Action of estradiol-17beta on the synthetic activity of the silk gland in Bombyx mori L

Vertebrate estrogen regulates the development of female characteristics in silkworm, Bombyx mori

The vertebrate estrogens include 17-β-estradiol (E2), which has an analog in silkworm ovaries. In this study, the Bombyx mori vitellogenin gene (BmVg) was used as a biomarker to analyze the function of the E2 in silkworm. In most oviparous animals, Vg has female-specific expression. However, BmVg expression was also detected in B. mori males. Stage specific fluctuation of BmVg expression was similar in males and females, but expression levels in males were lower than in females. E2 treatment by injection or feeding of male larvae in the final instar stage induced and stimulated male BmVg transcription and protein synthesis. When silkworm ovary primordia were transplanted into males, BmVg was induced in male fat bodies. Transplanted ovaries primordia were also able to develop into ovaries and produce mature eggs. When females were treated with E2 promoted BmVg/BmVn protein accumulation in hemolymph, ovaries and eggs. However, BmVg transcription was decreased in female fat bodies. An E2 analog was identified in the hemolymph of day 3 wandering silkworms using high-performance liquid chromatography. Estradiol titers from fifth late-instar larvae to pupal stage were determined by enzyme-linked immunosorbent assay. The results suggested that silkworms synthesized a vertebrate E2 analog. This study found that E2 promoted the synthesis of BmVg, a female typical protein in silkworms.

Effects of bisphenol A in the ring-legged earwig, Euborellia annulipes

Bisphenol A is a known endocrine disruptor in vertebrates that mimics the action of estrogens by interacting with hormone receptors. It also affects reproduction and development in many invertebrate animals, though mechanisms of action are unclear. Terrestrial insects, despite their abundance and profound ecological significance, have been largely overlooked as a group that might be affected by vertebrate endocrine disrupting chemicals. We evaluated potential effects of bisphenol A on the ring-legged earwig, Euborellia annulipes, as a model for terrestrial arthropods. Dosages of 0, 0.12, 1.2 and 12 microg bisphenol A were injected over a 6 day period into newly eclosed males and newly mated (7-day) females. The lowest dosage (0.12 microg) was most effective in eliciting significant effects including reducing weight gain while increasing testis size and seminal vesicle size; higher dosages were less effective or ineffective. In females, treatment with 0.12 microg bisphenol A enhanced clutch size but higher dosages were required to affect the duration of embryogenesis in offspring of treated mothers. Hatching success and the onset of the second reproductive cycle were not affected by treatments. No gross abnormalities were observed as a result of treatment in the reproductive structures of either males or females. Similarly, injection of varying concentrations of estradiol into males enhanced testis length, though it had no effect on seminal vesicle size. Lastly, we administered bisphenol A in drinking water for up to 2 weeks. Surprisingly, as little as 1 microg/L inhibited testis growth; 100 microg/L inhibited ovarian growth.

Estradiol-17β: Tracing its metabolic significance in female fatbody of fifth instar larvae of silkworm, Bombyx mori L (race: Nistari)

In recent years, various vertebrate peptide and steroid hormones have been identified in invertebrates, estradiol-17beta (E2) being a major one. We have specifically shown NADP-malate dehydrogenase (NADP-MDH) activity in fifth instar larval fatbody of female silkworm, Bombyx mori, as an E2 responsive parameter. Interestingly enough, estradiol-induced increase in the enzyme activity could be counteracted by simultaneous application of specific E2-inhibitor, ICI-182780. Further, a nice correlation was obtained among the E2 titre, specific *E2 binding and expression of NADP-MDH activity in fatbody during different days of normal fifth instar larval development. Though the nature of the binding sites is quite similar to known steroid receptors of vertebrate, the reported absence of estrogen receptor gene in some insects poses a question. A recent finding regarding the presence of an estrogen-related receptor ortholog in fruit fly may provide some answers. The specific effects elicited by estradiol in the female fatbody of this insect support its possibility of having an important metabolic function. This role played by E2, whether hormonal or not, is yet to be identified.

The presence of estradiol-17beta and its specific binding sites in posterior silk gland of Bombyx mori

Although estradiol-17beta (E2) is known to be a vertebrate sex steroid, its presence in insects, particularly in Bombyx mori, has recently created considerable sensation regarding its significance in insect life. The specificity of estradiol action on the posterior silk gland (PSG) of B. mori has been previously assessed by the use of ICI-182780 as a specific receptor antagonist for estradiol. The present study provides evidence for systematic changes in the titer of estradiol-17beta in B. mori and its specific binding sites in the posterior silk gland according to the physiological age of fifth instar larvae. The changes in the estradiol level in the PSG after E2 treatment have also been determined to assess whether the administered estradiol has been taken up by the silk gland cells and whether the effect of exogenously administered E2 on the PSG is direct. The correlation in the profile of E2 binding sites with the endogenous estradiol level of PSG on different days of fifth instar larvae suggests that the level of binding sites in the PSG is regulated by the estradiol titer and that E2 may have some biological function in B. mori. Scatchard analysis of E2 binding sites in the PSG on 9th day of fifth instar larvae reveals that it consists of only one type of binding component. The dissociation constant (Kd) and maximum binding capacity (Bmax) of this putative E2 receptor have been found to be 8.98 +/- 0.744 nM and 55.8 +/- 5.53 fmol/mg cytosolic protein, respectively. The study thus indicates an involvement of the putative receptor in E2-mediated action on the silk gland of B. mori and paves a pathway for further evaluation of the physiological relevance of the presence of vertebrate steroids in insects.

Estradiol-17beta in Bombyx mori: possible significance and its effect on silk production

Although estrogen is well known as a vertebrate sex steroid, its presence in insects, including Bombyx mori, raises questions about its precise role in the physiology of insects. It was reported earlier that estradiol-17beta (E(2)) exerts a specific effect on silk-gland function in B. mori and that it may act in a nuclear-mediated way. To evaluate further the effect of E(2) on cocoon characters, larval growth and development, 1µg/g of E(2) was applied topically to the first and second day of fifth instar larvae. This resulted in a significant enhancement of cocoon characters, such as cocoon shell weight, silk filament length per cocoon, denier per filament and reelability of the cocoons, without any adverse effect on fecundity and hatchability. In the present study, E(2) levels in the haemolymph were quantified on different days of the fifth instar larvae and age-dependent changes in the endogenous E(2) titre have been demonstrated. These age-dependent variations in E(2) content coincide with physiological events occurring during the fifth instar. Such observations exclude the possibility of a dietary origin for E(2), as a sudden and sharp rise of the E(2) level in the haemolymph was observed on the 10th day of the fifth instar, preceded by a small increase on the ninth day after an eight-day feeding period. The increased level of estradiol in the haemolymph of larvae treated topically with E(2) indicates effective penetration of this hormone through the larval cuticle. Moreover, similar patterns of alteration of E(2) levels on different days of the fifth instar in both control and treated groups suggests the existence of some internal metabolic pathway in the silkworm body to regulate the hormone titre. Thus, the present investigation offers a system for investigating the unique function of E(2) in B. mori and offers potential for improvement of silk production.