Novel polyisoprenyl phosphates block phospholipase D and human neutrophil activation in vitro and murine peritoneal inflammation in vivo

PLD2 deletion ameliorates sepsis-induced cardiomyopathy by suppressing cardiomyocyte pyroptosis via the NLRP3/caspase 1/GSDMD pathway

OBJECTIVE

Sepsis-induced cardiomyopathy (SICM) is a life-threatening complication. Phospholipase D2 (PLD2) is crucial in mediating inflammatory reactions and is associated with the prognosis of patients with sepsis. Whether PLD2 is involved in the pathophysiology of SICM remains unknown. This study aimed to investigate the effect of PLD2 knockout on SICM and to explore potential mechanisms.

METHODS

The SICM model was established using cecal ligation and puncture in wild-type and PLD2-knockout mice and lipopolysaccharide (LPS)-induced H9C2 cardiomyocytes. Transfection with PLD2-shRNA lentivirus and a PLD2 overexpression plasmid were used to interfere with PLD2 expression in H9C2 cells. Cardiac pathological alterations, cardiac function, markers of myocardial injury, and inflammatory factors were used to evaluate the SICM model. The expression of pyroptosis-related proteins (NLRP3, cleaved caspase 1, and GSDMD-N) was assessed using western blotting, immunofluorescence, and immunohistochemistry.

RESULTS

SICM mice had myocardial tissue damage, increased inflammatory response, and impaired heart function, accompanied by elevated PLD2 expression. PLD2 deletion improved cardiac histological changes, mitigated cTNI production, and enhanced the survival of the SICM mice. Compared with controls, PLD2-knockdown H9C2 exhibits a decrease in inflammatory markers and lactate dehydrogenase production, and scanning electron microscopy results suggest that pyroptosis may be involved. The overexpression of PLD2 increased the expression of NLRP3 in cardiomyocytes. In addition, PLD2 deletion decreased the expression of pyroptosis-related proteins in SICM mice and LPS-induced H9C2 cells.

CONCLUSION

PLD2 deletion is involved in SICM pathogenesis and is associated with the inhibition of the myocardial inflammatory response and pyroptosis through the NLRP3/caspase 1/GSDMD pathway.

Acadesine alleviates acute pancreatitis-related lung injury by mediating the barrier protective function of pulmonary microvascular endothelial cells

Severe acute pancreatitis (SAP) is a condition characterized by highly fatal acute inflammation and is usually associated with multiple organ dysfunction syndrome. Acute lung injury (ALI) is the most common complications of SAP, which is the accelerator of other organ dysfunction caused by SAP and the primary cause of early death due to SAP. Acadesine, an adenosine analog and an AMPK activator, has been discovered to modulate glucose and lipid metabolism, and inhibit the production of pro-inflammatory cytokines and iNOS. However, its role in SAP-ALI and its mechanism remains unclear and need to be explored. Herein, we discovered that acadesine mitigated the generation of reactive oxygen species (ROS) in human pulmonary microvascular endothelial cells (HPMECs), alleviated apoptosis and recovered barrier integrity, thereby contributing to anti-inflammatory effects in vitro and in vivo. Moreover, Nrf2 deficiency partially eliminated the effects of acadesine-induced antioxidant effects and thus weakened the protective effects on cells and Nrf2-knockout (Nrf2^-/-) mice. This study demonstrates that acadesine attenuated SAP-ALI associated inflammation and tissue damage by modulating the Nrf2-dependent antioxidant pathway by triggering AMPK. These findings are of great significance for the treatment of SAP-related lung injury.

