USP15 promotes the apoptosis of degenerative nucleus pulposus cells by suppressing the PI3K/AKT signalling pathway

Deubiquitylating Enzymes in Cancer and Immunity

Deubiquitylating enzymes (DUBs) maintain relative homeostasis of the cellular ubiquitome by removing the post-translational modification ubiquitin moiety from substrates. Numerous DUBs have been demonstrated specificity for cleaving a certain type of ubiquitin linkage or positions within ubiquitin chains. Moreover, several DUBs perform functions through specific protein-protein interactions in a catalytically independent manner, which further expands the versatility and complexity of DUBs' functions. Dysregulation of DUBs disrupts the dynamic equilibrium of ubiquitome and causes various diseases, especially cancer and immune disorders. This review summarizes the Janus-faced roles of DUBs in cancer including proteasomal degradation, DNA repair, apoptosis, and tumor metastasis, as well as in immunity involving innate immune receptor signaling and inflammatory and autoimmune disorders. The prospects and challenges for the clinical development of DUB inhibitors are further discussed. The review provides a comprehensive understanding of the multi-faced roles of DUBs in cancer and immunity.

Neglected PTM in Animal Adipogenesis: E3-mediated Ubiquitination

Ubiquitination is a widespread post-transcriptional modification (PTM) that occurs during protein degradation in eukaryotes and participates in almost all physiological and pathological processes, including animal adipogenesis. Ubiquitination is a cascade reaction regulated by the activating enzyme E1, conjugating enzyme E2, and ligase E3. Several recent studies have reported that E3 ligases play important regulatory roles in adipogenesis. However, as a key influencing factor for the recognition and connection between the substrate and ubiquitin during ubiquitination, its regulatory role in adipogenesis has not received adequate attention. In this review, we summarize the E3s' regulation and modification targets in animal adipogenesis, explain the regulatory mechanisms in lipogenic-related pathways, and further analyze the existing positive results to provide research directions of guiding significance for further studies on the regulatory mechanisms of E3s in animal adipogenesis.

Ubiquitin-specific proteases: Vital regulatory molecules in bone and bone-related diseases

Stabilization of bone structure and function involves multiple cell-to-cell and molecular interactions, in which the regulatory functions of post-translational modifications such as ubiquitination and deubiquitination shouldn't be underestimated. As the largest family of deubiquitinating enzymes, the ubiquitin-specific proteases (USPs) participate in the development of bone homeostasis and bone-related diseases through multiple classical osteogenic and osteolytic signaling pathways, such as BMP/TGF-β pathway, NF-κB/p65 pathway, EGFR-MAPK pathway and Wnt/β-catenin pathway. Meanwhile, USPs may also broadly regulate regulate hormone expression level, cell proliferation and differentiation, and may further influence bone homeostasis from gene fusion and nuclear translocation of transcription factors. The number of patients with bone-related diseases is currently enormous, making exploration of their pathogenesis and targeted therapy a hot topic. Pathological increases in the levels of inflammatory mediators such as IL-1β and TNF-α lead to inflammatory bone diseases such as osteoarthritis, rheumatoid arthritis and periodontitis. While impaired body metabolism greatly increases the probability of osteoporosis. Abnormal physiological activity of bone-associated cells results in a variety of bone tumors. The regulatory role of USPs in bone-related disease has received particular attention from academics in recent studies. In this review, we focuse on the roles and mechanisms of USPs in bone homeostasis and bone-related diseases, with the expectation of informing targeted therapies in the clinic.

TSG-6 inhibits IL-1β-induced inflammatory responses and extracellular matrix degradation in nucleus pulposus cells by activating the PI3K/Akt signaling pathway

PURPOSE

Tumor necrosis factor (TNF)-stimulated gene-6 (TSG-6), a secreted protein associated with inflammation, is believed to possess momentous and multiple anti-inflammatory and tissue-protective properties. However, the role and potential mechanism of TSG-6 in cervical disk degeneration (CDD) are still not clear. Hence, we aimed to explore the effect of TSG-6 on CDD.

