Mediator complex subunit 1 architects a tumorigenic Treg cell program independent of inflammation.

While immunotherapy has revolutionized cancer treatment, its safety has been hampered by immunotherapy-related adverse events. Unexpectedly, we show that Mediator complex subunit 1 (MED1) is required for T regulatory (Treg) cell function specifically in the tumor microenvironment. Treg cell-specific MED1 deletion does not predispose mice to autoimmunity or excessive inflammation. In contrast, MED1 is required for Treg cell promotion of tumor growth because MED1 is required for the terminal differentiation of effector Treg cells in the tumor. Suppression of these terminally differentiated Treg cells is sufficient for eliciting antitumor immunity. Both human and murine Treg cells experience divergent paths of differentiation in tumors and matched tissues with non-malignant inflammation. Collectively, we identify a pathway promoting the differentiation of a Treg cell effector subset specific to tumors and demonstrate that suppression of a subset of Treg cells is sufficient for promoting antitumor immunity in the absence of autoimmune consequences.

Ubiquitin-specific peptidase 22 controls integrin-dependent cancer cell stemness and metastasis.

Integrins plays critical roles in connecting the extracellular matrix and actin skeleton for cell adhesion, migration, signal transduction, and gene transcription, which upregulation is involved in cancer stemness and metastasis. However, the molecular mechanisms underlying how integrins are upregulated in cancer stem cells (CSCs) remain as a biomedical mystery. Herein, we show that the death from cancer signature gene USP22 is essential to maintain the stemness of breast cancer cells through promoting the transcription of a group of integrin family members in particular integrin ß1 (ITGB1). Both genetic and pharmacological USP22 inhibition largely impaired breast cancer stem cell self-renewal and prevented their metastasis. Integrin ß1 reconstitution partially rescued USP22-null breast cancer stemness and their metastasis. At the molecular level, USP22 functions as a bona fide deubiquitinase to protect the proteasomal degradation of the forkhead box M1 (FoxM1), a transcription factor for tumoral ITGB1 gene transcription. Importantly unbiased analysis of the TCGA database revealed a strong positive correlation between the death from cancer signature gene ubiquitin-specific peptidase 22 (USP22) and ITGB1, both of which are critical for cancer stemness, in more than 90% of human cancer types, implying that USP22 functions as a key factor to maintain stemness for a broad spectrum of human cancer types possibly through regulating ITGB1. To support this notion, immunohistochemistry staining detected a positive correlation among USP22, FoxM1 and integrin ß1 in human breast cancers. Collectively, our study identifies the USP22-FoxM1-integrin ß1 signaling axis critical for cancer stemness and offers a potential target for antitumor therapy.

SEL1L-HRD1 ER-associated degradation suppresses hepatocyte hyperproliferation and liver cancer.

Endoplasmic reticulum (ER) homeostasis has been implicated in the pathogenesis of various forms of cancer; however, our understanding of the role of ER quality control mechanisms in tumorigenesis remains incomplete. Here, we show that the SEL1L-HRD1 complex of ER-associated degradation (ERAD) suppresses hepatocyte proliferation and tumorigenesis in mice. Hepatocyte-specific deletion of Sel1L or Hrd1 predisposed mice to diet/chemical-induced tumors. Proteomics screen from SEL1L-deficient livers revealed WNT5A, a tumor suppressor, as an ERAD substrate. Indeed, nascent WNT5A was misfolding prone and degraded by SEL1L-HRD1 ERAD in a quality control capacity. In the absence of ERAD, WNT5A misfolds is largely retained in the ER and forms high-molecular weight aggregates, thereby depicting a loss-of-function effect and attenuating WNT5A-mediated suppression of hepatocyte proliferation. In humans, SEL1L-HRD1 ERAD expression correlated positively with survival time for patients with liver cancer. Overall, our data reveal a key role of SEL1L-HRD1 ERAD in suppressing hepatocyte proliferation and liver cancer.

The ubiquitin-specific peptidase 22 is a deubiquitinase of CD73 in breast cancer cells.

