MAPK-induced Ser727 phosphorylation promotes SUMOylation of STAT1

SUMOylation of ERp44 enhances Ero1α ER retention contributing to the pathogenesis of obesity and insulin resistance

OBJECTIVE

As the only E2 conjugating enzyme for the SUMO system, Ubc9-mediated SUMOylation has been recognized to regulate diverse biological processes, but its impact on adipocytes relevant to obesity and insulin resistance is yet to be elucidated.

METHODS

We established adipocyte-specific Ubc9 deficient mice to explore the effects of Ubc9 on obesity and metabolic disorders induced by high-fat diet (HFD) in adult mice. The molecular targets of SUMOylation were explored by liquid chromatography-mass spectrometry, and the regulatory mechanism of SUMOylation in T2D was analyzed.

RESULTS

Adipocyte-specific depletion of Ubc9 (AdipoQ-Cre-Ubc9^fl/fl, Ubc9^AKO) protected mice from HFD-induced obesity, insulin resistance, and hepatosteatosis. The Ubc9^AKO mice were featured by the reduced HFD-induced endoplasmic reticulum (ER) stress and inflammatory response. Mechanically, over nutrition rendered adipocytes to undergo a SUMOylation turnover characterized by the change of SUMOylation levels and substrates. ERp44 displayed the highest change in terms of SUMOylation levels of substrates involved in ER-related functions. The lack of ERp44 SUMOylation at lysine 76 (K76) located within the thioredoxin (TRX)-like domain by Ubc9 deficiency enhanced its degradation and suppressed its covalent binding to Ero1α, an oxidase that exists in the ER but lacks ER retention motif, thereby alleviating endoplasmic reticulum stress by promoting Ero1α secretion.

CONCLUSIONS

Our data suggest that modulation of ERp44 SUMOylation in adipocytes could be a feasible strategy against obesity and insulin resistance in clinical settings.

RNF220 mediates K63-linked polyubiquitination of STAT1 and promotes host defense

STAT1 is a master regulator that orchestrates type 1 and 2 interferon (IFN)-induced IFN-stimulated gene (ISG) expression. The mechanisms by which STAT1 is phosphorylated and activated upon IFN signaling remain elusive. Our work demonstrated that ubiquitination of STAT1 mediated by the E3 ligase RNF220 contributed significantly to STAT1 activation and innate immune responses. Rnf220 gene deficiency resulted in the downregulation of IFN signaling and decreased expression of ISGs in response to type 1 and 2 IFNs stimulation and Acinetobacter baumannii and HSV-1 infection. Mechanistically, RNF220 interacted with STAT1 and mediated the K63-linked polyubiquitination of STAT1 at residue K110, which promoted the interaction between STAT1 and the kinase JAK1. The expression of RNF220 was induced by pathogenic infection and IFN signaling. RNF220 promoted STAT1 ubiquitination and phosphorylation through a positive feedback loop. RNF220 haploinsufficiency impaired IFN signaling, and RNF220-defective mice were more susceptible to A. baumannii and HSV-1 infection than WT mice. Our work offers novel insights into the mechanisms of STAT1 modulation and provides potential therapeutic targets against bacterial and viral infection and inflammatory diseases.

Constitutive activation of the EGFR-STAT1 axis increases proliferation of meningioma tumor cells

Background

Meningiomas are the most frequent primary brain tumors of the central nervous system. The standard of treatment is surgery and radiotherapy, but effective pharmacological options are not available yet. The well-characterized genetic background stratifies these tumors in several subgroups, thus increasing diversification. We identified epidermal growth factor receptor–signal transducer and activator of transcription 1 (EGFR–STAT1) overexpression and activation as a common identifier of these tumors.

Methods

We analyzed STAT1 overexpression and phosphorylation in 131 meningiomas of different grades and locations by utilizing several techniques, including Western blots, qPCR, and immunocytochemistry. We also silenced and overexpressed wild-type and mutant forms of the gene to assess its biological function and its network. Results were further validated by drug testing.

Results

STAT1 was found widely overexpressed in meningioma but not in the corresponding healthy controls. The protein showed constitutive phosphorylation not dependent on the JAK–STAT pathway. STAT1 knockdown resulted in a significant reduction of cellular proliferation and deactivation of AKT and ERK1/2. STAT1 is known to be activated by EGFR, so we investigated the tyrosine kinase and found that EGFR was also constitutively phosphorylated in meningioma and was responsible for the aberrant phosphorylation of STAT1. The pharmaceutical inhibition of EGFR caused a significant reduction in cellular proliferation and of overall levels of cyclin D1, pAKT, and pERK1/2.

Conclusions

STAT1–EGFR-dependent constitutive phosphorylation is responsible for a positive feedback loop that causes its own overexpression and consequently an increased proliferation of the tumor cells. These findings provide the rationale for further studies aiming to identify effective therapeutic options in meningioma.

ALK and RET Inhibitors Promote HLA Class I Antigen Presentation and Unmask New Antigens within the Tumor Immunopeptidome

T-cell immunotherapies are often thwarted by the limited presentation of tumor-specific antigens abetted by the downregulation of human leukocyte antigen (HLA). We showed that drugs inhibiting ALK and RET produced dose-related increases in cell-surface HLA in tumor cells bearing these mutated kinases in vitro and in vivo, as well as elevated transcript and protein expression of HLA and other antigen-processing machinery. Subsequent analysis of HLA-presented peptides after ALK and RET inhibitor treatment identified large changes in the immunopeptidome with the appearance of hundreds of new antigens, including T-cell epitopes associated with impaired peptide processing (TEIPP) peptides. ALK inhibition additionally decreased PD-L1 levels by 75%. Therefore, these oncogenes may enhance cancer formation by allowing tumors to evade the immune system by downregulating HLA expression. Altogether, RET and ALK inhibitors could enhance T-cell-based immunotherapies by upregulating HLA, decreasing checkpoint blockade ligands, and revealing new, immunogenic, cancer-associated antigens.

