Mechanism of rapid elimination of lysophosphatidic acid and related lipids from the circulation of mice

Autotaxin Inhibition Reduces Post-Ischemic Myocardial Inflammation via Epigenetic Gene Modifications

Myocardial infarction (MI) triggers a complex inflammatory response that is essential for cardiac repair but can also lead to adverse outcomes if left uncontrolled. Recent studies have highlighted the importance of epigenetic modifications in regulating post-MI inflammation. This study investigated the role of the autotaxin (ATX)/lysophosphatidic acid (LPA) signaling axis in modulating myocardial inflammation through epigenetic pathways in a mouse model of MI. C57BL/6 J mice underwent left anterior descending coronary artery ligation to induce MI and were treated with the ATX inhibitor, PF-8380, or vehicle. Cardiac tissue from the border zone was collected at 6 h, 1, 3, and 7 days post-MI for epigenetic gene profiling using RT2 Profiler PCR Arrays. The results revealed distinct gene expression patterns across sham, MI + Vehicle, and MI + PF-8380 groups. PF-8380 treatment significantly altered the expression of genes involved in inflammation, stress response, and epigenetic regulation compared to the vehicle group. Notably, PF-8380 downregulated Hdac5, Prmt5, and Prmt6, which are linked to exacerbated inflammatory responses, as early as 6 h post-MI. Furthermore, PF-8380 attenuated the reduction of Smyd1, a gene important in myogenic differentiation, at 7 days post-MI. This study demonstrates that the ATX/LPA signaling axis plays a pivotal role in modulating post-MI inflammation via epigenetic pathways. Targeting ATX/LPA signaling may represent a novel therapeutic strategy to control inflammation and improve outcomes after MI. Further research is needed to validate these findings in preclinical and clinical settings and to elucidate the complex interplay between epigenetic mechanisms and ATX/LPA signaling in the context of MI.

Lpar1-mediated Effects in Endothelial Progenitor Cells Are Crucial for Lung Repair in Acute Respiratory Distress Syndrome/Acute Lung Injury

Acute respiratory distress syndrome/acute lung injury (ARDS/ALI) involves acute respiratory failure characterized by vascular endothelial and lung alveolar epithelial injury. Endothelial progenitor cells (EPCs) can mediate vasculogenesis. However, the limitations of EPCs, such as low survival and differentiation, are believed to inhibit the effectiveness of autologous cell therapies. This study demonstrated that lysophosphatidic acid (LPA), a bioactive small molecule without immunogenicity, is involved in the survival and antiapoptotic effects in human umbilical cord mesenchymal stem cells. This study aimed to explore whether LPA improves the survival of EPCs, enhancing the cellular therapeutic efficacy in ARDS, and these results will expand the application of LPA in stem cells and regenerative medicine. LPA promoted the colony formation, proliferation, and migration of EPCs and upregulated the expression of vascular endothelial-derived growth factor (VEGF) in EPCs. LPA pretreatment of transplanted EPCs improved the therapeutic effect by increasing EPC numbers in the rat lungs. LPA enhanced EPC proliferation and migration through Lpar1 coupled to G_i/o and G_q/11, respectively. Activation of extracellular signal-related kinase 1/2, or ERK1/2, was related to LPA-induced EPC proliferation but not migration. LPA/Lpar1-mediated G_i/o protein was also shown to be involved in promoting VEGF expression and inhibiting IL-1α expression in EPCs. Low LPA concentrations are present after lung injury; thus, the restoration of LPA may promote endothelial cell homeostasis and lung repair in ARDS. Inhalation of LPA significantly promoted the homing of endogenous EPCs to the lung and reduced lung injury in both rats with LPS-induced ALI and Streptococcus pneumoniae-infected mice. Taken together, these data indicated that LPA/Lpar1-mediated effects in EPCs are involved in maintaining endothelial cell homeostasis and lung tissue repair under physiological conditions.

Biological functions of bacterial lysophospholipids

Lysophospholipids (LPLs) are lipid-derived metabolic intermediates in the cell membrane. The biological functions of LPLs are distinct from their corresponding phospholipids. In eukaryotic cells LPLs are important bioactive signaling molecules that regulate many important biological processes, but in bacteria the function of LPLs is still not fully defined. Bacterial LPLs are usually present in cells in very small amounts, but can strongly increase under certain environmental conditions. In addition to their basic function as precursors in membrane lipid metabolism, the formation of distinct LPLs contributes to the proliferation of bacteria under harsh circumstances or may act as signaling molecules in bacterial pathogenesis. This review provides an overview of the current knowledge of the biological functions of bacterial LPLs including lysoPE, lysoPA, lysoPC, lysoPG, lysoPS and lysoPI in bacterial adaptation, survival, and host-microbe interactions.

Pathogenic Role of the Sphingosine 1-Phosphate (S1P) Pathway in Common Gynecologic Disorders (GDs): A Possible Novel Therapeutic Target

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid, noteworthy for its involvement both in the modulation of various biological processes and in the development of many diseases. S1P signaling can be either pro or anti-inflammatory, and the sphingosine kinase (SphK)–S1P–S1P receptor (S1PR) axis is a factor in accelerating the growth of several cells, including endometriotic cells and fibrosis. Gynecologic disorders, including endometriosis, adenomyosis, and uterine fibroids are characterized by inflammation and fibrosis. S1P signaling and metabolism have been shown to be dysregulated in those disorders and they are likely implicated in their pathogenesis and pathophysiology. Enzymes responsible for inactivating S1P are the most affected by the dysregulation of S1P balanced levels, thus causing accumulation of sphingolipids within these cells and tissues. The present review highlights the past and latest evidence on the role played by the S1P pathways in common gynecologic disorders (GDs). Furthermore, it discusses potential future approaches in the regulation of this signaling pathway that could represent an innovative and promising therapeutical target, also for ovarian cancer treatment.

Plasma membrane phospholipid phosphatase-related proteins as pleiotropic regulators of neuron growth and excitability

Neuronal plasma membrane proteins are essential for integrating cell extrinsic and cell intrinsic signals to orchestrate neuronal differentiation, growth and plasticity in the developing and adult nervous system. Here, we shed light on the family of plasma membrane proteins phospholipid phosphatase-related proteins (PLPPRs) (alternative name, PRGs; plasticity-related genes) that fine-tune neuronal growth and synaptic transmission in the central nervous system. Several studies uncovered essential functions of PLPPRs in filopodia formation, axon guidance and branching during nervous system development and regeneration, as well as in the control of dendritic spine number and excitability. Loss of PLPPR expression in knockout mice increases susceptibility to seizures, and results in defects in sensory information processing, development of psychiatric disorders, stress-related behaviors and abnormal social interaction. However, the exact function of PLPPRs in the context of neurological diseases is largely unclear. Although initially described as active lysophosphatidic acid (LPA) ecto-phosphatases that regulate the levels of this extracellular bioactive lipid, PLPPRs lack catalytic activity against LPA. Nevertheless, they emerge as atypical LPA modulators, by regulating LPA mediated signaling processes. In this review, we summarize the effects of this protein family on cellular morphology, generation and maintenance of cellular protrusions as well as highlight their known neuronal functions and phenotypes of KO mice. We discuss the molecular mechanisms of PLPPRs including the deployment of phospholipids, actin-cytoskeleton and small GTPase signaling pathways, with a focus on identifying gaps in our knowledge to stimulate interest in this understudied protein family.

Structure of the active Gi-coupled human lysophosphatidic acid receptor 1 complexed with a potent agonist

Lysophosphatidic acid receptor 1 (LPA₁) is one of the six G protein-coupled receptors activated by the bioactive lipid, lysophosphatidic acid (LPA). LPA₁ is a drug target for various diseases, including cancer, inflammation, and neuropathic pain. Notably, LPA₁ agonists have potential therapeutic value for obesity and urinary incontinence. Here, we report a cryo-electron microscopy structure of the active human LPA₁-G_i complex bound to ONO-0740556, an LPA analog with more potent activity against LPA₁. Our structure elucidated the details of the agonist binding mode and receptor activation mechanism mediated by rearrangements of transmembrane segment 7 and the central hydrophobic core. A structural comparison of LPA₁ and other phylogenetically-related lipid-sensing GPCRs identified the structural determinants for lipid preference of LPA₁. Moreover, we characterized the structural polymorphisms at the receptor-G-protein interface, which potentially reflect the G-protein dissociation process. Our study provides insights into the detailed mechanism of LPA₁ binding to agonists and paves the way toward the design of drug-like agonists targeting LPA₁.

LPA₁ is one of the GPCRs that are drug targets for various diseases. Here the authors report a cryo-EM structure of the active human LPA₁-G_i complex bound to an LPA analog with more potent activity against LPA₁ and clarified the ligand recognition mechanism.

