Ubiquitin receptors and protein quality control

Metabolite Neu5Ac triggers SLC3A2 degradation promoting vascular endothelial ferroptosis and aggravates atherosclerosis progression in ApoE-/-mice

Background: Atherosclerosis (AS) is still the major cause of cardiovascular disease (CVD) as well as stroke. Endothelial metabolic disorder has been found to be activated and then promote endothelial cells (ECs) injury, which is regarded to initiate AS progression. N-acetylneuraminic acid (Neu5Ac), a metabolite produced by hexosamine-sialic acid pathway branching from glucose metabolism, was presented as a notable biomarker of CVD and is positively correlated with ECs function. However, few studies explain whether Neu5Ac regulate AS progression by affecting EC function as well as its involved mechanisms are still unknown. Methods: Here, we mimicked an animal model in ApoE-/- mice which displaying similar plasma Neu5Ac levels with AS model to investigate its effect on AS progression. Results: We found that Neu5Ac exacerbated plaques area and increased lipids in plasma in absence of HFD feeding, and ECs inflammatory injury was supposed as the triggering factor upon Neu5Ac treatment with increasing expression of IL-1β, ICAM-1, and promoting ability of monocyte adhesion to ECs. Mechanistic studies showed that Neu5Ac facilitated SLC3A2 binding to ubiquitin and then triggered P62 mediated degradation, further leading to accumulation of lipid peroxidation in ECs. Fer-1 could inhibit ECs injury and reverse AS progression induced by Neu5Ac in ApoE-/- mice. Interestingly, mitochondrial dysfunction was also partly participated in ECs injury after Neu5Ac treatment and been reversed by Fer-1. Conclusions: Together, our study unveils a new mechanism by which evaluated metabolite Neu5Ac could promote SLC3A2 associated endothelial ferroptosis to activate ECs injury and AS plaque progression, thus providing a new insight into the role of Neu5Ac-ferroptosis pathway in AS. Also, our research revealed that pharmacological inhibition of ferroptosis may provide a novel therapeutic strategy for premature AS.

PSMD4 drives progression of hepatocellular carcinoma via Akt/COX2 pathway and p53 inhibition

The ubiquitin-dependent proteolytic pathway is crucial for cellular regulation, including control of the cell cycle, differentiation, and apoptosis. Proteasome 26S Subunit Ubiquitin Receptor, Non-ATPase 4, (PSMD4) is a member of the ubiquitin proteasome family that is upregulated in multiple solid tumors, including hepatocellular carcinoma (HCC), and the existence of PSMD4 is associated with unfavorable prognosis. In this study, transcriptome sequencing of HCC tissues and non-tumor hepatic tissues from the public database Cancer Genome Atlas (TGCA) revealed a high expression of PSMD4. Additionally, PSMD4 loss in HCC cells suppressed the tumor development in mouse xenograft model. PSMD4, which is maintained by inflammatory factors secreted from tumor matrix cells, positively mediates cell growth and is associated with Akt/GSK-3β/ cyclooxygenase2 (COX2) pathway activation, inhibition of p53 promoter activity, and increased p53 degradation. However, the domain without the C-terminus (VWA+UIM1/2) sustained the activation of p53 transcription. Thus, our findings suggest that PSMD4 is involved in HCC tumor growth through COX2 expression and p53 downregulation. Therapeutic strategies targeting PSMD4 and its downstream effectors could be used for the treatment of PSMD4-abundant HCC patients.

MicroRNA-183-5p regulates TAR DNA-binding protein 43 neurotoxicity via SQSTM1/p62 in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that selectively attacks motor neurons, and leads to progressive muscle weakness and death. A common pathological feature is the misfolding, aggregation, and cytoplasmic mislocalization of TAR DNA-binding protein 43 (TDP-43) proteins in more than 95% of ALS patients, suggesting a universal role TDP-43 proteinopathy in ALS. Mutations in SQSTM1/p62 have been identified in familial and sporadic cases of ALS. MicroRNAs (miRNAs) are small non-coding RNAs that post-transcriptionally regulate their target genes. Emerging evidence indicates that miRNA dysregulation is associated with neuronal toxicity and mitochondrial dysfunction, and also plays a pivotal role in ALS pathogenesis. Here, we report the first evidence that miR-183-5p is aberrantly upregulated in spinal cords of patients with ALS. Using luciferase reporter assays and miR-183-5p agomirs, we demonstrate that miR-183-5p regulates the SQSTM1/p62 3'-untranslated region to suppress expression. A miR-183-5p agomir attenuated SOSTM1/p62 expression and led to an increase in TDP-43 protein levels in neuronal and non-neuronal cells. In contrast, a miR-183-5p antagomir decreased TDP-43 but increased SQSTM1/p62 protein levels. The antagomir repressed formation of stress granules and aggregated TDP43 protein in neuronal cells under stress-induced conditions and protected against cytotoxicity. Knockdown of SQSTM1/p62 decreased total ubiquitination and increased TDP-43 protein aggregation, indicating that SQSTM1/p62 may play a protective role in cells. In summary, our study reveals a novel mechanism of TDP-43 proteinopathy mediated by the miR-183-5p and provides a molecular link between aberrant RNA processing and protein degradation, two major pillars in ALS pathogenesis.

The evolving views of the simplest pectic polysaccharides: homogalacturonan

Pectin is an important component of cell wall polysaccharides and is important for normal plant growth and development. As a major component of pectin in the primary cell wall, homogalacturonan (HG) is a long-chain macromolecular polysaccharide composed of repeated α-1,4-D-GalA sugar units. At the same time, HG is synthesized in the Golgi apparatus in the form of methyl esterification and acetylation. It is then secreted into the plasmodesmata, where it is usually demethylated by pectin methyl esterase (PME) and deacetylated by pectin acetylase (PAE). The synthesis and modification of HG are involved in polysaccharide metabolism in the cell wall, which affects the structure and function of the cell wall and plays an important role in plant growth and development. This paper mainly summarizes the recent research on the biosynthesis, modification and the roles of HG in plant cell wall.

CFTR Rescue by Lumacaftor (VX-809) Induces an Extensive Reorganization of Mitochondria in the Cystic Fibrosis Bronchial Epithelium

BACKGROUND

Cystic Fibrosis (CF) is a genetic disorder affecting around 1 in every 3000 newborns. In the most common mutation, F508del, the defective anion channel, CFTR, is prevented from reaching the plasma membrane (PM) by the quality check control of the cell. Little is known about how CFTR pharmacological rescue impacts the cell proteome.

METHODS

We used high-resolution mass spectrometry, differential ultracentrifugation, machine learning and bioinformatics to investigate both changes in the expression and localization of the human bronchial epithelium CF model (F508del-CFTR CFBE41o-) proteome following treatment with VX-809 (Lumacaftor), a drug able to improve the trafficking of CFTR.

RESULTS

The data suggested no stark changes in protein expression, yet subtle localization changes of proteins of the mitochondria and peroxisomes were detected. We then used high-content confocal microscopy to further investigate the morphological and compositional changes of peroxisomes and mitochondria under these conditions, as well as in patient-derived primary cells. We profiled several thousand proteins and we determined the subcellular localization data for around 5000 of them using the LOPIT-DC spatial proteomics protocol.

CONCLUSIONS

We observed that treatment with VX-809 induces extensive structural and functional remodelling of mitochondria and peroxisomes that resemble the phenotype of healthy cells. Our data suggest additional rescue mechanisms of VX-809 beyond the correction of aberrant folding of F508del-CFTR and subsequent trafficking to the PM.

Coenzyme Q10 Reduces Infarct Size in Animal Models of Myocardial Ischemia-Reperfusion Injury: A Meta-Analysis and Summary of Underlying Mechanisms

Objective

Effective interventions that might limit myocardial ischemia-reperfusion (I/R) injury are still lacking. Coenzyme Q₁₀ (CoQ₁₀) may exert cardioprotective actions that reduce myocardial I/R injury. We conducted this meta-analysis to assess the potential cardioprotective effect of CoQ₁₀ in animal models of myocardial I/R injury.

Methods

We searched PubMed and Embase databases from inception to February 2022 to identify animal studies that compared the effect of CoQ₁₀ with vehicle treatment or no treatment on myocardial infarct size in models of myocardial I/R injury. Means and standard deviations of the infarct size measurements were pooled as the weighted mean difference with 95% confidence interval (CI) using the random-effects model. Subgroup analyses were also conducted according to animals' species, models' type, and reperfusion time.

