Insulin and 20-hydroxyecdysone Oppose Each Other in the Regulation of Phosphoinositide-dependent Kinase-1 Expression During Insect Pupation

Insulin promotes larval growth of insects by stimulating the synthesis of the steroid hormone 20-hydroxyecdysone (20E), which induces pupation and apoptosis. However, the mechanism underlying the coordinate regulation of insect pupation and apoptosis by these two functionally opposing hormones is still unclear. Here, using the lepidopteran insect and serious agricultural pest (cotton bollworm) as a model, we report that phosphoinositide-dependent kinase-1 (PDK1) and forkhead box O (FoxO) play key roles in these processes. We found that the transcript levels of the gene are increased during the larval feeding stages. Moreover, expression was increased by insulin, but repressed by 20E. dsRNA-mediated knockdown in the larvae delayed pupation and resulted in small pupae and also decreased /protein kinase B expression and increased expression. Furthermore, the knockdown blocked midgut remodeling and decreased 20E levels in the larvae. Of note, injecting larvae with 20E overcame the effect of the knockdown and restored midgut remodeling. overexpression in an epidermal cell line (HaEpi) did not induce apoptosis, but promoted autophagy and repressed cell proliferation. These results reveal cross-talk between insulin and 20E and that both hormones oppose each other's activities in the regulation of insect pupation and apoptosis by controlling expression and, in turn, expression. We conclude that sufficiently high 20E levels are a key factor for inducing apoptosis during insect pupation.