Icariin Exerts Estrogen-Like Actions on Proliferation of Osteoblasts in Vitro via Membrane Estrogen Receptors-Mediated Non-nuclear Effects

BACKGROUND: According to reports, icariin (ICA) is a bone anabolic agent able to prevent osteoporosis in both ovariectomized rats and postmenopausal women. However, its effect on osteoblast proliferation remains to be determined, and the underlying mechanism remains to be elucidated. METHODS: Icariin-bovine serum albumin (BSA) conjugates were purified by Sephadex G-25 gel chromatography technology. Primary osteoblasts from neonatal rats were used to evaluate the effects of ICA, ICA-BSA, ICA-BSA + ICI182780, and ICA-BSA + PD98059. 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) and propidium iodide (PI)-staining assays were used to detect the proliferation of osteoblasts after drug exposure. The intracellular calcium ions were detected using a confocal microscope with Fluo-3/AM as the fluorescent indicator. Western blot was capitalized on to measure the relative content of phospho-extracellular signal-regulated kinase (p-ERK). RESULTS: Primary osteoblasts in culture were detected by histochemical staining of alkaline phosphatase, and calcified nodules were obtained by sequential digestion. Icariin and bovine serum albumin could form conjugate, which could be purified by Sephadex G-25 gel chromatography technology. MTT and flow cytometry results show that ICA-BSA conjugate significantly facilitated the proliferation of osteoblasts (P < 0.05). The intracellular calcium ions also ascended vastly in the cells treated with ICA-BSA conjugate (P < 0.01). Icariin-bovine serum albumin exposure rapidly activated the extracellular signal-regulated kinase (ERK) signaling. Furthermore, ICA- and ICA-BSA-mediated actions on osteoblasts were signally alleviated after dealing with ERK inhibitor PD98059 or estrogen receptor (ER) antagonist ICI182780, which might have a relation to the repression of ERK phosphorylation. CONCLUSIONS: Icariin could serve as estrogen in osteoblast cells by the rapid nongenomic ER signaling pathway independent of ligand and estrogen response element (ERE) and mediated by mitogen-activated protein kinase (MAPK).