Identification of two lipid phosphatases that regulate sphingosine-1-phosphate cellular uptake and recycling

Sphingosine-1-phosphate (S1P) is a sphingolipid metabolite that serves as a potent extracellular signaling molecule. Metabolic regulation of extracellular S1P levels impacts key cellular activities through altered S1P receptor signaling. Although the pathway through which S1P is degraded within the cell and thereby eliminated from reuse has been previously described, the mechanism used for S1P cellular uptake and the subsequent recycling of its sphingoid base into the sphingolipid synthesis pathway is not completely understood. To identify the genes within this S1P uptake and recycling pathway, we performed a genome-wide CRISPR/Cas9 KO screen using a positive-selection scheme with Shiga toxin, which binds a cell-surface glycosphingolipid receptor, globotriaosylceramide (Gb3), and causes lethality upon internalization. The screen was performed in HeLa cells with their sphingolipid de novo pathway disabled so that Gb3 cell-surface expression was dependent on salvage of the sphingoid base of S1P taken up from the medium. The screen identified a suite of genes necessary for S1P uptake and the recycling of its sphingoid base to synthesize Gb3, including two lipid phosphatases, PLPP3 (phospholipid phosphatase 3) and SGPP1 (S1P phosphatase 1). The results delineate a pathway in which plasma membrane–bound PLPP3 dephosphorylates extracellular S1P to sphingosine, which then enters cells and is rephosphorylated to S1P by the sphingosine kinases. This rephosphorylation step is important to regenerate intracellular S1P as a branch-point substrate that can be routed either for dephosphorylation to salvage sphingosine for recycling into complex sphingolipid synthesis or for degradation to remove it from the sphingolipid synthesis pathway.

Modified lipid metabolism and cytosolic phospholipase A2 activation in mesangial cells under pro-inflammatory conditions

Diabetic kidney disease is a consequence of hyperglycemia and other complex events driven by early glomerular hemodynamic changes and a progressive expansion of the mesangium. The molecular mechanisms behind the pathophysiological alterations of the mesangium are yet to be elucidated. This study aimed at investigating whether lipid signaling might be the missing link. Stimulation of human mesangial cells with high glucose primed the inflammasome-driven interleukin 1 beta (IL-1β) secretion, which in turn stimulated platelet-derived growth factor (PDGF-BB) release. Finally, PDGF-BB increased IL-1β secretion synergistically. Both IL-1β and PDGF-BB stimulation triggered the formation of phosphorylated sphingoid bases, as shown by lipidomics, and activated cytosolic phospholipase cPLA2, sphingosine kinase 1, cyclooxygenase 2, and autotaxin. This led to the release of arachidonic acid and lysophosphatidylcholine, activating the secretion of vasodilatory prostaglandins and proliferative lysophosphatidic acids. Blocking cPLA2 release of arachidonic acid reduced mesangial cells proliferation and prostaglandin secretion. Validation was performed in silico using the Nephroseq database and a glomerular transcriptomic database. In conclusion, hyperglycemia primes glomerular inflammatory and proliferative stimuli triggering lipid metabolism modifications in human mesangial cells. The upregulation of cPLA2 was critical in this setting. Its inhibition reduced mesangial secretion of prostaglandins and proliferation, making it a potential therapeutical target.

Lysophosphatidic Acid May Be a Novel Biomarker for Early Acute Aortic Dissection

Background: Misdiagnosis and delayed diagnosis of acute aortic dissection (AAD) significantly increase mortality. Lysophosphatidic acid (LPA) is a biomarker related to coagulation cascade and cardiovascular-injury. The extent of LPA elevation in AAD and whether it can discriminate sudden-onset of acute chest pain are currently unclear.

Methods: We measured the plasma concentration of LPA in a cohort of 174 patients with suspected AAD chest pain and 30 healthy participants. Measures to discriminate AAD from other acute-onset thoracalgia were compared and calculated.

Results: LPA was significantly higher in AAD than in the AMI, PE, and the healthy (344.69 ± 59.99 vs. 286.79 ± 43.01 vs. 286.61 ± 43.32 vs. 96.08 ± 11.93, P < 0.01) within 48 h of symptom onset. LPA level peaked at 12 h after symptom onset, then gradually decreased from 12 to 48 h in AAD. LPA had an AUC of 0.85 (0.80–0.90), diagnosis threshold of 298.98 mg/dl, a sensitivity of 0.81, specificity of 0.77, and the negative predictive value of 0.85. The ROC curve of LPA is better than D-dimer (P = 0.041, Delong test). The decision curve showed that LPA had excellent standardized net benefits.

Conclusion: LPA showed superior overall diagnostic performance to D-dimer in early AAD diagnosis may be a potential biomarker, but additional studies are needed to determine the rapid and cost-effective diagnostic tests in the emergency department.

Enpp2 Expression by Dendritic Cells Is a Key Regulator in Migration

Enpp2 is an enzyme that catalyzes the conversion of lysophosphatidylcholine (LPC) to lysophosphatidic acid (LPA), which exhibits a wide variety of biological functions. Here, we examined the biological effects of Enpp2 on dendritic cells (DCs), which are specialized antigen-presenting cells (APCs) characterized by their ability to migrate into secondary lymphoid organs and activate naïve T-cells. DCs were generated from bone marrow progenitors obtained from C57BL/6 mice. Enpp2 levels in DCs were regulated using small interfering (si)RNA or recombinant Enpp2. Expression of Enpp2 in LPS-stimulated mature (m)DCs was high, however, knocking down Enpp2 inhibited mDC function. In addition, the migratory capacity of mDCs increased after treatment with rmEnpp2; this phenomenon was mediated via the RhoA-mediated signaling pathway. Enpp2-treated mDCs showed a markedly increased capacity to migrate to lymph nodes in vivo. These findings strongly suggest that Enpp2 is necessary for mDC migration capacity, thereby increasing our understanding of DC biology. We postulate that regulating Enpp2 improves DC migration to lymph nodes, thus improving the effectiveness of cancer vaccines based on DC.

The development of modulators for lysophosphatidic acid receptors: A comprehensive review

Lysophosphatidic acids (LPAs) are bioactive phospholipids implicated in a wide range of cellular activities that regulate a diverse array of biological functions. They recognize two types of G protein-coupled receptors (LPARs): LPA_1-3 receptors and LPA_4-6 receptors that belong to the endothelial gene (EDG) family and non-endothelial gene family, respectively. In recent years, the LPA signaling pathway has captured an increasing amount of attention because of its involvement in various diseases, such as idiopathic pulmonary fibrosis, cancers, cardiovascular diseases and neuropathic pain, making it a promising target for drug development. While no drugs targeting LPARs have been approved by the FDA thus far, at least three antagonists have entered phase Ⅱ clinical trials for idiopathic pulmonary fibrosis (BMS-986020 and BMS-986278) and systemic sclerosis (SAR100842), and one radioligand (BMT-136088/¹⁸F-BMS-986327) has entered phase Ⅰ clinical trials for positron emission tomography (PET) imaging of idiopathic pulmonary fibrosis. This article provides an extensive review on the current status of ligand development targeting LPA receptors to modulate LPA signaling and their therapeutic potential in various diseases.

Visualization of the binding between gintonin, a Panax ginseng-derived LPA receptor ligand, and the LPA receptor subtypes and transactivation of the EGF receptor

Background

Gintonin is a ginseng-derived exogenous G-protein-coupled lysophosphatidic acid (LPA) receptor ligand. Gintonin exerts its neuronal and non-neuronal in vitro and in vivo effects through LPA receptor subtypes. However, it is unknown whether gintonin can bind to the plasma membrane of cells and can transactivate the epidermal growth factor (EGF) receptor. In the present study, we examined whether gintonin-biotin conjugates directly bound to LPA receptors and transactivated the EGF receptor.

Methods

We designed gintonin-biotin conjugates through gintonin biotinylation and examined whether gintonin-biotin conjugate binding sites co-localized with the LPA receptor subtype binding sites. We further examined whether gintonin-biotin transactivated the EGF receptor via LPA receptor regulation via phosphor-EGF and cell migration assays.

Results

Gintonin-biotin conjugates elicit [Ca²⁺]_i transient similar to that observed with unbiotinylated gintonin in cultured PC3 cells, suggesting that biotinylation does not affect physiological activity of gintonin. We proved that gintonin-biotin conjugate binding sites co-localized with the LPA1/6 receptor binding sites. Gintonin-biotin binding to the LPA1 receptor transactivates the epidermal growth factor (EGF) receptor through phosphorylation, while the LPA1/3 receptor antagonist, Ki16425, blocked phosphorylation of the EGF receptor. Additionally, an EGF receptor inhibitor AG1478 blocked gintonin-biotin conjugate-mediated cell migration.

