Deficiency of Cbl-b gene enhances infiltration and activation of macrophages in adipose tissue and causes peripheral insulin resistance in mice

T cell specific deletion of Casitas B lineage lymphoma-b reduces atherosclerosis, but increases plaque T cell infiltration and systemic T cell activation

Introduction

Atherosclerosis is a lipid-driven inflammatory disease of the arterial wall, and the underlying cause of the majority of cardiovascular diseases. Recent advances in high-parametric immunophenotyping of immune cells indicate that T cells constitute the major leukocyte population in the atherosclerotic plaque. The E3 ubiquitin ligase Casitas B-lymphoma proto-oncogene-B (CBL-B) is a critical intracellular regulator that sets the threshold for T cell activation, making CBL-B a potential therapeutic target to modulate inflammation in atherosclerosis. We previously demonstrated that complete knock-out of CBL-B aggravated atherosclerosis in Apoe-/- mice, which was attributed to increased macrophage recruitment and increased CD8+ T cell activation in the plaque.

Methods

To further study the T cell specific role of CBL-B in atherosclerosis, Apoe-/- CD4cre Cblb fl/fl (Cbl-bcKO) mice and Apoe-/-CD4WTCblbfl/fl littermates (Cbl-bfl/fl) were fed a high cholesterol diet for ten weeks.

Results

Cbl-bcKO mice had smaller atherosclerotic lesions in the aortic arch and root compared to Cbl-bfl/fl, and a substantial increase in CD3+ T cells in the plaque. Collagen content in the plaque was decreased, while other plaque characteristics including plaque necrotic core, macrophage content, and smooth muscle cell content, remained unchanged. Mice lacking T cell CBL-B had a 1.4-fold increase in CD8+ T cells and a 1.8-fold increase in regulatory T cells in the spleen. Splenic CD4+ and CD8+ T cells had increased expression of C-X-C Motif Chemokine Receptor 3 (CXCR3) and interferon-γ (IFN-γ), indicating a T helper 1 (Th1)-like/effector CD8+ T cell-like phenotype.

Conclusion

In conclusion, Cbl-bcKO mice have reduced atherosclerosis but show increased T cell accumulation in the plaque accompanied by systemic T cell activation.

Role of the ubiquitin-proteasome system on macrophages in the tumor microenvironment

Tumor-associated macrophages (TAMs) are key components of the tumor microenvironment, and their different polarization states play multiple roles in tumors by secreting cytokines, chemokines, and so on, which are closely related to tumor development. In addition, the enrichment of TAMs is often associated with poor prognosis of tumors. Thus, targeting TAMs is a potential tumor treatment strategy, in which therapeutic approaches such as reducing TAMs numbers, remodeling TAMs phenotypes, and altering their functions are being extensively investigated. Meanwhile, the ubiquitin-proteasome system (UPS), an important mechanism of protein hydrolysis in eukaryotic cells, participates in cellular processes by regulating the activity and stability of key proteins. Interestingly, UPS plays a dual role in the process of tumor development, and its role in TAMs deserve to be investigated in depth. This review builds on this foundation to further explore the multiple roles of UPS on TAMs and identifies a promising approach to treat tumors by targeting TAMs with UPS.

Rising Star in Immunotherapy: Development and Therapeutic Potential of Small-Molecule Inhibitors Targeting Casitas B Cell Lymphoma-b (Cbl-b)

Casitas B cell lymphoma-b (Cbl-b) is a vital negative regulator of TCR and BCR signaling pathways, playing a significant role in setting an appropriate threshold for the activation of T cells and controlling the tolerance of peripheral T cells via a variety of mechanisms. Overexpression of Cbl-b leads to immune hyporesponsiveness of T cells. Conversely, the deficiency of Cbl-b in T cells results in markedly increased production of IL-2, even in the lack of CD28 costimulation in vitro. And Cbl-b-/- mice spontaneously reject multifarious cancers. Therefore, Cbl-b may be associated with immune-mediated diseases, and blocking Cbl-b could be considered as a new antitumor immunotherapy strategy. In this review, the possible regulatory mechanisms and biological potential of Cbl-b for antitumor immunotherapy are summarized. Besides, the potential roles of Cbl-b in immune-mediated diseases are comprehensively discussed, with emphasis on Cbl-b immune-oncology agents in the preclinical stage and clinical trials.

Role of UPP pathway in amelioration of diabetes-associated complications

Type 2 diabetes (T2D) is one of the most widely spread metabolic disorder also called as "life style" disease. Due to the alarming number of patients, there is great need to therapies targeting functions which can help in maintaining the homeostasis of glucose levels and improving insulin sensitivity. Detailed analysis was done through various research and review papers which was searched using MEDLINE, BIOSIS, and EMBASE using various keywords. This search retrieved the most appropriate content on these molecules targeting UPP pathway. From this extensive review involving UPP pathway, it was concluded that the role of ubiquitin's is not only limited to neurodegenerative disorders but also plays a critical role in progression of diabetes including obesity, insulin resistance, and various neurogenerative disorders but it also targets proteasomal degradation including mediation of cellular signaling pathways. Thus, drugs targeting UPP not only may show effect against diabetes but also are therapeutically beneficial in the treatment of diabetes-associated complications which may be obtained. Thus, based on the available information and data on UPP functions, it can be concluded that regulation of UPP pathway via downstream regulators mainly E1, E2, and E3 may bring promising results. Drugs targeting these transcriptional factors may emerge as a novel therapy in the treatment of diabetes and diabetes-associated complications.

E3 ubiquitin ligase Casitas B lineage lymphoma-b and its potential therapeutic implications for immunotherapy

The distinction of self from non-self is crucial to prevent autoreactivity and ensure protection from infectious agents and tumors. Maintaining the balance between immunity and tolerance of immune cells is strongly controlled by several sophisticated regulatory mechanisms of the immune system. Among these, the E3 ligase ubiquitin Casitas B cell lymphoma-b (Cbl-b) is a newly identified component in the ubiquitin-dependent protein degradation system, which is thought to be an important negative regulator of immune cells. An update on the current knowledge and new concepts of the relevant immune homeostasis program co-ordinated by Cbl-b in different cell populations could pave the way for future immunomodulatory therapies of various diseases, such as autoimmune and allergic diseases, infections, cancers and other immunopathological conditions. In the present review, the latest findings are comprehensively summarized on the molecular structural basis of Cbl-b and the suppressive signaling mechanisms of Cbl-b in physiological and pathological immune responses, as well as its emerging potential therapeutic implications for immunotherapy in animal models and human diseases.

Comprehensive Analysis of mRNA and lncRNA Transcriptomes Reveals the Differentially Hypoxic Response of Preadipocytes During Adipogenesis

Local hypoxia has recently been reported to occur in the white adipose tissue (WAT) microenvironment during obesity. Adipocytes have a unique life cycle that reflects the different stages of adipogenesis in the WAT niche. Long non-coding RNAs (lncRNAs) play an important role in the cellular response to hypoxia. However, the differentially hypoxic responses of preadipocytes during adipogenesis and the potential role of lncRNAs in this process remain to be elucidated. Here, we evaluated the differentially hypoxic responses of primary hamster preadipocytes during adipogenesis and analyzed mRNA and lncRNA expression in same Ribo-Zero RNA-seq libraries. Hypoxia induced HIF-1α protein during adipogenesis and caused divergent changes of cell phenotypes. A total of 10,318 mRNAs were identified to be expressed in twenty libraries (five timepoints), and 3,198 differentially expressed mRNAs (DE mRNAs) were detected at five timepoints (hypoxia vs. normoxia). Functional enrichment analysis revealed the shared and specific hypoxia response pathways in the different stages of adipogenesis. Hypoxia differentially modulated the expression profile of adipose-associated genes, including adipokines, lipogenesis, lipolysis, hyperplasia, hypertrophy, inflammatory, and extracellular matrix. We also identified 4,296 lncRNAs that were expressed substantially and detected 1,431 DE lncRNAs at five timepoints. Two, 3, 5, 13, and 50 DE mRNAs at D0, D1, D3, D7, and D11, respectively, were highly correlated and locus-nearby DE lncRNAs and mainly involved in the cell cycle, vesicle-mediated transport, and mitochondrion organization. We identified 28 one-to-one lncRNA-mRNA pairs that might be closely related to adipocyte functions, such as ENSCGRT00015041780-Hilpda, TU2105-Cdsn, and TU17588-Ltbp3. These lncRNAs may represent the crucial regulation axis in the cellular response to hypoxia during adipogenesis. This study dissected the effects of hypoxia in the cell during adipogenesis, uncovered novel regulators potentially associated with WAT function, and may provide a new viewpoint for interpretation and treatment of obesity.

