Syringin prevents bone loss in ovariectomized mice via TRAF6 mediated inhibition of NF-κB and stimulation of PI3K/AKT

Syringin, also called eleutheroside B, is a main bioactive phenolic glycoside in Acanthopanax senticosus (Rupr. et Maxim.) Harms. Based on the "kidney dominates bone" theory of TCM, A. senticosus can strengthen bone and Syringin may be one of the responsibilities. The objectives of this study were to estimate the osteoporotic activity of Syringin and reveal the possible molecular mechanisms in vivo. Sixty female ICR mice were randomly assigned into sham operated group (SHAM, treated with vehicle) and five ovariectomized subgroups (n = 10 each), treated with vehicle as OVX group, estradiol valerate (EV, 1 mg/kg/day) as positive group, and Syringin (10, 20 and 40 mg/kg/day) as low, moderate and high dosage groups. The therapeutic effect of Syringin against osteoporosis was systematically analyzed by determining the bone mineral density (BMD), bone biomechanical properties, bone microarchitecture and serum biochemical parameters, and the molecular mechanism was also evaluated. After three months of orally administrated intervention, Syringin (10, 20 and 40 mg/kg/day) significantly improved the BMD, bone maximum load and trabecular bone microarchitecture in ovariectomized mice, evidenced by the increased bone mineral content, tissue mineral content, tissue mineral density, trabecular thickness and trabecular number, as well as the decreased trabecular separation in OVX mice. Meanwhile, the activities of tartrate-resistant acid phosphatase, deoxypyridinoline and cathepsin K in OVX mice were also inhibited by Syringin, while the increased body weight and decreased uterus weight seemed not influenced by Syringin administration. Concerning the underlying molecular mechanisms, Syringin significantly downregulated the expression of tumor-necrosis factor receptor-associated factor 6 (TRAF6), nuclear factor kappa B (NF-κB) and receptor activator of nuclear factor kappa B ligand (RANKL) proteins levels, upregulated the expression of osteoprotegerin (OPG), phosphoinositide 3-kinase (PI3K) and protein kinase B (AKT) levels, suggesting that Syringin prevented bone lost by TRAF6-mediated inhibition of NF-κB and stimulation of PI3K/AKT, and subsequently increasing the OPG/RANKL ratio and inhibiting the osteoclastogenesis, finally promoting bone formation. All of the data implied Syringin possessed the potent anti-osteoporosis activity on ovariectomized mice, and the underlying molecular mechanism may be related to the NF-κB and PI3K/AKT signaling pathways.