Cancer cells evade the immune system by expressing inhibitory immune checkpoint receptors such as ecto-5'-nucleotidase (NT5E), also known as CD73, which consequently suppress tumor neoantigen-specific immune response. Blockade of CD73 in mouse models of breast cancer showed a reduction in tumor growth and metastasis. CD73 expression is elevated in a variety of human tumors including breast cancer. While the regulation of CD73 expression at the transcriptional level has been well understood, the factors involved in regulating CD73 expression at the post-transcriptional level have not been identified. Herein, we discovered that the ubiquitin-specific peptidase 22 (USP22), a deubiquitinase associated with poor prognosis and overexpressed in breast cancers, is a positive regulator for CD73. Targeted USP22 deletion resulted in a statistically significant reduction in CD73 protein expression. In contrast, CD73 mRNA expression levels were not reduced, but even slightly increased by USP22 deletion. Further analysis demonstrated that USP22 is a deubiquitinase that specifically interacts with and inhibits CD73 ubiquitination. Consequently, USP22 protects CD73 from ubiquitin-mediated proteasomal degradation in breast cancer cells. Targeted USP22 deletion, inhibits syngeneic breast cancer growth. Collectively, our study reveals USP22 as a positive regulator to promote CD73 expression in breast cancer and provides a rationale to target USP22 in antitumor immune therapy.

HRD1-mediated METTL14 degradation regulates m6A mRNA modification to suppress ER proteotoxic liver disease.

Accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) lumen triggers an unfolded protein response (UPR) for stress adaptation, the failure of which induces cell apoptosis and tissue/organ damage. The molecular switches underlying how the UPR selects for stress adaptation over apoptosis remain unknown. Here, we discovered that accumulation of unfolded/misfolded proteins selectively induces N6-adenosine-methyltransferase-14 (METTL14) expression. METTL14 promotes C/EBP-homologous protein (CHOP) mRNA decay through its 3' UTR N6-methyladenosine (m6A) to inhibit its downstream pro-apoptotic target gene expression. UPR induces METTL14 expression by competing against the HRD1-ER-associated degradation (ERAD) machinery to block METTL14 ubiquitination and degradation. Therefore, mice with liver-specific METTL14 deletion are highly susceptible to both acute pharmacological and alpha-1 antitrypsin (AAT) deficiency-induced ER proteotoxic stress and liver injury. Further hepatic CHOP deletion protects METTL14 knockout mice from ER-stress-induced liver damage. Our study reveals a crosstalk between ER stress and mRNA m6A modification pathways, termed the ERm6A pathway, for ER stress adaptation to proteotoxicity.

Inositol-Requiring Enzyme 1α-Mediated Synthesis of Monounsaturated Fatty Acids as a Driver of B Cell Differentiation and Lupus-like Autoimmune Disease.

OBJECTIVE: To explore the molecular mechanisms underlying dysregulation of lipid metabolism in the pathogenesis of systemic lupus erythematosus (SLE). METHODS: B cells in peripheral blood from patients with SLE and healthy controls were stained with BODIPY dye for detection of lipids. Mice with targeted knockout of genes for B cell-specific inositol-requiring enzyme 1α (IRE-1α) and stearoyl-coenzyme A desaturase 1 (SCD-1) were used for studying the influence of the IRE-1α/SCD-1/SCD-2 pathway on B cell differentiation and autoantibody production. The preclinical efficacy of IRE-1α suppression as a treatment for lupus was tested in MRL.Faslpr mice. RESULTS: In cultures with mouse IRE-1α-null B cells, supplementation with monounsaturated fatty acids largely rescued differentiation of plasma cells from B cells, indicating that the compromised capacity of B cell differentiation in the absence of IRE-1α may be attributable to a defect in monounsaturated fatty acid synthesis. Moreover, activation with IRE-1α/X-box binding protein 1 (XBP-1) was required to facilitate B cell expression of SCD-1 and SCD-2, which are 2 critical enzymes that catalyze monounsaturated fatty acid synthesis. Mice with targeted Scd1 gene deletion displayed a phenotype that was similar to that of IRE-1α-deficient mice, with diminished B cell differentiation into plasma cells. Importantly, in B cells from patients with lupus, both IRE-1α expression and Xbp1 messenger RNA splicing were significantly increased, and this was positively correlated with the expression of both Scd1 and Scd2 as well as with the amount of B cell lipid deposition. In MRL.Faslpr mice, both genetic and pharmacologic suppression of IRE-1α protected against the pathologic development and progression of lupus-like autoimmune disease. CONCLUSION: The results of this study reveal a molecular link in the dysregulation of lipid metabolism in the pathogenesis of lupus, demonstrating that the IRE-1α/XBP-1 pathway controls plasma cell differentiation through SCD-1/SCD-2-mediated monounsaturated fatty acid synthesis. These findings provide a rationale for targeting IRE-1α and monounsaturated fatty acid synthesis in the treatment of patients with SLE.

