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Tumor-initiating cells are thought to be the source of metastasis. For these reasons, the Wnt pathway is viewed as a strong candidate for therapeutic intervention.
The Wnt signaling pathway was first characterized in the 1970s in Drosophila melanogaster development. It was later recognized in mammalian systems for its importance in cancer. Specifically, core components of this pathway were shown to be dysregulated in colorectal disease. It has since been shown to play a role in other forms of cancer, where it promotes proliferation and survival. It also plays an important role in the maintenance of the pool of tumor-initiating cells, which promote the regrowth of tumors after an insult like surgery or chemotherapy. Tumor-initiating cells are thought to be the source of metastasis. For these reasons, the Wnt pathway is viewed as a strong candidate for therapeutic intervention.The Wnt pathway takes on many forms that fall under the broad classifications of canonical and noncanonical. The canonical pathway deals with the regulation of β-catenin protein levels. Under normal conditions, a cytosolic scaffold known as the destruction complex binds and phosphorylates β-catenin, resulting in its ubiquitylation and degradation. The destruction complex includes the adenomatous polyposis coli (APC) protein, axin, and glycogen synthase kinase-3β. When Wnt ligand binds to the Frizzled receptor, its coreceptor LRP5/6 is recruited and phosphorylated in the intracellular domain, promoting the binding of Dishevelled protein and the sequestration of axin. This disintegrates the destruction complex, resulting in accumulation of β-catenin in the cytosol and upregulated trafficking into the nucleus. β-catenin promotes transcription of genes related to proliferation and survival by acting as a coactivator for the Tcf/Lef family of transcription factors in the nucleus.
Aside from canonical Wnt signaling, two major noncanonical pathways have been studied. In the first of these two pathways, Wnt ligand binding to the Frizzled receptor induces recruitment of Dishevelled protein and the Dishevelled-associated activator of morphogenesis 1 (Daam1). This complex initiates a cascade that activates the Rac and Rho GTPases to mediate cell polarity. The other most widely studied noncanonical Wnt signaling pathway is related to calcium signaling. Wnt ligand binding to the Frizzled receptor promotes the recruitment of Dishevelled in complex with a G protein. This complex promotes intracellular calcium levels to mediate other signaling pathways.
Biological systems tightly regulate the Wnt signaling pathway to prevent aberrant cell growth. It has been known for decades that dysregulation of Wnt signaling leads to cancer, where it was first recognized in familial colorectal disease with mutations in the APC gene. Since then, Wnt signaling has been found to act prominently in breast, liver, skin, and prostate cancers.
This illustration depicts the best-elucidated cancer-promoting routes of Wnt cell signaling, which draws its name from the wingless mutation in the fruitfly Drosophila melanogaster.
Aberrations in canonical Wnt signaling can manifest in many ways. For example, proteins involved in the destruction complex can become nonfunctional through mutations or truncations, inhibiting β-catenin downregulation. The β-catenin protein itself can be mutated to inhibit its recognition by the destruction complex. In addition, the production of Wnt ligand or receptors can be upregulated, resulting in excessive signaling. These different routes of activation complicate the use of a single therapeutic against the Wnt pathway.Wnt signaling is of great interest to cancer researchers because it is linked to many different forms of the disease. Preclinical models throughout the last decade have established this pathway as an attractive drug target. However, to date, therapies meant to attenuate the Wnt pathway have remained largely theoretical and preclinical. Thankfully, compounds are now starting to enter clinical trials.
Vitamin D has been postulated as a suitable anti-Wnt therapy. The vitamin D receptor binds and sequesters β-catenin at the plasma membrane, inhibiting its nuclear translocation. Mice bearing an APC mutation that promotes spontaneous colon cancer developed more disease when the vitamin D receptor was knocked out. Additionally, the association between sunlight exposure and decreased risk of colon cancer implies that the inhibition of Wnt signaling by vitamin D may be conserved in humans. This phenomenon has yet to be tested in a systematic trial, however.
