The B56γ3 regulatory subunit-containing protein phosphatase 2A outcompetes Akt to regulate p27KIP1 subcellular localization by selectively dephosphorylating phospho-Thr157 of p27KIP1

// Tai-Yu Lai 1 , Chia-Jui Yen 2, 3, * , Hung-Wen Tsai 4, * , Yu-San Yang 5 , Wei-Fu Hong 5 , Chi-Wu Chiang 1, 5, 6 1 Institute of Basic Medical Sciences, College of Medicine, National Cheng Kung University, Tainan, Taiwan 2 Department of Internal Medicine, College of Medicine, National Cheng Kung University, Tainan, Taiwan 3 Institute of Clinical Medicine, College of Medicine, National Cheng Kung University, Tainan, Taiwan 4 Department of Pathology, College of Medicine, National Cheng Kung University, Tainan, Taiwan 5 Institute of Molecular Medicine, College of Medicine, National Cheng Kung University, Tainan, Taiwan 6 Center for Infectious Disease and Signaling Research, National Cheng Kung University, Tainan, Taiwan * These authors have contributed equally to this work Correspondence to: Chi-Wu Chiang, e-mail: chiangcw@mail.ncku.edu.tw Keywords: PP2A, B56γ3, p27, subcellular localization, Akt Received: June 17, 2015     Accepted: December 02, 2015     Published: December 14, 2015 ABSTRACT The B56γ-containing protein phosphatase 2A (PP2A-B56γ) has been postulated to have tumor suppressive functions. Here, we report regulation of p27KIP1 subcellular localization by PP2A-B56γ3. B56γ3 overexpression enhanced nuclear localization of p27KIP1, whereas knockdown of B56γ3 decreased p27KIP1 nuclear localization. B56γ3 overexpression decreased phosphorylation at Thr157 (phospho-Thr157), whose phosphorylation promotes cytoplasmic localization of p27KIP1, whereas B56γ3 knockdown significantly increased the level of phospho-Thr157. In vitro , PP2A-B56γ3 catalyzed dephosphorylation of phospho-Thr157 in a dose-dependent and okadaic acid-sensitive manner. B56γ3 did not increase p27KIP1 nuclear localization by down-regulating the upstream kinase Akt activity and outcompeted a myristoylated constitutively active Akt (Aktca) in regulating Thr157 phosphorylation and subcellular localization of p27KIP1. In addition, results of interaction domain mapping revealed that both the N-terminal and C-terminal domains of p27 and a domain at the C-terminus of B56γ3 are required for interaction between p27 and B56γ3. Furthermore, we demonstrated that p27KIP1 levels are positively correlated with B56γ levels in both non-tumor and tumor parts of a set of human colon tissue specimens. However, positive correlation between nuclear p27KIP1 levels and B56γ levels was found only in the non-tumor parts, but not in tumor parts of these tissues, implicating a dysregulation in PP2A-B56γ3-regulated p27KIP1 nuclear localization in these tumor tissues. Altogether, this study provides a new mechanism by which the PP2A-B56γ3 holoenzyme plays its tumor suppressor role.