USP22 controls iNKT immunity through MED1 suppression of histone H2A monoubiquitination.

The ubiquitin pathway has been shown to regulate iNKT cell immunity, but the deubiquitinase involved in this process has not been identified. Herein we found that ubiquitin-specific peptidase 22 (USP22) is highly expressed in iNKT cells during their early developmental stage 1. USP22 deficiency blocked the transition from stage 1 to 2 during iNKT cell development in a cell-intrinsic manner. USP22 suppression also diminishes iNKT17 and iNKT1 differentiation but favors iNKT2 polarization without altering conventional T cell activation and differentiation. USP22 interacts with the Mediator complex subunit 1 (MED1), a transcription coactivator involved in iNKT cell development. Interestingly, while interacting with MED1, USP22 does not function as a deubiquitinase to suppress MED1 ubiquitination for its stabilization. Instead, USP22 enhances MED1 functions for IL-2Rß and T-bet gene expression through deubiquitinating histone H2A but not H2B monoubiquitination. Therefore, our study revealed USP22-mediated histone H2A deubiquitination fine-tunes MED1 transcriptional activation as a previously unappreciated molecular mechanism to control iNKT development and functions.

The E3 ligase Hrd1 stabilizes Tregs by antagonizing inflammatory cytokine-induced ER stress response.

Treg differentiation, maintenance, and function are controlled by the transcription factor FoxP3, which can be destabilized under inflammatory or other pathological conditions. Tregs can be destabilized under inflammatory or other pathological conditions, but the underlying mechanisms are not fully defined. Herein, we show that inflammatory cytokines induce ER stress response, which destabilizes Tregs by suppressing FoxP3 expression, suggesting a critical role of the ER stress response in maintaining Treg stability. Indeed, genetic deletion of Hrd1, an E3 ligase critical in suppressing the ER stress response, leads to elevated expression of ER stress-responsive genes in Treg and largely diminishes Treg suppressive functions under inflammatory condition. Mice with Treg-specific ablation of Hrd1 displayed massive multiorgan lymphocyte infiltration, body weight loss, and the development of severe small intestine inflammation with aging. At the molecular level, the deletion of Hrd1 led to the activation of both the ER stress sensor IRE1α and its downstream MAPK p38. Pharmacological suppression of IRE1α kinase, but not its endoribonuclease activity, diminished the elevated p38 activation and fully rescued the stability of Hrd1-null Tregs. Taken together, our studies reveal ER stress response as a previously unappreciated mechanism underlying Treg instability and that Hrd1 is crucial for maintaining Treg stability and functions through suppressing the IRE1α-mediated ER stress response.

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.

ER-associated ubiquitin ligase HRD1 programs liver metabolism by targeting multiple metabolic enzymes.

The HMG-CoA reductase degradation protein 1 (HRD1) has been identified as a key enzyme for endoplasmic reticulum-associated degradation of misfolded proteins, but its organ-specific physiological functions remain largely undefined. Here we show that mice with HRD1 deletion specifically in the liver display increased energy expenditure and are resistant to HFD-induced obesity and liver steatosis and insulin resistance. Proteomic analysis identifies a HRD1 interactome, a large portion of which includes metabolic regulators. Loss of HRD1 results in elevated ENTPD5, CPT2, RMND1, and HSD17B4 protein levels and a consequent hyperactivation of both AMPK and AKT pathways. Genome-wide mRNA sequencing revealed that HRD1-deficiency reprograms liver metabolic gene expression profiles, including suppressing genes involved in glycogenesis and lipogenesis and upregulating genes involved in glycolysis and fatty acid oxidation. We propose HRD1 as a liver metabolic regulator and a potential drug target for obesity, fatty liver disease, and insulin resistance associated with the metabolic syndrome.

