BRD4 promotes LPS-induced endothelial cells senescence via activating and cooperating STING-IRF3 pathway

Endothelial cells (ECs) senescence is closely associated with the initiation and development of multiple age-related cardiovascular diseases. It is necessary to explore the underlying molecular mechanisms of ECs senescence, which is not only the basis to decipher cellular senescence, but also a novel therapeutic target for the endothelial senescence-related diseases. BRD4, a key epigenetic regulator, is universally related to gene expression regulation and has been reported to accelerate cell senescence. Besides, emerging evidence has suggested that the stimulator of interferon genes protein (STING) can regulate inflammatory and senescence-related diseases. However, whether STING pathway activation is regulated by BRD4 in the context of ECs senescence remains largely unclear. Here, we observed that elevated BRD4 and activated STING-IRF3 signaling pathway during ECs senescence and further confirmed that BRD4 could abolish STING activation. We demonstrated that BRD4 could inhibit E3 ubiquitin ligase HRD1-mediated ubiquitination degradation of STING via inhibiting HRD1 transcription. In addition to the direct regulatory effect of BRD4 on STING activation, we have confirmed that BRD4 cooperates with IRF3 and P65 to promote SASP gene expression, thereby accelerating ECs senescence. Here, we proposed a novel mechanism underlying BRD4' key dual role in activating the STING pathway during ECs senescence.

Japanese encephalitis virus hijacks ER-associated degradation regulators for its replication

Flaviviruses target their replication on membranous structures derived from the ER, where both viral and host proteins play crucial structural and functional roles. Here, we have characterized the involvement of the ER-associated degradation (ERAD) pathway core E3 ligase complex (SEL1L-HRD1) regulator proteins in the replication of Japanese encephalitis virus (JEV). Through high-resolution immunofluorescence imaging of JEV-infected HeLa cells, we observe that the virus replication complexes marked by NS1 strongly colocalize with the ERAD adapter SEL1L, lectin OS9, ER-membrane shuttle factor HERPUD1, E3 ubiquitin ligase HRD1 and rhomboid superfamily member DERLIN1. NS5 positive structures also show strong overlap with SEL1L. While these effectors show significant transcriptional upregulation, their protein levels remain largely stable in infected cells. siRNA mediated depletion of OS9, SEL1L, HERPUD1 and HRD1 significantly inhibit viral RNA replication and titres, with SEL1L depletion showing the maximum attenuation of replication. By performing protein translation arrest experiments, we show that SEL1L, and OS9 are stabilised upon JEV infection. Overall results from this study suggest that these ERAD effector proteins are crucial host-factors for JEV replication.

An evolutionarily conserved ubiquitin ligase drives infection and transmission of flaviviruses

Mosquito-borne flaviviruses such as dengue (DENV) and Zika (ZIKV) cause hundreds of millions of infections annually. The single-stranded RNA genome of flaviviruses is translated into a polyprotein, which is cleaved equally into individual functional proteins. While structural proteins are packaged into progeny virions and released, most of the nonstructural proteins remain intracellular and could become cytotoxic if accumulated over time. However, the mechanism by which nonstructural proteins are maintained at the levels optimal for cellular fitness and viral replication remains unknown. Here, we identified that the ubiquitin E3 ligase HRD1 is essential for flaviviruses infections in both mammalian hosts and mosquitoes. HRD1 directly interacts with flavivirus NS4A and ubiquitylates a conserved lysine residue for ER-associated degradation. This mechanism avoids excessive accumulation of NS4A, which otherwise interrupts the expression of processed flavivirus proteins in the ER. Furthermore, a small-molecule inhibitor of HRD1 named LS-102 effectively interrupts DENV2 infection in both mice and Aedes aegypti mosquitoes, and significantly disturbs DENV transmission from the infected hosts to mosquitoes owing to reduced viremia. Taken together, this study demonstrates that flaviviruses have evolved a sophisticated mechanism to exploit the ubiquitination system to balance the homeostasis of viral proteins for their own advantage and provides a potential therapeutic target to interrupt flavivirus infection and transmission.

Cholesterol-induced HRD1 reduction accelerates vascular smooth muscle cell senescence via stimulation of endoplasmic reticulum stress-induced reactive oxygen species

Senescence of vascular smooth muscle cells (VSMCs) is a key contributor to plaque vulnerability in atherosclerosis (AS), which is affected by endoplasmic reticulum (ER) stress and reactive oxygen species (ROS) production. However, the crosstalk between ER stress and ROS production in the pathogenesis of VSMC senescence remains to be elucidated. ER-associated degradation (ERAD) is a complex process that clears unfolded or misfolded proteins to maintain ER homeostasis. HRD1 is the major E3 ligase in mammalian ERAD machineries that catalyzes ubiquitin conjugation to the unfolded or misfolded proteins for degradation. Our results showed that HRD1 protein levels were reduced in human AS plaques and aortic roots from ApoE-/- mice fed with high-fat diet (HFD), along with the increased ER stress response. Exposure to cholesterol in VSMCs activated inflammatory signaling and induced senescence, while reduced HRD1 protein expression. CRISPR Cas9-mediated HRD1 knockout (KO) exacerbated cholesterol- and thapsigargin-induced cell senescence. Inhibiting ER stress with 4-PBA (4-Phenylbutyric acid) partially reversed the ROS production and cell senescence induced by HRD1 deficiency in VSMCs, suggesting that ER stress alone could be sufficient to induce ROS production and senescence in VSMCs. Besides, HRD1 deficiency led to mitochondrial dysfunction, and reducing ROS production from impaired mitochondria partly reversed HRD1 deficiency-induced cell senescence. Finally, we showed that the overexpression of HDR1 reversed cholesterol-induced ER stress, ROS production, and cellular senescence in VSMCs. Our findings indicate that HRD1 protects against senescence by maintaining ER homeostasis and mitochondrial functionality. Thus, targeting HRD1 function may help to mitigate VSMC senescence and prevent vascular aging related diseases. TRIAL REGISTRATION: A real-world study based on the discussion of primary and secondary prevention strategies for coronary heart disease, URL:https://www.clinicaltrials.gov, the trial registration number is [2022]-02-121-01.

