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Wnt Signalling and the Actin Cytoskeleton Oncogene (2006) 25, 7538–7544 & 2006 Nature Publishing Group All rights reserved 0950-9232/06 $30.00 www.nature.com/onc REVIEW Wnt signalling and the actin cytoskeleton T Akiyama and Y Kawasaki Laboratory of Molecular and Genetic Information, Institute for Molecular and Cellular Biosciences, The University of Tokyo, Bunkyo-ku, Tokyo, Japan The tumour suppressor adenomatous polyposis coli (APC) that carry an APC truncation develop adenomas and is mutated in sporadic and familial colorectal tumours. tumours in the intestine (Oshima et al., 1997; Fodde APC binds to b-catenin, a key component of the Wnt et al., 2001). signalling pathway, and induces its degradation. In The productof the APC gene is a ubiquitously addition to this role, there is increasing evidence for expressed multifunctional protein, and interacts with additional roles of APC, including the organization of various cellular proteins, including b-catenin, Axin, cytoskeletal networks. APC interacts with microtubules Asef, IQGAP1, kinesin-2, end-binding protein 1 (EB1), and accumulates at their plus ends in membrane protru- microtubules and Drosophila discs large (DLG) sions. Also, it has been reported that APC is associated (Figure 1). The mostthoroughly studied function of APC with the plasma membrane in an actin-dependent manner. is its ability to induce degradation of b-catenin, a key Moreover, APC interacts with IQGAP1, an effector of Wntsignalling effector(Cadigan and Nusse, 1997; Bienz Rac1 and Cdc42, and APC-stimulated guanine nucleotide and Clevers, 2000; Polakis, 2000; Fodde et al., 2001). exchange factor (Asef), a Rac1-specific guanine nucleo- The specific sites in APC to which b-catenin binds are tide exchange factor (GEF). IQGAP1 mediates associa- the three 15-amino-acid repeats and the more carboxy- tion of APC with cortical actin in the leading edge of proximal seven 20-amino-acid repeats, the latter of migrating cell and both proteins are required for cell which are responsible for the degradation of b-catenin. polarization and directional migration. APC interacts The majority of the somatic mutations in APC found in with Asef and stimulates its activity, thereby regulating colon cancers are confined to its central region called the the actin cytoskeletal network, cell morphology, adhesion mutation cluster region, and they typically result in the and migration. Truncated mutant APCs present in generation of truncated gene products (Figure 1). In colorectal tumour cells activate Asef constitutively and these cancers, degradation of b-catenin is impaired, contribute to their aberrant migratory properties, which resulting in its accumulation and formation of a stable, may be important for adenoma formation as well as constitutively active complex with T cell factor (TCF), tumour progression to invasive malignancy. resulting in the activation of Wnt signalling target genes. Oncogene (2006) 25, 7538–7544. doi:10.1038/sj.onc.1210063 In addition, in a few percentage of colon tumours, b-catenin has been found to be mutated. Mutated Keywords: tumour suppressor; colon tumour; APC; b-catenin is resistant to APC-mediated degradation, and cytoskeleton; kinesin-2; Asef accumulates in the cytoplasm and constitutively trans- activates target genes. Thus, the accumulation of b-catenin per se is believed to be important for colorectal tumorigenesis. Although it is widely accepted that the ability of APC Introduction to induce degradation of b-catenin is essential for its tumour suppressor function, most mutations in colon Adenomatous polyposis coli (APC) was identified as a tumours occur in APC and only a few percentage of gene responsible for the onset of familial adenomatous mutations occur in b-catenin. In addition, it has been polyposis (FAP), an autosomal dominantly inherited reported that small adenomas with b-catenin mutations disease that predisposes patients to multiple colorectal do not appear to be likely to progress to larger polyps and cancers (for reviews, see Cadigan and Nusse, adenomas and invasive carcinomas (Samowitz et al., 1997; Bienz and Clevers, 2000; Polakis, 2000; Fodde 1999). It is therefore possible that inactivation of the et al., 2001). Inactivation of APC is also seen in the additional APC functions might also be important for majority of sporadic colorectal tumours and is an early colorectal tumorigenesis (Fodde et al., 2001). In this event in tumorigenesis. In mouse models, heterozygotes regard, it is interesting that APC has recently been revealed to interact with various cellular proteins other than b-catenin, including Asef, IQGAP and kinesin-2, Correspondence: Professor T Akiyama, Laboratory of Molecular and and to regulate cytoskeletal networks, thereby control- Genetic Information, Institute for Molecular and Cellular Biosciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, ling cell adhesion and motility (Jimbo et al., 2002; Japan. Kawasaki et al., 2003; Watanabe et al., 2005). In this E-mail: [email protected] review, I will focus on the function of APC in the Wnt signalling and the actin cytoskeleton T Akiyama and