Activation of mammalian retinoid X receptors by the insect growth regulator methoprene

Activation of transcription through the ligand-binding pocket of the orphan nuclear receptor ultraspiracle

The invertebrate nuclear receptor, ultraspiracle (USP), an ortholog of the vertebrate RXR, is typically modelled as an orphan receptor that functions without a ligand-binding activity. The identification of a ligand that can transcriptionally activate USP would provide heuristic leads to the structure of potentially high affinity activating compounds, with which to detect unknown regulatory pathways in which this nuclear receptor participates. We show here that the application of the sesquiterpenoid methyl epoxyfarnesoate (juvenile hormone III) to Sf9 cells induces transcription from a transfected heterologous core promoter, through a 5'-placed DR12 enhancer to which the receptor ultraspiracle (USP) binds. Isolated, recombinant USP from Drosophila melanogaster specifically binds methyl epoxyfarnesoate, whereupon the receptor homodimerizes and changes tertiary conformation, including the movement of the ligand-binding domain alpha-helix 12. Ligand-binding pocket point mutants of USP that do not bind methyl epoxyfarnesoate act as dominant negative suppressors of methyl epoxyfarnesoate-activation of the reporter promoter, and addition of wild-type USP rescues this activation. These data establish a paradigm in which the USP ligand-binding pocket can productively bind ligand with a functional outcome of enhanced promoter activity, the first such demonstration for an invertebrate orphan nuclear receptor. USP thus establishes the precedent that invertebrate orphan receptors are viable targets for development of agonists and antagonists with which to discern and manipulate transcriptional pathways dependent on USP or other orphan receptors. The demonstration here of these functional capacities of USP in a transcriptional activation pathway has significant implications for current paradigms of USP action that do not include for USP a ligand-binding activity.

Cloning and sequence analysis of Galleria mellonella juvenile hormone binding protein--a search for ancestors and relatives

Juvenile hormone binding proteins (JHBPs) serve as specific carriers of juvenile hormone (JH) in insect hemolymph. As shown in this report, Galleria mellonella JHBP is encoded by a cDNA of 1063 nucleotides. The pre-protein consists of 245 amino acids with a 20 amino acid leader sequence. The concentration of the JHBP mRNA reaches a maximum on the third day of the last larval instar, and decreases five-fold towards pupation. Comparison of amino acid sequences of JHBPs from Bombyx mori, Heliothis virescens, Manduca sexta and G. mellonella shows that 57 positions out of 226 are occupied by identical amino acids. A phylogeny tree was constructed from 32 proteins, which function could be associated to JH. It has three major branches: (i) ligand binding domains of nuclear receptors, (ii) JHBPs and JH esterases (JHEs), and (iii) hypothetical proteins found in Drosophila melanogaster genome. Despite the close positioning of JHEs and JHBPs on the tree, which probably arises from the presence of a common JH binding motif, these proteins are unlikely to belong to the same family. Detailed analysis of the secondary structure modeling shows that JHBPs may contain a beta-barrel motif flanked by alpha-helices and thus be evolutionary related to the same superfamily as calycins.

Assessment of the risk of solar ultraviolet radiation to amphibians. I. Dose-dependent induction of hindlimb malformations in the northern leopard frog (Rana pipiens)

A number of environmental stressors have been hypothesized as responsible for recent increases in limb malformations in several species of North American amphibians. The purpose of this study was to generate dose-response data suitable for assessing the potential role of solar ultraviolet (UV) radiation in causing limb malformations in a species in which this phenomenon seemingly is particularly prevalent, the northern leopard frog (Rana pipiens). Frogs were exposed from early embryonic stages through complete metamorphosis to varying natural sunlight regimes, including unaltered (100%) sunlight, sunlight subjected to neutral density filtration to achieve relative intensities of 85%, 75%, 65%, 50%, and 25% of unaltered sunlight, and sunlight filtered with glass or acrylamide to attenuate, respectively, the UVB (290-320 nm) and UVB plus UVA (290-380 nm) portions of the spectrum. The experiments were conducted in a controlled setting, with continual monitoring of UVB, UVA, and visible light to support a robust exposure assessment. Full sunlight caused approximately 50% mortality of the frogs during early larval development; no significant treatment-related mortality occurred under any of the other exposure regimes, including 100% sunlight with glass or acrylamide filtration. There was a dose-dependent (p < 0.0001) induction of hindlimb malformations in the frogs, with the percentage of affected animals ranging from about 97% under unaltered sunlight to 0% in the 25% neutral density treatment. Malformations were comprised mostly of missing or truncated digits, and generally were bilateral as well as symmetrical. Filtration of sunlight with either glass or acrylamide both significantly reduced the incidence of malformed limbs. The estimated sunlight dose resulting in a 50% limb malformation rate (ED50) was 63.5%. The limb ED50 values based on measured sunlight intensities corresponded to average daily doses of 4.5 and 100 Wh x m(-2) for UVB and UVA, respectively. Exposure to sunlight also resulted in increased eye malformations in R. pipiens, however, the dose-response relationship for this endpoint was not monotonic. The results of this study, in conjunction with measured or predicted exposure data from natural settings, provide a basis for quantitative prediction of the risk of solar UV radiation to amphibians.

Molecular recognition of agonist ligands by RXRs

The nuclear receptor RXR is an obligate partner in many signal transduction pathways. We report the high-resolution structures of two complexes of the human RXRalpha ligand-binding domain specifically bound to two different and chemically unrelated agonist compounds: docosa hexaenoic acid, a natural derivative of eicosanoic acid, present in mammalian cells and recently identified as a potential endogenous RXR ligand in the mouse brain, and the synthetic ligand BMS 649. In both structures the RXR-ligand-binding domain forms homodimers and exhibits the active conformation previously observed with 9-cis-RA. Analysis of the differences in ligand-protein contacts (predominantly van der Waals forces) and binding cavity geometries and volumes for the several agonist-bound RXR structures clarifies the structural features important for ligand recognition. The L-shaped ligand-binding pocket adapts to the diverse ligands, especially at the level of residue N306, which might thus constitute a new target for drug-design. Despite its highest affinity 9-cis-RA displays the lowest number of ligand-protein contacts. These structural results support the idea that docosa hexaenoic acid and related fatty acids could be natural agonists of RXRs and question the real nature of the endogenous ligand(s) in mammalian cells.

