Potential role of CYLD (Cylindromatosis) as a deubiquitinating enzyme in vascular cells

CYLD links the TRAF6/sNASP axis to TLR4 signaling in sepsis-induced acute lung injury

Sepsis-induced acute lung injury (ALI) involves severe lung dysfunction and leads to high morbidity and mortality rates due to the lack of effective treatments. The somatic nuclear autoantigenic sperm protein (sNASP)/tumor necrosis factor receptor-associated factor 6 (TRAF6) axis plays a crucial role in regulating inflammatory responses during sepsis through Toll-like receptor 4 (TLR4) signaling. However, it is unclear whether deubiquitinating enzymes affect the TRAF6/sNASP axis. In this study, we showed that cylindromatosis (CYLD) directly binds to the sNASP and prevents TRAF6 activation. When TLR4 is activated, phosphorylation of sNASP releases CYLD from the TRAF6/sNASP complex, leading to TRAF6 autoubiquitination and the production of proinflammatory cytokines. To stop TRAF6 activation, a complex of sNASP, TRAF6, and CYLD is reformed once dephosphorylation of sNASP occurs by protein phosphatase 4 (PP4). Silencing sNASP negated the inhibitory effects of CYLD on interleukin (IL)-6 and TNF-α production after lipopolysaccharide (LPS) treatment. Similarly, the absence of CYLD also reduced PP4's negatively regulated production of proinflammatory cytokines, indicating that phosphorylation is crucial for the interaction between sNASP and CYLD as well as TRAF6 activation. Finally, mice infected with a recombinant adenovirus carrying the CYLD gene (Ad-CYLD WT), but not a mutation, showed significant reductions in cecal ligation and puncture (CLP)-mediated lung injury and proinflammatory cytokine production. In conclusion, CYLD alleviated sepsis-induced inflammation by interacting with the TRAF6/sNASP axis. These findings suggest that CYLD could be a potential therapeutic target for treating sepsis-induced ALI.

A computational approach for structural and functional analyses of disease-associated mutations in the human CYLD gene

Tumor suppressor cylindromatosis protein (CYLD) regulates NF-κB and JNK signaling pathways by cleaving K63-linked poly-ubiquitin chain from its substrate molecules and thus preventing the progression of tumorigenesis and metastasis of the cancer cells. Mutations in CYLD can cause aberrant structure and abnormal functionality leading to tumor formation. In this study, we utilized several computational tools such as PANTHER, PROVEAN, PredictSNP, PolyPhen-2, PhD-SNP, PON-P2, and SIFT to find out deleterious nsSNPs. We also highlighted the damaging impact of those deleterious nsSNPs on the structure and function of the CYLD utilizing ConSurf, I-Mutant, SDM, Phyre2, HOPE, Swiss-PdbViewer, and Mutation 3D. We shortlisted 18 high-risk nsSNPs from a total of 446 nsSNPs recorded in the NCBI database. Based on the conservation profile, stability status, and structural impact analysis, we finalized 13 nsSNPs. Molecular docking analysis and molecular dynamic simulation concluded the study with the findings of two significant nsSNPs (R830K, H827R) which have a remarkable impact on binding affinity, RMSD, RMSF, radius of gyration, and hydrogen bond formation during CYLD-ubiquitin interaction. The principal component analysis compared native and two mutants R830K and H827R of CYLD that signify structural and energy profile fluctuations during molecular dynamic (MD) simulation. Finally, the protein-protein interaction network showed CYLD interacts with 20 proteins involved in several biological pathways that mutations can impair. Considering all these in silico analyses, our study recommended conducting large-scale association studies of nsSNPs of CYLD with cancer as well as designing precise medications against diseases associated with these polymorphisms.

The Potential of Cylindromatosis (CYLD) as a Therapeutic Target in Oxidative Stress-Associated Pathologies: A Comprehensive Evaluation

Oxidative stress (OS) arises as a consequence of an imbalance between the formation of reactive oxygen species (ROS) and the capacity of antioxidant defense mechanisms to neutralize them. Excessive ROS production can lead to the damage of critical biomolecules, such as lipids, proteins, and DNA, ultimately contributing to the onset and progression of a multitude of diseases, including atherosclerosis, chronic obstructive pulmonary disease, Alzheimer's disease, and cancer. Cylindromatosis (CYLD), initially identified as a gene linked to familial cylindromatosis, has a well-established and increasingly well-characterized function in tumor inhibition and anti-inflammatory processes. Nevertheless, burgeoning evidence suggests that CYLD, as a conserved deubiquitination enzyme, also plays a pivotal role in various key signaling pathways and is implicated in the pathogenesis of numerous diseases driven by oxidative stress. In this review, we systematically examine the current research on the function and pathogenesis of CYLD in diseases instigated by oxidative stress. Therapeutic interventions targeting CYLD may hold significant promise for the treatment and management of oxidative stress-induced human diseases.

