Since the discovery of the ubiquitin proteasome system (UPS), the roles of ubiquitinating and deubiquitinating enzymes (DUBs) have been widely elucidated. The ubiquitination of proteins regulates many aspects of cellular functions such as protein degradation and localization, and also modifies protein-protein interactions. DUBs cleave the attached ubiquitin moieties from substrates and thereby reverse the process of ubiquitination. The dysregulation of these two paramount pathways has been implicated in numerous diseases, including cancer. Attempts are being made to identify inhibitors of ubiquitin E3 ligases and DUBs that potentially have clinical implications in cancer, making them an important target in the pharmaceutical industry. Therefore, studies in medicine are currently focused on the pharmacological disruption of DUB activity as a rationale to specifically target cancer-causing protein aberrations. Here, we briefly discuss the pathophysiological and physiological roles of DUBs in key cancer-related pathways. We also discuss the clinical applications of promising DUB inhibitors that may contribute to the development of DUBs as key therapeutic targets in the future.

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Phenylketonuria (PKU) is an autosomal recessive metabolic disorder caused by the dysfunction of the enzyme phenylalanine hydroxylase (PAH). Alterations in the level of PAH leads to the toxic accumulation of phenylalanine in the blood and brain. Protein degradation mediated by ubiquitination is a principal cellular process for maintaining protein homeostasis. Therefore, it is important to identify the E3 ligases responsible for PAH turnover and proteostasis. Here, we report that anaphase-promoting complex/cyclosome-Cdh1 (APC/C)Cdh1 is an E3 ubiquitin ligase complex that interacts and promotes the polyubiquitination of PAH through the 26S proteasomal pathway. Cdh1 destabilizes and declines the half-life of PAH. In contrast, the CRISPR/Cas9-mediated knockout of Cdh1 stabilizes PAH expression and enhances phenylalanine metabolism. Additionally, our current study demonstrates the clinical relevance of PAH and Cdh1 correlation in hepatocellular carcinoma (HCC). Overall, we show that PAH is a prognostic marker for HCC and Cdh1 could be a potential therapeutic target to regulate PAH-mediated physiological and metabolic disorders.

Oct4 is an important mammalian POU family transcription factor expressed by early human embryonic stem cells (hESCs). The precise level of Oct4 governs the pluripotency and fate determination of hESCs. Several post-translational modifications (PTMs) of Oct4 including phosphorylation, ubiquitination, and SUMOylation have been reported to regulate its critical functions in hESCs. Ubiquitination and deubiquitination of Oct4 should be well balanced to maintain the pluripotency of hESCs. The protein turnover of Oct4 is regulated by several E3 ligases through ubiquitin-mediated degradation. However, reversal of ubiquitination by deubiquitinating enzymes (DUBs) has not been reported for Oct4. In this study, we generated a ubiquitin-specific protease 3 (USP3) gene knockout using the CRISPR/Cas9 system and demonstrated that USP3 acts as a protein stabilizer of Oct4 by deubiquitinating Oct4. USP3 interacts with endogenous Oct4 and co-localizes in the nucleus of hESCs. The depletion of USP3 leads to a decrease in Oct4 protein level and loss of pluripotent morphology in hESCs. Thus, our results show that USP3 plays an important role in controlling optimum protein level of Oct4 to retain pluripotency of hESCs.

The well-known androgen-regulated homeobox gene, NKX3.1, is located on the short arm of chromosome 8. It is the first known prostate epithelium-specific marker, and is a transcription factor involved in development of the testes and prostate. In addition to specifying the prostate epithelium and maintaining normal prostate secretory function, Nkx3.1 is an established marker for prostate cancer. Over the years, however, this gene has been implicated in various other cancers, and technological advances have allowed determination of its role in other cellular functions. Nkx3.1 has also been recently identified as a factor capable of replacing Oct4 in cellular reprogramming. This review highlights the role of this tumor suppressor and briefly describes its functions, ranging from prostate development to maintenance of stemness and cellular reprogramming.

Deubiquitinating enzymes play key roles in the precise modulation of Aurora B—an essential cell cycle regulator. The expression of Aurora B increases before the onset of mitosis and decreases during mitotic exit; an imbalance in these levels has a severe impact on the fate of the cell cycle. Dysregulation of Aurora B can lead to aberrant chromosomal segregation and accumulation of errors during mitosis, eventually resulting in cytokinesis failure. Thus, it is essential to identify the precise regulatory mechanisms that modulate Aurora B levels during the cell division cycle. Using a deubiquitinase knockout strategy, we identified USP48 as an important candidate that can regulate Aurora B protein levels during the normal cell cycle. Here, we report that USP48 interacts with and stabilizes the Aurora B protein. Furthermore, we showed that the deubiquitinating activity of USP48 helps to maintain the steady-state levels of Aurora B protein by regulating its half-life. Finally, USP48 knockout resulted in delayed progression of cell cycle due to accumulation of mitotic defects and ultimately cytokinesis failure, suggesting the role of USP48 in cell cycle regulation.

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