Luteolin Protects Against 6-Hydoroxydopamine-Induced Cell Death via an Upregulation of HRD1 and SEL1L

Parkinson's Disease (PD) is caused by many factors and endoplasmic reticulum (ER) stress is considered as one of the responsible factors for it. ER stress induces the activation of the ubiquitin-proteasome system to degrade unfolded proteins and suppress cell death. The ubiquitin ligase 3-hydroxy-3-methylglutaryl-coenzyme A reductase degradation 1 (HRD1) and its stabilizing molecule, the suppressor/enhancer lin-12-like (SEL1L), can suppress the ER stress via the ubiquitin-proteasome system, and that HRD1 can also suppress cell death in familial and nonfamilial PD models. These findings indicate that HRD1 and SEL1L might be key proteins for the treatment of PD. Our study aimed to identify the compounds with the effects of upregulating the HRD1 expression and suppressing neuronal cell death in a 6-hydroxydopamine (6-OHDA)-induced cellular PD model. Our screening by the Drug Gene Budger, a drug repositioning tool, identified luteolin as a candidate compound for the desired modulation of the HRD1 expression. Subsequently, we confirmed that low concentrations of luteolin did not show cytotoxicity in SH-SY5Y cells, and used these low concentrations in the subsequent experiments. Next, we demonsrated that luteolin increased HRD1 and SEL1L mRNA levels and protein expressions. Furthermore, luteolin inhibited 6-OHDA-induced cell death and suppressed ER stress response caused by exposure to 6-OHDA. Finally, luteolin did not reppress 6-OHDA-induced cell death when expression of HRD1 or SEL1L was suppressed by RNA interference. These findings suggest that luteolin might be a novel therapeutic agent for PD due to its ability to suppress ER stress through the activation of HRD1 and SEL1L.

HRD1 reduction promotes cholesterol-induced vascular smooth muscle cell phenotypic change via endoplasmic reticulum stress

Phenotypic change of vascular smooth muscle cells (VSMCs) is the main contributor of vascular pathological remodeling in atherosclerosis. The endoplasmic reticulum (ER) is critical for maintaining VSMC function through elimination of misfolded proteins that impair VSMC cellular function. ER-associated degradation (ERAD) is an ER-mediated process that controls protein quality by clearing misfolded proteins. One of the critical regulators of ERAD is HRD1, which also plays a vital role in lipid metabolism. However, the function of HRD1 in VSMCs of atherosclerotic vessels remains poorly understood. The level of HRD1 expression was analyzed in aortic tissues of mice fed with a high-fat diet (HFD). The H&E and EVG (VERHOEFF'S VAN GIESON) staining were used to demonstrate pathological vascular changes. IF (immunofluorescence) and WB (western blot) were used to explore the signaling pathways in vivo and in vitro. The wound closure and transwell assays were also used to test the migration rate of VSMCs. CRISPR gene editing and transcriptomic analysis were applied in vitro to explore the cellular mechanism. Our data showed significant reduction of HRD1 in aortic tissues of mice under HFD feeding. VSMC phenotypic change and HRD1 downregulation were detected by cholesterol supplement. Transcriptomic and further analysis of HRD1-KO VSMCs showed that HRD1 deficiency induced the expression of genes related to ER stress response, proliferation and migration, but reduced the contractile-related genes in VSMCs. HRD1 deficiency also exacerbated the proliferation, migration and ROS production of VSMCs induced by cholesterol, which promoted the VSMC dedifferentiation. Our results showed that HRD1 played an essential role in the contractile homeostasis of VSMCs by negatively regulating ER stress response. Thus, HRD1 in VSMCs could serve as a potential therapeutic target in metabolic disorder-induced vascular remodeling.

UFMylation of HRD1 regulates endoplasmic reticulum homeostasis

Ubiquitin fold modifier 1 is a small ubiquitin-like protein modifier that is essential for embryonic development of metazoans. Although UFMylation has been connected to endoplasmic reticulum homeostasis, the underlying mechanisms and the relevant cellular targets are largely unknown. Here, we show that HRD1, a ubiquitin ligase of ER-associated protein degradation (ERAD), is a novel substrate of UFM1 conjugation. HRD1 interacts with UFMylation components UFL1 and DDRGK1 and is UFMylated at Lys610 residue. In UFL1-depleted cells, the stability of HRD1 is increased and its ubiquitination modification is reduced. In the event of ER stress, the UFMylation and ubiquitination modification of HRD1 is gradually inhibited over time. Alteration of HRD1 Lys610 residue to arginine impairs its ability to degrade unfolded or misfolded proteins to disturb protein processing in ER. These results suggest that UFMylation of HRD1 facilitates ERAD function to maintain ER homeostasis.

Large HBV Surface Protein-Induced Unfolded Protein Response Dynamically Regulates p27 Degradation in Hepatocellular Carcinoma Progression

Up to 50% of hepatocellular carcinoma (HCC) is caused by hepatitis B virus (HBV) infection, and the surface protein of HBV is essential for the progression of HBV-related HCC. The expression of large HBV surface antigen (LHB) is presented in HBV-associated HCC tissues and is significantly associated with the development of HCC. Gene set enrichment analysis revealed that LHB overexpression regulates the cell cycle process. Excess LHB in HCC cells induced chronic endoplasmic reticulum (ER) stress and was significantly correlated with tumor growth in vivo. Cell cycle analysis showed that cell cycle progression from G1 to S phase was greatly enhanced in vitro. We identified intensive crosstalk between ER stress and cell cycle progression in HCC. As an important regulator of the G1/S checkpoint, p27 was transcriptionally upregulated by transcription factors ATF4 and XBP1s, downstream of the unfolded protein response pathway. Moreover, LHB-induced ER stress promoted internal ribosome-entry-site-mediated selective translation of p27, and E3 ubiquitin ligase HRD1-mediated p27 ubiquitination and degradation. Ultimately, the decrease in p27 protein levels reduced G1/S arrest and promoted the progress of HCC by regulating the cell cycle.

HRD1 functions as a tumor suppressor in ovarian cancer by facilitating ubiquitination-dependent SLC7A11 degradation

The E3 ubiquitin ligase 3-hydroxy-3-methylglutaryl reductase degradation (HRD1) was found to be a tumor suppressor in diverse types of cancers; we aimed to explore its expression pattern and biological function in ovarian cancer (OC). HRD1 expression in OC tumor tissues was detected using quantitative real-time polymerase chain reaction (qRT-PCR) and immunohistochemistry (IHC). The overexpression plasmid of HRD1 was transfected into OC cells. Cell proliferation, colony formation, and apoptosis were analyzed using bromodeoxy uridineassay, colony formation assay, and flow cytometry, respectively. OC mice models were established to explore the effect of HRD1 on OC in vivo. Ferroptosis was evaluated by malondialdehyde, reactive oxygen species, and intracellular ferrous iron. Expressions offerroptosis-related factors were examined using qRT-PCR and western blot. Erastin and Fer-1 were, respectively, employed to promote or inhibit ferroptosis in OC cells. Online bioinformatics tool and co-immunoprecipitation assay were performed to predict and verify the interactive genes of HRD1 in OC cells, respectively. Gain-of-function studies were carried out to determine the roles of HRD1 in cell proliferation, apoptosis, and ferroptosis in vitro. HRD1 was under-expressed in OC tumor tissues. The overexpression of HRD1 inhibited OC cell proliferation and colony formation in vitro and suppressed OC tumor growth in vivo. The overexpression of HRD1 promoted cell apoptosis and ferroptosis in OC cell lines. HRD1 interacted with the solute carrier family 7 member 11 (SLC7A11) in OC cells, and HRD1 regulated the stability and ubiquitination in OC. SLC7A11 overexpression recovered the effect of HRD1 overexpression in OC cell lines. HRD1 inhibited tumor formation and promoted ferroptosis in OC through enhancing SLC7A11 degradation.

