Targeting the p27 E3 ligase SCF(Skp2) results in p27- and Skp2-mediated cell-cycle arrest and activation of autophagy

Ubiquitination regulates autophagy in cancer: simple modifications, promising targets

Autophagy is an important lysosomal degradation process that digests and recycles bio-molecules, protein or lipid aggregates, organelles, and invaded pathogens. Autophagy plays crucial roles in regulation of metabolic and oxidative stress and multiple pathological processes. In cancer, the role of autophagy is dual and paradoxical. Ubiquitination has been identified as a key regulator of autophagy that can influence various steps in the autophagic process, with autophagy-related proteins being targeted for ubiquitination, thus impacting cancer progression and the effectiveness of therapeutic interventions. This review will concentrate on mechanisms underlying autophagy, ubiquitination, and their interactions in cancer, as well as explore the use of drugs that target the ubiquitin-proteasome system (UPS) and ubiquitination process in autophagy as part of cancer therapy.

Immune regulation and prognostic prediction model establishment and validation of PSMB6 in lung adenocarcinoma

Lung cancer is one of the most common malignant tumors, and patients are often diagnosed at an advanced stage, posing a substantial risk to human health, so it is crucial to establish a model to forecast the prognosis of patients with lung cancer. Recent research has indicated that proteasome 20S subunit 6 (PSMB6) may be closely associated with anti-apoptotic pathways, and proliferation transduction signals in tumor cells of different tumors. However, the precise role of PSMB6 in the immunoregulatory processes within lung adenocarcinoma (LUAD) is yet to be elucidated. By analyzing the TCGA database, we discovered a positive correlation between the expression of PSMB6 and tumor growth trends, and lung adenocarcinoma patients with elevated PSMB6 expression levels had a worse prognosis. Our findings suggest a close correlation between PSMB6 expression levels, immune cell infiltration and immune checkpoint gene expression, which suggests that PSMB6 may become a new independent prognostic indicator. In addition, we developed a prognostic model of PSMB6-regulated immune infiltration-associated genes by analyzing the link between PSMB6 and the immune microenvironment. This model can not only predict the prognosis of lung adenocarcinoma but also forecasts the patient's reaction to immunotherapy. The validity of this research outcome has been confirmed by the GSE31210 and IMvigor210 cohorts. Analysis of the Kaplan-Meier Plotter database indicates that individuals with elevated levels of PSMB6 expression exhibit a poorer prognosis. Additionally, in vitro experiments demonstrated that knockdown of PSMB6 inhibits the proliferation, migration, and invasion of lung adenocarcinoma cells while promoting their apoptosis. Overall, our findings suggest that PSMB6 could remarkably influence the management and treatment of lung adenocarcinoma, opening new avenues for targeted immunotherapeutic strategies.

Discovery of Novel [1,2,4]Triazolo[1,5-a]pyrimidine Derivatives as Novel Potent S-Phase Kinase-Associated Protein 2 (SKP2) Inhibitors for the Treatment of Cancer

Skp1-CUL1-ROC1-F-box E3 ubiquitin ligases' main component S-phase kinase-associated protein 2 (Skp2) is responsible for specifically recognizing ubiquitination-modified substrates to be degraded such as p27 and p21 in the case of binding with adaptor protein Cks1. Pharmacological inhibition of Skp2 has exhibited promising antitumor activity. Herein, we present the design and optimization of a series of [1,2,4]triazolo[1,5-a]pyrimidine-based small molecules targeting Skp2. Among them, E35 demonstrated excellent inhibitory activities against the binding of Skp2-Cks1. In addition, compound E35 significantly inhibited colony formation and migration, as well as arrested the cell cycle at the S-phase. Mechanistically, compound E35 markedly decreased the expression of Skp2, as well as increased the expression of its substrates p21 and p27. Furthermore, compound E35 showed an obvious inhibitory effect on MGC-803 xenograft mice without obvious toxicity. All of these results suggest that compound E35 might be a valuable lead compound for antitumor agents targeting Skp2.

Ubiquitination and deubiquitination in cancer: from mechanisms to novel therapeutic approaches

Ubiquitination, a pivotal posttranslational modification of proteins, plays a fundamental role in regulating protein stability. The dysregulation of ubiquitinating and deubiquitinating enzymes is a common feature in various cancers, underscoring the imperative to investigate ubiquitin ligases and deubiquitinases (DUBs) for insights into oncogenic processes and the development of therapeutic interventions. In this review, we discuss the contributions of the ubiquitin-proteasome system (UPS) in all hallmarks of cancer and progress in drug discovery. We delve into the multiple functions of the UPS in oncology, including its regulation of multiple cancer-associated pathways, its role in metabolic reprogramming, its engagement with tumor immune responses, its function in phenotypic plasticity and polymorphic microbiomes, and other essential cellular functions. Furthermore, we provide a comprehensive overview of novel anticancer strategies that leverage the UPS, including the development and application of proteolysis targeting chimeras (PROTACs) and molecular glues.

Non-canonical deubiquitination of OTUB1 induces IFNγ-mediated cell cycle arrest via regulation of p27 stability

The deubiquitinase OTUB1, implicated as a potential oncogene in various tumors, lacks clarity in its regulatory mechanism in tumor progression. Our study investigated the effects and underlying mechanisms of OTUB1 on the breast cancer cell cycle and proliferation in IFNγ stimulation. Loss of OTUB1 abrogated IFNγ-induced cell cycle arrest by regulating p27 protein expression, whereas OTUB1 overexpression significantly enhanced p27 expression even without IFNγ treatment. Tyr26 phosphorylation residue of OTUB1 directly bound to p27, modulating its post-translational expression. Furthermore, we identified crucial lysine residues (K134, K153, and K163) for p27 ubiquitination. Src downregulation reduced OTUB1 and p27 expression, suggesting that IFNγ-induced cell cycle arrest is mediated by the Src-OTUB1-p27 signaling pathway. Our findings highlight the pivotal role of OTUB1 in IFNγ-induced p27 expression and cell cycle arrest, offering therapeutic implications.

Targeting RCC1 to block the human soft-tissue sarcoma by disrupting nucleo-cytoplasmic trafficking of Skp2

Soft-tissue sarcomas (STS) emerges as formidable challenges in clinics due to the complex genetic heterogeneity, high rates of local recurrence and metastasis. Exploring specific targets and biomarkers would benefit the prognosis and treatment of STS. Here, we identified RCC1, a guanine-nucleotide exchange factor for Ran, as an oncogene and a potential intervention target in STS. Bioinformatics analysis indicated that RCC1 is highly expressed and correlated with poor prognosis in STS. Functional studies showed that RCC1 knockdown significantly inhibited the cell cycle transition, proliferation and migration of STS cells in vitro, and the growth of STS xenografts in mice. Mechanistically, we identified Skp2 as a downstream target of RCC1 in STS. Loss of RCC1 substantially diminished Skp2 abundance by compromising its protein stability, resulting in the upregulation of p27Kip1 and G1/S transition arrest. Specifically, RCC1 might facilitate the nucleo-cytoplasmic trafficking of Skp2 via direct interaction. As a result, the cytoplasmic retention of Skp2 would further protect it from ubiquitination and degradation. Notably, recovery of Skp2 expression largely reversed the phenotypes induced by RCC1 knockdown in STS cells. Collectively, this study unveils a novel RCC1-Skp2-p27Kip1 axis in STS oncogenesis, which holds promise for improving prognosis and treatment of this formidable malignancy.

