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TBL1–TBLR1 and β-catenin recruit each other to Wnt target-gene promoter for transcription activation and oncogenesisJiong Li1 and Cun-Yu Wang1,2

Aberrant Wnt signalling promotes oncogenesis by increasing the nuclear accumulation of β-catenin to activate downstream target genes. However, the mechanism of β-catenin recruitment to the Wnt target-gene promoter, a critical step for removing the co-repressor complex, is largely unknown. Here, we report that transducin β-like protein 1 (TBL1) and its highly related family member TBLR1

were required for Wnt–β-catenin-mediated transcription. Wnt signalling induced the interaction between β-catenin and TBL1–TBLR1, as well as their binding to Wnt target genes. Importantly, the recruitment of TBL1–TBLR1 and β-catenin to Wnt target-gene promoters was mutually dependent on each other. Furthermore, the depletion of TBL1–TBLR1 significantly inhibited Wnt–β-catenin-induced gene expression and oncogenic growth in vitro and in vivo. Our results unravel two new components required for nuclear β-catenin function, and have important implications in developing new strategies for inhibiting Wnt–β-catenin-mediated tumorigenesis.

The Wnt–β-catenin signalling pathway plays a critical role in development, specification of cell fate and adult stem cell proliferation1–4. The abnormal activation of the Wnt–β-catenin signalling pathway has been found to be associated with a variety of human cancers, including colorectal cancer and head and neck squamous cell carcinoma. Many genes associated with tumour growth (including c-Myc, matrix metalloproteinases (MMPs) and cyclin D1) have been identified as Wnt target genes5–8. Recently, signifi-cant progress has been made in understanding how β-catenin functions as a transcription co-activator. In the absence of β-catenin binding, the promoter of Wnt target genes is occupied by Tcf/Lef family proteins and co-repressors of Groucho/TLE1 and histone deacetylase 1 (HDAC1)9–12. Wnt signalling leads to the accumulation of cytosolic and nuclear β-cat-enin. As a first step in initiating transcription, nuclear β-catenin displaces Groucho/TLE from Tcf by binding Tcf in the promoter of Wnt target genes. Subsequently, β-catenin functions as a co-activator in recruiting chromatin remodelling complexes or other co-activators to stimulate tran-scription1–4,13,14. Recently, several transcription complexes or co-activators, including Bcl-9/Lgs and Pygopus (Pygo), polymerase-associated factor 1 (Paf1) and SET1 (trithorax), have been identified as being recruited by β-catenin to induce downstream target genes15–17. Despite these new find-ings, it remains a mystery how nuclear β-catenin is specifically recruited to the Wnt target-gene promoter for transcription activation.

Transducin β-like protein 1 (TBL1) is a mammalian homologue of Drosophila Ebi, which contains F-box and WD-40 domains18–23. TBL1 and TBL1-related protein (TBLR1) were initially identified as components of the co-repressor silencing mediator for retinoid and thyroid hormone

receptor (SMRT)–nuclear receptor co-repressor (N-CoR) complex22,23. Subsequently, TBL1 and TBLR1 were found to serve as specific adap-tors in mediating an exchange of the nuclear receptor co-repressors, SMRT–N-CoR, for co-activators on ligand binding24. They function as E3 ubiquitin ligase adaptors for the recruitment of specific ubiquitin/protea-some machinery to degrade co-repressors24. TBL1 was also identified as a key player involved in p53-mediated degradation of β-catenin induced by DNA-damage agents18. Interestingly, this novel pathway is glycogen synthase kinase 3β (GSK3β) phosphorylation-independent, and initi-ated by an increase in Siah-1 proteins, which are induced by p53. Siah-1 can interact sequentially with Siah interacting protein (SIP), Skp1 and TBL1 to induce β-catenin degradation18. While studying the precise role of TBL1 in β-catenin degradation, it was unexpectedly observed that TBL1–TBLR1 has an essential role in canonical Wnt signalling by recruit-ing β-catenin to the Wnt target-gene promoter to activate transcription.

RESULTSBoth TBL1 and TBLR1 are required for β-catenin–Tcf-mediated transcriptionAs TBL1-mediated β-catenin degradation is independent of GSK phosphorylation, a mutant form of β-catenin (β-catM) was trans-fected with TBL1 into HK293T cells to determine whether TBL1 could inhibit β-catM-stimulated transcription. β-catM carries four alanine substitutions in the GSK3β recognition site. As shown in Fig. 1a, overexpression of TBL1 significantly enhanced β-catM-stimulated transcription, as determined by a SuperTopflash reporter assay that

1Laboratory of Molecular Signalling, Division of Oral Biology and Medicine, UCLA School of Dentistry, Los Angeles, CA 90095, USA2Correspondence should be addressed to C.-Y.W. (e-mail: cwang@dentistry.ucla.edu)

Received 17 September 2007; accepted 18 December 2007; published online 13 January 2008; DOI: 10.1038/ncb1684

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specifically measured β-catenin–Tcf-mediated transcriptional activi-ties25. Similarly, TBLR1 also potentiated β-catM-induced transcription. These results suggest that, although TBL1 is involved in p53-induced degradation, TBL1 may also be involved in the activation of β-cat-enin–Tcf-mediated transcription. These unexpected findings led us to explore whether TBL1–TBLR1 played a critical role in the canonical Wnt–β-catenin signalling pathway. Small interference RNA (siRNA) was used to knockdown the expression of TBL1 or TBLR1. Western blot analysis confirmed that TBL1 siRNA or TBLR1 siRNA signifi-cantly reduced their expression in HK293T cells by 70 to 80% com-pared with control siRNA (non-targeting siRNA; Fig. 1b). As shown in Fig. 1c, the reduction in TBL1 or TBLR1 expression significantly inhibited SuperTopflash reporter activities induced by lithium (LiCl), which is the GSK3β inhibitor, and frequently induced Wnt signalling by stabilizing β-catenin1–4. Moreover, knockdown of TBL1 or TBLR1 also decreased SuperTopflash reporter activities induced by Wnt3a (Fig. 1d). As a negative control, the knockdown of TBL1 or TBLR1 did not affect SuperTopflash reporter activities and basal NF-κB activi-ties (Fig. 1c, d). Taken together, these observations argue that TBL1–TBLR1 may be required for β-catenin–Tcf-dependent transcription in the canonical Wnt signalling pathway.

β-catenin has been shown to interact with several transcription com-plexes to activate transcription. To determine whether β-catenin interacted with TBL1 or TBLR1, Flag-tagged TBL1 and TBLR1 and HA-tagged β-catenin were expressed either individually or in combination in HK293T cells. The whole cell lysates were prepared and immunoprecipitated with anti-HA antibodies. Flag–TBL1 or Flag–TBLR1 could be detected in the immunoprecipitates by western blot analysis with anti-Flag antibodies (Fig. 2a). Extensive mapping showed that the amino-terminal region 1–142 of TBL1 (including F-box domain) interacted with the arm repeats 1–8 of β-catenin (amino acids 133–467; see Supplementary Information, Fig. S1a, b). To further determine whether Wnt signalling induced the endogenous interaction, 293T cells were treated with lithium and nuclear extracts were isolated from these cells. On lithium stimulation, β-catenin was found to co-immunoprecipiate with TBL1 or TBLR1 (Fig. 2b). The reciprocal co-immunoprecipitation experiment also showed that TBL and TBLR1 were present in the β-catenin precipitate. To exclude the non-spe-cific effect of lithium on cells, we also examined whether Wnt3a stimulated the endogenous interaction between β-catenin and TBL1–TBLR1. The reciprocal co-immunoprecipitation experiment also revealed that Wnt3a treatment induced complex formation between β-catenin and TBL1–TBLR1 (Fig. 2c). Interestingly, TBL1, but not TBLR1, also interacted with

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Figure 1 Both TBL1 and TBLR1 are required for β-catenin–Tcf-mediated transcription. (a) TBL1 and TBLR1 enhanced β-catenin transactivation. 293T cells were cotransfected with a SuperTop-flash reporter, the lacZ expression plasmid pCMV–β-galactosidase, pcDNA3–β-CatM and increasing amounts of pQNCXII–TBL1 and TBLR1 as indicated. Luciferase activities were determined 40 h post transfection and normalized against β-galactosidase values. Values are mean ± s.d. for triplicate samples from a representative experiment. (b) TBL1 and TBLR1 were knocked down by siRNA in 293T cells. Cells were transfected with TBL1 or TBLR1 siRNA using Oliogolipofectamine. After transfection (48 h), the whole cell lysates were prepared and probed with anti-TBL1 or anti-TBLR1

antibodies. Uncropped images of the blots are shown in the Supplementary Information. (c) TBL1 and TBLR1 were required for SuperTopflash activities induced by LiCl. 293T cells were transfected with TBL1, TBLR1 or control siRNA. Two days after the first transfection, cells were transfected with a SuperTopflash, SuperFopflash or NF-κB luciferase reporter and pCMV-β-galactosidase for 24 h and then treated with 20 mM LiCl for 6 h. Values are mean ± s.d. for triplicate samples from a representative experiment. (d) TBL1 and TBLR1 were required for SuperTopflash activities induced by Wnt3a. Cells were transfected as described as in c and then treated with Wnt3a (200 ng ml–1) for 6 h. Values are mean ± s.d. for triplicate samples (n = 3) from a representative experiment.

