Cytokine signaling: STATS in plasma membrane rafts

Cyclic fasting bolsters cholesterol biosynthesis inhibitors' anticancer activity

Identifying oncological applications for drugs that are already approved for other medical indications is considered a possible solution for the increasing costs of cancer treatment. Under the hypothesis that nutritional stress through fasting might enhance the antitumour properties of at least some non-oncological agents, by screening drug libraries, we find that cholesterol biosynthesis inhibitors (CBIs), including simvastatin, have increased activity against cancers of different histology under fasting conditions. We show fasting's ability to increase CBIs' antitumour effects to depend on the reduction in circulating insulin, insulin-like growth factor-1 and leptin, which blunts the expression of enzymes from the cholesterol biosynthesis pathway and enhances cholesterol efflux from cancer cells. Ultimately, low cholesterol levels through combined fasting and CBIs reduce AKT and STAT3 activity, oxidative phosphorylation and energy stores in the tumour. Our results support further studies of CBIs in combination with fasting-based dietary regimens in cancer treatment and highlight the value of fasting for drug repurposing in oncology.

Central role of Prominin-1 in lipid rafts during liver regeneration

Prominin-1, a lipid raft protein, is required for maintaining cancer stem cell properties in hepatocarcinoma cell lines, but its physiological roles in the liver have not been well studied. Here, we investigate the role of Prominin-1 in lipid rafts during liver regeneration and show that expression of Prominin-1 increases after 2/3 partial hepatectomy or CCl₄ injection. Hepatocyte proliferation and liver regeneration are attenuated in liver-specific Prominin-1 knockout mice compared to wild-type mice. Detailed mechanistic studies reveal that Prominin-1 interacts with the interleukin-6 signal transducer glycoprotein 130, confining it to lipid rafts so that STAT3 signaling by IL-6 is effectively activated. The overexpression of the glycosylphosphatidylinsositol-anchored first extracellular domain of Prominin-1, which is the domain that binds to GP130, rescued the proliferation of hepatocytes and liver regeneration in liver-specific Prominin-1 knockout mice. In summary, Prominin-1 is upregulated in hepatocytes during liver regeneration where it recruits GP130 into lipid rafts and activates the IL6-GP130-STAT3 axis, suggesting that Prominin-1 might be a promising target for therapeutic applications in liver transplantation.

Arachidonic acid, a clinically adverse mediator in the ovarian cancer microenvironment, impairs JAK-STAT signaling in macrophages by perturbing lipid raft structures

Survival of ovarian carcinoma is associated with the abundance of immunosuppressed CD163^highCD206^high tumor‐associated macrophages (TAMs) and high levels of arachidonic acid (AA) in the tumor microenvironment. Here, we show that both associations are functionally linked. Transcriptional profiling revealed that high CD163 and CD206/MRC1 expression in TAMs is strongly associated with an inhibition of cytokine‐triggered signaling, mirrored by an impaired transcriptional response to interferons and IL‐6 in monocyte‐derived macrophages by AA. This inhibition of pro‐inflammatory signaling is caused by dysfunctions of the cognate receptors, indicated by the inhibition of JAK1, JAK2, STAT1, and STAT3 phosphorylation, and by the displacement of the interferon receptor IFNAR1, STAT1 and other immune‐regulatory proteins from lipid rafts. AA exposure led to a dramatic accumulation of free AA in lipid rafts, which appears to be mechanistically crucial, as the inhibition of its incorporation into phospholipids did not affect the AA‐mediated interference with STAT1 phosphorylation. Inhibition of interferon‐triggered STAT1 phosphorylation by AA was reversed by water‐soluble cholesterol, known to prevent the perturbation of lipid raft structure by AA. These findings suggest that the pharmacologic restoration of lipid raft functions in TAMs may contribute to the development new therapeutic approaches.

25-Hydroxycholesterol suppress IFN-γ-induced inflammation in microglia by disrupting lipid raft formation and caveolin-mediated signaling endosomes

Acute microglial activation plays an important role in neuroprotection. However, dysregulated, prolonged microgliosis exacerbates neurodegeneration through excessive release of pro-inflammatory cytokines and cytotoxic factors. Interferon-gamma (IFN-γ), an inflammatory cytokine, exacerbates the detrimental microglial response. Although various anti-inflammatory drugs have been evaluated as interventions for microglia-mediated neuroinflammation, no anti-inflammatories are in clinical use for microgliosis. The present study evaluated the anti-inflammatory mechanisms of oxysterols, blood brain barrier (BBB) penetrable bioactive lipids, revealing that this intervention suppresses neuroinflammation by disrupting membrane lipid raft formation and caveolae-mediated endosomal IFN-γ signaling. We find that 25-hydroxycholesterol (25-HC) rapidly repressed IFN-γ receptor trafficking to lipid rafts in microglia by disrupting raft formation, thereby suppressing microglial inflammatory response. IFN-γ treatment upregulated expression of Cav-1, a major component of caveolae, and IFN-γ signaling was sustained through Cav-1+ signaling endosomes. 25-HC repressed IFN-γ induction of Cav-1 expression in microglia, and subsequently suppressed the chronic inflammatory response. Taken together, these findings demonstrated that 25-HC effectively regulate the inflammatory status of microglia by mediating the formation of rafts and caveolae-dependent signaling endosomes. Given the important roles of IFN-γ and microglia in the pathology of neurodegenerative brain diseases, a novel anti-inflammatory mechanism of 25-HC that is not receptor-dependent, but rather is related to the regulation of membrane rafts and caveolae, suggests a new therapeutic target for inflammatory neurodegenerations.

Endosomes as Signaling Platforms for IL-6 Family Cytokine Receptors

Interleukin-6 (IL-6) is the name-giving cytokine of a family of eleven members, including IL-6, CNTF, LIF, and IL-27. IL-6 was first recognized as a B-cell stimulating factor but we now know that the cytokine plays a pivotal role in the orchestration of inflammatory processes as well as in inflammation associated cancer. Moreover, IL-6 is involved in metabolic regulation and it has been shown to be involved in major neural activities such as neuroprotection, which can help to repair and to reduce brain damage. Receptor complexes of all members formed at the plasma membrane contain one or two molecules of the signaling receptor subunit GP130 and the mechanisms of signal transduction are well understood. IL-6 type cytokines can also signal from endomembranes, in particular the endosome, and situations have been reported in which endocytosis of receptor complexes are a prerequisite of intracellular signaling. Moreover, pathogenic GP130 variants were shown to interfere with spatial activation of downstream signals. We here summarize the molecular mechanisms underlying spatial regulation of IL-6 family cytokine signaling and discuss its relevance for pathogenic processes.

Lipid Rafts Interaction of the ARID3A Transcription Factor with EZRIN and G-Actin Regulates B-Cell Receptor Signaling

Several diseases originate via dysregulation of the actin cytoskeleton. The ARID3A/Bright transcription factor has also been implicated in malignancies, primarily those derived from hematopoietic lineages. Previously, we demonstrated that ARID3A shuttles between the nucleus and the plasma membrane, where it localizes within lipid rafts. There it interacts with components of the B-cell receptor (BCR) to reduce its ability to transmit downstream signaling. We demonstrate here that a direct component of ARID3A-regulated BCR signal strength is cortical actin. ARID3A interacts with actin exclusively within lipid rafts via the actin-binding protein EZRIN, which confines unstimulated BCRs within lipid rafts. BCR ligation discharges the ARID3A-EZRIN complex from lipid rafts, allowing the BCR to initiate downstream signaling events. The ARID3A-EZRIN interaction occurs almost exclusively within unpolymerized G-actin, where EZRIN interacts with the multifunctional ARID3A REKLES domain. These observations provide a mechanism by which a transcription factor directly regulates BCR signaling via linkage to the actin cytoskeleton with consequences for B-cell-related neoplasia.

Oncogenic KIT Modulates Type I IFN-Mediated Antitumor Immunity in GIST

Type I IFNs are implicated in tumor immunogenicity and response to systemic therapy, but their interaction with oncogene signaling is not well understood. Here, we studied oncogenic KIT, which drives gastrointestinal stromal tumor (GIST), the most common sarcoma. Using mouse models of GIST, we found that KIT inhibition reduced type I IFN production and signaling, which downregulated tumor MHC class I expression. Absence of type I IFN signaling increased tumor size, in part due to CD8⁺ T-cell impairment. Oncogenic KIT was required for GIST type I IFN signal transduction via STAT1. In human GIST cell lines and surgical specimens, type I IFN signaling contributed to human lymphocyte antigen class I expression and correlated with tumor immunogenicity. Augmenting the type I IFN response partially compensated for the immunosuppressive effects of KIT inhibition. Thus, KIT signaling contributes to type I IFN signaling, whereas KIT inhibition attenuates tumor immunogenicity and is partly rescued by innate immune stimulation.See related Spotlight on p. 489.

