Beta-catenin is a signaling molecule that promotes cell proliferation by the induction of gene transcription through the activation of T-cell factor (TCF)/lymphoid enhancer factor (LEF) transcription factors. The canonical mechanism of the regulation of beta-catenin involves its phosphorylation by casein kinase 1 at the Ser-45 site and by glycogen synthase kinase 3 (GSK3) at the Thr-41, Ser-37, and Ser-33 sites. This phosphorylation targets beta-catenin to ubiquitination and degradation by the proteasome system. Mitogenic factors promote beta-catenin signaling through the inhibition of GSK3, resulting in reduced beta-catenin phosphorylation, its stabilization, and subsequent accumulation in the nucleus, where it stimulates TCF/LEF-dependent gene transcription. In the present study, we have shown that (i) beta-catenin can be phosphorylated by protein kinase A (PKA) in vitro and in intact cells at two novel sites, Ser-552 and Ser-675; (ii) phosphorylation by PKA promotes the transcriptional activity (TCF/LEF transactivation) of beta-catenin; (iii) mutation of Ser-675 attenuates the promoting effect of PKA; (iv) phosphorylation by PKA does not affect the GSK3-dependent phosphorylation of beta-catenin, its stability, or intracellular localization; and (v) phosphorylation at the Ser-675 site promotes the binding of beta-catenin to its transcriptional coactivator, CREB-binding protein. In conclusion, this study identifies a novel, noncanonical mechanism of modulation of beta-catenin signaling through direct phosphorylation of beta-catenin by PKA, promoting its interaction with CREB-binding protein.
"It remains to be clarified whether ghrelininduced í µí»½-catenin stabilization via cAMP-dependent protein kinase activation involves only the inhibition of GSK-3í µí»½. A number of recent reports showed that the phosphorylation of í µí»½-catenin on ser552 by PKA enhances the transcriptional ability of í µí»½-catenin independent of the phosphorylation of í µí»½-catenin in the N-terminal domain at canonical sites regulated by Wnt pathway  . This molecular mechanism of PKA-dependent í µí»½-catenin stabilization could be involved in ghrelin-induced í µí»½-catenin cytoplasmatic accumulation . "
[Show abstract][Hide abstract] ABSTRACT: Ghrelin, by binding growth hormone secretagogue receptor (GHS-R), promotes osteoblast proliferation but the signaling mechanism of GHS-R on these cells remains unclear. Since canonical Wnt/β-catenin pathway is critically associated with bone homeostasis, we investigated its involvement in mediating ghrelin effects in osteoblasts and in osteoblast-osteoclast cross talk. Ghrelin (10(-10)M) significantly increased β-catenin levels in rat osteoblasts (rOB). This stimulatory action on β-catenin involves a specific interaction with GHS-R1a, as it is prevented by the selective GHS-R1a antagonist, D-Lys(3)-GHRP-6 (10(-7)M). The effect of ghrelin on β-catenin involves the phosphorylation and inactivation of GSK-3β via protein kinase A (PKA). Inhibition of PKA activity reduces the facilitatory action of ghrelin on β-catenin stabilization. Ghrelin treatment of rOB significantly increases the expression of osteoprotegerin (OPG), which plays an important role in the regulation of osteoclastogenesis, and this effect is blocked by D-Lys(3)-GHRP-6. Furthermore, ghrelin reduced RANKL/OPG ratio thus contrasting osteoclastogenesis. Accordingly, conditioned media from rOB treated with ghrelin decreased the number of multinucleated TRAcP+ cells as compared with the conditioned media from untreated-control rOB. Our data suggest new roles for ghrelin in modulating bone homeostasis via a specific interaction with GHSR-1a in osteoblasts with subsequent enhancement of both β-catenin levels and OPG expression.
International Journal of Endocrinology 04/2015; 2015:547473. DOI:10.1155/2015/547473 · 1.95 Impact Factor
"Although this is an appealing hypothesis, it warrants very careful evaluation of the in vivo interactions between both pathways. Indeed, although some experimental data suggests a potential repressive effect of PKA on -catenin activity (Kang et al., 2002), most of the current literature shows that PKA can phosphorylate and inactivate GSK3 or directly phosphorylate -catenin serine residues 552 and 675, which results in stabilisation of catenin and potentially increased recruitment of cofactors (Fang et al., 2000; Hino et al., 2005; Taurin et al., 2006). Consistent with this scenario, some human adrenal tumours with constitutive PKA signalling showed accumulation of -catenin and overexpression of some potential WNT target genes. "
"Analyses of phospho-isoforms of b-CATENIN after treatment with Ivermectin or Selamectin under PP2A/PP1 protein phosphataseblocked conditions suggest that these drugs may act by enhancing, directly or indirectly, phosphatase activity involved in dephosphorylating P-Ser552/P-Ser675. This effect can help explain the phenotype of Ivermectin-treated cells since P-Ser552-and P-Ser675-b-CATENIN show enhanced transcriptional activity in cooperation with TCF factors and are essential for WNT signaling in colon cancer cells (Hino et al, 2005; Taurin et al, 2006; Fang et al, 2007; Zhu et al, 2012a). 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). "
[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
Data provided are for informational purposes only. Although carefully collected, accuracy cannot be guaranteed. The impact factor represents a rough estimation of the journal's impact factor and does not reflect the actual current impact factor. Publisher conditions are provided by RoMEO. Differing provisions from the publisher's actual policy or licence agreement may be applicable.