Inflammasome activation is a two-step process where step one, priming, prepares the inflammasome for its subsequent activation, by step two. Classically step one can be induced by LPS priming followed by step two, high dose ATP. Furthermore, when IL-18 processing is used as the inflammasome readout, priming occurs before new protein synthesis. In this context, how intracellular pathogens such as Francisella activate the inflammasome is incompletely understood, particularly regarding the relative importance of priming versus activation steps. To better understand these events we compared Francisella strains that differ in virulence and ability to induce inflammasome activation for their relative effects on step one vs. step two. When using the rapid priming model, i.e., 30 min priming by live or heat killed Francisella strains (step 1), followed by ATP (step 2), we found no difference in IL-18 release, p20 caspase-1 release and ASC oligomerization between Francisella strains (F. novicida, F. holarctica –LVS and F. tularensis Schu S4). This priming is fast, independent of bacteria viability, internalization and phagosome escape, but requires TLR2-mediated ERK phosphorylation. In contrast to their efficient priming capacity, Francisella strains LVS and Schu S4 were impaired in inflammasome triggering compared to F. novicida. Thus, observed differences in inflammasome activation by F. novicida, LVS and Schu S4 depend not on differences in priming but rather on their propensity to trigger the primed inflammasome.
Immunopathology plays important roles in the development of different life-threatening diseases, such as atherosclerosis and its consequences (acute myocardial infarction and stroke), cancer, chronic inflammatory diseases. Effective modulation of the immune system may significantly increase the efficacy of prevention and therapy efforts. Currently there are no marketed drugs capable of normalizing immune system function in an intrinsic and comprehensive way. Here, we describe a test system designed for complex analysis of monocyte activity in individuals to diagnose immunopathology and monitor treatment efficacy. This cell-based test system may also be useful for screening compounds with an immune-correcting effects. Both diagnostic and screening systems are based on primary culture of human monocytes and/or monocyte-derived macrophages. This is the first step in creating a method for assessment of macrophage activity, which is required for further development of immune-correcting drugs. The existing preliminary data provide the basis for realization of this idea.
Activation; atherosclerosis; chemokine; cytokine; diagnostics; immunopathology; macrophage; therapy.
Caspase-1 activation is a central event in innate immune responses to many pathogenic infections and tissue damage. The NLRP3 inflammasome, a multi-protein scaffolding complex that assembles in response to two distinct steps, priming and activation, is required for caspase-1 activation. However the detailed mechanisms of these steps remain poorly characterized. To investigate the process of LPS mediated NLRP3 inflammasome priming we used constitutively present proIL-18 as the caspase-1 specific substrate to allow study of the early events. We analyzed human monocyte caspase-1 activity in response to LPS priming followed by activation with ATP. Within minutes of endotoxin priming, the NLRP3 inflammasome is licensed for ATP induced release of processed IL-18, ASC, and active caspase-1, independent of new mRNA or protein synthesis. Moreover, extracellular-regulated kinase 1 (ERK1) phosphorylation is central to the priming process. ERK inhibition and siRNA mediated ERK1 knockdown profoundly impair priming. In addition, proteasome inhibition prevents ERK phosphorylation and blocks priming. Scavenging reactive oxygen species (ROS) with diphenylene-iodonium also blocks both priming and ERK phosphorylation. These findings suggest that ERK1-mediated post-translational modifications license the NLRP3 inflammasome to respond to the second signal ATP by inducing posttranslational events that are independent of new production of proIL-1β and NOD-like receptor components.
