Mitogen-Activated Protein Kinases Promote WNT/beta-CateninSignaling via Phosphorylation of LRP6

LDL-related protein 6 (LRP6) is a coreceptor of WNTs and a key regulator of the WNT/beta-catenin pathway. Upon activation LRP6 is phosphorylated within its intracellular PPPS/TP motifs. On the basis of a kinome-wide siRNA screen and confirmative biochemical analysis, we show that several proline-directed mitogen-activated protein kinases (MAPKs), such as p38, ERK1/2 and JNK1 are sufficient and required for the phosphorylation of PPPS/TP motifs of LRP6. External stimuli, which control the activity of MAPKs, such as phorbolesters and fibroblast growth factor 2 (FGF2) control the choice of the LRP6-PPPS/TP kinase and regulate the amplitude of LRP6 phosphorylation and WNT/beta-catenin-dependent transcription. Our findings suggest that cells recruit not only one dedicated LRP6 kinase but rather select their LRP6 kinase depending on cell type and the external stimulus.