Non-proteolytic ubiquitin modification of PPARγ by Smurf1 protects the liver from steatosis

YTHDC1 promotes postnatal brown adipose tissue development and thermogenesis by stabilizing PPARγ

Brown adipose tissue (BAT) plays a vital role in non-shivering thermogenesis and energy metabolism and is influenced by factors like environmental temperature, ageing, and obesity. However, the molecular mechanisms behind BAT development and thermogenesis are not fully understood. Our study identifies the m6A reader protein YTHDC1 as a crucial regulator of postnatal interscapular BAT development and energy metabolism in mice. YTHDC1 directly interacts with PPARγ through its intrinsically disordered region (IDR), thus protecting PPARγ from binding the E3 ubiquitin ligase ARIH2, and preventing its ubiquitin-mediated proteasomal degradation. Specifically, the ARIH2 RING2 domain is essential for PPARγ degradation, while PPARγ's A/B domain is necessary for their interaction. Deletion of Ythdc1 in BAT increases PPARγ degradation, impairing interscapular BAT development, thermogenesis, and overall energy expenditure. These findings reveal a novel mechanism by which YTHDC1 regulates BAT development and energy homeostasis independently of its m6A recognition function.

SDCCAG3 inhibits adipocyte hypertrophy and improves obesity-related metabolic disorders via SDCCAG3/SMURF1/PPARγ axis

Obesity is a prevalent global disease associated with various metabolic disorders. The expansion of white adipose tissue plays a pivotal role in regulating obesity-related metabolic dysfunctions. This study identified serum-defined colon cancer antigen 3 (SDCCAG3) as a novel key modulator of adipocyte metabolism. In adipose-specific SDCCAG3 knockout mice fed a high-fat diet, pathological expansion of adipose tissue, impaired glucose tolerance, insulin resistance, increased inflammatory markers, and augmented hepatic lipid accumulation were observed. Conversely, obesity models by specific overexpression of SDCCAG3 in adipose tissue confirmed that SDCCAG3 alleviated pathological expansion of adipose tissue, improved obesity-related metabolic disorders, with no observed changes in adipose tissue development under normal dietary conditions. Mechanistically, SDCCAG3 enhanced the stability of peroxisome proliferator-activated receptor gamma (PPARγ) by preventing its degradation via the ubiquitin-proteasome system through the SMAD specific E3 ubiquitin protein ligase 1 (SMURF1). Additionally, SDCCAG3 was subjected to negative transcriptional regulation by PPARγ, forming a SDCCAG3-PPARγ-SDCCAG3 loop that enhanced adipocyte lipid metabolism. Collectively, these findings demonstrated that SDCCAG3 functioned as a beneficial positive regulator of adipose tissue expansion and metabolic homeostasis, indicating its potential as a therapeutic target for metabolic diseases associated with nutrient excess.

The emerging role of E3 ubiquitin ligases and deubiquitinases in metabolic dysfunction-associated steatotic liver disease

Metabolic dysfunction-associated steatotic liver disease (MASLD) is the most common chronic liver disease worldwide, with a prevalence as high as 32.4%. MASLD encompasses a spectrum of liver pathologies, ranging from steatosis to metabolic dysfunction-associated steatohepatitis (MASH), fibrosis, and, in some cases, progression to end-stage liver disease (cirrhosis and hepatocellular carcinoma). A comprehensive understanding of the pathogenesis of this highly prevalent liver disease may facilitate the identification of novel targets for the development of improved therapies. E3 ubiquitin ligases and deubiquitinases (DUBs) are key regulatory components of the ubiquitin‒proteasome system (UPS), which plays a pivotal role in maintaining intracellular protein homeostasis. Emerging evidence implicates that aberrant expression of E3 ligases and DUBs is involved in the progression of MASLD. Here, we review abnormalities in E3 ligases and DUBs by (1) discussing their targets, mechanisms, and functions in MASLD; (2) summarizing pharmacological interventions targeting these enzymes in preclinical and clinical studies; and (3) addressing challenges and future therapeutic strategies. This review synthesizes current evidence to highlight the development of novel therapeutic strategies based on the UPS for MASLD and progressive liver disease.