HRD1 Promotes Non-small Cell Lung Carcinoma Metastasis by Blocking Autophagy-mediated MIEN1 Degradation

Dysregulation of autophagy has been implicated in the development of many diseases, including cancer. Here, we revealed a novel function of the E3 ubiquitin ligase HRD1 in non-small cell lung carcinoma (NSCLC) metastasis by regulating autophagy. Mechanistically, HRD1 inhibits autophagy by promoting ATG3 ubiquitination and degradation. Additionally, a pro-migratory and invasive factor, MIEN1 (migration and invasion enhancer 1), was found to be autophagically degraded upon HRD1 deficiency. Importantly, both HRD1 and MIEN1 expression are upregulated and positively correlated in lung tumors. Based on these results, we proposed a novel mechanism of HRD1 function that the degradation of ATG3 protein by HRD1 leads to autophagy inhibition and MIEN1 release, thus promoting NSCLC metastasis. Therefore, our findings provided new insights into the role of HRD1 in NSCLC metastasis and new therapeutic targets for lung cancer treatment.

Downregulation of miR-221-3p promotes the ferroptosis in gastric cancer cells via upregulation of ATF3 to mediate the transcription inhibition of GPX4 and HRD1

BACKGROUND

Gastric cancer (GC) is an aggressive gastrointestinal tumor. MiRNAs participate in the tumorigenesis of GC. Nevertheless, the function of miR-221-3p in GC remains largely unknown.

METHODS

RNA levels were assessed by RT-qPCR. Western blot was performed to test the protein levels. The relation between miR-221-3p and ATF3 was investigated by dual-luciferase reporter assay. ChIP and dual-luciferase reporter assay were applied to assess the interaction between ATF3 and HRD1 or GPX4. Meanwhile, cell proliferation was investigated by CCK8 and colony formation assay. The content of erastin-induced Fe²⁺ was investigated by iron assay kit. Erastin-induced lipid ROS level was assessed by C11-BODIPY 581/591. Co-immunoprecipitation was used to detect the interaction between HRD1 and ACSL4. In addition, xenograft mice model was established to detect the effect of miR-221-3p in GC.

RESULTS

Depletion of miR-221-3p greatly attenuated GC cell proliferation through promoting ferroptosis. Meanwhile, ATF3 was downregulated in GC, and it was identified to be the downstream mRNA of miR-221-3p. MiR-221-3p downregulation could promoted the ferroptosis in GC cells through upregulation of ATF3. HRD1 mediates ubiquitination and degradation of ACSL4 to inhibit ferroptosis. ATF3 upregulation could reduce GC cell proliferation via downregulating the transcription of GPX4 and HRD1. Furthermore, downregulation of miR-221-3p markedly attenuated the growth of GC in mice.

CONCLUSION

HRD1 mediates ubiquitination and degradation of ACSL4 to inhibit ferroptosis. MiR-221-3p depletion upregulates the ferroptosis in GC cells via upregulation of ATF3 to mediate the transcription inhibition of GPX4 and HRD1. Our study might provide a novel target for GC treatment.

HRD1 in human malignant neoplasms: Molecular mechanisms and novel therapeutic strategy for cancer

In tumor cells, the endoplasmic reticulum (ER) plays an essential role in maintaining cellular proteostasis by stimulating unfolded protein response (UPR) underlying stress conditions. ER-associated degradation (ERAD) is a critical pathway of the UPR to protect cells from ER stress-induced apoptosis and the elimination of unfolded or misfolded proteins by the ubiquitin-proteasome system (UPS). 3-Hydroxy-3-methylglutaryl reductase degradation (HRD1) as an E3 ubiquitin ligase plays an essential role in the ubiquitination and dislocation of misfolded protein in ERAD. In addition, HRD1 can target other normal folded proteins. In various types of cancer, the expression of HRD1 is dysregulated, and it targets different molecules to develop cancer hallmarks or suppress the progression of the disease. Recent investigations have defined the role of HRD1 in drug resistance in types of cancer. This review focuses on the molecular mechanisms of HRD1 and its roles in cancer pathogenesis and discusses the worthiness of targeting HRD1 as a novel therapeutic strategy in cancer.

Insight into the roles of the ER-associated degradation E3 ubiquitin ligase HRD1 in plant cuticular lipid biosynthesis

Cuticular lipids consisting of cutin and wax coat aerial plant surfaces providing protection against biotic and abiotic stresses. Although much progress has been made on comprehending the regulation of plant cuticular lipid biosynthesis, their functional relevance in plant protection merits further investigation of potential regulators of their synthesis. HRD1 and DOA10 mediate two major Endoplasmic Reticulum-Associated Degradation (ERAD) pathways in yeast and also regulate common pathways during lipid metabolism. However, their roles in plant lipid metabolism are not well studied. CER9, an Arabidopsis homolog of DOA10, is known to play important roles in cuticular lipid biosynthesis. This prompted us to determine if HRD1 also plays a role in regulation cuticular lipid biosynthesis. Here we report that an Arabidopsis hrd1a hrd1b double mutant is impacted in the accumulation of both cutin and cuticular waxes including a large increase in total stem cutin with a concomitant decrease in stem wax content. We further investigated genetic relationship between HRD1A/1B- and CER9-mediated ERAD pathways with regard to cuticular lipid synthesis. Surprisingly, simultaneous mutation of HRD1 and CER9 revealed additive effects on stem wax synthesis, but not stem cutin synthesis. Collectively, our study advances our understanding of the ERAD regulatory roles in cuticular lipid synthesis identifying HRD1 as an important player in the regulated deposition of Arabidopsis stem cuticular lipids.

CircNR3C2 promotes HRD1-mediated tumor-suppressive effect via sponging miR-513a-3p in triple-negative breast cancer

BACKGROUND

E3 ubiquitin ligase HRD1 (HMG-CoA reductase degradation protein 1, alias synoviolin with SYVN1 as the official gene symbol) was found downregulated and acting as a tumor suppressor in breast cancer, while the exact expression profile of HRD1 in different breast cancer subtypes remains unknown. Recent studies characterized circular RNAs (circRNAs) playing an regulatory role as miRNA sponge in tumor progression, presenting a new viewpoint for the post-transcriptional regulation of cancer-related genes.

