Identification of a nuclear receptor that is activated by farnesol metabolites

Ligand-bound RXR can mediate retinoid signal transduction during embryogenesis

Retinoids regulate various aspects of vertebrate development through the action of two types of receptors, the retinoic acid receptors (RARs) and the retinoid-X-receptors (RXRs). Although RXRs bind 9-cis-retinoic acid (9cRA) with high affinity, in vitro experiments suggest that RXRs are for the most part not liganded, but serve as auxiliary factors forming heterodimers with liganded partner receptors such as RAR. Here we have used RXR- and RAR-specific ligands 4-[1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydro-2-napthyl)ethenyl]b enzoic acid (LG69) and (E)-4-[2-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-2-naphthalenyl)-1-prope nyl]benzoic acid (TTNPB), and show that, in the context of an embryo, liganded RXR can mediate retinoid signal transduction. This conclusion emerges from examining the induction of several retinoid-responsive genes in the limb bud (Hoxb-6/-8, RARbeta) and in the developing central nervous system (Hoxb-1, otx-2). RARbeta and Hoxb-1 genes were most effectively activated by a combination of TTNPB and LG69, suggesting that the activation of these genes benefits from the presence of ligand-bound RAR and ligand-bound RXR. Hoxb-6/-8 genes were most efficiently induced by LG69, suggesting that liganded RXR can activate these genes. The regulation of the expression of the otx-2 gene was complex; expression was repressed by TTNPB, but such repression was relieved when LG69 was provided together with TTNPB, suggesting that ligand-bound RXR can overcome repression of transcription exerted by liganded RAR. Based on these findings, we propose that in our experimental system in which ligands are provided exogenously, transcriptional regulation of several genes involves liganded RXR.

The peroxisome proliferator-activated receptors: ligands and activators

PPAR alpha and PPAR gamma represent related but distinct members of the nuclear receptor superfamily. PPAR alpha signaling is modulated by long-chain fatty acids, whereas PPAR gamma ligands are potent antidiabetic agents.

Cell compartmentalization of cholesterol biosynthesis

Thus, the results showing the presence of cholesterol synthetic enzymes in peroxisomes (see references 1, 4, 5, 6, 7, 8, 12, 13, 20, 21, 22, 24, 25, and 26), the reduced levels of cholesterol synthesis enzymes and cholesterol synthetic capacity of cells and tissues lacking peroxisomes, 26, 37, 39 and the low serum cholesterol levels in patients suffering from peroxisomal deficiency diseases40-43 demonstrate that peroxisomes are essential for normal cholesterol synthesis. A number of metabolic pathways require co-participation of enzymes located in both peroxisomes as well as enzymes found in other intracellular compartments. For example, the first steps of plasmalogen synthesis occur in the peroxisomes, while the terminal reactions are completed in the endoplasmic reticulum. Similarly, the oxidation of cholesterol to bile acids requires the participation of enzymes localized in the endoplasmic reticulum as well as peroxisomes. Little is known about the regulation of such pathways or about the shuttling of intermediates between compartments. The physiological importance of peroxisomal enzymes in the regulation of sterol metabolism remains to be clarified.

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.

Cloning and upstream sequence of a juvenile hormone-regulated gene from the migratory locust

As a step toward analyzing the molecular mechanism of action of juvenile hormone (JH), the gene encoding a JH-inducible 21-kDa protein (Jhp21) produced in the fat body of the migratory locust has been cloned. Four exons, representing 750 nucleotides of cDNA sequence, were found to be distributed through 13 kb of genomic DNA. Upstream 2 kb of DNA has been sequenced and three potential hormone-response elements have been identified.

Synergistic binding of sterol regulatory element-binding protein and NF-Y to the farnesyl diphosphate synthase promoter is critical for sterol-regulated expression of the gene

