**3. DUBs and Cell Proliferation**

In addition to their role in regulating the cell cycle, DUBs have been reported to regulate cell proliferation through different cell signaling pathways, such as Wnt/β-catenin signaling, p53-mouse double minute 2 (MDM2) signaling, PI3K-Akt signaling, AR signaling, and transforming growth factor beta (TGF-β) signaling. Aberrant canonical Wnt/β-catenin signaling is tightly associated with many solid and liquid tumors [103]. Furthermore, alteration or loss of differentiation control could facilitate the development of metastatic traits during tumorigenesis [104,105]. Numerous studies have demonstrated the control of Wnt/β-catenin signaling by DUBs in cancer [48,106]. USP6NL is elevated in colorectal cancer (CRC) and regulates β-catenin accumulation. Knockdown of USP6NL results in inhibition of cell proliferation and G0/G1 cell cycle arrest in human CRC cell lines [53]. In addition, USP4 is a candidate for a β-catenin-specific DUB. There is a positive correlation between the levels of USP4 and β-catenin in human colon cancer tissues. Further, knockdown of USP4 reduces invasiveness and migration in colon cancer cells [48]. β-catenin is also stabilized by USP9X, leading to high-grade glioma cell growth. USP9X removes the Lys48-linked polyubiquitin chains from β-catenin to prevent its proteasomal degradation. Depletion of USP9X induces G1-S cell cycle arrest and inhibits cell proliferation in glioblastoma cells [59].

The tumor suppressor p53 is a transcription factor able to control important cellular pathways. It prevents genome mutation and plays protective roles in tumor onset and progression. It is mainly regulated by ubiquitylation, indicating the importance of DUBs in monitoring its ubiquitin cycle [107]. Both MDM2 and p53 are targeted by different DUBs (Figure 2). Suppression of USP2 leads to MDM2 destabilization and results in p53 activation [44]. USP7 plays a key role in the p53 pathway by stabilizing both MDM2 and p53 (Figure 2) [54–58]. Under normal conditions, USP7 has a higher binding affinity to MDM2, the major E3 ligase of p53 [56], and thus deubiquitylates MDM2 more efficiently to prevent its self-degradation and maintain stable protein levels for controlling p53 via the UBQ-proteasome pathway [108]. USP10 regulates p53 localization and stability by deubiquitinating p53. It reverses MDM2-induced p53 nuclear export and degradation [40]. Moreover, USP29 is reported to cleave poly-ubiqutin chains from p53 and thus stabilize it [47], while USP15 stabilizes the E3 UBQ ligase MDM2 in cancer cells and regulates p53 function and cancer cell survival. Inhibition of USP15 induces apoptosis and boosts antitumor T cell responses in tumor cells [42]. Furthermore, a large number of DUBs have been found to target p53 or p53-associated proteins directly, leading to proliferation. USP5

regulates p53 levels and alters cell growth and cell cycle distribution associated with p21 induction in melanoma cells [52]. OTUD1 is required for p53 stabilization, and OTUD1 overexpression increases the cleavage of caspase-3 and PARP and subsequently increases apoptosis [38]. Another p53-associated DUB, otubain 1 (OTUB1), is expressed in high-grade tumor types, such as lung, breast, and ovarian tumors. OTUB1 regulates p53 to promote tumor cell survival and proliferation [37]. USP42 controls the level of p53 ubiquitination during the early phase of the DDR to promote DNA repair, resulting in the activation of p53-dependent transcription and cell-cycle arrest in response to stress [45]. In addition, USP28 depletion leads to increased ubiquitinated H2A-K119 and decreased expression of p53, p21, and p16INK4a, suggesting a role for USP28 in cell proliferation via the control of p53 and p53-associated proteins [109]. Additionally, USP28 deubiquitinates TP53-binding protein 1 to promote p53-mediated transcription [46]. USP4 is a potential oncogene that inhibits p53 and NF-κB via histone deacetylases 2 (HDAC2) [49,50]. USP9X-dependent p53 degradation was observed in hepatocellular carcinoma (HCC) cells treated with the small molecule DUB inhibitor WP1130 [60].

**Figure 2.** DUBs in MDM2-p53 pathways. Ubiquitination is found on both p53 and MDM2 molecules; various DUBs could revise that via deubiquitination to regulate the p53 pathway. DUBs' targets on p53 are labeled in light brown; those that interact with MDM2 are labeled in light yellow. Detailed descriptions can be referred to the main text.

DUBs are also involved in other signaling pathways that promote tumor proliferation. USP15, which stabilizes the type I TGF-β receptor and enhances the TGF-β pathway, is upregulated in various cancers [43]. In addition, OTUD1 mitigates TGF-β-induced pro-oncogenic responses via deubiquitination of SMAD7 at lysine 220 in breast cancer [39]. USP49 regulates the Akt pathway through the stabilization of FKBP51. FKBP51 activates PH domain leucine-rich-repeats protein phosphatase (PHLPP) to dephosphorylate Akt, which inhibits pancreatic cancer cell proliferation [51]. The AR pathway is commonly activated in prostate cancer (PCa), and it plays a critical role in PCa growth and progression. USP14 was reported to bind with and stabilize AR in androgen-responsive PCa cells. Overexpression of USP14 promotes the proliferation of LNCaP cells [41]. Furthermore, DUBs control di fferent growth factors in tumor cells. For instance, USP8 prevents degradation of the epidermal growth factor receptor and thus promotes proliferation [110].
