Enhanced Clearance of Pseudomonas aeruginosa by Peroxisome Proliferator-Activated Receptor Gamma

How Staphylococcus aureus and Pseudomonas aeruginosa Hijack the Host Immune Response in the Context of Cystic Fibrosis

Cystic fibrosis (CF) is a serious genetic disease that leads to premature death, mainly due to impaired lung function. CF lungs are characterized by ongoing inflammation, impaired immune response, and chronic bacterial colonization. Staphylococcus aureus (SA) and Pseudomonas aeruginosa (PA) are the two most predominant bacterial agents of these chronic infections. Both can colonize the lungs for years by developing host adaptation strategies. In this review, we examined the mechanisms by which SA and PA adapt to the host immune response. They are able to bypass the physical integrity of airway epithelia, evade recognition, and then modulate host immune cell proliferation. They also modulate the immune response by regulating cytokine production and by counteracting the activity of neutrophils and other immune cells. Inhibition of the immune response benefits not only the species that implements them but also other species present, and we therefore discuss how these mechanisms can promote the establishment of coinfections in CF lungs.

Host cell responses against the pseudomonal biofilm: A continued tale of host-pathogen interactions

In biofilm formation, pathogens within the bacterial community coordinate a cell-cell communication system called quorum sensing (QS). This is achieved through various signalling pathways that regulate bacterial virulence and host immune response. Here, we reviewed the host responses, key clinical implications, and novel therapeutic approaches against the biofilms of P. aeruginosa. Given the high degree of intrinsic antibiotic resistance and biofilm formation by the pathogen, the ensuing treatment complications could result in high morbidity and mortality rates worldwide. Notwithstanding the availability of intervention strategies, there remains a paucity of effective therapeutic options to control biofilmogenesis. This review discusses the basic understanding of QS-associated virulence factors and several key therapeutic interventions to foil the biofilm menace of P. aeruginosa.

Pioglitazone Modifies Kupffer Cell Function and Protects against Escherichia coli-Induced Bacteremia in Burned Mice

Infectious complications and subsequent sepsis in severely burned patients lead to high morbidity and mortality in response to uncontrolled innate immune responses mediated by macrophages. Peroxisome proliferator-activated receptor gamma (PPARγ) has anti-inflammatory activity and acts as a master regulator of macrophage polarization. In this study, we investigated whether the administration of a PPARγ agonist could modulate the Kupffer cell phenotype and thereby ameliorate the dysregulated innate response during post-burn bacterial infection. C57BL/6 mice were subjected to severe burns and randomized to receive either the PPARγ agonist, pioglitazone, or the vehicle control five days after injury, followed by the subsequent analysis of hepatic macrophages. Survival from the bacterial infection was monitored for seven days. Pioglitazone protected burned mice against bacterial infection. A single treatment with pioglitazone significantly enhanced phagocytosis, phagosome acidification, bacterial clearance, and reduction in inflammatory mediators in Kupffer cells. In conclusion, PPARγ activation by pioglitazone prevents clinical deterioration due to post-burn bacterial infection and improves survival. Our findings suggest that pioglitazone may be an effective therapeutic candidate for post-burn infectious complications.

Repurposing of substances with lactone moiety for the treatment of γ-Hydroxybutyric acid and γ-Butyrolactone intoxication through modulating paraoxonase and PPARγ

GHB and GBL are highly accessible recreational drugs of abuse with a high risk of adverse effects and mortality while no specific antidotes exist. These components can also be found in the clinical setting, beverages, and cosmetic products, leading to unwanted exposures and further intoxications. As the structural analogue of GABA, GHB is suggested as the primary mediator of GHB/GBL effects. We further suggest that GBL might be as critical as GHB in this process, acting through PPARγ as its receptor. Moreover, PPARγ and PON (i.e., the GHB-GBL converting enzyme) can be targeted for GHB/GBL addiction and intoxication, leading to modulation of the GHB-GBL balance and blockage of their effects. We suggest that repurposing substances with lactone moiety such as bacterial lactones, sesquiterpene lactones, and statins might lead to potential therapeutic options as they occupy the active sites of PPARγ and PON and interfere with the GHB-GBL balance. In conclusion, this hypothesis improves the GHB/GBL mechanism of action, suggests potential therapeutic options, and highlights the necessity of classifying GBL as a controlled substance.

