Ecdysteroids and activation of epidermal cells in the locust, Locusta migratoria

The ecdysteroids control the biosynthesis of cuticular macromolecules, i.e. chitin and proteins, and the metabolic activity of epidermal cells during post-embryonic development of insects. The effects of some naturally occurring ecdysteroids (ecdysone, 20-hydroxyecdysone, makisterone A) and of various growth inhibitors on cuticle synthesis have been investigated in vitro, using wing-pad epidermis of 1-day, fifth-instar larvae of Locusta migratoria as a biological model system. The biosynthesis of chitin was studied by measuring incorporation of a radiolabelled precursor, [ 14C] N-acetyl-G-glucosamine (0.4 μCi/ml medium), after 6 h following a 3 day culture in synthetic medium for insects (S20 Landureau). Ecdysone (10 −8 M to 10 −5 M) had no effect on chitin synthesis, but 20-hydroxyecdysone exhibited a stimulatory effect after at least 36 h of presence. 20-hydroxyecdysone acted in a dose-dependent manner for concentrations ranging from 10 −7 M to 10 −5 M. Cuticle deposition required integrity of microtubular system as shown by inhibitory effect on colcemid on chitin synthesis. Diflubenzuron and azadirachitin also had an inhibitory effect on this process. For the study of protein synthesis, [ 3H]leucine (1 μCi/ml medium) was added during the first 5 h of 24- or 72-h cultures in S20 medium (Leu-) supplemented or not with ecdysteroids. Protein synthesis was stimulated by both 20-hydroxyecdysone and ecdysone, but the former was far more potent than the latter. Protein phosphorylations are generally involved in cellular activation. Protein kinase activity and levels of phosphorylation of various substrates change during the maturation of epidermis of last larval instar. Preliminary results support the hypothesis of a participation of 20-hydroxyecdysone in the regulation of some steps of these processes.