Worldwide, breast cancer is the most frequently diagnosed life-threatening cancer in women and is the leading cause of cancer-related mortality among women. It is extremely rare but highly lethal in men. The deubiquitinating enzyme USP39 plays important roles in mRNA processing, and our previous data showed that high levels of USP39 are selectively present in different types of human breast tumor cells. The potential of USP39 as a therapeutic target for breast cancer was investigated. The expression levels of USP39 protein in 23 breast cancer specimens were quantified using an immunohistochemical assay and were found to have high levels in human breast cancer tissues when compared to these levels in normal breast tissues. In the breast cancer cell line MCF-7, USP39 expression was knocked down by a lentiviral short hairpin RNA (shRNA) delivery system. The RNA interference (RNAi)-mediated downregulation of USP39 expression markedly reduced the proliferative and colony forming ability of MCF-7 cells. In addition, the inhibition of USP39 induced G0/G1-phase arrest and apoptosis of the cells. These results suggest that USP39 may act as an oncogenic factor in breast cancer and could be a potential molecular target for breast cancer gene therapy.

Increasing evidence indicates that circular RNAs (circRNAs) play a crucial role in regulating microRNAs (miRs) and mRNAs during breast cancer (BC) progression. Based on the in silico analysis of circRNA/miR/mRNA in BC, we aim to define an important role of circRNA_000554 in BC in relation to miR‐182 and zinc finger protein 36 (ZFP36). Low expression of circRNA_000554 and ZFP36, and high miR‐182 expression were determined in the clinical BC tissues. CircRNA_000554 acted as a sponge of miR‐182, and miR‐182 directly targeted ZFP36. After that, in order to evaluate the effects of circRNA_000554, miR‐182, and ZFP36 on cellular process, we evaluated in vitro epithelial‐mesenchymal transition (EMT) and in vivo tumor growth after delivering a series of overexpression plasmids, mimic, inhibitor, or shRNAs into BC cells. Increasing circRNA_000554 suppressed EMT, cell invasion and migration during BC by depleting miR‐182 and increasing ZFP36. The inhibitory effect of circRNA_000554 on tumor growth was validated in vivo. Taken together, the present study confirms that circRNA_000554 functioned as an inhibitor of EMT in BC and suggests a molecular mechanism that circRNA_000554 bound to miR‐182 to upregulate ZFP36 in this process.

Triple-negative breast cancer (TNBC), characterized by distinct biological and clinicopathological features, has a poor prognosis due to lack of effective therapeutic targets. Our previous data revealed that high levels of USP39 were selectively present in TNBC samples compared with their normal breast tissue samples and USP39 was also expressed at different levels in cultured TNBC cells and normal breast cells. Yet, the underlying cellular and molecular mechanisms of USP39 remain unclear. In the present study, we describe a doxycycline (DOX)-regulated lentiviral vector system expressing shRNA or cDNA of the USP39 gene in the TNBC cell line MDA-MB-231. USP39 expression was knocked down by the miR-30-based inducible lentiviral short hairpin RNA (shRNA) delivery system or overexpressed by the inducible cDNA system. The inducible shRNA-mediated downregulation of USP39 expression markedly reduced the proliferation and colony-forming ability of MDA-MB-231 cells, while overexpression of USP39 by the inducible system did not promote cancer cell proliferation. The lentiviral vector-mediated Tet-on system demonstrated efficient and inducible knockdown of USP39 or overexpression of USP39 in TNBC cells, facilitating a wide variety of applications for gene knockdown and overexpression experiments in gene functional studies in vitro and in vivo.