Triple-negative breast cancer (TNBC) accounts for 15–20% of breast cancers (BC) and is characterized by high aggressiveness and lack of specific therapeutic targets. The intratumor existence of cancer stem cells (CSCs) represents one of the main causes of tumor relapse and resistance to treatments. Among the signaling pathways responsible for maintaining CSC activity, the Notch pathway plays a role in regulating self-renewal and in promoting BC development and resistance to chemo- and radiotherapy. B-cell CLL/lymphoma 6 (BCL6) is a transcriptional repressor that has a role in germinal center formation. Recent studies demonstrated its involvement in supporting BC cell survival, proliferation, invasiveness and in inducing an epithelial-to-mesenchymal transition program.
TCGA data from RNA-seq (HiSeq counts) and somatic copy number (CN) alterations were accessed using the Xena browser. METABRIC-processed data were retrieved from cbioportal. Normalized data and clinical information including TNBC data sets were retrieved from GEO Omnibus. Mammosphere formation efficiency percentage (MFE%) was calculated following BCL6 inhibition. Immuno Precipitation (IP) and Chromatin Immuno Precipitation (ChIP) experiments demonstrated the functional cooperation between BCL6 and EZH2.
Through a bioinformatics-based analysis of benchmark BC data we evidenced the existence of a BCL6 altered expression in TNBC subtype. Further, we show that this feature plays a role in the maintenance/expansion of CSCs as showed by the significantly higher levels of BCL6 transcript in TNBC cells cultured under 3D versus 2D conditions. At the contrary, its loss of function significantly decreased MFE% by preferentially targeting CD44-positive cells versus ALDH-positive ones. Moreover, by taking advantage of the small molecule FX1, a BCL6 specific inhibitor, we revealed the existence of a functional interplay between BCL6 and EZH2 that triggers the activation of Notch signaling via Numb transcriptional repression.
Our findings provide insights towards the feasibility of pharmacological targeting of BCL6/Notch axis in combination with standard agents, to eradicate both the CSC compartment and the bulk of the tumor.
The authors.
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All authors have declared no conflicts of interest.