Distinct roles of small GTPases RAC1 and RAC2 in histamine H₄ receptor–mediated chemotaxis of mast cells

Histamine induces chemotaxis of mast cells through the H₄ receptor. However, little is known about the precise intracellular signaling pathway that mediates this process. In this study, we identified small GTPases Rac1 and Rac2 as intracellular binding partners of the H₄ receptor and characterized their roles in H₄ receptor signaling. We showed that histamine induced Rac GTPase activation via the H₄ receptor. A Rac inhibitor NSC23766 attenuated chemotaxis of mast cells toward histamine, as well as histamine-induced calcium mobilization and extracellular signal-regulated kinase (ERK) activation. Histamine-induced migration of mast cells was also sensitive to PD98059, an inhibitor of the mitogen-activated protein kinase kinase, indicating that the Rac-ERK pathway was involved in chemotaxis through the H₄ receptor. Inhibition of phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) by LY294002 suppressed the histamine-induced chemotaxis and activation of Rac GTPases, suggesting that PI3K regulates chemotaxis upstream of Rac activation. Specific knockdown of Rac1 and Rac2 by short-hairpin RNA revealed that both Rac GTPases are necessary for histamine-induced migration. Downregulation of Rac1 and Rac2 led to attenuated response in calcium mobilization and ERK activation, respectively. These observations suggested that Rac1 and Rac2 have distinct and essential roles in intracellular signaling downstream of H₄ receptor-PI3K in histamine-induced chemotaxis of mast cells.