The Wnt/β-catenin signaling pathway is up-regulated in mammary tumors that arise with 70% incidence in parous female BK5.ATF3 transgenic mice. Nuclear accumulation of β-catenin is seen in both basal and supra-basal tumor cells in this model, whereas nuclear expression of the transcription factor ATF3 is only seen in the basal cell layer of the tumors. If ATF3 acted as a positive regulator of one or more Wnt ligands in the basal cells, one might expect paracrine upregulation of the Wnt/β-catenin pathway in neighboring supra-basal cells. To test this, we measured mRNA expression levels for 9 Wnt genes in BK5.ATF3 induced mammary tumors and normal mammary glands. The genes for five known positive acting Wnt ligands, Wnt3, Wnt3a, Wnt5a, Wnt7b, and Wnt10b, were significantly upregulated in tumors as compared to transgenic mammary glands. One of these, Wnt3, was significantly down-regulated in non-transgenic glands. In a mouse mammary tumor cell line, EMT6, in which the endogenous Atf3 gene is overexpressed, the only Wnt gene for which expression could be detected by qPCR was Wnt7b. ChIP assays demonstrated binding of both Atf3 and Jun proteins to two putative AP-1 sites upstream of the Wnt7b transcription start site in these cells. ATF3 and JUN are known in other systems to form heterodimers that exhibit positive transactivation through AP-1 sites, and we have identified a physical interaction between ATF3 and JUN in BK5.ATF3-induced tumors.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2022. doi:10.1158/1538-7445.AM2011-2022