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

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.

Nonsteroidal ecdysone agonists

Nonsteroidal ecdysone agonists are novel compounds that have become attractive candidates not only as pest control agents in agriculture but also as tools for research. Their narrow spectrum of activity makes them relatively safe as pesticides, and their mode of action as ligands for gene expression has found application in gene therapy and inducing transgenic gene expression in plants. These diacylhydrazines (DAHs) are potent nonsteroidal ecdysone agonists, and four of them, tebufenozide, methoxyfenozide, chromafenozide, and halofenozide, have been developed as insecticides. Although these compounds are very toxic to insects, they are safe for mammals and are environmentally benign. Their action on insects is also selective, the first three are effective against Lepidoptera but weakly active or inactive on Diptera and Coleoptera. On the other hand, halofenozide is effective on Coleoptera but mildly active on Lepidoptera. Previous reviews on ecdysone agonists have concentrated on the biological response of some DAHs and their effects on pests. In this review, the chemistry, biological effects and their modes of action at the molecular level will be covered. In addition, a few studies on other nonsteroidal ecdysone agonists, such as 3,5-di-tert-butyl-4-hydroxy-N-iso-butylbenzamide, acylaminoketones, and benzoyl-1,2,3,4-tetrahydroquinolines, will be briefly reviewed.

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.

Early steps in ecdysteroid biosynthesis: evidence for the involvement of cytochrome P-450 enzymes

The first step in the biosynthesis of ecdysteroids by Manduca sexta prothoracic glands, the conversion of cholesterol to 7-dehydrocholesterol, is mediated by an enzyme with characteristics of a microsomal cytochrome P-450, i.e. sensitivity to CO and fenarimol, and a requirement for NADPH. The enzyme responsible for hydroxylation at C-25 of the putative 3-dehydroecdysone precursor, 14-hydroxy-5 beta-cholest-7-en-3,6-dione, is also microsomal, while those mediating hydroxylations at C-22 and C-2 of 3,14,25-trihydroxy-5 beta-cholest-7-en-6-one are mitochondrial. Indirect evidence revealed that the steps between 7-dehydrocholesterol and the trideoxyecdysteroids occur in the mitochondria, suggesting that extensive shuttling of intermediates between the endoplasmic reticulum and mitochondria takes place in the prothoracic gland cell during ecdysteroid biosynthesis. During the fifth larval instar, cholesterol 7,8-dehydrogenase activity is evident from days 2 to 9, while the conversion to [3H]ecdysteroids is not significant prior to the ecdysteroid commitment peak on day 4. Terminal hydroxylase activity shows little change throughout the instar. These data support the hypothesis that regulation of the biosynthetic pathway by PTTH occurs at the step immediately following the formation of 7-dehydrocholesterol. The steroid biosynthesis inhibitor, fenarimol, has been shown to inhibit each of these P-450 enzymes, as well as fat body ecdysone 20-monooxygenase, with an I50 of 10(-4) M in disrupted glands, suggesting that it is a general P-450 inhibitor. The secretion of ecdysteroids by the glands in vitro is very sensitive to fenarimol, i.e. I50 of 10(-6) M. RH5849, 1,2-dibenzoyl-1-tert-butylhydrazine, fails to inhibit any of these prothoracic gland reactions, yet strongly inhibits fat body ecdysone 20-monooxygenase activity. This suggests that RH5849 is a specific ecdysteroid substrate/product mimic in this reaction.

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.