Membrane-associated RING-CH (MARCH) 8 mediates the ubiquitination and lysosomal degradation of the transferrin receptor

O-GlcNAcylation regulates the stability of transferrin receptor (TFRC) to control the ferroptosis in hepatocellular carcinoma cells

Ferroptosis is an iron-dependent programmed cell death (PCD) enforced by lipid peroxidation accumulation. Transferrin receptor (TFRC), one of the signature proteins of ferroptosis, is abundantly expressed in hepatocellular carcinoma (HCC). However, post-translational modification (PTM) of TFRC and the underlying mechanisms for ferroptosis regulation remain less understood. In this study, we found that TFRC undergoes O-GlcNAcylation, influencing Erastin-induced ferroptosis sensitivity in hepatocytes. Further mechanistic studies found that Erastin can trigger de-O-GlcNAcylation of TFRC at serine 687 (Ser687), which diminishes the binding of ubiquitin E3 ligase membrane-associated RING-CH8 (MARCH8) and decreases polyubiquitination on lysine 665 (Lys665), thereby enhancing TFRC stability that favors labile iron accumulation. Therefore, our findings report O-GlcNAcylation on an important regulatory protein of ferroptosis and reveal an intriguing mechanism by which HCC ferroptosis is controlled by an iron metabolism pathway.

MARCH8 downregulation modulates profibrotic responses including myofibroblast differentiation

Interstitial lung diseases can result in poor patient outcomes, especially in idiopathic pulmonary fibrosis (IPF), a severe interstitial lung disease with unknown causes. The lack of treatment options requires further understanding of the pathological process/mediators. Membrane-associated RING-CH 8 (MARCH8) has been implicated in immune function regulation and inflammation, however, its role in the development of pulmonary fibrosis and particularly the fibroblast to myofibroblast transition (FMT) remains a gap in existing knowledge. In this study, we demonstrated decreased MARCH8 expression in patients with IPF compared with non-PF controls and in bleomycin-induced PF. TGF-β dose- and time-dependently decreased MARCH8 expression in normal and IPF human lung fibroblast (HLFs), along with induction of FMT markers α-SMA, collagen type I (Col-1), and fibronectin (FN). Interestingly, overexpression of MARCH8 significantly suppressed TGF-β-induced expression of α-SMA, Col-1, and FN. By contrast, the knockdown of MARCH8 using siRNA upregulated basal expression of α-SMA/Col-1/FN. Moreover, MARCH8 knockdown enhanced TGF-β-induced FMT marker expression. These data clearly show that MARCH8 is a critical "brake" for FMT and potentially affects PF. We further found that TGF-β suppressed MARCH8 mRNA expression and the proteasome inhibitor MG132 failed to block MARCH8 decrease induced by TGF-β. Conversely, TGF-β decreases mRNA levels of MARCH8 in a dose- and time-dependent manner, suggesting the transcriptional regulation of MARCH8 by TGF-β. Mechanistically, MARCH8 overexpression suppressed TGF-β-induced Smad2/3 phosphorylation, which may account for the observed effects. Taken together, this study demonstrated an unrecognized role of MARCH8 in negatively regulating FMT and profibrogenic responses relevant to interstitial lung diseases.NEW & NOTEWORTHY MARCH8 is an important modulator of inflammation, immunity, and other cellular processes. We found that MARCH8 expression is downregulated in the lungs of patients with idiopathic pulmonary fibrosis (IPF) and experimental models of pulmonary fibrosis. Furthermore, TGF-β1 decreases MARCH8 transcriptionally in human lung fibroblasts (HLFs). MARCH8 overexpression blunts TGF-β1-induced fibroblast to myofibroblast transition while knockdown of MARCH8 drives this profibrotic change in HLFs. The findings support further exploration of MARCH8 as a novel target in IPF.

Determining the antiviral mechanism of MARCH2

Membrane-associated RING-CH (MARCH) 2 protein is a member of the MARCH protein family of RING-CH finger E3 ubiquitin ligases that have important functions in regulating the levels of proteins found on the cell surface. MARCH1, 2 and 8 inhibit HIV-1 infection by preventing the incorporation of the envelope glycoproteins in nascent virions. However, a better understanding on the mechanism utilized by MARCH proteins to restrict HIV-1 is needed. In this report, we identify an amino acid in human MARCH2, that is absent in mouse MARCH2, critical for its antiretroviral function. Moreover, we map the domains of human MARCH2 critical for restricting as well as binding to the HIV-1 envelope glycoproteins. Our findings reveal important new aspects of the antiviral mechanism utilized by human MARCH2 to restrict HIV-1 that have potential implications to all MARCH proteins with antiviral functions.

Transferrin Receptor Protein 1 Is an Entry Factor for Rabies Virus

Rabies virus (RABV) is a prototypical neurotropic virus that causes rabies in human and animals with an almost 100% mortality rate. Once RABV enters the central nervous system, no treatment is proven to prevent death. RABV glycoprotein (G) interacts with cell surface receptors and then enters cells via clathrin-mediated endocytosis (CME); however, the key host factors involved remain largely unknown. Here, we identified transferrin receptor 1 (TfR1), a classic receptor that undergoes CME, as an entry factor for RABV. TfR1 interacts with RABV G and is involved in the endocytosis of RABV. An antibody against TfR1 or the TfR1 ectodomain soluble protein significantly blocked RABV infection in HEK293 cells, N2a cells, and mouse primary neuronal cells. We further found that the endocytosis of TfR1 is coupled with the endocytosis of RABV and that TfR1 and RABV are transported to early and late endosomes. Our results suggest that RABV hijacks the transport pathway of TfR1 for entry, thereby deepening our understanding of the entry mechanism of RABV. IMPORTANCE For most viruses, cell entry involves engagement with many distinct plasma membrane components, each of which is essential. After binding to its specific receptor(s), rabies virus (RABV) enters host cells through the process of clathrin-mediated endocytosis. However, whether the receptor-dependent clathrin-mediated endocytosis of RABV requires other plasma membrane components remain largely unknown. Here, we demonstrate that transferrin receptor 1 (TfR1) is a functional entry factor for RABV infection. The endocytosis of RABV is coupled with the endocytosis of TfR1. Our results indicate that RABV hijacks the transport pathway of TfR1 for entry, which deepens our understanding of the entry mechanism of RABV.

Membrane-associated RING-CH protein (MARCH8) is a novel glycolysis repressor targeted by miR-32 in colorectal cancer

BACKGROUND

Colorectal cancer (CRC) is the third most common cancer and leading cause of cancer-related deaths worldwide. Aberrant cellular metabolism is a hallmark of cancer cells, and disturbed metabolism showed clinical significance in CRC. The membrane-associated RING-CH 8 (MARCH8) protein, the first MARCH E3 ligase, plays an oncogenic role and serves as a prognostic marker in multiple cancers, however, the role of MARCH8 in CRC is unclear. In the present study, we aimed to investigate the biomarkers and their underlying mechanism for CRC.

METHOD

In this study, we first examined the function of MARCH8 in CRC by analysing public database. Besides, we performing gene silencing studies and generating cellular overexpression and xenograft models. Then its protein substrate was identified and validated. In addition, the expression of MARCH8 was investigated in tissue samples from CRC patients, and the molecular basis for decreased expression was analysed.

RESULTS

Systematic analysis reveals that MARCH8 is a beneficial prognostic marker in CRC. In CRC, MARCH8 exhibited tumor-suppressive activity both in vivo and in vitro. Furthermore, we found that MARCH8 is negatively correlated with hexokinase 2 (HK2) protein in CRC patients. MARCH8 regulates glycolysis and promotes ubiquitination-mediated proteasome degradation to reduces HK2 protein levels. Then HK2 inhibitor partially rescues the effect of MARCH8 knockdown in CRC. Poised chromatin and elevated miR-32 repressed MARCH8 expression.

CONCLUSION

In summary, we propose that in CRC, poised chromatin and miR-32 decrease the expression of MARCH8, further bind and add ubiquitin, induce HK2 degradation, and finally repress glycolysis to promote tumor suppressors in CRC.