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TCF-4 and this interaction could be enhanced by Wnt3a stimulation (see Supplementary Information, Fig. S1c). In most colorectal cancers, the APC tumour suppressor gene is mutated and β-catenin cannot be degraded, resulting in the accumulation of β-catenin in the nucleus and the constitu-tive activation of Wnt signalling5–8. Therefore, we also examined whether elevated nuclear β-catenin interacted with TBL1–TBLR1 in colorectal can-cer cells. As shown in Fig. 2d, Anti- β-catenin antibodies, but not control IgG, could pull-down TBL1 or TBL1R1 in a colorectal cancer cell line SW480 that contains an APC mutation5,7. The reverse co-immunoprecipi-tation also found that anti-TBL1 or anti-TBLR1 pulled down β-catenin. These results indicate that Wnt signalling induces the interaction between β-catenin and TBL1–TBLR1.

Wnt signalling is highly conserved from nematode through high ver-tebrates. In Drosophila, the homologue of TBL1, Ebi, is required for epi-dermal growth factor receptor (EGFR)-induced expression of the Notch ligand, Delta. Ebi promotes proteasomal degradation of Drosophila N-CoR homologue, SMRTER21. According to genomic database analysis, Ebi is the sole homologue for TBL1 and TBLR1 in Drosophila. Thus, we also determined whether Ebi has an essential role in Wingless (Wg) signalling in fly kc167 cells. The overexpression of Ebi also enhanced Topflash reporter activities induced by Arm* (a stable form of Armadillo; see Supplementary Information, Fig. S2a). To further validate that Ebi was required for Wg signalling, RNA interference (RNAi) was used to knockdown ebi to determine whether Ebi functioned in the expression of Wg target genes. nkd and CG6234 were selected as they have been identified as Wg target genes26, 27. ebi RNAi corresponding to two dif-ferent regions of ebi open reading frame (ORF) or their mixture, but not control RNAi, significantly suppressed ebi expression, as deter-mined by real-time PCR analysis (see Supplementary Information, Fig. S2b). Although nkd mRNA induced by Wg-conditioned media in Kc167 cells was induced approximately 20–40 fold, knockdown of ebi significantly reduced nkd transcription by over 70%. Similarly, the knockdown of ebi also significantly inhibited CG6234 expression by over 75% (see Supplementary Information, Fig. S2c). In contrast, ebi knock-down did not affect the expression of dpp, a hedgehog target gene (see Supplementary Information, Fig. S2d). As a control, knockdown of ebi had no effects on the Wg stabilization of Arm protein levels in kc167 cells, as determined by western blot analysis (see Supplementary Information, Fig. S2e). Furthermore, Wg also stimulated the interaction between Arm

and Ebi in kc167 cells, as determined by co-immunoprecipitation (see Supplementary Information, Fig. S2f). Taken together, these results sug-gest that Ebi is also required for Wg signalling in Drosophila cells.

Recruitment of TBLR1 and/or TBL1 to the Wnt target-gene promoterAs TBL1–TBLR1 are required for activation of Wnt target genes and can form a complex with β-catenin in the nucleus, it is possible that TBL1–TBLR1 might be recruited to a Wnt-regulated enhancer (WRE). To exam-ine this possibility, we determined whether or not TBL1–TBLR1 bound to the AXIN2 promoter, which contains WRE28–30 and is tightly regulated by Wnt–β-catenin signalling, using chromatin immunoprecipitation (ChIP) assays. The ChIP-enriched DNA was quantified by real-time PCR using AXIN2-specific primers. As a strict negative control, a region located in the ORF was also examined. In the absence of lithium stimulation, β-catenin and TBLR1 bound to the WRE region at a background level compared to the ORF region (Table 1). TBL1 could be modestly detected at the WRE compared to the ORF. Addition of LiCl caused a dramatic increase in β-catenin and TBLR1 binding to the AXIN2 promoter, with similar kinetics. TBL1 binding to the AXIN2 enhancer was also increased. Correlated with β-catenin binding, the co-repressors TLE1 and HDAC1 were removed from the AXIN2 promoter on lithium stimulation (Table 1 and see Supplementary Information, Fig. S3d). Wnt3a also induced the recruitment of β-catenin and TBLR1 to the AXIN2 promoter, coincident with the removal of the co-repressors TLE1 and HDAC1 (Table 1). As c-Myc is a well known Wnt target gene that plays a critical role in oncogen-esis16, we also examined whether Wnt3a induced TBL1–TBLR1 binding to the c-Myc promoter using ChIP. Similarly to AXIN2, β-catenin and TBLR1 were also rapidly recruited to the c-Myc promoter on Wnt3a stimulation, as determined by real-time PCR using c-Myc-specific prim-ers to quantify the ChIP-enriched DNA (Table 1). Unlike on the AXIN2 promoter, however, TBL1 was not detected on the c-Myc promoter with-out stimulation, and was recruited to the c-Myc promoter in the presence of Wnt3a stimulation. To exclude the secondary effect of Wnt-induced transcriptional activity, we also confirmed that LiCl or Wnt3a did not induce ChIP signals of histones on the AXIN2 or c-Myc promoter (see Supplementary Information, Fig. S3a, b). The ChIP signals of TBL1 or TBLR1 on the AXIN2 promoter induced by LiCl could be reduced by TBL1 or TBLR1 siRNA, respectively (see Supplementary Information, Fig. S3c).

Table 1 TBL1–TBLR1 and β-catenin were recruited to the Wnt target-gene promoter after LiCl treatment or Wnt3a stimulation

ChIP

Inducer LiCl Wnt3a

Gene AXIN2 AXIN2 c-Myc

Time 0 min 50 min 100 min 200 min 360 min 480 min 0 1 h 2 h 4 h 6 h 0 1 h 2 h 4 h 6 h

β-cat-enin

WRE 0.10 0.44 0.76 1.33 1.16 0.93 0.08 0.57 0.59 0.34 0.29 0.07 0.19 0.36 0.19 0.17

ORF 0.08 0.09 0.08 0.08 0.08 0.09 0.09 0.08 0.07 0.08 0.06 0.04 0.05 0.06 0.07 0.06

TBL1WRE 0.28 0.37 0.42 0.53 0.47 0.38 0.14 0.20 0.19 0.20 0.21 0.16 0.26 0.32 0.31 0.22

ORF 0.12 0.12 0.11 0.09 0.11 0.11 0.08 0.08 0.10 0.07 0.09 0.13 0.14 0.14 0.12 0.14

TBLR1WRE 0.13 0.22 0.25 0.40 0.24 0.20 0.09 0.19 0.37 0.16 0.14 0.10 0.26 0.31 0.38 0.31

ORF 0.09 0.08 0.12 0.11 0.10 0.11 0.09 0.11 0.11 0.13 0.11 0.13 0.12 0.14 0.15 0.15

HDAC1WRE 1.32 0.71 0.47 0.65 0.99 0.96 3.87 2.09 1.39 2.40 3.16 1.37 0.81 0.53 0.96 0.99

ORF 0.62 0.48 0.46 0.55 0.63 0.48 0.99 1.07 0.83 1.02 0.99 0.33 0.33 0.34 0.32 0.30

TLE1WRE 1.14 0.71 0.49 0.52 0.77 0.57 4.45 2.93 2.05 3.73 4.01 0.53 0.26 0.20 0.43 0.46

ORF 0.64 0.60 0.48 0.63 0.71 0.41 1.25 1.20 1.22 1.37 1.29 0.22 0.18 0.21 0.20 0.23

Each data point is the average of duplicates from a representative experiment. Experiments were repeated at least three times

