Tissue-Specific Effects of Reduced β-catenin Expression on Adenomatous Polyposis Coli Mutation-Instigated Tumorigenesis in Mouse Colon and Ovarian Epithelium

Adenomatous polyposis coli (APC) inactivating mutations are present in most human colorectal cancers and some other cancers. The APC protein regulates the β-catenin protein pool that functions as a co-activator of T cell factor (TCF)-regulated transcription in Wnt pathway signaling. We studied effects of reduced dosage of the Ctnnb1 gene encoding β-catenin in Apc-mutation-induced colon and ovarian mouse tumorigenesis and cell culture models. Concurrent somatic inactivation of one Ctnnb1 allele, dramatically inhibited Apc mutation-induced colon polyposis and greatly extended Apc-mutant mouse survival. Ctnnb1 hemizygous dose markedly inhibited increases in β-catenin levels in the cytoplasm and nucleus following Apc inactivation in colon epithelium, with attenuated expression of key β-catenin/TCF-regulated target genes, including those encoding the EphB2/B3 receptors, the stem cell marker Lgr5, and Myc, leading to maintenance of crypt compartmentalization and restriction of stem and proliferating cells to the crypt base. A critical threshold for β-catenin levels in TCF-regulated transcription was uncovered for Apc mutation-induced effects in colon epithelium, along with evidence of a feed-forward role for β-catenin in Ctnnb1 gene expression and CTNNB1 transcription. The active β-catenin protein pool was highly sensitive to CTNNB1 transcript levels in colon cancer cells. In mouse ovarian endometrioid adenocarcinomas (OEAs) arising from Apc- and Pten-inactivation, while Ctnnb1 hemizygous dose affected β-catenin levels and some β-catenin/TCF target genes, Myc induction was retained and OEAs arose in a fashion akin to that seen with intact Ctnnb1 gene dose. Our findings indicate Ctnnb1 gene dose exerts tissue-specific differences in Apc mutation-instigated tumorigenesis. Differential expression of selected β-catenin/TCF-regulated genes, such as Myc, likely underlies context-dependent effects of Ctnnb1 gene dosage in tumorigenesis.


Introduction
Colorectal cancers (CRCs) harbor accumulated mutations in tumor suppressor genes and oncogenes along with epigenetic alterations. Many CRCs arise from precursor lesions, such as adenomatous polyps or serrated epithelial lesions with dysplasia. Inactivating mutations in the APC (adenomatous polyposis coli) and TP53 tumor suppressor genes are found in roughly 80% and 60% of CRCs, respectively [1]. Oncogenic mutations activating the functions of the KRAS and PI3KCA (phosphoinositide-3-kinase, catalytic, alpha polypeptide) proteins are found in about 40% and 20% of CRCs, respectively [1]. Constitutional mutations inactivating one APC allele underlie the familial adenomatous polyposis (FAP) syndrome, where affected individuals often develop hundreds to thousands of colon adenomas during their second or third decades of life. The wild type APC allele is somatically inactivated in adenomas arising in those with FAP [1,2]. Mice carrying certain heterozygous, constitutional mutations inactivating Apc, such as the Apc Min mutation, may develop 50-100 small intestinal tumors and occasional colon tumors by 140 days of age and nearly all of the tumors are adenomas. Similar to the situation in FAP tumors, intestinal tumors in Apc Min mice show somatic inactivation of the wild type Apc allele [3].
The best understood function of the roughly 300 kD APC protein is regulation of the pool of β-catenin protein that functions in the canonical (β-catenin-dependent) Wnt signaling pathway [4][5][6]. In the absence of an activating Wnt ligand signal, the β-catenin destruction complex-comprised by the APC, AXIN, casein kinase I, and glycogen synthase kinase-3β factors and other proteins-promotes phosphorylation of conserved serine/threonine residues in the β-catenin amino (N)-terminal region. The N-terminally phosphorylated β-catenin can then be β ubiquitinated and degraded by the proteasome. Activating Wnt ligands inhibit degradation of the "free" or Wnt signaling pool of β-catenin via binding at the cell surface to the frizzled and LRP5/6 (low density lipoprotein-related proteins 5 and 6) cognate receptor complex, resulting in inhibition of β-catenin phosphorylation and/or ubiquitination by the destruction complex [4,6]. In colon adenomas and CRCs where both APC alíeles are defective, destruction of the free pool of β-catenin is impaired and active β-catenin accumulates in the cytoplasm and nucleus, where it can complex with DNA binding proteins of the TCF (T-cell factor family)/ Lef (lymphoid enhancer family) family. β-catenin functions as a transcriptional co-activator for TCFs [7]. Normally, β-catenin/TCF transcriptional activation is restricted to the crypt base, especially in the so-called crypt base columnar stem cells characterized by expression of the Wnt-regulated Lgr5 presumptive stem cell marker protein [8]. Constitutive activation of βcatenin/TCF transcription in Wnt pathway-defective adenomas and CRCs may promote a stem or progenitor cell phenotype in epithelial cells independent of cell position in the crypt [9,10]. Activation of β-catenin/TCF-dependent transcription also alters crypt compartmentalization and coordinated migration of cells, apparently through increased expression of the EphB2 and EphB3 receptors and via inhibition of the expression of their ligands ephrin B1 and B2 [11,12]. The MYC gene has been highlighted as a potentially key target gene regulated by β-catenin/TCF in CRCs. Genes encoding negative-feedback inhibitor proteins functioning in the Wnt/β-catenin/TCF pathway, such as AXIN2, DKK1, and NKD1, are also activated by β-catenin/TCF (see http://www.stanford.edu/~rnusse/pathways/targets.html for a list of candidates). In APC-mutant neoplastic cells, the ability of these induced regulator proteins to inhibit the Wnt signaling pathway is abrogated because the factors function upstream of or at the level of the APC protein in the pathway [13].
Besides these findings, other evidence indicates that APC inactivation may promote cancer development through β-catenin dysregulation. For instance, while most CRCs harbor APC mutations, a subset of CRCs and other cancers lacking APC mutations have CTNNB1 gene mutations resulting in production of oncogenic β-catenin proteins that are resistant to regulation by the destruction complex and that activate β-catenin/TCF transcription [6,13]. Also, some prior studies have used genetic approaches to study effects of Ctnnb1 gene dosage on liver, small intestine, and mammary gland tumor phenotypes in mouse models as well as effects of Ctnnb1 hemizygous inactivation state (Ctnnb1 +/-) in Apc-mutation induced mouse embryonic development phenotypes [14,15]. The prior studies indicated the Ctnnb1 +/constitutional state can inhibit intestinal and liver tumorigenesis in mice carrying mutations in the Apc gene (Apc 1638N , Apc Min , or Apc fl ) [14,15]. In contrast, mammary gland tumorigenesis was enhanced in Apc 1638N Ctnnb1 +/mice, perhaps because Ctnnb1 functions as a tumor suppressor gene in the mammary gland tumors via β-catenin's role in E-cadherin-dependent tumor suppression [14]. Nonetheless, while the prior studies yielded evidence that β-catenin signaling dosage impacts Apc mutation-induced tumorigenesis in some tissues, the prior work did not assess the role of Ctnnb1 dosage in Apc mutation-induced colon tumorigenesis, the chief site of APC mutation-dependent tumorigenesis in humans. Moreover, the work used mice constitutionally deficient in β-catenin, not just in Apc-mutant epithelial cells, and the findings did not highlight specific factors and mechanisms that might account for effects of Ctnnb1 dosage in Apc mutation-instigated tumorigenesis in different contexts. We report here on studies of the effects of Ctnnb1 gene dosage on β-catenin protein expression and β-catenin/TCF transcription in Apc mutation-induced colon and ovarian mouse tumors and cell culture models. We provide evidence that Apc mutation-induced tumorigenesis in the colon is inhibited by Ctnnb1 hemizygous gene status through marked effects on the free pool of β-catenin in the cytoplasm and nucleus and its ability to activate key β-catenin/TCF-regulated target genes, including those encoding key stem factors, such as Lgr5, and regulators of crypt compartmentalization, such as the EphB2/B3 receptors. We also uncovered a novel feed-forward mechanism where β-catenin protein stabilization and β-catenin/TCF transcription appear critical in regulating Ctnnb1/ CTNNB1 transcription in the setting of Apc inactivation in mouse colon and human colon cancer cells. Moreover, we found that differences in the ability to activate Myc expression may underlie colon versus ovary tissue-specific differences in Apc mutation-instigated tumorigenesis in the setting of Ctnnb1 hemizygous gene dosage.

