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Wnt/Fz Signaling and the Cytoskeleton: Potential Roles in Tumorigenesis

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npg Wnt/Fz signaling and cytoskeleton Cell Research (2009) 19:532-545. 532 © 2009 IBCB, SIBS, CAS All rights reserved 1001-0602/09 $ 30.00 npg REVIEW www.nature.com/cr Wnt/Fz signaling and the cytoskeleton: potential roles in tumorigenesis Shih-Lei Lai1, 2, Andy J Chien1, Randall T Moon1 1Howard Hughes Medical Institute, Department of Pharmacology, Institute for Stem Cell and Regenerative Medicine, University of Washington School of Medicine, Seattle, WA 98195, USA; 2Institute of Zoology, National Taiwan University, Taipei 10617 Wnt/β-catenin regulates cellular functions related to tumor initiation and progression, cell proliferation, differ- entiation, survival, and adhesion. β-Catenin-independent Wnt pathways have been proposed to regulate cell polarity and migration, including metastasis. In this review, we discuss the possible roles of both β-catenin-dependent and -independent signaling pathways in tumor progression, with an emphasis on their regulation of Rho-family GTPases, cytoskeletal remodeling, and relationships with cell-cell adhesion and cilia/ciliogenesis. Keywords: Wnt, cytoskeleton, cilia, cell adhesion, Rho Cell Research (2009) 19:532-545. doi: 10.1038/cr.2009.41; published online 14 April 2009 Introduction translocates to the nucleus, where it activates target gene transcription in cooperation with T-cell factor (TCF) and Wnts are a family of secreted glycoproteins with lymphoid enhancer-binding protein transcription factors members that activate various intracellular pathways (LEFs; Figure 1B, for reviews, see [3-6]). after binding to seven-pass transmembrane frizzled (Fz) The term non-canonical Wnt pathway has been used receptor family proteins or to a complex comprised of in the literature to refer to Wnt-activated signal transduc- Fz and LDL receptor-related proteins 5/6 (LRP5/6). The tion independent of b-catenin stabilization. At least two best studied Wnt pathway is the Wnt/b-catenin pathway, non-canonical pathways have been summarized in previ- also known as the “canonical” Wnt pathway. In the ab- ous reviews [7-9]. However, it is still unclear whether sence of Wnt ligands, b-catenin is recruited into a de- non-canonical Wnt pathways represent truly distinct struction complex comprised of adenomatous polyposis pathways or whether they are all parts of a large incom- coli (APC) and Axin, which induce the phosphorylation pletely characterized signaling network. of b-catenin by casein kinase 1 (CK1) and glycogen syn- One candidate b-catenin-independent Wnt pathway thase kinase 3 (GSK3). This phosphorylation leads to the is the planar cell polarity (PCP) pathway, which was ubiquitylation and proteasomal degradation of b-catenin first elucidated in Drosophila (Figure 2A). Genetic stud- (Figure 1A). Upon binding of Wnts to Fz and LRP5/6 ies revealed the asymmetric distribution of core PCP receptors, a signal is transduced to activate the cyto- gene products that include Frizzled (Fz), DVL, Prickle plasmic phosphoprotein Dishevelled (DVL). Activated (Pk), Flamingo (Fmi/CELSR) and Strabismus (Stbm/ DVL then recruits the destruction complex to the plasma VANGL2). PCP signaling is pivotal for establishing cell membrane, inhibiting GSK3 and thus preventing phos- polarity in the orientation of wing hairs, the organiza- phorylation of b-catenin. In addition, phosphorylation of tion of multifaceted eye structures, the orientation of LRP5/6 at intracellular PPPSP motifs by CK1 and GSK3 hair cells in the inner ear, and the orientation of bristles also stimulates Axin recruitment to the plasma membrane and other asymmetric polar structures. Activation of the [1, 2]. Consequently, b-catenin escapes proteasomal deg- PCP pathway leads to subcellular-localized activation radation, accumulates in the cytoplasm and subsequently of Rho-family GTPases and JNK, which are involved in processes such as embryonic convergent extension (CE) in Xenopus [10]. Correspondence: Randall T Moon Involvement of the established Wnt/b-catenin path- E-mail: [email protected] way members Fz and DVL, along with phenotypic simi- Cell Research | Vol 19 No 5 | May 2009 Shih-Lei Lai et al. npg 533 A B WNT Fz Fz LRP 5/6 LRP LRP 5/6 LRP P DVL DVL Axin CK1 GSK3 GSK3 APC P Ubiquitylation b-cat b-cat AXIN & proteolysis b-cat APC b-cat b-cat TCF TCF Target genes Target genes Nucleus Nucleus Figure 1 Overview of the canonical Wnt-b-catenin pathway. (A) In the absence of Wnt, b-catenin (b-cat) is recruited into the destruction complex comprised of adenomatous polyposis coli (APC) and Axin, and phosphorylated by casein kinase 1 (CK1) and glycogen synthase kinase 3 (GSK3). This finally leads to the ubiquitylation and proteolysis of b-catenin. (B) Upon bind- ing of Wnt to frizzled (Fz) and LDL receptor-related proteins 5/6 (LRP5/6), active Dishevelled (DVL) recruits the destruction complex to the plasma membrane and prevents b-catenin degradation. Accumulated cytosolic b-catenin subsequently trans- locates into the nucleus and activates target genes in cooperation with T-cell factor (TCF) and lymphoid enhancer-binding