Leukotriene B₄ stimulates Rac-ERK cascade to generate reactive oxygen species that mediates chemotaxis

Leukotriene B₄ is a potent chemoattractant known to be involved mainly in inflammation, immune responses, and host defense against infection, although the exact signaling mechanisms by which it exerts its effects are not well understood. Here we show that exogenous leukotriene B₄ induces reactive oxygen species (ROS) generation via a Rac-dependent pathway, and that stable expression of Rac^N17, a dominant negative Rac1 mutant, completely blocks leukotriene B₄-induced ROS generation. In addition, leukotriene B₄-induced ROS generation is selectively blocked by inhibition of ERK or cytosolic phospholipase A₂, but not p38 kinase, which is indicative of its dependence on ERK activation and synthesis of arachidonic acid. Consistent with those findings, leukotriene B₄ Rac-dependently stimulates ERK and cytosolic phospholipase A₂ activity, and transient transfection with plasmid expressing Rac^V12, a constitutively activated Racl mutant, also dose-dependently stimulates ERK activity. Our findings suggest that ERK and cytosolic phospholipase A₂ are situated downstream of Rac, and we conclude that Rac, ERK, and cytosolic phospholipase A₂ all play pivotal roles in mediating the ROS generation that appears to be a prerequisite for leukotriene B₄-induced chemotaxis and cell proliferation.