Summary

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The canonical Wnt signaling pathway plays critical roles in embryonic development1 but is also extremely important for adult life of all multicellular organisms including humans. The pathway controls cell proliferation and differentiation and thus is necessary for the support of stem cell proliferation and self-renewal as well as for regenerative processes.2 On the other hand, aberrant activation or hyperactivation of the Wnt pathway in an adult tissue can lead to carcinogenesis, most notably in the colon3 and breast.4

Breast cancer is typically divided into three major forms. The most frequent estrogen receptor-positive type is the most curable through application of hormonal therapy agents, such as tamoxifen.5 The second type lacks expression of estrogen receptors, but instead overexpresses the receptor tyrosine kinase HER2 and is efficiently treated with anti-HER2 antibodies (trastuzumab).6 The third form, relatively less frequent (ca. 10-15% of breast cancer cases) is known as the triple-negative breast cancer due to the lack of expression of estrogen receptor, HER2, and progesterone receptor. Consequently, this form does not respond to hormonal or trastuzumab therapies, leaving the treatment of patients with general chemotherapies (such as cyclophosphamide) as the only option. This absence of targeted therapies, along with the fast progression of TNBC to metastasis, make this form of breast cancer the deadliest, accounting for more than half of all breast cancer deaths.79

Development of a targeted anti-cancer therapy requires understanding of the signaling pathway overactivated in a given cancer type. As clearly established recently, this pro-proliferative signaling in TNBC is provided by overactivation of the Wnt signaling pathway.10 The Wnt pathway is a complex signaling cascade. In humans it operates with 19 types of Wnt proteins as ligands and 10 types of Frizzled proteins as receptors. The downstream components of the pathway involve a co-receptor LRP 5/6, G-proteins, a multiprotein β-catenin destruction complex, β-catenin itself (which serves as a transcription factor inducing expression of the Wnt target genes) and many cofactors of the latter. Taken together, it gives a variety of targets, both for the activation and the inhibition of the pathway.11

In TNBC, overexpression of Wnt ligands, the receptor Frizzled7, and the co-receptor LRP6 has been described.12,13 After that, development of agents targeting the Wnt signaling pathway as future anti-TNBC drugs (as well as drugs against other Wnt-dependent cancers) has been initiated. One approach involves development of human antibodies against upstream extracellular components of the pathway. OncoMed Pharmaceuticals has developed an antibody against the cysteine-rich (extracellular) domain of Frizzled7. This antibody later showed cross-reactivity with 4 other Frizzleds but efficiency against several solid cancers in vitro;14 phase I clinical studies are currently in place. Screening of libraries of synthetic small molecules is another approach to derive Wnt signaling inhibitors.15 Several hit compounds have been identified, among them the molecule XAV939 inhibiting tankyrase – a multifunctional enzyme among other things regulating the activity of the β-catenin-destruction complex, – developed by Novartis.16 Rational drug design and in silico screening is also applicable to the Wnt pathway, especially after the recent description of the crystal structure of a Wnt-Frizzled complex;17 an example of such in silico analysis is our recent identification of an anti-leprosy drug clofazimine as a potent inhibitor of the Wnt pathway and growth of TNBC cells in vitro.18 Finally, natural products such as medicinal plants can become a source of novel targeted anti-Wnt therapies, and attempts to develop such novel drugs are also on the way.

With the complexity of signaling pathways initiated by many possible Wnt/Frizzled ligand/receptor pair combinations in various tissues, the issues of specificity and toxicity of drugs targeting this type of signaling will likely become highly important. The available drug candidates do not discriminate between different types of Wnt signaling.19 Thus it will be imperative to target the individual components of the pathway characteristic for certain disease states – such as Frizzled7 in TNBC. In this regard, the discovery that Frizzleds, a family of 10 receptors in humans, not only rely in their signaling on heterotrimeric G proteins in some exceptional cases,21,22 but are functional G protein-coupled receptors (GPCRs) in organisms from insects to mammals,22,23 and moreover, are GPCRs by biochemical characterization,2426 provides a very promising advance. Indeed, the whole richness of drug discovery approaches tailored at GPCRs can now be applied to any of the ten Frizzleds.15 This endeavor is likely to produce drugs targeting specific Frizzled-dependent conditions, such as first-in-class therapies targeting Frizzled7 in the case of TNBC.