Identification of a functional nuclear localization signal within the human USP22 protein

Inhibition of SCFKDM2A/USP22-dependent nuclear β-catenin ubiquitylation mediates cerebral ischemic tolerance

Hypoxic postconditioning (HPC) was reported to stabilize nuclear β-catenin by inhibiting lysine (K)-specific demethylase 2 A (KDM2A) in hippocampal CA1 against transient global cerebral ischemia (tGCI). Herein we investigate how HPC inhibits the K48-linked poly-ubiquitination (K48-Ub)-related degradation of nuclear β-catenin in CA1 after tGCI. We confirmed that SCFKDM2A complex targets nuclear β-catenin for degradation via ubiquitin proteasome pathway in vitro. HPC reduced SCFKDM2A complex and the K48-Ub of β-catenin, and increased ubiquitin-specific peptidase 22 (USP22) in nucleus after tGCI. Furthermore, KDM2A knockdown decreased the K48-Ub of nuclear β-catenin and nuclear β-catenin-SCFKDM2A complex interaction after tGCI. Moreover, β-catenin knockdown suppressed nuclear survivin expression and attenuated neuroprotection induced by HPC. In contrast, the overexpression of USP22 promoted nuclear β-catenin deubiquitination and enhanced the neuroprotective effects offered by HPC. Taken together, this study supports that HPC downregulated the K48-Ub of nuclear β-catenin through suppressing SCFKDM2A and increasing USP22, thereby inducing cerebral ischemic tolerance.

USP32 confers cancer cell resistance to YM155 via promoting ER-associated degradation of solute carrier protein SLC35F2

Background: The most commonly preferred chemotherapeutic agents to treat cancers are small-molecule drugs. However, the differential sensitivity of various cancer cells to small molecules and untargeted delivery narrow the range of potential therapeutic applications. The mechanisms responsible for drug resistance in a variety of cancer cells are also largely unknown. Several deubiquitinating enzymes (DUBs) are the main determinants of drug resistance in cancer cells.

Methods: We used CRISPR-Cas9 to perform genome-scale knockout of the entire set of genes encoding ubiquitin-specific proteases (USPs) and systematically screened for DUBs resistant to the clinically evaluated anticancer compound YM155. A series of in vitro and in vivo experiments were conducted to reveal the relationship between USP32 and SLC35F2 on YM155-mediated DNA damage in cancer cells.

Results: CRISPR-based dual-screening method identified USP32 as a novel DUB that governs resistance for uptake of YM155 by destabilizing protein levels of SLC35F2, a solute-carrier protein essential for the uptake of YM155. The expression of USP32 and SLC35F2 was negatively correlated across a panel of tested cancer cell lines. YM155-resistant cancer cells in particular exhibited elevated expression of USP32 and low expression of SLC35F2.

Conclusion: Collectively, our DUB-screening strategy revealed a resistance mechanism governed by USP32 associated with YM155 resistance in breast cancers, one that presents an attractive molecular target for anti-cancer therapies. Targeted genome knockout verified that USP32 is the main determinant of SLC35F2 protein stability in vitro and in vivo, suggesting a novel way to treat tumors resistant to small-molecule drugs.

USP22 promotes IRF3 nuclear translocation and antiviral responses by deubiquitinating the importin protein KPNA2

USP22 is a cytoplasmic and nuclear deubiquitinating enzyme, and the functions of cytoplasmic USP22 are unclear. Here, we discovered that cytoplasmic USP22 promoted nuclear translocation of IRF3 by deubiquitianting and stabilizing KPNA2 after viral infection. Viral infection induced USP22-IRF3 association in the cytoplasm in a KPNA2-depedent manner, and knockdown or knockout of USP22 or KPNA2 impaired IRF3 nuclear translocation and expression of downstream genes after viral infection. Consistently, Cre-ER Usp22^fl/fl or Lyz2-Cre Usp22^fl/fl mice produced decreased levels of type I IFNs after viral infection and exhibited increased susceptibility to lethal viral infection compared with the respective control littermates. Mechanistically, USP22 deubiquitinated and stabilized KPNA2 after viral infection to facilitate efficient nuclear translocation of IRF3. Reconstitution of KPNA2 into USP22 knockout cells restored virus-triggered nuclear translocation of IRF3 and cellular antiviral responses. These findings define a previously unknown function of cytoplasmic USP22 and establish a mechanistic link between USP22 and IRF3 nuclear translocation that expands potential therapeutic strategies for infectious diseases.

