(69h) In vitro Model of Breast Cancer Cell Dormancy Under Hypoxia-Mimicking Microenvironments Using Cobalt Chloride

Hak Rae Lee¹, Faith Leslie¹, Pedram Motallebnejad¹, and Samira M. Azarin¹

1. Department of Chemical Engineering and Materials Science, University of Minnesota

The majority of cancer-related deaths among breast cancer patients result from metastatic disease that originates from disseminated tumor cells (DTCs). Clinical observations have indicated that some DTCs undergo a period of dormancy before they start colonization at the metastatic site, with this latent period sometimes lasting years to decades. Cancer cells can enter dormant states to survive unfavorable environmental factors such as metabolic or oxidative stress. While hypoxia has been well-studied in various tumor microenvironments, its role in DTC dormancy is not yet fully understood, in part due to a lack of well-established in vitro and in vivo models. Hypoxic conditions under conventional hypoxic chambers are extremely unstable and cannot be maintained during characterization outside the chamber since normoxic response is quickly established. Cobalt chloride (CoCl₂) is a hypoxia-mimetic agent which stabilizes hypoxia inducible factor 1-alpha (HIF1α), a major regulator protein of hypoxia signaling. Thus, we sought to develop an in vitro breast cancer dormancy model under hypoxia-mimicking microenvironments using CoCl₂. Western blot analysis demonstrated that addition of CoCl₂ to MCF-7 breast cancer cells was sufficient to increase HIF1α expression, and the level of HIF1α expression could be modulated by the dose and duration of CoCl₂ treatment. Immunofluorescence staining of HIF1α and glucose transporter 1 (Glut1), a key target protein of HIF1 signaling, also demonstrated that both proteins were upregulated and stabilized under CoCl₂ treatment. Growth curve analysis showed that CoCl₂ treatment led to inhibition of cell growth, with the cell number remaining constant for up to 24 days. The inhibition in cell growth by CoCl₂ treatment matched the growth inhibition observed in a hypoxic chamber control, indicating that the effects matched those of true hypoxia. Furthermore, the CoCl₂-treated cells exhibited additional hallmarks of dormancy, such as reduced expression of Ki67, a marker of actively cycling cells, and accumulation of cells in the G0/G1 phase of the cell cycle, as determined by flow cytometry following propidium iodide staining. Importantly, longitudinal tracking of cell growth and Ki67 status showed that these non-proliferative breast cancer cells resumed their growth and became Ki67-positive again after removal of CoCl₂, indicating the cells under hypoxia-mimicking conditions were quiescent and not senescent. Collectively, the results demonstrate that CoCl₂ treatment can effectively mimic dormancy under hypoxic conditions in vitro with high stability, thereby enabling robust characterization of dormant cells under hypoxia-mimicking conditions in vitro.