human; inflammation; cytokines; monocytes/macrophages; lipopolysaccharide; cell activation; bacterial infections
Relative to monocytes, human macrophages are deficient in their ability to process and release IL-1β. In an effort to explain this difference, we used a model of IL-1β processing and release that is dependent upon bacterial escape into the cytosol. Fresh human blood monocytes were compared with monocyte-derived macrophages (MDM) for their IL-1β release in response to challenge with Francisella novicida. Although both cell types produced similar levels of IL-1β mRNA and intracellular pro-IL-1β, only monocytes readily released processed mature IL-1β. Baseline mRNA expression profiling of candidate genes revealed a remarkable deficiency in the pyrin gene, MEFV, expression in MDM compared with monocytes. Immunoblots confirmed a corresponding deficit in MDM pyrin protein. To determine whether pyrin levels were responsible for the monocyte/MDM difference in mature IL-1β release, pyrin expression was knocked down by nucleofecting small interfering RNA against pyrin into monocytes or stably transducing small interfering RNA against pyrin into the monocyte cell line, THP-1. Pyrin knockdown was associated with a significant drop in IL-1β release in both cell types. Importantly, M-CSF treatment of MDM restored pyrin levels and IL-1β release. Similarly, the stable expression of pyrin in PMA-stimulated THP-1-derived macrophages induces caspase-1 activation, associated with increased IL-1β release after infection with F. novicida. In summary, intracellular pyrin levels positively regulate MDM IL-1β responsiveness to Francisella challenge.
Lung epithelial cell death is critical to the lung injury that occurs in the acute respiratory distress syndrome. It is known that FasL plays a prominent role in this lung cell death pathway and may work in part through activation of the receptor interacting protein-2 (RIP2). RIP2 is serine/threonine kinase with a C-terminal caspase activation and recruitment domain (CARD). This CARD contains a highly conserved, predicted tyrosine phosphorylation site. Thus, involvement of tyrosine phosphorylation in the CARD domain of RIP2 may play a critical role in Fas-mediated apoptosis in the human lung immune system. To test this hypothesis, human lung epithelial cells (BEAS-2B) were induced to undergo cell death in response to the Fas agonist antibody CH11 with and without manipulation of endogenous RIP2 concentrations. We show that CH11 increases lung epithelial cell death in a dose-dependent manner as determined by LDH release and nuclear condensation. Fas-induced LDH release was inhibited by RIP2 knock-down. Reduced levels of RIP2 in BEAS-2B cells after treatment with RIP2 siRNA were confirmed by immunoblot. Overexpression of RIP2 in BEAS-2B cells synergized with Fas ligand-induced LDH release in a dose-dependent manner. Finally, mutation of the tyrosine phosphorylation site in CARD of RIP2 protected BEAS-2B cells from Fas ligand induced cell death. Thus RIP2's CARD tyrosine phosphorylation may represent a new therapeutic target to promote the survival of human lung epithelial cells in disorders that lead to acute lung injury and ARDS.
Background: Human monocyte inflammatory responses differ between virulent and attenuated Francisella infection.
Results: A mixed infection model showed that the virulent F. tularensis Schu S4 can attenuate inflammatory cytokine responses to the less virulent F. novicida in human monocytes.
F. tularensis dampens inflammatory response by an active process.
Significance: This suppression may contribute to enhanced pathogenicity of F. tularensis. Francisella tularensis is a Gram-negative facultative bacterium that can cause the disease tularemia, even upon exposure to low numbers of bacteria. One critical characteristic of Francisella is its ability to dampen or subvert the host immune response. Previous work has shown that monocytes infected with highly virulent F. tularensis subsp. tularensis strain Schu S4 responded with a general pattern of quantitatively reduced pro-inflammatory signaling pathway genes and cytokine production in comparison to those infected with the less virulent related F. novicida. However, it has been unclear whether the virulent Schu S4 was merely evading or actively suppressing monocyte responses. By using mixed infection assays with F. tularensis and F. novicida, we show that F. tularensis actively suppresses monocyte pro-inflammatory responses. Additional experiments show that this suppression occurs in a dose-dependent manner and is dependent upon the viability of F. tularensis. Importantly, F. tularensis was able to suppress pro-inflammatory responses to earlier infections with F. novicida. These results lend support that F. tularensis actively dampens human monocyte responses and this likely contributes to its enhanced pathogenicity.
