Inhibitors of CYP6D1 in house fly microsomes

Developmental Toxicity Assessment of Piperonyl Butoxide Exposure Targeting Sonic Hedgehog Signaling and Forebrain and Face Morphogenesis in the Mouse: An in Vitro and in Vivo Study

BACKGROUND

Piperonyl butoxide (PBO) is a pesticide synergist used in residential, commercial, and agricultural settings. PBO was recently found to inhibit Sonic hedgehog (Shh) signaling, a key developmental regulatory pathway. Disruption of Shh signaling is linked to birth defects, including holoprosencephaly (HPE), a malformation of the forebrain and face thought to result from complex gene-environment interactions.

OBJECTIVES

The impact of PBO on Shh signaling in vitro and forebrain and face development in vivo was examined.

METHODS

The influence of PBO on Shh pathway transduction was assayed in mouse and human cell lines. To examine its teratogenic potential, a single dose of PBO (22-1,800mg/kg) was administered by oral gavage to C57BL/6J mice at gestational day 7.75, targeting the critical period for HPE. Gene-environment interactions were investigated using Shh+/- mice, which model human HPE-associated genetic mutations.

RESULTS

PBO attenuated Shh signaling in vitro through a mechanism similar to that of the known teratogen cyclopamine. In utero PBO exposure caused characteristic HPE facial dysmorphology including dose-dependent midface hypoplasia and hypotelorism, with a lowest observable effect level of 67mg/kg. Median forebrain deficiency characteristic of HPE was observed in severely affected animals, whereas all effective doses disrupted development of Shh-dependent transient forebrain structures that generate cortical interneurons. Normally silent heterozygous Shh null mutations exacerbated PBO teratogenicity at all doses tested, including 33mg/kg.

DISCUSSION

These findings demonstrate that prenatal PBO exposure can cause overt forebrain and face malformations or neurodevelopmental disruptions with subtle or no craniofacial dysmorphology in mice. By targeting Shh signaling as a sensitive mechanism of action and examining gene-environment interactions, this study defined a lowest observable effect level for PBO developmental toxicity in mice more than 30-fold lower than previously recognized. Human exposure to PBO and its potential contribution to etiologically complex birth defects should be rigorously examined. https://doi.org/10.1289/EHP5260.

Inhibition against mosquito cytochrome P450 enzymes by rhinacanthin-A, -B, and -C elicits synergism on cypermethrin cytotoxicity in Spodoptera frugiperda cells

Rhinacanthus nasutus (Acanthaceae) is a shrub reported to contain insecticidal activities. The current study was conducted to determine whether R. nasutus constituents could inhibit benzyloxyresorufin O-debenzylation (BROD) mediated by CYP6AA3 and CYP6P7. Both enzymes have shown pyrethroid degradation activity and been implicated to play role in pyrethroid resistance in Anopheles minumus (Theobald) mosquito, a malaria vector. Three compounds, rhinacanthin-A, -B, and -C that exhibited potent inhibitory activity were isolated and purified from aerial part of R. nasutus. Their kinetic parameters and modes of inhibition were determined. Rhinacanthin-B was the most potent inhibitor in in vitro inhibition assay and exhibited mechanism-based inhibition against both CYP6AA3 and CYP6P7. Rhinacanthin-C which is a major compound of R. nasutus reversibly inhibited both enzymes in vitro with 2-4 folds less inhibitory potency than rhinacanthin-B. In contrast, rhinacanthin-A reversibly inhibited CYP6AA3, but inhibition against CYP6P7 was a mechanism-based inhibition type. Where mechanism-based inhibition was found, the inhibition showed characteristic of time-, concentration-dependence, and requirement of NADPH. Using 3-(4, 5-dimethylthiazol-2-y-l)-2, 5-diphenyltetrazolium bromide (MTT) cytotoxicity assay in intact Spodoptera frugiperda (Sf9) cells, the three compounds increased susceptibility of CYP6AA3- and CYP6P7-expressing cells to cypermethrin cytotoxicity because of inhibition effect on mosquito enzymes. The combined inhibition effect on mosquito cytochrome P450 enzyme and synergistic effect on cypermethrin cytotoxicity of the three R. nasutus compounds could be beneficial for resistance management strategies in mosquito vector control.