METHODS

Quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) or enzyme-linked immunosorbent assay was applied to detect the expression level of TSG-6 and IL-1β in normal and degenerated nucleus pulposus (NP) tissues. Then, qRT-PCR and western blot were adopted to test the TSG-6 protein expression after IL-1β treatment (10 ng/mL) in human NP cells (HNPCs). After over-expressing TSG-6, qRT-PCR was also utilized to evaluate the expression of TNF-α, IL-8, and IL-6 and the synthesis of sulfated glycosaminoglycans (sGAGs), western blot to check the expression of extracellular matrix (ECM) proteins [collagen II, aggrecan, and matrix metalloproteinase-3 (MMP-3)], pain-related molecules (CGRP, calcitonin gene-related peptide; NGF, nerve growth factor; SP, substance P), and PI3K/Akt signaling pathway-related proteins.

RESULTS

Briefly speaking, TSG-6 and IL-1β expression levels were significantly increased in CDD patient tissues; and IL-1β treatment could significantly increase TSG-6 expression in HNPCs. Further research revealed that, in addition to greatly promoting sGAGs synthesis, TSG-6 over-expression also inhibited TNF-α, IL-8, and IL-6 expression and ECM degradation in IL-1β-induced HNPCs. (The collagen II and aggrecan expression was up-regulated and MMP-3 expression was down-regulated.) Furthermore, over-expression of TSG-6 could decrease the levels of CGRP, NGF, and SP protein expression and activate the PI3K/Akt signaling pathway in IL-1β-treated HNPCs.

CONCLUSION

TSG-6 inhibits inflammatory responses, ECM degradation, and expression of pain-related molecules in IL-1β-induced HNPCs by activating the PI3K/Akt signaling pathway.

USP15 regulates p66Shc stability associated with Drp1 activation in liver ischemia/reperfusion

Liver ischemia/reperfusion (I/R) injury is a major clinical concern of liver transplantation, which accounts for organ rejection and liver dysfunction. The adaptor protein p66Shc acts as a crucial redox enzyme and is implicated in liver I/R. Elevated p66Shc expression is associated with hepatocellular apoptosis in liver I/R, but the molecular mechanisms of p66Shc responsible for its aberrant expression and function remain unknown. In the present study, hepatocyte-specific p66Shc-knockdown mice exhibited clear inhibition in hepatocellular apoptosis and oxidative stress under liver I/R, while hepatocyte-specific p66Shc overexpression mice displayed the deteriorative impairment. Mechanistically, p66Shc-triggered mitochondrial fission and apoptosis in liver I/R by mediating ROS-driven Drp1 activation. Furthermore, a screening for p66Shc-interacting proteins identified ubiquitin-specific protease 15 (USP15) as a mediator critical for abnormal p66Shc expression. Specifically, USP15 interacted with the SH2 domain of p66Shc and maintained its stabilization by removing ubiquitin. In vivo, p66Shc knockdown abrogated USP15-driven hepatocellular apoptosis, whereas p66Shc overexpression counteracted the antiapoptotic effect of USP15 silencing in response to liver I/R. There was clinical evidence for the positive association between p66Shc and USP15 in patients undergoing liver transplantation. In summary, p66Shc contributes to mitochondrial fission and apoptosis associated with Drp1 activation, and abnormal p66Shc expression relies on the activity of USP15 deubiquitination under liver I/R. The current study sheds new light on the molecular mechanism of p66Shc, and identifies USP15 as a novel mediator of p66Shc to facilitate better therapeutics against liver I/R.

Inhibition of USP15 ameliorates high-glucose-induced oxidative stress and inflammatory injury in podocytes through regulation of the Keap1/Nrf2 signaling

Ubiquitin-specific peptidase 15 (USP15) is implicated in the pathogenesis of numerous diseases. However, whether USP15 plays a role in diabetic nephropathy remains undetermined. This project was designed to determine the potential role of USP15 in mediating high glucose (HG)-induced podocyte injury, a key event in the pathogenesis of diabetic nephropathy. We found that USP15 levels were elevated in podocytes after HG stimulation. Inhibition of USP15 led to decreases in HG-evoked apoptosis, oxidative stress, and inflammation in podocytes. Further investigation showed that inhibition of USP15 enhanced the activation of NF-E2-related factor 2 (Nrf2) and expression of Nrf2 target genes in HG-simulated podocytes. Moreover, depletion of Kelch-like ECH-associated protein 1 (Keap1) diminished the regulatory effect of USP15 inhibition on Nrf2 activation. In addition, Nrf2 suppression reversed USP15-inhibition-induced protective effects in HG-injured podocytes. Taken together, these data indicate that USP15 inhibition protects podocytes from HG-induced injury by enhancing Nrf2 activation via Keap1.