CYLD mediates human pulmonary artery smooth muscle cell dysfunction in congenital heart disease-associated pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a common complication of congenital heart disease (CHD). Deubiquitinase cylindromatosis (CYLD) has been reported to significantly aggravate vascular smooth muscle cell (VSMC) phenotypic transformation, proliferation, and migration. Here, we aimed to further investigate its roles and underlying mechanisms in the CHD-PAH development. The expression of CYLD in the lung tissues from CHD-PAH patients and monocrotaline (MCT) plus aortocaval (AV)-induced PAH rats, pulmonary artery smooth muscle cells (PASMCs) from MCT-AV-induced PAH rats, and human PASMCs (HPASMCs) was evaluated. After infection with CYLD siRNA or pcNDA3.1-CYLD, the proliferation, migration, and apoptosis of HPASMCs were measured using an EdU assay, transwell and scratch wound healing assays, and flow cytometric assay, respectively. An adeno-associated virus (AAV) vector encoding CYLD was used to suppress CYLD expression by being intratracheally instilled in rats 7 days before MCT-AV treatment. The results showed that CYLD was increased in the lung tissues from CHD-PAH patients and MCT-AV-induced PAH rats, and in PASMCs from MCT-AV-induced PAH rats. The contractile-type HPASMCs expressed low levels of CYLD, while the proliferative synthetic-type HPASMCs expressed high levels of CYLD. In addition, CYLD could mediate HPASMC dysfunction, which regulated HPASMC phenotypic transformation and proliferation via the modulation of p38 and ERK activation, while CYLD regulated HPASMC migration via the modulation of p38 activation. In vivo results demonstrated that the local suppression of CYLD expression could attenuate the increased levels of PAH and its associated pulmonary vascular remodeling in MCT-AV-induced PAH rats. Collectively, these results indicated that CYLD might be a potential novel therapeutic target for the prevention of PAH and pulmonary vascular remodeling in CHD-PAH through the modulation of HPASMC dysfunction.

The Genetic Dissection of Ace2 Expression Variation in the Heart of Murine Genetic Reference Population

Background: A high inflammatory and cytokine burden that induces vascular inflammation, myocarditis, cardiac arrhythmias, and myocardial injury is associated with a lethal outcome in COVID-19. The SARS-CoV-2 virus utilizes the ACE2 receptor for cell entry in a similar way to SARS-CoV. This study investigates the regulation, gene network, and associated pathways of ACE2 that may be involved in inflammatory and cardiovascular complications of COVID-19. Methods: Cardiovascular traits were determined in the one of the largest mouse genetic reference populations: BXD recombinant inbred strains using blood pressure, electrocardiography, and echocardiography measurements. Expression quantitative trait locus (eQTL) mapping, genetic correlation, and functional enrichment analysis were used to identify Ace2 regulation, gene pathway, and co-expression networks. Results: A wide range of variation was found in expression of Ace2 among the BXD strains. Levels of Ace2 expression are negatively correlated with cardiovascular traits, including systolic and diastolic blood pressure and P wave duration and amplitude. Ace2 co-expressed genes are significantly involved in cardiac- and inflammatory-related pathways. The eQTL mapping revealed that Cyld is a candidate upstream regulator for Ace2. Moreover, the protein-protein interaction (PPI) network analysis inferred several potential key regulators (Cul3, Atf2, Vcp, Jun, Ppp1cc, Npm1, Mapk8, Set, Dlg1, Mapk14, and Hspa1b) for Ace2 co-expressed genes in the heart. Conclusions: Ace2 is associated with blood pressure, atrial morphology, and sinoatrial conduction in BXD mice. Ace2 co-varies with Atf2, Cyld, Jun, Mapk8, and Mapk14 and is enriched in the RAS, TGFβ, TNFα, and p38α signaling pathways, involved in inflammation and cardiac damage. We suggest that all these novel Ace2-associated genes and pathways may be targeted for preventive, diagnostic, and therapeutic purposes in cardiovascular damage in patients with systemic inflammation, including COVID-19 patients.

The inhibitory effect of the deubiquitinase cylindromatosis (CYLD) on inflammatory responses in human gingival fibroblasts

OBJECTIVE

Experiments were performed to evaluate CYLD expression in human gingival tissue samples and to examine the effects of CYLD on inflammatory responses in lipopolysaccharide (LPS)- or TNF-α-stimulated human gingival fibroblasts (HGFs).