Francisella; monocytes; cytokines; signal transduction; innate immunity
Pyroptosis is a type of cell death in which danger associated molecular patterns (DAMPs) and pathogen associated molecular patterns (PAMPs) induce mononuclear phagocytes to activate caspase-1 and release mature IL-1β. Because the tyrosine kinase inhibitor AG126 can prevent DAMP/PAMP induced activation of caspase-1, we hypothesized that tipping the tyrosine kinase/phosphatase balance toward phosphorylation would promote caspase-1 activation and cell death. THP-1 derived macrophages were therefore treated with the potent specific tyrosine phosphatase inhibitor, sodium orthovanadate (OVN) and analyzed for caspase-1 activation and cell death. OVN induced generalized increase in phosphorylated proteins, IL-1β release and cell death in a time and dose dependent pattern. This OVN induced pyroptosis correlated with speck formations that contained the apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC). Culturing the cells in the presence of extracellular K+ (known to inhibit ATP dependent pyroptosis), a caspase inhibitor (ZVAD) or down regulating the expression of ASC with stable expression of siASC prevented the OVN induced pyroptosis. These data demonstrate that pyroptotic death is linked to tyrosine phosphatase activity providing novel targets for future pharmacologic interventions.
inflammasome; pyroptosis; phosphatases; ASC; caspase-1; IL-1β
Among human natural killer (NK) cell intermediates in secondary lymphoid tissue (SLT), stage 3 CD34− CD117+CD161+CD94− immature NK (iNK) cells uniquely express aryl hydrocarbon receptor (AHR) and interleukin-22 (IL-22), supporting a role in mucosal immunity. The mechanisms controlling proliferation and differentiation of these cells are unknown. Here we demonstrate that the IL-1 receptor IL-1R1 was selectively expressed by a subpopulation of iNK cells that localized proximal to IL-1β-producing conventional dendritic cells (cDCs) within SLT. IL-1R1hi iNK cells required continuous exposure to IL-1β to retain AHR and IL-22 expression, and proliferate in direct response to cDC-derived IL-15 and IL-1β. In the absence of IL-1β, a substantially greater fraction of IL-1R1hi iNK cells differentiated to stage 4 NK cells and acquired the ability to kill and secrete IFN-γ. Thus, cDC-derived IL-1β preserves and expands IL-1R1hiIL-22+AHR+ iNK cells, potentially influencing human mucosal innate immunity during infection.
Inflammasomes are multiprotein complexes that include members of the NLR (nucleotide-binding domain leucine-rich repeat containing) family and caspase-1. Once bacterial molecules are sensed within the macrophage, the inflammasome is assembled mediating the activation of caspase-1. Caspase-11 mediates caspase-1 activation in response to lipopolysaccharide and bacterial toxins. Yet, its role during bacterial infection is unknown. Here, we demonstrated that caspase-11 was dispensable for caspase-1 activation in response to Legionella, Salmonella, Francisella and Listeria. We also determined that active mouse caspase-11 was required for restriction of L. pneumophila infection. Similarly, human caspase-4 and 5, homologs of mouse caspase-11, cooperated to restrict L. pneumophila infection in human macrophages. Caspase-11 promoted the fusion of the L. pneumophila- vacuole with lysosomes by modulating actin polymerization through cofilin. However, caspase-11 was dispensable for the fusion of lysosomes with phagosomes containing non-pathogenic bacteria, uncovering a fundamental difference in the trafficking of phagosomes according to their cargo.
Burkholderia cenocepacia is an opportunistic pathogen that causes chronic infection and induces progressive respiratory inflammation in cystic fibrosis patients. Recognition of bacteria by mononuclear cells generally results in the activation of caspase-1 and processing of IL-1β, a major proinflammatory cytokine. In this study, we report that human pyrin is required to detect intracellular B. cenocepacia leading to IL-1β processing and release. This inflammatory response involves the host adapter molecule ASC and the bacterial type VI secretion system (T6SS). Human monocytes and THP-1 cells stably expressing either small interfering RNA against pyrin or YFP–pyrin and ASC (YFP–ASC) were infected with B. cenocepacia and analyzed for inflammasome activation. B. cenocepacia efficiently activates the inflammasome and IL-1β release in monocytes and THP-1. Suppression of pyrin levels in monocytes and THP-1 cells reduced caspase-1 activation and IL-1β release in response to B. cenocepacia challenge. In contrast, overexpression of pyrin or ASC induced a robust IL-1β response to B. cenocepacia, which correlated with enhanced host cell death. Inflammasome activation was significantly reduced in cells infected with T6SS-defective mutants of B. cenocepacia, suggesting that the inflammatory reaction is likely induced by an as yet uncharacterized effector(s) of the T6SS. Together, we show for the first time, to our knowledge, that in human mononuclear cells infected with B. cenocepacia, pyrin associates with caspase-1 and ASC forming an inflammasome that upregulates mononuclear cell IL-1β processing and release.