Glucose and TNF enhance expression of TNF and IL1B, and histone H3 acetylation and K4/K36 methylation, in juvenile macrophage cells

Hyperglycemia activates innate leukocytes such as monocytes and induces pro-inflammatory cytokine expression, resulting in increased monocyte adhesion to aortic endothelial cells. In this study, we investigated whether high glucose and/or tumor necrosis factor (TNF) would enhance pro-inflammatory cytokine expression of tumor necrosis factor (TNF) and interleukin (IL)-1β (IL1B) by altering histone modifications in U937, a juvenile macrophage cell line. The mRNA levels of TNF and IL1B in U937 cells were significantly affected by glucose concentration and TNF treatment. Mono-methylated histone H3K4 signals around TNF and IL1B were lower in cells treated with high glucose compared with low glucose. Conversely, tri-methylated histone H3K4 and H3K36 signals were higher in cells treated with high glucose compared with low glucose. TNF treatment of U937 cells cultured in high glucose enhanced histone H3K36 tri-methylation, particularly around the gene regions of TNF and IL1B. Histone acetylation was induced by treatment with TNF in high-glucose medium. The induction of acetylation and tri-methylation of K4 and K36 of histone H3 around TNF and IL1B by treatment with high glucose and/or TNF was positively associated with the induction of these genes in juvenile macrophage U937 cells.

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Culture with high glucose induced TNF and IL1B expression in U937 cells.

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TNF treatment enhanced high glucose inducible TNF expression in U937 cells.

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H3K4me3 around TNF and IL1B was induced by high glucose treatment.

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TNF treatment enhanced H3Ac in the gene body region of TNF and IL1B.

Cbl Proto-Oncogene B (CBLB) c.197A>T Mutation Induces Mild Metabolic Dysfunction in Partial Type I Multiple Symmetric Lipomatosis (MSL)

BACKGROUND

Multiple symmetric lipomatosis (MSL) is a rare disease showing chronic progression of multiple, symmetrical, and non-encapsulated subcutaneous lipoma. The cause of the disease remains unknown.

PATIENTS AND METHODS

This study reported and summarized 13 sporadic cases of Type I MSL patients in terms of histopathology and cellular and molecular biology and assessed the CBLB c.197A>T mutation in the IRS1-PI3K-Akt pathway.

RESULTS

The clinical data showed that these 13 Type I patients were all male with a mean age of 57.0 ± 6.6 years old and consumed alcohol heavily. The laboratory tests revealed that most of the patients had hyperuricemia, diabetes, hyperinsulinemia, or insulin resistance; however, their blood lipid levels were close to a normal range. The imaging data exhibited lipomas that only occurred subcutaneously but not viscerally, ie, Types Ia (15.4%), Ib (30.8%), and Ic (53.8%). The molecular analyses of adipocytes of isoprenaline stimulated human adipose tissue-derived mesenchymal stromal cells (hADSCs) isolated from the adipose tissue lipoma-like masses (ATLLM) demonstrated that these adipocytes did not express UCP-1. The Cbl proto-oncogene B (CBLB), an E3 ubiquitin-protein ligase, was associated with insulin resistance and obesity and was mutated (ie, CBLB c.197A>T) in four MSL patients after the whole genome and Sanger sequencing of the blood samples. Furthermore, the CBLB c.197A>T mutation induced hADSC resistance to insulin by inactivation of the IRS-1-PI3K-AKT pathway.

CONCLUSION

This study analyzed clinical, histopathological, and cellular and molecular biological characterizations of 13 Type I MSL patients and identified the CBLB c.197A>T heterozygous mutation that could be responsible for MSL metabolic dysfunction or even MSL development.

A novel CBL-Bflox/flox mouse model allows tissue-selective fully conditional CBL/CBL-B double-knockout: CD4-Cre mediated CBL/CBL-B deletion occurs in both T-cells and hematopoietic stem cells

CBL-family ubiquitin ligases are critical negative regulators of tyrosine kinase signaling, with a clear redundancy between CBL and CBL-B evident in the immune cell and hematopoietic stem cell studies. Since CBL and CBL-B are negative regulators of immune cell activation, elimination of their function to boost immune cell activities could be beneficial in tumor immunotherapy. However, mutations of CBL are associated with human leukemias, pointing to tumor suppressor roles of CBL proteins; hence, it is critical to assess the tumor-intrinsic roles of CBL and CBL-B in cancers. This has not been possible since the only available whole-body CBL-B knockout mice exhibit constitutive tumor rejection. We engineered a new CBL-Bflox/flox mouse, combined this with an existing CBLflox/flox mouse to generate CBLflox/flox; CBL-Bflox/flox mice, and tested the tissue-specific concurrent deletion of CBL and CBL-B using the widely-used CD4-Cre transgenic allele to produce a T-cell-specific double knockout. Altered T-cell development, constitutive peripheral T-cell activation, and a lethal multi-organ immune infiltration phenotype largely resembling the previous Lck-Cre driven floxed-CBL deletion on a CBL-B knockout background establish the usefulness of the new model for tissue-specific CBL/CBL-B deletion. Unexpectedly, CD4-Cre-induced deletion in a small fraction of hematopoietic stem cells led to expansion of certain non-T-cell lineages, suggesting caution in the use of CD4-Cre for T-cell-restricted gene deletion. The establishment of a new model of concurrent tissue-selective CBL/CBL-B deletion should allow a clear assessment of the tumor-intrinsic roles of CBL/CBL-B in non-myeloid malignancies and help test the potential for CBL/CBL-B inactivation in immunotherapy of tumors.

Torenia concolor Lindley var. formosana Yamazaki extracts improve inflammatory response and lipid accumulation via PPARs activation

Background/Introduction: At present, human diet is replete with sugar and fat. Abnormal metabolism and hyperglycemia or hyperlipidemia in the body induces the development of an overactive and continuous inflammatory response, resulting in obesity and metabolic syndromes, including hypertension, hyperlipidemia, and insulin resistance. Torenia concolor Lindley var. formosana Yamazaki (TC), a perennial creeping herbaceous plant, is a traditional Chinese medicinal herb widely used for the treatment of heat stroke, aching muscles and bones, cold, dysentery, and ambustion.

Purpose: This study evaluated the influence of TC on inflammation responses and lipid metabolism. Methods: In this study, ground TC powder was extracted with 95% ethanol. The ethanol was removed by vacuum concentration, and the resulting extract was further extracted with a number of solvents of different polarity to produce four final extracts: an ethanol extract (TCEE), an ethyl acetate extract (TCEAE), an n-butanol extract (TCBUE), and a water extract (TCWE). The anti-inflammatory efficacy of the extracts and their capability for lipid metabolism regulation was then explored.

Results: TCEE, TCEAE, and TCBUE exhibited good anti-inflammatory efficacy; TCEAE also simultaneously regulated lipid metabolism. In RAW264.7 cells, these three extracts suppressed the expression of iNOS and IL-6 via the signaling pathway activation of the transcription factor peroxisome proliferator activated receptor γ (PPARγ) and thereby showed anti-inflammatory efficacy. In 3T3-L1 cells, these three extracts promoted lipid metabolism and reduced lipid accumulation through the activation of PPARα and the increased expression of adiponectin, thus demonstrating regulation of lipid metabolism.

Conclusion: These results indicate that TC possesses anti-inflammatory efficacy and can regulate lipid metabolism through the activation of transcription factor PPARs. We speculate that these nutraceutical effects are attributable to betulin, an active ingredient in this herbal medicine.

