Hypoxia-reoxygenation cycle leads to context dependent phenotypic effects in breast cancer cell lines

Hypoxia is a common feature of solid tumors and is associated with cancer progression. We investigated the effect of hypoxia followed by reoxygenation on the phenotypic behavior of two breast cancer cell lines, MCF-7 and MDA-MB-231.

MCF-7 and MDA-MB-231 cells were exposed to controlled hypoxia (1% O_2, 1 h and 24 h) followed by reoxygenation (24 h to 96 h) and compared to their normoxic controls for cell proliferation, survival, migration and invasion, and chemoresistance. Image-iT™ Green hypoxia reagent confirmed the time-dependent induction of hypoxia in both cell lines. Hypoxia exerted cell-type specific effects on proliferation and cell-cycle regulation. MDA-MB-231 cells demonstrated marked G2/M arrest (p = 0.0016) following 24 h of hypoxia. Distinct cell-death pathways were observed, with hypoxia inducing apoptosis in MCF-7 cells (hypoxia vs. normoxia, 19.45% Vs 3.70%, p < 0.0001) and necrosis in MDA-MB-231 cells (hypoxia vs. normoxia, 52.43% Vs 22.86%, p < 0.0001). Hypoxia-reoxygenation increased proliferation in MCF-7 cells, particularly after long term (24 h) hypoxic preconditioning. In contrast, MDA-MB-231 cells showed increased proliferation after short-term (1 h) hypoxia but reduced proliferation following prolonged hypoxia. Moreover, hypoxia-reoxygenation significantly increased anchorage independent growth, migration and invasion in MCF-7 cells, particularly after short-term hypoxia, while MDA-MB-231 cells exhibited minimal changes in migration and reduced invasion after prolonged hypoxia. Lastly, hypoxia significantly increased chemoresistance, with MCF-7 cells exhibiting three-fold and MDA-MB-231 cells nine-fold increase in resistance to doxorubicin.

These findings suggest that hypoxia followed by reoxygenation results in transient alterations in multiple phenotypic traits, especially in less aggressive breast cancer cells, highlighting the need for strategies to mitigate hypoxia-driven tumour progression.