The ubiquitin-proteasome system: A potential target for the MASLD

Metabolic dysfunction-associated steatotic liver disease (MASLD), the most prevalent chronic liver condition globally, lacks adequate and effective therapeutic remedies in clinical practice. Recent studies have increasingly highlighted the close connection between the ubiquitin-proteasome system (UPS) and the progression of MASLD. This relationship is crucial for understanding the disease's underlying mechanism. As a sophisticated process, the UPS govern protein stability and function, maintaining protein homeostasis, thus influencing a multitude of elements and biological events of eukaryotic cells. It comprises four enzyme families, namely, ubiquitin-activating enzymes (E1), ubiquitin-conjugating enzymes (E2), ubiquitin-protein ligases (E3), and deubiquitinating enzymes (DUBs). This review aims to delve into the array of pathways and therapeutic targets implicated in the ubiquitination within the pathogenesis of MASLD. Therefore, this review unveils the role of ubiquitination in MASLD while spotlighting potential therapeutic targets within the context of this disease.

Targeting PPARγ via SIAH1/2-mediated ubiquitin-proteasomal degradation as a new therapeutic approach in luminal-type bladder cancer

Bladder cancer (BC) is the second most prevalent genitourinary malignancy worldwide. Despite recent approvals of immune checkpoint inhibitors and targeted therapy for muscle invasive or recurrent BC, options remain limited for patients with non-muscle invasive BC (NMIBC) refractory to Bacillus Calmette-Guérin (BCG) and chemotherapy. NMIBC is more frequently classified as a luminal subtype, in which increased PPARγ activity is a key feature in promoting tumor growth and evasion of immunosurveillance. Cinobufotalin is one of the major compound of bufadienolides, the primary active components of toad venom that has been utilized in the clinical treatment of cancer. We herein focused on cinobufotalin, examining its anticancer activity and molecular mechanisms in luminal-type NMIBC. Our results newly reveal that cinobufotalin strongly suppresses the viability and proliferation of luminal BC cells with minimal cytotoxic effects on normal uroepithelial cells, and exhibits significant antitumor activity in a RT112 xenograft BC model. Mechanistically, our sub-G1-phase cell accumulation, Annexin V staining, caspase-3/8/9 activation, and PARP activation analyses show that cinobufotalin induces apoptosis in luminal-type BC cells. Cinobufotalin significantly inhibited the levels of PPARγ and its downstream targets, as well as lipid droplet formation and free fatty acid levels in RT112 cells. PPARγ overexpression rescued RT112 cells from cinobufotalin-induced apoptosis and mitigated the downregulation of FASN and PLIN4. Finally, we show seemingly for the first time that cinobufotalin promotes SIAH1/2-mediated proteasomal degradation of PPARγ in luminal BC cells. Together, these findings compellingly support the idea that cinobufotalin could be developed as a promising therapeutic agent for treating luminal-type NMIBC.

Regulation of lipid metabolism by E3 ubiquitin ligases in lipid-associated metabolic diseases

Previous studies have progressively elucidated the involvement of E3 ubiquitin (Ub) ligases in regulating lipid metabolism. Ubiquitination, facilitated by E3 Ub ligases, modifies critical enzymes in lipid metabolism, enabling them to respond to specific signals. In this review, we aim to present a comprehensive analysis of the role of E3 Ub ligases in lipid metabolism, which includes lipid synthesis and lipolysis, and their influence on cellular lipid homeostasis through the modulation of lipid uptake and efflux. Furthermore, it explores how the ubiquitination process governs the degradation or activation of pivotal enzymes, thereby regulating lipid metabolism at the transcriptional level. Perturbations in lipid metabolism have been implicated in various diseases, including hepatic lipid metabolism disorders, atherosclerosis, diabetes, and cancer. Therefore, this review focuses on the association between E3 Ub ligases and lipid metabolism in lipid-related diseases, highlighting enzymes critically involved in lipid synthesis and catabolism, transcriptional regulators, lipid uptake translocators, and transporters. Overall, this review aims to identify gaps in current knowledge, highlight areas requiring further research, offer potential targeted therapeutic approaches, and provide a comprehensive outlook on clinical conditions associated with lipid metabolic diseases.

