Nuclear localization and DNA binding of ecdysone receptor and ultraspiracle

Mating induces switch from hormone-dependent to hormone-independent steroid receptor-mediated growth in Drosophila secondary cells

Male reproductive glands like the mammalian prostate and the paired Drosophila melanogaster accessory glands secrete seminal fluid components that enhance fecundity. In humans, the prostate, stimulated by environmentally regulated endocrine and local androgens, grows throughout adult life. We previously showed that in fly accessory glands, secondary cells (SCs) and their nuclei also grow in adults, a process enhanced by mating and controlled by bone morphogenetic protein (BMP) signalling. Here, we demonstrate that BMP-mediated SC growth is dependent on the receptor for the developmental steroid ecdysone, whose concentration is reported to reflect sociosexual experience in adults. BMP signalling appears to regulate ecdysone receptor (EcR) levels via one or more mechanisms involving the EcR's N terminus or the RNA sequence that encodes it. Nuclear growth in virgin males is dependent on ecdysone, some of which is synthesised in SCs. However, mating induces additional BMP-mediated nuclear growth via a cell type-specific form of hormone-independent EcR signalling, which drives genome endoreplication in a subset of adult SCs. Switching to hormone-independent endoreplication after mating allows growth and secretion to be hyperactivated independently of ecdysone levels in SCs, permitting more rapid replenishment of the accessory gland luminal contents. Our data suggest mechanistic parallels between this physiological, behaviour-induced signalling switch and altered pathological signalling associated with prostate cancer progression.

Physiological effect of mild thermal stress and its induction of gene expression in the common cutworm, Spodoptera litura

Heat shock protein (Hsp) and its cognate protein (Hsc) play important roles in helping insects survive extreme temperatures. However, high level of Hsp expression usually brings negative physiological effects on organisms. The mechanism of this trade-off is unclear. In this study, a lepidopteran insect, the common cutworm Spodoptera litura, was stressed at different temperatures, and the impact on both thermotolerance and fecundity was examined. The mRNA levels of four Hsp/Hscs (Hsp90, Hsc90, Hsp70 and Hsc70) and two ecdysone receptors (EcRs, EcRA and EcRB1) in different stresses and during the larval-pupal metamorphosis were determined. The results revealed that the pre-acclamation at mild stress increased the thermotolerance but decreased the egg production in adults. During the stress process, the mRNA levels of all the Hsp/Hsc and ecdysone receptor genes were significantly up-regulated. The two Hsp/Hsc70s and EcRs revealed consistent expression profiles with each other during the larval-pupal metamorphosis. Co-immunoprecipitation and Western blotting analysis indicated that Hsp/Hsc70 interacted with EcRs. RNAi of Hsc70 decreased the mRNA levels of two 20E-induced genes such as E74B and E75. Hsp70 transferred from the cytoplasm to nucleus in response to cold stress. These data together suggest that Hsp/Hsc70 might be involved in the regulation of 20E signaling, and the protein-protein interaction between Hsp/Hsc70 and EcRs probably act as a bridge mediating the trade-off between high thermotolerance and physiological defects.

JS-K, a glutathione/glutathione S-transferase-activated nitric oxide releasing prodrug inhibits androgen receptor and WNT-signaling in prostate cancer cells

BACKGROUND

Nitric oxide (NO) and its oxidative reaction products have been repeatedly shown to block steroid receptor function via nitrosation of zinc finger structures in the DNA-binding domain (DBD). In consequence NO-donors could be of special interest for the treatment of deregulated androgen receptor(AR)-signaling in castration resistant prostate cancer (CRPC).

METHODS

Prostate cancer (PCa) cells were treated with JS-K, a diazeniumdiolate derivate capable of generating large amounts of intracellular NO following activation by glutathione S-transferase. Generation of NO was determined indirectly by the detection of nitrate in tissue culture medium or by immunodetection of nitrotyrosine in the cytoplasm. Effects of JS-K on intracellular AR-levels were determined by western blotting. AR-dimerization was analyzed by mammalian two hybrid assay, nuclear translocation of the AR was visualized in PCa cells transfected with a green fluorescent AR-Eos fusion protein using fluorescence microscopy. Modulation of AR- and WNT-signalling by JS-K was investigated using reporter gene assays. Tumor cell proliferation following JS-K treatment was measured by MTT-Assay.

