Expression and characterization of an epoxide hydrolase from Anopheles gambiae with high activity on epoxy fatty acids

Knockdown of Anopheles dirus far upstream element-binding protein gene lower oocyst numbers of Plasmodium vivax

The modulation of gene expression levels of Anopheles dirus on Plasmodium vivax infection at the ookinete and oocyst stages was previously reported. In the present study, several upregulated An. dirus genes were selected based on their high expression levels and subcellular locations to examine their roles in P. vivax infection. Five An. dirus genes-carboxylesterase, cuticular protein RR-2 family, far upstream element-binding protein, kraken, and peptidase212-were knocked down by dsRNA feeding using dsRNA-lacZ as a control. The dsRNA-fed mosquitoes were later challenged by P. vivax-infected blood, and the oocyst numbers were determined. The expression of these five genes was examined in many organs of both male and female mosquitoes. The results showed that the decreased expression level of the far upstream element-binding protein gene could lower the oocyst numbers, whereas the others showed no effect on P. vivax infection. The expression levels of these genes in ovaries were found, and in many organs, they were similar between male and female mosquitoes. The reduction of these five gene expressions did not affect the lifespan of the mosquitoes. In addition, the malaria box compound, MMV000634, demonstrated the lowest binding energy to the far upstream element-binding protein using virtual screening. This protein might be a target to block malaria transmission.

Identification of four secretory phospholipase A2s in a lepidopteran insect, Acrolepiopsis sapporensis, and their functional association with cellular immune responses

Background

Eicosanoids are a group of the oxygenated C20 polyunsaturated fatty acids and play crucial roles in mediating various insect physiological processes. Catalytic activity of phospholipase A₂ (PLA₂) provides an initial substrate, arachidonic acid (AA), for subsequent eicosanoid biosynthesis.

Results

This study identified four different secretory PLA₂ (As-PLA₂A-As-PLA₂D) genes encoded in the Asian onion moth, Acrolepiopsis sapporensis. A phylogenetic analysis indicated that As-PLA₂A and As-PLA₂D are clustered with Group III PLA₂s while As-PLA₂B and As-PLA₂C are clustered with Group XII and Group X PLA₂s, respectively. Expression levels of these PLA₂ genes increased along with larval development, especially in the fat body. A bacterial immune challenge upregulated the basal expression levels of the four PLA₂ genes, which resulted in significant increases of the PLA₂ enzyme activity. The enzyme activity was susceptible to a calcium chelator or reducing agent, suggesting Ca²⁺ dependency and disulfide linkage required for the catalytic activities of the secretory type of PLA₂s. In addition, the PLA₂ activity was also susceptible to bromophenacyl bromide (BPB), a specific inhibitor to sPLA₂, but not to intracellular PLA₂ inhibitors. An addition of BPB to the immune challenge significantly prevented hemocyte-spreading behavior of A. sapporensis. BPB treatment also suppressed a cellular immune response measured by hemocyte nodule formation. However, the immunosuppression was significantly rescued by the AA addition. To determine the PLA₂(s) responsible for the immunity, individual RNA interference (RNAi) treatments specific to each of the four PLA₂s were performed. Injection of gene-specific double-stranded RNAs caused significant reductions in the transcript level in all four PLA₂s. In all four PLA₂s, the RNAi treatments prevented the cellular immune response even after the immune challenge.

Conclusion

This study reports four secretory PLA₂s encoded in A. sapporensis and their function in mediating cellular immunity.

Meloxicam methyl group determines enzyme specificity for thiazole bioactivation compared to sudoxicam

Meloxicam is a thiazole-containing NSAID that was approved for marketing with favorable clinical outcomes despite being structurally similar to the hepatotoxic sudoxicam. Introduction of a single methyl group on the thiazole results in an overall lower toxic risk, yet the group's impact on P450 isozyme bioactivation is unclear. Through analytical methods, we used inhibitor phenotyping and recombinant P450s to identify contributing P450s, and then measured steady-state kinetics for bioactivation of sudoxicam and meloxicam by the recombinant P450s to determine relative efficiencies. Experiments showed that CYP2C8, 2C19, and 3A4 catalyze sudoxicam bioactivation, and CYP1A2 catalyzes meloxicam bioactivation, indicating that the methyl group not only impacts enzyme affinity for the drugs, but also alters which isozymes catalyze the metabolic pathways. Scaling of relative P450 efficiencies based on average liver concentration revealed that CYP2C8 dominates the sudoxicam bioactivation pathway and CYP2C9 dominates meloxicam detoxification. Dominant P450s were applied for an informatics assessment of electronic health records to identify potential correlations between meloxicam drug-drug interactions and drug-induced liver injury. Overall, our findings provide a cautionary tale on assumed impacts of even simple structural modifications on drug bioactivation while also revealing specific targets for clinical investigations of predictive factors that determine meloxicam-induced idiosyncratic liver injury.