Characterization of crab EcR and RXR homologs and expression during limb regeneration and oocyte maturation

We report here complete coding sequences for the Uca pugilator homologs of the ecdysteroid (UpEcR) and retinoid-X receptors (UpRXR). Library screenings recovered cDNA clones containing a unique amino terminal open-reading frame (A/B domain) for each gene, most similar to insect B1 EcR and USP1/RXR isoforms. Splicing variants in the UpRXR ligand-binding domain were also identified, in a region critical for folding of Drosophila and lepidopteran USP. UpEcR and UpRXR proteins were able to associate, and both are required for binding to an ecdysteroid HRE; these interactions were not hormone-dependent. Ribonuclease protection assays (RPA) were conducted using A/B domain and 'common' (C or E) domain probes on RNA isolated from various stages of regenerating limb buds and ovaries. For several of the limb bud and ovarian stages examined, the relative level of A/B domain sequence protected was significantly less than common domain suggesting alternative amino terminal isoforms other than those isolated through cloning. This is the first report of UpEcR and UpRXR transcription during ovarian maturation, implicating the ovary as a potential target for hormonal control in Crustacea.

Juvenile hormone III-dependent conformational changes of the nuclear receptor ultraspiracle

The identification of potential endogenous or synthetic ligands for orphan receptors in the steroid receptor superfamily is important both for discerning endogenous regulatory pathways and for designing receptor inhibitors. The insect nuclear receptor Ultraspiracle (USP), an ortholog of vertebrate RXR, has long been treated as an orphan receptor. We have tested here the fit of terpenoid ligands to the JH III-binding site of monomeric and homo-oligomeric USP from Drosophila melanogaster (dUSP). dUSP specifically bound juvenile hormone III (JH III), but not control farnesol or JH III acid, and also specifically changed in conformation upon binding of JH III in a fluorescence binding assay. Juvenile hormone III binding caused intramolecular changes in receptor conformation, and stabilized the receptor's dimeric/oligomeric quaternary structure. In both a radiometric competition assay and the fluorescence binding assay the synthetic JH III agonist methoprene specifically competed with JH III for binding to dUSP, the first demonstration of specific binding of a biologically active JH III analog to an insect nuclear receptor. The recombinant dUSP bound with specificity to a DR12 hormone response element in a gel shift assay. The same DR12 element conferred enhanced transcriptional responsiveness of a transfected juvenile hormone esterase core promoter to treatment of transfected cells with JH III, but not to treatment with retinoic acid or T3. The activity of JH III or JH III-like structures, but not structures without JH III biological activity, to bind specifically to dUSP and activate its conformational change, provide evidence of a terpenoid endogenous ligand for Ultraspiracle, and offer the prospect that synthetic, terpenoid structures may be discovered that can agonize or antagonize USP function in vivo.

Frog decline, frog malformations, and a comparison of frog and human health

The decline in frog populations and the increase in the frequency of frog malformations are discussed. Topics considered for analysis include chytridiomycosis, retinoids, UV-B radiation, chemical contaminants, environmental threats, introduced invasive species and predation, unsustainable use, and enigmatic decline. Care must be taken to distinguish between hypotheses, laboratory experiments, and the findings in feral frog populations. Clearly, the causes of population decline and malformations are heterogeneous. The subject of frogs and humans is addressed under three subheadings: the importance of frogs to human societies, medical implications of frog studies, and a comparison of frog and human disease factors.

Quantitative axial profiles of retinoic acid in the embryonic mouse spinal cord: 9-cis retinoic acid only detected after all-trans-retinoic acid levels are super-elevated experimentally

Studies using bioassays in normal mice and gene activation in transgenic reporter mice have demonstrated peaks of retinoic acid receptor (RAR) signaling in the brachial and lumbar regions of the spinal cord. Recently, Solomin et al. (Solomin et al. [1998] Nature 395:398-402) detected a retinoid X receptor (RXR) signal in the same region of the developing spinal cord at a slightly later stage than the RAR signal. This finding raises the question of which retinoid ligands underlie RAR and RXR signaling in this part of the embryo. Quantitative measurements of regional differences in retinoid profiles have not been reported previously due to limitation in the sensitivity and specificity of available retinoid detection methods. Here, by using a recently developed ultrasensitive HPLC technique (Sakhi et al. [1998] J. Chromatogr. A 828:451-460), we address this question in an attempt to identify definitively the endogenous retinoids present in different regions of the spinal cord at the stages when regional differences in RAR and RXR signaling have been reported. We find a bimodal distribution of all-trans retinoic acid (at-RA), the ligand for RARs, and relate this to the expression of several retinoid-synthesizing enzymes. However, we do not detect 9-cis-retinoic acid (9-cis-RA), the putative RXR ligand, in any region of the spinal cord unless retinoid levels are massively increased experimentally by gavage feeding pregnant mice with teratogenic doses of at-RA. This study provides for the first time quantitative profiles of endogenous retinoids along the axis of the developing spinal cord, thereby establishing a foundation for more definitive studies of retinoid function in the future. It sets definite limits on how much 9-cis-RA potentially is present and demonstrates that at-RA predominates over 9-cis-RA by at least 30- to 180-fold in different spinal cord regions.