Results

Six animal studies (4 in vivo and 2 ex vivo) with 116 animals were included. Pooled analysis suggested that CoQ₁₀ significantly reduced myocardial infarct size by −11.36% (95% CI: −16.82, −5.90, p < 0.0001, I² = 94%) compared with the control group. The significance of the pooled effect estimate was maintained in rats, Hartley guinea pigs, and Yorkshire pigs. However, it became insignificant in the subgroup of rabbits −5.29% (95% CI: −27.83, 17.26; I² = 87%). Furthermore, CoQ₁₀ significantly reduced the myocardial infarct size regardless of model type (either in vivo or ex vivo) and reperfusion time (either ≤ 4 h or >4 h).

Conclusion

Coenzyme Q₁₀ significantly decreased myocardial infarct size by 11.36% compared with the control group in animal models of myocardial I/R injury. This beneficial action was retained regardless of model type and reperfusion time.

Ubiquitin Carboxyl-Terminal Hydrolase L1 of Cardiomyocytes Promotes Macroautophagy and Proteostasis and Protects Against Post-myocardial Infarction Cardiac Remodeling and Heart Failure

Ubiquitin carboxyl-terminal hydrolase L1 (UCHL1) is a deubiquitinase known to play essential roles in the nervous tissue. Myocardial upregulation of UCHL1 was observed in human dilated cardiomyopathy and several animal models of heart disease, but the (patho)physiological significance of UCHL1 in cardiomyocytes remains undefined. Hence, we conducted this study to fill this critical gap. We produced cardiomyocyte-restricted Uchl1 knockout (CKO) by coupling the Uchl1-floxed allele with transgenic Myh6-Cre in C57B/6J inbred mice. Mice transgenic for Myh6-Cre were used as controls (CTL). Myocardial Uchl1 proteins were markedly reduced in CKO mice but they did not display discernible abnormal phenotype. Ten-week old CTL or CKO mice were subjected to left anterior descending artery ligation (myocardial infarction, MI) or sham surgery (Sham) and characterized at 7- and 28-day after surgery. Compared with Sham mice, significant increases in myocardial UCHL1 proteins were detected in CTL MI but not in CKO MI mice. MI-induced left ventricular (LV) chamber dilation, reduction of ejection fraction (EF) and fractional shortening (FS), and LV anterior wall thinning detected by echocardiography were comparable between the CTL MI and CKO MI groups 7-day post-MI. However, by 28-day post-MI, MI-induced LV chamber dilatation, EF and FS reduction, increases of myocardial ubiquitin conjugates, and increases in the heart weight to body weight ratio and the ventricular weight to body weight ratio were significantly more pronounced in CKO MI than CTL MI mice. As further revealed by LV pressure-volume relationship analyses, CKO MI mice but not CTL MI mice displayed significant decreases in stroke volume, cardiac output, and the maximum rates of LV pressure rising or declining and of LV volume declining, as well as significant increases in LV end-diastolic pressure and Tau, compared with their respective Sham controls. LC3-II flux assays reveal that autophagic flux is decreased in CKO mouse myocardium as well as in cultured Uchl1-deficient cardiomyocytes. In conclusion, UCHL1 of cardiomyocytes is dispensable for development but promotes macroautophagy in cardiomyocytes. Upregulation of UCHL1 in post-MI hearts occurs primarily in the cardiomyocytes and protects against post-MI cardiac remodeling and malfunction likely through supporting autophagic flux and proteostasis during a stress condition.

Proteolysis-targeting chimeras and their implications in breast cancer

Breast cancer (BC) is a highly heterogeneous neoplasm of the mammary tissue, causing the deaths of a large number of women worldwide. Nearly 70% and 20% of BC cases are estrogen receptor alpha positive (ERα+) and human epidermal growth factor receptor 2-positive (HER2+), respectively; therefore, ER and HER2 targeted therapies have been employed in BC treatment. However, resistance to these therapies has been reported, indicating a need for developing novel therapeutic strategies. Proteolysis-targeting chimeras (PROTACs) are new, promising therapeutic tools designed with a bimodular structure: one module allows specific binding to target proteins, and the other module allows efficient degradation of these target proteins. In this paper, PROTACs and their potential in controlling the progression of ERα and HER2+ BC are discussed.

P62 Links the Autophagy Pathway and the Ubiquitin-Proteasome System in Endothelial Cells during Atherosclerosis

Among autophagy-related molecules, p62/SQSTM1 is an adaptor for identifying and delivering intracellular cargo for degradation. Since ubiquitination is reversible, it has a switch role in autophagy. Ubiquitination is also involved in regulating autophagy in a timely manner. This study aimed to elucidate how p62-mediated autophagy is regulated in human endothelial cells and macrophages under atherosclerotic conditions, focusing on the lysosomal and proteasomal pathways. Co-cultured HUVECs and THP-1 cells were exposed to oxLDL (50 μg/mL) and autophagy was assessed. To downregulate p62, siRNA was administered, and the E3 ligases were inhibited by Heclin or MLN4924 treatment under the condition that cellular inflammatory processes were stimulated by oxLDL simultaneously initiated autophagy. Downregulating p62 induced an alternative degradation system, and the E3 ligases were found to be involved in the progression of atherosclerosis. Collectively, the present study demonstrated that the endothelial lipid accumulation under atherosclerotic conditions was caused by lysosomal dysfunction associated with autophagy.

(-)-Epigallocatechin-3-O-gallate at a high concentration may induce lipolysis via ATP consumption by activation of stress defense mechanisms

Green tea catechins have thus far been demonstrated to have antiobesity effects in a variety of experimental models. However, upstream molecular events triggering those phenomena remain to be identified. In this study, we found that (-)-epigallocatechin-3-O-gallate (EGCG) promoted lipolysis in lipid-loaded Huh7 human hepatoma cells. Notably, EGCG at a high concentration induced both oxidative stress and protein stress (proteo-stress), leading to activation of stress defense mechanisms, such as mRNA expressions of antioxidant and phase-2 detoxifying enzymes, and heat shock proteins. Furthermore, EGCG decreased the level of intracellular ATP, while glucose uptake from culture media was promoted possibly for energy homeostasis. EGCG also upregulated the expression of adipose triglyceride lipase, and activated AMP-activated protein kinase. Collectively, these results suggest that EGCG induces lipolysis to compensate for ATP reduction derived from activation of stress defense systems against its oxidative and proteo-stress properties.

The Effect of Shikonin on U87 Cells Through Notch2 Signaling Pathway and Its Mechanism

Background: The paper explored the inhibitory effect of Shikonin on Notch2 signaling pathway of U87 cells and elucidated the mechanism. Material and methods: CCK-8 was used to determine the viability of U87 cells. The Kit was used to detect the levels of ROS and GSH in the cells. After Annexin V-FITC/PI staining, flow cytometry was used to detect the effect of Shikonin on U87 cell apoptosis. Western Blotting was used to detect the expressions of Notch2, Notch3, Hes1 and Hey1. The levels of NH4Cl and MG132 were determined to measure the effect of Shikonin inhibiting Notch2 protein level in U87 cells, and the effect of Shikonin on Itch inhibiting Notch2 protein level. Results: Shikonin can inhibit the expressions of Notch2 and Notch3 proteins and the levels of downstream signaling molecules Hes1 and Hey1 in U87 cells, and in a concentration- and time dependent manner. Shikonin can promote the degradation of Notch2 via the lysosomal pathway, which is associated with the up-regulation of the Itch expression. The inhibition of Notch2 and cell viability is related to the levels of GSH and ROS in cells, and Shikonin can down-regulate Notch2 to inhibit the proliferation of U87 cells. Conclusion: Shikonin inhibits the malignancy of glioma cells by promoting the degradation of Notch2 through the lysosomal pathway, which is related to the antioxidant effect. The results of our experiments provided certain experimental and theoretical basis for Shikonin treating glioma.

Atractylenolide III alleviates the apoptosis through inhibition of autophagy by the mTOR-dependent pathway in alveolar macrophages of human silicosis

Silica-induced apoptosis of alveolar macrophages (AMs) is an essential part of silicosis formation. Autophagy tends to present a bidirectional effect on apoptosis. Our previous study found that the blockade of autophagy degradation might aggravate the apoptosis of AMs in human silicosis. We presume that targeting the autophagic pathway is regarded as a promising new strategy for silicosis fibrosis. As a main active component of the Atractylodes rhizome, Atractylenolide III (ATL-III) has been widely applied in clinical anti-inflammation. However, the effect and mechanism of ATL-III on autophagy in AMs of silicosis are unknown. In this study, we found that ATL-III might inhibit autophagy by mTOR-dependent manner, thereby improving the blockage of autophagic degradation in AMs. ATL-III alleviated the apoptosis of AMs in human silicosis. Furthermore, Rapamycin reversed the protective effect of ATL-III in AMs. These results indicate that ATL-III may be a potentially protective ingredient targeting autophagy for workers exposed to silica dust. These findings also suggest that inhibition of autophagy may be an effective way to alleviate the apoptosis of AMs in silicosis.