The PPAR-γ agonist pioglitazone exerts proinflammatory effects in bronchial epithelial cells during acute Pseudomonas aeruginosa pneumonia

Pseudomonas aeruginosa is a common respiratory pathogen that causes injurious airway inflammation during acute pneumonia. Peroxisome proliferator-activated receptor (PPAR)-γ is involved in the regulation of metabolic and inflammatory responses in different cell types and synthetic agonists of PPAR-γ exert anti-inflammatory effects on myeloid cells in vitro and in models of inflammation in vivo. We sought to determine the effect of the PPAR-γ agonist pioglitazone on airway inflammation induced by acute P. aeruginosa pneumonia, focusing on bronchial epithelial cells. Mice pretreated with pioglitazone or vehicle (24 and 1 h) were infected with P. aeruginosa via the airways. Pioglitazone treatment was associated with increased expression of chemokine (Cxcl1, Cxcl2, and Ccl20) and cytokine genes (Tnfa, Il6, and Cfs3) in bronchial brushes obtained 6 h after infection. This pro-inflammatory effect was accompanied by increased expression of Hk2 and Pfkfb3 genes encoding rate-limiting enzymes of glycolysis; concurrently, the expression of Sdha, important for maintaining metabolite flux in the tricarboxylic acid cycle, was reduced in bronchial epithelial cells of pioglitazone treated-mice. Pioglitazone inhibited bronchoalveolar inflammatory responses measured in lavage fluid. These results suggest that pioglitazone exerts a selective proinflammatory effect on bronchial epithelial cells during acute P. aeruginosa pneumonia, possibly by enhancing intracellular glycolysis.

Pathogenesis of the Pseudomonas aeruginosa Biofilm: A Review

Pseudomonas aeruginosa is associated with several human infections, mainly related to healthcare services. In the hospital, it is associated with resistance to several antibiotics, which poses a great challenge to therapy. However, one of the biggest challenges in treating P. aeruginosa infections is that related to biofilms. The complex structure of the P. aeruginosa biofilm contributes an additional factor to the pathogenicity of this microorganism, leading to therapeutic failure, in addition to escape from the immune system, and generating chronic infections that are difficult to eradicate. In this review, we address several molecular aspects of the pathogenicity of P. aeruginosa biofilms.

The Effect of PGC-1alpha-SIRT3 Pathway Activation on Pseudomonas aeruginosa Infection

The innate immune response to P. aeruginosa pulmonary infections relies on a network of pattern recognition receptors, including intracellular inflammasome complexes, which can recognize both pathogen- and host-derived signals and subsequently promote downstream inflammatory signaling. Current evidence suggests that the inflammasome does not contribute to bacterial clearance and, in fact, that dysregulated inflammasome activation is harmful in acute and chronic P. aeruginosa lung infection. Given the role of mitochondrial damage signals in recruiting inflammasome signaling, we investigated whether mitochondrial-targeted therapies could attenuate inflammasome signaling in response to P. aeruginosa and decrease pathogenicity of infection. In particular, we investigated the small molecule, ZLN005, which transcriptionally activates peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), a master regulator of mitochondrial biogenesis, antioxidant defense, and cellular respiration. We demonstrate that P. aeruginosa infection promotes the expression of inflammasome components and attenuates several components of mitochondrial repair pathways in vitro in lung epithelial cells and in vivo in an acute pneumonia model. ZLN005 activates PGC-1α and its downstream effector, Sirtuin 3 (SIRT3), a mitochondrial-localized deacetylase important for cellular metabolic processes and for reactive oxygen species homeostasis. ZLN005 also attenuates inflammasome signaling induced by P. aeruginosa in bronchial epithelial cells and this action is dependent on ZLN005 activation of SIRT3. ZLN005 treatment reduces epithelial-barrier dysfunction caused by P. aeruginosa and decreases pathogenicity in an in vivo pneumonia model. Therapies that activate the PGC-1α—SIRT3 axis may provide a complementary approach in the treatment of P. aeruginosa infection.

Post-treatment with the PPAR-γ agonist pioglitazone inhibits inflammation and bacterial growth during Klebsiella pneumonia

Agonists of peroxisome proliferator-activated receptor (PPAR)-γ have been suggested as potential adjuvant therapy in bacterial pneumonia because of their capacity to inhibit inflammation and enhance bacterial clearance. Previous studies only assessed the effects of pretreatment with these compounds, thereby bearing less relevance for the clinical scenario. Moreover, PPAR-γ agonists have not been studied in pneumonia caused by Klebsiella pneumoniae, a common human respiratory pathogen of which antibiotic treatment is hampered by increasing antimicrobial resistance. Here we show that administration of the PPAR-γ agonist pioglitazone 6 or 8 h after infection of mice with a highly virulent strain of Klebsiella pneumoniae via the airways results in reduced cytokine and myeloperoxidase levels in the lungs at 24 h after infection, as well as reduced bacterial growth in the lungs and decreased dissemination to distant organs at 42 h post-infection. These results suggest that pioglitazone may be an interesting agent in the treatment of Klebsiella pneumonia.