Antagonistic regulation, yet synergistic defense: effect of bergapten and protease inhibitor on development of cowpea bruchid Callosobruchus maculatus

The furanocoumarin compound bergapten is a plant secondary metabolite that has anti-insect function. When incorporated into artificial diet, it retarded cowpea bruchid development, decreased fecundity, and caused mortality at a sufficient dose. cDNA microarray analysis indicated that cowpea bruchid altered expression of 543 midgut genes in response to dietary bergapten. Among these bergapten-regulated genes, 225 have known functions; for instance, those encoding proteins related to nutrient transport and metabolism, development, detoxification, defense and various cellular functions. Such differential gene regulation presumably facilitates the bruchids' countering the negative effect of dietary bergapten. Many genes did not have homology (E-value cutoff 10(-6)) with known genes in a BlastX search (206), or had homology only with genes of unknown function (112). Interestingly, when compared with the transcriptomic profile of cowpea bruchids treated with dietary soybean cysteine protease inhibitor N (scN), 195 out of 200 coregulated midgut genes are oppositely regulated by the two compounds. Simultaneous administration of bergapten and scN attenuated magnitude of change in selected oppositely-regulated genes, as well as led to synergistic delay in insect development. Therefore, targeting insect vulnerable sites that may compromise each other's counter-defensive response has the potential to increase the efficacy of the anti-insect molecules.

Isolation and functional characterization of CYP71AJ4 encoding for the first P450 monooxygenase of angular furanocoumarin biosynthesis

The biosynthesis of linear and angular furanocoumarins is still poorly understood at the molecular level, with only psoralen synthase (CYP71AJ1) identified from Ammi majus. Using cDNA probes inferred from CYP71AJ1, three orthologs were isolated from Apium graveolens (CYP71AJ2) and Pastinaca sativa (CYP71AJ3 and -4) and functionally expressed in yeast cells. CYP71AJ2 and CYP71AJ3 displayed psoralen synthase activity, whereas CYP71AJ4 only catalyzed the conversion of (+)-columbianetin to angelicin and negligible amounts of a hydroxylated columbianetin by-product. CYP71AJ4 thus constitutes the first fully characterized P450 monooxygenase specific for the angular furanocoumarin pathway. The angelicin synthase exhibited an apparent K(m) of 2.1 +/- 0.4 microm for (+)-columbianetin and a k(cat) of 112 +/- 14 min(-1). Moreover, the use of 3'-deuterated (+)-columbianetin as substrate led to an almost complete "metabolic switch," resulting in the synthesis of anti-3'-hydroxy-3'-deuterated(+)-columbianetin. This confirms that angelicin synthase attacks columbianetin by syn-elimination of hydrogen from C-3'. Sequence comparison between psoralen synthase (CYP71AJ3) and angelicin synthase (CYP71AJ4) showed 70% identity, whereas the identity dropped to 40% in those regions thought to provide the substrate recognition sites. Accordingly, CYP71AJ3 and CYP71AJ4 might be derived from a common ancestor of unknown functionality by gene duplication and subsequent molecular evolution.

Inhibition of CYP6B1-mediated detoxification of xanthotoxin by plant allelochemicals in the black swallowtail (Papilio polyxenes)

The structural and biosynthetic diversity of allelochemicals in plants is thought to arise from selection for additive toxicity as a consequence of toxin mixture or for enhanced toxicity as a result of synergism. In order to understand how insects cope with this type of plant defense, we tested the effects of some allelochemicals in host plants of the black swallowtail Papilio polyxenes on the xanthotoxin-metabolic activity of CYP6B1, the principal enzyme responsible for the detoxification of furanocoumarins in this caterpillar. Additionally, the effects of some synthetic compounds not normally encountered by P. polyxenes on CYP6B1 were tested. These studies demonstrate that the integrity of furanocoumarin structure is important for competitive binding to the active site of CYP6B1, even though the carbonyl group on the pyranone ring apparently does not affect its inhibitory capacity, as in the case of furanochromones. Angular furanocoumarins are generally less phototoxic to many organisms than linear furanocoumarins due to their reduced capacity for cross-linking DNA strands, yet they are more toxic than linear furanocoumarins to black swallowtail larvae. This enhanced toxicity in vivo may be due to the ability of angular furanocoumarins to bind to the active site of CYP6B1 without being rapidly metabolized. This binding reduces the availability of CYP6B1 to metabolize other linear furanocoumarins. The structure-activity relationships for methylenedioxyphenyl compounds, flavonoids, imidazole, and imidazole derivatives are also discussed in light of their capacity to inhibit the xanthotoxin-metabolic activity of CYP6B1.