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Overexpression of mouse TBL1 or TBLR1, which are not targeted by human TBL1 or TBLR1 siRNA, rescued the Topflash activity caused by RNAi knockdown of TBL1–TBLR1 (see Supplementary Information, Fig. S3e). Western blot analysis confirmed that LiCl or Wnt3a induced the nuclear level of β-catenin (Fig. 3a, b). Finally, TBL1 and TBLR1, similarly to β-catenin, were constitutively bound to the promoter of AXIN2 (Fig. 3c, d), immunoglobulin transfection factor-2 (ITF2) and dickkopf 1(DKK1) (see Supplementary Information, Fig. S4a, b) in the human colorectal can-cer cell lines SW480 and HT29. TBL1 and TBLR1 were also constitutively detected on the promoter of ITF2 and DKK1 in Wnt1-expressing 293 cells (see Supplementary Information, Fig. S4c). Taken together, these observa-tions indicate that TBLR1–TBL1 and β-catenin are recruited to the Wnt target-gene promoter on Wnt signalling.

Recruitment of TBL1–TBLR1 and β-catenin to the Wnt target-gene promoter is mutually dependentNext, we explored how TBL1–TBLR1 controlled β-catenin–Tcf-mediated transcription. Previously, TBL1–TBLR1 has been found to strongly bind histone, which may help to maintain the co-repressor complexes on chro-matin to repress transcription23. Given the opposite role of TBL1–TBLR1 in Wnt signalling, we hypothesized that TBL1–TBLR1 might target β-catenin to chromatin to remove the co-repressors TLE1 and HDAC1.

To test our hypothesis, we determined whether the depletion of TBL1 and TBLR1 by siRNA affected β-catenin binding to the promoter of Wnt target genes. Compared with control siRNA treatment, β-catenin binding to the AXIN2 promoter induced by lithium was significantly inhibited by reducing endogenous TBL1 or TBLR1 (Fig. 4a). Similarly, knock-down of ebi also inhibited Arm binding to the nkd promoter induced by Wg (Fig. 4b). It is known that β-catenin shuttles between cytoplasm and nucleus, a process that is regulated by several Wnt regulators (such as APC and Axin15,31). Bcl-9/Lgs and Pygopus play a role in the nuclear translocation of β-catenin, as well as in transcription32. Thus, it was pos-sible that TBL–TBLR1 might also function in maintaining β-catenin in the nucleus. Subsequently, the depletion of TBL–TBLR1 might reduce the nuclear level of β-catenin, resulting in the reduction of β-catenin bind-ing to the promoter indirectly. To exclude this possibility, western blot analysis was performed to examine the nuclear level of β-catenin. The nuclear accumulation of β-catenin induced by lithium was not affected by the knockdown of TBL1 or TBLR1 (Fig. 4c). Moreover, the knockdown of TBL1 or TBLR1 also suppressed β-catenin binding to the promoters of AXIN2 and c-Myc induced by Wnt3a (Fig. 4d,e). In the nuclear recep-tor signalling pathway, TBL1–TBLR1 promote transcription activation by recruiting the ubiquitin-conjugating machinery to degrade the co-repressors. The inhibition of the proteasome activity suppresses nuclear

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Figure 2 Wnt signalling stimulates the interaction between β-catenin and TBL1–TBLR1. (a) TBL1–TBLR1 and β-catenin interacted when overexpressed in 293T cells. Cells were transfected with plasmids expressing HA–β-catM, HA–Hox C6, Flag–TBL1 or Flag–TBLR1, as indicated. Forty-eight hours post transfection, cell extracts were prepared and immunoprecipitated with anti-HA antibodies. The precipitates were resolved on SDS–PAGE gels and analysed by western blot with anti-Flag antibodies. (b) Endogenous β-catenin interacted with TBL1–TBLR1 on LiCl stimulation. 293T cells were untreated or treated with LiCl for 2 h. The nuclear protein extracts were prepared and immunoprecipiated with anti-TBL1, anti-TBLR1 or anti-IgG, respectively. The precipitates were resolved on SDS–PAGE and analysed by western blot with the indicated antibodies. Uncropped images of the

blots are shown in the Supplementary Information. (c) Wnt3a induced the interaction between β-catenin and TBL1–TBLR1. Cells were untreated or treated with Wnt3a (200 ng ml–1) for 2 h. The nuclear extracts were prepared and immunoprecipiated with anti-TBL1, anti-TBLR1 or anti-β-catenin antibodies, respectively. The precipitates were analysed by western blot with the indicated antibodies. Uncropped images of the blots are shown in the Supplementary Information. (d) Endogenous TBL1–TBLR1 interacts with β-catenin in SW480 cells. The nuclear protein extracts were prepared from SW480 cells and immunoprecipiated with anti-TBL1, anti-TBLR1, anti-β-catenin or control IgG. The precipitates were analysed by western blot using the indicated antibodies. Uncropped images of the blots are shown in the Supplementary Information.

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receptor-mediated transcription. On the contrary, knockdown of the S1 subunit of the 19S proteasome, the E2 enzyme UbcH7, or the E3 ubiq-uitin ligase components Siah1 or Skp1 did not affect TBL1–TBLR1 or β-catenin binding to the c-Myc and Axin2 promoters (see Supplementary Information, Fig. S5a, b) in Wnt1-expressing 293 cells. The proteasome inhibitors, PS-341 and MG132, also did not affect their binding to the Wnt target gene promoter (see Supplementary Information, Fig. S5c). In addition, the knockdown of Siah1 and Skp1 did not inhibit the expression of Wnt target genes induced by Wnt1, as determined by real-time RT–PCR (see Supplementary Information, Fig. S5d). These results indicate that TBL1 and TBLR1 play a critical role in recruiting β-catenin to the Wnt target-gene promoter in the Wnt–β-catenin signalling pathway.

As the Wnt signal induced the interaction between β-catenin and TBL1–TBLR1, it is possible that β-catenin might affect TBL1–TBLR1 binding to the Wnt target-gene promoter. To examine this possibility, siRNA was used to knockdown β-catenin. Because the knockdown of β-catenin in 293 cells significantly affected cell growth and attachment, we could not obtain a sufficient amount of cells for ChIP assays. To over-come this problem, β-catenin was knocked down in Wnt-1-expressing 293 cells. Western blot analysis confirmed that β-catenin siRNA strongly reduced β-catenin levels (Fig. 4f). ChIP assays revealed that the deple-tion of β-catenin also significantly inhibited TBL1–TBLR1 binding to the AXIN2 or c-Myc promoter (Fig. 4g). Taken together, these results suggest that the recruitment of TBL1–TBLR1 and β-catenin to the Wnt target gene promoter is mutually dependent.

As described above, TBL1–TBLR1 constitutively occupied the Wnt tar-get-gene promoter in human colorectal cancer cells. Previously, an HT-29-APC colorectal cancer cell line has been established that expresses the full-length wild-type APC under the control of a zinc-inducible metal-lothionein promoter33. The induction of wild-type APC by zinc in these cells degrades β-catenin and inhibits Wnt target-gene transcription. Interestingly, it was demonstrated that wild-type APC could bind to the Wnt target-gene promoter and promote the dissociation of β-catenin and its co-activators from the c-Myc promoter in HT-29-APC cells15. Thus, we examined whether the induction of wild-type APC and the reduction of β-catenin could remove TBL1–TBLR1 from the AXIN2 and c-Myc pro-moter. Western blot analysis showed that zinc treatment induced β-catenin degradation in HT-29–APC cells, but not in control cells HT-29–β-Gal (Fig. 4h). ChIP analysis revealed that the induction of APC by zinc treat-ment strongly stimulated the dissociation of TBL1–TBLR1 and β-catenin from the axin promoter (Fig. 4i). Similarly, the induction of APC expres-sion also removed TBL1–TBLR1 and β-catenin from the c-Myc promoter (Fig. 4j). As a control, zinc treatment did not affect TBL1–TBLR1 binding to the AXIN2 and c-Myc promoters (data not shown).