Corrupted adipose tissue endogenous myelopoiesis initiates diet-induced metabolic disease

Activation and increased numbers of inflammatory macrophages, in adipose tissue (AT) are deleterious in metabolic diseases. Up to now, AT macrophages (ATM) accumulation was considered to be due to blood infiltration or local proliferation, although the presence of resident hematopoietic stem/progenitor cells (Lin-/Sca+/c-Kit+; LSK phenotype) in the AT (AT-LSK) has been reported. By using transplantation of sorted AT-LSK and gain and loss of function studies we show that some of the inflammatory ATM inducing metabolic disease, originate from resident AT-LSK. Transplantation of AT-LSK sorted from high fat diet-fed (HFD) mice is sufficient to induce ATM accumulation, and to transfer metabolic disease in control mice. Conversely, the transplantation of control AT-LSK improves both AT-inflammation and glucose homeostasis in HFD mice. Our results clearly demonstrate that resident AT-LSK are one of the key point of metabolic disease, and could thus constitute a new promising therapeutic target to fight against metabolic disease.

DOI:http://dx.doi.org/10.7554/eLife.23194.001

Hidden targets of ubiquitin proteasome system: To prevent diabetic nephropathy

Diabetic nephropathy (DN) is the major cause of end stage renal failure. Although, several therapeutic targets have emerged to prevent the progression of DN, the number of people with DN still continues to rise worldwide, suggesting an urgent need of novel targets to prevent DN completely. Currently, the role of ubiquitin proteasome system (UPS) has been highlighted in the pathogenesis and progression of various diseases like obesity, insulin resistance, atherosclerosis, cancers, neurodegerative disorders and including secondary complications of diabetes. UPS mainly involves in protein homeostatis through ubiquitination (post translational modification) and proteasomal degradation of various proteins. Ubiquitination, not only involves in proteasomal degradation, but also directs the substrate proteins to participate in multitude of cell signalling pathways. However, very little is known about ubiquitination and UPS in the progression of DN. This review mainly focuses on UPS and its components including E2 conjugating enzymes, E3 ligases and deubiquitinases (DUBs) in the development of DN and thus may help us to find novel therapeutic targets with in UPS to prevent DN completely in future.

Involvement of Cbl-b-mediated macrophage inactivation in insulin resistance

Aging and overnutrition cause obesity in rodents and humans. It is well-known that obesity causes various diseases by producing insulin resistance (IR). Macrophages infiltrate the adipose tissue (AT) of obese individuals and cause chronic low-level inflammation associated with IR. Macrophage infiltration is regulated by the chemokines that are released from hypertrophied adipocytes and the immune cells in AT. Saturated fatty acids are recognized by toll-like receptor 4 (TLR4) and induce inflammatory responses in AT macrophages (ATMs). The inflammatory cytokines that are released from activated ATMs promote IR in peripheral organs, such as the liver, skeletal muscle and AT. Therefore, ATM activation is a therapeutic target for IR in obesity. The ubiquitin ligase Casitas b-lineage lymphoma-b (Cbl-b) appears to potently suppress macrophage migration and activation. Cbl-b is highly expressed in leukocytes and negatively regulates signals associated with migration and activation. Cbl-b deficiency enhances ATM accumulation and IR in aging- and diet-induced obese mice. Cbl-b inhibits migration-related signals and SFA-induced TLR4 signaling in ATMs. Thus, targeting Cbl-b may be a potential therapeutic strategy to reduce the IR induced by ATM activation. In this review, we summarize the regulatory functions of Cbl-b in ATMs.

MARCH1 E3 Ubiquitin Ligase Dampens the Innate Inflammatory Response by Modulating Monocyte Functions in Mice

Ubiquitination was recently identified as a central process in the pathogenesis and development of numerous inflammatory diseases, such as obesity, atherosclerosis, and asthma. Treatment with proteasomal inhibitors led to severe side effects because ubiquitination is heavily involved in a plethora of cellular functions. Thus, new players regulating ubiquitination processes must be identified to improve therapies for inflammatory diseases. In addition to their role in adaptive immunity, endosomal MHC class II (MHCII) molecules were shown to modulate innate immune responses by fine tuning the TLR4 signaling pathway. However, the role of MHCII ubiquitination by membrane associated ring-CH-type finger 1 (MARCH1) E3 ubiquitin ligase in this process remains to be assessed. In this article, we demonstrate that MARCH1 is a key inhibitor of innate inflammation in response to bacterial endotoxins. The higher mortality of March1^-/- mice challenged with a lethal dose of LPS was associated with significantly stronger systemic production of proinflammatory cytokines and splenic NK cell activation; however, we did not find evidence that MARCH1 modulates LPS or IL-10 signaling pathways. Instead, the mechanism by which MARCH1 protects against endotoxic shock rests on its capacity to promote the transition of monocytes from Ly6C^Hi to Ly6C^+/- Moreover, in competitive bone marrow chimeras, March1^-/- monocytes and polymorphonuclear neutrophils outcompeted wild-type cells with regard to bone marrow egress and homing to peripheral organs. We conclude that MARCH1 exerts MHCII-independent effects that regulate the innate arm of immunity. Thus, MARCH1 might represent a potential new target for emerging therapies based on ubiquitination reactions in inflammatory diseases.

Role of E3 ubiquitin ligases in insulin resistance

E3 ubiquitin ligases are a large family of proteins that catalyse the ubiquitination of many proteins for degradation by the 26S proteasome. E3 ubiquitin ligases play pivotal roles in the process of insulin resistance and diabetes. In this review, we summarize the currently available studies to analyse the potential role of E3 ubiquitin ligases in the development of insulin resistance. We propose two mechanisms by which E3 ubiquitin ligases can affect the process of insulin resistance. First, E3 ubiquitin ligases directly degrade the insulin receptor, insulin receptor substrate and other key insulin signalling molecules via the UPS. Second, E3 ubiquitin ligases indirectly regulate insulin signalling by regulating pro-inflammatory mediators that are involved in the regulation of insulin signalling molecules, such as tumour necrosis factor-α, interleukin (IL)-6, IL-4, IL-13, IL-1β, monocyte chemoattractant protein-1 and hypoxia-inducible factor 1α. Determining the mechanism by which E3 ubiquitin ligases affect the development of insulin resistance can identify a novel strategy to protect against insulin resistance and diabetes.

E3 ubiquitin ligase Cbl-b negatively regulates C-type lectin receptor-mediated antifungal innate immunity

Innate immune responses mediated by C-type lectin receptors Dectin-2 and Dectin-3 against fungal infections are negatively regulated by Cbl-b ubiquitination.

Activation of various C-type lectin receptors (CLRs) initiates potent proinflammatory responses against various microbial infections. However, how activated CLRs are negatively regulated remains unknown. In this study, we report that activation of CLRs Dectin-2 and Dectin-3 by fungi infections triggers them for ubiquitination and degradation in a Syk-dependent manner. Furthermore, we found that E3 ubiquitin ligase Casitas B–lineage lymphoma protein b (Cbl-b) mediates the ubiquitination of these activated CLRs through associating with each other via adapter protein FcR-γ and tyrosine kinase Syk, and then the ubiquitinated CLRs are sorted into lysosomes for degradation by an endosomal sorting complex required for transport (ESCRT) system. Therefore, the deficiency of either Cbl-b or ESCRT subunits significantly decreases the degradation of activated CLRs, thereby resulting in the higher expression of proinflammatory cytokines and inflammation. Consistently, Cbl-b–deficient mice are more resistant to fungi infections compared with wild-type controls. Together, our study indicates that Cbl-b negatively regulates CLR-mediated antifungal innate immunity, which provides molecular insight for designing antifungal therapeutic agents.

Delphinidin prevents disuse muscle atrophy and reduces stress-related gene expression

Delphinidin is a member of the anthocyanidin class of plant pigments. We examined the effects of delphinidin on muscle atrophy. Oral administration of delphinidin suppressed the muscle weight loss induced by mechanical unloading. Microarray analysis showed that delphinidin suppresses the upregulation of oxidative stress-related gene expression, including the expression of Cbl-b. Thus, delphinidin may prevent unloading-mediated muscle atrophy.