Intestinal interplay of quorum sensing molecules and human receptors

Human gut is in permanent contact with microorganisms that play an important role in many physiological processes including metabolism and immunologic activity. These microorganisms communicate and manage themself by the quorum sensing system (QS) that helps to coordinate optimal growth and subsistence by activating signaling pathways that regulate bacterial gene expression. Diverse QS molecules produced by pathogenic as well as resident microbiota have been found throughout the human gut. However, even a host can by affected by these molecules. Intestinal and immune cells possess a range of molecular targets for QS. Our present knowledge on bacteria-cell communication encompasses G-protein-coupled receptors, nuclear receptors and receptors for bacterial cell-wall components. The QS of commensal bacteria has been approved as a protective factor with favourable effects on intestinal homeostasis and immunity. Signaling molecules of QS interacting with above-mentioned receptors thus parcipitate on maintaining of barrier functions, control of inflammation processes and increase of resistance to pathogen colonization in host organisms. Pathogens QS molecules can have a dual function. Host cells are able to detect the ongoing infection by monitoring the presence and changes in concentrations of QS molecules. Such information can help to set the most effective immune defence to prevent or overcome the infection. Contrary, pathogens QS signals can target the host receptors to deceive the immune system to get the best conditions for growth. However, our knowledge about communication mediated by QS is still limited and detailed understanding of molecular mechanisms of QS signaling is desired.

Label-free biosensor of phagocytosis for diagnosing bacterial infections

Phagocytic cells recognize and phagocytose invading microbes for destruction. However, bacterial pathogens can remain hidden at low levels from conventional detection or replicate intracellularly after being phagocytosed by immune cells. Current phagocytosis-detection approaches involve flow cytometry or microscopic search for rare bacteria-internalized phagocytes among large populations of uninfected cells, which poses significant challenges in research and clinical settings. Hence it is imperative to develop a rapid, non-disruptive, and label-free phagocytosis detection approach. Using deformability assays and microscopic imaging, we have demonstrated for the first time that the presence of intracellular bacteria in phagocytic blood cells led to aberrant physical properties. Specifically, human monocytes with internalized bacteria of various species were stiffer and larger compared with uninfected monocytes. Taking advantage of these physical differences, a novel microfluidics-based biosensor platform was developed to passively sort, concentrate and quantify rare monocytes with internalized pathogens (MIP) from uninfected monocyte populations for phagocytosis detection. The clinical utility of the MIP platform was demonstrated by enriching and detecting bacteria-internalized monocytes from spiked human blood samples within 1.5 h. Patient-derived clinical isolates were used to validate the utility of the MIP platform further. This proof-of-concept presents a phagocytosis detection platform that could be used to rapidly diagnose microbial infections, especially in bloodstream infections (BSIs), thereby improving the clinical outcomes for point-of-care management.

Erythropoietin Promotes Infection Resolution and Lowers Antibiotic Requirements in E. coli- and S. aureus-Initiated Infections

Endogenous mechanisms underlying bacterial infection resolution are essential for the development of novel therapies for the treatment of inflammation caused by infection without unwanted side effects. Herein, we found that erythropoietin (EPO) promoted the resolution and enhanced antibiotic actions in Escherichia coli (E. coli)- and Staphylococcus aureus (S. aureus)-initiated infections. Levels of peritoneal EPO and macrophage erythropoietin receptor (EPOR) were elevated in self-limited E. coli-initiated peritonitis. Myeloid-specific EPOR-deficient mice exhibited an impaired inflammatory resolution and exogenous EPO enhanced this resolution in self-limited infections. Mechanistically, EPO increased macrophage clearance of bacteria via peroxisome proliferator-activated receptor γ (PPARγ)-induced CD36. Moreover, EPO ameliorated inflammation and increased the actions of ciprofloxacin and vancomycin in resolution-delayed E. coli- and S. aureus-initiated infections. Collectively, macrophage EPO signaling is temporally induced during infections. EPO is anti-phlogistic, increases engulfment, promotes infection resolution, and lowers antibiotic requirements.

Long non-coding RNA LINC00488 facilitates thyroid cancer cell progression through miR-376a-3p/PON2

Objective: Long non-coding RNAs (lncRNAs) recently have been identified as influential indicators in a variety of malignancies. The aim of the present study was to identify a functional lncRNA LINC00488 and its effects on thyroid cancer in the view of cell proliferation and apoptosis.

