The morphological response of Kc-H cells to ecdysteroids: Hormonal specificity

Chemical Treatments for Insect Cell Differentiation: The Effects of 20-Hydroxyecdysone and Veratridine on Cultured Spodoptera frugiperda (Sf21) Insect Cell Ultrastructure

Simple Summary

Cultured insect Sf21 cells treated with the hormone 20-hydroxyecdysone grow long processes and resemble neurons. They also make physical contact with one another and appear to have the potential to form synapses, areas in which nerve cells are in close contact and communicate with one another electrically or by the release of chemical transmitters. This study uses electron microscopy to look for structural evidence of synapses in 20-hydroxyexdysone treated Sf21 cell cultures. Unfortunately, no evidence of synaptic structures were observed, suggesting that other factors are required for the formation of functional synapses in these cultures.

Abstract

Previous studies have shown that insect cell cultures stop dividing, form clumps, and can be induced to grow processes reminiscent of axons, when the culture medium is supplemented with 20-hydroxyecdysone, insulin, or an agent that mimics their action, such as the ecdysone agonist, methoxyfenozide. Those cell growing processes resemble nerve cells, and the present study evaluates the ultrastructure of these cultures by transmission electron microscopy. Sf21 cells treated with 20-hydroxyecdysone (with or without veratridine amendment) and subjected to ultrastructural analysis had a similar somatic appearance to control cells, with slight changes in organelles and organization, such as a greater number of cytoplasmic vacuoles and mitochondrial granules. Finger-like projections were observed between control and treated cells. However, no structural markers of synaptic contacts (e.g., vesicles or synaptic thickenings) were observed in controls, 20-hydroxyecdysone, or 20-hydroxyecdysone + veratridine treated cells. It is concluded that additional agents would be required to induce functional synaptogenesis in Sf21 cells.

Electrophysiological evidence of RML12 mosquito cell line towards neuronal differentiation by 20-hydroxyecdysdone

Continuous cell lines from insect larval tissues are widely used in different research domains, such as virology, insect immunity, gene expression, and bio pharmacology. Previous study showed that introduction of 20-hydroxyecdysone to Spodoptera cell line induced a neuron-like morphology with neurite extensions. Despite some results suggesting potential presence of neuro-receptors, no study so far has shown that these neuron-induced cells were functional. Here, using microelectrode arrays, we showed that the mosquito cell line, RML12, differentiated with 20-hydroxyecdysone, displays spontaneous electrophysiological activity. Results showed that these cells can be stimulated by GABAergic antagonist as well as nicotinic agonist. These results provide new evidence of neuron-like functionality of 20-hydroxyecdysone induced differentiated mosquito cell line. Finally, we used this new model to test the effects of two insecticides, temephos and permethrin. Our analysis revealed significant changes in the spiking activity after the introduction of these insecticides with prolonged effect on the neuronal activity. We believe that this differentiated mosquito neuronal cell model can be used for high-throughput screening of new pesticides on insect nervous system instead of primary neurons or in vivo studies.

ROLE OF SERUM AND ION CHANNEL BLOCK ON GROWTH AND HORMONALLY-INDUCED DIFFERENTIATION OF Spodoptera frugiperda (Sf21) INSECT CELLS

A neuronal morphological phenotype can be induced in cultured Spodoptera frugiperda insect cells (Sf21) by supplementing serum-containing media with 20-hydroxyecdysone (20-HE) and/or insulin. In this study, the primary objectives were to determine any role of ion channels in mediating the morphological change in cells treated with 20-HE and insulin, and whether serum was required to observe this effect. Results showed serum-free media also induced growth of processes in Sf21 cells, but at a lower percentage than that found previously in cells bathed in serum-containing media. Veratridine, a sodium channel activator, increased cell survival when applied in combination with 20-HE to Sf21 cells, and the effect was blocked by tetrodotoxin (1 μM) a known sodium channel blocker. Cobalt, a calcium channel blocker, showed significant inhibition of cell process growth when applied in combination with both 20-HE and 20-HE plus veratridine. Cobalt also showed significant inhibition of cell process growth when applied in combination with insulin. Thus, some type of sodium channel, as well as a mechanism for transmembrane calcium ion movement, are apparently expressed in Sf21 cells and are involved in the differentiation process. These cell lines may be used in a wide variety of endeavors, including the screening of insecticides, as well as foster basic studies of neurodevelopment and ecdysone action.

A cell-based reporter assay for screening for EcR agonist/antagonist activity of natural ecdysteroids in Lepidoptera (Bm5) and Diptera (S2) cell cultures, followed by modeling of ecdysteroid-EcR interactions and normal mode analysis

Ecdysteroid signal transduction is a key process in insect development and therefore an important target for insecticide development. We employed an in vitro cell-based reporter bioassay for the screening of potential ecdysone receptor (EcR) agonistic and antagonistic compounds. Natural ecdysteroids were assayed with ecdysteroid-responsive cell line cultures that were transiently transfected with the reporter plasmid ERE-b.act.luc. We used the dipteran Schneider S2 cells of Drosophila melanogaster and the lepidopteran Bm5 cells of Bombyx mori, representing important pest insects in medicine and agriculture. Measurements showed an EcR agonistic activity only for cyasterone both in S2 (EC50=3.3μM) and Bm5 cells (EC50=5.3μM), which was low compared to that of the commercial dibenzoylhydrazine-based insecticide tebufenozide (EC50=0.71μM and 0.00089μM, respectively). Interestingly, a strong antagonistic activity was found for castasterone in S2 cells with an IC50 of 0.039μM; in Bm5 cells this effect only became visible at much higher concentrations (IC50=18μM). To gain more insight in the EcR interaction, three-dimensional modeling of dipteran and lepidopteran EcR-LBD was performed. In conclusion, we showed that the EcR cell-based reporter bioassay tested here is a useful and practical tool for the screening of candidate EcR agonists and antagonists. The docking experiments as well as the normal mode analysis provided evidence that the antagonist activity of castasterone may be through direct binding with the receptor with specific changes in protein flexibility. The search for new ecdysteroid-like compounds may be particularly relevant for dipterans because the activity of dibenzoylhydrazines appears to be correlated with an extension of the EcR-LBD binding pocket that is prominent in lepidopteran receptors but less so in the modeled dipteran structure.