The E3 Ubiquitin Ligase c-Cbl Inhibits Microglia-Mediated CNS Inflammation by Regulating PI3K/Akt/NF-κB Pathway

BACKGROUND

Microglia-mediated inflammation may play an important role in the pathophysiology progression of neurodegenerative diseases, such as Parkinson's disease (PD), but the molecular mechanisms are poorly understood.

AIMS

This study sought to determine whether E3 ubiquitin ligase c-Cbl plays a role in the brain inflammation and to explore the relevant molecular mechanism.

METHODS

After BV2 microglial cells and c-Cbl-deficient mice were treated with lipopolysaccharide (LPS), neuroinflammation and microglial activation were evaluated by immunohistochemistry, ELISA and Western blot. We further investigated the possible mechanism of c-Cbl in regulating microglial activation.

RESULTS

Here, we showed that the E3 ubiquitin ligase c-Cbl had high expression in brain tissues including substantia nigra pars compacta (SNc), striatum and hippocampus, and it was abundantly expressed in microglia. Systemic LPS administration resulted in more severe microglial activation in CNS and increased expression of brain proinflammatory factors (TNF-α, IL-6, IL-1β and MCP-1) in c-Cbl knockout mice than wild type mice (WT). Downregulation of c-Cbl expression with c-Cbl siRNA in BV-2 microglial cells demonstrated a more robust increase in the proinflammatory factors release and NF-κB p65 nuclear translocation than that in control siRNA. Interestingly, Akt phosphorylation induced by LPS was also significantly augmented after c-Cbl knockdown. Moreover, blockade of PI3K/Akt activation by LY294002 significantly reduced inflammation response and NF-κB p65 nuclear translocation.

CONCLUSION

In sum, c-Cbl inhibits expression of LPS-stimulated proinflammatory cytokines and chemokines in microglia. We demonstrate an unprecedented role for c-Cbl in microglia-mediated neuroinflammation involving PI3K/Akt/NF-κB pathway.

E3 ubiquitin ligases as novel targets for inflammatory diseases

Ubiquitination is one of the post translational modifications which decide the fate of various proteins in the cells, by either directing them towards proteasomal degradation or participation in several cell signalling pathways. Recently, the role of ubiquitination has been unravelled in pathogenesis and progression of various diseases, where inflammation is critical, like obesity, insulin resistance, atherosclerosis, angiotensin-II induced cardiac inflammation and asthma. E3 ligases are known to be instrumental in regulation of the inflammatory cascade. This review focuses on the role of different E3 ligases in the development of inflammatory diseases and thus may help us to target these E3 ligases in future drug discovery to prevent inflammation.

Nedd4 haploinsufficient mice display moderate insulin resistance, enhanced lipolysis, and protection against high-fat diet-induced obesity

Neural precursor cell expressed developmentally down-regulated protein 4 (Nedd4) is the prototypical protein in the Nedd4 ubiquitin ligase (E3) family, which governs ubiquitin-dependent endocytosis and/or degradation of plasma membrane proteins. Loss of Nedd4 results in embryonic or neonatal lethality in mice and reduced insulin/IGF-1 signaling in embryonic fibroblasts. To delineate the roles of Nedd4 in vivo, we examined the phenotypes of heterozygous knockout mice using a high-fat diet-induced obesity (HFDIO) model. We observed that Nedd4+/- mice are moderately insulin resistant but paradoxically protected against HFDIO. After high-fat diet feeding, Nedd4+/- mice showed less body weight gain, less fat mass, and smaller adipocytes vs the wild type. Despite ameliorated HFDIO, Nedd4+/- mice did not manifest improvement in glucose tolerance vs the wild type in both genders. Nedd4+/- male, but not female, mice displayed significantly lower fasting blood glucose levels and serum insulin levels. Under obesogenic conditions, Nedd4+/- mice displayed elevated stimulated lipolytic activity, primarily through a β2-adrenergic receptor. Combined, these data support novel complex roles for Nedd4 in metabolic regulation involving altered insulin and β-adrenergic signaling pathways.

Modulation of Immune Cell Functions by the E3 Ligase Cbl-b

Maintenance of immunological tolerance is a critical hallmark of the immune system. Several signaling checkpoints necessary to balance activating and inhibitory input to immune cells have been described so far, among which the E3 ligase Cbl-b appears to be a central player. Cbl-b is expressed in all leukocyte subsets and regulates several signaling pathways in T cells, NK cells, B cells, and different types of myeloid cells. In most cases, Cbl-b negatively regulates activation signals through antigen or pattern recognition receptors and co-stimulatory molecules. In line with this function, cblb-deficient immune cells display lower activation thresholds and cblb knockout mice spontaneously develop autoimmunity and are highly susceptible to experimental autoimmunity. Interestingly, genetic association studies link CBLB-polymorphisms with autoimmunity also in humans. Vice versa, the increased activation potential of cblb-deficient cells renders them more potent to fight against malignancies or infections. Accordingly, several reports have shown that cblb knockout mice reject tumors, which mainly depends on cytotoxic T and NK cells. Thus, targeting Cbl-b may be an interesting strategy to enhance anti-cancer immunity. In this review, we summarize the findings on the molecular function of Cbl-b in different cell types and illustrate the potential of Cbl-b as target for immunomodulatory therapies.

E3 ubiquitin ligase Cbl-b in innate and adaptive immunity

Casitas B-lineage lymphoma proto-oncogene-b (Cbl-b), a RING finger E3 ubiquitin-protein ligase, has been demonstrated to play a crucial role in establishing the threshold for T-cell activation and controlling peripheral T-cell tolerance via multiple mechanisms. Accumulating evidence suggests that Cbl-b also regulates innate immune responses and plays an important role in host defense to pathogens. Understanding the signaling pathways regulated by Cbl-b in innate and adaptive immune cells is therefore essential for efficient manipulation of Cbl-b in emerging immunotherapies for human disorders such as autoimmune diseases, allergic inflammation, infections, and cancer. In this article, we review the latest developments in the molecular structural basis of Cbl-b function, the regulation of Cbl-b expression, the signaling mechanisms of Cbl-b in immune cells, as well as the biological function of Cbl-b in physiological and pathological immune responses in animal models and human diseases.

Modified Si-Miao-San () regulates adipokine expression and ameliorates insulin resistance by targeting IKKβ/Insulin receptor substrate-1 in mice

OBJECTIVE

To evaluate modified Si-Miao-San (mSMS, ) regulation of insulin sensitivity and explore the molecular mechanism by which mSMS inhibits inflammation and improves insulin action in mice.

METHODS

Insulin resistant model in mice was prepared by stimulation with macrophage-derived condition medium (Mac-CM) and the effects of mSMS on oral glucose tolerance, insulin sensitivity and liver glycogen content in mice was observed. The mice adipose tissue was isolated and the regulation of inflammation-related adipokine expression and insulin phosphatidylinositol 3-kinase (PI3K) signaling transduction by mSMS was investigated. Effect of mSMS on insulin-mediated glucose uptake was also investigated in adipocytes.

RESULTS

Oral administration of mSMS improved glucose tolerance in mice. Treatment of mice with Mac-CM resulted in glucose intolerance in mice and this change was effectively reversed by mSMS. Meanwhile, mSMS enhanced insulin sensitivity and increased glucose load-stimulated liver glycogen when mice were exposed to Mac-CM. Mac-CM stimulation induced dysregulation of adipokine expression in adipose tissue of mice. mSMS downregulated tumor necrosis factor α and interleukin 6 (IL-6) overexpression and upregulated adiponectin and peroxisomal proliferator activated receptor γ with inhibition of inhibitory kappa B kinase-β (IKKβ) and p65 phophsphorylation. Meanwhile, mSMS inhibited IL-6 production and increased adiponectin secretion in adipocytes against Mac-CM insult. Mac-CM challenge impaired insulin phosphatidylinositol 3 kinase (PI3K) signaling in adipose tissue. Oral administration mSMS inhibited inflammation-induced serine phosphorylation of insulin receptor substrate-1 (IRS-1) and restored insulin-mediated tyrosine phosphorylation, and thereby facilitated insulin PI3K signaling manifested by restoration of Akt phosphorylation. The resultant improvement of insulin sensitivity promoted insulin-stimulated glucose uptake when adipocytes were exposed to Mac-CM.