EpOMEs act as immune suppressors in a lepidopteran insect, Spodoptera exigua

Epoxyoctadecamonoenoic acids (EpOMEs) are epoxide derivatives of linoleic acid (9,12-octadecadienoic acid) and include 9,10-EpOME and 12,13-EpOME. They are synthesized by cytochrome P450 monooxygenases (CYPs) and degraded by soluble epoxide hydrolase (sEH). Although EpOMEs are well known to play crucial roles in mediating various physiological processes in mammals, their role is not well understood in insects. This study chemically identified their presence in insect tissues: 941.8 pg/g of 9,10-EpOME and 2,198.3 pg/g of 12,13-EpOME in fat body of a lepidopteran insect, Spodoptera exigua. Injection of 9,10-EpOME or 12,13-EpOME into larvae suppressed the cellular immune responses induced by bacterial challenge. EpOME treatment also suppressed the expression of antimicrobial peptide (AMP) genes. Among 139 S. exigua CYPs, an ortholog (SE51385) to human EpOME synthase was predicted and its expression was highly inducible upon bacterial challenge. RNA interference (RNAi) of SE51385 prevented down-regulation of immune responses at a late stage (> 24 h) following bacterial challenge. A soluble epoxide hydrolase (Se-sEH) of S. exigua was predicted and showed specific expression in all development stages and in different larval tissues. Furthermore, its expression levels were highly enhanced by bacterial challenge in different tissues. RNAi reduction of Se-sEH interfered with hemocyte-spreading behavior, nodule formation, and AMP expression. To support the immune association of EpOMEs, urea-based sEH inhibitors were screened to assess their inhibitory activities against cellular and humoral immune responses of S. exigua. 12-(3-adamantan-1-yl-ureido) dodecanoic acid (AUDA) was highly potent in suppressing the immune responses. The addition of AUDA to a pathogenic bacterium significantly increased bacterial pathogenicity by suppressing host immune defense. In sum, this study demonstrated that EpOMEs play a crucial role in facilitating anti-inflammatory responses in S. exigua.

Inhibitors of Eicosanoid Biosynthesis Reveal that Multiple Lipid Signaling Pathways Influence Malaria Parasite Survival in Anopheles gambiae

Eicosanoids are bioactive signaling lipids derived from the oxidation of fatty acids that act as important regulators of immune homeostasis and inflammation. As a result, effective anti-inflammatory drugs have been widely used to reduce pain and inflammation which target key eicosanoid biosynthesis enzymes. Conserved from vertebrates to insects, the use of these eicosanoid pathway inhibitors offer opportunities to evaluate the roles of eicosanoids in less-characterized insect systems. In this study, we examine the potential roles of eicosanoids on malaria parasite survival in the mosquito Anopheles gambiae. Using Plasmodium oocyst numbers to evaluate parasite infection, general or specific inhibitors of eicosanoid biosynthesis pathways were evaluated. Following the administration of dexamethasone and indomethacin, respective inhibitors of phospholipid A2 (PLA2) and cyclooxygenase (COX), oocyst numbers were unaffected. However, inhibition of lipoxygenase (LOX) activity through the use of esculetin significantly increased oocyst survival. In contrast, 12-[[(tricyclo[3.3.1.13,7]dec-1-ylamino)carbonyl]amino]-dodecanoic acid (AUDA), an inhibitor of epoxide hydroxylase (EH), decreased oocyst numbers. These experiments were further validated through RNAi experiments to silence candidate genes homologous to EH in An. gambiae to confirm their contributions to Plasmodium development. Similar to the results of AUDA treatment, the silencing of EH significantly reduced oocyst numbers. These results imply that specific eicosanoids in An. gambiae can have either agonist or antagonistic roles on malaria parasite survival in the mosquito host.