Ubiquitin ligase E3 HUWE1/MULE targets transferrin receptor for degradation and suppresses ferroptosis in acute liver injury

Hepatic ischemia followed by reperfusion (I/R), a major clinical problem during liver surgical procedures, can induce liver injury with severe cell death including ferroptosis which is characterized by iron-dependent accumulation of lipid peroxidation. The HECT domain-containing ubiquitin E3 ligase HUWE1 (also known as MULE) was initially shown to promote apoptosis. However, our preliminary study demonstrates that high expression of HUWE1 in the liver donors corelates with less injury and better hepatic function after liver transplantation in patients. Thus, we investigate the role of HUWE1 in acute liver injury, and identify HUWE1 as a negative ferroptosis modulator through transferrin receptor 1(TfR1). Deficiency of Huwe1 in mice hepatocytes (HKO) exacerbated I/R and CCl4-induced liver injury with more ferroptosis occurrence. Moreover, Suppression of Huwe1 remarkably enhances cellular sensitivity to ferroptosis in primary hepatocytes and mouse embryonic fibroblasts. Mechanistically, HUWE1 specifically targets TfR1 for ubiquitination and proteasomal degradation, thereby regulates iron metabolism. Importantly, chemical and genetic inhibition of TfR1 dramatically diminishes the ferroptotic cell death in Huwe1 KO cells and Huwe1 HKO mice. Therefore, HUWE1 is a potential protective factor to antagonize both aberrant iron accumulation and ferroptosis thereby mitigating acute liver injury. These findings may provide clinical implications for patients with the high-expression Huwe1 alleles.

Cellular and molecular mechanisms underlying plasma membrane functionality and integrity

The plasma membrane not only protects the cell from the extracellular environment, acting as a selective barrier, but also regulates cellular events that originate at the cell surface, playing a key role in various biological processes that are essential for the preservation of cell homeostasis. Therefore, elucidation of the mechanisms involved in the maintenance of plasma membrane integrity and functionality is of utmost importance. Cells have developed mechanisms to ensure the quality of proteins that inhabit the cell surface, as well as strategies to cope with injuries inflicted to the plasma membrane. Defects in these mechanisms can lead to the development or onset of several diseases. Despite the importance of these processes, a comprehensive and holistic perspective of plasma membrane quality control is still lacking. To tackle this gap, in this Review, we provide a thorough overview of the mechanisms underlying the identification and targeting of membrane proteins that are to be removed from the cell surface, as well as the membrane repair mechanisms triggered in both physiological and pathological conditions. A better understanding of the mechanisms underlying protein quality control at the plasma membrane can reveal promising and unanticipated targets for the development of innovative therapeutic approaches.

Advances in HIV-1 Assembly

The assembly of HIV-1 particles is a concerted and dynamic process that takes place on the plasma membrane of infected cells. An abundance of recent discoveries has advanced our understanding of the complex sequence of events leading to HIV-1 particle assembly, budding, and release. Structural studies have illuminated key features of assembly and maturation, including the dramatic structural transition that occurs between the immature Gag lattice and the formation of the mature viral capsid core. The critical role of inositol hexakisphosphate (IP6) in the assembly of both the immature and mature Gag lattice has been elucidated. The structural basis for selective packaging of genomic RNA into virions has been revealed. This review will provide an overview of the HIV-1 assembly process, with a focus on recent advances in the field, and will point out areas where questions remain that can benefit from future investigation.

Dual RNA sequencing reveals dendritic cell reprogramming in response to typhoidal Salmonella invasion

Salmonella enterica represent a major disease burden worldwide. S. enterica serovar Typhi (S. Typhi) is responsible for potentially life-threatening Typhoid fever affecting 10.9 million people annually. While non-typhoidal Salmonella (NTS) serovars usually trigger self-limiting diarrhoea, invasive NTS bacteraemia is a growing public health challenge. Dendritic cells (DCs) are key professional antigen presenting cells of the human immune system. The ability of pathogenic bacteria to subvert DC functions and prevent T cell recognition contributes to their survival and dissemination within the host. Here, we adapted dual RNA-sequencing to define how different Salmonella pathovariants remodel their gene expression in tandem with that of infected DCs. We find DCs harness iron handling pathways to defend against invading Salmonellas, which S. Typhi is able to circumvent by mounting a robust response to nitrosative stress. In parallel, we uncover the alternative strategies invasive NTS employ to impair DC functions.

Aulicino, Antanaviciute et al investigate the transcriptional response to invasive Salmonella strains in dendritic cells (DCs). They show that S. Typhi mount a response against nitrosative stress pathways and propose a role of iron uptake and transport in preventing infection, which the pathogen can bypass. In parallel, they find that invasive Salmonella employs several mechanisms targeting more classic aspects of immunity to impair DC function.

MARCH8 Targets Cytoplasmic Lysine Residues of Various Viral Envelope Glycoproteins

The host transmembrane protein MARCH8 is a RING finger E3 ubiquitin ligase that downregulates various host transmembrane proteins, such as MHC-II. We have recently reported that MARCH8 expression in virus-producing cells impairs viral infectivity by reducing virion incorporation of not only HIV-1 envelope glycoprotein but also vesicular stomatitis virus G-glycoprotein through two different pathways. However, the MARCH8 inhibition spectrum remains largely unknown. Here, we show the antiviral spectrum of MARCH8 using viruses pseudotyped with a variety of viral envelope glycoproteins. Infection experiments revealed that viral envelope glycoproteins derived from the rhabdovirus, arenavirus, coronavirus, and togavirus (alphavirus) families were sensitive to MARCH8-mediated inhibition. Lysine mutations at the cytoplasmic tails of rabies virus-G, lymphocytic choriomeningitis virus glycoproteins, SARS-CoV and SARS-CoV-2 spike proteins, and Chikungunya virus and Ross River virus E2 proteins conferred resistance to MARCH8. Immunofluorescence showed impaired downregulation of the mutants of these viral envelope glycoproteins by MARCH8, followed by lysosomal degradation, suggesting that MARCH8-mediated ubiquitination leads to intracellular degradation of these envelopes. Indeed, rabies virus-G and Chikungunya virus E2 proteins proved to be clearly ubiquitinated. We conclude that MARCH8 has inhibitory activity on a variety of viral envelope glycoproteins whose cytoplasmic lysine residues are targeted by this antiviral factor.

IMPORTANCE A member of the MARCH E3 ubiquitin ligase family, MARCH8, downregulates many different kinds of host transmembrane proteins, resulting in the regulation of cellular homeostasis. On the other hands, MARCH8 acts as an antiviral factor when it binds to and downregulates HIV-1 envelope glycoprotein and vesicular stomatitis virus G-glycoprotein that are viral transmembrane proteins. This study reveals that, as in the case of cellular membrane proteins, MARCH8 shows broad-spectrum inhibition against various viral envelope glycoproteins by recognizing their cytoplasmic lysine residues, resulting in lysosomal degradation.

Exploration of Aberrant E3 Ligases Implicated in Alzheimer's Disease and Development of Chemical Tools to Modulate Their Function

The Ubiquitin Proteasome System (UPS) is responsible for the degradation of misfolded or aggregated proteins via a multistep ATP-dependent proteolytic mechanism. This process involves a cascade of ubiquitin (Ub) transfer steps from E1 to E2 to E3 ligase. The E3 ligase transfers Ub to a targeted protein that is brought to the proteasome for degradation. The inability of the UPS to remove misfolded or aggregated proteins due to UPS dysfunction is commonly observed in neurodegenerative diseases, such as Alzheimer's disease (AD). UPS dysfunction in AD drives disease pathology and is associated with the common hallmarks such as amyloid-β (Aβ) accumulation and tau hyperphosphorylation, among others. E3 ligases are key members of the UPS machinery and dysfunction or changes in their expression can propagate other aberrant processes that accelerate AD pathology. The upregulation or downregulation of expression or activity of E3 ligases responsible for these processes results in changes in protein levels of E3 ligase substrates, many of which represent key proteins that propagate AD. A powerful way to better characterize UPS dysfunction in AD and the role of individual E3 ligases is via the use of high-quality chemical tools that bind and modulate specific E3 ligases. Furthermore, through combining gene editing with recent advances in 3D cell culture, in vitro modeling of AD in a dish has become more relevant and possible. These cell-based models of AD allow for study of specific pathways and mechanisms as well as characterization of the role E3 ligases play in driving AD. In this review, we outline the key mechanisms of UPS dysregulation linked to E3 ligases in AD and highlight the currently available chemical modulators. We present several key approaches for E3 ligase ligand discovery being employed with respect to distinct classes of E3 ligases. Where possible, specific examples of the use of cultured neurons to delineate E3 ligase biology have been captured. Finally, utilizing the available ligands for E3 ligases in the design of proteolysis targeting chimeras (PROTACs) to degrade aberrant proteins is a novel strategy for AD, and we explore the prospects of PROTACs as AD therapeutics.

