Regulation of Phenotypic Plasticity in Triple-Negative Breast Cancer
Breast cancers with a basal-like gene signature are primarily triple-negative, are frequently metastatic, and carry the worst prognosis. Basal-like breast cancers are frequently enriched for markers of breast cancer stem cells as well as markers of epithelial-mesenchymal transition (EMT). While EMT is generally thought to be important in the process of metastasis, direct in vivo evidence of EMT in human disease remains rare. Here we report a novel model of human triple-negative breast cancer, the DKAT cell line, which was isolated from an aggressive, treatment-resistant triple-negative breast cancer that demonstrated morphological and biochemical evidence of epithelial-mesenchymal plasticity in the patient.
In culture, the DKAT cell line exhibits a basal epithelial phenotype under normal culture conditions in serum-free MEGM, and can undergo a reversible EMT in response to serum-containing media, a unique property among the breast cancer cell lines we tested. This EMT is marked by increased expression of the transcription factor Zeb1, and Zeb1 is required for the enhanced migratory ability of DKAT cells in the mesenchymal state. Additionally, we find that expression of the cytokine IL-6 is dramatically increased in mesenchymal DAKT cells, and blocking IL-6 signaling reduces expression of Zeb1. DKAT cells also express progenitor-cell markers, and single DKAT cells are able to generate tumorspheres containing both epithelial and mesenchymal cell types. In vivo, as few as ten DKAT cells are capable of forming xenograft tumors which display a range of epithelial and mesenchymal phenotypes. Finally, we also show evidence of vimentin expression in mammary epithelial cell clusters from asymptomatic women at high risk for breast cancer, suggesting that changes characteristic of epithelial-mesenchymal plasticity may be inherent to some breast cancers from their earliest stages.
Our results provide evidence that the aggressive behavior of a subset of triple-negative breast cancers is driven by inherent epithelial-mesenchymal plasticity. The novel finding that IL-6 regulates Zeb1 expression adds further rationale for the development of anti-IL-6 therapeutics, which will have the potential to target pathways at the intersection of metastasis and tumor recurrence. The DKAT cell line represents a novel model for further study of the molecular mechanisms that regulate plasticity in highly aggressive triple-negative breast cancers. An improved understanding of the pathways that are critical for this plasticity may lead to improved treatment options for highly aggressive and deadly breast cancers.
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