[Show abstract][Hide abstract] ABSTRACT: In a previous study, we have shown that the molting hormone, 20-hydroxyecdysone (20E), reduces insect food consumption resulting in fat body lipolysis during the non-feeding molting and pupation stages, and assumed that the transcription factor FoxO is involved in this process. To verify this hypothesis, we cloned foxO from the silkworm, Bombyx mori. During molting and pupation, FoxO is highly expressed and predominantly localizes in the nuclei of fat body cells. 20E induced foxO mRNA expression and FoxO nuclear localization resulting in an increase of FoxO transcriptional activity. RNAi of foxO prior to the 4(th) larval molting downregulated two lipase genes - the insect adipose triacylglycerol lipase homologue, brummer, and an acid lipase, acid lipase-1, in the fat body. Overexpression of the constitutively-active form of foxO (foxO(CA)) upregulated brummer and acid lipase-1 in both the fat body and Bombyx Bm-12 cells. Putative FoxO-response elements (FREs) are present in the promoter regions of brummer and acid lipase-1, and mutation of the FREs attenuated their FoxO-induced luciferase activities. ChIP assay revealed that FoxO binds directly to those FREs. Moreover, foxO(CA) overexpression in vivo doubled lipid concentration in the hemolymph, increased total lipase activity, and slightly but significantly reduced lipid content in the fat body. Taken together, we conclude that 20E increases the transcriptional activity of FoxO which, in turn, upregulates brummer and acid lipase-1 and induces lipolysis in the Bombyx fat body during molting and pupation.
[Show abstract][Hide abstract] ABSTRACT: Sericulture has been greatly advanced by applying hybrid breeding techniques to the domesticated silkworm, Bombyx mori, but has reached a plateau during the last decades. For the first time, we report improved silk yield in a GAL4/UAS transgenic silkworm. Overexpression of the Ras1(CA) oncogene specifically in the posterior silk gland improved fibroin production and silk yield by 60%, while increasing food consumption by only 20%. Ras activation by Ras1(CA) overexpression in the posterior silk gland enhanced phosphorylation levels of Ras downstream effector proteins, up-regulated fibroin mRNA levels, increased total DNA content, and stimulated endoreplication. Moreover, Ras1 activation increased cell and nuclei sizes, enriched subcellular organelles related to protein synthesis, and stimulated ribosome biogenesis for mRNA translation. We conclude that Ras1 activation increases cell size and protein synthesis in the posterior silk gland, leading to silk yield improvement.
Cell Research 03/2011; 21(6):934-43. DOI:10.1038/cr.2011.36 · 12.41 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Genetic studies in the fruitfly, Drosophila melanogaster, have uncovered a conserved insulin/insulin growth factor signaling (IIS) pathway that regulates nutrition-dependent growth rates of insects. From the silkworm, Bombyx mori, we have identified and characterized several key genes involved in the IIS pathway, including InR, IRS, PI3K110, PI3K60, PTEN, PDK, and Akt. Tissue distribution analysis showed that most of these genes were highly expressed in the fat body implying that the IIS pathway is functionally important within insect adipose tissue. Developmental profile studies revealed that the expression levels of InR, IRS, PI3K110, and PDK were elevated in the fat body during molting and pupation, periods when animals ceased feeding and hemolymph levels of 20-hydroxyecdysone (20E) were high. Starvation rapidly up-regulated the mRNA levels of these same genes in the fat body, while 20E slowly induced their transcription. We conclude that 20E slowly reduces food consumption and then indirectly induces a state of starvation resulting in the elevation of the mRNA levels of InR, IRS, PI3K110, and PDK in the Bombyx fat body during molting and pupation.