The WNT signaling pathway from ligand secretion to gene transcription: Molecular mechanisms and pharmacological targets
Introduction
The WNT signaling pathways are highly conserved among species, are essential in developmental processes, and may be reactivated or suppressed during disease pathogenesis. A simplified and straight-lined representation of canonical (i.e. β-catenin-dependent) WNT signaling (Fig. 1) is that the pathway is activated by a secreted extracellular WNT ligand that binds to a seven transmembrane receptor, called Frizzled (FZD), and its co-receptors the low-density lipoprotein receptor related proteins 5 and 6 (LRP5/6). The activated FZD receptor in turn switches on the intracellular signaling cascade by activating the dishevelled proteins (DVL) which in combination with the phosphorylation of LRP5/6 results in the inactivation of a so called destruction complex consisting of Adenomatosis polyposis coli (APC), Axin, glycogen synthase kinase-3β (GSK-3β) and casein kinase-1 (CK-1) (Logan and Nusse, 2004, Clevers, 2006, Angers and Moon, 2009, MacDonald et al., 2009). In the absence of WNT ligands, the destruction complex phosphorylates cytosolic β-catenin, the key effector of canonical WNT signaling, and thereby targets it for proteosomal degradation. However, in the presence of WNT signaling the function of the destruction complex is inhibited allowing free unphosphorylated β-catenin to accumulate in the cytosol and subsequently translocate into the nucleus. The transcriptional co-activator β-catenin associates in the nucleus with the T-cell factor/lymphoid enhancer factor-1 (TCF/LEF) family of transcription factors and induces canonical gene transcription (Fig. 1) (Logan and Nusse, 2004, Clevers, 2006, Angers and Moon, 2009, MacDonald et al., 2009).
In addition to canonical WNT signaling, also various non-canonical WNT signaling pathways exist. These do not depend on β-catenin as downstream effector, but have mainly been associated with the activation of c-Jun-N-terminal kinase (JNK)-dependent or Ca2+-dependent signaling pathways (Logan and Nusse, 2004, Clevers, 2006, Angers and Moon, 2009, MacDonald et al., 2009). Additionally, non-canonical WNT signaling has been associated with the activation of rap1, atypical protein kinase C (aPKC), mammalian target of rapamycin (mTOR), protein kinase A (PKA) and phosphatidylinositide 3-kinases (PI3K) (Semenov et al., 2007). Originally, the non-canonical WNT pathway was synonymous for the planar cell polarity (PCP) pathway. The PCP pathway regulates tissue morphogenesis during development and the synchronous polarity of sheets of cells. This specific pathway is associated with the downstream activation of JNK and proteins involved in cytoskeleton rearrangement (Semenov et al., 2007, McNeill and Woodgett, 2010). On the other hand, the WNT/Ca2+ signaling pathway is associated with the activation of phospholipase C (PLC), which leads to the formation of inositol 1,4,5-triphosphate (IP3) and 1,2 diacylglycerol (DAG) from the membrane-bound phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2). The formation of IP3 and DAG results in an increase in intracellular Ca2+ levels. Subsequently, Ca2+ causes the activation of calmodulin-dependent protein kinase II (CAMKII), protein kinase C (PKC) and the nuclear factor of activated T cells (NFAT) transcription factor as well as a variety of other transcription factors (Semenov et al., 2007, McNeill and Woodgett, 2010, De, 2011). The WNT/Ca2+ signaling pathway is important for developmental processes; however it has recently also been linked to cancer development and it may contribute to inflammatory responses (De, 2011).
Over the last decades WNT signaling and the complex regulation of this pathway have been extensively investigated. Here the focus will be mainly on the components of canonical WNT/β-catenin signaling, which are still far better understood than the non-canonical WNT pathways. Several aspects of WNT signaling that are applicable for both non-canonical and canonical WNT signaling in general will be discussed.
Section snippets
WNT ligands
WNT ligand genes encode evolutionary conserved secreted glycoproteins (Table 1) that act as signaling molecules essential for a number of fundamental processes like embryonic patterning and cell-fate decisions in development. The name WNT is a fusion of the wingless (Wg) gene and the homologous vertebrate oncogene intergrase-1 (int-1) (Korzh, 2008). In 1973, a mutation in the Wg gene in Drosophila melanogaster was identified and the loss of function mutations resulted in impaired haltere
Cell surface receptors for WNT signaling: Frizzled (FZD) and low-density lipoprotein receptor related proteins (LRP) receptors
The interaction of WNT ligands with their receptors on the cellular surface is the first step in transducing the extracellular signal into an intracellular response. Two distinct receptor families are critical for (canonical) WNT signaling; the Frizzled family of seven transmembrane receptors (FZD receptors) and the low-density lipoprotein receptor related proteins 5 and 6 (LRP5/6 receptors; arrow in D. melanogaster). The initial connection of WNT ligands binding to FZD receptors resulting in
β-Catenin structure and cellular function
Central in the transmission of canonical WNT signaling is the stabilization of the multi-functional protein β-catenin. β-Catenin plays a crucial role both in cell adhesion, by being a component of cadherins based adherens junctions stabilizing cell–cell contacts, and in gene transcription as a transcriptional co-activator in canonical WNT signaling. The main members of the catenin family are; α-catenin, β-catenin, δ-catenin (p120 catenin) and γ-catenin (Shapiro and Weis, 2009, McCrea and Gu,
Inhibition of WNT signaling
Aberrant activation of canonical WNT signaling can result in development of cancers and contributes to various fibroproliferative diseases (Clevers, 2006, Gosens et al., 2008, Konigshoff et al., 2008, Konigshoff et al., 2009, Lam and Gottardi, 2011). The canonical WNT pathway is therefore a potentially attractive target for the treatment of these diseases in several organs, including the liver, skin, lung, kidney, intestine, prostate, and breast (Klaus and Birchmeier, 2008, Konigshoff and
Concluding remarks
Studies in diverse cell types, animal models and genetically modified organisms indicate that canonical and non-canonical WNT signaling is indispensable for several physiological processes. However, when aberrantly regulated it underlies various pathological conditions, including cancer and fibroproliferative diseases in several organs. Further insights into the dynamics of WNT-driven cellular processes and responses as well as the development of novel compounds targeting specific components of
Conflict of interest statement
The authors declare that there are no conflicts of interest.
Acknowledgments
HAB is supported by a postdoctoral fellowship from the European Respiratory Society (Fellowship LTRF 79-2012). MK is supported by a European Research Council Starting Grant (ERC-StG-LS7) and RG is supported by a ZonMW Vidi Grant from the Netherlands Organization of Scientific Research (016.126.307). We would like to acknowledge COST action BM1201 “The developmental origins of chronic lung disease”. We would like to thank Prof. Dr. Herman Meurs and Prof. Dr. Huib A.M. Kerstjens for critically
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