regulated by PTH, 1,25(OH)_2D_3, dexamethasone, TGF-β, IGF-I, and increased RANKL/OPG ratio was maintained up to 24h. IL-1β and TNF-α transiently they greatlhy decreased RANKL/OPG ratio. These results showed that RANKL and OPG could be potential targets for bone resorption regulation by osteotropic hormenes, cytokines, and growth factors. However, regulatory patterns were not alvays coincident with in vivo or in vitro effects on osteoclastogenesis, implying that RANKL and OPG are not the sole mediators of their action.