Transcriptional regulation of the insulin signaling pathway genes by starvation and 20-hydroxyecdysone in the Bombyx fat body

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.