CONCLUSIONS

mSMS improves glucose tolerance in mice by enhancing insulin sensitivity in mice. mSMS inhibits IKKβ/NF κ B (p65)-dependent inflammatory response with beneficial regulation of adipokine expression in adipose tissue. mSMS inhibits inflammation and improves insulin sensitivity by blocking inflammatory interaction between IKKβ/IRS-1.

Ubiquitin ligase Cbl-b and obesity-induced insulin resistance

Obesity causes type 2 diabetes, atherosclerosis and cardiovascular diseases by inducing systemic insulin resistance. It is now recognized that obesity is related to chronic low-grade inflammation in adipose tissue. Specifically, activated immune cells infiltrate adipose tissue and cause inflammation. There is increasing evidence that activated macrophages accumulate in the hypertrophied adipose tissue of rodents and humans and induce systemic insulin resistance by secreting inflammatory cytokines. Accordingly, a better understanding of the molecular mechanisms underlying macrophage activation in adipose tissue will facilitate the development of new therapeutic strategies. Currently, little is known about the regulation of macrophage activation, although E3 ubiquitin ligase Casitas B-lineage lymphoma (Cbl)-b was identified recently as a novel negative regulator of macrophage activation in adipose tissue. Cbl-b, which is a suppressor of T- and B-cell activation, inhibits intracellular signal transduction by targeting some tyrosine kinases. Notably, preventing Cbl-b-mediated macrophage activation improves obesity-induced insulin resistance in mice. c-Cbl is another member of the Cbl family that is associated with insulin resistance in obesity. These reports suggest that Cbl-b and c-Cbl are potential therapeutic targets for treating obesity-induced insulin resistance. In this review, we focus on the importance of Cbl-b in macrophage activation in aging-induced and high-fat diet-induced obesity.

Lack of oncostatin M receptor β leads to adipose tissue inflammation and insulin resistance by switching macrophage phenotype

Oncostatin M (OSM), a member of the IL-6 family of cytokines, plays important roles in a variety of biological functions, including inflammatory responses. However, the roles of OSM in metabolic diseases are unknown. We herein analyzed the metabolic parameters of OSM receptor β subunit-deficient (OSMRβ(-/-)) mice under normal diet conditions. At 32 weeks of age, OSMRβ(-/-) mice exhibited mature-onset obesity, severer hepatic steatosis, and insulin resistance. Surprisingly, insulin resistance without obesity was observed in OSMRβ(-/-) mice at 16 weeks of age, suggesting that insulin resistance precedes obesity in OSMRβ(-/-) mice. Both OSM and OSMRβ were expressed strongly in the adipose tissue and little in some other metabolic organs, including the liver and skeletal muscle. In addition, OSMRβ is mainly expressed in the adipose tissue macrophages (ATMs) but not in adipocytes. In OSMRβ(-/-) mice, the ATMs were polarized to M1 phenotypes with the augmentation of adipose tissue inflammation. Treatment of OSMRβ(-/-) mice with an anti-inflammatory agent, sodium salicylate, improved insulin resistance. In addition, the stimulation of a macrophage cell line, RAW264.7, and peritoneal exudate macrophages with OSM resulted in the increased expression of M2 markers, IL-10, arginase-1, and CD206. Furthermore, treatment of C57BL/6J mice with OSM increased insulin sensitivity and polarized the phenotypes of ATMs to M2. Thus, OSM suppresses the development of insulin resistance at least in part through the polarization of the macrophage phenotypes to M2, and OSMRβ(-/-) mice provide a unique mouse model of metabolic diseases.

Cbl-b is a critical regulator of macrophage activation associated with obesity-induced insulin resistance in mice

We previously reported the potential involvement of casitas B-cell lymphoma-b (Cbl-b) in aging-related murine insulin resistance. Because obesity also induces macrophage recruitment into adipose tissue, we elucidated here the role of Cbl-b in obesity-related insulin resistance. Cbl-b(+/+) and Cbl-b(-/-) mice were fed a high-fat diet (HFD) and then examined for obesity-related changes in insulin signaling. The HFD caused recruitment of macrophages into adipose tissue and increased inflammatory reaction in Cbl-b(-/-) compared with Cbl-b(+/+) mice. Peritoneal macrophages from Cbl-b(-/-) mice and Cbl-b-overexpressing RAW264.7 macrophages were used to examine the direct effect of saturated fatty acids (FAs) on macrophage activation. In macrophages, Cbl-b suppressed saturated FA-induced Toll-like receptor 4 (TLR4) signaling by ubiquitination and degradation of TLR4. The physiological role of Cbl-b in vivo was also examined by bone marrow transplantation and Eritoran, a TLR4 antagonist. Hematopoietic cell-specific depletion of the Cbl-b gene induced disturbed responses on insulin and glucose tolerance tests. Blockade of TLR4 signaling by Eritoran reduced fasting blood glucose and serum interleukin-6 levels in obese Cbl-b(-/-) mice. These results suggest that Cbl-b deficiency could exaggerate HFD-induced insulin resistance through saturated FA-mediated macrophage activation. Therefore, inhibition of TLR4 signaling is an attractive therapeutic strategy for treatment of obesity-related insulin resistance.

Soy Glycinin Contains a Functional Inhibitory Sequence against Muscle-Atrophy-Associated Ubiquitin Ligase Cbl-b

Background. Unloading stress induces skeletal muscle atrophy. We have reported that Cbl-b ubiquitin ligase is a master regulator of unloading-associated muscle atrophy. The present study was designed to elucidate whether dietary soy glycinin protein prevents denervation-mediated muscle atrophy, based on the presence of inhibitory peptides against Cbl-b ubiquitin ligase in soy glycinin protein. Methods. Mice were fed either 20% casein diet, 20% soy protein isolate diet, 10% glycinin diet containing 10% casein, or 20% glycinin diet. One week later, the right sciatic nerve was cut. The wet weight, cross sectional area (CSA), IGF-1 signaling, and atrogene expression in hindlimb muscles were examined at 1, 3, 3.5, or 4 days after denervation. Results. 20% soy glycinin diet significantly prevented denervation-induced decreases in muscle wet weight and myofiber CSA. Furthermore, dietary soy protein inhibited denervation-induced ubiquitination and degradation of IRS-1 in tibialis anterior muscle. Dietary soy glycinin partially suppressed the denervation-mediated expression of atrogenes, such as MAFbx/atrogin-1 and MuRF-1, through the protection of IGF-1 signaling estimated by phosphorylation of Akt-1. Conclusions. Soy glycinin contains a functional inhibitory sequence against muscle-atrophy-associated ubiquitin ligase Cbl-b. Dietary soy glycinin protein significantly prevented muscle atrophy after denervation in mice.

Isoflavones derived from soy beans prevent MuRF1-mediated muscle atrophy in C2C12 myotubes through SIRT1 activation

Proinflammatory cytokines are factors that induce ubiquitin-proteasome-dependent proteolysis in skeletal muscle, causing muscle atrophy. Although isoflavones, as potent antioxidative nutrients, have been known to reduce muscle damage during the catabolic state, the non-antioxidant effects of isoflavones against muscle atrophy are not well known. Here we report on the inhibitory effects of isoflavones such as genistein and daidzein on muscle atrophy caused by tumor necrosis factor (TNF)-α treatment. In C2C12 myotubes, TNF-α treatment markedly elevated the expression of the muscle-specific ubiquitin ligase MuRF1, but not of atrogin-1, leading to myotube atrophy. We found that MuRF1 promoter activity was mediated by acetylation of p65, a subunit of NFκB, a downstream target of the TNF-α signaling pathway; increased MuRF1 promoter activity was abolished by SIRT1, which is associated with deacetylation of p65. Of interest, isoflavones induced expression of SIRT1 mRNA and phosphorylation of AMP kinase, which is well known to stimulate SIRT1 expression, although there was no direct effect on SIRT1 activation. Moreover, isoflavones significantly suppressed MuRF1 promoter activity and myotube atrophy induced by TNF-α in C2C12 myotubes. These results suggest that isoflavones suppress myotube atrophy in skeletal muscle cells through activation of SIRT1 signaling. Thus, the efficacy of isoflavones could provide a novel therapeutic approach against inflammation-related muscle atrophy.

