Cystamine is a redox-dependent inverse agonist of the histamine H₄ receptor

Cystamine, the oxidized dimer of cysteamine, has been reported to exert anti-inflammatory actions, but the underlying molecular mechanism remains unclear. Because the histamine H₄ receptor (H4R) is a key regulator of mast cell chemotaxis and inflammatory signaling, we examined whether cystamine directly modulates H4R activity. Cystamine potently inhibited histamine-induced mast cell migration (IC₅₀ = 440 nM) without affecting stem cell factor-induced migration, indicating pathway specificity. Although cystamine suppressed transglutaminase activity, this required millimolar concentrations and did not account for its effect on migration. At low micromolar concentrations, cystamine attenuated histamine-dependent activation of Rac1 and Rac2 GTPases and extracellular signal-regulated kinase (ERK). In H4R-expressing HEK293A cells, cystamine reduced basal and agonist-induced cyclic AMP response element reporter activity, demonstrating competitive antagonism and inverse agonism. Molecular docking supported direct binding of cystamine-but not its reduced monomer, cysteamine-to H4R. These findings identify cystamine as a redox-dependent inverse agonist of the H4R and provide a mechanistic explanation for its reported anti-inflammatory properties. Notably, cystamine generated endogenously or reformed locally from cysteamine under oxidative conditions may act on H4R in a redox-dependent and microenvironment-specific manner. Together, these insights suggest the potential for developing redox-dependent, tissue-selective H4R modulators for inflammatory diseases.NEW & NOTEWORTHY This study identifies cystamine as a previously unrecognized modulator of H4R. Cystamine inhibits mast cell migration and suppresses H4R-mediated Rac-ERK signaling at concentrations far below those required to inhibit transglutaminase, and it exhibits inverse agonist activity. Docking simulations show that cystamine, but not reduced cysteamine, engages the receptor's orthosteric site, revealing a redox-dependent difference in receptor interaction.