Regulation of cytochrome P-450 dependent steroid hydroxylase activity in Manduca sexta: evidence for the involvement of a neuroendocrine-endocrine axis during larval-pupal development

Phospholipid dependence of the reversible, energy-linked, mitochondrial transhydrogenase in Manduca sexta

Midgut mitochondria from fifth larval instar Manduca sexta exhibit a membrane-associated transhydrogenase that catalyzes hydride ion transfer between NADP(H) and NAD(H). The NADPH-forming transhydrogenations occur as nonenergy- and energy-linked activities. The energy-linked activities couple with electron transport-dependent utilization of NADH/succinate, or with Mg(2+)-dependent ATPase. These energy-linked transhydrogenations have been shown to be physiologically and developmentally significant with respect to insect larval/pupal maturation. In the present study, isolated mitochondrial membranes were lyophilized and subjected to organic solvent or phospholipase treatments. Acetone extraction and addition of Phospholipase A(2) proved to be effective inhibitors of the insect transhydrogenase. Liberation of phospholipids was reflected by measured phosphorous release. Addition of phospholipids to organic solvent- and phospholipase-treated membranes was without effect. Employing a partially lipid-depleted preparation, phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine were reintroduced and transhydrogenase activity assessed. Of the phospholipids tested, only phosphatidylcholine significantly stimulated transhydrogenase activity. The results of this study suggest a phospholipid dependence of the M. sexta mitochondrial transhydrogenase.

Midgut and fatbody mitochondrial transhydrogenase activities during larval-pupal development of the tobacco hornworm, Manduca sexta

Midgut and fatbody mitochondria from fifth larval instar Manduca sexta display a membrane-associated transhydrogenase that catalyzes a reversible hydride ion transfer between NADP(H) and NAD(H). The NADPH-forming activity occurs as a nonenergy- or energy-linked activity with energy for the latter derived from either electron transport-dependent NADH or succinate utilization, or ATP hydrolysis by Mg(++)-dependent ATPase. During the ten-day developmental period preceding the larval-pupal molt (fifth larval instar), significant peaks in the mitochondrial transhydrogenase activities of midgut and fatbody tissues were noted and these peaks were coincident with the onset of wandering behavior and with the fifty-fold increase in ecdysone 20-monooxygenase (E20-M) activity previously reported for M. sexta midgut. Since E20-M preferentially uses NADPH in catalyzing ecdysone conversion to the physiologically active molting hormone, 20-hydroxyecdysone, the physiological and developmental significance of the mitochondrial, NADPH-forming energy-linked transhydrogenations were made apparent. Moreover, that the increases in all transhydrogenase activities resulted from de novo enzyme synthesis were indicated by the cycloheximide-dependent reductions in these activities.

Physiological requirements for 20-hydroxyecdysone-induced rectal sac distention in the pupa of the silkworm, Bombyx mori

Successful insect development is achieved via appropriate fluctuation of ecdysteroid levels. When an insect's ecdysteroid level is disrupted, physiological and developmental defects occur. In the pupa of the silkworm, Bombyx mori, the rectal sac is an essential organ that operates as a repository for degraded ecdysteroids, and it can be distended by administration of 20-hydroxyecdysone (20E). Our previous study showed that rectal sac distention appears 4 days after 20E administration. Hemolymph ecdysteroid levels, however, decrease to lower level during this period. Thus, the timing of the rectal sac distention does not match with that of ecdysteroid elevation. Here, we examine how 20E induces rectal sac distention. A ligature experiment and ecdysteroid quantification showed that continuous 20E stimulation induces rectal sac distention. Thorax tissue contributed to the continuous 20E stimulation needed to induce distention. Ecdysteroid released from the thorax tissue may be converted to 20E by ecdysone 20-hydroxylase to produce continuous 20E stimulation. Thus, the ecdysone metabolic pathway plays a critical role in rectal sac distention.