Development of a retinoic acid receptor-binding assay with rainbow trout tissue: characterization of retinoic acid binding, receptor tissue distribution, and developmental changes

Retinoic acid (RA) regulates the transcription of various genes required for several essential functions in vertebrates through binding to two classes of nuclear receptors, the retinoic acid receptors (RAR) and retinoid X receptors (RXR). We investigated nuclear RA binding in tissues from rainbow trout using the radiolabeled all-trans and 9-cis isomers of RA. Specific binding (indicative of receptor binding) of both all-trans- and 9-cis-RA was found in all tissues tested, including the adult trout ovary, testis, gill, liver, kidney, blood, white muscle, and heart. The kinetics and absolute amount of RA binding were dependent on both the tissue and the isomer of RA used. All-trans-RA bound with high affinity (K(d) approximately 1.0-3.9 nM), and low capacity (B(max) approximately 75-484 fmol RA/mg protein), while 9-cis-RA bound with lower affinity (K(d) approximately 7-56 nM), but with a greater capacity (B(max) approximately 214-1076 fmol RA/mg protein). The B(max) results were used to estimate RAR and RXR levels and revealed that the gill possesses primarily RARs while the liver possesses primarily RXRs. The RAR-specific competitor TTNPB was able to effectively displace all-trans-[3H]RA in most tissues, and the RXR-specific competitor AGN 194204 was able to effectively displace 9-cis-[3H]RA. However, TTNPB and AGN 194204 could not displace all of the RA in the kidney and testis, suggesting the existence of another nuclear RA binding protein. Binding of all-trans- and 9-cis-RA was also found in developing trout embryos and fry. Kinetic analysis revealed that RAR levels predominated at the eyed-embryo stage, but decreased 87% by the swim-up fry stage, while RXR levels remained relatively constant over the same time period. These findings suggest that RA and its receptors may play a key role in early trout development. This study has provided a simple and rapid radioligand binding assay that can identify RAR and RXRs in trout tissues.

Environmental signaling: what embryos and evolution teach us about endocrine disrupting chemicals

The term "endocrine disrupting chemicals" is commonly used to describe environmental agents that alter the endocrine system. Laboratories working in this emerging field-environmental endocrine research-have looked at chemicals that mimic or block endogenous vertebrate steroid hormones by interacting with the hormone's receptor. Environmental chemicals known to do this do so most often with receptors derived from the steroid/thyroid/retinoid gene family. They include ubiquitous and persistent organochlorines, as well as plasticizers, pharmaceuticals, and natural hormones. These chemicals function as estrogens, antiestrogens, and antiandrogens but have few, if any, structural similarities. Therefore, receptor-based or functional assays have the best chance of detecting putative biological activity of environmental chemicals. Three nuclear estrogen receptor forms-alpha, beta, and gamma-as well as multiple membrane forms and a possible mitochondrial form have been reported, suggesting a previously unknown diversity of signaling pathways available to estrogenic chemicals. Examples of environmental or ambient estrogenization occur in laboratory experiments, zoo animals, domestic animals, wildlife, and humans. Environmentally estrogenized phenotypes may differ depending upon the time of exposure-i.e., whether the exposure occurred at a developmental (organizational and irreversible) or postdevelopmental (activational and reversible) stage. The term "estrogen" must be defined in each case, since steroidal estrogens differ among themselves and from synthetic or plant-derived chemicals. An "estrogen-like function" seems to be an evolutionarily ancient signal that has been retained in a number of chemicals, some of which are vertebrate hormones. Signaling, required for symbiosis between plants and bacteria, may be viewed, therefore, as an early example of hormone cross-talk. Developmental feminization at the structural or functional level is an emerging theme in species exposed, during embryonic or fetal life, to estrogenic compounds. Human experience as well as studies in experimental animals with the potent estrogen diethylstilbestrol provide informative models. Advances in the molecular genetics of sex differentiation in vertebrates facilitate mechanistic understanding. Experiments addressing the concept of gene imprinting or induction of epigenetic memory by estrogen or other hormones suggest a link to persistent, heritable phenotypic changes seen after developmental estrogenization, independent of mutagenesis. Environmental endocrine science provides a new context in which to examine the informational content of ecosystem-wide communication networks. As common features come to light, this research may allow us to predict environmentally induced alterations in internal signaling systems of vertebrates and some invertebrates and eventually to explicate environmental contributions to human reproductive and developmental health.

Ecological developmental biology: developmental biology meets the real world

The production of phenotype is regulated by differential gene expression. However, the regulators of gene expression need not all reside within the embryo. Environmental factors, such as temperature, photoperiod, diet, population density, or the presence of predators, can produce specific phenotypes, presumably by altering gene-expression patterns. The field of ecological developmental biology seeks to look at development in the real world of predators, competitors, and changing seasons. Ecological concerns had played a major role in the formation of experimental embryology, and they are returning as the need for knowledge about the effects of environmental change on embryos and larvae becomes crucial. This essay reviews some of the areas of ecological developmental biology, concentrating on new studies of amphibia and Homo.

Transgenic zebrafish as sentinels for aquatic pollution

Using the golden mutant zebrafish having a decrease in interfering pigmentation, we are developing transgenic lines in which DNA motifs that respond to selected environmental pollutants are capable of activating a reporter gene that can be easily assayed. We have begun with three response elements that recognize three important classes of foreign chemicals. Aromatic hydrocarbon response elements (AHREs) respond to numerous polycyclic hydrocarbons and halogenated coplanar molecules such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dioxin) and polychlorinated biphenyls. Electrophile response elements (EPREs) respond to quinones and numerous other potent electrophilic oxidants. Metal response elements (MREs) respond to heavy metal cations such as mercury, copper, nickel, cadmium, and zinc. Soon, we will include estrogen response elements (EREs) to detect the effects of environmental endocrine disruptors, and retinoic acid response elements (RARE, RXRE) to detect the effects of retinoids in the environment. Each of these substances is known to be bioconcentrated in fish to varying degrees; for example, 10(-17) M TCDD in a body of water becomes concentrated to approximately 10(-12) M TCDD in a fish, where it would act upon the AHRE motif and turn on the luciferase (LUC) reporter gene. The living fish as a sentinel will not only be assayed intact in the luminometer, but--upon several days or weeks of depuration--would be usable again. To date, we have established that zebrafish transcription factors are able to recognize both mammalian and trout AHRE, EPRE, and MRE sequences in a dose-dependent and chemical-class-specific manner, and that expression of both the LUC and jellyfish green fluorescent protein (GFP) reporter genes is easily detected in zebrafish cell cultures and in the intact live zebrafish. Variations in sensitivity of this model system can be achieved by increasing the copy number of response elements and perhaps by altering the sequence of each core consensus response element and flanking regions. This transgenic technology should allow for a simple, exquisitely sensitive, and inexpensive assay for monitoring aquatic pollution. We have already initiated studies using sentinel zebrafish to monitor a public drinking water source.

