Cross-talk between the Toll-like receptor 4 and Notch1 pathways augments the inflammatory response in the interstitial cells of stenotic human aortic valves

BACKGROUND AND PURPOSE

Calcific aortic stenosis is a chronic inflammatory disease, and aortic valve interstitial cells (AVIC) play an important role in valvular inflammation. Whereas AVIC from stenotic aortic valves exhibit an augmented response to Toll-like receptor 4 (TLR4) stimulation, the underlying mechanism is unclear. This study tested the hypothesis that an excessive cross-talk between the TLR4 and Notch1 pathways is responsible for augmentation of the inflammatory response to lipopolysaccharide (LPS) in AVIC of stenotic valves.

METHODS AND RESULTS

Human AVIC were isolated from normal and stenotic leaflets. Nuclear factor kappa-B (NF-κB) activation and production of interleukin-8, monocyte chemoattactrant protein-1, and intercellular adhesion molecule-1 were analyzed after treatment with LPS. The role of Notch1 in the inflammatory response was determined using inhibitor, siRNA, and specific ligand. Cells from diseased valves produced greater levels of chemokines and intercellular adhesion molecule-1 that are associated with enhanced NF-κB activation. Interestingly, diseased cells exhibited augmented Jagged1 release and Notch1 activation after TLR4 stimulation. Inhibition and silencing of Notch1 each resulted in greater suppression of the TLR4-induced inflammatory response in diseased cells. Conversely, activation of Notch1 with a specific ligand, Jagged1, enhanced the LPS-induced inflammatory response in normal AVIC. Further, Notch1 intracellular domain was coimmunoprecipited with the inhibitor of NF-κB kinase after LPS stimulation, and inhibition of Notch1 abrogated the difference in the level of NF-κB activation between diseased and normal cells.

CONCLUSION

Notch1 enhances the inflammatory response to TLR4 stimulation in human AVIC through modulating NF-κB activation. Excessive cross-talk between the TLR4 and Notch1 pathways is responsible for augmentation of the TLR4 response in AVIC of stenotic valves.

Intestinal epithelial Toll-like receptor 4 regulates goblet cell development and is required for necrotizing enterocolitis in mice

BACKGROUND & AIMS

Little is known about factors that regulate intestinal epithelial differentiation; microbial recognition receptors such as Toll-like receptor (TLR)4 might be involved. We investigated whether intestinal TLR4 regulates epithelial differentiation and is involved in development of necrotizing enterocolitis (NEC) of the immature intestine.

METHODS

Mice with conditional disruption of TLR4 in the intestinal epithelium and TLR4 knockout (TLR4(-/-)) mice were generated by breeding TLR4(loxp/loxp) mice with villin-cre and Ella-cre, respectively. Enterocytes that did not express or overexpressed TLR4 were created by lentiviral or adenoviral transduction. Intestinal organoids were cultured on tissue matrices. Bile acids were measured by colorimetric assays, and microbial composition was determined by 16S pyrosequencing. NEC was induced in 7- to 10-day-old mice by induction of hypoxia twice daily for 4 days.

RESULTS

TLR4(-/-) mice and mice with enterocyte-specific deletion of TLR4 were protected from NEC; epithelial differentiation into goblet cells was increased via suppressed Notch signaling in the small intestinal epithelium. TLR4 also regulates differentiation of goblet cells in intestinal organoid and enterocyte cell cultures; differentiation was increased on deletion of TLR4 and restored when TLR4 was expressed ectopically. TLR4 signaling via Notch was increased in intestinal tissue samples from patients with NEC, and numbers of goblet cells were reduced. 16S pyrosequencing revealed that wild-type and TLR4-deficient mice had similar microbial profiles; increased numbers of goblet cells were observed in mice given antibiotics. TLR4 deficiency reduced levels of luminal bile acids in vivo, and addition of bile acids to TLR4-deficient cell cultures prevented differentiation of goblet cells.

CONCLUSIONS

TLR4 signaling and Notch are increased in intestinal tissues of patients with NEC and required for induction of NEC in mice. TLR4 prevents goblet cell differentiation, independently of the microbiota. Bile acids might initiate goblet cell development.

Noncanonical Notch signaling modulates cytokine responses of dendritic cells to inflammatory stimuli

Dendritic cell (DC)-derived cytokines play a key role in specifying adaptive immune responses tailored to the type of pathogen encountered and the local tissue environment. However, little is known about how DCs perceive the local environment. We investigated whether endogenous Notch signaling could affect DC responses to pathogenic stimuli. We demonstrate that concurrent Notch and TLR stimulation results in a unique cytokine profile in mouse bone-marrow derived DCs characterized by enhanced IL-10 and IL-2, and reduced IL-12 expression compared with TLR ligation alone. Unexpectedly, modulation of cytokine production occurred through a noncanonical Notch signaling pathway, independent of γ-secretase activity. Modulation required de novo protein synthesis, and PI3K, JNK, and ERK activity were necessary for enhanced IL-2 expression, whereas modulation of IL-10 required only PI3K activity. Further, we show that this γ-secretase-independent Notch pathway can induce PI3K activity. In contrast, expression of the canonical Notch target gene Hes1 was suppressed in DCs stimulated with Notch and TLR ligands simultaneously. Thus, our data suggest that Notch acts as an endogenous signal that modulates cytokine expression of DCs through a noncanonical pathway and therefore has the potential to tailor the subsequent adaptive immune response in a tissue- and/or stage-dependent manner.

Notch ligand delta-like 4 blockade attenuates atherosclerosis and metabolic disorders

Atherosclerosis and insulin resistance are major components of the cardiometabolic syndrome, a global health threat associated with a systemic inflammatory state. Notch signaling regulates tissue development and participates in innate and adaptive immunity in adults. The role of Notch signaling in cardiometabolic inflammation, however, remains obscure. We noted that a high-fat, high-cholesterol diet increased expression of the Notch ligand Delta-like 4 (Dll4) in atheromata and fat tissue in LDL-receptor-deficient mice. Blockade of Dll4-Notch signaling using neutralizing anti-Dll4 antibody attenuated the development of atherosclerosis, diminished plaque calcification, improved insulin resistance, and decreased fat accumulation. These changes were accompanied by decreased macrophage accumulation, diminished expression of monocyte chemoattractant protein-1 (MCP-1), and lower levels of nuclear factor-κB (NF-κB) activation. In vitro cell culture experiments revealed that Dll4-mediated Notch signaling increases MCP-1 expression via NF-κB, providing a possible mechanism for in vivo effects. Furthermore, Dll4 skewed macrophages toward a proinflammatory phenotype ("M1"). These results suggest that Dll4-Notch signaling plays a central role in the shared mechanism for the pathogenesis of cardiometabolic disorders.