RESULTS

The NO-releasing compound JS-K was shown to inhibit AR-mediated reporter gene activity in 22Rv1 CRPC cells. Inhibition of AR signaling was neither due to an inhibition of nuclear import nor to a reduction in AR-dimerization. In contrast to previously tested NO-donors, JS-K was able to reduce the intracellular concentration of functional AR. This could be attributed to the generation of extremely high intracellular levels of the free radical NO as demonstrated indirectly by high levels of nitrotyrosine in JS-K treated cells. Moreover, JS-K diminished WNT-signaling in AR-positive 22Rv1 cells. In line with these observations, castration resistant 22Rv1 cells were found to be more susceptible to the growth inhibitory effects of JS-K than the androgen dependent LNCaP which do not exhibit an active WNT-signaling pathway.

CONCLUSIONS

Our results suggest that small molecules able to inhibit WNT- and AR-signaling via NO-release represent a promising platform for the development of new compounds for the treatment of CRPC.

Ecdysone signaling in adult Drosophila melanogaster

The steroid hormone 20-hydroxyecdysone and its EcR/USP receptor are vital during arthropod development for coordinating molting and metamorphosis. Traditionally, little attention has been given to potential post-developmental functions for this hormone signaling system. However, recent studies in Drosophila melanogaster indicate that the hormone and receptor are present and active in adults and that mutations decreasing hormone or receptor levels affect diverse processes such as reproduction, behavior, stress resistance, and lifespan. We review the current state of knowledge regarding adult hormone production and titers and discuss receptor expression and activity in order to identify potential mechanisms which explain the observed mutant phenotypes. Finally, we describe future research directions focused on identifying isoform-specific functions of EcR, distinguishing effects from EcR/USP gene activation and repression, and determining how ecdysone signaling impacts different tissue types.

Ecdysone receptor and ultraspiracle proteins are tyrosine phosphorylated during adult development of silkmoths

20-hydroxy ecdysone (20E) is essential to promote adult development in diapausing silkmoth pupae. Increases in protein tyrosine/serine-phosphorylations observed soon after 20E administration supported the initial hypothesis that activation of receptor tyrosine kinase-ras-MAPK pathway could be responsible for the growth promoting effects of 20E. This report pertains to the high levels of protein tyrosine phosphorylations (PTP) that occurred later during the growth to differentiation transition because of its novelty and relevance to 20E dependence of adult development. Further analyses demonstrated that both ecdysone receptor (EcR) and ultraspiracle (USP), the two dimerizing partners of the functional ecdysone receptor, are tyrosine phosphorylated coincidental with high PTP. Enhanced PTP during growth to differentiation transition and concomitant tyrosine phosphorylation of EcR and USP was shown to occur in another silkmoth species pointing to the necessity of similar protein tyrosine phosphorylation pathways for adult development. Properly timed increases in tissue protein tyrosine kinase (PTK) activity could explain the enhancement of PTP in the wing epidermis of both the silkmoths. Thymidine incorporation measurements showed that cessation of DNA synthesis preceded the increase in PTK activity thus emphasizing a role for PTP in aspects of tissue physiology related to differentiative events rather than cell proliferation. Phosphatase and tyrosine kinase inhibitors (Tyrphostins) had minimal effects on adult wing development in vivo. However, the escape of the adult from the pupal case was blocked by a tyrphostin indicating the importance of PTKs in eclosion.

Review of the reproductive biology of amphipods and their endocrine regulation: identification of mechanistic pathways for reproductive toxicants

The reproductive biology of amphipods is reviewed to update the knowledge of the male and female reproductive processes of oogenesis and spermatogenesis as well as the endocrine systems of amphipods with the aim of advancing studies of reproductive toxicology. The ovarian and reproduction cycles of female gammaridean amphipods are closely correlated with the molt cycle, which is under direct control by the steroid hormone 20-hydroxyecdysone. The ability of males to copulate and subsequently for females to ovulate is restricted to the early postmolt period of the females. New developments in our understanding of the molt cycle and the endocrine regulatory pathways for reproduction using genomics techniques on other crustacean species are also discussed. The arthropod sterol ponasterone A or xenobiotics such as the fungicide fenarimol have been shown to elicit endocrine disruption in some crustaceans by acting as an agonist for 20-hydroxyecdysone at the ecdysone receptor or by inhibiting the synthesis of 20-hydroxyecdysone, respectively, resulting in disruption of molting and reproduction. Recent studies suggest that cadmium can inhibit secondary vitellogenesis in amphipods. Experimental approaches for examining the metabolic pathways associated with ecdysteroid hormonal signaling or metabolism, exoskeleton maintenance and molting, and the regulation of vitellogenin in amphipods are discussed. This information should aid in the identification of useful biomarkers for reproductive toxicity.