METHODS

Examination of the expression of HRD1 protein and mRNA was implemented using public microarray/RNA-sequencing datasets and breast cancer tissues/cell lines. Based on public RNA-sequencing results, online databases and enrichment/clustering analyses were used to predict the specific combinations of circRNA/miRNA that potentially govern HRD1 expression. Gain-of-function and rescue experiments in vitro and in vivo were executed to evaluate the suppressive effects of circNR3C2 on breast cancer progression through HRD1-mediated proteasomal degradation of Vimentin, which was identified using immunoblotting, immunoprecipitation, and in vitro ubiquitination assays.

RESULTS

HRD1 is significantly underexpressed in triple-negative breast cancer (TNBC) against other subtypes and has an inverse correlation with Vimentin, inhibiting the proliferation, migration, invasion and EMT (epithelial-mesenchymal transition) process of breast cancer cells via inducing polyubiquitination-mediated proteasomal degradation of Vimentin. CircNR3C2 (hsa_circ_0071127) is also remarkably downregulated in TNBC, negatively correlated with the distant metastasis and lethality of invasive breast carcinoma. Overexpressing circNR3C2 in vitro and in vivo leads to a crucial enhancement of the tumor-suppressive effects of HRD1 through sponging miR-513a-3p.

CONCLUSIONS

Collectively, we elucidated a bona fide circNR3C2/miR-513a-3p/HRD1/Vimentin axis that negatively regulates the metastasis of TNBC, suggesting that circNR3C2 and HRD1 can act as potential prognostic biomarkers. Our study may facilitate the development of therapeutic agents targeting circNR3C2 and HRD1 for patients with aggressive breast cancer.

HRD1 inhibits fatty acid oxidation and tumorigenesis by ubiquitinating CPT2 in triple-negative breast cancer

Dependence on glutamine and acceleration of fatty acid oxidation (FAO) are both metabolic characteristics of triple‐negative breast cancer (TNBC). With the rapid growth of tumors, accelerated glutamine catabolism depletes local glutamine, resulting in glutamine deficiency. Studies have shown that the use of alternative energy sources, such as fatty acids, enables tumor cells to continue to proliferate rapidly in a glutamine‐deficient microenvironment. However, the detailed mechanisms behind this metabolic change are still unclear. Herein, we identified HRD1 as a regulatory protein for FAO that specifically inhibits TNBC cell proliferation under glutamine‐deficient conditions. Furthermore, we observed that HRD1 expression is significantly downregulated under glutamine deprivation and HRD1 directly ubiquitinates and stabilizes CPT2 through K48‐linked ubiquitination. In addition, the inhibition of CPT2 expression dramatically suppresses TNBC cell proliferation mediated by HRD1 knockdown in vitro and in vivo. Finally, we found that the glutaminase inhibitor CB839 significantly inhibited TNBC cell tumor growth, but not in the HRD1 knock‐downed TNBC cells. These findings provide an invaluable insight into HRD1 as a regulator of lipid metabolism and have important implications for TNBC therapeutic targeting.

Role of the E3 ubiquitin ligase HRD1 in the regulation of serotonin transporter function

To elucidate the regulation of serotonin transporter (SERT) function via its membrane trafficking, we investigated the involvement of the ubiquitin E3 ligase HRD1 (HMG-CoA reductase degradation protein), which participates in endoplasmic reticulum (ER)-associated degradation (ERAD), in the functional regulation of SERT. Cells transiently expressing wild-type SERT or a SERT C-terminal deletion mutant (SERTΔCT), a SERT protein predicted to be misfolded, were used for experiments. Studies using HRD1-overexpressing or HRD1-knockdown cells demonstrated that HRD1 is involved in SERT proteolysis. Overexpression of HRD1 promoted SERT ubiquitination, the effect of which was augmented by treatment with the proteasome inhibitor MG132. Immunoprecipitation studies revealed that HRD1 interacts with SERT in the presence of MG132. In addition, HRD1 was intracellularly colocalized with SERT, especially with aggregates of SERTΔCT in the ER. HRD1 also affected SERT uptake activity in accordance with the expression levels of the SERT protein. These results suggest that HRD1 contributes to the membrane trafficking and functional regulation of SERT through its involvement in ERAD-mediated SERT degradation.

Derlin-3 Is Required for Changes in ERAD Complex Formation under ER Stress

Endoplasmic reticulum (ER)-associated protein degradation (ERAD) is a quality control system that induces the degradation of ER terminally misfolded proteins. The ERAD system consists of complexes of multiple ER membrane-associated and luminal proteins that function cooperatively. We aimed to reveal the role of Derlin-3 in the ERAD system using the liver, pancreas, and kidney obtained from different mouse genotypes. We performed coimmunoprecipitation and sucrose density gradient centrifugation to unravel the dynamic nature of ERAD complexes. We observed that Derlin-3 is exclusively expressed in the pancreas, and its deficiency leads to the destabilization of Herp and accumulation of ERAD substrates. Under normal conditions, Complex-1a predominantly contains Herp, Derlin-2, HRD1, and SEL1L, and under ER stress, Complex-1b contains Herp, Derlin-3 (instead of Derlin-2), HRD1, and SEL1L. Complex-2 is upregulated under ER stress and contains Derlin-1, Derlin-2, p97, and VIMP. Derlin-3 deficiency suppresses the transition of Derlin-2 from Complex-1a to Complex-2 under ER stress. In the pancreas, Derlin-3 deficiency blocks Derlin-2 transition. In conclusion, the composition of ERAD complexes is tissue-specific and changes in response to ER stress in a Derlin-3-dependent manner. Derlin-3 may play a key role in changing ERAD complex compositions to overcome ER stress.

E3 ubiquitin ligase HRD1 modulates the circadian clock through regulation of BMAL1 stability

Circadian rhythm serves an essential role in numerous physiological functions. Circadian oscillations are organized by circadian clock components at the molecular level. The precision of the circadian clock is controlled by transcriptional-translational negative feedback loops, as well as post-translational modifications of clock proteins, including ubiquitination; however, the influence of E3 ligases on clock protein ubiquitination requires further investigation. The results of co-immunoprecipitation and immunofluorescent localization, indicated that the endoplasmic reticulum transmembrane E3 ubiquitin ligase HRD1, encoded by the synoviolin 1 gene, interacted with brain and muscle ARNT-like 1 (BMAL1) and enhanced BMAL1 protein ubiquitination. In addition, the results of western blotting and reverse transcription-quantitative PCR suggested that HRD1 promoted K48-associated polyubiquitination of BMAL1 and thus mediated its degradation via the ubiquitin-proteasome system. Furthermore, gene knockdown and gene overexpression assays revealed that HRD1-dependent degradation of BMAL1 protein regulated the expression of BMAL1 target genes and the amplitude of circadian oscillations in mammalian cells. The findings of the current study indicate that HRD1 may influence the regulation of circadian rhythm via modulation of BMAL1 stability.

