SIX1 transmits signals for breast cancer progression via the ZEB1/IL6/STAT3 signaling axis.

Epithelial-mesenchymal transition (EMT) and breast cancer stem cells (BCSCs) are pivotal in breast cancer mechanism research. It was demonstrated that Sine oculis homeobox homolog 1 (SIX1) orchestrates breast cancer EMT and BCSCs, concurrently activating the Signal transducer and activator of transcription 3 (STAT3) signaling pathway. Yet, the mechanism by which SIX1 modulates STAT3 and its potential to regulate EMT and BCSCs through STAT3 signaling remain unexplored. Here, cellular, animal, organoid models, and integrated single-cell transcriptomic and ST-seq of human breast cancer specimens were conducted. The results revealed that SIX1 can enhance Zinc finger E-box binding homeobox 1 (ZEB1) expression and translation, which in turn binds to the Interleukin-6 (IL6) promoter (1138bp-1148bp) to stimulate its transcription, translation, and secretion. Subsequently, IL6 can activate the cell's own STAT3 signaling pathway, promote the phosphorylation of STAT3, promote the downstream signal c-Myc and Cyclin D1 transduction, and promote the expression of stem cell-related transcription factors such as ALDH1A1, OCT4, and SOX2, thereby promoting EMT and stemness. In addition, this study found a new cell interaction model, in which the above-mentioned secreted IL6 can promote the activation of STAT3 signaling pathway, EMT and stemness transformation in the surrounding cells with low expression of SIX1 in a paracrine manner. Our data favored that SIX1/ZEB1/IL6 axis activated the STAT3 signaling pathway of the breast cells themselves and surrounding cells with low SIX1 expression, thus promoting EMT and stemness transformation, activating the malignant progression of the whole breast cancer.