Pseudomonas aeruginosa biofilm-associated homoserine lactone C12 rapidly activates apoptosis in airway epithelia

Pseudomonas aeruginosa N-3-Oxododecanoyl Homoserine Lactone Disrupts Endothelial Integrity by Activating the Angiopoietin-Tie System

The activation of the angiopoietin (Angpt)-Tie system is linked to endothelial dysfunction during sepsis. Bacterial quorum-sensing molecules function as pathogen-associated molecular patterns. However, their impact on the endothelium and the Angpt-Tie system remains unclear. Therefore, this study investigated whether treatment with N-3-oxododecanoyl homoserine lactone (3OC12-HSL), a quorum-sensing molecule derived from Pseudomonas aeruginosa, impaired endothelial function in human umbilical vein endothelial cells. 3OC12-HSL treatment impaired tube formation even at sublethal concentrations, and immunocytochemistry analysis revealed that it seemed to reduce vascular endothelial-cadherin expression at the cell-cell interface. Upon assessing the mRNA expression patterns of genes associated with the Angpt-Tie axis, the expressions of Angpt2, Forkhead box protein O1, Tie1, and vascular endothelial growth factor 2 were found to be upregulated in the 3OC12-HSL-treated cells. Moreover, western blot analysis revealed that 3OC12-HSL treatment increased Angpt2 expression. A co-immunoprecipitation assay was conducted to assess the effect of 3OC12-HSL on the IQ motif containing GTPase activating protein 1 (IQGAP1) and Rac1 complex and the interaction between these proteins was consistently maintained regardless of 3OC12-HSL treatment. Next, recombinant human (rh)-Angpt1 was added to assess whether it modulated the effects of 3OC12-HSL treatment. rh-Angpt1 addition increased cellular viability, improved endothelial function, and reversed the overall patterns of mRNA and protein expression in endothelial cells treated with 3OC12-HSL. Additionally, it was related to the increased expression of phospho-Akt and the IQGAP1 and Rac1 complex. Collectively, our findings indicated that 3OC12-HSL from Pseudomonas aeruginosa can impair endothelial integrity via the activation of the Angpt-Tie axis, which appeared to be reversed by rh-Angpt1 treatment.

Pseudomonas aeruginosa quorum-sensing molecule N-(3-oxododecanoyl)-L-homoserine lactone mediates Ca+2 dysregulation, mitochondrial dysfunction, and apoptosis in human peripheral blood lymphocytes

N-(3-oxododecanoyl)-l-homoserine lactone is a Pseudomonas aeruginosa secreted quorum-sensing molecule that mediates the secretion of virulence factors, biofilm formation and plays a pivotal role in proliferation and persistence in the host. Apart from regulating quorum-sensing, the autoinducer signal molecule N-(3-oxododecanoyl)-l-homoserine lactone (3O-C12-HSL or C12) of a LasI-LasR circuit exhibits immunomodulatory effects and induces apoptosis in various host cells. However, the precise pathophysiological impact of C12 on human peripheral blood lymphocytes and its involvement in mitochondrial dysfunction remained largely elusive. In this study, the results suggest that C12 (100 μM) induces upregulation of cytosolic and mitochondrial Ca+2 levels and triggers mitochondrial dysfunction through the generation of mitochondrial ROS (mROS), disruption of mitochondrial transmembrane potential (ΔΨm), and opening of the mitochondrial permeability transition pore (mPTP). Additionally, it was observed that C12 induces phosphatidylserine (PS) exposure and promotes apoptosis in human peripheral blood lymphocytes. However, apoptosis plays a critical role in the homeostasis and development of lymphocytes, whereas enhanced apoptosis can cause immunodeficiency through cell loss. These findings suggest that C12 exerts a detrimental effect on lymphocytes by mediating mitochondrial dysfunction and enhancing apoptosis, which might further impair the effective mounting of immune responses during Pseudomonas aeruginosa-associated infections.

Surface growth of Pseudomonas aeruginosa reveals a regulatory effect of 3-oxo-C12-homoserine lactone in the absence of its cognate receptor, LasR

IMPORTANCE

The bacterium Pseudomonas aeruginosa colonizes and thrives in many environments, in which it is typically found in surface-associated polymicrobial communities known as biofilms. Adaptation to this social behavior is aided by quorum sensing (QS), an intercellular communication system pivotal in the expression of social traits. Regardless of its importance in QS regulation, the loss of function of the master regulator LasR is now considered a conserved adaptation of P. aeruginosa, irrespective of the origin of the strains. By investigating the QS circuitry in surface-grown cells, we found an accumulation of QS signal 3-oxo-C12-HSL in the absence of its cognate receptor and activator, LasR. The current understanding of the QS circuit, mostly based on planktonic growing cells, is challenged by investigating the QS circuitry of surface-grown cells. This provides a new perspective on the beneficial aspects that underline the frequency of LasR-deficient isolates.

Pseudomonas aeruginosa Cytotoxins: Mechanisms of Cytotoxicity and Impact on Inflammatory Responses

Pseudomonas aeruginosa is one of the most virulent opportunistic Gram-negative bacterial pathogens in humans. It causes many acute and chronic infections with morbidity and mortality rates as high as 40%. P. aeruginosa owes its pathogenic versatility to a large arsenal of cell-associated and secreted virulence factors which enable this pathogen to colonize various niches within hosts and protect it from host innate immune defenses. Induction of cytotoxicity in target host cells is a major virulence strategy for P. aeruginosa during the course of infection. P. aeruginosa has invested heavily in this strategy, as manifested by a plethora of cytotoxins that can induce various forms of cell death in target host cells. In this review, we provide an in-depth review of P. aeruginosa cytotoxins based on their mechanisms of cytotoxicity and the possible consequences of their cytotoxicity on host immune responses.

Pseudomonas aeruginosa quorum-sensing molecule N-(3-oxododecanoyl) homoserine lactone induces calcium signaling-dependent crosstalk between autophagy and apoptosis in human macrophages

N-(3-oxododecanoyl) homoserine lactone (3oc) is a Pseudomonas aeruginosa secreted quorum-sensing signal molecule playing a crucial role in regulating quorum-sensing (QS) dependent biofilm formation and secretion of virulence factors. In addition to regulating quorum sensing, 3oc also plays an immunomodulatory role in the host by triggering regulated cell death in immune cells. The molecular mechanisms of 3oc in modulating macrophage pathologies are still unclear. In this study, we hypothesized the novel 3oc mediated crosstalk between autophagy and apoptosis at the interphase of calcium signaling in human macrophages. The study showed that 3oc induces mitochondrial dysfunction and apoptosis in macrophages through elevating cytosolic Ca⁺² ([Ca⁺²]_cyt) levels. Pre-treatment with the calcium-specific chelator BAPTA-AM effectively abrogated 3oc-induced apoptotic events, like mitochondrial ROS generation (mROS), mitochondrial membrane potential (MMP) drop, and phosphatidylserine (PS) exposure. The study also showed that 3oc induces autophagy, as assessed by the accumulation of autophagic vacuoles, induction of lysosomal biogenesis, upregulation of autophagy genes (LC3, BECLIN 1, STX17, PINK1, and TFEB), autophagosomes formation, and LC3 lipidation. Mechanistically, our study proved that 3oc-induced autophagy was [Ca⁺²]_cyt dependent as BAPTA-AM pre-treatment reduced autophagosome formation. Furthermore, inhibiting autophagy with chloroquine attenuated 3oc-induced apoptosis, while autophagy induction with rapamycin aggravated cell death, suggesting autophagy plays a role in cell death in 3oc-treated macrophages. In conclusion, our findings indicate that 3oc activates a multifaceted death signaling by activating autophagy and apoptosis through Ca⁺² signaling, and we propose pharmacological modulation of Ca⁺² signaling may act as a combinatorial therapeutic intervention in patients with Pseudomonas aeruginosa-associated infections.

Impact of quorum sensing signaling molecules in gram-negative bacteria on host cells: current understanding and future perspectives

Quorum sensing is a molecular signaling-based communication mechanism in prokaryotes. In the basic mode, signaling molecules released by certain bacteria are sensed by intracellular receptors or membrane-bound receptors of other members in the community, leading to the collective isogenic signaling molecule synthesis and synchronized activities. This regulation is important for the symbiosis of the bacterium with the host, as well as virulence and biofilm formation. Notably, quorum sensing signaling molecules are not only able to control microbial community behavior but can likewise regulate the physiological status of host cells. Here, we provide a comprehensive review of the importance of quorum sensing signaling molecules in gram-negative bacteria in regulating host cell function and gut health, and suggest possible opportunities for application in combating human and animal diseases by blocking the pathways through which quorum sensing signaling molecules exert their functions.

