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

Y Liu,P Li,X Bu,J Ding,G Yang,G Liu,W Jia

doi:10.1038/cddis.2011.28

Abstract

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--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.

Highlights

The plasma membrane (PM), as a major site of Akt activation, contains multiple microdomains, and among these, a cholesterol-rich, detergent-resistant type of microdomain known as the lipid raft has been suggested as a critical platform for cell signaling.[8,9] Lipid rafts function as contributors to lateral membrane heterogeneity and are generated through lipid–lipid and specific lipid–protein interactions.[10]
To demonstrate the differential effects of aPPD on lipid rafts in the two cell lines, we looked at changes in cholesterol concentrations of various PM fractions of U87MG and N2a cells treated with aPPD or MbCD
Western blots of total Akt and its two phosphorylation residues, Thr[308] and Ser[473], were performed with total membrane proteins of U87MG and N2a cells treated with aPPD

Summary

Introduction

The plasma membrane (PM), as a major site of Akt activation, contains multiple microdomains, and among these, a cholesterol-rich, detergent-resistant type of microdomain known as the lipid raft has been suggested as a critical platform for cell signaling.[8,9] Lipid rafts function as contributors to lateral membrane heterogeneity and are generated through lipid–lipid and specific lipid–protein interactions.[10]. Received 04.1.11; revised 28.2.11; accepted 08.3.11; Edited by V De Laurenzi and effectively neutralizing its pro-apoptotic activity.[19,20] Recent studies indicated that raft-associated Akt could be an important determinant of oncogenicity.[21,22] Studies of small-cell lung cancer cells showed that specific PI3K isoforms reside in lipid rafts and that disruption of membrane rafts by methyl-b-cyclodextrin (MbCD) inhibits PI3K-mediated Akt activity.[23] Live-cell fluorescence imaging has shown that raft Akt is activated faster and more potently than non-raft Akt, presumably because of compartmentalization of various components of the signaling pathway, including the receptors, PI3K and Akt itself.[22] Given their saponin-like structure, ginsenosides are well known for their ability to increase the fluidity of cellular membranes.[11,24] As the structure of aPPD highly resembles that of cholesterol, we hypothesized that this compound may interfere with lipid rafts and alter lateral movement of raft resident proteins. The cellular outcomes to a toxic stimulus were opposite, indicating that the pharmacological effects of this compound can be completely cell-type-specific because of differences in its effect on the raft-associated cell signaling of different cells

Methods

Results

Conclusion