Energy supplementation rescues growth restriction and female infertility of mice with hepatic HRD1 ablation

Severe dietary restriction, catabolic states and even short-term caloric deprivation impair fertility in mammals including human, which is often reversible by restoration of the energy supplementation. The dysregulated crosstalk among multiple organs is possibly involved in this process. However, ideal experimental animal models are needed to illuminate functional crosstalk among distal organs during the starvation pathogenesis. We have recently discovered that conditional hepatic HRD1 gene deletion results in elevated energy expenditure and consequently leads to growth retardation and female fertility. Herein, we discovered that both growth retardation and female infertility of liver-specific HRD1 knockout mice could be fully rescued by additional energy supplementation upon HFD feeding. Hepatic HRD1 deletion appears to impair by the pituitary gland functions in secreting critical hormones in growth and female fertility including growth hormone (GH), follicle-stimulating hormone (FSH) and luteinizinghormone (LH) because a dramatic reduction in the sera levels of all three hormones were detected in liver HRD1 KO mice, which consequently shortened their tibia lengths and impaired the ovary functions in females. HFD feeding for six weeks largely restored all three hormones in liver HRD1 KO mice back to levels comparable with those in WT mice. In addition, the growth hormone induced activation of JAK-STAT5 pathway was inhibited by HRD1 deletion, and additional energy supplementation upon HFD feeding restored STAT5 transcriptional activation. Our studies establish a unique mouse model to study liver crosstalk with distal organs in regulating energy balance in growth and female fertility.

HRD1 prevents atherosclerosis-mediated endothelial cell apoptosis by promoting LOX-1 degradation

The 3-hydroxy-3-methylglutaryl reductase degradation (HRD1) is an E3 ubiquitin ligase that can preserve heart structure and function, but its role in endothelial dysfunction and atherosclerosis (AS) is unclear. The aim of this study was to explore the role and biological function of HRD1 in AS. HRD1 expression was significantly decreased in atherosclerotic intima and ox-LDL led to a decrease of HRD1 level in endothelial cells (ECs). Forced expression of HRD1 inhibited the endothelial apoptosis induced by ox-LDL. The transcription factor KLF2 specifically bound to the HRD1 promoter and positively regulated HRD1 expression. KLF2 up-regulation could reverse the decrease of HRD1 level in ECs treated with ox-LDL. Further analysis showed that HRD1 interacted with LOX-1 and promoted ubiquitination and degradation of LOX-1 by the proteasome. Deletion of LOX-1 attenuated the ECs apoptosis induced by HRD1 downregulation. Pravastatin, which protected EC from damage via a KLF2-dependent mechanism, could dose-dependently enhanced HRD1 expression in EC exposed to ox-LDL. Interestingly, interference of HRD1 abolished the cytoprotective effect of pravastatin. Collectively, our data indicate that decreased HRD1 expression leads to apoptosis of ECs and restoration of HRD1 expression could represent a novel strategy for human AS therapy.

E3 Ubiquitin Ligase HRD1 Promotes Lung Tumorigenesis by Promoting Sirtuin 2 Ubiquitination and Degradation

The NAD-dependent histone deacetylase sirtuin 2 (SIRT2) plays critical roles in mitosis and cell cycle progression and recently was shown to suppress tumor growth and to be downregulated in several types of cancers. However, the underlying mechanism of SIRT2 downregulation remains unknown. In this study, using bioinformatics, gene expression profiling, protein overexpression approaches, and cell migration assays, we showed that E3 ubiquitin ligase 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase degradation 1 (HRD1) interacts with SIRT2 and promotes its ubiquitination and degradation.

The NAD-dependent histone deacetylase sirtuin 2 (SIRT2) plays critical roles in mitosis and cell cycle progression and recently was shown to suppress tumor growth and to be downregulated in several types of cancers. However, the underlying mechanism of SIRT2 downregulation remains unknown. In this study, using bioinformatics, gene expression profiling, protein overexpression approaches, and cell migration assays, we showed that E3 ubiquitin ligase 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase degradation 1 (HRD1) interacts with SIRT2 and promotes its ubiquitination and degradation. Furthermore, we found that HRD1 deficiency induces SIRT2 upregulation and inhibits the growth and tumor formation of lung cancer cells both in vitro and in vivo. Of note, we observed that SIRT2 expression is downregulated in human lung cancer and also negatively correlates with HRD1 expression in these cancers. Additionally, we found that patients with lung adenocarcinoma having lower HRD1 or higher SIRT2 expression levels tend to survive longer. On the basis of these results, we propose a mechanism of lung tumorigenesis that involves HRD1-mediated downregulation of SIRT2 and suggest that interventions targeting HRD1 activity could be a potential therapeutic strategy to treat patients with lung cancer.

Endoplasmic reticulum-associated degradation and beyond: The multitasking roles for HRD1 in immune regulation and autoimmunity

Endoplasmic reticulum (ER)-associated degradation (ERAD) is a mechanism against ER stress, wherein unfolded/misfolded proteins accumulated in the ER are transported to the cytosol for degradation by the ubiquitin-proteasome system. The ER resident E3 ubiquitin ligase HRD1 has been identified as a key ERAD factor that directly catalyzes ubiquitin conjugation onto the unfolded or misfolded proteins for proteasomal degradation. The abnormally increased HRD1 expression was discovered in rheumatoid synovial cells, providing the first evidence for HRD1 dysregulation involved in human inflammatory pathogenesis. Further studies shown that inflammatory cytokines involved in rheumatoid pathogenesis including IL-1β, TNF-α, IL-17 and IL-26 induce HRD1 expression. Recent studies using mice with tissue-specific targeted deletion of HRD1 gene have revealed important functions of HRD1 in immune regulation and inflammatory diseases. HRD1 has been shown critical for dendritic cell expression of antigens to both CD4 and CD8 T cells. Both TCR and costimulatory receptor CD28 signaling induces HRD1 expression, which promotes T cell clonal expansion and IL-2 production. Together with the fact that HRD1 is required for maintaining the stability of regulatory T cell (Treg) stability, HRD1 appears to fine tone T cell immunity. In addition, HRD1 is involved in humoral immune response by regulating early B cell development and maintaining B cell survival upon recognition of specific antigen. HRD1 appears to target its substrates for ubiquitination through, either ERAD-dependent or -independent, at least two distinct molecular mechanisms in a cell or tissue specific manner to achieve its physiological functions. Dysregulation of HRD1 expression and/or it functions are involved in autoimmune inflammatory diseases in particular rheumatoid arthritis and lupus. Here, we review current findings on the mechanism of HRD1 protein in immune regulation and the involvement of HRD1 in the pathogenesis of autoimmune inflammatory diseases.