O-GlcNAcylation of E3 ubiquitin ligase SKP2 promotes hepatocellular carcinoma proliferation

O-linked-β-N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation) and ubiquitination are critical posttranslational modifications that regulate tumor development and progression. The continuous progression of the cell cycle is the fundamental cause of tumor proliferation. S-phase kinase-associated protein 2 (SKP2), an important E3 ubiquitin ligase, assumes a pivotal function in the regulation of the cell cycle. However, it is still unclear whether SKP2 is an effector of O-GlcNAcylation that affects tumor progression. In this study, we found that SKP2 interacted with O-GlcNAc transferase (OGT) and was highly O-GlcNAcylated in hepatocellular carcinoma (HCC). Mechanistically, the O-GlcNAcylation at Ser34 stabilized SKP2 by reducing its ubiquitination and degradation mediated by APC-CDH1. Moreover, the O-GlcNAcylation of SKP2 enhanced its binding ability with SKP1, thereby enhancing its ubiquitin ligase function. Consequently, SKP2 facilitated the transition from the G1-S phase of the cell cycle by promoting the ubiquitin degradation of cell cycle-dependent kinase inhibitors p27 and p21. Additionally, targeting the O-GlcNAcylation of SKP2 significantly suppressed the proliferation of HCC. Altogether, our findings reveal that O-GlcNAcylation, a novel posttranslational modification of SKP2, plays a crucial role in promoting HCC proliferation, and targeting the O-GlcNAcylation of SKP2 may become a new therapeutic strategy to impede the progression of HCC.

The cross talk of ubiquitination and chemotherapy tolerance in colorectal cancer

Ubiquitination, a highly adaptable post-translational modification, plays a pivotal role in maintaining cellular protein homeostasis, encompassing cancer chemoresistance-associated proteins. Recent findings have indicated a potential correlation between perturbations in the ubiquitination process and the emergence of drug resistance in CRC cancer. Consequently, numerous studies have spurred the advancement of compounds specifically designed to target ubiquitinates, offering promising prospects for cancer therapy. In this review, we highlight the role of ubiquitination enzymes associated with chemoresistance to chemotherapy via the Wnt/β-catenin signaling pathway, epithelial-mesenchymal transition (EMT), and cell cycle perturbation. In addition, we summarize the application and role of small compounds that target ubiquitination enzymes for CRC treatment, along with the significance of targeting ubiquitination enzymes as potential cancer therapies.

SKping cell cycle regulation: role of ubiquitin ligase SKP2 in hematological malignancies

SKP2 (S-phase kinase-associated protein 2) is a member of the F-box family of substrate-recognition subunits in the SCF ubiquitin-protein ligase complexes. It is associated with ubiquitin-mediated degradation in the mammalian cell cycle components and other target proteins involved in cell cycle progression, signal transduction, and transcription. Being an oncogene in solid tumors and hematological malignancies, it is frequently associated with drug resistance and poor disease outcomes. In the current review, we discussed the novel role of SKP2 in different hematological malignancies. Further, we performed a limited in-silico analysis to establish the involvement of SKP2 in a few publicly available cancer datasets. Interestingly, our study identified Skp2 expression to be altered in a cancer-specific manner. While it was found to be overexpressed in several cancer types, few cancer showed a down-regulation in SKP2. Our review provides evidence for developing novel SKP2 inhibitors in hematological malignancies. We also investigated the effect of SKP2 status on survival and disease progression. In addition, the role of miRNA and its associated families in regulating Skp2 expression was explored. Subsequently, we predicted common miRNAs against Skp2 genes by using miRNA-predication tools. Finally, we discussed current approaches and future prospective approaches to target the Skp2 gene by using different drugs and miRNA-based therapeutics applications in translational research.

Skp2: A critical molecule for ubiquitination and its role in cancer

The ubiquitin-proteasome system (UPS) is a multi-step process that serves as the primary pathway for protein degradation within cells. UPS activity also plays a crucial role in regulating various life processes, including the cell cycle, signal transduction, DNA repair, and others. The F-box protein Skp2, a crucial member of the UPS, plays a central role in the development of various diseases. Skp2 controls cancer cell growth and drug resistance by ubiquitinating modifications to a variety of proteins. This review emphasizes the multifaceted role of Skp2 in a wide range of cancers and the mechanisms involved, highlighting the potential of Skp2 as a therapeutic target in cancer. Additionally, we describe the impactful influence exerted by Skp2 in various other diseases beyond cancer.

E3 ubiquitin ligases in lung cancer: Emerging insights and therapeutic opportunities

Aim In this review, we have attempted to provide the readers with an updated account of the role of a family of proteins known as E3 ligases in different aspects of lung cancer progression, along with insights into the deregulation of expression of these proteins during lung cancer. A detailed account of the therapeutic strategies involving E3 ligases that have been developed or currently under development has also been provided in this review. MATERIALS AND METHODS: The review article employs extensive literature search, along with differential gene expression analysis of lung cancer associated E3 ligases using the DESeq2 package in R, and the Gene Expression Profiling Interactive Analysis (GEPIA) database (http://gepia.cancer-pku.cn/). Protein expression analysis of CPTAC lung cancer samples was carried out using the UALCAN webtool (https://ualcan.path.uab.edu/index.html). Assessment of patient overall survival (OS) in response to high and low expression of selected E3 ligases was performed using the online Kaplan-Meier plotter (https://kmplot.com/analysis/index.php?p=background). KEY FINDINGS: SIGNIFICANCE: The review provides an in-depth understanding of the role of E3 ligases in lung cancer progression and an up-to-date account of the different therapeutic strategies targeting oncogenic E3 ligases for improved lung cancer management.

Biology of Pellino1: a potential therapeutic target for inflammation in diseases and cancers

Pellino1 (Peli1) is a highly conserved E3 Ub ligase that exerts its biological functions by mediating target protein ubiquitination. Extensive evidence has demonstrated the crucial role of Peli1 in regulating inflammation by modulating various receptor signaling pathways, including interleukin-1 receptors, Toll-like receptors, nuclear factor-κB, mitogen-activated protein kinase, and phosphoinositide 3-kinase/AKT pathways. Peli1 has been implicated in the development of several diseases by influencing inflammation, apoptosis, necrosis, pyroptosis, autophagy, DNA damage repair, and glycolysis. Peli1 is a risk factor for most cancers, including breast cancer, lung cancer, and lymphoma. Conversely, Peli1 protects against herpes simplex virus infection, systemic lupus erythematosus, esophageal cancer, and toxic epidermolysis bullosa. Therefore, Peli1 is a potential therapeutic target that warrants further investigation. This comprehensive review summarizes the target proteins of Peli1, delineates their involvement in major signaling pathways and biological processes, explores their role in diseases, and discusses the potential clinical applications of Peli1-targeted therapy, highlighting the therapeutic prospects of Peli1 in various diseases.

Inhibitors Targeting the F-BOX Proteins

F-box proteins are involved in multiple cellular processes through ubiquitylation and consequent degradation of targeted substrates. Any significant mutation in F-box protein-mediated proteolysis can cause human malformations. The various cellular processes F-box proteins involved include cell proliferation, apoptosis, invasion, angiogenesis, and metastasis. To target F-box proteins and their associated signaling pathways for cancer treatment, researchers have developed thousands of F-box inhibitors. The most advanced inhibitor of FBW7, NVD-BK M120, is a powerful P13 kinase inhibitor that has been proven to bring about apoptosis in cancerous human lung cells by disrupting levels of the protein known as MCL1. Moreover, F-box Inhibitors have demonstrated their efficacy for treating certain cancers through targeting particular mutated proteins. This paper explores the key studies on how F-box proteins act and their contribution to malignancy development, which fabricates an in-depth perception of inhibitors targeting the F-box proteins and their signaling pathways that eventually isolate the most promising approach to anti-cancer treatments.