Ghrelin up-regulates cartilage-specific genes via the ERK/STAT3 pathway in chondrocytes of patients with adolescent idiopathic scoliosis

Adolescent idiopathic scoliosis (AIS) is a severe spinal deformity that often occurs during puberty. The occurrence of AIS is suggested to be related to abnormal development of cartilage. Our previous study found increased serum ghrelin levels in AIS patients that may linked to the development of AIS. However, whether ghrelin affects cartilage in AIS patients is unclear. We used quantitative real-time PCR (qRT-PCR) and immunohistochemistry to detect the expression of cartilage-specific genes and the ghrelin receptor, growth hormone secretagogue receptor (GHSR). The mRNA and protein levels of collagen II (COLII), SOX9, AGGRECAN (ACAN) and GHSR were higher in AIS patients than in controls. In addition, the protein levels of GHSR downstream signaling pathway members p-STAT3 (Ser727), and p-ERK1/2 were increased. Furthermore, we treated chondrocytes from AIS patients with 100 nM ghrelin, the cell proliferation assay and Western blotting showed that ghrelin promotes chondrocyte proliferation and enhances COLII, SOX9, ACAN, p-ERK1/2 and p-STAT3 expression, respectively. Interestingly, all these observed alterations were abolished by ghrelin + [D-Lys3]-GHRP-6 (a ghrelin receptor inhibitor) treatment. And after U0126 (an inhibitor of ERK1/2 phosphorylation) treatment, ERK1/2 and STAT3 (Ser727) phosphorylation was simultaneously suppressed indicating that ERK1/2 is an upstream pathway protein of STAT3 (Ser727). In conclusion, ghrelin plays an important role in upregulating cartilage-specific genes on AIS primary chondrocytes by activating ERK/STAT3 signaling pathway.

Luteolin suppresses the JAK/STAT pathway in a cellular model of intestinal inflammation

Current treatment strategies for inflammatory bowel diseases (IBDs) are associated with a lower efficacy and with several side effects that strongly affect the quality of life of IBD patients. Consequently, the development of new therapies, combining efficacy and safety is an important goal in the field of intestinal inflammation. In this context, evidence supports that polyphenols can be promising candidates due to their ability to modulate intracellular inflammatory signalling cascades. Luteolin, a naturally occurring flavonoid, exhibits anti-inflammatory properties in several models of inflammation. However, its action against intestinal inflammation has been poorly explored. Therefore, there is a lack of scientific knowledge about the potential impact of luteolin in the intestinal inflammation, particularly regarding the underlying molecular mechanisms by which luteolin can exert its anti-inflammatory action. We assessed the potential anti-inflammatory effect of luteolin in a cellular model of intestinal inflammation using cytokine-stimulated HT-29 colon epithelial cells, and the underlying key molecular mechanisms were identified. Luteolin significantly inhibited interleukine-8 (IL-8) production, cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) expression and nitric oxide (˙NO) overproduction induced by cytokines, indicating that luteolin negatively modulates key inflammatory signalling cascades underlying intestinal inflammation. Mechanistically, the inhibition of the JAK/STAT pathway was identified as a critical mechanism by which luteolin exerts its intestinal anti-inflammatory action. This study uncovers novel molecular mechanisms by which luteolin may act against intestinal inflammation, which might support the use of luteolin as a future therapeutic strategy in IBD.

Potential of anti-inflammatory agents for treatment of atherosclerosis

Chronic inflammation is a central pathogenic mechanism of atherosclerosis induction and progression. Vascular inflammation is associated with accelerated onset of late atherosclerosis complications. Atherosclerosis-related inflammation is mediated by a complex cocktail of pro-inflammatory cytokines, chemokines, bioactive lipids, and adhesion molecules, and blocking the key pro-atherogenic inflammatory mechanisms can be beneficial for treatment of atherosclerosis. Therapeutic agents that specifically target some of the atherosclerosis-related inflammatory mechanisms have been evaluated in preclinical and clinical studies. The most promising anti-inflammatory compounds for treatment of atherosclerosis include non-specific anti-inflammatory drugs, phospholipase inhibitors, blockers of major inflammatory cytokines, leukotrienes, adhesion molecules, and pro-inflammatory signaling pathways, such as CCL2-CCR2 axis or p38 MAPK pathway. Ongoing studies attempt evaluating therapeutic utility of these anti-inflammatory drugs for treatment of atherosclerosis. The obtained results are important for our understanding of atherosclerosis-related inflammatory mechanisms and for designing randomized controlled studies assessing the effect of specific anti-inflammatory strategies on cardiovascular outcomes.

Human Cytomegalovirus Utilizes a Nontraditional Signal Transducer and Activator of Transcription 1 Activation Cascade via Signaling through Epidermal Growth Factor Receptor and Integrins To Efficiently Promote the Motility, Differentiation, and Polarization of Infected Monocytes

Human cytomegalovirus (HCMV) infects peripheral blood monocytes and triggers biological changes that promote viral dissemination and persistence. We have shown that HCMV induces a proinflammatory state in infected monocytes, resulting in enhanced monocyte motility and transendothelial migration, prolonged monocyte survival, and differentiation toward a long-lived M1-like macrophage phenotype. Our data indicate that HCMV triggers these changes, in the absence of de novo viral gene expression and replication, through engagement and activation of epidermal growth factor receptor (EGFR) and integrins on the surface of monocytes. We previously identified that HCMV induces the upregulation of multiple proinflammatory gene ontologies, with the interferon-associated gene ontology exhibiting the highest percentage of upregulated genes. However, the function of the HCMV-induced interferon (IFN)-stimulated genes (ISGs) in infected monocytes remained unclear. We now show that HCMV induces the enhanced expression and activation of a key ISG transcriptional regulator, signal transducer and activator of transcription (STAT1), via an IFN-independent but EGFR- and integrin-dependent signaling pathway. Furthermore, we identified a biphasic activation of STAT1 that likely promotes two distinct phases of STAT1-mediated transcriptional activity. Moreover, our data show that STAT1 is required for efficient early HCMV-induced enhanced monocyte motility and later for HCMV-induced monocyte-to-macrophage differentiation and for the regulation of macrophage polarization, suggesting that STAT1 may serve as a molecular convergence point linking the biological changes that occur at early and later times postinfection. Taken together, our results suggest that HCMV reroutes the biphasic activation of a traditionally antiviral gene product through an EGFR- and integrin-dependent pathway in order to help promote the proviral activation and polarization of infected monocytes.IMPORTANCE HCMV promotes multiple functional changes in infected monocytes that are required for viral spread and persistence, including their enhanced motility and differentiation/polarization toward a proinflammatory M1 macrophage. We now show that HCMV utilizes the traditionally IFN-associated gene product, STAT1, to promote these changes. Our data suggest that HCMV utilizes EGFR- and integrin-dependent (but IFN-independent) signaling pathways to induce STAT1 activation, which may allow the virus to specifically dictate the biological activity of STAT1 during infection. Our data indicate that HCMV utilizes two phases of STAT1 activation, which we argue molecularly links the biological changes that occur following initial binding to those that continue to occur days to weeks following infection. Furthermore, our findings may highlight a unique mechanism for how HCMV avoids the antiviral response during infection by hijacking the function of a critical component of the IFN response pathway.