Results
Inactivation of a Ctnnb1 allele extends survival and inhibits adenomatous polyposis and epithelial abnormalities induced by somatic bi-allelic Apc inactivation in mouse colon We previously described CDX2P-G22Cre transgenic mice, in which human CDX2 regulatory sequences and an out-of-frame Cre transgene allele, carrying a 22-basepair guanine nucleotide repeat tract affecting the Cre open reading frame, manifest mosaic Cre recombinase expression in caudal embryonic tissues and in epithelium of the distal ileum, cecum, colon, and rectum during adult life [16]. We also previously described CDX2P-CreER T2 transgenic mice that express a tamoxifen (TAM)-regulated Cre protein (CreER T2 ) under control of human CDX2 regulatory sequences, allowing for TAM-inducible targeting of loxP-containing alleles in adult terminal ileum, cecum, colon, and rectal epithelium [17]. Using the CDX2P-G22Cre or CDX2P-CreER T2 transgenic mice, we have described the phenotypic consequences in colon epithelium of somatic, bi-allelic, inactivating mutations in Apc [16,17]. Consistent with our prior studies, we found CDX2P-G22Cre Apc fl/fl mice lived only for 8-20 days after birth (median survival = 13 d; Fig 1A). After three daily doses of TAM to inactivate both Apc alleles in distal intestinal epithelial tissues, CDX2P-CreER T2 Apc fl/fl adult mice lived on average for 22 days (Fig 1B). In marked contrast, concurrent somatic inactivation of one Ctnnb1 allele along with both Apc alleles, using either the CDX2P-G22Cre or CDX2P-CreER T2 transgene for somatic gene targeting, led to a dramatically increased life span relative to that seen in mice with Apc bi-allelic targeting, with median survival of 168 d of age in CDX2P-G22Cre Apc fl/fl Ctnnb1 fl/+ mice and for 134 d after TAM treatment in the CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice (Fig 1A and 1B).
Consistent with our prior reports [16,17], the proximal colon and cecum of both CDX2P-G22Cre Apc fl/fl mice (when moribund at 8-20 d of age) and CDX2P-CreER T2 Apc fl/fl mice (only 20 days after TAM induction) were dramatically thickened and many polypoid lesions were seen (S1 Fig). Histological analysis of proximal colon epithelial tissues from these mice showed significant hyperplastic and dysplastic (adenomatous) changes along with frequent crypt fission and branching ( Fig 1C). The dramatic polyposis in cecum and colon seen following bi-allelic Apc inactivation was significantly inhibited at both early and later time points by concurrent inactivation of one Ctnnb1 allele, with no grossly discernable epithelial phenotype seen in the proximal colon and only two to four polyps in the cecum per mouse as the CDX2P-G22Cre Apc fl/fl Ctnnb1 fl/+ and CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice were aged (S1 Fig). The cecal polyps arising in CDX2P-G22Cre Apc fl/fl Ctnnb1 fl/+ and CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice may contribute to their premature mortality relative to control mice, as no other grossly detectable intestinal lesions or pathology were noted in the mice. The Cremediated somatic inactivation of both Apc alleles and one Ctnnb1 allele in proximal colon epithelium was confirmed by genotyping. The rare cecal adenomas arising in CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice were found to have significant fractions of cells that escaped Cre-mediated Ctnnb1 targeting, even though Cre-mediated somatic inactivation of both Apc alleles occurred to the same extent in the rare adenomas and proximal colon mucosa (S1 Fig). Compared to the situation in CDX2P-CreER T2 Apc fl/fl mice, microscopic examination of proximal colon tissues of CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice revealed modest hyperplastic changes and minimal crypt branching (Fig 1C). Our efforts to inactivate both Ctnnb1 alleles in colon epithelium via either CDX2P-G22Cre-or CDXP-CreER T2 -mediated targeting with or without Apc inactivation indicated that colon epithelial cells completely lacking β-catenin expression and function could not be generated. This likely reflects a required role for β-catenin function in colon epithelium, perhaps not limited to Wnt signaling, but also in cadherin-mediated adhesion, centrosome assembly or other functions. Apc mutation-induced polyposis in mouse cecum and colon epithelium is dramatically inhibited by concurrent inactivation of one Ctnnb1 allele. (A) The survival of CDX2P-G22Cre Apc fl/fl Ctnnb1 fl/+ mice (blue; n = 24) is contrasted with that of CDX2P-G22Cre Apc fl/fl mice (red; n = 9). P <0.0001 by log-rank (Mantel-Cox) test. (B) The survival of CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice (blue; n = 12) is contrasted with that in CDX2P-CreER T2 Apc fl/fl mice (red; n = 14), where time zero represents the third day of three days when TAM was administered daily. P <0.0001 by log-rank (Mantel-Cox) test. (C) Hematoxylin and eosin (H&E) staining of proximal colon tissues from a CDX2P-CreER T2 Apc fl/fl mouse 20 days after the third of three daily doses of TAM (left, lower), and a control (non-gene-targeted) mouse (left, upper). Right panels show H&E staining of proximal colon tissues from CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice 20 days (right, upper) or 2 months (right, lower) after TAM dosing. Scale bars, 50 μm. Immunohistological analysis of colon sections from CDX2P-CreER T2 Apc fl/fl mice showed strong cytoplasmic and nuclear β-catenin expression in many epithelial cells, compared to the nearly uniform membrane β-catenin staining in colon epithelium of wild-type mice (Fig 2A). In CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice, we observed infrequent cells with elevated cytoplasmic and/or nuclear β-catenin expression (Fig 2A). Paneth cells, a specialized secretory cell linage that expresses lysozyme and other markers, are found at the crypt base in normal mouse small intestinal epithelium, but are absent in normal mouse colon epithelium. Paneth cells have been proposed to have a key role in generation and/or maintenance of the intestinal crypt stem cell niche [18]. Bi-allelic Apc inactivation has been associated with the generation of many ectopic lysozyme-expressing Paneth-like cells throughout the crypts of small intestine and colon [12,17,19,20]. We confirmed this finding in CDX2P-CreER T2 Apc fl/fl mice (Fig 2A). Whereas no Paneth-like cells were seen in normal mouse colon, modest numbers of lysozymeexpressing cells were seen in the colons of CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice (Fig 2A). We also used a transgenic mouse line carrying a Cre-activated enhanced yellow fluorescence protein (EYFP) reporter gene at the ubiquitously expressed Rosa26 locus to monitor colon epithelial cells and glands where Cre-mediated targeting had occurred. Ectopic lysozyme-expressing cells were found in nearly all of the EYFP-positive crypts in CDX2P-G22Cre Apc fl/fl Ctnnb1 fl/+ and CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice (Fig 2B and 2C). The rare occurrence of Paneth-like cells in crypts without EYFP expression likely reflects the possibility that Cre may more efficiently target the loxP sites at the Apc locus than at the Rosa26 locus.
Prior studies from other groups and ours have shown that Apc bi-allelic inactivation increases both cell proliferation and apoptosis in intestine and colon epithelium [12,17,19,21]. Following TAM-induced Apc bi-allelic inactivation in proximal colon epithelium, we confirmed significantly elongated crypts and increased cell proliferation and apoptosis relative to control epithelial tissues (Fig 2D and 2E). In contrast, only modestly increased crypt height, cell proliferation and apoptosis relative to control epithelium were seen in epithelium of CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice following combined Apc and Ctnnb1 gene inactivation (Fig 2D and 2E). Furthermore, although bi-allelic Apc inactivation induced extensive cell proliferation in the upper half of targeted colon crypts, cell proliferation following gene targeting in CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice was largely restricted to the bottom half of each crypt, with only a slight increase in cell proliferation compared to control colon epithelium (Fig 2D and 2E). The cell proliferation and apoptosis results were well correlated with the immunohistochemical studies of β-catenin levels and localization (Fig 2A), suggesting differences in the strength of β-catenin-dependent Wnt signaling in cells with bi-allelic Apc defects underlie the observed effects on colon epithelial morphology, cell fate and differentiation, and cell proliferation and apoptosis.