The endoplasmic reticulum-resident E3 ubiquitin ligase Hrd1 controls a critical checkpoint in B cell development in mice.

Humoral immunity involves multiple checkpoints that occur in B cell development, maturation, and activation. The pre-B-cell receptor (pre-BCR) is expressed following the productive recombination of the immunoglobulin heavy-chain gene, and sSignalsing through the pre-BCR are required for the differentiation of pre-B cells into immature B cells. However, the molecular mechanisms controlling the pre-BCR expression and signaling strength remain undefined. Herein, we probed the role of the endoplasmic reticulum-associated, stress-activated E3 ubiquitin ligase HMG-CoA reductase degradation 1 (Hrd1) in B cell differentiation. Using mice with a specific Hrd1 deletion in pro-B cells and subsequent B cell developmental stages, we showed that the E3 ubiquitin ligase Hrd1 governs a critical checkpoint during B cell development. We observed that Hrd1 is required for degradation of the pre-BCR complex during the early stage of B cell development. As a consequence, loss of Hrd1 in the B cell lineage resulted in increased pre-BCR expression levels and a developmental defect in the transition from large to small pre-B cells. This defect, in turn, resulted in reduced fewer mature B cells in bone marrow and peripheral lymphoid organs. Our results revealed a novel critical role of Hrd1 in controlling a critical checkpoint in B cell-mediated immunity and suggest that Hrd1 may functioning as an E3 ubiquitin ligase of the pre-BCR complex.

USP22 deficiency leads to myeloid leukemia upon oncogenic Kras activation through a PU.1-dependent mechanism.

Ras mutations are commonly observed in juvenile myelomonocytic leukemia (JMML) and chronic myelomonocytic leukemia (CMML). JMML and CMML transform into acute myeloid leukemia (AML) in about 10% and 50% of patients, respectively. However, how additional events cooperate with Ras to promote this transformation are largely unknown. We show that absence of the ubiquitin-specific peptidase 22 (USP22), a component of the Spt-Ada-GCN5-acetyltransferase chromatin-remodeling complex that is linked to cancer progression, unexpectedly promotes AML transformation in mice expressing oncogenic KrasG12D/+ USP22 deficiency in KrasG12D/+ mice resulted in shorter survival compared with control mice. This was due to a block in myeloid cell differentiation leading to the generation of AML. This effect was cell autonomous because mice transplanted with USP22-deficient KrasG12D/+ cells developed an aggressive disease and died rapidly. The transcriptome profile of USP22-deficient KrasG12D/+ progenitors resembled leukemic stem cells and was highly correlated with genes associated with poor prognosis in AML. We show that USP22 functions as a PU.1 deubiquitylase by positively regulating its protein stability and promoting the expression of PU.1 target genes. Reconstitution of PU.1 overexpression in USP22-deficient KrasG12D/+ progenitors rescued their differentiation. Our findings uncovered an unexpected role for USP22 in Ras-induced leukemogenesis and provide further insights into the function of USP22 in carcinogenesis.

JAK1-mediated Sirt1 phosphorylation functions as a negative feedback of the JAK1-STAT3 pathway.

The type III NAD-dependent histone deacetylase Sirt1 plays important roles in a variety of pathobiological functions through targeting either the acetylated histones or transcription factors. However, the molecular mechanisms underlying how the Sirt1 functions are regulated remain vague. Herein we identified that the Janus kinase 1 (JAK1) interacts with Sirt1 and catalyzes its phosphorylation at the tyrosine residues of 280 and 301, both of which are highly conserved and located in the histone deacetylase catalytic domain of Sirt1. IL-6 stimulation enhanced Sirt1 interaction with JAK1 and JAK1-mediated Sirt1 phosphorylation. Interestingly, JAK1-mediated Sirt1 phosphorylation did not alter Sirt1 deacetylase catalytic activity, but instead it is required for Sirt1 interaction with the downstream transcription factor STAT3. JAK1-mediated phosphorylation enhanced Sirt1 suppression of STAT3 acetylation and transcriptional activity. As a consequence, Sirt1 activation attenuates IL-6 activity in protecting cancer cells from chemotherapeutic drug-induced apoptosis. Our studies identify JAK1 as a previously unappreciated tyrosine kinase of Sirt1 and reveal a novel negative feedback of the JAK1-STAT3 pathway.

