Enrichment of breast cancer stem cells following cytotoxic chemotherapy is mediated by hypoxia-inducible factors

Breast cancers (BCs) that do not express the estrogen or progesterone receptor or human epidermal growth factor receptor 2 are known as triple negative breast cancers (TNBCs). Women with TNBC receive non-targeted chemotherapy with a durable response rate of less than 20%. BC stem cells (BCSCs) are a small subpopulation of BC cells that are characterized by the capacity for infinite self-renewal; are the only BC cells capable of forming a secondary (recurrent or metastatic) BC; and must be eliminated in order to eradicate BC. Hypoxia-inducible factors (HIFs) activate hundreds of genes in TNBCs and HIF-1 expression in the diagnostic tumor biopsy is associated with patient mortality. In this paper, we report that treatment of TNBC cells with cytotoxic chemotherapy increased HIF-1 and HIF-2 protein levels and HIF target gene expression. Chemotherapy also increased the percentage of BCSCs through pathways involving interleukin-6 (IL-6), IL-8, and multidrug resistance 1. HIF inhibitors blocked increased BCSC specification in response to cytotoxic chemotherapy and combination therapy led to tumor eradication. Increased HIF target gene expression in BC biopsies was correlated with increased mortality, especially in those patients treated with chemotherapy alone. Our results suggest that HIF-dependent BCSC enrichment provides a molecular and cellular basis for the high incidence of relapse in women with TNBC.