Surface Hydration Protects Cystic Fibrosis Airways from Infection by Restoring Junctional Networks

Defective hydration of airway surface mucosa is associated with recurrent lung infection in cystic fibrosis (CF), a disease caused by CF transmembrane conductance regulator (CFTR) gene mutations. Whether the composition and/or presence of an airway surface liquid (ASL) is sufficient to prevent infection remains unclear. The susceptibility to infection of polarized wild type and CFTR knockdown (CFTR-KD) airway epithelial cells was determined in the presence or absence of a healthy ASL or physiological saline. CFTR-KD epithelia exhibited strong ASL volume reduction, enhanced susceptibility to infection, and reduced junctional integrity. Interestingly, the presence of an apical physiological saline alleviated disruption of the airway epithelial barrier by stimulating essential junctional protein expression. Thus, rehydrated CFTR-KD cells were protected from infection despite normally intense bacterial growth. This study indicates that an epithelial integrity gatekeeper is modulated by the presence of an apical liquid volume, irrespective of the liquid's composition and of expression of a functional CFTR.

T2R bitter taste receptors regulate apoptosis and may be associated with survival in head and neck squamous cell carcinoma

Better management of head and neck squamous cell carcinomas (HNSCCs) requires a clearer understanding of tumor biology and disease risk. Bitter taste receptors (T2Rs) have been studied in several cancers, including thyroid, salivary, and GI, but their role in HNSCC has not been explored. We found that HNSCC patient samples and cell lines expressed functional T2Rs on both the cell and nuclear membranes. Bitter compounds, including bacterial metabolites, activated T2R‐mediated nuclear Ca²⁺ responses leading to mitochondrial depolarization, caspase activation, and ultimately apoptosis. Buffering nuclear Ca²⁺ elevation blocked caspase activation. Furthermore, increased expression of T2Rs in HNSCCs from The Cancer Genome Atlas is associated with improved overall survival. This work suggests that T2Rs are potential biomarkers to predict outcomes and guide treatment selection, may be leveraged as therapeutic targets to stimulate tumor apoptosis, and may mediate tumor‐microbiome crosstalk in HNSCC.

N-3-oxododecanoyl homoserine lactone exacerbates endothelial cell death by inducing receptor-interacting protein kinase 1-dependent apoptosis

Endothelial dysfunction is associated with the initiation of sepsis-associated organ failure. Bacterial quorum-sensing molecules act as pathogen-associated molecular patterns; however, the effects of quorum-sensing molecules on endothelial cells remain less understood. This study investigated the molecular mechanisms of quorum-sensing molecule-induced cell death and their interaction with lipopolysaccharide (LPS) in human umbilical vein endothelial cells. Endothelial cells were treated with N-3-oxododecanoyl homoserine lactone (3OC12-HSL) and LPS derived from Pseudomonas aeruginosa. Treatment with 3OC12-HSL reduced cell viability in a dose-dependent manner, and cotreatment with 3OC12-HSL and LPS enhanced cell death. Terminal deoxynucleotidyl transferase deoxyuridine triphosphate nick end labeling assay revealed an increase in apoptotic cell death following 3OC12-HSL treatment; furthermore, cotreatment with 3OC12-HSL and LPS enhanced apoptosis. Western blotting revealed that treatment with 3OC12-HSL activated the receptor-interacting protein kinase 1 (RIPK1) pathway, leading to an increase in the levels of cleaved caspase 8 and 3. In addition, we found that treatment with necrostatin-1, an RIPK1 inhibitor, reduced cell death and ameliorated the activation of the RIPK1-dependent apoptotic pathway in 3OC12-HSL-treated cells. In conclusion, 3OC12-HSL induced endothelial cell apoptosis via the activation of the RIPK1 pathway, independent of LPS toxicity. Inhibition of RIPK1 may act as a therapeutic option for preserving endothelial cell integrity in patients with sepsis by disrupting the mechanism by which quorum-sensing molecules mediate their toxicity.

N-(3-oxododecanoyl)-homoserine lactone disrupts intestinal barrier and induces systemic inflammation through perturbing gut microbiome in mice

As a quorum sensing signal molecule, N-(3-oxododecanoyl)-homoserine lactone (3OC12) regulate the population behavior of microorganisms. Many studies have proved that 3OC12 harm the physiological function of host intestinal epithelial cells. However, the detrimental effects of 3OC12 on intestinal health need verification in animals. Besides, the role of gut microbiome in 3OC12-induced intestinal damage also needs further understanding. In our study, 3OC12 was first administered to specific pathogen-free (SPF) mice, then the fecal microbiome of SPF mice was transplanted into germ-free (GF) mice to reveal the effects of 3OC12 on intestinal health and regulatory mechanisms of the intestinal microbiome. 3OC12 treatment significantly decreased body weight, shortened colonic length, disrupted the morphology of the colonic epithelium and increased the histopathological score of the colon in SPF mice. The levels of diamine peroxidase, d-lactate, TNF-α, IL-1β, and IL-8 were found to be significantly elevated in the serum of 3OC12 mice, while the levels of IL-10 were significantly reduced. Besides, the fecal microbial community of mice was also altered in the 3OC12-treated SPF mice. The results of fecal microbial transplantation (FMT) experiment showed that the phenotypes in SPF mice were almost reproduced in GF mice, manifested by body weight loss, colon damage and changed in serum chemical markers. More importantly, a joint analysis of fecal microbes in SPF and GF mice revealed Feature14_Elizabethkingia spp. was common differential bacteria in the feces of two kinds of mice treated with and without FMT. Our results demonstrated that 3OC12 challenge led to systemic inflammation and body weight loss in mice by disrupting intestinal barrier function, in which gut microbiome played a key role. These findings increased our understanding of the mechanism of intestinal injury caused by 3CO12, providing new ideas for the prevention and therapy of diseases caused by bacterial infection from the perspective of intestinal microbiome.

N-3-Hydroxy Dodecanoyl-DL-homoserine Lactone (OH-dDHL) Triggers Apoptosis of Bone Marrow-Derived Macrophages through the ER- and Mitochondria-Mediated Pathways

Quorum sensing of Acinetobacter nosocomialis for cell-to-cell communication produces N-3-hydroxy dodecanoyl-DL-homoserine lactone (OH-dDHL) by an AnoR/I two-component system. However, OH-dDHL-driven apoptotic mechanisms in hosts have not been clearly defined. Here, we investigated the induction of apoptosis signaling pathways in bone marrow-derived macrophages treated with synthetic OH-dDHL. Moreover, the quorum-sensing system for virulence regulation was evaluated in vivo using wild-type and anoI-deletion mutant strains. OH-dDHL decreased the viability of macrophage and epithelial cells in dose- and time-dependent manners. OH-dDHL induced Ca²⁺ efflux and caspase-12 activation by ER stress transmembrane protein (IRE1 and ATF6a p50) aggregation and induced mitochondrial dysfunction through reactive oxygen species (ROS) production, which caused cytochrome c to leak. Pretreatment with a pan-caspase inhibitor reduced caspase-3, -8, and -9, which were activated by OH-dDHL. Pro-inflammatory cytokine and paraoxonase-2 (PON2) gene expression were increased by OH-dDHL. We showed that the anoI-deletion mutant strains have less intracellular invasion compared to the wild-type strain, and their virulence, such as colonization and dissemination, was decreased in vivo. Consequently, these findings revealed that OH-dDHL, as a virulence factor, contributes to bacterial infection and survival as well as the modification of host responses in the early stages of infection.

Detection of gut microbiota and pathogen produced N-acyl homoserine in host circulation and tissues

Recent studies suggest that quorum-sensing molecules may play a role in gut microbiota-host crosstalk. However, whether microbiota produces quorum-sensing molecules and whether those molecules can trans-kingdom transport to the host are still unknown. Here, we develop a UPLC-MS/MS-based assay to screen the 27 N-acyl homoserine lactones (AHLs) in the gut microbiota and host. Various AHL molecules are exclusively detected in the cecal contents, sera and livers from conventionally-raised mice but cannot be detected in germ-free mice. Pathogen-produced C4-HSL is detected in the cecal contents and sera of Citrobacter rodentium (C. rodentium)-infected mice, but not found in uninfected controls. Moreover, C. rodentium infection significantly increases the level of multiple AHL molecules in sera. Our findings demonstrate that both commensal and pathogenic bacteria, can produce AHLs that can be detected in host bodies, suggesting that quorum-sensing molecules could be a group of signaling molecules in trans-kingdom microbiota-host crosstalk.