Pre-clinical investigation of STAT3 pathway in bladder cancer: Paving the way for clinical translation

Effective cancer therapy requires identification of signaling networks and investigating their potential role in proliferation and invasion of cancer cells. Among molecular pathways, signal transducer and activator of transcription 3 (STAT3) has been of importance due to its involvement in promoting proliferation, and invasion of cancer cells, and mediating chemoresistance. In the present review, our aim is to reveal role of STAT3 pathway in bladder cancer (BC), as one of the leading causes of death worldwide. In respect to its tumor-promoting role, STAT3 is able to enhance the growth of BC cells via inhibiting apoptosis and cell cycle arrest. STAT3 also contributes to metastasis of BC cells via upregulating of MMP-2 and MMP-9 as well as genes in the EMT pathway. BC cells obtain chemoresistance via STAT3 overexpression and its inhibition paves the way for increasing efficacy of chemotherapy. Different molecular pathways such as KMT1A, EZH2, DAB2IP and non-coding RNAs including microRNAs and long non-coding RNAs can function as upstream mediators of STAT3 that are discussed in this review article.

Lipid rafts as a therapeutic target

Lipid rafts regulate the initiation of cellular metabolic and signaling pathways by organizing the pathway components in ordered microdomains on the cell surface. Cellular responses regulated by lipid rafts range from physiological to pathological, and the success of a therapeutic approach targeting "pathological" lipid rafts depends on the ability of a remedial agent to recognize them and disrupt pathological lipid rafts without affecting normal raft-dependent cellular functions. In this article, concluding the Thematic Review Series on Biology of Lipid Rafts, we review current experimental therapies targeting pathological lipid rafts, including examples of inflammarafts and clusters of apoptotic signaling molecule-enriched rafts. The corrective approaches include regulation of cholesterol and sphingolipid metabolism and membrane trafficking by using HDL and its mimetics, LXR agonists, ABCA1 overexpression, and cyclodextrins, as well as a more targeted intervention with apoA-I binding protein. Among others, we highlight the design of antagonists that target inflammatory receptors only in their activated form of homo- or heterodimers, when receptor dimerization occurs in pathological lipid rafts. Other therapies aim to promote raft-dependent physiological functions, such as augmenting caveolae-dependent tissue repair. The overview of this highly dynamic field will provide readers with a view on the emerging concept of targeting lipid rafts as a therapeutic strategy.jlr;61/5/687/F1F1f1.

Lipid rafts in glial cells: role in neuroinflammation and pain processing

Activation of microglia and astrocytes secondary to inflammatory processes contributes to the development and perpetuation of pain with a neuropathic phenotype. This pain state presents as a chronic debilitating condition and affects a large population of patients with conditions like rheumatoid arthritis and diabetes, or after surgery, trauma, or chemotherapy. Here, we review the regulation of lipid rafts in glial cells and the role they play as a key component of neuroinflammatory sensitization of central pain signaling pathways. In this context, we introduce the concept of an inflammaraft (i-raft), enlarged lipid rafts harboring activated receptors and adaptor molecules and serving as an organizing platform to initiate inflammatory signaling and the cellular response. Characteristics of the inflammaraft include increased relative abundance of lipid rafts in inflammatory cells, increased content of cholesterol per raft, and increased levels of inflammatory receptors, such as toll-like receptor (TLR)4, adaptor molecules, ion channels, and enzymes in lipid rafts. This inflammaraft motif serves an important role in the membrane assembly of protein complexes, for example, TLR4 dimerization. Operating within this framework, we demonstrate the involvement of inflammatory receptors, redox molecules, and ion channels in the inflammaraft formation and the regulation of cholesterol and sphingolipid metabolism in the inflammaraft maintenance and disruption. Strategies for targeting inflammarafts, without affecting the integrity of lipid rafts in noninflammatory cells, may lead to developing novel therapies for neuropathic pain states and other neuroinflammatory conditions.

27-Hydroxycholesterol Impairs Plasma Membrane Lipid Raft Signaling as Evidenced by Inhibition of IL6-JAK-STAT3 Signaling in Prostate Cancer Cells

We recently reported that restoring the CYP27A1-27hydroxycholesterol axis had antitumor properties. Thus, we sought to determine the mechanism by which 27HC exerts its anti-prostate cancer effects. As cholesterol is a major component of membrane microdomains known as lipid rafts, which localize receptors and facilitate cellular signaling, we hypothesized 27HC would impair lipid rafts, using the IL6-JAK-STAT3 axis as a model given its prominent role in prostate cancer. As revealed by single molecule imaging of DU145 prostate cancer cells, 27HC treatment significantly reduced detected cholesterol density on the plasma membranes. Further, 27HC treatment of constitutively active STAT3 DU145 prostate cancer cells reduced STAT3 activation and slowed tumor growth in vitro and in vivo. 27HC also blocked IL6-mediated STAT3 phosphorylation in nonconstitutively active STAT3 cells. Mechanistically, 27HC reduced STAT3 homodimerization, nuclear translocation, and decreased STAT3 DNA occupancy at target gene promoters. Combined treatment with 27HC and STAT3 targeting molecules had additive and synergistic effects on proliferation and migration, respectively. Hallmark IL6-JAK-STAT gene signatures positively correlated with CYP27A1 gene expression in a large set of human metastatic castrate-resistant prostate cancers and in an aggressive prostate cancer subtype. This suggests STAT3 activation may be a resistance mechanism for aggressive prostate cancers that retain CYP27A1 expression. In summary, our study establishes a key mechanism by which 27HC inhibits prostate cancer by disrupting lipid rafts and blocking STAT3 activation. IMPLICATIONS: Collectively, these data show that modulation of intracellular cholesterol by 27HC can inhibit IL6-JAK-STAT signaling and may synergize with STAT3-targeted compounds.

Upregulation of ERp57 promotes clear cell renal cell carcinoma progression by initiating a STAT3/ILF3 feedback loop

Background

ERp57 dysfunction has been shown to contribute to tumorigenesis in multiple malignances. However, the role of ERp57 in clear cell renal carcinoma (ccRCC) remains unclear.

Methods

Cell proliferation ability was measured by MTT and colony forming assays. Western blotting and quantitative real-time PCR (qRT-PCR) were performed to measure protein and mRNA expression. Co-immunoprecipitation (CoIP) and proximity ligation assay (PLA) were performed to detect protein-protein interaction. Chromatin immunoprecipitation (ChIP), ribonucleoprotein immunoprecipitation (RIP), and oligo pull-down were used to confirm DNA–protein and RNA–protein interactions. Promoter luciferase analysis was used to detect transcription factor activity.

Results

Here we found ERp57 was overexpressed in ccRCC tissues, and the higher levels of ERp57 were correlated with poor survival in patients with ccRCC. In vivo and in vitro experiments showed that ccRCC cell proliferation was enhanced by ERp57 overexpression and inhibited by ERp57 deletion. Importantly, we found ERp57 positively regulated ILF3 expression in ccRCC cells. Mechanically, ERp57 was shown to bind to STAT3 protein and enhance the STAT3-mediated transcriptional activity of ILF3. Furthermore, ILF3 levels were increased in ccRCC tissues and associated with poor prognosis. Interestingly, we revealed that ILF3 could bind to ERp57 and positively regulate its expression by enhancing its mRNA stability. Furthermore, ccRCC cell proliferation was moderated via the ERp57/STAT3/ILF3 feedback loop.

Conclusions

In summary, our results indicate that the ERp57/STAT3/ILF3 feedback loop plays a key role in the oncogenesis of ccRCC and provides a potential therapeutic target for ccRCC treatment.