To further demonstrate that the interaction between TBL1–TBLR1 and β-catenin promotes the binding of the Wnt target-gene promoter, we exam-ined whether or not TBL1–TBLR1 and β-catenin co-occupied the Wnt tar-get-gene promoter, using sequential two-step ChIP or reChIP assays under conditions of activated Wnt signalling. The chromatin complexes from lithium-treated 293T cells were first isolated with anti-β-catenin and then subjected to reChIP with anti-TBL1 or anti-TBLR1. Compared with the non- β-catenin bound region, a significant amount of TBL1–TBLR1 bound chromatin complexes was detected from the initial β-catenin-associated chromatin complex, as determined by real-time PCR using the AXIN2-specific primers. The reverse reChIP experiment, first using anti-TBL1 or anti-TBLR1 and subsequently anti-β-catenin, also revealed that β-catenin

could be pulled down from the initial TBL1–TBLR1-containing chro-matin complex (Fig. 5a). Furthermore, we examined whether Wnt3a induced TBL1–TBLR1 and β-catenin to co-occupy the AXIN2 and c-Myc promoters. The reciprocal two-step ChIP assays demonstrated that TBL1–TBLR1 and β-catenin co-occupied the AXIN2 and c-Myc promoters on Wnt3a stimulation (Fig. 5b, c).

Critical role of TBL1–TBLR1 in β-catenin-mediated oncogenesisAs β-catenin is constitutively activated in human colorectal cancer cells, we examined whether the depletion of TBL1–TBLR1 also affected β-catenin binding to the promoters of the Wnt target genes in HT29 cells. To effi-ciently knockdown TBL1 and TBLR1 in HT29 cells, retrovirus-mediated delivery of short-hairpin RNA (shRNA) against TBL1 and TBLR1 was used. Western blot analysis confirmed that the expression levels of TBL1 or TBLR1 in HT29 cells was significantly reduced by TBL1 or TBLR1

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Figure 3 TBL1–TBLR1 constitutively occupied on the Wnt target-gene promoter in colorectal cancer cell lines. (a, b). Wnt3a and LiCl induced β-catenin stabilization. Uncropped images of the blots are shown in the Supplementary Information. (c, d) Both TBL1 and TBLR1 constitutively occupied the axin-2 promoter in colorectal cancer cell lines SW480 and HT29 cells. Each data point is the average of duplicates from a representative experiment. Experiments were repeated at least three times.

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Figure 4 Both TBL1 and TBLR1 are required for the recruitment of β-catenin to the Wnt target-gene promoter. (a) Knockdown of TBL1 or TBLR1 inhibited the recruitment of β-catenin to the AXIN2 promoter induced by LiCl in 293T cells. Chromatin and DNA complexes were immunoprecipitated with anti-β-catenin antibodies. (b) ChIP analysis of Arm in kc167 cells demonstrated that binding of Arm to the nkd promoter was decreased by the depletion of ebi with dsRNA. (c) Knockdown of TBL1 or TBLR1 did not affect the nuclear accumulation of β-catenin. The nuclear proteins were isolated and probed with anti-β-catenin. Uncropped images of the blots are shown in the Supplementary Information. (d) Knockdown of TBL1 or TBLR1 inhibited the recruitment of β-catenin to the AXIN2 promoter induced by Wnt3a in 293T cells. The ChIP assay was performed as described in the Methods. (e) Knockdown of TBL1

or TBLR1 inhibited the recruitment of β-catenin to the c-Myc promoter induced by Wnt3a in 293T cells. (f) Knockdown of β-catenin by siRNA in Wnt1-expressing 293 cells. Uncropped images of the blots are shown in the Supplementary Information. (g) Knockdown of β-catenin inhibited TBL1–TBLR1 recruitment to the AXIN2 or c-Myc promoter in Wnt1-expressing 293 cells. (h) The induction of wild-type APC induced β-catenin degradation. Uncropped images of the blots are shown in the Supplementary Information. (i) The induction of wild-type APC removed TBL1–TBLR1 and β-catenin from the axin promoter. Cells were treated with ZnCl2 and the ChIP assay was performed. (j) The induction of wild-type APC removed TBL1–TBLR1 and β-catenin from the c-Myc promoter. Each data point is the average of duplicates from a representative experiment. Experiments were repeated at least three times.

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shRNA when compared with the cells expressing control shRNA (Fig. 6a). Although the depletion of TBL1 or TBLR1 did not affect the nuclear level of β-catenin (Fig. 6b), it significantly reduced the recruitment of β-catenin to the AXIN2 and c-Myc promoters, as determined by ChIP assays (Fig. 6c, d). As TBL1 and TBLR1 were required for recruiting β-catenin to the Wnt target-gene promoter to initiate transcription, we wanted to deter-mine whether TBL1–TBLR1 played an essential role in the endogenous expression of Wnt target genes. Compared with cells infected with control shRNA, the knockdown of TBL1 or TBLR1 significantly reduced AXIN2 expression (by 60–70%), as determined by real-time RT–PCR (Fig. 6e). In colorectal cancer cells, c-Myc, MMP7 and ITF2, which have been identi-fied as Wnt target genes8,34,35, are highly expressed and play a critical role in tumour growth and progression. Thus, we also examined whether the knockdown of TBL1 or TBLR1 inhibited the endogenous expression of c-Myc, MMP7 and ITF2 in HT29 cells. Compared with control shRNA, knockdown of TBL1 or TBLR1 also significantly inhibited the endogenous expression of c-Myc, MMP7 and ITF2 by 60–80% (Fig. 6f–h). In contrast, knockdown TBL1 or TBLR1 did not affect the expression of non-Wnt target genes in HT-29 cells (see Supplementary Information, Fig. S4d, e). Interestingly, knockdown of TBL1 or TBLR1 significantly increased the spontaneous apoptosis in HT29 cells. HT29 cells expressing TBL1 or TBLR1 shRNA could not propagated for over two weeks, which is con-sistent with the anti-apoptotic function of Wnt–β-catenin signalling33,36,37. Thus, soft agar assays were performed to measure anchorage-independent survival and growth that typically correlates with a tumorigenic phenotype in vivo. Although HT29 cells expressing Luc shRNA could form clones in the soft agar, HT29 cells expressing either TBL1 or TBLR1 shRNA did not grow (Fig. 6i, j). Previously, we observed that overexpression of β-cat-enin in squamous cell carcinoma cells promoted invasive growth on the Matrigel38. Thus, we also determined whether TBL1–TBLR1 were required for β-catenin-mediated invasive growth. Knockdown of TBL1 or TBLR1 also strongly inhibited tumour cell invasion through the Matrigel-coated membrane (Fig. 6k, l). Moreover, knockdown of TBL1 or TBLR1 strongly suppressed tumour growth in nude mice (Fig. 6m).

DISCUSSIONOur results identify two new components required for nuclear β-catenin function. Unlike other β-catenin-interacting partners (such as Bcl9/Lgs and Pigopus, PAF-1 and SET1)15,17, TBL1–TBLR1 play a specific role in the recruitment of β-catenin to the Wnt target-gene promoter. Intriguingly, this recruitment seems to be mutually dependent, as the depletion of β-catenin also impairs TBL1–TBLR1 binding to the Wnt tar-get-gene promoter. Our findings provide new insights into the molecular regulation of β-catenin-mediated transcription.