The Lysine Acetyltransferase GCN5 Is Required for iNKT Cell Development through EGR2 Acetylation.

The development of CD1d-restricted invariant natural killer T (iNKT) cells, a population that is critical for both innate and adaptive immunity, is regulated by multiple transcription factors, but the molecular mechanisms underlying how the transcriptional activation of these factors are regulated during iNKT development remain largely unknown. We found that the histone acetyltransferase general control non-derepressible 5 (GCN5) is essential for iNKT cell development during the maturation stage. GCN5 deficiency blocked iNKT cell development in a cell-intrinsic manner. At the molecular level, GCN5 is a specific lysine acetyltransferase of early growth responsive gene 2 (EGR2), a transcription factor required for iNKT cell development. GCN5-mediated acetylation positively regulated EGR2 transcriptional activity, and both genetic and pharmacological GCN5 suppression specifically inhibited the transcription of EGR2 target genes in iNKT cells, including Runx1, promyelocytic leukemia zinc finger protein (PLZF), interleukin (IL)-2Rb, and T-bet. Therefore, our study revealed GCN5-mediated EGR2 acetylation as a molecular mechanism that regulates iNKT development.

The Histone Acetyltransferase Gcn5 Positively Regulates T Cell Activation.

Histone acetyltransferases (HATs) regulate inducible transcription in multiple cellular processes and during inflammatory and immune response. However, the functions of general control nonrepressed-protein 5 (Gcn5), an evolutionarily conserved HAT from yeast to human, in immune regulation remain unappreciated. In this study, we conditionally deleted Gcn5 (encoded by the Kat2a gene) specifically in T lymphocytes by crossing floxed Gcn5 and Lck-Cre mice, and demonstrated that Gcn5 plays important roles in multiple stages of T cell functions including development, clonal expansion, and differentiation. Loss of Gcn5 functions impaired T cell proliferation, IL-2 production, and Th1/Th17, but not Th2 and regulatory T cell differentiation. Gcn5 is recruited onto the il-2 promoter by interacting with the NFAT in T cells upon TCR stimulation. Interestingly, instead of directly acetylating NFAT, Gcn5 catalyzes histone H3 lysine H9 acetylation to promote IL-2 production. T cell-specific suppression of Gcn5 partially protected mice from myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis, an experimental model for human multiple sclerosis. Our study reveals previously unknown physiological functions for Gcn5 and a molecular mechanism underlying these functions in regulating T cell immunity. Hence Gcn5 may be an important new target for autoimmune disease therapy.

Endoplasmic reticulum-resident E3 ubiquitin ligase Hrd1 controls B-cell immunity through degradation of the death receptor CD95/Fas.

Humoral immunity involves multiple checkpoints during B-cell development, maturation, and activation. The cell death receptor CD95/Fas-mediated apoptosis plays a critical role in eliminating the unwanted activation of B cells by self-reactive antigens and in maintaining B-cell homeostasis through activation-induced B-cell death (AICD). The molecular mechanisms controlling AICD remain largely undefined. Herein, we show that the E3 ubiquitin ligase Hrd1 protected B cells from activation-induced cell death by degrading the death receptor Fas. Hrd1-null B cells exhibited high Fas expression during activation and rapidly underwent Fas-mediated apoptosis, which could be largely inhibited by FasL neutralization. Fas mutation in Hrd1 KO mice abrogated the increase in B-cell AICD. We identified Hrd1 as the first E3 ubiquitin ligase of the death receptor Fas and Hrd1-mediated Fas destruction as a molecular mechanism in regulating B-cell immunity.

The ER membrane-anchored ubiquitin ligase Hrd1 is a positive regulator of T-cell immunity.

