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© 2000 American Society for Clinical Oncology
Biology of the Adenomatous Polyposis Coli Tumor SuppressorFrom the Howard Hughes Medical Institute, Department of Molecular Genetics, Biochemistry, and Microbiology, University of Cincinnati College of Medicine, Cincinnati, OH. Address reprint requests to Joanna Groden, PhD, Department of Molecular Genetics, College of Medicine, University of Cincinnati, 231 Bethesda Ave, Cincinnati, OH 45267; email joanna.groden{at}uc.edu ABSTRACT ABSTRACT: The adenomatous polyposis coli (APC) gene was first identified as the gene mutated in an inherited syndrome of colon cancer predisposition known as familial adenomatous polyposis coli (FAP). Mutation of APC is also found in 80% of all colorectal adenomas and carcinomas and is one of the earliest mutations in colon cancer progression. Similar to other tumor suppressor genes, both APC alleles are inactivated by mutation in colon tumors, resulting in the loss of full-length protein in tumor cells. The functional significance of altering APC is the dysregulation of several physiologic processes that govern colonic epithelial cell homeostasis, which include cell cycle progression, migration, differentiation, and apoptosis. Roles for APC in some of these processes are in large part attributable to its ability to regulate cytosolic levels of the signaling molecule beta-catenin and to affect the transcriptional profile in cells. This article summarizes numerous genetic, biochemical, and cell biologic studies on the mechanisms of APC-mediated tumor suppression. Mouse models of FAP, in which the APC gene has been genetically inactivated, have been particularly useful in testing therapeutic and chemopreventive strategies. These data have significant implications for colorectal cancer treatment approaches as well as for understanding other disease genes and cancers of other tissue types. THE ADENOMATOUS polyposis coli (APC) tumor suppressor gene was identified 10 years ago through its association with an inherited syndrome of colorectal cancer known as familial adenomatous polyposis coli (FAP). The genetics of FAP and its similarity to other syndromes of site-specific tumor predisposition, such as retinoblastoma, led to the designation of APC as a tumor suppressor gene. This designation was confirmed by mutational inactivation of APC in both familial and sporadic tumors and by its ability to inhibit tumor cell growth. Functionally, the APC gene product modulates the oncogenic Wnt signal transduction cascade through its effects on cellular levels of beta-catenin. In addition, dependent or independent of its ability to titrate ß-catenin, APC affects diverse physiologic processes from cell growth to apoptosis in a number of cell types and organisms. This article summarizes many experiments that focus on the role of APC mutation in tumor development and the basic mechanisms of APC-mediated tumor suppression. Such research has far-reaching implications for our understanding of how tumors form and how they are treated, as well as for promoting our understanding of basic cell biology and biochemistry. THE ROLE OF APC IN TUMORIGENESIS: INHERITED SUSCEPTIBILITY TO CANCER Characterization of the etiology of FAP has provided insight into the cause of this inherited cancer predisposition and of sporadic colon cancer. FAP is a rare, autosomal, dominantly inherited syndrome affecting approximately one in 10,000 individuals and is characterized by the development of multiple (hundreds to thousands) colorectal adenomas at a young age (before 20 years). Because of the large number of adenomas and the well-established propensity of at least a subset of adenomas to become malignant, prophylactic resection of the colon is the recommended approach to managing the disease. Extracolonic manifestations of FAP are common and include adenoma and adenocarcinoma of the stomach, periampullary region, pancreas and thyroid, osteomas, desmoids, dental abnormalities, epidermal cysts, congenital hypertrophy of the retinal pigmented epithelium (CHRPE) and CNS tumors. Conventionally, the clinical association of FAP with desmoid tumors and osteomas is referred to as Gardner syndrome, whereas Turcot syndrome is characterized by the association of FAP with CNS tumors, in particular medulloblastoma. The calculated risk of medulloblastoma for an FAP patient is greater than 90-fold of that of a normal individual.1 Identification of the gene mutated in FAP was accomplished by linkage analysis of families with the syndrome2,3 and conventional positional cloning strategies.4-7 The APC gene was first localized to human chromosome 5q by the identification of a chromosome 5q deletion in a Gardner syndrome patient several years earlier.8 The APC gene includes 21 exons contained within a 98-kilobase locus.9 The largest exon, 15, comprises more than 75% of the 8,535 base pairs (bp) of coding sequence (Fig 1) and is the target of most germline mutations in FAP patients and somatic mutations in tumors. In addition to the conventional form of APC encoded by exons 1 to 15, alternatively expressed exons of the gene (0.3, BS, 0.1, 0.2, 1, 9, and 10A)10-12 encode alternate protein isoforms.13 Although the tissue-specific expression patterns of these isoforms differ from conventional APC,14 their function is not yet understood. They are found generally in postmitotic tissues and terminally differentiated cell lines14 and differ most significantly from conventional APC in their amino-termini and predicted ability to homo- or heterodimerize.
