Chronic hypoxia induces dormancy in breast cancer cell line MDA-MB-231

The tumor microenvironment (TME) is recognized as a key factor in multiple stages of disease progression, local resistance, immune-escaping and metastasis. TME is the product of developing crosstalk between different cells types and components, which provide an essential communication network through the secretion of growth factors and chemokines, and induce oncogenic signals enhancing cancer-cell proliferation and invasion. The rapid proliferation of tumor cells and the aberrant structure of the blood vessels within tumors result in a marked heterogeneity in the perfusion of the tumor tissue with regions of low oxygen or hypoxia. Although most of the tumor cells die in these hypoxic conditions, a part of them can adapt and survive for many days or months in a dormant state. Dormant tumor cells are characterized by cell cycle arrest in G0/G1 phase as well as low metabolism and, are refractive to common chemotherapy giving rise to metastasis. Despite these features, the cells retain their ability to proliferate when conditions improve. Exposing human breast cancer cell line exposure MDA-MB-231 to at least three cycles of 1% O2 hypoxia and reoxygenation, we select a subpopulation of hypoxia resistant cells. These cells, designed as chMDA-MB-231, stably survive under 1% O2 hypoxia condition by entering in dormant state. The reprogramming of cells into tumor dormancy results from the low p-ERK/p-p38 ratio that is described as the molecular switch of tumor dormancy in restrictive environment. This dormant state is reversible since once replaced in normoxia the cells recover the proliferation rate in 2 weeks. Moreover, the data in this thesis demonstrate that cycling hypoxic/reoxygenation stress selects dormant MDA-MB-231 cells that present the cancer stem-like phenotype characterized by CD24-/CD44+/ESA+ expression and spheroid forming capacity. Different reports recognize autophagy as a mechanism activated by microenvironment stresses that may contribute to survival of cells in tumor dormancy. Interestingly, we found that chMDA-MB-231 cells have a high level of autophagy, as measured by the detection of autophagolysome and LC3-II expression, suggesting that autophagy may be the survival mechanism of dormant chMDA-MB-231 cells. We believe that the proposed experimental approach to select dormant breast cancer cells could provide a rationale for the development of novel agents to target dormant tumor cells population.