Biosynthesis of a C21 steroid conjugate in an insect. The conversion of [14C]cholesterol to 5-[14C]pregnen-3 beta,20 beta-diol glucoside in the tobacco hornworm, Manduca sexta

Insect Sterols and Steroids

Insects are incapable of biosynthesising sterols de novo so they need to obtain them from their diets or, in certain cases, from symbiotic microorganisms. Sterols serve a structural role in cellular membranes and act as precursors for signalling molecules and defence compounds. Many phytophagous insects dealkylate phytosterols to yield primarily cholesterol, which is also the main sterol that carnivorous and omnivorous insects obtain in their diets. Some phytophagous species have secondarily lost the capacity to dealkylate and consequently use phytosterols for structural and functional roles. The polyhydroxylated steroid hormones of insects, the ecdysteroids, are derived from cholesterol (or phytosterols in non-dealkylating phytophagous species) and regulate many crucial aspects of insect development and reproduction by means of precisely regulated titres resulting from controlled synthesis, storage and further metabolism/excretion. Ecdysteroids differ significantly from vertebrate steroid hormones in their chemical, biochemical and biological properties. Defensive steroids (cardenolides, bufadienolides, cucurbitacins and ecdysteroids) can be accumulated from host plants or biosynthesised within the insect, depending on species, stored in significant amounts in the insect and released when it is attacked. Other allelochemical steroids serve as pheromones. Vertebrate-type steroids have also been conclusively identified from insect sources, but debate continues about their significance. Side chain dealkylation of phytosterols, ecdysteroid metabolism and ecdysteroid mode of action are targets of potential insect control strategies.

Testosterone maintains male longevity and female reproduction in Chrysopa pallens

Vertebrate testosterone, an androgen present in the testes, is essential for male fertility. Vertebrate-type steroid hormones have been identified in insects, but their function remains unknown. Insect vitellogenin (Vg) is usually a female-specific protein involved in reproductive processes. However, males of some species, such as the green lacewing Chrysopa pallens, have Vg. Here, we demonstrated that the knockdown of C. pallens male Vg by RNAi significantly shortened the lifespan of males, suppressed the reproduction of post-mating females, and strikingly reduced the abundance of several immune-related compounds, including testosterone. LC-MS/MS revealed that C. pallens male testosterone had the same structure and molecular mass as vertebrate testosterone. Topical testosterone application partially restored the lifespan of Vg-deficient males and the reproduction of post-mating females. These results suggest that vertebrate-type testosterone maintains male longevity and female reproduction under the control of the male Vg in C. pallens.

C21 steroidal glycosides and oligosaccharides from the root bark of Periploca sepium

Four new C₂₁ steroidal glycosides (1-4), named perisepiumosides FI (1-4) together with six known steroidal glycosides (5-10) and four oligosaccharides (11-14), were isolated from the root bark of Periploca sepium. Their structures were characterized on the basis of 1D and 2D-NMR spectroscopic data as well as HR-ESI-MS analysis. The evaluation of inhibition activity against human A-549 and HepG2 cell lines indicated that compounds 2, 8, 10 and 13 showed different levels of cytotoxic activities with IC₅₀ values ranging from 0.61 to 7.86μM.

Steroids in aquatic invertebrates

Steroid molecules are present in all invertebrates, and some of them have established hormonal roles: this is the case for ecdysteroids in arthropods and, to a lesser extent, for vertebrate-type steroids in molluscs. Steroids are not only hormones, they may also fulfill many other functions in chemical communication, chemical defense or even digestive physiology. The increasing occurrence of endocrine disruption problems caused by environmental pollutants, which interfere in particular with reproductive physiology of vertebrates but also of invertebrates has made necessary to better understand the endocrine physiology of the latter and the role of steroids in these processes. So many attempts are being made to better understand the endocrine roles of steroids in arthropods and molluscs, and to establish whether they also fulfill similar functions in other invertebrate phyla. At the moment, both the precise identification of these steroids, the determination of their origin (endogenous versus exogenous) and of their mechanism of action are under active investigation. This research takes profit of the development of genome sequencing programs on many invertebrate species, which allow the identification of receptors and/or biosynthetic enzymes, when related to their vertebrate counterparts, but the story is not so simple, as will be exemplified by estrogen receptors of molluscs.

Progesterone in Periplaneta americana and Neobellieria bullata adults from the procuticle phase until first progeny production

A significant amount of progesterone-like immunoreactive material (150 ng/g) was measured by EIA in the procuticle phase of adult of both sexes of Periplaneta americana. This peak markedly decreased to 1-10 ng/g during sclerotization and was unlikely to be of dietary origin. In the case of 0-hr-old P. americana adults 96-98% of progesterone-like material was localized in the digestive tract and Malpighian tubules. In contrast, a relatively low level of progesterone-like immunoreactive material was measured in 0-hr-old Neobellieria bullata adults. Activity of 3beta-HSD/isomerase converting pregnenolone to progesterone was high (22-43 fmol/mg protein/20 min) in 0-hr-old P. americana adults and significantly fell during sclerotization. High progesterone levels (13-16 ng/g), measured by HPLC-RIA, coexist with high levels of 3beta-HSD/isomerase activity. Orally active human contraceptives (ethisterone, ethynodiol, ethynodiol diacetate, lynestrenol, mestranol, norgestrel, norethynodrel, tamoxifen citrate, and mifepristone) which act on mammalian steroid receptors had no significant effects on progeny production in either polytrophic or meroistic insect ovaries even at concentration of 5000 mg/kg.