Sterol-regulated transcription of the farnesyl diphosphate (FPP) synthase gene is dependent on two cis elements in the proximal promoter. These elements, an inverted CCAAT box and sterol regulatory element 3 (SRE-3), bind NF-Y and sterol regulatory element-binding protein 1 (SREBP-1), respectively. We now demonstrate that the binding of recombinant SREBP-1 to its cognate site (SRE-3) within the FPP synthase promoter in vitro is enhanced by binding of NF-Y to the upstream inverted CCAAT box. Using an FPP synthase promoter fragment containing the binding sites for both NF-Y and SREBP-1 in gel mobility shift assays, we demonstrate that the addition of NF-Y increases the binding of SREBP-1 to SRE-3 over 20-fold. In contrast, NF-Y does not stimulate the binding of SREBP-1 to SRE-3 when the inverted CCAAT box is either mutated or 4 base pairs (bp) are inserted between the inverted CCAAT box and SRE-3. Promoter-reporter genes, containing either the wild-type FPP synthase promoter sequence or containing the 4-bp insertion between the inverted CCAAT box and SRE-3, were transiently transfected into cells. The activity of the wild-type promoter-reporter gene increased when the cells were either incubated in sterol-depleted medium or were co-transfected with an expression vector encoding transcriptionally active SREBP-1. This increase in activity was attenuated when the promoter contained the 4-bp insert, consistent with defective binding of SREBP to the promoter in vivo. These studies suggest that the binding of SREBP-1 to SRE-3 in the FPP synthase promoter, and subsequent stimulation of transcription, is dependent on synergistic binding and a functional interaction between SREBP-1 and NF-Y.

Retinoid receptors in transcriptional regulation

Our understanding of the mechanism of action of retinoids has been greatly expanded by a series of recent findings. First, the three-dimensional structure of the ligand-binding domain of two retinoid receptors has been solved and suggests that ligand binding induces marked allosteric changes. Second, several co-factors interacting with the receptors have been cloned, some of which are capable of regulating the function of receptors. Third, the advent of synthetic retinoids helped define the activities of the receptors. Fourth, the study of the in vivo receptor-DNA interactions has revealed a previously unrecognized role of the ligand in regulating the stability of receptor-DNA complexes. These advances have revealed complex molecular interactions operating at multiple levels, opening new avenues of research for addressing their mechanisms.

Role of CBP/P300 in nuclear receptor signalling

The nuclear receptor superfamily includes receptors for steroids, retinoids, thyroid hormone and vitamin D, as well as many related proteins. An important feature of the action of the lipophilic hormones and vitamins is that the maintenance of homeostatic function requires both intrinsic positive and negative regulation. Here we provide in vitro and in vivo evidence that identifies the CREB-binding protein (CBP) and its homologue P300 (refs 6,7) as cofactors mediating nuclear-receptor-activated gene transcription. The role of CBP/P300 in the transcriptional response to cyclic AMP, phorbol esters, serum, the lipophilic hormones and as the target of the E1A oncoprotein suggests they may serve as integrators of extracellular and intracellular signalling pathways.

Juvenile hormone regulation of an insect gene: a specific transcription factor and a DNA response element

We have used locust fat body nuclear protein extracts and upstream DNA of the juvenile hormone (JH)-inducible locust gene, jhp21, to examine the regulation of specific transcription by JH. Promoter activity was assayed with G-free cassette reporter constructs. Nuclear extracts from adult female fat body, previously exposed to JH or an analog, actively transcribe from the jhp21 promoter and a control adenovirus major late (AdML) promoter, whereas extracts from JH-deprived female fat body, or other tissues, transcribe strongly from the AdML promoter but weakly or not at all from the jhp21 promoter. Transcription is enhanced by sequences between -140 and -211 nt from the jhp21 transcription start point (tsp), which include a CAAT box, and also by sequences between -1056 and -1200. A 15-nt partially palindromic sequence element found at -1152, resembling known hormone response elements, was shown to stimulate transcription when restored to truncated jhp21 DNA. Two very similar sequences occur further upstream. In electrophoretic mobility shift assays (EMSA), the same sequence element was shown to specifically bind a protein that was present in nuclear extracts from JH-exposed, but not from JH-deprived, fat body. Several lines of evidence suggest that the DNA element may be a JH response element (JHRE). The JH-induced protein that binds to it appears to be a transcription factor that activates the initiation of JH target gene (jhp21) transcription, and could be a JH receptor.

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.

