Mice null for the deubiquitinase USP18 spontaneously develop leiomyosarcomas

Targeting ubiquitin specific proteases (USPs) in cancer immunotherapy: from basic research to preclinical application

Tumors have evolved in various mechanisms to evade the immune system, hindering the antitumor immune response and facilitating tumor progression. Immunotherapy has become a potential treatment strategy specific to different cancer types by utilizing multifarious molecular mechanisms to enhance the immune response against tumors. Among these mechanisms, the ubiquitin-proteasome system (UPS) is a significant non-lysosomal pathway specific to protein degradation, regulated by deubiquitinating enzymes (DUBs) that counterbalance ubiquitin signaling. Ubiquitin-specific proteases (USPs), the largest DUB family with the strongest variety, play critical roles in modulating immune cell function, regulating immune response, and participating in antigen processing and presentation during tumor progression. According to recent studies, the expressions of some USP family members in tumor cells are involved in tumor immune escape and immune microenvironment. This review explores the potential of targeting USPs as a new approach for cancer immunotherapy, highlighting recent basic and preclinical studies investigating the applications of USP inhibitors. By providing insights into the structure and function of USPs in cancer immunity, this review aims at assisting in developing new therapeutic approaches for enhancing the immunotherapy efficacy.

Interferon stimulated gene 15 (ISG15) in cancer: An update

Among the plethora of defense mechanisms which a host elicits after pathogen invasion, type 1 interferons play a central role in regulating the immune system's response. They induce several interferon-stimulated genes (ISGs) which play a diverse role once activated. Over the past few decades, there have been several studies exploring the role of ISGs in cancer and ISG15 is among the most studied for its pro and anti-tumorigenic role. In this review, we aim to provide an update on the recent observations and findings related to ISG15 in cancer. We provide a brief overview about the initial observations and important historical findings which helped scientists understand structure and function of ISG15. We aim to provide an overview of ISG15 in cancer with an emphasis on studies which delve into the molecular mechanism of ISG15 in modulating the tumor microenvironment. Further, the dysregulation of ISG15 in cancer and the molecular mechanisms associated with its pro and anti-tumor roles are discussed in respective cancer types. Finally, we discuss multiple therapeutic applications of ISG15 in current cancer therapy.

Downregulated Expression of USP18 Is Associated with a Higher Recurrence Risk of Papillary Thyroid Carcinoma

As a member of the deubiquitinating protease family, ubiquitin specific peptidase 18 (USP18) is well acknowledged for its roles in stabilizing downstream protein substrates and inhibiting type I interferon signaling. USP18 has been reported to exert distinct roles in different cancer types. However, its expression and function in papillary thyroid carcinoma (PTC) remain unknown. Here we collected 156 PTC patients and retrospectively retrieved their clinicopathological characteristics as well as their survival data. Among them, USP18 was hypoexpressed in 47 PTC samples (30.1%) and significantly correlated with oncogenic characteristics. According to univariate and multivariate analyses, low USP18 can act as an independent prognostic indicator for unfavorable progression-free survival of PTC patients. Ectopic overexpression and knockdown assays indicated that USP18 can negatively regulate the proliferation of PTC cell lines. The anti-tumor effect of USP18 was finally validated by xenografts results from nude mice. Taken together, PTC patients with low level of USP18 have worse survival compared to those possess high USP18 expression. Downregulated USP18 may be involved in the proliferation of PTC, and USP18 expression can serve as an independent survival predictor.

USP18-deficiency in cervical carcinoma is crucial for the malignant behavior of tumor cells in an ERK signal-dependent manner

