Pseudomonas aeruginosa autoinducer modulates host cell responses through calcium signalling

The opportunistic pathogen Pseudomonas aeruginosa utilizes a cell density-dependent signalling phenomenon known as quorum sensing (QS) to regulate several virulence factors needed for infection. Acylated homoserine lactones, or autoinducers, are the primary signal molecules that mediate QS in P. aeruginosa. The autoinducer N-3O-dodecanoyl-homoserine lactone (3O-C12) exerts effects on mammalian cells, including upregulation of pro-inflammatory mediators and induction of apoptosis. However, the mechanism(s) by which 3O-C12 affects mammalian cell responses is unknown. Here we report that 3O-C12 induces apoptosis and modulates the expression of immune mediators in murine fibroblasts and human vascular endothelial cells (HUVEC). The effects of 3O-C12 were accompanied by increases in cytosolic calcium levels that were mobilized from intracellular stores in the endoplasmic reticulum (ER). Calcium release was blocked by an inhibitor of phospholipase C, suggesting that release occurred through inositol triphosphate (IP3) receptors in the ER. Apoptosis, but not immunodulatory gene activation, was blocked when 3O-C12-exposed cells were co-incubated with inhibitors of calcium signalling. This study indicates that 3O-C12 can activate at least two independent signal transduction pathways in mammalian cells, one that involves increases in intracellular calcium levels and leads to apoptosis, and a second pathway that results in modulation of the inflammatory response.     

过氧化物酶体增殖物激活受体与动脉粥样硬化

过氧化物酶体增殖物激活受体 (peroxisomeproliferator activatedreceptors ,PPARs)是核受体超家族中的一类配体依赖的核转录因子 ,包括α、β/δ和γ三种亚型 ,在脂肪细胞分化、能量代谢和炎症过程中都发挥重要的作用。最近的研究显示 ,PPARs的活化不仅可以改善包括糖尿病、高血压和肥胖等在内的胰岛素抵抗综合征 ,而且还直接作用于血管壁 ,从而减缓动脉粥样硬化的进程。本综述将就PPARs的结构、功能、与动脉粥样硬化发病机制和治疗相关的最新研究进展进行简要介绍。     

Peroxisome proliferator-activated receptors and skin development

PPARs are nuclear hormone receptors. PPAR subtypes (alpha, gamma, delta, the latter a xPPARbeta homologue) were initially investigated in skin because of their known role in regulating lipid metabolism. Studies adding specific PPAR ligand activators to cultured skin or skin cells are compatible with the concepts that PPARalpha activation mediates early lipogenic steps common to the function of both skin epidermal cells (keratinocytes) and sebaceous cells (sebocytes), PPARgamma activation plays a unique role in stimulating sebocyte lipogenesis, and PPARdelta activation may contribute to lipid biosynthesis in both sebocytes and keratinocytes under certain circumstances. Epidermal keratinocytes appear to express small amounts of PPARalpha and PPARdelta mRNA and a trace of PPARgamma mRNA which is up-regulated with differentiation. Sebocytes express all subtypes; PPARgamma gene expression excedes that in epidermis. The emerging data on PPAR protein expression suggests that epidermis normally expresses predominantly PPARalpha, while sebocytes express more PPARgamma than PPARalpha. These expression patterns may change during hyperplasia, differentiation and inflammation. Gene disruption studies in mice are compatible with a contribution of PPARalpha to skin barrier function, suggest that PPARgamma is necessary for sebocyte differentiation, and indicate that PPARdelta can ameliorate inflammatory responses in skin. PPARs appear to play a role in keratinocyte synthesis of the lipids that they export to the intercellular space to form the skin permeability barrier. They also appear to be important for sebocyte formation of the intracellular fused lipid droplets that constitute the holocrine secretion of the sebaceous gland. In addition, they may play roles in keratinocyte growth and differentiation and the inhibition of skin inflammation by diverse mechanisms not necessarily related to fat metabolism.     

Peroxisome proliferator-activated receptors and cardiovascular remodeling

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors that heterodimerize with the retinoid X receptor and then modulate the function of many target genes. Three PPARs are known: alpha, beta/delta, and gamma. The better known are PPAR-alpha and PPAR-gamma, which may be activated by different synthetic agonists, although the endogenous ligands are unknown. PPAR-alpha is involved in fatty acid oxidation and expressed in the liver, kidney, and skeletal muscle, whereas PPAR-gamma is involved in fat cell differentiation, lipid storage, and insulin sensitivity. However, both have been shown to be present in variable amounts in cardiovascular tissues, including endothelium, smooth muscle cells, macrophages, and the heart. The activators of PPAR-alpha (fibrates) and PPAR-gamma (thiazolidinediones or glitazones) antagonized the actions of angiotensin II in vivo and in vitro and exerted cardiovascular antioxidant and anti-inflammatory effects. PPAR activators lowered blood pressure, induced favorable effects on the heart, and corrected vascular structure and endothelial dysfunction in several rodent models of hypertension. Activators of PPARs may become therapeutic agents useful in the prevention of cardiovascular disease beyond their effects on carbohydrate and lipid metabolism. Some side effects, such as weight gain, as well as documented aggravation of advanced heart failure through fluid retention by glitazones, may, however, limit their therapeutic application in prevention of cardiovascular disease.     