Clemen C, Eichinger L. Functional Characterisation of the Autophagy ATG12~5/16 Complex in Dictyostelium discoideum

Macroautophagy, a highly conserved and complex intracellular degradative pathway, involves more than 20 core autophagy (ATG) proteins, among them the hexameric ATG12~5/16 complex, which is part of the essential ubiquitin-like conjugation systems in autophagy. Dictyostelium discoideumatg5 single, atg5/12 double, and atg5/12/16 triple gene knock-out mutant strains displayed similar defects in the conjugation of ATG8 to phosphatidylethanolamine, development, and cell viability upon nitrogen starvation. This implies that ATG5, 12 and 16 act as a functional unit in canonical autophagy. Macropinocytosis of TRITC dextran and phagocytosis of yeast were significantly decreased in ATG5¯ and ATG5¯/12¯ and even further in ATG5¯/12¯/16¯ cells. In contrast, plaque growth on Klebsiella aerogenes was about twice as fast for ATG5¯ and ATG5¯/12¯/16¯ cells in comparison to AX2, but strongly decreased for ATG5¯/12¯ cells. Along this line, phagocytic uptake of Escherichia coli was significantly reduced in ATG5¯/12¯ cells, while no difference in uptake, but a strong increase in membrane association of E. coli, was seen for ATG5¯ and ATG5¯/12¯/16¯ cells. Proteasomal activity was also disturbed in a complex fashion, consistent with an inhibitory activity of ATG16 in the absence of ATG5 and/or ATG12. Our results confirm the essential function of the ATG12~5/16 complex in canonical autophagy, and furthermore are consistent with autophagy-independent functions of the complex and its individual components. They also strongly support the placement of autophagy upstream of the ubiquitin-proteasome system (UPS), as a fully functional UPS depends on autophagy.

Identification of the autophagy pathway in a mollusk bivalve, Crassostrea gigas

The Pacific oyster, Crassostrea gigas, is a mollusk bivalve commercially important as a food source. Pacific oysters are subjected to stress and diseases during culture. The autophagy pathway is involved in numerous cellular processes, including responses to starvation, cell death, and microorganism elimination. Autophagy also exists in C. gigas, and plays a role in the immune response against infections. Although this process is well-documented and conserved in most animals, it is still poorly understood in mollusks. To date, no study has provided a complete overview of the molecular mechanism of autophagy in mollusk bivalves. In this study, human and yeast ATG protein sequences and public databases (Uniprot and NCBI) were used to identify protein members of the C. gigas autophagy pathway. A total of 35 autophagy related proteins were found in the Pacific oyster. RACE-PCR was performed on several genes. Using molecular (real-time PCR) and protein-based (western blot and immunohistochemistry) approaches, the expression and localization of ATG12, ATG9, BECN1, MAP1LC3, MTOR, and SQSTM1, was investigated in different tissues of the Pacific oyster. Comparison with human and yeast counterparts demonstrated a high homology with the human autophagy pathway. The results also demonstrated that the key autophagy genes and their protein products were expressed in all the analyzed tissues of C. gigas. This study allows the characterization of the complete C. gigas autophagy pathway for the first time.

Abbreviations: ATG: autophagy related; Atg1/ULK: unc-51 like autophagy activating kinase; ATG7: autophagy related 7; ATG9: autophagy related 9; ATG12: autophagy related 12; BECN1: beclin 1; BSA: bovine serum albumin; cDNA: complementary deoxyribonucleic acid; DNA: deoxyribonucleic acid; GABARAP: GABA type A receptor-associated protein; IHC: immunohistochemistry; MAP1LC3/LC3/Atg8: microtubule associated protein 1 light chain 3; MTOR: mechanistic target of rapamycin kinase; NCBI: national center for biotechnology information; ORF: open reading frame; PBS: phosphate-buffered saline; PCR: polymerase chain reaction; PtdIns3K: class III phosphatidylinositol 3-kinase; RACE-PCR: rapid amplification of cDNA-ends by polymerase chain reaction; RNA: ribonucleic acid; SQSTM1: sequestosome 1; Uniprot: universal protein resource; WIPI: WD repeat domain, phosphoinositide interacting.

N-acetylcysteine decreases malignant characteristics of glioblastoma cells by inhibiting Notch2 signaling

BACKGROUND

Glioblastomas multiforme (GBM) is the most devastating primary intracranial malignancy lacking effective clinical treatments. Notch2 has been established to be a prognostic marker and probably involved in GBM malignant progression. N-acetylcysteine (NAC), a precursor of intracellular glutathione (GSH), has been widely implicated in prevention and therapy of several cancers. However, the role of NAC in GBM remains unclear and the property of NAC independent of its antioxidation is largely unknown.

METHODS

The mRNA and protein levels of Notch family and other related factors were detected by RT-PCR and western blot, respectively. In addition, intracellular reactive oxygen species (ROS) was measured by flow cytometry-based DCFH-DA. Moreover, cell viability was assessed by CCK8 and cell cycle was analyzed by flow cytometry-based PI staining. The level of apoptosis was checked by flow cytometry-based Annexin V/PI. Cell migration and invasion were evaluated by wound healing and transwell invasion assays. At last, U87 Xenograft model was established to confirm whether NAC could restrain the growth of tumor.

RESULTS

Our data showed that NAC could decrease the protein level of Notch2. Meanwhile, NAC had a decreasing effect on the mRNA and protein levels of its downstream targets Hes1 and Hey1. These effects caused by NAC were independent of cellular GSH and ROS levels. The mechanism of NAC-mediated Notch2 reduction was elucidated by promoting Notch2 degradation through Itch-dependent lysosome pathway. Furthermore, NAC could prevent proliferation, migration, and invasion and might induce apoptosis in GBM cells via targeting Notch2. Significantly, NAC could suppress the growth of tumor in vivo.

CONCLUSIONS

NAC could facilitate Notch2 degradation through lysosomal pathway in an antioxidant-independent manner, thus attenuating Notch2 malignant signaling in GBM cells. The remarkable ability of NAC to inhibit cancer cell proliferation and tumor growth may implicate a novel application of NAC on GBM therapy.

The Role of ATG16 in Autophagy and The Ubiquitin Proteasome System

Autophagy and the ubiquitin proteasome system (UPS) are the two major cellular degradation pathways, which are critical for the maintenance of cell homeostasis. The two pathways differ in their mechanisms and clients. The evolutionary conserved ATG16 plays a key role in autophagy and appears to link autophagy with the UPS. Here, we review the role of ATG16 in different species. We summarize the current knowledge of its functions in autophagosome membrane expansion and autophagosome formation, in Crohn’s disease, and in bacterial sequestration. In addition, we provide information on its autophagy-independent functions and its role in the crosstalk between autophagy and the UPS.

Inadequate ubiquitination-proteasome coupling contributes to myocardial ischemia-reperfusion injury

The ubiquitin-proteasome system (UPS) degrades a protein molecule via 2 main steps: ubiquitination and proteasomal degradation. Extraproteasomal ubiquitin receptors are thought to couple the 2 steps, but this proposition has not been tested in vivo with vertebrates. More importantly, impaired UPS performance plays a major role in cardiac pathogenesis, including myocardial ischemia-reperfusion injury (IRI), but the molecular basis of UPS impairment remains poorly understood. Ubiquilin1 is a bona fide extraproteasomal ubiquitin receptor. Here, we report that mice with a cardiomyocyte-restricted knockout of Ubiquilin1 (Ubqln1-CKO mice) accumulated a surrogate UPS substrate (GFPdgn) and increased myocardial ubiquitinated proteins without altering proteasome activities, resulting in late-onset cardiomyopathy and a markedly shortened life span. When subject to regional myocardial ischemia-reperfusion, young Ubqln1-CKO mice showed substantially exacerbated cardiac malfunction and enlarged infarct size, and conversely, mice with transgenic Ubqln1 overexpression displayed attenuated IRI. Furthermore, Ubqln1 overexpression facilitated proteasomal degradation of oxidized proteins and the degradation of a UPS surrogate substrate in cultured cardiomyocytes without increasing autophagic flux. These findings demonstrate that Ubiquilin1 is essential to cardiac ubiquitination-proteasome coupling and that an inadequacy in the coupling represents a major pathogenic factor for myocardial IRI; therefore, strategies to strengthen coupling have the potential to reduce IRI.

