STAT3/Snail signaling and progression of hypoxia tolerance in breast cancer cells

One of the hallmark features of malignant neoplasms is their ability to sustain growth under hypoxic conditions resulting from insufficient oxygenation of tumor tissues. Prolonged hypoxia is associated with the gradual adaptation of tumor cells to low oxygen levels, leading to enhanced survival, increased metastatic potential, and the development of resistance to anticancer therapies. The aim of this study was to investigate the mechanisms underlying breast cancer cell adaptation to prolonged hypoxia and the maintenance of the hypoxia-tolerant phenotype. Using long-term cultivation under low oxygen conditions (1% O2), we established hypoxia-adapted sublines of luminal (MCF-7/H) and triple-negative (MDA-MB-231/H) breast cancer cells, characterized by stable growth in a hypoxic environment. We demonstrated that the acquisition of hypoxia tolerance is accompanied by the activation of the HIF-1α-dependent transcription factor STAT3 and persistent overexpression of Snail, a key downstream effector of STAT3. The maintenance and stabilization of this phenotype are mediated by miR-181a-2, which targets the STAT3/Snail signaling axis in resistant cells. Analysis of DNA methylation status revealed no significant changes in the expression or activity of DNA methyltransferases (DNMTs) in the hypoxia-adapted cells. However, pharmacological inhibition of DNMTs using decitabine, or DNMT knockdown, increased cellular sensitivity to hypoxia and partially reversed the resistant phenotype, which was accompanied by the activation of pro-apoptotic p53 signaling. In conclusion, our findings suggest that acquired hypoxia tolerance in breast cancer cells is, at least in part, mediated by the activation of the miR-181a-2/STAT3/Snail signaling pathway. Furthermore, demethylating agents may represent a promising therapeutic approach to target hypoxia-tolerant cancer cell populations.