What Scales the T Cell Response?

T cells are known to scale their clonal expansion and effector cytokine response according to the dose and strength of antigenic signal so as to balance their role of affecting protection with the intertwined and immunologically driven tissue damage. How T cells achieve this is now beginning to be understood. We underscore temporal integration of digital T cell receptor (TCR) signaling as the basis for achieving scaled response by means of accumulating crucial mediators over time. We also discuss the role of temporally integrated crosstalk between TCR and IL2 signaling in mediating a scaled, coherent, collective response by T cells. Finally, we highlight numerous known and putative regulatory interactions in the transcriptional program that are expected to quantitatively scale the T cell response, and also offer new mechanisms to hitherto unexplained observations.

Complementary PTM Profiling of Drug Response in Human Gastric Carcinoma by Immunoaffinity and IMAC Methods with Total Proteome Analysis

Gaining insight into normal cellular signaling and disease biology is a critical goal of proteomic analyses. The ability to perform these studies successfully to extract the maximum value and discovery of biologically relevant candidate biomarkers is therefore of primary importance. Many successful studies in the past have focused on total proteome analysis (changes at the protein level) combined with phosphorylation analysis by metal affinity enrichment (changes at the PTM level). Here, we use the gastric carcinoma cell line MKN-45 treated with the c-Met inhibitor SU11274 and PKC inhibitor staurosporine to investigate the most efficient and most comprehensive strategies for both total protein and PTM analysis. Under the conditions used, total protein analysis yielded few changes in response to either compound, while analysis of phosphorylation identified thousands of sites that changed differentially between the two treatments. Both metal affinity and antibody-based enrichments were used to assess phosphopeptide changes, and the data generated by the two methods was largely complementary (non-overlapping). Label-free quantitation of peptide peak abundances was used to accurately determine fold-changes between control and treated samples. Protein interaction network analysis allowed the data to be placed in a biologically relevant context, and follow-up validation of selected findings confirmed the accuracy of the proteomic data. Together, this study provides a framework for start-to-finish proteomic analysis of any experimental system under investigation to maximize the value of the proteomic study and yield the best chance for uncovering actionable target candidates.

FSH-induced p38-MAPK-mediated dephosphorylation at serine 727 of the signal transducer and activator of transcription 1 decreases Cyp1b1 expression in mouse granulosa cells

Most mammalian follicles undergo atresia at various stages before ovulation, and granulosa cell apoptosis is a major cause of antral follicular atresia. Estradiol is an essential mitogen for granulosa cell proliferation in vivo and inhibition of apoptosis. The estradiol-producing capacity and metabolism levels are important for follicle health, and sufficient estradiol is necessary for follicle development and ovulation. Cyp1b1, a member of the cytochrome P450 1 subfamily, is responsible for the metabolism of a wide variety of halogenated and polycyclic aromatic hydrocarbons in diverse tissues. In mouse follicles, Cyp1b1 converts estradiol to 4-hydroxyestradiol. We investigated mouse granulosa cells (MGCs) in vivo and in vitro and found that Cyp1b1 played a crucial role in estradiol metabolism in dominant follicles. Follicle-stimulating hormone (FSH) decreased estrogen metabolism by reducing Cyp1b1 mRNA and protein levels in MGCs. Furthermore, FSH regulated signal transducer and activator of transcription 1 (STAT1), a significant transcription factor of Cyp1b1, by mediating the dephosphorylation of STAT1 on serine 727 (Ser(727)) in MGCs. p38 mitogen-activated protein kinase (MAPK) may be involved in the FSH-induced dephosphorylation of STAT1 on Ser(727) in MGCs. These results suggested that FSH functions via p38 MAPK-induced dephosphorylation at Ser(727) of STAT1 to downregulate Cyp1b1 expression and maintain the estradiol levels in mouse dominant follicles.

Resveratrol modulates cytokine-induced Jak/STAT activation more efficiently than 5-aminosalicylic acid: an in vitro approach

Background

Many advances have been recently made focused on the valuable help of dietary polyphenols in chronic inflammatory diseases. On the other hand, current treatment options for intestinal bowel disease patients are unsatisfying and, for this reason, it is estimated that many patients use dietary supplements to achieve extra benefits.

Aim

The aim of this work was to analyze under a mechanistic perspective the anti-inflammatory potential of resveratrol, a natural polyphenolic compound, and to compare it with a pharmaceutical agent, 5-aminosalicylic acid, using the intestinal HT-29 cell line, as a cellular model.

Methodology and Principal Findings

In the present study, HT-29 colon epithelial cells were pre-treated with 25 µM resveratrol and/or 500 µM 5-aminosalicylic acid and then exposed to a combination of cytokines (IL-1α, TNF-α, IFN-γ) for a certain period of time. Our data showed that resveratrol, used in a concentration 20 times lower than 5-aminosalicylic acid, was able to significantly reduce NO and PGE₂ production, iNOS and COX-2 expression and reactive oxidant species formation induced by the cytokine challenge. However, as already verified with 5-aminosalicylic acid, in spite of not exhibiting any effect on IkB-α degradation, resveratrol down-regulated JAK-STAT pathway, decreasing the levels of activated STAT1 in the nucleus. Additionally, resveratrol decreased the cytokine-stimulated activation of SAPK/JNK pathway but did not counteract the cytokine-triggered negative feedback mechanism of STAT1, through p38 MAPK.