Mimicking biofilm formation and development: Recent progress in in vitro and in vivo biofilm models

Biofilm formation in living organisms is associated to tissue and implant infections, and it has also been linked to the contribution of antibiotic resistance. Thus, understanding biofilm development and being able to mimic such processes is vital for the successful development of antibiofilm treatments and therapies. Several decades of research have contributed to building the foundation for developing in vitro and in vivo biofilm models. However, no such thing as an "all fit" in vitro or in vivo biofilm models is currently available. In this review, in addition to presenting an updated overview of biofilm formation, we critically revise recent approaches for the improvement of in vitro and in vivo biofilm models.

Pseudomonas aeruginosa: An Audacious Pathogen with an Adaptable Arsenal of Virulence Factors

Pseudomonas aeruginosa is a dominant pathogen in people with cystic fibrosis (CF) contributing to morbidity and mortality. Its tremendous ability to adapt greatly facilitates its capacity to cause chronic infections. The adaptability and flexibility of the pathogen are afforded by the extensive number of virulence factors it has at its disposal, providing P. aeruginosa with the facility to tailor its response against the different stressors in the environment. A deep understanding of these virulence mechanisms is crucial for the design of therapeutic strategies and vaccines against this multi-resistant pathogen. Therefore, this review describes the main virulence factors of P. aeruginosa and the adaptations it undergoes to persist in hostile environments such as the CF respiratory tract. The very large P. aeruginosa genome (5 to 7 MB) contributes considerably to its adaptive capacity; consequently, genomic studies have provided significant insights into elucidating P. aeruginosa evolution and its interactions with the host throughout the course of infection.

PAR-2-activated secretion by airway gland serous cells: role for CFTR and inhibition by Pseudomonas aeruginosa

Airway submucosal gland serous cells are important sites of fluid secretion in conducting airways. Serous cells also express the cystic fibrosis (CF) transmembrane conductance regulator (CFTR). Protease-activated receptor 2 (PAR-2) is a G protein-coupled receptor that activates secretion from intact airway glands. We tested if and how human nasal serous cells secrete fluid in response to PAR-2 stimulation using Ca²⁺ imaging and simultaneous differential interference contrast imaging to track isosmotic cell shrinking and swelling reflecting activation of solute efflux and influx pathways, respectively. During stimulation of PAR-2, serous cells exhibited dose-dependent increases in intracellular Ca²⁺. At stimulation levels >EC₅₀ for Ca²⁺, serous cells simultaneously shrank ∼20% over ∼90 s due to KCl efflux reflecting Ca²⁺-activated Cl^- channel (CaCC, likely TMEM16A)-dependent secretion. At lower levels of PAR-2 stimulation (<EC₅₀ for Ca²⁺), shrinkage was not evident due to failure to activate CaCC. Low levels of cAMP-elevating VIP receptor (VIPR) stimulation, also insufficient to activate secretion alone, synergized with low-level PAR-2 stimulation to elicit fluid secretion dependent on both cAMP and Ca²⁺ to activate CFTR and K⁺ channels, respectively. Polarized cultures of primary serous cells also exhibited synergistic fluid secretion. Pre-exposure to Pseudomonas aeruginosa conditioned media inhibited PAR-2 activation by proteases but not peptide agonists in primary nasal serous cells, Calu-3 bronchial cells, and primary nasal ciliated cells. Disruption of synergistic CFTR-dependent PAR-2/VIPR secretion may contribute to reduced airway surface liquid in CF. Further disruption of the CFTR-independent component of PAR-2-activated secretion by P. aeruginosa may also be important to CF pathophysiology.

N-(3-oxododecanoyl)-homoserine lactone regulates osteoblast apoptosis and differentiation by mediating intracellular calcium

Pseudomonas aeruginosa (P. aeruginosa) is associated with periapical periodontitis. The lesions are characterized by a disorder in osteoblast metabolism. Quorum sensing molecular N-(3-oxododecanoyl)-homoserine lactone (AHL) is secreted by P. aeruginosa and governs the expression of numerous virulence factors. AHL can trigger intracellular calcium ([Ca²⁺]_i) fluctuations in many host cells. However, it is unclear whether AHL can regulate osteoblast metabolism by affecting [Ca²⁺]_i changes or its spatial correlation. We explored AHL-induced apoptosis and differentiation in pre-osteoblastic MC3T3-E1 cells and evaluated [Ca²⁺]_i mobilization using several extraction methods. The spatial distribution pattern of [Ca²⁺]_i among cells was investigated by Moran's I, an index of spatial autocorrelation. We found that 30 μM and 50 μM AHL triggered opposing osteoblast fates. At 50 μM, AHL inhibited osteoblast differentiation by promoting mitochondrial-dependent apoptosis and negatively regulating osteogenic marker genes, including Runx2, Osterix, bone sialoprotein (Bsp), and osteocalcin (OCN). In contrast, prolonged treatment with 30 μM AHL promoted osteoblast differentiation concomitantly with cell apoptosis. The elevation of [Ca²⁺]_i levels in osteoblasts treated with 50 μM AHL was spatially autocorrelated, while no such phenomenon was observed in 30 μM AHL-treated osteoblasts. The blocking of cell-to-cell spatial autocorrelation in the osteoblasts provoked by 50 μM AHL significantly inhibited apoptosis and partially restored differentiation. Our observations suggest that AHL affects the fate of osteoblasts (apoptosis and differentiation) by affecting the spatial correlation of [Ca²⁺]_i changes. Thus, AHL acts as a double-edged sword for osteoblast function.

Human Single-chain Variable Fragments Neutralize Pseudomonas aeruginosa Quorum Sensing Molecule, 3O-C12-HSL, and Prevent Cells From the HSL-mediated Apoptosis

The quorum sensing (QS) signaling molecule, N-(3-oxododecanoyl)-L-homoserine lactone (3O-C12-HSL), contributes to the pathogenesis of Pseudomonas aeruginosa by regulating expression of the bacterial virulence factors that cause intense inflammation and toxicity in the infected host. As such, the QS molecule is an attractive therapeutic target for direct-acting inhibitors. Several substances, both synthetic and naturally derived products, have shown effectiveness against detrimental 3O-C12-HSL activity. Unfortunately, these compounds are relatively toxic to mammalian cells, which limits their clinical application. In this study, fully human single-chain variable fragments (HuscFvs) that bind to P. aeruginosa haptenic 3O-C12-HSL were generated based on the principle of antibody polyspecificity and molecular mimicry of antigenic molecules. The HuscFvs neutralized 3O-C12-HSL activity and prevented mammalian cells from the HSL-mediated apoptosis, as observed by Annexin V/PI staining assay, sub-G1 arrest population investigation, transmission electron microscopy for ultrastructural morphology of mitochondria, and confocal microscopy for nuclear condensation and DNA fragmentation. Computerized homology modeling and intermolecular docking predicted that the effective HuscFvs interacted with several regions of the bacterially derived ligand that possibly conferred neutralizing activity. The effective HuscFvs should be tested further in vitro on P. aeruginosa phenotypes as well as in vivo as a sole or adjunctive therapeutic agent against P. aeruginosa infections, especially in antibiotic-resistant cases.

Pseudomonas aeruginosa N-3-Oxo-Dodecanoyl-Homoserine Lactone Impacts Mitochondrial Networks Morphology, Energetics, and Proteome in Host Cells

Mitochondria play crucial roles in cellular metabolism, signaling, longevity, and immune defense. Recent evidences have revealed that the host microbiota, including bacterial pathogens, impact mitochondrial behaviors and activities in the host. The pathogenicity of Pseudomonas aeruginosa requires quorum sensing (QS) cell-cell communication allowing the bacteria to sense population density and collectively control biofilm development, virulence traits, adaptation and interactions with the host. QS molecules, like N-3-oxo-dodecanoyl-L-homoserine lactone (3O-C₁₂-HSL), can also modulate the behavior of host cells, e.g., epithelial barrier properties and innate immune responses. Here, in two types of cells, fibroblasts and intestinal epithelial cells, we investigated whether and how P. aeruginosa 3O-C₁₂-HSL impacts the morphology of mitochondrial networks and their energetic characteristics, using high-resolution transmission electron microscopy, fluorescence live-cell imaging, assay for mitochondrial bioenergetics, and quantitative mass spectrometry for mitoproteomics and bioinformatics. We found that 3O-C₁₂-HSL induced fragmentation of mitochondria, disruption of cristae and inner membrane ultrastructure, altered major characteristics of respiration and energetics, and decreased mitochondrial membrane potential, and that there are distinct cell-type specific details of these effects. Moreover, this was mechanistically accompanied by differential expression of both common and cell-type specific arrays of components in the mitochondrial proteome involved in their structural organization, electron transport chain complexes and response to stress. We suggest that this effect of 3O-C₁₂-HSL on mitochondria may represent one of the events in the interaction between P. aeruginosa and host mitochondria and may have an impact on the pathogens strategy to hijack host cell activities to support their own survival and spreading.

