USP44 promotes chemotherapeutic drug resistance of triple negative breast cancer through EZH2 protein stability.

Triple negative breast cancer (TNBC), a highly invasive breast cancer, is one of the leading causes of cancer-related mortality worldwide. Although chemotherapy remains the standard of care for TNBC, the development of chemotherapy resistance significantly limits its clinical efficacy. In this study, we identified the deubiquitinating enzyme USP44 as a contributor to chemoresistance in TNBC and investigated the potential regulatory feedback mechanisms involved. In this experimental study, we investigated the sensitivity of TNBC cells MDA-MB-231 and BT-549 to chemotherapy drugs after overexpression and knockdown of USP44 using CCK-8 reagent kit and flow cytometry analysis, respectively. Western blot was performed to evaluate the expression levels of relevant proteins. In vivo xenograft models were established to examine the effects of USP44 and its downstream targets on chemosensitivity. Co-immunoprecipitation assay and ubiquitination assay were conducted to identify interacting proteins and elucidate the underlying molecular mechanisms. Knockdown of USP44 increased the sensitivity of MDA-MB-231 and BT-549 cells to chemotherapeutic agents, accompanied by elevated levels of Cleaved PARP. In contrast, USP44 overexpression reduced drug sensitivity. Mechanistically, USP44 was found to interact with EZH2, preventing its ubiquitination and subsequent proteasomal degradation. Notably, treatment with GSK126, a specific EZH2 inhibitor, reversed the chemoresistance induced by USP44 overexpression. USP44/EZH2 signaling pathway is one of the key to causing the drug resistance of TNBC, warranting further clinical investigation.