The Mechanism of Lung and Intestinal Injury in Acute Pancreatitis: A Review

Acute pancreatitis (AP), as a common cause of clinical acute abdomen, often leads to multi-organ damage. In the process of severe AP, the lungs and intestines are the most easily affected organs aside the pancreas. These organ damages occur in succession. Notably, lung and intestinal injuries are closely linked. Damage to ML, which transports immune cells, intestinal fluid, chyle, and toxic components (including toxins, trypsin, and activated cytokines to the systemic circulation in AP) may be connected to AP. This process can lead to the pathological changes of hyperosmotic edema of the lung, an increase in alveolar fluid level, destruction of the intestinal mucosal structure, and impairment of intestinal mucosal permeability. The underlying mechanisms of the correlation between lung and intestinal injuries are inflammatory response, oxidative stress, and endocrine hormone secretion disorders. The main signaling pathways of lung and intestinal injuries are TNF-α, HMGB1-mediated inflammation amplification effect of NF-κB signal pathway, Nrf2/ARE oxidative stress response signaling pathway, and IL-6-mediated JAK2/STAT3 signaling pathway. These pathways exert anti-inflammatory response and anti-oxidative stress, inhibit cell proliferation, and promote apoptosis. The interaction is consistent with the traditional Chinese medicine theory of the lung being connected with the large intestine (fei yu da chang xiang biao li in Chinese). This review sought to explore intersecting mechanisms of lung and intestinal injuries in AP to develop new treatment strategies.

Lipid-Derived Mediators are Pivotal to Leukocyte and Lung Cell Responses in Sepsis and ARDS

Acute inflammation in the lung is essential for host defense against pathogens and other injuries but chronic or excessive inflammation can contribute to several common respiratory diseases. In health, the inflammatory response is controlled by several cellular and molecular mechanisms. In addition to anti-inflammatory processes, there are non-phlogistic pro-resolving mechanisms that are engaged to promote the resolution of inflammation and a return to homeostasis. Defects in the production or actions of specialized pro-resolving mediators are associated with diseases characterized by excess or chronic inflammation. In this article, we review cellular and biochemical mechanisms for specialized pro-resolving mediators in health and in sepsis and the acute respiratory distress syndrome as examples of unrestrained inflammatory responses that result in life-threatening pathology. We are honored to contribute to this special edition of the Journal to help celebrate Professor Viswanathan Natarajan's contributions to our understanding of lipid-derived mediators and metabolism in lung cell responses to inflammatory, infectious, or mechanical insults; his foundational discoveries in cell biochemistry and biophysics are continuing to catalyze further advances by the field to uncover the mechanistic underpinnings of important human diseases.

Preliminary Study: Proteomic Profiling Uncovers Potential Proteins for Biomonitoring Equine Melanocytic Neoplasm

Equine melanocytic neoplasm (EMN) is a cutaneous neoplasm and is mostly observed in aged grey horses. This preliminary study aimed to identify potential proteins to differentiate normal, mild and severe EMN from serum proteomic profiling. Serum samples were collected from 25 grey horses assigned to three groups: normal (free of EMN; n = 10), mild (n = 6) and severe EMN (n = 9). To explore the differences in proteins between groups, proteomic profiling and analysis were employed. Accordingly, 8241 annotated proteins out of 8725 total proteins were compared between normal and EMN groups and inspected based on differentially expressed proteins (DEPs). Through DEP analysis, 95 significant DEPs differed between normal and EMN groups. Among these DEPs, 41 significant proteins were categorised according to protein functions. Based on 41 significant proteins, 10 were involved in metabolism and 31 in non-metabolism. Interestingly, phospholipid phosphatase6 (PLPP6) and ATPase subunit alpha (Na+/K+-ATPase) were considered as potential proteins uniquely expressed in mild EMN and related to lipid and energy metabolism, respectively. Non-metabolism-related proteins (BRCA1, phosphorylase B kinase regulatory subunit: PHKA1, tyrosine-protein kinase receptor: ALK and rho-associated protein kinase: ROCK1) correlated to melanoma development differed among all groups. The results of our study provide a foundation for early EMN biomonitoring and prevention.

The Role of Specialized Pro-Resolving Mediators in Cystic Fibrosis Airways Disease

Cystic Fibrosis (CF) is a recessive genetic disease due to mutations of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene encoding the CFTR chloride channel. The ion transport abnormalities related to CFTR mutation generate a dehydrated airway surface liquid (ASL) layer, which is responsible for an altered mucociliary clearance, favors infections and persistent inflammation that lead to progressive lung destruction and respiratory failure. The inflammatory response is normally followed by an active resolution phase to return to tissue homeostasis, which involves specialized pro-resolving mediators (SPMs). SPMs promote resolution of inflammation, clearance of microbes, tissue regeneration and reduce pain, but do not evoke unwanted immunosuppression. The airways of CF patients showed a decreased production of SPMs even in the absence of pathogens. SPMs levels in the airway correlated with CF patients' lung function. The prognosis for CF has greatly improved but there remains a critical need for more effective treatments that prevent excessive inflammation, lung damage, and declining pulmonary function for all CF patients. This review aims to highlight the recent understanding of CF airway inflammation and the possible impact of SPMs on functions that are altered in CF airways.

