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

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 Ca 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 Ca 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.