Wnt Signaling in Canine Mammary Cancer

Female dogs have a high incidence of mammary carcinomas which can be prevented by early spaying indicating a prominent role of ovarian steroids in the pathogenesis of mammary carcinoma. Earlier studies have revealed that exposure to progesterone or synthetic progestins results in mammary expression of the mRNAs encoding growth hormone and Wnt4. Wnt signaling plays an important role in the maintenance and activation of mammary stem cells. Canonical Wnt signaling is also upregulated in more than 50% of human and canine breast cancers. In three canine mammary tumor cell lines with an activated Wnt signaling, rounded morphology and clear stem cell properties we investigated the role of EGFR-related receptors and of cSRC on canonical Wnt activation. Inhibition of the EGFR-activated PI3K/AKT pathway further enhanced Wnt signaling whereas dasatinib, a cSRC inhibitor, downregulated the enhanced Wnt activity in these cells and decreased the migration and metastasis of these cells in a zebrafish xenograft model. Exome-sequencing revealed unique mutations that could play a role in the enhanced Wnt signaling. From all candidates a mutated CDH3 gene, encoding P-cadherin, was related to high Wnt activity. Silencing of CDH3 mRNA expression not only decreased Wnt signaling but changed also the morphology to a more spindle cell like structure. Mutated CDH3 seems to be rather sensitive for proteolytic cleavage by MMPs that leads to the formation of soluble sP-cadherin and stabilization of free β-catenin in the cytoplasm resulting in enhanced canonical Wnt signaling. MMP inhibition resulted in a return to a spindle cell morphology preventing the cells to migrate and form metastasis.