Lipoxin A4 Ameliorates Acute Pancreatitis-Associated Acute Lung Injury through the Antioxidative and Anti-Inflammatory Effects of the Nrf2 Pathway

Acute lung injury (ALI) is a critical event involved in the pathophysiological process of acute pancreatitis (AP). Many methods have been widely used for the treatment of AP-ALI, but few are useful during early inflammation. Lipoxin A4 (LXA4), a potent available anti-inflammatory and novel antioxidant mediator, has been extensively studied in AP-ALI, but its underlying mechanism as a protective mediator is not clear. This research was conducted to identify the possible targets and mechanisms involved in the anti-AP-ALI effect of LXA4. First, we confirmed that LXA4 strongly inhibited AP-ALI in mice. Next, using ELISA, PCR, and fluorescence detection to evaluate different parameters, LXA4 was shown to reduce the inflammatory cytokine production induced by AP and block reactive oxygen species (ROS) generation in vivo and in vitro. In addition, TNF-α treatment activated the nuclear factor E2-related factor 2 (Nrf2) signaling pathway and its downstream gene heme oxygenase-1 (HO-1) in human pulmonary microvascular endothelial cells (HPMECs), and LXA4 further promoted their expression. This study also provided evidence that LXA4 phosphorylates Ser40 and triggers its nuclear translocation to activate Nrf2. Moreover, when Nrf2-knockout (Nrf2^−/−) mice and cells were used to further assess the effect of the Nrf2/HO-1 pathway, we found that Nrf2 expression knockdown partially eliminated the effect of LXA4 on the reductions in inflammatory factor levels while abrogating the inhibitory effect of LXA4 on the ROS generation stimulated by AP-ALI. Overall, LXA4 attenuated the resolution of AP-induced inflammation and ROS generation to mitigate ALI, perhaps by modulating the Nrf2/HO-1 pathway. These findings have laid a foundation for the treatment of AP-ALI.

Phospholipase D isoforms differentially regulate leukocyte responses to acute lung injury

Phospholipase D (PLD) plays important roles in cellular responses to tissue injury that are critical to acute inflammatory diseases, such as the acute respiratory distress syndrome (ARDS). We investigated the expression of PLD isoforms and related phospholipid phosphatases in patients with ARDS, and their roles in a murine model of self-limited acute lung injury (ALI). Gene expression microarray analysis on whole blood obtained from patients that met clinical criteria for ARDS and clinically matched controls (non-ARDS) demonstrated that PLD1 gene expression was increased in patients with ARDS relative to non-ARDS and correlated with survival. In contrast, PLD2 expression was associated with mortality. In a murine model of self-resolving ALI, lung Pld1 expression increased and Pld2 expression decreased 24 h after intrabronchial acid. Total lung PLD activity was increased 24 h after injury. Pld1^-/- mice demonstrated impaired alveolar barrier function and increased tissue injury relative to WT and Pld2^-/- , whereas Pld2^-/- mice demonstrated increased recruitment of neutrophils and macrophages, and decreased tissue injury. Isoform-specific PLD inhibitors mirrored the results with isoform-specific Pld-KO mice. PLD1 gene expression knockdown in human leukocytes was associated with decreased phagocytosis by neutrophils, whereas reactive oxygen species production and phagocytosis decreased in M2-macrophages. PLD2 gene expression knockdown increased neutrophil and M2-macrophage transmigration, and increased M2-macrophage phagocytosis. These results uncovered selective regulation of PLD isoforms after ALI, and opposing effects of selective isoform knockdown on host responses and tissue injury. These findings support therapeutic strategies targeting specific PLD isoforms for the treatment of ARDS.