Promoter methylation and enhanced SKP2 are associated with the downregulation of CDKN1C in cervical squamous cell carcinoma

PURPOSE

Cervical Squamous Cell Carcinoma (CSCC) is one of the significant causes of cancer deaths among women. Distinct genetic and epigenetic-altered loci, including chromosomal 11p15.5-15.4, have been identified. CDKN1C (Cyclin-Dependent Kinase Inhibitor 1C, p57KIP2), a member of the CIP/KIP family of cyclin-dependent kinase inhibitors (CDKIs), located at 11p15.4, is a putative tumor suppressor. Apart from transcriptional control, S-Phase Kinase Associated Protein 2 (SKP2), an oncogenic E3 ubiquitin ligase, regulates the protein turnover of CDKN1C. But the molecular status of CDKN1C in CSCC and the underlying mechanistic underpinnings have yet to be explored.

METHODS

TCGA and other publicly available datasets were analyzed to evaluate the expression of CDKN1C and SKP2. The expression (transcript/protein) was validated in independent CSCC tumors (n = 155). Copy number alteration and promoter methylation were correlated with the expression. Finally, in vitro functional validation was performed.

RESULTS

CDKN1C was down-regulated, and SKP2 was up-regulated at the transcript and protein levels in CSCC tumors and the SiHa cell line. Notably, promoter methylation (50%) was associated with the downregulation of the CDKN1C transcript. However, high expression of SKP2 was found to be associated with the decreased expression of CDKN1C protein. Independent treatments with 5-aza-dC, MG132, and SKP2i (SKPin C1) in SiHa cells led to an enhanced expression of CDKN1C protein, validating the mechanism of down-regulation in CSCC.

CONCLUSION

Collectively, CDKN1C was down-regulated due to the synergistic effect of promoter hyper-methylation and SKP2 over-expression in CSCC tumors, paving the way for further studies of its role in the pathogenesis of the disease.

Ethanol Extract of Citrus grandis 'Tomentosa' Exerts Anticancer Effects by Targeting Skp2/p27 Pathway in Non-Small Cell Lung Cancer

SCOPE

This study aims to investigate the anticancer properties of Citrus grandis 'Tomentosa' (CGT) in non-small cell lung cancer (NSCLC).

METHODS AND RESULTS

The ethanol extract of CGT (CGTE) is prepared by using anhydrous ethanol and analyzed by ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS), revealing that the main chemical components in CGTE are flavonoids and coumarins, such as naringin, rhoifolin, apigenin, bergaptol, and osthole. CGTE at concentrations without inducing cell death significantly inhibits cell proliferation via inducing cell cycle G1 phase arrest by MTT, colony formation, and flow cytometry assays, implying that CGT has anticancer potential. CGTE markedly inhibits the activity of Skp2-SCF E3 ubiquitin ligase, decreases the protein level of Skp2, and promotes the accumulation of p27 by co-immunoprecipitation (co-IP) and in vivo ubiquitination assay; whereas Skp2 overexpression rescues the effects of CGTE in NSCLC cells. In subcutaneous LLC allograft and A549 xenograft mouse models, CGTE, without causing obvious side effects in mice, significantly inhibits lung tumor growth by targeting the Skp2/p27 signaling pathway.

CONCLUSION

These findings demonstrate that CGTE efficiently inhibits NSCLC proliferation both in vitro and in vivo by targeting the Skp2/p27 signaling pathway, suggesting that CGTE may serve as a therapeutic candidate for NSCLC treatment.

Skp2-mediated MLKL degradation confers cisplatin-resistant in non-small cell lung cancer cells

Non-small cell lung cancer (NSCLC) is the most prevalent type of cancer and the leading cause of cancer-related death. Chemotherapeutic resistance is a major obstacle in treating NSCLC patients. Here, we discovered that the E3 ligase Skp2 is overexpressed, accompanied by the downregulation of necroptosis-related regulator MLKL in human NSCLC tissues and cell lines. Knockdown of Skp2 inhibited viability, anchorage-independent growth, and in vivo tumor development of NSCLC cells. We also found that the Skp2 protein is negatively correlated with MLKL in NSCLC tissues. Moreover, Skp2 is increased and accompanied by an upregulation of MLKL ubiquitination and degradation in cisplatin-resistant NSCLC cells. Accordingly, inhibition of Skp2 partially restores MLKL and sensitizes NSCLC cells to cisplatin in vitro and in vivo. Mechanistically, Skp2 interacts and promotes ubiquitination-mediated degradation of MLKL in cisplatin-resistant NSCLC cells. Our results provide evidence of an Skp2-dependent mechanism regulating MLKL degradation and cisplatin resistance, suggesting that targeting Skp2-ubiquitinated MLKL degradation may overcome NSCLC chemoresistance.

COP9 Signalosome Promotes Neointimal Hyperplasia via Deneddylation and CSN5-Mediated Nuclear Export

BACKGROUND

Neointimal hyperplasia (NH) is a common pathological response to vascular injury and mediated primarily by vascular smooth muscle cell (VSMC) migration and proliferation. The COP9 signalosome (CSN) is formed by 8 canonical subunits (CSN1 through CSN8) with its deneddylation activity residing in CSN5. Each or some of CSN subunits may have deneddylation-independent function. Despite strong evidence linking the CSN to cell cycle regulation in cancer cells, the role of the CSN in vascular biology remains obscure.

METHODS

Neointimal CSN5 expression in the lung tissue of pulmonary hypertension (PAH) patients was assessed with immunohistochemistry. Adult mice with smooth muscle cell-restricted CSN5 knockout (CSN5-SMKO) or CSN8 hypomorphism (CSN8-hypo) and cultured mouse VSMCs were studied to determine the role and governing mechanisms of the CSN in NH. NH was induced by ligation of the left common carotid artery (LCCA) and PDGF-BB stimulation was used to mimic the vascular injury in cell cultures.

RESULTS

Remarkably higher CSN5 levels were detected in the neointimal VSMCs of the pulmonary arteries of human PAH. LCCA ligation induced NH and significantly increased the mRNA and protein levels of CSN subunits in the LCCA wall of adult wild type mice. CSN5-SMKO impaired Cullin deneddylation and the nuclear export of p27 in vessel walls and markedly inhibited VSMC proliferation in mice. On the contrary, CSN8-hypo significantly exacerbated NH and VSMC proliferation in vivo and in cellulo . Cytoplasmic CSN5 mini-complexes and the nuclear export of p27 were significantly increased in CSN8-hypo mouse vessels and cultured CSN8-hypo VSMCs. Nuclear export inhibition with leptomycin attenuated the PDGF-BB-induced increases in VSMC proliferation in both CSN8-hypo and control VSMCs. Further, genetically disabling CSN5 nuclear export but not disabling CSN5 deneddylase activity suppressed the hyperproliferation and restored p27 nuclear localization in CSN8 hypomorphic VSMCs. Interestingly, CSN deneddylase inhibition by CSN5i-3 did not alter the hyperproliferation of cultured CSN8-hypo VSMCs but suppressed wild type VSMC proliferation in cellulo and in vivo and blocked neointimal formation in wild type mice.

CONCLUSION

The CSN promotes VSMC proliferation and NH in injured vessels through deneddylation activity and CSN5-mediated nuclear export.

Platycodin D suppresses proliferation, migration, and invasion of human glioblastoma cells through regulation of Skp2

BACKGROUND

Platycodin D (PD) is a major bioactive component of Platycodon grandiflorum, a medicinal herb that is widely used in China, and is effective against various human cancers, including glioblastoma multiforme (GBM). S phase kinase-related protein 2 (Skp2) is oncogenic and overexpressed in various human tumors. It is highly expressed in GBM and its expression is correlated with tumor growth, drug resistance and poor prognosis. In this study, we investigated whether inhibition of glioma progression by PD is mediated by decreasing expression of Skp2.

METHODS

Cell Counting Kit-8 (CCK-8) and Transwell assays were used to determine the effects of PD on GBM cell proliferation, migration, and invasion in vitro. mRNA and protein expression were determined by real time polymerase chain reaction (RT-PCR) and western blotting, respectively. The U87 xenograft model was used to verify the anti-glioma effect of PD in vivo. Expression levels of Skp2 protein were analyzed by immunofluorescence staining.

