Ubiquitination and deubiquitination play a fundamental role in the signaling pathways associated with innate and adaptive immune responses. Macrophages are key sentinels for the host defense, triggering antiviral and inflammatory responses against various invading pathogens. Macrophages recognize the genetic material of these pathogens as pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) through the activation of its pattern recognition receptors (PRRs), initiating the cascade of immune signaling, which leads to the production of pro- and anti-inflammatory cytokines that initiates the appropriate immune response. Macrophage-mediated immune response is highly regulated and tightly controlled by the ubiquitin system since its abnormal activation or dysregulation may result in the severe pathogenesis of numerous inflammatory and autoimmune diseases. Deubiquitinating enzymes (DUBs) play a crucial role in reversing the ubiquitination and controlling the magnitude of the immune response. During infection, pathogens manipulate the host defense system by regulating DUBs to obtain nutrients and increase proliferation. Indeed, the regulation of DUBs by small molecule inhibitors has been proposed as an excellent way to control aberrant activation of immune signaling molecules. This review is focused on the complex role of DUBs in macrophage-mediated immune response, exploring the potential use of DUBs as therapeutic targets in autoimmune and inflammatory diseases by virtue of small molecule DUB inhibitors.

Hematopoietic stem cells (HSCs) are responsible for the production of blood cells throughout the human lifespan. Single HSCs can give rise to at least eight distinct blood-cell lineages. Together, hematopoiesis, erythropoiesis, and angiogenesis coordinate several biological processes, i.e., cellular interactions during development and proliferation, guided migration, lineage programming, and reprogramming by transcription factors. Any dysregulation of these processes can result in hematological disorders and/or malignancies. Several studies of the molecular mechanisms governing HSC maintenance have demonstrated that protein regulation by the ubiquitin proteasomal pathway is crucial for normal HSC function. Recent studies have shown that reversal of ubiquitination by deubiquitinating enzymes (DUBs) plays an equally important role in hematopoiesis; however, information regarding the biological function of DUBs is limited. In this review, we focus on recent discoveries about the physiological roles of DUBs in hematopoiesis, erythropoiesis, and angiogenesis and discuss the DUBs associated with common hematological disorders and malignancies, which are potential therapeutic drug targets.

Cell division cycle 25A (Cdc25A) is a dual-specificity phosphatase that is overexpressed in several cancer cells and promotes tumorigenesis. In normal cells, Cdc25A expression is regulated tightly, but the changes in expression patterns in cancer cells that lead to tumorigenesis are unknown. In this study, we showed that ubiquitin-specific protease 29 (USP29) stabilized Cdc25A protein expression in cancer cell lines by protecting it from ubiquitin-mediated proteasomal degradation. The presence of USP29 effectively blocked polyubiquitination of Cdc25A and extended its half-life. CRISPR-Cas9-mediated knockdown of USP29 in HeLa cells resulted in cell cycle arrest at the G0/G1 phase. We also showed that USP29 knockdown hampered Cdc25A-mediated cell proliferation, migration, and invasion of cancer cells in vitro. Moreover, NSG nude mice transplanted with USP29-depleted cells significantly reduced the size of the tumors, whereas the reconstitution of Cdc25A in USP29-depleted cells significantly increased the tumor size. Altogether, our results implied that USP29 promoted cell cycle progression and oncogenic transformation by regulating protein turnover of Cdc25A.

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.

A delicate intracellular balance among protein synthesis, folding, and degradation is essential to maintaining protein homeostasis or proteostasis, and it is challenged by genetic and environmental factors. Molecular chaperones and the ubiquitin proteasome system (UPS) play a vital role in proteostasis for normal cellular function. As part of protein quality control, molecular chaperones recognize misfolded proteins and assist in their refolding. Proteins that are beyond repair or refolding undergo degradation, which is largely mediated by the UPS. The importance of protein quality control is becoming ever clearer, but it can also be a disease-causing mechanism. Diseases such as phenylketonuria (PKU) and hereditary tyrosinemia-I (HT1) are caused due to mutations in PAH and FAH gene, resulting in reduced protein stability, misfolding, accelerated degradation, and deficiency in functional proteins. Misfolded or partially unfolded proteins do not necessarily lose their functional activity completely. Thus, partially functional proteins can be rescued from degradation by molecular chaperones and deubiquitinating enzymes (DUBs). Deubiquitination is an important mechanism of the UPS that can reverse the degradation of a substrate protein by covalently removing its attached ubiquitin molecule. In this review, we discuss the importance of molecular chaperones and DUBs in reducing the severity of PKU and HT1 by stabilizing and rescuing mutant proteins.

Hematopoietic stem cells (HSCs) are responsible for the production of blood cells throughout the human life span. Single HSCs can give rise to at least eight distinct blood cell lineages. Together, hematopoiesis, erythropoiesis and angiogenesis coordinate several biological processes, such as cellular interactions in development and proliferation, guided migration, lineage programming and reprogramming by transcription factors. Any dysregulation of these processes may result in hematological disorders and/or malignancies. Several studies of the molecular mechanisms governing HSC maintenance have demonstrated that protein regulation by the ubiquitin proteasomal pathway is crucial for normal HSC function. Recent studies have shown that the reversal of ubiquitination by deubiquitinating enzymes (DUBs) plays an equally important role in hematopoiesis; however, there is only limited additional information regarding the biological function of DUBs. In this review, we focus on recent discoveries that have led to a better understanding of the physiological roles of DUBs in hematopoiesis, erythropoiesis and angiogenesis. In addition, we discuss the DUBs associated with common hematological disorders and malignancies, which may potentially be therapeutic drug targets.