Induction and inhibition of an apparent neuronal phenotype in Spodoptera frugiperda insect cells (Sf21) by chemical agents

The goal of this research was to induce neuron-like properties in Sf21 cells, an insect ovarian cell line, which could lead to a new high-throughput insecticide screening method and a way to mass produce insect neuronal material for basic research. This study applied differentiation agents to produce viable neuron-like cells. In the presence of the molting hormone 20-hydroxyecdysone (20-HE), or insulin, in the growth medium, a maximum of ca. 30 % of Sf21 cells expressed an apparent neuronal morphology of unipolar, bipolar, or multipolar axon-like processes within 2-3 days. Maximal differentiation occurred after 2 days in the presence of 50 μM 20-HE or 3 days in 10 μM insulin. Both 20-HE and insulin displayed time- and concentration-dependent differentiation with biphasic curves, suggesting that two binding sites or processes were contributing to the observed effects. In addition, combinations of 20-HE and insulin produced apparent synergistic effects on differentiation. Caffeine, a central nervous system stimulant, inhibited induction of elongated processes by 20-HE and/or insulin, with an IC(50) of 9 nM for 20-HE, and the inhibition was incomplete, resulting in about one-quarter of the differentiated cells remaining, even at high concentrations (up to 1 mM). The ability to induce a neural phenotype simplifies the studies of insect cells, compared to either the use of primary nervous tissue or genetic engineering techniques. The presence of ion channels or receptors in the differentiated cells remains to be determined.

Genome-wide examination of the transcriptional response to ecdysteroids 20-hydroxyecdysone and ponasterone A in Drosophila melanogaster

Background

The 20-hydroxyecdysone (20E) hierarchy of gene activation serves as an attractive model system for studying the mode of steroid hormone regulated gene expression and development. Many structural analogs of 20E exist in nature and among them the plant-derived ponasterone A (PoA) is the most potent. PoA has a higher affinity for the 20E nuclear receptor, composed of the ecysone receptor (EcR) and Ultraspiracle proteins, than 20E and a comparison of the genes regulated by these hormones has not been performed. Furthermore, in Drosophila different cell types elicit different morphological responses to 20E yet the cell type specificity of the 20E transcriptional response has not been examined on a genome-wide scale. We aim to characterize the transcriptional response to 20E and PoA in Drosophila Kc cells and to 20E in salivary glands and provide a robust comparison of genes involved in each response.

Results

Our genome-wide microarray analysis of Kc167 cells treated with 20E or PoA revealed that far more genes are regulated by PoA than by 20E (256 vs 148 respectively) and that there is very little overlap between the transcriptional responses to each hormone. Interestingly, genes induced by 20E relative to PoA are enriched in functions related to development. We also find that many genes regulated by 20E in Kc167 cells are not regulated by 20E in salivary glands of wandering 3^rd instar larvae and we show that 20E-induced levels of EcR isoforms EcR-RA, ER-RC, and EcR-RD/E differ between Kc cells and salivary glands suggesting a possible cause for the observed differences in 20E-regulated gene transcription between the two cell types.

Conclusions

We report significant differences in the transcriptional responses of 20E and PoA, two steroid hormones that differ by only a single hydroxyl group. We also provide evidence that suggests that PoA induced death of non-adapted insects may be related to PoA regulating different set of genes when compared to 20E. In addition, we reveal large differences between Kc cells and salivary glands with regard to their genome-wide transcriptional response to 20E and show that the level of induction of certain EcR isoforms differ between Kc cells and salivary glands. We hypothesize that the differences in the transcriptional response may in part be due to differences in the EcR isoforms present in different cell types.

Generation of stable Drosophila cell lines using multicistronic vectors

Insect cell culture is becoming increasingly important for applications including recombinant protein production and cell-based screening with chemical or RNAi libraries. While stable mammalian cell lines expressing a protein of interest can be efficiently prepared using IRES-based vectors or viral-based approaches, options for stable insect cell lines are more limited. Here, we describe pAc5-STABLEs, new vectors for use in Drosophila cell culture to facilitate stable transformation. We show that viral-derived 2A-like (or "CHYSEL") peptides function in Drosophila cells and can mediate the multicistronic expression of two or three proteins of interest under control of the Actin5C constitutive promoter. The current vectors allow mCherry and/or GFP fusions to be generated for positive selection by G418 resistance in cells and should serve as a flexible platform for future applications.

Ecdysone and the cell cycle: investigations in a mosquito cell line

Cell lines provide a tool for investigating basic biological processes that underlie the complex interactions among the tissues and organs of an intact organism. We compare the evolution of insect and mammalian populations as they progress from diploid cell strains to continuous cell lines, and review the history of the well-characterized Aedes albopictus mosquito cell line, C7-10. Like Kc and S3 cells from Drosophila melanogaster, C7-10 cells are sensitive to the insect steroid hormone, 20-hydroxyecdysone (20E), and express 20E-inducible proteins as well as the EcR and USP components of the ecdysteroid receptor. The decrease in growth associated with 20E treatment results in an accumulation of cells in the G1 phase of the cycle, and a concomitant decrease in levels of cyclin A. In contrast, 20E induces a G2 arrest in a well-studied imaginal disc cell line from the moth, Plodia interpunctella. We hypothesize that 20E-mediated events associated with molting and metamorphosis include effects on regulatory proteins that modulate the mitotic cell cycle and that differences between the 20E response in diverse insect cell lines reflect an interplay between classical receptor-mediated effects on gene expression and non-classical effects on signaling pathways similar to those recently described for the vertebrate steroid hormone, estrogen.

Drosophila cell lines as model systems and as an experimental tool

Given the power of Drosophila genetics, it may seem surprising to discover that many fly researchers are turning to Drosophila cell culture as an experimental system. However, as we will show in this chapter, there are many benefits to be gained by using cell lines as a complement to studies in a tissue and developmental context in the fly. Moreover, one can argue that Drosophila cell culture, in itself, provides an excellent model system for the study of many fundamental questions in molecular and cellular biology. In this review, we offer a summary of techniques that should be useful to researchers in the Drosophila community working with fly cell lines. These include techniques for growing and maintaining cell lines, transient and stable transfection, RNA interference, imaging, immunostaining, fluorescence-activated cell sorting, and for the isolation of RNA and protein from fly cells.

Action of 24-epibrassinolide on a cell line of the beet armyworm, Spodoptera exigua

The Spodoptera exigua cell line Se4 is sensitive for ecdysteroid activity stimulated by the insect molting hormone, 20-hydroxyecdysone (20E), showing a cease in cell proliferation (with 50% inhibition around 1 microM) and characteristic cell morphology changes with aggregation and formation of long filamentous cytoplasmic extensions. The bisacylhydrazine tebufenozide also triggered such typical cellular effects in Se4, and in addition, it showed an affinity for binding in competition with 3H-ponasterone A (PoA) that was similar to 20E (with 50% competition around 1 microM), confirming that such non-ecdysteroids display an ecdysteroid agonist activity. In contrast, when Se4 cells were incubated with the native plant hormone 24-epibrassinolide (24BR), none of the effects triggered by 20E were observed. Hence, a competition binding experiment with 3H-PoA demonstrated no affinity of 24BR for binding to the ecdysteroid receptor in the Se4 cell line. In another series of experiments, the Se4 cell line was tested in sensitivity response to increased acetylcholinesterase (AchE) activity after treatment with ecdysteroid active compounds. The AchE activity measured in the cell line is discussed in relation to inhibition by eserine. The obtained results suggest that 24BR exerted no ecdysteroid activity.