Molecular mechanism and therapeutic significance of essential amino acids in metabolically associated fatty liver disease

Non-alcoholic fatty liver disease (NAFLD), also known as metabolically associated fatty liver disease (MAFLD), is a systemic metabolic disease characterized by lipid accumulation in the liver, lipid toxicity, insulin resistance, intestinal dysbiosis, and inflammation that can progress from simple steatosis to nonalcoholic steatohepatitis (NASH) and even cirrhosis or cancer. It is the most prevalent illness threatening world health. Currently, there are almost no approved drug interventions for MAFLD, mainly dietary changes and exercise to control weight and regulate metabolic disorders. Meanwhile, the metabolic pathway involved in amino acid metabolism also influences the onset and development of MAFLD in the body, and most amino acid metabolism takes place in the liver. Essential amino acids are those amino acids that must be supplemented from outside the diet and that cannot be synthesized in the body or cannot be synthesized at a rate sufficient to meet the body's needs, including leucine, isoleucine, valine (collectively known as branched-chain amino acids), tryptophan, phenylalanine (which are aromatic amino acids), histidine, methionine, threonine and lysine. The metabolic balance of the body is closely linked to these essential amino acids, and essential amino acids are closely linked to the pathophysiological process of MAFLD. In this paper, we will focus on the metabolism of essential amino acids in the body and further explore the therapeutic strategies for MAFLD based on the studies conducted in recent years.

The E3 ligase SMURF1 stabilizes p27 via UbcH7 catalyzed K29-linked ubiquitin chains to promote cell migration SMURF1-UbcH7 K29 ubiquitination of p27 and cell migration

Ubiquitination is a key regulator of protein stability and function. The multifunctional protein p27 is known to be degraded by the proteasome following K48-linked ubiquitination. However, we recently reported that when the ubiquitin-conjugating enzyme UbcH7 (UBE2L3) is overexpressed, p27 is stabilized, and cell cycle is arrested in multiple diverse cell types including eye lens, retina, HEK-293, and HELA cells. However, the ubiquitin ligase associated with this stabilization of p27 remained a mystery. Starting with an in vitro ubiquitination screen, we identified RSP5 as the yeast E3 ligase partner of UbcH7 in the ubiquitination of p27. Screening of the homologous human NEDD4 family of E3 ligases revealed that SMURF1 but not its close homolog SMURF2, stabilizes p27 in cells. We found that SMURF1 ubiquitinates p27 with K29O but not K29R or K63O ubiquitin in vitro, demonstrating a strong preference for K29 chain formation. Consistent with SMURF1/UbcH7 stabilization of p27, we also found that SMURF1, UbcH7, and p27 promote cell migration, whereas knockdown of SMURF1 or UbcH7 reduces cell migration. We further demonstrated the colocalization of SMURF1/p27 and UbcH7/p27 at the leading edge of migrating cells. In sum, these results indicate that SMURF1 and UbcH7 work together to produce K29-linked ubiquitin chains on p27, resulting in the stabilization of p27 and promoting its cell-cycle independent function of regulating cell migration.

Ubiquitination and Metabolic Disease

The increasing incidence of metabolic diseases, including obesity, type 2 diabetes mellitus (T2DM), and non-alcoholic fatty liver disease (NAFLD), in the past decade is a serious concern worldwide. Disruption of cellular protein homeostasis has been considered as a crucial contributor to the pathogenesis of metabolic diseases. To maintain protein homeostasis, cells have evolved multiple dynamic and self-regulating quality control processes to adapt new environmental conditions and prevent prolonged damage. Among them, the ubiquitin proteasome system (UPS), the primary proteolytic pathway for degradation of aberrant proteins via ubiquitination, has an essential role in maintaining cellular homeostasis in response to intracellular stress. Correspondingly, accumulating evidences have shown that dysregulation of ubiquitination can aggravate various metabolic derangements in many tissues, including the liver, skeletal muscle, pancreas, and adipose tissue, and is involved in the initiation and progression of diverse metabolic diseases. In this part, we will summarize the role of ubiquitination in the pathogenesis of metabolic diseases, including obesity, T2DM and NAFLD, and discuss its potential as a therapeutic target.