MARCH8 Restricts Influenza A Virus Infectivity but Does Not Downregulate Viral Glycoprotein Expression at the Surface of Infected Cells

Membrane-associated RING-CH8 (MARCH8) impairs the cell surface expression of envelope glycoproteins from different viruses, reducing their incorporation into virions. Using stable cell lines with inducible MARCH8 expression, we show that MARCH8 did not alter susceptibility to influenza A virus (IAV) infection, but virions released from infected cells were markedly less infectious. Knockdown of endogenous MARCH8 confirmed its effect on IAV infectivity. The expression of MARCH8 impaired the infectivity of both H3N2 and H1N1 strains and was dependent on its E3 ligase activity. Although virions released in the presence of MARCH8 expressed smaller amounts of viral hemagglutinin (HA) and neuraminidase (NA) proteins, there was no impact on levels of the viral HA, NA, or matrix 2 (M2) proteins detected on the surface of infected cells. Moreover, mutation of lysine residues in the cytoplasmic tails of HA, NA, and/or M2, or in the viral M1 protein, did not abrogate MARCH8-mediated restriction. While MARCH1 and -8 target similar immunological ligands and both restrict HIV-1, only MARCH8 inhibited IAV infectivity. Deletion of the N-terminal cytoplasmic (N-CT) domain of MARCH8 confirmed it to be a critical determinant of IAV inhibition. Of interest, deletion of the MARCH1 N-CT or its replacement with the MARCH8 N-CT resulted in acquisition of IAV restriction. Together, these data demonstrate that MARCH8 restricts a late stage in IAV replication by a mechanism distinct to its reported activity against other viruses. Moreover, we show that the N-CT of MARCH8 is essential for anti-IAV activity, whereas the MARCH1 N-CT inhibits its ability to restrict IAV.

HSC70 Inhibits Spring Viremia of Carp Virus Replication by Inducing MARCH8-Mediated Lysosomal Degradation of G Protein

As a fierce pathogen, spring viremia of carp virus (SVCV) can cause high mortality in the common carp, and its glycoprotein (G protein) is a component of the viral structure on the surface of virion, which is crucial in viral life cycle. This report adopted tandem affinity purification (TAP), mass spectrometry analysis (LC-MS/MS), immunoprecipitation, and confocal microscopy assays to identify Heat shock cognate protein 70 (HSC70) as an interaction partner of SVCV G protein. It was found that HSC70 overexpression dramatically inhibited SVCV replication, whereas its loss of functions elicited opposing effects on SVCV replication. Mechanistic studies indicate that HSC70 induces lysosomal degradation of ubiquitinated-SVCV G protein. This study further demonstrates that Membrane-associated RING-CH 8 (MARCH8), an E3 ubiquitin ligase, is critical for SVCV G protein ubiquitylation and leads to its lysosomal degradation. Furthermore, the MARCH8 mediated ubiquitylation of SVCV G protein required the participation of HSC70 through forming a multicomponent complex. Taken together, these results demonstrate that HSC70 serves as a scaffold for MARCH8 and SVCV G, which leads to the ubiquitylation and degradation of SVCV G protein and thus inhibits viral replication. These findings have established a novel host defense mechanism against SVCV.

MARCH8 inhibits influenza A virus infection by targeting viral M2 protein for ubiquitination-dependent degradation in lysosomes

The membrane-associated RING-CH (MARCH) proteins are E3 ligases that regulate the stability of various cellular membrane proteins. MARCH8 has been reported to inhibit the infection of HIV-1 and a few other viruses, thus plays an important role in host antiviral defense. However, the antiviral spectrum and the underlying mechanisms of MARCH8 are incompletely defined. Here, we demonstrate that MARCH8 profoundly inhibits influenza A virus (IAV) replication both in vitro and in mice. Mechanistically, MARCH8 suppresses IAV release through redirecting viral M2 protein from the plasma membrane to lysosomes for degradation. Specifically, MARCH8 catalyzes the K63-linked polyubiquitination of M2 at lysine residue 78 (K78). A recombinant A/Puerto Rico/8/34 virus carrying the K78R M2 protein shows greater replication and more severe pathogenicity in cells and mice. More importantly, we found that the M2 protein of the H1N1 IAV has evolved to acquire non-lysine amino acids at positions 78/79 to resist MARCH8-mediated ubiquitination and degradation. Together, our data support the important role of MARCH8 in host anti-IAV intrinsic immune defense by targeting M2, and suggest the inhibitory pressure of MARCH8 on H1N1 IAV transmission in the human population.

MARCH8 Suppresses Tumor Metastasis and Mediates Degradation of STAT3 and CD44 in Breast Cancer Cells

Protein stability is largely regulated by post-translational modifications, such as ubiquitination, which is mediated by ubiquitin-activating enzyme E1, ubiquitin-conjugating enzyme E2, and ubiquitin ligase E3 with substrate specificity. Membrane-associated RING-CH (MARCH) proteins represent one novel family of transmembrane E3 ligases which target glycoproteins for lysosomal destruction. While most of the MARCH family members are known to degrade membrane proteins in immune cells, their tumor-intrinsic role is largely unknown. In this study, we found that the expression of one MARCH family member, MARCH8, is specifically downregulated in breast cancer tissues and positively correlated with breast cancer survival rate according to bioinformatic analysis of The Cancer Genomic Atlas (TCGA) dataset. MARCH8 protein expression was also lower in a variety of human breast cancer cell lines in comparison to immortalized human mammary epithelial MCF-12A cells. Restoration of MARCH8 expression induced apoptosis in human breast cancer cell lines MDA-MB-231 and BT549. Stable expression of MARCH8 inhibited tumorigenesis and lung metastases of MDA-MB-231 cells in mice. Moreover, we discovered that the breast cancer stem-cell marker and metastasis driver CD44, a membrane protein, interacts with MARCH8 and is one of the glycoprotein targets subject to MARCH8-dependent lysosomal degradation. Unexpectedly, we identified a nonmembrane protein, signal transducer and transcription activator 3 (STAT3), as another essential ubiquitination target of MARCH8, whose degradation through the proteasome pathway is responsible for the proapoptotic changes mediated by MARCH8. These findings highlight a novel tumor-suppressing function of MARCH8 in targeting both membrane and nonmembrane protein targets required for the survival and metastasis of breast cancer cells.

MARCH8: the tie that binds to viruses

Membrane-associated RING-CH (MARCH) family member proteins are RING-finger E3 ubiquitin ligases that are known to downregulate cellular transmembrane proteins. MARCH8 is a novel antiviral factor that inhibits HIV-1 envelope glycoprotein and vesicular stomatitis virus G by downregulating these envelope glycoproteins from the cell surface, resulting in their reduced incorporation into virions. More recently, we have found that MARCH8 reduces viral infectivity via two different mechanisms. Additionally, several groups have reported further antiviral or virus-supportive functions of the MARCH8 protein and its other cellular mechanisms. In this review, we summarize the current knowledge about the molecular mechanisms by which MARCH8 can regulate cellular homeostasis and inhibit and occasionally support enveloped virus infection.

WDR45 Mutation Impairs the Autophagic Degradation of Transferrin Receptor and Promotes Ferroptosis

WDR45 is an autophagy-related protein that involves in the formation of autophagosome. Mutations in WDR45 lead to the impairment of autophagy which is associated with the human β-propeller protein-associated neurodegeneration (BPAN). However, the relationship between autophagy and brain iron accumulation in patients with BPAN remains unclear. Here, we demonstrated that transferrin receptor (TfRC) which is critical for the iron import of cells was degraded via autophagy. TfRC was accumulated after the inhibition of autophagy by treatment with autophagic inhibitor chloroquine or knockdown of ATG2A. The intracellular iron content was increased in cells overexpressing TfRC or mutant WDR45, however, ferritin H (FTH) chain was decreased. Increased TfRC and simultaneously decreased FTH consequently resulted in an elevated level of ferrous iron (Fe²⁺) which further promoted cell ferroptosis, demonstrated by the increased lipid peroxidation and reactive oxygen species (ROS) and the decreased glutathione peroxidase 4 (GPX4) and cell viability. Taken together, these findings provide a piece of important evidence that WDR45 deficiency impairs autophagic degradation of TfRC, therefore leading to iron accumulation, and the elevated iron promotes ferroptosis which may contribute to the progression of BPAN.