Functional and comparative analysis of two distinct ecdysteroid-responsive gene expression constructs in Drosophila S2 cells

Inducible expression systems have proven to be of major interest when analysing the function of specific genes or when expressing cytotoxic proteins. In an effort to develop inducible switches allowing for flexible fine-tuning of gene expression levels in insect cells, we have compared the induction capacities of two Drosophila minimal promoters when linked to four consecutive ecdysone response elements. These minimal promoters, either containing a TATA-box or a downstream promoter element, drove the expression of a luciferase reporter gene. Potent induction capacities were observed with the insect moulting hormone, 20-hydroxyecdysone, and with ponasterone A, a plant ecdysteroid. The developed inducible switches further expand the repertoire of molecular tools for functional expression of proteins of interest in insect cells. In addition, the combination of an ecdysone switch with promoters that possess different structural elements can provide novel insights into ecdysteroid-induced transcription in an insect cell line.

Synchronization of Plodia interpunctella lepidopteran cells and effects of 20-hydroxyecdysone

We have investigated the molecular and cellular mechanisms involved in the control of insect cell cycle by 20-hydroxyecdysone (20E) using the IAL-PID2 cell line established from imaginal wing discs of Plodia interpunctella. We first defined conditions for use of hydroxyurea, a reversible inhibitor of DNA synthesis, in order to synchronize the IAL-PID2 cells in their division cycle. A high degree of synchrony was reached when cells were exposed to two consecutive hydroxyurea treatments at 1 mm for 36 h spaced 16 h apart. Under these conditions, flow cytometry analysis demonstrated that 20E at 10(-6) m induced an inhibition of cell growth by an arrest of 90% of the cells in G2/M phase. Using cDNA probes specifically designed from E75 and HR3 nuclear receptors of Plodia interpunctella, we showed that PiE75 and PHR3 were highly induced by 20E through S and G2 phases with maximal enhancement just before the G2/M arrest of cells. These findings suggest that PiE75 and PHR3 could be involved in a 20E-induced genetic cascade leading to G2/M arrest.

The 20-hydroxyecdysone-induced cellular arrest in G2 phase is preceded by an inhibition of cyclin expression

We have studied the effect of 20-hydroxyecdysone (20E) on cellular proliferation in IAL-PID2 cell line established from imaginal wing discs of Plodia interpunctella. Flow cytometry analysis demonstrated that 20E induced an arrest of cells in G2 phase. To determine whether this arrest was due to an effect of 20E on cyclin expression, we cloned two cDNA fragments, named PcycA and PcycB, encoding, respectively, Plodia cyclins A and B. Using PcycA and PcycB probes, we have demonstrated that 20E induced a sharp decrease in the levels of cyclin A and B expression. Studies of induction pattern of Plodia HR3 transcription factor by 20E revealed that its induction preceded the decrease of cyclins transcripts. An exposure of cells to 20E in the presence of juvenile hormone (JH) led to a change in the kinetic of PHR3 induction and prevented both the decline of cyclin A and B expression and the G2 arrest. This effect of JH provides an additional argument for the existence of a correlation between cyclin transcripts level and G2 arrest. For the first time in insects, these findings bring evidence that ecdysteroids regulate cellular proliferation by acting on cell cycle regulators as cyclins.

A cell‐based high‐throughput screening system for detecting ecdysteroid agonists and antagonists in plant extracts and libraries of synthetic compounds

Screening systems for ecdysteroid mimetic or antiecdysteroid substances in plant extracts or libraries of synthetic compounds are commonly based on the observation of morphological and/or growth responses in insect cell lines. Because these responses are slow and require careful monitoring, existing screening systems are considered limited regarding their applicability to analysis in high-throughput (HT) formats. Here we describe the generation of transformed silkmoth (Bombyx mori) cell lines that respond to the addition of ecdysone-like substances through the expression of the green fluorescent protein (GFP) and the appearance of green fluorescence. Because tests consist of three simple steps, i.e., 1) distribution of transformed cells in microtiter plates; 2) addition of compounds/extracts at different concentrations; and 3) quantification of fluorescence intensity by a fluorescence plate reader, they can be performed quickly and be easily adapted to a HT format. The generated reporter cell lines are used for the screening of extracts from available plant collections for the presence of compounds with ecdysone mimetic or antagonistic activities as well as for monitoring subsequent activity during enrichment and purification steps. The same cell lines are also used here for the determination of structure-activity relationships among available synthetic dibenzoylhydrazine derivatives. Finally, for the identified agonists, we show that their activity as determined by the cell-based screening assays parallels their bioactivity in growth inhibition and toxicity assays carried out on live insects.

The Drosophila orphan nuclear receptor DHR38 mediates an atypical ecdysteroid signaling pathway

Ecdysteroid pulses trigger the major developmental transitions during the Drosophila life cycle. These hormonal responses are thought to be mediated by the ecdysteroid receptor (EcR) and its heterodimeric partner Ultraspiracle (USP). We provide evidence for a second ecdysteroid signaling pathway mediated by DHR38, the Drosophila ortholog of the mammalian NGFI-B subfamily of orphan nuclear receptors. DHR38 also heterodimerizes with USP, and this complex responds to a distinct class of ecdysteroids in a manner that is independent of EcR. This response is unusual in that it does not involve direct binding of ecdysteroids to either DHR38 or USP. X-ray crystallographic analysis of DHR38 reveals the absence of both a classic ligand binding pocket and coactivator binding site, features that seem to be common to all NGFI-B subfamily members. Taken together, these data reveal the existence of a separate structural class of nuclear receptors that is conserved from fly to humans.