Methods: In order to evaluate the effects of LINC00488 on the cellular process of thyroid cancer, we performed a series of in vitro experiments, including cell counting kit-8 (CCK-8) assay, EdU (5-ethynyl-2′-deoxyuridine) assay, flow cytometry, transwell chamber assay, Western blot and RT-qPCR. The target gene of LINC00488 was then identified by bioinformatics analysis (DIANA and TargetScan). Finally, a series of rescue experiments was conducted to validate the effect of LINC00488 and its target genes on proliferation, migration, invasion and apoptosis of thyroid cancer.

Results: Our findings revealed that LINC00488 was highly expressed in thyroid cancer cell lines (BCPAP, BHP5-16, TPC-1 and CGTH-W3) and promoted the proliferation, migration and invasion, while inhibited the apoptosis of thyroid cancer cells (BCPAP and TPC-1). The results of bioinformatics analysis and dual luciferase reporter gene assay showed that LINC00488 could directly bind to miR-376a-3p and down-regulated the expression level of miR-376a-3p. In addition, Paraoxonase-2 (PON2) was a target gene of miR-376a-3p and negatively regulated by miR-376a-3p. Rescue experiment indicated that LINC00488 might enhance PON2 expression by sponging miR-376a-3p in thyroid cancer.

Conclusion: Taken together, our study revealed that lncRNA LINC00488 acted as an oncogenic gene in the progression of thyroid cancer via regulating miR-376a-3p/PON2 axis, which indicated that LINC00488-miR-376a-3p-PON2 axis could serve as novel biomarkers or potential targets for the treatment of thyroid cancer.

The Relevance of Noncoding DNA Variations of Paraoxonase Gene Cluster in Atherosclerosis-Related Diseases

The human paraoxonase (PON) gene cluster is comprised of three contiguous genes (PON1, PON2 and PON3) of presumably common origin coding three lactonases of highly similar structure and substrate specificity. The catalytic activity of PON proteins is directed toward artificial organophosphates and in physiological conditions toward thiolactones and oxidized phospholipids. Consequently, PON enzymes are regarded as an effective defense against oxidative stress and, as a result, against atherosclerosis development. Additionally, both PON's serum activity and its concentration are influenced by several polymorphic variations in coding and noncoding DNA regions of the PON gene cluster remaining in linkage disequilibrium. Hence, the genetic polymorphism of the PON gene cluster may contribute to atherosclerotic process progression or deceleration. In this review the authors analyzed the relevance of noncoding DNA polymorphic variations of PON genes in atherosclerosis-related diseases involving coronary and peripheral artery disease, stroke, diabetes mellitus, dementia and renal disease and concluded that the effect of PON gene cluster' polymorphism has a considerable impact on the course and outcome in these conditions. The following PON genetic variations may serve as additional predictors of the risk of atherosclerosis in selected populations and individuals.

UPR modulation of host immunity by Pseudomonas aeruginosa in cystic fibrosis

Cystic fibrosis (CF) is a progressive multiorgan autosomal recessive disease with devastating impact on the lungs caused by derangements of the CF transmembrane conductance regulator (CFTR) gene. Morbidity and mortality are caused by the triad of impaired mucociliary clearance, microbial infections and chronic inflammation. Pseudomonas aeruginosa is the main respiratory pathogen in individuals with CF infecting most patients in later stages. Despite its recognized clinical impact, molecular mechanisms that underlie P. aeruginosa pathogenesis and the host response to P. aeruginosa infection remain incompletely understood. The nuclear hormone receptor peroxisome proliferator-activated receptor (PPAR) γ (PPARγ), has shown to be reduced in CF airways. In the present study, we sought to investigate the upstream mechanisms repressing PPARγ expression and its impact on airway epithelial host defense. Endoplasmic reticulum-stress (ER-stress) triggered unfolded protein response (UPR) activated by misfolded CFTR and P. aeruginosa infection contributed to attenuated expression of PPARγ. Specifically, the protein kinase RNA (PKR)-like ER kinase (PERK) signaling pathway led to the enhanced expression of the CCAAT-enhancer-binding-protein homologous protein (CHOP). CHOP induction led to the repression of PPARγ expression. Mechanistically, we showed that CHOP induction mediated PPARγ attenuation, impacted the innate immune function of normal and ∆F508 primary airway epithelial cells by reducing expression of antimicrobial peptide (AMP) and paraoxanse-2 (PON-2), as well as enhancing IL-8 expression. Furthermore, mitochondrial reactive oxygen species production (mt-ROS) and ER-stress positive feedforward loop also dysregulated mitochondrial bioenergetics. Additionally, our findings implicate that PPARγ agonist pioglitazone (PIO) has beneficial effect on the host at the multicellular level ranging from host defense to mitochondrial re-energization.