Challenges and Limitations of Anti-quorum Sensing Therapies

Quorum sensing (QS) is a mechanism allowing microorganisms to sense population density and synchronously control genes expression. It has been shown that QS supervises the activity of many processes important for microbial pathogenicity, e.g., sporulation, biofilm formation, and secretion of enzymes or membrane vesicles. This contributed to the concept of anti-QS therapy [also called quorum quenching (QQ)] and the opportunity of its application in fighting against various types of pathogens. In recent years, many published articles reported promising results indicating the possibility of reducing pathogenicity of tested microorganisms and their easier eradication when co-treated with antibiotics. The aim of the present article is to point to the opposite, negative side of the QQ therapy, with particular emphasis on three fundamental properties attributed to anti-QS substances: the selectivity, virulence reduction, and lack of resistance against QQ. This point of view may highlight new directions of research, which should be taken into account in the future before the widespread introduction of QQ therapies in the treatment of people.

Pseudomonas aeruginosa Induced Host Epithelial Cell Mitochondrial Dysfunction

The pathogenicity of P. aeruginosa is dependent on quorum sensing (QS), an inter-bacterial communication system that can also modulate host biology. The innate immune function of the lung mucosal barrier is dependent on proper mitochondrial function. The purpose of this study was to define the mechanism by which bacterial factors modulate host lung epithelial cell mitochondrial function and to investigate novel therapies that ameliorate this effect. 3-oxo-C12-HSL disrupts mitochondrial morphology, attenuates mitochondrial bioenergetics, and induces mitochondrial DNA oxidative injury. Mechanistically, we show that 3-oxo-C12-HSL attenuates expression of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), a master regulator of mitochondrial biogenesis, antioxidant defense, and cellular respiration, and its downstream effectors in both BEAS-2B and primary lung epithelial cells. Overexpression of PGC-1α attenuates the inhibition in cellular respiration caused by 3-oxo-C12-HSL. Pharmacologic activation of PGC-1α restores barrier integrity in cells treated with 3-oxo-C12-HSL. These data demonstrate that the P. aeruginosa QS molecule, 3-oxo-C12-HSL, alters mitochondrial pathways critical for lung mucosal immunity. Genetic and pharmacologic strategies that activate the PGC-1α pathway enhance host epithelial cell mitochondrial function and improve the epithelial innate response to P. aeruginosa. Therapies that rescue PGC-1α function may provide a complementary approach in the treatment of P. aeruginosa infection.

Bacterial quorum-sensing signal IQS induces host cell apoptosis by targeting POT1-p53 signalling pathway

Pseudomonas aeruginosa, an opportunistic life-threatening human bacterial pathogen, employs quorum-sensing (QS) signal molecules to modulate virulence gene expression. 2-(2-hydroxyphenyl)-thiazole-4-carbaldehyde (IQS) is a recently identified QS signal that integrates the canonical lasR-type QS of P. aeruginosa and host phosphate stress response to fine-tune its virulence production for a successful infection. To address the role of IQS in pathogen-host interaction, we here present that IQS inhibits host cell growth and stimulates apoptosis in a dosage-dependent manner. By downregulating the telomere-protecting protein POT1 in host cells, IQS activates CHK1, CHK2, and p53 in an Ataxia telangiectasia mutated (ATM)/ATM and RAD3-related (ATR)-dependent manner and induces DNA damage response. Overexpression of POT1 in host cells presents a resistance to IQS treatment. These results suggest a pivotal role of IQS in host apoptosis, highlighting the complexity of pathogenesis mechanisms developed by P. aeruginosa during infection.

Caspase-1-dependent mechanism mediating the harmful impacts of the quorum-sensing molecule N-(3-oxo-dodecanoyl)-l-homoserine lactone on the intestinal cells

N-(3-oxododecanoyl)-l-homoserine lactone (3-oxo-C12-HSL), a quorum-sensing (QS) molecule produced by Gram-negative bacteria in the gastrointestinal tract, adversly impacts host cells. Our previous study demonstrated that 3-oxo-C12-HSL induced a decrease in cell viability via cell apoptosis and eventually disrupted mucin synthesis from LS174T goblet cells. However, the molecular mechanism underlying cell apoptosis and whether pyroptosis was involved in this process are still unknown. In this study, we emphasized on the caspases signal pathway and sterile inflammation to reveal the harmful effects of 3-oxo-C12-HSL on LS174T goblet cells. Our data showed that 3-oxo-C12-HSL is a major inducer of oxidative stress indicated by a high level of intracellular reactive oxygen species (ROS). However, TQ416, an inhibitor of paraoxonase 2, can effectively block oxidative stress. A higher ROS level is the trigger for activating the caspase-1 and 3 cascade signal pathways. Blockade of ROS synthesis and caspase-1 and 3 cascades can obviously rescue the viability of LS174T cells after 3-oxo-C12-HSL treatment. We also found that paralleled with a higher level of ROS and caspases activation, an abnormal expression of proinflammatory cytokines was induced by 3-oxo-C12-HSL treatment; however, the blockage of TLRs-NF-κB pathway cannot restore cell viability and secretary function. These data collectively indicate that 3-oxo-C12-HSL exposure induces damages to cell viability and secretary function of LS174T goblet cells, which is mediated by oxidative stress, cell apoptosis, and sterile inflammation. Overall, the data in this study will provide a better understanding of the harmful impacts of some QS molecules on host cells and their underlying mechanism.

Quorum sensing molecules regulate epithelial cytokine response and biofilm-related virulence of three Prevotella species

Quorum sensing (QS) signaling regulates the motility, adhesion, and biofilm formation of bacteria, and at the same time activates immune response in eukaryotic organisms. We recently demonstrated that the QS molecule, dihydroxy-2, 3-pentanedione (DPD), and its analogs significantly inhibit estradiol-regulated virulence of Prevotella aurantiaca, one of the four species in the Prevotella intermedia group. Here, we examined the combined effects of estradiol and QS signaling on 1) cytokine response of human gingival keratinocytes (HMK) against whole cell extract (WCE) of P. intermedia, Prevotella nigrescens, and Prevotella pallens, and 2) biofilm formation of these three Prevotella species. All experiments were performed in the presence or absence of estradiol, and with different QS molecules: DPD and its analogs (ethyl-DPD, butyl-DPD, and isobutyl-DPD). Concentrations of interleukin (IL)-1β, -6, and -8 were determined by the Luminex multiplex immunoassay, biofilm mass was quantitatively evaluated by measuring protein concentration via the Bradford method, and the microtopography of biofilms was assessed by scanning electron microscopy (SEM) imaging. Concentrations of IL-6 and IL-8 were elevated when HMK cells were incubated with estradiol and WCE of P. intermedia and P. nigrescens, but decreased when incubated with estradiol and WCE of P. pallens. Butyl-DPD neutralized the estradiol- and WCE-induced regulation of HMK interleukin expression and, at the same time, inhibited the biofilm formation of P. intermedia and P. nigrescens. SEM micrographs revealed a decrease in biofilm mass after application of butyl-DPD, which was most detectable among the P. intermedia ATCC 25611 and P. nigrescens ATCC 33563 and AHN 8293 strains. In conclusion, butyl-DPD analog is able to neutralize the WCE-induced epithelial cytokine response and, at the same time, to inhibit the biofilm formation of P. intermedia and P. nigrescens.