Tulathromycin exerts proresolving effects in bovine neutrophils by inhibiting phospholipases and altering leukotriene B4, prostaglandin E2, and lipoxin A4 production

The accumulation of neutrophils and proinflammatory mediators, such as leukotriene B4 (LTB4), is a classic marker of inflammatory disease. The clearance of apoptotic neutrophils, inhibition of proinflammatory signaling, and production of proresolving lipids (including lipoxins, such as lipoxin A4 [LXA4]) are imperative for resolving inflammation. Tulathromycin (TUL), a macrolide used to treat bovine respiratory disease, confers immunomodulatory benefits via mechanisms that remain unclear. We recently reported the anti-inflammatory properties of TUL in bovine phagocytes in vitro and in Mannheimia haemolytica-challenged calves. The findings demonstrated that this system offers a powerful model for investigating novel mechanisms of pharmacological immunomodulation. In the present study, we examined the effects of TUL in a nonbacterial model of pulmonary inflammation in vivo and characterized its effects on lipid signaling. In bronchoalveolar lavage (BAL) fluid samples from calves challenged with zymosan particles (50 mg), treatment with TUL (2.5 mg/kg of body weight) significantly reduced pulmonary levels of LTB4 and prostaglandin E2 (PGE2). In calcium ionophore (A23187)-stimulated bovine neutrophils, TUL inhibited phospholipase D (PLD), cytosolic phospholipase A2 (PLA2) activity, and the release of LTB4. In contrast, TUL promoted the secretion of LXA4 in resting and A23187-stimulated neutrophils, while levels of its precursor, 15(S)-hydroxyeicosatetraenoic acid [15(S)-HETE], were significantly lower. These findings indicate that TUL directly modulates lipid signaling by inhibiting the production of proinflammatory eicosanoids and promoting the production of proresolving lipoxins.

Chemical modulation of glycerolipid signaling and metabolic pathways

Thirty years ago, glycerolipids captured the attention of biochemical researchers as novel cellular signaling entities. We now recognize that these biomolecules occupy signaling nodes critical to a number of physiological and pathological processes. Thus, glycerolipid-metabolizing enzymes present attractive targets for new therapies. A number of fields-ranging from neuroscience and cancer to diabetes and obesity-have elucidated the signaling properties of glycerolipids. The biochemical literature teems with newly emerging small molecule inhibitors capable of manipulating glycerolipid metabolism and signaling. This ever-expanding pool of chemical modulators appears daunting to those interested in exploiting glycerolipid-signaling pathways in their model system of choice. This review distills the current body of literature surrounding glycerolipid metabolism into a more approachable format, facilitating the application of small molecule inhibitors to novel systems. This article is part of a Special Issue entitled Tools to study lipid functions.

15-Epi-lipoxin A4 inhibits human neutrophil superoxide anion generation by regulating polyisoprenyl diphosphate phosphatase 1

Regulation of leukocyte activation is critical to limit unintended tissue injury during acute inflammation. On neutrophil activation, polyisoprenyl diphosphate phosphatase 1 (PDP1) rapidly converts presqualene diphosphate to presqualene monophosphate to facilitate cell activation. Lipoxins are potent anti-inflammatory mediators for neutrophils, yet their counterregulatory signaling mechanisms remain to be determined. 15-Epi-lipoxin A4 (15-epi-LXA4) blocked agonist-initiated association of the nicotinamide adenine dinucleotide phosphate oxidase components p47(PHOX) and p22(PHOX) in human neutrophils. 15-Epi-LXA4 (0.1-100 nM) inhibited neutrophil superoxide anion (O2(-)) generation in a concentration- and ALX/FPR2 receptor-dependent manner that was disrupted by PDP1-specific antibodies. In differentiated HL60 cells, a myeloid cell line, agonist-initiated O2(-) generation was inhibited by PDP1 siRNA. Recombinant human PDP1 was directly phosphorylated in vitro by select protein kinase C (PKC) isoforms, including PKCβII. When neutrophils were exposed to formyl-methionyl-leucyl-phenylalanine (fMLP), PKCβII was rapidly phosphorylated and physically associated with PDP1. Agonist-initiated conversion of neutrophil presqualene diphosphate to presqualene monophosphate was blocked by PKCβII inhibition. Neutrophil exposure to 15-epi-LXA4 attenuated fMLP triggered PKCβII phosphorylation and its interactions with PDP1. Together, these findings indicate that PDP1 serves an integral signaling role in neutrophil proinflammatory responses and as a target for counter-regulatory mediators.

