B cell antigen receptor signaling enhances IFN-gamma-induced Stat1 target gene expression through calcium mobilization and activation of multiple serine kinase pathways

TLR7 Signaling in Lupus B Cells: New Insights into Synergizing Factors and Downstream Signals

PURPOSE OF THE REVIEW

Systemic lupus erythematosus (SLE) is driven by nucleic acid-containing antigens that stimulate endosomal TLRs. We review new advances in our understanding of how TLR7 signaling in B cells drives autoimmunity.

RECENT FINDINGS

Pathogenic B cell responses to TLR7 engagement are shaped by the disease-associated cytokine environment. TLR7, IFNγ, and IL-21 together promote the formation of autoreactive germinal centers and the ABC/DN2 B cell subset. BAFF and type 1 IFNs enhance autoantibody production from transitional B cells in concert with TLR7. TLR7 signaling components STAT1, BANK1, IRF5, SLC15A4, and CXorf21/TASL are associated genetically with SLE and important for lupus development in mice, while role of T-bet is controversial. Proper control of TLR7 trafficking by UNC93B1, syntenin-1, and αvβ3 integrin is critical for preventing autoimmunity. A better understanding of TLR7 signaling has revealed potential new therapeutic approaches for SLE, several of which are being tested in animal models or clinical trials.

Simulation of the Dynamics of Primary Immunodeficiencies in B Cells

Primary immunodeficiencies (PIDs) are a group of over 300 hereditary, heterogeneous, and mainly rare disorders that affect the immune system. Various aspects of immune system and PID proteins and genes have been investigated and facilitate systems biological studies of effects of PIDs on B cell physiology and response. We reconstructed a B cell network model based on data for the core B cell receptor activation and response processes and performed semi-quantitative dynamic simulations for normal and B cell PID failure modes. The results for several knockout simulations correspond to previously reported molecular studies and reveal novel mechanisms for PIDs. The simulations for CD21, CD40, LYN, MS4A1, ORAI1, PLCG2, PTPRC, and STIM1 indicated profound changes to major transcription factor signaling and to the network. Significant effects were observed also in the BCL10, BLNK, BTK, loss-of-function CARD11, IKKB, MALT1, and NEMO, simulations whereas only minor effects were detected for PIDs that are caused by constitutively active proteins (PI3K, gain-of-function CARD11, KRAS, and NFKBIA). This study revealed the underlying dynamics of PID diseases, confirms previous observations, and identifies novel candidates for PID diagnostics and therapy.

ERK is a negative feedback regulator for IFN-γ/STAT1 signaling by promoting STAT1 ubiquitination

BACKGROUND

We recently reported that STAT1 plays a tumor suppressor role, and ERK was inversely correlation with STAT1 expression in esophageal squamous cell carcinoma (ESCC). Here, we investigated the mechanism(s) that are responsible for the ERK regulates STAT1 in ESCC.

METHODS

We performed the immunoprecipitation (IP) to detect the ubiquitin of STAT1 upon MEK transfection or U0126 treatment and co-IP to confirm the binding of STAT1 and ERK in ESCC cell lines.

RESULTS

We found evidence that the ubiquitin-proteasome pathway can efficiently degrade STAT1 in ESCC cells, as MG132 treatment rapidly and dramatically increased STAT1 expression in these cells. This process is not dependent on the phosphorylation of the two important STAT1 residues, Y701 and S727, as site-directed mutagenesis of these two sites did not affect STAT1 degradation. We also found that ERK promotes proteasome degradation of STAT1, supported by the observations that pharmacologic inhibition of ERK resulted in a substantial increase of STAT1 whereas expression of constitutively active ERK further reduced the STAT1 protein level. In addition to suppressing STAT1 expression, ERK limited STAT1 signaling by decreasing the production of IFNγ.

CONCLUSION

To conclude, ERK is an effective negative regulator of STAT1 signaling in ESCC, by promoting its proteasome degradation and decreasing IFNγ production. Our data further supports that targeting ERK and/or STAT1 may be useful for treating ESCC.