The orientation of the mitotic spindle axis may impact on cell fate decisions in intestinal epithelium. At cytokinesis, the orientation of the spindle axis in a planar fashion (i.e., parallel to the crypt axis) is thought to generate two daughter cells with equivalent luminal (apical) and basement (extracellular matrix) surfaces. If the spindle axis is not oriented parallel to the crypt axis, cytokinesis generates daughter cells with differences in luminal and basement membrane surfaces and the potential for resultant differences in the fates adopted by the two daughter cells. We previously reported significant increases in the percentage of epithelial cells where the mitotic spindle axis was oriented orthogonal to the planar axis in Apc-mutant mouse colon crypts relative to wild type crypts [17]. Consistent with our prior results, in colon epithelium of CDX2P-CreER T2 Apc fl/fl mice treated with TAM to inactivate both Apc alleles, roughly 50% of the cells in mitosis had mitotic spindle axes !30°degrees out of the planar axis, with nearly 20% showing spindle axes between 60°and 90°out of planar alignment. In contrast, in epithelium of CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice and control mice, >75% of mitotic colon epithelial cells had their mitotic spindles aligned within 30°of the planar (crypt) axis (S2 Fig).
The findings indicate β-catenin levels have a key role in the altered mitotic spindle axis phenotype of Apc-mutant colon epithelium.
Ctnnb1 inactivation inhibits Apc mutation-induced colon tumorigenesis via maintenance of EphB/ephrinB signaling and restriction of presumptive stem cells to the crypt base As described above, bi-allelic Apc inactivation acutely induces hyperproliferation and dysplastic alterations in mouse proximal colon epithelium, with the altered epithelium arising in part from expansion of the crypt progenitor compartment at the expense of the differentiated compartment, along with frequent crypt fission/branching [12,17,21]. The EphB/ephrinB signaling axis has been implicated in control of intestinal epithelial cell compartmentalization along the crypt axis and in cell migration [11,22]. The EphB2 and EphB3 receptors are two key effectors of compartmentalization and cell migration in the crypt, and EphB2 and EphB3 are each encoded by a gene activated in intestinal tissues by β-catenin/Tcf transcription. The EphB2/B3 receptor ligands, ephrinB1 and ephrinB2, show highest expression levels in differentiated cells at the crypt surface, and expression of ephrins B1 and B2 is negatively regulated by β-catenin/ Tcf activity [11,23]. Of note, EphB-ephrinB interactions generate repulsive forces that separate and compartmentalize the EphB-and ephrinB-expressing cells to maintain crypt architecture [11,23]. In normal mouse colon epithelium, the EphB2 and EphB3 receptors were expressed only in progenitor cells at the crypt base (Fig 3A and 3B). We found bi-allelic Apc inactivation in colon epithelium not only increased EphB2 and EphB3 expression, but also perturbed the gradient of EphB2 and B3 receptor expression along the crypt axis, with EphB2/B3 expression seen even at the crypt surface in Apc-mutant crypts (Fig 3A and 3B). In the case of ephrin ligand expression, our studies demonstrated strong expression of ephrinB1 and B2 in normal colon surface epithelial cells and normal colon crypt cells other than the crypt base. The normal pattern of ephrinB1/B2 expression remained largely unaffected in Apc-mutant crypts with one Ctnnb1 allele inactivated ( Fig 3C). In contrast, in Apc-mutant crypts where Ctnnb1 dosage was intact, ephrinB1/B2 expression was markedly down-regulated in colon surface epithelial cells and throughout the crypt (Fig 3C). Our findings are consistent with those in a prior study that showed increased expression of EphB2/B3 and loss of ephrinB1/B2 expression in colon adenomas of Apc min/+ mice [23]. Although expression of EphB2/B3 was moderately elevated in some colon epithelial cells of CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice compared to control mice, Inactivation of a Ctnnb1 allele in mouse colon epithelium reduces apoptosis and hyperproliferation induced by bi-allelic Apc inactivation, and also reduces, but does not abrogate, the aberrant cell differentiation associated with Apc inactivation. (A) Representative immunohistochemical staining for β-catenin (left panels) and lysozyme (right panels) in a CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mouse (middle row) or a Cdx2P-CreER T2 Apc fl/fl mouse (bottom row) 20 days following three daily doses of TAM. Immunohistochemical staining is also shown for normal control colon mucosa from a Cre negative control littermate mouse (top row). Scale bars, 20 μm. (B and C) EYFP expression activated in targeted colon crypts by Cre-mediated recombination was imaged along with Immunofluorescence staining for lysozyme (lyso; red) in proximal colon tissues that were counter-stained with Hoechst 33342 for detection of nuclei. A 6-month old G22Cre Apc fl/fl Ctnnb1 fl/+ EYFP fl/+ mouse and a Cre negative control littermate are shown in panel B. Colon tissues from a CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ EYFP fl/+ mouse (panel C, middle) or a CDX2P-CreER T2 Apc fl/fl EYFP fl/+ mouse (panel C, bottom) were analyzed 38 days after the third of three daily doses of TAM. Colon tissues from a CDX2P-CreER T2 EYFP fl/+ mouse (panel C, top left) and a Cre-negative EYFP reporter mouse (panel C, top right) served as controls. Arrowheads indicate the EYFP-negative crypts that expressed the Paneth cell maker, lysozyme. An arrow in the middle panel indicates a crypt showing EYFP expression but no lysozyme staining. Scale bars, 50 μm. (D) TUNEL assay (middle column panels) and immunohistochemical staining for lysozyme (left column panels) and BrdU (right column panels) in proximal colon tissues of a CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mouse (middle row) or a Cdx2P-CreER T2 Apc fl/fl mouse (bottom row) 20 days after three daily doses of TAM. The normal colon mucosa from a cre negative mouse served as control (top row). Arrows indicate apoptotic cells in tissues from a CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mouse and the control mouse; arrowheads indicate the lysozyme-positive cells in tissues from a CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mouse. Scale bars, 25 μm. (E) Apoptotic cells per crypt in proximal colon, based on the TUNEL assay, were quantified (panel E, top; n = 3 and >100 crypts counted; *P < .001 compared to the control mouse). Proliferating cells per crypt, as assessed by BrdU incorporation, was quantified (panel E, bottom; n = 3 and >110 crypts counted; *P < .001 compared to the control mouse).
doi:10.1371/journal.pgen.1005638.g002 β-catenin Function in Apc-Mutant Tumors elevated EphB2/B3 expression remained restricted to the crypt base region, rather than spreading throughout the crypt as was seen in Apc-mutant crypts with intact Ctnnb1 gene dosage ( Fig 3A and 3B). This observation suggests the reduced β-catenin levels in CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice leads to a failure to induce enough β-catenin/TCF-regulated EphB2/B3 expression to overcome the repulsive effects of the retained expression of ephrinB1/B2 ligands in Apc-mutant crypts with reduced Ctnnb1 dosage.
A similar expression pattern to that seen for EphB2 and EphB3 was also found for Sox9, a transcription factor encoded by a β-catenin/Tcf target gene. Sox9 expression is restricted to stem/progenitor cells at the normal colon crypt base (S3 Fig). Sox9 expression was only modestly increased and expanded following gene targeting in crypts of CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice relative to the marked changes in Sox9 levels and the number of Sox9-expressing cells in crypts from CDX2P-CreER T2 Apc fl/fl mice (S3 Fig). In spite of the reduced increase in β-catenin levels in colon crypts of CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice relative to CDX2P-CreER T2 Apc fl/fl mice, the resultant signaling was still sufficient to generate some ectopic Paneth-like cells (Fig 3A and 3B). In addition, the modest increase in β-catenin levels in targeted crypts of CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice was sufficient to induce expression in targeted crypts of a β-galactosidase reporter gene integrated into the β-catenin/TCFregulated Axin2 locus. However, β-galactosidase expression was reduced in crypts and few if any colon surface epithelial cells expressed β-galactosidase in Apc-mutant epithelium with In contrast, uniformly strong β-galactosidase expression was seen throughout Apc-mutant colon crypts and surface epithelial cells with intact Ctnnb1 dosage (S3 Fig). Taken together, the findings indicate distinct β-catenin/Tcf target genes in colon epithelium display differing transcriptional responses to β-catenin levels, with Axin2 perhaps representing a target gene capable of being activated by modest to moderate levels of βcatenin in colon epithelium. The Sox9, EphB2, and EphB3 genes appear dependent on higher levels of β-catenin for transcriptional activation in colon epithelium.