Entomogenous fungus Nomuraea rileyi inhibits host insect molting by C22-oxidizing inactivation of hemolymph ecdysteroids

The entomogenous fungus Nomuraea rileyi reportedly secretes a proteinaceous substance inhibiting larval molt and metamorphosis in the silkworm Bombyx mori. We studied the possibility that N. rileyi controls B. mori development by inactivating hemolymph molting hormone, ecdysteroids. Incubation of ecdysone (E) and 20-hydroxyecdysone (20E) in fungal-conditioned medium resulted in their rapid modification into products with longer retention times in reverse-phase HPLC. Each modified product from E and 20E was purified by HPLC, and identified by NMR as 22-dehydroecdysone and 22-dehydro-20-hydroxyecdysone. Some other ecdysteroids with a hydroxyl group at position C22 were also modified. Injection of the fungal-conditioned medium into Bombyx mori larvae in the mid-4th instar inhibited larval molt but induced precocious pupal metamorphosis, and its injection into 5th instar larvae just after gut purge blocked pupal metamorphosis. In hemolymph of injected larvae, E and 20E disappeared and, in turn, 22-dehydroecdysone and 22-dehydro-20-hydroxyecdysone accumulated. These results indicate that N. rileyi secretes a specific enzyme that oxidizes the hydroxyl group at position C22 of hemolymph ecdysteroids and prevents molting in B. mori larvae.

Characterization of the ligand-binding domain of the ecdysteroid receptor from Drosophila melanogaster

Mutants created by site-directed mutagenesis were used to elucidate the function of amino acids involved in ligand binding to ecdysteroid receptor (EcR) and heterodimer formation with ultraspiracle (USP). The results demonstrate the importance of the C-terminal part of the D-domain and helix 12 of EcR for hormone binding. Some amino acids are involved either in ligand binding to EcR (E476, M504, D572, I617, N626) or ligand-dependent heterodimerization as determined by gel mobility shift assays (A612, L615, T619), while others are involved in both functions (K497, E648). Some amino acids are suboptimal for ligand binding (L615, T619), but mediate ligand-dependent dimerization. We conclude that the enhanced regulatory potential by ligand-dependent modulation of dimerization in the wild type is achieved at the expense of optimal ligand binding. Mutation of amino acids (K497, E648) involved in the salt bridge between helix 4 and 12 impair ligand binding to EcR more severely than hormone binding to the heterodimer, indicating that to some extent heterodimerization compensates for the deleterious effect of certain mutations. Different effects of the same point mutations on ligand binding to EcR and EcR/USP (R511, A612, L615, I617, T619, N626) indicate that the ligand-binding pocket is modified by heterodimerization.

A possible role of 20-hydroxyecdysone in embryonic development of the silkworm Bombyx mori

It has been well established that eggs of insects, including those of the silkworm Bombyx mori, contain various ecdysteroids and the amounts of these ecdysteroids fluctuate during embryonic development. In order to know the function of egg ecdysteroids in embryonic development of B. mori, we examined the biological activities of various egg ecdysteroids by in vitro ligand-binding assay and bioassay using B. mori eggs. First, using the ecdysteroid receptor of B. mori (BmEcR-B1/BmUSP heterodimer) prepared by yeast and Escherichia coli expression systems, the interaction between the ecdysteroid receptor and various egg ecdysteroids of B. mori was analyzed. The relative binding affinities of egg ecdysteroids to the BmEcR-B1/BmUSP heterodimer decreased in the order of 20-hydroxyecdysone > 2-deoxy-20-hydroxyecdysone > 22-deoxy-20-hydroxyecdysone > ecdysone > 2-deoxyecdysone > ecdysone 22-phosphate. Next, several egg ecdysteroids of B. mori were injected into the prospective diapause eggs, which show a very low level of free ecdysteroids at the onset of embryonic diapause (gastrula stage). Approximately 7% of them (P < 0.002, chi(2)-test) developed beyond the gastrula stage without entering diapause by the injection of 20-hydroxyecdysone (25 ng/egg). In contrast, the injection of other ecdysteroids was not effective in inducing embryonic development. These results suggest that 20-hydroxyecdysone, via the ecdysteroid receptor, is responsible for the developmental difference between diapause and non-diapause in B. mori embryos. Furthermore, it was suggested that continuous supply of 20-hydroxyecdysone may be required to induce embryonic development.

Ligand-induced heterodimerization between the ligand binding domains of the Drosophila ecdysteroid receptor and ultraspiracle

The insect ecdysteroid receptor consists of a heterodimer between EcR and the RXR-orthologue, USP. We addressed the question of whether this heterodimer, like all other RXR heterodimers, may be formed in the absence of ligand and whether ligand promotes dimerization. We found that C-terminal protein fragments that comprised the ligand binding, but not the DNA binding domain of EcR and USP and which were equipped with the activation or DNA binding region of GAL4, respectively, exhibit a weak ability to interact spontaneously with each other. Moreover, the heterodimer formation is greatly enhanced upon administration of active ecdysteroids in a dose-dependent manner. This was shown in vivo by a yeast two-hybrid system and in vitro by a modified electromobility shift assay. Furthermore, the EcR fragment expressed in yeast was functional and bound radioactively labelled ecdysteroid specifically. Ligand binding was greatly enhanced by the presence of a USP ligand binding domain. Therefore, ecdysteroids are capable of inducing heterodimer formation between EcR and USP, even when the binding of these receptor proteins to cognate DNA response elements does not occur. This capability may be a regulated aspect of ecdysteroid action during insect development.

Biological activities of a specific ecdysteroid dimer and of selected monomeric structural analogues in the B(II) bioassay

The biological activities of selected specific ecdysteroids obtained by photochemical or chemical transformation are compared in the B(II) bioassay, in which the potency reflects the affinity of binding to the ligand-binding site of the Drosophila melanogaster ecdysteroid receptor. The compounds tested represent 14-deoxy, 14-dehydroxy, 14-hydroperoxy and 14-epi derivatives of 20-hydroxyecdysone and were selected on the basis of their close structural relationship to elucidate the contribution of the 14-hydroxy group and the stereochemical configuration at C-14 to ecdysteroid agonist activity. The structure-activity relationship shows that a 14-hydroxy group is not required for activity. However, the alpha-configuration of -H, -OH or -OOH at C-14, which determines the C/D rings trans-annelation, is very significant for activity; it is as important for activity as the well studied A/B rings cis-annelation. Compounds containing a double bond involving C-14 showed low activity with the exception of the specific, and so far unique, ecdysteroid dimer 7,7'-bis-[14-deoxy-8(14)-ene-20-hydroxyecdysone], which was obtained as the main product of the photochemical transformation of 20-hydroxyecdysone. The relatively high biological activity of this dimeric compound is discussed.

