Ecdysone-inducible functions of larval fat bodies in Drosophila

Late in the third instar larval stage of Drosophila melanogaster, the titer of the steroid hormone ecdysone increases sharply. This increase is blocked in the temperature-sensitive mutant ecd(1) after a temperature shift from 20 degrees C to 29 degrees C. The mutant was used to prepare three samples of late third instar larvae with different titers of ecdysone; the titer was low in one sample because of an earlier temperature shift, high in a second sample because the larvae were subsequently transferred to ecdysone-supplemented food, and also high in a third sample that was kept at 20 degrees C, providing a control for normal development. The effect of the high titer of ecdysone on proteins of the larval fat bodies was examined by comparing two-dimensional gel electrophoresis patterns of total proteins in stained gels. There were proteins at five positions in the gels for the high-ecdysone samples that were not detected at the corresponding positions in the gel for the low-ecdysone sample. The effect of ecdysone on these proteins was further studied by injecting [(35)S]methionine into the larvae at both early and late third instar stages, in order to label proteins synthesized before and after the increase in ecdysone titer. The results indicate that ecdysone induces two major responses in the fat bodies; certain proteins that were synthesized earlier in the fat bodies and secreted into the hemolymph are incorporated back into the fat bodies, and other proteins are newly synthesized. Attempts to induce prematurely the synthesis of the new proteins by exposing early third instar larvae to exogenous ecdysone were unsuccessful, suggesting that development must proceed further before the fat bodies can respond to ecdysone. By in vitro translation of RNA isolated from fat bodies of low-and high-ecdysone samples of larvae, it was shown that ecdysone greatly increases the amount of translatable messenger RNA for one of the newly synthesized proteins. A clone of DNA complementary to the induced messenger RNA has been isolated from a population of lambda bacteriophage carrying segments of the Drosophila genome. Using the cloned DNA to measure amounts of complementary poly(A)-RNA in the fat bodies by DNA.RNA hybridization, we detected about 50 times more complementary poly(A)-RNA in the high-ecdysone sample of larvae than in the low-ecdysone sample. This finding provides direct evidence that ecdysone induces an increase in the amount of the messenger RNA. The ecdysone-induced appearance of a major messenger RNA in late third instar larval fat bodies represents a developmental response to ecdysone that appears to be gene-specific, tissue-specific, and stage-specific, and it has exceptionally favorable features for further molecular studies of the control of gene expression by a steroid hormone.