Involvement of Notch signaling pathway in regulating IL-12 expression via c-Rel in activated macrophages

Macrophages play an important role both in innate and adaptive immune responses. Treatment with interferon (IFN) γ together with lipopolysaccharide (LPS) activates pro-inflammatory macrophages which secrete various pro-inflammatory cytokines including IL-12. IL-12 promotes a Th1 type immune response by directly controlling the differentiation of CD4(+) T helper 1 cells. Activation of Notch signaling pathway was reported in activated macrophages but the involvement of this signaling pathway in IL-12 expression has not been documented. In this study, we investigated the role of Notch signaling in regulating expression of the IL-12/IL-23 subunit, IL-12p40. Using a gamma-secretase inhibitor (GSI) to inhibit Notch signaling, we observed a profound decrease in il12p40 mRNA levels and IL-12p70 secretion upon IFNγ/LPS stimulation. On the other hand, overexpression of activated form of Notch1 in activated RAW264.7 macrophage-like cell lines significantly increased the level of il12p40 mRNA. GSI treatment did not affect the expression of irf5, a master regulator of il12p40 transcription in macrophages. Detailed analysis of the signaling cascades that were affected by this inhibition showed that c-Rel nuclear translocation was inhibited and Erk1/2 activation was compromised by GSI treatment. Addition of exogenous tumor necrosis factor (TNF) α only partially rescued the expression of il12p40 in the presence of GSI. Unexpectedly, inhibition of Notch signaling using a dominant negative (DN) Mastermind-like (MAML) transcription co-activator, did not affect c-Rel nuclear localization upon activation or il12p40 mRNA levels, suggesting that the transcriptional activity of Notch signaling is dispensable for the activation of c-Rel. These results strongly suggest that Notch signaling in activated macrophages is involved in regulating the expression of il12p40 directly via c-Rel and indirectly via TNFα production.

MiR-34a inhibits lipopolysaccharide-induced inflammatory response through targeting Notch1 in murine macrophages

Inflammatory responses are complex events occurring when the host immune system fights against invading pathogens, which are double-edged swords requiring appropriate control. MicroRNAs (miRNAs), emerging as a new layer of gene-regulation mechanism, have been reported to have crucial effects on inflammation. In the current study, we identified miR-34a, previously known for its potent tumor suppressive role, to be a novel inflammation regulator. We found that the expression of miR-34a was downregulated in macrophages after lipopolysaccharide (LPS) stimulation. MiR-34a mimics decreased, while the inhibition of miR-34a increased, the expression of inflammatory cytokines tumor necrosis factor-<alpha> (TNF-<alpha>) and interleukin-6 (IL-6) in LPS treated RAW264.7 cells. Bioinformatics predictions revealed a potential binding site of miR-34a in 3' untranslated region (UTR) of Notch1 and it was further confirmed by luciferase assay. Moreover, both the mRNA and protein level of Notch1 were downregulated by miR-34a in RAW264.7. Subsequently, knockdown of Notch1 with either genetic or pharmacological inhibition exhibited similar effects as miR-34a mimics on LPS-induced macrophage inflammatory response. Furthermore, the NF-κB activation induced by LPS was also significantly suppressed by miR-34a. These results together identify, for the first time, miR-34a as a negative regulator in LPS-induced inflammation at least partially by targeting Notch1. Besides extending the knowledge of miR-34a from tumor suppressor to inflammation regulator, this study also provides an implication that compounds which can enhance miR-34a expression or miR-34a itself may hold a promise in anti-inflammatory drugs development.

Notch signal suppresses Toll-like receptor-triggered inflammatory responses in macrophages by inhibiting extracellular signal-regulated kinase 1/2-mediated nuclear factor κB activation

Multiple signaling pathways are involved in the tight regulation of Toll-like receptor (TLR) signaling, which is important for the tailoring of inflammatory response to pathogens in macrophages. It is widely accepted that TLR signaling can activate Notch pathway; however, whether full activation of Notch signaling can feedback modulate TLR signaling pathway so as to control inflammation response remains unclear. Here, we demonstrated that stimulation with TLR ligands up-regulated Notch1 and Notch2 expression in macrophages. The expression of Notch target genes including Hes1 and Hes5 was also induced in macrophages by LPS, suggesting that TLR4 signaling enhances the activation of Notch pathway. Importantly, overexpression of constituted active form of Notch1 (NICD1) and Notch2 (NICD2) suppressed production of TLR4-triggered proinflammatory cytokines such as TNF-α and IL-6 but promoted production of antiinflammatory cytokine IL-10, which is dependent on the PEST domain of NICD. In addition, NICD1 and NICD2 suppressed TLR-triggered ERK phosphorylation, which is indispensable for Notch-mediated inhibition of TLR4-triggered proinflammatory cytokine production. Furthermore, activation of Notch signaling inhibited NF-κB transcription activity by MyD88/TRAF6 and TRIF pathways, which was dependent on ERK activity. Therefore, our results showed that Notch signaling negatively regulates TLR-triggered inflammation responses, revealing a new mechanism for negative regulation of TLR signaling via Notch pathway.

Monocyte to macrophage differentiation-associated (MMD) positively regulates ERK and Akt activation and TNF-α and NO production in macrophages

Macrophage activation is modulated by both environmental cues and endogenous programs. In the present study, we investigated the role of a PAQR family protein, monocyte to macrophage differentiation-associated (MMD), in macrophage activation and unveiled its underlying molecular mechanism. Our results showed that while MMD expression could be detected in all tissues examined, its expression level is significantly up-regulated upon monocyte differentiation. Within cells, EGFP-MMD fusion protein could be co-localized to endoplasmic reticulum, mitochondria, Golgi apparatus, but not lysosomes and cytoplasm. MMD expression is up-regulated in macrophages after LPS stimulation, and this might be modulated by RBP-J, the critical transcription factor of Notch signaling. Overexpression of MMD in macrophages increased the production of TNF-α and NO upon LPS stimulation. We found that MMD overexpression enhanced ERK1/2 and Akt phosphorylation in macrophages after LPS stimulation. Blocking Erk or Akt by pharmacological agent reduced TNF-α or NO production in MMD-overexpressing macrophages, respectively. These results suggested that MMD modulates TNF-α and NO production in macrophages, and this process might involves Erk or Akt.

Crosstalk between the canonical NF-κB and Notch signaling pathways inhibits Pparγ expression and promotes pancreatic cancer progression in mice

The majority of human pancreatic cancers have activating mutations in the KRAS proto-oncogene. These mutations result in increased activity of the NF-κB pathway and the subsequent constitutive production of proinflammatory cytokines. Here, we show that inhibitor of κB kinase 2 (Ikk2), a component of the canonical NF-κB signaling pathway, synergizes with basal Notch signaling to upregulate transcription of primary Notch target genes, resulting in suppression of antiinflammatory protein expression and promotion of pancreatic carcinogenesis in mice. We found that in the Kras(G12D)Pdx1-cre mouse model of pancreatic cancer, genetic deletion of Ikk2 in initiated pre-malignant epithelial cells substantially delayed pancreatic oncogenesis and resulted in downregulation of the classical Notch target genes Hes1 and Hey1. Tnf-α stimulated canonical NF-κB signaling and, in collaboration with basal Notch signals, induced optimal expression of Notch targets. Mechanistically, Tnf-α stimulation resulted in phosphorylation of histone H3 at the Hes1 promoter, and this signal was lost with Ikk2 deletion. Hes1 suppresses expression of Pparg, which encodes the antiinflammatory nuclear receptor Pparγ. Thus, crosstalk between Tnf-α/Ikk2 and Notch sustains the intrinsic inflammatory profile of transformed cells. These findings reveal what we believe to be a novel interaction between oncogenic inflammation and a major cell fate pathway and show how these pathways can cooperate to promote cancer progression.

