Deubiquitinase (DUB) is an essential component in the ubiquitin-proteasome system (UPS) by removing ubiquitin chains from substrates, thus modulating the expression, activity, and localization of many proteins that contribute to tumor development and progression. DUBs have emerged as promising prognostic indicators and drug targets. DUBs have shown significant roles in regulating breast cancer growth, metastasis, resistance to current therapies, and several canonical oncogenic signaling pathways. In addition, specific DUB inhibitors have been identified and are expected to benefit breast cancer patients in the future.
Although cancer cells express antigens that can be recognized by T cells and activate the immune system, most tumors escape from immune surveillance through various mechanisms, including self-modification of cancer cells and alteration of tumor microenvironment. For instance, cancer cells highly express programmed death ligand 1 (PD-L1), bind with its receptor, and downregulate the activation of immune responses induced by T cells.
CSN5 was identified as a critical component in PD-L1-mediated immune evasion that inhibits PD-L1 poly-ubiquitination and protects it from proteasomal degradation. It is found that CSN5 is upregulated transcriptionally by NF-κB activation of p65. Additionally, lncRNA also functions as an upstream signal to regulate CSN5. LncRNA GATA3-AS1 enhances CSN5 expression via separation of miR-676-3p from CSN5, thus contributing to the immune escape of TNBC cells. According to clinical evidence, the level of CSN5 is positively related with PD-L1 in breast cancer tissues, and overexpression of CSN5 indicates poor prognosis in patients with breast cancers.
Recently, OTUB1 has been found as a novel DUB of PD-L1 in breast cancer. OTUB1 stabilizes PD-L1 and protects it from endoplasmic reticulum-associated degradation (ERAD) by removing its K48-linked ubiquitin chains. Consistently, loss of OTUB1 leads to PD-L1 reduction in breast cancer cells, enhancing their sensitivity to the cytotoxicity of immune cells.
In conclusion, the NF-κB/p65/CSN5/PD-L1, GATA3-AS1/miR-676-3p/CSN5/PD-L1, and OTUB1/PD-L1 axis promotes the immunosuppression of breast cancer.
The high incidence of breast cancer patients relapsing after chemotherapy indicates that breast cancer cells have complex mechanisms of chemoresistance.
Generally, breast cancer is an estrogen-dependent malignancy. Consequently, chemotherapy with tamoxifen, a representative drug of estrogen antagonists, possesses a good therapeutic effect on patients with breast cancer, and changes in the ERα signaling pathway intensify the tendency of endocrine resistance.
USP22 deubiquitinates and stabilizes ERα, enhancing ERα-induced transactivation in breast cancer cells. At the molecular level, USP22 is demonstrated as a coactivator of downstream genes, which interacts with the cis-acting element together with ERα. As a result, USP22 increases breast cancer resistance to ERα antagonists. In breast cancer cell lines, USP22 reduction enhances the inhibitory effects on proliferation of ERα antagonist ICI 182,780 and tamoxifen by increasing cell sensitivity to endocrine therapy.
USP1 is also an essential deubiquitinase in ERα signaling, which enhances ERα stability through cleaving its Lys48-linked ubiquitin chains. According to TCGA and KMPLOT databases, high expression of USP1 is relevant to poor prognosis in ERα+ breast cancer patients.
Knockdown of USP9X gives rise to tamoxifen resistance by enhancing ERα’s interaction with chromatin. Although there is a physical interaction between USP9X and ERα, ERα is not the direct substrate for USP9X, indicating USP9X may deubiquitinate ERα cofactors to regulate ERα binding with chromatin.
It is found that the epidermal growth factor receptor (EGFR) represses ERα transcription via hyperactivation of MAPK signaling. In addition, UCHL1 downregulates ERα by deubiquitinating and stabilizing EGFR, thus increasing tamoxifen resistance in ERα- breast cancer. UCHL1 inhibition offers a novel treatment for breast cancer patients with ERα shortage and decrease.
