Early stages of ecdysteroid biosynthesis

Control and biochemical nature of the ecdysteroidogenic pathway

Molting is elicited by a critical titer of ecdysteroids that includes the principal molting hormone, 20-hydroxyecdysone (20E), and ecdysone (E), which is the precursor of 20E but also has morphogenetic roles of its own. The prothoracic glands are the predominate source of ecdysteroids, and the rate of synthesis of these polyhydroxylated sterols is critical for molting and metamorphosis. This review concerns three aspects of ecdysteroidogenesis: (a) how the brain neuropeptide prothoracicotropic hormone (PTTH) initiates a transductory cascade in cells of the prothoracic gland, which results in an increased rate of ecdysteroid biosynthesis (upregulation); (b) how the concentrations of 20E in the hemolymph feed back on the prothoracic gland to decrease rates of ecdysteroidogenesis (downregulation); and (c) how the prothoracic gland cells convert cholesterol to the precursor of E and then 20E, a series of reactions only now being understood because of the use of a combination of classical biochemistry and molecular genetics.

Biosynthesis of 20-hydroxyecdysone in Ajuga hairy roots: the possibility of 7-ene introduction at a late stage

Administration of [3alpha-2H]-3beta-hydroxy-5beta-cholestan-6-one to hairy roots of Ajuga reptans var. atropurpurea followed by 2H-NMR spectroscopic analysis of the resulting 20-hydroxyecdysone so formed revealed that the substrate was efficiently incorporated into the latter. Additionally, [5beta,7alpha,7beta-2H3]-2beta,3beta-dihydroxy-+ ++5beta-cholestan-6-one was converted into 20-hydroxyecdysone. These findings clearly indicate that Ajuga hairy roots are capable of introducing a double bond at the 7-position at a late stage of 20-hydroxyecdysone biosynthesis, suggesting the possibility of an alternative biosynthetic pathway which does not involve 7-dehydrocholesterol as an obligatory intermediate.

Biosynthesis of ecdysteroid hormones by crustacean Y-organs: conversion of cholesterol to 7-dehydrocholesterol is suppressed by a steroid 5 alpha-reductase inhibitor

A pair of glands (Y-organs) in crustaceans synthesize and secrete ecdysteroid hormones; the obligate precursor for synthesis is circulating cholesterol. Ecdysteroid output by the Y-organs is regulated negatively by an eyestalk neurosecretory peptide, molt-inhibiting hormone (MIH). The question was addressed, does MIH suppress ecdysteroid synthesis by decreasing cholesterol supply (uptake) or its utilization or both? Experiments were conducted with Y-organs in vitro from the crab, Menippe mercenaria, in the presence of labeled cholesterol, with or without the steroid 5 alpha-reductase inhibitor, L-645390 (Merck). Other experiments superimposed the presence or absence of eyestalk extract containing MIH activity. L-645390 greatly depressed incorporation of cholesterol into an early intermediate, 7-dehydrocholesterol and into secreted ecdysteroids. At the same time, cholesterol accumulated in the Y-organs, to levels significantly higher than in untreated controls. MIH alone depressed both cholesterol uptake and incorporation. MIH together with L-645390 produced the greatest depression of cholesterol incorporation while also preventing the cholesterol accumulation seen with L-645390 alone. These results indicate that cholesterol uptake and its metabolic utilization in Y-organs are separable events representing separate sites of regulation by MIH.

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.

Formation of ecdysteroids by Y-organs of the crab, Menippe mercenaria. II. Incorporation of cholesterol into 7-dehydrocholesterol and secretion products in vitro

The conversion in vitro of cholesterol (Ch) to nonpolar metabolites and ecdysteroids was studied in Y-organs of the xanthid crab, Menippe mercenaria. In one set of experiments, Y-organs were prelabeled in vivo by injecting crabs with 100 microCi of [3H]Ch, and halved glands were then incubated for 24 and 48 hr in the presence of unlabeled Ch. In another set, unlabeled Y-organs were incubated in standard medium containing 10 microCi/ml of [3H]Ch. Both polar and nonpolar metabolites were surveyed by HPLC. The early metabolite, 7-dehydrocholesterol (7-dhCh), was the only labeled derivative of Ch detectable in Y-organ tissue after incubation; preincubation amounts of 7-dhCh were higher in glands from de-eyestalked crabs vs glands from intact crabs, and labeling was an order of magnitude higher in glands incubated with labeled Ch vs those prelabeled in vivo. Specific activity calculations indicate highly efficient conversion of 7-dhCh to ecdysteroid secretions. Analyses of the incubation media revealed two polar secretory products, synthesized from Ch in vitro. These coeluted with the authentic standards, 3-dehydroecdysone and 25-deoxyecdysone. Secretion of 3-dehydroecdysone always exceeded that of 25-deoxyecdysone (ratio range, 1.9 to 14.4), in both de-eyestalked and intact crabs.

Formation of ecdysteroids by Y-organs of the crab, Menippe mercenaria. I. Biosynthesis of 7-dehydrocholesterol in vivo

Y-organs of the xanthid crab, Menippe mercenaria, secrete ecdysteroid hormones in vitro, apparently both 3-dehydroecdysone and 25-deoxyecdysone. Studies were initiated on the biosynthetic path(s), in which cholesterol is converted to these ecdysteroids. Crabs were injected with [3H]cholesterol. Y-organs and hemolymph were removed 12 hr later and extracted directly and the extracts were analyzed by HPLC. Both polar and nonpolar sterols were surveyed. The only metabolite of cholesterol detectable in Y-organs was 7-dehydrocholesterol (identified by mass spectrometry). The total amount of 7-dehydrocholesterol and the amount that was labeled were generally greater than for cholesterol and were higher in Y-organs from de-eyestalked crabs than in those from intact crabs. Subcellular fractionation of the Y-organs showed that over 70% of total radioactivity was in cholesterol and 7-dehydrocholesterol of mitochondria and microsomes, distributed about equally between the two organellar fractions. In hemolymph, the only nonpolar sterols present were cholesterol and 7-dehydrocholesterol; the concentration ratio was 20:1. However, 7-dehydrocholesterol was not significantly labeled. Analyses of polar compounds revealed two prominent, uv-absorbing ecdysteroids which coeluted with the authentic standards, 3-dehydro-20-hydroxyecdysone and 25-deoxy-20-hydroxyecdysone (ponasterone A). The radioactivity profile showed, in addition, a third prominent peak that corresponded in retention time with 3-dehydroecdysone. These results indicate that the Y-organs in vivo form 7-dehydrocholesterol from cholesterol and convert the latter to secretion products without accumulation of other intermediates. At least two ecdysteroids are secreted and appear to be converted peripherally in this crab to their respective 20-hydroxy derivatives.