In the nuclear receptor or other signalling pathways, TBL1 and TBLR1 have been identified in the N-CoR co-repressor complex and are responsible for recruiting ubiquitin/proteasome machinery to degrade co-repressors on ligand binding19,24. However, unlike the nuclear receptor signalling, the co-repressors TLE1 and HDAC1 from the promoter of Wnt target genes are removed by β-catenin through competitive bind-ing to Tcf. Knockdown of the S1 subunit of the 19S proteasome or other component of ubiquitin systems did not affect β-catenin recruitment. Thus, it is unlikely that TBL1–TBLR1 uses a similar mechanism in the nuclear receptor to remove the co-repressors in Wnt–β-catenin signal-ling. In the absence of Wnt signalling, the promoter region of the Wnt target gene is hypoacetylated because it is occupied by a complex of Tcf

and TLE1–HDAC1 (ref. 10). Interestingly, TBL1 and TBLR1 have been found to preferentially bind to the hypoacetylated histones and have a critical function in targeting the co-repressor complexes to chromatin23. As knockdown of TBL1 and TBLR1 inhibited the recruitment of β-cat-enin to the Wnt target gene, it is possible that that β-catenin may use TBL1–TBLR1 bind to hypoacetylated chromatin in the region of the Wnt target-gene promoter on Wnt stimulation. Subsequently, β-catenin interacts with Tcf by dissociating TLE1–HDAC1 to induce chromatin hyperacetylation and to recruit co-activators to stimulate downstream target genes. However, it should be emphasized that TBL1–TBLR1 and

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Figure 5 TBL1–TBLR1 and β-catenin co-occupied the Wnt target-gene promoter. (a) TBL1–TBLR1 and β-catenin co-occupied the AXIN2 promoter on lithium stimulation, as determined by two-step ChIP assays. 293T cells were treated with lithium for 4 hr. The chromatin and DNA complexes were first precipitated with β-catenin and then subjected to reChIP with anti-TBL1 or anti-TBLR1 antibodies. The initial β-catenin-associated chromatin complex was determined by real-time PCR using AXIN2-specific primers. For the reverse two-step ChIP experiment, the chromatin complexes were first immunoprecipiated with anti-TBL1 or anti-TBLR1, and subsequently with anti-β-catenin. (b) TBL1–TBLR1 and β-catenin co-occupied the AXIN2 promoter on Wnt3a stimulation. Cells were treated with Wnt3a for 2 h and two-step ChIP assays were performed as described in a. (c) TBL1–TBLR1 and β-catenin co-occupied the c-Myc promoter on Wnt3a stimulation, as determined by two-step ChIP assays. Each data point is the average of duplicates from a representative experiment. Experiments were repeated at least three times.

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Figure 6 TBL1 and TBLR1 play a critical role in β-catenin-mediated oncogenesis. (a) Knockdown of TBL1 or TBLR1 in HT29 cells by shRNA. HT29 cells were transduced with retroviruses expressing TBL1 shRNA, TBLR1 shRNA or Luc shRNA and selected with puromycin for 5 days. Uncropped images of the blots are shown in the Supplementary Information. (b) Knockdown of TBL1 or TBLR1 did not affect the nuclear level of β-catenin. Uncropped images of the blots are shown in the Supplementary Information. (c) Knockdown of TBL1 or TBLR1 reduced β-catenin binding to the AXIN2 promoter in HT-29 cells. (d) Knockdown of TBL1 or TBLR1 reduced β-catenin binding to the c-Myc promoter in HT-29 cells. Each data point is the average of duplicates from a representative experiment. Experiments were repeated at least three times. (e) Knockdown of TBL1 or TBLR1 inhibited the endogenous level of AXIN2 in HT-29 cells. The expression of AXIN2 was determined by quantitative RT–PCR. Values are mean ± s.d. of triplicate samples from a representative experiment. (f–h) Knockdown of TBL1 or TBLR1 inhibited the endogenous level of c-Myc, MMP7 and ITF2 in HT-29 cells. Values

are mean ± s.d. of triplicate samples from a representative experiment. (i, j) Knockdown of TBL1 or TBLR1 inhibited HT-29 cell growth in soft agar. Values are mean ± s.d. of triplicate samples from a representative experiment. The scale bar represents 200 µm. (k) Knockdown of TBL1 or TBLR1 did not affect the nuclear level of β-catenin in SCC1–β-cat cells. Uncropped images of the blots are shown in the Supplementary Information. (l) Knockdown of TBL1 or TBLR1 inhibited the invasive growth of head and neck squamous cell carcinoma cells mediated by β-catenin. The number of invasive cells was counted from three different fields and averaged. The results represent mean ± s.d. from three independent experiments. (m) Knockdown of TBL1 or TBLR1 inhibited head and neck squamous cell carcinoma growth in vivo. SCC1–β-Cat–Con shRNA, SCC1–β-Cat–TBL1 shRNA and SCC1–β-Cat–TBLR1 shRNA cells were injected into nude mice and tumour growth was measured for two weeks. Values are mean ± s.d. for five mice from a representative experiment. Statistical significance was determined by Student’s t-test. The asterisk indicates P < 0.01.

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β-catenin are mutually dependent on each other. Knockdown of β-catenin also impairs TBL1–TBLR1 binding to the Wnt target-gene promoter. In this regard, the interaction between TBL1–TBLR1 and β-catenin collabo-ratively facilitates their binding and recruitment to the Wnt target-gene promoter on Wnt stimulation (Fig. 7).

Given that TBL1 has previously been shown to have a critical role in p53-induced β-catenin degradation, which uses a different mechanism from the canonical Wnt–β-catenin signalling pathway to degrade β-cat-enin18, the findings that TBL1–TBLR1 are required for β-catenin-mediated transcription are intriguing and unexpected. A recent genome-wide RNAi screen in Drosophila cells has identified many potential regulators of the Wnt/β-catenin signalling pathway39. In this study, ebi was listed as one of key regulators for Arm-dependent transcription, further supporting our results. Interestingly, GSK3β has been shown to have an opposing role in the canonical Wnt–β-catenin signalling pathway that depends on its localiza-tion and associated complex40,41. In the p53-mediated β-catenin pathway, TBL1 is associated with β-catenin-independent of GSK3β phosphorylation, p53-induced Siah, and Skp1 (ref. 18). In the canonical Wnt–β-catenin sig-nalling pathway, β-catenin degradation is controlled through phosphoryla-tion-dependent interactions with the F-box protein β-TrCP1–4. Our findings highlight the importance of signalling specificity in the biological function of a signalling molecule. Importantly, knockdown of TBL1 and TBLR1 sig-nificantly suppressed the invasive growth of head and neck sqaumous cell carcinoma cells. Our results suggest that targeting the interaction between β-catenin and TBL1–TBLR1 may be a novel strategy for inhibiting Wnt–β-catenin signalling in cancer therapy.

METHODSPlasmids, antibodies and oligonucleotides. The plasmids SuperTopflash report-ers, pActin–β-galactosidase, CMV–β-galactosidase and pActin–V5–Arm* were a kind gift from K. M. Cadigan (University of Michigan, Ann Arbor [AU: OK?], MI). ebi was isolated by RT–PCR and subcloned into the pActin–V5 vector (Invitrogen). Flag–TBL1, Flag–TBLR1 and the fragments were constructed in pQNCX2 vector by standard PCR subcloning. All antibodies are commercially available: β-catenin (BD), anti-V5 epitope (Invitrogen), anti-Flag (Sigma), anti-HA (Covance), TBL1 and HDAC1 (Abcam), TBLR1 (Bethyl Laboratories) and TLE1 (Santa Cruz). The sequences of RT–PCR primers were: β-tubulin56D (5´-AGACCTACTGCATCGACAAC-3´, 5´-GACAAGATGGTTCAGGTCAC-3´); nkd (5´-TAAATTCTCGGCGGCTACAA-3´, 5´-CGCACCTGGTGGTACAT-CAG-3´); CG6234 (5´-GCTGCTCTGCGTGATCGTCTTC-3´, 5´-TCTGGT-GTTGGTGAACTCTCCTCC-3´); ebi (5´-AGCTTCGACAAGTGCGTACA-3´, 5´-GAGTTCCAGCACACCTCAAA-3´); GAPDH (5´-ATCATCCCTGCCTC-TACTGG-3´, 5´-GTCAAGTCCACCACTGACAC-3´); ITF2 (5´-CCCTAGCT CCTTCTTCATGC-3´, 5´-GTAGCTGCTGGACTGTGGAA-3´); AXIN2 (5´-CTGGCTTTGGTGAACTGTTG-3´, 5´-AGTTGCTCACAGCCAA-GACA-3´); c-Myc (5´-CTACCCTCTCAACGACAGCA-3´, 5´-AGAGCA-GAGAATCCGAGGAC-3´); MMP7 (5´-GACATCATGATTGGCTTTGC-3´, 5´-TCCTCATCGAAGTGAGCATC-3´). The oligonucleotide sequences for ChIP assays were: nkd-WRE (5´-AATTTCCCAGACCGCTTTCC-3´, 5´-CGAAAAAGCCGCCAAACATAT-3´); nkd-ORF (5´-ACCTTCTGGCTTT-GGAGCAG-3´, 5´-TGGGCTCCTCATAAACTGGC-3´); AXIN2-WRE (5´-CTGGAGCCGGCTGCGCTTTGATAA-3´, 5´-CGGCCCCGAAATC-CATCGCTCTGA-3´); AXIN2-ORF (5´-CTGGCTTTGGTGAACTGTTG-3´, 5´-AGTT GCTCACAGCCAAGACA-3´); c-Myc-WRE (5´-GCGGGTTACATAC AGTGCACTTCA-3´, 5´-TGGAAATGCGGTCATGCACAAA-3´); c-Myc-ORF (5´-CTACCCTCTCAACGACAGCA-3´, 5´-AGAGCAG AGAATC-CGAGGAC-3´); DKK1-WRE (5´-CACATTAGCCCACCACTGAG-3´, 5´-CAGACGCGTGAGATCAAAGT-3´); DKK1 ORF (5´-AGCCTCT-TAACTCCTTGGCA-3´, 5´-TCCATGTCACTGGGTTCCTA-3´); ITF2-WRE (5´-CTGCGGAAGTCTGTGGAGT-3´, 5´-CATTTCCCTGCGCTCTCTTATT-3´) and ITF2-ORF (5´-CTCCCAAGACCGTTCTCTCT-3´, 5´-CTG GTC-CCAGCAAGCTAAG-3´). The siRNA oligonucleotide sequences were: TBL1