Ubiquitin-specific peptidase (USP)18 belongs to the USP family, and is involved in cleaving and removing ubiquitin or ubiquitin-like molecules from their target molecules. Recently, increasing evidence has suggested that USP18 is constitutively expressed in different types of human tumors, and ectopic expression or downregulation of USP18 expression may contribute to tumorigenesis. However, the role of USP18 in uterine cervical cancer (UCC) remains unclear. Thus, the present study aimed to investigate USP18 expression in a human tissue microarray constructed using UCC and non-cancer cervical tissues, and to determine the potential role and molecular mechanism by which USP18 is implicated in the tumor biology of human UCC HeLa cells. Microarray analysis demonstrated that USP18 protein expression was downregulated in tumor tissues compared with in normal tissues. In addition, in vitro analysis revealed that USP18-knockdown markedly promoted the proliferation, colony formation, migration and aggressiveness of HeLa cells. Mechanistic analysis demonstrated that USP18-knockdown increased the levels of Bcl-2, STAT3 and phosphorylated-ERK in HeLa cells. Notably, USP18 silencing-induced malignant phenotypes were interrupted following exogenous administration of the ERK1/2 inhibitor PD98059. Overall, the results of the present study suggested that USP18 may be a potent inhibitor involved in UCC tumor-associated biological behaviors, which are associated with the ERK signaling pathway.

The Ubiquitin-Specific Peptidase USP18 Promotes Lipolysis, Fatty Acid Oxidation, and Lung Cancer Growth

Ubiquitin specific peptidase 18 (USP18), previously known as UBP43, is the IFN-stimulated gene 15 (ISG15) deconjugase. USP18 removes ISG15 from substrate proteins. This study reports that USP18-null mice (vs. wild-type mice) exhibited lower lipolysis rates, altered fat to body weight ratios, and cold sensitivity. USP18 is a regulator of lipid and fatty acid metabolism. Prior work established that USP18 promotes lung tumorigenesis. We sought to learn whether this occurs through altered lipid and fatty acid metabolism. Loss of USP18 repressed adipose triglyceride lipase (ATGL) expression; gain of USP18 expression upregulated ATGL in lung cancer cells. The E1-like ubiquitin activating enzyme promoted ISG15 conjugation of ATGL and destabilization. Immunoprecipitation assays confirmed that ISG15 covalently conjugates to ATGL. Protein expression of thermogenic regulators was examined in brown fat of USP18-null versus wild-type mice. Uncoupling protein 1 (UCP1) was repressed in USP18-null fat. Gain of USP18 expression augmented UCP1 protein via reduced ubiquitination. Gain of UCP1 expression in lung cancer cell lines enhanced cellular proliferation. UCP1 knockdown inhibited proliferation. Beta-hydroxybutyrate colorimetric assays performed after gain of UCP1 expression revealed increased cellular fatty acid beta-oxidation, augmenting fatty acid beta-oxidation in Seahorse assays. Combined USP18, ATGL, and UCP1 profiles were interrogated in The Cancer Genome Atlas. Intriguingly, lung cancers with increased USP18, ATGL, and UCP1 expression had an unfavorable survival. These findings reveal that USP18 is a pharmacologic target that controls fatty acid metabolism. IMPLICATIONS: USP18 is an antineoplastic target that affects lung cancer fatty acid metabolism.

Emerging Roles of USP18: From Biology to Pathophysiology

Eukaryotic proteomes are enormously sophisticated through versatile post-translational modifications (PTMs) of proteins. A large variety of code generated via PTMs of proteins by ubiquitin (ubiquitination) and ubiquitin-like proteins (Ubls), such as interferon (IFN)-stimulated gene 15 (ISG15), small ubiquitin-related modifier (SUMO) and neural precursor cell expressed, developmentally downregulated 8 (NEDD8), not only provides distinct signals but also orchestrates a plethora of biological processes, thereby underscoring the necessity for sophisticated and fine-tuned mechanisms of code regulation. Deubiquitinases (DUBs) play a pivotal role in the disassembly of the complex code and removal of the signal. Ubiquitin-specific protease 18 (USP18), originally referred to as UBP43, is a major DUB that reverses the PTM of target proteins by ISG15 (ISGylation). Intriguingly, USP18 is a multifaceted protein that not only removes ISG15 or ubiquitin from conjugated proteins in a deconjugating activity-dependent manner but also acts as a negative modulator of type I IFN signaling, irrespective of its catalytic activity. The function of USP18 has become gradually clear, but not yet been completely addressed. In this review, we summarize recent advances in our understanding of the multifaceted roles of USP18. We also highlight new insights into how USP18 is implicated not only in physiology but also in pathogenesis of various human diseases, involving infectious diseases, neurological disorders, and cancers. Eventually, we integrate a discussion of the potential of therapeutic interventions for targeting USP18 for disease treatment.