African-centric TP53 variant increases iron accumulation and bacterial pathogenesis but improves response to malaria toxin

A variant at amino acid 47 in human TP53 exists predominantly in individuals of African descent. P47S human and mouse cells show increased cancer risk due to defective ferroptosis. Here, we show that this ferroptotic defect causes iron accumulation in P47S macrophages. This high iron content alters macrophage cytokine profiles, leads to higher arginase level and activity, and decreased nitric oxide synthase activity. This leads to more productive intracellular bacterial infections but is protective against malarial toxin hemozoin. Proteomics of macrophages reveal decreased liver X receptor (LXR) activation, inflammation and antibacterial defense in P47S macrophages. Both iron chelators and LXR agonists improve the response of P47S mice to bacterial infection. African Americans with elevated saturated transferrin and serum ferritin show higher prevalence of the P47S variant (OR = 1.68 (95%CI 1.07–2.65) p = 0.023), suggestive of its role in iron accumulation in humans. This altered macrophage phenotype may confer an advantage in malaria-endemic sub-Saharan Africa.

A polymorphism in human TP53 (P47S) that predominantly exists in individuals of African descent affects ferroptosis. Here, the authors show that this results in iron accumulation in macrophages leading to more productive infection by intracellular bacteria but improved anti-inflammatory response to the malarial toxin hemozoin.

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.

Macrophages exposed to HIV viral protein disrupt lung epithelial cell integrity and mitochondrial bioenergetics via exosomal microRNA shuttling

Antiretroviral therapy extends survival but does not eliminate HIV from its cellular reservoirs. Between immune and stromal cells in the tissue microenvironment, a dynamic intercellular communication might influence host viral immune responses via intercellular transfer of extracellular vehicles (EVs) (microvesicles, exosome, or apoptotic bodies). It is increasingly recognized that HIV-infected macrophage-secreted nucleotide-rich exosomes might play a critical role in mediating communication between macrophages and other structural cells; however, molecular mechanisms underlying cell–cell crosstalk remain unknown. Here we show that HIV-1-infected macrophages and HIV-1 proteins Tat or gp120-treated macrophages express high levels of microRNAs, including miR-23a and miR-27a. Identical miRNAs expression patterns were detected in macrophage-secreted exosomes isolated from bronchoalveolar lavage fluid of HIV transgenic rats. Tat-treated macrophage-derived exosomal miR-23a attenuated posttranscriptional modulation of key tight junction protein zonula occludens (ZO-1) 3′-UTR in epithelial cells. In parallel, exosomal miR-27a released from Tat-treated macrophages altered the mitochondrial bioenergetics of recipient lung epithelial cells by targeting peroxisome proliferator-activated receptor gamma (PPARγ), while simultaneously stimulating glycolysis. Together, exosomal miRNAs shuttle from macrophages to epithelial cells and thereby explain in part HIV-mediated lung epithelial barrier dysfunction. These studies suggest that targeting miRNAs may be of therapeutic value to enhance lung health in HIV.

Pseudomonas aeruginosa Biofilms: Host Response and Clinical Implications in Lung Infections

Pseudomonas aeruginosa is a major health challenge that causes recalcitrant multidrug-resistant infections, especially in immunocompromised and hospitalized patients. P. aeruginosa is an important cause of nosocomial and ventilator-associated pneumonia characterized by high prevalence and fatality rates. P. aeruginosa also causes chronic lung infections in individuals with cystic fibrosis. Multidrug- and totally drug-resistant strains of P. aeruginosa are increasing threats that contribute to high mortality in these patients. The pathogenesis of many P. aeruginosa infections depends on its ability to form biofilms, structured bacterial communities that can coat mucosal surfaces or invasive devices. These biofilms make conditions more favorable for bacterial persistence, as embedded bacteria are inherently more difficult to eradicate than planktonic bacteria. The molecular mechanisms that underlie P. aeruginosa biofilm pathogenesis and the host response to P. aeruginosa biofilms remain to be fully defined. However, it is known that biofilms offer protection from the host immune response and are also extremely recalcitrant to antimicrobial therapy. Therefore, development of novel therapeutic strategies specifically aimed at biofilms is urgently needed. Here, we review the host response, key clinical implications of P. aeruginosa biofilms, and novel therapeutic approaches to treat biofilms relevant to lung infections. Greater understanding of P. aeruginosa biofilms will elucidate novel avenues to improve outcomes for P. aeruginosa pulmonary infections.

