Estrogen and testosterone use different cellular pathways to inhibit osteoclastogenesis and bone resorption

Using human peripheral blood CD14(+) osteoclast precursors, we show that testosterone directly inhibits osteoclast formation and bone resorption at physiological concentrations. Instead, estrogen has no direct effects, whereas its action seems to be mediated through osteoblasts by producing osteoprotegerin. Both estrogen and testosterone acts through their cognate receptors.

Estrogen (E2) deficiency is associated with both the development of postmenopausal and senile form of osteoporosis in elderly women. Testosterone (Te) deficiency, on the other hand, may cause osteoporosis in men. In both sexes, osteoporosis is associated with disturbed bone turnover, including increased bone resorption caused by enhanced osteoclast formation and increased osteoclast activity. However, the mechanisms by which E2 or Te act on bone are not fully understood, and one of the central questions is whether these hormones act directly on osteoclast precursors or whether their action is mediated through osteoblastic cells.

We cultured human peripheral blood CD14(+) osteoclast precursors in the presence of RANKL, macrophage-colony stimulating factor (M-CSF), TNF-alpha, and dexamethasone to induce them to differentiate into osteoclasts. To study the possible osteoblast-mediated effects, osteoclast precursors were also co-cultured either with human MG-63 or SaOS-2 osteoblast-derived osteosarcoma cells. These cultures were treated with 10(-8)-10(-12) M of E2 or Te for 7 days.

E2 did not have any direct effect on osteoclast formation, whereas testosterone inhibited osteoclast formation and bone resorption in a dose-dependent manner. In co-cultures, where MG-63 or SaOS-2 cells were present, E2 and Te inhibited osteoclast formation in a dose-dependent manner. At the same time, E2 and Te treatment in MG-63 or SaOS-2 cell-containing cultures stimulated significantly the formation of osteoprotegerin (OPG) compared with untreated cultures measured by ELISA assay from the culture medium. The effects of E2 and Te on osteoclast formation and bone resorption were completely antagonized by an E2 receptor (ER) antagonist, ICI 182,780, and an androgen receptor (AR) antagonist, flutamide, suggesting ER- and AR-mediated mechanisms, respectively, in these cultures.

Te is likely to have direct and indirect inhibitory effects on human osteoclast formation and bone resorption, whereas the effect of E2 on osteoclast precursors and osteoclasts seems to be mediated by osteoblastic cells. Inhibitory effect of E2 is associated with the stimulated secretion of OPG by osteoblast-derived osteosarcoma cells. Mechanism of action of E2 and Te is mediated by ER and AR, respectively.