Background: Ulcerative colitis (UC) has demonstrated an escalating global incidence and prevalence, thereby posing substantial challenges to public health. Despite recent advancements in therapeutic interventions, the clinical management of UC remains suboptimal, underscoring the urgent need for novel treatment strategies. Saikosaponin A (SSa), a bioactive compound derived from the traditional Chinese herb Radix Bupleuri (RB), exhibits potent anti-inflammatory and immunomodulatory effects. However, its effects on UC and the underlying molecular mechanisms remain to be thoroughly explored.

Objective: This study aims to elucidate the underlying mechanisms of SSa in ameliorating UC and establish a pharmacological foundation for developing novel treatment modalities to address unmet clinical needs in UC treatment.

Methods: The protective effects of SSa against DSS-induced acute colitis were evaluated in a 3 % DSS-treated mouse model. Histological (H&E staining) and molecular analyses (RT-qPCR, ELISA, Western blotting, and flow cytometry) were performed to assess colonic tissue damage and inflammatory responses. Macrophage depletion via tail vein injection of clodronate liposomes confirmed the pivotal role of macrophages in UC pathogenesis and SSa's anti-inflammatory effects. The inhibitory effects of SSa on NLRP3 inflammasome activation were analyzed in vivo and in LPS/ATP-stimulated bone marrow-derived macrophages (BMDMs) using RT-qPCR, ELISA, and Western blotting. Bioinformatics analysis, targeted LC-MS/MS, and molecular docking were employed to identify potential molecular targets and mechanisms of SSa. Drug affinity responsive target stability (DARTS), cellular thermal shift assay (CETSA), and CH25H siRNA knockdown assays were used to validate CH25H as the direct target of SSa.

Results: SSa effectively attenuated DSS-induced colitis in mice by alleviating colonic inflammation, preserving intestinal barrier integrity, reducing LPS translocation, and mitigating systemic organ injury in the liver and spleen. The inflammatory response of macrophages and the production of IL-1β were identified as key pathogenic components in colitis, and the clearance of macrophages significantly ameliorated colitis progression while SSa administration post-macrophage clearance did not further alter the disease phenotype. Mechanistically, SSa inhibited the NLRP3 inflammasome activation-mediated IL-1β secretion in macrophages. The sterol metabolism played a crucial role in the activation of the NLRP3 inflammasome. SSa also restored the disturbed sterol homeostasis in the colon under inflammatory conditions, especially promoted the synthesis of 25-hydroxycholesterol (25-OHC). Further investigation revealed that SSa primarily exerts its therapeutic effects by directly targeting cholesterol 25-hydroxylase (CH25H), which promotes the production of 25-OHC and inhibits macrophage NLRP3 inflammasome activation.

Conclusions: This pioneering study demonstrated the therapeutic effect of SSa on DSS-induced colitis by targeting CH25H to enhance 25-OHC biosynthesis, which subsequently inhibited NLRP3 inflammasome activation in IL-1β-producing macrophages. These findings reveal a novel mechanism of SSa in UC treatment through cholesterol metabolism-regulated cascade immune modulation, providing strong pharmacological support for its development as a potential UC therapy.