Periodic expression of an ecdysteroid-induced nuclear receptor in a lepidopteran cell line (IAL-PID2)

A set of DNA primers was designed within the DNA-binding domain of the Manduca hormone receptor 3 (MHR3) cDNA. These primers were used in RT-PCR to isolate a 204 bp cDNA fragment from IAL-PID2 cells exposed to 10(-6) M 20-hydroxyecdysone (20E) for 12 h. The amino acid sequence deduced from the cDNA fragment presented 100% identity with the zinc finger domain of Manduca hormone receptor 3 (MHR3), Galleria hormone receptor 3 (GHR3) and Choristoneura hormone receptor 3 (CHR3). This cDNA fragment was used as a probe on total RNA from IAL-PID2 cells exposed to 20E and hybridized to mRNA, the size of which was close to 4.5 kb and named Plodia hormone receptor 3 (PHR3). Kinetics of induction of PHR3 mRNA were similar to that of HR3 genes but varied according to the position of cells in their cell cycle. The non-steroidal ecdysone agonist, RH-5992 induced the expression of PHR3 at lower concentrations than 20E. From sequence similarity, mRNA size, 20E and RH-5992 inducibilities, we conclude that PHR3 transcript could encode a Plodia hormone receptor 3 involved in the genetic signalling cascade of 20E. Thanks to its periodic expression, this putative orphan nuclear receptor could serve as a suitable cellular marker for studying changes of epidermal cell sensitivity to 20E during the cell cycle.

Phytoecdysteroids: biological aspects

Phytoecdysteroids are a family of about 200 plant steroids related in structure to the invertebrate steroid hormone 20-hydroxyecdysone. Typically, they are C27, C28 or C29 compounds possessing a 14alpha-hydroxy-7-en-6-one chromophore and A/B-cis ring fusion (5beta-H). In the present review, the distribution, biosynthesis, biological significance and potential applications of phytoecdysteroids are summarised.

Significance of absorption, oxidation, and binding to toxicity of four ecdysone agonists in multi-resistant cotton leafworm

Treatment of last-instar larvae of multi-resistant cotton leafworm Spodoptera littoralis with four dibenzoylhydrazines, methoxyfenozide (RH-2485), tebufenozide (RH-5992), halofenozide (RH-0345), and RH-5849, resulted in premature molting leading to death. Methoxyfenozide was the most toxic followed by tebufenozide, halofenozide, and RH-5849. To explain differences in toxicity, especially between multi-resistant and laboratory strains, absorption in the body tissues and oxidative metabolism were tested with 14C-labeled ecdysone agonist and a Lineweaver-Burk assay, respectively. Then to address different compound potencies in multi-resistant strains, the potency of the four ecdysone agonists was measured based on their ability to mimic the natural insect molting hormone, 20-hydroxyecdysone (20E) by inducing evagination in isolated imaginal wing discs. Using monoclonal antibody 9B9, the presence of ecdysteroid receptors in imaginal discs in vitro was confirmed. In parallel, Scatchard plot analysis with whole imaginal wing discs cultured with different concentrations of 3H-labeled ponasterone A indicated no significant difference in affinity and in number of target sites for binding between multi-resistant and susceptible laboratory strains. The four compounds tested caused the effect as agonists of 20E in vitro, and typically the order of their toxicities (LC50s) corresponded with that for evagination-induction with whole imaginal discs.

Transcriptional activation of the Drosophila ecdysone receptor by insect and plant ecdysteroids

A number of insect ecdysteroids, plant ecdysteroids and juvenoids were assayed for their ability to activate Drosophila nuclear receptors in transfected tissue culture cells. Discrete modifications to 20-hydroxyecdysone, the apparent natural ligand for the ecdysone receptor (EcR), conferred dramatic changes on the transcriptional activity of this receptor, suggesting that other biologically relevant EcR ligands may exist. Conversely, none of the compounds tested had a significant effect on the activity of three Drosophila orphan nuclear receptors: DHR38, DHR78 or DHR96. Taken together, these results demonstrate the selectivity of EcR for a series of natural and synthetic ecdysone agonists and suggest that as yet untested compounds may be responsible for activating DHR38, DHR78 and DHR96.

Establishment and characterization of an Ostrinia nubilalis cell line, and its response to ecdysone agonists

A cell line derived from embryonic tissues of the European corn borer, Ostrinia nubilalis (UMC-OnE), was established in EX-CELL 401 medium containing 10% fetal bovine serum. The cells grew in suspension, and were mainly spherical in shape. The cell doubling times at the 17th and 79th passages were 56 and 36 h, respectively. DNA amplification fingerprinting showed that the DNA profile of the OnE cell line was different from that of the southwestern corn borer, Diatraea grandiosella (UMC-DgE), and that of the cotton bollworm, Helicoverpa zea (BCIRL-HZ-AM1). The OnE cell line was responsive to treatments of 20-hydroxyecdysone and the ecdysone agonists, methoxyfenozide (RH-2485) and tebufenozide (RH-5992). These compounds caused similar effects on the cells, which included cell clumping and decreased cell proliferation. The clumps were observed on the third day of incubation, and became larger after 7 d of incubation. After 168 h of incubation, methoxyfenozide and tebufenozide were 35 and 11 times more effective, respectively, in inhibiting proliferation of the OnE cells than was 20-hydroxyecdysone.

Overlapping antiparallel transcripts induced by ecdysone in a Drosophila cell line

Treating Drosophila Kc tissue culture cells with 20-hydroxyecdysone (20E) causes a rapid 10-fold increase in the synthesis of three polypeptides, with molecular weights of 28 kD, 29 kD, and 40 kD (named EIP28, EIP29, and EIP 40, in reference to their molecular weights), as shown by Savakis et al. (1980, 1984). Genomic clones that include Eip40, a gene encoding the EIP40 polypeptide, have been isolated and used to characterize transcription in a 10 kilobase region. At least five different transcripts are produced from this region in Kc cells. One of these transcripts is overlapping and antiparallel to the Eip40 transcript. Although these genes are located quite near one another, their response to 20E varies from little, if any, induction to an induction that is at least 25 fold. Approximately 2.5 kb of DNA which encompasses the Eip40 gene has been sequenced. Analysis of this sequence shows that Eip40 encodes a Drosophila homologue of the enzyme cystathionase. The complementary DNA strand, which produces an 1800 N transcript, has a 645 N open reading frame (ORF). This ORF shows some similarity to cystathionase, the protein encoded by the complementary strand, but shows no detectable similarity to other reported protein sequences.