USP14 governs CYP2E1 to promote nonalcoholic fatty liver disease through deubiquitination and stabilization of HSP90AA1

Nonalcoholic fatty liver disease (NAFLD) begins with excessive triglyceride accumulation in the liver, and overly severe hepatic steatosis progresses to nonalcoholic steatohepatitis (NASH), which is characterized by lipid peroxidation, inflammation, and fibrosis. Ubiquitin-specific proteinase 14 (USP14) regulates inflammation, hepatocellular carcinoma and viral infection, but the effect of USP14 on NAFLD is unknown. The aim of this study was to reveal the role of USP14 in the progression of NAFLD and its underlying mechanism. We demonstrated that hepatic USP14 expression was significantly increased in NAFLD in both humans and mice. Hepatic USP14 overexpression exacerbated diet-induced hepatic steatosis, inflammation and fibrosis in mice, in contrast to the results of hepatic USP14 knockdown. Furthermore, palmitic/oleic acid-induced lipid peroxidation and inflammation in hepatocytes were markedly increased by USP14 overexpression but decreased by USP14 knockdown. Notably, in vivo or in vitro data show that USP14 promotes NAFLD progression in a cytochrome p4502E1 (CYP2E1)-dependent manner, which exacerbates hepatocyte oxidative stress, impairs the mitochondrial respiratory chain and inflammation by promoting CYP2E1 protein levels. Mechanistically, we demonstrated by immunoprecipitation and ubiquitination analysis that USP14 inhibits the degradation of heat shock protein 90 alpha family class A member 1 (HSP90AA1) by decreasing its lysine 48-linkage ubiquitination. Meanwhile, upregulation of HAP90AA1 protein promotes CYP2E1 protein accumulation. Collectively, our data indicate that an unknown USP14-HSP90AA1-CYP2E1 axis contributes to NAFLD progression, and we propose that inhibition of USP14 may be an effective strategy for NASH treatment.

Neglected PTM in Animal Adipogenesis: E3-mediated Ubiquitination

Ubiquitination is a widespread post-transcriptional modification (PTM) that occurs during protein degradation in eukaryotes and participates in almost all physiological and pathological processes, including animal adipogenesis. Ubiquitination is a cascade reaction regulated by the activating enzyme E1, conjugating enzyme E2, and ligase E3. Several recent studies have reported that E3 ligases play important regulatory roles in adipogenesis. However, as a key influencing factor for the recognition and connection between the substrate and ubiquitin during ubiquitination, its regulatory role in adipogenesis has not received adequate attention. In this review, we summarize the E3s' regulation and modification targets in animal adipogenesis, explain the regulatory mechanisms in lipogenic-related pathways, and further analyze the existing positive results to provide research directions of guiding significance for further studies on the regulatory mechanisms of E3s in animal adipogenesis.

High sugar but not high fat diet consumption induces hepatic metabolic disruption and up-regulation of mitochondrial fission-associated protein Drp1 in a model of moderate obesity

Identification of new modifications and the association with diet patterns are essential for the prevention of non-alcoholic fatty liver disease (NAFLD). To address this problem, we feed rats with high caloric diets based on high sucrose (HSD) and high fat (HFD) and analysed metabolic and mitochondrial alterations. Both diets induce moderated obesity and fat accumulation in the liver after 8, 10 and 12 months of diet. The HSD induces both hyperleptinemia and hyperinsulinemia, as well as up-regulation of transcription factors SRBEP1 and PPARγ along slight increase nitrosylation of proteins and increased mitochondrial fission. In contrast, HFD induced hyperleptinemia without changes in neither insulin levels nor oxidative stress, SREBP1, PPARγ, or mitochondrial dynamics. In conclusion, chronic consumption of high sucrose content diets induces more pathological and metabolic alteration in liver in comparison with consumption of high-fat content diets, although both induces obesity and liver steatosis in these animal models.

