Inhibition of the Wnt Signaling Pathway by the PR61 Subunit of Protein Phosphatase 2A

Abstract

Axin, a negative regulator of the Wnt signaling pathway, forms a complex with glycogen synthase kinase-3␤ (GSK-3␤), ␤-catenin, adenomatous polyposis coli (APC) gene product, and Dvl, and it regulates GSK-3␤-dependent phosphorylation in the complex and the stability of ␤-catenin. Using yeast two-hybrid screening, we found that regulatory subunits of protein phosphatase 2A, PR61␤ and -␥, interact with Axin. PR61␤ or -␥ formed a complex with Axin in intact cells, and their interaction was direct. The binding site of PR61␤ on Axin was different from those of GSK-3␤, ␤-catenin, APC, and Dvl. Although PR61␤ did not affect the stability of ␤-catenin, it inhibited Dvl-and ␤-catenin-dependent T cell factor activation in mammalian cells. Moreover, it suppressed ␤-catenin-induced axis formation and expression of siamois, a Wnt target gene, in Xenopus embryos, suggesting that PR61␤ acts either at the level of ␤-catenin or downstream of it. Taken together with the previous observations that PR61 interacts with APC and functions upstream of ␤-catenin, these results demonstrate that PR61 regulates the Wnt signaling pathway at various steps.

Controlled levels of canonical Wnt signaling are required for neural crest migration

“…Under unstimulated conditions cytoplasmic level of ß-catenin is kept low through the dual phosphorylation (Liu et al, 2002) by glycogen synthase kinase 3β (GSK-3β) (Hart et al, 1999;Yost et al, 1996) and casein kinase 1α (CK1α) (Price, 2006;Amit et al, 2002), which together with the adenomatous polyposis coli tumor suppressor protein (APC) (Hart et al, 1999;Ha et al, 2004), protein phosphatase 2A (PP2A) (Hsu et al, 1999;Seeling et al, 1999;Ratcliffe et al, 2000;Yamamoto et al, 2001) and Axin (Hart et al, 1999;Ikeda et al, 1998;Dajani et al, 2003) form a destruction complex (Stamos and Weis, 2013).…”

The -Catenin Destruction Complex

“…Kimelman, Xu, and colleagues (Xing et al 2003) proposed that CK1 and GSK-3 phosphorylate APC after phosphorylating b-catenin; the phosphorylated APC 20-mer would displace b-catenin from Axin, freeing Axin to interact with another b-catenin molecule. This model also postulates that PP2A, which has been reported to bind to Axin and APC (Hsu et al 1999;Seeling et al 1999;Yamamoto et al 2001) and can modulate APC phosphorylation (Ikeda et al 2000), dephosphorylates b-catenin-bound APC to reset the system for another round of b-catenin phosphorylation and release.…”

“…The region carboxy-terminal to the bcatenin and GSK-3-binding sites has been reported to bind to the catalytic domain of the PP2A or to the PR61 regulatory subunit of PP2A ( Fig. 2) (Hsu et al 1999;Ikeda et al 2000;Yamamoto et al 2001), as well as the phosphatase PP1 (Luo et al 2007). The simultaneous binding of Axin to a kinase and b-catenin enforces close proximity and thereby increases the effective concentration of enzyme and substrate.…”

“…1). Protein phosphatase 2A (PP2A) also associates with the complex (Hsu et al 1999;Seeling et al 1999;Ratcliffe et al 2000;Yamamoto et al 2001). Mutations in destruction complex components associated with various cancers result in inappropriate stabilization of b-catenin and Wnt target gene expression in the absence of a Wnt stimulus (Dominguez et al 1995;Jiang and Struhl 1998;Marikawa and Elinson 1998;Peters et al 1999;Liu et al 2000;Satoh et al 2000;Heisenberg et al 2001).…”

LB100, a small molecule inhibitor of PP2A with potent chemo- and radio-sensitizing potential

“…PP2A complexes with adenomatous polyposis coli (APC), axin, glycogen synthase kinase-3-b, and dishevelled (DVL) to target b-catenin for proteasomal degradation. 7,8 Conversely, the PP2A-C catalytic subunit, has also been shown to positively reinforce Wnt signaling as a downstream signal transducer of the Wnt/b-catenin pathway. 9 This has been recapitulated in aspirin-treated colorectal cancer cell lines, in which phosphorylative deactivation of PP2A, induced by aspirin treatment, mediated dose-dependent decreases in Wnt/b-catenin pathway activation.…”