Conclusion/Significance

Taken together, our results show that resveratrol may be considered a future nutraceutical approach, promoting remission periods, limiting the inflammatory process and preventing colorectal cancer, which is common in these patients.

Inducing autophagy: a comparative phosphoproteomic study of the cellular response to ammonia and rapamycin

Autophagy is a lysosomal-mediated catabolic process, which through degradation of different cytoplasmic components aids in maintaining cellular homeostasis and survival during exposure to extra- or intracellular stresses. Ammonia is a potential toxic and stress-inducing byproduct of glutamine catabolism, which has recently been found to induce autophagy in an MTOR independent way and support cancer cell survival. In this study, quantitative phosphoproteomics was applied to investigate the initial signaling events linking ammonia to the induction of autophagy. The MTOR inhibitor rapamycin was used as a reference treatment to emphasize the differences between an MTOR-dependent and -independent autophagy-induction. By this means 5901 phosphosites were identified of which 626 were treatment-specific regulated and 175 were coregulated. Investigation of the ammonia-specific regulated sites supported that MTOR activity was not affected, but indicated increased MAPK3 activity, regulation of proteins involved in Rho signal transduction, and a novel phosphorylation motif, serine-proline-threonine (SPT), which could be linked to cytoskeleton-associated proteins. MAPK3 could not be identified as the primary driver of ammonia-induced autophagy but instead the data suggested an upregulation of AMPK and the unfolded protein response (UPR), which might link ammonia to autophagy induction. Support of UPR induction was further obtained from the finding of increased protein levels of the ER stress markers DDIT3/CHOP and HSPA5 during ammonia treatment. The large-scale data set presented here comprises extensive high-quality quantitative information on phosphoprotein regulation in response to 2 very different autophagy inducers and should therefore be considered a general resource for the community.

Merlin sumoylation is required for its tumor suppressor activity

Merlin, encoded by the Neurofibromatosis 2 (NF2) gene, is a multifunctional tumor suppressor that integrates and regulates extracellular cues and intracellular signaling pathways, both at the plasma membrane and in the nucleus, to control cell proliferation, migration and invasion. Molecular mechanisms regulating merlin's tumor-suppressive activity have not been clearly defined. Here we report that merlin can be sumoylated on Lysine residue (K76) in vitro and in vivo. Sumoylation mediates merlin's intramolecular and intermolecular binding activities and regulates its cytoplasm/nucleus trafficking. Interestingly, sumoylation of merlin is regulated by its phosphorylation via Akt and PAK2 kinases. Mutation of K76 into arginine (R) abolishes its sumoylation, disrupts merlin cortical cytoskeleton residency and attenuates its stability. Using a K76R mutant merlin in a subcutaneous U87MG xenograft model, we demonstrate that merlin sumoylation is required for tumor-suppressive activity. Taken together, our findings indicate that merlin is sumoylated and that this post-translational modification is essential for tumor suppression.

Structure-function analysis indicates that sumoylation modulates DNA-binding activity of STAT1

Background

STAT1 is an essential transcription factor for interferon-γ-mediated gene responses. A distinct sumoylation consensus site (ψKxE) ⁷⁰²IKTE⁷⁰⁵ is localized in the C-terminal region of STAT1, where Lys703 is a target for PIAS-induced SUMO modification. Several studies indicate that sumoylation has an inhibitory role on STAT1-mediated gene expression but the molecular mechanisms are not fully understood.

Results

Here, we have performed a structural and functional analysis of sumoylation in STAT1. We show that deconjugation of SUMO by SENP1 enhances the transcriptional activity of STAT1, confirming a negative regulatory effect of sumoylation on STAT1 activity. Inspection of molecular model indicated that consensus site is well exposed to SUMO-conjugation in STAT1 homodimer and that the conjugated SUMO moiety is directed towards DNA, thus able to form a sterical hindrance affecting promoter binding of dimeric STAT1. In addition, oligoprecipitation experiments indicated that sumoylation deficient STAT1 E705Q mutant has higher DNA-binding activity on STAT1 responsive gene promoters than wild-type STAT1. Furthermore, sumoylation deficient STAT1 E705Q mutant displayed enhanced histone H4 acetylation on interferon-γ-responsive promoter compared to wild-type STAT1.

Conclusions

Our results suggest that sumoylation participates in regulation of STAT1 responses by modulating DNA-binding properties of STAT1.

Phosphorylation-dependent SUMOylation of the transcription factor NF-E2

Nuclear factor erythroid-derived 2 (NF-E2), a heterodimer composed of p45 and p18, is a transcriptional activator in hematopoietic progenitors. The transcriptional activity of NF-E2 is not only upregulated by SUMOylation but also stimulated by the cAMP-dependent protein kinase A (PKA). However, the relationship between SUMOylation and phosphorylation in the activation of NF-E2 is unclear. In the present studies, we have demonstrated that PKA enhances NF-E2 SUMOylation in an in vitro system using purified proteins, suggesting a possible mechanism for PKA-dependent activation of the NF-E2 transcription factor through SUMOylation.

Targeting the interleukin-6/Jak/stat pathway in human malignancies

The Janus kinase/signal transducer and activator of transcription (Jak/Stat) pathway was discovered 20 years ago as a mediator of cytokine signaling. Since this time, more than 2,500 articles have been published demonstrating the importance of this pathway in virtually all malignancies. Although there are dozens of cytokines and cytokine receptors, four Jaks, and seven Stats, it seems that interleukin-6-mediated activation of Stat3 is a principal pathway implicated in promoting tumorigenesis. This transcription factor regulates the expression of numerous critical mediators of tumor formation and metastatic progression. This review will examine the relative importance and function of this pathway in nonmalignant conditions as well as malignancies (including tumor intrinsic and extrinsic), the influence of other Stats, the development of inhibitors to this pathway, and the potential role of inhibitors in controlling or eradicating cancers.

