Steroid regulation of programmed cell death during Drosophila metamorphosis

During Drosophila metamorphosis, pulses of the steroid hormone ecdysone trigger the histolysis of larval tissues in a stage-specific manner, but the mechanism of this developmental response remains unclear. In these studies, we show that histolysis of both the larval midgut and salivary glands is accompanied by increased nuclei permeability, DNA fragmentation and caspase activation, indicative of apoptotic cell death. Two Drosophila cell death activators, reaper (rpr) and head involution defective (hid), are induced by ecdysone in the larval midgut and salivary glands immediately preceding their histolysis, suggesting that the larval tissues are destroyed through the activation of the same apoptotic pathway that is employed during Drosophila embryogenesis. In addition, the cell death inhibitor diap2 is transiently induced in mid-prepupal salivary glands, but repressed in late prepupae, as rpr and hid are induced. These expression patterns suggest that ecdysone directs salivary gland cell death by simultaneously repressing diap2 and inducing rpr and hid. This hypothesis is supported by the results of our molecular genetic analysis which indicates that ecdysone triggers the switch in death gene expression through the ecdysone receptor and ecdysone-regulated transcription factors. Two ecdysone-inducible orphan nuclear receptors, E75A and E75B, are sufficient to repress diap2. In addition, ecdysone induces the Broad-Complex (BR-C) and E74A, which function together with the ecdysone receptor to induce rpr and hid. Our preliminary studies also show that two stress-activated transcription factors, AP-1 and NF-kappa-B, may function as stage-specific activators of rpr and hid in larval salivary glands. Based on these studies, we propose that the ecdysone receptor, ecdysone-induced transcription factors and the stress-response pathways cooperate to precisely regulate larval salivary gland cell death.