Cell-type-specific regulation of raft-associated Akt signaling

20S-protopanaxadiol (aPPD) is a metabolite of ginseng saponins, which is reported to be pro-apoptotic in some cells but anti-apoptotic in neuronal cells by regulating Akt signaling. Owing to its cholesterol-like structure, we hypothesized that aPPD may regulate Akt signaling by interacting with lipid rafts. Here, we compared Akt signaling in glioblastoma U87MG and neuroblastoma Neuro-2a cells treated with aPPD. aPPD did not change Akt activity in the total plasma membranes of each cell type, but drastically altered the activity of raft-associated Akt. Strikingly, Akt activity was decreased in the rafts of U87MG cells but increased in N2a cells by aPPD through regulating raft-associated dephosphorylation. The bidirectional regulation of raft-associated Akt signaling by aPPD enhanced the chemotoxicity of Paclitaxel or Vinblastine in U87MG cells but attenuated the excitotoxicity of N-methyl-D-aspartate in N2a cells. Our results demonstrated that the activity of raft-associated but not total membrane Akt determines its cellular functions. Lipid rafts differ in different types of cells, which allows for the possibility of cell-type-specific targeting for which aPPD might prove to be a useful agent.