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

Arthropod nuclear receptors and their role in molting

The molting process in arthropods is regulated by steroid hormones acting via nuclear receptor proteins. The most common molting hormone is the ecdysteroid, 20-hydroxyecdysone. The receptors of 20-hydroxyecdysone have also been identified in many arthropod species, and the amino acid sequences determined. The functional molting hormone receptors consist of two members of the nuclear receptor superfamily, namely the ecdysone receptor and the ultraspiracle, although the ecdysone receptor may be functional, in some instances, without the ultraspiracle. Generally, the ecdysone receptor/ultraspiracle heterodimer binds to a number of ecdysone response elements, sequence motifs that reside in the promoter of various ecdysteroid-responsive genes. In the ensuing transcriptional induction, the ecdysone receptor/ultraspiracle complex binds to 20-hydroxyecdysone or to a cognate ligand that, in turn, leads to the release of a corepressor and the recruitment of coactivators. 3D structures of the ligand-binding domains of the ecdysone receptor and the ultraspiracle have been solved for a few insect species. Ecdysone agonists bind to ecdysone receptors specifically, and ligand-ecdysone receptor binding is enhanced in the presence of the ultraspiracle in insects. The basic mode of ecdysteroid receptor action is highly conserved, but substantial functional differences exist among the receptors of individual species. Even though the transcriptional effects are apparently similar for ecdysteroids and nonsteroidal compounds such as diacylhydrazines, the binding shapes are different between them. The compounds having the strongest binding affinity to receptors ordinarily have strong molting hormone activity. The ability of the ecdysone receptor/ultraspiracle complex to manifest the effects of small lipophilic agonists has led to their use as gene switches for medical and agricultural applications.

The 20-hydroxyecdysone-induced signalling pathway in G2/M arrest of Plodia interpunctella imaginal wing cells

The mechanisms involved in the control of cellular proliferation by the steroid hormone 20-hydroxyecdysone (20E) in insects are not known. We dissected the 20E signalling pathway responsible for G2/M arrest of imaginal cells from the IAL-PID2 cells of the Indian meal moth Plodia interpunctella. We first used a 5'-3' RACE-based strategy to clone a 4479bp cDNA encoding a putative P. interpunctella HR3 transcription factor named PiHR3. The deduced amino acid sequence of PiHR3 was highly similar to those of HR3 proteins from other lepidopterans, e.g. Manduca sexta and Bombyx mori. Using double-stranded RNA-mediated interference (dsRNAi), we then succeeded in blocking the ability of 20E to induce the expression of PiEcR-B1, PiUSP-2 and PiHR3 genes that encode the P. interpunctella ecdysone receptor B1-isoform, Ultraspiracle-2 isoform, the insect homologue of the vertebrate retinoid X receptor, and the HR3 transcription factor. We showed that inhibiting the 20E induction of PiEcR-B1, PiUSP-2 and PiHR3 mRNAs prevented the decreased expression of B cyclin and consequently the G2/M arrest of IAL-PID2 cells. Using this functional approach, we revealed the participation of EcR, USP and HR3 in a 20E signalling pathway that controls the proliferation of imaginal cells by regulating the expression of B cyclin.

The Drosophila nuclear receptor e75 contains heme and is gas responsive

Nuclear receptors are a family of transcription factors with structurally conserved ligand binding domains that regulate their activity. Despite intensive efforts to identify ligands, most nuclear receptors are still "orphans." Here, we demonstrate that the ligand binding pocket of the Drosophila nuclear receptor E75 contains a heme prosthetic group. E75 absorption spectra, resistance to denaturants, and effects of site-directed mutagenesis indicate a single, coordinately bound heme molecule. A correlation between the levels of E75 expression and the levels of available heme suggest a possible role as a heme sensor. The oxidation state of the heme iron also determines whether E75 can interact with its heterodimer partner DHR3, suggesting an additional role as a redox sensor. Further, the E75-DHR3 interaction is also regulated by the binding of NO or CO to the heme center, suggesting that E75 may also function as a diatomic gas sensor. Possible mechanisms and roles for these interactions are discussed.

