Colorectal Cancer and the Enigma Surrounding Non-Canonical Wnt Signaling

Since the discovery of truncated mutations of adenomatous polyposis coli proteins in familial adenoma patients in 1991, the mechanism of cytosolic β-catenin regulation has been intensively studied, and now it is very well known that the central role of the canonical Wnt/β-catenin is in colorectal cancer. However, Wnt signaling is very complicated because of the presence of almost 20 Wnt ligand genes, six Frizzled seven-transmembrane receptors, and three LRP co-receptors. In addition, research in the past two decades illuminated the existence of the other Wnt signaling—non-canonical Wnt signaling (Wnt/PCP and Wnt/Ca2+ pathways), and an increasing number of studies have shown the potential role of non-canonical Wnt signaling in cancer recently. One of the well-studied Wnt ligands in non-canonical Wnt signaling is Wnt-5a. However, the role of Wnt-5a and non-canonical pathways in cancer is mosaic—i.e., it may involve tumor-promoting or suppressing pathways. In certain cancers, non-canonical Wnt signaling may mainly act as a tumor promoter, yet the results are very controversial in colorectal cancer. Elucidating the role of non-canonical Wnt signaling in colorectal cancer may be very important to further reduce the risk of colorectal cancer, especially in patients who do not carry truncated mutations of adenomatous polyposis coli. In this review, I would like to mainly discuss the apparent controversy surrounding non-canonical Wnt signaling in colorectal cancer, and I would like to point out a few potential reasons contributing to the mysterious roles of Wnt5a-initiated non-canonical signaling in colorectal cancer.