Ligation of CD180 inhibits IFN-α signaling in a Lyn-PI3K-BTK-dependent manner in B cells

A hallmark of systemic lupus erythematosus (SLE) is the consistent production of various auto-antibodies by auto-reactive B cells. Interferon-α (IFN-α) signaling is highly activated in SLE B cells and plays a vital role in the antibody response by B cells. Previous studies have shown that CD180-negative B cells, which are dramatically increased in SLE patients, are responsible for the production of auto-antibodies. However, the association between CD180 and IFN-α signaling remains unknown. In the present study, we explored the effect of CD180 on regulating the activation of IFN-α signaling in B cells. We found that the number of CD180-negative B cells was increased in MRL/Mp-Fas(lpr/lpr) lupus-prone mice compared with wild-type mice. Phenotypic analysis showed that CD180-negative B cells comprised CD138+ plasmablast/plasma cells and GL-7+ germinal center (GC) B cells. Notably, ligation of CD180 significantly inhibited the IFN-α-induced phosphorylation of signal transducer and activator of transcription 2 (STAT-2) and expression of IFN-stimulated genes (ISGs) in a Lyn-PI3K-BTK-dependent manner in vitro. Moreover, ligation of CD180 could also inhibit IFN-α-induced ISG expression in B cells in vivo. Furthermore, the Toll-like receptor 7 and Toll-like receptor 9 signaling pathways could significantly downregulate CD180 expression and modulate the inhibitory effect of CD180 signaling on the activation of IFN-α signaling. Collectively, our results highlight the close association between the increased proportion of CD180-negative B cells and the activation of IFN-α signaling in SLE. Our data provide molecular insight into the mechanism of IFN-α signaling activation in SLE B cells and a potential therapeutic approach for SLE treatment.

Novel drug targets for personalized precision medicine in relapsed/refractory diffuse large B-cell lymphoma: a comprehensive review

Diffuse large B-cell lymphoma (DLBCL) is a clinically heterogeneous lymphoid malignancy and the most common subtype of non-Hodgkin's lymphoma in adults, with one of the highest mortality rates in most developed areas of the world. More than half of DLBLC patients can be cured with standard R-CHOP regimens, however approximately 30 to 40 % of patients will develop relapsed/refractory disease that remains a major cause of morbidity and mortality due to the limited therapeutic options.Recent advances in gene expression profiling have led to the identification of at least three distinct molecular subtypes of DLBCL: a germinal center B cell-like subtype, an activated B cell-like subtype, and a primary mediastinal B-cell lymphoma subtype. Moreover, recent findings have not only increased our understanding of the molecular basis of chemotherapy resistance but have also helped identify molecular subsets of DLBCL and rational targets for drug interventions that may allow for subtype/subset-specific molecularly targeted precision medicine and personalized combinations to both prevent and treat relapsed/refractory DLBCL. Novel agents such as lenalidomide, ibrutinib, bortezomib, CC-122, epratuzumab or pidilizumab used as single-agent or in combination with (rituximab-based) chemotherapy have already demonstrated promising activity in patients with relapsed/refractory DLBCL. Several novel potential drug targets have been recently identified such as the BET bromodomain protein (BRD)-4, phosphoribosyl-pyrophosphate synthetase (PRPS)-2, macrodomain-containing mono-ADP-ribosyltransferase (ARTD)-9 (also known as PARP9), deltex-3-like E3 ubiquitin ligase (DTX3L) (also known as BBAP), NF-kappaB inducing kinase (NIK) and transforming growth factor beta receptor (TGFβR).This review highlights the new insights into the molecular basis of relapsed/refractory DLBCL and summarizes the most promising drug targets and experimental treatments for relapsed/refractory DLBCL, including the use of novel agents such as lenalidomide, ibrutinib, bortezomib, pidilizumab, epratuzumab, brentuximab-vedotin or CAR T cells, dual inhibitors, as well as mechanism-based combinatorial experimental therapies. We also provide a comprehensive and updated list of current drugs, drug targets and preclinical and clinical experimental studies in DLBCL. A special focus is given on STAT1, ARTD9, DTX3L and ARTD8 (also known as PARP14) as novel potential drug targets in distinct molecular subsets of DLBCL.