Protein tyrosine kinase regulation by ubiquitination: critical roles of Cbl-family ubiquitin ligases

Protein tyrosine kinases (PTKs) coordinate a broad spectrum of cellular responses to extracellular stimuli and cell-cell interactions during development, tissue homeostasis, and responses to environmental challenges. Thus, an understanding of the regulatory mechanisms that ensure physiological PTK function and potential aberrations of these regulatory processes during diseases such as cancer are of broad interest in biology and medicine. Aside from the expected role of phospho-tyrosine phosphatases, recent studies have revealed a critical role of covalent modification of activated PTKs with ubiquitin as a critical mechanism of their negative regulation. Members of the Cbl protein family (Cbl, Cbl-b and Cbl-c in mammals) have emerged as dominant "activated PTK-selective" ubiquitin ligases. Structural, biochemical and cell biological studies have established that Cbl protein-dependent ubiquitination targets activated PTKs for degradation either by facilitating their endocytic sorting into lysosomes or by promoting their proteasomal degradation. This mechanism also targets PTK signaling intermediates that become associated with Cbl proteins in a PTK activation-dependent manner. Cellular and animal studies have established that the relatively broadly expressed mammalian Cbl family members Cbl and Cbl-b play key physiological roles, including their critical functions to prevent the transition of normal immune responses into autoimmune disease and as tumor suppressors; the latter function has received validation from human studies linking mutations in Cbl to human leukemia. These newer insights together with embryonic lethality seen in mice with a combined deletion of Cbl and Cbl-b genes suggest an unappreciated role of the Cbl family proteins, and by implication the ubiquitin-dependent control of activated PTKs, in stem/progenitor cell maintenance. Future studies of existing and emerging animal models and their various cell lineages should help test the broader implications of the evolutionarily-conserved Cbl family protein-mediated, ubiquitin-dependent, negative regulation of activated PTKs in physiology and disease.

Protein tyrosine kinase regulation by ubiquitination: critical roles of Cbl-family ubiquitin ligases

Protein tyrosine kinases (PTKs) coordinate a broad spectrum of cellular responses to extracellular stimuli and cell-cell interactions during development, tissue homeostasis, and responses to environmental challenges. Thus, an understanding of the regulatory mechanisms that ensure physiological PTK function and potential aberrations of these regulatory processes during diseases such as cancer are of broad interest in biology and medicine. Aside from the expected role of phospho-tyrosine phosphatases, recent studies have revealed a critical role of covalent modification of activated PTKs with ubiquitin as a critical mechanism of their negative regulation. Members of the Cbl protein family (Cbl, Cbl-b and Cbl-c in mammals) have emerged as dominant "activated PTK-selective" ubiquitin ligases. Structural, biochemical and cell biological studies have established that Cbl protein-dependent ubiquitination targets activated PTKs for degradation either by facilitating their endocytic sorting into lysosomes or by promoting their proteasomal degradation. This mechanism also targets PTK signaling intermediates that become associated with Cbl proteins in a PTK activation-dependent manner. Cellular and animal studies have established that the relatively broadly expressed mammalian Cbl family members Cbl and Cbl-b play key physiological roles, including their critical functions to prevent the transition of normal immune responses into autoimmune disease and as tumor suppressors; the latter function has received validation from human studies linking mutations in Cbl to human leukemia. These newer insights together with embryonic lethality seen in mice with a combined deletion of Cbl and Cbl-b genes suggest an unappreciated role of the Cbl family proteins, and by implication the ubiquitin-dependent control of activated PTKs, in stem/progenitor cell maintenance. Future studies of existing and emerging animal models and their various cell lineages should help test the broader implications of the evolutionarily-conserved Cbl family protein-mediated, ubiquitin-dependent, negative regulation of activated PTKs in physiology and disease.

Neuronal Cbl controls biosynthesis of insulin-like peptides in Drosophila melanogaster

The Cbl family proteins function as both E3 ubiquitin ligases and adaptor proteins to regulate various cellular signaling events, including the insulin/insulin-like growth factor 1 (IGF1) and epidermal growth factor (EGF) pathways. These pathways play essential roles in growth, development, metabolism, and survival. Here we show that in Drosophila melanogaster, Drosophila Cbl (dCbl) regulates longevity and carbohydrate metabolism through downregulating the production of Drosophila insulin-like peptides (dILPs) in the brain. We found that dCbl was highly expressed in the brain and knockdown of the expression of dCbl specifically in neurons by RNA interference increased sensitivity to oxidative stress or starvation, decreased carbohydrate levels, and shortened life span. Insulin-producing neuron-specific knockdown of dCbl resulted in similar phenotypes. dCbl deficiency in either the brain or insulin-producing cells upregulated the expression of dilp genes, resulting in elevated activation of the dILP pathway, including phosphorylation of Drosophila Akt and Drosophila extracellular signal-regulated kinase (dERK). Genetic interaction analyses revealed that blocking Drosophila epidermal growth factor receptor (dEGFR)-dERK signaling in pan-neurons or insulin-producing cells by overexpressing a dominant-negative form of dEGFR abolished the effect of dCbl deficiency on the upregulation of dilp genes. Furthermore, knockdown of c-Cbl in INS-1 cells, a rat β-cell line, also increased insulin biosynthesis and glucose-stimulated secretion in an ERK-dependent manner. Collectively, these results suggest that neuronal dCbl regulates life span, stress responses, and metabolism by suppressing dILP production and the EGFR-ERK pathway mediates the dCbl action. Cbl suppression of insulin biosynthesis is evolutionarily conserved, raising the possibility that Cbl may similarly exert its physiological actions through regulating insulin production in β cells.

The cellular and signaling networks linking the immune system and metabolism in disease

It is now recognized that obesity is driving the type 2 diabetes epidemic in Western countries. Obesity-associated chronic tissue inflammation is a key contributing factor to type 2 diabetes and cardiovascular disease, and a number of studies have clearly demonstrated that the immune system and metabolism are highly integrated. Recent advances in deciphering the various cellular and signaling networks that participate in linking the immune and metabolic systems together have contributed to understanding of the pathogenesis of metabolic diseases and may also inform new therapeutic strategies based on immunomodulation. Here we discuss how these various networks underlie the etiology of the inflammatory component of insulin resistance, with a particular focus on the central roles of macrophages in adipose tissue and liver.

Identification of a redox-modulatory interaction between uncoupling protein 3 and thioredoxin 2 in the mitochondrial intermembrane space

Uncoupling protein 3 (UCP3) is a member of the mitochondrial solute carrier superfamily that is enriched in skeletal muscle and controls mitochondrial reactive oxygen species (ROS) production, but the mechanisms underlying this function are unclear.

AIMS

The goal of this work focused on the identification of mechanisms underlying UCP3 functions.

RESULTS

Here we report that the N-terminal, intermembrane space (IMS)-localized hydrophilic domain of mouse UCP3 interacts with the N-terminal mitochondrial targeting signal of thioredoxin 2 (Trx2), a mitochondrial thiol reductase. Cellular immunoprecipitation and in vitro pull-down assays show that the UCP3-Trx2 complex forms directly, and that the Trx2 N-terminus is both necessary and sufficient to confer UCP3 binding. Mutation studies show that neither a catalytically inactivated Trx2 mutant, nor a mutant Trx2 bearing the N-terminal targeting sequence of cytochrome c oxidase (COXMTS-Trx2) bind UCP3. Biochemical analyses using permeabilized mitochondria, and live cell experiments using bimolecular fluorescence complementation show that the UCP3-Trx2 complex forms specifically in the IMS. Finally, studies in C2C12 myocytes stably overexpressing UCP3 (2.5-fold) and subjected to Trx2 knockdown show that Trx2 is required for the UCP3-dependent mitigation of complex III-driven mitochondrial ROS generation. UCP3 expression was increased in mice fed a high fat diet, leading to increased localization of Trx2 to the IMS. UCP3 overexpression also increased expression of the glucose transporter GLUT4 in a Trx2-dependent fashion.

