The Bile Acid Membrane Receptor TGR5 Regulates Osteoclast Differentiation And Function By Active CAMP-AMPK Signaling Pathway

Osteoporosis is a bone disease that characteristic with low bone volume,bone pain and bone fractures,and most elderly and postmenopausal women suffer from it.The numbers of patients of osteoporosis in China up to 90 million,ranking first in the world.The economic burden from osteoporosis increases to hundreds billion RMB per year,and with the arrival of aging society,the number of patients in China will rise sharply.Osteoporosis mainly due to excessive activation of osteoclasts.In clinical,the current drugs used to treat osteoporosis achieve therapeutic effect mostly by inhibiting osteoclasts activity or inhibiting its function.However,the current treatment for osteoporosis still have many side effects and a variety of other deficiencies,therefore,further research the mechanism of activation of osteoclasts has great significance whether for basic research or clinical application of osteoporosis.Bile acids have two kinds of receptors,one kind is nuclear receptors such as FXR,and the other is membrane receptor,namely TGR5.TGR5 belong to the rhodopsin family(family A)of G protein-coupled receptor superfamily.Numerous studies have demonstrated that TGR5 involved in the regulation of glucose metabolism,lipid metabolism,and energy metabolism and inflammatory response.However,whether TGR5 involved in regulation of bone metabolism has not been reported yet.Many studies have shown that osteoclasts activity is closely related to energy metabolism,and TGR5 has an important function in macrophages which come from the same source with osteoclasts.Therefore,we speculate that TGR5 may involved in regulation of bone metabolism by regulate osteoclast activity.In this study,we found that,in vitro,activate TGR5 inhibit osteoclast differentiation,knockout Tgr5 promote osteoclast differentiation,also promote the formation of Sealing circle,and promote bone resorption function of osteoclasts.In vivo,knockout Tgr5 can upregulate osteoclasts activity,while the activation of TGR5 inhibited RANKL-induced excessive activation of osteoclasts.In the pathological conditions simulated by ovariectomy,knockout Tgr5 can lead to more severe bone loss,.But,interestingly,when we analyz the bone phenotype of the wild-type and knockout mice which at the age of 2 months and 4 months,we found that bone mass and micro-structure of the two groups of mice almost unchanged.These results suggest that TGR5 may involved in the regulation of bone only in ovariectomized and other pathological conditions.Then we explored the molecular mechanism,and we found that activate TGR5 can significantly promote the phosphorylation of AMPK,while inhibiting RANKL-induced degradation of IkB-α.AMPK inhibitor Compound C can rescue the osteoclast differentiation inhibition which due to TGR5 specific agonist OA.It indicates that TGR5 inhibit osteoclast differentiation and function by activating AMPK via upregulating cAMP.Given TGR5 influence bone phenotype only in pathological conditions,TGR5 may be a promising therapeutic target for osteoporosis.