USP18 - a multifunctional component in the interferon response

Ubiquitin-specific proteases (USPs) represent the largest family of deubiquitinating enzymes (DUB). These proteases cleave the isopeptide bond between ubiquitin and a lysine residue of a ubiquitin-modified protein. USP18 is a special member of the USP family as it only deconjugates the ubiquitin-like protein ISG15 (interferon-stimulated gene (ISG) 15) from target proteins but is not active towards ubiquitin. Independent of its protease activity, USP18 functions as a major negative regulator of the type I interferon response showing that USP18 is – at least – a bifunctional protein. In this review, we summarise our current knowledge of protease-dependent and -independent functions of USP18 and discuss the structural basis of its dual activity.

The ISG15-specific protease USP18 regulates stability of PTEN

The ubiquitin-like modifier interferon-stimulated gene 15 (ISG15) is implicated in both oncogenic and tumor suppressive programs. Yet, few ISGylation substrates are known and functionally validated in cancer biology. We previously found specific oncoproteins were substrates of ISGylation and were stabilized by the ISG15-specific deubiquitinase (DUB) ubiquitin specific peptidase 18 (USP18). Using reverse-phase protein arrays (RPPAs), this study reports that engineered loss of the DUB USP18 destabilized the tumor suppressor protein phosphatase and tensin homologue (PTEN) in both murine and human lung cancer cell lines. In contrast, engineered gain of USP18 expression in these same lung cancer cell lines stabilized PTEN protein. Using the protein synthesis inhibitor cycloheximide (CHX), USP18 knockdown was shown to destabilize PTEN whereas USP18 overexpression stabilized PTEN protein. Interestingly, repression of USP18 decreased cytoplasmic PTEN relative to nuclear PTEN protein levels. We sought to identify mechanisms engaged in this PTEN protein destabilization using immunoprecipitation assays and found ISG15 directly conjugated with PTEN. To confirm translational relevance of this work, USP18 and PTEN immunohistochemical expression were compared in comprehensive lung cancer arrays. There was a significant (P < 0.0001) positive correlation and association between PTEN and USP18 protein expression profiles in human lung cancers. Taken together, this study identified PTEN as a previously unrecognized substrate of the ISGylation post-translational modification pathway. The deconjugase USP18 serves as a novel regulator of PTEN stability. This indicates inhibition of ISGylation is therapeutically relevant in cancers.

Evidence for the ISG15-Specific Deubiquitinase USP18 as an Antineoplastic Target

Ubiquitination and ubiquitin-like posttranslational modifications (PTM) regulate activity and stability of oncoproteins and tumor suppressors. This implicates PTMs as antineoplastic targets. One way to alter PTMs is to inhibit activity of deubiquitinases (DUB) that remove ubiquitin or ubiquitin-like proteins from substrate proteins. Roles of DUBs in carcinogenesis have been intensively studied, yet few inhibitors exist. Prior work provides a basis for the ubiquitin-specific protease 18 (USP18) as an antineoplastic target. USP18 is the major DUB that removes IFN-stimulated gene 15 (ISG15) from conjugated proteins. Prior work discovered that engineered loss of USP18 increases ISGylation and in contrast to its gain decreases cancer growth by destabilizing growth-regulatory proteins. Loss of USP18 reduced cancer cell growth by triggering apoptosis. Genetic loss of USP18 repressed cancer formation in engineered murine lung cancer models. The translational relevance of USP18 was confirmed by finding its expression was deregulated in malignant versus normal tissues. Notably, the recent elucidation of the USP18 crystal structure offers a framework for developing an inhibitor to this DUB. This review summarizes strong evidence for USP18 as a previously unrecognized pharmacologic target in oncology. Cancer Res; 78(3); 587-92. ©2018 AACR.

