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.

Medication-related osteonecrosis of the jaw (MRONJ) is a refractory disease for which no established treatment currently exists. Surfactin, a biosurfactant produced by Bacillus subtilis, exhibits antimicrobial activity, anticancer effects, and anti-inflammatory properties, suggesting its potential medical applications. This study aimed to elucidate the ability of surfactin to modulate the immune response induced by lipopolysaccharide (LPS) derived from periodontal pathogens (Aggregatibacter actinomycetemcomitans), clarify the underlying molecular mechanisms, and explore its potential utility in the treatment of MRONJ. Reverse transcription quantitative polymerase chain reaction demonstrated that surfactin suppresses LPS-induced interleukin-6 (IL-6) expression and secretion in J774.1 cells in a concentration-dependent manner. Western blot analysis showed that surfactin inhibited activation of the JNK-c-Jun-AP-1 axis and the JAK/STAT signaling pathways in J774.1 cells. The effects of surfactin administration were further evaluated in an in vivo MRONJ model. Co-treatment with surfactin significantly reduced the extent of LPS-induced bone necrosis. Overall, these findings suggest that surfactin suppresses LPS-induced IL-6 expression in macrophages and inhibits osteonecrosis induced by bisphosphonate preparations and LPS through negative regulation of the JNK-c-Jun-AP-1 axis and inhibition of the JAK/STAT pathway. Hence, surfactin may represent a promising candidate for MRONJ management.