Phospholipases A2 (PLA2s) represent a growing family of enzymes that catalyze the hydrolysis of phospholipids at the sn-2 position leading to the generation of free fatty acids and lysophospholipids. Mammalian PLA2s are divided into two major classes according to their molecular mass and location: intracellular PLA2 and secreted PLA2 (sPLA2). Type-IIA sPLA2 (sPLA2-IIA), the best studied enzyme of sPLA2, plays a role in the pathogenesis of various inflammatory diseases. Conversely, sPLA2-IIA can exert beneficial action in the context of infectious diseases since recent studies have shown that this enzyme exhibits potent bactericidal effects. Induction of the synthesis of sPLA2-IIA is generally initiated by endotoxin and a limited number of cytokines via paracrine and/or autocrine processes. If the mechanisms involved in the regulation of sPLA2-IIA gene expression have been relatively clarified, little is known on the mechanisms that regulate the expression of other sPLA2. There have been substantial progresses in understanding the transcriptional regulation of sPLA2-IIA expression. Recently, transcription factors including NF-kappaB, PPAR, C/EBP have been identified to play a prominent role in the regulation of sPLA2-IIA gene expression. The activation of these transcription factors is under the control of distinct signaling pathways (PKC, cAMP ...). Accumulating evidences in the literature suggest that cytosolic PLA2 together with two sPLA2 isozymes (sPLA2-IIA and sPLA2-V) are functionally coupled with cyclooxygenase-1 and 2 pathways, respectively, for immediate and delayed PG biosynthesis. This spatio-temporal coupling of cyclooxygenase enzymes with PLA2s may represent a key mechanism in the propagation of inflammatory reaction. Unraveling the mechanisms involved in the regulation of the expression of sPLA2s is important for understanding their pathophysiological roles in inflammatory diseases.

Severe hepatic inflammation is a common cause of acute liver injury following systemic infection with Ehrlichia, obligate Gram-negative intracellular bacteria that lack lipopolysaccharide (LPS). We have previously shown that type I IFN (IFN-I) and inflammasome activation are key host-pathogenic mediators that promote excessive inflammation and liver damage following fatal Ehrlichia infection. However, the underlying signals and mechanisms that regulate protective immunity and immunopathology during Ehrlichia infection are not well understood. To address this issue, we compared susceptibility to lethal Ixodes ovatus Ehrlichia (IOE) infection between wild type (WT) and MyD88-deficient (MyD88-/-) mice. We show here that MyD88-/- mice exhibited decreased inflammasome activation, attenuated liver injury, and were more resistant to lethal infection than WT mice, despite suppressed protective immunity and increased bacterial burden in the liver. MyD88-dependent inflammasome activation was also dependent on activation of the metabolic checkpoint kinase mammalian target of rapamycin complex 1 (mTORC1), inhibition of autophagic flux, and defective mitophagy in macrophages. Blocking mTORC1 signaling in infected WT mice and primary macrophages enhanced bacterial replication and attenuated inflammasome activation, suggesting autophagy promotes bacterial replication while inhibiting inflammasome activation. Finally, our data suggest TLR9 and IFN-I are upstream signaling mechanisms triggering MyD88-mediated mTORC1 and inflammasome activation in macrophages following Ehrlichia infection. This study reveals that Ehrlichia-induced liver injury and toxic shock are mediated by MyD88-dependent inflammasome activation and autophagy inhibition.

As an opportunistic bacterial pathogen, Pseudomonas aeruginosa mainly affects immunocompromised individuals as well as patients with cystic fibrosis. In a previous study, we showed that ExoS of P. aeruginosa, when injected into host cells through a type III secretion apparatus, functions as an effector molecule to trigger apoptosis in various tissue culture cells. Here, we show that injection of the ExoS into HeLa cells activates c-Jun NH 2 -terminal kinase (JNK) phosphorylation while shutting down ERK1/2 and p38 phosphorylation. Inhibiting JNK activation by expression of a dominant negative JNK1 or with a specific JNK inhibitor abolishes ExoS-triggered apoptosis, demonstrating the requirement for JNK-mediated signaling. Following JNK phosphorylation, cytochrome c is released into the cytosol, leading to the activation of caspase 9 and eventually caspase 3. Although c-Jun phosphorylation is also observed as a result of JNK activation, ongoing host protein synthesis is not essential for the apoptotic induction, suggesting that c-Jun-or other AP-1-driven activation of gene expression is dispensable in this process. Therefore, ExoS has opposing effects on different cellular pathways that regulate apoptosis: it shuts down host cell survival signal pathways by inhibiting ERK1/2 and p38 activation, and it activates proapoptotic pathways through activation of JNK1/2 leading ultimately to cytochrome c release and activation of caspases. These results highlight the modulation of host cell signaling by the type III secretion system during interaction between P. aeruginosa and host cells.

The aim of this study was to evaluate the presence of type-II secretory phospholipase A2 (sPLA2-IIA) in alveolar space and its possible role in the destruction of surfactant in three rat models of acute lung injury.Alveolar instillation of either lipopolysaccaride or live Pseudomonas aeruginosa resulted in a significant increase in lung oedema and in a decrease in static compliance of the respiratory system together with alveolar-neutrophil influx as compared with healthy control rats.The upregulation of messenger ribonucleic acid and sPLA2-IIA by the lung was evident. This was associated with surfactant degradation and a decrease in large:small ratio of surfactant aggregates in bacteria-instilled rats. A negative correlation between compliance and sPLA2-IIA activity in bronchoalveolar lavage fluid was shown. By contrast, during alpha naphthylthiourea-induced injury, neither alveolar-neutrophil influx nor increase in sPLA2-IIA activity was observed.Additional experiments in rats treated with a specific inhibitor of type-II secretory phospholipase A2 activity (3 acetamine-1-benzyl-2 ethylindolyl-5 oxy; propane phosphonic acid (LY311727)) demonstrated no improvement in physiological parameters despite a biochemical effect, suggesting that its activity is only one of the multiple factors involved in the pathophysiology of lung injury. Surfactant degradation has been shown to be one of the key features of acute respiratory distress syndrome (ARDS) leading to a decrease in both lung compliance and oxygenation [1,2]. Consequently, several clinical trials have evaluated the effect of exogenous surfactant administration. However, the relative inefficacy of this approach has been attributed to insufficient alveolar delivery of exogenous surfactant and/or to its inhibition/degradation by alveolar enzymatic activities [2]. Along this line, type-IIA secretory phospholipase A2 (sPLA2-IIA), an enzyme produced by alveolar inflammatory cells during acute lung injury (ALI), has previously been shown to participate in surfactant degradation [3][4][5]. sPLA2-IIA belongs to a superfamily of enzymes that hydrolyses the sn-2 fatty acyl ester bond of phosphoglycerides, generating lysophospholipids and free fatty acids [6]. These enzymes play a major role in the induction of inflammatory reactions via the synthesis of lipid mediators, such as platelet activating factor, leukotrienes and prostaglandins [5,6]. A number of studies suggest that sPLA2-IIA is involved in the pathogenesis of inflammatory diseases including ARDS. Indeed, a clinical trial demonstrated an increased sPLA2-IIA activity in bronchoalveolar lavage (BAL) fluid of patients with ARDS whose levels were positively correlated with disease severity [5].The assessment of the role of sPLA2-IIA activity in terms of both surfactant and compliance impairment is of major value, since its inhibition could constitute a therapeutic approach. Therefore, the aims of this study were to assess whether correlation could be established between indexes of injury (lung oedema, art...