20-hydroxyecdysone Promotes The Expression And Phosphorylation Of Yorkie To Induce Apoptosis In The Larval Midgut At Metamorphosis Stage Of Helicoverpa Armigera

The molting and metamorphosis of insects are regulated by 20-hydroxyecdysone (20E) and juvenile hormone (JH). Larval organs will go through programmed cell death (PCD) at metamorphosis stage. The mechanism is expression and activation of the apoptotic related proteins induced by high pulse of 20E. The larval midgut of lepidoptera insect Helicoverpa armigera(H.armigera) undergoes destruction at metamorphosis stage. Apoptosis was found in this process. Thus, the larval mdigut of H.armigera provides a model for researches of PCD in insects. The downregulation and inactivation of inhibitor of apoptosis 1 (IAP1) are essential for PCD of larval organs at metamorphosis. I API is the target protein of the transcriptional co-activator Yorkie (Yki). Yki is the downstream protein of Hippo pathway and determine the fate of cells. Unphosphorylated Yki translocates into nucleus and interacts with Scalloped (Sd) to promote the expression of IAP1. Masses of IAP1 inhibit cell apoptosis. These indicate Yki may be involved in the PCD 20E induced.What functions Yki plays in the PCD of larval organs of insects? How does Yki participate into the PCD 20E induced? There are little researches in these aspects. We investigated them using the model of larval midgut apoptosis in H.armigera.We found that 20E upregulates Yki expression, phosphorylation and cytoplasmic location to promote midgut PCD in H.armigera. Yki is highly expressed in midgut during metamorphosis and up-regulated by 20E induction. Knockdown of Yki in larvae blocked the larval-pupal transition and midgut PCD. Yki is located in the cytoplasm of larval midgut cells when it is performing PCD during larval-pupal transition.20E arrested Yki in the cytoplasm by increasing the phosphorylation level of Yki and the interaction between Yki and protein 14-3-3-ε. Cytoplasmic located-Yki could not induce the expression of inhibitor of apoptosis, therefore promoted PCD. These results suggest that 20E promotes Yki expression, phosphorylation and cytoplasmic location to promote insect midgut apoptosis.