Dual Targeting of Bile Acid Receptor-1 (TGR5) and Farnesoid X Receptor (FXR) Prevents Estrogen-Dependent Bone Loss in Mice

Osteoporosis is a global bone disease characterized by reduced bone mineral density (BMD) and increased risk of fractures. The risk of developing osteoporosis increases with aging, especially after menopause in women. Discovering the signaling pathways that play a significant role in aging- and menopause-induced osteoporosis should accelerate osteoporosis drug discovery. In this study, we found that bile acid membrane receptor Tgr5 knockout C57BL/6J mice had similar bone mass as wild-type mice during early and middle-age (before 4 months old) bone remodeling; however, Tgr5 ^-/- markedly decreased bone mass in aged (more than 7 months old) and ovariectomized (OVX) mice compared with wild-type mice. Moreover, Tgr5 knockout strongly induced osteoclast differentiation but had no effect on osteoblast activity. Treatment with different TGR5 agonists consistently inhibited osteoclast differentiation. Importantly, our results showed that Tgr5 regulates osteoclastogenesis by the AMP-activated protein kinase (AMPK) signaling pathway, which is a central metabolic pathway involved in the pathophysiology of aging and age-related diseases. The bile acid nuclear receptor FXR is an established regulator of bone metabolism. We screened the derivatives of betulinic acid (BA), a known TGR5 agonist, to identify novel dual agonists of FXR and TGR5. The derivative SH-479, a pentacyclic triterpene acid, could activate both TGR5 and FXR, with a better inhibitory effect on osteoclastogenesis compared with agonists solely activating FXR or TGR5 and additionally enhanced osteoblastogenesis. Furthermore, SH-479 therapeutically abrogated bone loss in C57BL/6J mice through the bone remodeling pathways. Together, our results demonstrate that dual targeting the bile acid membrane receptor TGR5 and nuclear receptor FXR is a promising strategy for osteoporosis.