Multiple steps in the regulation of transcription-factor level and activity

This review focuses on the regulation of transcription factors, many of which are DNA-binding proteins that recognize cis-regulatory elements of target genes and are the most direct regulators of gene transcription. Transcription factors serve as integration centres of the different signal-transduction pathways affecting a given gene. It is obvious that the regulation of these regulators themselves is of crucial importance for differential gene expression during development and in terminally differentiated cells. Transcription factors can be regulated at two, principally different, levels, namely concentration and activity, each of which can be modulated in a variety of ways. The concentrations of transcription factors, as of intracellular proteins in general, may be regulated at any of the steps leading from DNA to protein, including transcription, RNA processing, mRNA degradation and translation. The activity of a transcription factor is often regulated by (de) phosphorylation, which may affect different functions, e.g. nuclear localization DNA binding and trans-activation. Ligand binding is another mode of transcription-factor activation. It is typical for the large super-family of nuclear hormone receptors. Heterodimerization between transcription factors adds another dimension to the regulatory diversity and signal integration. Finally, non-DNA-binding (accessory) factors may mediate a diverse range of functions, e.g. serving as a bridge between the transcription factor and the basal transcription machinery, stabilizing the DNA-binding complex or changing the specificity of the target sequence recognition. The present review presents an overview of different modes of transcription-factor regulation, each illustrated by typical examples.

Ecdysone-inducible gene expression in mammalian cells and transgenic mice

During metamorphosis of Drosophila melanogaster, a cascade of morphological changes is triggered by the steroid hormone 20-OH ecdysone via the ecdysone receptor, a member of the nuclear receptor superfamily. In this report, we have transferred insect hormone responsiveness to mammalian cells by the stable expression of a modified ecdysone receptor that regulates an optimized ecdysone responsive promoter. Inductions reaching 4 orders of magnitude have been achieved upon treatment with hormone. Transgenic mice expressing the modified ecdysone receptor can activate an integrated ecdysone responsive promoter upon administration of hormone. A comparison of tetracycline-based and ecdysone-based inducible systems reveals the ecdysone regulatory system exhibits lower basal activity and higher inducibility. Since ecdysone administration has no apparent effect on mammals, its use for regulating genes should be excellent for transient inducible expression of any gene in transgenic mice and for gene therapy.

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.

Compartmentalization of cholesterol biosynthesis. Conversion of mevalonate to farnesyl diphosphate occurs in the peroxisomes

We have recently demonstrated that mevalonate kinase and farnesyl diphosphate (FPP) synthase are localized predominantly in peroxisomes. This observation raises the question regarding the subcellular localization of the enzymes that catalyze the individual steps in the pathway between mevalonate kinase and FPP synthase (phosphomevalonate kinase, mevalonate diphosphate decarboxylase, and isopentenyl diphosphate isomerase). These enzyme are found in the 100,000 x g supernatant fraction of cells or tissues and have been considered to be cytoplasmic proteins. In the current studies, we show that the activities of mevalonate kinase, phosphomevalonate kinase, and mevalonate diphosphate decarboxylase are equal in extracts prepared from intact cells and selectively permeabilized cells, which lack cytosolic enzymes. We also demonstrate structure-linked latency of phosphomevalonate kinase and mevalonate diphosphate decarboxylase that is consistent with a peroxisomal localization of these enzymes. Finally, we show that cholesterol biosynthesis from mevalonate can occur in selectively permeabilized cells lacking cytosolic components. These results suggest that the peroxisome is the major site of the synthesis of FPP from mevalonate, since all of the cholestrogenic enzymes involved in this conversion are localized in the peroxisome.

Abnormal spermatogenesis in RXR beta mutant mice

We have generated mouse lines in which the RXR beta gene was disrupted by homologous recombination. Approximately 50% of the RXR beta homozygous mutants died before or at birth, but those that survived appeared normal except that the males were sterile, owing to oligo-astheno-teratozoospermia. Failure of spermatid release occurred within the germinal epithelium, and the epididymis contained very few spermatozoa that, in addition, exhibited abnormal acrosomes and tails. There was a progressive accumulation of lipids within the mutant Sertoli cells, which were histochemically characterized as unsaturated triglycerides. In old mutant males, progressive degeneration of the germinal epithelium occurred, ending with the formation of acellular lipid-filled tubules. The selective expression of RXR beta in Sertoli cells, together with the timing of appearance of the histological abnormalities, suggests that the primary defect resulting from the mutation resides in these cells.

