Maintenance of adenomatous polyposis coli(APC)-mutant colorectal cancer is dependenton Wnt/-catenin signalingAlix Scholer-Dahirel1, Michael R. Schlabach, Alice Loo, Linda Bagdasarian, Ronald Meyer, Ribo Guo, Steve Woolfenden,Kristine K. Yu, Judit Markovits, Karen Killary, Dmitry Sonkin, Yung-Mae Yao, Markus Warmuth, William R. Sellers,Robert Schlegel, Frank Stegmeier, Rebecca E. Mosher, and Margaret E. McLaughlin2

Novartis Institutes for Biomedical Research, Cambridge, MA 02139

Edited by Bert Vogelstein, Johns Hopkins University, Baltimore, MD, and approved August 23, 2011 (received for review March 18, 2011)

Persistent expression of certain oncogenes is required for tumormaintenance. This phenotype is referred to as oncogene addictionand has been clinically validated by anticancer therapies thatspecifically inhibit oncoproteins such as BCR-ABL, c-Kit, HER2,PDGFR, and EGFR. Identifying additional genes that are requiredfor tumor maintenance may lead to new targets for anticancerdrugs. Although the role of aberrant Wnt pathway activation inthe initiation of colorectal cancer has been clearly established,it remains unclear whether sustained Wnt pathway activationis required for colorectal tumor maintenance. To address thisquestion, we used inducible -catenin shRNAs to temporally con-trol Wnt pathway activation in vivo. Here, we show that activeWnt/-catenin signaling is required for maintenance of colorectaltumor xenografts harboring APC mutations. Reduced tumorgrowth upon -catenin inhibition was due to cell cycle arrest anddifferentiation. Upon reactivation of the Wnt/-catenin pathwaycolorectal cancer cells resumed proliferation and reacquired a cryptprogenitor phenotype. In human colonic adenocarcinomas, highlevels of nuclear -catenin correlated with crypt progenitor butnot differentiation markers, suggesting that the Wnt/-cateninpathway may also control colorectal tumor cell fate during themaintenance phase of tumors in patients. These results supportefforts to treat human colorectal cancer by pharmacological inhi-bition of the Wnt/-catenin pathway.

Mutational activation of the Wnt pathway occurs in the vastmajority of colorectal cancers through truncating muta-tions in adenomatous polyposis coli (APC) or AXIN2 or muta-tions in GSK3-target residues in -catenin (15). In normalintestinal cells, APC associates with axin, glycogen synthase ki-nase-3 (GSK-3), and casein kinase 1 (CK1) to form a -catenindestruction complex. This complex phosphorylates -catenin,resulting in its ubiquitylation and subsequent degradation by theproteasome (6). In contrast, in cells harboring mutations in APC,AXIN2, or -catenin, -catenin accumulates and upon its trans-location to the nucleus, interacts with the T-cell factor/lymphoidenhancer factor (TCF/LEF) family of transcription factors toactivate specific Wnt target genes (7, 8).Considerable evidence from human genetic studies and mouse

genetic models suggests that mutational activation of the Wntpathway initiates the process of colon tumorigenesis. InheritedAPC mutations cause familial adenomatous polyposis, and ac-quired APCmutations represent the earliest genetic alteration sofar detected in the genesis of sporadic colorectal cancer (9). Raremutations in AXIN2 or -catenin can also be present in smallneoplastic lesions (5, 10). In experimental mouse models, loss ofAPC (11, 12) or expression of constitutively active -catenin (13)is sufficient to drive polyp formation.Inhibition of the Wnt pathway in colorectal cancer cells

in vitro by overexpression of dominant-negative TCF4 or in-ducible -catenin siRNA results in rapid cell cycle arrest andblocks a genetic program that is physiologically active in crypt

progenitors. Consequently, colorectal cancer cells undergo dif-ferentiation (7, 14, 15). By imposing a proliferative crypt pro-genitor phenotype, aberrant Wnt pathway activation may allowbenign tumors (polyps) to persist for many years, providing anopportunity for the acquisition of further mutations (e.g., inKRAS, SMAD-4, and P53 genes) required for the development ofmalignant colorectal tumors (16).Although the role of Wnt pathway activation in the initiation

