We have found aberrant expression of cytokeratins 5 and 8 in most tumor cells in the BK5.ATF3 model, as well as supra-basal expression of cytokeratins 6 and 10 and several cytokeratins that are characteristic of the inner root sheath of hair follicles. Interestingly, nuclear IHC staining for the ATF3 transgene is confined to the basal cell layer in the tumors. In both types of models, squamous metaplastic histopathology is seen, with the tendency to form cyst-like structures with a core of keratinaceous material and cell debris, surrounded by a multi-layered epithelium that exhibits several features of squamous differentiation. These phenotypic similarities suggested the possibility that the Wnt/b-catenin pathway is somehow activated in ATF3-induced mammary tumors, or alternatively that ATF3 is a downstream effector of Wnt/b-catenin signaling. However, functional links between ATF3 and Wnt/b-catenin signaling have not been described in the literature. The Wnt/b-catenin pathway is well known for its involvement in colon carcinogenesis. About 85% of both familial and sporadic colon cancers involve mutations in the APC gene that lead to activation of the Wnt/b-catenin pathway. Wnt/b-catenin signaling is absolutely required for mammary gland development, and acts at several Niraparib critical time periods during pre- and post-natal development. Over the past decade, several epigenetic abnormalities in Wnt pathway genes have also been identified in human breast cancer. Promoter methylation of the APC gene has been found in about 40% of breast cancer cases, and high levels of promoter methylation for several Evofosfamide Wnt-inhibitory genes in the SFRP and DKK families in breast cancer have also been reported. The canonical pathway of Wnt/b-catenin signaling begins with the interaction of an extracellular Wnt family protein with a transmembrane receptor of the Fz family; each of these gene families have more than a dozen members in mouse and human. This triggers formation of a complex with a second membrane coreceptor, Lrp5 or Lrp6, phosphorylation of both receptors, and binding of two cytoplasmic proteins, Disheveled and Axin, to the complex. In the unstimulated cell, cytoplasmic b-catenin associates with a so-called destruction complex, containing the proteins Axin, APC and Gsk3. In this complex, b-catenin is specifically phosphorylated by the kinase activity of Gsk3, which marks it for subsequent ubiquitylation and degradation by the proteasome. Following Wnt binding to the receptor, the destruction complex becomes tethered to the membrane ligand/receptor complex through Axin and loses its affinity for b-catenin, which then accumulates in the cytoplasm and is transported to the nucleus.