USP15 in Cancer and Other Diseases: From Diverse Functionsto Therapeutic Targets

The process of protein ubiquitination and deubiquitination plays an important role in maintaining protein stability and regulating signal pathways, and protein homeostasis perturbations may induce a variety of diseases. The deubiquitination process removes ubiquitin molecules from the protein, which requires the participation of deubiquitinating enzymes (DUBs). Ubiquitin-specific protease 15 (USP15) is a DUB that participates in many biological cell processes and regulates tumorigenesis. A dislocation catalytic triplet was observed in the USP15 structure, a conformation not observed in other USPs, except USP7, which makes USP15 appear to be unique. USP15 has been reported to be involved in the regulation of various cancers and diseases, and the reported substrate functions of USP15 are conflicting, suggesting that USP15 may act as both an oncogene and a tumor suppressor in different contexts. The importance and complexity of USP15 in the pathological processes remains unclear. Therefore, we reviewed the diverse biological functions of USP15 in cancers and other diseases, suggesting the potential of USP15 as an attractive therapeutic target.

Pro-apoptotic and anti-apoptotic regulation mediated by deubiquitinating enzymes

Although damaged cells can be repaired, cells that are considered unlikely to be repaired are eliminated through apoptosis, a type of predicted cell death found in multicellular organisms. Apoptosis is a structured cell death involving alterations to the cell morphology and internal biochemical changes. This process involves the expansion and cracking of cells, changes in cell membranes, nuclear fragmentation, chromatin condensation, and chromosome cleavage, culminating in the damaged cells being eaten and processed by other cells. The ubiquitin-proteasome system (UPS) is a major cellular pathway that regulates the protein levels through proteasomal degradation. This review proposes that apoptotic proteins are regulated through the UPS and describes a unique direction for cancer treatment by controlling proteasomal degradation of apoptotic proteins, and small molecules targeted to enzymes associated with UPS.

Acute Myeloid Leukemia-Related Proteins Modified by Ubiquitin and Ubiquitin-like Proteins

Acute myeloid leukemia (AML), the most common form of an acute leukemia, is a malignant disorder of stem cell precursors of the myeloid lineage. Ubiquitination is one of the post-translational modifications (PTMs), and the ubiquitin-like proteins (Ubls; SUMO, NEDD8, and ISG15) play a critical role in various cellular processes, including autophagy, cell-cycle control, DNA repair, signal transduction, and transcription. Also, the importance of Ubls in AML is increasing, with the growing research defining the effect of Ubls in AML. Numerous studies have actively reported that AML-related mutated proteins are linked to Ub and Ubls. The current review discusses the roles of proteins associated with protein ubiquitination, modifications by Ubls in AML, and substrates that can be applied for therapeutic targets in AML.

Role of PI3K/AKT Signaling Pathway in Nucleus Pulposus Cells

OBJECTIVE

To investigate the role of PI3K/AKT signaling pathway in nucleus pulposus (NP) cells.

METHODS

Nucleus pulposus (NP) cells were isolated from SD rat, and thereafter, passage three (P3) NP cells were divided into the following experimental groups: control, PI3K/AKT agonist IGF-1 (25 ng/ml, 50 ng/ml, and 100 ng/ml), and PI3K/AKT inhibitor LY294002 (5 μM, 10 μM, and 20 μM). Flow cytometry and BrdU cell proliferation assays were performed to assess apoptosis and the proliferation rate of NP cells. Western blot analysis was performed to examine the protein expression level of Col II, Col X, Aggrecan, and MMP13.

RESULTS

PI3K/AKT inhibitor LY294002 increased the rate of apoptosis in NP cells when compared to the control and decreased the proliferation rate when compared to control. Moreover, LY294002 decreased the protein expression level of Col-II and Aggrecan in NP cells. At the same time, LY294002 increased the protein expression level of MMP13 and Col-X in NP cells. Through activating PI3K/AKT, IGF-1 increased the proliferation rate when compared to control and decreased the rate of apoptosis when compared to control. Additionally, IGF-1 decreased the protein expression level of MMP13 and Col-X and increased Col-II and Aggrecan in NP cells.