Juvenile hormone: the status of its "status quo" action

Juvenile hormone (JH) allows larval molting in response to ecdysteroids but prevents the switching of gene expression necessary for metamorphosis. I first review our efforts to isolate the nuclear receptor for JH in the larval epidermis of Manduca sexta using photoaffinity analogs and our recent findings that the molecule isolated does not bind JH I with high affinity. The reported apparent high affinity binding of JH I by the recombinant 29 kDa protein (rJP29) was artifactual due to the presence of contaminating esterases. Purified rJP29 bound little detectable JH I, but its binding of the photoaffinity analog was prevented by JH I as well as other isoprenoids, indicating a low affinity for these compounds. Our recent studies focus on the effects of JH on the early molecular events induced by 20-hydroxyecdysone (20E). Culture of day 2 5th larval epidermis with 10(-6)M 20E for 24 h caused first pupal commitment, then the onset of the predifferentiative events necessary for pupation. Biphasic increases in the mRNAs of the two isoforms of the ecdysone receptor (EcR-A and EcR-B1) and of E75A, an ecdysteroid-induced transcription factor, coincided with these two phases. The mRNAs for Ultraspiracle (USP) and the metamorphosis-specific Broad-Complex (BR-C) increased only during the second phase. The presence of JH had no effect on the initial increases in EcR mRNAs but caused an increased accumulation of E75A mRNA. This JH also prevented the later changes in EcR, USP, and BR-C mRNAs. Thus, JH influences only certain of the early actions of 20E which then result in its preservation of the "status quo."

Biochemistry of proteins that bind and metabolize juvenile hormones

A diverse group of proteins has evolved to bind and metabolize insect juvenile hormones (JHs). Synthetic radiolabeled JHs and their photoaffinity analogs have enabled us to isolate and characterize JH binding proteins (JHBPs), a putative nuclear JH receptor, JH esterases (JHEs), JH epoxide hydrolases (JHEHs), and methyl farnesoate binding proteins (MFBPs). Highlights of recent progress on structural characterization of JHBPs and JHEHs of two lepidopterans will be described. Efforts to identify MFBPs of penaeid shrimp will be discussed, and the discovery of a possible vertebrate JHBP will be presented.

A model for farnesoid feedback control in the mevalonate pathway

Mevalonate is the rate-limiting substrate leading to farnesyl pyrophosphate (FPP), the central intermediate for isoprenoids such as cholesterol, dolichols, ubiquinone, and carotenoids. One major challenge has been to identify the isoprenoid effector molecules and transcription factors mediating negative regulation in this metabolic pathway. A nuclear receptor called FXR has recently been characterized that is activated by farnesyl pyrophosphate metabolites such as farnesol, farnesal, farnesoic acid, and methyl farnesoate. FXR expression in isoprenoidogenic tissues suggests a hypothesis that these intracellular "farnesoids" may be signals for transcriptional feedback control of cholesterol biosynthesis.

Identification of spinach farnesyl protein transferase. Dithiothreitol as an acceptor in vitro

Spinach seedlings were found to contain farnesyl protein transferase. The enzyme is activated by Zn2+, but not by Mg2+. The pH optimum is approximately 7.0 and maximal activity is obtained at 40-45 degrees C. The apparent Km for the farnesyl diphosphate substrate is 7 microM. Western blotting of soluble proteins with an antiserum raised against mammalian farnesyl protein transferase demonstrated a specific cross-reactivity with the spinach enzyme. The antiserum preferentially recognises the beta-subunit of the heterodimeric farnesyl protein transferase, and the corresponding spinach polypeptide has a molecular mass of 42 kDa on SDS/PAGE. The enzyme can employ dithiothreitol as an acceptor for the farnesyl moiety and catalyses the formation of a thioether linkage between these substrates. On the basis of this discovery, a new method was developed utilising the hydrophobicity of the reaction product, and its interaction with poly(propylene). During in vivo labelling, the plants took up dithiothreitol, which inhibited the incorporation of [3H]mevalonate metabolites into proteins, indicating that dithiothreitol might be isoprenylated in vivo as well as in vitro. However, isoprenylation of some proteins remains unaffected by dithiothreitol suggesting the existence of different isoprenylation mechanisms. Thus, it is demonstrated that plants possess farnesyl protein transferase, which resembles its mammalian and yeast homologues.

