Ligand-independent requirements of steroid receptors EcR and USP for cell survival.

The active form of the Drosophila steroid hormone ecdysone, 20-hydroxyecdysone (20E), binds the heterodimer EcR/USP nuclear receptor to regulate target genes that elicit proliferation, cell death and differentiation during insect development. Although the 20E effects are relatively well known, the physiological relevance of its receptors remains poorly understood. We show here that the prothoracic gland (PG), the major steroid-producing organ of insect larvae, requires EcR and USP to survive in a critical period previous to metamorphosis, and that this requirement is 20E-independent. The cell death induced by the downregulation of these receptors involves the activation of the JNK-encoding basket gene and it can be rescued by upregulating EcR isoforms which are unable to respond to 20E. Also, while PG cell death prevents ecdysone production, blocking hormone synthesis or secretion in normal PG does not lead to cell death, demonstrating further the ecdysone-independent nature of the receptor-deprivation cell death. In contrast to PG cells, wing disc or salivary glands cells do not require these receptors for survival, revealing their cell and developmental time specificity. Exploring the potential use of this feature of steroid receptors in cancer, we assayed tumor overgrowth induced by altered yorkie signaling. This overgrowth is suppressed by EcR downregulation in PG, but not in wing disc, cells. The mechanism of all these cell death features is based on the transcriptional regulation of reaper. These novel and context-dependent functional properties for EcR and USP receptors may help to understand the heterogeneous responses to steroid-based therapies in human pathologies.

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