Ushering in the cardiac role of Ubiquilin1

Protein quality control (PQC) mechanisms are essential for maintaining cardiac function, and alterations in this pathway influence multiple forms of heart disease. Since heart disease is the leading cause of death worldwide, understanding how the delicate balance between protein synthesis and degradation is regulated in the heart demands attention. The study by Hu et al. reveals that the extraproteasomal ubiquitin receptor Ubiquilin1 (Ubqln1) plays an important role in cardiac ubiquitination-proteasome coupling, particularly in response to myocardial ischemia/reperfusion injury, thereby suggesting that this may be a new avenue for therapeutics.

Patterns of desmin expression in idiopathic dilated cardiomyopathy are related to the desmin mRNA and ubiquitin expression

Desmin expression depends on desmin messenger RNA (mRNA) and ubiquitin proteasome system. This process is poorly understood in dilated cardiomyopathy. The aim of the study was to investigate whether changes of desmin mRNA and ubiquitin expression correlate with types of desmin expression in cardiomyocytes. Endomyocardial biopsy was performed in 60 patients (85% men, mean age 46±14 years) with heart failure (HF; left ventricular ejection fraction <45%). Desmin and ubiquitin expression were analysed in histological sections by immunohistochemistry and in Western blot. Desmin mRNA expression was determined by fluorescent in situ hybridization methods. In patients with weak/even desmin expression, weak/even expression of ubiquitin in the cytosol and low desmin mRNA expression in the cytosol and nuclei of cardiomyocytes were observed. Expression of ubiquitin and desmin mRNA increased along with the progression of desmin cytoskeleton remodeling. Desmin mRNA and ubiquitin were weakly expressed/absent in cardiomyocytes with low/lack of desmin expression. Variations in desmin mRNA, desmin and ubiquitin expression were associated with gradual changes in myocardial structure and clinical parameters. To conclude, changes in ubiquitin and desmin mRNA expression are related to patterns of desmin expression. An increase in the expression of ubiquitin and desmin mRNA may be a protective feature against unfavorable cell remodeling. This may reduce the adverse effects of cytoskeleton damage in the early stages of HF. Low/lack ubiquitin and/or desmin mRNA expression may be markers of end-stage HF.

Proteostasis in epicardial versus subcutaneous adipose tissue in heart failure subjects with and without diabetes

BACKGROUND

Cardiovascular diseases (CVDs) are leading cause of death and primary cause of morbidity and mortality in diabetic population. Epicardial adipose tissue (EAT) covers the heart's surface and is a source of biomolecules regulating heart and blood vessel physiology. The protective activation of the unfolded protein response (UPR) and autophagy allows the cardiomyocyte reticular network to restore energy and/or nutrient homeostasis and to avoid cell death. However, an excessive or prolonged UPR activation can trigger cell death. UPR activation is an early event of diabetic cardiomyopathies and deregulated autophagy is associated with CVDs.

RESULTS

An upregulation of UPR markers (glucose-regulated protein 78 KDa, glucose-regulated protein 94 KDa, inositol-requiring enzyme 1α, protein kinase RNA-like ER kinase and CCAAT/-enhancer-binding protein homologous protein (CHOP) gene) in EAT compared to subcutaneous adipose tissue (SAT), was observed as well as the UPR-related apoptosis marker caspase-4/procaspase-4 ratio but not in CHOP protein levels. Additionally, levels of ubiquitin and ubiquitinated proteins were decreased in EAT. Moreover, upregulation of autophagy markers (5' adenosine monophosphate-activated protein kinase, mechanistic target of rapamycin, Beclin 1, microtubule-associated protein light chain 3-II, lysosome-associated membrane protein 2, and PTEN-induced putative kinase 1) was observed, as well as an increase in the apoptotic Bim but not the ratio between Bim and the anti-apoptotic Bcl-2 in EAT. Diabetic patients show alterations in UPR activation markers but not in autophagy or apoptosis markers.

CONCLUSION

UPR and autophagy are increased in EAT compared to SAT, opening doors to the identification of early biomarkers for cardiomyopathies and novel therapeutic targets.

Histone acetyltransferase CBP promotes function of SCF FBXL19 ubiquitin E3 ligase by acetylation and stabilization of its F-box protein subunit

Ubiquitin E3 ligases mediate ubiquitination and degradation of intracellular proteins. We have shown that a relatively new Skp, Cullin, F-box (SCF) protein E3 ligase, SCF FBXL19, has an anti-inflammatory effect and controls actin cytoskeleton dynamics via targeting cell membrane receptor and small GTPases for their ubiquitination and degradation, but the molecular regulation of its subunit FBXL19 stability remains unclear. Here we show that FBXL19 degradation is controlled by the balance between its ubiquitination and acetylation. FBXL19 is an unstable protein with a half-life of ∼3 h. FBXL19 can be polyubiquitinated, and the proteasome inhibitor MG-132 prolongs FBXL19 half-life, suggesting that FBXL19 degradation is mediated in the ubiquitin-proteasome system. FBXL19 can also be acetylated, and enhancing acetylation of FBXL19 by a deacetylase inhibitor reduces FBXL19 ubiquitination levels. Acetylation-mimic FBXL19 mutant exhibits a longer half-life than wild type. An acetyltransferase CBP catalyzes acetylation of FBXL19. Inhibition or down-regulation of CBP reduces FBXL19 stability, whereas it is increased in CBP-overexpressing cells. Taken together, the data indicate that CBP-mediated acetylation reduces ubiquitination and stabilizes FBXL19. Further, we demonstrate that FBXL19 targets small GTPase Cdc42 for its ubiquitination and degradation, whereas this effect is reversed by inhibition of CBP, suggesting that CBP increases the effect of SCF FBXL19 E3 ligase through acetylation and stabilization of FBXL19. Our study reveals a new molecular model for regulation of SCF E3 ligase function by acetylation and stabilization of its subunit F-box protein.-Wei, J., Dong, S., Yao, K., Martinez, M. F. Y. M., Fleisher, P. R., Zhao, Y., Ma, H., Zhao, J. Histone acetyltransferase CBP promotes function of SCF FBXL19 ubiquitin E3 ligase by acetylation and stabilization of its F-box protein subunit.

High expression of ubiquitin-conjugating enzyme E2A predicts poor prognosis in hepatocellular carcinoma

The present study aimed to illustrate the association of the expression of ubiquitin-conjugating enzyme E2A (UBE2A) with the clinicopathological parameters and prognosis in hepatocellular carcinoma (HCC). The expression levels of UBE2A mRNA and protein in a total of 276 HCC tissues and six liver cell lines was detected by fluorescent quantitative polymerase chain reaction, western blotting and immunohistochemistry. Statistical analysis was also performed to assess the association of the expression of UBE2A with the clinicopathological parameters and prognosis by the GraphPad Prism and SPSS version 21.0 software. UBE2A mRNA and protein were highly expressed in HCC tissues compared with those in the adjacent normal tissue. Immunohistochemical analysis revealed that UBE2A protein was more strongly stained in the 276 paraffin-embedded HCC tissues as compared with the 63 adjacent normal tissue. Statistical analysis also demonstrated that UBE2A expression was significantly associated with histological differentiation, TNM stage and vascular invasion of HCC (P<0.05). Notably, HCC patients with a high expression of UBE2A had a shorter survival time as compared with those with a low expression of UBE2A. There results suggested that UBE2A may be involved in the pathogenesis of HCC and may serve as an important prognostic marker. Further exploration of the involvement of UBE2A in HCC development may provide novel therapeutic targets.

HSPA5 negatively regulates lysosomal activity through ubiquitination of MUL1 in head and neck cancer

HSPA5/GRP78/BiP plays an important role in cell survival or tumor progression. For these reasons, HSPA5 is an emerging therapeutic target in cancer development. Here we report that HSPA5 contributes to head and neck cancer (HNC) survival via maintenance of lysosomal activity; however, a nonthermal plasma (NTP, considered as a next-generation cancer therapy)-treated solution (NTS) inhibits HNC progression through HSPA5-dependent alteration of lysosomal activity. HSPA5 prevents NTS-induced lysosome inhibition through lysosomal-related proteins or regulation of gene expression. However, NTS-induced MUL1/MULAN/GIDE/MAPL (mitochondrial ubiquitin ligase activator of NFKB 1) leads to downregulation of HSPA5 via K48-linked ubiquitination at the lysine 446 (K446) residue. MUL1 knockdown hinders NTS-induced lysosome inhibition or cytotoxicity through the reduction of HSPA5 ubiquitination in HNC cells. While MUL1 was suppressed, HSPA5 was overexpressed in tissues of HNC patients. NTS strongly inhibited HNC progression via alterations of expression of MUL1 and HSPA5, in vivo in a xenograft model. However, NTS did not induce inhibition of tumor progression or HSPA5 reduction in MUL1 knockout (KO) HNC cells which were generated by CRISPR/Cas9 system. The data provide compelling evidence to support the idea that the regulation of the MUL1-HSPA5 axis can be a novel strategy for the treatment of HNC.

