Title: β-Catenin—A Linchpin in Colorectal Carcinogenesis?
Abstract: An important role for β-catenin pathways in colorectal carcinogenesis was first suggested by the protein's association with adenomatous polyposis coli (APC) protein, and by evidence of dysregulation of β-catenin protein expression at all stages of the adenoma-carcinoma sequence. Recent studies have, however, shown that yet more components of colorectal carcinogenesis are linked to β-catenin pathways. Pro-oncogenic factors that also release β-catenin from the adherens complex and/or encourage translocation to the nucleus include ras, epidermal growth factor (EGF), c-erbB-2, PKC-βII, MUC1, and PPAR-γ, whereas anti-oncogenic factors that also inhibit nuclear β-catenin signaling include transforming growth factor (TGF)-β, retinoic acid, and vitamin D. Association of nuclear β-catenin with the T cell factor (TCF)/lymphoid enhancer factor (LEF) family of transcription factors promotes the expression of several compounds that have important roles in the development and progression of colorectal carcinoma, namely: c-myc, cyclin D1, gastrin, cyclooxygenase (COX)-2, matrix metalloproteinase (MMP)-7, urokinase-type plasminogen activator receptor (aPAR), CD44 proteins, and P-glycoprotein. Finally, genetic aberrations of several components of the β-catenin pathways, eg, Frizzled (Frz), AXIN, and TCF-4, may potentially contribute to colorectal carcinogenesis. In discussing the above interactions, this review demonstrates that β-catenin represents a key molecule in the development of colorectal carcinoma. An important role for β-catenin pathways in colorectal carcinogenesis was first suggested by the protein's association with adenomatous polyposis coli (APC) protein, and by evidence of dysregulation of β-catenin protein expression at all stages of the adenoma-carcinoma sequence. Recent studies have, however, shown that yet more components of colorectal carcinogenesis are linked to β-catenin pathways. Pro-oncogenic factors that also release β-catenin from the adherens complex and/or encourage translocation to the nucleus include ras, epidermal growth factor (EGF), c-erbB-2, PKC-βII, MUC1, and PPAR-γ, whereas anti-oncogenic factors that also inhibit nuclear β-catenin signaling include transforming growth factor (TGF)-β, retinoic acid, and vitamin D. Association of nuclear β-catenin with the T cell factor (TCF)/lymphoid enhancer factor (LEF) family of transcription factors promotes the expression of several compounds that have important roles in the development and progression of colorectal carcinoma, namely: c-myc, cyclin D1, gastrin, cyclooxygenase (COX)-2, matrix metalloproteinase (MMP)-7, urokinase-type plasminogen activator receptor (aPAR), CD44 proteins, and P-glycoprotein. Finally, genetic aberrations of several components of the β-catenin pathways, eg, Frizzled (Frz), AXIN, and TCF-4, may potentially contribute to colorectal carcinogenesis. In discussing the above interactions, this review demonstrates that β-catenin represents a key molecule in the development of colorectal carcinoma. The protein β-catenin was first described in humans as a member of the cell membrane-bound adherens complex.1Kemler R Ozawa M Uvomorulin-catenin complex: cytoplasmic anchorage of a Ca2+-dependent cell adhesion molecule.Bioessays. 1989; 11: 88-91Crossref PubMed Google Scholar Comparative studies of signaling pathways in Xenopus and Drosophila subsequently led to the discovery of a second role for β-catenin in human cells; this cell-signaling role involves translocation of the protein from the cytoplasm into the nucleus.2Morin PJ β-Catenin signaling and cancer.Bioessays. 1999; 21: 1021-1030Crossref PubMed Scopus (570) Google Scholar, 3Kikuchi A Regulation of β-catenin signaling in the Wnt pathway.Biochem Biophys Res Commun. 