A role for the ITAM signaling module in specifying cytokine-receptor functions

Diverse cellular responses to external cues are controlled by a small number of signal-transduction pathways, but how the specificity of functional outcomes is achieved remains unclear. Here we describe a mechanism for signal integration based on the functional coupling of two distinct signaling pathways widely used in leukocytes: the ITAM pathway and the Jak-STAT pathway. Through the use of the receptor for interferon-γ (IFN-γR) and the ITAM adaptor Fcγ as an example, we found that IFN-γ modified responses of the phagocytic antibody receptor FcγRI (CD64) to specify cell-autonomous antimicrobial functions. Unexpectedly, we also found that in peritoneal macrophages, IFN-γR itself required tonic signaling from Fcγ through the kinase PI(3)K for the induction of a subset of IFN-γ-specific antimicrobial functions. Our findings may be generalizable to other ITAM and Jak-STAT signaling pathways and may help explain signal integration by those pathways.

Cytokines alter IgA1 O-glycosylation by dysregulating C1GalT1 and ST6GalNAc-II enzymes

IgA nephropathy (IgAN), the most common primary glomerulonephritis, is characterized by renal immunodeposits containing IgA1 with galactose-deficient O-glycans (Gd-IgA1). These immunodeposits originate from circulating immune complexes consisting of anti-glycan antibodies bound to Gd-IgA1. As clinical disease onset and activity of IgAN often coincide with mucosal infections and dysregulation of cytokines, we hypothesized that cytokines may affect IgA1 O-glycosylation. We used IgA1-secreting cells derived from the circulation of IgAN patients and healthy controls and assessed whether IgA1 O-glycosylation is altered by cytokines. Of the eight cytokines tested, only IL-6 and, to a lesser degree, IL-4 significantly increased galactose deficiency of IgA1; changes in IgA1 O-glycosylation were robust for the cells from IgAN patients. These cytokines reduced galactosylation of the O-glycan substrate directly via decreased expression of the galactosyltransferase C1GalT1 and, indirectly, via increased expression of the sialyltransferase ST6GalNAc-II, which prevents galactosylation by C1GalT1. These findings were confirmed by siRNA knockdown of the corresponding genes and by in vitro enzyme reactions. In summary, IL-6 and IL-4 accentuated galactose deficiency of IgA1 via coordinated modulation of key glycosyltransferases. These data provide a mechanism explaining increased immune-complex formation and disease exacerbation during mucosal infections in IgAN patients.

Identification and characterization of novel immunomodulatory bursal-derived pentapeptide-II (BPP-II)

The bursa of Fabricius, the acknowledged central humoral immune organ, plays a vital role in B lymphocyte differentiation. However, there are few reports of the molecular basis of the mechanism on immune induction and potential antitumor activity of bursal-derived peptides. In this paper, a novel bursal-derived pentapeptide-II (BPP-II, MTLTG) was isolated and exerted immunomodulatory functions on antibody responses in vitro. Gene microarray analyses demonstrated that BPP-II regulated expression of 2478 genes in a mouse-derived hybridoma cell line. Immune-related gene ontology functional procedures were employed for further functional analysis. Furthermore, the majority of BPP-II-regulated pathways were associated with immune responses and tumor processes. Moreover, BPP-II exhibited immunomodulatory effects on antigen-specific immune responses in vivo, including enhancement of avian influenza virus (H9N2 subtype)-specific antibody and cytokine production and modification of T cell immunophenotypes and lymphocyte proliferation. Finally, BPP-II triggered p53 expression and stabilization and selectively inhibited tumor cell proliferation. These data identified the multifunctional factor, BPP-II, as a novel biomaterial representing an important linking between the humoral central immune system and immune induction, including antitumor. Information generated in this study elucidates further the mechanisms involved in humoral immune system and represents the potential basis of effective immunotherapeutic strategies for treating human tumors and immune improvement.

