Toosendanin inhibits STING signaling pathway to alleviate ulcerative colitis via blocking LYN kinase and STING protein.

The stimulator of interferon genes (STING) pathway plays a critical role in inflammatory diseases like ulcerative colitis (UC), yet no STING inhibitors are clinically available. Toosendanin (TSN), a bioactive triterpenoid from fruits of Melia toosendan, has been shown to ameliorate UC, though its exact molecular targets and detailed mechanisms remain unclear. This study investigates whether TSN inhibits STING pathway activation to alleviate UC and identifies potential target proteins involved in suppressing STING signaling. Herein, we first confirmed TSN's suppression of STING-dependent genes, including interferon-beta1 (IFNB1), C-X-C motif chemokine 10 (CXCL10), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-α) in murine RAW264.7 and human THP-1 differentiated macrophages activated by herring testis DNA (HT-DNA), 5,6-dimethylxantheonone-4-acetic acid (DMXAA), or cyclic GMP-AMP (cGAMP). Furthermore, TSN also suppressed phosphorylation of STING, TANK-binding kinase 1 (TBK1), interferon regulatory factor 3 (IRF3), nuclear factor kappa-B (NF-κB) p65 induced by HT-DNA, DMXAA, or cGAMP, while preventing DMXAA-induced nuclear translocation of IRF3 and p65 in RAW264.7 cells. Mechanistically, TSN could not only directly bind Lck/Yes-related novel protein tyrosine kinase (LYN) to reduce DMXAA-induced phosphorylation of LYN and protein kinase B (AKT), but also directly interacted with STING protein to inhibit its oligomerization, thereby inhibiting STING pathway activation. In vivo, TSN ameliorated dextran sulfate sodium (DSS)-induced colitis in mice via downregulating STING pathway components. These findings demonstrated that TSN achieved its anti-inflammatory effects by suppressing the STING pathway through direct binding to both LYN and STING proteins, suggesting its therapeutic potential for UC and other conditions involving aberrant STING activation.