COMMENTARY The Journal of Clinical Investigation levels: the next target against gynecological cancers?

Diane L. Haakonsen and Michael Rape Department of Molecular and Cell Biology and Howard Hughes Medical Institute, University of California at Berkeley, Berkeley, California, USA.

tively, whereas UBB and UBC encode Ubiquitylation is a tightly regulated process that is essential for appropriate fusions of three and nine ubiquitin mol- ecules, respectively (4). The monomeric cell survival and function, and the ubiquitin pathway has shown promise ubiquitin modifier is generated from these as a therapeutic target for several forms of cancer. In this issue of the JCI, precursors by the activity of deubiquitylas- Kedves and colleagues report the identification of a subset of gynecological es (DUBs), such as USP5, which selective- cancers with repressed expression of the polyubiquitin UBB, which ly cleave ubiquitin molecules off of their renders these cancer cells sensitive to further decreases in ubiquitin fusion partners (5). DUBs also recycle ubiq- production by inhibition of the polyubiquitin gene UBC. Moreover, inducible uitin after its signaling function has been depletion of UBC in mice harboring tumors with low UBB levels dramatically fulfilled, and depletion of proteasome- or decreased tumor burden and prolonged survival. Together, the results of lysosome-associated DUBs leads to deg- this study indicate that there is a synthetic lethal relationship between UBB radation of ubiquitin along with its targets, and UBC that has potential to be exploited as a therapeutic strategy to fight resulting in depletion of cellular ubiquitin these devastating cancers. levels (6, 7). All four ubiquitin con- tribute to basal ubiquitin levels; however, UBB and UBC are upregulated in response to cellular stresses, such as heat shock or oxidative stress (8). Increased ubiquitin Ubiquitylation: a tightly teasome, whereas K63- or Met1-linked expression from these loci is essential for regulated process ubiquitin chains act as a molecular glue metazoan development, as UBC deletion All eukaryotic cells rely on ubiquitylation that allows reversible assembly of large in mice is embryonic lethal due to defec- — the attachment of ubiquitin to a target signaling complexes (2). Multiple ubiq- tive hepatocyte differentiation and UBB — to control a vast array of process- uitin linkages can also be combined in deletion causes infertility and adult-on- es, including protein degradation, DNA a single conjugate to generate mixed or set neurodegeneration (9, 10). Although repair, transcription, protein trafficking, branched chains, and K11/K48–branched ubiquitin is essential in all eukaryotic cells, cell-cycle regulation, and signal trans- chains were recently found to work as pri- the ubiquitin levels that guarantee cellular duction. As many of these processes are ority access signals for proteasomal deg- homeostasis and the contributions of UBB essential for the survival of cancer cells, radation of aggregation-prone and UBC may be different in distinct tis- components of the ubiquitin pathway have (3). While proteolytic ubiquitin chains are sues. Whether the cellular machinery that emerged as attractive therapeutic targets essential signals for cell cycle and protein establishes proper ubiquitin levels could for cancer treatment. Ubiquitin is a high- quality control, nonproteolytic ubiquitin be targeted for therapeutic benefit has ly conserved, 76–amino acid protein that marks control protein sorting as well as been unclear. is estimated to represent up to 5% of the inflammatory signaling pathways. This total cellular protein content (1). Ubiqui- cellular signaling language requires pre- Stable UBB suppression in tin can be transferred to its cellular targets cise regulation of the necessary enzymes, aggressive gynecological either as a single molecule, referred to as including the approximately 600 human cancers monoubiquitylation, or as polymeric ubiq- E3 ubiquitin ligases that recruit specific In this issue, Kedves et al. analyzed expres- uitin chains. Ubiquitin chains can differ substrates, as well as high concentrations sion data across 27 tumor types and found in topology and function. For example, of the modifier itself, ubiquitin. that expression of UBB was specifically homotypic ubiquitin chains connected The high micromolar concentration decreased in high-grade serous ovarian through Lys11 (K11) or Lys48 (K48) of one of ubiquitin in cells is achieved by expres- cancers (HGSOCs), while other ubiqui- ubiquitin and the carboxy terminus of the sion from four genes. RPS27A and UBA52 tin-encoding loci were not affected (11). next ubiquitin molecule trigger degrada- encode ubiquitin as in-frame fusions to a Moreover, UBB expression correlated with tion of substrate proteins by the 26S pro- small and large ribosomal protein, respec- the methylation status of the gene, impli- cating methylation as a UBB-silencing Related Article: p. 4554 mechanism. Using the UBC/UBB mRNA ratio as a measure of UBB silencing, Kedves Conflict of interest: M. Rape is cofounder and consultant to Nurix, a company working in the ubiquitin space. and colleagues showed that three gyneco- Reference information: J Clin Invest. 2017;127(12):4228–4230. https://doi.org/10.1172/JCI98262. logical cancers (uterine carcinosarcomas,

