Evidence for the ISG15-Specific Deubiquitinase USP18 As an Antineoplastic Target
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Published OnlineFirst January 17, 2018; DOI: 10.1158/0008-5472.CAN-17-1752 Cancer Review Research Evidence for the ISG15-Specific Deubiquitinase USP18 as an Antineoplastic Target Lisa Maria Mustachio1, Yun Lu2, Masanori Kawakami1, Jason Roszik3,4, Sarah J. Freemantle5, Xi Liu1, and Ethan Dmitrovsky1,6 Abstract Ubiquitination and ubiquitin-like posttranslational modifica- and in contrast to its gain decreases cancer growth by destabi- tions (PTM) regulate activity and stability of oncoproteins and lizing growth-regulatory proteins. Loss of USP18 reduced can- tumor suppressors. This implicates PTMs as antineoplastic targets. cer cell growth by triggering apoptosis. Genetic loss of USP18 One way to alter PTMs is to inhibit activity of deubiquitinases repressed cancer formation in engineered murine lung cancer (DUB) that remove ubiquitin or ubiquitin-like proteins from models. The translational relevance of USP18 was confirmed by substrate proteins. Roles of DUBs in carcinogenesis have been finding its expression was deregulated in malignant versus intensively studied, yet few inhibitors exist. Prior work provides a normal tissues. Notably, the recent elucidation of the USP18 basis for the ubiquitin-specific protease 18 (USP18) as an anti- crystal structure offers a framework for developing an inhibitor neoplastic target. USP18 is the major DUB that removes IFN- to this DUB. This review summarizes strong evidence for USP18 stimulated gene 15 (ISG15) from conjugated proteins. Prior work as a previously unrecognized pharmacologic target in oncology. discovered that engineered loss of USP18 increases ISGylation Cancer Res; 78(3); 1–6. Ó2018 AACR. Background stimulated gene 15 (ISG15), the first ubiquitin-like protein dis- covered, is activated by a three-step enzymatic cascade that Growth-regulatory proteins that drive carcinogenesis are engages a specific E1-activating enzyme (UBE1L), E2-conjugating altered by posttranslational modifications (PTM; ref. 1). An enzyme (typically UBCH8), and E3 ligase (commonly HERC5A) improved understanding of how PTMs affect oncogenic, to complex ISG15 to protein substrates, as shown in Fig. 1A (6). tumor-suppressive, and other critical tumorigenic pathways will Although it is known that polyubiquitination triggers protein uncover ways to counteract the consequences of those same degradation through the proteasome, monoubiquitination alters pathways (2). PTMs that affect ubiquitination and related pro- other cellular functions (7). In contrast, the functional conse- cesses are under active study because they are deregulated in quences of ISGylation are less well understood. Mono-ISGylation diverse cancers. One important outcome of regulated expression elicits specialized functions including regulating activity of of ubiquitin-specific protease 18 (USP18) is to change the stability growth-regulatory proteins (6, 8, 9). of key proteins that maintain or mediate the carcinogenesis Ubiquitination and ISGylation modifications are removed by process (3, 4). deubiquitinases (DUB), as reviewed in ref. 10. Ubiquitin-specific Ubiquitination is a tightly controlled process. It is regulated by proteases (USP) are a class of DUBs that remove ubiquitin, ISG15, the ubiquitin-activating enzyme (E1), ubiquitin-conjugating or other ubiquitin-related species from complexed proteins enzyme (E2), and ubiquitin ligase (E3) that collectively allow (10, 11). About 100 DUBs are now known to exist, and their ubiquitin to conjugate to target proteins (5). Similarly, IFN- precise biological roles are being elucidated (11). Individual DUBs likely confer specific actions (10, 11). Likewise, if a partic- 1Department of Thoracic/Head and Neck Medical Oncology, The University of ular DUB is pharmacologically affected, this should lead to Texas MD Anderson Cancer Center, Houston, Texas. 2Department of Pharma- desired outcomes on physiology or pharmacologic responses. cology and Toxicology, Geisel School of Medicine at Dartmouth, Hanover, New Here, we describe the known activities of USP18, emphasizing Hampshire. 3Department of Melanoma Medical Oncology, The University of its role as a candidate antineoplastic target. Recent work estab- 4 Texas MD Anderson Cancer Center, Houston, Texas. Department of Genomic lished that reduction of USP18 expression conferred a prominent Medicine, The University of Texas MD Anderson Cancer Center, Houston, Texas. antitumorigenic response. Repression of USP18 protein reduced 5Department of Comparative Biosciences, College of Veterinary Medicine, University of Illinois, Illinois. 