Conclusions

We observed the binding between ginseng-derived gintonin and the plasma membrane target proteins corresponding to the LPA1/6 receptor subtypes. Moreover, gintonin transactivated EGF receptors via LPA receptor regulation. Our results suggest that gintonin directly binds to the LPA receptor subtypes and transactivates the EGF receptor. It may explain the molecular basis of ginseng physiology/pharmacology in biological systems.

Disrupted Blood-Brain Barrier and Mitochondrial Impairment by Autotaxin-Lysophosphatidic Acid Axis in Postischemic Stroke

Background The loss of endothelial integrity increases the risk of intracerebral hemorrhage during ischemic stroke. Adjunct therapeutic targets for reperfusion in ischemic stroke are in need to prevent blood-brain barrier disruption. Recently, we have shown that endothelial permeability is mediated by lysophosphatidic acid (LPA), but the role of autotaxin, which produces LPA, remains unclear in stroke. We investigate whether autotaxin/LPA axis regulates blood-brain barrier integrity after cerebral ischemia. Methods and Results Ischemic stroke was induced in mice by middle cerebral artery occlusion for 90 minutes, followed by 24-hour reperfusion. The therapeutic efficacy of autotaxin/LPA receptor blockade was evaluated using triphenyl tetrazolium chloride staining, Evans blue permeability, infrared imaging, mass spectrometry, and XF24 analyzer to evaluate blood-brain barrier integrity, autotaxin activity, and mitochondrial bioenergetics. In our mouse model of ischemic stroke, the mRNA levels of autotaxin were elevated 1.7-fold following the cerebral ischemia and reperfusion (I/R) group compared with the sham. The enzymatic activity of autotaxin was augmented by 4-fold in the I/R group compared with the sham. Plasma and brain tissues in I/R group showed elevated LPA levels. The I/R group also demonstrated mitochondrial dysfunction, as evidenced by decreased (P<0.01) basal oxygen consumption rate, mitochondrial ATP production, and spare respiratory capacity. Treatment with autotaxin inhibitors (HA130 or PF8380) or autotaxin/LPA receptor inhibitor (BrP-LPA) rescued endothelial permeability and mitochondrial dysfunction in I/R group. Conclusions Autotaxin-LPA signaling blockade attenuates blood-brain barrier disruption and mitochondrial function following I/R, suggesting targeting this axis could be a new therapeutic approach toward treating ischemic stroke.

A Single Human-Relevant Fast Food Meal Rapidly Reorganizes Metabolomic and Transcriptomic Signatures in a Gut Microbiota-Dependent Manner

BACKGROUND

A major contributor to cardiometabolic disease is caloric excess, often a result of consuming low cost, high calorie fast food. Studies have demonstrated the pivotal role of gut microbes contributing to cardiovascular disease in a diet-dependent manner. Given the central contributions of diet and gut microbiota to cardiometabolic disease, we hypothesized that microbial metabolites originating after fast food consumption can elicit acute metabolic responses in the liver.

METHODS

We gave conventionally raised mice or mice that had their microbiomes depleted with antibiotics a single oral gavage of a liquified fast food meal or liquified control rodent chow meal. After four hours, mice were sacrificed and we used untargeted metabolomics of portal and peripheral blood, 16S rRNA gene sequencing, targeted liver metabolomics, and host liver RNA sequencing to identify novel fast food-derived microbial metabolites and their acute effects on liver function.

RESULTS

Several candidate microbial metabolites were enriched in portal blood upon fast food feeding, and were essentially absent in antibiotic-treated mice. Strikingly, at four hours post-gavage, fast food consumption resulted in rapid reorganization of the gut microbial community and drastically altered hepatic gene expression. Importantly, diet-driven reshaping of the microbiome and liver transcriptome was dependent on an intact microbial community and not observed in antibiotic ablated animals.

CONCLUSIONS

Collectively, these data suggest a single fast food meal is sufficient to reshape the gut microbial community in mice, yielding a unique signature of food-derived microbial metabolites. Future studies are in progress to determine the contribution of select metabolites to cardiometabolic disease progression and the translational relevance of these animal studies.

Autotaxin-LPA-LPP3 Axis in Energy Metabolism and Metabolic Disease

Besides serving as a structural membrane component and intermediate of the glycerolipid metabolism, lysophosphatidic acid (LPA) has a prominent role as a signaling molecule through its binding to LPA receptors at the cell surface. Extracellular LPA is primarily produced from lysophosphatidylcholine (LPC) through the activity of secreted lysophospholipase D, autotaxin (ATX). The degradation of extracellular LPA to monoacylglycerol is mediated by lipid phosphate phosphatases (LPPs) at the cell membrane. This review summarizes and interprets current literature on the role of the ATX-LPA-LPP3 axis in the regulation of energy homeostasis, insulin function, and adiposity at baseline and under conditions of obesity. We also discuss how the ATX-LPA-LPP3 axis influences obesity-related metabolic complications, including insulin resistance, fatty liver disease, and cardiomyopathy.

Metabolomic profile of milk fermented with Streptococcus thermophilus cocultured with Bifidobacterium animalis ssp. lactis, Lactiplantibacillus plantarum, or both during storage

In this study, the microbial interactions among cocultures of Streptococcus thermophilus (St) with potential probiotics of Bifidobacterium animalis ssp. lactis (Ba) and Lactiplantibacillus plantarum (Lp) in fermented milk were investigated during a storage period of 21 d at 4°C, in terms of acidifying activity (pH and titratable acidity), viable counts, and metabolites. A nontargeted metabolomics approach based on ultra-high-performance liquid chromatography coupled with mass spectrometry was employed for mapping the global metabolite profiles of fermented milk. Probiotic strains cocultured with St accelerated milk acidification, and improved the microbial viability compared with the single culture of St. The St-Ba/Lp treatment manifested a higher bacteria viability and acidification ability in comparison with the St-Ba or the St-Lp treatment. Relative quantitation of 179 significant metabolites was identified, including nucleosides, AA, short peptides, organic acids, lipid derivatives, carbohydrates, carbonyl compounds, and compounds related to energy metabolism. The principal component analysis indicated that St treatment and coculture treatments displayed a complete distinction in metabolite profiles, and Lp had a larger effect than Ba on metabolic profiles of fermented milk produced by cofermentation with St during storage. The heat map in combination with hierarchical cluster analysis showed that the abundance of metabolites significantly varied with the starter cultures over the storage, and high abundance of metabolites was observed in either St or coculture samples. The St-Ba/Lp treatment showed relatively high abundance for the vast majority of metabolites. These findings suggest that the profile of the metabolites characterizing fermented milk samples may depend on the starter cultures, and incorporation of probiotics may considerably influence the metabolomic activities of fermented milks.

Lysolipids in Vascular Development, Biology, and Disease

Membrane phospholipid metabolism forms lysophospholipids, which possess unique biochemical and biophysical properties that influence membrane structure and dynamics. However, lysophospholipids also function as ligands for G-protein-coupled receptors that influence embryonic development, postnatal physiology, and disease. The 2 most well-studied species-lysophosphatidic acid and S1P (sphingosine 1-phosphate)-are particularly relevant to vascular development, physiology, and cardiovascular diseases. This review summarizes the role of lysophosphatidic acid and S1P in vascular developmental processes, endothelial cell biology, and their roles in cardiovascular disease processes. In addition, we also point out the apparent connections between lysophospholipid biology and the Wnt (int/wingless family) pathway, an evolutionarily conserved fundamental developmental signaling system. The discovery that components of the lysophospholipid signaling system are key genetic determinants of cardiovascular disease has warranted current and future research in this field. As pharmacological approaches to modulate lysophospholipid signaling have entered the clinical sphere, new findings in this field promise to influence novel therapeutic strategies in cardiovascular diseases.

Emerging roles of lysophospholipids in health and disease

Lipids are abundant and play essential roles in human health and disease. The main functions of lipids are building blocks for membrane biogenesis. However, lipids are also metabolized to produce signaling molecules. Here, we discuss the emerging roles of circulating lysophospholipids. These lysophospholipids consist of lysoglycerophospholipids and lysosphingolipids. They are both present in cells at low concentration, but their concentrations in extracellular fluids are significantly higher. The biological functions of some of these lysophospholipids have been recently revealed. Remarkably, some of the lysophospholipids play pivotal signaling roles as well as being precursors for membrane biogenesis. Revealing how circulating lysophospholipids are produced, released, transported, and utilized in multi-organ systems is critical to understand their functions. The discovery of enzymes, carriers, transporters, and membrane receptors for these lysophospholipids has shed light on their physiological significance. In this review, we summarize the biological roles of these lysophospholipids via discussing about the proteins regulating their functions. We also discuss about their potential impacts to human health and diseases.