(5´-TCACTGGACTGGAATACCA-3´, 5´-AAGATGAGCATAACCAGTGAC-3´); TBLR1 (5´-AAGGCCCTATATTTGCATTAA-3´, 5´-GGACGCACAT-ACTGGTGAA-3´); ebi-1 (5´-CTGGCGTGGACAAGACGACGATCATC-3´, 5´-AACTGTGCACTAGCTGTCC CGTTTGCGT-3’); ebi-2 (5´-GGCCTACGT-GTTTGGCATTGAGTCAC-3´, 5´-TTG ATCCTGTTGCAGTCGAAGCTCCG-3´) and scrambled control (5´-ATGATTGAA CAAGATGGATTGCACGCA-3´, 5´-AATATCACGGGTAGCCACGCTATGTCCT-3´).

Cell culture, transfection and reporter gene assays. Human embryonic kidney 293T, SW480 and UMSCC1 cells were cultured in DMEM media containing 10% FBS at 37 °C in a 5% CO2/95% air atmosphere. HT29 cells were cultured in McCoy’s 5A media containing 10% FBS. For transient transfections, 1 × 106 293T cells were seeded into 12-well plates for 12 h and then transfected using Lipofectamine 2000 reagents according to the manufacturer’s protocol (Invitrogen). Fly Kc167 cells were cultured in Schneider’s Drosophila Media containing 5% FBS at room temperature. For transfections, 1 × 106 Kc167 cells were seeded into 12-well plates. Cells were transfected using FuGENE6 reagents (Roche Molecular Biochemicals). The total amount of DNA in each individual well was kept constant by adding pQNCX2 or pActin5.1 empty vector as appropriate. Luciferase and β-galactosidase activity of total cell lysates were determined using Luc-Screen and GalactoStar kits (Tropix).

RNAi knockdown in Kc167 cells and siRNA knockdown in mammalian cells. Double stranded RNA (dsRNA) corresponding to the ORF of ebi were synthe-sized using the MEGAscript T7 kit (Ambion). Primer sequences for amplifying dsRNA are shown in the Supplementary Information, Table S1. Cells (1 × 106) were seeded in 12-well plates in Drosophila serum-free media (Invitrogen) in the presence of 10 µg of specific dsRNAs or control dsRNAs. After the cells were cul-tured at room temperature for 2 h, 5% FBS was added and the cells were cultured for 5 days. Wg-conditioned media was collected using stable pTubwg S2 cells (gift from K. M. Cadigan), and was typically concentrated for ~50-fold using a Centricon tube (Millipore) and stored at –80 °C.

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Figure 7 Schematic representation of a model for the role of TBL1–TBLR1 in the recruitment of β-catenin to the Wnt target-gene promoter. In the absence of Wnt signalling, Tcf interacts with the corepressors TLE1 and HDAC1 on the chromatin to repress Wnt target genes. On Wnt stimulation, stabilized β-catenin is translocated to the nucleus to interact with TBL1–TBLR1. TBL1–TBLR1 and β-catenin mutually facilitate their binding to the Wnt target-gene promoter and then displace the corepressors TLE and HDAC1, resulting in the stimulation of Wnt target-gene transcription.

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For transfection of siRNA, 293T cells were plated at 40–50% confluence and trans-fected with various amount of siRNA using Oligofectamine reagents (Invitrogen). For siRNA combined with luciferase reporter experiments, 100 ng of SuperTopflash and 50 ng of CMV–β-galactosidase constructs were transfected using Lipofectamine 2000 (Invitrogen) 2 days after the transfection of siRNA. siRNAs were synthesized by Dharmacon Research and primer sequences are listed in the Supplementary Information, Table S1. To knock down TBL1 and TBLR1 in HT29 and SCC1–β-cat cells, TBL1 shRNA and TBLR1 shRNA or control shRNA were constructed in pSI-REN–RetroQ according to the manufacturer’s instruction (Clontech). The primer sequences are listed in the Supplementary Information, Table S1.

Real-time PCR. Total RNA from kc167 or HT29 cells was purified using Trizol reagents and cDNA was synthesized with oligo-dT primers using SuperScript II (Invitrogen). Quantitative RT–PCR analysis was carried out with iQ SYBR Green Supermix (BioRad) on an iCycler iQ real-time PCR detection system (BioRad). Tubulin or GAPDH levels were used as a loading control for real-time PCR in Kc167 and HT29, respectively. Sequences of the primer pairs used are listed in the Supplementary Information, Table S1.

Coimmunoprecipitation and Wwestern blot analysis. kc167 cells (1 × 107) or 5 × 106 293T cells were lysed in 500 µl lysis buffer for 30 min on ice. After centrifuga-tion at 10,600g at 4 °C, the supernatants were incubated with antibodies at 4 °C for 2 h and followed by incubation with protein A or protein G–Sepharose (GE Healthcare) for 1 h. For nuclear co-immunoprecipitation, the nuclear extracts were prepared from 1.5 × 107 of 293T or SW480 cells, as described previously. Immunoprecipitates were washed three times with lysis buffer at 4 °C. Proteins bound to the beads were eluted with SDS-loading buffer at 98 °C for 2 min and then subjected to SDS–PAGE. Western blot analysis was performed as described previously36,37.

ChIP and two-step ChIP assays. ChIP assays were performed using a ChIP assay kit (Upstate Biotechnology) according to the manufacturer’s protocol. Cells were incu-bated with a dimethyl 3,3´-dithiobispropionimidate-HCl (DTBP; Pierce) solution (5 mmol) for 30 min on ice before formaldehyde treatment. For each ChIP reaction, 2 × 106 cells were used. All resulting precipitated DNA samples were quantified by real-time PCR. Data are expressed as the percentage of input DNA. The primer sequences used for real-time PCR are listed in the Supplementary Information, Table S1. For two-step ChIP assays, 1 × 107 cells were used for the first-step ChIP.

Soft Agar, invasion assays and tumour growth. 1×104 HT29 cells (1 × 104) in 2× growth medium were mixed 1:1 with 0.6% agar and then grown at 37 °C for 3 weeks to allow clonal formation. BD BioCoat Matrige invasion chambers were used for tumour cell invasion assays. UMSCC1 cells (5 × 104) were plated on the Matrigel. After 3 days, the top Matrigel was removed and the invaded cells were stained with HEMA-3 kit (Fisher) and counted. For tumour growth, 1 × 106 cells (5 mice per group) were injected into mice for 2 weeks.