Inhibitor of κB kinase epsilon (IKK(epsilon)), STAT1, and IFIT2 proteins define novel innate immune effector pathway against West Nile virus infection

West Nile virus is an emerging virus whose virulence is dependent upon viral evasion of IFN and innate immune defenses. The actions of IFN-stimulated genes (ISGs) impart control of virus infection, but the specific ISGs and regulatory pathways that restrict West Nile virus (WNV) are not defined. Here we show that inhibitor of κB kinase ε (IKKε) phosphorylation of STAT1 at serine 708 (Ser-708) drives IFIT2 expression to mediate anti-WNV effector function of IFN. WNV infection was enhanced in cells from IKKε(-/-) or IFIT2(-/-) mice. In IKKε(-/-) cells, the loss of IFN-induced IFIT2 expression was linked to lack of STAT1 phosphorylation on Ser-708 but not Tyr-701 nor Ser-727. STAT1 Ser-708 phosphorylation occurs independently of IRF-3 but requires signaling through the IFN-α/β receptor as a late event in the IFN-induced innate immune response that coincides with IKKε-responsive ISGs expression. Biochemical analyses show that STAT1 tyrosine dephosphorylation and CRM1-mediated STAT1 nuclear-cytoplasmic shuttling are required for STAT1 Ser-708 phosphorylation. When compared with WT mice, WNV-infected IKKε(-/-) mice exhibit enhanced kinetics of virus dissemination and increased pathogenesis concomitant with loss of STAT1 Ser-708 phosphorylation and IFIT2 expression. Our results define an IFN-induced IKKε signaling pathway of specific STAT1 phosphorylation and IFIT2 expression that imparts innate antiviral immunity to restrict WNV infection and control viral pathogenesis.

Negative regulation of C/EBPbeta1 by sumoylation in breast cancer cells

Sumoylation is a post-translational modification that is oftentimes deregulated in diseases such as cancer. Transcription factors are frequent targets of sumoylation and modification by SUMO can affect subcellular localization, transcriptional activity, and stability of the target protein. C/EBPbeta1 is one such transcription factor that is modified by SUMO-2/3. Non-sumoylated C/EBPbeta1, p52-C/EBPbeta1, is expressed in normal mammary epithelial cells but not breast cancer cell lines and plays a role in oncogene-induced senescence, a tumor suppressive mechanism. Although p52-C/EBPbeta1 is not observed via immunoblot in breast cancer cell lines, higher molecular weight bands are observed when breast cancer cell lines are subjected to immunoblot analysis with a C/EBPbeta1-specific antibody. We show that exogenously expressed C/EBPbeta1 is sumoylated in breast cancer cells, and that the higher molecular weight bands we observe in anti-C/EBPbeta1 immunoblots of breast cancer cell lines is sumoylated C/EBPbeta1. Phosphorylation oftentimes enhances sumoylation, and phosphorylation cascades are activated in breast cancer cells. We demonstrate that phosphorylation of C/EBPbeta1Thr235 by Erk-2 enhances sumoylation of C/EBPbeta1 in vitro. In addition, sumoylated C/EBPbeta1 is phosphorylated on Thr235 and mutation of Thr235 to alanine leads to a decrease in sumoylation of C/EBPbeta1. Finally, using a C/EBPbeta1-SUMO fusion protein we show that constitutive sumoylation of C/EBPbeta1 completely blocks its capability to induce senescence in WI38 fibroblasts expressing hTERT. Thus, sumolylation of C/EBPbeta1 in breast cancer cells may be a mechanism to circumvent oncogene-induced senescence.

Interpreting mixed signals: the cell's cytokine conundrum

Cytokines are essential for the activation, differentiation and control of the immune system. Many cytokines, alone or in combination with other cytokines, have multiple functions and can often act on many different cell types with distinct developmental or functional consequences. Despite the myriad of cytokines and cytokine receptors, there are relatively few signaling molecules that transduce these diverse cytokine signals. In this review, we will discuss the potential mechanisms used by cytokines to mediate distinct cellular outcomes from a small number of signaling molecules. Understanding this paradigm in cytokine signaling can aid in the development of potential therapeutic approaches involving cytokine targeting or use.

Epstein-Barr virus latent membrane protein 1 (LMP1) C-terminal-activating region 3 contributes to LMP1-mediated cellular migration via its interaction with Ubc9

Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1), the principal viral oncoprotein and a member of the tumor necrosis factor receptor superfamily, is a constitutively active membrane signaling protein that regulates multiple signal transduction pathways via its C-terminal-activating region 1 (CTAR1) and CTAR2, and also the less-studied CTAR3. Because protein sumoylation among other posttranslational modifications may regulate many signaling pathways induced by LMP1, we investigated whether during EBV latency LMP1 regulates sumoylation processes that control cellular activation and cellular responses. By immunoprecipitation experiments, we show that LMP1 interacts with Ubc9, the single reported SUMO-conjugating enzyme. Requirements for LMP1-Ubc9 interactions include enzymatically active Ubc9: expression of inactive Ubc9 (Ubc9 C93S) inhibited the LMP1-Ubc9 interaction. LMP1 CTAR3, but not CTAR1 and CTAR2, participated in the LMP1-Ubc9 interaction, and amino acid sequences found in CTAR3, including the JAK-interacting motif, contributed to this interaction. Furthermore, LMP1 expression coincided with increased sumoylation of cellular proteins, and disruption of the Ubc9-LMP1 CTAR3 interaction almost completely abrogated LMP1-induced protein sumoylation, suggesting that this interaction promotes the sumoylation of downstream targets. Additional consequences of the disruption of the LMP1 CTAR3-Ubc9 interaction revealed effects on cellular migration, a hallmark of oncogenesis. Together, these data demonstrate that LMP1 CTAR3 does in fact function in intracellular signaling and leads to biological effects. We propose that LMP1, by interaction with Ubc9, modulates sumoylation processes, which regulate signal transduction pathways that affect phenotypic changes associated with oncogenesis.

