PP2A Regulates the Pro-apoptotic Activity of FOXO1

FOXO1, a member of the evolutionarily conserved forkhead family of transcription factors, regulates expression of a number of genes that play critical roles in cell cycle and apoptosis. A pivotal regulatory mechanism of FOXO is reversible phosphorylation, catalyzed by kinases and phosphatases. Phosphorylation of FOXO1 is associated with 14-3-3 binding and cytosolic localization, whereas dephosphorylated FOXO1 translocates to the nucleus and is transcriptionally active. Experiments were performed to identify the serine/threonine phosphatase that dephosphorylates FOXO1. PP2A inhibitors, okadaic acid and fostriecin, increased FOXO1 phosphorylation in vitro and in cells. Microcystin-agarose pull-downs suggested that a phosphatase binds to FOXO1, and PP2A catalytic subunit was identified in endogenous FOXO1 immunocomplexes, indicating that PP2A is a FOXO1 phosphatase. Purified PP2A interacted directly with FOXO1 and dephosphorylated FOXO1 in vitro. Silencing of PP2A protected FOXO1 from dephosphorylation and delayed FOXO1 nuclear translocation, confirming the physiologic role of PP2A in the regulation of FOXO1 function. Furthermore, inhibition of PP2A phosphatases rescued FOXO1-mediated cell death by regulating the level of the pro-apoptotic protein BIM. We conclude that PP2A is a physiologic phosphatase of FOXO1.