A comparison of Drosophila melanogaster detoxification gene induction responses for six insecticides, caffeine and phenobarbital

Does the clock make the poison? Circadian variation in response to pesticides

Background

Circadian clocks govern daily physiological and molecular rhythms, and putative rhythms in expression of xenobiotic metabolizing (XM) genes have been described in both insects and mammals. Such rhythms could have important consequences for outcomes of chemical exposures at different times of day. To determine whether reported XM gene expression rhythms result in functional rhythms, we examined daily profiles of enzyme activity and dose responses to the pesticides propoxur, deltamethrin, fipronil, and malathion.

Methodology/Principal Findings

Published microarray expression data were examined for temporal patterns. Male Drosophila were collected for ethoxycoumarin-O-deethylase (ECOD), esterase, glutathione-S-transferase (GST), and, and uridine 5′-diphosphoglucosyltransferase (UGT) enzyme activity assays, or subjected to dose-response tests at four hour intervals throughout the day in both light/dark and constant light conditions. Peak expression of several XM genes cluster in late afternoon. Significant diurnal variation was observed in ECOD and UGT enzyme activity, however, no significant daily variation was observed in esterase or GST activity. Daily profiles of susceptibility to lethality after acute exposure to propoxur and fipronil showed significantly increased resistance in midday, while susceptibility to deltamethrin and malathion varied little. In constant light, which interferes with clock function, the daily variation in susceptibility to propoxur and in ECOD and UGT enzyme activity was depressed.

Conclusions/Significance

Expression and activities of specific XM enzymes fluctuate during the day, and for specific insecticides, the concentration resulting in 50% mortality varies significantly during the day. Time of day of chemical exposure should be an important consideration in experimental design, use of pesticides, and human risk assessment.

A putative amino acid transporter of the solute carrier 6 family is upregulated by lithium and is required for resistance to lithium toxicity in Drosophila

Lithium is an efficacious drug for the treatment of mood disorders, and its application is also considered a potential therapy for brain damage. However, the mechanisms underlying lithium's therapeutic action and toxic effects in the nervous system remain largely elusive. Here we report on the use of a versatile genetic model, the fruit fly Drosophila melanogaster, to discover novel molecular components involved in the lithium-responsive neurobiological process. We previously identified CG15088, which encodes a putative nutrient amino acid transporter of the solute carrier 6 (SLC6) family, as one of the genes most significantly upregulated in response to lithium treatment. This gene was the only SLC6 gene induced by lithium, and was thus designated as Lithium-inducible SLC6 transporter or List. Either RNA interference (RNAi)-mediated knockdown or complete deletion of List resulted in a remarkable increase in the susceptibility of adult flies to lithium's toxic effects, whereas transgenic expression of wild-type List significantly suppressed the lithium hypersensitive phenotype of List-deficient flies. Other ions such as sodium, potassium and chloride did not induce List upregulation, nor did they affect the viability of flies with suppressed List expression. These results indicate that lithium's biochemical or physical properties, rather than general osmotic responses, are responsible for the lithium-induced upregulation of List, as well as for the lithium-susceptible phenotype observed in List knockdown flies. Interestingly, flies became significantly more susceptible to lithium toxicity when List RNAi was specifically expressed in glia than when it was expressed in neurons or muscles, which is consistent with potential glial expression of List. These results show that the List transporter confers resistance to lithium toxicity, possibly as a consequence of its amino acid transporter activity in CNS glia. Our results have provided a new avenue of investigation toward a better understanding of the molecular and cellular mechanisms that underlie lithium-responsive neurobiological process.

Characterization of Drosophila melanogaster cytochrome P450 genes

Cytochrome P450s form a large and diverse family of heme-containing proteins capable of carrying out many different enzymatic reactions. In both mammals and plants, some P450s are known to carry out reactions essential for processes such as hormone synthesis, while other P450s are involved in the detoxification of environmental compounds. In general, functions of insect P450s are less well understood. We characterized Drosophila melanogaster P450 expression patterns in embryos and 2 stages of third instar larvae. We identified numerous P450s expressed in the fat body, Malpighian (renal) tubules, and in distinct regions of the midgut, consistent with hypothesized roles in detoxification processes, and other P450s expressed in organs such as the gonads, corpora allata, oenocytes, hindgut, and brain. Combining expression pattern data with an RNA interference lethality screen of individual P450s, we identify candidate P450s essential for developmental processes and distinguish them from P450s with potential functions in detoxification.

Functional Genomics Approaches to Studies of the Cytochrome P450 Superfamily

Functional genomics approaches are widely implemented in current research and have found application in many areas of biology. This review will present research fields, novel findings and new tools developed in the cytochrome P450 field using the functional genomics techniques. The most widely used method is microarray technology, which has already greatly contributed to the understanding of the cytochromes P450 function and expression. Several focused CYP microarrays have been developed for genotyping, toxicogenomics and studies of CYP function of many different organisms. Our contribution to the CYP field by development of Steroltalk microarrays to study the cross-talk of cholesterol homeostasis and drug metabolism is also presented.