PAQR3 depletion accelerates diabetic wound healing by promoting angiogenesis through inhibiting STUB1-mediated PPARγ degradation

The pathogenesis of diabetic wounds is closely associated with the dysregulation of macrophage polarization. However, the underlying mechanism remains poorly understood. In this study, we aimed to investigate the potential effects of PAQR3 (progestin and adipoQ receptor 3) silencing in accelerating diabetic wound healing. We showed that PAQR3 silencing promoted skin wound healing and angiogenesis in diabetic mice, which was accompanied by enhanced M2 macrophage polarization and elevated expression of PPARγ (peroxisome proliferator-activated receptor γ). PAQR3 silencing also promoted M2 polarization and increased PPARγ protein level in PMA-treated THP-1 cells. Moreover, knockdown of PAQR3 in macrophages enhanced the migration of HaCaT cells and tube formation of HUVECs. The ubiquitination of PPARγ protein in macrophages was repressed by PAQR3 silencing. STUB1 (STIP1 homology and U-box-containing protein 1) binds with the PPARγ protein to mediate PPARγ ubiquitination and degradation in macrophages, which was impaired by PAQR3 silencing. The PPARγ inhibitor, GW9662, or STUB1 overexpression abrogated the enhanced M2 macrophage polarization induced by PAQR3 silencing. Therefore, these findings demonstrates that PAQR3 silencing accelerates diabetic wound healing by promoting M2 macrophage polarization and angiogenesis, which is mediated by the inhibition of STUB1-mediated PPARγ protein ubiquitination and degradation.

ITCH E3 Ubiquitin Ligase downregulation compromises hepatic degradation of branched-chain amino acids

Objective

Metabolic syndrome, obesity, and steatosis are characterized by a range of dysregulations including defects in ubiquitin ligase tagging proteins for degradation. The identification of novel hepatic genes associated with fatty liver disease and metabolic dysregulation may be relevant to unravelling new mechanisms involved in liver disease progression

Methods

Through integrative analysis of liver transcriptomic and metabolomic obtained from obese subjects with steatosis, we identified itchy E ubiquitin protein ligase (ITCH) as a gene downregulated in human hepatic tissue in relation to steatosis grade. Wild-type or ITCH knockout mouse models of non-alcoholic fatty liver disease (NAFLD) and obesity-related hepatocellular carcinoma were analyzed to dissect the causal role of ITCH in steatosis

Results

We show that ITCH regulation of branched-chain amino acids (BCAAs) degradation enzymes is impaired in obese women with grade 3 compared with grade 0 steatosis, and that ITCH acts as a gatekeeper whose loss results in elevation of circulating BCAAs associated with hepatic steatosis. When ITCH expression was specifically restored in the liver of ITCH knockout mice, ACADSB mRNA and protein are restored, and BCAA levels are normalized both in liver and plasma

Conclusions

Our data support a novel functional role for ITCH in the hepatic regulation of BCAA metabolism and suggest that targeting ITCH in a liver-specific manner might help delay the progression of metabolic hepatic diseases and insulin resistance.

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ITCH expression is reduced in liver during NAFLD.

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Transcriptomics analysis of liver in obese women highlighted the interplay between ITCH and genes involved in BCAA degradation.

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Modulation of ITCH in models of metabolic hepatic diseases supported the association between ITCH and BCAA metabolism.

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Targeting ITCH in a liver specific manner might help to delay the progression of metabolic hepatic diseases and insulin resistance.

Non-proteolytic ubiquitylation in cellular signaling and human disease

Ubiquitylation is one of the most common post-translational modifications (PTMs) of proteins that frequently targets substrates for proteasomal degradation. However it can also result in non-proteolytic events which play important functions in cellular processes such as intracellular signaling, membrane trafficking, DNA repair and cell cycle. Emerging evidence demonstrates that dysfunction of non-proteolytic ubiquitylation is associated with the development of multiple human diseases. In this review, we summarize the current knowledge and the latest concepts on how non-proteolytic ubiquitylation pathways are involved in cellular signaling and in disease-mediating processes. Our review, may advance our understanding of the non-degradative ubiquitylation process.

Evanthia Pangou and co-authors review recent insights into the important roles of non-proteolytic ubiquitylation in cellular signaling as well as in physiology and disease.

Neddylation inhibition ameliorates steatosis in NAFLD by boosting hepatic fatty acid oxidation via the DEPTOR-mTOR axis

OBJECTIVE

Neddylation is a druggable and reversible ubiquitin-like post-translational modification upregulated in many diseases, including liver fibrosis, hepatocellular carcinoma, and more recently, non-alcoholic fatty liver disease (NAFLD). Herein, we propose to address the effects of neddylation inhibition and the underlying mechanisms in pre-clinical models of NAFLD.