6Department of Cancer Biology, The University of stability of critical growth-regulatory species and inhibited cancer Texas MD Anderson Cancer Center, Houston, Texas. cell growth and tumor formation by augmenting apoptotic and chemotherapeutic agent responses (12–15). Taken together, these Note: Supplementary data for this article are available at Cancer Research fi Online (http://cancerres.aacrjournals.org/). and other ndings described here implicate USP18 as a previously unrecognized antineoplastic target. Corresponding Author: Ethan Dmitrovsky, Departments of Thoracic/Head and Neck Medical Oncology and Cancer Biology, The University of Texas MD Anderson Cancer Center, 1515 Holcombe Blvd, Houston, TX 77030. Phone: USP18 Functions 713-745-0200; Fax: 713-792-7864; E-mail: [email protected] The DUB USP18 (originally known as UBP43) was first cloned doi: 10.1158/0008-5472.CAN-17-1752 from leukemia fusion protein expressing AML1-ETO mice and Ó2018 American Association for Cancer Research. then independently identified by different groups using porcine www.aacrjournals.org OF1 Downloaded from cancerres.aacrjournals.org on October 2, 2021. © 2018 American Association for Cancer Research. Published OnlineFirst January 17, 2018; DOI: 10.1158/0008-5472.CAN-17-1752 Mustachio et al. A B ISG15 USP18 protein expression in malignant tissues USP18 UBE1L Elevated Reduced Lung ISG15Substrate HERC5A ISG15 UBE1L Breast Cervix Kidney Protein destabilization Esophagus Prostate Stomach Decreased activity ISG15 UBCH8 Colon Changes in localization Thyroid UBCH8 HERC5A Pancreas ISG15 UBCH8 C Kidney chromophobe Kidney lymphoma B-cell tissue Normal adrenal and parag. Pheochromocytoma leukemia myeloid Acute carcinoma Adrenocortical adenocarcinoma Rectum melanoma Uveal adenocarcinoma Colon Normal kidney papillary carcinoma Kidney Normal liver cancer Liver glioma grade Lower Mesothelioma carcinoma clear cell Kidney Normal bile duct Cholangiocarcinoma Normal prostate adenocarcinoma Prostate Normal soft tissue Sarcoma Normal pancreas cancer Pancreatic Normal stomach cancer Stomach Glioblastoma Normal bladder Bladder cancer Normal esophagus Esophageal carcinoma Normal uterus carcinosarcoma Uterine endometrial carcinoma Uterine tumor germ cell Testicular Normal skin Skin cutaneous melanoma Normal thyroid carcinoma Thyroid Normal lung carcinoma Lung squamous cell Normal thymus Thymoma Lung adenocarcinoma Normal head and neck Head and neck cancer Normal breast cancer Breast Normal cervix cancer Cervical cancer Ovarian 100 10 1 0.1 USP18 mRNA levels (TPM) USP18 mRNA levels 0.01 0.001 © 2018 American Association for Cancer Research Figure 1. USP18 is an antineoplastic target. A, USP18 activity once inhibited will increase ISGylation and decrease stability of critical growth-regulatory proteins that are destabilized by ISGylation. B, USP18 protein levels are elevated in most examined cancers (adapted from ref. 13) when compared with histopathologically normal tissues (other than for prostate cancer where variable USP18 expression was detected). Reduced USP18 protein expression profiles were observed in kidney and stomach cancers as compared with corresponding histopathologically normal tissues. C, USP18 mRNA levels are significantly elevated in diverse cancers, as indicated by analysis of The Cancer Genome Atlas (ÃÃ, P < 0.01; ÃÃÃ, P < 0.001, two-tailed t tests). and human models (16–18). ISG15 expression is markedly from evidence for its important role in hematopoiesis by mod- enhanced by treatment with type I IFN, all-trans-retinoic acid ulating ubiquitin-dependent pathways and ubiquitination (see (RA), lipopolysaccharide, or double-stranded RNA, indicating the Supplementary Fig. S1A), which regulate species that control diverse pharmacologic pathways that could be affected by USP18 myeloid cell differentiation (16–18). inhibition (18–20). ISG15 expression is also elevated in tumor Prior work revealed that USP18 predominately acts to remove cells exposed to epigenetic-modifying agents. This triggers cyto- ISG15 from substrate proteins (18). Initial studies that engineered solic sensing of double-stranded RNA, including endogenous and characterized USP18-deficient mice found that knockout of retroviruses, which can alter type I IFN response (21, 22). Protein USP18 preferentially increased levels of intracellular ISG15 con- sequence and biochemical analyses established that USP18 is a jugates (18, 23). Subsequent in vitro studies found that USP18 USP family member (16–18). USP18, like other USPs, has DUB hydrolyzed ISG15 carboxyl-terminal extension proteins as com- activity (16–18). An early role for USP18 in cancer biology came pared with other ubiquitin-like proteins (18). This highlighted OF2 Cancer Res; 78(3) February 1, 2018 Cancer Research Downloaded from cancerres.aacrjournals.org on October 2, 2021. © 2018 American Association for Cancer Research. Published OnlineFirst January 17, 2018; DOI: 10.1158/0008-5472.CAN-17-1752 USP18 as an Antineoplastic Target USP18