Postembryonic development of the midline glia in the CNS of Drosophila: proliferation, programmed cell death, and endocrine regulation

The development of Drosophila midline glia during larval and pupal stages was characterized by localizing beta-gal expression in enhancer trap lines, as well as with BrdU incorporation and pulse-chase experiments. At hatching about 40 to 50 glial cells are present along the midline of the ventral nerve cord (2 to 3 dorsal and 1 to 2 ventral cells per neuromere). The cells proliferate during the third larval instar and spread dorsoventrally within the midline, increasing in number to about 230 or more (around 20 cells per neuromere). Cell divisions cease shortly after pupariation, and the cells persist for the first half of pupal life with no apparent changes in numbers or positions. Between 50 and 80% of metamorphosis, however, virtually all of the midline glia undergo programmed cell death. Tissue culture experiments indicate that the peak of ecdysteroids occurring at pupariation is required for the cessation of proliferation of midline glia and their subsequent degeneration. Midline glia in central nervous systems (CNS) cultured with low or no ecdysteroids survive and continue to divide, whereas they cease proliferating and later degenerate with high ecdysteroids levels. The midline glial may play a role during CNS metamorphosis similar to that of their progenitors in the embryo, in stabilizing outgrowing neurites that cross or run along the midline.

Action of different ecdysteroids on the regulation of mRNAs for the ecdysone receptor, MHR3, dopa decarboxylase, and a larval cuticle protein in the larval epidermis of the tobacco hornworm, Manduca sexta

To determine which ecdysteroids may be biologically active in the larval epidermis of the tobacco hornworm, Manduca sexta, we studied the action of several known ecdysteroids and metabolites on the expression of the genes encoding the ecdysone receptor (EcR), Manduca hormone receptor 3 (MHR3), dopa decarboxylase (DDC), and a larval cuticle protein (LCP-14). Both Day 2 fourth- and Day 2 fifth-instar larval epidermis contained significant 3 beta-reductase activity which metabolized 3-dehydroecdysone (3DE) and 3-dehydro-20-hydroxyecdysone (3D20E) to ecdysone (E) and 20-hydroxyecdysone (20E), respectively, but had only very low amounts of ecdysone oxidase activity (E to 3DE) and no detectable ecdysone 20-monooxygenase activity (E to 20E). When the expression of the various genes was studied in the epidermis in vitro, 20E and 3D20E had similar effects, whereas E, 3DE, 26-hydroxyecdysone and 20,26-dihydroxyecdysone were ineffective. Exposure of Day 2 fifth-instar epidermis to 500 ng/ml of either 20E or 3D20E for 24 hr caused a rapid, biphasic increase in EcR-B1 mRNA. By contrast, EcR-A mRNA showed a less rapid initial increase followed by a slow steady rise and was less responsive to 3D20E. Ecdysone in a 1:1 mixture with 20E effectively halved the concentration of 20E needed to induce EcR-B1 mRNA but showed no synergism in the induction of EcR-A mRNA. The induction of MHR3 mRNA and of DDC mRNA in Day 2 fourth-instar epidermis as well as the suppression of DDC and LCP-14 gene expression by 3D20E was indistinguishable from that of 20E. Therefore, for Manduca larval epidermis, only 20E and 3D20E are biologically active ecdysteroids. Since the 3D20E can be converted to 20E by the epidermis, its effects are likely mediated by 20E.

Biological activity of natural and synthetic ecdysteroids in the BII bioassay

The potencies of 19 ecdysteroids are compared in the BII bioassay, which reflects the affinity of binding to the ligand binding site of the Drosophila melanogaster ecdysteroid receptor. The compounds tested represent either natural products isolated from plants (phytoecdysteroids) or fungi (mycoecdysteroids) or synthetic analogues based on insect metabolites (zooecdysteroids). None of the tested compounds showed any antagonistic activity, but all possessed quantifiable agonistic activity. All the mycoecdysteroids were less potent than the major insect ecdysteroid, 20-hydroxyecdysone (20E). Also, conjugation of 20E with a glucose moiety results, as expected, in considerable reduction in biological activity, but the remaining activity is dependent on the position of conjugation. The implications of these findings for the structure/activity relationship of ecdysteroids are discussed.

8-O-acetylharpagide is a nonsteroidal ecdysteroid agonist

We have identified a novel nonsteroidal ecdysteroid agonist. This compound was isolated from a methanol extract of Ajuga reptans L. (Lamiaceae) and the structure was identified by spectroscopic methods as 8-O-acetylharpagide. We have characterised this compound as an ecdysteroid agonist in a transactivation assay using beta-galactosidase as the reporter gene regulated by ecdysteroid response elements. In this assay, 8-O-acetylharpagide has an EC50 of 22 microM. The compound also competes with tritiated-ponasterone A for binding to the Drosophila ecdysteroid receptor. Finally, it induces differentiation of Drosophila Kc cells as would be expected of an ecdysteroid agonist. This iridoid glycoside is common to several plant species and may play a role in the natural defense mechanisms of plants.

Insect nuclear receptors: a developmental and comparative perspective

The appearance of puffs on the polytene chromosomes of insect salivary glands incubated with 20-hydroxyecdysone provided the first demonstration that steroids act directly at the gene transcriptional level to bring about subsequent cellular changes (Becker, 1959; Clever and Karlson, 1960). Despite that auspicious beginning, learning about the molecular mechanisms that underlie the hormonal regulation of insect development was impeded for many years by the difficulty associated with isolating and identifying rare regulatory factors from limited tissue sources. The advent of recombinant DNA methodology and powerful techniques such as the polymerase chain reaction (PCR) along with the recognition that many important endocrine factors are structurally conserved across a wide range of species has, however, all but eliminated the technical obstacles once facing the insect endocrinologist trying to isolate and study these regulatory molecules. This review will discuss recent progress and recall some earlier experiments concerning the molecular basis of hormonal action in insects focusing primarily on the members of the nuclear hormone receptor superfamily in Drosophila melanogaster. Two members of this family comprise the functional ecdysteroid receptor and at least a dozen other "orphans" have been identified in Drosophila for which no cognate ligand has yet been found. Many of these orphans are regulated by ecdysteroids. A discussion of juvenile hormone binding proteins that are not family members has been included because of their potential impact on nuclear receptor function. As receptor homologues have been identified in other insects, several general ideas concerning insect hormonal regulation have begun to emerge and these will be examined from a comparative point of view.

Morphological changes of BM-N4 cells induced by bombyxin, an insulin-related peptide of Bombyx mori

Bombyxin is a brain secretory peptide of the silkmoth Bombyx mori whose amino acid sequence shows considerable sequence homology with vertebrate insulin-family peptides. We found that a Bombyx cell line, named BM-N4, showed morphological changes when bombyxin was added to the culture medium at doses as low as 10(-10) M. Bombyxin-treated cells displayed a series of morphological modifications: 1 to 2 weeks after introduction of bombyxin, the cells increased in size, then they trended to aggregate, or took a spindle shape. These changes of the cells were bombyxin specific, not induced by other vertebrate insulin-family peptides. To prove that these modifications of the cells are mediated by receptors on the cell surface of the cells, we performed a receptor binding assay using 125I-labeled bombyxin. Scatchard analysis of the binding assay indicated that this cell line has a single class of receptors for bombyxin with a Kd = 2.36 +/- 0.56 nM and each cell has 15,800 +/- 1400 binding sites on the cell surface.

