Inhibition of B56-containing protein phosphatase 2As by the early response gene IEX-1 leads to control of Akt activity.
The importance of PP2A in the regulation of Akt/PKB activity has long been recognized but the nature of the holoenzyme involved and the mechanisms controlling dephosphorylation are not yet known. We identified IEX-1, an early gene product with proliferative and survival activities, as a specific inhibitor of B56 regulatory subunit-containing PP2A. IEX-1 inhibits B56-PP2A activity by allowing the phosphorylation of B56 by ERK. This leads to sustained ERK activation. IEX-1 has no effect on PP2A containing other B family subunits. Thus, studying IEX-1 contribution to signaling should help the discovery of new pathways controlled by B56-PP2A. By using overexpression and RNA interference, we show here that IEX-1 increases Akt/PKB activity in response to various growth factors by preventing Akt dephosphorylation on both Thr(308) and Ser(473) residues. PP2A-B56beta and gamma subunits have the opposite effect and reverse IEX-1-mediated Akt activation. The effect of IEX-1 on Akt is ERK-dependent. Indeed: (i) a IEX-1 mutant deficient in ERK binding had no effect on Akt; (ii) ERK dominant-negative mutants reduced IEX-1-mediated increase in pAkt; (iii) a B56beta mutant that cannot be phosphorylated in the ERK.IEX-1 complex showed an enhanced ability to compete with IEX-1. These results identify B56-containing PP2A holoenzymes as Akt phosphatases. They suggest that IEX-1 behaves as a general inhibitor of B56 activity, enabling the control of both ERK and Akt signaling downstream of ERK.
Rocher
Géraldine
G
Institut Cochin, Department of Hematology, Paris F-75014, France.
Letourneux
Claire
C
Lenormand
Philippe
P
Porteu
Françoise
F
D000818:Animals; D051017:Apoptosis Regulatory Proteins / Q000172:deficiency / Q000378:metabolism*; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D004789:Enzyme Activation / Q000502:physiology; D048049:Extracellular Signal-Regulated MAP Kinases / Q000235:genetics / Q000378:metabolism; D015870:Gene Expression; D017874:Immediate-Early Proteins / Q000235:genetics / Q000378:metabolism*; D036341:Intercellular Signaling Peptides and Proteins / Q000378:metabolism; D020935:MAP Kinase Signaling System / Q000502:physiology*; D051379:Mice; D046912:Multiprotein Complexes / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D041681:NIH 3T3 Cells; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation; D046188:Protein Modification, Translational / Q000502:physiology*; D021122:Protein Subunits / Q000235:genetics / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt / Q000235:genetics / Q000378:metabolism*; D034622:RNA Interference
D051017:Apoptosis Regulatory Proteins; C518820:IEX-1 protein, mouse; D017874:Immediate-Early Proteins; D036341:Intercellular Signaling Peptides and Proteins; D046912:Multiprotein Complexes; D021122:Protein Subunits; D051057:Proto-Oncogene Proteins c-akt; D048049:Extracellular Signal-Regulated MAP Kinases; D010749:Phosphoprotein Phosphatases