Cyclic 3', 5' guanosine monophosphate and larval midgut ecdysone 20-monooxygenase activity of the tobacco hornworm, Manduca sexta

During the last larval stadium of the tobacco hornworm, Manduca sexta, midgut ecdysone 20-monooxygenase (E20M) activity increases 50-fold coincident with wandering onset. In our study, the effects of the ecdysone agonist RH-5849 and second messengers, dibutyryl cyclic 3', 5' adenosine monophosphate (dibutyryl cAMP) and dibutyryl cyclic 3', 5' guanosine monophosphate (dibutyryl cGMP), either alone or combined with other additives, on midgut E20M were assessed in vitro and in vivo. Pre-incubation of midguts with dibutyryl cAMP was ineffective, but dibutyryl cGMP elevated E20M activity. In vivo, the 50-fold increase in E20M activity was diminished by injections of guanylate cyclase inhibitors alone or combined with RH-5849. Guanylate cyclase inhibitor effects were significantly diminished when co-injected with dibutyryl cGMP, or a guanylate cyclase activator. Although dibutyryl cGMP injections into head- or thorax-ligated pre-wandering larvae did not elevate midgut E20M activity, they did synergize when co-injected with a suboptimal dose of RH-5849, thereby eliciting significant activity increases. Radioimmunoassays of midgut cGMP content throughout the M. sexta fifth larval instar revealed that levels of cGMP significantly increased between days four and five consistent with the 50-fold E20M activity increase. These data indicate cGMP involvement in the ecdysone-mediated increase in midgut E20M activity.

Midgut mitochondrial transhydrogenase in wandering stage larvae of the tobacco hornworm, Manduca sexta

Midgut mitochondria from fifth larval instar Manduca sexta exhibited a transhydrogenase that catalyzes the following reversible reaction: NADPH + NAD(+) <--> NADP(+) + NADH. The NADPH-forming transhydrogenation occurred as a nonenergy- and energy-linked activity. Energy for the latter was derived from the electron transport-dependent utilization of NADH or succinate, or from Mg++-dependent ATP hydrolysis by ATPase. The NADH-forming and all of the NADPH-forming reactions appeared optimal at pH 7.5, were stable to prolonged dialysis, and displayed thermal lability. N,N'-dicyclohexylcarbodiimide (DCCD) inhibited the NADPH --> NAD(+) and energy-linked NADH --> NADP(+) transhydrogenations, but not the nonenergy-linked NADH --> NADP(+) reaction. Oligomycin only inhibited the ATP-dependent energy-linked activity. The NADH-forming, nonenergy-linked NADPH-forming, and the energy-linked NADPH-forming activities were membrane-associated in M. sexta mitochondria. This is the first demonstration of the reversibility of the M. sexta mitochondrial transhydrogenase and, more importantly, the occurrence of nonenergy-linked and energy-linked NADH --> NADP(+) transhydrogenations. The potential relationship of the transhydrogenase to the mitochondrial, NADPH-utilizing ecdysone-20 monooxygenase of M. sexta is considered.

Marine invertebrate cytochrome P450: emerging insights from vertebrate and insects analogies

Cytochrome P450 enzymes (P450s) are responsible for the oxidative metabolism of a plethora of endogenous and exogenous substrates. P450s and associated activities have been demonstrated in numerous marine invertebrates belonging to the phyla Cnidaria, Annelida (Polychaeta), Mollusca, Arthropoda (Crustacea) and Echinodermata. P450s of marine invertebrates and vertebrates show considerable sequence divergence and the few orthologs reveal the selective constraint on physiologically significant enzymes. P450s are present in virtually all tissues of marine invertebrates, although high levels usually are found in hepatic-like organs and steroidogenic tissues. High-throughput technologies result in the rapid acquisition of new marine invertebrate P450 sequences; however, the understanding of their function is poor. Based on analogy to vertebrates and insects, it is likely that P450s play a pivotal role in the physiology of marine invertebrates by catalyzing the biosynthesis of signal molecules including steroids such as 20-hydroxyecdysone (the molting hormone of crustaceans). The metabolism of many exogenous compounds including benzo(a)pyrene (BaP), pyrene, ethoxyresorufin, ethoxycoumarin and aniline is mediated by P450 enzymes in tissues of marine invertebrates. P450 gene expression, protein levels and P450 mediated metabolism of xenobiotics are induced by PAHs in some marine invertebrate species. Thus, regulation of P450 enzyme activity may play a central role in the adaptation of animals to environmental pollutants. Emphasis should be put on the elucidation of the function and regulation of the ever-increasing number of marine invertebrate P450s.

