Dysregulation of Wnt-beta-catenin signaling disrupts axis formation in vertebrate embryos and underlies multiple human malignancies. The adenomatous polyposis coli (APC) protein, axin, and glycogen synthase kinase 3beta form a Wnt-regulated signaling complex that mediates the phosphorylation-dependent degradation of beta-catenin. A protein phosphatase 2A (PP2A) regulatory subunit, B56, interacted with APC in the yeast two-hybrid system. Expression of B56 reduced the abundance of beta-catenin and inhibited transcription of beta-catenin target genes in mammalian cells and Xenopus embryo explants. The B56-dependent decrease in beta-catenin was blocked by oncogenic mutations in beta-catenin or APC, and by proteasome inhibitors. B56 may direct PP2A to dephosphorylate specific components of the APC-dependent signaling complex and thereby inhibit Wnt signaling.
"Support for an involvement of PP2A also derives from the finding that its Ba (PR55a) subunit is required to downregulate the levels of P-Ser552 and P-Ser675 C-terminal phosphoforms of b- CATENIN in colon cancer cells (Zhang et al, 2009). The role of PP2A and its multiple subunits is thus likely to be complex since it can also act positively on WNT signaling with its B56 subunit through the inhibition of N-terminal b-CATENIN phosphorylation and thus inhibition of b-CATENIN destruction (Seeling et al, 1999). Nevertheless, in cancer cells lacking APC destruction complex function (e.g. "
[Show abstract][Hide abstract] ABSTRACT: Constitutive activation of canonical WNT-TCF signaling is implicated in multiple diseases, including intestine and lung cancers, but there are no WNT-TCF antagonists in clinical use. We have performed a repositioning screen for WNT-TCF response blockers aiming to recapitulate the genetic blockade afforded by dominant-negative TCF. We report that Ivermectin inhibits the expression of WNT-TCF targets, mimicking dnTCF, and that its low concentration effects are rescued by direct activation by TCFVP16. Ivermectin inhibits the proliferation and increases apoptosis of various human cancer types. It represses the levels of C-terminal β-CATENIN phosphoforms and of CYCLIN D1 in an okadaic acid-sensitive manner, indicating its action involves protein phosphatases. In vivo, Ivermectin selectively inhibits TCF-dependent, but not TCF-independent, xenograft growth without obvious side effects. Analysis of single semi-synthetic derivatives highlights Selamectin, urging its clinical testing and the exploration of the macrocyclic lactone chemical space. Given that Ivermectin is a safe anti-parasitic agent used by > 200 million people against river blindness, our results suggest its additional use as a therapeutic WNT-TCF pathway response blocker to treat WNT-TCF-dependent diseases including multiple cancers.
EMBO Molecular Medicine 08/2014; 6(10). DOI:10.15252/emmm.201404084 · 8.67 Impact Factor
"β-catenin is the first identified target of PP2A-B56α. Overexpression of B56α decreased β-catenin expression in mammalian cells and Xenopus embryo explants (33). PP2A-B56α is thought to have targets within Axin1-mediated degradation complex for β-catenin and it has been found to be able to inhibit the Wnt signaling pathway (34). "
[Show abstract][Hide abstract] ABSTRACT: Cantharidin is an active constituent of mylabris, a traditional Chinese medicine, and presents strong anticancer activity in various cell lines. Cantharidin is a potent and selective inhibitor of serine/threonine protein phosphatase 2A (PP2A). Our previous studies revealed the prospect of application of cantharidin, as well as other PP2A inhibitors, in the treatment of pancreatic cancer. However, the mechanisms involved in the anticancer effect of PP2A inhibitors have not been fully explored. The Wnt/β‑catenin pathway is involved in cell migration and proliferation and participates in the progression of pancreatic cancer. If β‑catenin is phosphorylated and degraded, the Wnt/β‑catenin pathway is blocked. PP2A dephosphorylates β‑catenin and keeps the Wnt/β‑catenin pathway active. In the present study, we found that PP2A inhibitor treatment induced phosphorylation and degradation of β‑catenin. The suppression on the migration and growth of PANC‑1 pancreatic cancer cells could be attenuated by pretreatment with FH535, a β‑catenin pathway inhibitor. Microarray showed that PP2A inhibitor treatment induced expression changes in 13 of 138 genes downstream of the β‑catenin pathway. Real‑time PCR further confirmed that FH535 attenuated the expression changes induced by PP2A inhibitors in 6 of these 13 candidate genes. These 6 genes, VEGFB, Dkk3, KRT8, NRP1, Cacnalg and WISP2, have been confirmed to participate in the migration and/or growth regulation in previous studies. Thus, the phosphorylation- and degradation-mediated suppression on β‑catenin participates in the cytotoxicity of PP2A inhibitors. Our findings may provide insight into the treatment of pancreatic cancer using a targeting PP2A strategy.
"pCEP plasmids containing B56α1, B56β1 or B56γ1 cDNA with a 4HA tag (pCEP4HAB56α1, pCEP4HAB56β1, pCEP4HAB56γ1) were provided by Dr David Virshup . Mouse B56γ1 containing vector (pCX4bsr-B56γ1) was provided by Dr Ito . "
[Show abstract][Hide abstract] ABSTRACT: Extracellular signal-regulated kinase (ERK) signalling plays a central role in various biological processes, including cell migration, but it remains unknown what factors directly regulate the strength and duration of ERK activation. We found that, among the B56 family of protein phosphatase 2A (PP2A) regulatory subunits, B56γ1 suppressed EGF-induced cell migration on collagen, bound to phosphorylated-ERK, and dephosphorylated ERK, whereas B56α1 and B56β1 did not. B56γ1 was immunolocalized in nuclei. The IER3 protein was immediately highly expressed in response to costimulation of cells with EGF and collagen. Knockdown of IER3 inhibited cell migration and enhanced dephosphorylation of ERK. Analysis of the time course of PP2A-B56γ1 activity following the costimulation showed an immediate loss of phosphatase activity, followed by a rapid increase in activity, and this activity then remained at a stable level that was lower than the original level. Our results indicate that the strength and duration of the nuclear ERK activation signal that is initially induced by ERK kinase (MEK) are determined at least in part by modulation of the phosphatase activity of PP2A-B56γ1 through two independent pathways.
PLoS ONE 12/2013; 8(5):e63729. DOI:10.1371/journal.pone.0063729 · 3.23 Impact Factor
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