Effect of caveolin-1 on Stat3-ptyr705 levels in breast and lung carcinoma cells

We recently demonstrated that Cav1 (caveolin-1) is a negative regulator of Stat3 (signal transducer and activator of transcription-3) activity in mouse fibroblasts and human lung carcinoma SHP77 cells. We now examined whether the cellular context may affect their levels as well as the relationship between them, by assessing Cav1 and Stat3-ptyr705 amounts in different cell lines. In MDA-MB-231, A549, and HaCat cells, Cav1 levels were high and Stat3-ptyr705 levels were low, consistent with the notion of a negative effect of endogenous Cav1 on Stat3-ptyr705 levels in these lines. In addition, manipulation of Cav1 levels revealed a negative effect in MCF7 and mouse fibroblast cells, while Cav1 upregulation induced apoptosis in MCF7 cells. In contrast, however, line MRC9 had high Cav1 and high Stat3-ptyr705 levels, indicating that high Cav1 is insufficient to reduce Stat3-ptyr705 levels in this line. MCF7 and LuCi6 cells had very low Cav1 and Stat3-ptyr705 levels, indicating that the low Stat3-ptyr705 can be independent from Cav1 levels altogether. Our results reveal a further level of complexity in the relationship between Cav1 and Stat3-ptyr705 than previously thought. In addition, we demonstrate that in a feedback loop, Stat3 inhibition upregulates Cav1 in HeLa cells but not in other lines tested.

Mechanism of the JAK2/STAT3-CAV-1-NR2B signaling pathway in painful diabetic neuropathy

PURPOSE

The aim of the present study was to further elucidate the role of JAK2/STAT3-CAV-1-NR2B on painful diabetic neuropathy.

METHODS

In vivo, the mechanical withdrawal threshold and thermal withdrawal latency were measured to evaluate neuropathic pain behaviors (n= 8), while western blot (n= 5) and an immunofluorescence double staining experiment (n= 6) were performed to understand the molecular mechanism. In vitro, the individual culture of BV2 mouse microglia cell lines, the co-culture of BV2 mouse microglia cell lines and PC12 rat neuron cell lines, and western blot analysis were performed to understand the molecular mechanism between microglia and neurons.

RESULTS

The expression of p-JAK2, p-STAT3, t-CAV-1, and p-NR2B was upregulated in the dorsal horn of DNP rats throughout the experiment. Through the immunofluorescence double staining experiment, it was found that p-STAT3 was mainly expressed in activated microglia, and this condition can be stably maintained for approximately 2 weeks after the establishment of the DNP model. The intrathecal injection of JAK2 inhibitor AG490 can relieve the abnormal expression of p-JAK2, p-STAT3, t-CAV-1, and p-NR2B, and relieve pain. The remission of AG490 began on the third day, and it could be stably sustained for 14 days. In vitro high-glucose induced the activation of p-STAT3 in microglia, thereby upregulating the expression of p-CAV-1 and p-NR2B in neurons in the co-culture system. JAK2 inhibitor AG490 can alleviate the abnormal expression of these proteins in the JAK2/STAT3-CAV-1-NR2B signaling pathway in vitro.

CONCLUSIONS

Microglial JAK2/STAT3 signaling probably contributes to neuropathic pain by activating the CAV-1-NR2B pathway.

The Sweeping Role of Cholesterol Depletion in the Persistence of Helicobacter pylori Infections

The ability of Helicobacter pylori to persist lifelong in the human gastric mucosa is a striking phenomenon. It is even more surprising since infection is typically associated with a vivid inflammatory response. Recent studies revealed the mechanism by which this pathogen inhibits the epithelial responses to IFN-γ and other central inflammatory cytokines in order to abolish an effective antimicrobial defense. The mechanism is based on the modification and depletion of cholesterol by the pathogen's cholesterol-α-glucosyltransferase. It abrogates the assembly of numerous cytokine receptors due to the reduction of lipid rafts. Particularly, the receptors for IFN-γ, IL-22, and IL-6 then fail to assemble properly and to activate JAK/STAT signaling. Consequently, cholesterol depletion prevents the release of antimicrobial peptides, including the highly effective β-defensin-3. Intriguingly, the inhibition is spatially restricted to heavily infected cells, while the surrounding epithelium continues to respond normally to cytokine stimulation, thus providing a platform of the intense inflammation typically observed in H. pylori infections. It appears that pathogen and host establish a homeostatic balance between tightly colonized and rather inflamed sites. This homeostasis is influenced by the levels of available cholesterol, which potentially exacerbate H. pylori-induced inflammation. The observed blockage of epithelial effector mechanisms by H. pylori constitutes a convincing explanation for the previous failures of T-cell-based vaccination against H. pylori, since infected epithelial cells remain inert upon stimulation by effector cytokines. Moreover, the mechanism provides a rationale for the carcinogenic action of this pathogen in that persistent infection and chronic inflammation represent a pro-carcinogenic environment. Thus, cholesterol-α-glucosyltransferase has been revealed as a central pathogenesis determinant of H. pylori.

The Role of Host Cholesterol During Flavivirus Infection

In recent years the emergence and resurgence of arboviruses have generated a global health alert. Among arboviruses, Dengue (DENV), Zika (ZIKV), Yellow Fever (YFV), and West Nile (WNV) virus, belong to the genus Flavivirus, cause high viremia and occasionally fatal clinical disease in humans. Given the genetic austerity of the virus, they depend on cellular factors and organelles to complete its replication. One of the cellular components required for flavivirus infection is cholesterol. Cholesterol is an abundant lipid in biomembranes of eukaryotes cells and is necessary to maintain the cellular homeostasis. Recently, it has been reported, that cholesterol is fundamental during flavivirus infection in both mammal and insect vector models. During infection with DENV, ZIKV, YFV, and WNV the modulation of levels of host-cholesterol facilitates viral entry, replicative complexes formation, assembly, egress, and control of the interferon type I response. This modulation involves changes in cholesterol uptake with the concomitant regulation of cholesterol receptors as well as changes in cholesterol synthesis related to important modifications in cellular metabolism pathways. In view of the flavivirus dependence of cholesterol and the lack of an effective anti-flaviviral treatment, this cellular lipid has been proposed as a therapeutic target to treat infection using FDA-approved cholesterol-lowering drugs. This review aims to address the dependence of cholesterol by flaviviruses as well as the basis for anti flaviviral therapy using drugs which target is cholesterol synthesis or uptake.

Helicobacter pylori Depletes Cholesterol in Gastric Glands to Prevent Interferon Gamma Signaling and Escape the Inflammatory Response

BACKGROUND & AIMS

Despite inducing an inflammatory response, Helicobacter pylori can persist in the gastric mucosa for decades. H pylori expression of cholesterol-α-glucosyltransferase (encoded by cgt) is required for gastric colonization and T-cell activation. We investigated how cgt affects gastric epithelial cells and the host immune response.

METHODS

MKN45 gastric epithelial cells, AGS cells, and human primary gastric epithelial cells (obtained from patients undergoing gastrectomy or sleeve resection or gastric antral organoids) were incubated with interferon gamma (IFNG) or interferon beta (IFNB) and exposed to H pylori, including cagPAI and cgt mutant strains. Some cells were incubated with methyl-β-cyclodextrin (to deplete cholesterol from membranes) or myriocin and zaragozic acid to prevent biosynthesis of sphingolipids and cholesterol and analyzed by immunoblot, immunofluorescence, and reverse transcription quantitative polymerase chain reaction analyses. We compared gene expression patterns among primary human gastric cells, uninfected or infected with H pylori P12 wt or P12Δcgt, using microarray analysis. Mice with disruption of the IFNG receptor 1 (Ifngr1-/- mice) and C57BL6 (control) mice were infected with PMSS1 (wild-type) or PMSS1Δcgt H pylori; gastric tissues were collected and analyzed by reverse transcription quantitative polymerase chain reaction or confocal microscopy.

RESULTS

In primary gastric cells and cell lines, infection with H pylori, but not cgt mutants, blocked IFNG-induced signaling via JAK and STAT. Cells infected with H pylori were depleted of cholesterol, which reduced IFNG signaling by disrupting lipid rafts, leading to reduced phosphorylation (activation) of JAK and STAT1. H pylori infection of cells also blocked signaling by IFNB, interleukin 6 (IL6), and IL22 and reduced activation of genes regulated by these signaling pathways, including cytokines that regulate T-cell function (MIG and IP10) and anti-microbial peptides such as human β-defensin 3 (hBD3). We found that this mechanism allows H pylori to persist in proximity to infected cells while inducing inflammation only in the neighboring, non-infected epithelium. Stomach tissues from mice infected with PMSS1 had increased levels of IFNG, but did not express higher levels of interferon-response genes. Expression of the IFNG-response gene IRF1 was substantially higher in PMSS1Δcgt-infected mice than PMSS1-infected mice. Ifngr1-/- mice were colonized by PMSS1 to a greater extent than control mice.