HRD1-ERAD controls production of the hepatokine FGF21 through CREBH polyubiquitination

The endoplasmic reticulum-associated protein degradation (ERAD) is responsible for recognizing and retro-translocating protein substrates, misfolded or not, from the ER for cytosolic proteasomal degradation. HMG-CoA Reductase (HMGCR) Degradation protein-HRD1-was initially identified as an E3 ligase critical for ERAD. However, its physiological functions remain largely undefined. Herein, we discovered that hepatic HRD1 expression is induced in the postprandial condition upon mouse refeeding. Mice with liver-specific HRD1 deletion failed to repress FGF21 production in serum and liver even in the refeeding condition and phenocopy the FGF21 gain-of-function mice showing growth retardation, female infertility, and diurnal circadian behavior disruption. HRD1-ERAD facilitates the degradation of the liver-specific ER-tethered transcription factor CREBH to downregulate FGF21 expression. HRD1-ERAD catalyzes polyubiquitin conjugation onto CREBH at lysine 294 for its proteasomal degradation, bridging a multi-organ crosstalk in regulating growth, circadian behavior, and female fertility through regulating the CREBH-FGF21 regulatory axis.

Unique RING finger structure from the human HRD1 protein

Artificial RING fingers (ARFs) are created by transplanting active sites of RING fingers onto cross-brace structures. Human hydroxymethylglutaryl-coenzyme A reductase degradation protein 1 (HRD1) is involved in the degradation of the endoplasmic reticulum (ER) proteins. HRD1 possesses the RING finger domain (HRD1_RING) that functions as a ubiquitin-ligating (E3) enzyme. Herein, we determined the solution structure of HRD1_RING using nuclear magnetic resonance (NMR). Moreover, using a metallochromic indicator, we determined the stoichiometry of zinc ions spectrophotometrically and found that HRD1_RING binds to two zinc atoms. The Simple Modular Architecture Research Tool database predicted the structure of HRD1_RING as a typical RING finger. However, it was found that the actual structure of HRD1_RING adopts an atypical RING-H₂ type RING fold. This structural analysis unveiled the position and range of the active site of HRD1_RING that contribute to its specific ubiquitin-conjugating enzyme (E2)-binding capability.

Ubiquitin ligase HMG-CoA reductase degradation 1 (HRD1) prevents cell death in a cellular model of Parkinson's disease

Endoplasmic reticulum (ER) stress may play a role in the etiology of Parkinson's disease (PD). We have previously reported that ubiquitin ligase 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase degradation 1 (HRD1) involved in ER stress degrades unfolded protein that accumulates in the ER due to loss of function of Parkin, which is a causative factor in familial PD. We have also demonstrated that cell death is suppressed by the degradation of unfolded proteins. These findings indicate that HRD1 may serve as a compensatory mechanism for the loss of function of Parkin in familial PD patients. However, the role of HRD1 in sporadic PD has not yet been identified. This study aimed to reveal the roles of HRD1 and associated molecules in a cellular model of PD. We demonstrated that expressions of HRD1 and Suppressor/Enhancer Lin12 1-like (SEL1L: a HRD1 stabilizer) increased in SH-SY5Y human neuroblastoma cells upon exposure to 6-hydroxydopamine (6-OHDA). The 6-OHDA-induced cell death was suppressed in cells overexpressing wt-HRD1, whereas cell death was enhanced in cells with knockdown of HRD1 expression. These results suggest that HRD1 is a key molecule involved in 6-OHDA-induced cell death. By contrast, suppression of SEL1L expression decreased the amount of HRD1 protein. As a result, 6-OHDA-induced cell death was enhanced in cells suppressing SEL1L expression, and this cell death was much more evident than that in cells with suppression of HRD1 expression. These findings strongly indicate that SEL1L is necessary for maintaining and stabilizing the amount of HRD1 protein, and stabilizing the amount of HRD1 protein through SEL1L may serve to protect against 6-OHDA-induced cell death. Furthermore, the expression of Parkin was reinforced when HRD1 mRNA had been suppressed in cells, but was not observed when SEL1L mRNA had been restrained. It is possible that Parkin expression is induced as a compensatory mechanism when HRD1 mRNA decreases. This intracellular transduction may suppress the enhancement of 6-OHDA-induced cell death caused by the loss of HRD1. Taken together with these results, it is suggested that HRD1 and its stabilizer (SEL1L) are key molecules for elucidating the pathogenesis and treatment of PD.

Upregulation of HRD1 promotes cell migration and invasion in colon cancer

3-Hydroxy-3-methylglutaryl reductase degradation (HRD1) is an E3 ubiquitin ligase that functions by promoting degradation of misfolded proteins in processes such as embryogenesis and rheumatoid arthritis. However, little is known about the role of HRD1 in cancer. The aim of the present study was to investigate the expression pattern and functions of HRD1 in human colon cancer (CC). We found that HRD1 expression was increased significantly in human CC tissues, and its overexpression was associated with TNM stage, tumor differentiation, tumor invasive depth, and distant metastasis. Knockdown of HRD1 using small hairpin (sh) RNA plasmid significantly inhibited CC cell migration and invasion. Furthermore, the silencing of HRD1 decreased the expression of MMP-2 and MMP-9, which is critical for CC cell migration and invasion. Taken together, these results revealed that HRD1 is overexpressed in CC and promotes migration and invasion of CC cells. Inhibition of HRD1 may be considered as an effective anti-CC strategy.

Characterization of WDR20: A new regulator of the ERAD machinery

ERAD is an important process of protein quality control that eliminates misfolded or unassembled proteins from ER. Before undergoing proteasome degradation, the misfolded proteins are dislocated from ER membrane into cytosol, which requires the AAA ATPase p97/VCP and its cofactor, the NPL4-UFD1 dimer. Here, we performed a CRISPR-based screen and identify many candidates for ERAD regulation. We further confirmed four proteins, FBOX2, TRIM6, UFL1 and WDR20, are novel regulators for ERAD. Then the molecular mechanism for WDR20 in ERAD is further characterized. Depletion of WDR20 inhibits the degradation of TCRα, a typical ERAD substrate, while WDR20 overexpression reduces TCRα protein level. WDR20 associates with TCRα and central regulators of the ERAD system, p97, GP78 and HRD1. A portion of WDR20 localizes to the ER-containing microsomal membrane. WDR20 expression increases TCRα ubiquitination, and HRD1 E3 ligase is essential for the process. WDR20 seems to serve as an adaptor protein to mediate the interaction between p97 and TCRα. Our study provides novel candidates and reveals an unexpected role of WDR20 in ERAD regulation.

