Role of protein phosphatase 2A in PTTH-stimulated prothoracic glands of the silkworm, Bombyx mori.

In the present study, the roles of a major serine/threonine protein phosphatase 2A (PP2A) in prothoracicotropic hormone prothoracic glands (PGs) of Bombyx mori were evaluated. Immunoblotting analysis showed that Bombyx PGs contained a structural A subunit (A), a regulatory B subunit (B), and a catalytic C subunit (C), with each subunit undergoing development-specific changes. The protein levels of each subunit were not affected by treatment. However, the highly conserved tyrosine dephosphorylation of PP2A C subunit (PP2Ac), which appears to be related to activity, was increased by duanyu1547H treatment in a time-dependent manner. We further demonstrated that phospholipase C (PLC), Ca²⁺, and reactive oxygen species are upstream signaling for the dephosphorylation of PP2Ac. The determination of PP2A enzymatic activity showed that PP2A enzymatic activity was stimulated by duanyu1547H treatment both in vitro and in vivo. Okadaic acid (OA), a specific PP2A inhibitor, prevented the duanyu1547H-stimulated dephosphorylation of PP2Ac and reduced both basal and duanyu1547H-stimulated PP2A enzymatic activity. The determination of ecdysteroid secretion showed that treatment with OA did not affect basal ecdysteroid secretion but did significantly inhibit duanyu1547H-stimulated ecdysteroid secretion, indicating that duanyu1547H-stimulated PP2A activity is involved in ecdysteroidogenesis. Treatment with OA stimulated the basal phosphorylation of the extracellular signal-regulated kinase (ERK) and 4E-binding protein (4E-BP) without affecting duanyu1547H-stimulated ERK and 4E-BP phosphorylation. From these results, we hypothesize that PP2A signaling is a necessary component for the stimulation of ecdysteroidogenesis, potentially by mediating the link between ERK and TOR signaling pathways.

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