Forkhead at the Crossroads of Cancer and Healthy Lifespan.

摘要

At the heart of cancer progression and metabolism lies the insulin/Akt signaling network. It is well established that Akt activation under contextual limits leads to tumor growth or diabetes. Nuclear localization of the Akt target, forkhead transcription factor (FOXO1a), suppresses tumor cell growth through increased expression of genes that induce cell cycle arrest and apoptosis. As a consequence of Akt hyperactivity, FOXO1a is sequestered in the cytoplasm away from its transcriptional targets. Therefore FOXO1a localization serves as a visible indicator of aberrant Akt signaling.;In a genome-wide siRNA screen, I used cells expressing fluorescently-tagged FOXO1a to identify gene targets that when inhibited caused nuclear FOXO1a localization. The 90 high-confidence genes determined by a support vector machine statistical method were further separated into genes that influenced FOXO1a nuclear localization specifically. Silencing a subset of translational machinery elements influenced general nuclear transport, whereas knockdown of specific spliceosome and proteasome components controlled FOXO1a localization. Additional factors such as RNA/DNA binding protein SON, glutamate transporter SNAT3, and uncoupling protein 5 (UCP5) were found to reduce Akt activation. Amongst these genes, tetraspanin 9 (TSPAN9) was connected to Akt through the focal adhesion network.;Secondly, I investigated the link between the proteasome and nuclear FOXO1a through Akt phosphorylation. Inhibition of the proteasome revealed a negative feedback loop that connected FOXO dependent transcription of the E3-ligase, atrogin-1, to Akt phosphorylation through degradation of protein phosphatases.;Finally, I explored the connection of a novel mitochondrial uncoupling protein, UCP5, to the Akt/FOXO network. Through UCP5 silencing, we modified the cellular energy balance and increased free radical production (ROS) through up-regulation of uncoupling protein 3 (UCP3). Elevated UCP3 altered mitochondrial membrane potential and ROS promoting activation of c-Jun N-terminal kinase (JNK1). JNK1 activity blocked Akt activation which increased nuclear FOXO1a-dependent expression of mitochondrial superoxide dismutase (SOD2).;Through these findings, I illustrate a diverse web of interactions around Akt which encompass mitochondrial regulation and protein degradation. The role FOXO1a plays in both of these processes highlights its importance in metabolism and cancer progression. The balance between these two processes holds the root to identifying potential drug targets for cancer and diabetes treatment.