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.

Regulation of retinoic acid signaling in the embryonic nervous system: a master differentiation factor

This review describes some of the properties of retinoic acid (RA) in its functions as a locally synthesized differentiation factor for the developing nervous system. The emphasis is on the characterization of the metabolic enzymes that synthesize and inactivate RA, and which determine local RA concentrations. These enzymes create regions of autocrine and paracrine RA signaling in the embryo. One mechanism by which RA can act as a differentiation agent is through the induction of growth factors and their receptors. Induction of growth factor receptors in neural progenitor cells can lead to growth factor dependency, and the consequent developmental fate of the cell will depend on the local availability of growth factors. Because RA activates the early events of cell differentiation, which then induce context-specific differentiation programs, RA may be called a master differentiation factor.

Steroid regulation of postembryonic development and reproduction in Drosophila

Ecdysteroids trigger major developmental transitions in Drosophila, including larval molts and metamorphosis. Recent genetic studies strongly support a role for the Ecdysteroid receptor (EcR)/Ultraspiracle (USP) heterodimer as an ecdysteroid receptor at the onset of metamorphosis, functioning as both a transcriptional activator and repressor in vivo. Genetic analysis also indicates that USP, like its vertebrate homolog retinoid X receptor (RXR), might be involved in regulatory pathways independently of EcR. The ecdysteroid hierarchy was also shown recently to regulate Drosophila oogenesis and reproduction.

The isolation of two juvenile hormone-inducible genes in Drosophila melanogaster

Juvenile hormone (JH) is an important regulator of both insect development and reproductive maturation. Although the molecular mechanism of JH action is not yet known, there is growing circumstantial evidence that JH directly regulates gene expression. In the absence of a JH target gene, however, this suggestion has remained speculative. Cultured Drosophila S2 cells have been used to identify genes whose expression is regulated by JH. Employing differential display we identified several genes whose transcripts accumulate in cells treated with the JH agonist methoprene. Two of the genes-JhI-1 and JhI-26-were cloned and characterized in detail. For both genes, transcripts showed rapid and specific induction in the presence of either methoprene or JHIII, but not in the presence of other biologically inactive compounds of similar chemical structure. Accumulation of JhI-1 and JhI-26 RNAs requires continuous hormone presence. The developmental expression of the two JH-inducible genes corresponds to the abundance profile of JH in vivo. Furthermore, topical methoprene application to pupae leads to the ectopic accumulation of JhI-1 and JhI-26 transcripts.

Considerations on the structural evidence of a ligand-binding function of ultraspiracle, an insect homolog of vertebrate RXR

This analysis considers the structural evidence of a ligand-binding function of the nuclear receptor ultraspiracle (USP). The positions and nature of residues in the ligand-binding domain of USP from six higher insects is evaluated in comparison to the function of conserved residues vertebrate receptors that have been co-crystallized with ligand. USP appears to conserve residues that in vertebrate receptors (1) form the hydrophobic ligand-binding pocket, (2) contact oxygen-containing moieties on ligands, such as hydroxyl, keto and carboxyl groups, and (3) in response to ligand-binding conformationally change to form a multi-helix hydrophobic groove for recruitment of transcriptional co-activators. These structural features are consistent with the recent report that USP can bind the epoxymethylfarnesoates (juvenile hormones) and thereupon is induced to change conformation.

The modes of action of juvenile hormones: some questions we ought to ask

This paper argues that the current dogma that juvenile hormones are structurally unique and constitute a family of derivatives of farnesoic acid which are produced by the corpus allatum (CA), secreted into the hemolymph, frequently transported by binding proteins, enter cells by diffusion across the cell membrane and there the products of the CA interact in some way with the genome, probably via nuclear receptors of the steroid superfamily, may not be tenable. It does so by examining the following questions. How many JHs are there? Are there other sources of JH in insects? Are there non-farnesoids with JH activity in insects? How does JH get into cells? Is the product of the CA the effective hormone? How many modes of action are there? How many receptors are there?

Both retinoic-acid-receptor- and retinoid-X-receptor-dependent signalling pathways mediate the induction of the brown-adipose-tissue-uncoupling-protein-1 gene by retinoids

The intracellular pathways and receptors mediating the effects of retinoic acid (RA) on the brown-fat-uncoupling-protein-1 gene (ucp-1) have been analysed. RA activates transcription of ucp-1 and the RA receptor (RAR) is known to be involved in this effect. However, co-transfection of an expression vector for retinoid-X receptor (RXR) increases the action of 9-cis RA but not the effects of all-trans RA on the ucp-1 promoter in brown adipocytes. Either RAR-specific ¿p-[(E)-2-(5,6,7,8,-tetrahydro-5,5,8, 8-tetramethyl-2-naphthalenyl)-1-propenyl]benzoic acid¿ or RXR-specific [isopropyl-(E,E)-(R,S)-11-methoxy-3,7, 11-trimethyldodeca-2,4-dienoate, or methoprene] synthetic compounds increase the expression of UCP-1 mRNA and the activity of chloramphenicol acetyltransferase expression vectors driven by the ucp-1 promoter. The RXR-mediated action of 9-cis RA requires the upstream enhancer region at -2469/-2318 in ucp-1. During brown-adipocyte differentiation RXRalpha and RXRgamma mRNA expression is induced in parallel with UCP-1 mRNA, whereas the mRNA for the three RAR subtypes, alpha, beta and gamma, decreases. Co-transfection of murine expression vectors for the different RAR and RXR subtypes indicates that RARalpha and RARbeta as well as RXRalpha are the major retinoid-receptor subtypes capable of mediating the responsiveness of ucp-1 to retinoids. It is concluded that the effects of retinoids on ucp-1 transcription involve both RAR- and RXR-dependent signalling pathways. The responsiveness of brown adipose tissue to retinoids in vivo relies on a complex combination of the capacity of RAR and RXR subtypes to mediate ucp-1 induction and their distinct expression in the differentiated brown adipocyte.