AIDA Selectively Mediates Downregulation of Fat Synthesis Enzymes by ERAD to Retard Intestinal Fat Absorption and Prevent Obesity

The efficiency of intestinal absorption of dietary fat constitutes a primary determinant accounting for individual vulnerability to obesity. However, how fat absorption is controlled and contributes to obesity remains unclear. Here, we show that inhibition of endoplasmic-reticulum-associated degradation (ERAD) increases the abundance of triacylglycerol synthesis enzymes and fat absorption in small intestine. The C2-domain protein AIDA acts as an essential factor for the E3-ligase HRD1 of ERAD to downregulate rate-limiting acyltransferases GPAT3, MOGAT2, and DGAT2. Aida^-/- mice, when grown in a thermal-neutral condition or fed high-fat diet, display increased intestinal fatty acid re-esterification, circulating and tissue triacylglycerol, accompanied with severely increased adiposity without enhancement of adipogenesis. Intestine-specific knockout of Aida largely phenocopies its whole-body knockout, strongly indicating that increased intestinal TAG synthesis is a primary impetus to obesity. The AIDA-mediated ERAD system may thus represent an anti-thrifty mechanism impinging on the enzymes for intestinal fat absorption and systemic fat storage.

The ubiquitin-proteasome system functionally links neuronal Tomosyn-1 to dendritic morphology

Altering the expression of Tomosyn-1 (Tomo-1), a soluble, R-SNARE domain-containing protein, significantly affects behavior in mice, Drosophila, and Caenorhabditis elegans Yet, the mechanisms that modulate Tomo-1 expression and its regulatory activity remain poorly defined. Here, we found that Tomo-1 expression levels influence postsynaptic spine density. Tomo-1 overexpression increased dendritic spine density, whereas Tomo-1 knockdown (KD) decreased spine density. These findings identified a novel action of Tomo-1 on dendritic spines, which is unique because it occurs independently of Tomo-1's C-terminal R-SNARE domain. We also demonstrated that the ubiquitin-proteasome system (UPS), which is known to influence synaptic strength, dynamically regulates Tomo-1 protein levels. Immunoprecipitated and affinity-purified Tomo-1 from cultured rat hippocampal neurons was ubiquitinated, and the levels of ubiquitinated Tomo-1 dramatically increased upon pharmacological proteasome blockade. Moreover, Tomo-1 ubiquitination appeared to be mediated through an interaction with the E3 ubiquitin ligase HRD1, as immunoprecipitation of Tomo-1 from neurons co-precipitated HRD1, and this interaction increases upon proteasome inhibition. Further, in vitro reactions indicated direct, HRD1 concentration-dependent Tomo-1 ubiquitination. We also noted that the UPS regulates both Tomo-1 expression and functional output, as HRD1 KD in hippocampal neurons increased Tomo-1 protein level and dendritic spine density. Notably, the effect of HRD1 KD on spine density was mitigated by additional KD of Tomo-1, indicating a direct HRD1/Tomo-1 effector relationship. In summary, our results indicate that the UPS is likely to participate in tuning synaptic efficacy and spine dynamics by precise regulation of neuronal Tomo-1 levels.

Melatonin reduces endoplasmic reticulum stress and corneal dystrophy-associated TGFBIp through activation of endoplasmic reticulum-associated protein degradation

Endoplasmic reticulum (ER) stress is emerging as a factor for the pathogenesis of granular corneal dystrophy type 2 (GCD2). This study was designed to investigate the molecular mechanisms underlying the protective effects of melatonin on ER stress in GCD2. Our results showed that GCD2 corneal fibroblasts were more susceptible to ER stress-induced death than were wild-type cells. Melatonin significantly inhibited GCD2 corneal cell death, caspase-3 activation, and poly (ADP-ribose) polymerase 1 cleavage caused by the ER stress inducer, tunicamycin. Under ER stress, melatonin significantly suppressed the induction of immunoglobulin heavy-chain-binding protein (BiP) and activation of inositol-requiring enzyme 1α (IRE1α), and their downstream target, alternative splicing of X-box binding protein 1(XBP1). Notably, the reduction in BiP and IRE1α by melatonin was suppressed by the ubiquitin-proteasome inhibitor, MG132, but not by the autophagy inhibitor, bafilomycin A1, indicating involvement of the ER-associated protein degradation (ERAD) system. Melatonin treatment reduced the levels of transforming growth factor-β-induced protein (TGFBIp) significantly, and this reduction was suppressed by MG132. We also found reduced mRNA expression of the ERAD system components HRD1 and SEL1L, and a reduced level of SEL1L protein in GCD2 cells. Interestingly, melatonin treatments enhanced SEL1L levels and suppressed the inhibition of SEL1L N-glycosylation caused by tunicamycin. In conclusion, this study provides new insights into the mechanisms by which melatonin confers its protective actions during ER stress. The results also indicate that melatonin might have potential as a therapeutic agent for ER stress-related diseases including GCD2.

HRD1 sensitizes breast cancer cells to Tamoxifen by promoting S100A8 degradation

Estrogen receptor alpha positive (ER+) of breast cancer could develop resistance to antiestrogens including Tamoxifen. Our previous study showed that the E3 ubiquitin ligase HRD1 played an important role in anti-breast cancer. However, its role in chemotherapy resistance hasn't been reported. In this study, we found that HRD1 expression was downregulated in Tamoxifen-resistant breast cancer cell line MCF7/Tam compared to the Tamoxifen sensitive cell line MCF7. Moreover, S100A8 is the direct target of HRD1 by proteome analysis. Our data showed that HRD1 decreased the protein level of S100A8 through ubiquitination while HRD1 was regulated by acetylation of histone. More importantly, HRD1 knockdown significantly increased the cell survival of MCF7 cells to the Tamoxifen treatment. HRD1 overexpression sensitized MCF7/Tam cells to the Tamoxifen treatment in vitro and in vivo. In conclusion, the decrease of HRD1 expression contributed to Tamoxifen resistance in breast cancer.

