An indole derivative of Ursolic acid, inhibits RANKL-induced osteoclast differentiation via modulating the Nrf2 and NFATc1/c-Fos axis: Targeting oxidative stress and osteoclastogenesis in managing rheumatoid arthritis

Rheumatoid arthritis (RA) encapsulates chronic autoimmune pathology, characterised by dysregulated immune responses that drive progressive joint destruction and systemic complications. Central to this process is aberrant osteoclast activation driven by RANKL-RANK signalling, which drives excessive bone resorption. In this context, we evaluated UA-1, an indole derivative of ursolic acid. Ursolic acid is a phytochemical known for its anti-arthritic properties with limited clinical applicability (BCS class IV), which drives the need for derivatization. In our recent study, UA-1 showed improved anti-inflammatory potential in comparison to ursolic acid, which may be attributed to its improved physicochemical properties. These results prompted us to evaluate UA-1's anti-arthritic potential both in vitro using RANKL-stimulated osteoclastogenesis in RAW 264.7 macrophages and in vivo using the CIA model. UA-1 significantly reduced the levels of pro-osteoclastogenic cytokines, thereby alleviating subsequent downstream osteoclast differentiation, as corroborated by the decreased expression of osteoclast-specific markers (TRAP, CTSK, MMPs). It also modulated key signalling pathways, including NF-κB, MAPK, and JAK/STAT. Additionally, UA-1 inhibited the key adaptor protein TRAF6 and downregulated the NFATc1/c-Fos axis, major transcription factors involved in osteoclastogenesis. Furthermore, UA-1 also enhanced the antioxidant levels by modulating the NRF-2-mediated antioxidant pathway. Additionally, in the CIA model using C57BL/6, UA-1 alleviated disease markers and reduced pro-inflammatory cytokines, while minimizing joint damage as evidenced by H&E and picrosirius red staining. Overall, the results suggest the potential efficacy of UA-1 in managing RA, given its pronounced suppression of inflammation, oxidative stress and osteoclastogenesis.