Caffeine Acts as an Agonist of Siglec-6, Inhibits MRGPRX2-Triggered Mast Cell Degranulation and Anaphylactoid Reactions.

Background: Mast cells (MCs) are effectors of anaphylactoid reactions. Mas-related G-protein-coupled receptor X2 (MRGPRX2) receptor mediates the direct activation of MCs in anaphylactoid disease. Siglec-6 negatively regulates MC activation and is a promising target in the development of antianaphylactoid reaction drugs. While caffeine exhibits an inhibitory effect against anaphylactic shock, the molecular mechanisms underlying these activities remain unknown. Objectives: Our objective was to investigate the inhibitory effect of caffeine and its underlying molecular mechanism in MRPGRX2-induced MC activation and anaphylactoid reactions. Methods: Local and systemic anaphylactoid reactions in mice and in vitro MC activation experiments were conducted to investigate the effects of caffeine on anaphylactoid reactions. Molecular docking and surface plasmon resonance (SPR) experiments were used to predict and verify the molecular target of caffeine activity. siRNA silencing and western blot analyses were utilized to investigate the molecular mechanisms underlying caffeine activity. Results: Caffeine inhibited local and systemic anaphylactoid reactions in mice and attenuated MRGPRX2-induced MC activation. Release of β-hexosaminidase, histamine, and Ca²⁺ in siRNA-Siglec-6-laboratory allergic disease 2 (LAD2) cells was significantly higher than in NC-LAD2 cells. The binding affinity between caffeine and Siglec-6 protein is with a calculated K _D of 1.76 × 10^-7 mol/L. Caffeine increased Siglec-6 expression, phosphorylation of SHP-1, and dephosphorylation of PLC-γ1, IP3R, and ERK1/2 in the MRGPRX2 signaling pathway. Western blot demonstrated that phosphorylated SHP-1 (p-SHP-1) protein levels showed no increase, and MRGPRX2, phosphorylated PLCγ1 (p-PLCγ1), and phosphorylated ERK1/2 (p-ERK1/2) were abolished with caffeine treatment in Siglec-6-knockdown cells than in NC-knockdown cells. Caffeine suppressed the m-3M3FBS-induced upregulation of p-PLCγ1 and p-ERK1/2 levels. Conclusions: We have demonstrated that caffeine is an agonist of Siglec-6 and that subsequent activation of the ITIM motif of Siglec-6 phosphorylates SHP-1. This arrests MRGPRX2/PLC-γ1/IP3R signal transduction, thereby attenuating anaphylactoid reactions, including anaphylactic shock.