Expression of a wheat cytochrome P450 monooxygenase in yeast and its inhibition by glyphosate

Glyphosate is a non-selective herbicide which acts by inhibiting 5-enolpyruvylshikimate-3-phosphate synthase. Wheat cytochrome P450 monooxygenase specifically catalyzes the metabolism of some sulfonylurea herbicides such as chlorsulfuron and triasulfuron. Here we report that glyphosate is an inhibitor of a wheat cytochrome (CYP71C6v1), the cDNA of which was amplified by RT-PCR and heterologously expressed in yeast. The microsomal fractions derived from this strain had a Soret peak at 502 nm in the reduced carbon monoxide difference spectrum, which is a typical spectral characteristic. The addition of glyphosate to the microsomal protein resulted in a Type II spectrum indicative of binding via the nitrogen group to haem of cytochrome P450 as a sixth ligand. A spectral dissociation constant, K(s) of 70 micromol litre(-1) was observed and an IC50 of 11 micromol litre(-1) was found for glyphosate inhibition of CYP71C6v1 P450 activity.

Amino acids in SRS1 and SRS6 are critical for furanocoumarin metabolism by CYP6B1v1, a cytochrome P450 monooxygenase

CYP6B1v1 is the principal cytochrome P450 monooxygenase (P450) that detoxifies dietary furanocoumarins in the guts of Papilio polyxenes, the black swallowtail caterpillar. Sequence alignments and structure comparisons of CYP6B1v1 with the mouse CYP2A5 and bacterial CYP102 proteins, which are also capable of metabolizing the linear furanocoumarin xanthotoxin (8-methoxypsoralen), suggested that Phe116, His117, Val368 and Phe484 might be active site residues. In a homology model developed for CYP6B1v1, the side chains of Phe116 and His117 located in the B'-C loop of SRS1 are predicted to be positioned above the haem plane, while the side chain of Phe484 located in SRS6 is predicted near the entrance of the catalytic pocket. Site-directed mutagenesis of residues Phe116, His117 and Phe484 indicated that these residues represent several of those that determine this protein's stability and substrate specificity. Whereas all aromatic mutants of Phe116 and Phe484 generated CO-difference spectra with maxima at 450 nm indicative of correctly configured monooxygenases, aromatic mutants of Phe116 exhibited reduced reactivities toward some furanocoumarins and aromatic mutants of Phe484 eliminated all reactivities toward furanocoumarins. All single and double aliphatic mutants of Phe116, His117 and Phe484 and aromatic mutants of His117 generated carbon monoxide (CO) difference spectra with maxima at 420 nm (P420) indicative of incorrectly configured monooxygenases. These studies define residues Phe116, His117 and Phe484 as determinants of this insect P450's catalytic site integrity and residues Phe116 and Phe484 as determinants of its substrate specificity. Conservation of Phe116 and His117 in an array of lepidopteran CYP6B proteins implies that these amino acids serve a similar function in other monooxygenases of the insect CYP6B subfamily.

Developmental variation in cytochrome P450 expression in Papilio polyxenes in response to xanthotoxin, a hostplant allelochemical