Heterogeneity of Microbiota Dysbiosis in Chronic Rhinosinusitis: Potential Clinical Implications and Microbial Community Mechanisms Contributing to Sinonasal Inflammation

Recent studies leveraging next-generation sequencing and functional approaches to understand the human microbiota have demonstrated the presence of diverse, niche-specific microbial communities at nearly every mucosal surface. These microbes contribute to the development and function of physiologic and immunological features that are key to host health status. Not surprisingly, several chronic inflammatory diseases have been attributed to dysbiosis of microbiota composition or function, including chronic rhinosinusitis (CRS). CRS is a heterogeneous disease characterized by inflammation of the sinonasal cavity and mucosal microbiota dysbiosis. Inflammatory phenotypes and bacterial community compositions vary considerably across individuals with CRS, complicating current studies that seek to address causality of a dysbiotic microbiome as a driver or initiator of persistent sinonasal inflammation. Murine models have provided some experimental evidence that alterations in local microbial communities and microbially-produced metabolites influence health status. In this perspective, we will discuss the clinical implications of distinct microbial compositions and community-level functions in CRS and how mucosal microbiota relate to the diverse inflammatory endotypes that are frequently observed. We will also describe specific microbial interactions that can deterministically shape the pattern of co-colonizers and the resulting metabolic products that drive or exacerbate host inflammation. These findings are discussed in the context of CRS-associated inflammation and in other chronic inflammatory diseases that share features observed in CRS. An improved understanding of CRS patient stratification offers the opportunity to personalize therapeutic regimens and to design novel treatments aimed at manipulation of the disease-associated microbiota to restore sinus health.

N-(3-oxododecanoyl)-l-homoserine lactone modulates mitochondrial function and suppresses proliferation in intestinal goblet cells

AIMS

The quorum-sensing molecule N‑(3‑oxododecanoyl)‑l‑homoserine lactone (C12-HSL), produced by the Gram negative human pathogenic bacterium Pseudomonas aeruginosa, modulates mammalian cell behavior. Our previous findings suggested that C12-HSL rapidly decreases viability and induces apoptosis in LS174T goblet cells.

MAIN METHODS

In this study, the effects of 100 μM C12-HSL on mitochondrial function and cell proliferation in LS174T cells treated for 4 h were evaluated by real-time PCR, enzyme-linked immunosorbent assay (ELISA) and flow cytometry.

KEY FINDINGS

The results showed that the activities of mitochondrial respiratory chain complexes IV and V were significantly increased (P < 0.05) in LS174T cells after C12-HSL treatment, with elevated intracellular ATP generation (P < 0.05). Flow cytometry analysis revealed significantly increased intracellular Ca²⁺ levels (P < 0.05), as well as disrupted mitochondrial activity and cell cycle arrest upon C12-HSL treatment. Apoptosis and cell proliferation related genes showed markedly altered expression levels (P < 0.05) in LS174T cells after C12-HSL treatment. Moreover, the paraoxonase 2 (PON2) inhibitor TQ416 (1 μM) remarkably reversed the above C12-HSL associated effects in LS174T cells.

SIGNIFICANCE

These findings indicated that C12-HSL alters mitochondrial energy production and function, and inhibits cell proliferation in LS174T cells, with PON2 involvement.

N-(3-Oxo-acyl)-homoserine lactone induces apoptosis primarily through a mitochondrial pathway in fibroblasts

N-(3-Oxododecanoyl)-l-homoserine lactone (C12) is produced by Pseudomonas aeruginosa to function as a quorum-sensing molecule for bacteria-bacteria communication. C12 is also known to influence many aspects of human host cell physiology, including induction of cell death. However, the signalling pathway(s) leading to C12-triggered cell death is (are) still not completely known. To clarify cell death signalling induced by C12, we examined mouse embryonic fibroblasts deficient in "initiator" caspases or "effector" caspases. Our data indicate that C12 selectively induces the mitochondria-dependent intrinsic apoptotic pathway by quickly triggering mitochondrial outer membrane permeabilisation. Importantly, the activities of C12 to permeabilise mitochondria are independent of activation of both "initiator" and "effector" caspases. Furthermore, C12 directly induces mitochondrial outer membrane permeabilisation in vitro. Overall, our study suggests a mitochondrial apoptotic signalling pathway triggered by C12, in which C12 or its metabolite(s) acts on mitochondria to permeabilise mitochondria, leading to activation of apoptosis.

N-(3-oxododecanoyl)-L-homoserine lactone interactions in the breast tumor microenvironment: Implications for breast cancer viability and proliferation in vitro

It is well documented that the tumor microenvironment profoundly impacts the etiology and progression of breast cancer, yet the contribution of the resident microbiome within breast tissue remains poorly understood. Tumor microenvironmental conditions, such as hypoxia and dense tumor stroma, predispose progressive phenotypes and therapy resistance, however the role of bacteria in this interplay remains uncharacterized. We hypothesized that the effect of individual bacterial secreted molecules on breast cancer viability and proliferation would be modulated by these tumor-relevant stressors differentially for cells at varying stages of progression. To test this, we incubated human breast adenocarcinoma cells (MDA-MB-231, MCF-DCIS.com) and non-malignant breast epithelial cells (MCF-10A) with N-(3-oxododecanoyl)-L-homoserine lactone (OdDHL), a quorum-sensing molecule from Pseudomonas aeruginosa that regulates bacterial stress responses. This molecule was selected because Pseudomonas was recently characterized as a significant fraction of the breast tissue microbiome and OdDHL is documented to impact mammalian cell viability. After OdDHL treatment, we demonstrated the greatest decrease in viability with the more malignant MDA-MB-231 cells and an intermediate MCF-DCIS.com (ductal carcinoma in situ) response. The responses were also culture condition (i.e. microenvironment) dependent. These results contrast the MCF-10A response, which demonstrated no change in viability in any culture condition. We further determined that the observed trends in breast cancer viability were due to modulation of proliferation for both cell types, as well as the induction of necrosis for MDA-MB-231 cells in all conditions. Our results provide evidence that bacterial quorum-sensing molecules interact with the host tissue environment to modulate breast cancer viability and proliferation, and that the effect of OdDHL is dependent on both cell type as well as microenvironment. Understanding the interactions between bacterial signaling molecules and the host tissue environment will allow for future studies that determine the contribution of bacteria to the onset, progression, and therapy response of breast cancer.

Intra-Species and Inter-Kingdom Signaling of Legionella pneumophila

The ubiquitous Gram-negative bacterium Legionella pneumophila parasitizes environ mental amoebae and, upon inhalation, replicates in alveolar macrophages, thus causing a life-threatening pneumonia called “Legionnaires’ disease.” The opportunistic pathogen employs a bi-phasic life cycle, alternating between a replicative, non-virulent phase and a stationary, transmissive/virulent phase. L. pneumophila employs the Lqs (Legionella quorum sensing) system as a major regulator of the growth phase switch. The Lqs system comprises the autoinducer synthase LqsA, the homologous sensor kinases LqsS and LqsT, as well as a prototypic response regulator termed LqsR. These components produce, detect, and respond to the α-hydroxyketone signaling molecule LAI-1 (Legionella autoinducer-1, 3-hydroxypentadecane-4-one). LAI-1-mediated signal transduction through the sensor kinases converges on LqsR, which dimerizes upon phosphorylation. The Lqs system regulates the bacterial growth phase switch, pathogen-host cell interactions, motility, natural competence, filament production, and expression of a chromosomal “fitness island.” Yet, LAI-1 not only mediates bacterial intra-species signaling, but also modulates the motility of eukaryotic cells through the small GTPase Cdc42 and thus promotes inter-kingdom signaling. Taken together, the low molecular weight compound LAI-1 produced by L. pneumophila and sensed by the bacteria as well as by eukaryotic cells plays a major role in pathogen-host cell interactions.

Paraoxonase 2 modulates a proapoptotic function in LS174T cells in response to quorum sensing molecule N-(3-oxododecanoyl)-L-homoserine lactone

A mucus layer coats the gastrointestinal tract and serves as the first line of intestinal defense against infection. N-acyl-homoserine lactone (AHL) quorum-sensing molecules produced by gram-negative bacteria in the gut can influence the homeostasis of intestinal epithelium. In this study, we investigated the effects of two representative long- and short-chain AHLs, N-3-(oxododecanoyl)-homoserine lactone (C12-HSL) and N-butyryl homoserine lactone (C4-HSL), on cell viability and mucus secretion in LS174T cells. C12-HSL but not C4-HSL significantly decreased cell viability by inducing mitochondrial dysfunction and activating cell apoptosis which led to a decrease in mucin expression. Pretreatment with lipid raft disruptor (Methyl-β-cyclodextrin, MβCD) and oxidative stress inhibitor (N-acetyl-L-cysteine, NAC) slightly rescued the viability of cells damaged by C12-HSL exposure, while the paraoxonase 2 (PON2) inhibitor (Triazolo[4,3-a]quinolone, TQ416) significantly affected recovering cells viability and mucin secretion. When LS174T cells were treated with C12-HSL and TQ416 simultaneously, TQ416 showed the maximal positive effect on cells viability. However, if cells were first treated with C12-HSL for 40 mins, and then TQ46 was added, the TQ416 had no effect on cell viability. These results suggest that the C12-HSL-acid process acts at an early step to activate apoptosis as part of C12-HSL’s effect on intestinal mucus barrier function.

