Small GTPase Rab4b participates in the gene transcription of 20-hydroxyecdysone and insulin pathways to regulate glycogen level and metamorphosis

Abstract

The insulin and 20-hydroxyecdysone (20E) pathways coordinately regulate insect growth and metamorphosis. However, the molecular mechanism of the interaction of these two pathways in regulating insect development is not well understood. In the present study, we found that a small GTPase Rab4b from a lepidopteran insect Helicoverpa armigera participates in gene transcription in the two pathways. The results show that RNA interference of Rab4b in larvae results in a decrease in glycogen levels, small pupae, abnormal metamorphic transition, or larval death. The molecular mechanisms are demonstrated that knockdown of Rab4b in the larvae suppresses the transcription of glycogen synthase (GS), as well as the metamorphic-initiating factor (Br) and hormone receptor 3 (HR3), but increases the transcription of Forkhead box class O (FOXO). Further studies in the cell line confirm that Rab4b is necessary for gene transcription in the insulin and 20E pathways. Rab4b locates in the cytoplasm and takes part in regulation on FOXO cytoplasmic location by insulin induction, but travels toward the cell membrane upon 20E induction without affecting the FOXO location. The transcription of Rab4b could be upregulated by insulin injection or glucose feeding to the larvae, but not by 20E or juvenile hormone analogy methoprene. Our data suggest that Rab4b takes part in metamorphosis by regulating gene transcription and glycogen level in the insulin and 20E pathways.

20-hydroxyecdysone activates Forkhead box O to promote proteolysis during Helicoverpa armigera molting

“…Previous studies have shown that insulin suppresses FoxO expression and keeps FoxO in the cytoplasm, and that 20E increases FoxO expression in H. armigera (Hou et al, 2012). 20E promotes FoxO nuclear localization in D. melanogaster (Colombani et al, 2005;Hossain et al, 2013).…”

“…FoxO expression is upregulated by the steroid hormone 20-hydroxyecdysone (20E), and is located in the cytoplasm during insulin induction in the lepidopteran insect Helicoverpa armigera (Hou et al, 2012). Feeding larvae with 20E causes nuclear localization of FoxO in the fat body cells of Drosophila melanogaster (Colombani et al, 2005).…”

Diet-delivered RNAi in Helicoverpa armigera – Progresses and challenges

“…RNAi mediated by dsRNA ingestion requires higher dosage and multiple administrations until the onset of silencing in insects when compared to RNAi via injection in the haemolymph (Chen et al, 2008;Tian et al, 2009;Zhu et al, 2011), although dosages needed to elicit effect via injection in H. armigera remain measured in micrograms (Bober and Rafaeli, 2010;Cai et al, 2014;Du et al, 2012;Hou et al, 2012;Qiao et al, 2014;Sivakumar et al, 2007;Tang et al, 2012;Wang et al, 2010;Yang and Han, 2014a).…”

Eat to reproduce: a key role for the insulin signaling pathway in adult insects

“…Some reports describe molecular signal transduction components that take part in both pathways [e.g., the small GTPase Rab4b (Hou et al, 2012)], while others demonstrate functional interactions between components of both pathways, resulting in modulated activity of one of both pathways [e.g., interaction between FOXO and an ecdysone receptor coactivator (Francis et al, 2010); regulatory activity on insulin signaling by an ecdysone-repressed microRNA (Jin et al, 2012); ecdysone inhibiting insulin signaling (Colombani et al, 2005)]. Also in the regulation of insect reproductive physiology, multiple functional links between both pathways have been demonstrated.…”