RESULTS

PD suppressed proliferation and motility of GBM cells in vitro. The expression of Skp2 in U87 and U251 cells was significantly reduced by PD. PD mainly decreased the cytoplasmic expression of Skp2 in glioma cells. Skp2 protein expression was downregulated by PD, resulting in upregulation of its downstream targets, p21and p27. The inhibitory effect of PD was enhanced by Skp2 knockdown in GBM cells and reversed in cells with Skp2 overexpression.

CONCLUSION

PD suppresses glioma development by regulation of Skp2 in GBM cells.

The roles of E3 ubiquitin ligases in cancer progression and targeted therapy

Ubiquitination is one of the most important post-translational modifications which plays a significant role in conserving the homeostasis of cellular proteins. In the ubiquitination process, ubiquitin is conjugated to target protein substrates for degradation, translocation or activation, dysregulation of which is linked to several diseases including various types of cancers. E3 ubiquitin ligases are regarded as the most influential ubiquitin enzyme owing to their ability to select, bind and recruit target substrates for ubiquitination. In particular, E3 ligases are pivotal in the cancer hallmarks pathways where they serve as tumour promoters or suppressors. The specificity of E3 ligases coupled with their implication in cancer hallmarks engendered the development of compounds that specifically target E3 ligases for cancer therapy. In this review, we highlight the role of E3 ligases in cancer hallmarks such as sustained proliferation via cell cycle progression, immune evasion and tumour promoting inflammation, and in the evasion of apoptosis. In addition, we summarise the application and the role of small compounds that target E3 ligases for cancer treatment along with the significance of targeting E3 ligases as potential cancer therapy.

Pan-Cancer Analysis of the Expression and Prognostic Value of S-Phase Kinase-Associated Protein 2

BACKGROUND: S-Phase Kinase-Associated Protein 2 (SKP2) is essential in modulating metabolism processes, cell proliferation, and carcinogenesis DUE to its capacity to ubiquitinate and degrade various tumor-suppressive substrates. However, the actual biological and mechanism significance of SKP2 in the development of tumors and as a possible therapeutic target remains to be completely understood. AIM: This study aimed to explore the potential roles of the SKP2 gene in the oncologic pathogenesis of various cancers through an in-depth pan-cancer analysis including gene expression assessment, survival analysis, genetic alteration, and enrichment analysis. METHODS: Public databases including the Cancer Genome Atlas database, Genotype-Tissue Expression Project database, cBioPortal database, Gene Expression Profiling Interactive Analysis 2 database, Tumor Immune Estimation Resource version 2.0 database, and STRING database were used to detect the SKP2 expression, molecular mechanism, and its association with the prognosis across pan-cancer. RESULTS: SKP2 was significantly highly expressed in most types of cancers and was substantially correlated to the poor survival of patients with specific cancers based on the log-rank test. SKP2 had the highest frequency of alteration in lung cancer and amplification was the most common genetic alteration type. Finally, SKP2-related genes were identified and enrichment analyses were conducted. CONCLUSION: This study presented the first demonstration of the pan-cancer landscape of abnormal SKP2 expression, it could potentially serve as a predictive indicator and prospective therapeutic target.

Small-molecule compounds inhibiting S-phase kinase-associated protein 2: A review

S-phase kinase-associated protein 2 (Skp2) is a substrate-specific adaptor in Skp1-CUL1-ROC1-F-box E3 ubiquitin ligases and widely regarded as an oncogene. Therefore, Skp2 has remained as an active anticancer research topic since its discovery. Accordingly, the structure of Skp2 has been solved and numerous Skp2 inhibiting compounds have been identified. In this review, we would describe the structural features of Skp2, introduce the ubiquitination function of SCF^Skp2, and summarize the diverse natural and synthetic Skp2 inhibiting compounds reported to date. The IC₅₀ data of the Skp2 inhibitors or inhibiting compounds in various kinds of tumors at cellular levels implied that the cancer type, stage and pathological mechanisms should be taken into consideration when selecting Skp2-inhibiting compound for cancer treatment.

Akt: a key transducer in cancer

Growth factor signaling plays a pivotal role in diverse biological functions, such as cell growth, apoptosis, senescence, and migration and its deregulation has been linked to various human diseases. Akt kinase is a central player transmitting extracellular clues to various cellular compartments, in turn executing these biological processes. Since the discovery of Akt three decades ago, the tremendous progress towards identifying its upstream regulators and downstream effectors and its roles in cancer has been made, offering novel paradigms and therapeutic strategies for targeting human diseases and cancers with deregulated Akt activation. Unraveling the molecular mechanisms for Akt signaling networks paves the way for developing selective inhibitors targeting Akt and its signaling regulation for the management of human diseases including cancer.

O-GlcNAc tranferase regulates p21 protein levels and cell proliferation through the FoxM1-Skp2 axis in a p53-independent manner

The protein product of the CDKN1A gene, p21, has been extensively characterized as a negative regulator of the cell cycle. Nevertheless, it is clear that p21 has manifold complex and context-dependent roles that can be either tumor suppressive or oncogenic. Most well studied as a transcriptional target of the p53 tumor suppressor protein, there are other means by which p21 levels can be regulated. In this study, we show that pharmacological inhibition or siRNA-mediated reduction of O-GlcNAc transferase (OGT), the enzyme responsible for glycosylation of intracellular proteins, increases expression of p21 in both p53-dependent and p53-independent manners in nontransformed and cancer cells. In cells harboring WT p53, we demonstrate that inhibition of OGT leads to p53-mediated transactivation of CDKN1A, while in cells that do not express p53, inhibiting OGT leads to increased p21 protein stabilization. p21 is normally degraded by the ubiquitin-proteasome system following ubiquitination by, among others, the E3 ligase Skp-Cullin-F-box complex; however, in this case, we show that blocking OGT causes impairment of the Skp-Cullin-F-box ubiquitin complex as a result of disruption of the FoxM1 transcription factor–mediated induction of Skp2 expression. In either setting, we conclude that p21 levels induced by OGT inhibition correlate with cell cycle arrest and decreased cancer cell proliferation.

Functional roles of E3 ubiquitin ligases in prostate cancer

Prostate cancer (PCa) is a malignant epithelial tumor of the prostate gland with a high male cancer incidence. Numerous studies indicate that abnormal function of ubiquitin-proteasome system (UPS) is associated with the progression and metastasis of PCa. E3 ubiquitin ligases, key components of UPS, determine the specificity of substrates, and substantial advances of E3 ubiquitin ligases have been reached recently. Herein, we introduce the structures and functions of E3 ubiquitin ligases and summarize the mechanisms of E3 ubiquitin ligases-related PCa signaling pathways. In addition, some progresses in the development of inhibitors targeting E3 ubiquitin ligases are also included.

Potential Therapeutics Targeting Upstream Regulators and Interactors of EHMT1/2

Euchromatin histone lysine methyltransferases (EHMTs) are epigenetic regulators responsible for silencing gene transcription by catalyzing H3K9 dimethylation. Dysregulation of EHMT1/2 has been reported in multiple cancers and is associated with poor clinical outcomes. Although substantial insights have been gleaned into the downstream targets and pathways regulated by EHMT1/2, few studies have uncovered mechanisms responsible for their dysregulated expression. Moreover, EHMT1/2 interacting partners, which can influence their function and, therefore, the expression of target genes, have not been extensively explored. As none of the currently available EHMT inhibitors have made it past clinical trials, understanding upstream regulators and EHMT protein complexes may provide unique insights into novel therapeutic avenues in EHMT-overexpressing cancers. Here, we review our current understanding of the regulators and interacting partners of EHMTs. We also discuss available therapeutic drugs that target the upstream regulators and binding partners of EHMTs and could potentially modulate EHMT function in cancer progression.