The nuclear hormone-receptor family in the brain: classics and orphans

Nuclear hormone receptors comprise a superfamily of over 40 transcription factors. About half of them are classical receptors for lipophilic ligands such as steroids and vitamins. Almost all of these true receptors are present in the brain, where they transduce chemical signals from endocrine organs or signals of nutritional origin into cellular responses. The other members resemble the classical receptors in structure, but have no known ligands, and are hence called 'orphan receptors'. The issue of whether ligands for nuclear orphan receptors exist is controversial. Evidence is emerging that orphan receptors might be activated by signal transduction pathways or might be constitutive enhancers or repressors that interact with the classical receptors. Thus, nuclear orphan receptors are placed in strategic positions in the regulation of gene expression in the nervous system.

Seven-up inhibits ultraspiracle-based signaling pathways in vitro and in vivo

Seven-up (Svp), the Drosophila homolog of the chicken ovalbumin upstream transcription factor (COUP-TF); Ultraspiracle (Usp), the Drosophila homolog of the retinoid X receptor; and the ecdysone receptor are all members of the nuclear/steroid receptor superfamily. COUP-TF negatively regulates hormonal signaling involving retinoid X receptor in tissue culture systems. Here we demonstrate that Svp, like COUP-TF, can modulate Ultraspiracle-based hormonal signaling both in vitro and in vivo. Transfection assays in CV-1 cells demonstrate that Seven-up can inhibit ecdysone-dependent transactivation by the ecdysone receptor complex, a heterodimeric complex of Usp and ecdysone receptor. This repression depends on the dose of Svp and occurs with two different Drosophila ecdysone response elements. Ectopic expression of Svp in vivo induces lethality during early metamorphosis, the time of maximal ecdysone responsiveness. Concomitant overexpression of Usp rescues the larvae from the lethal effects of Svp. DNA binding studies show that Svp can bind to various direct repeats of the sequence AGGTCA but cannot bind to one of the ecdysone response elements used in the transient transfection assays. Our results suggest that Svp-mediated repression can occur by both DNA binding competition and protein-protein interactions.

15-Deoxy-delta 12, 14-prostaglandin J2 is a ligand for the adipocyte determination factor PPAR gamma

Regulation of adipose cell mass is a critical homeostatic process in higher vertebrates. The conversion of fibroblasts into cells of the adipose lineage is induced by expression of the orphan nuclear receptor PPAR gamma. This suggests that an endogenous PPAR gamma ligand may be an important regulator of adipogenesis. By assaying arachidonate metabolites for their capacity to activate PPAR response elements, we have identified 15-deoxy-delta 12, 14-prostaglandin J2 as both a PPAR gamma ligand and an inducer of adipogenesis. Similarly, the thiazolidinedione class of antidiabetic drugs also bind to PPAR gamma and act as potent regulators of adipocyte development. Thus, adipogenic prostanoids and antidiabetic thiazolidinediones initiate key transcriptional events through a common nuclear receptor signaling pathway. These findings suggest a pivotal role for PPAR gamma and its endogenous ligand in adipocyte development and glucose homeostasis and as a target for intervention in metabolic disorders.

NF-Y has a novel role in sterol-dependent transcription of two cholesterogenic genes

The transcription of farnesyl diphosphate (FPP) synthase is regulated up to 30-fold by the sterol status of the cell. Point mutations in a 6-base pair ATTGGC sequence in the promoter disrupt both sterol-dependent transcription in vivo as well as binding of the transcription factor NF-Y in vitro. Co-transfection of cells with NF-YA29, a dominant negative form of NF-Y, and various promoter-reporter genes specifically inhibits the sterol-dependent regulation of FPP synthase and 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase. In contrast, NF-YA29 does not affect the regulation of reporter genes under the control of promoters derived from either the HMG-CoA reductase or the low density lipoprotein receptor gene. Transient expression of the 68-kDa transcriptionally active fragment of sterol regulatory element-binding protein in cells stimulates an HMG-CoA synthase-reporter gene over 90-fold. This induction is blocked in cells co-expressing NF-YA29. We hypothesize that NF-Y plays a novel role in sterol-dependent regulation of two key genes in the cholesterol biosynthetic pathway and that this role requires a specific interaction with the sterol regulatory element-binding protein or related transcription factors.

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.