The activation of transcription factor NF-kappa B by TNF involves the stimu
lation of a novel signaling cascade. In this paper we show that phosphatidy
linositol 3-kinase (PI 3-kinase) may play a pivotal role in TNF-mediated ac
tivation of NF-kappa B-dependent genes, Consistent with its involvement in
TNF signaling, PI 3-kinase activities in HepG2 and U937 cells can be stimul
ated by TNF in a rapid but transient manner through a mechanism that may in
volve its association with the insulin receptor substrate-1. A dominant-neg
ative mutant of the p85 regulatory subunit of PI 3-kinase, which is a poten
t inhibitor of PI 3-kinase signaling, effectively blocked the TNF-induced e
xpression of an NF-kappa B-dependent reporter gene. Although PI 3-kinase ma
y be required for NF-kappa B activation, overexpression of its p110 catalyt
ic subunit alone was unable to induce an NF-kappa B/chloramphenicol acetylt
ransferase (CAT) reporter gene. However, when TNF was added to p110-overexp
ressing cells, there was a synergistic activation of the NF-kappa B/CAT rep
orter, suggesting that other TNF-inducible signals may cooperate with PI 3-
kinase to activate NF-kappa B, Consistent with its role in NF-kappa B activ
ation, inhibition of PI 3-kinase activity by wortmannin or LY294002 greatly
potentiated TNF-induced apoptosis, This TNF/wortmannin-induced apoptosis w
as markedly prevented in cells overexpressing Rel A, Taken together, our re
sults indicate that a PI 3-kinase-regulated step in TNF-signaling is critic
al for the expression of NF-kappa B-dependent genes.