Meeting Abstract
Wing disc growth in lepidopteran insects is jointly stimulated by ecdysteroids and insulin-like hormones such as bombyxin. While growth of the discs during the early part of the last larval stage is dependent upon nutrition, in Manduca sexta the majority of wing disc growth occurs during the prepupal stage after the insects have stopped feeding. In the present study, we explore the growth of discs removed from larvae just after the cessation of feeding, to better understand the nature of cross-talk between insulin and ecdysteroid signaling at this stage. Discs from wandering fifth instar larvae were cultured for two days in Grace’s medium containing 20-hydroxyecdysone, recombinant human insulin, or both hormones. At levels corresponding to peak prepupal levels of circulating ecdysteroids, 20-hydroxyecdysone on its own increased disc content of the ecdysteroid receptor and stimulated limited growth in a manner suggesting the involvement of insulin signaling pathways. Specifically, prolonged exposure to 20-hydroxyecdysone alone stimulated the phosphorylation of Akt/protein kinase B, and of Akt substrates such as glycogen synthase kinase (GSK), as measured using Western blots. Insulin enhanced the phosphorylation of Akt and GSK to a far greater extent than 20-hydroxyecdysone, yet insulin alone did not stimulate growth. The results suggest that, while growth is stimulated most effectively by insulin and ecdysteroids acting together, ecdysteroids are capable of limited activation of insulin signaling independently, by a mechanism that remains to be determined.