Transcription of a lepidopteran cytochrome P450 promoter is modulated by multiple elements in its 5' UTR and repressed by 20-hydroxyecdysone

The biochemical response to the phytochemical xanthotoxin encountered in the diet of black swallowtail larvae is the induction of P450s capable of detoxifying this and other toxic furanocoumarins. As the xenobiotic response element to xanthotoxin (XRE-xan) is necessary but not sufficient for transcription of the CYP6B1v3 gene in Sf9 cells, sequences upstream of it, such as a putative EcRE, and downstream of it, such as a putative C/EBP binding site and Inr, have been tested for their roles in regulation. Mutation of the putative EcRE has indicated that it affects basal transcription of this promoter but not repression by 20-hydroxyecdysone. Mutation of the more proximal promoter sequence, including the C/EBP and Inr, have indicated that many core promoter elements between the TATA box and translation start site modulate basal and xanthotoxin-inducible expression of this composite promoter.

Ecdysone 20-monooxygenase in eggs of the silkworm, Bombyx mori: enzymatic properties and developmental changes

Ecdysone 20-monooxygenase in eggs of the silkworm Bombyx mori was characterized in relation to embryonic development. First, subcellular fractions were prepared by means of differential centrifugation, and analyzed using marker enzymes and antibodies against NADPH-cytochrome P450 reductase. It was demonstrated that most ecdysone 20-monooxygenase activity was associated with microsomes, and that there was little or no intrinsic mitochondrial ecdysone 20-monooxygenase. Next, conditions for the measurement of ecdysone 20-monooxygenase activity were established for the microsomal fraction, and changes in the enzyme activity were measured in diapause eggs and non-diapause eggs during early embryogenesis. It was demonstrated that enzyme activity in diapause eggs remained at a low level, while that in the non-diapause eggs increased from the gastrula stage. The increase in egg ecdysone 20-monooxygenase activity was prevented by actinomycin D and alpha-amanitin, suggesting that gene transcription is required for eliciting an increase in ecdysone 20-monooxygenase activity.

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.

Induction of enzymes involved in molting hormone (ecdysteroid) inactivation by ecdysteroids and an agonist, 1,2-dibenzoyl-1-tert-butylhydrazine (RH-5849)

Molting in insects is regulated by molting hormones (ecdysteroids). The major active hormone, 20-hydroxyecdysone, is formed by ecdysone 20-monooxygenase-catalyzed hydroxylation of ecdysone. During times of decreasing hormone titers, inactivation occurs by several routes including (i) 26-hydroxylation and further oxidation to the 26-oic acid, (ii) formation of various conjugates (e.g. phosphates), and (iii) in Lepidoptera in particular, ecdysone oxidase-catalyzed formation of 3-dehydroecdysteroid, which is reduced to 3-epiecdysteroid, followed by phosphotransferase-catalyzed formation of phosphate conjugates. Administration of the nonsteroidal ecdysteroid agonist RH-5849 (1,2-dibenzoyl-1-tert-butylhydrazine), but not 20-hydroxyecdysone, to tobacco hornworm (Manduca sexta) resulted in induction of midgut cytosolic ecdysone oxidase and ecdysteroid phosphotransferase activities. In addition, both 20-hydroxyecdysone and RH-5849 caused induction of ecdysteroid 26-hydroxylase activity in midgut mitochondria and microsomes, whereas 20-hydroxylase was induced to a lesser extent by 20-hydroxyecdysone in mitochondria and by either RH-5849 or 20-hydroxyecdysone in microsomes. Commensurate with induction of the enzymes by ecdysteroid and RH-5849 is a requirement for RNA and protein synthesis, without precluding indirect mechanisms. These results indicate that molting hormone stimulates at least one universal route of its own inactivation by inducing ecdysteroid 26-hydroxylase activity and are discussed in relation to an analogous phenomenon observed for vitamin D inactivation in vertebrates.

