Role of the Cytosolic Phospholipase A₂-linked Cascade in Signaling by an Oncogenic, Constitutively Active Ha-Ras Isoform

Activation of Ras signaling by growth factors has been associated with gene regulation and cell proliferation. Here we characterize the contributory role of cytosolic phospholipase A₂ in the oncogenic Ha-Ras ^V12 signaling pathway leading to activation of c-fos serum response element (SRE) and transformation in Rat-2 fibroblasts. Using a c-fos SRE-luciferase reporter gene, we showed that the transactivation of SRE by Ha-Ras^V12 is mainly via a Rac-linked cascade, although the Raf-mitogen-activated protein kinase cascade is required for full activation. In addition, Ha-Ras^V12-induced DNA synthesis was significantly attenuated by microinjection of recombinant Rac^N17, a dominant negative mutant of Rac1. To identify the mediators downstream of Rac in the Ha-Ras^V12 signaling, we investigated the involvement of cytosolic phospholipase A₂. Oncogenic Ha-Ras^V12-induced SRE activation was significantly inhibited by either pretreatment with mepacrine, a phospholipase A₂ inhibitor, or cotransfection with the antisense oligonucleotide of cytosolic phospholipase A₂. We also found cytosolic phospholipase A₂ to be situated downstream of Ha-Ras ^V12 in a signal pathway leading to transformation. Together, these results are indicative of mediatory roles of Rac and cytosolic phospholipase A₂ in the signaling pathway by which Ha-Ras^V12 transactivates c-fos SRE and transformation. Our findings point to cytosolic phospholipase A₂ as a novel potential target for suppressing oncogenic Ha-Ras^V12 signaling in the cell.