The Wnt signal pathways have key roles in embryonic development. Defects in the pathway have also been implicated in cancer of the colon and other organs1,2. Two Wnt pathways have been identified: the canonical and noncanonical pathways. Activation of the canonical pathway induces transcription of a new set of genes through the β-catenin–T cell factor (TCF) complex, which regulates cell proliferation and differentiation1. Activation of the noncanonical pathway does not require β-catenin signaling and controls cell movement during morphogenesis2.

In the absence of Wnt ligands bound to their receptors, the cytoplasmic complex of APC and Axin provide a scaffold for GSK3β to phosphorylate β-catenin (Fig. 1a). Phosphorylated β-catenin is then rapidly degraded through the ubiquitin pathway. When Wnt ligands bind to the cell-surface receptor Frizzled (Fzd), they trigger the phosphorylation of a cytoplasmic effector, Dishevelled (Dsh), which then inhibits the activity of GSK3β on the APC-Axin complex. Unphosphorylated, and therefore stable, β-catenin can then accumulate in the cytoplasm and form a complex with TCF in the nucleus, which initiates transcription of Wnt target genes (Fig. 1b).

Figure 1: Quantitative control of the canonical Wnt pathway.
figure 1

(a) Basal level. In the absence of Wnt ligand binding to Fzd receptors, most β-catenin molecules are rapidly phosphorylated by GSK3β in the APC-Axin scaffold and degraded through the ubiquitin pathway. (b) Midrange activation. When SFRPs are reduced by promoter methylation, Wnt ligands bind to Fzd and induce phosphorylation of Dsh, which in turn inhibits GSK3β on the complex. A sizable fraction of β-catenin remains unphosphorylated and binds to TCF in the nucleus. (c) Boosted activation. In addition to Wnt ligand activation of Fzd, downstream effector β-catenin carries a stabilizing mutation. A much larger fraction of β-catenin remains unphosphorylated and induces strong transcription of target genes. Mutations in APC or AXIN1 can cause similar effects. (d) Suppressed activation. SFRPs trap Wnt ligands, which partially suppresses signaling, even when β-catenin carries a stabilizing mutation. LRP, co-receptor, low-density-lipoprotein receptor–related protein family13.

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Canonical Wnt signaling in cancer

Most colon cancers and other digestive cancers are associated with mutations in APC, AXIN1 or CTNNB1, and ∼90% of colon cancers are associated with defects in the canonical Wnt signaling pathway (Fig. 1c)1. Mutant APC and Axin are unable to assist GSK3β in phosphorylating β-catenin. Similarly, mutations that lead to amino acid substitutions in the phosphorylated residues of β-catenin stabilize the protein. Either type of disruption causes constitutive signaling independent of the upstream signal from Wnt.

On page 417–422, Hiromu Suzuki and colleagues add a new twist to this simplistic view on the canonical Wnt pathway3. In an earlier paper, they isolated genes that were preferentially hypermethylated in human colon and gastric cancers4. Among them, they identified a family of secreted Fzd-related proteins (SFRPs) that can compete with Fzd for the Wnt ligands. Now, the authors report on experiments in which they expressed SFRPs in colon cancer cell lines carrying mutations in CTNNB1 or APC. SFRP1, SFRP2 and SFRP5 suppressed Wnt-dependent transcription by ∼60% (Fig. 1d). They then expressed WNT1 in the β-catenin mutant cell line HCT116. Wnt pathway–dependent transcription was ∼3 times greater and was suppressed by cotransfection with constructs expressing SFRPs. Furthermore, Wnt-dependent transcription was 3.5 and 7 times greater when the authors coexpressed wild-type or stable mutant β-catenin, respectively, in these cells.

The results establish that Wnt signal activation by mutant β-catenin or APC can be partially suppressed by upstream ligand competitors, and that Wnt signaling can be boosted further in a β-catenin- or APC-mutant cell line by upstream Wnt ligands or by additional expression of β-catenin (Fig. 1d and c, respectively). Accordingly, it may be possible to suppress the tumor phenotype in Wnt-activated cancer cells by inhibiting the Fzd receptor through competition with antagonists. In fact, Hiromu Suzuki and colleagues show that colon cancer cells that overexpress SFRPs have less colony formation and a higher rate of apoptosis.

How much Wnt is required for tumorigenesis?

The authors suggest that β-catenin mutated at Ser45 in the HCT116 cells only partially stabilizes the protein, because phosphorylation may take place at three other residues, consistent with a recent report5. But phosphorylation at Ser45 seems to be essential for phosphorylation of the other residues6,7. In this regard, it would be helpful to determine if deleting the remaining wild-type allele8, or all four phosphorylated residues9, would have a substantial effect.

Looking at the bigger picture of the Wnt signaling system, we must consider what might be allowing this dynamic signal regulation. Expression of exogenously introduced wild-type and Ser45-mutant β-catenin increased Wnt signaling in HCT116. Thus, Wnt signal–dependent transcription is not saturated in cells that carry the Ser45 mutation in only one of the alleles10. The increased Wnt signaling by expression of extra ligand can also be explained by Dsh-dependent inhibition of GSK3β phosphorylation of wild-type β-catenin synthesized from the other allele. Regarding SW480 cells with biallelic APC mutations11, additional signal from Wnt ligands may be explained by the flexible nature of the APC-Axin complex. For example, overexpression of Axin can compensate for the lack of APC in phosphorylating β-catenin12. Dsh inhibits GSK3β phosphorylation of β-catenin on the APC-Axin scaffold, but its mode of inhibition is not fully understood. Slowly, the pieces of the Wnt signaling puzzle are coming together and suggest that signaling can be regulated at many levels in a quantitative manner, with a wide, dynamic range.

Approximately 90% of primary colon cancer tissues showed methylation of SFRP1, whereas only ∼70% had APC mutations. Notably, methylation of SFRP1, SFRP2 and SFRP5 was also found in ∼90% of early colonic adenomas. Accordingly, silencing SFRP genes may be one of the earliest events in tumorigenesis. The new results suggest that we may be able to suppress Wnt signaling in cancer even when APC or CTNNB1 is mutated.