METHODS

Immunohistochemistry for CYLD and p65 expression was performed with healthy and inflamed gingival tissue samples. siRNA was used to knock down the expression of CYLD in HGFs. Upon LPS or TNF-α stimulation, NF-κB activation was detected in control and CYLD-knockdown HGFs. RT-PCR was applied to determine gene expression. Western blot analyses were employed to assess protein expression. Immunofluorescence staining was carried out to evaluate the nuclear translocation of p65.

RESULTS

Immunohistochemical staining showed the expression of CYLD in human gingival tissues. In addition, CYLD protein expression was reduced in inflamed gingival tissue samples compared with healthy tissue samples. CYLD knockdown greatly enhanced the mRNA expression of proinflammatory cytokines in LPS- or TNF-α-stimulated HGFs. Furthermore, knocking down CYLD expression increased LPS-stimulated NF-κB activation in HGFs. Unexpectedly, CYLD knockdown did not affect TNF-α-induced NF-κB activation.

CONCLUSIONS

Our results suggest that CYLD participates in periodontal inflammatory responses by negatively regulating LPS-induced NF-κB signalling.

USP20 (Ubiquitin-Specific Protease 20) Inhibits TNF (Tumor Necrosis Factor)-Triggered Smooth Muscle Cell Inflammation and Attenuates Atherosclerosis

Objective- Signaling that activates NFκB (nuclear factor κB) in smooth muscle cells (SMCs) is integral to atherosclerosis and involves reversible ubiquitination that activates proteins downstream of proatherogenic receptors. Deubiquitination of these proteins is mediated by USP20 (ubiquitin-specific protease 20), among other deubiquitinases. We sought to determine whether USP20 activity in SMCs decreases atherosclerosis. Approach and Results- To address this question, we used male Ldlr^-/- mice without (control) or with SMC-specific expression of murine USP20 (SMC-USP20-transgenic) or its dominant-negative (DN; C154S/H643Q) mutant (SMC-DN-USP20-transgenic). Before the appearance of intimal macrophages, NFκB activation in aortic medial SMCs was greater in SMC-DN-USP20-transgenic than in control mice. After 16 weeks on a Western diet, SMC-DN-USP20-transgenic mice had 46% greater brachiocephalic artery atheroma area than control mice. Congruently, aortic atherosclerosis assessed en face was 21% greater than control in SMC-DN-USP20-transgenic mice and 13% less than control in SMC-USP20-transgenic mice. In response to TNF (tumor necrosis factor), SMCs from SMC-DN-USP20-transgenic mice showed ≈3-fold greater NFκB activation than control SMCs. Silencing USP20 in SMCs with siRNA (small interfering RNA) augmented NFκB activation by ≈50% in response to either TNF or IL-1β (interleukin-1β). Coimmunoprecipitation experiments revealed that USP20 associates with several components of the TNFR1 (TNF receptor-1) signaling pathway, including RIPK1 (receptor-interacting protein kinase 1), a critical checkpoint in TNF-induced NFκB activation and inflammation. TNF evoked ≈2-fold more RIPK1 ubiquitination in SMC-DN-USP20-transgenic than in control SMCs, and RIPK1 was deubiquitinated by purified USP20 in vitro. Conclusions- USP20 attenuates TNF- and IL-1β-evoked atherogenic signaling in SMCs, by deubiquitinating RIPK1, among other signaling intermediates.

CYLD deficiency exacerbates lipopolysaccharide (LPS)-induced pyroptosis in astrocytes of mice with sepsis

Sepsis is a clinical syndrome occurring in patients following infection or injury, and is a leading cause of mortality in the world. However, the pathogenesis that contributes to sepsis and its associated brain injury is still unclear. Cylindromatosis (CYLD) is integrally involved in various physiological processes, such as immune responses, inflammation and cell cycle control. In the study, in vitro and in vivo models with sepsis by lipopolysaccharide (LPS) were performed to investigate the role of CYLD in inflammasome and its related astrocyte (AST) pyroptosis. The in vitro results suggested that LPS treatment significantly resulted in inflammasome activation and pyroptosis in AST. CYLD expression was progressively degraded upon LPS incubation in AST. Importantly, CYLD knockout (KO) could further promote pyroptosis and accelerate histone release in LPS-stimulated AST. In addition, the in vivo experiments suggested that LPS treatment in mice enhanced Caspase-1 immunoreactivity and resulted in high expression of phosphorylated nuclear factor-κB p65 (NF-κB p65), interleukin-1β (IL-1β) and IL-18 in hippocampus tissue samples, which were markedly exacerbated by CYLD knockout. Moreover, mice with CYLD deficiency exhibited higher mortality rate induced by LPS treatment. In conclusion, these findings suggested that CYLD decrease might accelerate astrocyte activity by promoting pyroptosis to promote sepsis-associated brain injury, and CYLD might be a potential therapeutic target for the management of this disease.