PLD1 rather than PLD2 regulates phorbol-ester-, adhesion-dependent and Fc{gamma}-receptor-stimulated ROS production in neutrophils

The signalling lipid phosphatidic acid (PA) is generated by the hydrolysis of phosphatidylcholine (PC), which is catalysed by phospholipase D (PLD) enzymes. Neutrophils, important cells of the innate immune system, maintain the body's defence against infection. Previous studies have implicated PLD-generated PA in neutrophil function; these have relied heavily on the use of primary alcohols to act as inhibitors of PA production. The recent development of isoform-selective small molecule inhibitors and the generation of a knockout mouse model provide us with accurate tools to study the role of PLDs in neutrophil responses. We show that PLD1 is a regulator of phorbol-ester-, chemoattractant, adhesion-dependent and Fcγ-receptor-stimulated production of reactive oxygen species (ROS) in neutrophils. Significantly we found that this role of PLD is isoform specific: the absence of PLD2 does not negatively affect these processes. Contrary to expectation, other functions required for an efficient immune response operate effectively in Pld2-deficient neutrophils or when both isoforms are inhibited pharmacologically. We conclude that although PLD1 does have important regulatory roles in neutrophils, the field has been confused by the use of primary alcohols; now that gold standard Pld-knockout mouse models are available, previous work might need to be reassessed.

Design, synthesis, and biological evaluation of halogenated N-(2-(4-oxo-1-phenyl-1,3,8-triazaspiro[4.5]decan-8-yl)ethyl)benzamides: discovery of an isoform-selective small molecule phospholipase D2 inhibitor

Phospholipase D (PLD) catalyzes the conversion of phosphatidylcholine to the lipid second messenger phosphatidic acid. Two mammalian isoforms of PLD have been identified, PLD1 and PLD2, which share 53% sequence identity and are subject to different regulatory mechanisms. Inhibition of PLD enzymatic activity leads to increased cancer cell apoptosis, decreased cancer cell invasion, and decreased metastasis of cancer cells; therefore, the development of isoform-specific, PLD inhibitors is a novel approach for the treatment of cancer. Previously, we developed potent dual PLD1/PLD2, PLD1-specific (>1700-fold selective), and moderately PLD2-preferring (>10-fold preferring) inhibitors. Here, we describe a matrix library strategy that afforded the most potent (PLD2 IC(50) = 20 nM) and selective (75-fold selective versus PLD1) PLD2 inhibitor to date, N-(2-(1-(3-fluorophenyl)-4-oxo-1,3,8-triazaspiro[4.5]decan-8-yl)ethyl)-2-naphthamide (22a), with an acceptable DMPK profile. Thus, these new isoform-selective PLD inhibitors will enable researchers to dissect the signaling roles and therapeutic potential of individual PLD isoforms to an unprecedented degree.

Molecular circuits of resolution in airway inflammation

Inflammatory diseases of the lung are common, cause significant morbidity, and can be refractory to therapy. Airway responses to injury, noxious stimuli, or microbes lead to leukocyte recruitment for host defense. As leukocytes respond, they interact with lung resident cells and can elaborate specific mediators that are enzymatically generated from polyunsaturated fatty acids via transcellular biosynthesis. These bioactive, lipid-derived, small molecules serve as agonists at specific receptors and are rapidly inactivated in the local environment. This review will focus on the biosynthesis, receptors, cellular responses, and in vivo actions of lipoxins, resolvins, and protectins as exemplary molecular signaling circuits in the airway that are anti-inflammatory and proresolving.

