The Drosophila TNF Eiger activates caspase-dependent necrosis when apoptosis is blocked

Abstract

Eiger (Egr), the homolog of the mammalian tumor-necrosis factor (TNF), is the ligand of the c-Jun N-terminal kinase (JNK) stress response signaling pathway in Drosophila. Although expression of Egr frequently leads to apoptosis, it has also been implicated in activation of non-apoptotic cell death. However, it is not yet clear how Egr can induce both apoptosis and non-apoptotic cell death, and if so, how such processes are coordinated. Here, we show that expression of Egr in the developing Drosophila eye induces apoptosis and non-apoptotic developmental defects, both of which are JNK-dependent. Intriguingly, when apoptotic effector caspases DrICE and Dcp-1 are defective or inhibited, expression of Egr induces necrosis characterized by loss of cell membrane integrity, translucent cytoplasm and aggregation of cellular organelles. Surprisingly, the induction of necrosis depends on the catalytic activity of the initiator caspase Dronc and the input from JNK signaling independently of their roles in apoptosis. Therefore, similar to the mammalian caspase-8, caspases in Drosophila also have dual roles in promoting TNF-mediated apoptosis and inhibiting necrosis.