The roles of E3 ligases in Hepatocellular carcinoma

Hepatocarcinogenesis is a complex multistep biological process involving genetic and epigenetic alterations that are accompanied by activation of oncoproteins and inactivation of tumor suppressors, which in turn results in Hepatocellular carcinoma (HCC), one of the common tumors with high morbidity and mortality worldwide. The ubiquitin-proteasome system (UPS) is the key to protein degradation and regulation of physiological and pathological processes, and E3 ligases are key enzymes in the UPS that contain a variety of subfamily proteins involved in the regulation of some common signal pathways in HCC. There is growing evidence that many structural or functional dysfunctions of E3 are engaged in the development and progression of HCC. Herein, we review recent research advances in HCC-associated E3 ligases, describe their structure, classification, functional roles, and discuss some mechanisms of the abnormal activation or inactivation of the HCC-associated signal pathway due to the binding of E3 to known substrates. In addition, given the success of proteasome inhibitors in the treatment of malignant cancers, we characterize the current knowledge and future prospects for targeted therapies against aberrant E3 in HCC.

PI3K inhibitor 3-MA promotes the antiproliferative activity of esomeprazole in gastric cancer cells by downregulating EGFR via the PI3K/FOXO3a pathway

Gastric cancer is a common gastrointestinal malignancy worldwide, with a high mortality rate and poor prognosis. Esomeprazole (ESO) has been shown to have anticancer activity by affecting cell growth and autophagy and its mechanism in gastric cancer cells is evident. The PI3K/AKT/FOXO3a pathway is central in cancers. 3-Methyladenine (3-MA), a dual inhibitor of PI3K and autophagy, plays a synergistic role in combination with antitumor agents. In this study, we assessed the role of ESO on the PI3K/AKT/FOXO3a pathway and the beneficial effects of ESO combined with 3-MA in gastric cancer cells. Cell viability, proliferation, invasion, migration, apoptosis, autophagy, and protein expression were detected by CCK-8, EdU, Transwell, flow cytometry, immunofluorescence assay, and western blot. ESO decreased cell viability in a concentration- and time-dependent manner and increased autophagy with upregulation of LC3II and P62. Additionally, ESO inhibited the proliferation, migration, and invasion and induced the apoptosis of gastric cancer cells in a concentration-dependent manner. ESO inhibited PI3K/AKT/FOXO3a signaling and EGFR and SKP2 expression concentration-dependent. 3-MA enhanced the antiproliferative activity of ESO and synergistically inhibited PI3K/FOXO3a signaling and the expression of EGFR but not SKP2. Furthermore, pretreatment with the EGFR inhibitor AG1478 enhanced the antiproliferative activity of ESO in gastric cancer cells. In conclusion, our results suggested that the PI3K inhibitor 3-MA promotes the antiproliferative activity of ESO in gastric cancer cells by synergistically downregulating EGFR via the PI3K/FOXO3a pathway.

The E3 ubiquitin ligase, FBXW5, promotes the migration and invasion of gastric cancer through the dysregulation of the Hippo pathway

F-box and WD repeat domain-containing 5 (FBXW5), with WD40 repeats, can bind to the PPxY sequence of the large tumor suppressor kinases 1/2 (LATS1/2) kinase domain, resulting in ubiquitination. Ubiquitination and the subsequent degradation of LATS1/2 abrogate the Hippo pathway and worsen gastric cancer (GC). However, the effects and molecular mechanisms of FBXW5 in GC remain unexplored. To elucidate the clinical significance of FBXW5, immunohistochemistry was conducted to reveal the positive correlation between FBXW5 expression and lymph node metastasis (p < 0.001) and TNM stage (training cohort: p = 0.018; validation cohort: p = 0.001). Further, patients with high FBXW5 expression were found to have poor prognosis (training cohort: log-rank p = 0.020; validation cohort: log-rank p = 0.025). Cell experiments revealed the promoting effects of FBXW5 on the proliferation, invasion, metastasis, and chemoresistance of GC cells. Blocking LATS1-YAP1 leads to the loss of FBXW5-mediated regulation of the Hippo pathway and partial functions. Further, co-immunoprecipitation and in vivo ubiquitination assays revealed the interaction between FBXW5 and LATS1, which promoted the ubiquitination and degradation of LATS1. Based on mouse xenograft assays, FBXW5 silencing attenuated the growth of subcutaneous tumor xenografts. Altogether, FBXW5 was found to inactivate the Hippo signaling pathway by enhancing LATS1 ubiquitination and degradation, which promoted the invasion, metastasis, and drug resistance of GC cells.

The Intricate Interplay between Cell Cycle Regulators and Autophagy in Cancer

Simple Summary

Autophagy is an intracellular catabolic program regulated by multiple external and internal cues. A large amount of evidence unraveled that cell-cycle regulators are crucial in its control. This review highlights the interplay between cell-cycle regulators, including cyclin-dependent kinase inhibitors, cyclin-dependent kinases, and E2F factors, in the control of autophagy all along the cell cycle. Beyond the intimate link between cell cycle and autophagy, this review opens therapeutic perspectives in modulating together these two aspects to block cancer progression.

Abstract

In the past decade, cell cycle regulators have extended their canonical role in cell cycle progression to the regulation of various cellular processes, including cellular metabolism. The regulation of metabolism is intimately connected with the function of autophagy, a catabolic process that promotes the efficient recycling of endogenous components from both extrinsic stress, e.g., nutrient deprivation, and intrinsic sub-lethal damage. Mediating cellular homeostasis and cytoprotection, autophagy is found to be dysregulated in numerous pathophysiological contexts, such as cancer. As an adaptative advantage, the upregulation of autophagy allows tumor cells to integrate stress signals, escaping multiple cell death mechanisms. Nevertheless, the precise role of autophagy during tumor development and progression remains highly context-dependent. Recently, multiple articles has suggested the importance of various cell cycle regulators in the modulation of autophagic processes. Here, we review the current clues indicating that cell-cycle regulators, including cyclin-dependent kinase inhibitors (CKIs), cyclin-dependent kinases (CDKs), and E2F transcription factors, are intrinsically linked to the regulation of autophagy. As an increasing number of studies highlight the importance of autophagy in cancer progression, we finally evoke new perspectives in therapeutic avenues that may include both cell cycle inhibitors and autophagy modulators to synergize antitumor efficacy.

Potential of E3 Ubiquitin Ligases in Cancer Immunity: Opportunities and Challenges

Cancer immunotherapies, including immune checkpoint inhibitors and immune pathway–targeted therapies, are promising clinical strategies for treating cancer. However, drug resistance and adverse reactions remain the main challenges for immunotherapy management. The future direction of immunotherapy is mainly to reduce side effects and improve the treatment response rate by finding new targets and new methods of combination therapy. Ubiquitination plays a crucial role in regulating the degradation of immune checkpoints and the activation of immune-related pathways. Some drugs that target E3 ubiquitin ligases have exhibited beneficial effects in preclinical and clinical antitumor treatments. In this review, we discuss mechanisms through which E3 ligases regulate tumor immune checkpoints and immune-related pathways as well as the opportunities and challenges for integrating E3 ligases targeting drugs into cancer immunotherapy.

