Ecdysteroid signaling in ecdysteroid-resistant cell lines from the polyphagous noctuid pest Spodoptera exigua

Combined Transcriptomic Analysis and RNA Interference Reveal the Effects of Methoxyfenozide on Ecdysone Signaling Pathway of Spodoptera exigua

Spodoptera exigua is a worldwide pest afflicting edible vegetables and has developed varying levels of resistance to insecticides. Methoxyfenozide (MET), an ecdysteroid agonist, is effective against lepidopteran pests such as S. exigua. However, the mechanism of MET to S. exigua remains unclear. In this study, we analyzed the expression patterns of genes related to the ecdysone signaling pathway in transcriptome data treated with sublethal doses of MET and analyzed how expression levels of key genes affect the toxicity of MET on S. exigua. Our results demonstrated that 2639 genes were up-regulated and 2512 genes were down-regulated in S. exigua treated with LC₃₀ of MET. Of these, 15 genes were involved in the ecdysone signaling pathway. qPCR results demonstrated that ecdysone receptor A (EcRA) expression levels significantly increased in S. exigua when treated with different doses of MET, and that the RNAi-mediated silencing of EcRA significantly increased mortality to 55.43% at 72 h when L3 S. exigua larvae were exposed to MET at the LC₃₀ dose. Additionally, knocking down EcRA suppressed the most genes expressed in the ecdysone signaling pathway. The combination of MET and dsEcRA affected the expression of E74 and enhanced the expression of TREA. These results demonstrate that the adverse effects of sublethal MET disturb the ecdysone signaling pathway in S. exigua, and EcRA is closely related to MET toxic effect. This study increases our collective understanding of the mechanisms of MET in insect pests.

MiR-189942 regulates fufenozide susceptibility by modulating ecdysone receptor isoform B in Plutella xylostella (L.)

Although dibenzoylhydrazine-type non-steroidal ecdysone agonists, such as fufenozide, have an excellent performance record, the emergence of resistance could severely compromise the efficacy of these compounds in integrated pest management programs. To investigate possible mechanisms of resistance, we investigated the regulation of the expression of the PxEcR-B gene encoding the ecdysone receptor isoform B (PxEcR-B), which is the specific target of fufenozide in P. xylostella. Bioinformatics analysis revealed a putative miR-189942 binding site in the 3'-UTR of PxEcR-B mRNA. In a PxEcR-B 3'-UTR luciferase reporter system, miR-189942 downregulated the luciferase activity, and these effects were abolished by a deletion mutation in the putative miR-189942 binding site. Moreover, at 96 h after treatment with an agomir (mimic) or antagomir (inhibitor) of miR-189942, PxEcR-B expression was decreased by 71 ± 4% and increased by 4.19- fold respectively. Furthermore, overexpression or knockdown of miR-189942 changed the sensitivity of P. xylostella to fufenozide in vivo but had no influence on the sensitivity to chlorantraniliprole, which does not target PxEcR-B. These data indicate that miR-189942 suppressed PxEcR-B expression via binding at the 3'-UTR of PxEcR-B, thus increasing the tolerance of P. xylostella to fufenozide. These findings provide empirical evidence of the involvement of miRNAs in the regulation of insecticide resistance.

Establishment of a cell line from the ash and privet borer beetle Tylonotus bimaculatus Haldeman and assessment of its sensitivity to diacylhydrazine insecticides

A novel cell line, NRCAN-Tb521, was developed from larvae of the longhorn beetle Tylonotus bimaculatus (Coleoptera: Cerambycidae), a pest of North American ash trees. The cell line has been successfully passaged more than 50 times and displayed very strong attachment to the substrate and a modal chromosomal count distribution of 19. Sequencing of a 649 bp fragment of the mitochondrial cytochrome oxidase I gene confirmed the identity of NRCAN-Tb521 as T. bimaculatus. The response of the cell line to 20-hydroxyecdysone and diacylhydrazine ecdysone agonist insecticides was also studied. At 10(-6) M, 20-hydroxyecdysone, tebufenozide, methoxyfenozide and halofenozide triggered the production of numerous filamentous cytoplasmic extensions, and the cells tended to form aggregates, indicative of a cell differentiation response. This response was followed by a strong decrease in viability after 4 d. Reverse transcription polymerase chain reaction (PCR) experiments and sequencing of PCR fragments showed that the 20E receptor gene EcR is expressed in the cells and that 20E, tebufenozide, methoxyfenozide and halofenozide also induce the expression of the nuclear hormone receptor gene HR3. This report establishes that NRCAN-Tb521 is a valuable in vitro model to study effects of ecdysone agonists in wood-boring cerambycids.

