Sterol metabolism in insects and biosynthesis of ecdysone in the silkworm

Developmental changes in the sterol composition and the glycerol content of cuticular and internal lipids of three species of flies

The glycerol concentration and the composition of cuticular and internal sterols in three medically and forensically important fly species, viz., Musca domestica, Sarcophaga carnaria, and Calliphora vicina, were analyzed. The cuticular and internal lipid extracts were separated by HPLC-LLSD, after which the sterol fraction was characterized by GC/MS in total ion current (TIC) mode. The cuticular lipids of M. domestica larvae contained seven sterols, while in pupae and females, six sterols were identified. Five sterols were found in the cuticular lipids of M. domestica males. The internal lipids of M. domestica larvae and pupae contained six and seven sterols, respectively, while those of male and female flies contained only five sterols. Sitosterol, cholesterol, and campesterol were the dominant sterols in M. domestica, while campestanol, stigmasterol, sitostanol, and fucosterol were identified in low concentrations or in traces. In contrast, cuticular and internal lipids of S. carnaria and C. vicina contained only cholesterol. Glycerol was identified in all stages of M. domestica, S. carnaria, and C. vicina. For all the three examined fly species, the present study clearly showed species-specific developmental changes in the composition of cuticular and internal sterols as well as in the glycerol concentration.

Dietary sterols/steroids and the generalist caterpillar Helicoverpa zea: physiology, biochemistry and midgut gene expression

Sterols are essential nutrients for insects because, in contrast to mammals, no insect (or arthropod for that matter) can synthesize sterols de novo. Plant-feeding insects typically generate their sterols, commonly cholesterol, by metabolizing phytosterols. However, not all phytosterols are readily converted to cholesterol. In this study we examined, using artificial diets containing single sterols/steroids, how typical (cholesterol and stigmasterol) and atypical (cholestanol and cholestanone) sterols/steroids affect the performance of a generalist caterpillar (Helicoverpa zea). We also performed sterols/steroids analyses, using GC/MS techniques, to explore the metabolic fate of these different dietary sterols/steroids. Finally, we used a microarray approach to measure, and compare, midgut gene expression patterns that arise as a function of dietary sterols/steroids. In general, H. zea performed best on the cholesterol and stigmasterol diets, with cholesterol as the dominant tissue sterol on these two treatments. Compared to the cholesterol and stigmasterol diets, performance was reduced on the cholestanol and cholestanone diets; on these latter treatments stanols were the dominant tissue sterol. Finally, midgut gene expression patterns differed as a function of dietary sterol/steroid; using the cholesterol treatment as a reference, gene expression differences were smallest on stigmasterol, intermediate on cholestanol, and greatest on cholestanone. Inspection of our data revealed two broad insights. First, they identify a number of genes potentially involved in sterol/steroid metabolism and absorption. Second, they provide unique mechanistic insights into how variation in dietary sterol/steroid structure can affect insect herbivores.

Ecdysteroid and free amino acid content of eggs of the Colorado potato beetle, Leptinotarsa decemlineata

In order to identify components of the Colorado potato beetle (CPB) egg that may be required by Edovum puttleri, a parasitic wasp that parasitizes the CPB egg, to complete development, ecdysteroid and free amino acid content of CPB eggs were analyzed by reversed phase high pressure liquid chromatography followed by radioimmunoassay to identify ecdysteroids. Ecdysteroid titers were relatively low (<300 pg/egg) through day 2 post-oviposition and then increased sharply, reaching concentrations >2,500 pg/egg on day 3 post-oviposition. Ecdysone (E), 20-hydroxyecdysone (20E), and polar conjugates of E were prominent ecdysteroids present in eggs sampled on days 0 and 1 post-ecdysis, and E, 20E, three peaks containing more polar ecdysteroids (metabolic inactivation products), and polar conjugates of E were present in eggs sampled on day 2. Thus, at a time when parasitization of CPB eggs by E. puttleri is relatively high (0-48 h), physiologically-active ecdysteroids (20E and perhaps E are physiologically active) are present at concentrations between 50 and 200 pg/egg. Ecdysone and 20E reached their highest levels in day-3 eggs, indicating that ecdysteroid may direct physiological processes associated with the completion of CPB embryonic development. In day-4 eggs, the concentration of E and 20E fall dramatically and polar metabolites of E and/or 20E are now responsible for the high ecdysteroid content of the eggs. Interestingly, conjugates of E decrease to relatively low levels in day-3 eggs and are absent in day-4 eggs. Therefore, it is likely that the increase in E in day-3 eggs is due, in part, to the breakdown of polar conjugates of E. Nine amino acids were present in significant quantities in eggs sampled at various times between 0 and 48 h post-oviposition. These include histidine, glutamine, proline, asparagine, serine, glutamic acid, threonine, lysine, and tyrosine. The first three amino acids were present at concentrations that were approximately 2 to 6 times greater than the concentrations of the last six amino acids. Amounts of most of the free amino acids varied with the age of the eggs from which the extract was prepared, but in general, there was no correlation between the levels at times of maximum parasitization (0 and 30 h) and the levels at the less favored times of parasitization (16 and 48 h). This information should facilitate the development of diets for both parasites and predators of pest species of beetles. Arch. Insect Biochem. Physiol. 44:172-182, 2000. Published 2000 Wiley-Liss, Inc.