CONCLUSIONS

H pylori expression of cgt reduces cholesterol levels in infected gastric epithelial cells and thereby blocks IFNG signaling, allowing the bacteria to escape the host inflammatory response. These findings provide insight into the mechanisms by which H pylori might promote gastric carcinogenesis (persisting despite constant inflammation) and ineffectiveness of T-cell-based vaccines against H pylori.

STAT3: a multifaceted oncoprotein

Signal transducer and activator of transcription (STAT) 3 is a key signalling protein engaged by a multitude of growth factors and cytokines to elicit diverse biological outcomes including cellular growth, differentiation, and survival. The complete loss of STAT3 is not compatible with life and even partial loss of function mutations lead to debilitating pathologies like hyper IgE syndrome. Conversely, augmented STAT3 activity has been reported in as many as 50% of all human tumours. The dogma of STAT3 activity posits that it is a tyrosine phosphorylated transcription factor which modulates the expression of hundreds of genes. However, the regulation and biological consequences of STAT3 activation are far more complex. In addition to tyrosine phosphorylation, STAT3 is decorated with a plethora of post-translational modifications which regulate STAT3's nuclear function in addition to its non-genomic activities. In addition to these emerging complexities in the biochemical regulation of STAT3 activity, recent studies reveal that STAT3 is either oncogenic or a tumour suppressor. This review will explore these complexities.

Reciprocal regulation of the Cadherin-11/Stat3 axis by caveolin-1 in mouse fibroblasts and lung carcinoma cells

Caveolin-1 (Cav1) is an integral plasma membrane protein and a complex regulator of signal transduction. The Signal Transducer and Activator of Transcription-3 (Stat3) is activated by a number of receptor and non-receptor tyrosine kinases and is positively implicated in cancer. Despite extensive efforts, the relationship between Cav1 and Stat3 has been a matter of controversy. We previously demonstrated that engagement of E- or N-cadherin or cadherin-11 cell to cell adhesion molecules, as occurs with confluence of cultured cells, triggers a dramatic increase in the levels of tyr705 phosphorylated i.e. activated Stat3, by a mechanism requiring the cRac1 small GTPase. Since confluence was not taken into account in previous studies, we revisited the question of the relationship between Cav1 and Stat3-ptyr705 in non-transformed mouse fibroblasts and in human lung carcinoma cells, by examining their effect at different cell densities. Our results unequivocally demonstrate that Cav1 downregulates cadherin-11, by a mechanism which requires the Cav1 scaffolding domain. This cadherin-11 downregulation, in turn, leads to a reduction in cRac1 and Stat3 activity levels. Furthermore, in a feedback loop possibly through p53 upregulation, Stat3 downregulation increases Cav1 levels. Our data reveal the presence of a potent, negative regulatory loop between Cav1 and cadherin-11/Stat3, leading to Stat3 inhibition and apoptosis.

Interaction of suppressor of cytokine signalling 3 with cavin-1 links SOCS3 function and cavin-1 stability

Effective suppression of JAK-STAT signalling by the inducible inhibitor "suppressor of cytokine signalling 3" (SOCS3) is essential for limiting signalling from cytokine receptors. Here we show that cavin-1, a component of caveolae, is a functionally significant SOCS3-interacting protein. Biochemical and confocal imaging demonstrate that SOCS3 localisation to the plasma membrane requires cavin-1. SOCS3 is also critical for cavin-1 stabilisation, such that deletion of SOCS3 reduces the expression of cavin-1 and caveolin-1 proteins, thereby reducing caveola abundance in endothelial cells. Moreover, the interaction of cavin-1 and SOCS3 is essential for SOCS3 function, as loss of cavin-1 enhances cytokine-stimulated STAT3 phosphorylation and abolishes SOCS3-dependent inhibition of IL-6 signalling by cyclic AMP. Together, these findings reveal a new functionally important mechanism linking SOCS3-mediated inhibition of cytokine signalling to localisation at the plasma membrane via interaction with and stabilisation of cavin-1.

Interleukin (IL)-6 modulates transforming growth factor-β receptor I and II (TGF-βRI and II) function in epidermal keratinocytes

It been shown that IL-6 modulates TGF-β1 expression in fibroblasts, however, what role IL-6 plays concerning TGF-βR expression and function in skin is unknown. Therefore, the aim of this study was to investigate the mechanism by which IL-6 might modulates TGF-β receptors in skin. Skin from WT, IL-6 over-expressing mice and IL-6 treated keratinocyte cultures was analysed for TGF-βRI and TGF-βRII expression via histology, PCR and flow cytometry. Receptor function was assessed by cell migration, bromodeoxyuridine (BrdU) proliferation assays, and Smad7 expression and Smad2/3 phosphorylation. Receptor localization within the membrane was determined by co-immunoprecipitation. IL-6 overexpression and treatment increased TGF-βRII expression in the epidermis. IL-6 treatment of keratinocytes induced TGF-βRI and II expression and augmented TGF-β1-induced function as demonstrated through increased migration and decreased proliferation. Additionally, IL-6 treatment of keratinocytes altered receptor activity as indicated by altered Smad2/3 phosphorylation and increased Smad7 and membrane localization. These results suggest that IL-6 regulates keratinocyte function by modulating TGF-βRI and II expression and signal transduction via trafficking of the receptor to lipid raft pools.

Multifunctional molecule ERp57: From cancer to neurodegenerative diseases

The protein disulfide isomerase (PDI) gene family is a protein family classically characterized by endoplasmic reticulum (ER) localization and isomerase and redox activity. ERp57, a prominent multifunctional member of the PDI family, is detected at various levels in multiple cellular localizations outside of the ER. ERp57 has been functionally linked to a host of physiological processes and numerous studies have demonstrated altered expression and aberrant functionality of ERp57 in association with diverse pathological states. Here, we summarize available knowledge of ERp57's functions in subcellular compartments and the roles of dysregulated ERp57 in various diseases toward an emphasis on the potential utility of therapeutic development of ERp57.

Differential modulation of the cellular and humoral immune responses in Drosophila is mediated by the endosomal ARF1-Asrij axis

How multicellular organisms maintain immune homeostasis across various organs and cell types is an outstanding question in immune biology and cell signaling. In Drosophila, blood cells (hemocytes) respond to local and systemic cues to mount an immune response. While endosomal regulation of Drosophila hematopoiesis is reported, the role of endosomal proteins in cellular and humoral immunity is not well-studied. Here we demonstrate a functional role for endosomal proteins in immune homeostasis. We show that the ubiquitous trafficking protein ADP Ribosylation Factor 1 (ARF1) and the hemocyte-specific endosomal regulator Asrij differentially regulate humoral immunity. Asrij and ARF1 play an important role in regulating the cellular immune response by controlling the crystal cell melanization and phenoloxidase activity. ARF1 and Asrij mutants show reduced survival and lifespan upon infection, indicating perturbed immune homeostasis. The ARF1-Asrij axis suppresses the Toll pathway anti-microbial peptides (AMPs) by regulating ubiquitination of the inhibitor Cactus. The Imd pathway is inversely regulated- while ARF1 suppresses AMPs, Asrij is essential for AMP production. Several immune mutants have reduced Asrij expression, suggesting that Asrij co-ordinates with these pathways to regulate the immune response. Our study highlights the role of endosomal proteins in modulating the immune response by maintaining the balance of AMP production. Similar mechanisms can now be tested in mammalian hematopoiesis and immunity.

Maintenance of membrane organization in the aging mouse brain as the determining factor for preventing receptor dysfunction and for improving response to anti-Alzheimer treatments

Although a major risk factor for Alzheimer's disease (AD), the "aging" parameter is not systematically considered in preclinical validation of anti-AD drugs. To explore how aging affects neuronal reactivity to anti-AD agents, the ciliary neurotrophic factor (CNTF)-associated pathway was chosen as a model. Comparison of the neuroprotective properties of CNTF in 6- and 18-month old mice revealed that CNTF resistance in the older animals is associated with the exclusion of the CNTF-receptor subunits from rafts and their subsequent dispersion to non-raft cortical membrane domains. This age-dependent membrane remodeling prevented both the formation of active CNTF-receptor complexes and the activation of prosurvival STAT3 and ERK1/2 pathways, demonstrating that age-altered membranes impaired the reactivity of potential therapeutic targets. CNTF-receptor distribution and CNTF signaling responses were improved in older mice receiving dietary docosahexaenoic acid, with CNTF-receptor functionality being similar to those of younger mice, pointing toward dietary intervention as a promising adjuvant strategy to maintain functional neuronal membranes, thus allowing the associated receptors to respond appropriately to anti-AD agents.