Targeting SLC1A5 and SLC3A2/SLC7A5 as a Potential Strategy to Strengthen Anti-Tumor Immunity in the Tumor Microenvironment

Cancer cells are metabolically vigorous and are superior in the uptake of nutrients and in the release of the tumor microenvironment (TME)-specific metabolites. They create an acidic, hypoxic, and nutrient-depleted TME that makes it difficult for the cytotoxic immune cells to adapt to the metabolically hostile environment. Since a robust metabolism in immune cells is required for optimal anti-tumor effector functions, the challenges caused by the TME result in severe defects in the invasion and destruction of the established tumors. There have been many recent developments in NK and T cell-mediated immunotherapy, such as engineering them to express chimeric antigen receptors (CARs) to enhance tumor-recognition and infiltration. However, to defeat the tumor and overcome the limitations of the TME, it is essential to fortify these novel therapies by improving the metabolism of the immune cells. One potential strategy to enhance the metabolic fitness of immune cells is to upregulate the expression of nutrient transporters, specifically glucose and amino acid transporters. In particular, the amino acid transporters SLC1A5 and SLC7A5 as well as the ancillary subunit SLC3A2, which are required for efficient uptake of glutamine and leucine respectively, could strengthen the metabolic capabilities and effector functions of tumor-directed CAR-NK and T cells. In addition to enabling the influx and efflux of essential amino acids through the plasma membrane and within subcellular compartments such as the lysosome and the mitochondria, accumulating evidence has demonstrated that the amino acid transporters participate in sensing amino acid levels and thereby activate mTORC1, a master metabolic regulator that promotes cell metabolism, and induce the expression of c-Myc, a transcription factor essential for cell growth and proliferation. In this review, we discuss the regulatory pathways of these amino acid transporters and how we can take advantage of these processes to strengthen immunotherapy against cancer.

Mechanism of Viral Glycoprotein Targeting by Membrane-Associated RING-CH Proteins

Viral envelope glycoproteins are an important structural component on the surfaces of enveloped viruses that direct virus binding and entry and also serve as targets for the host adaptive immune response. In this study, we investigate the mechanism of action of the MARCH family of cellular proteins that disrupt the trafficking and virion incorporation of viral glycoproteins across several virus families.

An emerging class of cellular inhibitory proteins has been identified that targets viral glycoproteins. These include the membrane-associated RING-CH (MARCH) family of E3 ubiquitin ligases that, among other functions, downregulate cell surface proteins involved in adaptive immunity. The RING-CH domain of MARCH proteins is thought to function by catalyzing the ubiquitination of the cytoplasmic tails (CTs) of target proteins, leading to their degradation. MARCH proteins have recently been reported to target retroviral envelope glycoproteins (Env) and vesicular stomatitis virus G glycoprotein (VSV-G). However, the mechanism of antiviral activity remains poorly defined. Here we show that MARCH8 antagonizes the full-length forms of HIV-1 Env, VSV-G, Ebola virus glycoprotein (EboV-GP), and the spike (S) protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), thereby impairing the infectivity of virions pseudotyped with these viral glycoproteins. This MARCH8-mediated targeting of viral glycoproteins requires the E3 ubiquitin ligase activity of the RING-CH domain. We observe that MARCH8 protein antagonism of VSV-G is CT dependent. In contrast, MARCH8-mediated targeting of HIV-1 Env, EboV-GP, and SARS-CoV-2 S protein by MARCH8 does not require the CT, suggesting a novel mechanism of MARCH-mediated antagonism of these viral glycoproteins. Confocal microscopy data demonstrate that MARCH8 traps the viral glycoproteins in an intracellular compartment. We observe that the endogenous expression of MARCH8 in several relevant human cell types is rapidly inducible by type I interferon. These results help to inform the mechanism by which MARCH proteins exert their antiviral activity and provide insights into the role of cellular inhibitory factors in antagonizing the biogenesis, trafficking, and virion incorporation of viral glycoproteins.

When MARCH family proteins meet viral infections

Membrane-associated RING-CH (MARCH) ubiquitin ligases belong to a RING finger domain E3 ligases family. Recent studies have demonstrated that MARCH proteins play critical roles during various viral infections. MARCH proteins can directly antagonize different steps of the viral life cycle and promote individual viral infection. This mini-review will focus on the latest advances of MARCH family proteins' emerging roles during viral infections.

Elucidating the Antiviral Mechanism of Different MARCH Factors

The membrane-associated RING-CH (MARCH) proteins belong to a family of E3 ubiquitin ligases, whose main function is to remove transmembrane proteins from the plasma membrane. Recent work has shown that the human MARCH1, 2, and 8 are antiretroviral factors that target the human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins by reducing their incorporation in the budding virions. Nevertheless, the dearth of information regarding the antiviral mechanism of this family of proteins necessitates further examination. In this study, using both the human MARCH proteins and their mouse homologues, we provide a comprehensive analysis of the antiretroviral mechanism of this family of proteins. Moreover, we show that human MARCH proteins restrict to various degrees the envelope glycoproteins of a diverse number of viruses. This report sheds light on the important antiviral function of MARCH proteins and their significance in cell intrinsic immunity.IMPORTANCE This study examines the mechanism utilized by different MARCH proteins to restrict retrovirus infection. MARCH proteins block the incorporation of envelope glycoproteins to the budding virions. In this report, by comparing the human and mouse MARCH genes and using murine leukemia virus (MLV) and HIV-1, we identify differences in the mechanism of restriction among MARCH proteins. Furthermore, we perform a comprehensive analysis on a number of envelope glycoproteins and show that MARCH proteins have broad antiviral functions.

FLCN regulates transferrin receptor 1 transport and iron homeostasis

Birt–Hogg–Dubé (BHD) syndrome is a multiorgan disorder caused by inactivation of the folliculin (FLCN) protein. Previously, we identified FLCN as a binding protein of Rab11A, a key regulator of the endocytic recycling pathway. This finding implies that the abnormal localization of specific proteins whose transport requires the FLCN-Rab11A complex may contribute to BHD. Here, we used human kidney-derived HEK293 cells as a model, and we report that FLCN promotes the binding of Rab11A with transferrin receptor 1 (TfR1), which is required for iron uptake through continuous trafficking between the cell surface and the cytoplasm. Loss of FLCN attenuated the Rab11A–TfR1 interaction, resulting in delayed recycling transport of TfR1. This delay caused an iron deficiency condition that induced hypoxia-inducible factor (HIF) activity, which was reversed by iron supplementation. In a Drosophila model of BHD syndrome, we further demonstrated that the phenotype of BHD mutant larvae was substantially rescued by an iron-rich diet. These findings reveal a conserved function of FLCN in iron metabolism and may help to elucidate the mechanisms driving BHD syndrome.

iTRAQ-based analysis for the identification of MARCH8 targets in human esophageal squamous cell carcinoma

MARCH8 is an E3 ligase, primarily involved in immune-modulation. Recently, we reported its aberrant expression in human esophageal squamous cell carcinoma. However, exact mechanisms by which it regulates cancer have been poorly understood. We applied high-throughput quantitative proteomics approach to identify downstream protein targets of MARCH8. Silencing of endogenous MARCH8 in ESCC cells followed by LC-MS/MS analysis led to identification of 1,029 unique proteins showing altered expression post MARCH8 knockdown. Several previously reported MARCH8 target proteins viz. TFR1, syntaxin-4, e-cadherin and CD44 were found to be upregulated. Furthermore, new putative targets of MARCH8, including β2M, were identified in the present study. We demonstrated that MARCH8 interacts with and ubiquitinates CDH1 and β2M. Inhibiting proteasome activity with MG132 prevented CDH1 and β2M degradation, indicating that MARCH8 might be targeting CDH1 and β2M for proteasomal degradation. Further, loss of β2M and CDH1 expression significantly and inversely correlated with MARCH8 expression in ESCC tissues (r =  -0.737 and  - 0.651, respectively; p < 0.01). In conclusion, our present study has led to identification of new targets of MARCH8 and suggests the role of MARCH8 in regulating CDH1 and β2M turnover in esophageal cancer cells. SIGNIFICANCE: The use of quantitative proteomics carried out has led to the recognition of new targets of MARCH8. The present study gives a broad understanding of the molecular remodeling arising in the ESCC after MARCH8 knockdown. The study also solidifies the idea that role of MARCH8 is not just limited to immunomodulation as silencing of MARCH8 affects various other processes such as protein processing and localization. This study might help in understanding the regulation of MARCH8 in ESCCs and the mechanism by which MARCH8 might be facilitating cancer cells to evade immune surveillance.