METHODS

Hepatic neddylation measured by immunohistochemical analysis and NEDD8 serum levels measured by ELISA assay were evaluated in NAFLD clinical and pre-clinical samples. The effects of neddylation inhibition by using a pharmacological small inhibitor, MLN4924, or molecular approaches were assessed in isolated mouse hepatocytes and pre-clinical mouse models of diet-induced NAFLD, male adult C57BL/6 mice, and the AlfpCre transgenic mice infected with AAV-DIO-shNedd8.

RESULTS

Neddylation inhibition reduced lipid accumulation in oleic acid-stimulated mouse primary hepatocytes and ameliorated liver steatosis, preventing lipid peroxidation and inflammation in the mouse models of diet-induced NAFLD. Under these conditions, increased Deptor levels and the concomitant repression of mTOR signaling were associated with augmented fatty acid oxidation and reduced lipid content. Moreover, Deptor silencing in isolated mouse hepatocytes abolished the anti-steatotic effects mediated by neddylation inhibition. Finally, serum NEDD8 levels correlated with hepatic neddylation during the disease progression in the clinical and pre-clinical models CONCLUSIONS: Overall, the upregulation of Deptor, driven by neddylation inhibition, is proposed as a novel effective target and therapeutic approach to tackle NAFLD.

TGF-β Signaling in Liver, Pancreas, and Gastrointestinal Diseases and Cancer

Genetic alterations affecting transforming growth factor-β (TGF-β) signaling are exceptionally common in diseases and cancers of the gastrointestinal system. As a regulator of tissue renewal, TGF-β signaling and the downstream SMAD-dependent transcriptional events play complex roles in the transition from a noncancerous disease state to cancer in the gastrointestinal tract, liver, and pancreas. Furthermore, this pathway also regulates the stromal cells and the immune system, which may contribute to evasion of the tumors from immune-mediated elimination. Here, we review the involvement of the TGF-β pathway mediated by the transcriptional regulators SMADs in disease progression to cancer in the digestive system. The review integrates human genomic studies with animal models that provide clues toward understanding and managing the complexity of the pathway in disease and cancer.

Puerarin Exerts the Hepatoprotection from Chronic Alcohol-Induced Liver Injury via Inhibiting the Cyclooxygenase-2 and the 5-Lipoxygenase Pathway in Rats

BACKGROUND

Puerarin (PR) as one of the main ingredients of the root of the traditional herb Kudzu has been suggested to improve chronic alcohol-induced liver injury. We explore the specific mechanisms of PR on hepatocellular changes after administration of alcohol.

METHODS

Sprague-Dawley rats were treated with 55% alcohol for 12 weeks to induce a chronic alcoholic liver damage model. Then the rats in each group were administered by oral gavage with zileuton, celecoxib, and PR for 2 weeks, respectively.

RESULTS

In the PR group, the weight loss was markedly improved and the abnormal serum alanine aminotransferase and aspartate aminotransferase were significantly lowered after PR treatment when compared to the alcoholic liver injured model group. Pathological examination indicated that alcohol-induced hepatocellular injury was improved by the PR treatment. The 5-lipoxygenase (5-Lox) and cyclooxygenase-2 (Cox-2) at the protein level and the mRNA level were obviously downregulated accompanied with the PR treatment. Meanwhile, the peroxisome proliferator-activated receptor γ (PPAR-γ) at the protein and mRNA level was notably elevated and the tumor necrosis factor α at the protein and mRNA level was markedly decreased following the PR treatment.

CONCLUSION

The possible cytoprotective mechanisms of PR may be involved inhibition of the Cox-2 pathway and the 5-Lox pathway to suppress inflammatory response and regulate the protective factor PPAR-γ expression.

Limited expression of TLR9 on T cells and its functional consequences in patients with nonalcoholic fatty liver disease

Background/Aims

Toll-like receptors (TLRs) modulate T cell responses in diverse diseases. Co-stimulation of T cell activation via TLR9 induces production of interferon gamma (IFN-γ), priming of which is critical for differentiation of pro-inflammatory macrophages. These macrophages have a crucial role in nonalcoholic fatty liver disease (NAFLD). We aimed to evaluate the expression of TLR9 protein on T cells and the consequences of TLR9-mediated triggering of these cells in patients with NAFLD.