Most APC mutations are chain-terminating, the majority of mutations created by frameshifts, which result in truncated gene products without the carboxy-terminus. APC mutation follows the classical two-hit model of tumor suppressor inactivation, in that patients with FAP inherit one germline mutation and develop tumors from those cells in which a second hit, or loss of the other allele of APC, is somatically acquired. More than 1,400 mutations in APC, both germline and somatic, have been described.15 Two of the most common germline mutations are 5-bp deletions at nucleotides 3927 and 3183 (codons 1309 and 1061), which comprise 18% and 12% of all germline APC mutations, respectively.15 These deletions occur in short, direct repeats within APC and have been identified in patients with and without a family history of the disease, suggesting that they can occur as new mutations.16 Laken et al17 described a polymorphism of APC at nucleotide 3920 that is found in approximately 6% (47 of 766) of Ashkenazi Jews and 28% (seven of 25) of Ashkenazi Jewish colon cancer patients with a family history of colon cancer. This T  A transversion does not result directly in a truncated gene product but seems to generate a hypermutable poly-adenine tract theoretically prone to slippage during DNA replication, thereby accounting for a higher than expected frequency of somatic mutation within this general region (approximately 500 bp) of APC. The clinical significance of this polymorphism remains uncertain; however, these data point to a novel mechanism by which nucleotide polymorphisms can affect somatic mutations in APC and other disease-causing genes. The precise location of germline mutation within APC can predict disease phenotype (Fig 1). Mutations between bases 3747 and 3990 (codons 1249 to 1330) are associated with a profuse phenotype of colonic tumors (ie, greater than 5,000 adenomas), whereas mutations 5' or 3' to this region are correlated with a sparse phenotype (ie, fewer than 1,000 adenomas). Furthermore, mutations at the very 5' end of the gene (codons 78 to 163) result in an attenuated adenomatous polyposis coli syndrome (AAPC) characterized by the development of fewer adenomas (often less than 100) at a later age. Although it is not understood how more severe APC truncations result in a less severe phenotype, recent studies of AAPC kindreds suggest that AAPC alleles may encode protein with residual function because some AAPC alleles acquire somatic mutations or are lost entirely during tumor development.18 Some germline mutations in the 3' region of exon 15 are associated with a similarly attenuated phenotype.19 Extracolonic manifestations are also associated with mutation position. For example, CHRPE usually occurs in FAP patients with mutations 3' of exon 9A,20 whereas desmoids are more often associated with APC mutation between bases 4335 and 4734 (codons 1445 to 1578).21 Additionally, patients with high numbers of desmoids but without polyposis have been identified and carry germline APC mutations at the 3' end of the gene.22 THE ROLE OF APC IN TUMORIGENESIS: SPORADIC TUMOR FORMATION Study of FAP has provided insight into the cause of this rare inherited disease but, perhaps even more importantly, has unveiled a common mechanism for sporadic colorectal cancer. Colorectal cancer is one of the most common cancers in the developed world and this year will account for over 130,000 new cases and 57,000 deaths in the United States alone. APC mutation is extremely common in colorectal tumors; somatic inactivation of APC occurs in 50% and 80% of sporadic colon adenomas and adenocarcinomas, respectively.23 A mutation cluster region exists within the 5' end of exon 15, between nucleotides 3000 and 4800 (codons 1000 to 1600) and represents approximately 60% of reported somatic mutations.24 The majority of sporadic colon tumors carry mutations of both APC alleles, the frequency of which remains constant between benign and malignant tumors.25 Furthermore, APC mutations are the earliest known genetic alterations in colorectal cancer progression, as they have been identified in the smallest detectable adenomas