2000; 268: 243-248Crossref PubMed Scopus (187) Google Scholar There have been recent advances in characterizing regulators of β-catenin function in both its cell adhesion and cell signaling roles. However, the last 4 years have seen, in particular, a revelation of several important molecular pathways that may serve as downstream effectors of β-catenin signaling. Among all human cancers, the molecular pathogenesis of colorectal adenocarcinoma (CRC) has been one of the most extensively studied. At least three pathways are thought to exist, namely the adenoma-carcinoma sequence (as initially proposed by Fearon and Vogelstein4Potter JD Colorectal cancer: molecules and populations.J Natl Cancer Inst. 1999; 91: 916-932Crossref PubMed Google Scholar), the microsatellite instability pathway, and the ulcerative colitis dysplasia-carcinoma pathway.4Potter JD Colorectal cancer: molecules and populations.J Natl Cancer Inst. 1999; 91: 916-932Crossref PubMed Google Scholar As the first of these three pathways is accepted as being central to a majority of CRCs,4Potter JD Colorectal cancer: molecules and populations.J Natl Cancer Inst. 1999; 91: 916-932Crossref PubMed Google Scholar it will be the focus of this review and represents the process referred to by the term “colorectal carcinogenesis,” unless otherwise indicated. Although several oncogenes and oncosuppressor genes are known to be involved in colorectal carcinogenesis, mutation of the adenomatous polyposis coli gene (APC) is regarded as being particularly crucial as an instigator of this process.4Potter JD Colorectal cancer: molecules and populations.J Natl Cancer Inst. 1999; 91: 916-932Crossref PubMed Google Scholar, 5Ilyas M Tomlinson IPM The interactions of APC, E-cadherin and β-catenin in tumour development and progression.J Pathol. 1997; 182: 128-137Crossref PubMed Scopus (159) Google Scholar A potential role for β-catenin in colorectal carcinogenesis first seemed likely in view of the importance of disruption of cell to cell adhesion to cancer invasion and metastasis.6Behrens J Birchmeier W Cell-cell adhesion in invasion and metastasis of carcinomas.Cancer Treat Res. 1994; 71: 251-266Crossref PubMed Google Scholar However, a more primary role in the pathogenesis of CRC soon became apparent with the discoveries that APC protein binds to cytosolic β-catenin,7Su LK Vogelstein B Kinzler KW Association of the APC tumor suppressor protein with catenins.Science. 1993; 262: 1731-1734Crossref PubMed Google Scholar and that APC mutation leads to nuclear accumulation of β-catenin,8Hülsken J Birchmeier W Behrens J E-cadherin and APC compete for the interaction with β-catenin and the cytoskeleton.J Cell Biol. 1994; 127: 2061-2069Crossref PubMed Scopus (481) Google Scholar a feature associated with progression along the adenoma-carcinoma sequence.9Hao X Tomlinson I Ilyas M Palazzo JP Talbot IC Reciprocity between membranous and nuclear expression of β-catenin in colorectal tumors.Virchows Arch. 1997; 431: 167-172Crossref PubMed Scopus (99) Google Scholar The significance of this primary role now seems to be ever increasing as several of the regulators and effectors of the β-catenin signaling pathway are being found to represent molecular pathways that have well-characterized links with colorectal carcinogenesis. The following review discusses these regulators and effectors and also abnormalities of components of the β-catenin signaling pathway that are or may potentially be involved in colorectal carcinogenesis. As a consequence, this review demonstrates that β-catenin may represent a crucial molecule linking several components of this process. The following summary (see also Figure 1) is intended to help place into context the following discussions of how regulators and components of the β-catenin pathways may relate to colorectal carcinogenesis. More detailed reviews of the β-catenin pathways are provided elsewhere.2Morin PJ β-Catenin signaling and cancer.Bioessays. 