To address mechanisms underlying suppression of crypt fission and branching in Apc-deficient colon epithelium when one Ctnnb1 allele was inactivated, we compared expression of presumptive stem cell markers in Apc-deficient colon crypts where both Ctnnb1 alleles were intact or where only one Ctnnb1 allele was active. Consistent with our prior work [17], 20 days after TAM-induced bi-allelic Apc inactivation, we detected strong induction of enhanced green fluorescent protein (EGFP) expressed from the Lgr5 locus (Lgr5-EGFP) ( Fig 4A) in Apc-deficient colon epithelium generated by CDX2P-CreER T2 targeting. Lgr5 is a β-catenin/TCF-regulated gene and a marker of presumptive crypt base columnar stem cells in normal colon, and the Lgr5 allele that we used has a EGFP open reading frame integrated in the locus to allow for monitoring of endogenous Lgr5 expression [8]. In Apc-mutant epithelium, we also confirmed strong induction of the Msi1 RNA-binding protein (Fig 4B), another presumptive intestinal stem cell marker [24,25]. In contrast to a prior study where it was reported that Lgr5-expressing cells were only expanded at the lower part of the crypts in colon epithelium following mutant β-catenin induction [21], we detected EYFP-positive and Msi-positive cells essentially throughout the Apc-mutant dysplastic colon crypts when both Ctnnb1 alleles were active, though expression of EYFP was more prominent near the crypt base region, including in the de novo crypts. While the net number of EYFP-and Msi1-expressing cells per crypt were slightly increased (e.g. from 3-4 to 5-8 Lgr5-positive cells per crypt) in colon epithelium of TAMtreated CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice compared to the control mice, paralleling the subtle increase in crypt fission/budding seen, the expanded population of EYFP-positive cells remained restricted to the crypt base region (Fig 4A), consistent with the EphB2 and EphB3 data described above. EYFP expression patterns in colon similar to those seen in TAM-treated CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice were also obtained when we used TAM-treatment to activate the Lgr5-driven CreER T2 transgene to target Apc and Ctnnb1 alleles and EYFP expression was used to mark Lgr5-expressing cells (Fig 4A).
Consistent with our studies of Lgr5 and Msi expression patterns in colon epithelium, the levels of transcripts encoding presumptive stem cell markers, including Lgr5, CD44, Msi1, and Hopx, were also found to increase dramatically in the colon tissues of CDX2P-CreER T2 Apc fl/fl mice (S4 Fig). The induction of genes encoding stem cell markers and other selective β-catenin/Tcf target genes (such as Axin2, Nkd1, Ccnd1 and Irs1) observed in Apc-deficient colon epithelium was significantly suppressed in colon epithelium from CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice (S4 Fig). Taken together, our data indicate that the robust induction of many β-catenin/Tcf-regulated genes that is seen response to Apc inactivation was variably inhibited in reduced Ctnnb1 gene dosage and β-catenin protein levels in mouse colon epithelium. In the setting of inactivation of one Ctnnb1 allele, the inability of Apc inactivation to substantially activate certain key β-catenin/TCF-regulated genes with functions in colon crypt compartmentalization and cell migration (e.g., EphB2 and EphB3) or stem cell fate (e.g., Lgr5 and Msi) is likely to underlie the dramatic abrogation of adenoma formation in CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice.
Ctnnb1 gene dosage effects on Myc and Ctnnb1 gene expression in Apc-mutant mouse colon tissues and an apparent feed-forward mechanism for β-catenin/TCF in regulating Ctnnb1 transcription Myc is a well-known β-catenin/TCF-regulated target gene [10,26], and we found that the strong induction of Myc gene expression in mouse colon epithelium seen following Apc bi-allelic inactivation was abrogated when Apc bi-allelic inactivation occurred concurrently with somatic inactivation of one Ctnnb1 allele ( Fig 5A). Another interesting observation was that Ctnnb1 transcripts were increased roughly 2-fold in proximal colon tissues following Apc bi-allelic inactivation in colon epithelium with wild type Ctnnb1 gene dosage, compared to the levels of Ctnnb1 transcript in untargeted colon tissues of Apc fl/fl mice (Fig 5B). Hemizygous Ctnnb1 gene dosage was associated with an inability of Apc bi-allelic inactivation to activate Ctnnb1 transcript levels in proximal colon tissues (Fig 5B). The effects of Apc inactivation and Ctnnb1 gene dosage in mouse colon epithelium on Myc and Ctnnb1 transcript levels did not appear to simply reflect a change in the epithelial cell numbers in Apc-mutant colon epithelium, because transcripts for the epithelial markers epithelial cell adhesion molecule (Epcam) and E-cadherin (Cdh1) were similar in the mouse colon tissues independent of genotype ( Fig 5C and 5D). The findings indicate that not only does Apc inactivation lead to increased β-catenin protein levels in murine colon tissues, but Ctnnb1 transcript levels in the colon tissues are also increased by Apc inactivation, consistent with an apparent feed-forward mechanism for up-regulation of Ctnnb1 transcripts following Apc inactivation. Because the Apc mutation-dependent induction of Ctnnb1 transcripts in mouse colon epithelium was not seen in the setting of reduced Ctnnb1 gene dosage, the findings imply that the feed-forward mechanism for Ctnnb1 induction may require sufficient levels of β-catenin and β-catenin/TCF-dependent transcription.
Of interest with regard to a role for β-catenin and β-catenin/TCF transcription in regulating Ctnnb1 transcription is that chromatin immunoprecipitation (ChIP) studies from the ENCODE project indicate that the TCF4 protein, encoded by the TCF7L2 gene, is bound in the proximal promoter and exon 1 region of the CTNNB1 gene in selected cell lines. The mouse Ctnnb1 and human CTNNB1 promoter regions lack known TCF family protein consensus binding elements. Nonetheless, to further explore the role of CTNNB1 transcript and β-catenin levels in regulating CTNNB1 transcription, we generated a reporter gene construct in which a 555 bp fragment of human CTNNB1 upstream and exon 1 sequences (-336 to +219 relative to the transcription start site) were cloned upstream of a firefly luciferase sequence (Fig 6A). This region of the CTNNB1 gene corresponds to the region that the ENCODE ChIP data indicates is occupied by the TCF family member TCF4 in some cell lines. We used two different doxycycline-induced shRNAs against CTNNB1 to study effects of antagonizing CTNNB1 endogenous transcript levels in the DLD1 human colon cancer cell line, which harbor APC defects leading to constitutive activation of β-catenin/TCF signaling (Fig 6B). The doxycycline-mediated shRNA-mediated inhibition of CTNNB1 transcript levels in DLD1 cells also led to marked inhibition of MYC expression (Fig 6B). In addition, we found CTNNB1 reporter luciferase activity was inhibited by about 30-40% by the reduction in CTNNB1 transcripts in the cells, whereas, the prototypical Wnt/β-catenin/TCF reporter gene construct TOPflash was more robustly inhibited by the reduction in CTNNB1 levels (Fig 6C). These studies and data complement the primary mouse colon tissue findings presented above by showing that reduction of CTNNB1 levels in colon cancer cells has a demonstrable effect on CTNNB1 transcriptional activity.
