The B55α-containing PP2A holoenzyme dephosphorylates FOXO1 in islet β-cells under oxidative stress

The FOXO1 (forkhead box O1) transcription factor influences many key cellular processes, including those important in metabolism, proliferation and cell death. Reversible phosphorylation of FOXO1 at Thr²⁴ and Ser ²⁵⁶ regulates its subcellular localization, with phosphorylation promoting cytoplasmic localization, whereas dephosphorylation triggers nuclear import and transcriptional activation. In the present study, we used biochemical andmolecular approaches to isolate and link the serine/threonine PP2A (protein phosphatase 2A) holoenzyme containing the B55α regulatory subunit, with nuclear import of FOXO1 in pancreatic islet β-cells under oxidative stress, a condition associated with cellular dysfunction in Type 2 diabetes. The mechanism of FOXO1 dephosphorylation and nuclear translocationwas investigated in pancreatic islet INS-1 and βTC-3 cell lines subjected to oxidative stress. A combined chemical cross-linking andMS strategy revealed the association of FOXO1 with a PP2A holoenzyme composed of the catalytic C, structural A and B55β regulatory subunits. Knockdown of B55β in INS-1 cells reduced FOXO1 dephosphorylation, inhibited FOXO1 nuclear translocation and attenuated oxidative stress-induced cell death. Furthermore, both B55β and nuclear FOXO1 levels were increased under hyperglycaemic conditions in db/db mouse islets, an animal model of Type 2 diabetes. We conclude that B55α-containing PP2A is a key regulator of FOXO1 activity in vivo.