Co-up-regulation of three P450 genes in response to permethrin exposure in permethrin resistant house flies, Musca domestica

Background

Insects may use various biochemical pathways to enable them to tolerate the lethal action of insecticides. For example, increased cytochrome P450 detoxification is known to play an important role in many insect species. Both constitutively increased expression (overexpression) and induction of P450s are thought to be responsible for increased levels of detoxification of insecticides. However, unlike constitutively overexpressed P450 genes, whose expression association with insecticide resistance has been extensively studied, the induction of P450s is less well characterized in insecticide resistance. The current study focuses on the characterization of individual P450 genes that are induced in response to permethrin treatment in permethrin resistant house flies.

Results

The expression of 3 P450 genes, CYP4D4v2, CYP4G2, and CYP6A38, was co-up-regulated by permethrin treatment in permethrin resistant ALHF house flies in a time and dose-dependent manner. Comparison of the deduced protein sequences of these three P450s from resistant ALHF and susceptible aabys and CS house flies revealed identical protein sequences. Genetic linkage analysis located CYP4D4v2 and CYP6A38 on autosome 5, corresponding to the linkage of P450-mediated resistance in ALHF, whereas CYP4G2 was located on autosome 3, where the major insecticide resistance factor(s) for ALHF had been mapped but no P450 genes reported prior to this study.

Conclusion

Our study provides the first direct evidence that multiple P450 genes are co-up-regulated in permethrin resistant house flies through the induction mechanism, which increases overall expression levels of P450 genes in resistant house flies. Taken together with the significant induction of CYP4D4v2, CYP4G2, and CYP6A38 expression by permethrin only in permethrin resistant house flies and the correlation of the linkage of the genes with resistance and/or P450-mediated resistance in resistant ALHF house flies, this study sheds new light on the functional importance of P450 genes in response to insecticide treatment, detoxification of insecticides, the adaptation of insects to their environment, and the evolution of insecticide resistance.

Cross-induction of detoxification genes by environmental xenobiotics and insecticides in the mosquito Aedes aegypti: impact on larval tolerance to chemical insecticides

The effect of exposure of Aedes aegypti larvae to sub-lethal doses of the pyrethroid insecticide permethrin, the organophosphate temephos, the herbicide atrazine, the polycyclic aromatic hydrocarbon fluoranthene and the heavy metal copper on their subsequent tolerance to insecticides, detoxification enzyme activities and expression of detoxification genes was investigated. Bioassays revealed a moderate increase in larval tolerance to permethrin following exposure to fluoranthene and copper while larval tolerance to temephos increased moderately after exposure to atrazine, copper and permethrin. Cytochrome P450 monooxygenases activities were induced in larvae exposed to permethrin, fluoranthene and copper while glutathione S-transferase activities were induced after exposure to fluoranthene and repressed after exposure to copper. Microarray screening of the expression patterns of all detoxification genes following exposure to each xenobiotic with the Aedes Detox Chip identified multiple genes induced by xenobiotics and insecticides. Further expression studies using real-time quantitative PCR confirmed the induction of multiple CYP genes and one carboxylesterase gene by insecticides and xenobiotics. Overall, this study reveals the potential of xenobiotics found in polluted mosquito breeding sites to affect their tolerance to insecticides, possibly through the cross-induction of particular detoxification genes. Molecular mechanisms involved and impact on mosquito control strategies are discussed.

Caffeine induction of Cyp6a2 and Cyp6a8 genes of Drosophila melanogaster is modulated by cAMP and D-JUN protein levels

Cytochrome P450 monooxygenases or CYPs, a family of endobiotics and xenobiotics metabolizing enzymes, are found in all organisms. We reported earlier that the promoters of Drosophila Cyp6a2 and Cyp6a8 genes are induced by caffeine. Since caffeine antagonizes adenosine receptor (AdoR) and inhibits cAMP phosphodiesterase (PDE), we used luciferase reporter gene to examine whether in SL-2 cells and adult Drosophila, induction of the two Cyp6 genes is mediated via AdoR and/or PDE pathway. Results showed that AdoR is not involved because AdoR agonists or antagonists do not affect the Cyp6 promoter activities. However, inhibition of PDE by specific inhibitors including caffeine causes induction of both Cyp6 gene promoters. We also found that flies mutant for dunce gene coding for cAMP-PDE, have higher Cyp6a8 promoter activity than the wild-type flies. We demonstrate that caffeine treatment increases intracellular cAMP levels, and cAMP treatment induces the Cyp6 gene promoters. Since both Cyp6 genes have multiple sites for JUN transcription factors, which generally play a positive role in cAMP pathway, effect of Drosophila jun (D-jun) on the Cyp6a8 promoter activity was examined. Results showed that the expression of D-jun sense plasmid causes downregulation rather than activation of the Cyp6a8 promoter. Conversely, expression of antisense plasmid increased the promoter activity. Interestingly, caffeine treatment decreased the D-JUN protein level in SL-2 cells as well as in adult flies. These results suggest that D-jun acts as a negative regulator, and caffeine induction of Cyp6a8 and Cyp6a2 genes is mediated by the upregulation of cAMP pathway and downregulation of the D-JUN protein level.