The active pool of β-catenin protein and β-catenin-regulated target gene transcription are highly sensitive to changes in β-catenin transcript levels in colon epithelial cells Canonical (β-catenin-dependent) Wnt signaling is dependent on increases in the levels and localization of a hypo-or un-phosphorylated pool of β-catenin, which is often termed the "free" or "active" pool of β-catenin in the cytoplasm and nucleus of cells with Wnt pathway activation [27,28]. Active β-catenin can function as a co-activator for TCF-dependent reporter construct contains CTNNB1 sequences from −336 to +219 relative to the transcription start site at +1. (B) DLD1 stably transduced with two different doxycycline-inducible shRNAs targeting CTNNB1 (CTNNB1-1 and CTNNB1-2) or a non-silencing scramble shRNA (Scrmbl), were treated for 4 days with DOX ("+ DOX") or a solvent control ("-DOX"). Effects of CTNNB1 shRNA-mediated inhibition on CTNNB1 (left) and MYC (right) expression are shown. Gene expression was assessed by qRT-PCR and normalized to HPRT expression. Error bars denote S.D. **P < 0.01 and *P < 0.05. (C) DLD1 cells, stably expressing the two different doxycycline-inducible shRNAs targeting CTNNB1 or the non-silencing scrambled shRNA (Scrmbl), were transfected with the CTNNB1 reporter construct, and luciferase activity was measured (bottom left). The cells were treated for 4 days with DOX ("+ DOX") at 2 μg/ml or a solvent control ("-DOX"). The TOPflash reporter construct was also used to assess effects of CTNNB1 inhibition on β-catenin/TCF-dependent transcription. The assays were performed in triplicate, mean ± S.D. values are shown, and the control CMV-driven Renilla luciferase-expressing vector was used to correct for differences in transfection efficiency. Two asterisks denote P < 0.01 in Student's t test. transcription of endogenous Wnt/β-catenin/TCF target genes [27]. To further address how reduced CTNNB1 gene expression affects β-catenin protein levels and β-catenin/TCF-regulated target gene induction in colon epithelial cells, we used an shRNA approach to antagonize CTNNB1 transcript and β-catenin protein levels in colon cell lines. In the immortalized, nonneoplastic human colon epithelial cell (HCEC) line, through use of a doxycycline (DOX)-regulated shRNA against APC, we reduced endogenous APC gene and protein expression in the cells to less than 10% of control levels (S5 Fig). Following APC shRNA induction, the levels of active β-catenin, as detected with a previously described antibody against the hypo-phosphorylated or active form of β-catenin, were significantly increased, whereas only a minor increase in total β-catenin levels was seen (Fig 7A and S6 Fig). Concurrent DOX-mediated induction of the APC shRNA and either of the two independent CTNNB1 shRNAs, which reduced CTNNB1 transcript levels to about 20-30% of control levels in HCECs, led to dramatic inhibition of the APC inactivation-stimulated effects on active β-catenin protein levels, but only modest to moderate reduction in the levels of total β-catenin protein (Fig 7A and S6 Fig). The marked effects of the APC and CTNNB1 shRNA approaches on the active β-catenin pool, with only more modest to moderate effects on total β-catenin levels in HCEC cells were reproducible (S6 Fig). Following APC shRNA induction by DOX treatment, expression of multiple βcatenin/TCF-regulated target genes, such as AXIN2, BMP4, NKD1 and IRS1, was significantly induced in HCECs (Fig 7B-7E). These APC shRNA-mediated increases in β-catenin/TCF-regulated target gene expression were almost completely abolished by shRNA-mediated inhibition of β-catenin (Fig 7B-7E). We also studied sub-cellular localization of β-catenin in the HCEC cells following APC shRNA induction and combined APC and CTNNB1 shRNA induction by DOX. Consistent with the marked increase in active β-catenin levels following APC shRNA induction, we found β-catenin protein mainly accumulated in the cytosol and nucleus of HCECs (S7 Fig). Concurrent induction of both the APC and CTNNB1 shRNAs in HCECs dramatically reduced the levels of β-catenin protein in the nucleus and cytoplasm (S7 Fig), consistent with the notion that the active, signaling pool of β-catenin in the cytoplasm and nucleus is highly sensitive to changes in Ctnnb1 transcript levels.
The strong inhibitory effect on the active pool of β-catenin compared to that for total β-catenin when CTNNB1 transcript levels were reduced in HCECs was further studied in three human colon cancer cell lines stably transduced with the two DOX-inducible CTNNB1 shRNAs. These included a colon cancer cell line with a gain-of-function mutation in CTNNB1 (HCT116) and two colon cancer cell lines with APC loss-of-function mutations (DLD1 and SW480). At 7 days after DOX-induction of the CTNNB1 shRNAs, in the three colon cancer cell lines, we found moderate (HCT116) to dramatic (DLD1 and SW480) decreases in the active pool of β-catenin protein with only modest changes in total β-catenin protein levels (S8 Ctnnb1 hemizygous inactivation does not affect Apc-and Ptenmutation-dependent mouse ovarian endometrioid adenocarcinoma (OEA) development To assess the role of β-catenin function in another Apc mutation-dependent tumor model, we explored the role of Ctnnb1 gene dosage in a mouse model of ovarian endometrial adenocarcinoma (OEA) arising from bi-allelic inactivation of both the Apc and Pten genes [29]. Prior studies have shown that the Wnt/β-catenin/Tcf signaling pathway is deregulated by mutations in 16%-38% of human OEAs, and PTEN mutations are often seen in the OEAs with Wnt pathway mutations [29][30][31][32]. In the mouse OEA model, tumors are initiated by conditional inactivation of the Apc and Pten genes following injection of AdCre into the right ovarian bursa of Apc fl/fl Pten fl/fl mice [29]. Interestingly, in both Apc fl/fl Pten fl/fl mice and Apc fl/fl Pten fl/fl Ctnnb1 fl/+ mice, adenocarcinomas morphologically similar to human OEAs formed following AdCre injection, with 100% penetrance and no difference in tumor latency between mice with two wild type Ctnnb1 alleles or one wild type and one floxed Ctnnb1 allele (Table 1). In addition, no significant differences in survival rates, tumor volumes, and rates of liver metastasis were found between AdCre-injected Apc fl/fl Pten fl/fl mice and Apc fl/fl Pten fl/fl Ctnnb1 fl/+ Fig 7. Inhibition of β-catenin expression by CTNNB1 shRNAs dramatically reduces the increase in the active pool of β-catenin and β-catenin/TCFregulated target gene expression seen following APC inhibition in HCECs. HCECs stably transduced with a lentiviral vector (ShRNA V1) expressing an APC shRNA or a non-silencing scramble shRNA (Scrmbl) were further transduced with lentiviral vectors (ShRNA V2) driving expression of two different shRNAs targeting CTNNB1 (CTNNB1-1 and CTNNB1-2) or a non-silencing scramble shRNA (Scrmbl). Protein and RNA were collected from the HCECs following a 3-day induction of shRNA expression by addition of doxycycline (DOX, "+") at 2 μg/ml or no DOX-mediated induction (i.e., solvent control "-"). (A) ShRNA-mediated changes in the expression of the total and active (un-or hypophosphorylated) pools of β-catenin in the HCECs were assessed by Western blot analysis for active β-catenin and total β-catenin, with β-actin as a loading and transfer control (left) and by qRT-PCR for CTNNB1 transcripts normalized to U6 expression (right). The two asterisks denote P < 0.01 in Student's t test, and error bars denote S.D. Analysis of the expression of selected Wnt/βcatenin/TCF-regulated target genes was undertaken in the cell lines with or without DOX-mediated induction of the indicated shRNAs-AXIN2 (B), BMP4 (C), NKD1 (D) and IRS1 (E), as assessed by qRT-PCR and normalized to U6 expression. In panels (B), (C) and (E), **P < 0.01. In panel (D), *P < 0.05.  (Table 1), and OEAs arising in both lines of mice shared similar histological features and immunohistochemical staining patterns for cytokeratin-8 (CK8), E-cadherin and αinhibin ( Fig 8A and Table 1). Efficient Cre-mediated deletion of Ctnnb1 and Apc was confirmed in tumors from these mice, and no OEAs arose in the AdCre-injected right ovaries in Apc fl/fl Pten fl/fl Ctnnb1 fl/fl mice, indicating OEAs could not arise from cells completely lacking β-catenin.