Vitamin E δ-tocotrienol inhibits TNF-α-stimulated NF-κB activation by up-regulation of anti-inflammatory A20 via modulation of sphingolipid including elevation of intracellular dihydroceramides

Nuclear factor-κB (NF-κB) regulates inflammation and cell survival, and is considered a potential target for anti-inflammatory and anti-cancer therapy. δ-Tocotrienol (δTE), a vitamin E form, has been shown to inhibit NF-κB, but the mechanism underlying this action is not clear. In the present study, we show that δTE inhibited TNF-α-induced activation of NF-κB and LPS-stimulated IL-6 in a dose- and time-dependent manner in Raw 264.7 macrophages. δTE potently inhibited TNF-α-induced phosphorylation of transforming growth factor β-activated kinase 1 (TAK1), an upstream kinase essential for the activation of NF-κB. Interestingly, δTE significantly increased the expression of A20 and to a less extent, cylindromatosis (CYLD), both of which are inhibitors of NF-κB. The importance of induction of A20 in δTE's anti-NF-κB effect is validated in A20 knockout cells where δTE's inhibition of NF-κB was largely diminished. In pursuit of the cause for A20 induction, we found that δTE treatment caused rapid and persistent elevation of dihydroceramides, while decreased ceramides initially but increased ceramides during prolonged treatment. These changes of sphingolipids were accompanied by increased cellular stress markers. Importantly, δTE's induction of A20 and inhibition of NF-κB activation were partially counteracted by myriocin, a potent inhibitor of de novo synthesis of sphingolipids, indicating a critical role of sphingolipid modulation in δTE-mediated effects. Since dihydroceramide has been shown to induce A20 and inhibit NF-κB in RAW cells, we conclude that that δTE inhibits NF-κB activation by enhancing its negative regulator A20 as a result of modulating sphingolipids especially elevation of dihydroceramides.

Vascular Smooth Muscle Cells and Arterial Stiffening: Relevance in Development, Aging, and Disease

The cushioning function of large arteries encompasses distension during systole and recoil during diastole which transforms pulsatile flow into a steady flow in the microcirculation. Arterial stiffness, the inverse of distensibility, has been implicated in various etiologies of chronic common and monogenic cardiovascular diseases and is a major cause of morbidity and mortality globally. The first components that contribute to arterial stiffening are extracellular matrix (ECM) proteins that support the mechanical load, while the second important components are vascular smooth muscle cells (VSMCs), which not only regulate actomyosin interactions for contraction but mediate also mechanotransduction in cell-ECM homeostasis. Eventually, VSMC plasticity and signaling in both conductance and resistance arteries are highly relevant to the physiology of normal and early vascular aging. This review summarizes current concepts of central pressure and tensile pulsatile circumferential stress as key mechanical determinants of arterial wall remodeling, cell-ECM interactions depending mainly on the architecture of cytoskeletal proteins and focal adhesion, the large/small arteries cross-talk that gives rise to target organ damage, and inflammatory pathways leading to calcification or atherosclerosis. We further speculate on the contribution of cellular stiffness along the arterial tree to vascular wall stiffness. In addition, this review provides the latest advances in the identification of gene variants affecting arterial stiffening. Now that important hemodynamic and molecular mechanisms of arterial stiffness have been elucidated, and the complex interplay between ECM, cells, and sensors identified, further research should study their potential to halt or to reverse the development of arterial stiffness.

CYLD Deubiquitinates Nicotinamide Adenine Dinucleotide Phosphate Oxidase 4 Contributing to Adventitial Remodeling

OBJECTIVE

Transdifferentiation of adventitial fibroblasts (AFs) into myofibroblasts plays a critical role during the vascular remodeling that occurs during atherosclerosis, restenosis, and aortic aneurysm. The ubiquitination/deubiquitination regulatory system is essential for the quality control of proteins. The involvement of ubiquitination/deubiquitination during AF transdifferentiation remains largely unknown. In this study, we determined the role of cylindromatosis (CYLD), a deubiquitinase, in the process of AF differentiation and activation in vitro and in vivo.