Dissecting Redox Biology Using Fluorescent Protein Sensors

SIGNIFICANCE

Fluorescent protein sensors have revitalized the field of redox biology by revolutionizing the study of redox processes in living cells and organisms.

RECENT ADVANCES

Within one decade, a set of fundamental new insights has been gained, driven by the rapid technical development of in vivo redox sensing. Redox-sensitive yellow and green fluorescent protein variants (rxYFP and roGFPs) have been the central players.

CRITICAL ISSUES

Although widely used as an established standard tool, important questions remain surrounding their meaningful use in vivo. We review the growing range of thiol redox sensor variants and their application in different cells, tissues, and organisms. We highlight five key findings where in vivo sensing has been instrumental in changing our understanding of redox biology, critically assess the interpretation of in vivo redox data, and discuss technical and biological limitations of current redox sensors and sensing approaches.

FUTURE DIRECTIONS

We explore how novel sensor variants may further add to the current momentum toward a novel mechanistic and integrated understanding of redox biology in vivo. Antioxid. Redox Signal. 24, 680-712.

Adh enhances Actinobacillus pleuropneumoniae pathogenicity by binding to OR5M11 and activating p38 which induces apoptosis of PAMs and IL-8 release

Members of the Trimeric Autotransporter Adhesin (TAA) family play a crucial role in the adhesion of Gram-negative pathogens to host cells, but the immunopathogenesis of TAAs remains unknown. Our previous studies demonstrated that Adh from Actinobacillus pleuropneumoniae (A. pleuropneumoniae) is required for full bacterial pathogenicity. Alveolar macrophages are the first line of defense against respiratory infections. This study compared the interactions between porcine alveolar macrophages (PAMs) and wild-type A. pleuropneumoniae (5b WT) or an Adh-deletion strain (5b ΔAdh) via gene microarray, immunoprecipitation and other technologies. We found that Adh was shown to interact with the PAMs membrane protein OR5M11, an olfactory receptor, resulting in the high-level secretion of IL-8 by activation of p38 MAPK signaling pathway. Subsequently, PAMs apoptosis via the activation of the Fax and Bax signaling pathways was observed, followed by activation of caspases 8, 9, and 3. The immunological pathogenic roles of Adh were also confirmed in both murine and piglets infectious models in vivo. These results identify a novel immunological strategy for TAAs to boost the pathogenicity of A. pleuropneumoniae. Together, these datas reveal the high versatility of the Adh protein as a virulence factor and provide novel insight into the immunological pathogenic role of TAAs.

Chemotaxis and Binding of Pseudomonas aeruginosa to Scratch-Wounded Human Cystic Fibrosis Airway Epithelial Cells

Confocal imaging was used to characterize interactions of Pseudomonas aeruginosa (PA, expressing GFP or labeled with Syto 11) with CF airway epithelial cells (CFBE41o-, grown as confluent monolayers with unknown polarity on coverglasses) in control conditions and following scratch wounding. Epithelia and PAO1-GFP or PAK-GFP (2 MOI) were incubated with Ringer containing typical extracellular salts, pH and glucose and propidium iodide (PI, to identify dead cells). PAO1 and PAK swam randomly over and did not bind to nonwounded CFBE41o- cells. PA migrated rapidly (began within 20 sec, maximum by 5 mins) and massively (10–80 fold increase, termed “swarming”), but transiently (random swimming after 15 mins), to wounds, particularly near cells that took up PI. Some PA remained immobilized on cells near the wound. PA swam randomly over intact CFBE41o- monolayers and wounded monolayers that had been incubated with medium for 1 hr. Expression of CFTR and altered pH of the media did not affect PA interactions with CFBE41o- wounds. In contrast, PAO1 swarming and immobilization along wounds was abolished in PAO1 (PAO1ΔcheYZABW, no expression of chemotaxis regulatory components cheY, cheZ, cheA, cheB and cheW) and greatly reduced in PAO1 that did not express amino acid receptors pctA, B and C (PAO1ΔpctABC) and in PAO1 incubated in Ringer containing a high concentration of mixed amino acids. Non-piliated PAKΔpilA swarmed normally towards wounded areas but bound infrequently to CFBE41o- cells. In contrast, both swarming and binding of PA to CFBE41o- cells near wounds were prevented in non-flagellated PAKΔfliC. Data are consistent with the idea that (i) PA use amino acid sensor-driven chemotaxis and flagella-driven swimming to swarm to CF airway epithelial cells near wounds and (ii) PA use pili to bind to epithelial cells near wounds.

Modulation of Host Biology by Pseudomonas aeruginosa Quorum Sensing Signal Molecules: Messengers or Traitors

Bacterial cells sense their population density and respond accordingly by producing various signal molecules to the surrounding environments thereby trigger a plethora of gene expression. This regulatory pathway is termed quorum sensing (QS). Plenty of bacterial virulence factors are controlled by QS or QS-mediated regulatory systems and QS signal molecules (QSSMs) play crucial roles in bacterial signaling transduction. Moreover, bacterial QSSMs were shown to interfere with host cell signaling and modulate host immune responses. QSSMs not only regulate the expression of bacterial virulence factors but themselves act in the modulation of host biology that can be potential therapeutic targets.

Airway Epithelial Cell Integrity Protects from Cytotoxicity of Pseudomonas aeruginosa Quorum-Sensing Signals

Cell-to-cell communication via gap junctions regulates airway epithelial cell homeostasis and maintains the epithelium host defense. Quorum-sensing molecules produced by Pseudomonas aeruginosa coordinate the expression of virulence factors by this respiratory pathogen. These bacterial signals may also incidentally modulate mammalian airway epithelial cell responses to the pathogen, a process called interkingdom signaling. We investigated the interactions between the P. aeruginosa N-3-oxo-dodecanoyl-L-homoserine lactone (C12) quorum-sensing molecule and human airway epithelial cell gap junctional intercellular communication (GJIC). C12 degradation and its effects on cells were monitored in various airway epithelial cell models grown under nonpolarized and polarized conditions. Its concentration was further monitored in daily tracheal aspirates of colonized intubated patients. C12 rapidly altered epithelial integrity and decreased GJIC in nonpolarized airway epithelial cells, whereas other quorum-sensing molecules had no effect. The effects of C12 were dependent on [Ca(2+)]i and could be prevented by inhibitors of Src tyrosine family and Rho-associated protein kinases. In contrast, polarized airway cells grown on Transwell filters were protected from C12 except when undergoing repair after wounding. In vivo during colonization of intubated patients, C12 did not accumulate, but it paralleled bacterial densities. In vitro C12 degradation, a reaction catalyzed by intracellular paraoxonase 2 (PON2), was impaired in nonpolarized cells, whereas PON2 expression was increased during epithelial polarization. The cytotoxicity of C12 on nonpolarized epithelial cells, combined with its impaired degradation allowing its accumulation, provides an additional pathogenic mechanism for P. aeruginosa infections.

Paraoxonase 2 serves a proapopotic function in mouse and human cells in response to the Pseudomonas aeruginosa quorum-sensing molecule N-(3-Oxododecanoyl)-homoserine lactone

Pseudomonas aeruginosa use quorum-sensing molecules, including N-(3-oxododecanoyl)-homoserine lactone (C12), for intercellular communication. C12 activated apoptosis in mouse embryo fibroblasts (MEF) from both wild type (WT) and Bax/Bak double knock-out mice (WT MEF and DKO MEF that were responsive to C12, DKOR MEF): nuclei fragmented; mitochondrial membrane potential (Δψmito) depolarized; Ca(2+) was released from the endoplasmic reticulum (ER), increasing cytosolic [Ca(2+)] (Cacyto); and caspase 3/7 was activated. DKOR MEF had been isolated from a nonclonal pool of DKO MEF that were non-responsive to C12 (DKONR MEF). RNAseq analysis, quantitative PCR, and Western blots showed that WT and DKOR MEF both expressed genes associated with cancer, including paraoxonase 2 (PON2), whereas DKONR MEF expressed little PON2. Adenovirus-mediated expression of human PON2 in DKONR MEF rendered them responsive to C12: Δψmito depolarized, Cacyto increased, and caspase 3/7 activated. Human embryonic kidney 293T (HEK293T) cells expressed low levels of endogenous PON2, and these cells were also less responsive to C12. Overexpression of PON2, but not PON2-H114Q (no lactonase activity) in HEK293T cells caused them to become sensitive to C12. Because [C12] may reach high levels in biofilms in lungs of cystic fibrosis (CF) patients, PON2 lactonase activity may control Δψmito, Ca(2+) release from the ER, and apoptosis in CF airway epithelia. Coupled with previous data, these results also indicate that PON2 uses its lactonase activity to prevent Bax- and Bak-dependent apoptosis in response to common proapoptotic drugs like doxorubicin and staurosporine, but activates Bax- and Bak-independent apoptosis in response to C12.