1999; 21: 1021-1030Crossref PubMed Scopus (570) Google Scholar, 3Kikuchi A Regulation of β-catenin signaling in the Wnt pathway.Biochem Biophys Res Commun. 2000; 268: 243-248Crossref PubMed Scopus (187) Google Scholar β-catenin may be regarded as existing in three different subcellular forms: membrane-bound (as part of the adherens complex), cytosolic, and nuclear (Figure 2, a and e).2Morin PJ β-Catenin signaling and cancer.Bioessays. 1999; 21: 1021-1030Crossref PubMed Scopus (570) Google Scholar, 3Kikuchi A Regulation of β-catenin signaling in the Wnt pathway.Biochem Biophys Res Commun. 2000; 268: 243-248Crossref PubMed Scopus (187) Google Scholar Binding of the protein to other members of the adherens complex, ie, E-cadherin and α-catenin, is thought to be regulated by tyrosine phosphorylation.10Müller T Choidas A Reichmann E Ullrich A Phosphorylation and free pool of β-catenin are regulated by tyrosine kinases and tyrosine phosphatases during epithelial cell migration.J Cell Biol. 1999; 274: 10173-10183Google Scholar Indeed, physical association of the complex with tyrosine kinases and phosphatases10Müller T Choidas A Reichmann E Ullrich A Phosphorylation and free pool of β-catenin are regulated by tyrosine kinases and tyrosine phosphatases during epithelial cell migration.J Cell Biol. 1999; 274: 10173-10183Google Scholar, 11Hiscox S Jiang WG Association of the HGF/SF receptor, c-met, with the cell-surface adhesion molecule, E-cadherin, and catenins in human tumor cells.Biochem Biophys Res Commun. 1999; 261: 406-411Crossref PubMed Scopus (84) Google Scholar is in keeping with tight regulation of this process. Tyrosine phosphorylation of β-catenin leads to its dissociation from the adherens complex10Müller T Choidas A Reichmann E Ullrich A Phosphorylation and free pool of β-catenin are regulated by tyrosine kinases and tyrosine phosphatases during epithelial cell migration.J Cell Biol. 1999; 274: 10173-10183Google Scholar and probable transfer of the protein to the cytosol where it exists in a soluble, monomeric state.2Morin PJ β-Catenin signaling and cancer.Bioessays. 1999; 21: 1021-1030Crossref PubMed Scopus (570) Google Scholar, 3Kikuchi A Regulation of β-catenin signaling in the Wnt pathway.Biochem Biophys Res Commun. 2000; 268: 243-248Crossref PubMed Scopus (187) Google Scholar Cytosolic β-catenin may subsequently be degraded or be translocated into the nucleus. The degradation of β-catenin involves binding of the protein to a complex involving APC protein, and two further proteins, AXIN and glycogen synthase kinase (GSK)-3β.2Morin PJ β-Catenin signaling and cancer.Bioessays. 1999; 21: 1021-1030Crossref PubMed Scopus (570) Google Scholar, 3Kikuchi A Regulation of β-catenin signaling in the Wnt pathway.Biochem Biophys Res Commun. 2000; 268: 243-248Crossref PubMed Scopus (187) Google Scholar The latter serves to phosphorylate serine and threonine residues on β-catenin, a crucial step required to target the protein for ubiquitination and proteosomal degradation. Both APC and AXIN enhance this phosphorylation and are, therefore, promoters of β-catenin degradation. For APC protein to have this promoting effect, it must bind to β-catenin via two possible binding regions, one of which contains tandem repeats of 20 amino acids.2Morin PJ β-Catenin signaling and cancer.Bioessays. 