APPROACH AND RESULTS

Transforming growth factor-β1 and homocysteine, 2 known inducers of AF transdifferentiation, greatly upregulated CYLD expression in a time- and dose-dependent manner. The silencing of CYLD significantly inhibited AF transdifferentiation and activation as evidenced by the expression of contractile proteins, the production of the proinflammatory cytokines MCP-1 (monocyte chemotactic protein 1) and IL-6 (interleukin-6), the deposition of extracellular matrix, and cell migration. We further asked whether CYLD mediates AF activation via the regulation of nicotinamide adenine dinucleotide phosphate oxidase 4 (Nox4) as it is an essential factor during AF transdifferentiation. Indeed, the silencing of CYLD repressed transforming growth factor-β1-induced and homocysteine-induced Nox4 upregulation and reactive oxygen species production, whereas Nox4 overexpression greatly rescued the inhibitory effect on AF activation by CYLD silencing. Most interestingly, transforming growth factor-β1 and homocysteine repressed Nox4 ubiquitination and prolonged the half-life of Nox4. Moreover, Nox4 was deubiquitinated via a direct interaction with the ubiquitin-specific protease domain of CYLD. In accordance, hyperhomocysteinemia significantly increased adventitial CYLD and Nox4 expression, promoted AF transdifferentiation, and aggravated CaPO₄-induced abdominal aortic aneurysm in mice. These effects were abolished in CYLD^-/- mice.

CONCLUSIONS

CYLD contributes to the transdifferentiation of AFs via deubiquitinating Nox4 and may play a role in vascular remodeling.

Neuroprotectant androst-3β, 5α, 6β-triol suppresses TNF-α-induced endothelial adhesion molecules expression and neutrophil adhesion to endothelial cells by attenuation of CYLD-NF-κB pathway

Neuroinflammation is one of key pathologic element in neurological diseases including stroke, traumatic brain injury, Alzheimer' s Disease, Parkinson's Disease, and multiple sclerosis as well. Up-regulation of endothelial adhesion molecules, which facilitate leukocyte adhesion to the endothelium, is the vital process of endothelial cells mediated neuroinflammation. Androst-3β, 5α, 6β-triol (Triol) is a synthetic steroid which has been reported to have neuroprotective effects in hypoxia/re-oxygenation-induced neuronal injury model. In the present study, we firstly investigated whether Triol inhibited the TNF-α-induced inflammatory response in rat brain microvascular endothelial cells (RBMECs). Our data showed that Triol decreased TNF-α-induced expression of vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) and the adhesion of neutrophil to RBMECs. We also found that Triol inhibited TNF-α-induced degradation of IκBα and phosphorylation of NF-κBp65 that are required for NF-κB activation. Furthermore, Triol significantly reversed TNF-α-induced down-expression of CYLD, which is a deubiquitinase that negatively regulates activation of NF-κB. These results suggest that Triol displays an anti-inflammatory effect on TNF-α-induced RBMECs via downregulating of CYLD-NF-κB signaling pathways and might have a potential benefit in therapeutic neuroinflammation related diseases.

CYLD Promotes TNF-α-Induced Cell Necrosis Mediated by RIP-1 in Human Lung Cancer Cells

Lung cancer is one of the most common cancers in the world. Cylindromatosis (CYLD) is a deubiquitination enzyme and contributes to the degradation of ubiquitin chains on RIP1. The aim of the present study is to investigate the levels of CYLD in lung cancer patients and explore the molecular mechanism of CYLD in the lung cancer pathogenesis. The levels of CYLD were detected in human lung cancer tissues and the paired paracarcinoma tissues by real-time PCR and western blotting analysis. The proliferation of human lung cancer cells was determined by MTT assay. Cell apoptosis and necrosis were determined by FACS assay. The results demonstrated that low levels of CYLD were detected in clinical lung carcinoma specimens. Three pairs of siRNA were used to knock down the endogenous CYLD in lung cancer cells. Knockdown of CYLD promoted cell proliferation of lung cancer cells. Otherwise overexpression of CYLD induced TNF-α-induced cell death in A549 cells and H460 cells. Moreover, CYLD-overexpressed lung cancer cells were treated with 10 μM of z-VAD-fmk for 12 hours and the result revealed that TNF-α-induced cell necrosis was significantly enhanced. Additionally, TNF-α-induced cell necrosis in CYLD-overexpressed H460 cells was mediated by receptor-interacting protein 1 (RIP-1) kinase. Our findings suggested that CYLD was a potential target for the therapy of human lung cancers.

