Bhattacharyya R, Noch E K, Khalili K
Department of Neuroscience, Center for Neurovirology, Temple University School of Medicine, Philadelphia, PA 19122, USA.
Oncogene. 2007 Dec 6;26(55):7628-36. doi: 10.1038/sj.onc.1210576. Epub 2007 Jul 16.
Wnt signaling follows canonical and non-canonical pathways to regulate a variety of processes during cellular homeostasis and development. The large T-antigen (T-Ag) of the human neurotropic JC virus, has been shown to modulate the Wnt-signaling pathway via interaction with beta-catenin, one of the most important components of the canonical Wnt pathway. Here, we have identified an alternative non-canonical pathway that allows T-Ag to recruit Rac1 for stabilizing beta-catenin by inhibiting its ubiquitin-dependent proteasomal degradation. We demonstrate that inhibition of Rac1 by its dominant negative mutant, RacN17, abrogates T-Ag-mediated stabilization of beta-catenin yet exhibits no impact on the transcriptional activity of beta-catenin. Results from immunocytochemistry revealed that together with T-Ag, a pool of beta-catenin appears at the cell surface, particularly at the membrane ruffles where active Rac1 is positioned. Interestingly, cooperativity between T-Ag and beta-catenin leads to activation of Rac1, which in turn, stimulates its association with beta-catenin. These observations unravel the interplay between beta-catenin and Rac1 that is initiated by T-Ag and results in stabilization of beta-catenin and its presence in cell membrane ruffles.
Wnt信号通路通过经典和非经典途径来调节细胞内稳态和发育过程中的多种进程。人嗜神经JC病毒的大T抗原（T-Ag）已被证明可通过与β-连环蛋白相互作用来调节Wnt信号通路，β-连环蛋白是经典Wnt通路最重要的组成部分之一。在此，我们发现了一条替代性非经典途径，该途径使T-Ag能够招募Rac1，通过抑制其泛素依赖性蛋白酶体降解来稳定β-连环蛋白。我们证明，其显性负性突变体RacN17对Rac1的抑制消除了T-Ag介导的β-连环蛋白稳定作用，但对β-连环蛋白的转录活性没有影响。免疫细胞化学结果显示，与T-Ag一起，一部分β-连环蛋白出现在细胞表面，特别是在活性Rac1所在的膜皱褶处。有趣的是，T-Ag与β-连环蛋白之间的协同作用导致Rac1激活，进而刺激其与β-连环蛋白的结合。这些观察结果揭示了由T-Ag引发的β-连环蛋白与Rac1之间的相互作用，导致β-连环蛋白的稳定及其在细胞膜皱褶中的存在。