Although developmental variation in activity and inducibility is typical of cytochrome P450 monooxygenases (P450s) in insects, the adaptive significance of such variation is often unclear, in part because the natural function of insect P450s is rarely known. In this study, we examined developmental variation in expression of CYP6B1 and CYP6B3 in Papilio polyxenes, the black swallowtail. Enzymes encoded by these genes have been implicated in the metabolism of xanthotoxin, a furanocoumarin characteristic of the apiaceous hostplants of P. polyxenes. In each life stage-egg, five larval instars, pupa, and adult-we examined individuals exposed to foliage with and without supplemental xanthotoxin. For each stage, we conducted enzyme assays to estimate xanthotoxin metabolism, Northern analysis to detect constitutive and induced mRNA levels, and RT-PCR amplification and Southern analysis to differentiate among P450 genes expressed. Inducible xanthotoxin metabolism, previously reported in fifth instars, was observed in four of five larval stages but was absent or undetectable in all stages that do not feed on foliage; the highest levels of activity were in early larval instars. The same pattern was observed in both Northern and RT-PCR gel blot analyses. In inducible larval stages, inducibility of CYP6B1 transcripts by xanthotoxin was greater than the inducibility of CYP6B3 transcripts. These findings support earlier suggestions that these two P450s contribute to xanthotoxin metabolism in this species and that expression of these P450 genes is regulated in an adaptive fashion with respect to probability of exposure to hostplant toxins over the course of development.

Cytochromes P450 of insects: the tip of the iceberg

The cytochrome P450-dependent monooxygenases are an extremely important metabolic system involved in the metabolism of endogenous compounds and xenobiotics. Collectively, P450 monooxygenases can metabolize numerous substrates and carry out multiple oxidative reactions. The large number of substrates metabolized is due to the plethora of P450 isoforms and to the broad substrate specificity of some isoforms. Monooxygenases of insects have several functional roles, including growth, development, feeding and protection against xenobiotics, including resistance to pesticides and tolerance to plant toxins. This review begins with background information about P450s and their evolution, followed by a discussion of the extraordinary diversity of insect P450s. Given the enormous interest in studying individual P450s, we then provide a synopsis of the different methods that have been used in their isolation and the substrates that are known to be metabolized. We conclude by summarizing the lessons we have learned from the study of individual insect P450s, including their roles in insecticide resistance, plant-insect interactions and insect physiology. However, these studies are just the 'tip of the iceberg'. Our knowledge continues to expand at a rapid pace, suggesting that the next decade will outpace the last in terms of improving our understanding of the cytochromes P450 of insects.

Metabolism of phenanthrene by house fly CYP6D1 and dog liver cytochrome P450

Polycyclic aromatic hydrocarbons (PAHs) are a ubiquitous class of environmental contaminants. The compound phenanthrene is a model PAH. A novel fluorometric method for measuring phenanthrene metabolism in vitro was developed and verified with direct measurement of [14C]phenanthrene using dog liver microsomes. The fluorometric assay and direct measurement of [14C]phenanthrene metabolism were used to show that CYP6D1, a house fly cytochrome P450, is the major house fly P450 involved in phenanthrene metabolism. Phenanthrene was metabolized by microsomes from the LPR strain of house fly that overexpresses CYP6D1, but metabolism was not observed in the CS strain that has a lower level of CYP6D1. Furthermore, the majority of phenanthrene metabolism was inhibited by a CYP6D1-specific antibody. This study increases the number of known substrates of CYP6D1 and identifies polyaromatic hydrocarbons as potential substrates of CYP6D1. The utility of CYP6D1 as an agent in bioremediation and the utility of the new fluorometric assay for understanding PAH metabolism in insects and mammals are discussed.

Inhibitory effect of natural furanocoumarins on human microsomal cytochrome P450 3A activity

To investigate the possible drug interaction with herbal medicine, furanocoumarin derivatives isolated from several Umbelliferous crude drugs were examined for their inhibitory effects on a typical human drug metabolizing enzyme, cytochrome P450 3A (CYP3A). Most furanocoumarins tested at 0.1 mM reduced microsomal testosterone 6beta-hydroxylation as an index of CYP3A activity to less than 50% of the control. In particular, the dimer and trimer derivatives of furanocoumarins showed striking inhibition, whose potencies were similar to that of a typical CYP3A inhibitor, ketoconazole. Preincubation of dimer types of furanocoumarins increased suppression but not most of the monomer derivatives, suggesting that the inhibition on CYP3A activity was caused by at least plural mechanisms. These results raised the possibility that the furanocoumarin containing herbal medicines may alter pharmacokinetics of co-ingested drugs similar to the case with grapefruit juice.

