Summary of Session IV: Juvenile Hormone Effects at the Molecular Level (Binding and Transport)

Abstract

Soon after the elucidation of the structure of the juvenile hormones in the laboratories of Willer and Meyer, it was suggested that the hemolymph of insects may contain carrier proteins for these lipophilic molecules comparable to those found for many hormones in vertebrate blood. We now know that such carrier proteins are not necessary for solubilization of the juvenile hormones (JH) because JH solubility in an aqueous medium is surprisingly high (Kramer et al.). However, the carrier proteins appear to have several other important functions:

• 1) They protect JH from degradative attack by ubiquitous nonspecific esterases, e.g. in Manduca sexta (Kramer et al.; Nowock et al.) and in Plodia interpunctella (Ferkovich et al.). In the hemolymph of Manduca, however, a DFP-resistant, JH-specific esterase appears in the last larval instar, which is able to hydrolyze both free and carrier-bound JH. This esterase probably arises from the fat body (Nowock et al.) and plays an important role in diminishing the JH titer prior to the pupal ecdysis. It remains to be elucidated, however, whether the binding protein carrier has a protective function within target cells as well. The data of Ferkovich et al. demonstrate a reduced metabolism in homogenates of the larval epidermis from Plodia if a binding protein fraction from the hemolymph is added, but no data are available as to whether such an effect also occurs in vivo. Indeed, it has not been demonstrated that the carrier protein penetrates the target cell membrane.

• 2) The carrier protein system represents a mechanism enabling the storage of relatively large quantities of JH in the hemolymph. In Manduca, the carrier may aid in controlling the actual titer of these hormones since the titer of this protein changes during larval development and drops during the pupal and pharate adult stage (Nowock et al.).

  The titer of JH in the hemolymph of insects is therefore regulated in a very complicated manner by different systems: synthesis and release of the JH by the corpora allata (see Schooley et al., this volume; Tobe and Pratt, this volume), occurrence of nonspecific and specific esterases in the hemolymph (Nowock et al.); the amount of carrier protein in the hemolymph, uptake and subsequent degradation in the tissues and then excretion (Nowock et al.); reflux from tissue compartments (Nowock et al.) and by other factors.

• 3) The binding proteins may influence the biological activity of the JH present in the hemolymph. In vertebrates, carrier bound hormones are always in an inactive form and this may occur in insects too (Ferkovich et al.). The uptake of the hormone into Manduca fat body cells is reduced in the presence of the carrier protein (Nowock et al.). Binding constants of cellular JH receptors in the target cells are probably higher than those of the carrier proteins as indicated by the dose response curves for the biological activity of juvenoids. Therefore, in the vicinity of the target cell, binding to cellular receptors may lead to a local depletion of unbound hormone which elicits a rapid and spontaneous dissociation of the carrier hormone complexes. The presence of the carrier may even lead to the increased biological activity of a given hormone titer by protecting the hormone from hydrolytic degradation and by preventing excessive unspecific binding to lipophilic compartments in the insect cells. Such a function is indicated by the data of Ferkovich et al., although these experiments were conducted by adding JH binding protein fractions to epidermis homogenates of Plodia. This treatment may give rise to many artifacts due to the interference of specific and nonspecific binding as well as degradative metabolism.