Note: Supplementary Information is available on the Nature Cell Biology website.

ACKNOWLEDGMENTSWe thank K. Cadigan for suggestions and reagents, and D. Saims and J. Guan for reading the manuscript. This work was supported by National Institutes of Health (NIH) grants to C.Y.W.

AUTHOR CONTRIBUTIONSJ.L. performed the experiments. J.L. and C.Y.W. designed the experiments, analysed the data and wrote the manuscript.

Published online at http://www.nature.com/naturecellbiology/ Reprints and permissions information is available online at http://npg.nature.com/reprintsandpermissions

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17. Kramps, T. et al. Wnt/Wingless signalling requires BCL9/Legless-mediated recruitment of pygopus to the nuclear β-catenin–TCF complex. Cell 109, 47–60 (2002).

18. Matsuzawa, S. I. & Reed, J. C. Siah-1, SIP, and Ebi collaborate in a novel pathway for β-catenin degradation linked to p53 responses. Mol. Cell 7, 915–926 (2001).

19. Rosenfeld, M. G., Lunyak, V. V. & Glass, C. K. Sensors and signals: a coactivator/core-pressor/epigenetic code for integrating signal-dependent programs of transcriptional response. Genes Dev. 20, 1405–1428 (2006).

20. Barker, N. et al. The chromatin remodelling factor Brg-1 interacts with β-catenin to promote target gene activation. EMBO J. 20, 4935–4943 (2001).

21. Tsuda, L., Nagaraj, R., Zipursky, S. L. & Banerjee, U. An EGFR/Ebi/Sno pathway pro-motes delta expression by inactivating Su(H)/SMRTER repression during inductive notch signalling. Cell 110, 625–637 (2002).

22. Yoon, H. G. et al. Purification and functional characterization of the human N-CoR complex: the roles of HDAC3, TBL1 and TBLR1. EMBO J. 22, 1336–1346 (2003).

23. Yoon, H. G., Choi, Y., Cole, P. A. & Wong, J. Reading and function of a histone code involved in targeting corepressor complexes for repression. Mol. Cell. Biol. 25, 324–335 (2005).

24. Perissi, V., Aggarwal, A., Glass, C. K., Rose, D. W. & Rosenfeld, M. G. A corepres-sor/coactivator exchange complex required for transcriptional activation by nuclear receptors and other regulated transcription factors. Cell 116, 511–526 (2004).

25. Mao, J. Low-density lipoprotein receptor-related protein-5 binds to axin and regulates the canonical Wnt signalling pathway. Mol. Cell 7, 801–809 (2001).

26. Zeng, W. et al. naked cuticle encodes an inducible antagonist of Wnt signalling Nature 403, 789–795 (1998).

27. Fang, M., Li, J., Blauwkamp, T., Bhambhani, C., Campbell, N. & Cadigan, K. M. C-terminal-binding protein directly activates and represses Wnt transcriptional targets in Drosophila. EMBO J. 25, 2735–2745 (2006).

28. Leung, J. Y. et al. Activation of AXIN2 expression by β-catenin-T cell factor. A feedback repressor pathway regulating Wnt signalling. J. Biol. Chem. 277, 21657–21665 (2002).

29. Lustig, B. et al. Negative feedback loop of Wnt signalling through upregulation of con-ductin/axin2 in colorectal and liver tumors. Mol. Cell. Biol. 22, 1184–1193 (2002).

30. Jho, E. H. et al. Wnt/β-catenin/Tcf signalling induces the transcription of Axin2, a nega-tive regulator of the signalling pathway. Mol. Cell. Biol. 22, 1172–1183 (2002).

31. Rosin-Arbesfeld, R., Cliffe, A., Brabletz, T. & Bienz, M. Nuclear export of the APC tumour suppressor controls β-catenin function in transcription. EMBO J. 22, 1101–1113 (2003).

32. Townsley, F. M., Cliffem A. & Bienz, M. Pygopus and Legless target Armadillo/β-catenin to the nucleus to enable its transcriptional co-activator function. Nature Cell Biol. 6, 626–633 (2004).

33. Morin, P. J., Vogelstein, B. & Kinzler, K. W. Apoptosis and APC in colorectal tumori-genesis. Proc. Natl Acad. Sci. USA 93, 7950–7954 (1996).

34. Kolligs, F. T. et al. ITF-2, a downstream target of the Wnt/TCF pathway, is activated in human cancers with β-catenin defects and promotes neoplastic transformation. Cancer Cell 1, 145–155 (2002).

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36. You, Z. et al. Wnt signalling promotes oncogenic transformation by inhibiting c-Myc-induced apoptosis. J. Cell Biol. 157, 429–440 (2002).

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nature cell biology �volume�10�|�number�2�|�FebruArY�2008� 169

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Figure S1 Characterization of interaction between TBL1 and -catenin. a, The arm repeats 1-8 of β-catenin (Aa 133-467) interacted with TBL1. 293T cells were transfected with Flag-β-catenin mutants as indicated. 48 hours post transfection, cell extracts were prepared and IP-ed with anti-TBL1 antibodies. The precipitates were resolved on SDS-PAGE gels and analyzed by Western blot with anti-Flag antibodies. b, The N-termnal regions containing F-box of TBL1 interacted with β-catenin. 293T cells were co-transfected with HA-β-catenin and Flag-TBL1 mutants as indicated. 48 hr after transfection, cell lysates were IP-ed with anti-HA antibodies. The precipitates were resolved on SDS-PAGE gels and analyzed by Western blot with anti-Flag antibodies. c, TBL1 interacted with TCF4. 293T cells were transfected with Flag-TBL1, Flag-TBLR1 or Flag-AP2α as a negative control. 36 hr after transfection, cells were treated with LiCl or Wnt-3a. Cell lysates were IP-ed with anti-TCF4

antibodies and probed with anti-Flag antibodies. Uncropped images of the blots are shown in the Supplementary information. d, Characterization of TBL1/TBLR1 antibodies. 293T cells were transfected with TBL1, TBLR1 or control siRNA. As a positive control, cells were transfected with Flag-TBL1 or Flag-TBLR1. Cells lyastes were probed anti-TBL1 or anti-TBLR1 antibodies. TFIIB was used as a loading control. Uncropped images of the blots are shown in the Supplementary information. e, TBL1 interacted with TBLR1. 293T cells were co-transfected with HA-TBL1 and/or Flag-TBLR1. Cell lysates were IP-ed with anti-Flag antibodies and probed with anti-HA antibodies or vice versa. Uncropped images of the blots are shown in the Supplementary information. f, TBL1/TBLR1 with the F-box deletion could not enhance β-catenin-mediated transcription. Values are mean and s.d. for triplicate samples (n = 3) from a representative experiment.

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Figure S2 Drosophila Ebi is required for Wg signaling. a, The over-expression of Ebi enhanced Arm-dependent transcription. Kc167 cells were transfected with pActin-Arm* (100ng) and pActin-EBI (50, 100, 300, 600ng) along with the SuperTopflash (100ng) and pActin-β-galactosidase (10ng). 48 hr after transfection, luciferase activities were measured. Values are mean and s.d. for triplicate samples (n = 3) from a representative experiment. b, The knock-down of ebi by RNAi in Kc167 cells. Kc167 cells were treated for 5 days with control dsRNA duplexes or RNAi corresponding to two different portions of the Ebi CDR individually or in combination. The transcript level of ebi was measured by Real-time PCR. Values are mean and s.d. for triplicate samples (n = 3) from a representative experiment. c, The knock-down of Ebi by RNAi in Kc167 cells inhibited the expression of the Wnt target genes nkd and CG6234. Cells were treated with Ebi RNAi or control dsRNA duplexes for 5 days and then

incubated with Wg-conditioned media or control conditioned media for 4 hr. The transcript levels of nkd and CG6234 were measured by Real-time RT-PCR. Values are mean and s.d. for triplicate samples (n = 3) from a representative experiment. d, The knock-down of Ebi did not affect the expression of dpp, a Hehgehog target gene in kc167 cells. Values are mean and s.d. for triplicate samples (n = 3) from a representative experiment. e, The induction of Arm proteins were not affected by Ebi depletion, as determined by Western blot analysis. Uncropped images of the blots are shown in the Supplementary information. f, Wg induced the interaction between Ebi and Arm. Kc167 cells were transfected with plasmids expressing V5-EBI. After 48 hr, cells were incubated with Wg-conditioned media for 4 hr. The proteins were extracted and IP-ed with anti-V5, anti-Arm or control IgG and resolved on SDS-PAGE gels. Uncropped images of the blots are shown in the Supplementary information.

