The Presence of Estradiol-17β and Its Specific Binding Sites in Posterior Silk Gland of Bombyx mori

Abstract

Although estradiol-17β (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-17β 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.