Mechanism of Viral Glycoprotein Targeting by Membrane-associated-RING-CH Proteins

An emerging class of cellular inhibitory proteins has been identified that targets viral glycoproteins. These include the membrane-associated RING-CH (MARCH) family of E3 ubiquitin ligases that, among other functions, downregulate cell-surface proteins involved in adaptive immunity. The RING-CH domain of MARCH proteins is thought to function by catalyzing the ubiquitination of the cytoplasmic tails (CTs) of target proteins, leading to their degradation. MARCH proteins have recently been reported to target retroviral envelope glycoproteins (Env) and vesicular stomatitis virus G glycoprotein (VSV-G). However, the mechanism of antiviral activity remains poorly defined. Here we show that MARCH8 antagonizes the full-length forms of HIV-1 Env, VSV-G, Ebola virus glycoprotein (EboV-GP), and the spike (S) protein of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) thereby impairing the infectivity of virions pseudotyped with these viral glycoproteins. This MARCH8-mediated targeting of viral glycoproteins requires the E3 ubiquitin ligase activity of the RING-CH domain. We observe that MARCH8 protein antagonism of VSV-G is CT dependent. In contrast, MARCH8-mediated targeting of HIV-1 Env, EboV-GP and SARS-CoV-2 S protein by MARCH8 does not require the CT, suggesting a novel mechanism of MARCH-mediated antagonism of these viral glycoproteins. Confocal microscopy data demonstrate that MARCH8 traps the viral glycoproteins in an intracellular compartment. We observe that the endogenous expression of MARCH8 in several relevant human cell types is rapidly inducible by type I interferon. These results help to inform the mechanism by which MARCH proteins exert their antiviral activity and provide insights into the role of cellular inhibitory factors in antagonizing the biogenesis, trafficking, and virion incorporation of viral glycoproteins.

Unconventional endocytosis and trafficking of transferrin receptor induced by iron

The posttranslational regulation of transferrin receptor (TfR1) is largely unknown. We investigated whether iron availability affects TfR1 endocytic cycle and protein stability in HepG2 hepatoma cells exposed to ferric ammonium citrate (FAC). NH₄Cl and bafilomycin A1, but not the proteasomal inhibitor MG132, prevented the FAC-mediated decrease in TfR1 protein levels, thus indicating lysosomal involvement. Knockdown experiments showed that TfR1 lysosomal degradation is independent of 1) endocytosis mediated by the clathrin adaptor AP2; 2) Tf, which was suggested to facilitate TfR1 internalization; 3) H-ferritin; and 4) MARCH8, previously implicated in TfR1 degradation. Notably, FAC decreased the number of TfR1 molecules at the cell surface and increased the Tf endocytic rate. Colocalization experiments confirmed that, upon FAC treatment, TfR1 was endocytosed in an AP2- and Tf-independent pathway and trafficked to the lysosome for degradation. This unconventional endocytic regulatory mechanism aimed at reducing surface TfR1 may represent an additional posttranslational control to prevent iron overload. Our results show that iron is a key regulator of the trafficking of TfR1, which has been widely used to study endocytosis, often not considering its function in iron homeostasis.

MARCH Proteins Mediate Responses to Antitumor Antibodies

CD98, which is required for the rapid proliferation of both normal and cancer cells, and MET, the hepatocyte growth factor receptor, are potential targets for therapeutic antitumor Abs. In this study, we report that the antiproliferative activity of a prototype anti-CD98 Ab, UM7F8, is due to Ab-induced membrane-associated ring CH (MARCH) E3 ubiquitin ligase-mediated ubiquitination and downregulation of cell surface CD98. MARCH1-mediated ubiquitination of CD98 is required for UM7F8's capacity to reduce CD98 surface expression and its capacity to inhibit the proliferation of murine T cells. Similarly, CD98 ubiquitination is required for UM7F8's capacity to block the colony-forming ability of murine leukemia-initiating cells. To test the potential generality of the paradigm that MARCH E3 ligases can mediate the antiproliferative response to antitumor Abs, we examined the potential effects of MARCH proteins on responses to emibetuzumab, an anti-MET Ab currently in clinical trials for various cancers. We report that MET surface expression is reduced by MARCH1, 4, or 8-mediated ubiquitination and that emibetuzumab-induced MET ubiquitination contributes to its capacity to downregulate MET and inhibit human tumor cell proliferation. Thus, MARCH E3 ligases can act as cofactors for antitumor Abs that target cell surface proteins, suggesting that the MARCH protein repertoire of cells is a determinant of their response to such Abs.

Negative regulation of NEMO signaling by the ubiquitin E3 ligase MARCH2

NF‐κB essential modulator (NEMO) is a key regulatory protein that functions during NF‐κB‐ and interferon‐mediated signaling in response to extracellular stimuli and pathogen infections. Tight regulation of NEMO is essential for host innate immune responses and for maintenance of homeostasis. Here, we report that the E3 ligase MARCH2 is a novel negative regulator of NEMO‐mediated signaling upon bacterial or viral infection. MARCH2 interacted directly with NEMO during the late phase of infection and catalyzed K‐48‐linked ubiquitination of Lys326 on NEMO, which resulted in its degradation. Deletion of MARCH2 resulted in marked resistance to bacterial/viral infection, along with increased innate immune responses both in vitro and in vivo. In addition, MARCH2^−/− mice were more susceptible to LPS challenge due to massive production of cytokines. Taken together, these findings provide new insight into the molecular regulation of NEMO and suggest an important role for MARCH2 in homeostatic control of innate immune responses.

Lack of the E3 Ubiquitin Ligase March1 Affects CD8 T Cell Fate and Exacerbates Insulin Resistance in Obese Mice

Obesity is a major risk factor for the development of insulin resistance and type 2 diabetes. However, the mechanisms that trigger the underlying adipose tissues inflammation are not completely understood. Here, we show that the E3 ubiquitin ligase March1 controls the phenotypic and functional properties of CD8⁺ T cells in mice white adipose tissue. In a diet-induced obesity model, mice lacking March1 [March1 knockout (KO)] show increased insulin resistance compared to their WT counterparts. Also, in obese March1 KO mice, the proportions of effector/memory (Tem) and resident/memory (Trm) CD8⁺ T cells were higher in the visceral adipose tissue, but not in the spleen. The effect of March1 on insulin resistance and on the phenotype of adipose tissue CD8⁺ T cells was independent of major histocompatibility complex class II ubiquitination. Interestingly, we adoptively transferred either WT or March1 KO splenic CD8⁺ T cells into obese WT chimeras that had been reconstituted with Rag1-deficient bone marrow. We observed an enrichment of Tem and Trm cells and exacerbated insulin resistance in mice that received March1 KO CD8 T cells. Mechanistically, we found that March1 deficiency alters the metabolic activity of CD8⁺ T cells. Our results provide additional evidence of the involvement of CD8⁺ T cells in adipose tissue inflammation and suggest that March1 controls the metabolic reprogramming of these cells.

MARCH8 inhibits viral infection by two different mechanisms

Membrane-associated RING-CH 8 (MARCH8) inhibits infection with both HIV-1 and vesicular stomatitis virus G-glycoprotein (VSV-G)-pseudotyped viruses by reducing virion incorporation of envelope glycoproteins. The molecular mechanisms by which MARCH8 targets envelope glycoproteins remain unknown. Here, we show two different mechanisms by which MARCH8 inhibits viral infection. Viruses pseudotyped with the VSV-G mutant, in which cytoplasmic lysine residues were mutated, were insensitive to the inhibitory effect of MARCH8, whereas those with a similar lysine mutant of HIV-1 Env remained sensitive to it. Indeed, the wild-type VSV-G, but not its lysine mutant, was ubiquitinated by MARCH8. Furthermore, the MARCH8 mutant, which had a disrupted cytoplasmic tyrosine motif that is critical for intracellular protein sorting, did not inhibit HIV-1 Env-mediated infection, while it still impaired infection by VSV-G-pseudotyped viruses. Overall, we conclude that MARCH8 reduces viral infectivity by downregulating envelope glycoproteins through two different mechanisms mediated by a ubiquitination-dependent or tyrosine motif-dependent pathway.