Functional characterization of the atypical integral membrane lipid phosphatase PDP1/PPAPDC2 identifies a pathway for interconversion of isoprenols and isoprenoid phosphates in mammalian cells

The polyisoprenoid diphosphates farnesyl diphosphate (FPP) and geranylgeranyl diphosphate (GGPP) are intermediates in the synthesis of cholesterol and related sterols by the mevalonate pathway and precursors for the addition of isoprenyl anchors to many membrane proteins. We developed tandem mass spectrometry assays to evaluate polyisoprenoid diphosphate phosphatase activity of an unusual integral membrane lipid enzyme: type 1 polyisoprenoid diphosphate phosphatase encoded by the PPAPDC2 gene (PDP1/PPAPDC2). In vitro, recombinant PDP1/PPAPDC2 preferentially hydrolyzed polyisoprenoid diphosphates, including FPP and GGPP over a variety of glycerol- and sphingo-phospholipid substrates. Overexpression of mammalian PDP1/PPAPDC2 in budding yeast depletes cellular pools of FPP leading to growth defects and sterol auxotrophy. In mammalian cells, PDP1/PPAPDC2 localizes to the endoplasmic reticulum and nuclear envelope and, unlike the structurally related lipid phosphate phosphatases, is predicted to be oriented with key residues of its catalytic domain facing the cytoplasmic face of the membrane. Studies using synthetic isoprenols with chemical properties that facilitate detection by mass spectrometry identify a pathway for interconversion of isoprenols and isoprenoid diphosphates in intact mammalian cells and demonstrate a role for PDP1/PPAPDC2 in this process. Overexpression of PDP1/PPAPDC2 in mammalian cells substantially decreases protein isoprenylation and results in defects in cell growth and cytoskeletal organization that are associated with dysregulation of Rho family GTPases. Taken together, these results focus attention on integral membrane lipid phosphatases as regulators of isoprenoid phosphate metabolism and suggest that PDP1/PPAPDC2 is a functional isoprenoid diphosphate phosphatase.

Activation of polyisoprenyl diphosphate phosphatase 1 remodels cellular presqualene diphosphate

Polyisoprenyl diphosphates play diverse and vital roles in cell function in health and disease. The counter-regulatory lipid signaling molecule, presqualene diphosphate (PSDP), is rapidly converted to its monophosphate form (PSMP) upon cell activation [Levy, B. D., Petasis, N. A., and Serhan, C. N. (1997) Nature 389, 985-990]. The first PSDP phosphatase was recently identified and named polyisoprenyl diphosphate phosphatase 1 (PDP1) [Fukunaga, K., et al. (2006) J. Biol. Chem. 281, 9490-9497]. Here, we present evidence that PDP1 displays properties of a lipid phosphate phosphatase/phosphotransferase with distinct substrate preference for PSDP. Cell activation with PMA increased PSDP phosphatase activity in a concentration-dependent manner, and Western analysis suggested that PDP1 is directly phosphorylated by protein kinase C. Cellular PSDP phosphatase activity was also induced by the receptor-mediated agonists insulin and TNF-alpha. To address PDP1's contribution to cellular PSDP phosphatase activity, HEK293 cells were established that stably expressed PDP1 siRNA, leading to a 60% decrease in the level of PDP1 RNA, and concomitant decreases in PDP1 protein and PMA-initiated PSDP phosphatase activity. HEK293 cells harboring the PDP1 siRNA construct also displayed a marked decrease in the extent of PMA-initiated conversion of cellular PSDP to PSMP. Together, these findings are the first to indicate that PDP1 is activated during cell responses to soluble stimuli to convert PSDP to PSMP. Moreover, they provide evidence that PDP1 can serve as a new checkpoint for polyisoprenyl phosphate remodeling during cell activation.

Design and synthesis of isoform-selective phospholipase D (PLD) inhibitors. Part I: Impact of alternative halogenated privileged structures for PLD1 specificity

This Letter describes the synthesis and structure-activity-relationships (SAR) of isoform-selective PLD inhibitors. By virtue of the installation of alternative halogenated piperidinyl benzimidazolone privileged structures, in combination with a key (S)-methyl group, novel PLD inhibitors with low nM potency and unprecedented levels of PLD1 isoform selectivity (approximately 1700-fold) over PLD2 were developed.