The TRAF-interacting protein (TRAIP) is a novel E2F target with peak expression in mitosis

The TRAF-interacting protein (TRAIP) is an E3 ubiquitin ligase required for cell proliferation. TRAIP mRNA is downregulated in human keratinocytes after inhibition of the PI3K/AKT/mTOR signaling. Since E2F transcription factors are downstream of PI3K/AKT/mTOR we investigated whether they regulate TRAIP expression. E2F1 expression significantly increased the TRAIP mRNA level in HeLa cells. Reporter assays with the 1400bp 5′-upstream promoter in HeLa cells and human keratinocytes showed that E2F1-, E2F2- and E2F4-induced upregulation of TRAIP expression is mediated by 168bp upstream of the translation start site. Mutating the E2F binding site within this fragment reduced the E2F1- and E2F2-dependent promoter activities and protein-DNA complex formation in gel shift assays. Abundance of TRAIP mRNA and protein was regulated by the cell cycle with a peak in G2/M. Expression of GFP and TRAIP-GFP demonstrated that TRAIP-GFP protein has a lower steady-state concentration than GFP despite similar mRNA levels. Cycloheximide inhibition experiments indicated that the TRAIP protein has a half-life of around four hours. Therefore, the combination of cell cycle-dependent transcription of the TRAIP gene by E2F and rapid protein degradation leads to cell cycle-dependent expression with a maximum in G2/M. These findings suggest that TRAIP has important functions in mitosis and tumorigenesis.

CYLD-mediated signaling and diseases

The conserved cylindromatosis (CYLD) codes for a deubiquitinating enzyme and is a crucial regulator of diverse cellular processes such as immune responses, inflammation, death, and proliferation. It directly regulates multiple key signaling cascades, such as the Nuclear Factor kappa B [NFkB] and the Mitogen-Activated Protein Kinase (MAPK) pathways, by its catalytic activity on polyubiquitinated key intermediates. Several lines of emerging evidence have linked CYLD to the pathogenesis of various maladies, including cancer, poor infection control, lung fibrosis, neural development, and now cardiovascular dysfunction. While CYLD-mediated signaling is cell type and stimuli specific, the activity of CYLD is tightly controlled by phosphorylation and other regulators such as Snail. This review explores a broad selection of current and past literature regarding CYLD's expression, function and regulation with emerging reports on its role in cardiovascular disease.

Dysfunction of the ubiquitin-proteasome system in atherosclerotic cardiovascular disease

The ubiquitin-proteasome system (UPS) is an integral part of the protein metabolism and protein quality control in eukaryotic cells. It is involved in a number of biological processes of significance for vascular biology and pathology such as oxidative stress, inflammation, foam cell formation, and apoptosis. This review summarizes both indirect and direct lines of evidence for a role of the UPS in atherosclerosis from the initiation to the progression and complication stage and concludes with a future perspective.

Regulation of NF-κB by deubiquitinases

The nuclear factor-κB (NF-κB) pathway is a critical regulator of innate and adaptive immunity. Noncanonical K63-linked polyubiquitination plays a key regulatory role in NF-κB signaling pathways by functioning as a scaffold to recruit kinase complexes containing ubiquitin-binding domains. Ubiquitination is balanced by deubiquitinases that cleave polyubiquitin chains and oppose the function of E3 ubiquitin ligases. Deubiquitinases therefore play an important role in the termination of NF-κB signaling and the resolution of inflammation. In this review, we focus on NF-κB regulation by deubiquitinases with an emphasis on A20 and CYLD. Deubiquitinases and the ubiquitin/proteasome components that regulate NF-κB may serve as novel therapeutic targets for inflammatory diseases and cancer.

Physiological and pathological role of the ubiquitin-proteasome system in the vascular smooth muscle cell

Vascular smooth muscle cell (VSMC) plasticity implies a capacity for rapid change and adaptability through processes requiring protein turnover. The ubiquitin-proteasome system (UPS) is at the core of protein turnover as the main pathway for the degradation of proteins related to cell-cycle regulation, signalling, apoptosis, and differentiation. This review briefly addresses some structural UPS aspects under the perspective of VSMC (patho)biology. The UPS loss-of-function promotes direct cell effects and many indirect effects related to the adaptation to apoptosis/survival signalling, oxidative stress, and endoplasmic reticulum stress. The UPS regulates redox homeostasis and is redox-regulated. Also, the UPS closely interacts with endoplasmic reticulum (ER) homeostasis as the effector of un/misfolded protein degradation, and ER stress is strongly involved in atherosclerosis. Inhibition of cell cycle-controlling ubiquitin ligases or the proteasome reduces VSMC proliferation and prevents modulation of their synthetic phenotype. Proteasome inhibition also strongly promotes VSMC apoptosis and reduces neointima. In atherosclerosis models, proteasome inhibitors display vasculoprotective effects and reduce inflammation. However, worsening of atherosclerosis or vascular dysfunction has also been reported. Proteasome inhibitors sensitize VSMC to increased ER stress-mediated cell death and suppress unfolded protein response signalling. Taken together, these observations show that the UPS has powerful effects in the control of VSMC phenotype and survival signalling. However, more profound knowledge of mechanisms is needed in order to render the UPS an operational therapeutic target.