Cystic fibrosis (CF) is the most common inherited lethal disease in Caucasians which results in multiorgan dysfunction. However, 85% of the deaths are due to pulmonary infections. Infection by Burkholderia cenocepacia (B. cepacia) is a particularly lethal threat to CF patients because it causes severe and persistent lung inflammation and is resistant to nearly all available antibiotics. In CFTR ΔF508 (ΔF508) mouse macrophages, B. cepacia persists in vacuoles that do not fuse with the lysosomes and mediates increased production of IL-1β. It is believed that intracellular bacterial survival contributes to the persistence of the bacterium. Here we show for the first time that in wild-type but not in ΔF508 macrophages, many B. cepacia reside in autophagosomes that fuse with lysosomes at later stages of infection. Accordingly, association and intracellular survival of B. cepacia are higher in CFTR-ΔF508 macrophages than in WT macrophages. An autophagosome is a compartment that engulfs nonfunctional organelles and parts of the cytoplasm then delivers them to the lysosome for degradation to produce nutrients during periods of starvation or stress. Furthermore, we show that B. cepacia downregulates autophagy genes in WT and ΔF508 macrophages. However, autophagy dysfunction is more pronounced in ΔF508 macrophages since they already have compromised autophagy activity. We demonstrate that the autophagystimulating agent, rapamycin markedly decreases B. cepacia infection in vitro by enhancing the clearance of B. cepacia via induced autophagy. In vivo, rapamycin decreases bacterial burden in the lungs of CF mice and drastically reduces signs of lung inflammation. Together, our studies reveal that if efficiently activated, autophagy can control B. cepacia infection and ameliorate the associated inflammation. Therefore, autophagy is a novel target for new drug development for CF patients to control B. cepacia infection and accompanying inflammation.
autophagy; rapamycin; cystic fibrosis; host-pathogen interaction; Burkholderia cenocepacia; inflammation; macrophages
The apoptosis-associated speck-like protein containing a caspase recruitment domain (Asc) is an adaptor molecule that mediates inflammatory and apoptotic signals. Legionella pneumophila is an intracellular bacterium and the causative agent of Legionnaire's pneumonia. L. pneumophila is able to cause pneumonia in immuno-compromised humans but not in most inbred mice. Murine macrophages that lack the ability to activate caspase-1, such as caspase-1−/− and Nlrc4−/− allow L. pneumophila infection. This permissiveness is attributed mainly to the lack of active caspase-1 and the absence of its down stream substrates such as caspase-7. However, the role of Asc in control of L. pneumophila infection in mice is unclear. Here we show that caspase-1 is moderately activated in Asc−/− macrophages and that this limited activation is required and sufficient to restrict L. pneumophila growth. Moreover, Asc-independent activation of caspase-1 requires bacterial flagellin and is mainly detected in cellular extracts but not in culture supernatants. We also demonstrate that the depletion of Asc from permissive macrophages enhances bacterial growth by promoting L. pneumophila-mediated activation of the NF-κB pathway and decreasing caspase-3 activation. Taken together, our data demonstrate that L. pneumophila infection in murine macrophages is controlled by several mechanisms: Asc-independent activation of caspase-1 and Asc-dependent regulation of NF-κB and caspase-3 activation.
inflammasome; caspase-1; Legionella pneumophila; Asc
Pathogen recognition by intracellular sensors involves the assembly of a caspase-1 activation machine termed the inflammasome. Intracellular pathogens like Francisella that gain access to the cytosolic detection systems are useful tools to uncover the details of caspase-1 activation events. This review overviews Francisella function in the mononuclear phagocyte with particular attention to inflammasome versus pyroptosome roles and outlines differences between mouse and human caspase-1 activation pathways. Specific attention is placed on functional differences between human and murine pyrin as an intracellular recognition molecule for Francisella.