N-acetylcysteine negatively regulates Notch3 and its malignant signaling

Notch3 receptor is expressed in a variety of cancers and the excised active intracellular domain (N3ICD) initiates its signaling cascade. N-acetylcysteine (NAC) as an antioxidant has been implicated in cancer prevention and therapy. In this study, we demonstrated a negative regulation of Notch3 by NAC in cancer cells. HeLa cells treated with NAC exhibited a time- and concentration-dependent decrease in Notch3 levels and its downstream effectors Hes1 and HRT1 in a manner independent of f-secretase or glutathione. In contrast, NAC did not affect protein levels of Notch1, the full length Notch3 precursor, or ectopically expressed N3ICD. Although SOD, catalase and NAC suppressed reactive oxygen species in HeLa cells, the first two antioxidants did not impact on Notch3 levels. While the mRNA expression of Notch3 was not altered by NAC, functional inhibition of lysosome, but not proteasome, blocked the NAC-dependent reduction of Notch3 levels. Furthermore, results from Notch3 silencing and N3ICD overexpression demonstrated that NAC prevented malignant phenotypes through down-regulation of Notch3 protein in multiple cancer cells. In summary, NAC reduces Notch3 levels through lysosome-dependent protein degradation, thereby negatively regulates Notch3 malignant signaling in cancer cells. These results implicate a novel NAC treatment in sensitizing Notch3-expressing tumors.

SYNJ2BP promotes the degradation of PTEN through the lysosome-pathway and enhances breast tumor metastasis via PI3K/AKT/SNAI1 signaling

SYNJ2BP plays an important role in breast cancer metastasis. However, the molecular mechanism associated with the function of SYNJ2BP in metastasis remains unclear. In this study, we investigated the role of SYNJ2BP in tumor metastasis and established the associated underlying mechanism. Over-expression of SYNJ2BP promoted both cell migration and invasion. In contrast, silencing SYNJ2BP caused the suppression of cell migration and invasion. SYNJ2BP increased the levels of phosphorylation for AKT and GSK3β, which could be inhibited by the PI3K inhibitor, LY294002, and the GSK3β inhibitor, LiCl, and regulated the accumulation of SNAI1 in the nucleus and the expression of the SNAI1 target gene, E-cadherin (EMT marker). It is known that the stability of PTEN is regulated by ubiquitination. However, in this study, we additionally demonstrated that SYNJ2BP mediated the degradation of PTEN protein by the lysosome-pathway and induced the activation of PI3K/AKT signaling by promoting the co-localization of PTEN with autophagy-lysosomes and the expression of LC3-II and p62. In vivo study, the overexpression of SYNJ2BP significantly increased the metastasis of 4T1 cells in BALB/c mice. In addition, SYNJ2BP was highly expressed in breast carcinoma (p = 0.0031), but not in normal breast tissue, while analysis of tissue samples taken from SNAI1-positive human breast cancers showed a significant correlation between the expression of SYNJ2BP and that of p-AKT (p < 0.005). Collectively, our data identified a tumor inducer, SYNJ2BP, which could activate the PI3K/AKT/GSK3β/SNAI1 signaling pathway through the lysosome-mediated degradation of PTEN, and promote both EMT and tumor metastasis during the progression of breast cancer.

Proteostasis in cardiac health and disease

The incidence and prevalence of cardiac diseases, which are the main cause of death worldwide, are likely to increase because of population ageing. Prevailing theories about the mechanisms of ageing feature the gradual derailment of cellular protein homeostasis (proteostasis) and loss of protein quality control as central factors. In the heart, loss of protein patency, owing to flaws in genetically-determined design or because of environmentally-induced 'wear and tear', can overwhelm protein quality control, thereby triggering derailment of proteostasis and contributing to cardiac ageing. Failure of protein quality control involves impairment of chaperones, ubiquitin-proteosomal systems, autophagy, and loss of sarcomeric and cytoskeletal proteins, all of which relate to induction of cardiomyocyte senescence. Targeting protein quality control to maintain cardiac proteostasis offers a novel therapeutic strategy to promote cardiac health and combat cardiac disease. Currently marketed drugs are available to explore this concept in the clinical setting.

The two Dictyostelium discoideum autophagy 8 proteins have distinct autophagic functions

Autophagy is a highly conserved cellular degradation pathway which is crucial for various cellular processes. The autophagic process is subdivided in the initiation, autophagosome maturation and lysosomal degradation phases and involves more than forty core and accessory autophagy-related (ATG) proteins. Autophagy 8 (ATG8, in mammals LC3) is a well-established marker of autophagy and is linked to the autophagic membrane from initiation until fusion with the lysosome. We generated single and double knock-out mutants of the two Dictyostelium paralogues, ATG8a and ATG8b, as well as strains that expressed RFP-ATG8a and/or GFP-ATG8b, RFP-ATG8b, RFP-GFP-ATG8a or RFP-GFP-ATG8b in different knock-out mutants. The ATG8b¯ mutant displayed only subtle phenotypic changes in comparison to AX2 wild-type cells. In contrast, deletion of ATG8a resulted in a complex phenotype with delayed development, reduced growth, phagocytosis and cell viability, an increase in ubiquitinylated proteins and a concomitant decrease in proteasomal activity. The phenotype of the ATG8a¯/b¯ strain was, except for cell viability, in all aforementioned aspects more severe, showing that both proteins function in parallel during most analysed cellular processes. Immunofluorescence analysis of knock-out strains expressing either RFP-GFP-ATG8a or RFP-GFP-ATG8b suggests a crucial function for ATG8b in autophagosome-lysosome fusion. Quantitative analysis of strains expressing RFP-ATG8a, RFP-ATG8b, or RFP-ATG8a and GFP-ATG8b revealed that ATG8b generally localised to small and large vesicles, whereas ATG8a preferentially co-localised with ATG8b on large vesicles, indicating that ATG8b associated with nascent autophagosomes before ATG8a, which is supported by previous results (Matthias et al., 2016). Deconvoluted confocal fluorescence images showed that ATG8b localised around ATG8a and was presumably mainly present on the outer membrane of the autophagosome while ATG8a appears to be mainly associated with the inner membrane. In summary, our data show that ATG8a and ATG8b have distinct functions and are involved in canonical as well as non-canonical autophagy. The data further suggest that ATG8b predominantly acts as adapter for the autophagy machinery at the outer and ATG8a as cargo receptor at the inner membrane of the autophagosome.

p62 links the autophagy pathway and the ubiqutin-proteasome system upon ubiquitinated protein degradation

The ubiquitin–proteasome system (UPS) and autophagy are two distinct and interacting proteolytic systems. They play critical roles in cell survival under normal conditions and during stress. An increasing body of evidence indicates that ubiquitinated cargoes are important markers of degradation. p62, a classical receptor of autophagy, is a multifunctional protein located throughout the cell and involved in many signal transduction pathways, including the Keap1–Nrf2 pathway. It is involved in the proteasomal degradation of ubiquitinated proteins. When the cellular p62 level is manipulated, the quantity and location pattern of ubiquitinated proteins change with a considerable impact on cell survival. Altered p62 levels can even lead to some diseases. The proteotoxic stress imposed by proteasome inhibition can activate autophagy through p62 phosphorylation. A deficiency in autophagy may compromise the ubiquitin–proteasome system, since overabundant p62 delays delivery of the proteasomal substrate to the proteasome despite proteasomal catalytic activity being unchanged. In addition, p62 and the proteasome can modulate the activity of HDAC6 deacetylase, thus influencing the autophagic degradation.

Cardiac proteasome functional insufficiency plays a pathogenic role in diabetic cardiomyopathy

BACKGROUND

Diabetic cardiomyopathy is a major risk factor in diabetic patients but its pathogenesis remains poorly understood. The ubiquitin-proteasome system (UPS) facilitates protein quality control by degrading unnecessary and damaged proteins in eukaryotic cells, and dysfunction of UPS is implicated in various cardiac diseases. However, the overall functional status of the UPS and its pathophysiological role in diabetic cardiomyopathy have not been determined.