Beclin 1 Promotes Endosome Recruitment of Hepatocyte Growth Factor Tyrosine Kinase Substrate to Suppress Tumor Proliferation

Beclin 1 has nonautophagic functions that include its ability to regulate endocytic receptor trafficking. However, the contribution of this function to tumor suppression is poorly understood. Here, we provide in vivo evidence that Beclin 1 suppresses tumor proliferation by regulating the endocytic trafficking and degradation of the EGFR and transferrin (TFR1) receptors. Beclin 1 promoted endosomal recruitment of hepatocyte growth factor tyrosine kinase substrate (HRS), which was necessary for sorting surface receptors to intraluminal vesicles for signal silencing and lysosomal degradation. In tumors with low Beclin 1 expression, endosomal HRS recruitment was diminished and receptor function was sustained. Collectively, our results demonstrate a novel role for Beclin 1 in impeding tumor growth by coordinating the regulation of key growth factor and nutrient receptors. These data provide an explanation for how low levels of Beclin 1 facilitate tumor proliferation and contribute to poor cancer outcomes. SIGNIFICANCE: Beclin 1 controls the trafficking fate of growth regulatory receptors to suppress tumor proliferation.

HIV-1 Envelope Glycoprotein at the Interface of Host Restriction and Virus Evasion

Without viral envelope proteins, viruses cannot enter cells to start infection. As the major viral proteins present on the surface of virions, viral envelope proteins are a prominent target of the host immune system in preventing and ultimately eliminating viral infection. In addition to the well-appreciated adaptive immunity that produces envelope protein-specific antibodies and T cell responses, recent studies have begun to unveil a rich layer of host innate immune mechanisms restricting viral entry. This review focuses on the exciting progress that has been made in this new direction of research, by discussing various known examples of host restriction of viral entry, and diverse viral countering strategies, in particular, the emerging role of viral envelope proteins in evading host innate immune suppression. We will also highlight the effective cooperation between innate and adaptive immunity to achieve the synergistic control of viral infection by targeting viral envelope protein and checking viral escape. Given that many of the related findings were made with HIV-1, we will use HIV-1 as the model virus to illustrate the basic principles and molecular mechanisms on host restriction targeting HIV-1 envelope protein.

Membrane-associated RING-CH (MARCH) 1 and 2 are MARCH family members that inhibit HIV-1 infection

Membrane-associated RING-CH 8 (MARCH8) is one of 11 members of the MARCH family of RING finger E3 ubiquitin ligases and down-regulates several membrane proteins (e.g. major histocompatibility complex II [MHC-II], CD86, and transferrin receptor). We recently reported that MARCH8 also targets HIV-1 envelope glycoproteins and acts as an antiviral factor. However, it remains unclear whether other family members might have antiviral functions similar to those of MARCH8. Here we show that MARCH1 and MARCH2 are MARCH family members that reduce virion incorporation of envelope glycoproteins. Infectivity assays revealed that MARCH1 and MARCH2 dose-dependently suppress viral infection. Treatment with type I interferon enhanced endogenous expression levels of MARCH1 and MARCH2 in monocyte-derived macrophages. Expression of these proteins in virus-producing cells decreased the efficiency of viral entry and down-regulated HIV-1 envelope glycoproteins from the cell surface, resulting in reduced incorporation of envelope glycoproteins into virions, as observed in MARCH8 expression. With the demonstration that MARCH1 and MARCH2 are antiviral MARCH family members as presented here, these two proteins join a growing list of host factors that inhibit HIV-1 infection.

A serine in the first transmembrane domain of the human E3 ubiquitin ligase MARCH9 is critical for down-regulation of its protein substrates

The membrane-associated RING-CH (MARCH) family of membrane-bound E3 ubiquitin ligases regulates the levels of cell-surface membrane proteins, many of which are involved in immune responses. Although their role in ubiquitin-dependent endocytosis and degradation of cell-surface proteins is extensively documented, the features of MARCH proteins and their substrates that drive the molecular recognition events leading to ubiquitin transfer remain poorly defined. In this study, we sought to determine the features of human MARCH9 that are required for regulating the surface levels of its substrate proteins. Consistent with previous studies of other MARCH proteins, we found that susceptibility to MARCH9 activity is encoded in the transmembrane (TM) domains of its substrates. Accordingly, substitutions at specific residues and motifs within MARCH9's TM domains resulted in varying degrees of functional impairment. Most notably, a single serine-to-alanine substitution in the first of its two TM domains rendered MARCH9 completely unable to alter the surface levels of two different substrates: the major histocompatibility class I molecule HLA-A2 and the T-cell co-receptor CD4. Solution NMR analysis of a MARCH9 fragment encompassing the two TM domains and extracellular connecting loop revealed that the residues contributing most to MARCH9 activity are located in the α-helical portions of TM1 and TM2 that are closest to the extracellular face of the lipid bilayer. This observation defines a key region required for substrate regulation. In summary, our biochemical and structural findings demonstrate that specific sequences in the α-helical MARCH9 TM domains make crucial contributions to its ability to down-regulate its protein substrates.

Crossing the Iron Gate: Why and How Transferrin Receptors Mediate Viral Entry

Because both the host and pathogen require iron, the innate immune response carefully orchestrates control over iron metabolism to limit its availability during times of infection. Nutritional iron deficiency can impair host immunity, while iron overload can cause oxidative stress to propagate harmful viral mutations. An emerging enigma is that many viruses use the primary gatekeeper of iron metabolism, the transferrin receptor, as a means to enter cells. Why and how this iron gate is a viral target for infection are the focus of this review.

The transferrin receptor: the cellular iron gate

The transferrin receptor (TfR1), which mediates cellular iron uptake through clathrin-dependent endocytosis of iron-loaded transferrin, plays a key role in iron homeostasis. Since the number of TfR1 molecules at the cell surface is the rate-limiting step for iron entry into cells and is essential to prevent iron overload, TfR1 expression is precisely controlled at multiple levels. In this review, we have discussed the latest advances in the molecular regulation of TfR1 expression and we have considered current understanding of TfR1 function beyond its canonical role in providing iron for erythroid precursors and rapidly proliferating cells.

Salmonella escapes antigen presentation through K63 ubiquitination mediated endosomal proteolysis of MHC II via modulation of endosomal acidification in dendritic cells

CD4+ T-cell response is vital for successful clearance of Salmonella Typhimurium infection. Efficient antigen presentation is crucial for effective CD4+ T-cell response. Previous study has reported that Salmonella abrogates antigen presentation capacity of dendritic cells in order to escape host adaptive immune response. In this study, we have elucidated the mechanism of Salmonella-mediated downregulation of the total cellular Major Histocompatibility Complex (MHC) II pool in dendritic cells. Infected dendritic cells show upregulation of E3 ubiquitin ligase, MARCH1 expression and K63-linked ubiquitination of MHC II. Salmonella infection also enhances the internalisation of ubiquitin-tagged MHC II molecules that are subsequently degraded by endosomal proteases. In addition, Salmonella regulates the activation of endosomal proteases by lowering the pH of endosomes. In infected dendritic cells, Salmonella delays NOX2 recruitment to the phagosomes thereby preventing its alkalinisation. NOX2 is a significant part of innate immune response against pathogens as it is responsible for Reactive Oxygen Species (ROS) production. In this study, we have demonstrated how Salmonella evades MHC II-mediated adaptive immune response in dendritic cells through enhanced endosomal proteolysis. To escape host CD4+T response, Salmonella delays NOX2 recruitment, an innate immune response element to the phagosomes.

Increased expression of MARCH8, an E3 ubiquitin ligase, is associated with growth of esophageal tumor

Background

Herein, for the first time, we report aberrant expression of membrane-associated RING-CH8 (MARCH8) in human esophageal squamous cell carcinoma. MARCH8 is a member of the recently discovered MARCH family of really interesting new genes (RING) E3 ligases. Though initial studies primarily focused on its immunomodulatory role, the newly discovered targets of this E3 ligase point towards its possible role in other biological processes such as embryogenesis and inhibition of apoptosis. However, its relevance in cancers is yet to be elucidated.

Methods

We carried out quantitative real time PCR and immunohistochemistry to examine the levels of MARCH8 mRNA and protein in esophageal squamous cell carcinoma tissues. The role of MARCH8 in esophageal cancer cells was evaluated by cell proliferation, clonogenic and migration/invasion assays and flow cytometry with MARCH8 gene knockdown.