Methods

Our study included 34 patients with simple steatosis, 34 patients with nonalcoholic steatohepatitis, eight patients with NAFLD who met general diagnostic criteria but lacked histological diagnosis, and 51 control subjects. We used a synthetic TLR9 ligand to co-stimulate T cells. We measured TLR9 expression in liver and peripheral T cells and CD69 and IFN-γ as phenotypic markers of T cell activation and differentiation by flow cytometry.

Results

TLR9 expression on liver and peripheral T cells was lowest in patients with simple steatosis and was positively associated with anthropometric, biochemical, and histopathological features of NAFLD. In vitro co-stimulation of T cells from patients with simple steatosis induced a limited number of IFN-γ-producing CD8⁺ T cells. At baseline, these patients showed a low frequency of circulating type 1 CD8⁺ cells.

Conclusions

The positive associations between TLR9 and anthropometric, clinical, and histological features and the crucial role of IFN-γ-in NAFLD suggest that limited TLR9 expression and production of IFN-γ play a protective role in patients with simple steatosis.

Protective Effect of SMAD-Specific E3 Ubiquitin Protein Ligase 1 in Alcoholic Steatohepatitis in Mice

Excessive accumulation of lipids in the liver is crucial in the pathogenesis of alcoholic steatohepatitis and may be partly mediated by impaired degradation of lipid droplets by autophagy. The E3 ubiquitin ligase SMAD-specific E3 ubiquitin protein ligase 1 (SMURF1) regulates selective autophagy by ubiquitinating proteins on cargo destined for autophagic delivery to the lysosome for degradation. Here, we evaluated the role of SMURF1 in the regulation of hepatic lipid degradation in alcoholic steatohepatitis. In patients with severe alcoholic hepatitis, SMURF1 colocalized with lipid droplet membranes in liver explants. In a mouse model of alcoholic steatohepatitis, Smurf1 -/- mice fed an alcohol diet displayed increased hepatocyte accumulation of lipid droplets and triglycerides as well as more severe liver injury compared to wild-type mice. The increased severity of liver steatosis in alcohol-fed Smurf1 -/- mice was rescued by adeno-associated virus (AAV) serotype 8-mediated hepatic expression of wild-type Smurf1 protein but not by mutant Smurf1 proteins either lacking the catalytically active cysteine 699 required for ubiquitin transfer or the N-terminal C2 phospholipid membrane-binding domain. Conclusion: Smurf1 plays a protective role in the pathogenesis of alcoholic steatohepatitis through a mechanism that requires both its ubiquitin-ligase activity and C2 phospholipid-binding domains. These findings have implications for understanding the roles of ubiquitin ligases in fatty liver disease.

Ubiquitin Ligases Involved in the Regulation of Wnt, TGF-β, and Notch Signaling Pathways and Their Roles in Mouse Development and Homeostasis

The Wnt, TGF-β, and Notch signaling pathways are essential for the regulation of cellular polarity, differentiation, proliferation, and migration. Differential activation and mutual crosstalk of these pathways during animal development are crucial instructive forces in the initiation of the body axis and the development of organs and tissues. Due to the ability to initiate cell proliferation, these pathways are vulnerable to somatic mutations selectively producing cells, which ultimately slip through cellular and organismal checkpoints and develop into cancer. The architecture of the Wnt, TGF-β, and Notch signaling pathways is simple. The transmembrane receptor, activated by the extracellular stimulus, induces nuclear translocation of the transcription factor, which subsequently changes the expression of target genes. Nevertheless, these pathways are regulated by a myriad of factors involved in various feedback mechanisms or crosstalk. The most prominent group of regulators is the ubiquitin-proteasome system (UPS). To open the door to UPS-based therapeutic manipulations, a thorough understanding of these regulations at a molecular level and rigorous confirmation in vivo are required. In this quest, mouse models are exceptional and, thanks to the progress in genetic engineering, also an accessible tool. Here, we reviewed the current understanding of how the UPS regulates the Wnt, TGF-β, and Notch pathways and we summarized the knowledge gained from related mouse models.