Specnuezhenide Alleviates Senile Osteoporosis by Activating TGR5/FXR Signaling in Bone Marrow Mesenchymal Stem Cells and RANKL-Induced Osteoclasts.

Background: Specnuezhenide (SPN) is an iridoid glycoside isolated from , an herb prescribed for the treatment of senile osteoporosis. However, the direct role of SPN on bone metabolism remains unclear. In this study, the effects of SPN on d-galactose (d-gal)-induced mice, bone marrow mesenchymal stem cells (BMSCs), and nuclear factor-κB ligand-induced osteoclasts were examined.

Methods: Micro-computed tomography was used to observe the bone microstructure. Osteogenesis was examined using Western blotting and alkaline phosphatase staining. Osteoclastogenesis was examined using Western blotting and F-actin ring staining. Senescence-associated β-galactosidase was used to detect cell senescence. In addition, the expression of Takeda G protein-coupled receptor 5 (TGR5)/farnesoid X receptor (FXR) signaling pathway-related genes and proteins was determined through quantitative real-time polymerase chain reaction and immunofluorescence.

Results: Oral administration of SPN improved the bone microstructure in d-gal-induced mice and increased bone mineral density, bone volume, trabecular thickness, and trabecular number. SPN also upregulated the expression of the osteogenesis markers osteocalcin, bone morphogenetic protein 2, and runt-related transcription factor 2 and downregulated the expression of the osteoclasis markers tartrate-resistant acid phosphatase, nuclear factor-κB, and nuclear factor of activated T-cells in the d-gal-induced bone. Furthermore, SPN increased alkaline phosphatase staining, inhibited F-actin ring formation, and reduced the activity of senescence-associated β-galactosidase in vitro. Mechanistically, SPN activated the TGR5/FXR pathway in d-gal-induced BMSCs and osteoclasts. The protective effects of SPN were abolished after addition of the TGR5 inhibitor SBI-115 or FXR inhibitor DY268. Moreover, SPN could elevate the protein and mRNA levels of TGR5, FXR, and the downstream small heterodimer partner in d-gal-induced bone.

Conclusion: SPN alleviated senile osteoporosis and cell senescence by activating the TGR5/FXR pathway.