Autotaxin inhibition reduces cardiac inflammation and mitigates adverse cardiac remodeling after myocardial infarction

OBJECTIVE

Acute myocardial infarction (AMI) initiates pathological inflammation which aggravates tissue damage and causes heart failure. Lysophosphatidic acid (LPA), produced by autotaxin (ATX), promotes inflammation and the development of atherosclerosis. The role of ATX/LPA signaling nexus in cardiac inflammation and resulting adverse cardiac remodeling is poorly understood.

APPROACH AND RESULTS

We assessed autotaxin activity and LPA levels in relation to cardiac and systemic inflammation in AMI patients and C57BL/6 (WT) mice. Human and murine peripheral blood and cardiac tissue samples showed elevated levels of ATX activity, LPA, and inflammatory cells following AMI and there was strong correlation between LPA levels and circulating inflammatory cells. In a gain of function model, lipid phosphate phosphatase-3 (LPP3) specific inducible knock out (Mx1-Plpp3^Δ) showed higher systemic and cardiac inflammation after AMI compared to littermate controls (Mx1-Plpp3^fl/fl); and a corresponding increase in bone marrow progenitor cell count and proliferation. Moreover, in Mx1- Plpp3^Δ mice, cardiac functional recovery was reduced with corresponding increases in adverse cardiac remodeling and scar size (as assessed by echocardiography and Masson's Trichrome staining). To examine the effect of ATX/LPA nexus inhibition, we treated WT mice with the specific pharmacological inhibitor, PF8380, twice a day for 7 days post AMI. Inhibition of the ATX/LPA signaling nexus resulted in significant reduction in post-AMI inflammatory response, leading to favorable cardiac functional recovery, reduced scar size and enhanced angiogenesis.

CONCLUSION

ATX/LPA signaling nexus plays an important role in modulating inflammation after AMI and targeting this mechanism represents a novel therapeutic target for patients presenting with acute myocardial injury.

Sphingosine 1-Phosphate in Malaria Pathogenesis and Its Implication in Therapeutic Opportunities

Sphingosine 1-Phosphate (S1P) is a bioactive lipid intermediate in the sphingolipid metabolism, which exist in two pools, intracellular and extracellular, and each pool has a different function. The circulating extracellular pool, specifically the plasma S1P is shown to be important in regulating various physiological processes related to malaria pathogenesis in recent years. Although blood cells (red blood cells and platelets), vascular endothelial cells and hepatocytes are considered as the important sources of plasma S1P, their extent of contribution is still debated. The red blood cells (RBCs) and platelets serve as a major repository of intracellular S1P due to lack, or low activity of S1P degrading enzymes, however, contribution of platelets toward maintaining plasma S1P is shown negligible under normal condition. Substantial evidences suggest platelets loss during falciparum infection as a contributing factor for severe malaria. However, platelets function as a source for plasma S1P in malaria needs to be examined experimentally. RBC being the preferential site for parasite seclusion, and having the ability of trans-cellular S1P transportation to EC upon tight cell-cell contact, might play critical role in differential S1P distribution and parasite growth. In the present review, we have summarized the significance of both the S1P pools in the context of malaria, and how the RBC content of S1P can be channelized in better ways for its possible implication in therapeutic opportunities to control malaria.

Role of Adipose Tissue-Derived Autotaxin, Lysophosphatidate Signaling, and Inflammation in the Progression and Treatment of Breast Cancer

Autotaxin (ATX) is a secreted enzyme that produces lysophosphatidate (LPA), which signals through six G-protein coupled receptors, promoting tumor growth, metastasis, and survival from chemotherapy and radiotherapy. Many cancer cells produce ATX, but breast cancer cells express little ATX. In breast tumors, ATX is produced by tumor-associated stroma. Breast tumors are also surrounded by adipose tissue, which is a major bodily source of ATX. In mice, a high-fat diet increases adipocyte ATX production. ATX production in obesity is also increased because of low-level inflammation in the expanded adipose tissue. This increased ATX secretion and consequent LPA signaling is associated with decreased adiponectin production, which results in adverse metabolic profiles and glucose homeostasis. Increased ATX production by inflamed adipose tissue may explain the obesity-breast cancer association. Breast tumors produce inflammatory mediators that stimulate ATX transcription in tumor-adjacent adipose tissue. This drives a feedforward inflammatory cycle since increased LPA signaling increases production of more inflammatory mediators and cyclooxygenase-2. Inhibiting ATX activity, which has implications in breast cancer adjuvant treatments, attenuates this cycle. Targeting ATX activity and LPA signaling may potentially increase chemotherapy and radiotherapy efficacy, and decrease radiation-induced fibrosis morbidity independently of breast cancer type because most ATX is not derived from breast cancer cells.

Lysophosphatidic Acid Receptor Agonism: Discovery of Potent Nonlipid Benzofuran Ethanolamine Structures

Lysophosphatidic acid (LPA) is the natural ligand for two phylogenetically distinct families of receptors (LPA_1-3, LPA_4-6) whose pathways control a variety of physiologic and pathophysiological responses. Identifying the benefit of balanced activation/repression of LPA receptors has always been a challenge because of the high lability of LPA and the limited availability of selective and/or stable agonists. In this study, we document the discovery of small benzofuran ethanolamine derivatives (called CpX and CpY) behaving as LPA_1-3 agonists. Initially found as rabbit urethra contracting agents, their elusive receptors were identified from [³⁵S]GTPγS-binding and β-arrestin2 recruitment investigations and then confirmed by [³H]CpX binding studies (urethra, hLPA_1-2 membranes). Both compounds induced a calcium response in hLPA_1-3 cells within a range of 0.4-1.5-log lower potency as compared with LPA. The contractions of rabbit urethra strips induced by these compounds perfectly matched binding affinities with values reaching the two-digit nanomolar level. The antagonist, KI16425, dose-dependently antagonized CpX-induced contractions in agreement with its affinity profile (LPA₁≥LPA₃>>LPA₂). The most potent agonist, CpY, doubled intraurethral pressure in anesthetized female rats at 3 µg/kg i.v. Alternatively, CpX was shown to inhibit human preadipocyte differentiation, a process totally reversed by KI16425. Together with original molecular docking data, these findings clearly established these molecules as potent agonists of LPA_1-3 and consolidated the pivotal role of LPA₁ in urethra/prostate contraction as well as in fat cell development. The discovery of these unique and less labile LPA_1-3 agonists would offer new avenues to investigate the roles of LPA receptors. SIGNIFICANCE STATEMENT: We report the identification of benzofuran ethanolamine derivatives behaving as potent selective nonlipid LPA_1-3 agonists and shown to alter urethra muscle contraction or preadipocyte differentiation. Unique at this level of potency, selectivity, and especially stability, compared with lysophosphatidic acid, they represent more appropriate tools for investigating the physiological roles of lysophosphatidic acid receptors and starting point for optimization of drug candidates for therapeutic applications.

Crossing signals: bioactive lipids in the microvasculature

The primary function of the arterial microvasculature is to ensure that regional perfusion of blood flow is matched to the needs of the tissue bed. This critical physiological mechanism is tightly controlled and regulated by a variety of vasoactive compounds that are generated and released from the vascular endothelium. Although these substances are required for modulating vascular tone, they also influence the surrounding tissue and have an overall effect on vascular, as well as parenchymal, homeostasis. Bioactive lipids, fatty acid derivatives that exert their effects through signaling pathways, are included in the list of vasoactive compounds that modulate the microvasculature. Although lipids were identified as important vascular messengers over three decades ago, their specific role within the microvascular system is not well defined. Thorough understanding of these pathways and their regulation is not only essential to gain insight into their role in cardiovascular disease but is also important for preventing vascular dysfunction following cancer treatment, a rapidly growing problem in medical oncology. The purpose of this review is to discuss how biologically active lipids, specifically prostanoids, epoxyeicosatrienoic acids, sphingolipids, and lysophospholipids, contribute to vascular function and signaling within the endothelium. Methods for quantifying lipids will be briefly discussed, followed by an overview of the various lipid families. The cross talk in signaling between classes of lipids will be discussed in the context of vascular disease. Finally, the potential clinical implications of these lipid families will be highlighted.

Role of the autotaxin-lysophosphatidate axis in the development of resistance to cancer therapy

Autotaxin (ATX) is a secreted enzyme that hydrolyzes lysophosphatidylcholine to produce lysophosphatidate (LPA), which signals through six G-protein coupled receptors (GPCRs). Signaling through LPA is terminated by its degradation by a family of three lipid phosphate phosphatases (LPPs). LPP1 also attenuates signaling downstream of the activation of LPA receptors and some other GPCRs. The ATX-LPA axis mediates a plethora of activities such as cell proliferation, survival, migration, angiogenesis and inflammation, which perform an important role in facilitating wound healing. This wound healing response is hijacked by cancers where there is decreased expression of LPP1 and LPP3 and increased expression of ATX. This maladaptive regulation of LPA signaling also causes chronic inflammation, which has been recognized as one of the hallmarks in cancer. The increased LPA signaling promotes cell survival and migration and attenuates apoptosis, which stimulates tumor growth and metastasis. The wound healing functions of increased LPA signaling also protect cancer cells from effects of chemotherapy and radiotherapy. In this review, we will summarize knowledge of the ATX-LPA axis and its role in the development of resistance to chemotherapy and radiotherapy. We will also offer insights for developing strategies of targeting ATX-LPA axis as a novel part of cancer treatment. This article is part of a Special Issue entitled Lysophospholipids and their receptors: New data and new insights into their function edited by Susan Smyth, Viswanathan Natarajan and Colleen McMullen.