Regulatory mechanism of cyclins and cyclin-dependent kinases in post-mitotic neuronal cell division

Neurodegenerative diseases (NDDs) are the most common life-threatening disease of the central nervous system and it cause the progressive loss of neuronal cells. The exact mechanism of the disease's progression is not clear and thus line of treatment for NDDs is a baffling issue. During the progression of NDDs, oxidative stress and DNA damage play an important regulatory function, and ultimately induces neurodegeneration. Recently, aberrant cell cycle events have been demonstrated in the progression of different NDDs. However, the pertinent role of signaling mechanism, for instance, post-translational modifications, oxidative stress, DNA damage response pathway, JNK/p38 MAPK, MEK/ERK cascade, actively participated in the aberrant cell cycle reentry induced neuronal cell death. Mounting evidence has demonstrated that aberrant cell cycle re-entry is a major contributing factor in the pathogenesis of NDDs rather than a secondary phenomenon. In the brain of AD patients with mild cognitive impairment, post miotic cell division can be seen in the early stage of the disease. However, in the brain of PD patients, response to various neurotoxic signals, the cell cycle re-entry has been observed that causes neuronal apoptosis. On contrary, the contributing factors that leads to the induction of cell cycle events in mature neurons in HD and ALS brain pathology is remain unclear. Various pharmacological drugs have been developed to reduce the pathogenesis of NDDs, but they are still not helpful in eliminating the cause of these NDDs.

More Than Just Cleaning: Ubiquitin-Mediated Proteolysis in Fungal Pathogenesis

Ubiquitin-proteasome mediated protein turnover is an important regulatory mechanism of cellular function in eukaryotes. Extensive studies have linked the ubiquitin-proteasome system (UPS) to human diseases, and an array of proteasome inhibitors have been successfully developed for cancer therapy. Although still an emerging field, research on UPS regulation of fungal development and virulence has been rapidly advancing and has generated considerable excitement in its potential as a target for novel drugs. In this review, we summarize UPS composition and regulatory function in pathogenic fungi, especially in stress responses, host adaption, and fungal pathogenesis. Emphasis will be given to UPS regulation of pathogenic factors that are important for fungal pathogenesis. We also discuss future potential therapeutic strategies for fungal infections based on targeting UPS pathways.

Tilting MYC toward cancer cell death

MYC oncoprotein promotes cell proliferation and serves as the key driver in many human cancers; therefore, considerable effort has been expended to develop reliable pharmacological methods to suppress its expression or function. Despite impressive progress, MYC-targeting drugs have not reached the clinic. Recent advances suggest that within a limited expression range unique to each tumor, MYC oncoprotein can have a paradoxical, proapoptotic function. Here we introduce a counterintuitive idea that modestly and transiently elevating MYC levels could aid chemotherapy-induced apoptosis and thus benefit the patients as much, if not more than MYC inhibition.

Establishment of high-throughput screening HTRF assay for identification small molecule inhibitors of Skp2-Cks1

S-phase kinase associated protein 2 (Skp2), a member of the F-box family that constitute the largest known class of ubiquitin E3 specificity components, is responsible for recognizing and recruiting cyclin-dependent kinase inhibitor p27 for its ubiquitination in the presence of the small accessory protein cyclin-dependent kinase regulatory subunit 1(Cks1). Skp2 is overexpressed in esophageal carcinoma tissues and closely related with tumor poor prognosis, and perturbation of the Skp2-Cks1 interaction by inhibitors or RNAi could inhibit the proliferation and metastasis of tumor cells. Therefore, inhibition of Skp2 function by small-molecule compounds targeting Skp2-Cks1 interaction is emerging as a promising and novel anti-cancer strategy. In this study, we establish an improved high-throughput screening platform to screen Skp2 inhibitors targeting Skp2-Cks1interaction, which may provide a new therapeutic approach for the clinic.

Phytochemical library screening reveals betulinic acid as a novel Skp2-SCF E3 ligase inhibitor in non-small cell lung cancer

Skp2 is overexpressed in multiple cancers and plays a critical role in tumor development through ubiquitin/proteasome-dependent degradation of its substrate proteins. Drugs targeting Skp2 have exhibited promising anticancer activity. Here, we identified a plant-derived Skp2 inhibitor, betulinic acid (BA), via high-throughput structure-based virtual screening of a phytochemical library. BA significantly inhibited the proliferation and migration of non-small cell lung cancer (NSCLC) through targeting Skp2-SCF E3 ligase both in vitro and in vivo. Mechanistically, BA binding to Skp2, especially forming H-bonds with residue Lys145, decreases its stability by disrupting Skp1-Skp2 interactions, thereby inhibiting the Skp2-SCF E3 ligase and promoting the accumulation of its substrates; that is, E-cadherin and p27. In both subcutaneous and orthotopic xenografts, BA significantly inhibited the proliferation and metastasis of NSCLC through targeting Skp2-SCF E3 ligase and upregulating p27 and E-cadherin protein levels. Taken together, BA can be considered a valuable therapeutic candidate to inhibit metastasis of NSCLC.

E3 ubiquitin ligases: styles, structures and functions

E3 ubiquitin ligases are a large family of enzymes that join in a three-enzyme ubiquitination cascade together with ubiquitin activating enzyme E1 and ubiquitin conjugating enzyme E2. E3 ubiquitin ligases play an essential role in catalyzing the ubiquitination process and transferring ubiquitin protein to attach the lysine site of targeted substrates. Importantly, ubiquitination modification is involved in almost all life activities of eukaryotes. Thus, E3 ligases might be involved in regulating various biological processes and cellular responses to stress signal associated with cancer development. Thanks to their multi-functions, E3 ligases can be a promising target of cancer therapy. A deeper understanding of the regulatory mechanisms of E3 ligases in tumorigenesis will help to find new prognostic markers and accelerate the growth of anticancer therapeutic approaches. In general, we mainly introduce the classifications of E3 ligases and their important roles in cancer progression and therapeutic functions.

The Role of the Ubiquitin Proteasome System in Glioma: Analysis Emphasizing the Main Molecular Players and Therapeutic Strategies Identified in Glioblastoma Multiforme

Gliomas constitute the most frequent tumors of the brain. High-grade gliomas are characterized by a poor prognosis caused by a set of attributes making treatment difficult, such as heterogeneity and cell infiltration. Additionally, there is a subgroup of glioma cells with properties similar to those of stem cells responsible for tumor recurrence after treatment. Since proteasomal degradation regulates multiple cellular processes, any mutation causing disturbances in the function or expression of its elements can lead to various disorders such as cancer. Several studies have focused on protein degradation modulation as a mechanism of glioma control. The ubiquitin proteasome system is the main mechanism of cellular proteolysis that regulates different events, intervening in pathological processes with exacerbating or suppressive effects on diseases. This review analyzes the role of proteasomal degradation in gliomas, emphasizing the elements of this system that modulate different cellular mechanisms in tumors and discussing the potential of distinct compounds controlling brain tumorigenesis through the proteasomal pathway.

Targeting the Ubiquitin-Proteasome System for Cancer Therapeutics by Small-Molecule Inhibitors

The ubiquitin-proteasome system (UPS) is a critical regulator of cellular protein levels and activity. It is, therefore, not surprising that its dysregulation is implicated in numerous human diseases, including many types of cancer. Moreover, since cancer cells exhibit increased rates of protein turnover, their heightened dependence on the UPS makes it an attractive target for inhibition via targeted therapeutics. Indeed, the clinical application of proteasome inhibitors in treatment of multiple myeloma has been very successful, stimulating the development of small-molecule inhibitors targeting other UPS components. On the other hand, while the discovery of potent and selective chemical compounds can be both challenging and time consuming, the area of targeted protein degradation through utilization of the UPS machinery has seen promising developments in recent years. The repertoire of proteolysis-targeting chimeras (PROTACs), which employ E3 ligases for the degradation of cancer-related proteins via the proteasome, continues to grow. In this review, we will provide a thorough overview of small-molecule UPS inhibitors and highlight advancements in the development of targeted protein degradation strategies for cancer therapeutics.

