Nuclear GSK-3β inhibits the canonical Wnt signalling pathway in a β-catenin phosphorylation-independent manner

β-Catenin is the central signalling molecule of the canonical Wnt pathway, where it activates target genes in a complex with lymphoid enhancer factor/T-cell factor transcription factors in the nucleus. The regulation of β-catenin activity is thought to occur via a cytoplasmatic multiprotein complex that includes the serine/threonine kinase glycogen synthase kinase-3β (GSK-3β) that phosphorylates β-catenin, marking it for degradation by the proteasome. Here, we provide evidence showing that GSK-3β has a nuclear function in downregulating the activity of β-catenin. Using colorectal cell lines that express a mutant form of β-catenin, which cannot be phosphorylated by GSK-3β and ectopically expressed mutant β-catenin protein, we show that nuclear GSK-3β functions in a mechanism that does not involve β-catenin phosphorylation to reduce the levels of Wnt signalling. We show that GSK-3β enters the nucleus, forms a complex with β-catenin and lowers the levels of β-catenin/TCF-dependent transcription in a mechanism that involves GSK-3β-Axin binding.