Signal transducers and activators of transcription-from cytokine signalling to cancer biology

Signal transducers and activators of transcription (STATs) are, as the name indicates, both signal transducers and transcription factors. STATs are activated by cytokines and some growth factors and thus control important biological processes. These include cell growth, cell differentiation, apoptosis and immune responses. Dysregulation of STATs, either due to constitutive activation or function impairment, can have, therefore, deleterious biological consequences. This review places particular emphasis on their structural organization, biological activities and regulatory mechanisms most commonly utilized by cells to control STAT-mediated signalling. STATs also play important roles in cancer and immune deficiencies and are thus being exploited as therapeutic targets.

Inducible COX-2-dependent apoptosis in human ovarian cancer cells

Resveratrol is a naturally occurring trihydroxyl-diphenylethylene compound that has been shown experimentally to have beneficial effects in the treatment of cancer and cardiovascular disease. Resveratrol induces programmed cell death (apoptosis) in these cells and activates important signal transducing proteins including extracellular signal-regulated kinases (ERKs) 1 and 2 in cancer cells. Resveratrol also causes nuclear accumulation of the enzyme cyclooxygenase (COX)-2 and of the oncogene suppressor protein, p53. We have studied the molecular basis of the anticancer actions of resveratrol using human ovarian carcinoma (OVCAR-3) cells. Our findings include the following: (i) nuclear accumulation of COX-2 in resveratrol-treated cells is blocked by the ERK1/2 inhibitor, PD98059; (ii) an inhibitor of COX-2 activity, NS398, prevents accumulation of ERK1/2, COX-2, activated p53 and small ubiquitin-like modifier (SUMO-1) in the nucleus; (iii) apoptosis, quantitated by nucleosome enzyme-linked immunosorbent assay and the nuclear abundance of the pro-apoptotic protein, BcL-xs, were inhibited by NS398. This finding implicates nuclear COX-2 in p53-mediated apoptosis induced by resveratrol. Sumoylation is important to stabilization of p53 and a COX-2-SUMO-1 interaction suggests sumoylation of COX-2 in resveratrol-treated cells and (iv) chromatin immunoprecipitation studies showed binding of induced nuclear COX-2 to the promoter region of PIG3 and Bax, pro-apoptotic gene targets of transcriptionally active p53. Nuclear accumulation of activated ERK1/2 and sumolyated COX-2 are essential to resveratrol-induced pSer-15-p53-mediated apoptosis in human ovarian cancer cells.

Phosphorylation-acetylation switch in the regulation of STAT1 signaling

STAT1 signaling regulates the expression of important genes controlling cell growth, differentiation, apoptosis, and immune functions. Biochemical and genetic experiments have identified how this cascade is modulated. Phosphorylation of STAT1 tyrosine and serine moieties is induced rapidly by cytokines and growth factors. Upon nuclear translocation, phosphorylated STAT1 homo- and heterodimers activate gene expression. Inactivation of phosphorylated nuclear STAT1 has to be precisely regulated in order to allow signal transduction within limited time frames. Lysine acetylation has recently been appreciated as a novel mechanism regulating signal transduction events relying on STAT proteins. Here, we review these analyses and the finding that a switch from phosphorylated to acetylated STAT1 regulates acetylation-dependent dephosphorylation of STAT1 via the T cell tyrosine phosphatase. We discuss how these observations can be integrated into our current understanding of STAT-dependent cytokine signaling and its potential relevance for endocrine functions.

MAP Kinase: SUMO pathway interactions

The convergence and coordinated cross talk of different signalling pathways forms a regulatory network which determines the biological outcome to environmental cues. The MAPK pathways are one of the important routes by which extracellular signals are transduced into intracellular responses. Through protein phosphorylation mechanisms, they can play a pivotal role in regulating other posttranslational modifications such as protein acetylation and ubiquitination. In addition, protein sumoylation has emerged as an important pathway which also functions through post-translational modification. The SUMO pathway modulates a diverse range of cellular processes including signal transduction, chromosome integrity, and transcription. Interestingly, recent studies have provided links between the SUMO and MAPK signalling pathways which converge to modulate transcription factor activity. This was first demonstrated by the observation that the activation of the ERK pathway caused de-sumoylation of the transcription factor, Elk-1. Furthermore, a growing number of links are now being made between the MAPK pathway and protein sumoylation. Given the nature of protein sumoylation in diverse biological functions, it is not surprising that the effect of MAPK pathways on sumoylation varies between different proteins. Here, we describe protocols that can be used in studying the cross talk between the MAPK and SUMO pathways, particularly at the level of gene regulation.

Selective transcriptional down-regulation of human rhinovirus-induced production of CXCL10 from airway epithelial cells via the MEK1 pathway

Human rhinovirus (HRV) infections can trigger exacerbations of lower airway diseases. Infection of airway epithelial cells induces production of a number of proinflammatory chemokines that may exacerbate airway inflammation, including CXCL10, a chemoattractant for type 1 lymphocytes and NK cells. Primary human bronchial epithelial cells and the BEAS-2B human bronchial epithelial cell line were used to examine the role of MAPK pathways in HRV-16-induced production of CXCL10. Surprisingly, PD98059 and U0126, two inhibitors of the MEK1/2-ERK MAPK pathway, significantly enhanced HRV-16-induced CXCL10 mRNA and protein. This enhancement was not seen with IFN-beta-induced production of CXCL10. Studies using small interfering RNA revealed that knockdown of MEK1, but not MEK2, was associated with enhanced HRV-induced CXCL10 production. Promoter construct studies revealed that PD98059 and U0126 enhanced HRV-16-induced transcriptional activation of CXCL10. HRV-16-induced promoter activation was regulated by two NF-kappaB binding sites, kappaB1 and kappaB2, and by an IFN-stimulated response element. Inhibitors of the MEK1/2-ERK pathway did not alter HRV-16-induced activation of tandem repeat kappaB1 or kappaB2 constructs, nor did they alter HRV-16-induced nuclear translocation/binding of NF-kappaB to either kappaB1 or kappaB2 recognition sequences. Furthermore, PD98059 and U0126 did not alter phosphorylation or degradation of IkappaBalpha. In contrast, inhibitors of the MEK1/2-ERK pathway, and small interfering RNA knockdown of MEK1, enhanced nuclear translocation/binding of IFN regulatory factor (IRF)-1 to the IFN-stimulated response element recognition sequence in HRV-16 infected cells. We conclude that activation of MEK1 selectively down-regulates HRV-16-induced expression of CXCL10 via modulation of IRF-1 interactions with the gene promoter in human airway epithelial cells.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Structural basis for recruitment of CBP/p300 coactivators by STAT1 and STAT2 transactivation domains

