Unravelling epigenetic mechanisms of CAF-chemotherapy resistance in mammary carcinoma

Triple negative breast cancer (TNBC) is one of the most challenging cancers to treat as, despite the initial response to chemotherapy, relapse occurs frequently. In this project, we focused on the mechanisms that allow TNBC cell survival under chemotherapy treatment. Phenotypic changes in TNBC cells such as gain of mesenchymal transition and stemness during genotoxic stress adaptation have been reported. Based on our data, CAF (Cyclophosphamide, Doxorubicin and 5-Fluorouracil)-chemotherapy treatment in TNBC cells leads to downregulation of the Polycomb Repressive Complex containing the methyl transferase EZH2 (PRC2/EZH2) and upregulation of Histone Deacetylases (HDAC4, -7 and -8). EZH2 is responsible for the gene silencing through H3K27me3 catalysis and is known as an invasiveness marker. It appears that EZH2 can have also anti-tumorigenic function. We identified, among others, Nuclear factor of activated T cells (NFATc1), as a gene regulated by EZH2 loss upon chemotherapy treatment in TNBC cells, indicated epigenetic switch. NFATc1 showed H3K27me3 loss and H3K27ac gain upon chemotherapy, where H3K27ac works antagonistically to the repressive mark, H3K27me3. NFATc1 has been associated with drug resistance and its loss impairs proliferation, migration and mesenchymal properties in TNBC cells in vitro. Our results strongly indicate a negative regulation of EZH2 on NFATc1 expression, in vitro and in vivo. Additionally, in TNBC patient data, we observed that low EZH2 and high NFATc1 expression correlate with poor survival. Therefore, in patients showing low EZH2 expression in cancer, NFATc1 inhibition may represent an alternative treatment option. Furthermore, our preliminary studies on HDACs suggest that HDAC8 supports TNBC invasiveness, affecting cell growth and modulating EMT. Interestingly, selective HDAC8 inhibition sensitizes TNBC cells to chemotherapy. This provides a potential mechanism linking epigenetic adaptation and cancer state during TNBC chemotherapy resistance. In summary, this work demonstrates a previously unknown PRC2/EZH2 function in TNBC, where its downregulation contributes to NFAT pathway changes driving cancer progression. Hence, NFATc1 can be considered as potential PRC2/EZH2–dependent driver of TNBC invasiveness. Additionally, we identified HDAC8 and NFATc1 as an interesting targets for further investigation in potential anti-TNBC therapeutic approach.2020-12-1