Activation Status of Wnt/ß-Catenin Signaling in Normal and Neoplastic Breast Tissues: Relationship to HER2/neu Expression in Human and Mouse

<div><p>Wnt/ß-catenin signaling is strongly implicated in neoplasia, but the role of this pathway in human breast cancer has been controversial. Here, we examined Wnt/ß-catenin pathway activation as a function of breast cancer progression, and tested for a relationship with HER2/neu expression, using a human tissue microarray comprising benign breast tissues, ductal carcinoma <em>in situ</em> (DCIS), and invasive carcinomas. Cores were scored for membranous ß-catenin, a key functional component of adherens junctions, and for nucleocytoplasmic ß-catenin, a hallmark of Wnt/ß-catenin pathway activation. Only 82% of benign samples exhibited membrane-associated ß-catenin, indicating a finite frequency of false-negative staining. The frequency of membrane positivity was similar in DCIS samples, but was significantly reduced in carcinomas (45%, <em>P</em><0.001), consistent with loss of adherens junctions during acquisition of invasiveness. Negative membrane status in cancers correlated with higher grade (<em>P</em> = 0.04) and estrogen receptor-negative status (<em>P</em> = 0.03), both indices of poor prognosis. Unexpectedly, a substantial frequency of nucleocytoplasmic ß-catenin was observed in benign breast tissues (36%), similar to that in carcinomas (35%). Positive-staining basal nuclei observed in benign breast may identify putative stem cells. An increased frequency of nucleocytoplasmic ß-catenin was observed in DCIS tumors (56%), suggesting that pathway activation may be an early event in human breast neoplasia. A correlation was observed between HER2/neu expression and nucleocytoplasmic ß-catenin in node-positive carcinomas (<em>P</em> = 0.02). Furthermore, cytoplasmic ß-catenin was detected in HER2/neu-induced mouse mammary tumors. The <em>Axin2^NLSlacZ</em> mouse strain, a previously validated reporter of mammary Wnt/ß-catenin signaling, was utilized to define <em>in vivo</em> transcriptional consequences of HER2/neu-induced ß-catenin accumulation. Discrete hyperplastic foci observed in mammary glands from bigenic MMTV/<em>neu</em>, <em>Axin2^NLSlacZ</em> mice, highlighted by robust ß-catenin/TCF signaling, likely represent the earliest stage of mammary intraepithelial neoplasia in MMTV/<em>neu</em> mice. Our study thus provides provocative evidence for Wnt/ß-catenin signaling as an early, HER2/neu-inducible event in breast neoplasia.</p>