The Many Faces of the Flavivirus NS5 Protein in Antagonism of Type I Interferon Signaling

The vector-borne flaviviruses cause severe disease in humans on every inhabited continent on earth. Their transmission by arthropods, particularly mosquitoes, facilitates large emergence events such as witnessed with Zika virus (ZIKV) or West Nile virus in the Americas. Every vector-borne flavivirus examined thus far that causes disease in humans, from dengue virus to ZIKV, antagonizes the host type I interferon (IFN-I) response by preventing JAK-STAT signaling, suggesting that suppression of this pathway is an important determinant of infection. The most direct and potent viral inhibitor of this pathway is the nonstructural protein NS5. However, the mechanisms utilized by NS5 from different flaviviruses are often quite different, sometimes despite close evolutionary relationships between viruses. The varied mechanisms of NS5 as an IFN-I antagonist are also surprising given that the evolution of NS5 is restrained by the requirement to maintain function of two enzymatic activities critical for virus replication, the methyltransferase and RNA-dependent RNA polymerase. This review discusses the different strategies used by flavivirus NS5 to evade the antiviral effects of IFN-I and how this information can be used to better model disease and develop antiviral countermeasures.

Caveolin-1 Mediates Low-Intensity Ultrasound-Induced Apoptosis via Downregulation of Signal Transducer and Activator of Transcription 3 Phosphorylation in Laryngeal Carcinoma Cells

Low-intensity ultrasound therapy has been found to be a potential tool in the management of malignant tumors in recent years. However, the molecular mechanism underlying low-intensity ultrasound-induced apoptosis is still not clear. In this study, we investigated the effects of low-intensity ultrasound-induced apoptosis in HEp-2 cells. We found that low-intensity ultrasound significantly induced apoptosis, and the expression level of caveolin-1 (Cav-1) was dramatically increased after ultrasound treatment of HEp-2 cells. After inhibiting the expression level of Cav-1 using siRNA transfection, we found that the cellular apoptosis induced by low-intensity ultrasound was significantly suppressed. In addition, inhibition of Cav-1 expression promoted phosphorylation of signal transducer and activator of transcription 3 (STAT3), suggesting that the STAT3 signaling pathway was involved in low-intensity ultrasound-induced apoptosis via Cav-1 regulation. Our results indicate that Cav-1/STAT3 signaling pathway may mediate low-intensity ultrasound-induced apoptosis, and this technology could potentially be used clinically for the treatment of cancers.

Chromosome 8p tumor suppressor genes SH2D4A and SORBS3 cooperate to inhibit interleukin-6 signaling in hepatocellular carcinoma

Several chronic inflammatory liver diseases, e.g., chronic hepatitis B or C viral infection and steatohepatitis, have been shown to predispose to the development of hepatocellular carcinoma (HCC). In patients with chronic liver disease, interleukin-6 (IL-6) serum levels are elevated and increase even more when HCC develops. However, the impact and regulatory mechanisms of IL-6 signaling during hepatocarcinogenesis are still poorly defined. Here, we show that gene expression profiles of patients with chromosome 8p loss correlate with increased IL-6 signaling. In addition, the chromosome 8p tumor suppressor genes Src homology 2 domain containing 4A (SH2D4A) and Sorbin and Src homology 3 domain containing 3 (SORBS3) together exerted greater inhibition of cell growth and clonogenicity compared to a single gene. Overexpression of SH2D4A and SORBS3 in HCC cells led to decreased IL-6 target gene expression and reduced signal transducer and activator of transcription 3 (STAT3) signaling. In situ and in vitro coimmunoprecipitation assays revealed that SH2D4A directly interacts with STAT3, thereby retaining STAT3 in the cytoplasm and inhibiting STAT3 transcriptional activity. On the other hand, SORBS3 coactivated estrogen receptor α signaling, leading indirectly to repression of STAT3 signaling. In human HCC tissues, SH2D4A was positively associated with infiltrating regulatory and cytotoxic T-cell populations, suggesting distinct immunophenotypes in HCC subgroups with chromosome 8p loss. Thus, the genetically linked tumor suppressors SH2D4A and SORBS3 functionally cooperate to inhibit STAT3 signaling in HCC.

CONCLUSION

The chromosome 8p tumor suppressor genes SORBS3 and SH2D4A are physically and functionally linked and provide a molecular mechanism of inhibiting STAT3-mediated IL-6 signaling in HCC cells. (Hepatology 2016;64:828-842).

Intermolecular disulfide bond influences unphosphorylated STAT3 dimerization and function

Signal transducer and activator of transcription 3 (STAT3) is a transcription factor activated by the phosphorylation of tyrosine 705 in response to many cytokines and growth factors. Recently, the roles for unphosphorylated STAT3 (U-STAT3) have been described in response to cytokine stimulation, in cancers, and in the maintenance of heterochromatin stability. It has been reported that U-STAT3 dimerizes, shuttles between the cytoplasm and nucleus, and binds to DNA, thereby driving genes transcription. Although many reports describe the active role of U-STAT3 in oncogenesis in addition to phosphorylated STAT3, the U-STAT3 functional pathway remains elusive.In this report, we describe the molecular mechanism of U-STAT3 dimerization, and we identify the presence of two intermolecular disulfide bridges between Cys367 and Cys542 and Cys418 and Cys426, respectively. Recently, we reported that the same cysteines contribute to the redox regulation of STAT3 signaling pathway both in vitro and in vivo The presence of these disulfides is here demonstrated to largely contribute to the structure and the stability of U-STAT3 dimer as the dimeric form rapidly dissociates upon reduction in the S-S bonds. In particular, the Cys367-Cys542 disulfide bridge is shown to be critical for U-STAT3 DNA-binding activity. Mutation of the two Cys residues completely abolishes the DNA-binding capability of U-STAT3. Spectroscopic investigations confirm that the noncovalent interactions are sufficient for proper folding and dimer formation, but that the interchain disulfide bonds are crucial to preserve the functional dimer. Finally, we propose a reaction scheme of U-STAT3 dimerization with a first common step followed by stabilization through the formation of interchain disulfide bonds.

Inhibition of VEGF-dependent angiogenesis by the anti-CD82 monoclonal antibody 4F9 through regulation of lipid raft microdomains

CD82 (also known as KAI1) belongs to the tetraspanin superfamily of type III transmembrane proteins, and is involved in regulating cell adhesion, migration and proliferation. In contrast to these well-established roles of CD82 in tumor biology, its function in endothelial cell (EC) activity and tumor angiogenesis is yet to be determined. In this study, we show that suppression of CD82 negatively regulates vascular endothelial growth factor (VEGF)-induced angiogenesis. Moreover, we demonstrate that the anti-CD82 mAb 4F9 effectively inhibits phosphorylation of VEGF receptor 2 (VEGFR2), which is the principal mediator of the VEGF-induced angiogenic signaling process in tumor angiogenesis, by regulating the organization of the lipid raft microdomain signaling platform in human EC. Our present work therefore suggests that CD82 on EC is a potential target for anti-angiogenic therapy in VEGFR2-dependent tumor angiogenesis.

Electrostatics and N-glycan-mediated membrane tethering of SCUBE1 is critical for promoting bone morphogenetic protein signalling

We investigated the membrane-associating mechanisms of SCUBE1 (S1), a BMP co-receptor. Electrostatics and glycan-mediated membrane localization of S1 is essential for promoting BMP signalling and N-glycosylation is required for its function in zebrafish.

The amphiphilic nature of saponins and their effects on artificial and biological membranes and potential consequences for red blood and cancer cells

Saponins, amphiphiles of natural origin with numerous biological activities, are widely used in the cosmetic and pharmaceutical industry. Some saponins exhibit relatively selective cytotoxic effects on cancer cells but the tendency of saponins to induce hemolysis limits their anticancer potential. This review focused on the effects of saponin activity on membranes and consequent implications for red blood and cancer cells. This activity seems to be strongly related to the amphiphilic character of saponins that gives them the ability to self-aggregate and interact with membrane components such as cholesterol and phospholipids. Membrane interactions of saponins with artificial membrane models, red blood and cancer cells are reviewed with respect to their molecular structures. The review considered the mechanisms of these membrane interactions and their consequences including the modulation of membrane dynamics, interaction with membrane rafts, and membrane lysis. We summarized current knowledge concerning the mechanisms involved in the interactions of saponins with membrane lipids and examined the structure activity relationship of saponins regarding hemolysis and cancer cell death. A critical analysis of these findings speculates on their potential to further develop new anticancer compounds.