Epithelial cell polarity alters Rho-GTPase responses to Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic human pathogen that preferentially infects damaged epithelial tissues. Previous studies have failed to distinguish whether the increased susceptibility of injured epithelium results from the loss of cell polarity or increased access to the basolateral surface. We have used confluent monolayers of Madin-Darby canine kidney (MDCK) cells cultured on porous filter supports for 1-3 d as a model system to investigate whether the differentiation state of a polarized model epithelium affected the response of epithelial cells to this pathogen. Confluent incompletely polarized MDCK cell monolayers (day 1) efficiently internalized apically applied P. aeruginosa via a pathway that required actin polymerization and activation of Rho-family GTPases and was accompanied by an increase in the amount of activated RhoA. In contrast, P. aeruginosa entry into highly polarized MDCK monolayers (day 3) was 10- to 100-fold less efficient and was insensitive to inhibitors of actin polymerization or of Rho-family GTPase activation. There was no activation of RhoA; instead, Cdc42-GTP levels increased significantly. Basolateral infection of highly polarized MDCK monolayers was less efficient and insensitive to Clostridium difficile Toxin B, whereas basolateral infection of incompletely polarized MDCK monolayers was more efficient and required activation of Rho-family GTPases. Together, our findings suggest that as epithelial barrier differentiates and becomes highly polarized, it becomes resistant to P. aeruginosa infection. Nevertheless, polarized epithelial cells still sense the presence of apically infecting P. aeruginosa, but they may do so through a different group of surface proteins and/or downstream signaling pathways than do incompletely polarized cells.     

过氧化物酶体增长因子活化受体（PPARs）与microRNAs

在细胞中,过氧化物酶体增长因子活化受体（PPARs）和microRNAs相互制约和调控从而影响相关细胞、组织和器官的功能,在脂肪细胞分化与代谢、炎症、癌症和心血管疾病等生理和病理过程中发挥重要作用.本文首先简要总结了PPARs发挥作用的分子机制;并分别以PPARs家族每一个成员（PPARα、PPARβ和PPARγ）为对象,分析了PPARs调控下的microRNAs表达及功能,探讨了microRNAs调控下的PPARs表达活性变化,并归纳了PPARs与microRNAs之间的调控关系;最后对PPARs相关microRNAs的应用前景进行了简单探讨.研究PPARs与microRNAs间的网络调控关系,可以为PPARs与microRNAs在理论和实践中的深入研究和应用提供参考.     

Eicosanoid-mediated proinflammatory activity of Pseudomonas aeruginosa ExoU

As Pseudomonas aeruginosa ExoU possesses two functional blocks of homology to calcium-independent (iPLA(2)) and cytosolic phospholipase A(2) (cPLA(2)), we addressed the question whether it would exhibit a proinflammatory activity by enhancing the synthesis of eicosanoids by host organisms. Endothelial cells from the HMEC-1 line infected with the ExoU-producing PA103 strain exhibited a potent release of arachidonic acid (AA) that could be significantly inhibited by methyl arachidonyl fluorophosphonate (MAFP), a specific PLA(2) inhibitor, as well as significant amounts of the cyclooxygenase (COX)-derived prostaglandins PGE(2) and PGI(2). Cells infected with an isogenic mutant defective in ExoU synthesis did not differ from non-infected cells in the AA release and produced prostanoids in significantly lower concentrations. Infection by PA103 induced a marked inflammatory response in two different in vivo experimental models. Inoculation of the parental bacteria into mice footpads led to an early increase in the infected limb volume that could be significantly reduced by inhibitors of both COX and lipoxygenase (ibuprofen and NDGA respectively). In an experimental respiratory infection model, bronchoalveolar lavage (BAL) from mice instilled with 10(4) cfu of PA103 exhibited a marked influx of inflammatory cells and PGE(2) release that could be significantly reduced by indomethacin, a non-selective COX inhibitor. Our results suggest that ExoU may contribute to P. aeruginosa pathogenesis by inducing an eicosanoid-mediated inflammatory response of host organisms.     

Production and properties of an inhibitor of the Pseudomonas autoinducer by Pseudomonas aeruginosa

An inhibitor was found in the culture fluid of Pseudomonas aeruginosa PAO1, which could inhibit the activity of the Pseudomonas autoinducer (PAI). The maximal inhibitory activity occurred in stationary phase culture sup ernatant. The PAI inhibitor did not influence the cell growth and the PAI production by P. aeruginosa PAO1 when the PAI inhibitor was added into culture medium. The induced expression of lacZ in the reporter strain Agrobacterium tumefaciens NT1 was suppressed by this PAI inhibitor, whereas inhibition could be relieved by increasing the auto inducer concentration. The quorum sensing of P. aeruginosa was inhibited presumably by inhibiting the inducing activity of Pseudomonas autoinducer but not by inhibiting the production of Pseudomonas autoinducer. It was demonstrated that the structure of the PAI inhibitor was different from that of acyl-homoserine lactones.     

Innate immune responses to Pseudomonas aeruginosa infection

Innate immune responses play a critical role in controlling acute infections due to Pseudomonas aeruginosa in both mice and in humans. In this review we focus on innate immune recognition and clearance mechanisms that are important for controlling P. aeruginosa in the mammalian lung, with particular attention to those that influence the outcome of in vivo infection in murine models.     

Peroxisome proliferator-activated receptor-alpha activation inhibits Langerhans cell function

Epidermal Langerhans cells (LC) play a pivotal role in initiating and maintaining primary immune responses in the skin. In the present study, we asked whether peroxisome proliferator-activated receptor-alpha (PPARalpha) activation modulates LC function. Our results show that PPARalpha is expressed in immature LC and is down-regulated in mature LC suggesting that an early decrease of PPARalpha expression in LC may allow them to mature after contact with an Ag. We further show that pharmacologic PPARalpha activation inhibits LC maturation, migratory capacity, cytokine expression, and the ability to drive T cell proliferation. Moreover, PPARalpha activation inhibits NF-kappaB but not stress-activated protein kinase/JNK, p38MAPK, and ERK1/2. In conclusion, PPARalpha activation by endogenous ligands may provide a molecular signal that allows LC to remain in an immature state within the epidermis for extended periods of time despite minor environmental stimuli.