Modulation of the Wnt/β-catenin pathway is associated with the basal-like breast cancer phenotype.

CTRC-AACR San Antonio Breast Cancer Symposium: 2008 Abstracts Abstract #4066 Background: Basal-like breast tumors are aggressive breast cancers that are distinct from other types of breast tumors by gene expression signatures. The Wnt/β-catenin signal transduction pathway is deregulated in numerous tumor types, including breast cancer, and is implicated in promoting the mesenchymal phenotype characteristic of basal-like breast tumor cells. Nuclear β-catenin has been associated with poor survival in some breast cancer patients, and analysis of human sporadic breast cancers indicate that as many as 44% of sporadic breast cancers or cell lines demonstrate inactivation of the APC tumor suppressor, a Wnt/β-catenin regulator. However, it is unclear whether deregulation of this pathway occurs in a specific subclass of breast cancers and whether therapeutically targeting this pathway in patients might be warranted. We hypothesized that the Wnt/β-catenin pathway is activated specifically in basal-like breast tumors and is causally involved in the morphology and behavior of basal-like tumor cells. Materials and Methods: To test our hypothesis, we utilized three complementary approaches. 1) In EpH4 mammary epithelial cells, β-catenin was stabilized by treatment with a proteasome (ALLN) or GSK3β (SB216763) inhibitor or by overexpression of a non-degradable mutant (S37A β-catenin). Specificity of the drug effects was tested by transfection of β-catenin-specific siRNAs. 2) APC knockdown was achieved in 4T07 mammary tumor cells using transient transfection of APC-specific siRNAs compared to non-specific control siRNAs. Cell morphology and cell-cell interactions were analyzed using phase-contrast microscopy and immunofluorescence. 3) Using tissue microarrays containing 32 normal breast, 7 DCIS, 41 invasive breast cancers and 23 metastases specimens, β-catenin immunohistochemistry was performed and scored for nuclear, membranous and cystosolic localization. A colon cancer tissue microarray was used as positive control for nuclear β-catenin localization. Results: In mammary epithelial cells, stabilization of β-catenin was sufficient to induce a morphology that resembles mesenchymal cells, including flattening and extension of robust cell protrusions, an effect that was rescued by β-catenin knockdown. Metastatic murine mammary tumor cells with a robust mesenchymal phenotype were converted to an epithelial phenotype when APC expression was suppressed. Furthermore, we found that nuclear localization of β-catenin, a read-out of Wnt/β-catenin pathway activity, was found exclusively in a subset of basal-like breast tumors (7 of 18) but not in normal breast or other tumor subtypes. Discussion: Our data indicate that alterations in Wnt/β-catenin pathway components can dramatically modify the phenotype of both normal and transformed breast cells. Further, these results indicate that activation of the Wnt/β-catenin pathway is specifically associated with basal-like breast tumors. These findings suggest that pharmacologic modulation of this pathway might be useful for basal-like tumors in further pre-clinical studies. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 4066.