Direct regulation of interleukin-6 expression by Notch signaling in macrophages

Interleukin-6 (IL-6) is a pleiotropic, pro-inflammatory cytokine produced by various types of cells, including macrophages. Within the IL-6 gene promoter region, the signature binding motif of CBF1/Su(H)/Lag-1 (CSL), a key DNA-binding protein in the Notch signaling pathway, was identified and found to overlap with a consensus nuclear factor (NF)-κB-binding site. Notch signaling is highly conserved and is involved in the regulation of biological functions in immune cells. In this study, we investigated the role of Notch signaling in the regulation of the IL-6 transcript in murine macrophages. The upregulation of Notch1 protein levels and the appearance of cleaved Notch1 (Val1744) correlated well with the increased IL-6 mRNA expression levels in murine primary bone marrow-derived macrophages (BMMφ) after activation by lipopolysaccharide (LPS) together with interferon-gamma (IFN-γ). Treatment of BMMφ with the γ-secretase inhibitor IL-CHO to suppress the transduction of Notch signaling resulted in a partial decrease in the level of IL-6 mRNA and the amount of IL-6 protein produced. In contrast, the overexpression of a constitutively activated intracellular Notch1 protein (N(IC)) in the RAW264.7 macrophage-like cell line resulted in significantly higher IL-6 transcript expression levels than in cells transfected with the empty vector control. The NF-κB inhibitor completely abrogated IL-6 mRNA expression induced by the overexpression of N(IC). Chromatin immunoprecipitation (ChIP) using an anti-Notch1 antibody demonstrated that Notch1 is associated with the IL-6 promoter in RAW264.7 cells activated by LPS/IFN-γ but not in unstimulated cells. Taken together, these results strongly suggest that Notch1 positively regulates IL-6 expression via NF-κB in activated macrophages.

Effects of Toll-like receptor signals in T-cell neoplasms

T-cell neoplasms have poor prognosis and few effective therapeutic options. Therefore, identification of factors in T-cell leukemia/lymphoma that are associated with cancer progression may represent novel therapeutic targets. Recent studies have highlighted a previously unappreciated role for the expression of Toll-like receptors (TLRs) on T cells and their effects on cell survival and proliferation. TLRs can bind exogenous molecules derived from pathogens as well as endogenous self-ligands released from damaged cells. Recent reports demonstrate that TLR engagement on primary mouse or human T cells enhances proliferation and/or cell survival. The mechanisms by which TLR stimulation on T cells influences these parameters and the different T-cell subsets that are affected by TLR stimulation are currently under investigation. Furthermore, neither the biological importance of stimulating TLRs on neoplastic T cells nor the prevalence of TLR expression in T-cell malignancies have yet to be characterized. Based on published reports and compelling preliminary data, we propose that the activation of the TLR-MyD88 signaling pathway in neoplastic T cells contributes to disease progression by reducing cell death and enhancing cell division. In this article, we present both theoretical arguments and experimental data in support of this hypothesis.

Notch Activation Enhances the Microglia-Mediated Inflammatory Response Associated With Focal Cerebral Ischemia

Background and Purpose—

Activation of Notch worsens ischemic brain damage as antisense knockdown or pharmacological inhibition of the Notch pathway reduces the infarct size and improves the functional outcome in a mouse model of stroke. We sought to determine whether Notch activation contributes to postischemic inflammation by directly modulating the microglial innate response.

Methods—

The microglial response and the attendant inflammatory reaction were evaluated in Notch1 antisense transgenic (Tg) and in nontransgenic (non-Tg) mice subjected to middle cerebral artery occlusion with or without treatment with a γ-secretase inhibitor (GSI). To investigate the impact of Notch on microglial effector functions, primary mouse microglia and murine BV-2 microglial cell line were exposed to oxygen glucose deprivation or lipopolysaccharide in the presence or absence of GSI. Immunofluorescence labeling, Western blotting, and reverse-transcription polymerase chain reaction were performed to measure microglial activation and production of inflammatory cytokines. The nuclear translocation of nuclear factor-κB in microglia was assessed by immunohistochemistry. The neurotoxic potential of microglia was determined in cocultures.

Results—

Notch1 antisense mice exhibit significantly lower numbers of activated microglia and reduced proinflammatory cytokine expression in the ipsilateral ischemic cortices compared to non-Tg mice. Microglial activation also was attenuated in Notch1 antisense cultures and in non-Tg cultures treated with GSI. GSI significantly reduced nuclear factor-κB activation and expression of proinflammatory mediators and markedly attenuated the neurotoxic activity of microglia in cocultures.

Conclusions—

These findings establish a role for Notch signaling in modulating the microglia innate response and suggest that inhibition of Notch might represent a complementary therapeutic approach to prevent reactive gliosis in stroke and neuroinflammation-related degenerative disorders.

Lipopolysaccharide-induced Notch signaling activation through JNK-dependent pathway regulates inflammatory response

BACKGROUND

Notch and TLR pathways were found to act cooperatively to activate Notch target genes and to increase the production of TLR-induced cytokines in macrophages. However, the mechanism of LPS-induced Notch activation and its role in sepsis still remains unclear.

METHODS

We analyzed the expression patterns of Notch components in a LPS-stimulated murine macrophage cell line using real-time PCR and western blotting. The role of DAPT, a gamma-secretase inhibitor that is known to be a potent Notch inhibitor, in LPS-induced cytokine release and experimental sepsis in mice was also explored. Student's t-test was used to analyze the difference between the two groups.

RESULTS

We found that Notch signaling was activated after LPS stimulation. The expression of Jagged 1, a Notch ligand, induced by LPS occurred in a JNK-dependent manner. In addition, Notch target genes were upregulated by early Notch-independent activation followed by delayed Notch-dependent activation after LPS stimulation. Disruption of Notch signaling by DAPT attenuated the LPS-induced inflammatory responses, including vascular endothelial growth factor (VEGF) and high-mobility group box chromosomal protein 1 (HMGB1), both in vitro and in vivo and partially improved experimental sepsis survival.

CONCLUSIONS

These findings support the existence of a synergistic effect of Notch signaling and the LPS pathway both in vitro and in vivo. Therefore, in the future Notch inhibitors may be utilized as adjunctive agents for the treatment of sepsis syndrome.

