Involvement of RSK phosphorylation in PTTH‐stimulated ecdysone secretion in prothoracic glands of the silkworm, Bombyx mori

Abstract

It is well known that phosphorylation of extracellular signal‐regulated kinase (ERK) is involved in prothoracicotropic hormone (PTTH)‐stimulated ecdysteroidogenesis in insect prothoracic glands (PGs). In the present study, we further investigated the downstream signalling pathways. Our results showed that PTTH stimulated p90 ribosomal S6 kinase (RSK) phosphorylation at Thr573 in Bombyx mori PGs both in vitro and in vivo. The in vitro PTTH stimulation was stage‐ and dose‐dependent. The absence of Ca2+ reduced PTTH‐stimulated RSK phosphorylation. Stimulation of RSK phosphorylation was also observed after treatment with either A23187 or thapsigargin. A phospholipase C (PLC) inhibitor, U73122, blocked PTTH‐stimulated RSK phosphorylation. These results indicate the involvement of Ca2+ and PLC. Treatment with diphenylene iodonium (DPI), a mitochondrial oxidative phosphorylation inhibitor, blocked PTTH‐regulated RSK phosphorylation, indicating its redox regulation. A mitogen‐activated protein kinase (MAPK)/ERK kinase (MEK) inhibitor, U0126, but not a phosphatidylinositol 3‐kinase (PI3K) inhibitor, LY294002, decreased PTTH‐stimulated RSK phosphorylation, indicating that ERK is an upstream signalling. A protein kinase C (PKC) inhibitor, chelerythrine C, inhibited PTTH‐stimulated RSK phosphorylation, and a PKC activator, phorbol 12‐myristate acetate (PMA) stimulated RSK phosphorylation, indicating the involvement of PKC. BI‐D1870, a specific RSK inhibitor, partly prevented PTTH‐stimulated RSK phosphorylation and significantly inhibited PTTH‐stimulated ecdysteroid secretion, indicating that PTTH‐stimulated RSK phosphorylation is involved in ecdysteroidogenesis. Taken together, these data indicate that PTTH activates RSK phosphorylation which plays important roles in PTTH‐stimulated ecdysteroidogenesis.

Circadian clock regulates developmental time through ecdysone and juvenile hormones in Bombyx mori

“…Ecdysone, the precursor of 20‐hydroxyecdysone (20E), is synthesized by the prothoracic gland (PG) and secreted into the hemolymph to be activated as 20E, triggering larval moulting and metamorphosis. Ecdysone is primarily regulated by a neuropeptide known as prothoracicotropic hormone (PTTH) (Christensen et al, 2020; Gu & Chen, 2021; Kataoka et al, 1991). PTTH is synthesized in two pairs of dorsolateral PTTH neurons in the brain and is transported to the corpora allata (CA) by axons running through the contralateral hemisphere of the brain, for storage and later secretion (Uchibori‐Asano et al, 2017).…”

Insect hormone PTTH regulates lifespan through temporal and spatial activation of NF-κB signaling during metamorphosis

“…Prothoracicotropic hormone (Ptth) is a brain neuropeptide hormone that stimulates the secretion of the molting hormone, ecdysone, from the prothoracic glands (PG) in insect larvae 1 . Once ecdysone is released into the hemolymph, it is converted to an active form, 20-hydroxyecdysone (20E), by a P450 monooxygenase at downstream target tissues 2 . 20E is the primary molting hormone that binds to a nuclear receptor (EcR) and initiates various gene expression cascades, which ultimately lead to the physiological, morphological, and behavioral changes associated with molting and metamorphosis 3 .…”