Lactate dehydrogenase is associated with PTTH-stimulated ecdysteroidogenesis in Bombyx mori

Lactate dehydrogenase (LDH) converts pyruvate into lactate and plays a critical role in aerobic glycolysis. A previous study showed that increased expression of sugar transporter 1 (St1) and trehalase 1 (Treh1), along with elevated Treh activity, contributed to prothoracicotropic hormone (PTTH)-stimulated ecdysteroidogenesis in the prothoracic glands (PGs) of Bombyx mori. However, the role of LDH in PTTH-stimulated ecdysteroidogenesis remains poorly understood. In this study, I investigated the relationship between LDH expression and PTTH-stimulated ecdysteroidogenesis in silkworm PGs. Basal LDH activity exhibited developmental stage-specific changes, with high activity observed during the late phase of the last larval instar and on the day of pupation. LDH activity was stimulated by PTTH treatment in a time- and stage-dependent manner in vitro. Pretreatment with U0126 (a mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK) inhibitor) partially inhibited PTTH-stimulated LDH activity, suggesting that ERK likely functions upstream of LDH activation. Pretreatment with sodium oxamate, a chemical LDH inhibitor, suppressed PTTH-stimulated LDH activity, decreased ERK signaling, and inhibited ecdysteroid secretion, indicating that PTTH-stimulated LDH activity is involved in ecdysteroidogenesis. LDH gene expression exhibited developmental stage-specific changes, with a sharp increase detected on the day of pupation. Immunoblotting confirmed the presence of LDH protein in Bombyx PGs, with protein levels varying during the final larval instar and on the day of pupation. PTTH treatment did not affect LDH protein levels but significantly increased LDH gene expression in a time-dependent manner. These results suggest that PTTH-regulated LDH expression is associated with ecdysteroidogenesis. To my knowledge, this is the first study demonstrating that LDH, likely acting downstream of the PTTH/ERK pathway, contributes to PTTH-stimulated ecdysteroidogenesis in B. mori.