The findings on the lack of a demonstrable effect of Ctnnb1 hemizygous gene dosage in the mouse OEA model contrast with the findings above, where Apc-mutation-dependent polyposis in colon epithelium was dramatically suppressed by Ctnnb1 hemizygous inactivation. Nonetheless, similar to the situation in mouse colon, based on immunohistochemical staining, the presumptive Wnt pathway signaling-competent pool of β-catenin in the nucleus and cytoplasm was significantly reduced in the OEAs in Apc fl/fl Pten fl/fl Ctnnb1 fl/+ mice compared to OEAs in the Apc fl/fl Pten fl/fl mice (Fig 8B). We also examined the β-catenin/TCF-mediated gene transcription in the OEAs arising in the Apc fl/fl Pten fl/fl mice and Apc fl/fl Pten fl/fl Ctnnb1 fl/+ mice. Consistent with the β-catenin dosage-dependent effects of Ctnnb1 transcripts seen in mouse Apc-mutant colon tissues described above, Ctnnb1 transcripts were significantly reduced in the OEAs arising in Apc fl/fl Pten fl/fl Ctnnb1 fl/+ mice compared to the OEAs in Apc fl/fl Pten fl/fl mice ( Fig 8C). Interestingly, although the Ctnnb1 hemizygous state in the Apc-and Pten-mutant OEAs markedly suppressed the induction of some β-catenin/TCF-regulated target genes, such as Axin2 and Nkd1 (Fig 8C), hemizygous Ctnnb1 function did not abrogate induction of Myc transcripts in the OEAs (Fig 8C). Therefore, our findings showing that Ctnnb1 hemizgyous state did not prevent development of Apc-and Pten-mutant OEAs even though there was a reduction in β-catenin levels and expression of some β-catenin/TCF-regulated genes suggest that retention of Myc induction in OEAs with hemizygous Ctnnb1 function, but not in Apcdeficient colon epithelium with hemizygous Ctnnb1 function, may be a contributing factor in the observed differences in tumor development in the two tissues. We also studied the consequences of shRNA-mediated inhibition of CTNNB1 on MYC gene expression in the TOV112D human ovarian endometrioid carcinoma cell line that harbors a CTNNB1 oncogenic mutation leading to β-catenin/TCF dysregulation [33]. We found that doxycycline-mediated induction of the two CTNNB1 shRNAs in TOV112D cells reduced CTNNB1 levels to about 50% of baseline (Fig 8D), but no statistically significant effect on MYC transcript levels was seen.

Discussion
Mutations inactivating the APC tumor suppressor gene are believed to be critical initiating lesions in the majority of colon adenomas and carcinomas [6,34,35]. APC mutations are likely key contributing factors in the development of some other cancer types, including a subset of human OEAs. The best understood function of the APC protein is to act as a component of a phosphorylation-and ubiquitination-dependent destruction complex that regulates the free or active pool of β-catenin. This pool of β-catenin functions as a regulator of TCF transcription in the Wnt pathway signaling [1]. In the studies described above, we assessed the effects of Ctnnb1 gene dosage on Apc mutation-instigated tumorigenesis in mouse genetically engineered colon and ovarian tumor models and in cultured cells. We found the florid polyposis phenotype resulting from somatic Apc bi-allelic inactivation in mouse colon epithelium is potently inhibited by concurrent somatic inactivation of one Ctnnb1 allele. The few polyps arising in CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice were found to have escaped Ctnnb1 targeting, though Cre-mediated somatic inactivation of both Apc alleles occurred in the lesions, likely reflecting strong positive selection for maintenance of wild type Ctnnb1 gene dosage for Apcmutant colon adenomas to arise and persist. In contrast to the situation in colon epithelium, in a mouse model of the human OEAs that harbor inactivating mutations in the APC and PTEN genes, we found that, regardless of whether the mice had two wild type Ctnnb1 alleles or one wild type and one targeted Ctnnb1 allele, adenocarcinomas morphologically similar to human OEAs formed with 100% penetrance and no differences in latency, size, morphology, or metastatic potential of the lesions arising from AdCre-mediated targeting of the Apc and Pten genes.
In-depth mutational analyses of the germline and somatic mutations in adenomas arising in patients with FAP led to the proposal there was strong biological selection for a "just-right" level of β-catenin signaling that would be optimal for tumor formation [36]. Some prior studies have used genetic approaches to study experimentally the effects of Ctnnb1 gene dosage on Apc mutation-dependent tumorigenesis in the small intestine, liver, and mammary gland in mouse models [14,15]. The earlier work indicated that the Ctnnb1 +/constitutional hemizygous state can inhibit intestinal and liver tumorigenesis in mice carrying Apc mutations. In mammary gland tumorigenesis, tumorigenesis was enhanced in Apc 1638N Ctnnb1 +/mice relative to Apc 1638N Ctnnb1 +/+ mice, perhaps because Ctnnb1 functions as a tumor suppressor gene in Apc 1638N mammary gland tumors via β-catenin's role in E-cadherin-dependent tumor suppression [14]. While our findings in a mouse Apc mutation-dependent colon tumorigenesis model are consistent with the prior work on Apc mutation-instigated small intestine and liver tumorigenesis, some significant differences in the studies should be noted. The prior work emphasized models where the mice carried constitutional mutations in one Apc allele and tumors arose following stochastic loss or inactivation of the remaining wild type Apc allele. In addition, mice in β-catenin Function in Apc-Mutant Tumors the prior small intestine work were constitutionally hemizygous for Ctnnb1. In our Apc mutation-dependent colon tumorigenesis model, both Apc alleles are somatically inactivated in colon epithelium by Cre-mediated targeting, and the Ctnnb1 hemizygous deficiency state was also somatically generated only in the colon epithelial cells by Cre-mediated targeting. In addition, our OEA model work contrasts with the prior published work, as it also relies on somatic targeting of Apc and Ctnnb1. The OEA results also differ from our own colon tumorigenesis results, as the findings indicate Ctnnb1 hemizygous gene dosage had no demonstrable effect on cancer latency, size, or morphology or the metastatic potential of mouse OEAs arising from combined somatic inactivation of Apc and Pten.
Besides highlighting tissue-specific differences for Ctnnb1 gene dosage in Apc mutationinstigated colon and ovarian tumorigenesis, our studies and data have provided several unique and in-depth insights into cell and tissue mechanisms by which Ctnnb1 gene dosage likely contributes to Apc mutation-dependent phenotypes in mouse colon epithelium. Inactivation of one Ctnnb1 allele markedly inhibited the increases in β-catenin cytoplasmic and nuclear levels that result from bi-allelic Apc inactivation in mouse colon epithelium. In turn, there was strikingly attenuated expression of key β-catenin/TCF-regulated target genes, including those encoding the EphB2/B3 receptors, and the stem cell markers Lgr5, Msi1, and Hopx. Of significant interest in terms of a likely key mechanisms through which Ctnnb1 gene dosage inhibits adenoma formation, the inability of the Apc-mutant colon epithelial cells to up-regulate and alter the crypt (high)-surface (low) gradient of EphB2/B3 expression appears to restrict high levels of EphB2/3 expression to the crypt base. Activated β-catenin/TCF transcription has been implicated in repression of ephrinB expression [11,23]. We found that the robust ephrinB1/B2 expression seen in the upper two-thirds of normal colon crypts as well as in the normal colon surface epithelium was maintained in Apc-mutant crypts when one Ctnnb1 allele was inactivated. In contrast, Apc-mutant crypts with intact Ctnnb1 dosage markedly down-regulated ephrinB1/B2 expression and dramatically upregulated and expanded EphB2/B3 expression throughout the colon crypts. Because the EphB and ephrin molecules mediate critical repulsive interactions in intestinal crypts, the maintenance of the normal inverse EphB/ephrinB gradient from crypt base to cell surface in Apc-mutant crypts where one Ctnnb1 allele is inactive restricts the expansion of the Lgr5-positive crypt stem cell pool and the crypt fission/branching that would result from unrestrained crypt stem cell expansion and altered migration [11,23]. As a result of the preservation of the inverse gradient of EphB/ephrinB expression in Apc mutant crypts with reduced Ctnnb1 dosage, stem cell expansion and the dysplastic and adenomatous changes induced by Apc inactivation in colon epithelium are potently inhibited, even though Lgr5 and some other stem cell marker genes are modestly increased in expression in the targeted crypts of CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice relative to crypts in normal mice.