Cooperative regulation of NOTCH1 protein-phosphatidylinositol 3-kinase (PI3K) signaling by NOD1, NOD2, and TLR2 receptors renders enhanced refractoriness to transforming growth factor-beta (TGF-beta)- or cytotoxic T-lymphocyte antigen 4 (CTLA-4)-mediated impairment of human dendritic cell maturation

Dendritic cells (DCs) as sentinels of the immune system are important for eliciting both primary and secondary immune responses to a plethora of microbial pathogens. Cooperative stimulation of a complex set of pattern-recognition receptors, including TLR2 and nucleotide-binding oligomerization domain (NOD)-like receptors on DCs, acts as a rate-limiting factor in determining the initiation and mounting of the robust immune response. It underscores the need for "decoding" these multiple receptor interactions. In this study, we demonstrate that TLR2 and NOD receptors cooperatively regulate functional maturation of human DCs. Intriguingly, synergistic stimulation of TLR2 and NOD receptors renders enhanced refractoriness to TGF-β- or CTLA-4-mediated impairment of human DC maturation. Signaling perturbation data suggest that NOTCH1-PI3K signaling dynamics assume critical importance in TLR2- and NOD receptor-mediated surmounting of CTLA-4- and TGF-β-suppressed maturation of human DCs. Interestingly, the NOTCH1-PI3K signaling axis holds the capacity to regulate DC functions by virtue of PKCδ-MAPK-dependent activation of NF-κB. This study provides mechanistic and functional insights into TLR2- and NOD receptor-mediated regulation of DC functions and unravels NOTCH1-PI3K as a signaling cohort for TLR2 and NOD receptors. These findings serve in building a conceptual foundation for the design of improved strategies for adjuvants and immunotherapies against infectious diseases.

Ligation of Notch receptors in human conventional and plasmacytoid dendritic cells differentially regulates cytokine and chemokine secretion and modulates Th cell polarization

Notch signaling is involved in multiple cellular processes. Recent data also support the prominent role of Notch signaling in the regulation of the immune response. In this study, we analyzed the expression and function of Notch receptors and ligands on both human blood conventional dendritic cells (cDCs) and plasmacytoid DCs (pDCs). The expression and modulation upon TLR activation of Notch molecules partially differed between cDCs and pDCs, but functional involvement of the Notch pathway in both cell types was clearly revealed by specific inhibition using DAPT. Beyond the induction of Notch target genes and modulation of maturation markers, Notch pathway was also involved in a differential secretion of some specific cytokines/chemokines by DC subsets. Whereas Notch ligation induced IL-10 and CCL19 secretion in cDCs, Notch inhibition resulted in a diminished production of these proteins. With regard to pDCs, Notch activation induced TNF-α whereas Notch inhibition significantly abrogated the secretion of CCL19, CXCL9, CXCL10, and TNF-α. Additionally, Notch modulation of DC subsets differentially affected Th polarization of allostimulated T cells. Our results suggest that the Notch pathway may function as an additional mechanism controlling human DC responses, with differential activity on cDCs and pDCs. This control mechanism may ultimately contribute to define the local milieu promoted by these cells under the particular conditions of the immune response.

Innate immune responses to M. tuberculosis infection

A prerequisite for successful establishment of Mycobacterium tuberculosis in the host is its ability to survive after internalization in alveolar macrophages that they encounter after inhalation. The innate immune response protects some individuals to the extent that they remain uninfected. In others, the innate immune system is not sufficient and an adaptive immune response is generated. This is usually protective, but not sterilizing, and individuals remain latently infected. In susceptible individuals, M. tuberculosis successfully escapes immune surveillance. The interplay between the host innate immune response and the bacterial mechanisms in play to offset this response, is of considerable importance in dictating the course of the disease. In order to gain an understanding of this interplay it is of importance to analyze how M. tuberculosis interacts with innate immune receptors and makes its entry into macrophages, how it subverts the bactericidal effects of macrophages, and dampens processes required for protective immunity, including cytokine and chemokine induction. This review will focus on some of the Indian efforts in these areas, concentrating mainly on the interaction of M. tuberculosis with macrophages and dendritic cells (DCs). The role of the PE/PPE family of proteins in regulating the immune response, will not be discussed in this chapter. The genome-wide approaches of analyzing host-M. tuberculosis interactions will also be discussed elsewhere.

Presenilin 2 is the predominant γ-secretase in microglia and modulates cytokine release

Presenilin 1 (PS1) and Presenilin 2 (PS2) are the enzymatic component of the γ-secretase complex that cleaves amyloid precursor protein (APP) to release amyloid beta (Aβ) peptide. PS deficiency in mice results in neuroinflammation and neurodegeneration in the absence of accumulated Aβ. We hypothesize that PS influences neuroinflammation through its γ-secretase action in CNS innate immune cells. We exposed primary murine microglia to a pharmacological γ-secretase inhibitor which resulted in exaggerated release of TNFα and IL-6 in response to lipopolysaccharide. To determine if this response was mediated by PS1, PS2 or both we used shRNA to knockdown each PS in a murine microglia cell line. Knockdown of PS1 did not lead to decreased γ-secretase activity while PS2 knockdown caused markedly decreased γ-secretase activity. Augmented proinflammatory cytokine release was observed after knockdown of PS2 but not PS1. Proinflammatory stimuli increased microglial PS2 gene transcription and protein in vitro. This is the first demonstration that PS2 regulates CNS innate immunity. Taken together, our findings suggest that PS2 is the predominant γ-secretase in microglia and modulates release of proinflammatory cytokines. We propose PS2 may participate in a negative feedback loop regulating inflammatory behavior in microglia.

Targeting ADAM-17/notch signaling abrogates the development of systemic sclerosis in a murine model

OBJECTIVE

Systemic sclerosis (SSc) is characterized by the fibrosis of various organs, vascular hyperreactivity, and immunologic dysregulation. Since Notch signaling is known to affect fibroblast homeostasis, angiogenesis, and lymphocyte development, we undertook this study to investigate the role of the Notch pathway in human and murine SSc.

METHODS

SSc was induced in BALB/c mice by subcutaneous injections of HOCl every day for 6 weeks. Notch activation was analyzed in tissues from mice with SSc and from patients with scleroderma. Mice with SSc were either treated or not treated with the γ-secretase inhibitor DAPT, a specific inhibitor of the Notch pathway, and the severity of the disease was evaluated.

RESULTS

As previously described, mice exposed to HOCl developed a diffuse cutaneous SSc with pulmonary fibrosis and anti-DNA topoisomerase I antibodies. The Notch pathway was hyperactivated in the skin, lung, fibroblasts, and splenocytes of diseased mice and in skin biopsy samples from patients with scleroderma. ADAM-17, a proteinase involved in Notch activation, was overexpressed in the skin of mice and patients in response to the local production of reactive oxygen species. In HOCl-injected mice, DAPT significantly reduced the development of skin and lung fibrosis, decreased skin fibroblast proliferation and ex vivo serum-induced endothelial H(2)O(2) production, and abrogated the production of anti-DNA topoisomerase I antibodies.

