Surfactin Inhibits Osteoclast Differentiation by Negatively Regulating the Elk1-AP-1-NFATc1 Axis

Background/Objectives: Surfactin is a biosurfactant with various biological activities, including antibacterial and anti-inflammatory properties; however, its effects on bone metabolism remain poorly understood. This study aimed to investigate the effects of surfactin on osteoclast differentiation and elucidate its underlying molecular mechanisms. Methods: RAW264.7 cells were treated with receptor activator of nuclear factor-kappa B ligand (RANKL) and surfactin, and osteoclast differentiation and maturation were evaluated by tartrate-resistant acid phosphatase and F-actin staining, respectively. Gene expression of differentiation markers was assessed using real-time reverse transcription-quantitative polymerase chain reaction, while the kinetics of intracellular signaling molecules and transcription factors were analyzed using Western blot analysis. Results: Surfactin treatment significantly inhibited osteoclast differentiation and maturation, as well as the mRNA expression of Nfatc1, Acp5, and Cathepsin K. Although surfactin did not markedly affect RANKL-induced activation of the NF-κB or MAPK-mediated signaling, it significantly suppressed the expression of c-Fos at both the mRNA and protein levels. Furthermore, surfactin attenuated the phosphorylation of Elk1, a transcription factor involved in c-Fos induction. Conclusions: Surfactin inhibits RANKL-induced osteoclast differentiation by negatively regulating the Elk1-AP-1-NFATc1 axis. Surfactin may thus be a promising therapeutic candidate for the treatment of metabolic bone disorders and inflammatory bone destruction.