Intermediates of stigmasterol metabolism in Spodoptera littoralis

Stigmasterol-24,28-epoxide, 22E-stigmasta-5,22,24(28E)-trien-3 beta-ol, and 22E-cholesta-5,22,24-trien-3 beta-ol were identified as normal metabolites of [3H]stigmasterol in Spodoptera littoralis larvae. Relative concentrations of all three of these metabolites increased when a diazasterol inhibitor was fed in combination with stigmasterol in the artificial diet. Identification of these sterols as intermediates in the conversion of stigmasterol to cholesterol in this insect indicates that intermediates analogous to fucosterol and fucosterol-24,28-epoxide in the conversion of sitosterol to cholesterol are produced in the metabolism of stigmasterol. This is the first published identification of stigmasterol-24,28-epoxide and 22E-stigmasta-5,22,24(28E)-trien-3 beta-ol as intermediates in this pathway in an insect.

Fucosterol epoxide lyase of insects: synthesis of labeled substrates and development of a partition assay

Fucosterol epoxide labeled with tritium in the C-29 methyl has been synthesized and employed in the development of a partition assay which allows the rapid determination of fucosterol epoxide lyase activity in vitro in homogenates of insect tissues. An independent synthesis of [24-14C]fucosterol epoxide provided a control substrate to evaluate nondealkylative transfer of labeled steroid to the aqueous layer during the enzyme assay. The diastereomeric 24R,28R- and 24S,28S-[29-3H]fucosterol epoxides were obtained via HPLC separation of their benzoate esters. Homogenates of the midgut tissue of larval tobacco hornworms (Manduca sexta) were examined at pH 5 to 9 in several buffer systems, and at temperatures of 7 to 67 degrees C in phosphate buffer. Optimal activity was found using pH 7.4, 76 mM phosphate buffer at 37 degrees C. The 24R,28R diastereomer of fucosterol epoxide was metabolized at a rate at least 100 times that of the 24S,28S isomer by this enzyme system.

Inhibition of C28 and C29 phytosterol metabolism by N,N-dimethyldodecanamine in the nematode Caenorhabditis elegans

Effects on the metabolism of campesterol and stigmasterol in Caenorhabditis elegans were investigated using N,N-dimethyldodecanamine, a known inhibitor of growth, reproduction and the delta 24-sterol reductase of this nematode. 7-Dehydrocholesterol was the predominant sterol (51%) of C. elegans grown in stigmasterol-supplemented media, whereas addition of 25 ppm amine resulted in a large decrease in the relative percentage of 7-dehydrocholesterol (23%) and the accumulation of a substantial proportion (33%) of delta 24-sterols (e.g., cholesta-5,7,24-trienol) and delta 22,24-sterols (e.g., cholesta-5,7,22, 24-tetraenol) but yielded no delta 22-sterols. Dealkylation of stigmasterol by C. elegans proceeded in the presence of the delta 22-bond; reduction of the delta 22-bond occurred prior to delta 24-reduction. Addition of 25 ppm amine to campesterol-supplemented media altered the sterol composition of C. elegans by increasing the percentage of unmetabolized dietary campesterol from 39 to 60%, decreasing the percentage of 7-dehydrocholesterol from 26 to 12%, and causing the accumulation of several delta 24-sterols (6%). C. elegans also was shown to be capable of dealkylating a delta 24 (28)-sterol as it converted 24-methylenecholesterol to mostly 7-dehydrocholesterol. The proposed role of 24-methylenecholesterol as an intermediate between campesterol and 7-dehydrocholesterol was supported by the results.