Adjuvant drug-assisted bone healing: Part II - Modulation of angiogenesis

 The treatment of critical-size bone defects following complicated fractures, infections or tumor resections is a major challenge. The same applies to fractures in patients with impaired bone healing due to systemic inflammatory and metabolic diseases. Despite considerable progress in development and establishment of new surgical techniques, design of bone graft substitutes and imaging techniques, these scenarios still represent unresolved clinical problems. However, the development of new active substances offers novel potential solutions for these issues. This work discusses therapeutic approaches that influence angiogenesis or hypoxic situations in healing bone and surrounding tissue. In particular, literature on sphingosine-1-phosphate receptor modulators and nitric oxide (NO•) donors, including bi-functional (hybrid) compounds like NO•-releasing cyclooxygenase-2 inhibitors, was critically reviewed with regard to their local and systemic mode of action.

Mechanism of sphingosine 1-phosphate clearance from blood

The interplay of sphingosine 1-phosphate (S1P) synthetic and degradative enzymes as well as S1P exporters creates concentration gradients that are a fundamental to S1P biology. Extracellular S1P levels, such as in blood and lymph, are high relative to cellular S1P. The blood-tissue S1P gradient maintains endothelial integrity while local S1P gradients influence immune cell positioning. Indeed, the importance of S1P gradients was recognized initially when the mechanism of action of an S1P receptor agonist used as a medicine for multiple sclerosis was revealed to be inhibition of T-lymphocytes' recognition of the high S1P in efferent lymph. Furthermore, the increase in erythrocyte S1P in response to hypoxia influences oxygen delivery during high altitude acclimatization. However, understanding of how S1P gradients are maintained is incomplete. For example, S1P is synthesized but is only slowly metabolized by blood yet circulating S1P turns over quickly by an unknown mechanism. Prompted by the counterintuitive observation that blood S1P increases markedly in response to inhibition S1P synthesis (by sphingosine kinase 2 (SphK2)), we studied mice wherein several tissues were made deficient in either SphK2 or S1P degrading enzymes. Our data reveal a mechanism whereby S1P is de-phosphorylated at the hepatocyte surface and the resulting sphingosine is sequestered by SphK phosphorylation and in turn degraded by intracellular S1P lyase. Thus, we identify the liver as the primary site of blood S1P clearance and provide an explanation for the role of SphK2 in this process. Our discovery suggests a general mechanism whereby S1P gradients are shaped.

Pleotropic Roles of Autotaxin in the Nervous System Present Opportunities for the Development of Novel Therapeutics for Neurological Diseases

Autotaxin (ATX) is a soluble extracellular enzyme that is abundant in mammalian plasma and cerebrospinal fluid (CSF). It has two known enzymatic activities, acting as both a phosphodiesterase and a phospholipase. The majority of its biological effects have been associated with its ability to liberate lysophosphatidic acid (LPA) from its substrate, lysophosphatidylcholine (LPC). LPA has diverse pleiotropic effects in the central nervous system (CNS) and other tissues via the activation of a family of six cognate G protein-coupled receptors. These LPA receptors (LPARs) are expressed in some combination in all known cell types in the CNS where they mediate such fundamental cellular processes as proliferation, differentiation, migration, chronic inflammation, and cytoskeletal organization. As a result, dysregulation of LPA content may contribute to many CNS and PNS disorders such as chronic inflammatory or neuropathic pain, glioblastoma multiforme (GBM), hemorrhagic hydrocephalus, schizophrenia, multiple sclerosis, Alzheimer's disease, metabolic syndrome-induced brain damage, traumatic brain injury, hepatic encephalopathy-induced cerebral edema, macular edema, major depressive disorder, stress-induced psychiatric disorder, alcohol-induced brain damage, HIV-induced brain injury, pruritus, and peripheral nerve injury. ATX activity is now known to be the primary biological source of this bioactive signaling lipid, and as such, represents a potentially high-value drug target. There is currently one ATX inhibitor entering phase III clinical trials, with several additional preclinical compounds under investigation. This review discusses the physiological and pathological significance of the ATX-LPA-LPA receptor signaling axis and summarizes the evidence for targeting this pathway for the treatment of CNS diseases.

The Role of S1P and the Related Signaling Pathway in the Development of Tissue Fibrosis

Tissue fibrosis, including pulmonary fibrosis, hepatic fibrosis, and cardiac fibrosis, is an important stage in the development of many diseases. It can lead to structural damage and dysfunction and even severe carcinogenesis or death. There is currently no effective method for the treatment of fibrosis. At present, the molecular mechanism of tissue fibrosis has not yet been fully elucidated, but many studies have demonstrated that it is involved in conveying the complex messages between fibroblasts and various cytokines. Sphingosine 1-phosphate (S1P) is a naturally bioactive sphingolipid. S1P and the related signaling pathways are important intracellular metabolic pathways involved in many life activities, including cell proliferation, differentiation, apoptosis, and cellular signal transduction. Increasing evidence suggests that S1P and its signaling pathways play an important role in the development of tissue fibrosis; however, the mechanisms of these effects have not yet been fully elucidated, and even the role of S1P and its signaling pathways are still controversial. This article focuses on the role of S1P and the related signaling pathways in the development of fibrosis of lung, liver, heart, and other tissues, with emphasis on the application of inhibitors of some of molecules in the pathway in clinical treatment of fibrosis diseases.

Preventing acute asthmatic symptoms by targeting a neuronal mechanism involving carotid body lysophosphatidic acid receptors

Asthma accounts for 380,000 deaths a year. Carotid body denervation has been shown to have a profound effect on airway hyper-responsiveness in animal models but a mechanistic explanation is lacking. Here we demonstrate, using a rat model of asthma (OVA-sensitized), that carotid body activation during airborne allergic provocation is caused by systemic release of lysophosphatidic acid (LPA). Carotid body activation by LPA involves TRPV1 and LPA-specific receptors, and induces parasympathetic (vagal) activity. We demonstrate that this activation is sufficient to cause acute bronchoconstriction. Moreover, we show that prophylactic administration of TRPV1 (AMG9810) and LPA (BrP-LPA) receptor antagonists prevents bradykinin-induced asthmatic bronchoconstriction and, if administered following allergen exposure, reduces the associated respiratory distress. Our discovery provides mechanistic insight into the critical roles of carotid body LPA receptors in allergen-induced respiratory distress and suggests alternate treatment options for asthma.

Acute bronchoconstriction is the leading cause of asthmatic sudden death following allergen exposure. The authors show that the systemic increase of LPA following inhaled allergen or bradykinin challenge activates the carotid bodies through TRPV1 and LPA-specific receptors and that systemic TRPV1 and LPA-specific receptor antagonists ameliorate acute bronchoconstriction.

Panax ginseng as an adjuvant treatment for Alzheimer's disease

Longevity in medicine can be defined as a long life without mental or physical deficits. This can be prevented by Alzheimer's disease (AD). Current conventional AD treatments only alleviate the symptoms without reversing AD progression. Recent studies demonstrated that Panax ginseng extract improves AD symptoms in patients with AD, and the two main components of ginseng might contribute to AD amelioration. Ginsenosides show various AD-related neuroprotective effects. Gintonin is a newly identified ginseng constituent that contains lysophosphatidic acids and attenuates AD-related brain neuropathies. Ginsenosides decrease amyloid β-protein (Aβ) formation by inhibiting β- and γ-secretase activity or by activating the nonamyloidogenic pathway, inhibit acetylcholinesterase activity and Aβ-induced neurotoxicity, and decrease Aβ-induced production of reactive oxygen species and neuroinflammatory reactions. Oral administration of ginsenosides increases the expression levels of enzymes involved in acetylcholine synthesis in the brain and alleviates Aβ-induced cholinergic deficits in AD models. Similarly, gintonin inhibits Aβ-induced neurotoxicity and activates the nonamyloidogenic pathway to reduce Aβ formation and to increase acetylcholine and choline acetyltransferase expression in the brain through lysophosphatidic acid receptors. Oral administration of gintonin attenuates brain amyloid plaque deposits, boosting hippocampal cholinergic systems and neurogenesis, thereby ameliorating learning and memory impairments. It also improves cognitive functions in patients with AD. Ginsenosides and gintonin attenuate AD-related neuropathology through multiple routes. This review focuses research demonstrating that ginseng constituents could be a candidate as an adjuvant for AD treatment. However, clinical investigations including efficacy and tolerability analyses may be necessary for the clinical acceptance of ginseng components in combination with conventional AD drugs.