Y Kawasaki 7539 cells and polarized cultured epithelial cells (Miyashiro a -catenin-binding sites Oligomerization et al., 1995; Nathke et al., 1996; Rosin-Arbesfeld et al., Arm 15-a.a. 20-a.a. Basic TSV 2001; Langford et al., 2006). The association of APC repeats repeats with the plasma membrane is reported to depend on the 1 2843 actin cytoskeleton but not on microtubules: APC Axin-binding sites associated with the plasma membrane is not detected Asef Mutation Cluster Region EB1 DLG when the actin cytoskeleton is depolymerized by IQGAP1 Microtubules latrunculin A in Madin–Darby canine kidney (MDCK) Kinesin2 cells (Rosin-Arbesfeld et al., 2001; Langford et al., b 2006). On the other hand, microtubule depolymerization causes a redistribution of APC from microtubule plus ends to the plasma membrane. Furthermore, drug Figure 1 Structure of the APC protein. (a) Wild-type full-length washout experiments suggest that APC is delivered APC. (b) A typical truncated mutant APC in colorectal tumour continuously to the plasma membrane by a dynamic cells. actin-dependent process. Thus, it appears that there are two distinct peripheral APC pools that are in equili- regulation of cytoskeletal networks, the actin cyto- brium with each other in these cells: actin-dependent skeleton in particular, and discuss its relevance to membrane-associated APC and microtubule-dependent tumorigenesis. APC clusters. Interestingly, restoration of expression of full-length APC in a colorectal cancer cell line has been shown to promote cell–cell adhesion (Faux et al., 2004). Furthermore, genetic analysis of the Drosophila homo- Clustering of APC at membrane protrusions logue of APC, E-adenomatous polyposis coli/d adeno- matous polyposis coli (E-APC/dAPC2), indicates that APC has been detected at a number of intracellular sites: Drosophila APC is also associated with the plasma the plasma membrane, cytoplasm, centrosomes, kineto- membrane in an actin-dependent manner and is chores and nucleus (for a review, see Bienz, 2002). important for cellular adhesion (for a review, see Bienz Although there are a number of conflicting reports, re- and Hamada, 2004). In addition, truncated mutant evaluation of various commonly used anti-APC anti- APCs presentin colorectalcancer cells are mainly bodies has revealed that the bulk of APC resides in the cytoplasmic and are not able to associate with the cytoplasm (Brocardo et al., 2005). The moststriking plasma membrane (Polakis, 2000; Rosin-Arbesfeld feature of APC localization is its accumulation in et al., 2001; Brocardo et al., 2005), suggesting that clusters near the distal ends of microtubules at the APC associated with the plasma membrane may edges of migrating epithelial cells (Nathke et al., 1996; contribute to its tumour suppressor function. More Reilein and Nelson, 2005) (Fig. 2). Furthermore, APC recently, rather striking evidence has been provided that has been shown to move along microtubules and argues that much of the proposed cytoskeletal functions accumulate at their growing plus ends in an ATP- of APC (atleastin flies) were deduced from gain-of- dependentmanner (Mimori-Kiyosue et al., 2000a). function mutants, and may thus not be relevant in a APC interacts with microtubules directly via the basic normal physiological setting. Although complete loss of region in its carboxy terminus (Munemitsu et al., 1994; dAPC and E-APC/dAPC2 results in strong activation of Nathke et al., 1996; Zumbrunn et al., 2001) and Wingless signaling, it does not substantially alter indirectly via end-binding protein 1 (EB1), forming a cadherin-based adhesion as well as cell viability, spindle stabilizing complex at microtubule plus ends (Su et al., morphology, orientation and selection of division plane, 1995; Askham et al., 2000; Mimori-Kiyosue et al., as predicted from previous studies (McCartney et al., 2000b). Interestingly, APC and EB1 bind to mDia to 2006). In addition, truncated mutant APCs found in stabilize microtubules downstream of Rho and promote tumours have been shown to possess dominant effects cell migration (Wen et al., 2004). Also, APC interacts on cytoskeletal regulation. with kinesin-2 and this interaction is essential for transportation of APC along microtubules (see below; Jimbo et al., 2002). Phosphorylation of APC with glycogen synthase kinase-3beta (GSK-3b) can cause The mechanism of APC trafficking to membrane APC to dissociate from microtubules (Zumbrunn et al., protrusions 2001) and prevents its accumulation at cell protrusions (Etienne-Manneville and Hall, 2003). For migration of APC along microtubules to the tips of membrane protrusions, kinesin-2 is believed to play an essential role (Jimbo et al., 2002). Kinesin-2 is one of the Actin-dependent association of APC with the mostubiquitouslyexpressed thekinesin superfamily plasma membrane proteins (KIFs) and has been implicated in the intracellular transport of membrane-bound organelles It has also been reported that APC is associated with the and protein complexes in various tissue (for reviews, lateral plasma membrane in mouse intestinal epithelial see Scholey, 1996; Miki et al., 2005).
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