Immunometabolism at the interface between macrophages and pathogens

It is generally regarded that the progression of an infection within host macrophages is the consequence of a failed immune response. However, recent appreciation of macrophage heterogeneity, with respect to both development and metabolism, indicates that the reality is more complex. Different lineages of tissue-resident macrophages respond divergently to microbial, environmental and immunological stimuli. The emerging picture that the developmental origin of macrophages determines their responses to immune stimulation and to infection stresses the importance of in vivo infection models. Recent investigations into the metabolism of infecting microorganisms and host macrophages indicate that their metabolic interface can be a major determinant of pathogen growth or containment. This Review focuses on the integration of data from existing studies, the identification of challenges in generating and interpreting data from ongoing studies and a discussion of the technologies and tools that are required to best address future questions in the field.

Macrophage nuclear receptors: Emerging key players in infectious diseases

Nuclear receptors (NRs) are ligand-activated transcription factors that are expressed in a variety of cells, including macrophages. For decades, NRs have been therapeutic targets because their activity can be pharmacologically modulated by specific ligands and small molecule inhibitors. NRs regulate a variety of processes, including those intersecting metabolic and immune functions, and have been studied in regard to various autoimmune diseases. However, the complex roles of NRs in host response to infection are only recently being investigated. The NRs peroxisome proliferator-activated receptor γ (PPARγ) and liver X receptors (LXRs) have been most studied in the context of infectious diseases; however, recent work has also linked xenobiotic pregnane X receptors (PXRs), vitamin D receptor (VDR), REV-ERBα, the nuclear receptor 4A (NR4A) family, farnesoid X receptors (FXRs), and estrogen-related receptors (ERRs) to macrophage responses to pathogens. Pharmacological inhibition or antagonism of certain NRs can greatly influence overall disease outcome, and NRs that are protective against some diseases can lead to susceptibility to others. Targeting NRs as a novel host-directed treatment approach to infectious diseases appears to be a viable option, considering that these transcription factors play a pivotal role in macrophage lipid metabolism, cholesterol efflux, inflammatory responses, apoptosis, and production of antimicrobial byproducts. In the current review, we discuss recent findings concerning the role of NRs in infectious diseases with an emphasis on PPARγ and LXR, the two most studied. We also highlight newer work on the activity of emerging NRs during infection.

Cardiovascular Insufficiency, Abdominal Sepsis, and Patients' Age Are Associated with Decreased Paraoxonase-1 (PON1) Activity in Critically Ill Patients with Multiple Organ Dysfunction Syndrome (MODS)

Oxidative stress and uncontrolled inflammation are hallmarks of sepsis, leading to organ failure and death. As demonstrated in animal studies, oxidative stress can be alleviated by antioxidant therapies. Paraoxonase-1 (PON1) is a serum-based antioxidant, anti-inflammatory agent, detoxifier, and quorum-sensing factor found to be a prognostic marker in sepsis. However, its associations with multiple organ dysfunction syndrome (MODS), a complication of sepsis and the leading cause of death in the surgical intensive care units (ICU), as well as with specific organ dysfunction, infection site, and invading pathogen remain unknown. Therefore, we measured arylesterase activity of PON1 in 87 individuals (35 with MODS) and related it to the clinical type, organ failure, infection site, pathogens, and hematological and biochemical indices of inflammation at admission to ICU and during a five-day follow-up. Suitability of PON1 and its indices derived from a follow-up as biomarkers in MODS was evaluated as well. MODS was associated with decreased PON1, more so in patients with septic shock, displaying an excellent accuracy as a marker of MODS (91%) and a fair one as a marker in differentiating septic shock from severe sepsis (76%). Decreased admission PON1 accompanied cardiovascular insufficiency (CVI), and, as its marker, PON1 displayed a good accuracy (82%). It was also associated with the abdomen as a site of infection but not with an invading pathogen. In multivariate analysis, 50% of variability in PON1 activity in patients with MODS was explained by the patients' age, CVI, and abdomen as a site of infection. Patients with septic shock, CVI, and abdominal MODS had distinctly different dynamics of PON1 during a follow-up. Mean PON1 activity during the follow-up reflected the associations observed for admission PON1 but was also significantly associated with metabolic dysfunction. Our results show PON1 potential as a biomarker in MODS, particularly as an indicator of CVI.

Thiazolidinediones and reduced risk of incident bacterial abscess in adults with type 2 diabetes: A population-based cohort study

AIM

Previous research has suggested that peroxisome proliferator-activated receptor-gamma (PPAR-γ) may play an important role in immunomodulation. We aimed to examine the association between thiazolidinediones, PPAR-γ agonists and incidence of bacterial abscess among patients with type 2 diabetes.