Results

Significantly increased expression of MARCH8 mRNA was found in esophageal squamous cell carcinoma as compared to distant matched non-malignant tissues (p = 0.024, AUC = 0.654). Immunohistochemical analysis revealed overexpression of MARCH8 protein in 86% of esophageal squamous cell carcinoma tissues (p < 0.001, AUC = 0.908). Interestingly, intense nuclear staining of MARCH8 protein was detected in cancer cells in addition to its cytoplasmic expression. Knockdown of MARCH8 resulted in decreased proliferation, migration, invasion and clonogenic potential of esophageal cancer cells. In addition to this, silencing of MARCH8 induced apoptosis in esophageal cancer cells which was measured by cell cycle distribution assay which showed increase in sub G0 and G2/M populations (cell death) and decrease in S-phase population. To further check the type of apoptosis induced by MARCH8 silencing, annexin assay was performed which showed significant increase in the number of cells in early apoptotic phase.

Conclusions

Overall, increased expression of MARCH8 gene in preneoplastic and neoplastic esophageal tissues and its knockdown effect on cancer cell properties demonstrated herein points towards the potential role of this protein in esophageal tumorigenesis.

The Salmonella Effector SteD Mediates MARCH8-Dependent Ubiquitination of MHC II Molecules and Inhibits T Cell Activation

The SPI-2 type III secretion system (T3SS) of intracellular Salmonella enterica translocates effector proteins into mammalian cells. Infection of antigen-presenting cells results in SPI-2 T3SS-dependent ubiquitination and reduction of surface-localized mature MHC class II (mMHCII). We identify the effector SteD as required and sufficient for this process. In Mel Juso cells, SteD localized to the Golgi network and vesicles containing the E3 ubiquitin ligase MARCH8 and mMHCII. SteD caused MARCH8-dependent ubiquitination and depletion of surface mMHCII. One of two transmembrane domains and the C-terminal cytoplasmic region of SteD mediated binding to MARCH8 and mMHCII, respectively. Infection of dendritic cells resulted in SteD-dependent depletion of surface MHCII, the co-stimulatory molecule B7.2, and suppression of T cell activation. SteD also accounted for suppression of T cell activation during Salmonella infection of mice. We propose that SteD is an adaptor, forcing inappropriate ubiquitination of mMHCII by MARCH8 and thereby suppressing T cell activation.

Molecular cloning of the MARCH family in grass carp (Ctenopharyngodon idellus) and their response to grass carp reovirus challenge

Grass carp (Ctenopharyngodon idellus) is an economical aquaculture species in China, and the Grass Carp Reovirus (GCRV) that causes hemorrhagic disease seriously affects the grass carp cultivation industry. Substantial evidence indicates that there is an association between the membrane-associated RING-CH family of E3 ligase (MARCH) family and immune defense in mammals, while functional studies on non-mammalian MARCH proteins are limited. In order to know the characteristics of the MARCH genes in C. idellus, eight MARCH genes (MARCH1, 2, 5, 6, 7, 8, 9 and 11) were cloned and the open reading frames (ORF) were identified in grass carp. All MARCH proteins in grass carp contained an RING-CH domain, which is characteristic of the MARCH protein. The phylogenetic analysis revealed that different MARCH proteins gathered into their separate clusters. All eight members of the MARCH gene family were detected in all tissues sampled, but the relative expression level differed. In addition, the mRNA expression of all the MARCHs was regulated at different levels in the immune organs after a GCRV challenge, and they responded robustly in both the intestine and liver. The mRNA expression of MARCH8, MHC II, TfR, IL1RAP, EGR1, and DUSP1 in the intestine after GCRV infection was analyzed, and the results showed that MARCH8 could negatively regulate TfR, IL1RAP, EGR1, and DUSP1, which signaled via the MAPK or NF-κB-activation pathways that play vital roles in immunity. Our findings identified a novel gene family in C. idellus and provided novel evidence that MARCH genes are inducible and involved in the immune response. Moreover, MARCH8 might function to negatively regulate immune receptors in C. idellus. Therefore, the MARCH might play a vital role in regulating the immune response of C. idellus.

Identification of an antiviral host transmembrane protein MARCH8

Membrane-associated RING-CH 8 (MARCH8) is one of 11 members of the recently discovered MARCH family of RING-finger E3 ubiquitin ligases. MARCH8 downregulates several host transmembrane proteins; however, its physiological roles remain unknown. Here we identify MARCH8 as a novel antiviral factor. The overexpression of MARCH8 in virus producing cells did not affect levels of lentivirus production, but markedly reduced viral infectivity. MARCH8 blocked the incorporation of HIV-1 envelope glycoprotein into virions by downregulating it from the cell surface, probably through their interaction, resulting in reduced viral entry efficiency. The inhibitory effect of MARCH8 on vesicular stomatitis virus G-glycoprotein was even more remarkable, suggesting a broad-spectrum inhibition of enveloped viruses by MARCH8. Importantly, the endogenous expression of MARCH8 was high in monocyte-derived macrophages and dendritic cells, and MARCH8 depletion in macrophages significantly increased the infectivity of virions produced from these cells. Our findings thus indicate that MARCH8, which is highly expressed in terminally differentiated myeloid cells, is a potent antiviral host transmembrane protein that reduces virion incorporation of viral envelope glycoproteins.

Characterization of E3 ligases involved in lysosomal sorting of the HIV-1 restriction factor BST2

The cellular protein BST2 (also known as tetherin) acts as a major intrinsic antiviral protein that prevents the release of enveloped viruses by trapping nascent viral particles at the surface of infected cells. Viruses have evolved specific strategies to displace BST2 from viral budding sites in order to promote virus egress. In HIV-1, the accessory protein Vpu counters BST2 antiviral activity and promotes sorting of BST2 for lysosomal degradation. Vpu increases polyubiquitylation of BST2, a post-translation modification required for Vpu-induced BST2 downregulation, through recruitment of the E3 ligase complex SCF adaptors β-TrCP1 and β-TrCP2 (two isoforms encoded by BTRC and FBXW11, respectively). Herein, we further investigate the role of the ubiquitylation machinery in the lysosomal sorting of BST2. Using a small siRNA screen, we highlighted two additional regulators of BST2 constitutive ubiquitylation and sorting to the lysosomes: the E3 ubiquitin ligases NEDD4 and MARCH8. Interestingly, Vpu does not hijack the cellular machinery that is constitutively involved in BST2 ubiquitylation to sort BST2 for degradation in the lysosomes but instead promotes the recognition of BST2 by β-TrCP proteins. Altogether, our results provide further understanding of the mechanisms underlying BST2 turnover in cells.

Highlighted Article: We identify two E3 ubiquitin ligases, NEDD4 and MARCH8, as regulators of BST2 (tetherin) – a protein that restricts viral release; we thus provide further understanding of the mechanisms underlying BST2 turnover in cells.

Yersinia pestis acetyltransferase-mediated dual acetylation at the serine and lysine residues enhances the auto-ubiquitination of ubiquitin ligase MARCH8 in human cells

Lysine acetylation is known as a post translational modification (PTM) by histone acetyltransferases (HAT) that modifies histones and non-histone proteins to regulate gene expression. Serine acetylation, however, is reported in mammalian hosts by serine acetyltransferase of Yersinia pestis (YopJ) during infection. The protein target and cellular function of bacterial YopJ in mammalian systems are not fully addressed. Here we report dual acetylation at the serine and lysine residues by transiently expressed serine acetyltransferase YopJ mimicking Y. pestis infection in HeLa cells. Using shotgun proteomics followed by label-free quantification, we demonstrate an increase of dual acetylation in YopJ transfected human cells, including 10 Ser- (YopJ/non-YopJ 1.3-fold, p = 0.02) and 8 Lys- (YopJ/non-YopJ 3.5-fold, p = 0.00003) acetylation sites. Specifically, YopJ expression augments acetylation of membrane-associated E3 ubiquitin ligase MARCH8 at the serine residue Sac44, Sac71 and Sac253, and the lysine residue Kac247 and Kac252. YopJ-mediated Ser- and Lys-acetylation of MARCH8 is further confirmed by Western blotting using the specific antibodies against MARCH8 Sac71 and pan-acetyl lysine. Functional study demonstrates that YopJ-mediated Ser- and Lys-acetylation affects the auto-ubiquitination of MARCH8. The mutant C172A of YopJ previously shown to abolish the acetyltransferase activity also reduces Ser- and Lys-acetylation and diminishes the auto-ubiquitination of MARCH8. In support, MARCH8 is indeed acetylated at serine and lysine in vitro by purified YopJ but the activity is reduced by the C172A mutant in YopJ. Our study provides evidence that bacterial serine acetyltransferase YopJ mediates Ser- and Lys-acetylation and affects auto-ubiquitination of ubiquitin ligase MARCH8 in human cells.