1999; 21: 1021-1030Crossref PubMed Scopus (570) Google Scholar, 12Polakis P Hart M Rubinfeld B Defects in the regulation of β-catenin in colorectal cancer.Adv Exp Med Biol. 1999; 470: 23-32Crossref PubMed Google Scholar Phosphorylation of β-catenin is important in enabling binding to the F box protein β-TrCP and hence ubiquitin-mediated proteolysis.2Morin PJ β-Catenin signaling and cancer.Bioessays. 1999; 21: 1021-1030Crossref PubMed Scopus (570) Google Scholar, 13Matsuzawa S Reed JC Siah-1, SIP and Ebi collaborate in a novel pathway for β-catenin degradation linked to p53 responses.Mol Cell. 2001; 7: 915-926Abstract Full Text Full Text PDF PubMed Scopus (376) Google Scholar However, it has recently been shown that β-catenin may also be targeted for such degradation independent of GSK-3β-mediated phosphorylation.13Matsuzawa S Reed JC Siah-1, SIP and Ebi collaborate in a novel pathway for β-catenin degradation linked to p53 responses.Mol Cell. 2001; 7: 915-926Abstract Full Text Full Text PDF PubMed Scopus (376) Google Scholar, 14Liu J Stevens J Rote CA Yost HJ Hu Y Neufeld KL White RL Matsunami N Siah-1 mediates a novel β-catenin degradation pathway linking p53 to the adenomatous polyposis coli protein.Mol Cell. 2001; 7: 927-936Abstract Full Text Full Text PDF PubMed Scopus (286) Google Scholar This putative alternative pathway requires interaction between β-catenin, APC, and a complex of proteins including the p53-inducible protein, Siah-1.13Matsuzawa S Reed JC Siah-1, SIP and Ebi collaborate in a novel pathway for β-catenin degradation linked to p53 responses.Mol Cell. 2001; 7: 915-926Abstract Full Text Full Text PDF PubMed Scopus (376) Google Scholar, 14Liu J Stevens J Rote CA Yost HJ Hu Y Neufeld KL White RL Matsunami N Siah-1 mediates a novel β-catenin degradation pathway linking p53 to the adenomatous polyposis coli protein.Mol Cell. 2001; 7: 927-936Abstract Full Text Full Text PDF PubMed Scopus (286) Google Scholar An important regulator of GSK-3β activity is the Wnt family. Binding of these glycoproteins to their trans-membrane receptor, Frz, leads to increased activity of the protein Dishevelled (Dvl) that, in turn, inhibits GSK-3β phosphorylating activity. In the presence of increased cytosolic levels of β-catenin because of, for example, Wnt signal activation, the protein is translocated into the nucleus. Here, β-catenin binds with a member of the TCF/LEF family of transcription factors to form a complex that activates transcription of target genes by binding to their promoter sequences. Of this family of transcription factors, TCF-4, LEF-1, and TCF-1 have the most known relevance to colorectal carcinogenesis, as is discussed below. It was initially thought that all TCFs bind to β-catenin to promote gene transcription. However, at least two TCFs, LEF-1 and TCF-1, are now known to exist in truncated forms that repress β-catenin/TCF transcriptional activity.15Hovanes K Li TW Munguia JE Truong T Milovanovic T Lawrence Marsh J Holcombe RF Waterman ML Beta-catenin-sensitive isoforms of lymphoid enhancer factor-1 are selectively expressed in colon cancer.Nat Genet. 2001; 28: 53-57Crossref PubMed Google Scholar, 16Roose J Huls G van Beest M Moerer P van der Horn K Goldschmeding R Logtenberg T Clevers H Synergy between tumor suppressor APC and the β-catenin-Tcf4 target Tcf1.Science. 