AIM2; Francisella; IL-1β; inflammasome; M-CSF; monocytes; pyrin; pyroptosome
The ability of Legionella pneumophila to cause pneumonia is determined by its capability to evade the immune system and grow within human monocytes and their derived macrophages. Human monocytes efficiently activate caspase-1 in response to Salmonella but not to L. pneumophila. The molecular mechanism for the lack of inflammasome activation during L. pneumophila infection is unknown. Evaluation of the expression of several inflammasome components in human monocytes during L. pneumophila infection revealed that the expression of the apoptosis-associated speck-like protein (ASC) and the NOD-like receptor NLRC4 are significantly down-regulated in human monocytes. Exogenous expression of ASC maintained the protein level constant during L. pneumophila infection and conveyed caspase-1 activation and restricted the growth of the pathogen. Further depletion of ASC with siRNA was accompanied with improved NF-κB activation and enhanced L. pneumophila growth. Therefore, our data demonstrate that L. pneumophila manipulates ASC levels to evade inflammasome activation and grow in human monocytes. By targeting ASC, L. pneumophila modulates the inflammasome, the apoptosome, and NF-κB pathway simultaneously.
Caspase; Cellular Immune Response; Immunology; Inflammation; Innate Immunity; ASC; Legionella; NOD-like Receptors; Human Monocytes; Inflammasome
Rationale: Little is known about the genetic regulation of granulomatous inflammation in sarcoidosis.
Objectives: To determine if tissue gene array analysis would identify novel genes engaged in inflammation and lung remodeling in patients with sarcoidosis.
Methods: Gene expression analysis was performed on tissues obtained from patients with sarcoidosis at the time of diagnosis (untreated) (n = 6) compared with normal lung tissue (n = 6). Expression of select genes was further confirmed in lung tissue from a second series of patients with sarcoidosis and disease-free control subjects (n = 11 per group) by semi-quantitative RT-PCR. Interactive gene networks were identified in patients with sarcoidosis using Ingenuity Pathway Analysis (Ingenuity Systems, Inc., Redwood, CA) software. The expression of proteins corresponding to selected overexpressed genes was determined using fluorokine multiplex analysis, and immunohistochemistry. Selected genes and proteins were then analyzed in bronchoalveolar lavage fluid in an independent series of patients with sarcoidosis (n = 36) and control subjects (n = 12).
Measurements and Main Results: A gene network engaged in Th1-type responses was most significantly overexpressed in the sarcoidosis lung tissues, including genes not previously reported in the context of sarcoidosis (e.g., IL-7). MMP-12 and ADAMDEC1 transcripts were most highly expressed (> 25-fold) in sarcoidosis lung tissues, corresponding with increased protein expression by immunohistochemistry. MMP-12 and ADAMDEC1 gene and protein expression were increased in bronchoalveolar lavage samples from patients with sarcoidosis, correlating with disease severity.
Conclusions: Tissue gene expression analyses provide novel insights into the pathogenesis of pulmonary sarcoidosis. MMP-12 and ADAMDEC1 emerge as likely mediators of lung damage and/or remodeling and may serve as markers of disease activity.
genetic; gene array; granuloma; lung; BAL
Mycoplasma contamination of cultured cell lines is a serious problem in research, altering cellular response to different stimuli thus compromising experimental results. We found that chronic mycoplasma contamination of THP-1 cells suppresses responses of THP-1 cells to TLR stimuli. For example, E. coli LPS induced IL-1 β was suppressed by 6 fold and IL-8 by 10 fold in mycoplasma positive THP-1 cells. Responses to live F. novicida challenge were suppressed by 50-fold and 40-fold respectively for IL-1β and IL-8. Basal TLR4 expression level in THP-1 cells was decreased by mycoplasma by 2.4-fold (p = 0.0003). Importantly, cell responses to pathogen associated molecular patterns are completely restored by mycoplasma clearance with Plasmocin. Thus, routine screening of cell lines for mycoplasma is important for the maintenance of reliable experimental data and contaminated cell lines can be restored to their baseline function with antibiotic clearance of mycoplasma.
Rationale: Monocytes are central to the initiation of the inflammatory response in sepsis, with caspase-1 activation playing a key role. Monocyte deactivation during sepsis has been linked to poor outcomes.
Objectives: Given the importance of caspase-1 in the immune response, we investigated whether monocytes from patients early in septic shock demonstrate alterations in mRNAs for caspase-1–related molecules.