as well as aberrant crypt foci.26 These data strongly support a role for APC mutation in colorectal tumor initiation rather than in progression of a benign tumor to malignancy. THE ROLE OF APC IN TUMORIGENESIS: STRUCTURAL ANALYSIS OF THE APC PROTEIN AND PROTEIN PARTNERS The APC gene product, known as APC, is large and comprised of 2,843 amino acids with a molecular mass of approximately 310 kd (Fig 1). Our current understanding of APC function comes from a dissection of its protein structure and putative functional motifs and from analysis of its protein partners. The predicted tertiary structure of the first third of APC contains a coiled-coil motif, characterized by a series of heptad repeats of hydrophobic residues that most likely mediate oligomerization of the protein. The first 170 amino acids are sufficient for APC homodimerization in vitro; this association requires the first 45 amino acids, which correspond to the first heptad repeat.27,28 Homodimerization of APC at the amino-terminus implies a possible dominant-negative mode of action for mutant APC in heterozygous cells, in which shorter proteins can functionally inactivate the full-length, normal protein. Although this hypothesis is not supported by mutational data (ie, two APC mutations or loss of heterozygosity are usually observed in adenomas), it is supported by other findings. Mice carrying one mutant Apc allele display a significant decrease in enterocyte migration in the intestinal villus.29 In vitro studies demonstrate that normal APC activity is severely abrogated on introduction of mutant APC and, to a lesser extent, an AAPC mutant gene.30 Conversely, a dominant-negative mode of action for APC is not supported by other experiments. FAP patients carrying cytogenetic deletion, not just mutation, of the APC gene have been identified.2,8,31-33 In addition, transgenic mice overexpressing truncated APC in the intestinal epithelium fail to develop intestinal tumors.34 The smallest pathologic lesions in mice with mutant APC demonstrate loss of the wild-type allele, supporting the notion that loss of full-length APC, not the acquisition of mutant APC, is rate-limiting for tumor formation.35-37 The amino-terminus of APC includes at least one functional nuclear export signal (NES) sequence.38,39 This peptide (67-DLLGRLKGLNLD-78) is similar to other well-characterized NES sequences (such as in HIV Rev, FMRP, and MAPKK) and, by itself, can direct exclusion of green fluorescent protein (GFP) from the nuclei of transfected cells. The function of this NES is sequence-specific and temperature-sensitive, consistent with GTP-dependent, receptor-mediated export.38 Although nuclear localization of APC has been reported in some cell types,40 no functional nuclear localization signals have been identified. Because APC is far too large to diffuse passively into the nucleus, it is possible that APC is shuttled into the nucleus by an unconventional mechanism or that the import and export of APC are tightly regulated by protein conformation or binding partners. The first third of APC contains seven Arm repeats named for an amino acid motif repeated 13 times in the Drosophila homolog of ß-catenin, Armadillo. Implicated in mediating protein-protein interactions, Arm repeats are present in a number of other proteins, including the desmosomal proteins plakoglobin, plakophilin, and band 6 protein, p120cas, the importin family of nuclear import receptors, and the PF16 microtubule-associated protein. In the APC protein partner ß-catenin, similar repeats are required for binding APC, E-cadherin, and the architectural transcription factors belonging to the Tcf family.41-43 Structurally, the Arm repeats in ß-catenin form a superhelix resulting in a positively charged groove that associates with a stretch of acidic amino acids in its partner.44 The function of these arm repeats in APC is unknown because protein partners that specifically bind APC in this region have yet to be described. Three 15–amino acid and seven 20–amino acid repeats are present within the central third of APC. The 15–amino acid repeats associate with the multifunctional ß-catenin