Enhanced ETS1 stability by DNAPKcs orchestrates transcriptional changes during chemoresistance in triple negative breast cancer.

Triple Negative Breast Cancer (TNBC) accounts for ~20% of all breast cancers and results in thousands of deaths every year. The median survival of TNBC patients sharply declines with the development of chemoresistance and metastatic disease. Although high expression of ETS1 in TNBC has been associated with aggressiveness, the mechanisms of ETS1 in TNBC therapy relapse are poorly understood. Here, we show that ETS1 is responsible for driving acquired drug resistance in the TNBC cell line models resistant to 5'-Fluorouracil and doxorubicin. Protein kinase, DNAPKcs (aka PRKDC) mediated phosphorylation of ETS1 at Serine 251 residue enhances protein stability by preventing ETS1's degradation, thus enhancing ETS1-driven resistance mechanisms. Further, transcriptomic profiling of resistant cells and TNBC patients showed that phosphorylated-ETS1 could activate genes of the E2F, MYC and G2/M pathways, resulting in enhanced DNA synthesis and proliferation, leading to resistance. DNAPKcs inhibitors resulted in ETS1 degradation, inhibition of proliferation gene circuits and subsequent apoptosis in resistant TNBC cells. Phospho-S251 ETS1 and associated ETS1-driven proliferative gene signatures were observed in drug-resistant TNBC patients. Our findings suggest that DNAPKcs-mediated phosphorylation of ETS1 promotes chemoresistance in TNBC patients and can be targeted using DNAPKcs kinase inhibitors.