of colon tumorigenesis has been well established, it is unclearwhether tumors that have acquired additional mutations inoncogenes or tumor suppressor genes remain dependent on Wntpathway activation. Although -catenin siRNA inhibits engraft-ment of colorectal cancer cells (17), a recent study reports thatinhibition of Wnt signaling in established colorectal xenografts(mutant for the -catenin gene) by inducible -catenin shRNAhad no significant effect on tumor growth (18). Human co-lorectal tumors with mutations in -catenin are usually less ag-gressive and smaller than those with APC mutations (19),suggesting that APC and -catenin mutations are not functionallyequivalent. Consistently, in addition to its function in the -cat-enin degradation complex, APC can also directly contribute tothe regulation of mitosis and apoptosis (20). Such -cateninindependent APC functions may influence the degree of depen-dency on Wnt pathway activation for colorectal tumor mainte-nance. Given the large preponderance of APC mutations inhuman colorectal cancer, it is crucial to determine whethersustained Wnt pathway activation is required for maintenance ofAPC-mutant colorectal tumors.In this study, we provide evidence that Wnt/-catenin pathway

activation is required for maintenance of colorectal tumorsharboring APC mutations. We show that -catenin inhibitionin vivo strongly inhibited the growth of established APC-mutantcolorectal tumor xenografts. Upon -catenin inhibition, co-lorectal cancer cells underwent cell cycle arrest and rapidlystarted to differentiate into polarized intestinal epithelial cells,producing mucins and displaying microvilli. Such differentiatedcancer cells, when relieved from -catenin inhibition, could de-differentiate and regain their proliferative potential in vitro. In

Author contributions: A.S.-D., M.R.S., A.L., Y.-M.Y., M.W., W.R.S., R.S., F.S., R.E.M., andM.E.M. designed research; A.S.-D., M.R.S., A.L., L.B., R.M., R.G., S.W., K.K.Y., J.M., K.K.,D.S., and M.E.M. performed research; A.S.-D., M.R.S., and D.S. contributed new reagents/analytic tools; A.S.-D., M.R.S., A.L., L.B., R.M., R.G., J.M., D.S., and M.E.M. analyzed data;and A.S.-D., M.R.S., A.L., M.W., W.R.S., R.S., F.S., R.E.M., and M.E.M. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.1Present address: Institut National de la Sant et de la Recherche Mdicale, U830, Geneticand Biology of Cancers, Institut Curie, F-75248 Paris Cedex 05, France.

2To whom correspondence should be addressed. E-mail: margaret.mclaughlin@novartis.com.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1104182108/-/DCSupplemental.

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addition, we report that in human colonic adenocarcinomas highlevels of nuclear -catenin correlated with crypt progenitor butnot differentiation markers, suggesting that the Wnt/-cateninpathway may also control colorectal tumor cell fate during themaintenance phase of tumors in patients.

ResultsSustained Wnt/-Catenin Pathway Activation Is Required forMaintenance of Colorectal Tumors Harboring APC Mutations. Toanalyze the role of sustained Wnt pathway activation in main-tenance of colorectal tumors, we used doxycycline-inducible-catenin shRNAs to temporally control Wnt pathway activationin vivo. We infected LS411N and SW403 colorectal cancer cells(both APC mutant) with a robust inducible single-lentiviralvector pLKO-Tet-On (21), containing either control nontarget-ing (NTC) shRNA or two distinct -catenintargeting shRNAsequences (shRNA-35 or shRNA-36). Polyclonal stable lineswere obtained after puromycin selection and inoculated s.c. inNu/Nu mice. To confirm that the doxycycline treatment led toinhibition of the Wnt/-catenin pathway, we treated mice bearingestablished tumors (100300 mm3) with doxycycline for 3 d andanalyzed various molecular components of the Wnt/-cateninpathway. Upon doxycycline treatment, -catenin expression wasdramatically reduced at the level of both mRNA (knockdownefficiency >95%, Fig. 1A) and protein in the -catenin shRNAtumors but not in the NTC shRNA tumors (Fig. 1 B and C andFig. S1 A and B). As expected, silencing of -catenin causeda concomitant reduction of -catenin target genes AXIN2 and c-MYC at the mRNA and protein levels (Fig. 1A and Fig. S1 EH).We noted that -catenin inhibition had a more dramatic effecton c-MYC expression in SW403 versus LS411N cells (up to 99%and 50% reduction of nuclear c-MYC staining intensity, re-spectively). Specificity of the -catenin shRNAs was confirmedin vitro: Decreased cell viability was noted only in LS411N andSW403 colorectal cancer cell lines, not in RKO colorectal cancercells that are wild type for -catenin and APC (Fig. S1 IN).Taken together, these results indicate that the -catenin shRNAsefficiently and specifically inhibit the Wnt/-catenin pathway.We next investigated the effect of -catenin shRNAs on tumor

