Nuclear signalling by Rac GTPase: Essential role of phospholipase A₂

Rac, one member of Rho family GTPases, stimulates c-fos serum response element (SRE)-luciferase reporter gene in Rat-2 fibroblast cells. By transient transfection analysis, we demonstrated that the activation of phospholipase A₂ (PLA₂) and the subsequent production of arachidonic acid (AA) are essential for Rac-induced c-fos SRE activation, implying a critical role for PLA₂ in the Rac-signalling pathway to the nucleus. Either pretreatment with mepacrine, a specific inhibitor of PLA₂, or cotransfection with the expression plasmid of lipocortin-1, a proposed inhibitory protein of PLA₂, selectively abolished RacV12-induced SRE activation. Further, we demonstrated that subsequent metabolism of AA, a major product of Rac-activated PLA₂, by lipoxygenase (LO) is essential for Rac-induced c-fos SRE activation. In agreement with the role of the PLA₂-AA-LO cascade as a potential mediator of Rac signalling to the nucleus, the addition of exogenous AA stimulated c-fos SRE-luciferase activity in an LO-dependent manner. Together, our results demonstrate that 'Rac-activated PLA₂ and subsequent AA metabolism by LO' constitute a novel and specific pathway in Rac GTPase-induced c-fos SRE activation.