Influence of helix 12 of Ultraspiracle on Drosophila melanogaster ecdysone receptor function

Although it has no ligand, helix 12 in the ligand binding domain of Ultraspiracle (USP) is locked in an antagonistic position. To investigate whether this position is of functional importance, we enhanced the flexibility of helix 12 by mutating two amino acids (259, located in L1-3 and F491 in helix 12). Mutated USP reduces the stability of USP and all isoforms of the ecdysone receptor (EcR) and impairs nuclear localization and DNA binding of EcR/USP(L259A/F491/A), resulting in lower levels of basal transcriptional activity. Although the affinity of the ligand ponasterone A to EcR/USP(L259/F491) is moderately diminished, hormone-induced stimulation of transcriptional activity is normal. Potentiation of the ecdysone response by juvenile hormone (JH) is selectively increased in mutated heterodimers with EcR-B1, demonstrating that the antagonistic position impairs functional interaction of the EcR complex with JHIII.

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.

DNA affects ligand binding of the ecdysone receptor of Drosophila melanogaster

The heterodimers of all three ecdysone receptor (EcR) isoforms with Ultraspiracle (Usp), the invertebrate orthologue of RXR, bind Ponasterone A with the same affinity in the absence of DNA. Ligand binding is stimulated by ecdysone response elements (EcREs) to different degrees depending on the receptor isoform, the heterodimerization partner, and the type of EcRE. Ligand binding to heterodimers with wtUsp is enhanced 5-fold with hsp27, Pal-1 and DR-1. In the presence of DNA substantial differences in ligand binding were observed, when the AB-domain of wtUsp is replaced by the N-terminus of VP16, which is routinely used for the determination of transcriptional activity to overcome the inhibitory action of the AB-domain of Usp. Enhanced dimerization in the presence of hormone response elements increases mainly the number of binding sites resulting in improved ligand binding, which is observed even if the C-domain of Usp is deleted. RXR, which can partially replace Usp function, confers high affinity ligand binding only in the presence of an EcRE.

Influence of hormone on intracellular localization of the Drosophila melanogaster ecdysteroid receptor (EcR)

In the absence of hormone the ecdysteroid receptor (EcR) is distributed between the cytoplasm and the nucleus. Addition of the hormone muristerone A increases nuclear localization of wild type EcR within 5-10 min. Mutation of M504 to alanine, an amino acid, which is essential for ligand binding and which is situated in helix 5 of the ligand binding domain, abolishes hormone binding but still allows nuclear localization at only slightly reduced levels in the absence of hormone, whereas nuclear localization of EcR(M504R) is nearly abolished. Cotransfection with ultraspiracle (USP), the invertebrate ortholog of RXR, leads to exclusively nuclear localization of wild type EcR and EcR(M504A) indicating that basal heterodimerization in the absence of hormone is still possible. In the presence of Usp, EcR(M504R) is only partially localized in the nucleus. EMSA experiments show that the ligand muristerone A enhances binding of wild type EcR, but only slightly of mutated EcRs, to the canonical hsp 27 ecdysone response element. This is confirmed by transactivation studies. The results indicate that the architecture of the E-domain of EcR is important for nuclear localization even in the absence of a ligand.

Nucleocytoplasmic shuttling of the ecdysteroid receptor (EcR) and of ultraspiracle (Usp) from Drosophila melanogaster in mammalian cells: energy requirement and interaction with exportin

The small G protein Ran, which is important for nucleocytoplasmic shuttling of proteins is present, but does not interact with EcR, Usp, and EcR/Usp. As shown by oligomycin treatment, EcR, Usp, and EcR/Usp import is energy dependent. Export of EcR and EcR/Usp is mediated by exportin-1 (CRM-1) as shown by the inhibiting effect of leptomycin B (LMB). Usp remains in the nucleus for more than 24 h. Nuclear retainment of EcR and Usp is energy dependent as shown by treatment with oligomycin. No export signal could be identified for Usp. The data confirm that EcR and Usp can enter the nucleus independently and that intracellular localization is regulated individually for each receptor. It is also demonstrated that the export signal of EcR is inaccessible after heterodimerization with Usp.