growth and treated mice bearing established tumors with doxy-cycline for 14 d. Such treatment induced long-term silencing of-catenin as demonstrated by down-regulation of -catenin atday 14 (Fig. S1 C and D). Both LS411N and SW403 -cateninshRNA tumors showed a significant inhibition of tumor growthfollowing doxycycline treatment (Fig. 1 D and E). This effect wasdue to inhibition of the Wnt/-catenin pathway rather thandoxycycline treatment alone, as NTC shRNA tumors progressedrapidly despite treatment. Interestingly, nuclear -catenin de-pletion by -catenin shRNAs was more complete after shortdoxycycline treatment in LS411N tumors (6075% and 45%reduction of nuclear -catenin at days 3 and 14, respectively).This observation suggests that there might be some selectivepressure favoring cancer cells that either had mild initial -cat-enin depletion or managed to escape silencing by -cateninshRNAs. We conclude that sustained Wnt/-catenin pathwayactivation is required for the maintenance of colorectal tumorxenografts harboring APC mutations.

In Vivo Inhibition of -Catenin in APC-Mutant Colorectal Cancer CellsResults in Cell Cycle Arrest. We next studied the mechanisms un-derlying the requirement of the active Wnt pathway for tumormaintenance and analyzed the effect of -catenin shRNAs ontumor cell cycle in vivo. Because c-MYC represses expressionof the cyclin-dependent kinase inhibitor p21 (22), we evaluatedp21 expression 3 d after doxycycline treatment. Doxycycline in-duction of -catenin shRNA caused a significant induction of p21expression in vivo. Although p21 induction occurred in bothLS411N and SW403 -catenin shRNA xenografts, p21 induction

was stronger in SW403 tumors where -catenin shRNAs had themost profound effect on c-MYC expression (Fig. 2 A and B andFig. S2 A and B). To determine whether p21 induction resultedin proliferation effects, we next analyzed Ki67 expression.-Catenin silencing in tumors led to a decreased percentage ofnuclei positive for Ki67, as early as day 3, that was further de-creased at day 14 (Fig. 2 C and D). The effect of -catenin si-lencing on Ki67 expression was more robust in SW403 xenografts(Fig. S2 C and D). These results are consistent with previousstudies reporting that disruption of endogenous -catenin/TCF4activity in colorectal cancer cells in vitro induces rapid cell cyclearrest (7, 14, 15). In our study, no change in cleaved caspase 3staining intensity was detected at day 3 (Fig. S3 AD), suggestingthat the cell cycle arrest was not coupled to apoptotic cell death.Although the mechanisms are poorly understood, cell cycleregulation may also crosstalk with autophagy (23). Wnt pathwayinhibition may activate autophagy as total LC3 protein level rose

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Fig. 1. Tumor growth is inhibited by -catenin shRNA in vivo. (AC) LS411Ncancer cells stably expressing dox-inducible NTC, sh35, or sh36 -cateninshRNA were inoculated into mice. Tumor-bearing mice were treated for3 d with either vehicle or doxycycline (n = 3). (A) Quantitative RT-qPCR oftumor -catenin, AXIN2, and c-MYC after 3 d of treatment. Graphs representmean SEM values. Arbitrary units are shown. (B) Representative images of-catenin staining by IHC after 3 d of treatment. (C) Mean signal intensity oftotal -catenin (Upper) and nuclear -catenin (Lower) after 3 d of treatment.Graphs represent mean SEM values. (D and E) LS411N (D) or SW403 (E)cancer cells stably expressing dox-inducible NTC, sh35, or sh36 -catenin shRNAwere inoculated into mice. When tumor volume reached 100300 mm3, micewere treated continuously with either vehicle (gray circles) or doxycycline(white circles) and tumor growth was monitored. Graphs represent mean SEM values. Two independent experiments are represented (n = 68 pertreatment group).

17136 | www.pnas.org/cgi/doi/10.1073/pnas.1104182108 Scholer-Dahirel et al.

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at day 3 of -catenin silencing. However, no change in the LC3-II/LC3-I ratio or in p62 expression was observed (Fig. S3 EG).Our results demonstrate that a major consequence of inhibitionof -catenin in vivo is cell cycle arrest of APC-mutant colorectalcancer cells.