Deubiquitinase USP18 Loss Mislocalizes and Destabilizes KRAS in Lung Cancer

KRAS is frequently mutated in lung cancers and is associated with aggressive biology and chemotherapy resistance. Therefore, innovative approaches are needed to treat these lung cancers. Prior work implicated the IFN-stimulated gene 15 (ISG15) deubiquitinase (DUB) USP18 as having antineoplastic activity by regulating lung cancer growth and oncoprotein stability. This study demonstrates that USP18 affects the stability of the KRAS oncoprotein. Interestingly, loss of USP18 reduced KRAS expression, and engineered gain of USP18 expression increased KRAS protein levels in lung cancer cells. Using the protein synthesis inhibitor cycloheximide, USP18 knockdown significantly reduced the half-life of KRAS, but gain of USP18 expression significantly increased its stability. Intriguingly, loss of USP18 altered KRAS subcellular localization by mislocalizing KRAS from the plasma membrane. To explore the biologic consequences, immunohistochemical (IHC) expression profiles of USP18 were compared in lung cancers of KrasLA2/+ versus cyclin E engineered mouse models. USP18 expression was higher in Kras-driven murine lung cancers, indicating a link between KRAS and USP18 expression in vivo To solidify this association, loss of Usp18 in KrasLA2/+ /Usp18-/- mice was found to significantly reduce lung cancers as compared with parental KrasLA2/+ mice. Finally, translational relevance was confirmed in a human lung cancer panel by showing that USP18 IHC expression was significantly higher in KRAS-mutant versus wild-type lung adenocarcinomas.Implications: Taken together, this study highlights a new way to combat the oncogenic consequences of activated KRAS in lung cancer by inhibiting the DUB USP18. Mol Cancer Res; 15(7); 905-14. ©2017 AACR.

Novel Protective Role for Ubiquitin-Specific Protease 18 in Pathological Cardiac Remodeling

Ubiquitin-specific protease 18 (USP18), a USP family member, is involved in antiviral activity and cancer inhibition. Although USP18 is expressed in heart, the role of USP18 in the heart and in cardiac diseases remains unknown. Here, we show that USP18 expression is elevated in both human dilated hearts and hypertrophic murine models. Cardiomyocyte-specific overexpression of USP18 in mice significantly blunted cardiac remodeling as evidenced by mitigated myocardial hypertrophy, fibrosis, ventricular dilation, and preserved ejection function, whereas USP18-deficient mice displayed exacerbated cardiac remodeling under the same pathological stimuli. Similar results were observed for in vitro angiotensin II-induced neonatal rat cardiomyocyte hypertrophy. The antihypertrophic effects of USP18 under hypertrophic stimuli were associated with the blockage of the transforming growth factor-β-activated kinase 1-p38/c-Jun N-terminal kinase 1/2 signaling cascade. Blocking transforming growth factor-β-activated kinase 1-p38/c-Jun N-terminal kinase 1/2 signaling with a pharmacological inhibitor (5Z-7-oxozeaenol) greatly reversed the detrimental effects observed in USP18-knockout mice subjected to aortic banding. Our data indicate that USP18 inhibits cardiac hypertrophy and postpones cardiac dysfunction during the remodeling process, which is dependent on its modulation of the transforming growth factor-β-activated kinase 1-p38/c-Jun N-terminal kinase 1/2 signaling axis. Thus, USP18 is a potent therapeutic target for heart failure treatment.

Positive feedback regulation of p53 transactivity by DNA damage-induced ISG15 modification

p53 plays a pivotal role in tumour suppression under stresses, such as DNA damage. ISG15 has been implicated in the control of tumorigenesis. Intriguingly, the expression of ISG15, UBE1L and UBCH8 is induced by DNA-damaging agents, such as ultraviolet and doxorubicin, which are known to induce p53. Here, we show that the genes encoding ISG15, UBE1L, UBCH8 and EFP, have the p53-responsive elements and their expression is induced in a p53-dependent fashion under DNA damage conditions. Furthermore, DNA damage induces ISG15 conjugation to p53 and this modification markedly enhances the binding of p53 to the promoters of its target genes (for example, CDKN1 and BAX) as well as of its own gene by promoting phosphorylation and acetylation, leading to suppression of cell growth and tumorigenesis. These findings establish a novel feedback circuit between p53 and ISG15-conjugating system for positive regulation of the tumour suppressive function of p53 under DNA damage conditions.

The ‘genome guardian' p53 has a well-established role in suppressing tumour development after DNA damage. Here the authors show that expression of the ubiquitin-like protein ISG15 is regulated by p53 which in turn is modified by ISG15 to enhance binding to target gene promoters.