protein and the related protein, plakoglobin.45 The residues of the 20–amino acid repeats are highly conserved both between repeats and animal species and demonstrate sequence similarity to the 15–amino acid repeats. Small fragments of APC containing the 20–amino acid repeats can interact with ß-catenin and mediate its downregulation41 (Fig 2), which will be described further in the next section. To bind ß-catenin, these fragments of APC require phosphorylation by the serine-threonine kinase GSK3ß,46 suggesting that APC and GSK3ß together modulate levels of cytoplasmic ß-catenin. The axin family of proteins, including axin, axil and conductin, also are found in the APC/GSK/ß-catenin complex and regulate the degradation of ß-catenin.47-49 The F-box protein beta-TrCP is a component of an E3 ubiquitin ligase that facilitates ß-catenin degradation at the proteosome,50,51 although the specific role of APC in the degradation process is not known.
The importance of APC-mediated ß-catenin degradation is highlighted by the location of the mutation cluster region within the region of the APC gene encoding the 20–amino acid repeats (Fig 1). Activating mutations of ß-catenin in the GSKß phosphorylation site required for its degradation result in accumulation of ß-catenin52 and have been documented in approximately 50% of colorectal tumors that do not contain APC mutations.53 There are limitations, however, to a model in which the sole function of APC is to regulate cytoplasmic levels of ß-catenin. A narrower mutation spectrum of APC would be expected if this association were the only critical function for APC-mediated tumor suppression. Moreover, studies in manipulatable developmental systems indicate additional functions of APC. The Drosophila APC homolog is not required for Armadillo (the Drosophila homolog of ß-catenin) signaling,54 whereas in Xenopus, APC regulates ß-catenin signaling cooperatively rather than antagonistically.55 Whether or not the predominant function of APC is to regulate ß-catenin levels, one consequence of ß-catenin accumulation in the cytosol is the dysregulation of target gene transcription. The carboxy-terminal end of APC, particularly residues 2200 to 2400, is enriched in basic amino acids. These basic residues may confer binding to microtubules because this region is sufficient for microtubule colocalization when transiently it is overexpressed in colon cancer cells56,57 and promotes microtubule polymerization in vitro.57 Specifically, residues 2219 to 2580 of APC bind nonassembled tubulin and bundle microtubules in vitro.58 Although a direct interaction between APC and tubulin has not been shown, APC is likely to associate indirectly with microtubules through EB1. EB1, a member of the EB/RP family of tubulin-binding proteins, was identified as a protein partner of APC through a yeast two-hybrid library screen using residues 2186 to 2843 of APC as bait.59 Given that a S. cerevisiae homolog of EB1 is required for a microtubule-dependent cytokinesis checkpoint,60 it is exciting to postulate that APC and EB1 function to maintain genomic stability by governing mitotic spindle integrity or proper chromosome segregation. Recent experiments indicate that APC/EB1 complexes are indeed cell cycle-regulated.61 APC localization to the leading edge of migrating epithelial cells in culture where microtubules are concentrated62 also supports an additional role for APC/microtubule complexes in cell migration and/or adhesion. RP1, another member of the EB/RP protein family binds the carboxy-terminal region of APC,63 again suggesting that APC may exist in a multiprotein complex with microtubules. The carboxy-terminal 15 residues of APC bind the human homolog of the Drosophila tumor suppressor, discs large (DLG). Association of APC with DLG was identified through a yeast two-hybrid library screen and confirmed biochemically and by colocalization in vivo.64 DLG contains a PDZ domain that specifically associates with a carboxy-terminal XS/TXV peptide on a partnering protein. The carboxy-terminus of APC contains this motif, specifically VTSV. In mammalian epithelial cells, DLG localizes to