Figure 1.

A. When the co-receptor LRP and the signaling receptor Frizzled are not activated by WNTs, the heterotetrameric destruction complex made up of the four components Axin, adenomatous polyposis coli, casein kinase and the constitutively active enzyme GSK3, phosphorylates β-catenin in the cytosol. This makes β-catenin directed to proteasomal degradation. B. When WNTs bind to LRP and Frizzled, the destruction complex becomes inactivated and non-phosphorylated β-catenin accumulates in the cytosol and translocates to the nuclei. In the nuclei, β-catenin activates the transcription factor complex TCF/LEF which binds to promoter regions of many genes including genes in osteoblasts which are involved in new bone formation.