Methods: Patients with septic shock (n = 26; age >18 years), critically ill intensive care unit patients (n = 20), and healthy volunteers (n = 22) were enrolled in a prospective cohort study in a university intensive care unit. Demographic, biological, physiologic, and plasma cytokine measurements were obtained. Monocytes were assayed for ex vivo tumor necrosis factor-α production, and fresh monocyte mRNA was analyzed by quantitative reverse-transcription polymerase chain reaction for Toll-like receptors, NOD-LRR proteins, cytokines, and nuclear factor-κB–related genes.
Measurements and Main Results: Relative copy numbers for the inflammasome mRNAs for ASC, caspase-1, NALP1, and Pypaf-7 were significantly lower in patients with septic shock compared with critically ill control subjects. NALP1 mRNA levels were linked to survival in patients with sepsis (P = 0.0068) and correlated with SAPS II scores (r = −0.63).
Conclusions: These data suggest that monocyte deactivation occurs during the earliest stages of the systemic inflammatory response and that changes in inflammasome mRNA expression are part of this process.
inflammasome; monocytes; septic shock; messenger RNA; NALP1
Legionella pneumophila (L. pneumophila), the causative agent of a severe form of pneumonia called Legionnaires' disease, replicates in human monocytes and macrophages. Most inbred mouse strains are restrictive to L. pneumophila infection except for the A/J, Nlrc4−/− (Ipaf−/−), and caspase-1−/− derived macrophages. Particularly, caspase-1 activation is detected during L. pneumophila infection of murine macrophages while absent in human cells. Recent in vitro experiments demonstrate that caspase-7 is cleaved by caspase-1. However, the biological role for caspase-7 activation downstream of caspase-1 is not known. Furthermore, whether this reaction is pertinent to the apoptosis or to the inflammation pathway or whether it mediates a yet unidentified effect is unclear. Using the intracellular pathogen L. pneumophila, we show that, upon infection of murine macrophages, caspase-7 was activated downstream of the Nlrc4 inflammasome and required caspase-1 activation. Such activation of caspase-7 was mediated by flagellin and required a functional Naip5. Remarkably, mice lacking caspase-7 and its macrophages allowed substantial L. pneumophila replication. Permissiveness of caspase-7−/− macrophages to the intracellular pathogen was due to defective delivery of the organism to the lysosome and to delayed cell death during early stages of infection. These results reveal a new mechanism for caspase-7 activation downstream of the Nlrc4 inflammasome and present a novel biological role for caspase-7 in host defense against an intracellular bacterium.
Legionella pneumophila causes a severe form of pneumonia called Legionnaires' disease. In human macrophages, L. pneumophila establishes special vacuoles that do not fuse with the lysosome and grows intracellularly. However, in mouse macrophages, the bacteria are efficiently delivered to the lysosome for degradation. Importantly, caspase-1 is activated when L. pneumophila infects mouse macrophages, but not when it infects human cells. Caspase-1 activation promotes the fusion of the L. pneumophila vacuole with the lysosome and macrophage death. However, the caspase-1 substrate mediating such effects is unknown. Experiments performed in vitro demonstrate that caspase-7 is a substrate of caspase-1. Yet, it is not known if the reaction takes place within the macrophage, and it is unclear if it has any biological effect. In this study we show that, in mouse macrophages, caspase-7 is activated by L. pneumophila downstream of caspase-1 and requires the host receptors Nlrc4 and Naip5. Remarkably, caspase-7 activation during L. pneumophila infection restricts growth by promoting early macrophage death and efficient delivery of the organism to the lysosome. Consequently, L. pneumophila grows in the macrophages and the lungs of caspase-7−/− mice. Therefore, we demonstrate a novel caspase-7 activation pathway that contributes to the restriction of L. pneumophila infection.