The Endocytic Fate of the Transferrin Receptor Is Regulated by c-Abl Kinase

Clathrin-mediated endocytosis of transferrin (Tf) and its cognate receptor (TfR1) is a central pathway supporting the uptake of trophic iron. It has generally been assumed that this is a constitutive process. However, we have reported that the non-receptor tyrosine kinase, Src, is activated by Tf to facilitate the internalization of the Tf-TfR1 ligand-receptor complex. As an extension of these findings, we have tested whether subsequent trafficking steps might be regulated by additional kinase-dependent cascades, and we observed a significant endocytic block by inhibiting c-Abl kinase by a variety of methods. Importantly, Tf internalization was reduced significantly in all of these cell models and could be restored by re-expression of WT c-Abl. Surprisingly, this attenuated Tf-TfR1 endocytosis was due to a substantial drop in both the surface and total cellular receptor levels. Additional studies with the LDL receptor showed a similar effect. Surprisingly, immunofluorescence microscopy of imatinib-treated cells revealed a marked colocalization of internalized TfR1 with late endosomes/lysosomes, whereas attenuating the lysosome function with several inhibitors reduced this receptor loss. Importantly, inhibition of c-Abl resulted in a striking redistribution of the chaperone Hsc70 from a diffuse cytosolic localization to an association with the TfR1 at the late endosome-lysosome. Pharmacological inhibition of Hsc70 ATPase activity in cultured cells by the drug VER155008 prevents this chaperone-receptor interaction, resulting in an accumulation of the TfR1 in the early endosome. Thus, inhibition of c-Abl minimizes receptor recycling pathways and results in chaperone-dependent trafficking of the TfR1 to the lysosome for degradation. These findings implicate a novel role for c-Abl and Hsc70 as an unexpected regulator of Hsc70-mediated transport of trophic receptor cargo between the early and late endosomal compartments.

Ubiquitylation of CD98 limits cell proliferation and clonal expansion

CD98 heavy chain (SLC3A2) facilitates lymphocyte clonal expansion that enables adaptive immunity; however, increased expression of CD98 is also a feature of both lymphomas and leukemias and represents a potential therapeutic target in these diseases. CD98 is transcriptionally regulated and ectopic expression of the membrane-associated RING-CH (MARCH) E3 ubiquitin ligases MARCH1 or MARCH8 leads to ubiquitylation and lysosomal degradation of CD98. Here, we examined the potential role of ubiquitylation in regulating CD98 expression and cell proliferation. We report that blocking ubiquitylation by use of a catalytically inactive MARCH or by creating a ubiquitylation-resistant CD98 mutant, prevents MARCH-induced CD98 downregulation in HeLa cells. March1-null T cells display increased CD98 expression. Similarly, T cells expressing ubiquitylation-resistant CD98 manifest increased proliferation in vitro and clonal expansion in vivo. Thus, ubiquitylation and the resulting downregulation of CD98 can limit cell proliferation and clonal expansion.

MARCH8 inhibits HIV-1 infection by reducing virion incorporation of envelope glycoproteins

Membrane-associated RING-CH 8 (MARCH8) is one of 11 members of the recently discovered MARCH family of RING (really interesting new gene)-finger E3 ubiquitin ligases. MARCH8 downregulates several host transmembrane proteins, including major histocompatibility complex (MHC)-II, CD86, interleukin (IL)-1 receptor accessory protein, TNF-related apoptosis-inducing ligand (TRAIL) receptor 1 and the transferrin receptor. However, its physiological roles remain largely unknown. Here we identify MARCH8 as a novel antiviral factor. The ectopic expression of MARCH8 in virus-producing cells does not affect levels of lentivirus production, but it does markedly reduce viral infectivity. MARCH8 blocks the incorporation of HIV-1 envelope glycoprotein into virus particles by downregulating it from the cell surface, probably through their interaction, resulting in a substantial reduction in the efficiency of viral entry. The inhibitory effect of MARCH8 on vesicular stomatitis virus G-glycoprotein is even more remarkable, suggesting a broad-spectrum inhibition of enveloped viruses by MARCH8. Notably, the endogenous expression of MARCH8 is high in monocyte-derived macrophages and dendritic cells, and MARCH8 knockdown or knockout in macrophages significantly increases the infectivity of virions produced by these cells. Our findings thus indicate that MARCH8 is highly expressed in terminally differentiated myeloid cells, and that it is a potent antiviral protein that targets viral envelope glycoproteins and reduces their incorporation into virions.

Protein homeostasis at the plasma membrane

The plasma membrane (PM) and endocytic protein quality control (QC) in conjunction with the endosomal sorting machinery either repairs or targets conformationally damaged membrane proteins for lysosomal/vacuolar degradation. Here, we provide an overview of emerging aspects of the underlying mechanisms of PM QC that fulfill a critical role in preserving cellular protein homeostasis in health and diseases.

The Membrane Associated RING-CH Proteins: A Family of E3 Ligases with Diverse Roles through the Cell

Since the discovery that conjugation of ubiquitin to proteins can drive proteolytic degradation, ubiquitination has been shown to perform a diverse range of functions in the cell. It plays an important role in endocytosis, signal transduction, trafficking of vesicles inside the cell, and even DNA repair. The process of ubiquitination-mediated control has turned out to be remarkably complex, involving a diverse array of proteins and many levels of control. This review focuses on a family of structurally related E3 ligases termed the membrane-associated RING-CH (MARCH) ubiquitin ligases, which were originally discovered as structural homologs to the virals E3s, K3, and K5 from Kaposi's sarcoma-associated herpesvirus (KSHV). These proteins contain a catalytic RING-CH finger and are typically membrane-bound, with some having up to 14 putative transmembrane domains. Despite several lines of evidence showing that the MARCH proteins play a complex and essential role in several cellular processes, this family remains understudied.

TRE17/USP6 regulates ubiquitylation and trafficking of cargo proteins that enter cells by clathrin-independent endocytosis

Plasma membrane proteins that enter cells by clathrin-independent endocytosis (CIE) are sorted either to lysosomes for degradation or recycled back to the plasma membrane. Expression of some MARCH E3 ubiquitin ligases promotes trafficking of CIE cargo proteins to lysosomes by ubiquitylating the proteins. Here, we show that co-expression of the ubiquitin-specific protease TRE17/USP6 counteracts the MARCH-dependent targeting of CIE cargo proteins, but not that of transferrin receptor, to lysosomes, leading to recovery of the stability and cell surface level of the proteins. The ubiquitylation of CIE cargo proteins by MARCH8 was reversed by TRE17, suggesting that TRE17 leads to deubiquitylation of CIE cargo proteins. The effects of TRE17 were dependent on its deubiquitylating activity and expression of TRE17 alone led to a stabilization of surface major histocompatibility complex class I (MHCI) molecules, a CIE cargo, suggesting that deubiquitylation of endogenous CIE cargo proteins promotes their stability. This study demonstrates that cycles of ubiquitylation and deubiquitylation can determine whether CIE cargo proteins are degraded or recycled.

Cell adhesion in zebrafish embryos is modulated by March 8

March8 is a member of a family of transmembrane E3 ubiquitin ligases that have been studied mostly for their role in the immune system. We find that March8 is expressed in the zebrafish egg and early embryo, suggesting a role in development. Both knock-down and overexpression of March8 leads to abnormal development. The phenotype of zebrafish embryos and Xenopus animal explants overexpressing March8 implicates impairment of cell adhesion as a cause of the effect. In zebrafish embryos and in cultured cells, overexpression of March8 leads to a reduction in the surface levels of E-cadherin, a major cell-cell adhesion molecule. Experiments in cell culture further show that E-cadherin can be ubiquitinated by March8. On the basis of these observations we suggest that March8 functions in the embryo to modulate the strength of cell adhesion by regulating the localization of E-cadherin.