1999; 285: 1923-1926Crossref PubMed Scopus (310) Google Scholar Little is known about the fate of nuclear β-catenin. However, recent studies do suggest the protein may be translocated from the nucleus back to the cell membrane,17Sadot E Simcha I Shtutman M Ben-Ze'ev A Geiger B Inhibition of β-catenin-mediated transactivation by cadherin derivatives.Proc Natl Acad Sci USA. 1998; 95: 15339-15344Crossref PubMed Scopus (161) Google Scholar presumably through a soluble, cytosolic intermediate state. The regulation of this retrograde movement has not been extensively studied but is likely to at least involve E-cadherin, as is discussed later. Most of the regulators of β-catenin function that are linked to colorectal carcinogenesis may be allocated to one of three groups according to their modes of action. There are, however, two molecules (PPAR-γ and MUC-1) that have roles in colorectal carcinogenesis and seem to regulate β-catenin function but have, as yet, unknown mechanisms underlying their relation to the protein; these two molecules are, therefore, considered separately. Colorectal carcinoma cells in vivo have been shown to express several growth factors, including EGF and platelet-derived growth factor, and to express receptors for these and other growth factors, for example, EGF-R, c-erbB-2, and c-met (receptor for hepatocyte growth factor).18Hiscox SE Hallett MB Puntis MC Nakamura T Jiang WG Expression of the HGF/SF receptor, c-met, and its ligand in human colorectal cancers.Cancer Invest. 1997; 15: 513-521Crossref PubMed Google Scholar, 19Nakae S Shimada E Urakawa T Study of c-erbB-2 protein and epidermal growth factor receptor expression and DNA ploidy pattern in colorectal carcinoma.J Surg Oncol. 1993; 54: 246-251Crossref PubMed Scopus (22) Google Scholar, 20Ito M Yoshida K Kyo E Ayhan A Nakayama H Yasui W Ito H Tahara E Expression of several growth factors and their receptor genes in human colon carcinomas.Virchows Arch. 1990; 59: 173-178Crossref Google Scholar Further, it is well known that EGF and hepatocyte growth factor treatment leads to transformation of colonic epithelial cells into a more malignant phenotype (eg, increased invasive qualities and higher proliferation rates).21Chakrabarty S Rajagopal S Huang S Expression of antisense epidermal growth factor receptor mRNA downmodulates the malignant behaviour of human colon cancer cells.Clin Exp Metastasis. 1995; 13: 191-195Crossref PubMed Scopus (41) Google Scholar, 22Jiang WG Lloyds D Puntis MC Nakamura T Hallett MB Regulation of spreading and growth of colon cancer cells by hepatocyte growth factor.Clin Exp Metastasis. 1993; 11: 235-242Crossref PubMed Google Scholar This change has been, at least in part, attributed to the ability of both growth factors to promote tyrosine phosphorylation of β-catenin and, hence, disruption of the adherens complex.23Hazan RB Norton L The epidermal growth factor receptor modulates the interaction of E-cadherin with the actin cytoskeleton.J Biol Chem. 1998; 273: 9078-9084Crossref PubMed Scopus (181) Google Scholar, 24Hiscox S Jiang WG Hepatocyte growth factor/scatter factor disrupts epithelial tumour cell-cell adhesion: involvement of β-catenin.Anticancer Res. 1999; 19: 509-518PubMed Google Scholar It is uncertain whether this phosphorylation is performed by the growth factor receptors themselves or via soluble tyrosine kinases. However, the growth factor receptors, c-met11Hiscox S Jiang WG Association of the HGF/SF receptor, c-met, with the cell-surface adhesion molecule, E-cadherin, and catenins in human tumor cells.Biochem Biophys Res Commun. 1999; 261: 406-411Crossref PubMed Scopus (84) Google Scholar and c-erbB-2,25Kanai Y Ochiai A Shibata T Oyama T Ushijima S Akimoto S Hirohashi S c-erbB-2 gene product directly associates with β-catenin and plakoglobin.Biochem Biophys Res Commun. 1995; 208: 1067-1072Crossref PubMed Scopus (134) Google Scholar have both been shown to bind to β-catenin in vitro. Mutations of the ras oncogenes, which lead to overexpression of their gene products, are found in at least 50% of sporadic CRCs.26Bos JL p21ras: an oncoprotein functioning in growth factor-inducing signal transduction.Eur J Cancer. 