Stat3 directly controls the expression of Tbx5, Nkx2.5, and GATA4 and is essential for cardiomyocyte differentiation of P19CL6 cells

The transcription factor Stat3 (signal transducer and activator of transcription 3) mediates many physiological processes, including embryogenesis, stem cell self-renewal, and postnatal survival. In response to gp130 receptor activation, Stat3 becomes phosphorylated by the receptor-associated Janus kinase, forms dimers, and enters the nucleus where it binds to Stat3 target genes and regulates their expression. In this report, we demonstrate that Stat3 binds directly to the promoters and regulates the expression of three genes that are essential for cardiac differentiation: Tbx5, Nkx2.5, and GATA4. We further demonstrate that Tbx5, Nkx2.5, and GATA4 expression is dependent on Stat3 in response to ligand treatment and during ligand-independent differentiation of P19CL6 cells into cardiomyocytes. Finally, we show that Stat3 is necessary for the differentiation of P19CL6 cells into beating cardiomyocytes. All together, these results demonstrate that Stat3 is required for the differentiation of cardiomyocytes through direct transcriptional regulation of Tbx5, Nkx2.5, and GATA4.

STAT1 and pathogens, not a friendly relationship

STAT1 belongs to the STAT family of transcription factors, which comprises seven factors: STAT1, STAT2, STAT3, STAT4, STAT5A, STAT5B and STAT6. STAT1 is a 91 kDa protein originally identified as the mediator of the cellular response to interferon (IFN) α, and thereafter found to be a major component of the cellular response to IFNγ. STAT1 is, in fact, involved in the response to several cytokines and to growth factors. It is activated by cytokine receptors via kinases of the JAK family. STAT1 becomes phosphorylated and forms a dimer which enters the nucleus and triggers the transcription of its targets. Although not lethal at birth, selective gene deletion of STAT1 in mice leads to rapid death from severe infections, demonstrating its major role in the response to pathogens. Similarly, in humans who do not express STAT1, there is a lack of resistance to pathogens leading to premature death. This indicates a key, non-redundant function of STAT1 in the defence against pathogens. Thus, to successfully infect organisms, bacterial, viral or parasitic pathogens must overcome the activity of STAT1, and almost all the steps of this pathway can be blocked or inhibited by proteins produced in infected cells. Interestingly, some pathogens, like the oncogenic Epstein–Barr virus, have evolved a strategy which uses STAT1 activation.

Identification of novel direct Stat3 target genes for control of growth and differentiation

Signal transducer and activator of transcription 3 (Stat3) is a key regulator of gene expression in response to signaling of the glycoprotein 130 (gp130) family cytokines, including interleukin 6, oncostatin M, and leukemia inhibitory factor. Many efforts have been made to identify Stat3 target genes and to understand the mechanism of how Stat3 regulates gene expression. Using the microarray technique, hundreds of genes have been documented to be potential Stat3 target genes in different cell types. However, only a small fraction of these genes have been proven to be true direct Stat3 target genes. Here we report the identification of novel direct Stat3 target genes using a genome-wide screening procedure based on the chromatin immunoprecipitation method. These novel Stat3 target genes are involved in a diverse array of biological processes such as oncogenesis, cell growth, and differentiation. We show that Stat3 can act as both a repressor and activator on its direct target genes. We further show that most of the novel Stat3 direct target genes are dependent on Stat3 for their transcriptional regulation. In addition, using a physiological cell system, we demonstrate that Stat3 is required for the transcriptional regulation of two of the newly identified direct Stat3 target genes important for muscle differentiation.