Design of isoform-selective phospholipase D inhibitors that modulate cancer cell invasiveness

Phospholipase D (PLD) is an essential enzyme responsible for the production of the lipid second messenger phosphatidic acid. Phosphatidic acid participates in both G protein-coupled receptor and receptor tyrosine kinase signal transduction networks. The lack of potent and isoform-selective inhibitors has limited progress in defining the cellular roles of PLD. We used a diversity-oriented synthetic approach and developed a library of PLD inhibitors with considerable pharmacological characterization. Here we report the rigorous evaluation of that library, which contains highly potent inhibitors, including the first isoform-selective PLD inhibitors. Specific members of this series inhibit isoforms with >100-fold selectivity both in vitro and in cells. A subset of inhibitors was shown to block invasiveness in metastatic breast cancer models. These findings demonstrate the power of diversity-oriented synthesis combined with biochemical assays and mass spectrometric lipid profiling of cellular responses to develop the first isoform-selective PLD inhibitors--a new class of antimetastatic agents.

Chemical mediators and the resolution of airway inflammation

Asthma pathobiology is remarkable for chronic airway inflammation that fails to spontaneously resolve. No curative therapy is currently available. A growing body of evidence indicates that, in health, inflammation resolution is an active process orchestrated by specific chemical mediators that are elaborated to restore tissue homeostasis. Activated cell membranes release polyunsaturated fatty acids from phospholipids for enzymatic conversion to biologically active mediators with profound regulatory effects on innate and adaptive immunity. Some of these mediators carry anti-inflammatory and pro-resolving actions that are transduced in a cell-type specific manner via specific recognition sites that initiate regulatory intracellular signals, such as presqualene diphosphate remodeling, to limit pro-phlogistic cell activation. Some of these counter-regulatory lipid mediators have been identified in the airway during asthma and defects in their production are associated with disease severity. In this review, we describe the biosynthesis and bioactions of pro-resolving chemical mediators and provide examples of select mediators and their structural analogs with particular relevance to asthma.

Endogenous lipid mediators in the resolution of airway inflammation

Acute inflammation in the lung is fundamentally important to host defence, but chronic or excessive inflammation leads to several common respiratory diseases, including asthma and acute respiratory distress syndrome. The resolution of inflammation is an active process. In health, events at the onset of acute inflammation establish biosynthetic circuits for specific chemical mediators that later serve as agonists to orchestrate a return to tissue homeostasis. In addition to an overabundance of pro-inflammatory stimuli, pathological inflammation can also result from defects in resolution signalling. The understanding of anti-inflammatory, pro-resolution molecules and their counter-regulatory signalling pathways is providing new insights into the molecular pathophysiology of lung disease and opportunities for the design of therapeutic strategies. In the present review, the growing family of lipid mediators of resolution is examined, including lipoxins, resolvins, protectins, cyclopentenones and presqualene diphosphate. Roles are uncovered for these compounds, or their structural analogues, in regulating airway inflammation.

Lipid mediators as agonists for the resolution of acute lung inflammation and injury

Resolution of acute lung inflammation and injury is an active process; it is not merely the absence of proinflammatory signals. Restoration of homeostasis is coordinated by specific mediators and cellular events. In response to injury and inflammatory stimuli, infiltrating leukocytes and tissue-resident cells interact to generate lipoxins (LXs), which are bioactive eicosanoids derived from arachidonic acid. In contrast to proinflammatory leukotrienes and prostaglandins, LXs display potent antiinflammatory actions. LXA(4) interacts with a G protein-coupled receptor, termed ALX, that transduces counter-regulatory signals in part via intracellular polyisoprenyl phosphate remodeling. Presqualene diphosphate (PSDP) is a polyisoprenyl phosphate in human neutrophils that is rapidly converted to presqualene monophosphate (PSMP) upon cell activation. PSDP, but not PSMP, directly inhibits phospholipase D, phosphoinositol-3 kinase, and superoxide anion generation. LXs block PSDP turnover in neutrophil membranes to prevent proinflammatory responses. Hence, LX and polyisoprenyl phosphate signaling provide a counter-regulatory circuit to promote resolution of acute lung inflammation. LXA(4) and PSDP mimetics have been prepared with potent protective actions in murine models of asthma and acute lung injury.

