The Wnt pathway in a normal and in a tumour cell

In normal cells without a Wnt signal a cytosolic destruction complex phosphorylates the signaling protein b-catenin leading to its degradation. In the presence of the extracellular Wnt factor, activation of the membrane bound receptor FZD leads to the dissociation of the destruction complex. As a result, the level of b-catenin in the cytosol and also in the nucleus is increased. Nuclear b-catenin activates transcription factors that lead to the expression of proto-oncogenes such as MYC and CCND1. In cells that carry mutations in a gene coding for a protein of the destruction complex such as APCor AXIN the level of b-catenin is also increased, which leads to the activation of proliferation and inhibition of apoptosis.