In Vivo Inhibition of -Catenin Induces APC-Mutant Colorectal CancerCell Differentiation. Cell cycle arrest is often sufficient to inducedifferentiation. Similarly, induction of a differentiation programcan trigger cell cycle withdrawal (24). We, therefore, assessedwhether inhibition of -catenin induces differentiation of APC-mutant colorectal cancer cells in vivo. To do so, we analyzed theexpression of various markers of the colon crypt progenitor/stemcells and of differentiation of intestinal epithelial cells by RT-qPCR. Three genes that have been previously described as cryptprogenitor/stem cell markers, ephrin type-B receptor 2 (EPHB2)(Fig. S4A), bone morphogenetic protein 4 (BMP4) (14), andleucine-rich repeat-containing G-proteincoupled receptor 5(LGR5) (25), were significantly down-regulated in LS411N-catenin shRNA tumors after 3 d doxycycline treatment, sug-gesting that -catenin imposes a crypt progenitor/stem cell phe-notype on LS411N cells. Concomitantly, differentiation markerssuch as carbonic anhydrase 2 (CA2) (Fig. S4B), mucin 2 (MUC2)(26), and transmembrane 4 L six family member 4 (TM4SF4)(27) were significantly up-regulated (Fig. 3A). The changes inEPHB2, mucins, and CA2 protein levels were confirmed byimmunohistochemistry. Doxycycline treatment led to decreasedEPHB2 and increased CA2 protein expression as well as mucinproduction (Fig. 3B), confirming that -catenin inhibition in-duced LS411N cell differentiation. Similarly, -catenin silencinginduced differentiation of SW403 cells in vivo (Fig. S2 E and F).We also noted the appearance of gland-like structures upon-catenin shRNA induction, suggesting that as the colorectalcancer cells acquire phenotypic characteristics of mucosecretoryintestinal cells, they may undergo major morphological changes.Electron microscopy analysis of cellular morphology revealed

that cancer cells in the vehicle-treated tumors were unpolarizedand rarely formed gland-like structures. The rare gland-likestructures present had a paucity of microvilli and lacked apicallateral junctions (Fig. 3C). In contrast, after 3 d of doxycycline-induced -catenin knockdown, the tumor cells became polarizedand displayed numerous microvilli and apicallateral junctions,acquiring morphology similar to that of normal intestinal epi-thelial cells (Fig. 3C). Consistent with differentiation toward anintestinal epithelial phenotype, we found no evidence of an ep-ithelialmesenchymal transition (Fig. S5). Costaining for thedifferentiation marker KLF4 (28) (Fig. S4C) and p21 revealedthat differentiation and cell cycle arrest were highly correlatedon the cellular level (Fig. 3D). Immunohistochemical staining ofserial sections with CA2 and p21 showed a similar trend (Fig.S6). Thus, whereas -catenin imposes a proliferative crypt pro-genitor phenotype on APC-mutant colorectal cancer cells, itsdepletion induces a rapid and dramatic differentiation processaccompanied by cell cycle arrest.

Reversibility of Cell Cycle Arrest and Differentiation Induced by-Catenin Silencing. To determine whether the cell cycle arrestand differentiation induced by Wnt/-catenin pathway inhibitionare reversible, we took advantage of our inducible -cateninshRNAs that allow us to inhibit (upon doxycycline treatment)and then restore (upon doxycycline withdrawal) -catenin ex-pression in APC-mutant colorectal cancer cells. Similarly to whatwe observed in vivo, doxycycline treatment for 3 d in vitro in-duced marked down-regulation of -catenin, AXIN2, and c-MYCexpression in LS411N cells containing -catenin shRNA but notNTC shRNA (Fig. 4A and Fig. S7 AC). Consequently, LS411Ncells showed reduced in vitro proliferation (Fig. 4B and Fig. S7D)and significant decrease in expression of crypt progenitor/stemcell markers (EPHB2, BMP4, and LGR5) concomitant with in-creased expression of differentiation markers (CA2, MUC2, andTM4SF4) (Fig. 4C and Fig. S7 EG). To determine whether suchdifferentiated cancer cells could regain their proliferative po-tential and dedifferentiate upon Wnt/-catenin pathway reac-tivation, after 3 d of doxycycline treatment, LS411N cells wereeither kept in the presence of doxycycline or further cultured indoxycycline-free medium. Within 814 d after doxycycline with-drawal, -catenin expression was restored and the Wnt/-cateninsignaling pathway was reactivated as determined by AXIN2mRNA expression (Fig. 4A). Interestingly, upon doxycyclinewithdrawal, LS411N -catenin shRNA cells resumed their pro-liferation (Fig. 4B) and appeared to reacquire a crypt progenitor/stem cell phenotype, losing differentiation markers (CA2,MUC2, and TM4SF4) and reexpressing crypt progenitor/stemcell markers (EPHB2, BMP4, and LGR5) (Fig. 4C). In contrast,cells maintained under continuous doxycycline treatment did notproliferate. This observation excludes the possibility that thegrowth observed upon doxycycline withdrawal was due to cellsthat were resistant to knockdown, as those cells would also bepresent in the population treated continuously with doxycycline.We cannot exclude the possibility that knockdown within a sub-set of the LS411N -catenin shRNA cells was inefficient, givingrise to partially differentiated cells that may cycle slowly but notcompletely arrest, and that it is this subpopulation that appearsto revert upon doxycycline withdrawal. Although our resultssuggest that the cell cycle arrest and differentiation induced byWnt/-catenin pathway inhibition may be reversible, futurestudies designed to track the fate of individual tumor cells areneeded to definitively prove reversibility.