HRD1 suppresses the growth and metastasis of breast cancer cells by promoting IGF-1R degradation

HRD1 (3-hydroxy-3-methylglutaryl reductase degradation) is an E3 ubiquitin ligase. We found that HRD1 was significantly downregulated in 170 breast cancer tissues. Low tumoral HRD1 expression was correlated with clinicopathological characteristics and a shorter survival in breast cancer patients. P65 specifically bound to the HRD1 promoter and inhibited HRD1 expression. Suppression of NF-κB activity reversed IL-6-induced downregulation of HRD1 expression. HRD1 interacted with IGF-1R and promoted its ubiquitination and degradation by the proteasome. Overexpression of HRD1 resulted in the inhibition of growth, migration and invasion of breast cancer cells in vitro and in vivo. Furthermore, HRD1 attenuated IL-6-induced epithelial-mesenchymal transition in MCF10A cells. These findings uncover a novel role for HRD1 in breast cancer.

Neuroprotection by Endoplasmic Reticulum Stress-Induced HRD1 and Chaperones: Possible Therapeutic Targets for Alzheimer's and Parkinson's Disease

Alzheimer’s disease (AD) and Parkinson’s disease (PD) are neurodegenerative disorders with a severe medical and social impact. Further insights from clinical and scientific studies are essential to develop effective therapies. Various stresses on the endoplasmic reticulum (ER) cause unfolded/misfolded proteins to aggregate, initiating unfolded protein responses (UPR), one of which is the induction of neuronal cell death. Some of the pathogenic factors for AD and PD are associated with UPR. ER molecules such as ubiquitin ligases (E3s) and chaperones are also produced during UPR to degrade and refold aberrant proteins that accumulate in the ER. In this review, we examine the role of HMG-CoA reductase degradation protein 1 (HRD1) and the chaperone protein-disulfide isomerase (PDI), which are both produced in the ER in response to stress. We discuss the importance of HRD1 in degrading amyloid precursor protein (APP) and Parkin-associated endothelin receptor-like receptor (Pael-R) to protect against neuronal death. PDI and the chemical chaperone 4-phenyl-butyrate also exert neuroprotective effects. We discuss the pathophysiological roles of ER stress, UPR, and the induction and neuroprotective effects of HRD1 and PDI, which may represent significant targets for novel AD and PD therapies.

Selective multifaceted E3 ubiquitin ligases barricade extreme defense: Potential therapeutic targets for neurodegeneration and ageing

Efficient and regular performance of Ubiquitin Proteasome System and Autophagy continuously eliminate deleterious accumulation of nonnative protiens. In cellular quality control system, E3 ubiquitin ligases are significant employees for defense mechanism against abnormal toxic proteins. Few findings indicate that lack of functions of E3 ubiquitin ligases can be a causative factor of neurodevelopmental disorders, neurodegeneration, cancer and ageing. However, the detailed molecular pathomechanism implying E3 ubiquitin ligases in cellular functions in multifactorial disease conditions are not well understood. This article systematically represents the unique characteristics, molecular nature, and recent developments in the knowledge of neurobiological functions of few crucial E3 ubiquitin ligases. Here, we review recent literature on the roles of E6-AP, HRD1 and ITCH E3 ubiquitin ligases in the neuro-pathobiological mechanisms, with precise focus on the processes of neurodegeneration, and thereby propose new lines of potential targets for therapeutic interventions.

Identifying the ERAD ubiquitin E3 ligases for viral and cellular targeting of MHC class I

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E3 ubiquitin ligases play a central role in viral and cellular degradation of MHC-I.

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HCMV US2 and US11 hijack the mammalian ERAD machinery to induce MHC-I degradation.

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We identified the TRC8 and TMEM129 E3 ligases as crucial for US2/11 function.

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The US2/11 degradation hubs are flexible and enable viral evasion of different immune functions.

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Cellular quality control of MHC-I is controlled by the HRD1/SEL1L E3 ligase complex.

The human cytomegalovirus (HCMV) US2 and US11 gene products hijack mammalian ER-associated degradation (ERAD) to induce rapid degradation of major histocompatibility class I (MHC-I) molecules. The rate-limiting step in this pathway is thought to be the polyubiquitination of MHC-I by distinct host ERAD E3 ubiquitin ligases. TRC8 was identified as the ligase responsible for US2-mediated MHC-I degradation and shown to be required for the cleavage-dependent degradation of some tail-anchored proteins. In addition to MHC-I, plasma membrane profiling identified further immune receptors, which are also substrates for the US2/TRC8 complex. These include at least six α integrins, the coagulation factor thrombomodulin and the NK cell ligand CD112. US2’s use of specific HCMV-encoded adaptors makes it an adaptable viral degradation hub. US11-mediated degradation is MHC-I-specific and genetic screens have identified TMEM129, an uncharacterised RING-C2 E3 ligase, as responsible for US11-mediated degradation. In a unique auto-regulatory loop, US11 readily responds to changes in cellular expression of MHC-I. Free US11 either rebinds more MHC-I or is itself degraded by the HRD1/SEL1L E3 ligase complex. While virally encoded US2 and US11 appropriate mammalian ERAD, the MHC-I complex also undergoes stringent cellular quality control and misfolded MHC-I is degraded by the HRD1/SEL1L complex. We discuss the identification and central role of E3 ubiquitin ligases in ER quality control and viral degradation of the MHC-I chain.

Derlin2 protein facilitates HRD1-mediated retro-translocation of sonic hedgehog at the endoplasmic reticulum

Endoplasmic reticulum-associated degradation (ERAD) is an important system that eliminates misfolded proteins from the ER. Three derlins have been implicated in this process, but their precise function remains unknown. In this study, we report that although both derlin1 and derlin2 are capable of binding the ERAD-specific ubiquitin ligase HRD1, they associate with the HRD1-containing complex with different affinities. Accordingly, these derlins have nonredundant functions in ERAD with derlin2 being an essential functional partner for HRD1-mediated ERAD of SHH and NHK. We show that derlin2, but not derlin1 or derlin3, is required for ERAD of both glycosylated and nonglycosylated SHH, as well as NHK. Derlin2 appears to act at a post-targeting step for HRD1-dependent retro-translocation. Without derlin2, the assembly of HRD1 into a functional retro-translocation homo-oligomer proceeds normally, and substrate targeting to the HRD1 complex also occurs. However, the ERAD substrate SHH-C is largely trapped inside the ER lumen. These observations raise the possibility that derlin2 may regulate the movement of substrates through the HRD1-containing retro-translocon. Our study is the first to report that derlin2 functions with HRD1 in ERAD of certain substrates independent of their glycosylation status. The mammalian ERAD system may require multiple derlins that each functions with a distinct E3 partner to eliminate a specific subset of substrates. This is different from the model in Saccharomyces cerevisiae, in which Hrd1p alone is sufficient for retro-translocation.