Lysophosphatidic Acid Signaling in Obesity and Insulin Resistance

Although simple in structure, lysophosphatidic acid (LPA) is a potent bioactive lipid that profoundly influences cellular signaling and function upon binding to G protein-coupled receptors (LPA1-6). The majority of circulating LPA is produced by the secreted enzyme autotaxin (ATX). Alterations in LPA signaling, in conjunction with changes in autotaxin (ATX) expression and activity, have been implicated in metabolic and inflammatory disorders including obesity, insulin resistance, and cardiovascular disease. This review summarizes our current understanding of the sources and metabolism of LPA with focus on the influence of diet on circulating LPA. Furthermore, we explore how the ATX-LPA pathway impacts obesity and obesity-associated disorders, including impaired glucose homeostasis, insulin resistance, and cardiovascular disease.

Cardiac-specific inactivation of LPP3 in mice leads to myocardial dysfunction and heart failure

Lipid Phosphate phosphatase 3 (LPP3), encoded by the Plpp3 gene, is an enzyme that dephosphorylates the bioactive lipid mediator lysophosphatidic acid (LPA). To study the role of LPP3 in the myocardium, we generated a cardiac specific Plpp3 deficient mouse strain. Although these mice were viable at birth in contrast to global Plpp3 knockout mice, they showed increased mortality ~ 8 months. LPP3 deficient mice had enlarged hearts with reduced left ventricular performance as seen by echocardiography. Cardiac specific Plpp3 deficient mice had longer ventricular effective refractory periods compared to their Plpp3 littermates. We observed that lack of Lpp3 enhanced cardiomyocyte hypertrophy based on analysis of cell surface area. We found that lack of Lpp3 signaling was mediated through the activation of Rho and phospho-ERK pathways. There are increased levels of fetal genes Natriuretic Peptide A and B (Nppa and Nppb) expression indicating myocardial dysfunction. These mice also demonstrate mitochondrial dysfunction as evidenced by a significant decrease (P < 0.001) in the basal oxygen consumption rate, mitochondrial ATP production, and spare respiratory capacity as measured through mitochondrial bioenergetics. Histology and transmission electron microscopy of these hearts showed disrupted sarcomere organization and intercalated disc, with a prominent disruption of the cristae and vacuole formation in the mitochondria. Our findings suggest that LPA/LPP3-signaling nexus plays an important role in normal function of cardiomyocytes.

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PLPP3 plays a prominent role in the heart compared to other isoforms of PLPP.

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Lack of PLPP3 results in deteriorating cardiac function.

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PLPP3 regulates LPA signaling in cardiomyocytes.

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Presence of PLPP3 is required for optimal mitochondrial function.

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Increased free radical production is mitigated with activated PLPP3.

Liver-specific deletion of the Plpp3 gene alters plasma lipid composition and worsens atherosclerosis in apoE-/- mice

The PLPP3 gene encodes for a ubiquitous enzyme that dephosphorylates several lipid substrates. Genome-wide association studies identified PLPP3 as a gene that plays a role in coronary artery disease susceptibility. The aim of the study was to investigate the effect of Plpp3 deletion on atherosclerosis development in mice. Because the constitutive deletion of Plpp3 in mice is lethal, conditional Plpp3 hepatocyte-specific null mice were generated by crossing floxed Plpp3 mice with animals expressing Cre recombinase under control of the albumin promoter. The mice were crossed onto the athero-prone apoE^-/- background to obtain Plpp3^f/fapoE^-/-Alb-Cre⁺ and Plpp3^f/fapoE^-/-Alb-Cre^- offspring, the latter of which were used as controls. The mice were fed chow or a Western diet for 32 or 12 weeks, respectively. On the Western diet, Alb-Cre⁺ mice developed more atherosclerosis than Alb-Cre^- mice, both at the aortic sinus and aorta. Lipidomic analysis showed that hepatic Plpp3 deletion significantly modified the levels of several plasma lipids involved in atherosclerosis, including lactosylceramides, lysophosphatidic acids, and lysophosphatidylinositols. In conclusion, Plpp3 ablation in mice worsened atherosclerosis development. Lipidomic analysis suggested that the hepatic Plpp3 deletion may promote atherosclerosis by increasing plasma levels of several low-abundant pro-atherogenic lipids, thus providing a molecular basis for the observed results.

Measurement of Lysophosphatidic Acid and Sphingosine-1-Phosphate by Liquid Chromatography-Coupled Electrospray Ionization Tandem Mass Spectrometry

Lysophosphatidic acids and sphingosine-1-phosphate are bioactive lipids that regulate diverse cellular and physiological processes through actions that are largely mediated by cell surface receptors. The roles played by these lipids in multiple disease processes make the enzymes and receptors involved in their synthesis, inactivation, and signaling attractive targets for pharmacological therapies. In this chapter we describe methods for sensitive accurate quantitation of LPA and S1P levels in biological fluids using liquid chromatography-coupled electrospray ionization tandem mass spectrometry.

Plant Lysophosphatidic Acids: A Rich Source for Bioactive Lysophosphatidic Acids and Their Pharmacological Applications

Lysophosphatidic acid (1-acyl-2-lyso-sn-glycero-3-phosphatidic acid; LPA) is a simple and minor phospholipid in plants. Plant LPAs are merely metabolic intermediates in de novo lipid synthesis in plant cell membranes or for glycerophospholipid storage. The production and metabolisms of LPAs in animals are also well characterized and LPAs have diverse cellular effects in animal systems; i.e., from brain development to wound healing through the activation of G protein-coupled LPA receptors. Recent studies show that various foodstuffs such as soybean, cabbage and seeds such as sesame and sunflower contain bioactive LPAs. Some LPAs are produced from phosphatidic acid during the digestion of foodstuff. In addition, herbal medicines such as corydalis tuber, and especially ginseng, contain large amounts of LPAs compared to foodstuffs. Herbal LPAs bind to cell surface LPA receptors in animal cells and exert their biological effects. Herbal LPAs elicit [Ca(2+)]i transient and are coupled to various Ca(2+)-dependent ion channels and receptor regulations via the activation of LPA receptors. They also showed beneficial effects of in vitro wound healing, in vivo anti-gastric ulcer, anti-Alzheimer's disease, autotaxin inhibition and anti-metastasis activity. Thus, herbal LPAs can be useful agents for human health. Humans can utilize exogenous plant-derived LPAs for preventive or therapeutic purposes if plant-derived LPAs are developed as functional foods or natural medicine targeting LPA receptors. This brief review article introduces the known rich sources of herbal LPAs and herbal LPA binding protein, describes their biological effects, and further addresses possible clinical applications.

Sphingosine 1-Phosphate Activation of EGFR As a Novel Target for Meningitic Escherichia coli Penetration of the Blood-Brain Barrier

Central nervous system (CNS) infection continues to be an important cause of mortality and morbidity, necessitating new approaches for investigating its pathogenesis, prevention and therapy. Escherichia coli is the most common Gram-negative bacillary organism causing meningitis, which develops following penetration of the blood-brain barrier (BBB). By chemical library screening, we identified epidermal growth factor receptor (EGFR) as a contributor to E. coli invasion of the BBB in vitro. Here, we obtained the direct evidence that CNS-infecting E. coli exploited sphingosine 1-phosphate (S1P) for EGFR activation in penetration of the BBB in vitro and in vivo. We found that S1P was upstream of EGFR and participated in EGFR activation through S1P receptor as well as through S1P-mediated up-regulation of EGFR-related ligand HB-EGF, and blockade of S1P function through targeting sphingosine kinase and S1P receptor inhibited EGFR activation, and also E. coli invasion of the BBB. We further found that both S1P and EGFR activations occurred in response to the same E. coli proteins (OmpA, FimH, NlpI), and that S1P and EGFR promoted E. coli invasion of the BBB by activating the downstream c-Src. These findings indicate that S1P and EGFR represent the novel host targets for meningitic E. coli penetration of the BBB, and counteracting such targets provide a novel approach for controlling E. coli meningitis in the era of increasing resistance to conventional antibiotics.

