Supplementary Figure S1 from The Activation of the WNT Signaling Pathway Is a Hallmark in Neurofibromatosis Type 1 Tumorigenesis

PDF file 161K, A simplified representation of the three different Wnt signalling pathways: the canonical pathway, the planar cell polarity pathway, and the Wnt/calcium pathway. In the absence of Wnt ligand, the 'destruction complex' composed of the core proteins Axin, adenomatous polyposis coli (APC), and glycogen synthase kinase-3 (GSK3) rapidly phosphorylates cytosolic beta-catenin, targeting it for subsequent proteasome-mediated destruction. Binding of Wnt to Frizzled (FZD) and low-density lipoprotein receptor-related protein 5/6 (LRP5/6) activates the cytosolic protein Dishevelled (DVL), leading to inhibition of the destruction complex. The resulting accumulated beta-catenin can then translocate to the nucleus to activate Wnt-responsive target genes regulated by lymphoid enhancer factor (LEF) and T cell factor (TCF) family transcription factors, leading to various cellular effects. The secreted inhibitor Dickkopf (DKK) can antagonize Wnt signalling by competitively binding to LRP5/6. Secreted FZD-related proteins (SFRPs) and Wnt inhibitory factor (WIF) are thought to antagonize Wnt signalling by sequestering Wnt ligand in the extracellular space. Binding of Wnt isoforms to FZD can trigger beta-catenin-independent downstream signalling events, other so-called non-canonical Wnt pathways that do not require the transcriptional activity of beta-catenin. One branch of non-canonical pathways involves the activation of RHO and RAC small G proteins to regulate the actin cytoskeleton. DVL-associated activator of morphogenesis 1 (DAAM1), when complexed with DVL and RHO, acts through the regulation of RHO-associated protein kinases (ROCK) and the DVL-RAC GTPase complex to affect actin remodelling. Another branch, when activated, is defined by a phospholipase C (PLC)-mediated increase in intracellular Ca2+ levels and Ca2+ fluxes that lead to the activation of Ca2+/calmodulin-dependent protein kinase (CaMK), protein kinase C (PKC), and nuclear factor of activated T cells (NFAT)