Comparative effects of a non-steroidal ecdysone agonist RH-5992 and 20-hydroxyecdysone in a lepidopteran cell line (IAL-PID2)

The non-steroidal ecdysone agonist, RH-5992, exhibits ecdysteroid activities in vivo as well as in vitro more effectively than 20-hydroxyecdysone (20E). Using the IAL-PID2 cells derived from imaginal wing discs of last larval instar of Plodia interpunctella, we investigated the action of RH-5992 in the control of cell growth. Its effects on the proliferative activity of IAL-PID2 cells, the induction level in G2/M arrest and on the expression rate of Plodia B cyclin (PcycB), ecdysone B1-isoform (PIEcR-B1) and Ultraspiracle-2 isoform (PIUSP-2) were examined. From these cellular and molecular assays, our results brought evidence that RH-5992, like 20E, induced an inhibition on cell proliferation by blocking IAL-PID2 cells in G2/M phase. Moreover, this G2/M arrest was preceded by a decrease in the expression level of PcycB and a high induction of PIEcR-B1, PIUSP-2 mRNAs. Dose-response experiments revealed that RH-5992 was even more potent than 20E. On these parameters, we therefore suggest that the differential observed in the expression level of USP and EcR by RH-5992 and 20E could contribute to the difference observed for the biological potency of these two compounds.

Cell cycle profiles of EcR, USP, HR3 and B cyclin mRNAs associated to 20E-induced G2 arrest of Plodia interpunctella imaginal wing cells

Using the IAL-PID2 cell line established from pupally committed imaginal wing discs of Plodia interpunctella, we have investigated the dynamics of cellular and molecular events involved in the G2/M arrest. We have first cloned a cDNA sequence named PIUSP-2 that likely encodes a homologue of the Ultraspiracle-2 isoform of Manduca sexta. When the IAL-PID2 cells were exposed to a 8 h 20E treatment applied at different times of the cell cycle, an optimal period of sensitivity of cells to 20E, in inducing G2 arrest, was determined at the S/G2 transition. Using cDNA probes specifically designed from Plodia B cyclin (PcycB), ecdysone receptor B1-isoform (PIEcR-B1) and HR3 transcription factor (PHR3), we provide evidence that the 20E-induced G2 arrest was correlated to a high induction of PHR3, PIEcR-B1, PIUSP-2 mRNAs at the S/G2 transition and a decrease in PcycB mRNA level at the end of G2 phase.

Effects of juvenile hormone on 20-hydroxyecdysone-inducible EcR, HR3, E75 gene expression in imaginal wing cells of Plodia interpunctella lepidoptera

The IAL-PID2 cells derived from imaginal wing discs of the last larval instar of Plodia interpunctella were responsive to 20-hydroxyecdysone (20E). These imaginal cells respond to 20E by proliferative arrest followed by a morphological differentiation. These 20E-induced late responses were inhibited in presence of juvenile hormone (JH II). From these imaginal wing cells, we have cloned a cDNA sequence encoding a P. interpunctella ecdysone receptor-B1 isoform (PIEcR-B1). The amino acid sequence of PIEcR-B1 showed a high degree of identity with EcR-B1 isoforms of Bombyx mori, Manduca sexta and Choristoneura fumiferana. The pattern of PIEcR-B1mRNA induction by 20E was characterized by a biphasic response with peaks at 2 h and 18 h. The presence of the protein synthesis inhibitor anisomycin induced a slight reduction in level of PIEcR-B1 mRNA and prevented the subsequent declines observed in 20E-treated cells. Therefore, PIEcR-B1 mRNA was directly induced by 20E and its downregulation depended on protein synthesis. An exposure of imaginal wing cells to 20E in the presence of JH II caused an increased expression of Plodia E75-B and HR3 transcription factors but inhibited the second increase of PIEcR-B1 mRNA. These findings showed that in vitro JH II was able to prevent the 20E-induced differentiation of imaginal wing cells. This effect could result from a JH II action on the 20E-induced genetic cascade through a modulation of EcR-B1, E75-B and HR3 expression.