Inflammation, endoplasmic reticulum stress, autophagy, and the monocyte chemoattractant protein-1/CCR2 pathway

Numerous inflammatory cytokines have been implicated in the pathogenesis of cardiovascular diseases. Monocyte chemoattractant protein (MCP)-1/CCL2 is expressed by mainly inflammatory cells and stromal cells such as endothelial cells, and its expression is upregulated after proinflammatory stimuli and tissue injury. MCP-1 can function as a traditional chemotactic cytokine and also regulates gene transcription. The recently discovered novel zinc-finger protein, called MCPIP (MCP-1-induced protein), initiates a series of signaling events that causes oxidative and endoplasmic reticulum (ER) stress, leading to autophagy that can result in cell death or differentiation, depending on the cellular context. After a brief review of the basic processes involved in inflammation, ER stress, and autophagy, the recently elucidated role of MCP-1 and MCPIP in inflammatory diseases is reviewed. MCPIP was found to be able to control inflammatory response by inhibition of nuclear factor-κB activation through its deubiquitinase activity or by degradation of mRNA encoding a set of inflammatory cytokines through its RNase activity. The potential inclusion of such a novel deubiquitinase in the emerging anti-inflammatory strategies for the treatment of inflammation-related diseases such as cardiovascular diseases and type 2 diabetes is briefly discussed.

A pro-inflammatory role of deubiquitinating enzyme cylindromatosis (CYLD) in vascular smooth muscle cells

CYLD, a deubiquitinating enzyme (DUB), is a critical regulator of diverse cellular processes, ranging from proliferation and differentiation to inflammatory responses, via regulating multiple key signaling cascades such as nuclear factor kappa B (NF-κB) pathway. CYLD has been shown to inhibit vascular lesion formation presumably through suppressing NF-κB activity in vascular cells. However, herein we report a novel role of CYLD in mediating pro-inflammatory responses in vascular smooth muscle cells (VSMCs) via a mechanism independent of NF-κB activity. Adenoviral knockdown of Cyld inhibited basal and the tumor necrosis factor alpha (TNFα)-induced mRNA expression of pro-inflammatory cytokines including monocyte chemotactic protein-1 (Mcp-1), intercellular adhesion molecule (Icam-1) and interleukin-6 (Il-6) in rat adult aortic SMCs (RASMCs). The CYLD deficiency led to increases in the basal NF-κB transcriptional activity in RASMCs; however, did not affect the TNFα-induced NF-κB activity. Intriguingly, the TNFα-induced IκB phosphorylation was enhanced in the CYLD deficient RASMCs. While knocking down of Cyld decreased slightly the basal expression levels of IκBα and IκBβ proteins, it did not alter the kinetics of TNFα-induced IκB protein degradation in RASMCs. These results indicate that CYLD suppresses the basal NF-κB activity and TNFα-induced IκB kinase activation without affecting TNFα-induced NF-κB activity in VSMCs. In addition, knocking down of Cyld suppressed TNFα-induced activation of mitogen activated protein kinases (MAPKs) including extracellular signal-activated kinases (ERK), c-Jun N-terminal kinase (JNK), and p38 in RASMCs. TNFα-induced RASMC migration and monocyte adhesion to RASMCs were inhibited by the Cyld knockdown. Finally, immunochemical staining revealed a dramatic augment of CYLD expression in the injured coronary artery with neointimal hyperplasia. Taken together, our results uncover an unexpected role of CYLD in promoting inflammatory responses in VSMCs via a mechanism involving MAPK activation but independent of NF-κB activity, contributing to the pathogenesis of vascular disease.

The ubiquitin-proteasome system and cardiovascular disease

Over the past decade, the role of the ubiquitin-proteasome system (UPS) has been the subject of numerous studies to elucidate its role in cardiovascular physiology and pathophysiology. There have been many advances in this field including the use of proteomics to achieve a better understanding of how the cardiac proteasome is regulated. Moreover, improved methods for the assessment of UPS function and the development of genetic models to study the role of the UPS have led to the realization that often the function of this system deviates from the norm in many cardiovascular pathologies. Hence, dysfunction has been described in atherosclerosis, familial cardiac proteinopathies, idiopathic dilated cardiomyopathies, and myocardial ischemia. This has led to numerous studies of the ubiquitin protein (E3) ligases and their roles in cardiac physiology and pathophysiology. This has also led to the controversial proposition of treating atherosclerosis, cardiac hypertrophy, and myocardial ischemia with proteasome inhibitors. Furthering our knowledge of this system may help in the development of new UPS-based therapeutic modalities for mitigation of cardiovascular disease.