High-throughput screens have been used extensively to identify small-molecule targeted inhibitors of different Wnt pathway constituents. The binding of β-catenin to its nuclear targets T-cell factor (Tcf) or Creb-binding protein (CBP) is a prototypical example of study in this realm. Screens were performed to identify specific inhibitors of this interaction. This work has identified useful hits in cell-based assays, but translation into the clinic has remained difficult due to the potential off-target effects. First, the drugs identified so far fail to discriminate between the binding of β-catenin to Tcf or APC, so the drug may prevent degradation of β-catenin in addition to its intended effect on transcription. Disruption of the destruction complex binding to β-catenin could lead to severe side effects in normal tissue. Another deleterious effect identified with blockers of β-catenin binding is the inhibition of complex formation with E-cadherin at cell-cell junctions. The net effect of this phenomenon could be to impair cell adhesion on a grand scale. As a result of these challenges, only one compound that directly disrupts β-catenin function has moved into clinical trials.
Other drug candidates inhibit Wnt aberrations upstream of β-catenin function. Several of these compounds have recently begun clinical trials. They block either Wnt ligand secretion or recognition, and preclinical evidence has been encouraging so far. Targeting at this level can also lead to side effects, however. By blocking Wnt signaling at the level of the membrane, it is possible to inhibit noncanonical in addition to canonical signaling. The effect this will have on the body is unknown. In addition, blocking Wnt ligand-receptor interactions may not be sufficient to inhibit Wnt signaling since the activating event may be mutation within the destruction complex or β-catenin itself. In these settings, Wnt signaling can be rendered constitutive and independent of ligand, and the therapy would likely fail.No approved compounds exist for the treatment of Wnt signaling. Phase I trials for these inhibitors should be illustrative in the coming years. While many cancers are addicted to this signaling for long-term growth and renewal, high-turnover tissues like the gastric epithelium and hair follicles are similarly reliant. Therefore, the field is cautious when utilizing any blockade of Wnt signaling, as significant toxicity may result.
Phase I/II Trials
(OncoMed Pharmaceuticals/ Bayer)
This monoclonal antibody targets the Frizzled receptors to block association with Wnt ligands. It was recently shown to potently block the capabilities of pancreatic tumor-initiating cells in a serial dilution assay. In xenograft models of breast, lung, pancreatic, and colon cancer, OMP-18R5 demonstrated significant inhibition of tumor growth, and it synergized with standard-of-care treatment in these models (paclitaxel in breast cancer, for example). (NCT01345201)
(OncoMed Pharmaceuticals/ Bayer)
This agent is a fusion protein of the Frizzled8 ligand-binding domain with the Fc region of a human immunoglobulin. It binds and sequesters soluble Wnt ligand, impairing its recognition by receptors on tissues. (NCT01608867)
(Prism Pharma Co, Ltd/Eisai)
This is a small-molecule inhibitor of the interaction between β-catenin and CBP. Disrupting the interaction prevents activated transcription by aberrant Wnt signaling at many levels. It is being studied in both solid tumors and myeloid malignancies. (NCT01606579, NCT01302405)
This small molecule inhibits acyltransferase porcupine. Preclinical work demonstrated this enzyme's action is crucial in the secretion of Wnt ligands out of the cell; therefore, inhibiting porcupine can be a small-molecule—based method for inhibiting Wnt ligand-mediated activation. (NCT01351103)
This small-molecule poly(ADP ribose) polymerase (PARP) inhibitor has demonstrated efficacy in cellular models of cancer survival. In Wnt signaling, PARPs like the tankyrases promote the ribosylation and subsequent degradation of axin, a key scaffolding protein of the destruction complex. By inhibiting tankyrase, the axin protein is stabilized and can promote the degradation of β-catenin.
This selective tankyrase1/2 inhibitor has been shown to inhibit the growth of colon cancer cells in both cell and animal models.
Sources: ClinicalTrials.gov website, company websites.
If Wnt pathway inhibitors can be proven safe, it may represent a milestone in cancer research given the strong preclinical evidence for cancer cell cytotoxicity. Since this pathway is also crucial in the maintenance of tumor-initiating cells, inhibition could represent a powerful tool in our arsenal to target cells that are resistant to traditional chemotherapy and promote metastasis.