20-hydroxyecdysone induces apoptosis in the labial gland of Manduca sexta

The labial glands of Manduca sexta die over 5 days during larva to pupa metamorphosis. This cell death is presumably triggered by endocrine cues. The mechanisms by which steroid hormones induce apoptosis, however, are poorly understood. To investigate the role that the insect molting hormone, 20hydroxyecdysone (20HE), plays in apoptosis of larval structures, we injected animals with exogenous 20HE. Since metamorphosing Manduca larvae have high titers of 20HE in the hemolymph, we surgically removed the prothoracic glands, which secrete 20HE, in order to create hormone-free abdomens. Labial glands from whole animals were used to establish the baseline levels of apoptosis during metamorphosis. Lysosomes play a critical role in the degradation of insect tissues (salivary gland, intersegmental muscle, and fat body) and mammalian tissues (mammary gland, prostate gland, and uterus). Apoptosis was monitored using acid phosphatase activity and histochemistry to monitor lysosomes and TUNEL to detect DNA fragmentation. Glands that were exposed to 20HE displayed an increase in the number of lysosomes, movement of lysosomes, and activation of acid phosphatase compared to controls. Glands from abdomens that received a single injection of hormone demonstrated levels of apoptosis that were comparable to glands from abdomens that were subjected to multiple 20HE injections. Our results suggest that 20HE triggers apoptosis of the labial glands and that its continuous administration is not necessary to kill the cells.