INNOVATION

This is the first report of a mitochondrial protein-protein interaction with UCP3 and the first demonstration that UCP3 binds directly, and in cells and tissues with mitochondrial thioredoxin 2.

CONCLUSION

These studies identify a novel UCP3-Trx2 complex, a novel submitochondrial localization of Trx2, and a mechanism underlying UCP3-regulated mitochondrial ROS production.

Endothelial cell apoptosis in brown adipose tissue of rats induced by hyperinsulinaemia: the possible role of TNF-α

The aim of the present study was to investigate whether hyperinsulinaemia, which frequently precedes insulin resistance syndrome (obesity, diabetes), induces apoptosis of endothelial cells (ECs) in brown adipose tissue (BAT) and causes BAT atrophy and also, to investigate the possible mechanisms underlying ECs death. In order to induce hyperinsuli-naemia, adult male rats of Wistar strain were treated with high dose of insulin (4 U/kg, intraperitonely) for one or three days. Examinations at ultrastructural level showed apoptotic changes of ECs, allowing us to point out that changes mainly but not exclusively, occur in nuclei. Besides different stages of condensation and alterations of the chromatin, nuclear fragmentation was also observed. Higher number of ECs apoptotic nuclei in the BAT of hyperinsulinaemic rats was also confirmed by propidium iodide staining. Immunohistochemical localization of tumor necrosis factor-alpha (TNF-α) revealed increased expression in ECs of BAT of hyperinsulinaemic animals, indicating its possible role in insulin-induced apoptotic changes. These results suggest that BAT atrophy in hyperinsulinaemia is a result of endothelial and adipocyte apoptosis combined, rather than any of functional components alone.

Clodronate liposomes improve metabolic profile and reduce visceral adipose macrophage content in diet-induced obese mice

Background

Obesity-related adipose inflammation has been thought to be a causal factor for the development of insulin resistance and type 2 diabetes. Infiltrated macrophages in adipose tissue of obese animals and humans are an important source for inflammatory cytokines. Clodronate liposomes can ablate macrophages by inducing apoptosis. In this study, we aim to determine whether peritoneal injection of clodronate liposomes has any beneficial effect on systemic glucose homeostasis/insulin sensitivity and whether macrophage content in visceral adipose tissue will be reduced in diet-induced obese (DIO) mice.

Methodology/Principal Findings

Clodronate liposomes were used to deplete macrophages in lean and DIO mice. Macrophage content in visceral adipose tissue, metabolic parameters, glucose and insulin tolerance, adipose and liver histology, adipokine and cytokine production were examined. Hyperinsulinemic-euglycemic clamp study was also performed to assess systemic insulin sensitivity. Peritoneal injection of clodronate liposomes significantly reduced blood glucose and insulin levels in DIO mice. Systemic glucose tolerance and insulin sensitivity were mildly improved in both lean and DIO mice treated with clodronate liposomes by intraperitoneal (ip) injection. Hepatosteatosis was dramatically alleviated and suppression of hepatic glucose output was markedly increased in DIO mice treated with clodronate liposomes. Macrophage content in visceral adipose tissue of DIO mice was effectively decreased without affecting subcutaneous adipose tissue. Interestingly, levels of insulin sensitizing hormone adiponectin, including the high molecular weight form, were significantly elevated in circulation.

Conclusions/Significance

Intraperitoneal injection of clodronate liposomes reduces visceral adipose tissue macrophages, improves systemic glucose homeostasis and insulin sensitivity in DIO mice, which can be partially attributable to increased adiponectin levels.

Indispensable roles of mammalian Cbl family proteins as negative regulators of protein tyrosine kinase signaling: Insights from in vivo models

All higher eukaryotes utilize protein tyrosine kinases (PTKs) as molecular switches to control a variety of cellular signals. Notably, many PTKs have been identified as proto-oncogenes whose aberrant expression, mutations or co-option by pathogens can lead to human malignancies. Thus, it is obvious that PTK functions must be precisely regulated in order to maintain homeostasis of an organism. Investigations over the past fifteen years have revealed that members of the Cbl family proteins can serve as negative regulators of PTK signaling, and biochemical and cell biological studies have unraveled the mechanistic basis of this regulation. Yet, it is only recently that the field has begun to appreciate the real significance of this novel regulatory apparatus in shaping PTK-mediated signaling in organismic contexts and in human diseases. Here, we discuss recent progress in murine models that are beginning to provide insights into the critical roles of Cbl proteins in physiological pathways, with important implications in understanding how aberrations of Cbl proteins contribute to oncogenesis.

An inflammatory micro-environment promotes human adipocyte apoptosis

Obesity-associated macrophage infiltration into adipose tissue is responsible for both local and systemic inflammation. Recent findings suggest fat cell apoptosis as an initiator of macrophage recruitment. Here, we investigated the effects of an inflammatory micro-environment on fat cells using human THP-1 macrophages and SGBS adipocytes. Macrophage-secreted factors induced insulin resistance, inhibited insulin-stimulated Akt phosphorylation, and induced apoptosis of adipocytes. The apoptosis-inducing effect was even more pronounced in direct co-cultures of adipocytes and macrophages. Our data suggest a link between insulin resistance and apoptosis sensitivity. Accordingly, pharmacological and genetic inhibition of insulin signaling at the level of Akt2 sensitized adipocytes to apoptosis induction by macrophage-secreted factors. In conclusion, we describe here a novel interaction of macrophages and fat cells, i.e. induction of apoptosis. Our data suggest a feed-forward cycle in which macrophages further drive the inflammatory process by inducing insulin resistance and concomitant apoptosis of adipocytes.

FFA-induced adipocyte inflammation and insulin resistance: involvement of ER stress and IKKβ pathways

Free-fatty acids (FFAs) are well-characterized factor for causing production of inflammatory factors and insulin resistance in adipocytes. Using cultured adipocytes, we demonstrate that FFAs can activate endoplasmic reticulum (ER) stress pathway by examination of ER stress sensor activation and marker gene expression. Chemical chaperone tauroursodeoxycholic acid (TUDCA) can reduce FFA-induced adipocyte inflammation and improve insulin signaling whereas overexpression of spliced X-box protein 1 (XBP-1s) only attenuates FFA-induced inflammation. PKR-like eukaryotic initiation factor 2α kinase (PERK) is one of the three major ER stress sensor proteins and deficiency of PERK alleviates FFA-induced inflammation and insulin resistance. The key downstream target of FFA-induced ER stress is IκB kinase β (IKKβ), a master kinase for regulating expression of inflammatory genes. Deficiency of PERK attenuates FFA-induced activation of IKKβ and deficiency of IKKβ alleviates FFA-induced inflammation and insulin resistance. Consistently, overexpression of IKKβ in 3T3-L1 CAR adipocytes causes inflammation and insulin resistance. In addition, IKKβ overexpression has profound effect on adipocyte lipid metabolism, including inhibition of lipogenesis and promotion of lipolysis. Furthermore, increased endogenous IKKβ expression and activation is also observed in isolated primary adipocytes from mice injected with lipids or fed on high-fat diet (HFD) acutely. These results indicate that ER stress pathway is a key mediator for FFA-induced inflammation and insulin resistance in adipocytes with PERK and IKKβ as the critical signaling components.

Essential role of E3 ubiquitin ligase activity in Cbl-b-regulated T cell functions

E3 ubiquitin ligases have been placed among the essential molecules involved in the regulation of T cell functions and T cell tolerance. However, it has never been experimentally proven in vivo whether these functions indeed depend on the catalytic E3 ligase activity. The Casitas B-cell lymphoma (Cbl) family protein Cbl-b was the first E3 ubiquitin ligase directly implicated in the activation and tolerance of the peripheral T cell. In this study, we report that selective genetic inactivation of Cbl-b E3 ligase activity phenocopies the T cell responses observed when total Cbl-b is ablated, resulting in T cell hyperactivation, spontaneous autoimmunity, and impaired induction of T cell anergy in vivo. Moreover, mice carrying a Cbl-b E3 ligase-defective mutation spontaneously reject tumor cells that express human papilloma virus Ags. These data demonstrate for the first time, to our knowledge, that the catalytic function of an E3 ligase, Cbl-b, is essential for negative regulation of T cells in vivo. Thus, modulation of the E3 ligase activity of Cbl-b might be a novel modality to control T cell immunity in vaccination, cancer biology, or autoimmunity.