Sent to destroy: the ubiquitin proteasome system regulates cell signaling and protein quality control in cardiovascular development and disease

The ubiquitin proteasome system (UPS) plays a crucial role in biological processes integral to the development of the cardiovascular system and cardiovascular diseases. The UPS prototypically recognizes specific protein substrates and places polyubiquitin chains on them for subsequent destruction by the proteasome. This system is in place to degrade not only misfolded and damaged proteins, but is essential also in regulating a host of cell signaling pathways involved in proliferation, adaptation to stress, regulation of cell size, and cell death. During the development of the cardiovascular system, the UPS regulates cell signaling by modifying transcription factors, receptors, and structural proteins. Later, in the event of cardiovascular diseases as diverse as atherosclerosis, cardiac hypertrophy, and ischemia/reperfusion injury, ubiquitin ligases and the proteasome are implicated in protecting and exacerbating clinical outcomes. However, when misfolded and damaged proteins are ubiquitinated by the UPS, their destruction by the proteasome is not always possible because of their aggregated confirmations. Recent studies have discovered how these ubiquitinated misfolded proteins can be destroyed by alternative "specific" mechanisms. The cytosolic receptors p62, NBR, and histone deacetylase 6 recognize aggregated ubiquitinated proteins and target them for autophagy in the process of "selective autophagy." Even the ubiquitination of multiple proteins within whole organelles that drive the more general macro-autophagy may be due, in part, to similar ubiquitin-driven mechanisms. In summary, the crosstalk between the UPS and autophagy highlight the pivotal and diverse roles the UPS plays in maintaining protein quality control and regulating cardiovascular development and disease.

CYLD: a tumor suppressor deubiquitinase regulating NF-kappaB activation and diverse biological processes

Protein ubiquitination is a reversible reaction, in which the ubiquitin chains are deconjugated by a family of deubiquitinases (DUBs). The presence of a large number of DUBs suggests that they likely possess certain levels of substrate selectivity and functional specificity. Indeed, recent studies show that a tumor suppressor DUB, cylindromatosis (CYLD), has a predominant role in the regulation of NF-kappaB, a transcription factor that promotes cell survival and oncogenesis. NF-kappaB activation involves attachment of K63-linked ubiquitin chains to its upstream signaling factors, which is thought to facilitate protein-protein interactions in the assembly of signaling complexes. By deconjugating these K63-linked ubiquitin chains, CYLD negatively regulates NF-kappaB activation, which may contribute to its tumor suppressor function. CYLD also regulates diverse physiological processes, ranging from immune response and inflammation to cell cycle progression, spermatogenesis, and osteoclastogenesis. Interestingly, CYLD itself is subject to different mechanisms of regulation.

On to the road to degradation: atherosclerosis and the proteasome

Protein metabolism is a central element of every living cell. The ubiquitin-proteasome system (UPS) is an integral part of the protein metabolism machinery mediating post-transcriptional processing and degradation of the majority of intracellular proteins. Over the past few years, remarkable progress has been made in our understanding of the role of the UPS in vascular biology and pathobiology, particularly atherosclerosis. This review reflects on the recent developments from the effects on endothelial cells and the initial stage of atherosclerosis to the effects on vascular smooth muscle and the progression stage of atherosclerosis and finally to the effects on cell viability and the complication stage of atherosclerosis. It will conclude with the integration of the available information in a synoptic view of the involvement of the UPS in atherosclerosis.

Regulation of the endothelial cell cycle by the ubiquitin-proteasome system

Degradation of poly-ubiquitinated proteins by the 26S-proteasome complex represents a crucial quantitative control mechanism. The ubiquitin-proteasome system (UPS) plays a pivotal role in the complex molecular network regulating the progression both between and within each cell-cycle phase. Two major complexes are involved: the SKP1-CUL1-F-box-protein complex (SCF) and the anaphase-promoting complex/cyclosome (APC/C). Notwithstanding structural similarities, SCF and APC/C display different cellular functions and mechanisms of action. SCF modulates all cell-cycle stages and plays a prominent role at G1/S transition mainly through three regulatory subunits: Skp2, Fbw7, and beta-TRCP. APC/C, regulated by Cdc20 or Cdh1 subunits, has a crucial role in mitosis. In this review, we will describe how the endothelial cell cycle is regulated by the UPS. We will illustrate the principal SCF- and APC/C-dependent molecular mechanisms that modulate cell growth, allowing a unidirectional cell-cycle progression. Then, we will focus our attention on UPS modulation by oxidative stress, a pathogenic stimulus that causes endothelial dysfunction and is involved in numerous cardiovascular diseases.