Eicosanoid-mediated immunity in insects

Eicosanoid is a collective term for oxygenated metabolites of C20 polyunsaturated fatty acids. As seen in mammals, eicosanoids play crucial roles in mediating various physiological processes, including immune responses, in insects. Upon microbial pathogen infection, non-self recognition signals are propagated to nearly immune effectors such as hemocytes and fat body using various immune mediators, in which eicosanoid signals act as the ultimate downstream mediator. The chemical diversity of eicosanoids may operate to mediate various immune responses. Some entomopathogenic bacteria suppress eicosanoid biosynthesis, which inhibits host insect immunity and promotes their pathogenicity. This review introduces immune responses mediated by various eicosanoids. Then it explains the cross-talks of eicosanoids with other immune mediators including cytokines, biogenic monoamines, and nitric oxide to clarify the complexity of insect immune mediation. Finally, we highlight the biological significance of eicosanoids by demonstrating bacterial pathogenicity inhibiting a key enzyme - phospholipase A₂ - in eicosanoid biosynthesis using their secondary metabolites to defend host insect immune attack.

Nitric Oxide Mediates Insect Cellular Immunity via Phospholipase A2 Activation

After infection or invasion is recognized, biochemical mediators act in signaling insect immune functions. These include biogenic amines, insect cytokines, eicosanoids, and nitric oxide (NO). Treating insects or isolated hemocyte populations with different mediators often leads to similar results. Separate treatments with an insect cytokine, 2 biogenic amines, and an eicosanoid lead to a single result, hemocyte spreading, understood in terms of intracellular cross-talk among these signaling systems. This study focuses on the cross-talk between NO and eicosanoid signaling in our model insect, Spodoptera exigua. Bacterial injection increased NO concentrations in the larval hemocytes and fat body, and RNA interference (RNAi) of the S. exigua NO synthase (NOS) gene suppressed NO concentrations. RNAi treatment also led to a significant reduction in hemocyte nodulation following bacterial injection. Similar RNAi treatments led to significantly reduced PLA2 activities in the hemocytes and fat body compared to control larvae. Injection of L-NAME also prevented the induction of PLA2 activity following bacterial challenge. An injected NO donor, S-nitroso-N-acetyl-DL-penicillamine, increased PLA2 activity in a dose-dependent manner. However, eicosanoids did not influence NO concentrations in immune-challenged larvae. We infer that NO and eicosanoid signaling operate via cross-talk mechanisms in which the elevated NO concentrations activate PLA2 and eicosanoid biosynthesis, which finally mediates various immune responses.

An entomopathogenic bacterium, Xenorhabdus hominickii ANU101, produces oxindole and suppresses host insect immune response by inhibiting eicosanoid biosynthesis

An entomopathogenic bacterium, Xenorhabdus hominickii ANU101, was isolated from an entomopathogenic nematode, Steinernema monticolum. X. hominickii exhibited significant insecticidal activities at ≥6.6×10² colony-forming units per larva against a lepidopteran insect, Spodoptera exigua with hemocoelic injection. The insecticidal activity of X. hominickii was reduced by an addition of arachidonic acid (AA, a catalytic product of PLA₂), but enhanced by an addition by dexamethasone (DEX, a specific inhibitor of PLA₂). S. exigua could defend the bacterial infection by forming hemocyte nodules. However, live X. hominickii significantly reduced the hemocytic nodulation compared to similar treatment with heat-killed X. hominickii. An addition of AA to live X. hominickii significantly rescued the immunosuppression. X. hominickii also inhibited phenoloxidase activity in hemolymph of S. exigua larvae. Furthermore, the bacteria suppressed gene expressions of antimicrobial peptides, such as attacin-1, attacin-2, defensin, gallerimycin and transferrin-1 of S. exigua. An organic extract of X. hominickii-cultured broth with ethyl acetate possessed oxindole and significantly suppressed hemocyte nodulation. Again, an addition of AA diminished the inhibitory activity of the organic extract against hemocyte nodulation. Oxindole alone inhibited hemocyte nodulation and PLA₂ enzyme activity. These results suggest that the entomopathogenicity of X. hominickii comes from its inhibitory activity against eicosanoid biosynthesis of target insects.

