Roles of phosphatidylinositol 3-kinase and Rac in the nuclear signaling bytumor necrosis factor-alpha in rat-2 fibroblasts

We investigated the extent to which phosphatidylinositol S-kinase (PI 3-kin ase) and Rac, a member of the Rho family of small GTPases, are involved in the signaling cascade triggered by tumor necrosis factor (TNF)-alpha leadin g to activation of c-fos serum response element (SRE) and c-Jun amino-termi nal kinase (JNK) in Rat-a fibroblasts. Inhibition of PI 3-kinase by LY29400 2 or wortmannin, two specific PI 3-kinase antagonists, or co-transfection w ith a dominant negative mutant of PI 3-kinase dose-dependently blocked stim ulation of c-fos SRE: by TNF-alpha. Similarly, LY294002 significantly dimin ished TNF-alpha-induced activation of JNK; suggesting that nuclear signalin g triggered by TNF-alpha is dependent on PI S-kinase-mediated activation of both c-fos SRE and JNK We also found nuclear signaling by TNF-alpha to be Rac-dependent, as demonstrated by the inhibitory effect of transient co-tra nsfection with a dominant negative Rac mutant, RacN17. Our findings suggest that Rac is situated downstream of PI 3-kinase in the TNF-alpha signaling pathway to the nucleus, and we conclude that PI 3-kinase and Rac each plays a pivotal role in the nuclear signaling cascade triggered by TNF-alpha.