hsa-miR-5688 inhibits FOXC1-OCT4/SOX2 feedforward loop that drives chemoresistance in breast cancer stem cells.

Inherently chemotherapy-resistant breast cancer stem cells (CSCs) are responsible for tumor initiation, metastasis, and relapse. CSCs "acquire" more resistance and stemness upon chemotherapy, thereby making relapse-free survival extremely challenging. Here, we describe a novel role of FOXC1 in "acquired resistance" of breast CSCs during chemotherapy. Putative binding sites of pluripotency factors OCT4 and SOX2, but not NANOG, on FOXC1 promoter, were demonstrated by JASPAR and validated by a docking experiment. Significant decline in FOXC1 expression was noticed after OCT4 or SOX2 ablation in breast CSCs. Contrastingly, presence of putative FOXC1 binding sites on the promoters of stemness genes and drug-resistance marker ABCG2, along with downregulation of OCT4 and SOX2 in FOXC1-ablated CSCs, indicated the existence of a feedforward FOXC1-OCT4/SOX2 transactivation loop in CSCs. Chemotherapy-induced upregulation of FOXC1, stemness, as well as drug resistance in CSCs, and downregulation of the same by prior FOXC1-ablation in in-vitro and in-vivo models, endorsed the contribution of this loop in chemo-induced acquisition of stemness and drug resistance. Finally, over-expression of hsa-miR-5688 sensitized CSCs toward chemotherapy and decelerated recurrence. Accordingly, we demonstrate a hitherto unknown mechanism underpinning chemotherapy-induced resistance in breast CSCs, causing relapse and identified hsa-miR-5688 as a potential therapeutic candidate for relapse-free survival of breast cancer patients.