According to the results of tissue microarrays from 3093 patients, 77% of invasive breast carcinomas are androgen receptor (AR) positive, indicating AR is frequently expressed in breast tumors. The AR pathway is critical in AR+ breast cancer, functionally interacting with multiple classic oncogenic signaling pathways. Importantly, AR-targeted therapies, including the AR antagonist, enzalutamide, have been demonstrated to be effective against breast cancer. USP14 is required for enhancing AR+ breast cancer cell proliferation through deubiquitination and stabilization of AR. Moreover, USP14 expression has a positive correlation with AR expression according to the results from the TCGA database and is remarkably high in all subtypes of breast cancer. Thus, USP14 promotes resistance to enzalutamide in AR+ breast cancer.
OTULIN, a member of OTU family, selectively recognizes and removes linear polyubiquitin chains from proteins. OTULIN enhances TNBC resistance to Dox and CPT-11 through activation of the Wnt/β-catenin pathway, which contributes to chemoresistance by maintaining CSCs. Mechanistically, DNA damage promotes c-Abl translocation from nuclear to cytoplasm, where c-Abl promotes OTULIN phosphorylation at Tyr56. Then, OTULIN prompts the Wnt/β-catenin pathway by attenuating the linear ubiquitination of β-catenin and facilitates breast cancer cells alteration to a chemoresistant state. Moreover, clinical data show that increased levels of OTULIN and β-catenin significantly correlate with poor prognosis and chemoresistance in TNBC patients.
According to the results of the viability of different breast cancer cell lines after cisplatin treatment, ER- breast cancer is more resistant to cisplatin. The deubiquitinase USP9X stabilizes MCL1, whose overexpression contributes to chemoresistance and poor prognosis in breast cancer. Downregulation of USP9X reinforces cisplatin sensitivity in ER- breast cancer cells, which is speculated to be a result of the degradation of MCL1.
C-Jun activation domain-binding protein-1 (Jab1), also known as CSN5, which is negatively regulated and directly targeted by miR-17, increases cisplatin resistance in TNBC. Jab1 also contributes to cellular resistance to cisplatin by enhancing Rad51 activity in DNA damage repair with the assistance of p53.
EMT transcription factors are significant for the acquisition of chemoresistance in cancer cells. For example, radiation or chemotherapy induces the expression of the Snail/Slug family in ovarian cancers. This, in turn, enhances cell survival by weakening the expression of the p53-mediated apoptotic gene and derepressing the expression of self-renewal genes. Similarly, Snail1 may contribute to chemoresistance in breast cancer patients following the above-mentioned regulation. It has been demonstrated that USP27X is a putative deubiquitinase for Snail1, which enhances breast cancer cells resistance to cisplatin via stabilizing Snail1 and at least reinforcing repression of apoptosis-associated genes.
BRCA1/2 are key components in the process of homologous recombination (HR) targeting the repair of DNA double-strand breaks (DSBs). Additionally, Poly-(ADP-ribose) polymerase (PARP) functions as a critical enzyme for DNA single-strand breaks repair, making PARP inhibitors (PARPi) an effective therapeutic strategy for cancer patients with BRCA mutations. Therefore, it is necessary to find valid biomarkers identifying breast cancers that are sensitive to PARPi treatment.
A study found that USP15 affects breast cancer cell sensitivity to PARPi via regulation of HR. MDC1 recruits USP15 to DNA damage sites, where the BRCT domain of BARD1 is deubiquitinated by USP15, thereby enhancing BRCA1/BARD1 retention that facilitates DSB end resection. Investigators also speculated that breast cancer patients with USP15 M861V and D967H mutants are more sensitive to PARPi treatment, suggesting that these two sites contribute to the interaction with BARD1.