Crosstalk among Jak-STAT, Toll-like receptor, and ITAM-dependent pathways in macrophage activation

Macrophage phenotype and activation are regulated by cytokines that use the Jak-STAT signaling pathway, microbial recognition receptors that include TLRs, and immunoreceptors that signal via ITAM motifs. The amplitude and qualitative nature of macrophage activation are determined by crosstalk among these signaling pathways. Basal ITAM signaling restrains macrophage responses to TLRs and other activating ligands, whereas strong ITAM signals synergize with the same ligands to activate cells strongly. Similarly, basal ITAM signaling augments IFN signaling and function of receptor activator of NF-kappaB, but extensive ITAM activation inhibits Jak-STAT signaling. Thus, intensity and duration of ITAM signaling determine whether ITAM-coupled receptors augment or attenuate TLR and Jak-STAT responses. IFN-gamma synergizes with TLRs in part by suppressing TLR-induced feedback inhibition, mediated by IL-10 and Stat3, by a mechanism that depends on glycogen synthase kinase (GSK)3 regulation of AP-1 and CREB. IFN-gamma suppresses TLR2 and TLR4 induction/activation of AP-1 by overlapping mechanisms that include regulation of MAPKs, GSK3-dependent suppression of DNA binding, and decreased Fos and Jun protein expression and stability. IFN-gamma suppression of TLR-induced activation of AP-1 and downstream target genes challenges current concepts about the inflammatory role of AP-1 proteins in macrophage activation and is consistent with a role for AP-1 in the generation of noninflammatory osteoclasts. Jak-STAT, TLR, and ITAM pathways are basally active in macrophages and strongly induced during innate responses. Thus, signal transduction crosstalk is regulated in a dynamic manner, which differs under homeostatic and pathologic conditions, and dysregulation of signal transduction crosstalk may contribute to pathogenesis of chronic inflammatory diseases.

Guanosine Inhibits CD40 Receptor Expression and Function Induced by Cytokines and β Amyloid in Mouse Microglia Cells

Growing evidence implicates CD40, a member of the TNFR superfamily, as contributing to the pathogenesis of many neurodegenerative diseases. Thus, strategies to suppress its expression may be of benefit in those disorders. To this aim, we investigated the effect of guanosine, a purine nucleoside that exerts neurotrophic and neuroprotective effects. CD40 expression and function are increased by exposure of mouse microglia cultures or the N9 microglia cell line to IFN-gamma (10 ng/ml) plus TNF-alpha (50 ng/ml) or beta amyloid (Abeta) peptide (Abeta(1-42); 500 nM). Culture pretreatment with guanosine (10-300 microM), starting 1 h before cytokine or Abeta addition, dose-dependently inhibited the CD40-induced expression as well as functional CD40 signaling by suppressing IL-6 production promoted by IFN-gamma/TNF-alpha challenge in the presence of CD40 cross-linking. Moreover, guanosine abrogated IFN-gamma-induced phosphorylation on Ser(727) and translocation of STAT-1alpha to the nucleus as well as TNF-alpha-/Abeta-induced IkappaBalpha and NF-kappaB p65/RelA subunit phosphorylation, thus inhibiting NF-kappaB-induced nuclear translocation. Guanosine effects were mediated by an increased phosphorylation of Akt, a PI3K downstream effector, as well as of ERK1/2 and p38 in the MAPK system, because culture pretreatment with selective ERK1/2, p38 MAPK, and PI3K antagonists (U0126, SB203580, or LY294002, respectively) counteracted guanosine inhibition on IFN-gamma/TNF-alpha-induced CD40 expression and function as well as on STAT-1alpha or NF-kappaB nuclear translocation. These findings suggest a role for guanosine as a potential drug in the experimental therapy of neuroinflammatory/neurodegenerative diseases, particularly Alzheimer's disease.

