20-Hydroxyecdysone

20-Hydroxyecdysone (20E) is an important steroid hormone that controls embryonic development, molting, metamorphosis, and reproduction in members of the phylum Arthropoda. This subchapter deals mainly with three subjects: (1) Ecdysone, precursor of 20E, is synthesized from dietary cholesterol in insect prothoracic glands, secreted to hemolymph, and oxidized to 20E in the peripheral tissues such as fat body. Halloween genes, which encode the cytochrome P450 monooxygenases, are involved in several steps of 20E biosynthesis. (2) 20E activates the genes related to molting and metamorphosis via the EcR/USP heterodimer that belongs to the nuclear receptor superfamily (genomic action). 20E also modulates intracellular signaling cascades via specific transmembrane-spanning GPCRs (non-genomic action). (3) Major reactions contributing to the inactivation of 20E are epimerization, ecdysonic acid formation, phosphorylation, fatty acylation, and glucosylation.