Phosphoprotein phosphatase-2A docks to Dishevelled and counterregulates Wnt3a/β-catenin signaling

Background: Wnt3a stimulates cellular trafficking of key signaling elements (e.g., Axin, Dishevelled-2, β-catenin, and glycogen synthase kinase-3β) and primitive endoderm formation in mouse F9 embryonic teratocarcinoma cells.

Results: The role of phosphoprotein phosphatase-2A in signaling of the Wnt/β-catenin/Lef-Tcfsensitive gene activation pathway was investigated. Wnt3a action attenuates phosphoprotein phosphatase-2A activity and stimulates the Lef/Tcf-sensitive gene transcription. Inhibiting phosphoprotein phosphatase-2A by okadaic acid, by treatment with siRNA (targeting the Csubunit of the enzyme), or by expression of SV40 small t antigen mimics Wnt3a action, increasing the cellular abundance of Axin and phospho-glycogen synthase kinase-3β as well as the trafficking of signaling elements in the Wnt/β-catenin pathway. Although mimicking effects of Wnt3a on the cellular abundance and trafficking of key signaling elements in the Wnt canonical pathway, suppression of phosphatase-2A alone did not provoke activation of the Lef/Tcf-sensitive transcriptional response, but did potentiate its activation by Wnt3a. Phosphoprotein phosphatase- 2A and the scaffold phosphoprotein Dishevelled-2 display similarities in cellular trafficking in response to either Wnt3a or suppression of the phosphatase. A docking site for phosphoprotein phosphatase-2A in the DEP domain of Dishevelled-2 was identified.

Conclusion: In current study, we showed new roles of phosphoprotein phosphatase-2A in Wnt/ β-catenin signaling pathway: effect on protein expression, effect on protein trafficking, retention of molecules in subcellular compartments, and regulation of enzymatic activity of several key players. Docking of phosphoprotein phosphatase-2A by Dishevelled-2 suppresses phosphatase activity and explains in part the central role of this phosphatase in the counterregulation of the Wnt/β-catenin signaling pathway.