Serine-phosphorylated STAT1 is a prosurvival factor in Wilms' tumor pathogenesis

Wilms' tumor (WT), one of the most common pediatric solid cancers, arises in the developing kidney as a result of genetic and epigenetic changes that lead to the abnormal proliferation and differentiation of the metanephric blastema. As activation of signal transducers and activators of transcription (STATs) plays an important role in the maintenance/growth and differentiation of the metanephric blastema, and constitutively activated STATs facilitate neoplastic behaviors of a variety of cancers, we hypothesized that dysregulation of STAT signaling may also contribute to WT pathogenesis. Accordingly, we evaluated STAT phosphorylation patterns in tumors and found that STAT1 was constitutively phosphorylated on serine 727 (S727) in 19 of 21 primary WT samples and two WT cell lines. An inactivating mutation of S727 to alanine reduced colony formation of WT cells in soft agar by more than 80% and induced apoptosis under conditions of growth stress. S727-phosphorylated STAT1 provided apoptotic resistance for WT cells via upregulation of expression of the heat-shock protein (HSP)27 and antiapoptotic protein myeloid cell leukemia (MCL)-1. The kinase responsible for STAT1 S727 phosphorylation in WT cells was identified based upon the use of selective inhibitors as protein kinase CK2, not p38, MAP-kinase kinase (MEK)1/2, phosphatidylinositol 3'-kinase, protein kinase C or Ca/calmodulin-dependent protein kinase II (CaMKII). The inhibition of CK2 blocked the anchorage-independent growth of WT cells and induced apoptosis under conditions of growth stress. Our findings suggest that serine-phosphorylated STAT1, as a downstream target of protein kinase CK2, plays a critical role in the pathogenesis of WT and possibly other neoplasms with similar STAT1 phosphorylation patterns.

Gene expression changes and molecular pathways mediating activity-dependent plasticity in visual cortex

Two key models for examining activity-dependent development of primary visual cortex (V1) involve either reduction of activity in both eyes via dark-rearing (DR) or imbalance of activity between the two eyes via monocular deprivation (MD). Combining DNA microarray analysis with computational approaches, RT-PCR, immunohistochemistry and physiological imaging, we find that DR leads to (i) upregulation of genes subserving synaptic transmission and electrical activity, consistent with a coordinated response of cortical neurons to reduction of visual drive, and (ii) downregulation of parvalbumin expression, implicating parvalbumin-expressing interneurons as underlying the delay in cortical maturation after DR. MD partially activates homeostatic mechanisms but differentially upregulates molecular pathways related to growth factors and neuronal degeneration, consistent with reorganization of connections after MD. Expression of a binding protein of insulin-like growth factor-1 (IGF1) is highly upregulated after MD, and exogenous application of IGF1 prevents the physiological effects of MD on ocular dominance plasticity examined in vivo.

Stat1-dependent synergistic activation of T-bet for IgG2a production during early stage of B cell activation

During the adaptive phase of an immune response, naive B cells receive multiple signals to become activated. Among them are the engagement of the B cell Ag receptor and stimulation by cytokines. Specifically for an anti-microbial response, the recognition of viral or bacterial Ags by the BCR and the stimulation of IFN-gamma result in the predominant production of IgG2a. The T-bet protein has been shown to be required for class switching to IgG2a. In this report we further investigated the regulation of T-bet gene expression during the early stage of B cell activation. We show that there is a striking synergistic activation of T-bet in primary B cells when both the BCR and IFN-gamma signaling pathways are activated. The synergistic activation of T-bet correlates with a 100% increase in the number of B cells that produce IgG2a. This transcription synergy on T-bet is transient in the first 24 h of B cell activation. Furthermore, we demonstrate that the synergistic activation of T-bet is dependent on Stat1 and that Stat1 is required for the IgG2a germline transcription and the production of IgG2a in response to the simultaneous signaling of BCR and IFN-gamma. Finally, we show that Stat1 directly regulates the expression of T-bet by binding to the T-bet promoter. These results reveal the mechanism of regulation of T-bet expression and uncover a novel physiological function of Stat1 for B cell activation.