CONCLUSION

Our results show the pivotal role of the ADAM-17/Notch pathway in SSc following activation by reactive oxygen species. The inhibition of this pathway may represent a new treatment of this life-threatening disease.

Notch ligand delta-like 4 regulates development and pathogenesis of allergic airway responses by modulating IL-2 production and Th2 immunity

Activation of the canonical Notch pathways has been implicated in Th cell differentiation, but the role of specific Notch ligands in Th2-mediated allergic airway responses has not been completely elucidated. In this study, we show that delta-like ligand 4 (Dll4) was upregulated on dendritic cells in response to cockroach allergen. Blocking Dll4 in vivo during either the primary or secondary response enhanced allergen-induced pathogenic consequences including airway hyperresponsiveness and mucus production via increased Th2 cytokines. In vitro assays demonstrated that Dll4 regulates IL-2 in T cells from established Th2 responses as well as during primary stimulation. Notably, Dll4 blockade during the primary, but not the secondary, response increased IL-2 levels in lung and lymph node of allergic mice. The in vivo neutralization of Dll4 was associated with increased expansion and decreased apoptosis during the primary allergen sensitization. Moreover, Dll4-mediated Notch activation of T cells during primary stimulation in vitro increased apoptosis during the contraction/resting phase of the response, which could be rescued by exogenous IL-2. Consistent with the role for Dll4-mediated IL-2 regulation in overall T cell function, the frequency of IL-4-producing cells was also significantly altered by Dll4 both in vivo and in vitro. These data demonstrate a regulatory role of Dll4 both in initial Th2 differentiation and in Th2 cytokine production in established allergic responses.

Autoamplification of Notch signaling in macrophages by TLR-induced and RBP-J-dependent induction of Jagged1

Several signaling pathways, including the Notch pathway, can modulate TLR activation to achieve responses most appropriate for the environment. One mechanism of TLR-Notch cross-talk is TLR-induced expression of Notch ligands Jagged and Delta that feed back to engage Notch receptors on TLR-activated cells. In this study, we investigated mechanisms by which TLRs induce Notch ligand expression in primary macrophages. TLRs induced Jagged1 expression rapidly and independently of new protein synthesis. Jagged1 induction was augmented by IFN-γ, was partially dependent on canonical TLR-activated NF-κB and MAPK signaling pathways, and elevated Jagged1 expression augmented TLR-induced IL-6 production. Strikingly, TLR-induced Jagged1 expression was strongly dependent on the Notch master transcriptional regulator RBP-J and also on upstream components of the Notch pathway γ-secretase and Notch1 and Notch2 receptors. Thus, Jagged1 is an RBP-J target gene that is activated in a binary manner by TLR and Notch pathways. Early and direct cooperation between TLR and Notch pathways leads to Jagged1-RBP-J-mediated autoamplification of Notch signaling that can modulate later phases of the TLR response.

Notch1 deficiency results in decreased inflammation during wound healing and regulates vascular endothelial growth factor receptor-1 and inflammatory cytokine expression in macrophages

We investigated whether Notch signaling plays a role in regulating macrophage responses to inflammation. In a wound healing assay, macrophage recruitment was decreased in Notch1(+/-) mice, and the wounds were characterized by decreased TNF-α expression. As wound healing progressed, Notch1(+/-) wounds had increased vascularization and collagen deposition compared with wild-type wounds. In mice with myeloid-specific Notch1 deletion, wounds had decreased macrophage recruitment as well as decreased TNF-α expression, indicating the specific role of Notch1 in the inflammatory response in these cells. In vitro, we found that vascular endothelial growth factor receptor-1 (VEGFR-1) was upregulated in macrophages in response to LPS/IFN-γ and that this upregulation depended on Notch signaling. Furthermore, macrophages from Notch1(+/-) mice had decreased expression of VEGFR-1 compared with macrophages from wild-type mice, whereas VEGFR-1 expression in Notch4(-/-) macrophages was normal. Inhibition of Notch signaling decreased induction of the inflammatory cytokines IL-6, IL-12, CXCL10, MCP-1, monokine induced by IFN-γ, and TNF-α in macrophages in response to LPS/IFN-γ. Additionally, macrophages from Notch1(+/-) mice demonstrated decreased induction of IL-6, IL-12, and TNF-α in response to stimulation compared with wild-type mice. Thus, both pharmacological inhibition of Notch and genetic analysis demonstrate that Notch1 regulates VEGFR-1 and cytokine expression in macrophages. We have also established that Notch1 is important for the inflammatory response during wound healing in mice.

Early-stage blocking of Notch signaling inhibits the depletion of goblet cells in dextran sodium sulfate-induced colitis in mice

BACKGROUND

Goblet cells, which contribute to mucosal defense and repair in the intestinal epithelium, are depleted in human and rodent colitis. The Notch signal pathway regulates the differentiation of intestinal stem cells into epithelial cells and inhibits the differentiation of secretory lineages, including goblet cells. The aim of our study was to clarify whether the blocking of the Notch pathway at an early stage of colitis would preserve goblet cells and facilitate the healing process in dextran sulfate sodium (DSS)-induced colitis in mice.

METHODS

DSS was orally administered to C57/BL6 mice for 7 days, and dibenzazepine (DBZ), a Notch pathway blocker, was administered for 5 consecutive days, beginning on the first day of DSS treatment. Colonic mucosal inflammation was evaluated clinically, biochemically, and histologically. The expression of the goblet cell-associated genes Math1 and MUC2 and proinflammatory cytokines was evaluated by real-time reverse-transcriptase-PCR, with the expression of Math1 and MUC2 also visualized by immunohistochemical examination.

RESULTS

The administration of DBZ at 4 mumol/kg significantly reduced the severity of the colitis. Compared with the DSS only-treated intestine, the number of goblet cells was relatively sustained, and the expression of Math1 and MUC2 was also elevated in the DSS/DBZ-treated intestine. DBZ treatment suppressed the mRNA levels for interleukin-1beta and -6, and matrix metalloproteinases-3 and -9 in the DSS-treated intestine.

CONCLUSIONS

Early-stage blocking of Notch signaling may ameliorate acute DSS colitis by preventing reduction in the number of goblet cells.

Notch signaling determines the M1 versus M2 polarization of macrophages in antitumor immune responses

Macrophages are important tumor-infiltrating cells and play pivotal roles in tumor growth and metastasis. Macrophages participate in immune responses to tumors in a polarized manner: classic M1 macrophages produce interleukin (IL) 12 to promote tumoricidal responses, whereas M2 macrophages produce IL10 and help tumor progression. The mechanisms governing macrophage polarization are unclear. Here, we show that the M2-like tumor-associated macrophages (TAM) have a lower level of Notch pathway activation in mouse tumor models. Forced activation of Notch signaling increased M1 macrophages which produce IL12, no matter whether M1 or M2 inducers were applied. When Notch signaling was blocked, the M1 inducers induced M2 response in the expense of M1. Macrophages deficient in canonical Notch signaling showed TAM phenotypes. Forced activation of Notch signaling in macrophages enhanced their antitumor capacity. We further show that RBP-J-mediated Notch signaling regulates the M1 versus M2 polarization through SOCS3. Therefore, Notch signaling plays critical roles in the determination of M1 versus M2 polarization of macrophages, and compromised Notch pathway activation will lead to the M2-like TAMs. These results provide new insights into the molecular mechanisms of macrophage polarization and shed light on new therapies for cancers through the modulation of macrophage polarization through the Notch signaling.