Nuclear -Catenin Correlates with Crypt Progenitor and Differ-entiation Markers in Human Colonic Adenocarcinomas. Next, weasked whether activation of the Wnt/-catenin pathway corre-lates with a crypt progenitor/stem cell phenotype in colorectaltumors in patients. We analyzed the expression of nuclear

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Fig. 2. -Catenin shRNA induces cell cycle arrest of colorectal cancer cellsin vivo. (AD) LS411N cancer cells containing NTC, sh35, or sh36 -cateninshRNA were inoculated into mice. Tumor-bearing mice were treated for 3 or14 d with either vehicle or doxycycline. (A) Representative images of p21staining by IHC after 3 d of treatment. (B) Percentage of nuclei positive forp21 after 3 d of treatment. Graphs represent mean SEM values. (C) Rep-resentative images of Ki67 staining by IHC after 14 d of treatment. (D)Percentage of nuclei positive for Ki67 after 3 d (Left) or 14 d (Right) oftreatment. Graphs represent mean SEM values. Two independent experi-ments are represented (n = 38 per treatment group).

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-catenin, the crypt progenitor/stem cell marker EPHB2, and thedifferentiation marker CA2 in 52 human colorectal adenocarci-nomas. These parameters did not correlate with tumor stage or

histologic grade (Fig. S8). However, we observed that humancolorectal tumors with high nuclear -catenin often had strongexpression of the crypt progenitor/stem cell marker EPHB2 and

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Fig. 3. Colorectal cancer cell differentiation is induced by -catenin shRNA in vivo. (AD) LS411N cancer cells containing NTC, sh35, or sh36 -catenin shRNAwere inoculated into mice. Tumor-bearing mice were treated for 3 d with either vehicle or doxycycline. (A) Quantitative RT-qPCR of tumor expression of cryptprogenitor markers including EPHB2, LGR5, and BMP4 and differentiation markers including MUC2, CA2, and TM4SF4 after 3 d of treatment. Expression wasnormalized to 18S mRNA. Graphs represent mean SEM values. Arbitrary units are shown. (B) Representative images of EPHB2 staining (Left) by IHC after3 d of treatment. Percentages of positive pixels are indicated. Graphs represent mean SEM values. (Center) Representative images of Alcian blue stainingafter 3 d of treatment. Percentages of blue pixels are indicated. Graphs represent mean SEM values. (Right) Representative images of CA2 staining by IHCafter 3 d of treatment. Percentages of positive pixels are indicated. Graphs represent mean SEM values. Two independent experiments are represented (n =3 per treatment group). (C) Representative electron microscopy images of sh36 -catenin shRNA tumors treated for 3 d with either vehicle (Upper) ordoxycyline (Lower). Arrow indicates apicallateral junction. (D, Left) Representative images of KLF4 (green) and p21 (red) costaining by immunofluorescenceafter 3 d of treatment. (Right) Mean intensity of nuclear KLF4 as a function of mean intensity of nuclear p21 in the indicated conditions. Each dot representsone nucleus. At least 1,000 nuclei were analyzed per condition. Correlation between mean intensity of nuclear KLF4 and mean intensity of nuclear p21: P