14-3-3 sigma Regulates beta-Catenin-Mediated Mouse Embryonic Stem Cell Proliferation by Sequestering GSK-3 beta

Background: Pluripotent embryonic stem cells are considered to be an unlimited cell source for tissue regeneration and cell-based therapy. Investigating the molecular mechanism underlying the regulation of embryonic stem cell expansion is thus important. 14-3-3 proteins are implicated in controlling cell division, signaling transduction and survival by interacting with various regulatory proteins. However, the function of 14-3-3 in embryonic stem cell proliferation remains unclear. Methodology and Principal Findings: In this study, we show that all seven 14-3-3 isoforms were detected in mouse embryonic stem cells. Retinoid acid suppressed selectively the expression of 14-3-3 sigma isoform. Knockdown of 14-3-3 sigma with siRNA reduced embryonic stem cell proliferation, while only 14-3-3 sigma transfection increased cell growth and partially rescued retinoid acid-induced growth arrest. Since the growth-enhancing action of 14-3-3s was abrogated by beta-catenin knockdown, we investigated the influence of 14-3-3 sigma overexpression on beta-catenin/GSK-3 beta. 14-3-3 sigma bound GSK-3 beta and increased GSK-3 beta phosphorylation in a PI-3K/Akt-dependent manner. It disrupted beta-catenin binding by the multiprotein destruction complex. 14-3-3 sigma overexpression attenuated beta-catenin phosphorylation and rescued the decline of beta-catenin induced by retinoid acid. Furthermore, 14-3-3 sigma enhanced Wnt3a-induced beta-catenin level and GSK-3 beta phosphorylation. DKK, an inhibitor of Wnt signaling, abolished Wnt3a-induced effect but did not interfere GSK-3b/14-3-3 sigma binding. Significance: Our findings show for the first time that 14-3-3 sigma plays an important role in regulating mouse embryonic stem cell proliferation by binding and sequestering phosphorylated GSK-3 beta and enhancing Wnt-signaled GSK-3 beta inactivation. 14-3-3 sigma is a novel target for embryonic stem cell expansion