Blockade of Notch1 signaling alleviates murine lupus via blunting macrophage activation and M2b polarization

Patients with systemic lupus erythematosus (SLE) are found to be accompanied with innate immunity dysregulation including abnormally macrophage activation. But the functional polarization of the activated macrophages and its underlying molecular mechanism during the pathogenesis of SLE remains unknown. As an important local cellular interaction mechanism responsible for cell fate determination, Notch signaling is reported to exert crucial functions in the development and differentiation of various immunocytes, whereas its role in macrophage polarization is not fully understood. In this study, in the SLE murine model generated by immunization with activated lymphocyte-derived DNA (ALD-DNA), infiltrated macrophages in the nephritic tissues were found to exhibit activation and M2b functional polarization. Notch1 signaling activity was significantly upregulated in the ALD-DNA-induced M2b macrophages in vitro and in vivo. Furthermore, ALD-DNA-induced M2b polarization was found to be dependent on enhanced Notch1 signaling through accelerating NF-kappaB p50 translocation into the nucleus mediated by PI3K and MAPK pathways. Moreover, blockade of Notch1 signaling with gamma-secretase inhibitor treatment before or after the disease initiation could ameliorate murine lupus through impeding macrophage M2b polarization. Our results implied that Notch1 signaling-dependent macrophage M2b polarization might play a pivotal role in the pathogenesis of SLE, which could provide Notch1 signaling blockade as a potential therapeutic approach for SLE disease.

Notch signaling: key role in intrauterine infection/inflammation, embryonic development, and white matter damage?

The mechanisms or pathophysiologies that lead to cerebral white matter damage during development are complex and not fully understood. It is postulated that exposure of the preterm brain to inflammatory cytokines during intrauterine infection/inflammation contributes to brain white matter damage, and this damage may affect the function and differentiation of progenitor oligodendrocyte cells under physiological conditions. The Notch pathway, an important signaling pathway controlling various cells' differentiation, functions in the timing of oligodendrocyte differentiation, and Notch signaling may contribute to white matter damage and may mediate neurogenesis in a pathophysiological phase. Recent studies have led to recognition of the role of the Notch pathway in neurogenesis in cerebral ischemic damage and in myelination and axonal damage of neurodegenerative diseases. Moreover, Notch plays a critical role in steering an immune response toward inflammation by regulating expression of various cytokines and proinflammatory cytokines resulting in the activation of Notch signaling. Thus, the Notch signaling pathway likely plays a key role in intrauterine infection/inflammation, brain development, and white matter damage, and future research directed toward understanding its role will be important. Insofar as Notch signaling could have an important effect on neurogenesis, mobilization of progenitor cells is one strategy for compensating for the neuronal losses seen in white matter damage after intrauterine infection/inflammation.

Nitric oxide is involved in Mycobacterium bovis bacillus Calmette-Guérin-activated Jagged1 and Notch1 signaling

Pathogenic mycobacteria have evolved unique strategies to survive within the hostile environment of macrophages. Modulation of key signaling cascades by NO, generated by the host during infection, assumes critical importance in overall cell-fate decisions. We show that NO is a critical factor in Mycobacterium bovis bacillus Calmette-Guérin-mediated Notch1 activation, as the generation of activated Notch1 or expression of Notch1 target genes matrix metalloproteinase-9 (MMP-9) or Hes1 was abrogated in macrophages derived from inducible NO synthase (iNOS) knockout (iNOS(-/-)), but not from wild-type, mice. Interestingly, expression of the Notch1 ligand Jagged1 was compromised in M. bovis bacillus Calmette-Guérin-stimulated iNOS(-/-) macrophages, and loss of Jagged1 expression or Notch1 signaling could be rescued by NO donors. Signaling perturbations or genetic approaches implicated that robust expression of MMP-9 or Hes1 required synergy and cross talk between TLR2 and canonical Notch1-PI3K cascade. Further, CSL/RBP-Jk contributed to TLR2-mediated expression of MMP-9 or Hes1. Correlative evidence shows that, in a murine model for CNS tuberculosis, this mechanism operates in vivo only in brains derived from WT but not from iNOS(-/-) mice. Importantly, we demonstrate the activation of Notch1 signaling in vivo in granulomatous lesions in the brains of Mycobacterium tuberculosis-infected human patients with tuberculous meningitis. Current investigation identifies NO as a pathological link that modulates direct cooperation of TLR2 with Notch1-PI3K signaling or Jagged1 to regulate specific components of TLR2 responses. These findings provide new insights into mechanisms by which Notch1, TLR2, and NO signals are integrated in a cross talk that modulates a defined set of effector functions in macrophages.

Jagged1 regulates the activation of astrocytes via modulation of NFkappaB and JAK/STAT/SOCS pathways

The Notch pathway is implicated in many aspects of the central nervous system (CNS) development and functions. Recently, we and others identified the Notch pathway to be involved in inflammatory events of the CNS. To understand the implication of this pathway on astrocytes, we have studied the Jagged-Notch-Hes pathway under inflammatory conditions. LPS exposure induced an upregulation of Jagged1 expression on cultured astrocytes. To address the role of Jagged1 in the modulation of inflammation, we used a siRNA mediated silencing of Jagged1 (siRNA J1). Jagged1 inhibition induced important variations on the Notch pathway components like Hes1, Hes5, Notch3, and RBP-Jkappa. siRNA J1 repressed the mRNA expression of genes known as hallmarks of the gliosis like GFAP and endothelin(B) receptor. On activated astrocytes, the inhibition of Jagged1 had antiinflammatory effects and resulted in a decrease of LPS-induced proinflammatory cytokines (IL1beta, IL1alpha, and TNFalpha) as well as the iNOS expression. The inhibition of Jagged1 induced a modulation of the JAK/STAT/SOCS signaling pathway. Most interestingly, the siRNA J1 decreased the LPS-induced translocation of NFkappaB p65 and this could be correlated to the phosphorylation of IkappaBalpha. These results suggest that during inflammatory and gliotic events of the CNS, Jagged1/Notch signaling sustains the inflammation mainly through NFkappaB and in part through JAK/STAT/SOCS signaling pathways.

