Modulation of gene expression by nutritional state and hormones in Bombyx larvae in relation to its growth period

Insect growth and development are mainly regulated via synchronization of many extrinsic and intrinsic factors such as nutrition and hormones. Previously we have demonstrated that larval growth period influences the effect of insulin on the accumulation of glycogen in the fat body of Bombyx larvae. In the present study we demonstrate that Bombyx larvae at the terminal growth period (TGP, after critical weight) had a significantly greater increase in the expression level of Akt in the fat body than at the active growth period (AGP, before critical weight). The larvae at TGP also showed an increase in the expression level of ecdysone receptors (EcRB1 and USP1) and ecdysone-induced early genes (E75A and broad). The treatment of bovine insulin and methoprene to larvae at AGP induced the transcript levels of Akt, irrespective of the nutritional status of the larvae. However, in larvae at TGP, insulin repressed the transcript level of Akt. On contrary, 20-hydroxyecdysone (20E) induced the expression level of Akt in TGP larvae, but at feeding only. Insulin and 20E thus showed an antagonistic action on the Akt expression level in TGP larvae under feeding. The studies thus showed that larval growth period influences the expression level of Akt and ecdysone receptors in Bombyx. Further, the growth period and nutrition modulate the effect of exogenous hormones on Akt expression.