1995; 31A: 1051-1054Abstract Full Text PDF PubMed Scopus (57) Google Scholar Cytoplasmic expression of the trefoil peptide TFF-3 (previously known as ITF), which has mitogenic and prokinetic properties, is significantly higher in CRC tissue compared with normal colonic mucosa.27Taupin D Ooi K Yeomans N Giraud A Conserved expression of intestinal trefoil factor in the human colonic adenoma-carcinoma sequence.Lab Invest. 1996; 75: 25-32PubMed Google Scholar Inducing overexpression of ras oncogenes in cell lines28Kinch MS Clark GJ Der CJ Burridge K Tyrosine phosphorylation regulates the adhesions of ras-transformed breast epithelia.J Cell Biol. 1995; 130: 461-471Crossref PubMed Scopus (256) Google Scholar or treatment with TFF-329Efstathiou JA Noda M Rowan A Dixon C Chinery R Jawhari A Hattori T Wright NA Bodmer WF Pignatelli M Intestinal trefoil factor controls the expression of the adenomatous polyposis coli-catenin and the E-cadherin-catenin complexes in human colon carcinoma cells.Proc Natl Acad Sci USA. 1998; 95: 3122-3127Crossref PubMed Scopus (125) Google Scholar both have a similar effect in promoting tyrosine phosphorylation of β-catenin. From what is known of β-catenin pathways, release of β-catenin from its membrane-bound pool by tyrosine phosphorylation might be expected to increase nuclear translocation. However, in the case of TFF-3 treatment, increased tyrosine phosphorylation of β-catenin was seen without demonstrable nuclear accumulation of the protein.29Efstathiou JA Noda M Rowan A Dixon C Chinery R Jawhari A Hattori T Wright NA Bodmer WF Pignatelli M Intestinal trefoil factor controls the expression of the adenomatous polyposis coli-catenin and the E-cadherin-catenin complexes in human colon carcinoma cells.Proc Natl Acad Sci USA. 1998; 95: 3122-3127Crossref PubMed Scopus (125) Google Scholar It is possible that additional factors, eg, GSK-3β inhibition or loss of functioning APC protein (see below), are needed to transfer membrane-released β-catenin into the nucleus, although the studies that failed to show increased nuclear β-catenin with TFF-3 treatment had been performed in an APC mutated colorectal epithelial cell line.29Efstathiou JA Noda M Rowan A Dixon C Chinery R Jawhari A Hattori T Wright NA Bodmer WF Pignatelli M Intestinal trefoil factor controls the expression of the adenomatous polyposis coli-catenin and the E-cadherin-catenin complexes in human colon carcinoma cells.Proc Natl Acad Sci USA. 1998; 95: 3122-3127Crossref PubMed Scopus (125) Google Scholar Unfortunately, the aforementioned studies on growth factors, their receptors, or ras and β-catenin had not mentioned whether or not nuclear accumulation of the latter protein was seen. The growth factor EGF and ras gene product are similar in having a second effect on β-catenin regulation. Both activate the proto-oncogene Akt, whose gene product has been shown to inactivate GSK-3β and, therefore, suppress degradation of cytosolic β-catenin.30Porter AC Vaillancourt RR Tyrosine kinase receptor-activated signal transduction pathways which lead to oncogenesis.Oncogene. 1998; 16: 1343-1352Crossref Google Scholar Interestingly, this effect of ras gene product is mediated via the insulin-like growth factor receptor,30Porter AC Vaillancourt RR Tyrosine kinase receptor-activated signal transduction pathways which lead to oncogenesis.Oncogene. 1998; 16: 1343-1352Crossref Google Scholar levels of which are overexpressed in CRC.31Hakam A Yeatman TJ Lu L Mora L Marcet G Nicosia SV Karl RC Coppola D Expression of insulin-like growth factor-1 receptor in human colorectal cancer.Hum Pathol. 