Akt is a downstream target of NF-κB

The ubiquitously expressed transcription factor NF-kappaB and the serine-threonine kinase Akt both are involved in the promotion of cell survival. Although initially believed to operate as components of distinct signaling pathways, several studies have demonstrated that the NF-kappaB and Akt signaling pathways can converge. Indeed, IkappaB kinase, the kinase involved in NF-kappaB activation, is a substrate of Akt, and activation of Akt therefore stimulates NF-kappaB activity. Although these results place Akt upstream of NF-kappaB activation in the sequence of signaling events, we report that this may not necessarily be the case and that Akt is a downstream target of NF-kappaB. Treatment of NIH3T3 cells with the NF-kappaB activators, tumor necrosis factor (TNF) alpha and lipopolysaccharide, results in the stimulation of Akt phosphorylation. The stimulation of Akt is, however, detected only after IkappaB-alpha degradation is induced by these agents. The nuclear translocation of p65 and increased DNA binding activity of NF-kappaB also precede Akt phosphorylation. Treatment with two pharmacological inhibitors of NF-kappaB, SN50 and N-tosyl-L-phenylalanine chloromethyl ketone (TPCK), blocks TNF-induced Akt activation. On the other hand TNF-mediated NF-kappaB activation is not reduced by the phosphoinositide-3 kinase inhibitors wortmannin and LY294002, although these inhibitors completely block the activation of Akt. These results suggest that NF-kappaB is required for TNF-mediated Akt activation and that it lies upstream of the stimulation of Akt. Consistent with this conclusion is the finding that overexpression of p65/RelA leads to Akt phosphorylation in the absence of extracellular stimulatory factors, whereas overexpression of IkappaB-alpha reduces Akt phosphorylation below basal levels. Interestingly, in addition to stimulating the phosphorylation of Akt, overexpression of p65 causes an increase in the expression of Akt mRNA and protein.