Chemical Complexity and the Genetics of Aging

We examine how aging is impacted by various chemical challenges that organisms face and by the molecular mechanisms that have evolved to regulate lifespan in response to them. For example, environmental information, which is detected and processed through sensory systems, can modulate lifespan by providing information about the presence and quality of food as well as presence and density of conspecifics and predators. In addition, the diverse forms of molecular damage that result from constant exposure to damaging chemicals that are generated from the environment and from metabolism pose an informatic and energetic challenge for detoxification systems, which are important in ensuring longevity. Finally, systems of innate immunity are vital for recognizing and combating pathogens but are also seen as of increasing importance in causing the aging process. Integrating ideas of molecular mechanism with context derived from evolutionary considerations will lead to exciting new insights into the evolution of aging.

o,p'-DDT elicits PXR/CAR-, not ER-, mediated responses in the immature ovariectomized rat liver

Technical-grade dichlorodiphenyltrichloroethane (DDT) is an agricultural pesticide and malarial vector control agent that has been designated a potential human hepatocarcinogen. The o,p'-enantiomer exhibits estrogenic activity that has been associated with the carcinogenicity of DDT. The temporal and dose-dependent hepatic estrogenicity of o,p'-DDT was investigated using complementary DNA microarrays in immature ovariectomized Sprague-Dawley rats with complementary histopathology and tissue-level analysis. Animals were gavaged with 300 mg/kg o,p'-DDT either once or once daily for 3 consecutive days. Liver samples were examined 2, 4, 8, 12, 18, or 24 h after a single dose or following three daily doses. For dose-response studies, a single dose of 3, 10, 30, 100, or 300 mg/kg body weight o,p'-DTT was administered for 3 consecutive days. Genes associated with drug metabolism (Cyp2b2 and Cyp3a2), the nuclear receptors constitutive androstane receptor (CAR) and pregnane X receptor (PXR), cell proliferation (Ccnd1, Ccnb1, Ccnb2, and Stmn1), and oxidative stress (Gclm and Hmox1) were significantly induced. Cyp2b2 exhibited dose-dependent regulation and was significantly induced across all time points, while cell proliferation- and oxidative stress-related genes exhibited transient induction. The induction of Cyp2b2 and Cyp3a2 mRNA levels suggest PXR/CAR activation, consistent with expression of genes associated with oxidative stress. Few genes known to be estrogen receptor (ER) regulated were differentially expressed when compared to the hepatic gene expression profile elicited by ethynyl estradiol in immature ovariectomized C57BL/6 mice using the same study design and analysis methods. These data indicate that o,p'-DDT elicits PXR/CAR-, not ER-, mediated gene expression in the rat liver. Based on the species-specific differences in CAR regulation, the extrapolation of rodent DDT hepatocarcinogenicity to humans warrants further investigation.

Studies on the glutathione S-transferase proteome of adultDrosophila melanogaster: Responsiveness to chemical challenge

GSTs from adult Drosophila melanogaster have been partially purified using three different affinity chromatography media and separated by 2-DE. Nine GSTs have been identified by MALDI-TOF MS. In the absence of special treatments, eight GSTs could be positively identified. These were DmGSTs D1 (the dominant Delta isoform which was present in five protein zones of differing pI) and D3 (and possibly also D5); the Epsilon-class GSTs E3, 6, 7 and 9 and a previously uncharacterised, probable member of the class, CG16936. The Sigma-class DmGSTS1 was prominent. DmGSTD2 was detected only after pretreatment of the flies with Phenobarbital (PhB). Treatment with Paraquat (PQ) led to an increase in the total GST activity, as measured with the substrates 1-chloro-2,4-dinitrobenzene (CDNB) and 3,4-dichloro-nitrobenzene (DCNB) and an increase in the relative amounts of the D1, D3, E6 and E7 isoforms. PhB treatment led to increases in the relative amounts of the D1, D2, E3, E6, E7 and E9 isoforms detected with a possible depression in the relative amount of GSTS1. CG16936 was unaffected by either pretreatment.

Piperonyl butoxide induces the expression of cytochrome P450 and glutathione S-transferase genes in Drosophila melanogaster

Piperonyl butoxide (PBO) is an insecticide synergist known to inhibit the activity of cytochrome P450 enzymes. PBO is currently used in some insecticide formulations, and has also been suggested as a pretreatment for some pesticide applications. Little is known about how insects respond to PBO exposure at the gene transcription level. The authors have characterised the transcriptional response of the Drosophila melanogaster genome after PBO treatment, using both a custom-designed 'detox' microarray, containing cytochrome P450 (P450), glutathione S-transferase (GST) and esterase genes, and a full genome microarray. A subset of P450 and GST genes is identified, along with additional metabolic genes, that are induced by PBO. The gene set is an extremely similar gene set to that induced by phenobarbital, a compound for which pretreatment is known to confer tolerance to a range of insecticide compounds. The implications of the induction of gene families known to metabolise insecticides and the use of PBO in pest management programs are discussed.