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.
Highlights
Protein phosphatase 2A (PP2A) is a serine/ threonine phosphatase that plays an important role in regulating many aspects of cellular activities [1]
We found that the distribution of Flag-p27 was predominantly nuclear in nearly 80 % of control cells carrying vector only, whereas p27 distribution became mainly ubiquitous throughout the entire cells when B56γ3 was knocked down (Figure 1A)
We have identified B56γ3 as a regulatory subunit targeting the PP2A holoenzyme to regulate the subcellular localization of p27 as evidenced by down-regulation of cytoplasmic localization and phosphorylation of p27 at Thr157 by expression of exogenous B56γ3 gene, and increased cytoplasmic localization and phosphorylation of p27 by silencing endogenous B56γ3 gene expression
Summary
Protein phosphatase 2A (PP2A) is a serine/ threonine phosphatase that plays an important role in regulating many aspects of cellular activities [1]. The regulatory B subunits can be grouped into four families named B ( known as B55 or PR55) [1, 2], B’ ( known as B56 or PR61) [3, 4], B” ( known as PR72) [5] and B’” ( known as PR93 or PR110) [6]. Among www.impactjournals.com/oncotarget the diverse PP2A holoenzyme complexes, the PP2A holoenzymes containing the regulatory subunit B56 family have been shown to regulate key molecules involved in tumorigenesis. The B56γcontaining PP2A (PP2A-B56γ) holoenzymes have been shown to play an important role in the tumor suppressive activity of PP2A. Null or reduced expression of B56γ subunits was found in several human lung cancer cell lines [10] and human malignant melanomas [11], respectively
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