Notch signaling in the hematopoietic system

Because of its multiple effects in tissue homeostasis and cancer, Notch signaling is gaining increasing attention as a potential therapeutic target. Notch proteins belong to a family of highly conserved cell surface receptors. Ligand binding leads to proteolytic cleavage of Notch receptors by the gamma-secretase complex, followed by translocation of the active intracellular Notch domain into the nucleus and transcriptional activation. Multiple genetic and pharmacological methods are available to inhibit or activate the Notch pathway, some of which are entering human clinical trials. In this review, we discuss our current understanding of Notch signaling in the hematopoietic system. Canonical Notch signaling is essential for the generation of definitive embryonic hematopoietic stem cells, but dispensable for their maintenance during adult life. Notch controls several early steps of T cell development, as well as specific cell fate and differentiation decisions in other hematopoietic lineages. In addition, emerging evidence indicates that Notch is a potent, context-specific regulator of T cell immune responses, including in several disease models relevant to patients. This knowledge will constitute a framework to explore Notch modulation as a therapeutic strategy and to understand potential hematopoietic side effects of systemic Notch inhibition.

Transcriptional integration of TLR2 and TLR4 signaling at the NCoR derepression checkpoint

Activation of toll-like receptors (TLRs) leads to derepression and subsequent activation of inflammatory response genes that play essential roles in innate and acquired immunity. Derepression requires signal-dependent turnover of the nuclear receptor corepressor NCoR from target promoters, but the mechanisms remain poorly understood. Here, we report that TLR4 uses NFkappaB to deliver IKKepsilon to target promoters that contain "integrated circuits" of kappaB and AP-1 sites, resulting in local phosphorylation of c-Jun and subsequent NCoR clearance. In contrast, TLR2 signaling leads to rapid activation of CaMKII and phosphorylation of the TBLR1 component of NCoR complexes, bypassing the requirement for c-Jun phosphorylation and enabling NCoR clearance from promoters lacking integrated kappaB elements. Intriguingly, the IKKvarepsilon-dependent clearance pathway is sensitive to transrepression by liver X receptors, while the CaMKII-dependent pathway is not. These findings reveal mechanisms for integration of TLR, calcium, and nuclear receptor signaling pathways that underlie pathogen-specific responses and disease-specific programs of inflammation.

Notch: a new player in MS mechanisms

Notch is a family of four transmembrane receptors (Notch1-4) that orchestrate differentiation of various cell types, tissues and organs. Recent studies have revealed that Notch, among other processes, regulates immune responses of peripheral T cells, controls oligodendrocyte maturation and myelination of axons and under inflammatory conditions affects activation of macrophages and microglia. Therefore, Notch signaling has been implicated in the differentiation and function of all cell types considered crucial for the development and clinical progression of multiple sclerosis (MS). Importantly, inflammatory/demyelinating lesions in MS and its animal model, autoimmune experimental encephalomyelitis (EAE), abundantly express Notch receptors, their ligands and downstream activation targets. In EAE, in vivo modulation of Notch signaling affects immune responses of myelin-reactive T cells, enhances tissue repair and reduces clinical severity of the disease. In this review, we present recent findings on how Notch signaling may affect function of both immune and glial cells, analyze data implicating the Notch pathway in MS and EAE, and discuss the therapeutic potential of manipulating Notch signaling in MS patients.

Development and function of IL-10 IFN-gamma-secreting CD4(+) T cells

IL-10 IFN-gamma-secreting CD4(+) T cells were first found in the early 1990s. They are suppressive T cells able to inhibit cytotoxic T lymphocytes. These cells (Foxp3-T bet(+)) have a similar function but are distinct from conventional Tregs. The production of IL-10 in these cells requires IL-27 and TGF-beta and was regulated by several signal pathways including Notch, STAT, and NF-kappaB. The crosstalk among these pathways is critical for the generation and function of these cells. IL-10 IFN-gamma-secreting CD4(+) T cells are activated in chronic infection and are responsible for prolonged infection. Thus, their modulation has therapeutic implications for the treatment of infectious diseases. However, it is complicated, and fine-tuning of IFN-gamma and IL-10 secretion by these cells is needed for disease management, as inhibition of these cells will also lead to overimmune responses. On the other hand, increasing their numbers in autoimmune diseases may have beneficial effects.

CpG oligodeoxynucleotide triggers the liver inflammatory reaction and abrogates spontaneous tolerance

Liver allografts are spontaneously accepted in the liver transplantation mouse model; however, the basis for this tolerance and the conditions that abrogate spontaneous tolerance to liver allografts are incompletely understood. We examined the role of CpG oligodeoxynucleotide (ODN) in triggering the liver inflammatory reaction and allograft rejection. Bioluminescence imaging quantified the activation of nuclear transcriptional factor kappaB (NF-kappaB) at different time points post-transplantation. Intrahepatic lymphocyte subsets were analyzed by immunofluorescence assay and flow cytometry. The results showed that liver allografts survived for more than 100 days without a requirement for any immunosuppressive therapy. Donor-matched cardiac allografts were permanently accepted, whereas third-party cardiac grafts were rejected with delayed kinetics; this confirmed donor-specific tolerance. NF-kappaB activation in the liver allografts was transiently increased on day 1 and diminished by day 4; in comparison, it was elevated up to 10 days post-transplantation in the cardiac allografts. When CpG ODN was administered at a high dose (50 microg per mouse x 1) to the recipients on day 7 post-transplantation, it induced an acute liver inflammatory reaction with elevated NF-kappaB activation in both allogeneic and syngeneic liver grafts. Multiple doses of CpG ODN (10 microg per mouse x 3) elicited acute rejection of the liver allografts with significant T cell infiltration in the liver allografts, reduced T regulatory cells, and enhanced interferon gamma-producing cells in the intrahepatic infiltrating lymphocytes. These data demonstrate that CpG ODN initiates an inflammatory reaction and abrogates spontaneous tolerance in the liver transplantation mouse model. Liver Transpl 15:915-923, 2009. (c) 2009 AASLD.

PIM2 Induced COX-2 and MMP-9 expression in macrophages requires PI3K and Notch1 signaling

Activation of inflammatory immune responses during granuloma formation by the host upon infection of mycobacteria is one of the crucial steps that is often associated with tissue remodeling and breakdown of the extracellular matrix. In these complex processes, cyclooxygenase-2 (COX-2) plays a major role in chronic inflammation and matrix metalloproteinase-9 (MMP-9) significantly in tissue remodeling. In this study, we investigated the molecular mechanisms underlying Phosphatidyl-myo-inositol dimannosides (PIM2), an integral component of the mycobacterial envelope, triggered COX-2 and MMP-9 expression in macrophages. PIM2 triggers the activation of Phosphoinositide-3 Kinase (PI3K) and Notch1 signaling leading to COX-2 and MMP-9 expression in a Toll-like receptor 2 (TLR2)-MyD88 dependent manner. Notch1 signaling perturbations data demonstrate the involvement of the cross-talk with members of PI3K and Mitogen activated protein kinase pathway. Enforced expression of the cleaved Notch1 in macrophages induces the expression of COX-2 and MMP-9. PIM2 triggered significant p65 nuclear factor -κB (NF-κB) nuclear translocation that was dependent on activation of PI3K or Notch1 signaling. Furthermore, COX-2 and MMP-9 expression requires Notch1 mediated recruitment of Suppressor of Hairless (CSL) and NF-κB to respective promoters. Inhibition of PIM2 induced COX-2 resulted in marked reduction in MMP-9 expression clearly implicating the role of COX-2 dependent signaling events in driving the MMP-9 expression. Taken together, these data implicate PI3K and Notch1 signaling as obligatory early proximal signaling events during PIM2 induced COX-2 and MMP-9 expression in macrophages.

