**2. DUBs and Cell Cycle Control**

The cell cycle refers to a series of processes, including DNA synthesis, S phase; cell growth, G1 phase; evaluation of the accuracy of the genomic material, G2 phase; and cell division, M phase. The cycle is completed by duplicating the genetic information and equally segregating it into two daughter cells. Many cell cycle checkpoints are controlled by cyclins and cyclin-dependent kinases (CDKs) [19]. The E3 ligases participate at almost every phase, indicating the importance of ubiquitination and deubiquitination in regulating the cell cycle [20,21].


**Table 2.** Functional roles of DUBs in cancer properties.


**Table 2.** *Cont.*

The ability to advance through di fferent stages of the cell cycle regardless of inhibitory signals is one of the hallmarks of cancer. A large number of DUBs have been found to play roles in cell cycle control of cancers via the regulation of di fferent cell cycle checkpoints. OTUD6B-2 and USP17 were reported to control the G1 phase; USP3, USP10, USP14, USP17, USP20, and BAP1 played roles in the G1/S transition. In addition, S/G2 transition was controlled by OTUD7B and DUB3. USP7 and OTUD7B were necessary for the regulation of mitotic phase (Figure 1).

**Figure 1.** Roles of DUBs in cell cycle control in cancers. The eukaryotic cell cycle consists of the G1 phase (blue), the S-phase (brown), the G2 phase (yellow), and the M (mitosis) phase (green). Cells can enter a quiescent state, the G0 phase (grey). Cell cycle phases are indicated by different colored arrows. The cell cycle is regulated by complexes that are composed of cyclins (light purple), and its relative cyclin-dependent protein kinases (CDKs) (pink). The cyclin-CDK complex plays regulatory roles in the cell cycle. The red arrows indicate their targets, either within the designated cell cycle phase or in the transition state. Various DUBs have been shown to interact with the cyclin–cdk complex. DUBs that participate in G1 phase are labeled in light blue; S phase in light brown; G2 phase in light yellow; and M phase in light green. The detailed interaction partner of each individual DUB can be found in the main text and the Table 2.

For the G1 phase regulation, OTUD6B operates downstream of mTORC1 signaling in non-small cell lung cancer (NSCLC), and its isoform OTUD6B-2, was reported to control the stability of cyclin D1 and c-Myc [24]. USP17 is another cell cycle-regulating DUB. It was found to be highly expressed in colon, esophageal, and cervical cancers. The depletion of USP17 increases the levels of the CDK inhibitor p21 and impairs the G1-S transition, leading to cell cycle arrest [31]. In addition, USP17 deubiquitinates the transcription factor ELK-1. The stabilization of ELK-1 increases the expression of cyclin D1 [32]. USP17 further decreases Su(var)3-9, enhancer-of-zeste, and trithorax domain-containing protein 8 ubiquitination to trigger cellular senescence [33].

For the G1/S phase, USP20 deubiquitinates and stabilizes the DNA checkpoint protein claspin, and thus activates the ATR-Chk1 signaling in the DNA damage response pathway [98]. USP10 deubiquitinates SKP2 and augments the activation of Bcr-Abl by mediating deubiquitination and stabilization of SKP2 in chronic myelogenous leukemia cells [29]. An RNAi-based screening study discovered that USP21 binds and deubiquitinates FOXM1, leading to its increased stability, which induces cell cycle progression in basal-like breast cancer [34]. In addition, DUBs could regulate transcription factors for cell cycle control. The transcription factor Krüppel-like factor 5 (KLF5), which promotes cell proliferation by inhibiting the expression of the cell cycle inhibitor p27 [22], is highly expressed in breast cancer. A genome-wide siRNA library screen identified BAP1 and USP3 as KLF5 DUBs. Both BAP1 and USP3 bind to and stabilize KLF5 via deubiquitination [22,35], indicating the possible regulatory role of DUBs in cancer proliferation. Another example is the androgen receptor (AR), a key transcription factor in the development of breast cancer [99]. It has been reported that AR can be stabilized by USP14, and depletion of USP14 reduces cell proliferation by blocking the G0/G1–S phase transition in AR-responsive breast cancer cells [30].

For the S/G2/M phase, OTUD7B, also called cezanne, is frequently overexpressed in different cancer types, such as breast and lung cancer [100,101]. It is reported to be a cell cycle-dependent DUB because it deubiquitylates substrates of the mitotic cyclin anaphase-promoting complex/cyclosome (APC/C) and prevents their degradation during mitosis [25]. The APC/C is a key regulator of cell cycle progression through the regulation of CDK activity [26]. OTUD7B controls the cell cycle through HIF2α and E2F1 in response to oncogenic signaling [27]. In addition, it removes UBQ from GβL in the mTOR complex to regulate mTORC2 signaling in response to growth signals [28]. Besides, DUB3 can directly deubiquitinate cyclin A in NSCLC. The depletion of DUB3 decreases cyclin A levels, leading to cell cycle arrest at the G0/G1-S phase checkpoint in NSCLC cells [23]. Lastly, it is known that histone demethylases can regulate the cell cycle through transcriptional regulation [102]. The histone demethylase PHF8 is stabilized by USP7, leading to the upregulation of cyclin A2, which is critical for cell growth and proliferation in breast carcinomas [36].