ATXN7L3 and ENY2 Coordinate Activity of Multiple H2B Deubiquitinases Important for Cellular Proliferation and Tumor Growth

Histone H2B monoubiquitination (H2Bub1) is centrally involved in gene regulation. The deubiquitination module (DUBm) of the SAGA complex is a major regulator of global H2Bub1 levels, and components of this DUBm are linked to both neurodegenerative diseases and cancer. Unexpectedly, we find that ablation of USP22, the enzymatic center of the DUBm, leads to a reduction, rather than an increase, in global H2bub1 levels. In contrast, depletion of non-enzymatic components, ATXN7L3 or ENY2, results in increased H2Bub1. These observations led us to discover two H2Bub1 DUBs, USP27X and USP51, which function independently of SAGA and compete with USP22 for ATXN7L3 and ENY2 for activity. Like USP22, USP51 and USP27X are required for normal cell proliferation, and their depletion suppresses tumor growth. Our results reveal that ATXN7L3 and ENY2 orchestrate activities of multiple deubiquitinating enzymes and that imbalances in these activities likely potentiate human diseases including cancer.

The deubiquitinase Usp27x stabilizes the BH3-only protein Bim and enhances apoptosis

Bim is a pro-apoptotic Bcl-2 family member of the BH3-only protein subgroup. Expression levels of Bim determine apoptosis susceptibility in non-malignant and in tumour cells. Bim protein expression is downregulated by proteasomal degradation following ERK-dependent phosphorylation and ubiquitination. Here, we report the identification of a deubiquitinase, Usp27x, that binds Bim upon its ERK-dependent phosphorylation and can upregulate its expression levels. Overexpression of Usp27x reduces ERK-dependent Bim ubiquitination, stabilizes phosphorylated Bim, and induces apoptosis in PMA-stimulated cells, as well as in tumour cells with a constitutively active Raf/ERK pathway. Loss of endogenous Usp27x enhances the Bim-degrading activity of oncogenic Raf. Overexpression of Usp27x induces low levels of apoptosis in melanoma and non-small cell lung cancer (NSCLC) cells and substantially enhances apoptosis induced in these cells by the inhibition of ERK signalling. Finally, deletion of Usp27x reduces apoptosis in NSCLC cells treated with an EGFR inhibitor. Thus, Usp27x can trigger via its proteolytic activity the deubiquitination of Bim and enhance its levels, counteracting the anti-apoptotic effects of ERK activity, and therefore acts as a tumour suppressor.

The business of deubiquitination - location, location, location

A majority of proteins in the cell can be modified by ubiquitination, thereby altering their function or stability. This ubiquitination is controlled by both ubiquitinating and deubiquitinating enzymes (DUBs). The number of ubiquitin ligases exceeds that of DUBs by about eightfold, indicating that DUBs may have much broader substrate specificity. Despite this, DUBs have been shown to have quite specific physiological functions. This functional specificity is likely due to very precise regulation of activity arising from the sophisticated use of all mechanisms of enzyme regulation. In this commentary, we briefly review key features of DUBs with more emphasis on regulation. In particular, we focus on localization of the enzymes as a critical regulatory mechanism which when integrated with control of expression, substrate activation, allosteric regulation, and post-translational modifications results in precise spatial and temporal deubiquitination of proteins and therefore specific physiological functions. Identification of compounds that target the structural elements in DUBs that dictate localization may be a more promising approach to development of drugs with specificity of action than targeting the enzymatic activity, which for most DUBs is dependent on a thiol group that can react non-specifically with many compounds in large-scale screening.

Ubiquitin-specific protease 22 is a deubiquitinase of CCNB1

The elevated level of CCNB1 indicates more aggressive cancer and poor prognosis. However, the factors that cause CCNB1 upregulation remain enigmatic. Herein, we identify USP22 as a CCNB1 interactor and discover that both USP22 and CCNB1 are dramatically elevated with a strong positive correlation in colon cancer tissues. USP22 stabilizes CCNB1 by antagonizing proteasome-mediated degradation in a cell cycle-specific manner. Phosphorylation of USP22 by CDK1 enhances its activity in deubiquitinating CCNB1. The ubiquitin ligase anaphase-promoting complex (APC/C) targets USP22 for degradation by using the substrate adapter CDC20 during cell exit from M phase, presumably allowing CCNB1 degradation. Finally, we discover that USP22 knockdown leads to slower cell growth and reduced tumor size. Our study demonstrates that USP22 is a CCNB1 deubiquitinase, suggesting that targeting USP22 might be an effective approach to treat cancers with elevated CCNB1 expression.