Targeting the MYC Ubiquitination-Proteasome Degradation Pathway for Cancer Therapy

Deregulated MYC overexpression and activation contributes to tumor growth and progression. Given the short half-life and unstable nature of the MYC protein, it is not surprising that the oncoprotein is highly regulated via diverse posttranslational mechanisms. Among them, ubiquitination dynamically controls the levels and activity of MYC during normal cell growth and homeostasis, whereas the disturbance of the ubiquitination/deubiquitination balance enables unwanted MYC stabilization and activation. In addition, MYC is also regulated by SUMOylation which crosstalks with the ubiquitination pathway and controls MYC protein stability and activity. In this mini-review, we will summarize current updates regarding MYC ubiquitination and provide perspectives about these MYC regulators as potential therapeutic targets in cancer.

N6-methyladenosine demethyltransferase FTO-mediated autophagy in malignant development of oral squamous cell carcinoma

N6-methyladenosine (m⁶A) is the most abundant internal mRNA modification in eukaryotes and plays an important role in tumorigenesis. However, the underlying mechanism remains largely unclear. Here, we established a cell model of rapamycin-induced autophagy to screen m⁶A-modifying enzymes. We found that m⁶A demethylase fat mass and obesity-associated protein (FTO) plays a key role in regulating autophagy and tumorigenesis by targeting the gene encoding eukaryotic translation initiation factor gamma 1 (eIF4G1) in oral squamous cell carcinoma (OSCC). Knocked down of FTO expression in OSCC cell lines, resulting in downregulation of eIF4G1 along with enhanced autophagic flux and inhibition of tumorigenesis. Rapamycin inhibited FTO activity, and directly targeted eIF4G1 transcripts and mediated their expression in an m⁶A-dependent manner. Dual-luciferase reporter and mutagenesis assays confirmed that YTH N6-methyladenosine RNA-binding protein 2 (YTHDF2) targets eIF4G1. Conclusively, after FTO silencing, YTHDF2 captured eIF4G1 transcripts containing m⁶A, resulting in mRNA degradation and decreased expression of eIF4G1 protein, thereby promoting autophagy and reducing tumor occurrence. Therefore, rapamycin may regulate m⁶A levels, determining the autophagic flux of OSCC, thereby affecting the biological characteristics of cancer cells. This insight expands our understanding of the crosstalk between autophagy and RNA methylation in tumorigenesis, which is essential for therapeutic strategy development for OSCC.

Emerging Roles of SKP2 in Cancer Drug Resistance

More than half of all cancer patients receive chemotherapy, however, some of them easily acquire drug resistance. Resistance to chemotherapy has become a massive obstacle to achieve high rates of pathological complete response during cancer therapy. S-phase kinase-associated protein 2 (Skp2), as an E3 ligase, was found to be highly correlated with drug resistance and poor prognosis. In this review, we summarize the mechanisms that Skp2 confers to drug resistance, including the Akt-Skp2 feedback loop, Skp2-p27 pathway, cell cycle and mitosis regulation, EMT (epithelial-mesenchymal transition) property, enhanced DNA damage response and repair, etc. We also addressed novel molecules that either inhibit Skp2 expression or target Skp2-centered interactions, which might have vast potential for application in clinics and benefit cancer patients in the future.

ROC1 promotes the malignant progression of bladder cancer by regulating p-IκBα/NF-κB signaling

Background

Regulator of cullins 1 (ROC1) is an important catalytic subunit of cullin–RING E3 ligase. Nuclear factor-kappa B (NF-κB) signaling is closely related to tumor invasion and metastasis. Earlier, we reported that ROC1 was associated with a poor prognosis in patients with bladder cancer (BCa). However, it is unclear whether ROC1 is involved in the NF-κB signaling associated with malignant BCa progression.

Methods

The expression of ROC1 and p65 in bladder cancer and paracancerous tissues were detected by immunohistochemistry (IHC). Pearson correlation was used to assess correlation between ROC1 and p65 protein expressions. The wound-healing and transwell assays were used to monitor cell invasion and migration. The effect of ROC1 on the expression of key proteins in the NF-κB signaling was determined by immunofluorescence and western blot (WB). Cycloheximide (CHX), MG132 and immunoprecipitation assays were used to evaluate the effect of ROC1 on the ubiquitination of phosphorylated inhibitor of kappa B alpha (p-IκBα). A lung metastasis mouse model was generated to detect the role of ROC1 in tumor metastasis.

Results

We found that ROC1 was up-regulated in BCa tissues and cell lines, and high ROC1 levels were positively correlated with higher tumour grade, lymph node metastasis, distant metastasis and poor prognosis. Linear-regression analysis showed significant a Pearson correlation between ROC1 and nuclear p65 expression in BCa tissue microarray (TMA) samples. Functional studies demonstrated that ROC1 promoted BCa cell invasion and migration. In vitro and in vivo experiments showed that ROC1 activated NF-κB signaling by enhancing the ubiquitination of p-IκBα, which caused p65 nuclear translocation and promoted the transcription of some metastasis-related target genes, such as urokinase-type plasminogen activator receptor (uPAR), intracellular adhesion molecule 1 (ICAM1), vascular cell adhesion molecule 1 (VCAM1), and matrix metalloproteinase 9 (MMP9), resulting in promoting BCa metastasis.

Conclusion

ROC1 plays an important role in the progression of BCa and serves as a potential diagnostic and therapeutic target for patients with BCa.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-021-01935-5.

Ubiquitin-proteasome system and the role of its inhibitors in cancer therapy

Despite all the other cells that have the potential to prevent cancer development and metastasis through tumour suppressor proteins, cancer cells can upregulate the ubiquitin–proteasome system (UPS) by which they can degrade tumour suppressor proteins and avoid apoptosis. This system plays an extensive role in cell regulation organized in two steps. Each step has an important role in controlling cancer. This demonstrates the importance of understanding UPS inhibitors and improving these inhibitors to foster a new hope in cancer therapy. UPS inhibitors, as less invasive chemotherapy drugs, are increasingly used to alleviate symptoms of various cancers in malignant states. Despite their success in reducing the development of cancer with the lowest side effects, thus far, an appropriate inhibitor that can effectively inactivate this system with the least drug resistance has not yet been fully investigated. A fundamental understanding of the system is necessary to fully elucidate its role in causing/controlling cancer. In this review, we first comprehensively investigate this system, and then each step containing ubiquitination and protein degradation as well as their inhibitors are discussed. Ultimately, its advantages and disadvantages and some perspectives for improving the efficiency of these inhibitors are discussed.

The FBXW7-NOTCH interactome: A ubiquitin proteasomal system-induced crosstalk modulating oncogenic transformation in human tissues

BACKGROUND

Ubiquitin ligases or E3 ligases are well programmed to regulate molecular interactions that operate at a post-translational level. Skp, Cullin, F-box containing complex (or SCF complex) is a multidomain E3 ligase known to mediate the degradation of a wide range of proteins through the proteasomal pathway. The three-dimensional domain architecture of SCF family proteins suggests that it operates through a novel and adaptable "super-enzymatic" process that might respond to targeted therapeutic modalities in cancer.

RECENT FINDINGS

Several F-box containing proteins have been characterized either as tumor suppressors (FBXW8, FBXL3, FBXW8, FBXL3, FBXO1, FBXO4, and FBXO18) or as oncogenes (FBXO5, FBXO9, and SKP2). Besides, F-box members like βTrcP1 and βTrcP2, the ones with context-dependent functionality, have also been studied and reported. FBXW7 is a well-studied F-box protein and is a tumor suppressor. FBXW7 regulates the activity of a range of substrates, such as c-Myc, cyclin E, mTOR, c-Jun, NOTCH, myeloid cell leukemia sequence-1 (MCL1), AURKA, NOTCH through the well-known ubiquitin-proteasome system (UPS)-mediated degradation pathway. NOTCH signaling is a primitive pathway that plays a crucial role in maintaining normal tissue homeostasis. FBXW7 regulates NOTCH protein activity by controlling its half-life, thereby maintaining optimum protein levels in tissue. However, aberrations in the FBXW7 or NOTCH expression levels can lead to poor prognosis and detrimental outcomes in patients. Therefore, the FBXW7-NOTCH axis has been a subject of intense study and research over the years, especially around the interactome's role in driving cancer development and progression. Several studies have reported the effect of FBXW7 and NOTCH mutations on normal tissue behavior. The current review attempts to critically analyze these mutations prognostic value in a wide range of tumors. Furthermore, the review summarizes the recent findings pertaining to the FBXW7 and NOTCH interactome and its involvement in phosphorylation-related events, cell cycle, proliferation, apoptosis, and metastasis.