Moreover, BRCA2 recruits Rad51 to DSBs in the HR repair pathway to catalyze homologous pairing. In addition, the deubiquitinase activity of UCHL3 is essential in this process. Mechanistically, ATM activates UCHL3 after DNA damage, which in turn enhances Rad51 interaction with BRCA2 via deubiquitination. Thus, UCHL3 strengthens the HR signaling pathway in DNA repair, rendering breast cancer cells resistant to PARPi. Likewise, according to clinical cases, UCHL3 overexpression functions as a prognostic index for unfavorable outcomes in breast cancer patients.
RNF169 is an atypical regulator in DSB repair that augments the accurate HR pathway instead of the nonhomologous end-joining (NEHJ) pathway. USP7 interacts with RNF169 by UBL domains, then deubiquitinates and stabilizes RNF169, which effectively accumulates at DSBs in the promotion of HR. As a result, the USP7–RNF169 axis contributes to accurate DSB repair and facilitates breast cancer cells resistance to PARPi.
Radiation therapy has increasingly become critical and conventional in breast cancer management. However, the presence of radioresistant cancer cells makes patients suffer from local tumor recurrences. It is therefore important to observe factors involve in radioresistance and explore potent tumor radiosensitizers.
It is well known that cancer stem cells (CSCs) are able to prompt cell cycle checkpoints, thus leading to radioresistance in tumors. Meanwhile, EMT enables cells to obtain stem-like properties, indicating that EMT engages in radioresistance. Researchers identified that ZEB1, a core factor of EMT, is amplified in radioresistant subtypes of breast cancer. ZEB1 is phosphorylated by ATR, a component of the DNA damage repair (DDR) pathway. Then, ZEB1 combines with USP7 to increase its deubiquitinase and stabilization ability towards checkpoint kinase 1 (CHK1), thus facilitating the HR pathway that contributes to radioresistance. In addition to EMT transcription factor ZEB1, long noncoding RNA LINC02582 also promotes radioresistance through interacting with USP7 and stabilizing CHK1. LINC02582 functionally serves as a molecular target of miR-200c, which has been previously demonstrated as a radiosensitizer in breast cancer. PHF8 is also identified as another substrate of USP7, which involves in DSB repair via recruitment of BLM and KU70. In conclusion, interfering USP7 deubiquitinase activity elevates breast cancer sensitivity to radiation therapy.
Rad51, a component in DNA repair pathway, is regarded as a selective target to sensitize tumors to cytotoxic treatments. It is found that UCHL3 weakens radiosensitivity in breast cancer cells by deubiquitinating and activating Rad51. Interventions targeting UCHL3 may improve the curative effect in combination with radiation treatment.
USP52 stabilizes the histone chaperone ASF1A by removing K48-linked polyubiquitin chains, then ASF1A delivers classical S-phase histones H3.1-H4 dimer to replication-coupled chromatin. Therefore, USP52-mediated ASF1A deubiquitination is essential in sustaining genome stability upon DNA damage. Analysis of breast cancer cell viability showed that the USP52/ASF1A signaling promotes tumor cells resistance to ionizing radiation.
Moreover, the UCHL1/HIF-1 axis plays an important role in promoting breast cancer resistance to radiotherapy. UCHL1 upregulates the activity of HIF-1 via deubiquitination of its subunit HIF-1α. Then, HIF-1 activates reprogramming of glucose metabolism and the subsequent pentose phosphate pathway (PPP), thus increasing the level of reduced glutathione (GSH). It is widely recognized that intracellular antioxidants represented by GSH protect cancer cells from radiation-induced DNA lesions through scavenging free radicals and other oxidative products.
In summary, the ATM/ZEB1/USP7/CHK1, miR-200c/LINC02582/USP7/CHK1, USP7/PHF8, UCHL3/RAD51, USP52/ASF1A, and UCHL1/HIF-1 signaling axis are potential targets to improve the radiosensitivity of breast cancer.
Table 1. Roles of DUBs in breast cancer progression.