regions of cell-cell contact,65 as do other PDZ-containing proteins such as ZO-1 and ZO-2. This association provides another means by which APC may affect epithelial cell migration and/or motility. DLG and APC colocalize at synaptic junctions along neuronal processes.64 DLG/APC complexes may have an important role in the CNS, where both proteins are highly expressed and APC mutation is correlated with predisposition to brain tumors. As the NMDA receptor and voltage-gated K+ channel associate with DLG and the closely related SAP90 protein,66,67 it is exciting to postulate that APC modulates neuron function by associating or competing with DLG/NMDA/K+ channel complexes. APC is a phosphoprotein with consensus phosphorylation sites for GSK3ß, MAPK, cyclin-dependent kinases (CDKs), protein kinase A, casein kinase I and II, and calmodulin kinase. APC is phosphorylated on serine and threonine residues and is hyperphosphorylated during the M-phase of the cell cycle.68,69 This observation is consistent with an increase of histone H1-kinase activity associated with full-length APC during M-phase.70 The CDK p34cdc2 can be immunoprecipitated with APC and can phosphorylate APC in vitro.70 Mutation of consensus CDK sites within the carboxy-terminal 700 residues of APC suggests that phosphorylation of APC by p34cdc2 is a mechanism to disassociate APC/EB1 complexes, specifically during mitosis (J.G., unpublished data).61 These phosphorylation sites, as well as the EB1 binding site, are absent when APC is mutated, suggesting that regulation of the APC/EB1/microtubule complex may be critical to the tumor-suppressing activity of APC. Furthermore, APC can be phosphorylated by GSK3ß in vitro,69 modifications that are required for the interaction of APC with ß-catenin.46 THE ROLE OF APC IN TUMORIGENESIS: MECHANISMS OF TUMOR SUPPRESSION Our understanding of APC-mediated tumor suppression comes predominantly from the identification of its protein partners. Using the colonic crypt as a paradigm, one can envision numerous processes in which a regulator of epithelial homeostasis, such as APC, may be important. These processes include proliferation or cell cycle control, migration, differentiation, and apoptosis. Additionally, the regulation of ß-catenin–mediated transcription is another way for APC to affect each of these processes indirectly.
Transcription There are strong indications that the ß-catenin/Tcf pathway is critical to tumor development. As discussed, APC and ß-catenin are frequently mutated in colorectal cancer, and ß-catenin mutations have been found in other tumor types including skin, stomach, and pancreas.76,77 APC and ß-catenin are components of the Wnt signaling pathway, shown in Fig 2. Importantly, binding of the oncogenic Wnt ligand to its transmembrane receptor, Frizzled, activates a signal transduction pathway that results in GSK3ß inhibition and a similar accumulation of cytosolic ß-catenin as when APC is inactivated.78 Activation of the Wnt family of ligands is sufficient for tumorigenesis in mice; and, although a direct role for Wnt in human cancer has not been established, aberrant expression of Wnt has been detected in several tumor types.79 Lastly, Tcf4 mutations have been identified in 40% to 50% of colorectal cancer cell lines and tumors demonstrating microsatellite instability.80 The association of the Wnt signaling pathway and cancer suggests that transcriptional targets of this pathway govern cell growth, migration, differentiation, or apoptotic processes. To date, a handful of ß-catenin/Tcf transcriptional targets have been described, each activated by ß-catenin/Tcf binding. These include the proto-oncogene and cell cycle regulator c-myc,81 the G1/S-regulating cyclin D1 gene,82 and the gene encoding the matrix-degrading metalloproteinase, matrilysin.83 The AP-1 transcription factors c-jun and fra-1 and the urokinase-type plasminogen activator receptor are also upregulated by ß-catenin/Tcf signaling.84 The list of physiologically relevant targets will grow certainly, but the challenge will be to demonstrate that transcriptional changes are causally involved in early-stage tumorigenesis