Chemotherapy resistance is a major clinical challenge for the management of locally advanced breast cancer. Accumulating evidence suggests a major role of cancer stem cells (CSCs) in chemoresistance evoking the requirement of drugs that selectively target CSCs in combination with chemotherapy. Here, we report that mithramycin A, a known specificity protein (Sp)1 inhibitor, sensitizes breast CSCs (bCSCs) by perturbing the expression of drug efflux transporters, ATP-binding cassette sub-family G, member 2 (ABCG2) and ATP-binding cassette sub-family C, member 1 (ABCC1), survival factors, B-cell lymphoma 2 (Bcl-2) and X-linked inhibitor of apoptosis (XIAP), and, stemness regulators, octamer-binding transcription factor 4 (Oct4) and Nanog, which are inherently upregulated in these cells compared with the rest of the tumor population. In-depth analysis revealed that aberrant overexpression of Sp1 in bCSCs transcriptionally upregulates (1) resistance-promoting genes to protect these cells from genotoxic therapy, and (2) stemness regulators to sustain self-renewal potential of these cells. However, mithramycin A causes transcriptional suppression of these chemoresistant and self-renewal genes by inhibiting Sp1 recruitment to their promoters. Under such antisurvival microenvironment, chemotherapeutic agent doxorubicin induces apoptosis in bCSCs via DNA damage-induced reactive oxygen species generation. Cumulatively, our findings raise the possibility that mithramycin A might emerge as a promising drug in combinatorial therapy with the existing chemotherapeutic agents that fail to eliminate CSCs. This will consequently lead to the improvement of therapeutic outcome for the treatment-resistant breast carcinomas.
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