The roles of bile acids and sphingosine-1-phosphate signaling in the hepatobiliary diseases

Based on research carried out over the last decade, it has become increasingly evident that bile acids act not only as detergents, but also as important signaling molecules that exert various biological effects via activation of specific nuclear receptors and cell signaling pathways. Bile acids also regulate the expression of numerous genes encoding enzymes and proteins involved in the synthesis and metabolism of bile acids, glucose, fatty acids, and lipoproteins, as well as energy metabolism. Receptors activated by bile acids include, farnesoid X receptor α, pregnane X receptor, vitamin D receptor, and G protein-coupled receptors, TGR5, muscarinic receptor 2, and sphingosine-1-phosphate receptor (S1PR)2. The ligand of S1PR2, sphingosine-1-phosphate (S1P), is a bioactive lipid mediator that regulates various physiological and pathophysiological cellular processes. We have recently reported that conjugated bile acids, via S1PR2, activate and upregulate nuclear sphingosine kinase 2, increase nuclear S1P, and induce genes encoding enzymes and transporters involved in lipid and sterol metabolism in the liver. Here, we discuss the role of bile acids and S1P signaling in the regulation of hepatic lipid metabolism and in hepatobiliary diseases.

An obligatory role for neurotensin in high-fat-diet-induced obesity

Obesity and its associated comorbidities (for example, diabetes mellitus and hepatic steatosis) contribute to approximately 2.5 million deaths annually and are among the most prevalent and challenging conditions confronting the medical profession. Neurotensin (NT; also known as NTS), a 13-amino-acid peptide predominantly localized in specialized enteroendocrine cells of the small intestine and released by fat ingestion, facilitates fatty acid translocation in rat intestine, and stimulates the growth of various cancers. The effects of NT are mediated through three known NT receptors (NTR1, 2 and 3; also known as NTSR1, 2, and NTSR3, respectively). Increased fasting plasma levels of pro-NT (a stable NT precursor fragment produced in equimolar amounts relative to NT) are associated with increased risk of diabetes, cardiovascular disease and mortality; however, a role for NT as a causative factor in these diseases is unknown. Here we show that NT-deficient mice demonstrate significantly reduced intestinal fat absorption and are protected from obesity, hepatic steatosis and insulin resistance associated with high fat consumption. We further demonstrate that NT attenuates the activation of AMP-activated protein kinase (AMPK) and stimulates fatty acid absorption in mice and in cultured intestinal cells, and that this occurs through a mechanism involving NTR1 and NTR3 (also known as sortilin). Consistent with the findings in mice, expression of NT in Drosophila midgut enteroendocrine cells results in increased lipid accumulation in the midgut, fat body, and oenocytes (specialized hepatocyte-like cells) and decreased AMPK activation. Remarkably, in humans, we show that both obese and insulin-resistant subjects have elevated plasma concentrations of pro-NT, and in longitudinal studies among non-obese subjects, high levels of pro-NT denote a doubling of the risk of developing obesity later in life. Our findings directly link NT with increased fat absorption and obesity and suggest that NT may provide a prognostic marker of future obesity and a potential target for prevention and treatment.

Tetracyclines increase lipid phosphate phosphatase expression on plasma membranes and turnover of plasma lysophosphatidate

Extracellular lysophosphatidate and sphingosine 1-phosphate (S1P) are important bioactive lipids, which signal through G-protein-coupled receptors to stimulate cell growth and survival. The lysophosphatidate and S1P signals are terminated partly by degradation through three broad-specificity lipid phosphate phosphatases (LPPs) on the cell surface. Significantly, the expression of LPP1 and LPP3 is decreased in many cancers, and this increases the impact of lysophosphatidate and S1P signaling. However, relatively little is known about the physiological or pharmacological regulation of the expression of the different LPPs. We now show that treating several malignant and nonmalignant cell lines with 1 μg/ml tetracycline, doxycycline, or minocycline significantly increased the extracellular degradation of lysophosphatidate. S1P degradation was also increased in cells that expressed high LPP3 activity. These results depended on an increase in the stabilities of the three LPPs and increased expression on the plasma membrane. We tested the physiological significance of these results and showed that treating rats with doxycycline accelerated the clearance of lysophosphatidate, but not S1P, from the circulation. However, administering 100 mg/kg/day doxycycline to mice decreased plasma concentrations of lysophosphatidate and S1P. This study demonstrates a completely new property of tetracyclines in increasing the plasma membrane expression of the LPPs.

Sphingosine Kinase 2 Inhibition and Blood Sphingosine 1-Phosphate Levels

Sphingosine 1-phosphate (S1P) levels are significantly higher in blood and lymph than in tissues. This S1P concentration difference is necessary for proper lymphocyte egress from secondary lymphoid tissue and to maintain endothelial barrier integrity. Studies with mice lacking either sphingosine kinase (SphK) type 1 and 2 indicate that these enzymes are the sole biosynthetic source of S1P, but they play different roles in setting S1P blood levels. We have developed a set of drug-like SphK inhibitors, with differing selectivity for the two isoforms of this enzyme. Although all SphK inhibitors tested decrease S1P when applied to cultured U937 cells, only those inhibitors with a bias for SphK2 drove a substantial increase in blood S1P in mice and this rise was detectable within minutes of administration of the inhibitor. Blood S1P also increased in response to SphK2 inhibitors in rats. Mass-labeled S1P was cleared more slowly after intravenous injection into SphK2 inhibitor-treated mice or mice lacking a functional SphK2 gene; thus, the increased accumulation of S1P in the blood appears to result from the decreased clearance of S1P from the blood. Therefore, SphK2 appears to have a function independent of generating S1P in cells. Our results suggest that differential SphK inhibition with a drug might afford a method to manipulate blood S1P levels in either direction while lowering tissue S1P levels.

Lysophosphatidic Acid and Sphingosine-1-Phosphate: A Concise Review of Biological Function and Applications for Tissue Engineering

The presentation and controlled release of bioactive signals to direct cellular growth and differentiation represents a widely used strategy in tissue engineering. Historically, work in this field has primarily focused on the delivery of large cytokines and growth factors, which can be costly to manufacture and difficult to deliver in a sustained manner. There has been a marked increase over the past decade in the pursuit of lipid mediators due to their wide range of effects over multiple cell types, low cost, and ease of scale-up. Lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are two bioactive lysophospholipids (LPLs) that have gained attention for use as pharmacological agents in tissue engineering applications. While these lipids can have similar effects on cellular response, they possess distinct chemical backbones, mechanisms of synthesis and degradation, and signaling pathways using a discrete set of G-protein-coupled receptors (GPCRs). LPA and S1P predominantly act extracellularly on their GPCRs and can directly regulate cell survival, differentiation, cytokine secretion, proliferation, and migration--each of the important functions that must be considered in regenerative medicine. In addition to these potent physiological functions, these LPLs play pivotal roles in a number of pathophysiological processes. To capitalize on the promise of these molecules in tissue engineering, these lipids have been incorporated into biomaterials for in vivo delivery. Here, we survey the effects of LPA and S1P on both cellular- and tissue-level phenotypes, with an eye toward regulating stem/progenitor cell growth and differentiation. In particular, we examine work that has translational applications for cell-based tissue engineering strategies in promoting cell survival, bone and cartilage engineering, and therapeutic angiogenesis.

Sources, metabolism, and regulation of circulating sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that acts either as an intracellular messenger or as a ligand for its membrane receptors. S1P is a normal constituent of blood, where it is found both in plasma and blood cells. Compared with other cell types, sphingolipid metabolism in erythrocytes and platelets has unique features that allow the erythrocytes and platelets to accumulate S1P. In plasma, S1P is bound mainly to HDLs and albumin. Of note, metabolism and biological activity of S1P is to a large extent affected by the type of its carrier. Plasma S1P is characterized by a short half-life, indicating rapid clearance by degradative enzymes and the presence of high-capacity sources involved in maintaining its high concentration. These sources include blood cells, vascular endothelium, and hepatocytes. However, the extent to which each of these contributes to the plasma pool of S1P is a matter of debate. Circulating S1P plays a significant physiological role. It was found to be the key regulator of lymphocyte trafficking, endothelial barrier function, and vascular tone. The purpose of this review is to summarize the present state of knowledge on the metabolism, transport, and origin of plasma S1P, and to discuss the mechanisms regulating its homeostasis in blood.