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Figure 1. Further depletion of ubiquitin levels in cancer cells with low levels of UBB expres- sion results in synthetic lethality. Cancer cells with repressed expression of the UBB poly- ubiquitin gene (UBBLO) likely have lower cellular ubiquitin levels and, therefore, are sensitive to further depletion of ubiquitin by inhibition of the polyubiquitin gene UBC. In contrast, cells with normal ubiquitin levels (UBBWT) remain viable after depletion of UBC. Alternatively, proteasome and DUB inhibitors could constitute effective treatment options for cancers with reduced ubiquitin, as these inhibitors also lower ubiquitin levels.

uterine corpus endometrial carcinomas, of UBB silencing, indicating that down- favor lower ubiquitin levels, as those path- and HGSOC), all derived from Mülleri- regulation of UBB expression is a stable ways could provide additional therapeutic an epithelium, display a high prevalence repressive mechanism. Together, these targets against these cancers. of UBB silencing. Moreover, compared findings provide a solid foundation for tar- with those with average UBB expression, geting UBC as a therapeutic intervention Conclusions HGSOC samples with low UBB expression for these high-grade uterine cancers. Ubiquitin has recently taken center stage associated with poorer patient survival It is not clear why UBB silencing for cancer drug discovery. This spotlight has outcomes, suggesting UBB silencing may appears to predominantly affect gyneco- been especially driven by the discovery that play a role in progression of this cancer. logical cancers. Cancer cells evolve rapidly, thalidomide and its derivatives, which are Extraction of data from the Cancer often leading to aneuploidy or a high muta- effective against multiple myeloma, alter Cell Line Encyclopedia yielded similar tional load, both of which increase the need the substrates targeted by the cereblon-con- results, as the majority of cell lines with low for ubiquitin-dependent proteasomal or taining E3 ubiquitin ligase (16). To date, UBB expression were derived from gyne- autophagosomal degradation (12, 13). Sim- most approaches have focused specifically cological cancers. Based on these observa- ilarly, ubiquitylation controls many import- on targeting the enzymes involved in the tions, Kedves et al. wondered whether low ant cell fate specification events during ubiquitylation reaction, such as tumor-re- expression of UBB would sensitize cells to early development (14, 15). These observa- lated E3 ligases like MDM2. However, the a further decrease in ubiquitin levels, and tions suggest that different ubiquitin levels work by Kedves et al. indicates that strate- thereby provide a potential therapeutic may be critical in distinct cellular contexts, gies to target ubiquitin-encoding genes have strategy against these cancers. Indeed, providing a potential explanation for the potential for therapeutic benefit against depletion of the polyubiquitin UBC gene striking observation that gynecological some cancers. Specifically, this study points affected viability of cell lines with low UBB cancers in particular may profit from sta- to a synthetic lethal relationship between expression but not those in which UBB ble repression of the UBB gene. Although UBB and UBC that can serve as an entry to was expressed at normal levels (Figure 1). Kedves et al. did not directly show that low treat a subset of aggressive gynecological This observation suggested that cancer UBB expression alters cellular ubiquitin cancers (Figure 1). Strategies to target UBC cells with a repressed UBB locus could be levels, lack of UBB expression could alter at the mRNA level will need to be developed selectively eliminated by downregulation the dynamics of ubiquitin production, and and may include the use of siRNA therapeu- of the UBC gene. Kedves and colleagues most likely, leads to lower ubiquitin abun- tics or antisense oligonucleotides; however, tested this notion by implanting gyneco- dance. Protein quality control, cell cycle the systemic effects, such as toxicity, of UBC logical cancer cells with low levels of UBB progression, and DNA damage all depend silencing remain to be assessed. Alterna- and an inducible system to deplete UBC on extensive ubiquitylation, often involv- tively, inhibition of either the proteasome into immunodeficient mice. Depletion of ing branched ubiquitin chains with many or DUBs have been shown to lower the ubiq- UBC resulted in marked tumor regression ubiquitin molecules; therefore, a partial uitin levels in cells and might therefore be and a long-term survival benefit, as com- inhibition of such regulatory networks effective in gynecological cancers. Learning pared with control animals. Strikingly, all could result in an evolutionary advantage more about the connectivity of ubiquitin resistant tumors were the result of a lack of for certain cancer cells. Further studies are signaling networks in gynecological cancer UBC knockdown and not due to a release needed to identify the mechanisms that cells could therefore be a fruitful strategy

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