Comparison of the activity of non-steroidal ecdysone agonists between dipteran and lepidopteran insects, using cell-based EcR reporter assays

Background: Diacylhydrazine (DAH) analogues have been developed successfully as a new group of insect growth regulators, called ecdysone agonists or moulting accelerating compounds. These DAHs have been shown to manifest their toxicity via interaction with the ecdysone receptor (EcR) in susceptible insects, as does the natural insect moulting hormone 20-hydroxyecdysone (20E). A notable feature is their high activity and specificity, particularly against lepidopteran insects, raising the question as to whether non-lepidopteran-specific analogues can be isolated. However, for the discovery of ecdysone agonists that target other important insect groups such as Diptera, efficient screening systems that are based on the activation of the EcR are needed.

Results: In this study, a dipteran-specific reporter-based screening system with transfected S2 cells of Drosophila melanogaster Meig. was developed in order to discover and evaluate compounds that have ecdysone agonistic or antagonistic activity. A library of non-steroidal ecdysone agonists containing different mother structures with DAH and other related analogues such as acylaminoketone (AAK) and tetrahydroquinoline (THQ) was tested. None of the compounds tested was as active as 20E. This is in contrast to the very high activity of several DAH and AAK congeners in lepidopteran cells (Bombyx mori L.-derived Bm5 cells). The latter agrees with a successful docking of a DAH, tebufenozide, in the binding pocket of the lepidopteran EcR (B. mori), while this was not the case with the dipteran EcR (D. melanogaster). Of note was the identification of two THQ compounds with activity in S2 but not in Bm5 cells. Although marked differences in activity exist with respect to the activation of EcR between dipterans and lepidopterans, there exists a positive correlation (R = 0.724) between the pLC(50) values in S2 and Bm5 cells. In addition, it was found through protein modelling that a second lobe was present in the ligand-binding pocket of lepidopteran BmEcR but was lacking in the dipteran DmEcR protein, suggesting that this difference in structure of the binding pocket is a major factor for preferential activation of the lepidopteran over the dipteran receptors by DAH ligands.

Conclusions: The present study confirmed the marked specificity of DAH and AAK analogues towards EcRs from lepidopteran insects. THQ compounds did not show this specificity, indicating that dipteran-specific ecdysone-agonist-based insecticides based on the THQ mother structure can be developed. The differences in activity of ecdysone agonists in dipteran and lepidopteran ecdysone-reporter-based screening systems are discussed.