Lipid-induced epigenomic changes in human macrophages identify a coronary artery disease-associated variant that regulates PPAP2B Expression through Altered C/EBP-beta binding

Genome-wide association studies (GWAS) have identified over 40 loci that affect risk of coronary artery disease (CAD) and the causal mechanisms at the majority of loci are unknown. Recent studies have suggested that many causal GWAS variants influence disease through altered transcriptional regulation in disease-relevant cell types. We explored changes in transcriptional regulation during a key pathophysiological event in CAD, the environmental lipid-induced transformation of macrophages to lipid-laden foam cells. We used a combination of open chromatin mapping with formaldehyde-assisted isolation of regulatory elements (FAIRE-seq) and enhancer and transcription factor mapping using chromatin immuno-precipitation (ChIP-seq) in primary human macrophages before and after exposure to atherogenic oxidized low-density lipoprotein (oxLDL), with resultant foam cell formation. OxLDL-induced foam cell formation was associated with changes in a subset of open chromatin and active enhancer sites that strongly correlated with expression changes of nearby genes. OxLDL-regulated enhancers were enriched for several transcription factors including C/EBP-beta, which has no previously documented role in foam cell formation. OxLDL exposure up-regulated C/EBP-beta expression and increased genomic binding events, most prominently around genes involved in inflammatory response pathways. Variants at CAD-associated loci were significantly and specifically enriched in the subset of chromatin sites altered by oxLDL exposure, including rs72664324 in an oxLDL-induced enhancer at the PPAP2B locus. OxLDL increased C/EBP beta binding to this site and C/EBP beta binding and enhancer activity were stronger with the protective A allele of rs72664324. In addition, expression of the PPAP2B protein product LPP3 was present in foam cells in human atherosclerotic plaques and oxLDL exposure up-regulated LPP3 in macrophages resulting in increased degradation of pro-inflammatory mediators. Our results demonstrate a genetic mechanism contributing to CAD risk at the PPAP2B locus and highlight the value of studying epigenetic changes in disease processes involving pathogenic environmental stimuli.

Coronary artery disease is a complex disease where over 40 genomic loci contributing to genetic risk have been identified. However, identifying the precise variants, genomic elements and genes that mediate this risk at each locus has proved challenging. We hypothesized that some genetic risk variants may influence a key step in development of coronary artery disease, which occurs when macrophages encounter environmentally-derived lipid. These cells take up lipid and accumulate in atherosclerotic plaques in the walls of blood vessels where they contribute to the inflammatory atherosclerotic disease process. Therefore, we studied the effects of this lipid exposure on the genomic activity of these cells. Environmental lipid exposure triggered changes in transcriptional regulation and gene expression. Variants at coronary artery disease risk loci were enriched for genomic regions altered by lipid exposure. We studied one such risk variant rs72664324 in detail and found that it altered binding of the C/EBP-beta transcription factor and altered expression of the PPAP2B gene. PPAP2B encodes an enzyme that degrades pro-inflammatory substances. Our study demonstrates a hitherto unknown genetic mechanism underlying atherosclerotic heart disease and demonstrates the value of studying changes in transcriptional regulation in key disease processes involving environmental influences.

Lipid phosphate phosphatases and their roles in mammalian physiology and pathology

Lipid phosphate phosphatases (LPPs) are a group of enzymes that belong to a phosphatase/phosphotransferase family. Mammalian LPPs consist of three isoforms: LPP1, LPP2, and LPP3. They share highly conserved catalytic domains and catalyze the dephosphorylation of a variety of lipid phosphates, including phosphatidate, lysophosphatidate (LPA), sphingosine 1-phosphate (S1P), ceramide 1-phosphate, and diacylglycerol pyrophosphate. LPPs are integral membrane proteins, which are localized on plasma membranes with the active site on the outer leaflet. This enables the LPPs to degrade extracellular LPA and S1P, thereby attenuating their effects on the activation of surface receptors. LPP3 also exhibits noncatalytic effects at the cell surface. LPP expression on internal membranes, such as endoplasmic reticulum and Golgi, facilitates the metabolism of internal lipid phosphates, presumably on the luminal surface of these organelles. This action probably explains the signaling effects of the LPPs, which occur downstream of receptor activation. The three isoforms of LPPs show distinct and nonredundant effects in several physiological and pathological processes including embryo development, vascular function, and tumor progression. This review is intended to present an up-to-date understanding of the physiological and pathological consequences of changing the activities of the different LPPs, especially in relation to cell signaling by LPA and S1P.

Increasing adipocyte lipoprotein lipase improves glucose metabolism in high fat diet-induced obesity

Lipid accumulation in liver and skeletal muscle contributes to co-morbidities associated with diabetes and obesity. We made a transgenic mouse in which the adiponectin (Adipoq) promoter drives expression of lipoprotein lipase (LPL) in adipocytes to potentially increase adipose tissue lipid storage. These mice (Adipoq-LPL) have improved glucose and insulin tolerance as well as increased energy expenditure when challenged with a high fat diet (HFD). To identify the mechanism(s) involved, we determined whether the Adipoq-LPL mice diverted dietary lipid to adipose tissue to reduce peripheral lipotoxicity, but we found no evidence for this. Instead, characterization of the adipose tissue of the male mice after HFD challenge revealed that the mRNA levels of peroxisome proliferator-activated receptor-γ (PPARγ) and a number of PPARγ-regulated genes were higher in the epididymal fat pads of Adipoq-LPL mice than control mice. This included adiponectin, whose mRNA levels were increased, leading to increased adiponectin serum levels in the Adipoq-LPL mice. In many respects, the adipose phenotype of these animals resembles thiazolidinedione treatment except for one important difference, the Adipoq-LPL mice did not gain more fat mass on HFD than control mice and did not have increased expression of genes in adipose such as glycerol kinase, which are induced by high affinity PPAR agonists. Rather, there was selective induction of PPARγ-regulated genes such as adiponectin in the adipose of the Adipoq-LPL mice, suggesting that increasing adipose tissue LPL improves glucose metabolism in diet-induced obesity by improving the adipose tissue phenotype. Adipoq-LPL mice also have increased energy expenditure.

ORMDL/serine palmitoyltransferase stoichiometry determines effects of ORMDL3 expression on sphingolipid biosynthesis

The ORM1 (Saccharomyces cerevisiae)-like proteins (ORMDLs) and their yeast orthologs, the Orms, are negative homeostatic regulators of the initiating enzyme in sphingolipid biosynthesis, serine palmitoyltransferase (SPT). Genome-wide association studies have established a strong correlation between elevated expression of the endoplasmic reticulum protein ORMDL3 and risk for childhood asthma. Here we test the notion that elevated levels of ORMDL3 decrease sphingolipid biosynthesis. This was tested in cultured human bronchial epithelial cells (HBECs) (an immortalized, but untransformed, airway epithelial cell line) and in HeLa cells (a cervical adenocarcinoma cell line). Surprisingly, elevated ORMDL3 expression did not suppress de novo biosynthesis of sphingolipids. We determined that ORMDL is expressed in functional excess relative to SPT at normal levels of expression. ORMDLs and SPT form stable complexes that are not increased by elevated ORMDL3 expression. Although sphingolipid biosynthesis was not decreased by elevated ORMDL3 expression, the steady state mass levels of all major sphingolipids were marginally decreased by low level ORMDL3 over-expression in HBECs. These data indicate that the contribution of ORMDL3 to asthma risk may involve changes in sphingolipid metabolism, but that the connection is complex.

Source and role of intestinally derived lysophosphatidic acid in dyslipidemia and atherosclerosis

We previously reported that i) a Western diet increased levels of unsaturated lysophosphatidic acid (LPA) in small intestine and plasma of LDL receptor null (LDLR(-/-)) mice, and ii) supplementing standard mouse chow with unsaturated (but not saturated) LPA produced dyslipidemia and inflammation. Here we report that supplementing chow with unsaturated (but not saturated) LPA resulted in aortic atherosclerosis, which was ameliorated by adding transgenic 6F tomatoes. Supplementing chow with lysophosphatidylcholine (LysoPC) 18:1 (but not LysoPC 18:0) resulted in dyslipidemia similar to that seen on adding LPA 18:1 to chow. PF8380 (a specific inhibitor of autotaxin) significantly ameliorated the LysoPC 18:1-induced dyslipidemia. Supplementing chow with LysoPC 18:1 dramatically increased the levels of unsaturated LPA species in small intestine, liver, and plasma, and the increase was significantly ameliorated by PF8380 indicating that the conversion of LysoPC 18:1 to LPA 18:1 was autotaxin dependent. Adding LysoPC 18:0 to chow increased levels of LPA 18:0 in small intestine, liver, and plasma but was not altered by PF8380 indicating that conversion of LysoPC 18:0 to LPA 18:0 was autotaxin independent. We conclude that i) intestinally derived unsaturated (but not saturated) LPA can cause atherosclerosis in LDLR(-/-) mice, and ii) autotaxin mediates the conversion of unsaturated (but not saturated) LysoPC to LPA.

Drugging sphingosine kinases

The transfer of the gamma phosphate from ATP to sphingosine (Sph) to generate a small signaling molecule, sphingosine 1-phosphate (S1P), is catalyzed by sphingosine kinases (SphK), which exist as two isoforms, SphK1 and SphK2. SphK is a key regulator of S1P and the S1P:Sph/ceramide ratio. Increases in S1P levels have been linked to diseases including sickle cell disease, cancer, and fibrosis. Therefore, SphKs are potential targets for drug discovery. However, the current chemical biology toolkit needed to validate these enzymes as drug targets is inadequate. With this review, we survey in vivo active SphK inhibitors and highlight the need for developing more potent and selective inhibitors.