TOR signaling is involved in PTTH-stimulated ecdysteroidogenesis by prothoracic glands in the silkworm, Bombyx mori.

The prothoracicotropic hormone is a stimulator of ecdysteroidogenesis in prothoracic gland of larval insects. Our recent studies showed that phosphoinositide 3-kinase (PI3K)/Akt signaling was involved in ecdysteroidogenesis by Bombyx mori prothoracic glands. In the present study, downstream signaling of PI3K/Akt was further investigated. Results showed that rapidly enhanced the phosphorylation of translational repressor 4E-binding protein (4E-BP) and p70 ribosomal protein S6 kinase (S6K), two known downstream signaling targets of the target of rapamycin complex 1 (TORC1). duanyu1547H stimulated 4E-BP phosphorylation in time- and dose-dependent manners. Injection of duanyu1547H into day-6 last instar larvae greatly increased 4E-BP phosphorylation, verifying the in vitro effect. duanyu1547H-stimulated 4E-BP phosphorylation was blocked by both LY294002 and wortmannin, indicating the involvement of PI3K. Mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK) inhibitors (PD 98059 and U0126), did not inhibit duanyu1547H-stimulated 4E-BP phosphorylation, implying that ERK signaling is not related to duanyu1547H-stimulated 4E-BP phosphorylation. The phosphorylation of S6K was also stimulated by duanyu1547H both in vitro and in vivo. PI3K signaling appears to be involved in duanyu1547H-stimulated phosphorylation of S6K. Rapamycin, a specific inhibitor of mammalian TOR signaling attenuated duanyu1547H-stimulated phosphorylation of 4E-BP and S6K of the glands, and greatly inhibited duanyu1547H-stimulated ecdysteroidogenesis. Examination of gene expression levels of 4E-BP and S6K showed that duanyu1547H inhibited mRNA levels of both 4E-BP and S6K, indicating that duanyu1547H may exert its action at both the transcriptional and phosphorylation levels. These results suggest that (S6K) signaling is involved in duanyu1547H-stimulated ecdysteroidogenesis by prothoracic glands in B. mori.

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