METHODS AND RESULTS

Type I diabetes was induced in wild-type and transgenic mice expressing a UPS functional reporter (GFPdgn) by injections of streptozotocin (STZ). STZ-induced diabetes progressively impaired cardiac UPS function as evidenced by the accumulation of GFPdgn proteins beginning two weeks after diabetes induction, and by a buildup of total and lysine (K) 48-linked polyubiquitinated proteins in the heart. To examine the functional role of the UPS in diabetic cardiomyopathy, cardiac overexpression of PA28α (PA28αOE) was used to enhance proteasome function in diabetic mouse hearts. PA28αOE diabetic mice displayed exhibited restoration of cardiac UPS function, as demonstrated by the diminished accumulation of GFPdgn and polyubiquitinated proteins. Moreover, PA28αOE diabetic mice exhibited reduced myocardial collagen deposition, decreased cardiomyocyte apoptosis, and improved cardiac systolic and diastolic function.

CONCLUSION

Impairment of cardiac UPS function is an early event in STZ-induced diabetes. Overexpression of PA28α attenuates diabetes-induced proteotoxic stress and cardiomyopathy, suggesting a potential therapeutic role for enhancement of cardiac proteasome function in this disorder.

Possible Contribution of Zerumbone-Induced Proteo-Stress to Its Anti-Inflammatory Functions via the Activation of Heat Shock Factor 1

Zerumbone is a sesquiterpene present in Zinger zerumbet. Many studies have demonstrated its marked anti-inflammatory and anti-carcinogenesis activities. Recently, we showed that zerumbone binds to numerous proteins with scant selectivity and induces the expression of heat shock proteins (HSPs) in hepatocytes. To dampen proteo-toxic stress, organisms have a stress-responsive molecular machinery, known as heat shock response. Heat shock factor 1 (HSF1) plays a key role in this protein quality control system by promoting activation of HSPs. In this study, we investigated whether zerumbone-induced HSF1 activation contributes to its anti-inflammatory functions in stimulated macrophages. Our findings showed that zerumbone increased cellular protein aggregates and promoted nuclear translocation of HSF1 for HSP expression. Interestingly, HSF1 down-regulation attenuated the suppressive effects of zerumbone on mRNA and protein expressions of pro-inflammatory genes, including inducible nitric oxide synthase and interlukin-1β. These results suggest that proteo-stress induced by zerumbone activates HSF1 for exhibiting its anti-inflammatory functions.

Rapid Proteasomal Degradation of Mutant Proteins Is the Primary Mechanism Leading to Tumorigenesis in Patients With Missense AIP Mutations

CONTEXT

The pathogenic effect of mutations in the aryl hydrocarbon receptor interacting protein (AIP) gene (AIPmuts) in pituitary adenomas is incompletely understood. We have identified the primary mechanism of loss of function for missense AIPmuts.

OBJECTIVE

This study sought to analyze the mechanism/speed of protein turnover of wild-type and missense AIP variants, correlating protein half-life with clinical parameters.

DESIGN AND SETTING

Half-life and protein-protein interaction experiments and cross-sectional analysis of AIPmut positive patients' data were performed in a clinical academic research institution.

PATIENTS

Data were obtained from our cohort of pituitary adenoma patients and literature-reported cases.

INTERVENTIONS

Protein turnover of endogenous AIP in two cell lines and fifteen AIP variants overexpressed in HEK293 cells was analyzed via cycloheximide chase and proteasome inhibition. Glutathione-S-transferase pull-down and quantitative mass spectrometry identified proteins involved in AIP degradation; results were confirmed by coimmunoprecipitation and gene knockdown. Relevant clinical data was collected.

MAIN OUTCOME MEASURES

Half-life of wild-type and mutant AIP proteins and its correlation with clinical parameters.

RESULTS

Endogenous AIP half-life was similar in HEK293 and lymphoblastoid cells (43.5 and 32.7 h). AIP variants were divided into stable proteins (median, 77.7 h; interquartile range [IQR], 60.7-92.9 h), and those with short (median, 27 h; IQR, 21.6-28.7 h) or very short (median, 7.7 h; IQR, 5.6-10.5 h) half-life; proteasomal inhibition rescued the rapid degradation of mutant proteins. The experimental half-life significantly correlated with age at diagnosis of acromegaly/gigantism (r = 0.411; P = .002). The FBXO3-containing SKP1-CUL1-F-box protein complex was identified as the E3 ubiquitin-ligase recognizing AIP.

CONCLUSIONS

AIP is a stable protein, driven to ubiquitination by the SKP1-CUL1-F-box protein complex. Enhanced proteasomal degradation is a novel pathogenic mechanism for AIPmuts, with direct implications for the phenotype.

Mifepristone increases mRNA translation rate, triggers the unfolded protein response, increases autophagic flux, and kills ovarian cancer cells in combination with proteasome or lysosome inhibitors

The synthetic steroid mifepristone blocks the growth of ovarian cancer cells, yet the mechanism driving such effect is not entirely understood. Unbiased genomic and proteomic screenings using ovarian cancer cell lines of different genetic backgrounds and sensitivities to platinum led to the identification of two key genes upregulated by mifepristone and involved in the unfolded protein response (UPR): the master chaperone of the endoplasmic reticulum (ER), glucose regulated protein (GRP) of 78 kDa, and the CCAAT/enhancer binding protein homologous transcription factor (CHOP). GRP78 and CHOP were upregulated by mifepristone in ovarian cancer cells regardless of p53 status and platinum sensitivity. Further studies revealed that the three UPR-associated pathways, PERK, IRE1α, and ATF6, were activated by mifepristone. Also, the synthetic steroid acutely increased mRNA translation rate, which, if prevented, abrogated the splicing of XBP1 mRNA, a non-translatable readout of IRE1α activation. Moreover, mifepristone increased LC3-II levels due to increased autophagic flux. When the autophagic-lysosomal pathway was inhibited with chloroquine, mifepristone was lethal to the cells. Lastly, doses of proteasome inhibitors that are inadequate to block the activity of the proteasomes, caused cell death when combined with mifepristone; this phenotype was accompanied by accumulation of poly-ubiquitinated proteins denoting proteasome inhibition. The stimulation by mifepristone of ER stress and autophagic flux offers a therapeutic opportunity for utilizing this compound to sensitize ovarian cancer cells to proteasome or lysosome inhibitors.

Macromolecular Degradation Systems and Cardiovascular Aging

Aging-related cardiovascular diseases are a rapidly increasing problem worldwide. Cardiac aging demonstrates progressive decline of diastolic dysfunction of ventricle and increase in ventricular and arterial stiffness accompanied by increased fibrosis stimulated by angiotensin II and proinflammatory cytokines. Reactive oxygen species and multiple signaling pathways on cellular senescence play major roles in the process. Aging is also associated with an alteration in steady state of macromolecular dynamics including a dysfunction of protein synthesis and degradation. Currently, impaired macromolecular degradation is considered to be closely related to enhanced inflammation and be involved in the process and mechanism of cardiac aging. Herein, we review the role and mechanisms of the degradation system of intracellular macromolecules in the process and pathophysiology of cardiovascular aging.

COP9 signalosome controls the degradation of cytosolic misfolded proteins and protects against cardiac proteotoxicity

RATIONALE

Impaired degradation of misfolded proteins is associated with a large subset of heart diseases. Misfolded proteins are degraded primarily by the ubiquitin-proteasome system, but the ubiquitin ligases responsible for the degradation remain largely unidentified. The cullin deneddylation activity of the COP9 signalosome (CSN) requires all 8 CSN subunits (CSN1 through CSN8) and regulates cullin-RING ligases, thereby controlling ubiquitination of a large number of proteins; however, neither CSN nor cullin-RING ligases is known to regulate the degradation of cytosolic misfolded proteins.

OBJECTIVE

We sought to investigate the role of CSN8/CSN in misfolded protein degradation and cardiac proteinopathy.

METHODS AND RESULTS

Cardiac CSN8 knockout causes mouse premature death; hence, CSN8 hypomorphism (CSN8(hypo)) mice were used. Myocardial neddylated forms of cullins were markedly increased, and myocardial capacity of degrading a surrogate misfolded protein was significantly reduced by CSN8 hypomorphism. When introduced into proteinopathic mice in which a bona fide misfolded protein R120G missense mutation of αβ-crystallin (CryAB(R120G)) is overexpressed in the heart, CSN8 hypomorphism aggravated CryAB(R120G)-induced restrictive cardiomyopathy and shortened the lifespan of CryAB(R120G) mice, which was associated with augmented accumulation of protein aggregates, increased neddylated proteins, and reduced levels of total ubiquitinated proteins and LC3-II in the heart. In cultured cardiomyocytes, both CSN8 knockdown and cullin-RING ligase inactivation suppressed the ubiquitination and degradation of CryAB(R120G) but not native CryAB, resulting in accumulation of protein aggregates and exacerbation of CryAB(R120G) cytotoxicity.