MATERIALS AND METHODS

This retrospective cohort study between 2000 and 2010 included 46 986 propensity (PS)-matched patients diagnosed with type 2 diabetes. We compared the incidence of bacterial abscess, including liver and non-liver abscesses, between patients treated with metformin plus a thiazolidinedione (M + T, N = 7831) or metformin plus a sulfonylurea (M + S, N = 39 155). Data were retrieved from a population-based Taiwanese database. We applied Cox proportional hazard regression models to estimate hazard ratios (HRs) and 95% confidence intervals (CIs), comparing M + T and M + S after PS matching.

RESULTS

During a median follow-up of 4.5 years, the incidence rate of bacterial abscess was lower with M + T than with M + S treatment (1.89 vs 3.15 per 1000 person-years) in the PS-matched cohort. M + T was associated with a reduced risk of bacterial abscess (HRs after PS matching, 0.58; 95% CI, 0.42-0.80 for total bacterial abscess; 0.54; 95% CI, 0.28-1.07 for liver abscess; 0.59; 95% CI, 0.41-0.85 for non-liver abscess). Results did not change materially after accounting for unmeasured confounding factors using high-dimenional PS matching and differential censoring between regimen groups. Rosiglitazone and pioglitazone, in combination with metformin, produced similar reductions in risk of all abscess outcomes.

CONCLUSION

We found that M + T may provide a protective benefit in reducing the incidence of bacterial abscesses. These findings merit further investigation.

PPARγ is reduced in the airways of non-CF bronchiectasis subjects and is inversely correlated with the presence of Pseudomonas aeruginosa

BACKGROUND

Chronic airway inflammation in conditions such as cystic fibrosis (CF) and non-CF bronchiectasis is characterised by a predominant neutrophilic inflammatory response, commonly due to the presence of pathogenic bacteria such as Pseudomonas aeruginosa. We hypothesised that down-regulation of the anti-inflammatory nuclear transcription regulator peroxisome proliferator-activated receptor gamma (PPARγ in non-CF bronchiectasis subjects may explain why this exuberant neutrophilic inflammation is able to persist unchecked in the inflamed airway.

METHODS

PPARγ gene expression was assessed in bronchoalveolar lavage fluid (BAL) of 35 macrolide naïve non-CF bronchiectasis subjects and compared with that in 20 healthy controls. Human RNA was extracted from pelleted BAL and PPARγ expression was determined by reverse-transcription quantitative PCR. Bacterial DNA was extracted from paired induced sputum and total bacterial load was determined by 16S rRNA qPCR. Quantification of individual bacterial species was achieved by qPCR.

RESULTS

PPARγ expression was lower in subjects with non-CF bronchiectasis compared with healthy control subjects (control: 1.00, IQR 0.55-1.44, n = 20 vs. Bronchiectasis: 0.49, IQR 0.12-0.89; n = 35; p<0.001, Mann-Whitney U test). This lower PPARγ expression correlated negatively with Pseudomonas aeruginosa (r = -0.53, n = 31; p = 0.002). No significant association was seen between PPARγ and total bacterial levels or levels Haemophilus influenzae.

CONCLUSION

PPARγ is expressed in low levels in the airways of non-CF bronchiectasis subjects, despite an aggressive inflammatory response. This low level PPARγ expression is particularly associated with the presence of high levels of P. aeruginosa, and may represent an intrinsic link with this bacterial pathogen.

Peroxisome Proliferator-Activated Receptor γ Is Essential for the Resolution of Staphylococcus aureus Skin Infections

Skin/soft tissue infections (SSTIs) caused by methicillin-resistant Staphylococcus aureus (MRSA) represent serious healthcare burdens worldwide. The host initially controls these infections with a pro-inflammatory infiltrate. However, once established, MRSA viability remains constant. To clear established MRSA SSTIs, the host must transition into the post-inflammatory resolution phase marked by infiltration of alternatively activated macrophages. Here we show that the host nuclear receptor, peroxisome proliferation activator receptor γ (PPARγ), is essential for this transition and MRSA clearance. Chemical PPARγ inhibition or genetic ablation of PPARγ in myeloid cells results in an extended inflammatory phase and exacerbated MRSA SSTIs. Conversely, treating mice with PPARγ agonists hastens the onset of the resolution phase and improves MRSA clearance in a myeloid-dependent fashion. The resolving fibrotic abscess lacks abundant glucose and oxygen but is replete with antimicrobial peptides, which together contribute to MRSA clearance. Thus, PPARγ agonists may serve as viable treatment options for complicated MRSA SSTIs.