M. bovis BCG induced expression of COX-2 involves nitric oxide-dependent and -independent signaling pathways

In a multifaceted immunity to mycobacterial infection, induced expression of cyclooxygenase-2 (COX-2) by Mycobacterium bovis bacillus Calmette-Guerin (BCG) may act as an important influencing factor for the effective host immunity. We here demonstrate that M. bovis BCG-triggered TLR2-dependent signaling leads to COX-2 and PGE2 expression in vitro in macrophages and in vivo in mice. Further, the presence of PGE2 could be demonstrated in sera or cerebrospinal fluid of tuberculosis patients. The induced COX-2 expression in macrophages is dependent on NF-kappaB activation, which is mediated by inducible NO synthase (iNOS)/NO-dependent participation of the members of Notch1-PI-3K signaling cascades as well as iNOS-independent activation of ERK1/2 and p38 MAPKs. Inhibition of iNOS activity abrogated the M. bovis BCG ability to trigger the generation of Notch1 intracellular domain (NICD), a marker for Notch1 signaling activation, as well as activation of the PI-3K signaling cascade. On the contrary, treatment of macrophages with 3-morpholinosydnonimine, a NO donor, resulted in a rapid increase in generation of NICD, activation of PI-3K pathway, as well as the expression of COX-2. Stable expression of NICD in RAW 264.7 macrophages resulted in augmented expression of COX-2. Further, signaling perturbations suggested the involvement of the cross-talk of Notch1 with members with the PI-3K signaling cascade. These results implicate the dichotomous nature of TLR2 signaling during M. bovis BCG-triggered expression of COX-2. In this perspective, we propose the involvement of iNOS/NO as one of the obligatory, early, proximal signaling events during M. bovis BCG-induced COX-2 expression in macrophages.

A notch in the toll belt

Two cell response pathways, Toll-like receptor (TLR) and Notch, conserved from Drosophila to mammals are well characterized for distinct roles in innate immunity and cell development, respectively. In this issue of Immunity, Hu et al. (2008) describe and characterize the direct cooperation of these two pathways.

Integrated regulation of Toll-like receptor responses by Notch and interferon-gamma pathways

Toll-like receptor (TLR) responses are regulated to avoid toxicity and achieve coordinated responses appropriate for the cell environment. We found that Notch and TLR pathways cooperated to activate canonical Notch target genes, including transcriptional repressors Hes1 and Hey1, and to increase production of canonical TLR-induced cytokines TNF, IL-6, and IL-12. Cooperation by these pathways to increase target gene expression was mediated by the Notch-pathway component and transcription factor RBP-J, which also contributed to lethality after endotoxin injection. TLR- and Notch-induced Hes1 and Hey1 attenuated IL-6 and IL-12 production. This Hes1- and Hey1-mediated feedback inhibitory loop was abrogated by interferon-gamma (IFN-gamma), which blocked TLR-induced activation of canonical Notch target genes by inhibiting Notch2 signaling and downstream transcription. These findings identify new immune functions for RBP-J, Hes, and Hey proteins and provide insights into mechanisms by which Notch, TLR, and IFN-gamma signals are integrated to modulate specific effector functions in macrophages.

Inhibition of gamma-secretase affects proliferation of leukemia and hepatoma cell lines through Notch signaling

Notch signaling is a well-conserved pathway playing crucial roles in regulating cell fate decision, proliferation, and apoptosis during the development of multiple cell lineages. Aberration in Notch signaling is associated with tumorigenesis of tissues from various origins. To investigate the role Notch signaling plays in the proliferation of cancer cell lines, the expression profiles of Notch1 in six human cancer cell lines (Jurkat, HepG2, SW620, KATOIII, A375, BT474) were examined. All cell lines differentially expressed Notch1, and only Jurkat and SW620 expressed cleaved Notch1 (Val1744). Among the six cell lines tested, only Jurkat and HepG2 showed a decrease in cell proliferation during 4 days of treatment with a gamma-secretase inhibitor (GSI). This is the first report on the anti-proliferative effects of GSI on a human hepatoma cell line. These two cell lines expressed Notch1-3, Jagged1, Jagged2, Dlk1 and Hes1. GSI treatment led to a decrease in Hes1 expression in both cell lines. Surprisingly, GSI treatment resulted in the accumulation of Notch1 protein upon treatment. During this period, GSI treatment did not induce apoptosis, but caused cell cycle arrest in both cell lines. This was also correlated with decreased c-myc expression. Forced expression of activated intracellular Notch1 completely abrogated GSI sensitivity in both cell lines. These results clearly demonstrate that Notch signaling positively regulates cell proliferation in Jurkat and HepG2 cell lines and that GSI treatment inhibits tumor cell proliferation through the suppression of Notch signaling.

NOTCH1 up-regulation and signaling involved in Mycobacterium bovis BCG-induced SOCS3 expression in macrophages

Suppressor of cytokine signaling (SOCS) 3 is a critical negative regulator of cytokine signaling and is induced by Mycobacterium bovis Bacille Calmette-Guérin (M. bovis BCG) in mouse macrophages. However, little is known about the early receptor proximal signaling mechanisms underlying mycobacteria-mediated induction of SOCS3. We demonstrate here for the first time that M. bovis BCG up-regulates NOTCH1 and activates the NOTCH1 signaling pathway, leading to the expression of SOCS3. We show that perturbing Notch signaling in infected macrophages results in the marked reduction in the expression of SOCS3. Furthermore, enforced expression of the Notch1 intracellular domain in RAW 264.7 macrophages induces the expression of SOCS3, which can be further potentiated by M. bovis BCG. The perturbation of Toll-like receptor (TLR) 2 signaling resulted in marked reduction in SOCS3 levels and expression of the NOTCH1 target gene, Hes1. The down-regulation of MyD88 resulted in a significant decrease in SOCS3 expression, implicating the role of the TLR2-MyD88 axis in M. bovis BCG-triggered signaling. However, the SOCS3 inducing ability of M. bovis BCG remains unaltered also upon infection of macrophages from TLR4-defective C3H/HeJ mice. More importantly, signaling perturbation data suggest the involvement of cross-talk among members of the phosphoinositide 3-kinase and mitogen-activated protein kinase cascades with NOTCH1 signaling in SOCS3 expression. Furthermore, SOCS3 expression requires the NOTCH1-mediated recruitment of Suppressor of Hairless (CSL) and nuclear factor-kappaB to the Socs3 promoter. Overall, these results implicate NOTCH1 signaling during inducible expression of SOCS3 following infection of macrophages with an intracellular bacillus-like M. bovis BCG.