20‐HYDROXYECDYSONE UPREGULATES APOPTOTIC GENES AND INDUCES APOPTOSIS IN THE B OMBYX FAT BODY

During insect metamorphosis, obsolete larval tissues are removed by programed cell death ( PCD ), mainly apoptosis and autophagy, which is directed by the molting hormone, 20‐hydroxyecdysone (20 E ) and the 20 E ‐triggered transcriptional cascade. Here, we investigated how 20 E regulates apoptosis at the transcriptional level in the fat body of the silkworm, B ombyx mori. As detected by TdT‐mediated dUTP Nick‐End Labeling ( TUNEL ), apoptosis weakly occurred during the fourth larval molting, decreased to undetected levels during the early fifth instar, and gradually increased from day 4 of fifth instar to the wandering stage to the prepupal stage. Meanwhile, as determined by quantitative real‐time PCR , eight genes involved in apoptosis, including A paf‐1, N edd2 like1, N edd2 like2, ICE 1, ICE 3, ICE 5, A rp, and IAP , were highly expressed during molting and pupation, when the 20 E titer is high. Injection of 20 E into day 2 of fifth instar larvae significantly induced apoptosis and upregulated apoptotic genes after 6 h of treatment, and in vitro treatment of larval fat body tissues with 20 E upregulated all the eight apoptotic genes. Moreover, RNAi knockdown of USP , a component of the 20 E receptor complex EcR‐USP , at the early‐wandering stage reduced apoptosis and downregulated apoptotic genes after 24 h of treatment. Taken together, we infer that 20 E upregulates apoptotic genes and thus induces apoptosis in the B ombyx fat body during larval molting and the larval–pupal transition.