Cavin-1: caveolae-dependent signalling and cardiovascular disease

Caveolae are curved lipid raft regions rich in cholesterol and sphingolipids found abundantly in vascular endothelial cells, adipocytes, smooth muscle cells and fibroblasts. They are multifunctional organelles with roles in clathrin-independent endocytosis, cholesterol transport, mechanosensing and signal transduction. Caveolae provide an environment where multiple receptor signalling components are sequestered, clustered and compartmentalized for efficient signal transduction. Many of these receptors, including cytokine signal transducer gp130 (glycoprotein 130), are mediators of chronic inflammation during atherogenesis. Subsequently, disruption of these organelles is associated with a broad range of disease states including cardiovascular disease and cancer. Cavin-1 is an essential peripheral component of caveolae that stabilizes caveolin-1, the main structural/integral membrane protein of caveolae. Caveolin-1 is an essential regulator of eNOS (endothelial nitric oxide synthase) and its disruption leads to endothelial dysfunction which initiates a range of cardiovascular and pulmonary disorders. Although dysfunctional cytokine signalling is also a hallmark of cardiovascular disease, knowledge of caveolae-dependent cytokine signalling is lacking as is the role of cavin-1 independent of caveolae. The present review introduces caveolae, their structural components, the caveolins and cavins, their regulation by cAMP, and their potential role in cardiovascular disease.

Different associations of CD45 isoforms with STAT3, PKC and ERK regulate IL-6-induced proliferation in myeloma

In response to interleukin 6 (IL-6) stimulation, both CD45RO and CD45RB, but not CD45RA, translocate to lipid rafts. However, the significance of this distinct translocation and the downstream signals in CD45 isoforms-participated IL-6 signal are not well understood. Using sucrose fractionation, we found that phosphorylated signal transducer and activator of transcription (STAT)3 and STAT1 were mainly localized in lipid rafts in response to IL-6 stimulation, despite both STAT3 and STAT1 localizing in raft and non-raft fractions in the presence or absence of IL-6. On the other hand, extracellular signal-regulated kinase (ERK), and phosphorylated ERK were localized in non-raft fractions regardless of the existence of IL-6. The rafts inhibitor significantly impeded the phosphorylation of STAT3 and STAT1 and nuclear translocation, but had little effect on (and only postponing) the phosphorylation of ERK. This data suggests that lipid raft-dependent STAT3 and STAT1 pathways are dominant pathways of IL-6 signal in myeloma cells. Interestingly, the phosphorylation level of STAT3 but not STAT1 in CD45⁺ cells was significantly higher compared to that of CD45^- cells, while the phosphorylation level of ERK in CD45⁺ myeloma cells was relatively low. Furthermore, exogenously expressed CD45RO/RB significantly enhanced STAT3, protein kinase C (PKC) and downstream NF-κB activation; however, CD45RA/RB inhibited IL-6-induced ERK phosphorylation. CD45 also enhanced the nuclear localization of STAT3 but not that of STAT1. In response to IL-6 stimulation, CD45RO moved into raft compartments and formed a complex with STAT3 and PKC in raft fraction, while CD45RA remained outside of lipid rafts and formed a complex with ERK in non-raft fraction. This data suggests a different role of CD45 isoforms in IL-6-induced signaling, indicating that while CD45RA/RB seems inhibit the rafts-unrelated ERK pathway, CD45RO/RB may actually work to enhance the rafts-related STAT3 and PKC/NF-κB pathways.

Abnormal expression of vesicular transport proteins in pulmonary arterial hypertension in monocrotaline-treated rats

Intracellular vesicular transport is shown to be dysfunctional in pulmonary arterial hypertension (PAH). However, the expression of intracellular vesicular transport proteins in PAH remains unclear. To elucidate the possible role of these proteins in the development of PAH, the changes in the expressions of N-ethyl-maleimide-sensitive factor (NSF), α-soluble NSF attachment protein (α-SNAP), synaptosome-associated membrane protein 23 (SNAP23), type 2 bone morphogenetic receptor (BMPR2), caveolin-1 (cav-1), and endothelial nitric oxide synthase (eNOS) were examined in lung tissues of monocrotaline (MCT)-treated rats by real-time polymerase chain reaction and western blot analysis. In addition, caspase-3, also examined by western blot analysis, was used as an indicator of apoptosis. Our data showed that during the development of PAH, the expressions of NSF, α-SNAP, and SNAP23 were significantly increased before pulmonary arterial pressure started to increase and then significantly decreased after PAH was established. The expressions of BMPR2 and eNOS were similar to those of NSF, α-SNAP, and SNAP23; however, the expression of cav-1 was down-regulated after MCT treatment. Caspase-3 expression was increased after exposure to MCT. In conclusion, the expressions of NSF, α-SNAP, and SNPA23 changed greatly during the onset of PAH, which was accompanied by abnormal expressions of BMPR2, cav-1, and eNOS, as well as an increase in apoptosis. Thus, changes in NSF, α-SNAP, and SNAP23 expressions appear to be mechanistically associated with the development of PAH in MCT-treated rats.

Contribution of chaperones to STAT pathway signaling

Aberrant STAT signaling is associated with the development and progression of many cancers and immune related diseases. Recent findings demonstrate that proteostasis modulators under clinical investigation for cancer therapy have a significant impact on STAT signaling, which may be critical for mediating their anti-cancer effects. Chaperones are critical for protein folding, stability and function and, thus, play an essential role in the maintenance of proteostasis. In this review we discuss the role of chaperones in STAT and tyrosine kinase (TK) protein folding, modulation of STAT and TK activity, and degradation of TKs. We highlight the important role of chaperones in STAT signaling, and how this knowledge has provided a framework for the development of new therapeutic avenues of targeting STAT signaling related pathologies.

Proteomic analysis of protein palmitoylation in adipocytes

Protein palmitoylation, by modulating the dynamic interaction between protein and cellular membrane, is involved in a wide range of biological processes, including protein trafficking, sorting, sub-membrane partitioning, protein-protein interaction and cell signaling. To explore the role of protein palmitoylation in adipocytes, we have performed proteomic analysis of palmitoylated proteins in adipose tissue and 3T3-L1 adipocytes and identified more than 800 putative palmitoylated proteins. These include various transporters, enzymes required for lipid and glucose metabolism, regulators of protein trafficking and signaling molecules. Of note, key proteins involved in membrane translocation of the glucose-transporter Glut4 including IRAP, Munc18c, AS160 and Glut4, and signaling proteins in the JAK-STAT pathway including JAK1 and 2, STAT1, 3 and 5A and SHP2 in JAK-STAT, were palmitoylated in cultured adipocytes and primary adipose tissue. Further characterization showed that palmitoylation of Glut4 and IRAP was altered in obesity, and palmitoylation of JAK1 played a regulatory role in JAK1 intracellular localization. Overall, our studies provide evidence to suggest a novel and potentially regulatory role for protein palmitoylation in adipocyte function.

Possible protective effect of membrane lipid rafts against interleukin-1β-mediated anti-proliferative effect in INS-1 cells

We recently reported that pancreatic islets from pre-diabetic rats undergo an inflammatory process in which IL-1β takes part and controls β-cell function. In the present study, using the INS-1 rat pancreatic β-cell line, we investigated the potential involvement of membrane-associated cholesterol-enriched lipid rafts in IL-1β signaling and biological effects on insulin secretion, β-cell proliferation and apoptosis. We show that, INS-1 cells exposure to increasing concentrations of IL-1β leads to a progressive inhibition of insulin release, an increase in the number of apoptotic cells and a dose-dependent decrease in pancreatic β-cell proliferation. Disruption of membrane lipid rafts markedly reduced glucose-stimulated insulin secretion but did not affect either cell apoptosis or proliferation rate, demonstrating that membrane lipid raft integrity is essential for β-cell secretory function. In the same conditions, IL-1β treatment of INS-1 cells led to a slight further decrease in insulin secretion for low concentrations of the cytokine, and a more marked one, similar to that observed in normal cells for higher concentrations. These effects occurred together with an increase in iNOS expression and surprisingly with an upregulation of tryptophane hydroxylase and protein Kinase C in membrane lipid rafts suggesting that compensatory mechanisms develop to counteract IL-1β inhibitory effects. We also demonstrate that disruption of membrane lipid rafts did not prevent cytokine-induced cell death recorded after exposure to high IL-1β concentrations. Finally, concerning cell proliferation, we bring strong evidence that membrane lipid rafts exert a protective effect against IL-1β anti-proliferative effect, possibly mediated at least partly by modifications in ERK and PKB expression/activities. Our results 1) demonstrate that IL-1β deleterious effects do not require a cholesterol-dependent plasma membrane compartmentalization of IL-1R1 signaling and 2) confer to membrane lipid rafts integrity a possible protective function that deserves to be considered in the context of inflammation and especially T2D pathogenesis.

