Inhibition of B56-containing Protein Phosphatase 2As by the Early Response Gene IEX-1 Leads to Control of Akt Activity

Claire Letourneux,Gé;raldine Rocher,Françoise Porteu,Philippe Lenormand

doi:10.1074/jbc.m609712200

Abstract

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.

Highlights

Protein phosphatase 2A (PP2A)2 accounts for the majority of serine/threonine phosphatase activity in the cell)
Adams et al [3] showed that PP2As containing B␣ and B␦ are required for activation of the MAPK kinase kinase Raf-1, whereas we demonstrated that the B56 family members shut down the signal by dephosphorylating ERK1 and ERK2 [4]
IEX-1, known as IER3, DIF2, or Gly96, is an ubiquitous early response gene product involved in cell proliferation and survival, which is rapidly induced by various growth factors, cytokines, chemical carcinogens, or viral infections [16]

Summary

Introduction

Protein phosphatase 2A (PP2A)2 accounts for the majority of serine/threonine phosphatase activity in the cell (reviewed in Refs. 1 and 2)). The decreased Akt activation upon down-regulation of IEX-1 expression is not specific to TPO signaling or to the particular cell context of UT7 cells, as shown by the impaired serum-induced Akt phosphorylation at both Ser473 and Thr308 in HEK293 expressing IEX-1-shRNA (Fig. 2C).

Results

Conclusion