Two dehydroecdysone reductases act as fat body-specific 20E catalyzers in Bombyx mori

Periodic post-embryonic changes in insects, including growth, development and metamorphosis, are strictly controlled by many compounds, including steroid hormones. The biosynthesis and clearance of 20-hydroxyecdysone (20E), the major active form of the insect steroid hormone ecdysone, result in titer fluctuations that help control insect development. The inactivation of 20E in the silkworm Bombyx mori is highly tissue-specific, with CYP18A1 and ecdysone oxidase controlling 20E inactivation specifically in the mid-silk gland and midgut, respectively. Here, we characterized silkworm 3-dehydroecdysone 3α reductase (Bm3DE3α) and 3-dehydroecdysone 3β reductase (Bm3DE3β), two enzymes involved predominantly in the C-3-mediated catalysis of 20E in fat bodies. The ubiquitous and silk gland-specific overexpression of Bm3DE3α decreased the 20E titer, resulting in larval lethality and larval–pupal transition failure, respectively. In contrast, the ubiquitous and mid-silk gland-specific overexpression of Bm3DE3β increased the 20E titer, resulting in larval growth delays and lethality at the mid-fifth larval stage, respectively. Thus, Bm3DE3α and Bm3DE3β mediate fat body-specific steroid hormone metabolism in B. mori, indicating that highly diversified 20E metabolism-related mechanisms exist in different insect species.