Both retinoic-acid-receptor- and retinoid-X-receptor-dependent signalling pathways mediate the induction of the brown-adipose-tissue-uncoupling-protein-1 gene by retinoids

The intracellular pathways and receptors mediating the effects of retinoic acid (RA) on the brown-fat-uncoupling-protein-1 gene (ucp-1) have been analysed. RA activates transcription of ucp-1 and the RA receptor (RAR) is known to be involved in this effect. However, co-transfection of an expression vector for retinoid-X receptor (RXR) increases the action of 9-cis RA but not the effects of all-trans RA on the ucp-1 promoter in brown adipocytes. Either RAR-specific ¿p-[(E)-2-(5,6,7,8,-tetrahydro-5,5,8, 8-tetramethyl-2-naphthalenyl)-1-propenyl]benzoic acid¿ or RXR-specific [isopropyl-(E,E)-(R,S)-11-methoxy-3,7, 11-trimethyldodeca-2,4-dienoate, or methoprene] synthetic compounds increase the expression of UCP-1 mRNA and the activity of chloramphenicol acetyltransferase expression vectors driven by the ucp-1 promoter. The RXR-mediated action of 9-cis RA requires the upstream enhancer region at -2469/-2318 in ucp-1. During brown-adipocyte differentiation RXRalpha and RXRgamma mRNA expression is induced in parallel with UCP-1 mRNA, whereas the mRNA for the three RAR subtypes, alpha, beta and gamma, decreases. Co-transfection of murine expression vectors for the different RAR and RXR subtypes indicates that RARalpha and RARbeta as well as RXRalpha are the major retinoid-receptor subtypes capable of mediating the responsiveness of ucp-1 to retinoids. It is concluded that the effects of retinoids on ucp-1 transcription involve both RAR- and RXR-dependent signalling pathways. The responsiveness of brown adipose tissue to retinoids in vivo relies on a complex combination of the capacity of RAR and RXR subtypes to mediate ucp-1 induction and their distinct expression in the differentiated brown adipocyte.

Teratology of retinoids

Either an excess or a deficiency of vitamin A and related compounds (retinoids) causes abnormal morphological development (teratogenesis). Potential retinoid sources come from dietary intake, nutritional supplements, and some therapeutic drugs. Therefore, understanding the mechanisms of retinoid teratogenesis is important. This review first gives an overview of the principles of teratology as they apply to retinoid-induced malformations. It then describes relevant aspects of the biochemical pathway and signal transduction of retinoids. The teratogenic activity of various retinoid compounds, the role of the retinoid receptors, and important toxicokinetic parameters in teratogenesis are reviewed.

Differential gene expression between developing queens and workers in the honey bee, Apis mellifera

Many insects show polyphenisms, or alternative morphologies, which are based on differential gene expression rather than genetic polymorphism. Queens and workers are alternative forms of the adult female honey bee and represent one of the best known examples of insect polyphenism. Hormonal regulation of caste determination in honey bees has been studied in detail, but little is known about the proximate molecular mechanisms underlying this process, or any other such polyphenism. We report the success of a molecular-genetic approach for studying queen- and worker-specific gene expression in the development of the honey bee (Apis mellifera). Numerous genes appear to be differentially expressed between the two castes. Seven differentially expressed loci described here belong to at least five distinctly different evolutionary and functional groups. Two are particularly promising as potential regulators of caste differentiation. One is homologous to a widespread class of proteins that bind lipids and other hydrophobic ligands, including retinoic acid. The second locus shows sequence similarity to a DNA-binding domain in the Ets family of transcription factors. The remaining loci appear to be involved with downstream changes inherent to queen- or worker-specific developmental pathways. Caste determination in honey bees is typically thought of as primarily queen determination; our results make it clear that the process involves specific activation of genes in workers as well as in queens.

Morphological clues from multilegged frogs: are retinoids to blame?

Morphological analysis was performed on multilegged deformed frogs representing five species from 12 different localities in California, Oregon, Arizona, and New York. The pattern of duplicated limbs was consistent with mechanical perturbation by trematode infestation but not with the effects of retinoids.

Dorsal and ventral retinal territories defined by retinoic acid synthesis, break-down and nuclear receptor expression

Determination of the dorso-ventral dimension of the vertebrate retina is known to involve retinoic acid (RA), in that high RA activates expression of a ventral retinaldehyde dehydrogenase and low RA of a dorsal dehydrogenase. Here we show that in the early eye vesicle of the mouse embryo, expression of the dorsal dehydrogenase is preceded by, and transiently overlaps with, the RA-degrading oxidase CYP26. Subsequently in the embryonic retina, CYP26 forms a narrow horizontal boundary between the dorsal and ventral dehydrogenases, creating a trough between very high ventral and moderately high dorsal RA levels. Most of the RA receptors are expressed uniformly throughout the retina except for the RA-sensitive RARbeta, which is down-regulated in the CYP26 stripe. The orphan receptor COUP-TFII, which modulates RA responses, colocalizes with the dorsal dehydrogenase. The organization of the embryonic vertebrate retina into dorsal and ventral territories divided by a horizontal boundary has parallels to the division of the Drosophila eye disc into dorsal, equatorial and ventral zones, indicating that the similarities in eye morphogenesis extend beyond single molecules to topographical patterns.