Induction of an inactivation pathway for ecdysteroids in larvae of the cotton leafworm, Spodoptera littoralis

Treatment of the last-instar larvae of the cotton leafworm (Spodoptera littoralis) with ecdysteroids (moulting hormones) results in the induction of an ecdysteroid-inactivation pathway. Administration of ecdysone, 20-hydroxyecdysone or an ecdysteroid agonist, RH 5849, leads to induction of an ecdysteroid 26-hydroxylase activity. This induction occurred in both early sixth-instar larvae and in older larvae which had been head-ligated to prevent the normal developmental increase in ecdysone 20-mono-oxygenase activity. The induction of 26-hydroxylase activity requires both RNA and protein synthesis, as demonstrated by experiments involving actinomycin D and cycloheximide. The 26-aldehyde derivative of ecdysone and ecdyson-26-oic acid were also formed from ecdysone in the RH 5849-induced systems. Formation of the aldehyde and the corresponding 26-oic acid (ecdysonoic acid) from 26-hydroxyecdysone was directly demonstrated in a cell-free system, thus establishing the following inactivation pathway: Ecdysteroid-->26-hydroxyecdysteroid-->ecdysteroid 26-aldehyde-->ecdysteroid 26-oic acid.

Effects of plant flavonoids and other allelochemicals on insect cytochrome P-450 dependent steroid hydroxylase activity

The plant flavonoids flavone, chrysin, apigenin, kaempferol, morin, quercetin, myricetin and phloretin were found to inhibit in a dose-dependent manner the cytochrome P-450 dependent ecdysone 20-monooxygenase activity associated with adult female Aedes aegypti, wandering stage larvae of Drosophila melanogaster, and fat body and midgut from prewandering and wandering stage last instar larvae of Manduca sexta. The concentrations of these flavonoids required to elicit a 50% inhibition of the steroid hydroxylase activity in all the insects ranged from ca 1 x 10(-5) to 1 x 10(-3) M. In addition, lower concentrations (1 x 10(-6) to 1 x 10(-5) M) of the flavonols kaempferol, morin, quercetin and myricetin significantly stimulated (50-100% above control) M. sexta fat body ecdysone 20-monooxygenase activity. Other plant allelochemicals examined and found to significantly inhibit insect ecdysone 20-monooxygenase activity include corynanthine, quinidine, and quinine; whereas, indican and mimosine were found to significantly stimulate M. sexta fat body steroid hydroxylase activity. Several allelochemicals were without effect at all concentrations tested. Although none of the compounds tested in this study elicited effects at very low concentrations (1 x 10(-9) to 1 x 10(-8) M), the in vitro monooxygenase radioassay does hold considerable promise as a screening tool for the detection and identification of plant allelochemicals which may function as biopesticides affecting insect ecdysteroidogenesis.

Effects of the adenylate cyclase activator forskolin and its inactive derivative 1,9-dideoxyforskolin on insect cytochrome P-450 dependent steroid hydroxylase activity

The adenylate cyclase activator forskolin and its pharmacologically inactive derivative 1,9-dideoxyforskolin were found to inhibit in a dose-dependent fashion the ecdysone 20-monooxygenase activity associated with wandering stage larvae of Drosophila melanogaster and fat body and midgut from last instar larvae of the tobacco hornworm, Manduca sexta. The concentrations of these labdane diterpenes required to elicit a 50% inhibition of the cytochrome P-450 dependent steroid hydroxylase activity in the insect tissues ranged from approximately 5 x 10(-6) to 5 x 10(-4) M.

Regulation of cytochrome P-450 dependent steroid hydroxylase activity in Manduca sexta: effects of the ecdysone agonist RH 5849 on ecdysone 20-monooxygenase activity

The non-steroidal ecdysone agonist RH 5849 (1,2-dibenzoyl-1-tert-butylhydrazine) was found to inhibit in a dose-response and apparently competitive fashion the cytochrome P-450 dependent ecdysone 20-monooxygenase activity in the midgut of wandering stage last instar larvae of the tobacco hornworn, Manduca sexta. More effectively on a per molar basis than the naturally occurring molting hormones ecdysone and 20-hydroxyecdysone, RH 5849 was also found to elicit the dramatic 50-fold increase in midgut steroid hydroxylase activity (which normally occurs with the onset of the wandering stage) when injected into competent head or thoracic ligated pre-wandering last instar larvae. These data support and extend the potential usefulness of RH 5849 as a pharmacological probe for further investigating the actions of ecdysteroids and their role(s) in the regulation of ecdysteroid monooxygenases.