Cytochromes P450 and insecticide resistance

The cytochrome P450-dependent monooxygenases (monooxygenases) are an extremely important metabolic system involved in the catabolism and anabolism of xenobiotics and endogenous compounds. Monooxygenase-mediated metabolism is a common mechanism by which insects become resistant to insecticides as evidenced by the numerous insect species and insecticides affected. This review begins by presenting background information about P450s, the role of monooxygenases in insects, and the different techniques that have been used to isolate individual insect P450s. Next, insecticide resistance is briefly described, and then historical information about monooxygenase-mediated insecticide resistance is reviewed. For any case of monooxygenase-mediated resistance, identification of the P450(s) involved, out of the dozens that are present in an insect, has proven very challenging. Therefore, the next section of the review focuses on the minimal criteria for establishing that a P450 is involved in resistance. This is followed by a comprehensive examination of the literature concerning the individual P450s that have been isolated from insecticide resistant strains. In each case, the history of the strain and the evidence for monooxygenase-mediated resistance are reviewed. The isolation and characterization of the P450(s) from the strain are then described, and the evidence of whether or not the isolated P450(s) is involved in resistance is summarized. The remainder of the review summarizes our current knowledge of the molecular basis of monooxygenase-mediated resistance and the implications for the future. The importance of these studies for development of effective insecticide resistance management strategies is discussed.

Insect cytochromes P450: diversity, insecticide resistance and tolerance to plant toxins

In the last decade, studies of individual insect P450s have blossomed. This new information has furthered our understanding of P450 diversity, insecticide resistance and tolerance to plant toxins. Insect P450s can be adult specific, larval specific or life stage independent. Similarly, insect P450s vary as to the tissues where they are expressed and in their response to inducers. Insect P450s can now be rapidly sequenced using degenerate PCR primers. Given the huge diversity represented by the Class Insecta, this technique will provide vast amounts of new information about insect P450s and the evolution of the P450 gene superfamily. CYP6D1 is responsible for monooxygenase-mediated resistance to pyrethroid insecticides in the house fly. CYP6D1 is ubiquitously expressed in adults with 10-fold higher levels found in the resistant strain compared to susceptible strains. CYP6D1 is on autosome 1 in house fly. The high level of expression found in the resistant strain is due to genes on autosomes 1 and 2. Whether or not the different CYP6D1 alleles found in resistant and susceptible strains have any role in resistance remains to be elucidated. The CYP6B gene subfamily is involved in the metabolism of host plant toxins (i.e. furanocoumarins). CYP6B gene transcripts in two Papilio (swallowtail) species have been shown to be induced by host plant toxins and in turn to metabolize these toxins. CYP6B P450s play a critical role in allowing Papilio to adapt to furanocoumarin-containing host plants. Similarities in structural and promoter regions of the CYP6B genes suggest that they are derived from a common ancestral gene. Although the P450 monooxygenases of insects are important for the metabolism of hormones and phermones, no individual P450 has yet been shown to metabolize an endogenous compound. Advances in this area are critical because they will provide important new information about insect physiology, biochemistry and development.

Induction of cytochrome P450 activities in Drosophila melanogaster strains susceptible or resistant to insecticides

We analysed Drosophila melanogaster cytochrome P450s (P450) through the measurements of four enzymatic activities: ethoxycoumarin-O-deethylase, ethoxyresorufin-O-deethylase, lauric acid hydroxylation, and testosterone hydroxylation. We did these measurements in two Drosophila strains: one is susceptible to insecticides (Cantons) and the other is resistant to insecticides by enhanced P450 activities (RDDTR). In addition, we also treated the flies with eight chemicals (beta-naphtoflavone, benzo-alpha-pyrene, 3-methylcholanthrene, phenobarbital, aminopyrine, rifampicin, prochloraz, and clofibrate) known to induces genes from the families CYP1, CYP2, CYP3, CYP4, and CYP6. Metabolisation of all the substrates by P450 from flies microsomes was observed. The chemicals had different effects on these activities, ranging from induction to inhibition. The effects of these chemicals varied with the strains as most of them were ineffective on the RDDTR strain. The results showed that P450-dependent activities are numerous in Drosophila. Regulation features of these activities are complex. The availability of mutant strains as RDDTR should allow fundamental studies of P450 in insects.