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Figure S3 Controls for the specificity of ChIP assays. a and b, Wnt signaling did not induce the ChIP signals of Histone H3. Cells were treated with LiCl or Wnt-3a as Fig. 3. The chromatin and DNA complexes were ChIP-ed with anti-Histone H3 antibodies. Each data point is the average of duplicates from a representative experiment. Experiments were repeated at least three times. c, TBL1 or TBLR1 siRNA reduced the ChIP signals of TBL1 or TBLR1, respectively. Each data point is the average of duplicates from a representative experiment. Experiments were repeated at least three times. d, TBL1/TBLR1 along with β-catenin were recruited to the

AXIN2 promoter upon LiCl stimulation. The ChIP-enriched DNAs were amplified and resolved on 2% agarose gel. e, Mouse TBL1 or TBLR1, but not TBL1/TBLR1 mutants with F-box deletion, rescued β-catenin-mediated transcription in cells depleted of TBL1/TBLR1, respectively. 293 cells were first transfected with human TBL1/TBLR1 siRNA for 36 hr and then co-transfected with normal mouse TBL1/TBLR1 or TBL1/TBLR1 mutants and Topflash reporter for 16 hr. Cells were treated with LiCl for 4 hr and luciferase activities were determined. Values are mean and s.d. for triplicate samples (n = 3) from a representative experiment.

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Figure S4. a and b, Both TBL1 and TBLR1 constitutively occupied on the ITF2 and DKK1 promoters in colorectal cancer cell lines SW480 and HT29 cells. The ChIP-enriched DNAs were quantitatively measured using Real-time PCR with ITF2- or DKK1-specific primers. Each data point is the average of duplicates from a representative experiment. Experiments were repeated at least three times. c, TBL1 and TBLR1 constitutively occupied on the ITF2 and DKK1 promoters in Wnt-1-expressing 293T cells. Each

data point is the average of duplicates from a representative experiment. Experiments were repeated at least three times. d, The knock-down of TBL1/TBLR1 did not affect the expression of HOXB2 and GAPDH genes in HT-29 cells as determined by RT-PCR. e, The knock-down of TBL1/TBLR1 did not affect the basal level of IL-8 and CIAP2 as determined by Real-time. Values are mean and s.d. for triplicate samples (n = 3) from a representative experiment.

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Figure S5 TBL1/TBLR1 recruit β-catenin to the Wnt target gene promoter independent of the ubiquitin/proteasome machinery. a, The knock-down of the S1 subunit of the 19S proteasome, UbcH7, Siah1 or Skp1 did not affect the binding of β-catenin and TBL1/TBLR1 to the AXIN2 promoter in Wnt-1-expressing 293 cells. The chromatin and DNA complexes were IP-ed with anti-β-catenin, anti-TBL1 or anti-TBLR1 antibodies. The ChIP-enriched DNA was quantitatively measured using Real-time PCR with the AXIN2-specific primers. Each data point is the average of duplicates from a representative experiment. Experiments were repeated at least three times. b, The knock-down of S1, UbcH7,

Siah1 or Skp by siRNA as determined by Western blot or Real-time PCR. Uncropped images of the blots are shown in the Supplementary information. c, The proteasome inhibitors PS-341 and MG-132 did not did not affect the binding of β-catenin and TBL1/TBLR1 to the c-myc promoter. Cells were treated with the inhibitors for 6 hrs. Each data point is the average of duplicates from a representative experiment. d, The knock-down of Siah1 and Skp1 did not affect the expression of Wnt target genes. The expression of AXIN2, ITF2 and c-Myc was determined by Real-time RT-PCR. Values are mean and s.d. for triplicate samples (n=3) from a representative experiment.

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Figure S6. The full-scans of gel presented in the paper are shown with molecular weight markers.

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75kd

37kd

25kd20kd

β-catenin

α-tubulin

siRNA Con β-catenin

50kd

250kd

150kd

100kd

75kd

37kd

25kdα-tubulin

Fig. 6a Fig. 6a

50kd

250kd

150kd

100kd

75kd

37kd

25kd

shRNA Con TBLR1

TBLR1

shRNA Con TBLR1

50kd

250kd

150kd

100kd

75kd

37kd

25kdα-tubulin

Fig. 6k

shRNA: Con TBL1 TBLR1

Fig. 6k

Blot:α-Flag

50kd

250kd150kd

100kd

75kd

37kd

25kd20kdβ-catenin

Co

n

TB

L1

TB

LR

1

siRNA

LiCl - - - + + +

Co

n

TB

L1

TB

LR

1

siRNA

50kd

250kd150kd

100kd

75kd

37kd

25kd20kd

TFIIb

Co

n

TB

L1

TB

LR

1

LiCl - - - + + +

Co

n

TB

L1

TB

LR

1

Fig. 4c

50kd

250kd

150kd

100kd

75kd

37kd

25kd

HT29-gal HT29-APC

β-catenin

TFIIb

Zinc: - + - +

50kd

250kd

150kd

100kd

75kd

37kd

β-catenin

hnRNP

shRNA: Con TBL1 TBLR1

TBLR1

50kd

37kd

50kd

37kd

TBL1

Fig. S1b

Blot: α-Flag

- + - + -

+

LiCl

Fla

g-A

P2

α

Fla

g-T

BL1

Fla

g-T

BLR

1

- +

50kd

150kd100kd75kd

37kd

25kd

50kd

150kd100kd75kd

37kd

25kd

50kd

100kd75kd

37kd25kd

150kd

Fig. S1c

+

Lysate

Blot: α-Flag

IP: α-TCF

Blot: α-Flag

IP: α-TCF

- + - + -Wnt3A +

Blot: α-Flag

FL Δ-F

-box

WD

40

F-b

ox

Δ-W

D40

50kd

150kd100kd75kd

37kd

25kd

250kd

Blot: α-HA

HA-β-catenin

Lysate

50kd

100kd

75kd

37kd

150kd

25kd

Blot:α-Flag

IP: α-HA

Fig. S1d

50kd

250kd150kd

100kd75kd

37kd25kd

50kd

250kd

150kd

100kd75kd

37kd

25kd

50kd

150kd

100kd75kd

37kd

25kd

con TBL1 consiRNA

Flag-TBL1

Flag-TBLR1

- - +

- - -

TBL1

TBLR1

TFIIb

50kd

250kd150kd

100kd75kd

37kd25kd

50kd

250kd

150kd

100kd75kd

37kd

25kd

con TBLR1 con

50kd

150kd

100kd

75kd

37kd

25kdTFIIb

- - +

- - -

TBL1

TBLR1

Fig. S1d

Flag-TBLR1

50kd

100kd

75kd

37kd

150kd

25kd

IP: α-Flag

Blot:α-HA

50kd

100kd

75kd

37kd

150kd

Blot:α-Flag

Lysate

Fig. S1e

50kd

100kd

75kd

37kd

150kd

25kd

Lysate

HA-TBL1

- + +

+ - +

Blot:α-HA

- + +

+ - +

Fig. S2d

50kd

250kd150kd

100kd75kd

37kd

25kdα-Arm

Con ebi1 ebi2 ebi1 con ebi1 ebi2 ebi1

+ebi2 +ebi2

Control-CM Wg-CM

50kd

250kd150kd

100kd

75kd

37kd

25kd

Input IgG V5 Arm

IP

Blot: α-V5

Fig. S2e

S1

siRNA Con S1

75kd100kd

50kd

37kd

25kd

150kd

Fig. S5bFig. S5b Fig. S5b

α-tubulin

75kd

100kd

50kd

37kd

25kd

150kd

siRNA Con S1

siRNA Con UbcH7

UbcH7

α-tubulin

50kd

75kd

15kd

20kd

25kd

37kd

100kd

© 2008 Nature Publishing Group