Ligand regulation of juvenile hormone binding protein mRNA in mutant Manduca sexta

Insect hemolymph juvenile hormone binding protein (hJHBP) regulates peripheral titers of its ligands, the juvenile hormones. In larvae of the black (bl) strain of the tobacco hornworm, Manduca sexta, treatment with small doses of juvenile hormone I (JH I) can also regulate titers of hJHBP. To further investigate this regulation, responsiveness of hJHBP mRNA expression to JH I was characterized in vivo. RNA analyzes revealed that transcript levels in fat body, the site of hJHBP synthesis, increased fivefold within several hours of treatment with physiological doses of hormone and remained elevated for approximately 16 h. Sensitivity to JH treatment was found to vary temporally. To ensure transcript identity, a wild-type cDNA clone and a bl RT-PCR fragment were sequenced and found to be 99% homologous. Together, these results suggest that JH participates in regulating expression of its transport protein in bl larvae by modifying the in vivo abundance of hJHBP's mRNA transcript.

Lipid soluble vitamins in gene regulation

Vitamin A (retinol) and vitamin D are lipid soluble vitamins that are precursors of the nuclear hormones all-trans retinoic acid (RA) and 1alpha,25-dihydroxyvitamin D3 (VD) that bind with high affinity to their cognate nuclear receptors, referred to as retinoic acid receptor (RAR) and vitamin D receptor (VDR). Both types of nuclear receptors are structurally related and belong to the same subclass of the nuclear receptor superfamily, a large family of ligand-inducible transcription factors. Both RAR and VDR form heterodimers preferentially with the nuclear receptor for 9-cis RA, referred to as the retinoid X receptor (RXR), but functional RAR-VDR heterodimers have also been observed. Moreover, both types of nuclear receptors interact in a ligand-dependent fashion with members of the same class of co-activator, co-repressor and co-integrator proteins. These similar molecular mechanisms of action provide several possibilities for an interaction of RARs with VDR that are all based on allosteric protein-protein interactions. These interactions can result in either an additive or a transrepressive functional interference between RA and VD. The two remaining lipid soluble vitamins, vitamins E and K, are not known to interact with nuclear receptors, but their structure does not exclude this possibility. Moreover, for vitamin E modulatory effects on transcription factors, such as AP-1, have been described. This review will discuss briefly gene regulation by the four lipid soluble vitamins.

9-cis retinoic acid induces the expression of the uncoupling protein-2 gene in brown adipocytes

The expression of uncoupling protein-2 (UCP2) mRNA is up-regulated during the differentiation of brown adipocytes in primary culture. When differentiation of brown adipocytes is impaired, UCP2 mRNA expression is down-regulated. 9-cis Retinoic acid causes a dose-dependent induction of UCP2 mRNA levels in brown adipocytes, whereas all-trans retinoic acid has no effect. Specific agonists of retinoid X receptors (RXR) induce UCP2 mRNA expression, whereas specific activators of retinoic acid receptors do not. 9-cis Retinoic acid, acting through RXR receptors, is identified as a major regulator of the expression of the UCP2 gene in the brown fat cell.

Insecticide resistance resulting from an absence of target-site gene product

Genetic changes in insects that lead to insecticide resistance include point mutations and up-regulation/amplification of detoxification genes. Here, we report a third mechanism, resistance caused by an absence of gene product. Mutations of the Methoprene-tolerant (Met) gene of Drosophila melanogaster result in resistance to both methoprene, a juvenile hormone (JH) agonist insecticide, and JH. Previous results have demonstrated a mechanism of resistance involving an intracellular JH binding protein that has reduced ligand affinity in Met flies. We show that a gamma-ray induced allele, Met27, completely lacks Met transcript during the insecticide-sensitive period in development. Although Met27 homozygotes have reduced oogenesis, they are viable, demonstrating that Met is not a vital gene. Most target-site resistance genes encode vital proteins and thus have few mutational changes that permit both resistance and viability. In contrast, resistance genes such as Met that encode nonvital insecticide target proteins can have a variety of mutational changes that result in an absence of functional gene product and thus should show higher rates of resistance evolution.

Action of estradiol-17beta on the synthetic activity of the silk gland in Bombyx mori L

The effects of estradiol-17beta (E(2)) were studied on several metabolic parameters in the silk gland of Bombyx mori L. race Nistari. Topical application of different doses (0.05-4.0&mgr;g/g body weight) of E(2) on the first and second day of the fifth instar larvae showed a dose dependent effect when studied on the fifth day. A significant increase in silk gland weight and fibroin content was observed between the doses 0.05 and 0.1, and 0.1 and 1.0&mgr;g/g of E(2). A similar pattern of dose-dependent rise in DNA and RNA content of posterior silk gland (PSG) was observed with the doses of E(2) when the contents were expressed per pair of PSG. Higher doses of E(2) (2.0 or 4.0&mgr;g/g) demonstrated relatively less increase, unchanged level or a decrease in the above parameters in comparison to the control values. The glutamate-pyruvate transaminase of PSG showed a significant increase from 0.1 to 2.0&mgr;g/g of E(2) doses in comparison to the control value. Simultaneous injection of ICI-182780 (1.0&mgr;g/g), a very pure and specific antiestrogenic compound, with E(2) (1.0&mgr;g/g) caused a significant counteraction of E(2)-induced increase in silk gland activity, which was reflected in DNA and RNA content of PSG, wet weight and fibroin content of silk gland, and on glutamate-pyruvate transaminase activity. Cycloheximide (0.5&mgr;g/g), a protein synthesis blocker, caused a significant inhibition of the E(2) (1.0&mgr;g/g)-induced silk gland activity when treated along with estradiol. From this study it appears that estradiol has a specific effect on silk gland function and that it may act in a nuclear mediated way.