as a consequence of APC mutation. The balance of Tcf and ß-catenin available for DNA binding is most likely critical to net transcriptional activity. In fact, promoter binding by Tcf without ß-catenin represses transcription,82,83,85 perhaps by Tcf association with transcriptional corepressors such as Groucho86,87 and CBP.88 Such data suggest that this pathway has several levels of regulation and that small changes in local protein concentrations can dramatically change the transcription profile of critical regulatory genes. An illustration of this point is that the Tcf family member Tcf1 is a target of ß-catenin/Tcf4 transactivation and may act as a feedback repressor of transcription because it lacks the domain required for ß-catenin association.89
Cell Cycle Control
Migration The identification of ß-catenin and plakoglobin as protein partners of APC first implicated APC in epithelial cell adhesion and migration. ß-catenin localizes to the adherens junction, and plakoglobin localizes to the desmosome, and both structures are involved in cell-cell contact. More recent characterization of the ß-catenin/Tcf signaling pathway has left the role of APC in adhesion and migration all but forgotten. Nevertheless, several lines of evidence support such functions of APC, which may or may not be mediated by ß-catenin regulation. Localization of full-length APC is predominantly cytoplasmic. Näthke et al,62 as well as others,40 immunolocalized APC to the leading edges of epithelial cells. This staining of endogenous APC can be recapitulated with a GFP-fused exogenous APC (Fig 4). 38 Accumulation of APC at the leading edges of actively migrating cells is dependent on the integrity of the microtubules but not actin filaments,62 alluding to a function for APC in cell motility or adhesion through its association with microtubules. Additional support for this hypothesis comes from the analysis of intestinal tissue from mice with genetically altered levels of the Apc gene. Intestinal cell migration along the crypt-villus axis is altered in histologically normal heterozygous Apc animals.29 These epithelial cells show a concomitant increase in ß-catenin levels,29 suggesting that altered migration is facilitated by ß-catenin dysregulation. Migration of intestinal epithelial cells is similarly disordered in mice with overexpression of full-length APC.93 It is certainly possible that APC mutation contributes to tumorigenesis by altering the relative adhesiveness of colonic epithelial cells and misregulating the integrity of the cadherin-catenin complexes. An indirect role for APC in migration may result from transcriptional activation of target genes that modulate cell motility and adhesion, such as E-cadherin71 and matrix-remodeling enzymes.83 In many ways, embryonic development is analogous to tumor development in that it involves dramatic cell proliferation and migration and extracellular matrix remodeling. Mice with two mutant Apc alleles die very early during embryonic development from an inability to complete gastrulation,94 supporting a critical requirement for APC in cell migration and/or proliferation.
Differentiation Although a role for APC in regulating intestinal cell differentiation has not been proven, the pattern of APC expression along the colonic crypt axis supports such a function. Smith et al95 first demonstrated APC protein in epithelial cells of the lumenal half of the colonic crypt, a region of the crypt containing terminally differentiated, nondividing cells. Alternatively expressed APC isoforms (ie, containing any of the 5' exons 0.3, BS, 0.2, or 0.1 but not exon 1) are detectable only in postmitotic, differentiated tissues.13 Consistent with these results, APC expression is induced early during the differentiation of mouse myoblast cultures into myotubes.14 It is possible that genes regulating differentiation may be targets of ß-catenin/Tcf–mediated transcription. In fact, mice lacking Tcf4 have a defect in intestinal stem-cell proliferation,96 suggesting that APC may control differentiation and/or proliferation of the intestinal epithelium by preventing aberrant ß-catenin/Tcf signaling.