Effects of the steroid hormone, 20-hydroxyecdysone, on the growth of neurites by identified insect motoneurons in vitro

During metamorphosis in the hawkmoth, Manduca sexta, identified larval leg motoneurons survive the degeneration of their larval targets to innervate new muscles of the adult legs. The dendrites and axon terminals of these motoneurons regress at the end of the larval stage and then regrow during adult development. Previous studies have implicated the insect steroid, 20-hydroxyecdysone (20-HE), in similar examples of dendritic reorganization during metamorphosis. The present studies were undertaken to test whether 20-HE acts directly on the leg motoneurons to regulate dendritic growth. Larval leg motoneurons were labeled with a fluorescent dye to permit their identification in culture following the dissociation of thoracic ganglia at later stages of development. Leg motoneurons isolated from early pupal stage animals (just before the normal onset of dendritic regrowth) survived in vitro and grew processes regardless of whether 20-HE was added to the culture medium. The extent of process outgrowth, however, as measured by the total length of all processes and the number of branches, was significantly greater for motoneurons maintained in the presence of 20-HE. The enhancement could be blocked by the addition of a juvenile hormone analog. By contrast, larval leg motoneurons that were isolated just before the normal period of dendritic regression did not show enhanced growth of neurites in the presence of 20-HE. The results suggest that 20-HE acts directly on the leg motoneurons to regulate the growth of processes during metamorphosis.

The response of Drosophila imaginal disc cell lines to ecdysteroids

We have investigated the action of the moulting hormone 20-hydroxy ecdysone (20-HOE) on our leg and wing imaginal disc cell lines. At the morphological level, cells stop dividing and there is some cell death. The remaining cells elongate and aggregate, often producing long processes which form connections between different aggregates. 20-HOE acts within the first one or two days of a passage, at an optimum concentration of 10 ng/ml, this being about 1/100 of the optimum for ecdysone. One cloned wing cell line, C9, has been found to be relatively insensitive to the action of 20-HOE. We have been able to select for resistance to 20-HOE by growing cells in gradually increasing concentrations of hormone followed by passages in hormone-free medium. This has enabled us to isolate a wing cell line C1.8R from its parent cloned line C1.8+. This shows no response to 20-HOE, and cell growth continues even at hormone concentrations as high as 150 ng/ml. We have measured chitin synthesis by the incorporation of radioactive glucosamine into a cell fraction resistant to extensive alkali hydrolysis. The residue was incubated with chitinase, which resulted in a 50% reduction in labelled product. Treatment with 10 ng/ml of 20-HOE dramatically increased chitin synthesis in line C1.8+, but had no effect in the line C1.8R, selected for resistance to hormone.

Neither enhanced removal of cyclobutane pyrimidine dimers nor strand-specific repair is found after transcription induction of the beta 3-tubulin gene in a Drosophila embryonic cell line Kc

Nucleotide excision repair (NER) of ultraviolet (UV) light induced cyclobutane pyrimidine dimers (CPDs) was assayed in a Drosophila melanogaster Kc subline that responds to treatment with the steroid hormone 20-hydroxyecdysone (20-OH-E; beta-ecdysone, ecdysterone). In this cell line the hormone induces transcription of the beta 3-tubulin gene which is not expressed under standard culture conditions. Cells were exposed to either 10 or 15 J/m2 UV (predominantly 254-nm) and removal of CPDs from several genes, including beta 3-tubulin, and total cellular DNA was assayed. We show that upon induction of transcription of the beta 3-tubulin gene, its repair is not enhanced. In non-treated as well as 20-OH-E treated cells, repair kinetics in beta 3-tubulin resemble those in the active genes Gart and Notch, the inactive locus white and total cellular DNA. Moreover, in the presence as well as in the absence of transcription, the separate strands of the beta 3-tubulin gene are repaired with the same rate and to the same extent: about 90% after 24 h. It can be concluded from these observations that transcription is not a prerequisite for the efficient repair of CPDs in the Drosophila embryonic Kc cell line.

Endocrine strategies for the control of ectoparasites and insect pests

The increasing knowledge about endocrine mechanisms in arthropods facilitates the biorational search for drugs against insect pests and parasites that interfere with arthropod hormone action. Juvenile hormone mimics have been successfully applied for about 20 years; however, resistance to juvenile hormone analogues has developed. The introduction of moulting hormone agonists, which compete for binding to the ecdysteroid receptor, is expected in the near future. Despite the considerable progress that has been achieved in peptide hormone research during the last few years, no successful insecticide is currently available, although comparisons of drugs for medical use demonstrate that in principle, successful interference with peptide hormone action is possible. The search for new drugs has been facilitated by advances in cell-culture techniques, which improve the development of suitable screening systems, and by progress in genetic engineering, which could be an important tool in the creation of new strategies for insect pest control.

The Drosophila EcR gene encodes an ecdysone receptor, a new member of the steroid receptor superfamily

The steroid hormone ecdysone triggers coordinate changes in Drosophila tissue development that result in metamorphosis. To advance our understanding of the genetic regulatory hierarchies controlling this tissue response, we have isolated and characterized a gene, EcR, for a new steroid receptor homolog and have shown that it encodes an ecdysone receptor. First, EcR protein binds active ecdysteroids and is antigenically indistinguishable from the ecdysone-binding protein previously observed in extracts of Drosophila cell lines and tissues. Second, EcR protein binds DNA with high specificity at ecdysone response elements. Third, ecdysone-responsive cultured cells express EcR, whereas ecdysone-resistant cells derived from them are deficient in EcR. Expression of EcR in such resistant cells by transfection restores their ability to respond to the hormone. As expected, EcR is nuclear and found in all ecdysone target tissues examined. Furthermore, the EcR gene is expressed at each developmental stage marked by a pulse of ecdysone.