Environmental signaling: a biological context for endocrine disruption

Endogenous and exogenous chemical signals have evolved as a means for organisms to respond to physical or biological stimuli in the environment. Sensitivity to these signals can make organisms vulnerable to inadvertent signals from xenobiotics. In this review we discuss how various chemicals can interact with steroid-like signaling pathways, especially estrogen. Numerous compounds have estrogenic activity, including steroids, phytoestrogens, and synthetic chemicals. We compare bioavailability, metabolism, interaction with receptors, and interaction with cell-signaling pathways among these three structurally diverse groups in order to understand how these chemicals influence physiological responses. Based on their mechanisms of action, chemical steroid mimics could plausibly be associated with recent adverse health trends in humans and animals.

Juvenile hormone acid: evidence for a hormonal function in induction of vitellogenin in larvae of Manduca sexta

In the tobacco hornworm (Manduca sexta), vitellogenin (Vg), the major yolk protein precursor, and its mRNA are first detectable in the prepupal stage; and production of both can be enhanced by methoprene, a juvenile hormone (JH) analog. Competence to respond to methoprene is acquired after ecdysteroid-initiated commitment for metamorphosis. Here we show that acquisition of competence requires prior exposure to JH-II acid in addition to ecdysteroid. Application of 20-hydroxyecdysone or RH5992, an ecdysteroid analog, to isolated abdomens from feeding larvae (precommitment) results in exposure of the dorsal vessel (EDV), a sign of metamorphic commitment--but such abdomens do not make Vg in response to methoprene. However, injection of JH-II acid along with 20-hydroxyecdysone into isolated abdomens causes Vg production in response to methoprene. Methoprene acid similarly induces competence to respond to methoprene. Northern blot analysis confirmed that Vg transcripts are present in fat body only if isolated abdomens were pretreated with both ecdysteroid, and JH-II acid or methoprene acid. The latter two can induce competence even in precocious prepupae resulting from removal of the corpora allata (the glands that produce JH) from early penultimate larvae. JH-III acid and related metabolites such as farnesol, farnesoic acid, and methyl farnesoate do not induce competence. Hitherto, JH acids have been regarded as precursors or catabolites of JHs. Here we show for the first time that JH acid has a hormonal function that cannot be performed by JH itself.

New insecticides with ecdysteroidal and juvenile hormone activity

Agrochemical research over the last two decades has resulted in the discovery of chemically novel insecticides that mimic the action of the two insect growth and developmental hormones, the steroidal 20-hydroxyecdysone (20E) and the sesquiterpenoid juvenile hormone (JH). Bisacylhydrazines are non-steroidal agonists of 20E and exhibit their insecticidal activity via interaction with the ecdysteroid receptor proteins. Interestingly, two of the bisacylhydrazine (tebufenozide and RH-2485) insecticides are very selectively toxic to lepidopteran pests. These insecticides are safe to beneficial insects and have a benign ecotoxicological profile. Aromatic non-terpenoidal insecticides (fenoxycarb and pyriproxyfen) mimic the action of JHs. However, like the JHs, their exact mode of action is not well understood. These insecticides are toxic to a broad spectrum of insects during their embryonic, last larval, or reproductive stages. The insecticidal, ecotoxicological properties and the mode of action of the two groups of insecticides are reviewed in this article.

Ultraspiracle: an invertebrate nuclear receptor for juvenile hormones

Juvenile hormones (JH), a sesquiterpenoid group of ligands that regulate developmental transitions in insects, bind to the nuclear receptor ultraspiracle (USP). In fluorescence-based binding assays, USP protein binds JH III and JH III acid with specificity, adopting for each ligand a different final conformational state. JH III treatment of Saccharomyces cerevisiae expressing a LexA-USP fusion protein stabilizes an oligomeric association containing this protein, as detected by formation of a protein-DNA complex, and induces USP-dependent transcription in a reporter assay. We propose that regulation of morphogenetic transitions in invertebrates involves binding of JH or JH-like structures to USP.

Trypanosoma brucei: effects of methoprene and other isoprenoid compounds on procyclic and bloodstream forms in vitro and in mice

Drug therapy for the treatment of African sleeping sickness is limited by toxicity and resistance and in the last 50 years only one new drug has been introduced for the treatment of the human disease. We report that the juvenile hormone analog, methoprene, and several structurally related isoprenoid compounds kill Trypanosoma brucei in culture. Of the other isoprenoids tested, juvenile hormone III and mammalian retinoid X receptor ligands were the most potent trypanocides. Both the procyclic forms and the bloodstream trypomastigotes are killed by these compounds with LD50 values of 5-30 microM. Of the two methoprene stereoisomers, the EE form was the most active, suggesting that a protein target may be involved in mediating effects of these analogues against the parasite. Methoprene was not, however, able to clear trypanosomes from the blood of infected mice. Methoprene acid, the immediate downstream metabolite of methoprene, is not an effective anti-trypanosomal agent, suggesting that in the mice methoprene is converted to an inactive compound. Since methoprene and its analogues have low and well characterized toxicity in mammals these studies stress the importance of further exploring these isoprenoids as lead compounds for the treatment of African sleeping sickness.

Several synthetic chemicals inhibit progesterone receptor-mediated transactivation in yeast

The human progesterone receptor (hPR) B-form and a progesterone-sensitive reporter were expressed in yeast and used to screen a library of synthetic chemicals for their ability to function as agonists or antagonists of hPR. The transcriptional activity of hPR was not increased in the presence of over 40 individual chemicals. Seven chemicals decreased progesterone-dependent activity in yeast. The most effective chemicals were 6-hydroxychrysene, 1-hydroxypyrene, 4-hydroxy, 2',4',6'-trichloro biphenyl, and 4-hydroxy, 2',3',4',5'-tetrachloro biphenyl. The decrease of progesterone-mediated transactivation strongly correlated with their displacement of [3H]progesterone from hPR. The absence of the hydroxyl group on the above chemicals completely abolished their inhibitory activity. The other chemicals which decreased progesterone activity were endosulfan II, endosulfan sulfate, and lindane. These chemicals did not inhibit [3H]progesterone binding, suggesting that they inhibit progesterone action by interacting with a region of hPR distinct from binding [3H]progesterone or by a mechanism independent of hPR. These results highlight the utility of yeast for screening hormonally-active chemicals. In addition, hydroxylation appears to be essential for the interaction of some chemicals with hPR.