Synthesis and metabolism of vertebrate-type steroids by tissues of insects: a critical evaluation

This review covers the synthesis and the metabolism of vertebrate-type steroids (progesterone, testosterone, estradiol, corticosteroids) by insect tissues and discusses the significance of the reactions for insect physiology. Biosynthesis of vertebrate-type steroids from cholesterol hitherto has been demonstrated in only two insect species, i.e. the water beetle Acilius sulcatus (Coleoptera) and the tobacco hornworm Manduca sexta (Lepidoptera). In Acilius, steroid synthesis is associated with exosecretion (chemical defense). Nothing, however, is known about a physiological role of the C21 steroid conjugate present in ovaries and eggs of Manduca. No synthesis of vertebrate-type steroids was observed in any other insect investigated to date. Most metabolic conversions of steroids by insects concerned oxidoreduction of oxygen groups (hydroxysteroid dehydrogenase activity) and (polar and apolar) conjugate formation. All important enzymatic steps involved in synthesis and catabolism, as known from studies with tissues of vertebrates, were not, or hardly observed. The conclusion is drawn that typical vertebrate-type (C21, C19 and C18) steroids probably do not act as physiologically active substances in insects.

Comparative developmental physiology and molecular cytology of the polytrophic ovarian follicles of the blowfly Sarcophaga bullata and the fruitfly Drosophila melanogaster

1. The ovarian follicles of Sarcophaga and Drosophila consist of one oocyte and 15 nurse cells, the whole being surrounded by follicle cells. Although oocyte and nurse cells are genetically identical sibling cells, and although they are interconnected by cytoplasmic bridges, their physiology is very different. 2. The DNA content of the oocyte nucleus (germinal vesicle) never exceeds 4C, while values of polyploidisation up to 1024C have been measured in the nurse cells, this being dependent on their position within a follicle. 3. The nurse cell nuclei very actively synthesize RNA, while the germinal vesicle is almost completely inactive in this respect. 4. It has been possible to visualise the major cytoskeletal elements in the different ovarian cell types. Cellular markers of polarity and dorsoventral asymmetry have been described. 5. Electrophysiological measurements have been performed to find out whether or not the self-electrophoresis principle may be involved in polarised transport between nurse cells and oocyte. 6. Most of the vitellogenin is synthesized by the fat body but some follicle cells also synthesize small amounts. 7. The role of 20-OH ecdysone in the induction of vitellogenin synthesis in the fat body, as well as the presence of met-enkephalin like immunoreactivity in the gonads is well established in both species. Not so clear is the exact role of juvenile hormones and the nature of brain factors controlling ovarian development. 8. Drosophila has the advantage of its well documented genetics while the larger species Sarcophaga is preferable for the study of (electro-) physiological and cell biological mechanisms.

Pregnenolone, testosterone, and estradiol in the migratory locust Locusta migratoria; a gas chromatographical-mass spectrometrical study

Homogenates of ovaria as well as testes from the locust Locusta migratoria were analyzed for the presence of pregnenolone, testosterone, and estradiol by gas chromatography followed by mass spectrometry. Selected ion monitoring analyses revealed that the molecular ion and two characteristic fragment ions of pregnenolone, the molecular ion with its isotopes and two characteristic mass fragments of testosterone, and the molecular ion, its isotopes and four characteristic fragment ions of estradiol, were present at the proper retention times and with the correct abundance ratios.

Metabolism of [14C]cholesterol to C-20 isomeric [14C]pregn-5-ene-3,20-diols in the tobacco hornworm, Manduca sexta

After injection into male and female fifth-instar larvae of Manduca sexta, [14C]cholesterol was converted to C21 steroids, [14C]pregn-5-ene-3 beta,20-diols. These metabolites were isolated from 8-day-old pupae and were identified by TLC, HPLC, and GC-MS as the C-20 isomers of pregnene-3 beta,20-diol. They also were isolated from male and female meconium fluid (of 16-day-old pupae) following injection of [14C]cholesterol into 14-day-old pupae.

The presence of a pregnenolone-binding factor in the copulatory organ of the migratory locust, Locusta migratoria migratorioides R. & F

The presence of binding sites for nonecdysteroid steroids was investigated in the cytosol of several tissues of the migratory locust, Locusta migratoria migratorioides. Binding of androgens was not observed. Most tissues, however, showed nonsaturable binding of estrogens and in some tissues saturable progestin binding could be demonstrated. A pregnenolone binder, that was found to be present in the male copulatory organ, was further studied. It showed a dissociation constant of 4.4 (+/- 1.6) X 10(-8) M. This is the first report of a nonecdysteroid steroid-binding factor in an insect tissue.

Glucosylations of pregn-5-ene-3 beta,20R-diol

The four possible monoglucosides of the pregn-5-ene-3 beta,20R-diol were synthesized along with a mixture of the four possible 3,20-diglucosides. The glucosides were characterized by HPLC and mass spectrometry.

Study of the metabolism of steroids in larvae of the fleshfly Sarcophaga bullata

1. Larvae of the fleshfly Sarcophaga bullata were injected with several 3H C21 and C19 steroids. After different incubation times, the larvae were homogenized and the metabolites were extracted and fractionated by Sephadex LH 20-, paper- and thin-layer chromatography. The chromatographic mobility of the labeled zones was compared with that of standard steroids. 2. Progesterone and 17 alpha-hydroxypregnenolone were metabolized to 17 alpha-hydroxyprogesterone. Androstenedione, 17 alpha-hydroxyprogesterone and dehydroepiandrosterone were converted to testosterone. Transformation of pregnenolone to progesterone or 17 alpha-hydroxypregnenolone was not observed. 3. C21 or C19 steroid formation from cholesterol could not be demonstrated. 4. Sixteen metabolites, different from all our standard substances have been found. Their structure remains to be elucidated.