JNK signaling in Drosophila immunity and homeostasis

•JNK signaling determines stress responses and homeostasis in Drosophila.•Drosophila immunity is an ideal model to understand JNK activation and regulation.•JNK activation in Drosophila triggers tissue-specific cellular responses.•JNK responses in Drosophila are evolutionarily conserved. As members of the mitogen-activated protein kinase (MAPK) family, the c-Jun N-terminal kinases (JNKs) regulate cell responses to a wide range of extrinsic and intrinsic insults, including irradiation, reactive oxygen species (ROS), DNA damage, heat, bacterial antigens, and inflammatory cytokines. Particularly, JNK signaling regulates and promotes many important physiological processes that influence metabolic and tissue homeostasis, cell death/survival, and cell damage repair and ultimately impacts the lifespan of an organism. This diverse functionality causes a variety of tissue-specific and context-specific cellular responses, mediated by various cross talks between JNK and other cellular signaling pathways. Thus, highlighting its significance as a determinant of stress responses, JNK loss-of-function mutations have been implicated in a multitude of pathologies, including neurodegenerative diseases, diabetes, and cancer. Because JNK functions are specified in a context-dependent manner and can greatly vary, the underlying causes for these different outcomes remain largely unresolved despite the gained knowledge of many regulatory roles of JNK signaling during the past two decades. In Drosophila melanogaster, JNK signaling is conserved and required for immune responses, as well as the development for morphogenetic processes (embryonic dorsal closure and thorax closure). Therefore, Drosophila innate immunity provides the ideal model to understand the complex mechanisms underlying JNK activation and regulation. In the following, we review studies in Drosophila that highlight several mechanisms by which JNK signaling influences immunity and homeostasis.