Identification of ecdysone response elements by analysis of the Drosophila Eip28/29 gene

We have identified ecdysone-response elements (EcREs) by studying regulation of the steroid-responsive Drosophila Eip28/29 gene. First, functional assays of deletion mutants identified large sequence regions required for the response; then a blotting method using the specifically labeled steroid receptor as probe identified receptor-binding regions. Three short receptor-binding regions near Eip28/29 have been identified: Prox and Dist [521 and 2295 nucleotides, respectively, downstream of the poly(A) site] are probably required for the Eip28/29 response in cell lines; Upstream (-440) is unnecessary for that response. We have also demonstrated that an EcRE-containing region from hsp27 contains a receptor-binding site. Each of these four receptor-binding regions functions as an EcRE when placed upstream of an ecdysone nonresponsive promoter and each contains an imperfect palindrome, suggesting the consensus 5'-RG(GT)TCANTGA(CA)CY-3'. Furthermore, a synthetic 15-bp fragment containing an imperfect palindrome similar to the consensus is a fully functional EcRE. The presence of any of the EcREs leads, in the absence of hormone, to depressed gene expression. When hormone is added, it relieves this repression and causes additional activation. The similarity of the EcRE sequence to response elements for estrogen, thyroid hormone, and retinoic acid receptors suggests that the steroid receptors and their signal transduction mechanisms have been strongly and broadly conserved.

Short-loop negative and positive feedback on ecdysone secretion by prothoracic gland in the tobacco hornworm, Manduca sexta

Ecdysteroid production by the prothoracic glands of the tobacco hornworm, Manduca sexta was found to be under feedback control by the ecdysteroids in hemolymph using both culture in vivo in diapausing pupae lacking the brain-corpora cardiaca-corpora allata complex and the prothoracic glands and culture in vitro. Prothoracic glands having relatively high activity in larvae, prepupae, or developing pupae were inhibited by ecdysone or 20-hydroxyecdysone. By contrast, prothoracic glands with low activity from feeding larvae, day 1 non-diapausing pupae and diapausing pupae were activated by both ecdysone and 20-hydroxyecdysone in vivo and in vitro. Dose-response studies on diapausing pupal glands showed that ecdysone was the most effective activator. These findings suggest that prothoracic glands are either stimulated or inhibited by ecdysone or 20-hydroxyecdysone, depending on both the secretory activity of the gland and the effective level of ecdysteroids in hemolymph. Thus, when the glands are first activated, the ecdysteroids that are secreted show a positive feedback on the glands to increase ecdysteroid output. Then the activated glands are turned off by the increasing 20-hydroxyecdysone titer in the hemolymph leading to the rapid decrease in ecdysteroid titer at the end of the molt period.

Effects of juvenile hormone on the ecdysone response of Drosophila Kc cells

Drosophila Kc cells are ecdysone-responsive: hormone treatment leads rapidly to increased synthesis of several ecdysone-inducible polypeptides (EIPs) and to commitment to eventual proliferative arrest. Later, the treated cells undergo morphological transformation, cease to proliferate, and develop new enzymatic activities, notably, acetylcholinesterase (AChE) activity. These responses have proven useful as models for studying ecdysone action. Here we report the sensitivity of Kc cells to another important insect developmental regulator--juvenile hormone (JH). We find that JH inhibits some, but not all, aspects of the ecdysone response. When Kc cells are treated with ecdysone in the presence of either natural JHs or synthetic analogues, the morphological and proliferative responses are inhibited and AChE induction is blocked. Most striking is that JHs protect the cells from the rapid proliferative commitment induced by ecdysone alone. The JH effects exhibit reasonable dose-response curves with half-maximal responses occurring at very low JH concentrations. Nonetheless, even at high JH concentrations the inhibitory effects are incomplete. It is interesting that EIP induction appears to be refractory to JH. It seems clear that JH is not simply a generalized inhibitor of ecdysone-induced responses.

Induction of process outgrowth in vertebrate and invertebrate cell lines by a 2-pyridinyl thiosemicarbazone

Regulation of differentiation in cells of disparate origin is often mediated by widely differing molecular signals and receptor mechanisms. For example, two neuron-like cell lines used extensively as models for molecular control of differentiation, the steroid-sensitive Kc line from Drosophila and the polypeptide- and cyclic nucleotide-sensitive PC12 line from rat, share no obvious growth factor or hormone receptors. However, we have found that a thiosemicarbazone, 1-pyrrolidinecarbothioic acid [1-(2-pyridinyl)ethylidene] hydrazide, one of a class of synthetic antineoplastic agents, induces process outgrowth - a marker of cellular differentiation - in cells of both of these lines. Moreover, the thiosemicarbazone induces process outgrowth in cells of mutant clones of these lines that are refractory to treatment with growth factors or hormones. Activity of the thiosemicarbazone is dependent upon the alpha-(N)-heterocyclic ring. These findings show that the 2-pyridinyl thiosemicarbazone mimics the effects of diverse epigenetic factors in inducing process outgrowth similar to that seen in cellular differentiation of these cell lines induced by natural regulators. Regulation may be by a mechanism, common to both invertebrate and vertebrate cells, which occurs downstream from the receptors that have been previously shown to mediate epigenetically induced differentiation.

RH 5849, a nonsteroidal ecdysone agonist: effects on a Drosophila cell line

The steroid molting hormone 20-hydroxyecdysone is the physiological inducer of molting and metamorphosis in insects. In ecdysone-sensitive Drosophila Kc cells, the insecticide RH 5849 (1,2-dibenzoyl-1-tert-butylhydrazine) mimics the action of 20-hydroxyecdysone by causing the formation of processes, an inhibition of cell proliferation, and induction of acetylcholinesterase. RH 5849 also competes with [3H]ponasterone A for high-affinity ecdysone receptor sites from Kc cell extracts. Resistant cell populations selected by growth in the continued presence of either RH 5849 or 20-hydroxyecdysone are insensitive to both compounds and exhibit a decreased titer of measurable ecdysone receptors. Although it is less potent than 20-hydroxyecdysone in both whole-cell and cell-free receptor assays, RH 5849 is the first nonsteroidal ecdysone agonist.

Ecdysteroid receptors of the blowfly Calliphora vicina: partial purification and characterization of ecdysteroid binding

A macromolecule with high affinity for the ecdysteroid analogue ponasterone A was isolated from nuclei of larvae of the blowfly Calliphora vicina. The ecdysteroid-binding molecule revealed characteristics of the moulting hormone receptor. It was sensitive towards protease but not towards nucleases. The nuclear protein had a limited binding capacity (0.2 pmol ponasterone A/mg protein), showed hormone analogue specificity and high affinity for ecdysteroids. Enzyme activities were present in the nuclear extract that metabolized ecdysteroids and thereby interfered with the binding assay. After their removal by DEAE-cellulose chromatography the ecdysteroid receptor preparation was stable at 20 degrees C for hours. This allowed a reliable determination of dissociation constants at equilibrium conditions. The hormone receptor complex had a KD of 1 nM, 30 nM, and 2000 nM with ponasterone A, 20-hydroxyecdysone, and ecdysone, respectively. The apparent molecular mass of the ecdysteroid receptor was 105,000 as determined by gel filtration.