Chronic Protein Kinase B (PKB/c-akt) Activation Leads to Apoptosis Induced by Oxidative Stress–Mediated Foxo3a Transcriptional Up-regulation

Abstract

Increased protein kinase B (PKB; c-Akt) activation is a hallmark of diverse neoplasias providing both proliferative and antiapoptotic survival signals. In this study, we investigated the effect of chronic PKB activation on cellular survival and proliferation using cytokine-dependent bone marrow–derived Ba/F3 cells, in which PKBα activation can be directly, and specifically, induced by addition of 4-hydroxytamoxifen (4-OHT). Direct activation of PKB rescued Ba/F3 cells from cytokine withdrawal–induced apoptosis; however, surprisingly, these antiapoptotic effects were short lived, cells only being protected for up to 48 hours. We observed that activation of PKB in survival factor–deprived cells led to a dramatic increase of Foxo3a on both the transcriptional and protein level leading to expression of its transcriptional targets Bim and p27^kip1. High levels of PKB activity result in increased aerobic glycolysis and mitochondrial activity resulting in overproduction of reactive oxygen species. To determine whether oxidative stress might itself be responsible for Foxo3a up-regulation, we utilized hydrogen peroxide (H₂O₂) as an artificial inducer of oxidative stress and N-acetylcysteine (NAC), a thiol-containing radical oxygen scavenger. Addition of NAC to the culture medium prolonged the life span of cells treated with 4-OHT and prevented the up-regulation of Foxo3a protein levels caused by PKB activation. Conversely, treatment of Ba/F3 cells with H₂O₂ caused an increase of Foxo3a on both transcriptional and protein levels, suggesting that deregulated PKB activation leads to oxidative stress resulting in Foxo3a up-regulation and subsequently cell death. Taken together, our data provide novel insights into the molecular consequences of uncontrolled PKB activation. (Cancer Res 2006; 66(22): 10760-9)