1999; 30: 1128-1133Abstract Full Text PDF PubMed Scopus (161) Google Scholar Two other molecules that have more recently been found to inhibit GSK-3β are the βII isoform of protein kinase C (PKC-βII)32Murray NR Davidson LA Chapkin RS Gustafson WC Schattenberg DG Fields AP Overexpression of protein kinase C βII induces colonic hyperproliferation and increased sensitivity to colon carcinogenesis.J Cell Biol. 1999; 145: 699-711Crossref PubMed Scopus (123) Google Scholar and polycystin-1.33Kim E Arnould T Sellin LK Benzing T Fan MJ Grüning W Sokol SY Drummond I Walz G The polycystic kidney disease 1 gene product modulates Wnt signaling.J Biol Chem. 1999; 274: 4947-4953Crossref PubMed Scopus (189) Google Scholar Intracellular PKC-βII levels are raised after exposure to secondary bile acids, which are thought to be carcinogenic in the large bowel,34Pongracz J Clark P Neoptolemos JP Lord JM Expression of protein kinase C isoenzymes in colorectal cancer tissue and their differential activation by different bile acids.Int J Cancer. 1995; 61: 35-39Crossref PubMed Scopus (101) Google Scholar and higher levels of the isoform are found in CRC tissue compared with normal colorectal mucosa.35Davidson LA Jiang YH Derr JN Aukema HM Lupton JR Chapkin RS Protein kinase isoforms in human and rat colonic mucosa.Arch Biochem Biophys. 1994; 312: 547-553Crossref PubMed Scopus (68) Google Scholar Murray and colleagues32Murray NR Davidson LA Chapkin RS Gustafson WC Schattenberg DG Fields AP Overexpression of protein kinase C βII induces colonic hyperproliferation and increased sensitivity to colon carcinogenesis.J Cell Biol. 1999; 145: 699-711Crossref PubMed Scopus (123) Google Scholar have recently described transgenic mice that overexpress PKC-βII and, when treated with azoxymethane, develop a greater number of aberrant crypt foci (the earliest morphologically recognizable stage in the adenoma-carcinoma sequence) than their similarly treated nontransgenic litter mates. Further, the colonic epithelium of the transgenic mice showed an expansion of the crypt proliferative compartment and increased levels of β-catenin, measured by Western blotting.32Murray NR Davidson LA Chapkin RS Gustafson WC Schattenberg DG Fields AP Overexpression of protein kinase C βII induces colonic hyperproliferation and increased sensitivity to colon carcinogenesis.J Cell Biol. 1999; 145: 699-711Crossref PubMed Scopus (123) Google Scholar Polycystin-1 is the protein product of the gene whose mutation underlies type I adult polycystic kidney disease.36International Polycystic Kidney Disease Consortium Polycystic kidney disease: the complete structure of the PKD1 gene and its protein.Cell. 1995; 81: 289-298Abstract Full Text PDF PubMed Scopus (0) Google Scholar In vitro studies have shown that polycystin-1 inhibits GSK-3β,33Kim E Arnould T Sellin LK Benzing T Fan MJ Grüning W Sokol SY Drummond I Walz G The polycystic kidney disease 1 gene product modulates Wnt signaling.J Biol Chem. 1999; 274: 4947-4953Crossref PubMed Scopus (189) Google Scholar forms a complex with β-catenin,37Huan Y van Adelsberg J Polycystin-1, the PKD1 gene product, is in a complex containing E-cadherin and the catenins.J Clin Invest. 1999; 104: 1459-1468Crossref PubMed Google Scholar and also increases expression of target genes of the β-catenin/TCF complex.33Kim E Arnould T Sellin LK Benzing T Fan MJ Grüning W Sokol SY Drummond I Walz G The polycystic kidney disease 1 gene product modulates Wnt signaling.J Biol Chem. 1999; 274: 4947-4953Crossref PubMed Scopus (189) Google Scholar In contrast to adult polycystic kidney disease, there are, as yet, no known human hereditary diseases that are characterized by overexpression of polycystin-1 and that could be studied for an increased risk of CRC. The existence of antibodies to the protein37Huan Y van Adelsberg J Polycystin-1, the PKD1 gene product, is in a complex containing E-cadherin and the catenins.J Clin Invest. 