Exported Epoxide Hydrolases Modulate Erythrocyte Vasoactive Lipids during Plasmodium falciparum Infection

Erythrocytes are reservoirs of important epoxide-containing lipid signaling molecules, including epoxyeicosatrienoic acids (EETs). EETs function as vasodilators and anti-inflammatory modulators in the bloodstream. Bioactive EETs are hydrolyzed to less active diols (dihydroxyeicosatrienoic acids) by epoxide hydrolases (EHs). The malaria parasite Plasmodium falciparum infects host red blood cells (RBCs) and exports hundreds of proteins into the RBC compartment. In this study, we show that two parasite epoxide hydrolases, P. falciparum epoxide hydrolases 1 (PfEH1) and 2 (PfEH2), both with noncanonical serine nucleophiles, are exported to the periphery of infected RBCs. PfEH1 and PfEH2 were successfully expressed in Escherichia coli, and they hydrolyzed physiologically relevant erythrocyte EETs. Mutations in active site residues of PfEH1 ablated the ability of the enzyme to hydrolyze an epoxide substrate. Overexpression of PfEH1 or PfEH2 in parasite-infected RBCs resulted in a significant alteration in the epoxide fatty acids stored in RBC phospholipids. We hypothesize that the parasite disruption of epoxide-containing signaling lipids leads to perturbed vascular function, creating favorable conditions for binding and sequestration of infected RBCs to the microvascular endothelium.

The malaria parasite exports hundreds of proteins into the erythrocyte compartment. However, for most of these proteins, their physiological function is unknown. In this study, we investigate two “hypothetical” proteins of the α/β-hydrolase fold family that share sequence similarity with epoxide hydrolases (EHs)—enzymes that destroy bioactive epoxides. Altering EH expression in parasite-infected erythrocytes resulted in a significant change in the epoxide fatty acids stored in the host cell. We propose that these EH enzymes may help the parasite to manipulate host blood vessel opening and inflame the vessel walls as they pass through the circulation system. Understanding how the malaria parasite interacts with its host RBCs will aid in our ability to combat this deadly disease.

Ingestion of the epoxide hydrolase inhibitor AUDA modulates immune responses of the mosquito, Culex quinquefasciatus during blood feeding

Epoxide hydrolases (EHs) are enzymes that play roles in metabolizing xenobiotic epoxides from the environment, and in regulating lipid signaling molecules, such as juvenile hormones in insects and epoxy fatty acids in mammals. In this study we fed mosquitoes with an epoxide hydrolase inhibitor AUDA during artificial blood feeding, and we found the inhibitor increased the concentration of epoxy fatty acids in the midgut of female mosquitoes. We also observed ingestion of AUDA triggered early expression of defensin A, cecropin A and cecropin B2 at 6 h after blood feeding. The expression of cecropin B1 and gambicin were not changed more than two fold compared to controls. The changes in gene expression were transient possibly because more than 99% of the inhibitor was metabolized or excreted at 42 h after being ingested. The ingestion of AUDA also affected the growth of bacteria colonizing in the midgut, but did not affect mosquito longevity, fecundity and fertility in our laboratory conditions. When spiked into the blood, EpOMEs and DiHOMEs were as effective as the inhibitor AUDA in reducing the bacterial load in the midgut, while EETs rescued the effects of AUDA. Our data suggest that epoxy fatty acids from host blood are immune response regulators metabolized by epoxide hydrolases in the midgut of female mosquitoes, inhibition of which causes transient changes in immune responses, and affects growth of microbes in the midgut.

Epoxide hydrolase activities and epoxy fatty acids in the mosquito Culex quinquefasciatus

Culex mosquitoes have emerged as important model organisms for mosquito biology, and are disease vectors for multiple mosquito-borne pathogens, including West Nile virus. We characterized epoxide hydrolase activities in the mosquito Culex quinquefasciatus, which suggested multiple forms of epoxide hydrolases were present. We found EH activities on epoxy eicosatrienoic acids (EETs). EETs and other eicosanoids are well-established lipid signaling molecules in vertebrates. We showed EETs can be synthesized in vitro from arachidonic acids by mosquito lysate, and EETs were also detected in vivo both in larvae and adult mosquitoes by LC-MS/MS. The EH activities on EETs can be induced by blood feeding, and the highest activity was observed in the midgut of female mosquitoes. The enzyme activities on EETs can be inhibited by urea-based inhibitors designed for mammalian soluble epoxide hydrolases (sEH). The sEH inhibitors have been shown to play diverse biological roles in mammalian systems, and they can be useful tools to study the function of EETs in mosquitoes. Besides juvenile hormone metabolism and detoxification, insect epoxide hydrolases may also play a role in regulating lipid signaling molecules, such as EETs and other epoxy fatty acids, synthesized in vivo or obtained from blood feeding by female mosquitoes.