CBP/p300 transcriptional coactivators mediate gene expression by integrating cellular signals through interactions with multiple transcription factors. To elucidate the molecular and structural basis for CBP-dependent gene expression, we determined structures of the CBP TAZ1 and TAZ2 domains in complex with the transactivation domains (TADs) of signal transducer and activator of transcription 2 (STAT2) and STAT1, respectively. Despite the topological similarity of the TAZ1 and TAZ2 domains, subtle differences in helix packing and surface grooves constitute major determinants of target selectivity. Our results suggest that TAZ1 preferentially binds long TADs capable of contacting multiple surface grooves simultaneously, whereas smaller TADs that are restricted to a single contiguous binding surface form complexes with TAZ2. Complex formation for both STAT TADs involves coupled folding and binding, driven by intermolecular hydrophobic and electrostatic interactions. Phosphorylation of S727, required for maximal transcriptional activity of STAT1, does not enhance binding to any of the CBP domains. Because the different STAT TADs recognize different regions of CBP/p300, there is a potential for multivalent binding by STAT heterodimers that could enhance the recruitment of the coactivators to promoters.

Mutually exclusive STAT1 modifications identified by Ubc9/substrate dimerization-dependent SUMOylation

Post-translational modifications control the physiological activity of the signal transducer and activator of transcription STAT1. While phosphorylation at tyrosine Y701 is a prerequisite for STAT1 dimerization, its SUMOylation represses the transcriptional activity. Recently, we have demonstrated that SUMOylation at lysine K703 inhibits the phosphorylation of nearby localized Y701 of STAT1. Here, we analysed the influence of phosphorylation of Y701 on SUMOylation of K703 in vivo. For that reason, an Ubc9/substrate dimerization-dependent SUMOylation (USDDS) system was developed, which consists of fusions of the SUMOylation substrate and of the SUMO-conjugating enzyme Ubc9 to the chemically activatable heterodimerization domains FKBP and FRB, respectively. When FKBP fusion proteins of STAT1, p53, CRSP9, FOS, CSNK2B, HES1, TCF21 and MYF6 are coexpressed with Ubc9-FRB, treatment of HEK293 cells with the rapamycin-related dimerizer compound AP21967 induces SUMOylation of these proteins in vivo. For STAT1-FKBP and p53-FKBP we show that this SUMOylation takes place at their specific SUMOylation sites in vivo. Using USDDS, we then demonstrate that STAT1 phosphorylation at Y701 induced by interferon-β treatment inhibits SUMOylation of K703 in vivo. Thus, pY701 and SUMO-K703 of STAT1 represent mutually exclusive modifications, which prevent signal integration at this molecule and probably ensure the existence of differentially modified subpopulations of STAT1 necessary for its regulated nuclear cytoplasmic activation/inactivation cycle.

The CTCF insulator protein is posttranslationally modified by SUMO

The CTCF protein is a highly conserved zinc finger protein that is implicated in many aspects of gene regulation and nuclear organization. Its functions include the ability to act as a repressor of genes, including the c-myc oncogene. In this paper, we show that the CTCF protein can be posttranslationally modified by the small ubiquitin-like protein SUMO. CTCF is SUMOylated both in vivo and in vitro, and we identify two major sites of SUMOylation in the protein. The posttranslational modification of CTCF by the SUMO proteins does not affect its ability to bind to DNA in vitro. SUMOylation of CTCF contributes to the repressive function of CTCF on the c-myc P2 promoter. We also found that CTCF and the repressive Polycomb protein, Pc2, are colocalized to nuclear Polycomb bodies. The Pc2 protein may act as a SUMO E3 ligase for CTCF, strongly enhancing its modification by SUMO 2 and 3. These studies expand the repertoire of posttranslational modifications of CTCF and suggest roles for such modifications in its regulation of epigenetic states.

SUMOylation inhibits SF-1 activity by reducing CDK7-mediated serine 203 phosphorylation

Steroidogenic factor 1 (SF-1) is an orphan nuclear receptor selectively expressed in the adrenal cortex and gonads, where it mediates the hormonal stimulation of multiple genes involved in steroid hormone biosynthesis. SF-1 is the target of both phosphorylation and SUMOylation, but how these modifications interact or contribute to SF-1 regulation of endogenous genes remains poorly defined. We found that SF-1 is selectively SUMOylated at K194 in Y1 adrenocarcinoma cells and that although SUMOylation does not alter the subcellular localization of SF-1, the modification inhibits the ability of SF-1 to activate target genes. Notably, whereas SF-1 SUMOylation is independent of S203 phosphorylation and is unaffected by adrenocorticotropin (ACTH) treatment, loss of SUMOylation leads to enhanced SF-1 phosphorylation at serine 203. Furthermore, preventing SF-1 SUMOylation increases the mRNA and protein levels of multiple steroidogenic enzyme genes. Analysis of the StAR promoter indicates that blockade of SF-1 SUMOylation leads to an increase in overall promoter occupancy but does not alter the oscillatory recruitment dynamics in response to ACTH. Notably, we find that CDK7 binds preferentially to the SUMOylation-deficient form of SF-1 and that CDK7 inhibition reduces phosphorylation of SF-1. Based on these observations, we propose a coordinated modification model in which inhibition of SF-1-mediated transcription by SUMOylation in adrenocortical cancer cells is mediated through reduced CDK7-induced phosphorylation of SF-1.

