Objective: The aim of this study was to investigate the therapeutic effect of API on UC via the regulation of PAMP-MRGPRX2-mediated mast cells (MCs) degranulation.

Background: The pro-inflammatory positive feedback loop mediated by Mas-related G-protein-coupled receptor X2 (MRGPRX2) and its endogenous ligand, PAMP-12, is associated with ulcerative colitis (UC) progression. However, the therapeutic strategies that target MRGPRX2 in the treatment of UC are less reported. Apigenin (API), a natural flavonoid, can relieve inflammation.

Methods: A dextran sodium sulfate (DSS)-induced mouse UC model was used to elucidate the therapeutic effects of API. Animal behavior assessment, serological assays, and histological analysis were performed in wild-type (WT) and MC MrgprB2-conditional knockout (CKO) mouse model. mRNA sequencing analysis, PCR, ELISA, and western blotting were performed in vitro and in vivo to elucidate the mechanism underlying the effect of API by a PAMP-12 triggered MC degranulation model.

Results: MC degranulation via MrgprB2 was critical for the persistence of inflammation in colitis. API attenuated colonic tissue damage, splenomegaly, and myeloperoxidase (MPO) activity in the colonic tissues. It also ameliorated colonic crypt structure damage and inflammatory cell infiltration. Moreover, API suppressed MCs degranulation, and the level of carboxypeptidases A3 (CPA3), in DSS-induced colitis, thereby blocking the pro-inflammatory positive feedback loop induced by PAMP-MrgprB2. Lastly, API effectively inhibited PAMP-12-triggered mast cell degranulation by regulating Akt1/XBP-1S/CHOP/TXNIP and NF-κB/IL-1β signaling pathways.

Conclusions: API alleviates inflammatory symptoms in UC by suppressing PAMP-MRGPRX2/B2 mediated MC sustained degranulation feedback loop.