Involvement of PI3K/Akt signaling in PTTH-stimulated ecdysteroidogenesis by prothoracic glands of the silkworm, Bombyx mori

The prothoracicotropic hormone (PTTH) stimulates ecdysteroidogenesis by prothoracic gland in larval insects. Previous studies showed that Ca²⁺, cAMP, extracellular signal-regulated kinase (ERK), and tyrosine kinase are involved in PTTH-stimulated ecdysteroidogenesis by the prothoracic glands of both Bombyx mori and Manduca sexta. In the present study, the involvement of phosphoinositide 3-kinase (PI3K)/Akt signaling in PTTH-stimulated ecdysteroidogenesis by B. mori prothoracic glands was further investigated. The results showed that PTTH-stimulated ecdysteroidogenesis was partially blocked by LY294002 and wortmannin, indicating that PI3K is involved in PTTH-stimulated ecdysteroidogenesis. Akt phosphorylation in the prothoracic glands appeared to be moderately stimulated by PTTH in vitro. PTTH-stimulated Akt phosphorylation was inhibited by LY294002. An in vivo PTTH injection into day 6 last instar larvae also increased Akt phosphorylation of the prothoracic glands. In addition, PTTH-stimulated ERK phosphorylation of the prothoracic glands was not inhibited by either LY294002 or wortmannin, indicating that PI3K is not involved in PTTH-stimulated ERK signaling. A23187 and thapsigargin, which stimulated B. mori prothoracic gland ERK phosphorylation and ecdysteroidogenesis, could not activate Akt phosphorylation. PTTH-stimulated ecdysteroidogenesis was not further activated by insulin, indicating the absence of an additive action of insulin and PTTH on the prothoracic glands. The present study, together with the previous demonstration that insulin stimulates B. mori ecdysteroidogenesis through PI3K/Akt signaling, suggests that crosstalk exists in B. mori prothoracic glands between insulin and PTTH signaling, which may play a critical role in precisely regulated ecdysteroidogenesis during development.