Inositol pentakisphosphate mediates Wnt/β-catenin signaling

Wnt3a stimulates lymphoid enhancer factor/T-cell factor protein-sensitive transcription, i.e. the canonical pathway, in mouse F9 embryonal tetratocarcinoma cells expressing rat Frizzled-1. We explored the potential roles for inositol polyphosphates as mediators of Wnt signaling in the canonical pathway. Wnt3a triggers G-protein-linked phosphatidylinositol signaling, transiently generating inositol polyphosphates, especially inositol pentakisphosphate (IP5) accumulation. Knock-down of G alpha(q) abolishes, whereas expression of the Q209L constitutively active mutant of G alpha(q) mimics, the effects of Wnt3a on IP5 generation and downstream signaling. Phospholipase C beta-1 and C beta-3 mediate the G protein signal to the level of phosphatidylinositol signaling. Knockdown and inhibitor studies of the enzymes responsible for generating IP5 reveal inositol 1,4,5-trisphosphate 3-kinase and inositol polyphosphate multikinase as key mediators in the production of IP5. Wnt3a stimulation of the canonical pathway requires accumulation of IP5, which acts to inhibit the activity of glycogen synthase kinase-3 beta, whereas stimulating casein kinase 2. Blockade of Wnt3a stimulation of IP5 generation blocks beta-catenin accumulation, activation of lymphoid enhancer factor/T-cell factor protein-sensitive transcription, and promotion of primitive endoderm formation in response to Wnt3a. Phosphatidylinositol signaling mediates Wnt3a action in the canonical pathway, acting to generate inositol pentakisphosphate, a key second messenger of Wnt3a.