CONCLUSION

The inhibition of PI3K/AKT signaling pathway accelerated the apoptosis of NP cells and facilitated the extracellular matrix degradation. However, the activation of PI3K/AKT pathway partly prevented the NP cell from apoptosis and promoted their proliferation. Meanwhile, its activation also delayed the loss of extracellular matrix.

Bromodomain-containing protein 7 regulates matrix metabolism and apoptosis in human nucleus pulposus cells through the BRD7-PI3K-YAP1 signaling axis

Intervertebral disc degeneration (IDD) results from dysregulated metabolism of the extracellular matrix of the nucleus pulposus (NP) and involves the participation of inflammatory factors such as TNF-α. Bromodomain-containing protein 7 (BRD7) shows considerable potential for anti-inflammatory applications. Herein, we investigated the role of BRD7 in IDD. The immunohistochemistry results demonstrated decreased BRD7 expression in severely degenerated human NP tissues compared to those showing mild degeneration. Lentiviruses and adenoviruses were used to knock down or overexpress BRD7 and YAP1, respectively. Our results revealed that BRD7 knockdown promoted matrix degradation and suppressed PI3K and YAP1 expression, while BRD7 overexpression alleviated matrix degradation and promoted YAP1 and PI3K expression. In addition, PI3K inhibition augmented matrix degradation, enhanced apoptosis, and reduced YAP1 expression, whereas YAP1 overexpression promoted matrix synthesis, suppressed apoptosis and promoted PI3K expression. Besides, BRD7 overexpression reversed the reductions in sulfated glycosaminoglycan levels induced by TNF-α, but this effect was blocked by PI3K or YAP1 inhibitors. Moreover, YAP1 and PI3K were shown to interact through coimmunoprecipitation analysis. In summary, our results demonstrate that BRD7 can regulate matrix metabolism and apoptosis in human NP cells through the BRD7-PI3K-YAP1 signaling axis. This study might provide new insights into the prevention and treatment of IDD.

Positive feedback regulation between USP15 and ERK2 inhibits osteoarthritis progression through TGF-β/SMAD2 signaling

Background

The transforming growth factor-β (TGF-β) signaling pathway plays an essential role in maintaining homeostasis in joints affected by osteoarthritis (OA). However, the specific mechanism of non-SMAD and classical SMAD signaling interactions is still unclear, which needs to be further explored.

Methods

In ATDC5 cells, USP15 overexpression and knockout were performed using the transfected lentivirus USP15 and Crispr/Cas9. Western blotting and immunofluorescence staining were used to test p-SMAD2 and cartilage phenotype-related molecular markers. In rat OA models, immunohistochemistry, hematoxylin and eosin (HE)/Safranin-O fast green staining, and histology were used to examine the regulatory activity of USP15 in TGF-β/SMAD2 signaling and the cartilage phenotype. Then, ERK2 overexpression and knockout were performed. The expressions of USP15, p-SMAD2, and the cartilage phenotype were evaluated in vitro and in vivo. To address whether USP15 is required for ERK2 and TGF-β/SMAD2 signaling, we performed rescue experiments in vitro and in vivo. Immunoprecipitation and deubiquitination assays were used to examine whether USP15 could bind to ERK2 and affect the deubiquitination of ERK2. Finally, whether USP15 regulates the level of p-ERK1/2 was evaluated by western blotting, immunofluorescence staining, and immunohistochemistry in vitro and in vivo.

Results

Our results indicated that USP15 stimulated TGF-β/SMAD2 signaling and the cartilage phenotype. Moreover, ERK2 required USP15 to influence TGF-β/SMAD2 signaling for regulating the cartilage phenotype in vivo and in vitro. And USP15 can form a complex with ERK2 to regulate ubiquitination of ERK2. Interestingly, USP15 did not regulate the stability of ERK2 but increased the level of p-ERK1/2 to further enhance the TGF-β/SMAD2 signaling pathway.

Conclusions

Taken together, our study revealed positive feedback regulation between USP15 and ERK2, which played a critical role in TGF-β/SMAD2 signaling to inhibit OA progression. Therefore, this specific mechanism can guide the clinical treatment of OA.