Bacterial quorum sensing molecule N-3-oxo-dodecanoyl-L-homoserine lactone causes direct cytotoxicity and reduced cell motility in human pancreatic carcinoma cells

In spite of chemotherapeutic and surgical advances, pancreatic cancer continues to have a dismal prognosis. Metastasis due to tumor cell migration remains the most critical challenge in treating pancreatic cancer, and conventional chemotherapy is rarely curative. In the quest for more novel molecules to fight this disease, we tested the hypothesis that the Pseudomonas aeruginosa quorum sensing signal molecule N-3-oxo-dodecanoyl-L-homoserine lactone (O-DDHSL) would be cytotoxic to and reduce mobility of pancreatic carcinoma cells (Panc-1 and Aspc-1). Results showed a decrease in cell viability from apoptosis, diminished colony formation, and inhibition of migration of the evaluated pancreatic carcinoma cell lines. Also, cell viability decreased in the presence of O-DDHSL when cells were grown in matrigel basement membrane matrix. While messenger RNA for IQGAP-1 decreased in Panc-1 and HPDE cells upon exposure to O-DDHSL, no change was observed in Aspc-1 cells. Cofilin mRNA expression was found to be increased in both HPDE and Panc-1 cells with marginal decrease in Aspc-1 cells. RhoC, a Rho-family GTPase involved in cell motility, increased in the presence of O-DDHSL, suggesting a possible compensatory response to alteration in other migration associated genes. Our results indicate that O-DDHSL could be an effective biomolecule in eukaryotic systems with multimodal function for essential molecular targeting in pancreatic cancer.

Pseudomonas aeruginosa quorum-sensing molecule homoserine lactone modulates inflammatory signaling through PERK and eI-F2α

Pseudomonas aeruginosa secrete N-(3-oxododecanoyl)-homoserine lactone (HSL-C12) as a quorum-sensing molecule to regulate bacterial gene expression. Because HSL-C12 is membrane permeant, multiple cell types in P. aeruginosa-infected airways may be exposed to HSL-C12, especially adjacent to biofilms where local (HSL-C12) may be high. Previous reports showed that HSL-C12 causes both pro- and anti-inflammatory effects. To characterize HSL-C12's pro- and anti-inflammatory effects in host cells, we measured protein synthesis, NF-κB activation, and KC (mouse IL-8) and IL-6 mRNA and protein secretion in wild-type mouse embryonic fibroblasts (MEF). To test the role of the endoplasmic reticulum stress inducer, PERK we compared these responses in PERK(-/-) and PERK-corrected PERK(-/-) MEF. During 4-h treatments of wild-type MEF, HSL-C12 potentially activated NF-κB p65 by preventing the resynthesis of IκB and increased transcription of KC and IL-6 genes (quantitative PCR). HSL-C12 also inhibited secretion of KC and/or IL-6 into the media (ELISA) both in control conditions and also during stimulation by TNF-α. HSL-C12 also activated PERK (as shown by increased phosphorylation of eI-F2α) and inhibited protein synthesis (as measured by incorporation of [(35)S]methionine by MEF). Comparisons of PERK(-/-) and PERK-corrected MEF showed that HSL-C12's effects were explained in part by activation of PERK→phosphorylation of eI-F2α→inhibition of protein synthesis→reduced IκBα production→activation of NF-κB→increased transcription of the KC gene but reduced translation and secretion of KC. HSL-C12 may be an important modulator of early (up to 4 h) inflammatory signaling in P. aeruginosa infections.

Disruption of epithelial barrier by quorum-sensing N-3-(oxododecanoyl)-homoserine lactone is mediated by matrix metalloproteinases

The intestinal epithelium forms a selective barrier maintained by tight junctions (TJs) and separating the luminal environment from the submucosal tissues. N-acylhomoserine lactone (AHL) quorum-sensing molecules produced by gram-negative bacteria in the gut can influence homeostasis of the host intestinal epithelium. In the present study, we evaluated the regulatory mechanisms affecting the impact of two representative long- and short-chain AHLs, N-3-(oxododecanoyl)-homoserine lactone (C12-HSL) and N-butyryl homoserine lactone (C4-HSL), on barrier function of human intestinal epithelial Caco-2 cells. Treatment with C12-HSL, but not with C4-HSL, perturbed Caco-2 barrier function; the effect was associated with decreased levels of the TJ proteins occludin and tricellulin and their delocalization from the TJs. C12-HSL also induced matrix metalloprotease (MMP)-2 and MMP-3 activation via lipid raft- and protease-activated receptor (PAR)-dependent signaling. Pretreatment with lipid raft disruptors, PAR antagonists, or MMP inhibitors restored the C12-HSL-induced loss of the TJ proteins and increased permeability of Caco-2 cell monolayers. These results indicate that PAR/lipid raft-dependent MMP-2 and -3 activation followed by degradation of occludin and tricellulin are involved in C12-HSL-induced alterations of epithelial paracellular barrier functions.

Transport proteins promoting Escherichia coli pathogenesis

Escherichia coli is a genetically diverse species infecting hundreds of millions of people worldwide annually. We examined seven well-characterized E. coli pathogens causing urinary tract infections, gastroenteritis, pyelonephritis and haemorrhagic colitis. Their transport proteins were identified and compared with each other and a non-pathogenic E. coli K12 strain to identify transport proteins related to pathogenesis. Each pathogen possesses a unique set of protein secretion systems for export to the cell surface or for injecting effector proteins into host cells. Pathogens have increased numbers of iron siderophore receptors and ABC iron uptake transporters, but the numbers and types of low-affinity secondary iron carriers were uniform in all strains. The presence of outer membrane iron complex receptors and high-affinity ABC iron uptake systems correlated, suggesting co-evolution. Each pathovar encodes a different set of pore-forming toxins and virulence-related outer membrane proteins lacking in K12. Intracellular pathogens proved to have a characteristically distinctive set of nutrient uptake porters, different from those of extracellular pathogens. The results presented in this report provide information about transport systems relevant to various types of E. coli pathogenesis that can be exploited in future basic and applied studies.

Thapsigargin blocks Pseudomonas aeruginosa homoserine lactone-induced apoptosis in airway epithelia

Pseudomonas aeruginosa secretes N-(3-oxododecanoyl)-homoserine lactone (C12) as a quorum-sensing molecule to regulate gene expression. Micromolar concentrations are found in the airway surface liquid of infected lungs. Exposure of the airway surface to C12 caused a loss of transepithelial resistance within 1 h that was accompanied by disassembly of tight junctions, as indicated by relocation of the tight junction protein zonula occludens 1 from the apical to the basolateral pole and into the cytosol of polarized human airway epithelial cell cultures (Calu-3 and primary tracheal epithelial cells). These effects were blocked by carbobenzoxy-valyl-alanyl-aspartyl-[O-methyl]-fluoromethylketone, a pan-caspase blocker, indicating that tight junction disassembly was an early event in C12-triggered apoptosis. Short-duration (10 min) pretreatment of airway epithelial (Calu-3 and JME) cells with 1 μM thapsigargin (Tg), an inhibitor of Ca(2+) uptake into the endoplasmic reticulum (ER), was found to be protective against the C12-induced airway epithelial barrier breakdown and also against other apoptosis-related effects, including shrinkage and fragmentation of nuclei, activation of caspase 3/7 (the executioner caspase in apoptosis), release of ER-targeted redox-sensitive green fluorescent protein into the cytosol, and depolarization of mitochondrial membrane potential. Pretreatment of Calu-3 airway cell monolayers with BAPTA-AM [to buffer cytosolic Ca(2+) concentration (Cacyto)] or Ca(2+)-free solution + BAPTA-AM reduced C12 activation of apoptotic events, suggesting that C12-triggered apoptosis may involve Ca(2+). Because C12 and Tg reduced Ca(2+) concentration in the ER and increased Cacyto, while Tg increased mitochondrial Ca(2+) concentration (Camito) and C12 reduced Camito, it is proposed that Tg may reduce C12-induced apoptosis in host cells not by raising Cacyto, but by preventing C12-induced decreases in Camito.