Identification of positive regulators of T-cell immunity induced during autoimmune diseases is critical for developing novel therapies. The endoplasmic reticulum resident ubiquitin ligase Hrd1 has recently emerged as a critical regulator of dendritic cell antigen presentation, but its role in T-cell immunity is unknown. Here we show that genetic deletion of Hrd1 in mice inhibits T-cell proliferation, production of IL-2, and differentiation of Th1 and Th17 cells, and consequently protects mice from experimental autoimmune encephalomyelitis. Hrd1 facilitates T-cell proliferation by the destruction of cyclin-dependent kinase inhibitor p27(kip1), and deletion of p27(kip1) in Hrd1-null T-cells rescues proliferative capacity but not the production of cytokines, including IL-2, IFN-γ and IL-17. T-cell expression of Hrd1 is higher in patients with multiple sclerosis than in healthy individuals, and knockdown of Hrd1 in human CD4(+) T cells inhibits activation and differentiation to Th1 and Th17 cells. Our study identifies Hrd1 as a previously unappreciated positive regulator of T cells and implies that Hrd1 is a potential therapeutic target for autoimmune diseases.

The Histone Acetyltransferase GCN5 Expression Is Elevated and Regulated by c-Myc and E2F1 Transcription Factors in Human Colon Cancer.

The histone acetyltransferase GCN5 has been suggested to be involved in promoting cancer cell growth. But its role in human colon cancer development remains unknown. Herein we discovered that GCN5 expression is significantly upregulated in human colon adenocarcinoma tissues. We further demonstrate that GCN5 is upregulated in human colon cancer at the mRNA level. Surprisingly, two transcription factors, the oncogenic c-Myc and the proapoptotic E2F1, are responsible for GCN5 mRNA transcription. Knockdown of c-Myc inhibited colon cancer cell proliferation largely through downregulating GCN5 transcription, which can be fully rescued by the ectopic GCN5 expression. In contrast, E2F1 expression induced human colon cancer cell death, and suppression of GCN5 expression in cells with E2F1 overexpression further facilitated cell apoptosis, suggesting that GCN5 expression is induced by E2F1 as a possible negative feedback in suppressing E2F1-mediated cell apoptosis. In addition, suppression of GCN5 with its specific inhibitor CPTH2 inhibited human colon cancer cell growth. Our studies reveal that GCN5 plays a positive role in human colon cancer development, and its suppression holds a great therapeutic potential in antitumor therapy.

IRE1α is an endogenous substrate of endoplasmic-reticulum-associated degradation.

Endoplasmic reticulum (ER)-associated degradation (ERAD) represents a principle quality control mechanism to clear misfolded proteins in the ER; however, its physiological significance and the nature of endogenous ERAD substrates remain largely unexplored. Here we discover that IRE1α, the sensor of the unfolded protein response (UPR), is a bona fide substrate of the Sel1L-Hrd1 ERAD complex. ERAD-mediated IRE1α degradation occurs under basal conditions in a BiP-dependent manner, requires both the intramembrane hydrophilic residues of IRE1α and the lectin protein OS9, and is attenuated by ER stress. ERAD deficiency causes IRE1α protein stabilization, accumulation and mild activation both in vitro and in vivo. Although enterocyte-specific Sel1L-knockout mice (Sel1L(ΔIEC)) are viable and seem normal, they are highly susceptible to experimental colitis and inflammation-associated dysbiosis, in an IRE1α-dependent but CHOP-independent manner. Hence, Sel1L-Hrd1 ERAD serves a distinct, essential function in restraint of IRE1α signalling in vivo by managing its protein turnover.

Icariin induces synoviolin expression through NFE2L1 to protect neurons from ER stress-induced apoptosis.

By suppressing neuronal apoptosis, Icariin is a potential therapeutic drug for neuronal degenerative diseases. The molecular mechanisms of Icariin anti-apoptotic functions are still largely unclear. In this report, we found that Icariin induces the expression of Synoviolin, an endoplasmic reticulum (ER)-anchoring E3 ubiquitin ligase that functions as a suppressor of ER stress-induced apoptosis. The nuclear factor erythroid 2-related factor 1 (NFE2L1) is responsible for Icariin-mediated Synoviolin gene expression. Mutation of the NFE2L1-binding sites in a distal region of the Synoviolin promoter abolished Icariin-induced Synoviolin promoter activity, and knockdown of NFE2L1 expression prevented Icariin-stimulated Synoviolin expression. More importantly, Icariin protected ER stress-induced apoptosis of PC12 cells in a Synoviolin-dependent manner. Therefore, our study reveals Icariin-induced Synoviolin expression through NFE2L1 as a previously unappreciated molecular mechanism underlying the neuronal protective function of Icariin.