Our findings showing that Ctnnb1 transcripts are up-regulated in Apc-mutant mouse colon epithelium as well as in Apc-mutant mouse OEAs, together with our findings that Ctnnb1 hemizygous gene dosage inhibited Apc mutation-dependent Ctnnb1 transcript induction in colon and ovarian tumor models imply that the Ctnnb1 gene is subject to feed-forward activation by β-catenin levels and β-catenin/TCF-regulated transcription. Of interest with regard to a role for β-catenin and β-catenin/TCF transcription in regulating Ctnnb1 transcription are chromatin immunoprecipitation (ChIP) studies from the ENCODE project reporting that the TCF4 protein, encoded by the TCF7L2 gene, is bound in the promoter region of the CTNNB1 gene in selected cell lines. Based on our mouse colon tissue studies and the ENCODE project findings, we generated a CTNNB1 reporter gene construct containing 555 bp of human CTNNB1 upstream and exon 1 sequences and found that shRNA-mediated inhibition of CTNNB1 endogenous gene expression in the APC-mutant DLD1 human colon cancer cell line led to inhibition of the activity of the CTNNB1 reporter gene. These data demonstrate that CTNNB1 transcript levels affected CTNNB1 transcription in colon cancer cells. The lack of known TCF consensus element binding sites in the mouse Ctnnb1 and human CTNNB1 promoter regions currently limits support for the argument that β-catenin/TCF transcription directly regulates Ctnnb1 transcription, though further studies to address the point will need to be pursued.
In contrast to the near complete abrogation of Myc induction in Apc-mutant colon epithelium with one Ctnnb1 allele, Myc induction was retained in the Apc-mutant mouse OEAs with one functional Ctnnb1 allele. Of note, in prior studies, it has been shown that hemizyous inactivation of Myc dramatically inhibited Apc mutation-induced small intestine tumor phenotypes, but not Apc mutation-induced effects on liver cell proliferation and size [37,38]. Hence, the findings from our work and these prior studies [37][38][39] highlight Myc as perhaps one of the key β-catenin/TCF-regulated genes with tissue-specific differences in its regulation by β-catenin/TCF that may account for why Ctnnb1 hemizygous state abrogates Apc mutation-induced effects in some tissues (e.g., small intestine and colon epithelium) but not in other tissues (e.g., liver and ovarian epithelium). The identification of a possible feed-forward mechanism for βcatenin and β-catenin/TCF transcription in regulating Ctnnb1 transcript levels following Apc inactivation are also potentially interesting with regard to Myc, because the ENCODE project work also indicates that the Myc protein is bound in the promoter and intron one regions of the Ctnnb1 gene in selected cell lines. As such, β-catenin/TCF transcription may cooperate in some fashion with Myc, itself encoded by a β-catenin/TCF target gene, in a more complex feed-forward loop to activate Ctnnb1 transcription in certain cell types when Apc is inactivated.
Further studies are needed to better understand the details of the apparent feed-forward mechanisms through which β-catenin and β-catenin/TCF transcription may regulate Ctnnb1/ CTNNB1 transcription in the setting of Apc/APC inactivation. Besides Myc, other β-catenin/ TCF target genes may also be differentially regulated in a tissue-and context-dependent fashion, perhaps contributing in some fashion to the tissue-specific differences of Ctnnb1 hemizygous gene dosage on Apc mutation-instigated tumorigenesis observed. In addition, the basis for the dramatic changes in the free or active pool of β-catenin protein relative to the more modest effects on total β-catenin protein levels when Ctnnb1/CTNNB1 transcript levels are reduced in colon epithelial cells with Wnt pathway dysregulation remains to be elucidated. Nonetheless, our findings highlight the possibility that novel approaches and/or agents that can reduce CTNNB1 transcript levels and/or the free pool of β-catenin protein might have quite dramatic effects on the development and perhaps persistence of neoplastic cells with Wnt pathway defects.

Plasmids
DNA fragment containing human CTNNB1 sequences from −336 to +219 relative to the transcription start site was obtained by PCR amplification of genomic DNA, and was subcloned upstream from the luciferase reporter gene in the pGL3Basic reporter vector (Promega, Madison, WI), using the MluI and XhoI sites. The forward primer for generating the CTNNB1 reporter construct was 5 0 -ACGCGTGCTGCTCTCCCGGTTCG -3 0 ; the reverse primer for generating the CTNNB1 reporter construct was 5 0 -CTCGAGCAGGGGAACAGGCTCCTC-3 0 .

Tamoxifen (TAM) treatment and AdCre injection
Mice with the CDX2P-CreER T2 transgene or Lgr5-EGFP-IRES-CreER T2 were injected intraperitoneally with TAM (Sigma-Aldrich) dissolved in corn oil (Sigma-Aldrich). For two TAM daily dosing, we used 150mg/kg weight per dose; for three consecutive daily doses, we administered TAM at 100mg/kg weight per dose. Mice were injected with TAM at 2-to 3-months of age. For OEA induction, 5 x 10 7 plaque-forming units of replication-incompetent recombinant adenovirus expressing Cre recombinase (AdCre, from the University of Michigan's Vector Core) were injected into the right ovarian bursal cavities of 6-10 week old female mice as previously described [29].

Immunohistochemistry, immunofluorescence and β-gal analysis
Mouse tissues were prepared for paraffin-embedding or cryosectioning as described previously [16]. For assessment of cell proliferation, mice were pulsed with 5-bromo-2-deoxyuridine (BrdU; Sigma-Aldrich) for 1 hr before euthanasia. Sections of paraffin-embedded human or mouse tissues were subjected to immunohistochemical analysis as previously described [45]. ) by adding to the washing buffer at 5 μg/ml. β-gal analysis for mouse with Axin2-LacZ reporter was performed as described previously [16].

Terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick endlabeling (TUNEL) assay
To assess apoptosis, TUNEL assays were undertaken using 4-μm sections of formalin-fixed, paraffin-embedded mouse colon tissues, after the tissue sections were deparaffinized, rehydrated and treated with 20 μg/ml protease K (Roche Applied Sciences, Indianapolis, IN) at 37°C for 15 min. The nicked DNA was labeled by using terminal transferase (TdT) (New England Biolabs, Ipswich, MA) and Biotin-16-UTP (Roche Applied Sciences) according to the manufacturer's recommendation. The signal was detected by using the Vectastain ABC kit (Vector Laboratories, Burlingame, CA) according to the manufacturer's suggestion.

Mitotic spindle axis assessment
The spindle angles were defined by the orientation of mitotic spindles, based on α-tubulin staining, relative to the most adjacent apical membrane, as indicated by Crb3 staining. The mitotic spindle axis angle relative to the planar axis of the cells (defined by the most adjacent apical membrane) was measured by ImageJ (NIH).

Quantitative reverse transcription (RT)-PCR (qRT-PCR)
cDNA was synthesized using a high capacity cDNA reverse transcription kit (Applied Biosystems, Foster City, CA). qRT-PCR was performed with an ABI Prism 7300 Sequence Analyzer using a SYBR green fluorescence protocol (Applied Biosystems). See S1 Table for primer sequences used in qRT-PCR.
Reporter gene assays DLD1 cells, stably expressing two different doxycycline-inducible shRNAs targeting CTNNB1 (CTNNB1-1 and CTNNB1-2) or a non-silencing scramble shRNA (Scrmbl), were treated for 4 days with DOX at 2 μg/ml or a solvent control. At second day during DOX treatment, cells were plated in 35-mm six-well plates. After 12-24h cells were then transfected with 0.5 μg of CTNNB1 reporter or TOPflash, 1 μg of pCDNA3 (Invitrogen) and 0.05 μg of PRL-CMV Renilla luciferase reporter vector (Promega) using Mirus TransIT-LT1 transfection reagent (Mirus Bio, Madison, WI) according to the manufacturer's protocol. Cells were harvested 45h later and luciferase activities were measured using a Dual-luciferase kit and GloMax-Multi Detection System from Promega.