CONCLUSIONS

(1) CSN8/CSN promotes the ubiquitination and degradation of misfolded proteins and protects against cardiac proteotoxicity, and (2) cullin-RING ligases participate in degradation of cytosolic misfolded proteins.

Binding of SGTA to Rpn13 selectively modulates protein quality control

Rpn13 is an intrinsic ubiquitin receptor of the 26S proteasome regulatory subunit that facilitates substrate capture prior to degradation. Here we show that the C-terminal region of Rpn13 binds to the tetratricopeptide repeat (TPR) domain of SGTA, a cytosolic factor implicated in the quality control of mislocalised membrane proteins (MLPs). The overexpression of SGTA results in a substantial increase in steady-state MLP levels, consistent with an effect on proteasomal degradation. However, this effect is strongly dependent upon the interaction of SGTA with the proteasomal component Rpn13. Hence, overexpression of the SGTA-binding region of Rpn13 or point mutations within the SGTA TPR domain both inhibit SGTA binding to the proteasome and substantially reduce MLP levels. These findings suggest that SGTA can regulate the access of MLPs to the proteolytic core of the proteasome, implying that a protein quality control cycle that involves SGTA and the BAG6 complex can operate at the 19S regulatory particle. We speculate that the binding of SGTA to Rpn13 enables specific polypeptides to escape proteasomal degradation and/or selectively modulates substrate degradation.

Highlighted Article: Binding of SGTA to the proteasome delays substrate degradation, thereby providing a mechanism for potentially viable proteins to be rescued for reuse.

Amino-terminal arginylation targets endoplasmic reticulum chaperone BiP for autophagy through p62 binding

We show that ATE1-encoded Arg-transfer RNA transferase (R-transferase) of the N-end rule pathway mediates N-terminal arginylation of multiple endoplasmic reticulum (ER)-residing chaperones, leading to their cytosolic relocalization and turnover. N-terminal arginylation of BiP (also known as GRP78), protein disulphide isomerase and calreticulin is co-induced with autophagy during innate immune responses to cytosolic foreign DNA or proteasomal inhibition, associated with increased ubiquitylation. Arginylated BiP (R-BiP) is induced by and associated with cytosolic misfolded proteins destined for p62 (also known as sequestosome 1, SQSTM1) bodies. R-BiP binds the autophagic adaptor p62 through the interaction of its N-terminal arginine with the p62 ZZ domain. This allosterically induces self-oligomerization and aggregation of p62 and increases p62 interaction with LC3, leading to p62 targeting to autophagosomes and selective lysosomal co-degradation of R-BiP and p62 together with associated cargoes. In this autophagic mechanism, Nt-arginine functions as a delivery determinant, a degron and an activating ligand. Bioinformatics analysis predicts that many ER residents use arginylation to regulate non-ER processes.

The COP9 signalosome and vascular function: intriguing possibilities?

Disorders of vascular function contribute importantly to cardiovascular disease which represents a substantial cause of morbidity and mortality worldwide. An emerging paradigm in the study of cardiovascular diseases is that protein ubiquitination and turnover represent key pathological mechanisms. Our understanding of these processes in the vasculature is growing but remains incomplete. Since protein ubiquitination and turnover can represent a terminal event in the life of a given protein, entry into these pathways must be highly regulated. However, at present understanding of these regulatory mechanisms, particularly in the vasculature, is fragmentary. The COP9 (constitutive photomorphogenic mutant 9) signalosome (CSN) is a heteromeric protein complex implicated in the control of protein degradation. The CSN participates critically in the control of Cullin Ring Ligases (CRLs), at least in part via the detachment of a small protein, Nedd8 (deneddylation). CRLs are one of the largest groups of ubiquitin ligases, which represent the most selective control point for protein ubiquitination. Thus, the CSN by virtue of its ability to control the CRLs ubiquitin ligase activity is ideally positioned to effect selective modulation of protein turnover. This review surveys currently available data regarding the potential role of the CSN in control of vascular function. Data potentially linking the CSN to control of regulatory proteins involved in vascular smooth muscle proliferation and to vascular smooth muscle contraction are presented with the intent of providing potentially intriguing possibilities for future investigation.

The role of SIGMAR1 gene mutation and mitochondrial dysfunction in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) patients exhibit diverse pathologies such as endoplasmic reticulum (ER) stress and mitochondrial dysfunction in motor neurons. Five to ten percent of patients have familial ALS, a form of the disease caused by mutations in ALS-related genes, while sporadic forms of the disease occur in 90-95% of patients. Recently, it was reported that familial ALS patients exhibit a missense mutation in SIGMAR1 (c.304G > C), which encodes sigma-1 receptor (Sig-1R), substituting glutamine for glutamic acid at amino acid residue 102 (p.E102Q). Expression of that mutant Sig-1R(E102Q) protein reduces mitochondrial ATP production, inhibits proteasome activity and causes mitochondrial injury, aggravating ER stress-induced neuronal death in neuro2A cells. In this issue, we discuss mechanisms underlying mitochondrial impairment seen in ALS motor neurons and propose that therapies that protect mitochondria might improve the quality of life (QOL) of ALS patients and should be considered for clinical trials.

Human RAD6 promotes G1-S transition and cell proliferation through upregulation of cyclin D1 expression

Protein ubiquitinylation regulates protein stability and activity. RAD6, an E2 ubiquitin-conjugating enzyme, which that has been substantially biochemically characterized, functions in a number of biologically relevant pathways, including cell cycle progression. In this study, we show that RAD6 promotes the G1-S transition and cell proliferation by regulating the expression of cyclin D1 (CCND1) in human cells. Furthermore, our data indicate that RAD6 influences the transcription of CCND1 by increasing monoubiquitinylation of histone H2B and trimethylation of H3K4 in the CCND1 promoter region. Our study presents, for the first time, an evidence for the function of RAD6 in cell cycle progression and cell proliferation in human cells, raising the possibility that RAD6 could be a new target for molecular diagnosis and prognosis in cancer therapeutics.

The interplay between autophagy and the ubiquitin-proteasome system in cardiac proteotoxicity

Proteotoxicity refers to the detrimental effects of damaged/misfolded proteins on the cell. Cardiac muscle is particularly susceptible to proteotoxicity because sustained and severe proteotoxic stress leads to cell death and the cardiac muscle has very limited self-renewal capacity. The ubiquitin-proteasome system (UPS) and the autophagic-lysosomal pathway (ALP) are two major pathways responsible for degradation of most cellular proteins. Alterations of UPS and ALP functions are associated with the accumulation of proteotoxic species in the heart, a key pathological feature of common forms of heart disease including idiopathic, ischemic, and pressure-overloaded cardiomyopathies and a large subset of congestive heart failure. Emerging evidence suggests that proteasome inhibition or impairment activates autophagy and conversely, acute ALP inhibition may sometimes increase intrinsic proteasome peptidase activities but chronic ALP inhibition hinders UPS performance in ubiquitinated protein degradation. The exact molecular basis on which the two degradative pathways interact remains largely undefined. Here we review current understanding of the roles of the UPS and autophagy in the control of cardiac proteotoxicity, with a specific focus on the crosstalk between the two pathways. This article is part of a Special Issue entitled: Autophagy and protein quality control in cardiometabolic diseases.

Autophagy in microglia degrades extracellular β-amyloid fibrils and regulates the NLRP3 inflammasome

Accumulation of β-amyloid (Aβ) and resultant inflammation are critical pathological features of Alzheimer disease (AD). Microglia, a primary immune cell in brain, ingests and degrades extracellular Aβ fibrils via the lysosomal system. Autophagy is a catabolic process that degrades native cellular components, however, the role of autophagy in Aβ degradation by microglia and its effects on AD are unknown. Here we demonstrate a novel role for autophagy in the clearance of extracellular Aβ fibrils by microglia and in the regulation of the Aβ-induced NLRP3 (NLR family, pyrin domain containing 3) inflammasome using microglia specific atg7 knockout mice and cell cultures. We found in microglial cultures that Aβ interacts with MAP1LC3B-II via OPTN/optineurin and is degraded by an autophagic process mediated by the PRKAA1 pathway. We anticipate that enhancing microglial autophagy may be a promising new therapeutic strategy for AD.

