20-hydroxyecdysone Reduces Insect Food Consumption Resulting in Fat Body Lipolysis During Molting and Pupation

Abstract

The insect steroid hormone 20-hydroxyecdysone (20E) acts through a specific nuclear receptor complex, ecdysone receptor (EcR) and ultraspiracle (USP). EcR and USP are FXR/LXR and RXR orthologs, respectively, which play critical roles in the regulation of lipid metabolism in mammals. Lipid concentration in Bombyx hemolymph and lipase activity in fat body peaked during molting and pupation, suggesting that 20E induces lipolysis at these stages. Differing from their mammalian orthologs, the 20E-bound EcR-USP was not able to directly stimulate fat body lipolysis in both Bombyx and Drosophila. Instead in Bombyx, 20E slowly reduced food consumption and then induced starvation, resulting in fat body lipolysis. Molecular analysis revealed that the evolutionarily conserved adipose triacylglycerol lipase gene Brummer was transcriptionally up-regulated by 20E-induced starvation during molting and pupation. To our knowledge, this is the first report demonstrating that the steroid hormone 20E is a critical regulator of lipolysis in insects.

BmATG5 and BmATG6 mediate apoptosis following autophagy induced by 20-hydroxyecdysone or starvation

“…In Bombyx, 20E slowly reduces food consumption and then induces starvationlike conditions during molting and pupation, providing a correlation between 20E and starvation in the induction of both autophagy and apoptosis. 31 In a previous study, we have found that autophagy precedes apoptosis in the midgut during Bombyx metamorphosis, but the core mechanism is not illustrated. 40 Here, we found that starvation and 20E addition caused significant cell death after 4 to 16 h treatment, with autophagy proceeding apoptosis at least by 3 h in Bombyx Bm-12 cells (Figs.…”

“…In addition, a long period of starvation caused degradation of ATG8 in S1-HP cells under amino acid starvation (S. litura), 44 it appears the same case in Bm-12 cells at 16 h after starvation consistent with the change of LysoTracker Red staining. In vivo, 20E induces starvation-like condition in a complex manner; 31,43,45 in turn, starvation induces autophagy and apoptosis. These phenomena could explain why both 20E and starvation causes massive cell death in the larval midgut during Bombyx metamorphosis, with autophagy preceding apoptosis.…”

“…40 In 2 other lepidopteran insect cell lines, glucose starvation in Sl-1 cells (from Spodoptera litura) and amino acid starvation in SL-ZSU-1cells (S. litura) induces autophagy preceding apoptosis at least by 48 h. 41,42 In this work, we used PBS starvation and 20E treatment to mimic the conditions during larval-pupal transition, when the animals are severely starved and the 20E titer is extremely high. 31,43 Together, in certain circumstances, it is a common feature that autophagy precedes apoptosis induced by the common stimuli such as 20E or starvation.…”

“…These phenomena could explain why both 20E and starvation causes massive cell death in the larval midgut during Bombyx metamorphosis, with autophagy preceding apoptosis. 31,40 However, it is currently unclear whether 20E induces starvation-like conditions in cultured cells. Our preliminary experiments suggest that 20E might inhibit the PI3K-AMPK-TORC1 interactions to initiate autophagosome formation in cultured cells, but the underlying molecular mechanism requires further investigation.…”

“…28,30 Interestingly, 20E slowly reduces food consumption and then induces starvation-like conditions during molting and pupation in Bombyx, providing a correlation between 20E and starvation in the induction of both autophagy and apoptosis. 31 In Drosophila, the relationship between autophagy and caspases appears to be context specific, varying by developmental stage and cell type. 8 During metamorphosis, many obsolete larval tissues undergo massive destruction mainly by autophagic cell death.…”