Apoptosis
Our group has used cell-free Xenopus egg extract to study the effect of APC on apoptosis in vitro (Steigerwald et al, manuscript in preparation). Recombinant human APC accelerates the rate of apoptosis using exogenous nuclei as substrate. This effect can be mimicked by a region of APC sufficient for ß-catenin binding and downregulation and can be ablated by caspase-8 inhibitors. This effect is transcription independent because the RNA II polymerase inhibitor THE ROLE OF APC IN TUMORIGENESIS: THERAPEUTIC IMPLICATIONS OF APC BIOLOGY The goal of many investigations into the biologic function of APC and the consequences of APC mutation in colon tumorigenesis is to develop meaningful prognostic indicators and therapeutic strategies for managing colorectal cancer. APC remains an attractive target for therapeutic intervention because its mutation is a common and early event in the continuum of colorectal tumor progression.
Generation and Use of Mouse Models
A handful of drugs have been tested using these mouse models that have shown significant benefits with respect to tumor burden and life span. The nonsteroidal anti-inflammatory drugs (NSAIDs), piroxicam and sulindac, were administered to Apcmin mice and dramatically reduced adenoma formation.106-109 Historically, epidemiologic evidence indicated a chemopreventive role for NSAIDs in patients with sporadic colorectal tumors110 and familial polyposis.111,112 These data are concordant with the observation that tumor formation is suppressed in Apc Other therapeutic agents used to treat mouse models of FAP include the matrix metalloproteinase inhibitor batimastat,118 Bowman-Birk protease inhibitor,119 and the DNA methyltransferase inhibitor 5-azacytidine.120 All of these agents are effective at inhibiting adenoma formation in the mouse and are in clinical trials or presently being used as standard cancer therapy.
Gene Therapy In summary, the tumor suppressor APC participates in several cellular processes, from proliferation to apoptosis, in the colonic epithelium (Fig 5). Some of the functions of APC are attributable to its ability to control ß-catenin levels and the transcription of target genes. APC mutation is an early and common event in sporadic colorectal tumor formation and is present in the germline of patients with an inherited predisposition to colon cancer known as familial adenomatous polyposis coli. Therefore, APC and its gene product are attractive targets for the design of therapeutic and chemopreventive strategies for colorectal cancer patients. Additional investigation into the biology, biochemistry, and genetics of APC will no doubt result in the realization of these goals.
ACKNOWLEDGMENTS Supported in part by National Institutes of Health grant no. CA-63507 and the Howard Hughes Medical Institute. We thank Jennifer Kordich, Andrew Lowy, and Thérèse Tuohy for critical review of the manuscript. REFERENCES
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-catenin, p120cas, and indirectly with the cytoskeleton. GSK3ß phosphorylates ß-catenin in a complex that contains ß-catenin, APC, and axin family members, and ß-catenin is rapidly degraded by ubiquination at the proteosome. (B) When APC is mutated, ß-catenin accumulates in the cytoplasm and the nucleus. Similarly, binding of Wnt ligand to its receptor, known as frizzled, inactivates the GSK3ß kinase through dishevelled, generating a cytosolic pool of ß-catenin. ß-catenin associates with members of the Tcf family of transcription factors and modulates the transcription of target genes with Tcf recognition sequences. In some instances, ß-catenin increases transcription of target genes by competing for Tcf binding with corepressors, such as Groucho and CBP, to relieve transcriptional repression.
N67) abrogates this rescue. (C) APC can regulate the G1/S transition by controlling ß-catenin/Tcf-mediated transcription of target genes, such as cyclin D1 and c-myc, that affect the Rb pathway. In addition, it is possible that APC controls S-phase entry in a manner independent of ß-catenin–induced effects on the Rb pathway.
), columnar absorptive cells (
), mucin-producing goblet cells (
), and enteroendocrine cells (
). Cells differentiate and migrate to the lumenal surface of the crypt where they are extruded into the lumen of the intestine by programmed cell death (
). It is likely that APC participates in all of these processes directly or indirectly by modulating transcription profiles within the intestinal epithelial cells.