1999; 104: 1459-1468Crossref PubMed Google Scholar will, however, permit an informative study of whether the protein is overexpressed in CRCs. Molecular pathways that interfere with β-catenin/TCF activity in the nucleus would be predicted to have an anti-oncogenic effect. Although TGF-β may have opposing effects in different cell types,38Alevizopoulos A Mermod N Transforming growth factor-beta: the breaking open of a black box.Bioessays. 1997; 19: 581-591Crossref PubMed Google Scholar its main effects on colonic epithelial cells are to reduce proliferation and to induce differentiation.39Chakrabarty S Fan D Varani J Modulation of differentiation and proliferation of human colon carcinoma cells by transforming growth factor beta 1 and beta 2.Int J Cancer. 1990; 46: 493-499Crossref PubMed Scopus (39) Google Scholar One of the effector pathways of TGF-β involves TAK-1, which is a member of the MAP-kinase 3 family38Alevizopoulos A Mermod N Transforming growth factor-beta: the breaking open of a black box.Bioessays. 1997; 19: 581-591Crossref PubMed Google Scholar, 40Yamaguchi K Identification of a member of the MAPKKK family as a potential mediator of TGFβ signal transduction.Science. 1995; 270: 2008-2011Crossref PubMed Google Scholar and which promotes the activity of a second kinase, NLK.41Ishitani T Ninomiya-Tsuji J Nagai S Nishita M Meneghini M Barker N Waterman M Bowerman B Clevers H Shibuya H Matsumoto K The TAK1-NLK-MAPK-related pathway antagonizes signaling between β-catenin and transcription factor TCF.Nature. 1999; 399: 798-802Crossref PubMed Scopus (378) Google Scholar NLK has been shown, in embryonic kidney cells, to co-localize with and to phosphorylate TCF-4,41Ishitani T Ninomiya-Tsuji J Nagai S Nishita M Meneghini M Barker N Waterman M Bowerman B Clevers H Shibuya H Matsumoto K The TAK1-NLK-MAPK-related pathway antagonizes signaling between β-catenin and transcription factor TCF.Nature. 1999; 399: 798-802Crossref PubMed Scopus (378) Google Scholar which is the only TCF isoform consistently found in normal colorectal epithelium and every CRC cell line examined so far.2Morin PJ β-Catenin signaling and cancer.Bioessays. 1999; 21: 1021-1030Crossref PubMed Scopus (570) Google Scholar Phosphorylation of TCF-4 was subsequently associated with impaired binding of the β-catenin/TCF-4 complex to DNA.41Ishitani T Ninomiya-Tsuji J Nagai S Nishita M Meneghini M Barker N Waterman M Bowerman B Clevers H Shibuya H Matsumoto K The TAK1-NLK-MAPK-related pathway antagonizes signaling between β-catenin and transcription factor TCF.Nature. 1999; 399: 798-802Crossref PubMed Scopus (378) Google Scholar In sharp contrast, smad4, another effector of TGF-β, has been shown to form a complex with β-catenin and LEF-1 and to increase β-catenin/LEF-1 transcriptional activity in a hepatoma cell line.42Nishita M Hiashimoto M Ogata S Laurent MN Ueno N Shibuya H Cho KWY Interaction between Wnt and TGFβ signaling pathways during formation of Spemann's organizer.Nature. 2000; 403: 781-784Crossref PubMed Scopus (301) Google Scholar These dichotomous effects of TGF-β on the β-catenin signaling pathway may explain, at least in part, its differing effects in different cell types. We are aware of only one previous study of the effects of TGF-β treatment on β-catenin expression in a colorectal cell line. Ilyas and colleagues43Ilyas M Efstathiou JA Straub J Kim HC Bodmer WF Transforming growth factor β stimulation of colorectal cancer cell lines: type II receptor bypass and changes in adhesion molecule expression.Proc Natl Acad Sci USA. 1999; 96: 3087-3091Crossref PubMed Scopus (34) Google Scholar reported an increase in β-catenin protein levels in the HCA46 cell line that also showed growth inhibition. Unfortunately, as β-catenin had only been stu