Selection and evaluation of reference genes for expression analysis using qRT-PCR in the beet armyworm Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae)

Quantitative real-time PCR (qRT-PCR) is a reliable and reproducible technique for measuring and evaluating changes in gene expression. The most common method for analyzing qRT-PCR data is to normalize mRNA levels of target genes to internal reference genes. Evaluating and selecting stable reference genes on a case-by-case basis is critical. The present study aimed to facilitate gene expression studies by identifying the most suitable reference genes for normalization of mRNA expression in qRT-PCR analysis of the beet armyworm Spodoptera exigua (Lepidoptera: Noctuidae). For this purpose, three software tools (geNorm, NormFinder and BestKeeper) were used to investigate 10 candidate reference genes in nine developmental stages and five different tissues (epidermis, head, midgut, fat body and hemolymph) in three larval physiological stages (molting, feeding and wandering stages) of, S. exigua. With the exception of 18S ribosomal RNA (18S), all other candidate genes evaluated, β-actin1(ACT1), β-actin2 (ACT2), elongation factor1(EF1), elongation factor 2 (EF2), Glyceralde hyde-3-phosphate dehydrogenase (GAPDH), ribosomal protein L10 (L10), ribosomal protein L17A (L17A), superoxide dismutase (SOD), α-tubulin (TUB),proved to be acceptable reference genes. However, their suitability partly differed between physiological stages and different tissues. L10, EF2 and L17A ranked highest in all tissue sample sets. SOD, ACT2, GAPDH, EF1 and ACT1 were stably expressed in all developmental stage sample sets; ACT2, ACT1 and L10 for larvae sample sets; GAPDH, ACT1 and ACT2 for pupae and adults; SOD and L17A for males; and EF2 and SOD for females. The expression stability of genes varied in different conditions. The findings provided here demonstrated, with a few exceptions, the suitability of most of the 10 reference genes tested in tissues and life developmental stages. Overall, this study emphasizes the importance of validating reference genes for qRT-PCR analysis in S. exigua.

Effects of hormone agonists on Sf9 cells, proliferation and cell cycle arrest

Methoxyfenozide and methoprene are two insecticides that mimic the action of the main hormones involved in the control of insect growth and development, 20-hydroxyecdysone and juvenile hormone. We investigated their effect on the Spodoptera frugiperda Sf9 cell line. Methoxyfenozide was more toxic than methoprene in cell viability tests and more potent in the inhibition of cellular proliferation. Cell growth arrest occurred in the G2/M phase after a methoprene treatment and more modestly in G1 after methoxyfenozide treatment. Microarray experiments and real-time quantitative PCR to follow the expression of nuclear receptors ultraspiracle and ecdysone receptor were performed to understand the molecular action of these hormone agonists. Twenty-six genes were differentially expressed after methoxyfenozide treatment and 55 genes after methoprene treatment with no gene in common between the two treatments. Our results suggest two different signalling pathways in Sf9 cells.

Ecdysone signaling and transcript signature in Drosophila cells resistant against methoxyfenozide

Methoxyfenozide (RH-2485) is a non-steroidal ecdysteroid agonist with a dibenzoylhydrazine structure, representing a group used as novel biorational insecticides in the control of insect pests. Here we report on the selection of Drosophila melanogaster S2 cells for resistance to inhibition of cell proliferation by methoxyfenozide by ∼ 1000-fold over 4 months. Cells were exposed to gradually increasing concentrations of methoxyfenozide and selected out based on the ecdysteroid-sensitive response for cell proliferation. In the resistant cells, the ecdysteroid receptor (EcR/USP) complex was no longer active in the presence of methoxyfenozide. But when resistant cells were relaxed from pressure in methoxyfenozide-free medium, induction of the reporter construct was observed. In parallel, EcR/USP functionality was also restored when resistant cells were rescued by a Drosophila EcR plasmid. However, it was striking that in the resistant cells the ecdysteroid-sensitive response for cell proliferation was not restored upon methoxyfenozide withdrawal, indicating permanent changes in the physiology of the cells during selection. To investigate changes in gene expression caused by inactivation of the EcR/USP complex in resistant cells, Drosophila oligo 14kv1 microarrays were used and probed with cDNAs from resistant cells in the presence and absence of ecdysone agonist on one hand and from unselected sensitive cells on the other hand. A selection of 324 differentially expressed genes was assigned covering diverse functions as transport, enzyme activity, cytoskeleton organization, cell cycle machinery, transcription/translation and ecdysteroid signaling. Besides the identification of (primary and secondary) target genes of the EcR/USP signaling pathway, this analysis also allows to gain insights into the mechanism of resistance and on the crosstalk between ecdysteroid signaling and cell proliferation-linked processes.