protein transcription factor (LEF). larities between PCP mutants and gain-of-function or loss-of-function of genes implicated in Wnt/PCP signal- loss-of-function phenotypes for certain Wnt isoforms, has ing have led to the hypothesis that this pathway may led to the hypothesis that PCP signaling is a b-catenin- regulate cellular adhesion and/or cellular motility. In ad- independent Wnt pathway, and, consequently, the term dition to CE, another distinctive epithelial PCP seen in Wnt/PCP is often utilized. However, no specific Wnt vertebrates is the uniform orientation of stereocilia on isoform has definitively linked Fz-dependent activation sensory hair cells in the mammalian cochlea. Wnt5a is by a Wnt to PCP signaling in Drosophila. In vertebrates, required for proper cellular distribution of Ltap/Vangl2 Wnt5, Wnt11 and homologs of PCP genes are involved and genetically interacts with Ltap/Vangl2 for uniform in regulating CE movements in the gastrulation phase orientation of stereocilia, cochlear extension and neural of embryonic development [8]. Stbm/VANGL2 mutant tube closure [19]. Even though there is evidence suggest- trilobite fish have a severe CE defect that is mimicked ing that Wnt signaling is one of the signals to break the by both overexpression and morpholino knockdown of symmetry of these PCP components, the exact contribu- Stbm/VANGL2 in both zebrafish and Xenopus [11-13]. tion of Wnt is still unclear. In addition, mutation of Stbm/VANGL2 in loop-tail mice Another proposed b-catenin-independent pathway, the also results in defective neural tube closure, likely due Wnt-calcium (Wnt/Ca2+) pathway, is less understood, and to CE failure [14]. Furthermore, mutation of CELSR in is defined by cell physiology, biochemistry and genetics mice and morpholino gene knockdown of zebrafish and (Figure 2B, for reviews, see [8, 20, 21]). The involve- Xenopus Pk showed similar effects on CE movement ment of certain Wnt isoforms in activating both the Wnt/ [15-18]. The CE defects seen upon gain-of-function or Ca2+ and Wnt/PCP pathways in vertebrates, as well as the www.cell-research.com | Cell Research npg Wnt/Fz signaling and cytoskeleton 534 A Wnt? B WNT Stbm/VANGL2 Fmi/CELSR Fz Fz Kny or Ror2 G Pk DVL DVL DAAM1 2+ Rac [Ca ]i Cdc42 RhoA CAMK II PKC Cytoskeletal remodeling JNK Other downstream effectors Complex and dynamic cellular response ? AP1 Target genes X Nucleus Nucleus Figure 2 Overview of b-catenin-independent Wnt pathways. (A) The Wnt/PCP pathway is characterized by asymmetric dis- tribution of Frizzled (Fz), Flamingo (Fmi/CELSR), Prickle (Pk) and Strabismus (Stbm/VANGL2), resulting in cell polarity and the activation of Rho GTPases and JNK through DVL and Dishevelled-associated activator of morphogenesis 1 (DAAM1). (B) Activation of the Wnt-calcium pathway by interaction of Wnts with Fz and co-receptor Kny or Ror2 increases intracellular calcium level and subsequently activates calcium/calmodulin-dependent protein kinase II (CAMKII) and protein kinase C (PKC). involvement of shared intracellular components between calcium transient frequency [30], and CE defects [31]. the two pathways, offers some evidence that these two In addition, expression of CaMKII partially rescues CE pathways may be part of the same signaling network. defects of Wnt5/pipetail mutants, suggesting the involve- Certain Wnt ligands, particularly Wnt5a and Wnt11, trig- ment of CaMKII in this process [30]. In the context of ger the release of intracellular calcium and utilize both tumor progression, overexpression of Wnt5a in melano- G-proteins and DVL during this process in certain cel- ma cells induces actin reorganization and increased cell lular contexts [22, 23]. The signaling specificity between invasion, through the activation of PKC [32]. This PKC Wnt/b-catenin and Wnt/Ca2+ pathways relies partially on activation by Wnt5a is most likely through the calcium participation of co-receptors, such as LRP5/6 in the Wnt/b- release since Wnt5a-induced calcium release has also catenin pathway and Knypek and Ror2 in the Wnt/Ca2+ been shown in both human mammary epithelial cells [33] pathway [24, 25]. Increased levels of intracellular cal- and mouse F9 teratocarcinoma cells [34]. cium subsequently modulate calcium-sensitive proteins In the context of cancer biology, the Wnt/b-catenin such as calcium/calmodulin-dependent protein kinase II pathway is implicated in the initiation and progression of (CaMKII) and protein kinase C (PKC) [23, 26]. various types of tumors, most likely through the regula- Like the Wnt/PCP pathway, the Wnt/Ca2+ pathway has tion of b-catenin-responsive genes affecting cell fate, also been postulated to play a crucial role in cell adhe- growth and apoptosis [3]. The majority of research on sion and/or cell movements during gastrulation [27]. Wnt signaling and oncogenesis has focused on the role of Intracellular calcium waves have been observed at the mutations or aberrant expression of Wnt signaling com- margin of gastrulating zebrafish embryos [28] and dor- ponents in promoting dysregulation of the Wnt/b-catenin sal explants of gastrulating Xenopus embryos [29], and pathway and target gene expression.
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