The protein ERp57 contributes to EGF receptor signaling and internalization in MDA-MB-468 breast cancer cells

The disulfide isomerase ERp57 is a soluble protein mainly located in the endoplasmic reticulum, where it acts in the quality control of newly synthesized glycoproteins, in association with calreticulin and calnexin. It has been also detected in other cell compartments, such as the cytosol, the plasma membrane and the nucleus. In these locations it is implicated in various processes, participating in the rapid response to calcitriol, modulating the activity of STAT3 and being requested for the pre-apoptotic exposure of calreticulin on the plasma membrane. In the present work, the involvement of ERp57 in the activity of the EGF receptor was evaluated for the first time. EGFR is a tyrosine kinase receptor, which is able to activate numerous signaling cascades, leading to cell proliferation and inhibition of apoptosis. In the MDA-MB-468 breast adenocarcinoma cells, which overexpress EGFR, ERp57 expression has been knocked down by siRNA and the effects on EGFR have been studied. ERp57 silencing did not affect EGFR protein expression, cell membrane exposure or EGF binding, whereas the internalization and the phosphorylation of the receptor were impaired. The implication of ERp57 in the activity of EGFR, whose upregulation is known to be associated with tumors, could be relevant for cancer therapy.

Analytical use of multi-protein Fluorescence Resonance Energy Transfer to demonstrate membrane-facilitated interactions within cytokine receptor complexes

Experiments measuring Fluorescence Resonance Energy Transfer (FRET) between cytokine receptor chains and their associated proteins led to hypotheses describing their organization in intact cells. These interactions occur within a larger protein complex or within a given nano-environment. To illustrate this complexity empirically, we developed a protocol to analyze FRET among more than two fluorescent proteins (multi-FRET). In multi-FRET, we model FRET among more than two fluorophores as the sum of all possible pairwise interactions within the complex. We validated our assumption by demonstrating that FRET among pairs within a fluorescent triplet resembled FRET between each pair measured in the absence of the third fluorophore. FRET between two receptor chains increases with increasing FRET between the ligand-binding chain (e.g., IFN-γR1, IL-10R1 and IFN-λR1) and an acylated fluorescent protein that preferentially resides within subsections of the plasma membrane. The interaction of IL-10R2 with IFN-λR1 or IL-10R1 results in decreased FRET between IL-10R2 and the acylated fluorescent protein. Finally, we analyzed FRET among four fluorescent proteins to demonstrate that as FRET between IFN-γR1 and IFN-γR2 or between IFN-αR1 and IFN-αR2c increases, FRET among other pairs of proteins changes within each complex.

Interferon gamma receptor: the beginning of the journey

Our view of endocytosis and membrane trafficking of transmembrane receptors has dramatically changed over the last 20 years. Several new endocytic routes have been discovered and mechanistically characterized in mammalian cells. Long considered as a passive means to terminate signaling through down-regulation of the number of activated receptors at the plasma membrane, it is now established that receptor endocytosis and endosomal sorting can be directly linked to the regulation of intracellular signaling pathways. The functional links between membrane trafficking of interferon receptors and JAK/STAT signaling have recently begun to be unraveled. These studies raise the exciting possibility that a certain level of signal specificity can be achieved through endocytosis and selective localization of the activated complexes within cellular membranes. The ongoing development of high-resolution cell imaging techniques with better spatial and temporal resolution gives new means of deciphering the inherent complexity of membrane trafficking and signaling. This should help to better comprehend the molecular mechanisms by which endocytosis and endosomal sorting of interferon receptors can orchestrate signaling selectivity within the JAK/STAT pathway that can be activated by as many as 60 different cytokines, growth factors, and hormones.

Conserved regulation of the Jak/STAT pathway by the endosomal protein asrij maintains stem cell potency

Asrij/OCIAD1 is an endosomal protein expressed in stem cells and cardiovascular lineages and aberrantly expressed in several cancers. We show that dose-dependent modulation of cytokine-dependent JAK/STAT signaling by Asrij regulates mouse embryonic stem cell pluripotency as well as Drosophila hematopoietic stem cell maintenance. Furthermore, mouse asrij can substitute for Drosophila asrij, indicating that they are true homologs. We identify a conserved region of Asrij that is necessary and sufficient for vesicular localization and function. We also show that Asrij and STAT3 colocalize in endosomes and interact biochemically. We propose that Asrij provides an endosomal scaffold for STAT3 interaction and activation, and may similarly control other circuits that maintain stemness. Thus, Asrij provides a key point of control for spatial and kinetic regulation of stem cell signals.

HDL induces the expression of the M2 macrophage markers arginase 1 and Fizz-1 in a STAT6-dependent process

Our lab has previously shown in a mouse model that normalization of a low HDL level achieves atherosclerotic plaque regression. This included the shift from a pro ("M1") to an anti-inflammatory ("M2") phenotypic state of plaque macrophages. Whether HDL can directly cause this phenotypic change and, if so, what the signaling mechanism is, were explored in the present studies. Murine primary macrophages treated with HDL showed increased gene expression for the M2 markers Arginase-1 (Arg-1) and Fizz-1, which are classically induced by IL-4. HDL was able to potentiate the IL-4-induced changes in Arg-1, and tended to do the same for Fizz-1, while suppressing the expression of inflammatory genes in response to IFNγ. The effects of either IL-4 or HDL were suppressed when macrophages were from STAT6(-/-) mice, but inhibitor studies suggested differential utilization of JAK isoforms by IL-4 and HDL to activate STAT6 by phosphorylation. Overall, our results describe a new function of HDL, namely its ability to directly enrich macrophages in markers of the M2, anti-inflammatory, state in a process requiring STAT6.

IFN signaling: how a non-canonical model led to the development of IFN mimetics

The classical model of cytokine signaling dominates our view of specific gene activation by cytokines such as the interferons (IFNs). The importance of the model extends beyond cytokines and applies to hormones such as growth hormone (GH) and insulin, and growth factors such as epidermal growth factor (EGF) and fibroblast growth factor (FGF). According to this model, ligand activates the cell via interaction with the extracellular domain of the receptor. This results in activation of receptor or receptor-associated tyrosine kinases, primarily of the Janus activated kinase (JAK) family, phosphorylation and dimerization of the signal transducer and activator of transcription (STAT) transcription factors, which dissociate from the receptor cytoplasmic domain and translocate to the nucleus. This view ascribes no further role to the ligand, JAK kinase, or receptor in either specific gene activation or the associated epigenetic events. The presence of dimeric STATs in the nucleus essentially explains it all. Our studies have resulted in the development of a non-canonical, more complex model of IFNγ signaling that is akin to that of steroid hormone (SH)/steroid receptor (SR) signaling. We have shown that ligand, receptor, activated JAKs, and STATs are associated with specific gene activation, where the receptor subunit IFNGR1 functions as a co-transcription factor and the JAKs are involved in associated epigenetic events. We found that the type I IFN system functions similarly. The fact that GH receptor, insulin receptor, EGF receptor, and FGF receptor undergo nuclear translocation upon ligand binding suggests that they may also function similarly. The SH/SR nature of type I and II IFN signaling provides insight into the specificity of signaling by members of cytokine families. The non-canonical model could also provide better understanding to more complex cytokine families such as those of IL-2 and IL-12, whose members often use the same JAKs and STATs, but also have different functions and properties.

