Control of ovarian steroidogenesis by insulin-like peptides in the blowfly (Phormia regina)

Abstract

This study investigated the ability of insulin and of insect insulin-like peptides (ILPs) to stimulate ovarian steroidogenesis in the blowfly Phormia regina. Bovine insulin was active on ovaries isolated in vitro, which showed an age-dependent sensitivity; this peptide progressively stimulated steroidogenesis in ovaries isolated from the third day after adult molt, but not in younger ones, and had maximal activity after the fifth day. This stimulatory effect was observed equally from females reared in the presence or in the absence of males, excluding a regulatory effect of mating. The mode of action of insulin in blowflies did not involve cAMP, but triggered a specific and well-conserved transduction cascade. In particular, a peroxovanadium compound, known to activate specifically the insulin receptor in mammals, also stimulated blowfly ovarian steroidogenesis in vitro. Conversely, chemicals known to inhibit the mammalian insulin receptor or downstream elements of its signaling pathway, such as LY294002, an inhibitor of phosphatidylinositol 3-kinase (PI3K), were able to prevent the steroidogenic action of bovine insulin on fly ovaries. Extracts from the median neurosecretory cells (MNCs) of blowfly brains, which are known to contain endogenous ILPs, stimulated ovarian steroidogenesis very efficiently and were also sensitive to inhibition by LY294002. These experiments indicated the involvement of PI3K in the mode of action of MNC extracts and substantiated that their endogenous ILPs are involved in the regulation of ovarian steroidogenesis. This conclusion was corroborated by the effects of synthetic bombyxin II, an ILP originating from silkworm MNCs, which also stimulated steroidogenesis in isolated blowfly ovaries. Altogether, these data suggest that insulinlike neurohormones from MNCs play a crucial role as steroidogenic gonadotropins in female flies.

Control of Metabolism and Growth Through Insulin-Like Peptides inDrosophila

“…A similar direct activation has been reported in the adult ovaries of other insects (19,20). IIS activity may thus constitute a specific input for the regulation of basal ecdysteroid synthesis, allowing the growth program or nutritional information to be coupled with ecdysteroid production.…”

Peptide immunocytochemistry of neurons projecting to the retrocerebral complex in the blow fly, Protophormia terraenovae

“…Possible gonadotropic peptides, which promote the biosynthesis of JH or ecdysteroids, have been reported in some species. In P. regina, insulinlike peptides in the PI induce ecdysteroid production by the ovaries (Manière et al 2004). In the mosquito A. aegypti, OEH I promotes ecdysteroid production in the ovary (Brown et al 1998).…”

Insulin stimulates ecdysteroidogenesis by prothoracic glands in the silkworm, Bombyx mori

“…These results indicate multiple regulatory mechanisms of insect prothoracic gland ecdysteroidogenesis. Although it was previously reported that bovine insulin stimulates ecdysteroid production by ovaries in vitro in both Aedes aegypti and Phormia regina (Graf et al, 1997;Riehle and Brown, 1999;Maniè re et al, 2004), it is not clear whether or not insulin can stimulate ecdysteroidogenesis by lepidopteran insect prothoracic glands. It was demonstrated that bombyxin, an insulin-like peptide in B. mori, stimulated prothoracic glands isolated from Samia cynthia, but not those from B. mori (Ishizaki and Suzuki, 1994;Wu and Brown, 2006).…”