Regulation of phosphatidylinositol 3-kinase by polyisoprenyl phosphates in neutrophil-mediated tissue injury

Neutrophils play a central role in host defense, inflammation, and tissue injury. Recent findings indicate a novel role for polyisoprenyl phosphates (PIPPs) as natural down-regulatory signals in neutrophils. The relationship between PIPPs and neutrophil early activating signals, such as phosphoinositides, has not been previously determined. Here, we establish presqualene diphosphate (PSDP) as an endogenous PIPP regulator of phosphatidylinositol 3-kinase (PI3K). In human neutrophils, leukotriene B4 (LTB4) triggered rapid decreases in PSDP and reciprocal increases in PI3K activity. In addition, PSDP was identified by gas chromatography/mass spectrometry in p110gamma-PI3K immunoprecipitates obtained 30 s after LTB4, indicating a physical interaction between PSDP and PI3K in activated neutrophils. Moreover, PSDP (0.4-800 pmol) directly inhibited recombinant human p110gamma-PI3K activity. During an experimental model of lung injury and inflammation, a reciprocal relationship was also present in vivo for lung PSDP and PI3K activity. To investigate its therapeutic potential, we developed a new PSDP structural mimetic that blocked human neutrophil activation and mouse lung PI3K activity and inflammation. Together, our findings indicate that PSDP is an endogenous PI3K inhibitor, and suggest that in inflammatory diseases characterized by excessive neutrophil activation, PIPPs can serve as structural templates in a novel antineutrophil therapeutic strategy to limit tissue injury.

Identification and functional characterization of a presqualene diphosphate phosphatase

Presqualene diphosphate (PSDP) is a bioactive lipid that rapidly remodels to presqualene monophosphate (PSMP) upon cell activation (Levy, B. D., Petasis, N. A., and Serhan, C. N. (1997) Nature 389, 985-990). Here, we have identified and characterized a phosphatase that converts PSDP to PSMP. Unlike the related polyisoprenyl phosphate farnesyl diphosphate (FDP), PSDP was not a substrate for type 2 lipid phosphate phosphohydrolases. PSDP phosphatase activity was identified in activated human neutrophil (PMN) extracts and partially purified in the presence of Nonidet P-40 with gel filtration and anion exchange chromatography. Peptide sequencing of a candidate phosphatase was consistent with phosphatidic acid phosphatase domain containing 2 (PPAPDC2), an uncharacterized protein that contains a lipid phosphate phosphohydrolase consensus motif. Recombinant PPAPDC2 displayed diphosphate phosphatase activity with a substrate preference for PSDP > FDP > phosphatidic acid. PPAPDC2 activity was independent of Mg(2+) and optimal at pH 7.0 to 8.0. Incubation of [(14)C]FDP with recombinant human squalene synthase led to [(14)C]PSDP and [(14)C]squalene formation, and in the presence of PPAPDC2, [(14)C]PSMP was generated from [(14)C]PSDP. PPAPDC2 mRNA was detected in human PMN, and is widely expressed in human tissues. Together, these findings indicate that PPAPDC2 in human PMN is the first lipid phosphate phosphohydrolase identified for PSDP. Regulation of this activity of the enzyme may have important roles for PMN activation in innate immunity.

Design, synthesis and bioactions of novel stable mimetics of lipoxins and aspirin-triggered lipoxins

The lipoxins (LX) are a class of potent endogenous oxygenated products that are enzymatically generated from arachidonic acid and have novel anti-inflammatory properties and promote resolution. Elucidation of the biochemical pathways involved in the metabolic inactivation of LX and the discovery of the aspirin-triggered lipoxins (ATL) provided the basis for the design and synthesis of stable analogs of LX and ATL. This special issue review describes the efforts that led to the design and synthesis of stable LX/ATL mimetics, which permitted the detailed elucidation of their novel biological roles, leading to the development of new anti-inflammatory agents that mimic their actions. These synthetic molecules provided the means to uncover the physiologic roles of both the LX and the ATL biosynthetic pathways which led to several unexpected discoveries. Among these findings is the involvement of polyisoprenyl phosphates (PIPP) in intracellular signaling mediated by presqualene diphosphate (PSDP), and the recognition of the novel roles of these lipid mediators in regulating cell trafficking during inflammation as well as in promoting resolution of inflammatory processes. These efforts also provided the basis for examining the potential therapeutic role of LX/ATL stable mimetics and led to the development of new analogs with improved pharmacokinetics that opened the way to potentially new approaches to treating human diseases.