Non-genomic STAT5-dependent effects at the endoplasmic reticulum and Golgi apparatus and STAT6-GFP in mitochondria

STAT protein species are well-known as transcription factors that regulate nuclear gene expression. Recent novel lines of research suggest new non-genomic functions of STAT5A/B and STAT6. It was discovered in human pulmonary arterial endothelial cells that STAT5A, including STAT5A-GFP, constitutively associated with the Golgi apparatus, and both STAT5A and B with the endoplasmic reticulum. Acute siRNA-mediated knockdown of STAT5A/B led to the rapid development of a dramatic cystic change in the endoplasmic reticulum (ER) characterized by deposition of the ER structural protein reticulon-4 (RTN4; also called Nogo-B) and the ER-resident GTPase atlastin-3 (ATL3) along cyst membranes and cyst-zone boundaries, accompanied by Golgi fragmentation. Functional consequences included reduced anterograde trafficking, an ER stress response (increased GRP78/BiP) and eventual mitochondrial fragmentation. This phenotype was "non-genomic" in that it was elicited in enucleated cytoplasts. In cross-immunopanning assays STAT5A and B species associated with ATL3, and the ER-lumen spacer CLIMP63 (also called cytoskeleton-associated protein 4, CKAP4) but not RTN4. From a disease significance perspective we posit that STAT5, which is known to be affected by estradiol-17β and prolactin, represents the gender-sensitive determinant in the pathogenesis of idiopathic pulmonary hypertension (IPAH), a disease which includes ER/Golgi dysfunctions but with a 2- to 4-fold higher prevalence in postpubertal women. A separate line of recent research produced evidence for the association of STAT6-GFP, but not STAT3-GFP, STAT3-DsRed, or STAT3-Flag, with mitochondria in live-cell, immunofluorescence, and immunoelectron microscopy. An N-terminal truncation of STAT6-GFP (1-459), which lacked the SH2 domain and Tyr-phosphorylation site, constitutively associated with mitochondria. Thus, the emergent new of biology STAT proteins includes non-genomic roles-structurally and functionally-in the three closely related membrane organelles consisting of the endoplasmic reticulum, Golgi apparatus, and mitochondria.

Expression of caveolin-1 in the early phase of beta-TCP implanted in dog mandible

Caveolin is an essential and signature protein of caveolae. Caveolin-1 participates in signal transduction processes by acting as a scaffolding protein that concentrates, organizes and functional regulates signalling molecules within caveolar membranes. Beta-tricalcium phosphate (β-TCP) has been widely used for scaffold in tissue engineering due to its high biodegradability, osteoconductivity, easy manipulation, and lack of histotoxicity. To better understand the role of caveolin-1 in bone homeostasis and response to β-TCP scaffold, β-TCP was implanted into the dog mandible defects in beagle dogs, and gene expression profiles were examined focused on the molecular components involved in caveolin-1 regulation. Here we showed the quantitative imageology analysis characterized using in vivo micro-computed tomography (CT) images at 4 and 7 days after β-TCP implanted in dog mandibles. The bone reformation by using the β-TCP scaffolds began within 4 days of surgery, and was healing well at 7 days after surgery. Higher mRNA level of caveolin-1 was observed in β-TCP-implanted Beagle dog mandibles compared with controls at day 4 and day 7 post-surgery. The enhancement of caveolin-1 by β-TCP was further confirmed by immunohistochemistry and immunofluorescence analysis. We further revealed increased Smad7 and Phospho Stat3 expression in β-TCP-implanted specimens. Taken together, these results suggest that the enhancement of caveolin-1 play an important role in accelerating bone formation by β-TCP.

Live-cell imaging of the association of STAT6-GFP with mitochondria

The transcription factor STAT3 has been previously reported to be associated with mitochondria. However, we have been unable to visualize an association of STAT3-GFP, STAT3-DsRed or STAT3-Flag with mitochondria in human Hep3B hepatocytes thus far even though an association of these molecules with other cytoplasmic organelles (endosomes) was readily demonstrable. We then addressed the broader question of a possible association of other STAT-family of proteins with mitochondria by first using immunolocalization assays in Hep3B and human pulmonary arterial endothelial and smooth muscle cells. Strong anti-STAT6-immunolocalization with mitochondria was apparent in fluorescence and electron microscopy assays of cells first washed with a digitonin-sucrose buffer to remove bulk soluble STAT proteins. In live-cell imaging studies, STAT6-GFP, but not N1-GFP, was observed to constitutively colocalize with MitoTracker- and tetramethylrhodamine ethyl ester (TMRE)-positive mitochondria, and with mitochondrial F1-ATPase when assayed by immunofluorescence after fixation. This association was Tyr-phosphorylation independent in that a STAT6 truncated protein (STAT6^1-459-GFP) which lacked the SH2 domain (517–632) and the cytokine-activated Y641 phosphorylation site also accumulated in MitoTracker-positive mitochondria. This was consistent with the unexpected discovery that anti-STAT6-immunofluoresence also associated with mitochondria in mouse embryo fibroblasts (MEFs) from both wild-type and the STAT6^SH2-/SH2- mouse. MEFs from the latter mouse, which had been engineered in 1996 to be deleted in the STAT6 SH2 domain (amino acids 505–584) expressed an immune-specific ∼50 kDa protein detectable in whole cell and mitochondria-enriched fractions. Taken together, the present data provide the first definitive evidence of the association of any STAT-protein family member with mitochondria - that of STAT6.

Electrophilic nitro-fatty acids inhibit vascular inflammation by disrupting LPS-dependent TLR4 signalling in lipid rafts

AIMS

Electrophilic fatty acid nitroalkene derivatives, products of unsaturated fatty acid nitration, exert long-term cardiovascular protection in experimental models of metabolic and cardiovascular diseases. The goal of this study is to examine the effects of nitro-fatty acids in the regulation of upstream signalling events in nuclear factor-κB (NF-κB) activation and determine whether low-dose acute administration of nitro-fatty acids reduces vascular inflammation in vivo.

METHODS AND RESULTS

Using NF-κB-luciferase transgenic mice, it was determined that pre-emptive treatment with nitro-oleic acid (OA-NO2), but not oleic acid (OA) inhibits lipopolysaccharide (LPS)-induced NF-κB activation both in vivo and in isolated macrophages. Acute intravenous administration of OA-NO2 was equally effective to inhibit leukocyte recruitment to the vascular endothelium assessed by intravital microscopy and significantly reduces aortic expression of adhesion molecules. An acute treatment with OA-NO2 in vivo yielding nanomolar concentrations in plasma, is sufficient to inhibit LPS-induced Toll-like receptor 4 (TLR4)-induced cell surface expression in leukocytes and NF-κB activation. In vitro experiments reveal that OA-NO2 suppresses LPS-induced TLR4 signalling, inhibitor of κB (IκBα) phosphorylation and ubiquitination, phosphorylation of the IκB kinase (IKK), impairing the recruitment of the TLR4 and TNF receptor associated factor 6 (TRAF6) to the lipid rafts compartments.

CONCLUSION

These studies demonstrate that acute administration of nitro-fatty acids is effective to reduce vascular inflammation in vivo. These findings reveal a direct role of nitro-fatty acids in the disruption of the TLR4 signalling complex in lipid rafts, upstream events of the NF-κB pathway, leading to resolution of pro-inflammatory activation of NF-κB in the vasculature.

A novel internalization motif regulates human IFN-γ R1 endocytosis

This study tested the hypothesis that the IFN-γ R1 287-YVSLI-91 intracellular motif regulates its endocytosis. IFN-γ exerts its biological activities by interacting with a specific cell-surface RC composed of two IFN-γ R1 and two IFN-γ R2 chains. Following IFN-γ binding and along with the initiation of signal transduction, the ligand and IFN-γ R1 are internalized. Two major types of consensus-sorting signals are described in receptors, which are rapidly internalized from the plasma membrane to intracellular compartments: tyrosine-based and dileucine-based internalization motifs. Transfection of HEK 293 cells and IFN-γ R1-deficient fibroblasts with WT and site-directed, mutagenesis-generated mutant IFN-γ R1 expression vectors helped us to identify region IFN-γ R1 287-YVSLI-291 as the critical domain required for IFN-γ-induced IFN-γ R1 internalization and Y287 and LI290-291 as part of a common structure essential for receptor endocytosis and function. This new endocytosis motif, YxxLI, shares characteristics of tyrosine-based and dileucine-based internalization motifs and is highly conserved in IFN-γ Rs across species. The IFN-γ R1 270-LI-271 dileucine motif, previously thought to be involved in this receptor endocytosis, showed to be unnecessary for receptor endocytosis.