THP-1 Macrophages and SGBS Adipocytes - A New Human in vitro Model System of Inflamed Adipose Tissue

Obesity is associated with an accumulation of macrophages in adipose tissue. This inflammation of adipose tissue is a key event in the pathogenesis of several obesity-related disorders, particularly insulin resistance. Here, we summarized existing model systems that mimic the situation of inflamed adipose tissue in vitro, most of them being murine. Importantly, we introduce our newly established human model system which combines the THP-1 monocytic cell line and the preadipocyte cell strain Simpson-Golabi-Behmel syndrome (SGBS). THP-1 cells, which originate from an acute monocytic leukemia, differentiate easily into macrophages in vitro. The human preadipocyte cell strain SGBS was recently introduced as a unique tool to study human fat cell functions. SGBS cells are characterized by a high capacity for adipogenic differentiation. SGBS adipocytes are capable of fat cell-specific metabolic functions such as insulin-stimulated glucose uptake, insulin-stimulated de novo lipogenesis and β-adrenergic-stimulated lipolysis and they secrete typical adipokines including leptin, adiponectin, and RBP4. Applying either macrophage-conditioned medium or a direct co-culture of macrophages and fat cells, our model system can be used to distinguish between paracrine and cell-contact dependent effects. In conclusion, we propose this model as a useful tool to study adipose inflammation in vitro. It represents an inexpensive, highly reproducible human system. The methods described here can be easily extended for usage of primary human macrophages and fat cells.

Macrophage-secreted factors inhibit ZAG expression and secretion by human adipocytes

Zinc-alpha2-glycoprotein (ZAG), a novel adipokine, is downregulated in adipose tissue in obesity, a state characterized by increased adipose tissue macrophage infiltration and chronic low-grade inflammation. This study investigated whether macrophage-secreted factors and TNF-alpha, a major product of macrophages, modulate ZAG expression and secretion by human adipocytes. ZAG was produced primarily by adipocytes, and not by preadipocytes and macrophages. Incubation of preadipocytes with macrophage-conditioned medium for up to 12 days decreased ZAG mRNA and protein release, and the expression of adipogenic markers (PPARgamma and C/EBPalpha). Adipocytes treated with macrophage-conditioned medium for 24h displayed significant reductions in ZAG mRNA and release. Chronic TNF-alpha treatment let to significant decreases in ZAG expression and secretion, but marked upregulation of pro-inflammatory cytokines and chemokines (IL-6, leptin, IL-8, MCP-1 and RANTES) in adipocytes. These findings suggest that macrophage-associated inflammation may play a significant role in the downregulation of ZAG in adipose tissue in obesity.

Cbl-b regulates airway mucosal tolerance to aeroallergen

BACKGROUND

As an E3 ubiquitin ligase and a molecular adaptor, Cbl-b controls the activation threshold of the antigen receptor and negatively regulates CD28 costimulation, functioning as an intrinsic mediator of T cell anergy that maintains tolerance. However, the role of Cbl-b in the airway immune response to aeroallergens is unclear.

OBJECTIVE

To determine the contribution of Cbl-b in tolerance to aeroallergens, we examined ovalbumin (OVA)-induced lung inflammation in Cbl-b-deficient mice.

METHODS

Cbl-b(-/-) mice and wild-type (WT) C57BL/6 mice were sensitized and challenged with OVA intranasally, a procedure normally tolerated by WT mice. We analysed lung histology, bronchoalveolar lavage fluid total cell counts and differential, cytokines and chemokines in the airway, and cytokine response by lymphocytes after re-stimulation by OVA antigen.

RESULTS

Compared with WT mice, OVA-challenged Cbl-b(-/-) mice showed significantly increased neutrophilic and eosinophilic infiltration in the lung and mucus hyperplasia. The serum levels of IgG2a and IgG1, but not IgE, were increased. The levels of inflammatory mediators IFN-γ, IL-10, IL-12, IL-13, IP-10, MCP-1, MIP-1α, eotaxin, and RANTES, but not IL-17A or IL-6, were elevated in the airway of Cbl-b(-/-) mice. Lymphocytes from Cbl-b(-/-) mice released increased amount of IFN-γ, IL-10, IL-13, and IP-10 in response to OVA re-stimulation. However, no significant changes were noted in the CD4(+) CD25(+) T regulatory cell populations in the lung tissues after OVA stimulation and there was no difference between WT and Cbl-b(-/-) mice.

CONCLUSION

These results demonstrate that Cbl-b deficiency leads to a breakdown of tolerance to OVA allergen in the murine airways, probably through increased activation of T effector cells, indicating that Cbl-b is a critical factor in maintaining lung homeostasis upon environmental exposure to aeroallergens.

Functional heterogeneity of CD11c-positive adipose tissue macrophages in diet-induced obese mice

Obesity represents a state of chronic, low grade inflammation and is associated with infiltration of increased numbers of adipose tissue macrophages (ATMs). Diet-induced obesity leads to an increase in non-inflammatory M1-like ATMs displaying the CD11c surface marker. We assessed the function of CD11c-positive ATMs when insulin resistant high fat diet (HFD) mice become insulin-sensitive after switching from HFD to normal chow (NC). HFD mice rapidly become insulin-sensitive in all major insulin-target tissues, including muscle, liver, and adipose tissue, after the diet switch. In adipose tissue the CD11c-positive macrophages remain constant in number despite the presence of insulin sensitivity, but these macrophages now assume a new phenotype in which they no longer exhibit increased inflammatory pathway markers. Adipose tissue markers of apoptosis and necrosis were elevated on HFD and remain high after the HFD --> NC diet switch. Furthermore, ATM accumulation preceded detectable adipocyte necrosis at the early phase of HFD. Together, these results indicate that 1) CD11c-positive M1-like ATMs can exhibit phenotypic plasticity and that the polarization of these cells between inflammatory and non-inflammatory states is well correlated to the presence of absence of insulin resistance, and 2) adipocyte necrosis and apoptosis can be dissociated from ATM accumulation.

Cbl-b in T-cell activation

Peripheral activation of antigen-specific T cells is stringently controlled to prevent immune responses against self-antigens. Only after a T cell is presented with two signals, an antigen and a co-stimulatory signal, can they be fully activated. In case antigen presentation occurs without co-stimulation, T-cell receptor (TCR) signaling pathways are regulated to prevent T-cell activation and induce T-cell tolerance. Thus, for a productive T-cell response to occur, co-stimulatory receptors need to serve the dual role of amplifying the TCR signaling while concomitantly releasing T cells from suppression. Biochemical and genetic studies during the last 10 years have documented the critical role of the E3 ubiquitin-ligase Cbl-b in this fundamental two-signal modulation of T-cell responses. In this review, we will discuss our current understanding on how Cbl-b controls T-cell activation and tolerance, its in vivo implications, as well as mechanisms for tuning T-cell-mediated immune responses by this essential E3 ligase.

Mammary tumors with diverse immunological phenotypes show differing sensitivity to adoptively transferred CD8+ T cells lacking the Cbl-b gene

We tested the efficacy of CD8+ T cells lacking the Cbl-b gene against a panel of mammary tumor lines with different intrinsic sensitivities to T cells. Mice bearing established tumors expressing an ovalbumin-tagged version of HER-2/neu underwent adoptive transfer with Cbl-b-replete or -null CD8+ T cells from OT-I T cell receptor transgenic donor mice. In general, Cbl-b-null OT-I cells showed enhanced expansion, persistence, and capacity for tumor infiltration. This resulted in markedly enhanced efficacy against two tumor lines that normally demonstrate complete (NOP21) or partial (NOP23) regression. Moreover, a third tumor line (NOP6) that normally demonstrates progressive disease underwent complete regression in response to Cbl-b-null OT-I cells. However, a fourth tumor line (NOP18) was resistant to Cbl-b-null OT-I cells owing to a profound barrier to lymphocyte infiltration. Thus, Cbl-b-null CD8+ T cells are generally more efficacious but are nonetheless unable to mediate curative responses against all tumor phenotypes.