Autophagic-lysosomal inhibition compromises ubiquitin-proteasome system performance in a p62 dependent manner in cardiomyocytes

Intracellular protein degradation is primarily performed by the ubiquitin-proteasome system (UPS) and the autophagic-lysosomal pathway (ALP). The interplay between these two pathways has been rarely examined in intact animals and the mechanism underlying the interplay remains unclear. Hence, we sought to test in vivo and in vitro the impact of inhibition of the ALP on UPS proteolytic performance in cardiomyocytes and to explore the underlying mechanism. Transgenic mice ubiquitously expressing a surrogate UPS substrate (GFPdgn) were treated with bafilomycin-A1 (BFA) to inhibit the ALP. Myocardial and renal GFPdgn protein levels but not mRNA levels were increased at 24 hours but not 3 hours after the first injection of BFA. Myocardial protein abundance of key proteasome subunits and the activities of proteasomal peptidases were not discernibly altered by the treatment. In cultured neonatal rat ventricular myocytes (NRVMs), the surrogate UPS substrate GFPu and a control red fluorescence protein (RFP) were co-expressed to probe UPS performance. At 12 hours or 24 hours after ALP inhibition by 3-methyladenine (3-MA) or BFA, GFPu/RFP protein ratios and the protein half-life of GFPu were significantly increased, which is accompanied by increases in p62 proteins. Similar findings were obtained when ALP was inhibited genetically via silencing Atg7 or Rab7. ALP inhibition-induced increases in GFPu and p62 are co-localized in NRVMs. siRNA-mediated p62 knockdown prevented ALP inhibition from inducing GFPu accumulation in NRVMs. We conclude that in a p62-dependent fashion, ALP inhibition impairs cardiac UPS proteolytic performance in cardiomyocytes in vitro and in vivo.

BAG6 regulates the quality control of a polytopic ERAD substrate

BAG6 participates in protein quality control and, here, we address its role in endoplasmic-reticulum-associated degradation (ERAD) by using the polytopic membrane protein OpD, an opsin degron mutant. Both BAG6 knockdown and BAG6 overexpression delay OpD degradation; however, our data suggest that these two perturbations are mechanistically distinct. Hence, BAG6 knockdown correlates with reduced OpD polyubiquitylation, whereas BAG6 overexpression increases the level of polyubiquitylated OpD. The UBL- and BAG-domains of exogenous BAG6 are dispensable for OpD stabilisation and enhanced levels of polyubiquitylated OpD. Thus, although endogenous BAG6 normally promotes OpD degradation, exogenous BAG6 expression delays this process. We speculate that overexpressed BAG6 subunits might associate with the endogenous BAG6 complex, resulting in a dominant-negative effect that inhibits its function. Interestingly, cellular levels of BAG6 also correlate with total steady-state polyubiquitylation, with Rpn10 (officially known as PSMD4) overexpression showing a similar effect. These findings suggest that perturbations of the levels of ubiquitin-binding proteins can impact upon cellular ubiquitin homeostasis. We propose that exogenous BAG6 perturbs the function of the BAG6 complex at a stage subsequent to substrate recognition and polyubiquitylation, most likely the BAG6-dependent delivery of OpD to the proteasome.

Proteasomal and lysosomal protein degradation and heart disease

In the cell, the proteasome and lysosomes represent the most important proteolytic machineries, responsible for the protein degradation in the ubiquitin-proteasome system (UPS) and autophagy, respectively. Both the UPS and autophagy are essential to protein quality and quantity control. Alterations in cardiac proteasomal and lysosomal degradation are remarkably associated with most heart disease in humans and are implicated in the pathogenesis of congestive heart failure. Studies carried out in animal models and in cell culture have begun to establish both sufficiency and, in some cases, the necessity of proteasomal functional insufficiency or lysosomal insufficiency as a major pathogenic factor in the heart. This review article highlights some recent advances in the research into proteasome and lysosome protein degradation in relation to cardiac pathology and examines the emerging evidence for enhancing degradative capacities of the proteasome and/or lysosome as a new therapeutic strategy for heart disease. This article is part of a Special Issue entitled "Protein Quality Control, the Ubiquitin Proteasome System, and Autophagy".

Gambogic acid suppresses pressure overload cardiac hypertrophy in rats

Cardiac hypertrophy is a common response of the heart to a variety of cardiovascular stimuli. Pathological cardiac hypertrophy eventually leads to heart failure. Gambogic acid (GA) is a main active ingredient isolated from the gamboge resin of Garcinia hanburyi trees and has potent anti-tumor and anti-inflammatory effects that are associated with inhibition of the NF-κB pathway. We and others recently reported that GA can significantly inhibit the function of the proteasome with much less toxicity than conventional proteasome inhibitors. The increasing lines of evidence indicate that the inhibition of the proteasome can promote the regression of cardiac hypertrophy induced by pressure overload through the blockade of the NF-κB pathway. In the present study, we examined the effect of GA on pressure overload or isoproterenol infusion induced cardiac hypertrophy and fibrosis, and changes in myocardial NF-κB signaling. We observed that the heart weight/body weight ratio, the size of cardiomyocytes, interstitial fibrosis, and the reactivation of fetal genes (α-SK-actin and BNP mRNA) were markedly increased by abdominal aorta constriction (AAC) or isoproterenol infusion (ISO), all of which were effectively inhibited by GA treatment. Furthermore, GA treatment abolished proteasome chymotrypsin-like activity increases induced by AAC or ISO, led to increased myocardial IκB protein, decreased NF-κB p65 subunit levels in the nuclear fraction, decreased NF-κB DNA-binding activity, and reduced IL2 levels in the myocardium of rats subject to AAC or ISO. In conclusion, GA treatment can suppress cardiac hypertrophy and fibrosis induced by pressure overload or isoproterenol possibly through the inhibition of the proteasome and the NF-κB pathway, suggesting that GA treatment may provide a new strategy to treat cardiac hypertrophy.

Roles for ATF6 and the sarco/endoplasmic reticulum protein quality control system in the heart

The hypertrophic growth of cardiac myocytes is a highly dynamic process that underlies physiological and pathological adaptation of the heart. Accordingly, a better understanding of the molecular underpinnings of cardiac myocyte hypertrophy is required in order to fully appreciate the causes and functional consequences of the changes in the size of the healthy and diseased heart. Hypertrophy is driven by increases in cardiac myocyte protein, which must be balanced by cellular ability to maintain protein quality in order to avoid maladaptive accumulation of toxic misfolded proteins. Recent studies have shown that the endoplasmic reticulum (ER), which, in cardiac myocytes, comprises the sarco/endoplasmic reticulum (SR/ER), is the site of most protein synthesis. Thus, the protein quality control machinery located at the SR/ER is likely to be an important determinant of whether the heart responds adaptively to hypertrophic growth stimuli. The SR/ER-transmembrane protein, ATF6, serves a critical protein quality control function as a first responder to the accumulation of potentially toxic, misfolded proteins. Misfolded proteins transform ATF6 into a transcription factor that regulates a gene program that is partly responsible for enhancing protein quality control. Two ATF6-inducible genes that have been studied in the heart and shown to be adaptive are RCAN1 and Derl3, which encode proteins that decrease protein-folding demand, and enhance degradation of misfolded proteins, respectively. Thus, the ATF6-regulated SR/ER protein quality control system is important for maintaining protein quality during growth, making ATF6, and other components of the system, potentially attractive targets for the therapeutic management pathological cardiac hypertrophy. This article is part of a Special Issue entitled "Protein Quality Control, the Ubiquitin Proteasome System, and Autophagy".

The COP9 signalosome is required for autophagy, proteasome-mediated proteolysis, and cardiomyocyte survival in adult mice

BACKGROUND

The COP9 signalosome (CSN) is an evolutionarily conserved protein complex composed of 8 unique protein subunits (CSN1 through CSN8). We have recently discovered in perinatal mouse hearts that CSN regulates not only proteasome-mediated proteolysis but also macroautophagy. However, the physiological significance of CSN in a post-mitotic organ of adult vertebrates has not been determined. We sought to study the physiological role of CSN8/CSN in adult mouse hearts.

METHODS AND RESULTS

Csn8 was conditionally ablated in the cardiomyocytes of adult mice (CSN8(CKO)) using a temporally controlled Cre-LoxP system. Loss of CSN8 accumulated the neddylated forms of cullins and noncullin proteins, increased ubiquitinated proteins, and stabilized a surrogate substrate of the proteasome in the heart. Autophagic flux was significantly decreased, whereas autophagosomes were markedly increased in CSN8(CKO) hearts, indicative of impaired autophagosome removal. Furthermore, we observed increased oxidized proteins, massive necrotic cardiomyocytes, and morphological and functional changes characteristic of dilated cardiomyopathy in CSN8(CKO) mice.

CONCLUSIONS

CSN deneddylates substrates more than cullins and is indispensable to cardiomyocyte survival in not only perinatal hearts but also adult hearts. CSN8/CSN regulates both proteasome-mediated proteolysis and the autophagic-lysosomal pathway, critical to the removal of oxidized proteins in the heart.