PPARγ-activation increases intestinal M1 macrophages and mitigates formation of serrated adenomas in mutant KRAS mice

To identify novel hubs for cancer immunotherapy, we generated C57BL/6J mice with concomitant deletion of the drugable transcription factor PPARγ and transgenic overexpression of the mutant KRASG12V oncogene in enterocytes. Animals developed epithelial hyperplasia, transmural inflammation and serrated adenomas in the small intestine with infiltration of CD3+ FOXP3+ T-cells and macrophages into the lamina propria of the non-malignant mucosa. Within serrated polyps, CD3+ CD8+ T-cells and phosphorylated ERK1/2 were reduced and the senescence marker P21 and macrophage counts up-regulated, indicative of an immunosuppressive tissue microenvironment. Treatment of mutant KRASG12V mice with the PPARγ-agonist rosiglitazone augmented M1 macrophage numbers, reduced IL4 expression and diminished polyp load in mice. Rosiglitazone also promoted M1 polarisation of human THP1-derived macrophages and decreased Il4 mRNA in isolated murine lymphocytes. Thus, inhibition of the oncogenic driver mutant RAS by PPARγ in epithelial and immune cell compartments may be a future target for the prevention or treatment of human malignancies associated with intestinal inflammation.

Peroxisome proliferator-activated receptor-γ agonists attenuate biofilm formation by Pseudomonas aeruginosa

Pseudomonas aeruginosa is a significant contributor to recalcitrant multidrug-resistant infections, especially in immunocompromised and hospitalized patients. The pathogenic profile of P. aeruginosa is related to its ability to secrete a variety of virulence factors and to promote biofilm formation. Quorum sensing (QS) is a mechanism wherein P. aeruginosa secretes small diffusible molecules, specifically acyl homo serine lactones, such as N-(3-oxo-dodecanoyl)-l-homoserine lactone (3O-C12-HSL), that promote biofilm formation and virulence via interbacterial communication. Strategies that strengthen the host's ability to inhibit bacterial virulence would enhance host defenses and improve the treatment of resistant infections. We have recently shown that peroxisome proliferator-activated receptor γ (PPARγ) agonists are potent immunostimulators that play a pivotal role in host response to virulent P. aeruginosa Here, we show that QS genes in P. aeruginosa (strain PAO1) and 3O-C12-HSL attenuate PPARγ expression in bronchial epithelial cells. PAO1 and 3O-C12-HSL induce barrier derangements in bronchial epithelial cells by lowering the expression of junctional proteins, such as zonula occludens-1, occludin, and claudin-4. Expression of these proteins was restored in cells that were treated with pioglitazone, a PPARγ agonist, before infection with PAO1 and 3O-C12-HSL. Barrier function and bacterial permeation studies that have been performed in primary human epithelial cells showed that PPARγ agonists are able to restore barrier integrity and function that are disrupted by PAO1 and 3O-C12-HSL. Mechanistically, we show that these effects are dependent on the induction of paraoxonase-2, a QS hydrolyzing enzyme, that mitigates the effects of QS molecules. Importantly, our data show that pioglitazone, a PPARγ agonist, significantly inhibits biofilm formation on epithelial cells by a mechanism that is mediated via paraoxonase-2. These findings elucidate a novel role for PPARγ in host defense against P. aeruginosa Strategies that activate PPARγ can provide a therapeutic complement for treatment of resistant P. aeruginosa infections.-Bedi, B., Maurice, N. M., Ciavatta, V. T., Lynn, K. S., Yuan, Z., Molina, S. A., Joo, M., Tyor, W. R., Goldberg, J. B., Koval, M., Hart, C. M., Sadikot, R. T. Peroxisome proliferator-activated receptor-γ agonists attenuate biofilm formation by Pseudomonas aeruginosa.

Paraoxonases and infectious diseases

The paraoxonases (PON1, PON2 and PON3) are an enzyme family with a high structural homology. All of them have lactonase activity and degrade lipid peroxides in lipoproteins and cells. As such, they play a role in protection against oxidation and inflammation. Infectious diseases are often associated with oxidative stress and an inflammatory response. Infection and inflammation trigger a cascade of reactions in the host, known as the acute-phase response. This response is associated with dramatic changes in serum proteins and lipoproteins, including a decrease in serum PON1 activity. These alterations have clinical consequences for the infected patient, including an increased risk for cardiovascular diseases, and an impaired protection against the formation of antibiotic-resistant bacterial biofilms. Several studies have investigated the value of serum PON1 measurement as a biomarker of the infection process. Low serum PON1 activities are associated with poor survival in patients with severe sepsis. In addition, preliminary studies suggest that serum PON1 concentration and/or enzyme activity may be useful as markers of acute concomitant infection in patients with an indwelling central venous catheter. Investigating the associations between paraoxonases and infectious diseases is a recent, and productive, line of research.