Retinoic acid receptor/retinoid X receptor heterodimers can be activated through both subunits providing a basis for synergistic transactivation and cellular differentiation

The receptor for 9-cis-retinoic acid, retinoid X receptor (RXR), forms heterodimers with several nuclear receptors, including the receptor for all-trans-retinoic acid, RAR. Previous studies have shown that retinoic acid receptor can be activated in RAR/RXR heterodimers, whereas RXR is believed to be a silent co-factor. In this report we show that efficient growth arrest and differentiation of the human monocytic cell line U-937 require activation of both RAR and RXR. Also, we demonstrate that the allosteric inhibition of RXR is not obligatory and that RXR can be activated in the RAR/RXR heterodimer in the presence of RAR ligands. Remarkably, RXR inhibition by RAR can also be relieved by an RAR antagonist. Moreover, the dose response of RXR agonists differ between RXR homodimers and RAR/RXR heterodimers, indicating that these complexes are pharmacologically distinct. Finally, the AF2 activation domain of both subunits contribute to activation even if only one of the receptors is associated with ligand. Our data emphasize the importance of signaling through both subunits of a heterodimer in the physiological response to retinoids and show that the activity of RXR is dependent on both the identity and the ligand binding state of its partner.

Unique requirements for retinoid-dependent transcriptional activation by the orphan receptor LXR

LXR is an orphan nuclear receptor that confers retinoid responsiveness to the retinoid X receptor (RXR) by its interaction on a specific response element called an LXRE. To understand the mechanism of this response, three characteristics were identified that are crucial to activation of the RXR-LXR complex. First, the orientation of the RXR-LXR heterodimer on DNA indicates that as the ligand-binding partner, RXR occupies the 5' half-site of the response element. Next, the sequence specificity of the LXRE was determined in order to identify residues required for retinoid activation of the heterodimer. Remarkably, subtle changes in the nucleotide sequence of the LXRE half-sites that do not substantially alter DNA binding of the RXR-LXR heterodimer have a significant effect on the ability of the complex to be activated by ligand. Finally, we characterized the contributions of the activation domains of each receptor to the trans-activation potential of the RXR-LXR heterodimer. Surprisingly, our results show that only the activation domain of LXR is required for retinoid activation. Taken together, these results demonstrate the existence of a unique form of communication between heterodimer partners in which the activation potential of one receptor (LXR) is enabled by ligand binding to its partner (RXR). Furthermore, we conclude that RXR ligand activation potential is not dictated solely by its position on DNA, but is influenced by other factors such as the receptor partner and sequence of the response element.

An oxysterol signalling pathway mediated by the nuclear receptor LXR alpha

Cholesterol and its oxysterol congeners are important constituents of cell membranes and function as intermediates in several crucial biosynthetic pathways. These compounds autoregulate their metabolic fate by end-product repression and activation of downstream catabolism. Although end-product repression by oxysterols is relatively well understood, the mechanism by which these compounds act as positive transcription signalling molecules is unknown. Here we identify a specific group of endogenous oxysterols that activate transcription through the nuclear receptor LXR alpha. Transactivation of LXR alpha by oxysterols occurs at concentrations at which these compounds exist in vivo. The most potent activators also serve as intermediary substrates in the rate-limiting steps of three important metabolic pathways: steroid hormone biosynthesis, bile acid synthesis, and conversion of lanosterol to cholesterol. Our results demonstrate the existence of a nuclear receptor signalling pathway for oxysterols and suggest that LXR alpha may be important as a sensor of cholesterol metabolites.

Phytol metabolites are circulating dietary factors that activate the nuclear receptor RXR

RXR is a nuclear receptor that plays a central role in cell signaling by pairing with a host of other receptors. Previously, 9-cis-retinoic acid (9cRA) was defined as a potent RXR activator. Here we describe a unique RXR effector identified from organic extracts of bovine serum by following RXR-dependent transcriptional activity. Structural analyses of material in active fractions pointed to the saturated diterpenoid phytanic acid, which induced RXR-dependent transcription at concentrations between 4 and 64 microM. Although 200 times more potent than phytanic acid, 9cRA was undetectable in equivalent amounts of extract and cannot be present at a concentration that could account for the activity. Phytanic acid, another phytol metabolite, was synthesized and stimulated RXR with a potency and efficacy similar to phytanic acid. These metabolites specifically displaced [3H]-9cRA from RXR with Ki values of 4 microM, indicating that their transcriptional effects are mediated by direct receptor interactions. Phytol metabolites are compelling candidates for physiological effectors, because their RXR binding affinities and activation potencies match their micromolar circulating concentrations. Given their exclusive dietary origin, these chlorophyll metabolites may represent essential nutrients that coordinate cellular metabolism through RXR-dependent signaling pathways.

The vitamin D(3) receptor in the context of the nuclear receptor superfamily : The central role of the retinoid X receptor

The nuclear hormone 1 α,25-dihydroxyvitamin D(3) (VD) is an important regulator of calcium homeostasis and is also a modulator of the cell cycle. The genomic actions of the hormone are mediated by a single transcription factor, the vitamin D(3) receptor (VDR). On the majority of the known VD response elements, VDR binds as heterodimeric complex with the retinoid X receptor (RXR), which is a member of the nuclear receptor superfamily like VDR. RXR supports not only the DNA binding affinity and specificity of VDR, but allosterically also its transactivation properties. Moreover RXR is a partner in other hormone response systems, which supports the idea that the different nuclear hormone signaling pathways are functionally linked.