Assessment of species specificity of moulting accelerating compounds in Lepidoptera: comparison of activity between Bombyx mori and Spodoptera littoralis by in vitro reporter and in vivo toxicity assays

BACKGROUND

Dibenzoylhydrazine analogues have been developed successfully as a new group of insect growth regulators, called ecdysone agonists or moulting accelerating compounds. A notable feature is their high activity against lepidopteran insects, raising the question as to whether species-specific analogues can be isolated. In this study, the specificity of ecdysone agonists was addressed through a comparative analysis in two important lepidopterans, the silkworm Bombyx mori L. and the cotton leafworm Spodoptera littoralis (Boisd.).

RESULTS

When collections of non-steroidal ecdysone agonists containing different mother structures (dibenzoylhydrazine, acylaminoketone, tetrahydroquinoline) were tested, in vitro reporter assays showed minor differences using cell lines derived from both species. However, when compounds with high ecdysone agonist activity were examined in toxicity assays, larvicidal activity differed considerably. Of note was the identification of three dibenzoylhydrazine analogues with > 100-fold higher activity against Bombyx than against Spodoptera larvae.

CONCLUSION

The present study demonstrated that species-specific ecdysone-agonist-based insecticides can be developed, but their species specificity is not based on differences in the activation of the ecdysone receptor but rather on unidentified in vivo parameters such as permeability of the cuticle, uptake/excretion by the gut or metabolic detoxification.

Ecdysteroid hormone action

Several reviews devoted to various aspects of ecdysone research have been published during the last few years. Therefore, this article concentrates mainly on the considerable progress in ecdysone research observed recently, and will cover the results obtained during the last 2 years. The main emphasis is put on the molecular mode of ecdysteroid receptor-mediated hormone action. Two examples of interaction with other hormonal signalling pathways are described, namely crosstalk with juvenile hormone and insulin. Some selected, recently investigated examples of the multitude of hormonal responses are described. Finally, ecological aspects and some practical applications are discussed.

Towards Coleoptera-specific high-throughput screening systems for compounds with ecdysone activity: development of EcR reporter assays using weevil (Anthonomus grandis)-derived cell lines and in silico analysis of ligand binding to A. grandis EcR ligand-binding pocket

Molting in insects is regulated by ecdysteroids and juvenile hormones. Several synthetic non-steroidal ecdysone agonists are on the market as insecticides. These ecdysone agonists are dibenzoylhydrazine (DBH) analogue compounds that manifest their toxicity via interaction with the ecdysone receptor (EcR). Of the four commercial available ecdysone agonists, three (tebufenozide, methoxyfenozide and chromafenozide) are highly lepidopteran specific, one (halofenozide) is used to control coleopteran and lepidopteran insects in turf and ornamentals. However, compared to the very high binding affinity of these DBH analogues to lepidopteran EcRs, halofenozide has a low binding affinity for coleopteran EcRs. For the discovery of ecdysone agonists that target non-lepidopteran insect groups, efficient screening systems that are based on the activation of the EcR are needed. We report here the development and evaluation of two coleopteran-specific reporter-based screening systems to discover and evaluate ecdysone agonists. The screening systems are based on the cell lines BRL-AG-3A and BRL-AG-3C that are derived from the weevil Anthonomus grandis, which can be efficiently transduced with an EcR reporter cassette for evaluation of induction of reporter activity by ecdysone agonists. We also cloned the almost full length coding sequence of EcR expressed in the cell line BRL-AG-3C and used it to make an initial in silico 3D-model of its ligand-binding pocket docked with ponasterone A and tebufenozide.

Biochemical mechanisms of methoxyfenozide resistance in the cotton leafworm Spodoptera littoralis

BACKGROUND

Methoxyfenozide is a lepidopteran-specific insecticide that belongs to a new group of insecticides, the non-steroidal ecdysteroid agonists, also called moulting accelerating compounds (MACs). To investigate the risk of resistance and possible mechanisms conferring resistance to methoxyfenozide, the authors selected in the laboratory for a resistant strain of the cotton leafworm Spodoptera littoralis (Boisd.), which is a representative lepidopteran model and an important pest in cotton and vegetables worldwide, with a high risk for resistance development.

RESULTS

After selection with methoxyfenozide during 13 generations, toxicity data showed that the selected strain developed fivefold resistance to methoxyfenozide in comparison with the susceptible strain. Measurement of the detoxification enzymes demonstrated that the monooxygenase (MO) activity was 2.1 times higher in the selected strain, whereas there was no change for esterases and glutathione-S-transferases. When the inhibitors piperonyl butoxide (PBO), S,S,S-tributyl phosphorotrithioate (DEF) and diethyl maleate were tested as synergists, the respective synergistic ratios were 0.97, 0.96 and 1.0 for the susceptible strain, and 2.2, 0.96 and 1.1 for the resistant strain. The significant synergistic effect by PBO concurs with the increased MO activity in the selected strain.

CONCLUSION

Taken overall, the present study supports the importance of MO-mediated metabolism in resistance to methoxyfenozide, directing tactics to fight against resistance development for this novel group of insecticides.