Statistical analysis
All data for qRT-PCR were evaluated by Student's t test and asterisks denote significance with P < 0.05. Error bars denote standard deviations (S.D.). Kaplan-Meier survival curves were compared by log-rank (Mantel-Cox) test. Chi-Square test was used to determine significance when mitotic spindle angles were compared among mice with different genotypes. P < 0.05 is considered statistically significant.

Ethics statement
Animal husbandry and experimental procedures were carried out under approval from the University Committee on Use and Care of Animals, University of Michigan (PRO00005075) and according to Michigan state and US federal regulations. All the mice were housed in specific-pathogen free (SPF) conditions. After weaning, rodent 5001 chow and automatically supplied water were provided ad libitum to mice. Animals were euthanized and analyzed at the specified time points, based on particular study design parameters or defined humane treatment and euthanasia guidelines. Right panels, Cre-mediated recombination of Apc and Ctnnb1 genes was assessed by PCR using genomic DNA isolated from grossly normal-appearing proximal colon or cecum tissues and rare cecal tumors from three different CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice (labeled as obtained from mouse 1, 2, 3) at 6 months following TAM treatment of the mice. Proximal colon or cecal tissue from a CDX2P-CreER T2 Apc fl/fl mouse (labeled as CA), obtained 20 days after TAM induction, or from an Apc fl/fl Ctnnb1 fl/+ mouse (lacking the CDX2P-CreER T2 transgene) (labeled as AB) served as controls. All of the tissues were genotyped for Apc and Ctnnb1. Flox alleles represent alleles that did not undergo Cre-mediated targeting, whereas the floxdel alleles have undergone Cre-dependent targeting. Due to the PCR primers used to detect the Cre-targeted Ctnnb1 allele (i.e., foxdel allele), the PCR product for the floxdel allele is larger than that for the flox allele. Tissues were manually dissected. Cecum tumor DNA preparations likely include DNA from some adjacent colon epithelial cells with Ctnnb1 gene targeting by Cre to generate floxdel alleles. Both epithelial and non-epithelial elements were present in grossly normal-appearing proximal colon and cecum tissue samples from CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice. The Cre transgene is only expressed in epithelial elements. Both flox and floxdel alleles for Apc and Ctnnb1 are therefore seen in grossly normal-appearing tissue samples from CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice; and rare Apc flox alleles are seen in proximal colon and cecal tumors due to non-neoplastic elements in the tumors.  (Control, left), CDX2P-CreER T2 Apc fl/fl (middle), or CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ (right) mice, obtained at 27 days after TAM dosing, were co-stained for αtublin (green), lysozyme (red), and Crb3 (red), and counter-stained with Hoechst 33342 (Blue). Representative low magnification (upper row panels) or high magnification (lower row panels) images are shown. Scale bars, 20 μm for low magnification images, and 10 μm for high magnification images. Double-headed arrows indicate the orientation of mitotic spindles; solid lines indicate the most adjacent apical membrane of the mitotic cell, which was used as a reference to assess the spindle angle. (B) The spindle angles were defined by the orientation of mitotic spindles relative to the most adjacent apical membrane, as indicated by Crb3 staining. Quantification for the tissues is shown and for CDX2P-CreER T2 Apc fl/fl and CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice, only the lysozyme-positive crypts, indicating Apc gene targeting had occurred, were scored. Ã P < .01 compared CDX2P-CreER T2 Apc fl/fl mice to CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice; n.s., not significant, compared CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice to the normal tissues using Chi-Square test (n = 4 and >70 crypts were counted). Apc mutation-induced increases in the expression of Wnt/β-catenin/TCF pathway target genes and genes encoding stem cell markers. (A) Effects of Ctnnb1 inactivation on Apc mutation-induced expression of selected Wnt target genes, Axin2, Nkd1, Ccnd1, and Irs1. Gene expression was assessed by qRT-PCR in mouse proximal colon tissues obtained from Cre negative control mice (cntrl), CDX2P-CreER T2 Apc fl/fl Ctnnb1 fl/+ mice (Apc fl/fl Ctnnb1 fl/+ ) and CDX2P-CreER T2 Apc fl/fl mice (Apc fl/fl ) at 20 days after TAM dosing to activate Cre to target alleles. Gene expression was normalized to β-actin expression. (B) Effects of Ctnnb1 inactivation on Apc mutation-induced expression of selected candidate stem cell markers, Lgr5, CD44, Msi1 and Hopx (Lgr5 and CD44 are also Wn/β-catenin/TCF pathway target genes). RNA preparations used for the work in panel A were studied by qRT-PCR and normalized to β-actin expression. Two asterisks denote P < 0.01 and one asterisk denotes P < 0.05 in expressing an APC shRNA or a non-silencing scramble shRNA (Scrmbl) were further transduced with lentiviral vectors (ShRNA V2) driving expression of two different shRNAs targeting CTNNB1 (CTNNB1-1 and CTNNB1-2) or a non-silencing scramble shRNA (Scrmbl). Proteins were collected from the HCECs at 3 days in the presence of doxycycline (DOX, "+") at 2 μg/ml or simply in a solvent control lacking DOX ("-"). CTNNB1 shRNA-mediated changes in the expression of the total (right) and active (un-or hypophosphorylated, left) pools of β-catenin in the HCECs were assessed by Western blot analysis (Fig 7A) and the density of Western blotting bands was quantified using AlphaImager HP system (from ProteinSimple). The protein levels of total and active β-catenin were normalized to β-actin level, and the expression from HCEC/Scrmbl in the presence of DOX was set as 1. (B) Similar Western blot analysis was performed as described in (A) except that the HCECs were treated with doxycycline (DOX, "+") at 1 μg/ml or 2 μg/ml or simply in a solvent control lacking DOX ("-"). The protein levels of total and active β-catenin were shown (top) and the density of Western blotting bands was quantified and shown at the bottom panels. β-actin was used as a loading and transfer control. (TIF) S7 Fig. Immunohistochemical studies in HCECs demonstrate that the nuclear and cytoplasmic pools of β-catenin protein induced by APC inhibition are markedly affected by reduction of Ctnnb1 transcript levels. HCECs stably transduced with a lentiviral vector (ShRNA V1) expressing an APC shRNA or a non-silencing scramble shRNA (Scrmbl) were further transduced with lentiviral vectors (ShRNA V2) driving expression of two different shRNAs targeting CTNNB1 (CTNNB1-1 and CTNNB1-2) or a non-silencing scramble shRNA (Scrmbl). These cells were grown on chamber slides and were induced for 3 days for shRNA expression by addition of DOX at 2 μg/ml. Cells were then subjected to Immunohistochemical staining for β-catenin. Scale bar, 20 μm. (TIF) S8 Fig. The active (non-phosphorylated) pool of β-catenin that functions in canonical Wnt signaling through interactions with TCFs is more sensitive to shRNA-mediated inhibition of CTNNB1 transcripts than the total pool of β-catenin in human CRC cell lines with mutational defects in Wnt signaling. Western blot analysis of protein levels of non-phosphorylated, active β-catenin and total β-catenin in human colon cancer cell lines, DLD1 (A), HCT116 (B) and SW480 (C) (with APC inactivation for DLD1 and SW480; β-catenin gain of function for HCT116), stably expressing two different doxycycline-inducible shRNAs targeting CTNNB1 (CTNNB1-1 and CTNNB1-2) or a non-silencing scramble shRNA (Scrmbl). β-catenin protein levels were determined after 3-day exposure of the cells to DOX ("+") at 2 μg/ml or a solvent control ("-"). β-actin protein levels served as a loading and transfer control. The density of Western blotting bands for each cell lines was quantified using AlphaImager HP system (from ProteinSimple). The protein levels of active and total β-catenin were normalized to βactin level, and the expression from cells expressing scramble shRNA in the presence of DOX was set as 1.