Francisella tularensis is a gram-negative facultative bacterium that causes the disease tularemia, even upon exposure to low numbers of bacteria. One critical characteristic of Francisella is its ability to dampen or subvert the host immune response. In order to help understand the mechanisms by which this occurs, we performed Affymetrix microarray analysis on transcripts from blood monocytes infected with the virulent Type A Schu S4 strain. Results showed that expression of several host response genes were reduced such as those associated with interferon signaling, Toll-like receptor signaling, autophagy and phagocytosis. When compared to microarrays from monocytes infected with the less virulent F. tularensis subsp. novicida, we found qualitative differences and also a general pattern of quantitatively reduced pro-inflammatory signaling pathway genes in the Schu S4 strain. Notably, the PI3K / Akt1 pathway appeared specifically down-regulated following Schu S4 infection and a concomitantly lower cytokine response was observed. This study identifies several new factors potentially important in host cell subversion by the virulent Type A F. tularensis that may serve as novel targets for drug discovery.
Women are at high risk of dying from unrecognized cardiovascular disease. Many differences in cardiovascular disease between men and women appear to be mediated by vascular smooth muscle cells (SMC). Since estrogen reduces the proliferation of SMC, we hypothesized that activation of estrogen receptor alpha (ERα) by agonists or by growth factors altered SMC function. To determine the effect of growth factors, estrogen, and ERα expression on SMC differentiation, human aortic SMC were cultured in serum-free conditions for 10 days. SMC from men had lower spontaneous expression of ERα and higher levels of the differentiation markers calponin and smooth muscle α-actin than SMC from women. When SMC containing low expression of ERα were transduced with a lentivirus containing ERα, activation of the receptor by ligands or growth factors reduced differentiation markers. Conversely, inhibiting ERα expression by small interfering (si) RNA in cells expressing high levels of ERα enhanced the expression of differentiation markers. ERα expression and activation reduced the phosphorylation of Smad2, a signaling molecule important in differentiation of SMC, and initiated cell death through cleavage of caspase-3. We conclude that ERα activation switched SMC to a dedifferentiated phenotype and may contribute to plaque instability.
apoptosis; cardiovascular disease; gene expression; nuclear receptors; smooth muscle differentiation
Francisella tularensis, a Gram-negative facultative intracellular pathogen infecting principally macrophages and monocytes, is the etiological agent of tularemia. Macrophage responses to F. tularensis infection include the production of pro-inflammatory cytokines such as interleukin (IL)-12, which is critical for immunity against infection. Molecular mechanisms regulating production of these inflammatory mediators are poorly understood. Herein we report that the SH2 domain-containing inositol phosphatase (SHIP) is phosphorylated upon infection of primary murine macrophages with the genetically related F. novicida, and negatively regulates F. novicida–induced cytokine production. Analyses of the molecular details revealed that in addition to activating the MAP kinases, F. novicida infection also activated the phosphatidylinositol 3-kinase (PI3K)/Akt pathway in these cells. Interestingly, SHIP-deficient macrophages displayed enhanced Akt activation upon F. novicida infection, suggesting elevated PI3K-dependent activation pathways in absence of SHIP. Inhibition of PI3K/Akt resulted in suppression of F. novicida–induced cytokine production through the inhibition of NFκB. Consistently, macrophages lacking SHIP displayed enhanced NFκB-driven gene transcription, whereas overexpression of SHIP led to decreased NFκB activation. Thus, we propose that SHIP negatively regulates F. novicida–induced inflammatory cytokine response by antagonizing the PI3K/Akt pathway and suppressing NFκB-mediated gene transcription. A detailed analysis of phosphoinositide signaling may provide valuable clues for better understanding the pathogenesis of tularemia.
Francisella tularensis is an intracellular Gram-negative bacterium that causes the disease tularemia. The bacteria infect macrophages and replicate within the host cell. Macrophages respond to the infection by producing pro-inflammatory cytokines in an effort to combat the infection. The intracellular signaling events that are involved in host cell responses are not fully understood. Much less understood are mechanisms that regulate these responses. In this study, the authors define a negative regulatory role for the inositol phosphatase SHIP during Francisella infection of mouse macrophages. The study demonstrates that SHIP is activated in infected cells and serves to down-regulate pro-inflammatory cytokine production. The authors have examined the molecular mechanism underlying this negative regulation, and show that SHIP acts upstream of NFκB, an important transcription factor involved in inflammatory cytokine gene transcription, and dampens its activation. This effect of SHIP appears to be mediated through the regulation of the PI3 kinase pathway. This study establishes a novel and critical role for SHIP in the regulation of macrophage inflammatory response to Francisella bacteria.