Pseudomonas aeruginosa homoserine lactone triggers apoptosis and Bak/Bax-independent release of mitochondrial cytochrome C in fibroblasts

Pseudomonas aeruginosa use N-(3-oxododecanoyl)-homoserine lactone (C12) as a quorum-sensing molecule to regulate gene expression in the bacteria. It is expected that in patients with chronic infections with P. aeruginosa, especially as biofilms, local [C12] will be high and, since C12 is lipid soluble, diffuse from the airways into the epithelium and underlying fibroblasts, capillary endothelia and white blood cells. Previous work showed that C12 has multiple effects in human host cells, including activation of apoptosis. The present work tested the involvement of Bak and Bax in C12-triggered apoptosis in mouse embryo fibroblasts (MEF) by comparing MEF isolated from embryos of wild-type (WT) and Bax(-/-) /Bak(-/-) (DKO) mice. In WT MEF C12 rapidly triggered (minutes to 2 h): activation of caspases 3/7 and 8, depolarization of mitochondrial membrane potential (Δψmito ), release of cytochrome C from mitochondria into the cytosol, blebbing of plasma membranes, shrinkage/condensation of cells and nuclei and, subsequently, cell killing. A DKO MEF line that was relatively unaffected by the Bak/Bax-dependent proapoptotic stimulants staurosporine and etoposide responded to C12 similarly to WT MEF: activation of caspase 3/7, depolarization of Δψmito and release of cytochrome C and cell death. Re-expression of Bax or Bak in DKO MEF did not alter the WT-like responses to C12 in DKO MEF. These data showed that C12 triggers novel, rapid proapoptotic Bak/Bax-independent responses that include events commonly associated with activation of both the intrinsic pathway (depolarization of Δψmito and release of cytochrome C from mitochondria into the cytosol) and the extrinsic pathway (activation of caspase 8). Unlike the proapoptotic agonists staurosporine and etoposide that release cytochrome C from mitochondria, C12's effects do not require participation of either Bak or Bax.

Pseudomonas aeruginosa quorum-sensing molecule N-(3-oxododecanoyl) homoserine lactone attenuates lipopolysaccharide-induced inflammation by activating the unfolded protein response

N-3-oxododecanoyl homoserine lactone (3-oxo-C12-HSL), a quorum-sensing signal molecule produced by Pseudomonas aeruginosa (P. aeruginosa), is involved in the expression of bacterial virulence factors and in the modulation of host immune responses by directly disrupting nuclear factor-κB (NF-κB) signaling and inducing cell apoptosis. The unfolded protein response (UPR) triggered by endoplasmic reticulum (ER) stress may suppress inflammatory responses in the later phase by blocking NF-κB activation. It was recently demonstrated that 3-oxo-C12-HSL may induce UPR in human aortic endothelial cells (HAECs). Therefore, 3-oxo-C12-HSL may also inhibit NF-κB activation and suppress inflammatory responses by activating UPR. However, the possible underlying mechanism has not been fully elucidated. Accordingly, we investigated the effects of 3-oxo-C12-HSL on cellular viability, UPR activation, lipopolysaccharide (LPS)-induced NF-κB activation and inflammatory response in the RAW264.7 mouse macrophage cell line. Treatment with 6.25 μM 3-oxo-C12-HSL was not found to affect the viability of RAW264.7 cells. However, pretreating RAW264.7 cells with 6.25 μM 3-oxo-C12-HSL effectively triggered UPR and increased the expression of UPR target genes, such as CCAAT/enhancer-binding protein β (C/EBP β) and CCAAT/enhancer-binding protein-homologous protein (CHOP). The expression of C/EBP β and CHOP was found to be inversely correlated with LPS-induced NF-κB activation. 3-Oxo-C12-HSL pretreatment was also shown to inhibit LPS-stimulated proinflammatory cytokine production. Hence, 3-oxo-C12-HSL may attenuate LPS-induced inflammation via UPR-mediated NF-κB inhibition without affecting cell viability. This may be another mechanism through which P. aeruginosa evades the host immune system and maintains a persistent infection.

Small molecule screen yields inhibitors of Pseudomonas homoserine lactone-induced host responses

Pseudomonas aeruginosa infections are commonly associated with cystic fibrosis, pneumonias, neutropenia and burns. The P. aeruginosa quorum sensing molecule N-(3-oxo-dodecanoyl) homoserine lactone (C12) cause multiple deleterious host responses, including repression of NF-κB transcriptional activity and apoptosis. Inhibition of C12-mediated host responses is predicted to reduce P. aeruginosa virulence. We report here a novel, host-targeted approach for potential adjunctive anti-Pseudomonal therapy based on inhibition of C12-mediated host responses. A high-throughput screen was developed to identify C12 inhibitors that restore NF-κB activity in C12-treated, lipopolysaccharide (LPS)-stimulated cells. Triazolo[4,3-a]quinolines with nanomolar potency were identified as C12-inhibitors that restore NF-κB-dependent luciferase expression in LPS- and TNF-stimulated cell lines. In primary macrophages and fibroblasts, triazolo[4,3-a]quinolines inhibited C12 action to restore cytokine secretion in LPS-stimulated cells. Serendipitously, in the absence of an inflammatory stimulus, triazolo[4,3-a]quinolines prevented C12-mediated responses, including cytotoxicity, elevation of cytoplasmic calcium, and p38 MAPK phosphorylation. In vivo efficacy was demonstrated in a murine model of dermal inflammation involving intradermalC12 administration. The discovery of triazolo[4,3-a]quinolines provides a pharmacological tool to investigate C12-mediated host responses, and a potential host-targeted anti-Pseudomonal therapy.

X-box binding protein 1 (XBP1s) is a critical determinant of Pseudomonas aeruginosa homoserine lactone-mediated apoptosis

Pseudomonas aeruginosa infections are associated with high mortality rates and occur in diverse conditions including pneumonias, cystic fibrosis and neutropenia. Quorum sensing, mediated by small molecules including N-(3-oxo-dodecanoyl) homoserine lactone (C12), regulates P. aeruginosa growth and virulence. In addition, host cell recognition of C12 initiates multiple signalling responses including cell death. To gain insight into mechanisms of C12-mediated cytotoxicity, we studied the role of endoplasmic reticulum stress in host cell responses to C12. Dramatic protection against C12-mediated cell death was observed in cells that do not produce the X-box binding protein 1 transcription factor (XBP1s). The leucine zipper and transcriptional activation motifs of XBP1s were sufficient to restore C12-induced caspase activation in XBP1s-deficient cells, although this polypeptide was not transcriptionally active. The XBP1s polypeptide also regulated caspase activation in cells stimulated with N-(3-oxo-tetradecanoyl) homoserine lactone (C14), produced by Yersinia enterolitica and Burkholderia pseudomallei, and enhanced homoserine lactone-mediated caspase activation in the presence of endogenous XBP1s. In C12-tolerant cells, responses to C12 including phosphorylation of p38 MAPK and eukaryotic initiation factor 2α were conserved, suggesting that C12 cytotoxicity is not heavily dependent on these pathways. In summary, this study reveals a novel and unconventional role for XBP1s in regulating host cell cytotoxic responses to bacterial acyl homoserine lactones.

Chronic and acute infections associated with P. aeruginosa constitute a major healthcare burden. Antimicrobial approaches are currently used against P. aeruginosa; however, infections are typically refractory to treatment and drug resistant strains have been isolated. As such, there is urgent need to understand mechanisms of P. aeruginosa virulence and for new strategies to fight infections. The P. aeruginosa-derived quorum-sensing molecule C12 is recognized by host cells and initiates stress responses including cytotoxicity. In this study, the X-box binding protein 1 transcription factor (XBP1s) was identified as a host factor critical for apoptotic responses initiated by C12 and other similar quorum sensing molecules. Additional C12-initiated host responses, including phosphorylation of p38 MAPK and eIF2α were found to be of lesser importance for C12-initiated cytotoxicity. These studies have broad implications for our understanding of bacterial virulence mechanisms and for development of potential new strategies to combat infections.