CONCLUSION

The review concludes by positioning FBXW7 as an effective diagnostic marker in tumors and by listing out recent advancements made in cancer therapeutics in identifying protocols targeting the FBXW7-NOTCH aberrations in tumors.

The Hippo Pathway Effector YAP Promotes Ferroptosis via the E3 Ligase SKP2

Ferroptosis is a new form of regulated cell death resulting from the accumulation of lipid-reactive oxygen species. A growing number of studies indicate ferroptosis as an important tumor suppressor mechanism having therapeutic potential in cancers. Previously, we identified TAZ, a Hippo pathway effector, regulates ferroptosis in renal and ovarian cancer cells. Because YAP (Yes-associated protein 1) is the one and only paralog of TAZ, sharing high sequence similarity and functional redundancy with TAZ, we tested the potential roles of YAP in regulating ferroptosis. Here, we provide experimental evidence that YAP removal confers ferroptosis resistance, whereas overexpression of YAP sensitizes cancer cells to ferroptosis. Furthermore, integrative analysis of transcriptome reveals S-phase kinase-associated protein 2 (SKP2), an E3 ubiquitin ligase, as a YAP direct target gene that regulates ferroptosis. We found that the YAP knockdown represses the expression of SKP2. Importantly, the genetic and chemical inhibitions of SKP2 robustly protect cells from ferroptosis. In addition, knockdown of YAP or SKP2 abolishes the lipid peroxidation during erastin-induced ferroptosis. Collectively, our results indicate that YAP, similar to TAZ, is a determinant of ferroptosis through regulating the expression of SKP2. Therefore, our results support the connection between Hippo pathway effectors and ferroptosis with significant therapeutic implications. IMPLICATIONS: This study reveals that YAP promotes ferroptosis by regulating SKP2, suggesting novel therapeutic options for YAP-driven tumors.

Nobiletin downregulates the SKP2-p21/p27-CDK2 axis to inhibit tumor progression and shows synergistic effects with palbociclib on renal cell carcinoma

Objective:

Natural extracts, including nobiletin, have been reported to enhance the efficacy and sensitivity of chemotherapeutic drugs. However, whether and how nobiletin affects tumor growth and progression in renal cell carcinoma (RCC) are still unclear.

Methods:

Cell proliferation, cell cycle and apoptosis analyses, colony-formation assays, immunoblotting analysis, and qRT-PCR analysis were performed to investigate how nobiletin affected RCC cell proliferation in vitro. The nude mouse model was used to test the efficacy of nobiletin alone or in combination with palbociclib.

Results:

Nobiletin inhibited cell proliferation by inducing G1 cell cycle arrest and cell apoptosis in RCC cells. Mechanistically, nobiletin decreased SKP2 protein expression by reducing its transcriptional level. The downregulated SKP2 caused accumulation of its substrates, p27 and p21, which further inhibited the activity of the G1 phase-related protein, CDK2, leading to inhibition of cell proliferation and tumor formation. A higher SKP2 protein level indicated less sensitivity to the CDK4/6 inhibitor, palbociclib. A combination of nobiletin and palbociclib showed a synergistic tumor inhibition in vitro and in an in vivo model.

Conclusions:

Nobiletin downregulated the SKP2-p21/p27-CDK2 axis to inhibit tumor progression and showed synergistic tumor inhibition effects with the CDK4/6 inhibitor, palbociclib, on RCC, which indicates a potential new therapeutic strategy.

Ubiquitination of Nonhistone Proteins in Cancer Development and Treatment

Ubiquitination, a crucial post-translation modification, regulates the localization and stability of the substrate proteins including nonhistone proteins. The ubiquitin-proteasome system (UPS) on nonhistone proteins plays a critical role in many cellular processes such as DNA repair, transcription, signal transduction, and apoptosis. Its dysregulation induces various diseases including cancer, and the identification of this process may provide potential therapeutic targets for cancer treatment. In this review, we summarize the regulatory roles of key UPS members on major nonhistone substrates in cancer-related processes, such as cell cycle, cell proliferation, apoptosis, DNA damage repair, inflammation, and T cell dysfunction in cancer. In addition, we also highlight novel therapeutic interventions targeting the UPS members (E1s, E2s, E3s, proteasomes, and deubiquitinating enzymes). Furthermore, we discuss the application of proteolysis-targeting chimeras (PROTACs) technology as a novel anticancer therapeutic strategy in modulating protein target levels with the aid of UPS.

The Roles of Post-Translational Modifications on mTOR Signaling

The mechanistic target of rapamycin (mTOR) is a master regulator of cell growth, proliferation, and metabolism by integrating various environmental inputs including growth factors, nutrients, and energy, among others. mTOR signaling has been demonstrated to control almost all fundamental cellular processes, such as nucleotide, protein and lipid synthesis, autophagy, and apoptosis. Over the past fifteen years, mapping the network of the mTOR pathway has dramatically advanced our understanding of its upstream and downstream signaling. Dysregulation of the mTOR pathway is frequently associated with a variety of human diseases, such as cancers, metabolic diseases, and cardiovascular and neurodegenerative disorders. Besides genetic alterations, aberrancies in post-translational modifications (PTMs) of the mTOR components are the major causes of the aberrant mTOR signaling in a number of pathologies. In this review, we summarize current understanding of PTMs-mediated regulation of mTOR signaling, and also update the progress on targeting the mTOR pathway and PTM-related enzymes for treatment of human diseases.

A Destiny for Degradation: Interplay between Cullin-RING E3 Ligases and Autophagy

Autophagy and the ubiquitin-proteasome system (UPS) are two major pathways for protein degradation. The cullin-RING E3 ligases (CRLs) are the largest E3 ligase family and have key biological functions in maintaining protein homeostasis. We provide an updated review of the interactions between CRLs and autophagy, focusing on the regulatory effects of CRLs on the core autophagy machinery that consists of several autophagy-related protein (ATG) complexes and their key upstream signaling pathways. The involvement of such functional interactions in health and disease is also discussed. Understanding the role of CRLs in autophagy is helpful for the development of therapeutic strategies for diseases in which CRLs and autophagy are dysregulated, such as cancer and neurodegenerative conditions.

Targeted modulation of E3 ligases using engineered ubiquitin variants

Ubiquitination plays an essential role in signal transduction to regulate most if not all cellular processes. Among the enzymes that are involved in the ubiquitin (Ub) signaling cascade, tremendous efforts have been focused on elucidating the roles of E3 Ub ligases as they determine the complexity and specificity of ubiquitination. Not surprisingly, the malfunction of E3 ligases is directly implicated in many human diseases, including cancer. Therefore, there is an urgent need to develop potent and specific molecules to modulate E3 ligase activity as intracellular probes for target validation and as pharmacological agents in preclinical research. Unfortunately, the progress has been hampered by the dynamic regulation mechanisms for different types of E3 ligases. Here, we summarize the progress of using protein engineering to develop Ub variant (UbV) inhibitors for all major families of E3 ligases and UbV activators for homologous with E6-associated protein C terminus E3s and homodimeric RING E3s. We believe that this provides a general strategy and a valuable toolkit for the research community to inhibit or activate E3 ligases and these synthetic molecules have important implications in exploring protein degradation for drug discovery.