SUMO under stress

During the last decade, SUMOylation has emerged as a central regulatory post-translational modification in the control of the fate and function of proteins. However, how SUMOylation is regulated itself has just started to be delineated. It appears now that SUMO (small ubiquitin-related modifier) conjugation/deconjugation equilibrium is affected by various environmental stresses, including osmotic, hypoxic, heat, oxidative and genotoxic stresses. This regulation occurs either at the level of individual targets, through an interplay between stress-induced phosphorylation and SUMOylation, or via modulation of the conjugation/deconjugation machinery abundance or activity. The present review gives an overview of the connections between stress and SUMOylation, the underlying molecular mechanisms and their effects on cellular functions.

The spindle positioning protein Kar9p interacts with the sumoylation machinery in Saccharomyces cerevisiae

Accurate positioning of the mitotic spindle is important for the genetic material to be distributed evenly in dividing cells, but little is known about the mechanisms that regulate this process. Here we report that two microtubule-associated proteins important for spindle positioning interact with several proteins in the sumoylation pathway. By two-hybrid analysis, Kar9p and Bim1p interact with the yeast SUMO Smt3p, the E2 enzyme Ubc9p, an E3 Nfi1p, as well as Wss1p, a weak suppressor of a temperature-sensitive smt3 allele. The physical interaction between Kar9p and Ubc9p was confirmed by in vitro binding assays. A single-amino-acid substitution in Kar9p, L304P disrupted its two-hybrid interaction with proteins in the sumoylation pathway, but retained its interactions with the spindle positioning proteins Bim1p, Stu2p, Bik1p, and Myo2p. The kar9-L304P mutant showed defects in positioning the mitotic spindle, with the spindle located more distally than normal. Whereas wild-type Kar9p-3GFP normally localizes to only the bud-directed spindle pole body (SPB), Kar9p-L304P-3GFP was mislocalized to both SPBs. Using a reconstitution assay, Kar9p was sumoylated in vitro. We propose a model in which sumoylation regulates spindle positioning by restricting Kar9p to one SPB. These findings raise the possibility that sumoylation could regulate other microtubule-dependent processes.

Analysis of extracellular signal-regulated kinase 2 function in neural stem/progenitor cells via nervous system-specific gene disruption

Extracellular signal-regulated kinase 2 (ERK2) is involved in a variety of cell fate decisions during development, but its exact role in this process remains to be determined. To specifically focus on the role of ERK2 in the brain, and to avoid early lethalities, we used a conditional gene-targeting approach to preferentially inactivate Erk2 in the embryonic mouse brain. The resulting mutant mice were viable and were relatively normal in overall appearance. However, the loss of Erk2 resulted in a diminished proliferation of neural stem cells in the embryonic ventricular zone (VZ), although the survival and differentiation of these cells was unaffected. The multipotent neural progenitor cells (NPCs) isolated from ERK2-deficient brains also showed impaired proliferation, reduced self-renewal ability, and increased apoptosis. By neurosphere differentiation analysis we further observed that lineage-restricted glial progenitors were increased in ERK2-deficient mice. The decline in the self-renewal ability and multipotency of NPCs resulting from the loss of ERK2 was found to be caused at least in part by upregulation of the JAK-STAT signaling pathway and reduced G1/S cell cycle progression. Furthermore, by global expression analysis we found that neural stem cell markers, including Tenascin C NR2E1 (Tlx), and Lgals1 (Galectin-1), were significantly downregulated, whereas several glial lineage markers were upregulated in neurospheres derived from ERK2-deficient mice. Our results thus suggest that ERK2 is required both for the proliferation of neural stem cells in the VZ during embryonic development and in the maintenance of NPC multipotency by suppressing the commitment of these cells to a glial lineage.

Retinoic acid-stimulated sequential phosphorylation, PML recruitment, and SUMOylation of nuclear receptor TR2 to suppress Oct4 expression

We previously reported an intricate mechanism underlying the homeostasis of Oct4 expression in normally proliferating stem cell culture of P19, mediated by SUMOylation of orphan nuclear receptor TR2. In the present study, we identify a signaling pathway initiated from the nongenomic activity of all-trans retinoic acid (atRA) to stimulate complex formation of extracellular signal-regulated kinase 2 (ERK2) with its upstream kinase, mitogen-activated protein kinase kinase (MEK). The activated ERK2 phosphorylates threonine-210 (Thr-210) of TR2, stimulating its subsequent SUMOylation. Dephosphorylated TR2 recruits coactivator PCAF and functions as an activator for its target gene Oct4. Upon phosphorylation at Thr-210, TR2 increasingly associates with promyelocytic leukemia (PML) nuclear bodies, becomes SUMOylated, and recruits corepressor RIP140 to act as a repressor for its target, Oct4. To normally proliferating P19 stem cell culture, exposure to a physiological concentration of atRA triggers a rapid nongenomic signaling cascade to suppress Oct4 gene and regulate cell proliferation.

Recruitment of Stat1 to chromatin is required for interferon-induced serine phosphorylation of Stat1 transactivation domain

The transcription factor Stat1 plays an essential role in responses to interferons (IFNs). Activation of Stat1 is achieved by phosphorylation on Y701 that is followed by nuclear accumulation. For full transcriptional activity and biological function Stat1 must also be phosphorylated on S727. The molecular mechanisms underlying the IFN-induced S727 phosphorylation are incompletely understood. Here, we show that both Stat1 Y701 phosphorylation and nuclear translocation are required for IFN-induced S727 phosphorylation. We further show that Stat1 mutants lacking the ability to stably associate with chromatin are poorly serine-phosphorylated in response to IFN-gamma. The S727 phosphorylation of these mutants is restored on IFN-beta treatment that induces the formation of the ISGF3 complex (Stat1/Stat2/Irf9) where Irf9 represents the main DNA binding subunit. These findings indicate that Stat1 needs to be assembled into chromatin-associated transcriptional complexes to become S727-phosphorylated and fully biologically active in response to IFNs. This control mechanism, which may be used by other Stat proteins as well, restricts the final activation step to the chromatin-tethered transcription factor.